Emerging roles of DP and CRTH2 in allergic inflammation

Role of phospholipase A2s and lipid mediators in secondary damage after spinal cord injury

Inflammation is considered to be an important contributor to secondary damage after spinal cord injury (SCI). This secondary damage leads to further exacerbation of tissue loss and functional impairments. The immune responses that are triggered by injury are complex and are mediated by a variety of factors that have both detrimental and beneficial effects. In this review, we focus on the diverse effects of the phospholipase A(2) (PLA(2)) superfamily and the downstream pathways that generate a large number of bioactive lipid mediators, some of which have pro-inflammatory and demyelinating effects, whereas others have anti-inflammatory and pro-resolution properties. For each of these lipid mediators, we provide an overview followed by a discussion of their expression and role in SCI. Where appropriate, we have compared the latter with their role in other neurological conditions. The PLA(2) pathway provides a number of targets for therapeutic intervention for the treatment of SCI and other neurological conditions.

PGH1, the precursor for the anti-inflammatory prostaglandins of the 1-series, is a potent activator of the pro-inflammatory receptor CRTH2/DP2

Prostaglandin H₁ (PGH₁) is the cyclo-oxygenase metabolite of dihomo-γ-linolenic acid (DGLA) and the precursor for the 1-series of prostaglandins which are often viewed as “anti-inflammatory”. Herein we present evidence that PGH₁ is a potent activator of the pro-inflammatory PGD₂ receptor CRTH2, an attractive therapeutic target to treat allergic diseases such as asthma and atopic dermatitis. Non-invasive, real time dynamic mass redistribution analysis of living human CRTH2 transfectants and Ca²⁺ flux studies reveal that PGH₁ activates CRTH2 as PGH₂, PGD₂ or PGD₁ do. The PGH₁ precursor DGLA and the other PGH₁ metabolites did not display such effect. PGH₁ specifically internalizes CRTH2 in stable CRTH2 transfectants as assessed by antibody feeding assays. Physiological relevance of CRTH2 ligation by PGH₁ is demonstrated in several primary human hematopoietic lineages, which endogenously express CRTH2: PGH₁ mediates migration of and Ca²⁺ flux in Th2 lymphocytes, shape change of eosinophils, and their adhesion to human pulmonary microvascular endothelial cells under physiological flow conditions. All these effects are abrogated in the presence of the CRTH2 specific antagonist TM30089. Together, our results identify PGH₁ as an important lipid intermediate and novel CRTH2 agonist which may trigger CRTH2 activation in vivo in the absence of functional prostaglandin D synthase.

Isoquinoline derivatives as potent CRTH2 receptor antagonists: synthesis and SAR

Synthesis and structure-activity relationship of a novel series of isoquinoline CRTH2 receptor antagonists are described. One of the most potent compounds, TASP0376377 (6m), showed not only potent binding affinity (IC(50)=19 nM) but also excellent functional antagonist activity (IC(50)=13 nM). TASP0376377 was tested for its ability of a chemotaxis assay to show the effectiveness (IC(50)=23 nM), which was in good agreement with the CRTH2 antagonist potency. Furthermore, TASP0376377 showed sufficient selectivity for binding to CRTH2 over the DP1 prostanoid receptor (IC(50)>1 μM) and COX-1 and COX-2 enzymes (IC(50)>10 μM).

[Physiopathology of aspirin intolerant asthma]

Aspirin-exacerbated respiratory disease (AERD) refers to the development of bronchoconstriction in individuals with asthma following the ingestion of aspirin. AERD affects up to 20 % of adults with asthma. At present, no reliable in vitro test is available to confirm the diagnosis. The confirmation of the diagnosis of AERD therefore depends on the response to challenge testing with aspirin. The pathogenesis of AERD is linked to abnormalities in arachidonic acid metabolism. Prior to exposure to aspirin, respiratory mucosal inflammation is the result of a cell infiltration, an overproduction of leukotrienes, prostaglandins D2, 5-oxo-eicosatetraenoic acid and an underproduction of lipoxins. After aspirin ingestion, patients with AERD synthesize excessive amounts of cysteinyl leukotrienes and prostaglandin metabolites involved in bronchoconstriction. New hypotheses concerning AERD pathogenesis have been added to the initial cyclooxygenase theory. These propose that AERD may be linked to the complement system, adenosine metabolism or angiotensin converting enzyme gene and IgE receptor gene polymorphisms.

Prostagladin D2 is a mast cell-derived antiangiogenic factor in lung carcinoma

It is well established that prostaglandins (PGs) are involved in tumor angiogenesis and growth, yet the role of prostaglandin D(2) (PGD(2)) remains virtually unknown. Here, we show that host hematopoietic PGD(2) synthase (H-PGDS) deficiency enhances Lewis lung carcinoma (LLC) progression, accompanied by increased vascular leakage, angiogenesis, and monocyte/mast cell infiltration. This deficiency can be rescued by hematopoietic reconstitution with bone marrow from H-PGDS-naive (WT) mice. In tumors on WT mice, c-kit(+) mast cells highly express H-PGDS. Host H-PGDS deficiency markedly up-regulated the expression of proangiogenic factors, including TNF-α in the tumor. In mast cell-null Kit(W-sh/W-sh) mice, adoptive transfer of H-PGDS-deficient mast cells causes stronger acceleration in tumor angiogenesis and growth than in WT mast cells. In response to LLC growth, H-PGDS-deficient mast cells produce TNF-α excessively. This response is suppressed by the administration of a synthetic PGD(2) receptor agonist or a degradation product of PGD(2), 15-deoxy-Δ(12,14)-PGJ(2). Additional TNF-α deficiency partially counteracts the tumorigenic properties seen in H-PGDS-deficient mast cells. These observations identify PGD(2) as a mast cell-derived antiangiogenic factor in expanding solid tumors. Mast cell-derived PGD(2) governs the tumor microenvironment by restricting excessive responses to vascular permeability and TNF-α production.

