Influence of the somatostatin receptor sst2 on growth factor signal cascades in human glioma cells

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

The impact of SST2 trafficking and signaling in the treatment of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (Pan-NETs), are heterogeneous neoplasms, whose incidence and prevalence are increasing worldwide. Pan-NETs are characterized by the expression of somatostatin receptors (SSTs). In particular, SST2 is the most widely distributed SST in NETs, thus representing the main molecular target for somatostatin analogs (SSAs). SSAs are currently approved for the treatment of well-differentiated NETs, and radionuclide-labeled SSAs are used for diagnostic and treatment purposes. SSAs, by binding to SSTs, have been shown to inhibit hormone secretion and thus provide control of hypersecretion symptoms, when present, and inhibit tumor proliferation. After SSA binding to SST2, the fate of the receptor is determined by trafficking mechanisms, crucial for the response to endogenous or pharmacological ligands. Although SST2 acts mostly through G protein-dependent mechanism, receptor-ligand complex endocytosis and receptor trafficking further regulate its function. SST2 mediates the decrease of hormone secretion via a G protein-dependent mechanism, culminating with the inhibition of adenylyl cyclase and calcium channels; it also inhibits cell proliferation and increases apoptosis through the modulation of protein tyrosine phosphatases. Moreover, SST2 inhibits angiogenesis and cell migration. In this respect, the cross-talk between SST2 and its interacting proteins, including Filamin A (FLNA) and aryl hydrocarbon receptor-interacting protein (AIP), plays a crucial role for SST2 signaling and responsiveness to SSAs. This review will focus on recent studies from our and other groups that have investigated the trafficking and signaling of SST2 in Pan-NETs, in order to provide insights into the mechanisms underlying tumor responsiveness to pharmacological treatments.

Peptide receptor targeting in cancer: the somatostatin paradigm

Peptide receptors involved in pathophysiological processes represent promising therapeutic targets. Neuropeptide somatostatin (SST) is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors, named sst1–5, while in central nervous system, SST acts as a neurotransmitter/neuromodulator, regulating locomotory and cognitive functions. Critical points of SST/SST receptor biology, such as signaling pathways of individual receptor subtypes, homo- and heterodimerization, trafficking, and cross-talk with growth factor receptors, have been extensively studied, although functions associated with several pathological conditions, including cancer, are still not completely unraveled. Importantly, SST exerts antiproliferative and antiangiogenic effects on cancer cells in vitro, and on experimental tumors in vivo. Moreover, SST agonists are clinically effective as antitumor agents for pituitary adenomas and gastro-pancreatic neuroendocrine tumors. However, SST receptors being expressed by tumor cells of various tumor histotypes, their pharmacological use is potentially extendible to other cancer types, although to date no significant results have been obtained. In this paper the most recent findings on the expression and functional roles of SST and SST receptors in tumor cells are discussed.

Noncanonical suppression of GH-dependent isoforms of cytochrome P450 by the somatostatin analog octreotide

Octreotide is a potent somatostatin analog therapeutically used to treat several conditions including hyper GH secretion in patients with acromegaly. We infused, over 30 s, octreotide into male rats every 12 h for 6 days at levels considerably greater than typical human therapeutic doses. Unexpectedly, resulting circulating GH profile was characterized by pulses of higher amplitudes, longer durations, and greater total content than normal, but still contained an otherwise male-like episodic secretory profiles. In apparent disaccord, the normally elevated masculine expression levels (protein and/or mRNA) of CYP2C11 (accounting for >50% of the total hepatic cytochrome P450 content), CYP3A2, CYP2C7, and IGF1, dependent on the episodic GH profile, were considerably downregulated. We explain this contradiction by proposing that the requisite minimal GH-devoid interpulse durations in the masculine profile that solely regulate expression of at least CYP2C11 and IGF1 may be sufficiently reduced to suppress transcription of the hepatic genes. Alternatively, we observed that octreotide infusion may have acted directly on the hepatocytes to induce expression of immune response factors postulated to suppress CYP transcription and/or upregulate expression of several negative regulators (e.g. phosphatases and SOCS proteins) of the JAK2/STAT5B signaling pathway that normally mediates the upregulation of CYP2C11 and IGF1 by the masculine episodic GH profile.

