Etk/Bmx as a Tumor Necrosis Factor Receptor Type 2-Specific Kinase: Role in Endothelial Cell Migration and Angiogenesis

Tumor necrosis factor (TNF) is a cytokine that mediates many pathophysiologial processes, including angiogenesis. However, the molecular signaling involved in TNF-induced angiogenesis has not been determined. In this study, we examined the role of Etk/Bmx, an endothelial/epithelial tyrosine kinase involved in cell adhesion, migration, and survival in TNF-induced angiogenesis. We show that TNF activates Etk specifically through TNF receptor type 2 (TNFR2) as demonstrated by studies using a specific agonist to TNFR2 and TNFR2-deficient cells. Etk forms a preexisting complex with TNFR2 in a ligand-independent manner, and the association is through multiple domains (pleckstrin homology domain, TEC homology domain, and SH2 domain) of Etk and the C-terminal domain of TNFR2. The C-terminal 16-amino-acid residues of TNFR2 are critical for Etk association and activation, and this Etk-binding and activating motif in TNFR2 is not overlapped with the TNFR-associated factor type 2 (TRAF2)-binding sequence. Thus, TRAF2 is not involved in TNF-induced Etk activation, suggesting a novel mechanism for Etk activation by cytokine receptors. Moreover, a constitutively active form of Etk enhanced, whereas a dominant-negative Etk blocked, TNF-induced endothelial cell migration and tube formation. While most TNF actions have been attributed to TNFR1, our studies demonstrate that Etk is a TNFR2-specific kinase involved in TNF-induced angiogenic events.