Glycemic Changes Related to Arsenic Exposure: An Overview of Animal and Human Studies

Background: Arsenic (As) is a risk factor associated with glycemic alterations. However, the mechanisms of action and metabolic aspects associated with changes in glycemic profiles have not yet been completely elucidated. Therefore, in this review, we aimed to investigate the metabolic aspects of As and its mechanism of action associated with glycemic changes. Methods: We searched the PubMed (MEDLINE) and Google Scholar databases for relevant articles published in English. A combination of free text and medical subject heading keywords and search terms was used to construct search equations. The search yielded 466 articles; however, only 50 were included in the review. Results: We observed that the relationship between As exposure and glycemic alterations in humans may be associated with sex, smoking status, body mass index, age, occupation, and genetic factors. The main mechanisms of action associated with changes induced by exposure to As in the glycemic profile identified in animals are increased oxidative stress, reduced expression of glucose transporter type 4, induction of inflammatory factor expression and dysfunction of pancreatic β cells. Conclusions: Therefore, As exposure may be associated with glycemic alterations according to inter-individual differences.


Introduction
Endocrine disorders are among the harmful effects of heavy metal exposure on human health, resulting in changes in the hypothalamus-pituitary axis and the interruption of the secretion of hormones such as thyroid-stimulating, luteinizing, and adrenocorticotropic hormones, and prolactin [1].Exposure to endocrine-disrupting chemicals, including arsenic (As), is associated with diabetes mellitus (DM) etiology.As it accumulates in the liver, kidney, and pancreas, it has harmful effects on carbohydrate metabolism pathways, especially glycolysis, glycogenesis, and gluconeogenesis [2].
Environmental exposure to As has been observed in several countries, including the United States (USA) [3], Croatia [4], Mexico [5], and Spain [6].As contamination in Brazil has been recorded in soils, sediments, and water sources in the northern, southern, and southeastern regions, mainly arising from anthropogenic actions [7].
As is highly toxic and can be found in nature in the elemental forms As0, As3+, and As5+ and as organic arsenobetaine, arsenosugars, and arsenolipids, predominantly in seafood [8].The inorganic forms (iAs), arsenite and arsenate, are present in drinking water, food, dust, and ambient air [9].Considerable iAs concentrations have been detected in different types of rice depending on the cultivation method, processing, and country of production [10].
Organic forms of As are nontoxic because they are not metabolized and are rapidly excreted [11].In contrast, iAs undergoes several methylations and are converted into monomethyl arsenic (MAsIII and MAsV) and dimethyl arsenic (DMAsIII and DMAsV) compounds [12], which are excreted in the urine together with unchanged As [13].
Studies in animal models exposed to iAs have identified increased fasting glucose and insulin levels [12], glucose intolerance, reduced insulin resistance index (HOMA-IR) [13], and oxidative damage [14].Yang et al. [15] identified low to moderate concentrations of As that were not associated with the development of type 2 DM (T2DM) throughout life.Another study observed that the total urinary concentration of As was positively correlated with the prevalence of T2DM and prediabetes [16].
However, the mechanisms of action associated with these effects have not yet been elucidated.Therefore, in this review, we aimed to investigate the metabolic aspects of As and its relationship with glycemic changes.

Materials and Methods
The PubMed (MEDLINE) database was used as the primary source of potentially relevant studies.Google Scholar was used as the secondary source, with searches limited to 100 reports, sorted by relevance ranking [17].We searched databases for articles published between January 1998 and February 2023.The eligibility criteria included full-text articles on animal and human studies published in English.Narrative reviews, systematic reviews, commentaries, correspondences, editorials, in vitro studies, and studies with self-reported DM diagnoses were excluded.
A search was conducted using a combination of free text and medical subject heading (MeSH) search terms and keywords, namely "Diabetes mellitus", "Prediabetic states", "Hyperglycemia", "Glucose intolerance", "Arsenic", and "Arsenite", based on each database characteristic.Both the keywords and search terms were used to construct the search equations.The reports were transferred to the Rayyan-Intelligent Systematic Review application developed by the Qatar Computing Research Institute [18] for the selection procedures.Two investigators independently selected the studies by analyzing the titles, abstracts, and keywords.

Results
The search yielded 466 articles; however, only 50 were included in this narrative review (Figure 1).

