Evidence for differential effects of selective somatostatin receptor subtype agonists on alpha-subunit and chromogranin a secretion and on cell viability in human nonfunctioning pituitary adenomas in vitro

Octreotide and Pasireotide Combination Treatment in Somatotroph Tumor Cells: Predominant Role of SST2 in Mediating Ligand Effects

Simple Summary

First-generation somatostatin receptor ligands, such as octreotide, are the first-line medical therapy in acromegaly. Octreotide shows preferential binding for somatostatin receptor subtype 2 (SST₂), while the second-generation ligand, pasireotide, has high affinity for multiple SSTs. We aimed to elucidate whether pasireotide acts via other receptors than SST₂ in somatotroph tumors, and to investigate the potential role of the combination therapy octreotide plus pasireotide. We found that octreotide and pasireotide are superimposable in reducing GH secretion in cultured somatotroph tumor cells, as well as in inhibiting cell proliferation and intracellular pathway activity in rat GH4C1 cells (a model of somatotroph tumors). We did not find any additive/synergistic effect for the combination treatment. Furthermore, we observed that co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of both compounds. Therefore, the two drugs act mainly via SST₂ in somatotroph tumor cells, and their combination is not superior to single agent treatment.

Abstract

First-generation somatostatin receptor ligands (fg-SRLs), such as octreotide (OCT), represent the first-line medical therapy in acromegaly. Fg-SRLs show a preferential binding affinity for somatostatin receptor subtype-2 (SST₂), while the second-generation ligand, pasireotide (PAS), has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). Whether PAS acts via SST₂ in somatotroph tumors, or through other SSTs (e.g., SST₅), is a matter of debate. In this light, the combined treatment OCT+PAS could result in additive/synergistic effects. We evaluated the efficacy of OCT and PAS (alone and in combination) on growth hormone (GH) secretion in primary cultures from human somatotroph tumors, as well as on cell proliferation, intracellular signaling and receptor trafficking in the rat GH4C1 cell line. The results confirmed the superimposable efficacy of OCT and PAS in reducing GH secretion (primary cultures), cell proliferation, cAMP accumulation and intracellular [Ca²⁺] increase (GH4C1 cells), without any additive effect observed for OCT+PAS. In GH4C1 cells, co-incubation with a SST₂-selective antagonist reversed the inhibitory effect of OCT and PAS on cell proliferation and cAMP accumulation, while both compounds resulted in a robust internalization of SST₂ (but not SST₅). In conclusion, OCT and PAS seem to act mainly through SST₂ in somatotroph tumor cells in vitro, without inducing any additive/synergistic effect when tested in combination.

Somatostatin receptor expression and patients' response to targeted medical treatment in pituitary tumors: evidences and controversies

BACKGROUND

Somatostatin receptors (SSTs) are widely co-expressed in pituitary tumors. SST₂ and SST₅ are the most represented SST subtypes. First-generation somatostatin receptor ligands (SRLs) mainly target SST₂, while pasireotide, a multi-receptor ligand, shows high binding affinity for both SST₅ and SST₂. Therefore, SRLs are routinely used as medical treatment for GH-, TSH-, and ACTH-secreting pituitary tumors.

METHODS

Critical revision of literature data correlating SST expression with patients' response to SRLs.

RESULTS

SST₂ expression in somatroph tumors directly correlates with GH and IGF-1 decrease after first-generation SRL treatment. SST₂ immunohistochemistry represents a valuable tool to predict biochemical response to first-generation SRLs in acromegalic patients. Pasireotide seems to exert its biological effects via SST₂ in unselected patients. However, in those subjects resistant to first-generation SRLs, harbouring tumors with negligible SST₂ expression, pasireotide can act throughout SST₅. More than somatotroph tumors, TSH-omas represent the paradigm of tumors showing a satisfactory response to SRLs. This is probably due to the high SST₂ expression observed in nearly 100% of cases, as well as to the balanced amount of SST₅. In corticotroph tumors, pasireotide mainly act via SST₅, although there is a need for translational studies correlating its efficacy with SST expression in this peculiar tumor histotype.

CONCLUSIONS

The assumption "more target receptor, more drug efficacy" is not straightforward for SRLs. The complex pathophysiology of SSTs, and the technical challenges faced to translate research findings into clinical practice, still need our full commitment to make receptor evaluation a worthwhile procedure for individualizing treatment decisions.