Somatostatin and somatostatin analogues reduce PDGF-induced endometrial cell proliferation and motility

BACKGROUND

Endometriosis is characterized by ectopic implantation of endometrial cells, which show increased proliferation and migration. Somatostatin (SST) and its analogues inhibit normal and cancer cell growth and motility through the SST receptors, sst1-5. Cortistatin (CST), which displays high structural and functional homology with SST, binds all ssts, as well as MrgX2. Our objective was to investigate the gene expression of the SST/CST system and to determine the effect of SST and its analogues on platelet-derived growth factor (PDGF)-induced proliferation and motility in telomerase-immortalized human endometrial stromal cell (T HESC) line and in primary endometrial stromal cell (ESCs) isolated from human endometriotic tissues.

METHODS

Ectopic endometrial tissues were collected from women (n= 23) undergoing laparoscopic surgery for endometriosis (Stage III/IV). Gene expression was evaluated by real-time PCR, cell motility by wound healing assay, protein expression and β-actin rearrangement by immunofluorescence, cell proliferation by the Alamar blue assay and ERK1/2 and Akt phosphorylation by western blot.

RESULTS

Human endometriotic tissues, primary ESCs and T HESCs expressed SST, CST and ssts. SST, its analogues SOM230 and octreotide, as well as CST, counteracted PDGF-induced proliferation and migration in both ESCs and T HESCs. SST also inhibited vascular endothelial growth factor and metalloprotease-2 mRNA expression, and reduced basal and PDGF-induced ERK1/2 phosphorylation.

CONCLUSION

These results indicate that the SST/CST system is expressed in endometriotic tissues and cells. The inhibitory effects of SST and its analogues on PDGF-induced proliferation and motility suggest that these peptides may represent promising tools in the treatment of endometriosis.

Cross-talk and modulation of signaling between somatostatin and growth factor receptors

The process of homo- and/or heterodimerization of G-protein coupled receptors (GPCRs) and receptor tyrosine kinase (RTK) families are crucial for implicating the fundamental properties of receptor proteins including receptor expression, trafficking, and desensitization as well as signal transduction. The members of GPCR and RTK family constitute largest cell surface receptor proteins and regulate physiological functions of cells in response to external and internal stimuli. Notably, GPCRs and RTKs play major role in regulation of several key cellular functions which are associated with several pathological conditions including cancer biology, neurodegenerative and cardiovascular diseases. The focus of this review is to highlight the recent findings on the possible cross-talk between somatostatin receptors (members of GPCR family) and growth factor receptors like epidermal growth factor receptors (members of RTK family). Furthermore, functional consequences of such an interaction in modulation of signaling pathways linked to pathological conditions specifically in cancer are discussed.

Somatostatin receptor biology in neuroendocrine and pituitary tumours: part 1--molecular pathways

Neuroendocrine tumours (NETs) may occur at many sites in the body although the majority occur within the gastroenteropancreatic axis. Non-gastroenteropancreatic NETs encompass phaeochromocytomas and paragangliomas, medullary thyroid carcinoma, anterior pituitary tumour, broncho-pulmonary NETs and parathyroid tumours. Like most endocrine tumours, NETs also express somatostatin (SST) receptors (subtypes 1–5) whose ligand SST is known to inhibit endocrine and exocrine secretions and have anti-tumour effects. In the light of this knowledge, the idea of using SST analogues in the treatment of NETs has become increasingly popular and new studies have centred upon the development of new SST analogues. We attempt to review SST receptor (SSTR) biology primarily in neuroendocrine tissues, focusing on pituitary tumours. A full data search was performed through PubMed over the years 2000–2009 with keywords ‘somatostatin, molecular biology, somatostatin receptors, somatostatin signalling, NET, pituitary’ and all relevant publications have been included, together with selected publications prior to that date. SSTR signalling in non-neuroendocrine solid tumours is beyond the scope of this review. SST is a potent anti-proliferative and anti-secretory agent for some NETs. The successful therapeutic use of SST analogues in the treatment of these tumours depends on a thorough understanding of the diverse effects of SSTR subtypes in different tissues and cell types. Further studies will focus on critical points of SSTR biology such as homo- and heterodimerization of SSTRs and the differences between post-receptor signalling pathways of SSTR subtypes.

