Selective effects of somatostatin-14, -25 and -28 on in vitro insulin and glucagon secretion

Paracrine signalling by pancreatic δ cells determines the glycaemic set point in mice

While pancreatic β and α cells are considered the main drivers of blood glucose homeostasis through insulin and glucagon secretion, the contribution of δ cells and somatostatin (SST) secretion to glucose homeostasis remains unresolved. Here we provide a quantitative assessment of the physiological contribution of δ cells to the glycaemic set point in mice. Employing three orthogonal mouse models to remove SST signalling within the pancreas or transplanted islets, we demonstrate that ablating δ cells or SST leads to a sustained decrease in the glycaemic set point. This reduction coincides with a decreased glucose threshold for insulin response from β cells, leading to increased insulin secretion to the same glucose challenge. Our data demonstrate that β cells are sufficient to maintain stable glycaemia and reveal that the physiological role of δ cells is to provide tonic feedback inhibition that reduces the β cell glucose threshold and consequently lowers the glycaemic set point in vivo.

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

Strategic Approaches to Improvise Peptide Drugs as Next Generation Therapeutics

In recent years, the occurrence of a wide variety of drug-resistant diseases has led to an increase in interest in alternate therapies. Peptide-based drugs as an alternate therapy hold researchers' attention in various therapeutic fields such as neurology, dermatology, oncology, metabolic diseases, etc. Previously, they had been overlooked by pharmaceutical companies due to certain limitations such as proteolytic degradation, poor membrane permeability, low oral bioavailability, shorter half-life, and poor target specificity. Over the last two decades, these limitations have been countered by introducing various modification strategies such as backbone and side-chain modifications, amino acid substitution, etc. which improve their functionality. This has led to a substantial interest of researchers and pharmaceutical companies, moving the next generation of these therapeutics from fundamental research to the market. Various chemical and computational approaches are aiding the production of more stable and long-lasting peptides guiding the formulation of novel and advanced therapeutic agents. However, there is not a single article that talks about various peptide design approaches i.e., in-silico and in-vitro along with their applications and strategies to improve their efficacy. In this review, we try to bring different aspects of peptide-based therapeutics under one article with a clear focus to cover the missing links in the literature. This review draws emphasis on various in-silico approaches and modification-based peptide design strategies. It also highlights the recent progress made in peptide delivery methods important for their enhanced clinical efficacy. The article would provide a bird's-eye view to researchers aiming to develop peptides with therapeutic applications.

Graphical Abstract

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Progress in the Management of Pancreatic Neuroendocrine Tumors

Pancreatic neuroendocrine tumors (PNETs) are a heterogeneous and orphan group of neoplasms that vary in their histology, clinical features, prognosis, and management. The treatment of PNETs is highly dependent on the stage at presentation, tumor grade and differentiation, presence of symptoms from hormonal overproduction or from local growth, tumor burden, and rate of progression. The US Food and Drug Administration has recently approved many novel treatments, which have altered decision making and positively impacted the care and prognosis of these patients. In this review, we focus on the significant progress made in the management of PNETs over the past decade, as well as the active areas of research. Expected final online publication date for the Annual Review of Medicine, Volume 73 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

KATP channel blockers control glucagon secretion by distinct mechanisms: A direct stimulation of α-cells involving a [Ca2+]c rise and an indirect inhibition mediated by somatostatin

Objective

Glucagon is secreted by pancreatic α-cells in response to hypoglycemia and its hyperglycemic effect helps to restore normal blood glucose. Insulin and somatostatin (SST) secretions from β- and δ-cells, respectively, are stimulated by glucose by mechanisms involving an inhibition of their ATP-sensitive K⁺ (K_ATP) channels, leading to an increase in [Ca²⁺]_c that triggers exocytosis. Drugs that close K_ATP channels, such as sulfonylureas, are used to stimulate insulin release in type 2 diabetic patients. α-cells also express K_ATP channels. However, the mechanisms by which sulfonylureas control glucagon secretion are still largely debated and were addressed in the present study. In particular, we studied the effects of K_ATP channel blockers on α-cell [Ca²⁺]_c and glucagon secretion in the presence of a low (1 mM) or a high (15 mM) glucose concentration and evaluated the role of SST in these effects.

Methods

Using a transgenic mouse model expressing the Ca²⁺-sensitive fluorescent protein, GCaMP6f, specifically in α-cells, we measured [Ca²⁺]_c in α-cells either dispersed or within whole islets (by confocal microscopy). By measuring [Ca²⁺]_c in α-cells within islets and glucagon secretion using the same perifusion protocols, we tested whether glucagon secretion correlated with changes in [Ca²⁺]_c in response to sulfonylureas. We studied the role of SST in the effects of sulfonylureas using multiple approaches including genetic ablation of SST, or application of SST-14 and SST receptor antagonists.

Results

Application of the sulfonylureas, tolbutamide, or gliclazide, to a medium containing 1 mM or 15 mM glucose increased [Ca²⁺]_c in α-cells by a direct effect as in β-cells. At low glucose, sulfonylureas inhibited glucagon secretion of islets despite the rise in α-cell [Ca²⁺]_c that they triggered. This glucagonostatic effect was indirect and attributed to SST because, in the islets of SST-knockout mice, sulfonylureas induced a stimulation of glucagon secretion which correlated with an increase in α-cell [Ca²⁺]_c. Experiments with exogenous SST-14 and SST receptor antagonists indicated that the glucagonostatic effect of sulfonylureas mainly resulted from an inhibition of the efficacy of cytosolic Ca²⁺ on exocytosis. Although SST-14 was also able to inhibit glucagon secretion by decreasing α-cell [Ca²⁺]_c, no decrease in [Ca²⁺]_c occurred during sulfonylurea application because it was largely counterbalanced by the direct stimulatory effect of these drugs on α-cell [Ca²⁺]_c. At high glucose, i.e., in conditions where glucagon release was already low, sulfonylureas stimulated glucagon secretion because their direct stimulatory effect on α-cells exceeded the indirect effect by SST. Our results also indicated that, unexpectedly, SST-14 poorly decreased the efficacy of Ca²⁺ on exocytosis in β-cells.

