Phosphorylation of threonine 333 regulates trafficking of the human sst5 somatostatin receptor

Expression of free fatty acid receptor 2 in normal and neoplastic tissues

OBJECTIVE

Little information is available concerning protein expression of the free fatty acid receptor 2 (FFAR2), especially in tumours. Therefore, the aim of the present study was to comprehensively characterise the expression profile of FFAR2 in a large series of human normal and neoplastic tissues using immunohistochemistry thus providing a basis for further in-depth investigations into its potential diagnostic or therapeutic importance.

METHODS

We developed a novel rabbit polyclonal anti-FFAR2 antibody, 0524, directed against the C-terminal region of human FFAR2. Antibody specificity was confirmed via Western blot analyses and immunocytochemistry using the FFAR2-expressing cell line BON-1 and FFAR2-specific small interfering RNA as well as native and FFAR2-transfected HEK-293 cells. The antibody was then used for immunohistochemical analyses of various formalin-fixed, paraffin-embedded specimens of normal and neoplastic human tissues.

RESULTS

In normal tissues, FFAR2 was mainly present in distinct cell populations of the cerebral cortex, follicular cells and C cells of the thyroid, cardiomyocytes of the heart, bronchial epithelia and glands, hepatocytes and bile duct epithelia of the liver, gall bladder epithelium, exocrine and β-cells of the endocrine pancreas, glomerular mesangial cells and podocytes as well as collecting ducts of the kidney, intestinal mucosa (particularly enteroendocrine cells), prostate epithelium, seminiferous tubules of the testicles, and placental syncytiotrophoblasts. In neoplastic tissues, FFAR2 was particularly prevalent in papillary thyroid carcinomas, parathyroid adenomas, and gastric, colon, pancreatic, hepatocellular, cholangiocellular, urinary bladder, breast, cervical, and ovarian carcinomas.

CONCLUSIONS

We generated and characterised a novel rabbit polyclonal anti-human FFAR2 antibody that is well-suited for visualising FFAR2 expression in human routine pathology tissues. This antibody is also suitable for Western blot and immunocytochemistry experiments. To our knowledge, this antibody enabled the first broad FFAR2 protein expression profile in various normal and neoplastic human tissues.

Identification of a Novel SSTR3 Full Agonist for the Treatment of Nonfunctioning Pituitary Adenomas

Somatostatin receptor (SSTR) agonists have been extensively used for treating neuroendocrine tumors. Synthetic therapeutic agonists showing selectivity for SSTR2 (Octreotide) or for SSTR2 and SSTR5 (Pasireotide) have been approved for the treatment of patients with acromegaly and Cushing's syndrome, as their pituitary tumors highly express SSTR2 or SSTR2/SSTR5, respectively. Nonfunctioning pituitary adenomas (NFPAs), which express high levels of SSTR3 and show only modest response to currently available SSTR agonists, are often invasive and cannot be completely resected, and therefore easily recur. The aim of the present study was the evaluation of ITF2984, a somatostatin analog and full SSTR3 agonist, as a new potential treatment for NFPAs. ITF2984 shows a 10-fold improved affinity for SSTR3 compared to Octreotide or Pasireotide. Molecular modeling and NMR studies indicated that the higher affinity for SSTR3 correlates with a higher stability of a distorted β-I turn in the cyclic peptide backbone. ITF2984 induces receptor internalization and phosphorylation, and triggers G-protein signaling at pharmacologically relevant concentrations. Furthermore, ITF2984 displays antitumor activity that is dependent on SSTR3 expression levels in the MENX (homozygous mutant) NFPA rat model, which closely recapitulates human disease. Therefore, ITF2984 may represent a novel therapeutic option for patients affected by NFPA.

