The elucidation of somatostatin receptor functions: a current view

Internalization of somatostatin receptors in brain and periphery

Somatostatin (SRIF) is a neuropeptide that acts as an important regulator of both endocrine and exocrine secretion and modulates neurotransmission in the central nervous system (CNS). SRIF also regulates cell proliferation in normal tissues and tumors. The physiological actions of SRIF are mediated by a family of five G protein-coupled receptors, called somatostatin receptor (SST) SST₁, SST₂, SST₃, SST₄, SST₅. These five receptors share similar molecular structure and signaling pathways but they display marked differences in their anatomical distribution, subcellular localization and intracellular trafficking. The SST subtypes are widely distributed in the CNS and peripheral nervous system, in many endocrine glands and tumors, particularly of neuroendocrine origin. In this review, we focus on the agonist-dependent internalization and recycling of the different SST subtypes in vivo in the CNS, peripheral organs and tumors. We also discuss the physiological, pathophysiological and potential therapeutic effects of the intracellular trafficking of SST subtypes.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca²⁺ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Synthesis, in vitro biological activity, hydrolytic stability and docking of new analogs of BIM-23052 containing halogenated amino acids

One of the potent somatostatin analogs, BIM-23052 (DC-23-99) D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH₂, has established in vitro growth hormone inhibitory activity in nM concentrations. It is also characterized by high affinity to some somatostatin receptors which are largely distributed in the cell membranes of many tumor cells. Herein, we report the synthesis of a series of analogs of BIM-23052 containing halogenated Phe residues using standard solid-phase peptide method Fmoc/OtBu-strategy. The cytotoxic effects of the compounds were tested in vitro against two human tumor cell lines-breast cancer cell line and hepatocellular cancer cell line, as well as on human non-tumorigenic epithelial cell line. Analogs containing fluoro-phenylalanines are cytotoxic in μM range, as the analog containing Phe (2-F) showed better selectivity against human hepatocellular cancer cell line. The presented study also reveals that accumulation of halogenated Phe residues does not increase the cytotoxicity according to tested cell lines. The calculated selective index reveals different mechanisms of antitumor activity of the parent compound BIM-23052 and target halogenated analogs for examined breast tumor cell lines. All peptides tested have high antitumor activity against the HepG2 cell line (IC₅₀ ≈ 100 μM and SI > 5) compared to breast cells. This is probably due to the high permeability of the cell membrane and the higher metabolic activity of hepatocytes. In silico docking studies confirmed that all obtained analogs bind well with the somatostatin receptors with preference to ssrt3 and ssrt5. All target compounds showed high hydrolytic stability at acid and neutral pH, which mimic physiological condition in stomach and human plasma.

Synthesis, in vitro biological activity and docking of new analogs of BIM-23052 containing unnatural amino acids

Somatostatin (SST) is an endogenous cyclic tetradecapeptide hormone that exerts multiple biological activities via a family of five receptors. BIM-23052 (DC-23-99) D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH2 is a linear SST analog with established in vitro GH-inhibitory activity and high affinity to sstr5, sstr3 and sstr2. The different SSTR subtypes are expressed in different tissues and in some tumor cells. Based on this finding, a series of new analogs of BIM-23052 with expected antitumor activity have been synthesized. The Thr at position 6 in BIM-23052 was replaced by the conformationally hindered Tle, Aib, Ac5c and Ac6c of the new analogs. The peptides were synthesized by standard solid-phase peptide chemistry methods, Fmoc strategy. The cytotoxic effects of the compounds were tested in vitro against a panel of tumor cell lines: HT-29, MDA-MB-23, Hep-G2, HeLa and the normal human diploid cell line Lep-3. All five somatostatin receptor subtypes were modeled and docking was performed to determine the binding affinity of the analogs. The new peptides exhibited different concentration-dependent antiproliferative effect on the tumor cell lines after 24 h of treatment. The compound 3B (Aib⁶) demonstrated the most pronounced antiproliferative effects on HepG-2 cells with the IC₅₀ = 0.01349 nM. Docking confirmed that all compounds bind well to SST receptors with preference to sstr3 and sstr5, which is most probably the reason for the observed biological effects.

Inhibitory effect of somatostatin on insulin secretion is not mediated via the CNS

The inhibitory effect of somatostatin (SST) on insulin secretion in vivo is attributed to a direct effect on pancreatic beta cells, but this is inconsistent with some in vitro results in which exogenous SST is ineffective in inhibiting secretion from isolated islets. We therefore investigated whether insulin secretion from the pancreatic islets may partly be regulated by an indirect effect of SST mediated via the CNS. Islet hormone secretion was assessed in vitro by perifusion and static incubations of isolated islets and in vivo by i.v. or i.c.v. administration of the SST analogue BIM23014C with an i.v. glucose challenge to conscious, chronically catheterised rats. Hormone content of samples was assessed by ELISA or RIA and blood glucose levels using a glucose meter. Exogenous SST14/SST28 or BIM23014C did not inhibit the release of insulin from isolated rodent islets in vitro, whereas peripheral i.v. administration of BIM23014C (7.5 μg) with glucose (1 g/kg) led to decreased plasma insulin content (2.3±0.5 ng insulin/ml versus 4.5±0.5 ng/ml at t=5 min, P<0.001) and elevated blood glucose levels compared with those of the controls (29.19±1.3 mmol/l versus 23.5±1.7 mmol/l, P<0.05). In contrast, central i.c.v. injection of BIM23014C (0.75 μg) had no significant effect on either plasma insulin (3.3±0.4 ng/ml, P>0.05) or blood glucose levels (23.5±1.7 mmol/l, P>0.05) although i.v. administration of this dose increased blood glucose concentrations (32.3±0.7 mmol/l, P<0.01). BIM23014C did not measurably alter plasma glucagon, SST, GLP1 or catecholamine levels whether injected i.v. or i.c.v. These results indicate that SST does not suppress insulin secretion by a centrally mediated effect but acts peripherally on islet cells.

Porcine sst1 can physically interact with other somatostatin receptors, and its expression is regulated by metabolic/inflammatory sensors

The majority of the biological actions attributed to somatostatin (SST) are thought to be mediated by SST receptor 2 (sst2), the most ubiquitous sst, and, to a lesser extent, by sst5. However, a growing body of evidence suggests a relevant role of sst1 in mediating SST actions in (patho)physiological situations (i.e., endometriosis, type 2 diabetes). Moreover, sst1 together with sst2 and sst5 is involved in the well-known actions of SST on pituitary somatotropes in pig and primates. Here, we cloned the porcine sst1 (psst1) and performed a structural and functional characterization using both primary and heterologous models. The psst1 sequence presents the majority of signature motifs shared among G protein-coupled receptors and, specifically, among ssts and exhibits a high homology with other mammalian sst1, with only minor differences in the amino-terminal domain, reinforcing the idea of an early evolutive divergence between mammalian and nonmammalian sst1s. psst1 is functional in terms of decreasing cAMP levels in response to SST when transfected in heterologous models. The psst1 receptor is expressed in several tissues, and analyses of gene cis elements predict regulation by multiple transcription factors and metabolic stimuli. Finally, psst1 is coexpressed with other sst subtypes in various tissues, and in vitro data demonstrate that psst1 can interact with itself forming homodimers and with other ssts forming heterodimers. These data highlight the functional importance of sst1 on the SST-mediated effects and its functional interaction with different ssts, which point out the necessity of exploring the consequences of such interactions.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Somatostatin and its receptors: functional regulation in the development of mice Sertoli cells

