title: Glucagon-like peptide-1 improves insulin and proinsulin binding on RINm5F cells and human monocytes.
abstract: Glucagon-like peptide-1-(7---36) amide (GLP-1) is a potent incretin hormone secreted from distal gut. It stimulates basal and glucose-induced insulin secretion and proinsulin gene expression. The present study tested the hypothesis that GLP-1 may modulate insulin receptor binding. RINm5F rat insulinoma cells were incubated with GLP-1 (0.01-100 nM) for different periods (1 min-24 h). Insulin receptor binding was assessed by competitive ligand binding studies. In addition, we investigated the effect of GLP-1 on insulin receptor 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 receptor antagonist exendin-(9---39) amide inhibited the GLP-1-induced increase in insulin receptor binding. The GLP-1 effect was potentiated by the adenylyl cyclase activator forskolin and the stable cAMP analog Sp-5, 6-dichloro-1-beta-D-ribofuranosyl-benzimidazole-3', 5'-monophosphorothioate but was antagonized by the intracellular Ca(2+) 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 diminished number of high-capacity/low-affinity insulin binding sites per cell in type 1 diabetics (9,000+/-3,200 vs. 18,500+/-3,600) and in type 2 diabetics (15,700+/-2,100 vs. 28,900+/-1,800) compared with nondiabetic control 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-affinity/high-capacity insulin binding sites may be more specific for proinsulin (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 investigated the effect of GLP-1 on proinsulin binding in RINm5F cells and monocytes. 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.
authors.affiliation: Children's Hospital Mannheim, University of Heidelberg, 68735 Mannheim, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Ebinger
authors.forename: D R
authors.initials: DR
authors.lastname: Jehle
authors.forename: R D
authors.initials: RD
authors.lastname: Fussgaenger
authors.forename: H C
authors.initials: HC
authors.lastname: Fehmann
authors.forename: P M
authors.initials: PM
authors.lastname: Jehle
mesh_terms: D000328:Adult; D000818:Animals; D000077270:Exenatide; D005260:Female; D005934:Glucagon / Q000494:pharmacology*; D052216:Glucagon-Like Peptide 1; D000067757:Glucagon-Like Peptide-1 Receptor; D004763:Glucagon-Like Peptides; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D007340:Insulinoma / Q000378:metabolism* / Q000473:pathology; D008297:Male; D009000:Monocytes / Q000378:metabolism*; D010190:Pancreatic Neoplasms / Q000378:metabolism* / Q000473:pathology; D010446:Peptide Fragments / Q000378:metabolism / Q000494:pharmacology*; D010455:Peptides / Q000494:pharmacology; D011384:Proinsulin / Q000378:metabolism*; D011498:Protein Precursors / Q000494:pharmacology*; D051381:Rats; D018027:Receptors, Glucagon / Q000819:agonists / Q000037:antagonists & inhibitors; D014407:Tumor Cells, Cultured; D014688:Venoms
chemical_list: C000593833:GLP1R protein, human; C000593835:Glp1r protein, rat; D000067757:Glucagon-Like Peptide-1 Receptor; D007328:Insulin; D010446:Peptide Fragments; D010455:Peptides; D011498:Protein Precursors; D018027:Receptors, Glucagon; D014688:Venoms; C055453:glucagon-like peptide 1 (7-36)amide; C083773:exendin (9-39); D004763:Glucagon-Like Peptides; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D011384:Proinsulin; D000077270:Exenatide