Rapamycin induces tumor-specific thrombosis via tissue factor in the presence of VEGF

Abstract

Therapeutic strategies that target and disrupt the already-formed vessel networks of growing tumors are actively pursued. The goal of these approaches is to induce a rapid shutdown of the vascular function of the tumor so that blood flow is arrested and tumor cell death occurs. Here we show that the mammalian target of rapamycin (mTOR) inhibitor rapamycin, when administered to tumor-bearing mice, selectively induced extensive local microthrombosis of the tumor microvasculature. Importantly, rapamycin administration had no detectable effect on the peritumoral or normal tissue. Intravital microscopy analysis of tumors implanted into skinfold chambers revealed that rapamycin led to a specific shutdown of initially patent tumor vessels. In human umbilical vein endothelial cells vascular endothelial growth factor (VEGF)-induced tissue factor expression was strongly enhanced by rapamycin. We further show by Western blot analysis that rapamycin interferes with a negative feedback mechanism controlling this pathologic VEGF-mediated tissue factor expression. This thrombogenic alteration of the endothelial cells was confirmed in a one-step coagulation assay. The circumstance that VEGF is up-regulated in most tumors may explain the remarkable selectivity of tumor vessel thrombosis under rapamycin therapy. Taken together, these data suggest that rapamycin, besides its known antiangiogenic properties, has a strong tumor-specific, antivascular effect in tumors. IntroductionVascular endothelium normally provides a nonadhesive, nonthrombogenic surface for blood constituents. However, in response to inflammatory stimuli from cytokines and bacterial products, induction of adhesion molecules and the expression of endothelial cell surface procoagulant proteins can occur. Central to the conversion of normal endothelium to a procoagulant surface is the induction of tissue factor (TF) on endothelial cells. 1 TF is a transmembrane protein that functions as a high-affinity receptor for factor VIIa. Formation of complexes between TF and factor VIIa initiates the extrinsic blood coagulation cascade by activation of factors IX and X. 2 Consistent with a protective role of TF in the hemostatic response, it is constitutively expressed in several extravascular cell types surrounding blood vessels (eg, smooth muscle cells, monocytes) and on organ surfaces. 3 Under normal circumstances TF is not expressed on endothelial cells but is rapidly induced in response to inflammatory stimuli including tumor necrosis factor ␣ (TNF-␣), vascular endothelial growth factor (VEGF), interleukin-1␤, lipopolysaccharide, and thrombin, rendering the endothelial cell surface of blood vessels thrombogenic. 4 Cell surface expression of TF has long been implicated in many clinical scenarios such as inflammatory or infectious diseases (sepsis), reperfusion injury, and transplant graft rejection. 5 The up-regulation of TF under these pathophysiologic conditions may be responsible for thrombotic complications. In addition, alterations in TF expression on endothelial ce...

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Inhibition of mTOR pathway by everolimus cooperates with EGFR inhibitors in human tumours sensitive and resistant to anti-EGFR drugs

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