Analysis of the passage of the marine biotoxin okadaic acid through an in vitro human gut barrier

A multi-omics approach to elucidate okadaic acid-induced changes in human HepaRG hepatocarcinoma cells

Okadaic acid (OA), a prevalent marine biotoxin found in shellfish, is known for causing acute gastrointestinal symptoms. Despite its potential to reach the bloodstream and the liver, the hepatic effects of OA are not well understood, highlighting a significant research gap. This study aims to comprehensively elucidate the impact of OA on the liver by examining the transcriptome, proteome, and phosphoproteome alterations in human HepaRG liver cells exposed to non-cytotoxic OA concentrations. We employed an integrative multi-omics approach, encompassing RNA sequencing, shotgun proteomics, phosphoproteomics, and targeted DigiWest analysis. This enabled a detailed exploration of gene and protein expression changes, alongside phosphorylation patterns under OA treatment. The study reveals concentration- and time-dependent deregulation in gene and protein expression, with a significant down-regulation of xenobiotic and lipid metabolism pathways. Up-regulated pathways include actin crosslink formation and a deregulation of apoptotic pathways. Notably, our results revealed that OA, as a potent phosphatase inhibitor, induces alterations in actin filament organization. Phosphoproteomics data highlighted the importance of phosphorylation in enzyme activity regulation, particularly affecting proteins involved in the regulation of the cytoskeleton. OA's inhibition of PP2A further leads to various downstream effects, including alterations in protein translation and energy metabolism. This research expands the understanding of OA's systemic impact, emphasizing its role in modulating the phosphorylation landscape, which influences crucial cellular processes. The results underscore OA's multifaceted effects on the liver, particularly through PP2A inhibition, impacting xenobiotic metabolism, cytoskeletal dynamics, and energy homeostasis. These insights enhance our comprehension of OA's biological significance and potential health risks.

Rat tight junction proteins are disrupted after subchronic exposure to okadaic acid

Okadaic acid (OA), a lipophilic phycotoxin distributed worldwide, causes diarrheic shellfish poisoning and even leads to tumor formation. Currently, the consumption of contaminated seafood is the most likely cause of chronic OA exposure, but there is a serious lack of relevant data. Here, the Sprague-Dawley rats were exposure to OA by oral administration at 100 µg/kg body weight, and the tissues were collected and analyzed to assess the effect of subchronic OA exposure. The results showed that subchronic OA administration disturbed colonic mucosal integrity and induced colitis. The colonic tight junction proteins were disrupted and the cell cycle of colonic epithelial cells was accelerated. It is inferred that disruption of the colonic tight junction proteins might be related to the development of chronic diarrhea by affecting water and ion transport. Moreover, the accelerated proliferation of colonic epithelial cells indicated that subchronic OA exposure might promote the restitution process of gut barrier or induce tumor promoter activity in rat colon.

Role of enteric glial cells in the toxicity of phycotoxins: Investigation with a tri-culture intestinal cell model

Lipophilic phycotoxins are secondary metabolites produced by phytoplankton. They can accumulate in edible filtering-shellfish and cause human intoxications, particularly gastrointestinal symptoms. Up to now, the in vitro intestinal effects of these toxins have been mainly investigated on simple monolayers of intestinal cells such as the enterocyte-like Caco-2 cell line. Recently, the combination of Caco-2 cells with mucus secreting HT29-MTX cell line has been also used to mimic the complexity of the human intestinal epithelium. Besides, enteric glial cells (EGC) from the enteric nervous system identified in the gut mucosa have been largely shown to be involved in gut functions. Therefore, using a novel model integrating Caco-2 and HT29-MTX cells co-cultured on inserts with EGC seeded in the basolateral compartment, we examined the toxicological effects of two phycotoxins, pectenotoxin-2 (PTX2) and okadaic acid (OA). Cell viability, morphology, barrier integrity, inflammation, barrier crossing, and the response of some specific glial markers were evaluated using a broad set of methodologies. The toxicity of PTX2 was depicted by a slight decrease of viability and integrity as well as a slight increase of inflammation of the Caco-2/HT29-MTX co-cultures. PTX2 induced some modifications of EGC morphology. OA induced IL-8 release and decreased viability and integrity of Caco-2/HT29-MTX cell monolayers. EGC viability was slightly affected by OA. The presence of EGC reinforced barrier integrity and reduced the inflammatory response of the epithelial barrier following OA exposure. The release of GDNF and BDNF gliomediators by EGC could be implicated in the protection observed.