Effect of SOM230 (pasireotide) on corticotropic cells: action in dogs with Cushing's disease

SOM230 (pasireotide) is a multiligand somatostatin (SRIF) analog able to bind to somatostatin receptor (SSTR) subtypes 1, 2, 3 and 5, and trigger antisecretory and antiproliferative signaling cascades. Canines have become in vivo models to test the pharmacological treatment of corticotropinomas because they frequently develop Cushing's disease in a spontaneous manner, due to adrenocorticotropic hormone (ACTH)-producing pituitary adenomas. Different levels of expression of SSTR2 and SSTR5 have been shown in both mouse AtT20 cells and canine tumoral corticotropinoma cells. The objective of this study was to evaluate whether SOM230 controls both tumor cell growth and hormone synthesis, therefore controlling the disease. SOM230 was tested in dogs suffering from Cushing's disease (10 animals were treated continuously during 6 months, and another 10 were treated with 3 cycles consisting of 2 months of treatment followed by a 2-month rest period). A significant decrease in ACTH, urinary cortisol creatinine ratio, adenoma size (magnetic nuclear resonance) and improvement of clinical signs were obtained, without side effects. AtT20 cells treated with SOM230 suppressed pro-opiomelanocortin (POMC) promoter activity through SSTR2, via the G(i) α-subunit, and reduced Nur77/Nurr1 transcriptional activity. We conclude that SOM230, in addition to its well-described antisecretory effects, inhibits, as shown in AtT20 cells, ACTH synthesis at the POMC transcriptional level, an effect mediated mainly through SSTR2, and limits tumor growth. The controlled Cushing's disease in the dogs that received the treatment indicates that SOM230 has a potential therapeutic use in humans suffering from Cushing's disease.

Antiproliferative effect of somatostatin analogs in gastroenteropancreatic neuroendocrine tumors

Somatostatin analogs were initially developed for the control of hormonal syndromes associated with neuroendocrine tumors (NETs). In recent years, accumulating data has supported their role as antiproliferative agents, capable of stabilizing tumor growth in patients with metastatic neuroendocrine malignancies, including carcinoid and pancreatic endocrine tumors. A phase III, randomized, placebo-controlled trial has now demonstrated that octreotide long-acting repeatable (LAR) 30 mg can significantly prolong time to tumor progression among patients with metastatic midgut NETs regardless of functional status, chromogranin A level or age. In addition to significantly lengthening time to tumor progression in the overall study population, subset analysis suggests that patients with low tumor burden are most likely to experience disease stabilization with octreotide LAR 30 mg, supporting the early use of octreotide LAR in patients with metastatic disease. Further research efforts are underway to evaluate the use of somatostatin analogs as antiproliferative agents in other types of gastroenteropancreatic-NETs. Ongoing studies are also evaluating novel somatostatin analogs and somatostatin analogs in combination with other anti-tumor therapies.

Receptor activation and inhibition in cellular response to chemotherapeutic combinational mimicries: the concept of divergent targeting

The antiproliferative effect of tandem somatostatin receptor (SSTR) activation, epidermal growth factor receptor (EGFR) inhibition, and induction of DNA damage was analyzed using octreotide (OCT), a SSTR agonist, the clinical DNA methylating agent temozolomide (TMZ), Iressa, an EGFR inhibitor, and dual EGFR-DNA targeting agents termed "combi-molecules". Using SSTR-expressing glioma cells harbouring low levels of EGFR (U87MG) or transfected to overexpress EGFR (U87/EGFR) or a variant (U87/EGFRvIII), we showed that Iressa, alone or in combination with the DNA damaging agent TMZ, and combi-molecules RA2 and RA5 inhibited EGF-induced phosphorylation of EGFR in U87MG and more moderately in U87/EGFR and U87/EGFRvIII transfected cells. This translated into equivalent levels of Erk 1/2 inhibition. Activation of SSTRs with OCT did not modulate the effects of the various treatments on Erk 1/2 phosphorylation. Likewise, SSTR activation did not alter TMZ- or DNA-damaging combi-molecules, RA2 and RA5, induced p53 activation nor upregulation. However, SSTR activation significantly shifted TMZ-, RA2- and RA5-induced cell-cycle arrest to earlier phases (i.e., G2/M to late S, late S to S, S to G1). Further analysis showed that apoptosis was not induced. This was in agreement with the fact that p53 activation did not induce Bax upregulation nor did EGFR inhibition promote Bad dephosphorylation. Moreover, enhancement of survivin, an anti-apoptotic protein, expression was observed. The results in toto suggest that the combination of SSTR activation with EGFR inhibition and DNA damage affects cell-cycle progression but a disconnection between the targeted signalling pathways in these brain tumour cells precludes synergistic cell-killing by the triple growth inhibitory events.