Recently, Sertoli cells have been ascertained as the target for the regulatory peptide somatostatin (SST). Therefore, the present study investigated the expression of somatostatin receptors, their age-related alterations and homologous regulation by in vitro treatment with SRIF14 on mice Sertoli cells; furthermore, it dealt with SRIF14 action on growth progression, apoptotic activity and related gene expressions in these cells. We found that mice Sertoli cells expressed all SST1-5 receptors with differential intensities. Age-related real-time PCR of all somatostatin receptors identified abundance of SSTR2 and SSTR5 mRNA level during Sertoli cell developmental period. Furthermore, higher level of these two receptors was independent of SRIF14, as treatment with SRIF14 failed to induce both receptor expressions when compared with control. Somatostatin treatment elicited a dose-dependent decrease in forskolin stimulated cAMP production. Low (100pM and 10nM) and high dosage (1μM) groups of SRIF14 significantly promoted apoptosis, while all treatment groups led to dose dependent cessation (P<0.05) of G1 phase of cell cycle as further validated by increase in casp3, decrease in bcl2, elevation of P21 (all by western blot) and decrease in Igf1 expressions, similarly, SST treatment caused a dose dependent suppression in the mRNA level of kitl gene, which is important in the regulation of spermatogenesis. These findings suggest that somatostatin and its receptors (SSTR2 and SSTR5) are important markers in the regulation and development of Sertoli cell; furthermore, it portrays physiological inhibitory role in Sertoli cell development by inducing apoptosis and cell cycle arrest.

The evolution of vertebrate somatostatin receptors and their gene regions involves extensive chromosomal rearrangements

Background

Somatostatin and its related neuroendocrine peptides have a wide variety of physiological functions that are mediated by five somatostatin receptors with gene names SSTR1-5 in mammals. To resolve their evolution in vertebrates we have investigated the SSTR genes and a large number of adjacent gene families by phylogeny and conserved synteny analyses in a broad range of vertebrate species.

Results

We find that the SSTRs form two families that belong to distinct paralogons. We observe not only chromosomal similarities reflecting the paralogy relationships between the SSTR-bearing chromosome regions, but also extensive rearrangements between these regions in teleost fish genomes, including fusions and translocations followed by reshuffling through intrachromosomal rearrangements. These events obscure the paralogy relationships but are still tractable thanks to the many genomes now available. We have identified a previously unrecognized SSTR subtype, SSTR6, previously misidentified as either SSTR1 or SSTR4.

Conclusions

Two ancestral SSTR-bearing chromosome regions were duplicated in the two basal vertebrate tetraploidizations (2R). One of these ancestral SSTR genes generated SSTR2, -3 and -5, the other gave rise to SSTR1, -4 and -6. Subsequently SSTR6 was lost in tetrapods and SSTR4 in teleosts. Our study shows that extensive chromosomal rearrangements have taken place between related chromosome regions in teleosts, but that these events can be resolved by investigating several distantly related species.

Identification of critical residues involved in ligand binding and G protein signaling in human somatostatin receptor subtype 2

G protein signaling through human somatostatin receptor subtype 2 (SSTR2) is well known, but the amino acids involved in stimulation of intracellular responses upon ligand binding have not been characterized. We constructed a series of point mutants in SSTR2 at amino acid positions 89, 139, and 140 in attempts to disrupt G protein signaling upon ligand binding. The aspartic acid changes at position 89 to either Ala, Leu, or Arg generated mutant receptors with varying expression profiles and a complete inability to bind somatostatin-14 (SST). Mutations to Asp 139 and Arg 140 also led to varying expression profiles with some mutants maintaining their affinity for SST. Mutation of Arg 140 to Ala resulted in a mutated receptor that had a B(max) and dissociation constant (K(d)) similar to wild-type receptor but was still coupled to the G protein as determined in both a cAMP assay and a calcium-release assay. In contrast, mutation of Asp 139 to Asn resulted in a mutated receptor with B(max) and K(d) values that were similar to wild type but was uncoupled from G protein-mediated cAMP signaling, but not calcium release. Thus, we identified mutations in SSTR2 that result in either receptor expression levels that are similar to wild type but is completely ablated for ligand binding or a receptor that maintains affinity for SST and is uncoupled from G protein-mediated cAMP signaling.

Somatostatin receptor biology in neuroendocrine and pituitary tumours: part 1--molecular pathways

Neuroendocrine tumours (NETs) may occur at many sites in the body although the majority occur within the gastroenteropancreatic axis. Non-gastroenteropancreatic NETs encompass phaeochromocytomas and paragangliomas, medullary thyroid carcinoma, anterior pituitary tumour, broncho-pulmonary NETs and parathyroid tumours. Like most endocrine tumours, NETs also express somatostatin (SST) receptors (subtypes 1–5) whose ligand SST is known to inhibit endocrine and exocrine secretions and have anti-tumour effects. In the light of this knowledge, the idea of using SST analogues in the treatment of NETs has become increasingly popular and new studies have centred upon the development of new SST analogues. We attempt to review SST receptor (SSTR) biology primarily in neuroendocrine tissues, focusing on pituitary tumours. A full data search was performed through PubMed over the years 2000–2009 with keywords ‘somatostatin, molecular biology, somatostatin receptors, somatostatin signalling, NET, pituitary’ and all relevant publications have been included, together with selected publications prior to that date. SSTR signalling in non-neuroendocrine solid tumours is beyond the scope of this review. SST is a potent anti-proliferative and anti-secretory agent for some NETs. The successful therapeutic use of SST analogues in the treatment of these tumours depends on a thorough understanding of the diverse effects of SSTR subtypes in different tissues and cell types. Further studies will focus on critical points of SSTR biology such as homo- and heterodimerization of SSTRs and the differences between post-receptor signalling pathways of SSTR subtypes.

Resistance to somatostatin analogs in acromegaly

Somatostatin analogs (SA) are widely used in acromegaly, either as first-line or adjuvant treatment after surgery. First-line treatment with these drugs is generally used in the patients with macroadenomas or in those with clinical conditions so severe as to prevent unsafe reactions during anesthesia. Generally, the response to SA takes into account both control of GH and IGF-I excess, with consequent improvement of clinical symptoms directly related to GH and IGF-I excess, and tumor shrinkage. This latter effect is more prominent in the patients treated first-line and bearing large macroadenomas, but it is also observed in patients with microadenomas, even with little clinical implication. Predictors of response are patients' gender, age, initial GH and IGF-I levels, and tumor mass, as well as adequate expression of somatostatin receptor types 2 and 5, those with the highest affinity for octreotide and lanreotide. Only sporadic cases of somatostatin receptor gene mutation or impaired signaling pathways have been described in GH-secreting tumors so far. The response to SA also depends on treatment duration and dosage of the drug used, so that a definition of resistance based on short-term treatments using low doses of long-acting SA is limited. Current data suggest that response to these drugs is better analyzed taking together biochemical and tumoral effects because only the absence of both responses might be considered as a poor response or resistance. This latter evidence seems to occur in 25% of treated patients after 12 months of currently available long-acting SA.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Distinct phosphorylation sites in the SST2A somatostatin receptor control internalization, desensitization, and arrestin binding

