Long time incubation of monocytic U 937 cells with LDL increases specific paf-acether binding and the cellular acetylhydrolase activity

Abstract

Besides the well established role of low density lipoproteins (LDL), the phospholipid PAF-acether (paf) seems to be involved in atherogenesis. The effect of LDL (10 μg/ml for 24 h, n = 3) on paf binding characteristics of monocyte/macrophage-like U 937 cells was investigated using the radioligand [ 3H]paf, unlabeled paf and the paf receptor antagonist WEB 2086. The specific [ 3H]paf binding significantly increased at 1.4 nM ( P < 0.02) and 2.8 nM ( P < 0.01) added [ 3H]paf with an increased number of paf binding sites in the Scatchard plot analysis of the data. Specific paf binding was functionally active since paf mediated a cellular [Ca 2+] i rise. The protein kinase C (PKC) activator PMA (1 nM, 37°C) expressed specific [ 3H]paf binding already after a 15-min incubation period, indicating a PKC activation as the decisive step of paf receptor expression. LDL also stimulated the paf degrading cellular acetylhydrolase significantly by increasing both K m (9.4 ± 1.9 vs. 2.0 ± 0.5 μM, P < 0.02) and ν max (0.5 ± 0.2 vs. 0.2 ± 0.0 nmol/min per mg cell protein, P < 0.02). The data demonstrate that LDL increases the number of paf receptors on monocyte/macrophage-like U 937 cells and interferes with the dynamics and/or synthesis of the cellular acetyl hydrolase. These effects could be of importance in the pathogenesis of atherosclerosis.