TI - NF-kappaB - and C/EBPbeta-driven interleukin-1beta gene expression and PAK1 -mediated caspase-1 activation play essential roles in interleukin-1beta release from Helicobacter pylori lipopolysaccharide-stimulated macrophages . AB - Helicobacter pylori is a Gram-negative microaerophilic bacterium that causes chronic gastritis , peptic ulcer , and gastric carcinoma . Interleukin-1beta ( IL-1beta ) is one of the potent proinflammatory cytokines elicited by H.pylori infection . We have evaluated the role of H.pylori lipopolysaccharide ( LPS ) as one of the mediators of IL-1beta release and dissected the signaling pathways leading to LPS -induced IL-1beta secretion . We demonstrate that both the NF-kappaB and the C/EBPbeta -binding elements of the IL-1beta promoter drive LPS -induced IL-1beta gene expression . NF-kappaB activation requires the classical TLR4-initiated signaling cascade leading to IkappaB phosphorylation as well as PI-3K/Rac1/p21-activated kinase ( PAK ) 1 signaling , whereas C/EBPbeta activation requires PI-3K/Akt/p38 mitogen-activated protein ( MAP ) kinase signaling . We observed a direct interaction between activated p38 MAP kinase and C/EBPbeta , suggesting that p38 MAPK is the immediate upstream kinase responsible for activating C/EBPbeta . Most important , we observed a role of Rac1/PAK1 signaling in activation of caspase-1 , which is necessary for maturation of pro-IL-1beta . H.pylori LPS induced direct interaction between PAK1 and caspase-1 , which was inhibited in cells transfected with dominant-negative Rac1 . PAK1 immunoprecipitated from lysates of H.pylori LPS-challenged cells was able to phosphorylate recombinant caspase-1 , but not its S376A mutant . LPS -induced caspase-1 activation was abrogated in cells transfected with caspase-1 (S376A) . Taken together , these results suggested a role of PAK1 -induced phosphorylation of caspase-1 at Ser376 in activation of caspase-1 . To the best of our knowledge our studies show for the first time that LPS -induced Rac1/PAK1 signaling leading to caspase-1 phosphorylation is crucial for caspase-1 activation . These studies also provide detailed insight into the regulation of IL-1beta gene expression by H.pylori LPS and are particularly important in the light of the observations that IL-1beta gene polymorphisms are associated with increased risk of H.pylori-associated gastric cancer .