Conclusions

Sulfonylureas exert two opposite actions on α-cells: a direct stimulation as in β-cells and an indirect inhibition by SST. This suggests that any alteration of SST paracrine influence, as described in diabetes, will modify the effect of sulfonylureas on glucagon release. In addition, we suggest that δ-cells inhibit α-cells more efficiently than β-cells.

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K_ATP channel blockers control glucagon secretion by two mechanisms.

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The first one is the direct stimulation of α-cell by a [Ca²⁺]_c rise, as in β-cells.

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The second one is an indirect inhibition mediated by δ-cells releasing somatostatin.

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Somatostatin mainly reduces the efficacy of Ca²⁺ on exocytosis in α-cells.

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Somatostatin more potently inhibits glucagon than insulin secretion.

Vitamin-D-Binding Protein Contributes to the Maintenance of α Cell Function and Glucagon Secretion

Vitamin-D-binding protein (DBP) or group-specific component of serum (GC-globulin) carries vitamin D metabolites from the circulation to target tissues. DBP is highly localized to the liver and pancreatic α cells. Although DBP serum levels, gene polymorphisms, and autoantigens have all been associated with diabetes risk, the underlying mechanisms remain unknown. Here, we show that DBP regulates α cell morphology, α cell function, and glucagon secretion. Deletion of DBP leads to smaller and hyperplastic α cells, altered Na+ channel conductance, impaired α cell activation by low glucose, and reduced rates of glucagon secretion both in vivo and in vitro. Mechanistically, this involves reversible changes in islet microfilament abundance and density, as well as changes in glucagon granule distribution. Defects are also seen in β cell and δ cell function. Immunostaining of human pancreata reveals generalized loss of DBP expression as a feature of late-onset and long-standing, but not early-onset, type 1 diabetes. Thus, DBP regulates α cell phenotype, with implications for diabetes pathogenesis.

Changes in post-oral glucose challenge pancreatic polypeptide hormone levels following metabolic surgery: A comparison of gastric bypass and sleeve gastrectomy

BACKGROUND

Pancreatic polypeptide (PP) hormone is a 36-amino-acid peptide released from the pancreas, the serum levels of which have been shown to rise upon food intake. The underlying mechanism for metabolic surgery in the treatment of patients with type 2 diabetes mellitus (T2DM) remains intriguing. We compared post-oral glucose challenge PP levels between patients undergoing laparoscopic gastric bypass (GB) and sleeve gastrectomy (SG) at 1 year after surgery.

METHODS

This hospital-based, prospective study followed up a total of 12 laparoscopic GB and 12 laparoscopic SG patients and evaluated their glucose homeostasis. One year after metabolic surgery, 75-g oral glucose tolerance tests (OGTTs) were performed in the patients in the GB and SG groups and the blood levels of PP were evaluated.

RESULTS

The laparoscopic GB group had stable serum PP levels within 120 min after OGTT; however, the levels were significantly higher in the laparoscopic SG group at 30 min after OGTT. The patients with complete T2DM remission did not exhibit significantly different PP levels at fasting and post-OGTT than those in patients without remission after GB. Similarly, after SG, patients with T2DM remission did not show significantly different PP levels at fasting and post-OGTT than those in patients without T2DM remission.

CONCLUSIONS

No significant difference was found in plasma PP levels after OGTT in T2DM patients that received either GB or SG, or in T2DM remitters or non-remitters 1 year after metabolic surgery.

Peptide Receptor Radionuclide Therapy for the Treatment of Pancreatic Neuroendocrine Tumors: Recent Insights

Peptide receptor radionuclide therapy (PRRT) is a paradigm shifting approach to the treatment of neuroendocrine tumors. Although there are no prospective randomized trials directly studying PRRT in pancreatic neuroendocrine tumors (panNETs), there are data to suggest benefit in this patient population. Collectively, the data, consisting of two prospective and six retrospective studies, show a median PFS ranging from 20 to 39 months and a median OS ranging from 37 to 79 months. There are ongoing (and upcoming) prospective, randomized trials of PRRT in panNETs, which will provide further evidence to support this approach.

Somatostatin Analogues in the Treatment of Neuroendocrine Tumors: Past, Present and Future

In recent decades, the incidence of neuroendocrine tumors (NETs) has steadily increased. Due to the slow-growing nature of these tumors and the lack of early symptoms, most cases are diagnosed at advanced stages, when curative treatment options are no longer available. Prognosis and survival of patients with NETs are determined by the location of the primary lesion, biochemical functional status, differentiation, initial staging, and response to treatment. Somatostatin analogue (SSA) therapy has been a mainstay of antisecretory therapy in functioning neuroendocrine tumors, which cause various clinical symptoms depending on hormonal hypersecretion. Beyond symptomatic management, recent research demonstrates that SSAs exert antiproliferative effects and inhibit tumor growth via the somatostatin receptor 2 (SSTR2). Both the PROMID (placebo-controlled, prospective, randomized study in patients with metastatic neuroendocrine midgut tumors) and the CLARINET (controlled study of lanreotide antiproliferative response in neuroendocrine tumors) trial showed a statistically significant prolongation of time to progression/progression-free survival (TTP/PFS) upon SSA treatment, compared to placebo. Moreover, the combination of SSA with peptide receptor radionuclide therapy (PRRT) in small intestinal NETs has proven efficacy in the phase 3 neuroendocrine tumours therapy (NETTER 1) trial. PRRT is currently being tested for enteropancreatic NETs versus everolimus in the COMPETE trial, and the potential of SSTR-antagonists in PRRT is now being evaluated in early phase I/II clinical trials. This review provides a synopsis on the pharmacological development of SSAs and their use as antisecretory drugs. Moreover, this review highlights the clinical evidence of SSAs in monotherapy, and in combination with other treatment modalities, as applied to the antiproliferative management of neuroendocrine tumors with special attention to recent high-quality phase III trials.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Somatostatin Is Only Partly Required for the Glucagonostatic Effect of Glucose but Is Necessary for the Glucagonostatic Effect of KATP Channel Blockers

