High-glucose-induced apoptosis, ROS production and pro-inflammatory response in cardiomyocytes is attenuated by metformin treatment via PP2A activation.

Metformin has been shown to ameliorate diabetic cardiomyopathy. In the present research we investigated whether metformin would reduce cardiomyocyte apoptosis that was induced by high-glucose stimulation in vitro via activation of PP2A. Primary human and rat cardiomyocytes were subject to high-glucose stimulation. Okadaic acid was used to inhibit PP2A activity. Cell viability and apoptosis was assessed using CCK-8 and by flow cytometry, respectively. Release of HMGB1, TNFα or IL-6 was analyzed by ELISA. Oxidative stress was evaluated by measuring cellular ROS and mitochondrial superoxide level. PP2A activity was evaluated by Serine/ Threonine phosphatase assay system or analyzing Y307 phosphorylation level of PP2A catalytic domain (PP2Ac) by Western blot and the association between PP2Ac and α4 by co-immunoprecipitation. Activation of the NF-κB signaling pathway was assessed by detecting Ser32 phosphorylation level of IκBα as well as nuclear entry of p65 protein by Western blot. Activation of the GSK3ß/MCL1 signaling pathway was assessed by detecting Ser9 phosphorylation level of GSK3 ß and protein level of MCL1. We found Metformin pre-treatment attenuated human and rat cardiomyocytes apoptosis, HMGB1, TNFα and IL-6 release and ROS production that were induced by high-glucose stimulation, and these effects of metformin could be blocked by okadaic acid treatment. Metformin reduced the upregulation of PP2Ac pY307 and the PP2Ac-a4 association, which was not affected by okadaic acid treatment. Metformin pre-treatment reduced NF-κB activation in human and rat cardiomyocytes apoptosis that was elicited by high-glucose stimulation, and this effect of metformin could be blocked by okadaic acid treatment. GSK3 ß/MCL1 is not part of metformin activating PP2A induced myocardial cell death inhibition. In conclusion, metformin reduced apoptosis, ROS production and inflammatory response in primary human and rat cardiomyocytes in vitro in a PP2A dependent manner.

Inhibition of protein phosphatase 2A attenuates titanium-particle induced suppression of bone formation.

Peri-prosthetic osteolysis (PPO) often generates after total joint arthroplasty, which can bring implant failure and following revision surgery. Wear debris shed from prostheses strongly enhances bone resorption and attenuates bone formation in osteolytic process. We previously proved that suppression of protein phosphatase 2A (PP2A), a major serine-threonine phosphatase, inhibited wear-debris-induced osteoclastogenesis and alleviated local osteolysis. Whether PP2A inhibition facilitates osteoblastogenesis and bone formation in the osteolytic sites remains unclear. Here, we observed that PP2A inhibition with a selective inhibitor attenuated particle-induced bone destruction by accelerating osteoblast differentiation and promoting bone regeneration. Meanwhile, we proved inhibition of PP2A alleviated the inhibition of osteogenic differentiation by titanium particles in MC3T3-E1 cells. In addition, PP2A inhibition increased ß-catenin expression and enhanced ß-catenin nuclear translocation, compared with that in the vehicle group. ICG-001, a specific inhibitor of ß-catenin, was further applied and was found to weaken the effect of PP2A inhibition on ß-catenin expression and nuclear translocation. Therefore, we demonstrated PP2A inhibition exerts protective effects on osteogenic differentiation mainly by activating Wnt/ß-catenin signaling pathway. Thus, all the results further revealed PP2A could be a promising target for treating PPO and other bone related diseases.

IMM-H004 reduced okadaic acid-induced neurotoxicity by inhibiting Tau pathology in vitro and in vivo.

This study aimed to explore effects and mechanisms of 004 (IMM-H004), a novel coumarin derivative, in OKA (okadaic acid)-induced AD (Alzheimer's disease)-like model. In vitro, MTT, LDH, and Annexin V/FITC flow cytometry assay were used to test cell survival. In vivo, OKA microinjection was conducted to simulate AD-like neuropathology. Morris water maze and Nissl staining were used to detect spatial memory function and neuronal damage respectively. Western blot and immunohistochemistry were used to study the mechanisms of 004 in Tau pathology. The results showed that 004 reduced cell death and increased survival in PC12 cells, and decreased neuronal injury in the hippocampus in rats. 004 improved learning and memory functions in OKA-treated rats. The mechanistic studies indicated that 004 inhibited phosphorylation of Tau protein by down-regulating the activity of protein kinases CDK5 and GSK3ß and increasing PP2A activity. Overall, 004 improved spatial memory impairments and neuron cells injury induced by OKA; on the other hand, 004 inhibited Tau hyperphosphorylation by regulating CDK5, GSK3ß and PP2A.

