Low-level arsenic impairs glucose-stimulated insulin secretion in pancreatic beta cells: involvement of cellular adaptive response to oxidative stress

Arsenic exposure and oxidative damage to lipid, DNA, and protein among general Chinese adults: A repeated-measures cross-sectional and longitudinal study

Arsenic-related oxidative stress and resultant diseases have attracted global concern, while longitudinal studies are scarce. To assess the relationship between arsenic exposure and systemic oxidative damage, we performed two repeated measures among 5236 observations (4067 participants) in the Wuhan-Zhuhai cohort at the baseline and follow-up after 3 years. Urinary total arsenic, biomarkers of DNA oxidative damage (8-hydroxy-2'-deoxyguanosine (8-OHdG)), lipid peroxidation (8-isoprostaglandin F2alpha (8-isoPGF2α)), and protein oxidative damage (protein carbonyls (PCO)) were detected for all observations. Here we used linear mixed models to estimate the cross-sectional and longitudinal associations between arsenic exposure and oxidative damage. Exposure-response curves were constructed by utilizing the generalized additive mixed models with thin plate regressions. After adjusting for potential confounders, arsenic level was significantly and positively related to the levels of global oxidative damage and their annual increased rates in dose-response manners. In cross-sectional analyses, each 1% increase in arsenic level was associated with a 0.406% (95% confidence interval (CI): 0.379% to 0.433%), 0.360% (0.301% to 0.420%), and 0.079% (0.055% to 0.103%) increase in 8-isoPGF2α, 8-OHdG, and PCO, respectively. More importantly, arsenic was further found to be associated with increased annual change rates of 8-isoPGF2α (β: 0.147; 95% CI: 0.130 to 0.164), 8-OHdG (0.155; 0.118 to 0.192), and PCO (0.050; 0.035 to 0.064) in the longitudinal analyses. Our study suggested that arsenic exposure was not only positively related with global oxidative damage to lipid, DNA, and protein in cross-sectional analyses, but also associated with annual increased rates of these biomarkers in dose-dependent manners.

Environmental Heavy Metal Exposure and Associated Cardiovascular Diseases in Light of the Triglyceride Glucose Index

Cardiovascular diseases (CVD), primarily ischemic heart disease and stroke, remain leading global health burdens. Environmental risk factors have a major role in the development of CVD, particularly exposure to heavy metals. The Triglyceride Glucose Index (TyG), a measure of insulin resistance and CVD risk, is the primary focus of this study, which summarizes the most recent findings on the effects of lead (Pb), arsenic (As), and cadmium (Cd) on CVD risk. A higher risk of CVD is correlated with an elevated TyG index, which has been linked to insulin resistance. Exposure to Cd is associated with disturbance of lipid metabolism and oxidative stress, which increases the risk of CVD and TyG. Exposure reduces insulin secretion and signaling, which raises the TyG index and causes dyslipidemia. Pb exposure increases the risk of CVD and TyG index via causing oxidative stress and pancreatic β-cell destruction. These results highlight the need of reducing heavy metal exposure by lifestyle and environmental modifications in order to lower the risk of CVD. To comprehend the mechanisms and create practical management plans for health hazards associated with heavy metals, more study is required.

Prenatal arsenic exposure alters EZH2-H3K27me3 occupancy at TNF-α promoter leading to insulin resistance and metabolic syndrome in a mouse model

The global prevalence of Metabolic Syndrome (MetS) is continuously rising and exposure to environmental toxicants such as arsenic could be contributing to this rapid surge. In this study, we have assessed the effects of prenatal arsenic exposure on insulin resistance and MetS parameters in a mouse model, and an underlying mechanism was identified. We found that prenatal arsenic exposure promotes insulin resistance and adipocyte dysfunction which leads to the early onset of MetS in male offspring. Primary adipocytes isolated from 20-week-old arsenic-exposed offspring showed hypertrophy, elevated basal lipolysis, and impaired insulin response along with enhanced expression of Tumor necrosis factor-alpha (TNF-α). TNF-α levels were consistently high at gestational day 15.5 (GD15.5) as well as primary adipocytes of 6-week-old arsenic-exposed male offspring. Along with TNF-α, downstream p-JNK1/2 levels were also increased, which led to inhibitory phosphorylation of IRS1and reduced GLUT4 translocation upon insulin stimulation in adipocytes. Insulin response and downstream signaling were restored upon TNF-α inhibition, confirming its central role. The persistent overexpression of TNF-α in adipocytes of arsenic-exposed mice resulted from diminished EZH2 occupancy and reduced H3K27me3 (gene silencing histone marks) at the TNF-α promoter. This further led to chromatin relaxation, recruitment of c-Jun and CBP/p300, formation of an enhanceosome complex, and TNF-α expression. Our findings show how prenatal arsenic exposure can epigenetically modulate TNF-α expression to promote adipocyte dysfunction and insulin resistance which contributes to the early onset of MetS in offspring.

Association of exposure to multiple heavy metals during pregnancy with the risk of gestational diabetes mellitus and insulin secretion phase after glucose stimulation

BACKGROUND

Epidemiological evidence for the association between heavy metals exposure during pregnancy and gestational diabetes mellitus (GDM) is still inconsistent. Additionally, that is poorly understood about the potential cause behind the association, for instance, whether heavy metal exposure is related to the change of insulin secretion phase is unknown.

OBJECTIVES

We aimed to explore the relationships of blood levels of arsenic (As), lead (Pb), thallium (Tl), nickel (Ni), cadmium (Cd), cobalt (Co), barium (Ba), chromium (Cr), mercury (Hg) and copper (Cu) during early pregnancy with the odds of GDM, either as an individual or a mixture, as well as the association of the metals with insulin secretion phase after glucose stimulation.

METHODS

We performed a nested case-control study consisting of 302 pregnant women with GDM and 302 controls at the First Affiliated Hospital of Anhui Medical University in Hefei, China. Around the 12th week of pregnancy, blood samples of pregnant women were collected and levels of As, Pb, Tl, Ni, Cd, Co, Ba, Cr, Hg and Cu in blood were measured. An oral glucose tolerance test (OGTT) was done in each pregnant woman during the 24-28th week of pregnancy to diagnose GDM and C-peptide (CP) levels during OGTT were measured simultaneously. The four metals (As, Pb, Tl and Ni) with the highest effect on odds of GDM were selected for the subsequent analyses via the random forest model. Conditional logistic regression models were performed to analyze the relationships of blood As, Pb, Tl and Ni levels with the odds of GDM. The weighted quantile sum (WQS) regression and bayesian kernel machine regression (BKMR) were used to assess the joint effects of levels of As, Pb, Tl and Ni on the odds of GDM as well as to evaluate which metal level contributed most to the association. Latent profile analysis (LPA) was conducted to identify profiles of glycemic and C-peptide levels at different time points. Multiple linear regression models were employed to explore the relationships of metals with glycaemia-related indices (fasting blood glucose (FBG), 1-hour blood glucose (1h BG), 2-hour blood glucose (2h BG), fasting C-peptide (FCP), 1-hour C-peptide (1h CP), 2-hour C-peptide (2h CP), FCP/FBG, 1h CP/1h BG, 2h CP/2h BG, area under the curve of C-peptide (AUCP), area under the curve of glucose (AUCG), AUCP/AUCG and profiles of BGs and CPs, respectively. Mixed-effects models with repeated measures data were used to explore the relationship between As (the ultimately selected metal) level and glucose-stimulated insulin secretion phase. The mediation effects of AUCP and AUCG on the association of As exposure with odds of GDM were investigated using mediation models.

RESULTS

The odds of GDM in pregnant women increased with every ln unit increase in blood As concentration (odds ratio (OR) = 1.46, 95% confidence interval (CI) = 1.04-2.05). The joint effects of As, Pb, Tl and Ni levels on the odds of GDM was statistically significant when blood levels of four metals were exceeded their 50th percentile, with As level being a major contributor. Blood As level was positively associated with AUCG and the category of glucose latent profile, the values of AUCG were much higher in GDM group than those in non-GDM group, which suggested that As exposure associated with the odds of GDM may be due to that As exposure was related to the impairment of glucose tolerance among pregnant women. The significant and positive relationships of As level with AUCP, CP latent profile category, 2h CP and 2h CP/2h BG were observed, respectively; and the values of 1h CP/1h BG and AUCP/AUCG were much lower in GDM group than those in non-GDM group, which suggested that As exposure may not relate to the impairment of insulin secretion (pancreatic β-cell function) among pregnant women. The relationships between As level and 2h CP as well as 2h CP/2h BG were positive and significant; additionally, the values of 2h CP/2h BG in GDM group were comparable with those in non-GDM group; the peak value of CP occurred at 2h in GDM group, as well as the values of 2h CP/2h BG in high As exposure group were much higher than those in low As exposure group, which suggested that As exposure associated with the increased odds of GDM may be due to that As exposure was related to the change of insulin secretion phase (delayment of the peak of insulin secretion) among pregnant women. In addition, AUCP mediated 11% (p < 0.05) and AUCG mediated 43% (p < 0.05) of the association between As exposure and the odds of GDM.

