A Novel Butylated Caffeic Acid Derivative Protects HaCaT Keratinocytes from Squalene Peroxidation-Induced Stress

Squalene is a major sebum lipid which is easily peroxidized by ultraviolet A (UVA) irradiation, generating products that can stress keratinocytes. Prevention of squalene photooxidation with antioxidants could potentially help defend skin from environmental stress. Previously, we have systematically characterized butylated caffeic acid (BCA) as a more lipophilic alternative of caffeic acid (CA). The current study aimed to test the hypothesis that the lipophilic property of BCA makes it a good candidate as a skin antioxidant. The UVA-induced peroxidation of squalene and the antioxidative activity of CA and BCA were measured for peroxide value and further characterized with ultraperformance liquid chromatography coupled with ultraviolet and mass spectrometry analysis. Both CA and BCA showed strong antioxidant capacity during squalene peroxidation under direct UVA challenge. In HaCaT keratinocyte culture, BCA's and CA's impact on the damage induced by peroxidized squalene (P-SQ) was investigated through quantification of reactive oxygen species (ROS) generation and expression of interleukin-1 (IL-1) cytokines. Both BCA and CA decreased P-SQ-induced IL-1β secretion in HaCaT cells. However, only BCA could reduce P-SQ-induced ROS generation as well as expression of inflammatory cytokines in the IL-1 family. This advantage in biological efficacy could have been attributed to BCA's superior intracellular bioavailability due to its higher lipophilicity compared with CA, as indicated by higher intracellular concentration of BCA. Based on this observation, we propose using BCA as a functional antioxidant to prevent sebum lipid from peroxidation and its detrimental effect to skin.