Mitotic phosphorylation of DNA topoisomerase II alpha by protein kinase CK2 creates the MPM-2 phosphoepitope on Ser-1469

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Phosphoproteomics revealed cellular signals immediately responding to disruption of cancer amino acid homeostasis induced by inhibition of l-type amino acid transporter 1

Background

Cancer-upregulated l-type amino acid transporter 1 (LAT1; SLC7A5) supplies essential amino acids to cancer cells. LAT1 substrates are not only needed for cancer rapid growth, but involved in cellular signaling. LAT1 has been proposed as a potential target for cancer treatment—its inhibitor, JPH203, is currently in clinical trials and targets biliary tract cancer (BTC). Here, we revealed to what extent LAT1 inhibitor affects intracellular amino acid content and what kind of cellular signals are directly triggered by LAT1 inhibition.

Methods

Liquid chromatography assay combined with o-phthalaldehyde- and 9-fluorenyl-methylchloroformate-based derivatization revealed changes in intracellular amino acid levels induced by LAT1 inhibition with JPH203 treatment in three BTC cell lines. Tandem mass tag-based quantitative phosphoproteomics characterized the effect of JPH203 treatment on BTC cells, and suggested key regulators in LAT1-inhibited cells. We further studied one of the key regulators, CK2 protein kinase, by using Western blot, enzymatic activity assay, and co-immunoprecipitation. We evaluated anticancer effects of combination of JPH203 with CK2 inhibitor using cell growth and would healing assay.

Results

JPH203 treatment decreased intracellular levels of LAT1 substrates including essential amino acids of three BTC cell lines, immediately and drastically. We also found levels of some of these amino acids were partially recovered after longer-time treatment. Therefore, we performed phosphoproteomics with short-time JPH203 treatment prior to the cellular compensatory response, and revealed hundreds of differentially phosphorylated sites. Commonly downregulated phosphorylation sites were found on proteins involved in the cell cycle and RNA splicing. Our phosphoproteomics also suggested key regulators immediately responding to LAT1 inhibition. Focusing on one of these regulators, protein kinase CK2, we revealed LAT1 inhibition decreased phosphorylation of CK2 substrate without changing CK2 enzymatic activity. Furthermore, LAT1 inhibition abolished interaction between CK2 and its regulatory protein NOLC1, which suggests regulatory mechanism of CK2 substrate protein specificity controlled by LAT1 inhibition. Moreover, we revealed that the combination of JPH203 with CK2 inhibitor resulted in the enhanced inhibition of proliferation and migration of BTC cells.

Conclusion

This study provides new perspectives on LAT1-dependent cellular processes and a rationale for therapeutics targeting reprogrammed cancer metabolism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40170-022-00295-8.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Exploration of the Role of the C-Terminal Domain of Human DNA Topoisomerase IIα in Catalytic Activity

Human topoisomerase IIα (TOP2A) is a vital nuclear enzyme involved in resolving knots and tangles in DNA during replication and cell division. TOP2A is a homodimer with a symmetrical, multidomain structure. While the N-terminal and core regions of the protein are well-studied, the C-terminal domain is poorly understood but is involved in enzyme regulation and is predicted to be intrinsically disordered. In addition, it appears to be a major region of post-translational modification and includes several Ser and Thr residues, many of which have not been studied for biochemical effects. Therefore, we generated a series of human TOP2A mutants where we changed specific Ser and Thr residues in the C-terminal domain to Ala, Gly, or Ile residues. We designed, purified, and examined 11 mutant TOP2A enzymes. The amino acid changes were made between positions 1272 and 1525 with 1-7 residues changed per mutant. Several mutants displayed increased levels of DNA cleavage without displaying any change in plasmid DNA relaxation or DNA binding. For example, mutations in the regions 1272-1279, 1324-1343, 1351-1365, and 1374-1377 produced 2-3 times more DNA cleavage in the presence of etoposide than wild-type TOP2A. Further, several mutants displayed changes in relaxation and/or decatenation activity. Together, these results support previous findings that the C-terminal domain of TOP2A influences catalytic activity and interacts with the substrate DNA. Furthermore, we hypothesize that it may be possible to regulate the enzyme by targeting positions in the C-terminal domain. Because the C-terminal domain differs between the two human TOP2 isoforms, this strategy may provide a means for selectively targeting TOP2A for therapeutic inhibition. Additional studies are warranted to explore these results in more detail.

Mitotic chromosomes

Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. We also discuss some of the remaining questions. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called "chromosome scaffold" from which loops of DNA project radially. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. These proteins are concentrated along the axis of the chromatid. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.

3,6-Disubstituted 1,2,4-Triazolo[3,4-b]Thiadiazoles with Anticancer Activity Targeting Topoisomerase II Alpha

BACKGROUND

Topoisomerase IIα (topIIα) maintains the topology of DNA in order to ensure the proper functioning of numerous DNA processes. Inhibition of topIIα leads to the killing of cancer cells thus constituting such inhibitors as useful tools in cancer therapeutics. Triazolo[3,4-b]thiadiazole derivatives are known for their wide range of pharmacological activities while previous studies have documented their in vitro anticancer activity. The purpose of the current study was to investigate if these chemical compounds can act as topIIα inhibitors in cell-free and cell-based systems.

MATERIALS AND METHODS

The MTT assay was performed in DLD-1, HT-29, and LoVo cancer cells so as to evaluate the antiproliferative activity of KA25, KA26, and KA39 triazolo[3,4-b]thiadiazole derivatives. The KA39 compound was tested as a potential topIIα inhibitor using the plasmid-based topoisomerase II drug screening kit. The inhibitory effect of the three derivatives on topIIα phosphorylation was studied in HT-29 and LoVo cancer cells according to Human Phospho-TOP2A/Topoisomerase II Alpha Cell-Based Phosphorylation ELISA Kit. Moreover, flow cytometry was utilized in order to explore apoptotic induction and cell cycle growth arrest, upon treatment with KA39, in DLD-1 and HT-29 cells, respectively. In silico studies were also carried out for further investigation.

RESULTS

All three triazolo[3,4-b]thiadiazole derivatives showed an in vitro antiproliferative effect with the KA39 compound being the most potent one. Our results indicated that KA39 induced both early and late apoptosis as well as cell cycle growth arrest in S phase. In addition, the compound blocked the relaxation of supercoiled DNA while it also inhibited topIIα phosphorylation (upon treatment; P<0.001).

CONCLUSION

Among the three triazolo[3,4-b]thiadiazole derivatives, KA39 was shown to be the most potent anticancer agent and catalytic inhibitor of topIIα phosphorylation as well.

Cell Cycle-Dependent Control and Roles of DNA Topoisomerase II

Type II topoisomerases are ubiquitous enzymes in all branches of life that can alter DNA superhelicity and unlink double-stranded DNA segments during processes such as replication and transcription. In cells, type II topoisomerases are particularly useful for their ability to disentangle newly-replicated sister chromosomes. Growing lines of evidence indicate that eukaryotic topoisomerase II (topo II) activity is monitored and regulated throughout the cell cycle. Here, we discuss the various roles of topo II throughout the cell cycle, as well as mechanisms that have been found to govern and/or respond to topo II function and dysfunction. Knowledge of how topo II activity is controlled during cell cycle progression is important for understanding how its misregulation can contribute to genetic instability and how modulatory pathways may be exploited to advance chemotherapeutic development.

