Role of lipid rafts and flagellin in invasion of colonic epithelial cells by Shiga-toxigenic Escherichia coli O113:H21.

Shiga-toxigenic Escherichia coli (STEC) O113:H21 strains that lack the locus of enterocyte effacement (LEE) efficiently invade eukaryotic cells in vitro, unlike LEE-positive O157:H7 strains. We used a fliC deletion mutant of the O113:H21 STEC strain 98NK2 (98NK2ΔfliC) to show that invasion of colonic epithelial (HCT-8) cells is heavily dependent on production of flagellin, even though adherence to the cells was actually enhanced in the mutant. Flagellin binds and signals through Toll-like receptor 5 (TLR5), but there was no evidence that either TLR5, the adaptor protein myeloid differentiation primary response gene 88 (MyD88), or the serine kinase interleukin-1 receptor-associated kinase (IRAK) were required for invasion of HCT-8 cells by strain 98NK2, as judged by transfection, RNA knockdown, or inhibitor studies. However, pretreatment of cells with anti-asialo-GM1 significantly decreased 98NK2 invasion (by 40.8%), while neuraminidase treatment (which cleaves terminal sialic acid residues, thus converting GM1 into asialo-GM1) significantly increased invasion (by 70.7%). Pretreatment of HCT-8 cells with either the cholesterol-depleting agent methyl-β-cyclodextrin (MβCD) or the tyrosine kinase inhibitor genistein significantly decreased invasion by 98NK2, indicating a potential role for lipid rafts in the invasion mechanism. Confocal microscopy also showed invading 98NK2 colocalized with lipid raft markers caveolin-1 and GM1. Interestingly, anti-asialo-GM1, neuraminidase, MβCD, and genistein have similar effects on the vestigial level of STEC invasion seen for STEC strain 98NK2ΔfliC, indicating that lipid rafts mediate a common step in flagellin-dependent and flagellin-independent cellular invasion.