Role of lipid hydroperoxides in photo-oxidative stress signaling

Singlet molecular oxygen generated by biological hydroperoxides

The chemistry behind the phenomenon of ultra-weak photon emission has been subject of considerable interest for decades. Great progress has been made on the understanding of the chemical generation of electronically excited states that are involved in these processes. Proposed mechanisms implicated the production of excited carbonyl species and singlet molecular oxygen in the mechanism of generation of chemiluminescence in biological system. In particular, attention has been focused on the potential generation of singlet molecular oxygen in the recombination reaction of peroxyl radicals by the Russell mechanism. In the last ten years, our group has demonstrated the generation of singlet molecular oxygen from reactions involving the decomposition of biologically relevant hydroperoxides, especially from lipid hydroperoxides in the presence of metal ions, peroxynitrite, HOCl and cytochrome c. In this review we will discuss details on the chemical aspects related to the mechanism of singlet molecular oxygen generation from different biological hydroperoxides.

Protective effect of Vaccinium myrtillus extract against UVA- and UVB-induced damage in a human keratinocyte cell line (HaCaT cells)

Recently, the field of skin protection have shown a considerable interest in the use of botanicals. Vaccinium myrtillus contains several polyphenols and anthocyanins with multiple pharmacological properties. The purpose of our study was to examine whether a water-soluble V. myrtillus extract (dry matter 12.4%; total polyphenols 339.3mg/100 g fw; total anthocyanins 297.4 mg/100 g fw) was able to reduce UVA- and UVB-induced damage using a human keratinocyte cell line (HaCaT). HaCaT cells were pretreated for 1h with extract in a serum-free medium and then irradiated with UVA (8-40 J/cm(2)) and UVB (0.008-0.72 J/cm(2)) rays. All experiments were performed 24h after the end of irradiation, except for oxidative stress tests. The extract was able to reduce the UVB-induced cytotoxicity and genotoxicity (studied by comet and micronucleous assays) at lower doses. V. myrtillus extract reduced lipid peroxidation UVB-induced, but had no effect against the ROS UVB-produced. With UVA-induced damage V. myrtillus reduced genotoxicity as well as the unbalance of redox intracellular status. Moreover our extract reduced the UVA-induced apoptosis, but had no effect against the UVB one. V. myrtillus extract showed its free radical scavenging properties reducing oxidative stress and apoptotic markers, especially in UVA-irradiated cells.

Neurodegeneration in the diabetic eye: new insights and therapeutic perspectives

Diabetic retinopathy (DR), one of the leading causes of preventable blindness, has been considered a microcirculatory disease of the retina. However, there is emerging evidence to suggest that retinal neurodegeneration is an early event in the pathogenesis of DR, which participates in the development of microvascular abnormalities. Therefore, the study of the underlying mechanisms leading to neurodegeneration and the identification of the mediators in the crosstalk between neurodegeneration and microangiopathy will be essential for the development of new therapeutic strategies. In this review, an updated discussion of the mechanisms involved in neurodegeneration, as well as the link between neurodegeneration and microangiopathy, is presented. Finally, the therapeutic implications and new perspectives based on identifying those patients with retinal neurodegeneration are given.

Comparative analysis of peroxiredoxin activation in the brown macroalgae Scytosiphon gracilis and Lessonia nigrescens (Phaeophyceae) under copper stress

Among thiol-dependent peroxidases (TDPs) peroxiredoxins (PRXs) standout, since they are enzymes capable of reducing hydrogen peroxide, alkylhydroperoxides and peroxynitrite, and have been detected in a proteomic study of the copper-tolerant species Scytosiphon gracilis. In order to determine the importance of these enzymes in copper-stress tolerance, TDP activity and type II peroxiredoxin (II PRX) protein expression were compared between the opportunistic S. gracilis and the brown kelp Lessonia nigrescens, a species absent from copper-impacted sites due to insufficient copper-tolerance mechanisms. Individuals of both species were cultured with increasing copper concentrations (0-300 µg l(-1) Cu) for 96 h and TDP activity and lipoperoxides (LPXs) were determined together with II PRX expression by immunofluorescence and Western blot analysis. The results showed that TDP activity was higher in S. gracilis than L. nigrescens in all copper concentrations, independent of the reducing agent used (dithiothreitol, thioredoxin or glutaredoxin). This activity was copper inhibited in L. nigrescens at lower copper concentrations (20 µg l(-1) Cu) compared to S. gracilis (100 µg l(-1) Cu). The loss of activity coincided in both species with an increase in LPX, which suggests that TDP may control LPX production. Moreover, II PRX protein levels increased under copper stress only in S. gracilis. These results suggest that in S. gracilis TDP, particularly type II peroxiredoxin (II PRX), acts as an active antioxidant barrier attenuating the LPX levels generated by copper, which is not the case in L. nigrescens. Thus, from an ecological point of view these results help explaining the inability of L. nigrescens to flourish in copper-enriched environments.

Oxidative stress in mammalian cells impinges on the cysteines redox state of human XRCC3 protein and on its cellular localization

In vertebrates, XRCC3 is one of the five Rad51 paralogs that plays a central role in homologous recombination (HR), a key pathway for maintaining genomic stability. While investigating the potential role of human XRCC3 (hXRCC3) in the inhibition of DNA replication induced by UVA radiation, we discovered that hXRCC3 cysteine residues are oxidized following photosensitization by UVA. Our in silico prediction of the hXRCC3 structure suggests that 6 out of 8 cysteines are potentially accessible to the solvent and therefore potentially exposed to ROS attack. By non-reducing SDS-PAGE we show that many different oxidants induce hXRCC3 oxidation that is monitored in Chinese hamster ovarian (CHO) cells by increased electrophoretic mobility of the protein and in human cells by a slight decrease of its immunodetection. In both cell types, hXRCC3 oxidation was reversed in few minutes by cellular reducing systems. Depletion of intracellular glutathione prevents hXRCC3 oxidation only after UVA exposure though depending on the type of photosensitizer. In addition, we show that hXRCC3 expressed in CHO cells localizes both in the cytoplasm and in the nucleus. Mutating all hXRCC3 cysteines to serines (XR3/S protein) does not affect the subcellular localization of the protein even after exposure to camptothecin (CPT), which typically induces DNA damages that require HR to be repaired. However, cells expressing mutated XR3/S protein are sensitive to CPT, thus highlighting a defect of the mutant protein in HR. In marked contrast to CPT treatment, oxidative stress induces relocalization at the chromatin fraction of both wild-type and mutated protein, even though survival is not affected. Collectively, our results demonstrate that the DNA repair protein hXRCC3 is a target of ROS induced by environmental factors and raise the possibility that the redox environment might participate in regulating the HR pathway.

