Metabolism of Inorganic Arsenic in Intestinal Epithelial Cell Lines

Arsenic speciation and arsenic feed-to-fish transfer in Atlantic salmon fed marine low trophic feeds based blue mussel and kelp

BACKGROUND

Aquaculture aims to reduce the environmental and climate footprints of feed production. Consequently, low trophic marine (LTM) resources such as blue mussels and kelp are potential candidates to be used as ingredients in salmon feed. It is relevant to study potential undesirables associated with their use, as well as assessing food safety by investigating their transfer from feed-to-fish. The marine biota is well known to contain relatively high levels of arsenic (As), which may be present in different organic forms depending on marine biota type and trophic position. Thus, it is important to not only obtain data on the concentrations of As, but also on the As species present in the raw materials, feed and farmed salmon when being fed novel LTM feed resources.

METHODS

Atlantic salmon were fed experimental diets for 70 days. A total of nine diets were prepared: four diets containing up to 4 % fermented kelp, three diets containing up to 11 % blue mussel silage, and one diet containing 12 % blue mussel meal, in addition to a standard reference diet containing 25 % fish meal. Concentrations of As and As species in feeds, faeces, liver and fillet of Atlantic salmon were determined by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography coupled to ICP-MS (HPLC-ICP-MS), respectively.

RESULTS

The use of kelp or blue mussel-based feed ingredients increased the concentration of total As, but maximum level as defined in Directive 2002/32 EC and amendments was not exceeded. The concentrations found in the experimental feeds ranged from 3.4 mg kg-1 to 4.6 mg kg-1 ww. Arsenic speciation in the feed varied based on the ingredient, with arsenobetaine dominating in all feed samples (36-60 % of the total As), while arsenosugars (5.2-8.9 % of the total As) were abundant in kelp-included feed. The intestinal uptake of total As ranged from 67 % to 83 %, but retention in fillet only ranged from 2 % to 22 % and in liver from 0.3 % to 0.6 %, depending on the marine source used. Fish fed feeds containing blue mussel showed higher intestinal uptake of total As when compared with fish fed feeds containing fermented kelp. Fish fed fermented kelp-based feeds had higher retained concentrations of total As when comparing with fish fed feeds containing blue mussel. Despite relatively high intestinal uptake of total As, inorganic and organic As, the retained concentrations of As did not reflect the same trend.

CONCLUSION

Although the use of LTM feed ingredients increased the level of total As in this feeds, salmon reared on these diets did not show increased total As levels. The well-known toxic inorganic As forms were not detected in salmon muscle reared on LTM diets, and the non-toxic organic AsB was the dominant As species that was retained in salmon muscle, while the organic AsSug forms were not. This study shows that speciation analysis of the LTM resources provides valuable information of the feed-to-fish transfer of As, needed to assess the food safety of farmed Atlantic salmon reared on novel low trophic feeds.

Impact of Chronic Exposure to Arsenate through Drinking Water on the Intestinal Barrier

Chronic exposure to inorganic arsenic (As) [As(III) + As(V)], which affects millions of people, increases the incidence of some kinds of cancer and other noncarcinogenic pathologies. Although the oral pathway is the main source of exposure, in vivo studies conducted to verify the intestinal toxicity of this metalloid are scarce and are mainly focused on evaluating the toxicity of As(III). The aim of this study was to evaluate the effect of chronic exposure (6 months) of BALB/c mice to As(V) (15-60 mg/L) via drinking water on the different components of the intestinal barrier and to determine the possible mechanisms involved. The results show that chronic exposure to As(V) generates a situation of oxidative stress (increased lipid peroxidation and reactive species) and inflammation (increased contents of several proinflammatory cytokines and neutrophil infiltrations) in the intestinal tissues. There is also evidence of an altered expression of constituent proteins of the intercellular junctions (Cldn1, Cldn3, and Ocln) and the mucus layer (Muc2) and changes in the composition of the gut microbiota and the metabolism of short-chain fatty acids. All of these toxic effects eventually may lead to the disruption of the intestinal barrier, which shows an increased paracellular permeability. Moreover, signs of endotoxemia are observed in the serum of As(V)-treated animals (increases in lipopolysaccharide-binding protein LBP and the proinflammatory cytokine IL-1β). The data obtained suggest that chronic exposure to As(V) via drinking water affects the intestinal environment.

