Inhibition of cell proliferation by the somatostatin analogue RC-160 is mediated by somatostatin receptor subtypes SSTR2 and SSTR5 through different mechanisms

Structural basis for activation of somatostatin receptor 5 by cyclic neuropeptide agonists

Somatostatin receptor 5 (SSTR5) is an important G protein-coupled receptor and drug target for neuroendocrine tumors and pituitary disorders. This study presents two high-resolution cryogenicelectron microscope structures of the SSTR5-Gi complexes bound to the cyclic neuropeptide agonists, cortistatin-17 (CST17) and octreotide, with resolutions of 2.7 Å and 2.9 Å, respectively. The structures reveal that binding of these peptides causes rearrangement of a "hydrophobic lock", consisting of residues from transmembrane helices TM3 and TM6. This rearrangement triggers outward movement of TM6, enabling Gαi protein engagement and receptor activation. In addition to hydrophobic interactions, CST17 forms conserved polar contacts similar to somatostatin-14 binding to SSTR2, while further structural and functional analysis shows that extracellular loops differently recognize CST17 and octreotide. These insights elucidate agonist selectivity and activation mechanisms of SSTR5, providing valuable guidance for structure-based drug development targeting this therapeutically relevant receptor.

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

Regulation of FGF2-induced proliferation by inhibitory GPCR in normal pituitary cells

Introduction

Intracellular communication is essential for the maintenance of the anterior pituitary gland plasticity. The aim of this study was to evaluate whether GPCR-Gαi modulates basic fibroblast growth factor (FGF2)-induced proliferative activity in normal pituitary cell populations.

Methods

Anterior pituitary primary cell cultures from Wistar female rats were treated with FGF2 (10ng/mL) or somatostatin analog (SSTa, 100nM) alone or co-incubated with or without the inhibitors of GPCR-Gαi, pertussis toxin (PTX, 500nM), MEK inhibitor (U0126, 100µM) or PI3K inhibitor (LY 294002, 10 μM).

Results

FGF2 increased and SSTa decreased the lactotroph and somatotroph BrdU uptak2e (p<0.05) whereas the FGF2-induced S-phase entry was prevented by SSTa co-incubation in both cell types, with these effects being reverted by PTX, U0126 or LY294002 pre-incubation. The inhibition of lactotroph and somatotroph mitosis was associated with a downregulation of c-Jun expression, a decrease of phosphorylated (p) ERK and pAKT. Furthermore, SSTa was observed to inhibit the S-phase entry induced by FGF2, resulting in a further increase in the number of cells in the G1 phase and a concomitant reduction in the number of cells in the S phases (p< 0.05), effects related to a decrease of cyclin D1 expression and an increase in the expression of the cell cycle inhibitors p27 and p21.

Discussion

In summary, the GPCR-Gαi activated by SSTa blocked the pro-proliferative effect of FGF2 in normal pituitary cells via a MEK-dependent mechanism, which acts as a mediator of both anti and pro-mitogenic signals, that may regulate the principal effectors of the G1 to S-phase transition.

Transcriptomic analyses of NeuroD1-mediated astrocyte-to-neuron conversion

Ectopic expression of a single neural transcription factor NeuroD1 can reprogram reactive glial cells into functional neurons both in vitro and in vivo, but the underlying mechanisms are not well understood yet. Here, we used RNA-sequencing technology to capture the transcriptomic changes at different time points during the reprogramming process. We found that following NeuroD1 overexpression, astroglial genes (ACTG1, ALDH1A3, EMP1, CLDN6, SOX21) were significantly downregulated, whereas neuronal genes (DCX, RBFOX3/NeuN, CUX2, RELN, SNAP25) were significantly upregulated. NeuroD family members (NeuroD1/2/6) and signaling pathways (Wnt, MAPK, cAMP) as well as neurotransmitter receptors (acetylcholine, somatostatin, dopamine) were also significantly upregulated. Gene co-expression analysis identified many central genes among the NeuroD1-interacting network, including CABP7, KIAA1456, SSTR2, GADD45G, LRRTM2, and INSM1. Compared to chemical conversion, we found that NeuroD1 acted as a strong driving force and triggered fast transcriptomic changes during astrocyte-to-neuron conversion process. Together, this study reveals many important downstream targets of NeuroD1 such as HES6, BHLHE22, INSM1, CHRNA1/3, CABP7, and SSTR2, which may play critical roles during the transcriptomic landscape shift from a glial profile to a neuronal profile.

Invasive Breast Carcinoma with Neuroendocrine Differentiation: A Single-Center Analysis of Clinical Features and Prognosis

Introduction Invasive breast cancer with neuroendocrine differentiation is a rare subtype of breast malignancy. Due to frequent changes in the definition of these lesions, the correct diagnosis, estimation of exact prevalence, and clinical behaviour of this entity may be challenging. The aim of this study was to evaluate the prevalence, clinical features, and outcomes in a large cohort of patients with breast cancer with neuroendocrine differentiation.

Patients Twenty-seven cases of breast cancer with neuroendocrine differentiation have been included in this analysis. Twenty-one cases were identified by systematic immunohistochemical re-evaluation of 465 breast cancer specimens using the neuroendocrine markers chromogranin A and synaptophysin, resulting in a prevalence of 4.5%. A further six cases were identified by a review of clinical records.

Results Median age at the time of diagnosis was 61 years. 70% of patients had T2 – 4 tumors and 37% were node-positive. The most common immunohistochemical subtype was HR-positive/HER2-negative (85%). 93% were positive for synaptophysin and 48% for chromogranin A. Somatostatin receptor type 2A status was positive in 12 of 24 analyzed tumors (50%). Neuroendocrine-specific treatment with somatostatin analogues was administered in two patients. The 5-year survival rate was 70%.

Conclusions Breast cancer with neuroendocrine differentiation is mostly HR-positive/HER2-negative and the diagnosis is made at a higher TNM stage than in patients with conventional invasive breast carcinoma. Moreover, breast cancer with neuroendocrine differentiation was found to be associated with impaired prognosis in several retrospective trials. Due to somatostatin receptor 2A expression, somatostatin receptor-based imaging can be used and somatostatin receptor-targeted therapy can be offered in selected cases.

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.

Identification of key modules and genes associated with breast cancer prognosis using WGCNA and ceRNA network analysis

Breast cancer is one of the leading causes of cancer-associated mortality in women worldwide and has become a major public health problem. Although the definitive cause of breast cancer is not known, many genes sensitive to breast cancer have been detected using advanced technologies. Our study identified 3301 differentially expressed lncRNAs and mRNAs between tumor and normal samples from The Cancer Genome Atlas database. Based on the gene expression analysis and clinical traits as well as weighted gene co-expression network analysis, the co-expression Brown module was found to be key for breast cancer prognosis. A total of 453 genes in the Brown module were used for functional enrichment, protein-protein interaction analysis, lncRNA-miRNA-mRNA ceRNA network, and lncRNA-RNA binding protein-mRNA network construction. GRM4, SSTR2, PARD6B, PRR15, COX6C, and lncRNA DSCAM-AS1 were the hub genes according to protein-protein interaction, lncRNA-miRNA-mRNA and lncRNA-RNA binding protein-mRNA network. Their high expression was found to be correlated with breast cancer development, according to multiple databases. In conclusion, this study provides a framework of the co-expression gene modules of breast cancer and identifies several important biomarkers in breast cancer development and prognosis.

