Cloning, expression and pharmacological characterisation of the mouse somatostatin sst(5) receptor

The mouse somatostatin (somatotropin release inhibiting factor, SRIF) sst(5) receptor coding sequence was cloned from a mouse BALB/c genomic library. It shows 97% and 81% homology with the corresponding rat and human receptors, respectively. The msst(5) receptor messenger RNA (mRNA) is present at low levels in the adult mouse brain, with significant expression in a few nuclei only, e.g. in the septum (lateral septal nuclei) or the amygdala (medial amygdaloid nucleus); very few signals were observed in the mesencephalon, metencephalon, and myelencephalon (except the dorsal motor nucleus of the vagus nerve). The msst(5) receptor was stably expressed in the hamster fibroblast cell line CCL39-SRE-Luci, which harbours the luciferase reporter gene driven by the serum responsive element. [(125)I]LTT-SRIF-28 ([Leu(8), D-Trp(22), (125)I-Tyr(25)]-SRIF-28), [(125)I]Tyr(10)-CST, [(125)I]CGP 23996, and [(125)I]Tyr(3)-octreotide labelled msst(5) receptors with high affinity (pK(d) values: 11.0, 10.15, 9.75 and 9.43) and in a saturable manner, but defined different Bmax values: 697, 495, 540 and 144 fmoles/mg, respectively. [(125)I]LTT-SRIF-28-labelled sites displayed the following rank order: SRIF-28> rCST-14> somatuline > CGP-23996= SRIF-14= octreotide, whereas [(125)I]Tyr(3)-octreotide-labelled sites displayed a different profile: octreotide > SRIF-28> rCST-14= somatuline > SRIF-14> CGP-23996. The pharmacological profiles determined with [(125)I]LTT-SRIF-28, [(125)I]CGP 23996 and [(125)I]Tyr(10)-CST correlated highly significantly (r(2) =0.88-0.99), whereas [(125)I]Tyr(3)-octreotide binding was rather divergent (r(2) =0.77). Also, human and mouse sst(5) receptor profiles are very different, e. g. r(2) =0.385 for [(125)I]Tyr(10)-CST and r(2) =0.323 for [(125)I]LTT-SRIF-28-labelled sites. Somatostatin induces expression of luciferase reporter gene in CCL39-SRE-Luci cells. The profile was consistent with a msst(5) receptor-mediated effect although apparent potency in the luciferase assay was much reduced compared to radioligand binding data: Octreotide = SRIF-28> rCST-14= SRIF-14= CGP-23996. Octreotide, SRIF-28, BIM23052 and D Tyr Cyanamid 154806 behaved as full or nearly full agonists in comparison to SRIF-14, whereas the other compounds had relative efficacies of 40 to 70%. The present study shows that agonists radioligands define apparently different receptor populations in terms of number of sites and pharmacological profile in cells expressing a single recombinant receptor. These variations suggest that the conformation of the ligand receptor complex may vary depending on the agonist. Further, the msst(5) receptor, although primarily coupled to Gi/Go proteins, is able to stimulate luciferase gene expression driven by the serum responsive element. Finally, it is suggested that putative sst(2) selective agonists e.g. octreotide, RC160 or BIM23027 show similar or higher potency at msst(5) receptors than SRIF-14.

Identification and characterization of subtype selective somatostatin receptor agonists

High affinity, subtype selective non-peptide agonists of somatostatin receptor subtypes 1-5 were identified in combinatorial libraries constructed based on molecular modeling of known peptide agonists. Simultaneous traditional chemical synthesis yielded an additional series of somatostatin subtype-2 receptor (SSTR2) selective agonists. These compounds have been used to further define the physiological functions of the individual somatostatin receptor subtypes. In vitro experiments demonstrated the role of the SSTR2 in inhibition of glucagon release from mouse pancreatic alpha-cells and the somatostatin subtype-5 receptor (SSTR5) as a mediator of insulin secretion from pancreatic beta-cells. Both SSTR2 and SSTR5 regulated growth hormone release from the rat anterior pituitary gland. In vivo studies performed with SSTR2 receptor selective compounds demonstrated effective inhibition of pulsatile growth hormone release in rats. The SSTR2 selective compounds also lowered plasma glucose levels in normal and diabetic animal models. The availability of high affinity, subtype selective non-peptide agonists for each of the somatostatin receptors provides a direct approach to defining their physiological function both peripherally and in the central nervous system.

Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity

Somatostatin and dopamine are two major neurotransmitter systems that share a number of structural and functional characteristics. Somatostatin receptors and dopamine receptors are colocalized in neuronal subgroups, and somatostatin is involved in modulating dopamine-mediated control of motor activity. However, the molecular basis for such interaction between the two systems is unclear. Here, we show that dopamine receptor D2R and somatostatin receptor SSTR5 interact physically through hetero-oligomerization to create a novel receptor with enhanced functional activity. Our results provide evidence that receptors from different G protein (heterotrimeric guanine nucleotide binding protein)-coupled receptor families interact through oligomerization. Such direct intramembrane association defines a new level of molecular crosstalk between related G protein-coupled receptor subfamilies.