Artocarpin attenuates ultraviolet B-induced skin damage in hairless mice by antioxidant and anti-inflammatory effect

Artocarpin, a prenylated flavonoid isolated from an agricultural plant Artocarpus communis, has been documented to possess anti-inflammation and anticancer activities. As oxidative stress and inflammation promote the development of ultraviolet B (UVB) irradiation-induced photodamage, the aim of the present study was to evaluate the photoprotective effect of artocarpin on UVB-induced skin damage in hairless mice. Artocarpin at a topical dose of 0.05% and 0.1% showed a significant photoprotective effect by decreasing histopathological changes, such as desquamation, epidermal thicken and sunburn cell formation, but 0.1% of artocarpin administration did not show better effect. Regarding the antioxidant activities, artocarpin exhibited a significant effect (P<0.05) by decreasing levels of reactive species oxygen and lipid peroxidation. In addition, artocarpin can significant decrease the level of tumor necrosis factor-α and interleukin-1β for downregulating the inflammation protein, including the synthesis of cytosolic phospholipase A2 and cyclooxygenase-2 (P<0.05). In conclusion, these data suggest that artocarpin can prevent skin damage from UVB irradiation-induced photodamage in hairless mice and this is likely mediated through its antioxidant and anti-inflammation mechanisms. Therefore, we suggested that artocarpin could be a useful photoprotective agent in medicine and/or cosmetics.

Principles in redox signaling: from chemistry to functional significance

Reactive oxygen and nitrogen species are currently considered not only harmful byproducts of aerobic respiration but also critical mediators of redox signaling. The molecules and the chemical principles sustaining the network of cellular redox regulated processes are described. Special emphasis is placed on hydrogen peroxide (H(2)O(2)), now considered as acting as a second messenger, and on sulfhydryl groups, which are the direct targets of the oxidant signal. Cysteine residues of some proteins, therefore, act as sensors of redox conditions and are oxidized in a reversible reaction. In particular, the formation of sulfenic acid and disulfide, the initial steps of thiol oxidation, are described in detail. The many cell pathways involved in reactive oxygen species formation are reported. Central to redox signaling processes are the glutathione and thioredoxin systems controlling H(2)O(2) levels and, hence, the thiol/disulfide balance. Lastly, some of the most important redox-regulated processes involving specific enzymes and organelles are described. The redox signaling area of research is rapidly expanding, and future work will examine new pathways and clarify their importance in cellular pathophysiology.

UVA causes dual inactivation of cathepsin B and L underlying lysosomal dysfunction in human dermal fibroblasts

Cutaneous exposure to chronic solar UVA-radiation is a causative factor in photocarcinogenesis and photoaging. Recently, we have identified the thiol-dependent cysteine-protease cathepsin B as a novel UVA-target undergoing photo-oxidative inactivation upstream of autophagic-lysosomal dysfunction in fibroblasts. In this study, we examined UVA effects on a wider range of cathepsins and explored the occurrence of UVA-induced cathepsin inactivation in other cultured skin cell types. In dermal fibroblasts, chronic exposure to non-cytotoxic doses of UVA caused pronounced inactivation of the lysosomal cysteine-proteases cathepsin B and L, effects not observed in primary keratinocytes and occurring only to a minor extent in primary melanocytes. In order to determine if UVA-induced lysosomal impairment requires single or dual inactivation of cathepsin B and/or L, we used a genetic approach (siRNA) to selectively downregulate enzymatic activity of these target cathepsins. Monitoring an established set of protein markers (including LAMP1, LC3-II, and p62) and cell ultrastructural changes detected by electron microscopy, we observed that only dual genetic antagonism (targeting both CTSB and CTSL expression) could mimic UVA-induced autophagic-lysosomal alterations, whereas single knockdown (targeting CTSB or CTSL only) did not display 'UVA-mimetic' effects failing to reproduce the UVA-induced phenotype. Taken together, our data demonstrate that chronic UVA inhibits both cathepsin B and L enzymatic activity and that dual inactivation of both enzymes is a causative factor underlying UVA-induced impairment of lysosomal function in dermal fibroblasts.

Zeaxanthin protects plant photosynthesis by modulating chlorophyll triplet yield in specific light-harvesting antenna subunits

Plants are particularly prone to photo-oxidative damage caused by excess light. Photoprotection is essential for photosynthesis to proceed in oxygenic environments either by scavenging harmful reactive intermediates or preventing their accumulation to avoid photoinhibition. Carotenoids play a key role in protecting photosynthesis from the toxic effect of over-excitation; under excess light conditions, plants accumulate a specific carotenoid, zeaxanthin, that was shown to increase photoprotection. In this work we genetically dissected different components of zeaxanthin-dependent photoprotection. By using time-resolved differential spectroscopy in vivo, we identified a zeaxanthin-dependent optical signal characterized by a red shift in the carotenoid peak of the triplet-minus-singlet spectrum of leaves and pigment-binding proteins. By fractionating thylakoids into their component pigment binding complexes, the signal was found to originate from the monomeric Lhcb4-6 antenna components of Photosystem II and the Lhca1-4 subunits of Photosystem I. By analyzing mutants based on their sensitivity to excess light, the red-shifted triplet-minus-singlet signal was tightly correlated with photoprotection in the chloroplasts, suggesting the signal implies an increased efficiency of zeaxanthin in controlling chlorophyll triplet formation. Fluorescence-detected magnetic resonance analysis showed a decrease in the amplitude of signals assigned to chlorophyll triplets belonging to the monomeric antenna complexes of Photosystem II upon zeaxanthin binding; however, the amplitude of carotenoid triplet signal does not increase correspondingly. Results show that the high light-induced binding of zeaxanthin to specific proteins plays a major role in enhancing photoprotection by modulating the yield of potentially dangerous chlorophyll-excited states in vivo and preventing the production of singlet oxygen.

