12-O-tetradecanoylphorbol-13-acetate not only modulates proliferation rates, but also alters antigen expression and LAK-cell susceptibility of normal human melanocytes in vitro.

For serial cultivation of normal human melanocytes media supplemented with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) are largely employed. By using a culture medium that permits cultivation of melanocytes without TPA, the effects of TPA on melanocyte proliferation, phenotype, and susceptibility to lymphokine-activated killer cells were studied. Addition of 50 ng/ml TPA to the medium induced rapid dendrite formation and increased the cell proliferation rate by 16-63% in mitogen-rich media (four of seven cultures, p < 0.01), and by 237% in mitogen-reduced media (p < 0.001). Furthermore, several phenotypic changes indicating early stages of melanocyte transformation were induced by 50 ng/ml TPA. These included increased expression of melanoma progression-associated antigens such as A.1.43 and A.10.33, upregulation of nerve-growth factor receptor as well as of the melanocyte-activation marker HMB-45 and of histocompatibility class I antigens. In contrast, the expression of the differentiation marker K.1.2 and of intercellular adhesion molecule-1 was decreased in TPA-treated cultures. Most of these changes persisted even after removal of TPA from the culture medium (> or = 2 weeks). Staurosporine, a protein kinase C inhibitor, modulated melanocyte-antigen expression similar to TPA, suggesting that protein kinase C downmodulation rather than activation by TPA is involved. In addition to the antigenic alterations, the susceptibility of TPA-treated melanocytes to lymphokine-activated killer cell cytotoxicity decreased by 40% (p < 0.01), possibly due to their altered surface antigen expression. The presented data reveal that the tumor promoter TPA hitherto used as a supplement of melanocyte culture media induces profound phenotypic and functional changes of the cultured cells, indicating incipient transformation of normal human melanocytes in vitro.