UVA-induced Apoptosis Research Articles

1174 Drug-induced phototoxicity: Disruption of 6-formylindolo[3,2-b]carbazole metabolism sensitizes keratinocytes to UVA-induced apoptosis

1174 Drug-induced phototoxicity: Disruption of 6-formylindolo[3,2-b]carbazole metabolism sensitizes keratinocytes to UVA-induced apoptosis

Valorization of the Photo-Protective Potential of the Phytochemically Standardized Olive (Olea europaea L.) Leaf Extract in UVA-Irradiated Human Skin Fibroblasts.

Leaves of Olea europaea are a by-product of the olive oil industry and a dietary supplement with acknowledged antioxidant and anti-inflammatory activity but underestimated photoprotective potential. We investigated the protective effects of the LC-PDA-MS/MS standardized ethanol-water extract of olive leaves (OLE), containing 26.2% total phenols and 22.2% oleuropein, with underlying mechanisms against the UVA-induced oxidative damage in human dermal fibroblasts. Hs68 cells were pre-treated (24 h) with OLE (2.5–25 μg/mL) or the reference antioxidants, quercetin and ascorbic acid (25 μg/mL), followed by irradiation (8 J/cm2). OLE significantly reduced the UVA-induced DNA damage and reactive oxygen species (ROS) overproduction and increased the thioredoxin reductase (TrxR) expression and post-radiation viability of fibroblasts by inhibiting their apoptosis. Both intrinsic and extrinsic apoptotic signaling pathways appeared to be inhibited by OLE, but the activity of caspase 9 was the most reduced. We hypothesized that the TrxR up-regulation by OLE could have prevented the UVA-induced apoptosis of Hs68 cells. In addition, a significant decrease in UVA-induced secretion levels of tumor necrosis factor (TNF-α) and interleukin-2 (IL-2) was shown in human lymphocyte culture in response to OLE treatment. In summary, our results support the beneficial effect of OLE in an in vitro model and indicate its great potential for use in the cosmetic and pharmaceutical industry as a topical photoprotective, antioxidant, and anti-inflammatory agent.

Poly(ADP-ribose) polymerase inhibitor PJ34 protects against UVA-induced oxidative damage in corneal endothelium

Fuchs endothelial corneal dystrophy (FECD) is one of the main causes for corneal endothelial blindness, which is characterized by the progressive decline of corneal endothelial cells. Poly (ADP-ribose) polymerase (PARP) was reported to be involved in cell death and apoptosis of several diseases. However, the role of PARP1 in the progression of FECD remains elusive. In the present study, we reported that UVA irradiation caused the corneal endothelial damage and corneal edema in mice, which was accompanied with the elevated activity of PARP1 and PAR. The PARP1 inhibitor PJ34 resolved the corneal edema and protected corneal endothelium from UVA-induced oxidative damage, mitochondrial dysfunction, and cell apoptosis. Mechanistically, PARP1 inhibition exerted its anti-apoptotic effects through downregulation of the phosphorylation levels of JNK1/2 and p38 MAPK and subsequentlythe increase of MKP-1. Our results suggest that PARP1 inhibition protects corneal endothelium from UVA-induced oxidative damage, which provides a potential alternative strategy for the therapy of FECD.

Akt activation‐dependent protective effect of wild ginseng adventitious root protein against UVA‐induced NIH‐3T3 cell damage

Prolonged skin exposure to ultraviolet radiation can lead to development of several acute and chronic diseases, with UVA exposure considered a primary cause of dermal photodamage. We prepared a wild ginseng adventitious root extract (ARE) that could alleviate UVA irradiation-induced NIH-3T3 cell viability decline. After employing a series of purification methods to isolate main active components of ARE, adventitious root protein mixture (ARP) was identified then tested for protective effects against UVA irradiation-induced NIH-3T3 cell damage. The results showed that ARP treatment significantly reduced UVA-induced cell viability decline and confirmed that the active constituent of ARP was the protein, since proteolytic hydrolysis and heat treatment each eliminated ARP protective activity. Moreover, ARP treatment markedly inhibited UVA-induced apoptosis, cell cycle arrest and DNA fragmentation, while also significantly reversing UVA effects (elevated Bax levels, reduced Bcl-2 expression) by reducing Bax levels and increasing Bcl-2 expression. Mechanistically, ARP promoted Akt phosphorylation regardless of UVA exposure, thus confirming ARP resistance to inactivation by UVA light. Notably, in the presence of Akt inhibitor SC0227, ARP could no longer counteract UVA-induced cell viability decline and DNA fragmentation. Additionally, our results demonstrated that ARP treatment protected UVA-irradiated NIH-3T3 cells by preventing UVA-induced reduction of collagen-I expression. Taken together, these results suggest that ARP treatment of NIH-3T3 cells effectively mitigated UVA-induced cell viability decline by activating intracellular Akt to reduce UVA-induced DNA damage, leading to reduced rates of apoptosis and cell cycle arrest after UVA exposure and restoring collagen expression to normal levels.