SINGLET OXYGEN RESISTANT 1 links reactive electrophile signaling to singlet oxygen acclimation in Chlamydomonas reinhardtii

Acclimation of Chlamydomonas reinhardtii cells to low levels of singlet oxygen, produced either by photoreactive chemicals or high light treatment, induces a specific genetic response that strongly increases the tolerance of the algae to subsequent exposure to normally lethal singlet oxygen-producing conditions. The genetic response includes the increased expression of various oxidative stress response and detoxification genes, like the glutathione peroxidase homologous gene GPXH/GPX5 and the σ-class glutathione-S-transferase gene GSTS1. To identify components involved in the signal transduction and activation of the singlet oxygen-mediated response, a mutant selection was performed. This selection led to the isolation of the singlet oxygen resistant 1 (sor1) mutant, which is more tolerant to singlet oxygen-producing chemicals and shows a constitutively higher expression of GPXH and GSTS1. Map-based cloning revealed that the SOR1 gene encodes a basic leucine zipper transcription factor, which controls its own expression and the expression of a large number of oxidative stress response and detoxification genes. In the promoter region of many of these genes, a highly conserved 8-bp palindromic sequence element was found to be enriched. This element was essential for GSTS1 induction by increased levels of lipophilic reactive electrophile species (RES), suggesting that it functions as an electrophile response element (ERE). Furthermore, GSTS1 overexpression in sor1 requires the ERE, although it is unknown whether it occurs through direct binding of SOR1 to the ERE. RES can be formed after singlet oxygen-induced lipid peroxidation, indicating that RES-stimulated and SOR1-mediated responses of detoxification genes are part of the singlet oxygen-induced acclimation process in C. reinhardtii.

Chemical quenching of singlet oxygen by carotenoids in plants

Carotenoids are considered to be the first line of defense of plants against singlet oxygen ((1)O(2)) toxicity because of their capacity to quench (1)O(2) as well as triplet chlorophylls through a physical mechanism involving transfer of excitation energy followed by thermal deactivation. Here, we show that leaf carotenoids are also able to quench (1)O(2) by a chemical mechanism involving their oxidation. In vitro oxidation of β-carotene, lutein, and zeaxanthin by (1)O(2) generated various aldehydes and endoperoxides. A search for those molecules in Arabidopsis (Arabidopsis thaliana) leaves revealed the presence of (1)O(2)-specific endoperoxides in low-light-grown plants, indicating chronic oxidation of carotenoids by (1)O(2). β-Carotene endoperoxide, but not xanthophyll endoperoxide, rapidly accumulated during high-light stress, and this accumulation was correlated with the extent of photosystem (PS) II photoinhibition and the expression of various (1)O(2) marker genes. The selective accumulation of β-carotene endoperoxide points at the PSII reaction centers, rather than the PSII chlorophyll antennae, as a major site of (1)O(2) accumulation in plants under high-light stress. β-Carotene endoperoxide was found to have a relatively fast turnover, decaying in the dark with a half time of about 6 h. This carotenoid metabolite provides an early index of (1)O(2) production in leaves, the occurrence of which precedes the accumulation of fatty acid oxidation products.

Pomegranate fruit extract inhibits UVB-induced inflammation and proliferation by modulating NF-κB and MAPK signaling pathways in mouse skin

There is considerable interest in the identification of natural agents capable of affording protection to skin from the adverse effects of solar ultraviolet B (UVB) radiation. Pomegranate (Punica granatum L.) fruit possesses as strong antioxidant, anti-inflammatory and antiproliferative properties. Recently, we have shown that oral feeding of pomegranate fruit extract (PFE) to mice afforded substantial protection from the adverse effects of single UVB radiation via modulation in early biomarkers of photocarcinogenesis. This study was designed to investigate the photochemopreventive effects of PFE (0.2%, wt/vol) after multiple UVB irradiations (180 mJ cm(-2), on alternative day, for a total of seven treatments) to the skin of SKH-1 hairless mice. Oral feeding of PFE to SKH-1 mice inhibited UVB-induced epidermal hyperplasia, infiltration of leukocytes, protein oxidation and lipid peroxidation. Immunoblot analysis demonstrated that oral feeding of PFE to mice inhibited UVB-induced (1) nuclear translocation and phosphorylation of nuclear factor kappa B/p65, (2) phosphorylation and degradation of IκBα, (3) activation of IKKα/ΙΚΚβ and (4) phosphorylation of mitogen-activated protein kinase proteins and c-Jun. PFE consumption also inhibited UVB-induced protein expression of (1) COX-2 and iNOS, (2) PCNA and cyclin D1 and (3) matrix metalloproteinases-2,-3 and -9 in mouse skin. Taken together, these data show that PFE consumption afforded protection to mouse skin against the adverse effects of UVB radiation by modulating UVB-induced signaling pathways.

Porphyrin biosynthesis control under water stress: sustained porphyrin status correlates with drought tolerance in transgenic rice

A controlled flow of porphyrin metabolites is critical for organisms, but little is known about the control of porphyrin biosynthesis under environmental stress. We monitored transgenic rice (Oryza sativa) plants expressing Myxococcus xanthus protoporphyrinogen oxidase (PPO) for their response to drought stress. Transgenic plants showed significantly improved drought tolerance, as indicated by a higher shoot water potential, less oxidative damage, and a more favorable redox balance compared with wild-type plants. Both transgenic and wild-type plants responded to the onset of drought stress, even prior to changes in shoot water potential and oxidative metabolism, by drastically scavenging porphyrin intermediates in leaves, which was crucial for alleviating reactive oxygen species-induced stress. Protoporphyrin IX, protochlorophyllide, magnesium-protoporphyrin IX, and its methyl ester were absent or hardly detected with the intensification of water stress (-3.1 MPa) in the wild type, whereas transgenic plants retained these intermediates to some extent. Additionally, the expression and activity of most enzymes involved in porphyrin biosynthesis, particularly in the chlorophyll branch, were primarily down-regulated under dehydrating conditions, with stronger repression in the wild type than in transgenic plants. There was up-regulation of Glutamate 1-Semialdehyde Aminotransferase, PPO1, and Fe Chelatase2 transcripts in drought-stressed transgenic plants, enabling the transgenic plants to make larger pools of 5-aminolevulinic acid and protoporphyrin IX available for subsequent steps in the heme branch. Overexpression of PPO ultimately protected the transgenic plants from drought-induced cytotoxicity, demonstrating clearly that manipulation of porphyrin biosynthesis can produce drought-tolerant plants. Our results support a possible role for tetrapyrroles in signaling their metabolic state and in plant protection under drought stress conditions.