Quantitative Analysis of Somatostatin and Dopamine Receptors Gene Expression Levels in Non-functioning Pituitary Tumors and Association with Clinical and Molecular Aggressiveness Features

The primary treatment for non-functioning pituitary tumors (NFPTs) is surgery, but it is often unsuccessful. Previous studies have reported that NFPTs express receptors for somatostatin (SST_1-5) and dopamine (DRDs) providing a rationale for the use of dopamine agonists and somatostatin analogues. Here, we systematically assessed SST_1-5 and DRDs expression by real-time quantitative PCR (RT-qPCR) in a large group of patients with NFPTs (n = 113) and analyzed their potential association with clinical and molecular aggressiveness features. SST_1-5 expression was also evaluated by immunohistochemistry. SST₃ was the predominant SST subtype detected, followed by SST₂, SST₅, and SST₁. DRD2 was the dominant DRD subtype, followed by DRD4, DRD5, and DRD1. A substantial proportion of NFPTs displayed marked expression of SST₂ and SST₅. No major association between SST_s and DRDs expression and clinical and molecular aggressiveness features was observed in NFPTs.

A Somatostatin Receptor Subtype-3 (SST3) Peptide Agonist Shows Antitumor Effects in Experimental Models of Nonfunctioning Pituitary Tumors

PURPOSE

Somatostatin analogues (SSA) are efficacious and safe treatments for a variety of neuroendocrine tumors, especially pituitary neuroendocrine tumors (PitNET). Their therapeutic effects are mainly mediated by somatostatin receptors SST₂ and SST₅. Most SSAs, such as octreotide/lanreotide/pasireotide, are either nonselective or activate mainly SST₂. However, nonfunctioning pituitary tumors (NFPTs), the most common PitNET type, mainly express SST₃ and finding peptides that activate this particular somatostatin receptor has been very challenging. Therefore, the main objective of this study was to identify SST₃-agonists and characterize their effects on experimental NFPT models.

EXPERIMENTAL DESIGN

Binding to SSTs and cAMP level determinations were used to screen a peptide library and identify SST₃-agonists. Key functional parameters (cell viability/caspase activity/chromogranin-A secretion/mRNA expression/intracellular signaling pathways) were assessed on NFPT primary cell cultures in response to SST₃-agonists. Tumor growth was assessed in a preclinical PitNET mouse model treated with a SST₃-agonist.

RESULTS

We successfully identified the first SST₃-agonist peptides. SST₃-agonists lowered cell viability and chromogranin-A secretion, increased apoptosis in vitro, and reduced tumor growth in a preclinical PitNET model. As expected, inhibition of cell viability in response to SST₃-agonists defined two NFPT populations: responsive and unresponsive, wherein responsive NFPTs expressed more SST₃ than unresponsive NFPTs and exhibited a profound reduction of MAPK, PI3K-AKT/mTOR, and JAK/STAT signaling pathways upon SST₃-agonist treatments. Concurrently, SSTR3 silencing increased cell viability in a subset of NFPTs.

CONCLUSIONS

This study demonstrates that SST₃-agonists activate signaling mechanisms that reduce NFPT cell viability and inhibit pituitary tumor growth in experimental models that expresses SST₃, suggesting that targeting this receptor could be an efficacious treatment for NFPTs.

[Morphofunctional features of non-functioning pituitary adenomas]

Non-functioning pituitary adenomas (NFPAs) account for about 30% of all pituitary tumors. NFPAs are characterized by the lack of secretory potential or its weak expression insufficient for determination of the blood level of adenohypophyseal tropic hormones and for development of a specific clinical picture. Morphologically, NFPAs are a heterogeneous group of tumors, the classification of which was previously based only on immunoreactivity for pituitary tropic hormones. The WHO revised its Classification of Tumors of Endocrine Organs (4th edition) in 2017. The main changes relate to adenohypophysial-cell lineage for the designation of adenomas into subtypes. The introduction of transcription factor antibodies has become a fundamentally new approach to the classification of NFPAs, which is necessary to recognize less differentiated tumor types. This paper provides information on the new histopathological classification of pituitary adenomas, on the theories of silent adenomas, and on the proliferative and prognostic markers of NFPAs.

Is receptor profiling useful for predicting pituitary therapy?

