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

Glucagon-like peptide-1 receptor is expressed in human and rodent testis

BACKGROUND

The incretin hormone glucagon-like peptide-l (GLP-1) is an important regulator of post-prandial insulin secretion, acting through a G protein-coupled cell surface receptor (GLP-1R). In addition to its expression in pancreatic β-cells, several studies suggested that GLP-1R is located in extra-pancreatic tissues.

OBJECTIVES

In this study, we examined for the first time the testicular distribution of the GLP-1R, both in normal human and neoplastic testicular tissues as well as in rodent testis and rodent testicular cell lines.

METHODS AND METHODS

The GLP-1R distribution in testicular section has been evaluated by immunohistochemistry, the specificity of IHC was validated by demonstrating a positive staining for GLP-1RmRNA by RISH technology. While GLP-1R expression in terms of protein was detected by western blot analysis, Moreover, mRNA levels were determined in human testis, in rodent Leydig, and Sertoli cell lines.

RESULTS

Using immunohistochemistrya specific staining for GLP-1R was detected in Leydig cells. The specificity of IHC was validated by demonstrating a positive staining for GLP-1RmRNA only in these cell types. Species differences in the GLP-1R expression between humans and rodents were observed. Interestingly, a decreased expression of the receptor in rodent tumor Leydig cell line and an absence in human Leydig tumor samples was detected.

DISCUSSION

It may be hypothesized that GLP-1R acts like an oncosuppressor in Leydig tumors. A role in regulation of hormone secretion by GLP-1 has been shown in other endocrine cells, therefore we hypothesized that GLP-1R is able to modulate somehow the Leydig cell function.

CONCLUSION

In our findings, a careful evaluation of human testicular tissues and rodent testis revealed Leydig cells as a potential target for GLP-1. Collectively, an effect of GLP-1R in Leydig cell function may be presumed although future studies are needed to ascertain the GLP-1R's role both in normal and tumor Leydig cells.

Specific biology of neuroendocrine tumors: Peptide receptors as molecular targets

Neuroendocrine tumors (NET) are characterized by a high over-expression of many different peptide hormone receptors. These receptors represent important molecular targets for imaging and therapy, using either radiolabeled or cold peptide analogs. The clinically best established example is somatostatin receptor targeting. A relatively new application is glucagon-like peptide 1 (GLP-1) receptor-targeted imaging of insulinomas, which is highly sensitive. A potential future candidate for peptide receptor targeting is the gastric inhibitory peptide (GIP) receptor. It was recently found to exhibit a very wide expression in NET and may be a particularly suitable target in somatostatin and GLP-1 receptor negative tumors. With increasing use of peptide receptor targeting, reliable morphologic in vitro tools to assess peptide receptors in tissues are mandatory, such as in vitro receptor autoradiography or thoroughly established immunohistochemical procedures.

Management of endocrine disease: Diabetes and osteoporosis: cause for concern?

Diabetes and osteoporosis are both frequent conditions, and they may thus occur simultaneously by chance. However, a growing body of evidence suggests that hyperglycemia may impair bone matrix formation and biochemical competence. Decreased biomechanical competence may be present even in a setting of increased bone mineral density, as assessed by traditional dual energy X-ray absorptiometry or normal structural parameters by quantitative computed tomography. Also, the absence of endogenous insulin secretion in type 1 diabetes (T1D) and insulin resistance or, in some cases, frank hyperinsulinemia in T2D may play a role.

A novel peptide nanomedicine against acute lung injury: GLP-1 in phospholipid micelles

PURPOSE

Treatment of acute lung injury (ALI) observed in Gram-negative sepsis represents an unmet medical need due to a high mortality rate and lack of effective treatment. Accordingly, we developed and characterized a novel nanomedicine against ALI. We showed that when human glucagon-like peptide 1(7-36) (GLP-1) self-associated with PEGylated phospholipid micelles (SSM), the resulting GLP1-SSM (hydrodynamic size, ~15 nm) exerted effective anti-inflammatory protection against lipopolysaccharide (LPS)-induced ALI in mice.

METHODS

GLP1-SSM was prepared by incubating GLP-1 with SSM dispersion in saline and characterized using fluorescence spectroscopy and circular dichroism. Bioactivity was tested by in vitro cAMP induction, while in vivo anti-inflammatory effects were determined by lung neutrophil cell count, myeloperoxidase activity and pro-inflammatory cytokine levels in LPS-induced ALI mice.