Casein kinase II-dependent phosphorylation of DNA topoisomerase II suppresses the effect of a catalytic topo II inhibitor, ICRF-193, in fission yeast

DNA topoisomerase II (topo II) regulates the topological state of DNA and is necessary for DNA replication, transcription, and chromosome segregation. Topo II has essential functions in cell proliferation and therefore is a critical target of anticancer drugs. In this study, using Phos-tag SDS-PAGE analysis in fission yeast (Schizosaccharomyces pombe), we identified casein kinase II (Cka1/CKII)-dependent phosphorylation at the C-terminal residues Ser¹³⁶³ and Ser¹³⁶⁴ in topo II. We found that this phosphorylation decreases the inhibitory effect of an anticancer catalytic inhibitor of topo II, ICRF-193, on mitosis. Consistent with the constitutive activity of Cka1/CKII, Ser¹³⁶³ and Ser¹³⁶⁴ phosphorylation of topo II was stably maintained throughout the cell cycle. We demonstrate that ICRF-193-induced chromosomal mis-segregation is further exacerbated in two temperature-sensitive mutants, cka1-372 and cka1/orb5-19, of the catalytic subunit of CKII or in the topo II nonphosphorylatable alanine double mutant top2-S1363A,S1364A but not in cells of the phosphomimetic glutamate double mutant top2-S1363E,S1364E Our results suggest that Ser¹³⁶³ and Ser¹³⁶⁴ in topo II are targeted by Cka1/CKII kinase and that their phosphorylation facilitates topo II ATPase activity in the N-terminal region, which regulates protein turnover on chromosome DNA. Because CKII-mediated phosphorylation of the topo II C-terminal domain appears to be evolutionarily conserved, including in humans, we propose that attenuation of CKII-controlled topo II phosphorylation along with catalytic topo II inhibition may promote anticancer effects.

Post-translational modifications in DNA topoisomerase 2α highlight the role of a eukaryote-specific residue in the ATPase domain

Type 2 DNA topoisomerases (Top2) are critical components of key protein complexes involved in DNA replication, chromosome condensation and segregation, as well as gene transcription. The Top2 were found to be the main targets of anticancer agents, leading to intensive efforts to understand their functional and physiological role as well as their molecular structure. Post-translational modifications have been reported to influence Top2 enzyme activities in particular those of the mammalian Top2α isoform. In this study, we identified phosphorylation, and for the first time, acetylation sites in the human Top2α isoform produced in eukaryotic expression systems. Structural analysis revealed that acetylation sites are clustered on the catalytic domains of the homodimer while phosphorylation sites are located in the C-terminal domain responsible for nuclear localization. Biochemical analysis of the eukaryotic-specific K168 residue in the ATPase domain shows that acetylation affects a key position regulating ATP hydrolysis through the modulation of dimerization. Our findings suggest that acetylation of specific sites involved in the allosteric regulation of human Top2 may provide a mechanism for modulation of its catalytic activity.

Re-evaluation of protein kinase CK2 pleiotropy: new insights provided by a phosphoproteomics analysis of CK2 knockout cells

CK2 denotes a ubiquitous and pleiotropic protein kinase whose holoenzyme is composed of two catalytic (α and/or α') and two regulatory β subunits. The CK2 consensus sequence, S/T-x-x-D/E/pS/pT is present in numerous phosphosites, but it is not clear how many of these are really generated by CK2. To gain information about this issue, advantage has been taken of C2C12 cells entirely deprived of both CK2 catalytic subunits by the CRISPR/Cas9 methodology. A comparative SILAC phosphoproteomics analysis reveals that, although about 30% of the quantified phosphosites do conform to the CK2 consensus, only one-third of these are substantially reduced in the CK2α/α'(-/-) cells, consistent with their generation by CK2. A parallel study with C2C12 cells deprived of the regulatory β subunit discloses a role of this subunit in determining CK2 targeting. We also find that phosphosites notoriously generated by CK2 are not fully abrogated in CK2α/α'(-/-) cells, while some phosphosites unrelated to CK2 are significantly altered. Collectively taken our data allow to conclude that the phosphoproteome generated by CK2 is not as ample and rigidly pre-determined as it was believed before. They also show that the lack of CK2 promotes phosphoproteomics perturbations attributable to kinases other than CK2.

Identification of Candidate Casein Kinase 2 Substrates in Mitosis by Quantitative Phosphoproteomics

Protein phosphorylation is a crucial regulatory mechanism that controls many aspects of cellular signaling. Casein kinase 2 (CK2), a constitutively expressed and active kinase, plays key roles in an array of cellular events including transcription and translation, ribosome biogenesis, cell cycle progression, and apoptosis. CK2 is implicated in cancerous transformation and is a therapeutic target in anti-cancer therapy. The specific and selective CK2 ATP competitive inhibitor, CX-4945 (silmitaseratib), is currently in phase 2 clinical trials. While many substrates and interactors of CK2 have been identified, less is known about CK2 substrates in mitosis. In the present work, we utilize CX-4945 and quantitative phosphoproteomics to inhibit CK2 activity in mitotically arrested HeLa cells and determine candidate CK2 substrates. We identify 330 phosphorylation sites on 202 proteins as significantly decreased in abundance upon inhibition of CK2 activity. Motif analysis of decreased sites reveals a linear kinase motif with aspartic and glutamic amino acids downstream of the phosphorylated residues, which is consistent with known substrate preferences for CK2. To validate specific candidate CK2 substrates, we perform in vitro kinase assays using purified components. Furthermore, we identified CK2 interacting proteins by affinity purification-mass spectrometry (AP-MS). To investigate the biological processes regulated by CK2 in mitosis, we perform network analysis and identify an enrichment of proteins involved in chromosome condensation, chromatin organization, and RNA processing. We demonstrate that overexpression of CK2 in HeLa cells affects proper chromosome condensation. Previously, we found that phosphoprotein phosphatase 6 (PP6), but not phosphoprotein phosphatase 2A (PP2A), opposes CK2 phosphorylation of the condensin I complex, which is essential for chromosome condensation. Here, we extend this observation and demonstrate that PP6 opposition of CK2 is a more general cellular regulatory mechanism.

Identification of potential key genes associated with glioblastoma based on the gene expression profile

Gliomas are serious primary brain tumors. The aim of the present study was to identify potential key genes associated with the progression of gliomas. The GSE31262 gene expression profile data, which included 9 glioblastoma stem cells (GSCs) samples and 5 neural stem cell samples from adult humans, were downloaded from Gene Expression Omnibus (GEO) database. limma package was used to identify differentially expressed genes (DEGs). Based on STRING database and Pearson Correlation Coefficient (PCC), a co-expression network was constructed to comprehensively understand the interactions between DEGs, and function analysis of genes in the network was conducted. Furthermore, the DEGs that were associated with prognosis were analyzed. A total of 431 DEGs were identified, including 98 upregulated DEGs and 333 downregulated DEGs. Genes including PDZ binding kinase, topoisomerase (DNA) II α (TOP2A), cyclin dependent kinase (CDK) 1, cell division cycle 6 and NIMA related kinase 2 had a relatively high degree in the co-expression network. A set of genes including cyclin D1, CDK1 and CDK2 were significantly enriched in the cell cycle and p53 signaling pathway. Additionally, 69 DEGs were identified as genes involved in glioblastoma prognosis, such as CDK2 and TOP2A. The genes that had a higher degree and were associated with cell cycle and p53 signaling pathway may play pivotal roles in the progress of glioblastoma.

