Melanocytes and keratinocytes have distinct and shared responses to ultraviolet radiation and arsenic

Nutrition and melanoma: the contribution of trace elements in onset, progression, and treatment of melanoma

Melanoma is a highly malignant and drug-resistant disease that imposes a substantial economic burden on the world. There are many studies linking trace elements to diverse types of cancers, including melanoma. This review elucidates the relationship between trace elements exposure and melanoma. It was identified that copper, manganese, selenium, zinc, iron, and many other trace elements were associated with melanoma in humans. In terms of epidemiology, different elements have different correlations with melanoma. These trace elements affect the occurrence and development of melanoma through various mechanisms, such as oxidative stress and the MAPK pathway. The literature on the role of trace elements in the pathogenesis and treatment of melanoma depicts promising prospects for this field.

Arsenic is a potent co-mutagen of ultraviolet light

Arsenic enhances the carcinogenicity of ultraviolet radiation (UVR). However, the mechanisms of arsenic-driven oncogenesis are not well understood. Here, we utilize experimental systems to investigate the carcinogenic and mutagenic properties of co-exposure to arsenic and UVR. In vitro and in vivo exposures indicate that, by itself, arsenic is not mutagenic. However, in combination with UVR, arsenic exposure has a synergistic effect leading to an accelerated mouse skin carcinogenesis and to more than 2-fold enrichment of UVR mutational burden. Notably, mutational signature ID13, previously found only in UVR-associated human skin cancers, is observed exclusively in mouse skin tumors and cell lines jointly exposed to arsenic and UVR. This signature was not observed in any model system exposed purely to arsenic or purely to UVR, making ID13, to the best of our knowledge, the first co-exposure signature to be reported using controlled experimental conditions. Analysis of existing skin cancer genomics data reveals that only a subset of cancers harbor ID13 and these exhibit an elevated UVR mutagenesis. Our results report a unique mutational signature caused by a co-exposure to two environmental carcinogens and provide comprehensive evidence that arsenic is a potent co-mutagen and co-carcinogen of UVR.

Arsenic is a potent co-mutagen of ultraviolet light

Environmental co-exposures are widespread and are major contributors to carcinogenic mechanisms. Two well-established environmental agents causing skin cancer are ultraviolet radiation (UVR) and arsenic. Arsenic is a known co-carcinogen that enhances UVR's carcinogenicity. However, the mechanisms of arsenic co-carcinogenesis are not well understood. In this study, we utilized primary human keratinocytes and a hairless mouse model to investigate the carcinogenic and mutagenic properties of co-exposure to arsenic and UVR. In vitro and in vivo exposures revealed that, on its own, arsenic is neither mutagenic nor carcinogenic. However, in combination with UVR, arsenic exposure has a synergistic effect leading to an accelerated mouse skin carcinogenesis as well as to more than 2-fold enrichment of UVR mutational burden. Notably, mutational signature ID13, previously found only in UVR-associated human skin cancers, was observed exclusively in mouse skin tumors and cell lines jointly exposed to arsenic and UVR. This signature was not observed in any model system exposed purely to arsenic or purely to UVR, making ID13 the first co-exposure signature to be reported using controlled experimental conditions. Analysis of existing genomics data from basal cell carcinomas and melanomas revealed that only a subset of human skin cancers harbor ID13 and, consistent with our experimental observations, these cancers exhibited an elevated UVR mutagenesis. Our results provide the first report of a unique mutational signature caused by a co-exposure to two environmental carcinogens and the first comprehensive evidence that arsenic is a potent co-mutagen and co-carcinogen of UVR. Importantly, our findings suggest that a large proportion of human skin cancers are not formed purely due to UVR exposure but rather due to a co-exposure of UVR and other co-mutagens such as arsenic.

