Apoptotic events induced by the phosphatase inhibitor okadaic acid in normal human lung fibroblasts

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.

Effect of a trans fatty acid-enriched diet on mitochondrial, inflammatory, and oxidative stress parameters in the cortex and hippocampus of Wistar rats

PURPOSE

Previously showed that dietary trans fatty acids (TFAs) may cause systemic inflammation and affect the central nervous system (CNS) in Wistar rats by increased levels of cytokines in the cerebrospinal fluid (CSF) and serum (Longhi et al. Eur J Nutr 56(3):1003-1016, 1). Here, we aimed to clarifying the impact of diets with different TFA concentrations on cerebral tissue, focusing on hippocampus and cortex and behavioral performance.

METHODS

Wistar rats were fed either a normolipidic or a hyperlipidic diet for 90 days; diets had the same ingredients except for fat compositions, concentrations, and calories. We used lard in the cis fatty acid (CFA) group and PHSO in the TFA group. The intervention groups were as follows: (1) low lard (LL), (2) high lard (HL), (3) low partially hydrogenated soybean oil (LPHSO), and (4) high partially hydrogenated soybean oil (HPHSO). Mitochondrial parameters, tumor necrosis factor alpha (TNF-α), 2'7'-dichlorofluorescein (DCFH) levels in brain tissue, and open field task were analyzed.

RESULTS

A worse brain tissue response was associated with oxidative stress in cortex and hippocampus as well as impaired inflammatory and mitochondrial parameters at both PHSO concentrations and there were alterations in the behavioral performance. In many analyses, there were no significant differences between the LPHSO and HPHSO diets.

CONCLUSIONS

Partially hydrogenated soybean oil impaired cortical mitochondrial parameters and altered inflammatory and oxidative stress responses, and the hyperlipidic treatment caused locomotor and exploratory effects, but no differences on weight gain in all treatments. These findings suggest that quality is more important than the quantity of fat consumed in terms of CFA and TFA diets.

Neonatal malnutrition programs the oxidant function of macrophages in response to Candida albicans

Experimental maternal nutrition restriction models are used to investigate short or long-term consequences of nutritional deficiency on puppies' growth. By assuming that the immune function is directly related to host's nutritional status, the current study aims to investigate the effects of neonatal malnutrition on oxidative stress and on the cell death of the alveolar macrophage after in vitro infection by Candida albicans. Wistar rats were suckled by mothers fed on diets containing 17% protein (Nourished group) or 8% protein (Malnourished group) in the current assay. Both groups received the standard diet used in the vivarium until adulthood, after weaning. The results showed that the offspring from mothers fed on low-protein diet presented lower body weight from 5 days of life on. Their low weight remained until adulthood when it was compared to that of rats in the nourished group. Superoxide and nitric oxide production was lower in malnourished animals and it was accompanied by low inducible nitric oxide synthase gene expression levels in systems in which the alveolar macrophages were challenged by immunogenic stimulus. No significant differences were observed in comparisons performed between the nourished and malnourished groups in any of the analyzed cell viability (apoptosis/necrosis) parameters. The fungal inoculum-stimulated system induced higher oxidative stress and cell death by necrosis. The current study demonstrated that dietary restriction during lactation alters the oxidant function of alveolar macrophages in puppies; It happens from the gene transcription step to the release of mediators, thus compromising the host's defenses against Candida albicans. It raises the possibility that Candida albicans may cease to be a commensal fungus to become a pathogen in offspring that have suffered nutritional deficiency during critical developmental periods, due to impaired immune responses.