Neurotoxicity induced by okadaic acid in the human neuroblastoma SH-SY5Y line can be differentially prevented by α7 and ß2* nicotinic stimulation.

A good model of neuronal death that reproduces the characteristic tau (τ) hyperphosphorylation of Alzheimers disease is the use of okadaic acid (OA). The aim of this study was to determine the contribution of α7 and ß2* nicotinic acetylcholine receptor (nAChR) subtypes to neuroprotection against OA in the SH-SY5Y cell line by using the selective α7 and ß2* nAChR agonists PNU 282987 and 5-Iodo-A85380, respectively. The results of this study show that both α7 and ß2* nAChR can afford neuroprotection against OA-induced neurotoxicity. Protection mediated by α7 nAChRs was independent of Ca(2+) and involved the intracellular signaling pathway Janus Kinase-2/Phosphatidylinositol-3-kinase/Akt. When Ca(2+) entry was promoted through the α7 nAChR by using the α7-selective positive allosteric modulator PNU 120596, protection was lost. By contrast, protection mediated by ß2* nAChRs was Ca(2+) dependent and implicated the signaling pathways PI3K/Akt and extracellular regulated kinase 1/2. Both α7 and ß2* nAChR activation converged on downregulation of GSK-3ß and reduction of τ phosphorylation in cells undergoing cell death induced by OA. Therefore, targeting nAChR could offer a strategy for reducing neurodegeneration secondary to hyperphosphorylation of protein τ.

N-acylaminophenothiazines: neuroprotective agents displaying multifunctional activities for a potential treatment of Alzheimer's disease.

We have previously reported the multifunctional profile of N-(3-chloro-10H-phenothiazin-10-yl)-3-(dimethylamino)propanamide (1) as an effective neuroprotectant and selective butyrylcholinesterase inhibitor. In this paper, we have developed a series of N-acylaminophenothiazines obtained from our compound library or newly synthesised. At micro- and sub-micromolar concentrations, these compounds selectively inhibited butyrylcholinesterase (BuChE), protected neurons against damage caused by both exogenous and mitochondrial free radicals, showed low toxicity, and could penetrate into the CNS. In addition, N-(3-chloro-10H-phenothiazin-10-yl)-2-(pyrrolidin-1-yl)acetamide (11) modulated the cytosolic calcium concentration and protected human neuroblastoma cells against several toxics, such as calcium overload induced by an L-type Ca2+-channel agonist, tau-hyperphosphorylation induced by okadaic acid and Aß peptide.

[Protective effects of IGF-1 on cell injuries and tau hyperphosphorylation induced by okadaic acid].

OBJECTIVE: To investigate the effects of insulin-like growth factor-1 (IGF-1) on cell injuries and tau hyperphosphorylation induced by okadaic acid (OA). METHODS: The experimental groups were designed as follows: (1) SH-SY5Y culture (control group); (2) SH-SY5Y exposed to 40 nmol/L OA for 24 hours (OA group); (3) SH-SY5Y exposed to OA for 24 hours in the presence of 2 hour pretreatment with 100, 200 and 400 ng/ml IGF-1 (IGF-1 pretreatment groups). The changes of cell morphology were observed by inverted microscope. The viability of cells was detected by MTT. The injuries of cells were examined by Hoechst 33258 staining and the activity of caspase-3. Western-blot was applied to determine the expression of phosphorylation of tau protein. RESULTS: In IGF-1 pretreatment group, the cell morphology was improved, the viability of cells was increased, and caspase-3 activation and hyperphosphorylation of tau (Ser396) were reduced. CONCLUSION: IGF-1 can protect the SH-SY5Y cells from cell injuries induced by OA by inhibiting tau hyperphosphorylation.

[Protective effect of (-) clausenamide against neurotoxicity induced by okadaic acid and beta-amyloid peptide25-35].

