Somatostatin receptor subtype specificity in human fetal pituitary cultures. Differential role of SSTR2 and SSTR5 for growth hormone, thyroid-stimulating hormone, and prolactin regulation

Structural insights into somatostatin receptor 5 bound with cyclic peptides

Somatostatin receptor 5 (SSTR5) is highly expressed in ACTH-secreting pituitary adenomas and is an important drug target for the treatment of Cushing's disease. Two cyclic SST analog peptides (pasireotide and octreotide) both can activate SSTR5 and SSTR2. Pasireotide is preferential binding to SSTR5 than octreotide, while octreotide is biased to SSTR2 than SSTR5. The lack of selectivity of both pasireotide and octreotide causes side effects, such as hyperglycemia, gastrointestinal disturbance, and abnormal glucose homeostasis. However, little is known about the binding and selectivity mechanisms of pasireotide and octreotide with SSTR5, limiting the development of subtype-selective SST analog drugs specifically targeting SSTR5. Here, we report two cryo-electron microscopy (cryo-EM) structures of SSTR5-Gi complexes activated by pasireotide and octreoitde at resolutions of 3.09 Å and 3.24 Å, respectively. In combination with structural analysis and functional experiments, our results reveal the molecular mechanisms of ligand recognition and receptor activation. We also demonstrate that pasireotide preferentially binds to SSTR5 through the interactions between Tyr(Bzl)/DTrp of pasireotide and SSTR5. Moreover, we find that the Q2.63, N6.55, F7.35 and ECL2 of SSTR2 play a crucial role in octreotide biased binding of SSTR2. Our results will provide structural insights and offer new opportunities for the drug discovery of better selective pharmaceuticals targeting specific SSTR subtypes.

Selective ligand recognition and activation of somatostatin receptors SSTR1 and SSTR3

Somatostatin receptors (SSTRs) exert critical biological functions such as negatively regulating hormone release and cell proliferation, making them popular targets for developing therapeutics to treat endocrine disorders, especially neuroendocrine tumors. Although several panagonists mimicking the endogenous ligand somatostatin are available, the development of more effective and safer somatostatinergic therapies is limited due to a lack of molecular understanding of the ligand recognition and regulation of divergent SSTR subtypes. Here, we report four cryoelectron microscopy structures of Gi-coupled SSTR1 and SSTR3 activated by distinct agonists, including the FDA-approved panagonist pasireotide as well as their selective small molecule agonists L-797591 and L-796778. Our structures reveal a conserved recognition pattern of pasireotide in SSTRs attributed to the binding with a conserved extended binding pocket, distinct from SST14, octreotide, and lanreotide. Together with mutagenesis analyses, our structures further reveal the dynamic feature of ligand binding pockets in SSTR1 and SSTR3 to accommodate divergent agonists, the key determinants of ligand selectivity lying across the orthosteric pocket of different SSTR subtypes, as well as the molecular mechanism underlying diversity and conservation of receptor activation. Our work provides a framework for rational design of subtype-selective SSTR ligands and may facilitate drug development efforts targeting SSTRs with improved therapeutic efficacy and reduced side effects.

Characterization of the somatostatin system in tilapia: implications for growth and reproduction

Somatostatin (SST) plays diverse physiological roles in vertebrates, particularly in regulating growth hormone secretion from the pituitary. While the function of SST as a neuromodulator has been studied extensively, its role in fish and mammalian reproduction remains poorly understood. To address this gap, we investigated the involvement of the somatostatin system in the regulation of growth and reproductive hormones in tilapia. RNA sequencing of mature tilapia brain tissue revealed the presence of three SST peptides: SST6, SST3, and low levels of SST1. Four different isoforms of the somatostatin receptor (SSTR) subfamily were also identified in the tilapia genome. Phylogenetic and synteny analysis identified tiSSTR2-like as the root of the tree, forming two mega clades, with SSTR1 and SSTR4 in one and SSTR2a, SSTR3a, and SSTR5b in the other. Interestingly, the tiSSTR-5 isoforms 5x1, 5x2, and 5x3 were encoded in the sstr3b gene and were an artifact of misperception in the nomenclature in the database. RNA-seq of separated pituitary cell populations showed that SSTRs were expressed in gonadotrophs, with sstr3a enriched in luteinizing hormone (LH) cells and sstr3b significantly enriched in follicle-stimulating hormone (FSH) cells. Notably, cyclosomatostatin, an SSTR antagonist, induced cAMP activity in all SSTRs, with SSTR3a displaying the highest response, whereas octreotide, an SSTR agonist, showed a binding profile like that observed in human receptors. Binding site analysis of tiSSTRs from tilapia pituitary cells revealed the presence of canonical binding sites characteristic of peptide-binding class A G-protein-coupled receptors. Based on these findings, we explored the effect of somatostatin on gonadotropin release from the pituitary in vivo. Whereas cyclosomatostatin increased LH and FSH plasma levels at 2 h post-injection, octreotide decreased FSH levels after 2 h, but the LH levels remained unaffected. Overall, our findings provide important insights into the somatostatin system and its mechanisms of action, indicating a potential role in regulating growth and reproductive hormones. Further studies of the complex interplay between SST, its receptors, and reproductive hormones may advance reproductive control and management in cultured populations.

SSTR2 Mediates the Inhibitory Effect of SST/CST on Lipolysis in Chicken Adipose Tissue

Somatostatin shows an anti-lipolytic effect in both chickens and ducks. However, its molecular mediator remains to be identified. Here, we report that somatostatin type 2 receptor (SSTR2) is expressed at a high level in chicken adipose tissue. In cultured chicken adipose tissue, the inhibition of glucagon-stimulated lipolysis by somatostatin was blocked by an SSTR2 antagonist (CYN-154086), supporting an SSTR2-mediated anti-lipolytic effect. Furthermore, a significant pro-proliferative effect was detected in SST28-treated immortalized chicken preadipocytes (ICP-1), and this cell proliferative effect may be mediated through the MAPK/ERK signaling pathway activated by SSTR2. In summary, our results demonstrate that SSTR2 may regulate adipose tissue development by affecting the number and volume of adipocytes in chickens.

Transcriptional reprogramming during human osteoclast differentiation identifies regulators of osteoclast activity

Enhanced osteoclastogenesis and osteoclast activity contribute to the development of osteoporosis, which is characterized by increased bone resorption and inadequate bone formation. As novel antiosteoporotic therapeutics are needed, understanding the genetic regulation of human osteoclastogenesis could help identify potential treatment targets. This study aimed to provide an overview of transcriptional reprogramming during human osteoclast differentiation. Osteoclasts were differentiated from CD14+ monocytes from eight female donors. RNA sequencing during differentiation revealed 8 980 differentially expressed genes grouped into eight temporal patterns conserved across donors. These patterns revealed distinct molecular functions associated with postmenopausal osteoporosis susceptibility genes based on RNA from iliac crest biopsies and bone mineral density SNPs. Network analyses revealed mutual dependencies between temporal expression patterns and provided insight into subtype-specific transcriptional networks. The donor-specific expression patterns revealed genes at the monocyte stage, such as filamin B (FLNB) and oxidized low-density lipoprotein receptor 1 (OLR1, encoding LOX-1), that are predictive of the resorptive activity of mature osteoclasts. The expression of differentially expressed G-protein coupled receptors was strong during osteoclast differentiation, and these receptors are associated with bone mineral density SNPs, suggesting that they play a pivotal role in osteoclast differentiation and activity. The regulatory effects of three differentially expressed G-protein coupled receptors were exemplified by in vitro pharmacological modulation of complement 5 A receptor 1 (C5AR1), somatostatin receptor 2 (SSTR2), and free fatty acid receptor 4 (FFAR4/GPR120). Activating C5AR1 enhanced osteoclast formation, while activating SSTR2 decreased the resorptive activity of mature osteoclasts, and activating FFAR4 decreased both the number and resorptive activity of mature osteoclasts. In conclusion, we report the occurrence of transcriptional reprogramming during human osteoclast differentiation and identified SSTR2 and FFAR4 as antiresorptive G-protein coupled receptors and FLNB and LOX-1 as potential molecular markers of osteoclast activity. These data can help future investigations identify molecular regulators of osteoclast differentiation and activity and provide the basis for novel antiosteoporotic targets.