Somatostatin receptors 1 and 5 heterodimerize with epidermal growth factor receptor: agonist-dependent modulation of the downstream MAPK signalling pathway in breast cancer cells

The role of somatostatin (SST) and epidermal growth factor (EGF) in breast cancer is undisputed; however, the molecular mechanisms underlying their antiproliferative or proliferative effects are not well understood. We initially confirmed that breast tumour tissues express all five somatostatin receptors (SSTR1-5) and four epidermal growth factor receptors (ErbB1-4). Subsequently, to gain insight into the function of SSTRs and ErbBs in oestrogen receptor (ER)-positive (MCF-7) or ERalpha-negative (MDA-MB-231) breast cancer cells, we defined SSTR1, SSTR5 and ErbB1 mRNA and protein expression in these two tumour cell lines. Consistent with previous studies showing SSTR1/SSTR5 heterodimerization and having seen cell-specific and ligand-selective alterations in receptor expression, we next elucidated whether SSTR1 and SSTR5 functionally interact with ErbB1 using pbFRET analysis. We subsequently determined the effects of SST and EGF either alone, or in combination, on selected downstream signalling molecules such as erk1/2, p38 and JNK. Here, we showed that both SST and EGF influenced erk1/2 phosphorylation and that SST modulated the effects of EGF in a cell-specific manner. We also demonstrated agonist-, time and cell-dependent regulation of p38 phosphorylation. We further investigated modulation of Grb2, SOS, Shc, SH-PTP1 and SH-PTP2. ErbB1 adaptor proteins known to play a role in MAPK activation, Shc, Grb2 and SOS, changed in an agonist- and cell-specific manner whereas, SH-PTP1 and SH-PTP2, adaptor proteins reported to interact with SSTRs, translocated from the cytosol to membrane in a cell-specific manner following SST and/or EGF treatment. Although several previous studies have shown crosstalk between RTKs and GPCRs, there are no reports describing SSTR (GPCR) modulation of ErbBs (RTK) in breast cancer. To the best of our knowledge, this is the first report describing crosstalk/interactions between SSTRs and ErbBs.

Somatostatin/somatostatin receptor signalling: phosphotyrosine phosphatases

Activation of phosphotyrosine phosphatases (PTPs) by somatostatin receptor (SSTR) represents one of the main intracellular mechanisms involved in the antiproliferative effect of somatostatin (SST) and analogues. Since their molecular cloning, the role of PTPs is emerging as a major regulator of different cell functions including cell proliferation, differentiation, cell to cell interactions, cell matrix adhesion and cell migration. It was demonstrated that PTPs possess high substrate specificity and their activity is tightly regulated. Importantly, different G protein-coupled receptors transduce their biological activities through PTPs. PTPs were identified as down-stream effectors of SSTRs to transduce antiproliferative signals, and so far, three family members (SHP-1, SHP-2 and DEP-1/PTPeta) have been identified as selective SSTR intracellular effectors. Here, the molecular mechanisms leading SSTRs to regulate PTP activity are discussed, focusing on recent data showing a close interplay between PTPs and tyrosine kinases to transduce tumoral cell growth arrest following SST analogs administration.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

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.

