Differential phosphorylation of Cdc25C phosphatase in mitosis

Nuclear-cytoplasmic compartmentalization of cyclin B1-Cdk1 promotes robust timing of mitotic events

The cyclin-dependent kinase (Cdk1) oscillator is widely characterized in homogenized cytosolic extracts, leaving unclear the impact of nucleocytoplasmic compartmentalization. Here, by developing a Förster resonance energy transfer (FRET) biosensor, we track Cdk1 spatiotemporal dynamics in reconstituted cells with or without side by side and find compartmentalization significantly modulates clock properties previously found in bulk studies. Although nucleus-absent cells display highly tunable frequency, the nucleus-present cells maintain constant frequency against cyclin B1 variations. Despite high expression variability, cyclin degraded within the same duration, enabling a robust mitotic phase. Moreover, Cdk1 and cyclin B1 cycle rigorously out-of-phase, ensuring wide phase-plane orbits, essential for oscillation robustness. Although Cdk1 in homogeneous extracts is well known for delayed switch-like activation, we find active cyclin B1-Cdk1 accumulates in nuclei, without delay, until the nuclear envelope breakdown (NEB) when another abrupt activation triggers anaphase. Cdk1 biphasic activation and spatial compartmentalization may together coordinate the accurate ordering of different downstream events.

Quantifying single-cell ERK dynamics in colorectal cancer organoids reveals EGFR as an amplifier of oncogenic MAPK pathway signalling

Direct targeting of the downstream mitogen-activated protein kinase (MAPK) pathway to suppress extracellular-regulated kinase (ERK) activation in KRAS and BRAF mutant colorectal cancer (CRC) has proven clinically unsuccessful, but promising results have been obtained with combination therapies including epidermal growth factor receptor (EGFR) inhibition. To elucidate the interplay between EGF signalling and ERK activation in tumours, we used patient-derived organoids (PDOs) from KRAS and BRAF mutant CRCs. PDOs resemble in vivo tumours, model treatment response and are compatible with live-cell microscopy. We established real-time, quantitative drug response assessment in PDOs with single-cell resolution, using our improved fluorescence resonance energy transfer (FRET)-based ERK biosensor EKAREN5. We show that oncogene-driven signalling is strikingly limited without EGFR activity and insufficient to sustain full proliferative potential. In PDOs and in vivo, upstream EGFR activity rigorously amplifies signal transduction efficiency in KRAS or BRAF mutant MAPK pathways. Our data provide a mechanistic understanding of the effectivity of EGFR inhibitors within combination therapies against KRAS and BRAF mutant CRC.

Anticancer activity of a novel small molecule tubulin inhibitor STK899704

We have identified the small molecule STK899704 as a structurally novel tubulin inhibitor. STK899704 suppressed the proliferation of cancer cell lines from various origins with IC50 values ranging from 0.2 to 1.0 μM. STK899704 prevented the polymerization of purified tubulin in vitro and also depolymerized microtubule in cultured cells leading to mitotic arrest, associated with increased Cdc25C phosphorylation and the accumulation of both cyclin B1 and polo-like kinase 1 (Plk1), and apoptosis. Unlike many anticancer drugs such as Taxol and doxorubicin, STK899704 effectively displayed antiproliferative activity against multidrug-resistant cancer cell lines. The proposed binding mode of STK899704 is at the interface between αβ-tubulin heterodimer overlapping with the colchicine-binding site. Our in vivo carcinogenesis model further showed that STK 899704 is potent in both the prevention and regression of tumors, remarkably reducing the number and volume of skin tumor by STK899704 treatment. Moreover, it was significant to note that the efficacy of STK899704 was surprisingly comparable to 5-fluorouracil, a widely used anticancer therapeutic. Thus, our results demonstrate the potential of STK899704 to be developed as an anticancer chemotherapeutic and an alternative candidate for existing therapies.

