Cdk1-cyclin B1-mediated phosphorylation of tumor-associated microtubule-associated protein/cytoskeleton-associated protein 2 in mitosis

Human CKAP2L shows a cell cycle-dependent expression pattern and exhibits microtubule-stabilizing properties

Cytoskeleton-associated protein 2-like (CKAP2L) is a paralogue of cytoskeleton-associated protein 2 (CKAP2). We characterized the expression pattern, subcellular localization, and microtubule-stabilizing properties of human CKAP2L. The levels of both CKAP2L transcript and protein were cell cycle phase-dependent, peaking during the G2/M phase and relatively high in certain human tissues, including testis, intestine, and spleen. CKAP2L protein was detectable in all human cancer cell lines we tested. CKAP2L localized to the mitotic spindle apparatus during mitosis, as reported previously. During interphase, however, CKAP2L localized mainly to the nucleus. Ectopic overexpression of CKAP2L resulted in 'microtubule bundling', and, consequently, an elevated CKAP2L level led to prolonged mitosis. These findings support the mitotic role of CKAP2L during the human cell cycle.

Targeting CDK1 in cancer: mechanisms and implications

Cyclin dependent kinases (CDKs) are serine/threonine kinases that are proposed as promising candidate targets for cancer treatment. These proteins complexed with cyclins play a critical role in cell cycle progression. Most CDKs demonstrate substantially higher expression in cancer tissues compared with normal tissues and, according to the TCGA database, correlate with survival rate in multiple cancer types. Deregulation of CDK1 has been shown to be closely associated with tumorigenesis. CDK1 activation plays a critical role in a wide range of cancer types; and CDK1 phosphorylation of its many substrates greatly influences their function in tumorigenesis. Enrichment of CDK1 interacting proteins with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was conducted to demonstrate that the associated proteins participate in multiple oncogenic pathways. This abundance of evidence clearly supports CDK1 as a promising target for cancer therapy. A number of small molecules targeting CDK1 or multiple CDKs have been developed and evaluated in preclinical studies. Notably, some of these small molecules have also been subjected to human clinical trials. This review evaluates the mechanisms and implications of targeting CDK1 in tumorigenesis and cancer therapy.

Identification of Cyclin-dependent Kinase 1 Specific Phosphorylation Sites by an In Vitro Kinase Assay

Cyclin-dependent kinase 1 (Cdk1) is a master controller for the cell cycle in all eukaryotes and phosphorylates an estimated 8 - 13% of the proteome; however, the number of identified targets for Cdk1, particularly in human cells is still low. The identification of Cdk1-specific phosphorylation sites is important, as they provide mechanistic insights into how Cdk1 controls the cell cycle. Cell cycle regulation is critical for faithful chromosome segregation, and defects in this complicated process lead to chromosomal aberrations and cancer. Here, we describe an in vitro kinase assay that is used to identify Cdk1-specific phosphorylation sites. In this assay, a purified protein is phosphorylated in vitro by commercially available human Cdk1/cyclin B. Successful phosphorylation is confirmed by SDS-PAGE, and phosphorylation sites are subsequently identified by mass spectrometry. We also describe purification protocols that yield highly pure and homogeneous protein preparations suitable for the kinase assay, and a binding assay for the functional verification of the identified phosphorylation sites, which probes the interaction between a classical nuclear localization signal (cNLS) and its nuclear transport receptor karyopherin α. To aid with experimental design, we review approaches for the prediction of Cdk1-specific phosphorylation sites from protein sequences. Together these protocols present a very powerful approach that yields Cdk1-specific phosphorylation sites and enables mechanistic studies into how Cdk1 controls the cell cycle. Since this method relies on purified proteins, it can be applied to any model organism and yields reliable results, especially when combined with cell functional studies.

CKAP2 phosphorylation by CDK1/cyclinB1 is crucial for maintaining centrosome integrity

Previously, we have reported that CKAP2 is involved in the maintenance of centrosome integrity, thus allowing for proper mitosis in primary hepatocytes. To understand this biological process, we identified the mitosis-specific phosphorylation sites in mouse CKAP2 and investigated CKAP’s possible role in cell cycle progression. Because we observed mouse CKAP2 depletion in amplified centrosomes and aberrant chromosomal segregation, which was rescued by ectopic expression of wild-type CKAP2, we focused on the centrosome duplication process among the various aspects of the cell cycle. Among the identified phosphorylation sites, T603 and possibly S608 were phosphorylated by CDK1–cyclin B1 during mitosis, and the ectopic expression of both T603A and S608A mutants was unable to restore the centrosomal abnormalities in CKAP2-depleted cells. These results indicated that the phosphorylation status of CKAP2 during mitosis is critical for controlling both centrosome biogenesis and bipolar spindle formation.

