Glucose-induced Insulin Release Research Articles

The role of ghrelin in the regulation of glucose homeostasis.

Ghrelin is a 28-amino acid (aa) stomach-derived peptide discovered in 1999 as the endogenous ligand for growth hormone secretagogue-receptor (GHS-R). Ghrelin-producing cells constitute a distinct group of endocrine cells dispersed throughout the gastric mucosa and to a lesser extent in the small intestine and the endocrine pancreas. Ghrelin plasma levels rise during fasting and chronic caloric restriction to stimulate food intake and fat storage and to prevent life-threatening falls in blood glucose. Plasma ghrelin levels decrease after a meal is consumed and in conditions of energy surplus (such as obesity). Ghrelin has emerged as a key player in the regulation of appetite and energy homeostasis. Ghrelin achieves these functions through binding the ghrelin receptor GHS-R in appetite-regulating neurons and in peripheral metabolic organs including the endocrine pancreas. Ghrelin levels are negatively correlated with body mass index (BMI) and insulin resistance. In addition, ghrelin secretion is impaired in obesity and insulin resistance. Several studies highlight an important role for ghrelin in glucose homeostasis. Genetic, immunological, and pharmacological blockade of ghrelin signaling resulted in improved glucose tolerance and insulin sensitivity. Furthermore, exogenous ghrelin administration was shown to decrease glucose-induced insulin release and increase glucose level in both humans and rodents. GHS-R was shown to be expressed in pancreatic β-cells and ghrelin suppressed insulin release via a Ca2+-mediated pathway. In this review, we provide a detailed summary of recent advances in the field that focuses on the role of insulin and insulin resistance in the regulation of ghrelin secretion and on the role of ghrelin in glucose-stimulated insulin secretion (GSIS).

TRPM3 Exhibits Slight Temperature Sensitivity in the Planar Lipid Bilayer System and Requires the Presence of PIP2

Transient Receptor Potential Melastatin 3 (TRPM3) channel is critical for various physiological processes. In sensory nervous system TRPM3 has been implicated in temperature perception and inflammatory hyperalgesia, while in pancreatic β-cells the channel was linked to glucose-induced insulin release. In cells, TRPM3 is activated by heat and chemical agonists, including neurosteroid, pregnenolone sulfate (PS), and L-type Ca2+ channel blocker, nifedipine. To define the physical interactions and nuances of TRPM3 channel activity with its modulators, we succeeded to incorporate purified TRPM3 protein into the artificial membrane system - planar lipid bilayers. We found that channel opening by PS required the presence of phosphatidylinositol-4,5-bisphosphate (PIP2) or clotrimazole. In the presence of 5 μM PIP2 the channel exhibited EC50 for PS of ∼1.3 μM, which is lower than that observed in cell recordings (∼23 μM), due to higher agonist's sensitivity of the channel in bilayers. Unlike PS, the presence of nifedipine alone sufficed to induce TRPM3 activity (EC50 = ∼1.7 μM), in which case channel demonstrated distinct gating behavior. Similarly to PS, EC50 of TRPM3 to nifedipine in cell recordings was much higher (∼32 μM), than that observed in the bilayers.We also performed an extensive thermodynamic analysis on TRPM3 temperature dependence. Intriguingly, we found that TRPM3 exhibits slight temperature sensitivity in comparison to highly temperature-sensitive TRPM8 or TRPV1. TRPM3 remained closed upon heat-induced stimuli, but exhibited openings in the presence of PIP2 (Q10 = 5.3), although only with low open probability profile (Po = 0.048 ± 0.016).Together our results resolved the details on TRPM3 channel function in isolated system, which confirmed its direct gating by PS and PIP2, but indicated lack of strong intrinsic temperature sensitivity, common to other thermo-sensitive TRP channels.

Enzymes for Pancreatic Islet Isolation Impact Chemokine-Production and Polarization of Insulin-Producing β-Cells with Reduced Functional Survival of Immunoisolated Rat Islet-Allografts as a Consequence.

