Tissue Differences in Insulin Receptors: Acute Changes in Insulin Binding Characteristics Induced by Wheat Germ Agglutinin

Abstract

The plant lectin, wheat germ agglutinin (WGA), produced differential effects on insulin binding by insulin receptors from rat adipocytes, rat liver, human placenta, and human monocytes. Treatment of adipocyte membranes with WGA (1 μg/ml) markedly enhanced insulin binding (500% of control, 100%) conducted with 5 × 10-10 M 125I-insulin. The lectin's effect on insulin binding by liver membranes was significantly less (260% of control) and required a larger WGA concentration (5 μg/ml) to produce the stimulation. Insulin binding by placental membranes was increased only slightly (140%) by WGA, and monocytes failed to respond. The rate of insulin dissociation from the membrane preparations was decreased by lectin treatment. This effect was most prominent with adipocyte membranes, followed next by liver membranes, whereas only a slight inhibition was found with placental membranes. Scatchard analysis of the adipocyte receptor binding data indicated that WGA treatment completely linearizes the normally curvilinear plot to one high-affinity component (Kd of 0.22 nM) and decreases total insulin binding capacity by 60%. Similar effects were found with liver membranes, except that the extent of the changes was not as dramatic. WGA treatment did not linearize the Scatchard plot determined for placental membranes. Adipocyte and liver receptors solubilized by Triton X-100 responded to WGA, although the response was less in the former (350% above control, 100%) and greater in the latter (500%). Solubilization of placental receptors did not improve their response to the lectin. These results suggest differences in the structure of the insulin receptors from different tissues. Furthermore, the effects of WGA on adipocyte and liver receptors indicate the potential usefulness of this lectin in studies of acute changes in the the insulin binding properties of these insulin target tissues.