Open access peer-reviewed chapter
Abstract
The molecular and cell biology of cervical cancer will be covered in detail in this chapter, particularly emphasizing the disease’s etiology, brief epidemiology, risk factors, cervical cancer hallmarks, and the main signaling pathways involved. The chapter will go in-depth about the characteristics of cancer, such as changes in cell cycle regulation, apoptosis, and cell differentiation, as well as the tumoral microenvironment. Signal pathways like the PI3K/AKT/mTOR pathway and the Wnt/beta-catenin pathway will be highlighted for their significance in the development of cervical cancer. The chapter will thoroughly explain the molecular and cell biology underlying this terrible illness.
Keywords
- cervical cancer
- molecular biology
- cancer biology
- cancer cells
- tumor niche
- tumor microenvironment
- cancer signaling
- hpv
- cancer hallmarks
1. Introduction
Cervical cancer (CC) is a cellular alteration that originates in the cervix due to the persistent infection of oncogenic genotypes of the human papillomavirus (HPV) and initially manifests itself through precancerous lesions of slow and progressive evolution [1].
CC progression takes around 20 years to generate an invasive carcinoma; it begins with the precancerous lesions called cervical intraepithelial neoplasias (CIN) [2]. It is estimated that around 30% of women with advanced CIN premalignant lesions who do not receive treatment can progress to CC [3].
CC is recognized as a significant health burden in low- and middle-income countries, where it is ranked as the fourth most common type of female cancer [4, 5] and the second leading cause of cancer death in women worldwide [6]. In 2020, there were an estimated 604,000 new cases of cervical cancer and 324,000 deaths worldwide, with almost 90% of these cases occurring in low- and middle-income countries [6]. CC prevention is derived from vaccination schemes against HPV, the use of condoms, and timely detection of precancerous lesions through cervical cytology and HPV detection tests [7].
In October 2019, the World Health Organization (WHO) reported that 124 countries already had a timely and free vaccination program for 10-year-old girls. However, this is still insufficient to significantly reduce CC cases, given that the distribution of vaccines barely covers 30% of the world population, and screening schemes are not usually applied in all health sectors in low- and medium-income countries [8]. Because of this situation, in 2020, the WHO committee supported the Global Strategy towards the Elimination of Cervical Cancer, and it marked a significant milestone as it became the first-ever elimination strategy for a cancer in the history of the WHO.
The strategy sets forth three specific global targets aimed at preventing and treating CC; by 2030, 90% of girls should receive complete HPV vaccination before reaching 15 years of age; screening using a high-performance test should be conducted on 70% of women by age 35 and again by age 45; and 90% of identified CC patients should receive the suitable treatment [9]. These targets serve as measurable indicators to gauge progress in preventing and managing cervical cancer globally.
Known risk factors for developing CC are HPV, low socioeconomic status, smoking, young age at first intercourse, unprotected sexual intercourse, polygamy, long-term use of hormonal contraceptives, and multiple births [7, 10, 11, 12, 13]. However, it has been shown that the most critical risk factor for CC development is persistent infection by high-risk HPV genotypes, which is essential for cell transformation and can be detected in 99.7% of CC cases [14, 15].
HPVs belong to the
Low-risk HPVs, such as genotypes 6, 11, 42, 43, and 44, are classified as such because they commonly cause only benign epithelial lesions, such as warts and papillomas. On the other hand, high-risk genotypes, including 16, 18, 31, 33, 34, 39, 45, 52, 53, 58, 68, and 70, are strongly linked to the development of CC [17, 18]. Genotypes 16 and 18 are considered of the most significant clinical relevance due to their association with approximately 70% of the CC cases worldwide, in addition to their great oncogenic potential [19, 20].
The oncogenic nature of high-risk HPV is attributed to the activity of E6 and E7 oncoproteins, which are the only ones that remain active even when the transformation process of cervical cells has already begun [21]. Continuous expression of the E6 and E7 oncoproteins is essential to initiate and sustain the transformation of infected cells since they activate immunological and carcinogenic pathways to favor tumorigenesis and modulate local immunity [22, 23].
