Octreotide and the novel multireceptor ligand somatostatin receptor agonist pasireotide (SOM230) block the adrenalectomy-induced increase in mitotic activity in male rat anterior pituitary

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

Introduction

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

Methods

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

Results

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

Discussion

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

Sustained somatostatin gene expression reverses kindling-induced increases in the number of dividing Type-1 neural stem cells in the hippocampi of behaviorally responsive rats

Neurogenesis persists throughout life in the hippocampi of all mammals, including humans. In the healthy hippocampus, relatively quiescent Type-1 neural stem cells (NSCs) can give rise to more proliferative Type-2a neural progenitor cells (NPCs), which generate neuronal-committed Type-2b NPCs that mature into Type-3 neuroblasts. Many Type-3 neuroblasts survive and mature into functionally integrated granule neurons over several weeks. In kindling models of epilepsy, neurogenesis is drastically upregulated and many new neurons form aberrant connections that could support epileptogenesis and/or seizures. We have shown that sustained vector-mediated hippocampal somatostatin (SST) expression can both block epileptogenesis and reverse seizure susceptibility in fully kindled rats. Here we test whether adeno-associated virus (AAV) vector-mediated sustained SST expression modulates hippocampal neurogenesis and microglial activation in fully kindled rats. We found significantly more dividing Type-1 NSCs and a corresponding increased number of surviving new neurons in the hippocampi of kindled versus sham-kindled rats. Increased numbers of activated microglia were found in the granule cell layer and hilus of kindled rats at both time points. After intrahippocampal injection with either eGFP or SST-eGFP vector, we found similar numbers of dividing Type-1 NSCs and -2 NPCs and surviving BrdU⁺ neurons and glia in the hippocampi of kindled rats. Upon observed variability in responses to SST-eGFP (2/4 rats exhibited Grade 0 seizures in the test session), we conducted an additional experiment. We found significantly fewer dividing Type-1 NSCs in the hippocampi of SST-eGFP vector-treated responder rats (5/13 rats) relative to SST-eGFP vector-treated non-responders and eGFP vector-treated controls that exhibited high-grade seizures on the test session. The number of activated microglia was upregulated in the GCL and hilus of kindled rats, regardless of vector treatment. These data support the hypothesis that sustained SST expression exerts antiepileptic effects potentially through normalization of neurogenesis and suggests that abnormally high proliferating Type-1 NSC numbers may be a cellular mechanism of epilepsy.

Pasireotide in Acromegaly: An Overview of Current Mechanistic and Clinical Data

BACKGROUND

Acromegaly is an insidious neuroendocrine disorder caused by hypersecretion of growth hormone (GH) by a somatotroph adenoma. Somatostatin receptor ligands (SRLs) are recommended as first-line medical therapy in patients for whom surgery has failed or is contraindicated. There are 5 known somatostatin receptor subtypes (SSTRs), 2 of which, i.e. SSTR2 and SSTR5, are expressed by a majority of somatotroph adenomas. The currently available SRLs, i.e. octreotide and lanreotide, primarily bind to SSTR2. Pasireotide (SOM230) is a new multireceptor-targeted SRL which has a broader binding profile and an increased affinity for SSTR1, 2, 3, and 5.

METHODS

PubMed searches were performed to identify all of the available published English language data on pasireotide with regard to the mechanism of action, in vitro effects, and clinical data.

RESULTS

Preclinical studies have demonstrated that pasireotide has a broader range of functional activity than octreotide. Recently, the efficacy of pasireotide in attenuating GH and insulin-like growth factor 1 (IGF-1) levels in patients with acromegaly has been evaluated in phase III clinical trials. Pasireotide demonstrated superiority over octreotide in achieving biochemical control (i.e. GH ≤2.5 µg/l and age- and sex-matched IGF-1 normalization) in patients with acromegaly, as well as significant efficacy in treating patients who were previously inadequately controlled on the maximum allowed doses of octreotide and lanreotide. Pasireotide-induced hyperglycemia was the most concerning adverse event but was reversible upon discontinuation of pasireotide.

CONCLUSION

The clinical data support pasireotide as a promising new therapy for the treatment of acromegaly, and the long-acting formulation was recently approved in the US and Europe for the treatment of acromegaly.

