Casein kinase II is required for transition of G0/G1, early G1, and G1/S phases of the cell cycle

Casein kinase 2 complex: a central regulator of multiple pathobiological signaling pathways in Cryptococcus neoformans

The casein kinase 2 (CK2) complex has garnered extensive attention over the past decades as a potential therapeutic target for diverse human diseases, including cancer, diabetes, and obesity, due to its pivotal roles in eukaryotic growth, differentiation, and metabolic homeostasis. While CK2 is also considered a promising antifungal target, its role in fungal pathogens remains unexplored. In this study, we investigated the functions and regulatory mechanisms of the CK2 complex in Cryptococcus neoformans, a major cause of fungal meningitis. The cryptococcal CK2 complex consists of a single catalytic subunit, Cka1, and two regulatory subunits, Ckb1 and Ckb2. Our findings show that Cka1 plays a primary role as a protein kinase, while Ckb1 and Ckb2 have major and minor regulatory functions, respectively, in growth, cell cycle control, morphogenesis, stress response, antifungal drug resistance, and virulence factor production. Interestingly, triple mutants lacking all three subunits (cka1Δ ckb1Δ ckb2Δ) exhibited more severe phenotypic defects than the cka1Δ mutant alone, suggesting that Ckb1/2 may have Cka1-independent functions. In a murine model of systemic cryptococcosis, cka1Δ and cka1Δ ckb1Δ ckb2Δ mutants showed severely reduced virulence. Transcriptomic, proteomic, and phosphoproteomic analyses further revealed that the CK2 complex controls a wide array of effector proteins involved in transcriptional regulation, cell cycle control, nutrient metabolisms, and stress responses. Most notably, CK2 disruption led to dysregulation of key signaling cascades central to C. neoformans pathogenicity, including the Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin signaling pathways. In summary, our study provides novel insights into the multifaceted roles of the fungal CK2 complex and presents a compelling case for targeting it in the development of new antifungal drugs.IMPORTANCEThe casein kinase 2 (CK2) complex, crucial for eukaryotic growth, differentiation, and metabolic regulation, presents a promising therapeutic target for various human diseases, including cancer, diabetes, and obesity. Its potential as an antifungal target is further highlighted in this study, which explores CK2's functions in C. neoformans, a key fungal meningitis pathogen. The CK2 complex in C. neoformans, comprising the Cka1 catalytic subunit and Ckb1/2 regulatory subunits, is integral to processes like growth, cell cycle, morphogenesis, stress response, drug resistance, and virulence. Our findings of CK2's role in regulating critical signaling pathways, including Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin, underscore its importance in C. neoformans pathogenicity. This study provides valuable insights into the fungal CK2 complex, reinforcing its potential as a target for novel antifungal drug development and pointing out a promising direction for creating new antifungal agents.

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.

Essential role of CK2α for the interaction and stability of replication fork factors during DNA synthesis and activation of the S-phase checkpoint

The ataxia telangiectasia mutated and Rad3-related (ATR)-CHK1 pathway is the major signalling cascade activated in response to DNA replication stress. This pathway is associated with the core of the DNA replication machinery comprising CDC45, the replicative MCM2-7 hexamer, GINS (altogether forming the CMG complex), primase-polymerase (POLε, -α, and -δ) complex, and additional fork protection factors such as AND-1, CLASPIN (CLSPN), and TIMELESS/TIPIN. In this study, we report that functional protein kinase CK2α is critical for preserving replisome integrity and for mounting S-phase checkpoint signalling. We find that CDC45, CLSPN and MCM7 are novel CK2α interacting partners and these interactions are particularly important for maintenance of stable MCM7-CDC45, ATRIP-ATR-MCM7, and ATR-CLSPN protein complexes. Consistently, cells depleted of CK2α and treated with hydroxyurea display compromised replisome integrity, reduced chromatin binding of checkpoint mediator CLSPN, attenuated ATR-mediated S-phase checkpoint and delayed recovery of stalled forks. In further support of this, differential gene expression analysis by RNA-sequencing revealed that down-regulation of CK2α accompanies global shutdown of genes that are implicated in the S-phase checkpoint. These findings add to our understanding of the molecular mechanisms involved in DNA replication by showing that the protein kinase CK2α is essential for maintaining the stability of the replisome machinery and for optimizing ATR-CHK1 signalling activation upon replication stress.

