Cdc25B cooperates with Cdc25A to induce mitosis but has a unique role in activating cyclin B1-Cdk1 at the centrosome

Withaferin A, a steroidal lactone from Withania somnifera, induces mitotic catastrophe and growth arrest in prostate cancer cells

Cell cycle deregulation is strongly associated with the pathogenesis of prostate cancer. Clinical trials of cell cycle regulators that target either the G0/G1 or G2/M phase to inhibit the growth of cancers including prostate cancer are increasing. The present study focused on the cell cycle regulatory potential of the withanolide withaferin A (1) on prostate cancer cells. Compound 1 induced G2/M arrest in both prostate cancer cell lines (PC-3 and DU-145) when treated for 48 h. The G2/M arrest was accompanied by upregulation of phosphorylated Wee-1, phosphorylated histone H3, p21, and Aurora B. On the other hand, downregulation of cyclins (A2, B1, and E2) and a reduction in phosphorylated Cdc2 (Tyr15) were observed in 1-treated prostate cancer cells. In addition, decreased levels of phosphorylated Chk1 (Ser345) and Chk2 (Thr68) were evident in prostate cancer cells on treatment with 1. These results suggest that activation of Cdc2 leads to arrest in the M phase, with abnormal duplication, and initiation of mitotic catastrophe that results in cell death. In conclusion, these results show clearly the potential of 1 as a regulator of the G2/M phase of the cell cycle and as a therapeutic agent for prostate cancer.

Clinical significance of microRNA-148a in patients with early relapse of stage II stage and III colorectal cancer after curative resection

Development of robust prognostic/predictive biomarkers in patients with colorectal cancer (CRC) is imperative for advancing treatment strategies for this disease. We aimed to determine whether expression status of microRNAs might be a simple and reliable biomarker to detect postoperative early relapse in patients with CRC after radical resection. We used microRNA arrays and identified that microRNA-148a (miRNA-148a) had substantially different expression levels in early and nonearly relapsed stage II and III CRC tissues. The validation study, which included 55 early relapsed patients and 55 nonearly relapsed patients, further confirmed overexpression of miRNA-148a in nonearly relapsed samples. Subsequently, we explored whether the serum level of miRNA-148a can be used to predict early CRC recurrence. The in vitro and in vivo effects of miRNA-148a were examined by cell proliferation, migration, and invasion, as well as cell cycles, and xenograft in null mice. Last, miRNA-148a was investigated as a potential biomarker for identifying early relapse. Cellular studies demonstrated that the overexpression of miRNA-148a inhibited colon cancer cell proliferation and migration but not invasion. The cell cycle studies also revealed that miRNA-148a caused an accumulation of the G2 population. Moreover, lower levels of miRNA-148a expression were associated with significantly shorter disease-free survival rates and poorer overall survival rates. This study showed that miRNA-148a can inhibit tumorigenesis and reduce the early recurrence of CRC. These findings suggest that miRNA-148a may have potential clinical applications for predicting the early relapse of patients with CRC after radical resection.

The epithelium specific cell cycle regulator 14-3-3sigma is required for preventing entry into mitosis following ultraviolet B

BACKGROUND

Deoxyribonucleic acid damage activates cell cycle checkpoints in order to maintain genomic stability. We assessed the role of different checkpoint genes in response to ultraviolet B irradiation.

METHODS

Cell lines expressing a dominant negative mutant of ataxia telangiectasia and Rad3 related (Atr) protein or overexpressing Cdc25A, cells deficient for 14-3-3σ, Nijmegen breakage syndrome (Nbs), or Ataxia telangiectasia mutated (Atm) were treated with ultraviolet B (UVB) and harvested after 12 h, 24 h, or 48 h for analysis by flow cytometry.

RESULTS

Functional loss of Atm, Atr, or Nbs did not result in a significant alteration of the cell cycle profile. Overexpression of Cdc25A led to a delayed arrest at the G1/S transition in response to low doses of UVB. Loss of 14-3-3σ, a negative cell cycle regulator and downstream target of p53, caused a transient arrest at the G2/M boundary.

CONCLUSIONS

Loss of 14-3-3σ sensitizes cells to UVB. After a transient cell cycle arrest, 14-3-3σ-deficient cells die by undergoing mitotic catastrophe. Cdc25A overexpression causes a delayed arrest in response to low doses of UVB. After higher doses, Cdc25A is no longer able to overrun the checkpoint. Atm, Atr, or Nbs are not essential for the checkpoint response to UVB, suggesting the existence of redundant signaling pathways.

Cell cycle-dependent localization of CHK2 at centrosomes during mitosis

Background

Centrosomes function primarily as microtubule-organizing centres and play a crucial role during mitosis by organizing the bipolar spindle. In addition to this function, centrosomes act as reaction centers where numerous key regulators meet to control cell cycle progression. One of these factors involved in genome stability, the checkpoint kinase CHK2, was shown to localize at centrosomes throughout the cell cycle.

Results

Here, we show that CHK2 only localizes to centrosomes during mitosis. Using wild-type and CHK2^−/− HCT116 human colon cancer cells and human osteosarcoma U2OS cells depleted for CHK2 with small hairpin RNAs we show that several CHK2 antibodies are non-specific and cross-react with an unknown centrosomal protein(s) by immunofluorescence. To characterize the localization of CHK2, we generated cells expressing inducible GFP-CHK2 and Flag-CHK2 fusion proteins. We show that CHK2 localizes to the nucleus in interphase cells but that a fraction of CHK2 associates with the centrosomes in a Polo-like kinase 1-dependent manner during mitosis, from early mitotic stages until cytokinesis.

Conclusion

Our findings demonstrate that a subpopulation of CHK2 localizes at the centrosomes in mitotic cells but not in interphase. These results are consistent with previous reports supporting a role for CHK2 in the bipolar spindle formation and the timely progression of mitosis.

Role of cyclin B1 levels in DNA damage and DNA damage-induced senescence

The cyclin B1-Cdk1 complex is a key regulator of mitotic entry. A large number of proteins are phosphorylated by the cyclin B1-Cdk1 complex prior to mitotic entry. Regulation of the mitotic events is linked to the control of the activity of the cyclin B1-Cdk1 complex to make cells enter mitosis, arrest at G2-phase, or skip mitosis. The roles of cyclin B1 levels in DNA damage are described. The ATM/ATR pathway acts as a molecular switch for regulating cell fates, flipping between cell death via progress into mitosis and polyploidization via sustained G2 arrest upon DNA damage, where cyclin B1 degradation is important for inducing polyploidization. The decrease in cyclin B1 levels that is induced by DNA damage leads to polyploidization in DNA damage-induced senescence. A useful method for monitoring the expression level of cyclin B1 throughout cell cycle progression in living cells is also presented.

Novel interactions between FOXM1 and CDC25A regulate the cell cycle

FOXM1 is a critical regulator of the G1/S and G2/M cell cycle transitions, as well as of the mitotic spindle assembly. Previous studies have suggested that FOXM1 regulates CDC25A gene transcription, but the mechanism remains unknown. Here, we provide evidence that FOXM1 directly regulates CDC25A gene transcription via direct promoter binding and indirect activation of E2F-dependent pathways. Prior literature reported that CDC25B and CDC25C activate CDK1/cyclinB complexes in order to enable phosphorylation of FOXM1. It was unknown if CDC25A functions in a similar manner. We report that FOXM1 transcriptional activity is synergistically enhanced when co-expressed with CDC25A. The increase is dependent upon CDK1 phosphorylation of FOXM1 at T600, T611 and T620 residues. We also report a novel protein interaction between FOXM1 and CDC25A via the C-terminus of FOXM1. We demonstrate that the phosphorylation of Thr 600 and Thr 611 residues of FOXM1 enhanced this interaction, and that the interaction is dependent upon CDC25A phosphatase activity. Our work provides novel insight into the underlying mechanisms by which FOXM1 controls the cell cycle through its association with CDC25A.

