c-Jun NH2-Terminal Kinase-Mediated Signaling Is Essential forPseudomonas aeruginosaExoS-Induced Apoptosis

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Abstract
As an opportunistic bacterial pathogen,Pseudomonas aeruginosamainly affects immunocompromised individuals as well as patients with cystic fibrosis. In a previous study, we showed that ExoS ofP.aeruginosa, when injected into host cells through a type III secretion apparatus, functions as an effector molecule to trigger apoptosis in various tissue culture cells. Here, we show that injection of the ExoS into HeLa cells activates c-Jun NH2-terminal kinase (JNK) phosphorylation while shutting down ERK1/2 and p38 phosphorylation. Inhibiting JNK activation by expression of a dominant negative JNK1 or with a specific JNK inhibitor abolishes ExoS-triggered apoptosis, demonstrating the requirement for JNK-mediated signaling. Following JNK phosphorylation, cytochromecis released into the cytosol, leading to the activation of caspase 9 and eventually caspase 3. Although c-Jun phosphorylation is also observed as a result of JNK activation, ongoing host protein synthesis is not essential for the apoptotic induction, suggesting that c-Jun- or other AP-1-driven activation of gene expression is dispensable in this process. Therefore, ExoS has opposing effects on different cellular pathways that regulate apoptosis: it shuts down host cell survival signal pathways by inhibiting ERK1/2 and p38 activation, and it activates proapoptotic pathways through activation of JNK1/2 leading ultimately to cytochromecrelease and activation of caspases. These results highlight the modulation of host cell signaling by the type III secretion system during interaction betweenP.aeruginosaand host cells.