Augmented Insulin Secretion Research Articles

1155-P: Effect of Endoscopic Duodenal-Jejunal Bypass Liner on Parameters of Insulin Sensitivity and Secretion in Subjects with Type 2 Diabetes Mellitus and Obesity

Duodenal-jejunal by-pass liner (DJBL) is an endoscopically implantable device designed to non-invasively mimic the effects of surgical gastric by-pass. However, the exact mechanisms responsible for improved glucose control associated with DJBL are still unclear. The aim of our study was to assess the effect of DJBL on insulin sensitivity and secretion in subjects with type 2 diabetes mellitus (T2DM) and obesity. Twenty-one obese subjects with T2DM (15 males, age 52.3 ± 11.2 years) underwent the implantation of the EndoBarrier DJBL (GI Dynamics, U.S.). Anthropometric, biochemical and hormonal parameters were measured before and 10 months after the implantation of DJBL. Insulin sensitivity was assessed using euglycemic-hyperinsulinemic clamp. Standard parameters including glucose disposal rate (M) and metabolic clearance rate of glucose (MCG) were calculated based on clamp results. Over 10 months the implantation of DJBL decreased body weight (body mass index 41.6 ± 6.1 vs. 37.5 ± 4.2 kg/m2, p<0.001) and improved glucose control (fasting blood glucose 10.5 ± 2.5 vs. 8.4 ± 2.4 mmol/l, p<0.001; HbA1C 74.7 ± 17.8 vs. 55.6 ± 12.4 mmol/mol, p<0.001). These improvements were accompanied by a marked increase in insulin sensitivity as shown by the results of the euglycemic-hyperinsulinemic clamp (M 1.5 ± 0.8 vs. 2.6 ± 1.0 mg.kg-1.min-1, p<0.001; MCG 1.4 ± 0.7 vs. 2.6 ± 0.2 mg.kg-1.min-1, p<0.001). In contrast, insulin secretion was not affected by DJBL (fasting insulin 21.8 ± 13.0 vs. 18.5 ± 9.5 mIU/l, p=0.312; C-peptide 2.7 ± 1.1 vs. 2.4 ± 1.1 nmol/l, p=0.172). To conclude the improvement in glucose control associated with the implantation of Endobarrier DJBL is primarily caused by increased insulin sensitivity rather than augmented insulin secretion. Disclosure M. Kudlackova: None. M. Mraz: None. M. Haluzik: None. T. Pelikanova: None. M. Benes: None. D. Lapesova: None. D. Kobrova: None. Z. Vlasakova: None. O. Brizova: None. Z. Lacinova: None. Funding Institute for Clinical and Experimental Medicine (IN00023001)

From the Literature

From the Literature

Exogenous Cytokine-Free Differentiation of Human Pluripotent Stem Cells into Classical Brown Adipocytes.

We previously established a method for a directed differentiation of human pluripotent stem cells into classical brown adipocytes (BA) by forming aggregates via massive floating culture in the presence of a specific cytokine cocktail. However, use of recombinant cytokines requires significant cost. Moreover, an enforced differentiation by exogenously added cytokines may amend skewed differentiation propensity of patient’s pluripotent stem cells, providing unsatisfactory disease models. Therefore, an exogenous cytokine-free method, where cytokines required for differentiation are provided in an auto/paracrine manner mimicking natural developmental process, is beneficial. Here we show that, if human pluripotent stem cells are cultured as size-controlled spheroids (100–120 µm radius, 2000–2500 cells/spheroid) in a mutually segregated manner with half-change of the medium every other day, they differentiate into classical BA via an authentic MYF5-positive myoblast route in the absence of exogenous cytokines. Differentiated BA exerted thermogenic activity in transplanted mice in response to beta-adrenergic receptor agonist stimuli. The cytokine-free differentiation method has further advantages in exploring BATokines, BA-derived physiologically active substances. Indeed, we have found that BA produces an unknown small (<1000 Da), highly hydrophilic molecule that augments insulin secretion from pancreatic beta cells. Our upgraded technique will contribute to an advancement of stem cell study for diverse purposes.

A Review of Practical Issues on the Use of Glucagon-Like Peptide-1 Receptor Agonists for the Management of Type 2 Diabetes.

Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are well established as effective treatments for patients with type 2 diabetes. GLP-1 RAs augment insulin secretion and suppress glucagon release via the stimulation of GLP-1 receptors. Although all GLP-1 RAs share the same underlying mechanism of action, they differ in terms of formulations, administration, injection devices and dosages. With six GLP-1 RAs currently available in Europe (namely, immediate-release exenatide, lixisenatide, liraglutide; prolonged-release exenatide, dulaglutide and semaglutide), each with its own characteristics and administration requirements, physicians caring for patients in their routine practice face the challenge of being cognizant of all this information so they are able to select the agent that is most suitable for their patient and use it in an efficient and optimal way. The objective of this review is to bring together practical information on the use of these GLP-1 RAs that reflects their approved use.Funding: Eli Lilly and Company.Plain Language Summary: Plain language summary available for this article.

Multivalent activation of GLP-1 and sulfonylurea receptors modulates β-cell second-messenger signaling and insulin secretion.

Linking two pharmacophores that bind different cell surface receptors into a single molecule can enhance cell-targeting specificity to cells that express the complementary receptor pair. In this report, we developed and tested a synthetic multivalent ligand consisting of glucagon-like peptide-1 (GLP-1) linked to glibenclamide (Glb) (GLP-1/Glb) for signaling efficacy in β-cells. Expression of receptors for these ligands, as a combination, is relatively specific to the β-cell in the pancreas. The multivalent GLP-1/Glb increased both intracellular cAMP and Ca2+, although Ca2+ responses were significantly depressed compared with the monomeric Glb. Moreover, GLP-1/Glb increased glucose-stimulated insulin secretion in a dose-dependent manner. However, unlike the combined monomers, GLP-1/Glb did not augment insulin secretion at nonstimulatory glucose concentrations in INS 832/13 β-cells or human islets of Langerhans. These data suggest that linking two binding elements, such as GLP-1 and Glb, into a single bivalent ligand can provide a unique functional agent targeted to β-cells.

The regulation of insulin secretion via phosphoinositide-specific phospholipase Cβ signaling.

Phospholipase Cβ (PLCβ) is a membrane-associated enzyme activated by membrane receptors, especially G-protein coupled receptors (GPCRs). It propagates intracellular signaling by mediating phospholipid metabolism and generating key second messengers, such as inositol triphosphate and diacylglycerol, leading to intracellular Ca2+ mobilization and activation of kinases, such as protein kinases C. In pancreatic β-cells, PLCβ-mediated signaling activated by various factors, such as free fatty acids and neuronal and hormonal ligands, has been confirmed as being involved in the regulation of insulin secretion, and PLCβs have been regarded as essential mediators for augmenting insulin secretion. In this review, we describe the physiological function of PLCβs in the regulation of glucose-stimulated insulin secretion and discuss emerging data on GPCR/PLCβ signaling that is being developed as a target for the treatment of diabetes mellitus.

Chronic Adrenergic Signaling Causes Abnormal RNA Expression of Proliferative Genes in Fetal Sheep Islets.

Intrauterine growth restriction (IUGR) increases the risk of developing diabetes in later life, which indicates developmental programming of islets. IUGR fetuses with placental insufficiency develop hypoxemia, elevating epinephrine and norepinephrine (NE) concentrations throughout late gestation. To isolate the programming effects of chronically elevated catecholamines, NE was continuously infused into normally grown sheep fetuses for 7 days. High plasma NE concentrations suppress insulin, but after the NE infusion was terminated, persistent hypersecretion of insulin occurred. Our objective was to identify differential gene expression with RNA sequencing (RNAseq) in fetal islets after chronic adrenergic stimulation. After determining the NE-regulated genes, we identified the subset of differentially expressed genes that were common to both islets from NE fetuses and fetuses with IUGR to delineate the adrenergic-induced transcriptional responses. A portion of these genes were investigated in mouse insulinoma (Min6) cells chronically treated with epinephrine to better approximate the β-cell response. In islets from NE fetuses, RNAseq identified 321 differentially expressed genes that were overenriched for metabolic and hormone processes, and the subset of 96 differentially expressed genes common to IUGR islets were overenriched for protein digestion, vitamin metabolism, and cell replication pathways. Thirty-eight of the 96 NE-regulated IUGR genes changed similarly between models with functional enrichment for proliferation. In Min6 cells, chronic epinephrine stimulation slowed proliferation and augmented insulin secretion after treatment. These data establish molecular mechanisms underlying persistent adrenergic stimulation in hyperfunctional fetal islets and identify a subset of genes dysregulated by catecholamines in IUGR islets that may represent programming of β-cell proliferation capacity.