A new 68Ga-labeled somatostatin analog containing two iodo-amino acids for dual somatostatin receptor subtype 2 and 5 targeting

Background

Somatostatin receptor (SST) targeting, specifically of the subtype 2 (SST2), with radiolabeled somatostatin analogs, is established for imaging and treatment of neuroendocrine tumors. Owing to the concomitant and heterogeneous expression of several subtypes on the same tumor, analogs targeting more subtypes than SST2 potentially target a broader spectrum of tumors and/or increase the uptake of a given tumor. The analog ST8950 ((4-amino-3-iodo)-d-Phe-c[Cys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH₂), bearing 2 iodo-amino acids, exhibits sub-nanomolar affinity to SST2 and SST5. We report herein the development and preclinical evaluation of DOTA-ST8950 labeled with ⁶⁸Ga, for imaging SST2- and SST5-expressing tumors. Comparative in vitro and in vivo studies were performed with the de-iodinated DOTA-ST8951 ((4-amino)-d-Phe-c[Cys-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH₂) and with the reference compounds DOTA-TATE (SST2 selective) and DOTA-NOC (for SST2 and SST5).

Results

Compared with ^natGa-DOTA-NOC, ^natGa-DOTA-ST8950 exhibited higher affinity to SST2 and SST5 (IC₅₀ (95%CI), nM = 0.32 (0.20–0.50) and 1.9 (1.1–3.1) vs 0.70 (0.50-0.96) and 3.4 (1.8-6.2), respectively), while ^natGa-DOTA-ST8951 lost affinity for both subtypes. ^natGa-DOTA-ST8950 had the same potency for inducing SST2-mediated cAMP accumulation as ^natGa-DOTA-TATE and slightly better than ^natGa-DOTA-NOC (EC₅₀, nM = 0.46 (0.23–0.92) vs 0.47 (0.15–1.5) vs 0.59 (0.18–1.9), respectively). [⁶⁷Ga]Ga-DOTA-ST8950 had a similar internalization rate as [⁶⁷Ga]Ga-DOTA-NOC in SST2-expressing cells (12.4 ± 1.6% vs 16.6 ± 2.2%, at 4 h, p = 0.0586). In vivo, [⁶⁸Ga]Ga-DOTA-ST8950 showed high and specific accumulation in SST2- and SST5-expressing tumors, comparable with [⁶⁸Ga]Ga-DOTA-NOC (26 ± 8 vs 30 ± 8 %IA/g, p = 0.4630 for SST2 and 15 ± 6 vs 12 ± 5 %IA/g, p = 0.3282, for SST5, 1 h p.i.) and accumulation in the SST-positive tissues, the kidneys and the liver. PET/CT images of [⁶⁸Ga]Ga-DOTA-ST8950, performed in a dual HEK-SST2 and HEK-SST5 tumor xenografted model, clearly visualized both tumors and illustrated high tumor-to-background contrast.

Conclusions

[⁶⁸Ga]Ga-DOTA-ST8950 reveals its potential for PET imaging SST2- and SST5-expressing tumors. It compares favorably with the clinically used [⁶⁸Ga]Ga-DOTA-NOC in terms of tumor uptake; however, its uptake in the liver remains a challenge for clinical translation. In addition, this study reveals the essential role of the iodo-substitutions in positions 1 and 3 of [⁶⁸Ga]Ga-DOTA-ST8950 for maintaining affinity to SST2 and SST5, as the de-iodinated [⁶⁸Ga]Ga-DOTA-ST8951 lost affinity for both receptor subtypes.

The effect of strength and endurance training on SSTR2 and SSTR5 in patients with colorectal cancer

The roots of apoptosis caused by the use of certain drugs in patients with colorectal cancer and certain drugs could be induced the change in genes SSTR2 and SSTR5. SSTRs play a role in colorectal cancer and it’s also known that exercise training has similar effects to certain drugs. Therefore, this study aims to evaluate the effects of a combined training program on hormonal and physiological changes in patients with colorectal cancer. Twenty patients with colorectal cancer were included in this study and divided into two groups as experimental (n=10) and control (n=10). The experimental group performed an exercise training program 5 days a week for 14 weeks. Blood samples were taken from the patients before and after the training program, and analyzed for somatostatin and its receptors. When the pretest effect excluded as statistical, there were no significant differences in SSRT2 (P>0.05) and SSRT5 (P>0.05) between the experimental and control groups. The hypothesis for this study was “SSTR2 and SSTR5 will improve after combined training program” however, in accordance with the results it was rejected. It seems that the determined training program for these patients need to be extended and performed for at least 3 months in a gradual incremental structure so that it could affect their health indicators significantly.

Somatostatin Analogues in the Treatment of Neuroendocrine Tumors: Past, Present and Future

In recent decades, the incidence of neuroendocrine tumors (NETs) has steadily increased. Due to the slow-growing nature of these tumors and the lack of early symptoms, most cases are diagnosed at advanced stages, when curative treatment options are no longer available. Prognosis and survival of patients with NETs are determined by the location of the primary lesion, biochemical functional status, differentiation, initial staging, and response to treatment. Somatostatin analogue (SSA) therapy has been a mainstay of antisecretory therapy in functioning neuroendocrine tumors, which cause various clinical symptoms depending on hormonal hypersecretion. Beyond symptomatic management, recent research demonstrates that SSAs exert antiproliferative effects and inhibit tumor growth via the somatostatin receptor 2 (SSTR2). Both the PROMID (placebo-controlled, prospective, randomized study in patients with metastatic neuroendocrine midgut tumors) and the CLARINET (controlled study of lanreotide antiproliferative response in neuroendocrine tumors) trial showed a statistically significant prolongation of time to progression/progression-free survival (TTP/PFS) upon SSA treatment, compared to placebo. Moreover, the combination of SSA with peptide receptor radionuclide therapy (PRRT) in small intestinal NETs has proven efficacy in the phase 3 neuroendocrine tumours therapy (NETTER 1) trial. PRRT is currently being tested for enteropancreatic NETs versus everolimus in the COMPETE trial, and the potential of SSTR-antagonists in PRRT is now being evaluated in early phase I/II clinical trials. This review provides a synopsis on the pharmacological development of SSAs and their use as antisecretory drugs. Moreover, this review highlights the clinical evidence of SSAs in monotherapy, and in combination with other treatment modalities, as applied to the antiproliferative management of neuroendocrine tumors with special attention to recent high-quality phase III trials.

Somatostatin receptor SSTR2A and SSTR5 expression in neuroendocrine breast cancer

Neuroendocrine breast cancer (NEBC) is a group of rare tumors, which could benefit from therapy targeting the somatostatin receptors (SSTRs). In particular, SSTR2A and SSTR5 are potential targets given their consistent expression in gastrointestinal and pancreatic primary and metastatic neuroendocrine cancers. Currently, there are no studies describing the expression of SSTRs in NEBC. The purpose of our study was to characterize the immunohistochemical expression of SSTR2A and SSTR5 in a cohort of NEBC. Thirty-one primary NEBC cases were analyzed, and SSTR2A and SSTR5 immunohistochemistry performed and scored using the modified immunoreactive score proposed by Remmele and Stanger. All patients were females with a mean age of 66.6 years (SD = 14). 77% of cases were histological grade 2. SSTR2A showed a weak positivity in 11 cases (35.5%), moderate positivity in 6 cases (19.4%) and strong positivity in 5 cases (16.1%). Nine cases were negative for SSTR2A (29%). SSTR5 showed a weak positivity in 16 cases (51.6%), moderate positivity in 6 cases (19.4%), while no cases showed strong positivity. Nine cases were negative for SSTR5 (29%). Five cases were negative for both SSTR2A and SSTR5. A weak to moderate SSTR2A and SSTR5 expression was observed in 50-70% of the cases. A subset of NEBCs with strong SSR2A expression may benefit from SSTRs targeted therapy. These results need further validation in a larger series including metastatic NEBC, to provide potential therapeutic targets for patients with advanced disease.

Visualisation of interstitial lung disease by molecular imaging of integrin αvβ3 and somatostatin receptor 2

OBJECTIVE

To evaluate integrin αvβ3 (alpha-v-beta-3)-targeted and somatostatin receptor 2 (SSTR2)-targeted nuclear imaging for the visualisation of interstitial lung disease (ILD).

