Induction of a negative autocrine loop by expression of sst2 somatostatin receptor in NIH 3T3 cells

Human somatostatin receptor-3 distinctively induces apoptosis in MCF-7 and cell cycle arrest in MDA-MB-231 breast cancer cells

Somatostatin (SST) mediates cytostatic and pro-apoptotic effects through five somatostatin receptors (SSTR1-5). The modest clinical benefits of SST analogs in cancers of different origin such as breast cancer are attributed to diminished SSTRs expression at tumor sites. In the present study, SSTR3 was overexpressed in MCF-7 and MDA-MB-231, and analyzed for downstream signaling molecules associated with cytostatic and cytotoxic effect. Cells overexpressing SSTR3 displayed inhibition of EGF induced proliferation and enhanced antiproliferative effect of SSTR3-specific agonist in comparison to non-transfected cells. SSTR3 overexpression in MCF-7 cells (R3-MCF-7) constitutively enhanced TUNEL staining, PARP-1 and p27(Kip1) expression suggesting apoptosis and cell-cycle arrest. Conversely, R3-MB-231 cells with SSTR3 overexpression exerted cytostatic and were devoid of any cytotoxic effects. The expression of PTP-1C and the status of ERK1/2, p38 and PI3K phosphorylation was modulated in a cell-specific manner. These findings provide new insights in understanding the antiproliferative role of SSTR3 in breast tumor biology.

Somatostatin analogs: does pharmacology impact antitumor efficacy?

Somatostatin is an endogenous inhibitor of secretion and cell proliferation. These features render somatostatin a logical candidate for the management of neuroendocrine tumors that express somatostatin receptors. Synthetic somatostatin analogs (SSAs) have longer half-lives than somatostatin, but have similar activities, and are used for the treatment of these types of disorders. Interest has focused on novel multireceptor analogs with broader affinity to several of the five somatostatin receptors, thereby presenting putatively higher antitumor activities. Recent evidence indicates that SSAs cannot be considered mimics of native somatostatin in regulating signaling pathways downstream of receptors. Here we review this knowledge, discuss the concept of biased agonism, and highlight what considerations need to be taken into account for the optimal clinical use of SSAs.

Intermediary quantitative traits--an alternative in the identification of disease genes in asthma?

Intermediary quantitative traits are a possible alternative for the identification of disease genes. This may be particularly relevant when diagnostic criteria are not very well defined as described for asthma. We analyzed serum samples from 944 individuals of 218 asthma families for 17 cytokines (eotaxin, GM-CSF, IFNγ, IL1B, IL1RA, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12(p40), IL-13, IL-17, IL-23, IL-33, TSLP and TNF-α) and determined the heritability. Linked chromosomal regions were identified by a genome-wide analysis using 334 autosomal microsatellite marker and association tested by further 550 SNP marker at genes implicated earlier with immune response. Heritability varied with TNF-α and IL-8 levels having the highest and TSLP having the lowest heritability. Linkage was significantly increased only for IL-12(p40) at D17S949. There were multiple significant single-nucleotide polymorphisms (SNP) associations (P<0.05) as found in the transmission disequilibrium test, whereas only a few replicated in parents or children only. These include SNPs in IL1RN that were associated with IL-33 and TSLP levels, and a SNP in NR3C2 that was associated with eotaxin, IL-13 and IFN-γ levels. Circulating level of serum cytokines exhibits genetic associations with asthma traits that are otherwise not detected using clinical diagnosis or when the clinical details are ambiguous.

Efficacy and tolerability of long-acting octreotide in the treatment of thymic tumors: results of a pilot trial

OBJECTIVES

Octreotide is a somatostatin analog, long-acting formulations of which have been used experimentally for the treatment of patients with invasive tumors and/or residual disease after conventional therapies. The objective of this retrospective study was to evaluate the efficacy of long-acting octreotide (Sandostatin LAR) for the treatment of thymic tumors, with a primary efficacy end point of progression-free survival.

METHODS

Between 1994 and 2010, 44 patients with thymic malignancies were evaluated. Twenty-seven patients underwent an OctreoScan, and 12 OctreoScan-positive patients were treated with long-acting octreotide at a dose of 20 mg, given as an intramuscular injection, every 2 weeks.

RESULTS

Treatment with long-acting octreotide gave the following results: 3 cases of partial response (25%), 5 cases of stable disease (42%), and 4 cases of progressive disease (33%), with an average progression-free survival of 8 months (range, 3 to 21). Treatment compliance and tolerability were good for all evaluated patients.

CONCLUSIONS

The results of this study confirm the somatostatin receptor as a valid target for the treatment of thymic malignancies. Overall, therapy with long-acting somatostatin analogs seems to be safe and effective.

Somatostatin receptors in non-neuroendocrine malignancies: the potential role of somatostatin analogs in solid tumors

Somatostatin receptors (sstrs) are G-protein-coupled receptors that mediate various physiological effects when activated by the neuropeptide somatostatin or its synthetic analogs. In addition to the well-documented antisecretory effects of sstr2-preferential somatostatin analogs octreotide and lanreotide, ligand binding to sstr initiates an inhibitory action on tumor growth. This effect may result from both indirect actions (suppression of growth factors and growth-promoting hormones [e.g., GH/IGF-1 axis] and inhibition of angiogenesis) and direct actions (activation of antigrowth activities [e.g., apoptosis]). As solid tumor cells express multiple sstrs, there is a rationale to evaluate the potential antitumor effects of pasireotide (SOM230), a multireceptor-targeted somatostatin analog with high binding affinity for sstr1–3 and sstr5. Pasireotide reduces systemic IGF-1 levels more potently than currently available somatostatin analogs and has been well tolerated in clinical trials.