Medical treatment of pituitary tumours may present important challenges in the presence of resistance to first line therapy. In this setting, the availability of specific markers of responsiveness/resistance could be helpful to provide tailored patients' treatment. Pituitary receptor profiling has emerged as a potentially useful tool for predicting the response to specific pituitary-directed medical therapy, mainly somatostatin analogues and dopamine agonists. However, its utility is not always straightforward. In fact, agonist-receptor coupling to the consequent biological response is complex and sometimes jeopardizes the understanding of the molecular basis of pharmacological resistance. Defective expression of pituitary receptors, genetic alterations, truncated variants, impaired signal transduction or involvement of other proteins, such as cytoskeleton proteins or the Aryl hydrocarbon receptor interacting protein amongst others, have been linked to differential tumour phenotype or treatment responsiveness with conflicting results, keeping the debate on the utility of pituitary receptor profiling open. Why does this occur? How can we overcome the difficulties? Is there a true role for pituitary receptor profiling in the near future? All authors of this debate article agree on the need of prospective studies using standardized methods in order to assess the efficacy of receptor profiling as a reliable clinical predictive factor.

Management of NFAs: medical treatment

INTRODUCTION

Non-functioning pituitary adenomas (NFPAs) are in general large tumors that present with symptoms secondary to local pressure on adjacent structures. Transsphenoidal surgery is the first line of treatment but residual tumor mass is often detected post-operatively. Medical therapy, in any stage of tumor management, is not well established.

METHODS

A literature search was performed to review the available data on medical treatment of NFPAs.

RESULTS

Medications investigated for the treatment of NFPAs include dopamine receptor agonists (DA) and somatostatin receptor ligands. Randomized controlled trials are lacking, but available data suggest that DA have a positive effect on tumor remnant stabilization after surgery and could be considered in this setting. Temozolomide is reserved for aggressive tumors, although future studies are required.

CONCLUSIONS

NFPA are often not amenable to complete surgical resection. Conservative follow-up after surgery is associated with a high prevalence of tumor remnant progression. DA therapy may prevent residual tumor enlargement in over 85% of these patients, with a substantial consequent reduction in the need for repeat surgery or radiation therapy. It is our view that DA treatment should be routinely considered for the management of NFPA patients with incompletely resected tumors.

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.

MANAGEMENT OF ENDOCRINE DISEASE: Present and future perspectives for medical therapy of nonfunctioning pituitary adenomas

In contrast to the clear indication for surgical treatment in symptomatic patients with clinically nonfunctioning pituitary adenomas (NFPA), there are no randomized controlled studies comparing therapeutic strategies such as watchful waiting, irradiation or medical therapy for the management of NFPA after surgery. Further, no medical therapy is currently approved for the treatment of NFPA. In this review, we summarize accumulating data on medications currently approved for secreting pituitary adenomas, used off-label in patients with NFPA. Perspectives on overall treatment optimization and potential future therapies are also detailed.

Effects of somatostatin and its analogues on progenitor mesenchymal cells isolated from human pituitary adenomas

PURPOSE

Progenitor mesenchymal cells (PMCs) have been found also in epithelial tumors and may derive from cancer stem cells (CSCs) by EMT mechanism. In this scenario, the effects of traditionally drugs on PMCs become of primary concern for therapeutic approaches. Previously, we isolated PMCs from acromegalic (GHomas) and not-functioning pituitary adenomas (NFPAs). Here we evaluate: (1) the role of EMT on their origin; (2) the presence of the somatostatin receptors (SSTR1-5); (3) the effects of somatostatin (SST) and its analogues (SSAs) on PMCs proliferation, apoptosis and SSTR1-5 expression.

METHODS

PMCs were isolated from GHomas and NFPAs; the expression of E-CADHERIN and TGFβRII (referred to EMT), the expression of the SSTR1-5 as well as the proliferation and apoptosis were tested before and after drugs administration.

RESULTS

Results show a decrease of E-CADHERIN and an increase of TGFβRII, confirming an EMT involvement; SSTR1-5 are more expressed by PMCs from GHomas than from NFPAs. SST and SSAs administration does not affect cell proliferation and SSTR1-5 expression on PMCs from NFPAs while in PMCs from GHomas, cell proliferation showed a marked decrease and a corresponding increase in the expression of SSTR1-2. Apoptosis rate and EMT were not affected by drugs administration.

CONCLUSIONS

Results indicate as EMT may be related to the presence of PMCs on pituitary tumors; SSAs, currently used in the management of human GHomas, exert anti-proliferative effect also in PMCs that, because of their derivation from CSCs, may be a new meaningful target for drugs treatment.