Singlet oxygen stress in microorganisms

Singlet oxygen is the primary agent of photooxidative stress in microorganisms. In photosynthetic microorganisms, sensitized generation by pigments of the photosystems is the main source of singlet oxygen and, in nonphotosynthetic microorganisms, cellular cofactors such as flavins, rhodopsins, quinones, and porphyrins serve as photosensitizer. Singlet oxygen rapidly reacts with a wide range of cellular macromolecules including proteins, lipids, DNA, and RNA, and thereby further reactive substances including organic peroxides and sulfoxides are formed. Microorganisms that face high light intensities or exhibit potent photosensitizers have evolved specific mechanisms to prevent photooxidative stress. These mechanisms include the use of quenchers, such as carotenoids, which interact either with excited photosensitizer molecules or singlet oxygen itself to prevent damage of cellular molecules. Scavengers like glutathione react with singlet oxygen. Despite those protection mechanisms, damage by reactions with singlet oxygen on cellular macromolecules disturbs cellular functions. Microorganisms that regularly face photooxidative stress have evolved specific systems to sense singlet oxygen and tightly control the removal of singlet oxygen reaction products. Responses to photooxidative stress have been investigated in a range of photosynthetic and nonphotosynthetic microorganisms. However, detailed knowledge on the regulation of this response has only been obtained for the phototrophic alpha-proteobacterium Rhodobacter sphaeroides. In this organism and in related proteobacteria, the extracytoplasmic function (ECF) sigma factor RpoE is released from the cognate antisigma factor ChrR in the presence of singlet oxygen and triggers the expression of genes providing protection against photooxidative stress. Recent experiments show that singlet oxygen acts as a signal, which is sensed by yet unknown components and leads to proteolysis of ChrR. RpoE induces expression of a second alternative sigma factor, RpoH(II), which controls a large set of genes that partially overlaps with the heat-shock response controlled by RpoH(I). In addition to the transcriptional control of gene regulation by alternative sigma factors, a set of noncoding small RNAs (sRNAs) appear to affect the synthesis of several proteins involved in the response to photooxidative stress. The interaction of mRNA targets with those sRNAs is usually mediated by the RNA chaperone Hfq. Deletion of the gene encoding Hfq leads to a singlet oxygen-sensitive phenotype, which underlines the control of gene regulation on the posttranscriptional level by sRNAs in R. sphaeroides. Hence, a complex network of different regulatory components controls the defense against photooxidative stress in anoxygenic photosynthetic bacteria.

Photodynamic therapy based on 5-aminolevulinic acid and its use as an antimicrobial agent

Exogenous 5-aminolevulinic acid (ALA) is taken up directly by bacteria, yeasts, fungi, and some parasites, which then induces the accumulation of protoporphyrin IX (PPIX). Subsequent light irradiation of PPIX leads to the inactivation of these organisms via photodamage to their cellular structures. ALA uptake and light irradiation of PPIX produced by host cells leads to the inactivation of other parasites, along with some viruses, via the induction of an immune response. ALA-mediated PPIX production by host cells and light irradiation result in the inactivation of other viruses via either the induction of a host cell response or direct photodynamic attack on viral particles. This ALA-mediated production of light-activated PPIX has been extensively used as a form of photodynamic therapy (PDT) and has shown varying levels of efficacy in treating conditions that are associated with microbial infection, ranging from acne and verrucae to leishmaniasis and onychomycosis. However, for the treatment of some of these conditions by ALA-based PDT, the role of an antimicrobial effect has been disputed and in general, the mechanisms by which the technique inactivates microbes are not well understood. In this study, we review current understanding of the antimicrobial mechanisms used by ALA-based PDT and its role in the treatment of microbial infections along with its potential medical and nonmedical applications.

Conversion of 7-ketocholesterol to oxysterol metabolites by recombinant CYP27A1 and retinal pigment epithelial cells

Of the different oxygenated cholesterol metabolites, 7-ketocholesterol (7KCh) is considered a noxious oxy-sterol implicated in the development of certain pathologies, including those found in the eye. Here we elucidated whether sterol 27-hydroxylase cytochrome P450 27A1 (CYP27A1) is involved in elimination of 7KCh from the posterior part of the eye: the neural retina and underlying retinal pigment epithelium (RPE). We first established that the affinities of purified recombinant CYP27A1 for 7KCh and its endogenous substrate cholesterol are similar, yet 7KCh is metabolized at a 4-fold higher rate than cholesterol in the reconstituted system in vitro. Lipid extracts from bovine neural retina and RPE were then analyzed by isotope dilution GC-MS for the presence of the 7KCh-derived oxysterols. Two metabolites, 3β,27-dihydroxy-5-cholesten-7-one (7KCh-27OH) and 3β-hydroxy-5-cholesten-7-one-26-oic acid (7KCh-27COOH), were detected in the RPE but not in the neural retina. 7KCh-27OH was also formed when RPE homogenates were supplemented with NADPH and the mitochondrial redox system. Quantifications in human RPE showed that CYP27A1 is indeed expressed in the RPE at 2-4-fold higher levels than in the neural retina. The data obtained represent evidence for the role of CYP27A1 in retinal metabolism of 7KCh and suggest that, in addition to cholesterol removal, the functions of this enzyme could also include elimination of toxic endogenous compounds.

The light responsive transcriptome of the zebrafish: function and regulation

Most organisms possess circadian clocks that are able to anticipate the day/night cycle and are reset or “entrained” by the ambient light. In the zebrafish, many organs and even cultured cell lines are directly light responsive, allowing for direct entrainment of the clock by light. Here, we have characterized light induced gene transcription in the zebrafish at several organizational levels. Larvae, heart organ cultures and cell cultures were exposed to 1- or 3-hour light pulses, and changes in gene expression were compared with controls kept in the dark. We identified 117 light regulated genes, with the majority being induced and some repressed by light. Cluster analysis groups the genes into five major classes that show regulation at all levels of organization or in different subset combinations. The regulated genes cover a variety of functions, and the analysis of gene ontology categories reveals an enrichment of genes involved in circadian rhythms, stress response and DNA repair, consistent with the exposure to visible wavelengths of light priming cells for UV-induced damage repair. Promoter analysis of the induced genes shows an enrichment of various short sequence motifs, including E- and D-box enhancers that have previously been implicated in light regulation of the zebrafish period2 gene. Heterologous reporter constructs with sequences matching these motifs reveal light regulation of D-box elements in both cells and larvae. Morpholino-mediated knock-down studies of two homologues of the D-box binding factor Tef indicate that these are differentially involved in the cell autonomous light induction in a gene-specific manner. These findings suggest that the mechanisms involved in period2 regulation might represent a more general pathway leading to light induced gene expression.

