Requirement of E7 oncoprotein for viability of HeLa cells

Product safety aspects of plant molecular farming

Plant molecular farming (PMF) has been promoted since the 1990s as a rapid, cost-effective and (most of all) safe alternative to the cultivation of bacteria or animal cells for the production of biopharmaceutical proteins. Numerous plant species have been investigated for the production of a broad range of protein-based drug candidates. The inherent safety of these products is frequently highlighted as an advantage of PMF because plant viruses do not replicate in humans and vice versa. However, a more nuanced analysis of this principle is required when considering other pathogens because toxic compounds pose a risk even in the absence of replication. Similarly, it is necessary to assess the risks associated with the host system (e.g., the presence of toxic secondary metabolites) and the production approach (e.g., transient expression based on bacterial infiltration substantially increases the endotoxin load). This review considers the most relevant host systems in terms of their toxicity profile, including the presence of secondary metabolites, and the risks arising from the persistence of these substances after downstream processing and product purification. Similarly, we discuss a range of plant pathogens and disease vectors that can influence product safety, for example, due to the release of toxins. The ability of downstream unit operations to remove contaminants and process-related toxic impurities such as endotoxins is also addressed. This overview of plant-based production, focusing on product safety aspects, provides recommendations that will allow stakeholders to choose the most appropriate strategies for process development.

Biomedical application of chitosan-based nanoscale delivery systems: Potential usefulness in siRNA delivery for cancer therapy

Gene therapy is an emerging and promising strategy in cancer therapy where small interfering RNA (siRNA) system has been deployed for down-regulation of targeted gene and subsequent inhibition in cancer progression; some issues with siRNA, however, linger namely, its off-targeting property and degradation by enzymes. Nanoparticles can be applied for the encapsulation of siRNA thus enhancing its efficacy in gene silencing where chitosan (CS), a linear alkaline polysaccharide derived from chitin, with superb properties such as biodegradability, biocompatibility, stability and solubility, can play a vital role. Herein, the potential of CS nanoparticles has been discussed for the delivery of siRNA in cancer therapy; proliferation, metastasis and chemoresistance are suppressed by siRNA-loaded CS nanoparticles, especially the usage of pH-sensitive CS nanoparticles. CS nanoparticles can provide a platform for the co-delivery of siRNA and anti-tumor agents with their enhanced stability via chemical modifications. As pre-clinical experiments are in agreement with potential of CS-based nanoparticles for siRNA delivery, and these carriers possess biocompatibiliy and are safe, further studies can focus on evaluating their utilization in cancer patients.

Estrogen Attenuates the Growth of Human Papillomavirus-Positive Epithelial Cells

Human papillomaviruses cause around 5% of all human cancers, yet there are no specific antiviral therapeutic approaches available for combatting these cancers. These cancers are currently treated with standard chemoradiation therapy (CRT). Specific antiviral reagents are desperately required, particularly for HPV+HNSCC whose incidence is increasing and for which there are no diagnostic tools available for combatting this disease. Using data from The Cancer Genome Atlas (TCGA), we and others determined that the estrogen receptor alpha (ERα) is overexpressed in HPV+HNSCC and that elevated levels are associated with an improved disease outcome. This has led to the proposal that estrogen treatment could be a novel therapeutic approach for combatting HPV+cancers. Here, we demonstrate that estrogen attenuates the growth of HPV+epithelial cells using multiple mechanisms, supporting the idea that estrogen has potential as a therapeutic agent for the treatment of HPV+HNSCC.

