Acute and long-term effects of arsenite in HepG2 cells: modulation of insulin signaling

Selective activation of cellular stress response pathways by fumaric acid esters

The cellular response to oxidants or xenobiotics comprises two key pathways, resulting in modulation of NRF2 and FOXO transcription factors, respectively. Both mount a cytoprotective response, and their activation relies on crucial protein thiol moieties. Using fumaric acid esters (FAEs), known thiol-reactive compounds, we tested for activation of NRF2 and FOXO pathways in cultured human hepatoma cells by dimethyl/diethyl as well as monomethyl/monoethyl fumarate. Whereas only the diesters caused acute glutathione depletion and activation of the stress kinase p38MAPK, all four FAEs stimulated NRF2 stabilization and upregulation of NRF2 target genes. However, no significant FAE-induced activation of FOXO-dependent target gene expression was observed. Therefore, while both NRF2 and FOXO pathways are responsive to oxidants and xenobiotics, FAEs selectively activate NRF2 signaling.

Down-regulation of O-GlcNAcylation alleviates insulin signaling pathway impairment following arsenic exposure via suppressing the AMPK/mTOR-autophagy pathway

Impairment of the insulin signaling pathway is a key contributor to insulin resistance under arsenic exposure. Specifically, O-GlcNAcylation, an important post-translational modification, plays a crucial role in insulin resistance. Nevertheless, the concrete effect and mechanism of O-GlcNAcylation in arsenic-induced impairment of the insulin signaling pathway remain elusive. Herein, C57BL/6 mice were continuously fed arsenic-containing food, with a total arsenic concentration of 30 mg/kg. We observed that the IRS/Akt/GSK-3β insulin signaling pathway was impaired, and autophagy was activated in mouse livers and HepG2 cells exposed to arsenic. Additionally, O-GlcNAcylation expression in mouse livers and HepG2 cells was elevated, and the key O-GlcNAcylation homeostasis enzyme, O-GlcNAc transferase (OGT), was upregulated. In vitro, non-targeted metabolomic analysis showed that metabolic disorder was induced, and inhibition of O-GlcNAcylation restored the metabolic profile of HepG2 cells exposed to arsenic. In addition, we found that the compromised insulin signaling pathway was dependent on AMPK activation. Inhibition of AMPK mitigated autophagy activation and impairment of insulin signaling pathway under arsenic exposure. Furthermore, down-regulation of O-GlcNAcylation inhibited AMPK activation, thereby suppressing autophagy activation, and improving the impaired insulin signaling pathway. Collectively, our findings indicate that arsenic can impair the insulin signaling pathway by regulating O-GlcNAcylation homeostasis. Importantly, O-GlcNAcylation inhibition alleviated the impaired insulin signaling pathway by suppressing the AMPK/mTOR-autophagy pathway. This indicates that regulating O-GlcNAcylation may be a potential intervention for the impaired insulin signaling pathway induced by arsenic.

Arsenic and adipose tissue: an unexplored pathway for toxicity and metabolic dysfunction

Arsenic-contaminated drinking water can induce various disorders by disrupting lipid and glucose metabolism in adipose tissue, leading to insulin resistance. It inhibits adipocyte development and exacerbates insulin resistance, though the precise impact on lipid synthesis and lipolysis remains unclear. This review aims to explore the processes and pathways involved in adipogenesis and lipolysis within adipose tissue concerning arsenic-induced diabetes. Although arsenic exposure is linked to type 2 diabetes, the specific role of adipose tissue in its pathogenesis remains uncertain. The review delves into arsenic's effects on adipose tissue and related signaling pathways, such as SIRT3-FOXO3a, Ras-MAP-AP-1, PI(3)-K-Akt, endoplasmic reticulum stress proteins, CHOP10, and GPCR pathways, emphasizing the role of adipokines. This analysis relies on existing literature, striving to offer a comprehensive understanding of different adipokine categories contributing to arsenic-induced diabetes. The findings reveal that arsenic detrimentally impacts white adipose tissue (WAT) by reducing adipogenesis and promoting lipolysis. Epidemiological studies have hinted at a potential link between arsenic exposure and obesity development, with limited research suggesting a connection to lipodystrophy. Further investigations are needed to elucidate the mechanistic association between arsenic exposure and impaired adipose tissue function, ultimately leading to insulin resistance.