CONCLUSION

Our results suggested that joint exposure to As, Pb, Tl and Ni during early pregnancy was positively associated with the odds of GDM, As was a major contributor; and the association of environmental As exposure with the increased odds of GDM may be due to that As exposure was related to the impairment of glucose tolerance and change of insulin secretion phase after glucose stimulation (delayment of the peak of insulin secretion) among pregnant women.

Alleviation of arsenic-induced neurobehavioral defects with selenium in the larvae of Zaprionus indianus

Selenium is an essential antioxidative micronutrient. This study was conducted to characterize the arsenic toxicity induced on the African fig fly, Zaprionus indianus, and its possible amelioration by selenium. We used computational tools and in vivo experiments to elucidate the mechanism of action of arsenic and selenium on Z. indianus larvae. We conducted experiments to study neurobehavioral parameters including learning and memory ability test and crawling and contraction assays. Our in silico study revealed twelve primary targets of arsenic trioxide. The gene ontology annotation of primary and secondary targets of arsenic trioxide revealed selenocysteine metabolic processes as one of the most reliable targets. To validate our in silico data, we analyzed the effect of arsenic trioxide on larvae of Z. indianus and tested the possible amelioration by sodium selenite supplementation. Our data demonstrated that the arsenic trioxide deteriorated the learning and memory ability of 2nd instar larvae of Z. indianus and such effect was reversed by sodium selenite supplementation. Furthermore, crawling and contraction assay done on 3rd instar larvae showed that there was reduction in both parameters upon arsenic trioxide exposure, which was restored with sodium selenite supplementation. Altogether, our computational and in vivo results strongly indicated that the neurobehavioral defects induced by arsenic trioxide on the larvae of Z. indianus can be successfully alleviated in the presence of sodium selenite.

Arsenic induces metabolome remodeling in mature human adipocytes

Human lifetime exposure to arsenic through drinking water, food supply or industrial pollution leads to its accumulation in many organs such as liver, kidneys, lungs or pancreas but also adipose tissue. Recently, population-based studies revealed the association between arsenic exposure and the development of metabolic diseases such as obesity and type 2 diabetes. To shed light on the molecular bases of such association, we determined the concentration that inhibited 17% of cell viability and investigated the effects of arsenic acute exposure on adipose-derived human mesenchymal stem cells differentiated in vitro into mature adipocytes and treated with sodium arsenite (NaAsO2, 10 nM to 10 µM). Untargeted metabolomics and gene expression analyses revealed a strong dose-dependent inhibition of lipogenesis and lipolysis induction, reducing the cellular ability to store lipids. These dysregulations were emphasized by the inhibition of the cellular response to insulin, as shown by the perturbation of several genes and metabolites involved in the mentioned biological pathways. Our study highlighted the activation of an adaptive oxidative stress response with the strong induction of metallothioneins and increased glutathione levels in response to arsenic accumulation that could exacerbate the decreased insulin sensitivity of the adipocytes. Arsenic exposure strongly affected the expression of arsenic transporters, responsible for arsenic influx and efflux, and induced a pro-inflammatory state in adipocytes by enhancing the expression of the inflammatory interleukin 6 (IL6). Collectively, our data showed that an acute exposure to low levels of arsenic concentrations alters key adipocyte functions, highlighting its contribution to the development of insulin resistance and the pathogenesis of metabolic disorders.

Urinary arsenic metabolism, genetic susceptibility, and their interaction on type 2 diabetes

Growing studies investigated the association of arsenic metabolism with type 2 diabetes (T2D), however, the epidemiological evidence is inconsistent. In addition, the interaction of arsenic metabolism-related genetic risk score (GRS)-arsenic on T2D risk was unclear. The present study aimed to evaluate the association of arsenic metabolism efficiency [inorganic arsenic (iAs)%, monomethylarsonic acid (MMA)%, and dimethylarsinic acid (DMA%)] with T2D risk. Moreover, the relationship of GRS and arsenic metabolism efficiency and the interaction of GRS-arsenic on T2D were investigated. Age- and sex-matched new-onset diabetes case-control study derived from the Dongfeng-Tongji cohort was conducted and 996 pairs participants were included in this study. The leave-one-out approach was used to evaluate the association of arsenic metabolism efficiency with T2D risk. The GRS and weight GRS (wGRS) were calculated based on 79 candidate SNPs. We estimated the relationship of GRS with arsenic metabolism efficiency by linear regression model. The interaction of GRS-arsenic on T2D was assessed by adding a multiplicative interaction term (GRS × arsenic) in the logistic regression models. Urinary iAs% was positively associated with T2D risk, and the OR (95% CI) was 1.06 (1.01, 1.12). MMA% and PMI were negatively associated with T2D risk, and the ORs (95% CI) were 0.87 (0.78, 0.97) and 0.64 (0.47, 0.86), respectively. Urinary DMA, As3+, and As5+ were positively associated with T2D risk. Similar relationships were found between arsenic metabolites and levels of FPG and HbA1c. Moreover, arsenic metabolism-related GRS/wGRS was positively associated with MMA% but negatively associated with DMA%. Genetic predisposition to arsenic metabolism modified the association of inorganic arsenic with T2D risk (Pinteraction = 0.033). Taken together, lower arsenic primary metabolism efficiency (higher iAs% and lower MMA%) may increase T2D risk. Genetic predisposition to arsenic metabolism was associated with arsenic metabolism efficiency, and might modify the association of inorganic arsenic with T2D risk.

Urinary metal profiles in mother-offspring pairs and their association with early dysglycemia in the International Hyperglycemia and Adverse Pregnancy Outcome Follow Up Study (HAPO-FUS)

BACKGROUND

Variations in dietary intake and environmental exposure patterns of essential and non-essential trace metals influence many aspects of human health throughout the life span.

OBJECTIVE

To examine the relationship between urine profiles of essential and non-essential metals in mother-offspring pairs and their association with early dysglycemia.

METHODS

Herein, we report findings from an ancillary study to the international Hyperglycemia and Adverse Pregnancy Outcome Follow-Up Study (HAPO-FUS) that examined urinary essential and non-essential metal profiles from mothers and offspring ages 10-14 years (1012 mothers, 1013 offspring, 968 matched pairs) from 10 international sites.

RESULTS

Our analysis demonstrated a diverse exposure pattern across participating sites. In multiple regression modelling, a positive association between markers of early dysglycemia and urinary zinc was found in both mothers and offspring after adjustment for common risk factors for diabetes. The analysis showed weaker, positive, and negative associations of the 2-h glucose value with urinary selenium and arsenic respectively. A positive association between 2-h glucose values and cadmium was found only in mothers in the fully adjusted model when participants with established diabetes were excluded. There was a high degree of concordance between mother and offspring urinary metal profiles. Mother-to-offspring urinary metal ratios were unique for each metal, providing insights into changes in their homeostasis across the lifespan.

SIGNIFICANCE

Urinary levels of essential and non-essential metals are closely correlated between mothers and their offspring in an international cohort. Urinary levels of zinc, selenium, arsenic, and cadmium showed varying degrees of association with early dysglycemia in a comparatively healthy cohort with a low rate of preexisting diabetes.

IMPACT STATEMENT

Our data provides novel evidence for a strong correlation between mother and offspring urinary metal patterns with a unique mother-to-offspring ratio for each metal. The study also provides new evidence for a strong positive association between early dysglycemia and urinary zinc, both in mothers and offspring. Weaker positive associations with urinary selenium and cadmium and negative associations with arsenic were also found. The low rate of preexisting diabetes in this population provides the unique advantage of minimizing the confounding effect of preexisting, diabetes related renal changes that would alter the relationship between dysglycemia and renal metal excretion.

Arsenic exposure incurs hyperglycemia mediated by oxidative damage in urban adult population: A prospective cohort study with three repeated measures

The associations and potential mechanisms of low to moderate arsenic exposure with fasting plasma glucose (FPG) and type 2 diabetes mellitus (T2DM) are still unclear. To assess the effects of short-term and long-term arsenic exposure on hyperglycemia and the mediating effect of oxidative damage on such association, three repeated-measures studies with 9938 observations were conducted in the Wuhan-Zhuhai cohort. The levels of urinary total arsenic, FPG, urinary 8-iso-prostaglandin F2alpha (8-iso-PGF2α), urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG), and plasma protein carbonyls (PCO) were measured. Generalized linear mixed models were used to evaluate the exposure-response relationships of urinary total arsenic with FPG and the prevalent risks of impaired fasting glucose (IFG), T2DM, and abnormal glucose regulation (AGR). Cox regression models were applied to assess the associations of arsenic exposure with incident risks of IFG, T2DM, and AGR. Mediation analyses were performed to assess the mediating effects of 8-iso-PGF2α, 8-OHdG, and PCO. In cross-sectional analyses, each one-unit increase in natural log-transformed urinary total arsenic was associated with a 0.082 (95% CI: 0.047 to 0.118) mmol/L increase in FPG, as well as a 10.3% (95% CI: 1.4%-20.0%), 4.4% (95% CI: 5.3%-15.2%), and 8.7% (95% CI: 1.2%-16.6%) increase in prevalent risks of IFG, T2DM, and AGR, respectively. In longitudinal analyses, arsenic exposure was further associated with the annual increased rate of FPG with a β (95% CI) of 0.021 (95% CI: 0.010 to 0.033). The incident risks of IFG, T2DM, and AGR were increased without statistical significance when arsenic levels increased. Mediation analyses showed that 8-iso-PGF2α and PCO mediated 30.04% and 10.02% of the urinary total arsenic-associated FPG elevation, respectively. Our study indicated that arsenic exposure was associated with elevated level and progression rate of FPG among general Chinese adults, where lipid peroxidation and oxidative protein damage might be the potential mechanisms.

