Characterization of the Intestinal Absorption of Arsenate, Monomethylarsonic Acid, and Dimethylarsinic Acid Using the Caco-2 Cell Line

Effects of arsenic on gut microbiota and its bioaccumulation and biotransformation in freshwater invertebrate

This study aimed to investigate the impact of arsenic stress on the gut microbiota of a freshwater invertebrate, specifically the apple snail (Pomacea canaliculata), and elucidate its potential role in arsenic bioaccumulation and biotransformation. Waterborne arsenic exposure experiments were conducted to characterize the snail's gut microbiomes. The results indicate that low concentration of arsenic increased the abundance of gut bacteria, while high concentration decreased it. The dominant bacterial phyla in the snail were Proteobacteria, Firmicutes, Bacteroidota, and Actinobacteriota. In vitro analyses confirmed the critical involvement of the gut microbiota in arsenic bioaccumulation and biotransformation. To further validate the functionality of the gut microbiota in vivo, antibiotic treatment was administered to eliminate the gut microbiota in the snails, followed by exposure to waterborne arsenic. The results demonstrated that antibiotic treatment reduced the total arsenic content and the proportion of arsenobetaine in the snail's body. Moreover, the utilization of physiologically based pharmacokinetic modeling provided a deeper understanding of the processes of bioaccumulation, metabolism, and distribution. In conclusion, our research highlights the adaptive response of gut microbiota to arsenic stress and provides valuable insights into their potential role in the bioaccumulation and biotransformation of arsenic in host organisms. ENVIRONMENTAL IMPLICATION: Arsenic, a widely distributed and carcinogenic metalloid, with significant implications for its toxicity to both humans and aquatic organisms. The present study aimed to investigate the effects of As on gut microbiota and its bioaccumulation and biotransformation in freshwater invertebrates. These results help us to understand the mechanism of gut microbiota in aquatic invertebrates responding to As stress and the role of gut microbiota in As bioaccumulation and biotransformation.

Arsenic and iron bioavailability in Caco-2 cells: The influence of their co-existence and concentration

Arsenic (As) exposure in humans is primarily caused through food and drinking water. Iron (Fe) is one of the most common element of the human and can influence the toxicity and bioavailability of As. However, information on the interaction between As and Fe when present together is limited. In this study, the interaction effects of Fe(III) (0, 3, and 10 mg/L) and As (As(III) at 0, 0.05, 0.1 mg/L, and As(V) at 0, 0.1, and 2 mg/L, respectively) on their absorption and bioavailability in Caco-2 cells were analyzed. As(III) absorption significantly decreased with the addition of Fe, while Fe absorption significantly increased. Compared with 0.1 mg/L As(III) addition alone, 3 and 10 mg/L Fe(III) addition significantly reduced the As(III) absorption by 8.6 and 11 μg/L, respectively. The absorption of As and Fe(III) and the bioavailability of Fe(III) significantly increased with the addition of As(III/V). Compared with 10 mg/L Fe(III) alone, the absorption of As(III) was significantly increased by 1 and 1.3 mg/L with 0.05 and 0.1 mg/L As(III) addition, respectively. Furthermore, the absorption and bioavailability of Fe(III) were significantly increased by 1.2 mg/L and 8% and 1.2 mg/L and 8.2%, respectively, after adding 0.1 and 2 mg/L As(V).

Differential toxicity of potentially toxic elements to human gut microbes

Specific microorganisms in the human gut (i.e., gut microbes) provide mutually beneficial outcomes such as microbial balance by inhibiting the growth of pathogenic organisms, immune system modulation, fermentation of ingested products, and vitamin production. The intake of contaminants including potenially toxic elements (PTEs) can occur through food, air, water and some medicines. The gut microbes not only can be affected by environmental contaminants but they themselves can alter the speciation and bioavailability of these contaminants. This research work was designed to demonstrate the relationship between increasing level of selected PTEs including As, Cd, Pb and Hg on the growth of selected gut microbes. The toxicity of above mentioned PTEs to three gut bacteria (Lactobacillus rhamnosus, Lactobacillus acidophilus and Escherichia coli) was examined. While the toxicity of all the cationic PTEs including Cd, Pb and Hg towards gut bacteria decreased with increasing pH, the anionic As species exhibited an opposite effect. The order of toxicity was Hg > Cd > Pb > As(III)>As(V) for E. coli; and Hg > Cd > As(III)>Pb > As(V) for the two Lactobacillus sp. Arsenite (AsIII) showed higher toxicity than arsenate (AsV) to gut bacteria. While As is an anion, Cd, Pb and Hg are cations and hence their binding capacity to the bacterial cell wall varied based on the charge dependent functional groups. However, the toxic effects of PTEs for a bacteria are controlled by their speciation and bioavailability.

Arsenic Exposure and Compromised Protein Quality Control

Exposure to arsenic in contaminated drinking water is a worldwide public health problem that affects more than 200 million people. Protein quality control constitutes an evolutionarily conserved mechanism for promoting proper folding of proteins, refolding of misfolded proteins, and removal of aggregated proteins, thereby maintaining homeostasis of the proteome (i.e., proteostasis). Accumulating lines of evidence from epidemiological and laboratory studies revealed that chronic exposure to inorganic arsenic species can elicit proteinopathies that contribute to neurodegenerative disorders, cancer, and type II diabetes. Here, we review the effects of arsenic exposure on perturbing various elements of the proteostasis network, including mitochondrial homeostasis, molecular chaperones, inflammatory response, ubiquitin-proteasome system, autophagy, as well as asymmetric segregation and axonal transport of misfolded proteins. We also discuss arsenic-induced disruptions of post-translational modifications of proteins, for example, ubiquitination, and their implications in proteostasis. Together, studies in the past few decades support that disruption of protein quality control may constitute an important mechanism underlying the arsenic-induced toxicity.