The somatostatin subtype 2A (sst2A) receptor, a member of the G protein-coupled receptor superfamily, mediates many of the neuroendocrine and neuromodulatory actions of somatostatin, and it represents the primary target for somatostatin analogs used in cancer therapy and tumor localization. Agonist stimulation leads to the rapid phosphorylation, endocytosis, and desensitization of the sst2A receptor; however, little is known about the role of phosphorylation in sst2A regulation. sst2A phosphorylation occurs on serine and threonine residues in the third intracellular loop and carboxyl terminus. Therefore, we generated mutant receptors in which serine (Ser-), threonine (Thr-), or both (Ser-/Thr-) residues in these regions were mutated to alanine. In contrast to the wild-type receptor, somatostatin treatment did not stimulate the phosphorylation of the Ser-/Thr- mutant, and it did not produce desensitization. Furthermore, internalization of the Ser-/Thr- mutant occurred 5 times more slowly than with the wild-type receptor. Mutating only the Ser residues did not inhibit either internalization or desensitization. In contrast, mutating only the Thr residues inhibited receptor endocytosis to the same extent as in the full mutant, but it did not affect receptor desensitization. In both the wild-type and Ser- receptors, agonist binding produced a stable arrestin-receptor complex that was maintained during receptor trafficking, whereas arrestin was not recruited to either the Thr- or the Ser-/Thr- receptors. These results demonstrate that agonist-stimulated receptor phosphorylation is necessary for both desensitization and rapid internalization of the sst2A receptor. However, sst2A receptor internalization and uncoupling can occur independently, involve different receptor phosphorylation sites, and exhibit different requirements for stable arrestin association.

Inhibitory effects of synthetic somatostatin receptor subtype 4 agonists on acute and chronic airway inflammation and hyperreactivity in the mouse

Somatostatin released from activated capsaicin-sensitive afferents of the lung inhibits inflammation and related bronchial hyperreactivity presumably via somatostatin 4 receptors (sst(4)). The aim of this study was to examine the effects of TT-232, a heptapeptide sst(4)/sst(1) receptor agonist and J-2156, a high affinity sst(4) receptor-selective peptidomimetic agonist in airway inflammation models. Acute pneumonitis was evoked by intranasal lipopolysaccharide 24 h before measurement. Chronic inflammation was induced by ovalbumin inhalation on days 28, 29 and 30 after i.p. sensitization on days 1 and 14. Semiquantitative histopathological scoring was based on perivascular/peribronchial oedema, neutrophil/macrophage infiltration, goblet cell hyperplasia in the acute model and eosinophil infiltration, mucosal oedema, mucus production and epithelial cell damage in chronic inflammation. Myeloperoxidase activity of the lung was measured spectrophotometrically to quantify granulocyte accumulation and the broncoalveolar lavage fluid was analysed by flow cytometry. Carbachol-induced bronchoconstriction was assessed by unrestrained whole body plethysmography and its calculated indicator, enhanced pause (Penh) was determined. TT-232 and J-2156 induced similar inhibition on granulocyte recruitment and histopathological changes in both models, although macrophage infiltration in LPS-induced inflammation was unaltered by either compounds. Both agonists diminished inflammatory airway hyperresponsiveness. Since their single administration after the development of the inflammatory reactions also inhibited carbachol-induced bronchoconstriction, somatostatin sst(4) receptor activation on bronchial smooth muscle cells is likely to be involved in their anti-hyperreactivity effect. These results suggest that stable, somatostatin sst(4) receptor-selective agonists could be potential candidates for the development of a completely novel group of anti-inflammatory drugs for the treatment of airway inflammation and hyperresponsiveness.

Inhibitory effect of pasireotide and octreotide on lymphocyte activation

Somatostatin (SST) regulates the function of the central and peripheral nervous system, the endocrine and exocrine organs, as well as the vascular and immune system. These actions are mediated by five specific membrane somatostatin receptors. This study compares the effects on human lymphocytes of two long-acting somatostatin analogues that have different receptor affinity: octreotide and pasireotide. Both analogues have an antiproliferative effect on human lymphocyte proliferation, but they act at different concentration and, while octreotide enhances IL10 and inhibits gamma IFN pasireotide inhibits IL2 and gamma IFN. In both sets of experiment the different behaviour of the two analogues could be due to their different affinity to the SSTR subtypes. Finally this study suggest that the growth inhibitory action of somatostatin analogues is an apoptotic phenomenon and it can be mediated by SSTR2a, in the case of octreotide, and by SSTR3 when pasireotide is used or it can be mediated by the heterodimerization of the two receptor.

Inhibitory effect of somatostatin on inflammation and nociception

The present review focuses on promising new opportunities for anti-inflammatory and analgesic therapy. The theoretical background is an original observation based on our own experimental results. These data demonstrate that somatostatin is released from capsaicin-sensitive, peptidergic sensory nerve endings in response to noxious heat and chemical stimuli such as vanilloids, protons or lipoxygenase products. It reaches distant parts of the body via the circulation and exerts systemic anti-inflammatory and analgesic effects. Somatostatin binds to G-protein-coupled membrane receptors (sst(1)-sst(5)) and diminishes neurogenic inflammation by prejunctional action on sensory-efferent nerve terminals, as well as by postjunctional mechanisms on target cells. It decreases the release of pro-inflammatory neuropeptides from sensory nerve endings and also acts on receptors of vascular endothelial, inflammatory and immune cells. Analgesic effect is mediated by an inhibitory action on peripheral terminals of nociceptive neurons, since circulating somatostatin cannot exert central action. Somatostatin itself is not suitable for drug development because of its broad spectrum and short elimination half life, stable, receptor-selective agonists have been synthesized and investigated. The present overview is aimed at summarizing the physiological importance of somatostatin and sst receptors, pharmacological significance of synthetic agonists and their potential in the development of novel anti-inflammatory and analgesic drugs. These compounds might provide novel perspectives in the pharmacotherapy of acute and chronic painful inflammatory diseases, as well as neuropathic conditions.

Design, Synthesis, and Biological Evaluation of Somatostatin-Based Radiopeptides

The prototypes for tumor targeting with radiolabeled peptides are derivatives of somatostatin. Usually, they primarily have high affinity for somatostatin receptor subtype 2 (sst2), and they have moderate affinity for sst5. We aimed at developing analogs that recognize different somatostatin receptor subtypes for internal radiotherapy in order to extend the present range of accessible tumors. We synthesized DOTA-octapeptides based on octreotide by replacing Phe3 mainly with unnatural amino acids. The affinity profile was determined by using cell lines transfected with sst1-5. Internalization was determined by using AR42J, HEK-sst3, and HEK-sst5 cell lines, and biodistribution was studied in rat tumor models. Two of the derivatives thus obtained showed an improved binding affinity profile, enhanced internalization into cells expressing sst2 and sst3, respectively, and better tumor:kidney ratios in animals.

