Elsevier

Biochemical Pharmacology

Volume 63, Issue 5, 1 March 2002, Pages 993-996
Biochemical Pharmacology

Short communication
Exendin-4 increases insulin sensitivity via a PI-3-kinase-dependent mechanism: contrasting effects of GLP-1

https://doi.org/10.1016/S0006-2952(01)00924-8Get rights and content

Abstract

The insulinotropic agent, exendin-4, is a long-acting analogue of glucagon-like peptide-1 (GLP-1) which improves glucose tolerance in humans and animals with diabetes, but the underlying mechanisms and the effects of exendin-4 on peripheral (muscle/fat) insulin action are unclear. Previous in vivo and clinical studies have been difficult to interpret because of complex, simultaneous changes in insulin and glucagon levels and possible effects on hepatic metabolism. Thus, the comparative effects of exendin-4 and GLP-1 on insulin-stimulated 2-[3H]deoxyglucose (2-DOG) uptake were measured in fully differentiated L6 myotubes and 3T3-adipocytes, including co-incubation with inhibitors of the PI-3-kinase (wortmannin) and mitogen-activated protein (MAP) kinase (PD098059) pathways. In L6 myotubes, there was a concentration-dependent and PI-3-kinase-dependent increase in insulin-stimulated 2-DOG uptake with exendin-4 and GLP-1, e.g. for exendin-4 the CI-200 value (concentration of insulin required to increase 2-DOG uptake 2-fold) decreased from 1.3±1.4×10−7 M (insulin alone, n=16) to 5.9±1.3×10−8 M (insulin+exendin-4 0.1 nM, n=18, P<0.03). A similar insulin-sensitizing effect was observed with exendin-4 in 3T3-adipocytes, but GLP-1 had no effect on adipocyte insulin sensitivity. In conclusion, this is the first direct evidence showing that exendin-4 increases insulin-stimulated glucose uptake in muscle and fat derived cells via a pathway that involves PI-3-kinase activation. Furthermore, the contrasting responses of exendin and GLP-1 in 3T3-adipocytes suggest that the peripheral insulin-sensitizing effect of exendin-4 (in contrast to the insulinotropic effect) does not involve the GLP-1 receptor pathway.

Introduction

GLP-1 and gastric inhibitory polypeptide are the hormones secreted from gastrointestinal endocrine cells in response to food intake which account for the incretin effect, i.e. the enhanced insulin secretion after oral vs. intravenous glucose administration [1]. It has recently been shown that postprandial GLP-1 concentrations are reduced in patients with type 2 diabetes [2], while administration of exogenous GLP-1 has transient glucose-lowering effects via stimulation of glucose-dependent insulin secretion [3], inhibition of pancreatic glucagon release [4] and delayed gastric emptying [5]. GLP-1 is unsuitable for therapeutic use because it is rapidly cleared from the circulation (half-life 1.5 min) by the ubiquitous enzyme dipeptidyl peptidase-IV (DPP-IV).

Exendin-4 is an analogue of GLP-1 (53% structural homology) which was first isolated from the salivary secretions of a South American lizard known as the Gila monster (Heloderma suspectum venum) [6]; it is a distinct protein derived from a separate set of genes [7] and a potent agonist at the pancreatic GLP-1 receptor [8]. Exendin-4 is resistant to DPP-IV cleavage, and therefore has a long-acting antidiabetic profile that is potentially suitable for therapeutic use in humans [9], [10]. Synthetic exendin-4, AC2993, is undergoing phase III clinical trials, and other GLP-1 analogues resistant to DPP-IV, e.g. LY307161, show promising antidiabetic effects in animal models of diabetes.

The extent to which peripheral actions contribute to the antidiabetic activity of GLP-1 and exendin-4 remains unclear. Previous work from our group and others has shown that GLP-1 augments insulin-stimulated glucose uptake and metabolism in isolated cells [11], [12], [13], and in Zucker diabetic rats ‘whole-body’ insulin sensitivity increased after chronic administration of exendin-4 [10]. The nature and explanation for this insulin-sensitizing effect is still unclear [10], mainly because the interpretation of in vivo studies in humans and animals is complicated by simultaneous changes in insulin and glucagon levels, as well as possible effects on hepatic metabolism [14], and the longer-term changes in insulin sensitivity may be secondary to improved glycaemic control [5], [10], [15]. Thus, the purpose of this in vitro study was to investigate the direct effects of exendin-4 on insulin-stimulated glucose uptake in skeletal muscle derived (L6 myotubes) and fat derived cells (3T3-L1 adipocytes), and to compare exendin and GLP-1 responses.

Section snippets

Materials and methods

L6 myoblasts, a rat skeletal muscle derived cell line (ECACC), were grown in Dulbecco’s modified Eagle’s medium (DMEM), supplemented with 10% FCS in 5% CO2, at 37°. After the cells became confluent, the medium was changed to one containing 2% FCS for 5 days in order to transform the cells from L6 myoblasts to mature myotubes. 3T3-L1 fibroblasts were grown in DMEM containing 10% fetal bovine serum and maintained in 5% CO2 at 37°. The cells were incubated in the presence of DMEM containing 0.5 mM

Results

Exendin-4 had no effect on basal (non-insulin-mediated) glucose uptake, but significantly augmented insulin-stimulated glucose transport in L6 myotubes (Fig. 1). For example, the CI-200 value decreased from 1.3±1.4×10−7 M (insulin alone, n=16) to 5.9±1.3×10−8 M (insulin+exendin-4 0.1 nM, n=18) (p<0.03).

A similar, concentration-dependent increase in insulin sensitivity was observed with exendin-4 in 3T3-adipocytes, an effect that was attenuated by wortmannin but not PD098059 (Fig. 2). Although

Discussion

Exendin-4 is a potent agonist at GLP-1 receptors on pancreatic islets [8], [18], and the insulinotropic and glucagonostatic effects of exendin are similar to those of GLP-1. The main difference is in the time-course of the pancreatic responses, reflecting the much longer plasma half-life of exendin-4. In vivo studies have indicated that additional (extra-pancreatic) mechanisms may contribute to the antidiabetic activity of exendin-4, e.g. increased ‘whole-body’ insulin sensitivity was reported

Acknowledgements

Dr. Idris is supported, in part, by an unrestricted educational grant from Eli Lilly (UK).

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