Biotransformation of arsenic and toxicological implication of arsenic metabolites

Arsenic is a well-known environmental carcinogen and chronic exposure to arsenic through drinking water has been reported to cause skin, bladder and lung cancers, with arsenic metabolites being implicated in the pathogenesis. In contrast, arsenic trioxide (As₂O₃) is an effective therapeutic agent for the treatment of acute promyelocytic leukemia, in which the binding of arsenite (iAs^III) to promyelocytic leukemia (PML) protein is the proposed initial step. These findings on the two-edged sword characteristics of arsenic suggest that after entry into cells, arsenic reaches the nucleus and triggers various nuclear events. Arsenic is reduced, conjugated with glutathione, and methylated in the cytosol. These biotransformations, including the production of reactive metabolic intermediates, appear to determine the intracellular dynamics, target organs, and biological functions of arsenic.

Arsenic intoxication: general aspects and chelating agents

Arsenic (As) is widely used in the modern industry, especially in the production of pesticides, herbicides, wood preservatives, and semiconductors. The sources of As such as contaminated water, air, soil, but also food, can cause serious human diseases. The complex mechanism of As toxicity in the human body is associated with the generation of free radicals and the induction of oxidative damage in the cell. One effective strategy in reducing the toxic effects of As is the usage of chelating agents, which provide the formation of inert chelator–metal complexes with their further excretion from the body. This review discusses different aspects of the use of metal chelators, alone or in combination, in the treatment of As poisoning. Consideration is given to the therapeutic effect of thiol chelators such as meso-2,3-dimercaptosuccinic acid, sodium 2,3-dimercapto-1-propanesulfonate, 2,3-dimercaptopropanol, penicillamine, ethylenediaminetetraacetic acid, and other recent agents against As toxicity. The review also considers the possible role of flavonoids, trace elements, and herbal drugs as promising natural chelating and detoxifying agents.

Embryonic arsenic exposure reduces intestinal cell proliferation and alters hepatic IGF mRNA expression in killifish (Fundulus heteroclitus)

Arsenic (As) is a toxicant found in food and water throughout the world, and studies suggested that exposure early in life reduces growth. Thus, the goal of this study was to examine mechanisms by which As impacted organismal growth. Killifish (Fundulus heteroclitus) were exposed to 0, 10, 50, or 200 ppb As as embryos and, after hatching, were reared in clean water for up to 40 weeks. Metabolism studies revealed that killifish biotransform As such that monomethylated and dimethylated arsenicals account for 15-17% and 45-61%, respectively, of the total metal. Growth, as measured by condition factor (CF), was significantly and dose-dependently reduced at 8 weeks of age but was similar to controls by 40 weeks. To determine mechanisms underlying the observed initial decrease, intestinal proliferation and morphology were examined. Arsenic-exposed fish exhibited significant 1.3- to 1.5-fold reduction in intestinal villus height and 1.4- to 1.6-fold decrease in proliferating cell nuclear antigen (PCNA+) intestinal cells at all weeks examined. In addition, there were significant correlations between CF, PCNA+ cells, and intestinal villus height. Upon examining whether fish might compensate for the intestinal changes, it was found that hepatic mRNA expression of insulin-like growth factor 1 (IGF-1) and its binding protein (IGFBP-1) were dose-dependently increased. These results indicate that embryonic exposure initially diminished growth, and while intestinal cell proliferation remained reduced, fish appear to compensate by enhancing transcript levels of hepatic IGF-1 and IGFBP-1.

Using mathematical modeling to infer the valence state of arsenicals in tissues: A PBPK model for dimethylarsinic acid (DMAV) and dimethylarsinous acid (DMAIII) in mice

Chronic exposure to inorganic arsenic (iAs), a contaminant of water and food supplies, is associated with many adverse health effects. A notable feature of iAs metabolism is sequential methylation reactions which produce mono- and di-methylated arsenicals that can contain arsenic in either the trivalent (III) or pentavalent (V) valence states. Because methylated arsenicals containing trivalent arsenic are more potent toxicants than their pentavalent counterparts, the ability to distinguish between the +3 and +5 valence states is a crucial property for physiologically based pharmacokinetic (PBPK) models of arsenicals to possess if they are to be of use in risk assessment. Unfortunately, current analytic techniques for quantifying arsenicals in tissues disrupt the valence state; hence, pharmacokinetic studies in animals, used for model calibration, only reliably provide data on the sum of the +3 and +5 valence forms of a given metabolite. In this paper we show how mathematical modeling can be used to overcome this obstacle and present a PBPK model for the dimethylated metabolite of iAs, which exists as either dimethylarsinous acid, (CH3)2AsIIIOH (abbreviated DMAIII) or dimethylarsinic acid, (CH3)2AsV(O)OH (abbreviated DMAV). The model distinguishes these two forms and sets a lower bound on how much of an organ's DMA burden is present in the more reactive and toxic trivalent valence state. We conjoin the PBPK model to a simple model for DMAIII-induced oxidative stress in liver and use this extended model to predict cytotoxicity in liver in response to the high oral dose of DMAV. The model incorporates mechanistic details derived from in vitro studies and is iteratively calibrated with lumped-valence-state PK data for intravenous or oral dosing with DMAV. Model formulation leads us to predict that orally administered DMAV undergoes extensive reduction in the gastrointestinal (GI) tract to the more toxic trivalent DMAIII.

Inorganic arsenic causes intestinal barrier disruption

Exposure to inorganic arsenic, principally to As(iii), has an effect on intestinal permeability, causing a loss of intestinal barrier function.

