TLR5-mediated phosphoinositide 3-kinase activation negatively regulates flagellin-induced proinflammatory gene expression.
Epithelial cells detect motile pathogens via TLR5 ligation of flagellin, resulting in rapid induction of antibacterial/proinflammatory gene expression. Although such flagellin-induced gene expression is quite transient, likely to avoid the negative consequences of inflammation, little is known regarding the molecular mechanisms that mediate its shutdown. We hypothesized that, analogous to the case for TLR4, phosphoinositide 3-kinase (PI3K) might negatively regulate TLR5 signaling. However, because PI3K is an essential positive mediator of some pathways of TLR-mediated gene expression, the opposite hypothesis was also considered. Herein, we observed that flagellin stimulation of epithelial cells indeed induced rapid (<30 min) PI3K activation, as evidenced by Akt phosphorylation, via a TLR5-mediated mechanism. Blockade of PI3K with wortmannin resulted in marked enhancement of flagellin-induced gene expression as assessed by measuring levels of inducible NO synthase, IL-6, and IL-8. Such enhancement of gene expression by PI3K inhibition correlated with prolonged activation of MAPK (p38 and ERK1/2) and was ablated under MAPK inhibition. Such effect of inhibiting PI3K with wortmannin was mimicked by the PI3K inhibitor LY294002, and, conversely, a constitutively active PI3K prevented p38 activation in response to flagellin. Last, to test the significance of these results in vivo, we measured flagellin-induced gene expression in PI3K knockout mice. PI3K-null mice displayed increased levels of flagellin-induced serum IL-6, KC (IL-8 homolog), and nitrite as compared with heterozygous littermates. Thus, TLR5's rapid activation of PI3K serves to limit MAPK signaling, thus limiting proinflammatory gene expression and reducing the potential negative consequences of proinflammatory gene expression.