The diarrhetic shellfish-poisoning toxin, okadaic acid, provokes gastropathy, dysbiosis and susceptibility to bacterial infection in a non-rodent bioassay, Galleria mellonella

Diarrhetic shellfish-poisoning (DSP) toxins such as okadaic acid and dinophysistoxins harm the human gastrointestinal tract, and therefore, their levels are regulated to an upper limit of 160 μg per kg tissue to protect consumers. Rodents are used routinely for risk assessment and studies concerning mechanisms of toxicity, but there is a general move toward reducing and replacing vertebrates for these bioassays. We have adopted insect larvae of the wax moth Galleria mellonella as a surrogate toxicology model. We treated larvae with environmentally relevant doses of okadaic acid (80–400 μg/kg) via intrahaemocoelic injection or gavage to determine marine toxin-related health decline: (1) whether pre-exposure to a sub-lethal dose of toxin (80 μg/kg) enhances susceptibility to bacterial infection, or (2) alters tissue pathology and bacterial community (microbiome) composition of the midgut. A sub-lethal dose of okadaic acid (80 μg/kg) followed 24 h later by bacterial inoculation (2 × 10⁵Escherichia coli) reduced larval survival levels to 47%, when compared to toxin (90%) or microbial challenge (73%) alone. Histological analysis of the midgut depicted varying levels of tissue disruption, including nuclear aberrations associated with cell death (karyorrhexis, pyknosis), loss of organ architecture, and gross epithelial displacement into the lumen. Moreover, okadaic acid presence in the midgut coincided with a shift in the resident bacterial population over time in that substantial reductions in diversity (Shannon) and richness (Chao-1) indices were observed at 240 μg toxin per kg. Okadaic acid-induced deterioration of the insect alimentary canal resembles those changes reported for rodent bioassays.

Co-culture model of Caco-2/HT29-MTX cells: A promising tool for investigation of phycotoxins toxicity on the intestinal barrier

Most lipophilic phycotoxins have been involved in human intoxications but some of these toxins have never been proven to induce human gastro-intestinal symptoms, although intestinal damage in rodents has been documented. For investigating the in vitro toxicological profile of lipophilic phycotoxins on intestine, the epithelial Caco-2 cell line has been the most commonly used model. Nevertheless, considering the complexity of the intestinal epithelium, in vitro co-cultures integrating enterocyte-like and mucus-secreting cell types are expected to provide more relevant data. In this study, the toxic effects (viability, inflammation, cellular monolayer integrity, modulation of cell type proportion and production of mucus) of four lipophilic phycotoxins (PTX2, YTX, AZA1 and OA) were evaluated in Caco-2/HT29-MTX co-cultured cells. The four toxins induced a reduction of viability from 20% to 50% and affected the monolayer integrity. Our results showed that the HT29-MTX cells population were more sensitive to OA and PTX2 than Caco-2 cells. Among the four phycotoxins, OA induced inflammation (28-fold increase of IL-8 release) and also a slight increase of both mucus production (up-regulation of mucins mRNA expression) and mucus secretion (mucus area and density). For PTX2 we observed an increase of IL-8 release but weaker than OA. Intestinal cell models integrating several cell types can contribute to improve hazard characterization and to describe more accurately the modes of action of phycotoxins.

