Ablation of GPR56 Causes ß-Cell Dysfunction by ATP Loss through Mistargeting of Mitochondrial VDAC1 to the Plasma Membrane.

The activation of G Protein-Coupled Receptor 56 (GPR56), also referred to as Adhesion G-Protein-Coupled Ceceptor G1 (ADGRG1), by Collagen Type III (Coll III) prompts cell growth, proliferation, and survival, among other attributes. We investigated the signaling cascades mediating this functional effect in relation to the mitochondrial outer membrane voltage-dependent anion Channel-1 (VDAC1) expression in pancreatic ß-cells. GPR56KD attenuated the Coll III-induced suppression of P70S6K, JNK, AKT, NFκB, STAT3, and STAT5 phosphorylation/activity in INS-1 cells cultured at 20 mM glucose (glucotoxicity) for 72 h. GPR56-KD also increased Chrebp, Txnip, and Vdac1 while decreasing Vdac2 mRNA expression. In GPR56-KD islet ß-cells, Vdac1 was co-localized with SNAP-25, demonstrating its plasma membrane translocation. This resulted in ATP loss, reduced cAMP production and impaired glucose-stimulated insulin secretion (GSIS) in INS-1 and human EndoC ßH1 cells. The latter defects were reversed by an acute inhibition of VDAC1 with an antibody or the VDAC1 inhibitor VBIT-4. We demonstrate that Coll III potentiates GSIS by increasing cAMP and preserving ß-cell functionality under glucotoxic conditions in a GPR56-dependent manner by attenuating the inflammatory response. These results emphasize GPR56 and VDAC1 as drug targets in conditions with impaired ß-cell function.

Pathophysiology of type 2 diabetes and the impact of altered metabolic interorgan crosstalk.

Diabetes is a complex and multifactorial disease that affects millions of people worldwide, reducing the quality of life significantly, and results in grave consequences for our health care system. In type 2 diabetes (T2D), the lack of ß-cell compensatory mechanisms overcoming peripherally developed insulin resistance is a paramount factor leading to disturbed blood glucose levels and lipid metabolism. Impaired ß-cell functions and insulin resistance have been studied extensively resulting in a good understanding of these pathways but much less is known about interorgan crosstalk, which we define as signaling between tissues by secreted factors. Besides hormones and organokines, dysregulated blood glucose and long-lasting hyperglycemia in T2D is associated with changes in metabolism with metabolites from different tissues contributing to the development of this disease. Recent data suggest that metabolites, such as lipids including free fatty acids and amino acids, play important roles in the interorgan crosstalk during the development of T2D. In general, metabolic remodeling affects physiological homeostasis and impacts the development of T2D. Hence, we highlight the importance of metabolic interorgan crosstalk in this review to gain enhanced knowledge of the pathophysiology of T2D, which may lead to new therapeutic approaches to treat this disease.

Metformin enhances LDL-cholesterol uptake by suppressing the expression of the pro-protein convertase subtilisin/kexin type 9 (PCSK9) in liver cells.

PURPOSE: Metformin (MF) intake associates with reduced levels of circulating low-density lipoprotein-cholesterol (LDL-C). This has been attributed to the activation of AMPK, which differentially regulates the expression of multiple genes involved in cholesterol synthesis and trafficking. However, the exact mechanism underlying the LDL-C lowering effect of MF remains ambiguous. METHODS: MF-treated Hep-G2 and HuH7 cells were evaluated for cell viability and the expression status of key lipid metabolism-related genes along with LDL-C uptake efficiency. RESULTS: MF treatment resulted in decreased expression and secretion of PCSK9, increased expression of LDLR and enhanced LDL-C uptake in hepatocytes. It also resulted in increased expression of activated AMPK (p-AMPK) and decreased expression of SREBP2 and HNF-1α proteins. Transcriptomic analysis of MF-treated Hep-G2 cells confirmed these findings and showed that other key lipid metabolism-related genes including those that encode apolipoproteins (APOB, APOC2, APOC3 and APOE), MTTP and LIPC are downregulated. Lastly, MF treatment associated with reduced HMG-CoA reductase expression and activity. CONCLUSIONS: These findings suggest that MF treatment reduces circulating LDL-C levels by suppressing PCSK9 expression and enhancing LDLR expression; hence the potential therapeutic utility of MF in hypercholesterolemia.

