Somatostatin receptor subtype 1-selective activation reduces cell growth and calcitonin secretion in a human medullary thyroid carcinoma cell line

Integrative Clinical, Radiological, and Molecular Analysis for Predicting Remission and Recurrence of Cushing Disease

CONTEXT

Adrenocorticotropin (ACTH)-secreting pituitary tumors (ACTHomas) are associated with severe comorbidities and increased mortality. Current treatments mainly focus on remission and prevention of persistent disease and recurrence. However, there are still no useful biomarkers to accurately predict the clinical outcome after surgery, long-term remission, or disease relapse.

OBJECTIVES

This work aimed to identify clinical, biochemical, and molecular markers for predicting long-term clinical outcome and remission in ACTHomas.

METHODS

A retrospective multicenter study was performed with 60 ACTHomas patients diagnosed between 2004 and 2018 with at least 2 years' follow-up. Clinical/biochemical variables were evaluated yearly. Molecular expression profile of the somatostatin/ghrelin/dopamine regulatory systems components and of key pituitary factors and proliferation markers were evaluated in tumor samples after the first surgery.

RESULTS

Clinical variables including tumor size, time until diagnosis/first surgery, serum prolactin, and postsurgery cortisol levels were associated with tumor remission and relapsed disease. The molecular markers analyzed were distinctly expressed in ACTHomas, with some components (ie, SSTR1, CRHR1, and MKI67) showing instructive associations with recurrence and/or remission. Notably, an integrative model including selected clinical variables (tumor size/postsurgery serum cortisol), and molecular markers (SSTR1/CRHR1) can accurately predict the clinical evolution and remission of patients with ACTHomas, generating a receiver operating characteristic curve with an area under the curve of 1 (P < .001).

CONCLUSION

This study demonstrates that the combination of a set of clinical and molecular biomarkers in ACTHomas is able to accurately predict the clinical evolution and remission of patients. Consequently, the postsurgery molecular profile represents a valuable tool for clinical evaluation and follow-up of patients with ACTHomas.

Octreotide and pasireotide effects on medullary thyroid carcinoma (MTC) cells growth, migration and invasion

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine neoplasm of the parafollicular thyroid C cells. Although somatostatin receptors are expressed by MTCs, treatment with octreotide has shown poor efficacy, whereas recently pasireotide has demonstrated antiproliferative effects in persistent postoperative MTCs. Aim of this study was to test the effects of octreotide and pasireotide on MTC cells proliferation, cell cycle proteins expression, MAPK activation, apoptosis, calcitonin secretion, migration and invasion in TT cell line as well as in primary MTC cultured cells. Our results showed that both octreotide and pasireotide reduced TT cell proliferation (-35.2 ± 12.1%, p < 0.001, and -25.3 ± 24.8%, p < 0.05, at 10^-8 M, respectively), with concomitant inhibition of ERK phosphorylation and cyclin D1 expression. This cytostatic effect was accompanied by a proapoptotic action, with an increase of caspase3/7 activity of 1.5-fold. Moreover, both octreotide and pasireotide inhibited cell migration (-50.9 ± 11.3%, p < 0.01, and -40.5 ± 17%, p < 0.05, respectively) and invasion (-61.3 ± 35.1%, p < 0.05, and -49.7 ± 18%, p < 0.01, respectively). No effect was observed on calcitonin secretion. We then tried to extend these observations to primary cultures (n = 5). Octreotide and/or pasireotide were effective in reducing cells proliferation in 3 out of 5 tumors, and to induce cell apoptosis in 1 out of 3 MTCs. Both octreotide and pasireotide were able to reduce cell migration in all MTC tested. SST2, SST3 and SST5 were expressed in all MTC, with a tendency to increased expression of SST2 in RET mutated vs wild type MTCs. In agreement, inhibition of mutated RET in TT cells reduced SST2 expression. In conclusion, we demonstrated that octreotide and pasireotide inhibited cell proliferation and invasiveness in a subset of MTC, supporting their potential use in the control of tumor growth.

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.

Somatostatin receptor subtype 1 might be a predictor of better response to therapy in medullary thyroid carcinoma

PURPOSE

Medullary thyroid carcinoma (MTC) is a malignant neoplasm of parafollicular cells. Because it is a neuroendocrine tumor, it has known somatostatin receptors (SSTRs). The actual frequencies of the SSTR subtypes and their potential influences (by binding with endogenous somatostatin) on MTC cell proliferation have not been fully elucidated to date. The present study evaluated the occurrence of SSTR subtypes 1, 2, 3 and 5 as well as the possible role that each subtype plays in the clinical evolution of patients with MTC.

METHODS

This retrospective, longitudinal study analyzed thyroid surgical material from 42 patients with MTC. Immunohistochemical staining was performed with monoclonal antibodies against subtypes 1, 2, 3 and 5 of SSTR. The histological material was classified as negative, focal positive or diffuse positive, in relation to each of the SSTR subtypes. The initial response to treatment, clinical course and patient mortality rate were assessed and related to the presence of SSTR subtypes.

RESULTS

The most prevalent SSTR subtype was SSTR 3, which was found in 81% of the patients, when considering any pattern of positivity. However, subtype 2 had the lowest number of positive patients, with 28.6% demonstrating any positive pattern. Subtypes 1 and 5 had an intermediate prevalence of positivity, with subtype 1 present in 45.2% of the patients and subtype 5 positive in 54.8% of the patients, when considering any pattern of positivity. The presence of STR 1, in the form of diffuse positivity, independently predicted a better response to the initial therapy, with a hazard ratio (HR) of 4.80 (p = 0.03).

CONCLUSION

This is the first study to show the correlation of the presence of SSTR1, detected by monoclonal immunohistochemical techniques, and better response to initial treatment and possibly better long-term clinical response in patients with MTC. In addition, these patients had low positivity rates for SSTR2, which might explain the low sensitivity of diagnostic and limited therapeutic response to octrotide based radioisotopes.