METHODS

The pulmonary expression of integrin αvβ3 and SSTR2 was analysed in patients with different forms of ILD as well as in bleomycin (BLM)-treated mice and respective controls using immunohistochemistry. Single photon emission CT/CT (SPECT/CT) was performed on days 3, 7 and 14 after BLM instillation using the integrin αvβ3-targeting ¹⁷⁷Lu-DOTA-RGD and the SSTR2-targeting ¹⁷⁷Lu-DOTA-NOC radiotracer. The specific pulmonary accumulation of the radiotracers over time was assessed by in vivo and ex vivo SPECT/CT scans and by biodistribution studies.

RESULTS

Expression of integrin αvβ3 and SSTR2 was substantially increased in human ILD regardless of the subtype. Similarly, in lungs of BLM-challenged mice, but not of controls, both imaging targets were stage-specifically overexpressed. While integrin αvβ3 was most abundantly upregulated on day 7, the inflammatory stage of BLM-induced lung fibrosis, SSTR2 expression peaked on day 14, the established fibrotic stage. In agreement with the findings on tissue level, targeted nuclear imaging using SPECT/CT specifically detected both imaging targets ex vivo and in vivo, and thus visualised different stages of experimental ILD.

CONCLUSION

Our preclinical proof-of-concept study suggests that specific visualisation of molecular processes in ILD by targeted nuclear imaging is feasible. If transferred into clinics, where imaging is considered an integral part of patients' management, the additional information derived from specific imaging tools could represent a first step towards precision medicine in ILD.

Single-Molecule Microscopy Reveals Dynamic FLNA Interactions Governing SSTR2 Clustering and Internalization

The cytoskeletal protein filamin A (FLNA) has been suggested to play an important role in the responsiveness of GH-secreting pituitary tumors to somatostatin receptor subtype 2 (SSTR2) agonists by regulating SSTR2 expression and signaling. However, the underlying mechanisms are unknown. In this study, we use fast multicolor single-molecule microscopy to image individual SSTR2 and FLNA molecules at the surface of living cells with unprecedented spatiotemporal resolution. We find that SSTR2 and FLNA undergo transient interactions, which occur preferentially along actin fibers and contribute to restraining SSTR2 diffusion. Agonist stimulation increases the localization of SSTR2 along actin fibers and, subsequently, SSTR2 clustering and recruitment to clathrin-coated pits (CCPs). Interfering with FLNA-SSTR2 binding with a dominant-negative FLNA fragment increases SSTR2 mobility, hampers the formation and alignment of SSTR2 clusters along actin fibers, and impairs both SSTR2 recruitment to CCPs and SSTR2 internalization. These findings indicate that dynamic SSTR2-FLNA interactions critically control the nanoscale localization of SSTR2 at the plasma membrane and are required for coupling SSTR2 clustering to internalization. These mechanisms explain the critical role of FLNA in the control of SSTR2 expression and signaling and suggest the possibility of targeting SSTR2-FLNA interactions for the therapy of pharmacologically resistant GH-secreting pituitary tumors.

The Role of Adding Somatostatin Analogues to Peptide Receptor Radionuclide Therapy as a Combination and Maintenance Therapy

Purpose: Although somatostatin analogues (SSA) and peptide receptor radionuclide therapy (PRRT) are validated therapies in patients with advanced gastroenteropancreatic neuroendocrine tumors (GEP-NET), it remains unclear whether SSA combined with PRRT or as maintenance therapy can provide prolonged survival compared with patients treated with PRRT alone. In this retrospective study, we aimed to investigate whether there is a survival benefit to adding SSA to PRRT as a combination therapy and/or maintenance therapy.Patients and Methods: The investigation included 168 patients with unresectable GEP-NETs treated at the University Hospital Bonn, Bonn, Germany. The patients were divided into two main groups: PRRT monotherapy (N = 81, group 1) and PRRT plus SSA (N = 87, group 2) as combined therapy with PRRT and/or as maintenance therapy after PRRT.Results: Data for overall survival (OS) were available from 168 patients, of whom 160 had data for progression-free survival (PFS). The median PFS was 27 months in group 1 versus 48 months in group 2 (P = 0.012). The median OS rates were 47 months in group 1 and 91 months in group 2 (P < 0.001). The death-event rates were lower in group 2 (26%) than in group 1 (63%). SSA as a combination therapy with PRRT and/or as a maintenance therapy showed a clinical benefit rate (objective response or stable disease) of 95%, which was significantly higher than group 1 (79%).Conclusions: SSA as a combination therapy and/or maintenance therapy may play a significant role in tumor control in patients with GEP-NET who underwent a PRRT. Clin Cancer Res; 24(19); 4672-9. ©2018 AACR.

Cellular effects of AP102, a somatostatin analog with balanced affinities for the hSSTR2 and hSSTR5 receptors

BACKGROUND

Somatostatin analogs (SSAs) are first-line medical therapy for the treatment of acromegaly and neuroendocrine tumors that express somatostatin receptors (SSTR). Somatostatin suppresses secretion of a large number of hormones through the stimulation of the five SSTR. However, unbalanced inhibition of secretion as observed with the highly potent SSAs pasireotide causes hyperglycaemia mainly by inhibiting insulin secretion. In contrast, AP102 a new SSAs has neutral effect on blood glucose while suppressing GH secretion. Our objective was to establish the cellular effects of AP102 on SSTR2 and SSTR5 that may explain the differences observed between AP102 and other SSAs.

METHODS

We compared the binding and agonist activity of AP102 with somatostatin-14, octreotide and pasireotide in HEK293 cells transfected with human SSTR2 and SSTR5 receptors. SSAs signal transduction effects (cAMP concentrations) were measured in forskolin-treated cells in the presence of SSAs. Proliferation and apoptotic effects were determined and binding assays were performed using ¹²⁵I- somatostatin-14.

RESULTS

AP102 has comparable affinity and agonist effect to octreotide at SSTR2 (IC50's of 112 pM and 244 pM, respectively; EC50's of 230 pM and 210 pM, respectively) in contrast to pasireotide that exhibits a 12-27 fold higher IC50 (3110 pM) and about 5-fold higher EC50 (1097 pM). At SSTR5, AP102 has much higher affinity and stimulating effect than octreotide (IC50's of 773 pM and 16,737 pM, respectively; EC50's of 8526 pM and 26,800 pM), and an intermediate affinity and agonist effect between octreotide and pasireotide. AP102, octreotide and pasireotide have variable anti-proliferative effects on HEK cells transfected with SSTR2 and SSTR5.

CONCLUSION

AP102 is a new SSA that better reduces signaling at SSTR2 than SSTR5 and prevents cell proliferation at both receptors. The euglycaemic effect of AP102 observed in preclinical studies may be related to this intermediate agonistic potency between pasireotide and octreotide at SSTR2 and SSTR5.

Identification of human somatostatin receptor 2 domains involved in internalization and signaling in QGP-1 pancreatic neuroendocrine tumor cell line

Somatostatin exerts inhibitory effects on hormone secretion and cell proliferation via five receptor subtypes (SST1-SST5), whose internalization is regulated by β-arrestins. The receptor domains involved in these effects have been only partially elucidated. The aim of the study is to characterize the molecular mechanism and determinants responsible for somatostatin receptor 2 internalization and signaling in pancreatic neuroendocrine QGP-1 cell line, focusing on the third intracellular loop and carboxyl terminal domains. We demonstrated that in cells transfected with somatostatin receptor 2 third intracellular loop mutant, no differences in β-arrestins recruitment and receptor internalization were observed after somatostatin receptor 2 activation in comparison with cells bearing wild-type somatostatin receptor 2. Conversely, the truncated somatostatin receptor 2 failed to recruit β-arrestins and to internalize after somatostatin receptor 2 agonist (BIM23120) incubation. Moreover, the inhibitory effect of BIM23120 on cell proliferation, cyclin D1 expression, P-ERK1/2 levels, apoptosis and vascular endothelial growth factor secretion was completely lost in cells transfected with either third intracellular loop or carboxyl terminal mutants. In conclusion, we demonstrated that somatostatin receptor 2 internalization requires intact carboxyl terminal while the effects of SS on cell proliferation, angiogenesis and apoptosis mediated by somatostatin receptor 2 need the integrity of both third intracellular loop and carboxyl terminal.

