Recent Progress in the Cellular and Molecular Biology of Prostaglandin Synthesis

https://doi.org/10.1016/S1050-1738(98)00004-8Get rights and content

Abstract

The discovery of an inducible form of prostaglandin synthase initiated a reexamination of the biochemical pathways for ligand-induced prostaglandin synthesis. As a result, new pharmaceutical agents with potential activity against pain, fever, chronic and acute inflammation, cardiovascular disorders, and colon cancer have been developed and are currently under intense investigation. Careful biochemical and pharmacologic studies of the differences between the constitutive and inducible prostaglandin synthase enzymes have suggested a previously unexpected mechanism for some of the therapeutic effects of aspirin. Identification of a new phospholipase, and recognition of its role in mast cell prostaglandin production and in transcellular prostaglandin synthesis, have identified additional potential targets for pharmacologic intervention in inflammation and other prostaglandin-mediated disorders.

Section snippets

• Prostaglandins and Prostaglandin Synthases Play a Central Role in Many Biological Processes

Nonsteroidal antiinflammatory drugs (NSAIDs), such as aspirin and ibuprofen, are used to treat pain, fever, chronic inflammatory diseases, acute infection, and a variety of other illnesses. In addition, aspirin is able to reduce substantially the morbidity of colon cancer (Thun et al. 1991). With the discovery that aspirin and the other NSAIDs block the production of prostaglandins by inhibiting the activity of prostaglandin synthase [reviewed by Vane (1994)], this enzyme became implicated in a

• The Discovery of an Inducible Prostaglandin Synthase, PGS2, Created a Paradox Regarding Prostaglandin Synthesis

Until recently, the conventional view of prostanoid synthesis has been that the rate-limiting step for prostaglandin production is the ligand-stimulated activation of latent phospholipase(s), which then releases free arachidonic acid from membrane phospholipids. According to this model, constitutive prostaglandin synthase is available for conversion of free arachidonic acid, released from membrane phospholipids, to PGH2; PGS is present in excess, and the availability of free arachidonic acid is

• Prostaglandin Synthase 2 Plays a Major Role in Production of Prostanoids in Inflammatory Reactions and Is the Primary Target of Most Nonsteroidal Antiinflammatory Drugs

Studies with animal models (Masferrer et al. 1996) demonstrated that, in vivo, acute and chronic inflammatory responses are accompanied by PGS2 induction. The major side effects of aspirin and the majority of other NSAIDs, namely, gastric ulcers and nephrotoxicity, are now thought to occur as a result of their inhibition of the constitutive PGS1 enzyme. With the discovery of PGS2, and the demonstration that this enzyme is induced by inflammatory stimuli, the pharmaceutical industry entered into

• Ligand-Stimulated Prostaglandin Synthesis in Fibroblasts and Macrophages Requires PGS2 Synthesis and Activity

PGE2 production in mitogen-stimulated fibroblasts or endotoxin-treated macrophages begins slowly, after an initial lag phase, and accelerates during a 2- to 6-hour period (Kujubu and Herschman 1992, Herschman 1996). During our initial characterization of PGS2 enzyme production following ligand stimulation, we showed that mitogen-induced enzyme accumulation in 3T3 fibroblasts was, as expected, blocked by cycloheximide treatment to block protein synthesis. In addition, cycloheximide prevented

• Mast Cell Experiments Demonstrate Two Distinct Pathways of Prostaglandin Synthesis

Aggregation of mast cell IgE receptors leads to degranulation and the release of histamine and serotonin. In addition, activation of mast cells induces the synthesis and secretion of PGD2. We found that activated mast cells produce PGD2 in two distinct phases (Kawata et al. 1995). The early phase of PGD2 synthesis is complete within 10–15 minutes. The delayed phase begins at about 1–2 hours after activation and is completed within 6–8 hours (Fig. 1). Using PGS1 and PGS2 inhibitors, we

• Type V sPLA2 Mediates the Early Phase of PGD2 Production in Activated Mast Cells

Type IIa sPLA2 had been thought to be the isoform that mediates PGD2 release in activated macrophages (Fonteh et al. 1994). However, mast cells from mice homozygous for a mutation in the PLA2G2A gene, encoding type IIa sPLA2, produce PGD2 normally after activation (Bingham et al. 1996, Reddy et al. 1997). Moreover, sPLA2 activity is secreted by mast cells derived from type IIa sPLA2 mutant mice after activation (Reddy et al. 1997). Northern blot and polymerase chain reaction analysis

• sPLA2 Released from Activated Mast Cells Initiates Transcellular Prostaglandin Synthesis

We speculated that sPLA2, released from mast cells in response to aggregation of IgE receptors, might transcellularly activate prostaglandin synthesis in distal cells via a PGS1-mediated pathway (Fig. 3). Using mixed mast cell/fibroblast cell cultures, sPLA2 inhibitors, and mast cell supernatants, we demonstrated that type V sPLA2 released from activated mast cells can induce rapid, PGS1-dependent PGE2 synthesis in fibroblasts. Moreover, this “transcellular” prostaglandin synthesis uses

• Aspirin-Modified PGS2 Plays a Role in the Transcellular Synthesis of a New Class of Antiinflammatory and Antiproliferative Compounds

Aspirin inhibits both PGS1 and PGS2 by covalently modifying specific serine residues in these enzymes. In contrast, most other NSAIDs do not covalently modify the PGS enzymes. Aspirin acetylation of PGS1 blocks essentially all conversion of arachidonic acid to any product. In contrast, aspirin-acetylated PGS2 converts arachidonic acid to (15R)-15-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid (15R-HETE), rather than to PGG2 (Holtzman et al. 1992, Meade et al. 1993,

Mancini et al. 1994,

• Key Regulatory Elements of the PGS2 Gene, Transcription Factors Mediating Ligand-Induced PGS2 Gene Expression, and Signal Transduction Pathways Leading to PGS2 Gene Expression Have Been Identified

We cloned and sequenced the 5′ regulatory region of the murine PGS2 gene (Fletcher et al. 1992). Using deletion analysis, with chimeric luciferase reporter constructs, we identified the region of the PGS2 promoter required for v-src (Xie et al. 1994) and platelet-derived growth factor (Xie and Herschman 1996) induction. Mutational analysis identified the cAMP response element (CRE) present at nucleotides −56 to −48 from the transcription start site as the critical cis-acting element of the PGS2

Acknowledgements

The preparation of this article was supported by grants (H.R.H.) from the National Institute of Allergy and Infectious Diseases, the National Institute of Environmental Health Sciences, and the National Institute of General Medical Sciences.

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