UVA-induced peroxidation of lipid in the dried film state

The Skin Epilipidome in Stress, Aging, and Inflammation

Lipids are highly diverse biomolecules crucial for the formation and function of cellular membranes, for metabolism, and for cellular signaling. In the mammalian skin, lipids additionally serve for the formation of the epidermal barrier and as surface lipids, together regulating permeability, physical properties, acidification and the antimicrobial defense. Recent advances in accuracy and specificity of mass spectrometry have allowed studying enzymatic and non-enzymatic modifications of lipids-the epilipidome-multiplying the known diversity of molecules in this class. As the skin is an organ that is frequently exposed to oxidative-, chemical- and thermal stress, and to injury and inflammation, it is an ideal organ to study epilipidome dynamics, their causes, and their biological consequences. Recent studies uncover loss or gain in biological function resulting from either specific modifications or the sum of the modifications of lipids. These studies suggest an important role for the epilipidome in stress responses and immune regulation in the skin. In this minireview we provide a short survey of the recent developments on causes and consequences of epilipidomic changes in the skin or in cell types that reside in the skin.

Targeting the redox balance in inflammatory skin conditions

Reactive oxygen species (ROS) can be both beneficial and deleterious. Under normal physiological conditions, ROS production is tightly regulated, and ROS participate in both pathogen defense and cellular signaling. However, insufficient ROS detoxification or ROS overproduction generates oxidative stress, resulting in cellular damage. Oxidative stress has been linked to various inflammatory diseases. Inflammation is an essential response in the protection against injurious insults and thus important at the onset of wound healing. However, hampered resolution of inflammation can result in a chronic, exaggerated response with additional tissue damage. In the pathogenesis of several inflammatory skin conditions, e.g., sunburn and psoriasis, inflammatory-mediated tissue damage is central. The prolonged release of excess ROS in the skin can aggravate inflammatory injury and promote chronic inflammation. The cellular redox balance is therefore tightly regulated by several (enzymatic) antioxidants and pro-oxidants; however, in case of chronic inflammation, the antioxidant system may be depleted, and prolonged oxidative stress occurs. Due to the central role of ROS in inflammatory pathologies, restoring the redox balance forms an innovative therapeutic target in the development of new strategies for treating inflammatory skin conditions. Nevertheless, the clinical use of antioxidant-related therapies is still in its infancy.

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.

Identification of oxidatively truncated ethanolamine phospholipids in retina and their generation from polyunsaturated phosphatidylethanolamines

Oxidized (ox) phospholipids are receiving growing recognition as important messengers in oxidative stress signaling pathways and as endogenous electrophilic toxins that interfere with protein function through covalent modifications. Phosphatidylcholine lipids predominate in low-density lipoproteins (LDL). Our previous studies of oxLDL identified a family of biologically active oxidatively truncated phosphatidylcholines that are also present in atherosclerotic plaques. In contrast, phosphatidylethanolamine (PE) lipids are extraordinarily abundant in retina. Because photoreceptors contain the most highly unsaturated fatty acids found in vertebrate tissues, these membranes are expected to be especially susceptible to oxidative damage. Here, we report that oxidatively truncated ethanolamine phospholipids (oxPEs) are present in retina. As expected, the most abundant oxPEs, succinyl (2.2 +/- 0.8 pmol/retina) and omega-oxobutyryl (1.5 +/- 1.0 pmol/retina) esters of 2-lysophosphatidylethanolamine, are derived from the docosahexaenoyl ester, the most abundant polyunsaturated PE in retina. However, a large amount of the omega-oxononanoyl ester (1.3 +/- 0.6 pmol/retina), derived from linoleyl-PE, is also present even though linoleate is an order of magnitude less abundant than docosahexenoate in retina. There is a notable trend for the presence in retina of greater amounts, relative to the levels of their precursors, of longer chain homologous aldehydes and alkanedioate monoesters. We considered the possibility that this trend results from differences in the proclivities of various polyunsaturated fatty acyl (PUFA)-PEs to generate these homologous products. Therefore, we examined oxidative cleavage of various PUFA-PEs in small unilamellar vesicles. Alkanedioate monoesters are the major stable end products. Particularly notable is the fact that omega-oxononanoyl-PE levels either do not decline or decline less than those of the analogous aldehydes omega-oxobutyryl-PE or omega-oxovaleryl-PE during autoxidation for 33 h. The resistance of omega-oxononanoyl-PE, as compared with omega-oxobutyryl-PE and omega-oxovaleryl-PE, to further oxidation may contribute to the greater amount of this oxPE relative to its precursor, linoleyl-PE, in retina.

