Licochalcone A Activates Nrf2 in Vitro and Contributes to Licorice Extract-induced Lowered Cutaneous Oxidative Stress in Vivo

The retrochalcone licochalcone A (LicA) has previously been shown to possess antimicrobial and anti-inflammatory properties. In this study, we focused on pathways responsible for the antioxidative properties of LicA. In vitro, LicA protected from oxidative stress mediated by reactive oxygen species (ROS) by activating the expression of cytoprotective phase II enzymes. LicA induced nuclear translocation of NF-E2-related factor 2 (Nrf2) in primary human fibroblasts and elevated the expression of the cytoprotective and anti-inflammatory enzymes heme oxygenase 1 and glutamate-cysteine ligase modifier subunit. LicA-treated cells displayed a higher ratio of reduced to oxidized glutathione and decreased concentrations of ROS in UVA-irradiated human dermal fibroblasts, as well as in activated neutrophils. In vivo, ultraweak photon emission analysis of skin treated with LicA-rich licorice extract revealed a significantly lowered UVA-induced luminescence, indicative for a decrease in oxidative processes. We conclude from these data that topical application of licorice extract is a promising approach to induce Nrf2-dependent cytoprotection in human skin.