Lipid-cytokine-chemokine cascades orchestrate leukocyte recruitment in inflammation

Chemoattractants are pivotal mediators of host defense, orchestrating the recruitment of immune cells into sites of infection and inflammation. Chemoattractants display vast chemical diversity and include bioactive lipids, proteolytic fragments of serum proteins, and chemokines (chemotactic cytokines). All chemoattractants induce chemotaxis by activating seven-transmembrane-spanning GPCRs expressed on immune cells, establishing the concept that all chemoattractants are related in function. However, although chemoattractants have overlapping functions in vitro, recent in vivo data have revealed that they function, in many cases, nonredundantly in vivo. The chemically diverse nature of chemoattractants contributes to the fine control of leukocyte trafficking in vivo, with sequential chemoattractant use guiding immune cell recruitment into inflammatory sites. Lipid mediators frequently function as initiators of leukocyte recruitment, attracting the first immune cells into tissues. These initial responding immune cells produce cytokines locally, which in turn, induce the local release of chemokines. Local chemokine production then markedly amplifies subsequent waves of leukocyte recruitment. These new discoveries establish a paradigm for leukocyte recruitment in inflammation--described as lipid-cytokine-chemokine cascades--as a driving force in the effector phase of immune responses.

D-type prostanoid receptor enhances the signaling of chemoattractant receptor-homologous molecule expressed on T(H)2 cells

BACKGROUND

Prostaglandin (PG) D(2) is substantially involved in allergic responses and signals through the 7 transmembrane-spanning/G protein-coupled receptors, chemoattractant receptor-homologous molecule expressed on T(H)2 cells (CRTH2), and D-type prostanoid (DP) receptor.

OBJECTIVE

Although the proinflammatory function of CRTH2 is well recognized and CRTH2 is hence considered an important emerging pharmacotherapeutic target, the role of the DP receptor in mediating the biological effects of PGD(2) in patients with allergic inflammation has remained unclear.

METHODS

The cross-talk of CRTH2 and DP receptors was investigated by using both a recombinant HEK293 cell model and human eosinophils in Ca(2+) mobilization assays, coimmunoprecipitation, Western blotting, radioligand binding, and immunofluorescence.

RESULTS

We show that CRTH2 and DP receptors modulate one another's signaling properties and form CRTH2/DP heteromers without altering their ligand-binding capacities. We find that the DP receptor amplifies the CRTH2-induced Ca(2+) release from intracellular stores and coincidentally forfeits its own signaling potency. Moreover, desensitization or pharmacologic blockade of the DP receptor hinders CRTH2-mediated signal transduction. However, CRTH2 internalization occurs independently of the DP receptor. In cells that express both receptors, pharmacologic blockade of Gα(q/11) proteins abolishes the Ca(2+) response to both CRTH2 and DP agonists, whereas inhibition of Gα(i) proteins selectively attenuates the CRTH2-mediated response but not the DP signal.

CONCLUSION

Our data demonstrate the capacity of DP receptors to amplify the biological response to CRTH2 activation. Therefore the CRTH2/DP heteromer might not only represent a functional signaling unit for PGD(2) but also a potential target for the development of heteromer-directed therapies to treat allergic diseases.

Optimization of the Central Core of Indolinone-Acetic Acid-Based CRTH2 (DP2) Receptor Antagonists

New spiroindolinone antagonists of CRTH2 are described. Following identification of insufficient stability in human plasma as an important liability of the lead compounds, replacement of the spirosuccinimide core with a spirohydantoin or spiropyrrolidinone structure has yielded a compound that is fully stable in human plasma and with good potency in a human whole blood assay (IC50 = 69 nM) but shows a much lower oral bioavailability (6-9% in rodents) than the earlier compounds. Successive optimization aimed at restoring an acceptable oral bioavailability has yielded compound (S)-17a, which exhibits both stability in human plasma and a good oral bioavailability in rat (37%) and mouse (39%). This compound is also active in a mouse model of ovalbumin-induced lung inflammation following oral dosing at 30 mg/kg.

Discovery of selective indole-based prostaglandin D₂ receptor antagonist

A series of N-benzoyl-2-methylindole-3-acetic acids were synthesized and biologically evaluated as prostaglandin (PG) D₂ receptor antagonists. Some of the selected compounds significantly inhibited OVA-induced vascular permeability in guinea pig conjunctiva after oral dosing. Structure-activity relationship study is presented.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Involvement of Th2 cytokines in the mouse model of flutamide-induced acute liver injury

Drug-induced liver injury is a growing concern for pharmaceutical companies and patients because numerous drugs have been linked to hepatotoxicity and it is the most common reason for a drug to be withdrawn. Flutamide rarely causes liver dysfunction in humans, and immune allergic reactions have been suggested in some cases. In this study, we investigated the mechanisms of flutamide-induced liver injury in BALB/c mice. Plasma alanine aminotransferase and aspartate aminotransferase levels were significantly increased 3, 6 and 9 h after flutamide (1500 mg kg⁻¹ , p.o.) administration. The biomarker for oxidative stress was not changed, but Th2-dominant immune-related factors, such as interleukin (IL)-4, IL-5, STAT6 and GATA-binding protein (GATA)-3, were induced in flutamide-administered mice. The pre-administration of monoclonal-IL-4 antibody suppressed the hepatotoxicity of flutamide. In addition, we investigated the effect of 13,14-dihydro-15-keto-PGD₂ (DK-PGD₂; 10 µg per mouse, i.p.) administration on flutamide-induced acute liver injury. Coadministration of DK-PGD₂ and flutamide resulted in a significant increase in alanine aminotransferase and a remarkable increase of macrophage inflammatory protein-2. In conclusion, we demonstrated that flutamide-induced acute liver injury is mediated by Th2-dominant immune responses in mice.