Relationship Between Serum Levels of Arsenic, Cadmium, and Mercury and Body Mass Index and Fasting Plasma Glucose in a Mexican Adult Population

In Mexico, few studies have analyzed the associations between toxic elements and metabolic diseases. In the present study, we analyzed the associations between serum arsenic (As), cadmium (Cd), and mercury (Hg) levels and body mass index (BMI) and fasting plasma glucose (FPG) in a Mexican adult population. Anthropometric data corresponding to 86 Mexican healthy adults (59 females and 27 men) were analyzed. FPG was analyzed by an enzymatic colorimetric method, and serum As, Cd, and Hg levels were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). The data show that the median serum As, Cd, and Hg levels were relatively higher in females (As = 1.78 ng mL^-1, Cd = 1.00 ng mL^-1, Hg = 0.96 ng mL^-1) than those in males (As = 1.22 ng mL^-1, Cd = 0.91 ng mL^-1, Hg = 0.95 ng mL^-1). However, these differences were not statistically significant (p ≥ 0.097). We also found that the median level of As significantly increased with an increase in the body weight categories (normal weight = 1.08; overweight = 1.50; obesity = 2.75; p < 0.001). In addition, a positive association between serum As levels and FPG before and after adjustment for BMI was demonstrated (Rho_Unadjusted = 0.012; (RhoA_djusted = 0.243, p = 0.032). Serum As levels are positively associated with BMI and FPG in the adult population of Mexico. Nevertheless, these results need to be replicated and confirmed with a larger sample size.

Annexin A1 inhibition facilitates NLRP3 inflammasome activation in arsenic-induced insulin resistance in rat liver

Hepatic insulin resistance (IR) is the primary pathology of type 2 diabetes (T2D). The role of the NOD-like receptor protein 3 (NLRP3) inflammasome in arsenic-induced hepatic IR has been previously demonstrated. However, the mechanism of the arsenic-induced activation of the NLRP3 inflammasome is still unclear. Here, we demonstrate that NaAsO₂ downregulated the mRNA and protein level of Annexin A1 (AnxA1), an anti-inflammatory factor, in rat livers and L-02 cells. Moreover, AnxA1 overexpression significantly alleviated arsenic-induced NLRP3 inflammasome activation and IR in L-02 cells. Importantly, Co-immunoprecipitation (Co-IP) results showed that AnxA1 1-190 peptide could bind to the domain encompassing amino acids 1-210 and 211-550 of NLRP3. In conclusion, our experiments demonstrated that arsenic exposure could activate the NLRP3 inflammasome and IR by inhibiting the AnxA1 activity. These findings suggest that AnxA1 may be a promising therapeutic target of arsenicosis.

SOX2 modulated astrocytic process plasticity is involved in arsenic-induced metabolic disorders

Metabolic disorders induced by arsenic exposure have attracted great public concern. However, it remains unclear whether hypothalamus-based central regulation mechanisms are involved in this process. Here, we exposed mice to 100 μg/L arsenic in drinking water and established a chronic arsenic exposure model. Our study revealed that chronic arsenic exposure caused metabolic disorders in mice including impaired glucose metabolism and decreased energy expenditure. Arsenic exposure also impaired glucose sensing and the activation of proopiomelanocortin (POMC) neurons in the hypothalamus. In particular, arsenic exposure damaged the plasticity of hypothalamic astrocytic process. Further research revealed that arsenic exposure inhibited the expression of sex-determining region Y-Box 2 (SOX2), which decreased the expression level of insulin receptors (INSRs) and the phosphorylation of AKT. The conditional deletion of astrocytic SOX2 exacerbated arsenic-induced effects on metabolic disorders, the impairment of hypothalamic astrocytic processes, and the inhibition of INSR/AKT signaling. Furthermore, the arsenic-induced impairment of astrocytic processes and inhibitory effects on INSR/AKT signaling were reversed by SOX2 overexpression in primary hypothalamic astrocytes. Together, we demonstrated here that chronic arsenic exposure caused metabolic disorders by impairing SOX2-modulated hypothalamic astrocytic process plasticity in mice. Our study provides evidence of novel central regulatory mechanisms underlying arsenic-induced metabolic disorders and emphasizes the crucial role of SOX2 in regulating the process plasticity of adult astrocytes.