Unconventional Functions of Mitotic Kinases in Kidney Tumorigenesis

Human tumors exhibit a variety of genetic alterations, including point mutations, translocations, gene amplifications and deletions, as well as aneuploid chromosome numbers. For carcinomas, aneuploidy is associated with poor patient outcome for a large variety of tumor types, including breast, colon, and renal cell carcinoma. The Renal cell carcinoma (RCC) is a heterogeneous carcinoma consisting of different histologic types. The clear renal cell carcinoma (ccRCC) is the most common subtype and represents 85% of the RCC. Central to the biology of the ccRCC is the loss of function of the Von Hippel–Lindau gene, but is also associated with genetic instability that could be caused by abrogation of the cell cycle mitotic spindle checkpoint and may involve the Aurora kinases, which regulate centrosome maturation. Aneuploidy can also result from the loss of cell–cell adhesion and apical–basal cell polarity that also may be regulated by the mitotic kinases (polo-like kinase 1, casein kinase 2, doublecortin-like kinase 1, and Aurora kinases). In this review, we describe the “non-mitotic” unconventional functions of these kinases in renal tumorigenesis.

Quantitative phosphoproteomics reveals new roles for the protein phosphatase PP6 in mitotic cells

Protein phosphorylation is an important regulatory mechanism controlling mitotic progression. Protein phosphatase 6 (PP6) is an essential enzyme with conserved roles in chromosome segregation and spindle assembly from yeast to humans. We applied a baculovirus-mediated gene silencing approach to deplete HeLa cells of the catalytic subunit of PP6 (PP6c) and analyzed changes in the phosphoproteome and proteome in mitotic cells by quantitative mass spectrometry-based proteomics. We identified 408 phosphopeptides on 272 proteins that increased and 298 phosphopeptides on 220 proteins that decreased in phosphorylation upon PP6c depletion in mitotic cells. Motif analysis of the phosphorylated sites combined with bioinformatics pathway analysis revealed previously unknown PP6c-dependent regulatory pathways. Biochemical assays demonstrated that PP6c opposed casein kinase 2-dependent phosphorylation of the condensin I subunit NCAP-G, and cellular analysis showed that depletion of PP6c resulted in defects in chromosome condensation and segregation in anaphase, consistent with dysregulation of condensin I function in the absence of PP6 activity.

Protein kinase CK2 in breast cancer: the CK2β regulatory subunit takes center stage in epithelial plasticity

Structurally, protein kinase CK2 consists of two catalytic subunits (α and α') and two regulatory subunits (β), which play a critical role in targeting specific CK2 substrates. Compelling evidence shows the complexity of the CK2 cellular signaling network and supports the view that this enzyme is a key component of regulatory protein kinase networks that are involved in several aspects of cancer. CK2 both activates and suppresses the expression of a number of essential oncogenes and tumor suppressors, and its expression and activity are upregulated in blood tumors and virtually all solid tumors. The prognostic significance of CK2α expression in association with various clinicopathological parameters highlighted this kinase as an adverse prognostic marker in breast cancer. In addition, several recent studies reported its implication in the regulation of the epithelial-to-mesenchymal transition (EMT), an early step in cancer invasion and metastasis. In this review, we briefly overview the contribution of CK2 to several aspects of cancer and discuss how in mammary epithelial cells, the expression of its CK2β regulatory subunit plays a critical role in maintaining an epithelial phenotype through CK2-mediated control of key EMT-related transcription factors. Importantly, decreased CK2β expression in breast tumors is correlated with inefficient phosphorylation and nuclear translocation of Snail1 and Foxc2, ultimately leading to EMT induction. This review highlights the pivotal role played by CK2β in the mammary epithelial phenotype and discusses how a modest alteration in its expression may be sufficient to induce dramatic effects facilitating the early steps in tumor cell dissemination through the coordinated regulation of two key transcription factors.

Topoisomerase IIα in chromosome instability and personalized cancer therapy

Genome instability is a hallmark of cancer cells. Chromosome instability (CIN), which is often mutually exclusive from hypermutation genotypes, represents a distinct subtype of genome instability. Hypermutations in cancer cells are due to defects in DNA repair genes, but the cause of CIN is still elusive. However, because of the extensive chromosomal abnormalities associated with CIN, its cause is likely a defect in a network of genes that regulate mitotic checkpoints and chromosomal organization and segregation. Emerging evidence has shown that the chromosomal decatenation checkpoint, which is critical for chromatin untangling and packing during genetic material duplication, is defective in cancer cells with CIN. The decatenation checkpoint is known to be regulated by a family of enzymes called topoisomerases. Among them, the gene encoding topoisomerase IIα (TOP2A) is commonly altered at both gene copy number and gene expression level in cancer cells. Thus, abnormal alterations of TOP2A, its interacting proteins, and its modifications may play a critical role in CIN in human cancers. Clinically, a large arsenal of topoisomerase inhibitors have been used to suppress DNA replication in cancer. However, they often lead to the secondary development of leukemia because of their effect on the chromosomal decatenation checkpoint. Therefore, topoisomerase drugs must be used judiciously and administered on an individual basis. In this review, we highlight the biological function of TOP2A in chromosome segregation and the mechanisms that regulate this enzyme's expression and activity. We also review the roles of TOP2A and related proteins in human cancers, and raise a perspective for how to target TOP2A in personalized cancer therapy.

The α isoform of topoisomerase II is required for hypercompaction of mitotic chromosomes in human cells

As proliferating cells transit from interphase into M-phase, chromatin undergoes extensive reorganization, and topoisomerase (topo) IIα, the major isoform of this enzyme present in cycling vertebrate cells, plays a key role in this process. In this study, a human cell line conditional null mutant for topo IIα and a derivative expressing an auxin-inducible degron (AID)-tagged version of the protein have been used to distinguish real mitotic chromosome functions of topo IIα from its more general role in DNA metabolism and to investigate whether topo IIβ makes any contribution to mitotic chromosome formation. We show that topo IIβ does contribute, with endogenous levels being sufficient for the initial stages of axial shortening. However, a significant effect of topo IIα depletion, seen with or without the co-depletion of topo IIβ, is the failure of chromosomes to hypercompact when delayed in M-phase. This requires much higher levels of topo II protein and is impaired by drugs or mutations that affect enzyme activity. A prolonged delay at the G2/M border results in hyperefficient axial shortening, a process that is topo IIα-dependent. Rapid depletion of topo IIα has allowed us to show that its function during late G2 and M-phase is truly required for shaping mitotic chromosomes.

Dynamic distribution of HDAC1 and HDAC2 during mitosis: association with F-actin

During mitosis, histone deacetylase 2 (HDAC2) becomes highly phosphorylated through the action of CK2, and HDAC1 and 2 are displaced from mitotic chromosomes. HDAC1 and 2 are components of corepressor complexes, which function with lysine acetyltransferases to catalyze dynamic protein acetylation and regulate gene expression. In this study, we show that HDAC1 and 2 associate with F-actin in mitotic cells. Inhibition of Aurora B or protein kinase CK2 did not prevent the displacement of HDAC1 and 2 from mitotic chromosomes in HeLa cells. Further, proteins of the HDAC1 and 2 corepressor complexes and transcription factors recruiting these corepressors to chromatin were dissociated from mitotic chromosomes independent of Aurora B activity. HDAC1 and 2 returned to the nuclei of daughter cells during lamin A/C reassembly and before Sp1, Sp3, and RNA polymerase II. Our results show that HDAC1 and 2 corepressor complexes are removed from the mitotic chromosomes and are available early in the events leading to the re-establishment of the gene expression program in daughter cells.