Salivary and Gut Microbiomes Play a Significant Role in in Vitro Oral Bioaccessibility, Biotransformation, and Intestinal Absorption of Arsenic from Food

The release of a toxicant from a food matrix during the gastrointestinal digestion is a crucial determinant of the toxicant's oral bioavailability. We present a modified setup of the human simulator of the gut microbial ecosystem (SHIME), with four sequential gastrointestinal reactors (oral, stomach, small intestine, and colon), including the salivary and colonic microbiomes. Naturally arsenic-containing rice, mussels, and nori seaweed were digested in the presence of microorganisms and in vitro oral bioaccessibility, bioavailability, and metabolism of arsenic species were evaluated following analysis by using HPLC/mass spectrometry. When food matrices were digested with salivary bacteria, the soluble arsenic in the gastric digestion stage increased for mussel and nori samples, but no coincidence impact was found in the small intestinal and colonic digestion stages. However, the simulated small intestinal absorption of arsenic was increased in all food matrices (1.2-2.7 fold higher) following digestion with salivary microorganisms. No significant transformation of the arsenic species occurred except for the arsenosugars present in mussels and nori. In those samples, conversions between the oxo arsenosugars were observed in the small intestinal digestion stage whereupon the thioxo analogs became major metabolites. These results expand our knowledge on the likely metabolism and oral bioavailabiltiy of arsenic during human digestion, and provide valuable information for future risk assessments of dietary arsenic.

Development of a host-microbiome model of the small intestine

The intestinal epithelium plays an essential role in the balance between tolerant and protective immune responses to infectious agents. In vitro models do not typically consider the innate immune response and gut microbiome in detail, so these models do not fully mimic the physiologic aspects of the small intestine. We developed and characterized a long-term in vitro model containing enterocyte, goblet, and immune-like cells exposed to a synthetic microbial community representative of commensal inhabitants of the small intestine. This model showed differential responses toward a synthetic microbial community of commensal bacterial inhabitants of the small intestine in the absence or presence of LPS from Escherichia coli O111:B4. Simultaneous exposure to LPS and microbiota induced impaired epithelial barrier function; increased production of IL-8, IL-6, TNF-α, and C-X-C motif chemokine ligand 16; and augmented differentiation and adhesion of macrophage-like cells and the overexpression of dual oxidase 2 and TLR-2 and -4 mRNA. In addition, the model demonstrated the ability to assess the adhesion of specific bacterial strains from the synthetic microbial community-more specifically, Veillonella parvula-to the simulated epithelium. This novel in vitro model may assist in overcoming sampling and retrieval difficulties when studying host-microbiome interactions in the small intestine.-Calatayud, M., Dezutter, O., Hernandez-Sanabria, E., Hidalgo-Martinez, S., Meysman, F. J. R., Van de Wiele, T. Development of a host-microbiome model of the small intestine.

In vivo and in vitro methods for evaluating soil arsenic bioavailability: relevant to human health risk assessment

Arsenic (As) is the most frequently occurring contaminant on the priority list of hazardous substances, which lists substances of greatest public health concern to people living at or near U.S. National Priorities List site. Accurate assessment of human health risks from exposure to As-contaminated soils depends on estimating its bioavailability, defined as the fraction of ingested As absorbed across the gastrointestinal barrier and available for systemic distribution and metabolism. Arsenic bioavailability varies among soils and is influenced by site-specific soil physical and chemical characteristics and internal biological factors. This review describes the state-of-the science that supports our understanding of oral bioavailability of soil As, the methods that are currently being explored for estimating soil As relative bioavailability (RBA), and future research areas that could improve our prediction of the oral RBA of soil As in humans. The following topics are addressed: (1) As soil geochemistry; (2) As toxicology; (3) in vivo models for estimating As RBA; (4) in vitro bioaccessibility methods; and (5) conclusions and research needs.

Butyrate-producing bacteria supplemented in vitro to Crohn's disease patient microbiota increased butyrate production and enhanced intestinal epithelial barrier integrity

The management of the dysbiosed gut microbiota in inflammatory bowel diseases (IBD) is gaining more attention as a novel target to control this disease. Probiotic treatment with butyrate-producing bacteria has therapeutic potential since these bacteria are depleted in IBD patients and butyrate has beneficial effects on epithelial barrier function and overall gut health. However, studies assessing the effect of probiotic supplementation on microbe-microbe and host-microbe interactions are rare. In this study, butyrate-producing bacteria (three mono-species and one multispecies mix) were supplemented to the fecal microbial communities of ten Crohn’s disease (CD) patients in an in vitro system simulating the mucus- and lumen-associated microbiota. Effects of supplementation in short-chain fatty acid levels, bacterial colonization of mucus environment and intestinal epithelial barrier function were evaluated. Treatment with F. prausnitzii and the mix of six butyrate-producers significantly increased the butyrate production by 5–11 mol%, and colonization capacity in mucus- and lumen-associated CD microbiota. Treatments with B. pullicaecorum 25-3^T and the mix of six butyrate-producers improved epithelial barrier integrity in vitro. This study provides proof-of-concept data for the therapeutic potential of butyrate-producing bacteria in CD and supports the future preclinical development of a probiotic product containing butyrate-producing species.

In Vitro Reduction of Arsenic Bioavailability Using Dietary Strategies

The main route of human exposure to inorganic arsenic (As) is through the consumption of food and water. Continued exposure to inorganic As [As(III) and As(V)] may cause a variety of diseases, including various types of cancer. The removal of As from these sources is complex, especially for food. One way to decrease As exposure could be by reducing intestinal absorption of it. The aim of this study is to seek dietary strategies (pure compounds, extracts, or supplements) that are capable of reducing the amount of As that is absorbed and reaches systemic circulation. Standard solutions of As(III) and As(V) and bioaccessible fractions of food samples with or without the dietary strategies to be tested were added to colon-derived human cells (NCM460 and HT-29MTX) to determine the apparent permeability (P_app) of As. Results show that transport across the intestinal monolayers is substantial, and the passage of As(III) (P_app = 4.2 × 10^-5 cm/s) is greater than that of As(V) (P_app = 2.4 × 10^-5 cm/s). Some of the treatments used (iron species, cysteine, grape extract) significantly reduce the transport of both inorganic As standards across the intestinal monolayer, thus decreasing absorption of them. In food samples, the effect of the dietary compounds on inorganic As bioavailability was also observed, especially in the cases of curcumin and cysteine. Compounds that proved effective in these in vitro assays could be the basis for intervention strategies aimed at reducing As toxicity in chronically exposed populations or regular consumers of food products with high As contents.

