Endogenous human papillomavirus E6 and E7 proteins differentially regulate proliferation,senescence, and apoptosis in HeLa cervical carcinoma cells

Cervical cancer cells express high-risk human papillomavirus (HPV) E6 and E7 proteins, and repression of HPV gene expression causes the cells to cease proliferation and undergo senescence. However, it is not known whether both HPV proteins are required to maintain the proliferative state of cervical cancer cells, or whether mutations that accumulate during carcinogenesis eliminate the need for one or the other of them. To address these questions, we used the bovine papillomavirus E2 protein to repress the expression of either the E6 protein or the E7 protein encoded by integrated HPV18 DNA in HeLa cervical carcinoma cells. Repression of the E7 protein activated the Rb pathway but not the p53 pathway and triggered senescence, whereas repression of the E6 protein activated the p53 pathway but not the Rb pathway and triggered both senescence and apoptosis. Telomerase activity, cyclin-dependent kinase activity, and expression of c-myc were markedly inhibited by repression of either E6 or E7. These results demonstrate that continuous expression of both the E6 and the E7 protein is required for optimal proliferation of cervical carcinoma cells and that the two viral proteins exert distinct effects on cell survival and proliferation. Therefore, strategies that inhibit the expression or activity of either viral protein are likely to inhibit the growth of HPV-associated cancers.     

The human DEK proto-oncogene is a senescence inhibitor and an upregulated target of high-risk human papillomavirus E7

The human DEK proto-oncogene is a nucleic acid binding protein with suspected roles in human carcinogenesis, autoimmune disease, and viral infection. Intracellular DEK functions, however, are poorly understood. In papillomavirus-positive cervical cancer cells, downregulation of viral E6/E7 oncogene expression results in cellular senescence. We report here the specific repression of DEK message and protein levels in senescing human papillomavirus type 16- (HPV16-) and HPV18-positive cancer cell lines as well as in primary cells undergoing replicative senescence. Cervical cancer cell senescence was partially overcome by DEK overexpression, and DEK overexpression was sufficient for extending the life span of primary keratinocytes, supporting critical roles for this molecule as a senescence regulator. In order to determine whether DEK is a bona fide HPV oncogene target in primary cells, DEK expression was monitored in human keratinocytes transduced with HPV E6 and/or E7. The results identify high-risk HPV E7 as a positive DEK regulator, an activity that is not shared by low-risk HPV E7 protein. Experiments in mouse embryo fibroblasts recapitulated the observed E7-mediated DEK induction and demonstrated that both basal and E7-induced regulation of DEK expression are controlled by the retinoblastoma protein family. Taken together, our results suggest that DEK upregulation may be a common event in human carcinogenesis and may reflect its senescence inhibitory function.     

HPV16 oncoproteins induce MMPs/RECK-TIMP-2 imbalance in primary keratinocytes: possible implications in cervical carcinogenesis

Cervical cancer is the third most common cancer in women worldwide. Persistent infection with high-risk HPV types, principally HPV16 and 18 is the main risk factor for the development of this malignancy. However, the onset of invasive tumor occurs many years after initial exposure in a minority of infected women. This suggests that other factors beyond viral infection are necessary for tumor establishment and progression. Tumor progression is characterized by an increase in secretion and activation of matrix metalloproteinases (MMPs) produced by either the tumor cells themselves or tumor-associated fibroblasts or macrophages. Increased MMPs expression, including MMP-2, MMP-9 and MT1-MMP, has been observed during cervical carcinoma progression. These proteins have been associated with degradation of ECM components, tumor invasion, metastasis and recurrence. However, few studies have evaluated the interplay between HPV infection and the expression and activity of MMPs and their regulators in cervical cancer. We analyzed the effect of HPV16 oncoproteins on the expression and activity of MMP-2, MMP-9, MT1-MMP, and their inhibitors TIMP-2 and RECK in cultures of human keratinocytes. We observed that E7 expression is associated with increased pro-MMP-9 activity in the epithelial component of organotypic cultures, while E6 and E7 oncoproteins co-expression down-regulates RECK and TIMP-2 levels in organotypic and monolayers cultures. Finally, a study conducted in human cervical tissues showed a decrease in RECK expression levels in precancer and cancer lesions. Our results indicate that HPV oncoproteins promote MMPs/RECK-TIMP-2 imbalance which may be involved in HPV-associated lesions outcome.     

