Inhibitory Effect of the Lectin Wheat Germ Agglutinin on the Binding of 125I-CCK-8s to the CCK-A and -B Receptors of AR42J Cells

Cholecystokinin (CCK) is a peptide hormone and plays a major role both in the regulation of pancreatic enzyme secretion and growth of the gastrointestinal tract. The pancreatic CCK receptors are highly glycosylated membrane proteins that are able to bind plant lectins such as wheat germ agglutinin (WGA) and Ulex europaeus agglutinin (UEA-I).In preceding papers, we demonstrated an inhibition of CCK-8s induced Ca2+ signaling and secretion of rat pancreatic acini and AR42J cells by the lectins WGA and UEA-I (Pancreas 2001;23:368-374). Here we studied the influence of WGA, UEA-I, and 22 other lectins on 125I-CCK-8s binding on AR42J cells. A binding assay was used with 125I-CCK-8s and dexamethasone-stimulated AR42J cells, bearing CCK-A as well as CCK-B receptors.WGA inhibits 125I-CCK-8s binding in a dose-dependent manner. The binding is affected at concentrations of WGA >1 microg/mL. The EC50 for inhibition is 8 microg/mL. At a concentration of 25 microg/mL, WGA inhibits the hormone binding 70%. This inhibition can be abolished by the specific sugars for WGA N,N',N"-triacetylchitotriose and N-acetylglucosamine, but not by N-acetylneuraminic acid. UEA-I diminished hormone binding but without significance, although UEA-I binds to the fucose residues of receptor glycosylations. All other 22 lectins tested here were ineffective.The blockage of CCK receptors by WGA explains the inhibition of CCK-8s induced Ca2+ signaling and the secretion of pancreatic acinar cells and AR42J cells. Although the inhibitory effect of WGA is in agreement with the findings of Santer et al, the results with UEA-I are in contrast to those of Santer et al (1990), who described a strong increase in 125I-CCK-8s binding to isolated crude rat pancreatic cell membranes in the presence of UEA-I.