Individual and Combined Glucose-Lowering Effects of Glucagon Receptor Antagonism and Dipeptidyl Peptidase-4 Inhibition

Type 2 diabetes is characterized by absolute or relative hypoinsulinemia and hyperglucagonemia. Dipeptidyl peptidase 4 inhibitors (DPP-4i) augment insulin secretion and decrease glucagon secretion, but if glucagon is removed from the metabolic equation, the effect of DPP-4i is unknown. In a randomized, placebo-controlled, double-dummy, double-blinded, cross-over study, patients with type 2 diabetes (n=12, age [mean (SD)]: 60.9 (7.8) years, BMI 34.6 (7.1) kg/m2; HbA1c 50.3 (10.5) mmol/mol) underwent four 4-hour liquid mixed meal tests preceded by single-dose administration of 1) placebo, 2) DPP-4i (5 mg linagliptin) 2 hours before the meal, 3) glucagon receptor antagonist (GRA) (300 mg LY2409021) 12 hours before the meal, and 4) GRA + DPP-4i. Compared to placebo, fasting plasma glucose (FPG) was lowered by GRA but not DPP-4i. Adding DPP-4i to GRA did not lower FPG further. Compared to placebo, DPP-4i increased insulin responses to the meal (C-peptide AUC) (P&amp;lt;0.01), but did not result in a difference in plasma glucose excursions (Figure). GRA alone lowered glucose AUC, and the combination of GRA and DPP-4i lowered glucose AUC further. In conclusion, the combination of DPP-4i and GRA has additive effects on postprandial glucose excursions. This seems to be driven by the efficient reduction in FPG by GRA combined with additional reduction in postprandial glucose excursions by DPP-4i. Disclosure S. Haedersdal: None. A.B. Lund: Other Relationship; Self; Novo Nordisk A/S. E. Nielsen-Hannerup: None. H. Maagensen: None. J.J. Holst: Advisory Panel; Self; Novo Nordisk A/S. Board Member; Self; Zealand Pharma A/S. Speaker's Bureau; Self; Merck Sharp &amp; Dohme Corp., AstraZeneca. Research Support; Self; Danish Diabetes Academy, Novo Nordisk Foundation. Other Relationship; Self; Antag Therapeutics. Other Relationship; Spouse/Partner; Antag Therapeutics. F.K. Knop: Advisory Panel; Self; AstraZeneca. Consultant; Self; AstraZeneca. Research Support; Self; AstraZeneca. Speaker's Bureau; Self; AstraZeneca. Advisory Panel; Self; Eli Lilly and Company. Speaker's Bureau; Self; Eli Lilly and Company. Advisory Panel; Self; Amgen Inc.. Speaker's Bureau; Self; Merck Sharp &amp; Dohme Corp.. Advisory Panel; Self; Novo Nordisk A/S. Consultant; Self; Novo Nordisk A/S. Speaker's Bureau; Self; Novo Nordisk A/S. Consultant; Self; Amgen Inc.. Advisory Panel; Self; MedImmune, Sanofi. Consultant; Self; Sanofi. Research Support; Self; Sanofi. Speaker's Bureau; Self; Sanofi.

Study of the serum level of fasting glucagon-like peptide-1 in type 2 diabetics and its relation to the glycemic profile

BackgroundGlucagon-like peptide-1 (GLP-1) is a peptide formed of 30 amino acids. It is synthesized in and released from the enteroendocrine L cells that are present throughout the small and the large intestine. Because of the important physiological role of GLP-1 in augmenting insulin secretion, it is generally believed that GLP-1 release is deficient in type 2 diabetes mellitus (T2DM) patients.However, studies in adults have yielded conflicting results showing decreased, normal, or increased GLP-1 concentrations in prediabetics or T2DM after oral glucose or mixed meal.The aim of this work was to study the level of fasting total GLP-1 in T2DM patients and its relation to the glycemic profile.Patients and methodsThis study included 60 T2DM patients and 40 participants matched for age and sex as a control group, selected from the inpatient and outpatient clinics of the Internal Medicine Department in Menoufia University Hospital. After obtaining their informed consent, all participants were subjected to a full assessment of history and physical examination with estimation of BMI, insulin resistance with homeostasis model assessment-2, and investigations including fasting blood glucose (FBG), glycated hemoglobin (HbAIc), and the total fasting levels of GLP-1.ResultsThere was no significant difference in the level of GLP-1 between T2DM patients and the controls. There were significant negative correlations between the fasting total GLP-1 and FBG, HbAIc, serum insulin, and homeostasis model assessment-2 in both the total sample and the T2DM patients.ConclusionFasting total levels of GLP-1 are not reduced in T2DM patients and are negatively correlated with FBG, HbAIc, and insulin resistance.