Effects of the somatostatin receptor subtype 4 selective agonist J-2156 on sensory neuropeptide release and inflammatory reactions in rodents

BACKGROUND AND PURPOSE

Substance P (SP) and calcitonin gene-related peptide (CGRP) released from capsaicin-sensitive sensory nerves induce local neurogenic inflammation; somatostatin exerts systemic anti-inflammatory actions presumably via sst4/sst1 receptors. This study investigates the effects of a high affinity, sst4-selective, synthetic agonist, J-2156, on sensory neuropeptide release in vitro and inflammatory processes in vivo.

EXPERIMENTAL APPROACH

Electrically-induced SP, CGRP and somatostatin release from isolated rat tracheae was measured with radioimmunoassay. Mustard oil-induced neurogenic inflammation in rat hindpaw skin was determined by Evans blue leakage and in the mouse ear with micrometry. Dextran-, carrageenan- or bradykinin-induced non-neurogenic inflammation was examined with plethysmometry or Evans blue, respectively. Adjuvant-induced chronic arthritis was assessed by plethysmometry and histological scoring. Granulocyte accumulation was determined with myeloperoxidase assay and IL-1beta with ELISA.

KEY RESULTS

J-2156 (10-2000 nM) diminished electrically-evoked neuropeptide release in a concentration-dependent manner. EC50 for the inhibition of substance P, CGRP and somatostatin release were 11.6 nM, 14.3 nM and 110.7 nM, respectively. J-2156 (1-100 microg kg(-1) i.p.) significantly, but not dose-dependently, inhibited neurogenic and non-neurogenic acute inflammatory processes and adjuvant-induced chronic oedema and arthritic changes. Endotoxin-evoked myeloperoxidase activity and IL-1beta production in the lung, but not IL-1beta- or zymosan-induced leukocyte accumulation in the skin were significantly diminished by J-2156.

CONCLUSIONS AND IMPLICATIONS

J-2156 acting on sst4 receptors inhibits neuropeptide release, vascular components of acute inflammatory processes, endotoxin-induced granulocyte accumulation and IL-1beta synthesis in the lung and synovial and inflammatory cells in chronic arthritis. Therefore it might be a promising lead for the development of novel anti-inflammatory drugs.

The release of 3H-1-methyl-4-phenylpyridinium from bovine adrenal chromaffin cells is modulated by somatostatin

Besides cholinergic regulation, catecholamine secretion from adrenal chromaffin cells can be elicited and/or modulated by noncholinergic neurotransmitters and hormones. This study was undertaken to investigate the influence of somatostatin and octreotide on [3H]MPP+ secretion evoked by KCl or cholinergic agents, from bovine adrenal chromaffin cells. The release of [3H]MPP+ was markedly increased by excess KCl (50 mM), acetylcholine (50 microM-10 mM) and by the nicotinic agonists, nicotine (5-100 microM) and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 10-100 microM), but not by the muscarinic agonist, pilocarpine (10-100 microM). Acetylcholine-evoked release of [3H]MPP+ from these cells was mainly mediated by nicotinic receptors: a) nicotine and DMPP stimulated the release of [3H]MPP+, b) a nicotinic antagonist, hexamethonium, markedly blocked the acetylcholine-evoked response and c) pilocarpine was devoid of effect on [3H]MPP+ secretion. At all concentrations tested, somatostatin and octreotide interfered neither with [3H]MPP+ basal release nor with KCl-induced release of [3H]MPP+. However, somatostatin (0.01-0.3 microM) increased the release of [3H]MPP+ induced by a high concentration of acetylcholine (10 mM). Octreotide (1-10 microM) had no effect. These results, showing that somatostatin potentiates acetylcholine-induced [3H]MPP+ release, support the hypothesis that somatostatin may increase the release of catecholamines from adrenal medullary cells.

Antiproliferative effects of somatostatin analogs in pituitary adenomas

The antisecretory effects of somatostatin (SRIF) and its analogs are widely recognised and provide the basis for treatment of hormonal hypersecretion in pituitary adenomas, especially in the settings of acromegaly. Evidence for an antiproliferative effect of these compounds has also been provided. This review focuses on the mechanisms transducing the antiproliferative effects of SRIF and its analogs on pituitary adenomas, and on the clinical consequences on tumor volume of pharmacological treatment of pituitary adenomas with these drugs.

A vitamin A-free diet results in impairment of the rat hippocampal somatostatinergic system

Previous studies have revealed the presence of retinoid specific receptors in the hippocampus and have demonstrated that vitamin A deficiency produces a severe deficit in spatial learning and memory which are linked to a proper hippocampal functioning. It is also well known that the tetradecapeptide somatostatin binds to specific receptors in the hippocampus and, when injected into this brain area, facilitates the acquisition of spatial tasks. In addition, depletion of somatostatin by cysteamine impairs acquisition of these tasks. Taken together, these studies support the idea that the hippocampal somatostatinergic system might be regulated by vitamin A. Hence, we evaluated the effects of vitamin A deprivation and subsequent administration of vitamin A on the rat hippocampal somatostatinergic system. Rats fed a vitamin A-free diet exhibited a significant reduction of somatostatin-like immunoreactivity content in the hippocampus whereas the somatostatin mRNA levels were unaltered. Vitamin A deficiency increased the somatostatin receptor density and its dissociation constant. Functional Gi activity as well as the capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity was decreased in vitamin A deficiency rats as compared with the control animals. All these parameters were fully restored when vitamin A was replaced in the diet. Furthermore, we found that the Gialpha1, Gialpha2 and Gialpha3 protein levels were unaltered in hippocampal membranes from rats fed a vitamin A-free diet whereas subsequent vitamin A administration to these rats caused a significant increase in the levels of Gialpha1 and Gialpha2. Altogether, the present findings suggest that dietary vitamin A levels modulate the somatostatinergic system in the rat hippocampus.

Fish somatostatin sst3 receptor: comparison of radioligand and GTPgammaS binding, adenylate cyclase and phospholipase C activities reveals different agonist-dependent pharmacological signatures

