An overview of the oxytocin-oxytocin receptor signaling network

Oxytocin, a nine amino acid long neuropeptide hormone, is synthesized in the hypothalamus and stored and released from the neural lobe of the pituitary gland. Although commonly known for its central role in the regulation of parturition and lactation, oxytocin signaling also plays a key role in modulating social behavior, evoking contentment, initiating maternal behavior, inducing trust, generosity and bonding in humans and animals. Oxytocin signaling can prove to be of great importance in therapeutics and drug targeting because of its diverse range of actions. However, a well annotated map of oxytocin signaling pathway is currently lacking in the publicly available pathway resources. Therefore, we systematically curated the available signaling information of oxytocin from published literature and collated the data to develop a more complete map. We cataloged 66 molecules belonging to oxytocin signaling pathway, which included 9 protein-protein interactions, 39 post-translational modifications, 14 protein translocation events and 22 activation/inhibition events. Further, Oxytocin signaling network data is made freely available to academic fraternity by integrating this into NetPath (http://www.netpath.org/), a freely available human signaling pathway resource developed previously by our group.     

The 5-HT1A receptor: an overview of recent advances

Progress in the field of neuronal receptor research has accelerated during the last few years due to developments in pharmacology and molecular biology. This is particularly true in the case of the serotonin 5-HT_1A receptor. In 1983 the very selective, high affinity 5-HT_1A agonist 8-OH-DPAT was developed which allowed the pharmacology and distribution of the 5-HT_1A receptor in the central nervous system of the rat and man to be extensively characterized. By 1987, the gene encoding this receptor protein was cloned and sequenced, allowing not only elucidation of its structure, but also better insight into the nature of its coupling to transmembrane signal transduction systems. Thus in a short period of time considerable knowledge has accumulated on how serotonin exerts its functions in the central nervous system via the 5-HT_1A receptor. In the present review we will briefly discuss some of the latest developments regarding the 5-HT_1A receptor.     

5-Hydroxytryptamine receptor "families"

The identification of multiple receptor subtypes for 5-hydroxytryptamine (5-HT) made by using radioligand binding techniques proliferated at a brisk rate in the 1980s. The application of molecular biological techniques to 5-HT receptor studies is likely to lead to an expansion rather than a reduction in the number of distinct 5-HT receptor subtypes. Although the current status of 5-HT receptor pharmacology may appear to be overwhelmingly confusing to most investigators, the evolving data suggest that 5-HT receptor subtypes can be categorized into three major families. Each family consists of multiple receptor subtypes that share similarities in their molecular biological, pharmacological, biochemical, and/or physiological properties. This review provides a summary of recent data as well as a framework for the classification of 5-HT receptor subtypes.     

A network map of thrombopoietin signaling

Thrombopoietin (THPO), also known as megakaryocyte growth and development factor (MGDF), is a cytokine involved in the production of platelets. THPO is a glycoprotein produced by liver and kidney. It regulates the production of platelets by stimulating the differentiation and maturation of megakaryocyte progenitors. It acts as a ligand for MPL receptor, a member of the hematopoietic cytokine receptor superfamily and is essential for megakaryocyte maturation. THPO binding induces homodimerization of the receptor which results in activation of JAKSTAT and MAPK signaling cascades that subsequently control cellular proliferation, differentiation and other signaling events. Despite the importance of THPO signaling in various diseases and biological processes, a detailed signaling network of THPO is not available in any publicly available database. Therefore, in this study, we present a resource of signaling events induced by THPO that was manually curated from published literature on THPO. Our manual curation of thrombopoietin pathway resulted in identification of 48 molecular associations, 66 catalytic reactions, 100 gene regulation events, 19 protein translocation events and 43 activation/inhibition reactions that occur upon activation of thrombopoietin receptor by THPO. THPO signaling pathway is made available on NetPath, a freely available human signaling pathway resource developed previously by our group. We believe this resource will provide a platform for scientific community to accelerate further research in this area on potential therapeutic interventions.     

