Overexpression or hyper-activation of Cdc20 has been observed in a variety of human cancers. In adult T-cell leukemia (ATL) patients, the viral oncoprotein Tax promotes cellular transformation in part through activating APCCdc20, a critical M phase driver, to enhance proliferation, and as proposed in this study, to evade apoptosis. The goal of this proposal is to uncover a novel oncogenic role of the APCCdc20 ubiquitin E3 ligase complex in allowing cancer cells to evade chemotherapeutic agents-induced cellular apoptosis, thereby promoting leukemia-genesis and conferring multidrug resistance. My preliminary data showed that the protein levels of the pro-apoptotic protein, Bim, were low in M phase when Cdc20 is most active. Moreover, Bim protein abundance but not mRNA level was significantly elevated in multiple cell lines after depletion of endogenous Cdc20. Further studies revealed that Cdc20 directly binds to Bim and promotes its ubiquitination and subsequent degradation by the 26S proteasome. Strikingly, Bim expression is remarkably low in Human T-cell Lymphotropic Virus type I (HTLV-I)-infected T cell lines with elevated Cdc20 activity, and consistently, these cells are resistant to various anti-mitotic and DNA damage agents. In this proposal, I hypothesize that Cdc20- dependent repression of Bim is critical for ATL cells to evade apoptosis, especially under the challenge of therapeutic drugs. Therefore, inhibiting Cdc20 activity or introducing BH3-mimetic reagents could sensitize ATL cells to conventional chemotherapeutic drugs both in vitro and in vivo. In this proposal, I plan to: 1) Characterize APCCdc20 as an upstream E3 ligase that negatively regulates Bim stability; 2) Determine the physiological role of APCCdc20 activation in suppressing apoptosis; 3) Determine whether targeted inhibition of Cdc20 or introducing BH3-mimetic suppresses the development of ATL or other Cdc20-overexpressing cancer including head and neck cancer in vivo. The long-term goals of my career are to apply the insights of molecular and cellular biology to understand the physiological significance of deregulated proteolytic pathways that are important in the development of human malignancies, and to search for proper druggable targets. This K99/R00 award will provide protected time for me to pursue the novel hypotheses of this proposal, obtain new skill sets to execute experiments and solve problems. In addition, the award will allow me to focus my efforts on independently conducting basic and translational research, and to train future young scientists.