OMICs Approaches in Diarrhetic Shellfish Toxins Research

Diarrhetic shellfish toxins (DSTs) are among the most prevalent marine toxins in Europe's and in other temperate coastal regions. These toxins are produced by several dinoflagellate species; however, the contamination of the marine trophic chain is often attributed to species of the genus Dinophysis. This group of toxins, constituted by okadaic acid (OA) and analogous molecules (dinophysistoxins, DTXs), are highly harmful to humans, causing severe poisoning symptoms caused by the ingestion of contaminated seafood. Knowledge on the mode of action and toxicology of OA and the chemical characterization and accumulation of DSTs in seafood species (bivalves, gastropods and crustaceans) has significantly contributed to understand the impacts of these toxins in humans. Considerable information is however missing, particularly at the molecular and metabolic levels involving toxin uptake, distribution, compartmentalization and biotransformation and the interaction of DSTs with aquatic organisms. Recent contributions to the knowledge of DSTs arise from transcriptomics and proteomics research. Indeed, OMICs constitute a research field dedicated to the systematic analysis on the organisms' metabolisms. The methodologies used in OMICs are also highly effective to identify critical metabolic pathways affecting the physiology of the organisms. In this review, we analyze the main contributions provided so far by OMICs to DSTs research and discuss the prospects of OMICs with regard to the DSTs toxicology and the significance of these toxins to public health, food safety and aquaculture.

Toxins of Okadaic Acid-Group Increase Malignant Properties in Cells of Colon Cancer

Diarrhetic shellfish poisoning (DSP) is a syndrome caused by the intake of shellfish contaminated with a group of lipophilic and thermostable toxins, which consists of okadaic acid (OA), dinophysistoxin-1 (DTX-1) and dinophysistoxin-2 (DTX-2). These toxins are potent protein Ser/Thr phosphatase inhibitors, mainly type 1 protein phosphatase (PP1) and type 2A protein phosphatase (PP2A). Different effects have been reported at the cellular, molecular and genetic levels. In this study, changes in cell survival and cell mobility induced by OA, DTX-1 and DTX-2 were determined in epithelial cell lines of the colon and colon cancer. The cell viability results showed that tumoral cell lines were more resistant to toxins than the nontumoral cell line. The results of the functional assays for testing cell migration, evaluation of cell death and the expression of proteins associated with cell adhesion showed a dual effect of toxins since in the nontumoral cell line, a greater induction of cell death, presumably by anoikis, was detected. In the tumoral cell lines, there was an induction of a more aggressive phenotype characterized by increased resistance to toxins, increased migration and increased FAK activation. In tumoral cell lines of colon cancer, OA, DTX-1/DTX-2 induce a more aggressive phenotype.

Combined effects of okadaic acid and pectenotoxin-2, 13-desmethylspirolide C or yessotoxin in human intestinal Caco-2 cells

Lipophilic phycotoxins are secondary metabolites produced by phytoplanktonic species. They accumulate in filtering shellfish and can cause human intoxications. Humans can be exposed to combinations of several phycotoxins. The toxicological effects of phycotoxin mixtures on human health are largely unknown. Published data on phycotoxin co-exposure show that okadaic acid (OA) is simultaneously found with pectenetoxin-2 (PTX-2), 13-desmethylspirolide C (also known as SPX-1), or yessotoxin (YTX). Therefore, the aim of this study was to examine the effects of three binary mixtures, OA/PTX-2, OA/SPX-1 and OA/YTX on human intestinal Caco-2 cells. A multi-parametric approach for cytotoxicity determination was applied using a high-content analysis platform, including markers for cell viability, oxidative stress, inflammation, and DNA damage. Mixtures effects were analyzed using two additivity mathematical models. Our assays revealed that OA induced cytotoxicity, DNA strand breaks and interleukin 8 release. PTX-2 slightly induced DNA strand breaks, whereas SPX-1 and YTX did not affect the investigated endpoints. The combination of OA with another toxin resulted in reduced toxicity at low concentrations, suggesting antagonistic effects, but in increased effects at higher concentrations, suggesting additive or synergistic effects. Taken together, our results demonstrated that the cytotoxic effects of binary mixtures of lipophilic phycotoxins could not be predicted by additivity mathematical models. In conclusion, the present data suggest that combined effects of phycotoxins may occur which might have the potential to impact on risk assessment of these compounds.