The primary aim of this study was to determine whether normal variations in enzyme-activities of collagenases applied for rat-islet isolation impact longevity of encapsulated islet grafts. Also we studied the functional and immunological properties of rat islets isolated with different enzyme preparations to determine whether this impacts these parameters. Rat-islets were isolated from the pancreas with two different collagenases with commonly accepted collagenase, neutral protease, and clostripain activities. Islets had a similar and acceptable glucose-induced insulin-release profile but a profound statistical significant difference in production of the chemokines IP-10 and Gro-α. The islets were studied with nanotomy which is an EM-based technology for unbiased study of ultrastructural features of islets such as cell-cell contacts, endocrine-cell condition, ER stress, mitochondrial conditions, and cell polarization. The islet-batch with higher chemokine-production had a lower amount of polarized insulin-producing β-cells. All islets had more intercellular spaces and less interconnected areas with tight cell-cell junctions when compared to islets in the pancreas. Islet-graft function was studied by implanting encapsulated and free islet grafts in rat recipients. Alginate-based encapsulated grafts isolated with the enzyme-lot inducing higher chemokine production and lower polarization survived for a two-fold shorter period of time. The lower survival-time of the encapsulated grafts was correlated with a higher influx of inflammatory cells at 7 days after implantation. Islets from the same two batches transplanted as free unencapsulated-graft, did not show any difference in survival or function in vivo. Lack of insight in factors contributing to the current lab-to-lab variation in longevity of encapsulated islet-grafts is considered to be a threat for clinical application. Our data suggest that seemingly minor variations in activity of enzymes applied for islet-isolation might contribute to longevity-variations of immunoisolated islet-grafts.

Reduced AMPKα2 protein expression restores glucose-induced insulin secretion in islets from calorie-restricted rats.

In a state of caloric restriction (CR), improved insulin action was associated with the activation of AMP-activated kinase (AMPK). Here, we verified whether AMPK was involved in impaired β-cell function in islets from rats subjected to CR for 21 days. Eight-week-old male rats were distributed into a control (CTL) group that was fed an isocaloric diet ad libitum or a CR group that received 60% of the food consumed by the CTL group. From days 18-21, CTL and CR rats were treated with sense (CTLS and CRS) or antisense (CTLAS and CRAS) AMPKα2 oligonucleotides. Caloric restriction was associated with decreased body weight, perigonadal fat pads and insulinaemia, while higher glucose tolerance was observed in CRS rats. Antisense treatment normalized insulinaemia and glucose tolerance in CRAS rats and increased cholesterolaemia in CRAS and CTLAS groups. These effects were associated with reduced pAMPK/AMPK protein expression in the liver of rats treated with antisense oligonucleotides. Additionally, CRS islets showed higher pAMPK/AMPK content and lower glucose-induced insulin release. As expected, antisense oligonucleotides against AMPKα2 efficiently reduced pAMPK/AMPK protein in CRAS and CTLAS islets. The lower AMPK content in CRAS islets normalized the insulin secretion in islets exposed to 16.7 mM glucose. In addition, CTLAS islets presented higher insulin secretion at 2.8 and 16.7 mM glucose. These findings support the hypothesis that higher AMPK protein expression is involved in impaired β-cell function in islets from rats subjected to CR for 21 days.

Stimulation-dependent gating of TRPM3 channel in planar lipid bilayers.

