Normal Human Fibroblasts Immortalized by Introduction of Human Papillomavirus Type 16 (HPV-16) E6-E7 Genes

This report demonstrates that normal human fibroblasts can be immortalized by the introduction of HPV-16 E6-E7 genes. We designed zinc-inducible expression plasmids with HPV-16 E6, E7 or both. Each plasmid was introduced into normal human fibroblasts (TIG-3 cells) using lipofection methods. Only trans-fectants with the HPV-16 E6-E7 zinc-inducible expression plasmid, which were cultured in medium supplemented with 100 μM ZnSO₄, overcame crisis and could be cultured over 200 population doubling levels (PDLs). These cell lines showed the reactivation of telomerase after crisis, and morphological alterations were also observed.     

人乳头瘤病毒16型早期基因E6、E7的克隆及表达

高危人乳头瘤病毒16型的感染与宫颈癌的发病密切相关。HPV16E6和E7蛋白在大多数HPV16相关宫颈癌及其癌前病变中持续表达,因此E6和E7蛋白可作为制备HPV16相关肿瘤及其癌前病变治疗性疫苗的靶抗原〔2〕。采用套式PCR方法,从宫颈癌患者组织中扩增出E6、E7基因并成功构建了包含E6、E7的表达质粒PET-E6、PET-E7。SDS聚丙烯酰胺凝胶电泳和Western-blotting分析结果表明,外源基因E6,E7在T7启动子控制下可获得稳定表达。     

In vitro biological activities of the E6 and E7 genes vary among human papillomaviruses of different oncogenic potential.

Human papillomavirus type 16 (HPV-16) and HPV-18 are often detected in cervical carcinomas, while HPV-6, although frequently present in benign genital lesions, is only rarely present in cancers of the cervix. Therefore, infections with HPV-16 and HPV-18 are considered high risk and infection with HPV-6 is considered low risk. We found, by using a heterologous promoter system, that expression of the E7 transforming protein differs between high- and low-risk HPVs. In high-risk HPV-16, E7 is expressed from constructs containing the complete upstream E6 open reading frame. In contrast, HPV-6 E7 was efficiently translated only when E6 was deleted. By using clones in which the coding regions of HPV-6, HPV-16, and HPV-18 E7s were preceded by identical leader sequences, we found that the ability of the E7 gene products to induce anchorage-independent growth in rodent fibroblasts correlated directly with the oncogenic association of the HPV types. By using an immortalization assay of normal human keratinocytes that requires complementation of E6 and E7, we found that both E6 and E7 of HPV-18 could complement the corresponding gene from HPV-16. However, neither E6 nor E7 from HPV-6 was able to substitute for the corresponding gene of HPV-16 or HPV-18. Our results suggest that multiple factors, including lower intrinsic biological activity of E6 and E7 and differences in the regulation of their expression, account for the low activity of HPV-6, in comparison with HPV-16 and HPV-18, in in vitro assays. These same factors may, in part, account for the apparent difference in oncogenic potential between these viruses.     

Human papillomavirus type 16 E6 and HPV-16 E6/E7 sensitize human keratinocytes to apoptosis induced by chemotherapeutic agents: roles of p53 and caspase activation

We and others have previously reported that human papillomavirus (HPV)-16 E6 protein expression sensitizes certain cell types to apoptosis. To confirm that this sensitization occurred in HPV's natural host cells, and to explore the mechanism(s) of sensitization, we infected human keratinocytes (HKCs) with retroviruses containing HPV-6 E6, HPV-16 E6, HPV-16 E7, or HPV-16 E6/E7. Apoptosis was monitored by DNA fragmentation gel analysis and direct observation of nuclei in cells stained with DAPI. Exposure of HKCs to etoposide, cisplatin, mitomycin C (MMC), atractyloside, and sodium butyrate, resulted in a time and dose-dependent induction of apoptosis. Expression of HPV-16 E6 and HPV-16 E6/E7, but not HPV-6 E6 or HPV-16 E7, enhanced the sensitivity of HKCs to cisplatin-, etoposide- and MMC-, but not atractyloside- or sodium butyrate-induced apoptosis. Expression of both HPV-16 E6 and HPV-16 E6/E7 decreased, but did not abolish, p53 protein levels relative to normal HKCs, and resulted in cytoplasmic localization of wt p53. P53 induction occurred in HPV-16 E6 and HPV-16 E6/E7 expressing cells after exposure to cisplatin or MMC, though never to levels found in normal untreated HKCs. P21 levels were decreased in HPV-16 E6 and HPV-16 E6/E7 expressing HKCs, and no induction of p21 was seen in these cells following exposure to cisplatin or MMC. Caspase-3 activity was found to be elevated in HPV-16 E6-expressing HKCs following exposure to cisplatin and MMC as documented by fluorometric and Western Blot analysis. Expression of wt CrmA or treatment of HPV-16 E6 expressing HKCs with the caspase-3 inhibitor DEVD.fmk prevented HPV-16 E6-induced sensitization in HKCs. These results suggest that HPV-16 E6 and HPV-16 E6/E7 expression sensitizes HKCs to apoptosis caused by cisplatin, etoposide and MMC, but not atractyloside or sodium butyrate. The data also suggest that wt p53 and caspase-3 activity are required for HPV-16 E6 and HPV-16 E6/E7-induced sensitization of HKCs to DNA damaging agents.     

