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

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Somatostatin inhibits the excitability of rat small-diameter trigeminal ganglion neurons that innervate nasal mucosa and project to the upper cervical dorsal horn via activation of somatostatin 2a receptor

This study investigated whether somatostatin (SST) modulates the excitability of nociceptive trigeminal ganglion (TRG) neurons that innervate the nasal mucosa and project to the upper cervical (C(1)) dorsal horn by using perforated-patch clamping, retrograde-labeling, and immunohistochemistry. Fluorogold (FG) retrograde labeling was used to identify the rat TRG neurons innervating the nasal mucosa, while microbeads (MB) were used to label neurons projected onto the superficial layer of the C(1) dorsal horn. FG-labeled small-diameter TRG neurons exhibited SST(2A) receptor immunoreactivity (19%) and half of these neurons were also labeled with MB. In whole-cell current-clamp mode, most (72%) of the dissociated FG-/MB-labeled TRG neurons were hyperpolarized by application of SST. The hyperpolarization was evoked by SST in a concentration-dependent manner (0.1-10 microM) and the responses were associated with a decrease in the cell input resistance. The minimum concentration to elicit a significant hyperpolarization was 1 microM. The repetitive firings during a depolarizing pulse were significantly reduced by SST (1 microM) application. The hyperpolarization and decreased firing evoked by SST were both blocked by the SST(2) receptor antagonist, CYN154806 (1 microM). Under voltage-clamp conditions, SST (1 microM) significantly increased the voltage-gated K(+) transient (I(A)) and sustained (I(K)) currents and these increases were abolished by coapplication of CYN154806 (1 microM). In the presence of both 4-aminopyridine (6 mM) and tetraethylammonium (10 mM), no significant changes in the membrane potential in response to SST application were found. These results suggest that modulation of trigeminal nociceptive transmission in the C(1) dorsal horn by activation of SST(2A) receptors occurs at the level of small-diameter TRG cell bodies and/or their afferent terminals, and that this may be related to regulation of protective upper-airway reflexes.

Selective regulation of somatostatin receptor subtype signaling: evidence for constitutive receptor activation

Anterior pituitary hormone secretion is under tonic suppression by hypothalamic somatostatin signaling through somatostatin receptor subtypes (SSTs). Because some hormonal axes are known to be abnormally regulated by ligand-independent constitutively active G protein-coupled receptors, we tested pituitary SSTs for selective constitutive signaling. We therefore differentially silenced endogenous SST2, SST3, and SST5 in somatostatin-sensitive ACTH-secreting mouse AtT-20 pituitary corticotroph cells using small inhibitory RNA (siRNA) and analyzed downstream SSTs-regulated pathways. Transfection with siRNA reduced specific receptor subtype mRNA expression up to 82%. Specificity of receptor silencing was validated against negative controls with different gene-selective siRNAs, concordance of mRNA and cAMP changes, reduced potency of receptor-selective agonists, and phenotype rescue by overexpression of the silenced receptor. Mouse SST3 > SST5 > SST2 knockdown increased basal cAMP accumulation (up to 200%) and ACTH secretion (up to 60%). SST2- and SST5-selective agonist potencies were reduced by SST3- and SST5-silencing, respectively. SST5 > SST2 = SST3 silencing also increased basal levels of ERK1/2 phosphorylation. SST3- and SST5-knockdown increased cAMP was only partially blocked by pertussis toxin. The results show that SST2, SST3, and SST5 exhibit constitutive activity in mouse pituitary corticotroph cells, restraining adenylate cyclase and MAPK activation and ACTH secretion. SST3 mainly inhibits cAMP accumulation and ACTH secretion, whereas SST5 predominantly suppresses MAPK pathway activation. Therefore, SST receptor subtypes control pituitary cell function not only through somatostatin binding to variably expressed cell membrane receptor subtypes, but also by differential ligand-independent receptor-selective constitutive action.

