Somatostatin receptor subtype-2-deficient mice with diet-induced obesity have hyperglycemia, nonfasting hyperglucagonemia, and decreased hepatic glycogen deposition

Readily releasable β cells with tight Ca2+-exocytosis coupling dictate biphasic glucose-stimulated insulin secretion

Biphasic glucose-stimulated insulin secretion (GSIS) is essential for blood glucose regulation, but a mechanistic model incorporating the recently identified islet β cell heterogeneity remains elusive. Here, we show that insulin secretion is spatially and dynamically heterogeneous across the islet. Using a zinc-based fluorophore with spinning-disc confocal microscopy, we reveal that approximately 40% of islet cells, which we call readily releasable β cells (RRβs), are responsible for 80% of insulin exocytosis events. Although glucose up to 18.2 mM fully mobilized RRβs to release insulin synchronously (first phase), even higher glucose concentrations enhanced the sustained secretion from these cells (second phase). Release-incompetent β cells show similarities to RRβs in glucose-evoked Ca2+ transients but exhibit Ca2+-exocytosis coupling deficiency. A decreased number of RRβs and their altered secretory ability are associated with impaired GSIS progression in ob/ob mice. Our data reveal functional heterogeneity at the level of exocytosis among β cells and identify RRβs as a subpopulation of β cells that make a disproportionally large contribution to biphasic GSIS from mouse islets.

Human skin stem cell-derived hepatic cells as in vitro drug discovery model for insulin-driven de novo lipogenesis

Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), is characterized by intrahepatic triglyceride accumulation and can progress to metabolic dysfunction-associated steatohepatitis (MASH) and liver fibrosis. Hepatic de novo lipogenesis (DNL), activated by glucose and insulin, is a central pathway contributing to early-stage development of MASLD. The emerging global prevalence of MASLD highlights the urgent need for pharmaceutical intervention to combat this health threat. However, the identification of novel drugs that could inhibit hepatic DNL is hampered by a lack of reliable, insulin-sensitive, human, in vitro, hepatic models. Here, we report human skin stem cell-derived hepatic cells (hSKP-HPC) as a unique in vitro model to study insulin-driven DNL (iDNL), evidenced by both gene expression and lipid accumulation readouts. Insulin-sensitive hSKP-HPC showed increased sterol regulatory element-binding protein 1c (SREBP-1c) expression, a key transcription factor for DNL. Furthermore, this physiologically relevant in vitro human steatosis model allowed both inhibition and activation of the iDNL pathway using reference inhibitors and activators, respectively. Optimisation of the lipid accumulation assay to a high-throughput, 384-well format enabled the screening of a library of annotated compounds, delivering new insights on key players in the iDNL pathway and MASLD pathophysiology. Together, these results establish the value of the hSKP-HPC model in preclinical development of antisteatotic drugs to combat MASLD.

A selective nonpeptide somatostatin receptor 5 (SST5) agonist effectively decreases insulin secretion in hyperinsulinism

Congenital hyperinsulinism (HI), a beta cell disorder most commonly caused by inactivating mutations of beta cell K_ATP channels, results in dysregulated insulin secretion and persistent hypoglycemia. Children with K_ATP-HI are unresponsive to diazoxide, the only FDA-approved drug for HI, and utility of octreotide, the second line therapy, is limited because of poor efficacy, desensitization, and somatostatin receptor type 2 (SST2)-mediated side effects. Selective targeting of SST5, an SST receptor associated with potent insulin secretion suppression, presents a new avenue for HI therapy. Here, we determined that CRN02481, a highly selective nonpeptide SST5 agonist, significantly decreased basal and amino acid-stimulated insulin secretion in both Sur1^-/- (a model for K_ATP-HI) and wild type mouse islets. Oral administration of CRN02481 significantly increased fasting glucose and prevented fasting hypoglycemia compared to vehicle in Sur1^-/- mice. During a glucose tolerance test, CRN02481 significantly increased glucose excursion in both WT and Sur1^-/- mice compared to control. CRN02481 also reduced glucose- and tolbutamide-stimulated insulin secretion from healthy, control human islets similar to the effects observed with SS14 and peptide somatostatin analogs. Moreover, CRN02481 significantly decreased glucose- and amino acid-stimulated insulin secretion in islets from two infants with K_ATP-HI and one with Beckwith-Weideman Syndrome-HI. Taken together, these data demonstrate that a potent and selective SST5 agonist effectively prevents fasting hypoglycemia and suppresses insulin secretion not only in a K_ATP-HI mouse model, but also in healthy human islets and islets from HI patients.

Peptides in the regulation of glucagon secretion

Glucose homeostasis is maintained by the glucoregulatory hormones, glucagon, insulin and somatostatin, secreted from the islets of Langerhans. Glucagon is the body's most important anti-hypoglycemic hormone, mobilizing glucose from glycogen stores in the liver in response to fasting, thus maintaining plasma glucose levels within healthy limits. Glucagon secretion is regulated by both circulating nutrients, hormones and neuronal inputs. Hormones that may regulate glucagon secretion include locally produced insulin and somatostatin, but also urocortin-3, amylin and pancreatic polypeptide, and from outside the pancreas glucagon-like peptide-1 and 2, peptide tyrosine tyrosine and oxyntomodulin, glucose-dependent insulinotropic polypeptide, neurotensin and ghrelin, as well as the hypothalamic hormones arginine-vasopressin and oxytocin, and calcitonin from the thyroid. Each of these hormones have distinct effects, ranging from regulating blood glucose, to regulating appetite, stomach emptying rate and intestinal motility, which makes them interesting targets for treating metabolic diseases. Awareness regarding the potential effects of the hormones on glucagon secretion is important since secretory abnormalities could manifest as hyperglycemia or even lethal hypoglycemia. Here, we review the effects of each individual hormone on glucagon secretion, their interplay, and how treatments aimed at modulating the plasma levels of these hormones may also influence glucagon secretion and glycemic control.

