\r\n\tThe study of populations and plant communities in their different aspects; ecological, structural, functional and dynamic, it is essential to establish a posteriori models of forest and agricultural management.
\r\n\r\n\tFor this, the methodological approaches on the type of sampling are considered essential, since there are differences between the purely ecological and the phytosociological methods, despite the fact that both pursue the same objective.
\r\n\tAlthough the ecological method for the knowledge of the vegetation is widely extended, the phytosociological one is no less so, since in the European Union it has been developed as a consequence of policies on sustainability, through which regulations have been issued, such as the habitats directive.
\r\n\tOn the other hand, research on plant dynamics and knowledge of the landscape in an integral way, have multiplied in the last 30 years, which has favored a deep knowledge of the floristic and phytocenotic wealth, which is fundamental for agricultural management, livestock and forestry.
",isbn:"978-1-83969-386-1",printIsbn:"978-1-83969-385-4",pdfIsbn:"978-1-83969-387-8",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"0abf2a59ee63fc1ba4fb64d77c9b1be7",bookSignature:"Dr. Eusebio Cano Carmona, Dr. Ricardo Quinto Canas, Dr. Ana Cano Ortiz and Dr. Carmelo Maria Musarella",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9662.jpg",keywords:"Climatic Factors, Bioclimate, Thermotype, Flora, Conservation, Phytocenosis, Plant Dynamics, Landscape, Cartography, Vegetation Series, Crops, Reforestation",numberOfDownloads:54,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"November 23rd 2020",dateEndSecondStepPublish:"January 25th 2021",dateEndThirdStepPublish:"March 26th 2021",dateEndFourthStepPublish:"June 14th 2021",dateEndFifthStepPublish:"August 13th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Dr. Cano Carmona and colleagues have directed 12 doctoral theses and more than 200 publications among articles, books, and book chapters. He has participated in national and international congresses with about 250 papers. He has held a number of different academic positions, including Dean of the Faculty of Experimental Sciences at the University of Jaen, Spain, and founder and director of the International Seminar on Management and Conservation of Biodiversity.",coeditorOneBiosketch:"Ricardo Jorge Quinto Canas is currently an Invited Assistant Professor in the Faculty of Sciences and Technology at the University of Algarve – Portugal, and a member of the Centre of Marine Sciences (CCMAR), University of Algarve. His current research projects focus on Botany, Vegetation Science (Geobotany), Biogeography, Plant Ecology, and Biology Conservation, aiming to support Nature Conservation.",coeditorTwoBiosketch:"Ana Cano Ortiz's fundamental line of research is related to botanical bioindicators. She has worked in Spain, Italy, Portugal, and Central America. It presents more than one hundred works published in various national and international journals, as well as books and book chapters; and has presented a hundred papers to national and international congresses.",coeditorThreeBiosketch:"Carmelo Maria Musarella is a biologist, specialized in Plant Biology. He is a member of the permanent scientific committee of the International Seminar on “Biodiversity Conservation and Management” guested by several European universities. He has participated in several international and national congresses, seminars, and workshops and presented oral communications and posters.",coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"87846",title:"Dr.",name:"Eusebio",middleName:null,surname:"Cano Carmona",slug:"eusebio-cano-carmona",fullName:"Eusebio Cano Carmona",profilePictureURL:"https://mts.intechopen.com/storage/users/87846/images/system/87846.png",biography:"Eusebio Cano Carmona obtained a PhD in Sciences from the\nUniversity of Granada, Spain. He is Professor of Botany at the\nUniversity of Jaén, Spain. His focus is flora and vegetation and he\nhas conducted research in Spain, Italy, Portugal, Palestine, the\nCaribbean islands and Mexico. As a result of these investigations,\nDr. Cano Carmona and colleagues have directed 12 doctoral theses\nand more than 200 publications among articles, books and book\nchapters. He has participated in national and international congresses with about\n250 papers/communications. He has held a number of different academic positions,\nincluding Dean of the Faculty of Experimental Sciences at the University of Jaen,\nSpain and founder and director of the International Seminar on Management and\nConservation of Biodiversity, a position he has held for 13 years. He is also a member of the Spanish, Portuguese and Italian societies of Geobotany.",institutionString:"University of Jaén",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"5",totalChapterViews:"0",totalEditedBooks:"1",institution:{name:"University of Jaén",institutionURL:null,country:{name:"Spain"}}}],coeditorOne:{id:"216982",title:"Dr.",name:"Ricardo Quinto",middleName:null,surname:"Canas",slug:"ricardo-quinto-canas",fullName:"Ricardo Quinto Canas",profilePictureURL:"https://mts.intechopen.com/storage/users/216982/images/system/216982.JPG",biography:"Ricardo Quinto Canas, Phd in Analysis and Management of Ecosystems, is currently an Invited Assistant Professor in the Faculty\nof Sciences and Technology at the University of Algarve, Portugal, and member of the Centre of Marine Sciences (CCMAR),\nUniversity of Algarve. He is also the Head of Division of Environmental Impact Assessment - Algarve Regional Coordination\nand Development Commission (CCDR - Algarve). His current\nresearch projects focus on Botany, Vegetation Science (Geobotany), Biogeography,\nPlant Ecology and Biology Conservation, aiming to support Nature Conservation.\nDr. Quinto Canas has co-authored many cited journal publication, conference articles and book chapters in above-mentioned topics.",institutionString:"University of Algarve",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"3",totalChapterViews:"0",totalEditedBooks:"0",institution:null},coeditorTwo:{id:"203697",title:"Dr.",name:"Ana",middleName:null,surname:"Cano Ortiz",slug:"ana-cano-ortiz",fullName:"Ana Cano Ortiz",profilePictureURL:"https://mts.intechopen.com/storage/users/203697/images/system/203697.png",biography:"Ana Cano Ortiz holds a PhD in Botany from the University of\nJaén, Spain. She has worked in private enterprise, in university\nand in secondary education. She is co-director of four doctoral\ntheses. Her research focus is related to botanical bioindicators.\nDr. Ortiz has worked in Spain, Italy, Portugal and Central America. She has published more than 100 works in various national\nand international journals, as well as books and book chapters.\nShe has also presented a great number of papers/communications to national and\ninternational congresses.",institutionString:"University of Jaén",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"6",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"University of Jaén",institutionURL:null,country:{name:"Spain"}}},coeditorThree:{id:"276295",title:"Dr.",name:"Carmelo Maria",middleName:null,surname:"Musarella",slug:"carmelo-maria-musarella",fullName:"Carmelo Maria Musarella",profilePictureURL:"https://mts.intechopen.com/storage/users/276295/images/system/276295.jpg",biography:"Carmelo Maria Musarella, PhD (Reggio Calabria, Italy –\n23/01/1975) is a biologist, specializing in plant biology. He\nstudied and worked in several European Universities: Messina,\nCatania, Reggio Calabria, Rome (Italy), Valencia, Jaén, Almeria\n(Spain), and Evora (Portugal). He was the Adjunct Professor\nof Plant Biology at the “Mediterranea” University of Reggio\nCalabria (Italy). His research topics are: floristic, vegetation,\nhabitat, biogeography, taxonomy, ethnobotany, endemisms, alien species, and\nbiodiversity conservation. He has authored many research articles published in\nindexed journals and books. He has been the guest editor for Plant Biosystems and a\nreferee for this same journal and others. He is a member of the permanent scientific\ncommittee of International Seminar on “Biodiversity Conservation and Management”, which includes several European universities. He has participated in several\ninternational and national congresses, seminars, workshops, and presentations of\noral communications and posters.",institutionString:'"Mediterranea" University of Reggio Calabria',position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"6",totalChapterViews:"0",totalEditedBooks:"1",institution:null},coeditorFour:null,coeditorFive:null,topics:[{id:"5",title:"Agricultural and Biological Sciences",slug:"agricultural-and-biological-sciences"}],chapters:[{id:"75595",title:"Assessment of the State of Forest Plant Communities of Scots Pine (Pinus sylvestris L.) in the Conditions of Urban Ecosystems",slug:"assessment-of-the-state-of-forest-plant-communities-of-scots-pine-pinus-sylvestris-l-in-the-conditio",totalDownloads:31,totalCrossrefCites:0,authors:[null]},{id:"76010",title:"Predictive Models for Reforestation and Agricultural Reclamation: A Clearfield County, Pennsylvania Case Study",slug:"predictive-models-for-reforestation-and-agricultural-reclamation-a-clearfield-county-pennsylvania-ca",totalDownloads:24,totalCrossrefCites:0,authors:[null]}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"247865",firstName:"Jasna",lastName:"Bozic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/247865/images/7225_n.jpg",email:"jasna.b@intechopen.com",biography:"As an Author Service Manager, my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"6893",title:"Endemic Species",subtitle:null,isOpenForSubmission:!1,hash:"3290be83fff5bc015f5bd3d78ae9c6c7",slug:"endemic-species",bookSignature:"Eusebio Cano Carmona, Carmelo Maria Musarella and Ana Cano Ortiz",coverURL:"https://cdn.intechopen.com/books/images_new/6893.jpg",editedByType:"Edited by",editors:[{id:"87846",title:"Dr.",name:"Eusebio",surname:"Cano Carmona",slug:"eusebio-cano-carmona",fullName:"Eusebio Cano Carmona"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"6418",title:"Hyperspectral Imaging in Agriculture, Food and Environment",subtitle:null,isOpenForSubmission:!1,hash:"9005c36534a5dc065577a011aea13d4d",slug:"hyperspectral-imaging-in-agriculture-food-and-environment",bookSignature:"Alejandro Isabel Luna Maldonado, Humberto Rodríguez Fuentes and Juan Antonio Vidales Contreras",coverURL:"https://cdn.intechopen.com/books/images_new/6418.jpg",editedByType:"Edited by",editors:[{id:"105774",title:"Prof.",name:"Alejandro Isabel",surname:"Luna Maldonado",slug:"alejandro-isabel-luna-maldonado",fullName:"Alejandro Isabel Luna Maldonado"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1591",title:"Infrared Spectroscopy",subtitle:"Materials Science, Engineering and Technology",isOpenForSubmission:!1,hash:"99b4b7b71a8caeb693ed762b40b017f4",slug:"infrared-spectroscopy-materials-science-engineering-and-technology",bookSignature:"Theophile Theophanides",coverURL:"https://cdn.intechopen.com/books/images_new/1591.jpg",editedByType:"Edited by",editors:[{id:"37194",title:"Dr.",name:"Theophanides",surname:"Theophile",slug:"theophanides-theophile",fullName:"Theophanides Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"371",title:"Abiotic Stress in Plants",subtitle:"Mechanisms and Adaptations",isOpenForSubmission:!1,hash:"588466f487e307619849d72389178a74",slug:"abiotic-stress-in-plants-mechanisms-and-adaptations",bookSignature:"Arun Shanker and B. Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"72",title:"Ionic Liquids",subtitle:"Theory, Properties, New Approaches",isOpenForSubmission:!1,hash:"d94ffa3cfa10505e3b1d676d46fcd3f5",slug:"ionic-liquids-theory-properties-new-approaches",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/72.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"314",title:"Regenerative Medicine and Tissue Engineering",subtitle:"Cells and Biomaterials",isOpenForSubmission:!1,hash:"bb67e80e480c86bb8315458012d65686",slug:"regenerative-medicine-and-tissue-engineering-cells-and-biomaterials",bookSignature:"Daniel Eberli",coverURL:"https://cdn.intechopen.com/books/images_new/314.jpg",editedByType:"Edited by",editors:[{id:"6495",title:"Dr.",name:"Daniel",surname:"Eberli",slug:"daniel-eberli",fullName:"Daniel Eberli"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"57",title:"Physics and Applications of Graphene",subtitle:"Experiments",isOpenForSubmission:!1,hash:"0e6622a71cf4f02f45bfdd5691e1189a",slug:"physics-and-applications-of-graphene-experiments",bookSignature:"Sergey Mikhailov",coverURL:"https://cdn.intechopen.com/books/images_new/57.jpg",editedByType:"Edited by",editors:[{id:"16042",title:"Dr.",name:"Sergey",surname:"Mikhailov",slug:"sergey-mikhailov",fullName:"Sergey Mikhailov"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1373",title:"Ionic Liquids",subtitle:"Applications and Perspectives",isOpenForSubmission:!1,hash:"5e9ae5ae9167cde4b344e499a792c41c",slug:"ionic-liquids-applications-and-perspectives",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/1373.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"56861",title:"The Multiple Roles of Tyrosinase-Related Protein-2/L- Dopachrome Tautomerase in Melanoma: Biomarker, Therapeutic Target, and Molecular Driver in Tumor Progression",doi:"10.5772/intechopen.70513",slug:"the-multiple-roles-of-tyrosinase-related-protein-2-l-dopachrome-tautomerase-in-melanoma-biomarker-th",body:'Cutaneous malignant melanoma (CMM) is a neoplasm generated through the malignant transformation of epidermal melanocytes, the cells which normally reside in the basal layer of the epidermis and produce the skin pigment melanin (Figure 1A–C). Noncutaneous melanomas can also develop at other sites populated by melanocytes such as choroidal layer of the eye, respiratory, gastrointestinal, and genitourinary mucosal surfaces, or the meninges. The main incriminating agent for causing CMM remains the UV radiation in interaction with host characteristics (Figure 1D). However, CMM may appear in skin areas that are not directly exposed to sun such as palms, soles, or under the nails, which demonstrates a pathogenesis more related to the noncutaneous melanomas. The incidence of CMM has been rising for the last 30 years around the world. Key statistics on CMM released by The American Cancer Society estimate that during 2017, in the US, about 87,110 new melanomas will be diagnosed (about 52,170 in men and 34,940 in women) and about 9730 people are expected to die of melanoma (about 6380 men and 3350 women) [1]. Although CMM makes only 4–7% of skin cancers, this neoplasm causes approximately 80% of skin cancer deaths. CMM is characterized by insidious and fast progression, heterogenic evolution among patients, and significant resistance to diverse therapeutic strategies. CMM is thought to develop in a stepwise manner being initiated with a benign nevus containing cell populations with intense proliferative capacities. Some of these lesions overcome the senescence-inducing signals, exhibit dysplasia (dysplastic nevus), and can progress further toward the malignant stages. The radial growth phase (RGP) is limited to epidermis and has a low invasive potential. In a more advanced stage, the melanoma cells migrate vertically up into epidermis and down into papillary dermis entering a new stage, the vertical growth phase (VGP). In metastatic stage, the tumor cells invade through blood or lymph vessels the distal organs (liver, brain, and lung) where they proliferate, eventually, causing death (Figure 1D). The activity of tumor cells is modulated by the complex and dynamic tumor microenvironment that can be extremely heterogenous among tumors of different patients. The multistep process of CMM progression is defined by a plethora of molecular events that are continuously explored, revised, and updated [2, 3].
