The Phosphoinositol-3-Kinase–Protein Kinase B/Akt Pathway Is Critical forPseudomonas aeruginosaStrain PAK Internalization

Abstract

Several Pseudomonas aeruginosa strains are internalized by epithelial cells in vitro and in vivo, but the host pathways usurped by the bacteria to enter nonphagocytic cells are not clearly understood. Here, we report that internalization of strain PAK into epithelial cells triggers and requires activation of phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (Akt). Incubation of Madin-Darby canine kidney (MDCK) or HeLa cells with the PI3K inhibitors LY294002 (LY) or wortmannin abrogated PAK uptake. Addition of the PI3K product phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P 3 ] to polarized MDCK cells was sufficient to increase PAK internalization. PtdIns(3,4,5)P 3 accumulated at the site of bacterial binding in an LY-dependent manner. Akt phosphorylation correlated with PAK invasion. The specific Akt phosphorylation inhibitor SH-5 inhibited PAK uptake; internalization also was inhibited by small interfering RNA-mediated depletion of Akt phosphorylation. Expression of constitutively active Akt was sufficient to restore invasion when PI3K signaling was inhibited. Together, these results demonstrate that the PI3K signaling pathway is necessary and sufficient for the P. aeruginosa entry and provide the first example of a bacterium that requires Akt for uptake into epithelial cells. INTRODUCTIONPseudomonas aeruginosa is one of the leading causes of nosocomial infections in humans (reviewed in Engel, 2003). This Gram negative opportunistic pathogen causes acute infections of the respiratory and urinary tract, skin, and eye in the setting of preexisting epithelial tissue damage and/or host immunocompromise. P. aeruginosa is also a cause of chronic lung infections and ultimately death in patients with cystic fibrosis.Although usually considered an extracellular pathogen, ϳ50% of clinical, laboratory, and environmental P. aeruginosa isolates demonstrate measurable internalization in vivo as well as in vitro (Chi et al., 1991;Fleiszig et al., 1994Fleiszig et al., , 1995Fleiszig et al., , 1997bFleiszig et al., , 1998Hirakata et al., 1998;Grassmé et al., 2000). These two different phenotypes correlate with the differences in type III secreted effectors (reviewed in Engel, 2003). Both classes of strains are virulent in animal models of P. aeruginosa infection. The noninvasive, cytotoxic strains secrete ExoU (Hauser et al., 1998), a potent phospholipase (Sato et al., 2003), and ExoT, a bifunctional enzyme with N-terminal GAP activity toward Rho family GTPases (Krall et al., 2000;Kazmierczak and Engel, 2002) and C-terminal ADP ribosyltransferase (ADPRT) activity toward Crk (Sun and Barbieri, 2003). Both domains of ExoT contribute to its anti-internalization activity (Garrity-Ryan et al., 2000;Garrity-Ryan et al., 2004). The invasive strains are much less cytotoxic due to the loss of the ExoU gene (Allewelt et al., 2000). Interestingly, these strains secrete ExoT and a closely related protein ExoS that also possesses an N-terminal GAP domain whose substrates include Rho family GTPases (Goehring et al., 1999...

Francisella Targets Cholesterol-Rich Host Cell Membrane Domains for Entry into Macrophages

“…PI3K plays a role for the entry process of many bacteria (e.g., (41)(42)(43)(44) and viruses (45). PI3K activity effects changes in the cytoskeleton and affects actin polymerization, which is a key element in uptake of bacteria into host cells (41,44).…”

Shiga Toxin Produced by Enterohemorrhagic Escherichia coli Inhibits PI3K/NF-κB Signaling Pathway in Globotriaosylceramide-3-Negative Human Intestinal Epithelial Cells

“…Actually, the inhibition of Akt phosphorylation by ERK1/2 (32,33) as well as the decreased ERK1/2 activation by a PI3K/Akt-dependent pathway (34, 35) have been described. The activation of the PI3K/Akt pathway has been evidenced in response to different bacterial species (36 -39) and plays a major function in cell invasion (37,39). Additionally, the PI3K/Akt-dependent activation of NF-B has also been evidenced in rat intestinal epithelial cells in response to the commensal Gram-negative Bacteroides vulgatus (36).…”

Inactivation of Host Akt/Protein Kinase B Signaling by Bacterial Pore-forming Toxins

“…Pseudomonas aeruginosa can also protect target host cells from death, apparently via stabilization of the X-chromosome-linked inhibitor of apoptosis protein, downstream of Akt activation by as-yet unidentified bacterial factor(s) (Ashare et al, 2007). Interestingly, P. aeruginosa not only stimulates Akt phosphorylation but also requires Akt activation to effectively invade target host cells, as does N. gonorrhoeae (Kierbel et al, 2005;Edwards and Apicella, 2006). Stimulation of Akt by another invasive pathogen, Francisella novicida, promotes the expression of proinflammatory cytokines by infected macrophages via Akt-dependent activation of NF B (Rajaram et al, 2006).…”