Focal adhesion kinase signalling in endothelial cells

The angiogenic factor vascular endothelial growth factor (VEGF) induces migration and cytoskeletal changes in endothelial cells. The mechanism by which VEGF signals was not known but it was possible an adhesion associated protein such as focal adhesion kinase (FAK) was involved. Further investigation revealed VEGF caused a marked activation of tyrosine phosphorylation of FAK in HUVE cells, which was time and concentration dependent. The PI3K inhibitor wortmannin partially inhibited VEGF stimulated FAK phosphorylation and it inhibited phosphorylation in response to lysophosphatidic acid and VEGF induced membrane ruffling. The tyrosine kinases Src and Fyn were associated with FAK upon VEGF stimulation of HUVE cells. VEGF stimulated recruitment to FAK would allow Src family kinases to stimulate tyrosine phosphorylation of FAK thereby mediating the effects of VEGF on FAK tyrosine phosphorylation. Analysis of a functional consequence of VEGF stimulated FAK tyrosine phosphorylation was assessed by displacement of endogenous FAK by microinjection of a truncated C-terminal FAK. A reduction in the distance migrated by VEGF stimulated endothelial cells was demonstrated, indicating VEGF signals via FAK to induce migration. To investigate the role of FAK in other endothelial cell functions, epitope tagged FAK mutants were generated and characterised. Co-expression of the FAK mutants and in addition, a Fyn kinase inactive protein, gave some insight into the mechanism of FAK tyrosine phosphorylation. Expression of the mutant FAK proteins revealed neither the autophosphorylation site or the kinase ability of FAK are required to provide survival signals from FAK in adherent endothelial cells however the truncated C-terminal FAK did induce apoptosis.