The transient receptor potential melastatin (TRPM)-3 channel is critical for various physiologic processes. In somatosensory neurons, TRPM3 has been implicated in temperature perception and inflammatory hyperalgesia, whereas in pancreatic β-cells the channel has been linked to glucose-induced insulin release. As a typical representative of the TRP family, TRPM3 is highly polymodal. In cells, it is activated by heat and chemical agonists, including pregnenolone sulfate (PS) and nifedipine (Nif). To define the nuances of TRPM3 channel activity and its modulators, we succeeded in incorporating the TRPM3 protein into planar lipid bilayers. We found that phosphatidylinositol-4,5-bisphosphate (PIP2) or clotrimazole is necessary for channel opening by PS. Unlike PS, the presence of Nif alone sufficed to induce TRPM3 activity and demonstrated distinct gating behavior. We also performed an extensive thermodynamic analysis of TRPM3 activation and found that TRPM3 exhibited slight temperature sensitivity in the bilayers. In the absence of other agonists TRPM3 channels remained closed upon heat-induced stimulation, but opened in the presence of PIP2, although with only a low open-probability profile. Together, our results elucidate the details peculiar to TRPM3 channel function in an isolated system. We confirmed its direct gating by PS and PIP2, but found a lack of the strong intrinsic temperature sensitivity common to other thermosensitive TRP channels.

IL-10 gene transfer upregulates arcuate POMC and ameliorates hyperphagia, obesity and diabetes by substituting for leptin.

Obesity and metabolic syndrome are the major risk factors for cardiovascular disease. Obesity is caused by increased food intake and/or decreased energy expenditure. Leptin potently inhibits food intake and promotes energy expenditure. These effects of leptin involve the activation of proopiomelanocortin (POMC) neurons in the hypothalamus arcuate nucleus (ARC). Disruption of leptin signaling in POMC neuron is considered one of the major causes for obesity. The present study aimed to examine whether overexpression of interleukin-10 (IL-10) could substitute for the leptin action and ameliorate obesity in leptin-deficient Lep(ob/ob) mice. Adeno-associated virus (AAV) expressing murine IL-10 (AAV-mIL-10) was injected into the skeletal muscle to overexpress IL-10 in mice. These mice were subsequently subjected to analysis of body weight, food intake, glucose metabolism and underlying mechanisms. In Lep(ob/ob) mice, AAV-IL-10 ameliorated hyperphagia, obesity, glucose intolerance and insulin resistance, as well as attenuated tumor necrosis factor-α expression. The IL-10 treatment also improved glucose-induced insulin release. Furthermore, IL-10 treatment increased POMC mRNA expression in ARC and phosphorylation of signal transducer and activator of transcription-3 (STAT3) in ARC and white adipose tissue (WAT). In neuron-specific STAT3-null mice that exhibited obesity and hyperphagia, AAV-mIL-10 administration failed to affect food intake, body weight and phosphorylation of STAT3 in WAT. These results demonstrate that peripheral overexpression of IL-10 induces STAT3 phosphorylation in ARC POMC neurons, and thereby ameliorates hyperphagia and obesity caused by leptin deficiency. IL-10 gene transfer may provide an effective approach for preventing progression of metabolic syndrome due to leptin resistance.

The β-cell GHSR and downstream cAMP/TRPM2 signaling account for insulinostatic and glycemic effects of ghrelin

Gastric hormone ghrelin regulates insulin secretion, as well as growth hormone release, feeding behavior and adiposity. Ghrelin is known to exert its biological actions by interacting with the growth hormone secretagogue-receptor (GHSR) coupled to Gq/11-protein signaling. By contrast, ghrelin acts on pancreatic islet β-cells via Gi-protein-mediated signaling. These observations raise a question whether the ghrelin action on islet β-cells involves atypical GHSR and/or distinct signal transduction. Furthermore, the role of the β-cell GHSR in the systemic glycemic effect of ghrelin still remains to be defined. To address these issues, the present study employed the global GHSR-null mice and those re-expressing GHSR selectively in β-cells. We here report that ghrelin attenuates glucose-induced insulin release via direct interaction with ordinary GHSR that is uniquely coupled to novel cAMP/TRPM2 signaling in β-cells, and that this β-cell GHSR with unique insulinostatic signaling largely accounts for the systemic effects of ghrelin on circulating glucose and insulin levels. The novel β-cell specific GHSR-cAMP/TRPM2 signaling provides a potential therapeutic target for the treatment of type 2 diabetes.

Heterozygous inactivation of plasma membrane Ca(2+)-ATPase in mice increases glucose-induced insulin release and beta cell proliferation, mass and viability.

