Rapid identification of subtype-selective agonists of the somatostatin receptor through combinatorial chemistry

Somatostatin receptor subtype 4 couples to the M-current to regulate seizures

The K(+) M-current (I(M), Kv7) is an important regulator of cortical excitability, and mutations in these channels cause a seizure disorder in humans. The neuropeptide somatostatin (SST), which has antiepileptic properties, augments I(M) in hippocampal CA1 pyramidal neurons. We used SST receptor knock-out mice and subtype-selective ligands to investigate the receptor subtype that couples to I(M) and mediates the antiepileptic effects of SST. Using pentylenetetrazole as a chemoconvulsant, SST(2), SST(3), and SST(4) receptor knock-out mice all had shorter latencies to different seizure stages and increased seizure severity when compared with wild-type mice. However, the most robust differences were observed in the SST(4) knock-outs. When seizures were induced by systemic injection of kainate, only SST(4) knock-outs showed an increase in seizure sensitivity. We next examined the action of SST and subtype-selective SST agonists on electrophysiological parameters in hippocampal slices of wild-type and receptor knock-out mice. SST(2) and SST(4) appear to mediate the majority of SST inhibition of epileptiform activity in CA1. SST lacked presynaptic effects in mouse CA1, in contrast to our previous findings in rat. SST increased I(M) in CA1 pyramidal neurons of wild-type and SST(2) knock-out mice, but not SST(4) knock-out mice. Using M-channel blockers, we found that SST(4) coupling to M-channels is critical to its inhibition of epileptiform activity. This is the first demonstration of an endogenous enhancer of I(M) that is important in controlling seizure activity. SST(4) receptors could therefore be an important novel target for developing new antiepileptic and antiepileptogenic drugs.

X-ray imaging optimization of 3D tissue engineering scaffolds via combinatorial fabrication methods

We have developed a combinatorial method for determining optimum tissue scaffold composition for several X-ray imaging techniques. X-ray radiography and X-ray microcomputed tomography enable non-invasive imaging of implants in vivo and in vitro. However, highly porous polymeric scaffolds do not always possess sufficient X-ray contrast and are therefore difficult to image with X-ray-based techniques. Incorporation of high radiocontrast atoms, such as iodine, into the polymer structure improves X-ray radiopacity but also affects physicochemical properties and material performance. Thus, we have developed a combinatorial library approach to efficiently determine the minimum amount of contrast agent necessary for X-ray-based imaging. The combinatorial approach is demonstrated in a polymer blend scaffold system where X-ray imaging of poly(desaminotyrosyl-tyrosine ethyl ester carbonate) (pDTEc) scaffolds is improved through a controlled composition variation with an iodinated-pDTEc analog (pI(2)DTEc). The results show that pDTEc scaffolds must include at least 9%, 16%, 38% or 46% pI(2)DTEc (by mass) to enable effective imaging by microradiography, dental radiography, dental radiography through 0.75cm of muscle tissue or microcomputed tomography, respectively. Only two scaffold libraries were required to determine these minimum pI(2)DTEc percentages required for X-ray imaging, which demonstrates the efficiency of this new combinatorial approach for optimizing scaffold formulations.

Discovery of the first nonpeptidic, small-molecule, highly selective somatostatin receptor subtype 5 antagonists: a chemogenomics approach

We disclose the first selective, nonpeptidic, small-molecule somatostatin receptor subtype 5 (SST5R) antagonists that were identified by a chemogenomics approach based on the analysis of the homology of amino acids defining the putative consensus drug binding site of SST5R. With this strategy, opioid, histamine, dopamine, and serotonine receptors were identified as the closest neighbors of SST5R. The H1 antagonist astemizole was chosen as a seed structure and subsequently transformed into a SST5 receptor antagonist with nanomolar binding affinity devoid of the original target activity.

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

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

Identification and SAR of potent and selective non-peptide obeline somatostatin sst1 receptor antagonists

A novel class of non-peptide somatostatin receptor ligands bearing the octahydrobenzo[g]quinoline (obeline) structural element has been identified. SAR studies have been performed that led to the discovery of derivatives with high affinity (pK(d) r sst(1) > or = 9) and selectivity (> or = 150-fold for h sst(1) over h sst(2)-h sst(5)) for somatostatin receptor subtype sst(1). In a functional assay, the compounds act as antagonists at human recombinant sst(1) receptors.

Fabrication of combinatorial polymer scaffold libraries

We have designed a novel combinatorial research platform to help accelerate tissue engineering research. Combinatorial methods combine many samples into a single specimen to enable accelerated experimentation and discovery. The platform for fabricating combinatorial polymer scaffold libraries can be used to rapidly identify scaffold formulations that maximize tissue formation. Many approaches for screening cell-biomaterial interactions utilize a two-dimensional format such as a film or surface to present test substrates to cells. However, cells in vivo exist in a three-dimensional milieu of extracellular matrix and cells in vitro behave more naturally when cultured in a three-dimensional environment than when cultured on a two-dimensional surface. Thus, we have designed a method for fabricating combinatorial biomaterial libraries where the materials are presented to cells in the form of three-dimensional, porous, salt-leached, polymer scaffolds. Many scaffold variations and compositions can be screened in a single experiment so that optimal scaffold formulations for tissue formation can be rapidly identified. In summary, we have developed a platform technology for fabricating combinatorial polymer scaffold libraries that can be used to screen cell response to materials in a three-dimensional, scaffold format.

