THE E7 PROTEIN OF HUMAN-PAPILLOMAVIRUS TYPE-16 SENSITIZES PRIMARY HUMAN KERATINOCYTES TO APOPTOSIS

The 'high risk' human papillomaviruses are associated with the develop ment of anogenital carcinomas and their E6 and E7 genes possess immort alizing and transforming functions in several in vitro culture systems , Recently the E6 gene has also been shown to enhance the apoptosis of human mammary epithelial cells, To determine the apoptotic activity o f these oncogenes in the natural host cell, we infected genital kerati nocytes with retroviruses expressing either HPV-16 E6, E7, or both the E6 and E7 (E6/7) genes. Apoptosis was quantitated under normal growth conditions or when induced by tumor necrosis factor alpha/cycloheximi de or sulfur mustard. In contrast to previous findings with mammary ep ithelial cells, the E6 gene did not significantly augment either spont aneous or induced apoptosis, E6 also did not suppress apoptosis in nor mal keratinocytes (despite dramatically reducing their p53 levels), su ggesting that p53-independent events mediated this effect, In contrast , E7 increased both spontaneous and induced apoptosis as well as the c ellular levels of p53 and p21 protein. Interestingly, coexpression of E6 abrogated E7-facilitated apoptosis by tumor necrosis factor alpha n early completely, but had only a minor protective effect on sulfur mus tard induced apoptosis in these cells, demonstrating at least in part the p53-dependence and -independence of these two apoptotic pathways, Finally, our results indicate that the apoptosis of normal and E7-expr essing keratinocytes is differentially affected by E6 expression and t hat E7, when unaccompanied by E6, sensitizes keratinocytes to apoptosi s.