Human somatostatin receptor, SSTR2, is coupled to adenylyl cyclase in the presence of Gi alpha 1 protein

Somatostatin Receptors and Analogs in Pheochromocytoma and Paraganglioma: Old Players in a New Precision Medicine World

Neuroendocrine tumors overexpress somatostatin receptors, which serve as important and unique therapeutic targets for well-differentiated advanced disease. This overexpression is a well-established finding in gastroenteropancreatic neuroendocrine tumors which has guided new medical therapies in the administration of somatostatin analogs, both "cold", particularly octreotide and lanreotide, and "hot" analogs, chelated to radiolabeled isotopes. The binding of these analogs to somatostatin receptors effectively suppresses excess hormone secretion and tumor cell proliferation, leading to stabilization, and in some cases, tumor shrinkage. Radioisotope-labeled somatostatin analogs are utilized for both tumor localization and peptide radionuclide therapy, with ⁶⁸Ga-DOTATATE and ¹⁷⁷Lu-DOTATATE respectively. Benign and malignant pheochromocytomas and paragangliomas also overexpress somatostatin receptors, irrespective of embryological origin. The pattern of somatostatin receptor overexpression is more prominent in succinate dehydrogenase subunit B gene mutation, which is more aggressive than other subgroups of this disease. While the Food and Drug Administration has approved the use of ⁶⁸Ga-DOTATATE as a radiopharmaceutical for somatostatin receptor imaging, the use of its radiotherapeutic counterpart still needs approval beyond gastroenteropancreatic neuroendocrine tumors. Thus, patients with pheochromocytoma and paraganglioma, especially those with inoperable or metastatic diseases, depend on the clinical trials of somatostatin analogs. The review summarizes the advances in the utilization of somatostatin receptor for diagnostic and therapeutic approaches in the neuroendocrine tumor subset of pheochromocytoma and paraganglioma; we hope to provide a positive perspective in using these receptors as targets for treatment in this rare condition.

Unexpected role of a conserved domain in the first extracellular loop in G protein-coupled receptor trafficking

G protein-coupled receptors are the largest superfamily of cell surface receptors in the Metazoa and play critical roles in transducing extracellular signals into intracellular responses. This action is mediated through conformational changes in the receptor following ligand binding. A number of conserved motifs have critical roles in GPCR function, and here we focus on a highly conserved motif (WxFG) in extracellular loop one (EL1). A phylogenetic analysis documents the presence of the WxFG motif in ∼90% of Class A GPCRs and the motif is represented in 17 of the 19 Class A GPCR subfamilies. Using site-directed mutagenesis, we mutagenized the conserved tryptophan residue in eight receptors which are members of disparate class A GPCR subfamilies from different taxa. The modification of the Drosophila leucokinin receptor shows that substitution of any non-aromatic amino acid for the tryptophan leads to a loss of receptor function. Additionally, leucine substitutions at this position caused similar signaling defects in the follicle-stimulating hormone receptor (FSHR), Galanin receptor (GALR1), AKH receptor (AKHR), corazonin receptor (CRZR), and muscarinic acetylcholine receptor (mACHR1). Visualization of modified receptors through the incorporation of a fluorescent tag revealed a severe reduction in plasma membrane expression, indicating aberrant trafficking of these modified receptors. Taken together, these results suggest a novel role for the WxFG motif in GPCR trafficking and receptor function.

In liver metastases from small intestinal neuroendocrine tumors, SSTR2A expression is heterogeneous

