Limonene protects human skin keratinocytes against UVB‐induced photodamage and photoaging by activating the Nrf2‐dependent antioxidant defense system

Abstract Long term exposure to solar ultraviolet B (UVB) radiation is one of the primary factors of premature skin aging and is referred to as photoaging. Also, mammalian skin exposed to UVB triggers an increase in production of α‐melanocyte‐stimulating hormone (α‐MSH), which is critically involved in the pathogenesis of hyperpigmentary skin diseases. This study investigated the protective effect of limonene on UVB‐induced photodamage and photoaging in immortalized human skin keratinocytes (HaCaT) in vitro. Initially, we determined cell viability and levels of reactive oxygen species (ROS) in UVB‐irradiated HaCaT cells. Pretreatment with limonene increased cell viability followed by inhibition of intracellular ROS generation in UVB‐irradiated HaCaT cells. Interestingly, the antioxidative activity of limonene was directly correlated with an increase in expression of endogenous antioxidants, including heme oxygenase 1 (HO‐1), NAD(P)H:quinone oxidoreductase 1 (NQO‐1), and γ‐glutamylcysteine synthetase (γ‐GCLC), which was associated with enhanced nuclear translocation and activation of NF‐E2‐related factor‐2 (Nrf2). Indeed, Nrf2 knockdown reduced limonene's protective effects. Additionally, we observed that limonene treatment inhibited UVB‐induced α‐MSH secretion followed by inhibition of proopiomelanocortin (POMC) via suppression of p53 transcriptional activation. Moreover, limonene prevented UVB‐mediated depletion of tight junction regulatory proteins, including occludin and zonula occludens‐1. On the other hand, limonene treatment significantly decreased matrix metalloproteinase‐2 levels in UVB‐irradiated HaCaT cells. Based on these results, limonene may have a dermato‐protective effect in skin cells by activating the Nrf2‐dependent cellular antioxidant defense system.