ABSTRACT

Both radiation- and chemo-therapy are ineffective as treatments of glioblastoma multiforme due to the highly invasive nature of these primary brain tumors.  The plasminogen activator system components, including the urokinase plasminogen activator (uPA) and its receptor (uPAR), have previously been implicated in the invasiveness of glioblastoma cells.  Surprisingly, patients with glioblastomas expressing high levels of the inhibitor of uPA, Plasminogen Activator Inhibitor-1 (PAI-1), and the Epidermal Growth Factor (EGF) Receptor have a shorter overall prognosis of survival.  This can be explained by the PAI-1-induced internalization of the PAI-1/uPA/uPAR/integrin complex, detachment of individual cells from the tumor mass, and their migration.  We report that EGF enhances the expression of PAI-1 via sequential activation of c-Src, protein kinase C delta (PKCd), and sphingosine kinase 1 (SphK1), the enzyme that produces sphingosine-1-phosphate.  EGF induced rapid phosphorylation of c-Src and PKCd, as well as the concomitant translocation of PKCd and SphK1 to the plasma membrane.  S1P generated by SphK1 activates PAI-1 expression via intracellular mechanisms while exogenously added S1P activates PAI-1 (and uPAR) expression via the S1P2 receptor.  In addition, EGF, as well as proinflammatory cytokines, including IL-1, activates expression of the sphk1 gene via the JNK-c-jun pathway, which provides an increased supply of S1P.  Collectively, these results provide a functional link between three critical downstream targets of EGF, c-Src, PKCd, and SphK1 that have all been implicated in regulating the motility and invasion of glioblastoma cells.