Syk, a Protein-tyrosine Kinase, Suppresses the Cell Motility and Nuclear Factor κB-mediated Secretion of Urokinase Type Plasminogen Activator by Inhibiting the Phosphatidylinositol 3′-Kinase Activity in Breast Cancer Cells*

Tumor growth and metastasis are multifaceted processes that mainly involve cell adhesion, proteolytic degradation of the extracellular matrix, and cell migration. Syk is a member of a tyrosine kinase family that is expressed mostly in hematopoietic cells. Syk is expressed in cell lines of epithelial origin, but its function in these cells remains unknown. Here we report that Syk is expressed in MCF-7 cells but not in MDA-MB-231 cells. The overexpression of wild type Syk kinase but not kinase-negative Syk suppressed cell motility and inhibited the activation of phosphatidylinositol (PI) 3′-kinase in MDA-MB-231 cells. In contrast, when Syk-specific antisense S-oligonucleotide but not the sense S-oligonucleotide was transfected to MCF-7 cells the level of PI 3′-kinase activity as well as cell motility were increased. The MDA-MB-231 cells transfected with wild type Syk cDNA followed by treatment with piceatannol, a Syk inhibitor, enhanced cell motility and PI 3′-kinase activity. Pervanadate, a phosphotyrosine phosphatase inhibitor, induced PI 3′-kinase activity and stimulated the interaction between the inhibitor of nuclear factor κBα (IκBα) and the p85α domain of PI 3′-kinase through tyrosine phosphorylation of the IκBα, which ultimately resulted in nuclear factor κB (NFκB) activation. Pervanadate had no effect on the activation of Syk in these cells. However, Syk suppressed the NFκB transcriptional activation and interaction between IκBα and PI 3′-kinase by inhibiting the tyrosine phosphorylation of IκBα. Syk, PI 3′-kinase inhibitors, and NFκB inhibitory peptide inhibited urokinase type plasminogen activator (uPA) secretion and cell motility in these cells. To our knowledge, this is the first report that Syk suppresses the cell motility and inhibits the PI 3′-kinase activity and uPA secretion by blocking NFκB activity through tyrosine phosphorylation of IκBα. These data further demonstrate a functional molecular link between Syk-regulated PI 3′-kinase activity and NFκB-mediated uPA secretion, and all of these ultimately control the motility of breast cancer cells.