1. Introduction
Gliomas are the most common central nervous system neoplasms that arise from the transformation of astrocytes or their precursor cells. Glial tumors develop as a result of stepwise accumulation of genetic alterations, which disrupt the cell cycle arrest pathways or activate various signal transduction pathways. Despite recent advances in treatment modalities such as surgical techniques, radiation therapy, chemotherapy and targeted gene therapy, their prognosis remains poor. Gliomas are graded from I to IV according to the 2007 World Health Organization (WHO) malignancy scale. Grade I lesions are benign and quite constrained with a slow propagation rate and they constitute the most common glioma of children, pilocytic astrocytoma. Grade II tumors, called diffuse astrocytoma have a slow growth rate and a high degree of cellular differentiation, with their ability to diffuse into normal brain parenchyma and progress toward more malignant form. Grade III tumors include anaplastic astrocytoma, which are characterized by a higher cellular density and the plentiful persistence of atypia and mitotic cells. Grade IV tumors are the most frequent malignant gliomas and they are characterized as glioblastoma with high recurrence rate. GBMs include two subtypes, primary GBMs arise `
Cell polarity is an essential phenomenon in several biological processes that contribute to normal tissue integrity and maturity. Several studies in different genetic models has identified and revealed different roles of polarity complexes in maintenances of stem cell population and their asymmetric division (Knoblich., 2010), T-cell function like migration in response to chemokines and antigens (Krummel and Macara., 2006), neuronal cell axon and dendritic specification (Arimura and Kaibuchi., 2007) and cell polarity is crucial for epithelial cell and tissue polarization for maintenance of multicellular structures and perform normal physiological functions like secretion, absorption and distribution of cytoplasmic and membrane proteins in appropriate positions within the cell in order to conduct proper signals.
2. Cell polarity regulators and their deregulation in cancer
Proficient research work in organisms like
2.1. PAR polarity complex
Par complex was initially identified in
2.2. Crumb polarity complex
Crumbs polarity complex was identified in
2.3. SCRIB polarity complex:
Scribble, Dlg and Lgl were identified in Drosophila as tumor suppressors, in mammals SCRIB is also called VARTUL. Scribble is a member of LAP family with leucine-rich repeats and PDZ domains, while Dlg in mammals exists in five isoforms and it is a member of the MAGUK family containing PDZ domains. Lgl which exists as Lgl1 and Lgl2 in mammals does not contain PDZ domains but has WD40 domains, which is thought to mediate interactions with phosphorylated serine and tyrosine (Dow et al. 2007).
Interaction among these three complexes dictates them to localize in their respective positions, for example Lgl1 and 2 can compete with Par3 for binding to a module of Par6 and aPKC and aPKC phosphorylate Lgl releasing it from the Par6/aPKC dimer, ensuing its localization to the basolateral region of cells (Betschinger et al. 2003). These complexes induce their function by regulating cytoskeleton architecture and there is a substantial data showing direct link with small GTPases of the Rho family and Cdc42, Rac1 and RhoA, control the cytoskeletal changes in cells by switching between an active GTP-bound state and an inactive GDP-bound state. For example Par3 can bind the Rac-activator Tiam1 during tight junction formation and Par6/Par3 regulates Cdc42-mediated Rac1 activation through Tiam1 in neuronal cells (Nishimura et al. 2005). These multifaceted interactions between polarity proteins and with small GTPases is essential for maintenance of polarity and carrying out normal physiological functions of the cell and any aberrant regulation in any of these proteins is related to tumor genesis.
Cell polarity proteins regulate cancer cell properties like proliferation, apoptosis, and epithelial-mesenchymal transition. Studies have shown that Par6 induces growth factor independent proliferation of human mammary epithelial cells by activating MAPK signaling through aPKC and Cdc42/Rac (Muthuswamy et al. 2008). aPKC can regulate cell proliferation through ERK and SRC-3 dependent manner (Castoria et al. 2004 and Yi et al. 2008) and knock down of aPKC in MCF-7 breast cancer cell line inhibited cell proliferation. Down regulation of Scribble induces JNK-dependent cell death (Brumby and Richardson. 2003) and in similar manner inhibition of aPKC increases apoptosis of MDCK cell by activating GSK3β (Kim et al., 2007). Acquisition of mesenchymal property by cancer cells during metastasis is due to change in the cell architecture which is regulated by cell polarity proteins. Research in genetic model
Cancer is a multistep process whereby cells first acquire benign over proliferation due to genetic assaults, followed by inhibition of apoptosis and increased cell proliferation. The conversion of benign to malignant form is accompanied by loss in cell-cell to contact and apical-basal polarity which ultimately leads to EMT associated with invasion into different parts from the site of cancer. During cancer progression steps these different polarity complexes are either aberrantly expressed or mislocalised from their respective locations. Genetic studies in model organisms and
However contribution of Crumb complex for tumor progression is not completely elucidated. Few studies have shown that Crb-3 expression negatively correlates with metastatic behavior of cells as its decreased expression is associated with increased expression of vimentin and reduced expression of E-cadherin (Bhat et al., 1999). Assembly of Crumb complex leads to phosphorylation of transcription effector molecule of Hippo signaling pathway TAZ/YAP which in turn leads to inhibition of TGF-β-SMAD signaling essential for vimentin expression (Varels et al., 2010). ZEB1 and SNAIL represses the Crumb complex activity. PATJ and Pals1 are required for tight junction (TJ) assembly. PATJ is targeted for degradation by human papiloma virus (HPV) oncoprotein during development of cervical cancer (Javier. 2008). Till date there is no evidence for role of Pals1 in cancer, however few studies in mice model have shown that Pals1 is essential for survival since its loss results in embryonic lethality.