Phosphatases and kinases regulating CDC25 activity in the cell cycle: clinical implications of CDC25 overexpression and potential treatment strategies

Alterations in the cell-cycle regulatory genes result in uncontrolled cell proliferation leading to several disease conditions. Cyclin-dependent kinases (CDK) and their regulatory subunit, cyclins, are essential proteins in cell-cycle progression. The activity of CDK is regulated by a series of phosphorylation and dephosphorylation at different amino acid residues. Cell Division Cycle-25 (CDC25) plays an important role in transitions between cell-cycle phases by dephosphorylating and activating CDKs. CDC25B and CDC25C play a major role in G2/M progression, whereas CDC25A assists in G1/S transition. Different isomers of CDC25 expressions are upregulated in various clinicopathological situations. Overexpression of CDC25A deregulates G1/S and G2/M events, including the G2 checkpoint. CDC25B has oncogenic properties. Binding to the 14-3-3 proteins regulates the activity and localization of CDC25B. CDC25C is predominantly a nuclear protein in mammalian cells. At the G2/M transition, mitotic activation of CDC25C protein occurs by its dissociation from 14-3-3 proteins along with its phosphorylation at multiple sites within its N-terminal domain. In this article, we critically reviewed the biology of the activation/deactivation of CDC25 by kinases/phosphatases to maintain the level of CDK-cyclin activities and thus the genomic stability, clinical implications due to dysregulation of CDC25, and potential role of CDC25 inhibitors in diseases.

Stereospecific phosphorylation by the central mitotic kinase Cdk1-cyclin B

The cis vs trans conformation, or shape, of phosphoserine-proline (pSer-Pro), a prevalent motif in cell cycle proteins, may play a significant role in regulating mitosis. We demonstrate that Cdk1-cyclin B, the central mitotic kinase, is specific for the trans conformation, not cis, of synthetic, locked Ser-Pro 11-residue peptide substrates, using LC-MSMS detection and sequencing of phosphorylated products. This substrate stereospecificity may contribute an additional level of mitotic regulation.

The Catalytic Subunit of DNA-Dependent Protein Kinase Coordinates with Polo-Like Kinase 1 to Facilitate Mitotic Entry

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is the key regulator of the non-homologous end joining pathway of DNA double-strand break repair. We have previously reported that DNA-PKcs is required for maintaining chromosomal stability and mitosis progression. Our further investigations reveal that deficiency in DNA-PKcs activity caused a delay in mitotic entry due to dysregulation of cyclin-dependent kinase 1 (Cdk1), the key driving force for cell cycle progression through G2/M transition. Timely activation of Cdk1 requires polo-like kinase 1 (Plk1), which affects modulators of Cdk1. We found that DNA-PKcs physically interacts with Plk1 and could facilitate Plk1 activation both in vitro and in vivo. Further, DNA-PKcs-deficient cells are highly sensitive to Plk1 inhibitor BI2536, suggesting that the coordination between DNA-PKcs and Plk1 is not only crucial to ensure normal cell cycle progression through G2/M phases but also required for cellular resistance to mitotic stress. On the basis of the current study, it is predictable that combined inhibition of DNA-PKcs and Plk1 can be employed in cancer therapy strategy for synthetic lethality.

Cell cycle-dependent Cdc25C phosphatase determines cell survival by regulating apoptosis signal-regulating kinase 1

Cdc25C (cell division cycle 25C) phosphatase triggers entry into mitosis in the cell cycle by dephosphorylating cyclin B-Cdk1. Cdc25C exhibits basal phosphatase activity during interphase and then becomes activated at the G2/M transition after hyperphosphorylation on multiple sites and dissociation from 14-3-3. Although the role of Cdc25C in mitosis has been extensively studied, its function in interphase remains elusive. Here, we show that during interphase Cdc25C suppresses apoptosis signal-regulating kinase 1 (ASK1), a member of mitogen-activated protein (MAP) kinase kinase kinase family that mediates apoptosis. Cdc25C phosphatase dephosphorylates phospho-Thr-838 in the activation loop of ASK1 in vitro and in interphase cells. In addition, knockdown of Cdc25C increases the activity of ASK1 and ASK1 downstream targets in interphase cells, and overexpression of Cdc25C inhibits ASK1-mediated apoptosis, suggesting that Cdc25C binds to and negatively regulates ASK1. Furthermore, we showed that ASK1 kinase activity correlated with Cdc25C activation during mitotic arrest and enhanced ASK1 activity in the presence of activated Cdc25C resulted from the weak association between ASK1 and Cdc25C. In cells synchronized in mitosis following nocodazole treatment, phosphorylation of Thr-838 in the activation loop of ASK1 increased. Compared with hypophosphorylated Cdc25C, which exhibited basal phosphatase activity in interphase, hyperphosphorylated Cdc25C exhibited enhanced phosphatase activity during mitotic arrest, but had significantly reduced affinity to ASK1, suggesting that enhanced ASK1 activity in mitosis was due to reduced binding of hyperphosphorylated Cdc25C to ASK1. These findings suggest that Cdc25C negatively regulates proapoptotic ASK1 in a cell cycle-dependent manner and may play a role in G2/M checkpoint-mediated apoptosis.