Arsenic and Diabetes Mellitus: A Putative Role for the Immune System

Diabetes mellitus (DM) is an enormous public health issue worldwide. Recent data suggest that chronic arsenic exposure is linked to the risk of developing type 1 and type 2 DM, albeit the underlying mechanisms are unclear. This review discusses the role of the immune system as a link to possibly explain some of the mechanisms of developing T1DM or T2DM associated with arsenic exposure in humans, animal models, and in vitro studies. The rationale for the hypothesis includes: (1) Arsenic is a well-recognized modulator of the immune system; (2) arsenic exposures are associated with increased risk of DM; and (3) dysregulation of the immune system is one of the hallmarks in the pathogenesis of both T1DM and T2DM. A better understanding of DM in association with immune dysregulation and arsenic exposures may help to understand how environmental exposures modulate the immune system and how these effects may impact the manifestation of disease.

Role of Reactive Oxygen Species in Glucose Metabolism Disorder in Diabetic Pancreatic β-Cells

The dysfunction of pancreatic β-cells plays a central role in the onset and progression of type 2 diabetes mellitus (T2DM). Insulin secretory defects in β-cells are characterized by a selective impairment of glucose stimulation, and a reduction in glucose-induced ATP production, which is essential for insulin secretion. High glucose metabolism for insulin secretion generates reactive oxygen species (ROS) in mitochondria. In addition, the expression of antioxidant enzymes is very low in β-cells. Therefore, β-cells are easily exposed to oxidative stress. In islet studies using a nonobese T2DM animal model that exhibits selective impairment of glucose-induced insulin secretion (GSIS), quenching ROS generated by glucose stimulation and accumulated under glucose toxicity can improve impaired GSIS. Acute ROS generation and toxicity cause glucose metabolism disorders through different molecular mechanisms. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, is a master regulator of antioxidant defense and a potential therapeutic target in oxidative stress-related diseases, suggesting the possible involvement of Nrf2 in β-cell dysfunction caused by ROS. In this review, we describe the mechanisms of insulin secretory defects induced by oxidative stress in diabetic β-cells.

Candidate master microRNA regulator of arsenic-induced pancreatic beta cell impairment revealed by multi-omics analysis

Arsenic is a pervasive environmental toxin that is listed as the top priority for investigation by the Agency for Toxic Substance and Disease Registry. While chronic exposure to arsenic is associated with type 2 diabetes (T2D), the underlying mechanisms are largely unknown. We have recently demonstrated that arsenic treatment of INS-1 832/13 pancreatic beta cells impairs glucose-stimulated insulin secretion (GSIS), a T2D hallmark. We have also shown that arsenic alters the microRNA profile of beta cells. MicroRNAs have a well-established post-transcriptional regulatory role in both normal beta cell function and T2D pathogenesis. We hypothesized that there are microRNA master regulators that shape beta cell gene expression in pathways pertinent to GSIS after exposure to arsenicals. To test this hypothesis, we first treated INS-1 832/13 beta cells with either inorganic arsenic (iAsIII) or monomethylarsenite (MAsIII) and confirmed GSIS impairment. We then performed multi-omic analysis using chromatin run-on sequencing, RNA-sequencing, and small RNA-sequencing to define profiles of transcription, gene expression, and microRNAs, respectively. Integrating across these data sets, we first showed that genes downregulated by iAsIII treatment are enriched in insulin secretion and T2D pathways, whereas genes downregulated by MAsIII treatment are enriched in cell cycle and critical beta cell maintenance factors. We also defined the genes that are subject primarily to post-transcriptional control in response to arsenicals and demonstrated that miR-29a is the top candidate master regulator of these genes. Our results highlight the importance of microRNAs in arsenical-induced beta cell dysfunction and reveal both shared and unique mechanisms between iAsIII and MAsIII.

Arsenic exposure during pregnancy and postpartum maternal glucose tolerance: evidence from Bangladesh

BACKGROUND

Arsenic exposure has been associated with gestational diabetes mellitus. However, the extent to which arsenic exposure during pregnancy is associated with postpartum glucose intolerance is unknown.

METHODS

We studied 323 women in Bangladesh. We assessed arsenic exposure in early pregnancy via toenail and water samples. We measured fasting glucose and insulin in serum at a mean (SD) of 4.0 (3.5) weeks post-delivery. We ran covariate-adjusted, linear regression models to examine associations of arsenic concentrations with HOMA-IR, a marker of insulin resistance, and HOMA-β, a marker of beta cell function.

RESULTS

Median (IQR) arsenic concentration was 0.45 (0.67) μg/g in toenails and 2.0 (6.5) μg/L in drinking water. Arsenic concentrations during pregnancy were not associated with insulin resistance or beta cell function postpartum. HOMA-IR was 0.07% (- 3.13, 3.37) higher and HOMA-β was 0.96% (- 3.83, 1.99) lower per IQR increment in toenail arsenic, but effect estimates were small and confidence intervals crossed the null.

CONCLUSIONS

Although arsenic exposure during pregnancy has been consistently associated with gestational diabetes mellitus, we found no clear evidence for an adverse effect on postpartum insulin resistance or beta cell function.

Is Arsenic Exposure a Risk Factor for Metabolic Syndrome? A Review of the Potential Mechanisms

Exposure to arsenic in drinking water is a worldwide health problem. This pollutant is associated with increased risk of developing chronic diseases, including metabolic diseases. Metabolic syndrome (MS) is a complex pathology that results from the interaction between environmental and genetic factors. This condition increases the risk of developing type 2 diabetes, cardiovascular diseases, and cancer. The MS includes at least three of the following signs, central obesity, impaired fasting glucose, insulin resistance, dyslipidemias, and hypertension. Here, we summarize the existing evidence of the multiple mechanisms triggered by arsenic to developing the cardinal signs of MS, showing that this pollutant could contribute to the multifactorial origin of this pathology.

Correlation between urinary contents of some metals and fasting plasma glucose levels: A cross-sectional study in China

Many metals are involved in the pathogenesis of diabetes, but most of existing studies focused on single metals. The study of mixtures represents real-life exposure scenarios and deserves attention. This study aimed to explore the potential relationship of urinary copper (Cu), zinc (Zn), arsenic (As), selenium (Se), and strontium (Sr) contents with fasting plasma glucose (FPG) levels in 2766 participants. The levels of metals in urine were determined by inductively coupled plasma-mass spectrometry. We used linear regression models and the Bayesian kernel machine regression (BKMR) to evaluate the association between metals and FPG levels. In the multiple metals linear regression, Zn (β = 0.434), Se (β = 0.172), and Sr (β = -0.143) showed significant association with FPG levels (all P < 0.05). The BKMR model analysis showed that the results of single metal association were consistent with the multiple metals linear regression. The mixture of five metals had a positive over-all effect on FPG levels, and Zn (PIP = 1.000) contributed the most to the FPG levels. Cu and As were negatively correlated with FPG levels in women. The potential interaction effect between Cu and Sr was observed in participants aged ≥ 60 years old (Pinteraction = 0.035). In summary, our results suggested that multiple metals in urine are associated with FPG levels. Further studies are needed to confirm these findings and clarify the underlying mechanisms.

Arsenic exposure and non-carcinogenic health effects

Inorganic arsenic (iAs) exposure is a serious health problem that affects more than 140 million individuals worldwide, mainly, through contaminated drinking water. Acute iAs poisoning produces several symptoms such as nausea, vomiting, abdominal pain, and severe diarrhea, whereas prolonged iAs exposure increased the risk of several malignant disorders such as lung, urinary tract, and skin tumors. Another sensitive endpoint less described of chronic iAs exposure are the non-malignant health effects in hepatic, endocrine, renal, neurological, hematological, immune, and cardiovascular systems. The present review outlines epidemiology evidence and possible molecular mechanisms associated with iAs-toxicity in several non-carcinogenic disorders.