We examined somatostatin receptor type 2A (SSTR2A) expression in primary and metastatic small intestinal neuroendocrine tumors (SI-NETs). We retrieved 156 liver metastases from 26 patients (10 males, 16 females) who had two or more liver lesions resected. A representative formalin-fixed paraffin-embedded section of tumor tissue from each liver metastasis and from the primary tumor, when available, were immunohistochemically stained for SSTR2A. SSTR2A expression was evaluated by the Her2/neu-scoring system and the scoring system proposed by Volante et al. Based on the Her2/neu-scoring system, moderate to strong SSTR2A expression was observed in 121 of 156 (78%) liver metastases. In 15 (58%) subjects, all liver metastases showed moderate to strong SSTR2A expression, whereas in 11 (42%) one or more liver tumors had weak or no expression. Of the 16 stained primaries, 11 (69%) showed heterogeneous SSTR2A expression. The corresponding liver metastases showed only weak to no expression in one, moderate to strong in five, and both weak to no and moderate to strong expression in five of the 11 cases. Using the Volante scoring system, no tumor was scored 0 (0%), two were scored 1 (1%), 38 were scored 2 (24%), and 116 were scored 3 (74%). No statistically significant association was observed between SSTR2A expression and Ki67 index (p = 0.56). Fifteen of 18 (83%) metastatic tumors with a Ki67 index >20% showed moderate to strong SSTR2A. Most liver tumors with weak SSTR2A expression or an IHC score of 2 were detected by OctreoScan. SSTR2A expression in liver metastases of SI-NETs can be variable, even between lesions in the same patient. Expression in metastatic lesions is not always similar to that in the primary tumor. SSTR2A expression is not associated with the Ki67 index.

Somatostatin receptor type 2 contributes to the self-renewal of murine embryonic stem cells

AIM

The roles of G-protein coupled receptors (GPCRs) in stem cell biology remain unclear. In this study, we aimed to identify GPCRs that might contribute to the self-renewal of mouse embryonic stem cells (mESCs).

METHODS

The expression levels of pluripotent genes and GPCR gene were detected in E14 mESCs using PCR array and RT-PCR. Immunofluorescent staining was used to examine the expression of pluripotent markers and the receptor translocation. Western blot analysis was used to detect phosphorylation of signal proteins. Knock-down of receptor was conducted to confirm its role in pluripotency maintenance.

RESULTS

In leukemia inhibitory factor (LIF)-free medium, mESCs lost the typical morphology of pluripotency, accompanied by markedly decreases in expression of somatostatin receptor type 2 (SSTR2), as well as the pluripotency biomarkers Oct4, Sox2, Rex1 and Nanog. Addition of the SSTR2 agonist octreotide or seglitide (0.1-30 μmol/L) in LIF-free medium dose-dependently promoted the self-renewal of mESCs, whereas the SSTR2 antagonist S4 (0.03-3 μmol/L) dose-dependently blocked octreotide-induced self-renewal. Knock-down of SSTR2 significantly decreased the self-renewal of mESCs even in the presence of LIF. Addition of LIF (1000 U/mL) or octreotide (1 μmol/L) in LIF-free medium significantly increased both phosphorylation and nuclear ocalization of STAT3.

CONCLUSION

The activation of SSTR2 contributes to the self-renewal of mESCs via activation of the STAT3 pathway.

Somatostatin receptors: from signaling to clinical practice

Somatostatin is a peptide with a potent and broad antisecretory action, which makes it an invaluable drug target for the pharmacological management of pituitary adenomas and neuroendocrine tumors. Somatostatin receptors (SSTR1, 2A and B, 3, 4 and 5) belong to the G protein coupled receptor family and have a wide expression pattern in both normal tissues and solid tumors. Investigating the function of each SSTR in several tumor types has provided a wealth of information about the common but also distinct signaling cascades that suppress tumor cell proliferation, survival and angiogenesis. This provided the rationale for developing multireceptor-targeted somatostatin analogs and combination therapies with signaling-targeted agents such as inhibitors of the mammalian (or mechanistic) target of rapamycin (mTOR). The ability of SSTR to internalize and the development of rabiolabeled somatostatin analogs have improved the diagnosis and treatment of neuroendocrine tumors.