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

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

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

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

Feedback control of thyrotropin secretion in the chicken: thyroid hormones increase the expression of hypophyseal somatostatin receptor types 2 and 5

We have studied the involvement of the somatostatin receptor type 5 (SSTR5) in the control of thyrotropin (TSH) release in the chicken. Hypothalamic somatostatin (SS-14) is known to inhibit both thyrotropin-releasing hormone (TRH)- and corticotropin-releasing hormone (CRH)-induced TSH secretion. Studies using receptor-specific agonists have indicated that the inhibitory effect of SS-14 on TRH-induced TSH release is mediated by SSTR2 and SSTR5. Using the same agonists, we were able to demonstrate the involvement of SSTR5 in the inhibition of the in vitro CRH-induced TSH secretion by SS-14. Subsequently, we determined hypophyseal SSTR5 mRNA expression during the last week of embryonic development using real-time PCR. SSTR5 mRNA levels were low until day 19 of incubation, but between day 19 and hatching SSTR5 mRNA expression increased 3-fold. Since this increase coincides with the increasing plasma T(3) levels towards hatching, and a similar ontogenetic expression pattern was found for SSTR2, we quantified hypophyseal SSTR2 and SSTR5 mRNA expression levels in chicken embryos treated with thyroid hormones. Injection of thyroid hormones was indeed found to increase the expression of both mRNAs significantly. We hypothesize that the negative feedback exerted by the increasing plasma T(3) levels towards hatching is at least in part mediated by an increased expression of SSTR2 and SSTR5.

Coexpression of somatostatin receptor subtype 5 affects internalization and trafficking of somatostatin receptor subtype 2

The somatostatin [somatotropin release-inhibiting factor (SRIF)] receptor subtypes sst(2A) and sst(5) are frequently coexpressed in SRIF-responsive cells, including endocrine pituitary cells. We previously demonstrated that sst(2A) and sst(5) exhibit different subcellular localizations and regulation of cell surface expression, although they have similar signaling properties. We investigated here whether sst(2A) and sst(5) functionally interact in cells coexpressing the two receptor subtypes. We stimulated both transfected cells stably expressing sst(2A) alone (CHO-sst(2A)) or together with sst(5) (CHO-sst(2A+5)) and the pituitary cell line AtT20, which endogenously expresses the two receptor subtypes, with either the nonselective agonist [D-Trp(8)]-SRIF-14 or the sst(2)-selective agonist L-779,976. In CHO-sst(2A) cells, stimulation with either ligand resulted in the loss of approximately 75% of cell surface SRIF binding sites and massive internalization of sst(2A) receptors. The cells were desensitized to subsequent stimulation with [D-Trp(8)]-SRIF-14, which failed to inhibit forskolin-evoked cAMP accumulation. Similarly, in CHO-sst(2A+5) and AtT20 cells, [D-Trp(8)]-SRIF-14 induced the loss of 60-70% of SRIF binding sites as well as massive sst(2A) endocytosis. By contrast, in cells expressing both sst(2A) and sst(5), selective stimulation of sst(2A) with L-779,976 resulted in only 20-40% loss of cell surface binding and markedly reduced sst(2A) internalization. Consequently, whereas CHO-sst(2A+5) and AtT20 cells stimulated with [D-Trp(8)]-SRIF-14 were desensitized to a second stimulation with the same agonist, cells prestimulated with L-779,976 were not desensitized to subsequent [D-Trp(8)]-SRIF-14 stimulation. These findings indicate that the presence of sst(5) in the same cells modulates trafficking and cell surface regulation of sst(2A) and cellular desensitization to the effects of SRIF.