Gene discovery in cervical cancer : towards diagnostic and therapeutic biomarkers

Cervical cancer is a potentially preventable disease; however, it remains the second most common malignancy in women worldwide. The human papillomavirus (HPV) is the single most important etiological agent in cervical cancer. HPV contributes to neoplastic progression through the action of two viral oncoproteins E6 and E7, which interfere with critical cell cycle pathways, tumor protein p53, and retinoblastoma protein. However, evidence suggests that HPV infection alone is insufficient to induce malignant changes, and other host genetic variations are important in the development of cervical cancer. Advances in molecular biology and high throughput technologies have heralded a new era in biomarker discovery and identification of molecular targets related to carcinogenesis. These advancements have improved our understanding of carcinogenesis and will facilitate screening, early detection, management, and personalized targeted therapy. A number of these developments and molecular targets associated with cervical cancer will be addressed in this review.     

Role of proteomics in translational research in cervical cancer

Cervical cancer is one of the leading causes of cancer morbidity and mortality in women worldwide. More than 98% of cases are related to a human papillomavirus (HPV) infection. Infection with specific subtypes of HPV has been strongly implicated in cervical carcinogenesis. The identification and functional verification of host proteins associated with HPV E6 and E7 oncoproteins may provide useful information for understanding cervical carcinogenesis and the development of cervical cancer-specific markers. In addition, proteomic profiling of altered proteins by anticancer drugs on cervical cancer cells may contribute to providing the fundamental resources for investigation of disease-specific target proteins, elucidation of the novel mechanisms of action and development of new drugs. The advent of proteomics has provided the hope of discovering novel biological markers for use in the screening, early diagnosis and prediction of response to therapy. This review describes the studies where profiles of protein expression in cervical cancer have been generated.     

Molecular targets of HPV oncoproteins: Potential biomarkers for cervical carcinogenesis

Cervical cancer is the second most common cancer among women worldwide and is responsible for 275,000 deaths each year. Persistent infection with high-risk human papillomavirus (HR-HPV) is an essential factor for the development of cervical cancer. Although the process is not fully understood, molecular mechanisms caused by HPV infection are necessary for its development and reveal a large number of potential biomarkers for diagnosis and prognosis. These molecules are host genes and/or proteins, and cellular microRNAs involved in cell cycle regulation that result from disturbed expression of HR-HPV E5, E6 and E7 oncoproteins. One of the current challenges in medicine is to discover potent biomarkers that can correctly diagnose cervical premalignant lesions and standardize clinical management. Currently, studies are showing that some of these molecules are potential biomarkers of cervical carcinogenesis, and it is possible to carry out a more accurate diagnosis and provide more appropriate follow-up treatment for women with cervical dysplasia. In this paper, we review recent research studies on cell cycle molecules deregulated by HPV infections, as well as their potential use for cervical cancer screening.     

Interferon-β Induced microRNA-129-5p Down-Regulates HPV-18 E6 and E7 Viral Gene Expression by Targeting SP1 in Cervical Cancer Cells

