Dual action of cyclin-dependent kinase inhibitors: induction of cell cycle arrest and apoptosis. A comparison of the effects exerted by roscovitine and cisplatin

Cyclin-dependent kinases (CDKs) have recently raised considerable interest in view of their key role in the regulation of the cell cycle progression. In proliferating cells, distinct CDKs associated with specific cyclins coordinate in an orchestrated way the appropriate transition between different phases of the cell cycle. Mutations and/or aberrant expression of distinct CDKs and their regulatory components lead to uncontrolled proliferation and finally to carcinogenesis. However, in post-mitotic neurons, all CDKs with the exception of CDK5 are silent. CDK5, a proline-directed serine/threonine kinase exhibiting a close structural homology to the mitotic CDKs, binds to p35, the neuron-specific regulatory subunit of CDK5. CDK5 is very abundant in mature neurons and seems to regulate neurotransmitter release through phosphorylation and down-regulation of calcium channel activity. Therefore, the inhibition of CDKs in neurons after oxidative stress and in neurodegenerative disorders has a protective action. Selective CDKs inhibitors were developed as promising drugs for cancer therapy due to their ability to arrest cell cycle progression. The aim of this study was to compare the anti-proliferative effect of roscovitine (ROSC), a potent CDKs inhibitor, with that of cisplatin (CP) on human breast cancer MCF-7 cells. ROSC exerted stronger inhibitory effect on proliferation and cell cycle progression of MCF-7 than CP. Accumulation of G(2)/M arrested cells starting 6 h after onset of ROSC treatment coincided with a strong up-regulation of the p53. Reconstitution with caspase-3 sensitized MCF-7 cells to CP action. It implicates that ROSC inhibits more selectively and efficaciously the proliferation of human breast carcinoma cells.     

Antiproliferative effects of quercetin through cell cycle arrest and apoptosis in human breast cancer MDA-MB-453 cells

To explore the anticancer effects of the flavonoid quercetin on human breast cancer MDA-MB-453 cells via cell cycle regulation and the induction of apoptosis, the antiproliferative effect of quercetin was first examined by MTT assay. When MDA-MB-453 cells were treated with quercetin for various periods of time (3–24 hrs) and at various doses (1–100 μM), cell growth decreased significantly in a time-and dose-dependent manner. To elucidate the mechanism underlying the antiproliferative effect of quercetin, cell cycle progression and the induction of apoptosis in MDA-MB-453 cells exposed to 100 μM quercetin for 24 hrs were investigated. Quercetin caused a remarkable increase in the number of sub-G1 phase cells, and an Annexin-V assay revealed that exposure to quercetin affected apoptosis. Moreover, treatment with quercetin increased Bax expression but decreased Bcl-2 expression. Cleaved caspase-3 and PARP expression was also increased by quercetin. Thus, quercetin has probable anticancer activity. Our results suggest the existence of multiple pathways for the induction of cell cycle arrest and apoptosis by quercetin.     

Suppression of cell cycle progression by a fungal lectin: activation of cyclin-dependent kinase inhibitors

The antiproliferative activity of a fungal lectin (VVL) isolated from the mushroom, Volvariella volvacea, was studied using a battery of cultured tumor cell lines. It was revealed that [(3)H]thymidine incorporation into the cell lines was markedly reduced at 0.32 microM VVL. When S180 mouse sarcoma cells were incubated for 48 hr with doses of VVL ranging from 0.32 to 0.8 microM, prominent blebs on the cell surface and large vacuoles in the cytoplasm, but not apoptotic bodies, were observed under a fluorescence microscopy. VVL did not exert ribosome-inactivating activity or induce any changes in the expression of cyclins A, D1, and E. However, it did activate the expression of cyclin kinase inhibitors, namely p21, p27, p53, and Rb, in a dose-dependent manner. Flow cytometric analysis demonstrated an accumulation of cells in the G2/M phase in a time- and dose-dependent manner, indicating that VVL arrested cell proliferation by blocking cell cycle progression in the G2/M phase.     

