Somatostatin receptor (SSTR) subtype-selective analogues differentially suppress in vitro growth hormone and prolactin in human pituitary adenomas. Novel potential therapy for functional pituitary tumors

FTO-mediated DSP m6A demethylation promotes an aggressive subtype of growth hormone-secreting pituitary neuroendocrine tumors

BACKGROUND

Growth hormone-secreting pituitary neuroendocrine tumors can be pathologically classified into densely granulated (DGGH) and sparsely granulated types (SGGH). SGGH is more aggressive and associated with a poorer prognosis. While epigenetic regulation is vital in tumorigenesis and progression, the role of N6-methyladenosine (m6A) in aggressive behavior has yet to be elucidated.

METHODS

We performed m6A-sequencing on tumor samples from 8 DGGH and 8 SGGH patients, complemented by a suite of assays including ELISA, immuno-histochemistry, -blotting and -fluorescence, qPCR, MeRIP, RIP, and RNA stability experiments, aiming to delineate the influence of m6A on tumor behavior. We further assessed the therapeutic potential of targeted drugs using cell cultures, organoid models, and animal studies.

RESULTS

We discovered a significant reduction of m6A levels in SGGH compared to DGGH, with an elevated expression of fat mass and obesity-associated protein (FTO), an m6A demethylase, in SGGH subtype. Series of in vivo and in vitro experiments demonstrated that FTO inhibition in tumor cells robustly diminishes hypoxia resistance, attenuates growth hormone secretion, and augments responsiveness to octreotide. Mechanically, FTO-mediated m6A demethylation destabilizes desmoplakin (DSP) mRNA, mediated by the m6A reader FMR1, leading to prohibited desmosome integrity and enhanced tumor hypoxia tolerance. Targeting the FTO-DSP-SSTR2 axis curtailed growth hormone secretion, therefor sensitizing tumors to octreotide therapy.

CONCLUSION

Our study reveals the critical role of FTO in the aggressive growth hormone-secreting pituitary neuroendocrine tumors subtype and suggests FTO may represent a new therapeutic target for refractory/persistent SGGH.

Current role of pasireotide in the treatment of acromegaly

"First-generation" somatostatin receptor agonists (SSTRAs) octreotide and lanreotide are the most commonly used first-line pharmacological therapy for patients with acromegaly. A subset of patients respond only partially or not at all to the first-generation SSTRA, necessitating the use of additional pharmacological agents or other modes of therapy. Pasireotide is a "second-generation" SSTRA that has multi-receptor activity. Prospective studies have shown promise in the use of pasireotide in patients with poor response to first-generation SSTRA. Here we elucidate the molecular pathways of resistance to first-generation SSTRA, the mechanism of action, pre-clinical and clinical evidence of the use of pasireotide in patients having incomplete / lack of response to first-generation SSTRA. We also discuss the clinical, pathological, and radiological markers predicting response to pasireotide, and the difference in side-effect profiles of pasireotide, compared to first-generation SSTRA.

A novel somatostatin receptor ligand for human ACTH - and GH -secreting pituitary adenomas

OBJECTIVE

Somatostatin receptor ligands have come to play a pivotal role in the treatment of both ACTH- and GH-secreting pituitary adenomas. Clinical efficacy averages 30-50%, thus a considerable number of patients with Cushing's disease or acromegaly remain unresponsive to this therapeutic approach. HTL0030310 is a new somatostatin receptor ligand selective for subtype 5 over subtype 2, thus with a different receptor profile compared to clinical somatostatin receptor ligands.

DESIGN

Assessment of the effect of HTL0030310 on hormone secretion in human ACTH- and GH-secreting pituitary adenomas in vitro.

METHODS

Primary cultures from 3 ACTH-secreting and 5 GH-secreting pituitary adenomas were treated with 1, 10 and 100 nM HTL0030310 alone or with 10 nM CRH or GHRH, respectively. Parallel incubations with 10 nM pasireotide were also carried out. ACTH and GH secretion were assessed after 4 and 24 hour incubation; SSTR2, SSTR3, SSTR5, GH and POMC expression were evaluated after 24 hours.

RESULTS

HTL0030310 reduced unchallenged ACTH and POMC levels up to 50% in 2 ACTH-secreting adenomas and blunted CRH-stimulated ACTH/POMC by 20-70% in all 3 specimens. A reduction in spontaneous GH secretion was observed in 4 GH-secreting adenomas and in 2 specimens during GHRH co-incubation. SSTRs expression was detected in all specimens.

CONCLUSIONS

This first study on a novel somatostatin receptor 5-preferring ligand indicates that HTL0030310 can inhibit hormonal secretion in human ACTH- and GH-secreting pituitary adenomas. These findings suggest a potential new avenue for somatostatin ligands in the treatment of Cushing's disease and acromegaly.

