The Molecular Mechanisms of Prostaglandin E2 Receptor 3 and its associated G protein, G z , in the Pancreatic β‐cell

Signaling mediated by the G protein‐coupled receptor (GPCR), Prostaglandin E2 Receptor 3 (EP3), and the unique inhibitory G protein alpha subunit, Gα z , is emerging as a key player in the pathogenesis of beta‐cell dysfunction and loss in diabetes. β‐cell EP3 expression, as well as production of its endogenous ligand, PGE 2 , are up‐regulated in the context of diabetes and can be specifically targeted to improve insulin secretion from diabetic islets. Genetic ablation of Gα z protects against hyperglycemia in both Type 1 and Type 2 diabetes models by allowing for either a significant expansion in β‐cell mass or enhanced glucose stimulated insulin secretion, or both. In the rat, mouse, and human genomes, the Ptger3 gene encodes for multiple EP3 receptor splice variants that vary only in their cytoplasmic C‐terminal tail: a critical determinant for G‐protein coupling; beta‐arrestin‐mediated receptor desensitization, endocytosis and intracellular vesicular signaling; and ability to signal through other non‐canonical pathways. The mouse genome encodes for three EP3 splice variants: EP3α, EP3 β, and EP3γ. Little is known about how these splice variants differ with regards to Gα z coupling or their involvement in other signal transduction mechanisms, particularly in the beta‐cell. The overall goal of our research program is to fully characterize beta‐cell signaling mediated by the PGE2/EP3/Gα z , signaling pathway that is inhibitory towards insulin secretion, with the long‐term goal of being able to rationally target this pathway for preventative or therapeutic purposes. In this project, we focused on characterizing signaling mediated by EP3α and EP3γ, which both have human homologs. We demonstrate that EP3γ is constitutively active and coupled solely to Gα z in the beta‐cell. The ability of EP3γ to reduce insulin secretion is lost in Gα z ‐null islets and is not influenced by addition of either receptor agonist or antagonist. EP3a also couples to Gα z but can be further activated by sulprostone, an EP3‐selective agonist, and may couple to other G proteins in the absence of Gα z . In beta‐cell lines and islets, sulprostone mediates both cAMP‐dependent and cAMP‐independent signaling, the latter of which we suspect is mediated by the ability of activated Gα z to recruit Rap1GAP from the cytosol to the membrane, where it can bind and inactivate Rap1b. We show that Rap1GAP inhibits insulin secretion stimulated by glucose, KCl, or sulfonylureas, and that this inhibitory effect can be further potentiated by addition of sulprostone. Thus, we postulate that EP3 isoforms direct Gα z and other effectors to mediate a variety of signaling pathways in the pancreatic β‐cell, all of which converge to reduce insulin secretion. Support or Funding Information • R01 DK102598/DK/NIDDK NIH HHS/United States