The mechanisms of control of glucagon secretion are largely debated. In particular, the paracrine role of somatostatin (SST) is unclear. We studied its role in the control of glucagon secretion by glucose and K_ATP channel blockers, using perifused islets and the in situ perfused pancreas. The involvement of SST was evaluated by comparing glucagon release of control tissue or tissue without paracrine influence of SST (pertussis toxin-treated islets, or islets or pancreas from Sst^-/- mice). We show that removal of the paracrine influence of SST suppresses the ability of K_ATP channel blockers or K_ATP channel ablation to inhibit glucagon release, suggesting that in control islets, the glucagonostatic effect of K_ATP channel blockers/ablation is fully mediated by SST. By contrast, the glucagonostatic effect of glucose in control islets is mainly independent of SST for low glucose concentrations (0-7 mmol/L) but starts to involve SST for high concentrations of the sugar (15-30 mmol/L). This demonstrates that the glucagonostatic effect of glucose only partially depends on SST. Real-time quantitative PCR and pharmacological experiments indicate that the glucagonostatic effect of SST is mediated by two types of SST receptors, SSTR2 and SSTR3. These results suggest that alterations of the paracrine influence of SST will affect glucagon release.

Current knowledge on the sensitivity of the (68)Ga-somatostatin receptor positron emission tomography and the SUVmax reference range for management of pancreatic neuroendocrine tumours

Physiologically increased pancreatic uptake at the head/uncinate process is observed in more than one-third of patients after injection of one of the three ⁶⁸Ga-labelled octreotide-based peptides used for somatostatin (sst) receptor (r) imaging. There are minor differences between these ⁶⁸Ga-sstr-binding peptides in the imaging setting. On ⁶⁸Ga-sstr-imaging the physiological uptake can be diffuse or focal and usually remains stable over time. Differences in the maximal standardised uptake values (SUV_max) reported for the normal pancreas as well as for pancreatic neuroendocrine tumour (PNET) lesions may be related to several factors, including (a) differences in the peptide binding affinities as well as differences in sstr subtype expression of pancreatic α- and β-cells, and heterogeneity / density of tumour cells, (b) differences in scanner resolution, image reconstruction techniques and acquisition protocols, (c) mostly retrospective study designs, (d) mixed patient populations, or (e) interference with medications such as treatment with long-acting sst analogues. The major limitation in most of the studies lies in the lack of histopathological confirmation of abnormal findings. There is a significant overlap between the calculated SUV_max-values for physiological pancreas and PNET-lesions of the head/uncinate process that do not favour the use of quantitative parameters in the clinical setting. Anecdotal long-term follow-up studies have even indicated that increased uptake in the head/uncinate process still can turn out to be malignant over years of follow up. SUV_max-data for the pancreatic body and tail are limited. Therefore, any visible focal tracer uptake in the pancreas must be considered as suspicious for malignancy irrespective of quantitative parameters. In general, sstr-PET/CT has significant implications for the management of NET patients leading to a change in treatment decision in about one-third of patients. Therefore, follow-up with ⁶⁸Ga-sstr-PET/CT is mandatory in the clinical setting if uptake in the head/uncinate process is observed.

Asymmetrical distribution of δ and PP cells in human pancreatic islets

The aim of this study was to evaluate the location of PP and δ cells in relation to the vascularization within human pancreatic islets. To this end, pancreas sections were analysed by immunofluorescence using antibodies against endocrine islet and endothelial cells. Staining in different islet areas corresponding to islet cells adjacent or not to peripheral or central vascular channels was quantified by computerized morphometry. As results, α, PP and δ cells were preferentially found adjacent to vessels. In contrast to α cells, which were evenly distributed between islet periphery and intraislet vascular channels, PP and δ cells had asymmetric and opposite distributions: PP staining was higher and somatostatin staining was lower in the islet periphery than in the area around intraislet vascular channels. Additionally, frequencies of PP and δ cells were negatively correlated in the islets. No difference was observed between islets from the head and the tail of the pancreas, and from type 2 diabetic and non-diabetic donors. In conclusion, the distribution of δ cells differs from that of PP cells in human islets, suggesting that vessels at the periphery and at the centre of islets drain different hormonal cocktails.

Association of Pancreatic Polypeptide with Mild Cognitive Impairment Varies by APOE ε4 Allele

We conducted a preliminary case-control investigation of the association of pancreatic polypeptide (PP) with mild cognitive impairment (MCI) in 202 MCI cases (mean age, 81.6 years) and 202 age- and sex-matched cognitively normal controls in the Mayo Clinic Study of Aging. Plasma PP was measured and examined as the natural logarithm (continuous) and dichotomized at the median. The OR (95% CI) of MCI increased with increasing PP [1.46 (1.04-2.05)]. There was a negative interaction of PP with apolipoprotein E (APOE) ε4 allele; compared to the reference group (no APOE ε4 allele and low PP), the OR (95% CI) for combinations of ε4 and PP were: 2.64 (1.39-5.04) for APOE ε4 plus low PP; 2.09 (1.27-3.45) for no APOE ε4 plus high PP; and 1.91 (1.04-3.53) for no APOE ε4 plus high PP (P for interaction = 0.017). There was also a trend toward a negative interaction with type 2 diabetes (P for interaction = 0.058). Compared to no diabetes and low PP, the OR (95% CI) was 3.02 (1.22-7.46) for low PP plus diabetes but 1.80 (1.01-3.22) for high PP plus diabetes. Participants with high PP had a greater mean (SD) weight loss (kilograms per decade) than persons with low PP [-2.27 (4.07) vs. -1.61 (5.24); P = 0.016]. MCI cases had a non-significantly greater weight loss per decade compared to controls. These findings suggest that high PP alone or jointly with APOE ε4 allele or type 2 diabetes is associated with MCI, and that high PP may mitigate some effects of APOE ε4 allele and type 2 diabetes on cognition. Potential mechanisms may involve PP-related weight loss and centrally mediated effects of PP on cognition. These findings remain to be validated in other studies.