Cutaneous malignant melanoma. (A) Schematic representation of epidermal melanocytes with melanosomes (black dots) exported to the surrounding keratinocytes. One melanocyte and 30–40 keratinocytes form the “epidermal melanin unit”. (B) The image of a human melanocyte obtained by confocal fluorescence microscopy of a human skin specimen immunostained for TYR and DCT. The common TYR-DCT staining is in the perinuclear region, whereas TYR staining is visible in dendritic tips too. (C) The image of a human epidermal melanocyte (HEM) in culture, obtained by bright field microscopy. (D) The risk factors for developing cutaneous malignant melanoma and the steps of neoplastic transformation and malignant progression of epidermal melanocytes culminating with the metastatic stage. Several molecular markers and processes emblematic for each tumor stage are indicated.
The only cure for melanoma is the surgical removal of early-stage tumors. For metastatic patients having the median overall survival less than a year, there are different strategies, including combined chemo-/radio- and vaccine therapies, extremely rarely leading to total cure and whose success depends very much on the staging accuracy. Major improvements in the metastatic treatment have been achieved due to advances in understanding the molecularity of this neoplasm. The modern alternative for melanoma evaluation and management is the analysis based on key genes or biomarker(s), pathways, diagnostic technologies, and potentially relevant therapeutics. These tend to replace current limited histological and microscopical evaluation introducing concepts such as “molecular melanoma subtypes” [4], “melanoma disease model (MDM)” [5], or “molecular diagnostic of melanoma” [6], aiming to bring together clinicians, researchers, and pharma for more efficient diagnostic, prognostic, and therapeutic strategies [7, 8]. Tyrosinase-related protein-2 (TRP2, TYRP2) or L-Dopachrome tautomerase (L-DCT) is a member of tyrosinase-related protein (TRP) family known for many years only for its enzymatic activity in the distal steps of melanogenesis. Studies emerging from different groups identified TRP2/L-DCT in relation to processes distinct from melanin synthesis (cell protection from environmental and therapeutic stress), melanoma diagnostic (potential biomarker), and therapy (immunotherapeutic target). TRP2/L-DCT is also expressed in precursors of peripheral nervous system associated with developmental processes and in glioma, a brain cancer similar to melanoma in terms of aggressiveness and therapeutic resistance and more recently, unexpectedly, in nonmelanocytic or nonneuronal cellular phenotypes.
This chapter aims to provide an updated status of TRP2/L-DCT in order to demonstrate its multiple implications in melanoma molecularity and therapeutic potential as well as to open up new perspectives for a better understanding of other molecular processes and pathologies. For simplicity, we will further refer to TRP2/L-DCT as DCT.
TRPs are type I transmembrane N-glycoproteins. Their polypeptides share significant aminoacid sequence homology and similar patterns of polypeptide chain organization, an amino-terminal signal sequence (residues 1–23 in human DCT) followed by a lumenal domain (aa 24–439), a transmembrane (TM) hydrophobic region (aa 473–493) that inserts the protein into subcellular membranous structures and a carboxi-terminal cytoplasmic (CYT) tail (aa 494–519) interacting with the elements of the sorting and traffic machinery. The lumenal domain encompasses the enzymatic active site shaped by two highly conserved metal-binding regions (MeB1 and MeB2) molded at the core of a four-helical bundle. Interspersed with these two metal-binding regions are two Cys-rich regions (Cys1 and Cys2). Cys1 precedes MeB1 and contains 10 Cys residues conserved only in the human TRPs, and Cys2 located between MeB1 and MeB2 contains six Cys residues of which five are conserved in the human TRPs. Unfortunately, none of the human TRPs have been crystallized, but models of human tyrosinase have previously been developed [9]. Using a similar protocol and based on the high degree of sequence homology among TRPs (about 60% on the entire sequence and 66% in the lumenal domain only), we built a structural model for the lumenal domain of human DCT using as templates the available X-ray structures of tyrosinase proteins from
Sequence alignment of human TRPs (TYR, TRP1, and DCT) with the X-ray templates used for modeling DCT (PDB codes 3AX0 and 3NM8). Identical/similar residues between DCT and other sequences are highlighted dark/light gray, metal binding His residues are highlighted black. Assigned/predicted secondary structure elements for templates/DCT are shown above and below the alignment. Membrane pictogram indicates location of (predicted) transmembrane region in all proteins. The rectangles indicate the two Metal-binding regions (MeB1, MeB2). Symbols indicate various functionally relevant residues: stars = phosphorylated residues; diamond = methylated R409 residue; dark triangles = putative N-glycosylation sites, light triangles = experimentally confirmed occupied sites in DCT; arrows = Cys residues. Signal sequence in DCT is thin underlined. The DCT-derived peptides 60–74 [
Despite this high degree of sequence homology between DCT and other human TRPs, distinctive DCT features regarding overall hydrophobicity and charge profiles, active site stereochemistry and composition, N-glycosylation, or phosphorylation patterns generate significant differences in protein function, interaction partners, and sorting/trafficking pathways.
Although the two metal-binding regions in the lumenal domain represent a highly conserved feature of TRP family, DCT has a unique preference for zinc instead of copper, as is in the case of TYR. Purified DCT contains two Zn atoms per protein molecule as measured by atomic absorption spectroscopy and Zn2+ chelation experiments. Zn2+ is the crucial element that accounts for the tautomerization of L-Dopachrome tautomerase [12]. The enzyme DCT reconstituted with Cu2+, which is the cofactor for TYR, or with Fe2+, is inactive, whereas with Co2+ is partially active. Unlike the native DCT, which shows a very strict specificity for L-Dopachrome and for which neither dopaminochrome nor D-Dopachrome are suitable substrates, the reconstituted enzyme is stereospecific as well but is also able to rearrange D-Dopachrome into DHI [13]. At this point, it is important to specify that there is also a D-Dopachrome tautomerase (D-DCT, or D-DT) which is decarboxylating D-Dopachrome to DHI. There is no structural or functional relation between L-DCT and D-DT, which is a circulating cytokine, member of macrophage migration inhibitory factor (MIF) protein superfamily with an overlapping functional spectrum with MIF. Within lumenal domain of human DCT, there are 16 cysteine (Cys) residues, clustered into three regions, the first two located N-terminal to MeA and the third between MeA and MeB. In addition to these clustered Cys residues, single Cys residues may be found in the C-terminus cytoplasmic tails of TYR and TRP1 but not of DCT, which indicates a TYR-TRP1 interaction via intermolecular disulfides without DCT participation [14]. This finding is in agreement with our experimental data, showing that DCT does not share common subcellular structures with TYR or TRP1 (see Section 2.3.1.2) and does not support the early theory that all TRPs are possibly interconnected via intermolecular disulfides. Despite the fact that the number of N-glycosylation sites is almost the same in human TYR (seven sites) and DCT (six sites) and they are all located in the lumenal domain, glycosylation pattern is significantly different between TYR and DCT. In the case of human TYR, occupancy of six of the seven sites was demonstrated by site-directed mutagenesis [9], while in the case of DCT, only two sites (N300 and N342) have been experimentally confirmed to be occupied [15] by MALDI/TOF of a truncated version of protein expressed in insect cells. Both N-glycosylated sites in DCT are located in close vicinity (on opposite sides) of the metal containing active site, possibly influencing ligand access within, but only N300 is conserved in all human TRPs while equivalent of N342 is found only in TRP1 not in TYR. The first two N-sites of TYR, which are required for TYR entry in the CNX cycle [16] are not present in DCT, which further supports the idea that TYR and DCT take different intracellular processing pathways. Indeed, our experimental data confirmed that folding pathways, which in all TRPs are dependent on the step of N-glycan processing, are differently regulated within the same cell phenotype and have further distinct impact on their trafficking and stability (see Section 2.3.1.2). Additional unique characteristics of DCT post-translational modifications refer to the methylated residues. A recent large-scale mass spectrometry analysis of arginine-methylated peptides in human T cells [17] demonstrated methylation of R409 in DCT (indicated by a diamond in the alignment in Figure 2), located at the end of the second metal-binding region. Structurally, this positively charged residue is positioned in the luminal domain and oriented toward the melanosomal membrane (Figure 3), thus likely to interact with the negatively charged head groups of membrane phospholipids. Addition of a methyl group to R409 would shield the positive charge and decrease probability of luminal domain interacting with membrane. Surprisingly, although this residue is conserved in all human TRPs, the same study could not identify similar modification of corresponding residues in the other members of the family. This post-translational modification of DCT could have an impact on interactions between DCT and sorting/traffic machinery and subsequently on DCT intracellular routes. The same study [17] demonstrates that changes in arginine methylation stoichiometry during cellular stimulation in a subset of proteins are critical to T cell differentiation. DCT is a tumor antigen, and several peptides derived from it were identified as targets of CD4+ or CD8+ T-lymphocytes, and their position within DCT sequence is presented in Figure 2 [18, 19, 20, 21, 22]. Whether DCT-methylated peptides could be a part of the peptide-methylated pool involved in triggering T-cell differentiation in melanoma would represent a subject worthwhile to be further investigated. Other distinctive features of DCT TM domain are the presence of cholesterol (CRAC) and caveolin-binding motifs, which supports the idea of an interaction with these membrane components. Our detailed computational analysis using various sequence bioinformatics, structural modeling, and molecular simulation approaches allowed us to generate the first complete structural model of DCT in interaction with caveolin-1. This model revealed DCT-specific structural determinants involved in interaction with membranes having specific compositions and possibly regulating its enzymatic activity and intracellular trafficking, as well as its participation in complex processes as signaling pathways [23] (Figure 3). The overall model advocates for an interaction between Cav1 and DCT mediated by two distinct regions, one within the membrane (hydrophobicity-driven interaction) and the second cytosolic (electrostatics-driven interaction). The CYT DCT domain is predicted to adopt an extended, possibly disordered conformation and has a net positive charge (7 basic and 3 acidic residues out of 26) whose distribution is complementary to that of Cav1 cytosolic region carrying a negative formal charge, which strongly supports the electrostatic interaction between these regions, facilitated by salt bridges (Figure 3, thin lines). Interestingly, the DCT charge distribution in the CYT domain may be modified by the phosphorylation state of two adjacent serine residues (S511, S512 pointed by stars in Figure 2 and indicated by dotted van der Waals spheres in Figure 3) whose phosphorylation was experimentally confirmed by mass spectrometry [24]. We can speculate that phosphorylation of these unique sites may represent a control mechanism for modulating DCT interaction with Cav1 or with other molecules involved in trafficking/sorting/signaling pathways. However, the presence of these interactors would need to be confirmed by additional experimental approaches.
Structural model of DCT protein (cartoon representation) interacting with membrane bilayer and caveolin-1 (Cav1). In the lumenal domain (above membrane) the helical segments indicate the two metal-binding regions, containing two Zn2+ ions (shown as opaque spheres). Putative N-glycosylation sites are depicted using thick sticks. Representative structural models of N-glycans (shown as transparent spheres) are attached to glycosylation sites experimentally shown to be occupied (N300 and N342). Methylated R409 (within lumenal domain) and phosphorylated S511, S512 (within cytosolic membrane) are shown as dotted spheres. Within DCT transmembrane region, aromatic residues F487, F492 (thick sticks) and Y495 (behind helical structure) form the Cav1-binding motif. Charged residues in the cytosolic regions of DCT and Cav1 are labeled and shown as sticks, and putative salt bridges are depicted by thin gray lines connecting oppositely charged residues.
To understand more deeply the specific behavior of TRPs in interaction with cholesterol-rich membranes, we performed molecular dynamics simulations (60 ns) of TYR and DCT TM segments embedded in 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) lipid bilayers in the presence and absence of cholesterol. The 3D structures of TM domains were modeled ab initio as α-helices whose length was based on sequence hydrophobicity and helix propensity profiles which indicated that TYR TM is slightly longer (~4 residues, one helical turn) than DCT TM. Although the two TM domains had identical initial positions and orientations in the membrane bilayer, and the overall helical structure is maintained throughout the entire 60 ns simulations, the TYR TM adopted a more tilted inclination (measured by the angle between α-helix central axis and axis normal to bilayer plane) compared to DCT (upper panels in Figure 4A). The magnitude of this tilting effect is likely correlated with the length of the hydrophobic helix segment that needs to fit within the membrane thickness; therefore, the orientation of shorter DCT helix is closer to normal axis while TYR is more tilted (see plot in Figure 4B). As expected, tilting is less pronounced in cholesterol-containing membrane due to its increased thickness (lower panels in Figure 4A). Surprisingly, cholesterol affects helix translation within membrane in a different manner: while in the cholesterol-free membrane both proteins experience similar levels of helix translation, in cholesterol-containing membrane, DCT translation is highly restricted while TYR translation is only slightly affected, suggesting that cholesterol interacts more tightly with DCT, possibly due to the presence of CRAC signature. This would explain the preferential DCT sorting into CRD domains and distinct trafficking along the secretory pathway (see Section 2.3.1.2). This study, presented here for the first time, is one of the few simulation studies on the importance of cholesterol for TM type I protein stability and trafficking. The DCT structural determinants account for its distinct intracellular processing and biological functions.