Characterization of the Bystander Effect of Somatostatin Receptor sst2 After In Vivo Gene Transfer into Human Pancreatic Cancer Cells

Pancreatic cancer is one of the most aggressive and devastating human malignancies. The present study was conducted to determine whether in vivo sst2 gene transfer into human pancreatic tumors would impair tumor progression, and to characterize sst2 antitumoral bystander mechanisms. sst2 administration, using the synthetic vector PEI, strongly inhibited tumor progression of human pancreatic adenocarcinoma, in vivo. sst2 gene transfer induced intratumoral production of its ligand somatostatin. Disruption of this autocrine loop by RNA interference completely reversed sst2 antitumoral activity. Mice depleted of natural killer (NK) cells did not hamper sst2 tumor growth inhibition. However, microvessel density and vascular endothelial growth factor (VEGF) expression were markedly reduced in sst2-transfected tumors, whereas sst3 somatostatin receptor was upregulated. Depleting somatostatin by RNA interference completely abolished the sst2 inhibitory effect on VEGF expression and tumor angiogenesis, and sst2-induced sst3 expression in peripheral tumor vessels. We conclude that in vivo sst2 gene transfer elicited intratumoral somatostatin production and strongly impaired human pancreatic tumor growth. NK cells were not involved in this antitumoral bystander effect. VEGF and tumor vascularization were identified as novel targets for sst2-mediated antitumoral bystander effect. sst3 somatostatin receptor was upregulated in sst2-transfected tumors. Therefore, in vivo gene delivery of sst2 receptor to target the angiogenic process in pancreatic ductal adenocarcinoma might be a new therapeutic approach for treatment of pancreatic cancer in patients with unresectable disease.

VeloceGenomics: An Accelerated in Vivo Drug Discovery Approach to Rapidly Predict the Biologic, Drug-Like Activity of Compounds, Proteins, or Genes

PURPOSE

The aim of this study is to test the predictive power of in vivo multiorgan RNA expression profiling in identifying the biologic activity of molecules.

METHODS

Animals were treated with compound A or B. At the end of the treatment period, in vivo multiorgan microarray-based gene expression data were collected. Investigators masked to the identity of the compounds analyzed the transcriptome signatures to define the molecular pathways affected by treatment and to hypothesize the biologic activity and potential therapeutic indications of the blinded compounds.

RESULTS

For compound A, G-protein-coupled receptors and factors associated with cell growth were affected-growth hormone/insulin-like growth factor-1, glucagon/insulin axes, and general somatomedin-like activity. Deblinding showed the compound to be a somatostatin analog, SOM230, confirming the accuracy of the predicted biologic activity. For compound B, components of the inflammatory cascade potentially mediated by lipopolysaccharide, tumor necrosis factor, or proinflammatory cytokines were affected. The gene expression signatures were most consistent with an interleukin-6 family activity. Deblinding revealed that compound B was leukemia inhibitory factor.

CONCLUSIONS

VeloceGenomics is a strategy of coupling in vivo compound testing with genomic technologies. The process enables prediction of the mechanism of action and, coupled with other relevant data, prediction of the suitability of compounds for advancement in the drug development process.

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.

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.

Central and peripheral actions of somatostatin on the growth hormone-IGF-I axis

Somatostatin (SRIF) analogs provide safe and effective therapy for acromegaly. In a proportion of patients, however, SRIF analogs may lead to discordant growth hormone (GH) and IGF-I suppression, which suggests a more complex mechanism than attributable to inhibition of GH release alone. To elucidate whether SRIF acts peripherally on the GH-IGF-I axis, we showed that rat hepatocytes express somatostatin receptor subtypes-2 and -3 and that IGF-I mRNA and protein levels were suppressed in a dose-dependent manner by administration of octreotide. The inhibitory effect of SRIF was not apparent without added GH and in the presence of GH was specific for IGF-I induction and did not inhibit GH-induced c-myc or extracellular signal regulated kinase (ERK) phosphorylation. Pertussis toxin treatment of hepatocytes incubated with GH and SRIF, or with GH and octreotide, abrogated the inhibitory effect on GH-induced IGF-I, which confirms the requirement for the inhibitory G-protein. Treatment with SRIF and GH increased protein tyrosine phosphatase (PTP) activity and inhibited signal transducer and activator of transcription-5b (STAT5b) phosphorylation and nuclear localization. Octreotide also inhibited GH-stimulated IGF-I protein content of ex vivo-perfused rat livers. The results demonstrate that SRIF acts both centrally and peripherally to control the GH-IGF-I axis, providing a mechanistic explanation for SRIF analog action in treating patients with GH-secreting pituitary adenomas.

