Breast cancer-specific gene 1 interacts with the mitotic checkpoint kinase BubR1

Abstract

The abnormal expression of breast cancer-specific gene 1 (BCSG1) in malignant mammary epithelial cells is highly associated with the development and progression of breast cancer. A series of in vitro and in vivo studies performed in our laboratory and others have demonstrated that BCSG1 expression significantly stimulates proliferation, invasion, and metastasis of breast cancer cells. However, currently little is known about how BCSG1 exerts its oncogenic functions. To elucidate the cellular mechanisms underlying the effects of BCSG1 in breast cancer cells, we used a yeast two-hybrid system to screen for proteins that could associate with BCSG1. Through this screening, we identified the mitotic checkpoint protein BubR1 as a novel binding partner of BCSG1. The specific association of BCSG1 with BubR1 in breast cancer cells was demonstrated by immunoprecipitation and GST pull-down assays. Intriguingly, experiments conducted in four different cell lines all showed that exogenous expressions of BCSG1 consistently reduce the cellular levels of the BubR1 protein without affecting BubR1 mRNA expression. The tendency of endogenous BCSG1 expression coinciding with lower BubR1 protein levels was also observed in seven out of eight breast cancer cell lines. We further showed that the reducing effect of BCSG1 on BubR1 protein expression could be prevented by treating BCSG1-transfected cells with MG-132, a selective 26S proteasome inhibitor, implying that the proteasome machinery may be involved in the BCSG1-induced reduction of the BubR1 protein. Accompanied with a reduction of BubR1 protein level, BCSG1 expression resulted in multinucleation of breast cancer cells upon treatment with spindle inhibitor nocodazole, indicating an impaired mitotic checkpoint. Taken together, our novel findings suggest that BCSG1 may accelerate the progression of breast cancer at least in part by compromising the mitotic checkpoint control through inactivation of BubR1.

Synuclein γ Compromises Spindle Assembly Checkpoint and Renders Resistance to Antimicrotubule Drugs

“…Studies have shown that disruption of BubR1 activity results in a loss of checkpoint control, chromosomal instability, and/or early onset of malignancy (41). Previous studies using yeast two-hybrid screening revealed an interaction of SNCG with BubR1 (30). Because SNCG compromises SAC function and renders a resistance to antimicrotubule drug, we reason that SNCG-BubR1 interaction may contribute to the inhibition of SAC as a result of nonmutational inactivation.…”

“…The C-terminal kinase domain is involved in the phosphorylation of critical components of a mitotic checkpoint, whereas N-terminal is involved in interaction with SAC component Cdc20. Previous studies conducted through yeast two-hybrid screening revealed an interaction of SNCG with BubR1 (30). To gain more insight into their interacting mechanism and to determine whether such interaction will cause the structural changes of BubR1, a docking analysis was carried out to understand the interaction between SNCG and the crystallographic structure of human BubR1.…”

“…Previous studies conducted through yeast two-hybrid screening revealed an interaction of SNCG with the mitotic checkpoint kinase BubR1 (30). BubR1 was originally characterized as a kinase that controls the activation of the anaphase-promoting complex (APC) by binding and inhibiting Cdc20 (33), the major APC regulatory protein.…”

“…SNCG is a new unfavorable prognostic marker for breast cancer progression and a potential target for breast cancer treatment. At the cellular level, SNCG increases metastasis (26) and hormonedependent tumor growth (27)(28)(29), and promotes genetic instability (30,31). Investigations aimed to elucidate the molecular mechanisms underlying the oncogenic functions of this protein revealed that SNCG functions like a tumor-specific chaperone and regulates many pathways in cancer progression, which include cell motility (26) and estrogen receptor (ER) signaling (27)(28)(29)32).…”

SAC‐ing mitotic errors: How the spindle assembly checkpoint (SAC) plays defense against chromosome mis‐segregation

“…Likewise, Bub3 haploinsufficiency promotes aneuploidy, and Bub3þ/À mice develop tumors rapidly in response to carcinogen [Babu et al, 2003]. In addition, SAC inactivation may play a role in the development of leukemia and breast cancer because the TAX oncoprotein encoded by the human T cell leukemia virus binds and inhibits Mad1p [Jin et al, 1998], and expression of the oncogenic Breast Cancer Specific Gene 1 (BCSG-1) reduces BubR1p abundance [Gupta et al, 2003].…”