Cervical cancer is one of the most common gynecological malignant tumors, and its incidence ranks the third among female malignant tumors in the world. Cervical cancer is more common in middle-aged women . The E2F transcription factor family has been shown to be associated with cell cycle in vivo and to have significant effects on apoptosis, and proliferation. It has been confirmed that E2F overexpression is closely related to the manifestations of various malignant tumors, such as the occurrence and development of liver cancer and ovarian cancer [37-40]. However, the expression of different E2F family members and their exact role in cervical cancer are unclear. In this study, we attempted to systematically investigate mRNA expression, gene change, correlation, potential function, and prognostic value of different E2Fs in CC patients.
E2F1 is one of the classic transcription factor, on cell cycle regulation in vivo, the function such as cell proliferation, apoptosis, autophagy has significant influence. In recent years, the study found that in patients with a variety of malignant tumors in vivo detection of E2F1 overexpression, in cervical cancer research results show that the E2F1 may promote the miR - 136 in cervical cancer cells show lower expression, raised its expression inhibits cell proliferation, promote apoptosis, accelerate the course of cervical cancer [41-44]. In our study, ONCOMINE and GEPIA datasets showed that the expression of E2F1 was up-regulated in CC patients, and E2F1 expression was associated with the clinical characteristics of CC patients. By using Kaplan–Meier plotter, we found that E2F1 RNA expression level was increased, which was associated with better OS in all CC patients. E2F2 regulates many cellular processes, such as cell cycle, DNA synthesis, proliferation, and tumorigenesis [45-48]. According to the current research results, the reduced activity of E2F2 can be used as a targeted HPV link to inhibit cervical cancer . In our report, the expression of E2F2 in cervical cancer tissues was higher than that in normal tissues. Although, E2F2 expression was not correlated with tumor stage in patients with breast cancer, a high E2F2 expression was signiﬁcantly correlated with better OS in all of the patients with cervical cancer.
Recent studies have also shown that E2F3 is positively correlated with the regulation of cell cycle in organisms and it has an effect on biological functions such as cell proliferation and apoptosis [50-52]. Evidence points out that targeted therapy of cervical cancer can be achieved by reducing E2F3 . There is also evidence that lncRNA TTN-AS1 is involved in the progression of cervical cancer cells by regulating the miR-573-E2F3 axis , which provides a new perspective for our study of therapeutic strategies for cervical cancer cells. Similarly, in the present study, we demonstrated significantly higher expression of E2F3 in CC tissues and poorer RFS in cervical cancer patients, so we consider E2F3 as another therapeutic target for CC patients.
E2F4 transcription factor is a key factor in cell apoptosis and cell cycle. E2F4 is found in many diseases. E2F4 is also overexpressed in cervical cancer lesions. However, a recent study pointed to evidence that HAND2-AS1 recruits the transcription factor E2F4 to the C16orf74 promoter region and down-regulates C16orf74 expression to suppress cervical cancer development . Although there was no significant up- or down-regulation of E2F4 expression levels in CC, further studies on the role of E2F4 in CC are needed, considering the little evidence available.
E2F5 is an important member of the E2F family. It has growth inhibitory properties and has been observed in several solid cancers, such as osteosarcoma, colon cancer , and breast cancer . E2F5 has been proven to be a landmark indicator of prostate cancer . The current research also shows that increasing the barrier to E2F5 can reduce the carcinogenic activity of HPV and affect the transformation of cervical cancer . However, mRNA expression levels of E2F5 did not have prognostic values in CC patients according to our study.
E2F6 is an important factor that regulates mitotic events in the retinoblastoma-related gene (RB / E2F) pathway. The downstream genes that E2F6 can regulate are divided into activation and inhibition methods, which involve cell cycle regulation, growth and apoptosis, proliferation and Important physiological functions such as differentiation. E2F6 can inhibit DNA damage-induced apoptosis [61-63]. In recent years, it has been found to be related to hypoxia . The expression of E2F6 mRNA and protein in cells is regulated by oxygen concentration. E2F6 mRNA and/or protein expression is reduced under hypoxia. However, the expression of E2F6 in cervical tumor tissues has not been reported yet.