Molecular dynamics simulations (60ns) of transmembrane segments of human TYR/DCT embedded in pure POPC bilayer compared to cholesterol-containing membranes. (A) Structural representation of the transmembrane helix (shown as cylinder) every 2ns in the membrane bilayer; and (B) plot of the helix tilt angle variation during simulation, indicating higher tilt for TYR compared to DCT.
DCT is expressed preponderantly in melanocytes, which originate from neural crest cells (NCC) and migrate during embryonic development to different regions (Figure 5). There are also melanocytes in retinal pigmented epithelium (RPE) that originate from the forebrain neuroepithelium and in which DCT expression has also been confirmed [25]. DCT is detected in melanoblast, the progenitor of melanocyte, at embryonic day (E) E9.5, in a SOX10-melanoblast/glial bipotent progenitor, together with microphthalmia-associated transcription factor (MITF) and KIT, whereas TYR or TRP1 are expressed later in the development [26]. In hair follicle, DCT expression has been associated with a pool of melanocytes having stem cell traits of self-renewal and multipotency within the lower permanent proliferation portion of this tisssue [27]. In the precursors of peripheral nervous system which derive also from NCC, the spatial and temporal profiles of DCT expression correlate with neurogenesis during embryonic development and enhance the proliferation of cortical neural progenitor cells and neuroblast migration [28]. A unique cell population called melanocyte-like cells, found within murine and human hearts, that is distributed to the pulmonary veins, atria, and atrioventricular canal, also expresses DCT but has transcriptional profiles distinct from dermal melanocytes. The presence of these DCT-positive cells has been connected with the clinical syndrome of atrial ectopy initiating atrial fibrillation, autonomic dysregulation, and oxidative stress. It seems that DCT-cardiac melanocytes are involved in maintaining the normal balance of oxidative species in the myocardium [29]. The DCT expression is also retained in the malignat counterparts derived from melanocytes and neuronal cells as melanoma retinoblastoma [30], glioma [31], and glioblastoma [32]. Moreover, the neoplastic cells express different DCT transcripts and in higher amounts compared with the normal cells. For example, in patients with glioma, the DCT mRNA transcripts are in excces of 100,000-fold over that in healthy brain [33]. In amelanotic melanoma cells, in which TYR and TRP1 are downregulated or enzymatically inactive, DCT is well expressed [34] and during melanoma malignant progression, DCT expression, unlike TYR or TRP1, remains constant [35]. A recent study presents that DCT is endogenously expressed in HaCaT cells (basal keratinocytes) [36] which has an electrophoretic pattern comparable with DCT in RPE lysate, but distinct from the 68/80 kDa DCT doublet expressed by melanoma cell lines [37, 38]. A significant number of commercially available anti-DCT antibodies include in their technical data sheets, as positive controls for endogenous DCT, cell lysates, or histopathological specimens from cell lines or neoplasms in which DCT is not expected to be expressed such as A431-epidermoid carcinoma (Sigma-Atlas); NBT-II-Nara bladder tumor cells, WEHI-231 B cell line, CTLL-2-cytotoxic lymphocyte (Santa Cruz), human liver cancer tissue lysate, K562 (leukemia) lysate, K-562-chronic myelogenic leukemia, A549-lung carcinoma, HeLa-cervical cancer (Abcam); MCF7 cells-breast cancer, HL-60 cells-caucasian promyelocytic leukemia (Proteintech Group); human cervical cancer tissue (OriGene). Most of them show in WB analysis bands of approximately 50 kDa or/and 30 kDa. Two hypotheses can explain these data: (1) the 50/30 kDa bands are not DCT but possibly contaminants detected due to antibodies cross-reactivity. This would be very unlikely because these antibodies have been raised against different DCT sequences, by different technologies, in different laboratories. However, as many of these antibodies do not show data on these cells having “DCT gene” downregulated or amplified (with specific primers for DCT mRNA), their specificity is still questionable and may induce false-positive results with severe consequences especially in clinic; (2) the 50/30 kDa in nonmelanocytic/-neuronal cells or tissues are indeed derived from DCT (possibly isoforms or degradation products). DCT is expressed in neural crest progenitors that generate multiple cell lineages during development. The demonstrated DCT involvement in anti-apoptotic and stress-resistance pathways (Section 2.4) would qualify it for activated expression in cellular niches of different normal or transformed phenotypes where it would be requested to sustain specific processes. For example, osteopontin, primarily expressed in bone cells (osteoblasts) has become a well-known marker for various neoplasms, including melanoma, where its expression is associated with tumor progression [39]. HaCaT is an immortalized keratinocyte cell line with a high capacity to differentiate and proliferate in which endogenous DCT has detoxification biological activities similar to those already described in melanocytic lineage [36]. These new data consolidate the theory that DCT expression may encompass, indeed, multiple cell phenotypes where it accomplishes, very likely, functions related to cell protection. How is DCT expression activated and modulated in nonmelanocytic/-neuronal cells are questions whose clarification require additional studies. Morevoer, the DCT expression in nonmelanocytic lineages would raise the question whether DCT can still be considered a specific biomarker for the diagnosis of melanocytic lesions.
DCT cellular and tissular expression. DCT is primarly expressed by melanocytic (continous line) and neuronal (interrupted line) cells and by their malignant counterparts (dotted line). DCT possible expression in cells of nonmelanocytic origin is also indicated.
The human DCT gene (h-DCT) has 55-kb and was mapped to the chromosomal region 13q31-q32 with a coding region of eight exons all encompassing the open reading frame of the protein [40]. The h-DCT is controlled by the two separate regulatory regions: the 32-bp element and the proximal region [41]. The 32-bp element is a composite enhancer having potential binding sites for transcription factors that contain a basic helix-loop-helix structure (including Microphthalmia-associated transcription factor—MITF), a high-mobility-group (HMG) domain (the TCF/LEF-1 or SOX family), or an Ets domain [42]. MITF is a master regulator of pigmentary system [43], and there is a selective requirement for MITF-M isoform for melanocyte development. The promoter region of MITF-M contains CREB, SOX10, PAX3, and LEF-1 binding sites. The presence within DCT promotor of the 32-bp element containing a CAATTG motif do not produce significant transactivation by MITF, as in case of the other TRPs, suggesting that the mechanism for melanocyte-specific transcription of the DCT gene is different from that of the other TRPs [44]. In addition to MITF, DCT is regulated by SOX10, which is a high-mobility-group transcription factor that plays a critical role in many processes in neural crest cells, including multipotency, proliferation, apoptosis, survival, and commitment to defined neural crest-derived lineages. SOX10 transiently regulates DCT expression during early melanocyte development, independently of MITF function [45] and synergistically with MITF that enhances SOX10-dependent activation of the DCT promoter [46]. Another member of the SOX family, SOX5, inhibits the SOX10-stimulated activity of the DCT promoter in melanocytes [47]. A synergistic transactivation of DCT gene promotor results also from cooperation between TLEF-1 and MITF or between TLEF-1 and TFE3, a MITF-related protein [48]. The TCF/LEF-1 family regulates target gene transcription in response to Wnt signals. The transcriptional regulation of DCT involves also PAX3, a member of a highly conserved family of transcription factors essential to the development of many tissue types throughout embryogenesis and vital to the maintenance of several stem cell niches. Unlike MITF which is an activator of DCT expression, PAX3 inhibits both DCT expression and the ability of MITF to bind to the DCT promoter. PAX3 forms a repressor complex with LEF1 and GRG4 on the DCT enhancer sequence and actively blocks MITF binding. In the presence of beta-catenin, LEF1 forms a complex with MITF and beta catenin and displaces PAX3 from DCT enhancer [49]. Oppositely, SOX10 does not cooperate with PAX3 to activate DCT in combination with PAX3 [50]. OTX2 is a transcription factor that regulates the specific expression of DCT gene in REP. OTX2 binds to the DCT gene promoter
TRPs follow the general secretory pathway: TRP-polypeptide synthesis and folding in endoplasmic reticulum (ER), the N-glycan maturation along the Golgi complex and transport to the steady-state destination, the melanosomes, the site of melanin synthesis and storage. In parallel with our early research on TRP1 intracellular processing in murine melanoma cells [54], studies of other groups were presenting a specific drug-and UV-resistance mediated by TRP2/DCT in melanoma [55, 56, 57]. In this context, we considered that deciphering the intracellular processing pathways of DCT would bring fundamental knowledge and possible exploitable information into melanoma development and therapy. The immunofluorescence microscopy images and ultracentrifugation data reveal a unique pattern of DCT subcellular distribution. Unexpectedly, DCT is detected in high amounts in a perinuclear position, co-localizing with the TGN marker, syntaxin 6, and in substructures at plasma membrane (PM), showing weak overlapping with late melanosome markers TRP1 and Rab27a. The maturation kinetics and traffic along the secretory pathway show that ER DCT 68 kDa precursor containing high-mannose N-glycans moves along the Golgi where it acquires complex structures, gradually turning into the DCT 80 kDa mature protein, within approximately 3 h [37] compared to 45 min in which TRP1 becomes a fully glycosylated 75 kDa protein [54]. Similar to TYR and TRP1, DCT interacts with the ER lectin chaperone calnexin that assists normal polypeptide folding of all TRPs [37]. In N-glycoproteins, the glycan procesing in ER interferes with polypeptide folding. The step of N-glycan trimming by glucosidase I and II results in the formation of a monoglucosylated precursor that interacts with the ER lectin chaperones, calnexin, or calreticulin, which assist the polypeptide folding. The inhibition of glucosidase I and II with N-butyldeoxynojirimycin (NBDNJ) perturbs N-glycosylation, resulting in a triglucosylated precursor unable to interact with calnexin. In NBDNJ-treated cells, TRP1 folds in the absence of interaction with calnexin, being rescued by another ER chaperone BiP, leaves ER, and moves along Golgi [54], whereas in the same cells, TRP2/DCT conformation is severely altered, and the misfolded protein is targeted to proteasomal degradation [37]. A more recent study reports that the treatment of Melan-a cells, with the chemical compound, A3B5, results also in proteasomal degradation of DCT but not of TYR [58]. Whether DCT from A3B5-treated cells is targeted to proteasome from the ER, via the well-known retrotranslocation pathway or from a post-ER compartment remains to be further investigated. In any case, this is an additional proof that, indeed, DCT fate in melanoma is distinctly regulated from the other TRPs. Additional information about the DCT biosynthetic pathway came from our investigations of the two human amelanotic melanoma cell lines, MelJuSo (MJS) and SKMel28 (SK28) [23]. In SK28, as in other amelanotic cell lines, pH homeostasis is altered, and TYR is retained in the secretory pathway and prematurely, proteasomally degraded [59]. Importantly, in both MJS and SK28, amelanotic cell phenotypes DCT appears at steady state as a mix of the fully processed protein and the partially glycosylated precursor. This pattern indicates that a significant DCT amount is able to overcome the pH-induced blockade being sorted from the early steps of its biosynthetic pathway in a different cargo than TYR. Our experimental data demonstrate that DCT maturation between ER and Golgi is interrupted or pertured in the presence of nystatin [23] or monensin [60], two pharmacological agents that disrupt CRDs or insert in Golgi CRDs, respectively. A significant amount of DCT is detected by co-localization and co-immunoprecipitation experiments in complexes with Cav1, an abundant component of CRDs. The association of DCT with Cav1 and cholesterol is supported by our structural analysis (detailed in Section 2.1). Cav1 downregulation has a profound regulatory impact on DCT and subsequently on its entire biosynthetic pathway [23] (detailed in Section 5.3). Our theory is that a significant fraction of DCT is sorted in the early secretory pathway, possibly from ER, in CRDs with Cav1, in a cargo without TYR and trafficked on a route less sensitive to amelanotic acidic pH. Our data is supporting the concept of the selective ER exit sites and ER-Golgi transport [61] and that production of specific lipids might have a regulatory role in cargo recruitment and export from ER [62]. Another cellular parameter regulating DCT processing, between ER and Golgi is the intravesicular pH. The treatment of B16F1 pigmented melanoma cells with bafilomycin (Baf), a specific inhibitor of v-ATPases and pH corrector, slightly increases the amount of DCT mature complex protein [60]. This demonstrates that pH of the secretory pathway is altered in pigmented phenotypes as well, but to a less extent than in amelanotic cells and that only a DCT fraction is trafficked on a route sensitive to pH alterations too. We also found that DCT maturation between ER and Golgi is interrupted by microtubule depolymerization agent nocodazole (NCZ) when DCT is prevented to reach medial Golgi and remains in the form of the 68 kDa precursor [unpublished data]. Post-Golgi, the membrane composition and the interaction of the sorting and trafic machinery with the CYT tail of TRPs decide their destination [63, 64]. The di-Leu motif (QPLLMD) present in both cytoplasmic tails of TYR and TRP-1 and specifically requested for the interaction with the AP-3/AP-1 sorting elements in post-Golgi compartments is absent from DCT CYT domain which has Tyr-like motif (YRRL). The detection of DCT in TGN area and at PM in both murine and human melanoma cell lines with two distinct antibodies and the low amounts in mature melanosomes [23, 37] support the theory that post-Golgi DCT is trafficked on a distinct route than TYR or TRP-1, possibly being recycled from PM via a recycling endosomal (RE) compartment. Interestingly, in GL261 mouse glioma cell line DCT is also detected at PM, which may indicate a post-Golgi common route for DCT in different tumor cells [65]. We discovered an unexpected effect of the lysosomotropic agent chloroquine (CQ) on DCT stability, from both murine and human cell lines. CQ, a well-known pharmacologic agent that accumulates within acidic compartments, usually recommended as inhibitor of lysosomal enzymatic machinery [66] was expected to block DCT constitutive degradation. Instead, we found that DCT amount synthetized within 30 min (pulse), after 3 h (chase), in the presence of added CQ is diverted to a premature degradation pathway, whereas TRP1 stability is not affected in the same cell line. This is not an artifact, given that DCT degradation can be prevented in CQ-treated cells if Baf is present in the system. It is worth mentioning that DCT degradation is significantly decreased if CQ is added at 6 h chase, when probably DCT is in a more protected compartment. The effects of CQ in living systems are pleiotropic, and many of its action mechanisms or targets are still unknown. CQ interferes with the trafficking [67] and recycling processes from PM [68] or with the fusion vesicular processes, by enhancing the rate of the phagolysosomal fusion [69]. Our theory about CQ impact on DCT fate is that in our experimental conditions (mild CQ concentration, 50 μM and short time period treatment of 2 h), CQ potentiates the fusion between a DCT-positive post-Golgi endosomal compartment with a still proteolytically active one, most likely the lysosomes. It will be also interesting to identify which other proteins share the DCT fate in CQ- treated melanoma cells or if the effect of CQ is similar in other cells phenotypes expressing endogenous DCT.