Scribble complex, regarded as fly tumor suppressor genes and basolateral polarity complex are regulated at different levels in human cancers. Dlg and Lgl proteins are down regulated and mislocalised in tumors of breast, prostate, lung, ovary, cervical and liver.scribble and Dlg are targeted for degradation by viral oncoproteins from HPV(Thomas et al., 2005), Human T-cell leukemia (HTLV) (Okajima et al., 2008) and their expression levels are correlated with loss of tissue architecture. Scribble is miss localized in cervical, colon, endometrial and prostate cancer (Nakagawa et al., 2004, Gardiol et al., 2006, Ouyang et al., 2010 and Pearson et al., 2011). In
3. Cell polarity in glial cell
Glial cells include myelinating oligodendrocytes, Schwann cells and astrocytes and they execute diverse functions that are vital for the development, functioning and regeneration of neurons. Unlike epithelial cells which require cell polarity for polarization of cellular components and neurons for axon and dendrite specification, glial cells require cell polarity for migration and myelination.
Oligodendrocytes and Schwann cells are responsible for myelination of neurons, it requires proper sorting of proteins and lipids and polarized membrane trafficking to organize myelin domains and maintain this highly polarized phenotype (DeBruin and Harauz. 2007). Astrocytes are the cells which carry out major functions in brain including interactions with neurons and blood vessels (Schipke and Kettenmann., 2004), migration towards inflammation called astrogliosis (Ridet et al. 1997) which requires polarization of astrocytes into front-rear axis. Astrocytes polarize and migrate by interacting with extra cellular matrix components. Upon interaction of ECM with integrin receptors on astorcytes, activates intracellular signaling through small G proteins like Rac and Cdc42 for controlled polarization and orientation (Heasman and Ridley. 2008). Guanine exchange factors are in charge for the GDP–GTP exchange and therefore are the major regulators of small G proteins activity. It has been shown that cell polarity protein Scrib plays a crucial role in astrocyte migration by binding with Rac and Cdc42-specific exchange factor βPIX and controlling the localization of Cdc42 at leading edge of migratory processes (Osmani et al. 2006). Besides Scrib other evolutionary conserved polarity proteins like Par protein complex is implicated in astrocyte migration. Par6-aPKC complex controls and regulates the microtubule organization during astrocyte migration. The GTP-bound form of Cdc4 binds to Par6 and activates aPKC kinase at leading edge of astrocyte (Etienne-Manneville et al., 2005).
4. Loss of cell polarity during gliomagenesis
Previously loss of cell polarity was regarded as post effect of cancer, but recent research work led to discovery that cell polarity is lost and responsible for tumorigenesis and its progression. Loss of cell polarity in brain tumors is not well documented and very few works like Klezovitch et al. have show that loss of cell polarity causes severe brain dysplasia using Lgl1 knockout mice. We analyzed total of 50 astrocytic tumor samples from Krishna Institute of Medical Sciences (Hyderabad, India), they were classified histopathologically according to the WHO classification: 16 pilocytic astrocytoma (PILO; grade I), 8 diffuse astrocytomas (DA; grade II), 7 anaplastic astrocytomas (AA; grade III), and 19 glioblastoma multiforme (GBM; grade IV) and one normal brain sample was obtained during autopsy for the expression of Scribble polarity complex in astocytic tumors. All samples were obtained from patients after taking the informed consent form patients or their guardians. Using western blotting and immuno histochemical examination we found that protein levels of Scrib was negatively associated with increase in the tumor grade and in few GBM samples Scrib was completely absent. Immuno fluorescence staining as shown in Fig. 2, 3, 4 revealed that in control brain astrocytes, Scrib was localized at the end of astrocyte processes and it was mislocalised in low grade tumors with complete absence in GBM (Khamushavalli et al., unpublished data).