In situ visualization of opioid and cannabinoid drug effects using phosphosite-specific GPCR antibodies

G protein-coupled receptors (GPCRs) are important signal transducers that are phosphorylated upon activation at intracellular serine and threonine residues. Although antibodies that specifically recognize the phosphorylation state of GPCRs have been available for many years, efficient immunolocalization of phosphorylated receptors in their tissues of origin has not been possible. Here, we show that phosphorylation of receptors is highly unstable during routine immunohistochemical procedures, requiring the use of appropriate phosphatase inhibitors particular during tissue perfusion, post-fixation, and cryoprotection but not during immunostaining of tissue sections. We provide proof of concept using phosphorylation state-specific μ-opioid receptor (MOP) and cannabinoid receptor 1 (CB1) antibodies. Indeed, three of four well-characterized phosphosite-specific MOP antibodies, including pS375-MOP, pT376-MOP, and pT379-MOP, showed robust neuronal immunostaining in brain and spinal cord sections of opioid-treated mice only after inclusion of phosphatase inhibitors. We then extended this approach to the CB1 receptor and demonstrated that one of three newly-generated phosphosite-specific CB1 antibodies, namely pS425-CB1, showed striking staining of fibers and varicosities in brain slices from cannabinoid-treated mice. Although subsequent experiments showed that phospho-CB1 immunostaining was less sensitive to phosphatases, we conclude that the use of phosphatase inhibitors should always be considered in the development of immunohistochemical procedures for new phosphosite-specific GPCR antibodies. In summary, we anticipate that this improved protocol will facilitate the widespread use of phosphorylation state-specific antibodies to monitor the activation of endogenous GPCRs under physiological and pharmacological conditions. Our approach may also prove useful to confirm target engagement of GPCR drug candidates in native tissues.

A bead-based GPCR phosphorylation immunoassay for high-throughput ligand profiling and GRK inhibitor screening

Analysis of agonist-driven phosphorylation of G protein-coupled receptors (GPCRs) can provide valuable insights into the receptor activation state and ligand pharmacology. However, to date, assessment of GPCR phosphorylation using high-throughput applications has been challenging. We have developed and validated a bead-based immunoassay for the quantitative assessment of agonist-induced GPCR phosphorylation that can be performed entirely in multiwell cell culture plates. The assay involves immunoprecipitation of affinity-tagged receptors using magnetic beads followed by protein detection using phosphorylation state-specific and phosphorylation state-independent anti-GPCR antibodies. As proof of concept, five prototypical GPCRs (MOP, C5a1, D1, SST2, CB2) were treated with different agonizts and antagonists, and concentration-response curves were generated. We then extended our approach to establish selective cellular GPCR kinase (GRK) inhibitor assays, which led to the rapid identification of a selective GRK5/6 inhibitor (LDC8988) and a highly potent pan-GRK inhibitor (LDC9728). In conclusion, this versatile GPCR phosphorylation assay can be used extensively for ligand profiling and inhibitor screening.

A G protein-coupled receptors (GPCRs) phosphorylation assay for cell culture plates can be used for ligand profiling and inhibitor screening, as evidenced by the identification of two GRK inhibitor compounds.

Epigenetic-Like Stimulation of Receptor Expression in SSTR2 Transfected HEK293 Cells as a New Therapeutic Strategy

Simple Summary

Neuroendocrine tumors (NETs) expressing the somatostatin receptor subtype 2 (SSTR2) are promising targets for peptide receptor radionuclide therapy (PRRT) using the somatostatin analogue Lu-177-DOTATATE. Patients expressing low levels of SSTR2 do not benefit from PRRT. Therefore, an approach to increase the efficacy of PRRT utilizing the effects of 5-aza-2′-deoxycytidine (5-aza-dC) and valproic acid (VPA) on the SSTR2 expression levels is investigated. The cell lines HEKsst₂ and PC3 are incubated with 5-aza-dC and VPA in different combinations. The drug pretreatment of HEKsst₂ cells leads to increased Lu-177-DOTATATE uptake values (factor 28) and lower cell survival (factor 4) in comparison to unstimulated cells; in PC3 cells, the effects are negligible. Further, for the stimulated cell types, the maintenance of the intrinsic radiosensitivity in each cell type is confirmed by X-ray irradiation. The increased SSTR2 expression induced by VPA and 5-aza-dC stimulation in HEKsst₂ cells might improve treatment strategies for patients with NETs.