One of the early events in neoplastic transformation of melanocytes is the uncontrolled proliferation. During this step, tumor cells secrete numerous cytokines and growth factors, which can regulate back the tumor cells activities, by binding to self-receptors (autocrine stimulation) or receptors of neighboring cells (paracrine stimulation) and self-sustaining tumor growth signals. In addition, the nutrient deprivation and numerous homotypic cell-cell contacts, established as a result of the alterations that occurred in cell adhesion molecule repertoire, result in activation of multiple signaling cascades. A similar situation to autocrine/paracrine stimulation is simulated in an
The intracellular journey of DCT in melanoma cells. The DCT biosynthetic pathway within a melanoma cell is schematically presented. All checkpoints along this route are indicated by triangle symbol. The DCT polypeptide is synthetized and folded in ER assisted by lectin chaperone calnexin (Clx). The interruption of N-glycan processing in ER with NBDNJ prevents interaction with Clx. TRP1 is further processed beyond the ER, whereas DCT is targeted to proteasomal degradation (1st checkpoint). Between ER and Golgi, DCT maturation is blocked by disrupting agents of cholesterol-rich domains (CRD) (nystatin-Nys, monensin-Mon) and microtubules (nocodazole-NCZ), intravesicular pH (bafilomycin—Baf) and caveolin-1 (Cav1) downregulation (2nd checkpoint). Post-Golgi, DCT, unlike TRP1, is diverted into a premature degradation pathway induced by CQ treatment (3rd checkpoint). Nutrient deprivation, secreted factors during proliferation and Cav1 gene down regulation are activators of DCT, not of TYR or TRP1, expression (4th checkpoint). Possible DCT recycling route from PM is presented as segmented line. TRP1 post-Golgi route to melanosomes stage III/IV is shown as intrerupted line.
Melanins represent a group of polymers produced by both normal and transformed melanocytes. The skin melanins are synthetized and deposited within melanocyte-specialized cellular organelles called melanosomes that are finally transferred into epidermal keratinocytes ensuring not only skin pigmentation but also UV light absorption and scattering, free radical scavenging, coupled oxidation-reduction reactions, and ion storage [71]. TRPs are the main regulators of principal steps of melanin polymer formation (Figure 7). TYR is the key-enzyme of melanogenesis that catalyzes the hydroxylation of L-Tyrosine to L-3,4-dihydroxyphenyl alanine (L-DOPA). L-DOPA is rapidly oxidized to DOPAquinone that spontaneously undergoes cyclization to Dopachrome. In the absence of any enzymatic activity, Dopachrome loses carboxylic acid generating 5,6-dihydroxyindole (DHI). TRP2 or L-Dopachrome tautomerase (DCT) acts downstream of TYR by rearranging Dopachrome into DHI-2-carboxylic acid (DHICA) that is further oxidized to the corresponding quinone by the activity of TRP1 in mouse or by TYR in humans. In 1992, Jackson and colab reported the cloning and sequencing of mouse cDNA corresponding to the region of the mice coat color mutation slaty. The gene product was named tyrosinase-related protein-2 (TRP-2) due to its high degree of amino acid identity with the other TRPs [72] or Dopachrome tautomerase (DCT) due to enzymatic activity on Dopachrome [73]. DCT is now well acknowledged as the modulator of melanin qualities. L-Dopachrome is the second branch point which under the unique L-DCT action is transformed into DHICA (Figure 7). Melanin derived from oxidation and polymerization of DHI, formed in the absence of DCT are black and insoluble, whereas the DHICA-enriched melanins that contain a higher proportion of carboxylated versus noncarboxylated indolic monomers are brown and more soluble [74]. Despite of numerous mutations identified in other melanosomal proteins, with consequences on pigmentation, no mutations have been described in human DCT, suggesting this is a conserved protein. However, in mouse, mutant alleles of DCT are associated with pigment dilution, producing the slaty (R194Q substitution in the MeA binding domain) and slaty light (G486R substitution in the TM domain) phenotypes. DCT mutations increase pheomelanin and reduce eumelanin produced by melanocytes in culture showing that the enzymatic activity of DCT play a role in determining whether pheo-or eu-melanin pathway is preferred [75]. The intermediates generated during melanogenesis have genotoxic [71] and immunosuppressive properties [76]. DHI is a cytotoxic melanin precursor [77], whereas DHICA is an antioxidant molecule [78], a diffusible chemical messenger [79], and DHICA unlike DHI melanins exhibit potent hydroxyl radical-scavenging activity (Figure 7). Moreover, eumelanins bind calcium with an affinity similar to calmodulin and thus interfere with the intracellular calcium regulation [80]. DCT, as a specific limiting factor of DHI concentration and DHICA-eumelanins formation becomes thus a modulator of different processes in melanocyte in which DHICA and DHICA-melanins are involved. To establish the general impact of DCT on a living organism, the DCT gene was targeted during mouse embryonic development [81]. The DCT-KO mice are viable, have a diluted coat color phenotype, due to reduced melanin content in hair but do not show any decrease in melanocyte numbers. However, under chronic UVA-induced oxidative stress in skin of DCT-KO mice compared with wild-type, the level of reactive oxygen species (ROS) and the numbers of apoptotic cells are increased, whereas the amount of eumelanin is decreased [82]. This demonstrates that, in melanocytes, DCT is involved in regulating a protective pathway in response to environmental stressful conditions. The DCT protective effect seems not to be exerted only via its enzymatic activity. The extremely low growth rate for the DCT-slaty and DCT-slatylight melanocytes could not be abgrogated in the presence of catalase, added to culture medium to overcome effects of H2O2 resulted from DHI excess due to inactivity of mutated DCT [83]. In transformed melanocytes, DCT is a tumor protector as well. In pigmented melanoma, as in melanocytes, DCT generates DHICA and further DHICA-eumelanins, both exerting the antioxidant properties (Figure 7). However, DCT protective activity is independent of melanin pathway, and this is in good-agreement with finding that DCT is well-expressed in amelanotic cell lines and tumors [34, 35]. In a process of identification of genes associated with cis-diamminedichloroplatinum (II)(CDDP)-and X-ray resistance in the amelanotic melanoma cell line WM35, Bed-David’s group found that DCT expression was upregulated in both CDDP- and X-ray resistant mutants compared with the parental line [84]. On the other hand, DCT ectopic overexpression in melanoma cells abrogates UVB-induced apoptosis [57]. DCT-drug resistance-mediated pathway is related to antitumorals that interferes with DNA replication as CDDP, carboplatin, or methotrexate and is not effective to the ones acting on microtubule formation as paclitaxel. In correlation with our data about DCT intracellular processing, we can speculate that DCT-mediated tumor resistance to the microtubule depolymerizing agents, unlike the one to DNA-alkylating agents, requires mature DCT and not DCT precursor which is the only DCT glycoform in cells treated with microtubule depolymerization agents (Section 2.3.1.2). DCT-radiation resistance is addressed to both X- and UVB-radiation that act on DNA by creating DNA strands and causes the formation of pyrimidine dimers, respectively, and are independent of TYR or TRP1 expression or melanin content [85]. DCT protective effect may be explained by either interference with DNA repair mechanisms or the regulation of anti-apoptotic pathways. DCT anti-apoptotic activity has also been reported in AJS sensory neurons in
The processes mediated by DCT in different normal and malignant cell phenotypes.
Although the object of this chapter is DCT in melanoma, we consider that it is of importance to discuss the role of DCT in other cell lineages. We have argued about DCT expression in HaCaT cells (basal keratinocytes) [36] (Section 2.2). The effects of DCT downregulation in HaCaTs are similar to the ones reported so far in melanocytic cells, namely increased ROS levels, DNA damage, and altered cell cycle, which furthermore compromise the infection of these cells with HPV. There are several common processes, mainly related to cell protection, with which DCT interferes, regardless the cell phenotypes in which it is expressed. However, these processes are involved in cell-specific responses to different aggressors (e.g., therapeutic stressors in melanoma and viral infection in basal keratinocytes).
The diagnostic and prognostic of CMM is in general evaluated histopathologicaly. In particular cases, when it is difficult to discriminate between melanocytic lesions and other resembling tumors as sarcomas, lymphomas, or neuroendocrine tumors, the expression of melanocytic biomarkers is requested, and they are commonly assessed by immunohistochemistry. For patients with unambiguous tumor histologic features, the CMM prognostication relied on Breslow’s index, the level of invasion in skin layers (Clark’s level), growth pattern (nodular, superficial spreading, etc.), dimensions, and presence/absence of ulceration information proves to be statistically significant in very large clinical cohorts [91]. The panel of melanoma markers is continuously revised and improved in accordance with the new discoveries related to the molecular mechanisms and pathways in melanoma progression [92]. One of the most challenging is the thin melanoma subset, defined by Breslow depth, 1.0 mm representing patients with early-stage disease. Despite that most are thought to have an excellent clinical outcome (85% survival during a 10-year period) and can be treated effectively, 15% of melanoma deaths result from metastases of thin lesions. Furthermore, the clinical outcome of patients with melanoma of intermediate thickness (2.0–4.0 mm in Breslow depth) is less predictable. Clearly, identifying a high-risk population with thin melanomas remains a challenge, and new markers to assist this patient population are expected in order to establish more accurate risk groups with subsequent more aggressive therapeutic approach and tighter follow-up [93]. Our group assessed for the first time, the expression of DCT comparatively with the one of TYR in a panel of formalin-fixed, paraffin wax-embedded benign and malignant melanocytic lesions. The DCT and TYR proteins were analyzed by immunohistofluorescence microscopy in human specimens by simultaneous triple staining, with anti-DCT/-TYR antibodies, followed by secondary antibodies AlexaFluor-labelled and with DAPI for nuclei [38]. This technique allows to follow DCT and TYR expressions in identical cells within different tumor components. In tumor progression, the expressions of melanoma antigens are often lowered [94], and their immunodetection in histological specimens may be enhanced using antibody populations that recognize more than one epitope. In this study, the DCT expression was assessed with a novel anti-DCT antibody raised in our laboratory against the luminal domain of human DCT and in which the bioinformatic analysis identified multiple potential antigenic sites [38]. There is a heterogeneity in the expressions of the two antigens in benign tumors represented by junctional (JNs), compound (CNs), or dysplastic nevi (DNs) and malignant melanomas represented by superficial spreading (SSMs), nodular (NMs), achromic (ACMs), acralentiginous (ALMs) melanomas. Specimens expressing both antigens, only one and negative for both, were present in different numbers in each melanoma subgroup that was analyzed. The melanocyte neoplastic transformation and malignant progression is well correlated with the dissociation of DCT and TYR expression in distinct cell populations. In Figure 8A is presented an example of DCT and TYR dissociated expression in distinct tumor cells in a specimen representing a nodular melanoma. Within the double-positive category, we have identified in some specimens a subtype named by us “DCT-phenotype” in which DCT and TYR expressions specifically distributed within cell populations of tumor components create a tumor-specific architecture, with cells Tyr+/DCT- in the subepidermal layer, whereas DCT+/Tyr- cells segregate into deep dermis. The DCT-phenotype was found in benign specimens with high neurotization and also in some early malignant ones having low Breslow/Clark indexes but with ulceration. Our theory is that DCT-phenotype is emblematic for a long-lasting, “die-hard” phenotype. The DCT-intense expression is observed in large areas of compound neurotized nevi contributing probably to the well-acknowledged enhanced stability and low proliferation rate of these nevus cells [95] and may not represent a life-threatening problem in benign tumors. However, the superficial malignant melanomas, with low indexes Clark or Breslow but having DCT-phenotype could be a warning signal for considering those specimens as ones of high risk with a possible unfavorable prognostic. The DCT-clones selected in inner dermis of early malignant lesions acquire the expression and subcellular distribution of molecular markers reported to be associated with different types of neoplasms, including melanoma, with extended migratory capacities (caveolin-1-), survival in stressful conditions (cytoplasmic Hif-1α+), activated anti-apoptotic mechanisms (cytoplasmic cyclin D+ and Bcl-1+), angiogenic, and metastatic potential (cytoplasmic cyclin E+) (Figure 8B). Several ALMs or ACMs advanced melanomas diagnosed by anatomopathological analysis with bad prognostic detected DCT as the unique melanosomal antigen. The ALMs distinguish themselves from other melanoma types in terms of a worse prognosis, enhanced aggressiveness, and by a more advanced stage at diagnosis [96], whereas some ACMs are characterized by a peculiar and aggressive evolution [97]. It is very possible that DCT expression in ALMs and ACMs mediates tumor stress resistance pathways and contributes to the malignant characteristics of these melanoma categories. DCT could be an useful adjunct marker increasing sensitivity of tumor cell detection in specimens having downregulated other melanoma antigens, and the DCT-phenotype could represent a parameter associated with high-risk for bad disease outcome.