The expression of the truncated isoform of somatostatin receptor subtype 5 associates with aggressiveness in medullary thyroid carcinoma cells

The truncated somatostatin receptor variant sst5TMD4 associates with increased invasiveness and aggressiveness in breast cancer. We previously found that sst5 activation may counteract sst2 selective agonist effects in a medullary thyroid carcinoma (MTC) cell line, the TT cells, and that sst5TMD4 is overexpressed in poorly differentiated thyroid cancers. The purpose of this study is to evaluate sst5TMD4 expression in a series of human MTC and to explore the functional role of sst5TMD4 in TT cells. We evaluated sst5TMD4 and sst5 expression in 36 MTC samples. Moreover, we investigated the role of sst5TMD4 in TT cells evaluating cell number, DNA synthesis, free cytosolic calcium concentration ([Ca(2+)]i), calcitonin and vascular endothelial growth factor levels, cell morphology, protein expression, and invasion. We found that in MTC the balance between sst5TMD4 and sst5 expression influences disease stage. sst5TMD4 overexpression in TT cells confers a greater growth capacity, blocks sst2 agonist-induced antiproliferative effects, modifies the cell phenotype, decreases E-cadherin and phosphorylated β-catenin levels, increases vimentin, total β-catenin and phosphorylated GSK3B levels (in keeping with the development of epithelial to mesenchymal transition), and confers a greater invasion capacity. This is the first evidence indicating that sst5TMD4 is expressed in human MTC cells, where it associates with more aggressive behavior, suggesting that sst5TMD4 might play a functionally relevant role.

Deletion of exons 1-3 of the MEN1 gene in a large Italian family causes the loss of menin expression

Multiple endocrine neoplasia type 1 (MEN1) syndrome is an autosomal dominant disease, characterized by parathyroid adenomas, endocrine gastroenteropancreatic tumors and pituitary adenomas, due to inactivating mutations of the MEN1 gene (chromosome 11q13). MEN1 mutations are mainly represented by nonsense, deletions/insertions, splice site or missense mutations that can be detected by direct sequencing of genomic DNA. However, MEN1 patients with large heterozygous deletions may escape classical genetic screening and may be misidentified as phenocopies, thereby hindering proper clinical surveillance. We employed a real-time polymerase chain reaction application, the TaqMan copy number variation assay, to evaluate a family in which we failed to identify an MEN1 mutation by direct sequencing, despite a clear clinical diagnosis of MEN1 syndrome. Using the TaqMan copy number variation assay we identified a large deletion of the MEN1 gene involving exons 1 and 2, in three affected family members, but not in the other nine family members that were to date clinically unaffected. The same genetic alteration was not found in a group of ten unaffected subjects, without family history of endocrine tumors. The MEN1 deletion was further confirmed by multiplex ligation-dependent probe amplification, which showed the deletion extended from exon 1 to exon 3. This new approach allowed us to correctly genetically diagnose three clinical MEN1 patients that were previously considered as MEN1 phenocopies. More importantly, we excluded the presence of genetic alterations in the unaffected family members. These results underline the importance of using a variety of available biotechnology approaches when pursuing a genetic diagnosis in a clinically suggestive setting of inherited endocrine cancer.

Dopamine 2 and somatostatin 1-5 receptors coexpression in clinically non-functioning pituitary adenomas

This study investigated quantitated expression of dopamine 2 receptor (D2R) and somatostatin receptors of the five types (SSTR1-SSTR5) in a large series of clinically non-functioning pituitary adenomas (CNFAs). Co-expression of these receptors in individual adenomas was studied as well as correlation between receptor types. Adenoma tissue from 198 patients who underwent surgery for CNFAs was analyzed by immunohistochemistry and quantitative real-time PCR. D2R and SSTR1-3 mRNA was expressed in all 198 adenomas. SSTR4 and SSTR5 were detectable in 85 % and 61 % of adenomas, respectively. Expression of D2R was significantly higher than that of the somatostatin receptors. The median relative expressions were as follows from highest D2R >> SSTR3 > SSTR2 > SSTR1 > SSTR5 > SSTR4. High relative expression (ratio to beta-glucuronidase mRNA > 1) of D2R was found in 60 % of tumors, high expression of SSTR1 in 7.5 %, SSTR2 in 7 %, SSTR3 in 4 % and SSTR5 in 0.5 %. The quantity of D2R correlated positively with expression of SSTR2 and SSTR3, and negatively with SSTR1 and SSTR5. Among histological adenoma types, SSTR1 was significantly higher in null-cell adenomas and SSTR3 was lower in silent corticotroph adenomas. In conclusions, in CNFAs, high expression of somatostatin receptors is much less common than that of D2R, and co-expression of both these receptors is exceptional. D2R and SSTR3 seem to be the most promising targets for pharmacological treatment.

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.