Somatostatin Receptor Type 2 (SSTR2) Internalization and Intracellular Trafficking in Pituitary GH-Secreting Adenomas: Role of Scaffold Proteins and Implications for Pharmacological Resistance

Somatostatin receptor type 2 (SSTR2), together with SSTR5, represents the main target of medical treatment for growth hormone (GH)-secreting pituitary tumors, since it is expressed in most of these tumors and exerts both antiproliferative and cytostatic effects, and reduces hormone secretion, as well. However, clinical practice indicates a great variability in the frequency and entity of favorable responses of acromegalic patients to long-acting somatostatin analogues (SSAs), but the molecular mechanisms regulating this pharmacological resistance are not completely understood. So far, several potentially implied mechanisms have been suggested, including impaired expression of SSTRs, or post-receptor signal transduction alterations. More recently, new studies exploited the molecular factors involved in SSTRs intracellular trafficking regulation, this being a critical point for the modulation of the available active G-coupled receptors (GPCRs) amount at the cell surface. In this respect, the role of the scaffold proteins such as β-arrestins, and the cytoskeleton protein Filamin A (FLNA), have become of relevant importance for GH-secreting pituitary tumors. In fact, β-arrestins are linked to SSTR2 desensitization and internalization, and FLNA is able to regulate SSTR2 trafficking and stability at the plasma membrane. Therefore, the present review will summarize emerging evidence highlighting the role of β-arrestins and FLNA, as possible novel players in the modulation of agonist activated-SSTR2 receptor trafficking and response in GH-secreting pituitary tumors.

SSTR2 promoter hypermethylation is associated with the risk and progression of laryngeal squamous cell carcinoma in males

Background

Somatostatin receptor 2 (SSTR2) encodes somatostatin receptor that can inhibit the cell proliferation of solid tumors. Promoter hypermethylation is likely to silence the expression of SSTR2. The goal of our study was to investigate the association between SSTR2 promoter methylation and the risk and progression of laryngeal carcinoma.

Methods

In the current study, tumor tissues and their adjacent non-tumor tissues were collected from a total of 87 laryngeal squamous cell carcinoma (LSCC) male patients. DNA methylation levels of nine SSTR2 promoter CpGs were measured using the bisulphite pyrosequencing technology.

Results

Our results revealed that there was a significantly increased SSTR2 promoter methylation in LSCC tissues than in their adjacent non-cancerous tissues (adjusted P = 0.003). Breakdown analysis by age indicated that the significant association was mainly contributed by patients younger than 60 (adjusted P = 0.039) but not in patients older than 60. Meanwhile, the significant association was observed in the patients with moderately (adjusted P = 0.037) and well differentiated tissues (adjusted P = 0.028), as well as the patients with histological stage IV (adjusted P = 0.031). Multivariate Cox analysis suggested that SSTR2 promoter methylation was an independent prognostic factor of LSCC (HR = 1.127, 95 % CI = 1.034–1.228).

Conclusions

In conclusion, SSTR2 promoter hypermethylation might be associated with the risk and progression of LSCC in males.

Alteration of somatostatin receptor 2 expression in canine mammary gland tumor

Somatostatin receptor 2 (SSTR2) is a negative regulator of cell proliferation in human breast cancer. Since there is little information about SSTR2 in canine mammary gland tumor (MGT), we clarified its distribution and expression level in normal mammary gland, benign MGT and malignant MGT. SSTR2 expression determined by immunohistochemical staining was observed in the cytoplasm of luminal epithelial cells. The intensity was negatively correlated with malignancy: normal tissues and some of the benign tumors had the highest levels, while the malignant tumors had little or no SSTR2 expression. As for the Western blotting, SSTR2 protein level in benign tumors was significantly lower than the normal mammary gland. On the other hand, SSTR2 protein levels in two of three malignant tumors were higher than the other groups. These results suggest that SSTR2 expression alters according to the malignancy of canine MGT.

Somatostatin receptors in gastrointestinal stromal tumors: new prognostic biomarker and potential therapeutic strategy

Somatostatin receptors (SSTRs) already act as important roles in gastroenteropancreatic neuroendocrine tumors (GEP-NETs) with high expression levels for prognosis predicting and octreotide LAR treatment purposes but less noticed in gastrointestinal stromal tumors (GISTs). Our study aims to fully evaluate the expression levels and prognostic values of SSTRs in GIST patients. For SSTRs expression detection, qPCR were used in 25 fresh GIST specimens, and then, 453 GIST samples (405 GISTs with operation only and 48 with imatinib adjuvant therapy after surgery) were collected for tissue microarrays (TMAs) construction and confirmed by immunohistochemistry (IHC). Clinicopathological data were confirmed by pathological diagnosis and clinical recorders, recurrence-free survivals (RFS) were evaluated in 453 GIST patients. With IHC performed, SSTR1 and SSTR2 present high positive proportion (81.9% and 87.6%) in 453 GISTs in our study, and positive expression rates of SSTR3, SSTR4 and SSTR5 are 56.1%, 8.8% and 47.2%, respectively. SSTR2 and SSTR5 negative expression are associated with decreased RFS when compared to positive cases by Kaplan-Meier survival analyses with log-rank test and univariate analysis in GISTs, furthermore, SSTR2 was an independent prognostic indicator for GISTs by multivariate analysis. In our study, detection of SSRT2 and SSTR5 expression helps to predict different prognosis in GIST patients. SSTR2 is a novel independent prognostic biomarker for GISTs. With high expression performance of SSTRs in GISTs, new therapeutic strategies such as octreotide or pasireotide LAR could be taken into consideration in selected advanced GIST patients.

Filamin A (FLNA) plays an essential role in somatostatin receptor 2 (SST2) signaling and stabilization after agonist stimulation in human and rat somatotroph tumor cells

Somatostatin receptor type 2 (SST2) is the main pharmacological target of medical therapy for GH-secreting pituitary tumors, but molecular mechanisms regulating its expression and signaling are largely unknown. The aim of this study was to investigate the role of cytoskeleton protein filamin A (FLNA) in SST2 expression and signaling in somatotroph tumor cells. We found a highly variable expression of FLNA in human GH-secreting tumors, without a correlation with SST2 levels. FLNA silencing in human tumoral cells did not affect SST2 expression and localization but abolished the SST2-induced reduction of cyclin D1 (-37% ± 15% in control cells, P < .05 vs basal) and caspase-3/7 activation (+63% ± 31% in control cells, P < .05 vs basal). Overexpression of a FLNA dominant-negative mutant that specifically prevents SST2-FLNA binding reduced SST2 expression after prolonged agonist exposure (-55% ± 5%, P < .01 vs untreated cells) in GH3 cells. Moreover, SST2-induced apoptotic effect (77% ± 54% increase of caspase activity, P < .05 vs basal) and SST2-mediated ERK1/2 inhibition (48% ± 17% reduction of ERK1/2 phosphorylation, P < .01 vs basal) were abrogated in cells overexpressing another FLNA mutant that prevents FLNA interaction with partner proteins but not with SST2, suggesting a scaffold function of FLNA in somatotrophs. In conclusion, these data demonstrate that FLNA is involved in SST2 stabilization and signaling in tumoral somatotrophs, playing both a structural and functional role.

Octreotide modulates the effects on fibrosis of TNF-α, TGF-β and PDGF in activated rat hepatic stellate cells

BACKGROUND AND AIMS

Somatostatin and its analogs may influence hepatic fibrosis interfering through several mechanisms. The aim of this study was to investigate the effect of octreotide on cytokine activated hepatic stellate cells (HSC).

