Metastatic Non-small Cell Lung Cancer (NSCLC) is almost uniformly fatal, with a median survival of approximately 8-10 months using the most active combination of conventional chemotherapies. Recent treatment strategies for NSCLC have focused on the development of "targeted therapies" that aim to specifically disrupt critical oncogenic mechanisms. The phosphoinositide 3-kinase (PI3K)/Akt pathway has been highly implicated in a wide range of epithelial cancers including NSCLC. PI3K consists of a dimer between the regulatory subunit, p85, and the catalytic subunit, p110.PI3K is activated upon the binding of p85's SH2 domains to tyrosine-phosphorylated proteins, such as receptor tyrosine kinase (RTKs) or adaptors, at the plasma membrane. Recently, somatic EGFR mutations have been identified in a high proportion of patients with NSCLC who respond to clinically approved EGFR tyrosine kinase inhibitors (TKIs). We and others have shown that, in this unique subset of NSCLC cell lines, EGFR kinase activity is critical for activation of the PI3K/Akt signaling pathway. By immunoprecipitating p85 from EGFR TKIs- sensitive NSCLC cell lines, we discovered that ErbB-3, an EGFR family member, is the major phosphotyrosine protein bound to PI3K and couples EGFR to PI3K activity. Unfortunately, only a small minority of patients with NSCLC responds to EGFR TKIs and those that respond eventually develop resistance. The major goals of this grant are to identify the molecular mechanisms of activation of PI3K in NSCLCs with acquired and ctenovo resistance to EGFR TKIs. The experiments will utilize PI3K immunoprecipitations as was done to identify ErbB-3 as the mediator of PI3K/Akt signaling in EGFR TKIs- sensitive NSCLC cell lines. These studies will be performed on NSCLC cell lines, xenograft tumor models, and clinical specimens. Ultimately, once the pathways leading to PI3K activation are identified, they will be manipulated in vitro and in xenograft tumor models to assess the functional consequences on intracellular signaling cascades and cellular growth and survival. These studies should serve as the basis for future clinical development of therapeutic strategies and provide benefit to public health by improving treatments for lung cancer. These studies are perfectly suited to my career development as a physician scientist in the field of translational thoracic oncology.