GROWTH-INHIBITION OF HUMAN MELANOMA-DERIVED CELLS BY 12-O-TETRADECANOYL PHORBOL 13-ACETATE

In vitro growth of 6 human melanoma-derived cell lines was inhibited markedly by the phorbol-ester tumor promoter 12-O-tetradecanoyl phorbol 13-acetate (TPA), a potent activator of several isoforms of protein kinase C (PKC). Utilizing PKC isoform-specific antibodies in immunoblotting experiments, we found that the PKCalpha and PKCepsilon isoforms were expressed in all of the 6 melanoma cell lines tested, whereas the PKCbeta isoform was expressed at detectable levels in only 2 of the 6 cell lines. The SK-Mel-173 melanoma cell line, which had relatively high levels of PKCbeta mRNA and protein expression, and which was also the most sensitive to cell growth inhibition by TPA, was used to isolate clones whose growth was less inhibited by TPA. Immunoblotting experiments revealed that in parental SK-Mel 173 cells PKCbeta was rapidly down-regulated to below detectable levels after treatment for 48 hr with TPA, but that in TPA-resistant variant clones there was negligible down-regulation of PKCbeta by TPA. On the other hand, treatment of parental and TPA-resistant SK-Mel 173 cells with TPA led to partial down-regulation of PKCalpha in both cell lines. Total PKC enzyme activity was also greater in TPA-resistant cells than in parental SK-Mel 173 cells. Our results show that TPA might inhibit the growth of melanoma cells by causing down-regulation of specific isoforms of PKC that are required to maintain the growth of these cells. (C) 1994 Wiley-Liss, Inc.