1 The fish somatostatin receptor 3 (fsst3) is one of the few somatostatin (SRIF) receptors cloned from a non-mammalian species so far. Here we extended our earlier characterization of this receptor by investigating the guanine nucleotide sensitivity of agonist radioligand binding at the fsst3 receptor recombinantly expressed in CCL39 (Chinese hamster lung fibroblast) cells. Further, we measured somatostatin (SRIF) and cortistatin (CST) analogues stimulated GTPgammaS binding, inhibition of forskolin-stimulated adenylate cyclase (FSAC) and stimulation of phospholipase C (PLC) activities. The present transductional data were then compared with previous radioligand binding and/or second messenger features determined for fsst3 and/or human SRIF receptors (hsst2, hsst3 and hsst5). 2 The GTP analogue guanylylimidodiphosphate (GppNHp) inhibited binding of [125I]CGP 23996 and [125I][Tyr3octreotide by 72 and 83% suggesting preferential labelling of G-protein-coupled fsst3 receptors. By contrast, [125I]LTT-SRIF28 and [125I][Tyr10]CST14 binding was rather GppNHp insensitive (42 and 35% inhibition) suggesting labelling of both coupled and non-coupled receptor states. These results might explain the apparent higher receptor densities determined in saturation experiments with [125I]LTT-SRIF28 and [125I][Tyr10]CST14 (4470 and 4030 fmol mg(-1)) compared with [125I]CGP 23996 and [125I][Tyr3]octreotide (3420 and 1520 fmol mg(-1)). 3 SRIF14 (10 microm)-stimulated specific [35S]GTPgammaS binding by three-fold; SRIF28 and octreotide displayed full agonism, whereas most other ligands displayed 60-80% intrinsic activity compared with SRIF14. SRIF14 and SRIF28 inhibited forskolin-stimulated AC (FSAC) activity by 60%; all tested ligands except BIM 23056 inhibited FSAC with comparable high intrinsic activities. SRIF14 stimulated PLC activity five- to six-fold, as determined by measuring total [3H] IP(x) accumulation; it was rather insensitive to pertussis toxin (PTX, 100 ng ml(-1), 21% inhibition), which suggests the G(q)-family proteins couple to PLC activity. SRIF14, SRIF28 and [Tyr10]CST14 showed full agonism at PLC, whereas all other ligands behaved as partial agonists (20-70% intrinsic activity). BIM 23056, which showed weak partial or no agonism, antagonized SRIF14-induced total [3H]-IP(x) production (pK(B) = 6.83), but failed to block competitively agonist-stimulated [35S]GTPgammaS binding or agonist-induced inhibition of FSAC activity. 4 Comparison of the pharmacological profiles of fsst3 receptors established in GTPgammaS binding, FSAC inhibition and PLC stimulation resulted in low correlations (r = 0.410-0.594). Both rank orders of potency and rank orders of relative efficacy varied in the three second messenger experiments. Significant, although variable correlations were obtained comparing GTPgammaS binding and inhibition of FSAC activity with previously reported affinity profiles of [125I]LTT-SRIF28, [125I][Tyr10]CST14, [125I]CGP 23996, [125I][Tyr3]octreotide (r = 0.75-0.83; 0.68-0.89). By contrast, the PLC stimulation and radioligand-binding profiles did not correlate. 5 Comparison of the functional data (GTPgammaS binding, FSAC inhibition, PLC stimulation) of fsst3 receptors with those of human sst2, sst3, sst5 receptors expressed in CCL39 cells resulted in highest correlation with the hsst5 receptor (r = 0.94, 0.97, 0.49) > hsst2 (0.80, 0.50, n.d.) > hsst3 (0.25, 0.19, 0.17). 6 In summary, fsst3 receptors expressed in CCL39 cells are involved in signalling cascades similar to those reported for mammalian SRIF receptors, suggesting SRIF receptors to be highly conserved in evolution. Binding and functional data showed highest similarity of fsst3 receptors with the human sst5 receptor subtype. Different affinities, receptor densities and GppNHp-sensitivities determined with the four radioligands (agonists) are assumed to results from ligand-specific states of the fsst3-ligand complex. The differences in the rank orders of potency and relative efficacy in the various signalling cascades may be explained by agonist-induced receptor trafficking.

Somatostatin down-regulates the expression and release of endozepines from cultured rat astrocytes via distinct receptor subtypes

Endozepines, a family of regulatory peptides related to diazepam-binding inhibitor (DBI), are synthesized and released by astroglial cells. Because rat astrocytes express various subtypes of somatostatin receptors (sst), we have investigated the effect of somatostatin on DBI mRNA level and endozepine secretion in rat astrocytes in secondary culture. Somatostatin reduced in a concentration-dependent manner the level of DBI mRNA in cultured astrocytes. This inhibitory effect was mimicked by the selective sst4 receptor agonist L803-087 but not by the selective sst1, sst2 and sst3 receptor agonists L779-591, L779-976 and L797-778, respectively. Somatostatin was unable to further reduce DBI mRNA level in the presence of the MEK inhibitor U0126. Somatostatin and the sst1, sst2 and sst4 receptor agonists induced a concentration-dependent inhibition of endozepine release. Somatostatin and the sst1, sst2 and sst4 receptor agonists also inhibited cAMP formation dose-dependently. In addition, somatostatin reduced forskolin-induced endozepine release. H89 mimicked the inhibitory effect of somatostatin on endozepine secretion. In contrast the PLC inhibitor U73122, the PKC activator PMA and the PKC inhibitor calphostin C had no effect on somatostatin-induced inhibition of endozepine release. The present data demonstrate that somatostatin reduces DBI mRNA level mainly through activation of sst4 receptors negatively coupled to the MAPK pathway, and inhibits endozepine release through activation of sst1, sst2 and sst4 receptors negatively coupled to the adenylyl cyclase/PKA pathway.

Endocrine tumours of the gastrointestinal tract. Somatostatin receptors as tools for diagnosis and therapy: molecular aspects

Somatostatin is a neuropeptide that acts as an endogenous inhibitor of various cellular functions including endocrine and exocrine secretions and the proliferation of normal and tumour cells. Its action is mediated by a family of G-protein-coupled receptors (sst1-sst5) that are widely distributed in normal and tumour cells. Gastroenteropancreatic endocrine tumours express multiple somatostatin receptors, sst2 being clearly predominant. These receptors represent the molecular basis for the clinical use of somatostatin analogues in the treatment of endocrine tumours and their in vivo localisation. This review covers current knowledge in somatostatin receptor biology and signalling.

Display of somatostatin-related peptides in the complementarity determining regions of an antibody light chain

Peptide display in antibody complementarity determining regions (CDRs) offers several advantages over other peptide display systems including the potential to graft heterologous peptide sequences into multiple positions in the same backbone molecule. Despite the presence of six CDRs in an antibody variable domain, the majority of insertions reported have been made in heavy chain CDR3 (h-CDR3) which may be explained in part by the highly variable length and sequence diversity found in h-CDR3 in native antibodies. The ability to graft peptide sequences into CDRs is restricted by amino acids in these loops that make structural contacts to framework regions or are oriented towards the hydrophobic interior and are important for the proper folding of the antibody. To identify such positions in human kappa-light chain CDR1 (kappa-CDR1) and CDR2 (kappa-CDR2), we performed alignments of 1330 kappa-light chain variable region amino acid sequences and 19 variable region X-ray crystal structures. From analyses of these alignments, we predict insertion points where sequences can be grafted into kappa-CDR1 and kappa-CDR2 to prepare synthetic antibody molecules. We then tested these predictions by inserting somatostatin and somatostatin-related sequences into kappa-CDR1 and kappa-CDR2, and analyzing the expression and ability of the modified antibodies to bind to membranes containing somatostatin receptor 5. These results expand the repertoire of CDRs that can be used for the display of heterologous peptides in the CDRs of antibodies.

Octreotide in insulinoma patients: efficacy on hypoglycemia, relationships with Octreoscan scintigraphy and immunostaining with anti-sst2A and anti-sst5 antibodies

OBJECTIVE

We studied the efficacy of octreotide treatment on hypoglycaemia in patients with insulinoma and its relationships with Octreoscan scintigraphy and the presence of tumoral somatostatin receptors sst2A and sst5.

DESIGN AND METHODS

17 patients with insulinoma were evaluated using (i) evaluation of blood glucose, insulin and C-peptide during a short 100 mug octreotide test in fasting patients and/or treatment over 8 days-8 months with octreotide, (ii) Octreoscan scintigraphy and (iii) immunostaining of the tumor with anti-sst2A and anti-sst5.