Pathways for genome integrity in G2 phase of the cell cycle

The maintenance of genome integrity is important for normal cellular functions, organism development and the prevention of diseases, such as cancer. Cellular pathways respond immediately to DNA breaks leading to the initiation of a multi-facetted DNA damage response, which leads to DNA repair and cell cycle arrest. Cell cycle checkpoints provide the cell time to complete replication and repair the DNA damage before it can continue to the next cell cycle phase. The G2/M checkpoint plays an especially important role in ensuring the propagation of error-free copies of the genome to each daughter cell. Here, we review recent progress in our understanding of DNA repair and checkpoint pathways in late S and G2 phases. This review will first describe the current understanding of normal cell cycle progression through G2 phase to mitosis. It will also discuss the DNA damage response including cell cycle checkpoint control and DNA double-strand break repair. Finally, we discuss the emerging concept that DNA repair pathways play a major role in the G2/M checkpoint pathway thereby blocking cell division as long as DNA lesions are present.

Doxorubicin promotes transcriptional upregulation of Cdc25B in cancer cells by releasing Sp1 from the promoter

Cdc25B phosphatases have a key role in G2/M cell-cycle progression by activating the CDK1-cyclinB1 complexes and functioning as important targets of checkpoints. Overexpression of Cdc25B results in a bypass of the G2/M checkpoint and illegitimate entry into mitosis. It can also cause replicative stress, which leads to genomic instability. Thus, fine-tuning of the Cdc25B expression level is critical for correct cell-cycle arrest in response to DNA damage. In response to genotoxic stress, Cdc25B is mainly regulated by post-transcriptional mechanisms affecting either Cdc25B protein stability or translation. Here, we show that upon DNA damage Cdc25B can be regulated at the transcriptional level. Although ionizing radiation downregulates Cdc25B in a p53-dependent pathway, doxorubicin transcriptionally upregulates Cdc25B in p53-proficient cancer cells. We show that in the presence of wild-type p53, doxorubicin activates the Cdc25B promoter by preventing the binding of Sp1 and increasing the binding of NF-Y on the Cdc25B promoter, thus preventing p53 from downregulating this promoter. Our results highlight the mechanistically distinct regulation of the three Cdc25 phosphatases by checkpoint signalling following doxorubicin treatment.

MiR-210 disturbs mitotic progression through regulating a group of mitosis-related genes

MiR-210 is up-regulated in multiple cancer types but its function is disputable and further investigation is necessary. Using a bioinformatics approach, we identified the putative target genes of miR-210 in hypoxia-induced CNE cells from genome-wide scale. Two functional gene groups related to cell cycle and RNA processing were recognized as the major targets of miR-210. Here, we investigated the molecular mechanism and biological consequence of miR-210 in cell cycle regulation, particularly mitosis. Hypoxia-induced up-regulation of miR-210 was highly correlated with the down-regulation of a group of mitosis-related genes, including Plk1, Cdc25B, Cyclin F, Bub1B and Fam83D. MiR-210 suppressed the expression of these genes by directly targeting their 3'-UTRs. Over-expression of exogenous miR-210 disturbed mitotic progression and caused aberrant mitosis. Furthermore, miR-210 mimic with pharmacological doses reduced tumor formation in a mouse metastatic tumor model. Taken together, these results implicate that miR-210 disturbs mitosis through targeting multi-genes involved in mitotic progression, which may contribute to its inhibitory role on tumor formation.

CDC-25.1 controls the rate of germline mitotic cell cycle by counteracting WEE-1.3 and by positively regulating CDK-1 in Caenorhabditis elegans

In Caenorhabditis elegans, cdc-25.1 loss-of-function mutants display a lack of germline proliferation. We found that the proliferation defect of cdc-25.1 mutants was suppressed by wee-1.3 RNAi. Further, among the seven cdk and seven cyclin homologs examined, cdk-1 and cyb-3 RNAi treatment caused the most severe germline proliferation defects in an rrf-1 mutant background, which were similar to those of the cdc-25.1 mutants. In addition, while RNAi of cyd-1 and cye-1 caused significant germline proliferation defects, RNAi of cdk-2 and cdk-4 did not. Compared with the number of germ nuclei in wee-1.3(RNAi) worms, the number in wee-1.3(RNAi);cdk-1(RNAi) and wee-1.3(RNAi);cyb-3(RNAi) worms further decreased to the level of cdk-1(RNAi) and cyb-3(RNAi) worms, respectively, indicating that cdk-1 and cyb-3 are epistatic and function downstream of cdc-25.1 and wee-1.3 in the control of the cell cycle. BrdU labeling of adult worms showed that, while 100% of the wild-type germ nuclei in the mitotic region incorporated BrdU when labeled for more than 12 h at 20°C, a small fraction of the cdc-25.1 mutant germ nuclei failed to incorporate BrdU even when labeled for 68 h. These results indicate that CDC-25.1 is required for maintaining proper rate of germline mitotic cell cycle. We propose that CDC-25.1 regulates the rate of germline mitotic cell cycle by counteracting WEE-1.3 and by positively controlling CDK-1, which forms a complex primarily with CYB-3, but also possibly with CYD-1 and CYE-1.

Loss of cyclin-dependent kinase 2 (CDK2) inhibitory phosphorylation in a CDK2AF knock-in mouse causes misregulation of DNA replication and centrosome duplication

Cyclin-dependent kinase 1 (CDK1) inhibitory phosphorylation controls the onset of mitosis and is essential for the checkpoint pathways that prevent the G(2)- to M-phase transition in cells with unreplicated or damaged DNA. To address whether CDK2 inhibitory phosphorylation plays a similar role in cell cycle regulation and checkpoint responses at the start of the S phase, we constructed a mouse strain in which the two CDK2 inhibitory phosphorylation sites, threonine 14 and tyrosine 15, were changed to alanine and phenylalanine, respectively (CDK2AF). This approach showed that inhibitory phosphorylation of CDK2 had a major role in controlling cyclin E-associated kinase activity and thus both determined the timing of DNA replication in a normal cell cycle and regulated centrosome duplication. Further, DNA damage in G(1) CDK2AF cells did not downregulate cyclin E-CDK2 activity when the CDK inhibitor p21 was also knocked down. We were surprised to find that this was insufficient to cause cells to bypass the checkpoint and enter the S phase. This led to the discovery of two previously unrecognized pathways that control the activity of cyclin A at the G(1) DNA damage checkpoint and may thereby prevent S-phase entry even when cyclin E-CDK2 activity is deregulated.

Translationally controlled tumor protein induces mitotic defects and chromosome missegregation in hepatocellular carcinoma development

Emerging evidence implicates the chromodomain helicase/ATPase DNA binding protein 1-like gene (CHD1L) as a specific oncogene in human hepatocellular carcinoma (HCC). To better understand the molecular mechanisms underlying HCC cases carrying CHD1L amplification (>50% HCCs), we identified a CHD1L target, translationally controlled tumor protein (TCTP), and investigated its role in HCC progression. Here, we report that CHD1L protein directly binds to the promoter region (nt -733 to -1,027) of TCTP and activates TCTP transcription. Overexpression of TCTP was detected in 40.7% of human HCC samples analyzed and positively correlated with CHD1L overexpression. Clinically, overexpression of TCTP was significantly associated with the advanced tumor stage (P = 0.037) and overall survival time of HCC patients (P = 0.034). In multivariate analyses, TCTP was determined to be an independent marker associated with poor prognostic outcomes. In vitro and in vivo functional studies in mice showed that TCTP has tumorigenic abilities, and overexpression of TCTP induced by CHD1L contributed to the mitotic defects of tumor cells. Further mechanistic studies demonstrated that TCTP promoted the ubiquitin-proteasome degradation of Cdc25C during mitotic progression, which caused the failure in the dephosphorylation of Cdk1 on Tyr15 and decreased Cdk1 activity. As a consequence, the sudden drop of Cdk1 activity in mitosis induced a faster mitotic exit and chromosome missegregation, which led to chromosomal instability. The depletion experiment proved that the tumorigenicity of TCTP was linked to its role in mitotic defects.

