Effect of okadaic acid on the cytotoxic activity of Clostridium difficile toxin B and Clostridium sordellii toxin L

Clostridium difficle toxin B and Clostridium sordellii toxin L, which are immunologically related toxins, possess a cytotoxic activity inducing depolymerization of microfilaments and cellular retraction of cell bodies that are different for toxin B‐ and toxin‐L‐treated cells. The biological mechanisms responsible for these effects are unknown, but a previous study revealed that both toxins induce modification of phosphorylation of cellular proteins extracted from toxin B‐ and toxin L‐treated cells without changes in protein kinase C activity or cAMP concentration. In the present study, we have investigated the effect of okadaic acid, an inhibitor of protein phosphatases, on the cytotoxic activity of toxins B and L in MacCoy cells. Firstly, we reveal by cytotoxic assay and staining of F‐actin that okadaic acid (1 μM or higher) induces depolymerization of microfilaments and cellular morphological modifications which are similar to that of cells treated with toxin L. Secondly, we show that 1 μM okadaic acid potentiates the cytotoxic activity of toxin L but not of toxin B. These observations suggest that the cytotoxic mechanisms induced by okadaic acid and toxin L treatment are partly common, indicating that an increase in phosphorylation favors the cytotoxicity of toxin L. Since okadaic acid had no influence on the cytotoxicity of toxin B, we suggest that toxin B and L, alter the cells by different cellular biological mechanisms. © 1992 Wiley‐Liss, Inc.