Co-operative signalling through DP(1) and DP(2) prostanoid receptors is required to enhance leukotriene C(4) synthesis induced by prostaglandin D(2) in eosinophils

BACKGROUND AND PURPOSE

Prostaglandin (PG) D(2) has emerged as a key mediator of allergic inflammatory pathologies and, particularly, PGD(2) induces leukotriene (LT) C(4) secretion from eosinophils. Here, we have characterized how PGD(2) signals to induce LTC(4) synthesis in eosinophils.

EXPERIMENTAL APPROACH

Antagonists and agonists of DP(1) and DP(2) prostanoid receptors were used in a model of PGD(2) -induced eosinophilic inflammation in vivo and with PGD(2) -stimulated human eosinophils in vitro, to identify PGD(2) receptor(s) mediating LTC(4) secretion. The signalling pathways involved were also investigated.

KEY RESULTS

In vivo and in vitro assays with receptor antagonists showed that PGD(2) -triggered cysteinyl-LT (cysLT) secretion depends on the activation of both DP(1) and DP(2) receptors. DP(1) and DP(2) receptor agonists elicited cysLTs production only after simultaneous activation of both receptors. In eosinophils, LTC(4) synthesis, but not LTC(4) transport/export, was activated by PGD(2) receptor stimulation, and lipid bodies (lipid droplets) were the intracellular compartments of DP(1) /DP(2) receptor-driven LTC(4) synthesis. Although not sufficient to trigger LTC(4) synthesis by itself, DP(1) receptor activation, signalling through protein kinase A, did activate the biogenesis of eosinophil lipid bodies, a process crucial for PGD(2) -induced LTC(4) synthesis. Similarly, concurrent DP(2) receptor activation used Pertussis toxin-sensitive and calcium-dependent signalling pathways to achieve effective PGD(2) -induced LTC(4) synthesis.

CONCLUSIONS AND IMPLICATIONS

Based on pivotal roles of cysLTs in allergic inflammatory pathogenesis and the collaborative interaction between PGD(2) receptors described here, our data suggest that both DP(1) and DP(2) receptor antagonists might be attractive candidates for anti-allergic therapies.

Chitin particles induce size-dependent but carbohydrate-independent innate eosinophilia

Murine Mϕ that phagocytose CMP develop into M1; this response depends on the size and the chemical composition of the particles. In contrast, recent studies concluded that chitin particles induce M2 and eosinophil migration, promoting acquired Th2 immune responses against chitin-containing microbes or allergens. This study examined whether these apparently inconsistent responses to chitin could be induced by variation in the size and chemical composition of the chitin particles. We compared the responses of Mϕ with CMP, LCB, and Sephadex G-100 beads (>40 μm). Beads were given i.p. to WT mice and to mice deficient in a CRTH2, a receptor for the eosinophil chemoattractant PGD(2). In contrast to the M1 activation induced by CMP, i.p. administration of LCB or Sephadex beads induced within 24 h a CRTH2-dependent peritoneal eosinophilia, as well as CRTH2-independent activation of peritoneal Mϕ that expressed Arg I, an M2 phenotype. LCB-induced Mϕ exhibited elevated Arg I and a surface MR, reduced surface TLR2 levels, and no change in the levels of CHI3L1 or IL-10 production. Our results indicate that the effects of chitin in vivo are highly dependent on particle size and that large, nonphagocytosable beads, independent of their chemical composition, induce innate eosinophilia and activate Mϕ expressing several M2, but not M1, phenotypes.

IL-4 mediates dicloxacillin-induced liver injury in mice

Drug-induced liver injury (DILI) is a major problem in drug development and clinical drug therapy. In most cases, the mechanisms are still unknown. It is difficult to predict DILI in humans due to the lack of experimental animal models. Dicloxacillin, penicillinase-sensitive penicillin, rarely causes cholestatic or mixed liver injury, and there is some evidence for immunoallergic idiosyncratic reaction in human. In this study, we investigated the mechanisms of dicloxacillin-induced liver injury. Plasma ALT and total-bilirubin (T-Bil) levels were significantly increased in dicloxacillin-administered (600 mg/kg, i.p.) mice. Dicloxacillin administration induced Th2 (helper T cells)-mediated factors and increased the plasma interleukin (IL)-4 level. Neutralization of IL-4 suppressed the hepatotoxicity of dicloxacillin, and recombinant mouse IL-4 administration (0.5 or 2.0 μg/mouse, i.p.) exacerbated it. Chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTh2) is a cognate receptor for prostaglandin (PG) D(2), and is suggested to be involved in Th2-dependent allergic inflammation. We investigated the effect of 13,14-Dihydro-15-keto-PGD(2) (DK-PGD(2); 10 μg/mouse, i.p.) administration on dicloxacillin-induced liver injury. DK-PGD(2)/dicloxacillin coadministration resulted in a significant increase of alanine aminotransferases and a remarkable increase of macrophage inflammatory protein 2 expression. In conclusion, to the best of our knowledge, this is the first report to demonstrate that dicloxacillin-induced liver injury is mediated by a Th2-type immune reaction and exacerbated by DK-PGD(2).

