Rapid identification of subtype-selective agonists of the somatostatin receptor through combinatorial chemistry

Nonpeptide agonists of each of the five somatostatin receptors were identified in combinatorial libraries constructed on the basis of molecular modeling of known peptide agonists. In vitro experiments using these selective compounds demonstrated the role of the somatostatin subtype-2 receptor in inhibition of glucagon release from mouse pancreatic alpha cells and the somatostatin subtype-5 receptor as a mediator of insulin secretion from pancreatic beta cells. Both receptors regulated growth hormone release from the rat anterior pituitary gland. The availability of high-affinity, subtype-selective agonists for each of the somatostatin receptors provides a direct approach to defining their physiological functions.

Differential agonist activity of somatostatin and L-362855 at human recombinant sst4 receptors

1. The operational characteristics of somatostatin (SRIF) sst4 receptors are poorly understood. In this study, we have characterized human recombinant sst4 receptors expressed in CHO cells (CHOsst4) by radioligand binding and microphysiometry. 2. Increasing concentrations SRIF or other SRIF receptor ligands inhibited specific [125I]-Tyr11-SRIF binding in CHOsst4 cell membranes with respective pIC50 values of SRIF (8.82), L-362855 (7.40), BIM-23027 (<5.5) and MK-678 (<5.5). 3. These ligands displayed agonist activity, producing concentration-dependent increases in rates of extracellular acidification (EAR) with pEC50 values of SRIF (9.6) and L-362855 (8.0), respectively. BIM-23027 and MK-678 were at least 1000 times weaker than SRIF. The SRIF maximum was about 40% of that observed with L-362855. 4. In the presence of SRIF (0.1-1 nM), concentration-effect curves to L-362855 were displaced to the right with a progressive reduction in the L-362855 maximum. 5. When cells were only exposed to a single maximally effective concentration of SRIF or L-362855, there was no difference in the magnitude of the agonist-induced increase in EAR. However, a second agonist challenge, 30 min later showed that responses to SRIF but not L-362855 were markedly desensitized. 6. When concentration-effect curves to SRIF and L-362855 were obtained by combining data from cells exposed to only a single agonist concentration, SRIF (pEC50 9.2) was approximately 20 times more potent than L-362855 (pEC50 8.0) but the maxima were the same. Responses to both SRIF and L-362855 were abolished by pertussis toxin. 7. SRIF and L-362855-induced increases in EAR were inhibited by N-ethyl isopropyl amiloride (10 microM) but were not modified by inhibitors of PKC (Go-6976), MAP kinase (PD-98059), tyrosine kinase (genistein) or tyrosine phosphatase (sodium orthovanadate). 8. The results suggest that SRIF-induced increases in EAR in CHOsst4 cells involved activation of the Na+/H+ antiporter and were mediated via Gi/Go G proteins. Responses to SRIF, but not L-362855, were subject to marked desensitization which may be a consequence of differential activation of receptor-effector coupling pathways.

Somatostatin displayed on filamentous phage as a receptor-specific agonist

1. In search of methods to identify bio-active ligands specific for G protein-coupled receptors with seven transmembrane spanning regions, we have developed a filamentous phage-based selection and functional screening method. 2. First, methods for panning peptide phage on cells were established, using the hormone somatostatin as a model. Somatostatin was displayed on the surface of filamentous phage by cloning into phage(mid) vectors and fusion to either pIII or pVIII viral coat proteins. Peptide displaying phage bound to a polyclonal anti-somatostatin serum, and, more importantly, to several somatostatin receptor subtypes (Sst) expressed on transfected CHO-K1 cells, in a pattern which was dependent on the used display method. Binding was competed with somatostatin, with an IC50 in the nanomolar range. The phage were specifically enriched by panning on cells, establishing conditions for cell selections of phage libraries. 3. Binding of somatostatin displaying phage to sst2 on a reporter cell line, in which binding of natural ligand reduces secretion of alkaline phosphatase (via a cyclic AMP responsive element sensitive promoter), proved that the phage particles act as receptor-specific agonists. Less than 100 phage particles per cell were required for this activity, which is approximately 1000 fold less than soluble somatostatin, suggesting that phage binding interferes with normal receptor desensitization and/or recycling. 4. The combination of biopanning of phage libraries on cells with functional screening of phage particles for receptor triggering activity, may be used to select novel, bio-active ligands from phage libraries of random peptides, antibody fragments, or libraries based on the natural receptor ligand.

Synthesis and biological activities of potent peptidomimetics selective for somatostatin receptor subtype 2

A series of nonpeptide somatostatin agonists which bind selectively and with high affinity to somatostatin receptor subtype 2 (sst2) have been synthesized. One of these compounds, L-054,522, binds to human sst2 with an apparent dissociation constant of 0.01 nM and at least 3,000-fold selectivity when evaluated against the other somatostatin receptors. L-054,522 is a full agonist based on its inhibition of forskolin-stimulated adenylate cyclase activity in Chinese hamster ovary-K1 cells stably expressing sst2. L-054,522 has a potent inhibitory effect on growth hormone release from rat primary pituitary cells and glucagon release from isolated mouse pancreatic islets. Intravenous infusion of L-054,522 to rats at 50 microgram/kg per hr causes a rapid and sustained reduction in growth hormone to basal levels. The high potency and selectivity of L-054, 522 for sst2 will make it a useful tool to further characterize the physiological functions of this receptor subtype.

