Deletion of GαZ Protein Protects against Diet-induced Glucose Intolerance via Expansion of β-Cell Mass

Insufficient plasma insulin levels caused by deficits in both pancreatic β-cell function and mass contribute to the pathogenesis of type 2 diabetes. This loss of insulin-producing capacity is termed β-cell decompensation. Our work is focused on defining the role(s) of guanine nucleotide-binding protein (G protein) signaling pathways in regulating β-cell decompensation. We have previously demonstrated that the α-subunit of the heterotrimeric Gz protein, Gαz, impairs insulin secretion by suppressing production of cAMP. Pancreatic islets from Gαz-null mice also exhibit constitutively increased cAMP production and augmented glucose-stimulated insulin secretion, suggesting that Gαz is a tonic inhibitor of adenylate cyclase, the enzyme responsible for the conversion of ATP to cAMP. In the present study, we show that mice genetically deficient for Gαz are protected from developing glucose intolerance when fed a high fat (45 kcal%) diet. In these mice, a robust increase in β-cell proliferation is correlated with significantly increased β-cell mass. Further, an endogenous Gαz signaling pathway, through circulating prostaglandin E activating the EP3 isoform of the E prostanoid receptor, appears to be up-regulated in insulin-resistant, glucose-intolerant mice. These results, along with those of our previous work, link signaling through Gαz to both major aspects of β-cell decompensation: insufficient β-cell function and mass.