Cervical cancer (CC) is caused by the abnormal growth of cells in the cervix, which can spread to other parts of the body. No typical symptoms are observed in the early stage of CC, while symptoms including abnormal vaginal bleeding, pelvic pain, or pain during sexual intercourse occur as cancer proceed. Over 90% of cervical cancer are driven by the infection of a high-risk variety of the Human Papillomaviruses (HPV). The viruses infect the basal layer of the cervix, leading to deregulation of cellular proliferation in progenitors, and finally complete assembly in the upper differentiated epithelium layer. Development of cervical cancer from the precancerous stage typically takes 10 to 20 years. About 90% of cervical cancer patients are squamous cell carcinomas, 10% are adenocarcinoma, and a small group is atypical types. Cervical cancer is the fourth most common cancer and the fourth most cause of death from cancer in women all over the world. The mortality associated with cervical cancer is especially high in developing countries, accounting for 90% of the CC-associated death worldwide, which is caused by a deficiency in vaccinations and screening. While in developed countries, the promotion of cervical screening programs has significantly reduced the prevalence rate of CC.
Fig.1 Cervical cancer signaling pathway.
Epidermal growth factor receptor (EGFR) belongs to the tyrosine kinase receptor family, as well as its family members, Her2/Erb2, Her3/Erb3, and Her4/Erb4. The receptors are related to over 70% of all cancers. EGFR is proved to be associated with tumor cell proliferation, survival, angiogenesis, and metastatic. The activation of EGFR could be induced by the binding of epidermal growth factor (EGF), transforming growth factor alpha (TGF-α), heparin binding EGF-like growth factor (HB-EGF), amphiregulin, betacellulin, or epiregulin. The EGFR is then dimerized and autophosphorylated by the intrinsic kinase activity of the receptor, which finally leads to the activation of downstream intracellular pathways. Additionally, the EGF signal can also induce the synthesis of vascular endothelial growth factor (VEGF), which promotes tumor angiogenesis, growth, and metastasis. The expression of VEGF is proved to be associated with the severity of precursor lesions and invasion in cervical cancer.
The PI3K and the related PI3K/Akt/mTOR signaling pathway are found to be activated by growth factor receptor tyrosine kinase (RTKs) ligation in various human cancers. PI3K is a common lipid kinase that participates in receptor signal transduction, which also downregulates the Ras signaling pathway. The activated PI3K could convert the membrane lipid PtdIns(4,5)P2 to PtdIns(3,4,5)P3. Then the protein serine-threonine kinases Akt and phosphoinositide-dependent kinase 1 (PDK1) are recruited and bind to PtdIns(3,4,5)P3, which facilitate the phosphorylation of Akt and lead to the activation of downstream mTOR. Activated mTOR subsequently comprises two protein complexes and finally leads to the translation of downstream proteins that regulate cell growth, differentiation, proliferation, angiogenesis, and apoptosis. Studies have shown that the PI3K/Akt signaling pathway is playing a critical role in human cancers, including malignancies of the breast, endometrium, and cervix.
The canonical Wnt signaling pathway is initiated by the binding of specific Wnt ligands to the target frizzled membrane receptor and the LRP5/6 co-receptors, resulting in downstream gene transcription by β-catenin. In the absence of Wnt ligands, the multi-protein destruction complex composed of the scaffolding proteins Axin, adenomatous polyposis coli (APC), Casein kinase 1 alpha (CK1α), and glycogen synthase kinase 3β (GSK3β) bound to cytoplasmic β-catenin, and facilitate the phosphorylation of β-catenin. The phosphorylated β-catenin is then degraded by the proteasome and finally leads to the inhibition of the downstream transcription factor T-cell factor (TCF)/lymphoid enhancer-binding factor (LEF). Differently, non-canonical Wnt signaling is a β-catenin-independent pathway and classified into the planar cell polarity pathway (Wnt/PCP) and the Wnt/Ca2+ pathway. Both pathways are involved in the process of cell adhesion and migration. Multiple therapeutic strategies have been developed for the Wnt pathway. However, due to the complexity of the pathway, additional studies are needed to identify more interventions that target the Wnt for the treatment of cervical cancer.