Arsenic and cancer: Evidence and mechanisms

Arsenic is a potent carcinogen and poses a significant health concern worldwide. Exposure occurs through ingestion of drinking water and contaminated foods and through inhalation due to pollution. Epidemiological evidence shows arsenic induces cancers of the skin, lung, liver, and bladder among other tissues. While studies in animal and cell culture models support arsenic as a carcinogen, the mechanisms of arsenic carcinogenesis are not fully understood. Arsenic carcinogenesis is a complex process due its ability to be metabolized and because of the many cellular pathways it targets in the cell. Arsenic metabolism and the multiple forms of arsenic play distinct roles in its toxicity and contribute differently to carcinogenic endpoints, and thus must be considered. Arsenic generates reactive oxygen species increasing oxidative stress and damaging DNA and other macromolecules. Concurrently, arsenic inhibits DNA repair, modifies epigenetic regulation of gene expression, and targets protein function due its ability to replace zinc in select proteins. While these mechanisms contribute to arsenic carcinogenesis, there remain significant gaps in understanding the complex nature of arsenic cancers. In the future improving models available for arsenic cancer research and the use of arsenic induced human tumors will bridge some of these gaps in understanding arsenic driven cancers.

Human Variation in DNA Repair, Immune Function, and Cancer Risk

DNA damage constantly threatens genome integrity, and DNA repair deficiency is associated with increased cancer risk. An intuitive and widely accepted explanation for this relationship is that unrepaired DNA damage leads to carcinogenesis due to the accumulation of mutations in somatic cells. But DNA repair also plays key roles in the function of immune cells, and immunodeficiency is an important risk factor for many cancers. Thus, it is possible that emerging links between inter-individual variation in DNA repair capacity and cancer risk are driven, at least in part, by variation in immune function, but this idea is underexplored. In this review we present an overview of the current understanding of the links between cancer risk and both inter-individual variation in DNA repair capacity and inter-individual variation in immune function. We discuss factors that play a role in both types of variability, including age, lifestyle, and environmental exposures. In conclusion, we propose a research paradigm that incorporates functional studies of both genome integrity and the immune system to predict cancer risk and lay the groundwork for personalized prevention.

Ambient UVR and Environmental Arsenic Exposure in Relation to Cutaneous Melanoma in Iowa

Intermittent sun exposure is the major environmental risk factor for cutaneous melanoma (CM). Cumulative sun exposure and other environmental agents, such as environmental arsenic exposure, have not shown consistent associations. Ambient ultraviolet radiation (UVR) was used to measure individual total sun exposure as this is thought to be less prone to misclassification and recall bias. Data were analyzed from 1096 CM cases and 1033 controls in the Iowa Study of Skin Cancer and Its Causes, a population-based, case-control study. Self-reported residential histories were linked to satellite-derived ambient UVR, spatially derived environmental soil arsenic concentration, and drinking water arsenic concentrations. In men and women, ambient UVR during childhood and adolescence was not associated with CM but was positively associated during adulthood. Lifetime ambient UVR was positively associated with CM in men (OR for highest vs. lowest quartile: 6.09, 95% confidence interval (CI) 2.21–16.8), but this association was not as strong among women (OR for highest vs. lowest quartile: 2.15, 95% CI 0.84–5.54). No association was detected for environmental soil or drinking water arsenic concentrations and CM. Our findings suggest that lifetime and adulthood sun exposures may be important risk factors for CM.

Contribution of NADPH oxidase to the retention of UVR-induced DNA damage by arsenic

