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Retinoic acid is a potent inhibitor of inducible pigmentation in murine and hamster melanoma cell lines.

Melanocyte-stimulating hormone (MSH) induces melanogenesis in Cloudman mouse melanoma cells. The activities of two enzymes in the melanogenesis pathway, tyrosinase and dopachrome conversion factor, are increased as part of the induction process. Trans retinoic acid (RA), at concentrations as low as 0.1 nM, inhibited the induction of tyrosinase, dopachrome conversion factor, and melanogenesis, but had no effect on the basal levels of either enzyme or of cellular melanin content. Half-maximal effects of RA occurred at a concentration of 10 nM; maximal effects were observed at 1 microM. The effects of RA on melanogenesis were independent of its effects on cellular growth since one Cloudman line tested was growth-inhibited by RA and another was growth-stimulated by RA, but the induction of melanogenesis by MSH in both lines was inhibited by RA. Mixing experiments with cell lysates failed to demonstrate the induction of a tyrosinase inhibitor by RA. The effects of RA were not limited to MSH or to Cloudman melanoma cells since RA blocked cholera toxin-inducible melanogenesis in Cloudman cells, as well as the induction of tyrosinase activity by L-tyrosine in Bomirski hamster melanoma cells. The effects of RA were specific to melanogenesis, however, since RA did not interfere with MSH-induced changes in cellular morphology and growth. Thus, RA appears to be a new and potent tool for understanding mechanisms regulating induction of the pigmentary system.