Melatonin and its metabolites accumulate in the human epidermis in vivo and inhibit proliferation and tyrosinase activity in epidermal melanocytes in vitro

• Melatonin and its metabolites accumulate in the human epidermis.
• Epidermal production of melatonin/metabolites depends on gender, age and race.
• Melatonin and its metabolites inhibit proliferation of melanoma.
• Melatonin and its metabolites inhibit proliferation and tyrosinase in melanocytes.

Melatonin and its metabolites including 6-hydroxymelatonin (6(OH)M), N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and 5-methoxytryptamine (5MT) are endogenously produced in human epidermis. This production depends on race, gender and age. The highest melatonin levels are in African-Americans. In each racial group they are highest in young African-Americans [30–50 years old (yo)], old Caucasians (60–90 yo) and Caucasian females. AFMK levels are the highest in African-Americans, while 6(OH)M and 5MT levels are similar in all groups. Testing of their phenotypic effects in normal human melanocytes show that melatonin and its metabolites (10−5 M) inhibit tyrosinase activity and cell growth, and inhibit DNA synthesis in a dose dependent manner with 10−9 M being the lowest effective concentration. In melanoma cells, they inhibited cell growth but had no effect on melanogenesis, except for 5MT which enhanced L-tyrosine induced melanogenesis. In conclusion, melatonin and its metabolites [6(OH)M, AFMK and 5MT] are produced endogenously in human epidermis and can affect melanocyte and melanoma behavior.

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