A comparison of the effects of gastrin, somatostatin and dopamine receptor ligands on rat gastric enterochromaffin-like cell secretion and proliferation

Gastrin regulates ECL cell histamine release and is a critical determinant of acid secretion. ECL cell secretion and proliferation is inhibited by gastrin antagonists and somatostatin but little is known about the role of dopamine agonists in this process. Since the ECL cell exhibits all three classes of receptor we evaluated and compared the effects of the gastrin receptor antagonist, (YF476), lanreotide (SST agonist) and novel dopaminergic agents (BIM53061 and BIM27A760) on ECL cell histamine secretion and proliferation. Highly enriched (>98%) ECL cell preparations prepared from rat gastric mucosa using a FACS approach were studied. Real-time PCR confirmed presence of the CCK2, SS2 and SS5 and D1 receptors on ECL cells. YF476 inhibited histamine secretion and proliferation with IC(50)s of 1.25 nM and 1.3 x 10(-11) M respectively, values 10-1000x more potent than L365,260. Lanreotide inhibited secretion and proliferation (2.2 nM, 1.9 x 10(-10) M) and increased YF476-inhibited proliferation a further 5-fold. The dopamine agonist, BIM53061, inhibited gastrin-mediated ECL cell secretion and proliferation (17 nM, 6 x 10(-10) M) as did the novel dopamine/somatostatin chimera BIM23A760 (22 nM, 4.9 x 10(-10) M). Our studies demonstrate that the gastrin receptor antagonist, YF476, is the most potent inhibitor of ECL cell histamine secretion and proliferation. Lanreotide, a dopamine agonist and a dopamine/somatostatin chimera inhibited ECL cell function but were 10-1000x less potent than YF476. Agents that selectively target the CCK2 receptor may provide alternative therapeutic strategies for gastrin-mediated gastrointestinal cell secretion and proliferation such as evident in the hypergastrinemic gastric carcinoids associated with low acid states.

Somatostatin sst4 ligands: chemistry and pharmacology

Several classes of compounds (thioureas, ureas, beta-glucosides, sulfonamides, and cyclic peptides) show enhanced binding affinity and selectivity at somatostatin subtype 4 receptors (sst4). Pharmacophore models have recently been proposed to explain receptor subtype selectivity. The chemistry and therapeutic potential of sst4 ligands will be the subject of this review.

Novel insights in somatostatin receptor physiology

The experimental data reviewed in the present paper deal with the molecular events underlying the agonist-dependent regulation of the distinct somatostatin receptor subtypes and may suggest important clues about the clinical use of somatostatin analogs with different pattern of receptor specificity for the in vivo targeting of tumoral somatostatin receptors. Somatostatin receptor subtypes are characterized by differential beta-arrestin trafficking and endosomal sorting upon agonist binding due, at least in part, to the differences in their C-terminal tails. Moreover, the subcellular expression pattern of somatostatin receptor subtypes and their activity in response to agonist treatment are affected by intracellular complements, such as proteins involved in intracellular vesicle trafficking. Different somatostatin analogs may induce distinct conformations of the receptor/ligand complex, preferentially coupled to either receptor signaling or receptor endocytosis.

Agonist-induced endocytosis of rat somatostatin receptor 1

Somatostatin-receptor 1 (sst1) is an autoreceptor in the central nervous system that regulates the release of somatostatin. Sst1 is present intracellularly and at the cell surface. To investigate sst1 trafficking, rat sst1 tagged with epitope was expressed in rat insulinoma cells 1046-38 (RIN-1046-38) and tracked by antibody labeling. Confocal microscopic analysis revealed colocalization of intracellularly localized rat sst1-human simplex virus (HSV) with Rab5a-green fluorescent protein and Rab11a-green fluorescent protein, indicating the distribution of the receptor in endocytotic and recycling organelles. Somatostatin-14 induced internalization of cell surface receptors and reduction of binding sites on the cell surface. It also stimulated recruitment of intracellular sst1-HSV to the plasma membrane. Confocal analysis of sst1-HSV revealed that the receptor was initially transported within superficial vesicles. Prolonged stimulation of the cells with the peptide agonist induced intracellular accumulation of somatostatin-14. Because the number of cell surface binding sites did not change during prolonged stimulation, somatostatin-14 was internalized through a dynamic process of continuous endocytosis, recycling, and recruitment of intracellularly present sst1-HSV. Accumulated somatostatin-14 bypassed degradation via the endosomal-lysosomal route and was instead rapidly released as intact and biologically active somatostatin-14. Our results show for the first time that sst1 mediates a dynamic process of endocytosis, recycling, and re-endocytosis of its cognate ligand.