Arsenic is a naturally occurring element present in food, soil and water and human exposure is associated with increased cancer risk. Arsenic inhibits DNA repair at low, non-cytotoxic concentrations and amplifies the mutagenic and carcinogenic impact of other DNA-damaging agents, such as ultraviolet radiation (UVR). Arsenic exposure leads to oxidation of zinc coordinating cysteine residues, zinc loss and decreased activity of the DNA repair protein poly(ADP)ribose polymerase (PARP)-1. Because arsenic stimulates NADPH oxidase (NOX) activity leading to generation of reactive oxygen species (ROS), the goal of this study was to investigate the role of NOX in arsenic-induced inhibition of PARP activity and retention of DNA damage. NOX involvement in the arsenic response was assessed in vitro and in vivo. Keratinocytes were treated with or without arsenite, solar-simulated UVR, NOX inhibitors and/or isoform specific NOX siRNA. Knockdown or inhibition of NOX decreased arsenite-induced ROS, PARP-1 oxidation and DNA damage retention, while restoring arsenite inhibition of PARP-1 activity. The NOX2 isoform was determined to be the major contributor to arsenite-induced ROS generation and DNA damage retention. In vivo DNA damage was measured by immunohistochemical staining and analysis of dorsal epidermis sections from C57BI/6 and p91phox knockout (NOX2^-/-) mice. There was no significant difference in solar-simulated UVR DNA damage as detected by percent PH2AX positive cells within NOX2^-/- mice versus control. In contrast, arsenite-dependent retention of UVR-induced DNA damage was markedly reduced. Altogether, the in vitro and in vivo findings indicate that NOX is involved in arsenic enhancement of UVR-induced DNA damage.

Viability of cultured human skin cells treated with 1,6-hexamethylene diisocyanate monomer and its oligomer isocyanurate in different culture media

The isocyanate monomer 1,6-hexamethylene diisocyanate (HDI) and one of its trimers, HDI isocyanurate, are airway and skin sensitizers contained in polyurethane paint. The toxic response of cultured skin cells to these compounds was measured by evaluating the isocyanate concentrations at which 50% of the cells die (i.e., lethal concentration 50%, LC₅₀) because the relative toxicity of each form of HDI should be considered when exposure limits of HDI-based paints are set. By using a luminescent ATP-viability assay, we compared the cytotoxic effects of HDI monomer and HDI isocyanurate on cultured human skin cells (keratinocytes, fibroblasts, and melanocytes) after 4-h isocyanate exposures using culture media with varying levels of nutrients in order to also determine the effects of media composition on isocyanate toxicity. Before analysis, experimental wells were normalized to controls containing cells that were cultured with the same vehicle and media. The measured mean LC₅₀ values ranged from 5 to 200 µM across the experimental conditions, in which HDI isocyanurate in protein-devoid media was the most toxic to cells, producing the lowest LC₅₀ values. For HDI monomer, keratinocytes were the most resistant to its toxicity and melanocytes were the most susceptible. However, when exposed to HDI isocyanurate, the opposite was observed, with melanocytes being the most resilient and the keratinocytes and fibroblasts were more susceptible. Depending on the type of skin cells, dose–response data indicated that HDI isocyanurate was 2–6 times more toxic than HDI monomer when using protein-devoid media whereas HDI isocyanurate was 4–13 times more toxic than HDI monomer when protein-rich media was used. Therefore, if the protein-devoid saline medium alone were used for these experiments, then a significant under-estimation of their relative toxicities in protein-rich environments would have resulted. This difference is because HDI monomer toxicity was more attenuated by the presence of protein in the culture media than HDI isocyanurate toxicity. Thus, conclusions based on comparative toxicity studies and consequent inference applied to potential human toxicity can be affected by in vitro culture media conditions. The physiochemical difference in reactivity of the two forms of HDI to biological molecules most likely explains the observed toxicity differences and may have implications for skin penetration, adverse effects like skin sensitization, and systemic responses like asthma. Future studies are warranted to investigate differences in the biological availability, cellular toxicity, and immunologic sensitization mechanisms for HDI monomer and HDI isocyanurate.

Arsenic co-carcinogenesis: Inhibition of DNA repair and interaction with zinc finger proteins

Arsenic is widely present in the environment and is associated with various population health risks including cancers. Arsenic exposure at environmentally relevant levels enhances the mutagenic effect of other carcinogens such as ultraviolet radiation. Investigation on the molecular mechanisms could inform the prevention and intervention strategies of arsenic carcinogenesis and co-carcinogenesis. Arsenic inhibition of DNA repair has been demonstrated to be an important mechanism, and certain DNA repair proteins have been identified to be extremely sensitive to arsenic exposure. This review will summarize the recent advances in understanding the mechanisms of arsenic carcinogenesis and co-carcinogenesis, including DNA damage induction and ROS generation, particularly how arsenic inhibits DNA repair through an integrated molecular mechanism which includes its interactions with sensitive zinc finger DNA repair proteins.

