Glucagon and glucagon-like peptide-1 (GLP-1) are produced in pancreatic α-cells and enteroendocrine L-cells, respectively, in a tissue-specific manner from the same precursor, proglucagon, that is encoded by glucagon gene (Gcg), and play critical roles in glucose homeostasis. Here, we studied glucose homeostasis and β-cell function of Gcg-deficient mice that are homozygous for a Gcg-GFP knock-in allele (Gcggfp/gfp). The Gcggfp/gfp mice displayed improved glucose tolerance and enhanced insulin secretion, as assessed by both oral glucose tolerance test (OGTT) and intraperitoneal glucose tolerance test (IPGTT). Responses of glucose-dependent insulinotropic polypeptide (GIP) to both oral and intraperitoneal glucose loads were unexpectedly enhanced in Gcggfp/gfp mice, and immunohistochemistry localized GIP to pancreatic β-cells of Gcggfp/gfp mice. Furthermore, secretion of GIP in response to glucose was detected in isolated islets of Gcggfp/gfp mice. Blockade of GIP action in vitro and in vivo by cAMP antagonism and genetic deletion of the GIP receptor, respectively, almost completely abrogated enhanced insulin secretion in Gcggfp/gfp mice. These results indicate that ectopic GIP expression in β-cells maintains insulin secretion in the absence of proglucagon-derived peptides (PGDPs), revealing a novel compensatory mechanism for sustaining incretin hormone action in islets.
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