Diallyl disulfide selectively causes checkpoint kinase-1 mediated G2/M arrest in human MGC803 gastric cancer cell line

Previous studies have shown that diallyl disulfide (DADS), a naturally occurring anticancer agent in garlic, arrested human gastric cancer cells (MGC803) in the G2/M phase of the cell cycle. Due to the importance of cell cycle redistribution in DADS-mediated anticarcinogenic effects, we investigated the role of checkpoint kinases (Chk1 and Chk2) during DADS-induced cell cycle arrest. In the present study, the northern blot analysis showed that mRNA expression of for Chkl and Chk2 was unchanged. Notably, DADS induced the accumulation of phosphorylated Chk1, but not of Chk2, activated phospho-ATR (ATM-RAD3-related gene), and dowregulated CDC25C and cyclin B1 expression. Furthermore, CDC25C was immunoprecipitated by anti-Chk1 but not anti-Chk2. Results of the overexpression and knockdown studies, showed that Chk1 but not Chk2 regulated the DADS-induced G2/M arrest of MGC803 cells. The overexpression of Chk1 resulted in significantly increased DADS-induced G2/M arrest, increased DADS-induced Chk1 phosphorylation and inhibited CDC25C expression. Knockdown of Chk1 reduced DADS‑induced G2/M arrest and blocked the DADS-induced inhibition of CDC25C and cyclin B1 expression. These results suggested that Chk1 is important in DADS‑induced cell cycle G2/M arrest in the human MGC803 gastric cancer cell line. Furthermore, the DADS-induced G2/M checkpoint response is mediated by Chk1 signaling through ATR/Chk1/CDC25C/cyclin B1.

HS-173, a novel PI3K inhibitor, attenuates the activation of hepatic stellate cells in liver fibrosis

Hepatic stellate cells (HSCs) are the primary source of matrix components in liver disease such as fibrosis. Phosphatidylinositol 3-kinase (PI3K) signaling in HSCs has been shown to induce fibrogenesis. In this study, we evaluated the anti-fibrotic activity of a novel imidazopyridine analogue (HS-173) in human HSCs as well as mouse liver fibrosis. HS-173 strongly suppressed the growth and proliferation of HSCs and induced the arrest at the G2/M phase and apoptosis in HSCs. Furthermore, it reduced the expression of extracellular matrix components such as collagen type I, which was confirmed by an in vivo study. We also observed that HS-173 blocked the PI3K/Akt signaling pathway in vitro and in vivo. Taken together, HS-173 suppressed fibrotic responses such as cell proliferation and collagen synthesis by blocking PI3K/Akt signaling. Therefore, we suggest that this compound may be an effective therapeutic agent for ameliorating liver fibrosis through the inhibition of PI3K signaling.

2-methoxyestradiol induces mitotic arrest, apoptosis, and synergistic cytotoxicity with arsenic trioxide in human urothelial carcinoma cells

2-Methoxyestradiol (2-ME), an endogenous derivative of 17β-estradiol, has been reported to elicit antiproliferative responses in various tumors. In this study, we investigated the effects of 2-ME on cell viability, proliferation, cell cycle, and apoptosis in human urothelial carcinoma (UC) cell lines. We used two high-grade human bladder UC cell lines (NTUB1 and T24). After treatment with 2-ME, the cell viability and apoptosis were measured by MTT assay and flow cytometry (fluorescence-activated cell sorting), with annexin V-FITC staining and propidium iodide (PI) labeling. DNA fragmentation was analyzed by agarose gel electrophoresis. Flow cytometry with PI labeling was used for the cell cycle analyses. The protein levels of caspase activations, poly (ADP-ribose) polymerase (PARP) cleavage, phospho-histone H2A.X, phospho-Bad, and cell cycle regulatory molecules were measured by Western blot. The effects of the drug combinations were analyzed using the computer software, CalcuSyn. We demonstrated that 2-ME effectively induces dose-dependent cytotoxicity and apoptosis in human UC cells after 24 h exposure. DNA fragmentation, PARP cleavage, and caspase-3, 7, 8, 9 activations can be observed with 2-ME-induced apoptosis. The decreased phospho-Bad (Ser136 and Ser155) and mitotic arrest of the cell cycle in the process of apoptosis after 2-ME treatment was remarkable. In response to mitotic arrest, the mitotic forms of cdc25C, phospho-cdc2, cyclin B1, and phospho-histone H3 (Ser10) were activated. In combination with arsenic trioxide (As2O3), 2-ME elicited synergistic cytotoxicity (combination index <1) in UC cells. We concluded that 2-ME significantly induces apoptosis through decreased phospho-Bad and arrests bladder UC cells at the mitotic phase. The synergistic antitumor effect with As2O3 provides a novel implication in clinical treatment of UC.