Molecular modeling suggests conformational scaffolds specifically targeting five subtypes of somatostatin receptors

Several analogs of somatostatin with conformational constraints in their peptide backbones have been modeled to determine energetically feasible conformations. Comparison of low-energy backbone structures of these peptides suggested unique conformations of the central Phe/Ala(i)-D-Trp(i+1)-Lys(i+2)-Thr(i+3) fragment characteristic for specific interactions of somatostatin with each of the five distinct subtypes of somatostatin receptors (SSTRs). The conformations obtained were in good agreement with experimental data obtained earlier by NMR measurements and/or X-ray crystallography. The results help rationalize experimental observations on the specificity of binding of various somatostatin analogs with different subtypes of the SSTRs. They also serve as templates for the design of conformationally constrained non-peptide scaffolds that effectively and selectively interact with different subtypes of SSTRs. Such scaffolds can be convenient carriers of radiolabels and near-infrared labels in specific agents for imaging tumors expressing different SSTR subtypes.

New sst4/5-selective somatostatin peptidomimetics based on a constrained tryptophan scaffold

The synthesis and biological evaluation of four peptidomimetic analogs of somatostatin based on a constrained Trp residue, 3-amino-indolo[2,3-c]azepin-2-one (Aia), are reported. It is shown that dipeptidomimetics with a D-Aia-Lys sequence, functionalized with N- and C-terminal aromatic substituents, display a good selectivity for both sst4 and sst5. This study allowed us to identify a new highly potent sst5 agonist with good selectivity over the other receptors, except versus sst4.

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.

Application of a novel design paradigm to generate general nonpeptide combinatorial templates mimicking beta-turns: synthesis of ligands for melanocortin receptors

We report the further application of a novel approach to template and ligand design by the synthesis of agonists of the melanocortin receptor. This design method uses the conserved structural data from the three-dimensional conformations of beta-turn peptides to design rigid nonpeptide templates that mimic the orientation of the main chain C-alpha atoms in a peptide beta-turn. We report details on a new synthesis of derivatives of template 1 that are useful for the synthesis of exploratory libraries. The utility of this technique is further exemplified by several iterative rounds of high-throughput synthesis and screening, which result in new partially optimized nonpeptide agonists for several melanocortin receptors.

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.

Somatostatin, a negative-regulator of central leptin action in the rat hypothalamus

Leptin-responsive neurons of the hypothalamus constitute a heterogeneous population expressing a vast array of different neuropeptides and neurotransmitters, some of which participate in the regulation of hunger and satiety. Here we report that somatostatin modulates the efficacy of leptin-signalling in the rat hypothalamus. Using a two-pulse paradigm at 30-min intervals, we delivered somatostatin or somatostatin receptor subtype-selective agonists in combination with leptin into the lateral cerebral ventricle of stereotaxically cannulated rats. To monitor the effect of somatostatin on the leptin-signalling pathway, we quantified changes in the leptin-mediated activation of STAT3, the signal transducer and activator of transcription 3. Successive administration of somatostatin and leptin diminished the level of STAT3-phosphorylation and nuclear STAT3 translocation in the ventromedial and dorsomedial hypothalamic nuclei, the lateral hypothalamic area, and the arcuate nucleus by about 40% compared to leptin administration alone. Furthermore, application of subtype-selective somatostatin receptor agonists suggests that the observed reduction in leptin-responsiveness is predominantly mediated by the sst3 receptor-subtype, followed by sst1 and sst2. In addition, the intensity of the negative-regulatory effect of somatostatin on leptin-signalling displayed regional differences for the three receptor-subtypes involved. Addressing the functional consequences of the diminished leptin-signalling, behavioural analyses showed that centrally applied somatostatin counteracts the leptin-mediated suppression of food intake. These results suggest that the pleiotropic effector somatostatin also plays a role in the central regulation of energy homeostasis.

Medical treatment of diabetic retinopathy with somatostatin analogues

Traditional management strategies for retinal neovascularisation accompanying proliferative diabetic retinopathy include photocoagulation laser therapy. The development of preventative pharmacological treatments aimed at replacing or delaying this acute intervention has been an active research area and somatostatin analogues have shown promise in reducing the progression of retinal vascular pathologies. This review summarises the present knowledge on the molecular and cellular mechanisms of neovascularisation, and the rationale for the therapeutic use of somatostatin analogues as well as the results of two key recent clinical trials using octreotide. The potential use of octreotide and other somatostatin analogues in reducing the risk of severe visual impairment in proliferative diabetic retinopathy is discussed and pharmacological treatment regimens are proposed as an additional strategy or a less invasive alternative to laser therapy.