Effects of Somatostatin Analogs and Dopamine Agonists on Insulin-Like Growth Factor 2-Induced Insulin Receptor Isoform A Activation by Gastroenteropancreatic Neuroendocrine Tumor Cells

BACKGROUND

Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) express insulin-like growth factor (IGF)-related factors [IGF1, IGF2; insulin receptor (IR)-A, IR-B; IGF-binding protein (IGFBP) 1-3] as well as somatostatin (SSTRs) and dopamine D2 receptors (D2Rs).

OBJECTIVES

To (1) compare mRNA expression of IGF-related factors in human pancreatic NET (panNET) cell lines with that in human GEP-NETs to evaluate the usefulness of these cells as a model for studying the IGF system in GEP-NETs, (2) determine whether panNET cells produce growth factors that activate IR-A, and (3) investigate whether somatostatin analogs (SSAs) and/or dopamine agonists (DAs) influence the production of these growth factors.

METHODS

In panNET cells (BON-1 and QGP-1) and GEP-NETs, mRNA expression of IGF-related factors was measured by quantitative real-time PCR. Effects of the SSAs octreotide and pasireotide (PAS), the DA cabergoline (CAB), and the dopastatin BIM-23A760 (all 100 nM) were evaluated at the IGF2 mRNA and protein level (by ELISA) and regarding IR-A bioactivity (by kinase receptor activation assay) in panNET cells.

RESULTS

panNET cells and GEP-NETs had comparable expression profiles of IGF-related factors. Especially in BON-1 cells, IGF2 and IR-A were most highly expressed. PAS + CAB inhibited IGF2 (-29.5 ± 4.9%, p < 0.01) and IGFBP3 (-20.0 ± 4.0%, p < 0.01) mRNA expression in BON-1 cells. In BON-1 cells, IGF2 protein secretion was significantly inhibited with BIM-23A760 (-23.7 ± 3.8%). BON-1- but not QGP-1- conditioned medium stimulated IR-A bioactivity. In BON-1 cells, IR-A bioactivity was inhibited by BIM-23A760 and PAS + CAB (-37.8 ± 2.1% and -30.9 ± 4.1%, respectively, p < 0.0001).

CONCLUSIONS

(1) The BON-1 cell line is a representative model for studying the IGF system in GEP-NETs, (2) BON-1 cells produce growth factors (IGF2) activating IR-A, and (3) combined SSTR and D2R targeting with PAS + CAB and BIM-23A760 suppresses IGF2-induced IR-A activation.

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.

Therapeutic uses of somatostatin and its analogues: Current view and potential applications

Somatostatin is an endogeneous cyclic tetradecapeptide hormone that exerts multiple biological activities via five ubiquitously distributed receptor subtypes. Classified as a broad inhibitory neuropeptide, somatostatin has anti-secretory, anti-proliferative and anti-angiogenic effects. The clinical use of native somatostatin is limited by a very short half-life (1 to 3min) and the broad spectrum of biological responses. Thus stable, receptor-selective agonists have been developed. The majority of these somatostatin therapeutic agonists bind strongly to two of the five receptor subtypes, although recently an agonist of wider affinity has been introduced. Somatostatin agonists are established in the treatment of acromegaly with recently approved indications in the therapy of neuroendocrine tumours. Potential therapeutic uses for somatostatin analogues include diabetic complications like retinopathy, nephropathy and obesity, due to inhibition of IGF-1, VEGF together with insulin secretion and effects upon the renin-angiotensin-aldosterone system. Wider uses in anti-neoplastic therapy may also be considered and recent studies have further revealed anti-inflammatory and anti-nociceptive effects. This review provides a comprehensive, current view of the biological functions of somatostatin and potential therapeutic uses, informed by the wide range of pharmacological advances reported since the last published review in 2004 by P. Dasgupta. The pharmacology of somatostatin receptors is explained, the current uses of somatostatin agonists are discussed, and the potential future of therapeutic applications is explored.

Igf-I influences everolimus activity in medullary thyroid carcinoma

CONTEXT

Medullary thyroid carcinoma (MTC) is a rare tumor originating from thyroid parafollicular C cells. It has been previously demonstrated that insulin-like growth factor I (IGF-I) protects MTC from the effects of antiproliferative drugs. Everolimus, an mTOR inhibitor, has shown potent antiproliferative effects in a human MTC cell line, TT, and in two human MTC primary cultures.

OBJECTIVE

To verify whether IGF-I may influence the effects of everolimus in a group of human MTC primary cultures.

DESIGN

We collected 18 MTCs that were dispersed in primary cultures, treated without or with 10 nM-1 μM everolimus and/or 50 nM IGF-I. Cell viability was evaluated after 48 h, and calcitonin (CT) secretion was assessed after a 6 h incubation. IGF-I receptor downstream signaling protein expression profile was also investigated.

RESULTS

Everolimus significantly reduced cell viability in eight MTC [by ~20%; P < 0.01 vs. control; everolimus-responders (E-R) MTCs], while cell viability did not change in 10 MTCs [everolimus-non-responders (E-NR) MTCs]. In E-R MTCs, IGF-I blocked the antiproliferative effects of everolimus that did not affect CT secretion, but blocked the stimulatory effects of IGF-I on this parameter. IGF-I receptor downstream signaling proteins were expressed at higher levels in E-NR MTC as compared to E-R MTCs.

CONCLUSION

IGF-I protects a subset of MTC primary cultures from the antiproliferative effects of everolimus and stimulates CT secretion by an mTOR mediated pathway that, in turn, may represent a therapeutic target in the treatment of aggressive MTCs.