2. Oncogenes, tumor suppressor-genes, and mutations
The transition from proto-oncogenes (genes with normal biological functions in cell homeostasis) to oncogenes (mutated proto-oncogenes with a gain in their normal biological function) is considered one of the first molecular events that trigger subsequent carcinogenesis [24, 25]. Changes in function from proto-oncogenes to oncogenes allow the appearance of key signs of cancer in a cell population, such as loss of apoptosis, uncontrolled cell multiplication, angiogenesis, and metastasis to distant organs [26].
Another fundamental molecular transformation for carcinogenesis is the loss of function of TSG, which is the primary antagonistic mechanism of carcinogenesis by maintaining cell homeostasis, promoting the repair of cell damage, and inducing apoptosis when this repair fails to occur [24, 26]; it has been hypothesized that the loss of function of both alleles of these genes is necessary to cause a phenotypic change; this is known as Knudson’s phenomenon or the “two-hit theory” [27].
2.1 Cervical cancer and tumor suppressor genes
A sequence of steps in carcinogenesis onset related to high-risk HPV has been hypothesized in CC, presumably beginning with the decrease in the activity of tumor suppressor genes [26].
HPV infects the basal cells of the cervical epithelium, using the normal cycle of epithelial cells, which replicate in the basal region (where the virus replicates minimally), migrate to the apical region, and specialize (where viral replication increases) [28, 29, 30]. The process of carcinogenesis secondary to infection entails a process of several decades of persistent or repetitive infection, in which, also accidentally, the viral genome integrates into the host genome [26, 28, 30, 31], which is not part of the normal life cycle of the virus [30].
Upon integration into the genome, various molecular processes take place. These processes have two outcomes: Firstly, they interfere with the
Figure 1.
Schematic representation of the main action of E6 and E7 viral oncoproteins on tumor suppressor genes. E6 blocking the function of the tp53 protein and inducing its degradation. E7 blocking the function of the RB protein and inducing its degradation.
2.2 Proto-oncogenes and oncogenes
Once genomic instability and cell immortalization are established by mechanisms that will be described further, it is considered that oncogenesis continues with the transition from proto-oncogenes to oncogenes. One of the main pathways described in more than 90% of cells with CC is mediated by PI3K/Akt [35], which promotes mechanisms of cellular proliferation; the activation of this pathway has been described secondary to the suppression of the PTEN gene [35] or to the overexpression of PI3KCA [25].
Other important oncogenes related to the proliferation of CC are the
3. Cell survival and immortality in cervical cancer
In CC, cancer cells survive by avoiding apoptosis and acquiring the capacity to replicate indefinitely. These characteristics and other hallmarks of cancer allow its uncontrolled proliferation and the formation of tumor masses in the cervix [36].
It is thus a well-recognized fact that resistance to apoptosis is a hallmark of cancer in general, a requirement for the persistence of transformed cells that, through various strategies, manage to evade it [36].
Nevertheless, what allows CC cells to overcome apoptosis? Aforesaid, HPV infection is a necessary and predisposing factor for developing and progressing CC. Interestingly, HPV E6 and E7 oncoproteins have been shown to enable cancer cells to escape apoptosis by disrupting or modifying pathways that precipitate this mechanism and that are finely regulated by pro- and anti-apoptotic proteins [38]. The ubiquitin-mediated degradation of p53 by E6 oncoprotein prevents cell cycle arrest and, ultimately, the apoptotic machinery, mediated in part by transcriptional activation of BAX and PUMA [39]. Given this fact, it is worth mentioning that the presence of E6 leads to a significant increase in the activity of the
Regarding E7 oncoprotein, its role in cell cycle has been previously described; however, concerning apoptosis, it plays a dual role; it can induce or inhibit this process depending on its interaction with different proteins and mechanisms and significantly depending on the HPV present [43].
Moreover, considering that the E5 oncoprotein does not usually receive much attention due to its elimination at later stages of infection, it is essential to mention that its presence in early stages is crucial for survival and propagation of the virus in the cervical epithelium [44]. It modulates the epidermal growth factor signaling pathway, which induces the degradation of BAX, which is essential for triggering the intrinsic apoptosis pathway [45]. A relationship has also been found between its presence and the downregulation of death receptors such as CD95 in cervical tumors leading to impaired apoptosis [46].