Expression, pharmacology, and functional role of somatostatin receptor subtypes 1 and 2 in human macrophages

Somatostatin (SRIF)-14 is recognized as an important mediator between the nervous and the immune system, although the functional role of its receptors (sst(1)-sst(5)) is poorly understood in humans. In our study, we demonstrate that human macrophages, differentiated from PBMC-derived monocytes, express sst(1) and sst(2) mRNAs. sst(1) and sst(2) are mostly localized at the cell surface and display active binding sites. In particular, sst(1)/sst(2) activation results in a weak internalization of sst(1), and the sst(2) internalization appears more efficient. At the functional level, the activation of SRIF receptors by the multiligand analogs SOM230 and KE108, but not by SRIF-14 or cortistatin-14, reduces macrophage viability. Their effects are mimicked by the selective activation of sst(1) and sst(2) using CH-275 and SMS 201-995/L-779,976, respectively. Further, sst(1)- and sst(2)-mediated effects are reversed by the sst(1) antagonist SRA-880 or the sst(2) antagonist CYN 154806, respectively. CH-275, SMS 201-995, and L-779,976, but not SRIF-14, decrease mRNA expression and secretion of the MCP-1. In addition, SRIF-14, CH-275, SMS 201-995, and L-779,976 decrease IL-8 secretion, and they do not affect IL-8 mRNA expression. In contrast, SRIF-14 and sst(1)/sst(2) agonists do not affect the secretion of matrix metalloproteinase-9. Collectively, our results suggest that the SRIF system, through sst(1) and sst(2), exerts mainly an immunosuppressive effect in human macrophages and may, therefore, represent a therapeutic window that can be exploited for the development of new strategies in pharmacological therapy of inflammation.

A facile method for post-conjugation prosthetic radioiodination of “mini-peptides”

Prosthetic radioiodination methods were developed to facilitate the labeling of proteins devoid of tyrosine(s) or when these moieties are crucial for biological activity. This method involves the use of the so-called Bolton-Hunter-type reagents. However, the in vivo instability of the label prompted the search for more stable groups. Although these second generation reagents have worked well with proteins and peptides, the current reaction scheme takes a long time to perform. A simplified method may be more appropriate especially from radiation safety point of view. More importantly, for short-lived halogens, advantage may be gained utilizing a shorter reaction time. Recently we reported on the radioiodination of interleukin-8 (IL-8) using the pyridine carboxylate-derived activated ester. We have successfully conjugated this prosthetic group to tri- and tetrapeptides harboring the somatostatin (SST) receptor recognition units and characterized by HPLC and MS. The radioiodination was accomplished using the Iodogen method in a reasonable yield (mean=60%). The total synthesis time was approximately 60 min, which was 3-4 times shorter than the classical two-step method. Preliminary biodistribution of the radiolabeled peptide showed uptake in some of the organs known to express SST receptors. Injection of a low specific activity tracer significantly decreased the retention of radioactivity in these organs.

Somatostatin regulates aggressive behavior in an African cichlid fish

Animals respond to environmental and social change with plasticity in the neural substrates underlying particular behavioral states. In the African cichlid fish Astatotilapia burtoni, social dominance status in males is accompanied by reduced somatic growth rate as well as increased somatostatin neuron size in the preoptic area. Although somatostatin is commonly studied within the context of growth, we show here for the first time that this ancient neuropeptide also plays a role in controlling social behavior. Somatostatin antagonists increased aggressive behavior in a dose-dependent fashion and the potent somatostatin agonist octreotide decreased aggression. We cloned and sequenced the genes encoding two somatostatin receptor subtypes in this species to study transcription in the gonads. When we examined somatostatin receptor gene expression in testes, expression of the somatostatin type 3 receptor was negatively correlated with an aggressive display and androgen levels. However, octreotide treatment did not reduce plasma testosterone or 11-ketotestosterone levels, suggesting that the behavioral effects of somatostatin are not mediated by androgens. These results show that somatostatin has important effects on social behavior. In dominant male A. burtoni, somatostatin may function to contain energetically costly processes such as somatic growth and aggressive behavior.

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.

Novel sst2-selective somatostatin agonists. Three-dimensional consensus structure by NMR

The 3D NMR structures of six octapeptide agonist analogues of somatostatin (SRIF) in the free form are described. These analogues, with the basic sequence H-DPhe/Phe2-c[Cys3-Xxx7-DTrp8-Lys9-Thr10-Cys14]-Thr-NH2 (the numbering refers to the position in native SRIF), with Xxx7 being Ala/Aph, exhibit potent and highly selective binding to human SRIF type 2 (sst2) receptors. The backbone of these sst2-selective analogues have the usual type-II' beta-turn reported in the literature for sst2/3/5-subtype-selective analogues. Correlating the biological results and NMR studies led to the identification of the side chains of DPhe2, DTrp8, and Lys9 as the necessary components of the sst2 pharmacophore. This is the first study to show that the aromatic ring at position 7 (Phe7) is not critical for sst2 binding and that it plays an important role in sst3 and sst5 binding. This pharmacophore is, therefore, different from that proposed by others for sst2/3/5 analogues.