Mitotane enhances doxorubicin cytotoxic activity by inhibiting P-gp in human adrenocortical carcinoma cells

Mitotane is currently employed as adjuvant therapy as well as in the medical treatment of adrenocortical carcinoma (ACC), alone or in combination with chemotherapeutic agents. It was previously demonstrated that mitotane potentiates chemotherapeutic drugs cytotoxicity in cancer cells displaying chemoresistance due to P-glycoprotein (P-gp), an efflux pump involved in cancer multidrug resistance. The majority of ACC expresses high levels of P-gp and is highly chemoresistent. The aim of our study was to explore in vitro whether mitotane, at concentrations lower than those currently reached in vivo, may sensitize ACC cells to the cytotoxic effects of doxorubicin and whether this effect is due to a direct action on P-gp. NCI-H295 and SW13 cell lines as well as 4 adrenocortical neoplasia primary cultures were treated with mitotane and doxorubicin, and cell viability was measured by MTT assay. P-gp activity was measured by calcein and P-gp-Glo assays. P-gp expression was evaluated by Western blot. We found that very low mitotane concentrations sensitize ACC cells to the cytotoxic effects of doxorubicin, depending on P-gp expression. In addition, mitotane directly inhibits P-gp detoxifying function, allowing doxorubicin cytotoxic activity. These data provide the basis for the greater efficacy of combination therapy (mitotane plus chemotherapeutic drugs) on ACC patients. Shedding light on mitotane mechanisms of action could result in an improved design of drug therapy for patients with ACC.

Interferon alpha and rapamycin inhibit the growth of carcinoid and medullary thyroid cancer in vitro

Neuroendocrine tumors (NETs) are highly vascularized neoplasms characterized by rising incidence. Moreover, the neuroendocrine cells were shown to express vascular endothelial growth factor (VEGF) and VEGF receptors. Therefore, angiomodulators could be potentially a new group of drugs enhancing still unsatisfactory effectiveness of NET therapy. The aim of this study was to assess the direct influence of angiomodulators: VEGF and five endogenous and exogenous antiangiogenic compounds (endostatin, interferon alpha [IFNα], rapamycin, JV1-36, semaxinib [SU5416]) on the growth of two NET cell lines: lung carcinoid H727 cell line and medullary thyroid cancer TT cell line in vitro. IFNα and rapamycin induced the inhibitory effect on H727 and TT cell viability and proliferation, increasing apoptosis and arresting the cell cycle. Also semaxinib (10(-5)M) inhibited proliferation of both cell lines. VEGF and endostatin did not influence the growth of H727 and TT cells. The inhibitory effect of IFNα, rapamycin and semaxinib on carcinoid and medullary thyroid cancer growth was revealed in our in vitro study, although some other antiangiogenic agents did not directly influence H727 and TT cell growth. Thus, IFNα and mTOR inhibitors as multidirectionally acting drugs with antiangiogenic effect could be potentially efficient in treatment of neuroendocrine tumors and are worth further studies.

mTOR, p70S6K, AKT, and ERK1/2 levels predict sensitivity to mTOR and PI3K/mTOR inhibitors in human bronchial carcinoids

Bronchial carcinoids (BCs) are rare neuroendocrine tumors that are still orphans of medical treatment. Human BC primary cultures may display resistance to everolimus, an inhibitor of the mammalian target of rapamycin (mTOR), in terms of cell viability reduction. Our aim was to assess whether the novel dual phosphatidylinositol 3-kinase (PI3K)/mTOR inhibitor NVP-BEZ235 is effective in everolimus-resistant human BC tissues and cell lines. In addition, we searched for possible markers of the efficacy of mTOR inhibitors that may help in identifying the patients who may benefit from treatment with mTOR inhibitors, sparing them from ineffective therapy. We found that NVP-BEZ235 is twice as potent as everolimus in reducing cell viability and activating apoptosis in human BC tissues that display sensitivity to mTOR inhibitors, but is not effective in everolimus-resistant BC tissues and cell lines that bypass cyclin D1 downregulation and escape G0/G1 blockade. Rebound AKT activation was not observed in response to treatment with either mTOR inhibitor in the 'resistant' BC cells. In addition to total mTOR levels, putative markers of the sensitivity of BCs to mTOR inhibitors are represented by AKT, p70S6K (RPS6KB2), and ERK1/2 (MAPK3/1) protein levels. Finally, we validated these markers in an independent BC group. These data indicate that the dual PI3K/mTOR inhibitor NVP-BEZ235 is more potent than everolimus in reducing the proliferation of human BC cells. 'Resistant' cells display lower levels of mTOR, p70S6K, AKT, and ERK1/2, indicating that these proteins may be useful as predictive markers of resistance to mTOR and PI3K/mTOR inhibitors in human BCs.

Growth hormone receptor blockade inhibits growth hormone-induced chemoresistance by restoring cytotoxic-induced apoptosis in breast cancer cells independently of estrogen receptor expression

CONTEXT

GH and IGF-I play a role in breast cancer (BC) development. We previously demonstrated that GH protects the estrogen receptor (ER) positive BC-derived MCF7 cell line toward the cytotoxic effects of doxorubicin (D), independently of IGF-I. This issue may be important in ER negative BC cells that are more aggressive and more likely to develop chemoresistance.

AIM OF THE STUDY

The aim of this study was to evaluate whether GH may impact chemoresistance phenotype of ER-negative BC-derived MDA-MB-231 cell line and investigate the possible mechanisms implicated in the protective action of GH toward the cytotoxic effects of D in both ER-positive and ER-negative BC-derived cell lines.

RESULTS

GH protects ER-negative MDA-MB-231 cells from the cytotoxic effects of D and GH receptor antagonist pegvisomant reduces GH-induced DNA synthesis also in these cells. In both MDA-MB-231 and MCF7 cells, GH does not revert D-induced G2/M accumulation but significantly reduces basal and D-induced apoptosis, an effect blocked by pegvisomant. Glutathione S-transferase activity is not implicated in the protective effects of GH, whereas D-induced apoptosis depends on c-Jun N terminal kinase (JNK) activation. GH reduces both basal and D-stimulated JNK transcriptional activity and phosphorylation.