Androgens upregulate Cdc25C protein by inhibiting its proteasomal and lysosomal degradation pathways

Cdc25C is a cell cycle protein of the dual specificity phosphatase family essential for activating the cdk1/Cyclin B1 complex in cells entering into mitosis. Since altered cell cycle is a hallmark of human cancers, we investigated androgen regulation of Cdc25C protein in human prostate cancer (PCa) cells, including androgen-sensitive (AS) LNCaP C-33 cells and androgen-independent (AI) LNCaP C-81 as well as PC-3 cells. In the regular culture condition containing fetal bovine serum (FBS), Cdc25C protein levels were similar in these PCa cells. In a steroid-reduced condition, Cdc25C protein was greatly decreased in AS C-33 cells but not AI C-81 or PC-3 cells. In androgen-treated C-33 cells, the Cdc25C protein level was greatly elevated, following a dose- and a time-dependent manner, correlating with increased cell proliferation. This androgen effect was blocked by Casodex, an androgen receptor blocker. Nevertheless, epidermal growth factor (EGF), a growth stimulator of PCa cells, could only increase Cdc25C protein level by about 1.5-fold. Altered expression of Cdc25C in C-33 cells and PC-3 cells by cDNA and/or shRNA transfection is associated with the corresponding changes of cell growth and Cyclin B1 protein level. Actinomycin D and cycloheximide could only partially block androgen-induced Cdc25C protein level. Treatments with both proteasomal and lysosomal inhibitors resulted in elevated Cdc25C protein levels. Immunoprecipitation revealed that androgens reduced the ubiquitination of Cdc25C proteins. These results show for the first time that Cdc25C protein plays a role in regulating PCa cell growth, and androgen treatments, but not EGF, greatly increase Cdc25C protein levels in AS PCa cells, which is in part by decreasing its degradation. These results can lead to advanced PCa therapy via up-regulating the degradation pathways of Cdc25C protein.

R. T, Soung NK, Jang JH, Kim YS, Lee KS, Kwon YT, Asami Y, Ahn JS, Erikson RL, Kim BY. STK295900, a dual inhibitor of topoisomerase 1 and 2, induces G(2) arrest in the absence of DNA damage

STK295900, a small synthetic molecule belonging to a class of symmetric bibenzimidazoles, exhibits antiproliferative activity against various human cancer cell lines from different origins. Examining the effect of STK295900 in HeLa cells indicates that it induces G(2) phase arrest without invoking DNA damage. Further analysis shows that STK295900 inhibits DNA relaxation that is mediated by topoisomerase 1 (Top 1) and topoisomerase 2 (Top 2) in vitro. In addition, STK295900 also exhibits protective effect against DNA damage induced by camptothecin. However, STK295900 does not affect etoposide-induced DNA damage. Moreover, STK295900 preferentially exerts cytotoxic effect on cancer cell lines while camptothecin, etoposide, and Hoechst 33342 affected both cancer and normal cells. Therefore, STK295900 has a potential to be developed as an anticancer chemotherapeutic agent.

Suppression of tumor proliferation and angiogenesis of hepatocellular carcinoma by HS-104, a novel phosphoinositide 3-kinase inhibitor