Identification of dynamic changes in proteins associated with the cellular cytoskeleton after exposure to okadaic acid

Exposure of cells to the diarrhetic shellfish poison, okadaic acid, leads to a dramatic reorganization of cytoskeletal architecture and loss of cell-cell contact. When cells are exposed to high concentrations of okadaic acid (100–500 nM), the morphological rearrangement is followed by apoptotic cell death. Okadaic acid inhibits the broad acting Ser/Thr protein phosphatases 1 and 2A, which results in hyperphosphorylation of a large number of proteins. Some of these hyperphosphorylated proteins are most likely key players in the reorganization of the cell morphology induced by okadaic acid. We wanted to identify these phosphoproteins and searched for them in the cellular lipid rafts, which have been found to contain proteins that regulate cytoskeletal dynamics and cell adhesion. By using stable isotope labeling by amino acids in cell culture cells treated with okadaic acid (400 nM) could be combined with control cells before the isolation of lipid rafts. Protein phosphorylation events and translocations induced by okadaic acid were identified by mass spectrometry. Okadaic acid was shown to regulate the phosphorylation status and location of proteins associated with the actin cytoskeleton, microtubules and cell adhesion structures. A large number of these okadaic acid-regulated proteins have previously also been shown to be similarly regulated prior to cell proliferation and migration. Our results suggest that okadaic acid activates general cell signaling pathways that induce breakdown of the cortical actin cytoskeleton and cell detachment.

Induction of apoptosis in sea bream fibroblasts by Vibrio harveyi haemolysin and evidence for an anti-apoptotic role of heat shock protein 70

In this study, we exposed black sea bream, Mylio macrocephalus (Basilewsky), fibroblast (BSF) and silver sea bream, Sparus sarba Forsskål, fibroblast (SSF) cell lines to a recombinant Vibrio harveyi haemolysin (VHH) and investigated mechanisms involved in apoptosis. A decrease in mitochondrial membrane potential, followed by an increase in caspase 3 activity, occurred within 2-8 h of VHH exposure, in both cell lines; however, VHH did not alter cellular levels of reactive oxygen species. As heat shock protein 70 (HSP70) is known to prevent the onset of apoptosis in certain mammalian cells, we aimed to test whether such a protective effect is operative in VHH-exposed fibroblasts. The amounts of HSP70 were elevated in SSF and BSF via an acute heat shock or an acute heat shock followed by a 6 h recovery. It was found that the VHH-mediated reduction in mitochondrial membrane potential was suppressed in cells that had a 6 h post-heat shock recovery, and the protective effect of heat shock-induced HSP70 was attenuated following treatment of cells with the HSP70 inhibitor, quercetin. This study demonstrates how haemolysin causes cell death via induction of apoptosis and provides evidence as to the role of HSP70 as an anti-apoptotic factor.

Cytotoxic activity of extracts of marine sponges from NW Spain on a neuroblastoma cell line

Six species of marine sponges collected at intertidal and sublittoral sites of the coast of Galicia (NW Spain) were screened for potential cytotoxic properties on Neuroblastoma BE(2)-M17 cell line. Exposure to Halichondria panicea, Pachymatisma johnstonia, Ophlitaspongia seriata and Haliclona sp. aqueous extracts strongly affected cell appearance, inducing loss of neuron-like morphology and the formation of clumps. Extracts from these species also caused significant rates of cell detachment and decrease of mitochondrial membrane potential. Incubation with P. johnstonia, O. seriata and Suberites massa extracts also decreased the rate of cell proliferation. The increase of incubation time enhanced propidium iodide uptake by neuroblastoma cells. Toxic responses triggered by sponge extracts are compatible with apoptotic phenomena in neuroblastoma cells, even though increasing propidium uptake at long periods of exposure might indicate the induction of secondary necrosis. The cytotoxic properties of the tested extracts suggest the presence of compounds with potential pharmacological or biotechnological applications in the screened sponge species.

