REGULATION OF GLUCAGON-LIKE PEPTIDE-1 SYNTHESIS AND SECRETION IN THE GLUTAG ENTEROENDOCRINE CELL-LINE

Glucagon-like peptide-1 (GLP-1) released from the intestine is a poten t stimulator of glucose-dependent insulin secretion. To elucidate the factors regulating GLP-1 secretion, we have studied the enteroendocrin e GLUTag cell line. GLP-1 secretion was stimulated in a dose-dependent fashion by activation of protein kinase A or C with forskolin or phor bol 12,13-dibutyrate, respectively (by 2.3 +/- 0.5-fold at 100 mu M an d 4.3 +/- 0.6-fold at 0.3 mu M, respectively; P < 0.01-0.001). Of the regulatory peptides tested, only glucose-dependent insulinotropic pept ide stimulated the release of GLP-1 (by 2.3 +/- 0.2-fold at 0.1 mu M; P < 0.001); glucagon was without effect, and paradoxically, the inhibi tory neuropeptide somatostatin-14 increased secretion slightly (by 1.6 +/- 0.3-fold at 0.01 mu M; P < 0.05). In tests of several neurotransm itters, only the cholinergic agonists carbachol and bethanechol stimul ated peptide secretion in a dose-dependent fashion (by 2.3 +/- 0.5- an d 1.7 +/- 0.3-fold at 1000 mu M; P < 0.06-0.001); the beta-adrenergic agonist isoproterenol and the chloride channel inhibitor gamma-aminobu tyric acid did not affect release of GLP-1. Long chain monounsaturated fatty acids (18:1), but not saturated fatty acids (16:0), also stimul ated the release of GLP-1 (by 1.7 +/- 0.1-fold at 150 mu M; P < 0.001) . Consistent with the presence of a cAMP response element in the progl ucagon 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 re sults obtained using primary L cell cultures or in vivo models, GLUTag cells appear to respond appropriately to the regulatory mechanisms co ntrolling intestinal GLP-1 secretion.