Calcium plays an important role in the process of glucose-induced insulin release in pancreatic beta cells. These cells are equipped with a double system responsible for Ca(2+) extrusion--the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+)-ATPase (PMCA). We have shown that heterozygous inactivation of NCX1 in mice increased glucose-induced insulin release and stimulated beta cell proliferation and mass. In the present study, we examined the effects of heterozygous inactivation of the PMCA on beta cell function. Biological and morphological methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release and immunohistochemistry) were used to assess beta cell function and proliferation in Pmca2 (also known as Atp2b2) heterozygous mice and control littermates ex vivo. Blood glucose and insulin levels were also measured to assess glucose metabolism in vivo. Pmca (isoform 2) heterozygous inactivation increased intracellular Ca(2+) stores and glucose-induced insulin release. Moreover, increased beta cell proliferation, mass, viability and islet size were observed in Pmca2 heterozygous mice. However, no differences in beta cell glucose metabolism, proinsulin immunostaining and insulin content were observed. The present data indicates that inhibition of Ca(2+) extrusion from the beta cell and its subsequent intracellular accumulation stimulates beta cell function, proliferation and mass. This is in agreement with our previous results observed in mice displaying heterozygous inactivation of NCX, and indicates that inhibition of Ca(2+) extrusion mechanisms by small molecules in beta cells may represent a new approach in the treatment of type 1 and type 2 diabetes.

Postnatal β-cell maturation is associated with islet-specific microRNA changes induced by nutrient shifts at weaning.

Glucose-induced insulin secretion is an essential function of pancreatic β-cells that is partially lost in individuals affected by Type 2 diabetes. This unique property of β-cells is acquired through a poorly understood postnatal maturation process involving major modifications in gene expression programs. Here we show that β-cell maturation is associated with changes in microRNA expression induced by the nutritional transition that occurs at weaning. When mimicked in newborn islet cells, modifications in the level of specific microRNAs result in a switch in the expression of metabolic enzymes and cause the acquisition of glucose-induced insulin release. Our data suggest microRNAs have a central role in postnatal β-cell maturation and in the determination of adult functional β-cell mass. A better understanding of the events governing β-cell maturation may help understand why some individuals are predisposed to developing diabetes and could lead to new strategies for the treatment of this common metabolic disease.

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.

Role of Islet Glucokinase, Glucose Metabolism, and Insulin Pathway in the Enhancing Effect of Islet Neogenesis-Associated Protein on Glucose-Induced Insulin Secretion.

To demonstrate the role of islet glucokinase, glucose metabolism, and intracellular insulin mediators in the enhancing effect of islet neogenesis-associated protein pentadecapeptide (INGAP-PP) on glucose-induced insulin secretion. Islets from normal rats were cultured for 4 days in the absence or presence of 10 μg/mL INGAP-PP, with/without Wortmannin or LY294002. Islets were incubated with different glucose concentrations to measure insulin secretion and content, hexokinase and glucokinase activity, glucose oxidation and utilization, glucokinase, insulin receptor, insulin receptor substrate (IRS)-1/2, and PI3K concentration and phosphorylation. The INGAP-PP significantly increased insulin release at high but not at low glucose concentration, glucokinase activity, glucose metabolism, glucokinase, insulin receptor, IRS-2 and PI3K protein concentration, insulin receptor and IRS-1/2 tyrosine phosphorylation, and the association of p85 with IRS-1. Wortmannin and LY294002 blocked INGAP-PP effect on insulin secretion and glucokinase protein levels in a dose-dependent manner. The enhancing effect of INGAP-PP on glucose-induced insulin release could be partly ascribed to its effect on glucokinase activity and glucose metabolism and is mainly mediated by the PI3K/AKT pathway. These results, together with the low hypoglycemia risk associated with the use of INGAP-PP, offer a new alternative for diabetes prevention and treatment.

Regulation of the transient receptor potential channel TRPM3 by phosphoinositides.