Replication-deficient rSV40 mediate pancreatic gene transfer and long-term inhibition of tumor growth

Pancreatic cancer is one of the most aggressive and devastating human malignancies. There is an urgent need for more effective therapy for patients with advanced disease. In this context, genetic therapy potentially represents a rational new approach to treating pancreatic cancer, which could provide an adjunct to conventional options. Because of the promise of recombinant SV40 vectors, we tested their ability to deliver a transgene, and to target a transcript, so as to inhibit pancreatic tumors growth in vivo. BxPC3 and Capan-1 cells were efficiently transduced using SV40 vectors without selection, as compared to synthetic vectors PEI. SV40 vectors were as efficient as adenoviral vectors, and provided long-term transgene expression. Next, we devised a SV40-derived, targeted gene therapy approach of pancreatic cancer, by combining hTR tumor-specific promoter with sst2 somatostatin receptor tumor-suppressor gene. In vitro cell proliferation was strongly impaired following administration of SV(hTR-sst2). SV40-derived sst2-mediated antiproliferative effect was dependent on the local production of somatostatin. In vivo, intratumoral gene transfer of sst2 using rSV40 vectors resulted in a marked inhibition of Capan-1 tumor progression, and proliferation. These results represent the initial steps toward a novel approach to the gene therapy of pancreatic cancer using SV40 as a vector.

A genome-wide search identifies epigenetic silencing of somatostatin, tachykinin-1, and 5 other genes in colon cancer

BACKGROUND & AIMS

Gene silencing via promoter hypermethylation is a central event in the pathogenesis of cancers. To identify novel methylation targets in colon cancer, we conducted a genome-wide, microarray-based, in silico, and epigenetic search.

METHODS

Complementary DNA microarray experiments were first performed to identify genes down-regulated in primary colon cancers and up-regulated in colon cancer cell lines after global DNA demethylation by 5-aza-2'-deoxycitidine. Candidate methylation targets were then identified by combining these microarray data with in silico genetic and functional searches. Candidate genes recognized by these searches were further investigated for promoter hypermethylation in colon cancer using methylation-specific polymerase chain reaction.

RESULTS

We identified 51 novel and 3 known candidate methylation targets. Subsequent epigenetic analysis revealed that primary colon cancers demonstrated frequent methylation of somatostatin (SST, 30 of 34 cases, 88%) and the substance P precursor gene tachykinin-1 (TAC1; 16 of 34 cases, 47%). TAC1 methylation intensity was significantly higher in Dukes A/B than in Dukes C/D cancers (P = .01). SST methylation intensity was significantly higher in low-level microsatellite instability (MSI-L) than in non-MSI-L cancers (P = .02). Methylation was associated with messenger RNA down-regulation for both SST and TAC1. Furthermore, we isolated 5 additional novel promoter methylation targets: NELL1, AKAP12, caveolin-1, endoglin, and MAL.

CONCLUSIONS

These data strongly suggest that SST and TAC1 are involved in colon carcinogenesis. Further studies are now indicated to elucidate mechanisms underlying their involvement in colon cancer and their values as clinical biomarkers. NELL1, AKAP12, caveolin-1, endoglin, and MAL are also promising tumor suppressor gene candidates deserving of further study.

Inhibition of metastatic progression of SSTR2 gene transfection mediated by adenovirus in human pancreatic carcinoma cells

The inhibition of metastatic progression of Somatostatin receptor type 2 (SSTR2) gene transfection mediated by adenovirus in human pancreatic carcinoma cells and the mechanisms involved in this effect were studied. The full-length human SSTR2 cDNA was introduced into the pancreatic cancer cell line BXPC-3 by adenovirus-mediated transfection. Stable expression of mRNAs and protein of SSTR2 was detected by RT-PCR and Western-blot. The Matrigel-coated Transwell was used to detect the migratory and invasive ability of SSTR2-expressing cells, Adv-GFP control cells and mock control cells. Furthermore, the expression of matrix metalloproteinase-2 (MMP-2) and tissue inhibitor of metalloproteinase-2 (TIMP-2) was detected by RT-PCR in these cells. The stable expression of SSTR2 was detected in BXPC-3 transfected by Adv-GFP-SSTR2. A dramatic decrease of BXPC-3 expressing sst2 cells migrating through a Matrigel-coated filter was observed, as compared with Adv-GFP control and mock control cells (P < 0.01). Moreover, the expression of MMP-2 mRNA was significantly reduced in the SSTR2-expressing cells and conversely the expression of TIMP-2 mRNA was significantly increased in the SSTR2-expressing cells when compared with the Adv-GFP control and mock control (P < 0.01). The expression of reintroduced human SSTR2 gene in BXPC-3 cells by Adv-GFP-SSTR2 had the anti-migratory and anti-invasive effects, and the mechanisms involved in this effect may be due to the down-regulated expression of MMP-2 and up-regulated expression of TIMP-2.

Novel synergistic mechanism for sst2 somatostatin and TNFα receptors to induce apoptosis: crosstalk between NF-κB and JNK pathways

Somatostatin is a multifunctional hormone that modulates cell proliferation, differentiation and apoptosis. Mechanisms for somatostatin-induced apoptosis are at present mostly unsolved. Therefore, we investigated whether somatostatin receptor subtype 2 (sst2) induces apoptosis in the nontransformed murine fibroblastic NIH3T3 cells. Somatostatin receptor subtype 2 expression induced an executioner caspase-mediated apoptosis through a tyrosine phosphatase SHP-1 (Src homology domain phosphatase-1)-dependent stimulation of nuclear factor kappa B (NF-kappaB) activity and subsequent inhibition of the mitogen-activated protein kinase JNK. Tumor necrosis factor alpha (TNFalpha) stimulated both NF-kappaB and c-Jun NH2-terminal kinase (JNK) activities, which had opposite action on cell survival. Importantly, sst2 sensitized NIH3T3 cells to TNFalpha-induced apoptosis by (1) upregulating TNFalpha receptor protein expression, and sensitizing to TNFalpha-induced caspase-8 activation; (2) enhancing TNFalpha-mediated activation of NF-kappaB, resulting in JNK inhibition and subsequent executioner caspase activation and cell death. We have here unraveled a novel signaling mechanism for a G protein-coupled receptor, which directly triggers apoptosis and crosstalks with a death receptor to enhance death ligand-induced apoptosis.

