First-phase Glucose-stimulated Insulin Secretion Research Articles

Renal tubule-specific Atgl deletion links kidney lipid metabolism to glucagon-like peptide 1 and insulin secretion independent of renal inflammation or lipotoxicity

ObjectiveLipotoxic injury from renal lipid accumulation in obesity and type 2 diabetes (T2D) is implicated in associated kidney damage. However, models examining effects of renal ectopic lipid accumulation independent of obesity or T2D are lacking. We generated renal tubule-specific adipose triglyceride lipase knockout (RT-SAKO) mice to determine if this targeted triacylglycerol (TAG) over-storage affects glycemic control and kidney health. MethodsMale and female RT-SAKO mice and their control littermates were tested for changes in glycemic control at 10–12 and 16–18 weeks of age. Markers of kidney health and blood lipid and hormone concentrations were analyzed. Kidney and blood lysophosphatidic acid (LPA) levels were measured, and a role for LPA in mediating impaired glycemic control was evaluated using the LPA receptor 1/3 inhibitor Ki-16425. ResultsAll groups remained insulin sensitive, but 16- to 18-week-old male RT-SAKO mice became glucose intolerant, without developing kidney inflammation or fibrosis. Rather, these mice displayed lower circulating insulin and glucagon-like peptide 1 (GLP-1) levels. Impaired first-phase glucose-stimulated insulin secretion was detected and restored by Exendin-4. Kidney and blood LPA levels were elevated in older male but not female RT-SAKO mice, associated with increased kidney diacylglycerol kinase epsilon. Inhibition of LPA-mediated signaling restored serum GLP-1 levels, first-phase insulin secretion, and glucose tolerance. ConclusionsTAG over-storage alone is insufficient to cause renal tubule lipotoxicity. This work is the first to show that endogenously derived LPA modulates GLP-1 levels in vivo, demonstrating a new mechanism of kidney-gut-pancreas crosstalk to regulate insulin secretion and glucose homeostasis.

Insulin C-peptide secretion on-a-chip to measure the dynamics of secretion and metabolism from individual islets

First-phase glucose-stimulated insulin secretion is mechanistically linked to type 2 diabetes, yet the underlying metabolism is difficult to discern due to significant islet-to-islet variability. Here, we miniaturize a fluorescence anisotropy immunoassay onto a microfluidic device to measure C-peptide secretion from individual islets as a surrogate for insulin (InsC-chip). This method measures secretion from up to four islets at a time with ∼7s resolution while providing an optical window for real-time live-cell imaging. Using the InsC-chip, we reveal two glucose-dependent peaks of insulin secretion (i.e., a double peak) within the classically defined 1st phase (<10min). By combining real-time secretion and live-cell imaging, we show islets transition from glycolytic to oxidative phosphorylation (OxPhos)-driven metabolism at the nadir of the peaks. Overall, these data validate the InsC-chip to measure glucose-stimulated insulin secretion while revealing new dynamics in secretion defined by a shift in glucose metabolism.

Mitoribosome insufficiency in β cells is associated with type 2 diabetes-like islet failure

Genetic variations in mitoribosomal subunits and mitochondrial transcription factors are related to type 2 diabetes. However, the role of islet mitoribosomes in the development of type 2 diabetes has not been determined. We investigated the effects of the mitoribosomal gene on β-cell function and glucose homeostasis. Mitoribosomal gene expression was analyzed in datasets from the NCBI GEO website (GSE25724, GSE76894, and GSE76895) and the European Nucleotide Archive (ERP017126), which contain the transcriptomes of type 2 diabetic and nondiabetic organ donors. We found deregulation of most mitoribosomal genes in islets from individuals with type 2 diabetes, including partial downregulation of CRIF1. The phenotypes of haploinsufficiency in a single mitoribosomal gene were examined using β-cell-specific Crif1 (Mrpl59) heterozygous-deficient mice. Crif1beta+/− mice had normal glucose tolerance, but their islets showed a loss of first-phase glucose-stimulated insulin secretion. They also showed increased β-cell mass associated with higher expression of Reg family genes. However, Crif1beta+/− mice showed earlier islet failure in response to high-fat feeding, which was exacerbated by aging. Haploinsufficiency of a single mitoribosomal gene predisposes rodents to glucose intolerance, which resembles the early stages of type 2 diabetes in humans.