Inhibition of 6-formylindolo[3,2-b]carbazole metabolism sensitizes keratinocytes to UVA-induced apoptosis: Implications for vemurafenib-induced phototoxicity

Ultraviolet (UV) B irradiation of keratinocytes results in the formation of the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole (FICZ) which is a high-affinity ligand for the aryl hydrocarbon receptor (AHR). The resulting activation of AHR signaling induces the expression of cytochrome P450 (CYP) 1A1 which subsequently metabolizes FICZ. Importantly, FICZ is also a nanomolar photosensitizer for UVA radiation. Here, we assess whether a manipulation of the AHR-CYP1A1 axis in human epidermal keratinocytes affects FICZ/UVA-induced phototoxic effects and whether this interaction might be mechanistically relevant for the phototoxicity of the BRAF inhibitor vemurafenib. Treatment of keratinocytes with an AHR agonist enhanced the CYP1A1-catalyzed metabolism of FICZ and thus prevented UVA photosensitization, whereas an inhibition of either AHR signaling or CYP1A1 enzyme activity resulted in an accumulation of FICZ and a sensitization to UVA-induced oxidative stress and apoptosis. Exposure of keratinocytes to vemurafenib resulted in the same outcome. Specifically, CYP phenotyping revealed that vemurafenib is primarily metabolized by CYP1A1 and to a lesser degree by CYP2J2 and CYP3A4. Hence, vemurafenib sensitized keratinocytes to UVA-induced apoptosis by interfering with the CYP1A1-mediated oxidative metabolism of FICZ. In contrast to this pro-apoptotic effect, a treatment of UVB-damaged keratinocytes with vemurafenib suppressed apoptosis, a process which might contribute to the skin carcinogenicity of the drug. Our results provide insight into the mechanisms responsible for the photosensitizing properties of vemurafenib and deliver novel information about its metabolism which might be relevant regarding potential drug-drug interactions. The data emphasize that the AHR-CYP1A1 axis contributes to the pathogenesis of cutaneous adverse drug reactions.

753 The UVB-induced tryptophan photoproduct 6-formylindolo [3,2-]carbazole (FICZ) activates aryl hydrocarbon receptor (AHR) signaling in keratinocytes to prevent UVA-induced apoptosis

753 The UVB-induced tryptophan photoproduct 6-formylindolo [3,2-]carbazole (FICZ) activates aryl hydrocarbon receptor (AHR) signaling in keratinocytes to prevent UVA-induced apoptosis

Apoptosis Induced by Ultraviolet A Exposure in the Presence of Enoxacin in HL-60 Cells.

The ultraviolet A (UVA) spectrum mainly includes the region associated with the phototoxicity of fluoroquinolone antimicrobial agents. This study investigated apoptosis induced with UVA light and enoxacin in HL-60 cells. HL-60 cells were irradiated by UVA (1.1 mW/cm2) for 20 min in the presence or absence of enoxacin. The induction of apoptosis was investigated by analysing cell morphology, flow cytometry of annexin V-positive cells, DNA ladder formation, and caspase-3 activation. Significant induction of apoptosis, DNA fragmentation, and caspase-3 activation were observed in cells treated with both UVA and enoxacin. UVA-induced apoptosis was significantly suppressed when NaN3, a singlet oxygen scavenger, was present. Apoptosis was induced by the combination of UVA and enoxacin in HL-60 cells, and singlet oxygen appears to play an important role in photodynamically-induced apoptosis.