CONCLUSIONS

In human BC cell lines, GH directly promotes resistance to apoptosis induced by chemotherapeutic drugs independently of ER expression by modulating JNK, further broadening the concept that GH excess may hamper cytotoxic BC treatment. These findings support the hypothesis that blocking GH receptor may be viewed as a potential new therapeutic approach to overcome chemoresistance, especially in ER-negative BC.

Protein kinase C: a putative new target for the control of human medullary thyroid carcinoma cell proliferation in vitro

We investigate the role of protein kinase C (PKC) in the control of medullary thyroid carcinoma (MTC) cell proliferation by a PKC inhibitor, Enzastaurin, in human MTC primary cultures and in the TT cell line. We found that PKC inhibition reduces cell proliferation by inducing caspase-mediated apoptosis and blocks the stimulatory effect of IGF-I on calcitonin secretion. Enzastaurin reduces PKCβII (Thr500) phosphorylation, indicating a direct involvement of this isoform as well as the phosphorylated levels of Akt (Ser 473) and glycogen synthase kinase (Ser9), PKC pathway downstream targets and pharmacodynamic markers for PKC inhibition. PKCβII and PKCδ enzyme isoforms expression and localization were investigated. These data indicate that in vitro PKC is involved in the control of human MTC proliferation and survival by modulating apoptosis, with a mechanism that implicates PKCβII inhibition and translocation in different subcellular compartments. Targeting PKC may represent a useful therapeutic approach for controlling MTC proliferation.

The role of cell lines in the study of neuroendocrine tumors

Cell lines originating from neuroendocrine tumors (NETs) represent useful experimental models to assess the control of synthesis and release of different hormones and hormone-like peptides, to evaluate the mechanisms of action of these agents in target tissues at the cellular and subcellular levels, and to study cell proliferation and tumor development, as well as the effect of different drugs on these complex processes. To date, the understanding of NET biology (with regard to their mechanisms of hormone secretion, cell proliferation and metastatic spread) has been hampered by the lack of appropriate animal models or cell lines for their study. In the present review, we aim to summarize the recent in vitro/in vivo data regarding cell lines derived from NETs which are most frequently employed in experimental neuroendocrinology.

Targeting protein kinase C by Enzastaurin restrains proliferation and secretion in human pancreatic endocrine tumors

Dysregulation of the protein kinase C (PKC) signaling pathway has been implicated in tumor progression. In this study, we investigate the effects of a PKC inhibitor, Enzastaurin, in human pancreatic neuroendocrine neoplasms (PNN) primary cultures and in the human pancreatic endocrine cancer cell line, BON1. To this aim six human PNN dispersed in primary cultures and BON1 cells were treated without or with 1-10 μM Enzastaurin and/or 100 nM IGF1 in the presence or absence of serum. Cell viability and apoptosis were evaluated after 48-72 h; Chromogranin A (CgA) and/or insulin secretion was assessed after 6 h of incubation. PKC expression was investigated by immunofluorescence and western blot. We found that Enzastaurin significantly reduced human PNN primary culture cell viability, as well as CgA and insulin secretion. Moreover, in the BON1 cell line Enzastaurin inhibited cell proliferation at 5 and 10 μM by inducing caspase-mediated apoptosis, and reduced phosphorylation of glycogen synthetase kinase 3β (GSK3β) and of Akt, both downstream targets of PKC pathway and pharmacodynamic markers for Enzastaurin. In addition, Enzastaurin blocked the stimulatory effect of IGF1 on cell proliferation, and reduced CgA expression and secretion in BON1 cells. Two different PKC isoforms are expressed at different levels and have partially different subcellular localization in BON1 cells. In conclusion, Enzastaurin reduces cell proliferation by inducing apoptosis, with a mechanism likely involving GSK3β signaling, and inhibits secretory activity in PNN in vitro models, suggesting that Enzastaurin might represent a possible medical treatment of human PNN.

Magmas, a gene newly identified as overexpressed in human and mouse ACTH-secreting pituitary adenomas, protects pituitary cells from apoptotic stimuli

Pituitary tumors are mostly benign, being locally invasive in 5-35% of cases. Deregulation of several genes has been suggested as a possible alteration underlying the development and progression of pituitary tumors. We here report the identification of a cDNA, corresponding to Magmas gene (mitochondria-associated protein involved in granulocyte-macrophage colony-stimulating factor signal transduction), which is highly expressed in two different ACTH-secreting mouse pituitary adenoma cell lines as compared with normal pituitary as well as in two thirds of 64 examined pituitary adenomas as compared with human normal pituitary. Tim 16, the mitochondrial protein encoded by Magmas, was indeed expressed in a mouse ACTH-secreting pituitary adenoma cell line, AtT-20 D16v-F2 cells, in a subcellular compartment likely corresponding to mitochondria. Magmas silencing determined a reduced rate of DNA synthesis, an accumulation in G1 phase, and a concomitant decrease in S phase in At-T20 D16v-F2 cells. Moreover, Magmas-silenced cells displayed basal caspase 3/7 activity and DNA fragmentation levels similar to control cells, which both increased under proapoptotic stimuli. Our data demonstrate that Magmas is overexpressed in mouse and human ACTH-secreting pituitary adenomas. Moreover, our results show that Magmas protects pituitary cells from apoptosis, suggesting its possible involvement in neoplastic transformation.

Somatostatin and its receptors from fish to mammals

Somatostatin (SST) and its receptors (sst) make up a molecular family with unique functional complexity and versatility. Widespread distribution and frequent coexpression of sst subtypes underlies the multiplicity of (patho)physiological processes controlled by SST (central nervous system functions, endocrine and exocrine secretion, cell proliferation). This complexity is clearly reflected in the intricate evolutionary development of this molecular family. Recent studies postulate the existence of an ancestral somatostatin/urotensin II (SST/UII) gene, which originated two ancestral, SST and UII, genes by local duplication. Subsequently, segment duplication would have originated two diverging SST genes in both fish (SS1/SS2) and tetrapods [(SST/cortistatin(CST))]. SST/CST actions are mediated by a family of GPCRs (sst1-5) encoded by five different genes. sst1-4 sequences are highly conserved compared with sst5, suggesting unique evolutionary and functional relevance for the latter. Indeed, we recently identified novel truncated but functional sst5 variants in several species, which may help to explain part of the complexity of the SST/CST/sst family. Comparative and phylogenetic analysis of this molecular family would enhance our understanding of its paradigmatic evolutionary complexity and functional versatility.

