Melatonin protects keratinocytes from diesel-derived PM2.5-induced damage by suppressing JNK/p38 MAPK signaling and caspase activation.

Melatonin is a naturally produced hormone in the human body that primarily regulates the sleep-wake cycle. This study examined the role of melatonin in protecting human keratinocytes from diesel-derived particulate matter 2.5 (DPM2.5)-induced oxidative injury. HaCaT keratinocytes and male HR-1 hairless mice were exposed to DPM2.5 to induce oxidative injury and treated with various concentrations of melatonin, followed by various analyses, including reactive oxygen species (ROS) detection, mitochondrial membrane potential analysis, western blotting, and cell cycle analysis. Melatonin exhibited potent radical-scavenging activity. Consistently, melatonin attenuated DPM2.5-induced intracellular ROS, mitigated oxidative damage to cellular macromolecules, and restored cell viability. Furthermore, melatonin reduced DPM2.5-induced apoptotic cell death by alleviating mitochondrial dysfunction, including mitochondrial membrane potential disruption. It also partially restored the decreased Bcl-2 expression and increased Bax and Bim expression in DPM2.5-treated cells, suppressing DPM2.5-induced activation of caspase-9 and caspase-3. Given that ROS overproduction is a key activator of c-Jun N-terminal kinase (JNK)/p38 mitogen-activated protein kinase (MAPK) signaling, we determined whether melatonin-mediated antioxidant effects are linked to suppression of these pathways. Melatonin reduced DPM2.5-mediated JNK and p38 MAPK phosphorylation, and protective effects were observed in DPM2.5-exposed mouse skin tissue. Particularly, co-treatment of melatonin with pan-caspase, JNK, or p38 MAPK inhibitors did not significantly alter the anti-apoptotic effect of melatonin in DPM2.5-treated cells. In conclusion, melatonin protects human keratinocytes from DPM2.5-induced oxidative stress damage by scavenging ROS, thereby mitigating damage to cellular components. It also reduces damage to skin keratinocytes from DPM2.5-induced apoptosis by inhibiting caspase- and/or MAPK-dependent signaling pathways. Thus, melatonin has potential as an active ingredient for developing skin-care products to prevent the harmful effects of DPM2.5.