Prevalence of type 2 diabetes mellitus in relation to arsenic exposure and metabolism in Mexican women

BACKGROUND

Experimental studies have shown the diabetogenic potential of inorganic arsenic (iAs); however, the epidemiological evidence is still inconclusive. This could be explained by differences in exposure, metabolism efficiency, nutritional and genetic factors.

OBJECTIVE

To evaluate the association between type 2 diabetes mellitus (T2DM) prevalence with arsenic exposure and metabolism, considering one-carbon metabolism nutrient intake and arsenite methyltransferase (AS3MT) polymorphisms.

METHODS

From healthy controls of a case control study for female breast cancer in northern Mexico, 227 self-reported diabetic women were age-matched with 454 non-diabetics. Participants were interviewed about dietary, sociodemographic and clinical characteristics. Urinary iAs metabolites were determined by HPLC-ICP-MS, methylation efficiency parameters were calculated, and AS3MT c.860 T > C and c.529-56G > C genotypes were determined. Unconditional logistic regression models were used to evaluate associations.

RESULTS

Total arsenic in urine (TAs) ranged from 0.73 to 248.12 μg/L with a median of 10.48 μg/L. In unadjusted analysis, TAs (μg/g) was significantly higher in cases than controls, but not when expressed as TAs (μg/L). Cases had significantly lower urinary monomethylarsonic acid percentage (%MMA), first methylation ratio (FMR), creatinine, and choline and selenium intakes. In multi-adjusted models and in women without HTA history T2DM showed significant positive associations with %iAs and FMR, respectively, and a significant negative association with %DMA. In participants with HTA history there was a marginal positive association (p = 0.08) between T2DM and TAs concentrations (μg/g) without other significant associations.

CONCLUSIONS

Our results support an association between T2DM prevalence and iAs metabolism but not with urinary arsenic levels. However, elucidation of the interplay among iAs metabolism, T2DM and HTA merit further studies.

Effects of Stocking Density on the Growth Performance, Physiological Parameters, Redox Status and Lipid Metabolism of Micropterus salmoides in Integrated Rice–Fish Farming Systems

Stocking density has been identified as one of the main factors affecting fish growth, welfare and behavior. However, few studies have focused on the effects of stocking density on fish health in integrated rice–fish farming systems. Thus, the aim of this study was to evaluate the effects of different stocking densities on the growth performance, physiological parameters, redox status and lipid metabolism of Micropterus salmoides in an integrated rice–fish farming system. The fish were reared at three densities: low density (LD, 40 g/m³), medium density (MD, 80 g/m³) and high density (HD, 120 g/m³) for 90 days. At the end of the experiment, fish reared in the MD and HD groups showed lower growth performance than those from the LD group. The HD treatment significantly altered the physiological parameters, including glucose and lactate. Meanwhile, the HD treatment induced oxidative stress and lipid peroxidation after 90 days of farming. Furthermore, transcriptomic analysis revealed that HD treatment led to abnormal lipid metabolism. Interestingly, we found the suppression of three key pathways related to lipid metabolism, including the PPAR, insulin and adipocytokine signaling pathways, in the HD group. Overall, our data indicated that the HD treatment inhibited growth and caused physiological responses, oxidative stress and abnormal hepatic lipid metabolism in M. salmoides in an integrated rice–fish farming system.

Arsenic disrupts neuronal insulin signaling through increasing free PI3K-p85 and decreasing PI3K activity

