Expression of the Virulence Factor, BfpA, by Enteropathogenic Escherichia coli is Essential for Apoptosis Signalling but not for NF-kB Activation in Host Cells

Abstract

Localized adherence (LA) of enteropathogenic Escherichia coli (EPEC) to epithelial cells results in attaching and effacing of the surface of these cells. LA depends on the gene bfpA, which codes for the BfpA protein.Wefound that EPEC-E. coli adherence factor (EAF)(þ), expressing BfpA, significantly reduced HeLa cell viability in comparison with EPEC-EAF(–), as evaluated by the mitochondrial-dependent succinate dehydrogenase conversion of 30-[4,5,-dimethylthiazol-2yl]2,5-diphenyltetrazolium bromide (MTT) to its formazan. Apoptosis accounts for a substantial loss of the cell viability, because the cells incubated with EPEC-EAF(þ) or with cloned BfpA (data not shown), but not with EPEC-EAF(–), were positive for annexin-V binding, demonstrated chromatin condensation and nuclei fragmentation and exhibited a high level of caspase-3 activity. Because the blockade of bacterial cell-surfaceassociated BfpA by anti-BfpA immunoglobulin (Ig)Y antibody suppressed apoptotic death induced by EPEC-EAF(þ), BfpA may be the trigger for apoptosis. Both EPEC-EAF(þ) and EPEC-EAF(–), as well as recombinant BfpA (data not shown), activated nuclear factor (NF)-kB in a similar manner as analysed by the electrophoretic mobility shift assay (EMSA). EMSA supershift analysis demonstrated the presence of p65/RelA in a DNA-binding complex. In contrast to DNA binding, NF-kB-dependent reporter gene transactivation was stimulated more strongly by EPEC B171/EAF(þ), suggesting a role for this virulence factor in the regulation of transcriptional activity of NF-kB. Because suppression of NF-kB activation by BAY11-7085, a NF-kB inhibitor, neither induced apoptosis by itself nor blocked apoptosis induction by EPEC-EAF(þ), it may be suggested that apoptosis is not regulated by the NF-kB pathway in HeLa cells.