ANKRD13C acts as a molecular chaperone for G protein-coupled receptors

Although the mechanisms that regulate folding and maturation of newly synthesized G protein-coupled receptors are crucial for their function, they remain poorly characterized. By yeast two-hybrid screening, we have isolated ANKRD13C, a protein of unknown function, as an interacting partner for the DP receptor for prostaglandin D(2). In the present study we report the characterization of this novel protein as a regulator of DP biogenesis and trafficking in the biosynthetic pathway. Co-localization by confocal microscopy with an endoplasmic reticulum (ER) marker, subcellular fractionation experiments, and demonstration of the interaction between ANKRD13C and the cytoplasmic C terminus of DP suggest that ANKRD13C is a protein associated with the cytosolic side of ER membranes. Co-expression of ANKRD13C with DP initially increased receptor protein levels, whereas siRNA-mediated knockdown of endogenous ANKRD13C decreased them. Pulse-chase experiments indicated that ANKRD13C can promote the biogenesis of DP by inhibiting the degradation of newly synthesized receptors. However, a prolonged interaction between ANKRD13C and DP resulted in ER retention of misfolded/unassembled forms of the receptor and to their proteasome-mediated degradation. ANKRD13C also regulated the expression of other GPCRs tested (CRTH2, thromboxane A(2) (TPα), and β2-adrenergic receptor), whereas it did not affect the expression of green fluorescent protein, GRK2 (G protein-coupled receptor kinase 2), and VSVG (vesicular stomatitis virus glycoprotein), showing specificity toward G protein-coupled receptors. Altogether, these results suggest that ANKRD13C acts as a molecular chaperone for G protein-coupled receptors, regulating their biogenesis and exit from the ER.

Novel CRTH2 antagonists: a review of patents from 2006 to 2009

IMPORTANCE OF THE FIELD

The receptor CRTH2 (also known as DP₂) is an important mediator of the inflammatory effects of prostaglandin D₂ and has attracted much attention as a therapeutic target for the treatment of conditions such as asthma, COPD, allergic rhinitis and atopic dermatitis.

AREAS COVERED IN THIS REVIEW

The validation of CRTH2 as a therapeutic target and the early antagonists are summarized, CRTH2 antagonists published in the patent literature from 2006 to 2009 are comprehensively covered and a general update on the recent progress in the development of CRTH2 antagonists for the treatment of inflammatory diseases is provided.

WHAT THE READER WILL GAIN

Insight into the validation of CRTH2 as a therapeutic target, a comprehensive overview of the development of new CRTH2 ligands between 2006 and 2009, and a general overview of the state of the art.

TAKE HOME MESSAGE

Many diverse potent CRTH2 antagonists are now available, and several are in or on the way into the clinic. It is still early to draw final conclusions, but preliminary results give reason for optimism, and the prospect that we will see new CRTH2 antagonists reaching the market for the treatment of asthma, rhinitis, atopic dermatitis and/or COPD seems good.

Potent and highly selective DP1 antagonists with 2,3,4,9-tetrahydro-1H-carbazole as pharmacophore

We discovered that the introduction of a methyl group to the benzylic position of the N-benzyl group in lead compound 1a has a dramatic effect on improving the binding selectivity of this ligand for the prostanoid receptors DP1 (receptor for prostaglandin D(2)) as compared to TP (receptor for thromboxane A(2)). Based on this discovery, we have synthesized a series of potent and highly selective DP1 antagonists. Among them, compound 1h was identified as a highly selective DP1 antagonist with excellent overall properties. It has a K(i) of 0.43 nM to DP1 in binding assay and an IC(50) of 2.5 nM in the DP1 functional assay. Its selectivity for DP1 over TP (the most potent receptor after DP1) exceeds 750-fold based on both binding and functional assays. These properties make 1h a very potent and highly selective DP1 receptor antagonist suitable for investigating the biological functions of DP1 in normal physiology and models of disease.

The mRNA level of Charcot-Leyden crystal protein/galectin-10 is a marker for CRTH2 activation in human whole blood in vitro

CRTH2 is one of the prostaglandin D₂ receptors and plays a proinflammatory role in allergic diseases. Gene expression markers in whole blood induced by CRTH2 activation have not previously been reported. Using microarray analyses of 54 675 genes, we revealed modest gene expression changes in human whole blood stimulated in vitro by a selective CRTH2 agonist, DK-PGD₂. Five genes were found to exhibit 1.5- to 2.6-fold changes in expression. The expression of Charcot-Leyden crystal protein/galectin-10 (CLC/Gal-10) in particular was consistently enhanced in human whole blood stimulated by DK-PGD₂, as confirmed by quantitative real-time polymerase chain reaction analyses. DK-PGD(2)-induced increases in blood CLC/Gal-10 mRNA levels were largely attenuated by the CRTH2 antagonist CAY10471.Thus, the DK-PGD₂-induced CLC/Gal-10 mRNA level can serve as a potential marker for monitoring pharmacodynamic effects of blood exposure to CRTH2 modulating agents.

