Molecular pharmacology of somatostatin receptor subtypes

Advances in the therapy of gastroenteropancreatic-neuroendocrine tumours (GEP-NETs)

Neuroendocrine tumours (NET) of the digestive tract comprise a broad range of malignancies. The therapeutic approach to these tumours has not evolved as it did in other tumour types in the last two decades. The deeper knowledge of the underlying molecular biology behind the growth of neuroendocrine cells has brought much information to light. We now know that somatostatin analogues may not only be considered as symptomatic treatment but also as antitumour agents. Sunitinib, a tyrosine kinase (TK) inhibitor with antiangiogenic and antitumoural properties, has been shown to induce significant improvement in progression-free survival in a randomised trial conducted in well-differentiated pancreatic islet-cell NETs. The relevance of the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway seems to be crucial in gastroenteropancreatic (GEP)-NETs. In fact, mTOR inhibitors have shown activity in uncontrolled trials, and large, randomised trial results will be available shortly. In this article, we summarise the most recent available data on medical therapy for GEPNETs.

Somatostatin and somatostatin receptors: implications for neoplastic growth and cancer biology

Somatostatin agonists (SM-As) are capable of achieving durable symptomatic relief and significant clinical responses in certain tumours. Herein, we review the diverse direct and indirect mechanisms of antineoplastic activity elicited by SM-As as well as the hurdles that complicate their use as monotherapies in a broader range of malignancies. Emphasis is placed on recent clinical attempts to neutralise the IGF-mediated survival factor effects in the bone metastasis microenvironment in advanced prostate cancer. The first clinical trials of this 'anti-survival factor manipulation' strategy utilised the ability of SM-As to suppress the growth hormone-dependent liver-derived IGF-I bioavailability in combination with other drugs, such as dexamethasone, zolendronate and oestrogens, acting systemically and at the bone metastasis microenvironment. These regimens restored androgen ablation responsiveness in stage D3 prostate cancer patients and successfully produced objective clinical responses while only mild toxicities were observed. Furthermore, we focus on the preclinical experimental data of a targeted SM-A coupled to the super-potent doxorubicin derivative AN-201. The resulting conjugate (AN-238) has shown increased antitumour potency with a favourable toxicity profile. The potential use of novel SM-As as anticancer drugs is discussed in relation to data suggesting other direct and indirect treatment approaches pertaining to the somatostatin system.

Dosimetry for 131I-MIBG therapies in metastatic neuroblastoma, phaeochromocytoma and paraganglioma

PURPOSE

Radiation dosimetry is a basic requirement for targeted radionuclide therapies (TRT) which have become of increasing interest in nuclear medicine. Despite the significant role of the radiopharmaceutical (131)I-metaiodobenzylguanidine (MIBG) for the treatment of metastatic neuroblastoma, phaeochromocytoma and paraganglioma details for a reliable dosimetry are still sparse. This work presents our procedures, the dosimetric data and experiences with TRT using (131)I-MIBG.

METHODS

A total of 21 patients were treated with (131)I-MIBG between 2004 and 2008 according to a clearly defined protocol. Whole-body absorbed doses were determined by a series of scintillation probe readings for all 21 cases. Tumour absorbed doses were calculated on the basis of quantitative imaging for an entity of 25 lesions investigated individually using the region of interest (ROI) technique based on five scans each.

RESULTS

Typical whole-body absorbed doses are found in the region of 2 Gy (range: 1.0-2.9 Gy) whereas tumour absorbed doses in turn cover a span between 10 and 60 Gy. Nonetheless this variation of tumour absorbed doses is comparatively low.

CONCLUSION

The trial protocol in use is a substantial advancement in terms of reliable dosimetry. A clearly defined modus operandi for MIBG therapies should involve precisely described dosimetric procedures, e.g. a minimum of 20 whole-body measurements using a calibrated counter and at least four gamma camera scans over the whole period of the inpatient stay should be carried out. Calculation of tumour volumes is accomplished best via evaluation of SPECT and CT images.

Somatostatin receptor subtypes 2 and 5 mediate inhibition of egg yolk-induced gall bladder emptying in mice

BACKGROUND

Somatostatin inhibits gall bladder contraction. Impaired gall bladder emptying is associated with gall bladder stone formation. The incidence of cholecystolithiasis is high in patients treated with a somatostatin agonist octreotide, which predominantly interacts with somatostatin receptor subtype 2 (SSTR2). Therefore, it is believed that SSTR2 regulates gall bladder contraction; however, evidence has not been provided. Here, we evaluate the effects of SSTR1-SSTR5-selective agonists on egg yolk-induced gall bladder contraction in mice.

METHODS

Homozygous deletion of SSTR2 and SSTR5 was generated by cross-mating of SSTR2(-/-) with SSTR5(-/-) mice. Mice of different genotypes were injected with SSTR1-5-selective agonists or octreotide 15 min before induction of gall bladder emptying by egg yolk. One hour later, gall bladders were removed and weighed.

KEY RESULTS

Egg yolk-reduced gall bladder weights in all mice, irrespective of their genotype. Octreotide was the most potent inhibitor of gall bladder emptying in wild-type mice. In contrast, agonists with high selectivity for SSTR2 or SSTR5 inhibited gall bladder emptying by approximately 50-60%, whereas SSTR1-, SSTR3- and SSTR4-selective agonists failed to influence gall bladder contraction. In SSTR2(-/-) mice, octreotide and an SSTR5-selective agonist inhibited gall bladder emptying by approximately 50%, whereas SSTR2-selective agonists were inactive. Octreotide inhibited gall bladder emptying in SSTR5(-/-) mice by approximately 50%, without any effect in SSTR2(-/-)/SSTR5(-/-) mice.

CONCLUSIONS & INFERENCES

Our study provides evidence for the role of SSTR2 and SSTR5 in regulating gall bladder emptying in mice.

The beta-D-glucose scaffold as a beta-turn mimetic

Activity and selectivity are typically the first considerations when designing a drug. However, absorption, distribution, metabolism, excretion, and toxicity (ADMET) are equally important considerations. Peptides can provide a combination of potent binding and exquisite selectivity, as evidenced by their pervasive use as enzymes, hormones, and signaling agents within living systems. In particular, peptidic turn motifs are key elements of molecular recognition. They may be found at the exposed surfaces of globular proteins, where they are available for binding interactions with other peptides and small molecules. However, despite these advantages, peptides often make poor drugs. The amide backbone is subject to rapid enzymatic proteolysis, resulting in short half-lives. Furthermore, the ability of the amide backbone to hydrogen bond with water restricts its ability to cross membranes and, consequentially, results in poor oral bioavailability. Accordingly, the development of nonpeptidic scaffolds that mimic peptidic turn motifs represents a promising means of converting peptidic agents into more drugable molecules. In this Account, we describe the design and synthesis of beta-turn mimetics that use a beta-D-glucose scaffold, the first use of a sugar scaffold for this purpose. Somatostatin (SRIF) is a small protein (14 amino acid residues) human hormone; a shorter (6 amino acid residues) synthetic peptide, L-363,301, is a fully peptidal agonist. These two cyclic peptides share the beta-turn motif comprising Phe(7)-Trp(8)-Lys(9)-Thr(10) (d-Trp(8) in the case of L-363,301), of which the tryptophan and lysine residues in the i + 1 and i + 2 positions, respectively, are critical for binding. In 1988, we initiated a program that tested and validated the then-novel proposition that the beta-D-glucose scaffold can mimic the beta-turn in L-363,301. The beta-D-glucose scaffold proved to be an attractive mimic of a beta-turn in part because it permits the convenient attachment of amino acid side chains via facile etherification reactions, rather than carbon-carbon bond formations; it is also an inexpensive starting material with well-defined stereochemistry. From the beginning, biological assays were used alongside physical measurements to assess the relevance of the design. Our first two synthetic targets, compounds 6 and 7, bound the SRIF receptors on benchmark (AtT-20) cells, albeit weakly, consistent with the objective of the design. Subsequently, a better ligand (8) and two congeners were found to be agonists at the SRIF receptors, providing convincing evidence that the peptide backbone is not required for receptor binding or signal transduction. The unexpectedly high level of receptor affinity of selected analogs, as well as the fortuitous discovery that our peptidomimetics were active against several chemically distinct receptors, led us to hypothesize that these monosaccharides could access multiple potential binding modes. Our later studies of this sugar scaffold confirmed this property, which we termed pseudosymmetry, whereby multiple similar but nonidentical motifs are displayed within a single analog. We propose the presence of pseudosymmetry to be an element of privilege and an advantage for lead discovery.