Difference of cell cycle arrests induced by lidamycin in human breast cancer cells

Lidamycin (LDM) is a member of the enediyne antibiotic family. It is undergoing phase I clinical trials in China as a potential chemotherapeutic agent. In the present study, we investigated the mechanism by which LDM induced cell cycle arrest in human breast cancer cells. The results showed that LDM induced G1 arrest in p53 wild-type MCF-7 cells at low concentrations, and caused both G1 and G2/M arrests at higher concentrations. In contrast, LDM induced only G2/M arrest in p53-mutant MCF-7/DOX cells. Western blotting analysis indicated that LDM-induced G1 and G2/M arrests in MCF-7 cells were associated with an increase of p53 and p21, and a decrease of phosphorylated retinoblastoma tumor suppressor protein, cyclin-dependent kinase (Cdk), Cdc2 and cyclin B1 protein levels. However, LDM-induced G2/M arrest in MCF-7/DOX cells was correlated with the reduction of cyclin B1 expression. Further study indicated that the downregulation of cyclin B1 by LDM in MCF-7 cells was associated with decreasing cyclin B1 mRNA levels and promoting protein degradation, whereas it was only due to inducing cyclin B1 protein degradation in MCF-7/DOX cells. In addition, activation of checkpoint kinases Chk1 or Chk2 maybe contributed to LDM-induced cell cycle arrest. Taken together, we provide the first evidence that LDM induces different cell cycle arrests in human breast cancer cells, which are dependent on drug concentration and p53 status. These findings are helpful in understanding the molecular anti-cancer mechanisms of LDM and support its clinical trials.     

Src kinase inhibitors induce apoptosis and mediate cell cycle arrest in lymphoma cells

Src kinases are involved in multiple cellular contexts such as proliferation, adhesion, tumor invasiveness, angiogenesis, cell cycle control and apoptosis. We here demonstrate that three newly developed dual selective Src/Abl kinase inhibitors (SrcK-I) (AZM559756, AZD0530 and AZD0424) are able to induce apoptosis and cell cycle arrest in BCR-ABL, c-KIT and platelet-derived growth factor-negative lymphoma cell lines. Treatment of DOHH-2, WSU-NHL, Raji, Karpas-299, HUT78 and Jurkat cells with SrcK-I revealed that the tested substances were effective on these parameters in the cell lines DOHH-2 and WSU-NHL, whereas the other tested cell lines remained unaffected. Phosphorylation of Lyn and in particular Lck were affected most heavily by treatment with the SrcK-I. Extrinsic as well as intrinsic apoptosis pathways were activated and elicited unique expressional patterns of apoptosis-relevant proteins such as downregulation of survivin, Bcl-XL and c-FLIP. Protein levels of c-abl were downregulated and Akt phosphorylation was decreased by treatment with SrcK-I. Basal expression levels of c-Myc were notably lower in sensitive cell lines as compared with nonsensitive cell lines, possibly providing an explanation for sensitivity versus resistance against these novel substances. This study provides the first basis for establishing novel SrcK-I as weapons in the arsenal against lymphoma cells.     

A current and comprehensive review of cyclin-dependent kinase inhibitors for the treatment of metastatic breast cancer

Background: Resistance to endocrine treatment generally occurs over time, especially in the metastatic stage. In this paper, we aimed to review the mechanisms of cyclin-dependent kinase (CDK) 4/6 inhibition and clinical usage of new agents in the light of recent literature updates.

Scope: A literature search was carried out using PubMed, Medline and ASCO and ESMO annual-meeting abstracts by using the following search keywords; “palbociclib”, “abemaciclib”, “ribociclib”, “cyclin-dependent kinase inhibitors” and “CDK 4/6” in metastatic breast cancer (MBC). The last search was on 10 June 2017.

Findings: CDKs and cyclins are two molecules that have a key role in cell cycle progression. Today, there are three highly selective CDK4/6 inhibitors in clinical development – palbociclib, ribociclib and abemaciclib. Palbociclib and ribociclib were recently approved by the US FDA in combination with letrozole for the treatment of MBC in a first-line setting, as well as palbociclib in combination with fulvestrant for hormone-receptor (HR)-positive MBC that had progressed while on previous endocrine therapy according to the PALOMA-1, MONALEESA-2 and PALOMA-3 trials, respectively. In the recently published randomized phase III MONARCH 2 trial, abemaciclib plus letrozole had longer progression-free survival and higher objective response rates with less serious adverse events in advanced HR-positive breast cancer previously treated with hormonal treatment.