Mast cell-derived prostaglandin D2 controls hyaluronan synthesis in human orbital fibroblasts via DP1 activation: implications for thyroid eye disease

Thyroid eye disease (TED) is a debilitating disorder characterized by the accumulation of adipocytes and hyaluronan (HA). Production of HA by fibroblasts leads to remarkable increases in tissue volume and to the anterior displacement of the eyes. Prostaglandin D(2) (PGD(2)), mainly produced by mast cells, promotes orbital fibroblast adipogenesis. The mechanism by which PGD(2) influences orbital fibroblasts and their synthesis of HA is poorly understood. We report here that mast cell-derived PGD(2) is a key factor that promotes HA biosynthesis by orbital fibroblasts. Primary orbital fibroblasts from TED patients were isolated and used to test the effects of PGD(2), prostaglandin J(2), as well as prostaglandin D receptor (DP) agonists and antagonists on HA synthesis. The expression of HA synthase (HAS), hyaluronidase, DP1, and DP2 mRNA levels was assessed by PCR. Small interfering RNAs against HAS1 or HAS2 were used to assess the importance of HAS isoforms on HA production. Treatment of human orbital fibroblasts with PGD(2) and PGJ(2) increased HA synthesis and HAS mRNA. HAS2 was the dominant isoform responsible for HA production by PGD(2). The effect of PGD(2) on HA production was mimicked by the selective DP1 agonist BW245C. The DP1 antagonist MK-0524 completely blocked PGD(2)-induced HA synthesis. Human mast cells (HMC-1) produced PGD(2). Co-culture of HMC-1 cells with orbital fibroblasts induced HA production and inhibition of mast cell-derived PGD(2) prevented HA synthesis. Mast cell-derived PGD(2) increased HA production via activation of DP1. Selectively targeting the production of PGD(2) and/or activation of DP1 may prevent pathological changes associated with TED.

Prostaglandins in pathogenesis and treatment of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS) characterized by inflammation, demyelination, axonal loss, and gliosis. The inflammatory lesions are manifested by a large infiltration and a heterogeneous population of cellular and soluble mediators of the immune system, such as T cells, B cells, macrophages, and microglia, as well as a broad range of cytokines, chemokines, antibodies, complement, and other toxic substances. Prostaglandins (PGs) are arachidonic acid-derived autacoids that have a role in the modulation of many physiological systems including the CNS, respiratory, cardiovascular, gastrointestinal, genitourinary, endocrine, and immune systems. PG production is associated with inflammation, a major feature in MS that is characterized by the loss of myelinating oligodendrocytes in the CNS. With respect to the role of PGs in the induction of inflammation, they can be effective mediators in the pathophysiology of MS. Thus use of agonists or antagonists of PG receptors may be considered as a new therapeutic protocol in MS. In this review, we try to clarify the role of PGs in immunopathology and treatment of MS.

Increased Concentrations of Prostaglandin D2 During Post-Fracture Bone Remodeling

Objective.

To test the hypothesis that increased concentrations of prostaglandin D2 (PGD2) correlate with bone remodeling. Studies using isolated bone cells indicate that PGD2 may be implicated in the regulation of bone homeostasis, with a positive influence on bone anabolism. We studied patients with traumatic fractures and age- and sex-matched healthy controls as an in vivo model of increased bone remodeling.

Methods.

Thirty-five patients with bone fracture and matched controls were recruited. Urine and sera samples were collected. Urinary 11ß-PGF2α, a PGD2 metabolite, and PGE2 metabolites (PGEM), serum lipocalin-type PGD2 synthase (L-PGDS), bone alkaline phosphatase (bone ALP), and crosslinked C-telopeptides of type I collagen (CTX) were measured.

Results.

At 5–6 weeks post-fracture, 11ß-PGF2α, L-PGDS, bone ALP, and CTX were significantly increased in the fracture patients compared to controls. PGEM levels were not different between groups. Levels of 11ß-PGF2α and bone ALP were positively correlated, suggesting that PGD2 may be implicated in fracture repair.

Conclusion.

These results support our working hypothesis that PGD2 could be implicated in the control of bone anabolism in humans.

Clinical studies of the DP1 antagonist laropiprant in asthma and allergic rhinitis

BACKGROUND

Prostaglandin D(2) is a proinflammatory mediator believed to be important in asthma and allergic rhinitis (AR). Allelic variants in the prostaglandin D(2) receptor type 1 (DP1) gene (PTGDR) have been suggested to be associated with asthma susceptibility.

OBJECTIVES

We sought to investigate the efficacy of the DP1 antagonist laropiprant (alone or with montelukast) in asthma and seasonal AR and explore whether sequence variations in PTGDR are associated with asthma severity.

METHODS

For asthma, in a double-blind crossover study, 100 patients with persistent asthma were randomized to placebo or laropiprant, 300 mg/d for 3 weeks, followed by addition of montelukast, 10 mg/d for 2 weeks. PTGDR promoter haplotypes were categorized as high, medium, or low transcriptional efficiency. The primary efficacy end point was FEV(1). For AR, in a double-blind parallel-group study, 767 patients sensitized to a regionally prevalent fall allergen with symptomatic fall rhinitis were allocated to laropiprant, 25 mg/d or 100 mg/d; cetirizine, 10mg/d; or placebo for 2 weeks. The primary end point was the Daytime Nasal Symptoms Score.

RESULTS

For asthma, no significant differences in FEV(1) or asthma symptoms were noted for laropiprant versus placebo or laropiprant plus montelukast vs montelukast (differences between montelukast and placebo: P <or= .001). No clear association was seen between haplotype pair (ie, diplotype) and asthma severity. For AR, although cetirizine (vs placebo) demonstrated an improvement in the Daytime Nasal Symptoms Score (P < .001), laropiprant did not.