Previously, we reported that prolonged arsenic exposure impaired neuronal insulin signaling. Here we have further identified novel molecular mechanisms underlying neuronal insulin signaling impairment by arsenic. Arsenic treatment altered insulin dose-response curve and reduced maximum insulin response in differentiated human neuroblastoma SH-SY5Y cells, suggesting that arsenic hindered neuronal insulin signaling in a non-competitive like manner. Mechanistically, arsenic suppressed insulin receptor (IR) kinase activity, as witnessed by a decreased insulin-activated autophosphorylation of IR at Y1150/1151. Arsenic decreased the level of insulin receptor substrate 1 (IRS1) but increased the protein ratio between PI3K regulatory subunit, p85, and PI3K catalytic subunit, p110. Interestingly, co-immunoprecipitation demonstrated that arsenic did not alter a level of PI3K-p110/PI3K-p85 complex while increased PI3K-p85 levels in a PI3K-p110 depletion supernatant resulted from PI3K-p110 immunoprecipitation. These results indicated that arsenic increased PI3K-p85 which was free from PI3K-p110 binding. In addition, arsenic significantly increased interaction between IRS1 and PI3K-p85 but not PI3K-p110, suggesting that there may be a fraction of free PI3K-p85 interacting with IRS1. In vitro PI3K activity demonstrated that arsenic lowered PI3K activity in both basal and insulin-stimulated conditions. These results suggested that the increase in free PI3K-p85 by arsenic might compete with PI3K heterodimer for the same IRS1 binding site, in turn blocking the activation of its catalytic subunit, PI3K-p110. Taken together, our results provide additional insights into mechanisms underlying the impairment of neuronal insulin signaling by arsenic through the reduction of IR autophosphorylation, the increase in free PI3K-p85, and the impeding of PI3K activity.

Endocrine disruption and obesity: A current review on environmental obesogens

Obesity represents an important public health concern because it substantially increases the risk of multiple chronic diseases and thereby contributing to a decline in both quality of life and life expectancy. Besides unhealthy diet, physical inactivity and genetic susceptibility, environmental pollutants also contribute to the rising prevalence of obesity epidemic. An environmental obesogen is defined as a chemical that can alter lipid homeostasis to promote adipogenesis and lipid accumulation whereas an endocrine disrupting chemical (EDC) is defined as a synthetic chemical that can interfere with the endocrine function and cause adverse health effects. Many obesogens are EDCs that interfere with normal endocrine regulation of metabolism, adipose tissue development and maintenance, appetite, weight and energy balance. An expanding body of scientific evidence from animal and epidemiological studies has begun to provide links between exposure to EDCs and obesity. Despite the significance of environmental obesogens in the pathogenesis of metabolic diseases, the contribution of synthetic chemical exposure to obesity epidemic remains largely unrecognised. Hence, the purpose of this review is to provide a current update on the evidences from animal and human studies on the role of fourteen environmental obesogens in obesity, a comprehensive view of the mechanisms of action of these obesogens and current green and sustainable chemistry strategies to overcome chemical exposure to prevent obesity. Designing of safer version of obesogens through green chemistry approaches requires a collaborative undertaking to evaluate the toxicity of endocrine disruptors using appropriate experimental methods, which will help in developing a new generation of inherently safer chemicals.

•

Many environmental obesogens are endocrine disrupting chemicals that interfere with normal endocrine regulation of metabolism.

•

Understanding the role of environmental obesogens in the epidemics of obesity is in an infant stage.

•

Green chemistry approach aims to design a safer version of these chemicals by understanding their hazardous effects.

•

Further studies are necessary to fully establish the hazardous effects of obesogens and their association to human obesity.

Oleic Acid Protects from Arsenic-Induced Cardiac Hypertrophy via AMPK/FoxO/NFATc3 Pathway

Arsenic toxicity is one of the major environmental problems causing various diseases, cardiovascular disorders is one of them. Several epidemiological studies have shown that arsenic causes cardiac hypertrophy but the detailed molecular mechanism is to be studied yet. This study is designed to determine the molecules involved in the augmentation of arsenic-induced cardiac hypertrophy. Furthermore, the effects of oleic acid on arsenic-induced hypertrophy and cardiac injury have also been investigated. Our results show that arsenic induces cardiac hypertrophy both in vivo in mice and in vitro in rat H9c2 cardiomyocytes. Moreover, arsenic results in decreased activity of AMPK and FoxO1 along with increased NFATc3 expression, a known cardiac hypertrophy inducer. In addition, activation of AMPK and FoxO1 results in reduced NFATc3 expression causing attenuation of arsenic-induced cardiac hypertrophy in H9c2 cells. Interestingly, we have observed that oleic acid helps in ameliorating cardiac hypertrophy in arsenic-exposed mice. Our studies on protection from arsenic-induced cardiac hypertrophy by oleic acid in H9c2 cells shows that oleic acid activates AMPK along with increased nuclear FoxO1 localization, thereby reducing NFATc3 expression and attenuating cardiomyocyte hypertrophy. This study will help in finding out new avenues in treating arsenic-induced cardiac hypertrophy.