Transfection of SSTR-1 and SSTR-2 inhibits Panc-1 cell proliferation and renders Panc-1 cells responsive to somatostatin analogue

BACKGROUND

Somatostatin inhibits cell proliferation through interaction with its cellular receptor, somatostatin receptors (SSTRs). We have previously demonstrated that overexpression of individual SSTR-1 or SSTR-2 genes in receptor-negative pancreatic cancer cells inhibited cell proliferation. We hypothesize that reintroduction of SSTR genes back into pancreatic cancer cells might be an effective gene therapy strategy for pancreatic cancer.

STUDY DESIGN

We transfected human pancreatic cancer cell line (Panc-1) with human SSTR-1 and SSTR-2 genes and examined the expression by real-time reverse transcriptase-polymerase chain reaction and immunofluorescence. Panc-1 cell proliferation was determined by [(3)H]-thymidine incorporation assay. Activation of phosphorylated c-Jun N-terminal protein kinase (JNK) and cytokine secretion after SSTR-1 and SSTR-2 transfection were detected by Bio-Plex 4-plex phosphoprotein assay and cytokine assay.

RESULTS

Panc-1 cells did not express SSTR-1 or SSTR-2, although Panc-1 cells transfected with SSTR-1 and SSTR-2 genes showed a significant amount of SSTR expression. Cell growth rate in Panc-1 cells transfected with SSTR-1 and SSTR-2 was inhibited about 41%, and the cell proliferation of Panc-1 expressing SSTR-1 and SSTR-2 was further reduced about 12% on treatment with somatostatin analogue as compared with the control group. SSTR-1 and SSTR-2 cotransfected Panc-1 cells activated phosphorylation of JNK and increased secretion of interferon-gamma and interleukin-5.

CONCLUSIONS

These findings suggest, for the first time, a synergistic inhibitory effect of multiple SSTRs in response to a somatostatin analogue in Panc-1 cells. These studies may improve our understanding of the mechanism by which SSTR inhibits cell growth and lead to novel gene therapies for pancreatic cancer.

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.

Restoration of functional gap junctions through internal ribosome entry site-dependent synthesis of endogenous connexins in density-inhibited cancer cells

Gap junctions are composed of connexins and are critical for the maintenance of the differentiated state. Consistently, connexin expression is impaired in most cancer cells, and forced expression of connexins following cDNA transfection reverses the tumor phenotype. We have found that the restoration of density inhibition of human pancreatic cancer cells by the antiproliferative somatostatin receptor 2 (sst2) is due to overexpression of endogenous connexins Cx26 and Cx43 and consequent formation of functional gap junctions. Immunoblotting along with protein metabolic labeling and mRNA monitoring revealed that connexin expression is enhanced at the level of translation but is not sensitive to the inhibition of cap-dependent translation initiation. Furthermore, we identified a new internal ribosome entry site (IRES) in the Cx26 mRNA. The activity of Cx26 IRES and that of the previously described Cx43 IRES are enhanced in density-inhibited cells. These data indicate that the restoration of functional gap junctions is likely a critical event in the antiproliferative action of the sst2 receptor. We further suggest that the existence of IRESes in connexin mRNAs permits connexin expression in density-inhibited or differentiated cells, where cap-dependent translation is generally reduced.

Somatostatin, Somatostatin Receptors, and Pancreatic Cancer

Somatostatin may play an important role in the regulation of cancer growth including pancreatic cancer by interaction with somatostatin receptors (SSTRs) on the cell surface. Five SSTRs were cloned, and the function of these SSTRs is addressed in this review. SSTR-2, SSTR-5, and SSTR-1 are thought to play major roles in inhibiting pancreatic cancer growth both in vitro and in vivo. SSTR-3 may be involved in mediating apoptosis, but the role of SSTR-4 is not clear. In most pancreatic cancers, functional SSTRs are absent. Reintroduction of SSTR genes has been shown to inhibit pancreatic cancer growth in cell cultures and animal models.

[Phosphotyrosine phosphatase SHP-1, somatostatin and prostate cancer]

We review the mechanisms involved in prostatic growth based on androgens and product of neuroendocrine secretion, with special reference to the role of somatostatin (SS) in the inhibition of neoplastic growth. Our contributions in the field confirm the antiproliferative effect of SS on the prostate is mediated by phosphotyrosine phosphatase SHP-1, that is present in human prostate. This enzyme plays a role in the control of prostatic cell proliferation and in the progression of prostate cancer. Besides, we consider its presence may determine the therapeutic potential of SS in the control of prostate cancer.

Characterization of somatostatin receptor expression in human pancreatic cancer using real-time RT-PCR

BACKGROUND

Somatostatin inhibits cell proliferation and may act as a tumor suppressor by interacting with five different somatostatin receptors (SSTRs). We hypothesized that SSTR expression is down-regulated in human pancreatic cancer. In the current study, we used a powerful real-time RT-PCR technique to examine the mRNA expression levels of all five SSTR subtypes in human pancreatic cancer.