Forms and Sources of As
As is a toxic metalloid from natural and anthropogenic sources and is found in water, soil, and air [57].In nature, As is present in the elemental form As0, or in combination with other metalloids, in the trivalent (As3+), pentavalent (As5+), and organic forms [11] iAs, such as arsenite and arsenate, are found in drinking water, food, dust, and ambient air [9].
In contrast, organic forms such as arsenobetaine, arsenosugars, and arsenolipids are found mainly in seafood [35].
Industrial pollution [59], mineral extraction, and use of pig feed additives [10], fertilizers, and pesticides [60] are environmental sources of As contamination.A study carried out in the Middle Banat (Serbia) region identified a median As concentration of 56.1 µg/L in the water of public supply systems [37].
Based on the total dietary exposure to iAs, the maximum tolerated intake for humans is estimated to be between 2 and 7 g/kg body weight/day [61].Drinking water is a significant contributor to the dietary exposure to total iAs.Among foods with the highest As contamination, rice ranks at the top [39] and, when adulterated, milk can contain water containing As [62].
Depending on the concentration, As contamination can also occur through food preparation and crop irrigation.The urinary concentration of As may vary according to demographic characteristics and lifestyle, being higher in agricultural workers, forestry workers, fishers, artisans, operators of installations and machines, and assemblers [10].Regarding lifestyle, lower concentrations of urinary As have been detected among current smokers and non-drinkers [10].