The SUMO E3 Ligase MdSIZ1 Sumoylates a Cell Number Regulator MdCNR8 to Control Organ Size

Plant growth and organ size putatively associated with crop yield are regulated by a complex network of genes including ones for controlling cell proliferation. The gene fw2.2 was first identified in tomatoes and reported to govern fruit size variation through controlling cell division. In this study, we isolated a putative ortholog of the tomato fw2.2 gene from apple, Cell Number Regulator 8 (MdCNR8). Our functional analysis showed that MdCNR8 may control fruit size and root growth. MdCNR8 was mediated by the SUMO E3 ligase MdSIZ1, and SUMOylation of MdCNR8 at residue-Lys39 promoted the translocation of MdCNR8 from plasma membrane to the nucleus. The effect of MdCNR8 in inhibiting root elongation could be completely counteracted by the coexpression of MdSIZ1. Moreover, the lower cell proliferation of apple calli due to silencing MdSIZ1 could be rescued by silencing MdCNR8. Collectively, our results showed that the MdSIZ1-mediated SUMOylation is required for the fulfillment of MdCNR8 in regulating cell proliferation to control plant organ size. This regulatory interaction between MdSIZ1 and MdCNR8 will facilitate understanding the mechanism underlying the regulation of organ size.

Casein kinase II activates Bik to induce death of hyperplastic mucous cells in a cell cycle-dependent manner

Extensive inflammation causes epithelial cell hyperplasia in the airways and Bcl-2-interacting killer (Bik) reduces epithelial cell and mucous cell hyperplasia without affecting resting cells to restore homeostasis. These observations suggest that Bik induces apoptosis in a cell cycle-specific manner, but the mechanisms are not understood. Mice were exposed to an allergen for 3, 14, or 30 days and Bik expression was induced in airway epithelia of transgenic mice. Bik reduced epithelial and mucous cell hyperplasia when mice were exposed to an allergen for 3 or 14 days, but not when exposure lasted for 30 days, and Ki67-positivity was reduced. In culture, Bik expression killed proliferating cells but not quiescent cells. To capture the stage of the cell cycle when Bik induces cell death, airway cells that express fluorescent ubiquitin cell cycle indicators were generated that fluoresce red or green during the G0/G1 and S/G2/M phases of the cells cycle, respectively. Regardless of the cell cycle stage, Bik expression eliminated green-fluorescent cells. Also, Bik, when tagged with a blue-fluorescent protein, was only detected in green cells. Bik phosphorylation mutants at threonine 33 or serine 35 demonstrated that phosphorylation activated Bik to induce death even in quiescent cells. Immunoprecipitation and proteomic approaches identified casein kinase IIα to be responsible for phosphorylating and activating Bik to kill cells in S/G2/M. As casein kinase 2 alpha (CKIIα) is expressed only during the G2/M phase, we conclude that Bik activation in airway epithelial cells selectively targets hyperplastic epithelial cells, while leaving resting airway cells unaffected.

The Effect of Casein Kinase 2 Inhibition on three Leukemic Cell Lines

Background::

Casein Kinase 2 (CK2) is a Ser/Thr protein kinase that coregulates a great number of signalling pathways in the cell. It is involved in cell cycle regulation and cell proliferation, apoptosis, DNA damage response and gene transcription. Its substrates are numerous kinases and transcription factors. It was found to be upregulated in different tumours, and certain types of leukaemia are very sensitive to its inhibition.