Novel Insights on the Toxicity of Phycotoxins on the Gut through the Targeting of Enteric Glial Cells

In vitro and in vivo studies have shown that phycotoxins can impact intestinal epithelial cells and can cross the intestinal barrier to some extent. Therefore, phycotoxins can reach cells underlying the epithelium, such as enteric glial cells (EGCs), which are involved in gut homeostasis, motility, and barrier integrity. This study compared the toxicological effects of pectenotoxin-2 (PTX2), yessotoxin (YTX), okadaic acid (OA), azaspiracid-1 (AZA1), 13-desmethyl-spirolide C (SPX), and palytoxin (PlTX) on the rat EGC cell line CRL2690. Cell viability, morphology, oxidative stress, inflammation, cell cycle, and specific glial markers were evaluated using RT-qPCR and high content analysis (HCA) approaches. PTX2, YTX, OA, AZA1, and PlTX induced neurite alterations, oxidative stress, cell cycle disturbance, and increase of specific EGC markers. An inflammatory response for YTX, OA, and AZA1 was suggested by the nuclear translocation of NF-κB. Caspase-3-dependent apoptosis and induction of DNA double strand breaks (γH2AX) were also observed with PTX2, YTX, OA, and AZA1. These findings suggest that PTX2, YTX, OA, AZA1, and PlTX may affect intestinal barrier integrity through alterations of the human enteric glial system. Our results provide novel insight into the toxicological effects of phycotoxins on the gut.

The marine biotoxin okadaic acid affects intestinal tight junction proteins in human intestinal cells

Okadaic acid (OA) is a lipophilic phycotoxin that accumulates in the hepatopancreas and fatty tissue of shellfish. Consumption of highly OA-contaminated seafood leads to diarrhetic shellfish poisoning which provokes severe gastrointestinal symptoms associated with a disruption of the intestinal epithelium. Since the molecular mechanisms leading to intestinal barrier disruption are not fully elucidated, we investigated the influence of OA on intestinal tight junction proteins (TJPs) in differentiated Caco-2 cells. We found a concentration- and time-dependent deregulation of genes encoding for intestinal TJPs of the claudin family, occludin, as well as zonula occludens (ZO) 1 and 2. Immunofluorescence staining showed concentration-dependent effects on the structural organization of TJPs already after treatment with a subtoxic but human-relevant concentration of OA. In addition, changes in the structural organization of cytoskeletal F-actin as well as its associated protein ZO-1 were observed. In summary, we demonstrated effects of OA on TJPs in intestinal Caco-2 cells. TJP expressions were affected after treatment with food-relevant OA concentrations. These results might explain the high potential of OA to disrupt the intestinal barrier in vivo as its first target. Thereby the present data contribute to a better understanding of the OA-dependent induction of molecular effects within the intestinal epithelium.

Metabolism of okadaic acid by NADPH-dependent enzymes present in human or rat liver S9 fractions results in different toxic effects

The lipophilic marine biotoxin okadaic acid (OA) represents a natural contaminant produced by algae accumulating in seafood. Acute intoxications result in diarrhetic shellfish poisoning causing symptoms like nausea, vomiting and abdominal cramps. OA was preincubated with liver enzymes present in S9 fractions from humans, rats and rats pretreated with enzyme inducers in the presence or absence of the cofactor NADPH to investigate hepatic metabolism. Cytotoxicity was examined in HepG2 cells and metabolites of OA were determined by LC-MS/MS. Strong cytotoxicity was observed in HepG2 cells treated with OA that was preincubated in S9 fractions without NADPH. However, neither metabolites nor a decrease of OA itself were found. The addition of NADPH to the S9 fractions of rats resulted in a decreased cytotoxicity of OA, but a stronger toxicity in HepG2 cells was observed from OA preincubated in human S9 fractions with NADPH. Metabolite profiles of each S9 mix revealed that higher amounts of detoxified metabolites were formed by NADPH-dependent enzymes of rats compared to the same enzymes of humans. These differences in OA detoxification by NADPH-dependent liver enzymes of rats and humans may be of significance in the extrapolation of toxicological data from animal models (rats to humans, for example).