Pituitary mRNA expression of the growth hormone axis in the 1-year-old intrauterine growth restricted rat

Intrauterine growth restriction (IUGR) is one of the major causes of short stature in childhood. Abnormalities in the growth hormone (GH) axis have frequently been observed in children who are born intrauterine growth restricted and GH treatment is effective to improve final height. However, the way that the GH axis is involved is not fully understood. Previously, when investigating the effect of IUGR on the central somatotrophic axis, a hypothalamic effect was discovered with elevated somatostatin and decreased neuropeptide Y mRNA expression levels, whereas serum GH and insulin-like growth factor I (IGFI) were unaltered. These findings were thought to indicate a hypothalamic alteration of the GH axis due to IUGR, probably to compensate pituitary output, thereby normalising peripheral values of GH and IGFI. Therefore, the present study aimed to evaluate the effect of IUGR on the pituitary GH axis in this rat model. Pups from rats that underwent bilateral uterine artery ligation at day 17 of pregnancy were studied. Pituitary glands were collected from 1-year-old offspring for quantitative measurements of GH, GH-receptor (GH-R), GH-releasing hormone receptor (GHRH-R), somatostatin receptor subtype 2 and 5, IGFI and IGFI receptor mRNA levels using a real-time reverse transcriptase-polymerase chain reaction. In addition, liver GH-R and IGFI mRNA expression levels were measured and a radioimmunoassay was performed to determine serum IGFI levels. In the IUGR rat, levels of pituitary GH, GH-R and GHRH-R relative gene expression (RGE) were increased. No differences were found in the RGE level of all other pituitary growth factors, liver GH-R and IGFI, and serum IGFI concentration between IUGR and control rats. The present data show that intrauterine growth failure leads to changes in the pituitary that might counterbalance the effects found previously in the hypothalamus.

Synthesis and evaluation of radioiodinated substituted -naphthylalanine as a potential probe for pancreatic -cells imaging

A non-invasive imaging technique capable of relating a signal from the beta-cells to their mass will be of immense value in understanding the progression of diabetes. Several molecular markers have indeed been identified and investigations are ongoing aimed at accomplishing the said goal. These include pancreatic islet antigen (IC-2), somatostatin receptors (SSTRs), and sulfonylurea receptors (SURs) on the pancreatic beta-cells. Therefore investigations exploiting the potential application of the radiolabeled ligands for these receptors for beta-cell imaging are receiving intensive research attention. Radioiodinated peptidomimetic based on beta-naphthylalanine and n-hexanediamine has been synthesized. The molecule was subjected to in vitro and in vivo evaluation. Radioligand binding studies on CHO cell line expressing the SSTR2 showed very low affinity. Nonetheless, biodistribution in normal mice showed significant uptake in the pancreas. There was partial blockage of the pancreatic uptake when excess of the peptidomimetic was coinjected. The result implies that the pancreatic uptake was receptor mediated but may not involve the SSTR2 and therefore warrants further investigation.

Recent developments in the therapy of acromegaly

Acromegaly, a condition due to growth hormone hypersecretion usually from a benign pituitary tumour, is associated with significant morbidity and mortality. Disease control leads to normalisation of life expectancy with a reduction in signs and symptoms. Treatment modalities include surgery, radiotherapy and medical management. Surgery is the primary treatment in most of the patients, with success rates of 61-91% reported for those with microadenomas who are operated on by a specialist pituitary surgeon; however, most patients have macroadenomas and, although benefiting from surgery, are not cured and require additional medical therapy. This review will focus on emerging concepts in the medical treatment of acromegaly.

A chemogenomic analysis of the transmembrane binding cavity of human G-protein-coupled receptors

The amino acid sequences of 369 human nonolfactory G-protein-coupled receptors (GPCRs) have been aligned at the seven transmembrane domain (TM) and used to extract the nature of 30 critical residues supposed--from the X-ray structure of bovine rhodopsin bound to retinal--to line the TM binding cavity of ground-state receptors. Interestingly, the clustering of human GPCRs from these 30 residues mirrors the recently described phylogenetic tree of full-sequence human GPCRs (Fredriksson et al., Mol Pharmacol 2003;63:1256-1272) with few exceptions. A TM cavity could be found for all investigated GPCRs with physicochemical properties matching that of their cognate ligands. The current approach allows a very fast comparison of most human GPCRs from the focused perspective of the predicted TM cavity and permits to easily detect key residues that drive ligand selectivity or promiscuity.

