EFFECTS OF MELANOCYTE-STIMULATING HORMONE ON TYROSINASE EXPRESSION AND MELANIN SYNTHESIS IN HAIR FOLLICULAR MELANOCYTES OF THE MOUSE

Tyrosinase mRNA, synthesis and activity were measured in the skin duri ng the first 2 weeks of life Of C3H-HeA(vy) mice. Tyrosinase mRNA leve ls were found to peak on days 3-4 and were followed by increases in ty rosinase synthesis and activity which peaked on days 6-7 and 7-8 respe ctively. These changes in tyrosinase expression were presumably associ ated with the growth of the first coat of hair that in neonatal C3H-He A(vy) mice is yellow in colour as a result of the increased proportion of phaeomelanin. By the time hair growth had ceased there was no expr ession of tyrosinase at both mRNA and protein levels. Daily administra tion of alpha-melanocyte stimulating hormone (alpha-MSH) enhanced the expression of tyrosinase mRNA transcripts, tyrosinase synthesis and ac tivity. The increase in tyrosinase activity paralleled the change in t he amount of tyrosinase, suggesting that the primary action of alpha-M SH is to stimulate new synthesis of the enzyme. This induction of tyro sinase was associated with the growth of hair that was darker in colou r than that of the controls and contained an increased proportion of e umelanin. This increase in eumelanin reflected a decrease in phaeomela nin content. It was concluded that, through its actions on the enzyme tyrosinase, alpha-MSH is able to switch the synthesis of phaeomelanin to that of eumelanin in hair follicular melanocytes.