As Absorption, Metabolism, and Excretion
The different forms of As are metabolized in different ways: organic As is excreted unchanged through the urine [35], whereas iAs is methylated by the enzyme arsenic methyltransferase (AS3MT) [9].Initially, iAs is methylated and reduced to monomethyl arsenic (MAsIII and MAsV), and the process is repeated, forming dimethyl arsenic (DMAsIII and DMAsV) [35] (Figure 2).
Thus, As metabolism is directly associated with methylation reactions and, as a consequence, requires the generation of methyl groups, the availability of which depends on essential nutrients such as folate, vitamin B12, vitamin B6, vitamin B2, and methionine [6,44].
iAs poses a greater risk to human health, whereas organic As forms are considered nontoxic because they are rapidly excreted [10].Furthermore, other variables, such as valence, physical state, solubility, purity, absorption, and elimination rate, also influence As toxicity [63].
Inter-individual differences in the As methylation capacity have nongenetic determinants, including sex, smoking, diet, and BMI [64].Women have a higher % DMA than men, never-smokers generally have a higher % DMA than current smokers, dietary deficiencies of folate and vitamins are associated with lower % DMA, and obese individuals have a higher % DMA [9].
Regarding nutritional status and BMI, a lower intake of methyl-group-containing diet may result in lower As methylation [65].Individuals with higher BMI have been found to consume more cofactors used in As metabolism [66], and that body fat may interfere with As storage [64].
Hormonal differences, mainly in estrogen levels, likely explain the sex differences in As methylation capacity [64].With regard to smoking, the chemical substances in cigarettes compete for some enzymes or cofactors involved in As methylation processes [64].Furthermore, aging may be related to alterations in As metabolism owing to functional disturbances in metabolite excretion [64].Genetic factors are also involved in As metabolism, and higher urinary concentrations of DMAs have been identified in individuals harboring the M287T and G4965C variants of AS3MT [32].
As metabolism is speculated to differ under DM conditions.Diabetic mice (C57BKS/Lepr db) exposed to higher concentrations of iAs exhibited lower urinary excretion and a higher degree of As methylation than healthy mice [14].From this perspective, genetic variations could increase susceptibility to DM2 among individuals exposed to iAs [51].Thus, As metabolism is directly associated with methylation reactions and, as a consequence, requires the generation of methyl groups, the availability of which depends on essential nutrients such as folate, vitamin B12, vitamin B6, vitamin B2, and methionine [6,44].
iAs poses a greater risk to human health, whereas organic As forms are considered nontoxic because they are rapidly excreted [10].Furthermore, other variables, such as valence, physical state, solubility, purity, absorption, and elimination rate, also influence As toxicity [63].
Inter-individual differences in the As methylation capacity have nongenetic determinants, including sex, smoking, diet, and BMI [64].Women have a higher % DMA than men, never-smokers generally have a higher % DMA than current smokers, dietary deficiencies of folate and vitamins are associated with lower % DMA, and obese individuals have a higher % DMA [9].
Regarding nutritional status and BMI, a lower intake of methyl-group-containing diet may result in lower As methylation [65].Individuals with higher BMI have been found to consume more cofactors used in As metabolism [66], and that body fat may interfere with As storage [64].
Hormonal differences, mainly in estrogen levels, likely explain the sex differences in As methylation capacity [64].With regard to smoking, the chemical substances in cigarettes compete for some enzymes or cofactors involved in As methylation processes [64].
Peng et al. (2015) [43] found no changes in HOMA-IR in adolescents exposed to low As levels [43].Accordingly, in a population with low to moderate As exposure, As concentrations in the toenails were not associated with fasting blood glucose and insulin levels and HOMA-IR [15].Individuals from Bangladesh exposed to moderate and high doses of As were found to be more vulnerable to hyperglycemia [42].
In animal studies, acute exposure of Sprague-Dawley rats to 2, 4, and 8 mg/kg As for 15 to 120 min increased their blood glucose concentrations [30].Gong et al. (2019) [23] found that exposure to 0.25 ppm iAs caused glucose intolerance in C57BL/6 mice, whereas the group exposed to 2.5 ppm iAs did not show significant changes in glucose tolerance [23].The inconsistency between these findings implies a dose-time-response relationship between As exposure and glycemic alterations.
Interventions using antioxidant compounds can prevent the harmful effects of As toxicity.Rezaei et al. (2017) [30] found that pretreatment of Sprague-Dawley rats with 40 and 80 mg/kg N-acetylcysteine prevented As-induced glucose disturbances.The toxicity of As is related to its chemical form, oxidation state, and exposure dose [67].The biotransformation process of As involves methylation, which results in more toxic final metabolites.The increased acute toxicity of methylated trivalent As intermediates suggests that As methylation is not simply a detoxification mechanism [67].
Reactive Oxygen Species (ROS) mediated oxidative damage is a common denominator in As pathogenesis [68].As-triggered T2DM has been reported to contribute to the mitochondrial overproduction of ROS [12], and compensatory As-induced oxidative stress leads to an increase in the activity of antioxidant enzymes, such as total glutathione [27,29], superoxide dismutase, catalase, and glutathione-S-transferase activity [27].As a result of oxidative stress, tissue injury may occur, causing an increase in the inflammatory focus and release of tumor necrosis factor alpha (TNF-α) [28].
ROS are involved in intracellular signaling processes, regulation of cellular activity, and immune responses [69].Increased ROS stimulate inflammatory responses that damage key cellular components, including lipids, proteins, and deoxyribonucleic acid (DNA) [69,70].
With increased inflammation, TNF-α may play a role in causing fatty insulin resistance in patients with T2DM [71].Exposure to As results in increased oxidative stress, and consequently, apoptosis of human hepatocytes of the Chang lineage [72].
Additionally, a study conducted in Wistar rats showed that exposure to As 2 O 3 counteractingly increased the concentration of sirtuin 3, which is responsible for safeguarding the mitochondria against damage induced by free radicals.This illustrates the diabetogenic potential of As, as it disrupts mitochondrial respiration by reducing membrane potential, and consequently, cellular respiration and signaling [12,70].

Damage Caused to DNA
The potential genotoxic damage related to iAs exposure has also been studied.iAs may be associated with single-strand DNA breaks, the formation of apurinic and apyrimidine sites, oxidation of DNA bases, DNA-protein cross-linking, and chromosomal aberrations [68].Industrial workers exposed to As face a significant risk of genetic instability due to damage caused by oxidative stress, which is induced by the downregulation of the OGG1 and HPRT genes [73].
Furthermore, epigenetic changes have been suggested to play a significant role in the mechanism of action of iAs by altering methylation patterns [74].A relationship has been identified between exposure to low and moderate concentrations of As and the methylation of SLC7A11, a gene associated with the biosynthesis of glutathione, a crucial endogenous antioxidant that may provide protection against As-induced oxidative stress [74].
However, further studies are required to determine whether these DNA methylation profiles provide mechanistic insights into the development of iAs-associated diseases or serve as biomarkers for iAs exposure in humans [32].