MATERIALS AND METHODS

Total RNA was extracted from three pancreatic cancer cell lines (Panc-1, MIA PaCa-2, and Hs 766T), three surgical specimens of pancreatic cancer, and adjacent pancreatic tissue, and a pancreatic cancer cell line transfected with the SSTR-2 gene. Specific primers were designed and mRNA levels for the five SSTRs were analyzed with real-time quantitative RT-PCR using a Bio-Rad iCycler system.

RESULTS

The pancreatic tumor specimens had a 2.5- and 4.3-fold reduction of SSTR-2 and SSTR-5 mRNA levels, respectively, as compared to their adjacent normal pancreatic tissues. SSTR-1 and SSTR-3 were also detected in both the cancer specimens and the adjacent tissues, but SSTR-4 was absent. Human pancreatic cancer cell lines also expressed SSTR-2 and SSTR-5 mRNA, but not SSTR-1, SSTR-3, and SSTR-4. Up-regulation of SSTR-2 mRNA by 2.2 x 10(4)-fold in Panc-1 cells resulted in receptor expression and growth inhibition.

CONCLUSION

Expression of SSTR-2 and SSTR-5 could be important in the growth inhibitory effect of somatostatin in human pancreatic cancer. Down-regulation of SSTR transcription or SSTR mRNA instability may result in loss of a tumor suppressive effect of SSTRs in human pancreatic cancer.

Octreotide alone or with prednisone in patients with advanced thymoma and thymic carcinoma: an Eastern Cooperative Oncology Group Phase II Trial

PURPOSE

To determine the objective response rate, duration of remission and toxicity of octreotide alone or with the later addition of prednisone in patients with unresectable, advanced thymic malignancies in whom the pretreatment octreotide scan was positive.

PATIENTS AND METHODS

Forty-two patients with advanced thymoma or thymic carcinoma were entered onto the trial, of whom 38 were fully assessable (one patient had inconclusive histology; three patients had negative octreotide scan). Patients received octreotide 0.5 mg subcutaneously tid. At 2 months, patients were evaluated. Responding patients continued to receive octreotide alone; patients with progressive disease were removed from the study. All others received prednisone 0.6 mg/kg orally qid for a maximum of 1 year.

RESULTS

Two complete (5.3%) and 10 partial responses (25%) were observed (four partial responses with octreotide alone; the remainder with octreotide plus prednisone). None of the six patients without pure thymoma responded. The 1- and 2-year survival rates were 86.6% and 75.7%, respectively. Patients with an Eastern Cooperative Oncology Group performance status of 0 lived significantly longer than did those with a performance status of 1 (P =.031).

CONCLUSION

Octreotide alone has modest activity in patients with octreotide scan-positive thymoma. Prednisone improves the overall response rate but is associated with increased toxicity. Additional studies with the agent are warranted.

Antiangiogenic effect of somatostatin receptor subtype 2 on pancreatic cancer cell line: Inhibition of vascular endothelial growth factor and matrix metalloproteinase-2 expression in vitro

AIM: To investigate the anti-angiogenic effect of somatostatin receptor subtype 2 (SSTR2) gene transfer into pancreatic cancer cell line PC-3, and the mechanisms involved in this effect.

METHODS: The full length human SSTR2 cDNA was introduced into pancreatic cancer cell line PC-3 by lipofectamine-mediated transfection. Positive clones were screened by G418 and stable expression of SSTR2 was detected by immunohistochemistry SABC methods and RT-PCR. Enzyme-linked immunosorbent assay (ELISA) was used to detect vascular endothelial growth factor (VEGF) levels in the cell culture supernatants of SSTR2-expressing cells, vector control and mock control cells. Furthermore, the expressions of VEGF and matrix metalloproteinase-2 (MMP-2) were detected by immunohistochemistry SABC methods and RT-PCR in these cells.

RESULTS: VEGF levels in the cell culture supernatants were significantly reduced in the SSTR2-expressing cells (first week, 172.63 ± 21.2 ng/L and after two months, 198.85 ± 26.44 ng/L) compared with the vector control (first week, 790.39 ± 86.52 ng/L and after two months, 795.69 ± 72.35 ng/L) and mock control (first week, 786.42 ± 90.62 ng/L and after two months, 805.32 ± 84.36 ng/L) (P < 0.05). The immunohistochemical assay showed a significant reduction of the integral optical density of VEGF and MMP-2 in the SSTR2-expressing cells (42.25 ± 8.6 and 70.5 ± 6.25, respectively) compared with the vector control (85.75 ± 12.9 and 110.52 ± 13.5, respectively) and mock control (82.6 ± 9.28 and 113.56 ± 9.62, respectively) (P < 0.05). Conversely, the average gray value of VEGF and MMP-2 was significantly increased in the SSTR2-expressing cells (121.56 ± 8.43 and 134.46 ± 19.95, respectively) compared with the vector control (55.72 ± 5.6 and 62.26 ± 12.68, respectively) and mock control cells (58.48 ± 6.2 and 65.49 ± 9.16, respectively) (P < 0.05). Moreover, the expressions of VEGF mRNA and MMP-2 mRNA were significantly reduced in the SSTR2-expressing cells (0.1384 ± 0.017 and 0.2343 ± 0.070, respectively) compared with the vector control (1.024 ± 0.117 and 0.806 ± 0.119, respectively) and mock control (1.085 ± 0.105 and 0.714 ± 0.079, respectively) (P < 0.05).

CONCLUSION: The expression of reintroduced human SSTR2 gene exerts its antiangiogenic effects by down-regulating the expressions of the factors involved in tumor angiogenesis and metastasis, suggesting SSTR2 gene transfer as a new strategy of gene therapy for pancreatic cancer.

