Effects of 12-O-tetradecanoylphorbol-13-acetate on human papillomavirus type 16-positive keratinocytes at different stages of transformation.

Normal human keratinocytes grown under serum-free conditions can be triggered to differentiate by exposure to serum or to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). We found that TPA treatment of human papillomavirus (HPV) type 16-immortalized cells in culture induced formation of cornified envelopes indicative of squamous differentiation. Concurrent with differentiation, TPA inhibited the expression of HPV 16 E6 and E7 mRNA transcripts. Adaptation of the immortalized cells to growth in serum-containing medium led to the selection of a subpopulation of HPV-transformed cells that was resistant to TPA-induced differentiation. In this cell line, a transient suppression of HPV transcripts was observed at 5 h, whereas in differentiation-resistant, carcinoma-derived lines, TPA had little effect on HPV oncogene expression. c-myc transcripts were suppressed for the duration of exposure to TPA in only the differentiation-competent cells; c-fos and c-jun were transiently induced in all cell lines. Transforming growth factor-alpha mRNAs were also increased approximately eightfold as HPV 16-immortalized cells were induced to differentiate. These results demonstrate that, in HPV 16-immortalized keratinocytes, acquisition of resistance to inducers of squamous differentiation is accompanied by altered regulation of cell growth and gene expression.