Abstract
The human skin is not only a target for the protective actions of melatonin, but also a site of melatonin synthesis and metabolism, suggesting an important role for a local melatoninergic system in protection against ultraviolet radiation (UVR) induced damages. While melatonin exerts many effects on cell physiology and tissue homeostasis via membrane bound melatonin receptors, the strong protective effects of melatonin against the UVR-induced skin damage including DNA repair/protection seen at its high (pharmocological) concentrations indicate that these are mainly mediated through receptor-independent mechanisms or perhaps through activation of putative melatonin nuclear receptors. The destructive effects of the UVR are significantly counteracted or modulated by melatonin in the context of a complex intracutaneous melatoninergic anti-oxidative system with UVR-enhanced or UVR-independent melatonin metabolites. Therefore, endogenous intracutaneous melatonin production, together with topically-applied exogenous melatonin or metabolites would be expected to represent one of the most potent anti-oxidative defense systems against the UV-induced damage to the skin. In summary, we propose that melatonin can be exploited therapeutically as a protective agent or as a survival factor with anti-genotoxic properties or as a “guardian” of the genome and cellular integrity with clinical applications in UVR-induced pathology that includes carcinogenesis and skin aging.
Highlights
Overview of Melatonin SynthesisThe structure of melatonin (N-acetyl-5-methoxytrypamine) was defined over 50 years ago by Aaron
The human skin is a target for the protective actions of melatonin, and a site of melatonin synthesis and metabolism, suggesting an important role for a local melatoninergic system in protection against ultraviolet radiation (UVR) induced damages
arylalkylamine N-acetyltransferase (AANAT) is considered as the rate limiting step in melatonin synthesis [14], Liu and Borjigin have shown that rats with the H2BY mutation in the AANAT, which drastically reduces enzyme activities, have normal melatonin levels in their pineal gland [18]
Summary
The structure of melatonin (N-acetyl-5-methoxytrypamine) was defined over 50 years ago by Aaron. AANAT is considered as the rate limiting step in melatonin synthesis [14], Liu and Borjigin have shown that rats with the H2BY mutation in the AANAT, which drastically reduces enzyme activities, have normal melatonin levels in their pineal gland [18]. These findings are consistent with observations in mammalian skin showing that serotonin is acetylated in peripheral tissue by an alternative enzyme to AANAT, most likely arylamine N-acetyltransferase [9,10,19,20], including in the C57BL6 mouse [10,21]. The above information indicates that only HIOMT should be considered a rate limiting enzyme in melatonin synthesis [17]
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