Hyperinsulinemia: beneficial or harmful or both on glucose homeostasis.

Insulin, a principal anabolic hormone produced by pancreatic β-cells, has a primary function of storage of nutrients following excessive energy intake. Pre- or early type 2 diabetes stages present hyperinsulinemia (β-cell dysfunction) and insulin resistance. Initiation of hyperinsulinemia is triggered by a loss of first-phase glucose-stimulated insulin secretion with altered membrane ion channel distribution. More factors, including insulin resistance and excessive proliferation of β-cells, deteriorate the hyperinsulinemia, whereas the hyperinsulinemia contributes to further development of insulin resistance and type 2 diabetes; to develop eventually late-stage diabetes with absolute insulin deficiency. In this mini-review, the major focus was put on the causes and pathophysiology of hyperinsulinemia, and the metabolic consequences and current treatment of hyperinsulinemia were discussed. The data used in this narrative review were collected mainly from relevant discoveries in the past 3 years.

Intra-islet glucagon confers β-cell glucose competence for first-phase insulin secretion and favors GLP-1R stimulation by exogenous glucagon

We report that intra-islet glucagon secreted from α-cells signals through β-cell glucagon and GLP-1 receptors (GcgR and GLP-1R), thereby conferring to rat islets their competence to exhibit first-phase glucose-stimulated insulin secretion (GSIS). Thus, in islets not treated with exogenous glucagon or GLP-1, first-phase GSIS is abolished by a GcgR antagonist (LY2786890) or a GLP-1R antagonist (Ex[9–39]). Mechanistically, glucose competence in response to intra-islet glucagon is conditional on β-cell cAMP signaling because it is blocked by the cAMP antagonist prodrug Rp-8-Br-cAMPS-pAB. In its role as a paracrine hormone, intra-islet glucagon binds with high affinity to the GcgR, while also exerting a “spillover” effect to bind with low affinity to the GLP-1R. This produces a right shift of the concentration-response relationship for the potentiation of GSIS by exogenous glucagon. Thus, 0.3 nM glucagon fails to potentiate GSIS, as expected if similar concentrations of intra-islet glucagon already occupy the GcgR. However, 10 to 30 nM glucagon effectively engages the β-cell GLP-1R to potentiate GSIS, an action blocked by Ex[9–39] but not LY2786890. Finally, we report that the action of intra-islet glucagon to support insulin secretion requires a step-wise increase of glucose concentration to trigger first-phase GSIS. It is not measurable when GSIS is stimulated by a gradient of increasing glucose concentrations, as occurs during an oral glucose tolerance test in vivo. Collectively, such findings are understandable if defective intra-islet glucagon action contributes to the characteristic loss of first-phase GSIS in an intravenous glucose tolerance test, that is, diagnostic of type 2 diabetes in the clinical setting.

Microtubules regulate pancreatic β-cell heterogeneity via spatiotemporal control of insulin secretion hot spots.

Heterogeneity of glucose-stimulated insulin secretion (GSIS) in pancreatic islets is physiologically important but poorly understood. Here, we utilize mouse islets to determine how microtubules (MTs) affect secretion toward the vascular extracellular matrix at single cell and subcellular levels. Our data indicate that MT stability in the β-cell population is heterogenous, and that GSIS is suppressed in cells with highly stable MTs. Consistently, MT hyper-stabilization prevents, and MT depolymerization promotes the capacity of single β-cell for GSIS. Analysis of spatiotemporal patterns of secretion events shows that MT depolymerization activates otherwise dormant β-cells via initiation of secretion clusters (hot spots). MT depolymerization also enhances secretion from individual cells, introducing both additional clusters and scattered events. Interestingly, without MTs, the timing of clustered secretion is dysregulated, extending the first phase of GSIS and causing oversecretion. In contrast, glucose-induced Ca2+ influx was not affected by MT depolymerization yet required for secretion under these conditions, indicating that MT-dependent regulation of secretion hot spots acts in parallel with Ca2+ signaling. Our findings uncover a novel MT function in tuning insulin secretion hot spots, which leads to accurately measured and timed response to glucose stimuli and promotes functional β-cell heterogeneity.

