Role of Raf in Vascular Protection from Distinct Apoptotic Stimuli

Abstract

Raf kinases have been linked to endothelial cell survival. Here, we show that basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) differentially activate Raf, resulting in protection from distinct pathways of apoptosis in human endothelial cells and chick embryo vasculature. bFGF activated Raf-1 via p21-activated protein kinase-1 (PAK-1) phosphorylation of serines 338 and 339, resulting in Raf-1 mitochondrial translocation and endothelial cell protection from the intrinsic pathway of apoptosis, independent of the mitogen-activated protein kinase kinase-1 (MEK1). In contrast, VEGF activated Raf-1 via Src kinase, leading to phosphorylation of tyrosines 340 and 341 and MEK1-dependent protection from extrinsic-mediated apoptosis. These findings implicate Raf-1 as a pivotal regulator of endothelial cell survival during angiogenesis.

RKTG inhibits angiogenesis by suppressing MAPK-mediated autocrine VEGF signaling and is downregulated in clear-cell renal cell carcinoma

“…Upon VEGF binding, VEGFR2 undergoes dimerization and autophosphorylation at many tyrosine kinase residues (Dougher and Terman, 1999;Matsumoto et al, 2005), in which tyrosine phosphorylation at 1175 (or 1173 in mouse) activates the phospholipase Cg/protein kinase C pathway, leading to activation of the Raf/MEK/ERK signaling cascade (Guo et al, 1995;Xia et al, 1996;Matsumoto and Mugishima, 2006). The importance of the Raf/MEK/ERK signaling pathway in VEGFmediated angiogenesis is also supported by the finding that Raf-1 has a pivotal role in protecting endothelial cells from extrinsic-mediated apoptosis during angiogenesis (Alavi et al, 2003). In addition, our data are consistent with the finding that PD98059 is able to block the proliferation, migration and tube formation of endothelial cells in vitro (Wu et al, 2000;Srivastava et al, 2009).…”

Apoptosis of hematopoietic cells induced by growth factor withdrawal is associated with caspase-9 mediated cleavage of Raf-1

“…It has been suggested that Raf-1 may promote cell survival by antagonizing the proapoptotic apoptosis signal-regulating kinase 1 (ASK1) through a direct inhibitory interaction with the N-terminal domain of ASK1 (Chen et al, 2001). It was also reported that Raf-1 translocates to the mitochondria in response to survival stimuli, where it mounts an antiapoptotic response independently of ERK activation (Wang et al, 1996;Salomoni et al, 1998;Peruzzi et al, 1999;Alavi et al, 2003). It was suggested that, when located to the mitochondria, Raf-1 suppresses cell death by inactivating the proapoptotic Bcl-2 family member Bad (Wang et al, 1996;Salomoni et al, 1998).…”

Conferring specificity on the ubiquitous Raf/MEK signalling pathway

“…Using the Raf/MEK/ERK signalling pathway as paradigm, we have highlighted some of these mechanisms including differential protein interactions, subcellular compartmentalisation, different modes of activation, and differential targeting of downstream effectors. A recent study elegantly demonstrates how the cell orchestrates this repertoire of mechanisms (Alavi et al, 2003). In endothelial cells, vascular endothelial growth factor (VEGF) protects from apoptosis caused by serum starvation and DNA-damaging drugs, whereas basic fibroblast growth factor (bFGF) prevents the apoptosis induced by death receptor stimulation.…”