CONCLUSION

Collectively, we reveal a novel molecular pathway (CHD1L/TCTP/Cdc25C/Cdk1), which causes the malignant transformation of hepatocytes with the phenotypes of accelerated mitotic progression and the production of aneuploidy.

The Renaissance or the cuckoo clock

'…in Italy, for thirty years under the Borgias, they had warfare, terror, murder and bloodshed, but they produced Michelangelo, Leonardo da Vinci and the Renaissance. In Switzerland, they had brotherly love, they had five hundred years of democracy and peace-and what did that produce? The cuckoo clock'. Orson Welles as Harry Lime: The Third Man. Orson Welles might have been a little unfair on the Swiss, after all cuckoo clocks were developed in the Schwartzwald, but, more importantly, Swiss democracy gives remarkably stable government with considerable decision-making at the local level. The alternative is the battling city-states of Renaissance Italy: culturally rich but chaotic at a higher level of organization. As our understanding of the cell cycle improves, it appears that the cell is organized more along the lines of Switzerland than Renaissance Italy, and one major challenge is to determine how local decisions are made and coordinated to produce the robust cell cycle mechanisms that we observe in the cell as a whole.

Transcriptional repression of Cdc25B by IER5 inhibits the proliferation of leukemic progenitor cells through NF-YB and p300 in acute myeloid leukemia

The immediately-early response gene 5 (IER5) has been reported to be induced by γ-ray irradiation and to play a role in the induction of cell death caused by radiation. We previously identified IER5 as one of the 2,3,4-tribromo-3-methyl-1-phenylphospholane 1-oxide (TMPP)-induced transcriptional responses in AML cells, using microarrays that encompassed the entire human genome. However, the biochemical pathway and mechanisms of IER5 function in regulation of the cell cycle remain unclear. In this study, we investigated the involvement of IER5 in the cell cycle and in cell proliferation of acute myeloid leukemia (AML) cells. We found that the over-expression of IER5 in AML cell lines and in AML-derived ALDH^hi (High Aldehyde Dehydrogenase activity)/CD34⁺ cells inhibited their proliferation compared to control cells, through induction of G2/M cell cycle arrest and a decrease in Cdc25B expression. Moreover, the over-expression of IER5 reduced colony formation of AML-derived ALDH^hi/CD34⁺ cells due to a decrease in Cdc25B expression. In addition, over-expression of Cdc25B restored TMPP inhibitory effects on colony formation in IER5-suppressed AML-derived ALDH^hi/CD34⁺ cells. Furthermore, the IER5 reduced Cdc25B mRNA expression through direct binding to Cdc25B promoter and mediated its transcriptional attenuation through NF-YB and p300 transcriptinal factors. In summary, we found that transcriptional repression mediated by IER5 regulates Cdc25B expression levels via the release of NF-YB and p300 in AML-derived ALDH^hi/CD34⁺ cells, resulting in inhibition of AML progenitor cell proliferation through modulation of cell cycle. Thus, the induction of IER5 expression represents an attractive target for AML therapy.

Cyclin B1 expression regulated by cytoplasmic polyadenylation element binding protein in astrocytes

Astrocytes are the most abundant cells in the brain, playing vital roles in neuronal survival, growth, and function. Understanding the mechanism(s) regulating astrocyte proliferation will have important implications in brain development, response to injury, and tumorigenesis. Cyclin B1 is well known to be a critical regulator of mitotic entry via its interaction with cyclin-dependent kinase 1. In rat astrocytes, we now show that the mRNA binding protein cytoplasmic polyadenylation element binding protein 1 (CPEB1) is associated with cyclin B1 mRNA and that this interaction is enriched at the centrosome. In addition, if growth-arrested astrocytes are stimulated to divide, CPEB1 is phosphorylated and cyclin B1 mRNA is polyadenylated, both hallmarks of CPEB1 activation, resulting in an increase in cyclin B1 protein. CPEB1 binding to mRNA initially inhibits translation; therefore, removing CPEB1 from mRNA should result in an increase in translation due to derepression. Indeed, when we either knocked down CPEB1 protein with siRNA or sequestered it from endogenous mRNA by expressing RNA containing multiple CPEB1 binding sites, cyclin B1 protein was increased and cell proliferation was stimulated. Our data suggest a mechanism wherein CPEB1 is bound and represses cyclin B1 mRNA translation until a signal to proliferate phosphorylates CPEB1, resulting in an increase in cyclin B1 protein and progression into mitosis. Our results demonstrate for the first time a role for CPEB1 in regulating cell proliferation in the brain.

MicroRNA-148a is down-regulated in human pancreatic ductal adenocarcinomas and regulates cell survival by targeting CDC25B

MicroRNAs (miRNAs: short non-coding RNAs) are emerging as a class of potential novel tumor markers, as their dysregulation is being increasingly reported in various types of cancers. In the present study, we investigated the transcription status of miRNA-148a (miR-148a) in human pancreatic ductal adenocarcinoma (PDAC) and its role in the regulation of the dual specificity protein phosphatase CDC25B. We observed that miR-148a exhibited a significant 4-fold down-regulation in PDAC as opposed to normal pancreatic ductal cells. In addition, we observed that stable lentiviral-mediated overexpression of miR-148a in the pancreatic cancer cell line IMIM-PC2, inhibited tumor cell growth and colony formation. Furthermore, CDC25B was identified as a potential target of miR-148a by in silico analysis using PicTar, Targetscan and miRanda in conjunction with gene ontology analysis. The proposed interaction between miR-148a and the 3' untranslated region (UTR) of CDC25B was verified by in-vitro luciferase assays. We demonstrate that the activity of a luciferase reporter containing the 3'UTR of CDC25B was repressed in the presence of miR-148a mimics, confirming that miR-148a targets the 3'UTR of CDC25B. Finally, CDC25B was down-regulated at the protein level in miR-148a overexpressing IMIM-PC2-cells, and in transiently transfected pancreatic cell lines (as detected by Western blot analysis), as well as in patient tumor samples (as detected by immunohistochemistry). In summary, we identified CDC25B as a novel miR-148a target which may confer a proliferative advantage in PDAC.

Nuclear Chk1 prevents premature mitotic entry

Chk1 inhibits the premature activation of the cyclin-B1-Cdk1. However, it remains controversial whether Chk1 inhibits Cdk1 in the centrosome or in the nucleus before the G2-M transition. In this study, we examined the specificity of the mouse monoclonal anti-Chk1 antibody DCS-310, with which the centrosome was stained. Conditional Chk1 knockout in mouse embryonic fibroblasts reduced nuclear but not centrosomal staining with DCS-310. In Chk1(+/myc) human colon adenocarcinoma (DLD-1) cells, Chk1 was detected in the nucleus but not in the centrosome using an anti-Myc antibody. Through the combination of protein array and RNAi technologies, we identified Ccdc-151 as a protein that crossreacted with DCS-310 on the centrosome. Mitotic entry was delayed by expression of the Chk1 mutant that localized in the nucleus, although forced immobilization of Chk1 to the centrosome had little impact on the timing of mitotic entry. These results suggest that nuclear but not centrosomal Chk1 contributes to correct timing of mitotic entry.