Dimethyloxalylglycine (DMOG) and the Caspase Inhibitor "Ac-LETD-CHO" Protect Neuronal ND7/23 Cells of Gluocotoxicity.

It well known that long-lasting hyperglycaemia disrupts neuronal function and leads to neuropathy and other neurodegenerative diseases. The α-ketoglutarate analogue (DMOG) and the caspase-inhibitor "Ac-LETD-CHO are potential neuroprotective molecules. Whether their protections may also extend glucotoxicity-induced neuropathy is not known. Herein, we evaluated the possible cell-protective effects of DMOG and Ac-LETD-CHO against hyperglycaemia-induced reactive oxygen species and apoptosis in ND7/23 neuronal cells. The impact of glucotoxicity on the expression of HIF-1α and a panel of micro-RNAs of significance in hyperglycaemia and apoptosis was also investigated.ND7/23 cells cultured under hyperglycaemic conditions showed decreased cell viability and elevated levels of ROS production in a dose- and time-dependent manner. However, presence DMOG (500 µM) and/or Ac-LETD-CHO (50 µM) counteracted this effect and increase cell viability concomitant with reduction in ROS production, DNA damage and apoptosis. AcLETD-CHO suppressed hyperglycaemia-induced caspase 3 activation in ND7/23 cells. Both DMOG and Ac-LETD-CHO increased HIF-1α expression paralleled with the suppression of miR-126-5p, miR-128-3p and miR-181 expression and upregulation of miR-26b, 106a-5p, 106b-5p, 135a-5p, 135b-5p, 138-5p, 199a-5p, 200a-3p and 200c-3p expression.We demonstrate a mechanistic link for the DMOG and Ac-LETD-CHO protection against hyperglycaemia-induced neuronal dysfunction, DNA damage and apoptosis and thereby propose that pharmacological agents mimicking these effects may represent a promising novel therapy for the hyperglycaemia-induced neuropathy.

Reduced Expression of Chl1 gene Impairs Insulin Secretion by Down-Regulating the Expression of Key Molecules of ß-cell Function.

Silencing of Chl1 gene expression has been previously reported to reduce insulin secretion. Nevertheless, the mechanism underlying this effect remains unclear. In this study, we performed a serial of studies to investigate how Chl1 affects insulin secretion in INS-1 cells. RNA-sequencing was used to investigate the expression of CHL1 in human adipose, liver, muscle, and human islets. Silencing of Chl1 in INS-1 cells was done to assess its impact on the insulin secretion, content, cell viability, and apoptosis. In addition, gene set enrichment analysis (GSEA) was performed to identify possible molecular signatures that associate with Chl1 expression silencing.RNA sequencing data revealed a high expression of CHL1 in pancreatic islets and adipose tissues compared to liver and muscles tissues. Diabetic islets exhibited a lower expression of CHL1 as compared to non-diabetic islets. CHL1 expression was found to correlate positively with insulin secretory index, GLP1R but inversely with HbA1c and BMI. Silencing of Chl1 in INS-1 cells markedly reduced insulin content and secretion. The expression of key molecules of ß-cell function including Insulin, Pdx1, Gck, Glut2, and Insrß was down-regulated in Chl1-silenced cells at transcriptional and translational levels. Cell viability, apoptosis, and proliferation rate were not affected. GSEA showed that the insulin-signaling pathway was influenced in Chl1-silenced cells. Silencing of Chl1 impairs ß-cell function by disrupting the activity of key signaling pathways of importance for insulin biosynthesis and secretion.