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.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Sandostatin LAR (long-acting octreotide acetate) for malignant carcinoid syndrome: a 3-year experience

BACKGROUND

Somatostatin analogues are the best therapy for controlling the symptoms of malignant carcinoid syndrome. Octreotide acetate given as subcutaneous injection up to three times daily, intramuscular Lanreotide injection given once per 1-2 weeks and monthly intramuscular Sandostatin LAR have demonstrated similar efficacy in short-term studies.

AIM

To assess the long-term effect of Sandostatin LAR on the management of patients with malignant carcinoid syndrome.

METHODS

This was a 3-year retrospective study. Twenty-seven patients were assessed with a median follow-up of 23 months. Thirteen patients were switched from subcutaneous octreotide and 14 patients were octreotide naive. All patients showed avid uptake on indium-111 octreotide imaging.

RESULTS

Ten of the 13 patients previously on subcutaneous octreotide and 13 of the 14 patients who were octreotide naive had good symptom control on Sandostatin LAR. Over the period of follow-up, many patients showed progression of their tumour and required additional therapies. Patients expressed a preference for monthly intramuscular Sandostatin LAR as opposed to daily subcutaneous injections of octreotide. Although Sandostatin LAR was difficult to administer in certain instances, overall it was well tolerated.

CONCLUSIONS

Sandostatin LAR provides good long-term symptomatic control in patients with malignant carcinoid syndrome; it is well tolerated and patients expressed improved satisfaction in their management.

Somatostatin receptor subtype 2 sensitizes human pancreatic cancer cells to death ligand-induced apoptosis

Somatostatin receptor subtype 2 (sst2) gene expression is lost in 90% of human pancreatic adenocarcinomas. We previously demonstrated that stable sst2 transfection of human pancreatic BxPC-3 cells, which do not endogenously express sst2, inhibits cell proliferation, tumorigenicity, and metastasis. These sst2 effects occur as a consequence of an autocrine sst2-dependent loop, whereby sst2 induces expression of its own ligand, somatostatin. Here we investigated whether sst2 induces apoptosis in sst2-transfected BxPC-3 cells. Expression of sst2 induced a 4.4- +/- 0.05-fold stimulation of apoptosis in BxPC-3 through the activation of tyrosine phosphatase SHP-1. sst2 also sensitized these cells to apoptosis induced by tumor necrosis factor alpha (TNFalpha), enhancing it 4.1- +/- 1.5-fold. Apoptosis in BxPC-3 cells mediated by TNF-related apoptosis-inducing ligand (TRAIL) and CD95L was likewise increased 2.3- +/- 0.5-fold and 7.4- +/- 2.5-fold, respectively. sst2-dependent activation and cell sensitization to death ligand-induced apoptosis involved activation of the executioner caspases, key factors in both death ligand- or mitochondria-mediated apoptosis. sst2 affected both pathways: first, by up-regulating expression of TRAIL and TNFalpha receptors, DR4 and TNFRI, respectively, and sensitizing the cells to death ligand-induced initiator capase-8 activation, and, second, by down-regulating expression of the antiapoptotic mitochondrial Bcl-2 protein. These results are of interest for the clinical management of chemoresistant pancreatic adenocarcinoma by using a combined gene therapy based on the cotransfer of genes for both the sst2 and a nontoxic death ligand.

Antiproliferative activity induced by the somatostatin analogue, TT-232, in human pancreatic cancer cells

Somatostatin analogues have been developed as antiproliferative agents, but their administration as general antitumour agents is limited, mainly because of the wide distribution of somatostatin receptors throughout the human body. TT-232, a new somatostatin structural analogue, was reported to have tumour-selective antiproliferative activity without an antisecretory action. We examined whether TT-232 had antiproliferative activity in human pancreatic cancer cell lines, and compared its antiproliferative activity with that of RC-160 and other TT-232 derivatives. TT-232 inhibited the growth of all of the cell lines used in this study and induced apoptotic cell death. RC-160 showed no such growth inhibition. TT-232 also inhibited tumour formation in a xenograft model. A competitive binding assay was performed using the cell membrane fraction and 111In-DTPA-TT-232 in order to show the existence of a specific binding site on the cells. A specific binding site was detected in MIAPaCa-2 cells. It has been shown that the activation of protein tyrosine phosphatase (PTPase) is one of the main intracellular pathways responsible for somatostatinergic inhibition of cell growth. We found a significant PTPase stimulation after TT-232 administration using an immunoblot analysis assessing the level of protein tyrosine phosphorylation, and also a direct measurement of the PTPase activity. We also demonstrated that PTPase stimulation by TT-232 was involved in its antiproliferative activity as this activity was reversed by the addition of sodium orthovanadate, a PTPase inhibitor. Our results indicate that TT-232 could be a potentially useful therapeutic agent if these data are translated into clinical practice.

Cellular biology of somatostatin receptors

Somatostatin, and the recently discovered neuropeptide cortistatin, exert their physiological actions via a family of six G protein-coupled receptors (sst1, sst2A, sst2B, sst3, sst4, sst5). Following the cloning of somatostatin receptors significant advances have been made in our understanding of their molecular, pharmacological and signaling properties although much progress remains to be done to define their physiological role in vivo. In this review, the present knowledge regarding neuroanatomical localization, signal transduction pathways, desensitization and internalization properties of somatostatin receptors is summarized. Evidence that somatostatin receptors can form homo- and heterodimers and can physically interact with members of the SSTRIP/Shank/ProSAP1/CortBP1 family is also discussed.