Pancreatic polypeptide regulates glucagon release through PPYR1 receptors expressed in mouse and human alpha-cells

BACKGROUND

Plasma levels of pancreatic polypeptide (PP) rise upon food intake. Although other pancreatic islet hormones, such as insulin and glucagon, have been extensively investigated, PP secretion and actions are still poorly understood.

METHODS

The release of PP upon glucose stimulation and the effects of PP on glucagon and insulin secretion were analyzed in isolated pancreatic islets. Expression of PP receptor (PPYR1) was investigated by immunoblotting, quantitative RT-PCR on sorted pancreatic islet cells, and immunohistochemistry.

RESULTS

In isolated mouse pancreatic islets, glucose stimulation increased PP release, while insulin secretion was up and glucagon release was down. Direct exposure of islets to PP inhibited glucagon release. In mouse islets, PPYR1 protein was observed by immunoblotting and quantitative RT-PCR revealed PPYR1 expression in the FACS-enriched glucagon alpha-cell fraction. Immunohistochemistry on pancreatic sections showed the presence of PPYR1 in alpha-cells of both mouse and human islets, while the receptor was absent in other islet cell types and exocrine pancreas.

CONCLUSIONS

Glucose stimulates PP secretion and PP inhibits glucagon release in mouse pancreatic islets. PP receptors are present in alpha-cells of mouse and human pancreatic islets.

GENERAL SIGNIFICANCE

These data demonstrate glucose-regulated secretion of PP and its effects on glucagon release through PPYR1 receptors expressed by alpha-cells.

Pancreatic polypeptide inhibits somatostatin secretion

Pancreatic polypeptide (PP) is a major agonist for neuropeptide Y4 receptors (NPY4R). While NPY4R has been identified in various tissues, the cells on which it is expressed and its function in those cells has not been clearly delineated. Here we report that NPY4R is present in all somatostatin-containing cells of tissues that we tested, including pancreatic islets, duodenum, hippocampus, and hypothalamus. Its agonism by PP decreases somatostatin secretion from human islets. Mouse embryonic hippocampal (mHippo E18) cells expressed NPY4Rs and their activation by PP consistently decreased somatostatin secretion. Furthermore, central injection of PP in mice induced c-Fos immunoreactivity in somatostatin-containing cells in the hippocampus compared with PBS-injected mice. In sum, our results identify PP as a pivotal modulator of somatostatin secretion.

Diamine Derivatives as Novel Small-Molecule, Potent, and Subtype-Selective Somatostatin SST3 Receptor Agonists

A novel class of small-molecule, highly potent, and subtype-selective somatostatin SST3 agonists was discovered through modification of a SST3 antagonist. As an example, (1R,2S)-9 demonstrated not only potent in vitro SST3 agonist activity but also in vivo SST3 agonist activity in a mouse oral glucose tolerance test (OGTT). These agonists may be useful reagents for studying the physiological roles of the SST3 receptor and may potentially be useful as therapeutic agents.

Elucidating the role of disulfide bond on amyloid formation and fibril reversibility of somatostatin-14: relevance to its storage and secretion

The storage of protein/peptide hormones within subcellular compartments and subsequent release are crucial for their native function, and hence these processes are intricately regulated in mammalian systems. Several peptide hormones were recently suggested to be stored as amyloids within endocrine secretory granules. This leads to an apparent paradox where storage requires formation of aggregates, and their function requires a supply of non-aggregated peptides on demand. The precise mechanism behind amyloid formation by these hormones and their subsequent release remain an open question. To address this, we examined aggregation and fibril reversibility of a cyclic peptide hormone somatostatin (SST)-14 using various techniques. After proving that SST gets stored as amyloid in vivo, we investigated the role of native structure in modulating its conformational dynamics and self-association by disrupting the disulfide bridge (Cys(3)-Cys(14)) in SST. Using two-dimensional NMR, we resolved the initial structure of somatostatin-14 leading to aggregation and further probed its conformational dynamics in silico. The perturbation in native structure (S-S cleavage) led to a significant increase in conformational flexibility and resulted in rapid amyloid formation. The fibrils formed by disulfide-reduced noncyclic SST possess greater resistance to denaturing conditions with decreased monomer releasing potency. MD simulations reveal marked differences in the intermolecular interactions in SST and noncyclic SST providing plausible explanation for differential aggregation and fibril reversibility observed experimentally in these structural variants. Our findings thus emphasize that subtle changes in the native structure of peptide hormone(s) could alter its conformational dynamics and amyloid formation, which might have significant implications on their reversible storage and secretion.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Octreotide for acromegaly treatment: a reappraisal

INTRODUCTION

Acromegaly is a rare disorder characterized by excess secretion of growth hormone (GH) generally caused by a pituitary macroadenoma and associated with reduced life expectancy if the disease is untreated. This article covers the recent available evidences published on octreotide , the first somatostatin analog introduced into clinical practice for the medical treatment of acromegaly.

AREAS COVERED

This article discusses i) pharmacology of somatostatin and octreotide; ii) biochemical effects of regular octreotide and long-acting repeatable formulation; iii) tumor shrinkage effects of octreotide in acromegaly; iv) impact of octreotide on acromegalic clinical manifestations and chronic complications; v) safety of octreotide and vi) place of octreotide in the guidelines for acromegaly treatment. Full-text articles in the English language were selected from a PubMed search spanning 1984 - 2013, for keywords including 'octreotide,' 'acromegaly,' 'GH,' 'IGF-I,' and 'tumor shrinkage.' Reference lists in selected papers were also used to broaden the search.

EXPERT OPINION

Octreotide is a mature drug with a consolidated favorable benefit versus risks profile in the treatment of acromegaly.

