One out of every eight women is diagnosed with breast cancer, and it is the second leading cause of cancer-related death among women. In addition to genetic alterations of tumor suppressor proteins and oncoproteins, enhanced tyrosine kinase activity is also associated with breast cancer and is a prognostic indicator of decreased disease-free survival. The non-receptor protein tyrosine kinase (PTK), Brk, was cloned from a metastatic breast tumor and subsequently found expressed in approximately 65 percent of human breast tumors and breast cancer cell lines, but not in normal mammary tissue. Very little is known about how Brk promotes breast cancer progression. Our long range goal is to understand how Brk alters signal transduction pathways to promote breast cancer tumorigenesis. The objective of this proposal is to understand how Brk modulates ErbB receptor signaling pathways. The aims proposed in this project are; (1) Determine the specificity of Brk signaling and interactions with ErbB family members and (2) determine how Brk expression affects cellular proliferation, transformation and survival. These studies will be performed in the T47D breast cancer cell line, which express Brk and all ErbB family members, and normal human mammary epithelial cells (HMECs). Brk protein levels will be knocked-down in T47D cells by stably expressing shRNA, while wild-type and kinase activity mutants of Brk will be expressed alone and in combination with selected ErbB family members in HMECs. These cell models will be used to examine the influence of Brk expression on ErbB receptor signaling pathways and cellular proliferation, transformation, and apoptosis. Brk is expressed almost exclusively in tumor tissue and is therefore a potential therapeutic target to treat cancer. A better understanding of Brk's biochemical properties and role in breast cancer may provide clues to effective targeting of Brk or Brk-dependent signaling pathways. [unreadable] [unreadable] [unreadable] [unreadable]