Given that CC cells have effectively evaded the apoptosis mechanism, it is essential to mention that this enables them to achieve immortality, that is, an unlimited replication potential, thus contributing to the development and progression of cancer. Usually, as cells replicate, the ends of the chromosomes that protect the DNA during cell division, called telomeres, shorten with each successive cycle. This is part of cellular aging. However, in cancer cells, it is necessary to prevent the telomeres from shortening so the tumor can continue growing [47].
Telomerase is an enzyme responsible for telomere replication and is overexpressed in cancer cells but inactive in healthy cells [48]. In this regard, it has already been reported that HPV E6 and E7 persistently drive the expression of a catalytic unit in telomerase called hTERT, which gives these cells unlimited replicative capacity [49]. This is possible because E6 and E7 activate the hTERT promoter with the help of proteins such as c-myc and Sp1 that act as positive regulators and NFX1. NFX1 normally represses hTERT expression but is degraded by E6/E6AP, which activates the hTERT promoter [50].
4. Main signaling pathways (JAK/STAT, Ras/MEK/Erk, PI3K/Akt/mTOR, WNT/b-catenin)
Giving continuity to the critical regulation of external and internal factors on cell function, space is given to understand the signaling that makes this possible at the molecular level. Below are four signaling pathways relevant to the development of CC since their inappropriate regulation allows the cell to experience anabolic, proliferative, growth-promoting, and cell-survival effects.
4.1 Janus Kinase/Signal Transducer and Transcription Activator (JAK/STAT) signaling pathway
The Janus Kinase/Signal Transducer and Transcription Activator (JAK/STAT) signaling pathway is characterized by its physiological contribution to cell proliferation, differentiation, and death by causing a final gene transcription reaction in the cell nucleus. Its dysfunction can cause alterations in immune regulation and tumor processes [51].
The sequence of events begins with the interaction of cytokines and growth factors with their respective receptors, types I and II, on the cell membrane. The intracellular portion of the receptor interacts with the inactive JAK [52], which undergoes dimerization and oligomerization [53]. In both cases, a conformational change is induced in the cytoplasmic domain [52]. The described effect is the juxtaposition of JAKs for their phosphorylation and transphosphorylation by other JAKs or other families of tyrosine kinases.
Activation of JAKs phosphorylates the cytoplasmic domain, creating a binding site for other signaling molecules such as STAT proteins [51]. Cytoplasmic STAT binds to phosphorylated receptors, thus becoming substrates to be phosphorylated by JAKs. After phosphorylation, STATs form homodimers or heterodimers capable of translocating to the nucleus and activating gene transcription [23, 51]. Regulators of this pathway have been identified as suppressor signaling cytokines (SOCS), activated protein inhibitors of STAT (PIAS), and protein tyrosine phosphatases (PTP) [54].
4.2 Mitogen-activated protein kinase (MAPK) signaling pathway
Similarly, the dysfunction and incorrect regulation of the mitogen-activated protein kinase (MAPK) signaling cascade has been linked to carcinogenic events since it is also involved in cell proliferation, survival, differentiation, and migration [55, 56]. This cascade begins with stimuli, including growth factors, tumor-promoting substances, and differentiation factors. There is a stimulation of Ras-GDP to convert to Ras-GTP, resulting in its activation resulting in its activation and subsequent Raf phosphorylation. In turn, Raf is responsible for the phosphorylation of MEK1. MEK1 phosphorylates ERK, an extracellular receptor kinase responsible for regulating cytosolic proteins, transcription factors, and metastasis [55, 56].