人乳头瘤病毒16型亚基因DNA体外转化功能的细胞学研究

利用HZIP16和HZIP16K(见材料和方法)质粒,将人乳头瘤病毒16型(HPV-16)的全早期区基因及其开放读码框架(ORF)E6-E7分别转入ψ2细胞,所产生的重组病毒能诱导NIH3T3细胞发生转化。转化细胞具有恶性细胞的生物学和形态学特征,可在0.3％软琼脂中形成集落,可使裸鼠致瘤。Southern blot证明,HPV-16 E6-E7 ORFs序列以整合形式存在于转化细胞和裸鼠肿瘤细胞DNA中,表明HPV-16 DNA具有体外诱导NIH3T3细胞恶性转化的作用,E6-E7 ORFs是诱导细胞转化的关键基因。     

Identification of the HPV-16 E6 protein from transformed mouse cells and human cervical carcinoma cell lines.

Human cervical carcinoma cell lines that harbor human papillomavirus (HPV) have been reported to retain selectively and express HPV sequences which could encode viral E6 and E7 proteins. The potential importance of HPV E6 to tumors is suggested further by the observation that bovine papillomavirus (BPV) E6 can induce morphologic transformation of mouse cells in vitro. To identify HPV E6 protein, a polypeptide encoded by HPV-16 E6 was produced in a bacterial expression vector and used to raise antisera. The antisera specifically immunoprecipitated the predicted 18-kd protein in two human carcinoma cell lines known to express HPV-16 RNA and in mouse cells morphologically transformed by HPV-16 DNA. The 18-kd E6 protein was distinct from a previously identified HPV-16 E7 protein. The HPV-16 E6 antibodies were found to be type specific in that they did not recognize E6 protein in cells containing HPV-18 sequences and reacted weakly, if at all, to BPV E6 protein. The results demonstrate that human tumors containing HPV-16 DNA can express an E6 protein product. They are consistent with the hypothesis that E6 may contribute to the transformed phenotype in human cervical cancers that express this protein.     

The E5 oncoprotein of human papillomavirus type 16 transforms fibroblasts and effects the downregulation of the epidermal growth factor receptor in keratinocytes.

To determine the function of the E5 open reading frame (ORF) of the human papillomaviruses (HPVs), rodent fibroblast cell lines were transfected with the E5 ORF of HPV type 6 (HPV-6) and HPV-16 expressed from an exogenous promoter. Transfected fibroblasts were transformed to colony formation in soft agar, and the transformation frequency was increased by epidermal growth factor (EGF) but not by platelet-derived growth factor. In a transitory assay, the E5 ORFs from both HPV-6 and HPV-16 were mitogenic in primary human foreskin epithelial cells (keratinocytes) and acted synergistically with EGF. Investigation of keratinocytes expressing HPV-16 E5 showed that the number of endogenous EGF receptors (EGFRs) per cell was increased two- to fivefold. Immunofluorescence microscopy of HPV-16 E5-expressing keratinocytes indicated that there was an apparent delay in the internalization and degradation of EGFRs compared with controls. Kinetic studies with [125I]EGF showed that the ligand underwent normal internalization and degradation in both HPV-16 E5-expressing and control keratinocytes, but in E5-expressing cells, a greater number of receptors recycled back to the cell surface within 1 to 6 h of ligand binding. Finally, ligand-stimulated phosphorylation of the EGFR on tyrosine, an indication of receptor kinase activity, was of greater magnitude in the HPV-16 E5-expressing keratinocytes than in control cells, although the basal level of receptor phosphorylation was similar.     