Mitotic rate in melanoma: prognostic value of immunostaining and computer-assisted image analysis

The prognostic value of mitotic rate in melanoma is increasingly recognized, particularly in thin melanoma in which the presence or absence of a single mitosis/mm can change staging from T1a to T1b. Still, accurate mitotic rate calculation (mitoses/mm) on hematoxylin and eosin (H&E)-stained sections can be challenging. Antimonoclonal mitotic protein-2 (MPM-2) and antiphosphohistone-H3 (PHH3) are 2 antibodies reported to be more mitosis-specific than other markers of proliferation such as Ki-67. We used light microscopy and computer-assisted image analysis software to quantify MPM-2 and PHH3 staining in melanoma. We then compared mitotic rates by each method with conventional H&E-based mitotic rate for correlation with clinical outcomes. Our study included primary tissues from 190 nonconsecutive cutaneous melanoma patients who were prospectively enrolled at New York University Langone Medical Center with information on age, gender, and primary tumor characteristics. The mitotic rate was quantified manually by light microscopy of corresponding H&E-stained, MPM-2-stained, and PHH3-stained sections. Computer-assisted image analysis was then used to quantify immunolabeled mitoses on the previously examined PHH3 and MPM-2 slides. We then analyzed the association between mitotic rate and both progression-free and melanoma-specific survival. Univariate analysis of PHH3 found significant correlation between increased PHH3 mitotic rate and decreased progression-free survival (P=0.04). Computer-assisted image analysis enhanced the correlation of PHH3 mitotic rate with progression-free survival (P=0.02). Regardless of the detection method, neither MPM-2 nor PHH3 offered significant advantage over conventional H&E determination of mitotic rate.

Protein kinase CK2 regulates the dimerization of histone deacetylase 1 (HDAC1) and HDAC2 during mitosis

Histone deacetylase 1 (HDAC1) and HDAC2 are components of corepressor complexes that are involved in chromatin remodeling and regulation of gene expression by regulating dynamic protein acetylation. HDAC1 and -2 form homo- and heterodimers, and their activity is dependent upon dimer formation. Phosphorylation of HDAC1 and/or HDAC2 in interphase cells is required for the formation of HDAC corepressor complexes. In this study, we show that during mitosis, HDAC2 and, to a lesser extent, HDAC1 phosphorylation levels dramatically increase. When HDAC1 and -2 are displaced from the chromosome during metaphase, they dissociate from each other, but each enzyme remains in association with components of the HDAC corepressor complexes Sin3, NuRD, and CoREST as homodimers. Enzyme inhibition studies and mutational analyses demonstrated that protein kinase CK2-catalyzed phosphorylation of HDAC1 and -2 is crucial for the dissociation of these two enzymes. These results suggest that corepressor complexes, including HDAC1 or HDAC2 homodimers, might target different cellular proteins during mitosis.

Ataxia telangiectasia mutated-dependent regulation of topoisomerase II alpha expression and sensitivity to topoisomerase II inhibitor

Topoisomerase II alpha (TOP2A) has a crucial role in proper chromosome condensation and segregation. Here we report the interaction of TOP2A with ataxia telangiectasia mutated (ATM) and its phosphorylation in an ATM-dependent manner after DNA damage. In vitro kinase assay and site-directed mutagenesis studies revealed that serine 1512 is the target of phosphorylation through ATM. Serine 1512 to Alanine mutation of TOP2A showed increased stability of the protein, retaining TOP2A activity at least with regard to cell survival activity. Ataxia telangiectasia-derived cell lines showed high levels of TOP2A that were associated with hypersensitivity to the TOP2 inhibitor etoposide. These findings suggest that ATM-dependent TOP2A modification is required for proper regulation of TOP2 stability and subsequently of the sensitivity to TOP2 inhibitor. In a lymphoblastoid cell line derived from a patient who developed MLL rearrangement, positive infant leukemia, defective ATM expression, and increased TOP2A expression were shown. It was intriguing that hypersensitivity to TOP2 inhibitor and susceptibility to MLL gene rearrangement were shown by low-dose etoposide exposure in this cell line. Thus, our findings have clinically important implications for the pathogenesis of infantile acute leukemia as well as treatment-associated secondary leukemia following exposure to TOP2 inhibitors.

Using bacteria to determine protein kinase specificity and predict target substrates

The identification of protein kinase targets remains a significant bottleneck for our understanding of signal transduction in normal and diseased cellular states. Kinases recognize their substrates in part through sequence motifs on substrate proteins, which, to date, have most effectively been elucidated using combinatorial peptide library approaches. Here, we present and demonstrate the ProPeL method for easy and accurate discovery of kinase specificity motifs through the use of native bacterial proteomes that serve as in vivo libraries for thousands of simultaneous phosphorylation reactions. Using recombinant kinases expressed in E. coli followed by mass spectrometry, the approach accurately recapitulated the well-established motif preferences of human basophilic (Protein Kinase A) and acidophilic (Casein Kinase II) kinases. These motifs, derived for PKA and CK II using only bacterial sequence data, were then further validated by utilizing them in conjunction with the scan-x software program to computationally predict known human phosphorylation sites with high confidence.

Antitumoral activity of allosteric inhibitors of protein kinase CK2

INTRODUCTION

Due to its physiological role into promoting cell survival and its dysregulation in most cancer cells, protein kinase CK2 is a relevant physiopathological target for development of chemical inhibitors. We report the discovery of azonaphthalene derivatives, as a new family of highly specific CK2 inhibitors. First, we demonstrated that CK2 inhibition (IC50= 0.4 µM) was highly specific, reversible and non ATP-competitive. Small Angle X-ray Scattering experiments showed that this inhibition was due to large conformational change of CK2α upon binding of these inhibitors. We showed that several compounds of the family were cell-potent CK2 inhibitors promoting cell cycle arrest of human glioblastoma U373 cells. Finally, in vitro and in vivo assays showed that these compounds could decrease U373 cell tumor mass by 83 % emphasizing their efficacy against these apoptosis-resistant tumors. In contrast, Azonaphthalene derivatives inactive on CK2 activity showed no effect in colony formation and tumor regression assays. These findings illustrate the emergence of nonclassical CK2 inhibitors and provide exciting opportunities for the development of novel allosteric CK2 inhibitors.

BACKGROUND

CK2 is an emerging therapeutic target and ATP-competitive inhibitors have been identified. CK2 is endowed with specific structural features providing alternative strategies for inhibition.

RESULTS

Azonaphthalene compounds are allosteric CK2 inhibitors showing antitumor activity.

CONCLUSION

CK2 may be targeted allosterically.

SIGNIFICANCE

These inhibitors provide a foundation for a new paradigm for specific CK2 inhibition.

Phosphorylation and stabilization of topoisomerase IIα protein by p38γ mitogen-activated protein kinase sensitize breast cancer cells to its poisons

Cancer drugs suppress tumor cell growth by inhibiting specific cellular targets. However, most drugs also activate several cellular nonspecific stress pathways, and the implications of these off-target effects are mostly unknown. Here, we report that p38γ, but not p38α, MAPK is specifically activated by treatment of breast cancer cells with topoisomerase II (Topo II) drugs, whereas paclitaxel (Taxol) does not have this effect. The activated p38γ in turn phosphorylates and stabilizes Topo IIα protein, and this enhances the growth inhibition by Topo II drugs. Moreover, p38γ activity was shown to be necessary and sufficient for Topo IIα expression, the drug-p38γ-Topo IIα axis is only detected in intrinsically sensitive but not resistant cells, and p38γ is co-overexpressed with Topo IIα protein in primary breast cancers. These results reveal a new paradigm in which p38γ actively regulates the drug-Topo IIα signal transduction, and this may be exploited to increase the therapeutic activity of Topo II drugs.