Regeneration of pancreatic non-β endocrine cells in adult mice following a single diabetes-inducing dose of streptozotocin

The non-β endocrine cells in pancreatic islets play an essential counterpart and regulatory role to the insulin-producing β-cells in the regulation of blood-glucose homeostasis. While significant progress has been made towards the understanding of β-cell regeneration in adults, very little is known about the regeneration of the non-β endocrine cells such as glucagon-producing α-cells and somatostatin producing δ-cells. Previous studies have noted the increase of α-cell composition in diabetes patients and in animal models. It is thus our hypothesis that non-β-cells such as α-cells and δ-cells in adults can regenerate, and that the regeneration accelerates in diabetic conditions. To test this hypothesis, we examined islet cell composition in a streptozotocin (STZ)-induced diabetes mouse model in detail. Our data showed the number of α-cells in each islet increased following STZ-mediated β-cell destruction, peaked at Day 6, which was about 3 times that of normal islets. In addition, we found δ-cell numbers doubled by Day 6 following STZ treatment. These data suggest α- and δ-cell regeneration occurred rapidly following a single diabetes-inducing dose of STZ in mice. Using in vivo BrdU labeling techniques, we demonstrated α- and δ-cell regeneration involved cell proliferation. Co-staining of the islets with the proliferating cell marker Ki67 showed α- and δ-cells could replicate, suggesting self-duplication played a role in their regeneration. Furthermore, Pdx1(+)/Insulin(-) cells were detected following STZ treatment, indicating the involvement of endocrine progenitor cells in the regeneration of these non-β cells. This is further confirmed by the detection of Pdx1(+)/glucagon(+) cells and Pdx1(+)/somatostatin(+) cells following STZ treatment. Taken together, our study demonstrated adult α- and δ-cells could regenerate, and both self-duplication and regeneration from endocrine precursor cells were involved in their regeneration.

Anxiolytic-like effects of somatostatin isoforms SST 14 and SST 28 in two animal models (Rattus norvegicus) after intra-amygdalar and intra-septal microinfusions

RATIONALE AND OBJECTIVES

Somatostatin (SST) isoforms, SST 14 and SST 28, inhibit regulatory hormones in the periphery (e.g., growth hormone) and are widely distributed in the brain. In recent experiments, intracerebroventricular (ICV) SST produced anxiolytic-like effects in both behavioral and electrophysiological models. The sites of action of these anxiolytic effects in the brain, however, and the relative contributions of SST 14 and SST 28 to these effects are unknown.

MATERIALS AND METHODS

Anxiolytic effects were assessed in the plus-maze and shock-probe tests after (1) intra-amygdalar microinfusion of SST 14 (0.5 or 3 μg per hemisphere) or SST 28 (3 μg per hemisphere), (2) intra-septal microinfusion of SST 14 (0.5 or 1.5 μg per hemisphere) or SST 28 (1.5 μg per hemisphere), or (3) intra-striatal microinfusion of SST 14 (3 μg per hemisphere).

RESULTS

Intra-amygdalar and intra-septal microinfusions of SST 14 and SST 28 produced robust anxiolytic-like effects in the behavioral tests, unlike intra-striatal microinfusions. The magnitude of the anxiolytic effects in the amygdala and septum were comparable to those found previously with ICV SST 14, ICV L-779976, an SST (sst2) receptor agonist, and ICV diazepam, a classical benzodiazepine anxiolytic.

CONCLUSIONS

SST receptors in the septum and amygdala are responsive to both SST 14 and SST 28, but not those in the striatum. Although no obvious differences in the anxiolytic-like effects of the isoforms were detected, quantitative or even qualitative differences in their specific anxiolytic effects may occur in different sub-regions of the septum and amygdala, as has been found for benzodiazepine anxiolytics.

Malignant somatostatinoma presenting with diabetic ketoacidosis and inhibitory syndrome: pathophysiologic considerations

OBJECTIVE

To describe a patient with diabetic ketoacidosis secondary to a malignant somatostatinoma.

METHODS

We present the clinical, laboratory, radiologic, and pathologic findings of a patient with diabetic ketoacidosis secondary to a malignant somatostatinoma. We also review the potential effects of somatostatin on glucose homeostasis and discuss the underlying pathophysiologic mechanisms.

RESULTS

A 30-year-old woman presented with diabetic ketoacidosis and had a malignant somatostatinoma with hepatic, bone, and lymph node metastasis. She exhibited features of somatostatinoma "inhibitory syndrome" characterized by mild nonketotic hyperglycemia, hypochlorhydria, cholelithiasis, steatorrhea, anemia, and weight loss. In these tumors, the absence of ketoacidosis is thought to arise from the somatostatin-induced simultaneous suppression of the secretion of insulin and glucagon. The patient's primary tumor could not be located.

CONCLUSIONS

Diabetic ketoacidosis may occur in somatostatinomas. The secretion of larger molecular weight forms of somatostatin from the tumor may contribute to the ketogenesis.

Somatostatin receptor subtypes in human type 2 diabetic islets

OBJECTIVES

Somatostatin inhibits hormone release through 5 G protein-coupled somatostatin receptors (sst1-sst5). However, the role of somatostatin in islet physiology is not fully known. The immunoreactivity to sst1 to sst5 in normal human endocrine pancreas has been described. The present study reports the expression of sst1 to sst5 in human pancreatic islets with type 2 diabetes mellitus.

METHODS

Pancreatic autopsy specimens from individuals with type 2 diabetes mellitus and matched controls were double immunostained to demonstrate sst1 to sst5 in the major islet cell types.

RESULTS

Most apparent differences in type 2 diabetic islets were the lack of sst1 and sst4 in glucagon cells and sst1-3 and 4 in somatostatin cells, whereas minor changes were demonstrated in insulin cells. The pancreatic polypeptide cells showed a reversed staining pattern in diabetic islets compared with the controls.

