OBJECTIVE The atherosclerotic lesion is characterized by lipid peroxide accumulation. Paraoxonase 1 (PON1) reduces atherosclerotic lesion oxidative stress, whereas urokinase-type plasminogen activator (uPA) increases oxidative stress in atherosclerotic lesions and contributes to the progression and complications of atherosclerosis. We hypothesized that uPA may promote oxidative stress in the arterial wall via modulation of PON1 activity. Because the liver is the main site for PON1 production, in the present study, we tested whether uPA influences PON1 expression in hepatocytes. RESULTS HuH7 hepatocytes were incubated in culture with increasing concentrations of uPA. uPA decreased PON1 gene expression and activity in a dose-dependent manner and accordingly suppressed PON1 secretion from hepatocytes. This effect required uPA/uPA receptor interaction. uPA downregulated PON1 gene expression via inactivation of peroxisome proliferator-activated receptor-γ (PPARγ) activity, and this effect was dependent on uPA-mediated mitogen-activated protein kinase kinase activation. Mechanistic studies showed that uPA enhanced mitogen-activated protein kinase kinase-PPARγ interaction, resulting in PPARγ nuclear export to the cytosol. CONCLUSIONS This study provides the first evidence that uPA interferes with PPARγ transcriptional activity in hepatocytes, resulting in downregulation of PON1 expression and its secretion to the medium. This may explain, at least in part, the prooxidative effect of uPA in the vascular wall.