RESULTS

Amphipathic GLP-1 interacted spontaneously with SSM as indicated by increased α-helicity and fluorescence emission. This association elicited increased bioactivity as determined by in vitro cAMP production. Correspondingly, subcutaneous GLP1-SSM (5-30 nmol/mouse) manifested dose-dependent decrease in lung neutrophil influx, myeloperoxidase activity and interleukin-6 in ALI mice. By contrast, GLP-1 in saline showed no significant anti-inflammatory effects against LPS-induced lung hyper-inflammatory responses.

CONCLUSIONS

GLP1-SSM is a promising novel anti-inflammatory nanomedicine against ALI and should be further developed for its transition to clinics.

Cell-to-cell communication and cellular environment alter the somatostatin status of delta cells

INTRODUCTION

Somatostatin, released from pancreatic delta cells, is a potent paracrine inhibitor of insulin and glucagon secretion. Islet cellular interactions and glucose homeostasis are essential to maintain normal patterns of insulin secretion. However, the importance of cell-to-cell communication and cellular environment in the regulation of somatostatin release remains unclear.

METHODS

This study employed the somatostatin-secreting TGP52 cell line maintained in DMEM:F12 (17.5mM glucose) or DMEM (25mM glucose) culture media. The effect of pseudoislet formation and culture medium on somatostatin content and release in response to a variety of stimuli was measured by somatostatin EIA. In addition, the effect of pseudoislet formation on cellular viability (MTT and LDH assays) and proliferation (BrdU ELISA) was determined.

RESULTS

TGP52 cells readily formed pseudoislets and showed enhanced functionality in three-dimensional form with increased E-cadherin expression irrespective of the culture environment used. However, culture in DMEM decreased cellular somatostatin content (P<0.01) and increased somatostatin secretion in response to a variety of stimuli including arginine, calcium and PMA (P<0.001) when compared with cells grown in DMEM:F12. Configuration of TGP52 cells as pseudoislets reduced the proliferative rate and increased cellular cytotoxicity irrespective of culture medium used.

CONCLUSIONS

Somatostatin secretion is greatly facilitated by cell-to-cell interactions and E-cadherin expression. Cellular environment and extracellular glucose also significantly influence the function of delta cells.

Somatostatin, a negative-regulator of central leptin action in the rat hypothalamus

Leptin-responsive neurons of the hypothalamus constitute a heterogeneous population expressing a vast array of different neuropeptides and neurotransmitters, some of which participate in the regulation of hunger and satiety. Here we report that somatostatin modulates the efficacy of leptin-signalling in the rat hypothalamus. Using a two-pulse paradigm at 30-min intervals, we delivered somatostatin or somatostatin receptor subtype-selective agonists in combination with leptin into the lateral cerebral ventricle of stereotaxically cannulated rats. To monitor the effect of somatostatin on the leptin-signalling pathway, we quantified changes in the leptin-mediated activation of STAT3, the signal transducer and activator of transcription 3. Successive administration of somatostatin and leptin diminished the level of STAT3-phosphorylation and nuclear STAT3 translocation in the ventromedial and dorsomedial hypothalamic nuclei, the lateral hypothalamic area, and the arcuate nucleus by about 40% compared to leptin administration alone. Furthermore, application of subtype-selective somatostatin receptor agonists suggests that the observed reduction in leptin-responsiveness is predominantly mediated by the sst3 receptor-subtype, followed by sst1 and sst2. In addition, the intensity of the negative-regulatory effect of somatostatin on leptin-signalling displayed regional differences for the three receptor-subtypes involved. Addressing the functional consequences of the diminished leptin-signalling, behavioural analyses showed that centrally applied somatostatin counteracts the leptin-mediated suppression of food intake. These results suggest that the pleiotropic effector somatostatin also plays a role in the central regulation of energy homeostasis.

Regulation of Neuronal Nitric-oxide Synthase Activity by Somatostatin Analogs following SST5 Somatostatin Receptor Activation