Objective::

We analysed the effects of casein kinase 2 inhibition on three leukaemia cell lines of B and T cell origin: Jurkat, a T cell line, CLL, a chronic B lymphocytic leukaemia cell line and 697, a pre-B acute lymphocytic leukaemia cell line. Besides cell proliferation and cytotoxicity analysis, the aim was to investigate the influence of CK2 inhibition on elements of the Notch signalling pathway. Notch signalling has an important role in blood cell differentiation, and CK2 regulates Ikaros, a tumour suppressor interfering with Notch signalling

Methods::

and T leukaemia cells were treated with different concentrations of the CK2 inhibitor, CX-4945, for 6 days, and cell viability and proliferation were determined by Trypan Blue Exclusion Method. Analysis of gene expression was performed by RT-qPCR.

Results::

All three cell lines were sensitive to CK2 inhibition and among them, 697 cells had two times lower IC50. In Jurkat and CLL cells changes in c-Myc and Notch pathway gene expression were found.

Conclusion::

As CK2 is involved in numerous signalling circuits, we concluded that each cell type could have a cell-specific response in gene expression.

Regulation of the Mammalian SWI/SNF Family of Chromatin Remodeling Enzymes by Phosphorylation during Myogenesis

Myogenesis is the biological process by which skeletal muscle tissue forms. Regulation of myogenesis involves a variety of conventional, epigenetic, and epigenomic mechanisms that control chromatin remodeling, DNA methylation, histone modification, and activation of transcription factors. Chromatin remodeling enzymes utilize ATP hydrolysis to alter nucleosome structure and/or positioning. The mammalian SWItch/Sucrose Non-Fermentable (mSWI/SNF) family of chromatin remodeling enzymes is essential for myogenesis. Here we review diverse and novel mechanisms of regulation of mSWI/SNF enzymes by kinases and phosphatases. The integration of classic signaling pathways with chromatin remodeling enzyme function impacts myoblast viability and proliferation as well as differentiation. Regulated processes include the assembly of the mSWI/SNF enzyme complex, choice of subunits to be incorporated into the complex, and sub-nuclear localization of enzyme subunits. Together these processes influence the chromatin remodeling and gene expression events that control myoblast function and the induction of tissue-specific genes during differentiation.

CK2-Dependent Phosphorylation of the Brg1 Chromatin Remodeling Enzyme Occurs during Mitosis

Brg1 (Brahma-related gene 1) is one of two mutually exclusive ATPases that can act as the catalytic subunit of mammalian SWI/SNF (mSWI/SfigureNF) chromatin remodeling enzymes that facilitate utilization of the DNA in eukaryotic cells. Brg1 is a phospho-protein, and its activity is regulated by specific kinases and phosphatases. Previously, we showed that Brg1 interacts with and is phosphorylated by casein kinase 2 (CK2) in a manner that regulates myoblast proliferation. Here, we use biochemical and cell and molecular biology approaches to demonstrate that the Brg1-CK2 interaction occurred during mitosis in embryonic mouse somites and in primary myoblasts derived from satellite cells isolated from mouse skeletal muscle tissue. The interaction of CK2 with Brg1 and the incorporation of a number of other subunits into the mSWI/SNF enzyme complex were independent of CK2 enzymatic activity. CK2-mediated hyperphosphorylation of Brg1 was observed in mitotic cells derived from multiple cell types and organisms, suggesting functional conservation across tissues and species. The mitotically hyperphosphorylated form of Brg1 was localized with soluble chromatin, demonstrating that CK2-mediated phosphorylation of Brg1 is associated with specific partitioning of Brg1 within subcellular compartments. Thus, CK2 acts as a mitotic kinase that regulates Brg1 phosphorylation and subcellular localization.

