The present studies extend recent findings that mice null for the α2A adrenergic receptor (α2A AR KO mice) lack suppression of exogenous secretagogue-stimulated insulin secretion in response to α2 AR agonists by evaluating the endogenous secretagogue, glucose, ex vivo, and providing in vivo data that baseline insulin levels are elevated and baseline glucose levels are decreased in α2A AR KO mice. These latter findings reveal that the α2A AR subtype regulates glucose-stimulated insulin release in response to endogenous catecholamines in vivo. The changes in α2A AR responsiveness and resultant changes in insulin/glucose homeostasis encouraged us to utilize proteomics strategies to identify possible α2A AR downstream signaling molecules or other resultant changes due to perturbation of α2A AR expression. Although agonist stimulation of islets from wild type (WT) mice did not significantly alter islet protein profiles, several proteins were enriched in islets from α2A AR KO mice when compared with those from WT mice, including an enzyme participating in insulin protein processing. The present studies document the important role of the α2A AR subtype in tonic suppression of insulin release in response to endogenous catecholamines as well as exogenous α2 agonists and provide insights into pleiotropic changes that result from loss of α2A AR expression and tonic suppression of insulin release.
Xinran Hu, David Friedman, Salisha Hill, Richard Caprioli, Wendell Nicholson, Alvin C Powers, Lawrence Hunter and Lee E Limbird
Karin J Bosma, Mohsin Rahim, Kritika Singh, Slavina B Goleva, Martha L Wall, Jing Xia, Kristen E Syring, James K Oeser, Greg Poffenberger, Owen P McGuinness, Anna L Means, Alvin C Powers, Wen-hong Li, Lea K Davis, Jamey D Young and Richard M O’Brien
The G6PC1, G6PC2 and G6PC3 genes encode distinct glucose-6-phosphatase catalytic subunit (G6PC) isoforms. In mice, germline deletion of G6pc2 lowers fasting blood glucose (FBG) without affecting fasting plasma insulin (FPI) while, in isolated islets, glucose-6-phosphatase activity and glucose cycling are abolished and glucose-stimulated insulin secretion (GSIS) is enhanced at submaximal but not high glucose. These observations are all consistent with a model in which G6PC2 regulates the sensitivity of GSIS to glucose by opposing the action of glucokinase. G6PC2 is highly expressed in human and mouse islet beta cells however, various studies have shown trace G6PC2 expression in multiple tissues raising the possibility that G6PC2 also affects FBG through non-islet cell actions. Using real-time PCR we show here that expression of G6pc1 and/or G6pc3 are much greater than G6pc2 in peripheral tissues, whereas G6pc2 expression is much higher than G6pc3 in both pancreas and islets with G6pc1 expression not detected. In adult mice, beta cell-specific deletion of G6pc2 was sufficient to reduce FBG without changing FPI. In addition, electronic health record-derived phenotype analyses showed no association between G6PC2 expression and phenotypes clearly unrelated to islet function in humans. Finally, we show that germline G6pc2 deletion enhances glycolysis in mouse islets and that glucose cycling can also be detected in human islets. These observations are all consistent with a mechanism by which G6PC2 action in islets is sufficient to regulate the sensitivity of GSIS to glucose and hence influence FBG without affecting FPI.