Inhibitory effect of UDP-glucose on cAMP generation and insulin secretion.

Type-2 diabetes (T2D) is a global disease caused by the inability of pancreatic ß-cells to secrete adequate insulin. However, the molecular mechanisms underlying the failure of ß-cells to respond to glucose in T2D remains unknown. Here, we investigated the relative contribution of UDP-glucose (UDP-G), a P2Y14-specific agonist, in the regulation of insulin release using human isolated pancreatic islets and INS-1 cells. P2Y14 was expressed in both human and rodent pancreatic ß-cells. Dose-dependent activation of P2Y14 by UDP-G suppressed glucose-stimulated insulin secretion (GSIS) and knockdown of P2Y14 abolished the UDP-G effect. 12-h pretreatment of human islets with pertussis-toxin (PTX) improved GSIS and prevented the inhibitory effect of UDP-G on GSIS. UDP-G on GSIS suppression was associated with suppression of cAMP in INS-1 cells. UDP-G decreased the reductive capacity of nondiabetic human islets cultured at 5 mm glucose for 72 h and exacerbated the negative effect of 20 mm glucose on the cell viability during culture period. T2D donor islets displayed a lower reductive capacity when cultured at 5 mm glucose for 72 h that was further decreased in the presence of 20 mm glucose and UDP-G. Presence of a nonmetabolizable cAMP analog during culture period counteracted the effect of glucose and UDP-G. Islet cultures at 20 mm glucose increased apoptosis, which was further amplified when UDP-G was present. UDP-G modulated glucose-induced proliferation of INS-1 cells. The data provide intriguing evidence for P2Y14 and UDP-G's role in the regulation of pancreatic ß-cell function.

An Integrative Phenotype-Genotype Approach Using Phenotypic Characteristics from the UAE National Diabetes Study Identifies HSD17B12 as a Candidate Gene for Obesity and Type 2 Diabetes.

The United Arab Emirates National Diabetes and Lifestyle Study (UAEDIAB) has identified obesity, hypertension, obstructive sleep apnea, and dyslipidemia as common phenotypic characteristics correlated with diabetes mellitus status. As these phenotypes are usually linked with genetic variants, we hypothesized that these phenotypes share single nucleotide polymorphism (SNP)-clusters that can be used to identify causal genes for diabetes. Materials and We explored the National Human Genome Research Institute-European Bioinformatics Institute Catalog of Published Genome-Wide Association Studies (NHGRI-EBI GWAS) to list SNPs with documented association with the UAEDIAB-phenotypes as well as diabetes. The shared chromosomal regions affected by SNPs were identified, intersected, and searched for Enriched Ontology Clustering. The potential SNP-clusters were validated using targeted DNA next-generation sequencing (NGS) in two Emirati diabetic patients. RNA sequencing from human pancreatic islets was used to study the expression of identified genes in diabetic and non-diabetic donors. Eight chromosomal regions containing 46 SNPs were identified in at least four out of the five UAEDIAB-phenotypes. A list of 34 genes was shown to be affected by those SNPs. Targeted NGS from two Emirati patients confirmed that the identified genes have similar SNP-clusters. ASAH1, LRP4, FES, and HSD17B12 genes showed the highest SNPs rate among the identified genes. RNA-seq analysis revealed high expression levels of HSD17B12 in human islets and to be upregulated in type 2 diabetes (T2D) donors. Our integrative phenotype-genotype approach is a novel, simple, and powerful tool to identify clinically relevant potential biomarkers in diabetes. HSD17B12 is a novel candidate gene for pancreatic ß-cell function.

Somatostatin secretion by Na+-dependent Ca2+-induced Ca2+ release in pancreatic delta-cells.