CONCLUSION

Laropiprant did not demonstrate efficacy in asthmatic patients or patients with AR. Variations in PTGDR did not appear related to baseline asthma severity or treatment response (NCT00533208; NCT00783601).

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Mechanistic systems biology of inflammatory gene expression in airway smooth muscle as tool for asthma drug development

There is compelling evidence that airway smooth muscle cells may function as inflammatory cells in the airway system by producing multiple inflammatory cytokines in response to a large array of external stimuli such as acetylcholine, bradykinin, inflammatory cytokines, and toll-like receptor activators. However, how multiple extracellular stimuli interact in the regulation of inflammatory gene expression in an airway smooth muscle cell remains poorly understood. This review addresses the mechanistic systems biology of inflammatory gene expression in airway smooth muscle by discussing: a) redundancy underlying multiple stimulus-product relations in receptor-mediated inflammatory gene expression, and their regulation by convergent activation of Erk1/2 mitogen-activated protein kinase (MAPK), b) Erk1/2 MAPK-dependent induction of phosphatase expression as a negative feedback mechanism in the robust maintenance of inflammatory gene expression, and c) cyclooxygenase 2-dependent regulation of the differential temporal dynamics of early and late inflammatory gene expression. It is becoming recognized that a single-target approach is unlikely to be effective for the treatment of inflammatory airway diseases because airway inflammation is a result of complex interactions among multiple inflammatory mediators and cells types in the airway system. Understanding the mechanistic systems biology of inflammatory gene expression in airway smooth muscle and other cell types in the airway system may lead to the development of multi-target drug regimens for the treatment of inflammatory airway diseases such as asthma.

Role of prostaglandin D2 receptor DP as a suppressor of tumor hyperpermeability and angiogenesis in vivo

Although COX-dependent production of prostaglandins (PGs) is known to be crucial for tumor angiogenesis and growth, the role of PGD(2) remains virtually unknown. Here we show that PGD(2) receptor (DP) deficiency enhances tumor progression accompanied by abnormal vascular expansion. In tumors, angiogenic endothelial cells highly express DP receptor, and its deficiency accelerates vascular leakage and angiogenesis. Administration of a synthetic DP agonist, BW245C, markedly suppresses tumor growth as well as tumor hyperpermeability in WT mice, but not in DP-deficient mice. In a corneal angiogenesis assay and a modified Miles assay, host DP deficiency potentiates angiogenesis and vascular hyperpermeability under COX-2-active situation, whereas exogenous administration of BW245C strongly inhibits both angiogenic properties in WT mice. In an in vitro assay, BW245C does not affect endothelial migration and tube formation, processes that are necessary for angiogenesis; however, it strongly improves endothelial barrier function via an increase in intracellular cAMP production. Our results identify PGD(2)/DP receptor as a new regulator of tumor vascular permeability, indicating DP agonism may be exploited as a potential therapy for the treatment of cancer.

The C-terminal tail of CRTH2 is a key molecular determinant that constrains Galphai and downstream signaling cascade activation

Prostaglandin D(2) activation of the seven-transmembrane receptor CRTH2 regulates numerous cell functions that are important in inflammatory diseases, such as asthma. Despite its disease implication, no studies to date aimed at identifying receptor domains governing signaling and surface expression of human CRTH2. We tested the hypothesis that CRTH2 may take advantage of its C-tail to silence its own signaling and that this mechanism may explain the poor functional responses observed with CRTH2 in heterologous expression systems. Although the C terminus is a critical determinant for retention of CRTH2 at the plasma membrane, the presence of this domain confers a signaling-compromised conformation onto the receptor. Indeed, a mutant receptor lacking the major portion of its C-terminal tail displays paradoxically enhanced Galpha(i) and ERK1/2 activation despite enhanced constitutive and agonist-mediated internalization. Enhanced activation of Galpha(i) proteins and downstream signaling cascades is probably due to the inability of the tail-truncated receptor to recruit beta-arrestin2 and undergo homologous desensitization. Unexpectedly, CRTH2 is not phosphorylated upon agonist-stimulation, a primary mechanism by which GPCR activity is regulated. Dynamic mass redistribution assays, which allow label-free monitoring of all major G protein pathways in real time, confirm that the C terminus inhibits Galpha(i) signaling of CRTH2 but does not encode G protein specificity determinants. We propose that intrinsic CRTH2 inhibition by its C terminus may represent a rather unappreciated strategy employed by a GPCR to specify the extent of G protein activation and that this mechanism may compensate for the absence of the classical phosphorylation-dependent signal attenuation.

In vitro allergen challenge of peripheral blood induces differential gene expression in mononuclear cells of asthmatic patients: inhibition of cytosolic phospholipase A2alpha overcomes the asthma-associated response

BACKGROUND

Existing treatments for asthma are not effective in all patients and disease exacerbations are common, highlighting the need for increased understanding of disease mechanisms and novel treatment strategies. The leukotriene pathway including the enzyme responsible for arachidonic acid release from cellular phospholipids, cPLA(2)alpha, is a major contributor to asthmatic responses and an attractive target in asthma therapies.

OBJECTIVE

The study reported here investigates (a) the differential effects of in vitro exposure of peripheral blood mononuclear cells (PBMCs) to allergen between asthma and healthy subjects, and (b) the contribution of cPLA(2)alpha to these differences in gene expression.