Microfluidic Chip with Integrated Electrophoretic Immunoassay for Investigating Cell-Cell Interactions.

Microfluidics have been used to create "body-on-chip" systems to mimic in vivo cellular interactions with a high level of control. Most such systems rely on optical observation of cells as a readout. In this work we integrated a cell-cell interaction chip with online microchip electrophoresis immunoassay to monitor the effects of the interaction on protein secretion dynamics. The system was used to investigate the effects of adipocytes on insulin secretion. Chips were loaded with 190 000 3T3-L1 adipocytes and a single islet of Langerhans in separate chambers. The chambers were perfused at 300-600 nL/min so that adipocyte secretions flowed over the islets for 3 h. Adipocytes produced 80 μM of nonesterified fatty acids (NEFAs), a factor known to impact insulin secretion, at the islets. After perfusion, islets were challenged with a step change in glucose from 3 to 11 mM while monitoring insulin secretion at 8 s intervals by online immunoassay. Adipocyte treatment augmented insulin secretion by 6-fold compared to controls. The effect was far greater than comparable concentrations of NEFA applied to the islets demonstrating that adipocytes release multiple factors that can strongly potentiate insulin secretion. The experiments reveal that integration of chemical analysis with cell-cell interaction can provide valuable insights into cellular functions.

Characteristics of repaglinide effects on insulin secretion

The dynamics of insulin secretion stimulated by repaglinide, a glinide, and the combinatorial effects of repaglinide and incretin were investigated. At 4.4 mM glucose, repaglinide induced insulin secretion with a gradually increasing first phase, showing different dynamics from that induced by glimepiride, a sulfonylurea. In the presence of glucagon-like peptide-1 (GLP-1), insulin secretion by repaglinide was augmented significantly but to lesser extent and showed different dynamics from that by glimepiride. At 4.4 mM glucose, the intracellular Ca2+ level was gradually increased by repaglinide alone or repaglinide plus GLP-1, which differs from the Ca2+ dynamics by glimepiride alone or glimepiride plus GLP-1, suggesting that the difference in Ca2+ dynamics contributes to the difference in the dynamics of insulin secretion. At a higher concentration (8.8 mM) of glucose, the dynamics of insulin secretion stimulated by repaglinide was similar to that by glimepiride. Combination of repaglinide and GLP-1 significantly augmented insulin secretion, the amount of which was comparable to that by the combination of glimepiride and GLP-1. The Ca2+ dynamics was similar for repaglinide and glimepiride at 8.8 mM glucose. Our data indicate that repaglinide has characteristic properties in its effects on the dynamics of insulin secretion and intracellular Ca2+ and that the combination of repaglinide and GLP-1 stimulates insulin secretion more effectively than the combination of glimepiride and GLP-1 at a high concentration of glucose, providing a basis for its use in clinical settings.

A Novel Phenylchromane Derivative Increases the Rate of Glucose Uptake in L6 Myotubes and Augments Insulin Secretion from Pancreatic Beta-Cells by Activating AMPK.

A series of novel polycyclic aromatic compounds that augment the rate of glucose uptake in L6 myotubes and increase glucose-stimulated insulin secretion from beta-cells were synthesized. Designing these molecules, we have aimed at the two main pathogenic mechanisms of T2D, deficient insulin secretion and diminished glucose clearance. The ultimate purpose of this work was to create a novel antidiabetic drug candidate with bi-functional mode of action. All presented compounds were synthesized, and characterized in house. INS-1E cells and L6 myoblasts were used for the experiments. The rate of glucose uptake, mechanism of action, level of insulin secretion and the druggability of the lead compound were studied. The lead compound (6-(1,3-dithiepan-2-yl)-2-phenylchromane), dose- and time-dependently at the low μM range increased the rate of glucose uptake in L6 myotubes and insulin secretion in INS-1E cells. The compound exerted its effects through the activation of the LKB1 (Liver Kinase B1)-AMPK pathway. In vitro metabolic parameters of this lead compound exhibited good druggability. We anticipate that bi-functionality (increased rate of glucose uptake and augmented insulin secretion) will allow the lead compound to be a starting point for the development of a novel class of antidiabetic drugs.