4.3 PI3K/AKT/mTOR signaling cascade
The PI3K/AKT/mTOR signaling cascade also affects energy metabolism. The negative effects related to CC lie in the contribution to the creation and proliferation of malignant cell phenotypes [55, 56]. The cascade begins with a lipid kinase, PI3K, activated by the effect of extracellular stimuli recognized by receptors associated with G proteins (GPCRs) or receptor tyrosine kinases (RTKs), also using Ras-GTPases [55, 56]. The stimuli in question are given by growth factors or cytokines, which cause receptor dimerization and transphosphorylation. Once the receptor is phosphorylated, it is ready to bind to and activate proteins, in this case, PI3K. Activated PI3K produces PIP3, from PIP2, and PI(3,4)P2. PIP3, in turn, attracts the AKT molecule to the cell membrane, which is then phosphorylated and activated by PDK1 and mTORC2 [57]. AKT is responsible for activating mTORC1, the molecule responsible for the anabolic effects in the cell. This signaling cascade is important in regulating cell functions such as cell growth, motility, survival, metabolism, and angiogenesis [55, 56]. Dysfunction of this cascade, particularly its overactivation, is implicated in carcinogenesis.
4.4 WNT/beta-catenin signaling pathway
Lastly, the fourth relevant signaling pathway in the pathogenesis of CC is Wnt/beta-catenin, also called canonical. Wnt is proteins incorporated into exosomes for transport and have target-specific genes that can be transcribed through beta-catenin stabilization [58]. Beta-catenin in its inactive state is bound to the so-called destruction complex consisting of Axin, CKI, GSK3, APC, Dvl (or Dsh), and Beta-TrCP (Figure 2), which regulates it by degradation [59]. Beta-catenin phosphorylation serves as a stimulus for beta-TrCP to ubiquitinate it, resulting in its proteasomal. The effects of beta-catenin can be evidenced only by avoiding its degradation when its levels in the cytosol are high.
Figure 2.
Schematic representation of the molecular components taking part in the Wnt/beta-catenin pathway.
The beginning of the cascade occurs when Wnt activates its receptor complex formed by the Frizzled receptor and co-receptor LRP5/6; this induces the phosphorylation of LRP6 by CK1 and GSK3, allowing the translocation of the destruction complex from the cytosol to the membrane [58]. This process activates the Dvl (Dsh) component, which in its active state induces the sequestration or degradation of Axin in such a way that it inhibits the destruction complex by stabilizing cytosolic beta-catenin [59]. Wnt has target genes within the DNA in the nucleus and requires a molecule called TCF for its transcription. In the inactive Wnt state, TCF in the nucleus is inhibited by Groucho, preventing it from binding to DNA and starting gene transcription [60, 61]. In its active state, Wnt triggers the steps mentioned above, culminating in the increase in intracellular beta-catenin. As beta-catenin accumulates in the cytosol, it enters the nucleus, removes Groucho from its inhibitory position, and binds to TCF [60, 61]. This accumulation leads to the transcription of Wnt target genes. The effects of Wnt/beta-catenin signaling in CC promote epithelial-mesenchymal transition, migration, growth, and cell proliferation [62].
The beginning of the cascade occurs when Wnt activates its receptor complex formed by the Frizzled receptor and co-receptor LRP5/6; this induces the phosphorylation of LRP6 by CK1 and GSK3, allowing the translocation of the destruction complex from the cytosol to the membrane [58]. This process activates the Dvl (Dsh) component, which in its active state induces the sequestration or degradation of Axin in such a way that it inhibits the destruction complex by stabilizing cytosolic beta-catenin [59]. Wnt has target genes within the DNA in the nucleus and requires a molecule called TCF for its transcription. In the inactive Wnt state, TCF in the nucleus is inhibited by Groucho, preventing it from binding to DNA and starting gene transcription [60, 61]. In its active state, Wnt triggers the steps mentioned above, culminating in the increase in intracellular beta-catenin. As beta-catenin accumulates in the cytosol, it enters the nucleus, removes Groucho from its inhibitory position, and binds to TCF [60, 61].
This accumulation leads to the transcription of Wnt target genes. The effects of Wnt/beta-catenin signaling in CC promote epithelial-mesenchymal transition, migration, growth, and cell proliferation [62].
5. Cell cycle alterations and genomic instability
Viral infections are related to about 9.9% of cancers worldwide as they contribute to the oncogenesis of host cells [63]. The transformative process of the HPV-infected cell could be classified as a multifactorial process in its virulence mechanisms, which ensure the survival and proliferation of the virus at the expense of the integrity of the human genome, implying a loss of genomic stability through a mutation or an oncogenic exposure, thus being the center point of cancer [36].