Human exposure to organic arsenic species from seafood

Seafood, including finfish, shellfish, and seaweed, is the largest contributor to arsenic (As) exposure in many human populations. In contrast to the predominance of inorganic As in water and many terrestrial foods, As in marine-derived foods is present primarily in the form of organic compounds. To date, human exposure and toxicological assessments have focused on inorganic As, while organic As has generally been considered to be non-toxic. However, the high concentrations of organic As in seafood, as well as the often complex As speciation, can lead to complications in assessing As exposure from diet. In this report, we evaluate the presence and distribution of organic As species in seafood, and combined with consumption data, address the current capabilities and needs for determining human exposure to these compounds. The analytical approaches and shortcomings for assessing these compounds are reviewed, with a focus on the best practices for characterization and quantitation. Metabolic pathways and toxicology of two important classes of organic arsenicals, arsenolipids and arsenosugars, are examined, as well as individual variability in absorption of these compounds. Although determining health outcomes or assessing a need for regulatory policies for organic As exposure is premature, the extensive consumption of seafood globally, along with the preliminary toxicological profiles of these compounds and their confounding effect on assessing exposure to inorganic As, suggests further investigations and process-level studies on organic As are needed to fill the current gaps in knowledge.

In vitro study of soil arsenic release by human gut microbiota and its intestinal absorption by Caco-2 cells

Arsenic (As) speciation is essential in assessing health risks from As-contaminated soil. Release of soil-bound arsenic, As transformation by human gut microbiota, and the subsequent intestinal absorption of soil As metabolites were evaluated. A colon microbial community in a dynamic human gut model and the intestinal epithelial cell line Caco-2 were cultured. Arsenic speciation analysis and absorption of different As species were undertaken. In this study, soil As release (3.7-581.2 mg kg^-1) was observed in the colon. Arsenic in the colon digests was transformed more quickly than that in the soil solid phase. X-ray absorption near-edge spectroscopy (XANES) analysis showed that 44.2-97.6% of arsenite [As(III)] generated due to arsenate [As(V)] reduction was in the soil solid phase after the colon phase. We observed a high degree of cellular absorption of soil As metabolites, exhibiting that the intestinal absorption of monomethylarsonic acid and As(III) (33.6% and 30.2% resp.) was slightly higher than that of dimethylarsinic acid and As(V) (25.1% and 21.7% resp.). Our findings demonstrate that human gut microbiota can directly release soil-bound arsenic, particularly As-bearing amorphous Fe/Al-oxides. Determining As transformation and intestinal absorption simultaneously will result in an accurate risk assessment of human health with soil As exposures.

Accumulation and Transport of Roxarsone, Arsenobetaine, and Inorganic Arsenic Using the Human Immortalized Caco-2 Cell Line

Roxarsone (Rox), an organoarsenic compound, served as a feed additive in the poultry industry for more than 60 years. Residual amounts of Rox present in chicken meat could give rise to potential human exposure to Rox. However, studies on the bioavailability of Rox in humans are scarce. We report here the accumulation and transepithelial transport of Rox using the human colon-derived adenocarcinoma cell line (Caco-2) model. The cellular accumulation and transepithelial passage of Rox in Caco-2 cells were evaluated and compared to those of arsenobetaine (AsB), arsenite (As^III), and arsenate (As^V). When Caco-2 cells were exposed to 3 μM Rox, AsB, and As^III separately for 24 h, the maximum accumulation was reached at 12 h. After 24-h exposure, the accumulated Rox was 6-20 times less than AsB and As^III. The permeability of Rox from the apical to basolateral side of Caco-2 monolayers was similar to As^V but less than As^III and AsB. The results of lower bioavailability of Rox are consistent with previous observations of relatively lower amounts of Rox retained in the breast meat of Rox-fed chickens. These data provide useful information for assessing human exposure to and intestinal bioavailability of Roxarsone.

Cellular arsenic transport pathways in mammals

Natural contamination of drinking water with arsenic results in the exposure of millions of people world-wide to unacceptable levels of this metalloid. This is a serious global health problem because arsenic is a Group 1 (proven) human carcinogen and chronic exposure is known to cause skin, lung, and bladder tumors. Furthermore, arsenic exposure can result in a myriad of other adverse health effects including diseases of the cardiovascular, respiratory, neurological, reproductive, and endocrine systems. In addition to chronic environmental exposure to arsenic, arsenic trioxide is approved for the clinical treatment of acute promyelocytic leukemia, and is in clinical trials for other hematological malignancies as well as solid tumors. Considerable inter-individual variability in susceptibility to arsenic-induced disease and toxicity exists, and the reasons for such differences are incompletely understood. Transport pathways that influence the cellular uptake and export of arsenic contribute to regulating its cellular, tissue, and ultimately body levels. In the current review, membrane proteins (including phosphate transporters, aquaglyceroporin channels, solute carrier proteins, and ATP-binding cassette transporters) shown experimentally to contribute to the passage of inorganic, methylated, and/or glutathionylated arsenic species across cellular membranes are discussed. Furthermore, what is known about arsenic transporters in organs involved in absorption, distribution, and metabolism and how transport pathways contribute to arsenic elimination are described.

