Melatonin increases survival of HaCaT keratinocytes by suppressing UV-induced apoptosis

: Melatonin is a potent antioxidant and direct radical scavenger. As keratinocytes represent the major population in the skin and UV light causes damage to these cells, the possible protective effects of melatonin against UV‐induced cell damage in HaCaT keratinocytes were investigated in vitro. Cells were preincubated with melatonin at graded concentrations from 10−9 to 10−3 m for 30 min prior to UV irradiation at doses of 25 and 50 mJ/cm2. Biological markers of cellular viability such as DNA synthesis and colony‐forming efficiency as well as molecular markers of apoptosis were measured. DNA synthesis was determined by [3H]‐thymidine incorporation into insoluble cellular fraction, clonogenicity through plating efficiency experiments and apoptosis by the terminal deoxynucleotidyl transferase‐mediated dUTP nick‐end labeling (TUNEL) assay. DNA synthesis experiments showed a strong protective effect by preincubation with melatonin at concentrations of 10−4 m (P