Modulatory role of neuropeptides in seizures induced in rats by stimulation of glutamate receptors

Stimulation of glutamate receptors has been reported to modulate the expression of neuropeptides and their receptors in neurons. On the other hand, neuropeptides are known to regulate the presynaptic glutamate release and neuronal responses to excitatory neurotransmission. This evidence indicates a functional interaction between glutamatergic and neuropeptidergic transmission in the central nervous system (CNS). In this report, we provide pharmacologic evidence in experimental models of seizures, suggesting that somatostatin (SRIF) and neuropeptide Y (NPY) are endogenous modulators of glutamate-mediated hyperexcitability in the CNS. Electroencephalographic (EEG) and behavioral seizures were induced in rats by intrahippocampal or systemic injection of kainic acid, a glutamate analog. The number of EEG seizures and their total duration were inhibited significantly by intracerebral application of a SRIF(1) receptor agonist. Similarly, kainate seizures were reduced by N[-2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl-D-arginamide++ +] (BIBP 3226), a NPY Y(1) receptor antagonist. Enhanced seizure susceptibility to pentylentetrazol, ensuing in rats after a systemic administration of kainic acid, was reduced significantly by intracerebral application of RC 160, a SRIF(1) receptor agonist, or NPY 13-36, a Y(2)/Y(5) receptor agonist. This evidence suggests that neuropeptide analogs may be of value for controlling seizures and possibly in other pathologic conditions associated with excessive glutamate function.

Protein kinase C activation stimulates the phosphorylation and internalization of the sst2A somatostatin receptor

The sst2A receptor is expressed in the endocrine, gastrointestinal, and neuronal systems as well as in many hormone-sensitive tumors. This receptor is rapidly internalized and phosphorylated in growth hormone-R2 pituitary cells following somatostatin binding (Hipkin, R. W., Friedman, J., Clark, R. B., Eppler, C. M., and Schonbrunn, A. (1997) J. Biol. Chem. 272, 13869-13876). The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), also stimulates sst2A phosphorylation. Here we examine the mechanisms and consequences of PMA and agonist-induced sst2A phosphorylation. Like somatostatin, both PMA and bombesin increased sst2A receptor phosphorylation within 2 min. The PKC inhibitor GF109203X blocked PMA- and bombesin- stimulated sst2A phosphorylation, whereas stimulation by the somatostatin analog SMS 201-995 was unaffected. Agonist and PMA each stimulated phosphorylation in two receptor domains, the third intracellular loop and the C-terminal tail. Functionally, PMA dramatically increased the internalization of the sst2A receptor-ligand complex. This PMA stimulation was blocked by GF109203X, whereas basal internalization was unaffected. However, neither basal nor PMA-stimulated internalization was altered by pertussis toxin, whereas both were blocked by hypertonic sucrose. Therefore PKC activation and agonist binding stimulate sst2A phosphorylation by distinct mechanisms, and PKC potentiates internalization of the sst2A receptor via clathrin-coated pits. Thus, hormonal stimulation of PKC-coupled receptors may provide a mechanism for regulating the inhibitory actions of somatostatin in target tissue.

Human somatostatin receptor subtypes in acromegaly: distinct patterns of messenger ribonucleic acid expression and hormone suppression identify different tumoral phenotypes

Recently, studies using somatostatin (SRIF) analogs preferential for either the SRIF receptor 2 (SSTR2) or the SSTR5 subtype demonstrated a variable suppression of GH and PRL release from GH-secreting human adenomas. These data suggested the concept of SSTR subtype specificity in such tumors. In the present study the quantitative expression of messenger ribonucleic acid (mRNA) for the 5 SSTR subtypes and the inhibitory effects of SRIF14; SRIF28; octreotide; the SSTR2-preferential analog, BIM-23197; and the SSTR5-preferential analog, BIM-23268, on GH and PRL secretion were analyzed in cells cultured from 15 acromegalic tumors. RT-PCR analysis revealed a consistent pattern of SSTR2 and SSTR5 mRNA expression. SSTR5 mRNA was expressed at a higher level (1052 +/- 405 pg/pg glyceraldehyde-3-phosphate dehydrogenase) than SSTR2 mRNA (100 +/- 30 pg/pg glyceraldehyde-3-phosphate dehydrogenase). However, only SSTR2 mRNA expression correlated with the degree of GH inhibition induced by SRIF14, SRIF28, and BIM-23197. The SSTR5-preferential compound inhibited GH release in only 7 of 15 cases. In cells cultured from the 10 mixed adenomas that secreted both GH and PRL, RT-PCR analysis revealed a consistent coexpression of SSTR5, SSTR2, and SSTR1 mRNA. In all cases SRIF14, SRIF28, and the SSTR5-preferential analog, BIM-23268, significantly suppressed PRL secretion, with a mean maximal inhibition of 48 +/- 4%. In contrast, the SSTR2-preferential analogs, BIM-23197 and octreotide, were effective in suppressing PRL in only 6 of 10 cases. In cells cultured from adenomas taken from patients partially responsive to the SRIF analog, octreotide, partial additivity in suppressing both GH and PRL secretion was observed when the SSTR2- and SSTR5-preferring analogs, BIM-23197 and BIM-23268, were tested in combination. Our data show a highly variable ratio of the SSTR2 and SSTR5 transcripts, according to tumors. The SSTR2-preferring compound consistently inhibits GH release, whereas the SSTR5-preferring compound is the main inhibitor of PRL secretion. When both drugs are combined, the partial additivity observed in mixed GH- plus PRL-secreting adenomas may be of interest in the therapeutic approach of such tumors.

Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice

Somatostatin (SST) potently inhibits insulin and glucagon release from pancreatic islets. Five distinct membrane receptors (SSTR1-5) for SST are known, and at least two (SSTR2 and SSTR5) have been proposed to regulate pancreatic endocrine function. Our current understanding of SST physiology is limited by the receptor subtype selectivity of peptidyl SST analogs, making it difficult to assign a physiological function to an identified SST receptor subtype. To better understand the physiology of SSTRs we studied the in vitro effects of potent subtype-selective nonpeptidyl SST analogs on the regulation of pancreatic glucagon and insulin secretion in wild-type (WT) and in somatostatin receptor 2 knockout (SSTR2KO) mice. There was no difference in basal glucagon and insulin secretion between islets isolated from SSTR2KO and WT mice; however, potassium/arginine-stimulated glucagon secretion was approximately 2-fold higher in islets isolated from SSTR2KO mice. Neither SST nor any SSTR-selective agonist inhibited basal glucagon or insulin release. SST-14 potently inhibited stimulated glucagon secretion in islets from WT mice and much less effectively in islets from SSTR2KO mice. The SSTR2 selective analog L-779,976 inhibited glucagon secretion in islets from WT, but was inactive in islets from SSTR2KO mice. L-817,818, an SSTR5 selective analog, slightly reduced glucagon release in both animal groups, whereas SSTR1, -3, and -4 selective analogs were inactive. SST and L-817,818 inhibited glucose stimulated insulin release in islets from WT and SSTR2KO mice. L-779,976 much less potently reduced insulin secretion from WT islets. In conclusion, our data demonstrate that SST inhibition of glucagon release in mouse islets is primarily mediated via SSTR2, whereas insulin secretion is regulated primarily via SSTR5.

Characterisation of human recombinant somatostatin receptors. 1. Radioligand binding studies

Human somatostatin receptor subtypes 1-5 (sst1-5) were characterised using the agonist radioligands [125I]LTT-SRIF28, [125I][Tyr10]CST14, [125I]CGP 23996 and [125I][Tyr3]octreotide in stably transfected Chinese hamster lung fibroblast cells (CCL39 cells). The radioligands used labelled saturable and high-affinity populations of sites in each instance; at sst1-4 receptors maximum binding (Bmax) was roughly equivalent. By contrast, at sst5 receptors Bmax determined with [125I]CGP 23996 and [125I][Tyr3]octreotide was significantly lower (two-and eightfold) compared with [125I]LTT-SRIF28 and [125I][Tyr10]CST14. Experiments were performed with the stable GTP-analogue guanylylimidodiphosphate (GppNHp) to establish guanine nucleotide sensitivity of agonist binding to sst1-5 receptors. The sensitivity towards GppNHp was quite variable depending on receptor and/or ligand. At sst1 and sst4 receptors, GppNHp produced little effect overall, whereas binding to sst3 and sst2 receptors was reduced by 70 and >80%, respectively. At sst5 receptors, the binding of [125I]LTT-SRIF28 and [125I][Tyr10]CST14 was only slightly affected by GppNHp, while [125I]CGP 23996 and [125I][Tyr3]octreotide binding was almost entirely inhibited. Thus, [125I][Tyr3]octreotide labelled about 26-fold less sst5 receptors than [125I]LTT-SRIF28, in the presence of 10 microM GppNHp. These discrepancies in guanine nucleotide sensitivity, were confirmed in GppNHp competition experiments. Competition studies were performed at the five receptors labelled with the different radioligands to establish their respective pharmacological profiles: the rank order of affinity was largely radioligand-independent at sst1-4 receptors, in contrast to sst5 receptors where it was radioligand-dependent. Thus, the pharmacological profile of [125I]LTT-SRIF28- and [125I][Tyr10]CST14-labelled sst5 sites correlated highly significantly, but did not correlate with the affinity profiles defined with [125I]CGP 23996 and [125I][Tyr3]octreotide binding to sst5 receptors. Depending on the agonist radioligand used and the receptor studied, it would appear that binding can be essentially to a guanine nucleotide-sensitive state (e.g. sst2 or sst3), a guanine nucleotide-insensitive state (sst1 or sst4) or a mixture of both (sst5); in the latter case, each radioligand defining a more or less different rank order of affinity at the same receptor. In summary, the differences in agonist receptor binding and guanine nucleotide sensitivity cannot be explained by the ternary complex model or its variations, but rather suggest the existence of multiple agonist-specific receptor states which vary from one receptor to another.

Characterisation of human recombinant somatostatin receptors. 3. Modulation of adenylate cyclase activity

The five human somatostatin receptor subtypes (hsst1-5) were stably expressed in CCL39 cells (Chinese hamster lung fibroblast cells) to study the inhibition of forskolin-stimulated adenylate cyclase (FSAC) activity induced by somatostatin (somatotropin release inhibiting factor, SRIF), cortistatin (CST) and SRIF peptide analogues. Inhibition of FSAC was observed with all five receptors, although the maximal effects produced by SRIF14 varied from around 40% (sst1, sst2, sst4) to 67% (sst3, sst5) reflecting to some extent differences in receptor density (Bmax values published in accompanying paper, this journal). SRIF28 was slightly more potent than SRIF14 to inhibit FSAC at all five receptors, although the potency of the natural peptides SRIF14, SRIF28 and CST17 was generally similar with pEC50-values ranging from 7.5 to 8.7 depending on receptor and peptide. At SRIF1 receptors (sst2, sst3, sst5) most of the peptide analogues displayed full agonism (with some exceptions e.g. BIM 23056 at sst1-3 and sst5 receptors, and L362,855 and cycloantagonist SA at sst3 receptors), whereas at SRIF2 receptors these analogues tended to behave as partial agonists. BIM 23056 was an antagonist at sst3 receptors (antagonist binding constant pKB = 6.33), but not at other receptors. The AC inhibition profiles of sst1-5 receptors were compared with the different radioligand binding profiles as well as with [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding profile for sst2-5 receptors. High correlations were observed between FSAC inhibition, radioligand binding and [35S]GTPgammaS binding profiles at sst3, sst4 and sst5 receptors; by contrast, correlation coefficients at sst1 and sst2 receptors were low, and the binding profiles of [125I][Tyr10]CST14 correlated poorly. In line with these findings, the FSAC inhibition and [35S]GTPgammaS binding correlated poorly at sst2 receptors (sst1 receptors show no significant induction of [35S]GTPgammaS binding). The apparent lack of, or only weak, relationship between FSAC, radioligand or [35S]GTPgammaS binding observed for some SRIF receptors suggests that different active states may exist for these receptors, which may favour one transduction cascade over others.