Characterization of the dinophysistoxin-2 acute oral toxicity in mice to define the Toxicity Equivalency Factor

Ingestion of shellfish with dinophysistoxin-2 (DTX2) can lead to diarrheic shellfish poisoning (DSP). The official control method of DSP toxins in seafood is the liquid chromatography-mass spectrometry analysis (LC-MS). However in order to calculate the total toxicity of shellfish, the concentration of each compound must be multiplied by individual Toxicity Equivalency Factor (TEF). Considering that TEFs caused some controversy and the scarce information about DTX2 toxicity, the aim of this study was to characterize the oral toxicity of DTX2 in mice. A 4-Level Up and Down Procedure allowed the characterization of DTX2 effects and the estimation of DTX2 oral TEF based on determination of the lethal dose 50 (LD50). DTX2 passed the gastrointestinal barrier and was detected in urine and feces. Acute toxicity symptoms include diarrhea and motionless, however anatomopathology study and ultrastructural images restricted the toxin effects to the gastrointestinal tract. Nevertheless enterocytes microvilli and tight junctions were not altered, disconnecting DTX2 diarrheic effects from paracellular epithelial permeability. This is the first report of DTX2 oral LD₅₀ (2262 μg/kg BW) indicating that its TEF is about 0.4. This result suggests reevaluation of the present TEFs for the DSP toxins to better determine the actual risk to seafood consumers.

Comparative analysis of the cytotoxic effects of okadaic acid-group toxins on human intestinal cell lines

The phycotoxin, okadaic acid (OA) and dinophysistoxin 1 and 2 (DTX-1 and -2) are protein phosphatase PP2A and PP1 inhibitors involved in diarrhetic shellfish poisoning (DSP). Data on the toxicity of the OA-group toxins show some differences with respect to the in vivo acute toxicity between the toxin members. In order to investigate whether OA and congeners DTX-1 and -2 may induce different mechanisms of action during acute toxicity on the human intestine, we compared their toxicological effects in two in vitro intestinal cell models: the colorectal adenocarcinoma cell line, Caco-2, and the intestinal muco-secreting cell line, HT29-MTX. Using a high content analysis approach, we evaluated various cytotoxicity parameters, including apoptosis (caspase-3 activation), DNA damage (phosphorylation of histone H2AX), inflammation (translocation of NF-κB) and cell proliferation (Ki-67 production). Investigation of the kinetics of the cellular responses demonstrated that the three toxins induced a pro-inflammatory response followed by cell cycle disruption in both cell lines, leading to apoptosis. Our results demonstrate that the three toxins induce similar effects, as no major differences in the cytotoxic responses could be detected. However DTX-1 induced cytotoxic effects at five-fold lower concentrations than for OA and DTX-2.

Experimental basis for the high oral toxicity of dinophysistoxin 1: a comparative study of DSP

Okadaic acid (OA) and its analogues, dinophysistoxin 1 (DTX1) and dinophysistoxin 2 (DTX2), are lipophilic and heat-stable marine toxins produced by dinoflagellates, which can accumulate in filter-feeding bivalves. These toxins cause diarrheic shellfish poisoning (DSP) in humans shortly after the ingestion of contaminated seafood. Studies carried out in mice indicated that DSP poisonous are toxic towards experimental animals with a lethal oral dose 2–10 times higher than the intraperitoneal (i.p.) lethal dose. The focus of this work was to study the absorption of OA, DTX1 and DTX2 through the human gut barrier using differentiated Caco-2 cells. Furthermore, we compared cytotoxicity parameters. Our data revealed that cellular viability was not compromised by toxin concentrations up to 1 μM for 72 h. Okadaic acid and DTX2 induced no significant damage; nevertheless, DTX1 was able to disrupt the integrity of Caco-2 monolayers at concentrations above 50 nM. In addition, confocal microscopy imaging confirmed that the tight-junction protein, occludin, was affected by DTX1. Permeability assays revealed that only DTX1 was able to significantly cross the intestinal epithelium at concentrations above 100 nM. These data suggest a higher oral toxicity of DTX1 compared to OA and DTX2.