The intrinsic HPV proteins that are especially important in the transformative process of the cell are initially the E1 and E2 proteins and the E6 and E7 [64]. The process begins with the integration of the viral genome through the E1 helicase and the E2 binding protein; likewise, E1 facilitates the recruitment of DNA damage response (DDR) mechanisms, a set of proteins that signal the locations of damage to its repair and the signaling response by the ATM, which occurs in the event of a break in the double chain [65, 66]. Added to the expression of E1 and E2, it causes the formation of free radicals that damage DNA, causing changes in guanine to tyrosine kinase [67].
Finally, once the viral genome integrates with the host’s DNA, the hybrid gene presents alterations such as duplications, deletions, translocations, and inversions. Likewise, the integration of the viral genome itself can theoretically act as a
Ubiquitin-mediated degradation of p53 by E6 promotes genomic instability and the subsequent birth of cancer [64], since the absence of p53 activity allows the cell to proceed to the S phase of the cell cycle without having repaired the damaged areas of the DNA [21].
The progression from G1 to S without previous DNA revision and repair results in the accumulation of errors in the chromosomes [64]. E7 is the primary mediator for DDR activation, which is localized to the junction foci of the viral genome with the host genome. It is theorized that this DDR hijacking prevents the repair of further DNA damage [70]. In response to DDR, targeted transduction of the viral genome mediated by homologous recombination reduces the capacity to repair DNA double-strand breaks by 50% [71].
Other pro-carcinogenic activities of the E6 and E7 oncoproteins are the association with centrosome amplification and shortening, increasing cell cycle defects [71, 72]. This process drives the proliferation of infected cells from infected keratinocytes [22, 73]. The effect of E6 and E7 on telomeres causes lengthening and sometimes shortening capable of driving arrangements such as bridging in anaphase and “breakage-fusion-bridge” cycles or replication indefinitely [64, 74]; in CC, 64% of the cases show a high expression of alteration in telomeres [64, 75].
6. Metabolic switch
The “metabolic switch” or deregulation of cellular metabolism is considered one of the hallmarks of cancer and refers to the metabolic mutation that tumor cells present with an affinity for “aerobic” glycolysis to obtain energy and favor the synthesis of macromolecules for the cellular replacement [76, 77, 78], which will be explained in detail later.
Under normal conditions, healthy body cells in the presence of oxygen obtain energy through aerobic metabolism, that is, the citric acid cycle or Krebs cycle, where carbohydrates, amino acids, and lipids are metabolized [77]. Due to the oxidations in this cycle, three molecules of nicotinamide adenine dinucleotide (NAD) and one of flavin adenine dinucleotide (FAD) are produced (generating 2.5 ATP each NADH and 1.5 ATP each FADH) [77]. Those molecules (NADH and FADH) subsequently enter the respiratory chain and oxidative phosphorylation, resulting in a total of up to 36 ATP molecules through these aerobic routes [77].
Additionally, there is glycolysis, a metabolic pathway that occurs in normal cells without oxygen, where glucose is catabolized, obtaining 2 ATP molecules and two pyruvate molecules for each glucose molecule [79]. Both routes end with the same goal: to satisfy the energy needs of the tissues.
Most malignant cells alter these metabolic pathways to obtain energy (Figure 3), presenting greater glucose uptake compared to normal cells and metabolizing it through glycolysis, most of which is converted into lactate even in the presence of a high oxygen level. This metabolic deregulation is known as the Warburg Effect, where Otto Warburg postulated that malignant cells obtained their energy from “aerobic” glycolysis [76]. This restructuring favors and fosters an environment suitable for the use of cancer cells [76, 78]. Although glycolysis is less efficient to produce enough ATP molecules, cancer cells alter metabolic pathways using glycolysis as the main pathway and oxidative phosphorylation to a lesser extent, resulting in a fast production of ATP to meet the exuberant energy demands of cancer cells during their proliferation [80].
Figure 3.
Comparison of cancer cell metabolism vs. normal cell metabolism, both in the presence of oxygen.