Infection by human papillomavirus (HPV) can cause cervical intraepithelial neoplasia (CIN) and cancer. Down-regulation of E6 and E7 expression may be responsible for the positive clinical outcomes observed with IFN treatment, but the molecular basis has not been well determined. As miRNAs play an important role in HPV induced cervical carcinogenesis, we hypothesize that IFN-β can regulate the expressions of specific miRNAs in cervical cancer cells, and that these miRNAs can mediate E6 and E7 expression, thus modulate their oncogenic potential. In this study, we found that miR-129-5p to be a candidate IFN-β inducible miRNA. MiR-129-5p levels gradually decrease with the development of cervical intraepithelial lesions. Manipulation of miR-129-5p expression in Hela cells modulates HPV-18 E6 and E7 viral gene expression. Exogenous miR-129-5p inhibits cell proliferation in Hela cells, promotes apoptosis and blocks cell cycle progression in Hela cells. SP1 is a direct target of miR-129-5p in Hela cells. This study is the first report of a cellular miRNA with anti-HPV activity and provides new insights into regulatory mechanisms between the HPV and the IFN system in host cells at the miRNA level.     

HPV16 Down-Regulates the Insulin-Like Growth Factor Binding Protein 2 to Promote Epithelial Invasion in Organotypic Cultures

Cervical cancer is a multi-stage disease caused by human papillomaviruses (HPV) infection of cervical epithelial cells, but the mechanisms regulating disease progression are not clearly defined. Using 3-dimensional organotypic cultures, we demonstrate that HPV16 E6 and E7 proteins alter the secretome of primary human keratinocytes resulting in local epithelial invasion. Mechanistically, absence of the IGF-binding protein 2 (IGFBP2) caused increases in IGFI/II signalling and through crosstalk with KGF/FGFR2b/AKT, cell invasion. Repression of IGFBP2 is mediated by histone deacetylation at the IGFBP2 promoter and was reversed by treatment with histone deacetylase (HDAC) inhibitors. Our in vitro findings were confirmed in 50 invasive cancers and 79 cervical intra-epithelial neoplastic lesions caused by HPV16 infection, where IGFBP2 levels were reduced with increasing disease severity. In summary, the loss of IGFBP2 is associated with progression of premalignant disease, and sensitises cells to pro-invasive IGF signalling, and together with stromal derived factors promotes epithelial invasion.     

Dependence of Intracellular and Exosomal microRNAs on Viral E6/E7 Oncogene Expression in HPV-positive Tumor Cells

Specific types of human papillomaviruses (HPVs) cause cervical cancer. Cervical cancers exhibit aberrant cellular microRNA (miRNA) expression patterns. By genome-wide analyses, we investigate whether the intracellular and exosomal miRNA compositions of HPV-positive cancer cells are dependent on endogenous E6/E7 oncogene expression. Deep sequencing studies combined with qRT-PCR analyses show that E6/E7 silencing significantly affects ten of the 52 most abundant intracellular miRNAs in HPV18-positive HeLa cells, downregulating miR-17-5p, miR-186-5p, miR-378a-3p, miR-378f, miR-629-5p and miR-7-5p, and upregulating miR-143-3p, miR-23a-3p, miR-23b-3p and miR-27b-3p. The effects of E6/E7 silencing on miRNA levels are mainly not dependent on p53 and similarly observed in HPV16-positive SiHa cells. The E6/E7-regulated miRNAs are enriched for species involved in the control of cell proliferation, senescence and apoptosis, suggesting that they contribute to the growth of HPV-positive cancer cells. Consistently, we show that sustained E6/E7 expression is required to maintain the intracellular levels of members of the miR-17~92 cluster, which reduce expression of the anti-proliferative p21 gene in HPV-positive cancer cells. In exosomes secreted by HeLa cells, a distinct seven-miRNA-signature was identified among the most abundant miRNAs, with significant downregulation of let-7d-5p, miR-20a-5p, miR-378a-3p, miR-423-3p, miR-7-5p, miR-92a-3p and upregulation of miR-21-5p, upon E6/E7 silencing. Several of the E6/E7-dependent exosomal miRNAs have also been linked to the control of cell proliferation and apoptosis. This study represents the first global analysis of intracellular and exosomal miRNAs and shows that viral oncogene expression affects the abundance of multiple miRNAs likely contributing to the E6/E7-dependent growth of HPV-positive cancer cells.