Notch signaling pathways can function in multiple cell types in diverse ways and regulate cell division, differentiation, and survival. Studies have shown that Notch signaling plays crucial roles in cancer progression. Canonical Notch signaling is activated by the binding of transmembrane Notch receptors to ligands (Jagged 1, 2 and Delta-like 1, 3, and 4), leading to sequential cleavage in the receptor, and release the Notch intracellular domain (NICD). The NICD is then transported to the nucleus and activates target gene transcription. Notch signaling regulates the differentiation of basal cells. Given that HPV could infect the basal layer of the cervix, leading to deregulated cellular proliferation in progenitors, it is important to figure out the relationship between HPV oncoproteins and the Notch pathway regulation.
The transition from cervical pre-cancerous to invasive CC takes a long time, which allows early detection of CC when it is still in the safe and treatable stage. Researchers are devoted to finding molecule markers among viruses and the host that can identify the stage of the disease. DNA methylation is one of the most studied processes as a marker for the detection of cancer, including cervical cancer. It is of great significance in the progress of early carcinogenesis of several cancers and is also detectable in precursor HPV-related lesions and CC. DNA methylation refers to adding methyl groups to the GpG dinucleotide, which can regulate the expression of genes such as promoter genes. Hyper- and hypo-methylation can lead to overexpression of oncogenes or downregulation of tumor suppressor genes, and finally cause genomic instability in CC. TNFα is playing important roles in the pathogenesis and immune response caused by a viral infection, as well as viral-associated cancer such as gastric cancer and CC. TNFα polymorphism is considered a genetic marker for cancer development, which is helpful in the detection of local invasive CC in a very early stage. Besides, notch3 overexpression generally indicates squamous cervical cancer, which is more common than cervical adenocarcinomas.
With the development and application of proteomic techniques, the detection of protein biomarkers is increasingly important to diagnostic and prognostic medicine. Protein markers could discern diseases at an early, curable stage, and help to determine appropriate therapeutic strategies for those patients who respond well. Squamous cell cervical cancer constitutes 85-90% of all cervical cancers, and the increased squamous cell carcinoma antigen (SCC-Ag) level has been detected in 28-88% of cervical squamous cell cancer. SCC level is shown to be associated with the stage of CC, tumor size, and stromal invasion depth during the early treatment of CC. Cytokeratin 19 is an acidic subunit of cytokeratin, which is expressed in normal epithelium and CC. CYFRA is used for the measurement of serum cytokeratin 19 concentration and acts as a prognostic indicator in CC. Increased cytokeratin fragments levels have been detected in 42.52% of patients with squamous cell carcinoma of the uterine cervix. Additionally, cytokeratin fragments measurement can also be used to evaluate disease stage, tumor size, lymph vascular space involvement, and lymph node metastasis in CC.
Cetuximab is a human/murine chimeric immunoglobulin G2 mAb that targets EGFR. Studies showed that cetuximab can bind to EGFR and induce receptor dimerization, which finally inhibits the downstream signaling. Nimotuzumab is another EGFR inhibitor that inhibits the EGF-related/ligand-independent signaling and is a potential solution for epithelial origin tumors. A variety of clinical trials is currently undergoing to test nimotuzumab alone or in combination for the treatment of cervical tumors. Bevacizumab is a mAb that targets VEGF-A. It was the first anti-angiogenic agent for the treatment of colorectal cancer and also showed anti-tumor activity in pretreated cervical cancer. The tyrosine kinase inhibitors (TKIs) pazopanib and lapatinib that target VEGF and EGF, have shown antiangiogenic activity in a phase II clinical trial in patients with recurrent of advanced CC. Other TKIs, such as erlotinib, gefitinib, and imatinib are also under clinical trials for cervical cancer.