Prolactin and Growth Hormone Signaling and Interlink Focused on the Mammosomatotroph Paradigm: A Comprehensive Review of the Literature

Prolactin (PRL) and growth hormone (GH) are peptide hormones that bind to the class 1 cytokine receptor superfamily, a highly conserved cell surface class of receptors. Both hormones control their own secretion via a negative autocrine loop in their own mammosomatotroph, lactotroph or somatotroph. In this regard, GH and PRL are regulated by similar signaling pathways involving cell growth and hormone secretion. Thus, GH and PRL dysregulation and pituitary neuroendocrine tumor (PitNET) development may have common pathogenic pathways. Based on cell linage, lactotroph and somatotroph PitNETs come from pituitary-specific POU-class homeodomain transcription factor (Pit-1). Mammosomatotroph and plurihormonal PitNETs are a unique subtype of PitNETs that arise from a single-cell population of Pit-1 lineage. In contrast, mixed somatotroph-lactotroph PitNETs are composed of two distinct cell populations: somatotrophs and lactotrophs. Morphologic features that distinguish indolent PitNETs from locally aggressive ones are still unidentified, and no single prognostic parameter can predict tumor aggressiveness or treatment response. In this review, we aim to explore the latest research on lactotroph and somatotroph PitNETs, the molecular mechanisms involved in PRL and GH axis regulation and the signaling pathways involved in their aggressiveness, particularly focused on mammosomatotroph and mixed subtypes. Finally, we summarize epidemiological, clinical, and radiological features of these exceptional tumors. We aim to shed light, from basic to clinical settings, on new perspectives and scientific gaps in this field.

Novel Classification of Acromegaly in Accordance with Immunohistochemical Subtypes: Is There Really a Clinical Relevance?

According to the recent studies, immunohistochemical subtypes of growth hormone (GH) secreting adenomas have been considered as a predictive factor in determining the clinical outcomes including biochemical, radiologic, and endocrine remission. In a 20 year-of time period, acromegaly patients who were treated and followed at the Endocrinology Department of our University Hospital were screened for the study. Of total 98 patients, 65 patients who had been operated by transsphenoidal surgery and having postoperative specimens were included. Postoperative specimens of the surgery of the patients were classified into 3 groups based on the histochemical characteristics (densely, sparsely, and mixed). Parasellar extensions of pituitary tumors were classified into the five grades according to Knosp classification. The patients were investigated and evaluated for postoperative clinical progress, remission rates, comorbidities regarding with the histopathological patterns. Of total 65 patients, 31 were classified as densely granulated (group 1), 32 were classified as sparsely granulated (group 2), and 2 patients were assessed as mixed granulated (group 3). There was no difference between groups for age and gender. Pre-treatment of adenoma size in all groups was correlated with each other and the frequency of macroadenoma (1 vs. 2, 77.4 vs. 84.3%) was higher in two groups. Although mean initial GH levels in group 1 was higher than the other groups (p=0.03), IGF1 levels (age and gender matched) were similar in each group. Adenomas in all groups demonstrated noninvasive radiological characteristics (Knosp grade 0-1-2). Ki-67 proliferation index of both groups (64.5 vs. 50%) was predominantly 1%. With a similar follow-up period, the endocrine remission rates (GH<1 μg/l) in groups were 64 vs. 69%, respectively. In conclusion, classification according to immunohistochemical subtypes of growth hormone secreting adenomas may not be a qualified parameter to evaluate patients with patterns of aggressiveness, clinical outcomes, or treatment response.

Molecular Pathways in Prolactinomas: Translational and Therapeutic Implications

Prolactinoma has the highest incidence rate among patients with functional pituitary tumours. Although mostly benign, there is a subgroup that can be aggressive. Some clinical, radiological and pathology features have been associated with a poor prognostic. Therefore, it can be considered as a group of heterogeneous tumours. The aim of this paper is to give an overview of the molecular pathways involved in the behaviour of prolactinoma in order to improve our approach and gain deeper insight into the better understanding of tumour development and its management. This is essential for identifying patients harbouring aggressive prolactinoma and to establish personalised therapeutics options.

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.

Analgesic effect of long-acting somatostatin receptor agonist pasireotide in a patient with acromegaly and intractable headaches

A 22-year-old woman presented with worsening vision loss and headaches. A diagnosis of acromegaly was confirmed after detection of an invasive pituitary macroadenoma and biochemical testing. Despite two attempts of surgical debulking of the tumour and administration of long-acting octreotide and cabergoline, growth hormone (GH) and insulin-like growth factor 1 (IGF-1) levels were uncontrolled. The patient experienced persistent headaches despite surgery, gamma knife radiation and ventriculoperitoneal shunt placement; she was then enrolled in the ACCESS trial (ClinicalTrials.gov identifier, NCT01995734). Pasireotide (Signifor; Signifor LAR) was initiated, which led to reduced GH and IGF-1 levels and resolution of her intractable headaches. This highlights the use of monthly pasireotide in resolving headaches and improved biochemical control in a patient with acromegaly. We postulate that the headaches improved due to an analgesic and/or anti-inflammatory effect mediated by somatostatin receptors targeted by pasireotide. This may represent an additional benefit of pasireotide and requires further investigation.

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.

Somatostatin receptor ligands in the treatment of acromegaly

First-generation somatostatin receptors ligands (SRL) are the mainstay in the medical treatment of acromegaly, however the percentage of patients controlled with these drugs significantly varies in the different studies. Many factors are involved in the resistance to SRL. In this review, we update the physiology of somatostatin and its receptors (sst), the use of SRL in the treatment of acromegaly and the factors involved in the response to these drugs. The SRL act through interaction with the sst, which up to now have been characterized as five subtypes. The first-generation SRL, octreotide and lanreotide, are considered sst2 specific and have biochemical response rates varying from 20 to 70%. Tumor volume reduction can be found in 36-75% of patients. Several factors may determine the response to these drugs, such as sst, AIP, E-cadherin, ZAC1, filamin A and β-arrestin expression in the somatotropinomas. In patients resistant to first-generation SRL, alternative medical treatment options include: SRL high dose regimens, SRL in combination with cabergoline or pegvisomant, or the use of pasireotide. Pasireotide is a next-generation SRL with a broader pattern of interaction with sst. In the light of the recent increase of treatment options in acromegaly and the deeper knowledge of the determinants of response to the current first-line therapy, a shift from a trial-and-error treatment to a personalized one could be possible.

