Prolonged Exposure to Lipopolysaccharide Inhibits Macrophage 5-Lipoxygenase Metabolism Via Induction of Nitric Oxide Synthesis

Abstract

LPS from bacteria can result in the development of sepsis syndrome and acute lung injury. Although acute exposure to endotoxin primes leukocytes for enhanced synthesis of leukotrienes (LT), little is known about the effect of chronic exposure. Therefore, we determined the effect of prolonged LPS treatment on 5-lipoxygenase (5-LO) metabolism of arachidonic acid in alveolar macrophages (AM) and in peripheral blood monocytes. Pretreatment of AM with LPS caused time- and dose-dependent suppression of LT synthetic capacity. LPS pretreatment failed to inhibit arachidonic acid (AA) release. The fact that LPS inhibited LT synthesis from endogenous AA more than from exogenous AA suggested an effect on 5-LO-activating protein (FLAP). In addition, an inhibitory effect of LPS treatment on AM 5-LO activity was suggested by cell-free 5-LO enzyme assay. No effect on the expression of either 5-LO or FLAP proteins was observed. New protein synthesis was necessary for LPS-induced reduction of 5-LO metabolism in AM, and immunoblotting demonstrated marked induction of NO synthase (NOS). Inhibition by LPS was reproduced by an NO donor and was abrogated by inhibitors of constitutive and inducible NOS. Compared with AM, peripheral blood monocytes exhibited no suppression by LPS of 5-LO metabolism and no induction of inducible NOS. We conclude that prolonged exposure to LPS impairs AM 5-LO metabolism by NO-mediated suppression of both 5-LO and FLAP function. Because LT contribute to antimicrobial defense, this down-regulation of 5-LO metabolism may contribute to the increased susceptibility to pneumonia in patients following sepsis.

Modulation of Pulmonary Leukotriene B4 Production by Cyclooxygenase-2 Inhibitors and Lipopolysaccharide

“…Thus, in order for shunting to become significant in the induction of LTB 4 synthesis, the amount of AA precursors may need to exceed a certain threshold, within a specific time frame. Alternatively, LPS could induce the synthesis of inhibitory intermediary proteins that suppress 5-LOX and FLAP function, such as nitric oxide synthase, which has been described previously to inhibit 5-LOX metabolism in rat AMs (23). In any event, the cycloheximide experiments confirm the finding that LPS-induced LTB 4 synthesis is not mediated through up-regulation of 5-LOX or FLAP protein production.…”

“…To determine the effect of inflammatory stimuli that are potentially tumorigenic and known to modulate activity of the 5-LOX pathway in monocytes/macrophages (22,23), we further examined the responsiveness of AMs obtained from active and ex-smokers to LPS. Because mechanisms of inflammation associated with recurring infections may facilitate tumorigenesis (32), it is of great interest to determine whether or not COX-2 inhibition alters AM LTB 4 production when the cells are exposed to LPS.…”

“…We therefore investigated the effect of oral celecoxib (Celebrex) treatment on LTB 4 production in the lungs of active smokers, as part of a pilot study to evaluate the feasibility of celecoxib as a chemopreventive agent for lung cancer. Furthermore, because ongoing inflammation may contribute to the process of carcinogenesis (21) and lipopolysaccharide (LPS) is a proinflammatory stimulus known to modulate the production of AA metabolites by mononuclear cells (22,23), we also evaluated the effect of LPS on LTB 4 production by human AMs with or without COX-2 inhibition.…”

Leukotrienes: Underappreciated Mediators of Innate Immune Responses

“…Despite its participation in microbial killing, NO has the capacity to down-regulate inflammatory responses by reducing cytokine production (68) and neutrophil recruitment (69). Interestingly, NO has also been shown to reduce LT synthetic capacity in cultured alveolar macrophages (30,70) and mast cells (32). Such an impairment in macrophage LT synthesis in vitro and in vivo (71), attributable to LPS induction of NO generation, may contribute to the increased susceptibility to secondary infection (72) observed in patients who survive an episode of sepsis.…”

Human Group V Phospholipase A2 Induces Group IVA Phospholipase A2-independent Cysteinyl Leukotriene Synthesis in Human Eosinophils

“…(from 2 ϫ 10 6 cells; see above) was measured as described previously (25,26). The relative 5-LO activity was then expressed in terms of the percentage of increase over the control (unstimulated) value.…”