This study is to investigate the protective effect of (-) clausenamide against the neurotoxicity of okadaic acid in SH-SY5Y cell line, and injection beta-amyloid peptide25-35 (Abeta25-35) to the cerebral ventricle in ovariectomy (OVX) rats. MTT assay, LDH assay, and Hoechst 33258 staining were used to detect the effect of (-) clausenamide on the toxicity of okadaic acid in SH-SY5Y cell line. The animal model was induced by ovariectomized and injection of Abeta25-35 in the cerebroventricle of rats. The effect of (-) clausenamide on learning and memory deficiency was observed by step-through test. Electron microscope, Nissl body staining, and HE staining were used to examine the morphological changes in hippocampus and cerebral cortex neurons. Pretreatment of (-) clausenamide and LiCl decreased the rate of cell death from MTT, LDH release, and apoptosis from Hoechst 33258 staining in SH-SY5Y cell line. The step-through tests showed (-) clausenamide could improve the ability of learning and memory. The Nissl body staining and HE staining experiments also showed the neuroprotective effects of (-) clausenamide on the neurons of hippocampus and cerebral cortex. (-) Clausenamide has the protective effects against the neurotoxicity induced by okadaic acid and Abeta25-35.

Myosin II and Rho kinase activity are required for melanosome aggregation in fish retinal pigment epithelial cells.

In the retinal pigment epithelium (RPE) of fish, melanosomes (pigment granules) migrate long distances through the cell body into apical projections in the light, and aggregate back into the cell body in the dark. RPE cells can be isolated from the eye, dissociated, and cultured as single cells in vitro. Treatment of isolated RPE cells with cAMP or the phosphatase inhibitor, okadaic acid (OA), stimulates melanosome aggregation, while cAMP or OA washout in the presence of dopamine triggers dispersion. Previous studies have shown that actin filaments are both necessary and sufficient for aggregation and dispersion of melanosomes within apical projections of isolated RPE. The role of myosin II in melanosome motility was investigated using the myosin II inhibitor, blebbistatin, and a specific rho kinase (ROCK) inhibitor, H-1152. Blebbistatin and H-1152 partially blocked melanosome aggregation triggered by cAMP in dissociated, isolated RPE cells and isolated sheets of RPE. In contrast, neither drug affected melanosome dispersion. In cells exposed to either blebbistatin or H-1152, then triggered to aggregate using OA, melanosome aggregation was completely inhibited. These results demonstrate that (1) melanosome aggregation and dispersion occur through different, actin-dependent mechanisms; (2) myosin II and ROCK activity are required for full melanosome aggregation, but not dispersion; (3) partial aggregation that occurred despite myosin II or ROCK inhibition suggests a second component of aggregation that is dependent on cAMP signaling, but independent of ROCK and myosin II.

Involvement of mitogen-activated protein kinase in 2-hydroxyestradiol-17beta-induced oocyte maturation in the catfish Heteropneustes fossilis and a note on possible interaction with protein phosphatases.

Mitogen-activated protein kinase (MAPK) was demonstrated in the postvitellogenic follicles (theca-granulosa and oocyte) of catfish by Western blotting using a polyclonal anti-rabbit serum, which recognized both ERK1 and ERK2. Two distinct protein bands resolved in the 46-48 kDa range of 12% SDS-PAGE were immunoblotted. Incubation of the follicles with 5 microM 2-OHE2 elicited GVBD significantly in a duration-dependent manner with a concomitant increase in the expression of MAPK (ERK1 and ERK2). Densitometric analysis of the immunoblots showed significant variations in the intensity of staining. The ERK1 expression increased significantly from 6 h onwards but the changes were less pronounced. On the other hand, ERK2 registered a sharp significant increase after 3h, which paralleled the GVBD response. The MEK inhibitor PD098059 alone did not induce GVBD. Co-incubation of the follicles with 2-OHE2 and PD098059 significantly inhibited the steroid-induced GVBD at all concentrations. Immunoblot analysis showed that PD098059 inhibited MAPK activity significantly compared to the 2-OHE2 group. The addition of okadaic acid (OA) in the incubation medium containing both 2-OHE2 and PD098059 reversed the inhibitory effect of the latter and GVBD was elevated significantly over that of the 2-OHE2 group but significantly lower than that of the 2-OHE2 + OA group. The results suggest an involvement of MAPK in meiotic maturation but the site(s) of action: oocyte, follicular envelope or both needs further investigation.

