Tomoaki Inazumi

Prostaglandin E2–EP3 Signaling Induces Inflammatory Swelling by Mast Cell Activation

PGE2 has long been known as a potentiator of acute inflammation, but its mechanisms of action still remain to be defined. In this study, we employed inflammatory swelling induced in mice by arachidonate and PGE2 as models and dissected the role and mechanisms of action of each EP receptor at the molecular level. Arachidonate- or PGE2-induced vascular permeability was significantly reduced in EP3-deficient mice. Intriguingly, the PGE2-induced response was suppressed by histamine H1 antagonist treatment, histidine decarboxylase deficiency, and mast cell deficiency. The impaired PGE2-induced response in mast cell–deficient mice was rescued upon reconstitution with wild-type mast cells but not with EP3-deficient mast cells. Although the number of mast cells, protease activity, and histamine contents in ear tissues in EP3-deficient mice were comparable to those in wild-type mice, the histamine contents in ear tissues were attenuated upon PGE2 treatment in wild-type but not in EP3-deficient mice. Consistently, PGE2–EP3 signaling elicited histamine release in mouse peritoneal and bone marrow–derived mast cells, and it exerted degranulation and IL-6 production in a manner sensitive to pertussis toxin and a PI3K inhibitor and dependent on extracellular Ca2+ ions. These results demonstrate that PGE2 triggers mast cell activation via an EP3–Gi/o–Ca2+ influx/PI3K pathway, and this mechanism underlies PGE2-induced vascular permeability and consequent edema formation.

Roles of prostaglandin receptors in female reproduction

Prostaglandins (PGs) have long been known to play roles in various processes of female reproduction; however, the molecular mechanisms therein remained unsolved until recently. This review summarizes the recent progress towards understanding the molecular mechanisms underlying PG actions in fertilization and parturition. A series of studies using EP2-deficient mice demonstrated that after ovulation chemokine signalling in the cumulus cells stimulates integrin activation and cumulus extracellular matrix (ECM) assembly through the RhoA/ROCK/actomyosin pathway, although excessive chemokine signalling disturbs sperm penetration. PGE2-EP2 signalling suppresses such a chemokine signalling and stimulates cumulus ECM disassembly, which contributes to successful fertilization. A series of studies using FP-deficient mice revealed that PGF(2α)-FP signalling induces parturition at least by terminating progesterone production; however, some other EP signals are likely to be involved in parturition by inducing myometrial contraction. Therefore, it should be clarified as to which EP and/or FP receptor signals are physiologically essential for myometrial contraction and successful parturition.

Prostaglandin E2-EP4 signaling suppresses adipocyte differentiation in mouse embryonic fibroblasts via an autocrine mechanism

The prostaglandin (PG) receptors EP4 and FP have the potential to exert negative effects on adipogenesis, but the exact contribution of endogenous PG-driven receptor signaling to this process is not fully understood. In this study, we employed an adipocyte differentiation system from mouse embryonic fibroblasts (MEF) and compared the effects of each PG receptor-deficiency on adipocyte differentiation. In wild-type (WT) MEF cells, inhibition of endogenous PG synthesis by indomethacin augmented the differentiation, whereas exogenous PGE2, as well as an FP agonist, reversed the effect of indomethacin. In EP4-deficient cells, basal differentiation was upregulated to the levels in indomethacin-treated WT cells, and indomethacin did not further enhance differentiation. Differentiation in FP-deficient cells was equivalent to WT and was still sensitive to indomethacin. PGE2 or indomethacin treatment of WT MEF cells for the first two days was enough to suppress or enhance transcription of the Pparg2 gene as well as the subsequent differentiation, respectively. Differentiation stimuli induced COX-2 gene and protein expression, as well as PGE2 production, in WT MEF cells. These results suggest that PGE2-EP4 signaling suppresses adipocyte differentiation by affecting Pparg2 expression in an autocrine manner and that FP-mediated inhibition is not directly involved in adipocyte differentiation in the MEF system.

Expression profiling of cumulus cells reveals functional changes during ovulation and central roles of prostaglandin EP2 receptor in cAMP signaling

To understand the role of prostaglandin (PG) receptor EP2 (Ptger2) signaling in ovulation and fertilization, we investigated time-dependent expression profiles in wild-type (WT) and Ptger2(-/-) cumuli before and after ovulation by using microarrays. We prepared cumulus cells from mice just before and 3, 9 and 14 h after human chorionic gonadotropin injection. Key genes including cAMP-related and epidermal growth factor (EGF) genes, as well as extracellular matrix- (ECM-) related and chemokine genes were up-regulated in WT cumuli at 3 h and 14 h, respectively. Ptger2 deficiency differently affected the expression of many of the key genes at 3 h and 14 h. These results indicate that the gene expression profile of cumulus cells greatly differs before and after ovulation, and in each situation, PGE(2)-EP2 signaling plays a critical role in cAMP-regulated gene expression in the cumulus cells under physiological conditions.

