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

The role of exosome-shuttled miRNAs in heavy metal-induced peripheral tissues and neuroinflammation in Alzheimer's disease

Heavy metal-induced neuroinflammation is a significant pathophysiologic mechanism in Alzheimer's disease (AD). Microglia-mediated neuroinflammation plays a crucial role in the pathogenesis of AD. Multiple miRNAs are differentially expressed in peripheral tissues after heavy metal exposure, and increasing evidence suggests that they are involved in AD progression by regulating microglial homeostasis. Exosomes, which are capable of loading miRNAs and crossing the bloodbrain barrier, serve as mediators of communication between peripheral tissues and the brain. In this review, we summarize the current evidence on the link between miRNAs in peripheral tissues and neuroinflammation in AD after heavy metal exposure and propose a role for miRNAs in the microglial neurodegenerative phenotype (MGnD) of AD. This study will help to elucidate the link between peripheral tissue damage and MGnD-mediated neuroinflammation in AD after heavy metal exposure. Additionally, we summarize the regulatory effects of natural compounds on peripheral tissue-derived miRNAs, which could be potential therapeutic targets for natural compounds to regulate peripheral tissue-derived exosomal miRNAs to ameliorate heavy metal-induced MGnD-mediated neuroinflammation in patients with AD after heavy metal exposure.

The Role of microRNAs in Arsenic-Induced Human Diseases: A Review

MicroRNAs (miRNAs) are noncoding RNAs with 20-22 nucleotides, which are encoded by endogenous genes and are capable of targeting the majority of human mRNAs. Arsenic is regarded as a human carcinogen, which can lead to many adverse health effects including diabetes, skin lesions, kidney disease, neurological impairment, male reproductive injury, and cardiovascular disease (CVD) such as cardiac arrhythmias, ischemic heart failure, and endothelial dysfunction. miRNAs can act as tumor suppressors and oncogenes via directly targeting oncogenes or tumor suppressors. Recently, miRNA dysregulation was considered to be an important mechanism of arsenic-induced human diseases and a potential biomarker to predict the diseases caused by arsenic exposure. Endogenic miRNAs such as miR-21, the miR-200 family, miR-155, and the let-7 family are involved in arsenic-induced human disease by inducing translational repression or RNA degradation and influencing multiple pathways, including mTOR/Arg 1, HIF-1α/VEGF, AKT, c-Myc, MAPK, Wnt, and PI3K pathways. Additionally, exogenous miRNAs derived from plants, such as miR-34a, miR-159, miR-2911, miR-159a, miR-156c, miR-168, etc., among others, can be transported from blood to specific tissue/organ systems in vivo. These exogenous miRNAs might be critical players in the treatment of human diseases by regulating host gene expression. This review summarizes the regulatory mechanisms of miRNAs in arsenic-induced human diseases, including cancers, CVD, and other human diseases. These special miRNAs could serve as potential biomarkers in the management and treatment of human diseases linked to arsenic exposure. Finally, the protective action of exogenous miRNAs, including antitumor, anti-inflammatory, anti-CVD, antioxidant stress, and antivirus are described.

Association between Heavy Metals, Metalloids and Metabolic Syndrome: New Insights and Approaches

Metabolic syndrome (MetS) is an important public health issue that affects millions of people around the world and is growing to pandemic-like proportions. This syndrome is defined by the World Health Organization (WHO) as a pathologic condition characterized by abdominal obesity, insulin resistance, hypertension, and hyperlipidemia. Moreover, the etiology of MetS is multifactorial, involving many environmental factors, including toxicant exposures. Several studies have associated MetS with heavy metals exposure, which is the focus of this review. Environmental and/or occupational exposure to heavy metals are a major risk, contributing to the development of chronic diseases. Of particular note, toxic metals such as mercury, lead, and cadmium may contribute to the development of MetS by altering oxidative stress, IL-6 signaling, apoptosis, altered lipoprotein metabolism, fluid shear stress and atherosclerosis, and other mechanisms. In this review, we discuss the known and potential roles of heavy metals in MetS etiology as well as potential targeted pathways that are associated with MetS. Furthermore, we describe how new approaches involving proteomic and transcriptome analysis, as well as bioinformatic tools, may help bring about an understanding of the involvement of heavy metals and metalloids in MetS.

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.

Cadmium exposure suppresses insulin secretion through mtROS-mediated mitochondrial dysfunction and inflammatory response in pancreatic beta cells

BACKGROUND

Cadmium (Cd) exposure is a worldwide environmental threat to the public health and participates in the pathogenesis of multiple diseases. Epidemiologic research have established a direct relation between Cd exposure and diabetes development in humans. Although pancreatic β-cell dysfunction has been considered as the major culprit in the pathogenesis of diabetes, there is a paucity of studies to elucidate the molecular mechanism of Cd toxicity on β-cells.