Comprehensive network map of interferon gamma signaling

Interferon gamma (IFN-γ), is a cytokine, which is an important regulator of host defense system by mediating both innate and adaptive immune responses. IFN-γ signaling is primarily associated with inflammation and cell-mediated immune responses. IFN-γ is also represented as antitumor cytokine which facilitates immunosurveillance in tumor cells. In addition, IFN-γ mediated signaling also elicits pro-tumorigenic transformations and promotes tumor progression. Impact of IFN-γ signaling in mammalian cells has been widely studied which indicate that IFN-γ orchestrates distinct cellular functions including immunomodulation, leukocyte trafficking, apoptosis, anti-microbial, and both anti- and pro-tumorigenic role. However, a detailed network of IFN-γ signaling pathway is currently lacking. Therefore, we systematically curated the literature information pertaining to IFN-γ signaling and develop a comprehensive signaling network to facilitate better understanding of IFN-γ mediated signaling. A total of 124 proteins were catalogued that were experimentally proven to be involved in IFN-γ signaling cascade. These 124 proteins were found to participate in 81 protein-protein interactions, 94 post-translational modifications, 20 translocation events, 54 activation/inhibiton reactions. Further, 236 differential expressed genes were also documented in IFN-γ mediated signaling. IFN-γ signaling pathway is made freely available to scientific audience through NetPath at (http://www.netpath.org/pathways?path_id=NetPath_32). We believe that documentation of reactions pertaining to IFN-γ signaling and development of pathway map will facilitate further research in IFN-γ associated human diseases including cancer.     

Role of somatostatin receptor 1 and 5 on epidermal growth factor receptor mediated signaling

Epidermal growth factor (EGF) regulates normal and tumor cell proliferation via epidermal growth factor receptor (EGFR) phosphorylation, homo- or heterodimerization and activation of mitogen-activated protein kinases (MAPKs) and PI3K/AKT cell survival pathways. In contrast, SST via activation of five different receptor subtypes inhibits cell proliferation and has been potential target in tumor treatment. To gain further insight for the effect of SSTRs on EGFR activated signaling, we determine the role of SSTR1 and SSTR1/5 in human embryonic kidney (HEK) 293 cells. We here demonstrate that cells transfected with SSTR1 or SSTR1/5 negatively regulates EGF mediated effects attributed to the inhibition of EGFR phosphorylation, MAPKs as well as the cell survival signaling. Furthermore, SSTR effects were significantly enhanced in cells when EGFR was knock down using siRNA or treated with selective antagonist (AG1478). Most importantly, the presence of SSTR in addition to modulating signaling pathways leads to the dissociation of the constitutive and EGF induced heteromeric complex of EGFR/ErbB2. Furthermore, cells cotransfected with SSTR1/5 display pronounced effect of SST on the signaling and dissociation of the EGFR/ErbB2 heteromeric complex than the cells expressing SSTR1 alone. Taken together this study provides the first evidence that the presence of SSTR controls EGF mediated cell survival pathway via dissociation of ErbB heteromeric complex. We propose that the activation of SSTR and blockade of EGFR might serve novel therapeutic approach in inhibition of tumor proliferation.     

5-Hydroxytryptamine receptor subtypes in vertebrates and invertebrates

In the last few years, molecular biology has led to the cloning and characterization of several 5-HT receptors (serotonin receptors) in vertebrates and in invertebrates. These studies have allowed identification not only of 5-HT receptors already described but also of novel subtypes. The molecular cloning of 13 different mammalian receptor subtypes revealed an unexpected heterogeneity among 5-HT receptors. Except for the 5-HT₃ receptors which are ligand-gated ion channel receptors, all the other 5-HT receptors belong to the large family of receptors interacting with G proteins. Based on their amino acid sequence homology and coupling to second messengers these receptors can be divided into distinct families: the 5-HT₁ family contains receptors that are negatively coupled to adenylate cyclase; the 5-HT₁ family includes receptors that stimulate phospholipase C; the adenylyl cyclase stimulatory receptors are a heterogeneous group including the 5-HT₄ receptor which has not yet been cloned, the Drosophila 5-HT_drol receptor and two mammalian receptors tentatively named 5-HT₆ and 5-HT₇ receptors. The 5-HT_5A and 5-HT_5B receptors might constitute a new family of 5-HT receptors whose effectors are unknown. This review focusses on the molecular characteristics of the cloned 5-HT receptors such as their structure, their effector systems and their distribution within the central nervous system. The existence of a large number of receptors with distinct signalling properties and expression patterns might enable a single substance like 5-HT to generate simultaneously a large panel of effects in many brain structures. The availability of the genes encoding these receptors has already allowed a partial characterization of their structure-function relationship and will probably allow in the future a dissection of the contribution of each of these receptor subtypes to physiology and behaviour.     

