Involvement of pRB family in TGF beta-dependent epithelial cell hypertrophy

Although renal hypertrophy is often associated with the progressive loss of renal function, the mechanism of hypertrophy is poorly understood. In both primary cultures of rabbit proximal tubules and NRK- 52E cells (a renal epithelial cell line), transforming growth factor beta 1 (TGF beta) converted epidermal growth factor (EGF)-induced hyperplasia into hypertrophy. TGF beta did not affect EGF-induced increases in c-fos mRNA abundance or cyclin E protein abundance, but inhibited EGF-induced entry into S, G2, and M phases. EGF alone increased the amount of hyperphosphorylated (inactive) pRB; TGF beta blocked EGF-induced pRB phosphorylation, maintaining pRB in the active form. To determine the importance of active pRB in TGF beta-induced hypertrophy, NRK-52E cells were infected with SV40 large T antigen (which inactivates pRB and related proteins and p53), HPV16 E6 (which degrades p53), HPV16 E7 (which binds and inactivates pRB and related proteins), or both HPV16 E6 and E7. In SV40 large T antigen expressing clones, the magnitude of EGF + TGF beta-induced hypertrophy was inhibited and was inversely related to the magnitude of SV40 large T antigen expression. In the HPV16-infected cells, EGF + TGF beta-induced hypertrophy was inhibited in E7- and E6E7-expressing, but not E6- expressing cells. These results suggest a requirement for active pRB in the development of EGF + TGF beta-induced renal epithelial cell hypertrophy. We suggest a model of renal cell hypertrophy mediated by EGF-induced entry into the cell cycle with TGF beta-induced blockade at G1/S, the latter due to maintained activity of pRB or a related protein.     

Inhibition of growth, transformation, and expression of human papillomavirus type 16 E7 in human keratinocytes by alpha interferons.

We used a model system of normal human keratinocytes (HKc) and HKc immortalized with human papillomavirus type 16 DNA (HKc/HPV16) to investigate the effects of alpha interferons (IFN-alpha) on the growth of HPV16-immortalized human epithelial cells, on HPV16-mediated immortalization of normal HKc, and on HPV16 gene expression. Normal HKc and HKc/HPV16 were treated with several recombinant human IFN-alpha subtypes (IFN-alpha B, IFN-alpha D, and IFN-alpha B/D). These IFN-alpha subtypes inhibited proliferation of both normal HKc and HKc/HPV16 in a dose-dependent fashion; however, although 1,000 to 10,000 U of IFN-alpha per ml were required to inhibit growth of normal HKc, HKc/HPV16 were substantially growth inhibited by 100 U/ml. In addition, 100 U of IFN-alpha B/D per ml inhibited transformation of normal HKc by HPV16 DNA. Northern (RNA) blot analysis showed no effect of IFN-alpha on the mRNA levels of the HPV16 E6 and E7 open reading frames. However, immunofluorescence studies of the HPV16 E6 and E7 proteins with anti-E6 and anti-E7 monoclonal antibodies showed significant inhibition of E7 protein expression in cells treated with IFN-alpha, whereas E6 protein expression was not altered. The inhibition of E7 protein expression in cells treated with IFN-alpha was further confirmed by Western immunoblot analysis. These results suggest that IFN-alpha may inhibit HPV16-mediated transformation of HKc and proliferation of HKc/HPV16 through an inhibition of HPV16 E7 protein expression.     

Down-regulation of Id-1 expression is associated with TGF beta 1-induced growth arrest in prostate epithelial cells

Transforming growth factor beta1 (TGF beta 1) plays important roles in the regulation of cell growth and differentiation in both normal and malignant prostate epithelial cells. Although certain pathways have been suggested, the mechanisms responsible for the action of TGF beta 1 are not well understood. In the present study, using a human papilloma virus 16 E6/E7 immortalized prostate epithelial cell line, HPr-1, we report that TGF beta 1 was able to suppress the expression of Id-1, a helix-loop-helix (HLH) protein, which plays important roles in the inhibition of cell differentiation and growth arrest. In addition, a decrease at both Id-1 mRNA and protein expression levels was associated with TGF beta 1-induced growth arrest and differentiation, indicating that Id-1 may be involved in TGF beta 1 signaling pathway. The fact that up-regulation of p21(WAF1), one of the downstream effectors of Id-1, was observed after exposure to TGF beta 1 further indicates the involvement of Id-1 in the TGF beta 1-induced growth arrest in HPr-1 cells. However, increased expression of p27(KIP1) was also observed in the TGF beta 1-treated cells, suggesting that in addition to down-regulation of Id-1, other factors may be involved in the TGF beta 1-induced cell growth arrest and differentiation in prostate epithelial cells. Our results provide evidence for the first time that TGF beta 1 may be one of the upstream regulators of Id-1.     

Inhibition of cervical carcinoma cell line proliferation by the introduction of a bovine papillomavirus regulatory gene.

