We hypothesize that integrin signaling plays key roles during tumor initiation and progression. To study the role of the 4 integrin during ErbB2-mediated mammary tumorigenesis, we introduced a targeted deletion of the 4 signaling domain in MMTV-Neu mice. Loss of 4 signaling delayed tumor onset and inhibited invasive growth. Ex vivo studies indicated that 4 forms a complex with ErbB2 and enhances activation of STAT3 and c-Jun. STAT3 contributes to disrupt epithelial adhesion and promote invasion, whereas c-Jun is required for hyperproliferation. To examine the role of Focal Adhesion Kinase (FAK) in mammary tumorigenesis, we introduced a mammary gland-specific ablation of FAK in MMTV-PyMT mice. Notably, deletion of FAK completely suppressed mammary tumorigenesis. In addition, silencing of FAK induced Ras- and PI-3K- transformed mammary carcinoma cells to undergo growth arrest and senescence or apoptosis in vitro. These results suggest that 4 promotes tumor progression by amplifying ErbB2 signaling, whereas FAK is required for Ras- and PI-3K-mediated mammary tumorigenesis. We propose: 1) To Examine the Oncogene Specificity of the Pro-tumorigenic Effects of 4 and FAK. Normal mammary epithelial cells will be altered in vitro to suppress FAK or 4 signaling and then subjected to transformation assays with various oncogenes. Conversely, mammary tumor cells carrying distinct oncogenes will be genetically manipulated in vitro to suppress FAK or 4 signaling and subjected to assays designed to examine their ability to proliferate, to resist apoptosis, to invade in vitro, and to form orthotopic tumors in NOD/SCID mice. Key observations will be confirmed using selected human breast cancer cell lines and transgenic mouse models; 2) To Elucidate the Mechanisms through which 4 Signaling Disrupts Epithelial Adhesion and Promotes Invasion. The mechanism by which 4 amplifies ErbB2 signaling will be examined by using mutagenesis in combination with biochemical analysis and iRNA-mediated inhibition. The signaling pathways through which hyperactivation of Rac causes disassembly of adherens junctions will be studied by using biochemical analysis, silencing, and imaging methods; 3) To Study the Molecular Mechanisms through which FAK Promotes Mammary Tumorigenesis. Silencing will be used to confirm that CAS mediates the pro-tumorigenic effect of FAK. Biochemical analysis of control and CAS-silenced tumor cells will then be used to identify CAS-dependent pathways. Inhibition of key downstream effectors will be used to identify the major signaling pathways through which CAS promotes mammary tumorigenesis; 4) To Examine the Mechanisms through which 4 and FAK Cooperate to Sustain ErbB2-initiated Mammary Tumorigenesis. Biochemical experiments will be conducted to identify signaling components jointly activated by 4 and FAK in mammary tumor cells. Silencing will be used to test the pro- tumorigenic role of potential key signaling integrators. Mouse genetics will be used to determine if 4 and FAK cooperate in vivo to promote ErbB2-initiated mammary tumorigenesis.