DCT in melanocytic lesions. (A) A nodular melanoma specimen immunostained for DCT and TYR expressions and analyzed by fluorescence microscopy. Tumor cells co-expressing both markers and cells DCT+/TYR- or DCT-Tyr+ can be observed. Unlike TYR, DCT is well expressed in numerous cells. (B) Schematic representation of DCT and TYR dissociation in melanocyte transformation and melanoma progression and molecular anatomy of DCT-phenotype. The switch in molecular repertoire of markers of tumor progression and bad prognosis in DCT+ cells in intraepidermal (IE) layer is indicated. DCT+ cells in deep dermis acquire molecular parameters of metastatic phenotypes [
The surgical removal is the only cure for melanoma with the condition that the excised lesion be in an early stage. However, the micrometastases cannot be addressed exclusively by the surgery and therefore, combinatorial therapeutical strategies are applied in the attempt to extend survival rates. The treatment options in melanoma are continuously revised, and there are several excellent reviews about this topic [98, 99, 100]. The schematic representation of the treatment of metastatic melanoma including different approaches is shown in Figure 9.
The therapies in melanoma. The different anti-melanoma treatment strategies are presented. DCT-based therapies are integrated part of the targeted therapies. The solid lines indicate the already existing therapies, whereas the dotted lines are proposed as possible adjuvant therapies based on the molecular studies about DCT intracellular processing and stability in melanoma cells. The melanoma specimen is an ulcerated nodular melanoma of a 26-year-old man, from lumbar region (by courtesy of Dr. S. Zurac, Department of Pathology, Colentina University Hospital, Bucharest, Romania).
The identification of different T-cell clones in melanoma patients recognizing peptides derived from DCT (Figure 2) raised the interest for this antigen in the development of anti-melanoma immunotherapeutical strategies. The cellular vaccine engineered to co-express a DCT epitope, with IFN-γ in the same gene by replacing the IFN-γ signal peptide with a DCT epitope-expressing signal peptide, resulted in decreased B16 tumorigenicity and enhanced immunogenicity after gene transfer. More importantly, irradiated transiently, TRP-2 epitope-expressing, IFN-c gene-modified B16 cells worked efficiently as a cellular vaccine to protect animals from parental wild-type tumor challenge [101]. The VacciMax® (VM), a liposome-based antigen delivery platform, has been used to deliver DCT 181–188 in combination with p53-derived peptides. A single administration of VM was capable of inducing an effective CTL response to multiple tumor-associated antigens. The responses generated were able to reject 6-day old B16-F10 tumors [102]. Another plasmide liposome DNA vaccine targeting the DCT in combination with chemokine CCL2 as an adjuvant used xenogeneic (human) DCT in a mouse model and resulted in induction of strong anti-DCT cell-mediated immunity after two vaccinations [103]. A novel vaccine system designed from a long TRP2/DCT peptide with a CD8 epitope (TRP2/DCT 180-88) and a CD4 epitope (TRP2/DCT 88-102) together with α-galactosyl ceramide, a lipid antigen representing a new class of promissing vaccine adjuvants into cationic liposomes tested on mice tumors resulted in the enhanced production of IFN-ϒ and increased cytotoxic T-cell responses [104]. Importantly, the antitumor immune activity involving MDAs as immunotherapeutic targets may have as side effects the damage (depigmentation) of the normal tissues that also express the MDAs [105]. However, in a patient receiving infusion with TIL586 (recognizing the DCT 109–205 peptide), tumor regression was observed, but not depigmentation [20], which demonstrates that immunotherapy directed against some DCT epitopes is specific and does not affect normal tissues. In another study, the inoculation of plasmid DNA encoding murine DCT elicited antigen-specific CTLs that recognized the B16 mouse melanoma and protected the mice from challenge with tumor cells. Moreover, mice that rejected the tumor did not develop generalized vitiligo, indicating that autoimmunity is not automatically triggered by administrating therapeutic MDA-based vaccines [106]. The vaccination with bone marrow-derived dendritic cells loaded with DCT peptide resulted in activation of high avidity CTLs mediating protective antitumor immunity
Despite the already acknowledged DCT involvement in melanoma drug-resistance, there are no reports so far, to our knowledge, about melanoma therapies targeting directly the DCT gene or protein. There is, however, a patent claiming the treatment of melanoma cells
Our data about the intracellular processing and the main checkpoints in DCT fate in tumor cells (Section 2.3) indicate that pharmacological agents that impact DCT stability could represent also potential adjuvants in melanoma therapy. For example, NBDNJ or A3B5 produce specific DCT proteasomal degradation possibly sensitizing tumor cells to therapeutic stress and could also generate DCT-peptides suitable for MHCI presentation and immune response. The selective premature DCT degradation induced in melanoma cells following CQ treatment is another possible way to decrease tumor cell resistance to therapies. CQ has been found to strongly potentiate the inhibitory effect of radiation on tumor cell proliferation [111], to be effective in eliminating chemotherapy-resistant cancer cells and to significantly improve the median survival in glioblastomamultiformis patients [112]. Moreover, the DCT detection at PM by us in melanoma cells [23] and by others in glioma cells [65] introduces DCT as a suitable molecule for targeting tumor cells with specific antibodies. If studies will confirm that DCT is internalized from the PM, this will open interesting perspectives of coupling anti-DCT antibodies with nanocarriers loaded with various antitumor agents. And finally by downregulating DCT (by siRNA or CRISPR/Cas9 system), it can be targeted the Cav1 stability and architecture and possibly some Cav1-mediated pathways including ones involved in tumor progression. The DCT-mediated therapeutic strategies are presented as integrated part of anti-melanoma treatments in Figure 9.
Our most recent studies in two distinct amelanotic melanoma cell lines representing different tumor phenotypes, MJS and SK28, demonstrate a molecular crosstalk, between DCT and caveolin-1 (Cav1), with structural and functional implications [23].
DCT and Cav1 are present in common structures in cytoplasm or decorating segments of PM (Figure 10A). Both Cav1 monomers/oligomers and DCT-precursor/mature forms have the same distribution along a density gradient in an ultracentrifugation experiment. Moreover, Cav1 has been identified in western blot and mass spectrometry analysis of the immunoprecipitates obtained with anti-DCT antibody from MJS cell lysates [23]. These experimental data are strongly supported by the structural analysis of DCT and Cav1 and by DCT-Cav1 structural model presented in Section 2.1.
The structural and functional relationship between DCT and Cav-1. (A) MJS and SK28 amelanotic melanoma cells immunostained for DCT and Cav1 and analyzed by confocal fluorescence microscopy demonstrate DCT and Cav1 in cytoplasmic and PM common structures; in DCT downregulated cells, the morphologies of Cav1 positive structures are severely altered. The fourth and the sixth panels represent the enlarged details of the indicated insets; (B) the DCT-high clones in MJS having downregulated Cav1 expression analyzed by tissue FAXS. In the upper part of quadrant are shown the cells with high DCT expression; (C) the crosstalk between DCT and Cav1. The impact of si-DCT on Cav1 and of si-Cav1 on DCT is indicated. Possible processes mediated by either DCT or Cav1 are indicated in dotted boxes; (D) DCT, unlike TYR or TRP1 is overexpressed during transition from subconfluent (48 h) to semi-confluent (72 h) and confluent (96 h). Medium was not replenished for 96 h (MR−) or replenished every 24 h (MR+). Autocrine/paracrine stimulation (starvation, secreted factors by proliferative MJS tumor cells within 48 h) decrease Cav1, increase DCT expressions, and change the cell morphology. The cells at 48 h are polygonal with visible contacts between adjacent cells, whereas cells at 96 h are elongated with no cell-cell contacts and form large clusters.
The transient downregulation of DCT expression (si-DCT) in MJS and SK28 cells increased the amount of Cav1 protein by its redistribution into more stable, insoluble membrane aggregates with altered morphologies [23] (Figure 10A). This is the first report about a melanosomal protein that regulates Cav1 assembly. We postulate that DCT may regulate Cav1-and/or lipid raft structures by competing either with different signaling molecules for Cav1 binding or with Cav1 monomers for Cav1 oligomerization domain or for cholesterol binding. Both caveolae and Cav1-scaffolds are associated with lipid rafts, which are membrane domains with a very dynamic structure abundant in cholesterol, sphingolipids recruiting different molecular players of signaling platforms, and controlling numerous and diverse cellular processes [113]. Either directly or indirectly, DCT as a major regulator of Cav1- or cholesterol-membrane architecture is thus expected to impact also different cellular events mediated by Cav1 (Figure 10C). For example, the interaction of membrane/lipid rafts, with the cytoskeleton, has impact on trafficking and sorting mechanisms, formation of platforms for cell anchorage to ECM, transduction of signaling cascades across the PM, cell growth and migration, entry of microorganisms (viruses/bacteria), and toxins or nanoparticles [114]. Indeed, we also observed that in MJS cells having downregulated DCT expression, there was an increase in cell volume, a significant redistribution of actin filaments in cell periphery, and a dramatic decrease in cell proliferation by 20 at 48, 60 at 72, and 75% at 96 h coupled with the cell cycle arrest in G1 [unpublished data]. Interestingly, these effects were less prominent in SK28 phenotype that indicates that DCT-mediated processes are tumor phenotype specific. Importantly, our mass spectrometry analysis of immunoprecipitates obtained from MJS cell lysates with anti-DCT antibodies against N- or C-terminus epitopes has identified as potential DCT interactors, regulators of small GTPases (Arf, Rho and Ras) and numerous proteins involved in anti-apoptotic, proliferative, migration, and invasion mechanisms and pathways [unpublished data]. The structural analysis pointed also the possibility that two Ser residues within DCT CYT subdomain to be phosphorylated (Section 2.1). Our theory based on all these data and preliminary information is that DCT, placed in a molecular environment with Cav1, is a key-molecular player acting on one or more signaling pathways involved in tumor cell survival and morphology, either by itself, as a potential target of the phosphorylation cascades, or as modulator of Cav1 or other participants in regulatory processes (Figure 10C). The numerous potential interactors present DCT as a possible new molecular scaffold. Further experimental studies are required to validate these interactions and place DCT in the exact pathway(s) where it operates.
The Cav1 downregulation (si-Cav1) has a dramatic impact on DCT in MJS cells. There is a 20-fold increase over 96 h of Cav1 silencing on DCT mRNA level. Accordingly, there is also a protein increase detected by western blot, and the deglycosylation experiments showed that DCT synthetized in si-Cav1 cells is mainly DCT-precursor. The imagistic studies of confocal immunofluorescence microscopy and Tissue FAXS cytometry quantitative analysis revealed a 7-fold increase in a DCT-population with intense cytoplasmic, but no PM, DCT staining, the “DCT-high clones” (Figure 10B). This is the first report about a melanosomal protein/melanoma antigen-regulated by Cav1 and a novel target gene for Cav1. Cav1 is a regulator of several genes as CyclinD or folate receptor promoters [115] or for survivin, a member of the Inhibitor Apoptosis Protein-family [116]. In melanoma, Cav1 function is still ambiguous. In some studies, Cav1 is associated with tumorigenicity [117], whereas others present Cav1 as a tumor suppressor by inhibiting Wnt-β-catenin-TCF/LEF [118], Src/FAK [119] pathways, or attenuating tumor cell motility by disrupting glycosphingolipid GD3-mediated malignant signaling [120]. In the context of DCT-mediating pro-survival and resistance pathways and the upregulation of DCT in si-Cav1 cells, we consider that Cav1 acts as a tumor suppressor gene, at least in this early malignant phenotype. The exact mechanism of how Cav1 controls DCT gene expression and how this intersects DCT-mediated processes (Figure 10C) needs to be deciphered and validated in one or more melanoma cell line(s) in addition to MJS.