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.

Preoperative detection of functional somatostatin receptors in a patient with an insulinoma

Octreotide is used in patients with insulinomas to treat hypoglycemia, and somatostatin receptor (SSTR) 2 expression is important for its efficacy. We report a case of insulinoma in a 50-year-old woman that responded to an octreotide test, showed accumulation in somatostatin scintigraphy, and was positive for SSTR2A on immunostaining.

Influence of Leptin on the Secretion of Growth Hormone in Ewes under Different Photoperiodic Conditions

Leptin is an adipokine with a pleiotropic impact on many physiological processes, including hypothalamic-pituitary-somatotropic (HPS) axis activity, which plays a key role in regulating mammalian metabolism. Leptin insensitivity/resistance is a pathological condition in humans, but in seasonal animals, it is a physiological adaptation. Therefore, these animals represent a promising model for studying this phenomenon. This study aimed to determine the influence of leptin on the activity of the HPS axis. Two in vivo experiments performed during short- and long-day photoperiods were conducted on 12 ewes per experiment, and the ewes were divided randomly into 2 groups. The arcuate nucleus, paraventricular nucleus, anterior pituitary (AP) tissues, and blood were collected. The concentration of growth hormone (GH) was measured in the blood, and the relative expression of GHRH, SST, GHRHR, SSTR1, SSTR2, SSTR3, SSTR5, LEPR, and GH was measured in the collected brain structures. The study showed that the photoperiod, and therefore leptin sensitivity, plays an important role in regulating HPS axis activity in the seasonal ewe. However, leptin influences the release of GH in a season-dependent manner, and its effect seems to be targeted at the posttranscriptional stages of GH secretion.

Molecular targets in acromegaly

Molecular therapeutic targets in growth hormone (GH)-secreting adenomas range from well-characterized surface receptors that recognize approved drugs, to surface and intracellular markers that are potential candidates for new drug development. Currently available medical therapies for patients with acromegaly bind to somatostatin receptors, GH receptor, or dopamine receptors, and lead to attainment of disease control in most patients. The degree of control is variable: however, correlates with both disease aggressiveness and tumor factors that predict treatment response including somatostatin receptor subtype expression, granulation pattern, kinases and their receptors, and other markers of proliferation. A better understanding of the mechanisms underlying these molecular markers and their relationship to outcomes holds promise for expanding treatment options as well as a more personalized approach to treating patients with acromegaly.

Identification of SSTR5 Gene Polymorphisms and Their Association With Growth Traits in Hulun Buir Sheep

The aim of this study was to locate SSTR5 polymorphisms and evaluate their association with growth traits in Hulun Buir sheep. The study followed up 884 Hulun Buir sheep from birth to 16 months of age, which were born in the same pasture and the same year, and a consistent grazing management strategy was maintained. The birth weight (BRW) was recorded at birth, and body weight (BW), body height (BH), body length (BL), chest circumference (ChC), chest depth (ChD), chest width (ChW), hip width (HW), and cannon circumference (CaC) were measured at 4 and 9 months of age. BW, BH, BL, ChD, HW, and CaC were also recorded at 16 months of age. Based on the growth traits, 233 sheep were selected as experimental animals. Sanger sequencing was performed, and seven single-nucleotide polymorphisms (SNPs) were identified. Association analyses of the SNPs and the growth traits were then conducted. Seven SNPs of the SSTR5 exhibited moderate polymorphism (0.25 Collapse

Key Words

• Hulun Buir sheep
• SSTR5
• association
• growth traits
• haplotypes

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MESH Headings Collapse

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Affiliation(s)

Xue Li Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China University of Chinese Academy of Sciences, Beijing, China
Ning Ding Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China University of Chinese Academy of Sciences, Beijing, China
Zhichao Zhang Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China University of Chinese Academy of Sciences, Beijing, China
Dehong Tian Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China
Buying Han Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China University of Chinese Academy of Sciences, Beijing, China
Dehui Liu Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China University of Chinese Academy of Sciences, Beijing, China
Sijia Liu Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China
Fei Tian Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China
Dejun Fu Inner Mongolia Daxing 'anling Agricultural Reclamation Group Co. LTD., Hulun Buir, China
Xiaoliang Song Inner Mongolia Daxing 'anling Agricultural Reclamation Group Co. LTD., Hulun Buir, China
Kai Zhao Key Laboratory of Adaptation and Evolution of Plateau Biota, Qinghai Provincial Key Laboratory of Animal Ecological Genomics, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, China

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The value of [68Ga]Ga-DOTA-TATE PET/CT in diagnosis and management of suspected pituitary tumors

Background

Gallium 68-tetraazacyclododecane-tetraacetic acid-octreotate ([68Ga]Ga-DOTA-TATE) is a selective somatostatin analogue ligand, which shows increased affinity for somatostatin receptor subtype (SSTR) 2 and has been used routinely for imaging neuroendocrine tumors with PET/CT. We investigated the utility of [68Ga]Ga-DOTA-TATE positron emission tomography/computed tomography (PET/CT) in patients with suspected pituitary pathology. We reviewed imaging for twenty consecutive patients (8 men, 12 women, mean age of 48.2, range 14–78) with suspected pituitary pathology who were referred for [68Ga]Ga-DOTA-TATE PET/CT.

Results

Nine patients presented with recurrent Cushing’s syndrome following surgical resection of pituitary adenomas due to recurrent Cushing’s disease (seven patients) and ectopic ACTH secreting tumor (2 patients). All seven patients with recurrent Cushing’s disease showed positive pituitary [68Ga]Ga-DOTA-TATE uptake while both cases of ectopic hormonal secretion had absented pituitary uptake. In 1 of these 2 patients, [68Ga]Ga-DOTA-TATE was able to localize the source of ectopic ACTH tumor.

Six patients presented de novo with Cushing’s due to ectopic ACTH secretion; [68Ga]Ga-DOTA-TATE PET/CT was able to localize ectopic tumors in six of eight patients (3 lungs, 2 pancreases, 1 mid-gut)

There was high uptake [68Ga]Ga-DOTA-TATE in 3 cases of recurrent central hyperthyroidism (SUVmax 6.6–14.3) and 2 cases of prolactinoma (SUVmax 5.5 and 11.3).

Conclusion

Absent [68Ga]Ga-DOTA-TATE activity in the pituitary fossa is useful in excluding pituitary disease in recurrent Cushing’s. Recurrent pituitary thyrotropinomas and prolactinomas showed moderate to high pituitary activity. In addition, in Cushing’s syndrome, [68Ga]Ga-DOTA-TATE is useful for detection of ectopic sources of ACTH production, especially where anatomic imaging is negative.