Keratinocyte death by ferroptosis initiates skin inflammation after UVB exposure

The ultraviolet B radiation (UVB) causes skin inflammation, which contributes to the causality and the exacerbation of a number of cutaneous diseases. However, the mechanism of UVB-driven inflammation in the skin remains poorly understood. We show that ferroptosis, a non-apoptotic programmed cell death pathway that is promoted by an excessive phospholipid peroxidation, is activated in the epidermal keratinocytes after their exposure to UVB. The susceptibility of the keratinocytes to UVB-induced ferroptosis depends on the extent of pro-ferroptosis death signal generation and the dysregulation of the glutathione system. Inhibition of ferroptosis prevents the release of HMGB1 from the human epidermal keratinocytes, and blocks necroinflammation in the UVB-irradiated mouse skin. We show that while apoptosis and pyroptosis are also detectable in the keratinocytes after UVB exposure, ferroptosis plays a significant role in initiating UVB-induced inflammation in the skin. Our results have important implications for the prevention and the treatment of a broad range of skin diseases which are fostered by UVB-induced inflammation.

N6-methyladenosine mediates arsenite-induced human keratinocyte transformation by suppressing p53 activation

N⁶-methyladenosine (m⁶A), the most abundant and reversible RNA modification, plays critical a role in tumorigenesis. However, whether m⁶A can regulate p53, a leading antitumor protein remains poorly understood. In this study, we explored the regulatory role of m⁶A on p53 activation using an arsenite-transformed keratinocyte model, the HaCaT-T cell line. We created the cell line by exposing human keratinocyte HaCaT cells to 1 μM arsenite for 5 months. We found that the cells exhibited an increased m⁶A level along with an aberrant expression of the methyltransferases, demethylase, and readers of m⁶A. Moreover, the cells exhibited decreased p53 activity and reduced p53 phosphorylation, acetylation, and transactivation with a high nucleus export rate of p53. Knockdown of the m⁶A methyltransferase, METTL3 significantly decreased m⁶A level, restoring p53 activation and inhibiting cellular transformation phenotypes in the arsenite-transformed cells. Further, using both a bioinformatics analysis and experimental approaches, we demonstrated that m⁶A downregulated the expression of the positive p53 regulator, PRDM2, through the YTHDF2-promoted decay of PRDM2 mRNAs. We showed that m⁶A upregulated the expression of the negative p53 regulator, YY1 and MDM2 through YTHDF1-stimulated translation of YY1 and MDM2 mRNA. Taken together, our study revealed the novel role of m⁶A in mediating arsenite-induced human keratinocyte transformation by suppressing p53 activation. This study further sheds light on the mechanisms of arsenic carcinogenesis via RNA epigenetics.

Peroxynitrite contributes to arsenic-induced PARP-1 inhibition through ROS/RNS generation

Arsenic, in the trivalent form (AsIII), is a human co-carcinogen reported to enhance mutagenesis effects of other carcinogens such as UV radiation by inhibiting DNA repair. The zinc finger DNA repair protein Poly (ADP-ribose) polymerase 1 (PARP-1) is a sensitive target of AsIII and both reactive oxygen and nitrogen species (ROS/RNS) generated by AsIII contribute to PARP-1 inhibition. However, the mechanisms of ROS/RNS-mediated PARP inhibition and how AsIII-generated ROS/RNS may be interconnected are still unclear. In this study, we found AsIII exposure of normal human keratinocyte (HEKn) cells generated peroxynitrite through superoxide and nitric oxide production in an AsIII concentration dependent manner. Peroxynitrite inhibited PARP-1 activity and caused zinc loss from PARP-1 protein while scavenging peroxynitrite was protective of the impacts on PARP-1. We identified peroxynitrite was responsible for S-nitrosation on cysteine residues resulting in PARP-1 zinc finger conformational changes. Taken together, the evidence indicates AsIII generates peroxynitrite through superoxide and nitric oxide production, induces S-nitrosation on PARP-1, leading to zinc loss and activity inhibition of PARP-1, thus enhancing DNA damage caused by UV radiation. These findings highlight a role for peroxynitrite as a key molecule of ROS/RNS mediated DNA repair inhibition by AsIII which should inform the development of prevention and intervention strategies against AsIII co-carcinogenesis.

