Melatonin and its metabolites ameliorate ultraviolet B-induced damage in human epidermal keratinocytes

We investigated the protective effects of melatonin and its metabolites: 6-hydroxymelatonin (6-OHM), N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), N-acetylserotonin (NAS), and 5-methoxytryptamine (5-MT) in human keratinocytes against a range of doses (25, 50, and 75 mJ/cm²) of ultraviolet B (UVB) radiation. There was significant reduction in the generation of reactive oxygen species (50-60%) when UVB-exposed keratinocytes were treated with melatonin or its derivatives. Similarly, melatonin and its metabolites reduced the nitrite and hydrogen peroxide levels that were induced by UVB as early as 30 min after the exposure. Moreover, melatonin and its metabolites enhanced levels of reduced glutathione in keratinocytes within 1 hr after UVB exposure in comparison with control cells. Using proliferation assay, we observed a dose-dependent increase in viability of UVB-irradiated keratinocytes that were treated with melatonin or its derivatives after 48 hr. Using the dot-blot technique and immunofluorescent staining we also observed that melatonin and its metabolites enhanced the DNA repair capacity of UVB-induced pyrimidine photoproducts (6-4)or cyclobutane pyrimidine dimers generation in human keratinocytes. Additional evidence for induction of DNA repair in cells exposed to UVB and treated with the indole compounds was shown using the Comet assay. Finally, melatonin and its metabolites further enhanced expression of p53 phosphorylated at Ser-15 but not at Ser-46 or its nonphosphorylated form. In conclusion, melatonin, its precursor NAS, and its metabolites 6-OHM, AFMK, 5-MT, which are endogenously produced in keratinocytes, protect these cells against UVB-induced oxidative stress and DNA damage.