Octreotide and pasireotide (dis)similarly inhibit pituitary tumor cells in vitro

Somatostatin analogs (SSA) are the mainstay of pharmacological treatment for pituitary adenomas. However, some patients escape from therapy with octreotide, a somatostatin receptor 2 (sst2)-preferring SSA, and pasireotide, a novel multi-sst-preferring SSA, may help to overcome this problem. It has been proposed that correspondence between sst1-sst5 expression pattern and SSA-binding profile could predict patient's response. To explore the cellular/molecular features associated with octreotide/pasireotide response, we performed a parallel comparison of their in vitro effects, evaluating sst1-sst5 expression, intracellular Ca²⁺ signaling ([Ca²⁺]_i), hormone secretion and cell viability, in a series of 85 pituitary samples. Somatotropinomas expressed sst5>sst2, yet octreotide reduced [Ca²⁺]_i more efficiently than pasireotide, while both SSA similarly decreased growth hormone release/expression and viability. Corticotropinomas predominantly expressed sst5, but displayed limited response to pasireotide, while octreotide reduced functional endpoints. Non-functioning adenomas preferentially expressed sst3 but, surprisingly, both SSA increased cell viability. Prolactinomas mainly expressed sst1 but were virtually unresponsive to SSA. Finally, both SSA decreased [Ca²⁺]_i in normal pituitaries. In conclusion, both SSA act in vitro on pituitary adenomas exerting both similar and distinct effects; however, no evident correspondence was found with the sst1-sst5 profile. Thus, it seems plausible that additional factors, besides the simple abundance of a given sst, critically influence the SSA response.

Therapeutic innovations in endocrine diseases - Part 4 : Pasireotide: Long-acting release somatostatin analogue

Pasireotide, the latest long-acting release somatostatin analogue, is distributed more widely to the various somatostatin receptors, which theoretically increases its strength and broadens its scope. Does this reflect genuine therapeutic progress? Or rather does its reduced specificity cause too many adverse reactions to make it a significant therapeutic achievement?

BMP-6 modulates somatostatin effects on luteinizing hormone production by gonadrotrope cells

The effects of somatostatin analogs and roles of BMP-6 in the regulation of luteinizing hormone (LH) secretion were investigated using mouse gonadotrope LβT2 cells. LH mRNA expression and LH secretion induced by GnRH were suppressed by treatments with somatostatin analogs, including octreotide and pasireotide, in LβT2 cells. Of note, the inhibitory effects of somatostatin analogs on LH secretion were enhanced by the action of BMP-6. BMP-6 increased the expression levels of somatostatin receptor (SSTR)5, suggesting that BMP-6 upregulates SSTR activity that leads to reduction of GnRH-induced LH secretion. In addition, GnRH-induced phosphorylation of MAPKs including ERK, but not P38 or SAPK, was suppressed by pasireotide in the presence of BMP-6. Given that each inhibitor of ERK, JNK or P38 signaling suppressed GnRH-induced LH transcription, MAPKs are individually involved in the induction of LH production by LβT2 cells. Somatostatin analogs also impaired BMP-6-induced Smad1/5/8 phosphorylation by suppressing BMPRs and augmenting Smad6/7 expression. Collectively, the results indicate that somatostatin analogs have dual effects on the modulation of GnRH-induced MAPK signaling and BMP activity. The pituitary BMP system may play a regulatory role in GnRH-induced LH secretion by tuning the responsiveness to somatostatin analogs in gonadotrope cells.

Bromocriptine, a dopamine agonist, increases growth hormone secretion in a patient with acromegaly

Bromocriptine, a potent D2-dopamine agonist, suppresses growth hormone (GH) secretion in most patients with acromegaly and has been approved for the treatment of acromegaly. Here we report a patient with acromegaly who showed increased GH secretion after administration of bromocriptine. A 70-year-old man with acromegalic manifestation was admitted to our hospital because of a pituitary tumor invading to the right cavernous sinus detected by brain magnetic resonance imaging. Serum GH and insulin-like growth factor-I (IGF-I) levels were elevated in several occasions (GH: 15.0-51.7 ng/mL, reference range: <2.47 ng/mL; and IGF-I: 776-856 ng/mL, reference range: 57-175 ng/mL). Effect of bromocriptine on serum GH levels was then studied because pre-operative treatment with a D2-dopamine agonist was planned in order to reduce the tumor size and serum GH levels before surgery. After oral administration of 2.5 mg of bromocriptine, serum GH levels were unexpectedly increased from 30.7 ng/mL to 189 ng/mL, despite the fact that the levels of prolactin (PRL) were decreased from 4.2 ng/mL to 0.6 ng/mL. By contrast, serum GH levels were decreased by a somatostatin analogue, octreotide. Transsphenoidal surgery of the pituitary tumor was performed after treatment of octreotide. Histological analysis and immunohistochemistry revealed a GH-producing pituitary adenoma positive for D2-dopamine receptor. This case of acromegaly suggests that the preliminary test with a single administration of a short-acting D2-dopamine agonist, bromocriptine, is mandatory before the long-term therapy with a D2-dopamine agonist in patients with GH-secreting pituitary tumors.

