β-Catenin and NF-κB cooperate to regulate the uPA/uPAR system in cancer cells

Expression of the urokinase plasminogen activator (uPA) and urokinase plasminogen activator receptor (uPAR) has recently been shown to be directly regulated by the Wnt/β-catenin signaling pathway in colon cancer cells, through β-catenin binding to T-cell factor binding element motifs present in their gene promoters. In our study, we present evidence that inhibition of β-catenin causes upregulation of uPA/uPAR gene expression enhancing invasive potential. Using MCF-7, MDA-MB-231 (breast cancer cells) and SW480 (colon cancer cells), we found that siRNA-mediated silencing of β-catenin increased uPA, uPAR and plasminogen activator inhibitor-1 (PAI-1) expression at the mRNA and protein levels. This increase was responsible for the observed enhanced invasive capacity of MDA-MB-231 and SW480 cancer cells. In addition, β-catenin stabilization and accumulation by lithium chloride treatment, a well-known inhibitor of glycogen synthase kinase-3β (GSK-3β), or by β-catenin/T-cell factor-4 expression vectors transfection led to a decrease in uPA, uPAR and PAI-1 mRNA expression in the studied cancer models. Treatment of β-catenin siRNA-transfected cells with a specific inhibitor of nuclear factor-kappaB (NF-κB), SN50, significantly reduced enhancement of uPA, uPAR and PAI-1 expression and cancer cell invasion, observed in β-catenin siRNA-transfected cells. Furthermore, β-catenin siRNA-treated cells exhibited NF-κB nuclear accumulation. These data suggest that β-catenin regulates the uPA/uPAR system in cooperation with NF-κB transcription factor, which constitutes a novel mechanism of regulation.