Molecular signaling of somatostatin receptors

Somatostatin is a neuropeptide family that is produced by neuroendocrine, inflammatory, and immune cells in response to different stimuli. Somatostatin acts as an endogenous inhibitory regulator of various cellular functions including secretions, motility, and proliferation. Its action is mediated by a family of G-protein-coupled receptors (called sst1-sst5) that are widely distributed in the brain and periphery. The five receptors bind the natural peptides with high affinity, but only sst2, sst5, and sst3 bind the short synthetic analogs used to treat acromegaly and neuroendocrine tumors. This review covers the current knowledge in somatostatin receptor biology and signaling.

Somatostatin receptors and regulation of cell proliferation

Somatostatin is an inhibitory neuropeptide, which acts on various targets throughout the body to regulate a variety of physiological functions including inhibition of endocrine and exocrine secretions, modulation of neurotransmission, motor and cognitive functions, inhibition of intestinal motility, absorption of nutrients and ions, vascular contractility and inhibition of normal and tumour cell proliferation. It exerts its effects through interaction with five somatostatin receptors (sst1-sst5), which belong to the family of G-protein-coupled receptors with seven transmembrane spanning domains and are variably expressed in a variety of tumours such as gastroenteropancreatic tumours, pituitary tumours, and carcinoid tumours. This review covers the present knowledge regarding the molecular mechanisms involved in somatostatin antineoplastic activity. Evidence that sst2 receptor acts as a tumour suppressor is also discussed.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.

Inhibitory effect of octreotide on gastric cancer growth via MAPK pathway

AIM: Somatostatin and its analogues may suppress the growth of various tumor cells. However, the effect of octreotide on growth of gastric adenocarcinoma is still largely unknown. This study was to explore if octreotide could inhibit the growth of gastric adenocarcinoma and its probable mechanisms.

METHODS: Proliferation of gastric cancer cell line affected by octreotide was determined by ³H-thymidine incorporation. After xenografts of human gastric cancer were implanted orthotopically in stomach, nude mice were administrated octreotide for 8 weeks. The mRNA of somatostatin receptor in the SGC-7901 cells was detected by reverse transcription polymerase chain reaction technique. Extracellular signal-regulated protein kinase and c-Fos in gastric cancer tissues were measured by immunohistochemistry and Western blot. Activator protein-1 binding activity was examined by electrophoretic mobility sift assay.

RESULTS: ³H-thymidine incorporation into SGC-7901 cells was significantly decreased by octreotide in a concentration dependent manner. Either size or weight of tumors treated with octreotide was significantly reduced in vivo. The inhibition rate for tumor was 62.3% in octreotide group. The genes of somatostatin receptors 2 and 3 were expressed in SGC-7901 gastric cancer cell lines. Extracellular signal-regulated protein kinase and c-Fos protein level were decreased in gastric adenocarcinoma treated with octreotide. Moreover, fetal calf serum stimulated activator protein-1 binding activity could be suppressed by octreotide potentially.

CONCLUSION: Inhibition of sequential molecular events in MAPK pathway may interpret the mechanisms underlying the effect of octreotide on the growth of gastric adenocarcinoma.

Interaction of transforming growth factor-beta (TGF-beta) and epidermal growth factor (EGF) in human glioma cells

Gliomas are characterized by a deregulation of growth factor production and growth factor receptors expression, e.g. overproduction of the cytokine transforming growth factor-beta (TGF-beta) and overexpression/constitutive activation of receptors for the epidermal growth factor (EGF). Potential interactions of such growth factors and their signaling cascades could enhance the malignancy of these tumors. Therefore, we investigated the effects of TGF-beta and EGF alone and in combination on the proliferation of glioma cells cultivated from eight solid human WHO grade IV gliomas and one glioma cell line, analyzed the expression and intactness of the TGF-beta-signaling molecules Samd-4 and -2, and the phosphorylation of the EGF-signaling kinases ERK 1/2. The effects were divergent and complex: Whereas EGF mostly stimulated glioma cell proliferation, TGF-beta either enhanced, inhibited or had no significant effect on proliferation. In combination, co-stimulation and inhibition of the EGF-induced mitogenic activity could be observed. Smad-4/-2 were expressed in all glioma cells, one point mutation at base 1595 in Smad-4 did not affect its protein sequence. In part of the glioma cells, reduced phosphorylation of ERK 1/2 and expression of cyclin-dependent kinase inhibitor 1 or p21 was observed in co-stimulation experiments. These experiments show that TGF-beta can inhibit EGF-mediated effects only in some gliomas, whereas it enhances it in others. The interaction of both factors is very complex and varies between different gliomas.