Copper-mediated shifts in transcriptomic responses of intestines in Bufo gargarizans tadpoles to lead stress

The differential transcriptomic responses of intestines in Bufo gargarizans tadpoles to Pb alone or in the presence of Cu were evaluated. Tadpoles were exposed to 30 μg/L Pb individually and in combination with Cu at 16 or 64 μg/L from Gosner stage (Gs) 26 to Gs 38. After de novo assembly, 105,107 unigenes were generated. Compared to the control group, 7387, 6937, and 11139 differentially expressed genes (DEGs) were identified in the treatment of Pb + Cu0, Pb + Cu16, and Pb + Cu64, respectively. In addition, functional annotation and enrichment analysis of DEGs revealed substantial transcriptional reprogramming of diverse molecular and biological pathways were induced in all heavy metal treatments. The relative expression levels of genes associated with intestinal epithelial barrier and bile acids (BAs) metabolism, such as mucin2, claudin5, ZO-1, Asbt, and Ost-β, were validated by qPCR. This study demonstrated that Pb exposure induced transcriptional responses in tadpoles, and the responses could be modulated by Cu.

Bioactive compounds, antibiotics and heavy metals: effects on the intestinal structure and microbiome of monogastric animals – a non-systematic review

The intestinal structure and gut microbiota are essential for the animals‘ health. Chemical components taken with food provide the right environment for a specific microbiome which, together with its metabolites and the products of digestion, create an environment, which in turn is affects the population size of specific bacteria. Disturbances in the composition of the gut microbiota can be a reason for the malformation of guts, which has a decisive impact on the animal‘ health. This review aimed to analyse scientific literature, published over the past 20 years, concerning the effect of nutritional factors on gut health, determined by the intestinal structure and microbiota of monogastric animals. Several topics have been investigated: bioactive compounds (probiotics, prebiotics, organic acids, and herbal active substances), antibiotics and heavy metals (essentaial minerals and toxic heavy metals).

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.

In vitro toxicity of arsenic rich waters from an abandoned gold mine in northeast Portugal

This is a follow-up study of physicochemical water monitoring data from the abandoned Freixeda gold mine in Portugal, where arsenic (As) has remained above drinking water and irrigation limits over the years. The main objective of the current work was to investigate the toxicological potential of As-containing water on human cell line as an indicator of a potential health risk to humans. Six water samples collected in February 2018 were analysed for arsenic, major anions, cations and trace elements. Toxicity experiments were carried out on the human gastrointestinal cell line Caco-2 with five water samples containing As above 10 μg L^-1. The results show that groundwater contains higher amounts of dissolved minerals than surface water, particularly with higher concentrations of SO₄^2-, Fe and HCO₃^- and also higher As(III), reaching 336 μg L^-1 (As(T) = 607 μg L^-1). In surface waters As concentration decreased and reached 150 μg L^-1, mainly as As(V). Metabolic activity was generally lower in Caco-2 cells exposed to As-containing water samples compared to pure As(III) solution, adapted to As concentrations, while production of reactive oxygen species (ROS) was higher. Short-term exposure to As-contaminated water samples also resulted in increased genotoxicity. This study suggests that mixture of As with various chemical elements in water may have a synergistic effect in promoting cytotoxicity. It is likely that prolonged exposure, as is common in areas with contaminated water, would have even more harmful effects.

Bioavailability of arsenic, cadmium, lead and mercury as measured by intestinal permeability

In this study, the intestinal permeability of metal(loid)s (MLs) such as arsenic (As), cadmium (Cd), lead (Pb) and mercury (Hg) was examined, as influenced by gut microbes and chelating agents using an in vitro gastrointestinal/Caco-2 cell intestinal epithelium model. The results showed that in the presence of gut microbes or chelating agents, there was a significant decrease in the permeability of MLs (As-7.5%, Cd-6.3%, Pb-7.9% and Hg-8.2%) as measured by apparent permeability coefficient value (Papp), with differences in ML retention and complexation amongst the chelants and the gut microbes. The decrease in ML permeability varied amongst the MLs. Chelating agents reduce intestinal absorption of MLs by forming complexes thereby making them less permeable. In the case of gut bacteria, the decrease in the intestinal permeability of MLs may be associated to a direct protection of the intestinal barrier against the MLs or indirect intestinal ML sequestration by the gut bacteria through adsorption on bacterial surface. Thus, both gut microbes and chelating agents can be used to decrease the intestinal permeability of MLs, thereby mitigating their toxicity.

