Thymoquinone, extract from Nigella sativa seeds, protects human skin keratinocytes against UVA-irradiated oxidative stress, inflammation and mitochondrial dysfunction

Ultraviolet A (UVA) irradiation caused skin keratinocytes to accumulate reactive oxygen species (ROS) leading to the skin injury. Thymoquinone (TQ) was identified as the prominent bioactive ingredient in Nigella sativa seeds which was applied in therapying various human diseases. This study aimed to illustrate the role and mechanism of TQ in UVA-induced skin injury. We pre-treated HaCaT cells with TQ and irradiated them by UVA. MTT and Elisa assays were used to evaluate cell viability and apoptosis, as well as cytokine levels. To detect the related parameters of oxidative stress and mitochondrial function, colorimetry, spectrophotometry, bioluminescence, and dual-luciferase reporter methods were used. RT-qPCR and western blotting were performed for expressions of related mRNAs and proteins. TQ significantly improved the UVA-induced cytotoxicity on HaCaT cells. TQ treatment alleviated the oxidative stress and inflammation in UVA-irradiated keratinocytes. Besides, UVA irradiation promoted mitochondrial dysregulation in HaCaT cells leading to cell apoptosis, which could be reversed by TQ treatment. More importantly, NrF2/ARE pathway was activated in TQ-treated cells, while COX-2 was depressed, and inhibiting the pathway or activating COX-2 blocked the therapeutic effect of TQ on UVA-induced skin cell injury. Our study suggested that TQ treatment attenuated the UVA-induced oxidative and inflammatory responses, as well as mitochondrial apoptosis in keratinocytes by COX-2 inhibition via activating NrF2/ARE pathway. This might be a novel sight for preventing the solar radiation damage to the skin.