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Flagellin/TLR5 responses in epithelia reveal intertwined activation of inflammatory and apoptotic pathways

¹Epithelial Pathobiology Unit, Department of Pathology and Laboratory Medicine, and ²Division of Neonatal-Perinatal Medicine, Department of Pediatrics, Emory University School of Medicine, Atlanta, Georgia

Submitted 15 June 2005 ; accepted in final form 15 September 2005

Flagellin, the primary structural component of bacterial flagella, is recognized by Toll-like receptor 5 (TLR5) present on the basolateral surface of intestinal epithelial cells. Utilizing biochemical assays of proinflammatory signaling pathways and mRNA expression profiling, we found that purified flagellin could recapitulate the human epithelial cell proinflammatory responses activated by flagellated pathogenic bacteria. Flagellin-induced proinflammatory activation showed similar kinetics and gene specificity as that induced by the classical endogenous proinflammatory cytokine TNF-, although both responses were more rapid than that elicited by viable flagellated bacteria. Flagellin, like TNF-, activated a number of antiapoptotic mediators, and pretreatment of epithelial cells with this bacterial protein could protect cells from subsequent bacterially mediated apoptotic challenge. However, when NF-B-mediated or phosphatidylinositol 3-kinase/Akt proinflammatory signaling was blocked, flagellin could induce programmed cell death. Consistently, we demonstrate that flagellin and viable flagellate Salmonella induces both the extrinsic and intrinsic caspase activation pathways, with the extrinsic pathway (caspase 8) activated by purified flagellin in a TLR5-dependant fashion. We conclude that interaction of flagellin with epithelial cells induces caspase activation in parallel with proinflammatory responses. Such intertwining of proinflammatory and apoptotic signaling mediated by bacterial products suggests roles for host programmed cell death in the pathogenesis of enteric infections.

Salmonella; inflammation; Toll-like receptor; cDNA microarray

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