Pancreatic islets are complex micro-organs consisting of at least three different cell types: glucagon-secreting α-, insulin-producing ß- and somatostatin-releasing δ-cells1. Somatostatin is a powerful paracrine inhibitor of insulin and glucagon secretion2. In diabetes, increased somatostatinergic signalling leads to defective counter-regulatory glucagon secretion3. This increases the risk of severe hypoglycaemia, a dangerous complication of insulin therapy4. The regulation of somatostatin secretion involves both intrinsic and paracrine mechanisms5 but their relative contributions and whether they interact remains unclear. Here we show that dapagliflozin-sensitive glucose- and insulin-dependent sodium uptake stimulates somatostatin secretion by elevating the cytoplasmic Na+ concentration ([Na+]i) and promoting intracellular Ca2+-induced Ca2+ release (CICR). This mechanism also becomes activated when [Na+]i is elevated following the inhibition of the plasmalemmal Na+-K+ pump by reductions of the extracellular K+ concentration emulating those produced by exogenous insulin in vivo 6. Islets from some donors with type-2 diabetes hypersecrete somatostatin, leading to suppression of glucagon secretion that can be alleviated by a somatostatin receptor antagonist. Our data highlight the role of Na+ as an intracellular second messenger, illustrate the significance of the intraislet paracrine network and provide a mechanistic framework for pharmacological correction of the hormone secretion defects associated with diabetes that selectively target the δ-cells.

COVID-19 and Possible Pharmacological Preventive Options.

The dreadful fear of the coronavirus disease 2019 (COVID-19), which is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with the deadly consequences, requires rapid development of pharmacological cures. The objective of this review is to speculate about possible pharmacological options, already available today to prevent or treat the COVID-19 in the early stage of its outbreak. A literature search across PubMed and internet was conducted. A number of studies dealing with COVID-19 were identified. The data elucidated that increased pro-inflammatory and decreased anti-inflammatory cytokines in combination with hypoxia, thromboembolism and pneumonia are involved in the pathogenesis of SARS-CoV-2 infection. Although many drugs has been tested in monotherapy regimen with varying outcome or without desirable effect, there is still hope for better results by simultaneously targeting the virus itself and its symptoms. Theoretically, a mixture of at least two available antiviral drugs in combination with other anti-pathogenic and immune system-enhancing drugs or combination of antiviral drugs with convalescent plasma seems likely to have much better effect than the monotherapy regimen of either of these drugs.

Orphan G-protein coupled receptor 183 (GPR183) potentiates insulin secretion and prevents glucotoxicity-induced ß-cell dysfunction.

The expression and functional impact of most orphan G-protein coupled receptors (GPCRs) in ß-cell is not fully understood. Microarray expression indicated that 36 orphan GPCRs are restricted in human islets, while 55 receptors overlapped between human islets and INS-1 cells. GPR183 showed higher expression in diabetic compared to non-diabetic human islets. GPR183 expression co-localized with ß-cells while it was lacking in α-cells in human islets. The GPR183 agonist (7α-25-DHC) potentiated insulin secretion and protected against glucotoxicity-induced ß-cell damage in human islets. Silencing of GPR183 in INS-1 cells decreased the expression of proinsulin genes, Pdx1, Mafa and impaired insulin secretion with a concomitant decrease in cAMP generation. Cultured INS-1 cells with 7α-25-DHC were associated with increased proliferation and expression of GPR183, INS2, PDX1, NeuroD, and INSR. In conclusion, the beneficial impact of GPR183 activation on ß-cell function makes it a potential therapeutic target to prevent or reverse ß-cell dysfunction.

Reduced Expression of PLCXD3 Associates With Disruption of Glucose Sensing and Insulin Signaling in Pancreatic ß-Cells.