Prostaglandin EP3 receptor superactivates adenylyl cyclase via the Gq/PLC/Ca2+ pathway in a lipid raft-dependent manner.

We previously demonstrated that prostaglandin EP3 receptor augments EP2-elicited cAMP formation in COS-7 cells in a G(i/o)-insensitive manner. The purpose of our current study was to identify the signaling pathways involved in EP3-induced augmentation of receptor-stimulated cAMP formation. The enhancing effect of EP3 receptor was irrespective of the C-terminal structure of the EP3 isoform. This EP3 action was abolished by treatment with inhibitors for phospholipase C and intracellular Ca(2+)-related signaling molecules such as U73122, staurosporine, 2-APB and SK&F 96365. Indeed, an EP3 agonist stimulated IP(3) formation and intracellular Ca(2+) mobilization, which was blocked by U73122, but not by pertussis toxin. The enhancing effect by EP3 on cAMP formation was mimicked by both a Ca(2+) ionophore and the activation of a typical G(q)-coupled receptor. Moreover, EP3 was exclusively localized to the raft fraction in COS-7 cells and EP3-elicited augmentation of cAMP formation was abolished by cholesterol depletion and introduction of a dominant negative caveolin-1 mutant. These results suggest that EP3 elicits adenylyl cyclase superactivation via G(q)/phospholipase C activation and intracellular Ca(2+) mobilization in a lipid raft microdomain-dependent manner.

Prostanoid receptors and acute inflammation in skin

Prostanoids such as prostaglandins (PGs) and thromboxanes exert a wide variety of actions via nine types of G protein-coupled receptors, including four PGE2 receptors (EPs) and two PGD2 receptors (DPs). Recent studies have revealed that prostanoids trigger or modulate acute inflammation in the skin via multiple mechanisms involving distinct receptors and molecules; PGE2 elicits vascular permeability and edema formation via EP3 receptor on mast cells, and PGE2 increases blood flow by eliciting vasodilatation via EP2/EP4 receptors on smooth muscle cells. PGD2-DP1 signaling plays a role in mast cell maturation and mast cell-mediated inflammation. Therefore, the local inhibition of specific prostanoid receptor signaling is expected to be an effective strategy for the prevention and treatment of acute inflammation.

An aromatic amino acid within intracellular loop 2 of the prostaglandin EP2 receptor is a prerequisite for selective association and activation of Gαs

We previously demonstrated that the aromatic moiety of Tyr143 within the intracellular loop 2 (ICL2) region of the prostaglandin EP2 receptor plays a crucial role in Gs coupling. Here we investigated whether the ICL2 of the EP2 receptor directly binds to Gαs and whether an aromatic moiety affects this interaction. In Chinese hamster ovary cells, mutations of Tyr143 reduced the ability of the EP2 receptor to interact with G proteins as demonstrated by GTPγS sensitivity, as well as the ability of agonist-induced cAMP formation, with the rank order of Phe>Tyr (wild-type)=Trp>Leu>Ala (=0). We found that the wild-type ICL2 peptide (i2Y) and its mutant with Phe at Tyr143 (i2F) inhibited receptor-G protein complex formation of wild-type EP2 in membranes, whereas the Ala-substituted mutant (i2A) did not. Specific interactions between these peptides and the Gαs protein were detected by surface plasmon resonance, but Gαs showed different association rates, with a rank order of i2F>i2Y≫i2A, with similar dissociation rates. Moreover, i2F and i2Y, but not i2A activated membrane adenylyl cyclase. These results indicate that the ICL2 region of the EP2 receptor is its potential interaction site with Gαs, and that the aromatic side chain moiety at position 143 is a determinant for the accessibility of the ICL2 to the Gαs protein.

Pathophysiological Roles of Prostanoid Receptors in the Central Nervous System

Prostanoids comprising prostaglandins (PGs) and thromboxanes exert diverse actions by acting on their specific receptors. Recently, physiological roles of these receptors have been clarified using knockout mice for each receptor as well as receptor-selective agonists and antagonists. In the central nervous system (CNS), prostanoids have been shown to regulate not only fever, but also neuroinflammation, and to play a role in the pathogenesis of many neurodegenerative diseases. In this report, we review the recent research on the roles and molecular mechanisms of prostanoids and their receptors in the CNS and discuss their possibilities as therapeutic targets.