Expression of somatostatin and somatostatin receptor subtypes in Apolipoprotein D (ApoD) knockout mouse brain: An immunohistochemical analysis

Apolipoprotein D (ApoD) is widely distributed in central and peripheral nervous system. ApoD expression has been shown to increase in several neurodegenerative and neuropsychiatric disorders, as well as during regeneration in the nervous system. Like ApoD, in the central nervous system somatostatin (SST) is widely present and functions as neurotransmitter and neuromodulator. The biological effects of SST are mediated via binding to five high-affinity G-protein coupled receptors termed SSTR1-5. Mice lacking ApoD exhibit reduced SST labeling in cortex and hippocampus and increased expression in striatum and amygdala without any noticeable changes in substantia nigra. Changes in SSTRs expressions have been described in several neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. In the present study, using SSTR1-5 receptor-specific antibodies, we mapped their distribution in wild type (wt) and ApoD knockout (ApoD(-/-)) mouse brain. SSTR1-5 expression was observed both as membrane and cytoplasmic protein and display regions and receptor specific differences between wt and ApoD(-/-) mice brains. In cortex and hippocampus, SSTR subtypes like immunoreactivity are decreased in ApoD(-/-) mice brain. Unlike cortex and hippocampus, in the striatum of ApoD(-/-) mice, projection neurons showed increased SSTR immunoreactivity, as compared to wt. Higher SSTR subtypes immunoreactivity is seen in substantia nigra pars compacta (SNpc) whereas lower in substantia nigra pars reticulata (SNpr) of ApoD(-/-) mice brains as compared to wt. Whereas, amygdala displayed SSTR subtypes changes in different nuclei of ApoD(-/-) mice in comparison to wt mice brain. Taken together, our results describe receptor and region specific changes in SST and SSTR subtypes expression in ApoD(-/-) mice brain, which may be linked to specific neurological disorders.

Expression of somatostatin receptor subtypes in human thyroid tumors: the immunohistochemical and molecular biology (RT-PCR) investigation

Human endocrine tumors often express the somatostatin receptors SSTR 1-5 with different intensity. It has been widely investigated their distribution in pituitary adenomas, brain tumors, adrenal tumors and neuroendocrine tumors in gastrointestinal tract (NET). Some of studies also concern the expression of SSTRs in thyroid tumors but they are mainly limited to parafollicular C cells - derived medullary thyroid carcinomas (MTC). Results of SSTR 1-5 detection in other thyroid pathologies like follicular adenomas and papillary cancers are still scarce and often controversial, depending of investigation method used. The aim of this study was to report the presence of all the 5 subtypes of SSTR (including 2A and 2B SSTR isoforms) in some surgically treated human thyroid tumors by means of immunohistochemistry and real-time PCR method and to correlate the results obtained with both techniques. SSTR 1 protein was expressed in 88.8% of investigated cases, SSTR 2A and 2B both in 44.4%, SSTR 3 in 55.5%, SSTR 4 in 11.2% and SSTR 5 in 33.3%. SSTR 1 is the dominant form in the thyroid gland tumor and hyperplasia. We found positive confirmation of both methods in 88.8% for SSTR 1, 2A, 3 subtypes, in 22.2% for SSTR 4 and in 100% for SSTR 5. It suggests that somatostatin multiligand analogs or selective SSTR 1 agonists may be used in thyroid tumors treatment.

Use of somatostatin analogues in obesity

Obesity is a condition that results from dysregulation of energy balance. Insulin, a component of the efferent pathway of the energy-regulatory circuit, promotes storage of energy substrates in adipose tissue and is, therefore, a potential target for pharmacotherapy. Somatostatin and its analogues (octreotide and lanreotide) bind to somatostatin subtype 5 receptors on the beta-cell membrane, which limits insulin release and, consequently, may decrease adipogenesis. Somatostatin and its analogues have been used in trials in patients with paediatric hypothalamic obesity. These children have hypothalamic dysfunction, mainly due to brain tumours such as craniopharyngiomas, which are thought to generate increased vagal output, leading to hyperinsulinaemia and weight gain. Two small trials, each of 6 months' duration, in children with paediatric hypothalamic obesity showed either a minimal weight loss or stabilization of bodyweight. In children with Prader-Willi syndrome, the most common genetic hypothalamic disorder associated with hyperphagia, hyperghrelinaemia, massive obesity and other endocrine disturbances, somatostatin failed to control hyperphagia and weight gain in a small number of patients, although it lowered the levels of the anorexigenic hormone ghrelin. Long-acting release octreotide was recently used in hyperinsulinaemic obese adults without cranial pathology. Insulin suppression was associated with small decreases in the body mass indexes of obese subjects receiving the higher dosages of the drug, with an acceptable safety profile, similar to that in previous studies. In conclusion, somatostatin and its analogues, by suppressing beta-cell insulin secretion, can retard weight gain in children with hypothalamic obesity and induce a small amount of weight loss in some adults with hyperinsulinaemic obesity.

Octreotide LAR for the treatment of acromegaly

BACKGROUND

Somatostatin analogs previously considered as adjuvant therapy in acromegaly are increasingly used as a first-line therapy in selected cases.

OBJECTIVE

To review the octreotide LAR pharmacological and clinical data, and discuss the impact of this agent on current treatment regimens.

METHODS

We reviewed PubMed publications since the first use of octreotide LAR in acromegaly, and historical articles related to the discovery and development of this molecule. We chose, for efficacy and safety data, reviews, clinical and randomized controlled trials that included >or=10 patients.

RESULTS/CONCLUSION

Octreotide LAR controls acromegaly in approximately 50-60% of patients by inhibiting GH and IGF-I secretion, and by reducing tumor size. This drug is well tolerated in most patients.

Spotlight on lanreotide Autogel in acromegaly

Lanreotide Autogel (ATG) [Somatuline Depot]The use of trade names is for product identification purposes only and does not imply endorsement. is a novel, long-acting preparation of the somatostatin analog lanreotide acetate that acts via somatostatin receptors to reduce both growth hormone (GH) and insulin-like growth factor-I (IGF-I) levels. It is indicated for the management of acromegaly and, relative to most other licensed agents, it has a favorable pharmacokinetic profile that permits administration once every 28-42 days. Subcutaneous lanreotide ATG was an effective and generally well tolerated treatment in patients with acromegaly in well designed trials and extension studies of up to 4 years' duration. It was shown to be no less effective than intramuscular lanreotide long-acting (LA) microparticle treatment in these studies, with more limited data showing that lanreotide ATG therapy was as effective as intramuscular octreotide long-acting repeating (octreotide LAR) treatment. While both of these latter agents offer the advantage of treatment once every 28 days, lanreotide ATG also has the advantage of being available in a convenient pre-filled syringe and is given subcutaneously rather than intramuscularly like other somatostatin analogs. Thus it provides a valuable first-line option for the management of patients with acromegaly.