Dysregulation of the phosphoinositide 3-kinase (PI3K)/AKT/mTOR signaling pathway frequently instigates tumorigenesis leading to hepatocellular carcinoma (HCC). We synthesized N-(5-(3-(3-methyl-1,2,4-oxadiazol-3-yl)imidazo[1,2-a]pyridin-6-yl)pyridin-3-yl)benzenesulfonamide (HS-104), a novel PI3K inhibitor, and investigated its in vitro anticancer effect and in vivo capacity in an animal xenograft model. The inhibition of cell growth by HS-104 revealed that it was effective against HCC cell lines. Also, the activation of the AKT/mTOR signal cascade was inhibited by HS-104 treatment in a dose dependent manner. Flow cytometry analysis showed an accumulation of HCC cells in the G2/M phase with concomitant loss of cells in the S phase. The apoptotic effect of HS-104 was accompanied by increased evidence of cleaved caspase-3 and PARP, as well as DNA fragmentation. In angiogenesis studies, HS-104 inhibited the tube formation of vascular endothelial growth factor (VEGF)-induced human umbilical vein endothelial cells (HUVECs), and suppressed microvessel sprouting from a rat aortic ring, ex vivo, and blood vessel formation in the Matrigel plug assay in mice. HS-104 inhibited the expression of the downstream proteins of PI3K including p-AKT, p-mTOR and p-p70S6K in VEGF-induced HUVECs. In the xenograft animal model, HS-104 significantly delayed tumor growth in a dose dependent manner and suppressed the expression of PCNA, CD34 and cleaved caspase-3 in tumor tissue. These studies show that HS-104 inhibited the PI3K/AKT/mTOR signaling pathway resulting in cell growth/angiogenesis inhibition and apoptosis induction. Therefore, HS-104 is considered as a novel drug candidate for the treatment of HCC.

Reactive-oxygen-species-mediated Cdc25C degradation results in differential antiproliferative activities of vanadate, tungstate, and molybdate in the PC-3 human prostate cancer cell line

The differential antiproliferative effects of vanadate, tungstate, and molybdate on human prostate cancer cell line PC-3 were compared and the underlying mechanisms were investigated. The results demonstrate that all of the three oxoanions can cause G(2)/M cell cycle arrest, which is evidenced by the increase in the level of phosphorylated Cdc2 at its inactive Tyr-15 site. Moreover, even if the difference in cellular uptake among the three oxoanions is excluded from the possible factors affecting their antiproliferative activity, vanadate exerted a much more potent effect in PC-3 cells than the other two oxoanions. Our results also reveal that reactive oxygen species (ROS)-mediated degradation of Cdc25C rather than Cdc25A or Cdc25B is responsible for vanadate-induced G(2)/M cell cycle arrest. We propose a possible mechanism to clarify the differential effect of the three oxoanions in biological systems beyond just considering that they are structural analogs of phosphate. We suggest that ROS formation is unlikely to be involved in the biological function of tungstate and molybdate, whereas the redox properties of vanadium may be important factors for it to exert pharmacological effects. Further, given the evidence from epidemiology studies of the association between diabetes and prostate cancer, the possibility of vanadate as a good candidate as both an antidiabetic and an anticancer agent or a chemopreventive agent is indicated.

KX-01, a novel Src kinase inhibitor directed toward the peptide substrate site, synergizes with tamoxifen in estrogen receptor α positive breast cancer

KX-01 is the first clinical Src inhibitor of the novel peptidomimetic class that targets the peptide substrate site of Src providing more specificity toward Src kinase. The present study was designed to evaluate the effects of KX-01 as a single agent and in combination with tamoxifen (TAM) on cell growth and apoptosis of ERα positive breast cancer in vitro and in vivo. Flow cytometry demonstrated that KX-01 induced cell cycle arrest in G2/M phase. Immunofluorescent staining for mitotic phase markers and TUNEL staining indicated that cells had arrested in the mitotic phase and mitotic arrested cells were undergoing apoptosis. KX-01 induced nuclear accumulation of cyclin B1, and activation of CDK1, MPM2, and Cdc25C that is required for progression past the G2/M checkpoint. Apoptosis resulted from activation of caspases 6, 7, 8, and 9. Combinational index analysis revealed that combinations of KX-01 with TAM resulted in synergistic growth inhibition of breast cancer cell lines. KX-01 combined with TAM resulted in decreased ERα phosphorylation at Src-regulated phosphorylation sites serines 118 and 167 that were associated with reduced ERα transcriptional activity. Orally administered KX-01 resulted in a dose dependent growth inhibition of MCF-7 tumor xenografts, and in combination with TAM exhibited synergistic growth inhibition. Immunohistochemical analysis revealed that combinational treatment reduced angiogenesis, and ERα signaling in tumors compared to either drug alone that may underlie the synergistic tumor growth inhibition. Combinations of KX-01 with endocrine therapy present a promising new strategy for clinical management of ERα positive breast cancer.

Spatholobus suberectus inhibits cancer cell growth by inducing apoptosis and arresting cell cycle at G2/M checkpoint

AIM OF THE STUDY

Although herbs have long been alternatively applied for cancer treatment in China, its treatment effects and their potential mechanisms have not been sufficiently investigated. The chinese herb Spatholobus suberectus (SS) is commonly prescribed to cancer patients. In this study, the anti-cancer effect of SS and its molecular mechanisms have been investigated.

