Bacterial invasion is not required for activation of NF-κB in enterocytes

Pathogenic enteric microorganisms induce the NF-κB-dependent expression of proinflammatory genes in intestinal epithelial cells. The purpose of the present study was to clarify the contribution of microbial invasion to the degradation of the regulatory protein IκBα and the subsequent activation of NF-κB in cultured intestinal epithelial cells. Caco-2BBe cells were incubated with Salmonella dublin, Salmonella typhimurium, or a weakly invasive strain of E. coli. S. dublin and S. typhimurium (10⁷ organisms/ml) induced equivalent concentration-dependent gel mobility shifts of an NF-κB consensus sequence that was preceded by IκBα degradation. E. coli (10⁷ organisms/ml) did not induce IκBα degradation or NF-κB translocation. Pretreatment with cytochalasin D blocked invasion of all three strains but had no effect on IκBα degradation or NF-κB activation. S. dublin and S. typhimurium adhered to Caco-2BBe cells 3- to 10-fold more than E. coli. NF-κB activation was prevented by physical separation of S. dublin from Caco-2BBe cells by a 0.4- μm-pore-size filter. Our results imply that bacterial adhesion, rather than invasion or release of a secreted factor, is sufficient to induce IκBα degradation and NF-κB activation in intestinal epithelial cells. Our data suggest that strategies to reduce enteric inflammation should be directed to the reduction of bacterial enterocyte adhesion.