Role of CK2 inhibitor CX-4945 in anti-cancer combination therapy - potential clinical relevance

BACKGROUND

Protein kinase CK2 inhibition has long been considered as an attractive anti-cancer strategy based on the following considerations: CK2 is a pro-survival kinase, it is frequently over-expressed in human tumours and its over-expression correlates with a worse prognosis. Preclinical evidence strongly supports the feasibility of this target and, although dozens of CK2 inhibitors have been described in the literature so far, CX-4945 (silmitasertib) was the first that entered into clinical trials for the treatment of both human haematological and solid tumours. However, kinase inhibitor monotherapies turned out to be effective only in a limited number of malignancies, probably due to the multifaceted causes that underlie them, supporting the emerging view that multi-targeted approaches to treat human tumours could be more effective.

CONCLUSIONS

In this review, we will address combined anti-cancer therapeutic strategies described so far which involve the use of CX-4945. Data from preclinical studies clearly show the ability of CX-4945 to synergistically cooperate with different classes of anti-neoplastic agents, thereby contributing to an orchestrated anti-tumour action against multiple targets. Overall, these promising outcomes support the translation of CX-4945 combined therapies into clinical anti-cancer applications.

Down-regulation of CK2α correlates with decreased expression levels of DNA replication minichromosome maintenance protein complex (MCM) genes

Protein kinase CK2 is a serine/threonine kinase composed of two catalytic subunits (CK2α and/or CK2α’) and two regulatory subunits (CK2β). It is implicated in every stage of the cell cycle and in the regulation of various intracellular pathways associated with health and disease states. The catalytic subunits have similar biochemical activity, however, their functions may differ significantly in cells and in vivo. In this regard, homozygous deletion of CK2α leads to embryonic lethality in mid-gestation potentially due to severely impaired cell proliferation. To determine the CK2α-dependent molecular mechanisms that control cell proliferation, we established a myoblast-derived cell line with inducible silencing of CK2α and carried out a comprehensive RNA-Seq analysis of gene expression. We report evidence that CK2α depletion causes delayed cell cycle progression through the S-phase and defective response to replication stress. Differential gene expression analysis revealed that the down-regulated genes were enriched in pathways implicated in cell cycle regulation, DNA replication and DNA damage repair. Interestingly, the genes coding for the minichromosome maintenance proteins (MCMs), which constitute the core of the replication origin recognition complex, were among the most significantly down-regulated genes. These findings were validated in cells and whole mouse embryos. Taken together, our study provides new evidence for a critical role of protein kinase CK2 in controlling DNA replication initiation and the expression levels of replicative DNA helicases, which ensure maintenance of proliferative potential and genome integrity in eukaryotic cells.

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.

Casein kinase 2-mediated phosphorylation of Brahma-related gene 1 controls myoblast proliferation and contributes to SWI/SNF complex composition

Transcriptional regulation is modulated in part by chromatin-remodeling enzymes that control gene accessibility by altering chromatin compaction or nucleosome positioning. Brahma-related gene 1 (Brg1), a catalytic subunit of the mammalian SWI/SNF chromatin-remodeling enzymes, is required for both myoblast proliferation and differentiation, and the control of Brg1 phosphorylation by calcineurin, PKCβ1, and p38 regulates the transition to differentiation. However, we hypothesized that Brg1 activity might be regulated by additional kinases. Here, we report that Brg1 is also a target of casein kinase 2 (CK2), a serine/threonine kinase, in proliferating myoblasts. We found that CK2 interacts with Brg1, and mutation of putative phosphorylation sites to non-phosphorylatable (Ser to Ala, SA) or phosphomimetic residues (Ser to Glu, SE) reduced Brg1 phosphorylation by CK2. Although BRG1-deleted myoblasts that ectopically express the SA-Brg1 mutant proliferated similarly to the parental cells or cells ectopically expressing wild-type (WT) Brg1, ectopic expression of the SE-Brg1 mutant reduced proliferation and increased cell death, similar to observations from cells lacking Brg1. Moreover, pharmacological inhibition of CK2 increased myoblast proliferation. Furthermore, the Pax7 promoter, which controls expression of a key transcription factor required for myoblast proliferation, was in an inaccessible chromatin state in the SE-Brg1 mutant, suggesting that hyperphosphorylated Brg1 cannot remodel chromatin. WT-, SA-, and SE-Brg1 exhibited distinct differences in interacting with and affecting expression of the SWI/SNF subunits Baf155 and Baf170 and displayed differential sub-nuclear localization. Our results indicate that CK2-mediated phosphorylation of Brg1 regulates myoblast proliferation and provides insight into one mechanism by which composition of the mammalian SWI/SNF enzyme complex is regulated.

