Dual roles of human BubR1, a mitotic checkpoint kinase, in the monitoring of chromosomal instability

p53: guardian of ploidy

Aneuploidy, often preceded by tetraploidy, is one of the hallmarks of solid tumors. Indeed, both aneuploidy and tetraploidy are oncogenic occurrences that are sufficient to drive neoplastic transformation and cancer progression. True to form, the tumor suppressor p53 obstructs propagation of these dangerous chromosomal events by either instigating irreversible cell cycle arrest or apoptosis. The tumor suppressor Lats2, along with other tumor inhibitory proteins such as BRCA1/2 and BubR1, are central to p53-dependent elimination of tetraploid cells. Not surprisingly, these proteins are frequently inactivated or downregulated in tumors, synergizing with p53 inactivation to establish an atmosphere of "tolerance" for a non-diploid state.

Mitosis in vertebrates: the G2/M and M/A transitions and their associated checkpoints

In this review, I stress the importance of direct data and accurate terminology when formulating and communicating conclusions on how the G2/M and metaphase/anaphase transitions are regulated. I argue that entry into mitosis (i.e., the G2/M transition) is guarded by several checkpoint control pathways that lose their ability to delay or stop further cell cycle progression once the cell becomes committed to divide, which in vertebrates occurs in the late stages of chromosome condensation. After this commitment, progress through mitosis is then mediated by a single Mad/Bub-based checkpoint that delays chromatid separation, and exit from mitosis (i.e., completion of the cell cycle) in the presence of unattached kinetochores. When cells cannot satisfy the mitotic checkpoint, e.g., when in concentrations of spindle poisons that prohibit the stable attachment of all kinetochores, they are delayed in mitosis for many hours. In normal cells, the duration of this delay depends on the organism and ranges from ∼4 h in rodents to ∼22 h in humans. Recent live cell studies reveal that under this condition, many cancer cells (including HeLa and U2OS) die in mitosis by apoptosis within ∼24 h, which implies that biochemical studies on cancer cell populations harvested in mitosis after a prolonged mitotic arrest are contaminated with dead or dying cells.

Caspase activity is not required for the mitotic checkpoint or mitotic slippage in human cells

Biochemical studies suggest that caspase activity is required for a functional mitotic checkpoint (MC) and mitotic slippage. To test this directly, we followed nontransformed human telomerase immortalized human retinal pigment epithelia (RPE-1) cells through mitosis after inhibiting or depleting selected caspases. We found that inhibiting caspases individually, in combination, or in toto did not affect the duration or fidelity of mitosis in otherwise untreated cells. When satisfaction of the MC was prevented with 500 nM nocodazole or 2.5 μM dimethylenastron (an Eg5 inhibitor), 92-100% of RPE-1 cells slipped from mitosis in the presence of pan-caspase inhibitors or after simultaneously depleting caspase-3 and -9, and they did so with the same kinetics (~21-22 h) as after treatment with nocodazole or Eg5 inhibitors alone. Surprisingly, inhibiting or depleting caspase-9 alone doubled the number of nocodazole-treated, but not Eg5-inhibited, cells that died in mitosis. In addition, inhibiting or depleting caspase-9 and -3 together accelerated the rate of slippage ~40% (to ~13-15 h). Finally, nocodazole-treated cells that recently slipped through mitosis in the presence or absence of pan-caspase inhibitors contained numerous BubR1 foci in their nuclei. From these data, we conclude that caspase activity is not required for a functional MC or for mitotic slippage.

Caspase-3-dependent mitotic checkpoint inactivation by the small-molecule inducers of mitotic slippage SU6656 and geraldol

Microtubule-targeting cancer drugs such as paclitaxel block cell-cycle progression at mitosis by prolonged activation of the mitotic checkpoint. Cells can spontaneously escape mitotic arrest and enter interphase without chromosome segregation by a process termed mitotic slippage that involves the degradation of cyclin B1 without mitotic checkpoint inactivation. Inducing mitotic slippage with chemicals causes cells to die after multiple rounds of DNA replication without cell division, which may enhance the antitumor activity of microtubule-targeting drugs. Here, we explore pathways leading to mitotic slippage by using SU6656 and geraldol, two recently identified chemical inducers of mitotic slippage. Mitotic slippage induced by SU6656 or geraldol was blocked by the proteasome inhibitor MG-132 and involved proteasome-dependent degradation of cyclin B1 and the mitotic checkpoint proteins budding uninhibited by benzimidazole related 1 (BubR1) and cell division cycle 20 (Cdc20) in T98G cells. Mitotic slippage and the degradation of BubR1 and Cdc20 were also inhibited by the caspase-3 and -7 inhibitor DEVD-CHO. MCF-7 cells lacking caspase-3 expression could not degrade BubR1 or undergo mitotic slippage in response to SU6656 or geraldol. Introduction of caspase-3 completely restored the ability of MCF-7 cells to degrade BubR1 and undergo mitotic slippage. However, lack of expression of caspase-3 did not affect cell death after exposure to paclitaxel, with or without mitotic slippage induction. The requirement for caspase-3 for chemically induced mitotic slippage reveals a new mechanism for mitotic exit and a link between mitosis and apoptosis that has implications for the outcome of cancer chemotherapy.

Hydrophobic bile acid-induced micronuclei formation, mitotic perturbations, and decreases in spindle checkpoint proteins: relevance to genomic instability in colon carcinogenesis

We show, for the first time, that hydrophobic bile acids cause aberrations of the mitotic machinery of colon cells that can give rise to aneuploidy, the chromosomal perturbations common in colon tumors. First, we show that DOC induces a statistically significant fourfold increase in the number of micronuclei in NCM-460 cells (a noncancerous colon cell line) and a threefold increase in the number of micronuclei in binucleated HT-29 colon cancer cells using the cytokinesis block micronucleus assay. Second, we observed mitotic aberrations after DOC treatment, including improper alignment of chromosomes at the metaphase plate, lagging chromosomes during anaphase, anaphase/telophase chromatin bridges, multipolar divisions, and formation of polynucleated cells. It was determined that there was a statistically significant threefold increase in the number of aberrant metaphases after short-term and long-term exposure of HT-29 and HCT-116 cells, respectively. Third, we showed with Western blots and immunohistochemistry that a likely basis for these mitosis-related perturbations included decreased expression of the spindle checkpoint proteins, Mad2, BubR1, and securin. Fourth, results of DOC treatment on nocodazole-challenged cells further indicated deficiencies in activation of the spindle assembly checkpoint. This study provides mechanisms by which hydrophobic bile acids can induce genomic instability in colon epithelial cells.