Somatostatin receptor SST5 is an inhibitory G protein-coupled receptor that exerts a strong cytostatic effect on various cell types. We reported previously that the SST5 anti-proliferative effect results in the inhibition of mitogen-induced increases in intracellular cGMP levels and MAPK activity. This study was conducted to define the early molecular events accountable for the SST5-mediated anti-proliferative effect. Here, we demonstrate that, in Chinese hamster ovary cells expressing SST5 (CHO/SST5 cells), somatostatin inhibited cell proliferation induced by nitric oxide donors and overexpression of the neuronal nitric-oxide synthase (nNOS) protein isoform. Accordingly, nNOS activity and dimerization were strongly inhibited following SST5 activation by the somatostatin analog RC-160. In CHO/SST5 cells, nNOS was dynamically recruited by the SST5 receptor and phosphorylated at tyrosyl residues following RC-160 treatment. RC-160 induced SST5-p60(src) kinase complex formation and subsequent p60(src) kinase activation. Coexpression of an inactive p60(src) kinase mutant with SST5 blocked RC-160-induced nNOS phosphorylation and inactivation and prevented the SST5-mediated anti-proliferative effect. In CHO/SST5 cells, p60(src) kinase associated with nNOS to induce its inactivation by phosphorylation at tyrosyl residues following RC-160 treatment. Using recombinant proteins, we demonstrated that such phosphorylation prevented nNOS homodimerization. Next, surface plasmon resonance and mutation analysis revealed that p60(src) directly associated with nNOS phosphorylated Tyr604. SST5-mediated inhibition of nNOS activity was demonstrated to be essential to the RC-160 anti-proliferative effect on pancreatic endocrine tumor-derived cells. We therefore identified nNOS as a new p60(src) kinase substrate essential for SST5-mediated anti-proliferative action.

Effect of epiregulin on pancreatic beta cell growth and insulin secretion

The aim of this study was to determine whether epiregulin, a novel member of EGF-related growth factor family, was able to affect proliferation and secretory function of rat insulinoma INS-1E and RINm5F cell lines. A 24 h treatment with epiregulin resulted in a stimulation of INS-1E and RINm5F cells proliferation; this effect was completely blocked in the presence of an anti-epiregulin antibody which did not affect basal DNA synthesis in the absence of added ligand. In acute experiments, epiregulin was able to potentiate insulin release in the presence of glucose or arginine, in the two cell lines. Finally, in the two cell lines expressing ErbB receptors, we demonstrated that only EGFR/ErbB1 was activated by epiregulin. Thus, epiregulin appears as a new growth and insulinotropic factor in pancreatic beta cell lines.

A possible role of GLP-1 in the pathophysiology of early dumping syndrome

Exaggerated plasma concentrations of GLP-1 precede reactive hypoglycemia after oral glucose in gastrectomy patients, resulting in late dumping syndrome. Recently, we showed that GLP-1 elicits the activation of sympathetic outflow. Because sympathetic activation is thought to be a cause of early dumping, we hypothesized that exaggerated GLP-1 may contribute to the pathophysiology of early dumping syndrome. In 11 patients after gastrectomy and 14 controls, blood pressure, heart rate, and plasma concentrations of norepinephrine, epinephrine, GLP-1, glucagon, insulin, and glucose were measured after oral glucose. In gastrectomy patients, GLP-1, norepinephrine, and heart rate peaked 15 to 30 min after oral glucose. Significant positive correlations were found among GLP-1, norepinephrine, and heart rate at 30 min, and these parameters at 30 min were significantly higher in patients with early dumping syndrome. These results suggest that GLP-1 is involved in the pathophysiology of early dumping syndrome.

The incretin effect and its potentiation by glucagon-like peptide 1-based therapies: a revolution in diabetes management

The incretin effect is a phenomenon in which enteral glucose administration provokes greater insulin secretion than intravenous administration. The main incretins, glucose-dependent insulinotropic peptide and glucagon-like peptide (GLP)-1 are defective in Type 2 diabetes; whereas glucose-dependent insulinotropic peptide displays diminished effectiveness, GLP-1 secretion is decreased; thus, GLP-1 was a stronger candidate for a new class of anti-diabetic agents designed to potentiate the incretin effect. In the past decade, GLP-1 mimetics, peptidase inhibitors and GLP-1 have been developed. Early randomised trials show that these agents contribute to glucose homeostasis and enhance beta-cell function, without causing hypoglycaemia or weight gain. This review includes an historical perspective, physiology of incretins, and discussions of the pathophysiology in Type 2 diabetes, pharmacology of the main agents and randomised clinical trials published to date.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Glucagon-like peptide 1 modulates calcium responses to glutamate and membrane depolarization in hippocampal neurons

Glucagon-like peptide 1 (GLP-1) activates receptors coupled to cAMP production and calcium influx in pancreatic cells, resulting in enhanced glucose sensitivity and insulin secretion. Despite evidence that the GLP-1 receptor is present and active in neurons, little is known of the roles of GLP-1 in neuronal physiology. As GLP-1 modulates calcium homeostasis in pancreatic beta cells, and because calcium plays important roles in neuronal plasticity and neurodegenerative processes, we examined the effects of GLP-1 on calcium regulation in cultured rat hippocampal neurons. When neurons were pre-treated with GLP-1, calcium responses to glutamate and membrane depolarization were attenuated. Whole-cell patch clamp analyses showed that glutamate-induced currents and currents through voltage-dependent calcium channels were significantly decreased in neurons pre-treated with GLP-1. Pre-treatment of neurons with GLP-1 significantly decreased their vulnerability to death induced by glutamate. Acute application of GLP-1 resulted in a transient elevation of intracellular calcium levels, consistent with the established effects of GLP-1 on cAMP production and activation of cAMP response element-binding protein. Collectively, our findings suggest that, by modulating calcium responses to glutamate and membrane depolarization, GLP-1 may play important roles in regulating neuronal plasticity and cell survival.