Molecular basis of pharmacological therapy in Cushing's disease

Cushing's disease (CD) is a severe endocrine condition caused by an adrenocorticotropin (ACTH)-producing pituitary adenoma that chronically stimulates adrenocortical cortisol production and with potentially serious complications if not or inadequately treated. Active CD may produce a fourfold increase in mortality and is associated with significant morbidities. Moreover, excess mortality risk may persist even after CD treatment. Although predictors of risk in treated CD are not fully understood, the importance of early recognition and adequate treatment is well established. Surgery with resection of a pituitary adenoma is still the first line therapy, being successful in about 60-70 % of patients; however, recurrence within 2-4 years may often occur. When surgery fails, medical treatment can reduce cortisol production and ameliorate clinical manifestations while more definitive therapy becomes effective. Compounds that target hypothalamic-pituitary axis, glucocorticoid synthesis or adrenocortical function are currently used to control the deleterious effects of chronic glucocorticoid excess. In this review we describe and analyze the molecular basis of the drugs targeting the disease at central level, suppressing ACTH secretion, as well as at peripheral level, acting as adrenal inhibitors, or glucocorticoid receptor antagonists. Understanding of the underlying molecular mechanisms in CD and of glucocorticoid biology should promote the development of new targeted and more successful therapies in the future. Indeed, most of the drugs discussed have been tested in limited clinical trials, but there is potential therapeutic benefit in compounds with better specificity for the class of receptors expressed by ACTH-secreting tumors. However, long-term follow-up with management of persistent comorbidities is needed even after successful treatment of CD.

Radiation induces up-regulation of somatostatin receptors 1, 2, and 5 in small cell lung cancer in vitro also at low absorbed doses

BACKGROUND

Radiation can be used to up-regulate the expression of the somatostatin receptor (sstr) subtype 2 in small cell lung cancer (SCLC) cells at absorbed doses of 2-8 Gy. Increased sstr expression results in increased binding of radiolabeled somatostatin analogs to the tumor cell, which enhances the efficacy of systemic radionuclide therapy. The aim of this study was to determine if lower absorbed doses could up-regulate sstr2 expression, and possibly influence other sstr subtypes.

METHODS

Human H69 SCLC cells were irradiated with an absorbed dose of 0.12-6.0 Gy and the sstr mRNA expression 3 days after irradiation was measured by quantitative real-time polymerase chain reaction for sstr1-5. At the same time point was the binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate to the cells measured after irradiation to an absorbed dose of 0.12-2.0 Gy and compared to the binding to nonirradiated cells.

RESULTS

mRNA expression of sstr1, sstr2, and sstr5 was increased by a factor of 1.5-2 in cells irradiated with absorbed doses≥4 Gy and the binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate was, accordingly, 2-3 times higher to irradiated cells for all absorbed doses, except 0.25 Gy.

CONCLUSION

The binding of [(177)Lu]-DOTA(0)-Tyr(3)-octreotate was increased after radiation exposure. This increase was observed at low absorbed doses in parallel with up-regulation of sstr1, sstr2, and sstr5 mRNA.

Effects of bone morphogenetic protein (BMP) on adrenocorticotropin production by pituitary corticotrope cells: involvement of up-regulation of BMP receptor signaling by somatostatin analogs

The mechanism by which somatostatin analogs suppress ACTH production by corticotropinomas has yet to be fully elucidated. We here studied the effects of somatostatin analogs on ACTH secretion using mouse corticotrope AtT20 cells focusing on the biological activity of bone morphogenetic proteins (BMPs). BMP ligands, receptors and Smads, and somatostatin receptors (SSTRs)-2, -3, and -5 were expressed in AtT20 cells. BMP-2, -4, -6, and -7 decreased basal ACTH production with BMP-4 effects being the most prominent. BMP-4 also inhibited CRH-induced ACTH production and proopiomelanocortin (POMC) transcription. However, the decrease in CRH-induced cAMP accumulation caused by BMP-4 was not sufficient to completely account for BMP-4 actions, indicating that ACTH suppression by BMPs was not directly linked to cAMP inhibition. CRH-activated ERK1/ERK2, p38-MAPK, stress-activated protein kinase/c-Jun NH(2)-terminal kinase, protein kinase C, and Akt pathways and CRH-induced ACTH synthesis was significantly decreased in the presence of U0126 or SB203580. Because BMPs attenuated CRH-induced ERK and p38 phosphorylation, it was suggested that BMP-4 suppresses ACTH production by inhibiting CRH-induced ERK and p38 phosphorylation. Somatostatin analogs octreotide and pasireotide (SOM230) significantly suppressed CRH-induced ACTH and cAMP production in AtT20 cells and reduced ERK and p38 phosphorylation. Notably, CRH-induced ACTH production was enhanced in the presence of noggin, a BMP-binding protein. The inhibitory effects of octreotide and SOM230 on CRH-induced ACTH production were also attenuated by noggin, implying that the endogenous BMP system plays a key role in inhibiting CRH-induced ACTH production by AtT20 cells. The findings that OCT and SOM230 up-regulated BMP-Smad1/Smad5/Smad8 signaling and ALK-3 and BMPRII and down-regulated inhibitory Smad6/7 establish that the activation of endogenous BMP system is functionally involved in the mechanism by which somatostatin analogs suppress CRH-induced ACTH production.