Okadaic acid: more than a diarrheic toxin

Okadaic acid (OA) is one of the most frequent and worldwide distributed marine toxins. It is easily accumulated by shellfish, mainly bivalve mollusks and fish, and, subsequently, can be consumed by humans causing alimentary intoxications. OA is the main representative diarrheic shellfish poisoning (DSP) toxin and its ingestion induces gastrointestinal symptoms, although it is not considered lethal. At the molecular level, OA is a specific inhibitor of several types of serine/threonine protein phosphatases and a tumor promoter in animal carcinogenesis experiments. In the last few decades, the potential toxic effects of OA, beyond its role as a DSP toxin, have been investigated in a number of studies. Alterations in DNA and cellular components, as well as effects on immune and nervous system, and even on embryonic development, have been increasingly reported. In this manuscript, results from all these studies are compiled and reviewed to clarify the role of this toxin not only as a DSP inductor but also as cause of alterations at the cellular and molecular levels, and to highlight the relevance of biomonitoring its effects on human health. Despite further investigations are required to elucidate OA mechanisms of action, toxicokinetics, and harmful effects, there are enough evidences illustrating its toxicity, not related to DSP induction, and, consequently, supporting a revision of the current regulation on OA levels in food.

Alterations in metabolism-related genes induced in SHSY5Y cells by okadaic acid exposure

Okadaic acid (OA) is a widely distributed marine toxin produced by several phytoplanktonic species and responsible for diarrheic shellfish poisoning in humans. At the molecular level OA is a specific inhibitor of several types of serine/threonine protein phosphatases. Due to this enzymic inhibition, OA was reported to induce numerous alterations in relevant cellular physiological processes, including several metabolic pathways such as glucose uptake, lipolysis and glycolysis, heme metabolism, and glycogen and protein synthesis. In order to further understand the underlying mechanisms involved in OA-induced effects on cellular metabolism, the expression levels of six genes related to different catabolic and anabolic metabolism-related processes were analyzed by real-time polymerase chain reaction. Specifically, the expression patterns of GAPDH, TOMM5, SLC25A4, COII, QARS, and RGS5 genes were determined in SHSY5Y human neuroblastoma cells exposed to OA for 3, 24, or 48 h. All these genes showed alterations in their expression levels after at least one of the OA treatments tested. These alterations provide a basis to understand the mechanisms underlying the previously described OA-induced effects on different metabolic processes, mainly regarding glucose and mitochondrial metabolism. However, other OA-induced affected genes can not be ruled out, and further studies are required to more comprehensively characterize in the mechanisms of OA-induced interaction on cell metabolism.

The cytotoxic effect of palytoxin on Caco-2 cells hinders their use for in vitro absorption studies

Palytoxin (PLTX), found in Palythoa zoanthids and Ostreopsis dinoflagellates, has also been detected in crabs and fish, through which it can enter into the food chain. Indeed, PLTX is considered the causative agent of several cases of human seafood poisoning resulting in systemic symptoms. Available epidemiological data on PLTX human toxicity suggest that the intestinal tract may be one of its in vivo targets and its potential site of access into the bloodstream. Hence, the purpose of this study was to investigate the suitability of the human intestinal Caco-2 cell line for evaluating PLTX oral absorption. A detailed analysis of PLTX cytotoxicity revealed a high sensitivity of Caco-2 cells: 4h toxin exposure reduced mitochondrial activity (MTT assay, EC(50) of 8.9±3.7×10(-12)M), cell density (SRB assay, EC(50) of 2.0±0.6×10(-11)M) and membrane integrity (LDH release, EC(50) of 4.5±1.4×10(-9)M and PI uptake, EC(50) of 1.0±0.8×10(-8)M). After low PLTX concentration (1.0×10(-11)M) exposure for 1-8h, followed by 24h recovery time in toxin-free medium, cell density reduction was only partially reversible. These results indicate that, due to the high susceptibility to PLTX cytotoxic effects, Caco-2 cells do not represent an appropriate and reliable model for investigating intestinal barrier permeation by this toxin.