Epigenetic influence of environmentally neurotoxic metals

Continuous globalization and industrialization have ensured metals are an increasing aspect of daily life. Their usefulness in manufacturing has made them vital to national commerce, security and global economy. However, excess exposure to metals, particularly as a result of environmental contamination or occupational exposures, has been detrimental to overall health. Excess exposure to several metals is considered environmental risk in the aetiology of several neurological and neurodegenerative diseases. Metal-induced neurotoxicity has been a major health concern globally with intensive research to unravel the mechanisms associated with it. Recently, greater focus has been directed at epigenetics to better characterize the underlying mechanisms of metal-induced neurotoxicity. Epigenetic changes are those modifications on the DNA that can turn genes on or off without altering the DNA sequence. This review discusses how epigenetic changes such as DNA methylation, post translational histone modification and noncoding RNA-mediated gene silencing mediate the neurotoxic effects of several metals, focusing on manganese, arsenic, nickel, cadmium, lead, and mercury.

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

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

Assessing the Viability of a Synthetic Bacterial Consortium on the In Vitro Gut Host-microbe Interface

The interplay between host and microbiota has been long recognized and extensively described. The mouth is similar to other sections of the gastrointestinal tract, as resident microbiota occurs and prevents colonisation by exogenous bacteria. Indeed, more than 600 species of bacteria are found in the oral cavity, and a single individual may carry around 100 different at any time. Oral bacteria possess the ability to adhere to the various niches in the oral ecosystem, thus becoming integrated within the resident microbial communities, and favouring growth and survival. However, the flow of bacteria into the gut during swallowing has been proposed to disturb the balance of the gut microbiota. In fact, oral administration of P. gingivalis shifted bacterial composition in the ileal microflora. We used a synthetic community as a simplified representation of the natural oral ecosystem, to elucidate the survival and viability of oral bacteria subjected to simulated gastrointestinal transit conditions. Fourteen species were selected, subjected to in vitro salivary, gastric, and intestinal digestion processes, and presented to a multicompartment cell model containing Caco-2 and HT29-MTX cells to simulate the gut mucosal epithelium. This model served to unravel the impact of swallowed bacteria on cells involved in the enterohepatic circulation. Using synthetic communities allows for controllability and reproducibility. Thus, this methodology can be adapted to assess pathogen viability and subsequent inflammation-associated changes, colonization capacity of probiotic mixtures, and ultimately, potential bacterial impact on the presystemic circulation.

Mucus: An Underestimated Gut Target for Environmental Pollutants and Food Additives

Synthetic chemicals (environmental pollutants, food additives) are widely used for many industrial purposes and consumer-related applications, which implies, through manufactured products, diet, and environment, a repeated exposure of the general population with growing concern regarding health disorders. The gastrointestinal tract is the first physical and biological barrier against these compounds, and thus their first target. Mounting evidence indicates that the gut microbiota represents a major player in the toxicity of environmental pollutants and food additives; however, little is known on the toxicological relevance of the mucus/pollutant interplay, even though mucus is increasingly recognized as essential in gut homeostasis. Here, we aimed at describing how environmental pollutants (heavy metals, pesticides, and other persistent organic pollutants) and food additives (emulsifiers, nanomaterials) might interact with mucus and mucus-related microbial species; that is, “mucophilic” bacteria such as mucus degraders. This review highlights that intestinal mucus, either directly or through its crosstalk with the gut microbiota, is a key, yet underestimated gut player that must be considered for better risk assessment and management of environmental pollution.

