Etk/Bmx Transactivates Vascular Endothelial Growth Factor 2 and Recruits Phosphatidylinositol 3-Kinase to Mediate the Tumor Necrosis Factor-induced Angiogenic Pathway

Niklas Ekman,Yingqian Xu,Rong Zhang,Kari Alitalo,Zhenhua Wu,Wang Min,Jiong Wu

doi:10.1074/jbc.m310678200

Abstract

Tumor necrosis factor (TNF), via its receptor 2 (TNFR2), induces Etk (or Bmx) activation and Etk-dependent endothelial cell (EC) migration and tube formation. Because TNF receptor 2 lacks an intrinsic kinase activity, we examined the kinase(s) mediating TNF-induced Etk activation. TNF induces a coordinated phosphorylation of vascular endothelial growth factor (VEGF) receptor 2 (VEGFR2) and Etk, which is blocked by VEGFR2-specific inhibitors. In response to TNF, Etk and VEGFR2 form a complex resulting in a reciprocal activation between the two kinases. Subsequently, the downstream phosphatidylinositol 3-kinase (PI3K)-Akt signaling (but not signaling through phospholipase C-gamma) was initiated and directly led to TNF-induced EC migration, which was significantly inhibited by VEGFR2-, PI3K-, or Akt-specific inhibitors. Phosphorylation of VEGFR2 at Tyr-801 and Tyr-1175, the critical sites for VEGF-induced PI3K-Akt signaling, was not involved in TNF-mediated Akt activation. However, TNF induces phosphorylation of Etk at Tyr-566, directly mediating the recruitment of the p85 subunit of PI3K. Furthermore, TNF- but not VEGF-induced activation of VEGFR2, Akt, and EC migration are blunted in EC genetically deficient with Etk. Taken together, our data demonstrated that TNF induces transactivation between Etk and VEGFR2, and Etk directly activates PI3K-Akt angiogenic signaling independent of VEGF-induced VEGFR2-PI3K-Akt signaling pathway.

Highlights

Angiogenic factors such as vascular endothelial growth factor (VEGF)1 promote angiogenesis in vitro and in vivo, it has been demonstrated that inflammatory responses play an important role in stimulating angiogenesis [1,2,3,4]
VEGFR2 phosphorylation induced by Tumor necrosis factor (TNF) was blocked by VEGFR2 inhibitors (Fig. 1d), suggesting that TNF induces a transactivation of VEGFR2 primarily through VEGFR2 autophosphorylation
We show that TNF induces a complex formation of Etk and VEGFR2, leading to a reciprocal activation between Etk and VEGFR2, as both kinase activities are required for activation one another

Summary

Introduction

Angiogenic factors such as vascular endothelial growth factor (VEGF) promote angiogenesis in vitro and in vivo, it has been demonstrated that inflammatory responses (as defined by the presence of infiltrated macrophages and proinflammatory cytokines) play an important role in stimulating angiogenesis [1,2,3,4]. Etk and three other members of this family (Btk, Itk, and Tec), participate in signal transduction in response to virtually all types of extracellular stimuli that are transmitted by growth factor receptors, cytokine receptors, G-protein-coupled receptors, antigen receptors, and integrins [27,28,29,30,31]. They share a common structure domain including a pleckstrin homology (PH) domain, TEC homology domain, which has a PXXP motif (with exception of Etk), an SH3 and an SH2 domain, and a kinase domain. Etk acts upstream of VEGFR2 and PI3K-Akt to mediate TNF-induced angiogenic signaling pathway distinct from that induced by VEGF

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