Conclusion: CDK4/6 inhibition is a new and promising target for patients with hormone-receptor-positive MBC. Both palbociclib and ribociclib showed significant additive benefit for patients receiving first-line treatment for HR-positive, epidermal growth factor receptor-2-negative advanced breast cancer. Palbociclib and abemaciclib also had significant activity in combination with fulvestrant for patients with MBC that progressed on previous endocrine therapy.     

Inhibition of cyclin-dependent kinase 5 by roscovitine impairs memory consolidation and reconsolidation in the day-old chick

Cyclin-dependent kinase 5 (CDK-5) is reported to phosphorylate the NMDA receptor prior to the induction of long-term potentiation (LTP), among its many other effects. Application of CDK-5 inhibitors disrupts LTP and results in impaired task acquisition in behaving animals. In this study, we investigated the effect of exogenously applied roscovitine, a potent CDK-5 inhibitor, on consolidation and reconsolidation processes in day-old male chicks. New HampshirexWhite leghorn cockerels were trained using a modified version of the passive avoidance learning task. Intracranial injections of roscovitine (2.5 microM) administered immediately after training induced a memory deficit that evolved from 5-minute post-training and persisted until at least 24 h following training. Injections of roscovitine (2.75 microM) administered immediately after the reminder trial induced a memory deficit observed by 30-minute post-reminder which had resolved by 24 h following the reminder. The comparison between consolidation and reconsolidation demonstrates differences both in the time of the onset of the memory deficit as well as in the permanence of this deficit. The results suggest an important, although different role for CDK-5 in consolidation and reconsolidation processes following passive avoidance learning.     

Mechanism of cell cycle arrest caused by histone deacetylase inhibitors in human carcinoma cells

Inhibitors of histone deacetylase (HDAC) block cell cycle progression at G1 in many cell types. We investigated the mechanism by which trichostatin A (TSA), a specific inhibitor of HDAC, induces G1 arrest in human cervix carcinoma HeLa cells. TSA treatment induced histone hyperacetylation followed by growth arrest in G as well as hypophosphorylation of pRb. The Cdk4 kinase activity was essentially unchanged during the TSA-induced G1 arrest. On the other hand, the arrest was accompanied by down-regulation of kinase activity of Cdk2, although the total protein levels of Cdk2 and its activator Cdc25A were unaffected. Upon TSA treatment, amounts of cyclin E and the CDK inhibitor p21WAF1/Cip1 were markedly increased, while that of cyclin A was reduced. The induction of p21 and down-regulation of cyclin A correlated well with the decreased Cdk2 activity and cell cycle arrest. Furthermore, gel filtration chromatography showed the association of p21 with the cyclin E-Cdk2 complex, suggesting that the activation of Cdk2 by the enhanced expression of cyclin E is blocked by the increased p21. The elevated expression of p2 is also observed in cells treated with trapoxin and FR901228, structurally unrelated histone deacetylase inhibitors. A human colorectal carcinoma cell line lacking both alleles of the p21 gene (p21-/-) was resistant to TSA several times more than the parental line (p21+/+). These results suggest that the suppression of Cdk2 kinase activity due to p21 overexpression play a critical role in HDAC inhibitor-induced growth inhibition.     

Resveratrol modulates roscovitine-mediated cell cycle arrest of human MCF-7 breast cancer cells.

Human MCF-7 breast cancer cells are relatively resistant to anti-cancer drugs. Recently, we reported that roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, arrested human MCF-7 breast cancer cells in G2 phase of the cell cycle and concomitantly induced apoptosis. Moreover, we observed that the effect of the CDK inhibitor was dependent on the content of the culture medium. The cell cycle inhibiting action of ROSC was markedly diminished in human MCF-7 cells cultivated in medium supplemented with phenol red. These observations indicated that the therapeutic effects of ROSC can be affected by the components of the tissue medium. Recently, a number of epidemiological and experimental studies indicated that polyphenols (e.g. resveratrol, epicatechins etc.), abundant micronutrients in food, are anti-oxidant agents and could have strong anti-mitotic as well as pro-apoptotic activities. In the present contribution we raised the question whether the ROSC-mediated cell cycle arrest could be additionally modulated by compounds of natural origin, especially by polyphenols. Considering the potential benefits of the dietary components during the post-chemotherapy period, we focused our attention on the effects of resveratrol administration after treatment with ROSC. We analyzed whether the combined treatment with resveratrol would exert any additional effect on the cell cycle status of ROSC-treated human cancer cells. Resveratrol exhibited low direct cytotoxicity. The combined treatment with ROSC enhanced the ROSC-mediated inhibition of cell proliferation and cell cycle arrest. These results indicate that targeted combination of anti-cancer drugs with distinct naturally occurring compounds could increase the efficacy of the therapy and concomitantly reduce the undesired side effects exerted by cytostatic drugs.     

