Evaluation of the use of two human cell lines for okadaic acid and DTX-1 determination by cytotoxicity assays and damage characterization

Differences in Toxic Response Induced by Three Variants of the Diarrheic Shellfish Poisoning Phycotoxins in Human Intestinal Epithelial Caco-2 Cells

Diarrheic shellfish poisoning (DSP) is caused by the consumption of shellfish contaminated with a group of phycotoxins that includes okadaic acid (OA), dinophysistoxin-1 (DTX-1), and dinophysistoxin-2 (DTX-2). These toxins are inhibitors of serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A), but show distinct levels of toxicity. Aside from a difference in protein phosphatases (PP) inhibition potency that would explain these differences in toxicity, others mechanisms of action are thought to be involved. Therefore, we investigated and compared which mechanisms are involved in the toxicity of these three analogues. As the intestine is one of the target organs, we studied the transcriptomic profiles of human intestinal epithelial Caco-2 cells exposed to OA, DTX-1, and DTX-2. The pathways specifically affected by each toxin treatment were further confirmed through the expression of key genes and markers of toxicity. Our results did not identify any distinct biological mechanism for OA and DTX-2. However, only DTX-1 induced up-regulation of the MAPK transduction signalling pathway, and down-regulation of gene products involved in the regulation of DNA repair. As a consequence, based on transcriptomic results, we demonstrated that the higher toxicity of DTX-1 compared to OA and DTX-2 was consistent with certain specific pathways involved in intestinal cell response.

Okadaic acid promotes epithelial-mesenchymal transition of hepatocellular carcinoma cells by inhibiting protein phosphatase 2A

As a specific inhibitor of serine/threonine protein phosphatases, okadaic acid (OA) has been found to be a tumor promoter. However, whether OA plays a role in metastasis of hepatocellular carcinoma (HCC) has not been well elucidated. In this study, Hep3B and HepG2 cells were treated with different doses of OA and the cell viability was determined by CCK8 test. As a result, Hep3B and HepG2 cells showed no obvious cytotoxicity after OA treatment below 20 or 25 nM for 12 or 24 hours. However, wound healing, invasion, and migration abilities of HCC cells were significantly enhanced in the OA-treated groups than those of the control group (P < .05), measured by cell scratching and BD transwell assays. Moreover, we found that the expression of epithelial-mesenchymal transition (EMT)-related key factors was changed upon OA treatment in a dose-dependent manner. In addition, the activity of protein phosphatase 2A (PP2A) in OA-treated cells was also decreased significantly compared with the control cells (P < .05). Interfering of PP2A subunit A or C caused a similar expression change of EMT-related key factors as the OA treatment in HCC cells. Our results suggest that OA promotes the EMT process of HCC cells by inhibiting the activity of PP2A.

Evaluation of cytotoxic compounds in different organs of the sea bream Sarpa salpa as related to phytoplankton consumption: an in vitro study in human liver cell lines HepG2 and WRL68

The present study was aimed to assess the cytotoxic effects of not-yet identified compounds present in organ extracts of Sarpa salpa, collected in autumn, the period with a peak in health problems. In addition, we studied the cytotoxicity of extracts of epiphytes found in the stomach content of S. salpa collected in summer and of epiphytes collected from the sea in the Sfax area at the end of spring. We tested these fractions in two human hepatic cell lines: HepG2 and WRL68. We observed a significant loss of viable cells when HepG2 cells were exposed for 72 h to acetone extracts of livers of S. salpa at a concentration of 2.5 mg/ml protein. Proteins extracted from brain or muscle did not significantly induce cell death at the studied concentrations (≤10 mg/ml). There was a significant loss of viable cells when treated with liver extract of S. salpa dissolved in DMSO. Extracts of epiphytes collected in late spring showed a cytotoxic effect in a concentration-dependent manner. Moreover, we observed a significantly decreased cell viability of HepG2 at a dilution (1/40) of epiphyte extracts from stomach contents of two fish we had collected. The cytotoxic effect of the observed epiphyte extracts confirms the transfer of toxins originating from toxic dinoflagellates which live in epiphyte on the Posidonia oceanica leaves to fish organs by grazing. Hence, the liver of this fish can cause a threat to human health and consumption should for this reason be dissuaded.