CONCLUSIONS

In type 2 diabetes mellitus, the sst pattern differed from that of the controls in somatostatin, pancreatic polypeptide, and glucagon cells, to a minor extent in insulin cells. It is unclear whether the changes in sst patterns are primarily due to the diabetes or secondary to metabolic disturbances. However, this study may be the basis for further functional studies to evaluate the role of sst1 to sst5 in the diabetic state.

Octreotide LAR for the treatment of acromegaly

BACKGROUND

Somatostatin analogs previously considered as adjuvant therapy in acromegaly are increasingly used as a first-line therapy in selected cases.

OBJECTIVE

To review the octreotide LAR pharmacological and clinical data, and discuss the impact of this agent on current treatment regimens.

METHODS

We reviewed PubMed publications since the first use of octreotide LAR in acromegaly, and historical articles related to the discovery and development of this molecule. We chose, for efficacy and safety data, reviews, clinical and randomized controlled trials that included >or=10 patients.

RESULTS/CONCLUSION

Octreotide LAR controls acromegaly in approximately 50-60% of patients by inhibiting GH and IGF-I secretion, and by reducing tumor size. This drug is well tolerated in most patients.

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.

Spontaneous fibrillation of the native neuropeptide hormone Somatostatin-14

Natural Somatostatin-14 is a small cyclic neuropeptide hormone with broad inhibitory effects on endocrine secretions. Here we show that natural Somatostatin-14 spontaneously self-assembles in water and in 150 mM NaCl into liquid crystalline nanofibrils, which follow characteristic structural features of amyloid fibrils. These non-covalent highly stable structures are based on the Somatostatin native backbone conformation and are formed under non-denaturing conditions. Our results support the hypothesis that self-assembly into amyloid fibrils is a generic property of the polypeptide chain under appropriate conditions. Given recent advances on the mechanisms of biological storage and sorting modes of peptide/protein hormones into secretory granules, we propose that Somatostatin-14 fibrillation could be relevant to the regulated secretion pathway of this neuropeptide hormone. Such a hypothesis is consistent with the emerging concept of the existence of non-disease related but functional amyloids.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Somatostatin and its receptors in the development of the endocrine pancreas

The development of the endocrine pancreas is regulated by numerous transcription and growth factors. Somatostatin (SST) is present in many tissues and acts as a neurotransmitter and autocrine/paracrine/endocrine regulator in response to ions, nutrients, peptides, and hormones as well as neurotransmitters. In the pancreas, there is evidence that SST acts an inhibitory paracrine regulator of hormone secretion. Somatostatin receptors (SSTRs) are a family of 5 transmembrane G protein-coupled receptors, which are widely expressed in mammals including humans. SSTRs regulate multiple downstream signal transduction pathways that mediate inhibitory effects. These receptors also exhibit age- and tissue-specific expression patterns. Interactions of SST and SSTRs are not only important during normal pancreas development, but have also been implicated in many pancreatic diseases such as diabetes mellitus and pancreatic cancer. In this review article, we use evidence from recently published animal studies to present the critical roles of SST and SSTRs proteins in the development of the endocrine pancreas.

Alterations in glucose homeostasis in SSTR1 gene-ablated mice

SSTR1 is found on the majority of human pancreatic beta cells, however, its role in insulin secretion has yet to be elucidated. In this study, we used the SSTR1 knockout mouse model to examine the role of SSTR1 in insulin secretion and glucose homeostasis in mice. Despite the reported effect of SSTR1 in inhibiting growth hormone secretion, SSTR1-/- mice had significantly reduced body weight with growth retardation. Perfusion of isolated mouse pancreata at 3 months of age demonstrated a significant increase in insulin secretion in SSTR1-/- mice compared with that of WT controls. We also found that at 3 months of age, SSTR1-/- mice had significantly decreased levels of systemic insulin secretion and were glucose intolerant. However, SSTR1 gene-ablated mice had a much higher rate of insulin clearance compared to WT mice at the same age. When challenged at 12 months of age, we found SSTR1-/- mice had increased glucose tolerance with exaggerated increase of insulin levels at the end of the experiment. Immunochemical analysis showed that the pancreatic islets of SSTR1-/- mice had significantly decreased levels of somatostatin staining and a significant decrease of SSTR5 expression. These results demonstrate that SSTR1 plays an important role in the regulation of insulin secretion in the endocrine pancreas in mice.

The PDZ/coiled-coil domain containing protein PIST modulates insulin secretion in MIN6 insulinoma cells by interacting with somatostatin receptor subtype 5

The multi-domain protein PIST (protein interacting specifically with Tc10) interacts with the SSTR5 (somatostatin receptor 5) and is responsible for its intracellular localization. Here, we show that PIST is expressed in pancreatic beta-cells and interacts with SSTR5 in these cells. PIST expression in MIN6 insulinoma cells is reduced by somatostatin (SST). After stimulation with SST, SSTR5 undergoes internalization together with PIST. MIN6 cells over-expressing PIST display enhanced glucose-stimulated insulin secretion and a decreased sensitivity to SST-induced inhibition of insulin secretion. These data suggest that PIST plays an important role in insulin secretion by regulating SSTR5 availability at the plasma membrane.

3-Thio-1,2,4-triazoles, novel somatostatin sst2/sst5 agonists

Novel 3-thio-1,2,4-triazoles have been obtained via a solution-phase parallel synthesis strategy, affording potent non-peptidic human somatostatin receptor subtypes 2 and 5 agonists.