Human papillomavirus (HPV) E6 and E7 oncogenes are expressed in the great majority of human cervical carcinomas, whereas the viral E2 regulatory gene is usually disrupted in these cancers. To investigate the roles of the papillomavirus E2 genes in the development and maintenance of cervical carcinoma, the bovine papillomavirus (BPV) E2 gene was acutely introduced into cervical carcinoma cell lines by infection with high-titer stocks of simian virus 40-based recombinant viruses. Expression of the BPV E2 protein in HeLa, C-4I, and MS751 cells results in specific inhibition of the expression of the resident HPV type 18 (HPV18) E6 and E7 genes and in inhibition of cell growth. HeLa cells, in which HPV gene expression is nearly completely abolished, undergo a dramatic and rapid inhibition of proliferation, which appears to be largely a consequence of a block in progression from the G1 to the S phase of the cell cycle. Loss of HPV18 gene expression in HeLa cells is also accompanied by a marked increase in the level of the cellular p53 tumor suppressor protein, apparently as a consequence of abrogation of HPV18 E6-mediated destabilization of p53. The proliferation of HT-3 cells, a human cervical carcinoma cell line devoid of detectable HPV DNA, is also inhibited by E2 expression, whereas two other epithelial cell lines that do not contain HPV DNA are not inhibited. Thus, a number of cervical carcinoma cell lines are remarkably sensitive to growth inhibition by the E2 protein. Although BPV E2-mediated inhibition of HPV18 E6 and E7 expression may contribute to growth inhibition in some of the cervical carcinoma cell lines, the BPV E2 protein also appears to exert a growth-inhibitory effect that is independent of its effects on HPV gene expression.     

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.     

Identification of early proteins of the human papilloma viruses type 16 (HPV 16) and type 18 (HPV 18) in cervical carcinoma cells.

We have sequenced 1730 bp of human papilloma virus type 18 (HPV 18) DNA containing the open reading frames (ORF) E6, E7, the N-terminal part of E1 and, additionally, 120 bp of the N-terminal part of L1. Based on these sequencing data, together with the human papilloma virus type 16 (HPV 16) DNA sequence published recently, we identified and cloned the ORF E6, E7, E1 and L1 of HPV 18 and the ORF E6, E7, E1, E4, E5, L2 and L1 of HPV 16 into prokaryotic expression vectors. The expression system used provides fusions to the N-terminal part of the MS2 polymerase gene controlled by the heat-inducible lambda PL promoter. Using the purified fusion proteins as immunogens we raised antisera against the proteins encoded by the ORF E6, E7 and E1 of HPV 18 as well as those encoded by the ORF E6, E7, E4 and L1 of HPV 16. By Western blot analysis we could show that the E7 gene product is the most abundant protein in cell lines containing HPV 16 or HPV 18 DNA. It is a cytoplasmic protein of 15 kd in the SiHa and the CaSki cell lines which contain HPV 16 DNA, and 12 kd in the HeLa, the C4-1 and the SW756 cell lines which contain HPV 18 DNA. These results were confirmed by in vitro translation of hybrid-selected HPV 16 and HPV 18 specific poly(A)+ RNA from SiHa, CaSki and HeLa cells. Additionally, these experiments led to the identification of an 11-kd E6 and a 10-kd E4 protein in the CaSki cell line as well as a 70-kd E1 protein in HeLa cells.     

RbAp48 is a critical mediator controlling the transforming activity of human papillomavirus type 16 in cervical cancer

Although human papillomavirus (HPV) infections are the primary cause of cervical cancer, the molecular mechanism by which HPV induces cervical cancer remains largely unclear. We used two-dimensional electrophoresis with mass spectrometry to study protein expression profiling between HPV16-positive cervical mucosa epithelial H8 cells and cervical cancer Caski cells to identify 18 differentially expressed proteins. Among them, retinoblastoma-binding protein 4 (RbAp48) was selected, and its differentiation expression was verified with both additional cervical cancer-derived cell lines and human tissues of cervical intraepithelial neoplasia and cervical cancer. Suppression of RbAp48 using small interfering RNA approach in H8 cells significantly stimulated cell proliferation and colony formation and inhibited senescence-like phenotype. Remarkably, H8 cells acquired transforming activity if RpAp48 was suppressed, because H8 cells stably transfected with RbAp48 small interfering RNA led to tumor formation in nude mice. In addition, overexpression of RbAp48 significantly inhibited cell growth and tumor formation. This RbAp48-mediated transformation of HPV16 is probably because of the regulation by RbAp48 of tumor suppressors retinoblastoma and p53, apoptosis-related enzymes caspase-3 and caspase-8, and oncogenic genes, including E6, E7, cyclin D1 (CCND1), and c-MYC. In brief, RbAp48, previously unknown in cervical carcinogenesis, was isolated in a global screen and identified as a critical mediator controlling the transforming activity of HPV16 in cervical cancer.     