Human papillomaviruses (HPVs) are small, double-stranded DNA viruses that are significant risk factors in the development of cancer, and HPV accounts for approximately 5% of all worldwide cancers. Recent studies using data from The Cancer Genome Atlas (TCGA) have demonstrated that elevated levels of estrogen receptor alpha (ERα) are associated with improved survival in oropharyngeal cancers, and these elevated receptor levels were linked with human papillomavirus-positive cancers (HPV+cancers). There has been a dramatic increase in HPV-related head and neck squamous cell carcinomas (HPV+HNSCCs) over the last 2 decades, and therapeutic options for this ongoing health crisis are a priority; currently, there are no antiviral therapeutics available for combatting HPV+cancers. During our TGCA studies on head and neck cancer, we had also discovered the overexpression of ERα in HPV+cancers. Here, we demonstrate that 17β-estradiol (estrogen) attenuates the growth/cell viability of HPV+cancers in vitro, but not HPV-negative cancer cells. In addition, N/Tert-1 cells (foreskin keratinocytes immortalized with human telomerase reverse transcriptase [hTERT]) containing human papillomavirus 16 (HPV16) have elevated levels of ERα and growth sensitivity after estrogen treatment compared with parental N/Tert-1 cells. Finally, we demonstrate that there are potentially two mechanisms contributing to the attenuation of HPV+ cell growth following estrogen treatment. First, estrogen represses the viral transcriptional long control region (LCR) downregulating early gene expression, including E6/E7. Second, expression of E6 and E7 by themselves sensitizes cells to estrogen. Overall, our results support the recent proposal that estrogen could be exploited therapeutically for the treatment of HPV-positive oral cancers.

IMPORTANCE Human papillomaviruses cause around 5% of all human cancers, yet there are no specific antiviral therapeutic approaches available for combatting these cancers. These cancers are currently treated with standard chemoradiation therapy (CRT). Specific antiviral reagents are desperately required, particularly for HPV+HNSCC whose incidence is increasing and for which there are no diagnostic tools available for combatting this disease. Using data from The Cancer Genome Atlas (TCGA), we and others determined that the estrogen receptor alpha (ERα) is overexpressed in HPV+HNSCC and that elevated levels are associated with an improved disease outcome. This has led to the proposal that estrogen treatment could be a novel therapeutic approach for combatting HPV+cancers. Here, we demonstrate that estrogen attenuates the growth of HPV+epithelial cells using multiple mechanisms, supporting the idea that estrogen has potential as a therapeutic agent for the treatment of HPV+HNSCC.

Proteasome-Rich PaCS as an Oncofetal UPS Structure Handling Cytosolic Polyubiquitinated Proteins. In Vivo Occurrence, in Vitro Induction, and Biological Role

In this article, we outline and discuss available information on the cellular site and mechanism of proteasome interaction with cytosolic polyubiquitinated proteins and heat-shock molecules. The particulate cytoplasmic structure (PaCS) formed by barrel-like particles, closely reproducing in vivo the high-resolution structure of 26S proteasome as isolated in vitro, has been detected in a variety of fetal and neoplastic cells, from living tissue or cultured cell lines. Specific trophic factors and interleukins were found to induce PaCS during in vitro differentiation of dendritic, natural killer (NK), or megakaryoblastic cells, apparently through activation of the MAPK-ERK pathway. Direct interaction of CagA bacterial oncoprotein with proteasome was shown inside the PaCSs of a Helicobacter pylori-infected gastric epithelium, a finding suggesting a role for PaCS in CagA-mediated gastric carcinogenesis. PaCS dissolution and autophagy were seen after withdrawal of inducing factors. PaCS-filled cell blebs and ectosomes were found in some cells and may represent a potential intercellular discharge and transport system of polyubiquitinated antigenic proteins. PaCS differs substantially from the inclusion bodies, sequestosomes, and aggresomes reported in proteinopathies like Huntington or Parkinson diseases, which usually lack PaCS. The latter seems more linked to conditions of increased cell proliferation/differentiation, implying an increased functional demand to the ubiquitin–proteasome system.

A novel HPV16 E7-affitoxin for targeted therapy of HPV16-induced human cervical cancer

Cervical cancer, the second most common cause of cancer death in women worldwide, is significantly associated with infection of high-risk human papillomaviruses (HPVs), especially the most common genotype, HPV 16. To date, there is no established noninvasive therapy to treat cervical cancer.