Antagonizing somatostatin receptor subtype 2 and 5 reduces blood glucose in a gut- and GLP-1R-dependent manner

Somatostatin (SS) inhibits glucagon-like peptide-1 (GLP-1) secretion in a paracrine manner. We hypothesized that blocking somatostatin subtype receptor 2 (SSTR2) and 5 (SSTR5) would improve glycemia by enhancing GLP-1 secretion. In the perfused mouse small intestine, the selective SSTR5 antagonist (SSTR5a) stimulated glucose-induced GLP-1 secretion to a larger degree than the SSTR2 antagonist (SSTR2a). In parallel, mice lacking the SSTR5R showed increased glucose-induced GLP-1 secretion. Both antagonists improved glycemia in vivo in a GLP-1 receptor-dependent (GLP-1R-dependent) manner, as the glycemic improvements were absent in mice with impaired GLP-1R signaling and in mice treated with a GLP-1R-specific antagonist. SSTR5a had no direct effect on insulin secretion in the perfused pancreas, whereas SSTR2a increased insulin secretion in a GLP-1R-independent manner. Adding a dipeptidyl peptidase 4 inhibitor (DPP-4i) in vivo resulted in additive effects on glycemia. However, when glucose was administered intraperitoneally, the antagonist was incapable of lowering blood glucose. Oral administration of SSTR5a, but not SSTR2a, lowered blood glucose in diet-induced obese mice. In summary, we demonstrate that selective SSTR antagonists can improve glucose control primarily through the intestinal GLP-1 system in mice.

Obesity dysregulates fasting-induced changes in glucagon secretion

Hyperglucagonemia, a hallmark in obesity and insulin resistance promotes hepatic glucose output, exacerbating hyperglycemia and thus predisposing to the development type 2 diabetes. As such, glucagon signaling is a key target for new therapeutics to manage insulin resistance. We evaluated glucagon homeostasis in lean and obese mice and people. In lean mice, fasting for 24 h caused a rise in glucagon. In contrast, a decrease in serum glucagon compared to baseline was observed in diet-induced obese mice between 8 and 24 h of fasting. Fasting decreased serum insulin in both lean and obese mice. Accordingly, the glucagon:insulin ratio was unaffected by fasting in obese mice but increased in lean mice. Re-feeding (2 h) restored hyperglucagonemia in obese mice. Pancreatic perfusion studies confirm that fasting (16 h) decreases pancreatic glucagon secretion in obese mice. Consistent with our findings in the mouse, a mixed meal increased serum glucagon and insulin concentrations in obese humans, both of which decreased with time after a meal. Consequently, fasting and re-feeding less robustly affected glucagon:insulin ratios in obese compared to lean participants. The glucoregulatory disturbance in obesity may be driven by inappropriate regulation of glucagon by fasting and a static glucagon:insulin ratio.

How to Position Pasireotide LAR Treatment in Acromegaly

CONTEXT

Pasireotide long-acting release (LAR) is a somatostatin multireceptor ligand, and in the current consensus criteria pasireotide LAR is considered the second-line medical treatment for acromegaly. We present in this article our recommendations to define the position of pasireotide LAR in the treatment of acromegaly and provide recommendations for the management of pasireotide-induced hyperglycemia.

EVIDENCE ACQUISITION

Our recommendations are based on our experiences with the pasireotide LAR and pegvisomant (PEGV) combination study and the available basic or clinical articles published in peer-reviewed international journals on pasireotide LAR and acromegaly.

EVIDENCE SYNTHESIS

In accordance with the current consensus criteria, we recommend pasireotide LAR monotherapy as a second-line therapy in young patients who show tumor growth during first-generation somatostatin receptor ligand (SRL) therapy and in patients who show tumor growth during PEGV therapy. In addition, we recommend pasireotide LAR monotherapy in patients with headache not responsive to first-generation SRL therapy and in patients who experience side effects or are intolerant to PEGV monotherapy. In contrast to the current consensus criteria, we recommend considering combination therapy with pasireotide LAR and PEGV as third-line treatment in patients without diabetes at low PEGV dosages (≤80 mg/week) and in patients with tumor growth or symptoms of active acromegaly during first-generation SRL and PEGV combination therapy. With respect to pasireotide-induced hyperglycemia, we recommend a more liberal strategy of blood glucose monitoring during pasireotide treatment.

CONCLUSIONS

In contrast to the current consensus criteria, we recommend a more reluctant use of pasireotide LAR therapy for the treatment of acromegaly.

miR-132-3p is a positive regulator of alpha-cell mass and is downregulated in obese hyperglycemic mice

Objective

Diabetes is a complex disease implicating several organs and cell types. Within the islets, dysregulation occurs in both alpha- and beta-cells, leading to defects of insulin secretion and increased glucagon secretion. Dysregulation of alpha-cells is associated with transcriptome changes. We hypothesized that microRNAs (miRNAs) which are negative regulators of mRNA stability and translation could be involved in alpha-cell alterations or adaptations during type 2 diabetes.