Alterations in glucose homeostasis in SSTR1 gene-ablated mice

SSTR1 is found on the majority of human pancreatic beta cells, however, its role in insulin secretion has yet to be elucidated. In this study, we used the SSTR1 knockout mouse model to examine the role of SSTR1 in insulin secretion and glucose homeostasis in mice. Despite the reported effect of SSTR1 in inhibiting growth hormone secretion, SSTR1-/- mice had significantly reduced body weight with growth retardation. Perfusion of isolated mouse pancreata at 3 months of age demonstrated a significant increase in insulin secretion in SSTR1-/- mice compared with that of WT controls. We also found that at 3 months of age, SSTR1-/- mice had significantly decreased levels of systemic insulin secretion and were glucose intolerant. However, SSTR1 gene-ablated mice had a much higher rate of insulin clearance compared to WT mice at the same age. When challenged at 12 months of age, we found SSTR1-/- mice had increased glucose tolerance with exaggerated increase of insulin levels at the end of the experiment. Immunochemical analysis showed that the pancreatic islets of SSTR1-/- mice had significantly decreased levels of somatostatin staining and a significant decrease of SSTR5 expression. These results demonstrate that SSTR1 plays an important role in the regulation of insulin secretion in the endocrine pancreas in mice.

Pharmaceutical Drug Discovery: Designing the Blockbuster Drug

Twenty years ago, drug discovery was a somewhat plodding and scholastic endeavor; those days are gone. The intellectual challenges are greater than ever but the pace has changed. Although there are greater opportunities for therapeutic targets than ever before, the costs and risks are great and the increasingly competitive environment makes the pace of pharmaceutical drug hunting range from exciting to overwhelming. These changes are catalyzed by major changes to drug discovery processes through application of rapid parallel synthesis of large chemical libraries and high-throughput screening. These techniques result in huge volumes of data for use in decision making. Besides the size and complex nature of biological and chemical data sets and the many sources of data “noise”, the needs of business produce many, often conflicting, decision criteria and constraints such as time, cost, and patent caveats. The drive is still to find potent and selective molecules but, in recent years, key aspects of drug discovery are being shifted to earlier in the process. Discovery scientists are now concerned with building molecules that have good stability but also reasonable properties of absorption into the bloodstream, distribution and binding to tissues, metabolism and excretion, low toxicity, and reasonable cost of production. These requirements result in a high-dimensional decision problem with conflicting criteria and limited resources. An overview of the broad range of issues and activities involved in pharmaceutical screening is given along with references for further reading.

Chemogenomics in drug discovery

Chemogenomics is a new strategy in drug discovery which, in principle, searches for all molecules that are capable of interacting with any biological target. Because of the almost infinite number of drug-like organic molecules, this is an impossible task. Therefore chemogenomics has been defined as the investigation of classes of compounds (libraries) against families of functionally related proteins. In this definition, chemogenomics deals with the systematic analysis of chemical-biological interactions. Congeneric series of chemical analogs are probes to investigate their action on specific target classes, e.g., GPCRs, kinases, phosphodiesterases, ion channels, serine proteases, and others. Whereas such a strategy developed in pharmaceutical industry almost 20 years ago, it is now more systematically applied in the search for target- and subtype-specific ligands. The term "privileged structures" has been defined for scaffolds, such as the benzodiazepines, which very often produce biologically active analogs in a target family, in this case in the class of G-protein-coupled receptors. The SOSA approach is a strategy to modify the selectivity of biologically active compounds, generating new drug candidates from the side activities of therapeutically used drugs.

Somatostatin modulates dopamine release in rat retina

The aim of the present study was to determine the possible role of somatostatin as a modulator of dopamine release in rat retina. Basal release of dopamine, and how this is influenced by somatostatin receptor (sst) selective ligands, was examined ex vivo in rat retinal explants. Dopamine levels were quantified by high-pressure liquid chromatography (HPLC) with electrochemical detection. Basal levels of dopamine were measured over 120 min of tissue incubation and found to be 1.17+/-0.35 ng/ml. Somatostatin (10(-6), 10(-5), 10(-4)M) increased dopamine levels in a concentration-dependent manner, while the sst(2) antagonist CYN154806 (10(-4)M) reversed its actions. BIM23014 (sst(2) agonist) increased dopamine levels in a statistically significant manner only at the concentration of 10(-5)M. The sst(1) agonist L797.591 (10(-5), 10(-4)M) also increased dopamine levels, while activation of the sst(3) receptor (sst(3) agonist, L796.778, 10(-4)M) had no effect. These data substantiate a neuromodulatory role for sst(1) and sst(2) somatostatin receptors in the retina and show for the first time somatostatin's influence on dopamine release.

