The E7 protein of human papillomavirus type 16 sensitizes primary human keratinocytes to apoptosis

Abstract

The 'high risk' human papillomaviruses are associated with the development of anogenital carcinomas and their E6 and E7 genes possess immortalizing 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 of these oncogenes in the natural host cell, we infected genital keratinocytes 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/cycloheximide or sulfur mustard. In contrast to previous findings with mammary epithelial cells, the E6 gene did not significantly augment either spontaneous or induced apoptosis. E6 also did not suppress apoptosis in normal keratinocytes (despite dramatically reducing their p53 levels), suggesting that p53-independent events mediated this effect. In contrast, E7 increased both spontaneous and induced apoptosis as well as the cellular levels of p53 and p21 protein. Interestingly, co-expression of E6 abrogated E7-facilitated apoptosis by tumor necrosis factor alpha nearly completely, but had only a minor protective effect on sulfur mustard 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-expressing keratinocytes is differentially affected by E6 expression and that E7, when unaccompanied by E6, sensitizes keratinocytes to apoptosis.