TR3, also called NGFI-B or nur77, is an immediate-early response gene and an orphan member of the steroid/thyroid/retinoid receptor superfamily. TR3/nur77 exerts not only mitogenic but also apoptotic effects in cancer cells in response to different stimuli. How TR3/nur77 mediates the opposing activities, survival and death, is interesting and remains unknown. Recently, we observed that TR3/nur77 acts in the nucleus to induce cell proliferation by inhibiting expression of retinoic acid receptor beta (RARbeta), a potent growth inhibitor. In addition, we found that TR3/nur77, in response to apoptosis-inducing agents, translocates from the nucleus to the cytoplasm, where it resides in mitochondria to induce cytochrome c release and apoptosis. These results led us to propose that TR3/nur77 acts in the nucleus to induce cell proliferation by inhibiting RARbeta expression. whereas it acts in mitochondria to trigger cytochrome c release and apoptosis. In the proposed studies, we plan to: 1. Investigate whether cellular localization of TR3/nur77 defines its biological functions in various cell types. 2. Study the inhibitory effect of TR3/nur77 on RARbeta expression. 3. Determine whether and how mitochondrial localization of TR3/nur77 triggers cytochrome c release and apoptosis. 4. Explore the possibility that Bcl-2 acts to mediate TR3/nur77 mitochondrial targeting and its induction of cytochrome c release through its physical interaction with TR3/nur77. 5. Determine the effects of TR3/nur77 phosphorylation by Jun N-terminal kinase (JNK) on its nuclear export and mitochondrial targeting. 6. Evaluate mitogenic and apoptotic effects of TR3/nur77 on tumor growth in nude mice. Results from these studies will enhance our understanding of the mechanisms by which TR3/nur77 exerts mitogenic and apoptotic activities and its role in regulating tumor development, and may provide valuable information to determine the feasibility of using TR3/nur77 as a molecular target for developing new generation of anti-cancer drugs.