Somatostatin, cortistatin and their receptors in tumours

Somatostatin (SS) and its synthetic analogs have a role in the treatment of neuroendocrine tumours both in terms of symptoms control and antiproliferative activities. These effects are mediated by five SS receptors, widely expressed in both human neuroendocrine and non-neuroendocrine tumours, which were demonstrated to be diagnostically and therapeutically valuable targets. Cortistatin (CST), a brain cortex peptide, partially homologous to SS and having similar functions is also expressed in peripheral tissues and tumours. CST binds all SS receptors, and, differently from SS, also the ghrelin receptor GHSR1a and the CST specific receptor MrgX2. The expression profile of CST is mostly restricted to neuroendocrine tumours (gastrointestinal, pancreas, lung, parathyroid, thyroid, adrenal). In these tumours, CST probably acts via the SS or ghrelin receptor, the MrgX2 receptor being absent. Thus, in comparison to SS analogs, CST synthetic analogs may represent additional diagnostic/therapeutic tools in those tumours expressing the receptors for SS, for ghrelin or for both peptides.

Genetic abnormalities of somatostatin receptors in pituitary tumors

Somatostatin exerts antisecretive and antiproliferative effects on different endocrine cells by acting through a family of G protein-coupled receptors that includes five subtypes (SST1-5). Normal human pituitary and pituitary adenomas have been shown to express almost all SST subtypes, with the exception of SST4. Consistent with the observation that octreotide and other somatostatin analogs bind to SST2 and SST5 with high affinity, these genes have been screened for quantitative/qualitative abnormalities in tumors removed from patients with poor responsiveness to somatostatin analogs treatment. Data obtained in GH-secreting adenomas suggested that resistance to octreotide was frequently associated with low expression of SST2 mRNA, although other authors failed to confirm this finding. To date, the only mutational change involving SST2 and SST5 is the Arg to Trp substitution in codon 240 of the SST5 gene that was found in one acromegalic patient resistant to octreotide. Similarly, loss of heterozygosis at SST5 gene locus in pituitary adenomas has been described in individual tumors. In recent years, molecular studies investigated the possible association of gene polymorphisms and susceptibility to diseases and/or resistance to drugs. As far as polymorphic variants of SST genes are concerned, a possible role of SST5 C1004T and T-461C alleles in influencing GH and IGF-I levels in patients with acromegaly has been proposed. Nevertheless, polymorphic variants in SST2 and SST5 genes seem to have a minor, if any, role in determining the different responsiveness to somatostatin analogs in patients with acromegaly.

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.

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.

Somatostatinoma: clinico-pathological features of three cases and literature reviewed

Somatostatinoma is a rare endocrine tumor that comprises around 1% of all gastroenteropancreatic endocrine neoplasms. This paper gives an updated review on somatostatinoma and describes three sporadic cases of somatostatinoma located in the pancreas, duodenum, and jejunum. Approximately 200 case histories of somatostatinoma have been published, with the duodenum being the most frequent site, followed by the pancreas. Somatostatinomas may be sporadic or associated with neurofibromatosis type 1, Multiple Endocrine Neoplasia type 1, and Von Hippel-Lindau syndromes. Functional somatostatinomas release excessive amounts of somatostatin suppressing gallbladder motility and inhibiting the secretory activity of various endocrine and exocrine cell types. A triad of mild diabetes mellitus, cholelithiasis, and diarrhea/steatorrhoea characterizes the somatostatinoma or 'inhibitory' syndrome. Non-functional somatostatinomas tend either to be asymptomatic or to present with obstructive symptoms. These tumors are often malignant and by the time they are detected, nearly two-thirds have already metastasized to the regional lymph nodes or the liver. A comparison between our three cases and those in the literature provides useful insights into the clinical management of these patients. Interestingly, the jejunal somatostatinoma described here is the second case ever reported.

Cerebellar granule cell migration and the effects of alcohol

In the developing brain the majority of neurons migrate from their birthplace to their final destination. This active movement is essential for the formation of cortical layers and nuclei. The impairment of migration does not affect the viability of neurons but often results in abnormal differentiation. The proper migration of neurons requires the orchestrated activities of multiple cellular and molecular events, such as pathway selection, the activation of specific receptors and channels, and the assembly and disassembly of cytoskeletal components. The migration of neurons is very vulnerable to exposure to environmental toxins, such as alcohol. In this article, we will focus on recent developments in the migration of cerebellar granule cells. First, we will describe when, where and how granule cells migrate through different cortical layers to reach their final destination. Second, we will present how internal programs control the sequential changes in granule cell migration. Third, we will review the roles of external guidance cues and transmembrane signals in granule cell migration. Finally, we will reveal mechanisms by which alcohol exposure impairs granule cell migration.

New treatment approaches for prostate cancer based on peptide analogues

OBJECTIVES

New therapy modalities for the treatment of advanced prostate cancer based on peptide analogues are reviewed.

RESULTS

Agonists and antagonists of luteinising hormone-releasing hormone (LHRH) lead to androgen deprivation, but direct effects on tumours may also play a role. Radiolabeled somatostatin analogues can be targeted to tumours expressing receptors for somatostatin and have been successfully applied for the localization of these tumours. Tumoural LHRH, growth hormone-releasing hormone (GHRH), and bombesin/gastrin-releasing peptide (BN/GRP) and their receptors appear to be involved in the proliferation of prostate cancer. On the basis of the recent advances in the understanding of the role of neuropeptides in tumour growth and progression, new therapeutic modalities are being developed that are based on antagonists of GHRH and of BN/GRP, which inhibit growth factors or their receptors. Another promising approach for the therapy of prostate cancer consists of the use of cytotoxic analogues of LHRH, bombesin, and somatostatin, which can be targeted to receptors for these peptides in prostate cancers and their metastases.

CONCLUSIONS

New promising forms of hormone therapy and targeted chemotherapy may improve therapy of advanced stage prostate cancer.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Somatostatin decreases voltage-gated Ca2+ currents in GH3 cells through activation of somatostatin receptor 2

The secretion of growth hormone (GH) is inhibited by hypothalamic somatostatin (SRIF) in somatotropes through five subtypes of the somatostatin receptor (SSTR1-SSTR5). We aimed to characterize the subtype(s) of SSTRs involved in the Ca2+ current reduction in GH3 somatotrope cells using specific SSTR subtype agonists. We used nystatin-perforated patch clamp to record voltage-gated Ca2+ currents, using a holding potential of -80 mV in the presence of K+ and Na+ channel blockers. We first established the presence of T-, L-, N-, and P/Q-type Ca2+ currents in GH3 cells using a variety of channel blockers (Ni+, nifedipine, omega-conotoxin GVIA, and omega-agatoxin IVA). SRIF (200 nM) reduced L- and N-type but not T- or P/Q-type currents in GH3 cells. A range of concentrations of each specific SSTR agonist was tested on Ca2+ currents to find the maximal effective concentration. Activation of SSTR2 with 10(-7) and 10(-8) M L-797,976 decreased the voltage-gated Ca2+ current and abolished any further decrease by SRIF. SSTR1, SSTR3, SSTR4, and SSTR5 agonists at 10(-7) M did not modify the voltage-gated Ca2+ current and did not affect the Ca2+ current response to SRIF. These results indicate that SSTR2 is involved mainly in regulating voltage-gated Ca2+ currents by SRIF, which contributes to the decrease in intracellular Ca2+ concentration and GH secretion by SRIF.