Safety and efficacy of oral octreotide in acromegaly: results of a multicenter phase III trial

BACKGROUND

A novel oral octreotide formulation was tested for efficacy and safety in a phase III, multicenter, open-label, dose-titration, baseline-controlled study in patients with acromegaly.

METHODS

We enrolled 155 complete or partially controlled patients (IGF-1 <1.3 × upper limit of normal [ULN], and 2-h integrated GH <2.5 ng/mL) receiving injectable somatostatin receptor ligand (SRL) for ≥ 3 months. Subjects were switched to 40 mg/d oral octreotide capsules (OOCs), and the dose escalated to 60 and then up to 80 mg/d to control IGF-1. Subsequent fixed doses were maintained for a 7-month core treatment, followed by a voluntary 6-month extension.

RESULTS

Of 151 evaluable subjects initiating OOCs, 65% maintained response and achieved the primary endpoint of IGF-1 <1.3 × ULN and mean integrated GH <2.5 ng/mL at the end of the core treatment period and 62% at the end of treatment (up to 13 mo). The effect was durable, and 85 % of subjects initially controlled on OOCs maintained this response up to 13 months. When controlled on OOCs, GH levels were reduced compared to baseline, and acromegaly-related symptoms improved. Of 102 subjects completing the core treatment, 86% elected to enroll in the 6-month extension. Twenty-six subjects who were considered treatment failures (IGF-1 ≥ 1.3 × ULN) terminated early, and 23 withdrew for adverse events, consistent with those known for octreotide or disease related.

CONCLUSIONS

OOC, an oral therapeutic peptide, achieves efficacy in controlling IGF-1 and GH after switching from injectable SRLs for up to 13 months, with a safety profile consistent with approved SRLs. OOC appears to be effective and safe as an acromegaly monotherapy.

Somatostatin receptor 1-5; expression profiles during rat development

BACKGROUND

Somatostatin acts through five receptor subtypes (SSTRs 1-5). We aimed to investigate SSTRs mRNA expression and protein distribution in whole rat embryos, with special emphasis on the pancreas.

MATERIAL AND METHODS

Rat embryos were collected on embryonal days 10, 11, 12, 14, 15, 17, 19, 21, and at birth. Presence of SSTRs was investigated with RT-PCR techniques and immunohistochemistry.

RESULTS

There was no SSTR5 mRNA expression in the whole rat embryos. All SSTR1-5 proteins were observed at embryonal day 10, but the localization varied between the different subtypes. From day 11 to birth SSTRs protein presence increased with time in major structures such as skin and cartilage. It remained similar over time in the heart and liver. In the fetal pancreas mRNA expression of SSTR2 and 4 was detected at day 14, and there was an increase up to birth. Only SSTR1 protein co-localized to a higher extent with the islet hormones studied. SSTR2 was present in all islet endocrine cells except for β-cells. In contrast, the immunostaining for SSTR3-4 was co-localized with insulin and PP, and, finally, SSTR5 with glucagon and pancreatic polypeptide. In mRNA isolated from whole rat embryos SSTR1-2 and SSTR4 expression showed a peak at day 14, while SSTR3 mRNA was not present until day 15.

CONCLUSION

The present data suggest a role for SSTRs during the development of the rat embryo. Subsequent functional studies may elucidate regulatory roles of specific SSTRs for the growth and differentiation of the pancreas as well as other organs.

Medical management of functioning pituitary adenoma: an update

The treatment of functioning pituitary adenoma (FPA) must achieve endocrinological remission as well as tumor size reduction. The first-line treatment of FPA except prolactinoma is transsphenoidal surgery (TSS). Medical treatments and/or radiation will be applied as adjuvant therapies succeeding to TSS. In patients with prolactinoma, dopamine agonists, especially cabergoline, are quite efficient. Dopamine agonists decrease plasma prolactin levels and induce shrinkage in most patients and can be ceased in some of them. In patients with acromegaly, dopamine agonists, somatostatin analogues, and growth hormone receptor antagonist have been used as a monotherapy or the combination, and the high remission rate can be achieved. Pasireotide having high affinity to type 5 somatostatin receptors will be available for the patients presenting resistance against type 2 receptor agonists, such as octreotide and lanreotide. The preceding treatment with somatostatin analogues is beneficial for improving the success rate of TSS. The chimera compounds of somatostatin analogues and dopamine agonists have been investigated. The medical treatments of Cushing's disease are challenging, if TSS is not successful. To suppress ACTH secretion, dopamine agonists and somatostatin analogues have been examined, but neither came to show a sufficient effect. Pasireotide reduces urinary cortisol excretion with a high remission rate. Adrenal enzyme inhibitors (AEIs), such as metyrapone, can inhibit cortisol synthesis form adrenal glands promptly and sufficiently in most of patients. LCI699, a newly developed AEI, is more potent than metyrapone and will be available. We should use available medical treatments for improving the prognosis and quality of life.