The adaptor protein APPL2 controls glucose-stimulated insulin secretion via F-actin remodeling in pancreatic β-cells

Filamentous actin (F-actin) cytoskeletal remodeling is critical for glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells, and its dysregulation causes type 2 diabetes. The adaptor protein APPL1 promotes first-phase GSIS by up-regulating soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein expression. However, whether APPL2 (a close homology of APPL1 with the same domain organization) plays a role in β-cell functions is unknown. Here, we show that APPL2 enhances GSIS by promoting F-actin remodeling via the small GTPase Rac1 in pancreatic β-cells. β-cell specific abrogation of APPL2 impaired GSIS, leading to glucose intolerance in mice. APPL2 deficiency largely abolished glucose-induced first- and second-phase insulin secretion in pancreatic islets. Real-time live-cell imaging and phalloidin staining revealed that APPL2 deficiency abolished glucose-induced F-actin depolymerization in pancreatic islets. Likewise, knockdown of APPL2 expression impaired glucose-stimulated F-actin depolymerization and subsequent insulin secretion in INS-1E cells, which were attributable to the impairment of Ras-related C3 botulinum toxin substrate 1 (Rac1) activation. Treatment with the F-actin depolymerization chemical compounds or overexpression of gelsolin (a F-actin remodeling protein) rescued APPL2 deficiency-induced defective GSIS. In addition, APPL2 interacted with Rac GTPase activating protein 1 (RacGAP1) in a glucose-dependent manner via the bin/amphiphysin/rvs-pleckstrin homology (BAR-PH) domain of APPL2 in INS-1E cells and HEK293 cells. Concomitant knockdown of RacGAP1 expression reverted APPL2 deficiency-induced defective GSIS, F-actin remodeling, and Rac1 activation in INS-1E cells. Our data indicate that APPL2 interacts with RacGAP1 and suppresses its negative action on Rac1 activity and F-actin depolymerization thereby enhancing GSIS in pancreatic β-cells.

XPR1 Mediates the Pancreatic β-Cell Phosphate Flush.

Glucose-stimulated insulin secretion is the hallmark of the pancreatic β-cell, a critical player in the regulation of blood glucose concentration. In 1974, the remarkable observation was made that an efflux of intracellular inorganic phosphate (Pi) accompanied the events of stimulated insulin secretion. The mechanism behind this “phosphate flush,” its association with insulin secretion, and its regulation have since then remained a mystery. We recapitulated the phosphate flush in the MIN6m9 β-cell line and pseudoislets. We demonstrated that knockdown of XPR1, a phosphate transporter present in MIN6m9 cells and pancreatic islets, prevented this flush. Concomitantly, XPR1 silencing led to intracellular Pi accumulation and a potential impact on Ca2+ signaling. XPR1 knockdown slightly blunted first-phase glucose-stimulated insulin secretion in MIN6m9 cells, but had no significant impact on pseudoislet secretion. In keeping with other cell types, basal Pi efflux was stimulated by inositol pyrophosphates, and basal intracellular Pi accumulated following knockdown of inositol hexakisphosphate kinases. However, the glucose-driven phosphate flush occurred despite inositol pyrophosphate depletion. Finally, while it is unlikely that XPR1 directly affects exocytosis, it may protect Ca2+ signaling. Thus, we have revealed XPR1 as the missing mediator of the phosphate flush, shedding light on a 45-year-old mystery.

Decreased Plasma Maresin 1 Concentration Is Associated with Diabetic Foot Ulcer.