Previous work has shown that reduced expression of PLCXD3, a member of the phosphoinositide-specific phospholipases (PI-PLC) family, impaired insulin secretion with an unclear mechanism. In the current study, we aim to investigate the mechanism underlying this effect using human islets and rat INS-1 (832/13) cells. Microarray and RNA sequencing data showed that PLCXD3 is among the highly expressed PI-PLCs in human islets and INS-1 (832/13) cells. Expression of PLCXD3 was reduced in human diabetic islets, correlated positively with Insulin and GLP1R expression and inversely with the donor's body mass index (BMI) and glycated hemoglobin (HbA1c). Expression silencing of PLCXD3 in INS-1 (832/13) cells was found to reduce glucose-stimulated insulin secretion (GSIS) and insulin content. In addition, the expression of Insulin, NEUROD1, GLUT2, GCK, INSR, IRS2, and AKT was downregulated. Cell viability and apoptosis rate were unaffected. In conclusion, our data suggest that low expression of PLCXD3 in pancreatic ß-cells associates with downregulation of the key insulin signaling and insulin biosynthesis genes as well as reduction in glucose sensing.

Expression levels of enzymes generating NO and CO in islets of murine and human diabetes.

The possible implication of the gasotransmitters NO and CO for the development of diabetes remains unresolved. Our previous investigations in rodents suggested NO being inhibitory, and CO stimulatory, to glucose-stimulated insulin secretion (GSIS). Here we studied the possible role of these gasotransmitters in both murine and human type 2 diabetes (T2D) by mapping the expression pattern of neural nitric oxide synthase (nNOS), inducible NOS (iNOS), constitutive heme oxygenase (HO-2), and inducible HO (HO-1) in isolated pancreatic islets. Two variants of obese murine diabetes with distinct phenotype, the db/db and the ob/ob mouse, were studied at the initiation of the diabetic condition. Plasma glucose and plasma insulin were recorded and ß-cell expression levels of the different enzymes were measured with confocal microscopy and fluorescence intensity recordings. In human islets taken from nondiabetic controls (ND) and type 2 diabetes (T2D) the expression of the enzymes was analyzed by RNA-sequencing and qPCR. At the initiation of murine diabetes plasma glucose was slightly increased, whereas plasma insulin was extremely enhanced in both db/db and ob/ob mice. The ß-cell expression of nNOS and iNOS was markedly increased over controls in db/db mice, known to develop severe diabetes, while it was very low in ob/ob mice, known to develop mild diabetes. HO-2 expression was unaffected in db/db and modestly decreased in ob/ob mice. HO-1 expression was slightly enhanced in ob/ob, but, in contrast, extremely enhanced in db/db mice, suggesting a counteracting, antidiabetic action by CO. Moreover, the diabetic pattern of highly increased nNOS, iNOS and HO-1 expression seen in db/db mice was also fully recognized in human T2D islets. These results suggest that increased expression of the NOS-enzymes, especially an early upregulation of nNOS, could be involved in the initial development of the severe diabetes of db/db mice as well as in human T2D. Hence, nNOS, iNOS and HO-1 might be regarded as interesting targets to take into consideration in the early treatment of a diabetic condition in different variants of T2D.

GNAS gene is an important regulator of insulin secretory capacity in pancreatic ß-cells.

BACKGROUND: Type 2 diabetes (T2D) is a complex polygenic disease with unclear mechanism. In an attempt to identify novel genes involved in ß-cell function, we harness a bioinformatics method called Loss-of-function tool (LoFtool) gene score. METHODS: RNA-sequencing data from human islets were used to cross-reference genes within the 1st quartile of most intolerant LoFtool score with the 100th most expressed genes in human islets. Out of these genes, GNAS and EEF1A1 genes were selected for further investigation in diabetic islets, metabolic tissues along with their correlation with diabetic phenotypes. The influence of GNAS and EEF1A1 on insulin secretion and ß-cell function were validated in INS-1 cells. RESULTS: A comparatively higher expression level of GNAS and EEF1A1 was observed in human islets than fat, liver and muscle tissues. Furthermore, diabetic islets displayed a reduced expression of GNAS, but not of EEF1A, compared to non-diabetic islets. The expression of GNAS was positively correlated with insulin secretory index, GLP1R, GIPR and inversely correlated with HbA1c. Diabetic human islets displayed a reduced cAMP generation and insulin secretory capacity in response to glucose. Moreover, siRNA silencing of GNAS in INS-1 cells reduced insulin secretion, insulin content, and cAMP production. In addition, the expression of Insulin, PDX1, and MAFA was significantly down-regulated in GNAS-silenced cells. However, cell viability and apoptosis rate were unaffected. CONCLUSION: LoFtool is a powerful tool to identify genes associated with pancreatic islets dysfunction. GNAS is a crucial gene for the ß-cell insulin secretory capacity.