Influence of proline-rich inositol polyphosphate 5-phosphatase, on early development of fertilized mouse eggs, via inhibition of phosphorylation of Akt

OBJECTIVES

Proline-rich inositol polyphosphate 5-phosphatase (PIPP) is one of the signal-modifying enzymes that play pivotal regulatory roles in PI3K signalling pathway. The aim of this study was to determine the role of PIPP in early development of fertilized mouse eggs, via inhibition of Akt activity and subsequent downstream signalling events.

MATERIALS AND METHODS

The mRNA transcript levels of endogenous PIPP and Akt1, Akt2, Akt3 were detected in G(1) , S, G(2) and M phases of fertilized mouse eggs by RT-PCR. Levels of exogenous PIPP, phosphorylated Akt at Ser473, dephosphorylated cdc2 at Tyr15 and levels of CCNB1, were detected respectively by immunoblotting. Changes in Akt localization were observed by fluoroimmunoassay; meanwhile, changes in activity of Akt and its downstream MPF were detected. Percentages of cells undergoing division were determined by counting, using a dissecting microscope.

RESULTS

PIPP and Akt1 transcripts were detectable in G(1), S, G(2) and M phases of fertilized mouse eggs, but Akt2 and Akt3 were not. We also observed that overexpression of PIPP in fertilized eggs decreased expression of phosphorylated Akt at Ser473 and altered membrane localization of phosphorylated Akt at Ser473 specifically. Furthermore, overexpression of PIPP resulted in decreases in mitosis-phase promoting factor activity, level of dephosphorylated cdc2 at Tyr15 and cleavage rate of fertilized mouse eggs.

CONCLUSIONS

Our data suggest, for the first time, that PIPP may affect development of fertilized mouse eggs by inhibition of level of phosphorylated Akt at Ser473 and subsequent inhibition of downstream signal cascades.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

Identification of N-terminally truncated stable nuclear isoforms of CDC25B that are specifically involved in G2/M checkpoint recovery

CDC25B phosphatases must activate cyclin B-CDK1 complexes to restart the cell cycle after an arrest in G2 phase caused by DNA damage. However, little is known about the precise mechanisms involved in this process, which may exert considerable impact on cancer susceptibility and therapeutic responses. Here we report the discovery of novel N-terminally truncated CDC25B isoforms, referred to as ΔN-CDC25B, with an exclusively nuclear and nonredundant function in cell cycle re-initiation after DNA damage. ΔN-CDC25B isoforms are expressed from a distinct promoter not involved in expression of canonical full-length isoforms. Remarkably, in contrast to the high lability and spatial dynamism of the full-length isoforms, ΔN-CDC25B isoforms are highly stable and exclusively nuclear, strongly suggesting the existence of two pools of CDC25B phosphatases in the cell that have functionally distinct properties. Using isoform-specific siRNA, we found that depleting full-length isoforms, but not ΔN-CDC25B isoforms, delays entry into mitosis. Thus, in an unperturbed cell cycle, the full-length isoforms are exclusively responsible for activating cyclin B-CDK1. Strikingly, in the late response to DNA damage, we found a CHK1-dependent shift in accumulation of CDC25B isoforms toward the ΔN-CDC25B species. Under this physiological stress condition, the ΔN-CDC25B isoform was found to play a crucial, nonredundant function in restarting the cell cycle after DNA damage-induced G2 phase arrest. Our findings reveal the existence of a previously unrecognized CDC25B isoform that operates specifically in the nucleus to reinitiate G2/M transition after DNA damage.

Human Cdc14B promotes progression through mitosis by dephosphorylating Cdc25 and regulating Cdk1/cyclin B activity

Entry into and progression through mitosis depends on phosphorylation and dephosphorylation of key substrates. In yeast, the nucleolar phosphatase Cdc14 is pivotal for exit from mitosis counteracting Cdk1-dependent phosphorylations. Whether hCdc14B, the human homolog of yeast Cdc14, plays a similar function in mitosis is not yet known. Here we show that hCdc14B serves a critical role in regulating progression through mitosis, which is distinct from hCdc14A. Unscheduled overexpression of hCdc14B delays activation of two master regulators of mitosis, Cdc25 and Cdk1, and slows down entry into mitosis. Depletion of hCdc14B by RNAi prevents timely inactivation of Cdk1/cyclin B and dephosphorylation of Cdc25, leading to severe mitotic defects, such as delay of metaphase/anaphase transition, lagging chromosomes, multipolar spindles and binucleation. The results demonstrate that hCdc14B-dependent modulation of Cdc25 phosphatase and Cdk1/cyclin B activity is tightly linked to correct chromosome segregation and bipolar spindle formation, processes that are required for proper progression through mitosis and maintenance of genomic stability.

Cdc25B is negatively regulated by p53 through Sp1 and NF-Y transcription factors

Cdc25B phosphatases function as key players in G2/M cell cycle progression by activating the CDK1-cyclinB1 complexes. They also have an essential role in recovery from the G2/M checkpoint activated in response to DNA damage. Overexpression of Cdc25B results in bypass of the G2/M checkpoint and illegitimate entry into mitosis, and also causes replicative stress, leading to genomic instability. Thus, fine-tuning of Cdc25B expression level is critical for correct cell cycle progression and G2 checkpoint recovery. However, the transcriptional regulation of Cdc25B remains largely unknown. Earlier studies have shown that the tumor suppressor p53 overexpression transcriptionally represses Cdc25B; however, the molecular mechanism of this repression has not yet been elucidated, although it was suggested to occur through the induction of p21. Here we show that Cdc25B is downregulated by the basal level of p53 in multiple cell types. This downregulation also occurs in p21-/- cell lines, indicating that p21 is not required for p53-mediated regulation of Cdc25B. Deletion and mutation analyses of the Cdc25B promoter revealed that downregulation by p53 is dependent on the presence of functional Sp1/Sp3 and NF-Y binding sites. Furthermore, chromatin immunoprecipitation analyses show that p53 binds to the Cdc25B promoter and mediates transcriptional attenuation through the Sp1 and NF-Y transcription factors. Our results suggest that the inability to downregulate Cdc25B after loss of p53 might contribute to tumorigenesis.

Spatiotemporal regulations of Wee1 at the G2/M transition

Wee1 is highly dynamic at the SPB during the G2/M transition. Wee1 accumulates at the nuclear face of the SPB when cyclin B–Cdc2 peaks at the SPB and disappears from the SPB during spindle assembly. This dynamic behavior of Wee1 at the SPB is important for regulation of cyclin B–Cdc2 activity and proper mitotic entry and progression.

Wee1 is a protein kinase that negatively regulates mitotic entry in G2 phase by suppressing cyclin B–Cdc2 activity, but its spatiotemporal regulations remain to be elucidated. We observe the dynamic behavior of Wee1 in Schizosaccharomyces pombe cells and manipulate its localization and kinase activity to study its function. At late G2, nuclear Wee1 efficiently suppresses cyclin B–Cdc2 around the spindle pole body (SPB). During the G2/M transition when cyclin B–Cdc2 is highly enriched at the SPB, Wee1 temporally accumulates at the nuclear face of the SPB in a cyclin B–Cdc2-dependent manner and locally suppresses both cyclin B–Cdc2 activity and spindle assembly to counteract a Polo kinase–dependent positive feedback loop. Then Wee1 disappears from the SPB during spindle assembly. We propose that regulation of Wee1 localization around the SPB during the G2/M transition is important for proper mitotic entry and progression.