Chronicles in drug discovery

Chronicles in Drug Discovery features special interest reports on advances in drug discovery. This month we highlight agents that target and deplete immunosuppressive regulatory T cells, which are produced by tumor cells to hinder innate immunity against, or chemotherapies targeting, tumor-associated antigens. Antiviral treatments for respiratory syncytial virus, a severe and prevalent infection in children, are limited due to their side effect profiles and cost. New strategies currently under clinical development include monoclonal antibodies, siRNAs, vaccines and oral small molecule inhibitors. Recent therapeutic lines for Huntington's disease include gene therapies that target the mutated human huntingtin gene or deliver neuroprotective growth factors and cellular transplantation in apoptotic regions of the brain. Finally, we highlight the antiinflammatory and antinociceptive properties of new compounds targeting the somatostatin receptor subtype sst4, which warrant further study for their potential application as clinical analgesics.

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.

Effects of somatostatin-14 and the receptor-specific somatostatin analogs on chromogranin A and alpha-subunit (alpha-SU) release from "clinically nonfunctioning" pituitary adenoma cells incubated in vitro

The aim of the study was to examine the effect of somatostatin (SST) and its analogs on the release of chromogranin A (CgA) and alpha-subunit (alpha-SU) from clinically non-functioning pituitary adenomas incubated in vitro. Seven pituitary macroadenomas surgically removed were investigated. All of the tumors were diagnosed before surgery as non-functioning, but they expressed either gonadotropins or their subunits as detected by immunohistochemistry. Two tumors additionally expressed prolactin and growth hormone. All adenomas also expressed chromogranin A (CgA) and at least 3 of 5 subtypes of somatostatin receptors. The cells isolated from the examined tumors were exposed in vitro to either native SST-14 or the following receptor-specific SST analogs: BIM-23926 (agonist of sst1 receptor), BIM-23120 (agonist of sst2 receptor), BIM-23206 (agonist of sst5 receptor) and BIM23A387 (somatostatin/dopamine chimera). The concentration of CgA was measured by means of ELISA method and of alpha-SU was measured by an immunoradiometric method. It was found that the exposure on SST-14 resulted in the decrease of CgA and alpha-SU release from tumor cells in majority of samples, and the effect on CgA was positively correlated with the expression of sst3 and also with the sst2A/sst2B expressions ratio. The inhibitory effect of SST-14 on CgA and alpha-SU seems also to correlate negatively with the expression of sst2B. CgA inhibition also correlates positively with sst5 expression. Among the other compounds studied, only the sst2 agonist decreased the release in all the investigated samples. The remaining substances (agonists of sst1 and sst5 and SST/DA chimera) produced the divergent changes (increased or decreased release, depending on the sample). The data suggest that the inhibition of CgA (and possibly of alpha-SU) release by SST is mediated via subtypes sst2A, sst3 and sst5, whereas sst2B subtype may induce the opposite effect.

Characterization of somatostatin receptor subtype-specific regulation of insulin and glucagon secretion: an in vitro study on isolated human pancreatic islets

INTRODUCTION

Pancreatic A- and B-cells express somatostatin receptors (SSTRs). Five pharmacologically distinct SSTR subtypes are known (SSTR1-SSTR5). In rodents, SSTR2 inhibits glucagon secretion, whereas SSTR5 suppresses the release of insulin. Human pancreatic A- and B-cells express SSTR1-3 and SSTR5; however, their contribution to the regulation of glucagon and insulin secretion is not well known.

