PROJECT SUMMARY Despite half a century of social and economic investments, an unprecedented understanding of cancer genes and their pathways, and an arsenal of new molecular and immunological therapies, a diagnosis of malignancy still carries significant morbidity and mortality. Heralded as a breakthrough for cancer “cures”, the promise of personalized medicine, where every patient receives the right drug for the right type of cancer based on genetic makeup is yet to be realized. In fact, most molecularly-targeted drugs have been disappointing in the clinic, producing only short-lived responses, often at staggering costs and financial hardship for the patients, only to be supplanted by the emergence of drug-resistant and metastatic disease. We know that the extraordinary heterogeneity of human tumors, with hundreds of malignant clones in constant competition and cooperation with each other, is a major reason for treatment failure, but an in-depth understanding of this process has remained surprisingly elusive. Work carried out by our group over the past ten years has focused on mechanisms of tumor adaptation, or plasticity as novel, fundamental drivers of disease heterogeneity and worse patient outcome. We found that stress conditions typical of the tumor microenvironment, whether hypoxia, shortage of nutrients, protein toxicity or exposure to molecular therapy activate a coordinated set of cellular responses, a network that sustains cell proliferation, promotes survival, reconfigures metabolism, stimulates gene expression, and heightens cell motility and invasion. The net effect for the malignant population is not only improved fitness to cope with an unfavorable microenvironment, but also the acquisition of new traits characteristic of aggressive disease, including drug-resistance and metastatic competence. Unexpectedly, we identified reprogramming of mitochondrial functions as an obligatory hub for this process, enabling organelle-cytoskeleton dynamics, assembly of novel apoptosis-regulatory complex(es), and retrograde gene expression. Therefore, the hypothesis that tumor plasticity imparts cellular diversity in response to stress, propagates tumor heterogeneity and promotes the acquisition of aggressive disease traits through mitochondrial reprogramming can be formulated, and will constitute the focus of the present application. The proposed studies will dissect the cellular and molecular requirements of tumor plasticity as a novel hallmark of cancer, credential its relevance in xenograft and genetic mouse models of localized and metastatic disease, and exploit emerging vulnerabilities of these pathways for innovative cancer drug discovery. The results will establish tumor plasticity as a novel driver of disease progression, reach a comprehensive blueprint of the role of mitochondrial homeostasis in cancer, and validate new, actionable therapeutic targets for patients with late-stage disease.