Regulation of adenylate cyclase by beta-melanotropin in the M2R melanoma cell line.

We have examined adenylate cyclase (AC) in the M2R melanoma cell line, a novel clone of transplantable B16 melanoma cells. It has been found that activity of this enzyme is highly responsive to beta-melanotropin (beta-MSH) and other hormones possessing melanotropic activity (e.g., alpha-melanotropin (alpha-MSH) and adrenocorticotrophic hormone (ACTH1-24)). beta-MSH stimulation of adenylate cyclase, both in the intact cell and in a plasma membrane-enriched fraction derived thereof, was shown to be saturable and dose-dependent. In addition, prostaglandin E1 (PGE1) was found to be a potent stimulator of AC activity in these cells. Hormone stimulation of enzyme activity in the intact cell was strongly potentiated by forskolin which not only enhanced maximal AC activity 3-fold, but lowered by 40-fold the concentration of beta-MSH required for half-maximal stimulation. Using biologically active [125I]iodo-beta-MSH prepared in our laboratory we have examined the specificity of beta-MSH binding to its receptor in both intact M2R cells and plasma membranes derived thereof. Among a series of hormones tested only alpha-MSH and ACTH1-24 competed with [125I]iodo-beta-MSH for binding to the melanotropin receptor in accordance with the results obtained with AC. In contrast to the strong effect on cyclic 3',5'-adenosine monophosphate (cAMP) accumulation in M2R cells forskolin has no effect on [125I]iodo-beta-MSH binding. It appears that the kinetic properties of beta-MSH binding and beta-MSH stimulation of adenylate cyclase activity are essentially identical, the half-maximal effects of which are demonstrated at approximately 20 nM beta-MSH.