A general framework for modeling growth and division of mammalian cells

BACKGROUND

Modeling the cell-division cycle has been practiced for many years. As time has progressed, this work has gone from understanding the basic principles to addressing distinct biological problems, e.g., the nature of the restriction point, how checkpoints operate, the nonlinear dynamics of the cell cycle, the effect of localization, etc. Most models consist of coupled ordinary differential equations developed by the researchers, restricted to deal with the interactions of a limited number of molecules. In the future, cell-cycle modeling--and indeed all modeling of complex biologic processes--will increase in scope and detail.

RESULTS

A framework for modeling complex cell-biologic processes is proposed here. The framework is based on two constructs: one describing the entire lifecycle of a molecule and the second describing the basic cellular machinery. Use of these constructs allows complex models to be built in a straightforward manner that fosters rigor and completeness. To demonstrate the framework, an example model of the mammalian cell cycle is presented that consists of several hundred differential equations of simple mass action kinetics. The model calculates energy usage, amino acid and nucleotide usage, membrane transport, RNA synthesis and destruction, and protein synthesis and destruction for 33 proteins to give an in-depth look at the cell cycle.

CONCLUSIONS

The framework presented here addresses how to develop increasingly descriptive models of complex cell-biologic processes. The example model of cellular growth and division constructed with the framework demonstrates that large structured models can be created with the framework, and these models can generate non-trivial descriptions of cellular processes. Predictions from the example model include those at both the molecular level--e.g., Wee1 spontaneously reactivates--and at the system level--e.g., pathways for timing-critical processes must shut down redundant pathways. A future effort is to automatically estimate parameter values that are insensitive to changes.

Protein kinases and protein phosphatases that regulate meiotic maturation in mouse oocytes

Oocytes arrest at prophase of meiosis I (MI) and in vivo do not resume meiosis until they receive ovulatory cues. Meiotic resumption entails two rounds of chromosome segregation without an intervening round of DNA replication and an arrest at metaphase of meiosis II (MII); fertilization triggers exit from MII and entry into interphase. During meiotic resumption, there is a burst of protein phosphorylation and dephosphorylation that dramatically changes during the course of oocyte meiotic maturation. Many of these phosphorylation and dephosphorylation events are key to regulating meiotic cell cycle arrest and/or progression, chromosome dynamics, and meiotic spindle assembly and disassembly. This review, which is subdivided into sections based upon meiotic cell cycle stages, focuses on the major protein kinases and phosphatases that have defined requirements during meiosis in mouse oocytes and, when possible, connects these regulatory pathways.

Human Cdc14A phosphatase modulates the G2/M transition through Cdc25A and Cdc25B

The Cdc14 family of serine-threonine phosphatases antagonizes CDK activity by reversing CDK-dependent phosphorylation events. It is well established that the yeast members of this family bring about the M/G1 transition. Budding yeast Cdc14 is essential for CDK inactivation at the end of mitosis and fission yeast Cdc14 homologue Flp1/Clp1 down-regulates Cdc25 to ensure the inactivation of mitotic CDK complexes to trigger cell division. However, the functions of human Cdc14 homologues remain poorly understood. Here we have tested the hypothesis that Cdc14A might regulate Cdc25 mitotic inducers in human cells. We found that increasing levels of Cdc14A delay entry into mitosis by inhibiting Cdk1-cyclin B1 activity. By contrast, lowering the levels of Cdc14A accelerates mitotic entry. Biochemical analyses revealed that Cdc14A acts through key Cdk1-cyclin B1 regulators. We observed that Cdc14A directly bound to and dephosphorylated Cdc25B, inhibiting its catalytic activity. Cdc14A also regulated the activity of Cdc25A at the G2/M transition. Our results indicate that Cdc14A phosphatase prevents premature activation of Cdk1 regulating Cdc25A and Cdc25B at the entry into mitosis.

Mitotic phosphorylation of Cdc25B Ser321 disrupts 14-3-3 binding to the high affinity Ser323 site

Cdc25B is a key regulator of entry into mitosis, and its activity and localization are regulated by binding of the 14-3-3 dimer. There are three 14-3-3 binding sites on Cdc25B, with Ser(323) being the highest affinity binding and is highly homologous to the Ser(216) 14-3-3 binding site on Cdc25C. Loss of 14-3-3 binding to Ser(323) increases cyclin/Cdk substrate access to the catalytic site, thereby increasing its activity. It also affects the localization of Cdc25B. Thus, phosphorylation and 14-3-3 binding to this site is essential for down-regulating Cdc25B activity, blocking its mitosis promoting function. The question of how this inhibitory signal is relieved to allow Cdc25B activation and entry into mitosis is yet to be resolved. Here, we show that Ser(323) phosphorylation is maintained into mitosis, but phosphorylation of Ser(321) disrupts 14-3-3 binding to Ser(323), mimicking the effect of inhibiting Ser(323) phosphorylation on both Cdc25B activity and localization. The unphosphorylated Ser(321) appears to have a role in stabilizing 14-3-3 binding to Ser(323), and loss of the Ser hydroxyl group appears to be sufficient to significantly reduce 14-3-3 binding. A consequence of loss of 14-3-3 binding is dephosphorylation of Ser(323). Ser(321) is phosphorylated in mitosis by Cdk1. The mitotic phosphorylation of Ser(321) acts to maintain full activation of Cdc25B by disrupting 14-3-3 binding to Ser(323) and enhancing the dephosphorylation of Ser(323) to block 14-3-3 binding to this site.

Gene expression profiling of monkeypox virus-infected cells reveals novel interfaces for host-virus interactions

Monkeypox virus (MPV) is a zoonotic Orthopoxvirus and a potential biothreat agent that causes human disease with varying morbidity and mortality. Members of the Orthopoxvirus genus have been shown to suppress antiviral cell defenses, exploit host cell machinery, and delay infection-induced cell death. However, a comprehensive study of all host genes and virus-targeted host networks during infection is lacking. To better understand viral strategies adopted in manipulating routine host biology on global scale, we investigated the effect of MPV infection on Macaca mulatta kidney epithelial cells (MK2) using GeneChip rhesus macaque genome microarrays. Functional analysis of genes differentially expressed at 3 and 7 hours post infection showed distinctive regulation of canonical pathways and networks. While the majority of modulated histone-encoding genes exhibited sharp copy number increases, many of its transcription regulators were substantially suppressed; suggesting involvement of unknown viral factors in host histone expression. In agreement with known viral dependence on actin in motility, egress, and infection of adjacent cells, our results showed extensive regulation of genes usually involved in controlling actin expression dynamics. Similarly, a substantial ratio of genes contributing to cell cycle checkpoints exhibited concerted regulation that favors cell cycle progression in G1, S, G2 phases, but arrest cells in G2 phase and inhibits entry into mitosis. Moreover, the data showed that large number of infection-regulated genes is involved in molecular mechanisms characteristic of cancer canonical pathways. Interestingly, ten ion channels and transporters showed progressive suppression during the course of infection. Although the outcome of this unusual channel expression on cell osmotic homeostasis remains unknown, instability of cell osmotic balance and membrane potential has been implicated in intracellular pathogens egress. Our results highlight the role of histones, actin, cell cycle regulators, and ion channels in MPV infection, and propose these host functions as attractive research focal points in identifying novel drug intervention sites.