Differential effects of doxorubicin treatment on cell cycle arrest and Skp2 expression in breast cancer cells

Overexpression of Skp2, the ubiquitin ligase subunit that targets p27 for degradation, is often observed in cancers, and is associated with aggressive tumor proliferation and poor prognosis. As there is no drug at present that specifically targets Skp2, studies were undertaken to examine the effects of commonly used drugs on Skp2 regulation. Doxorubicin is among the most effective antitumor agents used for the management of breast cancer, but its effect on Skp2 expression is unknown. The objective of this study was to examine the effect of doxorubicin on Skp2 expression regulation in breast cancer cell lines. The expression of Skp2 mRNA and the protein levels of Skp2, p27, p21 and cyclin B were examined in doxorubicin-treated MCF-7 and MDA-MB-231 breast cancer cells. The effect of doxorubicin on the cell cycle profile was assessed by fluorescence-activated cell sorting analysis. Doxorubicin decreased Skp2 mRNA and protein levels in MCF-7 cells, but had the opposite effect in MDA-MB-231 cells. p27 levels were slightly decreased, whereas p53 and p21 levels were significantly upregulated in doxorubicin-treated MCF-7 cells. In contrast, p27 levels were unaffected by doxorubicin treatment in MDA-MB-231 cells, but cyclin B levels were markedly increased. Doxorubicin arrested MCF-7 cells at G1/S and G2/M checkpoints, whereas MDA-MB-231 cells were arrested at G2/M only. The differential effects of doxorubicin on Skp2 expression in breast cancer cells depend upon the specific cell cycle checkpoints activated by the drug. These changes induced by doxorubicin, however, do not significantly affect p27 expression in these cell lines, suggesting that the potential of a given drug to alter p27 expression through Skp2 modulation might depend on its specific action on cell cycle arrest.     

冬凌草甲素诱导胃癌BGC-823细胞凋亡及G2／M细胞周期阻滞作用研究

目的研究冬凌草甲素体外诱导胃癌BGC-823细胞凋亡和细胞周期阻滞的作用，阐明其作用机理，为临床应用提供科学依据。方法用MTT方法测定冬凌草甲素体外抑制BGC-823细胞生长的作用。用共聚焦荧光显微镜和流式细胞仪分别观察诱导凋亡的情况。线粒体膜电位和细胞内钙离子浓度分别用荧光探针标记后流式细胞仪测定。凋亡和细胞周期相关蛋白的表达用Westem blotting进行测定。结果冬凌草甲素对BGC-823细胞的半数生长抑制浓度IC50值为22．21μmol.L^-1，并诱导细胞凋亡，呈现浓度依赖性。此外，能够降低线粒体膜电位，升高细胞内钙离子浓度，激活caspase-3前体。同时，冬凌草甲素能够使得BGC-823细胞阻滞在细胞周期的G2／M期，伴随有cyclin A蛋白的表达下降。在发生凋亡和细胞阻滞之前，观察到p53蛋白的表达增加。结论冬凌草甲素能够诱导BGC-823细胞产生凋亡，并使细胞周期阻滞在G2／M期。其凋亡机理与caspase-3的激活，p53蛋白表达上调及cyclin A表达下调相关。     

Inhibition of mitogen-activated protein kinase kinase enhances apoptosis induced by arsenic trioxide in human breast cancer MCF-7 cells