636 The tryptophan photoproduct 6-formylindolo[3,2-b]carbazole sensitizes keratinocytes to UVA-induced apoptosis: Clues for a cytoprotective role of the aryl hydrocarbon receptor

636 The tryptophan photoproduct 6-formylindolo[3,2-b]carbazole sensitizes keratinocytes to UVA-induced apoptosis: Clues for a cytoprotective role of the aryl hydrocarbon receptor

HOPX functions as a tumour suppressor in head and neck cancer.

Head and neck squamous cell carcinoma (HNSCC) is generalized term that encompasses a diverse group of cancers that includes tumours of the oral cavity (OSCC), oropharynx (OPSCC) and nasopharynx (NPC). Genetic alterations that are common to all HNSCC types are likely to be important for squamous carcinogenesis. In this study, we have investigated the role of the homeodomain-only homeobox gene, HOPX, in the pathogenesis of HNSCC. We show that HOPX mRNA levels are reduced in OSCC and NPC cell lines and tissues and there is a general reduction of HOPX protein expression in these tumours and OPSCCs. HOPX promoter methylation was observed in a subset of HNSCCs and was associated with a worse overall survival in HPV negative tumours. RNAseq analysis of OSCC cells transfected with HOPX revealed a widespread deregulation of the transcription of genes related to epithelial homeostasis and ectopic over-expression of HOPX in OSCC and NPC cells inhibited cell proliferation, plating efficiency and migration, and enhanced sensitivity to UVA-induced apoptosis. Our results demonstrate that HOPX functions as a tumour suppressor in HNSCC and suggest a central role for HOPX in suppressing epithelial carcinogenesis.

Propolis Inhibits UVA-Induced Apoptosis of Human Keratinocyte HaCaT Cells by Scavenging ROS.

Propolis is a resinous material collected by honeybees from several plant sources. This research aimed at showing its protective effect against UVA-induced apoptosis of human keratinocyte HaCaT cells. Using Hoechst staining, it was demonstrated that propolis (5 and 10 μg/mL) significantly inhibited the apoptosis of HaCaT cells induced by UVA-irradiation. Propolis also showed the protective effect against loss of mitochondrial membrane potential induced by UVA-irradiaiton in HaCaT cells. Propolis also inhibited the expression of activated caspase-3 induced by UVA-irradiation. To investigate the role of ROS in UVA-induced apoptosis and protection by propolis, the generation of ROS was determined in cells. The results showed that the generation of ROS was markedly reduced in cells pretreated with propolis. Consequently, propolis protected human keratinocyte HaCaT cells against UVA-induced apoptosis, which might be related to the reduction of ROS generation by UVA-irradiation.

Increased susceptibility to oxidative stress- and ultraviolet A-induced apoptosis in fibroblasts in atypical progeroid syndrome/atypical Werner syndrome with LMNA mutation.

Atypical progeroid syndrome (APS), including atypical Werner syndrome (AWS), is a disorder of premature ageing caused by mutation of the lamin A gene, the same causal gene involved in Hutchinson-Gilford syndrome (HGS). We previously reported the first Japanese case of APS/AWS with a LMNA mutation (p.D300N). Recently, it has been reported that UVA induced abnormal truncated form of lamin A, called progerin, as well as HGS-like abnormal nuclear structures in normal human fibroblasts, being more frequent in the elderly, suggesting that lamin A may be involved in the regulation of photoageing. The objective of this study was to elucidate the sensitivity to cell damage induced by oxidative stress or UVA in fibroblasts from APS/AWS patient. Using immunofluorescence staining and flow cytometry analysis, the amount of early apoptotic cells and degree of intra-cellular reactive oxygen species (ROS) generation were higher in H2 02 - or UVA-treated APS/AWS fibroblasts than in normal fibroblasts, suggesting that repeated UV exposure may induce premature ageing of the skin in APS/AWS patients and that protecting against sunlight is possibly important for delaying the emergence of APS/AWS symptoms. In addition, we demonstrated that H2 O2 -, or UVA-induced apoptosis and necrosis in normal and APS/AWS fibroblasts were enhanced by farnesyltransferase inhibitor (FTI) treatment, indicating that FTI might not be useful for treating our APS/AWS patient.