Medical management of functioning pituitary adenoma: an update

The treatment of functioning pituitary adenoma (FPA) must achieve endocrinological remission as well as tumor size reduction. The first-line treatment of FPA except prolactinoma is transsphenoidal surgery (TSS). Medical treatments and/or radiation will be applied as adjuvant therapies succeeding to TSS. In patients with prolactinoma, dopamine agonists, especially cabergoline, are quite efficient. Dopamine agonists decrease plasma prolactin levels and induce shrinkage in most patients and can be ceased in some of them. In patients with acromegaly, dopamine agonists, somatostatin analogues, and growth hormone receptor antagonist have been used as a monotherapy or the combination, and the high remission rate can be achieved. Pasireotide having high affinity to type 5 somatostatin receptors will be available for the patients presenting resistance against type 2 receptor agonists, such as octreotide and lanreotide. The preceding treatment with somatostatin analogues is beneficial for improving the success rate of TSS. The chimera compounds of somatostatin analogues and dopamine agonists have been investigated. The medical treatments of Cushing's disease are challenging, if TSS is not successful. To suppress ACTH secretion, dopamine agonists and somatostatin analogues have been examined, but neither came to show a sufficient effect. Pasireotide reduces urinary cortisol excretion with a high remission rate. Adrenal enzyme inhibitors (AEIs), such as metyrapone, can inhibit cortisol synthesis form adrenal glands promptly and sufficiently in most of patients. LCI699, a newly developed AEI, is more potent than metyrapone and will be available. We should use available medical treatments for improving the prognosis and quality of life.

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.

Constitutive somatostatin receptor subtype-3 signaling suppresses growth hormone synthesis

Somatostatin signals through somatostatin receptor subtypes (SSTR) 2 and 5 to attenuate GH secretion. Although expressed in normal pituitary glands and in GH-secreting pituitary tumors, SSTR3 function was unclear, and we have now determined the role of SSTR3 in somatotroph function. Stable rat pituitary tumor cell (GC) transfectants of human SSTR3 (GpSSTR3(WT)) showed suppression of rat (r) GH promoter activity, GH mRNA expression, and secreted GH concordant with suppressed cAMP/protein kinase A (PKA) signaling. In contrast, cAMP levels and GH expression were unchanged in cells expressing a mutant SSTR3 DRY motif (GpSSTR3(R141A)). GH expression was rescued by treatment of GpSSTR3(WT) with forskolin and 8-bromo-cAMP. GpSSTR3(WT) exhibited activation of glycogen synthase kinase3-β (GSK3-β), a PKA substrate, which was also reversed by 8-Bromo-cAMP treatment. Moreover, SSTR3-dependent GH transcriptional inhibition was rescued by inhibition of GSK3-β. GpSSTR3(WT) exhibited elevated Pit-1 serine phosphorylation and decreased Pit-1 occupancy of the rGH promoter with sustained Pit-1 expression. GSK3-β and Pit-1 physically interacted with each other, indicating that Pit-1 may be a GSK3-β phosphorylation substrate. In conclusion, constitutive SSTR3 activity mediates transcriptional repression of GH through cAMP/PKA, leading to subsequent activation of GSK3-β and increased Pit-1 phosphorylation and ultimately attenuating Pit-1 binding to the rGH promoter.

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.

Somatostatin analogs and chimeric somatostatin-dopamine molecules differentially regulate human growth hormone and prolactin gene expression and secretion in vitro

We tested effects of selective somatostatin receptor 2 (SST2) agonist BIM-23120, SST5 agonist BIM-23206 and chimeric somatostatin-dopamine molecules (SRIF/DA) BIM-23A760 and BIM-23A761 on GH and PRL secretion and gene expression in human GH/PRL-secreting pituitary tumors in vitro. In "responders" group BIM-23120 suppressed GH levels by 26±4%, BIM-23206 by 31±5%, BIM-23A760 by 23±4%, BIM-23A761 by 39±8% and D(2)-dopamine agonist BIM-53097 by 31±5%. Using real-time PCR we demonstrated that GH inhibition was not accompanied by decreased GH mRNA levels. PRL secretion was inhibited by BIM-23A760 (29±5%), BIM-23A761 (34±4%), BIM-23206 (26±4%) and BIM-53097 (36±2%). SRIF/DA and BIM-53097 also suppressed PRL mRNA levels. Concluding, SST2 and SST5 agonists and SRIF/DA inhibit GH secretion, but do not suppress GH gene transcription. SRIF/DA and BIM-53097 inhibit both PRL secretion and PRL gene expression. SST5 agonist inhibits PRL secretion, but does not suppress PRL gene expression. D(2) affinity is crucial in SRIF/DA action on PRL gene expression.

Italian Society of Endocrinology Career Award Lecture: from somatostatin to…somatomedin

Somatostatin plays different parts in hormonal regulation through 5 specific receptors in human body. It has two interesting actions such as an antisecretory activity, mostly on the gastrointestinal system and pituitary level, and an antiproliferative action on tumor cells. Many synthetic somatostatin analogues, more stable than the natural one, have been developed and two are already used in different clinical settings, including endocrine oncology. The inhibitory action on tumor growth may result from both indirect actions, namely the suppression of growth factors and growth-promoting hormones (e.g., GH/IGF-I axis) and inhibition of angiogenesis, as well as modulation of the immune system, and direct actions, such as activation of anti-growth activities (e.g., apoptosis). Recently, the development of specific polyclonal antibodies allowed the precise identification of the 5 specific somatostatin receptors and their localization in different cell species. Somatostatin receptor subtypes belong to the G protein-coupled receptor family, share a common molecular topology, and can traffic not only in vitro within different cell types but also in vivo. A picture of the pathways and proteins involved in these processes is beginning to emerge. Moreover, the process of homo- and/or heterodimerization of G-protein coupled receptors and receptor tyrosine kinase families are crucial for implicating the fundamental properties of receptor proteins including receptor expression, trafficking, and desensitization, as well as signal transduction. Furthermore, functional consequences of such an interaction in modulation of signaling pathways linked to pathological conditions specifically in cancer are discussed.

