The Glucagon-Like Peptide-1 Receptor Agonist Oxyntomodulin Enhances β-Cell Function but Does Not Inhibit Gastric Emptying in Mice

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Abstract
The proglucagon gene gives rise to multiple peptides that play diverse roles in the control of energy intake, gut motility, and nutrient disposal. Glucagon-like peptide-1 (GLP-1), a 30-amino-acid peptide regulates glucose homeostasis via control of insulin and glucagon secretion and by inhibition of gastric emptying and food intake. Oxyntomodulin (OXM) a 37-amino-acid peptide also derived from the proglucagon gene, binds to both the glucagon and GLP-1 receptor (GLP-1R); however, a separate OXM receptor has not yet been identified. Here we show that OXM, like other GLP-1R agonists, stimulates cAMP formation and lowers blood glucose after both oral and ip glucose administration, actions that require a functional GLP-1R. OXM also directly stimulates insulin secretion from murine islets and INS-1 cells in a glucose- and GLP-1R-dependent manner. Moreover, OXM ameliorates hyperglycemia and significantly reduces apoptosis in murine β-cells after streptozotocin administration and directly reduces apoptosis in thapsigargin-treated INS-1 cells. Unexpectedly, OXM, but not the GLP-1R agonist exendin-4, increased plasma levels of insulin after oral glucose administration. Moreover, OXM administered at doses that potently lower blood glucose had no effect on inhibition of gastric emptying but reduced food intake in WT mice. Taken together, these findings illustrate that although structurally distinct proglucagon-derived peptides such as GLP-1 and OXM engage the GLP-1R, OXM mimics some but not all of the actions of GLP-1R agonists in vivo. These findings may have implications for therapeutic efforts using OXM as a long-acting GLP-1R agonist for the treatment of metabolic disorders.

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