Oxygen reactive radicals production in cell culture by okadaic acid and their implication in protein synthesis inhibition

Abstract

<jats:p> Okadaic acid (OA), a diarrhetic shellfish toxin is a potent promoter of tumours in mouse skin and a specific inhibitor of protein phosphatases 1 and 2A. Recently it has been shown that OA inhibited protein synthesis in a cell-free system, with 50% inhibitory concentration of 6.3610<jats:sup>7</jats:sup><jats:sup>12</jats:sup> M but the mechanism whereby this inhibition is mediated was still unclear. In the present study, the effect of OA on protein synthesis in Vero cell cultures was investigated. Protein synthesis was inhibited by OA alone in Vero cells in a concentration-dependent manner (IC<jats:sub>50</jats:sub>=27 ng/ml i.e. 3.3610<jats:sup>7</jats:sup><jats:sup>8</jats:sup> M). Since OA also induced lipid peroxidation and likely oxygen reactive radicals, it was interesting to know whether these radicals impair the protein synthesis process. Therefore, SOD+catalase known as scavenger of active oxygen radicals were added in the culture medium in the presence of OA and labelled leucine. These enzymes partially prevented the inhibition of protein synthesis induced by OA, indicating that the formation of high reactive oxygen free radicals could be one of the pathways this marine toxin induces its toxicity. Since the prevention by SOD+catalase was only partial (the IC<jats:sub>50</jats:sub> increased from 27 ng/ml to 48 ng/ml i.e. 3.3610<jats:sup>7</jats:sup><jats:sup>8</jats:sup> M to 5.9610<jats:sup>7</jats:sup><jats:sup>8</jats:sup> M)itwas speculated that the production of oxygen reactive radical scavengered by SOD+catalase is not the main mechanism whereby OA induces its cytotoxicity. Vitamins E and C completely prevent the lipid peroxidation induced by OA in cells, but failed to reduce the inhibition of protein synthesis to the same level, indicating that a more specific mechanism might be responsible for protein synthesis inhibition. That is the hyperphosphorylation of elongation factor EF-2 in the protein synthesis machinery. However our results pointed to lipid peroxidation being a precocious phenomenon following the OA exposure, since a concentrationwithenhancedMDAproductionwaslower than that inducing significant cellular protein synthesis inhibition. </jats:p>

Olanzapine Attenuates the Okadaic Acid-Induced Spatial Memory Impairment and Hippocampal Cell Death in Rats

“…Thus our own previous data, together with the findings by other investigators that OA-induced apoptosis is associated with downregulation of Bcl-2 and can be prevented by upregulation of Bcl-2 (Benito et al, 1997;Nuydens et al, 2000;Cabado et al, 2004), suggest that Bcl-2 plays an important role in the neuroprotective effects of olanzapine on OA-induced neurodegeneration and apoptosis. Olanzapine may also attenuate OA-induced neurotoxicity by upregulating superoxide dismutase (Manna et al, 1998;Li et al, 1999;Matias et al, 1999), and perform protective effects on OA-induced apoptotic cell death by modulating the expression of pro-and antiapoptotic proteins such as Bax and Bcl-X L (Wei et al, 2003b;Cabado et al, 2004). However, further studies are necessary to elucidate whether olanzapine attenuated OA-induced neurotoxicity by directly affecting the activation of phosphatases or caspases.…”

“…In cultured rat cortical neurons, OA may induce cell death by increasing phosphorylation of microtubuleassociated protein 2 and tau concomitant with early changes in the neuronal cytoskeleton (Arias et al, 1993). OA may also induce cellular injury and death by inactivating Bcl-2 by phosphorylation (Haldar et al, 1995), activating caspase-3 (Hong et al, 2002), or forming high reactive free radicals (Matias et al, 1999) in vitro. Infusion of OA into the rat brain results in severe memory impairment, accompanied by remarkable neuropathological changes including hippocampal neurodegeneration, a paired helical filament-like phosphorylation of tau protein, and the formation of b/A4-amyloid containing plaque-like structures in gray and white matter areas (Arendt et al, 1995(Arendt et al, , 1998Zhao et al, 1995;He et al, 2001;Tian et al, 2004).…”

Combined cytotoxicity and genotoxicity of a marine toxin and seafood contaminant metal ions (chromium and cadmium)

“…Our findings are in full agreement with this hypothesis. The reactive oxygen species produced also contribute to the cytotoxicity of OA (Matias et al, 1999a;Traoré et al, 2000).…”

“…OA is known to be a potent inhibitor of serine/ threonine phosphatases (Bagu et al, 1997;Biolojan and Takai, 1998) and a specific inhibitor of protein synthesis through hyperphosphorylation of elongation factor EF2 (Matias et al, 1996(Matias et al, , 1999a. Matias and Creppy (1998) showed that at low concentrations, OA increases DNA bases oxidation and methylation.…”

“…This toxin is also a tumor promoter (Fujiki and Suganuma, 1999), and an inducer of lipid peroxidation and apoptosis in cultured mammalian cells (Guzman and Castro, 1991;Boe et al, 1991;Matias et al, 1999a;Traoré et al, 1999Traoré et al, , 2000Lerga et al, 1999;Berven et al, 2001).…”