Methods: Here, we report a novel affitoxin that targets HPV16 E7 protein, one of the primary target proteins in molecular targeted therapy for HPV-induced cervical cancer. The affitoxin, Z_HPV16E7 affitoxin384 was generated by fusing the modified Pseudomonas Exotoxin A (PE38KDEL) to the HPV16 E7-specific affibody. The expressed and purified Z_HPV16E7 affitoxin384 was characterized using numerous methods. SPR assay, indirect immunofluorescence assay, and near-infrared (NIR) optical imaging were respectively performed to assess the targeting ability of Z_HPV16E7 affitoxin384 to HPV16 E7 protein both in vitro and in vivo. Cell viability assays and SiHa tumor-bearing nude mice were used to evaluate the efficacy of Z_{HPV16 E7} affitoxin384 in vitro and in vivo, respectively.

Results: Using in vitro methods the SPR assay and indirect immunofluorescence assay showed that Z_HPV16E7 affitoxin384 targeted HPV16 E7 with high binding affinity and specificity. Significant reduction of cell viability in HPV16 positive cells was observed in the presence of Z_{HPV16 E7} affitoxin384. By NIR optical imaging, Z_{HPV16 E7} affitoxin384 specifically targeted HPV16 positive tumors in vivo. Z_HPV16E7 affitoxin384 showed significant in vivo antitumor efficacy in two kinds of tumor-bearing nude mouse models.

Conclusions: Z_HPV16E7 affitoxin384 is a potent anti-cervical cancer therapeutic agent that could be effective against HPV16 positive tumors in humans.

In Vitro and In Vivo Synergistic Therapeutic Effect of Cisplatin with Human Papillomavirus16 E6/E7 CRISPR/Cas9 on Cervical Cancer Cell Line

PURPOSE: Human papillomavirus (HPV) type 16 is one of the major etiologic factors of cervical cancer. Our study aims to investigate the potentiality of the antiviral clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated Cas9 system (CRISPR/Cas9) targeting the E6 and E7 oncogenes of HPV16 as a potential chemosensitizer of cisplatin (cis-diaminedichloroplatinum II; CDDP) for cervical cancer. METHODS: Specifically, the therapeutic efficacy of combination of CDDP and HPV16 E6 + E7-CRISPR/Cas9 was assessed in cervical cancer cells and cervical cancer xenograft models. RESULTS: In vitro experiments showed that long-term exposure of SiHa cells to the HPV16 E6 + E7-CRISPR/Cas9 induced apoptosis, and its pro-apoptosis effect became more obvious when combined with CDDP. In vivo study found the efficacy of the combination of HPV16 E6 + E7-CRISPR/Cas9 and CDDP were superior to either of the treatments in term of apoptosis induction and metastasis inhibition. CONCLUSION: Collectively, our results suggested that HPV16 E6 + E7-CRISPR/Cas9 could be an effective sensitizer of CDDP chemotherapy in cervical cancer.

In vitro and in vivo growth suppression of human papillomavirus 16-positive cervical cancer cells by CRISPR/Cas9

Deregulated expression of high-risk human papillomavirus oncogenes (E6 and E7) is a pivotal event for pathogenesis and progression in cervical cancer. Both viral oncogenes are therefore regarded as ideal therapeutic targets. In the hope of developing a gene-specific therapy for HPV-related cancer, we established CRISPR/Cas9 targeting promoter of HPV 16 E6/E7 and targeting E6, E7 transcript, transduced the CRISPR/Cas9 into cervical HPV-16-positive cell line SiHa. The results showed that CRISPR/Cas9 targeting promoter, as well as targeting E6 and E7 resulted in accumulation of p53 and p21 protein, and consequently remarkably reduced the abilities of proliferation of cervical cancer cells in vitro. Then we inoculated subcutaneously cells into nude mice to establish the transplanted tumor animal models, and found dramatically inhibited tumorigenesis and growth of mice incubated by cells with CRISPR/Cas9 targeting (promoter+E6+E7)-transcript. Our results may provide evidence for application of CRISPR/Cas9 targeting HR-HPV key oncogenes, as a new treatment strategy, in cervical and other HPV-associated cancer therapy.

