It is well recognized that deregulation of endogenous programmed cell death is fundamentally important in malignant transformation and tumor progression. The cell surface protein Fas can elicit apoptosis when engaged by its cognate ligand, FasL. First recognized as a critical regulator of immunohomeostasis and immunologic privilege, Fas has been recently implicated in the control of tumor development, growth, and metastasis. Our laboratory has documented that primary NSCLC in vivo express low to non-detectable levels of Fas while potently expressing FasL (Preliminary Data). As the induction of Fas expression can lead to apoptosis in FasL-expressing cells (1), the ability to upregulate Fas may be of major consequence for the development of novel therapeutic strategies aimed toward the treatment of malignancies (such as NSCLC) that are refractory to conventional treatments. In this regard, we have previously documented that both p53 and radiation can activate the transcription of Fas in human tumor cell lines in vitro (2). Wild-type p53 gene replacement therapy resulting in tumor apoptosis has been documented to have clinical utility for the treatment of patients with lung cancer (3). A combination treatment consisting of p53/radiation, shown to have super-additive therapeutic properties for tumor growth inhibition in pre-clinical nude mouse models, will now be tested in clinical trials. Such treatment may not be effective in all patients and individual patient responsiveness (such as the capacity to upregulate pro-apoptotic proteins including Fas) may influence clinical outcome. This proposal will address the overall hypothesis that wild-type p53/radiation-induced tumor apoptosis involves the Fas/FasL receptor ligand system. In the studies outlined below, we will systematically evaluate the molecular mechanisms underlying p53/radiation-induced Fas upregulation, the effects of bcl-2 overexpression on the upregulation of Fas after Ad-p53 nad Ad- p53/radiation, the role of Fas and FasL in Ad-p53/radiation- induced tumor cell killing in vitro and in vivo using a nude mouse model, the ability of Ad-Fas to induce tumor cell apoptosis in vitro and in vivo, and finally, Fas upregulation as a predictor of individual patient responses to radiation/p53 gene therapy for the treatment of lung cancer.