Subchronic Arsenism Disorders mRNA Expression of Cytokines and Immunoglobulins in the Intestinal Tract of the Cock

Provision of feed containing arsenic may cause intestinal flora imbalance and consequently, the dysfunction of immunological protection of the cock. To understand the intricate tuning of immune responses in the intestinal tract elicited by subchronic arsenism, a cock model (1-day-old Hy-line cocks) was established by subjecting cocks to different environmentally relevant concentrations of arsenic in the diet for 90 days. Intestinal cytokine and immunoglobulin (Ig) messenger RNA (mRNA) expression levels were determined using real-time PCR on days 30, 60, and 90. Results showed that, compared to those of the control groups, the mRNA levels of interleukin (IL)-1β, IL-2, IL-4, and interferon (IFN)-γ displayed increases on day 30 then decreases on days 60 and 90 dose-dependently in every tissue. Except for the decrease in the jejunum, the mRNA levels of IL-6 and IL-8 were increased in the duodenum, ileum, cecum, and rectum. However, the mRNA levels of IL-12β decreased in every tissue and every time point compared to those of the control groups. In contrast, chicks showed considerably higher expression levels of IgA, IgM, and IgG after exposure to arsenic. These results demonstrated that immune strategies of cocks were disturbed when suffered from subchronic arsenism, at least on the intestinal level.

Metabolism, toxicity and anticancer activities of arsenic compounds

A variety of studies indicated that inorganic arsenic and its methylated metabolites have paradoxical effects, namely, carcinogenic and anticancer effects. Epidemiological studies have shown that long term exposure to arsenic can increase the risk of cancers of lung, skin or bladder in man, which is probably associated with the arsenic metabolism. In fact, the enzymatic conversion of inorganic arsenic by Arsenic (+3 oxidation state) methyltransferase (AS3MT) to mono- and dimethylated arsenic species has long been considered as a major route for detoxification. However, several studies have also indicated that biomethylation of inorganic arsenic, particularly the production of trivalent methylated metabolites, is a process that activates arsenic as a toxin and a carcinogen. On the other hand, arsenic trioxide (As2O3) has recently been recognized as one of the most effective drugs for the treatment of APL. However, elaboration of the cytotoxic mechanisms of arsenic and its methylated metabolites in eradicating cancer is sorely lacking. To provide a deeper understanding of the toxicity and carcinogenicity along with them use of arsenic in chemotherapy, caution is required considering the poor understanding of its various mechanisms of exerting toxicity. Thereby, in this review, we have focused on arsenic metabolic pathway, the roles of the methylated arsenic metabolites in toxicity and in the therapeutic efficacy for the treatments of solid tumors, APL and/or non-APL malignancies.

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.

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.

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.

Green tea (Camellia sinensis) alleviates arsenic-induced damages to DNA and intestinal tissues in rat and in situ intestinal loop by reinforcing antioxidant system

This study elucidates the protective role of Green tea (Camellia sinensis or CS) against arsenic-induced mutagenic DNA-breakage/intestinal (small) damages in female rats. Intestinal epithelial cells receive ingested arsenic initially. Though, the possibility of damages in this tissue is immense and the therapeutic strategies against this damage are of great concern, reports on either issue are scanty. Our earlier study on arsenic-exposed human unveils a link between carcinogenesis and mutagenic DNA damage. Here, we demonstrate that supplementation of CS-extract (10 mg/mL water) with NaAsO2 (0.6 ppm)/100 g b.w. for 28 days to rats offered a significant protection against arsenic-induced oxidative damages to DNA and intestinal (small) tissues by buttressing antioxidant systems. Necrotic and apoptotic damages and their CS-protection are shown in DNA-fragmentation, comet-assay, and histoarchitecture (hematoxylin and eosin and periodic acid-schiff staining) results. Only arsenic exposure significantly decreased intestinal superoxide dismutase, catalase activities, and level of soluble thiol with a concomitant increase in malondialdehyde/conjugated dienes. Alteration of serum necrotic marker lactate dehydrogenase and the metabolic inflammatory marker c-reactive protein also indicate the impairment may be occurring at transcription and/or cellular signal transduction level. In addition, in situ incubation in rat intestinal loop filled for 24 h with NaAsO2 alone (250 µM) or with aqueous CS-extract (250 mg/mL) suggests that small intestinal epithelial cells are significantly protected by CS against arsenic-associated necrotic/mutagenic damages, which is observed in DNA-breakage studies. In conclusion, besides intensifying endogenous antioxidant system, CS polyphenols also offer a direct role on free radical scavenging activity that is associated to the protection from mutagenic DNA-breakages and prevention of tissue necrosis/carcinogenesis generated by arsenic.