RESULTS

Octreotide was effective on hypoglycaemia in 10/17 patients. Octreoscan scintigraphy detected 4/17 insulinomas. sst2A receptor was detected in 7/17 insulinomas and sst5 in 15/17 insulinomas. Octreotide was effective on hypoglycaemia in those seven patients with sst2A receptor-expressing insulinoma, and in three patients with undetectable sst2A receptor and detectable sst5; it was ineffective in six patients whose tumor expressed the sst5 receptor with undetectable sst2A and in one patient with undetectable sst2A and sst5 receptor.

CONCLUSIONS

Octreotide is an effective treatment of hypoglycaemia in more than 50% of patients with insulinoma. Detection of responsive patients was better based on a positive short test with subcutaneous octreotide than on the results of Octreoscan scintigraphy. Positive anti-sst2 receptor immunostaining is associated with efficacy of octreotide treatment, but does not account for all cases of responsiveness to octreotide. Expression of sst5 receptor does not appear to explain per se the efficacy of octreotide on sst2A-negative insulinomas.

Activity of the hippocampal somatostatinergic system following daily administration of melatonin

If melatonin or its analogs are to be used therapeutically in humans, their chronic effects on responsiveness of melatonin target cells need to be assessed. We have previously demonstrated that acute melatonin treatment regulates the somatostatinergic system in the rat hippocampus. In the present study, we have investigated the effects of subchronic and chronic daily treatment with melatonin on the somatostatinergic system in the rat hippocampus. Male Wistar rats (200-250 g) were injected with melatonin (25 microg/kg body weight, subcutaneously) daily for 4, 7 or 14 days and sacrificed 24 h after the last injection. Melatonin administration for 4 days induced a decrease in the hippocampal somatostatin (SRIF)-like immunoreactivity content as well as a decrease in the number of SRIF receptors and an increase in their apparent affinity. The decreased number of SRIF receptors in the melatonin (4 days)-treated rats was associated with a decreased capacity of SRIF to inhibit both basal and forskolin-stimulated adenylyl cyclase activity. These melatonin-induced effects reversed to control values after 7 or 14 days of treatment. Hippocampal membranes from control and melatonin-treated rats showed similar Gi and Gs activities. Melatonin treatment altered neither the functional Gi activity nor the Gialpha 1 or Gialpha 2 levels at any of the time periods studied. The present results suggest that chronic exposure to melatonin results in a tolerance of the hippocampus to this hormone.

Molecular signaling of somatostatin receptors

Somatostatin is a neuropeptide family that is produced by neuroendocrine, inflammatory, and immune cells in response to different stimuli. Somatostatin acts as an endogenous inhibitory regulator of various cellular functions including secretions, motility, and proliferation. Its action is mediated by a family of G-protein-coupled receptors (called sst1-sst5) that are widely distributed in the brain and periphery. The five receptors bind the natural peptides with high affinity, but only sst2, sst5, and sst3 bind the short synthetic analogs used to treat acromegaly and neuroendocrine tumors. This review covers the current knowledge in somatostatin receptor biology and signaling.

The effect of drug dose and drug exposure time on the binding, internalization, and cytotoxicity of radiolabeled somatostatin analogs

BACKGROUND

Creation of protease-resistant somatostatin analogs has allowed development of these peptides as clinically useful drugs. Widespread diagnostic use of radiolabeled somatostatin analogs has enhanced interest in the binding and intracellular distribution of these peptides. The degree of drug internalization and length of drug retention may be critical for drug-induced cytotoxicity. We hypothesized that the ability of a radiolabeled peptide to bind to a cell, be internalized, and induce cytotoxicity is proportional to both the radioligand concentration and the exposure time.

MATERIALS AND METHODS

To test this hypothesis, somatostatin receptor-expressing cells (IMR-32) were incubated with (111)In-pentetreotide, a sst 2 preferring somatostatin analogue. Radioligand exposure time and/or concentration were varied.

RESULTS

Prolonged exposure to a fixed concentration of radioligand resulted in progressive increases in whole cell binding and internalization over time. Cells exposed to a relatively fixed number of microCi-Hr yielded constant whole cell binding and internalization. Increasing the microCi-Hr resulted in a proportionate increase in binding. Cytotoxicity was also proportional to the dose of radiation regardless of whether the exposure was internalized radiation (microCi-Hr from (111)In-pentetreotide) or from external beam radiation (cGy).

CONCLUSION

Both drug exposure time and drug concentration contribute to cell binding and cytotoxicity in this model and their relative contributions are inversely related.

Comparison of functional profiles at human recombinant somatostatin sst2 receptor: simultaneous determination of intracellular Ca2+ and luciferase expression in CHO-K1 cells

1. Somatostatin (somatotropin release inhibiting factor; SRIF) acts via five G protein-coupled receptors (sst(1)-sst(5)) that modulate multiple cellular effectors. The aim of this study was to compare two functional effects of the human sst(2) receptor stably expressed in CHO-K1 cells in a single experiment using a duplex assay for intracellular calcium and serum response element (SRE)-driven luciferase expression. 2. Intracellular calcium was measured using a fluorometric imaging plate reader II (FLIPR II). SRIF-14 rapidly and transiently increased intracellular calcium with a pEC(50) of 8.74+/-0.03 (n=52). At 5 h after FLIPR II measurements, luciferase expression was determined. SRIF-14 concentration-dependently increased luciferase expression (pEC(50)=9.06+/-0.03, n=52). 3. Natural and synthetic agonist/antagonist ligands for SRIF receptors were tested in the duplex assay. Correlation of agonist potencies and efficacies between the two responses were significant (r(2)=0.83 and 0.90, pEC(50) and E(max), respectively). 4. Pertussis toxin pretreatment reduced SRIF-14/octreotide-mediated intracellular calcium increases by 45-47% and luciferase expression by 95-98%. 5. Thapsigargin pretreatment abolished the SRIF-14/octreotide-mediated intracellular calcium increase but had no effect on luciferase expression. 6. In conclusion, SRIF stimulates an increase in intracellular calcium and SRE-luciferase expression via human sst(2) receptors in CHO-K1 cells. The increase in luciferase is mediated via G(i)/G(o) while intracellular calcium increase is mediated by both G(i)/G(o) proteins and pertussis toxin-insensitive G proteins, and is mainly via release of calcium from intracellular stores. SRIF ligands display a similar recognition profile suggesting that the ligand/receptor/G protein/effector interaction is similar for the two parameters.

Somatostatin receptors and regulation of cell proliferation

Somatostatin is an inhibitory neuropeptide, which acts on various targets throughout the body to regulate a variety of physiological functions including inhibition of endocrine and exocrine secretions, modulation of neurotransmission, motor and cognitive functions, inhibition of intestinal motility, absorption of nutrients and ions, vascular contractility and inhibition of normal and tumour cell proliferation. It exerts its effects through interaction with five somatostatin receptors (sst1-sst5), which belong to the family of G-protein-coupled receptors with seven transmembrane spanning domains and are variably expressed in a variety of tumours such as gastroenteropancreatic tumours, pituitary tumours, and carcinoid tumours. This review covers the present knowledge regarding the molecular mechanisms involved in somatostatin antineoplastic activity. Evidence that sst2 receptor acts as a tumour suppressor is also discussed.

