Prostaglandin D₂: A biochemical perspective

Although it has been suggested that prostaglandin (PG) D₂ is involved in the pathogenesis of allergic rhinitis, whether the inhibition of hematopoietic PGD₂ synthase (H-PGDS) shows beneficial effects on allergic rhinitis has been unclear. We evaluated the effects of a selective H-PGDS inhibitor, TFC-007, on nasal symptoms on Japanese cedar pollen-induced allergic rhinitis of guinea pigs. Sensitized animals were challenged with the pollen once a week. TFC-007 (30 mg/kg, p.o.) given once before a challenge almost completely suppressed PGD₂ production in the nasal tissue early and late after the challenge. Although pre-treatment did not affect the incidences of sneezing and early phase nasal blockage, late phase nasal blockage was partially but significantly attenuated; however, nasal eosinophilia was not suppressed. In contrast, when TFC-007 was given once 1.5 h after the challenge, the late phase response was not affected. Collectively, PGD₂ produced by H-PGDS early after an antigen challenge can participate in the induction of late phase nasal blockage, although the mechanism may be independent of eosinophil infilatration. The strategy for H-PGDS inhibition may be beneficial for allergic rhinitis therapy.

L-type prostaglandin synthase (L-PGDS) produces PGD₂, a lipid mediator involved in neuromodulation and inflammation. Here, we show that L-PGDS and arrestin-3 (Arr3) interact directly and can be co-immunoprecipitated endogenously from MG-63 osteoblasts. Perinuclear L-PGDS/Arr3 co-localization is observed in PGD₂-producing MG-63 cells and is induced by the addition of the L-PGDS substrate or co-expression of COX-2 in HEK293 cells. Inhibition of L-PGDS activity in MG-63 cells triggers redistribution of Arr3 and L-PGDS to the cytoplasm. Perinuclear localization of L-PGDS is detected in wild-type mouse embryonic fibroblasts (MEFs) but is more diffused in MEFs-arr-2^−/−-arr-3^−/−. Arrestin-3 promotes PGD₂ production by L-PGDS in vitro. IL-1β-induced PGD₂ production is significantly lower in MEFs-arr-2^−/−-arr-3^−/− than in wild-type MEFs but can be rescued by expressing Arr2 or Arr3. A peptide corresponding to amino acids 86–100 of arrestin-3 derived from its L-PGDS binding domain stimulates L-PGDS-mediated PGD₂ production in vitro and in MG-63 cells. We report the first characterization of an interactor/modulator of a PGD₂ synthase and the identification of a new function for arrestin, which may open new opportunities for improving therapies for the treatment of inflammatory diseases.

Since the discovery of CRTH2 as a PGD₂ receptor almost 10 years ago, a large number of highly potent carboxylic acid-based CRTH2 antagonists have been reported. A number of them have been advanced into phase II clinical trials. The data from these trials will validate whether CRTH2 selective antagonists alone or CRTH2 and DP dual antagonists will be beneficial for treating asthma and other allergic conditions. Receptor binding assays may be used to measure the ability of an antagonist to inhibit ligand binding to a receptor. PGD2 binds human CRTH2 with a dissociation constant (K_D) of 12.9 nM and human DP with a KD of 1.5 nM. There are additional agonists that are highly selective for either CRTH2 or DP that have been used for investigating receptor functions in various assays. Indomethacin was identified as a CRTH2 receptor agonist shortly after the discovery of the receptor. This agent could induce Ca²⁺ mobilization in CRTH2-transfected K562 cells and in cultured Th2 cells, and induce the chemotaxis of Th2 cells, eosinophils and basophils. The effects of indomethacin were blocked by treatment with a neutralizing anti-CRTH2 monoclonal antibody BM7.

The ability of Bothrops moojeni venom (BmV) to induce oedema in mice, the involvement of principal inflammatory mediators and mast cells (MCs) were investigated. The intraplantar injection of BmV (0.3–6 μg/paw) caused a dose- and time-dependent oedema with a peak between 30 and 60 min after venom injection (0.3–1 μg/paw), disappearing within 24 h. Either MCs granule inhibition or depletion by cromoglycate or C48/80, respectively, markedly reduced BmV-induced oedema. MCs depletion by imatinib also reduced oedema. Intraperitoneal BmV injection (2.5–10 μg/site) induced MCs degranulation and release of PGD₂. Treatment with promethazine, cimetidine or thioperamide, histamine H1, H2 and H3/H4 receptor antagonists, respectively, markedly reduced the initial phase of oedema. Combined treatment with these antagonists further reduced, but not abrogated oedema. Indomethacin or eterocoxib (cyclooxygenase inhibitors) reduced oedema until 180 min, whereas zileuton (lipoxygenase inhibitor) affected this event until 60 min. Dexamethazone caused a long lasting reduction of oedema. However, L-NAME and aminoguanidine (NO synthase inhibitors) significantly increased BmV-induced oedema. In conclusion, BmV induces oedema, mediated by MCs degranulation, histamine by H1, H2, H3/H4 receptors, prostaglandins and leukotrienes, and down-regulated by NO. Partial neutralization of oedema was observed even when polyspecific bothropic antivenom was injected immediately after venom.

Eicosanoids have been implicated in a vast number of devastating inflammatory conditions, including arthritis, atherosclerosis, pain, and cancer. Currently, over a hundred different eicosanoids have been identified, with many having potent bioactive signaling capacity. These lipid metabolites are synthesized de novo by at least 50 unique enzymes, many of which have been cloned and characterized. Due to the extensive characterization of eicosanoid biosynthetic pathways, this field provides a unique framework for integrating genomics, proteomics, and metabolomics toward the investigation of disease pathology. To facilitate a concerted systems biology approach, this review outlines the proteins implicated in eicosanoid biosynthesis and signaling in human, mouse, and rat. Applications of the extensive genomic and lipidomic research to date illustrate the questions in eicosanoid signaling that could be uniquely addressed by a thorough analysis of the entire eicosanoid proteome.

In this study, the ability of Bothrops asper snake venom (BaV) to increase the production of prostaglandins PGE₂ and PGD₂ was assessed in a mouse model in vivo and in inflammatory cells in vitro. In addition, the expressions of COX-1 and COX-2 were assessed. BaV induced an increment in the in vivo synthesis of PGE₂ and PGD₂, together with an enhanced expression of COX-2, but not of COX-1. However, enzymatic activities of COX-1 and COX-2 were increased. Incubation of isolated macrophages and neutrophils with a sub-cytotoxic concentration of BaV in vitro resulted in increased release of PGE₂ and PGD₂ by macrophages and PGE₂ by neutrophils, concomitantly with an increment in the expression of COX-2, but not of COX-1 by both cell types. Our results demonstrate the ability of BaV to promote the expression of COX-2 and to induce the synthesis of proinflammatory prostaglandins. Macrophages and neutrophils may be important targets for this venom under in vivo situation.