Evidence that somatostatin sst2 receptors mediate striatal dopamine release

1 Somatostatin (SRIF) is a cyclic tetradecapeptide present in medium-sized aspiny interneurones in the rat striatum. We have previously shown that exogenous SRIF potently stimulates striatal dopamine (DA) release via a glutamate-dependent mechanism. We now report the ability of the selective sst2 receptor agonist, BIM-23027, to mimic this effect of SRIF. 2 In vivo microdialysis studies were performed in anaesthetized male Wistar rats. In most experiments, compounds were administered by retrodialysis into the striatum for 15 min periods, 90 min and 225 min after sampling commenced, with levels of neurotransmitters being measured by HPLC with electrochemical and fluorescence detection. 3 BIM-23027 (50 and 100 nM) stimulated DA release with extracellular levels increasing by up to 18 fold. 4 Prior retrodialysis of BIM-23027 (50 nM) abolished the effects of subsequent administration of SRIF (100 nM). 5 The agonist effects of both BIM-23027 and SRIF were abolished by the selective sst2 receptor antagonist, L-Tyr8-CYN-154806 (100 nM). 6 The AMPA/kainate receptor antagonist, DNQX (100 microM), abolished the agonist effects of BIM-23027 as previously shown for SRIF. 7 This study provides evidence that the sst2 receptor mediates the potent dopamine-releasing actions observed with SRIF in the rat striatum. Dopamine release evoked by both peptides appears to be mediated indirectly via a glutamatergic pathway. Other subtype-specific somatostatin receptor ligands were unable to elicit any effects and therefore we conclude that no other somatostatin receptor types are involved in mediating the dopamine-releasing actions of SRIF in the striatum.

Characterisation of human recombinant somatostatin receptors. 4. Modulation of phospholipase C activity

Total [3H]phosphoinositide (IPx) accumulation, a measure of phospholipase C (PLC) activity, induced by somatostatin (somatotropin release-inhibiting factor, SRIF) and cortistatin (CST) analogues was studied at human somatostatin receptor subtypes 1-5 (hsst1-5) recombinantly expressed in CCL39 (Chinese hamster lung fibroblast) cells. SRIF14 (10 microM) stimulated total [3H]-IPx production 200% and 1070% over basal levels, and increased intracellular Ca2+ ([Ca2+]i) 1600% and 2790%, in cells expressing hsst3 and hsst5 receptors, respectively. The SRIF14-stimulated IPx production was partly blocked by 100 ng/ml pertussis toxin (PTX) (30% and 15% inhibition, respectively). At hsst1, hsst2, and hsst4 receptors, only weak or no stimulation of PLC activity was found (Emax = 114%, 122%, and 102%, respectively). Consequently, hsst3 and hsst5 receptors were subjected to more detailed studies to establish pharmacological profiles of PLC stimulation. At hsst3 receptors, the relative efficacies of most ligands were in the same range (maximum response Emax = 218-267%). At hsst5 receptors Emax varied over a broad range, seglitide, CST17, SRIF28 displaying almost full agonism compared to SRIF14, whereas octreotide and BIM 23052 showed very low partial agonism. BIM 23056 behaved as an antagonist on SRIF14-induced total [3H]-IPx accumulation with a pKB (negative logarithm of antagonist binding constant) of 6.74 at hsst3 receptors, and of 6.94 at hsst5 receptors. The putative cycloantagonist SA showed weak antagonist activity on SRIF14-induced total [3H]-IPx levels at hsst3 (pKB = 5.85), but not at hsst5 receptors. The [3H]-IPx accumulation profiles at sst3/sst5 receptors were compared to their respective radioligand binding ([125I]LTT-SRIF28, [125I][Tyr10]CST14, [125I]CGP 23996, [125I][Tyr3]octreotide binding), to [35S]GTPgammaS binding, and to forskolin-stimulated adenylate cyclase (FSAC) inhibition profiles determined previously in CCL39 cells. The different affinity profiles correlated relatively well at both receptor subtypes with PLC activation (sst3: r = 0.90-0.97; sst5: r = 0.80-0.87). However, [35S]GTPgammaS binding correlated only minimally with stimulation of [3H]-IPx levels at sst5 receptors (r = 0.59), but rather well at sst3 receptors (r = 0.80). A moderate correlation was also observed between inhibition of FSAC activity and stimulation of PLC activity for hsst3 and hsst5 receptors with correlation coefficients of 0.85 and 0.70, respectively. In summary, most SRIF analogues behave as full agonists at hsst3 receptors and agonist-induced phosphoinositide turnover correlates well with radioligand binding, [35S]GTPgammaS binding and inhibition of adenylate cyclase activity, all measured in CCL39 cells. By contrast, at hsst5 receptors, most SRIF analogues behave as intermediate or very low partial agonists (although receptor levels are comparatively high, 7000 vs. 400 fmol/mg), and the agonist-induced phosphoinositide turnover correlates rather poorly with radioligand binding, [35S]GTPgammaS binding or inhibition of adenylate cyclase activity, all measured in the same cell line. Agonist-induced phosphoinositide turnover, [35S]GTPgammaS binding and inhibition of adenylate cyclase activity, show differences both in the rank orders of potency and relative efficacy at hsst3 and markedly at hsst5 receptors, suggesting either that PLC activity is functionally irrelevant or, more probably, that agonist-dependent receptor trafficking is taking place in CCL39 cells.