Insulin secretion is inhibited by subtype five somatostatin receptor in the mouse

BACKGROUND

Recently five somatostatin receptor subtypes (SSTRs) were cloned, allowing the development of highly specific agonists to these SSTRs. Previous studies have shown a species specificity phenomenon with respect to the inhibition of insulin secretion by these selective agonists. This study was undertaken to determine which SSTR (2 or 5) is responsible for the inhibitory effect of somatostatin on glucose-stimulated mouse insulin secretion.

METHODS

Intact mouse islets (n = 10) were stimulated with D-glucose in the presence or absence of receptor-specific somatostatin agonists.

RESULTS

D-glucose (16.7 mmol/L) augmented insulin secretion by 158% above that seen with 3.9 mmol/L D-glucose. In the presence of DC 32-92 (SSTR5) selective agonist, D-glucose (16.7 mmol/L) augmented insulin secretion by 64% above that seen with 3.9 mmol/L D-glucose. The presence of SSTR 5 selective agonist resulted in a significant (P < .05) inhibition of glucose-stimulated insulin secretion. The identification of SSTR5 within the mouse pancreas was established by reverse transcriptase polymerase chain reaction and confirmed by Southern blot analysis.

CONCLUSIONS

These results suggest that the inhibitory effect of somatostatin on insulin secretion is mediated through the subtype 5 receptor within the mouse islet.

High basal gastric acid secretion in somatostatin receptor subtype 2 knockout mice

BACKGROUND & AIMS

Somatostatin receptor subtype 2 (sst2) agonists inhibit gastric secretion. The role of sst2 in the regulation of acid secretion was assessed using sst2 knockout mice and urethane to induce somatostatin release.

METHODS

Acid secretion was monitored every 10 minutes by gastric perfusion and backtitration of perfusates in fasted, urethane-anesthetized C57/129 sst2 (-/-) mice and wild-type (+/+) mice. The ileal vein was cannulated for drug injection. Intragastric pH and serum gastrin were monitored 1 hour after anesthesia without perfusion.

RESULTS

Gastric pH values were lower in sst2 (-/-) mice (3.8 +/- 0.3) than in wild-type mice (7.1 +/- 0.1, P < 0.05), and there was no difference in gastrin levels. Basal acid output per 2 hours was 10-fold higher in sst2 knockout mice compared with wild-type mice. The gastrin antibody abolished the high basal acid secretion in sst2 (-/-) mice and had no effect in wild-type mice. The somatostatin antibody increased basal secretion by 4-fold in wild-type and had no effect in knockout mice. Somatostatin 14 or the sst2 agonist DC 32-87 inhibited pentagastrin-stimulated acid secretion in wild-type mice, but did not alter basal secretion in knockout mice.

CONCLUSIONS

These results indicate that sst2 is the main subtype whereby endogenous somatostatin suppresses gastric acid secretion through inhibition of gastrin action.

Modulation of receptor and receptor subtype affinities using diastereomeric and enantiomeric monosaccharide scaffolds as a means to structural and biological diversity. A new route to ether synthesis

We show that carbohydrates constitute an attractive source of readily available, stereochemically defined scaffolds for the facile attachment of side chains contained in genetically encoded and other amino acids. beta-D- and beta-L-glucose, L-mannose, and the 6-deoxy-6-N-analogue of beta-D-glucose have been employed to synthesize peptidomimetics that bind the SRIF receptors on AtT-20 mouse pituitary cells, five cloned human receptor subtypes (hSSTRs), and the NK-1 receptor. The affinity profile of various sugar-based ligands at the hSSTRs is compared with that of SRIF. Compound 19 bound hSSTR4 with a Ki of 100 nM. Subtle structural changes affect affinities. Evidence is presented that suggests that one compound (8) binds both the AtT-20 cell receptors and the five hSSTRs via a unique mode. The SARs of the glycosides at SRIF receptors differ markedly from those at the NK-1 receptor. For example a 4-benzyl substituent is important for SRIF receptor binding, but the 4-desbenzyl analogue 27 was highly potent (IC50 of 27 nM) at the NK-1 receptor. A new, nonbasic method for the synthesis of base-sensitive ethers from primary and secondary alcohols is also described.

Coupling of rat somatostatin receptor subtypes to a G-protein gated inwardly rectifying potassium channel (GIRK1)

The five different rat somatostatin receptor subtypes (SSTR1-SSTR5) were coexpressed with a subunit of G-protein gated inwardly rectifying potassium channel (GIRK1) in Xenopus oocytes. SSTR2-SSTR5, but not SSTR1 coupled efficiently to the activation of GIRK currents when stimulated by SST14 or SST28. A comparison of the dose-response curves and of the maximum currents obtained indicates that SSTR2 couples most efficiently to this effector, supporting the notion that SSTR2 is involved in activation of potassium conductances by SST in vivo.

