Urokinase activates macrophage PON2 gene transcription via the PI3K/ROS/MEK/SREBP-2 signalling cascade mediated by the PDGFR-β

Abstract

We have recently shown that urokinase plasminogen activator (uPA) increases oxidative stress (OS), cholesterol biosynthesis, and paraoxonase 2 (PON2) expression in macrophages via binding to its receptor, the uPAR. Since PON2 is regulated by both OS and cholesterol content, we hypothesized that uPA elicits a cascade of signal transduction events shared by NADPH oxidase and cholesterol biosynthesis that culminates in PON2 gene expression. Here, we investigated the signalling pathway that leads to the expression of PON2 in macrophages in response to uPA. The increase in macrophage PON2 mRNA levels in response to uPA was shown to depend on PON2 gene promoter activation and mRNA transcription. LDL abolished these effects, suggesting a possible role for a transcription factor involved in cellular cholesterogenesis. Indeed, uPA upregulated PON2 expression in a sterol regulatory binding protein-2 (SREBP-2)-dependent manner, since blocking SREBP-2 maturation by 4-(2-aminoethyl)-benzenesulfonyl fluoride abolished uPA-stimulation of PON2, whereas inhibition of SREBP-2 catabolism by N-acetyl-leucyl-norleucinal had an opposite effect. The upstream signalling mechanisms include uPA activation of extracellular signal-regulated kinases (ERK1/2), which was dependent on NADPH oxidase and phosphatidylinositol 3-kinase activation, and these latter effects were mediated by the tyrosine kinase activity of the platelet-derived growth factor receptor-beta. These findings provide a framework linking interactions among cellular signalling pathways associated with reactive oxygen species production, macrophage cholesterol biosynthesis, and cellular PON2 expression in vascular pathophysiology.