5. Conclusion
Loss of cell architecture and cell-cell contacts is the hall mark of metastasis, and these process are regulated by cell polarity proteins. Recent work in different cancers has revealed the expressional loss and mislocalisation of cell polarity proteins and their potential role as tumor suppressors. Till date there is no report explaining the role of these polarity proteins in glioma and the above results indicate that cell polarity proteins do have role in glioamgenesis and further research is necessary to elucidate how these proteins are deregulated during glioma prosression.
Acknowledgments
The authors would like to acknowledge funding from DBT, CSIR, ICMR, DST and DST nano UoH project. We thank CSIR for fellowship to Khamushavalli Geevimaan.
References
- 1.
Ohgaki H Kleihues P 2007 Genetic pathways to primary and secondary glioblastoma.170 1445 1453 - 2.
Holland E. C 2001 Gliomagenesis: genetic alterations and mouse models.2 120 129 - 3.
Cavenee W. K 1992 Accumulation of genetic defects during astrocytoma progression.70 1788 1793 - 4.
Hayashi Y Ueki K Waha A Wiestler O. D Louis D. N Von Deimling A 1997 Association of EGFR gene amplification and CDKN2 (16 MTS1) gene deletion in glioblastoma multiforme. 7:871-875. - 5.
Knoblich J. A 2010 Asymmetric cell division: recent developments and their implications for tumour biology.11 849 860 - 6.
Krummel M. F Macara I 2006 Maintenance and modulation of T cell polarity.7 1143 1149 - 7.
Arimura N Kaibuchi K 2007 Neuronal polarity: from extracellular signals to intracellular mechanisms. .8 194 205 - 8.
Bilder D Li M Perrimon N 2000 Cooperative regulation of cell polarity and growth by Drosophila tumor suppressors.289 113 116 - 9.
The Scribble and Par complexes in polarity and migration: friends or foes?Humbert P. O Dow L. E Russell S. M 16 622 630 - 10.
Horikoshi Y Suzuki A Yamanaka T Sasaki K Mizuno K Sawada H et al 2009 Interaction between PAR-3 and the aPKC-PAR-6 complex is indispensable for apical domain development of epithelial cell.122 1595 1606 - 11.
Tepass U Knust E 1993 Crumbs and stardust act in a genetic pathway that controls the organization of epithelia in .159 311 326 - 12.
Dow L. E Kauffman J. S Caddy J Zarbalis K Peterson A. S Jane S. M Russell S. M Humbert P. O 2007 The tumour-suppressor Scribble dictates cell polarity during directed epithelial migration: regulation of Rho GTPase recruitment to the leading edge.26 2272 2282 - 13.
Betschinger J Mechtler K Knoblich J. A 2003 The Par complex directs asymmetric cell division by phosphorylating the cytoskeletal protein Lgl.422 326 330 - 14.
Nishimura T Yamaguchi T Kato K Yoshizawa M Nabeshima Y Ohno S et al 2005 PAR-6-PAR-3 mediates Cdc42-induced Rac activation through the Rac GEFs STEF/Tiam1.7 270 277 - 15.
Nolan M. E Aranda V Lee S Lakshmi B Basu S Allred D. C Muthuswamy S. K 2008 The polarity protein Par6 induces cell proliferation and is over expressed in breast cancer. Cancer Res.68 8201 8209 - 16.
Di Domenico, M., Lombardi, M., De Falco, A., Varricchio, L., Bilancio, A., Barone, M.V., Auricchio, F.Castoria G Migliaccio A 2004 Role of atypical protein kinase C in estradiol-triggered G1/S progression of Mcf-7 cells.24 7643 7653 - 17.
B.W.Yi P Feng Q Amazit L Lonard D. M Tsai S. Y Tsai M. J O Malley 2008 Atypical protein kinase C regulates dual pathways for degradation of the oncogenic coactivator Src-3/Aib1.29 465 476 - 18.
Brumby A. M Richardson H. E 2003 Scribble mutants cooperate with oncogenic Ras or notch to cause neoplastic overgrowth in Drosophila.22 5769 5779 - 19.
Kim M Datta A Brakeman P Yu W Mostov K. E 2007 Polarity proteins Par6 and APKC regulate cell death through GSK-3beta in 3d epithelial morphogenesis. ,120 2309 2317 - 20.
Pagliarini R. A Xu T 2003 A genetic screen in Drosophila for metastatic behavior.302 1227 1231 - 21.