Ligand binding pocket of the human somatostatin receptor 5: mutational analysis of the extracellular domains

The ligand binding domain of G protein-coupled receptors for peptide ligands consists of a pocket formed by extracellular and transmembrane domain (TM) residues. In the case of somatostatin (SRIF), however, previous studies have suggested that the binding cavity of the octapeptide analog SMS201-995 (SMS) is lined by residues in TMs III-VII. The additional involvement of the extracellular domains for binding SMS or the natural SRIF ligands (SRIF-14, SRIF-28) has not been clarified. Using a cassette construct cDNA for the human somatostatin 5 receptor (sst5R), we systematically examined the role of exofacial structures in ligand binding by creating a series of mutants in which the extracellular portions have been altered by conservative segment exchange (CSE) mutagenesis for the extracellular loops (ECLs) and by deletion (for the NH2-terminal segment) or truncation analysis (ECL3). CHO-K1 cells were stably transfected with wild type or mutant human sst5R constructs, and agonist binding was assessed using membrane binding assays with 125I-LTT SRIF-28 ligand. Deletion of the NH2 terminus or CSE mutagenesis of ECL1 and ECL3 produced minor 2-8-fold decreases in affinity for SRIF-14, SRIF-28, and SMS ligands. Truncation of ECL3 to mimic the size of this loop in sst1R and sst4R (the two subtypes that do not bind SMS) did not interfere with the binding of SMS, SRIF-14, or SRIF-28. In contrast, both ECL2 mutants failed to bind 125I-LTT SRIF-28. Immunocytochemical analysis of nonpermeabilized cells with a human sst5R antibody revealed that the mutant receptors were targeted to the plasma membrane. Labeled SMS (125I-Tyr3 SMS) also failed to bind to the mutant ECL2 receptors. These results suggest a potential contribution of ECL2 (in addition to the previously identified residues in TMs III-VII) to the SRIF ligand binding pocket.

Distinct agonist-mediated endocytosis of cloned rat somatostatin receptor subtypes expressed in insulinoma cells

Endocytosis of somatostatin receptors could regulate cellular responses to the two natural peptides, somatostatin-14 and somatostatin-28, and to synthetic ligands used in the clinical diagnosis and symptomatic therapy of neuroendocrine tumours. The five cloned SSTRs with or without epitope tags at their carboxyl-termini were expressed in rat insulinoma 1046-38 cells. Application of the two natural peptides or octreotide, at 37 degrees C but not at 4 degrees C, to cells transfected with somatostatin receptor subtype 2 or 3 cDNA resulted in a significant decrease of cell surface binding-sites for 125I-Tyr11-somatostatin-14. In contrast, cells transfected with subtype 5 cDNA only responded to stimulation with octreotide or somatostatin-28. Cells transfected with subtype 1 cDNA responded to somatostatin-14 and 28, while cells expressing subtype 4 cDNA showed no response. Confocal microscopy revealed that 6 min after stimulation with somatostatin-14 at 37 degrees C, tagged somatostatin receptor subtypes 1, 2 and 3 were internalized into vesicles. Internalization was not observed at 4 degrees C in the presence of 0.4 M sucrose and 80 microM phenylarsine oxide and hence proceeded via endocytosis through clathrin-coated pits and vesicles. After 20 min the internalized receptors appeared in perinuclear vesicles and after 120 min they reappeared at the plasma membrane. This recycling was not sensitive to cycloheximide and, hence, not dependent on de novo protein synthesis. Recovery of cell surface receptors was, however, inhibited by brefeldin A, monensin and bafilomycin A1, indicating that receptor recycling proceeded through vesicular traffic of acidified compartments. The data are consistent with the assumption that the observed agonist and subtype specific internalization of somatostatin receptors in a neuroendocrine cell line may be important for tumour diagnosis and therapy and, thus, suggest a manifold control in cellular signalling.

Characterization of the antiproliferative signal mediated by the somatostatin receptor subtype sst5

We investigated cell proliferation modulated by cholecystokinin (CCK) and somatostatin analogue RC-160 in CHO cells bearing endogenous CCKA receptors and stably transfected by human subtype sst5 somatostatin receptor. CCK stimulated cell proliferation of CHO cells. This effect was suppressed by inhibitor of the soluble guanylate cyclase, LY 83583, the inhibitor of the cGMP dependent kinases, KT 5823, and the inhibitor of mitogen-activated protein (MAP) kinase kinase, PD 98059. CCK treatment induced an increase of intracellular cGMP concentrations, but concomitant addition of LY 83583 virtually suppressed this increase. CCK also activated both phosphorylation and activity of p42-MAP kinase; these effects were inhibited by KT 5823. All the effects of CCK depended on a pertussis toxin-dependent G protein. Somatostatin analogue RC-160 inhibited CCK-induced stimulation of cell proliferation but it did not potentiate the suppressive effect of the inhibitors LY 83583 and KT 5823. RC-160 inhibited both CCK-induced intracellular cGMP formation as well as activation of p42-MAP kinase phosphorylation and activity. This inhibitory effect was observed at doses of RC-160 similar to those necessary to occupy the sst5 recombinant receptor and to inhibit CCK-induced cell proliferation. We conclude that, in CHO cells, the proliferation and the MAP kinase signaling cascade depend on a cGMP-dependent pathway. These effects are positively regulated by CCK and negatively influenced by RC-160, interacting through CCKA and sst5 receptors, respectively. These studies provide a characterization of the antiproliferative signal mediated by sst5 receptor.