Arsenic trioxide (As2O3) has recently been used to treat acute promyelocytic leukaemia and has activity in vitro against several solid tumour cell lines where the induction of differentiation and apoptosis are the prime effects. The mechanism of As2O3-induced cell death has yet to be clarified, especially in solid cancers. In the present study, the human breast cancer cell line MCF-7 was examined as a cellular model for As2O3 treatment. The involvement of extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) was investigated in As2O3-induced cell death. 3. It was found that As2O3 activates the prosurvival mitogen-activated protein kinase kinase (MEK)/ERK pathway in MCF-7 cells, which, conversely, may compromise the efficacy of As2O3. Hence, a combination treatment of As2O3 and MEK inhibitors was investigated to determine whether this treatment could lead to enhanced growth inhibition and apoptosis in MCF-7 cells. 4. Inhibition of MEK/ERK with the pharmacological inhibitors U0126 (10 micromol/L) or PD98059 (20 micromol/L) together with As2O3 (2 and 5 micromol/L) resulted in a significant enhancement of growth inhibition in breast cancer MCF-7 cells as determined by the 3-(4,5-dimethyl-2 thiazoyl)-2,5-diphenyl-2H-tetrazolium bromide assay and [Methyl-3H]-thymidine incorporation. Furthermore, the results demonstrated that combined treatment with As2O3 and the MEK1/2 inhibitor U0126 could augment breast cancer MCF-7 cell apoptosis approximately twofold compared with the effects of the two drugs alone, as determined by Hoechst 33258 or annexin V/propidium iodide (PI) staining and flow cytometry. 5. In addition, As2O3 activated p38 in a dose-dependent manner, but had no effect on JNK1/2. Treatment with a p38 inhibitor did not prevent As2O3-induced apoptosis. 6. In conclusion, the results of the present study showed that enhanced apoptosis is detected in breast cancer MCF-7 cells in the presence of As2O3 and an MEK inhibitor, which may be a new promising adjuvant to current breast cancer treatments.     

Cell cycle arrest and proapoptotic effects of the anticancer cyclodepsipeptide serratamolide (AT514) are independent of p53 status in breast cancer cells

In a search for new anticancer agents, we have identified serratamolide (AT514), a cyclodepsipeptide from Serratia marcescens 2170 that induces cell cycle arrest and apoptosis in various cancer cell lines. A cell viability assay showed that the concentrations that cause 50% inhibition (IC50) in human cancer cell lines range from 5.6 to 11.5 microM depending on the cell line. Flow cytometry analysis revealed that AT514 caused cell cycle arrest in G0/G1 or cell death, depending on the cell type and the length of time for which the cells were exposed to the drug. Subsequent studies revealed that AT514-induced cell death is caused by apoptosis, as indicated by caspases activation (8, 9, 2 and 3) and cleavage of poly (ADP-ribose) polymerase (PARP), release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria, and the appearance of apoptotic bodies and DNA laddering. Alterations in protein levels of Bcl-2 family members might be involved in the mitochondrial disruption observed. AT514 induced p53 accumulation in wild-type p53 cells but cell death was observed in both deficient and wild-type p53 cells. Our results indicate that AT514 induces cell cycle arrest and apoptosis in breast cancer cells irrespectively of p53 status, suggesting that it might represent a potential new chemotherapeutic agent.     

Cell cycle arrest,extracellular matrix changes and intrinsic apoptosis in human melanoma cells are induced by Boron Neutron Capture Therapy

Boron Neutron Capture Therapy (BNCT) involves the selective accumulation of boron carriers in tumor tissue followed by irradiation with a thermal or epithermal neutron beam. This therapy is therefore a cellular irradiation suited to treat tumors that have infiltrated into healthy tissues. BNCT has been used clinically to treat patients with cutaneous melanomas which have a high mortality. Human normal melanocytes and melanoma cells were treated with BNCT at different boronophenylalanine concentrations for signaling pathways analysis. BNCT induced few morphological alterations in normal melanocytes, with a negligible increase in free radical production. Melanoma cells treated with BNCT showed significant extracellular matrix (ECM) changes and a significant cyclin D1 decrease, suggesting cell death by necrosis and apoptosis and cell cycle arrest, respectively. BNCT also induced a significant increase in cleaved caspase-3 and a decrease in the mitochondrial electrical potential with selectivity for melanoma cells. Normal melanocytes had no significant differences due to BNCT treatment, confirming the data from the literature regarding the selectivity of BNCT. The results from this study suggest that some signaling pathways are involved in human melanoma treatment by BNCT, such as cell cycle arrest, ECM changes and intrinsic apoptosis.     

