Light-driven electron and energy transfer involving non-
DNA skin chromophores as endogenous
photosensitizers induces oxidative stress in UVA-exposed human skin, a process relevant to photoaging and photocarcinogenesis.
Malondialdehyde is an electrophilic dicarbonyl-species derived from
membrane lipid peroxidation. Here, we present experimental evidence suggesting that the
malondialdehyde-derived
protein epitope dihydropyridine (DHP)-
lysine is a potent endogenous UVA-
photosensitizer of human skin cells. Immunohistochemical analysis revealed the abundant occurrence of
malondialdehyde-derived and DHP-
lysine epitopes in human skin. Using the chemically protected
dihydropyridine-derivative (2S)-Boc-2-amino-6-(3,5-diformyl-4-methyl-4H-pyridin-1-yl)-hexanoic
acid-t-butylester as a model of
peptide-bound DHP-
lysine, photodynamic inhibition of proliferation and induction of cell death were observed in human skin Hs27 fibroblasts as well as primary and HaCaT keratinocytes exposed to the combined action of UVA and DHP-
lysine. DHP-
lysine photosensitization induced intracellular oxidative stress, p38 MAPkinase activation, and upregulation of
heme oxygenase-1 expression. Consistent with UVA-driven ROS formation from DHP-
lysine, formation of
superoxide,
hydrogen peroxide, and
singlet oxygen was detected in chemical assays, but little protection was achieved using SOD or
catalase during cellular
photosensitization. In contrast, inclusion of NaN(3) completely abolished DHP-
photosensitization. Taken together, these data demonstrate photodynamic activity of DHP-
lysine and support the hypothesis that
malondialdehyde-derived
protein-
epitopes may function as endogenous sensitizers of UVA-induced oxidative stress in human skin.