Distinct pools of cdc25C are phosphorylated on specific TP sites and differentially localized in human mitotic cells

Background

The dual specificity phosphatase cdc25C was the first human cdc25 family member found to be essential in the activation of cdk1/cyclin B1 that takes place at the entry into mitosis. Human cdc25C is phosphorylated on Proline-dependent SP and TP sites when it becomes active at mitosis and the prevalent model is that this phosphorylation/activation of cdc25C would be part of an amplification loop with cdk1/cyclin B1.

Methodology/Principal Findings

Using highly specific antibodies directed against cdc25C phospho-epitopes, pT67 and pT130, we show here that these two phospho-forms of cdc25C represent distinct pools with differential localization during human mitosis. Phosphorylation on T67 occurs from prophase and the cdc25C-pT67 phospho-isoform closely localizes with condensed chromosomes throughout mitosis. The phospho-T130 form of cdc25C arises in late G2 and associates predominantly with centrosomes from prophase to anaphase B where it colocalizes with Plk1. As shown by immunoprecipitation of each isoform, these two phospho-forms are not simultaneously phosphorylated on the other mitotic TP sites or associated with one another. Phospho-T67 cdc25C co-precipitates with MPM2-reactive proteins while pT130-cdc25C is associated with Plk1. Interaction and colocalization of phosphoT130-cdc25C with Plk1 demonstrate in living cells, that the sequence around pT130 acts as a true Polo Box Domain (PBD) binding site as previously identified from in vitro peptide screening studies. Overexpression of non-phosphorylatable alanine mutant forms for each isoform, but not wild type cdc25C, strongly impairs mitotic progression showing the functional requirement for each site-specific phosphorylation of cdc25C at mitosis.

Conclusions/Significance

These results show for the first time that in human mitosis, distinct phospho-isoforms of cdc25C exist with different localizations and interacting partners, thus implying that the long-standing model of a cdc25C/cdk1 multi-site auto amplification loop is implausible.

Cell cycle G2/M arrest through an S phase-dependent mechanism by HIV-1 viral protein R

Background

Cell cycle G2 arrest induced by HIV-1 Vpr is thought to benefit viral proliferation by providing an optimized cellular environment for viral replication and by skipping host immune responses. Even though Vpr-induced G2 arrest has been studied extensively, how Vpr triggers G2 arrest remains elusive.

Results

To examine this initiation event, we measured the Vpr effect over a single cell cycle. We found that even though Vpr stops the cell cycle at the G2/M phase, but the initiation event actually occurs in the S phase of the cell cycle. Specifically, Vpr triggers activation of Chk1 through Ser³⁴⁵ phosphorylation in an S phase-dependent manner. The S phase-dependent requirement of Chk1-Ser³⁴⁵ phosphorylation by Vpr was confirmed by siRNA gene silencing and site-directed mutagenesis. Moreover, downregulation of DNA replication licensing factors Cdt1 by siRNA significantly reduced Vpr-induced Chk1-Ser³⁴⁵ phosphorylation and G2 arrest. Even though hydroxyurea (HU) and ultraviolet light (UV) also induce Chk1-Ser³⁴⁵ phosphorylation in S phase under the same conditions, neither HU nor UV-treated cells were able to pass through S phase, whereas vpr-expressing cells completed S phase and stopped at the G2/M boundary. Furthermore, unlike HU/UV, Vpr promotes Chk1- and proteasome-mediated protein degradations of Cdc25B/C for G2 induction; in contrast, Vpr had little or no effect on Cdc25A protein degradation normally mediated by HU/UV.

Conclusions

These data suggest that Vpr induces cell cycle G2 arrest through a unique molecular mechanism that regulates host cell cycle regulation in an S-phase dependent fashion.

Aurora kinases A and B are up-regulated by Myc and are essential for maintenance of the malignant state

Myc oncoproteins promote continuous cell growth, in part by controlling the transcription of key cell cycle regulators. Here, we report that c-Myc regulates the expression of Aurora A and B kinases (Aurka and Aurkb), and that Aurka and Aurkb transcripts and protein levels are highly elevated in Myc-driven B-cell lymphomas in both mice and humans. The induction of Aurka by Myc is transcriptional and is directly mediated via E-boxes, whereas Aurkb is regulated indirectly. Blocking Aurka/b kinase activity with a selective Aurora kinase inhibitor triggers transient mitotic arrest, polyploidization, and apoptosis of Myc-induced lymphomas. These phenotypes are selectively bypassed by a kinase inhibitor-resistant Aurkb mutant, demonstrating that Aurkb is the primary therapeutic target in the context of Myc. Importantly, apoptosis provoked by Aurk inhibition was p53 independent, suggesting that Aurka/Aurkb inhibitors will show efficacy in treating primary or relapsed malignancies having Myc involvement and/or loss of p53 function.

Prophase I arrest and progression to metaphase I in mouse oocytes: comparison of resumption of meiosis and recovery from G2-arrest in somatic cells

Mammalian oocytes are arrested at prophase I until puberty when luteinizing hormone (LH) induces resumption of meiosis of follicle-enclosed oocytes. Resumption of meiosis is tightly coupled with regulating cyclin-dependent kinase 1 (CDK1) activity. Prophase I arrest depends on inhibitory phosphorylation of CDK1 and anaphase-promoting complex-(APC-CDH1)-mediated regulation of cyclin B levels. Prophase I arrest is maintained by endogenously produced cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA) that in turn phosphorylates (and activates) the nuclear kinase WEE2. In addition, PKA-mediated phosphorylation of the phosphatase CDC25B results in its cytoplasmic retention. The combined effect maintains low levels of CDK1 activity that are not sufficient to initiate resumption of meiosis. LH triggers synthesis of epidermal growth factor-like factors in mural granulosa cells and leads to reduced cGMP transfer from cumulus cells to oocytes via gap junctions that couple the two cell types. cGMP inhibits oocyte phosphodiesterase 3A (PDE3A) and a decline in oocyte cGMP results in increased PDE3A activity. The ensuing decrease in oocyte cAMP triggers maturation by alleviating the aforementioned phosphorylations of WEE2 and CDC25B. As a direct consequence CDC25B translocates into the nucleus. The resulting activation of CDK1 also promotes extrusion of WEE2 from the nucleus thereby providing a positive amplification mechanism for CDK1 activation. Other kinases, e.g. protein kinase B, Aurora kinase A and polo-like kinase 1, also participate in resumption of meiosis. Mechanisms governing meiotic prophase I arrest and resumption of meiosis share common features with DNA damage-induced mitotic G2-checkpoint arrest and checkpoint recovery, respectively. These common features include CDC14B-dependent activation of APC-CDH1 in prophase I arrested oocytes or G2-arrested somatic cells, and CDC25B-dependent cell cycle resumption in both oocytes and somatic cells.

Quantitative reconstitution of mitotic CDK1 activation in somatic cell extracts

The regulation of mitotic entry in somatic cells differs from embryonic cells, yet it is only for embryonic cells that we have a quantitative understanding of this process. To gain a similar insight into somatic cells, we developed a human cell extract system that recapitulates CDK1 activation and nuclear envelope breakdown in response to mitotic cyclins. As cyclin B concentrations increase, CDK1 activates in a three-stage nonlinear response, creating an ordering of substrate phosphorylations. This response is established by dual regulatory feedback loops involving WEE1/MYT1, which impose a cyclin B threshold, and CDC25, which allows CDK1 to escape the WEE1/MYT1 inhibition. This system also exhibits a complex response to cyclin A. Cyclin A promotes WEE1 phosphorylation to weaken the negative feedback loop and primes mitotic entry through cyclin B. This observation explains the requirement of both cyclin A and cyclin B to initiate mitosis in somatic cells.

