MELATONIN ANTAGONIZES ALPHA-MELANOCYTE-STIMULATING HORMONE ENHANCEMENT OF MELANOGENESIS IN MOUSE MELANOMA-CELLS BY BLOCKING THE HORMONE-INDUCED ACCUMULATION OF THE C-LOCUS TYROSINASE

Melatonin was found to have a small inhibitory effect on tyrosinase ac tivity and a slight stimulatory action on dopachrome tautomerase activ ity in B16 mouse melanoma eels. These effects were time and dose depen dent, with the maximal response being observed after 24-48 h treatment and at concentrations of melatonin higher than the physiologic levels of the circulating hormone. Although these effects on the melanogenic activities were modest, incubation of melanocytes with melatonin prio r to the addition of the melanotropin mediated a dramatic inhibition o f melanocyte-stimulating-hormone-(alpha-MSH)-induced melanogenesis. Th is inhibitory effect was evident at melatonin concentrations as low as 10 nM. Inhibition was nearly total at 0.1 mM melatonin, even at high concentrations of alpha-MSH (1 mu M). The inhibitory effect of melaton in on alpha-MSH stimulation of melanogenesis was investigated. Melaton in appeared to act at least at two stages. Pharmacological concentrati ons of melatonin diminished the number of alpha-MSH receptors to about 75% of the control values without an apparent effect on receptor affi nity, as determined by receptor-binding studies using I-125-[N-Leu4-D- Phe7]alpha-MSH as a probe. Physiological concentrations of melatonin a lso appeared to interfere with the intracellular events coupling incre ased cAMP levels and induction of the c locus tyrosinase, since it str ongly inhibited the theophylline-mediated stimulation of melanogenesis . The inhibiton of tyrosinase stimulation was higher in the microsomal than in the melanosomal fractions of cells which were treated with me latonin, then exposed to either alpha-MSH (1 mu M) or theophylline (1 mM), suggesting that one of the main effects of melatonin might be inh ibition of the induction of tyrosinase de novo synthesis.