Abstract

The aim of the study was to increase the uptake of the SSTR2-targeted radioligand Lu-177-DOTATATE using the DNA methyltransferase inhibitor (DNMTi) 5-aza-2′-deoxycytidine (5-aza-dC) and the histone deacetylase inhibitor (HDACi) valproic acid (VPA). The HEKsst₂ and PC3 cells were incubated with variable concentrations of 5-aza-dC and VPA to investigate the uptake of Lu-177-DOTATATE. Cell survival, subsequent to external X-rays (0.6 or 1.2 Gy) and a 24 h incubation with 57.5 or 136 kBq/mL Lu-177-DOTATATE, was investigated via colony formation assay to examine the effect of the epidrugs. In the case of stimulated HEKsst₂ cells, the uptake of Lu-177-DOTATATE increased by a factor of 28 in comparison to the unstimulated cells. Further, stimulated HEKsst₂ cells demonstrated lower survival fractions (factor 4). The survival fractions of the PC3 cells remained almost unchanged. VPA and 5-aza-dC did not induce changes to the intrinsic radiosensitivity of the cells after X-ray irradiation. Clear stimulatory effects on HEKsst₂ cells were demonstrated by increased cell uptake of the radioligand and enhanced SST2 receptor quantity. In conclusion, the investigated approach is suitable to stimulate the somatostatin receptor expression and thus the uptake of Lu-177-DOTATATE, enabling a more efficient treatment for patients with poor response to peptide radionuclide therapy (PRRT).

Pasireotide in the Personalized Treatment of Acromegaly

The delay in controlling the disease in patients who do not respond to first-line treatment with first generation somatostatin receptor ligands (first-generation SRLs) can be quantified in years, as every modification in the medical therapy requires some months to be fully evaluated. Considering this, acromegaly treatment should benefit from personalized medicine therapeutic approach by using biomarkers identifying drug response. Pasireotide has been positioned mostly as a compound to be used in first-generation SRLs resistant patients and after surgical failure, but sufficient data are now available to indicate it is a first line therapy for patients with certain characteristics. Pasireotide has been proved to be useful in patients in which hyperintensity T2 MRI signal is shown and in those depicting low SST2 and high expression of SST5, low or mutated AIP condition and sparsely granulated immunohistochemical pattern. This combination of clinical and pathological characteristics is unique for certain patients and seems to cluster in the same cases, strongly suggesting an etiopathogenic link. Thus, in this paper we propose to include this clinico-pathologic phenotype in the therapeutic algorithm, which would allow us to use as first line medical treatment those compounds with the highest potential for achieving the fastest control of GH hypersecretion as well as a positive effect upon tumor shrinkage, therefore accelerating the implementation of precision medicine for acromegaly. Moreover, we suggest the development, validation and clinical use of a pasireotide acute test, able to identify patients responsive to pasireotide LAR as the acute octreotide test is able to do for SRLs.

Agonist-induced phosphorylation bar code and differential post-activation signaling of the delta opioid receptor revealed by phosphosite-specific antibodies

The δ-opioid receptor (DOP) is an attractive pharmacological target due to its potent analgesic, anxiolytic and anti-depressant activity in chronic pain models. However, some but not all selective DOP agonists also produce severe adverse effects such as seizures. Thus, the development of novel agonists requires a profound understanding of their effects on DOP phosphorylation, post-activation signaling and dephosphorylation. Here we show that agonist-induced DOP phosphorylation at threonine 361 (T361) and serine 363 (S363) proceeds with a temporal hierarchy, with S363 as primary site of phosphorylation. This phosphorylation is mediated by G protein-coupled receptor kinases 2 and 3 (GRK2/3) followed by DOP endocytosis and desensitization. DOP dephosphorylation occurs within minutes and is predominantly mediated by protein phosphatases (PP) 1α and 1β. A comparison of structurally diverse DOP agonists and clinically used opioids demonstrated high correlation between G protein-dependent signaling efficacies and receptor internalization. In vivo, DOP agonists induce receptor phosphorylation in a dose-dependent and agonist-selective manner that could be blocked by naltrexone in DOP-eGFP mice. Together, our studies provide novel tools and insights for ligand-activated DOP signaling in vitro and in vivo and suggest that DOP agonist efficacies may determine receptor post-activation signaling.

