Phototoxicity of melatonin

Constitutive and UV‐induced metabolism of melatonin in keratinocytes and cell‐free systems

Melatonin, which can be produced in the skin, exerts a protective effect against damage induced by UV radiation (UVR). We have investigated the effect of UVB, the most damaging component of UVR, on melatonin metabolism in HaCaT keratinocytes and in a cell-free system. Four metabolites were identified by HPLC and LC-MS: 6-hydroxymelatonin, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), 2-hydroxymelatonin (the main intermediate between melatonin and AFMK), and 4-hydroxymelatonin. Concentrations of these photoproducts were directly proportional to UVR-dose and to melatonin substrate content, and their accumulation was time-dependent. The UVR-dependent increase of AFMK and 2-hydroxymelatonin was also detected in keratinocytes, where it was accompanied by simultaneous consumption of intracellular melatonin. Of note, melatonin and its two major metabolites, 2-hydroxymelatonin and AFMK, were also detected in untreated keratinocytes, neither irradiated nor preincubated with melatonin. Thus, intracellular melatonin metabolism is enhanced under exposure to UVR. The additional biological activity of these individual melatonin metabolites increases the spectrum of potential actions of the recently identified cutaneous melatoninergic system.

Interaction of melatonin with model membranes and possible implications in its photoprotective activity

It is well known that administration of antioxidants represents a successful strategy for preventing the occurrence and for reducing the severity of UV-mediated oxidative damage. Melatonin was recently shown to be an efficacious photoprotective agent. The aim of the present study was to better investigate the interaction of melatonin with model membranes and the possible implications in its photoprotective activity. The antioxidant activity of melatonin was tested in two 'in vitro' experimental models: (a) UV radiation-induced peroxidation in phosphatidylcholine multilamellar vesicles (MLVs); (b) scavenging activity against nitric oxide (NO). Furthermore, we investigated the melatonin/biomembrane interaction by differential scanning calorimetry (DSC) on dimyristoylphosphatidylcholine (DMPC) MLVs and unilamellar vesicles (LUVs). The findings of in vitro antioxidant tests suggest that the photoprotective effect of melatonin should be due, partially at least, to the drug scavenging activity against aqueous and lipophilic free radicals, including NO; besides, melatonin might provide its protective effect against UV radiation-induced damage also by acting as a UV-absorbing screen. The results of DSC experiments have evidenced a good capability of melatonin to interact with DMPC bilayers, causing a significant fluidifying effect; however, the transfer of melatonin in the LUVs is faster than that observed for MLVs, even if both values tend to the maximum values reachable. Our present data allow us to emphasize two points: (1) the fluidifying effect induced by melatonin on lipidic bilayers might act as a cooperative mechanism in its protective effect against peroxidative membrane damage; (2) melatonin appears able to cross biomembranes, so that it could protect intracellular components against peroxidative insult.