The effect of octreotide on healing of injured colonic anastomosis with immediate postoperative intraperitoneal administration of 5-Fluorouracil

PURPOSE

This study was designed to investigate the effect of octreotide on side effects of immediate usage of 5-fluorouracil after colonic anastomosis.

METHODS

Forty male Wistar rats were randomly assigned into four groups and underwent standardized left colonic anastomosis. The rats served as control or received intraperitoneal 5-fluorouracil (20 mg/kg daily), subcutaneous octreotide (20 mug/kg daily), or both. Diarrhea and wound complications were noted during the experiment. The colonic anastomoses were assessed for healing on postoperative Day 7 by determining the anastomotic bursting pressure, performing histologic examination, and measuring the tissue hydroxyproline content, serum malondialdehyde, and nitric oxide levels. Intraperitoneal adhesions and anastomotic leakage were also noted.

RESULTS

No statistical significant difference was found between the control and octreotide groups for each of the parameters measured. Immediate 5-fluorouracil use resulted with higher adhesion score (P = 0.002), significant depression in anastomotic bursting pressure (P = 0.0001), histopathologic score (P = 0.0001), hydroxyproline content (P = 0.0001), and increasing nitric oxide (P = 0.0001) and malondialdehyde levels (P = 0.0001) compared with the control group. Diarrhea was seen in 80 percent of the 5-fluorouracil group but in neither the control nor octreotide groups (P = 0.0001 for each comparison). However, all these parameters were ameliorated by use of concomitant octreotide and 5-fluorouracil (P = 0.019, P = 0.023, P = 0.0001, P = 0.006, P = 0.0001, and P = 0.013, respectively). In addition, diarrhea was found to be prevented (P = 0.0001).

CONCLUSIONS

The results of this study showed that concomitant octreotide use might prevent the side effects of 5-fluorouracil, such as diarrhea, postoperative adhesion, and delaying the anastomotic healing parameters. In addition, it might reduce tissue damage and inflammation.

Chemo-hormone therapy of non-well-differentiated endocrine tumours from different anatomic sites with cisplatinum, etoposide and slow release lanreotide formulation

We report the results of a phase II trial in patients with metastatic endocrine tumours from different sites, which aimed to evaluate the anti-tumour activity and toxicity of a cisplatinum and etoposide regimen administered in combination with the somatostatin agonist lanreotide given in slow release formulation. Between January 1999 and November 2003, 27 patients with histological diagnoses of endocrine tumours with different degrees of differentiation, excluding well differentiated carcinoid neoplasms, received intravenous (i.v.) administration of cisplatinum (30 mg m⁻²) and etoposide (100 mg m⁻²) on days 1–3 and intramuscular administration of 60 mg lanreotide on day 1, in a 21-day cycle. All of the patients were evaluable for toxicity and response. The treatment was very well tolerated as no grade 4 toxicity was observed. Four patients achieved a complete response, six a partial response, 12 experienced disease stabilisation and five disease progression. The average time to progression and to survival were 9 and 24 months respectively. These results suggest that this chemo-hormone therapy regimen is well tolerated and active in patients with non-well differentiated endocrine tumours.

Nuclear imaging of neuroendocrine tumours

The diagnosis of neuroendocrine tumours (NETs) and monitoring of therapy in many patients relies mainly on morphological imaging techniques such as computed tomography (CT), ultrasound (US) and magnetic resonance imaging (MRI). However, functional imaging modalities--such as somatostatin receptor scintigraphy (SRS)--have great impact on patient management by providing tools for better staging of the disease, visualization of occult tumour, and evaluation of eligibility for somatostatin analogue treatment. Positron emission tomography (PET) using (18)F-fluoro-deoxy-glucose (FDG) is a powerful functional modality for oncological imaging. Unfortunately, FDG is not accumulated in NETs except in the case of dedifferentiated tumours and tumours with high proliferative activity. Based on the concept of amine precursor uptake and decarboxylation (APUD), the (18)F- and (11)C-labelled amine precursors L-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan (5-HTP) have been utilized for PET imaging of NETs. In comparative studies of patients with a variety of NETs, (11)C5-HTP-PET proved better than CT and SRS by visualizing additional small lesions. With carbidopa premedication orally before (11)C5-HTP-PET examination the tumour uptake could be increased and the urinary radioactivity concentration considerably reduced. This concept may also be applied to (18)F-L-DOPA-PET, a method which in a limited number of studies has gained additional diagnostic information in NET patients compared to SRS and morphological imaging. (68)Ga is available from an in-house generator and has been utilized for labelling of somatostatin analogues for PET imaging of NETs with promising results in a small number of patients. However, SRS is an established functional imaging method for patients with NETs, whereas the role for PET in the clinical routine needs further evaluation in comparative studies in larger groups of patients.

Short-term exposure to somatostatin or muscarinic agonists reduce acetylcholine-induced 3H-MPP+ release from bovine adrenal medullary cells

The aim of this work was to investigate the effect of a short-term exposure to somatostatin (SS), its receptors (SSTR) selective agonists as well as muscarinic receptors agonists upon acetylcholine-induced release of (3)H-MPP(+ )from bovine adrenal medullary cells. Acetylcholine (ACH, 100, 500 microM) was found to increase the release of (3)H-MPP(+ )by these cells (to 175 and 171% of basal release, respectively). ACH-elicited (3)H-MPP(+) release was significantly reduced by hexamethonium (100 microM) and atropine (100 microM), selective nicotinic and muscarinic antagonists, respectively. Previous exposure to any of two muscarinic agonists, oxotremorine or pilocarpine, led to a significant reduction of (3)H-MPP(+) release in response to 100 microM ACH, to about a maximum of 51% and 78% of control, respectively. Somatostatin (SS, 0.01-0.1 microM), previously applied to the preparation, depressed ACH-elicited (3)H-MPP(+ )release by 25-27%, but only when a 500 microM ACH concentration was used. The inhibition exerted by SS upon ACH-evoked (3)H-MPP(+) release appeared to be mediated by its SSTR: (1) SSTR2, 3 and 4 subtype agonists mimicked the effects seen with SS, and (2) the SSTR non-selective antagonist, cyclo-SS, counteracted the SS inhibitory effect. When SS was tested in the presence of any of the muscarinic agonists, oxotremorine or pilocarpine, its inhibitory effect on 500 microM ACH-induced (3)H-MPP(+) release was no longer detectable. These results, showing a somewhat similar effect of short-term exposure to SS and muscarinic agonists over ACH-induced release of (3)H-MPP(+), as well as the loss of effect of SS by the presence of the muscarinic agonists, suggest that these compounds may share signalling pathways.

Inhibitory effect of somatostatin on inflammation and nociception

The present review focuses on promising new opportunities for anti-inflammatory and analgesic therapy. The theoretical background is an original observation based on our own experimental results. These data demonstrate that somatostatin is released from capsaicin-sensitive, peptidergic sensory nerve endings in response to noxious heat and chemical stimuli such as vanilloids, protons or lipoxygenase products. It reaches distant parts of the body via the circulation and exerts systemic anti-inflammatory and analgesic effects. Somatostatin binds to G-protein-coupled membrane receptors (sst(1)-sst(5)) and diminishes neurogenic inflammation by prejunctional action on sensory-efferent nerve terminals, as well as by postjunctional mechanisms on target cells. It decreases the release of pro-inflammatory neuropeptides from sensory nerve endings and also acts on receptors of vascular endothelial, inflammatory and immune cells. Analgesic effect is mediated by an inhibitory action on peripheral terminals of nociceptive neurons, since circulating somatostatin cannot exert central action. Somatostatin itself is not suitable for drug development because of its broad spectrum and short elimination half life, stable, receptor-selective agonists have been synthesized and investigated. The present overview is aimed at summarizing the physiological importance of somatostatin and sst receptors, pharmacological significance of synthetic agonists and their potential in the development of novel anti-inflammatory and analgesic drugs. These compounds might provide novel perspectives in the pharmacotherapy of acute and chronic painful inflammatory diseases, as well as neuropathic conditions.

