Cell type-dependent internalization of the Escherichia coli STb enterotoxin

Previous studies have suggested that internalization of the Escherichia coli STb enterotoxin in human and rat intestinal epithelial cells is involved in STb pathogenesis, but toxin uptake in porcine jejunum epithelium, the in vivo target tissue, still remains elusive. Using flow cytometry, we studied the internalization of fluorescein isothiocyanate-labelled STb in porcine intestinal epithelial IPEC-J2 and murine fibroblast NIH-3T3 cell lines. In contrast to the selective pronase resistance of STb in NIH-3T3 cells at 37 °C, but not at 4 °C, indicative of toxin internalization, most of the toxin was pronase-sensitive at both temperatures in IPEC-J2 cells, indicating reduced uptake, but significant cell surface binding. Actin reorganization is required for STb internalization by NIH-3T3 cells, confirming STb endocytosis in these cells. The toxin receptor, sulfatide, could not explain these internalization differences because both cell lines possessed surface sulfatide and internalized antisulfatide antibodies over time at 37 °C. Inhibition of lipid rafts endocytosis, known to contain sulfatide, with methyl-β-cyclodextrin or genistein, did not influence toxin uptake by either cell line. STb internalization is therefore differentially regulated depending on the cell type, possibly by factors other than sulfatide. Although a small STb fraction could be internalized by porcine intestinal epithelial cells, our findings suggest the ability of STb to induce, from the cell surface, intracellular signalling leading to fluid secretion in porcine intestinal epithelium.