The pituitary-adrenal response to CRF-41 is unaltered by intravenous somatostatin in normal subjects

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.

The Treatment of Cushing's Disease

Cushing's disease (CD), or pituitary-dependent Cushing's syndrome, is a severe endocrine disease caused by a corticotroph pituitary tumor and associated with increased morbidity and mortality. The first-line treatment for CD is pituitary surgery, which is followed by disease remission in around 78% and relapse in around 13% of patients during the 10-year period after surgery, so that nearly one third of patients experience in the long-term a failure of surgery and require an additional second-line treatment. Patients with persistent or recurrent CD require additional treatments, including pituitary radiotherapy, adrenal surgery, and/or medical therapy. Pituitary radiotherapy is effective in controlling cortisol excess in a large percentage of patients, but it is associated with a considerable risk of hypopituitarism. Adrenal surgery is followed by a rapid and definitive control of cortisol excess in nearly all patients, but it induces adrenal insufficiency. Medical therapy has recently acquired a more important role compared to the past, due to the recent employment of novel compounds able to control cortisol secretion or action. Currently, medical therapy is used as a presurgical treatment, particularly for severe disease; or as postsurgical treatment, in cases of failure or incomplete surgical tumor resection; or as bridging therapy before, during, and after radiotherapy while waiting for disease control; or, in selected cases, as primary therapy, mainly when surgery is not an option. The adrenal-directed drug ketoconazole is the most commonly used drug, mainly because of its rapid action, whereas the glucocorticoid receptor antagonist, mifepristone, is highly effective in controlling clinical comorbidities, mainly glucose intolerance, thus being a useful treatment for CD when it is associated with diabetes mellitus. Pituitary-directed drugs have the advantage of acting at the site responsible for CD, the pituitary tumor. Among this group of drugs, the dopamine agonist cabergoline and the somatostatin analog pasireotide result in disease remission in a consistent subgroup of patients with CD. Recently, pasireotide has been approved for the treatment of CD when surgery has failed or when surgery is not an option, and mifepristone has been approved for the treatment of Cushing's syndrome when associated with impairment of glucose metabolism in case of the lack of a surgical indication. Recent experience suggests that the combination of different drugs may be able to control cortisol excess in a great majority of patients with CD.

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.

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.

Neuropeptidomics of mouse hypothalamus after imipramine treatment reveal somatostatin as a potential mediator of antidepressant effects

Excessive activation of the hypothalamic-pituitary-adrenal (HPA) axis has been associated with numerous diseases, including depression, and the tricyclic antidepressant imipramine has been shown to suppress activity of the HPA axis. Central hypothalamic control of the HPA axis is complex and involves a number of neuropeptides released from multiple hypothalamic subnuclei. The present study was therefore designed to determine the effects of imipramine administration on the mouse hypothalamus using a peptidomics approach. Among the factors found to be downregulated after acute (one day) or chronic (21 days) imipramine administration were peptides derived from secretogranin 1 (chromogranin B) as well as peptides derived from cerebellin precursors. In contrast, peptides SRIF-14 and SRIF-28 (1-11) derived from somatostatin (SRIF, somatotropin release inhibiting factor) were significantly upregulated by imipramine in the hypothalamus. Because diminished SRIF levels have long been known to occur in depression, a second part of the study investigated the roles of individual SRIF receptors in mediating potential antidepressant effects. SRA880, an antagonist of the somatostatin-1 autoreceptor (sst1) which positively modulates release of endogenous SRIF, was found to synergize with imipramine in causing antidepressant-like effects in the tail suspension test. Furthermore, chronic co-administration of SRA880 and imipramine synergistically increased BDNF mRNA expression in the cerebral cortex. Application of SRIF or L054264, an sst2 receptor agonist, but not L803807, an sst4 receptor agonist, increased phosphorylation of CaMKII and GluR1 in cerebrocortical slices. Our present experiments thus provide evidence for antidepressant-induced upregulation of SRIF in the brain, and strengthen the notion that augmented SRIF expression and signaling may counter depressive-like symptoms. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Somatostatin and dopamine receptors as targets for medical treatment of Cushing's Syndrome