Agonist-selective NOP receptor phosphorylation correlates in vitro and in vivo and reveals differential post-activation signaling by chemically diverse agonists

Agonists of the nociceptin/orphanin FQ opioid peptide (NOP) receptor, a member of the opioid receptor family, are under active investigation as novel analgesics, but their modes of signaling are less well characterized than those of other members of the opioid receptor family. Therefore, we investigated whether different NOP receptor ligands showed differential signaling or functional selectivity at the NOP receptor. Using newly developed phosphosite-specific antibodies to the NOP receptor, we found that agonist-induced NOP receptor phosphorylation occurred primarily at four carboxyl-terminal serine (Ser) and threonine (Thr) residues, namely, Ser³⁴⁶, Ser³⁵¹, Thr³⁶², and Ser³⁶³, and proceeded with a temporal hierarchy, with Ser³⁴⁶ as the first site of phosphorylation. G protein-coupled receptor kinases 2 and 3 (GRK2/3) cooperated during agonist-induced phosphorylation, which, in turn, facilitated NOP receptor desensitization and internalization. A comparison of structurally distinct NOP receptor agonists revealed dissociation in functional efficacies between G protein-dependent signaling and receptor phosphorylation. Furthermore, in NOP-eGFP and NOP-eYFP mice, NOP receptor agonists induced multisite phosphorylation and internalization in a dose-dependent and agonist-selective manner that could be blocked by specific antagonists. Our study provides new tools to study ligand-activated NOP receptor signaling in vitro and in vivo. Differential agonist-selective NOP receptor phosphorylation by chemically diverse NOP receptor agonists suggests that differential signaling by NOP receptor agonists may play a role in NOP receptor ligand pharmacology.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

G Protein-Coupled Receptor Resensitization Paradigms

Cellular responses to extracellular milieu/environment are driven by cell surface receptors that transmit the signal into the cells resulting in a synchronized and measured response. The ability to provide such exquisite responses to changes in external environment is mediated by the tight and yet, deliberate regulation of cell surface receptor function. In this regard, the seven transmembrane G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that regulate responses like cardiac contractility, vision, and olfaction including platelet activation. GPCRs regulate these plethora of events through GPCR-activation, -desensitization, and -resensitization. External stimuli (ligands or agonists) activate GPCR initiating downstream signals. The activated GPCR undergoes inactivation or desensitization by phosphorylation and binding of β-arrestin resulting in diminution of downstream signals. The desensitized GPCRs are internalized into endosomes, wherein they undergo dephosphorylation or resensitization by protein phosphatase to be recycled back to the cell membrane as naïve GPCR ready for the next wave of stimuli. Despite the knowledge that activation, desensitization, and resensitization shoulder an equal role in maintaining GPCR function, major advances have been made in understanding activation and desensitization compared to resensitization. However, increasing evidence shows that resensitization is exquisitely regulated process, thereby contributing to the dynamic regulation of GPCR function. In recognition of these observations, in this chapter we discuss the key advances on the mechanistic underpinning that drive and regulate GPCR function with a focus on resensitization.

Functional Amyloids and their Possible Influence on Alzheimer Disease

Amyloids play critical roles in human diseases but have increasingly been recognized to also exist naturally. Shared physicochemical characteristics of amyloids and of their smaller oligomeric building blocks offer the prospect of molecular interactions and crosstalk amongst these assemblies, including the propensity to mutually influence aggregation. A case in point might be the recent discovery of an interaction between the amyloid β peptide (Aβ) and somatostatin (SST). Whereas Aβ is best known for its role in Alzheimer disease (AD) as the main constituent of amyloid plaques, SST is intermittently stored in amyloid-form in dense core granules before its regulated release into the synaptic cleft. This review was written to introduce to readers a large body of literature that surrounds these two peptides. After introducing general concepts and recent progress related to our understanding of amyloids and their aggregation, the review focuses separately on the biogenesis and interactions of Aβ and SST, before attempting to assess the likelihood of encounters of the two peptides in the brain, and summarizing key observations linking SST to the pathobiology of AD. While the review focuses on Aβ and SST, it is to be anticipated that crosstalk amongst functional and disease-associated amyloids will emerge as a general theme with much broader significance in the etiology of dementias and other amyloidosis.

