Melanocyte-stimulating hormone promotes activation of pre-existing tyrosinase molecules in Cloudman S91 melanoma cells

Abstract

TYROSINASE activity is greatly enhanced in cultured Cloudman S91 melanoma cells following addition of melanocyte-stimulating hormone (MSH) to the culture medium¹. The increased activity occurs in the G2 phase of the cell cycle² because membrane receptors for MSH are available only in this phase³. The response to MSH is mediated through cyclic AMP (refs 1–4). It is well documented that many peptide hormones function by activating membrane-bound adenyl cyclase molecules, causing net increases in intracellular cyclic AMP concentrations. These increases in turn have profound effects on cell division, morphology, and the expression of differentiated functions in a wide variety of cells and tissues. Little is known, however, about the levels at which genetic expression is regulated or the molecular intermediates involved in the regulation. For example, it is generally assumed that cyclic AMP-dependent protein kinases are involved in many of the responses because most tissues contain such enzymes⁵. The regulation of glycogenolysis is the best known example supporting this assumption^6–7. But it cannot be taken as fact that in eukaryotes all cyclic AMP-mediated processes are post-translational in nature.