Transformation of arsenic species during in vitro gastrointestinal digestion of vegetables

Arsenic is an element widely distributed in the environment, and the diet is the main source of arsenic exposure for most people. However, many of the processes related to steps before intestinal absorption are unknown. This study evaluates the effect of in vitro gastrointestinal digestion on pentavalent arsenic forms [As(V), MMA(V), DMA(V)] present in various vegetables (garlic, broccoli, asparagus, spinach) after soaking or boiling in aqueous solutions of these species. The results showed that the gastrointestinal digest contained trivalent or thiolated arsenic forms different from the pentavalent species added initially. Transformation percentages varied, depending on sample, treatment, and arsenic species. Results showed transformation of up to 22% to As(III), 35% to MMA(III)/MMAS, and 26% to DMA(III)/DMAS. These data indicate that more toxic arsenic species are present in the gastrointestinal digest, and they highlight the need to consider this process when evaluating the toxicological risk associated with ingestion of this metalloid.

Trivalent arsenic species induce changes in expression and levels of proinflammatory cytokines in intestinal epithelial cells

Chronic arsenic (As) toxicity in humans has been documented in many countries where exposure mostly occurs through drinking water. The As immunotoxic effects have been demonstrated in animal models as well as in humans. The studies of the immunotoxicity of As have centered on organs related to immune response or target organs, with few data being available at intestinal level. The present study has evaluated the changes in the expression and release of cytokines in Caco-2 cells, widely used as an intestinal epithelial model. Differentiated cells were exposed to 1 μM of As(III), 0.1 μM of monomethylarsonous acid [MMA(III)] and 1 μM of dimethylarsinous acid [DMA(III)] during 2, 4, 6 and 24 h. Additionally, the effect of As coexposure with lipopolysaccharide (LPS, 10 ng/mL) has been evaluated. The results show trivalent species to induce increases in the expression and release of the proinflammatory cytokines tumor necrosis factor alpha (TNFα), IL6, IL8 - the magnitude and time of response being different for each As species. The response of greatest magnitude corresponds to DMA(III), followed by As(III), while MMA(III) generates a limited response. Furthermore, the presence of LPS in the co-exposed cells could affect the expression and secretion of cytokines compared with individual exposure to arsenicals, especially for As(III)/LPS and DMA(III)/LPS.

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.

Differential toxicity and gene expression in Caco-2 cells exposed to arsenic species

Inorganic arsenic [As(V)+As(III)] and its metabolites, especially the trivalent forms [monomethylarsonous acid, MMA(III), and dimethylarsinous acid, DMA(III)], are considered the forms of arsenic with the highest degree of toxicity, linked to certain types of cancer and other pathologies. The gastrointestinal mucosa is exposed to these forms of arsenic, but it is not known what toxic effect these species may have on it. The aim of the present work was to evaluate the toxicity and some mechanisms of action of inorganic arsenic and its metabolites [monomethylarsonic acid, MMA(V), dimethylarsinic acid, DMA(V), MMA(III) and DMA(III)] in intestinal epithelial cells, using the Caco-2 human cell line as a model. The results show that the pentavalent forms do not produce toxic effects on the intestinal monolayer, but the trivalent species have a different degree of toxicity. As(III) induces death mainly by necrosis, whereas only apoptotic cells are detected after exposure to MMA(III), and for DMA(III) the percentages of apoptosis and necrosis are similar. The three forms produce reactive oxygen species, accompanied by a reduction in intracellular GSH and lipid peroxidation, the latter being especially notable in the dimethylated form. They also alter the enzyme activity of glutathione peroxidase and catalase and induce expression of stress proteins and metallothioneins. The results indicate that the trivalent forms of arsenic can affect cell viability of intestinal cells by mechanisms related to the induction of oxidative stress. Further studies are needed to evaluate how the effects observed in this study affect the structure and functionality of the intestinal epithelium.