The transforming growth factor-β-activated kinase 1 (TAK1) is a mitogen-activated protein 3 kinase and also suggested to be an AMP-activated protein kinase (AMPK) kinase. TAK1 is activated in response to cytokines and growth factors, mediating inflammation and oxidative stress response. In endothelial cells, these effects are relevant for angiogenesis and although TAK1^-/- mice display defects in developmental vasculogenesis, the functional role of TAK1 in endothelial cells has not been investigated in detail.

TAK1 knockdown (siRNA) delayed proliferation, migration (scratch wound) and angiogenesis (tube formation and spheroid sprouting) in endothelial cells. Endothelial sprouting from aortic rings was markedly reduced in mice lacking TAK1 in endothelial cells compared to wild-type mice. The stimulatory effect of VEGF on angiogenesis was dependent on TAK1 expression and the VEGF-stimulated phosphorylation of the c-Jun N-terminal kinase, p38 and AMPK could be prevented by the pharmacological TAK1 inhibitor oxozeaenol. A proteomics analysis identified superoxide dismutases (SOD) as potential TAK1 targets. A reduced protein expression of the mitochondrial superoxide dismutase 2 could be confirmed (Western blot) in endothelial cells lacking TAK1. Hydrogen peroxide production (Amplex Red) was reduced in TAK1-deficient endothelial cells. The mitochondrial membrane potential was also abnormal in cells lacking TAK1 (flow cytometry of JC-1). Mitochondrial superoxide production was increased in these cells (flow cytometry for MitoSOX).

These results establish TAK1 as an intermediate of VEGF-signalling and subsequent endothelial angiogenesis. A disturbed VEGF-stimulated hydrogen peroxide production as a consequence of TAK1-deletion may mediate the delayed angiogenesis.