AIM OF THE STUDY

The goal of this study was to characterize the role of individual SSTR subtypes in regulating human glucagon and insulin secretion in vitro.

METHODS

Human pancreatic islets were isolated from healthy donors and incubated with somatostatin, SSTR1-3-selective and SSTR5-selective agonists, or an SSTR2-selective antagonist (DC-41-33). Stimulation of insulin secretion was induced by glucose (10, 20 mm) alone or in combination with 10 nm exendin-4 or 10 mm L-arginine. Glucagon secretion was induced by 20 mm L-arginine. Basal secretion of insulin and glucagon was measured at 2.8 or 3.3 mm glucose.

RESULTS

SSTR1-, SSTR2-, and SSTR5-selective agonists inhibited insulin secretion with the following order of potency: SSTR2 (EC50, 0.08 nm) > SSTR5 (EC50, 5.3 nm) > SSTR1 (EC50, 35 nm). Glucagon secretion was inhibited by SSTR-selective agonists with the following order of potency: SSTR2 (EC50, 0.05 nm) > SSTR1 (EC50, 1.8 nm) > SSTR5 (EC50, 28 nm). DC-41-33 dose-dependently reversed the effects of the SSTR2-selective agonist on insulin and glucagon secretion.

CONCLUSION

Our study demonstrates that SSTR2-agonist is the most potent inhibitor of insulin and glucagon secretion from isolated human pancreatic islets. Furthermore, we identify SSTR1- and SSTR5-selective agonists as additional inhibitors of insulin and glucagon secretion from human pancreas.

Porcine somatostatin receptor 2 displays typical pharmacological sst2 features but unique dynamics of homodimerization and internalization

Somatostatin (SRIF) exerts its multiple actions, including inhibition of GH secretion and of tumoral growth, through a family of five receptor subtypes (sst1-sst5). We recently reported that an sst2-selective agonist markedly decreases GH release from pig somatotropes, suggesting important roles for this scarcely explored receptor, psst2. Here, functional expression of psst2 in Chinese hamster ovary-K1 and human embryonic kidney-293-AD cell lines was employed to determine its pharmacological features and functional ability to reduce cAMP, and to examine its homodimerization and internalization dynamics in real time in single living cells. Results show that psst2 is a high-affinity receptor (dissociation constant = 0.27 nM) displaying a typical sst2 profile (nM affinity for SRIF-14> or =SRIF-28>cortistatin>MK678>octreotide) and high selectivity (EC(50) = 1.1 nM) for the sst2 agonist l-779,976, but millimolar or undetectable affinity to other sst-specific agonists (sst3>sst1>sst5>>>sst4). Accordingly, SRIF dose-dependently inhibited forskolin-stimulated cAMP with high potency (EC(50) = 6.55 pm) and modest efficacy (maximum 29.1%) via psst2. Cotransfection of human embryonic kidney-293 and Chinese hamster ovary-K1 cells with two receptor constructs modified with distinct fluorescent tags (psst2-YFP/psst2-CFP) enabled fluorescence resonance energy transfer measurement of physical interaction between psst2 receptors and also receptor internalization in single living cells. This revealed that under basal conditions, psst2 forms constitutive homodimers/homomultimers, which dissociate immediately (11 sec) upon SRIF binding. Interestingly, contrary to human sst2, psst2 rapidly reassociates (110.5 sec) during a subsequent process that temporally overlaps with receptor internalization (half-maximal = 95.1 sec). Therefore, psst2 is a potent inhibitory receptor displaying a unique set of interrelated dynamic features of agonist-dependent dimerization, dissociation, internalization, and reassociation, a cascade of events that might be critical for receptor function.

