Pancreatic islet β-cells produce large amounts of γ-aminobutyric acid (GABA), which is co-released with insulin. GABA inhibits glucagon secretion by hyperpolarizing α-cells via type-A GABA receptors (GABAARs). We and others recently reported that islet β-cells also express GABAARs and that activation of GABAARs increases insulin release. Here we investigate the effects of insulin on the GABA-GABAAR system in the pancreatic INS-1 cells using perforated-patch recording. The results showed that GABA produces a rapid inward current and depolarizes INS-1 cells. However, pre-treatment of the cell with regular insulin (1 µM) suppressed the GABA-induced current (IGABA) by 43%. Zinc-free insulin also suppressed IGABA to the same extent of inhibition by regular insulin. The inhibition of IGABA occurs within 30 seconds after application of insulin. The insulin-induced inhibition of IGABA persisted in the presence of PI3-kinase inhibitor, but was abolished upon inhibition of ERK, indicating that insulin suppresses GABAARs through a mechanism that involves ERK activation. Radioimmunoassay revealed that the secretion of C-peptide was enhanced by GABA, which was blocked by pre-incubating the cells with picrotoxin (50 µM, p<0.01) and insulin (1 µM, p<0.01), respectively. Together, these data suggest that autocrine GABA, via activation of GABAARs, depolarizes the pancreatic β-cells and enhances insulin secretion. On the other hand, insulin down-regulates GABA-GABAAR signaling presenting a feedback mechanism for fine-tuning β-cell secretion.

Insulin therapy using insulin detemir (d-INS) has demonstrated weight-sparing effects compared with other insulin formulations. Mechanisms underlying these effects, however, remain largely unknown. Here we postulate that the intestinal tissues' selective preference allows d-INS to exert enhanced action on proglucagon (Gcg) expression and the production of glucagon-like peptide (GLP)-1, an incretin hormone possessing both glycemia-lowering and weight loss effects. To test this hypothesis, we used obese type 2 diabetic db/db mice and conducted a 14-day intervention with daily injection of a therapeutic dose of d- insulin; protein kinase B; glycogen synthase kinase-3; extracellular signal-regulated kinase; ␤-catenin; adenosine 3=,5=-cyclic monophosphate response element-binding protein; Wnt; proglucagon; gut; L cells INSULIN THERAPY IS A CRITICAL treatment for type 1 diabetes patients, and it is also becoming increasingly accepted for type 2 diabetes for the reduction of diabetic complications (48). Weight gain is a primary adverse effect in both type 1 and type 2 diabetic patients on insulin therapy (7, 50). However, patients using long-acting insulin analogs, particularly insulin detemir (d-INS), often show decreased weight gain (16,39).Insulin exerts a variety of biological effects through activation of its cell surface receptor (43). Activation of phosphatidylinositol 3-kinase (PI3-K) and protein kinase B (Akt) is a critical event that triggers diverse downstream signaling cascades and effector responses (23). In the periphery, activation of this pathway conveys insulin actions such as glucose transport, gene transcription, and protein synthesis (23, 53). In the brain, integral insulin signaling via the PI3-K/Akt pathway is related to nutrient homeostasis and appetite regulation (36).d-INS has a fatty acid chain addition at LysB29 that permits reversible albumin binding (15). Diabetic patients with d-INS therapy usually show both improved glycemic control and weight stability (16, 18) compared with other basal insulin therapies. The molecular mechanisms underlying the weightsparing benefit of d-INS remain largely unknown. It has been proposed that d-INS could cross the blood-brain barrier faster and in higher quantities than other types of insulin to induce stronger effect in the brain (17). As a result, reduced appetite may contribute to its weight-sparing benefit (4).We have demonstrated recently that insulin stimulates proglucagon (Gcg) expression in the intestinal endocrine L cells but not in the pancreatic ␣-cells (53). In colon cancer cells, insulin stimulates ␤-catenin (␤-cat) Ser 675 phosphorylation, which was associated with enhanced nuclear localization of ␤-cat and the binding of ␤-cat/TCF7L2 to Wnt target promoters (41). The bipartite transcription factor ␤-cat/TCF is the key downstream effector in the Wnt signaling pathway while the phosphorylation of ␤-cat at Ser 675 increases its nuclear translocation (44). Interestingly, insulin appears to use the same cisand trans-elements that are used by the ...