METHODS

To unveil the toxic effect and its underlying mechanism of Cd exposure on β-cells, we used an in vitro MIN6 cell model of environment-relevant Cd exposure to elucidate the crucial role of mtROS-mediated mitochondrial dysfunction and inflammatory response in suppression of pancreatic β-cell insulin secretion.

RESULTS

We uncovered that Cd treatment suppresses cell viability and induces insulin secretion dysfunction in a dose-dependent manner. Moreover, Cd exposure elicits the inflammatory response, as indicated by increased IL-1β, IL-6 and TNF-α expressions. Significant elevations of intracellular ROS and mitochondrial ROS levels were detected as early as 3 h after Cd treatment. In mitochondrial function analysis, we demonstrated that Cd treatment induced mitochondrial dysfunction and disorder of mitochondrial fission indicated by the significant decline in ATP production, the marked depolarization of mitochondrial membrane potential, the decrease in mtDNA copy numbers, the suppressions of mitochondrial transcription factor A (Tfam) and mitochondrial fission-related gene Drp1 expressions. Pretreatment with TEMPO, a specific mitochondrial ROS (mtROS) scavenger, efficiently antagonizes Cd cytotoxicity, which is indicated by attenuating Cd-induced mitochondrial dysfunction, suppressing IL-1β, IL-6 and TNF-α expressions, ameliorating insulin production dysfunction and preserving cell viability in MIN6 cells.

CONCLUSION

Our study demonstrates that Cd exposure induces an inflammatory response through mtROS-mediated mitochondrial dysfunction. Antagonism of mtROS production might be an effective strategy to prevent pancreatic toxicity from environment-relevant Cd exposure.

Role of the Transcription Factor MAFA in the Maintenance of Pancreatic β-Cells

Pancreatic β-cells are specialized to properly regulate blood glucose. Maintenance of the mature β-cell phenotype is critical for glucose metabolism, and β-cell failure results in diabetes mellitus. Recent studies provide strong evidence that the mature phenotype of β-cells is maintained by several transcription factors. These factors are also required for β-cell differentiation from endocrine precursors or maturation from immature β-cells during pancreatic development. Because the reduction or loss of these factors leads to β-cell failure and diabetes, inducing the upregulation or inhibiting downregulation of these transcription factors would be beneficial for studies in both diabetes and stem cell biology. Here, we discuss one such factor, i.e., the transcription factor MAFA. MAFA is a basic leucine zipper family transcription factor that can activate the expression of insulin in β-cells with PDX1 and NEUROD1. MAFA is indeed indispensable for the maintenance of not only insulin expression but also function of adult β-cells. With loss of MAFA in type 2 diabetes, β-cells cannot maintain their mature phenotype and are dedifferentiated. In this review, we first briefly summarize the functional roles of MAFA in β-cells and then mainly focus on the molecular mechanism of cell fate conversion regulated by MAFA.

MafA Regulation in β-Cells: From Transcriptional to Post-Translational Mechanisms

β-cells are insulin-producing cells in the pancreas that maintain euglycemic conditions. Pancreatic β-cell maturity and function are regulated by a variety of transcription factors that enable the adequate expression of the cellular machinery involved in nutrient sensing and commensurate insulin secretion. One of the key factors in this regulation is MAF bZIP transcription factor A (MafA). MafA expression is decreased in type 2 diabetes, contributing to β-cell dysfunction and disease progression. The molecular biology underlying MafA is complex, with numerous transcriptional and post-translational regulatory nodes. Understanding these complexities may uncover potential therapeutic targets to ameliorate β-cell dysfunction. This article will summarize the role of MafA in normal β-cell function and disease, with a special focus on known transcriptional and post-translational regulators of MafA expression.

Ameliorative mechanisms of turmeric-extracted curcumin on arsenic (As)-induced biochemical alterations, oxidative damage, and impaired organ functions in rats

Arsenic (As) is known for its carcinogenic and hepatorenal toxic effects causing serious health problems in human beings. Turmeric (Curcuma longa L.) extracted curcumin (Cur) is a polyphenolic antioxidant which has ability to combat hazardous environmental toxicants. This study (28 days) was carried out to investigate the therapeutic efficacy of different doses of Cur (Cur: 80, 160, 240 mg kg^-1) against the oxidative damage in the liver and kidney of male rats caused by sodium arsenate (Na₃AsO₄) (10 mg L^-1). As exposure significantly elevated the values of organ index, markers of hepatic injury (i.e., alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP)) and renal functions (i.e., total bilirubin, urea and creatinine, total cholesterol, total triglycerides, and lipid peroxidation malondialdehyde (MDA)). Moreover, different antioxidant markers such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR) activities in the liver and kidney tissues were reduced after As-induced toxicity. However, Na₃AsO₄ induced histopathological changes in various organs were minimized after the treatment with Cur. The alleviation effect of Cur was dosage dependent with an order of 240>160>80 mg kg^-1. The oral administration of Cur prominently alleviated the As-induced toxicity in liver and kidney tissues by reducing lipid peroxidation, ALT, AST, ALP, total bilirubin, urea, creatinine, total cholesterol, total triglycerides, and low-density lipoproteins (LDL). In addition, Cur being an antioxidant improved defense system by enhancing activities of SOD, CAT, GPx, and GR. Overall, the findings explain the capability of Cur to counteract the oxidative alterations as well as hepatorenal injuries due to As intoxication.