Reagent Selector: using Synthon Analysis to visualize reagent properties and assist in combinatorial library design

Reagent Selector is an intranet-based tool that aids in the selection of reagents for use in combinatorial library construction. The user selects an appropriate reagent group as a query, for example, primary amines, and further refines it on the basis of various physicochemical properties, resulting in a list of potential reagents. The results of this selection process are, in turn, converted into synthons: the fragments or R-groups that are to be incorporated into the combinatorial library. The Synthon Analysis interface graphically depicts the chemical properties for each synthon as a function of the topological bond distance from the scaffold attachment point. Displayed in this fashion, the user is able to visualize the property space for the universe of synthons as well as that of the synthons selected. Ultimately, the reagent list that embodies the selected synthons is made available to the user for reagent procurement. Application of the approach to a sample reagent list for a G-protein coupled receptor targeted library is described.

Somatostatin receptor type 2 undergoes plastic changes in the human epileptic dentate gyrus

Temporal lobe epilepsy (TLE) is characterized by hippocampal sclerosis together with profound losses and phenotypic changes of different classes of interneurons, including those expressing somatostatin (SRIF). To understand the functional significance of the plasticity of SRIF transmission in TLE, unraveling the status of SRIF receptors is, however, a prerequisite. To address this issue, we characterized expression and distribution of the major SRIF receptor, the sst2 subtype, in hippocampal tissue resected in patients with TLE using complementary neuroanatomic approaches. In patients with hippocampal sclerosis, the number of cells expressing sst2 receptor mRNA as well as sst2 receptor-binding sites and immunoreactivity decreased significantly in the CA1-3, reflecting neuronal loss. By contrast, in the dentate gyrus, sst2 receptor mRNA expression was strongly increased in the granule cell layer, and sst2 receptor-binding sites and immunoreactivity was preserved in the inner but decreased significantly in the outer molecular layer. In this latter region, pronounced changes in SRIF terminal fields were observed. Decreased receptor density in the distal dendrites of granule cells is likely to reflect downregulation of sst2 receptors in response to physiopathologic release of SRIF. Because sst2 receptors have anticonvulsant and antiepileptogenic properties, this phenomenon may contribute to the etiology of TLE seizures.

Discovery of iodinated somatostatin analogues selective for hsst2 and hsst5 with excellent inhibition of growth hormone and prolactin release from rat pituitary cells

Inhibition of growth hormone (GH) and prolactin (PRL) release from the anterior pituitary gland is mediated through somatostatin receptor subtypes sst2 and sst5. It has been found that somatostatin (SS) analogues that are selective for both receptor subtypes are more effective at inhibiting GH and PRL release than monospecific analogues alone. We synthesized several disulfide-bridged octapeptide SS analogues. Iodinated compounds 7, (4-amino-3-iodo)-d-Phe-c[Cys-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH2, and 9, (4-amino-3-iodo)-d-Phe-c[Cys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH2, were as potent as somatostatin in binding at receptors hsst2 and hsst5 and inhibited GH and PRL release from rat pituitary cells as potently as somatostatin.

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

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

Somatostatin receptors (sst2) regulate cGMP production in rat retina

The present study investigated the effect of somatostatin in the regulation of cGMP levels in rat retina and the mechanisms involved in this process. Isolated rat retinas were treated alone or in the presence of somatostatin (0.01-10 microM), BIM23014 (sst2 agonist, 0.01-10 microM), L-796,778 (sst3 agonist, 10 microM), somatostatin (0.1 microM) in combination with CYN154806 (sst2 antagonist, 1 microM), N(G)-methyl-L-arginine acetate salt (NMMA, inhibitor of the nitric oxide synthase (NOS), 250 microM), orthovanadate (inhibitor of tyrosine phosphatase, SHP-1, 1 microM), and arginine alone (250 microM). cGMP levels were quantified by ELISA. Immunohistochemistry studies were performed for the detection of cGMP and nNOS, while Western blot analysis was employed for the detection of SHP-1. Somatostatin increased cGMP levels in a concentration-dependent manner. This increase was inhibited by CYN154806. BIM23014 increased cGMP levels only at the concentration of 10 microM, while L-796,778 had no effect. NMMA blocked completely the somatostatin stimulated increase of cGMP levels and nNOS was detected in rat retina. cGMP immunoreactivity was observed primarily in bipolar cells only of nitroprusside-treated retinas. SHP-1 inhibition by orthovanadate reduced the somatostatin effect in a statistically significant manner. These results suggest that a SRIF/SHP-1/NO/cGMP mechanism underlies the actions of somatostatin in the retina and in its influence of retinal circuitry.

The somatostatin sst1 receptor: an autoreceptor for somatostatin in brain and retina?

The sst1 receptor was the first of the 5 somatostatin receptors to be cloned by homology with the glucagon receptor 13 years ago. It is a 7-transmembrane domain G-protein-coupled receptor that is negatively coupled to adenylyl cyclase, but can also trigger other transduction pathways. The distribution of sst1 mRNA, immunolabeling, and radioligand binding are not entirely overlapping, but the recent availability of knockout (KO) mice and a (still limited) number of selective agonists/antagonists has increased our knowledge about this receptor. These new tools have helped to reveal a role for the sst1 receptor in hippocampal, hypothalamic, basal ganglia, and retinal functions. In at least the latter 3 structures, the sst1 receptor appears to act as an inhibitory autoreceptor located on somatostatin neurons, whereas in the hippocampus such a role is still based on circumstantial evidence.