Somatostatin receptor subtypes: basic pharmacology and tissue distribution

The heptahelical receptor superfamily constitutes the largest single family of transmembrane-signalling molecules that regulate a wide range of physiological processes. The five somatostatin receptors represent a distinct subgroup of this seven transmembrane receptor superfamily. They range in size from 356 to 391 amino acids with 39-57% protein identity between the subtypes with 100 residues strictly conserved among the somatostatin receptor sequences. A high grade of mRNA homology exists in somatostatin receptor subtypes cloned from different species. Following somatostatin receptor binding and functional activity studies, two alternative models of ligand-binding interaction have been hypothesised. One relies on the presence of a binding pocket within the receptor structure constituted by specific amino acids residues, the other denies the presence of such binding structures and suggests that it is the interaction of agonists with specific extracellular and/or transmembrane domains that determine stable receptor structure conformation. Somatostatin receptors, as, indeed, all G-protein-coupled receptors are able to regulate their responsiveness to agonist exposure. This agonist-specific regulation includes three main events, namely, desensitisation, receptor internalisation and receptor degradation. The cell expression of somatostatin receptor subtypes, at the mRNA level, has been characterised in rodent and in human organs with multiple subtype expression in brain and peripheral tissues.

A reappraisal of the blood glucose homeostat which comprehensively explains the type 2 diabetes mellitus-syndrome X complex

Blood glucose concentrations are unaffected by exercise despite very high rates of glucose flux. The plasma ionised calcium levels are even more tightly controlled after meals and during lactation. This implies 'integral control'. However, pairs of integral counterregulatory controllers (e.g. insulin and glucagon, or calcitonin and parathyroid hormone) cannot operate on the same controlled variable, unless there is some form of mutual inhibition. Flip-flop functional coupling between pancreatic alpha- and beta-cells via gap junctions may provide such a mechanism. Secretion of a common inhibitory chromogranin by the parathyroids and the thyroidal C-cells provides another. Here we describe how the insulin:glucagon flip-flop controller can be complemented by growth hormone, despite both being integral controllers. Homeostatic conflict is prevented by somatostatin-28 secretion from both the hypothalamus and the pancreatic islets. Our synthesis of the information pertaining to the glucose homeostat that has accumulated in the literature predicts that disruption of the flip-flop mechanism by the accumulation of amyloid in the pancreatic islets in type 2 diabetes mellitus will lead to hyperglucagonaemia, hyperinsulinaemia, insulin resistance, glucose intolerance and impaired insulin responsiveness to elevated blood glucose levels. It explains syndrome X (or metabolic syndrome) as incipient type 2 diabetes in which the glucose control system, while impaired, can still maintain blood glucose at the desired level. It also explains why it is characterised by high plasma insulin levels and low plasma growth hormone levels, despite normoglycaemia, and how this leads to central obesity, dyslipidaemia and cardiovascular disease in both syndrome X and type 2 diabetes.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.

Pancreatic somatostatin inhibits insulin secretion via SSTR-5 in the isolated perfused mouse pancreas model

INTRODUCTION

The function of pancreatic somatostatin in insulin secretion is controversial, and the receptor(s) mediating such event has not been exclusively investigated.

AIM AND METHODOLOGY

To differentiate the specific role of SSTR5 in the mouse pancreas, we generated a mouse SSTR5 gene ablation model. Mice homozygous for the deletion (SSTR5-/-) and wild type (WT) littermate controls underwent whole pancreas perfusion to determine the effect of SSTR5 gene ablation on glucose-stimulated insulin secretion. The perfusion was done with and without octreotide added to the infusion buffer. Furthermore, pancreatic somatostatin was immunoneutralized by using a potent somatostatin monoclonal antibody to determine whether pancreatic somatostatin regulates insulin secretion in these mice.

RESULTS

Results showed that at 3 months of age, there were no alterations in insulin secretion compared with WT controls. However, glucose-stimulated insulin secretion was significantly enhanced in 12-month-old SSTR5-/- mice compared with WT controls. The addition of octreotide to the perfusion significantly suppressed insulin secretion in WT controls, while it had no effect on SSTR5-/- mice. Immunoneutralization of pancreatic somatostatin resulted in enhanced glucose-stimulated insulin secretion in WT controls, but decreased levels of insulin secretion in SSTR5-/- mice.

CONCLUSION

These results suggest that, in the mouse, pancreatic somatostatin regulates insulin secretion through SSTR5, and that the effect is age-specific.

Somatostatin receptors in pituitary and development of somatostatin receptor subtype-selective analogs

Somatostatin receptor (SSTR) subtypes 1, 2, and 5 are expressed in the normal human pituitary. SSTR2 and SSTR5 are expressed in almost all growth hormone (GH) cell adenomas, and prolactin (PRL)-secreting tumors express SSTR5 more than SSTR2. SSTR4 is not detected in all pituitary adenoma subtypes, and SSTR1 and SSTR3 are expressed in about 50% of tumors. Human GH is regulated through ligand binding to both SSTR2 and SSTR5, but octreotide and lanreotide, the two clinically available somatostatin analogs, bind to human SSTR2 much better than to SSTR5. Novel SSTR2- and SSTR5- selective analogs with improved binding affinity for these receptor subtypes are highly potent in suppressing GH release from cultures of human fetal pituitaries or GH-cell adenomas. Only SSTR5-selective analogs suppress in vitro PRL secretion from cultured prolactinomas. A new SSTR2+5 bispecific analog with high affinity and selectivity for both SSTR2 and SSTR5, and a somatostatin analog with a unique broad receptor (SSTR1, 2, 3, and 5) binding profile, are both able to inhibit in vitro GH release in GH cell adenomas partially sensitive to octreotide. Recently, a somatostatindopamine hybrid molecule was introduced with potentially functional synergy on GH and PRL release. Using the expanding knowledge on SSTRs and their ligand activation, the development of novel pharmacologic concepts may open new opportunities for effective manipulation of this complex intracellular signaling system. These concepts may achieve better control of pituitary hormone hypersecretion, pituitary size, as well as antitumor effects in patients with SSTR-expressing tumors.