Evolution of thyrotropin-releasing factor extracellular communication units

Thyroid hormones (THs) are ancient signaling molecules that contribute to the regulation of metabolism, energy homeostasis and growth. In vertebrates, the hypothalamus-pituitary-thyroid (HPT) axis links the corresponding organs through hormonal signals, including thyrotropin releasing factor (TRF), and thyroid stimulating hormone (TSH) that ultimately activates the synthesis and secretion of THs from the thyroid gland. Although this axis is conserved among most vertebrates, the identity of the hypothalamic TRF that positively regulates TSH synthesis and secretion varies. We review the evolution of the hypothalamic factors that induce TSH secretion, including thyrotropin-releasing hormone (TRH), corticotrophin-releasing hormone (CRH), urotensin-1-3, and sauvagine, and non-mammalian glucagon-like peptide in metazoans. Each of these peptides is part of an extracellular communication unit likely composed of at least 3 elements: the peptide, G-protein coupled receptor and bioavailability regulator, set up on the central neuroendocrine articulation. The bioavailability regulators include a TRH-specific ecto-peptidase, pyroglutamyl peptidase II, and a CRH-binding protein, that together with peptide secretion/transport rate and transduction coupling and efficiency at receptor level shape TRF signal intensity and duration. These vertebrate TRF communication units were coopted from bilaterian ancestors. The bona fide elements appeared early in chordates, and are either used alternatively, in parallel, or sequentially, in different vertebrate classes to control centrally the activity of the HPT axis. Available data also suggest coincidence between apparition of ligand and bioavailability regulator.

Somatostatin-Dopamine Chimeric Molecules in Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) are a widely heterogeneous family of neoplasms arising from neuroendocrine cells, which are interspersed throughout the body. Despite NENs are relatively rare, their incidence and prevalence are constantly increasing probably due to the improvement in earlier diagnosis and patients’ management. When surgery is not curative, particularly for patients with metastatic disease, several medical options are available. Somatostatin analogues (SSA) are the first-line medical therapy for well-differentiated NENs. Interestingly, the heterodimerization of somatostatin receptors (SSTs) with dopamine receptors (DRs) has been discovered in NENs. This phenomenon results in hybrid receptors with enhanced functional activity. On these bases, chimeric molecules embracing somatostatin and dopamine features have been recently developed. The aim of this review is to provide a comprehensive overview of the available preclinical and clinical data regarding chimeric somatostatin-dopamine agonists as a new class of “magic bullet” in the therapy of NENs.

Glucocorticoid Receptor Antagonism Upregulates Somatostatin Receptor Subtype 2 Expression in ACTH-Producing Neuroendocrine Tumors: New Insight Based on the Selective Glucocorticoid Receptor Modulator Relacorilant

Somatostatin exhibits an inhibitory effect on pituitary hormone secretion, including inhibition of growth hormone and adrenocorticotropic hormone (ACTH), and it can have antisecretory and antitumor effects on neuroendocrine tumors (NETs) that express somatostatin receptors. Although the precise mechanism remains unclear, the finding that glucocorticoids downregulate somatostatin receptor subtype 2 (SSTR2) expression has been used to explain the lack of efficacy of traditional SSTR2-targeting analogs in patients with ACTH-secreting NETs. Glucocorticoid receptor (GR) antagonism with mifepristone has been shown to reverse the glucocorticoid-induced downregulation of SSTR2; however, the effects of GR modulation on SSTR2 expression in ACTH-secreting NETs, particularly corticotroph pituitary tumors, are not well known. The current study presents new insight from in vitro data using the highly selective GR modulator relacorilant, showing that GR modulation can overcome dexamethasone-induced suppression of SSTR2 in the murine At-T20 cell line. Additional data presented from clinical case observations in patients with ACTH-secreting NETs suggest that upregulation of SSTR2 via GR modulation may re-sensitize tumors to endogenous somatostatin and/or somatostatin analogs. Clinical, laboratory, and imaging findings from 4 patients [2 ACTH-secreting bronchial tumors and 2 ACTH-secreting pituitary tumors (Cushing disease)] who were treated with relacorilant as part of two clinical studies (NCT02804750 and NCT02762981) are described. In the patients with ectopic ACTH secretion, SSTR2-based imaging (Octreoscan and ⁶⁸Ga-DOTATATE positron emission tomography) performed before and after treatment with relacorilant showed increased radiotracer uptake by the tumor following treatment with relacorilant without change in tumor size at computed tomography. In the patients with Cushing disease who received relacorilant prior to scheduled pituitary surgery, magnetic resonance imaging after a 3-month course of relacorilant showed a reduction in tumor size. Based on these findings, we propose that GR modulation in patients with ACTH-secreting NETs upregulates previously suppressed SSTR2s, resulting in tumor-specific antisecretory and anti-proliferative effects. The effect of relacorilant on pituitary corticotroph tumors is being investigated in an ongoing phase 3 study (NCT03697109; EudraCT 2018-003096-35).

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.

Comparative distribution of somatostatin and somatostatin receptors in PTU-induced hypothyroidism

PURPOSE

Propylthiouracil (PTU)-induced hypothyroidism is a well-established model for assessing hormonal and morphological changes in thyroid as well as other central and peripheral tissues. Somatostatin (SST) is known to regulate hormonal secretion and synthesis in endocrine tissues; however, nothing is currently known about the distribution of SST and its receptor in hypothyroidism.

METHOD

In the present study, the comparative immunohistochemical distribution of SST and somatostatin receptors (SSTRs) were analyzed in PTU-induced hypothyroid rats. Rats were treated with PTU for 15 days followed by a co-administration of levothyroxine (LVT) for 15 days. After PTU and LVT treatments (day 30), rats were further administered LVT alone for 15 more days (day 45). The subcellular distribution of SST and SSTR subtypes was determined by peroxidase immunohistochemistry in the thyroid gland collected from control and treated rats.

RESULTS

SST and SSTR subtypes were found to be moderately expressed in control thyroid tissues. SST and SSTR subtypes like immunoreactivity increased significantly in follicular and parafollicular epithelial cells in the thyroid of PTU-treated rats. The PTU-induced changes in the expression of SST and SSTR subtypes were suppressed by the administration of the LVT. In addition to thyroid tissues, SST and SSTRs expression was also changed in non-follicular tissues including blood vessels, smooth muscle cells, and connective tissue following treatments.

CONCLUSION

The present study revealed a distinct subcellular distribution of SST and SSTR subtypes in the thyroid and provides a new insight for the role of SST and SSTR subtypes in hypothyroidism in addition to its well-established role in negative regulation of hormonal secretion.

Molecular-Level Understanding of the Somatostatin Receptor 1 (SSTR1)-Ligand Binding: A Structural Biology Study Based on Computational Methods

Somatostatin receptor 1 (SSTR1), a subtype of somatostatin receptors, is involved in various signaling mechanisms in different parts of the human body. Like most of the G-protein-coupled receptors (GPCRs), the available information on the structural features of SSTR1 responsible for the biological activity is scarce. In this study, we report a molecular-level understanding of SSTR1-ligand binding, which could be helpful in solving the structural complexities involved in SSTR1 functioning. Based on a three-dimensional quantitative structure-activity relationship (3D-QSAR) study using comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA), we have identified that an electronegative, less-bulkier, and hydrophobic atom substitution can substantially increase the biological activity of SSTR1 ligands. A density functional theory (DFT) study has been followed to study the electron-related properties of the SSTR1 ligands and to validate the results obtained via the 3D-QSAR study. 3D models of SSTR1-ligand systems have been embedded in lipid-lipid bilayer membranes to perform molecular dynamics (MD) simulations. Analysis of the MD trajectories reveals important information about the crucial residues involved in SSTR1-ligand binding and various conformational changes in the protein that occur after ligand binding. Additionally, we have identified the probable ligand-binding site of SSTR1 and validated it using MD. We have also studied the favorable conditions that are essential for forming the most stable and lowest-energy bioactive conformation of the ligands inside the binding site. The results of the study could be useful in constructing more potent and novel SSTR1 antagonists and agonists.