BACE inhibitor reduces APP-beta-C-terminal fragment accumulation in axonal swellings of okadaic acid-induced neurodegeneration.

Emerging evidence suggests that not only beta-amyloid but also other amyloid precursor protein (APP) fragments, such as the beta-C-terminal fragment (betaCTF), might be involved in Alzheimer's disease (AD). Treatment of neurons with okadaic acid (OA), a protein phosphatase-2A inhibitor, has been used to induce tau phosphorylation and neuronal death to create a research model of AD. In this study, we analyzed axonopathy and APP regulation in cultured rat neurons treated with OA. After OA treatment, the neurons presented with axonal swellings filled with vesicles, microtubule fragments, and transport molecules such as kinesin and synapsin-I. Western blotting showed that intracellular APP levels were increased and immunocytochemistry using antibodies against the APP C-terminus showed that APP accumulated in the axonal swellings. This APP C-terminus immunoreactivity disappeared when neurons were cotreated with a beta-secretase inhibitor, but not with alpha- or gamma-secretase inhibitors, indicating that the accumulation was primarily composed of APP-betaCTF. These findings provide the first evidence that APP-betaCTF can accumulate in the axons of OA-treated neurons, and may suggest that APP-betaCTF is involved in the pathogenesis of AD.

Geldanamycin activates Hsp70 response and attenuates okadaic acid-induced cytotoxicity in human retinal pigment epithelial cells.

Reversible protein phosphorylation regulates the biological activities of many human proteins involved in crucial cellular processes, e.g., protein-protein interactions, cell signaling, gene transcription, cell growth, and death. A malfunction of cellular homeostasis in retinal pigment epithelial (RPE) cells is involved in the age-related retinal degeneration. In this study, we examined cytotoxicity in human RPE cells subjected to the protein phosphatase inhibitor, okadaic acid (OA). Moreover, the influence of Hsp90 inhibitor geldanamycin (GA), a benzoquinone ansamycin, in cytoprotection was assessed. Hsp70 protein levels were analyzed by Western blot. Cellular viability was determined by LDH and MTT assays. To study apoptotic cell death, caspase-3 enzyme activity was measured by assaying the cleavage of a fluorescent peptide substrate and Hoechst dye was used to visualize nuclear morphology. OA treatment caused morphological changes and induced cytotoxicity by caspase-3-independent manner in the RPE cells. No evidence of nuclear fragmentation was observed in response to OA. Interestingly, GA treatment accumulated Hsp70 protein and attenuated OA-induced cytotoxicity. This study suggests that Hsp70 and Hsp90 are closely related to cytoprotection of RPE cells in response to protein phosphatase inhibition.

Olanzapine attenuates the okadaic acid-induced spatial memory impairment and hippocampal cell death in rats.

It is hypothesized that olanzapine, an atypical antipsychotic drug, has beneficial effects on cognitive impairment and neuropathological changes in treating neurodegenerative diseases. In the present study, we investigated the effects of chronic administration of olanzapine on the spatial memory impairment and hippocampal cell death induced by the direct injection of okadaic acid (OA), a potent neurotoxin, into the rat hippocampus. After being pretreated with olanzapine (0.5 or 2 mg/kg/day, i.p.) for 2 weeks, the rats were unilaterally microinjected with OA (100 ng) into the hippocampus, and then were continuously administrated with olanzapine for an additional week The rats were trained on a spatial memory task in an eight-arm radial maze before OA administration, and tested on the same task 18 h after the last olanzapine injection. After the behavioral test, the rats were killed for Nissl staining and terminal deoxynucleutidyl transferase-mediated biotinylated UTP nick end labeling staining. OA significantly induced spatial memory impairment, and caused pyramidal cell loss in the CAI and apoptotic cell death in the hippocampus. Olanzapine significantly attenuated OA-induced spatial memory impairment and the OA-induced neuropathological changes in the hippocampus. These findings suggest that olanzapine may have therapeutic effects in treatment of cognitive impairment and neuropathological changes of schizophrenia and other neurodegenerative diseases.

Synthesis and neuroprotective activity of analogues of glycyl-L-prolyl-L-glutamic acid (GPE) modified at the alpha-carboxylic acid.

