Objective: Hyperglycemia and systemic inflammation, hallmarks of Type 2 Diabetes (T2D), can induce the production of the inflammatory signaling molecule Prostaglandin E2 (PGE2) in islets. The effects of PGE2 are mediated by its four receptors, E-Prostanoid Receptors 1-4 (EP1-4). EP3 and EP4 play opposing roles in many cell types due to signaling through different G proteins, Gi and GS, respectively. We previously found that EP3 and EP4 expression are reciprocally regulated by activation of the FoxM1 transcription factor, which promotes β-cell proliferation and survival. Our goal was to determine if EP3 and EP4 regulate β-cell proliferation and survival and, if so, to elucidate the downstream signaling mechanisms.
Methods: β-cell proliferation was assessed in mouse and human islets ex vivo treated with selective agonists and antagonists for EP3 (sulprostone and DG-041, respectively) and EP4 (CAY10598 and L-161,982, respectively). β-cell survival was measured in mouse and human islets treated with the EP3- and EP4-selective ligands in conjunction with a cytokine cocktail to induce cell death. Changes in gene expression and protein phosphorylation were analyzed in response to modulation of EP3 and EP4 activity in mouse islets.
Results: Blockade of EP3 enhanced β-cell proliferation in young, but not old, mouse islets in part through phospholipase C (PLC)-γ1 activity. Blocking EP3 also increased human β-cell proliferation. EP4 modulation had no effect on ex vivo proliferation alone. However, blockade of EP3 in combination with activation of EP4 enhanced human, but not mouse, β-cell proliferation. In both mouse and human islets, EP3 blockade or EP4 activation enhanced β-cell survival in the presence of cytokines. EP4 acts in a protein kinase A (PKA)-dependent manner to increase mouse β-cell survival. In addition, the positive effects of FoxM1 activation on β-cell survival are inhibited by EP3 and dependent on EP4 signaling.
Conclusions: Our results identify EP3 and EP4 as novel regulators of β-cell proliferation and survival in mouse and human islets ex vivo.
Keywords: COX-2, cyclooxygenase-2; Cell death; DAG, diacylglycerol; EP1-4, E-Prostanoid Receptors 1-4; GPCR, G protein-coupled receptor; IP3, inositol 1,4,5-trisphosphate; PGE2, prostaglandin E2; PKA, protein kinase A; PL, placental lactogen; PLC, phospholipase C; PT, pertussis toxin; Pancreatic β-cell; Proliferation; Prostaglandin E2.