Aims To assess the maresin 1 (MaR1) contents in type 2 diabetic patients with or without diabetic foot ulcer and to analyze the association of MaR1 concentrations with several metabolism-related parameters. Methods Plasma MaR1 concentrations were analyzed in 96 subjects with normal glucose tolerant (NC, n = 43), type 2 diabetes (T2DM, n = 40), or diabetic foot ulcer (DFU, n = 13). The intravenous glucose tolerance test (IVGTT) and biochemical parameters were measured in all participants. Results Plasma MaR1 concentrations were significant decreased in type 2 diabetes patient with or without DFU compared with NC (both P < 0.001) and were lowest in DFU patients among these 3 groups. (DFU vs. T2DM, P < 0.05). Plasma MaR1 concentrations were negatively correlated with BMI, waist circumference (Wc), waist hip ratio (WHR), systolic blood pressure (SBP), diastolic blood pressure (DBP), LDL-c, FPG, 2hPG, HbA1c, and homeostasis model assessment for insulin resistance (HOMA-IR) (all P < 0.05) and were positively correlated with HDL-c, acute insulin response (AIR), area under the curve of the first-phase (0-10 min) insulin secretion (AUC), and homeostasis model assessment for beta-cell function (HOMA-β) (all P < 0.05). After adjusting for age and sex, Wc, WHR, TG, FPG, 2hPG, HbA1c, HOMA-IR, AIR, AUC, and HOMA-β remain statistically significant (all P < 0.05). Conclusions Plasma MaR1 concentration were decreased in T2DM with or without DFUs and were the lowest in DFU patients. The decreased plasma MaR1 strongly associated with obesity, impaired glucose and lipid metabolism, reduced first-phase of glucose-stimulated insulin secretion, and enhanced insulin resistance.

DOC2b Protects Functional Beta‐cell Mass via A Novel Tyrosine Phosphorylation Dependent Mechanism

Type 2 diabetes (T2D) has reached the stage of a global pandemic. Approximately 10% of the total US population (~30 million people) are impacted by T2D and 86 million more are in the stage of prediabetes. Accumulating evidence suggests that intrinsic beta‐cell stresses, secretory dysfunction, and inflammation are the early players during the progression of the T2D and leads to the loss of functional beta‐cell mass. Hence, reduction of cell stress and enhancement of beta‐cell secretory function hold promise towards delayed disease progression and prevention. Our lab has revealed the crucial role of the SNARE‐regulator protein DOC2b (double C2 domain‐containing protein) on stimulating the glucose‐stimulated insulin secretion (GSIS) and protecting the beta‐cell from the diabetogenic stresses. The objective of this study is to develop DOC2b as a therapeutic target for the treatment of T2D by elucidating the mechanism by which DOC2b preserves functional beta‐cell mass. We hypothesize that tyrosine phosphorylation of DOC2b’s functionally indispensable C2AB domain is required for DOC2b’s beneficial function during the pathological conditions related to T2D.To test this hypothesis we have used human and rodent clonal beta‐cell lines and primary human islets procured from non‐diabetic and T2D donors. Our results showed a marked loss of DOC2b mRNA and protein (~50% reduction) in the T2D human islets and a resurrected first phase of GSIS in otherwise dysfunctional T2D islets following DOC2b overexpression. Multiple studies confirmed elevated expressions of the CXCL9 and 10 (chemokine receptor 3 ligands) in diabetics. Our qPCR and biochemical analysis revealed significant attenuation of cytokine‐induced elevation of CXCL9 and 10 gene expressions following DOC2b overexpression in the clonal and primary beta‐cells. These results again signified the beneficial effect of DOC2b enrichment in the beta‐cell. In islet beta‐cells, glucose‐stimulated tyrosine phosphorylation events regulate SNARE complex assembly and GSIS. Using biochemical approaches, we determined that DOC2b is phosphorylated following glucose stimulation and pervanadate (a tyrosine phosphatase inhibitor) treatment in the clonal beta‐cells. In silico evaluation from the crystal structure of the C2B domain of DOC2b protein revealed three putative tyrosine phosphorylation sites: Y301 (within the loop region), and Y305, Y309 (within the ordered domain) and our results using point mutants of each putative site (Y to F) indicated the requirement for all three phosphorylation sites for the boosting effect of DOC2b on GSIS in the beta‐cells. We further identified that the mutations in at least Y301 and Y305 residues significantly blunted DOC2b’s ability to protect beta‐cells from cytokine‐induced beta‐cell stress and apoptosis. These results unveil a novel mechanism by which DOC2b enrichment boosts GSIS and protects beta‐cells against diabetogenic stimuli, the knowledge of which would be crucial for pursuing DOC2b as a novel drug target to prevent and/treat diabetes.Support or Funding Information5R01DK112917‐02

Roux-en-Y gastric bypass enhances insulin secretion in type 2 diabetes via FXR-mediated TRPA1 expression