A network map of Interleukin-10 signaling pathway

Interleukin-10 (IL-10) is an anti-inflammatory cytokine with important immunoregulatory functions. It is primarily secreted by antigen-presenting cells such as activated T-cells, monocytes, B-cells and macrophages. In biologically functional form, it exists as a homodimer that binds to tetrameric heterodimer IL-10 receptor and induces downstream signaling. IL-10 is associated with survival, proliferation and anti-apoptotic activities of various cancers such as Burkitt lymphoma, non-Hodgkins lymphoma and non-small scell lung cancer. In addition, it plays a central role in survival and persistence of intracellular pathogens such as Leishmania donovani, Mycobacterium tuberculosis and Trypanosoma cruzi inside the host. The signaling mechanisms of IL-10 cytokine are not well explored and a well annotated pathway map has been lacking. To this end, we developed a pathway resource by manually annotating the IL-10 induced signaling molecules derived from literature. The reactions were categorized under molecular associations, activation/inhibition, catalysis, transport and gene regulation. In all, 37 molecules and 76 reactions were annotated. The IL-10 signaling pathway can be freely accessed through NetPath, a resource of signal transduction pathways previously developed by our group.     

Syncytial apoptosis signaling network induced by the HIV-1 envelope glycoprotein complex: an overview

Infection by human immunodeficiency virus-1 (HIV-1) is associated with a progressive decrease in CD4 T-cell numbers and the consequent collapse of host immune defenses. The major pathogenic mechanism of AIDS is the massive apoptotic destruction of the immunocompetent cells, including uninfected cells. The latter process, also known as by-stander killing, operates by various mechanisms one of which involves the formation of syncytia which undergo cell death by following a complex pathway. We present here a detailed and curated map of the syncytial apoptosis signaling network, aimed at simplifying the whole mechanism that we have characterized at the molecular level in the last 15 years. The map was created using Systems Biology Graphical Notation language with the help of CellDesigner software and encompasses 36 components (proteins/genes) and 54 interactions. The simplification of this complex network paves the way for the development of novel therapeutic strategies to eradicate HIV-1 infection. Agents that induce the selective death of HIV-1-elicited syncytia might lead to the elimination of viral reservoirs and hence constitute an important complement to current antiretroviral therapies.     

5-Hydroxytryptamine 2A receptor signaling cascade modulates adiponectin and plasminogen activator inhibitor 1 expression in adipose tissue

Knowledge of the regulatory factors associated with down-regulation of adiponectin gene expression and up-regulation of PAI-1 gene expression is crucial to understand the pathophysiological basis of obesity and metabolic diseases, and could establish new treatment strategies for these conditions. We showed that expression of 5-HT(2A) receptors was up-regulated in hypertrophic 3T3-L1 adipocytes, which exhibited decreased expression of adiponectin and increased expression of PAI-1. 5-HT(2A) receptor antagonists and suppression of 5-HT(2A) receptor gene expression enhanced adiponectin expression. Activation of Gq negatively regulated adiponectin expression, and inhibition of mitogen-activated protein kinase reversed the Gq-induced effect. Moreover, the 5-HT(2A) receptor blockade reduced PAI-1 expression. These findings indicate that antagonism of 5-HT(2A) receptors in adipocytes could improve the obesity-linked decreases in adiponectin expression and increases in PAI-1 expression.     

Navigating the chaperone network: an integrative map of physical and genetic interactions mediated by the hsp90 chaperone

Physical, genetic, and chemical-genetic interactions centered on the conserved chaperone Hsp90 were mapped at high resolution in yeast using systematic proteomic and genomic methods. Physical interactions were identified using genome-wide two hybrid screens combined with large-scale affinity purification of Hsp90-containing protein complexes. Genetic interactions were uncovered using synthetic genetic array technology and by a microarray-based chemical-genetic screen of a set of about 4700 viable yeast gene deletion mutants for hypersensitivity to the Hsp90 inhibitor geldanamycin. An extended network, consisting of 198 putative physical interactions and 451 putative genetic and chemical-genetic interactions, was found to connect Hsp90 to cofactors and substrates involved in a wide range of cellular functions. Two novel Hsp90 cofactors, Tah1 (YCR060W) and Pih1 (YHR034C), were also identified. These cofactors interact physically and functionally with the conserved AAA(+)-type DNA helicases Rvb1/Rvb2, which are key components of several chromatin remodeling factors, thereby linking Hsp90 to epigenetic gene regulation.     