Circulating expression levels of CircHIPK3 and CDR1as circular-RNAs in type 2 diabetes patients

BACKGROUND

Recent investigations suggested that deregulated levels of Circular RNAs (circRNAs) could be associated with type 2 diabetes mellitus (T2DM) pathogenesis. Accordingly, this study aimed to determine the expression levels of circulating CircHIPK3, CDR1as and their correlation with biochemical parameters in patients with T2DM, pre-diabetes and control subjects.

METHODS AND RESULTS

The expression of circRNAs in peripheral blood was determined using QRT-PCR in 70 patients with T2DM, 60 pre-diabetes and in 69 age and sex matched healthy controls. Moreover, bioinformatics tools were applied to explore and predict the potential interactions between circRNAs and other non-coding RNAs (ncRNAs). Our analysis revealed that the expression level of CircHIPK3 was significantly elevated in T2DM patients compared to healthy participants (P < 0.001) and pre-diabetes subjects (P = 0.018). In addition, ROC analysis suggested that at the cutoff value of 0.24 and the sensitivity and specificity of 50% and 88.4%, respectively, CircHIPK3 could distinguish between T2DM patients and control subjects. Furthermore, it was observed that the expression level of CDR1as is higher in pre-diabetic individuals than healthy individuals (P = 0.004). Finally, Spearman correlation analysis showed that there was a significant correlation between CircHIPK3 and CDR1as expression levels and clinical and anthropometrical parameters such as BMI, systolic and diastolic blood pressure, HbA1c and fasting blood glucose (P < 0.005).

CONCLUSIONS

The data of this study provided evidence that the expression levels of CircHIPK3, CDR1as increased in T2DM and pre-diabetes subjects, respectively.

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.

MicroRNA-191 blocking the translocation of GLUT4 is involved in arsenite-induced hepatic insulin resistance through inhibiting the IRS1/AKT pathway

Environmental exposure to arsenic can cause a variety of health problems. Epidemiological and experimental studies have established a diabetogenic role for arsenic, but the mechanisms responsible for arsenic-induced impairment of insulin action are unclear. MicroRNAs (miRNAs) are involved in various metabolic disorders, particularly in the development of insulin resistance. The present study investigated whether arsenite, an active form of arsenic, induces hepatic insulin resistance and the mechanisms underlying it. After male C57BL/6J mice were exposed to arsenite (0 or 20 ppm) in drinking water for 12 months, intraperitoneal glucose tolerance tests (IPGTTs) and insulin tolerance tests (ITTs) revealed an arsenite-induced glucose metabolism disorder. Hepatic glycogen levels were lower in arsenite-exposed mice. Further, for livers of mice exposed to arsenite, miR-191 levels were higher, and protein levels of insulin receptor substrate 1 (IRS1), p-IRS1, and phospho-protein kinase B (p-AKT) were lower. Further, glucose transporter 4 (GLUT4) had lower levels on the plasma membrane. For insulin-treated L-02 cells, arsenite decreased glucose consumption and glycogen levels, increased miR-191 levels, and inhibited the IRS1/AKT pathway and the translocation of GLUT4 from the cytoplasm to the plasma membrane. For insulin-treated L-02 cells, the decreases of glucose consumption, glycogen levels, GLUT4 on the plasma membrane, and p-AKT levels induced by arsenite were reversed by SC79 (agonist of AKT) and an miR-191 inhibitor; these effects caused by miR-191 inhibitor were restored by IRS1 siRNA. In insulin-treated L-02 cells, miR-191, via IRS1, was involved in the arsenite-induced decreases of glucose consumption and glycogen levels and in inhibition of the translocation of GLUT4. Thus, miR-191 blocking the translocation of GLUT4 was involved in arsenite-induced hepatic insulin resistance through inhibiting the IRS1/AKT pathway. Our study reveals a mechanism for arsenite-induced hepatic insulin resistance, which provides clues for discovering biomarkers for the development of type 2 diabetes and for prevention and treatment of arsenic poisoning.

