Chemoresistance of endothelial cells induced by basic fibroblast growth factor depends on Raf-1-mediated inhibition of the proapoptotic kinase, ASK1

Genotoxic stress induced by anticancer drugs can lead to apoptosis, of both angiogenic endothelial cells (ECs) and proliferating tumor cells. However, growth factors such as basic fibroblast growth factor (bFGF) and vascular endothelial cell growth factor (VEGF) present within the tumor microenvironment can promote chemoresistance by suppressing apoptotic mechanisms in these cells. Here, we have identified apoptosis signal-regulating kinase I (ASK1), a proapoptotic member of the MAP3K family, as a target of bFGF-mediated survival signaling in ECs. Evidence is provided that ASK1 is required for EC apoptosis in response to the genotoxic chemotherapeutic agent doxorubicin, and that bFGF, but not VEGF, neutralizes the death-promoting activity of ASK1. Specifically, bFGF stimulation promotes the formation of a Raf-1/ASK1 complex at the mitochondria, inhibits ASK1 kinase activity, and protects ECs. from genotoxic stress. Mutation of the Ralf-1 activation domain (SS338/9AA) not only prevents Raf-1/ASK1 complex formation but abolishes bFGF-mediated EC protection from genotoxic stress. In line with these observations, bFGF, but not VEGF, neutralizes the antiangiogenic effects of doxorubicin in vivo. These findings reveal a new pathway of EC survival signaling and define a molecular mechanism for chemoresistance induced by bFGF.