Threonine 48 in the BIR domain of survivin is critical to its mitotic and anti-apoptotic activities and can be phosphorylated by CK2 in vitro

In this study we report that the protein kinase CK2 phosphorylates survivin specifically on threonine 48 (T48) within its BIR domain, and that T48 is critical to both the mitotic and anti-apoptotic roles of survivin. Interestingly, during mitosis T48 mutants localise normally, but are unable to support cell growth when endogenous survivin is removed by siRNA. In addition, while overexpression of survivin normally confers inhibition of TRAIL-mediated apoptosis, this protection is abolished by mutation of T48. Furthermore in interphase cells depletion of endogenous survivin causes redistribution of T48 mutants from the cytoplasm to the nucleus and treatment of cells expressing survivin-GFP with the CK2 inhibitor TBB phenocopies this nuclear redistribution. Finally, we show T48 mutants have increased affinity for borealin, and that this association and cell proliferation can be restored by introduction of a second mutation at T97. To our knowledge these data are the first to identify T48 as a key regulatory site on survivin, and CK2 as a mediator of its mitotic and anti-apoptotic functions.

Tyrosine 656 in topoisomerase IIβ is important for the catalytic activity of the enzyme: Identification based on artifactual +80-Da modification at this site

Topoisomerase (topo) II catalyzes topological changes in DNA. Although both human isozymes, topo IIα and β are phosphorylated, site-specific phosphorylation of topo IIβ is poorly characterized. Using LC-MS/MS analysis of topo IIβ, cleaved with trypsin, Arg C or cyanogen bromide (CNBr) plus trypsin, we detected four +80-Da modified sites: tyr656, ser1395, thr1426 and ser1545. Phosphorylation at ser1395, thr1426 and ser1545 was established based on neutral loss of H(3) PO(4) (-98 Da) in the CID spectra and on differences in 2-D-phosphopeptide maps of (32) P-labeled wild-type (WT) and S1395A or T1426A/S1545A mutant topo IIβ. However, phosphorylation at tyr656 could not be verified by 2-D-phosphopeptide mapping of (32) P-labeled WT and Y656F mutant protein or by Western blotting with phosphotyrosine-specific antibodies. Since the +80-Da modification on tyr656 was observed exclusively during cleavage with CNBr and trypsin, this modification likely represented bromination, which occurred during CNBr cleavage. Re-evaluation of the CID spectra identified +78/+80-Da fragment ions in CID spectra of two peptides containing tyr656 and tyr711, confirming bromination. Interestingly, mutation of only tyr656, but not ser1395, thr1326 or ser1545, decreased topo IIβ activity, suggesting a functional role for tyr656. These results, while identifying an important tyrosine in topo IIβ, underscore the importance of careful interpretation of modifications having the same nominal mass.

Combretastatin A-4 inhibits cell growth and metastasis in bladder cancer cells and retards tumour growth in a murine orthotopic bladder tumour model

BACKGROUND AND PURPOSE

Bladder cancer is a highly recurrent cancer after intravesical therapy, so new drugs are needed to treat this cancer. Hence, we investigated the anti-cancer activity of combretastatin A-4 (CA-4), an anti-tubulin agent, in human bladder cancer cells and in a murine orthotopic bladder tumour model.

EXPERIMENTAL APPROACH

Cytotoxicity of CA-4 was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, propidium iodide (PI) staining assay and clonogenic survival assay. In vivo microtubule assembly assay, cell cycle analyses, Western blot and cell migration assay were used to study the mechanism of CA-4. The effect of intravesical CA-4 therapy on the development of tumours was studied in the murine orthotopic bladder tumour model.

KEY RESULTS

CA-4 inhibited microtubule polymerization in vivo. Cytotoxic IC(50) values of CA-4 in human bladder cancer cells were below 4 nM. Analyses of cell-cycle distribution showed CA-4 obviously induced G(2)-M phase arrest with sub-G(1) formation. The analyses of apoptosis showed that CA-4 induced caspase-3 activation and decreased BubR1 and Bub3 in cancer cells. In addition to apoptosis, CA-4 was also found to induce the formation of multinucleated cells. CA-4 had a significantly reduced cell migration in vitro. Importantly, the in vivo study revealed that intravesical CA-4 therapy retarded the development of murine bladder tumours.

CONCLUSIONS AND IMPLICATIONS

These data demonstrate that CA-4 kills bladder cancer cells by inducing apoptosis and mitotic catastrophe. It inhibited cell migration in vitro and tumour growth in vivo. Hence, CA-4 intravesical therapy could provide another strategy for treating superficial bladder cancers.

SUMO modification of DNA topoisomerase II: trying to get a CENse of it all

DNA topoisomerase II (topo II) is an essential determinant of chromosome structure and function, acting to resolve topological problems inherent in recombining, transcribing, replicating and segregating DNA. In particular, the unique decatenating activity of topo II is required for sister chromatids to disjoin and separate in mitosis. Topo II exhibits a dynamic localization pattern on mitotic chromosomes, accumulating at centromeres and axial chromosome cores prior to anaphase. In organisms ranging from yeast to humans, a fraction of topo II is targeted for SUMO conjugation in mitotic cells, and here we review our current understanding of the significance of this modification. As we shall see, an emerging consensus is that in metazoans SUMO modification is required for topo II to accumulate at centromeres, and that in the absence of this regulation there is an elevated frequency of chromosome non-disjunction, segregation errors, and aneuploidy. The underlying molecular mechanisms for how SUMO controls topo II are as yet unclear. In closing, however, we will evaluate two possible interpretations: one in which SUMO promotes enzyme turnover, and a second in which SUMO acts as a localization tag for topo II chromosome trafficking.

Casein kinase I delta/epsilon phosphorylates topoisomerase IIalpha at serine-1106 and modulates DNA cleavage activity

We previously reported that phosphorylation of topoisomerase (topo) IIα at serine-1106 (Ser-1106) regulates enzyme activity and sensitivity to topo II-targeted drugs. In this study we demonstrate that phosphorylation of Ser-1106, which is flanked by acidic amino acids, is regulated in vivo by casein kinase (CK) Iδ and/or CKIɛ, but not by CKII. The CKI inhibitors, CKI-7 and IC261, reduced Ser-1106 phosphorylation and decreased formation of etoposide-stabilized topo II–DNA cleavable complex. In contrast, the CKII inhibitor, 5,6-dichlorobenzimidazole riboside, did not affect etoposide-stabilized topo II–DNA cleavable complex formation. Since, IC261 specifically targets the Ca²⁺-regulated isozymes, CKIδ and CKIɛ, we examined the effect of down-regulating these enzymes on Ser-1106 phosphorylation. Down-regulation of these isozymes with targeted si-RNAs led to hypophosphorylation of the Ser-1106 containing peptide. However, si-RNA-mediated down-regulation of CKIIα and α′ did not alter Ser-1106 phosphorylation. Furthermore, reduced phosphorylation of Ser-1106, observed in HRR25 (CKIδ/ɛ homologous gene)-deleted Saccharomyces cerevisiae cells transformed with human topo IIα, was enhanced following expression of human CKIɛ. Down-regulation of CKIδ and CKIɛ also led to reduced formation of etoposide stabilized topo II–DNA cleavable complex. These results provide strong support for an essential role of CKIδ/ɛ in phosphorylating Ser-1106 in human topo IIα and in regulating enzyme function.