METHODS

In vitro responses of asthma (N=26) and healthy (N=11) subject PBMC samples to allergen stimulation in the presence and absence of cPLA(2)alpha inhibition or 5-lipoxygenase inhibition were compared at the gene expression level using oligonucleotide arrays and at the protein level using ELISA.

RESULTS

Subject samples within both asthma and healthy groups showed allergen-dependent cytokine production and allergen-dependent gene expression changes, although transcriptional profiling identified 153 genes that were modulated significantly differently by allergen between asthma and healthy subjects. Among these were genes previously associated with asthma, but the majority (about 80%) have not previously been associated with asthma.

CONCLUSIONS

Transcriptional profiling elucidated novel gene expression differences between the asthmatic and healthy subject samples. Although 5-lipoxygenase inhibition did not significantly affect allergen-modulated gene expression, the inhibition of cPLA(2)alpha activity affected many of the allergen-dependent, asthma-associated gene expression changes.

Different effects of spinally applied prostaglandin D2 on responses of dorsal horn neurons with knee input in normal rats and in rats with acute knee inflammation

Prostaglandin D2(PGD2) is the most produced prostanoid in the CNS of mammals, and in behavioral experiments it has been implicated in the modulation of spinal nociception. In the present study we addressed the effects of spinal PGD2 on the discharge properties of nociceptive spinal cord neurons with input from the knee joint using extracellular recordings in vivo, both in normal rats and in rats with acute inflammation in the knee joint. Topical application of PGD2 to the spinal cord of normal rats did not influence responses to mechanical stimulation of the knee and ankle joint except at a high dose. Specific agonists at either the prostaglandin D2 receptor 1 (DP1) or the prostaglandin D2 receptor 2 (DP2) receptor had no effect on responses to mechanical stimulation of the normal knee. By contrast, in rats with inflamed knee joints either PGD2 or a DP1 receptor agonist decreased responses to mechanical stimulation of the inflamed knee and the non-inflamed ankle thus reducing established inflammation-evoked spinal hyperexcitability. Vice versa, spinal application of an antagonist at DP1 receptors increased responses to mechanical stimulation of the inflamed knee joint and the non-inflamed ankle joint suggesting that endogenous PGD2 attenuated central sensitization under inflammatory conditions, through activation of DP1 receptors. Spinal application of a DP2 receptor antagonist had no effect. The conclusion that spinal PGD2 attenuates spinal hyperexcitability under inflammatory conditions is further supported by the finding that spinal coapplication of PGD2 with prostaglandin E2 (PGE2) attenuated the PGE2-induced facilitation of responses to mechanical stimulation of the normal joint.

CRTH2 plays an essential role in the pathophysiology of Cry j 1-induced pollinosis in mice

PGD(2) is the major prostanoid produced during the acute phase of allergic reactions. Two PGD(2) receptors have been isolated, DP and CRTH2 (chemoattractant receptor-homologous molecule expressed on Th2 cells), but whether they participate in the pathophysiology of allergic diseases remains unclear. We investigated the role of CRTH2 in the initiation of allergic rhinitis in mice. First, we developed a novel murine model of pollinosis, a type of seasonal allergic rhinitis. Additionally, pathophysiological differences in the pollinosis were compared between wild-type and CRTH2 gene-deficient mice. An effect of treatment with ramatroban, a CRTH2/T-prostanoid receptor dual antagonist, was also determined. Repeated intranasal sensitization with Cry j 1, the major allergen of Cryptomeria japonica pollen, in the absence of adjuvants significantly exacerbated nasal hyperresponsive symptoms, Cry j 1-specific IgE and IgG1 production, nasal eosinophilia, and Cry j 1-induced in vitro production of IL-4 and IL-5 by submandibular lymph node cells. Additionally, CRTH2 mRNA in nasal mucosa was significantly elevated in Cry j 1-sensitized mice. Following repeated intranasal sensitization with Cry j 1, CRTH2 gene-deficient mice had significantly weaker Cry j 1-specific IgE/IgG1 production, nasal eosinophilia, and IL-4 production by submandibular lymph node cells than did wild-type mice. Similar results were found in mice treated with ramatroban. These results suggest that the PGD(2)-CRTH2 interaction is elevated following sensitization and plays a proinflammatory role in the pathophysiology of allergic rhinitis, especially pollinosis in mice.

Prostaglandin D(2) induces contraction via thromboxane A(2) receptor in rat liver myofibroblasts

Increased intrahepatic resistance is one of the major characteristics of cirrhotic liver, in which extravascular cells including liver myofibroblasts (MFs) abnormally contract. Although several studies provided evidence that various prostaglandins (PG) are involved in liver cirrhosis, the role of PGD(2) remains unknown. In this study, we investigated the effect of PGD(2) on the contractile properties of liver MFs. Cultured rat liver MFs were used at passages 4-7. A collagen gel contraction assay was used for the evaluation of the MFs contraction. mRNA expression was assessed by semi-quantitative RT-PCR. Intracellular Ca(2+) concentrations ([Ca(2+)](i)) were measured by monitoring the fluorescence intensity of fura-2. PGD(2) (1-10 microM) induced liver MF contraction in a dose-dependent manner with [Ca(2+)](i) elevation. Pretreatment with 300 nM LaCl(3), a nonselective Ca(2+) channel blocker abolished the 10 microM PGD(2)-induced MFs contraction. RT-PCR revealed that three distinct PGD(2) responsive receptors, prostanoid DP receptor, chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) and thromboxane A(2) receptor (prostanoid TP receptor), were expressed in liver MFs. While prostanoid DP receptor agonist and CRTH2 agonist didn't induce contraction, 0.01-1 microM U46619 (11alpha, 9alpha-epoxymethano-PGH(2), prostanoid TP receptor agonist) caused robust contraction with [Ca(2+)](i) elevation. Furthermore, pretreatment with prostanoid TP receptor antagonists ramatroban (1 microM) or SQ29548 ([1S-[1alpha, 2alpha(Z), 3alpha, 4alpha]]-7-[3-[[2-[(phenyl amino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid, 1 microM) completely suppressed PGD(2)-induced contraction and [Ca(2+)](i) elevation. Additionally, we observed that BW245C (1-10 microM) decreased basal MF contraction. These results suggest that PGD(2) induces rat liver MF contraction with an increase in [Ca(2+)](i) through prostanoid TP receptor.

