Keratinocytes exposed to Ozone induce a bystander oxinflammatory response in dermal fibroblasts

While ozone (O₃) is widely recognized for its respiratory effects and its detrimental impact on skin health has been well demonstrated, the mechanisms underlying dermal damage remain incompletely understood. This study investigates the biological effects of O₃ on human skin using both clinical and in vitro approaches, focusing on the interactions between O₃-exposed keratinocytes and unexposed fibroblasts, to investigate a potential bystander effect. Fifteen healthy volunteers underwent repeated forearm exposure to O₃, and skin biopsies were analyzed for extracellular matrix components and oxidative stress markers. In parallel, keratinocytes were exposed to O₃ in vitro, and their conditioned medium was applied to dermal fibroblasts to evaluate viability, oxidative stress, DNA damage, aryl hydrocarbon receptor (AhR) activation, and matrix metalloproteinase-9 (MMP-9) activity. After O₃ exposure, human skin biopsies revealed decreased expression of key extracellular matrix (ECM) components, including tropoelastin, elastin and collagen III, as well as increased levels of 8-OHdG and AhR. Mechanistic in vitro investigations revealed that conditioned medium from O₃-treated keratinocytes reduced fibroblast viability, and triggered oxidative stress, mitochondrial ROS production, lipid peroxidation, and DNA damage in unexposed dermal fibroblasts. These changes were accompanied by AhR activation and increased MMP-9 activity, leading to ECM protein degradation. Collectively, these findings identify a previously underappreciated paracrine pathway through which O₃-induced oxidative mediators propagate damage from the epidermis to the dermis. This mechanism provides new insight into how environmental O₃ contributes to skin aging and structural deterioration, highlighting the broader cutaneous health implications of air pollution exposure.