Presynaptic inhibition of glutamate release by neuropeptides: use-dependent synaptic modification

Neuropeptides are signaling molecules that interact with G-protein coupled receptors located both pre- and postsynaptically. Presynaptically, these receptors are localized in axons and terminals away from presynaptic specializations. Neuropeptides are stored in dense core vesicles that are distinct from the clear synaptic vesicles containing classic neurotransmitters such as glutamate and GABA. Because they require a stronger Ca(2+) signal than synaptic vesicles, dense core vesicles do not release neuropeptides with single action potentials but rather require high-frequency trains. Thus, neuropeptides only modulate strongly stimulated synapses, providing negative or positive feedback. Many neuropeptides have been found to inhibit glutamate release from presynaptic terminals, and the major mechanism is likely direct interaction of betagamma G-protein subunits with presynaptic proteins such as SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). The use of mouse genetic models and specific receptor antagonists are beginning to unravel the function of inhibitory neuropeptides. The opioid receptors kappa and mu, which are activated by endogenous opioid peptides such as dynorphin, enkephalin, and possibly the endomorphins, are important in modulating pain transmission. Dynorphin, nociceptin/orphanin FQ, and somatostatin and its related peptide cortistatin appear to play a role in modulation of learning and memory. Neuropeptide Y has important functions in ingestive behavior and also in entraining circadian rhythms. The existence of neuropeptides greatly expands the computational ability of the brain by providing additional levels of modulation.

Analgesic activity of a non-peptide imidazolidinedione somatostatin agonist: in vitro and in vivo studies in rat

Several lines of evidence support an important role for somatostatin receptors (SSTRs) in pain modulation. The therapeutic use of established SSTR peptide agonists for this indication is limited by their broad range of effects, need for intrathecal delivery, and short half-life. Therefore, the goal of the present study was to investigate the analgesic effect of SCR007, a new, highly selective SSTR2 non-peptide agonist. Behavioral studies demonstrated that paw withdrawal latencies to heat were significantly increased following intraplantar SCR007. Furthermore, both intraperitoneal and intraplantar injection of SCR007 significantly reduced formalin- and capsaicin-induced flinching and lifting/licking nociceptive behaviors. Recordings from nociceptors using an in vitro glabrous skin-nerve preparation showed that SCR007 reduced heat responses in a dose-dependent fashion, bradykinin-induced excitation, heat sensitization and capsaicin-induced excitation. In both the behavioral and single fiber studies, the SCR007 effects were reversed by the SSTR antagonist cyclo-somatostatin, demonstrating receptor specificity. In the single fiber studies, the opioid antagonist naloxone did not reverse SCR007-induced anti-nociception suggesting that SCR007 did not exert its effects through activation of opioid receptors. Analysis of cAMP/protein kinase A (PKA) involvement demonstrated that SCR007 prevented forskolin- and Sp-8-Br-cAMPS (a PKA activator)-induced heat sensitization, supporting the hypothesis that SCR007-induced inhibition could involve a down-regulation of the cAMP/PKA pathway. These data provide several lines of evidence that the non-peptide imidazolidinedione SSTR2 agonist SCR007 is a promising anti-nociceptive and analgesic agent for the treatment of pain of peripheral and/or central origin.

Somatostatin receptors

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

Cellular biology of somatostatin receptors

Somatostatin, and the recently discovered neuropeptide cortistatin, exert their physiological actions via a family of six G protein-coupled receptors (sst1, sst2A, sst2B, sst3, sst4, sst5). Following the cloning of somatostatin receptors significant advances have been made in our understanding of their molecular, pharmacological and signaling properties although much progress remains to be done to define their physiological role in vivo. In this review, the present knowledge regarding neuroanatomical localization, signal transduction pathways, desensitization and internalization properties of somatostatin receptors is summarized. Evidence that somatostatin receptors can form homo- and heterodimers and can physically interact with members of the SSTRIP/Shank/ProSAP1/CortBP1 family is also discussed.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

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.