Naturally contaminated shellfish samples: quantification of diarrhetic shellfish poisoning toxins in unhydrolysed and hydrolysed extracts and cytotoxicity assessment

Contamination of shellfish from the Portuguese coast with diarrhetic shellfish poisoning (DSP) toxins is a recurrent event, with most of the commercial bivalves contaminated with high percentages of esters of okadaic acid (OA) and dinophysistoxin-2 (DTX2). This report describes the quantification of DSP toxins in unhydrolysed and hydrolysed extracts of several cockle and mussel samples naturally contaminated and the evaluation of their cytotoxicity profiles in V79 cells. The quantification of the acyl esters in the shellfish samples involved the cleavage of the ester bond through alkaline hydrolysis and the release of the parent toxins OA and DTX2. Unhydrolysed and hydrolysed extracts were then analyzed by liquid chromatography (LC) coupled with mass spectrometry (MS) for the detection and quantification of DSP toxins. The cytotoxicity of the analysed extracts was evaluated using the MTT reduction assay and compared with the cytotoxicity presented by different concentrations of OA standard (1-100 nM). OA exhibited marked cytotoxic effects and decreased cell viability in a dose dependent mode, with an IC₅₀ of 27 nM. The cytotoxicity pattern of unhydrolysed extracts was clearly dependent on the concentration of free toxins. Moreover, the cytotoxicity of the esterified toxins present was revealed after their conversion into free toxins by alkaline hydrolysis. For the hydrolysed extracts of cockles and mussels, the cytotoxicity presented was mainly related to the concentration of OA and DTX2.

Suitability of the Neuro-2a cell line for the detection of palytoxin and analogues (neurotoxic phycotoxins)

Palytoxin and related compounds are neurotoxic phycotoxins produced by benthic microalgae belonging to the genus Ostreopsis. For several years this family of phycotoxins has been posing a threat to human health since they can bioaccumulate in shellfish. With the aim of replacing current biological assays, such as the mouse or hemolytic assays, we investigated using the Neuro-2a neuroblastoma cell line to detect palytoxin and related compounds. Cell death induced by the effects of PlTX and analogues on Na+, K+-ATPase were measured using the 1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT) assay for mitochondrial reductase activity as a surrogate for cell number. The specificity of the Neuro-2a cell-based assay for palytoxin detection was confirmed by using ouabain, which also acts on Na+, K+-ATPase. Pre-treatment of the Neuro-2a cells with ouabain minimizes the effects of palytoxin. The specificity of the Neuro-2a assay was confirmed by the finding that cell death was not detected when Neuro-2a cells were exposed to other phycotoxins with unrelated cellular targets. When the Neuro-2a assay was used to detect palytoxin in mussel extracts spiked with levels of palytoxin around the proposed regulatory value of 250 microg palytoxin/kg shellfish, a good correlation was observed between the levels found and the expected values. We conclude by proposing an experimental design for functional assays using the Neuro-2a cell line for the specific detection of four neurotoxic phycotoxin families: saxitoxins, brevetoxins, ciguatoxins and palytoxins.