MATERIALS AND METHODS

The effect of SS on cell proliferation was studied by cell growth assay and flow cytometry on breast cancer cell lines MCF-7 and colon cancer cell line HT-29. The role of SS in apoptosis was studied by flow cytometry, DNA fragmentation assay and mitochondrial membrane potential assay. Expression of proteins associated with cell cycle and apoptosis was determined by Western blot analysis. The in vivo effect of SS was tested in nude mouse cancer xenografts.

RESULTS

Cell growth assay showed that SS effectively inhibits tumor cell growth in a dose-dependent manner. Flow cytometry analysis showed that SS could arrest the cell cycle at G2/M checkpoint, which is associated with DNA damage and activation of phosphor-Chk1/Chk2. The pro-apoptotic effect of SS was demonstrated by Annexin V-PI staining and mitochondrial membrane potential assay. In vivo experiments show that the efficiency of SS alone group was superior to docetaxel or to docetaxel and SS combined. No obvious body weight loss or blood toxicity was observed in SS tested animals.

CONCLUSIONS

Our data demonstrates that SS is a potential herb for cancer treatment by inhibiting tumor growth via induction of apoptosis and arrest of the cell cycle at G2/M phase.

Positive-feedback loops in cell cycle progression

A positive-feedback loop is a simple motif that is ubiquitous to the modules and networks that comprise cellular signaling systems. Signaling behaviors that are synonymous with positive feedback include amplification and rapid switching, maintenance, and the coherence of outputs. Recent advances have been made towards understanding how positive-feedback loops function, as well as their mechanistic basis in controlling eukaryotic cell cycle progression. Some of these advances will be reviewed here, including: how cyclin controls passage through Start and maintains coherence of G1/S regulon expression in yeast; how Polo-like kinase 1 activation is driven by Bora and Aurora A, and its expression is stimulated by Forkhead Box M1 in mammalian cells; and how some of the various dynamic behaviors of spindle assembly and anaphase onset can be produced.

Twilight effects of low doses of ionizing radiation on cellular systems: a bird's eye view on current concepts and research

The debate about the health risks from low doses of radiation is vigorous and often acrimonious since many years and does not appear to weaken. Being far from completeness, this review presents only a bird's eye view on current concepts and research in the field. It is organized and divided in two parts. The first is dedicated to molecular responses determined by radiation-induced DNA ruptures. It focuses its attention on molecular pathways that are activated by ATM and tries to describe the variegated functions and specific roles of Chk2 and p53 and other proteins in sensing, promoting and executing DNA repair. The second part is more concerned with the risk associated with exposure to low dose radiation and possible effects that the radiation-affected cell may undergo. These effects include induction of apoptosis and mitotic catastrophe, bystander effect and genomic instability, senescence and hormetic response. Current hypotheses and research on these issues are briefly discussed.

Downregulation of EZH2 decreases growth of estrogen receptor-negative invasive breast carcinoma and requires BRCA1

Increased levels of enhancer of zeste homolog 2 (EZH2), a critical regulator of cellular memory, are associated with negative estrogen receptor (ER) expression and disease progression in breast cancer. High levels of EZH2 signal the presence of metastasis and poor outcome in breast cancer patients. To test the hypothesis that deregulation of EZH2 contributes to ER-negative breast cancer progression, EZH2 expression was inhibited in ER-negative breast cancer cells MDA-MB-231 and CAL51 using a lentivirus system. EZH2 knockdown decreased proliferation and delayed the G(2)/M cell-cycle transition, although not affecting apoptosis. In vivo, EZH2 downregulation significantly decreased breast xenograft growth and improved survival. EZH2 knockdown upregulated BRCA1 protein. Of note, BRCA1 knockdown was sufficient to rescue the effects of EZH2 downregulation on proliferation, G(2)/M arrest, and on the levels of hyperphosphorylated mitotic Cdc25C and Cyclin B1 proteins, crucial for entry into mitosis. Invasive ER-negative breast carcinomas show significant overexpression of EZH2 and downregulation of BRCA1 proteins. Taken together, we show that EZH2 is important in ER-negative breast cancer growth in vivo and in vitro, and that BRCA1 is required for the proliferative effects of EZH2. Blockade of EZH2 may provide a prime target to prevent and/or halt ER-negative breast cancer progression.