Binding of cholera toxin B subunit to intestinal epithelial cells

We have prepared I-125-labeled cholera toxin B subunit (I-125-labeled CT-B, a specific activity of 98 Ci/mmol) and found that it binds to rat IEC-6 and human Caco-2 intestinal epithelial cells with high affinity (K-d 3.6 and 3.7 nM, respectively). The binding of labeled protein was completely inhibited by unlabeled thymosin-alpha(1) (TM-alpha(1)), interferon -alpha(2) (IFN-alpha(2)), and the synthetic peptide LKEKK that corresponds to residues 16-20 in TM-alpha(1) and 131-135 in IFN-alpha(2), but was not inhibited by the synthetic peptide KKEKL with inverted amino acid sequence (K-1 > 10 mu M). Thus, TM-alpha(1), IFN-alpha(2), and the peptide: LKEKK bind with high affinity and specificity to the cholera toxin receptor on IEC-6 and Caco-2 cells. It was found that CT-B and the peptide: LKEKK at concentrations of 10-1000 nM increased in a dose-dependent manner the nitric oxide production and the soluble guanylate cyclase activity in IEC-6 and Caco-2 cells.