Regulation Of Insulin Release Research Articles

Involvement of the Adhesion GPCRs Latrophilins in the Regulation of Insulin Release

Insulin secretion from pancreatic β cells is a highly complex and tightly regulated process. Its dysregulation is one characteristic of type 2 diabetes, and thus, an in-depth understanding of the mechanisms controlling insulin secretion is essential for rational therapeutic intervention. G-protein-coupled receptors (GPCRs) have been established as major regulators of insulin exocytosis. Recent studies also suggest the involvement of adhesion GPCRs, a non-prototypical class of GPCRs. Here, we identify latrophilins, which belong to the class of adhesion GPCRs, to be highly expressed in different cell types of pancreatic islets. Invitro and exvivo analyses show that distinct splice variants of the latrophilin LPHN3/ADGRL3 decrease insulin secretion from pancreatic β cells by reducing intracellular cyclic AMP levels via the Gi-mediated pathway. Our data highlight the key role of LPHN3 in modulating insulin secretion and its potential as therapeutic target for type 2 diabetes.

Does the Time of Drug Administration Alter the Metabolic Risk of Aripiprazole?

Antipsychotic drugs cause metabolic abnormalities through a mechanism that involves antagonism of D2 dopamine receptors (D2R). Under healthy conditions, insulin release follows a circadian rhythm and is low at night, and in pancreatic beta-cells, D2Rs negatively regulate insulin release. Since they are sedating, many antipsychotics are dosed at night. However, the resulting reduction in overnight D2R activity may disrupt 24 h rhythms in insulin release, potentially exacerbating metabolic dysfunction. We examined retrospective clinical data from patients treated over approximately 1 year with the antipsychotic drug aripiprazole (ARPZ), a D2R partial agonist. To identify effects of timing on metabolic risk, we found cases treated with ARPZ either in the morning (n = 90) or at bedtime (n = 53), and compared hemoglobin A1c, and six secondary metabolic parameters across the two groups. After controlling for demographic and clinical factors, patients treated with ARPZ at night had a significant decrease in HDL cholesterol, while in patients who took ARPZ in the morning had no change. There was a non-significant trend toward higher serum triglycerides in the patients treated with ARPZ at night vs. morning. There were no group differences in hemoglobin A1c, BMI, total cholesterol, LDL cholesterol, or blood pressure. Patients taking APPZ at night developed a worse lipid profile, with lower HDL cholesterol and a trend toward higher triglycerides. These changes may pose additional metabolic risk factors compared to those who take ARPZ in the morning. Interventions based on drug timing may reduce some of the adverse metabolic consequences of antipsychotic drugs.

Glucose homeostasis dependency on acini\u2013islet\u2013acinar (AIA) axis communication: a new possible pathophysiological hypothesis regarding diabetes mellitus

Studies have highlighted the existence of two intra-pancreatic axes of communication: one involved in the regulation of enzyme production by insulin—the insular–acinar axis; and another involved in the regulation of insulin release by pancreatic enzymes—the acini–insular axis. Previous studies by our laboratory show that pancreatic enzymes can affect blood glucose homeostasis and insulin secretion independently of their digestive functions, both from the gut lumen and probably from the blood. As a result we would like to introduce here the concept of acini–islet–acinar (AIA) axis communication (feedback), which could play an important role in the development of obesity and diabetes type 2. The AIA feedback links the endocrine and exocrine parts of the pancreas and emphasizes the essential role that the pancreas plays, as a single organ, in the regulation of glucose homeostasis by amylase most probably in gut epithelium and by insulin and glucagon in peripheral blood.