Somatostatin receptor subtype 5 regulates insulin secretion and glucose homeostasis

Somatostatin (SRIF) regulates pancreatic insulin and glucagon secretion. In the present study we describe the generation of SRIF receptor subtype 5 knockout (sst(5) KO) mice to examine the role of SRIF receptor subtypes (sst) in regulating insulin secretion and glucose homeostasis. Mice deficient in sst(5) were viable, fertile, appeared healthy, and displayed no obvious phenotypic abnormalities. Pancreatic islets isolated from sst(5) KO mice displayed increased total insulin content as compared with islets obtained from wild-type (WT) mice. Somatostatin-28 (SRIF-28) and the sst(5)/sst(1)-selective agonist compound 5/1 potently inhibited glucose-stimulated insulin secretion from WT islets. SRIF-28 inhibited insulin secretion from sst(5) KO islets with 16-fold less potency while the maximal effect of compound 5/1 was markedly diminished when compared with its effects in WT islets. sst(5) KO mice exhibited decreased blood glucose and plasma insulin levels and increased leptin and glucagon concentrations compared with WT mice. Furthermore, sst(5) KO mice displayed decreased susceptibility to high fat diet-induced insulin resistance. The results of these studies suggest sst(5) mediates SRIF inhibition of pancreatic insulin secretion and contributes to the regulation of glucose homeostasis and insulin sensitivity. Our findings suggest a potential beneficial role of sst(5) antagonists for alleviating metabolic abnormalities associated with obesity and insulin resistance.

Somatostatin inhibits glucagon-like peptide-1-induced insulin secretion and proliferation of RINm5F insulinoma cells

Glucagon-like peptide-1 [GLP-1; formerly GLP-1(7-36)amide] and somatostatin (SS) are two postprandially or paracrine released peptide hormones that regulate insulin secretion from pancreatic islets. Using the rat insulinoma cell line RINm5F as a model, we investigated the effects of both peptides alone and in combination on insulin release, proliferation, and intracellular signal transduction. In addition, we determined the SS receptor subtypes expressed and involved by reverse transcription-polymerase chain reaction and use of selective SS agonists. GLP-1 stimulated insulin release, cell proliferation, intracellular cAMP accumulation and activation of the transcription factor cAMP-response element binding protein (CREB) which all could be reduced to basal values by co-incubation with SS. Incubation with SS alone did not affect basal levels. RINm5F cells express the somatostatin receptor (sst) subtypes sst1 and sst2 as well as traces of sst3. In accordance, the sst1- or sst2-selective non-peptide agonists L-797591 or L-054522 and peptide agonist octreotide (SMS 201995; sst2, sst3, and sst5 selective) potently inhibited GLP-1-induced insulin secretion whereas the sst3-selective agonist L-796778 showed little effect. Moreover, the sst1- and sst2-selective agonists slightly reduced also basal insulin release. The experiments show that GLP-1 and SS are perfect opponents for regulating pancreatic beta-cell insulin secretion.

Antiangiogenic effects of somatostatin analogues

Inhibition of angiogenesis has become a target for antineoplastic therapy and for treatment of retinal neovascularization. The presence of somatostatin receptors on tumour cells and on the proliferating vascular endothelium has led to several in vitro and in vivo studies to investigate the antiproliferative and antiangiogenic effects of somatostatin analogues. Currently available data suggest that somatostatin analogues might inhibit angiogenesis directly through somatostatin receptors present on endothelial cells and also indirectly through the inhibition of growth factor secretion such as IGF-I and vascular endothelial growth factor (VEGF) and reducing monocyte chemotaxis. However, beneficial effects on inhibition of neovascularization have been questioned by some studies. More work is therefore required to firmly establish the role of somatostatin analogues as potential antiangiogenic therapy. The currently available somatostatin analogues have high affinity for somatostatin receptor subtype 2 (sst2) and, to a lesser extent, sst5 and sst3. However, because vascular endothelial cells express several types of somatostatin receptors, it will be important to investigate somatostatin analogues with different receptor subtype affinities, which might increase the spectrum of available therapy for tumours.

High-throughput receptor-binding methods for somatostatin receptor 2

Three high-throughput screening methods for quantitating 125I-SS14 binding to human somatostatin receptor 2 (hSST2) have been developed. Microplate-based separation assays were performed in Packard Unifilter and Millipore Multiscreen plates. A homogeneous ligand-binding assay was developed by employing wheat germ agglutinin (WGA)-coated Flashplates. Apparent dissociation constants for 125I-SS14 binding to hSST2 were obtained with each method. IC(50) values were determined for 12 compounds using each of the methods. Similar IC(50) values were obtained for each compound with all of the methods. The WGA-Flashplate is suitable for fully automated high-throughput screening whereas the Unifilter and Multiscreen methods are more suitable for semiautomated and manual screening applications.

SSTR2 mediates the somatostatin-induced increase in intracellular Ca(2+) concentration and insulin secretion in the presence of arginine vasopressin in clonal beta-cell HIT-T15

The effects of somatostatin (SRIF) are mediated through the seven transmembrane receptor family that signals via Gi/Go. To date, five distinct SRIF receptors have been characterized and designated SSTR1-5. We have characterized the SRIF receptor that mediates the increase in [Ca(2+)](i) and insulin secretion in HIT-T15 cells (Simian virus 40-transformed Syrian hamster islets) using high affinity, subtype selective agonists for SSTR1 (L-797,591), SSTR2 (L-779,976), SSTR3 (L-796,778), SSTR4 (L-803,087), SSTR5 (L-817,818) and PRL-2903, a specific SSTR2 antagonist. In the presence of arginine vasopressin (AVP), SRIF increased [Ca(2+)](i) and insulin secretion. Treatment with the SSTR2 agonist L-779,976 resulted in similar responses to SRIF. In addition, L-779,976 increased both [Ca(2+)](i) and insulin secretion in a dose-dependent manner. Treatment with L-779,976 alone did not alter [Ca(2+)](i) or basal insulin secretion. In the presence of AVP, all other SRIF receptor agonists failed to increase [Ca(2+)](i) and insulin secretion. The effects of SRIF and L-779,976 were abolished by the SSTR2 antagonist PRL-2903. Our results suggest that the mechanism underlying SRIF-induced insulin secretion in HIT-T15 cells be mediated through the SSTR2.