Lethal toxin from Clostridium sordellii induces apoptotic cell death by disruption of mitochondrial homeostasis in HL‐60 cells

Summary Lethal toxin (LT) from Clostridium sordellii (strain IP82) inactivates in glucosylating the small GTPases Ras, Rap, Ral and Rac. In the present study we show that LT‐IP82 induces cell death via an intrinsic apoptotic pathway in the myeloid cell‐line HL‐60. LT‐IP82 was found to disrupt mitochondrial homeostasis as characterized by a decrease in mitochondrial transmembrane potential and cardiolipin alterations, associated with the release of cytochrome c in the cytosol. Time‐course studies of caspase activation revealed that caspase‐9 and caspase‐3 were activated before caspase‐8. Moreover, although LT‐IP82‐induced cell death was abrogated by caspase‐inhibitors, these inhibitors did not suppress mitochondrial alterations, indicating that caspase activation occurs downstream of mitochondria. Protection of mitochondria by Bcl‐2 overexpression prevented mitochondrial changes as well as apoptosis induction. Furthermore, evidence is provided that LT‐IP82‐induced apoptosis is not a consequence of cortical actin disorganization, suggesting that Rac inactivation does not initiate the apoptotic process. Cell exposure to LT‐IP82 leads to a co‐localization of the toxin with a mitochondrial marker within 2 h. Therefore, we suggest that LT‐IP82 could act at the mitochondrion level independently of its enzymatic effect on small GTPases.