In glycolysis, there is a leak of substrates useful for other metabolic routes with intermediate products that will serve for the subsequent synthesis of proteins, nucleic acids, and lipids that are fundamental for the development and division of malignant cells [81, 82]. In turn, this dysregulation prompts cellular adaptation to metabolic stress and protects cancer cells from reactive oxygen species (ROS)-related damage [82].
The specific role of HPV occurs in the microenvironment after the infection by this virus helps to evade the immune response and promote local immunosuppression [80]; in addition, it contributes to the restructuring of the metabolism of infected cells in the immune microenvironment. There is evidence of metabolism changes related to glutamine, taurine, and lysine in positive cases of HPV, which are related to the interaction of this pathogen and normal cellular metabolism [83]. Also, E6 and E7 may favor the Warburg effect by contributing to chemoresistance [29, 84].
When lactate accumulates in abnormal cells, the acid environment facilitates the deregulation of normal metabolism and signaling pathways of immune cells such as dendritic cells, macrophages, and T lymphocytes, producing the immune response. While all these changes are favorable to continue viral propagation and the persistence of the malignant cellular process, T lymphocytes are forced to change by such a high lactate environment, decreasing their functionality [80].
7. Cervical cancer and immune system
CC has two essential elements for its establishment and progression: the parenchyma, comprised of cancer cells, and the stroma, which includes connective tissue, blood vessels, extracellular matrix (ECM), nutrients, and the immune system [77]. These elements contribute through three situations: immune escaping, angiogenesis activation, and tumor progression (proliferation, invasion, and metastasis) [85].
The tumor microenvironment (TME) is defined as a complex ecosystem surrounding a tumor inside the body; the TME is in a constant battle between a tumor suppression response and a tumor-promoting response [86].
Cancer cells represent the core of the TME, and they are responsible for manipulating the cell and non-cell components through signaling mechanisms to take advantage of the non-tumoral cells to promote carcinogenesis and metastasis [87].
Intercellular communication is generated by the synthesis of cytokines, chemokines, growth factors, and inflammatory mediators, among others, but novel mechanisms recently reported include cell-free DNA, exosomes, circulating tumor cells, and apoptotic bodies [88, 89, 90, 91].
Near the tumoral cell core, cancer-associated fibroblasts (CAFs) are located; these populations play a crucial role in carcinogenesis as they promote the proliferation, migration, and survival of cancer cells [91]; however, it is essential to acknowledge that as a heterogeneous cell population, some are related to antitumoral activities. In the tumoral niche, some CAFs promote the recruitment of immunosuppressive cells through the synthesis of ECM proteins [92], and others promote angiogenesis by producing fibroblast growth factor 2 (FGF2) and vascular endothelial growth factor A (VEGFA); they also offer nutrients to tumoral cells such as ketone and cytokines necessary for mitochondrial biogenesis and autophagy [93].
Among the non-malignant cells located at TME are the tumor-infiltrating lymphocytes (TILs), which comprise one of the primary mediators of the dynamic yet ambiguous immune response. This group comprises T cell subsets such as CD4+, CD8+, regulatory T cells (Tregs), and Natural Killer (NK) cells [94]. Even though most of these cell populations are specialized in antitumoral functions, the overall action is insufficient to eradicate the tumor effectively due to the low levels of antitumoral cells, and the TME promotes an immuno-suppressor stimulus that induces a senescent state among those cells. In CC, the presence of cytotoxic CD8+ T cells activated by tumoral antigens is recognized as an excellent prognostic marker due to their killing activities and suppression of angiogenesis through IL-2 and IFN-γ secretion. On the other hand, CD4+ T cells coordinate an ambiguous immune response; Th1 cells synthesize potent modulators of cell-mediated immune responses [95]. However, recent studies have demonstrated that CIN or CC patients had a lower proportion of Th1 subtype and a higher proportion of Th2, Th17, and Treg cells when compared against healthy controls, and this imbalance aggravates along with the progression of the disease [96].
