Abstract
Tissue factor is the prime initiator of blood coagulation. Expression of tissue factor in tumor endothelial cells leads to thrombus formation, occlusion of vessels and development of hemorrhagic infarctions in the tumor tissue, often followed by regression of the tumor. Tumor cells produce endogenous vascular endothelial growth factor (VEGF), which sensitizes endothelial cells for systemically administered tumor necrosis factor α (TNF α) and synergistically enhances the TNF‐induced expression of tissue factor. We have analyzed the pathways involved in the induction of tissue factor in human umbilical cord vein endothelial cells (HUVECs) after combined stimulation with TNF and VEGF. By using specific low molecular weight inhibitors, we demonstrated that protein kinase C (PKC), p44/42 and p38 mitogen‐activated protein (MAP) kinases, and stress‐activated protein kinase (JNK) are essentially involved in the induction of tissue factor. In contrast, the application of wortmannin, an inhibitor of phosphatidylinositol 3 (PI3)‐kinase, led to strongly enhanced expression of tissue factor in TNF‐ and VEGF‐treated cells, implicating a negative regulatory role for PI3‐kinase. In vivo, the application of wortmannin promoted the formation of TNF‐induced hemorrhages and intratumoral necroses in murine meth A tumors. The co‐injection of wortmannin lowered the effective dose of applied TNF. Therefore, it is conceivable that the treatment of TNF‐sensitive tumors with a combination of TNF and wortmannin will ensure the selective damage of the tumor endothelium and minimize the risk of systemic toxicity of TNF. TNF‐treatment in combination with specific inhibition of PI3‐kinase is a novel concept in anti‐cancer therapy. © 2003 Wiley‐Liss, Inc.