Association of Urinary and Blood Concentrations of Heavy Metals with Measures of Bone Mineral Density Loss: a Data Mining Approach with the Results from the National Health and Nutrition Examination Survey

Osteoporosis and its consequence of fragility fracture represent a major public health problem. Human exposure to heavy metals has received considerable attention over the last decades. However, little is known about the influence of co-exposure to multiple heavy metals on bone density. The present study aimed to examine the association between exposure to metals and bone mineral density (BMD) loss. Blood and urine concentrations of 20 chemical elements were selected from 3 cycles (2005-2010) NHANES (National Health and Nutrition Examination Survey), in which we included white women over 50 years of age and previously selected for BMD testing (N = 1892). The bone loss group was defined as participants having T-score < - 1.0, and the normal group was defined as participants having T-score ≥ - 1.0. We developed classification models based on support vector machines capable of determining which factors could best predict BMD loss. The model which included the five-best features-selected from the random forest were age, body mass index, urinary concentration of arsenic (As), cadmium (Cd), and tungsten (W), which have achieved high scores for accuracy (92.18%), sensitivity (90.50%), and specificity (93.35%). These data demonstrate the importance of these factors and metals to the classification since they alone were capable of generating a classification model with a high prediction of accuracy without requiring the other variables. In summary, our findings provide insight into the important, yet overlooked impact that arsenic, cadmium, and tungsten have on overall bone health.

Pigment epithelium-derived factor (PEDF) ameliorates arsenic-induced vascular endothelial dysfunction in rats and toxicity in endothelial EA.hy926 cells

Although the harmful effects of arsenic exposure on the cardiovascular system have received great attention, there is still no effective treatment. Vascular endothelial dysfunction (VED) is the initial step of cardiovascular diseases, where pigment epithelium-derived factor (PEDF) plays an important role in maintaining endothelial function. Here, we explored the protective role of PEDF in VED induced by arsenic, and its underlying molecular mechanism, designing an in vivo rat model of arsenic exposure recovery and in vitro endothelial EA. hy926 cell-based assays. The edema of aortic endothelial cells in rats significantly improved during recovery from arsenite exposure compared with rats exposed to 10 and 50 mg/L arsenite continuously. In addition, serum levels of nitric oxide (NO), von Willebrand factor, and nitric oxide synthase (inducible and total activities) in rats, which were greatly affected by arsenite exposure, returned to levels similar to those in the control group after recovery with distilled water. The recovery from arsenite exposure was associated with increased levels of PEDF; decreased protein levels of Fas, FasL, P53, and phospho-p38; and inhibited apoptosis in aortic endothelial cells in vivo. Recombinant human PEDF treatment (100 nM) prevented the toxic effects of arsenite (50 μM) on endothelial cells in vitro by increasing NO content, decreasing reactive oxygen species (ROS) levels, and inhibiting apoptosis, as well as increasing cell viability and decreasing levels of P53 and phospho-p38. Our findings suggest that PEDF protects endothelial cells from arsenic-induced VED by increasing NO release and inhibiting apoptosis, where P53 and p38MAPK are its main targets.

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.

Overexpression of miR-297b-5p protects against stearic acid-induced pancreatic β-cell apoptosis by targeting LATS2

Chronic exposure to high concentrations of stearic acid (C18:0) can result in β-cell dysfunction, leading to development of type 2 diabetes. However, the molecular mechanisms underlying the destructive effects of stearic acid on β-cells remain largely unknown. In this study, we aimed to investigate the role of miR-297b-5p on stearic acid-induced β-cell apoptosis. Differential expression of microRNAs (miRNAs) was assessed in a β-TC6 cell line exposed to stearic acid, palmitic acid, or a normal culture medium by high-throughput sequencing. The apoptosis rate was measured by flow cytometry after miR-297b-5p mimic/inhibitor transfection, and large-tumor suppressor kinase 2 (LATS2) was identified as a target of miR-297b-5p using a luciferase activity assay. In vivo, C57BL/6 mice were fed with normal and high-stearic-acid diet, respectively. Mouse islets were used for similar identification of miR-297b-5p and Lats2 in β-TC6 cell. We selected two differentially expressed miRNAs in stearic acid compared with those in the palmitic acid and control groups. miR-297b-5p expression was significantly lower in β-TC6 cells and mouse islets in stearic acid than in control group. Upregulation of miR-297b-5p alleviated the stearic acid-induced cell apoptosis and reduction in insulin secretion by inhibiting Lats2 expression in vitro. Meanwhile, silencing Lats2 significantly reversed the stearic acid-stimulated β-cell dysfunction in both β-TC6 cells and islets. Our findings indicate a suppressive role for miR-297b-5p in stearic acid-induced β-cell apoptosis, which may reveal a potential target for the treatment of β-cell dysfunction in the pathogenesis of type 2 diabetes.

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.