The dermato-protective effects of lucidone from Lindera erythrocarpa through the induction of Nrf2-mediated antioxidant genes in UVA-irradiated human skin keratinocytes

This study investigated the dermato-protective effects of lucidone, a cyclopentenedione from Lindera erythrocarpa dried fruits, against UVA-induced oxidative stress and apoptosis in human keratinocytes (HaCaT). Pre-treatment with lucidone (1–4 µM) significantly protects keratinocytes from UVA (15 J/cm2)-induced cell death, excessive ROS generation, LDH release, lipid peroxidation, and DNA damage. Notably, lucidone inhibited the UVA-induced apoptosis of HaCaT cells as measured by a reduction of DNA fragmentation, mitochondria dysfunction, and Bcl-2/Bax dysregulation. Furthermore, antioxidant potential of lucidone was directly correlated with the induction of antioxidant genes, including HO-1, NQO-1, and γ-GCLC by transcriptional activation of Nrf2. Treatment with hydrogen peroxide showed cellular byproducts of UVA irradiation-induced cell death, LDH release, and ROS generation, but these phenomena were inhibited by pre-treatment of lucidone. Besides, in Nrf2 knock-down cells, lucidone failed to protect UVA-induced oxidative stress or cell death. Our findings suggest that lucidone is capable of protecting skin cells from UVA-irradiated damage.

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/100gfw; total anthocyanins 297.4mg/100gfw) 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 (8–40J/cm2) and UVB (0.008–0.72J/cm2) 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.

Molecular mechanism of polypeptides from Chlamys farreri (PCF)’s anti-apoptotic effect in UVA-exposed HaCaT cells involves HSF1/HSP70, JNK, XO, iNOS and NO/ROS

This study investigated the molecular mechanisms of polypeptides from Chlamys farreri (PCF)’s anti-apoptotic effects in ultraviolet A-rays (UVA) exposed HaCaT cells. UVA-induced apoptosis in HaCaT cells was confirmed with Hoechst 33258 fluorescent staining; PCF treatment inhibited UVA-induced apoptosis in HaCaT cells, increased transcriptional activities of heat shock factor protein 1 (HSF1) and the expression of heat shock protein 70 (HSP70), whereas inhibited activation of c-Jun N-terminal kinases (JNK), expression of xanthine oxidese (XO), inducible nitric oxide synthase (iNOS) and release of nitric oxide (NO)/reactive oxygen species (ROS). Meanwhile, the HSF1 transcription inhibitor quercetin increased UVA-induced apoptosis, activation of JNK, expression of XO and iNOS and release of NO/ROS. Among the two NO release peaks we found in UVA exposed HaCaT cells, XO inhibitor oxypurinol was found to be able to inhibit NO release at 3h post UVA exposure but not 18h, while iNOS inhibitor S-methylisothiourea sulfate (SMT) was found to inhibit iNOS expression and NO release at 18h but not 3h. PCF’s protection against UVA-induced apoptosis in HaCaT cells involves increased transcriptional activity of HSF1, increased expression of HSP70, and the subsequential inhibition of JNK pathway, XO and iNOS expression and ROS/NO release.

Baicalin inhibits ultraviolet A-induced oxidative damage and apoptosis in human fibroblasts