Methods

miRNA microarray analyses were performed on pure alpha- and beta-cells from high-fat diet fed obese hyperglycemic mice and low-fat diet fed controls. Then, the most regulated miRNA was overexpressed or inhibited in primary culture of mouse and human alpha-cells to determine its molecular and functional impact.

Results

16 miRNAs were significantly regulated in alpha-cells of obese hyperglycemic mice and 28 in beta-cells. miR-132-3p had the strongest regulation level in alpha-cells, where it was downregulated, while we observed an opposite upregulation in beta-cells. In vitro experiments showed that miR-132-3p, which is inversely regulated by somatostatin and cAMP, is a positive modulator of alpha-cell proliferation and implicated in their resistance to apoptosis. These effects are associated with the regulation of a series of genes, including proliferation and stress markers Mki67 and Bbc3 in mouse and human alpha-cells, potentially involved in miR-132-3p functions.

Conclusions

Downregulation of miR-132-3p in alpha-cells of obese diabetic mice may constitute a compensatory mechanism contributing to keep glucagon-producing cell number constant in diabetes.

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Alpha- and beta-cells present specific microRNA signatures.

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16 microRNAs are significantly regulated in alpha-cells of obese hyperglycemic mice.

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miR-132-3p is downregulated in alpha-cells of obese hyperglycemic mice.

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miR-132-3p stimulates alpha-cells proliferation and resistance to apoptosis.

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miR-132-3p is regulated by somatostatin in alpha-cells.

Comparisons of Effects on Intestinal Short-Chain Fatty Acid Concentration after Exposure of Two Glycosidase Inhibitors in Mice

Acarbose and voglibose are the most widely used diabetes drugs as glycosidase inhibitors. In this study, the use of these two inhibitors significantly increased the content of starch in large intestine, and altered the concentration of short-chain fatty acids (SCFAs) by affecting the intestinal microbiota. However, there are some differences in the intestinal microbiome of the two groups of mice, mainly in bacteria such as Bacteroidaceae bacteroides and Desulfovibrionaceae desulfovibrio. The productions of acetate and propionate in caecum in voglibose group were significantly higher than those in acarbose group and two kinds of glycosidase inhibitors were close in the production of butyrate in caecum. The Tax4Fun analysis based on Kyoto Encyclopedia of Genes and Genomes (KEGG) data indicated that different productions of acetate and propionate between acarbose group and voglibose group may be related to 2-oxoisovalerate dehydrogenase and pyruvate oxidase. In addition, in-vitro experiments suggested that voglibose had less effect on epithelial cells than acarbose after direct stimulation. According to the recent researches of SCFAs produced by intestinal microbiota, our comparative study shown higher concentration of these beneficial fatty acids in the lumen of voglibose-treated mice, which implied a lower level of inflammation.

Hyperglycemia induced by pasireotide in patients with Cushing's disease or acromegaly

PURPOSE

Cushing's disease (CD) and acromegaly are characterized by excessive hormone secretion resulting in comorbidities such as impaired glucose metabolism, diabetes and hypertension. Pasireotide is a new-generation, multireceptor-targeted somatostatin receptor ligand approved for CD (subcutaneous [SC] injection formulation) and acromegaly (long-acting release [LAR] formulation). In clinical studies of pasireotide, hyperglycemia-related adverse events (AEs) were frequently observed. This review highlights differences in reported rates of hyperglycemia in pasireotide trials and discusses risk factors for and management of pasireotide-associated hyperglycemia.

METHODS

Clinical trials evaluating pasireotide in patients with CD or acromegaly were reviewed.

RESULTS

The frequency of hyperglycemia-related AEs was lower in patients with acromegaly treated with pasireotide LAR (57.3-67.0 %) than in patients with CD treated with pasireotide SC (68.4-73.0 %). Fewer patients with acromegaly treated with pasireotide LAR discontinued therapy because of hyperglycemia-related AEs (Colao et al. in J Clin Endocrinol Metab 99(3):791-799, 2014, 3.4 %; Gadelha et al. in Lancet Diabetes Endocrinol 2(11):875-884, 2014, 4.0 %) than did patients with CD treated with pasireotide SC (Boscaro et al. in Pituitary 17(4):320-326, 2014, 5.3 %; Colao et al. in N Engl J Med 366(10):914-924, 2012, 6.0 %). Hyperglycemia-related AEs occurred in 40.0 % of patients with acromegaly treated with pasireotide SC, and 10.0 % discontinued treatment because of hyperglycemia. Ongoing studies evaluating pasireotide LAR in patients with CD and management of pasireotide-induced hyperglycemia in patients with CD or acromegaly (ClinicalTrials.gov identifiers NCT01374906 and NCT02060383, respectively) will address these key safety issues.

CONCLUSIONS

Disease pathophysiology, drug formulation, and physician experience potentially influence the differences in reported rates of pasireotide-induced hyperglycemia in CD and acromegaly. Hyperglycemic effects associated with pasireotide have a predictable pattern, can be managed with antidiabetic agents, and are reversible upon discontinuation.