The transient receptor potential (TRP) channel TRPM3 is a calcium-permeable cation channel activated by heat and by the neurosteroid pregnenolone sulfate (PregS). TRPM3 is highly expressed in sensory neurons, where it plays a key role in heat sensing and inflammatory hyperalgesia, and in pancreatic β cells, where its activation enhances glucose-induced insulin release. However, despite its functional importance, little is known about the cellular mechanisms that regulate TRPM3 activity. Here, we provide evidence for a dynamic regulation of TRPM3 by membrane phosphatidylinositol phosphates (PIPs). Phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) and ATP applied to the intracellular side of excised membrane patches promote recovery of TRPM3 from desensitization. The stimulatory effect of cytosolic ATP on TRPM3 reflects activation of phosphatidylinositol kinases (PI-Ks), leading to resynthesis of PIPs in the plasma membrane. Various PIPs directly enhance TRPM3 activity in cell-free inside-out patches, with a potency order PI(3,4,5)P3 > PI(3,5)P2 > PI(4,5)P2 ≈ PI(3,4)P2 >> PI(4)P. Conversely, TRPM3 activity is rapidly and reversibly inhibited by activation of phosphatases that remove the 5-phosphate from PIPs. Finally, we show that recombinant TRPM3, as well as the endogenous TRPM3 in insuloma cells, is rapidly and reversibly inhibited by activation of phospholipase C-coupled muscarinic acetylcholine receptors. Our results reveal basic cellular mechanisms whereby membrane receptors can regulate TRPM3 activity.

Epigenetic programming of glucose-regulated insulin release.

Pancreatic β cells support glucose homeostasis with great precision by matching insulin release to the metabolic needs of the moment. Previous gene-expression analysis indicates that adult β cells not only produce cell-specific proteins, but also repress a small set of housekeeping genes - such as those encoding lactate dehydrogenase A (LDHA), solute transporter MCT1, and hexokinase 1 (HK1) - that would otherwise interfere with normal β cell function. In this issue of the JCI, Dhawan et al. elucidate a molecular mechanism involved in β cell-specific repression of Ldha and Hk1 that is mediated by induction of the de novo DNA methyltransferase DNMT3A during the first weeks after birth. Failure to induce DNMT3A-dependent methylation disrupts normal glucose-induced insulin release in adult life. The results of this study reinforce the idea that the phenotype of adult β cells has two faces and that failure to achieve selective gene repression undermines β cell support of normal glucose homeostasis.

5,7-Dihydroxy-3,4,6-Trimethoxyflavone Attenuates Ischemic Damage and Apoptosis in Mouse Islets

BackgroundThe transplantation of isolated pancreatic islets is a promising treatment for diabetes. 5,7-dihydroxy-3,4,6-trimethoxyflavone (Eupatilin), a pharmacologically active flavone derived from the Artemisia plant species, has been reported to have antioxidant and anti-inflammatory activities. This study examines the hypothesis that preoperative eupatilin treatment can attenuate ischemic damage and apoptosis before islet transplantation. MethodsIslets isolated from Balb/c mice were randomly divided into 2 groups, and cultured in medium supplemented with or without eupatilin. In vitro islet viability and function were assessed. After treatment with a cytokine cocktail consisting of tumor necrosis factor (TNF)-α, interferon (INF)-γ, and interleukin (IL)-1β, islet cell viability, function, and apoptotic status were determined. The glutathione (GSH) and nitrous oxide (NO) levels were also measured. Proteins related to apoptosis were analyzed using Western blotting. ResultsThere was no difference in cell viability between the 2 groups. Islets cultured in the medium supplemented with eupatilin showed 1.4-fold higher glucose-induced insulin secretion than the islets cultured in the medium without eupatilin. After treatment with a cytokine cocktail, glucose-induced insulin release and the total insulin content of the islets were significantly improved in eupatilin-pretreated islets compared with islets not treated with eupatilin. Apoptosis was significantly decreased, and GSH levels were elevated in the eupatilin-pretreated group. Cytokine-only treated islets produced significantly higher levels of NO, iNOS, and caspase-3 than islets pretreated with eupatilin before cytokine treatment. ConclusionsThese results suggest that preoperative eupatilin administration enhances islet function before transplantation and attenuates the cytokine-induced damage associated with NO production and apoptosis.