Pituitary somatostatin receptor signaling

Somatotropin-release inhibitory factor (SRIF) is a major regulator of pituitary function, mostly inhibiting hormone secretion and to a lesser extent pituitary cell growth. Five SRIF receptor subtypes (SSTR1-5) are ubiquitously expressed G-protein coupled receptors. In the pituitary, SSTR1, 2, 3 and 5 are expressed, with SSTR2 and SSTR5 predominating. As new SRIF analogs have recently been introduced for treatment of pituitary disease, we evaluate the current knowledge of cell-specific pituitary SRIF receptor signaling and highlight areas of future research for comprehensive understanding of these mechanisms. Elucidating pituitary SRIF receptor signaling enables understanding of pituitary hormone secretion and cell growth, and also encourages future therapeutic development for pituitary disorders.

Drugs in the medical treatment of Cushing's syndrome

Cushing's syndrome is a complex endocrine condition with potential serious complications if untreated or inadequately treated. Transsphenoidal surgery with resection of a pituitary adenoma is successful in 75 - 80% of patients, but approximately 20 - 25% show persistence of Cushing's, and a similar proportion may experience recurrence within 2 - 4 years post-op. When surgery fails, medical treatment can temporarily suppress excessive cortisol production and ameliorate its clinical manifestations while more definitive therapy becomes effective. We describe pharmacological approaches to the treatment of Cushing's syndrome. Drugs used to suppress cortisol secretion are mostly inhibitors of steroidogenesis. Ketoconazole, fluconazole aminoglutethimide, metyrapone, mitotane and etomidate are in that category. Ketoconazole is in current use while other drugs, although mostly available in the past, continue to have a potential role either alone or in combination. Drugs that suppress adrenocorticotropic hormone (ACTH) secretion are less popular as standard treatment and include cyproheptadine, valproic acid, cabergoline, somatostatin analogs, PPAR-gamma agonists, vasopressin antagonists. Some of these drugs have been tested in limited clinical trials but there is potential therapeutic benefit in analogs with better specificity for the class of receptors present in ACTH-secreting tumors. A third category of drugs is glucocorticoid receptor antagonists. Mifepristone is currently being tested in clinical trials in patients with persistent or recurrent Cushing's disease and in patients with metastatic adrenal cortical carcinoma or ectopic ACTH syndrome not amenable to surgery. We also review replacement therapy after surgery and non-specific drugs to treat complications in patients with severe hypercortisol. The review provides a complete survey of the drugs used in the medical treatment of Cushing's, and new advances in the development of pituitary-active drugs as well as receptor blockers of glucocorticoid action. It also provides avenues for exploration of new drugs active on somatostatin, dopamine and vasopressin receptors. There are effective pharmacological agents capable of chronically reversing biochemical and clinical manifestations of hypercortisolemia in Cushing's syndrome but new drugs are needed with action at the pituitary level.

Differential ligand-mediated pituitary somatostatin receptor subtype signaling: implications for corticotroph tumor therapy

OBJECTIVE

Pituitary targeted pharmacotherapy for Cushing's disease is challenging and ineffective. Unlike octreotide and lanreotide, the multisomatostatin receptor (SST) analog pasireotide that exhibits SST5 greater than SST2 binding affinity offers potential for treating Cushing's disease. Because corticotroph cells express SST5 more abundantly than SST2, pasireotide likely exerts superior corticotroph action mainly through SST5. However, there is no direct evidence for this assumption, and moreover, the ligand effect on corticotroph SST2 is not known.

RESULTS

We used AtT20 mouse pituitary corticotroph tumor cells stably overexpressing SST2 or SST5 and TtT/GF mouse pituitary folliculostellate cells stably or transiently expressing SST receptors to examine ligand-receptor activation by SST2- and SST5-selective agonists. We show that pasireotide was more potent than either octreotide or somatostatin-14 in mouse corticotroph cells. Pasireotide potency is not affected by SST2 abundance, SST2 antagonist treatment, or octreotide cotreatment in SST2-overexpressing cells. Pasireotide also does not induce SST2 internalization and attenuates octreotide or SRIF14-induced SST2 internalization only at superphysiological dose ranges. In contrast, octreotide attenuates pasireotide potency in SST5-overexpressing cells. Moreover, short exposure to pasireotide causes prolonged inhibition of forskolin or CRH-induced cAMP accumulation, in contrast to somatostatin-14- and SST2-selective agonists that induced postwithdrawal cAMP rebound. Long-term pasireotide signaling effects are enhanced by SST5 overexpression.