Therapeutic concentrations of mitotane (o,p'-DDD) inhibit thyrotroph cell viability and TSH expression and secretion in a mouse cell line model

Mitotane therapy is associated with many side effects, including thyroid function perturbations mimicking central hypothyroidism, possibly due to laboratory test interference or pituitary direct effects of mitotane. We investigated whether increasing concentrations of mitotane in the therapeutic range might interfere with thyroid hormone assays and evaluated the effects of mitotane on a mouse TSH-producing pituitary cell line. TSH, free T(4), and free T(3) levels do not significantly change in sera from hypo-, hyper-, or euthyroid patients after addition of mitotane at concentrations in the therapeutic window. In the mouse TalphaT1 cell line, mitotane inhibits both TSH expression and secretion, blocks TSH response to TRH, and reduces cell viability, inducing apoptosis at concentrations in the therapeutic window. TRH is not capable of rescuing TalphaT1 cells from the inhibitory effects of mitotane on TSH expression and secretion, which appear after short time treatment and persist over time. Our results demonstrate that mitotane does not interfere with thyroid hormone laboratory tests but directly reduces both secretory activity and cell viability on pituitary TSH-secreting mouse cells. These data represent a possible explanation of the biochemical picture consistent with central hypothyroidism in patients undergoing mitotane therapy and open new perspectives on the direct pituitary effects of this drug.

Growth hormone excess promotes breast cancer chemoresistance

CONTEXT

GH and IGF-I are known to promote breast carcinogenesis. Even if breast cancer (BC) incidence is not increased in female acromegalic patients, mortality is greater as compared with general population.

OBJECTIVE

The objective of the study was to evaluate whether GH/IGF-I excess might influence BC response to chemotherapy.

DESIGN

We evaluated GH and IGF-I effects on cell proliferation of a BC cell line, MCF7 cells, in the presence of doxorubicin (Doxo), frequently used in BC chemotherapy, and the possible mechanisms involved.

RESULTS

GH and IGF-I induce MCF7 cell growth in serum-free conditions and protect the cells from the cytotoxic effects of Doxo. GH effects are direct and not mediated by IGF-I because they are apparent also in the presence of an IGF-I receptor blocking antibody and disappear in the presence of the GH antagonist pegvisomant. The expression of the MDR1 gene, involved in resistance to chemotherapeutic drugs, was not induced by GH. In addition, c-fos transduction was reduced by Doxo, which prevented GH stimulatory effects. Pegvisomant inhibited basal and GH-induced c-fos promoter transcriptional activity. Autocrine GH action is ruled out by the lack of endogenous GH expression in this MCF7 cell strain.

CONCLUSIONS

These data indicate that GH can directly induce resistance to chemotherapeutic drugs with a mechanism that might involve GH-induced early gene transcription and support the hypothesis that GH excess can hamper BC treatment, possibly resulting in an increased mortality.

Novel, potent, and radio-iodinatable somatostatin receptor 1 (sst1) selective analogues

The proposed sst(1) pharmacophore (J. Med. Chem. 2005, 48, 523-533) derived from the NMR structures of a family of mono- and dicyclic undecamers was used to design octa-, hepta-, and hexamers with high affinity and selectivity for the somatostatin sst(1) receptor. These compounds were tested for their in vitro binding properties to all five somatostatin (SRIF) receptors using receptor autoradiography; those with high SRIF receptor subtype 1 (sst(1)) affinity and selectivity were shown to be agonists when tested functionally in a luciferase reporter gene assay. Des-AA(1,4-6,10,12,13)-[DTyr(2),DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (25) was radio-iodinated ((125)I-25) and specifically labeled sst(1)-expressing cells and tissues. 3D NMR structures were calculated for des-AA(1,4-6,10,12,13)-[DPhe(2),DTrp(8),IAmp(9)]-SRIF-Thr-NH(2) (16), des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (23), and des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9),Tyr(11)]-SRIF-NH(2) (27) in DMSO. Though the analogues have the sst(1) pharmacophore residues at the previously determined distances from each other, the positioning of the aromatic residues in 16, 23, and 27 is different from that described earlier, suggesting an induced fit mechanism for sst(1) binding of these novel, less constrained sst(1)-selective family members.

Are somatostatin and cortistatin two siblings in regulating endocrine secretions? In vitro work ahead

Somatostatin (SRIF) and cortistatin (CST) are two cyclic peptides sharing remarkable structural, pharmacological and functional similarities. Both peptides bind all somatostatin receptors subtypes (sst1-5) with comparable affinities, which may explain the considerable similitude between their actions, particularly on endocrine targets. However, the expression patterns of both peptides do not overlap in human tissues, and they are regulated by different stimuli, suggesting that SRIF and CST can exert unique roles. In fact, CST can bind other receptors, different to ssts (e.g. ghrelin receptor, GHS-R and the MrgX2 receptor), which may be involved in those differential actions. In this review, we have summarized the limited knowledge gathered so far regarding the in vitro actions exerted by CST in different endocrine systems under normal and pathophysiological conditions, and have compared them with the well established functions known for SRIF on these systems. Available data suggests that CST substantially reproduces, but not fully mimics the "in vitro" effects of SRIF on pituitary secretions of human and animal models. Conversely, the functions of CST in the majority of peripheral endocrine (and non-endocrine) tissues are still unknown. Notwithstanding this, the differential tissue expression pattern of SRIF, CST and their receptors suggests that CST may act as a mere natural SRIF analogue in a number of tissues but in some endocrine tissues it may play a predominant, unique regulatory role with potential pathophysiological relevance. The challenge is now to find the genuine differences between these seemingly identical endocrine siblings.