Cdc25 phosphatases are required for timely assembly of CDK1-cyclin B at the G2/M transition

Progression through mitosis requires the coordinated regulation of Cdk1 kinase activity. Activation of Cdk1 is a multistep process comprising binding of Cdk1 to cyclin B, relocation of cyclin-kinase complexes to the nucleus, activating phosphorylation of Cdk1 on Thr(161) by the Cdk-activating kinase (CAK; Cdk7 in metazoans), and removal of inhibitory Thr(14) and Tyr(15) phosphorylations. This dephosphorylation is catalyzed by the dual specific Cdc25 phosphatases, which occur in three isoforms in mammalian cells, Cdc25A, -B, and -C. We find that expression of Cdc25A leads to an accelerated G(2)/M phase transition. In Cdc25A-overexpressing cells, Cdk1 exhibits high kinase activity despite being phosphorylated on Tyr(15). In addition, Tyr(15)-phosphorylated Cdk1 binds more cyclin B in Cdc25A-overexpressing cells compared with control cells. Consistent with this observation, we demonstrate that in human transformed cells, Cdc25A and Cdc25B, but not Cdc25C phosphatases have an effect on timing and efficiency of cyclin-kinase complex formation. Overexpression of Cdc25A or Cdc25B promotes earlier assembly and activation of Cdk1-cyclin B complexes, whereas repression of these phosphatases by short hairpin RNA has a reverse effect, leading to a substantial decrease in amounts of cyclin B-bound Cdk1 in G(2) and mitosis. Importantly, we find that Cdc25A overexpression leads to an activation of Cdk7 and increase in Thr(161) phosphorylation of Cdk1. In conclusion, our data suggest that complex assembly and dephosphorylation of Cdk1 at G(2)/M is tightly coupled and regulated by Cdc25 phosphatases.

Unscheduled expression of CDC25B in S-phase leads to replicative stress and DNA damage

Background

CDC25B phosphatase is a cell cycle regulator that plays a critical role in checkpoint control. Up-regulation of CDC25B expression has been documented in a variety of human cancers, however, the relationships with the alteration of the molecular mechanisms that lead to oncogenesis still remain unclear. To address this issue we have investigated, in model cell lines, the consequences of unscheduled and elevated CDC25B levels.

Results

We report that increased CDC25B expression leads to DNA damage in the absence of genotoxic treatment. H2AX phosphorylation is detected in S-phase cells and requires active replication. We also report that CDC25B expression impairs DNA replication and results in an increased recruitment of the CDC45 replication factor onto chromatin. Finally, we observed chromosomal aberrations that are also enhanced upon CDC25B expression.

Conclusion

Overall, our results demonstrate that a moderate and unscheduled increase in CDC25B level, as observed in a number of human tumours, is sufficient to overcome the S-phase checkpoint efficiency thus leading to replicative stress and genomic instability.

MAPK pathway activation delays G2/M progression by destabilizing Cdc25B

Activation of the mitogen-activated protein kinase (MAPK) pathway by growth factors or phorbol esters during G(2) phase delays entry into mitosis; however, the role of the MAPK pathway during G(2)/M progression remains controversial. Here, we demonstrate that activation of the MAPK pathway with either epidermal growth factor or 12-O-tetradecanoylphorbol-13-acetate induces a G(2) phase delay independent of known G(2) phase checkpoint pathways but was specifically dependent on MAPK/extracellular signal-regulated kinase kinase (MEK1). Activation of MAPK signaling also blocked exit from a G(2) phase checkpoint arrest. Both the G(2) phase delay and blocked exit from the G(2) checkpoint arrest were mediated by the MEK1-dependent destabilization of the critical G(2)/M regulator cdc25B. Reintroduction of cdc25B overcame the MEK1-dependent G(2) phase delay. Thus, we have demonstrated a new function for MEK1 that controls G(2)/M progression by regulating the stability of cdc25B. This represents a novel mechanism by which factors that activate MAPK signaling can influence the timing of entry into mitosis, particularly exit from a G(2) phase checkpoint arrest.

Sequential phosphorylation of Nedd1 by Cdk1 and Plk1 is required for targeting of the gammaTuRC to the centrosome

Nedd1 is a new member of the gamma-tubulin ring complex (gammaTuRC) and targets the gammaTuRC to the centrosomes for microtubule nucleation and spindle assembly in mitosis. Although its role is known, its functional regulation mechanism remains unclear. Here we report that the function of Nedd1 is regulated by Cdk1 and Plk1. During mitosis, Nedd1 is firstly phosphorylated at T550 by Cdk1, which creates a binding site for the polo-box domain of Plk1. Then, Nedd1 is further phosphorylated by Plk1 at four sites: T382, S397, S637 and S426. The sequential phosphorylation of Nedd1 by Cdk1 and Plk1 promotes its interaction with gamma-tubulin for targeting the gammaTuRC to the centrosome and is important for spindle formation. Knockdown of Plk1 by RNAi decreases Nedd1 phosphorylation and attenuates Nedd1 accumulation at the spindle pole and subsequent gamma-tubulin recruitment at the spindle pole for microtubule nucleation. Taken together, we propose that the sequential phosphorylation of Nedd1 by Cdk1 and Plk1 plays a pivotal role in targeting gammaTuRC to the centrosome by promoting the interaction of Nedd1 with the gammaTuRC component gamma-tubulin, during mitosis.

Functional dynamics of Polo-like kinase 1 at the centrosome

Polo-like kinase 1 (Plk1) functions as a key regulator of mitotic events by phosphorylating substrate proteins on centrosomes, kinetochores, the mitotic spindle, and the midbody. Through mechanisms that are incompletely understood, Plk1 is released from and relocalizes to different mitotic structures as cells proceed through mitosis. We used fluorescence recovery after photobleaching to examine the kinetics of this process in more detail. We observed that Plk1 displayed a range of different recovery rates that differ at each mitotic substructure and depend on both the Polo-box domain and a functional kinase domain. Upon mitotic entry, centrosomal Plk1 becomes more dynamic, a process that is directly enhanced by Plk1 kinase activity. In contrast, Plk1 displays little dynamic exchange at the midbody, a process that again is modulated by the kinase activity of Plk1. Our findings suggest that the intrinsic kinase activity of Plk1 triggers its release from early mitotic structures and its relocalization to late mitotic structures. To assess the importance of Plk1 dynamic relocalization, Plk1 was persistently tethered to the centrosome. This resulted in a G(2) delay, followed by a prominent prometaphase arrest, as a consequence of defective spindle formation and activation of the spindle checkpoint. The dynamic release of Plk1 from early mitotic structures is thus crucial for mid- to late-stage mitotic events and demonstrates the importance of a fully dynamic Plk1 at the centrosome for proper cell cycle progression. This dependence on dynamic Plk1 was further observed during the mitotic reentry of cells after a DNA damage G(2) checkpoint, as this process was significantly delayed upon centrosomal tethering of Plk1. These results indicate that mitotic progression and control of mitotic reentry after DNA damage resides, at least in part, on the dynamic behavior of Plk1.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

The polo-like kinase 1 regulates CDC25B-dependent mitosis entry

Activation of cyclin-dependent kinase complexes (CDK) at key cell cycle transitions is dependent on their dephosphorylation by CDC25 dual-specificity phosphatases (CDC25A, B and C in human). The CDC25B phosphatase plays an essential role in controlling the activity of CDK1-cyclin B complexes at the entry into mitosis and together with polo-like kinase 1 (PLK1) in regulating the resumption of cell cycle progression after DNA damage-dependent checkpoint arrest in G2. In this study, we analysed the regulation of CDC25B-dependent mitosis entry by PLK1. We demonstrate that PLK1 activity is essential for the relocation of CDC25B from the cytoplasm to the nucleus. By gain and loss of function analyses, we show that PLK1 stimulates CDC25B-induced mitotic entry in both normal conditions and after DNA-damage induced G2/M arrest. Our results support a model in which the relocalisation of CDC25B to the nucleus at the G2-M transition by PLK1 regulates its mitotic inducing activity.