Characterisation of human recombinant somatostatin receptors. 2. Modulation of GTPgammaS binding

G protein activation by somatostatin (somatotropin release inhibiting factor, SRIF), cortistatin (CST) and analogues of these neuropeptides was investigated at human somatostatin receptor subtypes 1-5 (sst1-5) stably expressed in CCL39 Chinese hamster lung fibroblast cells by measuring agonist-stimulated [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding. [35S]GTPgammaS binding was assessed in the presence of 100 mM NaCl and 1 microM GDP, although higher Emax and/or pEC50 values may have been obtained under other conditions, but at the expense of lower absolute stimulation or signal/noise ratio. SRIF14 stimulated [35S]GTPgammaS binding to 162, 220, 148 and 266% of control levels via sst2, sst3, sst4 and sst5 receptors, respectively. At sst1 receptors, SRIF14 produced only a limited stimulation (Emax 115%). Hence sst1 receptors were not subjected to further [35S]GTPgammaS binding experiments. [35S]GTPgammaS binding assays were then performed with sst2-5 receptors. Most of the peptide analogues stimulated [35S]GTPgammaS binding in sst2-5 receptor-expressing cells. BIM 23056 behaved as an antagonist on SRIF14-induced [35S]GTPgammaS binding with an apparent pKBs of 6.33 and 5.84 at hsst3 and hsst5 receptors respectively, whereas neither agonism nor antagonism could be shown (at 1 microM) at sst2 or sst4 receptors. The effect at sst5 receptors was not surmountable and needs further investigations. The so-called "antagonist" SA, was devoid of antagonist activity at sst2 or sst3 receptors, whereas it was almost a full agonist at sst4 and sst5 receptor-mediated [35S]GTPgammaS binding. The [35S]GTPgammaS-binding profiles of hsst2-5 receptors were compared with their respective radioligand binding profiles. For sst4 and sst5 receptors, the rank order of affinity of all tested radioligands correlated highly significantly with [35S]GTPgammaS binding (r = 0.814-0.897). At sst3 receptors, [35S]GTPgammaS binding correlated somewhat less with binding profiles obtained with [125I][Tyr10]CST14 and [125I]CGP 23996 than with [125I]LTT-SRIF28 (r = 0.743, 0.757 and 0.882, respectively). At sst2 receptors, [35S]GTPgammaS binding correlated with [125I]LTT-SRIF28, [125I]CGP 23996 and [125I][Tyr3]octreotide binding profiles (r = 0.596-0.699), but not with [125I][Tyr10]CST14 binding. The present [35S]GTPgammaS binding data combined to previous radioligand binding results obtained in cells expressing human SRIF receptors, suggest that at any given receptor, agonists' rank orders of potency (not to mention absolute affinity values which vary profoundly) are not as strictly ordered as may be anticipated. We are investigating these aspects further by analysing additional signalling pathways.

Nonpeptidyl somatostatin agonists demonstrate that sst2 and sst5 inhibit stimulated growth hormone secretion from rat anterior pituitary cells

Somatostatin (SST) regulates growth hormone (GH) secretion from pituitary somatotrophs by interacting with members of the SST family of G-protein-coupled receptors (sst1-5). We have used potent, nonpeptidyl SST agonists with sst2 and sst5 selectivity to determine whether these receptor subtypes are involved in regulating growth hormone releasing hormone (GHRH) stimulated secretion. GHRH stimulated GH release from pituitary cells in a dose-dependent manner, and this secretion was inhibited by Tyr(11)-SST-14, a nonselective SST analog. A sst2 selective agonist, L-779,976, potently inhibited GHRH-stimulated GH release. In addition, L-817, 818, a potent sst5 receptor selective agonist, also inhibited GH secretion, but was approximately 10-fold less potent (P < 0.01, ANOVA) in inhibiting GH release than either Tyr(11)-SST-14 or L-779, 976. These results show that both sst2 and sst5 receptor subtypes regulate GHRH-stimulated GH release from rat pituitary cells.

Agonist-dependent up-regulation of human somatostatin receptor type 1 requires molecular signals in the cytoplasmic C-tail

We have previously reported that the human somatostatin receptor type 1 (hSSTR1) stably expressed in Chinese hamster ovary-K1 cells does not internalize but instead up-regulates at the membrane during continued agonist treatment (1 microM somatostatin (SST)-14 x 22 h). Here we have investigated the molecular basis of hSSTR1 up-regulation. hSSTR1 was up-regulated by SST in a time-, temperature-, and dose-dependent manner to saturable levels, in intact cells but not in membrane preparations. Although hSSTR1 was acutely desensitized to adenylyl cyclase coupling after 1 h SST-14 treatment, continued agonist exposure (22 h) restored functional effector coupling. Up-regulation was unaffected by cycloheximide but blocked by okadaic acid. Confocal fluorescence immunocytochemistry of intact and permeabilized cells showed progressive, time-dependent increase in surface hSSTR1 labeling, associated with depletion of intracellular SSTR1 immunofluorescent vesicles. To investigate the structural domains of hSSTR1 responsible for up-regulation, we constructed C-tail deletion (Delta) mutants and chimeric hSSTR1-hSSTR5 receptors. Human SSTR5 was chosen because it internalizes readily, displays potent C-tail internalization signals, and does not up-regulate. Like wild type hSSTR1, Delta C-tail hSSTR1 did not internalize and additionally lost the ability to up-regulate. Swapping the C-tail of hSSTR1 with that of hSSTR5 induced internalization (27%) but not up-regulation. Substitution of hSSTR5 C-tail with that of hSSTR1 converted the chimeric receptor to one resembling wild type hSSTR1 (poor internalization, 71% up-regulation). These results show that ligand-induced up-regulation of hSSTR1 occurs by a temperature-dependent active process of receptor recruitment from a pre-existing cytoplasmic pool to the plasma membrane. It does not require new protein synthesis or signal transduction, is sensitive to dephosphorylation events, and critically dependent on molecular signals in the receptor C-tail.