The regulation of alcohol intake by melanin-concentrating hormone in rats

Given into the brain, melanin-concentrating hormone (MCH) increases alcohol consumption, but the mechanism and physiological relevance of this effect are unclear. We hypothesized that endogenous MCH will enhance alcohol drinking and that MCH increases alcohol's reinforcing properties. An MCH receptor 1 (MCHR1) antagonist, or saline was administered centrally alone, or preceding MCH or saline to rats trained to drink 10% alcohol using sucrose fading. Blocking MCHR1 neither reduced alcohol intake (saline=0.4+/-0.1 g, 30 microg MCHR1 antagonist=0.4+/-0.1 g/kg alcohol), nor attenuated MCH-induced alcohol drinking (MCHR1 antagonist/saline=0.7+/-0.1 g/kg, MCHR1 antagonist/MCH=0.9+/-0.1 g/kg alcohol). Another cohort of rats was trained to lever press for alcohol on a progressive ratio schedule. MCH or saline was administered centrally and lever presses were measured. MCH had no effect prior to the break point, but increased total responding during the session (saline=87.2+/-32.0, MCH=315.4+/-61.0 presses). In conclusion, these data suggest that MCH augments alcohol drinking partly by enhancing the drug's reinforcing value. Further, endogenous MCH does not seem to regulate alcohol drinking, however because the antagonist failed to attenuate MCH-induced alcohol intake this conclusion is tentative.

The analysis of quantitative expression of somatostatin and dopamine receptors in gastro-entero-pancreatic tumours opens new therapeutic strategies

OBJECTIVE

Somatostatin (sst) are present in the majority of gastro-entero-pancreatic (GEP) tumours. Effects of somatostatin receptor (sst) analogues are partial and of limited duration. Cell lines derived from GEP express dopaminergic receptors D(2). New chimeric analogues simultaneously recognising sst(2) and sst(5) or sst(2) and D(2) have additive effects in inhibition of GH and prolactin secretion in pituitary adenomas. Our aim was to quantify the expression of sst and D(2) mRNA in human GEP tumours.

DESIGN AND METHODS

mRNA expression of sst(1), sst(2), sst(3) and sst(5) as well as D(2), was analysed using real-time PCR (TaqMan probe) in a series of 35 patients with GEP tumours (pancreas (n = 19) and intestinal (n = 16)). Levels of expression were compared with a group of 13 somatotroph adenomas.

RESULTS

All GEP tumours express sst(1), sst(2) and D(2). Expression of sst(3) and sst(5) was observed in 89 and 76% of tumours respectively with highly variable levels. sst(2) mRNA expression was higher in nonfunctional tumours (P < 0.009) and sst5 was higher in pancreatic than in intestinal tumours (P < 0.02). Whereas sst(2) levels were similar between GEP and somatotroph tumours, levels of sst(5) and D(2) were higher in the former (394.9 +/- 156.1 x 10(-2) vs 69.7 +/- 19.5 x 10(-2) copy/copy beta-Gus (P < 0.0036) and 519.6 +/- 121.2 x 10(-2) vs 50.0 +/- 21.6 x 10(-2) copy/copy beta-Gus (P < 0.0001) respectively). In small tumours ( < 30 mm), sst(2) density appeared as a crucial parameter in somatostatin receptor scintigraphy results, whereas in big tumours, a consistent bias in SRS results was introduced by the size. In pancreatic GEP, high-level sst(3) expression was found in tumours with more active angiogenesis (higher microvessel density and vascular endothelial growth factor expression (P < 0.03)).

CONCLUSIONS

GEP tumours co-express sst(2) and D(2) in 100% of cases and sst(5) in 89% thus supporting the testing of bi-specific agonists (sst(2)/sst(5) or sst(2)/D(2)) in these tumours.