Okadaic acid induces apoptosis through the PKR, NF-κB and caspase pathway in human osteoblastic osteosarcoma MG63 cells

Okadaic acid (OA) is the major component of diarrheic shellfish poisoning toxins and a potent inhibitor of protein phosphatase 1 and 2A. However, the underlying regulatory mechanisms involved in OA-induced cell death are not well understood. In the present study, we examined the effects of OA on apoptosis of MG63 cells by characterizing apoptotic morphological changes of the cells and DNA fragmentation. The roles of double-stranded RNA-dependent protein kinase (PKR), nuclear factor-κB (NF-κB) and caspase in OA-mediated apoptosis in MG63 cells were also examined. Results showed that OA induced cytotoxicity and apoptosis in MG63 cells at IC50 of 75 nM. A functional PKR pathway is required to induce apoptosis in response to OA treatment. Blockade of NF-κB by ammonium pyrrolidinedithiocarbamate (PDTC) resulted in down-regulation of apoptosis. The caspase-3 and caspase-8 inhibitors blocked apoptosis in MG63 cells. In conclusion, our results imply that OA can induce MG63 cell apoptosis through the PKR, NF-κB and caspase pathway.

Morphological and biochemical changes associated with apoptosis induced by okadaic acid in human amniotic FL cells

The marine toxin okadaic acid (OA) is an apoptosis inducer and a tumor promoter. During recent years, extensive studies have demonstrated that OA can induce apoptosis in a wide variety of cell types. In contrast to the relatively longer incubation time or higher treatment concentrations of OA in apoptosis shown previously, relatively lower concentrations (<or=100 nM) and shorter time (4 h) were designed in the current study to observe the toxic effects of OA in human amniotic cells (FL cells). The present study was undertaken to determine the morphological and biochemical changes of FL cells induced by OA. Results indicated that externalization of phosphatidylserine, cytoskeletal disruption, DNA strand breaks and decrease of Bcl-2 protein expression levels as well as increase of PP2A-A subunit protein were all involved in the apoptosis of FL cells induced by OA. This work not only provided further evidence of apoptosis induced by OA but also suggested that PP2A might play a pivotal role in apoptosis induced by protein phosphatases inhibitors.

Cell-based assay coupled with chromatographic fractioning: a strategy for marine toxins detection in natural samples

Cell-based assays (CBA) have been proposed for the evaluation of toxicity caused by marine toxins in natural samples (fish, shellfish and microalgae). However, their application has been hindered due to the interferences present in biological matrices that may cause cellular response and interfere in toxicity evaluation. This work reviews in an extensive introduction the use of CBA for toxicity evaluation of marine toxins. Afterwards, the coupling of chromatographic fractioning with neuroblastoma Neuro-2a CBA is presented to enhance the applicability of CBA for complex matrices. Examples of application are provided for mussel samples (Mytilus galloprovincialis) and microalgae (Gambierdiscus sp.), and the results demonstrated the great potential of the combined strategy for reliable toxicological evaluation without ethical concern. Fractioning of an equivalent of 72 mg eq mL(-1) of mussel sample allowed the identification of non-toxic and toxic fractions whereas only 2.5mg eq mL(-1) of non-purified mussel sample was responsible for 20% of cell mortality. Furthermore, the application of CBA allowed selectively distinguishing between ciguatoxin-like and other unspecific toxicity in Gambierdiscus sp. extract.

Relationship Between Phosphatase Activity and Cytotoxic Effect of Two Protein Phosphatase Inhibitors, Okadaic Acid and Pervanadate, on Human Myeloid Leukemia Cell Line

Protein phosphatases are signalling molecules that regulate a variety of fundamental cellular processes including cell growth, metabolism and apoptosis. The aim of this work was to correlate the cytotoxicity of pervanadate and okadaic acid on HL60 cells and their effect on the phosphatase obtained from these cells. The cytotoxicity of these protein phosphatase inhibitors was evaluated on HL60 cells using phosphatase activity, protein quantification and MTT reduction as indices. The major phosphatase presents in the cellular extract showed high activity (80%) and affinity (Km = 0.08 mM) to tyrosine phosphate in relation to p-nitrophenyl phosphate (pNPP)-(Km = 0.51 mM). Total phosphatase (pNPP) was inhibited in the presence of 10 mM vanadate (98%), 200 microM pervanadate (95%) and 100 microM p-chloromercuribenzoate (80%) but okadaic acid caused a slight increase in enzyme activity (25%). When the HL60 cells were treated with the phosphatase inhibitors (pervanadate and okadaic acid) for 24hours, only 20% residual activity was observed in presence of 200 microM pervanadate, whereas in the presence of okadaic acid this inhibitory effect was not observed. However, in respect to mitochondrial function, cell viability decreased about 80% in the presence of 100 nM okadaic acid. The total protein content was decreased 25% when the cells were treated with 100 nM okadaic acid in combination with 200 microM pervanadate. Our results suggest that both phosphatase inhibitors presented different mechanisms of action on HL60 cells. However, their effect on the cell redox status have to be considered.