Arsenic induces hepatic insulin resistance via mtROS-NLRP3 inflammasome pathway

Hepatic insulin resistance (IR) is the key event for arsenic-caused type 2 diabetes (T2D). However, the unequivocal mechanism of arsenic-induced hepatic IR remains unclear. The current study determined the role of NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome activation in arsenic-induced IR and revealed the underlying mechanism. Three-month NaAsO₂ gavage led to glucose intolerance and insulin insensitivity, impaired hepatic insulin signaling. Additionally, NaAsO₂ upregulated the level of oxidized mitochondrial DNA (ox-mtDNA) and mitophagy, thereby activating the NLRP3 inflammasome in SD rat liver. In vitro, we demonstrated that NaAsO₂-induced IR depended upon the NLRP3 inflammasome activation. Moreover, inhibiting mitophagy mitigated the NLRP3 inflammasome activation and impaired insulin signaling induced by NaAsO₂. Furthermore, mitochondrial reactive oxygen species (mtROS) scavenger alleviated the upregulated ox-mtDNA and mitophagy, thereby inhibiting the NLRP3 inflammasome activation, and improving insulin signaling. Taken together, these data demonstrated that mtROS-triggered ox-mtDNA, mitophagy, and the activation of NLRP3 inflammasome was involved in arsenic-induced hepatic IR.

lncRNA Eif4g2 improves palmitate-induced dysfunction of mouse β-cells via modulation of Nrf2 activation

Chronic oxidative stress resulting from hyperlipidemia is thought to be a key pathogenic driver of pancreatic β-cell dysfunction in leading to the onset of type 2 diabetes mellitus (T2DM). Long non-coding RNAs (lncRNAs) have been increasingly recognized to regulate dysfunction within pancreatic β-cells in the context of T2DM. In the present study, we sought to comprehensively analyze the roles of lncRNAs in dysfunctional β-cells and mouse islets. Analyses of INS-1E cells were performed by RNA-seq and qRT-PCR after treating with or without 0.5 mM palmitate for 4 days, leading us to identify the novel lncRNA Eif4g2 (lncEif4g2) as a functional regulator within these cells. When we overexpressed lncEif4g2 in INS-1E β-cells and mouse islets, this was sufficient for the reversal of palmitate-mediated reductions in cell viability, insulin production, ATP production by mitochondria, and creation of intracellular reactive oxygen species (ROS) and the dysfunction of mouse islets, with nuclear factor erythroid 2 related factor 2 (Nrf2) activation also being observed. In contrast, when lncEif4g2 was knocked down this led INS-1E cells and mouse islets to become more sensitive to palmitate-induced dysfunction, with reduced Nrf2 nuclear translocation also being detected. When antioxidants were used to treat INS-1E cells and mouse islets, however, these negative effects were reversed. Additional functional analyses revealed lncEif4g2 to be capable of directly binding to miR-3074-5p in β-cells, with the expression of lncEif4g2 and miR-3074-5p being negatively correlated with one another. We further found that cAMP-responsive element binding-protein (CREB) was a miR-3074-5p target gene in these cells, thus at least in part serving as a functional mediator of the lncEif4g2/miR-3074-5p axis within dysfunctional β-cells. In summary, our results thus reveal that lncEif4g2 is able to indirectly regulate the expression of CREB via targeting miR-3074-5p in INS-1E cells and mouse islets, thereby leading to enhanced Nrf2 activation.

Assessment of arsenic-induced modifications in the DNA methylation of insulin-related genes in rat pancreatic islets

Extended exposure to inorganic arsenic through contaminated drinking water has been linked with increased incidence of diabetes mellitus. The most common exposure occurs through the consumption of contaminated drinking water mainly through geogenic sources of inorganic arsenic. Epigenetic modifications are important mechanisms through which environmental pollutants could exert their toxic effects. Bisulfite sequencing polymerase chain reaction method followed by Sanger sequencing was performed for DNA methylation analysis. Our results showed that sodium arsenite treatment significantly reduced insulin secretion in pancreatic islets. It was revealed that the methylation of glucose transporter 2 (Glut2) gene was changed at two cytosine-phosphate-guanine (CpG) sites (-1743, -1734) in the promoter region of the sodium arsenite-treated group comparing to the control. No changes were observed in the methylation status of peroxisome proliferator-activated receptor-gamma (PPARγ), pancreatic and duodenal homeobox 1 (Pdx1) and insulin 2 (Ins2) CpG sites in the targeted regions. Measuring the gene expression level showed increase in Glut2 expression, while the expression of insulin (INS) and Pdx1 were significantly affected by sodium arsenite treatment. This study revealed that exposure to sodium arsenite changed the DNA methylation pattern of Glut2, a key transporter of glucose entry into the pancreatic beta cells (β-cells). Our data suggested possible epigenetic-mediated toxicity mechanism for arsenite-induced β-cells dysfunction. Further studies are needed to dissect the precise epigenetic modulatory activity of sodium arsenite that affect the biogenesis of insulin.

Redox toxicology of environmental chemicals causing oxidative stress

Living organisms are surrounded with heavy metals such as methylmercury, manganese, cobalt, cadmium, arsenic, as well as pesticides such as deltamethrin and paraquat, or atmospheric pollutants such as quinone. Extensive studies have demonstrated a strong link between environmental pollutants and human health. Redox toxicity is proposed as one of the main mechanisms of chemical-induced pathology in humans. Acting as both a sensor of oxidative stress and a positive regulator of antioxidants, the nuclear factor erythroid 2-related factor 2 (NRF2) has attracted recent attention. However, the role NRF2 plays in environmental pollutant-induced toxicity has not been systematically addressed. Here, we characterize NRF2 function in response to various pollutants, such as metals, pesticides and atmospheric quinones. NRF2 related signaling pathways and epigenetic regulations are also reviewed.

Association between arsenic exposure and biomarkers of type 2 diabetes mellitus in a Croatian population: A comparative observational pilot study

Chronic exposure to high inorganic As levels in drinking water has been related to many diseases, including type 2 diabetes mellitus (T2D). The association with low and moderate As levels, however, remains controversial and has yet not been studied in European populations. This study aimed to investigate possible association between As exposure and biomarkers of T2D in Croatian population. Observation recruited 86 adults from Eastern Croatia, where groundwater is contaminated with inorganic As, and 116 adults from Western Croatia, where As levels in drinking water are low. Both populations were divided in patient groups (T2D or prediabetes) and healthy controls. Exposure was assessed by determining total As in blood and urine and As metabolites in urine. Eastern Croatian population had a significantly higher content of As in urine than Western, whereas the opposite was true for arsenobetain. Total As and As metabolites in urine positively correlated with hemoglobin A1c (HbA1c) and negatively with albuminuria. This study provides important preliminary data on the levels of As in urine and blood and their association with biomarkers of T2D in Croatian population exposed to low or moderate levels of As through drinking water as a solid basis for further research of the pathophysiological effects of such As exposure on the status and complications of diabetes.

Biotransformation of dietary inorganic arsenic in a freshwater fish Carassius auratus and the unique association between arsenic dimethylation and oxidative damage

The metabolic process and toxicity mechanism of dietary inorganic arsenic (iAs) in freshwater fish remain unclear to date. The present study conducted two iAs [arsenate (As(V)) and arsenite (As(III))] dietary exposures in freshwater fish crucian carp (Carassius auratus). The fish were fed on As supplemented artificial diets at nominal concentrations of 50 and 100 μg As(III) or As(V) g^-1 (dry weight) for 10 d and 20 d. We found that the liver, kidney, spleen, and intestine of fish accumulated more As in As(V) feeding group than that in As(III), while the total As levels in muscle were similar between As(V) and As(III) group at the end of exposure. Reduction of As(V) to As(III) and oxidation of As(III) to As(V) occurred in fish fed with As(V) and As(III), respectively, indicating that toxicity of iAs was likely elevated or reduced when iAs was absorbed by fish before entering into human body through diet. Biomethylation to monomethylarsonic acid and dimethylarsinic acid and transformation to arsenocholine and arsenobetaine were also found in the fish. The linear regression analysis showed a positive correlation between secondary methylation index and the malondialdehyde content in tissues, highlighting the vital role of arsenic dimethylation in the oxidative damages in fish.

Exposure to inorganic arsenic and its methylated metabolites alters metabolomics profiles in INS-1 832/13 insulinoma cells and isolated pancreatic islets

Inorganic arsenic (iAs) is an environmental diabetogen, but mechanisms underlying its diabetogenic effects are poorly understood. Exposures to arsenite (iAsIII) and its methylated metabolites, methylarsonite (MAsIII) and dimethylarsinite (DMAsIII), have been shown to inhibit glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells and isolated pancreatic islets. GSIS is regulated by complex mechanisms. Increase in ATP production through metabolism of glucose and other substrates is the ultimate trigger for GSIS in β-cells. In the present study, we used metabolomics to identify metabolites and pathways perturbed in cultured INS-1 832/13 rat insulinoma cells and isolated murine pancreatic islets by exposures to iAsIII, MAsIII and DMAsIII. We found that the exposures perturbed multiple metabolites, which were enriched primarily in the pathways of amino acid, carbohydrate, phospholipid and carnitine metabolism. However, the effects of arsenicals in INS-1 832/13 cells differed from those in the islets and were exposure specific with very few overlaps between the three arsenicals. In INS-1 832/13 cells, all three arsenicals decreased succinate, a metabolite of Krebs cycle, which provides substrates for ATP synthesis in mitochondria. Acetylcarnitine was decreased consistently by exposures to arsenicals in both the cells and the islets. Acetylcarnitine is usually found in equilibrium with acetyl-CoA, which is the central metabolite in the catabolism of macronutrients and the key substrate for Krebs cycle. It is also thought to play an antioxidant function in mitochondria. Thus, while each of the three trivalent arsenicals perturbed specific metabolic pathways, which may or may not be associated with GSIS, all three arsenicals appeared to impair mechanisms that support ATP production or antioxidant defense in mitochondria. These results suggest that impaired ATP production and/or mitochondrial dysfunction caused by oxidative stress may be the mechanisms underlying the inhibition of GSIS in β-cells exposed to trivalent arsenicals.