The synthesis of nine GPE* analogues, wherein the alpha-carboxylic acid group of glutamic acid has been modified, is described by coupling readily accessible N-benzyloxycarbonyl-glycyl-L-proline 2 with various analogues of glutamic acid. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glutamate residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Synthesis and pharmacological evaluation of side chain modified glutamic acid analogues of the neuroprotective agent glycyl-L-prolyl-L-glutamic acid (GPE).

The synthesis of eight GPE* analogues, wherein the gamma-carboxylic moiety of the glutamic residue has been modified, is described by coupling readily accessible N-benzyloxycarbonyl-glycyl-L-proline with various analogues of glutamic acid. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glutamate residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Synthesis and pharmacological evaluation of glycine-modified analogues of the neuroprotective agent glycyl-L-prolyl-L-glutamic acid (GPE).

The synthesis of 10 G*PE analogues, wherein the glycine residue has been modified, is described by coupling readily accessible dibenzyl-L-prolyl-L-glutamate 2 with various analogues of glycine. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glycine residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Comparison of melatonin versus vitamin C on oxidative stress and antioxidant enzyme activity in Alzheimer's disease induced by okadaic acid in neuroblastoma cells.

We demonstrated that exposure of cells to 50 nM okadaic acid for 2 h induced a reduction in cellular glutathione transferase, glutathione reductase and catalase activity. Likewise, this acid prompted an increase in lipid peroxidation. Treatment of cells with 10(-5) M melatonin or 0.5 microg/ml vitamin C prevented the effects of okadaic acid. These results indicate that okadaic acid induces an oxidative stress imbalance, while melatonin and vitamin C prevent the oxidative stress induced by okadaic acid. Likewise, these data indicate the great importance of oxidative stress in both this experimental model and in the development and course of neurodegenerative disease, especially Alzheimer's disease. They show that melatonin is much more efficient than vitamin C in reducing the extent of oxidative stress. This phenomenon was demonstrated by the smaller dose of melatonin needed to obtain effects similar to those obtained with vitamin C on lipid peroxidation and by the protective effect of melatonin on antioxidant enzyme activity.

A dominant negative c-jun specifically blocks okadaic acid-induced skin tumor promotion.

Okadaic acid (OA) is a prototypical non-phorbol ester skin tumor-promoting agent that works by inhibiting protein phosphatases, leading to an increase in protein phosphorylation. Increased protein phosphorylation can lead to stimulated signaling through various signal transduction pathways. One or more of the pathways affected by OA leads to increased signaling via the activator protein 1 (AP-1) transcription factor. Because AP-1 signaling has been shown to be required for skin tumor promotion by phorbol ester, studies were undertaken to determine whether AP-1 signaling is also required for 7,12-dimethylbenz(a)anthracene (DMBA)-initiated/OA-promoted skin tumorigenesis. Transgenic mice expressing a dominant negative c-jun (TAM-67) controlled by the keratin 14 promoter in ICR mice were used to determine the effects of OA on AP-1 signaling. By crossing the TAM-67 mice with mice expressing an AP-1-responsive luciferase, it was shown that TAM-67 decreases AP-1 activation in response to OA treatment by 95%. After 7,12-dimethylbenz(a)anthracene initiation, the TAM-67 mice and nontransgenic littermates were promoted with twice weekly applications of OA. These experiments showed that TAM-67 expression decreased tumor multiplicity by 90%. Additional experiments with TAM-67 mice showed that the hyperplastic response to OA is not impaired in these mice, nor were there differences in OA-induced transcription of various genes known to be AP-1 responsive under other conditions. This result suggests that only a subset of AP-1-regulated genes is targeted by TAM-67 when it prevents tumor promotion by OA. A determination of the mechanism by which TAM-67 can block tumor promotion without affecting hyperplasia will be important.

Phosphoinositol 3 kinase inhibitor, LY294002 increases bcl-2 protein and inhibits okadaic acid-induced apoptosis in Bcl-2 expressing renal epithelial cells.