An integrated comparative phosphoproteomic and bioinformatic approach reveals a novel class of MPM-2 motifs upregulated in EGFRvIII-expressing glioblastoma cells

Glioblastoma (GBM, WHO grade IV) is an aggressively proliferative and invasive brain tumor that carries a poor clinical prognosis with a median survival of 9 to 12 months. In a prior phosphoproteomic study performed in the U87MG glioblastoma cell line, we identified tyrosine phosphorylation events that are regulated as a result of titrating EGFRvIII, a constitutively active mutant of the epidermal growth factor receptor (EGFR) associated with poor prognosis in GBM patients. In the present study, we have used the phosphoserine/phosphothreonine-specific antibody MPM-2 (mitotic protein monoclonal #2) to quantify serine/threonine phosphorylation events in the same cell lines. By employing a bioinformatic tool to identify amino acid sequence motifs regulated in response to increasing oncogene levels, a set of previously undescribed MPM-2 epitope sequence motifs orthogonal to the canonical "pS/pT-P" motif was identified. These motifs contain acidic amino acids in combinations of the -5, -2, +1, +3, and +5 positions relative to the phosphorylated amino acid. Phosphopeptides containing these motifs are upregulated in cells expressing EGFRvIII, raising the possibility of a general role for a previously unrecognized acidophilic kinase (e.g. casein kinase II (CK2)) in cell proliferation downstream of EGFR signaling.

The impact of the human DNA topoisomerase II C-terminal domain on activity

Background

Type II DNA topoisomerases (topos) are essential enzymes needed for the resolution of topological problems that occur during DNA metabolic processes. Topos carry out an ATP-dependent strand passage reaction whereby one double helix is passed through a transient break in another. Humans have two topoII isoforms, α and β, which while enzymatically similar are differentially expressed and regulated, and are thought to have different cellular roles. The C-terminal domain (CTD) of the enzyme has the most diversity, and has been implicated in regulation. We sought to investigate the impact of the CTD domain on activity.

Methodology/Principle Findings

We have investigated the role of the human topoII C-terminal domain by creating constructs encoding C-terminally truncated recombinant topoIIα and β and topoIIα+β-tail and topoIIβ+α-tail chimeric proteins. We then investigated function in vivo in a yeast system, and in vitro in activity assays. We find that the C-terminal domain of human topoII isoforms is needed for in vivo function of the enzyme, but not needed for cleavage activity. C-terminally truncated enzymes had similar strand passage activity to full length enzymes, but the presence of the opposite C-terminal domain had a large effect, with the topoIIα-CTD increasing activity, and the topoIIβ-CTD decreasing activity.

Conclusions/Significance

In vivo complementation data show that the topoIIα C-terminal domain is needed for growth, but the topoIIβ isoform is able to support low levels of growth without a C-terminal domain. This may indicate that topoIIβ has an additional localisation signal. In vitro data suggest that, while the lack of any C-terminal domain has little effect on activity, the presence of either the topoIIα or β C-terminal domain can affect strand passage activity. Data indicates that the topoIIβ-CTD may be a negative regulator. This is the first report of in vitro data with chimeric human topoIIs.

Plk1-dependent phosphorylation regulates functions of DNA topoisomerase IIalpha in cell cycle progression

Plk1 (Polo-like kinase 1) has been documented as a critical regulator of many mitotic events. However, increasing evidence supports the notion that Plk1 might also have functions outside of mitosis. Using biochemical fractionation and RNA interference approaches, we found that Plk1 was required for both G(1)/S and G(2)/M phases and that DNA topoisomerase IIalpha (topoIIalpha) was a potential target for Plk1 in both interphase and mitosis. Plk1 phosphorylates Ser(1337) and Ser(1524) of topoIIalpha. Overexpression of an unphosphorylatable topoIIalpha mutant led to S phase arrest, suggesting that Plk1-associated phosphorylation first occurs in S phase. Moreover, overexpression of the unphosphorylatable topoIIalpha mutant activated the ATM/R-dependent DNA damage checkpoint, probably due to reduced catalytic activity of topoIIalpha, and resulted in accumulation of catenated DNA. Finally, we showed that wild type topoIIalpha, but not the unphosphorylatable mutant, was able to rescue topoIIalpha depletion-induced defects in sister chromatid segregation, indicating that Plk1-associated phosphorylation is essential for the functions of topoIIalpha in mitosis.

Large-scale identification of novel mitosis-specific phosphoproteins

Systematic identification of phosphoproteins is essential for understanding cellular signalling pathways since phosphorylation plays important roles in cellular regulation. Monoclonal antibody MPM-2 recognizes a discrete set of mitosis-specific phosphoproteins and constitutes a specific tool to investigate the significance of phosphorylation in cell cycle. However, due to the difficulties in identifying antigens revealed on immunoblot membrane, only minority of MPM-2 antigens have been identified. Here we originated proteomics approaches for large-scale identification of MPM-2 phosphoproteins. Mitotic extracts were run on several two-dimensional gel electrophoresis (2D) in parallel, and stained by Coomassie Blue. Each individual spot on one of the gels was excised, and proteins in it were further resolved by regular SDS-electrophoresis and blotted on membrane for MPM-2 stain. Counterparts of the positive proteins were selected on another parallel 2D gel and identified by mass-spectrometry. Using this strategy, 100 spots were excised from Coomassie-stained 2D gel and screened by 1D immunoblots for MPM-2 reactivity, and 22 proteins containing potential MPM-2 epitope were identified in addition to a known MPM-2 antigen, laminin-binding protein. These results were further validated by immunofluorescence, co-immunoprecipitation and in vitro phosphorylation assay. The identification of an unprecedented number of potential MPM-2 phosphoprotein antigens gives new insight into the range of proteins involved in the regulation of the early stages of cell division. Meanwhile, this strategy could be used wherever unknown antigens are explored, especially for antibodies that can recognize more than one antigen.

The complex p25/Cdk5 kinase in neurofibrillary degeneration and neuronal death: the missing link to cell cycle

Emergence of the cell cycle hypothesis in neurodegenerative disease comes from the numerous lines of evidence showing a tight link between "cell cycle-like reactivation" and neuronal death. Terminally differentiated neurons remain in G0 phase and display, compared to proliferating cells, an opposite regulation pattern of cell cycle markers in that most of the key activators and inhibitors are respectively down- and up-regulated. It has been clearly established that any experimental attempt to force terminally differentiated neurons to divide ultimately leads to their death. Conversely, cell cycle blockade in experimental models of neuronal death is able to rescue neurons. Hence, cell cycle deregulation is certainly among mechanisms governing neuronal death. However, many questions remain unresolved, especially those related to which molecular mechanisms trigger cell cycle deregulation and how this deregulation leads to cell death. In the present review, we focus on neurodegeneration in Alzheimer's disease and discuss the cell cycle deregulation related to this neurodegenerative pathology. Finally, we emphasize the role of p25/Cdk5 kinase complex in this pathological process through retinoblastoma protein phosphorylation and derepression of E2F-responsive genes and other actors such as cdc2, cyclins, and MCM proteins.