Rationale for the use of somatostatin analogs as antitumor agents

BACKGROUND

There is a need for novel antitumor agents that demonstrate efficacy in currently refractory tumors without adding to the toxicity of therapy. The somatostatin analogs, which have demonstrated antineoplastic activities in experimental tumor models, and good tolerability and safety profiles are attractive candidates.

MATERIALS AND METHODS

Data from preclinical studies provide evidence for direct and indirect mechanisms by which somatostatin analogs exert antitumor effects.

RESULTS

Direct antitumor activities, mediated through somatostatin receptors (sst(1)-sst(5)) expressed in tumor cells, include blockade of autocrine/paracrine growth-promoting hormone and growth factor production, inhibition of growth factor-mediated mitogenic signals and induction of apoptosis. Indirect antitumor effects include inhibition of growth-promoting hormone and growth factor secretion, and antiangiogenic actions. Many human tumors express more than one somatostatin receptor subtype, with sst(2) being predominant. Somatostatin analogs such as octreotide and lanreotide, which present a high affinity for sst(2), are in current clinical use to alleviate symptoms in patients with endocrine tumors, and radiolabeled somatostatin analogs have been developed for diagnosis and radiotherapy.

CONCLUSIONS

While the rationale exists for the use of somatostatin analogs as antitumor agents, studies are ongoing to identify analogs with activity across the range of receptor subtypes to maximize the potential of such treatment.

Interferon-alpha and somatostatin analog in patients with gastroenteropancreatic neuroendocrine carcinoma: single agent or combination?

In most cases gastro-enteropancreatic neuroendocrine tumors grow slowly. Interferon-alpha and somatostatin analogs have shown symptomatic, biochemical, and, in a minority of cases, antiproliferative activity. Generally, they are proposed as single-agent therapy. However, based on in vitro and in vivo evidence, the combined use of these drugs was proposed in several non-randomized trials, indicating that there is an additive effect of the combination. Nevertheless, the three randomized trials published so far did not show a statistically significant survival benefit for the combination compared to the same agents alone, even though an advantage for the combination came out in all three studies. On the other hand, data from non-randomized trials would justify the sequential use of the two drugs or the combination after progression on single agent therapy. Therefore, at present the up-front combined use of interferon-alpha and somatostatin analog is not justified, whereas it could be indicated after progression to single-agent therapy. Further larger, international, prospective, randomized, multicentric clinical trials studying homogeneous populations would be necessary to give a final answer, but the rarity and heterogeneity of this malignancy does not assure that it will be possible.

Mapping of somatostatin receptor types in GH or/and PRL producing pituitary adenomas

BACKGROUND

Somatostatin is a tetradecapeptide exerting inhibitory action on endocrine and exocrine cell secretion and proliferation. Somatostatin receptors (SST) are widely expressed in various neoplasms including endocrine tumours. Using immunohistochemistry, the expression of SST(1), SST(2A), SST(2B), SST(3), SST(4), and SST(5) was studied in tissue microarrays (TMAs), using a series of 90 human pituitary adenomas producing growth hormone and/or prolactin, including 30 of each somatotroph, lactotroph, and mixed somatotroph/lactotroph adenoma type.

METHODS

For immunohistochemistry, the standard avidin biotin complex method enhanced by tyramide was used, using polyclonal antisera for all SST types. A four point scoring system was used to assess the membranous immunopositivity.

RESULTS

All SST types were positive in all tumour types, showing varying immunoreactivity scores. SST(5) and SST(2A) were the predominant receptors, showing strong expression in high frequency in all three adenoma types. Strong expression of SST(1) was higher in lactotroph adenomas than in other tumour types.

CONCLUSIONS

The immunohistochemical results of SST expression are in agreement with most findings of previous molecular studies. The fact that SST(2A) expression is predominant suggests that pharmaceutical octapeptide somatostatin analogues may act through this receptor, while the role of SST(2B) may be merely synergistic.

Treatment of neuroendocrine tumors with somatostatin analogs

Neuroendocrine tumors constitute a group of hormone producing tumors originating from neuroendocrine cells in different organs. Most tumors have a low proliferation index measured by Ki67 and the progression of the tumor is slow. However, many patients suffer from endocrine symptoms induced by the hormones produced and released by the tumor cells. For some patients these symptoms can be life- threatening as in midgut carcinoid patients suffering from carcinoid crises with extensive flushes and hypotension or in patients with severe diarrhea induced by tumors producing vasointestinal polypeptide. In many other patients the hormone-induced symptoms interfere with the ability to carry out ordinary daily activities. The introduction of somatostatin analogs in the treatment of these hormone related symptoms has made it possible to control most of them and has added significantly to the quality of life for this group of patients. Unfortunately, the clinical inhibitory effect on tumor growth has not been very good with only 5-10% of the patients showing an objective response. However, stabilization of tumor growth may be achieved in a significant number of patients. In the future, the hope is that development of new somatostatin analogs with broader receptor-binding profiles will give us new analogs which are more efficient with regard to their antiproliferative effect. This possibility will be studied in future trials.

Differential efficacies of somatostatin receptor agonists for G-protein activation and desensitization of somatostatin receptor subtype 4-mediated responses

Although desensitization represents an important physiological feedback mechanism that protects against overstimulation, it can significantly limit the therapeutic usefulness of drugs. In the current investigation, we have employed Cytosensor microphysiometry for the purpose of determining the propensity of somatostatin receptor agonists to induce desensitization of the human somatostatin receptor subtype 4 (h sst4)-mediated extracellular acidification rate (EAR) response in intact Chinese hamster ovary (CHO) cells. We have compared this propensity with the efficacies of the agonists as measured in a [35S]guanosine-5'-O-(3-thio)triphosphate binding assay with membranes of the same CHO-h sst4 cell line. We observed that (1'S,2S)-4-amino-N-(1'-carbamoyl-2'-phenylethyl)-2-(4''-methyl-1''-naphthalenesulfonylamino)butanamide (J-2156), a superagonist at the h sst4 with higher efficacy than somatostatin-14 itself (Engström et al., 2005), was considerably less prone to cause desensitization of the EAR response than somatostatin-14, somatostatin-28, and cortistatin-17. In contrast, compound A (methyl (2S)-5-{[amino(imino)methyl]amino}-2-{[4-[5-7-difluoro-2-phenyl-1H-indol-3-yl)butanoyl]amino}-pentanoate), which we also found to be an h sst(4) superagonist, albeit to a lesser degree than J-2156, demonstrated a high propensity to cause desensitization. Our results indicate that there is no relationship between the efficacy of the agonists to cause G-protein activation and their ability to induce desensitization of the h sst4-mediated EAR responses. The finding that on the h sst4, J-2156 is not only a superagonist but also shows a low propensity to cause desensitization, might offer therapeutic advantages. At a minimum, the compound will be a powerful tool to study the mechanisms connected to efficacy and desensitization of h sst4-mediated responses.