Increasing GLP-1 Circulating Levels by Bariatric Surgery or by GLP-1 Receptor Agonists Therapy: Why Are the Clinical Consequences so Different?

The "incretin effect" is used to describe the observation that more insulin is secreted after the oral administration of glucose compared to that after the intravenous administration of the same amount of glucose. During the absorption of meals, the gut is thought to regulate insulin secretion by secreting a specific factor that targets pancreatic beta cells. Additional research confirmed this hypothesis with the discovery of two hormones called incretins: gastric inhibitory peptide (GIP) and glucagon-like peptide 1 (GLP-1). During meals, specific cells in the gut (L and K enteroendocrine cells) secrete incretins, causing an increase in the blood concentrations of, respectively, GLP-1 and GIP. Bariatric surgery is now proposed during the therapeutic management of type 2 diabetes in obese or overweight populations. It has been hypothesized that restoration of endogenous GLP-1 secretion after the surgery may contribute to the postsurgical resolution of diabetes. In 2005, the commercialization of GLP-1 receptor agonists gave the possibility to test this hypothesis. A few years later, it is now accepted that GLP-1 receptor agonists and bariatric surgery differently improve type 2 diabetes. These differences between endogenous and exogenous GLP-1 on glucose homeostasis emphasized the dual properties of GLP-1 as a peptide hormone and as a neurotransmitter.

Alpha-, Delta- and PP-cells: Are They the Architectural Cornerstones of Islet Structure and Co-ordination?

Islet non-β-cells, the α- δ- and pancreatic polypeptide cells (PP-cells), are important components of islet architecture and intercellular communication. In α-cells, glucagon is found in electron-dense granules; granule exocytosis is calcium-dependent via P/Q-type Ca(2+)-channels, which may be clustered at designated cell membrane sites. Somatostatin-containing δ-cells are neuron-like, creating a network for intra-islet communication. Somatostatin 1-28 and 1-14 have a short bioactive half-life, suggesting inhibitory action via paracrine signaling. PP-cells are the most infrequent islet cell type. The embryologically separate ventral pancreas anlage contains PP-rich islets that are morphologically diffuse and α-cell deficient. Tissue samples taken from the head region are unlikely to be representative of the whole pancreas. PP has anorexic effects on gastro-intestinal function and alters insulin and glucagon secretion. Islet architecture is disrupted in rodent diabetic models, diabetic primates and human Type 1 and Type 2 diabetes, with an increased α-cell population and relocation of non-β-cells to central areas of the islet. In diabetes, the transdifferentiation of non-β-cells, with changes in hormone content, suggests plasticity of islet cells but cellular function may be compromised. Understanding how diabetes-related disordered islet structure influences intra-islet cellular communication could clarify how non-β-cells contribute to the control of islet function.

The diabetes gene Hhex maintains δ-cell differentiation and islet function

The homeodomain transcription factor HHEX (hematopoietically expressed homeobox) has been linked to type 2 diabetes mellitus in genome-wide association studies. Zhang et al. discover that Hhex is selectively expressed in the somatostatin-secreting δ cell of the adult pancreas. Hhex was required for δ-cell differentiation, and the reduced somatostatin levels in Hhex-deficient islets caused disrupted paracrine inhibition of insulin release from δ cells. This study identifies Hhex as the first transcriptional regulator specifically required for islet δ cells and suggests compromised paracrine control as a contributor to type 2 diabetes.

The homeodomain transcription factor HHEX (hematopoietically expressed homeobox) has been repeatedly linked to type 2 diabetes mellitus (T2DM) using genome-wide association studies. We report here that within the adult endocrine pancreas, Hhex is selectively expressed in the somatostatin-secreting δ cell. Using two mouse models with Hhex deficiency in the endocrine pancreas, we show that Hhex is required for δ-cell differentiation. Decreased somatostatin levels in Hhex-deficient islets cause disrupted paracrine inhibition of insulin release from β cells. These findings identify Hhex as the first transcriptional regulator specifically required for islet δ cells and suggest compromised paracrine control as a contributor to T2DM.

Logistic model of glucose-regulated C-peptide secretion: hysteresis pathway disruption in impaired fasting glycemia

The present analysis tests the hypothesis that quantifiable disruption of the glucose-stimulated insulin-secretion dose-response pathway mediates impaired fasting glycemia (IFG) and type 2 diabetes mellitus (DM). To this end, adults with normal and impaired fasting glycemia (NFG, n = 30), IFG (n = 32), and DM (n = 14) were given a mixed meal containing 75 g glucose. C-peptide and glucose were measured over 4 h, 13 times in NFG and IFG and 16 times in DM (age range 50-57 yr, body mass index 28-32 kg/m(2)). Wavelet-based deconvolution analysis was used to estimate time-varying C-peptide secretion rates. Logistic dose-response functions were constructed analytically of the sensitivity, potency, and efficacy (in the pharmacological sense of slope, one-half maximal stimulation, and maximal effect) of glucose's stimulation of prehepatic insulin (C-peptide) secretion. A hysteresis changepoint time, demarcating unequal glucose potencies for onset and recovery pathways, was estimated simultaneously. According to this methodology, NFG subjects exhibited distinct onset and recovery potencies of glucose in stimulating C-peptide secretion (6.5 and 8.5 mM), thereby defining in vivo hysteresis (potency shift -2.0 mM). IFG patients manifested reduced glucose onset potency (8.6 mM), and diminished C-peptide hysteretic shift (-0.80 mM). DM patients had markedly decreased glucose potency (18.8 mM), reversal of C-peptide's hysteretic shift (+4.5 mM), and 30% lower C-peptide sensitivity to glucose stimulation. From these data, we conclude that a dynamic dose-response model of glucose-dependent control of C-peptide secretion can identify disruption of in vivo hysteresis in patients with IFG and DM. Pathway-defined analytic models of this kind may aid in the search for prediabetes biomarkers.

