Protein kinase C mediates induced secretion of vascular endothelial growth factor by human glioma cells

To understand how vascular endothelial growth factor (VEGF) production is activated in malignant glioma cells, we employed protein tyrosine kinase (PTK) and protein kinase C (PKC) inhibitors to evaluate the extent to which these protein kinases were involved in signal transduction leading to VEGF production. PTK inhibitors blocked glioma proliferation and epidermal growth factor (EGF)-induced VEGF secretion, while H-7, a PKC inhibitor, inhibited both EGF-induced and baseline VEGF secretion. Phorbol 12-myristate 13-acetate (PMA), a non-specific activator of PKC, induced VEGF secretion by glioma cells, which was enhanced by calcium ionophore A23187, but completely blocked after prolonged treatment of cells with 1 microM PMA, by presumably depleting PKC. All inhibitors (genistein, AG18, AG213, H-7, prolonged PMA treatment) which inhibited EGF-induced VEGF secretion in glioma cells also inhibited cell proliferation at similar concentrations. However, PKC inhibition only blocked 50% of the VEGF secretion induced by growth factors (EGF, platelet-derived growth factor-BB, or basic fibroblast growth factor). This reserve capacity could be ascribed to a PKC-independent effect, or to PKC isoenzymes not down-regulated by PMA. These findings extend our previous assertion that VEGF secretion is tightly coupled with proliferation by suggesting that activation of convergent growth factor signaling pathways will lead to increased glioma VEGF secretion. Understanding of signal transduction of growth factor-induced VEGF secretion should provide a rational basis for the development of novel strategies for therapy.