Sulfonylurea signal transduction

In the pancreatic beta cells the proximal step in sulfonylurea signal transduction is the binding of these clinically important drugs to high-affinity receptors in the beta cell membrane. Using HIT cells as a model system, we have established an extremely close correlation between the affinity of binding of glyburide and its analog, iodoglyburide, and the activation of various steps in stimulus-secretion coupling--inhibition of 86Rb+ efflux, increase in [Ca2+]i resulting from gating of voltage-gated calcium channels by cell depolarization, and the exocytosis of insulin. Two different L-type channel cDNAs have been identified in an HIT cell library, one neuroendocrine in type and one more cardiac-like. A HIT cell membrane protein of Mr 140,000, which we believe to be the high-affinity sulfonylurea receptor, can be covalently linked to 5(125)-iodo-2-hydroxyglyburide by ultraviolet irradiation. The receptor has been solubilized and retains binding activity and the same rank order of displacement of the 5(125)-iodo-2-hydroxyglyburide as observed with the native receptor. The Mr 140,000 protein has been partially purified and the amino acid sequences of three proteolytic fragments have been used to design oligonucleotides to screen HIT cell cDNA libraries. Since the binding constant of glyburide or iodoglyburide is closely correlated with the ability of these compounds to inhibit the ATP-sensitive K+ channel, increase [Ca2+]i, and elicit insulin secretion, we have identified the Mr 140,000 protein as the sulfonylurea receptor. Expression of the cloned cDNA should allow us to test this hypothesis directly.