Exposure to Trace Elements and Risk of Skin Cancer: A Systematic Review of Epidemiologic Studies

Exposure to environmental trace elements has been studied in relation to many cancers. However, an association between exposure to trace elements and skin cancer remains less understood. Therefore, we conducted a systematic review of published epidemiologic literature examining the association between exposure to trace elements, and risk of melanoma and keratinocyte carcinoma in humans. We identified epidemiologic studies investigating exposure to arsenic, cadmium, chromium, copper, iron, selenium, and zinc and risk of skin cancer in humans. Among the minerals, arsenic, selenium, and zinc had more than five studies available. Exposure to arsenic was associated with increased risk of keratinocyte carcinoma, while too few studies existed on melanoma to draw conclusions. Exposure to selenium was associated with possible increased risk of keratinocyte carcinoma. Studies of zinc and skin cancer were case-control in design and were found to have inconsistent associations. The data on the association between cadmium, chromium, copper, and iron and risk of skin cancer remain too sparse to draw any conclusions. In summary, epidemiologic studies on exposure to trace elements and cutaneous malignancies are limited. Studies with larger sample sizes and prospective designs are warranted to improve our knowledge of trace elements and skin cancer.

Hydroquinone induces TK6 cell growth arrest and apoptosis through PARP-1/p53 regulatory pathway

Hydroquinone (HQ), one of the most important metabolites derived from benzene, induces cell cycle arrest and apoptosis. Poly(ADP-ribose) polymerase-1 (PARP-1) participates in various biological processes, including DNA repair and cell cycle regulation. To explore whether PARP-1 regulatory pathway mediated HQ-induced cell cycle arrest and apoptosis, we assessed the effect of PARP-1 suppression on induction of apoptosis analyzed by FACSCalibur flow cytometer in PARP-1 deficientTK6 cells (TK6-shPARP-1). We observed an increase in the fraction of cells in G1 phase by 7.6% and increased apoptosis by 4.5% in PARP-1-deficient TK6 cells (TK6-shPARP-1) compared to those negative control cells (TK6-shNC cells) in response to HQ treatment. Furthermore, HQ might activate the extrinsic pathways of apoptosis via up-regulation of Fas expression, followed by caspase-3 activation, apoptotic body, and sub G1 accumulation. Enhanced p53 expression was observed in TK6-shPARP-1 cells than in TK6-shNC cells after HQ treatment. In contrast, Fas expression was lower in TK6-shPARP-1 cells than in TK6-shNC cells. Therefore, we conclude that HQ may activate apoptotic signals via Fas up-regulation and p53-mediated apoptosis in TK6-shNC cells. The reduction of PARP-1 expression further intensified up-regulation of p53 in TK6-shPARP-1 cells, resulting in an increased G1→S phase cell arrest and apoptosis in TK6-shPARP-1 cells compared to TK6-shNC cells.

Induction of retinal-dependent calcium influx in human melanocytes by UVA or UVB radiation contributes to the stimulation of melanosome transfer

OBJECTIVES

The transfer of melanosomes from melanocytes to neighbouring keratinocytes is critical to protect the skin from the deleterious effects of ultraviolet A (UVA) and ultraviolet B (UVB) irradiation; however, the initial factor(s) that stimulates melanosome transfer remains unclear. In this study, we investigated the induction of retinal-dependent calcium (Ca²⁺ ) influx in melanocytes (MCs) by UVA or UVB irradiation and the effect of transient receptor potential cation channel subfamily M member 1 (TRPM1) (melastatin1)-related Ca²⁺ influx on melanosome transfer.