Characterization and function of TWIK-related acid sensing K+ channels in a rat nociceptive cell

We examined the properties of a proton sensitive current in acutely dissociated, capsaicin insensitive nociceptive neurons from rat dorsal root ganglion (DRG). The current had features consistent with K(+) leak currents of the KCNK family (TASK-1, TASK-3; TWIK-related acid sensing K(+)). Acidity and alkalinity induced inward and outward shifts in the holding current accompanied by increased and decreased whole cell resistance consistent with a K(+) current. We used alkaline solutions to open the channel and examine its properties. Alkaline evoked currents (AECs; pH 10.0-10.75), reversed near the K(+) equilibrium potential (-74 mV), and were suppressed 85% in 0 mM K(+). AECs were insensitive to Cs(+) (1 mM) and anandamide (1 microM), but blocked by Ba(++) (1 mM), quinidine (100 microM) or Ruthenium Red (10 microM). This pharmacology was identical to that of rat TASK-3 and inconsistent with that of TASK-1 or TASK-2. The TASK-like AEC was not modulated by PKA (forskolin, kappa opioid agonists U69593 and GR8696, somatostatin) but was inhibited by PKC activator phorbol-12-myristate-13 acetate (PMA). When acidic solutions were used, we were able to isolate a Ba(++) and Ruthenium Red insensitive current that was inhibited by Zn(++). This Zn(++) sensitive component of the proton sensitive current was consistent with TASK-1. In current clamp studies, acidic pH produced sensitive changes in resting membrane potential but did not influence excitability (pH 7.2-6.8). In contrast, Zn(++) produced substantial changes in excitability at physiological pH. Alkaline solutions produced hyperpolarization followed by proportional burst discharges (pH 10.75-11.5) and increased excitability (at pH 7.4). In conclusion, multiple TASK currents were present in a DRG nociceptor and differentially contributed to distinct discharge mechanisms.

In vivo phosphorylation of the somatostatin 2A receptor in human tumors

Hormone-stimulated receptor internalization and desensitization occur widely in the G protein-coupled receptor (GPCR) family. A critical first step in both these processes is thought to be receptor phosphorylation, a reaction which has been extensively characterized in cell culture. However, little is known about GPCR phosphorylation in vivo. The somatostatin (SS) receptor subtype (sst)2A is widely distributed in human neuroendocrine tumors, and SS analogs are commonly used to target this receptor for both therapy and diagnosis. In cultured pituitary cells sst2A is rapidly phosphorylated and internalized after hormone binding. The aim of the present study was to go one crucial step further and characterize the phosphorylation state of this receptor in human neuroendocrine tumors using a newly developed gel-shift assay. The receptor from a somatostatinoma was completely phosphorylated. In contrast, only unphosphorylated sst2A was present in human tumors that were not exposed to autocrine stimulation. Both in vivo and in cultured cells, the phosphorylation state of the sst2A receptor was correlated with its subcellular localization: phosphorylated receptor was mostly intracellular, whereas unphosphorylated receptor was localized at the cell surface. These results are the first to demonstrate ligand-stimulated GPCR phosphorylation in human tissue in situ, providing a crucial step toward a better understanding of receptor regulation in vivo. Analysis of sst2A phosphorylation promises to provide a sensitive indicator of the effectiveness of SS analogs in diagnostic and therapeutic situations in tumor patients.

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.

Critical role of Src and SHP-2 in sst2 somatostatin receptor-mediated activation of SHP-1 and inhibition of cell proliferation

The G protein-coupled sst2 somatostatin receptor acts as a negative cell growth regulator. Sst2 transmits antimitogenic signaling by recruiting and activating the tyrosine phosphatase SHP-1. We now identified Src and SHP-2 as sst2-associated molecules and demonstrated their role in sst2 signaling. Surface plasmon resonance and mutation analyses revealed that SHP-2 directly associated with phosphorylated tyrosine 228 and 312, which are located in sst2 ITIMs (immunoreceptor tyrosine-based inhibitory motifs). This interaction was required for somatostatin-induced SHP-1 recruitment and activation and consequent inhibition of cell proliferation. Src interacted with sst2 and somatostatin promoted a transient Gbetagamma-dependent Src activation concomitant with sst2 tyrosine hyperphosphorylation and SHP-2 activation. These steps were abrogated with catalytically inactive Src. Both catalytically inactive Src and SHP-2 mutants abolished somatostatin-induced SHP-1 activation and cell growth inhibition. Sst2-Src-SHP-2 complex formation was dynamic. Somatostatin further induced sst2 tyrosine dephosphorylation and complex dissociation accompanied by Src and SHP-2 inhibition. These steps were defective in cells expressing a catalytically inactive Src mutant. All these data suggest that Src acts upstream of SHP-2 in sst2 signaling and provide evidence for a functional role for Src and SHP-2 downstream of an inhibitory G protein-coupled receptor.

Pharmacological rationale for the use of somatostatin and analogues in portal hypertension

Somatostatin and its analogue octreotide have been used for two decades to treat oesophageal variceal haemorrhage. The drug was introduced because of its capacity to decrease portal venous pressure without major side effects. In clinical trials assessing the efficacy of somatostatin and long-acting analogues in arresting variceal haemorrhage, conflicting results have been obtained. Furthermore, in haemodynamic studies evaluating the effects of somatostatin and analogues in patients with cirrhosis, divergent effects were observed. The main reason for these differences is probably related to different affinities of the drugs for different somatostatin receptor subtypes. The effects of somatostatin and analogues are mediated via five different G-protein coupled receptors (somatostatin receptor subtypes 1-5), which regulate the activity of ion channels (Ca2+, K+, Na+ and Cl-) and enzymes (adenyl cyclase, phospholipase C, phospholipase A2, phosphoinositide 3-kinase and guanylate cyclase) responsible for the synthesis or degradation of intracellular second messengers including cyclic AMP, inositol 1,4,5-trisphosphate, diacylglycerol and cyclic GMP. Despite universal use of somatostatin, the cellular and biochemical mechanisms of its effects in portal hypertension are relatively poorly studied and remain incompletely understood. In this review, we summarize relevant signal transduction of somatostatin and analogues, the haemodynamic effects of the drugs and the possible mechanisms by which these effects are mediated.

Effects of subchronic and chronic melatonin treatment on somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex

Melatonin and somatostatin are known to exert similar effects on locomotor activity. We have previously demonstrated that acute melatonin treatment regulates somatostatin receptor function in the rat frontoparietal cortex. However, the effects of subchronic and chronic melatonin treatment on the somatostatin receptor-G protein-adenylyl cyclase system in the rat frontoparietal cortex are unknown. Melatonin was administered subcutaneously at a daily dose of 25 microg/kg for 4 days, 1 wk or 2 wk. Twenty-four hours after the last injection, the animals were sacrificed. Melatonin did not alter the somatostatin-like immunoreactivity content in the frontoparietal cortex from control and melatonin-treated rats during any of the previously indicated periods. Four days of melatonin administration induced both an increase in the number of [(125)I]-Tyr11-somatostatin receptors and a decrease in the affinity of somatostatin for its receptors in frontoparietal cortical membranes. The increased number of somatostatin receptors in the melatonin-treated rats was associated with an increased capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity. Melatonin administration for 4 days induced a higher adenylyl cyclase activity both under basal conditions and after direct stimulation of the enzyme with forskolin. No significant differences were observed in the function of Gi proteins in the 4-day melatonin-treated rats. Western blot analyses showed that the 4-day melatonin treatment reduced Gialpha(2) levels, without altering the amount of Gialpha(1). These melatonin-induced changes reverted to control values after 7 or 14 days of treatment. Altogether, the present findings suggest that subchronic melatonin treatment modulates the somatostatin receptor/effector system in the rat frontoparietal cortex.