BUBR1 expression in oral squamous cell carcinoma and its relationship to tumor stage and survival

BACKGROUND

Defects in the mitotic spindle checkpoint have been proposed to contribute to the chromosomal instability observed in human cancers, including oral squamous cell carcinoma (OSCC). BUBR1 is a key component of the spindle checkpoint, whose role in oral carcinogenesis still needs to be clarified.

METHODS

We have analyzed the expression of BUBR1 in 49 cases of OSCC by immunohistochemistry and compared the findings with clinicopathologic parameters, proliferative activity, and DNA ploidy.

RESULTS

BUBR1 was overexpressed in 11 cases (22.4%). Tumors with overexpression of BUBR1 were associated with a less advanced pathologic stage (p = .05) and showed longer survival periods (p = .38) but shorter recurrence-free survival periods (p = .13) than those without it.

CONCLUSIONS

Our data imply the possibility that BUBR1 may be involved in the progression of OSCC, and suggest that BUBR1 may be a promising prognostic marker in patients with OSCC.

Replication-compromised cells require the mitotic checkpoint to prevent tetraploidization

Replication stress often induces chromosome instability. In this study, we explore which factors in replication-compromised cells promote abnormal chromosome ploidy. We expressed mutant forms of either polymerase α (Polα) or polymerase δ (Polδ) in normal human fibroblasts to compromise DNA replication. Cells expressing the mutant Polα-protein failed to sustain mitotic arrest and, when propagated progressively, down-regulated Mad2 and BubR1 and accumulated 4N-DNA from the 2N-DNA cells. Significantly, a population of these cells became tetraploids. The Polα mutant expressing cells also exhibited elevated cellular senescence markers, suggesting as a mechanism to limit proliferation of the tetraploids. Expression of the Polδ mutant also caused cells to accumulate 4N-DNA. In contrast to the Polα mutant expressing cells, the Polδ mutant expressing cells expressed sufficient levels of Mad2, BubR1, and cyclin B1 to sustain mitotic arrest, and these cells had normal chromosome ploidy. Together, these results suggest that replication-compromised cells depend on the mitotic checkpoint to prevent mitotic slippage that could result in tetraploidization.

Cross-talk between BubR1 expression and the commitment to differentiate in adipose-derived mesenchymal stem cells

BubR1 mitotic checkpoint kinase monitors attachment of microtubules to kinetochores and links regulation of the chromosome-spindle attachment to mitotic checkpoint signaling. Defects in BubR1-mediated signaling severely perturb checkpoint control and are linked to diseases such as cancer. Studies using BubR1 mouse models suggest that BubR1 activities prevent premature aging and infertility. In this study, we show that BubR1 depletion in human adipose-derived mesenchymal stem cells (ASCs) precedes loss of the differentiation potential and induction of replicative senescence. These effects occur independently of p16(INK4A) expression and may involve DNA methylation. Our results reveal a new and unsuspected feature of BubR1 expression in regulation of adult stem cell differentiation.

Glyfoline induces mitotic catastrophe and apoptosis in cancer cells

Glyfoline exhibits cytotoxic activity in vitro and antitumor activity in mice bearing murine or human solid tumors, but the underlying mechanisms are unknown. In our study, we found that glyfoline inhibited cell growth and induced accumulation of mitotic cells in human cancer cell lines. Glyfoline induced the appearance of spindle abnormalities, chromosome mis-segregation, multipolar cell division and multiple nuclei, all of which are indicative of mitotic catastrophe. However, glyfoline did not bind to DNA and did not inhibit or stabilize tubulin polymerization, but slightly increased the resistance of mitotic spindles to nocodazole-induced disassembly. In addition, microtubule aster formation was significantly enhanced in the extract prepared from glyfoline-arrested mitotic cells compared to that from synchronized mitotic cells. When Eg5, a mitotic kinesin that plays an essential role in establishing mitotic spindle bipolarity, was inhibited using S-trityl-cysteine in glyfoline-treated cells, formation of spindle multipolarity, multipolar cell division, and multinuclei was significantly reduced. After glyfoline-mediated arrest of cells at mitosis, considerable poly(ADP-ribose) polymerase degradation was induced and the number of annexin V-positive cells significantly increased, indicating that glyfoline ultimately induces apoptosis. Small interfering RNA-mediated silencing of the spindle checkpoint proteins BUBR1 and MAD2 markedly reduced induction of mitotic cell accumulation, but did not affect glyfoline-induced mitotic catastrophe and apoptosis. Thus, glyfoline induces mitotic catastrophe probably by enhancing microtubule aster formation and subsequent apoptosis in cancer cells independently of spindle checkpoint function.

BUB3 that dissociates from BUB1 activates caspase-independent mitotic death (CIMD)

The cell death mechanism that prevents aneuploidy caused by a failure of the spindle checkpoint has recently emerged as an important regulatory paradigm. We previously identified a new type of mitotic cell death, termed caspase-independent mitotic death (CIMD), which is induced during early mitosis by partial BUB1 (a spindle checkpoint protein) depletion and defects in kinetochore-microtubule attachment. In this study, we have shown that survived cells that escape CIMD have abnormal nuclei, and we have determined the molecular mechanism by which BUB1 depletion activates CIMD. The BUB3 protein (a BUB1 interactor and a spindle checkpoint protein) interacts with p73 (a homolog of p53), specifically in cells wherein CIMD occurs. The BUB3 protein that is freed from BUB1 associates with p73 on which Y99 is phosphorylated by c-Abl tyrosine kinase, resulting in the activation of CIMD. These results strongly support the hypothesis that CIMD is the cell death mechanism protecting cells from aneuploidy by inducing the death of cells prone to substantial chromosome missegregation.

Expression of mitotic checkpoint proteins BUB1B and MAD2L1 in salivary duct carcinomas

OBJECTIVE

Defects in the mitotic checkpoint lead to aneuploidy and might facilitate tumorigenesis. However, the ploidy status in salivary duct carcinoma (SDC) has been reported to play limited role in prediction of prognosis. Thus, we need more reliable markers to reflect the rapid tumor progression in SDCs. We aimed here to investigate the expression of mitotic checkpoint proteins benzimidazole 1 homolog beta (BUB1B) and mitosis arrest-deficient 2 like 1 (MAD2L1) in SDCs and to determine their possible role as surrogate prognostic markers.

