Prolonged Stimulation of S91 Melanoma Tyrosinase by [Nle⁴, D-Phe⁷]-substituted α-Melanotropins

a-Melanotropin (a-melanocyte stimulating hormone, a-MSH) stimulates tyrosinase activity in Cloudman S91 murine melanoma cells. Three [Nle⁴, D-Phe⁷]-substituted a-melanotropin analogues, [Nle⁴, D-Phe⁷]-a-MSH, Ac-[Nle⁴, D-Phe⁷]-a-MSH4-n-NH₂, and Ac-[Nle⁴, D-Phe⁷]-a-MSH4-io-NH₂, are at least 100-fold more effective than a-MSH in stimulating melanoma tyrosinase, the rate-limiting enzyme in melanin biosynthesis. These [Nle⁴, D-Phe⁷]-substituted melanotropin analogues induce tyrosinase activity in melanoma cells with shorter contact times than required by the native hormone, a-MSH. [Nle⁴, D-Phe⁷]-substituted me-lanotropins also induce a prolonged (residual) stimulation of melanoma tyrosinase. Following incubation of melanoma cells in the presence of [Nle⁴, D-Phe⁷]-a-MSH for 24 h, tyrosinase activity is maintained for up to 6 days in the absence of the melanotropin. The shorter 4-10 and 4-11 fragment analogues also exhibit residual melanotropic activity. The prolonged stimulation of tyrosinase in the absence of the analogues is maintained even though melanoma cells continue to divide about every 24 h. These results suggest that melanoma cells possess spare melanotropin receptors and that [Nle⁴, D-Phe⁷]-substituted analogues bind almost irreversibly to these receptors or to some other component of the adenylate cyclase enzyme complex responsible for enhancing tyrosinase activity and melanin production.