MATERIALS AND METHODS

Primary human epidermal MCs were exposed to physiological doses of UVB or UVA light and loaded with a calcium indicator Fluo-4 dye. The change of intracellular calcium of MCs was monitored using a two-photon confocal fluorescence microscopy. MCs were co-cultured with human epidermal keratinocytes (KCs) in the absence or presence of voriconazole (a TRPM1 blocker) or calcium chelators. MCs were also transfected with TRPM1 siRNA for silencing the expression of TRPM1 gene. The melanosome transfer in the co-cultured cells was quantitatively analysed using flow cytometry and was further confirmed by immunofluorescent double-staining. The protein levels and distributions of TRPM1, OPN3 and OPN5 in MCs were measured by Western blotting or immunofluorescent staining.

RESULTS

The retinal-dependent Ca²⁺ influx of UVA-exposed melanocytes differed greatly from that of UVB-exposed melanocytes in the timing-phase. The protein expression of TRPM1 in mono- and co-cultured MCs was dose-dependently up-regulated by UVA and UVB. TRPM1 siRNA-mediated knockdown and the blockage of TRPM1 channel using a putative antagonist (voriconazole) significantly inhibited melanosome transfer in co-cultures following UVA or UVB exposure.

CONCLUSIONS

The distinct time-phases of Ca²⁺ influx in MCs induced by UVA or UVB contribute to the consecutive stimulation of melanosome transfer, thereby providing a potent photoprotection against harmful UV radiation.

Inorganic arsenic inhibits the nucleotide excision repair pathway and reduces the expression of XPC

Chronic exposure to arsenic, most often through contaminated drinking water, has been linked to several types of cancer in humans, including skin and lung cancer. However, the mechanisms underlying its role in causing cancer are not well understood. There is evidence that exposure to arsenic can enhance the carcinogenicity of UV light in inducing skin cancers and may enhance the carcinogenicity of tobacco smoke in inducing lung cancers. The nucleotide excision repair (NER) pathway removes different types of DNA damage including those produced by UV light and components of tobacco smoke. The aim of the present study was to investigate the effect of sodium arsenite on the NER pathway in human lung fibroblasts (IMR-90 cells) and primary mouse keratinocytes. To measure NER, we employed a slot-blot assay to quantify the introduction and removal of UV light-induced 6-4 photoproducts (6-4 PP) and cyclobutane pyrimidine dimers (CPDs). We find a concentration-dependent inhibition of the removal of 6-4 PPs and CPDs in both cell types treated with arsenite. Treatment of both cell types with arsenite resulted in a significant reduction in the abundance of XPC, a protein that is critical for DNA damage recognition in NER. The abundance of RNA expressed from several key NER genes was also significantly reduced by treatment of IMR-90 cells with arsenite. Finally, treatment of IMR-90 cells with MG-132 abrogated the reduction in XPC protein, suggesting an involvement of the proteasome in the reduction of XPC protein produced by treatment of cells with arsenic. The inhibition of NER by arsenic may reflect one mechanism underlying the role of arsenic exposure in enhancing cigarette smoke-induced lung carcinogenesis and UV light-induced skin cancer, and it may provide some insights into the emergence of arsenic trioxide as a chemotherapeutic agent.

Distinctive molecular responses to ultraviolet radiation between keratinocytes and melanocytes

Solar ultraviolet radiation (UVR) is the major risk factor for skin carcinogenesis. To gain new insights into the molecular pathways mediating UVR effects in the skin, we performed comprehensive transcriptomic analyses to identify shared and distinctive molecular responses to UVR between human keratinocytes and melanocytes. Keratinocytes and melanocytes were irradiated with varying doses of UVB (10, 20 and 30 mJ/cm(2) ) then analysed by RNA-Seq at different time points post-UVB radiation (4, 24 and 72 h). Under basal conditions, keratinocytes and melanocytes expressed similar number of genes, although they each expressed a distinctive subset of genes pertaining to their specific cellular identity. Upon UVB radiation, keratinocytes displayed a clear pattern of time- and dose-dependent changes in gene expression that was different from melanocytes. The early UVB-responsive gene set (4 h post-UVR) differed significantly from delayed UVB-responsive gene sets (24 and 72 h). We also identified multiple novel UVB signature genes including PRSS23, SERPINH1, LCE3D and CNFN, which were conserved between melanocyte and keratinocyte lines from different individuals. Taken together, our findings elucidated both common and distinctive molecular features between melanocytes and keratinocytes and uncovered novel UVB signature genes that might be utilized to predict UVB photobiological effects on the skin.

