7-Dehydrocholesterol enhances ultraviolet A-induced oxidative stress in keratinocytes: Roles of NADPH oxidase, mitochondria, and lipid rafts

Long wavelength solar UVA radiation stimulates formation of reactive oxygen species (ROS) and prostaglandin E 2 (PGE 2), which are involved in skin photosensitivity and tumor promotion. High levels of 7-dehydrocholesterol (7-DHC), the precursor to cholesterol, cause exaggerated photosensitivity to UVA in patients with Smith-Lemli-Opitz syndrome (SLOS). Partially replacing cholesterol with 7-DHC in keratinocytes rapidly (< 5 min) increased UVA-induced ROS, intracellular calcium, phospholipase A 2 activity, PGE 2, and NADPH oxidase activity. UVA-induced ROS and PGE 2 production were inhibited in these cells by depleting the Nox1 subunit of NADPH oxidase using siRNA or using a mitochondrial radical quencher, MitoQ. Partial replacement of cholesterol with 7-DHC also disrupted membrane lipid raft domains, although depletion of cholesterol, which also disrupts lipid rafts, did not affect UVA-induced increases in ROS and PGE 2. Phospholipid liposomes containing 7-DHC were more rapidly oxidized by a free radical mechanism than those containing cholesterol. These results indicate that 7-DHC enhances rapid UVA-induced ROS and PGE 2 formation by enhancing free radical-mediated membrane lipid oxidation and suggests that this mechanism might underlie the UVA photosensitivity in SLOS.