Function and expression of somatostatin receptors of the endocrine pancreas

Somatostatin (SST) regulates multiple biological processes via five genetically distinct, G-protein coupled receptors. Clinical interest in therapy for neuroendocrine and metabolic disorders has resulted in the development of new tools for exploring the function of somatostatin receptors (SSTRs). The development of highly SSTR-selective agonists and antagonists, animal models with the deletion of individual SSTRs, as well as SSTR-specific antibodies have all been utilized in delineating SSTR functions. In the pancreas, SST is a potent regulator of insulin and glucagon secretion. Indeed, the inappropriate regulation of pancreatic A- and B-cell function in metabolic diseases provides an impetus to evaluate the SSTRs as therapeutic targets. By combining the results obtained from molecular biology, pharmacology and immunochemical studies the current review provides a summary of important recent developments which have extended our knowledge of SST actions in the endocrine pancreas.

In vitro antiangiogenic activity of selective somatostatin subtype-1 receptor agonists

BACKGROUND

Endothelial cells of human blood vessels (arteries and veins) show high levels of somatostatin subtype-1 receptor (sst(1)). The aim of the present study is to investigate the inhibitory effects of novel somatostatin analogs, highly selective for human sst(1), on in vitro angiogenesis and their modulation of vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor-2 (VEGFR-2) expression.

MATERIALS AND METHODS

Somatostatin analogs BIM-23745 and BIM-23926 were tested for their ability to prevent proliferation and migration of human endothelial HMEC-1 cells, to modulate VEGF and VEGFR-2 expression and to inhibit sprouting of microvessels from cultured human placental vessel explants in fibrin matrix for 28 days.

RESULTS

The somatostatin sst(1 )receptor-selective agonists, BIM-23745 and BIM-23926 showed a suppression of endothelial proliferation (e.g. 10(-6) M BIM-23475, 40.0 +/- 2.1% vs. 100% of controls; 10(-7) M BIM-23926, 55.3 +/- 3.3% vs. 100% of controls), migration (e.g. 10(-7) M BIM-23475, 35.0 +/- 1.56% vs. 100% of controls; 10(-7) M BIM-23926, 53.7 +/- 1.77% vs. 100% of controls) and microvessel sprouting (e.g. 10(-8) M BIM-23475, 42.8 +/- 5.6% vs. 100% of controls; 10(-7) M BIM-23926, 17.2 +/- 11.8% vs. 100% of controls). A small but significant percentage of cells exposed to BIM-23745 and BIM-23926 for 24 h and for 72 h presented typical apoptotic morphology. Moreover, both the analogs significantly inhibit VEGF and VEGFR-2 gene expression in endothelial cells grown for 144 h in a fibrin matrix and the VEGF secretion in conditioned media.

CONCLUSIONS

The inhibition of endothelial activities suggests potential therapeutic utility for administration of somatostatin sst(1 )receptor-selective agonists in the proliferative diseases involving angiogenesis.

Role of complex cyclin d1/cdk4 in somatostatin subtype 2 receptor-mediated inhibition of cell proliferation of a medullary thyroid carcinoma cell line in vitro

Somatostatin (SRIH) inhibits cell proliferation by interacting with five distinct SRIH receptor subtypes (SSTRs) activating several pathways in many tissues. We previously demonstrated that SRIH, by activating Src homology-2-containing protein, inhibits cell proliferation of the human medullary thyroid carcinoma cell line, TT, which expresses all SSTRs. However, the effects of SRIH on cell cycle proteins have not been investigated so far. We therefore evaluated the effects of SRIH and a selective SSTR2 agonist on cell cycle protein expression, mainly focusing on cyclin D1 and its associated kinases. Our data show that SRIH and the selective SSTR2 agonist, BIM-23120, reduce cell proliferation and DNA synthesis as well as induce a delay of the cell cycle in G(2)/M phase. Moreover, treatment with both SRIH and BIM-23120 decreases cyclin D1 levels, with a parallel increase in phosphocyclin D1 levels, suggesting protein degradation. Moreover, our data show an increase in glycogen synthase kinase-3beta activity, which triggers phosphorylation-dependent cyclin D1 degradation. Indeed, we observed a reduction in cyclin D1 protein half-life under treatment with SRIH or the SSTR2 selective agonist. A reduction in cdk4 protein levels is also observed with a parallel reduction in Rb phosphorylation levels at Ser-780. Our data indicate that the subtype 2 receptor-mediated antiproliferative effect of SRIH on TT cell proliferation may be exerted through a decrease in cyclin D1 levels.

Treatment of advanced carcinoid tumors

PURPOSE OF REVIEW

Carcinoid tumors often present with metastatic disease. Generally, these tumors can be treated conservatively. New evidence exists, however, that stage IV disease may be better managed with more aggressive medical and surgical treatment. Headway is also being made into understanding the associated fibrosis seen with advanced disease and in better understanding signaling pathways with the hope of offering future treatment options.

RECENT FINDINGS

Recent literature has advocated for more aggressive surgical treatment of carcinoid tumors, especially in the setting of hepatic metastases and peritoneal carcinomatosis. Octreotide and lanreotide are further being described for treating metastatic carcinoids. Radiolabeled somatostatin analogues may prove to be as effective for treating carcinoids as for visualizing them. Other potential treatment modalities include pharmacologic activation of signaling pathways to control excess hormone production. Research into fibrosis - a cause of pain, bowel obstruction, retroperitoneal vascular constriction and right heart failure - has shown that serotonin and tachykinins may be the key mediators.

SUMMARY

Patients with stage IV carcinoid tumors may benefit from more aggressive surgical management and new treatment modalities. The growing body of knowledge regarding important molecular signaling pathway may lead to new medical therapies and further understanding of the sequelae of excess hormone production.