Atrial Natriuretic Peptide Affects Stimulus-Secretion Coupling of Pancreatic β-Cells.

Atrial natriuretic peptide (ANP) influences glucose homeostasis and possibly acts as a link between the cardiovascular system and metabolism, especially in metabolic disorders like diabetes. The current study evaluated effects of ANP on β-cell function by the use of a β-cell-specific knockout of the ANP receptor with guanylate cyclase activity (βGC-A-KO). ANP augmented insulin secretion at the threshold glucose concentration of 6 mmol/L and decreased KATP single-channel activity in β-cells of control mice but not of βGC-A-KO mice. In wild-type β-cells but not β-cells lacking functional KATP channels (SUR1-KO), ANP increased electrical activity, suggesting no involvement of other ion channels. At 6 mmol/L glucose, ANP readily elicited Ca2+ influx in control β-cells. This effect was blunted in β-cells of βGC-A-KO mice, and the maximal cytosolic Ca2+ concentration was lower. Experiments with inhibitors of protein kinase G (PKG), protein kinase A (PKA), phosphodiesterase 3B (PDE3B), and a membrane-permeable cyclic guanosine monophosphate (cGMP) analog on KATP channel activity and insulin secretion point to participation of the cGMP/PKG and cAMP/PKA/Epac (exchange protein directly activated by cAMP) directly activated by cAMP Epac pathways in the effects of ANP on β-cell function; the latter seems to prevail. Moreover, ANP potentiated the effect of glucagon-like peptide 1 (GLP-1) on glucose-induced insulin secretion, which could be caused by a cGMP-mediated inhibition of PDE3B, which in turn reduces cAMP degradation.

Optogenetic regulation of insulin secretion in pancreatic \u03b2-cells

Pancreatic β-cell insulin production is orchestrated by a complex circuitry involving intracellular elements including cyclic AMP (cAMP). Tackling aberrations in glucose-stimulated insulin release such as in diabetes with pharmacological agents, which boost the secretory capacity of β-cells, is linked to adverse side effects. We hypothesized that a photoactivatable adenylyl cyclase (PAC) can be employed to modulate cAMP in β-cells with light thereby enhancing insulin secretion. To that end, the PAC gene from Beggiatoa (bPAC) was delivered to β-cells. A cAMP increase was noted within 5 minutes of photostimulation and a significant drop at 12 minutes post-illumination. The concomitant augmented insulin secretion was comparable to that from β-cells treated with secretagogues. Greater insulin release was also observed over repeated cycles of photoinduction without adverse effects on viability and proliferation. Furthermore, the expression and activation of bPAC increased cAMP and insulin secretion in murine islets and in β-cell pseudoislets, which displayed a more pronounced light-triggered hormone secretion compared to that of β-cell monolayers. Calcium channel blocking curtailed the enhanced insulin response due to bPAC activity. This optogenetic system with modulation of cAMP and insulin release can be employed for the study of β-cell function and for enabling new therapeutic modalities for diabetes.

OP-33 - Lipid peroxidation, lipohormesis and lipotoxicity in diabetes

Little is known about the impact of lipids, phospholipid turnover and metabolism within membranes of insulin granules in pancreatic beta cells. We have discovered that high levels of glucose and saturated fatty acids affect glucose-stimulated insulin secretion and cell viability in a biphasic manner: non-cytotoxic levels of both induce a hormetic response that augments insulin secretion, which increases the rate of glucose and fatty acids disposal from the circulation to storage organs. This hormetic response is lost at very high levels of both nutrients, which synergistically attenuate insulin secretion and progressively causes apoptotic beta cell death. These hormetic and detrimental processes are tightly linked to membrane phospholipid remodeling and enhanced lipid peroxidation. The basis of these interactions includes enzymatic and non-enzymatic metabolism of polyunsaturated fatty acids and the emergence of hormetic mediators (e.g., 4‑hydroxynonenal) that act at low levels as activating ligands of PPARδ and as cytotoxic agents at high levels. In addition, monounsaturated fatty acids protect beta cells by enhancing the incorporation of saturated fatty acids into triglycerides and their sequestration in newly formed lipid droplets.

