Stimulation of the receptor for melanocyte‐stimulating hormone by retinoic acid

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Abstract
Treatment of Cloudman S91 melanoma cells with retinoic acid (RA) inhibits MSH‐induced tyrosinase activity and melanin formation [(1990) J. Invest. Dermatol. 94, 461‐464]. We report here, however, that in spite of inhibiting MSH‐induced pigmentation, RA treatment caused a marked increase in MSH binding capacity for both cell surface and internal MSH binding sites. The stimulation was dose‐ and time‐dependent and reversible, with half‐maximal effects seen at 2 μM RA. Stimulation of MSH binding was seen as early as 3 h after exposure of cells to RA. Cell surface and internal binding activity increased in concert. Scatchard analyses indicated that increased MSH binding resulted from a 3–4‐fold increase in the number of sites with no significant difference in their affinity for MSH. It appears that in suppressing MSH‐induced melanogenesis, RA elicited a compensatory up‐regulation of the MSH receptor system.