THE EXPRESSION LEVELS OF THE HUMAN PAPILLOMAVIRUS TYPE-16 E7 CORRELATE WITH ITS TRANSFORMING POTENTIAL

The transforming potential of the human papillomavirus (HPV) type 16 h as been defined largely in the E7, E6, and E5 oncoproteins, with the m ajor transforming capability residing in the E7 gene. In this paper, w e found that in cooperation with the activated res the HPV16 E7 gene w hen expressed in a retroviral vector could fully transform baby rat ki dney (BRK) cells in transfections, whereas the same construct could on ly immortalize the BRK cells following retroviral infection. This inab ility to transform correlated with the low levels of E7 gene RNA expre ssion in the viral infected cells, which harbor a lower number of copi es of the E7 gene constructs. Cotransfection of the expression vector FV2E7, which gives high levels of E7 gene expression, and activated ra s lead to rapid and efficient morphological transformation of BRK cell s which grew easily in soft agar and induced large tumors in athymic n ude mice. In contrast, cotransfections of the expression vector FV1E7, which gives lower levels of 67 gene expression, produced much lower n umbers of transformed colonies which took longer to form, showed a ret arded growth on soft agar, and induced smaller tumors in nude mice. Un der these conditions, colonies of immortalized, but morphologically un transformed cells formed in large numbers. These results indicate that the transforming potential is directly correlated to the expression l evels of the oncoprotein and that a threshold level of the E7 oncoprot ein may be required before the cells can be fully transformed. This su pports the hypothesis that the transformation processes include at lea st two separate and continuous steps which first lead to immortalizati on and then to metastasis, in agreement with the clinical progression of genital tumors from benign to malignancy. Such a progression may in volve enhanced expression of the oncoproteins. (C) 1995 Academic Press , Inc.