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

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

Somatostatin inhibits insulin release via SSTR2 in hamster clonal beta-cells and pancreatic islets

Somatostatin (SST) inhibits pancreatic endocrine secretion. It is generally accepted that SSTR2 and SSTR5 mediate the inhibition of glucagon and insulin release, respectively. The present study was performed to test the hypothesis that SSTR2, but not SSTR5, mediates SST-induced inhibition of insulin release in hamster beta-cells. Both hamster clonal beta-cells HIT-T15 and pancreatic islets were used to test this hypothesis. Both SST and a nonpeptide SSTR2 agonist L-779,976 (1-100 nM) inhibited insulin release from HIT-T15 and islets in a concentration-dependent manner. In contrast, nonpeptide agonists for SSTR1, 3, 4 and 5 at the highest concentration studied (1 microM) failed to inhibit insulin release. PRL-2903, a peptide SSTR2 antagonist (0.1-1 muicroM), antagonized SST-induced inhibition of insulin release in a concentration-dependent manner. Taken together, we conclude that, in hamster beta-cells, SST inhibits insulin release via SSTR2 but not SSTR5.

Neurobiological Effects of Bisphenol A May Be Mediated by Somatostatin Subtype 3 Receptors in Some Regions of the Developing Rat Brain

Considerable attention has been focused on environmental disruptors such as the xenoestrogen bisphenol A, which influences reproductive, developmental, and cognitive activities through its interaction with specific neuromediating systems in an estrogen-like fashion. In the present study, the effects of this xenoestrogen proved to be preferentially directed toward hypothalamic and extrahypothalamic somatostatin receptor subtype 3, which displayed a higher binding affinity of its specific nonpeptide agonist L-796-778 than that of L-779-976 (subtype 2). One type of action, with respect to animals treated with vehicle alone, consisted of a very strong (p < 0.001) decrease of somatostatin receptor subtype 3 mRNA levels in layer V of the frontoparietal cortex of adult rats (Sprague-Dawley) after transplacental and lactational exposure to bisphenol A (400 microg/kg/day). Similarly, such treatment in 7-day-old rats was responsible for a very strong reduction of the subtype 3 mRNA levels in the hypothalamic periventricular nuclei and a strong (p < 0.01) increase of the subtype 3 mRNA levels in the ventromedial nuclei. Moreover, even greater upregulated and downregulated activities were reported when subtype 3 mRNA levels were determined in the presence of receptor agonists specific for distinct alpha GABA(A) receptor subunits (alpha(1,5)). The predominant effects of bisphenol A on somatostatin receptor subtype 3 mRNA levels occurring in an alpha GABA(A) subunit-dependent manner tend to suggest the early modulatory importance of this environmental disruptor on cross-talking mechanisms that are implicated in the plasticity of neural circuits, with consequential influence on neuroendocrine/sociosexual behaviors.

Utilizing the intramolecular Fukuyama–Mitsunobu reaction for a flexible synthesis of novel heterocyclic scaffolds for peptidomimetic drug design

We report the synthesis of the novel scaffolds pyrazino[1,2-b]isoquinoline and pyrrolo[1,2-a]pyrazine displaying the somatostatin pharmacophores. Both classes of compounds contain a pyrazine heterocycle, which can be prepared in a straightforward manner utilizing an intramolecular Fukuyama-Mitsunobu reaction. As both the families derive from amino acids, they can be accessed in high optical purity.

Somatostatin receptors in primary human breast cancer: quantitative analysis of mRNA for subtypes 1–5 and correlation with receptor protein expression and tumor pathology

Somatostatin receptors (SSTRs) have been identified in most hormone-producing tumors as well as in breast cancer. In the present study, we determined SSTR1-5 expression in primary ductal NOS breast tumors through semi-quantitative RT-PCR and immunocytochemistry. The results from the analysis of 98 samples were correlated with several key histological markers and receptor expression. All five SSTR subtypes are variably expressed at the mRNA level in breast tumors with 91% of samples showing SSTR1, 98% SSTR2, 96% SSTR3, 76% SSTR4, and 54% SSTR5. SSTR1-5 are localized to both tumor cells and the surrounding peritumoral regions as detected by immunocytochemistry. Levels of SSTR mRNA, when corrected for beta-actin levels, were highest for SSTR3 followed by SSTR1, SSTR2, SSTR5, and SSTR4. Furthermore, there was good correlation between mRNA and protein expression with 84% for SSTR1, 79% for SSTR2, 89% for SSTR3, 68% for SSTR4, 68% for SSTR5, and 78% for all five receptors. SSTR1, 2 and 4 were correlated with ER levels whereas SSTR2 showed an additional correlation with PR levels. These correlations were independent of patient age and histological grade. Moreover, using immunocytochemistry, blood vessels exhibited receptor-specific localization for SSTR2 and SSTR5. Our results indicate significant correlations between mRNA and protein expression along with receptor-specific correlations with histological markers as well as ER and PR levels. Differential distribution of SSTR subtypes in tumors and receptor-specific expression in vascular structures may be considered as a novel diagnosis for breast tumors with receptor subtype agonists.