Microsomal prostaglandin E synthase-1 inhibition in cardiovascular inflammatory disease

Prostaglandins (PGs), particularly PGE2 and prostacyclin (PGI2), are potent mediators of pain and inflammation. Both atherosclerosis and aortic aneurysm exhibit the hallmarks of inflammation. However, randomized trials of inhibitors of PG synthesis--nonsteroidal anti-inflammatory drugs--reveal that they predispose to cardiovascular risk. This appears to be consequent to inhibition of PGI2 and PGE2 formed by cyclooxygenase-2 (COX-2). Inhibitors of microsomal PGE synthase-1 (mPGES-1) are being developed for relief of pain and interest has focused on their potential impact on the cardiovascular system. Deletion of mPGES-1 retards atherogenesis and limits aortic aneurysm formation in hyperlipidaemic mice. However, it does not predispose to thrombogenesis and has a limited impact on blood pressure compared to inhibition of COX-2. This occurs despite the potential of the suppressed PGE2 in affording cardioprotection via its EP2 and EP4 receptors. However, deletion of mPGES-1 permits rediversion of the PGH2 substrate to other PG synthases and augmented formation of PGI2 and PGD2 mitigates this effect. However, increased PGI2 may also attenuate relief of pain. Pain relief seems likely to be a nuanced indication for mPGES-1 inhibitors, but they have therapeutic potential in syndromes of cardiovascular inflammation, cancer and perhaps in neurodegenerative disease. However, as the products of substrate rediversion vary according to cell type, these drugs may have contrasting impact amongst individuals at varied stages of disease evolution.

Prostaglandin D2 inhibits the production of IFN-gamma by invariant NK T cells: consequences in the control of B16 melanoma

Invariant NK T (iNKT) cells are a subset of innate/memory lymphocytes that recognize lipid Ags presented by CD1d-expressing APCs such as dendritic cells (DCs). Upon primary stimulation through their TCR, iNKT cells promptly produce large amounts of IFN-gamma and/or IL-4 that play critical roles in the regulation of innate and adaptive immune responses. To date, the role of environmental factors on iNKT cell functions has been poorly investigated. In this study, we addressed the question of whether PGD2, a potent eicosanoid lipid mediator involved in immune responses and inflammation, could be important in DC/iNKT cell cross-talk. We show that PGD2 dramatically reduced the production of IFN-gamma, but not IL-4, by iNKT cells in response to the superagonist alpha-galactosylceramide (alpha-GalCer) both in vitro and in vivo. This effect is mediated by the D prostanoid receptor 1 (DP1) expressed by DCs and iNKT cells and requires protein kinase A activation. We also report that PGD2 and BW245C (a selective DP1 agonist) reduce the protective effects of alpha-GalCer in B16F10-induced melanoma metastasis, an effect that depends on IFN-gamma production by iNKT cells. As a whole, these data reveal novel pathways regulating iNKT cell biologic functions and confirm the immunoregulatory roles of PGD2 on the innate response.

A novel antagonist of CRTH2 blocks eosinophil release from bone marrow, chemotaxis and respiratory burst

BACKGROUND

Chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) has been revealed to be a novel receptor for prostaglandin (PG) D(2), which is a major mast cell product released during the allergic response. The aim of this study was to analyze the effects of a newly developed small molecule antagonist of CRTH2, Cay10471, on eosinophil function with respect to recruitment, respiratory burst and degranulation.

METHODS

Chemotaxis of guinea pig bone marrow eosinophils and human peripheral blood eosinophils were determined using microBoyden chambers. Eosinophil release from bone marrow was investigated in the in situ perfused guinea pig hind limb preparation. Respiratory burst and degranulation were measured by flow cytometry.

RESULTS

Cay10471 bound with high affinity to recombinant human and guinea pig CRTH2, but not DP, receptors. The antagonist prevented the PGD(2)-induced release of eosinophils from guinea pig bone marrow, and inhibited the chemotaxis of guinea pig bone marrow eosinophils and human peripheral blood eosinophils. Pretreatment with PGD(2) primed eosinophils for chemotaxis towards eotaxin, and this effect was prevented by Cay10471. In contrast, PGD(2) inhibited the C5a-induced up-regulation of CD63, a cellular marker of degranulation, in a Cay10471-sensitive manner. Finally, Cay10471 abolished the respiratory burst of eosinophils upon stimulation by PGD(2).

CONCLUSION

These data further emphasize the importance of CRTH2 in eosinophil function and show that Cay10471 is a highly potent and selective antagonist of PGD(2)-induced eosinophil responses. Cay10471 might hence be a useful compound for the treatment of allergic diseases.