Activated G protein-coupled receptor induces tyrosine phosphorylation of STAT3 and agonist-selective serine phosphorylation via sustained stimulation of mitogen-activated protein kinase. Resultant effects on cell proliferation

The peptide hormone somatostatin exhibits antiproliferative activity by interacting with the G protein-coupled sst2 or sst5 receptor types. We show here that somatostatin at the human recombinant sst4 receptor induced a concentration-dependent increase in proliferation (EC50 20 nM) with a maximal response 5-fold greater than that produced by its synthetic analog, L-362,855. Analysis of the phosphorylation status of extracellular signal-regulated kinase (ERK)1 and ERK2 showed temporal differences in the changes evoked by the agonists. Phosphorylation induced by somatostatin (100 nM) peaked 10 min after the application and produced a response that continued for at least 4 h. In contrast, L-362,855 (1 microM) showed transient phosphorylation that had declined to basal levels by 1 h. However, both agonists induced rapid and sustained tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3) which was pertussis toxin-insensitive. Serine phosphorylation of STAT3 was only apparent after somatostatin treatment and was abolished by pertussis toxin or PD 98059, together with the associated increases in proliferation. Mitogen-activated protein/ERK kinase-1 inhibition also decreased the time interval over which somatostatin-induced ERK phosphorylation was observed (<2 h). We conclude that the difference in the magnitude of the proliferative response evoked by the two agonists at the sst4 receptor can be accounted for by their differential ability to phosphorylate STAT3 on serine residues and supports the concept that selective signaling can be achieved through pharmacological diversity.

Highly potent cyclic disulfide antagonists of somatostatin

The search for synthetic analogues of somatostatin (SRIF) which exhibit selective affinities for the five known receptor subtypes (sst1-5) has generated a large number of potent agonist analogues. Many of these agonists display good subtype selectivities and affinities for the subtypes 2, 3, and 5, with very few selective for sst1 or sst4. Until the recent report by Bass and co-workers (Mol. Pharmacol. 1996, 50, 709-715; erratum Mol. Pharmacol. 1997, 51, 170), no true antagonists of somatostatin had been discovered, let alone any displaying differential receptor subtype selectivity. In this present study, we further explore the effect of this putative L,5D6 antagonist motif on somatostatin octapeptide analogues with a cyclic hexapeptide core. The most potent antagonist found to date is H-Cpa-cyclo[DCys-Tyr-DTrp-Lys-Thr-Cys]-Nal-NH2, PRL-2970 (21), which has an IC50 of 1.1 nM in a rat pituitary growth hormone in vitro antagonist assay versus SRIF (1 nM). This analogue bound to cloned human somatostatin subtype 2 receptors with a Ki of 26 nM. The highest hsst2 affinity analogue was H-Cpa-cyclo[DCys-Pal-DTrp-Lys-Tle-Cys]-Nal-NH2, PRL-2915 (15), with a Ki of 12 nM (IC50 = 1.8 nM). This analogue was also selective for hsst2 over hsst3 and hsst5 by factors of 8 and 40, respectively, and had no agonist activity when tested alone at concentrations up to 10 microM. Regression analysis of the binding affinities versus the observed antagonist potencies revealed high correlations for hsst2 (r = 0.65) and hsst3 (r = 0.52) with a less significant correlation to hsst5 (r = 0.40). This is quite different from the somatostatin agonist analogues which show a highly significant correlation to hsst2 (r > 0.9). Receptor-selective somatostatin antagonists should provide valuable tools for characterizing the many important physiological functions of this neuropeptide.

Inhibition of human pancreatic islet insulin release by receptor-selective somatostatin analogs directed to somatostatin receptor subtype 5

Somatostatin (SS)-14 and SS28 are produced by pancreatic D cells and gut mucosa and inhibit pancreatic islet insulin and glucagon release. There are five distinct SS receptor (SSTR) subtypes, namely SSTR1-5, which show different affinities for SS14 and SS28. In order to identify the subtype responsible for inhibition of insulin release by human B cells, SSTR-selective SS analogs were tested in isolated human islets. Glucose-stimulated insulin secretion in human islets incubated for 1 hr at 20 mM glucose, and in islets cultured for 24 hr at a near-physiological (6.1 mM) glucose concentration, was inhibited (<50% of the control) by SSTR5-specific analogs and by SS14 and SS28. SS14, SS28, and different SSTR5 preferential analogs also inhibited islet amyloid polypeptide release during the 24-hr culture. On the other hand, a group of SSTR2-selective analogs failed to inhibit insulin release. Analysis by reverse transcription-polymerase chain reaction indicated that human islets express similar amounts of SSTR2 and SSTR5 mRNAs, while human pancreatic ductal cells express much lower levels of these mRNAs. In conclusion, our data suggest that SSTR5 is an important mediator of the insulin inhibitory action of SS in cultured human islets.