Radiolabeled somatostatin receptor antagonists are preferable to agonists for in vivo peptide receptor targeting of tumors

Targeting neuroendocrine tumors expressing somatostatin receptor subtypes (sst) with radiolabeled somatostatin agonists is an established diagnostic and therapeutic approach in oncology. While agonists readily internalize into tumor cells, permitting accumulation of radioactivity, radiolabeled antagonists do not, and they have not been considered for tumor targeting. The macrocyclic chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to two potent somatostatin receptor-selective peptide antagonists [NH(2)-CO-c(DCys-Phe-Tyr-DAgl(8)(Me,2-naphthoyl)-Lys-Thr-Phe-Cys)-OH (sst(3)-ODN-8) and a sst(2)-selective antagonist (sst(2)-ANT)], for labeling with (111/nat)In. (111/nat)In-DOTA-sst(3)-ODN-8 and (111/nat)In-DOTA-[4-NO(2)-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH(2)] ((111/nat)In-DOTA-sst(2)-ANT) showed high sst(3)- and sst(2)-binding affinity, respectively. They did not trigger sst(3) or sst(2) internalization but prevented agonist-stimulated internalization. (111)In-DOTA-sst(3)-ODN-8 and (111)In-DOTA-sst(2)-ANT were injected intravenously into mice bearing sst(3)- and sst(2)-expressing tumors, and their biodistribution was monitored. In the sst(3)-expressing tumors, strong accumulation of (111)In-DOTA-sst(3)-ODN-8 was observed, peaking at 1 h with 60% injected radioactivity per gram of tissue and remaining at a high level for >72 h. Excess of sst(3)-ODN-8 blocked uptake. As a control, the potent agonist (111)In-DOTA-[1-Nal(3)]-octreotide, with strong sst(3)-binding and internalization properties showed a much lower and shorter-lasting uptake in sst(3)-expressing tumors. Similarly, (111)In-DOTA-sst(2)-ANT was injected into mice bearing sst(2)-expressing tumors. Tumor uptake was considerably higher than with the highly potent sst(2)-selective agonist (111)In-diethylenetriaminepentaacetic acid-[Tyr(3),Thr(8)]-octreotide ((111)In-DTPA-TATE). Scatchard plots showed that antagonists labeled many more sites than agonists. Somatostatin antagonist radiotracers therefore are preferable over agonists for the in vivo targeting of sst(3)- or sst(2)-expressing tumors. Antagonist radioligands for other peptide receptors need to be evaluated in nuclear oncology as a result of this paradigm shift.

The selective effects of somatostatin- and GABA-mediated transmissions on voltage gated Ca2+ channel immunoreactivity in the gerbil hippocampus

To identify whether altered expressions of voltage gated Ca(2+) channel (VGCC) are linked to inhibitory transmission abnormalities in the gerbil hippocampus, we investigated the effects of GABA receptor or somatostatin receptor (SST) antagonists/agonists on VGCC immunoreactivity in vivo. VGCC immunoreactivities in the hippocampus were significantly higher in seizure sensitive (SS) gerbils than in seizure resistant (SR) gerbils. P/Q-type VGCC immunoreactivity in the gerbil hippocampus was reduced by enhancement in GABA(A) and GABA(B) receptor-mediated transmission, but not by SST-mediated transmission. N-type VGCC immunoreactivity was reduced only by a SST agonist, whereas L-type (alpha1C) VGCC immunoreactivity was reduced only by a GABA(A) receptor agonist, and L-type (alpha1D) VGCC immunoreactivity was modulated by the GABA(B) receptor acting drugs. These findings provide a comprehensive description of the differential responses of VGCC subunits to alteration in GABAergic or somatostatinergic transmission. These findings also suggest that up-regulated VGCC immunoreactivity may be consequence of the neuronal excitability caused by a reduction in inhibitory neurotransmission in the gerbil hippocampus.