PaCS is a novel cytoplasmic structure containing functional proteasome and inducible by cytokines/trophic factors

A variety of ubiquitinated protein-containing cytoplasmic structures has been reported, from aggresomes to aggresome-like induced structures/sequestosomes or particle-rich cytoplasmic structures (PaCSs) that we recently observed in some human diseases. Nevertheless, the morphological and cytochemical patterns of the different structures remain largely unknown thus jeopardizing their univocal identification. Here, we show that PaCSs resulted from proteasome and polyubiquitinated protein accumulation into well-demarcated, membrane-free, cytoskeleton-poor areas enriched in glycogen and glycosaminoglycans. A major requirement for PaCS detection by either electron or confocal microscopy was the addition of osmium to aldehyde fixatives. However, by analyzing living cells, we found that proteasome chymotrypsin-like activity concentrated in well-defined cytoplasmic structures identified as PaCSs by ultrastructural morphology and immunocytochemistry of the same cells. PaCSs differed ultrastructurally and cytochemically from sequestosomes which may coexist with PaCSs. In human dendritic or natural killer cells, PaCSs were induced in vitro by cytokines/trophic factors during differentiation/activation from blood progenitors. Our results provide evidence that PaCS is indeed a novel distinctive cytoplasmic structure which may play a critical role in the ubiquitin–proteasome system response to immune, infectious or proneoplastic stimuli.

Gene silencing with siRNA targeting E6/E7 as a therapeutic intervention against head and neck cancer-containing HPV16 cell lines

CONCLUSION

Our results indicate that siRNA E6 and/or E7 may have potential as a gene-specific therapy for human papillomavirus (HPV) type 16 (HPV16)-related squamous cell carcinoma of the head and neck (HNSCC).

OBJECTIVES

To evaluate the effectiveness of siRNA targeting E6 and/or E7 on the in vitro and in vivo growth suppression of HPV16-related HNSCC.

METHODS

HPV16-related HNSCC (UM-SCC47) cell lines were used for the present study. Expression of HPV viral oncogenes E6 and/or E7 and their cellular targets, p53 and pRb, was evaluated by real-time PCR, Western blotting, and immunofluorescence staining. To study the effect of siRNA on tumor growth in vivo, we developed animal models. Representative tumors harvested from each group were processed for apoptosis analyses (TUNEL assay) and immunofluorescence staining for p53 and pRb.

RESULTS

E6 and E7 oncogenes of HPV16 were down-regulated by E6 and/or E7 targeting siRNAs, respectively. The expression of p53 and pRb proteins in both the E6 siRNA group and E7 siRNA group was up-regulated compared with those of control groups. The cellular proliferation and apoptosis indexes of E6 and/or E7 siRNA groups were higher than those of controls. In vivo studies showed significant inhibitory effect of E6 and/or E7 siRNA compared with those of control groups, which was consistent with in vitro studies.

Requirement for stromal estrogen receptor alpha in cervical neoplasia

The major etiological factor for cervical cancer is the high-risk human papillomavirus (HPV), which encodes E6 and E7 oncogenes. However, HPV is not sufficient, and estrogen has been proposed as an etiological cofactor for the disease. Its requirement has been demonstrated in mouse models for HPV-associated cervical cancer (e.g., K14E7 transgenic mice). Although germline knockout of estrogen receptor alpha (ERα) renders mice resistant to cervical cancer, the cell-type-specific requirement for ERα is not known. In this study, we demonstrate that temporal deletion of stromal ERα induced complete regression of cervical dysplasia in K14E7 mice. Our results strongly support the hypothesis that stromal ERα is necessary for HPV-induced cervical carcinogenesis and implicate paracrine mechanisms involving ERα signaling in the development of estrogen-dependent cervical cancers. This is the first evidence to support the importance of stromal ERα in estrogen-dependent neoplastic disease of the female reproductive tract.