Effect of AP102, a subtype 2 and 5 specific somatostatin analog, on glucose metabolism in rats

PURPOSE

Somatostatin analogs are widely used to treat conditions associated with hormonal hypersecretion such as acromegaly and metastatic neuroendocrine tumors. First generation somatostatin analogs, such as octreotide and lanreotide, have high affinity for somatostatin receptor subtype 2 (SSTR2), but have incomplete efficacy in many patients. Pasireotide targets multiple SSTRs, having the highest affinity for SSTR5, but causes hyperglycemia and diabetes mellitus in preclinical and clinical studies. AP102 is a new somatostatin analogs with high affinity at both SSTR2 and SSTR5. We aimed to characterize the effects of AP102 vs. pasireotide on random and dynamic glucose levels, glucoregulatory hormone concentrations and growth axis measures in healthy Sprague-Dawley rats.

METHODS

Three doses of each compound were evaluated under acute conditions (1, 10, and 30 µg/kg s.c.), and two doses during a chronic (4-week) infusion (3 and 10 µg/kg/h s.c.).

RESULTS

Neither acute nor chronic AP102 administration altered blood glucose concentrations or dynamic responses following an intraperitoneal glucose tolerance test. In contrast, acute and chronic pasireotide dosing increased random and post-intraperitoneal glucose tolerance test blood glucose measures, compared to vehicle-treated controls. Both AP102 and pasireotide acutely suppressed growth hormone levels, although insulin-like growth factor-1 and somatic growth was suppressed to a greater extent with pasireotide.

CONCLUSIONS

AP102 is a new dual SSTR2/SSTR5-specific somatostatin analog that acutely reduces growth hormone but does not cause hyperglycemia during acute or chronic administration in a healthy rat model. Further studies in diabetic animals and in humans are necessary to determine the potential utility of AP102 in the clinical setting.

Paradoxical and atypical responses to pasireotide in aggressive ACTH-secreting pituitary tumors

PURPOSE

Pasireotide is the only pituitary targeted medication registered for the treatment of Cushing's disease. Drug efficacy data are largely based on a major prospective study in which the vast majority of patients had microadenomas. The purpose of this study was to summarize results of pasireotide treatment of ACTH secreting macroadenomas from our center.

METHODS

Retrospective review of data extracted from clinical files.

RESULTS

Three patients presented with large and invasive macroadenomas that required several surgical interventions and radiotherapy treatments. Patient 1 is a 57 year-old male who developed an extreme (27-fold) paradoxical response of urinary free cortisol (UFC) levels as measured 2 weeks after pasireotide institution, which increased further (71-fold) in response to dose increment but decreased to baseline levels after treatment interruption. Patient 2 is a 44 year old woman with a long standing (26 years) ACTH-secreting carcinoma metastatic to bone and after bilateral adrenalectomy. After an initial excellent response to pasireotide treatment, ACTH levels escaped suppression and a further rebound was noted 6 weeks after treatment interruption. Patient 3 is a 53 year old man that after escape from temozolomide therapy was started on pasireotide and rapidly responded by almost normalizing UFC excretion after 4 weeks, but returned to baseline UFC levels after four additional weeks of treatment.

CONCLUSIONS

We describe as yet unreported atypical responses to pasireotide treatment in patients with aggressive ACTH-secreting tumors. Increased vigilance is recommended during pasireotide treatment of such patients.

Somatostatin regulates NHE8 protein expression via the ERK1/2 MAPK pathway in DSS-induced colitis mice

Previous studies reported that administration of somatostatin (SST) to human patients mitigated their diarrheal symptoms. Octreotide (an analog of SST) treatment in animals resulted in upregulation of sodium/hydrogen exchanger 8 (NHE8). NHE8 is important for water/sodium absorption in the intestine, and loss of NHE8 function results in mucosal injury. Thus we hypothesized that NHE8 expression is inhibited during colitis and that SST treatment during pathological conditions can restore NHE8 expression. Our data showed for the first time that NHE8 is expressed in the human colonic tissue and that NHE8 expression is decreased in ulcerative colitis (UC) patients. We also found that octreotide could stimulate colonic NHE8 expression in colitic mice. Furthermore, the somatostatin receptor 2 (SSTR2) agonist seglitide and the somatostatin receptor 5 (SSTR5) agonist L-817,818 could restore NHE8 expression via its role in suppressing ERK1/2 phosphorylation. Our study uncovered a novel mechanism of SST stimulation of NHE8 expression in colitis.