Clinical significance of filamin A in patients with acromegaly and its association with somatostatin and dopamine receptor profiles

Filamin-A (FLNA) plays a crucial role in somatostatin receptor (sst) subtype-2 signaling in somatotropinomas. Our objective was to investigate the in vivo association between FLNA and sst2 expression, sst5 expression, dopamine receptor subtype-2 (D2) expression, somatostatin receptor ligand (SRL) responsiveness and tumor invasiveness in somatotropinomas. Quantitative real-time PCR was used to evaluate the absolute mRNA copy numbers of FLNA/sst2/sst5/D2 in 96 somatotropinomas. FLNA, sst2 and sst5 protein expression levels were also evaluated using immunohistochemistry. The Knosp-Steiner criteria were used to evaluate tumor invasiveness. Median FLNA, sst2, sst5 and D2 copy numbers were 4,244, 731, 156 and 3,989, respectively. Thirty-one of the 35 available tumors (89%) were immune positive for FLNA in the cytoplasm and membrane but not in the nucleus. FLNA and sst5 expression were positively correlated at the mRNA and protein levels (p < 0.001 and p = 0.033, respectively). FLNA was positively correlated with sst2 mRNA in patients who were responsive to SRL (p = 0.014, R = 0.659). No association was found between FLNA and tumor invasiveness. Our findings show that in somatotropinomas FLNA expression positively correlated with in vivo sst5 and D2 expression. Notably, FLNA was only correlated with sst2 in patients who were controlled with SRL. FLNA was not associated with tumor invasiveness.

Systemic Complications of Acromegaly and the Impact of the Current Treatment Landscape: An Update

Acromegaly is a chronic systemic disease with many complications and is associated with increased mortality when not adequately treated. Substantial advances in acromegaly treatment, as well as in the treatment of many of its complications, mainly diabetes mellitus, heart failure, and arterial hypertension, were achieved in the last decades. These developments allowed change in both prevalence and severity of some acromegaly complications and furthermore resulted in a reduction of mortality. Currently, mortality seems to be similar to the general population in adequately treated patients with acromegaly. In this review, we update the knowledge in complications of acromegaly and detail the effects of different acromegaly treatment options on these complications. Incidence of mortality, its correlation with GH (cumulative exposure vs last value), and IGF-I levels and the shift in the main cause of mortality in patients with acromegaly are also addressed.

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.

Toward a better understanding of the interaction between somatostatin receptor 2 and its ligands: a structural characterization study using molecular dynamics and conceptual density functional theory

This study is a part of the extensive research intending to provide the structural insights on somatostatin and its receptor. Herein, we have studied the structural complexity involved in the binding of somatostatin receptor 2 (SSTR2) with its agonists and antagonist. A 3D QSAR study based on comparative molecular field analysis and comparative molecular similarity analysis (CoMSIA) discerned that a SSTR2 ligand with electronegative, less-bulkier, and hydrogen atom donating/accepting substitutions is important for their biological activity. A conceptual density functional theory (DFT) study was followed to study the chemical behavior of the ligands based on the molecular descriptors derived using the Fukui's molecular orbital theory. We have performed molecular dynamics simulations of receptor-ligand complexes for 100 ns to analyze the dynamic stability of the backbone Cα atoms of the receptor and strength and approachability of the receptor-ligand complex. The findings of this study could be efficacious in the further studies understanding intricate structural features of the somatostatin receptors and in discovering novel subtype-specific ligands with higher affinity. Communicated by Ramaswamy H. Sarma.

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

AIP and the somatostatin system in pituitary tumours

Classic somatostatin analogues aimed at somatostatin receptor type 2, such as octreotide and lanreotide, represent the mainstay of medical treatment for acromegaly. These agents have the potential to decrease hormone secretion and reduce tumour size. Patients with a germline mutation in the aryl hydrocarbon receptor-interacting protein gene, AIP, develop young-onset acromegaly, poorly responsive to pharmacological therapy. In this review, we summarise the most recent studies on AIP-related pituitary adenomas, paying special attention to the causes of somatostatin resistance; the somatostatin receptor profile including type 2, type 5 and truncated variants; the role of G proteins in this pathology; the use of first and second generation somatostatin analogues; and the role of ZAC1, a zinc-finger protein with expression linked to AIP in somatotrophinoma models and acting as a key mediator of octreotide response.

Clinical Importance of Somatostatin Receptor 2 (SSTR2) and Somatostatin Receptor 5 (SSTR5) Expression in Thyrotropin-Producing Pituitary Adenoma (TSHoma)

Background

Thyrotropin-secreting pituitary adenomas (TSHomas) are a rare cause of hyperthyroidism. Somatostatin analogs have proved to be effective for inhibiting pituitary hormones secretion, working via interactions with somatostatin receptors (SSTRs). Moreover, antiproliferative activity of somatostatin analog is now demonstrated in several studies. In the present study, we determined the relative predominance of SSTR2 and SSTR5 subtypes among the different types of adenomas, especially TSHoma, and investigated the relationship between efficacy of short-term octreotide (OCT) treatment and SSTR expression.

Material/Methods

Serum hormone determinations and histological findings in resected tissue resulted in 5 diagnoses: 16 TSHomas, 8 acromegaly, 3 prolactinomas, 3 corticotropinomas, 4 clinically nonfunctioning adenomas (NFPAs), and 4 normal pituitary specimens. IHC was performed on formalin-fixed and paraffin-embedded tissue in tissue microarrays.

Results

IHC of SSTR subtypes in the different cohorts showed SSTR2 staining intensity scores higher than SSTR5 in TSHoma, acromegaly and prolactinoma, whereas the expression of SSTR5 was stronger than SSTR2 in corticotropinoma and NFPA. SSTR2 and SSTR5 expressions were significantly higher in TSHoma than in other pituitary adenomas. OCT treatment for a median of 8.4 days (range: 3–18 days) and with a total median dose of 1.9 mg (range: 0.9–4.2 mg) showed a significant decrease of thyroid hormone levels (TSH [μIU/ml] in all patients. Patients with low SSTR5 expression presented a significantly higher TSH suppression rate (P values <0.05).

Conclusions

The present data confirm that somatostatin analogs should be considered as a medical alternative to surgical treatment, especially in patients with TSHoma, and short-term response to OCT therapy may be related to the expression of SSTR5.

Theoretical analysis of somatostatin receptor 5 with antagonists and agonists for the treatment of neuroendocrine tumors

We report on SSTR5 receptor modeling and its interaction with reported antagonist and agonist molecules. Modeling of the SSTR5 receptor was carried out using multiple templates with the aim of improving the precision of the generated models. The selective SSTR5 antagonists, agonists and native somatostatin SRIF-14 were employed to propose the binding site of SSTR5 and to identify the critical residues involved in the interaction of the receptor with other molecules. Residues Q2.63, D3.32, Q3.36, C186, Y7.34 and Y7.42 were found to be highly significant for their strong interaction with the receptor. SSTR5 antagonists were utilized to perform a 3D quantitative structure-activity relationship study. A comparative molecular field analysis (CoMFA) was conducted using two different alignment schemes, namely the ligand-based and receptor-based alignment methods. The best statistical results were obtained for ligand-based ([Formula: see text], [Formula: see text] = 0.988, noc = 4) and receptor-guided methods (docked mode 1:[Formula: see text], [Formula: see text], noc = 5), (docked mode 2:[Formula: see text] = 0.555, [Formula: see text], noc = 5). Based on CoMFA contour maps, an electropositive substitution at [Formula: see text], [Formula: see text] and [Formula: see text] position and bulky group at [Formula: see text] position are important in enhancing molecular activity.