Apoptosis induced by an antagonist peptide against HPV16 E7 in vitro and in vivo via restoration of p53

Human papilloma virus type 16 (HPV16) E7 is a viral oncoprotein that is believed to play a major role in cervical neoplasia. A novel antagonist peptide against HPV16 E7 was previously selected by phage display screening and the selected peptide was found to have anti-tumor efficacy against HPV16-positive cervical carcinoma through induction of cell cycle arrest. In the current study, to further elucidate the mechanisms of the antagonist peptide, the effects of the peptide on apoptosis are investigated by RT-PCR, Western blotting, MTT assay, TUNEL staining, Annexin V apoptosis assay, flow cytometry, and animal experiments. The antagonist peptide showed obvious anti-tumor efficacy through apoptosis induction, both in HPV16-positive cervical cancer cell lines and tumor xenografts. Our results also revealed that the peptide induced accumulation of cellular p53 and p21, and led to HPV16 E7 protein degradation. In the case of mRNA levels, it resulted in unaltered p53 and HPV16 E7 expression, but increased expression of p21. In contrast, the induction of apoptosis and p53 reactivation effects by the selected peptide were abolished after E7 knocked down with siRNA. These results demonstrate that the selected peptide can induce E7 degradation and lead to marked apoptosis in HPV16-related cancer cells by activating cellular p53 and its target genes, such as p21. Furthermore, the evident therapeutic efficacy obtained from the subcutaneous tumor model experiments in nude mice suggests a therapeutic potential for HPV16-related cancers of the selected peptide. Therefore, this specific peptide may be used to create specific biotherapies for the treatment of HPV 16-positive cervical cancers.

Small molecule inhibitors of human papillomavirus protein - protein interactions

Human papillomaviruses (HPV) have now been identified as a necessary cause of benign and malignant lesions of the differentiating epithelium, particularly cervical cancer, the second most prevalent cancer in women worldwide. While two prophylactic HPV vaccines and screening programs are available, there is currently no antiviral drug for the treatment of HPV infections and associated diseases. The recent progress toward the identification and characterization of specific molecular targets for small molecule-based approaches provides prospect for the development of effective HPV antiviral compounds. Traditionally, antiviral therapies target viral enzymes. HPV encode for few proteins, however, and rely extensively on the infected cell for completion of their life cycle. This article will review the functions of the viral E1 helicase, which encodes the only enzymatic function of the virus, of the E2 regulatory protein, and of the viral E6 and E7 oncogenes in viral replication and pathogenesis. Particular emphasis will be placed on the recent progress made towards the development of novel small molecule inhibitors that specifically target and inhibit the functions of these viral proteins, as well as their interactions with other viral and/or cellular proteins.

Potent anti-tumor effect generated by a novel human papillomavirus (HPV) antagonist peptide reactivating the pRb/E2F pathway

Human papillomavirus type 16 (HPV16) E7 is a viral oncoprotein believed to play a major role in cervical cancer. In this study, an antagonist peptide against HPV16E7 protein was first identified from screening the c7c phage display peptide library. The binding specificity and affinity of the selected peptide to HPV16E7 were tested by competitive enzyme-linked immunosorbent assay (ELISA). The antagonist peptide showed obvious anti-tumor efficacy both in cell lines and animal tumor models. Significant cell proliferation inhibition with high specificity was noted when HPV16-positive cells were treated with the peptide. This anti-tumor efficacy was resulted from overriding the activities of HPV16E7 and reactivating the pRb/E2F pathway, as shown by a series of experiments. Flow cytometry analysis revealed that the selected peptide induced G1 arrest in a dose-dependent manner. Competitive ELISA, pull down, and Co-IP experiments indicated that the selected peptide disrupted the interaction between HPV16E7 and pRb proteins both in vitro and in vivo. Luciferase reporter assay verified that transcription activities of E2F were suppressed by the peptide through restoration of pRb. RT-PCR and Western blot revealed that it reduced cyclins A, D1, and E1 expression, and led to HPV16E7 protein degradation, but pRb protein stabilization. The current study suggests that this specific peptide may serve as a potential therapeutic agent for HPV16-positive cervical cancer.