Discovery of novel 5-oxa-2,6-diazaspiro[3.4]oct-6-ene derivatives as potent, selective, and orally available somatostatin receptor subtype 5 (SSTR5) antagonists for treatment of type 2 diabetes mellitus

Somatostatin receptor subtype 5 (SSTR5) has emerged as a novel attractive drug target for type 2 diabetes mellitus. Starting from N-benzyl azetidine derivatives 1 and 2 as in-house hit compounds, we explored the introduction of a carboxyl group into the terminal benzene of 1 to enhance SSTR5 antagonistic activity by the combination of the substituents at the 3-position of the isoxazoline. Incorporation of a carboxyl group at the 4-position of the benzene ring resulted in a significant enhancement in potency, however, the 4-benzoic acid derivative 10c exhibited moderate human ether-a-go-go related gene (hERG) inhibitory activity. A subsequent optimization study revealed that replacement of the 4-benzoic acid with an isonipecotic acid dramatically reduced hERG inhibition (5.6% inhibition at 30μM) by eliminating π-related interaction with hERG K+ channel, which resulted in the identification of 1-(2-((2,6-diethoxy-4′-fluorobiphenyl-4-yl)methyl)-5-oxa-2,6-diazaspiro[3.4]oct-6-en-7-yl)piperidin-4-carboxylic acid 25a (hSSTR5/mSSTR5 IC50=9.6/57nM). Oral administration of 25a in high-fat diet fed C57BL/6J mice augmented insulin secretion in a glucose-dependent manner and lowered blood glucose concentration.

Sunitinib specifically augments glucose-induced insulin secretion

The tyrosine kinase inhibitor sunitinib is used for the treatment of numerous cancers in humans. In diabetic patients, sunitinib lowers blood glucose levels and improves glycaemic control. This study aims to analyse whether sunitinib has specific and direct effects on insulin secreting β-cells. Regulation of insulin secretion, of cellular cAMP levels and activation of signalling pathways were examined upon exposure of rat insulinoma INS-1E cells to sunitinib under specific stimulatory and inhibitory conditions. Secreted insulin and cellular cAMP levels were measured using RIA and ELISA, respectively. Protein phosphorylations were examined on western blots. Sunitinib enhanced glucose-induced insulin secretion (GIIS) concentration-dependently, reaching a maximal stimulation at 2μM. Sunitinib further augmented insulin secretion in the presence of elevated cAMP levels and the FFAR1 agonists. Adrenaline and the PKA inhibitor H89 counteracted the stimulatory effect of sunitinib on secretion. However, sunitinib altered neither the cellular levels of cAMP nor the phosphorylation of PKA. Sunitinib did not reduce IGF-1-induced phosphorylation of AKT/PKB and ERK1/2. In conclusion, these results suggest that sunitinib stimulates GIIS by a direct effect on β-cells, which may contribute to the glucose-lowering action of the tyrosine kinase inhibitor in humans.

Cutting the brakes to prevent Type 2 Diabetes onset: augmenting insulin secretion and conserving pancreatic beta‐cell mass by deleting Gαz