Somatostatin (SS) and dopamine (DA) receptors are widely expressed in neuroendocrine tumours that cause Cushing's Syndrome (CS). Increasing knowledge of specific subtype expression within these tumours and the ability to target these receptor subtypes with high-affinity compounds, has driven the search for new SS- or DA-based medical therapies for the various forms of CS. In Cushing's disease, corticotroph adenomas mainly express dopamine receptor subtype 2 (D(2)) and somatostatin receptor subtype 5 (sst(5)), whereas sst(2) is expressed at lower levels. Activation of these receptors can inhibit ACTH-release in primary cultured corticotroph adenomas and compounds that target either sst(5) (pasireotide, or SOM230) or D(2) (cabergoline) have shown significant efficacy in subsets of patients in recent clinical studies. Combination therapy, either by administration of both types of compounds separately or by treatment with novel somatostatin-dopamine chimeric molecules (e.g. BIM-23A760), appears to be a promising approach in this respect. In selected cases of Ectopic ACTH-producing Syndrome (EAS), the sst(2)-preferring compound octreotide is able to reduce cortisol levels effectively. A recent study showed that D(2) receptors are also significantly expressed in the majority of EAS and that cabergoline may decrease cortisol levels in subsets of these patients. In both normal adrenal tissue as well as in adrenal adenomas and carcinomas that cause CS, sst and DA receptor expression has been demonstrated. Although selected cases of adrenal CS may benefit from sst or DA-targeted treatment, its total contribution to the treatment of these patients is likely to be low as surgery is effective in most cases.

Pituitary-directed medical treatment of Cushing's disease

The treatment of Cushing's disease is very complex and represents a challenge for clinicians. Transphenoidal surgical excision of adrenocorticotropic hormone (ACTH)-secreting pituitary adenoma remains the treatment of choice but, unfortunately, the rate of cure at long-term follow-up is suboptimal and recurrences are high, even in the hands of skilled neurosurgeons. Other treatment options, such as bilateral adrenalectomy and pituitary radiotherapy, are currently in use but no treatment has proven fully satisfactory during the lengthy progress of this chronic and devastating disease. Nelson's syndrome and hypopituitarism are of particular concern as affected patients need lifelong hormone-replacement therapy and have notably increased mortality. Although medical treatment represents a second-line treatment option in patients with Cushing's disease, so far pharmacological therapy has been considered a transient and palliative treatment. Many drugs have been employed: they may act at the hypothalamic-pituitary level, decreasing ACTH secretion; at the adrenal level, inhibiting cortisol synthesis (steroidogenesis inhibitors); or at the peripheral level by competing with cortisol (glucocorticoid receptor antagonists). Recently, there has been renewed interest in the medical therapy of Cushing's disease and pituitary-directed drugs include old compounds commercially available for other diseases, such as cabergoline, and new promising compounds, such as pasireotide (SOM230) or retinoic acid. This review focuses on the tumor-directed pharmacological approaches for the management of Cushing's disease based on the recent identification of possibile targets at a pituitary level.

Pituitary-targeted medical therapy of Cushing's disease

BACKGROUND

The goals of ideal medical therapy for Cushing's disease should be to target the aetiology of the disorder, as is the case for surgery, which is the current 'gold standard' treatment. However, no effective drug that directly and reliably targets the adrenocorticotropin-secreting pituitary adenoma has yet been found.

OBJECTIVE

To summarise pituitary-targeted medical treatment of Cushing's disease.

METHODS

Compounds with neuromodulatory properties and ligands of different nuclear hormone receptors involved in hypothalamo-pituitary regulation have been investigated.

RESULTS

The somatostatin analogue pasireotide and the dopamine agonist cabergoline, as well as their combination, show some therapeutic promise in the medical therapy of Cushing's disease. Other treatments such as retinoic acid analogues look promising and may be a possible option for further investigation. No other medical therapies seem to be reliably effective currently.