Dietary Strategies To Reduce the Bioaccessibility of Arsenic from Food Matrices

The main route of exposure to arsenic (As) is the consumption of water and foods, in which the forms with greatest toxicity are inorganic As and dimethylarsinic acid, DMA(V). The objective of this study was to search for dietary components that reduce the bioaccessibility of As from food and water, in order to reduce the amount of As available for absorption. For this purpose, 35 compounds were assayed by use of a static in vitro model of gastrointestinal digestion. Sulfates of Fe(II) and Fe(III) reduced the solubility of inorganic As (86-99%) and DMA(V) in aqueous solution (40-66%). This reduction was also observed in rice (100%) and seaweed (60%). Aluminum, titanium, and tannic acid also reduced the bioaccessibility of As from food (42-70%). These data show that the use of dietary components may be a good strategy to reduce the entry of As into systemic circulation.

Oxidative stress, epigenetics, and cancer stem cells in arsenic carcinogenesis and prevention

The carcinogenic role of arsenic has been extensively studied for more than half century. How arsenic causes human cancer, however, remains to be fully elucidated. In this brief review, we focus our attentions on the most recent discoveries by us and others on the capabilities of arsenic in inducing generation of reactive oxygen species (ROS), expression of microRNAs (miRNAs) and the generation of the cancer stem cells. We believe that these new understandings on the mechanisms of arsenic-induced carcinogenesis will shed light on the prevention and treatment of human cancers resulted from environmental or occupational arsenic exposure. Furthermore, these latest findings on arsenic-induced cellular responses will also have an important impact on the investigation of the carcinogenic effects of other environmental or occupational carcinogens or hazards.

Toxic trace elements at gastrointestinal level

Many trace elements are considered essential [iron (Fe), zinc (Zn), copper (Cu)], whereas others may be harmful [lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As)], depending on their concentration and chemical form. In most cases, the diet is the main pathway by which they enter our organism. The presence of toxic trace elements in food has been known for a long time, and many of the food matrices that carry them have been identified. This has led to the appearance of legislation and recommendations concerning consumption. Given that the main route of exposure is oral, passage through the gastrointestinal tract plays a fundamental role in their entry into the organism, where they exert their toxic effect. Although the digestive system can be considered to be of crucial importance in their toxicity, in most cases we do not know the events that occur during the passage of these elements through the gastrointestinal tract and of ascertaining whether they may have some kind of toxic effect on it. The aim of this review is to summarize available information on this subject, concentrating on the toxic trace elements that are of greatest interest for organizations concerned with food safety and health: Pb, Cd, Hg and As.

Westernized diets lower arsenic gastrointestinal bioaccessibility but increase microbial arsenic speciation changes in the colon

Arsenic (As) is an important contaminant present in food and water. Several studies have indicated that the occurrence of As based skin lesions is significantly different when root and gourd rich diets are consumed compared to meat rich diets. Additionally, urinary As speciation from orally exposed individuals appears to depend on the composition of the diet. These observations imply that diet composition can affect both the bioavailable As fraction as the As speciation in the body. In this study, we used the in vitro gastrointestinal method (IVG) to evaluate how an Asian type diet (fiber rich) and a Western type diet (fat and protein rich), differ in their capability to release inorganic As (iAs(V)) and dimethyl arsinate (DMA(V)) from a rice matrix following gastrointestinal digestion. Moreover, we used a validated dynamic gut simulator to investigate whether diet background affects As metabolism by gut microbiota in a colon environment. An Asian diet background resulted in a larger As bioaccessibility (81.2%) than a Western diet background (63.4%). On the other hand, incubation of As contaminated rice with human colon microbiota in the presence of a Western type diet resulted in a larger amount of hazardous As species - monomethyl arsonite and monomethylmonothio arsonate - to be formed after 48 h. The permeability of these As species (60.5% and 50.5% resp.) across a Caco-2 cell line was significantly higher compared to iAs(V) and DMA(V) (46.5% and 28% resp.). We conclude that dietary background is a crucial parameter to incorporate when predicting bioavailability with bioaccessibility measurements and when assessing health risks from As following oral exposure.

Proinflammatory effect of trivalent arsenical species in a co-culture of Caco-2 cells and peripheral blood mononuclear cells

Chronic exposure to inorganic arsenic (As) is associated with type 2 diabetes, cardiovascular diseases and cancer. Ingested inorganic As is transformed within the gastrointestinal tract and can give rise to more toxic species such as monomethylarsonous acid [MMA(III)] and dimethylarsinous acid [DMA(III)]. Thus, the intestinal epithelium comes into contact with toxic arsenical species, and the effects of such exposure upon epithelial function are not clear. The present study has evaluated the effect of 1 µM arsenite [As(III)], 0.1 µM MMA(III) and 1 µM DMA(III) upon the release of cytokines [interleukin-6 (IL6), IL8, tumor necrosis factor alpha (TNFα)], using a compartmentalized co-culture model with differentiated Caco-2 cells in the apical compartment and peripheral blood mononuclear cells in the basolateral compartment. In addition, the combined effect of arsenical species and lipopolysaccharide (LPS), both added into the apical compartment, has been analyzed. The results indicate that exposure to the arsenical forms induces a proinflammatory response. An increase in cytokine secretion into the basolateral compartment was observed, particularly as regards TNFα (up to 1,600 %). The cytokine levels on the apical side also increased, though to a lesser extent. As/LPS co-exposure significantly affected the proinflammatory response as compared to treatment with As alone. Treatment with DMA(III) and As/LPS co-exposure increased the permeability of the intestinal monolayer. In addition, As/LPS treatments enhanced As(III) and MMA(III) transport through the intestinal monolayer.