Comparison of octreotide LAR and lanreotide autogel as post-operative medical treatment in acromegaly

Long-acting somatostatin analogs are frequently used as adjuvant treatment of acromegaly patients after noncurative surgery. This sudy aims to compare the efficacy of octreotide long-acting release (OCT) and lanreotide Autogel (LAN) in acromegaly patients. Sixty-eight patients not cured by transsphenoidal endoscopic or microscopic pituitary surgery between 2003 and 2009 were retrospectively analyzed (25 men; 43 women; mean age 41.1 ± 10.9 years [range 18-65 years]). The patients were assigned randomly to OCT (n = 36) and LAN (n = 32) groups. Evaluations included insulin-like growth factor I (IGF-I) and growth hormone (GH) after oral glucose tolerance test (OGTT) 3, 6, 12 and 18 months after starting medical treatment; pituitary magnetic resonance imaging was performed before treatment and after 3 and 12 months. Patients achieving IGF-I levels within the age and gender normal range and GH level <1 μg/l following OGTT were considered a 'biochemical cure'. Mean IGF-I and GH values and tumor volumes (cm(3)) in the LAN and OCT groups were similar in the post-operative period before initiation of medical treatment. A statistically significant decrease in GH and IGF-I levels was obtained for both treatment groups at each follow-up visit compared to the previous value. Tumor shrinkage after 12 months of treatment was statistically significant in both groups but the percentage tumor shrinkage (28.5% vs. 34.9%, P = 0.166) and rate of patients achieving biochemical cure (63.9 and 78.1%, P = 0.454) were similar between OCT and LAN groups, respectively. OCT and LAN treatment options have similar efficacy for ensuring biochemical cure and tumor shrinkage in acromegaly patients who had noncurative surgery.

Pasireotide (SOM230) protects the retina in animal models of ischemia induced retinopathies

The neuropeptide somatostatin and selective analogs for the sst(2/5) receptor subtypes provided neuroprotection against retinal chemical ischemia ex vivo and AMPA [(RS)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide] induced retinal toxicity in vivo, when employed in micromolar concentrations (Mastrodimou et al., 2005; Kiagiadaki and Thermos, 2008). The aim of the present study was to investigate the neuroprotective properties of a new metabolically stable agent pasireotide (SOM230) in the above mentioned retinal models of ischemia. Adult Sprague Dawley (250-350 g) rats were employed. For the ex vivo experiments, retinal eye cups were incubated with PBS or the chemical ischemia mixture [iodoacetic acid (5 mM)/sodium cyanide (25 mM)] in the absence or presence of SOM230 (10(-7)-10(-5) M) alone or in the presence of the sst(2) antagonist CYN-154806 (10(-7) or 10(-5) M). In the in vivo model, the animals received intravitreally: PBS (50 mM), AMPA (42 nmol/eye) or AMPA (42 nmol) in combination with SOM230 (10(-7)-10(-5) M). Immunohistochemistry studies using antisera against bNOS, a marker for brain/neuronal NOS containing amacrine cells, protein kinase C (PKC) a marker for rod bipolar cells, and TUNEL studies in conjunction with FACS analysis were employed to examine retinal cell loss and protection. Chemical ischemia led to a loss of bNOS and PKC immunoreactivity which was reversed by SOM230. Partial and full protection of bNOS and PKC immunoreactive neurons, respectively, was observed even at the low concentration of 10(-7) M. The neuroprotective actions of SOM230 (10(-7) or 10(-5) M) were reversed by CYN-154806 (10(-7) or 10(-5) M, respectively). Similarly, SOM230 (10(-7), 10(-6), 10(-5) M) provided neuroprotection in the in vivo model. The dose of 10(-7) M prevented the loss of the bNOS cells and provided almost full protection. These data were substantiated by TUNEL staining and fluorescence-activated cell sorting (FACS) analysis. SOM230 appears very efficacious in its neuroprotective properties in both models of retinal ischemia affording neuroprotection at the concentration or dose of 100 nM. These data suggest that SOM230 might represent a useful pharmacological compound for the treatment of retinal disease.

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.

BMP action in the pituitary: its possible role in modulating somatostatin sensitivity in pituitary tumor cells

The existence of a functional bone morphogenetic protein (BMP) system in the pituitary has been recognized. Recent studies have provided evidence that BMPs elicit differential actions in the regulation of prolactin (PRL) and adrenocorticotropin (ACTH) release in lactotropinoma and corticotropinoma cells, respectively. BMPs play a key role in the modulation of somatostatin receptor (SSTR) sensitivity of lactosomatotrope cells in an autocrine/paracrine manner. In addition, SSTR action enhances BMP responsiveness in corticotrope cells. The functional link between BMP receptor signaling and SSTR actions may be crucial for individual tolerance to somatostatin analogs for controlling PRL and ACTH production. Adjustment of the endogenous SSTR sensitivity may be an effective strategy to inhibit the growth activity and hormonal productivity of intractable pituitary tumors.