Cabergoline reduces cell viability in non functioning pituitary adenomas by inhibiting vascular endothelial growth factor secretion

Dopamine (DA) therapy of non-functioning pituitary adenomas (NFA) can result in tumor stabilization and shrinkage. However, the mechanism of action is still unknown. Previous evidence showed that DA can inhibit pituitary vascular endothelial growth factor expression (VEGF), that may be involved in pituitary tumor growth. The aim of our study was to clarify whether VEGF secretion modulation might mediate the effects of DA agonists on cell proliferation in human NFA. We assessed DA receptor subtype 2 (DR2) expression in 20 NFA primary cultures, where we also investigated the effects of a selective DR2 agonist, cabergoline (Cab), on VEGF secretion and on cell viability. All NFA samples expressed α-subunit and DR2 was expressed in 11 samples. In DR2 expressing tumors, Cab significantly reduced cell viability (-25%; P < 0.05) and VEGF secretion (-20%; P < 0.05). These effects were counteracted by treatment with the DA antagonist sulpiride. Cab antiproliferative effects were blocked by VEGF. Our data demonstrate that Cab, via DR2, inhibits cell viability also by reducing VEGF secretion in a selected group of NFA, supporting that DA agonists can be useful in the medical therapy of DR2 expressing NFA.

Effect of everolimus on cell viability in nonfunctioning pituitary adenomas

CONTEXT

Pituitary adenomas can cause specific syndromes due to hormone excess and/or determine sellar mass symptoms. Pituitary cell growth can sometimes be influenced by medical therapy, such as for somatotroph adenomas treated with somatostatin analogs or prolactinomas treated with dopaminergic drugs. However, nonfunctioning pituitary adenomas (NFAs) are still orphans of medical therapy. Everolimus (RAD001), a derivative of rapamycin, is a well-known immunosuppressant drug, which has been recently shown to have antineoplastic activity in several human cancers.

OBJECTIVE

The objective of the study was to investigate the possible antiproliferative effects of RAD001 in human NFAs.

DESIGN

We collected 40 NFAs that were dispersed in primary cultures, treated without or with 1 nm to 1 microm RAD001, 10 nm cabergoline, 10 nm SOM230 (a somatostatin receptor multiligand), and/or 50 nm IGF-I. Cell viability and apoptosis were evaluated after 48 h, and vascular endothelial growth factor (VEGF) secretion was assessed after an 8-h incubation. Somatostatin and dopamine subtype 2 receptor expression was investigated by quantitative PCR.

RESULTS

In 28 cultures (70%), Everolimus significantly reduced cell viability (by approximately 40%; P < 0.05 vs. control), promoted apoptosis (+30%; P < 0.05 vs. control), inhibited p70S6K activity (-20%), and blocked IGF-I proliferative and antiapoptotic effects. In selected tissues cotreatment with SOM230, but not cabergoline, exerted an additive effect. Everolimus did not affect VEGF secretion but blocked the stimulatory effects of IGF-I on this parameter.

CONCLUSIONS

Everolimus reduced NFA cell viability by inducing apoptosis, with a mechanism likely involving IGF-I signaling but not VEGF secretion, suggesting that it might represent a possible medical treatment of invasive/recurrent NFAs.

Non-functioning pituitary adenomas

Non-functioning pituitary tumours are mostly of gonadotroph cell origin and are devoid of humoral hypersecretory syndromes. They are usually large at the time of diagnosis, commonly presenting with headaches, visual field defects and hypopituitarism. Trans-sphenoidal surgery remains the treatment of choice for rapid decompression of neighbouring structures, often bringing to normalisation or improvement of visual and pituitary function. The management of patients with postoperative residual tumours is still a matter of debate and may include observation alone, the use of dopamine agonists or radiation therapy. There are no controlled or comparative studies of the available therapeutic options; therefore, recommendations are not evidence based. Patients need long-term follow-up for the detection and treatment of hypopituitarism, visual dysfunction and tumour growth that may develop over time.

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

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Somatostatin-dopamine ligands in the treatment of pituitary adenomas

Somatostatin receptors (sst1-5) and dopamine receptor 2 (D2DR) are well expressed and co-localized in several human pituitary adenomas, suggesting possible functional interactions in the control of hormonal hypersecretion and tumor cell growth. The present review describes the expression and functionality of these receptors in the different classes of human pituitary adenomas. The sst2 agonists, octreotide and lanreotide, control GH hypersecretion and tumor growth in about 65% of somatotropinomas. The D2DR agonists, bromocriptine and cabergoline, control about 90% of prolactinomas. Such drugs are much less effective in the control of the others pituitary adenomas also expressing ssts and D2DR receptors. The second part summarizes the current knowledge on new chimeric compounds with sst2, sst5, and D2DR affinity. Such ligands bearing distinct ssts and DRD2 pharmacophores may synergistically produce an increased control of secretion and/or of proliferation in the different types of pituitary adenomas. The mechanisms of action of such chimeric molecules through increased binding affinities, prolonged bioavailability, ligand-induced modulation of receptors heterodimerization, are discussed.