The angiogenic peptide vascular endothelial growth factor (VEGF) is expressed in chronic sacral pressure ulcers

Chronic decubital lesions have a limited potential to heal. Evidence suggests that a lack of local revascularization is involved in this aetiology. The present study investigated the expression of one of the most important angiogenic factors, the vascular endothelial growth factor (VEGF), in different regions of sacral chronic decubital lesions and in normal skin by immunohistochemical, biochemical, molecular, and cell biology methods. To elucidate some of the factors responsible for the induction of VEGF in chronic skin ulcers, cultured fibroblasts were exposed to hypoxia and/or growth factors. In the central part (zone I) of chronic ulcers and in normal skin, immunostaining for VEGF remained largely negative. However, VEGF could be immunostained in cells in the granulation tissue adjacent to central necrosis (zone II). VEGF receptor 2 (VEGFR-2; KDR) could also be identified in microvessels. High VEGF levels were present in homogenates from granulation tissue by enzyme-linked immunosorbent assay (ELISA) and western blot experiments: low concentrations were found in areas of central necrosis and negligible amounts were present in normal skin. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed that only the splice variants VEGF(121) and VEGF(165) were expressed. In cultured fibroblasts, hypoxia or platelet-derived growth factor (PDGF) raised VEGF production. The angiogenic peptide VEGF is present in all zones of chronic decubital ulcers. Its strong expression within the adjacent granulation tissue supports the view that there is no deficiency of VEGF. VEGF may be involved in the healing process of chronic skin lesions, but it seems that loss of another factor may be responsible for the poor healing response.

Vascular endothelial growth factor induces chemotaxis and proliferation of microglial cells

Vascular endothelial growth factor (VEGF) is an angiogenic peptide that is produced in the brain after ischemia, injury or in malignant gliomas. Since these pathological conditions are associated with the infiltration of microglial cells, we investigated the expression of VEGF receptors (VEGFR) and possible effects of VEGF on cultivated microglial cells. As shown by reverse transcription-polymerase chain reaction and immunocytochemistry, rat microglial cells as well as the murine cell line BV-2 express the VEGFR-1, but not VEGFR-2. Murine VEGF induced 3H-thymidine incorporation into DNA of murine and rat microglial cells as well as chemotaxis in Boyden chamber assays. However, VEGF did not alter the phosphorylation of mitogen-activated protein kinases and only slightly that of the kinase Akt. These results show that microglial cells are targets for VEGF which induces migration and proliferation of these immunocompetent cells in the brain.

Somatostatin inhibits glucagon-like peptide-1-induced insulin secretion and proliferation of RINm5F insulinoma cells

Glucagon-like peptide-1 [GLP-1; formerly GLP-1(7-36)amide] and somatostatin (SS) are two postprandially or paracrine released peptide hormones that regulate insulin secretion from pancreatic islets. Using the rat insulinoma cell line RINm5F as a model, we investigated the effects of both peptides alone and in combination on insulin release, proliferation, and intracellular signal transduction. In addition, we determined the SS receptor subtypes expressed and involved by reverse transcription-polymerase chain reaction and use of selective SS agonists. GLP-1 stimulated insulin release, cell proliferation, intracellular cAMP accumulation and activation of the transcription factor cAMP-response element binding protein (CREB) which all could be reduced to basal values by co-incubation with SS. Incubation with SS alone did not affect basal levels. RINm5F cells express the somatostatin receptor (sst) subtypes sst1 and sst2 as well as traces of sst3. In accordance, the sst1- or sst2-selective non-peptide agonists L-797591 or L-054522 and peptide agonist octreotide (SMS 201995; sst2, sst3, and sst5 selective) potently inhibited GLP-1-induced insulin secretion whereas the sst3-selective agonist L-796778 showed little effect. Moreover, the sst1- and sst2-selective agonists slightly reduced also basal insulin release. The experiments show that GLP-1 and SS are perfect opponents for regulating pancreatic beta-cell insulin secretion.