Cyclins of phases G1, S and G2/M are overexpressed in aneuploid mammary carcinomas

Increased expression of tumor proliferation genes in Hispanic women with early-stage breast cancer

Hispanic women have higher breast cancer mortality compared to non-Hispanic whites. We evaluated for Proliferation Axis Score differences, as determined by Oncotype Dx, in Hispanic and non-Hispanic white women with newly diagnosed breast cancer. We matched 219 women, based upon age, stage, and nodal status. Compared to non-Hispanic whites, Hispanic women with hormone-sensitive, HER2-negative early-stage breast cancer had a higher Proliferation Axis Score. No differences were seen in Recurrence Score, ER, PR, or HER2 by Oncotype DX. CCNB1 and AURKA were significantly higher in Hispanic women. These tumor differences may help explain breast cancer outcome differences between the two ethnicities.

Constitutive Cdk2 activity promotes aneuploidy while altering the spindle assembly and tetraploidy checkpoints

The cell has many mechanisms for protecting the integrity of its genome. These mechanisms are often weakened or absent in many cancers, leading to high rates of chromosomal instability in tumors. Control of the cell cycle is crucial for the function of these checkpoints, and is frequently lost in cancers as well. Overexpression of Cyclin D1 in a large number of breast cancers causes overactivation of the cyclin-dependent kinases, including Cdk2. Constitutive Cdk2 activation through Cyclin D1 generates tumors in mice that are aneuploid and have many characteristics indicative of chromosomal instability. Expression of these complexes in the MCF10A cell line leads to retinoblastoma protein (Rb) hyperphosphorylation, a subsequent increase in proliferation rate, and increased expression of the spindle assembly checkpoint protein Mad2. This results in a strengthening of the spindle assembly checkpoint and renders cells more sensitive to the spindle poison paclitaxel. Constitutive Rb phosphorylation also causes a weakening of the p53-dependent tetraploidy checkpoint. Cells with overactive Cdk2 fail to arrest after mitotic slippage in the presence of paclitaxel or cytokinesis failure during treatment with cytochalasin-B, generating 8N populations. This additional increase in DNA content appears to further intensify the tetraploidy checkpoint in a step-wise manner. These polyploid cells are not viable long-term, either failing to undergo division or creating daughter cells that are unable to undergo subsequent division. This study raises intriguing questions about the treatment of tumors with overactive Cdk2.

Three Observations That Have Changed Our Understanding of Cyclin D1 and p27 in Cell Cycle Control

Our understanding of cell cycle control has been based largely upon studies of synchronized cultures, often focused upon the early stages of the cell cycle following stimulation of quiescent cultures. These studies showed that cyclin D1 and p27(Kip1) (p27) each respond to the growth environment of the cell and together control entry into the cell cycle. In contrast, all cell cycle phases were considered in these studies of actively growing cultures, including events leading to withdrawal from the cell cycle. This approach relies upon the techniques of microinjection, quantitative image analysis, and time-lapse microscopy. The results provide critical new detail to our understanding of the roles of cyclin D1 and p27 in cell cycle regulation. Three critical observations resulting from this work will be described here to demonstrate that 1) cyclin D1 levels oscillate through the normal cell cycle, 2) checkpoint kinases are able to suppress cyclin D1 during S phase, and 3) the level of p27 is determined by a dynamic interaction between cyclin D1 and p27 so as to determine the rate of cell cycle progression. Based upon these observations, a model of cell cycle control is presented in which ras activity stimulates cyclin D1 during G2 phase, resulting in commitment of the cell to continued cell cycle progression. During G1 phase, ras activity suppresses the level of p27 protein, most of which is bound to cyclin D1, resulting in regulation of the rate of proliferation. This model predicted the involvement of checkpoint kinases in regulating cyclin D1 and the role of checkpoint kinases in the protection of neural cells against reactive oxygen. The substantiation of these 2 predictions serves as general validation of the model.