E2F7 is an atypical E2F family member that acts as a transcriptional repressor of E2F target genes, thereby contributing to cell cycle arrest for DNA repair and genomic integrity [65-66]. E2F7 is related to the manifestations of multiple cancers, and studies have shown that it can promote the proliferation and migration of breast cancer cells . The overexpression of E2F7 protein also shows a correlation with the development of gastric cancer . In addition, studies have shown that the long non-coding RNA NEAT1 regulates the miR-889-3p / E2F7 axis by activating the PI3K / AKT pathway, thereby accelerating the cell process of cervical cancer . Excessive activation of E2F7 accelerates the transformation of cervical cancer. Although, ONCOMINE and GEPIA datasets revealed that the expression of E2F7was up-regulated in human CC, the Prognostic Values of E2F7 were not related with OS or RFS in CC.
The transcription factor E2F8 is an important regulator of the cell cycle, and the unrestricted activation of the dependent transcription of the E2F family is considered to be an important driving force for tumor formation and progression. Studies have shown that E2F8 plays an important role in embryonic development and cell cycle control by inhibiting E2F1 [70-71]. However, it is not yet known whether E2F8 is involved in the progression of cervical cancer. In the near future, if it is proved to knock down E2F8 in cervical cancer cell lines, E2F8 can effectively induce the expression of epithelial-mesenchymal transition (EMT) markers . Compared with patients with low E2F8 expression, cervical cancer patients with high E2F8 expression have higher FIGO staging and recurrence rates. In conclusion, studies have shown that E2F8 is highly correlated with progression-free survival in patients with cervical cancer. In our study, E2F8 had higher mRNA expression and worse RFS in cervical cancer compared with normal tissues, which seemed consistent with the role of E2F3 as an oncogene.
More and more evidences show that the eight members of E2F family interact with each other to affect cell cycle, cell proliferation, apoptosis and carcinogenesis. We also analyzed the coexpression and correlation of E2F family at gene and protein levels, the results show that in the topological heterogeneity analysis of targets associated with family genes, MSH2, E2F1, and E2F2 had high degree values, and existing studies confirmed that the positive expression of MSH2 in cervical tissues showed a significant positive correlation with the degree of pathological differentiation of cervical squamous cell carcinoma patients, and the increased expression of MSH2 was closely related to the development of cervical cancer, and to some extent reflected the malignancy of cervical cancer .From the enrichment analysis results，HPV is the leading cause of cervical cancer, but does not necessarily convert to malignancy .It has been shown that high-risk HPV subtypes can act directly on the tumor suppressor proteins p53 and PRb, which can inhibit the two pathways that regulate the cell cycle and promote the malignancy of CC;TGF-β /smads signaling pathway, as an important negative regulatory system of epithelial cell proliferation, is believed to control cell proliferation and induction of apoptosis, and is the central segment of the body's tumor suppression system. Alteration of TGF-β1 is an important mechanism in the formation of HPV-related cervical cancer. Although we have recognized the importance of E2F family interaction in the pathogenesis and development of OC, the specific molecular mechanism of E2F family interaction remains to be further studied. Our study adds to the growing evidence regarding the complexity of the E2F family members and their associated signaling pathways, which oﬀer clues into the rational development of multi-targeted and E2F-mediated targeted therapy.
To further clarify the genetic alteration, potential function, and carcinogenic mechanism of the E2F family members, we calculated the percentages of genetic alterations in E2F family members for CC and found that they varied from 5 to 18% for individual genes based on TCGA Pancancer dataset. Further, cases with alterations in one of the query gene had worse OS and DFS than those without any alterations in the query genes, although the difference of OS was not statistically signiﬁcant.