Molecular Mechanisms Linking Autonomic Dysfunction and Impaired Cardiac Contractility in Critical Illness

OBJECTIVES

Molecular mechanisms linking autonomic dysfunction with poorer clinical outcomes in critical illness remain unclear. We hypothesized that baroreflex dysfunction alone is sufficient to cause cardiac impairment through neurohormonal activation of (nicotinamide adenine dinucleotide phosphate oxidase dependent) oxidative stress resulting in increased expression of G-protein-coupled receptor kinase 2, a key negative regulator of cardiac function.

DESIGN

Laboratory/clinical investigations.

SETTING

University laboratory/medical centers.

SUBJECTS

Adult rats; wild-type/nicotinamide adenine dinucleotide phosphate oxidase subunit-2-deficient mice; elective surgical patients.

INTERVENTIONS

Cardiac performance was assessed by transthoracic echocardiography following experimental baroreflex dysfunction (sino-aortic denervation) in rats and mice. Immunoblots assessed G-protein-coupled receptor recycling proteins expression in rodent cardiomyocytes and patient mononuclear leukocytes. In surgical patients, heart rate recovery after cardiopulmonary exercise testing, time/frequency measures of parasympathetic variables were related to the presence/absence of baroreflex dysfunction (defined by spontaneous baroreflex sensitivity of <6 ms mm Hg). The associations of baroreflex dysfunction with intraoperative cardiac function and outcomes were assessed.

MEASUREMENTS AND MAIN RESULTS

Experimental baroreflex dysfunction in rats and mice resulted in impaired cardiac contractility and upregulation of G-protein-coupled receptor kinase 2 expression. In mice, genetic deficiency of gp91 nicotinamide adenine dinucleotide phosphate oxidase subunit-2 prevented upregulation of G-protein-coupled receptor kinase 2 expression in conditions of baroreflex dysfunction and preserved cardiac function. Baroreflex dysfunction was present in 81 of 249 patients (32.5%) and was characterized by lower parasympathetic tone and increased G-protein-coupled receptor kinase 2 expression in mononuclear leukocytes. Baroreflex dysfunction in patients was also associated with impaired intraoperative cardiac contractility. Critical illness and mortality were more frequent in surgical patients with baroreflex dysfunction (relative risk, 1.66 [95% CI, 1.16-2.39]; p = 0.006).

CONCLUSIONS

Reduced baroreflex sensitivity is associated with nicotinamide adenine dinucleotide phosphate oxidase subunit-2-mediated upregulation of G-protein-coupled receptor kinase 2 expression in cardiomyocytes and impaired cardiac contractility. Autonomic dysfunction predisposes patients to the development of critical illness and increases mortality.

Identification of Phosphorylation Sites Regulating sst3 Somatostatin Receptor Trafficking

The human somatostatin receptor 3 (sst3) is expressed in about 50% of all neuroendocrine tumors and hence a promising target for multireceptor somatostatin analogs. The sst3 receptor is unique among ssts in that it exhibits a very long intracellular C-terminal tail containing a huge number of potential phosphate acceptor sites. Consequently, our knowledge about the functional role of the C-terminal tail in sst3 receptor regulation is very limited. Here, we have generated a series of phosphorylation-deficient mutants that enabled us to determine crucial sites for its agonist-induced β-arrestin mobilization, internalization, and down-regulation. Based on this information, we generated phosphosite-specific antibodies for C-terminal Ser(337)/Thr(341), Thr(348), and Ser(361) that enabled us to investigate the temporal patterns of sst3 phosphorylation and dephosphorylation. We found that the endogenous ligand somatostatin induced a rapid and robust phosphorylation that was completely blocked by the sst3 antagonist NVP-ACQ090. The stable somatostatin analogs pasireotide and octreotide promoted clearly less phosphorylation compared with somatostatin. We also show that sst3 phosphorylation occurred within seconds to minutes, whereas dephosphorylation of the sst3 receptor occurred at a considerable slower rate. In addition, we also identified G protein-coupled receptor kinases 2 and 3 and protein phosphatase 1α and 1β as key regulators of sst3 phosphorylation and dephosphorylation, respectively. Thus, we here define the C-terminal phosphorylation motif of the human sst3 receptor that regulates its agonist-promoted phosphorylation, β-arrestin recruitment, and internalization of this clinically relevant receptor.