Knockdown of cyclin D1 inhibits proliferation, induces apoptosis, and attenuates the invasive capacity of human glioblastoma cells

Elevated cyclin D1 (CCND1) in human glioblastoma correlates with poor clinical prognosis. In this study, the human glioblastoma cell lines SHG-44 and U251 were stably transfected with short hairpin RNA (shRNA) targeting cyclin D1 or with ectogenic cyclin D1 by lentivirus-mediated transfection. Glioblastoma cells overexpressing or underexpressing cyclin D1 were then examined by in vitro growth assays, apoptosis assays, cell cycle analysis, and invasion assays. Cyclin D1 knockdown in SHG-44 cells inhibited cell proliferation, induced apoptosis, and attenuated migration across Matrigel, a model of invasive capacity. Western blot analysis and quantitative reverse-transcription polymerase chain reaction (RT-PCR) revealed that cells underexpressing CCND1 exhibited decreased multidrug resistance protein 1 (MDR1) and B-cell lymphoma-2 (Bcl-2) expression, but enhanced apoptosis effector caspase-3 expression. In contrast, cyclin D1 overexpression promoted cell proliferation, attenuated apoptosis, and enhanced invasive capacity. Furthermore, cyclin D1 overexpression was associated with increased expression of MDR1 and Bcl-2, and decreased caspase-3 expression. Results using the U251 cell line confirmed the effects of CCND1-targeted shRNA and lentivirus-mediated overexpression on proliferation and apoptosis of glioblastoma cells. Overexpression of cyclin D1 enhanced the proliferation and invasive potential of human glioblastoma cells, while reducing apoptosis. The ability to suppress the malignant phenotype by downregulating cyclin D1 expression may provide a new gene therapy approach for patients with malignant glioma.

Targeting cyclin B1 inhibits proliferation and sensitizes breast cancer cells to taxol

BACKGROUND

Cyclin B1, the regulatory subunit of cyclin-dependent kinase 1 (Cdk1), is essential for the transition from G2 phase to mitosis. Cyclin B1 is very often found to be overexpressed in primary breast and cervical cancer cells as well as in cancer cell lines. Its expression is correlated with the malignancy of gynecological cancers.

METHODS

In order to explore cyclin B1 as a potential target for gynecological cancer therapy, we studied the effect of small interfering RNA (siRNA) on different gynecological cancer cell lines by monitoring their proliferation rate, cell cycle profile, protein expression and activity, apoptosis induction and colony formation. Tumor formation in vivo was examined using mouse xenograft models.

RESULTS

Downregulation of cyclin B1 inhibited proliferation of several breast and cervical cancer cell lines including MCF-7, BT-474, SK-BR-3, MDA-MB-231 and HeLa. After combining cyclin B1 siRNA with taxol, we observed an increased apoptotic rate accompanied by an enhanced antiproliferative effect in breast cancer cells. Furthermore, control HeLa cells were progressively growing, whereas the tumor growth of HeLa cells pre-treated with cyclin B1 siRNA was strongly inhibited in nude mice, indicating that cyclin B1 is indispensable for tumor growth in vivo.

CONCLUSION

Our data support the notion of cyclin B1 being essential for survival and proliferation of gynecological cancer cells. Concordantly, knockdown of cyclin B1 inhibits proliferation in vitro as well as in vivo. Moreover, targeting cyclin B1 sensitizes breast cancer cells to taxol, suggesting that specific cyclin B1 targeting is an attractive strategy for the combination with conventionally used agents in gynecological cancer therapy.

Cell cycle-dependent expression of potassium channels and cell proliferation in rat mesenchymal stem cells from bone marrow

OBJECTIVE

Recently, our team has demonstrated that voltage-gated delayed rectifier K(+) current (IK(DR)) and Ca(2+)-activated K(+) current (I(KCa)) are present in rat bone marrow-derived mesenchymal stem cells; however, little is known of their physiological roles. The present study was designed to investigate whether functional expression of IK(DR) and I(KCa) would change with cell cycle progression, and whether they could regulate proliferation in undifferentiated rat mesenchymal stem cells (MSCs).

MATERIALS AND METHODS

Membrane potentials and ionic currents were recorded using whole-cell patch clamp technique, cell cycling was analysed by flow cytometry, cell proliferation was assayed with DNA incorporation method and the related genes were down-regulated by RNA interference (RNAi) and examined using RT-PCR.