Glucose stimulates somatostatin secretion in pancreatic δ-cells by cAMP-dependent intracellular Ca2+ release.

Somatostatin secretion from pancreatic islet δ-cells is stimulated by elevated glucose levels, but the underlying mechanisms have only partially been elucidated. Here we show that glucose-induced somatostatin secretion (GISS) involves both membrane potential-dependent and -independent pathways. Although glucose-induced electrical activity triggers somatostatin release, the sugar also stimulates GISS via a cAMP-dependent stimulation of CICR and exocytosis of somatostatin. The latter effect is more quantitatively important and in mouse islets depolarized by 70 mM extracellular K+ , increasing glucose from 1 mM to 20 mM produced an ∼3.5-fold stimulation of somatostatin secretion, an effect that was mimicked by the application of the adenylyl cyclase activator forskolin. Inhibiting cAMP-dependent pathways with PKI or ESI-05, which inhibit PKA and exchange protein directly activated by cAMP 2 (Epac2), respectively, reduced glucose/forskolin-induced somatostatin secretion. Ryanodine produced a similar effect that was not additive to that of the PKA or Epac2 inhibitors. Intracellular application of cAMP produced a concentration-dependent stimulation of somatostatin exocytosis and elevation of cytoplasmic Ca2+ ([Ca2+]i). Both effects were inhibited by ESI-05 and thapsigargin (an inhibitor of SERCA). By contrast, inhibition of PKA suppressed δ-cell exocytosis without affecting [Ca2+]i Simultaneous recordings of electrical activity and [Ca2+]i in δ-cells expressing the genetically encoded Ca2+ indicator GCaMP3 revealed that the majority of glucose-induced [Ca2+]i spikes did not correlate with δ-cell electrical activity but instead reflected Ca2+ release from the ER. These spontaneous [Ca2+]i spikes are resistant to PKI but sensitive to ESI-05 or thapsigargin. We propose that cAMP links an increase in plasma glucose to stimulation of somatostatin secretion by promoting CICR, thus evoking exocytosis of somatostatin-containing secretory vesicles in the δ-cell.

The functional impact of G protein-coupled receptor 142 (Gpr142) on pancreatic ß-cell in rodent.

We have recently shown that the G protein-coupled receptor 142 (GPR142) is expressed in both rodent and human pancreatic ß-cells. Herein, we investigated the cellular distribution of GPR142 within islets and the effects of selective agonists of GPR142 on glucose-stimulated insulin secretion (GSIS) in the mouse islets and INS-1832/13 cells. Double-immunostaining revealed that GPR142 immunoreactivity in islets mainly occurs in insulin-positive cells. Potentiation of GSIS by GPR142 activation was accompanied by increased cAMP content in INS-1832/13 cells. PKA/Epac inhibition markedly suppressed the effect of GPR142 activation on insulin release. Gpr142 knockdown (Gpr142-KD) in islets was accompanied by elevated release of MCP-1, IFNγ, and TNFα during culture period and abolished the modulatory effect of GPR142 activation on the GSIS. Gpr142-KD had no effect on Ffar1, Ffar2, or Ffar3 mRNA while reducing Gpr56 and increasing Tlr5 and Tlr7 mRNA expression. Gpr142-KD was associated with an increased expression of Chrebp, Txnip, RhoA, and mitochondrial Vdac1 concomitant with a reduced Pdx1, Pax6, and mitochondrial Vdac2 mRNA levels. Long-term exposure of INS-1832/13 cells to hyperglycemia reduced Gpr142 and Vdac2 while increased Chrebp, Txnip, and Vdac1 mRNA expression. GPR142 agonists or Bt2-cAMP counteracted this effect. Glucotoxicity-induced decrease of cell viability in Gpr142-KD INS-1 cells was not affected by GPR142-agonists while Bt2-cAMP prevented it. The results show the importance of Gpr142 in the maintenance of pancreatic ß-cell function in rodents and that GPR142 agonists potentiate GSIS by an action, which most likely is due to increased cellular generation of second messenger molecule cAMP.