HPV-16 E2 contributes to induction of HPV-16 late gene expression by inhibiting early polyadenylation

We provide evidence that the human papillomavirus (HPV) E2 protein regulates HPV late gene expression. High levels of E2 caused a read-through at the early polyadenylation signal pAE into the late region of the HPV genome, thereby inducing expression of L1 and L2 mRNAs. This is a conserved property of E2 of both mucosal and cutaneous HPV types. Induction could be reversed by high levels of HPV-16 E1 protein, or by the polyadenylation factor CPSF30. HPV-16 E2 inhibited polyadenylation in vitro by preventing the assembly of the CPSF complex. Both the N-terminal and hinge domains of E2 were required for induction of HPV late gene expression in transfected cells as well as for inhibition of polyadenylation in vitro. Finally, overexpression of HPV-16 E2 induced late gene expression from a full-length genomic clone of HPV-16. We speculate that the accumulation of high levels of E2 during the viral life cycle, not only turns off the expression of the pro-mitotic viral E6 and E7 genes, but also induces the expression of the late HPV genes L1 and L2.     

Genomic and proteomic expression patterns in HPV-16 E6 gene transfected stable human carcinoma cell lines

cDNA microarray and proteomics studies were performed to analyze the genomic and proteomic expression patterns in HPV-16 E6 gene transfected stable human carcinoma cell lines. Among 1024 known genes and ESTs tested by cDNA microarray, we found 50 upregulated and 35 downregulated genes in RC10.1 HPV-16 E6 transfected human colon adenocarcinoma cells compared to RKO cells, and 27 upregulated and 43 downregulated genes in A549E6 HPV-16 E6 transfected human lung adenocarcinoma cells compared to A549 cells. Employing two dimensional gel electrophoresis and MALDI-TOF-MS, the global pattern of protein expressions in RC10.1 human colon adenocarcinoma and A549E6 human lung adenocarcinoma cell lines stably expressing the HPV 16-E6 gene were compared with those of RKO and A549 cell lines to generate a differential protein expression catalog. We found 13 upregulated and 13 downregulated proteins in RC10.1 (E6-expressing RKO) cells compared to RKO cells and 12 upregulated and 14 downregulated proteins in A549E6 (E6-expressing A549) cells compared to A549 cells. The identified genes and proteins were classified into several groups according to the subcellular function. Expressing pattern of three genes and proteins (CDK5, Bak, and I-TRAF) were matched in both analyses of cDNA microarray and proteomics. These powerful approaches using cDNA microarray and proteomics could provide in-depth information on the impact of HPV-16 E6-related genes and proteins. Differential gene and protein expression patterns by transfection of HPV-16 E6 will provide the nucleus of valuable resource for investigation of the biochemical basis of cervical carcinogenesis. Further understanding of this data base may provide valuable resources for developing novel diagnostic markers and therapeutic targets of cervical cancer.     

The E6 and E7 genes of human papillomavirus type 6 have weak immortalizing activity in human epithelial cells.

Previous studies have shown that the E7 gene of human papillomavirus (HPV) type 16 or 18 alone was sufficient for immortalization of human foreskin epithelial cells (HFE) and that the efficiency was increased in cooperation with the respective E6 gene, whereas the HPV6 E6 or E7 gene was not active in HFE. To detect weak immortalizing activities of the HPV6 genes, cells were infected with recombinant retroviruses containing HPV genes, alone and in homologous and heterologous combinations. The HPV6 genes, alone or together (HPV6 E6 plus HPV6 E7), were not able to immortalize cells. However the HPV6 E6 gene, in concert with HPV16 E7, increased the frequency of immortalization threefold over that obtained with HPV16 E7 alone. Interestingly, 6 of 20 clones containing the HPV16 E6 gene and the HPV6 E7 gene were immortalized, whereas neither gene alone was sufficient. Thus, the HPV6 E6 and E7 genes have weak immortalizing activities which can be detected in cooperation with the more active transforming genes of HPV16. Acute expression of the HPV6 and HPV16 E6 and E7 genes revealed that only HPV16 E7 was able to stimulate the proliferation of cells in organotypic culture, resulting in increased expression of the proliferative cell nuclear antigen and the formation of a disorganized epithelial layer. Additionally, combinations of genes that immortalized HFE cells (HPV16 E6 plus HPV16 E7, HPV16 E6 plus HPV6 E7, and HPV6 E6 plus HPV16 E7) also stimulated proliferation.     