Exposure of low-concentration arsenic induces myotube atrophy by inhibiting an Akt signaling pathway

Arsenic, a widely distributed toxic metalloid, has been found to be associated with the low-birth-weight infants and the impairment of muscle regenerative capacity in areas with high levels of arsenic in drinking water. The distal muscular atrophy is one of side effects of arsenic trioxide (As₂O₃) for acute promyelocytic leukemia therapy. We hypothesized that arsenic may be a potential risk factor for skeletal muscle atrophy. Here, we investigated the action and molecular mechanism of low-dose arsenic on the induction of skeletal muscle atrophy in a skeletal muscle cell model. The differentiated C2C12 myotubes were treated with As₂O₃ (0.25-1 μM) for 48 h without apparent effects on cell viability. The signaling molecules for myotube atrophy were assessed. Submicromolar-concentration As₂O₃ dose-dependently triggered C2C12 myotube atrophy and increased the protein expressions of atrogenes Atrogin1 and MuRF1 and inhibited the upstream phosphorylated proteins Akt and FoxO1, while As₂O₃ dose-dependently increased AMPK phosphorylation in myotubes. Akt activator SC79 could significantly reverse the As₂O₃-induced myotube atrophy. These results suggest that arsenic is capable of inducing myotube atrophy by inhibiting an Akt signaling pathway.

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.

Dietary leucine affects glucose metabolism and lipogenesis involved in TOR/PI3K/Akt signaling pathway for juvenile blunt snout bream Megalobrama amblycephala

The present study evaluated the mechanisms governing insulin signaling, glucose metabolism, and lipogenesis in juvenile fish fed with different dietary leucine levels. Fish were fed six practical diets with graded leucine levels ranging from 0.90 to 2.94% of dry basis for 8 weeks. The trial results showed that, compared to the control group (0.90%), optimal dietary leucine level (1.72%) resulted in the up-regulation of mRNA expression related to insulin signaling pathway, including target of rapamycin (TOR), insulin receptor substrate 1 (IRS-1), phosphoinositide 3-kinase (PI3K), and protein kinase B (Akt). However, an excessive leucine level (2.94%) led to protein S6 kinase 1 (S6K1) overexpression and inhibited TOR, IRS-1, PI3K, and Akt mRNA expressions. The protein level of TOR, S6K1, IRS-1, PI3K, and Akt showed a similar result with mRNA level of these genes. Optimal dietary leucine level (1.72%) significantly improved plasma insulin content, while high level of leucine showed an inhibiting phenomenon. Optimal dietary leucine level (1.72%) could reduce plasma glucose by enhancing the ability of glycometabolism including improving glucose transporter 2 (GLUT2), glucokinase (GK) expressions and down-regulating phosphoenolpyruvate carboxykinase (PEPCK) expression. While an excessive leucine level (2.94%) resulted in high plasma glucose by inhibiting the ability of glycometabolism including lowering GLUT2 and GK expressions, and improving glucose-6-phosphatase (G6Pase) and PEPCK expressions. The relative expressions of pyruvate kinase (PK) and glycogen synthase (GS) were not significantly affected by dietary leucine levels. Dietary leucine level of 1.33% could improve plasma triglyceride content (TG) by enhancing lipogenesis including improving sterol-response element-binding protein 1 (SREBP1), fatty acid synthase (FAS), acetyl CoA carboxylase (ACC), and glucose-6-phosphate dehydrogenase (G6PDH) expressions compared to the control group (0.90%). Total cholesterol (TC) was not significantly affected by dietary leucine levels. The present results indicate that optimal leucine level could improve glycolysis and fatty acid synthesis through improving insulin sensitivity in juvenile blunt snout bream. However, excessive dietary leucine level resulted in high plasma glucose, which led to insulin resistance by inhibiting the gene expressions of insulin signaling pathway and activating gluconeogenesis-related gene expression.