METHODS

We analyzed the clinical courses, pathologic findings and immunohistochemical profiles of mitotic checkpoint proteins (BUB1B and MAD2L1) in 27 pathologically confirmed SDCs. The expression status of BUB1B and MAD2L1 was compared with clinicopathologic factors and other molecular markers, such as TGF-beta, c-erb-B2, androgen receptor, vascular endothelial growth factor, and epidermal growth factor receptor, for prognostic significance.

RESULTS

High BUB1B expression was detected in 25.9% of subjects, and high MAD2L1 expression was in 55.6% of subjects. However, survival analysis revealed that mitotic checkpoint expression did not have prognostic significance in SDCs, nor did the other studied markers. Rather, the clinical variable of N classification at diagnosis (in N+ status, hazard ratio 5.19, 95% CI 1.26-21.32 for disease-free survival and hazard ratio 7.18, 95% CI 1.09-46.99 for overall survival) was strongly associated with survival and prognosis based on the Cox proportional hazard model.

CONCLUSIONS

Mitotic checkpoint proteins appeared to play a limited role in predicting prognosis in SDCs. Further study is required to elucidate the exact role of mitotic checkpoint proteins in SDCs.

Regulation of ploidy and senescence by the AMPK-related kinase NUAK1

Senescence is an irreversible cell-cycle arrest that is elicited by a wide range of factors, including replicative exhaustion. Emerging evidences suggest that cellular senescence contributes to ageing and acts as a tumour suppressor mechanism. To identify novel genes regulating senescence, we performed a loss-of-function screen on normal human diploid fibroblasts. We show that downregulation of the AMPK-related protein kinase 5 (ARK5 or NUAK1) results in extension of the cellular replicative lifespan. Interestingly, the levels of NUAK1 are upregulated during senescence whereas its ectopic expression triggers a premature senescence. Cells that constitutively express NUAK1 suffer gross aneuploidies and show diminished expression of the genomic stability regulator LATS1, whereas depletion of NUAK1 with shRNA exerts opposite effects. Interestingly, a dominant-negative form of LATS1 phenocopies NUAK1 effects. Moreover, we show that NUAK1 phosphorylates LATS1 at S464 and this has a role in controlling its stability. In summary, our work highlights a novel role for NUAK1 in the control of cellular senescence and cellular ploidy.

Evidence that mitotic exit is a better cancer therapeutic target than spindle assembly

Current antimitotics work by perturbing spindle assembly, which activates the spindle assembly checkpoint, causes mitotic arrest, and triggers apoptosis. Cancer cells can resist such killing by premature exit, before cells initiate apoptosis, due to a weak checkpoint or rapid slippage. We reasoned blocking mitotic exit downstream of the checkpoint might circumvent this resistance. Using single-cell approaches, we showed that blocking mitotic exit by Cdc20 knockdown slowed cyclin B1 proteolysis, thus allowed more time for death initiation. Killing by Cdc20 knockdown did not require checkpoint activity and can occur by intrinsic apoptosis or an alternative death pathway when Bcl2 was overexpressed. We conclude targeting Cdc20, or otherwise blocking mitotic exit, may be a better cancer therapeutic strategy than perturbing spindle assembly.

Polo-like kinase 1 inhibition suppresses hepatitis B virus X protein-induced transformation in an in vitro model of liver cancer progression

Chronic hepatitis B virus (HBV) infection is linked to development of hepatocellular carcinoma (HCC). The HBV X protein (pX) is implicated in HCC pathogenesis acting as a weak oncogene or a cofactor in hepatocarcinogenesis. pX induces DNA re-replication, DNA damage, and partial polyploidy in a poorly differentiated, immortalized hepatocyte cell line. In this study we employed sorted, pX-induced polyploid cells to investigate their growth and oncogenic transformation potential over the course of 70 cell doublings. Immediately after live cell-sorting, nearly 40% of pX-induced polyploid cells undergo apoptosis, whereas the surviving cells exhibit proliferation sensitive to p53. After 40 cell generations the pX-expressing polyploid cultures exhibit loss of p53 function and become growth factor- and anchorage-independent, indicative of oncogenic transformation. The pX-induced polyploid cultures in the course of 70 cell generations undergo progressively increasing DNA damage, propagate damaged DNA to daughter cells, and display increased expression of a cluster of proliferation genes shown to be elevated in human HCC, including HBV-HCC. One of these genes is the mitotic kinase Polo-like kinase 1 (Plk1). Oncogenic transformation is suppressed in the absence of pX expression, and significantly, by inhibition of Plk1. These results identify Plk1 as crucial in pX-mediated oncogenic transformation.

CONCLUSION

Partial polyploidy induced by pX is not immediately associated with oncogenic transformation. Continued DNA damage for 40 cell generations is reproducibly associated with loss of p53 function, enhanced expression of Plk1, and oncogenic transformation. Because Plk1 expression is also elevated in HBV-HCC tumors, this in vitro cellular model simulates liver cancer progression and pathogenesis in chronic HBV patients. Inhibition of Plk1 activity suppresses pX-mediated oncogenic transformation, identifying Plk1 as a promising therapeutic target for HBV-mediated HCC.

BubR1 as a prognostic marker for recurrence-free survival rates in epithelial ovarian cancers

BACKGROUND

Epithelial ovarian cancer is one of the most lethal malignancies, and has a high recurrence rate. Thus, prognostic markers for recurrence are crucial for the care of ovarian cancer. As ovarian cancers frequently exhibit chromosome instability, we aimed at assessing the prognostic significance of two key mitotic kinases, BubR1 and Aurora A.

METHODS

We analysed paraffin-embedded tissue sections from 160 ovarian cancer patients whose clinical outcomes had been tracked after first-line treatment.

RESULTS

The median recurrence-free survival in patients with a positive and negative expression of BubR1 was 27 and 83 months, respectively (P<0.001). A positive BubR1 expression was also associated with advanced stage, serous histology and high grade. In contrast, Aurora A immunostaining did not correlate with any of the clinical parameters analysed.

CONCLUSION

BubR1, but not Aurora A, is a prognostic marker for recurrence-free survival rates in epithelial ovarian cancers.