Activation of human somatostatin receptor type 2 causes inhibition of cell growth in transfected HEK293 but not in transfected CHO cells

Somatostatin (SS) is known to have an antiproliferative effect on cell growth via somatostatin receptors (SSTR). The purpose of this study was to transfect cell lines with human SSTR2 and determine the subsequent effect on cell growth in response to SSTR agonist. Heterologous Chinese hamster ovary (CHO-K1) and human embryonic kidney 293 (HEK) cells were transfected with SSTR2 cDNA using lipofectin. Stable transformants were selected by G418 and confirmed by 125I-SS binding and RT-PCR. Binding studies were performed in the presence of 10(-6) to 10(-12) M SS-14, SS-28, SS analogue RC-160, SSTR2 agonist NC-9-74, and SSTR5 agonist DC-37-39. Cell growth was determined by counting cell numbers after 48 hr incubation in the presence of 10(-6) to 10(-12) M SSTR2 agonist NC-9-74. Binding of 125I-SS-14 to transfected CHO and transfected HEK293 cells showed that the cells had high affinity for SS-14, SS-28, NC-9-74, and RC-160 but low affinity for DC-37-39. Incubation with 10(-6) to 10(-12) M NC-9-74, showed that 1 nM to 1 microM NC-9-74 significantly inhibited transfected HEK293 cell growth but did not affect growth on transfected CHO cells (n = 4 for each dose, P < 0.01). The two cell lines transfected with the human SSTR2 showed similar high affinity for SS-14, SS-28, RC-160, and SSTR2 agonist but not SSTR5 agonist. The SSTR2 agonist NC-9-74 significantly inhibited transfected HEK293 cell growth but not CHO cells. These data suggest that activation of SSTR2 was more efficiently coupled to the signal transduction pathway of antiproliferation in the transfected HEK293 cells.

Somatostatin5 receptor-mediated [35S]guanosine-5'-O-(3-thio)triphosphate binding: agonist potencies and the influence of sodium chloride on intrinsic activity

We studied the activation of the human somatostatin5 receptor recombinantly expressed in CHO-K1 cells by using some newly available agonists and antagonists. Somatostatin-28 bound to this receptor with a higher affinity than somatostatin-14 and was more potent in increasing [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding. Somatostatin-14-induced [35S]GTPgammaS binding to membranes from this cell line was decreased in a concentration-related manner by increasing concentrations of GDP and sodium chloride. At 50 mM (low) sodium, agonist EC50 values for stimulating [35S]GTPgammaS binding were lower than those at 150 mM (high) sodium and were closer to their respective affinity estimates (dissociation equilibrium constants) for binding to the receptor in the absence of sodium. Both agonist binding to the high affinity state of the receptor and agonist-induced [35S]GTPgammaS binding were abolished by pertussis toxin pretreatment. The putative somatostatin5 receptor-selective ligand L-362,855, unlike somatostatin-14 and somatostatin-28, showed differential intrinsic activity for stimulation of [35S]GTPgammaS binding, behaving as a partial agonist in high sodium and a full agonist in low sodium. In contrast, BIM-23056 did not behave as an agonist under any conditions studied but was able to antagonize somatostatin-14-induced [35S]GTPgammaS binding. We conclude that measurement of [35S]GTPgammaS binding mediated by somatostatin receptor activation in the presence of different concentrations of sodium chloride provides a useful functional assay for assessing the relative agonist efficacies of novel ligands identified from radioligand binding studies.

Characterization of human recombinant somatostatin sst5 receptors mediating activation of phosphoinositide metabolism

1. We have functionally characterized the human recombinant somatostatin (SRIF) sst5 receptor in Chinese hamster ovary-K1 (CHOsst5) cells by measuring total [3H]-inositol phosphate ([3H]-InsPx) accumulation, in the presence of 10 mM LiCl, in cells labelled with [3H]-myo-inositol. 2. In CHOsst5 cells, SRIF, SRIF-28 and the cyclic hexapeptide, L-362,855, produced time- and concentration-related increases in [3H]-InsPx accumulation, with similar potency (pEC50 values of 6.5, 6.8 and 7.2, respectively). L-362,855 behaved as a partial agonist, producing approximately 30% of the SRIF maximum response. The other peptide analogues of SRIF, BIM-23027 and BIM-23056, were inactive as agonists. 3. Increasing concentrations of L-362,855 increased [3H]-InsPx accumulation and simultaneously produced rightward shifts of SRIF concentration-effect curves, with an estimated pKp value of 7.6, confirming that it was acting as a partial agonist. 4. BIM-23056, but not BIM-23027, potently antagonized SRIF-induced [3H]-InsPx accumulation, with an estimated pKB value of 7.4. BIM-23056 did not antagonize [3H]-InsPx accumulation induced by uridine 5'-triphosphate (UTP). 5. SRIF- but not UTP-induced [3H]-InsPx accumulation was inhibited by increasing concentrations of pertussis toxin (0.01-100 ng ml-1), indicating the involvement of pertussis toxin-sensitive G-proteins. 6. These findings show that the human recombinant sst5 receptor, when stably expressed in CHO-K1 cells, is able to mediate activation of phosphoinositide metabolism in a pertussis toxin-sensitive manner. In this system L-362,855 behaved as a partial agonist while BIM-23056 was a specific antagonist. These agents should provide useful tools for functionally characterizing endogenous SRIF receptors.