Theoretical analysis of somatostatin receptor 5 with antagonists and agonists for the treatment of neuroendocrine tumors

We report on SSTR5 receptor modeling and its interaction with reported antagonist and agonist molecules. Modeling of the SSTR5 receptor was carried out using multiple templates with the aim of improving the precision of the generated models. The selective SSTR5 antagonists, agonists and native somatostatin SRIF-14 were employed to propose the binding site of SSTR5 and to identify the critical residues involved in the interaction of the receptor with other molecules. Residues Q2.63, D3.32, Q3.36, C186, Y7.34 and Y7.42 were found to be highly significant for their strong interaction with the receptor. SSTR5 antagonists were utilized to perform a 3D quantitative structure-activity relationship study. A comparative molecular field analysis (CoMFA) was conducted using two different alignment schemes, namely the ligand-based and receptor-based alignment methods. The best statistical results were obtained for ligand-based ([Formula: see text], [Formula: see text] = 0.988, noc = 4) and receptor-guided methods (docked mode 1:[Formula: see text], [Formula: see text], noc = 5), (docked mode 2:[Formula: see text] = 0.555, [Formula: see text], noc = 5). Based on CoMFA contour maps, an electropositive substitution at [Formula: see text], [Formula: see text] and [Formula: see text] position and bulky group at [Formula: see text] position are important in enhancing molecular activity.

AB3-Loaded and Tumor-Targeted Unimolecular Micelles for Medullary Thyroid Cancer Treatment

Medullary thyroid cancer (MTC) is often resistant to standard therapies, emphasizing the need for the development of other treatments. A new histone deacetylase inhibitor, AB3, can effectively inhibit MTC cell proliferation in vitro. However, its poor aqueous solubility and stability, fast clearance, and lack of tumor targeting ability limit its in vivo application. Therefore, multifunctional unimolecular micelles were developed for targeted delivery of AB3 for MTC therapy. The unimolecular micelles exhibited a spherical core-shell structure, uniform size distribution, and excellent stability. AB3 was encapsulated into the hydrophobic core of the unimolecular micelles, thus significantly enhancing its aqueous solubility and stability. KE108, a somatostatin analog possessing high affinity to all five subtypes of SSTR, was used as an MTC-targeting ligand. In vitro cellular uptake analyses demonstrated that the KE108 exhibited superior targeting ability in MTC cells compared to octreotide, the first clinically used somatostatin analog. Moreover, the AB3-loaded and KE108-conjugated unimolecular micelles exhibited the best efficacy in suppressing MTC cell growth and tumor marker expression in vitro. Furthermore, AB3-loaded, KE108-conjugated micelles demonstrated the best anticancer efficacy in vivo without any apparent systemic toxicity, thereby offering a promising approach for targeted MTC therapy.

KE108-conjugated unimolecular micelles loaded with a novel HDAC inhibitor thailandepsin-A for targeted neuroendocrine cancer therapy

Neuroendocrine (NE) cancers can cause significant patient morbidity. Besides surgery, there are no curative treatments for NE cancers and their metastases, emphasizing the need for the development of other forms of therapy. In this study, multifunctional unimolecular micelles were developed for targeted NE cancer therapy. The unimolecular micelles were formed by multi-arm star amphiphilic block copolymer poly(amidoamine)-poly(valerolactone)-poly(ethylene glycol) conjugated with KE108 peptide and Cy5 dye (abbreviated as PAMAM-PVL-PEG-KE108/Cy5). The unimolecular micelles with a spherical core-shell structure exhibited a uniform size distribution and excellent stability. The hydrophobic drug thailandepsin-A (TDP-A), a recently discovered HDAC inhibitor, was physically encapsulated into the hydrophobic core of the micelles. KE108 peptide, a somatostatin analog possessing high affinity for all five subtypes of somatostatin receptors (SSTR 1-5), commonly overexpressed in NE cancer cells, was used for the first time as an NE cancer targeting ligand. KE108 exhibited superior targeting abilities compared to other common somatostatin analogs, such as octreotide, in NE cancer cell lines. The in vitro assays demonstrated that the TDP-A-loaded, KE108-targeted micelles exhibited the best capabilities in suppressing NE cancer cell growth. Moreover, the in vivo near-infrared fluorescence imaging on NE-tumor-bearing nude mice showed that KE108-conjugated micelles exhibited the greatest tumor accumulation due to their passive targeting and active targeting capabilities. Finally, TDP-A-loaded and KE108-conjugated micelles possessed the best anticancer efficacy without detectable systemic toxicity. Thus, these novel TDP-A-loaded and KE108-conjugated unimolecular micelles offer a promising approach for targeted NE cancer therapy.

Nutrient Sensing Overrides Somatostatin and Growth Hormone-Releasing Hormone to Control Pulsatile Growth Hormone Release

Pharmacological studies reveal that interactions between hypothalamic inhibitory somatostatin and stimulatory growth hormone-releasing hormone (GHRH) govern pulsatile GH release. However, in vivo analysis of somatostatin and GHRH release into the pituitary portal vasculature and peripheral GH output demonstrates that the withdrawal of somatostatin or the appearance of GHRH into pituitary portal blood does not reliably dictate GH release. Consequently, additional intermediates acting at the level of the hypothalamus and within the anterior pituitary gland are likely to contribute to the release of GH, entraining GH secretory patterns to meet physiological demand. The identification and validation of the actions of such intermediates is particularly important, given that the pattern of GH release defines several of the physiological actions of GH. This review highlights the actions of neuropeptide Y in regulating GH release. It is acknowledged that pulsatile GH release may not occur selectively in response to hypothalamic control of pituitary function. As such, interactions between somatotroph networks, the median eminence and pituitary microvasculature and blood flow, and the emerging role of tanycytes and pericytes as critical regulators of pulsatility are considered. It is argued that collective interactions between the hypothalamus, the median eminence and pituitary vasculature, and structural components within the pituitary gland dictate somatotroph function and thereby pulsatile GH release. These interactions may override hypothalamic somatostatin and GHRH-mediated GH release, and modify pulsatile GH release relative to the peripheral glucose supply, and thereby physiological demand.

Efficacy of somatostatin analogues in the treatment of neuroendocrine tumours based on the results of recent clinical trials

Due to their inhibitory effects on hormone secretion, somatostatin analogues are of pivotal importance in the symptomatic treatment of hormone-secreting neuroendocrine tumours. Although several earlier clinical observations supported the view that these biological agents are capable of inhibiting the growth of neuroendocrine tumours, the PROMID study published in 2009 was the first to confirm the inhibitory effect of octreotide on tumour growth and demonstrated the prolongation of progression free survival. These findings have been confirmed and extended by the most recent CLARINET trial with lanreotide published in 2014. Somatostatin analogues are capable of inhibiting tumour growth and stabilizing disease irrespective of the hormonal activity of the tumour and, therefore, their applicability is expected to be extended to the treatment of hormonally inactive neuroendocrine tumours, as well. Orv. Hetil., 2014, 155(48), 1908–1912.