Cyclooxygenase-2 inhibitors reverse chemoresistance phenotype in medullary thyroid carcinoma by a permeability glycoprotein-mediated mechanism

OBJECTIVE

Medullary thyroid carcinoma (MTC) is a highly chemoresistant malignant neoplasia deriving from parafollicular C cells. Chemotherapy failure has been ascribed, at least in part, to the overexpression by MTC of the multidrug resistance 1 (MDR1) gene, encoding a transmembrane glycoprotein [permeability glycoprotein (P-gp)] that antagonizes intracellular accumulation of cytotoxic agents. P-gp expression and function in a rat model have been demonstrated to depend on cyclooxygenase (COX)-2 isoform levels, which are found elevated in many human cancers. The aim of our study was to investigate the role of the COX-2 pathway in modulating chemoresistance.

DESIGN AND RESULTS

We investigated P-gp and COX-2 expression and then evaluated the sensitizing effects of COX-2 inhibitors on the cytotoxic effects of doxorubicin in the presence or in the absence of prostaglandin E2 in primary cultures and in a human MTC cell line, TT. Moreover, P-gp function has been studied. Our data show that TT cells express both MDR1 and COX-2 and that rofecoxib, a selective COX-2 inhibitor, sensitizes TT cells to the cytotoxic effects of doxorubicin, reducing P-gp expression and function.

CONCLUSIONS

Our data suggest that these effects are mediated by a mechanism not involving the generation of prostaglandin E2, possibly implicating the synthesis of other COX-2 products.

Differential regulation of somatostatin receptors 1 and 2 mRNA and protein expression by tamoxifen and estradiol in breast cancer cells

Somatostatin (SST) inhibition of hormone hypersecretion from tumors is mediated by somatostatin receptors (SSTRs). SSTRs also play an important role in controlling tumor growth through specific antiproliferative actions. These receptors are well expressed in numerous normal and tumor tissues and are susceptible to regulation by a variety of factors. Estradiol, a potent trophic and mitogenic hormone in its target tissues, is known to modulate the expression of SST and its receptors. Accordingly, in the present study, we determined the effects of tamoxifen, a selective estrogen receptor (ER) modulator (SERM), and estradiol on SSTR1 and SSTR2 expression at the mRNA and protein levels in ER-positive and -negative breast cancer cells. We found that SSTR1 was upregulated by tamoxifen in a dose-dependent manner but no effect was seen with estradiol. In contrast, SSTR2 was upregulated by both tamoxifen and estradiol. Combined treatment caused suppression of SSTR1 below control levels but had no significant effect on SSTR2. Treatment with SSTR1-specific agonist was significantly more effective in suppressing cell proliferation of cells pre-treated with tamoxifen. Taking these data into consideration, we suggest that tamoxifen and estradiol exert variable effects on SSTR1 and SSTR2 mRNA and protein expression and distributional pattern of the receptors. These changes are cell subtype-specific and affect the ability of SSTR agonists to inhibit cell proliferation.

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.

Regression of liver metastases of occult carcinoid tumor with slow release Lanreotide therapy

Few clinical studies have demonstrated an anti-proliferative activity of somatostatin (SST) analogs in carcinoids. We report the case of a woman with liver metastases of neuroendocrine tumor and no evidence of the primary tumor. The liver metastases were characterized by high proliferation index, immunoreactiviy for somatostatin receptor (SSTR)-1, 2, 3 and 5 and positive octreoscan. Urinary 5-hydroxyindolacetic acid, serum serotonin and chromogranin A were elevated. Slow release lanreotide (SR-LAN) therapy for 3 mo controlled clinical and biochemical signs of carcinoid tumor and caused a clear-cut reduction in the diameter of two liver metastases and disappearance of another lesion, with further reduction after 6 and 18 mo. We demonstrated a clear-cut long-lasting anti-proliferative effect of SR-LAN on liver metastases of occult carcinoid with high proliferation index and immunoreactivity for SSTR-1, 2, 3, and 5. Immuno-histochemistry for SSTRs could be a suitable method for the selection of patients with metastatic carcinoid that may benefit from SST analog therapy.

Global gene expression in neuroendocrine tumors from patients with the MEN1 syndrome

Background

Multiple Endocrine Neoplasia type 1 (MEN1, OMIM 131100) is an autosomal dominant disorder characterized by endocrine tumors of the parathyroids, pancreatic islets and pituitary. The disease is caused by the functional loss of the tumor suppressor protein menin, coded by the MEN1 gene. The protein sequence has no significant homology to known consensus motifs. In vitro studies have shown menin binding to JunD, Pem, Smad3, NF-kappaB, nm23H1, and RPA2 proteins. However, none of these binding studies have led to a convincing theory of how loss-of-menin leads to neoplasia.

Results

Global gene expression studies on eight neuroendocrine tumors from MEN1 patients and 4 normal islet controls was performed utilizing Affymetrix U95Av2 chips. Overall hierarchical clustering placed all tumors in one group separate from the group of normal islets. Within the group of tumors, those of the same type were mostly clustered together. The clustering analysis also revealed 19 apoptosis-related genes that were under-expressed in the group of tumors. There were 193 genes that were increased/decreased by at least 2-fold in the tumors relative to the normal islets and that had a t-test significance value of p < = 0.005. Forty-five of these genes were increased and 148 were decreased in the tumors relative to the controls. One hundred and four of the genes could be classified as being involved in cell growth, cell death, or signal transduction. The results from 11 genes were selected for validation by quantitative RT-PCR. The average correlation coefficient was 0.655 (range 0.235–0.964).