A State-of-the-Science Review of Arsenic's Effects on Glucose Homeostasis in Experimental Models

BACKGROUND

The prevalence of type 2 diabetes (T2D) has more than doubled since 1980. Poor nutrition, sedentary lifestyle, and obesity are among the primary risk factors. While an estimated 70% of cases are attributed to excess adiposity, there is an increased interest in understanding the contribution of environmental agents to diabetes causation and severity. Arsenic is one of these environmental chemicals, with multiple epidemiology studies supporting its association with T2D. Despite extensive research, the molecular mechanism by which arsenic exerts its diabetogenic effects remains unclear.

OBJECTIVES

We conducted a literature search focused on arsenite exposure in vivo and in vitro, using relevant end points to elucidate potential mechanisms of oral arsenic exposure and diabetes development.

METHODS

We explored experimental results for potential mechanisms and elucidated the distinct effects that occur at high vs. low exposure. We also performed network analyses relying on publicly available data, which supported our key findings.

RESULTS

While several mechanisms may be involved, our findings support that arsenite has effects on whole-body glucose homeostasis, insulin-stimulated glucose uptake, glucose-stimulated insulin secretion, hepatic glucose metabolism, and both adipose and pancreatic β -cell dysfunction.

DISCUSSION

This review applies state-of-the-science approaches to identify the current knowledge gaps in our understanding of arsenite on diabetes development. https://doi.org/10.1289/EHP4517.

miR-149 Negative Regulation of mafA Is Involved in the Arsenite-Induced Dysfunction of Insulin Synthesis and Secretion in Pancreatic Beta Cells

Chronic exposure to arsenic, a potent environmental oxidative stressor, is associated with the incidence of diabetes. However, the mechanisms for arsenite-induced reduction of insulin remain largely unclear. After CD1 mice were treated with 20 or 40 ppm arsenite in the drinking water for 12 months, the mice showed reduced fasting insulin levels, a depression in glucose clearance, and lower insulin content in the pancreas. The levels of glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells isolated from arsenite-exposed mice were low compared with those for control mice. Immunohistochemistry studies showed that arsenite exposure resulted a reduction of insulin content in the pancreas of mice. Exposure of Min6 cells, a pancreatic beta cell line, to low levels of arsenite led to lower GSIS in a dose- and time-dependent fashion. Since microRNAs (miRNAs) are involved in pancreatic β-cell function and the pathogenesis of diabetes, we hypothesized that arsenite exposure activates miR-149, decreases insulin transcription factor v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (mafA), and induces an insulin synthesis and secretion disorder. In arsenite-exposed Min6 cells, mafA activity was lowered by the increase of its target miRNA, miR-149. Luciferase assays illustrated an interaction between miR-149 and the mafA 3' untranslated region. In Min6 cells transfected with an miR-149 inhibitor, arsenite did not regulate GSIS and mafA expression. In control cells, however, arsenite decreased GSIS or mafA expression. Our results suggest that low levels of arsenite affect β-cell function and regulate insulin synthesis and secretion by modulating mafA expression through miR-149.

Nrf2 Activation Protects Mouse Beta Cells from Glucolipotoxicity by Restoring Mitochondrial Function and Physiological Redox Balance

Influencing the redox balance of pancreatic beta cells could be a promising strategy for the treatment of diabetes. Nuclear factor erythroid 2p45-related factor 2 (Nrf2) is present in beta cells and regulates numerous genes involved in antioxidant defense. As reactive oxygen species (ROS) are important for beta cell signaling but induce oxidative stress when present in excess, this study elucidates the influence of Nrf2-activating compounds on different kinds of ROS and correlates changes in redox balance to effects on mitochondrial function, insulin release, and cell viability. Acute glucose stimulation (15 mmol/L) of murine islet cells of C57Bl/6N mice affects ROS and redox status of the cells differently. Those ROS monitored by dihydroethidium, which detects superoxide radical anions, decrease. By contrast, oxidant status, monitored by dichlorodihydrofluorescein, as well as intracellular H₂O₂, increases. Glucolipotoxicity completely prevents these fast, glucose-mediated alterations and inhibits glucose-induced NAD(P)H production, mitochondrial hyperpolarization, and ATP synthesis. Oltipraz (10 μmol/L) or dimethyl fumarate (DMF, 50 μmol/L) leads to nuclear accumulation of Nrf2, restores mitochondrial activity and glucose-dependent ROS turnover, and antagonizes glucolipotoxicity-induced inhibition of insulin release and apoptosis. Importantly, these beneficial effects only occur when beta cells are challenged and damaged by high lipid and carbohydrate supply. At physiological conditions, insulin release is markedly reduced in response to both Nrf2 activators. This is not associated with severe impairment of glucose-induced mitochondrial hyperpolarization or a rise in apoptosis but coincides with altered ROS handling. In conclusion, Nrf2 activators protect beta cells against glucolipotoxicity by preserving mitochondrial function and redox balance. As our data show that this maintains glucose-stimulated insulin secretion, targeting Nrf2 might be suited to ameliorate progression of type 2 diabetes mellitus. By contrast, nonstressed beta cells do not benefit from Nrf2 activation, thus underlining the importance of physiological shifts in ROS homeostasis for the regulation of beta cell function.

Enhanced p62-NRF2 Feedback Loop due to Impaired Autophagic Flux Contributes to Arsenic-Induced Malignant Transformation of Human Keratinocytes

Chronic exposure to arsenic induces a variety of cancers, particularly in the skin. Autophagy is a highly conserved process which plays a dual role in tumorigenesis. In the present study, we found that chronic exposure to an environmentally relevant dose of arsenite induced malignant transformation of human keratinocytes (HaCaT) with dysregulated autophagy as indicated by an increased number of autophagosomes, activation of mTORC1 pathway, and elevated protein levels of p62 and LC3II. Meanwhile, arsenite-transformed cells showed lower intracellular levels of reactive oxygen species compared with control. Silencing p62 ameliorated elevation in mRNA levels of NRF2 downstream genes (AKR1C1 and NQO1) and malignant phenotypes (acquired invasiveness and anchor-independent growth) induced by chronic arsenite exposure. On the other hand, silencing NRF2 abrogated the increase in mRNA and protein levels of p62 and malignant phenotypes induced by arsenite. In response to acute arsenite exposure, impaired autophagic flux with an increase in p62 protein level and interrupted autophagosome-lysosome fusion was observed. The increase in p62 protein levels in response to arsenite was not completely dependent on NRF2 activation and at least partially attributed to protein degradation. Our data indicate that accumulation of p62 by impaired autophagic flux is involved in the activation of NRF2 and contributes to skin tumorigenesis due to chronic arsenite exposure.

Arsenite and its trivalent methylated metabolites inhibit glucose-stimulated calcium influx and insulin secretion in murine pancreatic islets

Chronic exposure to inorganic arsenic (iAs), a common drinking water and food contaminant, has been associated with an increased risk of type 2 diabetes in population studies worldwide. Several mechanisms underlying the diabetogenic effects of iAs have been proposed through laboratory investigations. We have previously shown that exposure to arsenite (iAs(III)) or its methylated trivalent metabolites, methylarsonite (MAs(III)) and dimethylarsinite (DMAs(III)), inhibits glucose-stimulated insulin secretion (GSIS) in pancreatic islets, without significant effects on insulin expression or insulin content. The goal of the present study was to determine if iAs(III) and/or its metabolites inhibit Ca²⁺ influx, an essential mechanism that regulates the release of insulin from β cells in response to glucose. We found that in vitro exposures for 48 h to non-cytotoxic concentrations of iAs(III), MAs(III), and DMAs(III) impaired Ca²⁺ influx in isolated murine pancreatic islets stimulated with glucose. MAs(III) and DMAs(III) were more potent inhibitors of Ca²⁺ influx than iAs(III). These arsenicals also inhibited Ca²⁺ influx and GSIS in islets treated with depolarizing levels of potassium chloride in the absence of glucose. Treatment with Bay K8644, a Ca_v1.2 channel agonist, did not restore insulin secretion in arsenical-exposed islets. Tolbutamide, a K_ATP channel blocker, prevented inhibition of insulin secretion in MAs(III)- and DMAs(III)-exposed islets, but only marginally in islets exposed to iAs(III). Our findings suggest that iAs(III), MAs(III), and DMAs(III) inhibit glucose-stimulated Ca²⁺ influx in pancreatic islets, possibly by interfering with K_ATP and/or Ca_v1.2 channel function. Notably, the mechanisms underlying inhibition of GSIS by iAs(III) may differ from those of its trivalent methylated metabolites.