Differential expression of somatostatin and dopamine receptor subtype genes in adrenocorticotropin (ACTH)-secreting pituitary tumors and silent corticotroph adenomas

Somatostatin analogs and dopamine agonists are clinically used for medical therapy of functioning pituitary tumors, such as growth hormone- and prolactin-secreting tumors, however, their effects on ACTH-secreting tumors are controversial. This study was aimed to determine whether somatostatin receptor (SSTR) subtype (1-5) and dopamine receptor type 2 (D2R) are differentially expressed in pituitary tumors causing Cushing's disease (CD), silent corticotroph adenoma (SCA), and non-functioning pituitary tumor (NFT). Tissue specimens were obtained from 35 pituitary tumors during transsphenoidal surgery. The steady-state mRNA levels of SSTR1-5 and D2R genes were determined by real-time reverse-transcription polymerase chain reaction. Both SSTR1 and 2 mRNA levels in SCA were greater than CD, while SSTR1 mRNA levels, but not SSTR2, in SCA were also greater than NFT. SSTR5 mRNA levels in CD were greater than SCA, but did not differ between NFT and SCA. SSTR4 mRNA expression was undetectable. D2R mRNA levels were markedly lower in CD and SCA than in NFT. The present study suggests that somatostatin analogs more selective for SSTR5 and for SSTR1 and/or 2may have the therapeutic potential for medical treatment of CD and SCA, respectively, whereas clinical application of dopamine agonists selective for D2R is very limited in either CD or SCA.

In vitro testing of new somatostatin analogs on pituitary tumor cells

Somatostatin has been discovered as a somatotroph release inhibitory factor (SRIF), and, indeed, it has been demonstrated that SRIF and its analogs can inhibit pituitary tumor hormone secretion and control neoplastic bulk. Several in vitro studies have contributed to the current knowledge of the mechanisms by which SRIF and its analogs may influence pituitary adenomas, opening the way to new possible therapeutic strategies. This review focuses on the results obtained by testing several SRIF analogs in vitro on pituitary adenomas, concerning both secretory activity and cell viability. These studies provide the basis for further investigations, both at basic and clinical level, of the application of SRIF analogs in the pituitary field.

Antitumor effects of somatostatin

Since its discovery three decades ago as an inhibitor of GH release from the pituitary gland, somatostatin has attracted much attention because of its functional role in the regulation of a wide variety of physiological functions in the brain, pituitary, pancreas, gastrointestinal tract, adrenals, thyroid, kidney and immune system. In addition to its negative role in the control of endocrine and exocrine secretions, somatostatin and analogs also exert inhibitory effects on the proliferation and survival of normal and tumor cells. Over the past 15 years, studies have begun to reveal some of the molecular mechanisms underlying the antitumor activity of somatostatin. This review covers the present knowledge in the antitumor effect of somatostatin and analogs and discusses the perspectives of novel clinical strategies based on somatostatin receptor sst2 gene transfer therapy.

Effects of selective somatostatin analogs and cortistatin on cell viability in cultured human non-functioning pituitary adenomas

Clinically "non-functioning" human pituitary adenomas (NFPA) constitute about 35% of pituitary adenomas. Somatostatin receptors (SSTR) expression in these adenomas has previously been described both in vitro and in vivo, without evidence for a correlation with tumor volume or the therapeutic efficacy of somatostatin analogs. This study was performed on 13 surgically removed pituitary macroadenomas, diagnosed before surgery as "non-functioning". In addition, 3 growth hormone (GH)-secreting adenomas served as controls. A specimen from each tumor was dispersed and digested to isolate and culture the tumor cells, and the in vitro effects of SSTR2 and SSTR5 selective analogs and Cortistatin (CST) (100nM) on cell viability were studied. The quantity of viable cells was estimated using the XTT method. RNA purification of tumor samples and subsequent RT-PCR studies for SSTR2 and SSTR5 expression were performed. Somatostatin analog with high affinity for SSTR2 reduced cell viability by 20-80% in 8 of 13 NFPAs studied, all expressing the SSTR2. The inhibitory effect on cell viability of SSTR5-selective analog was 15-80% in 10 of 13 NFPAs studied, all but three expressing the SSTR5. CST, however, effectively reduced cell viability in only 6 NFPAs. Cell viability was inhibited by all peptides studied in 2 out of 3 GH-secreting adenomas, expressing both receptors. The third adenoma responded to SSTR2 analog and expressed only SSTR2. These results suggest the involvement of SSTR2 and SSTR5 in the anti-proliferative effects of somatostatin; however, CST is less potent in reducing cell viability in these tumors.