Molecular mechanisms of human papillomavirus-induced carcinogenesis

Approximately 20% of all cancers are associated with infectious agents. Among them, human papillomaviruses (HPVs) are very common and are now recognized as the etiological agent of cervical cancer, the second most common cancer in women worldwide, and they are increasingly linked with other forms of dysplasia. Carcinogenesis is a complex and multistep process requiring the acquisition of several genetic and/or epigenetic alterations. HPV-induced neoplasia, however, is in part mediated by the intrinsic functions of the viral proteins. In order to replicate its genome, HPV modulates the cell cycle, while deploying mechanisms to escape the host immune response, cellular senescence and apoptosis. As such, HPV infection leads directly and indirectly to genomic instability, further favouring transforming genetic events and progression to malignancy. This review aims to summarize our current understanding of the molecular mechanisms exploited by HPV to induce neoplasia, with an emphasis on the role of the 2 viral oncoproteins E6 and E7. Greater understanding of the role of HPV proteins in these processes will ultimately aid in the development of antiviral therapies, as well as unravel general mechanisms of oncogenesis.

Persistence of high-grade cervical dysplasia and cervical cancer requires the continuous expression of the human papillomavirus type 16 E7 oncogene

Several mucosotropic human papillomaviruses (HPV), including HPV type 16 (HPV-16), are etiologic agents of a subset of anogenital cancers and head and neck squamous cell carcinomas. In mice, HPV-16 E7 is the most potent of the papillomaviral oncogenes in the development of cervical disease. Furthermore, interfering specifically with the expression of E7 in HPV-positive cell lines derived from human cervical cancers inhibits their ability to proliferate, indicating that the expression of E7 is important in maintaining the transformed phenotype in vitro. To assess the temporal role of E7 in maintaining HPV-associated tumors and precancerous lesions in vivo, we generated Bi-L E7 transgenic mice that harbor a tetracycline-inducible transgene that expresses both HPV-16 E7 and firefly luciferase. When we crossed Bi-L E7 mice to a K5-tTA transgene-inducing line of mice, which expresses a tetracycline-responsive transactivator selectively in the stratified squamous epithelia, the resulting Bi-L E7/K5-tTA bitransgenic mice expressed E7 and luciferase in the skin and cervical epithelium, and doxycycline repressed this expression. Bitransgenic mice displayed several overt and acute epithelial phenotypes previously shown to be associated with the expression of E7, and these phenotypes were reversed on treatment with doxycycline. Repressing the expression of E7 caused the regression of high-grade cervical dysplasia and established cervical tumors, indicating that they depend on the continuous expression of E7 for their persistence. These results suggest that E7 is a relevant target not only for anticancer therapy but also for the treatment of HPV-positive dysplastic cervical lesions.

HPV E7 contributes to the telomerase activity of immortalized and tumorigenic cells and augments E6-induced hTERT promoter function