HPV16 E6 and E7 proteins cooperate to immortalize human foreskin keratinocytes.

The human papillomavirus types (HPVs) most often associated with cancer of the cervix, such as HPV16, have been reported previously to immortalize normal human foreskin keratinocytes in vitro, while the types that are primarily associated with benign cervical lesions failed to do so. In this study we have determined the HPV16 genes that are responsible for the immortalizing activity of the viral genome. Transfection with a plasmid in which E6 and E7 were the only intact open reading frames (ORFs) induced an indefinite life-span in the keratinocytes with an efficiency similar to that of the entire early region of the viral DNA. Mutants in the E6E7 clone with inactivating lesions in E6 or E7 failed to induce immortalization. When transfected alone, E7 could induce hyperproliferation, but these cells eventually senesced. By itself, E6 exhibited no activity, Co-transfection of a plasmid with an intact E6 ORF and a second plasmid with an intact E7 ORF generated keratinocyte lines with indefinite growth potential. The E6 and E7 proteins were detected in the lines induced by the E6E7 DNA and by co-transfection of the E6 and E7 plasmids. Therefore, we conclude that HPV16 E6 and E7 cooperative to immortalize human keratinocytes in vitro. Changes in cellular gene expression are probably also required for immortalization since all of the keratinocyte lines examined were aneuploid. Serum and calcium resistant sublines were isolated from the E6E7 induced lines, indicating that other HPV genes do not play an obligatory role in the generation of resistance to differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tumorigenicity by human papillomavirus type 16 E6 and E7 in transgenic mice correlates with alterations in epithelial cell growth and differentiation.

The human papillomavirus type 16 (HPV-16) E6 and E7 oncogenes are thought to play a role in the development of most human cervical cancers. These E6 and E7 oncoproteins affect cell growth control at least in part through their association with and inactivation of the cellular tumor suppressor gene products, p53 and Rb. To study the biological activities of the HPV-16 E6 and E7 genes in epithelial cells in vivo, transgenic mice were generated in which expression of E6 and E7 was targeted to the ocular lens. Expression of the transgenes correlated with bilateral microphthalmia and cataracts (100% penetrance) resulting from an efficient impairment of lens fiber cell differentiation and coincident induction of cell proliferation. Lens tumors formed in 40% of adult mice from the mouse lineage with the highest level of E6 and E7 expression. Additionally, when lens cells from neonatal transgenic animals were placed in tissue culture, immortalized cell populations grew out and acquired a tumorigenic phenotype with continuous passage. These observations indicate that genetic changes in addition to the transgenes are likely necessary for tumor formation. These transgenic mice and cell lines provide the basis for further studies into the mechanism of action of E6 and E7 in eliciting the observed pathology and into the genetic alterations required for HPV-16-associated tumor progression.     

Induction of cytotoxic T lymphocytes specific for a syngeneic tumor expressing the E6 oncoprotein of human papillomavirus type 16.

Human papillomavirus (HPV) type 16 has been implicated in the etiology of cervical carcinomas, but it is unknown whether HPV-specific immunity can function in controlling the growth of HPV-associated carcinomas. We previously demonstrated that CD8+ T lymphocytes can inhibit the in vivo outgrowth of murine tumor cells transfected with the HPV-16 E7 gene and have now established a murine model to study the CTL responses to the E6 oncoprotein of HPV-16. Immunization of C3H/HeN mice with syngeneic fibroblasts expressing a transfected HPV-16 E6 gene induced regression of transplanted tumors expressing this gene. Populations of CTL isolated from the spleens of mice whose E6+ tumors had regressed were shown to specifically lyse E6+ target cells. The cytolytic activity was mediated by CD8+ CTL in a MHC restricted pattern. These data and our previous findings with transfected tumor cells expressing the E7 gene, support the conclusion that tumor cells associated with HPV-16 can be inhibited by CTL specific for molecules encoded by the HPV-16 E6 and E7 genes.