Somatostatin increases voltage-gated K+ currents in GH3 cells through activation of multiple somatostatin receptors

The secretion of GH by somatotropes is inhibited by somatostatin (SRIF) through five specific membrane receptors (SSTRs). SRIF increases both transient outward (IA) and delayed rectifying (IK) K+ currents. We aim to clarify the subtype(s) of SSTRs involved in K+ current enhancement in GH3 somatotrope cells using specific SSTR subtype agonists. Expression of all five SSTRs was confirmed in GH3 cells by RT-PCR. Nystatin-perforated patch clamp was used to record voltage-gated K+ currents. We first established the presence of IA and IK type K+ currents in GH3 cells using different holding potentials (-40 or -70 mV) and specific blockers (4-aminopirimidine and tetraethylammonium chloride). SRIF (200 nM) increased the amplitude of both IA and IK in a fully reversible manner. Various concentrations of each specific SRTR agonist were tested on K+ currents to find the maximal effective concentration. Activation of SSTR2 and SSTR4 by their respective agonists, L-779,976 and L-803,087 (10 nM), increased K+ current amplitude without preference to IA or IK, and abolished any further increase by SRIF. Activation of SSTR1 and SSTR5 by their respective agonists, L-797,591 or L-817,818 (10 nM), increased K+ current amplitude, but SRIF evoked a further increase. The SSTR3 agonist L-797,778 (10 nM) did not affect the K+ currents or the response to SRIF. These results indicate that SSTR1, -2, -4, and -5 may all be involved in the enhancement of K+ currents by SRIF but that only the activation of SSTR2 or -4 results in the full activation of K+ current caused by SRIF.

Fish somatostatin sst3 receptor: comparison of radioligand and GTPgammaS binding, adenylate cyclase and phospholipase C activities reveals different agonist-dependent pharmacological signatures

1 The fish somatostatin receptor 3 (fsst3) is one of the few somatostatin (SRIF) receptors cloned from a non-mammalian species so far. Here we extended our earlier characterization of this receptor by investigating the guanine nucleotide sensitivity of agonist radioligand binding at the fsst3 receptor recombinantly expressed in CCL39 (Chinese hamster lung fibroblast) cells. Further, we measured somatostatin (SRIF) and cortistatin (CST) analogues stimulated GTPgammaS binding, inhibition of forskolin-stimulated adenylate cyclase (FSAC) and stimulation of phospholipase C (PLC) activities. The present transductional data were then compared with previous radioligand binding and/or second messenger features determined for fsst3 and/or human SRIF receptors (hsst2, hsst3 and hsst5). 2 The GTP analogue guanylylimidodiphosphate (GppNHp) inhibited binding of [125I]CGP 23996 and [125I][Tyr3octreotide by 72 and 83% suggesting preferential labelling of G-protein-coupled fsst3 receptors. By contrast, [125I]LTT-SRIF28 and [125I][Tyr10]CST14 binding was rather GppNHp insensitive (42 and 35% inhibition) suggesting labelling of both coupled and non-coupled receptor states. These results might explain the apparent higher receptor densities determined in saturation experiments with [125I]LTT-SRIF28 and [125I][Tyr10]CST14 (4470 and 4030 fmol mg(-1)) compared with [125I]CGP 23996 and [125I][Tyr3]octreotide (3420 and 1520 fmol mg(-1)). 3 SRIF14 (10 microm)-stimulated specific [35S]GTPgammaS binding by three-fold; SRIF28 and octreotide displayed full agonism, whereas most other ligands displayed 60-80% intrinsic activity compared with SRIF14. SRIF14 and SRIF28 inhibited forskolin-stimulated AC (FSAC) activity by 60%; all tested ligands except BIM 23056 inhibited FSAC with comparable high intrinsic activities. SRIF14 stimulated PLC activity five- to six-fold, as determined by measuring total [3H] IP(x) accumulation; it was rather insensitive to pertussis toxin (PTX, 100 ng ml(-1), 21% inhibition), which suggests the G(q)-family proteins couple to PLC activity. SRIF14, SRIF28 and [Tyr10]CST14 showed full agonism at PLC, whereas all other ligands behaved as partial agonists (20-70% intrinsic activity). BIM 23056, which showed weak partial or no agonism, antagonized SRIF14-induced total [3H]-IP(x) production (pK(B) = 6.83), but failed to block competitively agonist-stimulated [35S]GTPgammaS binding or agonist-induced inhibition of FSAC activity. 4 Comparison of the pharmacological profiles of fsst3 receptors established in GTPgammaS binding, FSAC inhibition and PLC stimulation resulted in low correlations (r = 0.410-0.594). Both rank orders of potency and rank orders of relative efficacy varied in the three second messenger experiments. Significant, although variable correlations were obtained comparing GTPgammaS binding and inhibition of FSAC activity with previously reported affinity profiles of [125I]LTT-SRIF28, [125I][Tyr10]CST14, [125I]CGP 23996, [125I][Tyr3]octreotide (r = 0.75-0.83; 0.68-0.89). By contrast, the PLC stimulation and radioligand-binding profiles did not correlate. 5 Comparison of the functional data (GTPgammaS binding, FSAC inhibition, PLC stimulation) of fsst3 receptors with those of human sst2, sst3, sst5 receptors expressed in CCL39 cells resulted in highest correlation with the hsst5 receptor (r = 0.94, 0.97, 0.49) > hsst2 (0.80, 0.50, n.d.) > hsst3 (0.25, 0.19, 0.17). 6 In summary, fsst3 receptors expressed in CCL39 cells are involved in signalling cascades similar to those reported for mammalian SRIF receptors, suggesting SRIF receptors to be highly conserved in evolution. Binding and functional data showed highest similarity of fsst3 receptors with the human sst5 receptor subtype. Different affinities, receptor densities and GppNHp-sensitivities determined with the four radioligands (agonists) are assumed to results from ligand-specific states of the fsst3-ligand complex. The differences in the rank orders of potency and relative efficacy in the various signalling cascades may be explained by agonist-induced receptor trafficking.

SMS 201-995 enhances S-phase block induced by 5-fluorouracil in a human colorectal cancer cell line

The action of the somatostatin analog SMS-201.995 (SMS) was tested in monotherapy and in combined therapy with the cytotoxic agent 5-fluorouracil (5-FU) on cell cycle kinetics of the human colon cancer cell line WiDr, expressing a mutant p53 (mp53). The data, obtained by flow cytometric DNA analysis, showed that SMS at 0.2 microg/ml increased apoptosis, augmenting the proportion of cells with subdiploid DNA content by 65 and 48% after 3 and 6 h, respectively. In cultures lasting 24 and 36 h, it also decreased the percentages of cells in G0/G1 phase by 22.9 and 14.3%; whereas at a dose of 0.1 microg/ml, SMS decreased the percentage of cells in G2/M by 14.3%. In contrast to SMS, 5-FU (0.1 microg/ml) augmented the apoptosis at 12 h, and markedly increased the fraction of cells in S phase, increasing its value from 24 and 72 h by 108 and 234%, respectively, in comparison to the control. The most evident finding after the combination of SMS (0.2 microg/ml) and 5-FU (0.1 microg/ml) was a potentiation of 5-FU-induced S-phase block by a further 7.9, 12.9 and 42.1% at 24, 36 and 72 h, respectively. Treatment with 5-FU also upregulated HLA class I expression of the cancer cells. In this sense, SMS was less effective and when given in combination with 5-FU did not change the effects induced by 5-FU. The data emphasize that SMS exhibits pro-apoptotic and anti-proliferative effects, which in proper dose combinations might enhance the effects of 5-FU on human colorectal cancer cells expressing mp53.