Somatostatin receptor imaging, therapy and new strategies in patients with neuroendocrine tumours

BACKGROUND

Somatostatin receptors have been found on a variety of neuroendocrine tumours, such as carcinoids and paragangliomas, as well as on most pancreatic endocrine and breast tumours. Somatostatin receptor scintigraphy with a radionuclide-labelled somatostatin analogue, [111Indium- diethylenetriaminopenta-acetic acid]octreotide, is a sensitive and specific technique for visualizing in vivo the presence of somatostatin receptors on various tumours.

METHODS

Material was identified from previous review articles, references cited in original papers and a Medline search of the literature. Additional material was obtained from recently published abstracts of meetings.

RESULTS AND CONCLUSION

Somatostatin receptor imaging of neuroendocrine tumours is essential in the diagnostic evaluation of most of these tumours. The expression of somatostatin receptors in vivo not only predicts the outcome of somatostatin analogue treatment but also opens the possibility of new therapeutic strategies. Because better information about spread of the disease can be obtained, more justifiable options for therapy can be proposed.

DOTA-lanreotide: a novel somatostatin analog for tumor diagnosis and therapy

Long acting somatostatin-14 (SST) analogs are used clinically to inhibit tumor growth and proliferation of various tumor types via binding to specific receptors (R). We have developed a 111In-/90Y-labeled SST analog, DOTA-(D)betaNal1-lanreotide (DOTALAN), for tumor diagnosis and therapy. 111In-/90Y-DOTALAN bound with high affinity (dissociation constant, Kd, 1-12 nM) to a number of primary human tumors (n = 31) such as intestinal adenocarcinoma (n = 17; 150-4000 fmol/mg protein) or breast cancer (n = 4; 250-9000 fmol/mg protein). 111In-/90Y-DOTALAN exhibited a similar high binding affinity (Kd, 1-15 nM) for the human breast cancer cell lines T47D and ZR75-1, the prostate cancer cell lines PC3 and DU145, the colonic adenocarcinoma cell line HT29, the pancreatic adenocarcinoma cell line PANC1, and the melanoma cell line 518A2. When expressed in COS7 cells, 111In-DOTALAN bound with high affinity to hsst2 (Kd, 4.3 nM), hsst3 (Kd, 5.1 nM), hsst4 (Kd, 3.8 nM), and hsst5 (Kd, 10 nM) and with lower affinity to hsst1 (Kd, approximately 200 nM). The rank order of displacement of [125I]Tyr11-SST binding to hsst1 was: SST (IC50, 0.5 nM) >> DOTALAN (IC50, 154 nM) > lanreotide (LAN) approximate to Tyr3-octreotide (TOCT) approximate to DOTA-Tyr3-octreotide (DOTATOCT) approximate to DOTA-vapreotide (DOTAVAP; IC50, >1000 nM); that to hsst2 was: DOTATOCT approximate to TOCT approximate to DOTALAN approximate to SST approximately LAN approximate to DOTAVAP (IC50, 1.4 nM); that to hsst3 was: SST (IC50, 1.2 nM) > DOTALAN = LAN (IC50, 15 nM) approximate to TOCT (IC50, 20 nM) approximate to DOTAVAP (IC50, 28 nM) > DOTATOCT (IC50, 73 nM); that to hsst4 was: SST (IC50, 1.8 nM) approximate to DOTALAN (IC50, 2.5 nM) > LAN (IC50, 22 nM) >> DOTATOCT approximate to DOTAVAP approximate to TOCT (IC50, >500 nM); and that to hsst5 was: DOTALAN (IC50, 0.45 nM) > SST (IC50, 0.9 nM) > TOCT (IC50, 1.5 nM) > DOTAVAP (IC50, 5.4 nM) >> LAN (IC50, 21 nM) > DOTATOCT (IC50 260 nM). In Sprague Dawley rats (n = 10), 90Y-DOTALAN was rapidly cleared from the circulation and concentrated in hsst-positive tissues such as pancreas or pituitary. Taken together, our results indicate that 111In-/90Y-DOTALAN binds to a broad range of primary human tumors and tumor cell lines, probably via binding to hsst2-5. We conclude that this radiolabeled peptide can be used for hsst-mediated diagnosis (111In-DOTALAN) as well as systemic radiotherapy (90Y-DOTALAN) of human tumors.

Somatostatin receptor scintigraphy in thymoma imaging method and clinical application

Somatostatin receptor scintigraphy with 111In-[DTPA-D-Phe1]-octreotide has the potential for visualizing primary and recurrent thymomas in patients with myasthenia gravis, whereas thymic hyperplasias fail to accumulate somatostatin analog peptides. We demonstrate somatostatin receptor imaging findings in a patient with a mixed encapsulated thymoma which exhibited intense 111In-[DTPA-D-Phe1]-octreotide uptake in early and late scans. In another patient with a history of malignant thymoma 111In-[DTPA-D-Phe1]-octreotide accumulation was clearly seen in a mass suspected to be a recurrence. This paper describes the imaging protocol including Single Photon Emission Computed Tomography (SPECT) and discusses the clinical applications of this feasible functional imaging method in patients with thymomas.