Mechanisms of somatostatin-evoked responses in neurons of the rat lateral amygdala

The effects of somatostatin in the rat lateral amygdala (LA) in vitro were investigated through whole cell recording techniques. Somatostatin induced an inwardly rectifying K+ current in approximately 98% of LA projection neurons. Half-maximal effects were obtained by 189 nM somatostatin. The effects of somatostatin were insensitive to tetrodotoxin, reduced by Ba2+, occluded or abolished by the presence of nonhydrolysable GTP or GDP analogues, respectively, and blocked or mimicked by a somatostatin receptor type 2 antagonist (BIM-23627) or somatostatin receptor type 2 agonist (L-779,976), respectively, while somatostatin receptor type 1, 3 and 4 agonists were ineffective (L-797,591, L-796,778, L-803,087). Responses to somatostatin were associated with membrane hyperpolarization and decrease in input resistance, resulting in a dampening of cell excitability. It is suggested that these cellular mechanisms contribute to the role of somatostatin in decreasing anxiety behaviour as well as to anticonvulsant and antiepileptogenic actions of somatostatin or somatostatin agonists in the amygdala.

Development of growth hormone secretagogues

The GH secretagogues (GHS) were developed by reverse pharmacology. The objective was to develop small molecules with pharmacokinetics suitable for once-daily oral administration that would rejuvenate the GH/IGF-I axis. Neither the receptor nor the ligand that controlled pulse amplitude of hormone release was known; therefore, identification of lead structures was based on function. I reasoned that GH pulse amplitude could be increased by four possible mechanisms: 1) increasing GHRH release; 2) amplifying GHRH signaling in somatotrophs of the anterior pituitary gland; 3) reducing somatostatin release; and 4) antagonizing somatostatin receptor signaling. Remarkably, the GHS act through all four mechanisms to reproduce a young adult physiological GH profile in elderly subjects that was accompanied by increased bone mineral density and lean mass, modest improvements in strength, and improved recovery from hip fracture. Furthermore, restoration of thymic function was induced in old mice. The GHS receptor (GHS-R) was subsequently identified by expression cloning and found to be a previously unknown G protein-coupled receptor expressed predominantly in brain, pituitary gland, and pancreas. Reverse pharmacology was completed when the cloned GHS-R was exploited to identify an endogenous agonist (ghrelin) and a partial agonist (adenosine); ghsr-knockout mice studies confirmed that GHS are ghrelin mimetics.

Receptor Signaling and Endocytosis Are Differentially Regulated by Somatostatin Analogs

Upon hormone stimulation, the sst2 somatostatin receptor couples to adenylyl cyclase through G(i/o) proteins and undergoes rapid endocytosis via clathrin-coated pits. In this study, we determined the relationship between the ability of ligands to induce sst2 receptor internalization and inhibit adenylyl cyclase. Immunocytochemical studies demonstrated that peptide agonists [such as somatostatin-14, cortistatin-17, octreotide, vapreotide, KE108 (Tyr0-cyclo[d-diaminobutyric acid-Arg-Phe-Phe-d-Trp-Lys-Thr-Phe]), and SOM230 (cyclo[diaminoethylcarbamoyl-hydroxyproline-phenylglycine-d-Trp-Lys-(4-O-benzyl)-l-Tyr-Phe])] and nonpeptide agonists (such as L-779,976), stimulated the rapid endocytosis of sst2 receptors in human embryonic kidney 293 and CHO-K1 cells. In contrast, two antagonists did not induce receptor endocytosis by themselves and completely blocked agonist stimulation. Using a quantitative enzyme-linked immunosorbent assay to measure sst2 receptor sequestration, we found that peptide agonists varied by more than 100-fold in their potencies but exhibited the same efficacy as somatostatin14. In contrast, L-779,976 did not induce maximal receptor internalization. It is interesting that although betaarrestin-2 was recruited to cell surface sst2 receptors after stimulation with either somatostatin14 or L-779,976, the betaarrestin-receptor complex dissociated earlier in the endocytic pathway with the nonpeptide ligand. Although all agonists, including L-779,976, produced the same maximal inhibition of cyclic AMP, the potency ratio for inhibition of cyclic AMP and stimulation of receptor endocytosis varied by 15-fold. In general, native peptides showed similar potencies for cyclic AMP inhibition and receptor endocytosis, whereas short therapeutic analogs were substantially more potent at inhibiting cyclic AMP synthesis. These results demonstrate that the activity of somatostatin analogs to regulate receptor endocytosis and signaling are not tightly linked and provide compelling evidence for the induction of agonist specific states of the sst2 receptor.