2-pyridylthioureas: novel nonpeptide somatostatin agonists with SST4 selectivity

Somatostatin [somatotropin release-inhibiting factor (SRIF)] is a cyclic tetradecapeptide that is a potent inhibitor of growth hormone (GH) secretion from the anterior pituitary. In addition to the inhibitory effects on GH-release, SRIF-14 and SRIF-28, a 28-amino acid form of SRIF extended from the N-terminal end, inhibit the release of a variety of other peptides including glucagon, insulin, and gastrin, and both peptides act as neurotransmitters and neuromodulators in the central nervous system and the periphery. SRIF exerts its potent inhibitory effects following binding to high affinity SRIF receptors (ssts) that have been identified on target tissues. The recent cloning of five ssts has confirmed that the effects of SRIF are mediated by a family of G protein-coupled receptors (sst1-5). Based on structural and pharmacological properties sst2, sst3, and sst5 belong to the SRIF1 receptor subclass, and the sst1 and sst4 subtypes comprise the SRIF2 subclass. The major difference between these two subclasses is that SRIF1 receptors bind octapeptide and hexapeptide SRIF-14 analogs with high affinity, while SRIF2 receptors bind these analogs with drastically reduced affinity. A screening program was initiated to identify a lead nonpeptide with affinity for sst1-5 receptors. The search focused on a scaffold with the following attachments: (1) a heteroaromatic nucleus to mimic the Trp8 residue, (2) a nonheteroaromatic nucleus to mimic Phe7, and (3) a primary amine or other basic group to mimic the Lys9 residue of SRIF-14. Using these criteria, a novel thiourea (NNC 26-9100, 17) was discovered as a structural lead. The key fragments in this compound are a heteroaromatic moiety (pyridine), an aromatic group, and a basic imidazole group connected through a thiourea scaffold. Compound 17 exhibited a Ki = 6 nM at sst4 receptors with a 100-fold sst4/sst2 selectivity and was shown to be a full agonist at this receptor subtype. This article will review the literature on the design and development of nonpeptide somatostatin receptor ligands and the therapeutic potential of these agents. Furthermore, our work on the development of 2-pyridylthioureas as sst4 receptor agonists will be described.

Agonist-independent internalization and activity of a C-terminally truncated somatostatin receptor subtype 2 (delta349)

The rat somatostatin receptor subtype 2 (SSTR2) is rapidly internalized and phosphorylated in the presence of somatostatin 14 (SST14). Several C-terminal deletion constructs of SSTR2 have been investigated for their ability to undergo agonist-dependent internalization by using biochemical ligand binding assays and confocal microscopic analysis. Whereas mutant receptors lacking either 10 (delta359), 30 (delta339), or 44 (delta325) amino acid residues at the C terminus required SST14 for internalization, a construct lacking the last 20 amino acids (delta349) was detected mostly intracellularly and independently of the presence of the agonist. When internalization was blocked by sucrose, the delta349 receptor remained at the cell surface, strongly indicating that this mutant is internalized in an agonist-independent fashion. An increased affinity for agonists as measured in membrane binding assays and a reduced level of forskolin-stimulated cyclic AMP accumulation in human embryonic kidney cells expressing delta349 are properties that are characteristic of agonist-independent receptor activity. Delta349 is not phosphorylated detectably in the absence of agonist, demonstrating that phosphorylation per se is not a prerequisite for internalization of SSTR2. This observation is in line with data obtained for the delta325 mutant, which was internalized in an agonist-dependent manner, but not phosphorylated in either the presence or absence of SST14. We conclude that truncation of the SSTR2 C terminus at position 349 leads to agonist-independent, constitutive activity and internalization.

Internalization of [DOTA degrees,125I-Tyr3]Octreotide by somatostatin receptor-positive cells in vitro and in vivo: implications for somatostatin receptor-targeted radio-guided surgery

We compared internalization of three radioiodinated octreotide (OCT) somatostatin (SS) analogs-[125I-Tyr3]OCT, [DTPA degrees, 125I-Tyr3]OCT, and [DOTA degrees,125I-Tyr3]OCT-by somatostatin receptor (SSR)-positive mouse AtT20 pituitary tumor cells and human insulinoma cells. The three SS analogs were internalized in a specific, time-dependent manner. Internalization was significantly inhibited by pertussis toxin (100 microg/l) by 38%, 43%, and 31%, and by an inhibitor of receptor-mediated endocytosis (phenyl arsine oxide; 10 microM) by 98%, 94%, and 92%, respectively. Binding affinities of the three radioligands were comparable (0.2, 0.2, and 0.3 nM, respectively). However, [DOTA degrees,125I-Tyr3]OCT was internalized in a five-fold higher amount in comparison with the two other radioligands. A comparably high uptake of [DOTA degrees, 125I-Tyr3]OCT was found in SSR-positive organs (pituitary, pancreas, and adrenals) in vivo in rats (a ten-fold, five-fold, and eight-fold higher uptake 4 hr post injection, respectively, compared with the two other radioligands). This resulted in very high target-background ratios for [DOTA degrees,125I-Tyr3]OCT 4 hr post injection amounting to 274, 566, and 623 in the pituitary, adrenals, and pancreas, respectively. Both in vivo and in vitro there was a rapid dissociation of radioactivity from the SSR-positive cells. Main conclusions are that: 1) coupling of chelating groups like DTPA or DOTA to the SS analog [Tyr3]OCT does not prevent the internalization of OCT after binding to SSRs; 2) [DOTA degrees, 125I-Tyr3]OCT is internalized in a significantly higher amount by AtT20 and human insulinoma cells and in vivo in rats in SSR-positive organs, in comparison with [DTPA degrees,125I-Tyr3]OCT and [125I-Tyr3]OCT; and 3) the very high target-background ratios in vivo make radioiodinated [DOTA degrees,Tyr3]OCT a very suitable ligand for SSR-targeted radioguided surgery of SSR-positive human neuroendocrine tumors.