Lanreotide autogel in acromegaly - a decade on

INTRODUCTION

The novel formulation of lanreotide, lanreotide (LAN) autogel (ATG), has been available in Europe since 2001 and USA from 2006 for the treatment of acromegaly. It is one of only two clinically available somatostatin analogs available for use in acromegaly. Data relating to the use of ATG in acromegaly, specifically relating to comparison to octreotide (OCT) LAR and patient acceptability and preference, have been slow to accumulate.

AREAS COVERED

We performed a comprehensive review of the original literature relating to development, pharmacokinetics, acceptability and clinical efficacy of ATG.

EXPERT OPINION

LAN ATG is a novel formulation of LAN consequent on self-assembly of nanotubules in water. Diffusion between molecules within the nanotubules and surrounding tissue fluid in vivo leads to pseudo first-order pharmacokinetics. Efficacy is equivalent to the alternate long-acting somatostatin analog, OCT LAR, normalizing growth hormone and IGF-I levels in around 60 and 50% respectively. Control of tumor growth is observed in over 95% of patients, with 64% seeing a clinically significant reduction in tumor size. ATG is provided in a prefilled syringe for deep subcutaneous injection, allowing self-injection, and may be administered up to 8 weeks greatly improving convenience for the patient. The data strongly support consideration of ATG as the medical therapy of choice for patients with acromegaly.

Somatostatin system: molecular mechanisms regulating anterior pituitary hormones

The somatostatin (SRIF) system, which includes the SRIF ligand and receptors, regulates anterior pituitary gland function, mainly inhibiting hormone secretion and to some extent pituitary tumor cell growth. SRIF-14 via its cognate G-protein-coupled receptors (subtypes 1-5) activates multiple cellular signaling pathways including adenylate cyclase/cAMP, MAPK, ion channel-dependent pathways, and others. In addition, recent data have suggested SRIF-independent constitutive SRIF receptor activity responsible for GH and ACTH inhibition in vitro. This review summarizes current knowledge on ligand-dependent and independent SRIF receptor molecular and functional effects on hormone-secreting cells in the anterior pituitary gland.

Somatostatin receptor ligands and resistance to treatment in pituitary adenomas

Somatostatin (SST), an inhibitory polypeptide with two biologically active forms SST14 and SST28, inhibits GH, prolactin (PRL), TSH, and ACTH secretion in the anterior pituitary gland. SST also has an antiproliferative effect inducing cell cycle arrest and apoptosis. Such actions are mediated through five G-protein-coupled somatostatin receptors (SSTR): SSTR1-SSTR5. In GH-secreting adenomas, SSTR2 expression predominates, and somatostatin receptor ligands (SRLs; octreotide and lanreotide) directed to SSTR2 are presently the mainstays of medical therapy. However, about half of patients show incomplete biochemical remission, but the definition of resistance per se remains controversial. We summarize here the determinants of SRL resistance in acromegaly patients, including clinical, imaging features as well as molecular (mutations, SSTR variants, and polymorphisms), and histopathological (granulation pattern, and proteins and receptor expression) predictors. The role of SSTR5 may explain the partial responsiveness to SRLs in patients with adequate SSTR2 density in the cell membrane. In patients with ACTH-secreting pituitary adenomas, i.e. Cushing's disease (CD), SSTR5 is the most abundant receptor expressed and tumors show low SSTR2 density due to hypercortisolism-induced SSTR2 down-regulation. Clinical studies with pasireotide, a multireceptor-targeted SRL with increased SSTR5 activity, lead to approval of pasireotide for treatment of patients with CD. Other SRL delivery modes (oral octreotide), multireceptor-targeted SRL (somatoprim) or chimeric compounds targeting dopamine D2 receptors and SSTR2 (dopastatin), are briefly discussed.

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.

Somatostatin receptors type 2 and 5 expression and localization during human pituitary development

Somatostatin (SRIF), by acting mainly through sst2 and sst5 receptors, is a potent inhibitor of hormonal secretion by the human anterior pituitary gland. However, the pattern of protein expression of these SRIF receptors remains unknown during pituitary development. To get further insights into the physiological role of SRIF receptors in human development and pituitary function, the present study examined the developmental expression of the sst2 and sst5 receptors in the individual cell types of the anterior human pituitary. Thirteen fetal human pituitaries were investigated between 13 to 38 weeks of gestation (WG) by double-labeling immunofluorescence with antibodies raised against sst2 or sst5 receptors and GH, LH, FSH, TSH, or pro-opiomelanocortin proteins. SRIF immunoreactivity in the hypothalamus and median eminence was investigated at the same developmental ages. Immunoreactivity for the sst2 receptor was evident as early as 13 to 15 WG and onward mainly in TSH-, LH-, and FSH-expressing cells, whereas sst5 immunoreactivity was apparent at the late development stages (35-38 WG). GH-expressing cells mainly expressed sst5 immunoreactivity. SRIF-positive fibers and cells were detected as soon as 13 to 16 WG in the hypothalamus and median eminence and their densities increased with gestational age. The early appearance of hypothalamic SRIF cells and fibers suggests a physiological link between SRIF and its receptors during pituitary development. Whereas sst2 receptors might play a primary role in the differentiation and regulation of TSH, LH, and FSH cells, sst5 receptors appear to be mainly involved in GH regulation from birth onward.

Growth hormone granulation pattern and somatostatin receptor subtype 2A correlate with postoperative somatostatin receptor ligand response in acromegaly: a large single center experience

Acromegaly is associated with serious morbidity and mortality, if not well controlled. Approved somatostatin receptor ligands (SRLs) are a mainstay of medical therapy and exhibit preferential affinity for somatostatin receptor (SSTR) subtype 2. Our objective was to assess whether characteristic features of individual growth hormone (GH)-secreting adenomas at diagnosis, correlated with SRL sensitivity, using defined tumor markers. A retrospective review of 86 consecutive acromegaly surgeries (70 patients) performed between January 2006 and December 2011 was undertaken. Patients with any preoperative medical treatment were excluded. Response to SRL therapy was defined as normalization of insulin-like growth factor 1 (IGF1) and random GH < 1.0 ng/dl. Immunohistochemical staining pattern: sparsely granulated, densely granulated, mixed growth hormone-prolactin (GH/PRL) and SSRT2 positivity (+) were correlated with clinicopathologic features, adenoma recurrence, and SRL treatment response. Two-tailed t test, univariate ANOVA, Kruskal-Wallis and bivariate correlation were performed using PAWS 18. The cohort eligible for analysis comprised 59 patients (41 female and 18 male). Based on pre-surgery adenoma imaging dimensions, 81.3% (48) were macroadenomas and average maximum tumor diameter was 18.1 ± 9.9 mm. Patients on SRLs were followed for 13.4 ± 15.8 (mean ± SD) months. Sparsely granulated adenomas were significantly larger at diagnosis, exhibited lower SSTR2 positivity and had a lower rate of biochemical normalization to SRLs. Densely granulated adenomas were highly responsive to SRLs. Overall, patients with SSTR2A+ adenomas responded more favorably to SRL treatment than those with SSTR2A- adenomas. Eighty-one percent of patients with SSTR2A+ adenomas were biochemically controlled (both GH and IGF1) on SRL treatment, e.g. a much higher normalization rate than that reported in the unselected acromegaly population (20-30%). Detailed knowledge of adenoma GH granularity and the immunohistochemical SSTR2A+ status is a predictor of SRL response. These immunoreactive markers should be assessed routinely on surgical specimens to assess subsequent SRL responsiveness and potential need for adjunctive therapy after surgery.

