Previous studies have revealed that the glucoincretin hormone glucagon-like peptide-1 (GLP-1)(7-36)amide is metabolized by dipeptidyl peptidase-IV (DPP-IV) and neutral endopeptidase 24.11 (NEP) to yield GLP-1(9-36)amide and GLP-1(28-36)amide, respectively, as the principal metabolites. Contrary to the previous notion that GLP-1(7-36)amide metabolites are pharmacologically inactive, recent studies have demonstrated cardioprotective and insulinomimetic effects with both GLP-1(9-36)amide and GLP-1(28-36)amide in animals and humans. In the present work, we examined the metabolic stability of the two GLP-1(7-36)amide metabolites in cryopreserved hepatocytes, which have been used to demonstrate the in vitro insulin-like effects of GLP-1(9-36)amide and GLP-1(28-36)amide on gluconeogenesis. To examine the metabolic stability of the GLP-1(7-36)amide metabolites, a liquid chromatography-tandem mass spectrometry assay was developed for the quantitation of the intact peptides in hepatocyte incubations. GLP-1(9-36)amide and GLP-1(28-36)amide were rapidly metabolized in mouse [GLP-1(9-36)amide: t 1/2 = 52 minutes; GLP-1(28-36)amide: t 1/2 = 13 minutes] and human hepatocytes [GLP-1(9-36)amide: t 1/2 = 180 minutes; GLP-1(28-36)amide: t 1/2 = 24 minutes), yielding a variety of N-terminal cleavage products that were characterized using mass spectrometry. Metabolism at the C terminus was not observed for either peptides. The DPP-IV and NEP inhibitors diprotin A and phosphoramidon, respectively, did not induce resistance in the two peptides toward proteolytic cleavage. Overall, our in vitro findings raise the intriguing possibility that the insulinomimetic effects of GLP-1(9-36)amide and GLP-1(28-36)amide on gluconeogenesis and oxidative stress might be due, at least in part, to the actions of additional downstream metabolites, which are obtained from the enzymatic cleavage of the peptide backbone in the parent compounds.
IntroductionThe glucoincretin hormone glucagon-like peptide-1 (GLP-1) is derived from a proglucagon precursor and secreted by intestinal enteroendocrine L cells in response to oral nutrient ingestion (Kieffer and Habener, 1999;Holst, 2007;Lovshin and Drucker, 2009;Mundil et al., 2012). The majority of circulating GLP-1 levels comprise the 30-amino acid peptide GLP-1(7-36)amide, which acts through a seventransmembrane-spanning, heterotrimeric class B G protein-coupled receptor on pancreatic b cells to exert glucoregulatory and insulinotropic actions (Thorens, 1992). However, after its secretion from the intestine, native GLP-1(7-36)amide is rapidly degraded [half-life (t 1/2 ) = 1-2 minutes) on its N and C termini by the ubiquitously expressed enzymes dipeptidyl peptidase-IV (DPP-IV) and neutral endopeptidase 24.11 (NEP) (Hansen et al., 1999;Holst, 2007), respectively, to yield GLP-1(9-36)amide and nonapeptide GLP-1(28-36)amide as metabolites (Deacon et al., 1995a,b;Hupe-Sodmann et al., 1995;Mentlein, 1999). In patients with type 2 diabetes mellitus, secretion of GLP-1 is diminished, and administration of e...