Conclusion

This is the first analysis of global gene expression in MEN1-associated neuroendocrine tumors. Many genes were identified which were differentially expressed in neuroendocrine tumors arising in patients with the MEN1 syndrome, as compared with normal human islet cells. The expression of a group of apoptosis-related genes was significantly suppressed, suggesting that these genes may play crucial roles in tumorigenesis in this syndrome. We identified a number of genes which are attractive candidates for further investigation into the mechanisms by which menin loss causes tumors in pancreatic islets. Of particular interest are: FGF9 which may stimulate the growth of prostate cancer, brain cancer and endometrium; and IER3 (IEX-1), PHLDA2 (TSS3), IAPP (amylin), and SST, all of which may play roles in apoptosis.

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

Somatostatin (SRIF) analogs interacting with SRIF receptor (SSTR) subtypes SSTR2 and SSTR5 reduce hormone secretion of pituitary adenomas, but their antiproliferative effects are still controversial. We investigated the in vitro effects of SRIF and SSTR-selective agonists interacting with SSTR1 (BIM-23926), SSTR2 (BIM-23120), SSTR5 (BIM-23206), or both SSTR2 and SSTR5 (BIM-23244) on alpha-subunit and chromogranin A secretion and on cell viability of 12 nonfunctioning pituitary adenomas (NFA) expressing SSTR1, SSTR2, and SSTR5, as assessed by RT-PCR. Treatment with SRIF or BIM-23206 did not modify alpha-subunit and chromogranin A secretion, which was significantly inhibited by BIM-23926, BIM-23120, and BIM-23244. SRIF and BIM-23120 did not influence cell viability, which was significantly promoted by BIM-23206 and BIM-23244 and reduced by treatment with BIM-23926. These results demonstrate that, in the selected NFA, the SSTR1-selective agonist inhibits secretory activity and cell viability, the SSTR2-selective agonist inhibits secretion but not cell viability, and the SSTR5-selective agonist does not influence secretion but promotes cell viability. These data can explain the lack of inhibitory effects of currently used SRIF analogs and suggest that drugs acting potently and preferentially on SSTR1 might be useful for medical treatment of NFA.

The cholecystokinin2-receptor mediates calcitonin secretion, gene expression, and proliferation in the human medullary thyroid carcinoma cell line, TT

Gastrin-induced release of calcitonin from medullary thyroid carcinomas (MTC) is based on the expression of the cholecystokinin(2)-receptor (CCK(2)R) in these tumors. Recently, we have shown that the CCK(2)R is expressed not only in MTC but also in C-cells within the normal thyroid gland. The functions of the CCK(2)R in MTC and C-cells are largely unknown. We therefore explored the effects of gastrin-induced CCK(2)R stimulation in the highly differentiated MTC cell line, TT. CCK(2)R expression in TT-cells is detectable by RT-PCR as well as immunocytochemistry. Stimulation of the CCK(2)R by gastrin induces immediate release of calcitonin from TT-cells. Moreover, quantitative (LightCycler) RT-PCR demonstrates that gastrin stimulates transcription of the calcitonin and chromogranin A genes in TT-cells. TT-cell proliferation, assessed by counting of viable cells and (3)H-thymidine uptake, is markedly increased by gastrin. This effect is inhibited by the CCK(2)R-specific antagonist L-365,260. Our findings suggest physiological functions for the CCK(2)R in calcitonin-secretion and gene expression as well as a pathophysiological role in MTC proliferation. CCK(2)R antagonists might have therapeutic potential in these tumors.

Evolving concepts in the quest for advanced therapeutic analogues of somatostatin

A major challenge in developing somatostatin-based therapies, as well as an important question of basic physiology, is how to achieve functional specificity with an agent that has widespread actions. The natural somatostatin system achieves functional specificity in part through local somatostatin production at the site of action. Further selectivity may be achieved through the recently elucidated somatostatin receptor subtypes. To explore the relationship between the receptor subtypes and somatostatin-mediated functions, we have tested panels of selective somatostatin analogs in specific biological assays. Our studies have demonstrated complex, functional interactions between the somatostatin receptor subtypes. Specific combinations of somatostatin receptor subtypes in specific tissues may be either synergistic or antagonistic. By altering the expression ratio of the interacting receptors, the biological response to somatostatin can be influenced by environmental, hormonal and physiological status. In addition, inappropriate or unbalanced receptor expression may provide a novel mechanism of disease. These concepts take on an even broader context with the demonstration that somatostatin receptor subtypes also interact with other G-protein-coupled receptors. The insight gained from these studies has already resulted in several novel approaches to acromegaly therapy. Our further understanding of these complex cellular codes will provide the conceptual basis for future therapeutics with greatly enhanced efficacy and specificity.

Somatostatin receptors: from basic science to clinical approach--thyroid

Somatostatin and its receptors are expressed in the thyroid gland, but somatostatin analogs which are currently available have provided contradictory results in the diagnosis and treatment of thyroid neoplasia. Somatostatin and its analogs fail to influence follicular thyroid function, whereas their administration in patients with medullary thyroid carcinoma induces a reduction of serum calcitonin concentrations and clinical symptoms, but fails to influence tumour size and patient survival rate. Radiolabelled somatostatin analogs can localise tumours expressing somatostatin receptors, but somatostatin receptor-targeted radiotherapy of thyroid malignancies has provided conflicting and inconclusive results. Our recent results indicate that somatostatin receptor 2 activation by somatostatin receptor 2 agonists inhibits cell proliferation in the human medullary thyroid carcinoma cell line, TT. This effect can be hampered by concurrent somatostatin receptor 5 selective agonist treatment, which fails to influence TT cell proliferation, suggesting an antagonism between somatostatin receptors 5 and 2 agonists in medullary thyroid carcinoma cells. Moreover, somatostatin receptors 2 or 5 agonists fail to inhibit calcitonin secretion and calcitonin gene expression. On the other hand, somatostatin receptor 1 agonists inhibit proliferation, calcitonin secretion and calcitonin gene expression in parafollicular C cells, suggesting that analogs with enhanced somatostatin receptor 1 affinity and selectivity besides having great potentiality as pharmacological tools to control neoplastic growth, may also be used to reduce symptoms in patients with medullary thyroid carcinoma.