The E6 and E7 proteins of high-risk HPVs are both required for the immortalization of primary human keratinocytes and the maintenance of the malignant phenotype of HPV-positive cancer cell lines. Our previous studies have shown that E6 protein binds Myc protein and that both E6 and Myc associate with and cooperatively activate the hTERT promoter, thereby increasing cellular telomerase activity. In this study, we evaluated the role of E7 in the maintenance and activation of telomerase in immortalized and tumorigenic cells. siRNA knockdown of either E6 or E7 (or both) in HPV-immortalized cells or an HPV-positive cancer cell line reduced hTERT transcription and telomerase activity. Since telomerase was inhibited by E7 siRNA in cells that independently expressed the E6 and E7 genes, our results reveal an independent role for E7 in the maintenance of telomerase activity. However, E7 alone was insufficient to increase endogenous hTERT mRNA or telomerase activity, although it significantly augmented E6-induced hTERT transcription and telomerase activity. To further explore this apparent E7-induced promoter augmentation, we analyzed an exogenous hTERT core promoter in transduced keratinocytes. E7 alone induced the wt hTERT promoter and augmented E6-induced hTERT promoter activity. Mutation of the E2F site in the hTERT promoter abrogated the ability of E7 to induce the hTERT promoter or to enhance the ability of E6 to induce the promoter. Correspondingly, keratinocytes expressing E6 and a mutant E7 (defective for binding pRb pocket proteins) showed lower telomerase activity than cells expressing wt E6 and wt E7. Thus, HPV E7 plays a role in the maintenance of telomerase activity in stable cell lines and augments acute, E6-induced hTERT promoter activity.

Recent Advances in the Search for Antiviral Agents against Human Papillomaviruses

Infection by human papillomavirus (HPV) is extremely common and associated with the development of benign warts or malignant lesions of the skin and mucosa. Infection by a high-risk (oncogenic) anogenital HPV type, most often through sexual contacts, is the starting point of virtually all cases of cervical cancers and the majority of anal cancers. The same viral types are also increasingly being linked with a subset of head-and-neck and non-melanoma skin cancers. Although prophylactic vaccines are now available to protect against the four types most commonly found in cervical and anal cancers (HPV16 and HPV18) and anogenital warts (HPV6 and HPV11), these neither protect against all genital HPVs nor are of therapeutic utility for already infected patients. Thus, the need for antiviral agents to treat HPV-associated diseases remains great, but none currently exist. This article reviews the recent progress made towards the development of antiviral agents to treat HPV infections, from target identification and validation to the discovery of lead compounds with therapeutic potential. Emphasis has been placed on novel low-molecular-weight compounds that antagonize HPV proteins or, alternatively, inhibit cellular proteins which have been usurped by papillomaviruses and are mediating their pathogenic effects.

Human papillomavirus E7 repression in cervical carcinoma cells initiates a transcriptional cascade driven by the retinoblastoma family, resulting in senescence

This work demonstrates a central role for the retinoblastoma (Rb) family in driving the transcriptional program of induced and replicative senescence. HeLa cervical carcinoma cells rapidly undergo senescence when the human papillomavirus (HPV) type 18 E7 gene in these cells is repressed by the bovine papillomavirus (BPV) E2 protein. This senescence response requires the endogenous Rb pathway but not the p53 pathway. Microarray analysis 6 days after BPV E2 introduction into HeLa cells identified 224 cellular genes induced by E7 repression and 354 repressed genes. Many repressed genes were involved in cell cycle progression, and numerous induced genes encoded lysosomal proteins. These gene expression changes were blocked by constitutive expression of the wild-type HPV16 E7 or adenovirus E1A gene, but not by E7 or E1A mutants defective for Rb binding. Short hairpin RNAs targeting the Rb family also inhibited these gene expression changes and blocked senescence. Therefore, surprisingly, the transcriptional response to BPV E2 expression was entirely dependent on E7 repression and activation of the Rb family, and the BPV E2 protein did not directly affect the expression of cellular genes. Activation of the Rb family repressed E2F-responsive genes and stimulated transcriptional activators, thereby mobilizing multiple signals, such as repression of B-MYB and DEK, that were independently sufficient to induce senescence. There was extensive overlap between the transcriptional profiles of senescent, late-passage primary human fibroblasts and senescent cervical carcinoma cells, suggesting that this Rb family-mediated transcriptional cascade also plays a central role in replicative senescence.