Nuclear interactions of topoisomerase II alpha and beta with phospholipid scramblase 1

DNA topoisomerase (topo) II modulates DNA topology and is essential for cell division. There are two isoforms of topo II (α and β) that have limited functional redundancy, although their catalytic mechanisms appear the same. Using their COOH-terminal domains (CTDs) in yeast two-hybrid analysis, we have identified phospholipid scramblase 1 (PLSCR1) as a binding partner of both topo II α and β. Although predominantly a plasma membrane protein involved in phosphatidylserine externalization, PLSCR1 can also be imported into the nucleus where it may have a tumour suppressor function. The interactions of PLSCR1 and topo II were confirmed by pull-down assays with topo II α and β CTD fusion proteins and endogenous PLSCR1, and by co-immunoprecipitation of endogenous PLSCR1 and topo II α and β from HeLa cell nuclear extracts. PLSCR1 also increased the decatenation activity of human topo IIα. A conserved basic sequence in the CTD of topo IIα was identified as being essential for binding to PLSCR1 and binding of the two proteins could be inhibited by a synthetic peptide corresponding to topo IIα amino acids 1430-1441. These studies reveal for the first time a physical and functional interaction between topo II and PLSCR1.

Mitotic phosphorylation: breaking the balance of power by a tactical retreat

Profound changes in the phosphorylation state of many proteins occur during mitosis. It is well established that many of these mitotic phosphorylations are carried out by archetypal mitotic kinases that are activated only during mitosis, shifting the equilibrium of kinases and phosphatases towards phosphorylation. However, many studies have also detailed the phosphorylation of proteins at mitosis by kinases that are constitutively active throughout the cell cycle. In most cases, it is uncertain how kinases and phosphatases that appear to be constitutively active can induce phosphorylations specifically at mitosis. In this issue of the Biochemical Journal, Escargueil and Larsen provide evidence of an interesting alternative mechanism to attain specific mitotic phosphorylation. A mitosis-specific phosphorylation site in DNA topoisomerase IIalpha, which is recognized by the MPM-2 antibody, is phosphorylated by protein kinase CK2. The authors found that phosphorylation of this site is suppressed during interphase due to competing dephosphorylation by protein phosphatase 2A. Interestingly, protein phosphatase 2A is excluded from the nucleus during early mitosis, allowing CK2 to phosphorylate topoisomerase IIalpha. It is possible that similar mechanisms are used to regulate the phosphorylation of other proteins.

Mitosis-specific MPM-2 phosphorylation of DNA topoisomerase IIalpha is regulated directly by protein phosphatase 2A

Recent results suggest a role for topoIIalpha (topoisomerase IIalpha) in the fine-tuning of mitotic entry. Mitotic entry is accompanied by the formation of specific phosphoepitopes such as MPM-2 (mitotic protein monoclonal 2) that are believed to control mitotic processes. Surprisingly, the MPM-2 kinase of topoIIalpha was identified as protein kinase CK2, otherwise known as a constitutive interphase kinase. This suggested the existence of alternative pathways for the creation of mitotic phosphoepitopes, different from the classical pathway where the substrate is phosphorylated by a mitotic kinase. In the present paper, we report that topoIIalpha is co-localized with both CK2 and PP2A (protein phosphatase 2A) during interphase. Simultaneous incubation of purified topoIIalpha with CK2 and PP2A had minimal influence on the total phosphorylation levels of topoIIalpha, but resulted in complete disappearance of the MPM-2 phosphoepitope owing to opposite sequence preferences of CK2 and PP2A. Accordingly, short-term exposure of interphase cells to okadaic acid, a selective PP2A inhibitor, was accompanied by the specific appearance of the MPM-2 phosphoepitope on topoIIalpha. During early mitosis, PP2A was translocated from the nucleus, while CK2 remained in the nucleus until pro-metaphase thus permitting the formation of the MPM-2 phosphoepitope. These results underline the importance of protein phosphatases as an alternative way of creating cell-cycle-specific phosphoepitopes.

Mitotic phosphorylation of tankyrase, a PARP that promotes spindle assembly, by GSK3

The assembly and function of mitotic spindles require poly(ADP-ribosyl)ation of spindle components by tankyrase, a poly(ADP-ribose) polymerase that aggregates to spindle poles during mitosis. Tankyrase itself is phosphorylated during mitosis, but the kinases involved remain undefined. Herein we report that mitotic phosphorylation of tankyrase is abrogated in cells treated with the GSK3 inhibitors LiCl and indirubin. Moreover, the electrophoretic mobility-shift of tankyrase arising from mitotic phosphorylation can be reproduced in vitro by GSK3-mediated phosphorylation. Lastly, mutagenesis study suggested that GSK3 in vitro phosphorylates tankyrase on S978, T982, S987, and S991, residues that comprise two adjacent copies of the canonical GSK3 phospho-acceptor motif [S/T]-X-X-X-[S/T]. Collectively, our data suggest that GSK3 contributes to mitotic tankyrase phosphorylation, raising the possibility that this phosphorylation might mediate some of the established roles of GSK3 in spindle assembly and mitotic progression.

Germline variations of the topoisomerase IIα gene as risk factors for primary gastric B-cell lymphoma

We investigated if germline variations of the Topoisomerase II alpha gene could predispose patients with chronic Helicobacter pylori infection to develop gastric lymphoma and conducted a mutation detection of the entire promoter region. Single marker and haplotype analysis did not reveal any associations with development of gastric lymphoma in general, histological grade or stage of disease (P>0.05). No genetic variations in the promotor region were found in 92 chromosomes of lymphoma patients and controls and linkage disequilibrium indicated a highly conserved genomic region. The results of our work exclude genetic variations as predisposing factors of primary gastric B-cell lymphoma development.

The topoisomerase II-Hsp90 complex: a new chemotherapeutic target?

The modulation of DNA topology by topoisomerase II plays a crucial role during chromosome condensation and segregation in mitosis and has thus become a highly attractive target for chemotherapeutic drugs. However, these drugs are highly toxic, and so new approaches are required. One such strategy is to target topoisomerase II-interacting proteins. Here we report the identification of potential topoisomerase II-associated proteins using immunoprecipitation, followed by 1-D and 2-D gel electrophoresis and MALDI-TOF mass spectrometry. A total of 23 proteins were identified and, of these, 17 were further validated as topoisomerase IIalpha-associated proteins by coimmunoprecipitation and Western blot. Six of the interacting proteins were cellular chaperones, including 3 members of the heat shock protein-90 (Hsp90) family, and so the effect of Hsp90 modulation on the antitumor activity of topoisomerase II drugs was tested using the sulforhodamine B assay, clonogenic assays and a xenograft model. The Hsp90 inhibitors geldanamycin, 17-AAG (17-allylamino-17-demethoxygeldanamycin) and radicicol significantly enhanced the activity of the topoisomerase II poisons etoposide and mitoxantrone in vitro and in vivo. Thus, our method of identifying topoisomerase II-interacting proteins appears to be effective, and at least 1 novel topoisomerase IIalpha-associated protein, Hsp90, may represent a valid drug target in the context of topoisomerase II-directed chemotherapy.

CK2 interacting proteins: emerging paradigms for CK2 regulation?

