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1 Epithelial Pathobiology Unit, Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, GA, USA
2 Department of Pediatrics, Division of Neonatal-Perinatal Medicine, Emory University School of Medicine, Atlanta, GA, USA
* To whom correspondence should be addressed. E-mail: aneish{at}emory.edu.
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 anti-apoptotic 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 PI3K/Akt pro-inflammatory 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.
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