Gene regulation by ultraviolet A radiation and singlet oxygen

Cellular response to infrared radiation involves retrograde mitochondrial signaling

Infrared A radiation (IRA) is a major component of sunlight. Similar to ultraviolet (UV) B and UVA, IRA induces gene transcription. In contrast to the UV response very little is known about the IRA response. In the present study, IRA-induced expression of matrix metalloproteinase-1 (MMP-1) was found to be mediated by the formation of intracellular reactive oxygen species (ROS). Staining of IRA-irradiated cells with MitoSox revealed an increase in mitochondrial superoxide anion production and treatment of fibroblasts with the mitochondrial targeted antioxidant MitoQ completely abrogated the IRA, but not the UVB or UVA1, response. ROS relevant for IRA-induced signaling originated from the mt electron transport chain, because (i) chemical inhibition of the electron transport chain prevented IRA, but not UVB or UVA1, radiation-induced MMP-1 expression, (ii) rho0 fibroblasts specifically failed to increase MMP-1 expression in response to IRA, and (iii) peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) overexpressing fibroblasts with increased electron transport chain content were hypersensitive to IRA radiation-induced gene expression. Thus, IRA, in contrast to UV, elicits a retrograde signaling response in human skin.

Photooxidation generates biologically active phospholipids that induce heme oxygenase-1 in skin cells

Heme oxygenase-1 (HO-1) is a key enzyme in the cellular response to tissue injury and oxidative stress. HO-1 enzymatic activity results in the formation of the cytoprotective metabolites CO and biliverdin. In the skin, HO-1 is strongly induced after long wave ultraviolet radiation (UVA-1). Here we show that UVA-1 irradiation generates oxidized phospholipids derived from 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) that mediate the expression of HO-1 in skin cells. Using EO6 antibodies that recognize oxidized phospholipids, we show that UVA-1 irradiation of dermal fibroblasts generates oxidation-specific epitopes. Irradiation of arachidonate-containing phospholipids with UVA-1 led to formation of defined lipid oxidation products including epoxyisoprostane-phosphatidylcholine that induced HO-1 expression in dermal fibroblasts, in keratinocytes, and in a three-dimensional epidermal equivalent model. In addition, we demonstrate that the oxidation of PAPC by UVA-1 is a singlet oxygen-dependent mechanism. Together, we present a novel mechanism of UVA-1-induced HO-1 expression that is mediated by the generation of biologically active phospholipid oxidation products. Because UVA-1 irradiation is a mainstay treatment of several inflammatory skin diseases, structural identification of UVA-1-generated biomolecules with HO-1-inducing capacity should lead to the development of drugs that could substitute for irradiation.

Beta-carotene inhibits UVA-induced matrix metalloprotease 1 and 10 expression in keratinocytes by a singlet oxygen-dependent mechanism

UVA exposure causes skin photoaging by singlet oxygen (1)O(2)-mediated induction of, e.g., matrix metalloproteases (MMPs). We assessed whether pretreatment with beta-carotene, a (1)O(2) quencher and retinoic acid (RA) precursor, interferes with UVA-induced gene regulation. HaCaT keratinocytes were precultured with beta-carotene at physiological concentrations (0.5, 1.5, and 3.0 microM) prior to exposure to UVA from a Hönle solar simulator (270 kJ/m(2)). HaCaT cells accumulated beta-carotene in a time- and dose-dependent manner. UVA irradiation massively reduced the cellular beta-carotene content. Beta-carotene suppressed UVA-induction of MMP-1, MMP-3, and MMP-10, three major matrix metalloproteases involved in photoaging. We show that regulation by not only MMP-1, but also MMP-10, involves (1)O(2)-dependent mechanisms. Beta-carotene dose-dependently quenched (1)O(2)-mediated induction of MMP-1 and MMP-10. Thus, as in chemical solvent systems, beta-carotene quenches (1)O(2) also in living cells. Vitamin E did not cooperate with beta-carotene to further inhibit MMP induction. HaCaT cells produced weak retinoid activity from beta-carotene, as demonstrated by mild upregulation of RAR beta and activation of an RARE-dependent reporter gene. Beta-carotene did not regulate the genes encoding other RARs, RXRs, or the two beta-carotene cleavage enzymes. These results demonstrate that beta-carotene acts photoprotectively, and that this effect is mediated by (1)O(2) quenching.