Health effects inflicted by chronic low-level arsenic contamination in groundwater: A global public health challenge

Groundwater arsenic (As) contamination is a global public health concern. The high level of As exposure (100-1000 μg/L or even higher) through groundwater has been frequently associated with serious public health hazards, e.g., skin disorders, cardiovascular diseases, respiratory problems, complications of gastrointestinal tract, liver and splenic ailments, kidney and bladder disorders, reproductive failure, neurotoxicity and cancer. However, reviews on low-level As exposure and the imperative health effects are far less documented. The World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA) has set the permissible standard of As in drinking water at 10 μg/L. Considering the WHO and USEPA guidelines, most of the developed countries have established standards at or below this guideline. Worldwide many countries including India have millions of aquifers with low-level As contamination (≤50 μg/L). The exposed population of these areas might not show any As-related skin lesions (hallmark of As toxicity particularly in a population consuming As contaminated groundwater >300 μg/L) but might be subclinically affected. This review has attempted to encompass the wide range of health effects associated with chronic low-level As exposure ≤50 μg/L and the probable mechanisms that might provide a better insight regarding the underlying cause of these clinical manifestations. Therefore, there is an urgent need to create mass awareness about the health effects of chronic low-level As exposure and planning of proper mitigation strategies.

Evaluation of the association between urinary cadmium levels below threshold limits and the risk of diabetes mellitus: a dose-response meta-analysis

As cadmium levels are increasing in the environment, the adverse effects of cadmium exposure specifically associated with chronic diseases are receiving increasing attention. Several population-based studies have been conducted on the association between cadmium and diabetes mellitus (DM) but have reported controversial results. Here, we aimed to evaluate the association between cadmium exposure and DM. In this meta-analysis, a random effects model was used because there was evidence of heterogeneity among studies. A dose-response relationship was assessed through a restricted cubic spline model with three knots. The results showed a positive association between cadmium levels in the body and DM (OR = 1.27; 95% CI, 1.07-1.52). The cadmium levels in the body were defined on the basis of combined urinary and blood cadmium. Subgroup analysis further indicated a positive association between urinary cadmium levels and DM (OR = 1.31; 95% CI, 1.02-1.69). The dose-response analysis results showed a positive association between levels of urinary cadmium above 2.43 μg/g creatinine and DM, and the risk of DM increased by 16% for each l μg/g creatinine increase in urinary cadmium levels. The results from our meta-analysis indicate that cadmium levels in the body are positively associated with DM, and urinary cadmium levels above 2.43 μg/g creatinine are associated with an increased risk of DM.

Non-monotonic dose-response effects of arsenic on glucose metabolism

BACKGROUND

Inorganic arsenic (iAs) is a widespread environmental toxin. In addition to being a human carcinogen, its effect on diabetes has started to gain recognition recently. Insulin is the key hormone regulating systemic glucose metabolism. The in vivo effect of iAs on insulin sensitivity has not been directly addressed.

OBJECTIVES

Here we use mouse models to dissect the dose-dependent effects of iAs on glucose metabolism in vivo.

METHODS

We performed hyperinsulinemic-euglycemic clamp, the gold standard analysis of systemic insulin sensitivity. We also performed dynamic metabolic testings and RNA-seq analysis.

RESULTS

We found that a low-dose exposure (0.25 ppm iAs in drinking water) caused glucose intolerance in adult male C57BL/6 mice, likely by disrupting glucose-induced insulin secretion without affecting peripheral insulin sensitivity. However, a higher-dose exposure (2.5 ppm iAs) had diminished effects on glucose tolerance despite disrupted pancreatic insulin secretion. Insulin Clamp analysis showed that 2.5 ppm iAs actually enhanced systemic insulin sensitivity by simultaneously enhancing insulin-stimulated glucose uptake in skeletal muscles and improved insulin-mediated suppression of endogenous glucose production. RNA-seq analysis of skeletal muscles revealed that 2.5 ppm iAs regulated expression of many genes involved in the metabolism of fatty acids, pyruvate, and amino acids.

CONCLUSION

These findings suggest that iAs has opposite glycemic effects on distinct metabolic tissues at different dose thresholds. Such non-monotonic dose-response effects of iAs on glucose tolerance shed light on the complex interactions between iAs and the systemic glucose metabolism, which could potentially help reconcile some of the conflicting results in human epidemiological studies.

Pterostilbene Activates the Nrf2-Dependent Antioxidant Response to Ameliorate Arsenic-Induced Intracellular Damage and Apoptosis in Human Keratinocytes

The NF-E2 p45-related factor 2 (Nrf2), a transcription factor that regulates the cellular adaptive response to oxidative stress, is a target for limiting tissue damage from exposure to environmental toxins, including arsenic. In the current study, we determine whether Pterostilbene (Pts), as a potent activator of Nrf2, has a protective effect on arsenic-induced cytotoxicity and apoptosis in human keratinocytes. Human keratinocytes (HaCaT) or mouse epidermal cells (JB6) were pretreated with Pts for 24 h prior to arsenic treatment. Harvested cells were analyzed by MTT, DCFH-DA, commercial kits, Flow cytometry assay and western blot analysis. Our results demonstrated that Pts effectively regulated the viability in HaCaT and JB6 cells, decreased the reactive oxygen species (ROS) generation and lipid peroxidation (MDA), and improved the NaAsO₂-induced depletion of superoxide dismutase (SOD). Moreover, Pts treatment further dramatically inhibited NaAsO₂-induced apoptosis, specifically the mitochondrial mediation of apoptosis, which coincided with the effective recovery of NaAsO₂-induced mitochondrial membrane potential (ΔΨm) depolarization and cytochrome c release from the mitochondria. Furthermore, arsenic-induced decrease of anti-apoptotic factor Bcl-2 and Bcl-xl, and increase of pro-apoptotic factor Bax and Bad, as well as survival signal related factor caspase 3 activation were reversed by Pts treatment. Further mechanistic studies confirmed that Pts increased antioxidant enzyme expression in a dose-dependent manner, which was related to Nrf2 nuclear translocation. In addition, the effects of Pts on NaAsO₂-induced cell viability were largely weakened when Nrf2 was knocked down. Together, our results provide evidence for the use of Pts to activate the Nrf2 pathway to alleviate arsenic-induced dermal damage.

Arsenic modifies serotonin metabolism through glucuronidation in pancreatic β-cells

In arsenic-endemic regions of the world, arsenic exposure correlates with diabetes mellitus. Multiple animal models of inorganic arsenic (iAs, as As³⁺) exposure have revealed that iAs-induced glucose intolerance manifests as a result of pancreatic β-cell dysfunction. To define the mechanisms responsible for this β-cell defect, the MIN6-K8 mouse β-cell line was exposed to environmentally relevant doses of iAs. Exposure to 0.1-1 µM iAs for 3 days significantly decreased glucose-induced insulin secretion (GIIS). Serotonin and its precursor, 5-hydroxytryptophan (5-HTP), were both decreased. Supplementation with 5-HTP, which loads the system with bioavailable 5-HTP and serotonin, rescued GIIS, suggesting that recovery of this pathway was sufficient to restore function. Exposure to iAs was accompanied by an increase in mRNA expression of UDP-glucuronosyltransferase 1 family, polypeptide a6a (Ugt1a6a), a phase-II detoxification enzyme that facilitates the disposal of cyclic amines, including serotonin, via glucuronidation. Elevated Ugt1a6a and UGT1A6 expression levels were observed in mouse and human islets, respectively, following 3 days of iAs exposure. Consistent with this finding, the enzymatic rate of serotonin glucuronidation was increased in iAs-exposed cells. Knockdown by siRNA of Ugt1a6a during iAs exposure restored GIIS in MIN6-K8 cells. This effect was prevented by blockade of serotonin biosynthesis, suggesting that the observed iAs-induced increase in Ugt1a6a affects GIIS by targeting serotonin or serotonin-related metabolites. Although it is not yet clear exactly which element(s) of the serotonin pathway is/are most responsible for iAs-induced GIIS dysfunction, this study provides evidence that UGT1A6A, acting on the serotonin pathway, regulates GIIS under both normal and pathological conditions.