RNAi-mediated functional analysis of pathways influencing cancer cell drug resistance

Acquired drug resistance limits the efficacy of cytotoxics used in the management of haematological and solid tumours and is responsible for the declining clinical benefit following successive treatment regimens in metastatic cancers. Treatment failure has a major impact on quality of life and survival in advanced disease. Defining pathways of intrinsic and acquired drug resistance may provide new targets to prolong drug efficacy and time to disease progression. Predicting the intrinsic drug sensitivity of human tumours in advance of cytotoxic therapy is of paramount importance in order to limit unnecessary toxicity and optimise treatment outcome. RNA interference (RNAi) provides a powerful tool to annotate gene function and systematically define drug-resistance pathways. High-throughput screening RNAi technology has provided evidence for drug-specific resistance pathways as well as novel pathways implicated in multidrug sensitivity. The challenge is how to integrate these data with biological samples to define relevant drug-resistant pathways in vivo.

The consequences of tetraploidy and aneuploidy

Polyploidy, an increased number of chromosome sets, is a surprisingly common phenomenon in nature, particularly in plants and fungi. In humans, polyploidy often occurs in specific tissues as part of terminal differentiation. Changes in ploidy can also result from pathophysiological events that are caused by viral-induced cell fusion or erroneous cell division. Tetraploidization can initiate chromosomal instability (CIN), probably owing to supernumerary centrosomes and the doubled chromosome mass. CIN, in turn, might persist or soon give way to a stably propagating but aneuploid karyotype. Both CIN and stable aneuploidy are commonly observed in cancers. Recently, it has been proposed that an increased number of chromosome sets can promote cell transformation and give rise to an aneuploid tumor. Here, we review how tetraploidy can occur and describe the cellular responses to increased ploidy. Furthermore, we discuss how the specific physiological changes that are triggered by polyploidization might be used as novel targets for cancer therapy.

Heat shock protein inhibitors, 17-DMAG and KNK437, enhance arsenic trioxide-induced mitotic apoptosis

Arsenic trioxide (ATO) has recently emerged as a promising therapeutic agent in leukemia because of its ability to induce apoptosis. However, there is no sufficient evidence to support its therapeutic use for other types of cancers. In this study, we investigated if, and how, 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG), an antagonist of heat shock protein 90 (HSP90), and KNK437, a HSP synthesis inhibitor, potentiated the cytotoxic effect of ATO. Our results showed that cotreatment with ATO and either 17-DMAG or KNK437 significantly increased ATO-induced cell death and apoptosis. siRNA-mediated attenuation of the expression of the inducible isoform of HSP70 (HSP70i) or HSP90alpha/beta also enhanced ATO-induced apoptosis. In addition, cotreatment with ATO and 17-DMAG or KNK437 significantly increased ATO-induced mitotic arrest and ATO-induced BUBR1 phosphorylation and PDS1 accumulation. Cotreatment also significantly increased the percentage of mitotic cells with abnormal mitotic spindles and promoted metaphase arrest as compared to ATO treatment alone. These results indicated that 17-DMAG or KNK437 may enhance ATO cytotoxicity by potentiating mitotic arrest and mitotic apoptosis possibly through increased activation of the spindle checkpoint.

Cell-cycle control in the face of damage--a matter of life or death

Cells respond to DNA damage or defects in the mitotic spindle by activating checkpoints that arrest the cell cycle. Alternatively, damaged cells can undergo cell death by the process of apoptosis. The correct balance between these pathways is important for the maintenance of genomic integrity while preventing unnecessary cell death. Although the molecular mechanisms of the cell cycle and apoptosis have been elucidated, the links between them have not been clear. Recent work, however, indicates that common components directly link the regulation of apoptosis with cell-cycle checkpoints operating during interphase, whereas in mitosis, the control of apoptosis is directly coupled to the cell-cycle machinery. These findings shed new light on how the balance between cell-cycle progression and cell death is controlled.

Mitotic catastrophe is the predominant response to histone acetyltransferase depletion

Histone acetylation induces chromatin opening by perturbing higher-order chromatin compaction and folding, suggesting that histone acetylation and deacetylation dynamics are central to chromosome condensation or decondensation. The condensation of chromosomes during mitosis is an essential prerequisite for successful chromosome segregation. In this study, we depleted three representative histone acetyltransferases (HATs; p300, CBP, and P/CAF) using shRNAs to explore their role in regulating mitotic progression and chromosome segregation. We showed that HAT depletion severely interfered with the normal timing of mitotic progression, and it reduced condensin subunit levels. The predominant response to HAT depletion, in both human primary and cancer cells, was a mitotic catastrophe following aberrant mitotic arrest. Alternatively, adaptation to HAT depletion, particularly in cancer cells, led to multinucleation and aneuploidy. Interestingly, mitotic catastrophe induced by HAT depletion appeared to be coupled to the signaling process of H2AX phosphorylation and foci formation, independently of DNA double-strand breaks and DNA damage. Taken together, our results provide novel molecular evidence that HAT proteins maintain mitotic chromatin assembly and integrity as a cellular determinant of mitotic cell death.

Genomic and epigenetic instability in colorectal cancer pathogenesis

Colorectal cancer arises as a consequence of the accumulation of genetic alterations (gene mutations, gene amplification, and so on) and epigenetic alterations (aberrant DNA methylation, chromatin modifications, and so on) that transform colonic epithelial cells into colon adenocarcinoma cells. The loss of genomic stability and resulting gene alterations are key molecular pathogenic steps that occur early in tumorigenesis; they permit the acquisition of a sufficient number of alterations in tumor suppressor genes and oncogenes that transform cells and promote tumor progression. Two predominant forms of genomic instability that have been identified in colon cancer are microsatellite instability and chromosome instability. Substantial progress has been made to identify causes of chromosomal instability in colorectal cells and to determine the effects of the different forms of genomic instability on the biological and clinical behavior of colon tumors. In addition to genomic instability, epigenetic instability results in the aberrant methylation of tumor suppressor genes. Determining the causes and roles of genomic and epigenomic instability in colon tumor formation has the potential to yield more effective prevention strategies and therapeutics for patients with colorectal cancer.

Reduced level of the spindle checkpoint protein BUB1B is associated with aneuploidy in colorectal cancers

OBJECTIVES

The majority of solid human malignancies demonstrate DNA aneuploidy as a consequence of chromosomal instability. We wanted to investigate whether Aurora A, Aurora B, BUB1B and Mad2 were associated with the development of aneuploidy in colorectal adenocarcinomas as suggested by several in vitro studies, and if their protein levels were related to alterations at the corresponding chromosomal loci.