Protein phosphatase modulation of somatostatin receptor signaling in the mouse hippocampus

Many inhibitory interneurones in the hippocampus release the neuropeptide somatostatin (SST) which inhibits neuronal excitability through Gi/Go-coupled receptors. To investigate the signaling pathways underlying the SST inhibition of neuronal excitability in the hippocampus, we performed perforated patch-clamp recordings from CA1 pyramidal neurones in acute brain slices from P14-P18 mice. Bath application of 1 μM SST reversibly reduces the frequency of action potential firing in response to depolarising current steps, and is associated with neuronal hyperpolarisation and a reduction in membrane resistance. This effect is mediated by potassium channels with KCNK-like pharmacology. In addition, in slices that have been cultured in vitro for seven days or more, SST also produces a hyperpolarisation independent reduction in action potential firing, which can be also observed in acute slices when the Ser/Thr protein phosphatases PP2A and PP4 are inhibited selectively with fostriecin. This hyperpolarisation independent effect of SST appears to be mediated by G-protein-activated inwardly rectifying K+ (GIRK) channels. Knockdown of protein phosphatase 5, by Cre recombinase mediated deletion of the floxed Ppp5c gene, blocks the hyperpolarisation independent effect of SST, and reduces the hyperpolarisation dependent effect in a manner consistent with increased SST receptor desensitisation. Thus, reversible protein phosphorylation provides a mechanism to enhance or diminish the inhibitory effect of SST, which could allow system level regulation of circuit excitability in the hippocampus.

Reassessment of endothelin receptor A expression in normal and neoplastic human tissues using the novel rabbit monoclonal antibody UMB-8

The endothelins ET-1, ET-2 and ET-3 are a family of peptides, which exert their actions via two G protein-coupled receptor subtypes, ETA and ETB. ET-1 is a potent vasoconstrictor and is involved in the development of different cardiovascular and renal disorders. Additionally, ET-1 and the ETA receptor have been shown to be important mediators of cancer growth and metastasis. We have extensively characterized the novel monoclonal rabbit anti-ETA antibody UMB-8 using transfected cells as well as mouse, rat and human tissues. UMB-8 was then tested in a large series of formalin-fixed and paraffin-embedded human normal and neoplastic tissue specimens. Specificity of UMB-8 was demonstrated by detection of a broad band migrating at 70-80kDa in Western blot analyses of ETA-transfected HEK-293 cells and of different mouse tissues and by agonist-dependent translocation of the immunosignal from the surface of ETA-transfected cells into the cytosol. In tissue samples, UMB-8 yielded an efficient immunostaining of distinct cell populations with a predominance of plasma membrane staining, which was abolished after preadsorption of the antibody with its immunizing peptide. In normal tissue, ETA was present in the heart, blood vessels, gut and kidneys. ETA was also detected with a hitherto unappreciated high prevalence in all types of sarcomas and in glioblastomas, but also in various epithelial tumor entities and in tumor stroma. All in all, UMB-8 may prove of great value in the identification of ETA-expressing tumors during routine histopathological examinations.

Fine-tuning somatostatin receptor signalling by agonist-selective phosphorylation and dephosphorylation: IUPHAR Review 5

The biological actions of somatostatin are mediated by a family of five GPCRs, named sst1 to sst5 . Somatostatin receptors exhibit equally high-binding affinities to their natural ligand somatostatin-14 and largely overlapping distributions. The overexpression of somatostatin receptors in human tumours is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analogues octreotide, lanreotide and pasireotide. The efficiency of somatostatin receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation and dephosphorylation of intracellular carboxyl-terminal serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific antibodies for human sst2 and sst5 receptors. These phosphosite-specific antibodies are unique tools to monitor the spatial and temporal dynamics of receptors phosphorylation and dephosphorylation. Using a combined approach of phosphosite-specific antibodies and siRNA knock-down screening, relevant kinases and phosphatases were identified. Emerging evidence suggests distinct mechanisms of agonist-selective fine-tuning for individual somatostatin receptors. The recently uncovered differences in phosphorylation and dephosphorylation of these receptors may hence be of physiological significance in mediating responses to acute, persistent or repeated stimuli in a variety of target tissues.