METHODS

Primary HSCs were isolated from rats and were cultured on plastic for activation. Expression of somatostatin receptors (SSTR) was investigated in cultured HSCs by immunofluorescence and western blot. The effect of octreotide on cellular proliferation was studied with the MTT assay and western blot for α1-procollagen (α1-PROC) production in TNFα, TGF-β1 or PDGF treated HSCs. Phosphotyrosine (PTP) and phosphoserine-phosphothreonine (STP) phosphatases inhibition was performed with sodium orthovanadate and okadaic acid respectively.

RESULTS

Activated HSC express SSTR subtypes 1, 2A, 2B, 3 and 4 and their expression is enhanced by further HSC activation. Octreotide did not have an effect on HSC proliferation but inhibited plastic induced α1-PROC production. Interestingly, it enhanced PDGF-induced HSC proliferation but inhibited PDGF and TGFβ1 dependent expression of α1-PROC, while an opposite effect was observed in TNFα-induced cell proliferation and collagen production. PTP inhibition reversed the inhibitory effect of octreotide on α1-PROC, but potentiated its effect on PDGF and TGFβ1 dependent α1-PROC production. Finally, STP inhibition profoundly inhibited α1-PROC expression in all cases suggesting that both STP and PTP phosphatases are important regulators of pro-fibrotic mechanisms.

CONCLUSIONS

The net effect of octreotide on HSCs and therefore liver fibrosis is subject to the cytokine microenvironment of these cells. This effect is modulated by PTPs and STPs inhibition. Especially in the case of STPs their profibrotic effects could be an interesting new therapeutic target in liver fibrosis.

Inhibition of tumor promoting signals by activation of SSTR2 and opioid receptors in human breast cancer cells

Background

Somatostatin receptors (SSTRs) and opioid receptors (ORs) belong to the superfamily of G-protein coupled receptors and function as negative regulators of cell proliferation in breast cancer. In the present study, we determined the changes in SSTR subtype 2 (SSTR2) and μ, δ and κ-ORs expression, signaling cascades and apoptosis in three different breast cancer cells namely MCF-7, MDA-MB231 and T47D.

Methods

Immunocytochemistry and western blot analysis were employed to study the colocalization and changes in MAPKs (ERK1/2 and p38), cell survival pathway (PI3K/AKT) and tumor suppressor proteins (PTEN and p53) in breast cancer cell lines. The nature of cell death upon activation of SSTR2 or OR was analysed using flow cytometry analysis.

Results

The activation of SSTR2 and ORs modulate MAPKs (ERK1/2 and p38) in cell dependent and possibly estrogen receptor (ER) dependent manner. The activation of tumor suppressor proteins phosphatase and tensin homolog (PTEN) and p53 antagonized the PI3K/AKT cell survival pathway. Flow cytometry analyses reveal increased necrosis as opposed to apoptosis in MCF-7 and T47D cells when compared to ER negative MDA-MB231 cells. Furthermore, receptor and agonist dependent expression of ORs in SSTR2 immunoprecipitate suggest that SSTR2 and ORs might interact as heterodimers and inhibit epidermal growth factor receptor phosphorylation.

Conclusion

Taken together, findings indicate a new role for SSTR2/ORs in modulation of signaling pathways involved in cancer progression and provide novel therapeutic approaches in breast cancer treatment.

Clinicopathologic characteristics of pancreatic neuroendocrine tumors and relation of somatostatin receptor type 2A to outcomes

BACKGROUND

The impact of somatostatin receptor type 2 (SSTR-2a) expression levels on outcomes in patients with pancreatic neuroendocrine tumors (PNETs) has not been evaluated.

METHODS

Correlations between clinicopathologic characteristics, including SSTR-2a expression and outcomes, were retrospectively studied in 79 patients with pancreatic neuroendocrine tumors (PNETs).

RESULTS

The SSTR-2a score was 0 in 27% of patients, 1 in 24% of patients, 3 in 30% of patients, and 4 in 18% of patients. The overall survival rate was 87% at 1 year, 77% at 3 years, and 71% at 5 years. On univariate analysis, a pancreatic tumor that measured ≥ 20 mm in greatest dimension, stage IV disease, vascular invasion, neuroendocrine carcinoma (NEC), and an SSTR-2a score of 0 were associated significantly with poor outcomes. On multivariate analysis, NEC (P = .000; hazard ratio, 28.8; 95% confidence interval, 7.502-111.240) and an SSTR-2a score of 0 (P = .001; hazard ratio, 3.611; 95% confidence interval, 1.344-9.702) were related independently to poor outcomes.

CONCLUSIONS

The current analysis of prognostic factors in patients with PNETs demonstrated that NEC and an SSTR-2a score of 0 both were significant independent predictors of poor outcomes. The results suggest that the assessment of SSTR-2a may facilitate the selection of treatment regimens and the prediction of outcomes. Because a considerable proportion of patients with NEC have SSTR-2a-positive tumors, further analyses of the usefulness of somatostatin analogues are warranted in patients who have SSTR-2a-positive NEC.

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.

Peptide receptor targeting in cancer: the somatostatin paradigm

Peptide receptors involved in pathophysiological processes represent promising therapeutic targets. Neuropeptide somatostatin (SST) is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors, named sst1–5, while in central nervous system, SST acts as a neurotransmitter/neuromodulator, regulating locomotory and cognitive functions. Critical points of SST/SST receptor biology, such as signaling pathways of individual receptor subtypes, homo- and heterodimerization, trafficking, and cross-talk with growth factor receptors, have been extensively studied, although functions associated with several pathological conditions, including cancer, are still not completely unraveled. Importantly, SST exerts antiproliferative and antiangiogenic effects on cancer cells in vitro, and on experimental tumors in vivo. Moreover, SST agonists are clinically effective as antitumor agents for pituitary adenomas and gastro-pancreatic neuroendocrine tumors. However, SST receptors being expressed by tumor cells of various tumor histotypes, their pharmacological use is potentially extendible to other cancer types, although to date no significant results have been obtained. In this paper the most recent findings on the expression and functional roles of SST and SST receptors in tumor cells are discussed.

Analog of somatostatin vapreotide exhibits biological effects in vitro via interaction with neurokinin-1 receptor

OBJECTIVES

Vapreotide, a synthetic analog of somatostatin, has analgesic activity most likely mediated through the blockade of neurokinin-1 receptor (NK1R), the substance P (SP)-preferring receptor. The ability of vapreotide to interfere with other biological effects of SP has yet to be investigated.

METHODS

We studied the ability of vapreotide to antagonize NK1R in three different cell types: immortalized U373MG human astrocytoma cells, human monocyte-derived macrophages (MDM) and a human embryonic kidney cell line, HEK293. Both U373MG and MDM express endogenous NK1R while HEK293 cells, which normally do not express NK1R, are stably transformed to express human NK1R (HEK293-NK1R).

RESULTS

Vapreotide attenuates SP-triggered intracellular calcium increases and nuclear factor-κB activation in a dose-dependent manner. Vapreotide also inhibits SP-induced interleukin-8 and monocyte chemotactic protein-1 production in HEK293-NK1R and U373MG cell lines. Vapreotide inhibits HIV-1 infection of human MDM in vitro, an effect that is reversible by SP pretreatment.

CONCLUSIONS

Our findings indicate that vapreotide has NK1R antagonist activity and may have a potential application as a therapeutic intervention in HIV-1 infection.

