α-Tocopherol inhibits agonist-induced monocytic cell adhesion to cultured human endothelial cells

Abstract

Antioxidants have been proposed to be anti-atherosclerotic agents; however, the mechanisms underlying their beneficial effects are poorly understood. We have examined the effect of α-tocopherol (α-tcp) on one cellular event in atherosclerotic plaque development, monocyte adhesion to stimulated endothelial cells (ECs). Human umbilical vein ECs were pretreated with α-tcp before stimulation with known agonists of monocyte adhesion: IL- 1 (10 ng/ml), LPS (10 ng/ml), thrombin (30 U/ml), or PMA (10 nM). Agonist- induced monocytic cell adhesion, but not basal adhesion, was inhibited in a time- and concentration-dependent manner by α-tcp. The IC50 of α-tcp on an IL-1-induced response was 45 μM. The inhibition correlated with a decrease in steady state levels of E-selectin mRNA and cell surface expression of E-selectin which is consistent with the ability of a monoclonal antibody to E-selectin to inhibit monocytic cell adhesion in this system. Probucol (50 μM) and N-acetylcysteine (20 mM) also inhibited agonist- induced monocytic cell adhesion; whereas, several other antioxidants had no significant effect. Protein kinase C (PKC) does not appear to play a role in the α-tcp effect since no suppression of phosphorylation of PKC substrates was observed. Activation of the transcription factor NF-κB is reported to be necessary but not sufficient for E-selectin expression in EC. Electrophoretic mobility shift assays failed to show an α-tcp-induced decrease in activation of this transcription factor after cytokine stimulation. It has been hypothesized that α-tcp acts as an anti-atherosclerotic molecule by inhibiting generation of oxidized LDL - a putative triggering molecule in the atherosclerotic process. Our results point to a novel alternative mechanism of action of α-tcp.