MATERIALS AND METHODS

Expression levels of these spindle proteins were investigated by immunohistochemistry using tissue micro-arrays in a series of DNA aneuploid and diploid colorectal adenocarcinomas previously examined for genomic aberrations by comparative genomic hybridization.

RESULTS

All proteins were overexpressed in malignant tissues compared to controls (P < 0.001 for all). BUB1B level was significantly reduced in aneuploid compared to diploid cancers (P = 0.001), whereas expression of the other proteins was not associated with DNA ploidy status. High levels of Aurora A (P = 0.049) and low levels of Aurora B (P = 0.031) were associated with poor prognosis, but no associations were revealed between protein expression and genomic aberration.

CONCLUSIONS

A significant reduction of BUB1B level was detected in aneuploid compared to diploid colorectal cancers, consistent with earlier studies showing that loss of spindle checkpoint function may be involved in development of DNA aneuploidy. Our data also show that spindle proteins are overexpressed in colorectal cancers, and that expression of the Aurora kinases is associated with prognosis in colorectal cancer.

Cancer cells display profound intra- and interline variation following prolonged exposure to antimitotic drugs

Drugs targeting the mitotic spindle are used extensively during chemotherapy, but surprisingly, little is known about how they kill tumor cells. This is largely because many of the population-based approaches are indirect and lead to vague and confusing interpretations. Here, we use a high-throughput automated time-lapse light microscopy approach to systematically analyze over 10,000 single cells from 15 cell lines in response to three different classes of antimitotic drug. We show that the variation in cell behavior is far greater than previously recognized, with cells within any given line exhibiting multiple fates. We present data supporting a model wherein cell fate is dictated by two competing networks, one involving caspase activation, the other protecting cyclin B1 from degradation.

Validating the mitotic kinesin Eg5 as a therapeutic target in pancreatic cancer cells and tumor xenografts using a specific inhibitor

Pancreatic cancer is a devastating disease with a high mortality rate. Treatment of this malignancy remains a big challenge in oncology, and none of the currently available chemotherapeutic agents has a remarkable impact on improving patient survival. Consequently, it is important to explore new targets and find effective drugs for the management of this disease. Here we report that inhibition of the mitotic kinesin Eg5 by a pharmacological compound effectively prevents the proliferation of pancreatic cancer cells by halting mitotic progression, resulting in robust apoptosis. The mitotic arrest induced by this agent is attributed to its interference with spindle formation and activation of the spindle checkpoint. Impairment of the spindle checkpoint significantly compromises both mitotic arrest and apoptosis induced by the Eg5 inhibitor, suggesting the importance of the spindle checkpoint in monitoring Eg5 inhibitor sensitivity. Furthermore, treatment of nude mice bearing tumor xenografts of human pancreatic cancer results in pronounced tumor regression by triggering apoptosis. These data thus indicate Eg5 as a potential target for pancreatic cancer treatment.

Preventing aneuploidy: the contribution of mitotic checkpoint proteins

Aneuploidy, an abnormal number of chromosomes, is a trait shared by most solid tumors. Chromosomal instability (CIN) manifested as aneuploidy might promote tumorigenesis and cause increased resistance to anti-cancer therapies. The mitotic checkpoint or spindle assembly checkpoint is a major signaling pathway involved in the prevention of CIN. We review current knowledge on the contribution of misregulation of mitotic checkpoint proteins to tumor formation and will address to what extent this contribution is due to chromosome segregation errors directly. We propose that both checkpoint and non-checkpoint functions of these proteins contribute to the wide array of oncogenic phenotypes seen upon their misregulation.

Targeting synuclein-gamma to counteract drug resistance in cancer

BACKGROUND

Expression of synuclein-gamma (SNCG) protein is elevated in the advanced stages of many types of cancers, including ovarian, lung, liver, esophagus, colon, prostate and, in particular, breast. In breast carcinoma, SNCG is causatively linked to stimulated proliferation, metastasis and drug resistance.

OBJECTIVE

To establish SNCG as a potential therapeutic target and to discuss clinical use of SNCG inhibiting peptide.

METHODS

This review focuses on the plausible mechanisms of SNCG activity, SNCG mediated drug resistance and its inhibition.

RESULTS/CONCLUSION

Evidence based research shows that the aberrant expression of SNCG has a strong correlation with breast cancer progression and poor clinical outcome. A peptide based inhibitor counters activity of SNCG, which may be developed as an adjuvant therapy.

Aneuploidy: instigator and inhibitor of tumorigenesis

Aneuploidy, an aberrant chromosome number, has been recognized as a common characteristic of cancer cells for more than 100 years and has been suggested as a cause of tumorigenesis for nearly as long. However, this proposal had remained untested due to the difficulty of selectively generating aneuploidy without causing other damage. Using Cenp-E heterozygous animals, which develop whole chromosome aneuploidy in the absence of other defects, we have found that aneuploidy promotes tumorigenesis in some contexts and inhibits it in others. These findings confirm that aneuploidy can act oncogenically and reveal a previously unsuspected role for aneuploidy as a tumor suppressor.

Functional analysis of the transcription repressor PLU-1/JARID1B

The PLU-1/JARID1B nuclear protein, which is upregulated in breast cancers, belongs to the ARID family of DNA binding proteins and has strong transcriptional repression activity. To identify the target genes regulated by PLU-1/JARID1B, we overexpressed or silenced the human PLU-1/JARID1B gene in human mammary epithelial cells by using adenovirus and RNA interference systems, respectively, and then applied microarray analysis to identify candidate genes. A total of 100 genes showed inversely correlated differential expression in the two systems. Most of the candidate genes were downregulated by the overexpression of PLU-1/JARID1B, including the MT genes, the tumor suppressor gene BRCA1, and genes involved in the regulation of the M phase of the mitotic cell cycle. Chromatin immunoprecipitation assays confirmed that the metallothionein 1H (MT1H), -1F, and -1X genes are direct transcriptional targets of PLU-1/JARID1B in vivo. Furthermore, the level of trimethyl H3K4 of the MT1H promoter was increased following silencing of PLU-1/JARID1B. Both the PLU-1/JARID1B protein and the ARID domain selectively bound CG-rich DNA. The GCACA/C motif, which is abundant in metallothionein promoters, was identified as a consensus binding sequence of the PLU-1/JARID1B ARID domain. As expected from the microarray data, cells overexpressing PLU-1/JARID1B have an impaired G(2)/M checkpoint. Our study provides insight into the molecular function of the breast cancer-associated transcriptional repressor PLU-1/JARID1B.