RESULTS

It was found that membrane potential hyperpolarized, and cell size increased during the cell cycle. In addition, IK(DR) decreased, while I(KCa) increased during progress from G(1) to S phase. RT-PCR revealed that the mRNA levels of Kv1.2 and Kv2.1 (likely responsible for IK(DR)) reduced, whereas the mRNA level of KCa3.1 (responsible for intermediate-conductance I(KCa)) increased with the cell cycle progression. Down-regulation of Kv1.2, Kv2.1 or KCa3.1 with the specific RNAi, targeted to corresponding gene inhibited proliferation of rat MSCs.

CONCLUSION

These results demonstrate that membrane potential, IK(DR) and I(KCa) channels change with cell cycle progression and corresponding alteration of gene expression. IK(DR) and intermediate-conductance I(KCa) play an important role in maintaining membrane potential and they participate in modulation of proliferation in rat MSCs.

Variations in cyclin D1 levels through the cell cycle determine the proliferative fate of a cell

We present evidence that variations in cyclin D1 levels through the cell cycle are essential for continuing proliferation. Cyclin D1 levels must be high during G1 phase for a cell to initiate DNA synthesis, but then must be suppressed to low levels during S phase to allow for efficient DNA synthesis. This suppression during S phase is apparently regulated by cell cycle position alone and occurs automatically during each cell cycle. If the cell is to continue proliferating, cyclin D1 levels must be induced once again during G2 phase. This induction depends upon the activity of proliferative signaling molecules, and ensures that the extracellular environment continues to be conducive for growth. We propose that the suppression of cyclin D1 levels during each S phase ensures that the subsequent induction during G2 phase, and the resulting commitment to continuing proliferation, is closely linked to the cellular growth environment.

Identification of mutations that disrupt phosphorylation-dependent nuclear export of cyclin D1

Although cyclin D1 is overexpressed in a significant number of human cancers, overexpression alone is insufficient to promote tumorigenesis. In vitro studies have revealed that inhibition of cyclin D1 nuclear export unmasks its neoplastic potential. Cyclin D1 nuclear export depends upon phosphorylation of a C-terminal residue, threonine 286, (Thr-286) which in turn promotes association with the nuclear exportin, CRM1. Mutation of Thr-286 to a non-phosphorylatable residue results in a constitutively nuclear cyclin D1 protein with significantly increased oncogenic potential. To determine whether cyclin D1 is subject to mutations that inhibit its nuclear export in human cancer, we have sequenced exon 5 of cyclin D1 in primary esophageal carcinoma samples and in cell lines derived from esophageal cancer. Our work reveals that cyclin D1 is subject to mutations in primary human cancer. The mutations identified specifically disrupt phosphorylation of cyclin D1 at Thr-286, thereby enforcing nuclear accumulation of cyclin D1. Through characterization of these mutants, we also define an acidic residue within the C-terminus of cyclin D1 that is necessary for recognition and phosphorylation of cyclin D1 by glycogen synthase kinase-3 beta. Finally, through construction of compound mutants, we demonstrate that cell transformation by the cancer-derived cyclin D1 alleles correlates with their ability to associate with and activate CDK4. Our data reveal that cyclin D1 is subject to mutations in primary human cancer that specifically disrupt phosphorylation-dependent nuclear export of cyclin D1 and suggest that such mutations contribute to the genesis and progression of neoplastic growth.

Expression and gene amplification of primary (A, B1, D1, D3, and E) and secondary (C and H) cyclins in colon adenocarcinomas and correlation with patient outcome

BACKGROUND/AIMS

Deregulation of cell cycle control is a hallmark of cancer. The primary cyclins (A, B1, D1, D3, and E) are crucial for cell cycle progression. Secondary cyclins (C and H) have putative indirect effects on cell cycle progression and have not previously been evaluated in colon cancer. This study examined cyclin protein expression and gene amplification in colon adenocarcinoma and the correlation with patient outcome.

