The second intracellular loop of the calcitonin gene-related peptide receptor provides molecular determinants for signal transduction and cell surface expression

The calcitonin gene-related peptide (CGRP) receptor is a heterodimer of a family B G-protein-coupled receptor, calcitonin receptor-like receptor (CLR), and the accessory protein receptor activity modifying protein 1. It couples to Gs, but it is not known which intracellular loops mediate this. We have identified the boundaries of this loop based on the relative position and length of the juxtamembrane transmembrane regions 3 and 4. The loop has been analyzed by systematic mutagenesis of all residues to alanine, measuring cAMP accumulation, CGRP affinity, and receptor expression. Unlike rhodopsin, ICL2 of the CGRP receptor plays a part in the conformational switch after agonist interaction. His-216 and Lys-227 were essential for a functional CGRP-induced cAMP response. The effect of (H216A)CLR is due to a disruption to the cell surface transport or surface stability of the mutant receptor. In contrast, (K227A)CLR had wild-type expression and agonist affinity, suggesting a direct disruption to the downstream signal transduction mechanism of the CGRP receptor. Modeling suggests that the loop undergoes a significant shift in position during receptor activation, exposing a potential G-protein binding pocket. Lys-227 changes position to point into the pocket, potentially allowing it to interact with bound G-proteins. His-216 occupies a position similar to that of Tyr-136 in bovine rhodopsin, part of the DRY motif of the latter receptor. This is the first comprehensive analysis of an entire intracellular loop within the calcitonin family of G-protein-coupled receptor. These data help to define the structural and functional characteristics of the CGRP-receptor and of family B G-protein-coupled receptors in general. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.

Adrenomedullin: receptor and signal transduction

Adrenomedullin is a vascular tissue peptide and a member of the calcitonin family of peptides, which includes calcitonin calcitonin-gene-related peptide (CGRP) and amylin. Its many biological actions are mediated via CGRP type 1 (CGRP(1)) receptors and by specific adrenomedullin receptors. Although the pharmacology of these receptors is distinct, they are both represented in molecular terms by the type II family G-protein-coupled receptor, calcitonin-receptor-like receptor (CRLR). The specificity here is defined by co-expression of receptor-activity-modifying proteins (RAMPs). CGRP(1) receptors are represented by CRLR and RAMP1, and specific adrenomedullin receptors by CRLR and RAMP2 or 3. Here we discuss how CRLR/RAMP2 relates to adrenomedullin binding, pharmacology and pathophysiology, and how chemical cross-linking of receptor-ligand complexes in tissue relates to that in CRLR/RAMP2-expressing cells. CRLR, like other type II family G-protein-coupled receptors, signals via G(s) and adenylate cyclase activation. We demonstrated that adrenomedullin signalling in cell lines expressing specific adrenomedullin receptors followed this expected pattern.

Signal transduction pathways involved in proteolysis-inducing factor induced proteasome expression in murine myotubes

The proteolysis-inducing factor (PIF) is produced by cachexia-inducing tumours and initiates protein catabolism in skeletal muscle. The potential signalling pathways linking the release of arachidonic acid (AA) from membrane phospholipids with increased expression of the ubiquitin-proteasome proteolytic pathway by PIF has been studied using C2C12 murine myotubes as a surrogate model of skeletal muscle. The induction of proteasome activity and protein degradation by PIF was blocked by quinacrine, a nonspecific phospholipase A2 (PLA2) inhibitor and trifluroacetyl AA, an inhibitor of cytosolic PLA2. PIF was shown to increase the expression of calcium-independent cytosolic PLA2, determined by Western blotting, at the same concentrations as those inducing maximal expression of 20S proteasome α-subunits and protein degradation. In addition, both U-73122, which inhibits agonist-induced phospholipase C (PLC) activation and D609, a specific inhibitor of phosphatidylcholine-specific PLC also inhibited PIF-induced proteasome activity. This suggests that both PLA 2 and PLC are involved in the release of AA in response to PIF, and that this is important in the induction of proteasome expression. The two tyrosine kinase inhibitors genistein and tryphostin A23 also attenuated PIF-induced proteasome expression, implicating tyrosine kinase in this process. PIF induced phosphorylation of p44/42 mitogen-activated protein kinase (MAPK) at the same concentrations as that inducing proteasome expression, and the effect was blocked by PD98059, an inhibitor of MAPK kinase, as was also the induction of proteasome expression, suggesting a role for MAPK activation in PIF-induced proteasome expression. © 2003 Cancer Research UK.