Estrogen receptor mediated inhibition of cancer cell invasion and motility: An overview

In this overview of results from our laboratory, we address the question of the role of estrogens during early steps of metastasis, involving cell invasion through the basement membrane and cell motility. The motility of several estrogen receptor (ER) positive breast (MCF7, T47D) and ovarian (BG-1, SKOV3, PEO4) cancer cell lines was studied using a modified Boyden chamber assay. We observed, in all cases, estradiol induced inhibition of cancer cell invasion and motility. A similar inhibitory effect of estradiol was found when the wild-type ER was stably transfected in the ER-negative MDA-MB231 cells and 3Y1-Ad12 cancer cells. The mechanism of this inhibitory effect is unknown. In ovarian cancer, however, it may involve intermediary proteins such as fibulin-1, an extracellular matrix protein that strongly interacts with fibronectin and which is induced by estrogen and secreted by ovarian cancer cells. We conclude that estrogens in ER-positive breast and ovarian cancers have a dual effect, since they stimulate tumor growth but inhibit invasion and motility. This may be consistent with the good initial prognostic value of ER-positive breast cancers compared to ER negative breast cancers noted in several clinical studies.     

5-Hydroxytryptamine affects rat migrating myoelectric complexes through different receptor subtypes: evidence from 5-hydroxytryptophan administration

The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on jejunal migrating myoelectric complexes (MMCs) was investigated in conscious rats. Subcutaneous administration of low doses of 5-HTP (1-2 mg/kg) shortened the period between migrating complexes, whereas high doses of the compound (4-8 mg/kg) disrupted the MMC pattern. The serotonin (5-HT2) antagonist methysergide (8 mg/kg s.c.) did not alter basal MMC, neither did it prevent the effect of a low dose of 5-HTP; conversely, it antagonized the disruption due to the high dose. The 5-HT3 antagonist ICS 205-930 (30 micrograms/kg s.c.) decreased MMC frequency; administration of 2 mg/kg 5-HTP following ICS 205-930 brought the frequency of myoelectric complexes back to basal values. Both effects of 5-HTP were prevented by the decarboxylase inhibitor benserazide (85 mg/kg i.p.), which per se caused a transient inhibition of spiking activity. The results suggest that rat MMCs can be influenced in a composite fashion by progressively increasing concentrations of 5-HT, which in turn activate different receptor subtypes. A peripheral neuronal receptor, probably belonging to the 5-HT3 subclass, mediates the increase in MMC frequency observed after low doses of 5-HTP; higher levels of serotonin activate 5-HT2 receptors, causing disruption of cycling activity. Additionally, 5-HT3 receptors, but not 5-HT2, appear to be relevant for the regulation of the MMC pattern by the endogenous amine.     

1-Deoxy-D-xylulose 5-phosphate reductoisomerase: an overview

The methylerythritol phosphate pathway to isoprenoids, an alternate biosynthetic route present in many bacteria, algae, plants, and the malarial parasite Plasmodium falciparum, has become an attractive target for the development of new antimalarial and antibacterial compounds. The second enzyme in this pathway, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR; EC 1.1.1.267), has been shown to be the molecular target for fosmidomycin, a promising antimalarial drug. This enzyme converts 1-deoxy-D-xylulose 5-phosphate (DXP) into the branched compound 2-C-methyl-D-erythritol 4-phosphate (MEP). The transformation of DXP into MEP requires an isomerization, followed by a NADPH-dependent reduction. The discovery of DXR, its subsequent characterization, and the identification of inhibitors will be presented.     

The role of tryptophan residues in the 5-Hydroxytryptamine(3) receptor ligand binding domain

Aromatic amino acids are important components of the ligand binding site in the Cys loop family of ligand-gated ion channels. To examine the role of tryptophan residues in the ligand binding domain of the 5-hydroxytryptamine(3) (5-HT(3)) receptor, we used site-directed mutagenesis to change each of the eight N-terminal tryptophan residues in the 5-HT(3A) receptor subunit to tyrosine or serine. The mutants were expressed as homomeric 5-HT(3A) receptors in HEK293 cells and analyzed with radioligand binding, electrophysiology, and immunocytochemistry. Mutation of Trp(90), Trp(183), and Trp(195) to tyrosine resulted in functional receptors, although with increased EC(50) values (2-92-fold) to 5-HT(3) receptor agonists. Changing these residues to serine either ablated function (Trp(90) and Trp(183)) or resulted in a further increase in EC(50) (Trp(195)). Mutation of residue Trp(60) had no effect on ligand binding or receptor function, whereas mutation of Trp(95), Trp(102), Trp(121), and Trp(214) ablated ligand binding and receptor function, and all but one of the receptors containing these mutations were not expressed at the plasma membrane. We propose that Trp(90), Trp(183), and Trp(195) are intimately involved in ligand binding, whereas Trp(95), Trp(102), Trp(121), and Trp(214) have a critical role in receptor structure or assembly.