Cellular targets of the oncoproteins encoded by the cancer associated human papillomaviruses.

Insight into the mechanisms by which DNA tumor viruses transform cells has come from the recognition that the virus-encoded oncoproteins interact specifically with important cell regulatory proteins. The "high risk" human papillomaviruses such as HPV-16 and HPV-18 which are associated with human anogenital carcinomas encode two transforming genes (E6 and E7) which are expressed in HPV positive cancers and derived cell lines. E7 shares functional and structural features with the adenovirus E1A proteins. Like Ad E1A and the large T proteins of the polyomaviruses, E7 can complex pRB. The E7 proteins of the "high risk" HPVs associate with pRB with approximately a 10-fold higher affinity than do the E7 proteins of the "low risk" HPVs, and important biological differences between the E7 proteins of these two groups of HPVs are determined by amino-terminal sequences which include the pRB binding domain. Like SV40 large T and Ad 5 E1B, the E6 oncoprotein encoded by the "high risk" HPVs can form a complex with p53. In vitro, E6 promotes the degradation of p53 and this degradation involves the ubiquitin-dependent protease system. The selective degradation of cellular proteins such as p53 with negative regulatory functions provides a novel mechanism of action for dominant acting oncoproteins. The relevance of the inactivation of the normal functions of pRB and p53 in human cervical carcinogenesis has recently been demonstrated by the analysis of these two genes and their products in a series of HPV-positive and HPV-negative cell lines. Each of five HPV-positive cervical cancer cell lines expressed normal pRB and low levels of wild type p53 proteins, which are presumed to be altered in function as a consequence of association with the HPV oncoproteins. In contrast, mutations were identified in the p53 and RB genes expressed in the HPV-negative cervical carcinoma cell lines, C33-A and HT-3. These results support the hypothesis that the inactivation of the normal functions of the tumor suppressor proteins pRB and p53 are important steps in human cervical carcinogenesis, either by mutation or through complex formation with HPV E6 and E7 oncoproteins.