Role of ERβ and GPR30 in the endocrine pancreas: A matter of estrogen dose

The endocrine pancreas has emerged as a target for estrogens. The functions of pancreatic α-, β- and δ-cells are modulated by the endogenous hormone, 17β-estradiol (E2). Low physiological concentrations (100pM-1nM) of E2 rapidly decrease the activity of the ATP-sensitive potassium channel (K(ATP)) and enhance glucose-induced insulin release in β-cells in an estrogen receptor β (ERβ)-dependent manner. In addition to the insulinotropic action of ERβ, the newly described estrogen receptor, GPR30, is involved in the insulinotropic effects of high doses of E2 (100nM-5μM). The specific GPR30 agonist G1 also increases insulin secretion in β-cells. Low glucose-induced calcium oscillations and glucagon secretion are suppressed by E2. The effects on glucagon secretion may be mediated by GPR30. Somatostatin release is also decreased by E2 and G1. In this review we summarize all the data published up to date on the rapid insulinotropic effects of estrogens in the endocrine pancreas and propose a model to integrate the estrogen actions mediated through both receptors.

Effects of somatostatin analogs on glucose homeostasis in rats

Pasireotide (SOM230) is a multireceptor-targeted somatostatin analog with high binding affinity for sstr(1,2,3) and sstr(5). The effects of pasireotide and octreotide on blood glucose, insulin, and glucagon levels in rats were evaluated alone and in combination. Single-dose s.c. pasireotide acutely elevated plasma glucose, whereas single-dose s.c. octreotide had no or a small hypoglycemic effect. Glucose elevation with s.c. pasireotide was transient with tachyphylaxis after repeated or continuous administration. Pasireotide and octreotide caused similar inhibitory effects on insulin secretion, whereas pasireotide had a weaker inhibitory effect on glucagon secretion than octreotide. Continuous infusion of pasireotide or injection of pasireotide long-acting release (LAR) resulted in only small and transient elevations of plasma glucose. Based on these results, and differences in the sstr binding affinity of pasireotide vs octreotide, it was hypothesized that the sstr(5) vs sstr(2) receptor activation ratio is the main driver of hyperglycemia after pasireotide. The results also suggest that stronger activation of sstr(2) may counteract the hyperglycemic effect. Indeed, co-administration of octreotide, which has a high affinity for sstr(2), with a hyperglycemic dose of pasireotide did not cause significant changes in plasma glucose levels. In conclusion, although pasireotide and octreotide inhibited insulin to a similar degree, only pasireotide administration was associated with hyperglycemia. The strong glucagon inhibitory effect exhibited by octreotide but not pasireotide may explain this observation. The lack of hyperglycemia during co-administration of pasireotide and octreotide may be explained by the greater activation of sstr(2) compared with pasireotide alone, causing the insulin-glucagon balance to shift within the normoglycemic range. Extrapolation of these data to humans must account for species differences in islet cell sstr expression.

Somatostatin receptor type 2 antagonism improves glucagon and corticosterone counterregulatory responses to hypoglycemia in streptozotocin-induced diabetic rats

Diminished responsiveness to hypoglycemia contributes to defective counterregulation in diabetes. Pancreatic and/or circulating somatostatin are elevated in diabetes, which may inhibit counterregulatory hormone release during hypoglycemia. Thus, a selective somatostatin receptor type 2 antagonist (SSTR2a) should improve hormone counterregulation to hypoglycemia. Nondiabetic (N) and streptozotocin-induced diabetic (D) rats underwent 4-h infusion of saline or SSTR2a with insulin-induced hypoglycemia clamped at 2.5 ± 0.5 mmol/L. To evaluate the effect of the SSTR2a in the absence of hypoglycemia, rats underwent a 4-h infusion of saline (Ctrl:N, Ctrl:D) or SSTR2a (Ctrl:D+SSTR2a) only. The attenuated glucagon response to hypoglycemia in D (P < 0.0002) was fully restored by SSTR2a (P < 0.0001). Furthermore, the attenuated corticosterone response in D (P < 0.002) was also enhanced by SSTR2a (P < 0.05). In the absence of hypoglycemia, SSTR2a did not alter basal blood glucose levels. D exhibited 62% more pancreatic somatostatin than N after hypoglycemia. In N rats, SSTR2a did not augment the glucagon or corticosterone response to hypoglycemia. Thus, somatostatin may contribute to impaired glucagon responsiveness to hypoglycemia in diabetes. We demonstrate that SSTR2 antagonism enhances hypoglycemia-stimulated glucagon and corticosterone release in D but not in N rats. SSTR2 antagonism does not affect basal glycemia in D rats.