CONCLUSION

Since a percentage of patients treated with surgery are not cured, or improve and subsequently relapse, there is an urgent need for effective medical therapies for this disorder. At present, only cabergoline and pasireotide are under active investigation.

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.

Pharmacological management of Cushing's syndrome: an update

The treatment of choice for Cushing's syndrome remains surgical. The role for medical therapy is twofold. Firstly it is used to control hypercortisolaemia prior to surgery to optimize patient's preoperative state and secondly, it is used where surgery has failed and radiotherapy has not taken effect. The main drugs used inhibit steroidogenesis and include metyrapone, ketoconazole, and mitotane. Drugs targeting the hypothalamic-pituitary axis have been investigated but their roles in clinical practice remain limited although PPAR-gamma agonist and somatostatin analogue som-230 (pasireotide) need further investigation. The only drug acting at the periphery targeting the glucocorticoid receptor remains Mifepristone (RU486). The management of Cushing syndrome may well involve combination therapy acting at different pathways of hypercortisolaemia but monitoring of therapy will remain a challenge.

Putative somatostatin suppression potentiates adrenocorticotropin secretion driven by ghrelin and human corticotropin-releasing hormone

CONTEXT

Ghrelin is a 28-amino-acid Ser(3)-octanoylated peptide, and CRH is a 41-amino-acid peptide, both of which stimulate ACTH secretion. In principle, actions of these agonists could be subject to inhibitory modulation by hypothalamic somatostatin (SS).

OBJECTIVE

Our objective was to test the hypothesis that endogenous SS restrains ghrelin and CRH-stimulated ACTH secretion, thereby linking all three, ghrelin, CRH, and SS, with ACTH secretion.

DESIGN AND SETTING

We conducted a randomized, double-blind, placebo-controlled, crossover interventional study at an academic medical center.

PARTICIPANTS

Ten healthy postmenopausal women participated in the study.

INTERVENTIONS

Interventions included iv injection of saline, ghrelin, human CRH, or both after an infusion of saline vs. l-arginine to putatively inhibit SS outflow (eight visits per subject).

OUTCOME MEASURES

ACTH concentrations quantified by repetitive blood sampling and immunochemiluminometry.

RESULTS

Infusion of ghrelin induced peak ACTH concentrations [median (range)] of 21 (17-28) compared with 16 (11-20) ng/liter after saline (P = 0.037). CRH and l-arginine infusion evoked ACTH peaks of 23 (14-48) and 31 (21-286) ng/liter, respectively (P = 0.037 and P = 0.005 vs. saline). l-Arginine enhanced stimulation by ghrelin by 1.43-fold (P = 0.028) and that by CRH by 1.91-fold (P = 0.005). Triple stimulation with ghrelin, CRH, and l-arginine potentiated the effect of combined ghrelin/CRH by 1.45-fold (P = 0.028). Downstream cortisol responses mimicked those of ACTH but were time delayed.

CONCLUSIONS

The present outcomes indicate that the peptide ensemble comprising ghrelin, CRH, and SS (inferred by l-arginine infusion) can regulate ACTH and cortisol secretion in healthy adults.

The role of somatostatin analogs in Cushing's disease

Somatostatin (SRIF) has been proposed to be of therapeutic interest in the medical treatment of Cushing's disease. While in vitro data demonstrate the presence of SRIF-receptor subtype (sst) expression in corticotroph adenomas, the current clinically available SRIF-analog Octreotide, predominantly targeting sst(2), is ineffective in lowering ACTH levels in Cushing's disease and only appears to inhibit the release of ACTH in Nelson's syndrome. In the present review, current knowledge on the physiological role of SRIF in the regulation of ACTH secretion by the anterior pituitary gland, as well as by corticotroph tumor cells is summarized. In addition, the role of glucocorticoids in regulating sst-mediated inhibition of ACTH release by corticotroph adenoma cells is discussed. Recently, it was reported that the novel multiligand SRIF-analog SOM230 might have the potential to be of therapeutic interest for Cushing's disease. On the basis of the potent suppressive effects on ACTH release by SRIF-analogs with high binding affinity to sst(5) and the observation that sst(5) expression and action is relatively resistant to glucocorticoid treatment, including the recent observation that sst(5) is the predominant sst expressed in human corticotroph adenomas, it is hypothesized that sst(5) may be a new therapeutic target for the control of ACTH and cortisol hypersecretion in patients with pituitary dependent Cushing's disease.