Specific roles of G(i) protein family members revealed by dissecting SST5 coupling in human pituitary cells

Despite intensive investigation over the past 20 years, the specific role played by individual G(i) protein family members in mediating complex cellular effects is still largely unclear. Therefore, we investigated the role of specific G(i) proteins in mediating somatostatin (SS) effects in somatotroph cells. Because our previous data showed that SS receptor type 5 (SST5) carrying a spontaneous R240W mutation in the third intracellular loop had a similar ability to inhibit intracellular cAMP levels to the wild-type protein but failed to mediate inhibition of growth hormone (GH) release and cell proliferation, we used this model to check specific receptor-G-protein coupling by a bioluminescent resonance energy transfer analysis. In HEK293 cells, wild-type SST5 stimulated the activation of Gα(i1-3) and Gα(oA), B, whereas R240W SST5 maintained the ability to activate Gα(i1-3) and Gα(oB), but failed to activate the splicing variant Gα(oA). To investigate the role of the selective deficit in Gα(oA) coupling, we co-transfected human adenomatous somatotrophs with SST5 and a pertussis toxin (PTX)-resistant Gα(oA) (Gα(oA(PTX-r))) protein. In PTX-treated cells, Gα(oA(PTX-r)) rescued the ability of the selective SST5 analog BIM23206 to inhibit extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation, GH secretion and intracellular cAMP levels. Moreover, we demonstrated that silencing of Gα(oA) completely abolished SST5-mediated inhibitory effects on GH secretion and ERK1/2 phosphorylation, but not on cAMP levels. In conclusion, by analysing the coupling specificity of human SST5 to individual Gα(i) and Gα(o) subunits, we identified a crucial role for Gα(oA) signalling in human pituitary cells.

Efficacy and tolerability of long-acting octreotide in the treatment of thymic tumors: results of a pilot trial

OBJECTIVES

Octreotide is a somatostatin analog, long-acting formulations of which have been used experimentally for the treatment of patients with invasive tumors and/or residual disease after conventional therapies. The objective of this retrospective study was to evaluate the efficacy of long-acting octreotide (Sandostatin LAR) for the treatment of thymic tumors, with a primary efficacy end point of progression-free survival.

METHODS

Between 1994 and 2010, 44 patients with thymic malignancies were evaluated. Twenty-seven patients underwent an OctreoScan, and 12 OctreoScan-positive patients were treated with long-acting octreotide at a dose of 20 mg, given as an intramuscular injection, every 2 weeks.

RESULTS

Treatment with long-acting octreotide gave the following results: 3 cases of partial response (25%), 5 cases of stable disease (42%), and 4 cases of progressive disease (33%), with an average progression-free survival of 8 months (range, 3 to 21). Treatment compliance and tolerability were good for all evaluated patients.

CONCLUSIONS

The results of this study confirm the somatostatin receptor as a valid target for the treatment of thymic malignancies. Overall, therapy with long-acting somatostatin analogs seems to be safe and effective.

Microarray Analyses of Genes Differentially Expressed by Diet (Black Beans and Soy Flour) during Azoxymethane-Induced Colon Carcinogenesis in Rats

We previously demonstrated that black bean (BB) and soy flour (SF)-based diets inhibit azoxymethane (AOM)-induced colon cancer. The objective of this study was to identify genes altered by carcinogen treatment in normal-appearing colonic mucosa and those attenuated by bean feeding. Ninety-five male F344 rats were fed control (AIN) diets upon arrival. At 4 and 5 weeks, rats were injected with AOM (15 mg/kg) or saline and one week later administered an AIN, BB-, or SF-based diet. Rats were sacrificed after 31 weeks, and microarrays were conducted on RNA isolated from the distal colonic mucosa. AOM treatment induced a number of genes involved in immunity, including several MHC II-associated antigens and innate defense genes (RatNP-3, Lyz2, Pla2g2a). BB- and SF-fed rats exhibited a higher expression of genes involved in energy metabolism and water and sodium absorption and lower expression of innate (RatNP-3, Pla2g2a, Tlr4, Dmbt1) and cell cycle-associated (Cdc2, Ccnb1, Top2a) genes. Genes involved in the extracellular matrix (Col1a1, Fn1) and innate immunity (RatNP-3, Pla2g2a) were induced by AOM in all diets, but to a lower extent in bean-fed animals. This profile suggests beans inhibit colon carcinogenesis by modulating cellular kinetics and reducing inflammation, potentially by preserving mucosal barrier function.

A switch of G protein-coupled receptor binding preference from phosphoinositide 3-kinase (PI3K)-p85 to filamin A negatively controls the PI3K pathway

Frequent oncogenic alterations occur in the phosphoinositide 3-kinase (PI3K) pathway, urging identification of novel negative controls. We previously reported an original mechanism for restraining PI3K activity, controlled by the somatostatin G protein-coupled receptor (GPCR) sst2 and involving a ligand-regulated interaction between sst2 with the PI3K regulatory p85 subunit. We here identify the scaffolding protein filamin A (FLNA) as a critical player regulating the dynamic of this complex. A preexisting sst2-p85 complex, which was shown to account for a significant basal PI3K activity in the absence of ligand, is disrupted upon sst2 activation. FLNA was here identified as a competitor of p85 for direct binding to two juxtaposed sites on sst2. Switching of GPCR binding preference from p85 toward FLNA is determined by changes in the tyrosine phosphorylation of p85- and FLNA-binding sites on sst2 upon activation. It results in the disruption of the sst2-p85 complex and the subsequent inhibition of PI3K. Knocking down FLNA expression, or abrogating FLNA recruitment to sst2, reversed the inhibition of PI3K and of tumor growth induced by sst2. Importantly, we report that this FLNA inhibitory control on PI3K can be generalized to another GPCR, the mu opioid receptor, thereby providing an unprecedented mechanism underlying GPCR-negative control on PI3K.

Resistance to somatostatin analogs in acromegaly

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

GPCR somatostatin receptor extracellular loop 2 is a key ectodomain for making subtype-selective antibodies with agonist-like activities in the pancreatic neuroendocrine tumor BON cell line

OBJECTIVES

The extracellular loop 2 (ECL2) ectodomain of the G protein-coupled receptor class A is thought to function like an inactivation "lid." We created polyclonal somatostatin receptor ECL2 (anti-SSTR ECL2) antibodies to target this lid and to examine if these antibodies can selectively activate the SSTR.

METHODS

Western blots and live-cell immunofluorescence microscopy determined anti-SSTR ECL2 antibody receptor binding selectivity. 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay (MTS assay) and cell cycle assay (fluorescence-activated cell sorting) checked for antibody effect on antiproliferation. Nexin assay examined the antibody's ability to induce apoptosis. LANCE cAMP kit (Perkin Elmer) detected antibody-dependent cAMP decrease. Enzyme-linked immunosorbent assay measured antibody effect on suppressing serotonin secretion. Ligand-receptor binding interference assay with the fluorescent somatostatin (FAM-SST) was used to examine antibody interference to SST-SSTR binding.

RESULTS

Anti-SSTR ECL2 antibodies are SSTR subtype selective and agonist-like, and they suppress cell proliferation via cell cycle arrest and apoptosis. In addition, these antibodies decrease cAMP production and inhibit serotonin secretion. Interestingly, these antibodies do not interfere with SST-SSTR binding.

CONCLUSIONS

The ECL2 is an important ectodomain for G protein-coupled receptor activation and required for ligand binding selectivity. The anti-SSTR2, anti-SSTR3, and anti-SSTR5 ECL2 antibodies independently inhibited BON proliferation and decreased hormone secretion. Unlike octreotide, our antibodies do not interfere with SST-SSTR binding.

Ten-year adjuvant treatment with somatostatin analogs in a patient with atypical carcinoid of the lung

Both typical carcinoid and atypical carcinoid (AC) of the lung are surgically curable, but AC carries a considerably worse prognosis because of a relatively high rate of recurrence. Adjuvant therapy can be conducted with radiotherapy, chemotherapy, and somatostatin analogs (SST-As), but its effectiveness in preventing locoregional and distant recurrences is yet to be fully investigated. A 48-year-old woman, presenting with AC, was free of both radiographical and biochemical signs of residual disease, after surgery and chemotherapy. To prevent disease recurrence, she underwent long-term adjuvant treatment based on SST-As. During the 10-year follow-up period, no side effects referable to SST-As have been reported and no evidence of recurrence of the disease has been detected. In consideration of the relatively high recurrence rate of the disease and of excellent tolerance for SST-As, long-term adjuvant treatment based on SST-As could become a therapeutic option for surgically cured patients with AC. Clinical investigations, conducted in large samples of patients, are necessary to evaluate the effectiveness of such an approach.