Inhibition of casein kinase 2 blocks G2/M transition in early embryo mitosis but not in oocyte meiosis in mouse

Casein kinase 2 (CK2) is a highly conserved, ubiquitously expressed serine/threonine protein kinase with hundreds of substrates. The role of CK2 in the G₂/M transition of oocytes, zygotes, and 2-cell embryos was studied in mouse by enzyme activity inhibition using the specific inhibitor 4, 5, 6, 7-tetrabromobenzotriazole (TBB). Zygotes and 2-cell embryos were arrested at G₂ phase by TBB treatment, and DNA damage was increased in the female pronucleus of arrested zygotes. Further developmental ability of arrested zygotes was reduced, but that of arrested 2-cell embryos was not affected after releasing from inhibition. By contrast, the G₂/M transition in oocytes was not affected by TBB. These results indicate that CK2 activity is essential for mitotic G₂/M transition in early embryos but not for meiotic G₂/M transition in oocytes.

Identification of kinases phosphorylating 13 sites in the nuclear, DNA-binding protein NUCKS

NUCKS is a vertebrate specific, nuclear and DNA-binding phospho protein. The protein is highly expressed in rapidly dividing cells, and is overexpressed in a number of cancer tissues. The phosphorylation of NUCKS is cell cycle and DNA-damage regulated, but little is known about the responsible kinases. By utilizing in vitro and in vivo phosphorylation assays using isolated NUCKS as well as synthetic NUCKS-derived peptides in combination with mass spectrometry, phosphopeptide mapping, phosphphoamino acid analyses, phosphospecific antibodies and the use of specific kinase inhibitors, we found that NUCKS is phosphorylated on 11 sites by CK2. At least 7 of the CK2 sites are phosphorylated in vivo. We also found that NUCKS is phosphorylated on two sites by ATM kinase and DNA-PK in vitro, and is phosphorylated in vivo by ATM kinase in γ-irradiated cells. All together, we identified three kinases phosphorylating 13 out of 39 in vivo phosphorylated sites in mammalian NUCKS. The identification of CK2 and PIKK kinases as kinases phosphorylating NUCKS in vivo provide further evidence for the involvement of NUCKS in cell cycle control and DNA repair.

A Novel Interaction of Ecdysoneless (ECD) Protein with R2TP Complex Component RUVBL1 Is Required for the Functional Role of ECD in Cell Cycle Progression

Ecdysoneless (ECD) is an evolutionarily conserved protein whose germ line deletion is embryonic lethal. Deletion of Ecd in cells causes cell cycle arrest, which is rescued by exogenous ECD, demonstrating a requirement of ECD for normal mammalian cell cycle progression. However, the exact mechanism by which ECD regulates cell cycle is unknown. Here, we demonstrate that ECD protein levels and subcellular localization are invariant during cell cycle progression, suggesting a potential role of posttranslational modifications or protein-protein interactions. Since phosphorylated ECD was recently shown to interact with the PIH1D1 adaptor component of the R2TP cochaperone complex, we examined the requirement of ECD phosphorylation in cell cycle progression. Notably, phosphorylation-deficient ECD mutants that failed to bind to PIH1D1 in vitro fully retained the ability to interact with the R2TP complex and yet exhibited a reduced ability to rescue Ecd-deficient cells from cell cycle arrest. Biochemical analyses demonstrated an additional phosphorylation-independent interaction of ECD with the RUVBL1 component of the R2TP complex, and this interaction is essential for ECD's cell cycle progression function. These studies demonstrate that interaction of ECD with RUVBL1, and its CK2-mediated phosphorylation, independent of its interaction with PIH1D1, are important for its cell cycle regulatory function.