Gamma-irradiation and UV-C light-induced lipid peroxidation: a Fourier transform-infrared absorption spectroscopic study

Fourier transform-infrared spectroscopy of dry, multibilayer films has been used to study gamma-radiation and UV-C light induced lipid peroxidation in 1,2-dilinoleoyl-sn-glycero-3-phosphocholine liposomes. The observed spectral changes were compared with the results obtained from measurement of hydroperoxides, conjugated dienes and to the formation of thiobarbituric acid reactive substances, such as malondialdehyde (MDA) or MDA-like substances. Upon irradiation a decrease in intensity of the asymmetric C - H stretching vibration (va(CH2)) of the isolated cis C = C - H groups (3010 cm-1) was observed. Directly correlated with the decrease of the va(CH2) absorption was a shift of the asymmetric phosphate ester stretching vibration (va(P = O)) towards smaller wavenumbers (1260-->1244 cm-1), indicating that the lipid peroxidation induced molecular alterations in the fatty acid chains influence the packing of the phospholipids in dry multibilayer films. In addition, the formation of a new absorption band at 1693 cm-1 could be detected, the intensity of which was comparable with the formation of thiobarbituric acid reactive substances and, therefore, attributed to the (C = O) stretching of alpha, beta unsaturated aldehydes. Dose-dependent studies using ionizing radiation showed that the decrease of va(CH2) was directly correlated with an increase in absorption of the conjugated dienes at 234 nm and with the formation of hydroperoxides suggesting that the absorption at 3010 cm-1 is solely due to isolated cis C = C - H groups and hence subject to the early stages of the radical chain reaction. UV-C light induced lipid peroxidation revealed a non-linear decrease of I3010, which was directly correlated with the formation of hydroperoxides. The observed early saturation of the conjugated dienes was attributed to an early photodecomposition of the conjugated double bonds.

Evaluation of retinal susceptibility to light damage in pigmented rats supplemented with beta-carotene

The present study evaluated the influence of beta-carotene supplementation on the susceptibility of the retina to light damage. Long-Evans pigmented rats were supplemented with beta-carotene by either dietary or intraperitoneal administration, and beta-carotene levels in plasma, liver and retina were determined by high performance liquid chromatography. Other animals from each group were exposed to ultraviolet-A light at a dose of 8.1 J/cm2 in their right eye only, and photoreceptor cell losses determined by light microscopic morphometry. In supplemented animals, beta-carotene levels increased markedly in the liver, and were elevated from non-detectable to detectable in the plasma and retina, relative to nonsupplemented controls. In each tissue, beta-carotene levels were found to be higher in animals receiving intraperitoneal supplementation as compared to dietary. Beta-carotene supplementation by either route did not protect the retina against photoreceptor cell loss measured at two weeks following UVA exposure. Preliminary observations indicated that beta-carotene supplementation decreased the incidence of light-induced retinal pigment epithelium destruction.

Correlation between UVA-induced changes in microviscosity, permeability and malondialdehyde formation in liposomal membrane

The lipid peroxidation, 14C-glucose leakage and microviscosity of liposomal membrane increased linearly with increasing UVA fluence. A positive and highly significant correlation was found between these properties of the UVA-exposed liposomal membrane. The possible involvement of singlet oxygen in the UVA-induced damage of liposomal membrane is discussed.

On the induction of protective responses in Salmonella typhimurium strain TA1535/pSK1002 by UVA (365 nm)

Exposure to UVA (365 nm) led to growth delay, loss of viability and inhibition of 3H-thymidine incorporation into the cells of Salmonella typhimurium strain TA1535 containing multiple copies of a plasmid pSK1002 carrying a umuC'-'lacZ fusion gene. Ultraviolet-A induced umu gene expression, as monitored by the estimation of beta-galactosidase, in a linear fluence-dependent manner. The induction of umu gene expression increased with the increase of postirradiation incubation period of the cells in the LB-ampicillin (LBA) medium at 37 degrees C and leveled off from 2 h onward. The induction of gene expression depended on concomitant protein synthesis and represented the induction of the SOS response in the particular S. typhimurium cells used. The exposure to low fluences (sublethal) of UVA also led to the induction of an adaptive response in the same bacterial cells, which made them resistant to subsequent challenge by a much higher fluence of the same radiation. The adaptive response, as monitored by the assays of viability and beta-galactosidase units, increased with the period of exposure to sublethal fluences of UVA, attained a maximum at the UVA exposure of 4.5 kJ/m2 (15 min) and thereafter gradually decreased with further increase of UVA exposure period. Modulation studies involving D2O, LBA growth medium, different scavengers of free radicals and quenchers of activated oxygen species indicated the involvement of both hydroxyl free radicals and singlet oxygen in the UVA-induced umu gene expression.