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.

Morphological and functional alterations in human proximal tubular cell line induced by low level inorganic arsenic: evidence for targeting of mitochondria and initiated apoptosis

The kidney is a known target organ for arsenic and is critical for both arsenic biotransformation and elimination. Previous studies have demonstrated that at high doses (ppm) inorganic arsenic is toxic to mitochondria primarily by affecting cellular respiration. However, the effect of inorganic arsenic on mitochondria after low level exposures is not known, particularly in the kidney. Thus the functional and morphological effects of low level inorganic arsenic were investigated in a human proximal tubular cell line, HK-2. Mitochondrial function was assessed at subcytotoxic concentrations of arsenite (< or = 10 microm) by examining the alteration of the mitochondrial membrane potential using MitoTracker Red, a mitochondrion selective dye. In a subset of cells, subcytotoxic arsenite led to mitochondrial membrane depolarization, which could subsequently lead to permeability transition and apoptosis. Subcytotoxic arsenite also induced translocation of phosphatidylserine, indicative of early-stage apoptosis. To confirm whether subcytotoxic arsenite induces cellular and/or mitochondrial morphological alterations consistent with initiated apoptosis, HK-2 cells were evaluated with transmission electron microscopy. Classic morphology of apoptosis was not observed with subcytotoxic arsenite exposures; however, evidence of necrotic changes in the cytoplasmic structure and mitochondrial morphology were apparent. Therefore, based on depolarization of mitochondria and the externalization of phosphatidylserine, HK-2 cells appear to initiate apoptosis following subcytotoxic arsenite insult, but morphological changes indicate that HK-2 cells fail to complete apoptosis and ultimately undergo necrosis. Therefore, subcytotoxic arsenite can be sufficiently toxic to mitochondria that they lose their ability to keep the cell on course for apoptotic cell death.

Apoptotic events induced by yessotoxin in myoblast cell lines from rat and mouse

This study reports apoptotic events after yessotoxin (YTX) exposure in L6 (rat) and BC3H1 (mouse) skeletal muscle myoblast cell lines. These cell lines are relevant targets to study the cytotoxic effect since this toxin has been reported as cardiotoxic. Mechanisms of action of YTX in multicellular organisms are not fully elucidated. Cell culture studies can contribute to find some of these mechanisms and trace the molecular pathways involved. The present work shows results from exposing cells to 100 nM purified YTX for 72 h. Morphological and biochemical changes characteristic of apoptotic cell death were evaluated in the two cell lines. Immunofluorescence and western blot techniques showed caspase-3 and caspase-9 activation. Western blot analysis of poly(ADP-ribose)-polymerase (PARP) confirmed caspase-3 activation in both cell lines. DNA fragmentation was not detected in these cell lines. This evidence reflect that oligonucleosomal DNA fragmentation is not a biochemical event that can be used as a definitive apoptotic marker in L6 and BC3H1 myoblast cell lines. The results indicate that the time-course and degree of apoptotic events induced by YTX depend on cell line sensitivity.