Objective To investigate the inhibitory effect of baicalin on ultraviolet A (UVA)-induced oxidative damage and apoptosis in human diploid fibroblasts (HDFs) and its possible mechanism.Methods Diploid fibroblasts were isolated from healthy adult male foreskin tissue and subjected to primary culture and subculture.To determine the working concentrations of baicalin,CCK-8 assay was performed to evaluate the proliferation of HDFs treated with various concentrations (3.125-100 mg/L) of baicalin alone or in combination with UVA irradiation.Then,some HDFs were divided into 5 groups:the control group receiving no treatment,UVA group irradiated with UVA alone,3 combination groups irradiated with UVA followed by treatment with baicalin at 6.25 mg/L,12.5 mg/L and 25 mg/L,respectively.After additional culture for 24 hours,fluorescence microscopy and flow cytometry were performed to detect and quantify reactive oxygen species,superoxide anion and nitric oxide,flow cytometry to determine mitochondrial membrane potentials and cell apoptosis rate,and Western blot to measure the expression of caspase 3.One way analysis of variance was carried out to compare these parameters among these groups.Results Compared with the control group,the UVA group showed a significantly higher level of reactive oxygen species,superoxide anion and nitric oxide (813.1 ± 30.29 vs.442.5 ± 29.45,353.3 ± 19.13 vs.223.4 ± 10.67,107.1 ± 11.44 vs.42.17 ± 2.59,all P ＜ 0.05),together with elevated mitochondrial membrane potential (771.6 ± 68.27 vs.429.5 ± 27.14,P＜ 0.05) and apoptosis rate (18.14 ± 2.492 vs.2.90 ± 0.574,P＜0.05).There was a significant decrease in the level of reactive oxygen species,superoxide anion and nitric oxide,mitochondrial membrane potential and apoptosis rate in the 3 combination groups compared with the UVA group (all P ＜ 0.01).The relative expression level of caspase-3 was significantly higher in the UVA group than in the control group (1.2680 ± 0.0091 vs.0.3675 ± 0.1115,P ＜ 0.05),but statistically lower in the 3 combination groups irradiated with UVA followed by treatment with baicalin of 6.25,12.5 and 25 mg/L,respectively than in the UVA group (0.5588 ± 0.1705,0.5365 ± 0.1718 and 0.5454 ± 0.2083 vs.1.2680 ± 0.0091,all P ＜ 0.01).Conclusions Baicalin can attenuate UVA-induced oxidative damage and apoptosis in HDFs,likely by scavenging reactive radicals,inhibiting mitochondrial membrane depolarization,and blocking the activation and expression of the downstream effector of apoptosis,caspase-3. Key words: Ultraviolet rays; Fibroblasts; BAICALIN; Oxidative stress; Apoptosis

Protective role of AQP3 in UVA-induced NHSFs apoptosis via Bcl2 up-regulation

Aquaporin-3 (AQP3), a water/glycerol-transporting protein that facilitates water, urea, and glycerol transport, can inhibit arsenite-induced apoptosis by up-regulating Bcl-2. However, whether it has a protective role in ultraviolet A (UVA)-induced apoptosis in normal human skin fibroblasts is not known. In this study, we demonstrate that mild UVA treatment fails to induce oxidative cell stress and apoptosis in normal human skin fibroblasts (NHSFs) overexpressing AQP3. After severe UVA irradiation, there was an increase in oxidative cell stress and apoptosis when AQP3 levels decreased. We also found that silencing AQP3 sensitized NHSFs to low-dose UVA. Overexpressing AQP3 was protective against high-dose UVA-induced oxidative stress and apoptosis. Besides, we observed that Bcl-2 may be involved in UVA-induced apoptosis. Our findings suggested that the water/glycerol-transporting protein AQP3 plays a role in resistance to UVA-induced apoptosis.

Cyclophilin 40 alters UVA-induced apoptosis and mitochondrial ROS generation in keratinocytes

The CyP40 protein encoded by PPID gene is a member of the peptidyl-prolyl cis–trans isomerase (PPIase) family. PPIases catalyze the cis–trans isomerization of proline imidic peptide bonds in oligopeptides and accelerate the folding of proteins. The CyP40 protein has been shown to possess PPIase activity and, similar to other family members, can bind to the immunosuppressant drug cyclosporin A (CsA). In this study, we created keratinocyte cell lines with CyP40 being stably knocked down using viral particles containing shRNA for CyP40 which knocked down the expression level of CyP40 transcripts by 90–99%. The proliferation rates of the cell lines with silenced CyP40 were decreased compared to the control cells. After UVA irradiation, the rate of apoptosis was found to be significantly lower in CyP40 silenced cell lines than it was in control cells. Moreover, mitochondrial membrane potential (MMP) was found to be less dissipated and mitochondrial permeability transition pore (MPTP) less active in cells with knocked down CyP40 than in control cells after UVA irradiation. Also, less mitochondrial superoxide was detected in the cells with silenced CyP40 compared to control cells after UVA exposure. Moreover, silencing of CyP40 partially modulates expression of key genes involved in mitochondrial pore formation including CyPD, ANTs and VDAC family members. The ability of CyP40 to regulate UV induced apoptosis implicates this protein as a potential target for therapy in cancer cells.