Protein kinase CK2 represents a small family of highly conserved protein kinases involved in a complex series of cellular events. Furthermore, CK2 has been localised to many discrete cellular sites and has an extensive and diverse array of substrates and interaction partners in cells. Despite considerable investigation, the precise mechanism(s) of regulation of CK2 in cells remains poorly understood. In consideration of the prospect that cells contain many distinct sub-populations of CK2 that are distinguished on the basis of localisation and/or interactions with other cellular components, one possibility is that there may be differential regulation of specific sub-populations of CK2. With this in mind, some of the individual sub-populations of CK2 may be regulated through particular protein-protein interactions that may play a role in recruiting CK2 into the vicinity of its substrates and/or modulating its ability to phosphorylate specific cellular targets. In this respect, here we examine two CK2-interacting proteins, namely Pin1 and CKIP-1 that have been shown to participate in the modulation of CK2 specificity or the subcellular localisation of CK2, respectively. One aspect of this work has been focused on the prospect that Pin1 interacts with CK2 in response to UV stimulation in a manner analogous to the phosphorylation-dependent interactions of CK2 that occur following the mitotic phosphorylation of CK2. A second aspect of this work involves an examination of the structural basis for interactions between CK2 and CKIP-1 with emphasis on a putative HIKE domain in CK2.

Fip1--an essential component of the Saccharomyces cerevisiae polyadenylation machinery is phosophorylated by protein kinase CK2

Since Fip1 is phosphoprotein we investigated whether it is a substrate for protein kinase CK2. According to the amino acid sequence Fip1 harbours twenty putative CK2 phosphorylation sites. Here we have report characterization of Fip1 as a substrate for both forms of CK2. Fip1 serves as a substrate for both the recombinant CK2alpha ' (Km 1.28 microM) and holoenzyme (Km 1.4 microM) but not for CK1. By MALDI-MS we identified the two serine residues at positions 73 and 77 as the possible in vitro phosphorylation sites. These data may help to elucidate the role of Fip1 in the mRNA 3'-OH polyadenylation process and the involvement of CK2 mediated phosphorylation in regulation of interactions and activity members of cleavage/polyadenylation factor (CPF) complex.

The 'regulatory' beta-subunit of protein kinase CK2 negatively influences p53-mediated allosteric effects on Chk2 activation

The 'regulatory' beta-subunit of protein kinase CK2 has previously been shown to interact with protein kinases such as A-Raf, c-Mos, Lyn and Chk1 in addition to the catalytic subunit of CK2. Sequence alignments suggest that these interactions have a structural basis, and hence other protein kinases harboring corresponding sequences may be potential interaction partners for CK2beta. We show here that Chk2 specifically interacts with CK2beta in vitro and in cultured cells, and that activation of Chk2 leads to a reduction of this interaction. Additionally, we show that the presence of the CK2beta-subunit significantly reduces the Chk2-catalysed phosphorylation of p53 in vitro. These findings support the notion that CK2beta can act as a general modulator of remote docking sites in protein kinase--substrate interactions.

Down-Regulation of DNA Topoisomerase IIα Leads to Prolonged Cell Cycle Transit in G2and Early M Phases and Increased Survival to Microtubule-Interacting Agents

Microtubule binders are cell cycle-specific agents with preferential cytotoxicity toward mitotic cells. We have characterized vincristine-selected human leukemia cells to establish whether development of vincristine resistance was accompanied by changes in cell cycle kinetics and distribution. Our results indicate that vincristine resistance is accompanied by delayed G2 transit and prolonged early mitosis in both the absence and the presence of the microtubule binder nocodazole. The altered G2/M regulation is accompanied by resistance to short-term (12 h) but not continuous nocodazole exposure in agreement with the transient nature of the observed cell cycle alterations. Western blot analysis indicates that vincristine-selection is accompanied by down-regulation of topoisomerase IIalpha without detectable alterations of the other mitotic regulators studied, including Cdk1, p21, 14-3-3sigma, and 14-3-3epsilon. This was associated with at least 7-fold less chromosome-associated topoisomerase IIalpha, decreased catalytic activity, and cross-resistance to topoisomerase II inhibitors. Characterization of isogenic cell lines expressing different levels of topoisomerase II proteins shows that cellular levels of topoisomerase IIalpha, but not the closely related topoisomerase IIbeta, directly influence the cell cycle kinetics in G2 and early mitosis as well as the resistance to nocodazole. These results underline the importance of topoisomerase IIalpha in late G2 and early M phases and provide evidence for an as-yet-unsuspected interaction between topoisomerase II and microtubule-directed agents.

Class I histone deacetylase-selective novel synthetic inhibitors potently inhibit human tumor proliferation

We have developed previously a class of synthetic hybrid histone deacetylase (HDAC) inhibitors, which were built from hydroxamic acid of trichostatin A and pyridyl ring of MS-275. In this study we evaluated the antitumor effects of these novel hybrid synthetic HDAC inhibitors, SK-7041 and SK-7068, on human cancer cells. Both SK-7041 and SK-7068 effectively inhibited cellular HDAC activity at nanomolar concentrations and induced the time-dependent hyperacetylation of histones H3 and H4. These HDAC inhibitors preferentially inhibited the enzymatic activities of HDAC1 and HDAC2, as compared with the other HDAC isotypes, indicating that class I HDAC is the major target of SK-7041 and SK-7068. We found that these compounds exhibited potent antiproliferative activity against various human cancer cells in vitro. Growth inhibition effect of SK-7041 and SK-7068 was related with the induction of aberrant mitosis and apoptosis in human gastric cancer cells. Both compounds induced the accumulation of cells at mitosis after 6 h of treatment, which was demonstrated by accumulation of tetraploid cells, lack of G(2) cyclin/cyclin-dependent kinase inactivation, and higher mitotic index. After 12 h of treatment, apoptotic cells were increased through mitochondrial and caspase-mediated pathway. Finally, in vivo experiment showed that SK-7041 or SK-7068 was found to reduce the growth of implanted human tumors in nude mice. Therefore, based on isotype specificity and antitumor activity, SK-7041 and SK-7068 HDAC inhibitors are expected to be promising anticancer therapeutic agents and need additional clinical development.

The antitumor triazoloacridone C-1305 is a topoisomerase II poison with unusual properties

C-1305 [S-[[3-(dimethylamino)propyl]amino]-8-hydroxy-6H-v-triazolo[4,5,1-de]acridin-6-one] is a triazoloacridone with excellent activity in colon cancer models. The mechanism of C-1305 is unknown, although similarities in the chemical structure between C-1305 and amsacrine suggest common cellular targets. Here, we report that C-1305 is a topoisomerase II poison that is able to induce cleavable complexes with topoisomerase II in vitro as well as in living cells. Even at optimal concentrations, C-1305 is a much weaker inducer of cleavable complexes than amsacrine. Because the cytotoxic activities of the two compounds after continuous drug exposure are comparable, these findings suggest that the low levels of cleavable complexes induced by C-1305 may be unusually toxic. In contrast to amsacrine, the cytotoxicity of C-1305 is strongly time-dependent, with at least 24 h of drug exposure required for optimal cytotoxicity. The p53 tumor suppressor is inactivated in the majority of human tumors, including colorectal cancers. We therefore compared the long-term cytotoxic effects of C-1305, amsacrine, and doxorubicin on human cell lines in which the p53 or p21 pathways have been specifically disrupted by targeted homologous recombination. Disruption of p53 and p21 had minor influence on the cytotoxicity of doxorubicin, whereas p53 but not p21 disruption was associated with increased resistance to amsacrine. In marked contrast, disruption of p53 and p21 was associated with increased sensitivity to C-1305. Taken together, our results show that exposure to C-1305 is accompanied by the formation of low levels of potent cleavable complexes that are selectively toxic toward tumor cells with defective p53 function.