Lack of stress responses to long-term effects of corticosterone in Caps2 knockout mice.

Chronic stress is associated with anxiety and depressive disorders, and can cause weight gain. Ca2+-dependent activator protein for secretion 2 (CAPS2) is involved in insulin release. Caps2 knockout (KO) mice exhibit decreased body weight, reduced glucose-induced insulin release, and abnormal psychiatric behaviors. We chronically administered the stress hormone corticosterone (CORT), which induces anxiety/depressive-like behavior and normally increases plasma insulin levels, via the drinking water for 10 weeks, and we examined the stress response in KO mice. Chronic CORT exposure inhibited stress-induced serum CORT elevation in wild-type (WT) mice, but not in KO mice. Poor weight gain in CORT-treated animals was observed until week 6 in WT mice, but persisted for the entire duration of the experiment in KO mice, although there is no difference in drug*genotype interaction. Among KO mice, food consumption was unchanged, while water consumption was higher, over the duration of the experiment in CORT-treated animals, compared with untreated animals. Moreover, serum insulin and leptin levels were increased in CORT-treated WT mice, but not in KO mice. Lastly, both WT and KO mice displayed anxiety/depressive-like behavior after CORT administration. These results suggest that Caps2 KO mice have altered endocrine responses to CORT administration, while maintaining CORT-induced anxiety/depressive-like behavior.

ARA290 Improves Insulin Release and Glucose Tolerance in Type 2 Diabetic Goto-Kakizaki Rats.

Effects of ARA290 on glucose homeostasis were studied in type 2 diabetic Goto-Kakizaki (GK) rats. In GK rats receiving ARA290 daily for up to 4 wks, plasma glucose concentrations were lower after 3 and 4 wks, and hemoglobin A1c (Hb A1c) was reduced by ~20% without changes in whole body and hepatic insulin sensitivity. Glucose-stimulated insulin secretion was increased in islets from ARA290-treated rats. Additionally, in response to glucose, carbachol and KCl, islet cytoplasmic free Ca2+ concentrations, [Ca2+]i, were higher and the frequency of [Ca2+]i oscillations enhanced compared with placebo. ARA290 also improved stimulus-secretion coupling for glucose in GK rat islets, as shown by an improved glucose oxidation rate, ATP production and acutely enhanced glucose-stimulated insulin secretion. ARA290 also exerted an effect distal to the ATP-sensitive potassium (KATP) channel on the insulin exocytotic pathway, since the insulin response was improved following islet depolarization by KCl when KATP channels were kept open by diazoxide. Finally, inhibition of protein kinase A completely abolished effects of ARA290 on insulin secretion. In conclusion, ARA290 improved glucose tolerance without affecting hematocrit in diabetic GK rats. This effect appears to be due to improved γ-cell glucose metabolism and [Ca2+]i handling, and thereby enhanced glucose-induced insulin release.

Crosstalk between advanced glycation end products (AGEs)-receptor RAGE axis and dipeptidyl peptidase-4-incretin system in diabetic vascular complications.