Y1 receptor deficiency in \u03b2-cells leads to increased adiposity and impaired glucose metabolism

Insulin secretion from pancreatic β-cells is critical for maintaining glucose homeostasis and deregulation of circulating insulin levels is associated with the development of metabolic diseases. While many factors have been implicated in the stimulation of insulin secretion, the mechanisms that subsequently reduce insulin secretion remain largely unexplored. Here we demonstrate that mice with β-cell specific ablation of the Y1 receptor exhibit significantly upregulated serum insulin levels associated with increased body weight and adiposity. Interestingly, when challenged with a high fat diet these β-cell specific Y1-deficient mice also develop hyperglycaemia and impaired glucose tolerance. This is most likely due to enhanced hepatic lipid synthesis, resulting in an increase of lipid accumulation in the liver. Together, our study demonstrates that Y1 receptor signaling negatively regulates insulin release, and pharmacological inhibition of Y1 receptor signalling for the treatment of non-insulin dependent diabetes should be taken into careful consideration.

Glutamate dehydrogenase: role in regulating metabolism and insulin release in pancreatic β-cells.

Regulation of insulin release and glucose homeostasis by pancreatic β-cells is dependent on the metabolism of glucose by glucokinase (GK) and the influence of that activity on oxidative phosphorylation. Genetic alterations that result in hyperactivity of mitochondrial glutamate dehydrogenase (GDH-1) can cause hypoglycemia-hyperammonemia following high protein meals, but the role of GDH-1 remains poorly understood. GDH-1 activity is strongly inhibited by GTP, to near zero in the absence of ADP, and cooperatively activated ( n = 2.3) by ADP. The dissociation constant for ADP is near 200 µM in vivo, but leucine and its nonmetabolized analog 2-amino-2-norbornane-carboxylic acid (BCH) can activate GDH-1 by increasing the affinity for ADP. Under physiological conditions, as [ADP] increases GDH-1 activity remains very low until ~35 µM (threshold) and then increases rapidly. A model for GDH-1 and its regulation has been combined with a previously published model for glucose sensing that coupled GK activity and oxidative phosphorylation. The combined model (GK-GDH-core) shows that GK activity determines the energy state ([ATP]/[ADP][Pi]) in β-cells for glucose concentrations > 5 mM ([ADP] < 35 µM). As glucose falls < 5 mM the [ADP]-dependent increase in GDH-1 activity prevents [ADP] from rising above ~70 µM. Thus, GDH-1 dynamically buffers β-cell energy metabolism during hypoglycemia, maintaining the energy state and the basal rate of insulin release. GDH-1 hyperactivity suppresses the normal increase in [ADP] in hypoglycemia. This leads to hypoglycemia following a high protein meal by increasing the basal rate of insulin release (β-cells) and decreasing glucagon release (α-cells). NEW & NOTEWORTHY A model of β-cell metabolism and regulation of insulin release is presented. The model integrates regulation of oxidative phosphorylation, glucokinase (GK), and glutamate dehydrogenase (GDH-1). GDH-1 is near equilibrium under physiological conditions, but the activity is inhibited by GTP. In hypoglycemia, however, GK activity is low and [ADP], a potent activator of GDH-1, increases. Reducing equivalents from GDH dynamically buffers the intramitochondrial [NADH]/[NAD+], and thereby the energy state, preventing hypoglycemia-induced substrate deprivation.

Collective Sensing of β-Cells Generates the Metabolic Code.

Major part of a pancreatic islet is composed of β-cells that secrete insulin, a key hormone regulating influx of nutrients into all cells in a vertebrate organism to support nutrition, housekeeping or energy storage. β-cells constantly communicate with each other using both direct, short-range interactions through gap junctions, and paracrine long-range signaling. However, how these cell interactions shape collective sensing and cell behavior in islets that leads to insulin release is unknown. When stimulated by specific ligands, primarily glucose, β-cells collectively respond with expression of a series of transient Ca2+ changes on several temporal scales. Here we reanalyze a set of Ca2+ spike trains recorded in acute rodent pancreatic tissue slice under physiological conditions. We found strongly correlated states of co-spiking cells coexisting with mostly weak pairwise correlations widespread across the islet. Furthermore, the collective Ca2+ spiking activity in islet shows on-off intermittency with scaling of spiking amplitudes, and stimulus dependent autoassociative memory features. We use a simple spin glass-like model for the functional network of a β-cell collective to describe these findings and argue that Ca2+ spike trains produced by collective sensing of β-cells constitute part of the islet metabolic code that regulates insulin release and limits the islet size.