Somatostatin receptor desensitization in NG108-15 cells. A consequence of receptor sequestration

In NG108-15 cells inhibition of both N-type calcium channel current and adenylyl cyclase by somatostatin (SRIF) was not sustained but rapidly desensitized in the continued presence of the drug. The degree and rate of desensitization were concentration-dependent, and the desensitization was homologous with respect to the delta-opioid receptor. We have been unable to obtain evidence for the involvement of G protein-coupled receptor kinases (GRKs) in this desensitization. SRIF-induced desensitization of N-type calcium channel currents was not reduced in cells stably overexpressing a dominant negative mutant of GRK2 or following intracellular dialysis with GRK2- and GRK3-blocking peptides or with heparin. Inhibitors of protein kinase A, protein kinase C, and protein kinase G were also without effect. In contrast, both the rate and degree of SRIF-induced desensitization were reduced by pretreatment with phenylarsine oxide or concanavalin A, both inhibitors of receptor endocytosis. Furthermore, SRIF-induced desensitization was enhanced by monensin, which prevents receptor recycling back to the plasma membrane. Similarly, SRIF-induced desensitization of adenylyl cyclase inhibition was not reduced in cells stably overexpressing dominant negative mutant GRK2 but was reduced in cells pretreated with the receptor endocytosis inhibitor hyperosmotic sucrose or concanavalin A. These data are consistent with the view that SRIF-induced desensitization in NG108-15 cells results from receptor internalization.

Response to treatment with yttrium 90-DOTA-lanreotide of a patient with metastatic gastrinoma

1,4,7,10-tetraazacyclododecane-N,N',N",N'''-tetraacetic acid (DOTA)-lanreotide is a universal somatostatin (SST) receptor subtype ligand that binds to a large variety of human tumors. We report the case of a patient with metastatic gastrinoma who was treated with 90Y-DOTA-lanreotide. Before treatment, dosimetry with 111In-DOTA-lanreotide (150 MBq, 10 nmol) indicated a dose of 5.8 mGy/MBq for the recurrent abdominal gastrinoma, and a mean dose of approximately 1.0 mGy/MBq for liver metastases (i.e., 56 and approximately 10 mGy/MBq for 90Y-DOTA-lanreotide, respectively). After four infusions of 90Y-DOTA-lanreotide (each 1 GBq, approximately 30 nmol) over a 6-mo period, the 111In-DOTA-lanreotide scintigraphy of the liver had returned to a nearly normal condition and a remarkably decreased uptake by the recurrent gastrinoma was calculated (approximately 5 mGy/MBq for 90Y-DOTA-lanreotide). The imaging results were well-correlated with a 25% regression of the liver metastases as indicated by CT. Blood, urine and whole-body clearances of 111In-DOTA-lanreotide and 90Y-DOTA-lanreotide were very similar. The DOTA-lanreotide promises to be useful for functional tumor diagnosis (111In-DOTA-lanreotide) and receptor-mediated tumor radiotherapy (90Y-DOTA-lanreotide).

Fates of endocytosed somatostatin sst2 receptors and associated agonists

Somatostatin agonists are rapidly and efficiently internalized with the somatostatin sst2 receptor. The fate of internalized agonists and receptors is of critical importance because the rate of ligand recycling back to the cell surface can limit the amount of radioligand accumulated inside the cells, whereas receptor recycling might be of vital importance in providing the cell surface with dephosphorylated, resensitized receptors. Furthermore the accumulation of radioisotope-conjugated somatostatin agonists inside cancer cells resulting from receptor-mediated internalization has been used as a treatment for cancers that overexpress somatostatin receptors. In the present study, radio-iodinated agonists at the sst2 somatostatin receptor were employed to allow quantitative analysis of the fate of endocytosed agonist. After endocytosis, recycling back to the cell surface was the main pathway for both 125I-labelled somatostatin-14 (SRIF-14) and the more stable agonist 125I-labelled cyclo(N-Me-Ala-Tyr-d-Trp-Lys-Abu-Phe) (BIM-23027; Abu stands for aminobutyric acid), accounting for 75-85% of internalized ligand when re-endocytosis of radioligand was prevented. We have shown that there is a dynamic cycling of both somatostatin agonist ligands and receptors between the cell surface and internal compartments both during agonist treatment and after surface-bound agonist has been removed, unless steps are taken to prevent the re-activation of receptors by recycled agonist. Internalization leads to increased degradation of 125I-labelled SRIF-14 but not 125I-labelled BIM-23027. The concentration of recycled agonist accumulating in the extracellular medium was sufficient to re-activate the receptor, as measured both by the inhibition of forskolin-stimulated adenylate cyclase and the recovery of surface receptor number after internalization.

Nonpeptide somatostatin agonists with sst4 selectivity: synthesis and structure-activity relationships of thioureas

Utilizing NNC 26-9100 (11) as a structural lead, a variety of nonpeptide derivatives of somatostatin were synthesized and evaluated for sst2 and sst4 receptor binding affinity. A novel thiourea scaffold was utilized to attach (1) a heteroaromatic nucleus to mimic the Trp8 residue, (2) a nonheteroaromatic nucleus to mimic Phe7, and (3) a primary amine or other basic group to mimic the Lys9 residue of somatostatin. Displacement studies were carried out using membranes from cell lines expressing ssts [BHK cells (sst4) and HEK 293 cells (sst2)] utilizing [125I]Tyr11-SRIF as the radioligand. Several thioureas (11, 38, 39, 41, and 42) and the urea 66 exhibited Ki values of less than 100 nM. The thioureas 11 (Ki = 6 nM) and 41 (Ki = 16 nM) and the urea 66 (Ki = 14 nM) are believed to be the most potent nonpeptide sst4 agonists known. Since the thiourea 11 and the urea 66 exhibit high sst4 selectivity, these novel nonpeptide derivatives may be useful tools for studying the sst4 receptor. Studies are currently in progress to evaluate the therapeutic potential of NNC 26-9100 (11) in the treatment of glaucoma.