GLP-1 derivative liraglutide in rats with beta-cell deficiencies: influence of metabolic state on beta-cell mass dynamics

(1) Liraglutide is a long-acting GLP-1 derivative, designed for once daily administration in type II diabetic patients. To investigate the effects of liraglutide on glycemic control and beta-cell mass in rat models of beta-cell deficiencies, studies were performed in male Zucker diabetic fatty (ZDF) rats and in 60% pancreatectomized rats. (2) When liraglutide was dosed s.c. at 150 microg kg-1 b.i.d. for 6 weeks in ZDF rats 6-8 weeks of age at study start, diabetes development was markedly attenuated. Blood glucose was approximately 12 mm lower compared to vehicle (P<0.0002), and plasma insulin was 2-3-fold higher during a normal 24-h feeding period (P<0.001). Judged by pair feeding, approximately 53% of the antihyperglycemic effect observed on 24-h glucose profiles was mediated by a reduction in food intake, which persisted throughout the study and averaged 16% (P<0.02). (3) Histological analyses revealed that beta-cell mass and proliferation were significantly lower in prediabetic animals still normoglycemic after 2 weeks treatment compared to vehicle-treated animals that had begun to develop diabetes. When the treatment period was 6 weeks, the liraglutide-treated animals were no longer completely normoglycemic and the beta-cell mass was significantly increased compared to overtly diabetic vehicle-treated animals, while beta-cell proliferation was unaffected. (4) In the experiments with 60% pancreatectomized rats, 8 days treatment with liraglutide resulted in a significantly lower glucose excursion in response to oral glucose compared to vehicle treatment. Again, part of the antihyperglycemic effect was due to reduced food intake. No effect of liraglutide on beta-cell mass was observed in these virtually normoglycemic animals. (5) In conclusion, treatment with liraglutide has marked antihyperglycemic effects in rodent models of beta-cell deficiencies, and the in vivo effect of liraglutide on beta-cell mass may in part depend on the metabolic state of the animals.

Biological activity of somatostatin receptors in GC rat tumour somatotrophs: evidence with sst1-sst5 receptor-selective nonpeptidyl agonists

The physiological actions of somatostatin-14 (SRIF: somatotrophin release inhibitory factor) receptor subtypes (sst(1)-sst(5)), which are endogenously expressed in growth cells (GC cells), have not yet been elucidated, although there is evidence that sst(2) receptors are negatively coupled to cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and adenosine 3,5'-cyclic monophosphate (cAMP) accumulation. In addition, both sst(1) and sst(2) receptors are negatively coupled to growth hormone (GH) secretion in GC cells. Here we report on studies concerning the expression, the pharmacology and the functional role of native SRIF receptors in GC cells with the use of five nonpeptidyl agonists, highly selective for each of the SRIF receptors. Radioligand binding studies show that sst(2) and sst(5) receptors are present at different relative densities, while the presence of sst(3) and sst(4) receptors appears to be negligible. The absence of sst(1) receptor binding was unexpected in view of sst(1) receptor functional effects on GH secretion. This suggests very efficient receptor-effector coupling of a low-density population of sst(1) receptors. Functionally, only sst(2) receptors are coupled to the inhibition of [Ca(2+)](i) and cAMP accumulation and the selective activation of sst(5) receptors facilitates the stimulation of adenylyl cyclase activity through G(i/o) proteins. This effect was not observed when sst(2) and sst(5) receptors were simultaneously activated, suggesting that there is a functional interaction between sst(2) and sst(5) receptors. In addition, sst(1), sst(2) and sst(5) receptor activation inhibits GH release, further indicating that SRIF can modulate GH secretion in GC cells through mechanisms both dependent and independent on [Ca(2+)](i) and cAMP-dependent pathways. The present data suggest SRIF-mediated functional effects in GC cells to be very diverse and provides compelling arguments to propose that multiple native SRIF receptors expressed in the same cells are not simply redundant, but contribute to marked signalling diversity.