Brk (for breast tumor kinase) is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk was originally identified from a human metastatic breast tumor, and its overexpression is frequently observed in breast cancer and several other cancer types. However, the molecular mechanism by which this kinase participates in tumorigenesis remains poorly characterized. In the present study, we not only identified paxillin as the binding partner and substrate of Brk but also discovered a novel signaling pathway by which Brk mediates epidermal growth factor (EGF)-induced paxillin phosphorylation. We show that EGF stimulation activates the catalytic activity of Brk, which in turn phosphorylates paxillin at Y31 and Y118. These phosphorylation events promote the activation of small GTPase Rac1 via the function of CrkII. Through this pathway, Brk is capable of promoting cell motility and invasion and functions as a mediator of EGF-induced migration and invasion. In accordance with these functional roles, Brk translocates to membrane ruffles, where it colocalizes with paxillin during cell migration. Together, our findings identify novel signaling and biological roles of Brk and indicate the first potential link between Brk and metastatic malignancy.Unraveling the signaling pathways responsible for the establishment of a metastatic phenotype in carcinoma cells is of crucial importance for the understanding of the pathology of cancer. The process of metastasis includes several components, such as the ability to invade through acquisition of cell motility, degradation of extracellular matrix and basement membrane, cell proliferation, and survival signaling. Aberrant tyrosine kinase signaling via stimulation of growth factor receptors or intracellular tyrosine kinases has been shown to contribute to various steps of tumor development and progression, including metastasis (6). Brk is an intracellular tyrosine kinase that was identified in a study for screening kinases expressed in human metastatic breast tumors (36). In addition to a typical tyrosine kinase domain, Brk possesses both SH3 and SH2 domains and thus is related to Src family kinases (36). However, unlike Src family kinases, Brk lacks an N-terminal consensus sequence for myristoylation and membrane association (36). Its genomic structure is also distinct from that of Src family kinases, suggesting that Brk has diverged significantly from Src kinases in evolution (37). The expression pattern and subcellular localization of Brk have suggested its role in tumorigenesis. In normal tissues, the expression of Brk or its mouse ortholog Sik is restricted to differentiating epithelial cells of the skin and gastrointestinal tract (27). However, it is highly expressed in many breast carcinoma cell lines and a significant portion of breast tumor tissues but not in human mammary epithelial cells (3, 34, 36) and mouse mammary glands at various developmental stages (27). Elevated expression of Brk has also been detected in metastatic melanoma cell li...

Etk (also calledBmxEtk (also called Bmx) is a member of the Btk nonreceptor tyrosine kinase family (48,51,57). This family of kinases is characterized by a modular structure, including an N-terminal pleckstrin homology (PH) domain, a Tec homology domain, Src homology 2 (SH2) and SH3 domains, and a C-terminal kinase domain. Many members of the Btk family are predominantly expressed in cells of hematopoietic origin. Btk is expressed in B cells and myeloid cell lineages and is essential for B-cell development and signaling. Loss-of-function mutations in Btk result in human X-linked agammaglobulinemia and murine X-linked B-cell immunodeficiency (50,53