Carboxyl-terminal receptor domains control the differential dephosphorylation of somatostatin receptors by protein phosphatase 1 isoforms

We have recently identified protein phosphatase 1β (PP1β) as G protein-coupled receptor (GPCR) phosphatase for the sst₂ somatostatin receptor using siRNA knockdown screening. By contrast, for the sst₅ somatostatin receptor we identified protein phosphatase 1γ (PP1γ) as GPCR phosphatase using the same approach. We have also shown that sst₂ and sst₅ receptors differ substantially in the temporal dynamics of their dephosphorylation and trafficking patterns. Whereas dephosphorylation and recycling of the sst₂ receptor requires extended time periods of ∼30 min, dephosphorylation and recycling of the sst₅ receptor is completed in less than 10 min. Here, we examined which receptor domains determine the selection of phosphatases for receptor dephosphorylation. We found that generation of tail-swap mutants between sst₂ and sst₅ was required and sufficient to reverse the patterns of dephosphorylation and trafficking of these two receptors. In fact, siRNA knockdown confirmed that the sst₅ receptor carrying the sst₂ tail is predominantly dephosphorylated by PP1β, whereas the sst₂ receptor carrying the sst₅ tail is predominantly dephosphorylated by PP1γ. Thus, the GPCR phosphatase responsible for dephosphorylation of individual somatostatin receptor subtypes is primarily determined by their different carboxyl-terminal receptor domains. This phosphatase specificity has in turn profound consequences for the dephosphorylation dynamics and trafficking patterns of GPCRs.

Somatostatin analogs: does pharmacology impact antitumor efficacy?

Somatostatin is an endogenous inhibitor of secretion and cell proliferation. These features render somatostatin a logical candidate for the management of neuroendocrine tumors that express somatostatin receptors. Synthetic somatostatin analogs (SSAs) have longer half-lives than somatostatin, but have similar activities, and are used for the treatment of these types of disorders. Interest has focused on novel multireceptor analogs with broader affinity to several of the five somatostatin receptors, thereby presenting putatively higher antitumor activities. Recent evidence indicates that SSAs cannot be considered mimics of native somatostatin in regulating signaling pathways downstream of receptors. Here we review this knowledge, discuss the concept of biased agonism, and highlight what considerations need to be taken into account for the optimal clinical use of SSAs.

Illuminating somatostatin analog action at neuroendocrine tumor receptors

Somatostatin analogs for the diagnosis and therapy of neuroendocrine tumors (NETs) have been used in clinical applications for more than two decades. Five somatostatin receptor subtypes have been identified and molecular mechanisms of somatostatin receptor signaling and regulation have been elucidated. These advances increased understanding of the biological role of each somatostatin receptor subtype, their distribution in NETs, as well as agonist-specific regulation of receptor signaling, internalization, and phosphorylation, particularly for the sst2 receptor subtype, which is the primary target of current somatostatin analog therapy for NETs. Various hypotheses exist to explain differences in patient responsiveness to somatostatin analog inhibition of tumor secretion and growth as well as differences in the development of tumor resistance to therapy. In addition, we now have a better understanding of the action of both first generation (octreotide, lanreotide, Octreoscan) and second generation (pasireotide) FDA-approved somatostatin analogs, including the biased agonistic character of some agonists. The increased understanding of somatostatin receptor pharmacology provides new opportunities to design more sophisticated assays to aid the future development of somatostatin analogs with increased efficacy.

Regulated Control of Melanin-Concentrating Hormone Receptor 1 through Posttranslational Modifications

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that plays an important role in feeding behavior. It activates two G-protein-coupled receptors, MCHR1 and MCHR2, of which MCHR1 is the primary regulator of food intake and energy homeostasis in rodents. In mammalian cells transfected with MCHR1, MCH is able to activate multiple signaling pathways including calcium mobilization, extracellular signal-regulated kinase activation, and inhibition of cyclic AMP generation through Gi/o- and Gq-coupled pathways. Further evidence suggests that MCHR1 is regulated through posttranslational modifications, which control its intracellular localization and provide appropriate cellular responses involving G-protein signaling. This review summarizes the current data on the control of MCHR1 function through glycosylation and phosphorylation, as related to cell function. Especially, a series of mutagenesis study highlights the importance of complete glycosylation of MCHR1 for efficient trafficking to the plasma membrane.