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.

Glucagon-Like Peptide-1 Synthetic Analogs: New Therapeutic Agents for Use in the Treatment of Diabetes Mellitus

“…The cAMPGEFs couple cAMP production to the activation of Rap1, a small molecular weight G protein. Novel signaling properties of the GLP-1 receptor include its ability to activate immediate early genes (IEGs) [80], to increase the number of insulin receptors on insulin-secreting cells [81,82], to stimulate mitogen-activated protein kinases (p38 MAPK, ERK), phosphatidylinositol 3-kinase [83], atypical protein kinase C-ζ [84], Ca 2+ /calmodulin-regulated protein kinase [85], protein kinase B (Akt), and hormonesensitive lipase [86]. It seems likely that at least some of these effects result from activation of Epac by GLP-1.…”

NFAT Regulates Insulin Gene Promoter Activity in Response to Synergistic Pathways Induced by Glucose and Glucagon-Like Peptide-1

“…The second pathway also involves glucose metabolism but appears to be driven by glucosederived factors that target insulin gene transcription independently of [Ca 2ϩ ] i . Glucose metabolism is central to the regulation of ␤-cell function (11), and GLP-1 is a potent hormonal incretin that has been shown to markedly enhance the stimulatory effects of glucose on ␤-cells (12,13). For example, GLP-1 potentiates glucose-induced insulin exocytosis.…”

Type 1 Diabetes Mellitus: Redefining the Future of Cardiovascular Complications with Novel Treatments

“…Exendin-4 treatment increases insulin receptor affinity at the CE in both diabetic and normal rat groups; whereas insulin tends to normalize the affinity irrespective of exendin-4 absence or presence (Table 1). Exendin-4 treatment probably augments insulin receptor affinity both in normal and diabetic state through exerting its insulinotropic actions (Nikolaidis et al, 2005) and/or by improving insulin sensitivity (Ebinger et al, 2000). The dramatic increase in the affinity of insulin to its receptor in the normal group treated with exendin-4 might be attributed to both of the previously mentioned causes.…”

“…2 and 3). The regression in insulin receptor density with exendin-4 treatment is suggested to be attributable to the insulinomimetic effects of exendin-4 (Sokos et al, 2006) which result in a cross-talk between GLP-1 and insulin signaling pathways (Ebinger et al, 2000). There is no further attenuation in the level of insulin receptor upon insulin administration in combination with exendin-4 in diabetic rat groups.…”

“…This is its insulinotropic effect. Second, it improves insulin sensitivity possibly by inducing a conformational change in the insulin receptor (Ebinger et al, 2000). On the other hand, DPP-IV inhibitor (KR-62436, Sigma Chemical Company) treatment solely does not seem to be implicated in modulating insulin receptor affinity at the endothelial site in diabetic rats treated with DPP-IV inhibitor as compared to diabetics.…”