Angiotensin-converting enzyme 2 regulates mitochondrial function in pancreatic β-cells

Mitochondrial metabolism plays an essential role in the regulation of insulin release and glucose homeostasis. Evidence demonstrated that the angiotensin-converting enzyme 2 (ACE2) participates in the regulation of glucose metabolism, however, its role in mitochondrial metabolism remains unclear. The purpose of our study was to determine if ACE2 can regulate mitochondrial function in pancreatic β-cells. We found that ACE2 over-expression restored glucose-stimulated insulin secretion (GSIS) and mitochondrial membrane potential (MMP) in the presence of H2O2 in INS-1 cells. PCR array demonstrated that ACE2 over-expression up-regulated 67 mitochondria-related genes in INS-1 cells. In pancreatic islets, ACE2 ablation attenuated intracellular calcium influx with a decrease in GSIS. Ace2−/y mice islets exhibited impaired mitochondrial respiration and lower production of ATP, along with decreased expression of genes involved in mitochondrial oxidation. In islets from db/db mice, ACE2 over-expression increased intracellular calcium influx and restored impaired mitochondrial oxidation, potentially causing an increase in GSIS. These results shed light on the potential roles of ACE2 in mitochondrial metabolism, moreover, may improve our understanding of diabetes.

Synthetic "smart gel" provides glucose-responsive insulin delivery in diabetic mice.

Although previous studies have attempted to create "electronics-free" insulin delivery systems using glucose oxidase and sugar-binding lectins as a glucose-sensing mechanism, no successful clinical translation has hitherto been made. These protein-based materials are intolerant of long-term use and storage because of their denaturing and/or cytotoxic properties. We provide a solution by designing a protein-free and totally synthetic material-based approach. Capitalizing on the sugar-responsive properties of boronic acid, we have established a synthetic polymer gel-based insulin delivery device confined within a single catheter, which exhibits an artificial pancreas-like function in vivo. Subcutaneous implantation of the device in healthy and diabetic mice establishes a closed-loop system composed of "continuous glucose sensing" and "skin layer"-regulated insulin release. As a result, glucose metabolism was controlled in response to interstitial glucose fluctuation under both insulin-deficient and insulin-resistant conditions with at least 3-week durability. Our "smart gel" technology could offer a user-friendly and remarkably economic (disposable) alternative to the current state of the art, thereby facilitating availability of effective insulin treatment not only to diabetic patients in developing countries but also to those patients who otherwise may not be strongly motivated, such as the elderly, infants, and patients in need of nursing care.

Pediatric Endocrinology: Global Spread of Type 2 Diabetes Mellitus in Neonatal and Permanent Treatment Using Sulfonylurea Drug Regimen Glibenclamide

Pediatric endocrinology is a medical subspecialty dealing with disorders of the endocrine glands, such as variations of physical growth and sexual development in childhood, diabetes and many more. Permanent neonatal diabetes mellitus (PNDM) is a rare form of diabetes characterized by insulin-requiring hyperglycemia that is diagnosed within the first months of life.Type 2 diabetes mellitus is emerging as a new clinical problem within pediatric practice. Aim of the Study An explosion of work over the last decade has produced insight into the multiple hereditary causes of a non-immunological form of diabetes diagnosed most frequently within the first 6 months of life. These studies are providing increased understanding of genes involved in the entire chain of steps that control glucose homeostasis. Neonatal diabetes is now understood to arise from mutations in genes that play critical roles in the development of the pancreas, of β-cell apoptosis and insulin processing, as well as the regulation of insulin release.

Numerical simulation of a glucose sensitive composite membrane closed-loop insulin delivery system.