Characterization of singlet oxygen-accumulating mutants isolated in a screen for altered oxidative stress response in Chlamydomonas reinhardtii

BACKGROUND

When photosynthetic organisms are exposed to harsh environmental conditions such as high light intensities or cold stress, the production of reactive oxygen species like singlet oxygen is stimulated in the chloroplast. In Chlamydomonas reinhardtii singlet oxygen was shown to act as a specific signal inducing the expression of the nuclear glutathione peroxidase gene GPXH/GPX5 during high light stress, but little is known about the cellular mechanisms involved in this response. To investigate components affecting singlet oxygen signaling in C. reinhardtii, a mutant screen was performed.

RESULTS

Mutants with altered GPXH response were isolated from UV-mutagenized cells containing a GPXH-arylsulfatase reporter gene construct. Out of 5500 clones tested, no mutant deficient in GPXH induction was isolated, whereas several clones showed constitutive high GPXH expression under normal light conditions. Many of these GPXH overexpressor (gox) mutants exhibited higher resistance to oxidative stress conditions whereas others were sensitive to high light intensities. Interestingly, most gox mutants produced increased singlet oxygen levels correlating with high GPXH expression. Furthermore, different patterns of altered photoprotective parameters like non-photochemical quenching, carotenoid contents and α-tocopherol levels were detected in the various gox mutants.

CONCLUSIONS

Screening for mutants with altered GPXH expression resulted in the isolation of many gox mutants with increased singlet oxygen production, showing the relevance of controlling the production of this ROS in photosynthetic organisms. Phenotypic characterization of these gox mutants indicated that the mutations might lead to either stimulated triplet chlorophyll and singlet oxygen formation or reduced detoxification of singlet oxygen in the chloroplast. Furthermore, changes in multiple protection mechanisms might be responsible for high singlet oxygen formation and GPXH expression, which could either result from mutations in multiple loci or in a single gene encoding for a global regulator of cellular photoprotection mechanisms.

Enhanced photoprotection by protein-bound vs free xanthophyll pools: a comparative analysis of chlorophyll b and xanthophyll biosynthesis mutants

When light absorbed by plants exceeds the capacity of photosynthesis, the xanthophyll violaxanthin is reversibly de-epoxidized to zeaxanthin in the so-called xanthophyll cycle. Zeaxanthin plays a key role in the protection of photosynthetic organisms against excess light, by promoting rapidly reversible (qE) and long-term (qI) quenching of excited chlorophylls, and preventing lipid oxidation. The photoprotective role of zeaxanthin, either free or bound to light-harvesting complexes (Lhcs), has been investigated by using mutants lacking Chl b (ch1) and/or specific xanthophyll species (npq, lut2). The ch1 mutation causes (1) the absence of Lhcb proteins; (2) strong reduction of the feedback de-excitation (qE); and (3) accumulation of xanthophylls as free pigments into thylakoids. Ch1 mutants showed extreme sensitivity to photo-oxidative stress in high light, due to higher singlet oxygen (¹O₂) release. The double mutant ch1npq1 was more sensitive to photo-oxidation than ch1, showing that zeaxanthin does protect lipids even when free in the membrane. Nevertheless, lack of zeaxanthin had a much stronger impact on the level of lipid peroxidation in Lhcs-containing plants (WT vs npq1) with respect to Lhc-less plants (ch1 vs ch1npq1), implying that its protective effect is enhanced by interaction with antenna proteins. It is proposed that the antioxidant capacity of zeaxanthin is empowered in the presence of PSII-LHCs-Zea complexes, while its effect on enhancement of qE only provides a minor contribution. Comparison of the sensitivity of WT vs npq1 plants to exogenous ¹O₂ suggests that besides the scavenging of ¹O₂, at least one additional mechanism is involved in chloroplast photoprotection.

Generation and biological activities of oxidized phospholipids

Glycerophospholipids represent a common class of lipids critically important for integrity of cellular membranes. Oxidation of esterified unsaturated fatty acids dramatically changes biological activities of phospholipids. Apart from impairment of their structural function, oxidation makes oxidized phospholipids (OxPLs) markers of "modified-self" type that are recognized by soluble and cell-associated receptors of innate immunity, including scavenger receptors, natural (germ line-encoded) antibodies, and C-reactive protein, thus directing removal of senescent and apoptotic cells or oxidized lipoproteins. In addition, OxPLs acquire novel biological activities not characteristic of their unoxidized precursors, including the ability to regulate innate and adaptive immune responses. Effects of OxPLs described in vitro and in vivo suggest their potential relevance in different pathologies, including atherosclerosis, acute inflammation, lung injury, and many other conditions. This review summarizes current knowledge on the mechanisms of formation, structures, and biological activities of OxPLs. Furthermore, potential applications of OxPLs as disease biomarkers, as well as experimental therapies targeting OxPLs, are described, providing a broad overview of an emerging class of lipid mediators.

The retinal pigment epithelium: something more than a constituent of the blood-retinal barrier--implications for the pathogenesis of diabetic retinopathy

The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina. An overview of these functions will be given. Most of the research on the physiopathology of diabetic retinopathy has been focused on the impairment of the neuroretina and the breakdown of the inner BRB. By contrast, the effects of diabetes on the RPE and in particular on its secretory activity have received less attention. In this regard, new therapeutic strategies addressed to modulating RPE impairment are warranted.