CONCLUSION

The results indicate that SST5 determines short- and long-term enhanced pasireotide action in corticotroph cells, whereas the ligand action on SST2 is negligible.

Current therapy and drug pipeline for the treatment of patients with acromegaly

INTRODUCTION

Acromegaly is a multisystem disease resulting from chronic exposure to supraphysiological levels of growth hormone (GH), and is associated with significant morbidity and excess mortality. The etiology is almost exclusively an underlying pituitary adenoma. Current therapeutic interventions include surgery, radiotherapy, and medical therapy.

RESULTS

Despite surgery, around 50% of patients fail to achieve the biochemical targets shown to correlate with normalization of mortality rates. Radiotherapy is efficacious in controlling tumor growth and GH secretion; still, achievement of biochemical targets may take up to a decade and a number of safety issues have been raised with this treatment modality. Medical therapy, therefore, has an important role as adjuvant therapy in patients who fail to achieve control with surgery, or while awaiting the effects of radiotherapy to be realized. Furthermore, medical therapy is increasingly being used as primary therapy. Current medical therapies include dopaminergic agonists, somatostatin analogs, and GH receptor (GHR) antagonists. Dopaminergic agonists achieve biochemical targets in up to 30% of patients, and somatostatin analogs in around 60%. The currently available GHR antagonist pegvisomant effectively controls insulin-like growth factor-I levels in over 90% of patients; however, it has no effect on the tumor itself and has considerable financial implications. Research into optimizing the somatostatin and dopaminergic systems has led to promising advances in agonist development. Moieties with selectivity for various combinations of somatostatin receptor subtype receptors have been examined, along with molecules that additionally show high affinity for the dopaminergic D2 receptor. Of the molecules studied in vitro, only pasireotide (SOM230) and BIM-23A760 are currently undergoing further development. Other innovations to improve convenience of currently available drugs are also being investigated.

CONCLUSION

Significant advances in under standing of the somatostatin and dopaminergic system have aided drug development. This may lead to new clinically available therapies enabling control of acromegaly in a larger proportion of patients, and at an earlier stage in their disease management.

Physiology and pathology of somatostatin in the mammalian retina: a current view

In the retina, peptidergic signalling participates in multiple circuits of visual information processing. The neuropeptide somatostatin (SRIF) is localised to amacrine cells and, in some instances, in a subset of ganglion cells. The variegated expression patterns of SRIF receptors (sst(1)-sst(5)) and the variety of signalling mechanisms activated by retinal SRIF suggest that this peptide may exert multiple actions on retinal neurons and on retinal physiology, although our current understanding reflects a rather complicated picture. SRIF, mostly through sst(2), may act as a positive factor in the retina by regulating retinal homeostasis and protecting neurons against damage. In this respect, SRIF analogues seem to constitute a promising therapeutic arsenal to cure different retinal diseases, as for instance, ischemic and diabetic retinopathies. However, further investigations are needed not only to fully understand the functional role of the SRIF system in the retina but also to exploit new chemical space for drug-like molecules.

Somatostatin and somatostatin receptors in Cushing's disease

Cushing's disease is caused by an ACTH secreting pituitary adenoma. Surgery is the treatment of choice and cure rates between 60 and 90% are reported. For patients in which surgery fails, effective medical treatment options are needed. Somatostatin (SS) receptors (sst) are expressed on normal and tumoral corticotroph cells. However, the role of somatostatin and in particular the current clinically available sst(2)-preferring SS analogs in the regulation of normal ACTH secretion, as well as in lowering ACTH and cortisol hypersecretion in patients with Cushing's disease, has been shown to be limited. Recent studies have provided renewed insights into the expression of sst subtypes, as well as into the functional role of SS-analogs in the regulation of ACTH secretion by corticotroph tumors. Sst(2) and sst(5) seem the predominantly expressed sst in corticotroph adenoma cells and targeting both these receptors with a new generation of multiligand SS analogs showed promising effects in terms of lowering ACTH release and urinary free cortisol (UFC) levels in patients with Cushing's disease. In this review an overview of the current insights into the role of SS and sst in the regulation of normal and pathological ACTH secretion is provided.

Molecular and trophic mechanisms of tumorigenesis

A significant proportion of pituitary macroadenomas, and by definition all microadenomas, regain trophic stability after an initial period of deregulated growth. Classical proto-oncogene activation and tumor suppressor mutation are rarely responsible, and no histologic or molecular markers reliably predict behavior. GNAS1 activation and the mutations associated with multiple endocrine neoplasia type 1 and Carney complex, aryl hydrocarbon receptor interacting protein gene mutations, and a narrowing region of chromosome 11q13 in familial isolated acromegaly together account for such a small proportion of pituitary adenomas that the pituitary adenoma pathogenic epiphany is surely yet to come.