The hyperglycemia of Type 2 Diabetes (T2D) results from a combination of factors, including insulin resistance, impaired and inefficient insulin secretion, and destruction of pancreatic beta‐cells. The second messenger molecule, cyclic AMP (cAMP), amplifies glucose‐stimulated insulin secretion and has been suggested to preserve beta‐cell mass. Pathways that inhibit beta‐cell cAMP production are pathologically upregulated in T2D. One such pathway is mediated by Prostaglandin E2 (PGE2) receptor 3 (EP3), a G protein‐coupled receptor whose expression, as well as that of its natural ligand, PGE2, are both up‐regulated in islets collected from T2D mice and humans. While EP3 is ubiquitously expressed, in the beta‐cell it couples to a unique and non‐ubiquitously expressed G protein, Gz. We hypothesized the genetic elimination of the catalytic a‐subunit of Gz (Gαz) would augment beta‐cell function and mass, preventing or delaying the onset of hyperglycemia in the BTBR ob/ob mouse model of T2D. As expected, Gαz‐null ob/ob mice were as obese and as insulin resistant as wild‐type ob/ob mice, consistent with little to no liver, adipose, or muscle Gaz expression, but displayed near‐normal fasting blood glucose levels. Fasting euglycemia in Gaz‐null ob/ob mice correlated directly with elevated plasma insulin levels, suggesting that Gαz‐null ob/ob beta‐cells were able to better compensate for the increased insulin demand of obesity and insulin resistance. To test this directly, we isolated islets from Gaz‐null and wild‐type ob/ob mice, and found higher insulin content and insulin secretion from Gaz‐null ob/ob islets. To further elucidate the molecular mechanisms augmenting beta cell function in Gaz‐null ob/ob islets, we considered the influence of Gaz on glucagon‐like peptide‐1 (GLP‐1), which is a cAMP‐stimulatory gut/islet hormone. We postulated that, in the beta‐cell, Gaz signaling competes with GLP‐1 receptor signling to block the full effects of GLP‐1 on beta‐cell function and/or mass. To test this hypothesis, we subjected wild‐type and Gaz‐null ob/ob mice to intraperitoneal and oral glucose challenge, and found the Gaz‐null mutation partially restored glucose tolerance after an oral glucose challenge but not an intraperitoneal glucose challenge. Plasma levels of the GLP‐1 were not different between the genotypes, but intra‐islet GLP‐1 produced and secreted by alpha cells was altered in Gaz‐null ob/ob islets. Increased GLP‐1 responsiveness of Gaz‐null ob/ob islets was tested directly ex‐vivo by treatment with the stable GLP‐1 analog, exendin‐4. In sum, we identified a key role for Gαz in the islet pathophysiology of T2D and confirmed that deleting Gaz allowed for a more efficient response of the beta‐cell to a physiologic potentiator of insulin secretion and beta‐cell mass, GLP‐1.Support or Funding InformationMolecular and Applied Nutrition Training Program T32

Pancreas and Not Gut Mediates the GLP-1-Induced Glucoincretin Effect

The gut is believed to be the source of GLP-1 that augments insulin secretion in response to oral nutrients. In this issue of Cell Metabolism, Chambers etal. (2017) shift the paradigm by finding that GLP-1 produced within the islets of the pancreas, and not the gut, is responsible for the incretin effect in mice.

Pharmacological approach to understanding the control of insulin secretion in human islets.

To understand better the control of insulin secretion by human β cells and to identify similarities to and differences from rodent models. Dynamic insulin secretion was measured in perifused human islets treated with pharmacological agents of known modes of action. Glucokinase activation (Ro28-1675) lowered the glucose threshold for stimulation of insulin secretion to 1 mmol/L (G1), augmented the response to G3-G5 but not to G8-G15, whereas tolbutamide remained active in G20, which indicates that not all KATP channels were closed by high glucose concentrations. An almost 2-fold greater response to G15 than to supramaximal tolbutamide in G3 or to KCl+diazoxide in G15 vs G3 quantified the contribution of metabolic amplification to insulin secretion. Both disruption (latrunculin-B) and stabilization (jasplakinolide) of microfilaments augmented insulin secretion without affecting metabolic amplification. Tolbutamide-induced insulin secretion was consistently greater in G10 than G3, with a threshold at 1 and maximum at 10 µmol/L tolbutamide in G10, vs 10 and 25 µmol/L in G3. Sulphonylurea effects were thus clearly glucose-dependent. Insulin secretion was also increased by inhibiting K channels other than KATP channels: Kv or BK channels (tetraethylammonium), TASK-1 channels (ML-365) and SK4 channels (TRAM-34). Opening KATP channels with diazoxide inhibited glucose-induced insulin secretion with half maximum inhibitory concentrations of 9.6 and 24 µmol/L at G7 and G15. Blockade of L-type Ca channels (nimodipine) abolished insulin secretion, whereas a blocker of T-type Ca channels (NNC-55-0396) was ineffective at specific concentrations. Blockade of Na channels (tetrodotoxin) did not affect glucose-induced insulin secretion. In addition to sharing a KATP channel-dependent triggering pathway and a metabolic amplifying pathway, human and rodent β cells were found to display more similarities than differences in the control of insulin secretion.