Nuclear trapping of inactive FOXO1 by the Nrf2 activator diethyl maleate

Diethyl maleate (DEM), a thiol-reactive α,β-unsaturated carbonyl compound, depletes glutathione (GSH) in exposed cells and was previously shown by us to elicit a stress response in Caenorhabditis elegans that, at lower concentrations, results in enhanced stress resistance and longer lifespan. This hormetic response was mediated through both the Nrf2 ortholog, SKN-1, and the forkhead box O (FOXO) family transcription factor DAF-16. As FOXO signaling is evolutionarily conserved, we analyzed here the effects of DEM exposure on FOXO in cultured human cells (HepG2, HEK293). DEM elicited nuclear accumulation of GFP-coupled wild-type human FOXO1, as well as of a cysteine-deficient FOXO1 mutant. Despite the nuclear accumulation of FOXO1, neither FOXO1 DNA binding nor FOXO target gene expression were stimulated, suggesting that DEM causes nuclear accumulation but not activation of FOXO1. FOXO1 nuclear exclusion elicited by insulin or xenobiotics such as arsenite or copper ions was attenuated by DEM, suggesting that DEM interfered with nuclear export. In addition, insulin-induced FOXO1 phosphorylation at Thr-24, which is associated with FOXO1 nuclear exclusion, was attenuated upon exposure to DEM. Different from FOXO-dependent expression of genes, Nrf2 target gene mRNAs were elevated upon exposure to DEM. These data suggest that, different from C. elegans, DEM elicits opposing effects on the two stress-responsive transcription factors, Nrf2 and FOXO1, in cultured human cells.

•

Diethyl maleate (DEM) causes FOXO1 nuclear accumulation in HepG2 or HEK293 cells.

•

DEM also elicits nuclear accumulation of the FOXO-ortholog, DAF-16, in C. elegans.

•

Nevertheless, DEM does not elicit expression of FOXO, but of Nrf2 target genes.

•

Insulin-induced FOXO1 phosphorylation and nuclear exclusion are attenuated by DEM.

•

Nuclear accumulation of FOXO1 is due to interference with nuclear export machinery.

Effects of arsenic on adipocyte metabolism: Is arsenic an obesogen?

The environmental obesogen model proposes that in addition to a high-calorie diet and diminished physical activity, other factors such as environmental pollutants and chemicals are involved in the development of obesity. Although arsenic has been recognized as a risk factor for Type 2 Diabetes with a specific mechanism, it is still uncertain whether arsenic is also an obesogen. The impairment of white adipose tissue (WAT) metabolism is crucial in the onset of obesity, and distinct studies have evaluated the effects of arsenic on it, however only in some of them for obesity-related purposes. Thus, the known effects of arsenic on WAT/adipocytes were integrated based on the diverse metabolic and physiological processes that occur in WAT and are altered in obesity, specifically: adipocyte growth, adipokine secretion, lipid metabolism, and glucose metabolism. The currently available information suggests that arsenic can negatively affect WAT metabolism, resulting in arsenic being a potential obesogen.

Cellular adaptation to xenobiotics: Interplay between xenosensors, reactive oxygen species and FOXO transcription factors

Cells adapt to an exposure to xenobiotics by upregulating the biosynthesis of proteins involved in xenobiotic metabolism. This is achieved largely via activation of cellular xenosensors that modulate gene expression. Biotransformation of xenobiotics frequently comes with the generation of reactive oxygen species (ROS). ROS, in turn, are known modulators of signal transduction processes. FOXO (forkhead box, class O) transcription factors are among the proteins deeply involved in the cellular response to stress, including oxidative stress elicited by the formation of ROS. On the one hand, FOXO activity is modulated by ROS, while on the other, FOXO target genes include many that encode antioxidant proteins – thereby establishing a regulatory circuit. Here, the role of ROS and of FOXOs in the regulation of xenosensor transcriptional activities will be discussed. Constitutive androstane receptor (CAR), pregnane X receptor (PXR), peroxisome proliferator-activated receptors (PPARs), arylhydrocarbon receptor (AhR) and nuclear factor erythroid 2-related factor 2 (Nrf2) all interact with FOXOs and/or ROS. The two latter not only fine-tune the activities of xenosensors but also mediate interactions between them. As a consequence, the emerging picture of an interplay between xenosensors, ROS and FOXO transcription factors suggests a modulatory role of ROS and FOXOs in the cellular adaptive response to xenobiotics.

