Protective Role of Mitochondrial Peroxiredoxin III against UVB-Induced Apoptosis of Epidermal Keratinocytes

UVB light induces generation of reactive oxygen species, ultimately leading to skin cell damage. Mitochondria are a major source of reactive oxygen species in UVB-irradiated skin cells, with increased levels of mitochondrial reactive oxygen species having been implicated in keratinocyte apoptosis. Peroxiredoxin III (PrxIII) is the most abundant and potent H₂O₂-removing enzyme in the mitochondria of most cell types. Here, the protective role of PrxIII against UVB-induced apoptosis of epidermal keratinocytes was investigated. Mitochondrial H₂O₂ levels were differentiated from other types of ROS using mitochondria-specific fluorescent H₂O₂ indicators. Upon UVB irradiation, PrxIII-knockdown HaCaT human keratinocytes and PrxIII-deficient (PrxIII^–/–) mouse primary keratinocytes exhibited enhanced accumulation of mitochondrial H₂O₂ compared with PrxIII-expressing controls. Keratinocytes lacking PrxIII were subsequently sensitized to apoptosis through mitochondrial membrane potential loss, cardiolipin oxidation, cytochrome c release, and caspase activation. Increased UVB-induced epidermal tissue damage in PrxIII^–/– mice was attributable to increased caspase-dependent keratinocyte apoptosis. Our findings show that mitochondrial H₂O₂ is a key mediator in UVB-induced apoptosis of keratinocytes and that PrxIII plays a critical role in protecting epidermal keratinocytes against UVB-induced apoptosis through eliminating mitochondrial H₂O₂. These findings support the concept that reinforcing mitochondrial PrxIII defenses may help prevent UVB-induced skin damage such as inflammation, sunburn, and photoaging.