Asymmetric oxidation of giant vesicles triggers curvature-associated shape transition and permeabilization

Oxidation of unsaturated lipids is a fundamental process involved in cell bioenergetics as well as in cell death. Using giant unilamellar vesicles and a chlorin photosensitizer, we asymmetrically oxidized the outer or inner monolayers of lipid membranes. We observed different shape transitions such as oblate to prolate and budding, which are typical of membrane curvature modifications. The asymmetry of the shape transitions is in accordance with a lowered effective spontaneous curvature of the leaflet being targeted. We interpret this effect as a decrease in the preferred area of the targeted leaflet compared to the other, due to the secondary products of oxidation (cleaved-lipids). Permeabilization of giant vesicles by light-induced oxidation is observed after a lag and is characterized in relation with the photosensitizer concentration. We interpret permeabilization as the opening of a pore above a critical membrane tension, resulting from the budding of vesicles. The evolution of photosensitized giant vesicle lysis tension was measured and yields an estimation of the effective spontaneous curvature at lysis. Additionally photo-oxidation was shown to be fusogenic.

Singlet oxygen inhibits ATPase and proton translocation activity of the thylakoid ATP synthase CF1CFo

Singlet oxygen ((1)O(2)) produced in plants during photosynthesis has a strong damaging effect not only on both photosystems but also on the whole photosynthetic machinery. This is also applicable for the adenosine triphosphate (ATP) synthase. Here we describe the impact of (1)O(2) generated by the photosensitizer Rose Bengal on the ATP hydrolysis and ATP-driven proton translocation activity of CF1CFo. Both activities were reduced dramatically within 1min of exposure. Interestingly, it is shown that oxidized thylakoid ATP synthase is more susceptible to (1)O(2) than CF1CFo in its reduced state, a new insight on the mechanism of (1)O(2) interaction with the gamma subunit.

Signaling events in apoptotic photokilling of 5-aminolevulinic acid-treated tumor cells: inhibitory effects of nitric oxide

Antitumor photodynamic therapy (PDT) employs a photosensitizing agent, molecular oxygen, and visible light to produce reactive oxygen species that can destroy tumor and tumor vasculature cells. NO produced by these cells could be procarcinogenic by inhibiting apoptosis and promoting angiogenesis and tumor growth. We recently showed that NO from a chemical donor or activated macrophages makes COH-BR1 breast tumor cells more resistant to photokilling sensitized by 5-aminolevulinic acid (ALA)-generated protoporphyrin IX (PpIX). Signaling events associated with this hyperresistance have now been examined. ALA-treated COH-BR1 cells containing mitochondria-localized PpIX died mainly by apoptosis after being irradiated. Underlying redox signaling associated with MAP kinase (ERK1/2, p38, JUN) phosphorylation-activation, and heme oxygenase-1 (HO-1) upregulation was studied using immunoprecipitation and Western blot methodology. ALA/light treatment resulted in activation of proapoptotic JNK and p38 alpha, and deactivation of prosurvival p38 beta and ERK1/2. Involvement of both JNK and p38 in apoptosis was established by using a specific inhibitor for each. Spermine NONOate-derived NO, introduced immediately before irradiation, provided substantial protection against apoptosis. This was accompanied by greater HO-1 induction and a strong inhibition of each MAP kinase effect seen in the absence of NO. Downstream of JNK and p38 alpha activation, a marked upregulation/activation of proapoptotic Bax and Bid was observed along with down-regulation of antiapoptotic Bcl-xL, each response being reversed by NO. These findings provide new insights into signaling activity associated with the intrinsic apoptotic pathway in ALA-PDT and how this activity can be modulated by NO.

Induction of oxidative cell damage by photo-treatment with zincmetaN-methylpyridylporphyrin

We have previously reported that isomeric Zn(II) N-methylpyridylporphyrins (ZnTM-2(3,4)-PyP4 + ) can act as photosensitizers with efficacy comparable to that of hematoporphyrin derivative (HpD) in preventing cell proliferation and causing cell death in vitro. To better understand the biochemical basis of this activity, the effects of photo-activated ZnTM-3-PyP4 + on GSH/GSSG ratio, lipid peroxidation, membrane permeability, oxidative DNA damage, and the activities of SOD, catalase, glutathione reductase, and glutathione peroxidase were evaluated. Light exposure of ZnTM-3-PyP4 + -treated colon adenocarcinoma cells caused a wide spectrum of oxidative damage including depletion of GSH, inactivation of glutathione reductase and glutathione peroxidase, oxidative DNA damage and peroxidation of membrane lipids. Cell staining with Hoechst-33342 showed morphological changes consistent with both necrotic and apoptotic death sequences, depending upon the presence of oxygen.

Prunella vulgaris extract and rosmarinic acid suppress lipopolysaccharide-induced alteration in human gingival fibroblasts

Periodontitis is a chronic disease associated with inflammation of the tooth-supporting tissues. The inflammation is initiated by a group of gram-negative anaerobic bacteria. These express a number of irritating factors including a lipopolysaccharide (LPS), which plays a key role in periodontal disease development. Plant extracts with anti-inflammatory and anti-microbial properties have been shown to inhibit bacterial plaque formation and thus prevent chronic gingivitis. In this study we tested effects of Prunella vulgaris L. extract (PVE; 5, 10, 25microg/ml) and its component rosmarinic acid (RA; 1microg/ml) on LPS-induced oxidative damage and inflammation in human gingival fibroblasts. PVE and RA reduced reactive oxygen species (ROS) production, intracellular glutathione (GSH) depletion as well as lipid peroxidation in LPS-treated cells. Treatment with PVE and RA also inhibited LPS-induced up-regulation of interleukin 1beta (IL-1beta), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-alpha) and suppressed expression of inducible nitric oxide synthase (iNOS). The results indicate that PVE and RA are able to suppress LPS-induced biological changes in gingival fibroblasts. The effects of PVE and RA are presumably linked to their anti-inflammatory activities and thus use of PVE and RA may be relevant in modulating the inflammation process, including periodontal disease.

Apolipoprotein A1 is overexpressed in the retina of diabetic patients

PURPOSE

To compare apolipoprotein A1 (ApoA1) expression in the retina from diabetic and nondiabetic donors.

DESIGN

Case-control study.

METHODS

Diabetic postmortem eyes (n = 8) were compared with eyes (n = 8) from nondiabetic donors matched by age. Messenger ribonucleic acid (mRNA) of ApoA1 (quantitative reverse-transcriptase polymerase chain reaction) was measured separately in the neuroretina and retinal pigment epithelium (RPE). ApoA1 was assessed by immunofluorescence (confocal laser microscopy) and Western blot analysis. The presence of early diabetic retinal damage was evaluated by measuring the rate of apoptosis and glial activation.

