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

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 6.3 x 10(-12) 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 (IC50=27 ng/ml i.e. 3.3 x 10(-8) 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,, increased from 27 ng/ml to 48 ng/ml i.e, 3.3 x 10(-8) M to 5.9 x 10(-8) M) it was 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 concentration with enhanced MDA production was lower than that inducing significant cellular protein synthesis inhibition.