Arachidonic Acid Metabolism in Normal Human Alveolar Macrophages: Stimulus Specificity for Mediator Release and Phospholipid Metabolism, and Pharmacologic Modulation In Vitro and In Vivo

Abstract Arachidonic acid (AA) metabolism in normal human alveolar macrophages including phospholipid turnover, stimulus specificity for mediator release including trigger synergy, and the pharmacologic control of the release of AA metabolites was explored. Macrophages labeled overnight with [3H]AA, then activated, released three major AA metabolites, thromboxane B2 (TxB2), leukotriene B4 (LTB4), and 5-hydroxyeicosatetraenoic acid (5-HETE), as characterized by thin layer chromatography, high performance liquid chromatography, and radioimmunoassay. Although all triggers (phorbol myristate acetate [PMA], serum-activated zymosan, and ionophore A23187) resulted in the release of TxB2 and free AA, efficient synthesis of lipoxygenase products, particularly LTB4, required A23187. A23187 was the most effective single agent in producing LTB4 synthesis, was synergistic with PMA in causing LTB4 release, and was associated with significant turnover of phosphatidylcholine and phosphatidylinositol. Incubation of macrophages in vitro with cyclooxygenase inhibitors resulted in an inhibition of the formation of cyclooxygenase products; however, no shunting of metabolites into products of the lipoxygenase pathway was observed. Although overnight incubation of macrophages in vitro with dexamethasone (1 µM) resulted in an inhibition of both the spontaneous and A23187/PMA-triggered release of all AA metabolites, treatment of 5 volunteers with dexamethasone (4 mg po bid × 7 doses, in a single-blind, placebo-controlled, crossover protocol) resulted in no significant inhibition of the release of AA metabolites from macrophages triggered ex vivo. We conclude that activation of normal human alveolar macrophages results in phospholipid turnover (phosphatidylcholine and phosphatidylinositol) and the release of three major AA metabolites (TxB2, LTB4, and 5-HETE); that optimal synthesis of lipoxygenase product requires the presence of a calcium signal (A23187), although PMA can synergize with A23187 in the production of lipoxygenase products; and that glucocorticoids may have a different effect on the release of AA metabolites from alveolar macrophages when administered in vitro versus in vivo.