REGULATION OF TYROSINASE MESSENGER-RNA CELLS BY ALPHA-MELANOCYTE-STIMULATING HORMONE

Cloudman S-91 mouse melanoma cells respond to alpha-melanocyte-stimula ting hormone) by demonstrating a marked increase in tyrosinase activit y (O-diphenol-O-2 oxidoreductase, EC 1.14.18.1), This increase is the result of increased levels of tyrosinase mRNA with a subsequent increa se in tyrosinase abundance, Our studies were carried out to determine the effect of melanocyte-stimulating hormone on tyrosinase gene transc ription and to measure the kinetics of the hormone-induced increase in tyrosinase mRNA. When melanoma cells were exposed continuously to mel anocyte-stimulating hormone for 6 d, a large but transient increase in both tyrosinase mRNA abundance and enzyme activity were observed, The maximum increase in tyrosinase mRNA occurred 60 h after melanocyte-st imulating hormone stimulation and was followed by a decline in message levels even though cells were continuously exposed to hormone. Result s of nuclear run-off transcription assays showed that melanocyte-stimu lating hormone caused a slow increase in the rate of transcription of the tyrosinase gene with a maximal 6-fold stimulation occurring at 48 h. In cells treated with the ribonucleic acid synthesis inhibitor, 5,6 -dichloro-1-beta-D-ribofuranosyl-benzimidazole, tyrosinase mRNA levels decayed with a half-life of 4-5 h, This decay rate was unaffected by treatment of cells with melanocyte-stimulating hormone, indicating tha t the hormone does not act to stabilize tyrosinase ribonucleic acid. I nhibition of protein synthesis by treatment with cycloheximide had no effect on the melanocyte-stimulating hormone-induced increase in tyros inase messenger ribonucleic acid levels suggesting that ongoing protei n synthesis is not, required for, at least, the initial stimulation of tyrosinase gene transcription by melanocyte-stimulating hormone.