Somatostatin treatment reduces the exaggerated response of adrenocorticotropin hormone and cortisol to corticotropin-releasing hormone in polycystic ovary syndrome

OBJECTIVE

To evaluate the influence of somatostatin analogue (octreotide) in the function of hypothalamic-pituitary-adrenal (HPA) axis in women with polycystic ovary syndrome (PCOS).

SETTING

Women referred to the Department of Obstetrics and Gynecology, Università Cattolica del Sacro Cuore.

PATIENT(S)

Twelve PCOS women and 12 normo-ovulatory controls.

INTERVENTION(S)

In early follicular phase, I microgram/kg human corticotrophin-releasing hormone (CRH) was injected at 9:00 A.M. and blood samples were collected for 90 minutes after stimulus; ACTH and cortisol plasma levels were measured. The following day at 8:00 A.M., PCOS patients received an ACTH test (250 micrograms IV) and samples were collected 60 minutes after injection. After 6 weeks of octreotide treatment (100 mg s.c. twice daily), PCOS patients repeated the same study.

MAIN OUTCOME MEASURE(S)

Plasma cortisol and ACTH concentrations.

RESULT(S)

The ACTH and cortisol baseline levels were similar in PCOS and control patients. The responses to human CRH of ACTH (incremental area = 437.86 +/- 188.7 versus 175.78 +/- 87.6 pmol/L; mean +/- SD) and cortisol (incremental area = 17,293.6 +/- 4,320.3 versus 5,885 (912.1 nmol/L) were significantly greater in PCOS with respect to control subjects. After octreotide treatment, ACTH response significantly decreased and no difference was observed with respect to controls (incremental area = 176.94 +/- 91.4). Cortisol responses were decreased by treatment. However, they remained significantly higher than in controls. Treatment did not modify adrenal response to IV ACTH.

CONCLUSION(S)

Data suggest that, in the HPA axis, hyperfunction of PCOS somatostatin could be involved partially.

Hormones and obesity

This chapter has reviewed the evidence for obesity being characterized by distinct patterns of hormonal changes related to both the degree of obesity and the distribution of fat tissue. Many of these changes are also seen in subjects with Cushing's and polycystic ovary syndromes, in particular hyperinsulinaemia, alterations in adrenocortical activity and sex steroid secretion and binding. Animal models of obesity provide evidence to suggest the possibility of a primary abnormality of hypothalamic-pituitary function as a basis to corpulence and this cannot be excluded in the human situation. Nevertheless, abdominal distribution of adiposity plays a significant role in establishing a vicious cycle of metabolic events which may perpetuate both the obese state and PCOS. It is of interest that the additive genetic effect for total body fat is about 25% whereas the heritability of subcutaneous truncal-abdominal fat is about 30-35%, and may possibly be higher (Bouchard et al, 1993). Upper body obesity is characterized by large adipose cells with higher LPL activity, elevated basal and stimulated lipolysis but a low antilipolytic effect of insulin. The results from preliminary investigations of potential candidate genes suggest a possible genetic basis to hyperinsulinaemia/insulin resistance found in upper body obesity but further studies of greater numbers are required for confirmation. It is hoped that the findings from such molecular studies will shed additional light on both the genetic background to obesity and the complex hormonal alterations seen at the tissue level. This should provide the confirmation of a unifying theory for the causal factors associated with obesity and related conditions.