Evaluation of somatostatin receptor subtype expression in human neuroendocrine tumors using two sets of new monoclonal antibodies

INTRODUCTION

The expression and reliable detection of somatostatin receptor subtypes (SSTR1-5) is a prerequisite for the successful use of somatostatin analogs in neuroendocrine tumors (NETs). Two sets of monoclonal antibodies (mAbs) against human SSTR1, 2A, 3 and 5 have recently been developed by two independent laboratories using rabbit and mouse hybridomas. Our aim was to evaluate the usefulness of both sets of mAbs for detection of SSTRs in NET samples as they are routinely collected in clinical practice.

METHODS

Mouse and rabbit mAbs were characterized in SSTR1, 2A, 3 and 5-transfected HEK293 cells and human archival samples of pancreatic tissue and NET. Comparative analysis of mAbs was also conducted by immunostaining of a tissue microarray composed of 75 cores of NET.

RESULTS

Immunohistochemical analysis of HEK293 cells showed that both rabbit and mouse mAbs specifically detect their cognate receptor subtype, with mild cytoplasmic cross-reactivity observed for rabbit mAbs. Both sets of mAbs labeled normal pancreatic islets and showed similar patterns of immunoreactivity in NET controls. Direct comparison of mAb sets using a NET tissue microarray revealed strong correlation between rabbit and mouse mAbs against SSTR1 and 5, and moderate correlation for SSTR3. The rabbit mAb against SSTR2A showed higher affinity for its cognate receptor than the corresponding mouse mAb, resulting in a more reliable detection of this SSTR.

CONCLUSIONS

mAbs from both sets are reliable tools for the detection of SSTR1, 3 and 5, whereas the rabbit mAb against SSTR2A is recommended for use in routine clinical testing due to its superior binding affinity.

Molecular and functional properties of densely and sparsely granulated GH-producing pituitary adenomas

OBJECTIVE

GH-producing pituitary adenomas display two distinct morphological patterns of cytoplasmic GH-containing secretory granules, namely the densely and sparsely granulated somatotroph adenoma subtype. It is unknown whether these morphological variants reflect distinct pathophysiological entities at the molecular level.

METHODS

In 28 GH-producing adenoma tissues from a consecutive set of patients undergoing pituitary surgery for acromegaly, we studied the GH granulation pattern, the expression of somatostatin receptor subtypes (SSTR) as well as the calcium, cAMP and ZAC1 pathways in primary adenoma cell cultures.

RESULTS

The expression of GSP oncogene was similar between densely and sparsely granulated somatotroph adenoma cells. There were no differences in the calcium, cAMP and ZAC1 pathways as well as in their regulation by SSTR agonists. SSTR2 was exclusively expressed in densely but not in sparsely granulated tumours (membrane expression 86 vs 0%; cytoplasmic expression 67 vs 0%). By contrast, expression of SSTR5 was only found in sparsely but not in densely granulated somatotroph adenomas (membrane expression 29 vs 0%; cytoplasmic expression 57 vs 0%).

CONCLUSIONS

Our results indicate that different granulation patterns in GH-producing adenomas do not reflect differences in pathways and factors pivotal for somatotroph differentiation and function. In vitro, the vast majority of both densely and sparsely granulated tumour cells were responsive to SSTR activation at the molecular level. Sparsely granulated adenomas lacking SSTR2, but expressing SSTR5, might be responsive to novel SSTR agonists with increased affinity to SSTR5.

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.

Biotherapies for GEP-NETs

Biological treatment for GI neuroendocrine tumours (NETs) includes treatment with somatostatin analogues and alpha interferons. Both of these therapies were developed in the early 1980's and initially for treatment of a carcinoid syndrome in patients with small intestinal NETs. Later on tumour biology studies indicated that well differentiated NETs (G1-tumours) benefit from treatment with somatostatin analogues and alpha interferons. Both agents give symptomatic improvement in patients with functioning tumours in 40-60% of the patients, biochemical responses in 50-70% of the patients and significant tumour shrinkage in 5-10% of the patients. Combination therapy with somatostatin analogues and alpha interferon has demonstrated some clinical benefit.

IN CONCLUSION

Somatostatin analogues and alpha interferons are still playing an important role and considered to be first-line treatment in functioning and in non-functioning well-differentiated NETs, (G1-tumours) and somatostatin analogues might also be applied to control clinical symptoms in G2-tumours with higher proliferation.

Somatotroph pituitary adenoma with acromegaly and autosomal dominant polycystic kidney disease: SSTR5 polymorphism and PKD1 mutation

A 39-year-old woman with autosomal dominant polycystic kidney disease (ADPKD) presented with acromegaly and a pituitary macroadenoma. There was a family history of this renal disorder. She had undergone surgery for pituitary adenoma 6 years prior. Physical examination disclosed bitemporal hemianopsia and elevation of both basal growth hormone (GH) 106 ng/mL (normal 0-5) and insulin-like growth factor (IGF-1) 811 ng/mL (normal 48-255) blood levels. A magnetic resonance imaging scan disclosed a 3.0 cm sellar and suprasellar mass with both optic chiasm compression and left cavernous sinus invasion. Pathologic, cytogenetic, molecular and in silico analysis was undertaken. Histologic, immunohistochemical and ultrastructural studies of the lesion disclosed a sparsely granulated somatotroph adenoma. Standard chromosome analysis on the blood sample showed no abnormality. Sequence analysis of the coding regions of PKD1 and PKD2 employing DNA from both peripheral leukocytes and the tumor revealed the most common PKD1 mutation, 5014_5015delAG. Analysis of the entire SSTR5 gene disclosed the variant c.142C>A (p.L48M, rs4988483) in the heterozygous state in both blood and tumor, while no pathogenic mutations were noted in the MEN1, AIP, p27Kip1 and SSTR2 genes. To our knowledge, this is the fourth reported case of a GH-producing pituitary adenoma associated with ADPKD, but the first subjected to extensive morphological, ultrastructural, cytogenetic and molecular studies. The physical proximity of the PKD1 and SSTR5 genes on chromosome 16 suggests a causal relationship between ADPKD and somatotroph adenoma.

Somatostatin dramatically stimulates growth hormone release from primate somatotrophs acting at low doses via somatostatin receptor 5 and cyclic AMP

Somatostatin and cortistatin have been shown to act directly on pituitary somatotrophs to inhibit growth hormone (GH) release. However, previous results from nonprimate species indicate that these peptides can also directly stimulate GH secretion, at low concentrations. The relevance of this phenomenon in a nonhuman primate model was investigated in the present study by testing the impact of somatostatin/cortistatin on GH release in primary pituitary cell cultures from baboons. High doses (> 10(-10) m) of somatostatin/cortistatin did not alter basal GH secretion but blocked GH-releasing hormone (GHRH)- and ghrelin-induced GH release. However, at low concentrations (10(-17)-10(-13) m), somatostatin/cortistatin dramatically stimulated GH release to levels comparable to those evoked by GHRH or ghrelin. Use of somatostatin receptor (sst) specific agonists/antagonists, and signal transduction blockers indicated that sst2 and sst1 activation via intact adenylate cylcase and mitogen-activated protein kinase systems mediated the inhibitory actions of high-concentration somatostatin. By contrast, the stimulatory actions of low-dose somatostatin on GH release were mediated by sst5 signalling through adenylate cylcase/cAMP/protein kinase A and intracellular Ca(2+) pathways, and were additive with ghrelin (not GHRH). Notably, low-concentrations of somatostatin, similar to sst5-agonists, inhibited prolactin release. These results clearly demonstrate that the ultimate impact of somatostatin/cortistatin on hormone release is dose-dependent, cell type-selective and receptor-specific, where the stimulatory effects of low-concentration somatostatin/cortistatin on GH release extend to primates, thereby supporting the notion that this action is relevant in regulating GH secretion in humans.

