Characterization by photoaffinity labelling of the human platelet thromboxane A2/prostaglandin H2 receptor: evidence for N-linked glycosylation

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

Chemical Tools for Studying Lipid-Binding Class A G Protein-Coupled Receptors

Cannabinoid, free fatty acid, lysophosphatidic acid, sphingosine 1-phosphate, prostanoid, leukotriene, bile acid, and platelet-activating factor receptor families are class A G protein-coupled receptors with endogenous lipid ligands. Pharmacological tools are crucial for studying these receptors and addressing the many unanswered questions surrounding expression of these receptors in normal and diseased tissues. An inherent challenge for developing tools for these lipid receptors is balancing the often lipophilic requirements of the receptor-binding pharmacophore with favorable physicochemical properties to optimize highly specific binding. In this study, we review the radioligands, fluorescent ligands, covalent ligands, and antibodies that have been used to study these lipid-binding receptors. For each tool type, the characteristics and design rationale along with in vitro and in vivo applications are detailed.

A simple, quick, and high-yield preparation of the human thromboxane A2 receptor in full size for structural studies

Human thromboxane A2 receptor (TP), a G protein-coupled receptor (GPCR), is one of the most promising targets for developing the next generation of anti-thrombosis and hypertension drugs. However, obtaining a sufficient amount of the full-sized and active membrane protein has been the major obstacle for structural elucidation that reveals the molecular mechanisms of the receptor activation and drug designs. Here we report an approach for the simple, quick, and high-yield preparation of the purified and active full-sized TP in an amount suitable for structural studies. Glycosylated human TP was highly expressed in Sf-9 cells using an optimized baculovirus (BV) expression system. The active receptor was extracted and solubilized by different detergents for comparison and was finally purified to a nearly single band with a ratio of 1:0.9 +/- 0.05 (ligand:receptor molecule) in ligand binding using a Ni column with a relatively low yield. However, a high-yield purification (milligram quantity) of the TP protein, from a modulate scale of transfected Sf-9 cell culture, has been achieved by quick and simple purification steps, which include DNA digestion, dodecyl-maltoside detergent extraction, centrifugation, and FPLC purification. The purity and quantity of the purified TP, using the high-yield approach, were suitable for protein structural studies as evidenced by SDS-PAGE, Western blot analyses, ligand binding assays, and a feasibility test using high-resolution one-dimensional and two-dimensional (1)H NMR spectroscopic analyses. These studies provide a basis for the high-yield expression and purification of the GPCR for the structural and functional characterization using biophysics approaches.

Prostanoid receptors: ontogeny and implications in vascular physiology

Prostanoids exert significant effects on circulatory beds. They play a role in the response of the vasculature to adjustments in perfusion pressure and oxygen and carbon dioxide tension, and they mediate the actions of numerous factors. The role of prostanoids in governing circulation of the perinate is suggested to surpass that in the adult. Prostanoids are abundantly generated in the perinate. They have been implicated in autoregulation of blood flow as studied in brain and eyes. Prostaglandins are also dominant regulators of ductus arteriosus tone. The effects of these autacoids are mediated through specific G protein-coupled receptors. In addition to the pharmacological characterization of the prostanoid receptors, important advances in understanding the biology of these receptors have been made in the last decade. Their cloning and the development of animals with disrupted genes of these receptors have been very informative. The involvement of prostanoid receptors in the developing subject, especially on brain and ocular vasculature and on ductus arteriosus, has also begun to be investigated; the expression of these receptors changes with development. Some but not all of the ontogenic changes in these receptors are attributed to homologous regulation. Interestingly, in the process of elucidating their effects, functional perinuclear prostaglandin E2 receptors have been uncovered. This article reviews prostanoid receptors and addresses implications on the developing subject with attention to vascular physiology.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Prostaglandin E2 receptor EP3alpha subtype: the role of N-glycosylation in ligand binding as revealed by site-directed mutagenesis