ObjectiveRoux-en-Y gastric bypass surgery (RYGB) improves the first phase of glucose-stimulated insulin secretion (GSIS) in patients with type 2 diabetes. How it does so remains unclear. Farnesoid X receptor (FXR), the nuclear receptor of bile acids (BAs), is implicated in bariatric surgery. Moreover, the transient receptor potential ankyrin 1 (TRPA1) channel is expressed in pancreatic β-cells and involved in insulin secretion. We aimed to explore the role of BAs/FXR and TRPA1 in improved GSIS in diabetic rats after RYGB. MethodsRYGB or sham surgery was conducted in spontaneous diabetic Goto-Kakizaki (GK) rats, or FXR or TRPA1 transgenic mice. Gene and protein expression of islets were assessed by qPCR and western blotting. Electrophysiological properties of single β-cells were studied using patch-clamp technique. Binding of FXR and histone acetyltransferase steroid receptor coactivator-1 (SRC1) to the TRPA1 promoter, acetylated histone H3 (ACH3) levels at the TRPA1 promoter were determined using ChIP assays. GSIS was measured using enzyme-linked immunosorbent assays or intravenous glucose tolerance test (IVGTT). ResultsRYGB increases GSIS, particularly the first-phase of GSIS in both intact islets and GK rats in vivo, and ameliorates hyperglycemia of GK rats. Importantly, the effects of RYGB were attenuated in TRPA1-deficient mice. Moreover, GK β-cells displayed significantly decreased TRPA1 expression and current. Patch-clamp recording revealed that TRPA1−/− β-cells displayed a marked hyperpolarization and decreased glucose-evoked action potential firing, which was associated with impaired GSIS. RYGB restored TRPA1 expression and current in GK β-cells. This was accompanied by improved glucose-evoked electrical activity and insulin secretion. Additionally, RYGB-induced TRPA1 expression involved BAs/FXR-mediated recruitment of SRC1, promoting ACH3 at the promoter of TRPA1. ConclusionsThe BAs/FXR/SRC1 axis-mediated restoration of TRPA1 expression plays a critical role in the enhanced GSIS and remission of diabetes in GK rats after RYGB.

Targeted Derivation of Organotypic Glucose- and GLP-1-Responsive β Cells Prior to Transplantation into Diabetic Recipients.

SummaryGeneration of functional β cells from pluripotent sources would accelerate diagnostic and therapeutic applications for diabetes research and therapy. However, it has been challenging to generate competent β cells with dynamic insulin-secretory capacity to glucose and incretin stimulations. We introduced transcription factors, critical for β-cell development and function, in differentiating human induced pluripotent stem cells (PSCs) and assessed the impact on the functionality of derived β-cell (psBC) progeny. A perifusion system revealed stepwise transduction of the PDX1, NEUROG3, and MAFA triad (PNM) enabled in vitro generation of psBCs with glucose and GLP-1 responsiveness within 3 weeks. PNM transduction upregulated genes associated with glucose sensing, insulin secretion, and β-cell maturation. In recipient diabetic mice, PNM-transduced psBCs showed glucose-responsive insulin secretion as early as 1 week post transplantation. Thus, enhanced pre-emptive β-cell specification of PSCs by PNM drives generation of glucose- and incretin-responsive psBCs in vitro, offering a competent tissue-primed biotherapy.

Retinol Binding Protein 4 Impairs Pancreatic Beta-Cell Function, Leading to the Development of Type 2 Diabetes

Circulating Retinol binding protein 4 (RBP4), the specific transport protein for vitamin A, has been implicated in insulin resistance and type 2 diabetes. Pancreatic beta cell failure plays a decisive role in the onset and the progression of type 2 diabetes. However, the effect of RBP4 on beta cell function and the molecular mechanisms are far from clear. The direct effect of RBP4 on islet function was assessed by treating primary isolated islets from C57BL/J mice with different dose of RBP4 (0, 20, 40, 60 μg/ml) for 24 hour or with 60ug/ml RBP4 for different time points (0, 12, 18,24 hours). We found that RBP4 could dose and time dependently inhibit the glucose stimulated insulin secretion (GSIS). To further identify the role of RBP4 on islet beta cell function. RBP4-overexpressing mice (RBP4-Tg) were generated. Compared to the wild type group, the islets isolated from RBP4-Tg mice showed a significant decrease of insulin secretion in response to glucose (∼50%) and KCl (∼70%). The Dynamic insulin secretion of the isolated islets of RBP4-tg mice in response to glucose was further characterized. RBP4 could dramatically inhibit first-phase GSIS by up to 80%. In consistent to these in vitro findings, RBP4-tg mice showed a progressive decrease of GSIS (8weeks, 12weeks, 16weeks) which appeared as early as 8weeks. While the glucose tolerance of these mice was not significantly impaired until 12 weeks by intraperitoneal glucose tolerance test and the insulin sensitivity was not changed until 16 weeks by intraperitoneal insulin tolerance test respectively. In conclusion, RBP4 could impair pancreatic beta cell function which may lead to the onset and the development of T2DM. Disclosure R. Huang: None. X. Bai: None. X. Li: None. L. Zhao: None. M. Xia: None.