Marine toxins and the cytoskeleton: okadaic acid and dinophysistoxins

Okadaic acid (OA) and its analogs, the dinophysistoxins, are potent inhibitors of protein phosphatases 1 and 2A. This action is well known to cause diarrhea and gastrointestinal symptons when the toxins reach the digestive tract by ingestion of mollusks. A less well-known effect of these group of toxins is their effect in the cytoskeleton. OA has been shown to stimulate cell motility, loss of stabilization of focal adhesions and a consequent loss of cytoskeletal organization due to an alteration in the tyrosine-phosphorylated state of the focal adhesion kinases and paxillin. OA causes cell rounding and loss of barrier properties through mechanisms that probably involve disruption of filamentous actin (F-actin) and/or hyperphosphorylation and activation of kinases that stimulate tight junction disassembly. Neither methyl okadaate (a weak phosphatase inhibitor) nor OA modify the total amount of F-actin, but both toxins cause similar changes in the F-actin cytoskeleton, with strong retraction and rounding, and in many cases cell detachment. OA and dinophysistoxin-1 (35S-methylokadaic acid) cause rapid changes in the structural organization of intermediate filaments, followed by a loss of microtubules, solubilization of intermediate filament proteins, and disruption of desmosomes. The detailed pathways that coordinate all these effects are not yet known.

In vitro approaches to evaluate palytoxin-induced toxicity and cell death in intestinal cells

Palytoxin isolated from the genus Palythoa is the most potent marine toxin known. The aim of the present study was to quantify palytoxin-induced cellular injury in the human intestinal cell line Caco-2. Cellular damage was measured by evaluating cell proliferation, cell membrane permeability, cell morphology and apoptotic markers. Furthermore, changes in F-actin were studied after exposure of cells to increasing amounts of palytoxin. The results show that cell proliferation decreased in a concentration-dependent manner with a mean IC(50) value of about 0.1 nM. A noticeable increase of cell detachment correlated with cell rounding and F-actin depolymerization was observed in palytoxin-treated cells. Moreover LDH was released from the cells in a dose and time dependent manner, although under these conditions there was no propidium iodide uptake. On the other hand, palytoxin impaired mitochondrial activity but other apoptotic markers, such as DNA fragmentation or caspases activation, were not observed. The results obtained in this paper suggest that the effects of palytoxin in Caco-2 cells were very potent and unspecific, since a primary necrosis and a secondary apoptosis seem to occur under these conditions.

Characteristics of palytoxin-induced cytotoxicity in neuroblastoma cells

Cation fluxes appear to play a key role in palytoxin-induced signal. There are other cellular targets that have not been described as well as the biochemical signaling cascades that transmit palytoxin-stimulated signals remain to be clarified. Since modifications of cations, mainly calcium, are generally associated to cell death or apoptosis, we wanted to further evaluate the effect of palytoxin on cell death. Then, in vitro cytotoxic effects of palytoxin were characterized on human neuroblastoma cells. By using several techniques, we studied markers of cell death and apoptosis, such as cell detachment, mitochondrial membrane potential, caspases, DNA damage, LDH leakage, propidium iodide uptake, F-actin depolymerization and inhibition of cellular proliferation. Results show that palytoxin triggers a series of toxic responses; it inhibits cell proliferation, induces cell rounding, detachment from the substratum and F-actin disruption. Among the apoptotic markers studied we only detected fall in mitochondrial membrane potential. Neither caspases activation nor chromatin condensation or DNA fragmentation were observed in palytoxin-treated cells.

Collapse of mitochondrial membrane potential and caspases activation are early events in okadaic acid-treated Caco-2 cells

Diarrhetic Shellfish Poisoning (DSP) results from the consumption of shellfish contaminated with okadaic acid (OA) or one of the dinophysistoxins (DTX). It has been reported that this toxin induces apoptosis in several cell models, but the molecular events involved in this process have not been clarified. In this report we studied intracellular signals induced by OA in Caco-2 cells: mitochondrial membrane potential, F-actin depolymerization, caspases activation, cell proliferation and cell membrane integrity. Results indicate that caspases-8 and -9 increased their activity after 30 min of OA treatment according to their role as initiator caspases. In contrast, activation of the downstream caspase-3 is a later event in the execution phase of apoptosis. Mitochondrial membrane potential changes are detected at 30 min of OA exposure indicating that this is an early response in the apoptotic cascade. F-actin depolymerization occurs after 24h of incubation with OA and this effect is significant at low doses of the toxin. LDH is released into the culture medium, although there is not PI uptake, indicative of a significant cell death in addition to apoptosis. Moreover, OA led to a dose- and time-dependent decrease in cellular proliferation.

