EFFECT OF PHORBOL ESTER TUMOR PROMOTERS ON THE EXPRESSION OF MELANOGENESIS IN B-16 MELANOMA-CELLS

Abstract

Cells of the C3 clone of B-16 [mouse] melanoma synthesize melanin only at confluence after which they senesce and can no longer be passaged. Addition to the cultures of 10-8 to 10-7 M 12-O-tetradecanoylphorbol-13-acetate (TPA) shortly after plating delayed by about 2 days the onset of melanogenesis. TPA did not affect the growth of the cells or the time at which they reached confluence. The ability of a series of phorbol esters to delay melanogenesis correlated with their tumor-promoting activity on mouse skin. The optimum time for addition of TPA was within 24 h after plating; the inhibitory effect decreased when TPA was added later. .alpha.-Melanocyte-stimulating hormone (5 .times. 10-7 M) added to B-16 cultures 24 h after plating slowed the growth of the cells and caused them to differentiate when still subconfluent. TPA also inhibited this .alpha.-melanocyte-stimulating hormone-induced melanogenesis. TPA inhibits a very early stage in a stepwise process that leads to the differentiation of these cultures but the cells eventually escape from this inhibition. The B-16 melanoma cell culture system may be useful for studying the mechanism by which TPA and related tumor promoters affect cellular differentiation.