Plasma fetuin-B concentrations are associated with insulin resistance and first-phase glucose-stimulated insulin secretion in individuals with different degrees of glucose tolerance

AimsTo assess circulating fetuin-B concentrations in subjects with different degrees of glucose tolerance and to analyze the association of fetuin-B concentrations with insulin resistance and the first phase of glucose-stimulated insulin secretion. MethodsPlasma fetuin-B concentrations were analyzed in 149 subjects with normal glucose tolerance (NGT, n=54), impaired glucose regulation (preDM, n=42) and newly diagnosed type-2 diabetes mellitus (nT2DM, n=53). Intravenous glucose tolerance tests (IVGTTs) and biochemical parameters were also assessed in all participants. ResultsPlasma fetuin-B concentrations were significantly higher in nT2DM patients compared with NGT and preDM subjects (both P<0.001) and positively correlated with FPG, 2hPG, HOMA-IR, HbA1c, hsCRP, FINS and TG (P<0.05), but negatively correlated with AIR, AUC, GDI and fasting Belfiore index (P<0.01). After adjusting for age and gender, all correlations remained statistically significant (P<0.05). Multivariate logistic regression analysis revealed that plasma fetuin-B concentrations were significantly correlated with nT2DM after controlling for age, gender, BMI, WHR, blood pressure and lipid profiles. ConclusionPatients with nT2DM have significantly higher concentrations of plasma fetuin-B compared with NGT subjects and plasma fetuin-B is strongly associated with glucose and lipid metabolism, chronic inflammation and first-phase glucose-stimulated insulin secretion and insulin resistance.

New Roles of Syntaxin-1A in Insulin Granule Exocytosis and Replenishment

In type-2 diabetes (T2D), severely reduced islet syntaxin-1A (Syn-1A) levels contribute to insulin secretory deficiency. We generated β-cell-specific Syn-1A-KO (Syn-1A-βKO) mice to mimic β-cell Syn-1A deficiency in T2D. Glucose tolerance tests showed that Syn-1A-βKO mice exhibited blood glucose elevation corresponding to reduced blood insulin levels. Perifusion of Syn-1A-βKO islets showed impaired first- and second-phase glucose-stimulated insulin secretion (GSIS) resulting from reduction in readily releasable pool and granule pool refilling. To unequivocally determine the β-cell exocytotic defects caused by Syn-1A deletion, EM and total internal reflection fluorescence microscopy showed that Syn-1A-KO β-cells had a severe reduction in the number of secretory granules (SGs) docked onto the plasma membrane (PM) at rest and reduced SG recruitment to the PM after glucose stimulation, the latter indicating defects in replenishment of releasable pools required to sustain second-phase GSIS. Whereas reduced predocked SG fusion accounted for reduced first-phase GSIS, selective reduction of exocytosis of short-dock (but not no-dock) newcomer SGs accounted for the reduced second-phase GSIS. These Syn-1A actions on newcomer SGs were partly mediated by Syn-1A interactions with newcomer SG VAMP8.