Closed-loop insulin delivery system works on pH modulation by gluconic acid production from glucose, which in turn allows regulation of insulin release across membrane. Typically, the concentration variation of gluconic acid can be numerically modeled by a set of non-linear, non-steady state reaction diffusion equations. Here, we report a simpler numerical approach to time and position dependent diffusivity of species using finite difference and differential quadrature (DQ) method. The results are comparable to that obtained by analytical method. The membrane thickness directly determines the concentrations of the glucose and oxygen in the system, and inversely to the gluconic acid. The advantage with the DQ method is that its parameter values need not be altered throughout the analysis to obtain the concentration profiles of the glucose, oxygen and gluconic acid. Our work would be useful for modeling diabetes and other systems governed by such non-linear and non-steady state reaction diffusion equations.

Synergic effects of sugar and caffeine on insulin-mediated metabolomic alterations after an acute consumption of soft drinks.

High sugar consumption elicits numerous deleterious effects on health by inducing insulin resistance, which is closely associated with the development of metabolic disorders such as obesity or type-2 diabetes. Furthermore, there is also growing evidence that caffeine may play an important role in the regulation of insulin release and the appearance of related metabolic impairments. Thus, the aim of this work was to investigate the impact of acute sugar and caffeine intake on the metabolic health status by using a metabolomic multi-platform based on the combination of flow injection mass spectrometry and ultra-high performance liquid chromatography mass spectrometry. To this end, we performed a randomized, crossover and double-blind intervention study with different soft drinks from the same brand. Numerous metabolomic changes were detected in serum samples over time after the intake of sugar-sweetened beverages, including energy-related metabolites, amino acids and lipids, thus demonstrating the intense effects provoked by acute sugar consumption on the organism during 3 h of follow-up. However, the most significant findings were observed after the co-ingestion of caffeine, which could be indicative of a synergic effect of this psychostimulant on insulin-mediated perturbations.

Secretagogin Is a Redox-Responsive Ca2+ Sensor.

Secretagogin (SCGN), a multifunctional, Ca2+ binding, regulatory protein, known to regulate insulin release, has recently been implicated in the control of stress-related corticotropin-releasing hormone (CRH) secretion. Localization of SCGN to multiple intracellular (such as cytosol, nucleus, and endoplasmic reticulum) and extracellular sites appears to provide multifunctional capabilities; however, the structural elements conferring such a widespread cellular distribution to SCGN remain unidentified. We report that the spatial and functional attributes of SCGN plausibly originate from the interplay between Ca2+ and its redox state. The mutation of selective Cys residues provides further insights into the origin and mode of redox responsiveness. In the reducing milieu, SCGN exhibits a higher affinity for Ca2+, and more stability than in the oxidizing environment, suggesting it is a redox-responsive Ca2+ sensor protein, which is further supported by its response to dithiothreitol (reducing stress) in MIN6 cells. Our data provide a biophysical and biochemical explanation for the diverse localization of SCGN in the cellular scenario and beyond the cell.

Regulation of L-type Ca2+ Channel Activity and Insulin Secretion by Huntingtin-associated Protein 1

Huntingtin-associated protein 1 (Hap1) was originally identified as a protein that binds to the Huntington disease protein, huntingtin. Growing evidence has shown that Hap1 participates in intracellular trafficking via its association with various microtubule-dependent transporters and organelles. Recent studies also revealed that Hap1 is involved in exocytosis such as insulin release from pancreatic β-cells. However, the mechanism underlying the action of Hap1 on insulin release remains to be investigated. We found that Hap1 knock-out mice had a lower plasma basal insulin level than control mice. Using cultured pancreatic β-cell lines, INS-1 cells, we confirmed that decreasing Hap1 reduces the number of secreted vesicles and inhibits vesicle exocytosis. Electrophysiology and imaging of intracellular Ca2+ measurements demonstrated that Hap1 depletion significantly reduces the influx of Ca2+ mediated by L-type Ca2+ channels (Cav). This decrease is not due to reduced expression of Cav1.2 channel mRNA but results from the decreased distribution of Cav1.2 on the plasma membrane of INS-1 cells. Fluorescence recovery after photobleaching showed a defective movement of Cav1.2 in Hap1 silencing INS-1 cells. Our findings suggest that Hap1 is important for insulin secretion of pancreatic β-cells via regulating the intracellular trafficking and plasma membrane localization of Cav1.2, providing new insight into the mechanisms that regulate insulin release from pancreatic β-cells.