Physalis angulata induced G2/M phase arrest in human breast cancer cells.

Physalis angulata (PA) is employed in herbal medicine around the world. It is used to treat diabetes, hepatitis, asthma and malaria in Taiwan. We have evaluated PA as a cancer chemopreventive agent in vitro by studying the role of PA in regulation of proliferation, cell cycle and apoptosis in human breast cancer cell lines. PA inhibited cell proliferation and induced G2/M arrest and apoptosis in human breast cancer MAD-MB 231 and MCF-7 cell lines. In this study, under treatment with various concentrations of PA in MDA-MB 231 cell line, we checked mRNA levels for cyclin A and cyclin B1 and the protein levels of cyclin A and cyclin B1, Cdc2 (cyclin-dependent kinases), p21(waf1/cip1) and P27(Kip1) (cyclin-dependent kinase inhibitors), Cdc25C, Chk2 and Wee1 kinase (cyclin-dependent kinase relative factors) in cell cycle G2/M phase. From those results, we determined that PA arrests MDA-MB 231 cells at the G2/M phase by (i) inhibiting synthesis or stability of mRNA and their downstream protein levels of cyclin A and cyclin B1, (ii) increasing p21(waf1/cip1) and P27(kip1) levels, (iii) increasing Chk2, thus causing an increase in Cdc25C phosphorylation/inactivation and inducing a decrease in Cdc2 levels and an increase in Wee1 level. According to the results obtained, PA appears to possess anticarcinogenic properties; these results suggest that the effect of PA on the levels of phosphorylated/inactivated Cdc25C are mediated by Chk2 activation, at least in part, via p21(waf1/cip1) and P27(kip1) cyclin-dependent kinase inhibitors pathway to arrest cells at G2/M phase in breast cancer carcinoma cells.     

紫杉醇诱导人乳腺癌MCF-7细胞周期阻断及凋亡的基因表达谱分析

目的研究紫杉醇诱导人MCF-7细胞周期阻断及凋亡的分子机制。方法用流式细胞仪分析紫杉醇对MCF-7细胞周期变化的影响，用自制的含9 984个已知基因和EST的高密度基因芯片检测MCF-7细胞在不同浓度紫杉醇作用下的基因表达变化。结果MCF-7细胞在100 nmol·L^-1紫杉醇作用24 h，流式细胞仪结果显示77.8%细胞阻断在G₂/M期和1.3%细胞发生凋亡；基因表达谱分析发现：在12.5 nmol·L^-1 (IC₅₀)及100 nmol·L^-1紫杉醇作用下，分别有27及77个基因差异表达。结论紫杉醇可诱导MCF-7细胞周期阻断在G₂/M期并引起部分细胞凋亡，该作用与药物浓度有关。基因表达谱分析显示部分差异表达基因参与细胞微管及骨架结构、细胞周期调控、以及DNA损伤修复和凋亡过程。     

Induction of cell cycle arrest in human MCF-7 breast cancer cells by cis-stilbene derivatives related to VIOXX

In our present study, 12 new cis-stilbene derivatives (CRI-1–CRI-13) related to VIOXX^® were synthesized and studied for their inhibitory effects on cell cycle progression and anti-estrogenicity in human adenoma breast cancer MCF-7 cells. Based on the different substituents in the cis-stilbene molecule, we studied a possible structure activity relationship (SAR) for the inhibition of the cell cycle, cytotoxicity and (anti-) estrogenicity. The results showed that some cis-stilbenes have a pronounced effect on cell cycle distribution. CRI-5, 7, 10 and 12 caused an arrest of G2/M phase and reduction of G1/S phase in all tested doses (1–50 μM). In addition, some of these cis-stilbenes, have a moderate anti-estrogenic effect around 10 μM. Based on these results a preliminary SAR for cis-stilbene derivatives is suggested in which the presence and position of methoxy or thiomethoxy groups play an essential role in this cell cycle arrest. For this substitution on the para position of the left aromatic ring appears to be a prerequisite. However, the SAR for anti-estrogenicity appears to be different, but experimental information was too limited to define a possible SAR. In conclusion, our study shows that some synthetic cis-stilbene related to VIOXX^® might have chemopreventive properties that can effectively interfere with the cell cycle distribution of breast tumor cells.