Functional mouse prostaglandin E2 (PGE2) receptor EP3alpha subtype has been expressed in insect cells using a baculovirus system (Huang C. and Tai H.-H. Biochem J 1995; 307: 493-498). EP3alpha receptor has two potential sites (Asn-X-Ser/Thr), Asn 16 and Asn 193, for N-glycosylation. The role of glycosylation in ligand binding of the EP3alpha receptor was investigated by site-directed mutagenesis. Asn was mutated to Gln in each of the two potential glycosylation sites in the EP3alpha receptor. Recombinant wild-type and mutant EP3alpha receptors were expressed in insect cells using baculovirus. Ligand binding assay indicated that the affinity of PGE2 binding was reduced by 50% in the Gln 193 mutant EP3alpha receptor, while the specificity of ligand binding was unaltered. The affinity for PGE2 binding was not affected in the Gln 16 mutant EP3alpha receptor. However, its specificity was partially changed as the EP3-specific agonist became less effective in displacing the [3H]-PGE2 binding to the mutant receptor. These results indicated that N-glycosylation of the EP3alpha receptor could partially affect the affinity and specificity of the ligand binding.

Mechanism of platelet inhibition by nitric oxide: in vivo phosphorylation of thromboxane receptor by cyclic GMP-dependent protein kinase

Nitric oxide (NO) is a potent vasodilator and inhibitor of platelet activation. NO stimulates production of cGMP and activates cGMP-dependent protein kinase (G kinase), which by an unknown mechanism leads to inhibition of Galphaq-phospholipase C-inositol 1, 4,5-triphosphate signaling and intracellular calcium mobilization for several important agonists, including thromboxane A2 (TXA2). To explore the mechanism of platelet inhibition by NO, activation of platelet TXA2 receptors in the presence of cGMP was studied. The nonhydrolyzable analog 8-bromo-cyclic GMP (8-Br-cGMP) potently inhibited activation of the TXA2-specific GTPase in platelet membranes in a concentration-dependent fashion, suggesting that G kinase catalyzes the phosphorylation of some proximal component of the receptor-G protein signaling pathway. Nanomolar concentrations of G kinase were found to catalyze the phosphorylation of platelet TXA2 receptors in vitro, but not Galphaq copurifying with the TXA2 receptors in these experiments. Using immunoaffinity methods, in vivo phosphorylation of TXA2 receptors by cyclic GMP was demonstrated from 32P-labeled cells treated with 8-Br-cGMP. Peptide mapping studies of in vivo phosphorylated TXA2 receptors demonstrated cGMP mediates phosphorylation of the carboxyl terminus of the TXA2 receptor. G kinase also catalyzed the phosphorylation of peptides corresponding to the cytoplasmic tails of both alpha and beta forms of the receptor but not control peptide or a peptide corresponding to the third intracytoplasmic loop of the TXA2 receptor. These data identify TXA2 receptors as cGMP-dependent protein kinase substrates and support a novel mechanism for the inhibition of cell function by NO in which activation of G kinase inhibits signaling by G protein-coupled receptors by catalyzing their phosphorylation.

The role of N-glycosylation of human thromboxane A2 receptor in ligand binding

Thromboxane A2 receptor (TXA2R) was expressed in insect Sf21 cells and demonstrated to interact with 8-iso-PGF2 alpha and 9 alpha, 11 beta-PGF2 alpha with a potency similar to that of TXA2 agonist U46619. TXA2R was shown to be a glycoprotein. The role of N-glycosylation of TXA2R in ligand binding was investigated in the insect cells over-expressed with recombinant TXA2R. Deletion of the carbohydrate moiety by adding tunicamycin during infection of Sf21 cells or mutation of both potential N-glycosylation sites (Asn-4 and Asn-16) abolished the ligand binding of TXA2R, suggesting that N-glycosylation is crucial for binding function. Mutation of either Asn-4 or Asn-16 to a leucine did not have much effect on maximal binding. However, the mutant receptors possess lower binding affinity toward TXA2R antagonist [3H]SQ29548. Furthermore, the binding specificity of the mutant receptors was shown to be altered. Our data suggest that both Asn-4 and Asn-16 are glycosylated and glycosylation on either site is sufficient for ligand recognition. However, glycosylation on both sites is required to maintain binding affinity and specificity.