p53 Activation in Response to Mitotic Spindle Damage Requires Signaling via BubR1-Mediated Phosphorylation

The mitotic spindle checkpoint plays a crucial role in regulating accurate chromosome segregation and preventing the adaptation of multiploid progeny cells. Recent reports have indicated that the induction of p53 by mitotic checkpoint activation is essential for protecting cells from abnormal chromosome ploidization caused by mitotic failure. However, although studies have shown that p53 deficiencies arrest mitosis, compromise apoptosis, and may cause profound aneuploidy, the molecular mechanisms leading to p53 induction following mitotic checkpoint activation remain unknown. Here, we show that the BubR1 mitotic checkpoint kinase interacts with p53 both in vitro and in vivo, with higher levels of interaction in mitotic cells. This interaction contributes to p53 phosphorylation. Silencing of BubR1 expression reduces the phosphorylation and stability of p53, whereas exogenous introduction of BubR1 proteins into BubR1-depleted cells recovers p53 stability. In addition, inhibition of BubR1 expression in the presence of a microtubule inhibitor accelerates chromosomal instability and polyploidy in p53-null cells. These results collectively suggest that p53 activation in response to mitotic spindle damage requires signaling via BubR1-mediated phosphorylation.

Gamma-synuclein and the progression of cancer

The synucleins are a small, soluble, highly conserved group of neuronal proteins that have been implicated in both neurodegenerative diseases and cancer. The synuclein family consists of alpha-, beta-, and gamma-synucleins (gamma-syn). They are a natively unfolded group of proteins that share sequence homologies and structural properties. So far, the biological functions of the synucleins are still unclear, but their involvement in neurodegenerative diseases and cancer may provide insights into the pathological processes that result from these two groups of debilitating diseases, and present the possibility to use them as potential targets for early diagnosis and treatment. Recently, elevated levels of gamma-syn proteins have been detected in various types of cancer, especially in advanced stages of the disease. Furthermore, studies to date indicate that overexpression of gamma-syn compromises normal mitotic checkpoint controls, resulting in multinucleation as well as faster cell growth. Gamma-syn has also been shown to promote invasion and metastasis in in vitro assays as well as in animal models. Overexpression of gamma-syn also interferes with drug-induced apoptotic responses. These observations raise questions about the involvement of gamma-syn in the process of tumorigenesis and metastasis, and efforts have already been made to use gamma-syn as a marker for assessing breast cancer progression. This review will discuss the involvement of gamma-syn in cancer progression, metastasis and its potential as a marker.

Regulators of mitotic arrest and ceramide metabolism are determinants of sensitivity to paclitaxel and other chemotherapeutic drugs

Cytotoxic drug resistance is a major cause of cancer treatment failure. We report an RNA interference screen to identify genes influencing sensitivity of different cancer cell types to chemotherapeutic agents. A set of genes whose targeting leads to resistance to paclitaxel is identified, many of which are involved in the spindle assembly checkpoint. Silencing these genes attenuates paclitaxel-induced mitotic arrest and induces polyploidy in the absence of drug. We also identify a ceramide transport protein, COL4A3BP or CERT, whose downregulation sensitizes cancer cells to multiple cytotoxic agents, potentiating endoplasmic reticulum stress. COL4A3BP expression is increased in drug-resistant cell lines and in residual tumor following paclitaxel treatment of ovarian cancer, suggesting that it could be a target for chemotherapy-resistant cancers.

Loss of APC induces polyploidy as a result of a combination of defects in mitosis and apoptosis

Mutations in the adenomatous polyposis coli (APC) tumor suppressor gene initiate a majority of colorectal cancers. Acquisition of chromosomal instability is an early event in these tumors. We provide evidence that the loss of APC leads to a partial loss of interkinetochore tension at metaphase and alters mitotic progression. Furthermore, we show that inhibition of APC in U2OS cells compromises the mitotic spindle checkpoint. This is accompanied by a decrease in the association of the checkpoint proteins Bub1 and BubR1 with kinetochores. Additionally, APC depletion reduced apoptosis. As expected from this combination of defects, tetraploidy and polyploidy are consequences of APC inhibition in vitro and in vivo. The removal of APC produced the same defects in HCT116 cells that have constitutively active beta-catenin. These data show that the loss of APC immediately induces chromosomal instability as a result of a combination of mitotic and apoptotic defects. We suggest that these defects amplify each other to increase the incidence of tetra- and polyploidy in early stages of tumorigenesis.

Oncogenic regulators and substrates of the anaphase promoting complex/cyclosome are frequently overexpressed in malignant tumors

The fidelity of cell division is dependent on the accumulation and ordered destruction of critical protein regulators. By triggering the appropriately timed, ubiquitin-dependent proteolysis of the mitotic regulatory proteins securin, cyclin B, aurora A kinase, and polo-like kinase 1, the anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase plays an essential role in maintaining genomic stability. Misexpression of these APC/C substrates, individually, has been implicated in genomic instability and cancer. However, no comprehensive survey of the extent of their misregulation in tumors has been performed. Here, we analyzed more than 1600 benign and malignant tumors by immunohistochemical staining of tissue microarrays and found frequent overexpression of securin, polo-like kinase 1, aurora A, and Skp2 in malignant tumors. Positive and negative APC/C regulators, Cdh1 and Emi1, respectively, were also more strongly expressed in malignant versus benign tumors. Clustering and statistical analysis supports the finding that malignant tumors generally show broad misregulation of mitotic APC/C substrates not seen in benign tumors, suggesting that a "mitotic profile" in tumors may result from misregulation of the APC/C destruction pathway. This profile of misregulated mitotic APC/C substrates and regulators in malignant tumors suggests that analysis of this pathway may be diagnostically useful and represent a potentially important therapeutic target.

p53-independent abrogation of a postmitotic checkpoint contributes to human papillomavirus E6-induced polyploidy

Polyploidy is often an early event during cervical carcinogenesis, and it predisposes cells to aneuploidy, which is thought to play a causal role in tumorigenesis. Cervical and anogenital cancers are induced by the high-risk types of human papillomavirus (HPV). The HPV E6 oncoprotein induces polyploidy in human keratinocytes, yet the mechanism is not known. It was believed that E6 induces polyploidy by abrogating the spindle checkpoint after mitotic stress. We have tested this hypothesis using human keratinocytes in which E6 expression induces a significant amount of polyploidy. We found that E6 expression does not affect the spindle checkpoint. Instead, we provide direct evidence that E6 is capable of abrogating the subsequent G(1) arrest after adaptation of the mitotic stress. E6 targets p53 for degradation, and previous studies have shown an important role for p53 in modulation of the G(1) arrest after mitotic stress. Importantly, we have discovered that E6 mutants defective in p53 degradation also induce polyploidy, although with lower efficiency. These results suggest that E6 is able to induce polyploidy via both p53-dependent and p53-independent mechanisms. Therefore, our studies highlight a novel function of HPV E6 that may contribute to HPV-induced carcinogenesis and improve our understanding of the onset of the disease.

