The spindle assembly checkpoint (SAC) ensures accurate segregation of chromosomes by monitoring kinetochore attachment of spindles during mitosis. Proper progression of mitosis depends on orderly ubiquitination and subsequent degradation of various mitotic inhibitors. At the molecular level, upon removal of SAC, Cdc20 activates E3 ubiquitin ligase anaphasepromoting complex/cyclosome that, along with E2 ubiquitinconjugating enzyme UbcH10, executes this function. Both Cdc20 and UbcH10 are overexpressed in many cancer types and are associated with defective SAC function leading to chromosomal instability. The precise mechanism of correlated overexpression of these two proteins remains elusive. We show that Cdc20 transcriptionally up-regulates UbcH10 expression. The WD40 domain of Cdc20 is required for this activity. Physical interaction between Cdc20 and anaphase-promoting complex/ cyclosome-CBP/p300 complex and its subsequent recruitment to the UBCH10 promoter are involved in this transactivation process. This transcriptional regulatory function of Cdc20 was observed to be cell cycle-specific. We hypothesize that this coregulated overexpression of both proteins contributes to chromosomal instability.

Aberrant restoration of AR activity is linked with prostate tumor growth, therapeutic failures and development of castrate-resistant prostate cancer. Understanding the processes leading to AR-reactivation should provide the foundation for novel avenues of drug discovery. A differential gene expression study was conducted using biopsies from CaP and BPH patients to identify the components putatively responsible for reinstating AR activity in CaP. From the set of genes upregulated in CaP, FKBP52, an AR co-chaperone, was selected for further analysis. Expression of FKBP52 was positively correlated with that of c-Myc. The functional cross-talk between c-Myc and FKBP52 was established using c-Myc specific-siRNA to LNCaP cells that resulted in reduction of FKBP52. A non-canonical E-box sequence housing a putative c-Myc binding site was detected on the FKBP4 promoter using in silico search. LNCaP cells transfected with the FKBP52 promoter cloned in pGL3 basic showed increased luciferase activity which declined considerably when the promoter-construct was co-transfected with c-Myc specific-siRNA. ChIP-PCR confirmed the binding of c-Myc with the conserved E-box located in the FKBP52 promoter. c-Myc downregulation concomitantly affected expression of FGF8. Since expression of FGF8 is controlled by AR, our study unveiled a novel functional axis between c-Myc, AR and FGF8 operating through FKBP52.

Progression through mitosis requires precisely timed ubiquitin-dependent degradation of specific substrates. UbcH10, an ubiquitin-conjugating enzyme, plays a crucial role with anaphase promoting complex/ cyclosome (APC/C) in progression of and exit from mitotis. Similarly, spindle assembly checkpoint (SAC) protein Cdc20 plays an important role in transition from metaphase to anaphase by activating APC/C. Cdc20 itself gets ubiquitinated by UbcH10 and thus activated to bind with APC/C. Any defect in chromosome alignment activates SAC by blocking Cdc20 and thus anaphase transition is arrested. Earlier our lab reported that Cdc20 overexpression led to defective SAC resulting in aneuploidy. This prompted us to examine the regulation of UbcH10 expression in Cdc20 upregulated condition. Initially, we observed a correlation in the expression of both Cdc20 and UbcH10 in different primary tumors and cancer cell lines. Next, a dose-dependent increase in endogenous UbcH10 levels was found upon ectopic expression of Cdc20. Alternatively, CDC20 knockdown by siRNA led to downregulation of endogenous UbcH10. Activation of UBCH10 promoter-luciferase activity by Cdc20 indicated the regulation at transcriptional level. ChIP assay showed the presence of Cdc20 on UBCH10 promoter leading to chromatin remodelling by interacting with CBP/p300. APC/C-CBP/p300 complex is reported to be involved in transcription regulation. Here, we established the presence of APC/C on the UBCH10 promoter using ChIP assay. Further from ChIP and Co-IP assay we established that Cdc20 regulates APC/C and CBP/p300 association and thus modulates APC/C-CBP/p300 mediated transcriptional regulation of UBCH10. Also, we found cell cycle specific regulation of expression of UbcH10 by Cdc20. Finally, Cyclin B1 degradation assay showed physiological relevance of Cdc20 mediated UbcH10 regulation on mitotic progression. In summary, our data showed a novel role of Cdc20 as a transcription regulator and it regulates UbcH10 expression in a cell cycle specific manner. The co-regulated expression of both these proteins maintains proper mitotic progression which otherwise may results in aneuploidy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3075. doi:10.1158/1538-7445.AM2011-3075