Sulfonylurea binding to adipocyte membranes and potentiation of insulin-stimulated hexose transport

We have previously shown that the sulfonylureas increase insulin-stimulated glucose transport in adipocytes mainly by enhancing the insulin-induced recruitment of glucose transporter from its intracellular storage pool to the plasma membrane (Jacobs, D.B., and Jung, C.Y. (1985) J. Biol. Chem. 260, 2593-2596). In order to determine if this sulfonylurea effect is mediated by a specific membrane-associated sulfonylurea-binding protein, in the present report we measured exact dose dependence of the transport enhancement activities of different sulfonylureas in adipocytes in primary culture and equilibrium binding affinities of these agents to various adipocyte membrane fractions. Glyburide was found to increase the insulin-stimulated, 3-O-methyl-D-glucose equilibrium exchange in cultured rat adipocytes by up to 60% with little effect in the absence of insulin. The effect developed gradually reaching the maximum level at 24 h of incubation. The effect was concentration dependent showing a simple, one-to-one stoichiometry and an apparent activation constant (K(a)) of approximately 1 μM. Glypizide, tolazamide, and tolbutamide also enhanced the insulin-stimulated hexose transport by up to 60%, but with K(a) of approximately 2, 11, and 25 μM, respectively. HB-699 and ciglitazone, non-sulfonylureas, were without effect under the same condition. In equilibrium binding experiments, [³H] glyburide was found to bind to adipocyte membranes at two or more protein-specific, saturable sites, with similar apparent dissociation constants (K(D)) ranging 1-3 μM. These protein-specific glyburide bindings were displaced not only by tolazamide and tolbutamide, but also by ciglitazone and HB-699, with indicated K(D) of 11-16, 80-85, 20-25, and 85-95 μM, respectively. However, with the plasma membrane fraction, the displacements by ciglitazone and HB-699 were partial and did not exceed 56-61% at maximum. Based on these findings, we propose that there is a sulfonylurea-specific-binding protein in the plasma membrane of adipocytes, and that this sulfonylurea-binding protein may play a key role in the enhancement of insulin-stimulated hexose transport by sulfonylureas, probably via potentiation of the insulin-induced recruitment of glucose transporter.