3-Hydroxypyridine Chromophores Are Endogenous Sensitizers of Photooxidative Stress in Human Skin Cells

Abstract

Photocarcinogenesis and photoaging are established consequences of chronic exposure of human skin to solar irradiation. Accumulating evidence supports a causative involvement of UVA irradiation in skin photodamage. UVA photodamage has been attributed to photosensitization by endogenous skin chromophores leading to the formation of reactive oxygen species and organic free radicals as key mediators of cellular photooxidative stress. In this study, 3-hydroxypyridine derivatives contained in human skin have been identified as a novel class of potential endogenous photosensitizers. A structure-activity relationship study of skin cell photosensitization by endogenous pyridinium derivatives (pyridinoline, desmosine, pyridoxine, pyridoxamine, pyridoxal, pyridoxal-5 -phosphate) and various synthetic hydroxypyridine isomers identified 3-hydroxypyridine and N-alkyl-3-hydroxypyridinium cation as minimum phototoxic chromophores sufficient to effect skin cell sensitization toward UVB and UVA, respectively. Photosensitization of cultured human skin keratinocytes (HaCaT) and fibroblasts (CF3) by endogenous and synthetic 3-hydroxypyridine derivatives led to a dose-dependent inhibition of proliferation, cell cycle arrest in G 2 /M, and induction of apoptosis, all of which were reversible by thiol antioxidant intervention. Enhancement of UVAinduced intracellular peroxide formation and p38 mitogen-activated protein kinase-dependent stress signaling suggest a photooxidative mechanism of skin cell photosensitization by 3-hydroxypyridine derivatives. 3-Hydroxypyridine derivatives were potent photosensitizers of macromolecular damage, effecting protein (RNase A) photocross-linking and peptide (melittin) photooxidation with incorporation of molecular oxygen. Based on these results, we conclude that 3-hydroxypyridine derivatives comprising a wide range of skin biomolecules, such as enzymatic collagen cross-links, B 6 vitamers, and probably advanced glycation end products in chronologically aged skin constitute a novel class of UVA photosensitizers, capable of skin photooxidative damage.Most of the solar UV energy incident on human skin is in the deeply penetrating UVA region (Ͼ95% from 320 to 400 nm).Increasing experimental evidence supports a causative involvement of UVA irradiation in photoaging and carcinogenesis of human skin by photooxidative mechanisms (1-5). In contrast to the formation of mutagenic pyrimidine-base photoproducts through direct absorption of UVB (290 -320 nm) radiation by skin cell DNA (6), UVA radiation results in little photoexcitation of DNA directly, and generation of reactive oxygen species (ROS) 1 and organic free radicals is a widely accepted mechanism of UVA-phototoxicity (reviewed in Ref. 7). The formation of ROS as mediators of photooxidative stress in UV-irradiated skin seems to be dependent on non-DNA chromophores acting as endogenous photosensitizers (2, 3,8). Photosensitization occurs as a consequence of initial formation of excited states of chromophores and their subsequent interaction with substrat...

Riboflavin activated by ultraviolet A1 irradiation induces oxidative DNA damage-mediated mutations inhibited by vitamin C

“…DNA footprinting mapped the formation of most induced lesions to guanine residues along the cII transgene. It is known that the lower ionization potential of guanine relative to other DNA bases makes it a favorable target for type-I photosensitized one-electron oxidation and/or type-II photooxidation, mediated through singlet oxygen generation (37,38). The resulting radical cations of guanine can undergo hydration to produce 8-oxo-dG and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (39), both lesions being substrates of the DNA repair protein, Fpg (40).…”

NQO1-activated phenothiazinium redox cyclers for the targeted bioreductive induction of cancer cell apoptosis

“…Viability and induction of cell death (early and late apoptosis/necrosis) were examined by annexin-V-FITC/propidium iodide (PI) dual staining of cells followed by flow cytometric analysis as published previously [32,33]. Cells (150,000) were seeded on 35 mm dishes and received drug treatment 24 hours later.…”

Cinnamoyl-based Nrf2-activators targeting human skin cell photo-oxidative stress

“…Following 5 min incubation in the dark after irradiation, the cells were washed with PBS, fresh culture medium was added, and cells were returned to the incubator. Induction of cell death (early and late apoptosis/necrosis) was then examined 24 h after exposure by annexin-V-FITC/propidium iodide (PI) dual staining followed by flow cytometric analysis using an apoptosis detection kit (APO-AF, Sigma, St. Louis, MO) as published previously [16,48].…”

“…Accumulative evidence suggests that UVA-induced photodamage and carcinogenesis of human skin occur by photo-oxidative mechanisms mediated by reactive oxygen species (ROS) [9][10][11][12][13][14]. Recent research indicates that light-driven redoxcycling of non-DNA skin chromophores (including porphyrins, riboflavin, and collagen crosslinks) acting as endogenous photosensitizers is a major source of ROS in UVA-exposed human skin [2,4,12,[14][15][16][17][18]47]. The molecular consequences downstream of light-driven ROS production on skin structural integrity, signal transduction, gene expression and ultimately tumorigenic initiation and progression are well established [6].…”