Differences in the operational characteristics of the human recombinant somatostatin receptor types, sst1 and sst2, in mouse fibroblast (Ltk-) cells

1. The human recombinant somatostatin (SRIF) receptors, sst1 and sst2, have been stably expressed in mouse fibroblast (Ltk-) cells. Two stable clones, LSSR 1/20 and LSSR 11/13, expressing sst1 and sst2 receptors, respectively, have been used to characterize these receptor types using radioligand binding assays as well as measurements of changes in extracellular acidification rates using microphysiometry. 2. [125I]-[Tyr11]-SRIF bound to sst1 and sst2 receptors expressed in Ltk- cells with high affinity, Kd values being 1.52 nM, and 0.23 nM respectively. 3. In Ltk- cells expressing sst1 receptors, SRIF, SRIF-28, [D-Trp8]-SRIF and CGP 23996 all displaced [125I]-[Tyr11]-SRIF binding with high potency (IC50 values of 0.43 - 1.27 nM) whilst seglitide, BIM-23027, BIM-23056 and L-362855 were either weak inhibitors of binding or were ineffective. 4. In contrast MK-678 (seglitide) and BIM-23027 were the most potent inhibitors of [125I]-[Tyr11]-SRIF binding in Ltk- cells expressing sst2 receptors with IC50 values of 0.014 and 0.035 nM, respectively. 5. SRIF and a number of SRIF agonists, including seglitide and BIM-23027, caused concentration-dependent increases in extracellular acidification rates in Ltk- cells expressing sst2 receptors but not in Ltk- cells expressing sst1 receptors. The maximum increase in acidification rate produced by SRIF was 11.3 +/- 0.7% above baseline (0.1-0.28 pH unit min-1). The relative potencies of the SRIF agonists examined in causing increases in extracellular acidification rates in Ltk- cells expressing sst2 receptors correlated well with their relative potencies in inhibiting [125I]-[Tyr11] -SRIF binding (r = 0.94). 6. The increase in extracellular acidification produced by SRIF was markedly inhibited by pretreatment of cells with pertussis toxin (100 ng ml-1) indicating the involvement of pertussis toxin-sensitive G proteins. 7. SRIF (1 microM) had no effect on basal cyclic AMP levels in Ltk- cells expressing sst1 or sst2 receptors nor did it inhibit forskolin stimulated increases in cyclic AMP levels in either cell type. 8. The results from the present study describe the operational characteristics of human sst2 receptors expressed in Ltk- cells where receptor activation causes increases in extracellular acidification rates. This receptor is coupled to a pertussis toxin-sensitive G protein. In contrast, activation of sst1 receptors, at a similar transfection density, did not cause increases in extracellular acidification rates.

Molecular pharmacology of somatostatin receptors

Somatostatin was discovered for its ability to inhibit growth hormone (GH) secretion. Later, it was found to be widely distributed in other brain regions, in which it fulfills a neuromodulatory role, and in several organs of the gastrointestinal tract where it can act as a paracrine factor or as a true circulating factor. In mammals, two molecules of 14 (somatostatin 14) and 28 (somatostatin 28) amino acids are the only biologically active members of the family. They originate from a single gene which gives rise to a single propeptide alternately cleaved in different tissues. In 1992, a major breakthrough in our understanding of somatostatin functions was made with the cloning of five different receptor genes (sstr1 to sstr5) which belong to the seven transmembrane domain receptor family. Their closer relatives are opioid receptors. In first approximation, the tissular expression of the sstrs matches quite well with the distribution of somatostatin binding sites in the "classical" targets of the peptide ie brain, pituitary pancreatic islets and adrenals. The pharmacology of GH inhibition is very close to sstr2 binding but other actions of somatostatins have not yet been attributed clearly to a single receptor subtype. All clinically relevant agonists tested so far (octreotide, lanreotide and vapreotide) are selective of sstr2 being less potent on sstr3 and inactive for sstr1 and sstr4. Surprisingly, rat sstr5 displays nanomolar affinities for octreotide and vapreotide while these agonists are only active at much higher concentrations on human sstr5. All five receptors can be more or less efficiently coupled to inhibition of adenylate cyclase activity in transfected cell systems.(ABSTRACT TRUNCATED AT 250 WORDS)