Arsenic undergoes significant speciation changes upon incubation of contaminated rice with human colon micro biota

Cellular and animal studies involving MMA(III) (monomethyl arsonous acid) and DMA(III) (dimethyl arsinous acid) have indicated that their toxicities meet or exceed that of iAs. Thiolated arsenic metabolites were observed in urine after oral exposure of inorganic arsenic in some studies. For these species, the toxicological profile was not yet fully characterized in human cells. Some studies revealed that trivalent organoarsenic species are well absorbed in the intestine compared to iAs. However, other studies also indicated that a significant amount of rice-bound As reaches the colon, which may be attributed to the fibre-rich nature of the rice. Studies have revealed that microorganisms from the gut environment are important contributors to arsenic speciation changes. We aimed to study how the gut microbial metabolism affects As in different rice matrices. This was done in vitro using colon suspension from the Simulator of the Human Intestinal Microbial Ecosystem (SHIME system). Significant amounts of MMA(III), DMA(III) and MMMTA(V) were formed due to microbial metabolic processes like methylation and thiolation. These results suggested that presystemic metabolism by human gut micro biota should not be neglected in risk assessment studies. In this context, also toxicity and absorption of thiolated species by mammalian cells should be further investigated.

SLCO1B1 variants and urine arsenic metabolites in the Strong Heart Family Study

Arsenic species patterns in urine are associated with risk for cancer and cardiovascular diseases. The organic anion transporter coded by the gene SLCO1B1 may transport arsenic species, but its association with arsenic metabolites in human urine has not yet been studied. The objective of this study is to evaluate associations of urine arsenic metabolites with variants in the candidate gene SLCO1B1 in adults from the Strong Heart Family Study. We estimated associations between % arsenic species biomarker traits and 5 single-nucleotide polymorphisms (SNPs) in the SLCO1B1 gene in 157 participants, assuming additive genetics. Linear regression models for each SNP accounted for kinships and were adjusted for sex, body mass index, and study center. The minor allele of rs1564370 was associated with lower %MMA (p = .0003) and higher %DMA (p = .0002), accounting for 8% of the variance for %MMA and 9% for %DMA. The rs1564370 minor allele homozygote frequency was 17% and the heterozygote frequency was 43%. The minor allele of rs2291075 was associated with lower %MMA (p = .0006) and higher %DMA (p = .0014), accounting for 7% of the variance for %MMA and 5% for %DMA. The frequency of rs2291075 minor allele homozygotes was 1% and of heterozygotes was 15%. Common variants in SLCO1B1 were associated with differences in arsenic metabolites in a preliminary candidate gene study. Replication of this finding in other populations and analyses with respect to disease outcomes are needed to determine whether this novel candidate gene is important for arsenic-associated disease risks.

In vitro intestinal bioavailability of arsenosugar metabolites and presystemic metabolism of thio-dimethylarsinic acid in Caco-2 cells

Whereas inorganic arsenic is classified as a human carcinogen, risks to human health related to the presence of arsenosugars in marine food are still unclear. Since studies indicate that human inorganic arsenic metabolites contribute to inorganic arsenic induced carcinogenicity, a risk assessment for arsenosugars should also include a toxicological characterization of their respective metabolites. Here we assessed intestinal bioavailability of the human arsenosugar metabolites oxo-DMAA(V), thio-DMAA(V), oxo-DMAE(V), thio-DMAE(V) and thio-DMA(V) in relation to arsenite in the Caco-2 intestinal barrier model. Whereas arsenite and thio-DMA(V) caused barrier disruption at concentrations ≥10 μM, all other metabolites did not cause a barrier leakage, even when applied at 50 times higher concentrations than arsenite and thio-DMA(V). The transfer studies point to a strong intestinal bioavailability of thio-DMA(V) and thio-DMAE(V), whereas oxo-DMAA(V), thio-DMAA(V) and oxo-DMAE(V) passed the in vitro intestinal barrier only to a very small extent. Detailed influx and efflux studies indicate that arsenite and thio-DMA(V) cross the intestinal barrier most likely by passive diffusion (paracellular) and facilitated (transcellular) transport. LC-ICP-QMS based arsenic speciation studies during the transfer experiments demonstrate transfer of thio-DMA(V) itself across the intestinal barrier and suggest metabolism of thio-DMA(V) using the in vitro intestinal barrier model to its oxygen-analogue DMA(V). In the case of arsenite no metabolism was observed. In summary the two arsenosugar metabolites thio-DMA(V) and thio-DMAE(V) showed intestinal bioavailability similar to that of arsenite, and about 10-fold higher than that reported for arsenosugars (Leffers et al., Mol. Nutr. Food Res., 2013, DOI: 10.1002/mnfr.201200821) in the same in vitro model. Thus, a presystemic metabolism of arsenosugars might strongly impact arsenic intestinal bioavailability after arsenosugar intake and should therefore be considered when assessing the risks to human health related to the consumption of arsenosugar-containing food.

In vitro toxicological characterization of two arsenosugars and their metabolites

SCOPE

In their recently published Scientific Opinion on Arsenic in Food, the European Food Safety Authority concluded that a risk assessment for arsenosugars is currently not possible, largely because of the lack of relevant toxicological data. To address this issue, we carried out a toxicological in vitro characterization of two arsenosugars and six arsenosugar metabolites.

METHODS AND RESULTS

The highly pure synthesized arsenosugars, DMA(V) -sugar-glycerol and DMA(V) -sugar-sulfate, investigated in this study, as well as four metabolites, oxo-dimethylarsenoacetic acid (oxo-DMAA(V) ), oxo-dimethylarsenoethanol (oxo-DMAE(V) ), thio-DMAA(V) and thio-DMAE(V) , exerted neither cytotoxicity nor genotoxicity up to 500 μM exposure in cultured human bladder cells. However, two arsenosugar metabolites, namely dimethyl-arsinic acid (DMA(V) ) and thio-dimethylarsinic acid (thio-DMA(V) ), were toxic to the cells; thio-DMA(V) was even slightly more cytotoxic than arsenite. Additionally, intestinal bioavailability of the arsenosugars was assessed applying the Caco-2 intestinal barrier model. The observed low, but significant transfer rates of the arsenosugars across the barrier model provide further evidence that arsenosugars are intestinally bioavailable.