Ubiquitin-specific protease 22 is a deubiquitinase of CCNB1

The elevated level of CCNB1 indicates more aggressive cancer and poor prognosis. However, the factors that cause CCNB1 upregulation remain enigmatic. Herein, we identify USP22 as a CCNB1 interactor and discover that both USP22 and CCNB1 are dramatically elevated with a strong positive correlation in colon cancer tissues. USP22 stabilizes CCNB1 by antagonizing proteasome-mediated degradation in a cell cycle-specific manner. Phosphorylation of USP22 by CDK1 enhances its activity in deubiquitinating CCNB1. The ubiquitin ligase anaphase-promoting complex (APC/C) targets USP22 for degradation by using the substrate adapter CDC20 during cell exit from M phase, presumably allowing CCNB1 degradation. Finally, we discover that USP22 knockdown leads to slower cell growth and reduced tumor size. Our study demonstrates that USP22 is a CCNB1 deubiquitinase, suggesting that targeting USP22 might be an effective approach to treat cancers with elevated CCNB1 expression.

CDK2-dependent phosphorylation of Suv39H1 is involved in control of heterochromatin replication during cell cycle progression

Although several studies have suggested that the functions of heterochromatin regulators may be regulated by post-translational modifications during cell cycle progression, regulation of the histone methyltransferase Suv39H1 is not fully understood. Here, we demonstrate a direct link between Suv39H1 phosphorylation and cell cycle progression. We show that CDK2 phosphorylates Suv39H1 at Ser391 and these phosphorylation levels oscillate during the cell cycle, peaking at S phase and maintained during S-G2-M phase. The CDK2-mediated phosphorylation of Suv39H1 at Ser391 results in preferential dissociation from chromatin. Furthermore, phosphorylation-mediated dissociation of Suv39H1 from chromatin causes an enhanced occupancy of JMJD2A histone demethylase on heterochromatin and alterations in inactive histone marks. Overexpression of phospho-mimic Suv39H1 induces early replication of heterochromatin, suggesting the importance of Suv39H1 phosphorylation in the replication of heterochromatin. Moreover, overexpression of phospho-defective Suv39H1 caused altered replication timing of heterochromatin and increases sensitivity to replication stress. Collectively, our data suggest that phosphorylation-mediated modulation of Suv39H1-chromatin association may be an initial step in heterochromatin replication.

CKAP2 ensures chromosomal stability by maintaining the integrity of microtubule nucleation sites

Integrity of the microtubule spindle apparatus and intact cell division checkpoints are essential to ensure the fidelity of distributing chromosomes into daughter cells. Cytoskeleton-associated protein 2, CKAP2, is a microtubule-associated protein that localizes to spindle poles and aids in microtubule stabilization, but the exact function and mechanism of action are poorly understood. In the present study, we utilized RNA interference to determine the extent to which the expression of CKAP2 plays a role in chromosome segregation. CKAP2-depleted cells showed a significant increase of multipolar mitoses and other spindle pole defects. Notably, when interrogated for microtubule nucleation capacity, CKAP2-depleted cells showed a very unusual phenotype as early as two minutes after release from mitotic block, consisting of dispersal of newly polymerized microtubule filaments through the entire chromatin region, creating a cage-like structure. Nevertheless, spindle poles were formed after one hour of mitotic release suggesting that centrosome-mediated nucleation remained dominant. Finally, we showed that suppression of CKAP2 resulted in a higher incidence of merotelic attachments, anaphase lagging, and polyploidy. Based on these results, we conclude that CKAP2 is involved in the maintenance of microtubule nucleation sites, focusing microtubule minus ends to the spindle poles in early mitosis, and is implicated in maintaining genome stability.