Endocrine tumours of the gastrointestinal tract. Somatostatin receptors as tools for diagnosis and therapy: molecular aspects

Somatostatin is a neuropeptide that acts as an endogenous inhibitor of various cellular functions including endocrine and exocrine secretions and the proliferation of normal and tumour cells. Its action is mediated by a family of G-protein-coupled receptors (sst1-sst5) that are widely distributed in normal and tumour cells. Gastroenteropancreatic endocrine tumours express multiple somatostatin receptors, sst2 being clearly predominant. These receptors represent the molecular basis for the clinical use of somatostatin analogues in the treatment of endocrine tumours and their in vivo localisation. This review covers current knowledge in somatostatin receptor biology and signalling.

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.

Initial Staging of Lymphoma With Octreotide and Other Receptor Imaging Agents

Somatostatin receptor scintigraphy is useful in diagnosing tumors with increased expression of somatostatin receptors. The correct use of this technique reveals the localization of neuroendocrine primary tumors and unknown metastases in approximately 90% of patients. However, somatostatin receptor scintigraphy also can image many other human tumors expressing somatostatin receptors, including malignant lymphomas and thymomas. The sensitivity of somatostatin receptor scintigraphy to image somatostatin receptor-positive tumors is very high, but due to the variable expression of specific receptor subtypes, the specificity can be relatively low. This drawback is crucial in evaluating lymphoproliferative diseases, or, in general, when immune cells are involved. The sensitivity of somatostatin receptor scintigraphy for Hodgkin's lymphoma is 95%-100%, whereas for non-Hodgkin's lymphoma it is around 80%. It has been shown that the uptake of [(111)In-DTPA(0)]octreotide in lymphomas is lower compared to the uptake in neuroendocrine tumors. This is mainly attributed to the low number of receptors on immune cells compared to neuroendocrine cells; however, ligand-induced internalization and differential receptor regulation may also participate in determining this phenomenon. Therefore, caution should be taken when interpreting data from some studies. Several new ligands are currently under study to improve these limits and the expression of other neuropeptide receptors is being investigated to provide a molecular basis for in vivo multireceptor targeting of tumors. With the use of currently available somatostatin analogs, somatostatin receptor scintigraphy does not seem to have a significant impact in patients with lymphomas for diagnostic purposes. There are a few exceptions, however. Among these, the staging and restaging of extragastric lymphoma MALT-type may present some advantages. Conversely, somatostatin receptor scintigraphy in the imaging of thymic malignancies could enhance both our diagnostic and therapeutic capabilities. Somatostatin receptor scintigraphy is diagnostically relevant in differentiating malignant from benign lesions, especially in those patients with associated paraneoplastic syndromes, and is the main criterion to select patients suitable for therapy with somatostatin analogs. Recent findings emerging from in vitro studies on somatostatin receptor physiology in immune cells will certainly reopen and expand the potential applications of somatostatin analogs for in vivo diagnostic and therapeutic options.

Regulation of somatostatins and their receptors in fish

The multifunctional nature of the somatostatin (SS) family of peptides results from a multifaceted signaling system consisting of many forms of SS peptides that bind to a variety of receptor (SSTR) subtypes. Research in fish has contributed important information about the components, function, evolution, and regulation of this system. Somatostatins or mRNAs encoding SSs have been isolated from over 20 species of fish. Peptides and deduced peptides differ in their amino acid chain length and/or composition, and most species of fish possess more than one form of SS. The structural heterogeneity of SSs results from differential processing of the hormone precursor, preprosomatostatin (PPSS), and from the existence of multiple genes that give rise to multiple PPSSs. The PPSS genes appear to have arisen through a series of gene duplication events over the course of vertebrate evolution. The numerous PPSSs of fish are differentially expressed, both in terms of the distribution among tissues and in terms of the relative abundance within a tissue. Accumulated evidence suggests that nutritional state, season/stage of sexual maturation, and many hormones [insulin (INS), glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), and 17beta-estradiol (E2)] regulate the synthesis and release of particular SSs. Fish and mammals possess multiple SSTRs; four different SSTRs have been described in fish and several of these occur as isoforms. SSTRs are also wide spread and are differentially expressed, both in terms of distribution of tissues as well as in terms of relative abundance within tissues. The pattern of distribution of SSTRs may underlie tissue-specific responses of SSs. The synthesis of SSTR mRNA and SS-binding capacity are regulated by nutritional state and numerous hormones (INS, GH, IGF-I, and E2). Accumulated evidence suggests the possibility of both tissue- and subtype-specific mechanisms of regulation. In many instances, there appears to be coordinate regulation of PPSS and of SSTR; such regulation may prove important for many processes, including nutrient homeostasis and growth control.

Somatostatin Analogue Mimics Acute Ischemic Preconditioning in a Rat Model of Myocardial Infarction

We tested the hypothesis that octreotide, a somatostatin analogue, can mimic ischemic preconditioning (PC) to provide cardioprotection against myocardial infarction. An ischemia-reperfusion model of adult Wistar rats was used. Infarct size was expressed as a percentage of the area at risk under different treatment protocols. Octreotide PC (35 microg/Kg 20 minutes before ischemia-reperfusion) significantly decreased infarct size (18 +/- 4%) versus control (60 +/- 7%). The somatostatin receptor antagonist cyclo-somatostatin (0.5 mg/Kg) could blunt the above cardioprotection. Administration of either chelerythrine (a protein kinase C inhibitor, 2 mg/Kg) or genistein (a tyrosine kinase inhibitor, 5 mg/Kg) could also block octreotide PC (54 +/- 7% and 58 +/- 6%, respectively). Pretreatment with the mitochondrial ATP-sensitive potassium channel antagonist 5-hydroxydecanoic acid (5-HD) and the sarcolemmal ATP-sensitive potassium channel antagonist glibenclamide could abolish the effects of octreotide PC (54 +/- 6% and 52 +/- 6%). Chelerythrine, however, had no effect on octreotide PC. In conclusion, the present study demonstrates that octreotide can mimic ischemic PC to reduce infarct size. Acute effects of octreotide PC involve the activation of protein kinase C, tyrosine kinase C, and mitochondrial ATP-sensitive potassium channels, but not systemic IGF-I activation.

Subtype selective interactions of somatostatin and somatostatin analogs with sst1, sst2, and sst5 in BON-1 cells

Somatostatin is a polypeptide hormone acting as an inhibitor of pituitary, pancreatic, and gastrointestinal secretion through specific membrane receptors of which five subtypes have been cloned (sst(1-5)). Somatostatin analogs are used in the clinic to treat patients with excessive hormone production due to a neuroendocrine tumor. The aim of this study was to investigate the biological activity of three new somatostatin receptor subtype selective analogs (BIM-23926, sst(1)-selective; BIM-23120, sst(2)-selective; and BIM-23206, sst(5)-selective) in the human neuroendocrine tumor cell line, BON-1, which expresses sst(1), sst(2), and sst(5) natively. Somatostatin-14 and octreotide were used as reference substances. Forskolin-induced cAMP accumulation and chromogranin A (CgA) secretion were inhibited by BIM-23120, BIM-23206, and somatostatin-14 in a dose-dependent manner. Cholecystokinin (CCK-8) stimulated activation of mitogen-activated protein (MAP) kinase was inhibited by BIM-23120 and BIM-23206, while BIM-23926 stimulated the activity. Selective BIM analogs showed a more efficient inhibitory effect on cAMP accumulation, CgA secretion, and MAP kinase activity than octreotide in BON-1 cells. This may be explained by the differences in affinity of the ligand to the receptor or by interaction between different sst subtypes. We conclude that increasing knowledge about sst physiology and expression in malignant disease indicates a need for new analogs that can be incorporated into the therapeutic arsenal.