Baicalin protects human skin fibroblasts from ultraviolet A radiation-induced oxidative damage and apoptosis

Reactive oxygen species (ROS) are an important factor in the development of skin photodamage after ultraviolet A (UVA) radiation. A flavonoid antioxidant, baicalin, can selectively neutralize super-oxide anion (O2−) while having no significant effect on •OH. Fibroblasts are a key component of skin dermis. In the present study, we investigated the protective effects of baicalin on human skin fibroblasts (HSFs) under UVA induced oxidative stress. Fluorescence microscopy and flow cytometry were used to assay the intracellular O2−, NO, ROS concentrations and the mitochondrial membrane potential. Cell viability was determined using the Cell Counting Kit-8 (CCK-8). The concentrations of cellular MDA, SOD, GSH, T-AOC, and 8-oxo-dG were also measured. Cellular apoptosis was measured by flow cytometry and caspase-3 detection. The results revealed that UVA radiation could cause oxidative stress and apoptosis in HSFs. Interestingly, the use of baicalin after UVA radiation significantly reduced the level of intracellular O2−, NO, and ROS, stabilized the mitochondrial membrane potential, and attenuated production of MDA and 8-oxo-dG. These efficiently enhanced the antioxidative defense system and protected the HSFs from subsequent oxidative stress damage and apoptosis. In other words, baicalin decreased the excessive generation of intracellular ROS and NO, and elevated the cellular antioxidative defense, which eventually mitigate the UVA-induced apoptosis. Based on our results, baicalin may have applications in the treatment of skin photodamage caused by UVA irradiation.

Ellagic acid protects human keratinocyte (HaCaT) cells against UVA-induced oxidative stress and apoptosis through the upregulation of the HO-1 and Nrf-2 antioxidant genes

UV radiation from the sun is a potent environmental risk factor in the pathogenesis of skin damage. Much of the skin damage caused by ultraviolet A (UVA) irradiation from the sun is associated with oxidative stress. The aim of this study was to investigate the protective role of ellagic acid (25–75μM), a natural antioxidant, against UVA (5–20J/cm2)-induced oxidative stress and apoptosis in human keratinocyte (HaCaT) cells and to reveal the possible mechanisms underlying this protective efficacy. Ellagic acid pre-treatment markedly increased HaCaT cell viability and suppressed UVA-induced ROS generation and MDA formation. Moreover, ellagic acid pre-treatment prevented UVA-induced DNA damage as evaluated by the comet assay. Ellagic acid treatment also significantly inhibited the UVA-induced apoptosis of HaCaT cells, as measured by a reduction of DNA fragmentation, mitochondria dysfunction, ER stress, caspase-3 activation, and Bcl-2/Bax deregulation. Notably, the antioxidant potential of ellagic acid was directly correlated with the increased expression of HO-1 and SOD, which was followed by the downregulation of Keap1 and the augmented nuclear translocation and transcriptional activation of Nrf2 with or without UVA irradiation. Nrf2 knockdown diminished the protective effects of ellagic acid. Therefore, ellagic acid may be useful for the treatment of UVA-induced skin damage.

Silibinin Is a Potent Sensitizer of UVA Radiation‐induced Oxidative Stress and Apoptosis in Human Keratinocyte HaCaT Cells†

UVA radiation (315-400 nm), which constitutes ca 95% of the UV irradiation in natural sunlight reaching earth surface, is a major environmental risk factor associated with human skin cancer pathogenesis. UVA is an oxidizing agent that causes significant damage to cellular components through the release of reactive oxygen species (ROS) and leads to photoaging and photocarcinogenesis. Here we investigate the effect of silibinin, the flavonolignan from Silybum marianum, on UVA-induced ROS and cell death in human keratinocyte cell line HaCaT. In addition, the effect of silibinin on UVA-induced intracellular ROS-mediated endoplasmic reticulum (ER) stress was also analyzed. UVA irradiation resulted in ROS production and apoptosis in HaCaT cells in a dose-dependent manner, and the ROS levels and apoptotic index were found to be elevated significantly when the cells were treated with 75 μmsilibinin for 2 h before UVA exposure. When the cells were pretreated with 10 mmN-acetyl cysteine, the enhancement of UVA-induced apoptosis by silibinin was compromised. Furthermore, we found that silibinin enhances ER stress-mediated apoptosis in HaCaT cells by increasing the expression of CHOP protein. These results suggest that silibinin may be beneficial in the removal of UVA-damaged cells and the prevention of skin cancer.