| Cancer occurs when normal regulation of the cell division cycle is disrupted by genetic or environmental factors. Hence, understanding the molecular mechanisms that regulate cell division is essential for the development of anti-cancer therapeutics. Borealin/Dasra B/CDCA8 (Cell Division Cycle Associated 8) is a member of the chromosomal passenger complex (CPC) also composed of Aurora B, INCENP and Survivin. The CPC exhibits a dynamic pattern of localization during mitosis and plays important roles in chromosome segregation, spindle assembly checkpoint (SAC) and cytokinesis.;We identified Borealin to be an E2F/Rb target; several genes repressed by Rb dependent pathways are highly expressed in various cancers. We observed that Borealin expression was elevated in lymphomas, brain, and colon cancers and down regulated in response to DNA damage in a p53 dependent manner. Borealin is regulated in a cell cycle dependent manner and we show that it is degraded by the 26S proteasome during the cell cycle. We identified a putative stability region between amino acids 141-168 that protects Borealin from proteolytic degradation. Further, over expression of CDH1, an activator of the Anaphase promoting complex (APC/C) caused a minimal decrease in Borealin levels. Therefore Borealin may be targeted by an E3 ligase other than APC/C.;Post-translational modifications of the passenger proteins are essential in the regulation of the CPC. We observed that Borealin is phosphorylated in vivo, in response to increased expression of constitutively active Cdk1/Cyclin B1. In addition, we observed a reduced level of slow migrating phosphorylated Borealin species upon treatment with a combination of purvalanol (Cdk1 inhibitor) and ZM447439 (Aurora B inhibitor). However, Borealin was inefficiently phosphorylated by CDK1 in vitro. We further investigated candidate phosphatases belonging to the Cdc14 family. Immunofluorescence analysis revealed that Cdc14B and Borealin co-localize to the nucleolus of interphase cells. Conversely, overexpression of Cdc14B only caused a subtle decrease in mobility shift characteristic of Borealin phosphorylation. Also, Borealin was still dephosphorylated in cells lacking Cdc14B. However, shRNA mediated depletion of Cdc14A did induce phosphorylation of endogenous Borealin. Hence, Borealin maybe dephosphorylated by Cdc14A, while CDK1 and Aurora B may exert a combinatorial effect to induce phosphorylation of mitotic Borealin. |