Protein phosphorylation plays an indispensable role in cellular regulation of mitosis, metabolism, differentiation, and death. We previously reported that the protein phosphatase inhibitor okadaic acid (OKA) induces apoptosis in renal epithelial cells in culture. In the present study, we examined the role of phosphotidylinositol 3 (PI3) kinase signaling in okadaic acid-induced apoptosis by pre-treating normal rat kidney renal epithelial cells expressing human bcl-2 with the PI3 kinase inhibitors, LY294002 and wortmannin, followed by apoptosis-inducing concentrations of okadaic acid. Given the reported cell survival activity of PI3 kinase signaling mostly attributed to Akt kinase activation, we hypothesized that inhibition of PI3 kinase would enhance okadaic-induced apoptosis. Surprisingly, our data show that pretreatment with LY294002, but not wortmannin, attenuated okadaic acid-induced apoptosis. In contrast, to LY294002, wortmannin enhanced apoptosis. Interestingly, we also found that LY294002 treatment increased bcl-2 protein levels in normal rat kidney epithelial cells expressing bcl-2 (NRK-bcl-2). In untreated cells, bcl-2 appeared to be mainly perinuclear, coincident with the nuclear membrane, or in the cytosol. In OKA treated cells that were pre-treated with Ly294002, bcl-2 was highly co-localized with mitochondria, but in cells treated with okadaic acid alone, bcl-2 was associated with fragmented chromatin. In this model, it appears that LY294002 may exert anti-apoptotic effects by a previously unreported treatment related increase in bcl-2. Although it is widely accepted that bcl-2 protein can inhibit apoptosis, we propose that the subcellular location of bcl-2 is an important determinant in whether bcl-2 effectively inhibits apoptosis.

Phorbol myristate acetate inhibits okadaic acid-induced apoptosis and downregulation of X-linked inhibitor of apoptosis in U937 cells.

Okadaic acid is a specific inhibitor of serine/threonine protein phosphatase 1 (PP-1) and 2A (PP-2A). The phosphorylation and dephosphorylation at the serine/threonine residues on proteins play important roles in regulating gene expression, cell cycle progression, and apoptosis. In this study, phosphatase inhibitor okadaic acid induces apoptosis in U937 cells via a mechanism that appears to involve caspase 3 activation, but not modulation of Bcl-2, Bax, and Bcl-X(L) expression levels. Treatment with 20 or 40 nM okadaic acid for 24 h produced DNA fragmentation in U937 cells. This was associated with caspase 3 activation and PLC-gamma1 degradation. Okadaic acid-induced caspase 3 activation and PLC-gamma1 degradation and apoptosis were dose-dependent with a maximal effect at a concentration of 40 nM. Moreover, PMA (phorbol myristate acetate), PKC (protein kinase C) activator, protected U937 cells from okadaic acid-induced apoptosis, abrogated okadaic acid-induced caspase 3 activation, and specifically inhibited downregulation of XIAP (X-linked inhibitor of apoptosis) by okadaic acid. PMA cotreated U937 cells exhibited less cytochrome c release and sustained expression levels of the IAP (inhibitor of apoptosis) proteins during okadaic acid-induced apoptosis. In addition, these findings indicate that PMA inhibits okadaic acid-induced apoptosis by a mechanism that interferes with cytochrome c release and activity of caspase 3 that is involved in the execution of apoptosis.

Effects of the diarrhetic shellfish toxin, okadaic acid, on cytoskeletal elements, viability and functionality of rat liver and intestinal cells.

The diarrhetic shellfish toxin, okadaic acid, administered to rats by intragastric intubation, caused intestinal damage, diarrhea and death, but had no detectable effect on the liver. In contrast, okadaic acid administered intravenously had little effect on intestinal function, but caused a rapid dissolution of hepatic bile canalicular actin sheaths, congestion of blood in the liver, hypotension and death at high doses. In isolated rat hepatocytes, okadaic acid induced disruption of the canalicular sheaths as well as of the keratin intermediate filament network. Both of these cytoskeletal changes could be prevented by addition of a cytoprotective flavonoid, naringin, to the isolated hepatocytes, whereas intravenously or intragastrically administered naringin failed to protect against the effects of okadaic acid in vivo. Freshly isolated colonocytes already had fragmented keratin and tubulin cytoskeletons, died rapidly and were not further afflicted by okadaic acid. Naringin had no protective effect on isolated colonocytes or on intestinal function in vivo, but the nonspecific protein kinase inhibitor, K-252a, and the protein-tyrosine-phosphatase inhibitor, vanadate, significantly reduced the extent of colonocytic keratin fragmentation, and an inhibitor of apoptotic caspases, zVAD.fmk, was strongly protective. Further studies of hepatic and intestinal cytoprotectants should focus on conditions that limit their effectiveness in vivo.