Treatment of endocrine pancreatic tumors

Endocrine pancreatic tumors are rare with an incidence of 4 per million inhabitants. Most tumors are malignant except for insulinomas that usually are benign. They are slowly growing in the majority of cases but there are exceptions with rapidly progressing malignant carcinomas. Because of the rarity of these tumors large randomized trials are difficult to accomplish. However, most physicians treating these patients agree that surgery should be considered in all cases and that medical treatment with chemotherapy and biotherapy is well established for this group of patients.

Characterization of intracellular signaling mediated by human somatostatin receptor 5: role of the DRY motif and the third intracellular loop

Somatostatin (SST) exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5) linked to multiple cellular effectors. The receptor structural domains involved in these effects have been only partially elucidated. The aim of the study was to investigate the molecular determinants mediating the interaction of the human SST5 with intracellular signaling in the pituitary cell line GH3, focusing on the BBXXB domain in the third intracellular loop and the DRY motif in the second intracellular loop. We analyzed the effects of the SST5 agonist BIM23206 on cAMP accumulation, intracellular calcium, GH secretion, cell proliferation, and ERK1/2 phosphorylation in cells expressing either wild-type SST5 or mutant receptors, in particular the naturally occurring mutant R240W in the BBXXB domain and the D136A and R137A mutants in the DRY motif. We found that residues D136 and R137 were critical for SST5 signaling because their substitutions abolished all the intracellular responses. Conversely, third intracellular loop mutations resulted in receptor that inhibited intracellular cAMP levels similar to the wild-type (50 +/- 9 vs. 53 +/- 12% inhibition) but failed to mediate the other responses elicited by wild-type SST5, i.e. reduction of intracellular calcium levels as well as inhibition of ERK1/2. These events resulted in an absent inhibition of GH release and an impaired reduction of cell proliferation (38 +/- 7 vs. 76 +/- 6% inhibition in wild type, P < 0.05). These data indicate that different regions of SST5 are required for the activation of different signaling pathways.

The antiproliferative effects of somatostatin receptor subtype 2 in breast cancer cells

AIM

Somatostatin receptor subtype 2 (SSTR2) is the principal mediator of somatostatin's (SST) antiproliferative effects on normal and cancer cells. Therefore, we investigated whether the enhanced expression of SSTR2 could inhibit the proliferation of tumor cells, and, if so, the mechanisms that might be involved.

METHODS

SSTR2 expression levels were determined by qRT-PCR in several tumor cell lines. Then, a plasmid pIRES2-EGFP-SSTR2 (pSIG) was constructed and stably transfected into MCF-7 cells (MCF-7/pSIG). After SSTR2 overexpression was identified by qRT-PCR, immunofluorescence staining and a receptor binding assay, the MCF-7/pSIG cells were analyzed by PI staining for apoptosis and cell cycle arrest was tested by flow cytometry for epidermal growth factor receptor (EGFR) expression. The EGF-stimulated proliferation of MCF-7 cells was assayed by MTT.

RESULTS

The human breast cancer cell line MCF-7 expresses a lower level of SSTR2, thereby partly accounting for the decreased response to SST. The overexpression of SSTR2 in MCF-7 cells resulted in apoptosis, cytostasis and G(1)/S cell cycle arrest. Furthermore, the expression of EGFR, together with EGF-stimulated proliferation, was markedly decreased in the MCF-7/pSIG cells.

CONCLUSION

Enhanced SSTR2 expression played an antiproliferative role in MCF-7 cells through inducing apoptosis and G(1)/S cell cycle arrest, and also by decreasing EGFR expression, thereby counteracting the growth-stimulating effect of EGF. Our data seem to indicate that developing a new therapeutic agent capable of upregulating SSTR expression could potentially be a way to block tumor progression.Acta Pharmacologica Sinica (2009) 30: 1053-1059; doi: 10.1038/aps.2009.59.

Somatostatin and opioid receptors do not regulate proliferation or apoptosis of the human multiple myeloma U266 cells

BACKGROUND

opioid and somatostatin receptors (SSTRs) that can assemble as heterodimer were individually reported to modulate malignant cell proliferation and to favour apoptosis.

MATERIALS AND METHODS

SSTRs and opioid receptors expression were examined by RT-PCR, western-blot and binding assays, cell proliferation was studied by XTT assay and propidium iodide (PI) staining and apoptosis by annexin V-PI labelling.

RESULTS

almost all human malignant haematological cell lines studied here expressed the five SSTRs. Further experiments were conducted on the human U266 multiple myeloma cells, which express also micro-opioid receptors (MOP-R). XTT assays and cell cycle studies provide no evidence for a significant effect upon opioid or somatostatin receptors stimulation. Furthermore, neither direct effect nor potentiation of the Fas-receptor pathway was detected on apoptosis after these treatments.

CONCLUSION

these data suggest that SSTRs or opioid receptors expression is not a guaranty for an anti-tumoral action in U266 cell line.

Somatostatin receptors 1, 2, and 5 cooperate in the somatostatin inhibition of C6 glioma cell proliferation in vitro via a phosphotyrosine phosphatase-eta-dependent inhibition of extracellularly regulated kinase-1/2

Somatostatin inhibits cell proliferation through the activation of five receptors (SSTR1-5) expressed in normal and cancer cells. We analyzed the role of individual SSTRs in the antiproliferative activity of somatostatin in C6 rat glioma cells. Somatostatin dose-dependently inhibited C6 proliferation, an effect mimicked, with different efficacy or potency, by BIM-23745, BIM-23120, BIM-23206 (agonists for SSTR1, -2, and -5) and octreotide. The activation of SSTR3 was ineffective, although all SSTRs are functionally active, as demonstrated by the inhibition of cAMP production. All SSTRs induced cytostatic effects through the activation of the phosphotyrosine phosphatase PTPeta and the inhibition of ERK1/2. For possible synergism between SSTR subtypes, we tested the effects of the combined treatment with two agonists (SSTR1+2 or SSTR2+5) or bifunctional compounds. The simultaneous activation of SSTR1 and SSTR2 slightly increased the efficacy of the individual compounds with an IC50 in between the single receptor activation. SSTR2+5 activation displayed a pattern of response superimposable to that of the SSTR5 agonist alone (low potency and higher efficacy, as compared with BIM-23120). The simultaneous activation of SSTR1, -2, and -5 resulted in a response similar to somatostatin. In conclusion, the cytostatic effects of somatostatin in C6 cells are mediated by the SSTR1, -2, and -5 through the same intracellular pathway: activation of PTPeta and inhibition of ERK1/2 activity. Somatostatin is more effective than the individual agonists. The combined activation of SSTR1 and -2 shows a partial synergism as far as antiproliferative activity, whereas SSTR2 and -5 activation results in a response resembling the SSTR5 effects.