•

Exposure of cells to xenobiotics may trigger formation of reactive oxygen species.

•

Xenosensors respond to xenobiotics by upregulation of xenobiotic metabolism.

•

FOXO transcription factors modulate the activities of several xenosensors.

•

ROS affect FOXO activity, and FOXO target genes include antioxidant proteins.

•

FOXOs bridge xenobiotic-induced ROS generation and xenosensor regulation.

Genetic and epigenetic mechanisms underlying arsenic-associated diabetes mellitus: a perspective of the current evidence

Chronic exposure to arsenic has been associated with the development of diabetes mellitus (DM), a disease characterized by hyperglycemia resulting from dysregulation of glucose homeostasis. This review summarizes four major mechanisms by which arsenic induces diabetes, namely inhibition of insulin-dependent glucose uptake, pancreatic β-cell damage, pancreatic β-cell dysfunction and stimulation of liver gluconeogenesis that are supported by both in vivo and in vitro studies. Additionally, the role of polymorphic variants associated with arsenic toxicity and disease susceptibility, as well as epigenetic modifications associated with arsenic exposure, are considered in the context of arsenic-associated DM. Taken together, in vitro, in vivo and human genetic/epigenetic studies support that arsenic has the potential to induce DM phenotypes and impair key pathways involved in the regulation of glucose homeostasis.

Redox regulation of FoxO transcription factors

Transcription factors of the forkhead box, class O (FoxO) family are important regulators of the cellular stress response and promote the cellular antioxidant defense. On one hand, FoxOs stimulate the transcription of genes coding for antioxidant proteins located in different subcellular compartments, such as in mitochondria (i.e. superoxide dismutase-2, peroxiredoxins 3 and 5) and peroxisomes (catalase), as well as for antioxidant proteins found extracellularly in plasma (e.g., selenoprotein P and ceruloplasmin). On the other hand, reactive oxygen species (ROS) as well as other stressful stimuli that elicit the formation of ROS, may modulate FoxO activity at multiple levels, including posttranslational modifications of FoxOs (such as phosphorylation and acetylation), interaction with coregulators, alterations in FoxO subcellular localization, protein synthesis and stability. Moreover, transcriptional and posttranscriptional control of the expression of genes coding for FoxOs is sensitive to ROS. Here, we review these aspects of FoxO biology focusing on redox regulation of FoxO signaling, and with emphasis on the interplay between ROS and FoxOs under various physiological and pathophysiological conditions. Of particular interest are the dual role played by FoxOs in cancer development and their key role in whole body nutrient homeostasis, modulating metabolic adaptations and/or disturbances in response to low vs. high nutrient intake. Examples discussed here include calorie restriction and starvation as well as adipogenesis, obesity and type 2 diabetes.

Arsenic Induces Insulin Resistance in Mouse Adipocytes and Myotubes Via Oxidative Stress-Regulated Mitochondrial Sirt3-FOXO3a Signaling Pathway

Chronic exposure to arsenic via drinking water is associated with an increased risk for development of type 2 diabetes mellitus (T2DM). This study investigates the role of mitochondrial oxidative stress protein Sirtuin 3 (Sirt3) and its targeting proteins in chronic arsenic-induced T2DM in mouse adipocytes and myotubes. The results show that chronic arsenic exposure significantly decreased insulin-stimulated glucose uptake (ISGU) in correlation with reduced expression of insulin-regulated glucose transporter type 4 (Glut4). Expression of Sirt3, a mitochondrial deacetylase, was dramatically decreased along with its associated transcription factor, forkhead box O3 (FOXO3a) upon arsenic exposure. A decrease in mitochondrial membrane potential (Δψm) was observed in both 3T3L1 adipocytes and C2C12 myotubes treated by arsenic. Reduced FOXO3a activity by arsenic exhibited a decreased binding affinity to the promoters of both manganese superoxide dismutase (MnSOD) and peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α, a broad and powerful regulator of reactive oxygen species (ROS) metabolism. Forced expression of Sirt3 or MnSOD in mouse myotubes elevated Δψm and restored ISGU inhibited by arsenic exposure. Our results suggest that Sirt3/FOXO3a/MnSOD signaling plays a significant role in the inhibition of ISGU induced by chronic arsenic exposure.