Oxidized LDL and lysophosphatidylcholine stimulate plasminogen activator inhibitor-1 expression in vascular smooth muscle cells

Plasminogen activator inhibitor-1 (PAI-1) functions as an important regulat or of fibrinolysis by inhibiting both tissue-type and urokinase-type plasmi nogen activator. PAI-I is produced by smooth muscle cells (SMCs) in atheros clerotic arteries, but the mechanisms responsible for induction of PAI-1 in SMCs are less well understood. In cultured human aortic SMCs, PAI-1 mRNA e xpression and protein secretion were increased after incubation with oxidiz ed low-density lipoprotein (LDL) and the lipid peroxidation product lysopho sphatidylcholine, whereas the effects of native LDL on PAI-1 production and release were more variable and did not reach statistical significance. The effect of LDL on arterial expression of PAI-1 in vivo was also studied in an animal model. Intravenous injection of human LDL in Sprague-Dawley rats resulted in accumulation of apolipoprotein B in the aorta within 12 hours a s assessed by immunohistochemical testing. Epitopes specific for oxidized L DL began to develop in the aorta 12 hours after injection of LDL and peaked at 24 hours; this peak was accompanied by intense expression of PAI-1 immu noreactivity in the media, Also, increased aortic expression of PAI-1 mRNA after LDL injection was detected by using in situ hybridization, The transc ription factor activator protein-1, which is known to bind to the promoter of the PAI-1 gene, was activated in the aortic wall 24 hours after LDL inje ction as assessed by electrophoretic mobility shift assay. Pretreatment of LDL with the antioxidant probucol decreased expression of oxidized LDL and PAI-1 immunoreactivity and activator protein-1 induction in the aorta but d id not affect expression of apolipoprotein B immunoreactivity. These findin gs demonstrate that LDL oxidation enhances secretion of PAI-1 from cultured SMCs and that a similar mechanism may be involved in vascular expression o f PAI-I.