Somatostatin and its receptor family

Somatostatin (SST), a regulatory peptide, is produced by neuroendocrine, inflammatory, and immune cells in response to ions, nutrients, neuropeptides, neurotransmitters, thyroid and steroid hormones, growth factors, and cytokines. The peptide is released in large amounts from storage pools of secretory cells, or in small amounts from activated immune and inflammatory cells, and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells that are widely distributed in the brain and periphery. These actions are mediated by a family of seven transmembrane (TM) domain G-protein-coupled receptors that comprise five distinct subtypes (termed SSTR1-5) that are endoded by separate genes segregated on different chromosomes. The five receptor subtypes bind the natural SST peptides, SST-14 and SST-28, with low nanomolar affinity. Short synthetic octapeptide and hexapeptide analogs bind well to only three of the subtypes, 2, 3, and 5. Selective nonpeptide agonists with nanomolar affinity have been developed for four of the subtypes (SSTR1, 2, 3, and 4) and putative peptide antagonists for SSTR2 and SSTR5 have been identified. The ligand binding domain for SST ligands is made up of residues in TMs III-VII with a potential contribution by the second extracellular loop. SSTRs are widely expressed in many tissues, frequently as multiple subtypes that coexist in the same cell. The five receptors share common signaling pathways such as the inhibition of adenylyl cyclase, activation of phosphotyrosine phosphatase (PTP), and modulation of mitogen-activated protein kinase (MAPK) through G-protein-dependent mechanisms. Some of the subtypes are also coupled to inward rectifying K(+) channels (SSTR2, 3, 4, 5), to voltage-dependent Ca(2+) channels (SSTR1, 2), a Na(+)/H(+) exchanger (SSTR1), AMPA/kainate glutamate channels (SSTR1, 2), phospholipase C (SSTR2, 5), and phospholipase A(2) (SSTR4). SSTRs block cell secretion by inhibiting intracellular cAMP and Ca(2+) and by a receptor-linked distal effect on exocytosis. Four of the receptors (SSTR1, 2, 4, and 5) induce cell cycle arrest via PTP-dependent modulation of MAPK, associated with induction of the retinoblastoma tumor suppressor protein and p21. In contrast, SSTR3 uniquely triggers PTP-dependent apoptosis accompanied by activation of p53 and the pro-apoptotic protein Bax. SSTR1, 2, 3, and 5 display acute desensitization of adenylyl cyclase coupling. Four of the subtypes (SSTR2, 3, 4, and 5) undergo rapid agonist-dependent endocytosis. SSTR1 fails to be internalized but is instead upregulated at the membrane in response to continued agonist exposure. Among the wide spectrum of SST effects, several biological responses have been identified that display absolute or relative subtype selectivity. These include GH secretion (SSTR2 and 5), insulin secretion (SSTR5), glucagon secretion (SSTR2), and immune responses (SSTR2).

High expression of somatostatin receptor subtype 2 (sst2) in medulloblastoma: implications for diagnosis and therapy

Medulloblastoma is a pediatric malignancy, which arises in cerebellum. The neuropeptide somatostatin (SS-14) is a neuromodulator and growth regulator in the developing cerebellum. SS-14 has previously been demonstrated in medulloblastomas with immunohistochemical techniques, but somatostatin receptor (sst) expression is less well understood. We analyzed somatostatin and sst subtype expression (sst1-5) in central primitive neuroectodermal tumors (cPNET), including 23 medulloblastomas, 6 supratentorial PNET, and 10 cPNET cell lines. The expression of SS-14 and sst genes in cPNET was compared with expression of these genes in 17 tumors of the Ewing's sarcoma family of tumors using reverse transcriptase-PCR, Southern hybridization, quantitative in vitro receptor autoradiography, and competitive membrane binding assays. The sst1 subtype was expressed in similar frequency in cPNET (83%) and Ewing's sarcoma family of tumors (71%). Nine of the 10 cell lines and 76% of the cPNET expressed mRNA for sst2 compared with 35% of the Ewing's sarcoma family of tumors. High-affinity binding of SS-14 was demonstrated in cPNET by quantitative autoradiography as well as by competitive binding assays. The cPNET cell line D283 Med bound SS-14 and octreotide with high affinity; SS-14 inhibited proliferation of D283 Med cells as measured by a decrease in [3H]thymidine uptake. We conclude that both sst1 and sst2 are highly expressed in cPNET and suggest that somatostatin may regulate proliferation and differentiation in these developmental tumors.

Gene therapy for pancreatic carcinoma: local and distant antitumor effects after somatostatin receptor sst2 gene transfer

Human pancreatic adenocarcinomas lose the ability to express sst2, the somatostatin receptor, which mediates the antiproliferative effect of somatostatin. Reintroducing sst2 into human pancreatic cancer cells by stable expression evokes an autocrine negative feedback loop leading to a constitutive activation of the sst2 gene and an inhibition of cell proliferation and tumorigenicity. In vivo studies have been conducted in athymic mice to investigate the antitumor bystander effects resulting from the transfer of the sst2 gene into human pancreatic cancer cell line BxPC-3. In mixing experiments, a local bystander effect was observed: mixed tumors containing a ratio of sst2-expressing cells to control cells of 25:75, 50:50, and 75:25 grew with a time delay of 31, 44, and 50 days, respectively, when compared with control tumors derived from control cells. Tumors containing 100% sst2-expressing cells remained quiescent for up to 80 days. A significant increase in apoptosis and a decrease in the Ki67 index were detected in mixed and sst2 tumor when compared with control tumors. In combined experiments, mice were separately xenografted with control cells on one flank and with sst2-expressing cells on the other flank. A distant antitumor effect was induced: growth of control tumors was delayed by 33 days, the Ki67 index decreased significantly, and apoptosis increased when compared with control tumors that grew alone. The distant bystander effect may be explained in part by a significant increase in serum somatostatin-like immunoreactivity levels resulting from the autocrine feedback loop produced by sst2-expressing cells and inducing an upregulation of the type 1 somatostatin receptor, sst1, which also mediates the antiproliferative effect of somatostatin. In conclusion, the local and distant antitumor bystander effects obtained in this experimental model suggest that sst2 gene transfer may represent a new therapy for pancreatic cancer.

Somatostatin receptor scintigraphy and somatostatin therapy in the evaluation and treatment of malignant thymoma

PURPOSE

This report illustrates the potential diagnostic and therapeutic utility of somatostatin receptor scintigraphy and therapy with somatostatin.

