Vascular Endothelial Growth Factor (VEGF) Induces Cell Survival Via an Additional Urokinase (uPA)-Dependent Mechanism.

VEGF, the most important angiogenic growth factor, is known to induce cell-survival mainly via phosphorylation of the pro-apoptotic proteins BAD, PED/PEA-2 or pro-caspase-9 or inhibition of SAPKs (stress activated kinases). These mechanisms are all dependent on the PI3-kinase/Akt pathway. We could show recently that VEGF also induces pro-uPA activation via the PI3-kinase signaling pathway beside and independent of the transcriptional upregulation of uPA. Active uPA does not only contribute to angiogenesis via its proteolytic properties, but also effectuates itself pro-angiogenic signalling by the induction of endothelial cell migration, proliferation and differentiation. We were interested whether generation of uPA upon VEGF is inducing an additional effect on endothelial cell survival. First, we compared VEGF with urokinase in respect to cell survival in apoptosis assays and observed a pivotal anti-apoptotic effect of both stimuli, dose dependently and dependent on the type of matrix used. In addition, cell survival effects were additive, when both stimuli were added simultaneously. While VEGF-induced cell survival was PI3-kinase dependent, because it could be inhibited by the specific PI3-kinase inhibitor LY294002, the uPA-induced cell survival was PI3-kinase independent. Furthermore, uPA was able to rescue apoptosis induced by PI3-kinase inhibition in VEGF stimulated endothelial cells. From these data we conclude that uPA is indeed inducing an additional - PI3-kinase independent - cell survival mechanism. While VEGF led to a PI3-kinase dependent phosphorylation of Akt, which resulted in CDC42 activation, uPA activated CDC42 and its downstream effectors PAK leading to IKK-1 phosphorylation in a PI3-kinase/Akt independent manner. This indicates that the anti-apoptotic properties of uPA are not Akt, but NF-kappaB mediated. Indeed, when we used adenovirus overexpressing I-kappaB to block the NF-kappaB pathway, uPA was ineffective to support cell survival. In addition VEGF and uPA, both induced a transcriptional upregulation of inhibitor of apoptosis proteins (IAPs) in an NF-kappaB-dependent manner, among them most significantly the X-linked inhibitor of apoptosis protein (XIAP); again VEGF in a PI3-kinase dependent, uPA in a PI3-kinase independent manner.

From these data we conclude that VEGF is inducing cell survival in a strictly PI3-kinase dependent manner, on the one hand via its known mitochondrial pathway, but also via the PI3-kinase dependent pro-uPA activation leading to an NFkappa B dependent upregulation of inhibitor of antiapoptosis proteins.