Finally, it is essential to mention the role of macrophages since they represent one of the significant components of tumor infiltrates and are responsible for producing high amounts of inflammatory molecules, IL-1𝛽, IL-6, IL-23, and TNF-𝛼, ROS, hypoxia-induced factor (HIF), necessary for inflammatory processes [97]. At the TME, macrophages are identified as Tumor-Associated Macrophages (TAMs) and are often associated with tumor progression and worsening of CC patients. TAMs can polarize into two phenotypes, M1 macrophages that release inflammatory factors, promote immune responses, and inhibit the CC occurrence [98], and the M2 phenotype, which is correlated with poor prognosis, chemoresistance, and diminished patient survival [99].
The growing evidence demonstrates the relevance of the TME in CC progression since the interaction between tumoral cells and their surroundings promotes their proliferation, resistance to apoptosis, chemoresistance, aggressiveness, and immune evasion. Thus, TME is crucial in determining CC patients’ therapeutic response and clinical outlook.
8. Angiogenesis
Angiogenesis is the formation of new blood vessels as an essential mechanism for the growth, survival, and spread of solid tumors [100]. This process not only occurs in neoplasias but also is involved in the growth of the endometrium and fetal development, among other processes [101, 102].
For the formation of new vessels, in tumor development, there is an imbalance between stimulating and inhibitory factors, where there is an increase in angiogenic factors such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), and angiopoietins (Ang), which are produced by the same tumor cells, macrophages, and lymphocytes that may be attached to the tumor [103, 104]. In addition, a decrease in antiangiogenic agents such as Thrombospodin-1 (TSP-1) is detected, allowing the transition from an avascular phase to a vascular phase, also called “angiogenic switch” [105, 106, 107].
This imbalance begins in the tumor cells, where there are areas with little irrigation and, therefore, low oxygenation; this tumoral tissue starts producing HIF1, which is a regulator of the recruitment of endothelial progenitor cells (EPC), pericyte progenitor cells (PPC), and monocytes, and thus carries out vascular restructuring; it also increases VEGF activity [108, 109, 110].
VEGF, the major angiogenic factor, consists of a family of proteins, VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and the placental growth factor. These proteins interact with three tyrosine kinase receptors, with VEGFR1 and VEGFR2, to trigger the signaling cascade promoting migration, proliferation, and survival of endothelial cells, also increasing the permeability of existing blood vessels, and this allows the leakage of multiple plasma proteins and the formation of new vessels (Figure 4). VEGF also inhibits apoptosis of newly formed blood vessels [111, 112].
Figure 4.
Schematic representation of the intracellular response to VEGF stimulation, in tumor angiogenesis.
FGF-1 and FGF-2 are also important angiogenic factors; FGF-2 increases the expression of other angiogenic agents, such as VEGF, and regulates the balance of Ang, leading to a predominance of Ang-2 [113, 114, 115]. On the other hand, Ang-2 is secreted by endothelial cells at sites of active vascular remodeling and engages in tumor initiation. Multiple studies indicate that the imbalance of Ang-2 and Ang-1 is associated with vascular instability, a key point in the initiation of angiogenesis in tumors [115, 116].
In recent years, efforts have been made to inhibit the signaling cascade in CC as a possible therapeutic target. There are several drugs in use, such as bevacizumab, which is a drug that inhibits the formation of new vessels by neutralizing VEGF activity, among others. However, these drugs must be in conjunction with chemotherapy or radiation therapy [117, 118, 119, 120].
9. Epithelial-mesenchymal transition & metastasis
As the fourth most common cancer among females worldwide, lymph node metastasis (LNM) is a key prognostic factor and a leading cause of death in patients with CC [121]. In metastasis, the ability to undergo reversible cellular and phenotypic changes is crucial for disseminating cancer cells to adapt to the changing microenvironments and stress during this pathological process. One main form of cellular plasticity is the epithelial-mesenchymal transition (EMT); in this process, epithelial cells lose tight cell-cell connections and polarity, which confers migratory and invasive properties (Figure 5); in its reverse process, mesenchymal-epithelial transition (MET) cells loose migratory freedom; they begin expressing junction complexes and adopt apicobasal polarity. This form of cellular plasticity may occur in physiological processes like human embryonic development (e.g., neural crest formation) and pathological conditions such as organ fibrosis, cancer progression, and metastasis [122, 123].