Wang H. R Zhang Y Ozdamar B Ogunjimi A. A Alexandrova E Thomsen G. H Wrana J. L 2003 Regulation of cell polarity and protrusion formation by targeting Rhoa for degradation.302 1775 1779 - 22.
Aigner K Dampier B Descovich L Mikula M Sultan A Schreiber M Mikulits W Brabletz T Strand D Obrist P et al 2007 The transcription factor Zeb1 ([Delta]Ef1) promotes tumour cell dedifferentiation by repressing master regulators of epithelial polarity.26 6979 6988 - 23.
Regala R. P Weems C Jamieson L Khoor A Edell E. S Lohse C. M et al 2005 Atypical protein kinase C iota is an oncogene in human non-small cell lung cancer.65 8905 8911 - 24.
Eder A. M Sui X Rosen D. G Nolden L. K Cheng K. W Lahad J. P et al 2005 Atypical PKCiota contributes to poor prognosis through loss of apical-basal polarity and cyclin E overexpression in ovarian cancer.102 12519 12524 - 25.
Zen K Yasui K Gen Y Dohi O Wakabayashi N Mitsufuji S et al 2009 Defective expression of polarity protein PAR-3 gene (PARD3) in esophageal squamous cell carcinoma.28 2910 2918 - 26.
Ozdamar B Bose R Barrios-rodiles M Wang H. R Zhang Y Wrana J. L 2005 Regulation of the polarity protein Par6 by TGFbeta receptors controls epithelial cell plasticity. .307 1603 1609 - 27.
Bhat M. A Izaddoost S Lu Y Cho K. O Choi K. W Bellen H. J 1999 Discs Lost, a novel multi-PDZ domain protein, establishes and maintains epithelial polarity.96 833 845 - 28.
Varelas X Samavarchi-tehrani P Narimatsu M Weiss A Cockburn K Larsen B. G Rossant J Wrana J. L 2010 The Crumbs complex couples cell density sensing to Hippo-dependent control of the TGF-β-SMAD pathway. Dev Cell.6 831 44 - 29.
Javier R. T 2008 Cell polarity proteins: common targets for tumorigenic human viruses’.27 7031 7046 - 30.
Thomas M Massimi P Navarro C Borg J. P Banks L 2005 The hScrib/Dlg apico-basal control complex is differentially targeted by HPV-16 and HPV-18 E6 proteins..24 6222 6230 - 31.
Okajima M Takahashi M Higuchi M Ohsawa T Yoshida S Yoshida Y et al 2008 Human T-cell leukemia virus type 1 Tax induces an aberrant clustering of the tumor suppressor Scribble through the PDZ domain-binding motif dependent and independent interaction.37 231 240 - 32.
Nakagawa S Yano T Nakagawa K Takizawa S Suzuki Y Yasugi T et al 2004 Analysis of the expression and localisation of a LAP protein, human scribble, in the normal and neoplastic epithelium of uterine cervix. .90 194 199 - 33.
Gardiol D Zacchi A Petrera F Stanta G Banks L 2006 Human discs large and scrib are localized at the same regions in colon mucosa and changes in their expression patterns are correlated with loss of tissue architecture during malignant progression. .119 1285 1290 - 34.
Ouyang Z Zhan W Dan L 2010 hScrib, a human homolog of neoplastic tumor suppressor, is involved in the progress of endometrial cancer. Oncol Res.18 593 599 - 35.
Pearson H. B Perez-mancera P. A Dow L. E Ryan A Tennstedt P Bogani D et al 2011 SCRIB expression is deregulated in human prostate cancer and its deficiency in mice promotes prostate neoplasia. .121 4257 4267 - 36.
Klezovitch O Fernandez T. E Tapscott S. J Vasioukhin V 2004 Loss of cell polarity causes severe brain dysplasia in Lgl1 knockout mice.18 559 71 - 37.
Debruin L. S Harauz G 2007 White matter rafting- membrane microdomains in myelin.32 213 228 - 38.
Schipke C. G Kettenmann H 2004 Astrocyte responses to neuronal activity. .47 226 232 - 39.
Ridet J. L Malhotra S. K Privat A Cage F. H 1997 Reactive astrocytes: cellular and molecular cues to biological function. ,20 570 577 - 40.
Heasman S. J Ridley A. J 2008 Mammalian Rho GTPases: new insights into their functions from in vivo studies.9 690 701 - 41.
Osmani N Vitale N Borg J. P Etienne-manneville S 2006 Scrib controls Cdc42 localization and activity to promote cell polarization during astrocyte migration. ,16 2395 2405 - 42.
Etienne-manneville S Manneville J. B Nicholls S Ferenczi M Hall A 2005 Cdc42 and Par6-PKCζ regulate the spatially localized association of Dlg1 and APC to control cell polarization. .170 895 901