Physalis floridana Cell Number Regulator1 encodes a cell membrane-anchored modulator of cell cycle and negatively controls fruit size

Physalis species show a significant variation in berry size; however, the underlying molecular basis is unknown. In this work, we showed that cell division difference in the ovaries might contribute to the ultimate berry size variation within Physalis species, and that mRNA abundance of Physalis floridana Cell Number Regulator1 (PfCNR1), the putative orthologue of the tomato fruit weight 2.2 (FW2.2), was negatively correlated with cell division in the ovaries. Moreover, heterochronic expression variation of the PfCNR1 genes in the ovaries concomitantly correlated with berry weight variation within Physalis species. In transgenic Physalis, multiple organ sizes could be negatively controlled by altering PfCNR1 levels, and cell division instead of cell expansion was primarily affected. PfCNR1 was shown to be anchored in the plasma membrane and to interact with PfAG2 (an AGAMOUS-like protein determining ovary identity). The expression of PfCYCD2;1, a putative orthologue of the mitosis-specific gene CyclinD2;1 in the cell cycle was negatively correlated with the PfCNR1 mRNA levels. PfAG2 was found to selectively bind to the CArG-box in the PfCYCD2;1 promoter and to repress PfCYCD2;1 expression, thus suggesting a PfAG2-mediated pathway for PfCNR1 to regulate cell division. The interaction of PfCNR1 with PfAG2 enhanced the repression of PfCYCD2;1 expression. The nuclear import of PfAG2 was essential in the proposed pathway. Our data provide new insights into the developmental pathways of a cell membrane-anchored protein that modulates cell division and governs organ size determination. This study also sheds light on the link between organ identity and organ growth in plants.

The Arabidopsis Kinome: phylogeny and evolutionary insights into functional diversification

Background

Protein kinases constitute a particularly large protein family in Arabidopsis with important functions in cellular signal transduction networks. At the same time Arabidopsis is a model plant with high frequencies of gene duplications. Here, we have conducted a systematic analysis of the Arabidopsis kinase complement, the kinome, with particular focus on gene duplication events. We matched Arabidopsis proteins to a Hidden-Markov Model of eukaryotic kinases and computed a phylogeny of 942 Arabidopsis protein kinase domains and mapped their origin by gene duplication.

Results

The phylogeny showed two major clades of receptor kinases and soluble kinases, each of which was divided into functional subclades. Based on this phylogeny, association of yet uncharacterized kinases to families was possible which extended functional annotation of unknowns. Classification of gene duplications within these protein kinases revealed that representatives of cytosolic subfamilies showed a tendency to maintain segmentally duplicated genes, while some subfamilies of the receptor kinases were enriched for tandem duplicates. Although functional diversification is observed throughout most subfamilies, some instances of functional conservation among genes transposed from the same ancestor were observed. In general, a significant enrichment of essential genes was found among genes encoding for protein kinases.

Conclusions

The inferred phylogeny allowed classification and annotation of yet uncharacterized kinases. The prediction and analysis of syntenic blocks and duplication events within gene families of interest can be used to link functional biology to insights from an evolutionary viewpoint. The approach undertaken here can be applied to any gene family in any organism with an annotated genome.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-548) contains supplementary material, which is available to authorized users.