Compensatory changes in the hippocampus of somatostatin knockout mice: upregulation of somatostatin receptor 2 and its function in the control of bursting activity and synaptic transmission

Somatostatin-14 (SRIF) co-localizes with gamma-aminobutyric acid (GABA) in the hippocampus and regulates neuronal excitability. A role of SRIF in the control of seizures has been proposed, although its exact contribution requires some clarification. In particular, SRIF knockout (KO) mice do not exhibit spontaneous seizures, indicating that compensatory changes may occur in KO. In the KO hippocampus, we examined whether specific SRIF receptors and/or the cognate peptide cortistatin-14 (CST) compensate for the absence of SRIF. We found increased levels of both sst2 receptors (sst2) and CST, and we explored the functional consequences of sst2 compensation on bursting activity and synaptic responses in hippocampal slices. Bursting was decreased by SRIF in wild-type (WT) mice, but it was not affected by either CST or sst2 agonist and antagonist. sst4 agonist increased bursting frequency in either WT or KO. In WT, but not in KO, its effects were blocked by agonizing or antagonizing sst2, suggesting that sst2 and sst4 are functionally coupled in the WT hippocampus. Bursting was reduced in KO as compared with WT and was increased upon application of sst2 antagonist, while SRIF, CST and sst2 agonist had no effect. At the synaptic level, we observed that in WT, SRIF decreased excitatory postsynaptic potentials which were, in contrast, increased by sst2 antagonist in KO. We conclude that sst2 compensates for SRIF absence and that its upregulation is responsible for reduced bursting and decreased excitatory transmission in KO mice. We suggest that a critical density of sst2 is needed to control hippocampal activity.

Antidiabetic activity of a highly potent and selective nonpeptide somatostatin receptor subtype-2 agonist

Somatostatin inhibits both glucagon and insulin secretion. Glucagon significantly contributes to hyperglycemia in type 2 diabetes. Despite its function in the inhibition of glucagon secretion, somatostatin fails to reduce hyperglycemia in type 2 diabetes, due to a parallel suppression of insulin secretion. Five pharmacologically distinct somatostatin receptor subtypes (sst(1)-sst(5)) mediate the effects of somatostatin on a cellular level. Pancreatic A cells express sst(2), whereas B cells express sst(5). In this study, we describe a novel approach to the treatment of type 2 diabetes using a highly sst(2)-selective, nonpeptide agonist (compound 1). Compound 1 effectively inhibited glucagon secretion from pancreatic islets isolated from wild-type mice, whereas glucagon secretion from sst(2)-deficient islets was not suppressed. Compound 1 did not influence nonfasted insulin concentration. In sst(2)-deficient mice, compound 1 did not have any effects on glucagon or glucose levels, confirming its sst(2) selectivity. In animal models of type 2 diabetes in the nonfasted state, circulating glucagon and glucose levels were decreased after treatment with compound 1. In the fasting state, compound 1 lowered blood glucose by approximately 25%. In summary, small-molecule sst(2)-selective agonists that suppress glucagon secretion offer a novel approach toward the development of orally bioavailable drugs for treatment of type 2 diabetes.

Effects of chimeric somatostatin–dopamine molecules on human peripheral blood lymphocytes activation

BIM 23A761, selective for somatostatin receptors subtypes 2, 5 and the dopamine receptor subtype 2, and BIM 23A757 with affinity for SSTR2 and DAR2 were studied on human PBL proliferation and activation. BIM 23A761 was significantly more potent than specific SSTR and DAR2 agonists in suppressing lymphocyte proliferation induced by mitogen or alloantigen, while BIM 23A757 was more potent than specific SSTR2 and DAR2 agonists in suppressing antigen induced proliferation only. Both molecules displayed enhanced potency in suppressing IFNgamma and IL-6 secretion compared with the SSTR and DAR2 analogs, while only BIM 23A761 was able to inhibit IL-2 secretion and its effect is more potent than the control analogs. Furthermore BIM 23A761 inhibit cell progression into the S phase and then into the G2/M, while BIM 23A757 inhibited bromodeoxyuridine incorporation only during the S phase. Both chimeric molecules resulted significantly more effective than the respective controls.