Glucagon-like peptide-1 improves insulin and proinsulin binding on RINm5F cells and human monocytes

Glucagon-like peptide-1-(7-36) amide (GLP-1) is a potent incretin hormone s ecreted from distal gut. It stimulates basal and glucose-induced insulin se cretion and proinsulin gene expression. The present study tested the hypoth esis that GLP-1 may modulate insulin receptor binding. RINm5F rat insulinom a cells were incubated with GLP-1 (0.01-100 nM) for different periods (1 mi n-24 h). Insulin receptor binding was assessed by competitive ligand bindin g studies. In addition, we investigated the effect of GLP-1 on insulin rece ptor binding on monocytes isolated from type 1 and type 2 diabetes patients and healthy volunteers. In RINm5F cells, GLP-1 increased the capacity and affinity of insulin binding in a time- and concentration-dependent manner. The GLP-1 receptor agonist exendin-4 showed similar effects, whereas the re ceptor antagonist exendin-(9-39) amide inhibited the GLP-1-induced increase in insulin receptor binding. The GLP-1 effect was potentiated by the adeny lyl cyclase activator forskolin and the stable cAMP analog Sp-5,6-dichloro- 1-beta-D-ribofuranosylbenzimidazole- 3',5'-monophosphorothioate but was ant agonized by the intracellular Ca2+ chelator 1,2-bis(0-aminophenoxy) ethane- N,N,N',N'-tetraacetic acid-AM. Glucagon, gastric inhibitory peptide (GIP), and GIP-(1-30) did not affect insulin binding. In isolated monocytes, 24 h incubation with 100 nM GLP-1 significantly (P< 0.05) increased the diminish ed number of high-capacity/low-affinity insulin binding sites per cell in t ype 1 diabetics (9,000 +/- 3,200 vs. 18,500 +/- 3,600) and in type 2 diabet ics (15,700 +/- 2,100 vs. 28,900 +/- 1,800) compared with nondiabetic contr ol subjects (25,100 +/- 2,700 vs. 26,200 +/- 4,200). Based on our previous experiments in IEC-6 cells and IM-9 lymphoblasts indicating that the low-af finity/high-capacity insulin binding sites may be more specific for proinsu lin (Jehle, PM, Fussgaenger RD, Angelus NK, Jungwirth RJ, Saile B, and Lutz MP. Am J Physiol Endocrinol Metab 276: E262-E268, 1999 and Jehle, PM, Lutz MP, and Fussgaenger RD. Diabetologia 39: 421-432, 1996), we further invest igated the effect of GLP-1 on proinsulin binding in RINm5F cells and monocy tes. In both cell types, GLP-1 induced a significant increase in proinsulin binding. We conclude that, in RINm5F cells and in isolated human monocytes , GLP-1 specifically increases the number of high-capacity insulin binding sites that may be functional proinsulin receptors.