METHODS

Immunohistochemistry and real time quantitative polymerase chain reaction were used to determine cyclin expression and gene amplification in 219 tumours. The results were compared with clinical variables and patient outcomes.

RESULTS

Cyclin H was overexpressed in all tumours, cyclin C in 88%, cyclin B1 in 58%, cyclin A in 83%, cyclin D3 in 36%, cyclin E in 25%, and cyclin D1 in 11% of the tumours. Extra gene copies of cyclin A were seen in 6.2% of the tumours, cyclin B1 in 9%, cyclin C in 26.9%, cyclin D1 in 55%, cyclin D3 in 20.5%, cyclin E in 19.1%, and cyclin H in 5.1%. A significant correlation between protein overexpression and gene amplification was seen for cyclin C only. High expression of cyclin A was independently associated with improved survival. Amplification of cyclin C was independently associated with an unfavourable prognosis.

CONCLUSIONS

Amplification of the cyclin C gene was related to an unfavourable prognosis and high protein expression of cyclin A was associated with a better outcome in colon adenocarcinoma.

Expression of cyclins A, E and topoisomerase II alpha correlates with centrosome amplification and genomic instability and influences the reliability of cytometric S-phase determination

BACKGROUND

The progression of normal cells through the cell cycle is meticulously regulated by checkpoints guaranteeing the exact replication of the genome during S-phase and its equal division at mitosis. A prerequisite for this achievement is synchronized DNA-replication and centrosome duplication. In this context the expression of cyclins A and E has been shown to play a principal role.

RESULTS

Our results demonstrated a correlation between centrosome amplification, cell cycle fidelity and the level of mRNA and protein expression of cyclins A and E during the part of the cell cycle defined as G1-phase by means of DNA content based histogram analysis. It is shown that the normal diploid breast cell line HTB-125, the genomically relatively stable aneuploid breast cancer cell line MCF-7, and the genomically unstable aneuploid breast cancer cell line MDA-231 differ remarkably concerning both mRNA and protein expression of the two cyclins during G1-phase. In MDA-231 cells the expression of e.g. cyclin A mRNA was found to be ten times higher than in MCF-7 cells and about 500 times higher than in HTB-125 cells. Topoisomerase II alpha showed high mRNA expression in MDA compared to MCF-7 cells, but the difference in protein expression was small. Furthermore, we measured centrosome aberrations in 8.4% of the MDA-231 cells, and in only 1.3% of the more stable aneuploid cell line MCF-7. MDA cells showed 27% more incorporation of BrdU than reflected by S-phase determination with flow cytometric DNA content analysis, whereas these values were found to be of the same size in both HTB-125 and MCF-7 cells.

CONCLUSIONS

Our data indicate that the breast cancer cell lines MCF-7 and MDA-231, although both DNA-aneuploid, differ significantly regarding the degree of cell cycle disturbance and centrosome aberrations, which partly could explain the different genomic stability of the two cell lines. The results also question the reliability of cytometric DNA content based S-phase determination in genomically unstable tumor cell populations.

Regulation of G(2)/M events by Cdc25A through phosphorylation-dependent modulation of its stability

DNA replication in higher eukaryotes requires activation of a Cdk2 kinase by Cdc25A, a labile phosphatase subject to further destabilization upon genotoxic stress. We describe a distinct, markedly stable form of Cdc25A, which plays a previously unrecognized role in mitosis. Mitotic stabilization of Cdc25A reflects its phosphorylation on Ser17 and Ser115 by cyclin B-Cdk1, modifications required to uncouple Cdc25A from its ubiquitin-proteasome-mediated turnover. Cdc25A binds and activates cyclin B-Cdk1, accelerates cell division when overexpressed, and its downregulation by RNA interference (RNAi) delays mitotic entry. DNA damage-induced G(2) arrest, in contrast, is accompanied by proteasome-dependent destruction of Cdc25A, and ectopic Cdc25A abrogates the G(2) checkpoint. Thus, phosphorylation-mediated switches among three differentially stable forms ensure distinct thresholds, and thereby distinct roles for Cdc25A in multiple cell cycle transitions and checkpoints.