REVERSIBLE REPRESSION OF PAPILLOMAVIRUS ONCOGENE EXPRESSION IN CERVICAL-CARCINOMA CELLS - CONSEQUENCES FOR THE PHENOTYPE AND E6-P53 AND E7-PRB INTERACTIONS

The transforming genes E6 and E7 of high-risk human papillomaviruses a re consistently expressed in papillomavirus-associated neoplasms of th e anogenital tract. In papillomavirus type 18-associated SW 756 cervic al carcinoma cells, transcription of the viral E6-E7 genes is blocked by dexamethasone. Herein ive show that dexamethasone-mediated repressi on of the E6-E7 genes results in loss of the neoplastic phenotype of S W 756 cells. Withdrawal of dexamethasone restores E6-E7 expression and neoplastic growth. Moreover, reconstitution of E6-E7 gene expression by a dexamethasone-inducible expression vector renders the neoplastic phenotype resistant to dexamethasone. These results clearly indicate t hat the continuous expression of the viral E6-E7 oncogenes is required to maintain the neoplastic growth properties of SW 756 cervical cance r cells. The viral E6 protein destabilizes the p53 tumor suppressor ge ne product in vitro. Since low levels of p53 have been observed in pap illomavirus-transformed keratinocyte cell lines, it was speculated tha t degradation of p53 by E6 contributes to papillomavirus-associated gr owth deregulation. Consistent with this hypothesis, we detected a sign ificant increase in p53 levels upon dexamethasone-induced repression o f papillomavirus E6-E7 oncogene expression. No p53 increase was observ ed in dexamethasone-treated cells in which the viral oncogene expressi on was restored. The viral E7 protein has been shown to complex with t he retinoblastoma tumor suppressor gene product (pRB). In some cells, this interaction has been shown to release the transcription factor E2 F from its complex with pRB, and it has been hypothesized that E7-indu ced, increased levels of free E2F contribute to the transforming poten tial of the viral oncogenes. In gel shift experiments, we detected rel atively stable complexes of pRB and E2F in all SW 756-derived cells, i ndependent of the level of E7 expression. This suggests that E7-mediat ed release of E2F from its complex with pRB might not be required to m aintain the neoplastic phenotype of human papillomavirus-associated ca ncer cells, although a possibly relevant partial E7-mediated release o f E2F from pRB cannot be excluded.