Activation of somatostatin 2 receptors in the brain and the periphery induces opposite changes in circulating ghrelin levels: functional implications

Somatostatin is an important modulator of neurotransmission in the central nervous system and acts as a potent inhibitor of hormone and exocrine secretion and regulator of cell proliferation in the periphery. These pleiotropic actions occur through interaction with five G protein-coupled somatostatin receptor subtypes (sst(1) (-) (5)) that are widely expressed in the brain and peripheral organs. The characterization of somatostatin's effects can be investigated by pharmacological or genetic approaches using newly developed selective sst agonists and antagonists and mice lacking specific sst subtypes. Recent evidence points toward a divergent action of somatostatin in the brain and in the periphery to regulate circulating levels of ghrelin, an orexigenic hormone produced by the endocrine X/A-like cells in the rat gastric mucosa. Somatostatin interacts with the sst(2) in the brain to induce an increase in basal ghrelin plasma levels and counteracts the visceral stress-related decrease in circulating ghrelin. By contrast, stimulation of peripheral somatostatin-sst(2) signaling results in the inhibition of basal ghrelin release and mediates the postoperative decrease in circulating ghrelin. The peripheral sst(2)-mediated reduction of plasma ghrelin is likely to involve a paracrine action of D cell-derived somatostatin acting on sst(2) bearing X/A-like ghrelin cells in the gastric mucosa. The other member of the somatostatin family, named cortistatin, in addition to binding to sst(1) (-) (5) also directly interacts with the ghrelin receptor and therefore may simultaneously modulate ghrelin release and actions at target sites bearing ghrelin receptors representing a link between the ghrelin and somatostatin systems.

Homologous and heterologous in vitro regulation of pituitary receptors for somatostatin, growth hormone (GH)-releasing hormone, and ghrelin in a nonhuman primate (Papio anubis)

Secretion of GH by pituitary somatotrophs is primarily stimulated by GHRH and ghrelin and inhibited by somatostatin through the activation of specific receptors [GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R) and somatostatin receptors (sst1-5), respectively]. However, we have shown that somatostatin, at low doses, can also stimulate GH release, directly and specifically, in primary pituitary cultures from a nonhuman primate (baboons, Papio anubis) and pigs. To determine whether somatostatin, GHRH, and ghrelin can also regulate the expression of their receptors in primates, pituitary cultures from baboons were treated for 4 h with GHRH or ghrelin (10(-8) m) or with high (10(-7) m) and low (10(-15) m) doses of somatostatin, and GH release and expression levels of all receptors were measured. GHRH/ghrelin decreased the expression of their respective receptors (GHRH-R and GHS-R). Both peptides increased sst1, only GHRH decreased sst5 expression, whereas sst2 expression remained unchanged. The effects of GHRH/ghrelin were completely mimicked by forskolin (adenylate cyclase activator) and phorbol 12-myristate 13-acetate (protein kinase C activator), respectively, indicating the regulation of receptor subtype levels by GHRH and ghrelin involved distinct signaling pathways. In contrast, high-dose somatostatin did not alter GH release but increased sst1, sst2, and sst5 expression, whereas GHRH-R and GHS-R expression were unaffected. Interestingly, low-dose somatostatin increased GH release and sst1 mRNA but decreased sst5 and GHRH-R expression, similar to that observed for GHRH. Altogether, our data show for the first time in a primate model that the primary regulators of somatotroph function (GHRH/ghrelin/somatostatin) exert both homologous and heterologous regulation of receptor synthesis which is dose and subtype dependent and involves distinct signaling pathways.

Altered expression of somatostatin receptors in pancreatic islets from NOD mice cultured at different glucose concentrations in vitro and in islets transplanted to diabetic NOD mice in vivo

Somatostatin acts via five receptors (sst_1–5). We investigated if the changes in pancreatic islet sst expression in diabetic NOD mice compared to normoglycemic mice are a consequence of hyperglycemia or the ongoing immune reaction in the pancreas. Pancreatic islets were isolated from NOD mice precultured for 5 days and further cultured for 3 days at high or low glucose before examined. Islets were also isolated from NOD mice and transplanted to normal or diabetic mice in a number not sufficient to cure hyperglycemia. After three days, the transplants were removed and stained for sst_1–5 and islet hormones. Overall, changes in sst islet cell expression were more common in islets cultured in high glucose concentration in vitro as compared to the islet transplantation in vivo to diabetic mice. The beta and PP cells exhibited more frequent changes in sst expression, while the alpha and delta cells were relatively unaffected by the high glucose condition. Our findings suggest that the glucose level may alter sst expressed in islets cells; however, immune mechanisms may counteract such changes in islet sst expression.

Effect of somatostatin infusion on peptide YY secretion: studies in the acute and recovery phase of anorexia nervosa and in obesity

OBJECTIVE

Changes in many gastrointestinal peptides, including the anorexigenic peptide YY (PYY), which is produced by L cells, occur in both anorexia nervosa (AN) and obesity (OB). High PYY levels are present in AN, whereas in morbid OB fasting and postprandial PYY secretion is blunted. Somatostatin (somatotropin release-inhibiting factor (SRIF)) reportedly inhibits plasma PYY concentrations in animals and healthy humans, but the effect of a SRIF infusion on spontaneous PYY secretion in AN and OB is unknown.

METHODS

A total of 18 young women, seven with acute AN (A-AN), four with AN in the recovery phase (R-AN), and seven with morbid OB, were studied. All subjects underwent an infusion of SRIF (9 μg/kg i.v./h, over 60 min), with blood samples drawn before and at different time intervals after SRIF administration. Plasma PYY levels were measured at each time point.

RESULTS

SRIF significantly inhibited plasma PYY concentrations in R-AN and OB, without affecting PYY titers in A-AN. In OB, the inhibitory effect of SRIF also persisted at 90 min. Withdrawal of SRIF infusion in R-AN resulted in a prompt restoration of basal plasma PYY levels, whereas termination of SRIF infusion in OB was followed by a slower increase of PYY titers toward baseline levels. After infusion, PYY Δ area under the curve (ΔAUC) in R-AN was significantly higher than those in A-AN and OB patients. A significant difference in PYY ΔAUC between A-AN and OB was present.

CONCLUSIONS

These results suggest the existence of a hypo- and hyper-sensitivity of L cells to the inhibitory effect of SRIF in A-AN and OB respectively.

Growth hormone-releasing hormone: not only a neurohormone

Growth hormone-releasing hormone (GHRH) is mostly thought to act by stimulating the production and release of growth hormone from the pituitary. However, this neuropeptide emerges as a rather pleiotropic hormone in view of the identification of various extrapituitary sources for GHRH production, as well as the demonstration of a direct action of GHRH on several tissues other than the pituitary. Non-pituitary GHRH has a wide spectrum of activity, exemplified by its ability to modulate cell proliferation, especially in malignant tissues, to regulate differentiation of some cell types, and to promote healing of skin wounds. These findings extend the role of GHRH and its analogs beyond its accepted regulation of somatotropic activity and indicate new possibilities for therapeutic intervention.