In vivo roles of CDC25 phosphatases: biological insight into the anti-cancer therapeutic targets

CDC25 phosphatases are not only rate-limiting activators of cyclin-dependent kinases (CDKs) but also important targets of the CHK1/CHK2-mediated checkpoint pathway. Each isoform of the mammalian CDC25 family seems to exert unique biological functions. CDC25A is a critical regulator for both G1-S and G2-M transitions and essential for embryonic cell proliferation after the blastocyst stage. CDC25B is dispensable for embryogenesis but required for meiotic progression of oocytes in a manner analogous to Drosophila Twine or C. elegans cdc-25.1. Moreover, CDC25A and CDC25B appear to regulate different events or stages of mitosis. CDC25B may mediate the activation of CDK1/Cyclin B at the centrosome during prophase, while CDC25A may be required for the subsequent full activation of nuclear CDK1/Cyclin B. CDC25C is dispensable for both mitotic and meiotic divisions, although it is highly regulated during the processes. Excessive levels of CDC25A and CDC25B are often observed in various human cancer tissues. Deregulated expression of these phosphatases allows cells to overcome DNA damage-induced checkpoint, leading to genomic instability. Studies using mouse models demonstrated that deregulated expression of CDC25A significantly promotes RAS- or NEU-induced mammary tumor development with chromosomal aberrations, whereas decreased CDC25A expression in heterozygous knockout mice delays tumorigenesis. These biological properties of CDC25 phosphatases provide significant insight into the pathobiology of cancer and scientific foundation for anti-CDC25 therapeutic intervention.

Human Cdc25A phosphatase has a non-redundant function in G2 phase by activating Cyclin A-dependent kinases

Cdc25 phosphatases activate Cdk/Cyclin complexes by dephosphorylation and thus promote cell cycle progression. We observed that the peak activity of Cdc25A precedes the one of Cdc25B in prophase and the maximum of Cyclin/Cdk kinase activity. Furthermore, Cdc25A activates both Cdk1-2/Cyclin A and Cdk1/Cyclin B complexes while Cdc25B seems to be involved only in activation of Cdk1/Cyclin B. Concomitantly, repression of Cdc25A led to a decrease in Cyclin A-associated kinase activity and attenuated Cdk1 activation. Our results indicate that Cdc25A acts before Cdc25B - at least in cancer cells, and has non-redundant functions in late G2/early M-phase as a major regulator of Cyclin A/kinase complexes.

Inappropriate activation of cyclin-dependent kinases by the phosphatase Cdc25b results in premature mitotic entry and triggers a p53-dependent checkpoint

The dual specificity phosphatase Cdc25B is capable of inhibiting cellular proliferation, and this occurs in a manner dependent upon its catalytic activity. Here it is shown that this is accompanied by inappropriate cyclin-dependent kinase activation and premature mitotic entry, leading to both p53-dependent and independent checkpoints. Forced expression of Cdc25B inappropriately up-regulated the activity of Cdk1 and Cdk2, by reducing levels of inhibitory phosphorylation. In cells lacking p14ARF, p53 is induced, and components of the ATM and ATR pathways are activated. Cdc25B triggers cell cycle arrest in the G(1) and G(2) phases that is p53- and p21-dependent and is inhibited by caffeine. Cdc25B also causes cells with an S phase DNA content to enter mitosis prematurely in a p53-independent manner. Synchronization of cells with aphidicolin results in these cells undergoing apoptosis. Thus, inappropriate cell cycle progression and premature mitotic entry via dysregulation of cyclin-dependent kinases results in activation of both p53-dependent and independent responses. Because Cdc25B is known to have oncogenic activity, this provides insight into the multistep nature of cancer development and why there is p53 loss during tumorigenesis.

Molecular pathways involved in cell death after chemically induced DNA damage

DNA damage is at the center of the genesis, progression and treatment of cancer. We review here the molecular mechanisms of the DNA damage inducing small molecules most commonly used in cancer therapy. Cell cycle control and DNA repair mechanisms are known to be activated after DNA damage. Here, we revise recent discoveries related to the cell cycle control and DNA repair processes and how these findings are being utilized for the more efficient, powerful and selective therapies for cancer treatment.

Bioactivities of simplified adociaquinone B and naphthoquinone derivatives against Cdc25B, MKP-1, and MKP-3 phosphatases

Some simplified adociaquinone B analogs and a series of 1,4-naphthoquinone derivatives were synthesized and tested against the three enzymes Cdc25B, MKP-1, and MKP-3. Cdc25B and MKP-1 in particular are enzymes overexpressed in human cancer cells, and they represent potential molecular targets for novel cancer chemotherapeutic treatments. A number of analogs exhibited significant inhibitory activity against these enzymes, and the bioassay data in addition to structure-activity relationships of these compounds will be discussed.

Multimodal control of Cdc25A by nitrosative stress

Cdc25A propels cell cycle progression, is overexpressed in numerous human cancers, and possesses oncogenic and antiapoptotic activities. Reactive oxygen species, such as hydrogen peroxide, regulate Cdc25A, but the physiologic and pathologic effects of nitric oxide (*NO) and *NO-derived reactive species are not well defined. Herein, we report novel independent mechanisms governing Cdc25A in response to nitrosative insult. We observed direct and rapid inhibition of Cdc25A phosphatase activity after in vitro treatment with the low molecular mass cell-permeable S-nitrosothiol S-nitrosocysteine ethyl ester (SNCEE). In addition, treatment of cancer cells with SNCEE induced nitrosative stress and decreased Cdc25A protein levels in a time-dependent and concentration-dependent manner. Similarly, iNOS-derived *NO was sufficient to suppress Cdc25A expression, consistent with its role in mediating nitrosative stress. Whereas a decrease in Cdc25A half-life was not observed in response to SNCEE, we found the translational regulator eukaryotic initiation factor 2alpha (eIF2alpha) was hyperphosphorylated and total protein translation was decreased with kinetics consistent with Cdc25A loss. Inhibition of eIF2alpha decreased Cdc25A levels, supporting the hypothesis that SNCEE suppressed Cdc25A translation through inhibition of eIF2alpha. Nitrosative stress decreased the Cdc25A-bound fraction of apoptosis signal-regulating kinase-1 (ASK-1) and sensitized cells to apoptosis induced by the ASK-1-activating chemotherapeutic cis-diaminedichloroplatinum (II), suggesting that nitrosative stress-induced suppression of Cdc25A primed cells for ASK-1-dependent apoptosis. Together these data reveal novel *NO-dependent enzymatic and translational mechanisms controlling Cdc25A, and implicate Cdc25A as a mediator of *NO-dependent apoptotic signaling.

The Aurora kinase family in cell division and cancer

The Aurora protein kinase family (consisting of Aurora-A, -B and -C) is an important group of enzymes that controls several aspects of cell division in mammalian cells. Dysfunction of these kinases has been associated with a failure to maintain a stable chromosome content, a state that can contribute to tumourigenesis. Additionally, Aurora-A is frequently found amplified in a variety of tumour types and displays oncogenic activity. On the other hand, therapeutic inhibition of these kinases has shown great promise as potential anti-cancer treatment, most likely because of their essential roles during cell division. This review will focus on our present understanding of the different roles played by these kinases, their regulation throughout cell division, their deregulation in human cancers and on the progress that is made in targeting these important regulators in the treatment of cancer.