The oncogenic epithelial-mesenchymal transition (EMT) is a multistep process by which epithelial cells acquire invasive mesenchymal phenotype characteristics essential in metastatic spread [121]. EMT is regulated and characterized by molecular mechanisms involving specific transcription factors, signaling pathways, and biomarkers. In melanoma cells which do not have epithelial origin, there is a phenotype switching, with similitudes between the EMT program from development, and this EMT-like switch is a major determinant in tumor metastasis [122]. The role of Cav1 in the oncogenic EMT process is significant but controversial and depends on the type of cancer. In bladder cancer cells, Cav1 promotes invasive phenotypes by inducing EMT [123] in A431 human epidermoid carcinoma cells, the Cav1 downregulation by EGF (an EMT inducer) results in E-cadherin loss, and increased tumor cell invasion [124], whereas in primary tumors of head and neck, squamous cell carcinoma increases EMT and prometastatic properties [125]. During transition from subconfluent (48 h) to confluent (96 h) cultures in MJS, SK28, or MNT-1 cell lines, there is an increase in DCT expression, not observed for either TYR or TRP1 and more abrupt in MJS (VGP) than in MNT or SK28 (metastatic) cells (Figure 10D). Oppositely, in the same MJS culture, Cav1 was severely downregulated, in the same cells highly expressing DCT. The most stimulating agent for DCT overexpression is the culture medium exhausted in nutrients but rich in cytokines and growth factors secreted by the tumor cells during 96 h proliferation, whereas changing medium every 24 h has a lower impact on DCT increase (Figure 10D). EMT can result from multiple extracellular stimuli; for instance, a synergistic effect on EMT has been observed with combined stimulation of EGF and TGF-β [126]. Interestingly, the cell morphology of MJS, but not SK28 cells was dramatically changed during transition from subconfluent to confluent stage from a polygonal, low-expressing DCT/high-Cav1 to an elongated phenotype high-DCT/low- or negative Cav1 (Figure 10D). The same phenotype switching has been observed in si-Cav1 cells highly expressing cytoplasmic DCT. Oppositely, si-DCT cells adopt a wider morphology. We consider that, in MJS phenotype, the DCT and Cav1 crosstalk is a possible part of the EMT program. In subconfluent MJS culture (48 h), groups of 2–4 polygonal cells are interconnected via fine filaments and express low DCT and high Cav1. In confluent culture (96 h), the environmental signals trigger probably, independently, the DCT increase and Cav1 decrease. Furthermore, Cav1 downregulation itself sustains even more the DCT increase. The dynamic analysis of tumor cell populations with Tissue FAXS system demonstrates the perpetuation of a subset of DCT-high/Cav1-low, elongated fibroblast-like cells with long extensions, and forming large clusters (Figure 10D). This metamorphosis is an
TRP2/L-DCT is, undoubtedly, a benefit for the cell expressing it. In melanocytes, the detoxification processes involve the conversion of DCT natural substrate, DHICA into less toxic products. In nonmelanocytic cells, exogenous DCT is able to decrease the effects of oxidative stress acting on substrate analogs. In melanoma, the “preservation” of the expression of certain melanosomal antigens able to ensure tumor cell viability prevails over that of the key-enzymes for pigment production, and TRP2/L-DCT qualifies for this selection. For this prosurvival molecule, the tumor cells reserve complex transcriptional and post-translational mechanisms distinct from the other TRPs. DCT functions as a sensor in case of the autocrine stimulation/stressful conditions when its expression is highly increased, no matter whether the melanogenic pathway is active or not. There is a molecular crosstalk between DCT and Cav1, a master regulator of numerous cellular processes. The members of signaling platforms identified by mass-spectrometry analysis as potential DCT interactors, as well as the impact of DCT expression on cell proliferation, morphology, and cytoskeleton remodeling are strong proofs that DCT is a key player in cellular processes, acting, in our opinion, as a molecular scaffold within one or more signaling hubs. The recent findings about DCT expression pattern in the tumor architecture in correlation with a stable, longlasting/“die-hard” phenotype in benign lesions and with bad prognostic parameters in malignant lesions advocate for considering DCT as a warning indicative of possibly tumor unfavorable outcome.
On the other hand, TRP2/L-DCT has its own vulnerabilities in terms of stability that can be exploited for therapeutic purposes.
In spite of all these information, the role of DCT in melanoma is far from being elucidated or fully exploited and several issues still need clarification: the molecularity behind DCT regulation by Cav1 and DCT impact on Cav1 structural organization; the decipherment of the signaling pathways in which DCT activates, in amelanotic versus pigmented phenotypes in different stages of tumor progression; how are the DCT structural subdomains involved in DCT tumor cell regulatory mechanisms; the DCT role in tumor cell phenotype switching process; the value of DCT phenotype as prognostic indicative; the efficiency of NBDNJ, CQ, as possible adjuvants in melanoma therapeutic strategies; the clarification of DCT expression in nonmelanocytic/nonneuronal cell lines or tumors.
In melanoma, DCT is a double-edged sword, a lethal weapon for cancer cells serving the tumor progression or an exploitable molecular tool for scientists and clinicians to eradicate the malignant cells.
This work was supported by Grant Application 156, Exploratory Research Projects PN-II-ID-PCE-2011-3-0492-1, funded by Ministerul Educației și Cercetării Științifice and by the Academia Română Project 1/2011 of the Institute of Biochemistry. Molecular simulations were performed using the high-performance computational capabilities of the HPC Linux cluster at IBAR and the High-Performance Computing Infrastructure for South East Europe’s Research Communities (HP-SEE), a project cofunded by the European Commission (under contract number 261499) through the Seventh Framework Programme. Gabriela Negroiu acknowledges Dr. Sabina Zurac, Department of Pathology, Colentina University Hospital, Bucharest, Romania for providing the image of the specimen in Figure 9 and for sharing her valuable expertise in melanoma pathology during our collaborative research. Adina Milac is grateful to Dr. Andriy Anishkin, Department of Biology, University of Maryland, College Park, MD, USA for advice and discussions on molecular simulations of cholesterol-containing membranes.
Human herpesviruses, which include HSV-1 (Herpes simplex virus type-1), HSV-2 (Herpes simplex virus type-2), HZV (Herpes zoster virus), EBV (Epstein–Barr virus), CMV (Cytomegalovirus), HHV-6 (Human herpesvirus-6), HHV-7 (Human herpesvirus-7), HHV-8/KSHV (Human herpesvirus-8, Kaposi’s sarcoma-associated herpesvirus) are the causative factor of various diseases, including mononucleosis, roseola, chickenpox and many forms of ocular involvement, such as conjunctivitis, blepharitis, keratitis, uveitis and retinitis. The common features of all human herpesviruses include a double-stranded DNA genome, a 20-faceted icosahedral capsid, a surrounding proteinaceous tegument, and an external glycoprotein-laden lipid envelope. All herpesviruses are able to achieve a state of the latency, where the virus remains inactive in cells and occasionally reactivates. Recurrence could be described as the most characteristic feature of corneal infections caused by HSV, subsequently leading to visual impairment and blindness. According to epidemiological data, HSV keratitis remains a leading infectious cause of blindness in the world. The estimated global incidence of HSV keratitis is roughly 1,5 million, including 40,000 new cases of each year. Additionally the recurrence rate is high. It was estimated as 9.6% at 1 year, 22.9% at 2 years, and 63.2% at 20 years after the first episode of documented HSV keratitis [1, 2, 3, 4]. Also the worldwide seroprevalence rate is high and estimated above 50%, but recently it was reported declining in the United States [5].
\nIn this chapter we will focus on Herpes simplex virus 1 keratitis - the detailed corneal characteristics based on slit-lamp examination, optical coherence tomography scans and confocal microscopy results. The chapter also discusses recent methods of diagnosis based on PCR testing as well as established and future methods of treatment based on the latest research results.
\nGeneral pathogenesis of herpesvirus infections include: active viral replication, state of latency and reactivation. Primary infection, usually in the childhood could be asymptomatic, oral, but also could affect upper respiratory track or ocular surface in the form of the conjunctivitis or blepharoconiunctivitis. After a primary infection, HSV-1 begins a life-long latency in the trigeminal ganglia, where abundant viral RNAs are constantly produced. In order to establish latency, HSV-1 has evolved several mechanism to evade the host immune response. The process is complex based on HSV-1 several viral proteins targeting multiple steps of the cellular DNA-sensor-mediated antiviral signal pathway of the host. Moreover, it is believed, that viral protein activation varies between immediate period after infection and the late phase of infection. Inhibition of the type I interferon (IFN-I) activity has been described as the main pathogenetic pathway of downregulating the host immune response. Numerous mechanism including: inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation, modulating interferon regulatory factor 3 (IRF3), interferon regulatory factor 7 (IRF7) or stimulator of IFN genes (STING) function were identified. Recently a broad attention was brought to the HSV-1 immediate early (IE) protein infected-cell polypeptide 0 (ICP0), which is an E3 ubiquitin ligase, a nuclear phosphoprotein that was described to play an essential role in inhibition of IFN-I production through IRF7 protein expression reduction, thus promoting viral replication, latency, and reactivation. Certain triggering agents, physiological and environmental stress, including ultraviolet exposure, fever, injury, hormonal disruption or immunosuppression could cause viral reactivation in the tissues innervated by the trigeminal ganglion, causing different forms of the HSV keratitis: epithelial, stromal or endothelial. Epithelial keratitis is the most common form of HSV keratitis, but the recurrence infection may also affect other corneal layers. Recurrence varies in frequency between subjects and throughout the life and could cause irreversible corneal damage and decrease in visual acuity, ranging from superficial opacities to serious complications such as corneal perforation and endophthalmitis [6, 7, 8, 9, 10].
\nThe diagnosis of HSV keratitis is mainly based on the presence of typical unilateral corneal lesions on the slit lamp examination. However, the clinical diagnosis may be guided by modern imaging techniques, such as optical coherence tomography or confocal microscopy. Also, laboratory testing including polymerase chain reaction (PCR) and novel techniques based on multiplex dot hybridization (MDH) assay or immunochromatographic assay (ICGA) may serve as a potential guide in the diagnostic process.
\nPatients symptoms depend on the clinical form and stage of the disease. Primary infection may be asymptomatic. Recurrent infections symptoms include: foreign body sensation, ocular or ocular adnexa pain, lacrimation, photophobia, decreased vision and conjunctival hyperemia. Symptoms are usually not specific. Although, patient with recurrent keratitis are aware of the symptoms of the recurrent keratitis, which allows for the rapid referral and treatment. Patients with neutrotrophic keratitis due to HSV keratitis may experience only mild symptoms despite the advanced corneal involvement.
\nHerpetic keratitis is usually classified by anatomical localization in regards to affected corneal layers. Although the inflammation process may overlaps different layers. Also, recurrent keratitis is not only limited to one layer and can subsequently affect different corneal parts [11, 12].
\nEpithelial keratitis is the result of the active HSV replication in corneal, epithelial cells. The most characteristic form is the dendritic ulcer containing small branches with terminal bulbs. The borders of the branches are raised above the corneal surface. The ulcer may be single or multiple. Several dendritic ulcers may form a geographic ulcer, especially in patients with immune system deficiency, treated with topical steroids or in the long course of the disease. Other forms of epithelial involvement include punctate keratitis or epithelial vesicles. On the slit lamp examination, epithelial defects stain with fluorescein and become evident with the use of a blue filter (450 nm) with or without additional yellow barrier filter between 1 and 3 min after the dye instillation. Other symptoms in epithelial keratitis may include: bulbar conjunctival and limbal hyperaemia, subepithelial stromal edema at the ulcer site and subepithelial infiltration of inflammation cells. Epithelial keratitis in the form of the dendritic ulcer may also be present in the stromal recurrent keratitis. However, if multiple recurrence occur, the neurotrophic ulcer is definitely more probable clinical form compared to the dendritic ulcer. Characteristic features of the different forms of the epithelial keratitis are presented in Figure 1.
\nRepresentative images of the slit-lamp photograph of the epithelial HSV keratitis. (A, B) Central, single dendritic ulcer before and after fluorescein installation. Branches with terminal bulbs visible. (C, D) Single dendritic ulcer with branches are raised above the corneal surface. Stromal haze accompanying the ulcer is noticeable. (E) Multiple, small dendritic ulcers visible under blue light after fluorescein installation. (F) Geographic, paracentral ulcer visible under blue light after fluorescein installation.
Stromal involvement in case of herpetic keratitis develops on an immune related basis. Inflammatory response to the HSV is connected with the activation and infiltration of myeloid-derived cells, CD4+ T-cell and NK cells. Stromal inflammation may lead to the reduced corneal transparency, persistent scar formation, may also cause an irreversible tissue pathology including vascularization and stromal necrosis. The inflammation process is often accompanied by stromal localized or extensive corneal edema and a mild anterior chamber reaction. Several recurrences may lead to the lack of the corneal innervation. Moreover, the severity of disease may increase with each subsequent episode, as inflammatory reaction becomes stronger despite no detectable viral activity.
\nThroughout the years multiple clinical forms in terms of stromal keratitis were described, being the source of confusion in diagnostic terminology, including: immune stromal, interstitial, necrotizing, nonnecrotizing, disciform, focal, multifocal, diffuse. This could contributed to misdiagnosis, especially in early phases of the disease and misapplications of therapy in clinical practice. For example, in Japan, “disciform” keratitis is considered a type of stromal keratitis. “Immune stromal” term also is misleading, suggesting, that other forms of HSV stromal keratitis do not involve immune reaction. That is why a simplified classification of the stromal keratitis was proposed, dividing the keratitis into two distinct forms: stromal with and without an overlying epithelial ulceration. Stromal keratitis without ulceration, is the more common form, historically described as “nonnecrotizing,” “immune-stromal,” and “interstitial.” Stromal keratitis with ulceration is the effect of severe inflammation and relates to historical description of “necrotizing” keratitis. The form with the ulcer is more probably the result of stromal HSV reactivation, although the neurotrophic pathogenesis of the ulcer also cannot be ruled out. This terminology could be easily implemented in clinical practice and allows ophthalmologists to properly counsel patients regarding diagnosis, treatment and prognosis [12]. Figure 2 contains clinical presentations of the range stromal keratitis. Figure 3 present a clinical case of a patient diagnosed with stromal keratitis with ulceration throughout the treatment process.