Relevance of coexpression of somatostatin and dopamine D2 receptors in pituitary adenomas

Dopamine and somatostatin are both involved in the negative control of normal pituitary cells. Dopamine subtype 2 receptor (D2DR) and somatostatin receptor (sst) agonists, mainly directed to sst2, are used in the treatment of pituitary adenomas. Nevertheless, a majority of corticotroph and gonadotroph adenomas and a third of somatotroph adenomas are still not sufficiently controlled by these treatments. D2DR and sst1, 2, 3 and 5 are present in most pituitary adenomas. These receptors may interact by heterodimerization as shown for sst1-sst5, sst5-D2DR, sst2-sst3 and sst2-D2DR suggesting possible additive effects. D2DR and sst2 agonist cotreatment showed limited additivity on GH secretion in acromegaly. Moreover, new chimeric compounds with sst2, D2DR and sst5 affinity have shown an increased control of secretion and/or proliferation of different types of pituitary adenomas in cell culture. Together with the multi-sst ligand drugs recently developed, these dopamine-somatostatin ligands represent a new opportunity in the combinatory treatment of pituitary adenomas.

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

The novel somatostatin receptor agonist pasireotide binds with high affinity to somatostatin receptors SSTR1, 2, 3, and 5. Acting principally through the latter, it inhibits basal and CRH-stimulated ACTH secretion from the AtT20 corticotroph cell line and ACTH release from a proportion of human corticotroph adenomas both in vitro and in vivo. Data supporting an additional antiproliferative effect has led to pasireotide being explored as a potential therapy for patients with Cushing's disease. We have compared the effects of pasireotide and octreotide on adrenalectomy-induced mitotic and apoptotic activity in the male rat anterior pituitary. Adrenalectomized rats were treated with daily sc injections of vehicle, pasireotide, or octreotide. Changes in proliferation and apoptosis were determined 2-6 d postoperatively. Pasireotide and octreotide had no effect on baseline pituitary cell turnover and no measurable effects on apoptosis. However, the wave of increased mitotic activity normally seen in the pituitary after adrenalectomy was completely abolished. Nevertheless, pasireotide and octreotide did not diminish the increase in ACTH-immunopositive cell index after adrenalectomy, indicating that cell division and differentiation of hormonally null cells in the pituitary are under independent control. In conclusion, basal cell turnover in the pituitary is not inhibited by pasireotide or octreotide. Bilateral adrenalectomy stimulates differentiation of preexisting null cells into ACTH-positive cells. Cell division after bilateral adrenalectomy occurs in a specific subpopulation of hormonally null cells that are equally sensitive to the antiproliferative effects of pasireotide and octreotide, implicating SSTR2 receptors in this antimitotic response.

Control of pituitary adenoma cell proliferation by somatostatin analogs, dopamine agonists and novel chimeric compounds

The antisecretory effects of somatostatin (SRIH) and its analogs are widely recognized and provide the basis for treatment of hormonal hypersecretion in patients with pituitary adenomas, especially in the settings of acromegaly. Dopamine (DA) agonists have also been used for medical treatment of prolactin and/or GH hypersecretion, and recent evidence points to an even greater antisecretory effect for a chimeric molecule, having high affinity for both SRIH and DA receptors. Evidence for an antiproliferative effect of these compounds has also been provided. This review focuses on the antiproliferative effects of SRIH and its analogs, of DA and chimeric compounds on pituitary adenomas, and on the clinical consequences on tumor volume of pharmacological treatment of pituitary adenomas with these drugs.