Ceramide and reactive oxygen species (ROS) as signal transduction molecules in inflammation

Reactive oxygen species (ROS) and the sphingolipid ceramide are each partly responsible for the intracellular signal transduction of a variety of physiological, pharmacological or environmental agents. Furthermore, the enhanced production of many of these agents, that utilise ROS and ceramide as signalling intermediates, is associated with the aetiologies of several vascular diseases (e.g. atherosclerosis) or disorders of inflammatory origin (e.g. rheumatoid arthritis; RA). Excessive monocyte recruitment and uncontrolled T cell activation are both strongly implicated in the chronic inflammatory responses that are associated with these pathologies. Therefore the aims of this thesis are (1) to further elucidate the cellular responses to modulations in intracellular ceramide/ROS levels in monocytes and T cells, in order to help resolve the mechanisms of progression of these diseases and (2) to examine both existing agents (methotrexate) and novel targets for possible therapeutic manipulation. Utilising synthetic, short chain ceramide to mimic the cellular responses to fluctuations in natural endogenous ceramide or, stimulation of CD95 to induce ceramide formation, it is described here that ceramide targets and manipulates two discrete sites responsible for ROS generation, preceding the cellular responses of growth arrest in U937 monocytes and apoptosis in Jurkat T-cells. In both cell types, transient elevations in mitochondrial ROS generation were observed. However, the prominent redox altering effects appear to be the ceramide-mediated reduction in cytosolic peroxide, the magnitude of which dictates in part the cellular response in U937 monocytes, Jurkat T-cells and primary human peripheral blood resting or PHA-activated T-cells in vitro. The application of synthetic ceramides to U937 monocytes for short (2 hours) or long (16 hours) treatment periods reduced the membrane expression of proteins associated with cell-cell interaction. Furthermore, ceramide treated U937 monocytes demonstrated reduced adhesion to 5 or 24 hour LPS activated human umbilical vein endothelial cells (HUVEC) but not resting HUVEC. Consequently it is hypothesised that the targeted treatment of monocytes from patients with cardiovascular diseases with short chain synthetic ceramide may reduce disease progression. Herein, the anti-inflammatory and immunosuppressant drug, methotrexate, is described to require ROS production for the induction of cytostasis or cytotoxicity in U937 monocytes and Jurkat T-cells respectively. Further, ROS are critical for methotrexate to abrogate monocyte interaction with activated HUVEC in vitro. The histological feature of RA of enhanced infiltration, survivability and hyporesponsiveness of T-cells within the diseased synovium has been suggested to arise from aberrant signalling. No difference in the concentrations of endogenous T-cell ceramide, the related lipid diacylglycerol (DAG) and cytosolic peroxide ex vivo was observed. TCR activation following PHA exposure in vitro for 72 hours did not induced maintained perturbations in DAG or ceramide in T-cells from RA patients or healthy individuals. However, T-cells from RA patients failed to upregulate cytosolic peroxide in response to PHA, unlike those from normals, despite expressing identical levels of the activation marker CD25. This inability to upregulate cytosolic peroxide may contribute to the T-cell pathology associated with RA by affecting the signalling capacity of redox sensitive biomolecules. These data highlight the importance of two distinctive cellular pools of ROS in mediating complex biological events associated with inflammatory disease and suggest that modulation of cellular ceramides represents a novel therapeutic strategy to minimise monocyte recruitment.

