Co-compartmentalization of MAP kinases and cytosolic phospholipase A2 at cytoplasmic arachidonate-rich lipid bodies

Lipid bodies are inducible lipid domains abundantly present in leukocytes engaged in inflammation. They are rich in esterified arachidonate and are also potential sites for eicosanoid-forming enzyme localization. It is therefore of interest to know whether arachidonate-releasing cytosolic phospholipase A2 (cPLA2) localizes at lipid bodies. Here, we present evidence that cPLA2 and its activating protein kinases, mitogen-activated protein (MAP) kinases, co-localize at lipid bodies. U937 cells express high levels of cPLA2 and contain numerous cytoplasmic lipid bodies. Using double-labeling immunocytochemistry we demonstrated punctate cytoplasmic localizations of both cPLA2 and MAP kinases in U937 cells that were perfectly concordant with fluorescent fatty-acid-labeled lipid bodies. The co-localization of cPLA2 and MAP kinases at lipid bodies was confirmed by subcellular fractionation and immunoblot. Lipid body fractions free of cytosol and other organelles contained significant amounts of [14C]arachidonate-labeled phosphatidylcholine and cPLA2 enzymatic activities. Immunoblotting with specific antibodies identified cPLA2 as well as MAP kinases, including ERK1, ERK2, p85, and p38, in lipid bodies. The co-compartmentalization within arachidonate-rich lipid bodies of cPLA2 and its potentially activating protein kinases suggests that lipid bodies may be structurally distinct intracellular sites active in extracellular ligand-induced arachidonate release and eicosanoid formation.

Prostaglandin endoperoxide synthase (cyclooxygenase): ultrastructural localization to nonmembrane-bound cytoplasmic lipid bodies in human eosinophils and 3T3 fibroblasts

Lipid bodies are nonmembrane-bound cytoplasmic inclusions which are prominent in cells engaged in inflammatory responses. Using postembedding immunogold localization with a primary anti-PGH (prostaglandin endoperoxide) synthase monoclonal antibody, PGH synthase was localized by electron microscopy to lipid bodies of two prostaglandin-forming cells: human eosinophils and murine 3T3 fibroblasts. Freshly isolated eosinophils from a hypereosinophilic syndrome donor and peripheral blood eosinophils maintained in coculture with 3T3 fibroblasts and granulocyte-macrophage colony-stimulating factor were studied. In both cell types, lipid bodies were the predominant structures labeled with anti-PGH synthase. Labeling was absent in controls. In both human eosinophils and murine fibroblasts, lipid bodies represent sites of localization of PGH synthase--the rate-limiting initial enzyme in the formation of prostaglandins--and may serve as specific sites of formation of cyclooxygenase-pathway-derived eicosanoids.

Ultrastructural immunogold localization of tumor necrosis factor-alpha to the matrix compartment of eosinophil secondary granules in patients with idiopathic hypereosinophilic syndrome

Peripheral blood eosinophils from two normal donors and two patients with the hypereosinophilic syndrome (HES) were analyzed with a post-embedding immunogold method to detect the substructural location of tumor necrosis factor-alpha (TNF-alpha). In eosinophils of HES patients, TNF-alpha was localized to the matrix compartment of 64% of the specific secondary granules. Other structures in the HES eosinophils were unlabeled. No TNF-alpha was detected in eosinophils of normal donors. These studies document the first ultrastructural subcellular localization of any cytokine within the major population of secretory granules in human eosinophils and support other lines of evidence indicating that the expression of TNF-alpha may be greater in the eosinophils of HES patients than in those of normal donors.