Overexpression of BUBR1 is associated with chromosomal instability in bladder cancer

BUBR1, a mitotic checkpoint protein, is a key component of the mitotic spindle checkpoint machinery. Defective BUBR1 has been proposed to contribute to chromosomal instability (CIN). To elucidate the relationship of BUBR1 expression with CIN, expression of BUBR1, numbers of centrosomes, numerical aberrations of chromosomes, and DNA ploidy were examined, and BUBR1 expression status was compared with clinicopathological parameters in 104 human urothelial bladder carcinomas. Expression of BUBR1 and numbers of centrosomes were assessed by immunohistochemistry. Numerical aberrations of chromosomes 7, 9, and 17 were evaluated by fluorescence in situ hybridization. Cancers with a large intercellular variation in centromere copy number were designated as CIN cancers. Tumors with BUBR1 overexpression were associated with CIN, DNA aneuploidy, and centrosome amplification. Array CGH revealed that BUB1B amplification and loss rarely occurred, indicating that the overexpression of BUBR1 in these bladder cancers was independent of BUB1B copy number. Overexpression of BUBR1 significantly correlated with higher histological grade, advanced pathological stage, and high cell proliferation. Overexpression of BUBR1 predicted tumor recurrence and disease progression. These data suggest that overexpression of BUBR1 is potentially a new tumor marker for estimating biological characteristics of bladder cancer.

Chk1 is required for spindle checkpoint function

The spindle checkpoint delays anaphase onset in cells with mitotic spindle defects. Here, we show that Chk1, a component of the DNA damage and replication checkpoints, protects vertebrate cells against spontaneous chromosome missegregation and is required to sustain anaphase delay when spindle function is disrupted by taxol, but not when microtubules are completely depolymerized by nocodazole. Spindle checkpoint failure in Chk1-deficient cells correlates with decreased Aurora-B kinase activity and impaired phosphorylation and kinetochore localization of BubR1. Furthermore, Chk1 phosphorylates Aurora-B and enhances its catalytic activity in vitro. We propose that Chk1 augments spindle checkpoint signaling and is required for optimal regulation of Aurora-B and BubR1 when kinetochores produce a weakened signal. In addition, Chk1-deficient cells exhibit increased resistance to taxol. These results suggest a mechanism through which Chk1 could protect against tumorigenesis through its role in spindle checkpoint signaling.

Spindle proteins Aurora A and BUB1B, but not Mad2, are aberrantly expressed in dysplastic mucosa of patients with longstanding ulcerative colitis

BACKGROUND

Long term ulcerative colitis (UC) increases the risk of colorectal cancer (CRC). DNA aneuploidy is a common feature of both dysplastic and non-dysplastic colonic epithelia from patients with longstanding UC, and is regarded as an early sign of possible malignant transformation. The spindle proteins Aurora A, BUB1B and Mad2 have been implicated as contributors to aneuploidy and carcinogenesis.

AIMS

To investigate the role of these spindle proteins in relation to DNA aneuploidy and during the progressive morphological changes in ulcerative colitis associated colorectal cancer (UCCRC).

METHODS

Tissue microarrays were made from 31 colectomy specimens from patients with longstanding UC. Expression of Aurora A, BUB1B and Mad2 was investigated by immunohistochemistry and their relation to ploidy status, mucosal morphology and Ki67 levels was explored.

RESULTS

Expression of Aurora A and BUB1B was significantly associated with the progressive morphological changes of UCCRC. In the progression from non-dysplastic to dysplastic mucosa, Aurora A expression decreased while BUB1B expression increased. There was an increasing incidence of aneuploidy with progression towards cancer; expression of all spindle proteins was associated with the level of Ki67 but not with aneuploidy.

CONCLUSION

Due to the significant differences in Aurora A and BUB1B expression in dysplastic compared non-dysplastic mucosa, these proteins may serve as putative biological markers for the progressive morphological changes in UC associated carcinogenesis. The close relationship to Ki67 levels reflect that spindle proteins are expressed in tissues with a high proliferative rate; a role for these proteins in the development of aneuploidy was not found.

Synuclein-gamma targeting peptide inhibitor that enhances sensitivity of breast cancer cells to antimicrotubule drugs

Synuclein-gamma (SNCG) plays oncogenic roles in breast carcinogenesis. Although the expression of SNCG is abnormally high in advanced and metastatic breast carcinomas, SNCG is not expressed in normal or benign breast tissues. SNCG is an intrinsically disordered protein known to interact with BubR1, a mitotic checkpoint kinase. The SNCG-BubR1 interaction inhibits mitotic checkpoint control upon spindle damage caused by anticancer drugs, such as nocodazole and taxol. Antimicrotubule drugs that cause mitotic arrest and subsequent apoptosis of cancer cells are frequently used to treat breast cancer patients with advanced or metastatic diseases. However, patient response rates to this class of chemotherapeutic agents vary significantly. In this study, we have designed a novel peptide (ANK) and shown its interaction with SNCG using fluorometry, surface plasmon resonance, and isothermal titration calorimetry. Binding of the ANK peptide did not induce folding of SNCG, suggesting that SNCG can function biologically in its intrinsically disordered state. Microinjection of the ANK peptide in breast cancer cell line overexpressing SNCG (MCF7-SNCG) exhibited a similar cell killing response by nocodazole as in the SNCG-negative MCF7 cells. Overexpression of enhanced green fluorescent protein-tagged ANK reduces SNCG-mediated resistance to paclitaxel treatment by approximately 3.5-fold. Our coimmunoprecipitation and colocalization results confirmed the intracellular association of the ANK peptide with SNCG. This is likely due to the disruption of the interaction of SNCG with BubR1 interaction. Our findings shed light on the molecular mechanism of the ANK peptide in releasing SNCG-mediated drug resistance.