Aspects of signal transduction in the gastrointestinal tract

This study was undertaken to increase knowledge of the mechanisms of inter- and intracellular signalling in the gastrointestinal tract. Specific aims were: to use cell lines to elucidate factors affecting growth of gastric cells, to investigate the distribution and aspects of function of isoforms of protein kinase C in a gastric cell line and in the rat gastrointestinal tract and to determine the presence and regulation of nitric oxide synthase in gastrointestinal tissues from the rat and in cell lines. The gastric cancer cell line HGT-1 was used to investigate control of growth. Increases in cell number were found to be dependent on the seeding density of the cells. In cells plated at low density insulin, epidermal growth factor and gastrin all increased cell number. Gastrin produced a bell-shaped dose response curve with a maximum activity at 5nM. No effect of gastrin was apparent in cells plated at high density. α and β isoforms of protein kinase C were found, by immunoblotting procedures, to be widespread in the gastrointestinal tract of the rat, but protein kinase Cε was confined to the gastric mucosa and gastrointestinal smooth muscle. HGT-1 cells contained protein kinase C α and ε but β or γ were not detected. Preincubation of HGT-1 cells for 24h with 1μM phorbol-12,13-dibutyrate down-regulated protein kinase C α but not ε. The inhibition by the activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate (TPA) of the histamine-stimulated increase in cAMP in HGT-1 cells was down regulated by phorbol-12,13-dibutyrate. Inhibition of histamine-stimulation of adenylate cyclase by TPA was Ca2+-dependent and inhibited by the addition of an antibody to protein kinase C α. A role for protein kinase C α in modulating the effect of histamine on adenylate cyclase in HGT-1 cells is suggested. No nitric oxide synthase activity was detected in the gastrointestinal cell lines HGT-l, MKN-45 or CaCo-2. Ca2+-dependent nitric oxide synthase activity was observed in the gastric mucosa and the gastrointestinal smooth muscle from stomach to colon. The gastric: mucosal enzyme was soluble and showed half-maximal activity at 400nM Ca2+. Pretreatment of rats with endotoxin (3mg/kg body weight) induced nitric oxide synthase activity in both jejunal, ileal and colonic mucosa and muscle. A major portion of the induced activity in ileal and colonic mucosa was Ca2+-independent. Nitric oxide synthase activity in a high-density fraction of gastric mucosal cells was inhibited in a dose-dependent fashion by L-nitroarginine, NG-monomethyl-L-arginine, trifluoperazine and L-canavanine (in descending order of potency). Preincubation with okadaic acid and addition of ATPlMg2+ to the homogenisation buffer inhibited enzyme activity, which implies that phosphorylation inhibits gastric mucosal nitric oxide synthase.

Regulation of signal transduction by calcitonin gene-related peptide receptors

Calcitonin gene-related peptide (CGRP) plays a pivotal role in migraine, activating its cognate receptor to initiate intracellular signalling. This atypical receptor comprises a distinct assembly, made up of a G protein-coupled receptor (GPCR), a single transmembrane protein, and an additional protein that is required for Ga(s) coupling. By altering the expression of individual receptor components, it might be possible to adjust cellular sensitivity to CGRP. In recognition of the increasing clinical significance of CGRP receptors, it is timely to review the signalling pathways that might be controlled by this receptor, how the activity of the receptor itself is regulated, and our current understanding of the molecular mechanisms involved in these processes. Like many GPCRs, the CGRP receptor appears to be promiscuous, potentially coupling to several G proteins and intracellular pathways. Their precise composition is likely to be cell type-dependent, and much work is needed to ascertain their physiological significance.

The signal transduction pathways involved in the respiratory burst of murine peritoneal macrophages

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Proteomic analysis of phosphorylation, oxidation and nitrosylation in signal transduction

Signal transduction pathways control cell fate, survival and function. They are organized as intricate biochemical networks which enable biochemical protein activities, crosstalk and subcellular localization to be integrated and tuned to produce highly specific biological responses in a robust and reproducible manner. Post translational Modifications (PTMs) play major roles in regulating these processes through a wide variety of mechanisms that include changes in protein activities, interactions, and subcellular localizations. Determining and analyzing PTMs poses enormous challenges. Recent progress in mass spectrometry (MS) based proteomics have enhanced our capability to map and identify many PTMs. Here we review the current state of proteomic PTM analysis relevant for signal transduction research, focusing on two areas: phosphorylation, which is well established as a widespread key regulator of signal transduction; and oxidative modifications, which from being primarily viewed as protein damage now start to emerge as important regulatory mechanisms.