Functional interplay between protein kinase CK2 and salicylic acid sustains PIN transcriptional expression and root development

We have previously reported that CK2-defective Arabidopsis thaliana plants (CK2mut plants) were impaired severely in root development and auxin polar transport, and exhibited transcriptional misregulation of auxin-efflux transporters (Plant J., 67, 2011a, 169). In this work we show that CK2mut roots accumulate high levels of salicylic acid (SA) and that the gene that encodes isochorismate synthase (SID2) is overexpressed, strongly suggesting that CK2 activity is required for SA biosynthesis via the shikimate pathway. Moreover, SA activates transcription of CK2-encoding genes and, thus, SA and CK2 appear to be part of an autoregulatory feed-back loop to fine-tune each other's activities. We also show that exogenous SA and constitutive high SA levels in cpr mutants reproduce the CK2mut root phenotypes (decrease of root length and of number of lateral roots), whereas inhibition of CK2 activity in SA-defective and SA-signalling mutants lead to less severe phenotypes, suggesting that the CK2mut root phenotypes are SA-mediated effects. Moreover, exogenous SA mediates transcriptional repression of most of PIN-FORMED (PIN) genes, which is the opposite effect observed in CK2mut roots. These results prompted us to propose a model in which CK2 acts as a link between SA homeostasis and transcriptional regulation of auxin-efflux transporters. We also show that CK2 overexpression in Arabidopsis has neither impact on SA biosynthesis nor on auxin transport, but it improves the Arabidopsis root system. Thus, unlike the outcome in mammals, an excess of CK2 in plant cells does not produce neoplasia, but it might be advantageous for plant fitness.

Missense variants of uncertain significance (VUS) altering the phosphorylation patterns of BRCA1 and BRCA2

Mutations in BRCA1 and BRCA2 are responsible for a large proportion of breast-ovarian cancer families. Protein-truncating mutations have been effectively used in the clinical management of familial breast cancer due to their deleterious impact on protein function. However, the majority of missense variants identified throughout the genes continue to pose an obstacle for predictive informative testing due to low frequency and lack of information on how they affect BRCA1/2 function. Phosphorylation of BRCA1 and BRCA2 play an important role in their function as regulators of DNA repair, transcription and cell cycle in response to DNA damage but whether missense variants of uncertain significance (VUS) are able to disrupt this important process is not known. Here we employed a novel approach using NetworKIN which predicts in vivo kinase-substrate relationship, and evolutionary conservation algorithms SIFT, PolyPhen and Align-GVGD. We evaluated whether 191 BRCA1 and 43 BRCA2 VUS from the Breast Cancer Information Core (BIC) database can functionally alter the consensus phosphorylation motifs and abolish kinase recognition and binding to sites known to be phosphorylated in vivo. Our results show that 13.09% (25/191) BRCA1 and 13.95% (6/43) BRCA2 VUS altered the phosphorylation of BRCA1 and BRCA2. We highlight six BRCA1 (K309T, S632N, S1143F, Q1144H, Q1281P, S1542C) and three BRCA2 (S196I, T207A, P3292L) VUS as potentially clinically significant. These occurred rarely (n<2 in BIC), mutated evolutionarily conserved residues and abolished kinase binding to motifs established in the literature involved in DNA repair, cell cycle regulation, transcription or response to DNA damage. Additionally in vivo phosphorylation sites identified via through-put methods are also affected by VUS and are attractive targets for studying their biological and functional significance. We propose that rare VUS affecting phosphorylation may be a novel and important mechanism for which BRCA1 and BRCA2 functions are disrupted in breast cancer.

Localization of phosphorylated CK2α to the mitotic spindle requires the peptidyl-prolyl isomerase Pin1

CK2 is a serine/threonine kinase with many substrates, largely unknown modes of regulation and essential roles in mitotic progression. CK2α, a catalytic subunit of CK2, is phosphorylated in mitosis, and here we examine the effect of phosphorylation on CK2α localization. Using phosphospecific antibodies, we show that CK2α localizes to the mitotic spindle in a phosphorylation-dependent manner. Mitotic spindle localization requires the unique C-terminus of CK2α, and involves a novel regulatory mechanism in which phosphorylation of CK2α facilitates binding to the peptidyl-prolyl isomerase Pin1, which is required for CK2α mitotic spindle localization. This could explain how the constitutive activity of CK2α might be targeted towards mitotic substrates. Furthermore, because Pin1 has many important spindle substrates, this might represent a general mechanism for localization of mitotic signalling proteins.