Cleavage of tensin during cytoskeleton disruption in YTX-induced apoptosis

Yessotoxin (YTX) is a marine algal toxin previously shown to induce apoptosis in L6 and BC3H1 myoblast cell lines. Disassembly of the F-actin cytoskeleton and cleavage of tensin, a cytoskeletal protein localised at the focal adhesion contacts, appear during this apoptotic process. Tensin binds to actin filaments at the focal adhesion contacts and it links the actin cytoskeleton to the extracellular matrix (ECM). This binding occurs via integrin receptors and it makes tensin a potential link between the actin cytoskeleton and signal transduction. This study evaluates disruption in the F-actin cytoskeleton and change of tensin in myoblast cell lines exposed to 100 nM YTX up to 72 h. YTX treatment cleaves tensin and makes it translocate to the cell centre. Tensin has normally a role in the maintenance of cell shape and YTX-treatment may therefore alter the shape of the cells. YTX exposure also induces formation of lamellas associated with pseudopodia. Alternative linkages and cytoskeletal proteins anchoring the actin filaments to focal contacts remain to be identified.

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.

Effect of okadaic acid on integrins and structural proteins in BE(2)-M17 cells

Okadaic acid (OA), an algal toxin, is known to induce Diarrhetic Shellfish Poisoning and apoptosis in a variety of cell lines. One of the main targets of OA is the actin cytoskeleton which can be modulated by integrins and other structural proteins. In this paper we studied the role of these proteins and skeletal structures on OA-induced apoptosis in neuroblastoma cells. Results show that beta1 integrin and vinculin are down-regulated when cells were exposed to OA. We observed an interaction between talin and beta1 integrin that is impaired in OA treated cells.

Degradation and dephosphorylation of focal adhesion kinase during okadaic acid-induced apoptosis in human neuroblastoma cells

Focal adhesion kinase (FAK) prevents apoptosis in many cell types. We have reported that tyrosine residues in FAK are dephosphorylated and FAK is degraded during mannitol-induced apoptosis in human neuroblastoma cells. Several studies suggest that FAK dephosphorylation and degradation are separate events. The current study defines the relationship between FAK dephosphorylation and degradation in neuroblastoma cells using okadaic acid (OA). OA, a serine phosphatase inhibitor, promotes serine/threonine phosphorylation, which in turn blocks tyrosine phosphorylation. OA induced focal adhesion loss, actin cytoskeleton disorganization, and cellular detachment, which corresponded to a loss of FAK Tyr397 phosphorylation. These changes preceded caspase-3 activation, Akt and MAP kinase activity loss, protein ubiquitination, and cellular apoptosis. Insulin-like growth factor-I prevented mannitol-induced, but not OA-induced, substrate detachment and FAK Tyr397 dephosphorylation, and the effects of OA on FAK Tyr397 phosphorylation were irreversible. The proteolytic degradation of FAK is temporally distinct from its tyrosine dephosphorylation, occurring when apoptotic pathways are already initiated and during a generalized destruction of signaling proteins. Therefore, agents resulting in the dephosphorylation of FAK may be beneficial for therapeutic treatment, irrespective of FAK protein levels, as this may result in apoptosis, which cannot be prevented by growth factor signaling.

Caspase-1 and caspase-8 cleave and inactivate cellular parkin

Lesions in the parkin gene cause early onset Parkinson's disease by a loss of dopaminergic neurons, thus demonstrating a vital role for parkin in the survival of these neurons. Parkin is inactivated by caspase cleavage, and the major cleavage site is after Asp126. Caspases responsible for parkin cleavage were identified by several experimental paradigms. Transient coexpression of caspases and wild type parkin in HEK-293 cells identified caspase-1, -3, and -8 as efficient inducers of parkin cleavage whereas caspase-2, -7, -9, and -11 did not induce cleavage. A D126A parkin mutation abrogates cleavage induced by caspase-1 and -8, but not by caspase-3. In anti-Fas-treated Jurkat T cells, parkin cleavage was inhibited by caspase inhibitors hFlip and CrmA (but not by X-linked inhibitor of apoptosis (XIAP)), indicating that caspase-8 (but not caspase-3) is responsible for the parkin cleavage in this model. Moreover, induction of apoptosis in caspase-3-deficient MCF7 cells, either by caspase-1 or -8 overexpression or by tumor necrosis factor-alpha treatment, led to parkin cleavage. These results demonstrate that caspase-1 and -8 can directly cleave parkin and suggest that death receptor activation and inflammatory stress can cause loss of the ubiquitin ligase activity of parkin, thus causing accumulation of toxic parkin substrates and triggering dopaminergic cell death.

