The B₆-vitamer Pyridoxal is a Sensitizer of UVA-induced Genotoxic Stress in Human Primary Keratinocytes and Reconstructed Epidermis

UVA-driven photooxidative stress in human skin may originate from excitation of specific endogenous chromophores acting as photosensitizers. Previously, we have demonstrated that 3-hydroxypyridine-derived chromophores including B₆-vitamers (pyridoxine, pyridoxamine and pyridoxal) are endogenous photosensitizers that enhance UVA-induced photooxidative stress in human skin cells. Here, we report that the B₆-vitamer pyridoxal is a sensitizer of genotoxic stress in human adult primary keratinocytes (HEKa) and reconstructed epidermis. Comparative array analysis indicated that exposure to the combined action of pyridoxal and UVA caused upregulation of heat shock (HSPA6, HSPA1A, HSPA1L, HSPA2), redox (GSTM3, EGR1, MT2A, HMOX1, SOD1) and genotoxic (GADD45A, DDIT3, CDKN1A) stress response gene expression. Together with potentiation of UVA-induced photooxidative stress and glutathione depletion, induction of HEKa cell death occurred only in response to the combined action of pyridoxal and UVA. In addition to activational phosphorylation indicative of genotoxic stress [p53 (Ser15) and γ-H2AX (Ser139)], comet analysis indicated the formation of Fpg-sensitive oxidative DNA lesions, observable only after combined exposure to pyridoxal and UVA. In human reconstructed epidermis, pyridoxal preincubation followed by UVA exposure caused genomic oxidative base damage, procaspase 3 cleavage and TUNEL positivity, consistent with UVA-driven photooxidative damage that may be relevant to human skin exposed to high concentrations of B₆-vitamers.