Arsenic and ultraviolet radiation exposure: melanoma in a New Mexico non-Hispanic white population

Cases of cutaneous melanoma and controls were enrolled in a New Mexico population-based study; subjects were administered questionnaires concerning ultraviolet (UV) and inorganic arsenic (iAs) exposure. Historical iAs exposure was estimated. UV exposure estimates were also derived using geospatial methods. Drinking water samples were collected for iAs analysis. Blood samples were collected for DNA repair (Comet) and DNA repair gene polymorphism assays. Arsenic concentrations were determined in urine and toenail samples. UV exposures during the previous 90 days did not vary significantly between cases and controls. Mean (±SD) current home iAs drinking water was not significantly different for cases and controls [3.98 μg/L (±3.67) vs. 3.47 μg/L (±2.40)]. iAs exposure showed no effect on DNA repair or association with melanoma. Results did not corroborate a previously reported association between toenail As and melanoma risk. Arsenic biomarkers in urine and toenail were highly significantly correlated with iAs in drinking water. A UV-DNA repair interaction for UV exposure over the previous 7-90 days was shown; cases had higher DNA damage than controls at low UV values. This novel finding suggests that melanoma cases may be more sensitive to low-level UV exposure than are controls. A UV-APEX1 interaction was shown. Subjects with the homozygous rare APEX1 DNA repair gene allele had a higher risk of early melanoma diagnosis at low UV exposure compared with those with the homozygous wild type or the heterozygote. Notably, a UV-arsenic interaction on inhibition of DNA repair was not observed at iAs drinking water concentrations below 10 ppb (μg/L).

MiR-21: an environmental driver of malignant melanoma?

Since the mid-1950’s, melanoma incidence has been rising steadily in industrialized Caucasian populations, thereby pointing to the pivotal involvement of environmental factors in melanomagenesis. Recent evidence underlines the crucial role of microRNA (miR) signaling in cancer initiation and progression. Increased miR-21 expression has been observed during the transition from a benign melanocytic lesion to malignant melanoma, exhibiting highest expression of miR-21. Notably, common BRAF and NRAS mutations in cutaneous melanoma are associated with increased miR-21 expression. MiR-21 is an oncomiR that affects critical target genes of malignant melanoma, resulting in sustained proliferation (PTEN, PI3K, Sprouty, PDCD4, FOXO1, TIPE2, p53, cyclin D1), evasion from apoptosis (FOXO1, FBXO11, APAF1, TIMP3, TIPE2), genetic instability (MSH2, FBXO11, hTERT), increased oxidative stress (FOXO1), angiogenesis (PTEN, HIF1α, TIMP3), invasion and metastasis (APAF1, PTEN, PDCD4, TIMP3). The purpose of this review is to provide translational evidence for major environmental and individual factors that increase the risk of melanoma, such as UV irradiation, chemical noxes, air pollution, smoking, chronic inflammation, Western nutrition, obesity, sedentary lifestyle and higher age, which are associated with increased miR-21 signaling. Exosomal miR-21 induced by extrinsic and intrinsic stimuli may be superimposed on mutation-induced miR-21 pathways of melanoma cells. Thus, oncogenic miR-21 signaling may be the converging point of intrinsic and extrinsic stimuli driving melanomagenesis. Future strategies of melanoma treatment and prevention should thus aim at reducing the burden of miR-21 signal transduction.