Caspase activation and death induced by yessotoxin in HeLa cells

We have studied the death response induced by yessotoxin (YTX) in cultured HeLa cells, and have compared it to that triggered by okadaic acid (OA) in the same experimental system. Sub-nanomolar concentrations of YTX were found to induce HeLa cell death after a 48-96-h incubation. YTX caused loss of intact poly(ADP-ribose)-polymerase (PARP) in HeLa cells, and detection of the 85kDa fragment, which is indicative of proteolytic attack by caspases. Measurements of caspase activities using extracts prepared from YTX-treated cells and substrates of the caspase-3/7 and caspase-2 isoforms, showed that the relative proteolysis of caspase-3/7 substrate was about eight-fold higher than that of caspase-2, the levels of which were about twice those measured with extracts from control cells. These findings were matched by Western blot analyses of caspase-2, -3 and -7 in HeLa cell extracts, which showed that the levels of pro-caspase-2 were not greatly affected by YTX treatment, whereas pro-caspase-3 and -7 were activated in YTX-treated cells. Taken together, these data complement others previously obtained with OA, and support the notion that caspase isoforms involved in cell death induced by OA and YTX are cell- and toxin-specific.

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.

Characterization of F-actin depolymerization as a major toxic event induced by pectenotoxin-6 in neuroblastoma cells

Pectenotoxins are a group of marine toxins produced by dinoflagellates and formerly included within the group of diarrhetic shellfish poison or toxins (DSP or DST) because of their physico-chemical properties. However, toxicological data on pectenotoxins are still very scarce and its mechanism of action is largely unknown, but toxicity in laboratory animals has been demonstrated by intraperitoneal injection. In this report, we present results of in vitro toxicological assessment of pectenotoxin-6, a derivative of the parental toxin pectenotoxin-2 first isolated from toxic scallops. Results obtained demonstrate an specific time- and dose-dependent depolymerization of F-actin in neuroblastoma cells exposed to pectenotoxin-6 (half-maximal effect about 700 nM at 24 hr). The change in the state of polymerization of actin was not accompanied by other major effects on specific signal transduction pathways or cell survival rate. Pectenotoxin-6 does not modify cytosolic calcium levels either in a calcium containing or calcium-free medium in human lymphocytes. Only when capacitative calcium influx was first activated, the toxin addition significantly decreased the following calcium influx. In these cells, pectenotoxin-6 only modifies cAMP (adenosine 3',5'-cyclic monophosphate) levels in calcium-free conditions. In addition, no effect on cell attachment or apoptosis induction was observed at micromolar concentrations of pectenotoxin-6. Therefore, we conclude that cytoskeletal disruption is a key mechanism of PTX6-induced toxicity in eukaryotic cells.

Characterization of distinct apoptotic changes induced by okadaic acid and yessotoxin in the BE(2)-M17 neuroblastoma cell line

Apoptotic changes induced by okadaic acid and yessotoxin in BE(2)-M17 neuroblastoma cells have been evaluated and quantified by combining classical methods and fast and sensitive fluorimetric microplate assays. The phosphatase inhibitor okadaic acid induced rapid time- and dose-dependent apoptotic changes in this cell line, which were evident after 1h at concentrations equal or higher than 500 nM. Decreased mitochondrial membrane potential by okadaic acid (IC(50)=350 nM at 1h) was followed by cell detachment (IC(50)=400 nM at 1h), changes in total nucleic acids content (50% of controls after 1h with 1000 nM okadaic acid), caspase-3 activation (3- to 4-fold increase at 6h) and increased Annexin-V binding (1.5-fold at 6h). Yessotoxin induced similar changes in BE(2)-M17 cells, although significant differences were found in the time-course and degree of apoptotic events induced by this phycotoxin, indicating a lower potency for yessotoxin when compared with okadaic acid. This is the first report on apoptogenic activity of yessotoxin.