Reduced GLUT4 Expression and Reduced PPARγ Expression
Li et al. (2021) investigated the response of adipose cells to exposure to iAs and MAs and observed the suppression of PKB/Akt phosphorylation and interference with GLUT4 translocation.Therefore, when GLUT4 recruitment to the membrane becomes unviable, insulin-stimulated glucose uptake is compromised [25].
Prolonged exposure to iAs results in glucose intolerance, insulin resistance and lower PPARγ expression in mice.Impaired expression of PPARγ results in repression of adipocyte differentiation, increased lipolysis, and decreased insulin sensitivity [63].

Increased Gluconeogenesis and Pancreatic β-Cell Dysfunction
In individuals with T2DM, an inverse association was identified between urinary As concentration and the function of β-cells, which are possibly more susceptible to damage caused by iAs exposure than those in healthy individuals [50].It is noteworthy that pancreatic β cells are highly sensitive to oxidative stress, resulting in the induction of chronic inflammation and cell apoptosis [57].Experimental studies have demonstrated that As induces β-cell destruction, thereby impairing insulin production and release and glucose-driven insulin secretion [14].
Furthermore, As exposure is associated with increased gluconeogenesis, which may contribute to increased fasting blood glucose levels and lower glucose tolerance [14].The detrimental effects associated with carbohydrate metabolism pathways, such as glycolysis, glycogenesis, and gluconeogenesis, may occur due to the tendency of As to primarily accumulate in the liver, kidneys, and pancreas [75,76].
This accumulation induces alterations in enzymatic configuration, resulting in the modification of the active site and, consequently, enzyme activity [75,76].Notably, As has the capacity to alter the enzymatic activity of pyruvate dehydrogenase, thereby interfering with the Krebs cycle and inhibiting oxidative phosphorylation, ultimately resulting in cellular damage [76] 2019) identified that exposure to heavy metals, including As, is associated with significant global alterations in the intestinal microbiome, affecting bacterial genera associated with T2DM [22].Exposure to heavy metals slows growth and modifies the structure of phyla within the intestinal microbiome, affecting the biological functions of the microbiota, including metabolism and immunity [77].Immunomediated reactions triggered by changes in the microbiota composition are likely to facilitate the development of DM in predisposed individuals [78].
In a recent Strong Heart Family Study involving 59 participants, it was hypothesized that the one-carbon metabolic (OCM) pathway could influence the relationship between As metabolism and diabetes.After metabolomic analyses, eight metabolites of interest correlated with DM-related outcomes, including LPS 18:2, which is strongly associated with As metabolism and central obesity [44].Even with promising discoveries, there is still a lack of robust evidence on the subject that elucidates the mechanisms that interconnect exposure to As with changes in the microbiota, metabolomics, and metabolism in DM.
Finally, owing to the cross-sectional design characteristics of most of the articles included in this review, the causal relationship between As exposure and glycemic alterations is unclear.In addition, exposure to As was mostly based on the quantification of total As in a single urine and/or blood sample.Therefore, future studies with a longitudinal design are suggested to quantify the concentrations of not only total As but also of other forms of As at different time points.

Conclusions
Exposure to As may be associated with glycemic alterations, such as hyperglycemia and insulin resistance, in animals and humans.In addition, there is an increase in hemoglobin A1c level and the risk of DM and prediabetes in humans, according to interindividual factors.The main mechanisms of action associated with glycemic profiles identified in animals change due to As exposure are increased oxidative stress, reduced GLUT4 expression, induction of expression of inflammatory factors, and pancreatic β cell dysfunction.However, more studies are needed to elucidate the relationship between the dose and duration of exposure to As for outcomes related to changes in the glycemic profile.Of note, an animal model study has shed light on the role of antioxidants in preventing glycemic changes associated with As exposure.

Figure 2 .
Figure 2. Metabolism of different forms of As.The organic form is excreted and unchanged, whereas iAs undergoes methylation and reduction processes by the enzyme arsenic methyltransferase (AS3MT).Created with Canva.com.(Figure illustration by Rosendo, G.B.R).

Figure 2 .
Figure 2. Metabolism of different forms of As.The organic form is excreted and unchanged, whereas iAs undergoes methylation and reduction processes by the enzyme arsenic methyltransferase (AS3MT).Created with https://www.Canva.com.(Figure illustration by Rosendo, G.B.R).

Table 1 .
Characteristics of animal studies included in the review.

Table 2 .
Characteristics of the human studies included in the review.