Melatonin reduces X-ray irradiation-induced oxidative damages in cultured human skin fibroblasts

Melatonin is a hormone with multiple functions in humans, produced by the p ineal gland and stimulated by beta-adrenergic receptors. Melatonin has been shown to have radioprotection properties, but there has been little progre ss toward identifying the specific mechanisms of its action. To clarify the role of melatonin as a radioprotective compound. in response to X-ray irra diation, we investigated the effects of X-ray irradiation and melatonin on cytotoxicity, lipid peroxidation and alteration of the cell cycle in cultur ed skin fibroblast. An 8 Gy dose of X radiation resulted in cell death in 6 3% of irradiated cells, i.e. the cell viability was 37%. The damage was ass ociated with lipid peroxidation of the cell membrane, as shown by the accum ulation of malondialdehyde (MDA). By pre-incubation with melatonin (10(-5) M), a significant preventive effect was noted on the increase in the absolu te number of surviving cells (up to 68% of cells were survived), and the le vels of MDA were markedly decreased. These findings suggest a close correla tion between an increase of lipid peroxidation and a rate of cell death. Mo rphological changes associated with apoptotic cell death were demonstrated by TEM. DNA flow-cytometry analysis revealed that X radiation increased pre -G1 apoptotic population by 7.6% compared to a very low level (1.3%) of non -irradiated cells. However, in the presence of melatonin, this apoptotic po pulation decreased up to 4.5% at 10(-5) M. The p53 and p21 protein levels o f skin fibroblasts increased 4 h after 8 Gy irradiation, but melatonin pret reatment did not change those levels, This study suggests that melatonin pr etreatment inhibits radiation-induced apoptosis, and melatonin exerts its r adioprotective effect by inhibition of lipid peroxidation and without any i nvolvement of the p53/p21 pathway. (C) 2001 Elsevier Science Ireland Ltd. A ll rights reserved.