Efficacy of the combined cabergoline and octreotide treatment in a case of a dopamine-agonist resistant macroprolactinoma

Prolactinomas in males can be voluminous macroadenomas invading the surrounding structures. Medical therapy with dopamine agonists (the treatment of choice for these tumours) may be ineffective in the case of pharmacological resistance. In such cases, even surgical and/or radiation therapy cannot be curative due to the invasive potential of the adenoma. Hence, the appropriate therapeutic approach for these tumours is still a relevant clinical problem for endocrinologists. We report the history of an adolescent male who was diagnosed with a large invasive macroprolactinoma in 2002. He had severe bitemporal hemianopsia and hypopituitarism; prolactin levels at diagnosis were higher than 8,000 ng/ml. Medical therapy with cabergoline was initiated and resulted in decreased prolactin levels but not complete normalisation (maximal tolerated dose 3 mg/day). However, due to the worsening of the visual defect, the patient was operated in July 2004 through the trans-nasal approach and 2 years later through both the transcranial and the transphenoidal approaches. After the second surgery, a significant reduction of tumour mass was obtained. Immunohistochemistry for somatostatin receptors (sstr) subtypes showed a positive staining with the anti-sstr5 antibody. A scintigraphy with 111In-pentetreotide (Octreoscan) revealed a very intense tracer uptake in the sellar region. The administration of long-acting octreotide was initiated. After 12 months of therapy, prolactin levels normalised for the first time. Pituitary MRI did not reveal any tumor progression during a 2-year follow-up. This is a case of an invasive dopamine-resistant macroprolactinoma that was successfully controlled by extensive surgery and combined treatment with cabergoline and octreotide. The expression and functionality of sstr should be investigated in these tumours since a combined therapy with cabergoline and octreotide may be a good therapeutic course of action for select cases.

Somatostatin receptors in non-neuroendocrine malignancies: the potential role of somatostatin analogs in solid tumors

Somatostatin receptors (sstrs) are G-protein-coupled receptors that mediate various physiological effects when activated by the neuropeptide somatostatin or its synthetic analogs. In addition to the well-documented antisecretory effects of sstr2-preferential somatostatin analogs octreotide and lanreotide, ligand binding to sstr initiates an inhibitory action on tumor growth. This effect may result from both indirect actions (suppression of growth factors and growth-promoting hormones [e.g., GH/IGF-1 axis] and inhibition of angiogenesis) and direct actions (activation of antigrowth activities [e.g., apoptosis]). As solid tumor cells express multiple sstrs, there is a rationale to evaluate the potential antitumor effects of pasireotide (SOM230), a multireceptor-targeted somatostatin analog with high binding affinity for sstr1–3 and sstr5. Pasireotide reduces systemic IGF-1 levels more potently than currently available somatostatin analogs and has been well tolerated in clinical trials.

Medical treatment of prolactinomas

Prolactinomas, the most prevalent type of neuroendocrine disease, account for approximately 40% of all pituitary adenomas. The most important clinical problems associated with prolactinomas are hypogonadism, infertility and hyposexuality. In patients with macroprolactinomas, mass effects, including visual field defects, headaches and neurological disturbances, can also occur. The objectives of therapy are normalization of prolactin levels, to restore eugonadism, and reduction of tumor mass, both of which can be achieved in the majority of patients by treatment with dopamine agonists. Given their association with minimal morbidity, these drugs currently represent the mainstay of treatment for prolactinomas. Novel data indicate that these agents can be successfully withdrawn in a subset of patients after normalization of prolactin levels and tumor disappearance, which suggests the possibility that medical therapy may not be required throughout life. Nevertheless, multimodal therapy that involves surgery, radiotherapy or both may be necessary in some cases, such as patients who are resistant to the effects of dopamine agonists or for those with atypical prolactinomas. This Review reports on efficacy and safety of pharmacotherapy in patients with prolactinomas.

Chronic injection of pansomatostatin agonist ODT8-SST differentially modulates food intake and decreases body weight gain in lean and diet-induced obese rats

The aim of this study was to investigate the central actions of the stable pansomatostatin peptide agonist, ODT8-SST on body weight. ODT8-SST or vehicle was acutely (1μg/rat) injected or chronically infused (5μg/rat/d, 14d) intracerebroventricularly and daily food intake, body weight and composition were monitored. In lean rats, neither acute nor chronic ODT8-SST influenced daily food intake while body weight was reduced by 2.2% after acute injection and there was a 14g reduction of body weight gain after 14d compared to vehicle (p<0.01). In diet-induced obese (DIO) rats, chronic ODT8-SST increased cumulative 2-week food intake compared to vehicle (+14%, p<0.05) and also blunted body weight change (-11g, p<0.05). ODT8-SST for 14d reduced lean mass (-22g and -25g respectively, p<0.001) and total water (-19g and -22g respectively, p<0.001) in lean and DIO rats and increased fat mass in DIO (+16g, p<0.001) but not lean rats (+1g, p>0.05) compared to vehicle. In DIO rats, ODT8-SST reduced ambulatory (-27%/24h, p<0.05) and fine movements (-38%, p<0.01) which was associated with an increased positive energy balance compared to vehicle (+50g, p<0.01). Chronic central somatostatin receptor activation in lean rats reduces body weight gain and lean mass independently of food intake which is likely related to growth hormone inhibition. In DIO rats, ODT8-SST reduces lean mass but promotes food intake and fat mass, indicating differential responsiveness to somatostatin under obese conditions.

