Prolonged exposure to lipopolysaccharide inhibits macrophage 5-lipoxygenase metabolism via induction of nitric oxide synthesis

LPS from bacteria can result in the development of sepsis syndrome and acut e lung injury. Although acute exposure to endotoxin primes leukocytes for e nhanced synthesis of leukotrienes (LT), little is known about the effect of chronic exposure. Therefore, we determined the effect of prolonged LPS tre atment 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 cel l-free 5-LO enzyme assay. No effect on the expression of either 5-LO or FLA P proteins was observed. New protein synthesis was necessary for LPS-induce d reduction of 5-LO metabolism in AM, and immunoblotting demonstrated marke d induction of NO synthase (NOS), Inhibition by LPS was reproduced by an NO donor and was abrogated by inhibitors of constitutive and inducible NOS. C ompared with AM, peripheral blood monocytes exhibited no suppression by LPS of 5-LO metabolism and no induction of inducible NOS. We conclude that pro longed exposure to LPS impairs AM 5-LO metabolism by NO-mediated suppressio n of both 5-LO and FLAP function. Because LT contribute to antimicrobial de fense, this down-regulation of 5-LO metabolism may contribute to the increa sed susceptibility to pneumonia in patients following sepsis.