Braving the Element: Pancreatic β-Cell Dysfunction and Adaptation in Response to Arsenic Exposure

Type 2 diabetes mellitus (T2DM) is a serious global health problem, currently affecting an estimated 451 million people worldwide. T2DM is characterized by hyperglycemia and low insulin relative to the metabolic demand. The precise contributing factors for a given individual vary, but generally include a combination of insulin resistance and insufficient insulin secretion. Ultimately, the progression to diabetes occurs only after β-cells fail to meet the needs of the individual. The stresses placed upon β-cells in this context manifest as increased oxidative damage, local inflammation, and ER stress, often inciting a destructive spiral of β-cell death, increased metabolic stress due to further insufficiency, and additional β-cell death. Several pathways controlling insulin resistance and β-cell adaptation/survival are affected by a class of exogenous bioactive compounds deemed endocrine disrupting chemicals (EDCs). Epidemiological studies have shown that, in several regions throughout the world, exposure to the EDC inorganic arsenic (iAs) correlates significantly with T2DM. It has been proposed that a lifetime of exposure to iAs may exacerbate problems with both insulin sensitivity as well as β-cell function/survival, promoting the development of T2DM. This review focuses on the mechanisms of iAs action as they relate to known adaptive and maladaptive pathways in pancreatic β-cells.

Impact of in vitro heavy metal exposure on pancreatic β-cell function

Susceptibility to type-2 diabetes mellitus (DM) is determined, in part, by a variety of environmental factors, including exposure to metals. Heavy metals including inorganic arsenic (iAs), zinc (Zn), manganese (Mn), and cadmium (Cd) have been reported to affect glucose homeostasis or DM risk in population-based and/or laboratory studies. Previous evidence from our lab has shown that iAs can increase DM risk by impairing mitochondrial metabolism, one of the key steps in the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells. The goal of the current study was to compare the effects of iAs on GSIS and mitochondrial function in INS-1 832/13 β-cells with those of Cd, Mn, and Zn, and to evaluate effects of binary mixtures of these metals. As expected, 24-hour exposure to iAs (arsenite, ≥1 μM) significantly inhibited GSIS as did Cd (5 μM) and Mn (12.5, 25, or 50 μM). Zn had no effects on GSIS at concentrations up to 50 μM. Mitochondrial function was assessed by measuring oxygen consumption rate (OCR) after glucose stimulation and during simulated mitochondrial stress. While both iAs and Mn impaired mitochondrial function (inhibiting OCR, maximal respiration, and/or spare respiratory capacity of mitochondria), no significant effects were found in cells exposed to Cd. Interestingly, no additive or synergistic effects on GSIS or OCR were observed in binary mixtures of iAs with either Mn or Cd. These data suggest that Mn, like iAs, may inhibit GSIS by impairing mitochondrial function, whereas Cd may target other mechanisms that regulate GSIS in β-cells.

Underutilized and Under Threat: Environmental Policy as a Tool to Address Diabetes Risk

PURPOSE OF REVIEW

Diabetes is a burgeoning threat to public health in the USA. Importantly, the burden of diabetes is not equally borne across society with marked disparities based on geography, race/ethnicity, and income. The etiology of global and population-specific diabetes risk remains incompletely understood; however, evidence linking environmental toxicants acting as endocrine-disrupting chemicals (EDCs), such as particulate matter and arsenic, with diabetes suggests that environmental policies could play an important role in diabetes risk reduction.

RECENT FINDINGS

Evidence suggests that disproportionate exposures to EDCs may contribute to subgroup-specific diabetes risk; however, no federal policies regulate EDCs linked to diabetes based upon diabetogenic potential. Nevertheless, analyses of European Union data indicate that such regulation could reduce diabetes-associated costs and disease burden. Federal laws only regulate EDCs indirectly. The accumulating evidence linking these chemicals with diabetes risk should encourage policymakers to adopt stricter environmental standards that consider both health and economic impacts.

A comparative study on subacute toxicity of arsenic trioxide and dimethylarsinic acid on antioxidant status in Crandell Rees feline kidney (CRFK), human hepatocellular carcinoma (PLC/PRF/5), and epithelioma papulosum cyprini (EPC) cell lines

Arsenic (As) is a global contaminant of terrestrial and aquatic environments posing concern for environmental and human health. The effects of subacute concentrations of arsenic trioxide (As^III) and dimethylarsinic acid (DMA^V) were examined using Crandell Rees feline kidney (CRFK), human hepatocellular carcinoma (PLC/PRF/5), and epithelioma papulosum cyprini (EPC). Whole monolayer with suffering cells (confluence 100%, pyknosis and refractive cells; value scale = 2) led to identification of subacute As concentrations for the three cell lines. The selected As^III concentrations were 1.33 µM for CRFK and 33.37 µM for PLC/PRF/5 and EPC, at 48 hr time point. The selected DMA^V concentrations were 0.67 mM for PLC/PRF/5, 1.33 mM for CRFK, and 2.67 mM for EPC for 48 hr. Unlike the As^III test, the three cell lines did not exhibit marked susceptibility to DMA^V-mediated toxicity. Several oxidative stress biomarker levels, directly or indirectly associated with reactive oxygen species (ROS) elimination including superoxide dismutase, catalase, glutathione peroxidases, glutathione reductase, glutathione S-transferase, glyoxalase I, glyoxalase II, and total glutathione, were determined in the three cell lines at 24 and 48 hr. Antioxidant responses in metal-treated cells were significantly altered compared to controls, suggesting a perturbation of redox state. The weakening of antioxidant pathway in either healthy or tumoral cells was greater using As^III than DMA^V. Differences in level of several oxidative stress biomarkers suggest that the oxidative stress mechanism induced by As^III is distinctly different from DMA^V. Multifaceted mechanisms of action underlying ROS generation in tumor and nontumor cells versus As^III and DMA^V exposure are thus involved. Since As-mediated toxicity is quite complex, more data regarding both oxidant-enhancement and oxidant-lowering strategies may be useful to improve knowledge regarding the influence of As on human and animal cells.

Arsenite and methylarsonite inhibit mitochondrial metabolism and glucose-stimulated insulin secretion in INS-1 832/13 β cells

Growing evidence suggests that exposure to environmental contaminants contributes to the current diabetes epidemic. Inorganic arsenic (iAs), a drinking water and food contaminant, is one of the most widespread environmental diabetogens according to epidemiological studies. Several schemes have been proposed to explain the diabetogenic effects of iAs exposure; however, the exact mechanism remains unknown. We have shown that in vitro exposure to low concentrations of arsenite (iAsIII) or its trivalent methylated metabolites, methylarsonite (MAsIII) and dimethylarsinite (DMAsIII), inhibits glucose-stimulated insulin secretion (GSIS) from isolated pancreatic islets, with little effect on insulin transcription or total insulin content. The goal of this study was to determine if exposure to trivalent arsenicals impairs mitochondrial metabolism, which plays a key role in the regulation of GSIS in β cells. We used a Seahorse extracellular flux analyzer to measure oxygen consumption rate (OCR), a proxy for mitochondrial metabolism, in cultured INS-1 832/13 β cells exposed to iAsIII, MAsIII, or DMAsIII and stimulated with either glucose or pyruvate, a final product of glycolysis and a substrate for the Krebs cycle. We found that 24-h exposure to 2 μM iAsIII or 0.375-0.5 μM MAsIII inhibited OCR in both glucose- and pyruvate-stimulated β cells in a manner that closely paralleled GSIS inhibition. In contrast, 24-h exposure to DMAsIII (up to 2 µM) had no effects on either OCR or GSIS. These results suggest that iAsIII and MAsIII may impair GSIS in β cells by inhibiting mitochondrial metabolism, and that at least one target of these arsenicals is pyruvate decarboxylation or downstream reactions.

Arsenic exposure induces glucose intolerance and alters global energy metabolism

Environmental pollutants acting as endocrine-disrupting chemicals (EDCs) are recognized as potential contributors to metabolic disease pathogenesis. One such pollutant, arsenic, contaminates the drinking water of ~100 million people globally and has been associated with insulin resistance and diabetes in epidemiological studies. Despite these observations, the precise metabolic derangements induced by arsenic remain incompletely characterized. In the present study, the impact of arsenic on in vivo metabolic physiology was examined in 8-wk-old male C57BL/6J mice exposed to 50 mg/l inorganic arsenite in their drinking water for 8 wk. Glucose metabolism was assessed via in vivo metabolic testing, and feeding behavior was analyzed using indirect calorimetry in metabolic cages. Pancreatic islet composition was assessed via immunofluorescence microscopy. Arsenic-exposed mice exhibited impaired glucose tolerance compared with controls; however, no difference in peripheral insulin resistance was noted between groups. Instead, early insulin release during glucose challenge was attenuated relative to the rise in glycemia. Despite decreased insulin secretion, pancreatic β-cell mass was not altered, suggesting that arsenic primarily disrupts β-cell function. Finally, metabolic cage analyses revealed that arsenic exposure induced novel alterations in the diurnal rhythm of food intake and energy metabolism. Taken together, these data suggest that arsenic exposure impairs glucose tolerance through functional impairments in insulin secretion from β-cells rather than by augmenting peripheral insulin resistance. Further elucidation of the mechanisms underlying arsenic-induced behavioral and β-cell-specific metabolic disruptions will inform future intervention strategies to address this ubiquitous environmental contaminant and novel diabetes risk factor.

