Glucagon-Like Peptide-1 Enhances Glucokinase Activity in Pancreatic β-Cells through the Association of Epac2 with Rim2 and Rab3A

Abstract

Glucokinase (GK), which phosphorylates D-glucose, is a major glucose sensor in β-cells for glucose-stimulated insulin secretion (GSIS) and is a promising new drug target for type 2 diabetes (T2D). In T2D, pancreatic β-cells exhibit defective glucose sensitivity, which leads to impaired GSIS. Although glucagon-like peptide-1-(7-36)-amide (GLP-1) is known to enhance β-cell glucose sensitivity, the effect of GLP-1 on GK activity is still unknown. The present study demonstrated that GLP-1 pretreatment for 30 min significantly enhanced GK activity in a glucose-dependent manner, with a lower Michaelis-Menten constant (K(m)) but unchanged maximal velocity (V(max)). Thus, GLP-1 acutely enhanced cellular glucose uptake, mitochondrial membrane potential, and cellular ATP levels in response to glucose in rat INS-1 and native β-cells. This effect of GLP-1 occurred via its G protein-coupled receptor pathway in a cAMP-dependent but protein kinase A-independent manner with evidence of exchange protein activated by cAMP (Epac) involvement. Silencing Epac2, interacting molecule of the small G protein Rab3 (Rim2), or Ras-associated protein Rab3A (Rab3A) significantly blocked the effect of GLP-1. These results suggested that GLP-1 can further potentiate GSIS by enhancing GK activity through the signaling of Epac2 to Rim2 and Rab3A, which is the similar pathway for GLP-1 to potentiate Ca(2+)-dependent insulin granule exocytosis. The present finding may also be an important mechanism of GLP-1 for recovery of GSIS in T2D.

Regulation of Glucokinase by Intracellular Calcium Levels in Pancreatic β Cells

“…We also found that increasing cAMP levels with forskolin or isobutylmethylxanthine activated the FRET-GCK sensor (Fig. 3I), consistent with previous work tying cAMP levels to GLP-1 activation of GCK (38). The ability of these treatments to affect cAMP levels in ␤TC3 cells was confirmed in cells expressing the ICUE3 cAMP FRET indicator (39) (Fig.…”

β-Adrenergic Receptors Activate Exchange Protein Directly Activated by cAMP (Epac), Translocate Munc13-1, and Enhance the Rab3A-RIM1α Interaction to Potentiate Glutamate Release at Cerebrocortical Nerve Terminals

“…Such behavior would contribute to the formation of the tripartite Rab3A-RIM1␣-Munc13 complex, which in turn would increase the priming of vesicles to a release-competent state. In support of this hypothesis, Epac proteins activate the small G proteins Rap1 and Rab3A to induce exocytosis in non-neuronal systems (24), whereas Epac2 interacts with RIM2 to promote insulin secretion in a cAMP-dependent, PKA-independent manner in pancreatic ␤ cells (27,45,63).…”

“…Epac Activation Enhances the Interaction between Rab3 and RIM Proteins-In non-neuronal preparations, Epac proteins activate small G proteins like Rap1 and Rab3 (24) and then bind to the active zone protein RIM (27,45). Small G proteins cycle between active GTP-bound and inactive GDP-bound states (46).…”

RFamide Peptides 43RFa and 26RFa Both Promote Survival of Pancreatic β-Cells and Human Pancreatic Islets but Exert Opposite Effects on Insulin Secretion

“…Glucose uptake by pancreatic b-cells, through the glucose transporter GLUT2 and glucokinase activity, is essential for the normal insulin secretory response to hyperglycemia (35,36). To further ascertain the stimulatory and inhibitory roles of 43RFa and 26RFa, respectively, on GSIS, glucose uptake was determined in INS-1E b-cells and human pancreatic islets.…”