Regulation of Glucagon-Like Peptide-1 Synthesis and Secretion in the GLUTag Enteroendocrine Cell Line

Abstract

Glucagon-like peptide-1 (GLP-1) released from the intestine is a potent stimulator of glucose-dependent insulin secretion. To elucidate the factors regulating GLP-1 secretion, we have studied the enteroendocrine GLUTag cell line. GLP-1 secretion was stimulated in a dose-dependent fashion by activation of protein kinase A or C with forskolin or phorbol 12,13-dibutyrate, respectively (by 2.3 ± 0.5-fold at 100μ m and 4.3 ± 0.6-fold at 0.3 μm, respectively; P < 0.01–0.001). Of the regulatory peptides tested, only glucose-dependent insulinotropic peptide stimulated the release of GLP-1 (by 2.3 ± 0.2-fold at 0.1μ m; P < 0.001); glucagon was without effect, and paradoxically, the inhibitory neuropeptide somatostatin-14 increased secretion slightly (by 1.6 ± 0.3-fold at 0.01μ m; P < 0.05). In tests of several neurotransmitters, only the cholinergic agonists carbachol and bethanechol stimulated peptide secretion in a dose-dependent fashion (by 2.3 ± 0.5- and 1.7 ± 0.3-fold at 1000 μm; P < 0.05–0.001); the β-adrenergic agonist isoproterenol and the chloride channel inhibitor γ-aminobutyric acid did not affect release of GLP-1. Long chain monounsaturated fatty acids (18:1), but not saturated fatty acids (16:0), also stimulated the release of GLP-1 (by 1.7 ± 0.1-fold at 150 μm; P < 0.001). Consistent with the presence of a cAMP response element in the proglucagon gene, activation of the protein kinase A-dependent pathway with forskolin increased proglucagon messenger RNA transcript levels by 2-fold (P < 0.05); glucose-dependent insulinotropic peptide and phorbol 12,13-dibutyrate were without effect. Therefore, by comparison with results obtained using primary L cell cultures or in vivo models, GLUTag cells appear to respond appropriately to the regulatory mechanisms controlling intestinal GLP-1 secretion.