Mike A. Clark

Leukotriene B4 Modulates Phospholipid Methylation and Chemotaxis in Human Polymorphonuclear Leukocytes

Formation of phosphatidylcholine from phosphatidylethanolamine via the S‐adenosylmethionine (AdoMet) pathway has been shown to be required for signal transduction of receptor–ligand interactions in a variety of cells. These interactions result in the remodeling of phospholipid pools and phospholipase activation. To extend these observations and to explore the role of the phosphatidylcholine synthesis pathway in transduction of the leukotriene B4 (LTB4) receptor–ligand response, we examined phospholipid methylation in human polymorphonuclear leukocytes (PMN) following stimulation by LTB4, a potent chemotactic agent that is a metabolite of arachidonic acid. At early time points (~3–10 min), formation of methylated phospholipids was enhanced following LTB4 stimulation. The LTB4 analogs 6‐trans LTB4 as well as LTB4 epimers induced less methylation compared with LTB4, and the potencies of these analogs in inducing methylation correlated with their diminished ability to induce chemotaxis. Furthermore, the ability of these agonists to induce methylation also correlated with the binding affinity of these agents to the LTB4 receptors on these cells. Synthesis of phosphatidylcholine by the choline transferase pathway was not affected by LTB4. Inhibition of the AdoMet reaction with 3‐ deazaadenosine, L‐homocysteine homolactone, or erythro‐9‐[2‐hydroxy‐3‐nonyl] adenine (EHNA) abrogated LTB4‐induced phospholipid methylation and the chemotactic response. The potencies of these inhibitors in blocking phospholipid methylation also correlated with their ability to abrogate the LTB4‐induced chemotactic response. These data suggest that phospholipid methylation and phospholipase activation play an important role in transduction of the LTB4 receptor–ligand interaction in PMN, which results in chemotaxis.

The role of phospholipase A2 activating protein (PLAP) in regulating prostanoid production in smooth muscle and endothelial cells following leukotriene D4 treatment.

Leukotrienes are a family of compounds originally termed the slow-reacting substances of anaphylaxis and are recognized as important mediators of anaphylaxis (1). In particular, leukotriene C4 (LTC4 ) and leukotriene D4 (LTD4) are potent spasmogens in a variety of smooth muscle tissues including trachea, lung, ileum and the vasculature (2.4). The mechanism by which contraction is induced by the leukotrienes (LTs) is not yet known. However, in some experimental systems, but not all, a cyclooxygenase product of arachidonic acid metabolism, possibly thromboxane B2 (TxB2 ), may be involved in mediating leukotrieneinduced effects (3,4). This is based on the observation that inhibitors of cyclooxygenase, such as indomethacin and meclofenamic acid, can block leukotriene-induced contraction of the lung parenchyma, vasculature and the ileum (2.4). In other tissues, most notably guinea pig trachea, the cyclooxygenase products appear to be relatively unimportant mediators of leukotriene effects (5). Furthermore, in the tissues in which cyclooxygenase products appear to be important, thromboxane synthesis may be crucial for the leukotriene responses (4). Although TxB2 is usually assumed to be of platelet origin, the cellular source of LT-induced thromboxane synthesis is not known.

Activation Of Phospholipase A2 in Rheumatoid Arthritis

Rheumatoid arthritis (RA), a common and often disabling systemic disease with a predilection for joints, is characterized by an inflammatory and proliferative reaction of synovial cells associated with infiltration of immunocompetent cells and fluid into the synovial tissue, as well as the destruction of articular cartilage. Prostaglandins and related eicosanoids are thought to be important mediators and regulators of these immune and inflammatory responses (1, 2, 3). For example, prostaglandin E2 induces bone resorption, and leukotriene B4 stimulates vasodilitation and chemotaxis (1, 4). Increased quantities of eicosanoids are produced by rheumatoid synovium in both organ and cell culture (2, 5, 6) and by freshly isolated or cultured peripheral blood monocytes isolated from RA patients as compared to cells obtained from normal donors (7, 8, 9, 10). In addition, high concentrations of eicosanoids have been shown to be present in rheumatoid synovial fluid (2, 11, 12). Because eicosanoids are important mediators of this disease, numerous investigators have sought to understand the mechanisms of enhanced eicosanoid biosynthesis in this illness. The rate-limiting step in eicosanoid biosynthesis is the release of the precursor fatty acid from membrane phospholipids (4, 13, 14, 15, 16, 17). Once liberated, the unsaturated fatty acid, usually arachidonic acid, is then oxygenated to form prostaglandins, leukotrienes and related lipid metabolites.