Figure 5.
Stepwise epithelial-mesenchymal transitions. 1. Tumor cells acquire the ability to dissociate themselves from the primary tumor mass through EMT; these epithelial cells lose their cell-cell junctions to become motile. 2. Now these cells can migrate and invade through the ECM. 3. Intravasation into blood vessels or lymphatic vessels occur; in this phase, tumor cells pass through the endothelial lamina and enter systemic circulation. 4. During extravasation, tumor cells extravasate through the capillary endothelium of distant organs into its parenchyma. 5. Tumor cells establish themselves and proliferate, forming micrometastases. 6. Colonization of distant organs and re-activation of epithelial properties occur at the secondary site
9.1 Metastasis cascade
Tumor cells that have undergone EMT exhibit stem-cell-like properties, including the ability to self-renew, tumor-initiation properties, and resistance to chemotherapy and radiotherapy, which explains why metastasis is the primary cause of death in cancer patients and why advances in tumor expression markers can help identify and prevent metastasis from happening [123, 124].
There are different markers to identify the state in which a tumor cell can be found; in the case of tumor cells in the epithelial state, researchers look out for the expression of E-cadherin, EpCAM, claudins, occludins, and cytokeratins; in the case of cells in the mesenchymal state, they can look for expression of vimentin (VIM), fibronectin, and α-SMA [125].
While EMT does not require changes in DNA sequence and can be reversible [124], the same cannot be said about other pathways; for example, due to high energy demand during EMT, several morphological and metabolic changes are made by 5’AMP-activated kinase (AMPK), which is a cellular energy homeostasis sensor that controls the balance between energy intake and demand; it modulates processes such as carbohydrate and lipid metabolism, biosynthesis, autophagy, and cell cycle.
Specifically, in the case of CC metastasis, a study by Konieczny et al. demonstrated that AMPK expression is related to malignant behavior in CC cells [126]. Another example is the case of protein tyrosine phosphatase receptor type M (PTPRM); Liu et al. demonstrated that PTPRM was upregulated in CC with LNM; as a result, it promoted tumor cell proliferation, migration, and lymphangiogenesis (a critical early metastasis event important in LNM and a prognosis factor in patients with cervical cancer), as well as EMT
10. Conclusion
In the previous chapter, the currently known mechanisms that promote CC oncogenesis were explained. It is essential to emphasize the evident relationship between infection by high-risk HPV serotypes and the development of CC. There are well-defined molecular mechanisms by years of research since the relationship was discovered by Zur Hausen et al. regarding CC and HPV. Although there are still other mechanisms pending elucidation, what is clear is that the more its etiopathogenesis and the complexity of its molecular mechanics are known, the more are the possibilities of developing effective treatments.
Appendices and nomenclature
adenocarcinoma | |
adenosquamous carcinoma | |
5’AMP-activated kinase | |
angiopoietins | |
cervical cancer | |
cervical intraepithelial neoplasia | |
DNA damage response | |
extracellular matrix | |
epithelial-mesenchymal transition | |
endothelial progenitor cells | |
flavin adenine dinucleotide | |
fibroblast growth factor | |
receptors associated with G protein. | |
hypoxia-induced factor-1 | |
human papillomavirus | |
human telomerase reverse transcriptase | |
janus kinase/ signal transducer and transcription activator | |
lymph node metastasis | |
mitogen activated protein kinase | |
mesenchymal-epithelial transition | |
nicotinamide adenine dinucleotide | |
platelet-derived growth factor | |
activated protein inhibitors of stat. | |
pericyte progenitor cells | |
protein tyrosine phosphatase | |
protein tyrosine phosphatase receptor type M | |
reactive oxygen species | |
receptor tyrosine kinase | |
squamous cell carcinoma | |
suppressor signaling cytokines. | |
tumor-associated macrophages | |
tumor-infiltrating lymphocytes | |
tumor microenvironment | |
regulatory T cells | |
tumor suppressor genes | |
Thrombospondin-1 | |
vascular endothelial growth factor | |
vimentin | |
World Health Organization |
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