Novel chimeric somatostatin analogs: facts and perspectives

Dopamine and somatostatin receptor agonists inhibit hormone secretion by normal pituitary cells and pituitary adenomas. Indeed, initially several dopaminergic drugs, and lately somatostatin analogs, have been developed for the treatment of pituitary adenomas. Recently, it has been demonstrated that subtypes of somatostatin and dopamine receptors may form homo- and hetero-dimers at the membrane level, as part of their normal trafficking and function. Interestingly, a specific ligand for a given receptor may influence the activity of an apparently unrelated receptor, and the association between the two different receptors could be induced by addition of either dopamine or somatostatin. The new properties of these families of G-protein coupled receptors (GPCRs) offer a potential explanation for the apparent conflicting results observed both in vivo and in vitro in human cell systems treated with the presently available analogs. Moreover, this observation not only increases the possibilities of modulating the activities of these receptors, but also raises new questions on the role of associations of specific receptors in the control of cell functions. In fact, results from preclinical studies have shown that receptor activation may not only trigger different intracellular signaling pathways, but also induce a distinct response depending upon the specific cell type. Recently, a number of new interesting compounds (subtype selective analogs and antagonists, as well as bi-specific and hybrid somatostatin/dopamine compounds) have been developed. The effects of these new molecules have been explored in few animal and human cell lines and primary cultures from human tumors, revealing a heterogeneous, but broader, profile of activities. Further studies are certainly needed to fully elucidate the complex interplay between the GPCRs and consequent biological effects, to identify suitable therapies for controlling hormonal secretion of pituitary tumors. However, these recent observations form the basis for the application of new interesting strategies for the treatment of not only pituitary tumors but also other human malignancies.

Identification of differentially expressed microRNAs by microarray: a possible role for microRNA genes in pituitary adenomas

MicroRNAs (miRNAs) are small non-coding RNAs that control gene expression by targeting mRNA. It has been demonstrated that miRNA expression is altered in many human cancers, suggesting that they may play a role in human neoplasia. To determine whether miRNA expression is altered in pituitary adenomas, we analyzed the entire miRNAome in 32 pituitary adenomas and in 6 normal pituitary samples by microarray and by Real-Time PCR. Here, we show that 30 miRNAs are differentially expressed between normal pituitary and pituitary adenomas. Moreover, 24 miRNAs were identified as a predictive signature of pituitary adenoma and 29 miRNAs were able to predict pituitary adenoma histotype. miRNA expression could differentiate micro- from macro-adenomas and treated from non-treated patient samples. Several of the identified miRNAs are involved in cell proliferation and apoptosis, suggesting that their deregulated expression may be involved in pituitary tumorigenesis. Predictive miRNAs could be potentially useful diagnostic markers, improving the classification of pituitary adenomas.

Antiproliferative effects of somatostatin analogs in pituitary adenomas

The antisecretory effects of somatostatin (SRIF) and its analogs are widely recognised and provide the basis for treatment of hormonal hypersecretion in pituitary adenomas, especially in the settings of acromegaly. Evidence for an antiproliferative effect of these compounds has also been provided. This review focuses on the mechanisms transducing the antiproliferative effects of SRIF and its analogs on pituitary adenomas, and on the clinical consequences on tumor volume of pharmacological treatment of pituitary adenomas with these drugs.

SRC homology-2-containing protein tyrosine phosphatase-1 restrains cell proliferation in human medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a rare tumor originating from thyroid parafollicular C cells, where, in the inherited form, constitutive activation of the RET protooncogene is responsible for unrestrained cell proliferation. We previously demonstrated that somatostatin (SRIF) reduces cell growth in the human MTC cell line TT, which expresses all SRIF receptor (SSTR) subtypes and responds differently to selective SSTR agonists. The antiproliferative mechanism of SRIF and its analogs in MTC is still unclear. Src homology-2-containing protein tyrosine phosphatase-1 (SHP-1), a cytoplasmic protein tyrosine phosphatase (PTP), is activated by somatotropin release-inhibiting factor and reduces mutated RET autophosphorylation in a heterologous system. In this study, we explore the role of PTP activation, in particular of SHP-1, in TT cells, where RET is constitutively activated. In TT cells, SRIF stimulated the PTP activity of SHP-1, which was associated with proliferation inhibition and with reduction in the MAPK pathway activation. Blockade of PTP activity with sodium orthovanadate induced cell proliferation and MAPK phosphorylation and blunted the inhibitory effects of SRIF. Moreover, SHP-1 associates with SSTR2 depending on its activation. By using a MAPK kinase inhibitor, we demonstrated that TT cell growth depends on MAPK pathway activation. Furthermore, in TT cells overexpressing SHP-1, cell proliferation and MAPK signaling were strongly down-regulated, whereas in TT cells transfected with a dominant negative form of SHP-1, cell proliferation and MAPK signaling were markedly induced. Our data demonstrate that SRIF inhibitory effects on TT cell proliferation are mediated, at least in part, by SHP-1, which acts through a MAPK-dependent mechanism.