Intraislet somatostatin inhibits insulin (via a subtype-2 somatostatin receptor) but not islet amyloid polypeptide secretion in the isolated perfused human pancreas

It is our hypothesis that intraislet somatostatin regulates beta cell secretion in the isolated perfused human pancreas. The present study was designed to determine the relative influence of intraislet somatostatin on the regulation of islet amyloid polypeptide (IAPP) and insulin secretion, and to determine the effect of specific somatostatin receptor (SSTR) agonists on beta cell secretion during immunoneutralization of endogenous somatostatin in the isolated perfused human pancreas. Single-pass perfusion was performed in pancreata obtained from seven cadaveric organ donors using a modified Krebs medium with 3.9 mmol/L glucose. Sequential test periods were separated by basal periods and experiments were performed by infusion of any of the following: (1) somatostatin monoclonal antibody (S-Ab); (2) S-Ab + SSTR2 agonist (DC32-87); or (3) S-Ab + SSTR5 agonist (DC32-92). The changes in insulin and IAPP secretion from basal levels during each stimulation were calculated. Infusion of S-Ab resulted in a significant increase in insulin secretion (2033 +/- 429 pmol/L; P <0.05) but not IAPP. In the presence of S-Ab, infusion of the SSTR2 agonist resulted in a significant inhibition of insulin secretion (-1128 +/- 457 pmol/L; P <0.05) but not IAPP. In the presence of S-Ab, infusion of the SSTR5 agonist had no significant effect on insulin or IAPP secretion. We conclude that intraislet somatostatin inhibits insulin secretion via SSTR2, but not IAPP secretion, in the isolated perfused human pancreas model and that this effect occurs via SSTR2. These results also suggest that insulin and IAPP secretion are regulated by different mechanisms despite being co-localized to the beta cell.

Neuropeptide Y Y2 receptor and somatostatin sst2 receptor coupling to mobilization of intracellular calcium in SH-SY5Y human neuroblastoma cells

1. In this study we have investigated neuropeptide Y (NPY) and somatostatin (SRIF) receptor-mediated elevation of intracellular Ca2+ concentration ([Ca2+]i) in the human neuroblastoma cell line SH-SY5Y. 2. The Ca(2+)-sensitive dye fura 2 was used to measure [Ca2+]i in confluent monolayers of SH-SY5Y cells. Neither NPY (30-100 nM) nor SRIF (100 nM) elevated [Ca2+]i when applied alone. However, when either NPY (300 pM-1 microM) or SRIF (300 pM-1 microM) was applied in the presence of the cholinoceptor agonist carbachol (1 microM or 100 microM) they evoked an elevation of [Ca2+]i above that caused by carbachol alone. 3. The elevation of [Ca2+]i by NPY was independent of the concentration of carbachol. In the presence of 1 microM or 100 microM carbachol NPY elevated [Ca2+]i with a pEC50 of 7.80 and 7.86 respectively. 4. In the presence of 1 microM carbachol the NPY Y2 selective agonist peptide YY(3-36) (PYY(3-36)) elevated [Ca2+]i with a pEC50 of 7.94, the NPY Y1 selective agonist [Leu31, Pro34]-NPY also elevated [Ca2+]i when applied in the presence of carbachol, but only at concentrations > 300 nM. The rank order of potency, PYY(3-36) > or = NPY > > [Leu31, Pro34]-NPY indicates that an NPY Y2-like receptor is involved in the elevation of [Ca2+]i. 5. In the presence of 1 microM carbachol, SRIF elevated [Ca2+]i with a pEC50 of 8.24. The sst2 receptor-preferring analogue BIM-23027 (c[N-Me-Ala-Tyr-D-Trp-Lys-Abu-Phe]) elevated [Ca2+]i with a pEC50 of 8.63, and the sst5-receptor preferring analogue L-362855 (c[Aha-Phe-Trp-D-Trp-Lys-Thr-Phe]) elevated [Ca2+]i with a pEC50 of approximately 6.1. Application of the sst3 receptor-preferring analogue BIM-23056 (D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH2, 1 microM) to SH-SY5Y cells in the presence of carbachol neither elevated [Ca2+]i nor affected the elevations of [Ca2+]i caused by a subsequent coapplication of SRIF. The rank order of potency, BIM-23026 > or = SRIF > > L-362855 > > > BIM-23026 suggests that an sst2-like receptor is involved in the elevation of [Ca2+]i. 6. Block of carbachol activation of muscarinic receptors with atropine (1 microM) abolished the elevation of [Ca2+]i by the SRIF and NPY. 7. Muscarinic receptor activation, not a rise in [Ca2+]i, was required to reveal the NPY or SRIF response. The Ca2+ channel activator maitotoxin (2 ng ml-1) also elevated [Ca2+]i but subsequent application of either NPY or SRIF in the presence of maitotoxin caused no further changes in [Ca2+]i. 8. The elevations of [Ca2+]i by NPY and SRIF were abolished by pretreatment of the cells with pertussis toxin (200 ng-ml-1, 16 h). This treatment did not significantly affect the response of the cells to carbachol. 9. NPY and SRIF appeared to elevate [Ca2+]i by mobilizing Ca2+ from intracellular stores. Both NPY and SRIF continued to elevate [Ca2+]i when applied in nominally Ca(2+)-free external buffer. Thapsigargin (100 nM), an agent which discharges intracellular Ca2+ stores, also blocked the NPY and SRIF elevations of [Ca2+]i. 10. Delta-Opioid receptor agonists applied in the presence of carbachol also elevate [Ca2+]i in SH-SY5Y cells. When NPY (30 nM) or SRIF (100 nM) was applied together with a maximally effective concentration of the delta-opioid receptor agonist DPDPE ([D-Pen2,5]-enkephalin) (1 microM), the resulting elevations of [Ca2+]i were not greater than those caused by application of DPDPE alone. 11. Thus, in SH-SY5Y cells, NPY and SRIF can mobilize Ca2+ from intracellular stores via activation of NPY Y2 and sst2-like receptors, respectively. Neither NPY nor SRIF elevated [Ca2+]i when applied alone. The requirements for the elevations of [Ca2+]i by NPY and SRIF are the same as those for delta- and mu-opioid receptor and nociceptin receptor mobilization of [Ca2+]i in SH-SY5Y cells.