Bufalin increases sensitivity to AKT/mTOR-induced autophagic cell death in SK-HEP-1 human hepatocellular carcinoma cells

Bufalin is the major component of Chan-Su (a traditional Chinese medicine, TCM) extracts from the venom of Bufo bufo gargarizan. In the present study, we investigated the pharmacological mechanisms of cell cycle arrest and autophagic cell death induced by bufalin in SK-HEP-1 human hepatocellular carcinoma cells in vitro. Bufalin inhibited cell survival by MTT assay and increased cell death by trypan blue exclusion assay in a concentration-dependent manner. In addition, bufalin induced G2/M phase arrest by reducing CDK1 activity. Bufalin triggered DNA fragmentation and apoptotic cell death in SK-HEP-1 cells by DNA gel electrophoresis, TUNEL and caspase-3 activity assay, while bufalin induced autophagic cell death by double-membrane vacuoles (transmission electron microscopy, TEM), acidic vesicular organelles (acridine orange staining) and cleavage of microtubule-associated protein 1 light chain 3 (LC3). Protein expression levels of cyclin A and B, CDK1, phospho-CDK1 (Thr161), Cdc25c, phospho-Cdc25c (Ser198), phospho-AKT (Thr308), phospho-AKT (Ser473), phospho‑mTOR (Ser2481) were downregulated. In contrast, protein expression levels of the Chk1, Wee1, LC3-II, Beclin-1, Atg 5, Atg 7 and Atg 12 were upregulated in SK-HEP-1 cells after bufalin treatment. Inhibition of autophagy by 3-methyladenine (an inhibitor of class III phosphatidylinositol-3 kinase; 3-MA) or bafilomycin A1 (an inhibitor of the vacuolar proton pump of lysosomes and endosomes) reduced the effect of bufalin on cell viability and enhanced the effect of bufalin on apoptosis. In conclusion, bufalin triggered autophagic cell death and G2/M phase arrest through the AKT/mTOR signaling pathway in SK-HEP-1 cells. Our findings showed that bufalin may be potentially efficacious in the treatment of human hepatocellular carcinoma.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

Condensed chromatin staining of CKAP2 as surrogate marker for mitotic figures

PURPOSE

Proliferation activity has long been known to be one of the strongest prognostic factors in many different cancers. Nevertheless, microscopic evaluation of mitotic figures remains time-consuming and, furthermore, is relatively subjective. As the expression of cytoskeleton-associated protein 2 (CKAP2) is closely related to the mitotic phase, CKAP2 was evaluated as a surrogate mitotic figure (MF) marker.

METHODS

A monoclonal antibody specific to human CKAP2 was produced, and immunohistochemistry was performed on normal tissue array sections and 30 breast cancer tissues.

RESULTS

The expression of CKAP2 in the normal human tissues was limited to well-known cell proliferation zones. Strong, readily visible, condensed chromatin staining of CKAP2 was observed specifically in mitotic cells, and the number of these cells was tightly correlated with the MF count in breast cancer tissues (P < 0.001, ρ = 0.743), suggesting its usefulness as a surrogate marker for MF counting.

CONCLUSION

Immunohistochemical staining with CKAP2 monoclonal antibody can be considered to be a new, effective approach to the assessment of proliferation activity in cancer tissues.

Silencing of tubulin binding cofactor C modifies microtubule dynamics and cell cycle distribution and enhances sensitivity to gemcitabine in breast cancer cells

Tubulin binding cofactor C (TBCC) is essential for the proper folding of α- and β-tubulins into microtubule polymerizable heterodimers. Because microtubules are considered major targets in the treatment of breast cancer, we investigated the influence of TBCC silencing on tubulin pools, microtubule dynamics, and cell cycle distribution of breast cancer cells by developing a variant MCF7 cells with reduced content of TBCC (MC-). MC- cells displayed decreased content in nonpolymerizable tubulins and increased content of polymerizable/microtubule tubulins when compared with control MP6 cells. Microtubules in MC- cells showed stronger dynamics than those of MP6 cells. MC- cells proliferated faster than MP6 cells and showed an altered cell cycle distribution, with a higher percentage in S-phase of the cell cycle. Consequently, MC- cells presented higher sensitivity to the S-phase-targeting agent gemcitabine than MP6 cells in vitro. Although the complete duration of mitosis was shorter in MC- cells and their microtubule dynamics was enhanced, the percentage of cells in G(2)-M phase was not altered nor was there any difference in sensitivity to antimicrotubule-targeting agents when compared with MP6 cells. Xenografts derived from TBCC variants displayed significantly enhanced tumor growth in vivo and increased sensitivity to gemcitabine in comparison to controls. These results are the first to suggest that proteins involved in the proper folding of cytoskeletal components may have an important influence on the cell cycle distribution, proliferation, and chemosensitivity of tumor cells.