Table 1 Clinical trials of EGFR mAB Cetuximab
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT00997009 | Active, not recruiting | National Cancer Institute, Naples | October, 2009 |
| NCT00957411 | Completed | Institut Curie | March, 2009 |
| NCT00101192 | Completed | Gynecologic Oncology Group | September, 2004 |
| NCT00499031 | Completed | Gynecologic Oncology Group | June, 2007 |
Table 2 Clinical trials of EGFR mAB Nimotuzumab
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT03469531 | Recruiting | Zhujiang Hospital | March, 2018 |
| NCT04664244 | Recruiting | Peking Union Medical College Hospital | December, 2020 |
| NCT03413579 | Completed | El Kendi Pharmaceuticals Manufacturing Company | November, 2015 |
| NCT02083211 | Completed | National Institute of Cancerología | July, 2010 |
| NCT02095119 | Completed | National Institute of Cancerología | July, 2008 |
Table 3 Clinical trials of VEGF mAB Bevacizumab
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT03367871 | Recruiting | University of Miami | September, 2018 |
| NCT04974944 | Recruiting | Sun Yat-sen University | July, 2021 |
| NCT03912415 | Recruiting | Biocad | October, 2019 |
| NCT04138992 | Recruiting | Air Force Military Medical University | August, 2020 |
| NCT04651127 | Recruiting | Sun Yat-sen University | November, 2020 |
| NCT03476798 | Active, not recruiting | University of Oklahoma | June, 2018 |
| NCT03556839 | Active, not recruiting | Grupo Español de Investigación en Cáncer de Ovario | September, 2018 |
| NCT03786081 | Active, not recruiting | Seagen Inc. | February, 2019 |
Table 4 Clinical trials of VEGF TKIs Pazopanib
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT02348398 | Withdrawn | M.D. Anderson Cancer Center | August, 2016 |
| NCT00430781 | Completed | GlaxoSmithKline | November, 2006 |
The first PI3K inhibitors are Wortmannin (Wm) and LY294002, both of which target the p110 catalytic subunit of PI3K and inhibit the binding of ATP. The agents have been proved to cause apoptosis in human cervical cancer derived cells in vitro by reducing PI3K mediated phosphorylation. The phytochemical indole-3-carbinol (I3C) could upregulate the tumor suppressor protein PTEN and inactivate the Akt pathway in tumor cells. A study of I3C in a mouse model showed inhibition of laryngeal tissue tumor caused by HPV-16. mTOR is a downstream component of the PI3K/Akt pathway, the mTOR targeted agents such as CCI-779 and RAD-001 are designed for anti-cancer treatment. Both of the drugs have been approved for the treatment of renal cell carcinoma, and are now recruiting patients with CC for clinical trials.
Table 5 Clinical trials of mTOR inhibitor CCI-779
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT01026792 | Completed | National Cancer Institute (NCI) | December, 2009 |
| NCT01217177 | Completed | Novartis Pharmaceuticals | December, 2011 |
| NCT01065662 | Completed | Susana M. Campos, MD | February, 2010 |
Despite the Wnt signaling pathway is more complex and difficult to target, multiple combined therapeutic strategies have been proposed for Wnt in CC. Phytochemicals with antioxidant, anti-inflammatory, and chemo preventive properties have been tested in cervical cancer derived cell lines. Some compounds such as molecules, antibodies, and peptides targeting Wnt ligands, FZD, or β-catenin have been tested in clinical assays.
Given that notch can play both a tumor suppressor and oncogenic role, it is of great significance to test the expression of the notch and its downstream signals during the progression of CC. Studies have shown that overexpression of notch receptors and their ligands are detected in various malignancies including CC. Several clinical trials are currently investigating the use of notch inhibition in patients with metastatic cancer.
References