Inhibition of ligand binding to thromboxane A2/prostaglandin H2 receptors by diethylpyrocarbonate. Protection by receptor ligands and reversal by hydroxylamine

The potential of histidines to modulate the binding of agonists and antagonists to human platelet thromboxane A2 (TXA2) receptors was investigated. TXA2 receptors were purified from crude platelet membranes via affinity and wheat germ lectin chromatography. Radioligand binding studies were conducted using the TXA2, mimetic [125I]BOP (I-BOP (I-BOP = [1S-(1 alpha,2 beta(5Z),3 alpha(1E, 3R*),4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)7-oxabicyclo- [2.2.1]heptan-2-yl]-5-heptenoic acid) and the TXA2 receptor antagonist [125I]SAP (I-SAP = 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-iodobenzene- sulfonylamino)-bicyclo-[3.1.1]hept-2-yl]-(5Z)-heptenoic acid). The histidine modifying reagent diethyl-pyrocarbonate (DEPC) produced a concentration (30-100 microM) dependent inhibition of binding of both [125I]BOP and [125I]SAP. DEPC treatment significantly (P < 0.05, N = 6) decreased the affinity of the receptor for [125I]SAP (Kd = 2.4 +/- 0.4 and 5.4 +/- 0.4 nM, control and DEPC, respectively) without significantly decreasing the Bmax. The effects of DEPC were reversed by hydroxylamine. The inhibition of [125I]BOP and [125I]SAP binding produced by DEPC was reduced significantly by prior incubation of the purified receptors with the TXA2 receptor agonist U-46619 or the TXA2 receptor antagonist SQ 29548. The results strongly support the notion that one or more histidines reside in a domain that can modulate ligand binding to the TXA2 receptor.

Development of dual-acting agents for thromboxane receptor antagonism and thromboxane synthase inhibition--I. Synthesis, structure-activity relationship, and evaluation of substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids

A series of arylsulfonamido-substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids were synthesized and evaluated in vitro for their ability to act as both a thromboxane A2 receptor antagonist (TRA) and thromboxane synthase inhibitor (TSI). Variations of alkenoic acid chain length, olefin geometry, substituent effect on the benzenesulfonamido group, and conformational flexibility of the substituted arylsulfonamido group were examined. Among the various substituents, iodo-substitution gave the most potent compound. Conformational flexibility between the arylsulfonamido group and the phenyl ring attached to the alkenoic acid side chain significantly enhanced the dual activities. The compound (E)-21c was identified as the most potent TRA/TSI (TRA: Kd = 53 nM; TSI: IC50 = 23 nM) in the series studied. The compounds 9c and 10c have indicated that these series of compounds are orally active and are specific TSIs as exhibited by the so-called 'shunt' effect on prostacyclin synthesis in vitro.

Localization of [125I]SAP-N3 binding in the human platelet thromboxane A2/prostaglandin H2 receptor by proteolytic cleavage analysis

Photo-affinity labeling studies of purified human platelet thromboxane A2/prostaglandin H2 receptor by the ligand 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-azido-3- iodobenzenesulfonylamino)bicyclo[3.1.1]hept-2-yl]-5(Z)-heptenoic acid ([125I]SAP-N3) combined with proteolytic cleavage studies were performed to initiate studies aimed at localizing the binding domain of this ligand. Two endoproteinases, endoproteinase Asp-N (Asp-N) and endoproteinase Lys-C (Lys-C), and the endoglycosidase, N-glycosidase F (endo-F), were employed to generate fragments for analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) autoradiography. Computational analysis of the published sequence was then employed to predict cleavage products and then compared to the observed digestion results. Results of this work suggest that the majority of the binding domain of [125I]SAP-N3 includes putative transmembrane regions M-3 and M-4 (amino acids 99-192) with a minor component at the amino and carboxyl terminus.