Hyperglucagonemia precedes a decline in insulin secretion and causes hyperglycemia in chronically glucose-infused rats

Islet damage from glucose toxicity is implicated in the pathogenesis of type 2 diabetes, but the sequence of events leading to islet cell dysfunction and hyperglycemia remains unclear. To examine the early stages of islet pathology resulting from increased basal glucose loads, normal awake rats were infused with glucose continuously for 10 days. Plasma glucose and markers of islet and liver function were monitored throughout the infusion. After initial hyperglycemia, rats adapted to the infusion and maintained euglycemia for approximately 4 days. Continued infusion led to worsening hyperglycemia in just 5% of rats after 6 days, but 69% after 8 days and 89% after 10 days, despite unchanged basal and stimulated plasma insulin and C-peptide concentrations. In contrast, plasma glucagon concentrations increased fivefold. Endogenous glucose production (EGP) was appropriately suppressed after 4 days (2.8 ± 0.7 vs. 6.1 ± 0.4 mg·kg(-1)·min(-1) on day 0, P < 0.001) but tripled between days 4 and 8 (9.9 ± 1.7 mg·kg(-1)·min(-1), P < 0.01). Surprisingly, the increase in EGP was accompanied by increased mitochondrial phosphoenolpyruvate carboxykinase expression with appropriate suppression of the cytosolic isoform. Infusion of anti-glucagon antibodies normalized plasma glucose to levels identical to those on day 4 and ∼300 mg/dl lower than controls. This improved glycemia was associated with a 60% reduction in EGP. These data support the novel concept that glucose toxicity may first manifest as α-cell dysfunction prior to any measurable deficit in insulin secretion. Such hyperglucagonemia could lead to excessive glucose production overwhelming the capacity of the β-cell to maintain glucose homeostasis.

NO-1886 ameliorates glycogen metabolism in insulin-resistant HepG2 cells by GSK-3β signalling

OBJECTIVES

The aim of the study was to elucidate the possible role and mechanism of NO-1886 (ibrolipim, a lipoprotein lipase activator) in ameliorating insulin resistance induced by high palmitate.

METHODS

HepG2 cells were cultured in RPMI 1640 medium and were treated with palmitate to induce insulin resistance. Free fatty acids (FFAs), glucose, glycogen, cell viability and mRNA and protein levels were analysed separately.

KEY FINDINGS

We found that HepG2 cells treated with 0.5 mm palmitate for 48 h led to a significant decrease of insulin-induced glucose consumption (from 2.89 ± 0.85 mm in the control to 0.57 ± 0.44 mm in palmitate). Insulin resistance (IR) of HepG2 cells was induced by 0.5 mm palmitate for 48 h. NO-1886 stimulated glucose consumption, glycogen synthesis and FFA absorption in insulin-resistant HepG2 cells. Maximum stimulation effects were observed with 10 µm NO-1886 for 24 h. Compared with the dimethyl sulfoxide-treated group, 2.5 µm NO-1886 or higher could induce the mRNA expression of lipoprotein lipase. Meanwhile, NO-1886 increased the protein content of P-GSK-3βser(9) and decreased the protein level of GSK-3β in insulin-resistant HepG2 cells, but NO-1886 didn't change the protein levels of PI3-Kp85 and Akt2.

CONCLUSION

Lipoprotein lipase activator NO-1886 could increase glycogen synthesis in HepG2 cells and could ameliorate the insulin resistance, which was associated with GSK-3 signalling.

Central administration of pan-somatostatin agonist ODT8-SST prevents abdominal surgery-induced inhibition of circulating ghrelin, food intake and gastric emptying in rats

BACKGROUND

  Activation of brain somatostatin receptors (sst(1-5) ) with the stable pan-sst(1-5) somatostatin agonist, ODT8-SST blocks acute stress and central corticotropin-releasing factor (CRF)-mediated activation of endocrine and adrenal sympathetic responses. Brain CRF signaling is involved in delaying gastric emptying (GE) immediately post surgery. We investigated whether activation of brain sst signaling pathways modulates surgical stress-induced inhibition of gastric emptying and food intake.

METHODS

Fasted rats were injected intracisternally (i.c.) with somatostatin agonists and underwent laparotomy and 1-min cecal palpation. Gastric emptying of a non-nutrient solution and circulating acyl and desacyl ghrelin levels were assessed 50min post surgery. Food intake was monitored for 24 h.

KEY RESULTS

The abdominal surgery-induced inhibition of GE (65%), food intake (73% at 2h) and plasma acyl ghrelin levels (67%) was completely prevented by ODT8-SST (1μg per rat, i.c.). The selective sst(5) agonist, BIM-23052 prevented surgery-induced delayed GE, whereas selective sst(1) , sst(2) , or sst(4) agonists had no effect. However, the selective sst(2) agonist, S-346-011 (1μg per rat, i.c.) counteracted the abdominal surgery-induced inhibition of acyl ghrelin and food intake but not the delayed GE. The ghrelin receptor antagonist, [D-Lys(3) ]-GHRP-6 (0.93mg kg(-1) , intraperitoneal, i.p.) blocked i.p. ghrelin-induced increased GE, while not influencing i.c. ODT8-SST-induced prevention of delayed GE and reduced food intake after surgery.

