Etk/Bmx tyrosine kinase activates Pak1 and regulates tumorigenicity of breast cancer cells

Etk/Bmx, a member of the Tee family of nonreceptor protein-tyrosine kinases , is characterized by an N-terminal pleckstrin homology domain and has been shown to be a downstream effector of phosphatidylinositol 3-kinase. P21-ac tivated kinase 1 (Pak1), another well characterized effector of phosphatidy linositol. 3-kinase, has been implicated in the progression of breast cance r cells. In this study, we characterized the role of Etk in mammary develop ment and tumorigenesis and explored the functional interactions between Etk and Pak1. We report that Etk expression is developmentally regulated in th e mammary gland. Using transient transfection, coimmunoprecipitation and gl utathione S-transferase-pull down assays, we showed that Etk directly assoc iates with Pak1 via its N-terminal pleckstrin homology domain and also phos phorylates Pak1 on tyrosine residues. The expression of wild-type Etk in a non-invasive human breast cancer MCF-7 cells significantly increased prolif eration and anchorage-independent growth of epithelial cancer cells. Conver sely, expression of kinase-inactive mutant Etk-KQ suppressed the proliferat ion, anchorage-independent growth, and tumorigenicity of human breast cance r MDA-MB435 cells. These results indicate that Pak1 is a target of Etk and that Etk controls the proliferation as well as the anchorage-independent an d tumorigenic growth of mammary epithelial cancer cells.