Somatostatin receptors in Neuro2A neuroblastoma cells: ligand internalization

1. Receptor-dependent internalization of somatostatin (SRIF) agonists has been a matter of controversy probably because [125I]Tyr11-SRIF-14 is rapidly degraded. We have studied the internalization of a stable somatostatin analogue, [125I]-BIM-23027, in a neuronal cell line, Neuro2A, which natively expresses somatostatin sst2 receptors. 2. Incubation of Neuro2A cells with [125I]-BIM-23027 at 37 degrees C resulted in a time-dependent internalization of the ligand, which reached a maximum at 30 min. Acid-washing showed that cell-surface binding of the ligand accounted for only 34% of total binding at this time. Internalization was dramatically reduced at 15 degrees C. 3. Internalization of [125I]-BIM-23027 was prevented by inclusion of unlabelled somatostatin receptor agonists in a concentration-dependent manner. The IC50 values for inhibition of [125I]-BIM-23027 internalization were approximately 100 fold lower than for inhibition of [125I]-BIM-23027 binding to membrane homogenates but followed the same rank order of potencies. 4. Disruption of G-protein coupling by treatment with pertussis toxin caused a 60% reduction in internalization of ligand. A combination of antimycin (50 nM) and deoxyglucose (50 mM) pretreatment, which leads to a depletion of cellular ATP, decreased internalization of [125I]-BIM-23027 by 66% of control and increased the proportion of surface-bound ligand. Hypertonic sucrose, which prevents clathrin-mediated endocytosis, reversibly abolished the internalization of ligand without increasing the proportion bound at the cell surface. 5. After internalization of [125I]-BIM-23027, approximately half of the ligand was recycled back to the extracellular medium within 20 min at 37 degrees C. This finding suggests that the intracellular content of [125I]-BIM-23027 reaches a steady state which is determined by the rates of both internalization and recycling of the ligand. In contrast to studies in which the internalization of [125I]-Tyr11-SRIF-14 was examined, neither internalized nor recycled [125I]-BIM-23027 was degraded to its component amino acids. 6. These findings indicate that the somatostatin agonist, [125I]-BIM-23027, is internalized in a receptor-dependent manner which involves clathrin-coated pits in Neuro2A cells. Furthermore, much of the internalized ligand is rapidly recycled back to the extracellular medium without undergoing significant degradation.

Somatostatin receptors in Neuro2A neuroblastoma cells: operational characteristics

1. We have used somatostatin (SRIF) receptor subtype-selective ligands to determine some of the operational characteristics of somatostatin receptors in Neuro2A mouse neuroblastoma cells. The potent SRIF1-receptor selective ligand, BIM-23027, was able to displace completely the specific binding of radioiodinated somatostatin, [125I]-Tyr11-SRIF-14, with a pIC50 of 10.3, suggesting that Neuro2A cells contain predominantly receptors of the SRIF1 receptor group. The rank order of affinities for several somatostatin analogues tested in competition studies, together with the high affinity of BIM-23027, indicate that the majority of receptors in Neuro2A cells are of the sst2 subtype. 2. The stable radioligand, [125I]-BIM-23027, bound with high affinity (Kd = 13 pM, Bmax = 0.2 pmol mg-1 protein) to Neuro2A cell membranes, but its binding was only partially reversible at room temperature and below. Thus at 4 degrees C, only 36% of the bound ligand dissociated within 2 h. In contrast, 60% of the ligand dissociated at 15 degrees C and 89% of the ligand dissociated at 37 degrees C. 3. Equilibrium binding of [125I]-BIM-23027 was partially (25%) inhibited by 10 microM GTP, and by 120 mM NaCl (42% inhibition) but this inhibition was increased to 75% when sodium chloride and GTP were added together. This effect of GTP and sodium chloride was also seen in dissociation experiments. After incubation to equilibrium with [125I]-BIM-23027, dissociation was initiated with excess unlabelled ligand in the presence of GTP (10 microM) and sodium chloride (120 mM). Under these conditions 67% of the ligand dissociated at 4 degrees C, 81% at 15 degrees C and 93% at 37 degrees C. Binding was totally inhibited by pretreatment of cells with pertussis toxin. 4. Functionally, BIM-23027 inhibited forskolin-stimulated cyclic AMP accumulation in a concentration-dependent manner with an IC50 of 1.0 nM and a maximal inhibition of 37%. This effect was abolished by pretreatment of the cells with pertussis toxin. However, unlike in studies reported with the recombinant sst2 receptor, no rise in intracellular calcium concentration was observed with SRIF-14. 5. We conclude that Neuro2A cells provide a stable neuronal cell line for the study of functionally coupled endogenous somatostatin receptors of the sst2 type. In addition, we have found that activation of the receptor is associated with ligand-receptor internalisation.

