Melatonin enhances mitochondrial ATP synthesis, reduces reactive oxygen species formation, and mediates translocation of the nuclear erythroid 2‐related factor 2 resulting in activation of phase‐2 antioxidant enzymes ( γ ‐ GCS , HO ‐1, NQO 1) in ultraviolet radiation‐treated normal human epidermal keratinocytes (NHEK)

Melatonin is an ubiquitous molecule with a variety of functions including potent antioxidative properties. Due to its lipophilic character, it easily crosses cellular and intracellular membranes and reaches all subcellular organelles. Because of its ability to scavenge free radicals, melatonin protects against oxidative stress, for example, induced by ultraviolet radiation ( UVR ). Here, we investigated, in a dose‐dependent (0, 10, 25, and 50 mJ/cm 2 ) and time‐dependent (0, 4, 24, 48 hr post‐ UVR ) manner, whether melatonin prevents the UVR ‐mediated alterations in ATP synthesis and the generation of reactive oxygen species ( ROS ) in normal human epidermal keratinocytes ( NHEK ). Additionally, we evaluated the molecular mechanism of action of melatonin with regard to activation of phase‐2 antioxidative enzymes via nuclear erythroid 2‐related factor (Nrf2). We found that (i) melatonin counteracted UVR ‐induced alterations in the ATP synthesis and reduced free radical formation; (ii) melatonin induced the translocation of Nrf2 transcription factor from the cytosol into the nucleus resulting in, (iii) melatonin enhanced gene expression of phase‐2 antioxidative enzymes including γ ‐glutamylcysteine synthetase ( γ ‐ GCS ), heme oxygenase‐1 ( HO ‐1), and NADPH : quinone dehydrogenase‐1 ( NQO 1) representing an elevated antioxidative response of keratinocytes. These results suggest that melatonin not only directly scavenges ROS , but also significantly induces the activation of phase‐2 antioxidative enzymes via the Nrf2 pathway uncovering a new action mechanism that supports the ability of keratinocytes to protect themselves from UVR ‐mediated oxidative stress.