RESULTS

ApoA1 mRNA levels and ApoA1 immunofluorescence obtained in RPEs and in neuroretinas from diabetic donors were significantly higher than those obtained from nondiabetic donors. ApoA1 was expressed in all retina layers and it was more abundant in RPE than in the neuroretina in both diabetic and nondiabetic donors. In addition, ApoA1 immunofluorescence was significantly higher in all the layers of the neuroretina from diabetic patients. Densitometric analysis of immunoblots showed higher ApoA1 in the retinas from diabetic donors in comparison with nondiabetic donors, but the differences were at significant levels only for the RPE.

CONCLUSIONS

ApoA1 overexpression is an early event in the retina of diabetic patients and can be involved in the physiopathology of diabetic retinopathy. In addition, RPE is the main source of ApoA1 within the retina. These findings my be relevant to aiming new treatment strategies toward reducing the development of diabetic retinopathy.

RLIP76 in defense of radiation poisoning

PURPOSE

To determine the role of RLIP76 in providing protection from radiation and chemotherapy. In the present report, we used RLIP76 to refer to both the mouse (Ralbp1) and the human (RLIP76) 76-kDa splice variant proteins (RLIP76) for convenience and to avoid confusion. In other reports, Ralbp1 refers to the mouse enzyme (encoded by the Ralbp1 gene), which is structurally and functionally homologous to RLIP76, the human protein encoded by the human RALBP1 gene.

METHODS AND MATERIALS

Median lethal dose studies were performed in RLIP76(-/-) and RLIP76(+/+) C57B mice after treatment with a single dose of RLIP76 liposomes 14 h after whole body radiation. The radiosensitivity of the cultured mouse embryonic fibroblasts and the effects of buthionine sulfoximine (BSO), amifostine, c-jun N-terminal kinase (JNK), protein kinase B (Akt), and MAPK/ERK kinase (MEK) were determined by colony-forming assays. Glutathione-linked enzyme activities were measured by spectrophotometric assays, glutathione by dithiobis-2-nitrobenzoic acid (DTNB), lipid hydroperoxides by iodometric titration, and aldehydes and metabolites by thiobarbitauric acid reactive substances and liquid chromatography-mass spectrometry (LCMS).

RESULTS

RLIP76(-/-) mice were significantly more sensitive to radiation than were the wild-type, and RLIP76 liposomes prolonged survival in a dose-dependent manner in both genotypes. The levels of 4-hydroxynonenal and glutathione-conjugate of 4-hydroxynonenal were significantly increased in RLIP76(-/-) tissues compared with RLIP76(+/+). RLIP76(-/-) mouse embryonic fibroblasts were markedly more radiosensitive than RLIP76(+/+) mouse embryonic fibroblasts, despite increased glutathione levels in the former. RLIP76 augmentation had a remarkably greater protective effect compared with amifostine. The magnitude of effects of RLIP76 loss on radiation sensitivity was greater than those caused by perturbations of JNK, MEK, or Akt, and the effects of RLIP76 loss could not be completely compensated for by modulating the levels of these signaling proteins.

CONCLUSION

The results of our study have shown that RLIP76 plays a central role in radiation resistance.

Singlet oxygen is the major reactive oxygen species involved in photooxidative damage to plants

Reactive oxygen species act as signaling molecules but can also directly provoke cellular damage by rapidly oxidizing cellular components, including lipids. We developed a high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry-based quantitative method that allowed us to discriminate between free radical (type I)- and singlet oxygen ((1)O(2); type II)-mediated lipid peroxidation (LPO) signatures by using hydroxy fatty acids as specific reporters. Using this method, we observed that in non-photosynthesizing Arabidopsis (Arabidopsis thaliana) tissues, nonenzymatic LPO was almost exclusively catalyzed by free radicals both under normal and oxidative stress conditions. However, in leaf tissues under optimal growth conditions, (1)O(2) was responsible for more than 80% of the nonenzymatic LPO. In Arabidopsis mutants favoring (1)O(2) production, photooxidative stress led to a dramatic increase of (1)O(2) (type II) LPO that preceded cell death. Furthermore, under all conditions and in mutants that favor the production of superoxide and hydrogen peroxide (two sources for type I LPO reactions), plant cell death was nevertheless always preceded by an increase in (1)O(2)-dependent (type II) LPO. Thus, besides triggering a genetic cell death program, as demonstrated previously with the Arabidopsis fluorescent mutant, (1)O(2) plays a major destructive role during the execution of reactive oxygen species-induced cell death in leaf tissues.

Toxic tetrapyrrole accumulation in protoporphyrinogen IX oxidase-overexpressing transgenic rice plants

We generated transgenic rice plants (Oryza sativa cv. Dongjin) over-expressing human protoporphyrinogen IX oxidase (PPO) with the aim to increase mitochondrial PPO activity and confer herbicide resistance (Lee et al., Pestic Biochem Physiol 80:65-74, 2004). The transgenic plants showed during further leaf development the formation of severe necrotic spots and growth retardation. Several experiments were performed to examine the reasons for the formation of necrotic leaf lesions. Human PPO is normally located in mitochondria. An in vitro organellar import experiment revealed translocation of human PPO into pea chloroplasts, but not into mitochondria. Using a specific antibody raised against human PPO confirmed its plastidic localisation. The heme and chlorophyll contents were lower in necrotic leaves than wild-type leaves. Interestingly, mature and necrotic leaves of 12-week-old transgenic plants contained up to 14- and 24-fold more protoporphyrin IX, respectively, than mature wild-type leaves. Enhanced levels of Mg-Protoporphyrin IX, Mg-Protoporphyrin IX monomethyl ester and protochlorophyllide were concurrently observed in transgenic plants relative to wild type. Accumulated porphyrins and Mg-porphyrins likely act as photosensitizers and cause high formation of the reactive oxygen species. These high levels of tetrapyrrole intermediates correlated with increased rates of 5-aminolevulinic acid synthesis in transgenic plants. Tetrapyrrole-induced photooxidation was confirmed by increased lipid peroxidation and subsequent cell death. The transgenic phenotype is the consequence of a highly modified tetrapyrrole metabolism due to additional expression of human PPO. A possible regulatory role of PPO in graminaceous seedlings is discussed.

