Binding of the human E2F transcription factor to the retinoblastoma protein but not to cyclin A is abolished in HPV-16-immortalized cells.

The adenovirus E1A, SV40 large T and papillomavirus E7 proteins immortalize primary cells by virtue of their ability to bind the retinoblastoma gene product (pRB) and other cellular proteins, including cyclin A and the prRB-related protein, p107. It has been demonstrated that these viral oncogene products will prevent the inhibition of positive growth regulators by pRB, one of them being the E2F transcription factor. Here we show that the interactions of pRB and cyclin A with E2F are present also in normal keratinocytes and in primary human fibroblasts. In human keratinocytes immortalized by human papillomavirus 16 (HPV-16), expressing high levels of HPV-16 E7 protein, complexes between E2F and pRB are disrupted. In this cell line, as well as in HeLa cells which express HPV-18 E7, complexes containing E2F and cyclin A are maintained, indicating that this interaction is not sensitive to the viral oncoprotein and that cyclin A can associate with E2F independently of pRB. In vitro binding experiments suggest that the E7 gene product is able to preferentially abolish the interaction of pRB with E2F, leaving the cyclin A complexes intact. Our findings suggest that E7-dependent immortalization of human cells is associated with modifications of E2F multiprotein complexes.