ISOLATION OF A UNIQUE MELANOGENIC INHIBITOR FROM HUMAN SKIN XENOGRAFTS - INITIAL IN-VITRO AND IN-VIVO CHARACTERIZATION

Previously, split-thickness human skin grafted onto athymic mice has b een shown to become markedly hyperpigmented, but the factor(s) respons ible for this hyperpigmentation had not been isolated, The present stu dy describes the isolation and characterization of a potent melanogeni c inhibitor from grafted human skin. Extracts from grafted skin inhibi ted, in a concentration-dependent manner, tyrosinase activity of norma l human melanocytes and of Cloudman S91 murine melanoma in culture. So dium dodecylsulfate-polyacrylamide gel electrophoresis analysis of ext racts from pre- and post-grafted skin demonstrated the presence of a p rotein doublet of approximately 14 kD exclusively in the post-grafted skin. This protein inhibited both tyrosinase activity and cellular pro liferation in a concentration-dependent manner. The inhibition of tyro sinase activity in normal human melanocytes was 53% at 0.5 mu g/ml con centration, whereas this inhibition was almost complete in murine mela noma cultures at 1.0 mu g/ml. The protein did not inhibit either cellu lar proliferation or protein synthesis in normal human fibroblast cult ures, and therefore may act specifically on melanocytes. Injections of the inhibitor corresponded with a delay and reduction in the quantity of pigment in human skin 2 weeks after grafting. Multiple injections of the inhibitor into the hyperpigmented xenografts (20 weeks after gr afting) reversed the hyperpigmentation with no observable inflammatory or toxic responses. The results indicate that hyperpigmented human sk in xenografts contain a potent inhibitor of melanogenesis and melanocy te proliferation.