The peptide hormone, glucagon-like peptide 1 (GLP-1), has been shown to increase glucose-dependent insulin secretion, enhance insulin gene transcription, expand islet cell mass, and inhibit beta-cell apoptosis in animal models of diabetes. The aim of the present study was to evaluate whether GLP-1 could improve function and inhibit apoptosis in freshly isolated human islets. Human islets were cultured for 5 d in the presence, or absence, of GLP-1 (10 nm, added every 12 h) and studied for viability and expression of proapoptotic (caspase-3) and antiapoptotic factors (bcl-2) as well as glucose-dependent insulin production. We observed better-preserved three-dimensional islet morphology in the GLP-1-treated islets, compared with controls. Nuclear condensation, a feature of cell apoptosis, was inhibited by GLP-1. The reduction in the number of apoptotic cells in GLP-1-treated islets was particularly evident at d 3 (6.1% apoptotic nuclei in treated cultures vs. 15.5% in controls; P < 0.01) and at d 5 (8.9 vs. 18.9%; P < 0.01). The antiapoptotic effect of GLP-1 was associated with the down-regulation of active caspase-3 (P < 0.001) and the up-regulation of bcl-2 (P < 0.01). The effect of GLP-1 on the intracellular levels of bcl-2 and caspase-3 was observed at the mRNA and protein levels. Intracellular insulin content was markedly enhanced in islets cultured with GLP-1 vs. control (P < 0.001, at d 5), and there was a parallel GLP-1-dependent potentiation of glucose-dependent insulin secretion (P < 0.01 at d 3; P < 0.05 at d 5). Our findings provide evidence that GLP-1 added to freshly isolated human islets preserves morphology and function and inhibits cell apoptosis.
A constant remodeling of islet cell mass mediated by proliferative and apoptotic stimuli ensures a dynamic response to a changing demand for insulin. In this study, we investigated the effect of glucagon-like peptide-1 (GLP-1) in Zucker diabetic rats, an animal model in which the onset of diabetes occurs when the proliferative potential and the rate of beta-cell apoptosis no longer compensate for the increased demand for insulin. We subjected diabetic rats to a 2-d infusion of GLP-1 and tested their response to an ip glucose tolerance test. GLP-1 produced a significant increase of insulin secretion, which was paralleled by a decrease in plasma glucose levels (P < 0.001 and P < 0.01, respectively). Four days after the removal of the infusion pumps, rats were killed and the pancreas harvested to study the mechanism by which GLP-1 ameliorated glucose tolerance. Ex vivo immunostaining with the marker of cell proliferation, Ki-67, showed that the metabolic changes observed in rats treated with GLP-1 were associated with an increase in cell proliferation of the endocrine and exocrine component of the pancreas. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling staining, a marker of cellular apoptosis, indicated a reduction of apoptotic cells within the islet as well in the exocrine pancreas in GLP-1-treated rats. Double immunostaining for the apoptotic marker caspase-3 and for insulin showed a significant reduction of caspase-3 expression and an increase in insulin content in GLP-1-treated animals. Finally, staining of pancreatic sections with the nuclear dye 4,6-Diaminidino-2-phenyl-dihydrochloride demonstrated a marked reduction of fragmented nuclei in the islet cells of rats treated with GLP-1. Our findings provide evidence that the beneficial effects of GLP-1 in Zucker diabetic rats is mediated by an increase in islet cell proliferation and a decrease of cellular apoptosis.
The activation of the glucagon-like peptide-1 (GLP-1) receptor has been shown to have an important role in the functional activity of islet beta-cells and in the expansion of the islet cell mass. Constant remodeling of islet cell mass is mediated in vivo by proliferative and apoptotic stimuli to ensure a dynamic response to a changing demand for insulin. The present study was undertaken to investigate the biological activity of GLP-1 when cells were challenged by a proapoptotic stimulus. We have shown that activation of the GLP-1 receptor inhibits H(2)O(2)-induced apoptosis in a cultured mouse insulinoma cell line, termed MIN6. GLP-1 reduced DNA fragmentation and improved cell survival. This was mediated by an increased expression of the antiapoptotic proteins Bcl-2 and Bcl-xL. GLP-1 also prevented the H(2)O(2)-dependent cleavage of poly-(ADP-ribose)-polymerase. Inhibition of the GLP-1-dependent increase of cAMP by Rp-cAMP blocked the antiapoptotic action of GLP-1, as determined by DNA fragmentation and poly-(ADP-ribose)-polymerase assays and by detection of Bcl-2 and Bcl-xL protein levels. Investigation of the role of the protein kinases, PI-3 kinase (PI3K) and MAPK, by use of the inhibitors PD098059 and LY294002 demonstrated that the activation of PI3K, but not MAPK, was required to prevent proapoptotic events in cells exposed to H(2)O(2). The present study provides evidence that GLP-1 has an antiapoptotic action mediated by a cAMP- and PI3K-dependent signaling pathway.
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