Integrated genomics of susceptibility to alkylator-induced leukemia in mice

Background

Therapy-related acute myeloid leukemia (t-AML) is a secondary, generally incurable, malignancy attributable to chemotherapy exposure. Although there is a genetic component to t-AML susceptibility in mice, the relevant loci and the mechanism(s) by which they contribute to t-AML are largely unknown. An improved understanding of susceptibility factors and the biological processes in which they act may lead to the development of t-AML prevention strategies.

Results

In this work we applied an integrated genomics strategy in inbred strains of mice to find novel factors that might contribute to susceptibility. We found that the pre-exposure transcriptional state of hematopoietic stem/progenitor cells predicts susceptibility status. More than 900 genes were differentially expressed between susceptible and resistant strains and were highly enriched in the apoptotic program, but it remained unclear which genes, if any, contribute directly to t-AML susceptibility. To address this issue, we integrated gene expression data with genetic information, including single nucleotide polymorphisms (SNPs) and DNA copy number variants (CNVs), to identify genetic networks underlying t-AML susceptibility. The 30 t-AML susceptibility networks we found are robust: they were validated in independent, previously published expression data, and different analytical methods converge on them. Further, the networks are enriched in genes involved in cell cycle and DNA repair (pathways not discovered in traditional differential expression analysis), suggesting that these processes contribute to t-AML susceptibility. Within these networks, the putative regulators (e.g., Parp2, Casp9, Polr1b) are the most likely to have a non-redundant role in the pathogenesis of t-AML. While identifying these networks, we found that current CNVR and SNP-based haplotype maps in mice represented distinct sources of genetic variation contributing to expression variation, implying that mapping studies utilizing either source alone will have reduced sensitivity.

Conclusion

The identification and prioritization of genes and networks not previously implicated in t-AML generates novel hypotheses on the biology and treatment of this disease that will be the focus of future research.

Genome-wide microarray evidence that 8-cell human blastomeres over-express cell cycle drivers and under-express checkpoints

PURPOSE

To understand cell cycle controls in the 8-Cell human blastomere.

METHODS

Data from whole human genome (43,377 elements) microarray analyses of RNAs from normal 8-Cell human embryos were compiled with published microarrays of RNAs from human fibroblasts, before and after induced pluripotency, and embryonic stem cells. A sub database of 3,803 genes identified by high throughput RNA knock-down studies, plus genes that oscillate in human cells, was analyzed.

RESULTS

Thirty-five genes over-detected at least 7-fold specifically on the 8-Cell arrays were enriched for cell cycle drivers and for proteins that stabilize chromosome cohesion and spindle attachment and limit DNA and centrosome replication to once per cycle.

CONCLUSIONS

These results indicate that 8-cell human blastomere cleavage is guided by cyclic over-expression of key proteins, rather than canonical checkpoints, leading to rapidly increasing gene copy number and a susceptibility to chromosome and cytokinesis mishaps, well-noted characteristics of preimplantation human embryos.

Characterization of mitosis-specific phosphorylation of tumor-associated microtubule-associated protein

Tumor-associated microtubule-associated protein (TMAP), also known as cytoskeleton associated protein 2 (CKAP2), has been recently shown to be involved in the assembly and maintenance of mitotic spindle and also plays an essential role in maintaining the fidelity of chromosome segregation during mitosis. We have previously reported that TMAP is phosphorylated at multiple residues specifically during mitosis, and characterized the mechanism and functional importance of phosphorylation at one of the mitosis-specific phosphorylation residues (i.e., Thr-622). However, the phosphorylation events at the remaining mitotic phosphorylation sites of TMAP have not been fully characterized in detail. Here, we report on generation and characterization of phosphorylated Thr-578- and phosphorylated Thr-596-specific antibodies. Using the antibodies, we show that phosphorylation of TMAP at Thr-578 and Thr-596 indeed occurs specifically during mitosis. Immunofluorescent staining using the antibodies shows that these residues become phosphorylated starting at prophase and then become rapidly dephosphorylated soon after initiation of anaphase. Subtle differences in the kinetics of phosphorylation between Thr-578 and Thr-596 imply that they may be under different mechanisms of phosphorylation during mitosis. Unlike the phosphorylation-deficient mutant form for Thr-622, the mutant in which both Thr-578 and Thr-596 had been mutated to alanines did not induce significant delay in progression of mitosis. These results show that the majority of mitosis-specific phosphorylation of TMAP is limited to pre-anaphase stages and suggest that the multiple phosphorylation may not act in concert but serve diverse functions.