RORB and RORC associate with human islet dysfunction and inhibit insulin secretion in INS-1 cells.

Little is known about the expression and function of Retinoic acid-related orphan receptors (RORA, B, and C) in pancreatic ß cells. Here in, we utilized cDNA microarray and RNA sequencing approaches to investigate the expression pattern of ROR receptors in normal and diabetic human pancreatic islets. Possible correlations between RORs expression and HbA1c levels as well as insulin secretory capacity in isolated human islets were evaluated. The impact of RORB and RORC expression on insulin secretion in INS-1 (832/13) cells was validated as well. While RORA was the highest expressed gene among the three RORs in human islet cells, RORC was the highest expressed in INS-1 cells (832/13) and while RORB was the lowest expressed gene in human islet cells, RORA was the highest expressed in INS-1 cells (832/13). The expression of RORB and RORC was significantly lower in diabetic/hyperglycemic donors as compared with non-diabetic counterparts. Furthermore, while the expression of RORB correlated positively with insulin secretion and negatively with HbA1c, that of RORC correlated negatively with HbA1c. The expression pattern of RORA did not correlate with either of the two parameters. siRNA silencing of RORB or RORC in INS-1 (832/13) cells resulted in a significant downregulation of insulin mRNA expression and insulin secretion. These findings suggest that RORB and RORC are part of the molecular cascade that regulates insulin secretion in pancreatic ß cells; and insight that provides for further work on the potential therapeutic utility of RORB and RORC genes in ß cell dysfunction in type 2 diabetes.

Insulin inhibits glucagon release by SGLT2-induced stimulation of somatostatin secretion.

Hypoglycaemia (low plasma glucose) is a serious and potentially fatal complication of insulin-treated diabetes. In healthy individuals, hypoglycaemia triggers glucagon secretion, which restores normal plasma glucose levels by stimulation of hepatic glucose production. This counterregulatory mechanism is impaired in diabetes. Here we show in mice that therapeutic concentrations of insulin inhibit glucagon secretion by an indirect (paracrine) mechanism mediated by stimulation of intra-islet somatostatin release. Insulin's capacity to inhibit glucagon secretion is lost following genetic ablation of insulin receptors in the somatostatin-secreting δ-cells, when insulin-induced somatostatin secretion is suppressed by dapagliflozin (an inhibitor of sodium-glucose co-tranporter-2; SGLT2) or when the action of secreted somatostatin is prevented by somatostatin receptor (SSTR) antagonists. Administration of these compounds in vivo antagonises insulin's hypoglycaemic effect. We extend these data to isolated human islets. We propose that SSTR or SGLT2 antagonists should be considered as adjuncts to insulin in diabetes therapy.

Preserving Insulin Secretion in Diabetes by Inhibiting VDAC1 Overexpression and Surface Translocation in ß Cells.