\nRepresentative images of the slit-lamp photograph of eyes with the different involvement of the stromal keratitis or with corneal scars following HSV keratitis. (A) Paracentral stromal infiltration with profound, active limbal vascularization. (B) Epithelial, dendritic ulcer accompanied by active stromal keratitis with vascularization. (C) Central stromal scarring with deep, peripheral vascularization. (D) Stromal haze in the course of recurrent stromal HSV keratitis. (E) Excessive corneal scarring with significant, deep, peripheral vascularization. (F) Significant area of corneal scar accompanied by lipid keratopathy and deep vascularization.
Slit-lamp photographs presenting the follow up of a 65-year old patient with HSV stromal keratitis with ulcer. (A, B) Baseline, at diagnosis. Recurrent stromal keratitis with significant ulcer, stromal infiltration, vascularization and corneal thinning. Patient treated with the combination of antiviral medication (Oral acyclovir 800 mg, 5 times daily at baseline; topical 3% acyclovir ointment 4 times daily) combined with 0,1% dexamethasone (3 times daily) and preservative free lubricant eye drops (hourly). (C, D) At 1 month in the course of treatment. Significant decrease of the area of the ulcer. Remaining significant corneal infiltration with vascularization. Oral acyclovir dosage tapered gradually to 400 mg 4 times daily. Topical acyclovir discontinued. (E, F) At 3 months in the course of treatment. Ulcer healed completely. Punctate keratopathy visible under blue light. Decreased stromal infiltration, but stromal haze, thinning and vascularization visible. Oral acyclovir and 0,1% dexamethasone doses tapered very carefully within months to prevent active keratitis recurrence. Patient was recommended a frequent use of the preservative free eye lubricant drops.
Marginal keratitis is a special, rarely occurring form of stromal and epithelial keratitis. Clinically it is difficult to differentiate from other forms of marginal keratitis, thus laboratory testing may be helpful in establishing the final diagnosis. The lack of corneal sensitivity could also be used as a clinical clue in differential diagnosis.
\nThis form is believed to be a result of endothelial cells viral infection coexisting with immune reaction. Usually, the endothelitis is localized with a distinct area of the corneal edema. Therefore, it was historically described as disciform endothelial keratitis. Focal keratic precipitates, as well as Descemet membrane folds may be spotted in the affected area. Rarely, diffuse stromal edema, accompanied by trabeculitis with elevated intraocular pressure occurs. Various range of endothelitis is presented in Figure 4.
\nRepresentative images of the slit-lamp photograph of the different forms of HSV endothelitis. (A, B, C, D) The slit lamp photographs of the eye of a 34-year old patient with recurrent, excessive endothelitis with significant corneal edema and Descemet folds. (A, B) At baseline. Diffuse corneal edema with Descemet folds and punctate keratopathy. Patient treated with the combination of antiviral medication (oral acyclovir 800 mg, 5 times daily at baseline combined with 0,1% dexamethasone (7 times daily) and preservative free lubricant eye drops (5 times daily). (C, D) At 2 months in the course of treatment. Significant decrease in stromal edema, with only subtle stromal haze. Improvement of the punctate epitheliopathy. (E) Distinct area of the corneal edema - disciform endothelial keratitis. (F) Distinct area of the corneal edema - disciform endothelial keratitis at retroillumination. Ghost, profound vessels visible.
This should be considered as a different entity, because there is no virus activation in case of neurotrophic ulcer. Also, the inflammation level compared to active HSV keratitis is lower. The most characteristic feature is the absence of corneal innervation and a non-healing corneal ulcer with smooth margins. As HSV keratitis alters the corneal nerves, the disease is one of the leading causes of neurotrophic keratopathy, among others, such severe dry eye disease, ocular burns or denervation post neurosurgical procedures. The pathogenesis is complex and include toxicity from antiviral medications, lack of nerve growth factors, the nerve damage as a result of recurrent keratitis. The neurotrophic keratitis is characterized by three stages of the severity: stage 1, punctate epithelial keratitis (PEK); stage 2, a nonhealing corneal persistent epithelial defect (PED); and stage 3 involving stromal involvement in the form of the neurotrophic ulceration. Possible accompanying signs are neovascularization, stromal haze and scarring. Consequently corneal poor ability to heal may result in corneal melting, prolonged ulceration, corneal perforation and endophthalmitis. A corneal sensitivity test is essential to confirm a diagnosis of neurotrophic keratitis. The test should be performed in regards to corneal location (central, peripheral), using a cotton-tipped swab or an esthesiometer. Figure 5 presents forms of the neurotrophic keratitis.
\nRepresentative images of the slit-lamp photograph of the different forms of neurotrophic keratitis. (A) Neurotrophic keratitis stage 2. A nonhealing corneal persistent epithelial defect (PED) after fluorescein installation. (B) A single, central corneal ulcer with stromal infiltration and peripheral corneal vascularization. (C) Central corneal perforation in the course of the corneal thinning and scarring and vascularization. (D) Neurotrophic keratitis stage 3. Neurotrophic ulceration with elevated borders and significant stromal haze.
Confocal microscopy (IVCM - in vivo confocal microscopy) is the imaging technique developed to analyze corneal layers with the resolution of 1 μm. Imaging with confocal microscopy is used in clinical practice in differential diagnosis of microbial keratitis, corneal dystrophies and degenerations. The technique allows microscopic analysis of the cornea layer by layer and detailed assessment of keratocytes and inflammation cells. Features characteristic for HSV-1 keratitis depending on the stage and form include: microerosions, distortion of the superficial and basal epithelium, changes in superficial epithelial cell density, increase in epithelial cell size, squamous metaplasia, subepithelial infiltration of highly reflective dendritic structures (correspondind to Langerhans cells), keratocytes activation, sub-basal nerve plexus alteration or absence, stromal fibrosis and endothelial precipitates. Figure 6 presents the example of confocal microscopy results in case of patients with HSV keratitis. Confocal microscopy could guide in the disease diagnosis and monitoring the treatment results. In patients with stromal involvement the mean subbasal nerve density was proved to be significantly lower compared to healthy eyes. Also, in patients qualifying for surgical interventions, the technique has a potential role in assessing the sub-basal nerve plexus anatomy, helping the surgeons to procced with intervention decisions. The prognosis of patients with significantly altered corneal nerve plexus is poor after traditional transplant surgery [13, 14, 15, 16].
\nRepresentative images of the confocal microscopy scans revealing significant features characteristic for HSV keratitis. (A) Epithelial, healed dendritic ulcer with noticeable fibrotic borders (arrows). (B) Multiple infiltration of small dendritic structures as the level of the epithelium. Clusters of inflammation cells (arrows). (C) Multiple infiltration of pronounced dendritic cells forming a lattice pattern (arrows) at the level of the basal epithelial cells. (D) Marked fibrosis at the level of the Bowman layer (arrow) with inflammation cells infiltration (stars). (E) Excessive fibrosis and inflammation cells infiltration forming clusters at the level of the Bowman layer (arrows). Dendritic structures visible (star). (F) Anterior stromal keratocytes activation with accompanying haze (arrow). (G) Stromal infiltration and haze accompanied by multiple crystalline structures due to the lipid degeneration (stars). (H) Multiple endothelial opacities. Examples marked with stars.
Anterior eye segment imaging with 830 nm optical coherence tomography (AS OCT) was first demonstrated and published in 1994. Changing the light wavelength from 830 nm to 1310 nm allowed the direct transcleral anterior eye segment structures including trabecular-iris angle visualization in 2000. OCT provides in vivo anterior eye segment imaging with the axial resolution from 18 μm with time domain OCT (TD OCT) to 5 μm with spectral domain OCT (SD OCT) and to 5 μm with ultra high resolution spectral domain OCT. OCT is proven to provide reliable anterior eye segment morphology and morphometry results with high reproducibility and repeatability. Application of OCT in herpetic keratitis patients include: assisting in diagnosis of patients at active stage and assessing the scars in patients qualified for laser or surgical interventions. Active keratitis could be characterized by the presence of the ulceration, stromal edema and inflammatory hyperreflective infiltrates. Corneas with inactive keratitis are characterized by stromal scarring and thinning, and epithelial remodeling [17, 18, 19, 20, 21, 22]. Characteristic OCT features are presented in the Figure 7.
\nRepresentative images of the anterior segment swept source optical coherence scans revealing significant features characteristic for HSV corneal scars. (A) Slit lamp photograph of the central post herpetic keratitis scar. (B) High resolution scan. Hyperreflective tissue within corneal stroma with irregular borders (arrows). (C) Pachymetry map. Marked paracentral corneal thinning to 398 μm. (D) Slit lamp photograph of the central post herpetic keratitis scar. (E) High resolution scan. Hyperreflective tissue within corneal stroma with irregular borders. Note the relatively smooth corneal surface and epithelial compensation over the irregular corneal stroma (arrows). (F) Pachymetry map. Marked irregular, paracentral corneal thinning to 382 μm.
There are several laboratory techniques, which may help in the diagnostic process. Clinical samples for the analysis may be obtained through collection of tears, corneal epithelial cells, and conjunctival cells. Tear samples are usually obtained using Schirmer test. Epithelial or conjunctival cells may be collected through corneal scrapings, corneal impression membranes (CIM) or using conjunctival or corneal swab. The less invasive the technique the lesser probability of obtaining a clinically detectable material.
\nThe isolation of the HSV from the cornea and performing a viral culture remains a conventional, gold standard technique, however the main disadvantages of this methods are low sensitivity and a time consuming process. Giemsa staining of the epithelial corneal cells may visualize multinucleated giant cells, resulting from coalescence of HSV infected epithelial cells and intranuclear HSV inclusions. Immunofluorescence assay (IFA) is one of the modern techniques developed to diagnose HSV keratitis. The principle of the method is to introduce antibodies, that bind to HSV antigens specifically to gain fluorescence based immunological detection of HSV-1 antigen through color visualization under microscopy. Disadvantages of the method include: required subjective interpretation by an experienced technician and the risk of obtaining false positives results due to cross-reactivity between other microorganisms.
\nAdvanced diagnostic techniques include: Polymerase Chain Reaction (PCR) - conventional PCR, reverse transcriptase PCR (RT-PCR), real-time PCR (qPCR) and multiplex PCR. qPCR overcomes the disadvantages of conventional PCR by acquiring more rapid and sensitive results. Guda SJM. et al. assessed sensitivity and specificity of the conventional and real-time PCR compared to IFA performed on corneal scrapings. The sensitivity and specificity of conventional PCR was 100% and 76.9% and 100% and 28.2% of qPCR respectively. Satpathy et al. assessed and concluded, that specificity and positive predictive value (PPV) of PCR was higher in tear (90.6% and 37.5%). compared to cornea scrapings (71.3% and 30.3%). Moreover, Akbarian A. et al. reported, that conventional PCR with added internal amplification control (IAC) had higher sensitivity (100%) vs. culture method (66.66%), while the specificity was 100% for both diagnostic methods.
\nAlso novel methods, such as multiplex dot hybridization (MDH) assay, immunochromatographic assay (ICGA, AmpliVue) or Infected cell protein 0 (ICP0) detection in tears are either tested or incorporated into a clinical practice. AmpliVue is a commercially available immunochromatographic assay, office-based diagnostic test characterized by a 64.7% positive detection rate. Sensitivity and specificity of AmpliVue was assessed as 84% and 100% respectively, based on true positives from culture and PCR combined. The MDH assay is a rapid technique, that involves a series of oligonucleotide probes specific for HSV genes. Compared to the real-time PCR, the MDH assay is characterized by very high values of sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 93.3%, 100%, 100% and 98.4%, respectively. The infected cell protein 0 (ICP0) is an acute phase protein during HSV infection and plays a significant role in the virus gene expression activation. ICP0 could be potentially defected in tears of affected subjects [23, 24, 25, 26, 27, 28].
\nDifferential diagnosis is dependent on the corneal layer affected by the keratitis. Epithelial keratitis should be differentiated with epithelial regeneration line after traumatic epithelial defect, epithelial corneal dystrophies, such as epithelial basement membrane corneal dystrophy (EBMCD; map-finger-dot dystrophy, Cogan microcystic dystrophy), epitheliopathy associated with excessive contact lens wear or iatrogenic epitheliopathy after topical drops containing preservatives. Stromal involvement requires differentiation with other microbial keratitis (bacterial, fungal or amoebic), vaccinia virus keratitis (VACVK), Varicella Zoster virus keratitis, Thygeson superficial punctate keratopathy, stromal or Bowman layer corneal dystrophies, such as TGFBI corneal dystrophies. Marginal keratitis should be differentiated with other forms of marginal ulcers, such as staphylococcal marginal keratitis or related to atopic or autoimmune diseases, such as rheumatoid arthritis, systemic lupus or granulomatosis with polyangiitis (GPA). Also, neurotrophic keratitis may be initiated by multiple other causes, such as surgical and laser procedures, chemical burns, excessive contact lens wear and preservative-containing topical medicines, diabetes mellitus, multiple sclerosis and congenital or acquired abnormalities of the trigeminal nerve. Examples of the diseases, which require differential diagnosis with HSV keratitis are presented in Figure 8.