Disallowed and Allowed Gene Expression: Two Faces of Mature Islet Beta Cells.

Glucose homeostasis greatly depends on the match between fluctuating insulin demands and adjusted rates of insulin secretion, which is the function of pancreatic beta cells. Emerging evidence suggests that when neonatal beta cells mature, they acquire two faces of differentiated function: an expected "visible face" that depends on specific beta cell proteins needed for regulated insulin release, but also a "hidden face" that represses ubiquitous proteins to prevent inappropriate beta cell function such as elevated basal hormone secretion or insulin release triggered by exercise. This review highlights this novel concept, and we first propose that hidden faces may also be relevant for other specialized tissue functions, such as ketogenesis in the liver. Next, we discuss three scenarios in which aberrant gene expression causes abnormal glucose-induced insulin release and the epigenetic regulation of the hidden face in beta cells. We conclude with perspectives for new research, including beta cell replacement to cure diabetes.

Theoretical Investigations into the Quantitative Mechanisms Underlying the Regulation of [cAMP]i, Membrane Excitability and [Ca(2+)]i during GLP-1 Stimulation in Pancreatic β Cells

Upon elevation of plasma glucose concentration, pancreatic β-cells generate bursts of action potentials to induce cyclic changes in [Ca(2+)]i regulating insulin release. Glucose-dependent insulin secretion is synergistically enhanced by glucagon-like peptide-1 (GLP-1), which increases [cAMP]i and activates protein kinase A (PKA) and exchange protein activated by cAMP (Epac). The insulinotropic effect of GLP-1 is mediated, at least in part, by modulating multiple ion channels/transporters at the plasma membrane and ER through PKA- and EPAC-dependent mechanisms, which increase membrane excitability and intracellular Ca(2+) release. However, because of complex interactions between multiple cellular factors involved in the GLP-1 effects, quantitative aspects of the molecular/ionic mechanisms have not yet been determined. We thus performed simulation studies and mathematical analysis to investigate how GLP-1 signals control [cAMP]i and subsequently modify the bursting activities and Ca(2+) dynamics. First, a GLP-1 receptor signal transduction model was developed and introduced to our β-cells model. Secondly, modulatory effects of PKA/Epac on ion channels/transporters were incorporated based on experimental studies. Increases in the frequency and duration of the bursting activity observed during GLP-1 stimulation were well reconstructed by our model, and lead potential analysis quantitatively determined the functional role of each ion channel/transporter in modifying the burst pattern. Finally, an IP3R model was developed to reproduce GLP-1-induced Ca(2+) transients/oscillations. Instantaneous equilibrium analysis and bifurcation analysis also elucidated the quantitative mechanisms involved in generating IP3R-mediated Ca(2+) mobilization. The results of this theoretical analysis of the effects of GLP-1 on membrane excitability/Ca(2+) dynamics are discussed in this review.

Role of prolyl hydroxylase domain proteins in the regulation of insulin secretion.