Type 2 diabetes (T2D) develops after years of prediabetes during which high glucose (glucotoxicity) impairs insulin secretion. We report that the ATP-conducting mitochondrial outer membrane voltage-dependent anion channel-1 (VDAC1) is upregulated in islets from T2D and non-diabetic organ donors under glucotoxic conditions. This is caused by a glucotoxicity-induced transcriptional program, triggered during years of prediabetes with suboptimal blood glucose control. Metformin counteracts VDAC1 induction. VDAC1 overexpression causes its mistargeting to the plasma membrane of the insulin-secreting ß cells with loss of the crucial metabolic coupling factor ATP. VDAC1 antibodies and inhibitors prevent ATP loss. Through direct inhibition of VDAC1 conductance, metformin, like specific VDAC1 inhibitors and antibodies, restores the impaired generation of ATP and glucose-stimulated insulin secretion in T2D islets. Treatment of db/db mice with VDAC1 inhibitor prevents hyperglycemia, and maintains normal glucose tolerance and physiological regulation of insulin secretion. Thus, ß cell function is preserved by targeting the novel diabetes executer protein VDAC1.

Activation of imidazoline receptor I2, and improved pancreatic ß-cell function in human islets.

AIM: The impact of BL11282, an imidazoline receptor (NISCH) agonist, on potentiation of glucose-stimulated insulin secretion (GSIS) from isolated human non-diabetic (ND) and type 2 diabetic (T2D) islets was investigated. METHODS: Analysis of mRNA was performed by RNA-sequencing and qPCR. Insulin and cAMP by RIA and ELISA respectively. RESULTS: RNA-sequencing data revealed that NISCH is highly expressed in fat tissues, islets, liver and muscles, with eight detectable splice variants of transcripts in islets. NISCH had a positive correlation with GLP-1 (GLP1R) and GIP (GIPR) receptor transcripts. The expression of NISCH was confirmed by qPCR in human islets. NISCH and GLP1R were comparably higher expressed in mouse islets compared to human islets. GSIS was dose-dependently potentiated by BL11282 from incubated islets of ND and T2D human islet donors. The insulinotropic action of BL11282 was associated with increased cAMP. While the harmful effect of high glucose on reductive capacity of islet cells was enhanced by glibenclamide during long-term culture, it was counteracted by BL11282 or Bt2-cAMP. BL11282 also increased proliferation of INS-1 cells during long-time culture. CONCLUSION: Our data suggest that BL11282 potentiates GSIS by an action involving cAMP/PKA system and BL11282 could be an attractive insulinotropic and ß-cell protective agent.

Adrenaline Stimulates Glucagon Secretion by Tpc2-Dependent Ca2+ Mobilization From Acidic Stores in Pancreatic α-Cells.

Adrenaline is a powerful stimulus of glucagon secretion. It acts by activation of ß-adrenergic receptors, but the downstream mechanisms have only been partially elucidated. Here, we have examined the effects of adrenaline in mouse and human α-cells by a combination of electrophysiology, imaging of Ca2+ and PKA activity, and hormone release measurements. We found that stimulation of glucagon secretion correlated with a PKA- and EPAC2-dependent (inhibited by PKI and ESI-05, respectively) elevation of [Ca2+]i in α-cells, which occurred without stimulation of electrical activity and persisted in the absence of extracellular Ca2+ but was sensitive to ryanodine, bafilomycin, and thapsigargin. Adrenaline also increased [Ca2+]i in α-cells in human islets. Genetic or pharmacological inhibition of the Tpc2 channel (that mediates Ca2+ release from acidic intracellular stores) abolished the stimulatory effect of adrenaline on glucagon secretion and reduced the elevation of [Ca2+]i Furthermore, in Tpc2-deficient islets, ryanodine exerted no additive inhibitory effect. These data suggest that ß-adrenergic stimulation of glucagon secretion is controlled by a hierarchy of [Ca2+]i signaling in the α-cell that is initiated by cAMP-induced Tpc2-dependent Ca2+ release from the acidic stores and further amplified by Ca2+-induced Ca2+ release from the sarco/endoplasmic reticulum.