Selective Sensitization of Zinc Finger Protein Oxidation by Reactive Oxygen Species through Arsenic Binding

Cysteine oxidation induced by reactive oxygen species (ROS) on redox-sensitive targets such as zinc finger proteins plays a critical role in redox signaling and subsequent biological outcomes. We found that arsenic exposure led to oxidation of certain zinc finger proteins based on arsenic interaction with zinc finger motifs. Analysis of zinc finger proteins isolated from arsenic-exposed cells and zinc finger peptides by mass spectrometry demonstrated preferential oxidation of C3H1 and C4 zinc finger configurations. C2H2 zinc finger proteins that do not bind arsenic were not oxidized by arsenic-generated ROS in the cellular environment. The findings suggest that selectivity in arsenic binding to zinc fingers with three or more cysteines defines the target proteins for oxidation by ROS. This represents a novel mechanism of selective protein oxidation and demonstrates how an environmental factor may sensitize certain target proteins for oxidation, thus altering the oxidation profile and redox regulation.

Poly(ADP-ribosyl)ation enhances H-RAS protein stability and causes abnormal cell cycle progression in human TK6 lymphoblastoid cells treated with hydroquinone

Hydroquinone (HQ), one of the most important benzene-derived metabolites, can induce aberrant cell cycle progression; however, the mechanism of this induction remains unclear. Poly(ADP-ribosyl)ation (PARylation), which is catalysed primarily by poly(ADP-ribose) polymerase-1 (PARP-1), participates in various biological processes, including cell cycle control. The results of the present study show an accumulation in G1 phase versus S phase of TK6 human lymphoblast cells treated with HQ for 48h compared with PBS-treated cells; after 72h of HQ treatment, the cells transitioned from G1 arrest to S phase arrest. We examined the expression of six genes related to the cell cycle or leukaemia to further explore the reason for this phenomenon. Among these genes, H-RAS was found to be associated with this phenomenon because its mRNA and protein expression decreased at 48h and increased at 72h. Experiments for PARP activity induction and inhibition revealed that the observed PARylation was positively associated with H-RAS expression. Moreover, in cells treated with HQ in conjunction with PARP-1 knockdown, expression of the H-RAS protein decreased and the number of cells in G1 phase increased. The degree of poly(ADP-ribosyl) modification of the H-RAS protein increased in cells treated with HQ for 72h, further supporting that changes in PARylation contributed to the rapid alteration of H-RAS protein expression, followed by abnormal progression of the cell cycle. Co-immunoprecipitation (co-IP) assays were employed to determine whether protein complexes were formed by PARP-1 and H-RAS proteins, and the direct interaction between these proteins indicated that PARylation regulated H-RAS expression. As detected by confocal microscopy, the H-RAS protein was found in the nucleus and cytoplasm. To our knowledge, this study is the first to reveal that H-RAS protein can be modified by PARylation.

Metallothionein isoform 3 expression in human skin, related cancers and human skin derived cell cultures

Human skin is a well known target site of inorganic arsenic with effects ranging from hyperkeratosis to dermal malignancies. The current study characterizes the expression of a protein known to bind inorganic, As(3+), metallothionein 3 (MT-3). Expression of this protein was assessed immunohistochemically with a specific MT-3 antibody on human formalin-fixed, paraffin-embedded biopsy specimens in normal skin, squamous cell carcinoma (SCC), basal cell carcinoma (BCC) and melanoma. Assessment in normal skin using nine normal specimens showed moderate to intense MT-3 staining in epidermal karatinocytes with staining extending into the basal cells and moderate to intense staining in melanocytes of nevi. MT-3 immunoexpression was shown to be moderate to intense in 12 of 13 of SCC, low to moderate in 8 of 10 BCC, and moderate to intense in 12 melanoma samples. MT-3 expression in cell culture models (normal human epidermal keratinocytes, normal human melanocytes, and HaCaT cells) showed only trace expression of MT-3, while exposures to the histone deacytalase inhibitor, MS-275, partially restored expression levels. These results indicate that the epidermis of human skin and resulting malignancies express high level of MT-3 and potentially impact on the known association of arsenic exposure and the development of skin disorders and related cancers.