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

ABSTRACTAs an opportunistic bacterial pathogen, Pseudomonas aeruginosamainly affects immunocompromised individuals as well as patients with cystic fibrosis. In a previous study, we showed that ExoS of P. 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, cytochrome cis 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 cytochrome crelease and activation of caspases. These results highlight the modulation of host cell signaling by the type III secretion system during interaction between P. aeruginosaand host cells.