Type 2 diabetes is associated with impaired nutrient‐regulated anaplerosis and insulin secretion in pancreatic β‐cells. One key anaplerotic substrate that may be involved in regulating insulin release is α‐ketoglutarate (αKG). Since prolyl hydroxylase domain proteins (PHDs) can metabolize cytosolic αKG, we sought to explore the role of this enzyme in the regulation of β‐cell function. The oxygen‐sensing PHDs regulate the stability of hypoxia‐inducible factor 1α (HIF1α) as well as other proline‐containing proteins by catalyzing the hydroxylation of proline residues. This reaction is dependent on sufficient levels of oxygen, iron, and αKG. In the present study, we utilized both pharmacological and genetic approaches to assess the impact of inhibiting PHD activity on β‐cell function. We demonstrate that ethyl‐3,4‐dihydroxybenzoate (EDHB), a PHD inhibitor, significantly blunted glucose‐stimulated insulin secretion (GSIS) from 832/13 clonal cells, rat, and human islets. EDHB reduced glucose utilization, ATP/ADP ratio, and key TCA cycle intermediates such as pyruvate, citrate, fumarate, and malate. siRNA‐mediated knockdown of PHD1 and PHD3 inhibited GSIS, whereas siRNA‐mediated knockdown of PHD2 had no effect on GSIS. Taken together, the current results demonstrate an important role for PHDs as mediators of islet insulin secretion.

Development of a Rapid Insulin Assay by Homogenous Time-Resolved Fluorescence.

Direct measurement of insulin is critical for basic and clinical studies of insulin secretion. However, current methods are expensive and time-consuming. We developed an insulin assay based on homogenous time-resolved fluorescence that is significantly more rapid and cost-effective than current commonly used approaches. This assay was applied effectively to an insulin secreting cell line, INS-1E cells, as well as pancreatic islets, allowing us to validate the assay by elucidating mechanisms by which dopamine regulates insulin release. We found that dopamine functioned as a significant negative modulator of glucose-stimulated insulin secretion. Further, we showed that bromocriptine, a known dopamine D2/D3 receptor agonist and newly approved drug used for treatment of type II diabetes mellitus, also decreased glucose-stimulated insulin secretion in islets to levels comparable to those caused by dopamine treatment.

Enteral Pancreatic-like Enzymes of Microbial Origin affect Insulin Release during an Intravenous Glucose Tolerance Test

We have previously shown that the presence of pancreatic enzymes in the gut lumen of exocrine pancreatic insufficient pigs influences blood glucose and insulin levels during an intravenous glucose tolerance test (IVGTT). The present study aims to highlight the effects of orally applied pancreatic-like enzymes on blood glucose and plasma insulin levels during an IVGTT in young intact pigs. Five, 7-week old pigs were fed with pancreatic-like enzymes of microbial origin, a proteinase (from Aspergillus melleus), α-amylase (from Aspergillus oryzae) or lipase (from Burkholderia cepacia) alone or in combination with the Ca/Na salts of α-ketoglutaric acid (AKG). One hour following administration of the various supplements an IVGTT was performed. Blood samples were withdrawn during the 2 hours of IVGTT for glucose and insulin analyses. Blood glucose during the IVGTT was identical following administration of all combinations of the various enzymes or enzyme mixtures. Enteral loading of amylase or any amylase containing mixture resulted in reduced insulin secretion while administration of proteinase or any proteinase containing mixture resulted in enhanced insulin secretion during IVGTT, as compared to the control water vehicle. Lipase or AKG and lipase or AKG containing mixtures did not affect insulin secretion. Thus, it can be suggested that host amylase/protease ratio and their amount in pancreatic juice can participate in regulation of insulin release, thus, possibly affecting development of obesity and diabetes type 2.

Calcium release channel RyR2 regulates insulin release and glucose homeostasis

Calcium release channel RyR2 regulates insulin release and glucose homeostasis

The Presence of Human Chorionic Gonadotropin/Luteinizing Hormone Receptors in Pancreatic β-Cells.

We investigated the possible presence of functional human chorionic gonadotropin (hCG)/luteinizing hormone (LH) receptors in β-cells of pancreas, using a combination of techniques on hCG/LH receptor knockout mice, immortalized rat insulinoma cells, and human pancreatic islets. The results showed the presence of receptors and their activation resulted in a dose-dependent increase in glucose-induced release of insulin. These findings place hCG and LH among the regulators of insulin release with potential implications for insulin-level changes during the periods of altered hCG and LH secretion.