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.

SOM230: a new somatostatin peptidomimetic with potent inhibitory effects on the growth hormone/insulin-like growth factor-I axis in rats, primates, and dogs

The goal of this project was to find a somatostatin (SRIF) analog with superior therapeutic potential. Receptor binding studies of new SRIF analogs were used to reveal SRIF substructures that interact with individual human SRIF receptor subtypes (sst1-sst5). Incorporation of these substructures into a stable cyclohexapeptide template led to SOM230, which binds with nanomolar affinity to sst1, sst2, sst3, and sst5. In rats, the inhibitory effect of SOM230 on GH was similar to SMS 201-995 (octreotide) at 1 h, but was 4-fold more potent at 6 h post injection, indicating increased metabolic stability. Treatment of rats with SOM230, at 1 and 10 micro g/kg.h, decreased IGF-I plasma levels, on d 2, by 68% and 90% (P < 0.01); whereas, under SMS 201-995 treatment, plasma IGF-I levels decreased by 28% and 49%, respectively. After a 2-wk infusion of rats, the suppression of IGF-I levels by SOM230 was still pronounced, whereas the response to SMS 201-995 was largely lost. This enhanced effect of SOM230 on IGF-I plasma levels was confirmed in an 8-wk study where both analogs were infused at 50 micro g/kg/h in rats. In rhesus monkey, SOM230 and SMS 201-995 treatment resulted in GH inhibition, with half-maximal inhibitory dose values of 0.5 and 0.4 micro g/kg, respectively, but plasma IGF-I levels were only lowered by SOM230 (-53%). In cynomolgus monkeys, a 2-wk infusion of SOM230, but to a much lesser extent of SMS 201-995, lowered plasma GH levels significantly (from 16.3 to 1.8 ng/ml, P = 0.007). Both in cynomolgus monkeys and beagle dogs, infusion of SOM230, but not SMS 201-995, lowered IGF-I levels significantly. In conclusion, SOM230 has a unique structure, binds almost universally to human ssts, and inhibits potently the GH/IGF-I axis cross-species. SOM230 is a candidate drug for clinical use.

Localization and mRNA expression of somatostatin receptor subtypes in human prostatic tissue and prostate cancer cell lines

Somatostatin (SST) plays an important regulatory role in the physiological control of various organs including the prostate. Somatostatin receptors (SSTRs) and SST analogs are potential targets for prostate cancer treatment, especially since it has been shown that SST analogues are clinically effective in the treatment of advanced prostate cancer. The presence of SST containing neuroendocrine (NE) cells in the epithelium of the human prostate and their suggested role in the paracrine regulation of this gland prompted us to study the potential expression of somatostatin receptors (SSTRs) in human prostatic tissue and prostate cancer cell lines. Using the reverse transcriptase polymerase chain reaction (RT-PCR), we found the SSTR subtypes 1-3 in stromal cells and in prostate cancer cell lines LNCaP, PC-3 and DU 145. Immunohistochemical analysis of 27 radical prostatectomy specimens demonstrated the presence of hSSTR1 in a subpopulation of cancerous and NE cells, whereas hSSTR2 was found in the stroma, peritumoral blood vessels and tumor cells. Receptor subtype 3 was demonstrated to be present on the cell membrane of BPH and malignant areas. A strong immunoreaction (IR) of hSSTR4 was found in tumor cells, as compared with a less intense IR in adjacent BPH areas. Somatostatin receptor subtype 5 was not detectable. Western blot analysis revealed immunoreactive bands of molecular weight between 44-60 kDa. In summary, the present study clearly demonstrates the presence of hSSTR1-3 in tumoral and nontumoral epithelial cells as well as in the stromal compartment, whereas hSSTR4 was found to be confined to epithelial cells, and SSTR5 was not detectable.

Presence of a plasma membrane targeting sequence in the amino-terminal region of the rat somatostatin receptor 3

Although peptide hormone receptors commonly exert their actions at the plasma membrane the cellular mechanisms that route the receptor proteins to the cell surface during biosynthesis are not well characterized. Here we report on the identification of a plasma membrane targeting sequence of rat somatostatin receptor subtype 3. While type 3 somatostatin receptors are present almost exclusively at the cell surface, type 1 receptors localize in addition largely in intracellular vesicular compartments. Chimeric receptors were constructed between rat somatostatin receptors 3 and 1. They were tagged by recombinant DNA techniques with a herpes simplex virus glycoprotein D epitope at the carboxyl-termini to facilitate their detection using fluorescence microscopic methods. Following transfection of the constructs in human embryonic kidney and rat insulinoma cells the chimeric receptors were analyzed by indirect immunofluorescence using anti-epitope monoclonal antibody and confocal laser scanning microscopy. The results demonstrate that the amino-terminal domain of somatostatin receptor 3 suffices to guide chimeric receptors to the cell surface. In marked contrast, chimeric receptors that lack this sequence but contain instead the amino-terminus of somatostatin type 1 receptor localize in an intracellular vesicular compartment.

Somatostatins and their role in thyroid cancer

Somatostatins are neuropeptides that exert a downregulatory effect on various physiological processes. Somatostatin analogues are used in the imaging and management of various tumour types. Their role in thyroid cancer has not as yet been fully elucidated. A systematic review of the literature using the keywords thyroid, cancer and somatostatin revealed 263 references. This paper summarizes the current knowledge of the role of somatostatins in thyroid cancer and assesses their future potential.

Identification and characterization of a type five-like somatostatin receptor in goldfish pituitary

In this study, a somatostatin receptor (Sst) cDNA was cloned and sequenced from goldfish pituitary. The cDNA encodes a 390-amino acid type five-like Sst (designated as gfSst(5)). The amino acid sequence of the receptor has slightly higher homology to mammalian Sst(5'), compared with other mammalian Sst subtypes and recently identified fish Sst(1), Sst(2) and Sst(3). In CCL39-SRE-Luci cells stably expressing the cloned receptor, agonist radioligand [125I]LTT-SRIF(28'), a mammalian SRIF(28) analog, bound to a homogenous population of receptors with high affinity (nM K(d)). Competition binding studies showed that all three natural goldfish SRIF ligands, SRIF(14), [Pro(2)]SRIF(14), and goldfish SRIF(28) (gfSRIF(28)), and LTT-SRIF(28) bind the cloned gfSst(5) with high affinity and significantly stimulate [35S]GTPgammaS binding, with SRIF(28) peptides showing higher affinity in receptor binding and potency in [35S]GTPgammaS binding compared with SRIF(14) peptides. The receptor gene is highly and predominately expressed in pituitary; lower levels of the receptor mRNA were also detected in different brain regions by reverse transcriptor-polymerase chain reaction (RT-PCR) followed by Southern blot analysis.