Hypothalamic AMP-Activated Protein Kinase Regulates Biphasic Insulin Secretion from Pancreatic β Cells during Fasting and in Type 2 Diabetes

Glucose-stimulated insulin secretion (GSIS) by pancreatic β cells is biphasic. However, the physiological significance of biphasic GSIS and its relationship to diabetes are not yet fully understood. This study demonstrated that impaired first-phase GSIS follows fasting, leading to increased blood glucose levels and brain glucose distribution in humans. Animal experiments to determine a possible network between the brain and β cells revealed that fasting-dependent hyperactivation of AMP-activated protein kinase in the hypothalamus inhibited first-phase GSIS by stimulating the α-adrenergic pancreatic nerve. Furthermore, abnormal excitability of this brain-β cell neural axis was involved in diabetes-related impairment of first-phase GSIS in diabetic animals. Finally, pancreatic denervation improved first-phase GSIS and glucose tolerance and ameliorated severe diabetes by preventing β cell loss in diabetic animals. These results indicate that impaired first-phase GSIS is critical for brain distribution of dietary glucose after fasting. Furthermore, β cells in individuals with diabetes mistakenly sense that they are under conditions that mimic prolonged fasting. The present study provides additional insight into both β cell physiology and the pathogenesis of β cell dysfunction in type 2 diabetes.

Relationship between serum betatrophin levels and the first-phase of glucose-stimulated insulin secretion

To examine circulating betatrophin concentrations in subjects with different glucose tolerance status and to investigate the relationship between serum betatrophin levels and first-phase of glucose-stimulated insulin secretion. Serum betatrophin concentrations were measured in 110 age- and sex-matched subjects: 47 newly diagnosed type 2 diabetes mellitus (T2DM), 29 impaired glucose tolerance (IGT) and 34 normal glucose tolerance (NGT). Oral glucose tolerance test and intravenous glucose tolerance test were performed to assess glucose tolerance and first-phase of glucose-stimulated insulin secretion. Serum betatrophin levels were significantly higher in the T2DM and IGT group than in the NGT group (2.10±1.16ng/mL vs 0.77±0.44ng/mL, 1.73±1.28ng/mL vs 0.77±0.44ng/mL; P<0.01). The AIR and AUC among the three groups showed a progressive decrease from the NGT to IGT group with the lowest value in the T2DM group (P<0.01). Betatrophin were found to be positively correlated with BMI, waist circumference (WC), homeostatic model assessment of insulin resistance (HOMA-IR) and triglyceride (TG), and inversely associated with HDL-c (all P<0.01), but not significantly correlated with 0-10min insulin the area under the curve (AUC) and acute insulin response (AIR) (P>0.05). Stepwise multiple regression analysis showed that HOMA-IR and TG were independently related to betatrophin (P<0.05). Serum betatrophin concentrations were higher in T2DM and IGT, and were closely related to glucolipid disorder, insulin resistance, but not related to the first-phase of glucose-stimulated insulin secretion.

Lipids and immunoinflammatory pathways of beta cell destruction.

Islet inflammation contributes to beta cell demise in both type 1 and type 2 diabetes. 12-Lipoxygenase (12-LO, gene expressed as ALOX12 in humans and 12-Lo in rodents in this manuscript) produces proinflammatory metabolites such as 12(S)-hydroxyeicosatetraenoic acids through dioxygenation of polyunsaturated fatty acids. 12-LO was first implicated in diabetes when the increase in 12-Lo expression and 12(S)-hydroxyeicosatetraenoic acid was noted in rodent models of diabetes. Subsequently, germline 12-Lo (-/-) was shown to prevent the development of hyperglycemia in mouse models of type 1 diabetes and in high-fat fed mice. More recently, beta cell-specific 12-Lo (-/-) was shown to protect mice against hyperglycaemia after streptozotocin and a high-fat diet. In humans, 12-LO expression is increased in pancreatic islets of autoantibody-positive, type 1 diabetic and type 2 diabetic organ donors. Interestingly, the high expression of ALOX12 is associated with the alteration in first-phase glucose-stimulated insulin secretion in human type 2 diabetic islets. To further clarify the role of islet 12-LO in diabetes and to validate 12-LO as a therapeutic target of diabetes, we have studied selective pharmacological inhibitors for 12-LO. The compounds we have identified show promise: they protect beta cell lines and human islets from apoptosis and preserve insulin secretion when challenged by proinflammatory cytokine mixture. Currently studies are underway to test the compounds in mouse models of diabetes. This review summarises a presentation given at the 'Islet inflammation in type 2 diabetes' symposium at the 2015 annual meeting of the EASD. It is accompanied two other mini-reviews on topics from this symposium (by Simone Baltrusch, DOI: 10.1007/s00125-016-3891-x and Marc Donath, DOI: 10.1007/s00125-016-3873-z ) and a commentary by the Session Chair, Piero Marchetti (DOI: 10.1007/s00125-016-3875-x ).