Agonist-dependent regulation of cloned human somatostatin receptor types 1-5 (hSSTR1-5): subtype selective internalization or upregulation

Agonist regulation of somatostatin receptors (SSTRs) was investigated in stable CHO-K1 cells individually expressing the 5 human (h) SSTR subtypes. hSSTR 2,3,4, and 5 displayed rapid agonist-dependent internalization of [125I] LTT SST-28 ligand in a time- and temperature-dependent manner over 60 min. Maximum internalization of radioligand occurred with hSSTR3 (78%) followed by hSSTR5 (66%), hSSTR4 (29%) and hSSTR2 (20%). In contrast, hSSTR1 displayed virtually no internalization. Prolonged agonist treatment led to differential upregulation of some of the SSTRs. After 22 h, hSSTR1 was upregulated at the membrane by 110%, hSSTR2 and hSSTR4 by 26% and 22% respectively, whereas hSSTR3 and hSSTR5 showed little change. Agonist-induced recruitment of hSSTR1 to the membrane was confirmed by immunocytochemistry with hSSTR1 antibodies. These results show that SST regulates all 5 hSSTRs by differential subtype selective internalization or upregulation. Subtype selectivity for internalization and upregulation is inversely related.

Molecular biology of somatostatin receptor subtypes

Somatostatin (SRIF) receptors (ssts) comprise a family of heptahelical membrane proteins encoded by five related genes that map to separate chromosomes and which, with the exception of sst1, are intronless. The ssts1-4 display weak selectivity for SRIF-14 binding, whereas sst5 is SRIF-28-selective. Based on structural similarity and reactivity for octapeptide and hexapeptide sst analogs, ssts2,3 and sst5 belong to a similar sst subclass; ssts1-4 react poorly with these analogs and belong to a separate subclass. All five ssts are functionally coupled to inhibition of adenylyl cyclase via pertussis toxin-sensitive guanosine triphosphate (GTP)-binding proteins. mRNA for ssts1-5 is widely expressed in brain and peripheral organs and displays an overlapping but characteristic pattern that is subtype-selective and tissue- and species-specific. All pituitary cell subsets express sst2 and sst5, with sst5 being more abundant. Individual pituitary cells coexpress multiple sst subtypes. The binding pocket for SRIF-14 ligand lies deep within the membrane in transmembrane domains (TMDs) 3 to 7. Except for extracellular loop 2, it does not involve the other exofacial structures. Human (h)sst2A and hsst5 undergo agonist-mediated desensitization, associated with receptor internalization. The C-tail segment of hsst5 displays positive molecular internalization signals. The ssts inhibit the growth of tumor cells directly, through blockade of mitogenic signaling leading to growth arrest and through induction of apoptosis. This process is associated with translocation of phosphotyrosine phosphatase (PTP) 1C from the cytosol to the membrane.

Effector coupling of somatostatin receptor subtypes on human endocrine tumors

Effector coupling of somatostatin receptor subtypes sst1 and sst2 was examined in a reconstituted system. Forskolin-stimulated cyclic adenosine monophosphate (cAMP) formation was inhibited 66% by somatostatin (SRIF-14) in CHO cells expressing somatostatin receptor 1(sst1) (CHO-SR1), but not sst2, in a dose-dependent manner with an ED50 of 1 x 10(-9) mol/L SRIF-14. The inhibition was blocked by pertussis toxin (PTX), indicating that sst1 is coupled to adenylyl cyclase via PTX-sensitive Gi protein. In CHO cells, Gi alpha 2 and Gi alpha 3 mRNAs were detected. In adenylyl cyclase assays, 1 mumol/L SRIF-14 caused a 16% inhibition of forskolin-stimulated adenyly cyclase activity. Preincubation with Gi alpha 3, but not Gi alpha 1/Gi alpha 2, antiserum blocked this inhibition. By contrast, sst2 is coupled to adenylyl cyclase via Gi alpha 1. In cells expressing sst2 with Gi alpha 1(CHO-SR2G1), SRIF-14 significantly inhibited forskolin-stimulated cAMP formation by 53% and with an ED50 at 4 x 10(-9)mmol/L SRIF-14, which was completely blocked by PTX; ED50 values for sst1 and sst2 agree with the IC50 values in binding assays. In CHO-SR1, the rank of potency of agonists affecting adenyl cyclase was SRIF-14 = SRIF-28 > RC 160 > SMS 201-995. In CHO-SR2G1, the rank was RC-160 > SRIF-14 = SRIF-28 > SMS 201-995.