Agonist-induced up-regulation of human somatostatin receptor type 1 is regulated by beta-arrestin-1 and requires an essential serine residue in the receptor C-tail

We have previously shown that the human somatostatin receptor type 1 (hSSTR1) does not undergo agonist-induced internalization, but is instead up-regulated at the membrane upon prolonged somatostatin (SST) exposure. The deletion of the carboxyterminal C-tail of the receptor completely abolishes up-regulation. To identify molecular signals that mediate hSSTR1 up-regulation, we created mutant receptors with progressive C-tail deletions. Up-regulation was found to be absent in mutants lacking residues Lys359-Ser360-Arg361. Moreover, point mutation of Ser360 to Ala completely abolished up-regulation. The coexpression of wild type hSSTR1 with V53D, a dominant negative mutant of beta-arrestin-1, completely blocked hSSTR1 up-regulation. Further analysis demonstrated that calcium-calmodulin (CaM) dependent kinases were essential for the SST-induced up-regulation response. Like wild type receptors, all mutants failed to internalize after agonist exposure and were able to inhibit forskolin-stimulated cAMP accumulation. Taking these data together, we suggest that SST-induced hSSTR1 up-regulation is critically dependent upon a specific Lys-Ser-Arg sequence in the C-tail of the receptor, with Ser360 being essential. Up-regulation also requires the participation of CaM protein kinases and interactions with beta-arrestins. In contrast, coupling to adenyl cyclase (AC) and internalization occur independently of molecular signals in the receptor's C-tail.

Potent induction of apoptosis in human hepatoma cell lines by targeted cytotoxic somatostatin analogue AN-238

BACKGROUND/AIMS

The efficacy of a targeted cytotoxic hybrid somatostatin analogue AN-238 and of its superactive radical 2-pyrrolinodoxorubicin (AN-201) to induce apoptosis of HepG2 and Hep3B human hepatoma cell lines were studied. AN-238 was designed to selectively target tumor cells expressing somatostatin receptor subtypes (sst(s)). Its effects on HepG2 or Hep3B cells displaying or lacking tumor suppressor p53, respectively, were compared. Normal rat isolated hepatocytes were also tested.

METHODS

sst(s) were characterized by binding assays and RT-PCR. Cytotoxicity was quantified by flow cytometry. DNA fragmentation was studied by gel electrophoresis, PARP cleavage by Western blot and ROS formation using fluorescent probes.

RESULTS

Specific binding of iodinated RC-160 to HepG2 and Hep3B cells, and its displacement by AN-238 was characterized. mRNA for hsst(2A) was found in both cell lines. Flow cytometry showed a stronger effect of AN-238 than AN-201 to induce sub-G1 phase. DNA fragmentation, nuclear bodies, and PARP cleavage were observed. In addition, AN-238 increased formation of ROS more potently than AN-201. However, no inductions of DNA fragmentation by AN-201 or AN-238 were observed on rat hepatocytes.

CONCLUSIONS

Our results indicate that, in liver cancer, the cytotoxic somatostatin analogue AN-238 is a powerful agent that can induce apoptosis, through sst(s) and independently of p53.

Role of somatostatin receptors on gastric acid secretion in wild-type and somatostatin receptor type 2 knockout mice

Somatostatin, probably acting through somatostatin type 2 receptors (SSTR2), is the main inhibitor of gastric acid secretion. We characterized gastric acid secretion in SSTR2 knockout mice, and used preferential somatostatin receptor agonists to assess the relative role of SSTR1, 2, 3, 4, and 5 on gastric acid secretion. Basal gastric acid secretion and the secretory response to a meal were similar in conscious wild-type and knockout mice. However, under urethane anesthesia, which releases endogenous somatostatin, SSTR2 knockout mice had a basal secretion 11-15-fold higher than wild-type animals (micromol/10 min:1.40+/-0.09 vs. 0.10+/-0.01, p<0.05). Gastrin immunoneutralization or H(2) receptors blockade (cimetidine), but not cholinergic blockade (atropine), reduced the high basal secretion in SSTR2 knockout mice. In SSTR2 knockout mice, gastrin and histamine stimulated acid secretion with similar efficacy, while in wild-type mice histamine was more effective than gastrin. SSTR2 knockout mice showed also a hypersecretory response to pylorus ligation compared with wild-type animals. In wild-type mice, somatostatin-14, SMS 201-995, and the SSTR2-preferential agonist, DC 32-87, inhibited gastrin-stimulated acid secretion with an order of potency SMS 201-995>DC 32-87>somatostatin-14. Preferential agonists for the SSTR1, 3, 4, and 5 were devoid of any effect. None of the compounds tested affected the high basal secretion observed under urethane anesthesia in SSTR2 knockout mice. These results show that gastric antisecretory effects of peripheral somatostatin are mediated solely through SSTR2. In the absence of functional SSTR2 other somatostatin receptors do not compensate for the lack somatostatin-SSTR2-mediated inhibition. Basal acid secretion and the response to a meal are normal in conscious SSTR2 knockout mice, suggesting the presence of somatostatin-independent mechanisms that compensate for the lack of somatostatin-SSTR2-mediated inhibitory responses.

Double-gene ablation of SSTR1 and SSTR5 results in hyperinsulinemia and improved glucose tolerance in mice

BACKGROUND

Previous studies conducted in our laboratory showed that single-gene ablation of somatostatin receptor (SSTR)1 or 5 results in diabetes in mice. The objective of this study was to determine the effect of double-gene ablation of SSTR1 and SSTR5 on insulin secretion and glucose homeostasis in mice.

METHODS

SSTR1/5 -/- mice and wild-type (WT) control mice were generated and their genotype verified via polymerase chain reaction. Insulin secretion and glucose levels in these mice were examined with the use of an intraperitoneal glucose tolerance test (1.2-2.0 g/kg body weight). In vitro glucose-stimulated insulin secretion was studied with the use of the isolated perfused mouse pancreas model and islet culture techniques. Pancreata morphologic alterations were determined, and an immunohistochemistry analysis was performed.

RESULTS

In vitro incubation of isolated islets from WT mice with somatostatin peptides resulted in significant reduction in insulin secretion, whereas SSTR1/5 -/- mouse islets had no response to somatostatin peptides confirming SSTR1/5 gene ablation. SSTR1/5 -/- mice also had significant increase of both basal and glucose-stimulated insulin levels in vitro. During the intraperitoneal glucose tolerance test, SSTR1/5 -/- mice had significantly improved glucose tolerance and sustained an increase in late-phase insulin secretion in vivo. Histological analysis demonstrated significant islet hyperplasia in the SSTR 1/5 -/- mouse pancreas. Immunostaining revealed an overall increase of glucagon and pancreatic polypeptide-producing cells in the islets of SSTR1/5 -/- mice.

CONCLUSIONS

Double-gene ablation of SSTR1 and SSTR5 in mice resulted in a distinct phenotype with islet cell hyperplasia, hyperinsulinemia, and improved glucose tolerance. This form of diabetes differs from that seen in mice in which only the SSTR1 or SSTR5 gene was ablated. These results demonstrate that SSTR1 and SSTR5 are important regulators of insulin secretion and glucose regulation, and suggest that SSTR1 and SSTR5 are coordinately regulated.

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.