Somatostatin inhibits hepatic growth hormone receptor and insulin-like growth factor I mRNA expression by activating the ERK and PI3K signaling pathways

Previously, we reported that somatostatins (SS) inhibit organismal growth by reducing hepatic growth hormone (GH) sensitivity and by inhibiting insulin-like growth factor I (IGF-I) production. In this study, we used hepatocytes isolated from rainbow trout to elucidate the mechanism(s) associated with the extrapituitary growth-inhibiting actions of SS. SS-14, a predominant SS isoform, stimulated tyrosine phosphorylation of several endogenous proteins, including extracellular signal-regulated kinase (ERK), a member the mitogen-activated protein kinase (MAPK) family, and protein kinase B (Akt), a downstream target of phosphatidylinositol 3-kinase (PI3K). SS-14 specifically stimulated the phosphorylation of both ERK 1/2 and Akt in a concentration-dependent fashion. This activation occurred within 5–15 min, then subsided after 1 h. The ERK inhibitor U0126 retarded SS-14-stimulated phosphorylation of ERK 1/2, whereas the PI3K inhibitor LY294002 blocked SS-14-stimulated phosphorylation of Akt. SS-14-inhibited expression of GH receptor (GHR) mRNA was blocked by U0126 but not by LY294002. By contrast, U1026 had no effect on SS-14 inhibition of GH-stimulated IGF-I mRNA expression, whereas LY294002 partially blocked the inhibition of GH-stimulated IGF-I mRNA expression by SS-14. These results indicate that SS-14-inhibited GHR expression is mediated by the ERK signaling pathway and that the PI3K/Akt pathway mediates, at least in part, SS-14 inhibition of GH-stimulated IGF-I expression.

Somatostatin inhibits colon cancer cell growth through cyclooxygenase-2 downregulation

BACKGROUND AND PURPOSE

Cyclooxygenase-2 (COX-2) is expressed in colonic neoplasms, where it supports cell proliferation via prostaglandin E(2) (PGE(2)) production. This study investigated the effects of somatostatin-14 on COX-2 expression, PGE(2) production and proliferation in colon cancer cells.

EXPERIMENTAL APPROACH

Human colon adenocarcinoma cell lines Caco-2, HT-29 and HCT116 were used. The following techniques were employed: colourimetric assay for cell growth; 5-bromo-2'-deoxyuridine assay for DNA synthesis; enzyme immunoassay for PGE(2); COX-2 mRNA silencing; RT-PCR or Western blot for somatostatin receptor subtypes, cyclooxygenase isoforms, phosphorylated-ERK-1/ERK-2 and phosphorylated-Akt.

KEY RESULTS

HT-29 and Caco-2 cells expressed COX-2 and somatostatin receptors (sst(3/4/5) and sst(3/5), respectively). HCT116 cells did express somatostatin receptors (sst(2/3/5)), but not COX-2. Somatostatin-14 inhibited basal COX-2 expression, PGE(2) production, DNA synthesis and growth in Caco-2 cells and these effects were prevented by BN81658 (sst(3) receptor antagonist). Basal proliferation of HT-29, HCT116 and COX-2-silenced Caco-2 cells was not affected by somatostatin-14. Stimulation of HT-29 cells with gastrin-17 elicited increments of ERK-1/ERK-2 and Akt phosphorylation, COX-2 expression, PGE(2) production, DNA synthesis and cell growth, which were all counteracted by somatostatin-14. Somatostatin-14-induced inhibition of COX-2 expression, PGE(2) production and DNA synthesis were blocked by BIM23056 (sst(5) receptor antagonist).

CONCLUSIONS AND IMPLICATIONS

Somatostatin decreases COX-2 expression and function in colon cancer cells via activation of sst(3) or sst(5) receptors, and these effects contribute to the inhibitory action of somatostatin on cell proliferation. These findings can be relevant to the development of therapeutic strategies based on the modulation of the COX-2 pathway.

Identification of an upstream promoter of the human somatostatin receptor, hSSTR2, which is controlled by epigenetic modifications

Somatostatin is a neuropeptide that inhibits exocrine and endocrine secretions of several hormones and negatively regulates cell proliferation. These events are mediated through somatostatin engagement on one of five G protein-coupled receptors named SSTR1 to STTR5. Somatostatin binding to SSTR2 mediates predominantly antisecretory and antiproliferative effects; two important biological activities in the gastroenteropancreatic endocrine and exocrine system. Herein we demonstrate novel regulatory sequences for human (h) SSTR2 transcription. By genomic DNA sequence analysis, we reveal two CpG islands located 3.8 kb upstream from the transcription start site. We identify a novel transcription start site and a promoter region within one of these CpG islands. We demonstrate that two epigenetic modifications, DNA methylation and histone acetylation, regulate the activation of hSSTR2 upstream promoter. Furthermore, we show that the transcription from this upstream promoter region directly correlates to hSSTR2 mRNA expression in various human cell lines. A combined treatment of a demethylating agent, 5-aza-2-deoxycytidine and a histone deacetylase inhibitor, trichostatin A, leads to increased expression of hSSTR2 mRNA in cell lines in which the CpG island is methylated. The epigenetic regulation of this promoter region results in differential expression of hSSTR2 mRNA in human cell lines. This study reveals the existence of a novel upstream promoter for the hSSTR2 gene that is regulated by epigenetic modifications, suggesting for complex control of the hSSTR2 transcription.

Genetic abnormalities of somatostatin receptors in pituitary tumors

Somatostatin exerts antisecretive and antiproliferative effects on different endocrine cells by acting through a family of G protein-coupled receptors that includes five subtypes (SST1-5). Normal human pituitary and pituitary adenomas have been shown to express almost all SST subtypes, with the exception of SST4. Consistent with the observation that octreotide and other somatostatin analogs bind to SST2 and SST5 with high affinity, these genes have been screened for quantitative/qualitative abnormalities in tumors removed from patients with poor responsiveness to somatostatin analogs treatment. Data obtained in GH-secreting adenomas suggested that resistance to octreotide was frequently associated with low expression of SST2 mRNA, although other authors failed to confirm this finding. To date, the only mutational change involving SST2 and SST5 is the Arg to Trp substitution in codon 240 of the SST5 gene that was found in one acromegalic patient resistant to octreotide. Similarly, loss of heterozygosis at SST5 gene locus in pituitary adenomas has been described in individual tumors. In recent years, molecular studies investigated the possible association of gene polymorphisms and susceptibility to diseases and/or resistance to drugs. As far as polymorphic variants of SST genes are concerned, a possible role of SST5 C1004T and T-461C alleles in influencing GH and IGF-I levels in patients with acromegaly has been proposed. Nevertheless, polymorphic variants in SST2 and SST5 genes seem to have a minor, if any, role in determining the different responsiveness to somatostatin analogs in patients with acromegaly.

Biology of somatostatin in breast cancer

The biological effects of the neuropeptide somatostatin (SST) are mediated via a family of five somatostatin receptors (SSTRs) belonging to a family of G-protein-coupled receptors (GPCRs). SSTR regulate the secretion of hormones, growth factors, neurotransmission and cell growth in receptor-specific manner. In addition, SST plays an inhibitory role in several mammary cancer models. These effects are mediated both indirectly through inhibition of hormones and growth factors which promote tumor growth as well as directly via SSTRs present on tumor cells to inhibit mitogenic signaling of growth factor receptor kinases leading to growth arrest and induction of apoptosis. Here, we present an overview on the role of SST and its analogs in breast cancer.

Urotensin II and urotensin II-related peptide activate somatostatin receptor subtypes 2 and 5

The UII and urotensin II-related peptide (URP) genes belong to the same superfamily as the somatostatin gene. It has been previously shown that somatostatin activates the UII-receptor (UTR). In contrast, the possible interaction between UII and URP and somatostatin receptors has remained scarcely analyzed. Herein, we have investigated the effects of UII and URP on cell proliferation and free cytosolic Ca2+ concentration ([Ca2+]i) in CHO-K1 cells stably expressing the porcine somatostatin receptor subtypes sst2 and sst5. Results show that both UII and URP induce stimulation of cell proliferation mediated by sst2 receptors and UII provokes inhibition of cell proliferation mediated by sst5 receptors. UII and URP also provoked an increase of [Ca2+]i in both sst2- and sst5-transfected cells. Together, our present data demonstrate that UII and URP directly activate sst2 and sst5 and thus mimic the effect of somatostatin on its cognate receptors.