Inhibition of human papillomavirus type 16 E7 phosphorylation by the S100 MRP-8/14 protein complex

The human papillomavirus type 16 (HPV16) E7 is a major viral oncoprotein that is phosphorylated by casein kinase II (CKII). Two S100 family calcium-binding proteins, macrophage inhibitory-related factor protein 8 (MRP-8) and MRP-14, form a protein complex, MRP-8/14, that inactivates CKII. The MRP-8/14 protein complex may inhibit CKII-mediated E7 phosphorylation and therefore may alter its interaction with cellular ligands and reduce E7 oncogenic activity. We examined the inhibitory effect of the MRP-8/14 complex on CKII activity and HPV16 E7 phosphorylation. We have shown that CKII activity and HPV16 E7 phosphorylation were inhibited by uptake of exogenous MRP-8/14 and activation of endogenous MRP-8/14. MRP-8/14-mediated inhibition of E7 phosphorylation occurred at the G1 phase of the cell cycle. Analysis of MRP expression in primary keratinocytes and in HPV16- and 18-transformed cervical and foreskin epithelial cell lines showed that expression of MRP-8, MRP-14, and the MRP-8/14 complex was detected only in primary untransformed keratinocytes and not in the HPV-infected immortalized epithelial cells. CKII activity in HPV-immortalized keratinocytes was approximately fourfold higher than in HPV-negative primary keratinocytes. Treatment of HPV-positive immortalized epithelial cells with exogenous MRP-8/14 resulted in E7 hypophosphorylation and complete inhibition of cell growth within 2 weeks, compared with HPV-negative primary and immortalized HPV-negative cervical epithelial cells, which showed 25 and 40% growth inhibition, respectively. Together these results suggests that the MRP-8/14 protein complex in HPV-infected epithelial cells may play an important role in regulation of CKII-mediated E7 phosphorylation and inhibition of its oncogenic activity.     

Triggering of death receptor apoptotic signaling by human papillomavirus 16 E2 protein in cervical cancer cell lines is mediated by interaction with c-FLIP

Human papillomavirus (HPV) E2 gene disruption is one of the key features of HPV-induced cervical malignant transformation. Though it is thought to prevent progression of carcinogenesis, the pro-apoptotic function of E2 protein remains poorly understood. This study shows that expression of HPV16 E2 induces apoptosis both in HPV-positive and -negative cervical cancer cell lines and leads to hyperactivation of caspase-8 and caspase-3. Activation of these signaling factors is responsible for the observed sensitivity to apoptosis upon treatment with anti-Fas antibody or TNF-α. In addition, immunoprecipitation experiments clearly show an interaction between HPV16 E2 and c-FLIP, a key regulator of apoptotic cell death mediated by death receptor signaling. Moreover, c-FLIP and a caspase-8 inhibitor protect cells from HPV16 E2-mediated apoptosis. Overexpression of c-FLIP rescues cervical cancer cells from apoptosis induced by HPV16 E2 protein expression. The data suggest that HPV16 E2 abrogates the apoptosis-inhibitory function of c-FLIP and renders the cell hypersensitive to the Fas/FasL apoptotic signal even below threshold concentration. This suggests a novel mechanism for deregulation of cervical epithelial cell growth upon HPV-induced transformation, which is of great significance in developing therapeutic strategies for intervention of cervical carcinogenesis.     

人乳头瘤病毒（HPV）致癌机制研究进展

人乳头瘤病毒（Human Papillomavirus,HPV）在人群中广泛传播,能引起皮肤和黏膜的异常增生,某些型的感染与生殖道恶性病变关系密切。HPV在致癌过程中,E2基因通常整合到宿主细胞基因组内,E2基因的失活和E5基因对EGFR的干预都能引起E6、E7基因过表达,E6、E7蛋白分别通过抑制p53、pRb基因的活性,从而激活人细胞端粒酶基因（hTERT）的转录,引起细胞分化异常,导致正常细胞癌化。HPV致癌是一个多因素、多步骤的渐进过程,其中协同因素也发挥重要作用。随着研究的深入,HPV的致癌机制越来越受到国内外研究者的重视,对HPV致癌机制的探索也逐渐成为研究热点。     

Transforming growth factor beta 1 inhibition of p34cdc2 phosphorylation and histone H1 kinase activity is associated with G1/S-phase growth arrest.

Transforming growth factor beta 1 (TGF beta 1) is a potent inhibitor of epithelial cell proliferation. We present data which indicate that epithelial cell proliferation is inhibited when TGF beta 1 is added throughout the prereplicative G1 phase. Cultures become reversibly blocked in late G1 at the G1/S-phase boundary. The inhibitory effects of TGF beta 1 on cell growth occur in the presence of the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Associated with this inhibitory effect is a decrease in the phosphorylation and histone H1 kinase activity of the p34cdc2 protein kinase. These data suggest that TGF beta 1 growth inhibition in epithelial cells involves the regulation of p34cdc2 activity at the G1/S transition.