X-Linked Inhibitor of Apoptosis Protein (XIAP) Supports Urokinase (uPA)-Induced Endothelial Cell Survival

High uPA expressing tumors are associated with poor prognosis. While a direct effect on tumor cell behavior is described, uPA has especially been shown to mediate (tumor-) angiogenesis. Originally, the urokinase system has been implicated to assist the angiogenic process by it’s proteolytic activities. It is now becoming increasingly evident that uPA additionally elicits a whole array pro-angiogenic responses like differentiation, proliferation and cell migration, independent of its proteolytic activity by inducing intracellular signal transduction.

Here we show that uPA induces upregulation of inhibitor of apoptosis proteins (IAPs), which protects endothelial cells against apoptosis. Thereby, uPA-induced endothelial cell survival is mediated by transcriptional upregulation the X-linked inhibitor of apoptosis protein (XIAP), because downregulation of XIAP by small interfering RNA techniques significantly reduces uPA mediated cell survival efficiencies of uPA in endothelial cells. The antiapoptotic activity of uPA was dependent on the presence of uPAR and LRP, but independent of the PI3kinase pathway, while VEGF-dependent antiapoptosis is mainly PI3kinase dependent. uPA-induced cell survival is dependent on the type of extracellular matrix on which cells are attached used indicating the involvement of integrin adhesion receptors. TherebyConsistently, uPA induces phosphorylation of the CDC42 downstream effector p21-activated kinase 1 (PAK1), which leads to IkappaB kinase alpha (IKKa) phosphorylation, a prerequisite for NFkappaB activation. As a consequence, p52/p50 but not p65 is are translocated into the nucleus. Blocking NFkappaB by using the specific NFkappaB inhibitor BAY 11–7082 or by adenoviral-mediated overexpression of its inhibitor, IkB, inhibits uPA-induced XIAP expression as well as uPA-induced cell survival.

From these data we conclude that uPA, which is a main player in endothelial cell migration and invasion, provides an additional, PI3-kinase independent but NFkappaB dependent cell survival mechanism.