Bilberry extract reduces UVA-induced oxidative stress in HaCaT keratinocytes: a pilot study

Exposure to UVA radiation is known to cause many adverse biological effects by inducing the stricken cells to produce reactive oxygen species (ROS). In recent years the use of botanicals has received considerable interest in the skin protection. Bilberry (Vaccinium myrtillus L.) fruit contains several polyphenols with strong antioxidant and anti-inflammatory properties. In this study we evaluated potential UVA preventive effect of V. myrtillus fruit extract (VME; anthocyanins, 25% w/w) in HaCaT keratinocytes. Pre-treatment (1 h) or post-treatment (4 h) of HaCaT with VME resulted in attenuation of UVA-caused damage. Application of the extract significantly reduced UVA-stimulated ROS formation in keratinocytes. VME also prevented/reduced UVA-caused peroxidation of membrane lipids and depletion of intracellular GSH. The observed cytoprotective effect may be linked to the antioxidant activity of the plant constituents, namely anthocyanins.

Elevated zeaxanthin bound to oligomeric LHCII enhances the resistance of Arabidopsis to photooxidative stress by a lipid-protective, antioxidant mechanism

The xanthophyll cycle has a major role in protecting plants from photooxidative stress, although the mechanism of its action is unclear. Here, we have investigated Arabidopsis plants overexpressing a gene encoding beta-carotene hydroxylase, containing nearly three times the amount of xanthophyll cycle carotenoids present in the wild-type. In high light at low temperature wild-type plants exhibited symptoms of severe oxidative stress: lipid peroxidation, chlorophyll bleaching, and photoinhibition. In transformed plants, which accumulate over twice as much zeaxanthin as the wild-type, these symptoms were significantly ameliorated. The capacity of non-photochemical quenching is not significantly different in transformed plants compared with wild-type and therefore an enhancement of this process cannot be the cause of the stress tolerant phenotype. Rather, it is concluded that it results from the antioxidant effect of zeaxanthin. 80-90% of violaxanthin and zeaxanthin in wild-type and transformed plants was localized to an oligomeric LHCII fraction prepared from thylakoid membranes. The binding of these pigments in intact membranes was confirmed by resonance Raman spectroscopy. Based on the structural model of LHCII, we suggest that the protein/lipid interface is the active site for the antioxidant activity of zeaxanthin, which mediates stress tolerance by the protection of bound lipids.

A reporter system for the individual detection of hydrogen peroxide and singlet oxygen: its use for the assay of reactive oxygen species produced in vivo

A reporter system for the assay of reactive oxygen species (ROS) was developed in Chlamydomonas reinhardtii, a plant model organism well suited for the application of inhibitors and generators of various types of ROS. This system employs various HSP70A promoter segments fused to a Renilla reniformis luciferase gene as a reporter. Transformants with the complete HSP70A promoter were inducible by both hydrogen peroxide and singlet oxygen. Constructs that lacked upstream heat-shock elements (HSEs) were inducible by hydrogen peroxide, indicating that this induction does not require such HSEs. Rather, downstream elements located between positions -81 to -149 with respect to the translation start site appear to be involved. In contrast, upstream sequences are essential for the response to singlet oxygen. Thus, activation by singlet oxygen appears to require promoter elements that are different from those used by hydrogen peroxide. ROS generated endogenously by treatment of the alga with metronidazole, protoporphyrin IX, dinoterb or high light intensities were detected by this reporter system, and distinguished as production of hydrogen peroxide (metronidazole) and singlet oxygen (protoporphyrin IX, dinoterb, high light). This system thus makes it possible to test whether, under varying environmental conditions including the application of abiotic stress, hydrogen peroxide or singlet oxygen or both are produced.

Acclimation to singlet oxygen stress in Chlamydomonas reinhardtii

In an aerobic environment, responding to oxidative cues is critical for physiological adaptation (acclimation) to changing environmental conditions. The unicellular alga Chlamydomonas reinhardtii was tested for the ability to acclimate to specific forms of oxidative stress. Acclimation was defined as the ability of a sublethal pretreatment with a reactive oxygen species to activate defense responses that subsequently enhance survival of that stress. C. reinhardtii exhibited a strong acclimation response to rose bengal, a photosensitizing dye that produces singlet oxygen. This acclimation was dependent upon photosensitization and occurred only when pretreatment was administered in the light. Shifting cells from low light to high light also enhanced resistance to singlet oxygen, suggesting an overlap in high-light and singlet oxygen response pathways. Microarray analysis of RNA levels indicated that a relatively small number of genes respond to sublethal levels of singlet oxygen. Constitutive overexpression of either of two such genes, a glutathione peroxidase gene and a glutathione S-transferase gene, was sufficient to enhance singlet oxygen resistance. Escherichia coli and Saccharomyces cerevisiae exhibit well-defined responses to reactive oxygen but did not acclimate to singlet oxygen, possibly reflecting the relative importance of singlet oxygen stress for photosynthetic organisms.

7-Dehydrocholesterol enhances ultraviolet A-induced oxidative stress in keratinocytes: roles of NADPH oxidase, mitochondria, and lipid rafts

Long wavelength solar UVA radiation stimulates formation of reactive oxygen species (ROS) and prostaglandin E(2) (PGE(2)), which are involved in skin photosensitivity and tumor promotion. High levels of 7-dehydrocholesterol (7-DHC), the precursor to cholesterol, cause exaggerated photosensitivity to UVA in patients with Smith-Lemli-Opitz syndrome (SLOS). Partially replacing cholesterol with 7-DHC in keratinocytes rapidly (<5 min) increased UVA-induced ROS, intracellular calcium, phospholipase A(2) activity, PGE(2), and NADPH oxidase activity. UVA-induced ROS and PGE(2) production were inhibited in these cells by depleting the Nox1 subunit of NADPH oxidase using siRNA or using a mitochondrial radical quencher, MitoQ. Partial replacement of cholesterol with 7-DHC also disrupted membrane lipid raft domains, although depletion of cholesterol, which also disrupts lipid rafts, did not affect UVA-induced increases in ROS and PGE(2). Phospholipid liposomes containing 7-DHC were more rapidly oxidized by a free radical mechanism than those containing cholesterol. These results indicate that 7-DHC enhances rapid UVA-induced ROS and PGE(2) formation by enhancing free radical-mediated membrane lipid oxidation and suggests that this mechanism might underlie the UVA photosensitivity in SLOS.