Dido disruption leads to centrosome amplification and mitotic checkpoint defects compromising chromosome stability

Numerical and/or structural centrosome abnormalities have been correlated with most solid tumors and hematological malignancies. Tumorigenesis also is linked to defects in the mitotic or spindle assembly checkpoint, a key control mechanism that ensures accurate segregation of chromosomes during mitosis. We have reported that targeted disruption of the Dido gene causes a transplantable myelodysplastic/myeloproliferative disease in mice. Here, we report that Dido3, the largest splice variant of the Dido gene, is a centrosome-associated protein whose disruption leads to supernumerary centrosomes, failure to maintain cellular mitotic arrest, and early degradation of the mitotic checkpoint protein BubR1. These aberrations result in enhanced aneuploidy in the Dido mutant cells. Dido gene malfunction thus is reported to be part of an impaired signaling cascade that results in a defective mitotic checkpoint, leading to chromosome instability.

KEN-Box-dependent Degradation of the Bub1 Spindle Checkpoint Kinase by the Anaphase-promoting Complex/Cyclosome

The spindle checkpoint is a cell cycle surveillance mechanism that ensures the fidelity of chromosome segregation during mitosis and meiosis. Bub1 is a protein serine-threonine kinase that plays multiple roles in chromosome segregation and the spindle checkpoint. In response to misaligned chromosomes, Bub1 directly inhibits the ubiquitin ligase activity of the anaphase-promoting complex or cyclosome (APC/C) by phosphorylating its activator Cdc20. The protein level and the kinase activity of Bub1 are regulated during the cell cycle; they peak in mitosis and are low in G1/S phase. Here we show that Bub1 is degraded during mitotic exit and that degradation of Bub1 is mediated by APC/C in complex with its activator Cdh1 (APC/C(Cdh1)). Overexpression of Cdh1 reduces the protein levels of ectopically expressed Bub1, whereas depletion of Cdh1 by RNA interference increases the level of the endogenous Bub1 protein. Bub1 is ubiquitinated by immunopurified APC/C(Cdh1) in vitro. We further identify two KEN-box motifs on Bub1 that are required for its degradation in vivo and ubiquitination in vitro. A Bub1 mutant protein with both KEN-boxes mutated is stable in cells but fails to elicit a cell cycle phenotype, indicating that degradation of Bub1 by APC/C(Cdh1) is not required for mitotic exit. Nevertheless, our study clearly demonstrates that Bub1, an APC/C inhibitor, is also an APC/C substrate. The antagonistic relationship between Bub1 and APC/C may help to prevent the premature accumulation of Bub1 during G1.

Induction of apoptosis by monastrol, an inhibitor of the mitotic kinesin Eg5, is independent of the spindle checkpoint

Spindle poisons such as paclitaxel are widely used as cancer therapeutics. By interfering with microtubule dynamics, paclitaxel induces mitotic arrest and apoptosis. Targeting the kinesin Eg5, which is required for the formation of a bipolar spindle, is a promising therapeutic alternative to drugs that interfere with microtubule dynamics. Recent data suggest that the spindle checkpoint can determine the response of tumor cells to microtubule poisons. The relationship between checkpoint function and Eg5 inhibition, however, has not yet been fully investigated. Here, we used time-lapse video microscopy and biochemical analysis to study the effect of spindle checkpoint abrogation on the response of HeLa cells to monastrol, a selective Eg5 inhibitor. In HeLa cells, monastrol activated the spindle checkpoint, leading to mitotic arrest and apoptosis. Small interfering RNA-mediated depletion of the spindle checkpoint proteins BubR1 or Mad2 significantly shortened drug-induced arrest, causing premature mitotic exit without cell division. Time-lapse microscopy as well as analysis of caspase activation shows that these checkpoint-deficient cells initiate apoptosis after mitotic exit in response to monastrol. Checkpoint-deficient cells treated with paclitaxel, on the other hand, yielded a higher frequency of cells with >4N DNA content and a decreased incidence of apoptotic events, particularly in Mad2-depleted cells. These results indicate that the immediate fate of postmitotic cells is influenced by both the nature of the checkpoint defect and the type of drug used. Furthermore, these results show that inactivation of the kinesin Eg5 can induce apoptosis in tumor cells in the absence of critical spindle checkpoint components.

Bub1 up-regulation and hyperphosphorylation promote malignant transformation in SV40 tag-induced transgenic mouse models

Rodents do not naturally develop prostate cancer. Currently, most widely used genetically engineered mouse prostate cancer models use SV40 T/tag oncogene. To understand the mechanism underlying prostate cancer development in transgenic and knock-in SV40 Tag mouse models, we did cDNA microarray analyses, comparing gene expression profiles of prostate cancer tissues from early-, late-, and advance-stage androgen-independent prostate cancers. Of the 67 genes that were up-regulated by > or = 10-fold, 40 are known to be required for chromosome stability. In particular, the spindle checkpoint component Bub1 was persistently up-regulated from early to advanced androgen-independent prostate cancer lesions. Significantly, Bub1, which is required for accurate chromosome segregation during mitosis, has recently been reported to bind SV40 Tag. Consistent with a spindle checkpoint defect, flow cytometry experiments indicate that advanced androgen-independent prostate cancer tumors exhibit aneuploidy, along with up-regulation of levels of both Bub1 mRNA and Bub1 protein or hyperphosphorylation. Importantly, up-regulation and hyperphosphorylation of Bub1 were also observed in established human prostate cancer cell lines and in clinical studies. Furthermore, analysis of human prostate cancer lines showed impaired spindle checkpoint function and endoreduplication following exposure to spindle toxins. Small interfering RNA-mediated repression of Bub1 in the human prostate cancer line PC-3 restrained cell proliferation, an effect mimicked by inhibition of mitogen-activated protein kinase, an upstream activator of Bub1. Thus, by perturbing Bub1 function, our observations suggest a new mechanism whereby the SV40 Tag oncoprotein promotes chromosomal instability and aneuploidy in transgenic mouse prostate cancer models. Whereas the exact details of this mechanism remain unclear, our novel findings raise the possibility of exploiting Bub1 as a new therapeutic target in the treatment of prostate cancer, the most common cancer in adult men in North America.