CONCLUSIONS & INFERENCES

ODT8-SST acts in the brain to prevent surgery-induced delayed GE likely via activating sst(5) . ODT8-SST and the sst(2) agonist prevent the abdominal surgery-induced decrease in food intake and plasma acyl ghrelin indicating dissociation between brain somatostatin signaling involved in preventing surgery-induced suppression of GE and feeding response.

Prep1 controls insulin glucoregulatory function in liver by transcriptional targeting of SHP1 tyrosine phosphatase

OBJECTIVE

We investigated the function of the Prep1 gene in insulin-dependent glucose homeostasis in liver.

RESEARCH DESIGN AND METHODS

Prep1 action on insulin glucoregulatory function has been analyzed in liver of Prep1-hypomorphic mice (Prep1(i/i)), which express 2-3% of Prep1 mRNA.

RESULTS

Based on euglycemic hyperinsulinemic clamp studies and measurement of glycogen content, livers from Prep1(i/i) mice feature increased sensitivity to insulin. Tyrosine phosphorylation of both insulin receptor (IR) and insulin receptor substrate (IRS)1/2 was significantly enhanced in Prep1(i/i) livers accompanied by a specific downregulation of the SYP and SHP1 tyrosine phosphatases. Prep1 overexpression in HepG2 liver cells upregulated SYP and SHP1 and inhibited insulin-induced IR and IRS1/2 phosphorylation and was accompanied by reduced glycogen content. Consistently, overexpression of the Prep1 partner Pbx1, but not of p160MBP, mimicked Prep1 effects on tyrosine phosphorylations, glycogen content, and on SYP and SHP1 expression. In Prep1 overexpressing cells, antisense silencing of SHP1, but not that of SYP, rescued insulin-dependent IR phosphorylation and glycogen accumulation. Both Prep1 and Pbx1 bind SHP1 promoter at a site located between nucleotides -2,113 and -1,778. This fragment features enhancer activity and induces luciferase function by 7-, 6-, and 30-fold, respectively, in response to Prep1, Pbx1, or both.

CONCLUSIONS

SHP1, a known silencer of insulin signal, is a transcriptional target of Prep1. In liver, transcriptional activation of SHP1 gene by Prep1 attenuates insulin signal transduction and reduces glucose storage.

Activation of brain somatostatin 2 receptors stimulates feeding in mice: analysis of food intake microstructure

We recently reported that the oligosomatostatin receptor agonist, ODT8-SST increases food intake in rats via the somatostatin 2 receptor (sst(2)). We characterized ingestive behavior following intracerebroventricular (icv) injection of a selective sst(2) agonist in freely fed mice during the light phase. The sst(2) agonist (0.01, 0.03, 0.1, 0.3 or 1μg/mouse) injected icv under short inhalation anesthesia dose-dependently increased cumulative light phase food intake over 4h compared to vehicle with a 3.1-times increase at 1μg/mouse (p<0.05). Likewise, the sst(2,3,5) agonist octreotide (0.3 or 1μg/mouse) dose-dependently increased 4-h food intake, whereas selective sst(1) or sst(4) agonists at 1μg/mouse did not. In vehicle-treated mice, high fat diet increased caloric intake/4h by 2.8-times compared to regular diet (p<0.05) and values were further increased 1.4-times/4h by the sst(2) agonist. Automated continuous assessment of food intake established a 6.6-times higher food intake during the dark phase due to increased number of meals, meal size, meal duration and rate of ingestion compared to non-treated mice during the light phase. During the first 4h post icv sst(2) agonist injection, mice had a 57% increase in number of meals with a 60% higher rate of ingestion, and a 61% reduction in inter-meal intervals, whereas meal sizes were not altered compared to vehicle. These data indicate that the activation of brain sst(2) receptors potently stimulates the light phase ingestive behavior under basal or high fat diet-stimulated conditions in mice. The shortened inter-meal interval suggests an inhibitory effect of the sst(2) agonist on "satiety", whereas "satiation" is not altered as indicated by normal meal size.

Somatostatin and the gastrointestinal tract

PURPOSE OF REVIEW

Somatostatin influences motility, secretion, and absorption and often has in vivo a modulating, indirect effect on target cells in the gastrointestinal tract. Knowledge on tissue-specific expression of the five somatostatin receptors (SSTRs), their capacities for internalization and downregulation, their subtype-specific intracellular messengers, and the possibility of forming functionally distinct homodimers or heterodimers, has further complicated the actual in-vivo mechanism of action of somatostatin. This review reports recent in-vivo and in-vitro studies on somatostatin effects on the gastrointestinal tract and pancreas, most of them using a new engineered animal model able to define specific roles of somatostatin and/or its receptor subtypes.

RECENT FINDINGS

SSTR2 knockout mice showed normal circulating gastrin and unchanged acid output, suggesting a high degree of plasticity behind gastric acid secretion. Intestinal inflammation significantly increased somatostatin mRNA in SSTR2 null compared to wild type suggesting that somatostatin mediates inflammation also in SSTR2 null mice. In pancreatic islets of SSTR1-5 null mice no variations of islet size, cellular organization or glucagon or insulin content was shown when compared with null SSTRs and control mice.

SUMMARY

Although none of the recent findings produced on somatostatin seem ready to be considered for clinical application, recent developments of animal models such as SSTR knockout mice have highlighted promising results to better understand the direct and indirect effects of somatostatin on gastrointestinal tract functions.