Advanced glycation end products (AGEs) consist of heterogenous group of macroprotein derivatives, which are formed by non-enzymatic reaction between reducing sugars and amino groups of proteins, lipids and nucleic acids, and whose process has progressed at an accelerated rate under diabetes. Non-enzymatic glycation and cross-linking of protein alter its structural integrity and function, contributing to the aging of macromolecules. Furthermore, engagement of receptor for AGEs (RAGE) with AGEs elicits oxidative stress generation and subsequently evokes proliferative, inflammatory, and fibrotic reactions in a variety of cells. Indeed, accumulating evidence has suggested the active involvement of accumulation of AGEs in diabetes-associated disorders such as diabetic microangiopathy, atherosclerotic cardiovascular diseases, Alzheimer’s disease and osteoporosis. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretins, gut hormones secreted from the intestine in response to food intake, both of which augment glucose-induced insulin release, suppress glucagon secretion, and slow gastric emptying. Since GLP-1 and GIP are rapidly degraded and inactivated by dipeptidyl peptidase-4 (DPP-4), inhibition of DPP-4 and/or DPP-4-resistant GLP-1 analogues have been proposed as a potential target for the treatment of diabetes. Recently, DPP-4 has been shown to cleave multiple peptides, and blockade of DPP-4 could exert diverse biological actions in GLP-1- or GIP-independent manner. This article summarizes the crosstalk between AGEs-RAGE axis and DPP-4-incretin system in the development and progression of diabetes-associated disorders and its therapeutic intervention, especially focusing on diabetic vascular complications.

Impaired Islet Function in Commonly Used Transgenic Mouse Lines due to Human Growth Hormone Minigene Expression

The human growth hormone (hGH) minigene is frequently used in the derivation of transgenic mouse lines to enhance transgene expression. Although this minigene is present in the transgenes as a secondcistron, and thus not thought to be expressed, we found that three commonly used lines, Pdx1-Cre(Late), RIP-Cre, and MIP-GFP, each expressed significant amounts of hGH in pancreatic islets. Locally secreted hGH binds to prolactin receptors on β cells, activates STAT5 signaling, and induces pregnancy-like changes in gene expression, thereby augmenting pancreatic β cell mass and insulin content. In addition, islets of Pdx1-Cre(Late) mice have lower GLUT2 expression and reduced glucose-induced insulin release and are protected against the β cell toxin streptozotocin. These findings may be important when interpreting results obtained when these and other hGH minigene-containing transgenic mice are used.

PHYTOCHEMISTRY, MODE OF ACTION AND PHARMACOLOGICAL APPLICATIONS OF 4-HYDROXYISOLEUCINE FROM TRIGONELLA FOENUM GRAECUM

Herbal products are emerging as promising anti-diabetic drugs due to lower cost, lesser side effects and renewable natural sources. 4-hydroxyisoleucine is reported as the bioactive compound, which is responsible for anti-diabetic activity of fenugreek. This review illustrates the overall properties, synthesis, mode of action and pharmacological profile of 4-hydroxyisoleucine. From the seeds of Trigonella foenum graecum an unusual amino acid, 4-hydroxyisoleucine has been isolated which significantly improved lipid profile and it increased glucose-induced insulin release in human and rat pancreatic islet cells. No adverse effect or toxic effect of 4-hydroxyisoleucine has been found. Marketed preparations are also available.

Ghrelin signalling in β-cells regulates insulin secretion and blood glucose.

Insulin secretion from pancreatic islet β-cells is stimulated by glucose. Glucose-induced insulin release is potentiated or suppressed by hormones and neural substances. Ghrelin, an acylated 28-amino acid peptide, was isolated from the stomach in 1999 as the endogenous ligand for the growth hormone (GH) secretagogue-receptor (GHS-R). Circulating ghrelin is produced predominantly in the stomach and to a lesser extent in the intestine, pancreas and brain. Ghrelin, initially identified as a potent stimulator of GH release and feeding, has been shown to suppress glucose-induced insulin release. This insulinostatic action is mediated by Gα(i2) subtype of GTP-binding proteins and delayed outward K⁺ (Kv) channels. Interestingly, ghrelin is produced in pancreatic islets. The ghrelin originating from islets restricts insulin release and thereby upwardly regulates the systemic glucose level. Furthermore, blockade or elimination of ghrelin enhances insulin release, which can ameliorate glucose intolerance in high-fat diet fed mice and ob/ob mice. This review focuses on the insulinostatic action of ghrelin, its signal transduction mechanisms in islet β-cells, ghrelin's status as an islet hormone, physiological roles of ghrelin in regulating systemic insulin levels and glycaemia, and therapeutic potential of the ghrelin-GHS-R system as the target to treat type 2 diabetes.