Association of Low-Moderate Arsenic Exposure and Arsenic Metabolism with Incident Diabetes and Insulin Resistance in the Strong Heart Family Study

BACKGROUND

High arsenic exposure has been related to diabetes, but at low-moderate levels the evidence is mixed. Arsenic metabolism, which is partly genetically controlled and may rely on certain B vitamins, plays a role in arsenic toxicity.

OBJECTIVE

We evaluated the prospective association of arsenic exposure and metabolism with type 2 diabetes and insulin resistance.

METHODS

We included 1,838 American Indian men and women free of diabetes (median age, 36 y). Arsenic exposure was assessed as the sum of inorganic arsenic (iAs), monomethylarsonate (MMA), and dimethylarsinate (DMA) urine concentrations (ΣAs). Arsenic metabolism was evaluated by the proportions of iAs, MMA, and DMA over their sum (iAs%, MMA%, and DMA%). Homeostasis model assessment for insulin resistance (HOMA2-IR) was measured at baseline and follow-up visits. Incident diabetes was evaluated at follow-up.

RESULTS

Median ΣAs, iAs%, MMA%, and DMA% was 4.4 μg/g creatinine, 9.5%, 14.4%, and 75.6%, respectively. Over 10,327 person-years of follow-up, 252 participants developed diabetes. Median HOMA2-IR at baseline was 1.5. The fully adjusted hazard ratio [95% confidence interval (CI)] for incident diabetes per an interquartile range increase in ΣAs was 1.57 (95% CI: 1.18, 2.08) in participants without prediabetes at baseline. Arsenic metabolism was not associated with incident diabetes. ΣAs was positively associated with HOMA2-IR at baseline but negatively with HOMA2-IR at follow-up. Increased MMA% was associated with lower HOMA2-IR when either iAs% or DMA% decreased. The association of arsenic metabolism with HOMA2-IR differed by B-vitamin intake and AS3MT genetics variants.

CONCLUSIONS

Among participants without baseline prediabetes, arsenic exposure was associated with incident diabetes. Low MMA% was cross-sectional and prospectively associated with higher HOMA2-IR. Research is needed to confirm possible interactions of arsenic metabolism with B vitamins and AS3MT variants on diabetes risk. https://doi.org/10.1289/EHP2566.

Serum lipid, lipoprotein and apolipoprotein profiles in workers exposed to low arsenic levels: Lipid profiles and occupational arsenic exposure

Epidemiologic studies have reported that exposure to arsenic (As) is associated with higher risk of cardiovascular disease (i.e., coronary heart disease and peripheral arterial heart disease) and mortality. This cross-sectional study aimed to compare serum lipid, lipoprotein, and apolipoprotein profiles in workers exposed to As. The subjects of this study included 57 workers exposed to As and 57 controls. Demographic characteristics and occupational information were collected through questionnaires. Exposure to As was assessed in indoor air of a workplace and determined using the creatinine values in the urine. Blood samples were collected using immunochemistry and nephelometry to measure the levels of total cholesterol (CHOL), triglycerides (TRIG), high-density lipoprotein (HDL), low-density lipoprotein (LDL), lipoprotein(a) (Lp(a)), apolipoprotein-A1 (Apo-A1), and apolipoprotein-B (Apo-B). No significant difference in the demographic data was detected between the two groups. Urinary As concentration was significantly (p<0.001) higher in exposed subjects than in the controls (13.4±6.1 and 4.4±6.1μg/gCreat, respectively). No statistically significant differences were observed in CHOL, TRIG, HDL, and LDL concentrations between the two groups. Lp(a), Apo-B, and Apo-B/Apo-A1 ratio values were significantly higher and the Apo-A1 level was significantly lower in the exposed group than in the control subjects. Regression analysis highlighted a significant (p<0.001) association between urinary As and Lp(a), Apo-A1, and Apo-B concentration, and Apo-B/Apo-A1 ratio. This study revealed the influence of As on apolipoproteins, suggesting a potential risk of cardiovascular diseases in subjects exposed to low levels of As.

Polluted Pathways: Mechanisms of Metabolic Disruption by Endocrine Disrupting Chemicals

PURPOSE OF REVIEW

Environmental toxicants are increasingly implicated in the global decline in metabolic health. Focusing on diabetes, herein, the molecular and cellular mechanisms by which metabolism disrupting chemicals (MDCs) impair energy homeostasis are discussed.

RECENT FINDINGS

Emerging data implicate MDC perturbations in a variety of pathways as contributors to metabolic disease pathogenesis, with effects in diverse tissues regulating fuel utilization. Potentiation of traditional metabolic risk factors, such as caloric excess, and emerging threats to metabolism, such as disruptions in circadian rhythms, are important areas of current and future MDC research. Increasing evidence also implicates deleterious effects of MDCs on metabolic programming that occur during vulnerable developmental windows, such as in utero and early post-natal life as well as pregnancy. Recent insights into the mechanisms by which MDCs alter energy homeostasis will advance the field's ability to predict interactions with classical metabolic disease risk factors and empower studies utilizing targeted therapeutics to treat MDC-mediated diabetes.

Exposures to arsenite and methylarsonite produce insulin resistance and impair insulin-dependent glycogen metabolism in hepatocytes

Environmental exposure to inorganic arsenic (iAs) has been shown to disturb glucose homeostasis, leading to diabetes. Previous laboratory studies have suggested several mechanisms that may underlie the diabetogenic effects of iAs exposure, including (i) inhibition of insulin signaling (leading to insulin resistance) in glucose metabolizing peripheral tissues, (ii) inhibition of insulin secretion by pancreatic β cells, and (iii) dysregulation of the methylation or expression of genes involved in maintenance of glucose or insulin metabolism and function. Published studies have also shown that acute or chronic iAs exposures may result in depletion of hepatic glycogen stores. However, effects of iAs on pathways and mechanisms that regulate glycogen metabolism in the liver have never been studied. The present study examined glycogen metabolism in primary murine hepatocytes exposed in vitro to arsenite (iAs³⁺) or its methylated metabolite, methylarsonite (MAs³⁺). The results show that 4-h exposures to iAs³⁺ and MAs³⁺ at concentrations as low as 0.5 and 0.2 µM, respectively, decreased glycogen content in insulin-stimulated hepatocytes by inhibiting insulin-dependent activation of glycogen synthase (GS) and by inducing activity of glycogen phosphorylase (GP). Further investigation revealed that both iAs³⁺ and MAs³⁺ inhibit insulin-dependent phosphorylation of protein kinase B/Akt, one of the mechanisms involved in the regulation of GS and GP by insulin. Thus, inhibition of insulin signaling (i.e., insulin resistance) is likely responsible for the dysregulation of glycogen metabolism in hepatocytes exposed to iAs³⁺ and MAs³⁺. This study provides novel information about the mechanisms by which iAs exposure impairs glucose homeostasis, pointing to hepatic metabolism of glycogen as one of the targets.

Association of arsenobetaine with beta-cell function assessed by homeostasis model assessment (HOMA) in nondiabetic Koreans: data from the fourth Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2009

Background

Arsenic is known as an endocrine disruptor that people are exposed to through various sources such as drinking water and indigestion of marine products. Although some epidemiological and animal studies have reported a correlation between arsenic exposure and diabetes development, there are limited studies regarding the toxic effects of organic arsenic including arsenobetaine on the human body. Here, we analyzed the association between urine arsenobetaine and the homeostasis model assessment of β-cell function (HOMA-β), which is an index for predicting diabetes development and reflecting the function of pancreatic β-cells.

Methods

In the fourth Korea National Health and Nutrition Examination Survey (KNHANES), health and nutrition surveys and screening tests were performed. Of the total survey population, people with confirmed values for urine total arsenic and arsenobetaine were included, and known diabetic patients were excluded. A total 369 participants were finally included in the study. We collected surveys on health, height, body weight, body mass index, blood mercury level, fasting glucose level, and serum insulin level and calculated HOMA index. Owing to sexual discrepancy, we performed sexually stratified analysis.

Results

Urine total arsenic and total arsenic minus arsenobetaine was not associated with HOMA-IR and HOMA-β in univariate analysis or in sexually stratified analysis. However, urine arsenobetaine showed a statistically significant relationship with HOMA-β in univariate analysis, and only male participants showed a significant correlation in sexually stratified analysis. In the analysis adjusted for age, BMI, smoking, alcohol drinking, physical activity and blood mercury, the HOMA-β value in the group below the 25th percentile of arsenobetaine was significantly higher than the group between 50 and 75th percentile, while no difference was shown for HOMA-IR. In sexually stratified analysis, The value of HOMA-β was significantly higher in male participants with below the 25th percentile urine arsenobetaine than the group between 25 and 50th and between 50 and 75th, while no difference was shown for HOMA-IR. However, female participants did not demonstrate a relationship between HOMA–IR, HOMA-β and urine arsenobetaine.

Conclusion

This study revealed the association between urine arsenobetaine and pancreatic β-cell function assessed by HOMA-β in the normal population (without diabetes), especially in males, despite adjusting for factors affecting pancreatic β-cell function and diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s40557-017-0181-0) contains supplementary material, which is available to authorized users.