\nRepresentative images of the slit-lamp photograph of the different forms of corneal diseases, which should be differentiated with HSV-keratitis. (A) Slit-lamp photograph of the epithelial basement membrane corneal dystrophy (EBMCD; map-finger-dot dystrophy, Cogan microcystic dystrophy). Superficial white dots visible. (B) The slit lamp photograph after fluorescein installation under blue light with additional yellow barrier filter of the patient 9A. An irregular area of the disrupted epithelium visible. (C) Slit-lamp photograph of the lattice corneal dystrophy (LCD). A dystrophy was confirmed by the
Major advances in the treatment of HSV keratitis have been provided by the evidence-based results and conclusions of the Herpetic Eye Disease Study (HEDS) randomized clinical trials, which were multicenter, characterized by double-masking with placebo controls studies. Based on this knowledge, further treatment guidelines were proposed and published [12, 29, 30, 31, 32, 33, 34, 35, 36, 37]. Although the HEDS clinical trials directly addresses multiple clinical concerns, the studies have also several limitations. These include: inadequate sample size in case of HSV stromal keratitis with epithelial ulceration to determine the optimal course of therapy, relatively high rate of follow up failure within the study group. Also, the corticosteroid regimen was standardized and fixed in the study group, thus lacking the evidence of benefit of delivery of personalized care. Finally, the concerns regarding the dose and the optimal period of antiviral prophylaxis have not been resolved.
\nNowadays, the main treatment line of the active keratitis is a combination of the antiviral and corticosteroids drugs, depending on the epithelial and stromal involvement. The general rule to follow is to avoid corticosteroids in epithelial keratitis, because the entity of this form is virus activation and to treat with corticosteroids in stromal and endothelial keratitis without epithelial involvement, because those forms are strongly connected with the significant reaction of the immune system.
\nAntiviral drugs are used in two main forms: topical and oral. Topical anti HSV-1 drugs include: trifluridine solution (1%), ganciclovir gel (0.15%), and acyclovir ointment (3%). Oral anti HSV-1 drugs include: acyclovir, valacyclovir, and famciclovir. Historically, other systemic drugs were also used, such as idoxuridine, vidarabine, valganciclovir, foscarnet, and cidofovir, but they were withdrawn from the market or are relatively too toxic in combination with the achieved therapeutic effect.
\nMost common antiviral drug worldwide is acyclovir used either orally or topically or in combination. Common side effects of the prolonged oral acyclovir include nausea, vomiting, diarrhea, headache and weakness. Potentially serious, but very rare side effects include renal failure and hematology complications, such as: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS).
\nIn recent years, there have been an increasing interest in valacyclovir, due to its proven improved bioavailability and steadier plasma concentration compared to acyclovir. Valacyclovir is considered a prodrug of acyclovir. The lower frequency of dosing (2 times daily versus 5 times daily) may be a strong benefit for some patients. However, there is a lack of strong evidence, that treatment with valaciclovir provides leads to better results and less ocular and systemic complications. Comparing to herpes zoster ophthalmicus, the authors of the systematic Cochrane report indicated uncertainty of the relative benefits and harms of valacyclovir over acyclovir [38]. All topical antiviral drugs are characterized by ocular surface toxicity, could cause allergic reactions, and punctal and nasolacrimal duct stenosis, therefore the prolonged usage of those formulas is not advised. Authors of the Cochrane systematic review on HSV keratitis treatment, published in 2015 also assessed other methods of HSV keratitis treatment, such as manual debridement of the corneal epithelium or experimental biologic agents. Manual debridement alone has been proved to be not effective. Also, topical treatment with interferon has only a modest benefit over placebo [39]. In case of epithelial keratitis, the mainstay of treatment is antiviral agents. Corticosteroids excessive usage may lead to geographical ulcers and delay healing of the epithelium. When stromal involvement is present, the mainstay of treatment is the use of corticosteroids with the combination of antiviral agents. The HEDS clinical trials have brought solid rationale for the treatment of the stromal keratitis with corticosteroids. Nowadays, there are several available topical corticosteroids formulas with different anti-inflammatory potency and different potential for adverse reactions: Dexamethasone 0,1%, Betamethasone at concentrations ranging from 0.01% to 0.1%, Prednisolone 1%, Loteprednol Etabonate 0.5%, Rimexolone 1%, Dexamethasone 0.1%, Hydrocortisone 0.335%, Fluorometholone 0,1%. The strongest anti-inflammatory effect is demonstrated by dexamethasone, the weakest by hydrocortisone. This should be taken into consideration, when choosing the medication depending on the level of the corneal inflammation. Moreover, steroid medication must be withdrawn gradually, tapering the doses generally over few weeks’ time. During the drug withdrawal, instead of sudden discontinuation of the stronger corticosteroid, one may consider replacing it with a relatively weaker one to avoid a rebound increase in inflammation and a disease recurrence immediately after drug cessation. The recommended treatment for HSV stromal keratitis without ulceration should include a topical corticosteroid for at least a period exceeding ten weeks in conjunction with a prophylactic oral antiviral. A treatment period greater than ten weeks has been recommended, because of the high treatment failure rates six weeks after a ten-week prednisolone taper in the HEDS clinical trial. The most concerning side effects of topical steroids include: increase of the intraocular pressure, cataract and secondary infections (including bacterial, fungal, and also viral infections). Therefore, patients must be monitored carefully when treating with topical steroids.
\nThe HEDS study on recurrence rate clearly demonstrated that short-course oral during an active HSV epithelial keratitis does not prevent later stromal keratitis or iritis. On the other hand, a 12-month course of prophylactic oral acyclovir (400 mg) twice daily significantly decreased a recurrence rate of the stromal involvement. Although the HEDS study authors did not recommend a prolonged, beyond 12 months acyclovir prophylaxis, clinical practice recommendations and observations seem to postulate a positive role of a long-term prophylaxis, especially in patients with a high recurrence rate, significant corneal thinning at risk of corneal perforation, with comorbidities, such as atopy, autoimmune diseases or in immunocompromised patients. Also patients with history of HSV keratitis undergoing surgical procedures, such as corneal transplant, photorefractive procedures or cataract surgery may benefit from acyclovir prophylaxis, until the level of inflammation associate with the procedure and the risk of recurrence is decreased [12, 29, 30, 31, 32, 33, 34, 35, 36, 37].
\nOne of the future treatment strategies is to enhance patient’s immune system resistance to the infection through a vaccine against HSV-1. Nowadays there are no approved vaccine available, but there are ongoing studies regarding this subject. In the recently published study in 2020, the authors identified 15 viral-encoded proteins, which could serve as candidates for further testing for the HSV-1 vaccine [40].
\nThere are several methods of treatment depending on the severity level of keratitis. First line therapy includes discontinuing potentially toxic topical medications, tear replacement products and oral supplementation with omega-3 fatty acids. The next step of treatment is immunomodulatory therapy including: lifitegrast, cyclosporine and steroids at different frequency and concentrations, and also autologous serum eye drops at concentrations from 20–100%. Autologous serum eye drops are characterized by multiple benefits: biochemical characteristics, including pH, nutrient content, vitamins, fibronectin, growth factors such as epithelial growth factor (EGF) or nerve growth factor (NGF), are similar to that of human tears, the serum eye drops also inhibit the release of inflammatory cytokines and increase the number of goblet cells and mucin expression in the conjunctiva. Prolonged use of serum eye drops is proved to restore homeostasis of the ocular surface.
\nIn the last few years, there have been an increasing interest in the implementation of the Nerve Growth Factor (NGF) in the sub-basal nerve plexus regeneration, leading to the complete healing of the neurotrophic ulcers. NGF is an endogenous protein involved in the differentiation and maintenance of all systemic neurons, while in corneal tissue it is established to play a role in corneal innervation, tear secretion mechanism, and corneal epithelial cell growth and stability. Cenegermin is a recombinant human Nerve Growth Factor (rhNGF) that is structurally identical to the human NGF protein made in ocular tissues, it was introduced in the ophthalmic solution at concentration of 0.002% (20 mcg/mL). Two controlled clinical trails in Europe (REPARO) and USA (NGF0214) provided strong evidence on its effectiveness. 72% and 65% of patients with neurotrophic keratitis receiving cenegermin were completely healed in Europe and USA trails respectively [41, 42, 43, 44]. Matrix regenerating agent (ReGenerating Agent; RGTA), mimicking natural heparan sulfate within the corneal tissue, is also a recent topical agent showing promising results in the treatment. RGTA eye drops (Cacicol; Thea) are preservative-free, well-tolerated, proved to promote regeneration of damaged tissues and to enhance corneal tissue healing [45, 46].
\nNovel emerging treatment approaches also include thymosine β4, CODA001, topical insulin, Substance P and insulin-like growth factor 1 (IGF-1). Thymosine β4 and CODA001 are in the most advanced evaluation undergoing clinical trials. Thymosin beta 4 is a 43-amino acid peptide, a major constituent protein of macrophages, and platelets. Currently, third-phase, multi-center, randomized, double masked, placebo controlled clinical study is ongoing regarding its role in ocular surface healing. Insulin at 3 different concentrations. CODA001 is an antisense oligonucleotide (antisense deoxynucleotide oligomer) that modulates and downregulates the expression of the gap junction protein Cx43 (Connexin-43), which is increased in persistent epithelial defects [47].
\nOther procedures implemented at different severity levels of neurotrophic keratitis include: therapeutic contact lenses, lacrimal punctual occlusion, amniotic membrane contact lens or transplantation, partial or complete tarsorraphy, corneal transplant, conjunctival flap transplant or direct neurotization.
\nAmniotic membrane transplantation (AMT) is proved to provide many benefits in the treatment of neurotrophic keratitis. AMT inhibits the activity of inflammatory cells, extends the life of corneal epithelial stem cells and maintains their ability to regenerate epithelial cells, promotes healing of the corneal wounds, blocks the TGF-ß cytokine system activation and the transformation of fibroblasts into myofibroblasts, also creates a protective membrane covering the affected ocular surface tissues. In dry eye disease, it is used in case of serious complications, such as corneal ulcer or microperforation. An interesting solution to consider is a sutureless, adhesiveless amniotic membrane transplant (AMT; ProKera; Bio-Tissue, Inc.) implantation. It is a corneal–epithelial device that consists of a polycarbonate ring conformer containing cryopreserved amniotic membrane. Advantages of this design include: shorter surgical time and prevention of suture-related complications [48].
\nTo summarize neurotrophic keratitis treatment: a stepwise approach should be implemented with careful exclusion of the active infection. Topical treatments should be the first line therapy over the surgical interventions.
\nSurgical interventions in active HSV keratitis are limited to the severe stromal involvement with the increased risk of corneal perforation. Those may include: application of cyanoacrylate glue, amniotic membrane transplantation or therapeutic keratoplasty.
\nOther indications for surgical procedures include inactive corneal scarring after keratitis or cataract formation mainly due to prolonged treatment with topical steroids. Superficial opacifications could be considered as an indication for phototherapeutic keratectomy (PTK), although the corneal thinning is usual after HSV keratitis and therefore it limits the use of this method. The PTK ablation should always be limited to anterior one-third of stromal layers and leave a minimum residual stromal bed thickness (RSBT) of 250 μm to avoid further corneal ectasia. Also, spontaneous reactivation of HSV keratitis is well known after PTK, because laser ablation stimulates viral shedding in tears and reactivates the virus [49, 50].
\nWhen an extensive scar with corneal thinning is present a deep anterior lamellar keratoplasty (DALK) or penetrating keratoplasty (PK) should be considered. DALK eliminates the risk of endothelial immunologic rejection, but due to advanced corneal scarring and thinning may be difficult to perform. An obligatory preoperative assessment before keratoplasty procedures include the corneal sensitivity analysis and the exclusion of the active viral infection with neovascularization. It is well established, that the presence of deep stromal vascularization exceeding 2 or more quadrants, creates a significant risk for a graft immunologic rejection and graft failure. Another factor, strongly connected to the increased risk of the graft failure is a herpetic infection recurrence. To address those issues, the combination of the antiviral prophylaxis with the prophylaxis of a immunologic rejection should be implemented. Antiviral prophylaxis includes the use of high-dose oral acyclovir as recommended by American Academy of Ophthalmology (AAO guidelines recommended 800 mg 3 times daily for at least 1 year) [37]. The prophylaxis of a immunologic rejection includes usually systemic steroids combined with topical therapy. Despite the prophylaxis, there is a relatively high rate of graft failure performed in eyes after herpetic keratitis reported in the literature: 26% at 3 years, 15% at 5 years and 53.7% at 8 years [51, 52, 53]. In the last years, there have been an increasing interest in keratorosthesis surgery, as a viable option allowing a long term restoration of vision in patients with high risk for corneal transplantation. Boston type I keratoprosthesis (BKPro) is the most commonly implanted keratoprosthesis worldwide. BKPro was first used in 1965 by Professor Claes H. Dohlman [54, 55]. The BKPro surgery is usually complex with the high incidence of intraocular complications. Also the rate of postoperative complications is high and includes: glaucoma, retroprosthetic membrane formation, keratolysis, endophthalmitis, vitreoretinal complications, such as retinal detachment, cystoid macular edema, uveitis and hypotony/phthisis. In the latest study of the long term BKPro outcomes published in 2020, the probability of maintaining or improving vision was 75,0% at 5 years and 66,7% at 10 years [56].
\nIn summary, surgical intervention in HSV keratits is challenging and high-risk procedure, therefore a special attention should be brought when referring such patients.
\nHSV keratitis due to its multiform occurrence remains a challenging diagnostic in clinical practice. Modern imaging technique, such as optical coherence tomography or confocal microscopy as well as modern laboratory testing including multiplex dot hybridization (MDH) assay, immunochromatographic assay (ICGA, AmpliVue) are useful in guiding the diagnostic process.
\nOcular surface homeostasis should be always considered when treating HSV keratitis, especially in the neurotrophic keratitis at different severity grades.
\nThe author has no conflict of interest.
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