CONCLUSION

In a cellular system that metabolizes arsenosugars, cellular toxicity likely arises. Thus, in strong contrast to arsenobetaine, arsenosugars cannot be categorized as nontoxic for humans and a risk to human health cannot be excluded.

Analysis of the transport of and cytotoxic effects for nalbuphine solution in corneal cells

OBJECTIVE

To assess the in vitro effects of various nalbuphine concentrations on viability and wound healing ability of corneal cells and potential drug transport through the corneal epithelium.

SAMPLE

Cultured canine and human corneal epithelial cells (CECs) and cultured canine corneal stromal fibroblasts.

PROCEDURES

CECs and stromal fibroblasts were exposed to nalbuphine (concentration of solutions ranged from 0% to 1.2%) for up to 30 minutes, and viability was assessed with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. A standard scratch test technique was used. Wound healing of CECs and stromal fibroblasts was evaluated following treatment with nalbuphine solutions < 0.1%. Liquid chromatography-mass spectrometry-mass spectrometry analysis was used to evaluate drug transport across a monolayer and a multilayer of human CECs.

RESULTS

A progressive decrease in viability was detected in canine CECs for all nalbuphine treatment groups, whereas treatment with only 0.5% or 1.2% nalbuphine significantly reduced corneal stromal fibroblast viability, compared with results for control cells. Within 24 hours, treatment with 0.1% nalbuphine solution significantly altered the healing rate of both canine CECs and stromal fibroblasts. Continuous increases in transport rates of nalbuphine were detected with time for both the monolayer and multilayer of human CECs.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro, nalbuphine potentially could penetrate through corneal tissue, but it may cause damage to the corneal epithelium and stromal fibroblasts. Therefore, nalbuphine potentially may impair corneal wound healing.

In vitro study of intestinal transport of inorganic and methylated arsenic species by Caco-2/HT29-MTX cocultures

This study characterizes intestinal absorption of arsenic species using in vitro system Caco-2/HT29-MTX cocultures in various proportions (100/0 to 30/70). The species assayed were As(V), As(III), monomethylarsonic acid [MMA(V)], monomethylarsonous acid [MMA(III)], dimethylarsinic acid [DMA(V)], and dimethylarsinous acid [DMA(III)]. The results show that the apparent permeability (P(app)) values of pentavalent species increase significantly in the Caco-2/HT29-MTX cocultures in comparison with the Caco-2 monoculture, probably because of enhancement of paracellular transport. For MMA(III) and DMA(III), P(app) decreases in the Caco-2/HT29-MTX cell model, and for As(III), there is no change in P(app) between the two culture models. Transport studies of arsenic solubilized from cooked foods (rice, garlic, and seaweed) after applying an in vitro gastrointestinal digestion showed that arsenic absorption also varies with the model used, increasing with the incorporation of HT29-MTX in the culture. These results show the importance of choosing a suitable in vitro model when evaluating intestinal arsenic absorption processes.

Development and characterization of a new oral dapsone nanoemulsion system: permeability and in silico bioavailability studies

BACKGROUND

Dapsone is described as being active against Mycobacterium leprae, hence its role in the treatment of leprosy and related pathologies. Despite its therapeutic potential, the low solubility of dapsone in water results in low bioavailability and high microbial resistance. Nanoemulsions are pharmaceutical delivery systems derived from micellar solutions with a good capacity for improving absorption. The aim of this work was to develop and compare the permeability of a series of dapsone nanoemulsions in Caco-2 cell culture against that of effective permeability in the human body simulated using Gastroplus™ software.

METHODS AND RESULTS

The release profiles of the dapsone nanoemulsions using different combinations of surfactants and cosolvent showed a higher dissolution rate in simulated gastric and enteric fluid than did the dispersed dapsone powder. The drug release kinetics were consistent with a Higuchi model.

CONCLUSION

This comparison of dapsone permeability in Caco-2 cells with effective permeability in the human body simulated by Gastroplus showed a good correlation and indicates potential improvement in the biodisponibility of dapsone using this new system.

Metabolism of arsenic and its toxicological relevance

Arsenic is a worldwide environmental pollutant and a human carcinogen. It is well recognized that the toxicity of arsenicals largely depends on the oxidoreduction states (trivalent or pentavalent) and methylation levels (monomethyl, dimethyl, and trimethyl) that are present during the process of metabolism in mammals. However, presently, the specifics of the metabolic pathway of inorganic arsenicals have yet to be confirmed. In mammals, there are two possible mechanisms that have been proposed for the metabolic pathway of inorganic arsenicals, oxidative methylation, and glutathione conjugation. Oxidative methylation, which was originally proposed in fungi, is based on findings that arsenite (iAs(III)) is sequentially converted to monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)) in both humans and in laboratory animals such as mice and rats. However, recent in vitro observations have demonstrated that arsenic is only methylated in the presence of glutathione (GSH) or other thiol compounds, which strongly suggests that arsenic is methylated in trivalent forms. The glutathione conjugation mechanism is supported by findings that have shown that most intracellular arsenicals are trivalent and excreted from cells as GSH conjugates. Since non-conjugated trivalent arsenicals are highly reactive with thiol compounds and are easily converted to less toxic corresponding pentavalent arsenicals, the arsenic-glutathione conjugate stability may be the most important factor for determining the toxicity of arsenicals. In addition, "being a non-anionic form" also appears to be a determinant of the toxicity of oxo-arsenicals or thioarsenicals. The present review discusses both the metabolism of arsenic and the toxicity of arsenic metabolites.