Microarray analysis of somatostatin receptor 5-regulated gene expression profiles in murine pancreas

BACKGROUND

We previously demonstrated that somatostatin receptor type 5 (SSTR5) gene ablation results in alterations in insulin secretion and glucose metabolism, accompanied by morphologic alterations in the islets of Langerhans. The underlying mechanism(s) by which SSTR5 exerts its cellular functions remain(s) unknown. We hypothesized that SSTR5 mediates the inhibitory effect of somatostatin (SST) on insulin secretion and islet proliferation by regulating a specific set of pancreatic genes.

METHODS

To identify SSTR5-regulated pancreatic genes, gene expression microarray analysis was performed on the whole pancreas of 1- and 3-month-old wild-type (WT) and SSTR5 knockout (SSTR5-/-) male mice. Real-time RT-PCR and immunofluorescence were performed to validate selected differentially expressed genes.

RESULTS

A set of 143 probes were identified to be differentially expressed in the pancreas of 1-month-old SSTR5-/- mice, 72 of which were downregulated and 71 upregulated. At 3 months of age, SSTR5 gene ablation resulted in downregulation of a set of 30 probes and upregulation of a set of 37 probes. Among these differentially expressed genes, there were 15 and 5 genes that were upregulated and downregulated, respectively, in mice at both 1 and 3 months of age. Three genes, PAP/INGAP, ANG, and TDE1, were selected to be validated by real-time RT-PCR and immunofluorescence.

CONCLUSIONS

A specific set of genes linked to a wide range of cellular functions such as islet proliferation, apoptosis, angiogenesis, and tumorigenesis were either upregulated or downregulated in SSTR5-deficient male mice compared with their expression in wild-type mice. Therefore, these genes are potential SSTR5-regulated genes during normal pancreatic development and functional maintenance.

The role(s) of somatostatin, structurally related peptides and somatostatin receptors in the gastrointestinal tract: a review

Extensive functional and morphological research has demonstrated the pivotal role of somatostatin (SOM) in the regulation of a wide variety of gastrointestinal activities. In addition to its profound inhibitory effects on gastrointestinal motility and exocrine and endocrine secretion processes along the entire gastrointestinal tract, SOM modulates several organ-specific activities. In contrast to these well-known SOM-dependent effects, knowledge on the SOM receptors (SSTR) involved in these effects is much less conclusive. Experimental data on the identities of the SSTRs, although species- and tissue-dependent, point towards the involvement of multiple receptor subtypes in the vast majority of gastrointestinal SOM-mediated effects. Recent evidence demonstrating the role of SOM in intestinal pathologies has extended the interest of gastrointestinal research in this peptide even further. More specifically, SOM is supposed to suppress intestinal inflammatory responses by interfering with the extensive bidirectional communication between mucosal mast cells and neurons. This way, SOM not only acts as a powerful inhibitor of the inflammatory cascade at the site of inflammation, but exerts a profound antinociceptive effect through the modulation of extrinsic afferent nerve fibres. The combination of these physiological and pathological activities opens up new opportunities to explore the potential of stable SOM analogues in the treatment of GI inflammatory pathologies.

Benefits and limitations of reducing glucagon action for the treatment of type 2 diabetes

Glucagon is secreted from the alpha-cells of the pancreatic islets and regulates glucose homeostasis through modulation of hepatic glucose production. As elevated glucagon levels contribute to the pathophysiology of hyperglycemia in subjects with type 2 diabetes, reduction of glucagon receptor gene (Gcgr) activity represents a potential target for the treatment of T2DM. Herein, we review current concepts of glucagon action in hepatic and extrahepatic tissues and evaluate the therapeutic potential, mechanisms of action, and safety of reducing Gcgr signaling for the treatment of T2DM.

Null mutant mouse models of somatostatin and cortistatin, and their receptors

Somatostatin (somatotropin release inhibitory factor, SRIF) and the related cortistatin (CST) are multifunctional peptide molecules attributed with neurohormone, neurotransmitter/modulator, and autocrine/paracrine actions. The physiological responses of SRIF and CST are mediated by five widely distributed G protein-coupled receptors (sst1-5) which have been implicated in regulating numerous biological processes. Much of the information on the effects of somatostatin has been gained through pharmacological studies with analogs and antagonists. The possibility of targeted mutagenesis in the mouse has resulted, over the last 10 years, in the generation of mouse models which genetically lack somatostatin ligands or receptors. We will review here the mouse models generated, the studies undertaken with them, and what has been learned so far.

Function and expression of somatostatin receptors of the endocrine pancreas

Somatostatin (SST) regulates multiple biological processes via five genetically distinct, G-protein coupled receptors. Clinical interest in therapy for neuroendocrine and metabolic disorders has resulted in the development of new tools for exploring the function of somatostatin receptors (SSTRs). The development of highly SSTR-selective agonists and antagonists, animal models with the deletion of individual SSTRs, as well as SSTR-specific antibodies have all been utilized in delineating SSTR functions. In the pancreas, SST is a potent regulator of insulin and glucagon secretion. Indeed, the inappropriate regulation of pancreatic A- and B-cell function in metabolic diseases provides an impetus to evaluate the SSTRs as therapeutic targets. By combining the results obtained from molecular biology, pharmacology and immunochemical studies the current review provides a summary of important recent developments which have extended our knowledge of SST actions in the endocrine pancreas.