Syntaxin-3 Binds and Regulates Both R- and L-Type Calcium Channels in Insulin-Secreting INS-1 832/13 Cells.

Syntaxin (Syn)-1A mediates exocytosis of predocked insulin-containing secretory granules (SGs) during first-phase glucose-stimulated insulin secretion (GSIS) in part via its interaction with plasma membrane (PM)-bound L-type voltage-gated calcium channels (Cav). In contrast, Syn-3 mediates exocytosis of newcomer SGs that accounts for second-phase GSIS. We now hypothesize that the newcomer SG Syn-3 preferentially binds and modulates R-type Cav opening, which was postulated to mediate second-phase GSIS. Indeed, glucose-stimulation of pancreatic islet β-cell line INS-1 induced a predominant increase in interaction between Syn-3 and Cavα1 pore-forming subunits of R-type Cav2.3 and to lesser extent L-type Cavs, while confirming the preferential interactions between Syn-1A with L-type (Cav1.2, Cav1.3) Cavs. Consistently, direct binding studies employing heterologous HEK cells confirmed that Syn-3 preferentially binds Cav2.3, whereas Syn-1A prefers L-type Cavs. We then used siRNA knockdown (KD) of Syn-3 in INS-1 to study the endogenous modulatory actions of Syn-3 on Cav channels. Syn-3 KD enhanced Ca2+ currents by 46% attributed mostly to R- and L-type Cavs. Interestingly, while the transmembrane domain of Syn-1A is the putative functional domain modulating Cav activity, it is the cytoplasmic domain of Syn-3 that appears to modulate Cav activity. We conclude that Syn-3 may mimic Syn-1A in the ability to bind and modulate Cavs, but preferring Cav2.3 to perhaps participate in triggering fusion of newcomer insulin SGs during second-phase GSIS.

Relationship between serum secreted Frizzled-related protein 4 and the pancreatic β cell function

Objective To investigate the relationship between serum secreted frizzled-related protein 4(SFRP4) and the first-phase of glucose-stimulated insulin secretion from pancreatic β cell under different glucose tolerance statuses. Methods Fifty-six patients with newly diagnosed type 2 diabetes mellitus(T2DM group), 52 patients with impaired glucose tolerance(IGT group), and 42 subjects with normal glucose tolerance(NGT group)underwent intravenous glucose tolerance test. Fasting serum SFRP4 and interleukin(IL)-1β were assayed by ELISA. Acute insulin response(AIR), the area under the curve of the first-phase(0-10 min)insulin secretion(AUC), glucose disposition index(GDI), homeostasis model assessment for β cell function index(HOMA-β), and insulin resistance index(HOMA-IR)were calculated. Results (1)The levels of SFRP4 and IL-1β in T2DM group and IGT group were significantly higher than that in NGT group[(184.38±61.34 or 141.64±40.46 or 95.46±20.13)ng/ml, P<0.01]. AIR, AUC, and GDI in T2DM group and IGT group were significantly lower than those in NGT group(P<0.01), and these results were more significantly reduced in T2DM group compared with those in IGT group.(2)SFRP4 was negatively correlated with AIR, AUC, GDI, HOMA-β(P<0.01), and positively correlated with fasting plasma glucose, 2 h plasma glucose after glucose loading, HbA1C, IL-1β, and high sensitive C-reactive protein(P<0.01). (3)Multiple stepwise regression analysis showed that AUC, HOMA-IR, and serum IL-1β level were independently associated with SFRP4. Conclusion The concentration of serum SFRP4 is closely correlated with the glycolipid metabolic disorder, the first-phase of glucose-stimulated insulin secretion, and chronic low-grade inflammation. SFRP4 may be involved in the mechanism of β cell dysfunction in type 2 diabetes mellitus. Key words: Secreted frizzled-related protein 4; β-cell function; Intravenous glucose tolerance test; Interleukin-1β; Diabetes mellitus, type 2