METHODS

In-111 pentetreotide (In-111 octreotide), a somatostatin analog, was used to define the receptor status and the extent of disease in a case of malignant thymoma.

RESULTS

Subsequent treatment with nonradioactive somatostatin inhibited tumor growth.

CONCLUSION

In-111 octreotide may be useful to define tumor receptor status and may provide prognostic information useful in determining subsequent therapy.

Immunohistochemical and cytochemical localization of the somatostatin receptor subtype sst1 in the somatostatinergic parvocellular neuronal system of the rat hypothalamus

Somatostatin is known to mediate its actions through five G-protein-coupled receptors (sst1-sst5). We have studied the expression of the sst1 receptor in the rat hypothalamus by using a subtype-specific antiserum. In Western blotting, the antiserum reacted specifically with a band with an apparent molecular weight of 80,000 in membranes prepared from hypothalamic tissue. The localization of the sst1 receptor was investigated by immunohistochemistry in hypothalamus sections. Additionally, an immunofluorescent double-labeling was performed for the sst1 receptor and somatostatin. Light microscopy revealed that the sst1 receptor is located in perikarya and nerve fibers in the rostral periventricular area surrounding the third ventricle as well as in nerve fibers projecting from the perikarya to the external layer of the median eminence. In these neuronal structures, sst1 immunoreactivity was found to be colocalized with somatostatin. Furthermore, the location of sst1 receptors was studied by immunoelectron microscopy in the median eminence. In the external layer, receptor immunoreactivity was confined to nerve terminals. Immunoreactive nerve terminals were seen to make synapse-like junctions with other both stained and unstained nerve terminals. Thus, the sst1 receptor is present in the classic somatostatinergic hypothalamic parvocellular system inhibiting hormone secretion from the anterior pituitary gland. These findings indicate that the sst1 receptor may act as an autoreceptor and inhibit the release of somatostatin from periventricular neurons projecting to the median eminence.

Immunohistochemical and cytochemical localization of the somatostatin receptor subtype sst1 in the somatostatinergic parvocellular neuronal system of the rat hypothalamus

Somatostatin is known to mediate its actions through five G-protein-coupled receptors (sst1-sst5). We have studied the expression of the sst1 receptor in the rat hypothalamus by using a subtype-specific antiserum. In Western blotting, the antiserum reacted specifically with a band with an apparent molecular weight of 80,000 in membranes prepared from hypothalamic tissue. The localization of the sst1 receptor was investigated by immunohistochemistry in hypothalamus sections. Additionally, an immunofluorescent double-labeling was performed for the sst1 receptor and somatostatin. Light microscopy revealed that the sst1 receptor is located in perikarya and nerve fibers in the rostral periventricular area surrounding the third ventricle as well as in nerve fibers projecting from the perikarya to the external layer of the median eminence. In these neuronal structures, sst1 immunoreactivity was found to be colocalized with somatostatin. Furthermore, the location of sst1 receptors was studied by immunoelectron microscopy in the median eminence. In the external layer, receptor immunoreactivity was confined to nerve terminals. Immunoreactive nerve terminals were seen to make synapse-like junctions with other both stained and unstained nerve terminals. Thus, the sst1 receptor is present in the classic somatostatinergic hypothalamic parvocellular system inhibiting hormone secretion from the anterior pituitary gland. These findings indicate that the sst1 receptor may act as an autoreceptor and inhibit the release of somatostatin from periventricular neurons projecting to the median eminence.

S49 cells endogenously express subtype 2 somatostatin receptors which couple to increase protein tyrosine phosphatase activity in membranes and down-regulate Raf-1 activity in situ

S49 cells expressed type 2 somatostatin receptors (sstr2) by immunoblotting. Analysis by reverse transcription and polymerase chain reaction (RT-PCR) methodologies showed that S49 cells express predominantly sstr2A and sstr2B mRNAs; other subtypes were either not detected, in the case of sstr1, sstr3, sstr4, or variably detected, in the case of sstr5. No mutations were present in S49 cells at codon 12, 13, or 61 of the N-, K-, or H-ras genes. Nevertheless, randomly growing S49 cells contained Raf-1 activity by specific immune complex kinase assays. Treatment of S49 cells with somatostatin transiently inactivated the basal activity of Raf-1, but not that of B-Raf. Addition of somatostatin plus guanyl-5'-yl imidodiphosphate (GMPPNP) to S49 membranes stimulated PTPase activity. The concentration dependence for stimulation of PTPase activity correlated with high affinity binding of [125I-Tyr11]somatostatin-14. Both the effect of somatostatin to stimulate PTPase activity and to inactivate Raf-1 were abrogated by PTx. PTPase activity stimulated by somatostatin plus GMPPNP was recovered in a peak of high apparent M(r) (670,000) after solubilisation with Triton X-100 and Superose 6 chromatography. Furthermore, addition of activated, brain G alpha i/o subunits to fractions from control membranes stimulated PTPase activity in the high M(r) peak. Thus, S49 membranes contain a G-protein regulated PTPase (PTPase-G), and PTPase-G in these cells may reside in a high molecular weight complex.

Relevance of Somatostatin Receptors and Other Peptide Receptors in Pathology

Receptors for regulatory peptides can be overexpressed by several diseases, in particular by neoplasms. This review summarizes the current status of knowledge in this field, on the basis of in vitro receptor studies and with emphasis on receptors for somatostatin as well as for substance P (SP), vasoactive intestinal polypeptide (VIP), and cholecystokinin. It evaluates the existing and potential clinical implications of the findings for diagnosis and therapy and discusses the role of the pathologist in this context.