Relative bioavailability and bioaccessibility and speciation of arsenic in contaminated soils

BACKGROUND

Assessment of soil arsenic (As) bioavailability may profoundly affect the extent of remediation required at contaminated sites by improving human exposure estimates. Because small adjustments in soil As bioavailability estimates can significantly alter risk assessments and remediation goals, convenient, rapid, reliable, and inexpensive tools are needed to determine soil As bioavailability.

OBJECTIVES

We evaluated inexpensive methods for assessing As bioavailability in soil as a means to improve human exposure estimates and potentially reduce remediation costs.

METHODS

Nine soils from residential sites affected by mining or smelting activity and two National Institute of Standards and Technology standard reference materials were evaluated for As bioavailability, bioaccessibility, and speciation. Arsenic bioavailability was determined using an in vivo mouse model, and As bioaccessibility was determined using the Solubility/Bioavailability Research Consortium in vitro assay. Arsenic speciation in soil and selected soil physicochemical properties were also evaluated to determine whether these parameters could be used as predictors of As bioavailability and bioaccessibility.

RESULTS

In the mouse assay, we compared bioavailabilities of As in soils with that for sodium arsenate. Relative bioavailabilities (RBAs) of soil As ranged from 11% to 53% (mean, 33%). In vitro soil As bioaccessibility values were strongly correlated with soil As RBAs (R² = 0.92). Among physicochemical properties, combined concentrations of iron and aluminum accounted for 80% and 62% of the variability in estimates of RBA and bioaccessibility, respectively.

CONCLUSION

The multifaceted approach described here yielded congruent estimates of As bioavailability and evidence of interrelations among physicochemical properties and bioavailability estimates.

Effects of dissolution kinetics on bioaccessible arsenic from tailings and soils

Dissolution kinetics of arsenic from soils and tailings were studied under simulated gastrointestinal conditions to determine the effects of residence time, pH and soil composition on the bioaccessibility of arsenic. The samples were sieved to four particle size fractions from bulk to <45 μm, and included arsenic minerals, soils and tailings with total arsenic concentrations ranging from 19 to 42000 mg kg(-1). The bioaccessible arsenic concentrations varied from 2.8 to 10000 mg kg(-1), and the highest concentrations were associated with the smallest particle size fractions. Kinetic parameters were determined for each sample extracted under gastric conditions (pH=1.8) followed by intestinal conditions (pH=7.0). Under gastric pH conditions, dissolution appeared to be diffusion-controlled and followed an exponential curve, whereas a logarithmic or linear model was used to describe the mixed dissolution mechanisms observed under intestinal conditions. Nine of the 13 samples tested reached a steady state bioaccessible arsenic concentration within the 5-h physiologically-based extraction test (PBET). However the bioaccessible arsenic concentrations in four tailings samples increased significantly (p=0.034) between the 5-h and the extended 24-h extraction under intestinal conditions. Since arsenic absorption may occur along the entire digestive tract, assessments based on the standard 5-h PBET extraction may not adequately estimate the risks associated with arsenic absorption in such cases. The slow dissolution kinetics associated with secondary arsenic minerals in some tailings samples may require extending the PBET extractions to longer periods, or extrapolating using the proposed kinetic models, to reach steady state concentrations in simulated gastrointestinal fluids.

Mathematical model insights into arsenic detoxification

Background

Arsenic in drinking water, a major health hazard to millions of people in South and East Asia and in other parts of the world, is ingested primarily as trivalent inorganic arsenic (iAs), which then undergoes hepatic methylation to methylarsonic acid (MMAs) and a second methylation to dimethylarsinic acid (DMAs). Although MMAs and DMAs are also known to be toxic, DMAs is more easily excreted in the urine and therefore methylation has generally been considered a detoxification pathway. A collaborative modeling project between epidemiologists, biologists, and mathematicians has the purpose of explaining existing data on methylation in human studies in Bangladesh and also testing, by mathematical modeling, effects of nutritional supplements that could increase As methylation.

Methods

We develop a whole body mathematical model of arsenic metabolism including arsenic absorption, storage, methylation, and excretion. The parameters for arsenic methylation in the liver were taken from the biochemical literature. The transport parameters between compartments are largely unknown, so we adjust them so that the model accurately predicts the urine excretion rates of time for the iAs, MMAs, and DMAs in single dose experiments on human subjects.

Results

We test the model by showing that, with no changes in parameters, it predicts accurately the time courses of urinary excretion in mutiple dose experiments conducted on human subjects. Our main purpose is to use the model to study and interpret the data on the effects of folate supplementation on arsenic methylation and excretion in clinical trials in Bangladesh. Folate supplementation of folate-deficient individuals resulted in a 14% decrease in arsenicals in the blood. This is confirmed by the model and the model predicts that arsenicals in the liver will decrease by 19% and arsenicals in other body stores by 26% in these same individuals. In addition, the model predicts that arsenic methyltransferase has been upregulated by a factor of two in this population. Finally, we also show that a modification of the model gives excellent fits to the data on arsenic metabolism in human cultured hepatocytes.

Conclusions

The analysis of the Bangladesh data using the model suggests that folate supplementation may be more effective at reducing whole body arsenic than previously expected. There is almost no data on the upregulation of arsenic methyltransferase in populations chronically exposed to arsenic. Our model predicts upregulation by a factor of two in the Bangladesh population studied. This prediction should be verified since it could have important public health consequences both for treatment strategies and for setting appropriate limits on arsenic in drinking water. Our model has compartments for the binding of arsenicals to proteins inside of cells and we show that these comparments are necessary to obtain good fits to data. Protein-binding of arsenicals should be explored in future biochemical studies.