9-Hydroxyoctadecadienoic acid plays a crucial role in human skin photoaging

Human skin is regularly exposed to ultraviolet (UV) rays from sunlight, leading to photoaging, which differs from intrinsic aging. Although the acute effects of UV exposure have been extensively studied, limited research has addressed the long-term consequences of chronic UV exposure. This study aimed to investigate the underlying causes of chronic photoaging. A questionnaire-based assessment of sunlight exposure was conducted among volunteers in their 20s and 50s, and the stratum corneum of their skin was analyzed for bioactive lipid content. Volunteers were categorized into low and high UV exposure groups based on the questionnaire scores. The analysis results revealed a significant increase in 9-hydroxyoctadecadienoic acid (9-HODE) levels in the skin of individuals in their 50s with high UV exposure. However, UV exposure did not affect 9-HODE levels in the skin of individuals in their 20s. In vitro experiments further indicated that 9-HODE contributes to chronic inflammation, pigmentary changes, and extracellular matrix alterations during photoaging. Specifically, 9-HODE stimulated cytokine production [interleukin-6 (IL6), IL8, and granulocyte-macrophage colony-stimulating factor (GM-CSF)] and reduced dickkopf-1 (DKK1) production in keratinocytes. In fibroblasts, 9-HODE stimulated matrix metalloproteinase-1 (MMP1) and MMP3 production while reducing collagen I (COL1) production. The expression of G2A, the receptor for 9-HODE, was also confirmed in fibroblasts, suggesting that 9-HODE exerts its effects via G2A, as observed in keratinocytes. Overall, these findings indicate that 9-HODE is a mediator of chronic photoaging and highlight its potential significance in photoaging prevention.