Effects of malachite green (MG) and its major metabolite, leucomalachite green (LMG), in two human cell lines

Malachite green (MG) is still illegally used as a fungicide in aquaculture. In fish it is absorbed and metabolised to its major reduced metabolite, leucomalachite green (LMG). This latter represents the main residue found in fish tissues and may persist for several months. Since MG, suspected to act as a tumour promoter in vitro and in vivo, might be also present as a residue in fish, the present study was undertaken to ascertain the in vitro toxicity of both compounds in two human tumour cell lines (Caco-2 and HEp-2). After 24h incubation with MG, significant decreases of cell viability, measured by neutral red uptake (NRU) or total protein content (TPC) as well as proliferation capability (colony-forming ability, CFA) were noticed in HEp-2 cells; the mean IC(50) value was about 2 microM. As regards the differentiated Caco-2 cells, MG caused a dose-related significant cytotoxicity, measured either by MTT test, the LDH leakage or NRU, with a mean IC(50) value of about 15 microM. By contrast, LMG disclosed, in both cell lines, a lower cytotoxicity compared to MG. These results also show that HEp-2 cells are more sensitive than intestinal cells to the toxic action of both compounds.

Fluorescent microplate cell assay to measure uptake and metabolism of glucose in normal human lung fibroblasts

This is the first report of a fluorimetric microplate assay to assess glucose uptake and metabolism in eukaryotic cells. The assay was carried out incubating normal human lung fibroblasts in the wells of microtiter trays with a fluorescent D-glucose derivative, 2-N-7-(nitrobenz-2-oxa-1,3-diazol-4-yl)amino-2-deoxy-D-glucose (2-NBDG). This dye could be incorporated by glucose transporting systems in living cells. Substrate uptake was determined by analysing the data obtained with a fluorescence microplate reader. Variables studied in the development of the assay included dye concentration and incubation period. We found that this cell assay is very sensitive, reproducible, provides fast results and graphical display of data. It requires small sample volumes and allows handling of a large number of samples simultaneously. Okadaic acid was used to assess this microplate assay in the field of cytotoxicity. This diarrhetic shellfish toxin is a tumour promoter and a specific inhibitor of protein phosphatases 1 and 2A. The exposition of cells to okadaic acid (0.1 nM-1 microM) at different time intervals causes a decrease in intracellular glucose (40-50% over controls). Results obtained with okadaic acid are the starting point to evaluate application of the method to routine toxicity probes.

Nitric oxide production in Caco-2 cells exposed to different inducers, inhibitors and natural toxins

The involvement of the NO pathway in several intestinal inflammatory diseases is under investigation. In vitro models may provide a useful approach to better characterise this pathway at the cellular level. For this purpose, we have used Caco-2 cells, which are able to spontaneously differentiate in long-term culture to small intestine enterocytes. The effect of different NO pathway inducers [gamma-interferon (IFN-gamma) and phorbol myristate acetate (PMA)] has been studied. Our results demonstrate that Caco-2 cells constitutively express NOS at very low levels, while the induction with PMA+IFN-gamma triggers the expression of the inducible isoform with a stronger effect starting from day 14 of differentiation. The use of specific inhibitors of gene expression, at transcriptional and translational level, suggests that new synthesis of iNOS mRNA is required, through direct activation of the gene or new synthesis of transcription-required factors, as indicated by CHX inhibition. The morphological alteration induced by PMA+IFN-gamma is reversed by iNOS inhibitor, suggesting that the NO pathway may be involved in the cytoskeletal alterations. The DSP toxins, OA and DTX-1, induce NO production at levels corresponding to their different toxicity, previously detected in Caco-2 cells.