Somatostatin receptor subtypes SSTR2 and SSTR5 couple negatively to an L-type Ca2+ current in the pituitary cell line AtT-20

The somatostatin receptor subtypes SSTR2 and SSTR5 mediate distinct endocrine and exocrine functions of somatostatin and may also be involved in mediating the neuromodulatory actions of somatostatin in the brain. To investigate whether these receptors couple to voltage-sensitive Ca2+ channels, SSTR2 and SSTR5 selective agonists were tested for their effects on AtT-20 cells using whole cell patch clamp techniques. The SSTR2 selective agonist MK 678 inhibited Ca2+ currents in AtT-20 cells. The effects of MK 678 were reversible and blocked by pertussis toxin pretreatment, suggesting that SSTR2 couples to the L-type Ca2+ channels via G proteins. Other SSTR2-selective agonists, including BIM 23027 and NC8-12, were able to inhibit the Ca2+ currents in these cells. The SSTR5 selective agonist BIM 23052 also inhibited the Ca2+ currents in these cells and this effect was reversible and blocked by pertussis toxin treatment. The ability of SSTR5 to mediate inhibition of the Ca2+ current was greatly attenuated by pretreatment with the SSTR5-selective agonist BIM 23052, whereas SSTR2-mediated inhibition of the Ca2+ current was not altered by pretreatment with the SSTR2-selective agonist MK 678. Thus, the SSTR2 and SSTR5 couplings to the Ca2+ current are differentially regulated. The peptide L362,855, which we previously have shown to have high affinity for the cloned SSTR5, had minimal effects on Ca2+ currents in AtT-20 cells at concentrations up to 100 nM and did not alter the ability of MK 678 to inhibit Ca2+ currents. However, it completely antagonized the effects of the SSTR5-selective agonist BIM 23052 on the Ca2+ currents. L362,855 is an antagonist/partial agonist at SSTR5 since it can reduce Ca2+ currents in these cells at concentrations above 100 nM. L362,855 is also an antagonist/partial agonist at the cloned rat SSTR5 expressed in CHO cells since it is able to block the inhibition of cAMP accumulation induced by somatostatin at concentrations below 100 nM but at higher concentrations can inhibit cAMP formation itself. Structural analysis of L362,855 reveals that only a single hydroxyl group at residue seven in the peptide is needed to convert the compound from an antagonist/partial agonist to a full agonist at SSTR5. These studies reveal that two different somatostatin receptor subtypes, SSTR2 and SSTR5, can mediate the inhibition of an L-type Ca2+ channel in AtT-20 cells by somatostatin. The receptor subtype responses can be distinguished by selective agonists and antagonists and are regulated differently by agonist pretreatment. The inhibition of Ca2+ influx into endocrine cells and neurons may be a major cellular mechanism by which somatostatin modulates hormone and neurotransmitter release. Our results reveal that at least two receptor subtypes can mediate this cellular response.

Intracerebroventricular injection of somatostatin sst5 receptor agonist inhibits gastric acid secretion in rats

Somatostatin and its analogs act in the brain to influence gastric acid secretion. Five different somatostatin receptor subtypes have been characterized (sst1 to sst5). We studied the influence of somatostatin (0.18-0.6 nmol/rat) and selective sst2, sst3 and sst5 receptor ligands on basal gastric acid secretion in conscious rats equipped with chronic gastric and intracerebroventricular (i.c.v.) cannulae. Somatostatin-14 (0.36 nmol/rat), the sst2, sst3 and sst5 receptor agonist, Des-AA1,2,4,5,12,13-[D-Tryp8,D-Cys14]somatostatin (SMS 201-995) (0.18-0.36 nmol/rat) and the sst5 receptor agonist, BIM-23052, (0.8-1.2 nmol/rat) injected i.c.v. inhibited gastric acid secretion. Maximal inhibition reaching 42%, 60% and 42% was induced by somatostatin-14 (0.36 nmol/rat), SMS 201-995 (0.18 nmol/rat) and BIM-23052 (0.8 nmol/rat) respectively. The sst2 receptor agonist, DC 32-87 (0.2-0.8 nmol/rat) and sst3 receptor agonist, BIM-23056 (0.2-1.2 nmol/rat), did not modify gastric acid secretion, except the sst3 receptor agonist at 0.4 nmol/rat which increased acid output at 20 min post-injection. The sst2 receptor agonists (0.4 nmol/rat) co-injected i.c.v. with a subthreshold dose of sst5 (0.4 nmol/rat) inhibited gastric acid secretion. These results show that i.c.v. injection of somatostatin-14 inhibits basal gastric acid secretion in conscious rats through an action on sst5 receptor subtype which can be potentiated by sst2 receptor subtype.

A quantitative autoradiographical study on the distribution of somatostatin sst2 receptors in the rat central nervous system using [125I]-BIM-23027

The kinetic properties, steady state binding characteristics and autoradiographic distribution of the somatostatin (SRIF) sst2 receptor-selective ligand, [125I]-BIM-23027, have been investigated in the rat central nervous system. Analysis of kinetic, saturation and competition binding data in rat hippocampal membranes was consistent with [125I]-BIM-23207 binding to a single population of non-interacting binding sites. Competition studies, using different SRIF ligands suggested that [125I]-BIM-23027 was binding to sites similar to that of the recombinant sst2 receptor. The rank order of affinity for displacing specific binding was BIM-23027 = SRIF > L-362855 > > BIM-23056. There was a widespread distribution of [125I]-BIM-23027 binding sites in the rat central nervous system. The highest density of binding was observed in the dentate gyrus, medial habenular, amygdala, claustrum and lateral septum as well as in the piriform, cingulate and parietal cortex. The cervical and lumbar spinal cord also displayed moderate levels of binding localized to the substantia gelatinosa. The cellular localization of [125I]-BIM-23027 binding was found to be associated with dendritic terminal fields. In contrast, the cellular signal for sst2 receptor mRNA was restricted to cell somata. The widespread distribution of [125I]-BIM-23027 binding sites within the brain suggests that receptors similar to the recombinant sst2 receptor may mediate a variety of different physiological effects within the central nervous system.