Activities of bone morphogenetic proteins in prolactin regulation by somatostatin analogs in rat pituitary GH3 cells

Involvement of the pituitary BMP system in the modulation of prolactin (PRL) secretion regulated by somatostatin analogs, including octreotide (OCT) and pasireotide (SOM230), and a dopamine agonist, bromocriptine (BRC), was examined in GH3 cells. GH3 cells are rat pituitary somato-lactotrope tumor cells that express somatostatin receptors (SSTRs) and BMP system molecules including BMP-4 and -6. Treatment with BMP-4 and -6 increased PRL and cAMP secretion by GH3 cells. The BMP-4 effects were neutralized by adding a BMP-binding protein Noggin. These findings suggest the activity of endogenous BMPs in augmenting PRL secretion by GH3 cells. BRC and SOM230 reduced PRL secretion, but OCT failed to reduce the PRL level. In GH3 cells activated by forskolin, BRC suppressed forskolin-induced PRL secretion with reduction in cAMP levels. OCT did not affect forskolin-induced PRL level, while SOM230 reduced PRL secretion and PRL mRNA expression induced by forskolin. BMP-4 treatment enhanced the reducing effect of SOM230 on forskolin-induced PRL level while BMP-4 did not affect the effects of OCT or BRC. Noggin treatment had no significant effect on the BRC actions reducing PRL levels by GH3 cells. However, in the presence of Noggin, OCT elicited an inhibitory effect on forskolin-induced PRL secretion and PRL mRNA expression, whereas the SOM230 effect on PRL reduction was in turn impaired. It was further found that BMP-4 and -6 suppressed SSTR-2 but increased SSTR-5 mRNA expression of GH3 cells. These findings indicate that Noggin rescues SSTR-2 but downregulates SSTR-5 by neutralizing endogenous BMP actions, leading to an increase in OCT sensitivity and a decrease in SOM230 sensitivity of GH3 cells. In addition, BMP signaling was facilitated in GH3 cells treated with forskolin. Collectively, these findings suggest that BMPs elicit differential actions in the regulation of PRL release dependent on cellular cAMP-PKA activity. BMPs may play a key role in the modulation of SSTR sensitivity of somato-lactotrope cells in an autocrine/paracrine manner.

SSTR1- and SSTR3-selective somatostatin analogues

We prepared the two enantiomers of 3-(3'-quinolyl)-alanine (Qla, 1) in multigram scale by asymmetric hydrogenation. These amino acids, protected as Fmoc derivatives, were then used in the solid-phase synthesis of two new somatostatin 14 (SRIF-14) analogues 8 a and 8 b, tetradecapeptides in which the tryptophan residue (Trp8) is replaced by one of the two enantiomers of 3-(3'-quinolyl)-alanine (Qla8) and therefore lack the N--H bond in residue 8. The selectivity of these new analogues for the somatostatin receptors, SSTR1-5, was measured. Substitution with L-Qla8 yielded peptide 8 a, which was highly selective for SSTR1 and SSTR3, with an affinity similar to that of SRIF-14. Substitution by D-Qla gave the relatively selective analogue 8 b, which showed high affinity for SSTR3 and significant affinity for SSTR1, SSTR2 and SSTR5. The biological results demonstrate that bulky and electronically poor aromatic amino acids at position 8 are compatible with strong activity with SSTR1 and SSTR3. Remarkably, these high affinity levels were achieved with peptides in which the conformational mobility was increased with respect to that of SRIF-14. This observation suggests that conformational rigidity is not required, and might be detrimental to the interaction with receptors SSTR1 and SSTR3. The absence of an indole N proton in Qla8 might also contribute to the increased flexibility observed in these analogues.

The somatostatin receptor subtype 5 in neuroendocrine tumours

IMPORTANCE OF THE FIELD

In recent years, scientific work has been intensified to unravel new (patho-) physiological insights, particularly regarding the functional role of somatostatin (SRIF) receptor subtype 5 (sst) and the development of novel sst(5)-targeted SRIF analogues, in order to broaden medical therapeutic opportunities in patients suffering from neuroendocrine diseases.

AREAS COVERED IN THIS REVIEW

The scope of this review is primarily focused upon recent insights in sst(5)-receptor physiology, novel sst(5)-targeted treatment options predominantly directed towards pituitary adenomas, and gastroenteropancreatic neuroendocrine tumours.

WHAT THE READER WILL GAIN

An understanding of the potential that novel sst(5)-targeted SRIF analogues might have in the medical treatment of Cushing's disease and acromegaly, as demonstrated by translational research, based on pathophysiological data combined with results from clinical trials.

TAKE HOME MESSAGE

The role of targeting sst(5) in gastroenteropancreatic neuroendocrine tumours remains to be established. The sst(5) subtype might function as sst(2) modulator in terms of receptor internalization and desensitization, and seems less important compared with sst(2)-preferring SRIF analogues in the regulation of human insulin secretion by the pancreas. Finally, absence of sst(5) in corticotroph adenomas could be related to tumour aggressiveness in Cushing's disease.