CK2 inhibitors enhance the radiosensitivity of human non-small cell lung cancer cells through inhibition of stat3 activation

CK2 interacts and phosphorylates >300 proteins, including Stat3, and is linked to a number of human cancers. Constitutively activated Stat3 has been reported in 50% of human lung cancers. Inhibition of CK2 activity can induce apoptosis and suppression of Stat3 activation in cancer cells. This study examined the effects of CK2 inhibitors on growth inhibition of lung cancer cells and the therapeutic potential on lung cancer. The CK2 inhibitor and radiation both suppressed cancer cell growth in a dose-dependent manner. Besides, the cytotoxic effect of irradiation could be augmented by CK2 inhibitors (p<0.05, two-way analysis of variance and Tukey's Honestly Significant Difference). Moreover, the growth inhibition of CK2 inhibitor and irradiation was both associated with suppression of Stat3 activation. Taken together, inhibition of CK2 activity appears to be a promising treatment strategy for non-small cell lung cancer and CK2 inhibition results in reduced Stat3 activation. Our data warrant further effort to develop CK2-targeted radiosensitizer for lung cancer treatment.

Coordinate regulation of histone mRNA metabolism and DNA replication: cyclin A/cdk1 is involved in inactivation of histone mRNA metabolism and DNA replication at the end of S phase

S phase is characterized by the replication of DNA and assembly of chromatin. This requires the synthesis of large amounts of histone proteins to package the newly replicated DNA. Histone mRNAs are the only mRNAs that do not have polyA tails, ending instead in a conserved stemloop sequence. The stemloop binding protein (SLBP) that binds the 3' end of histone mRNA is cell cycle regulated and SLBP is required in all steps of histone mRNA metabolism. Activation of cyclin E/cdk2 prior to entry into S-phase is critical for initiation of DNA replication and histone mRNA accumulation. At the end of S phase SLBP is rapidly degraded as a result of phosphorylation of SLBP by cyclin A/cdk1 and CK2 effectively shutting off histone mRNA biosynthesis. E2F1, which is required for expression of many S-phase genes, is regulated in parallel with SLBP and its degradation also requires a cyclin binding site, suggesting that it may also be regulated by the same pathway. It is likely that activation of cyclin A/cdk1 helps inhibit both DNA replication and histone mRNA accumulation, marking the end of S phase and entry into G(2)-phase.

Evolution of FW2.2-like (FWL) and PLAC8 genes in eukaryotes

The tomato FW2.2 quantitative trait locus, which regulates tomato fruit size, was genetically and physically mapped around 15 years ago. Subsequently, the FW2.2 gene was cloned and shown to contain a PLAC8 domain, originally identified in mammalian placental proteins. Data suggest that FW2.2 likely controls tomato cell size, perhaps by direct interaction with casein kinase II. Several FW2.2-like (FWL) genes have now been identified from a variety of plant species, but until recently only the tomato FW2.2 gene had been the subject of detailed investigation. Recently, soybean and maize FWL genes were identified and shown to have a role in plant organogenesis. It is now apparent that the FWL genes in plants are a large gene family, which is even larger given inclusion of genes for the various eukaryotic PLAC8-domain proteins. Although overall the protein sequence identity/similarity among the family members is relatively low, there is strong conservation of key domains, suggesting a conservation of the core biochemical function of these proteins. In this Addendum Article, we highlight the similarities and differences exiting between plant FWL genes and enlarge this comparison to the mammalian PLAC8 genes. These comparisons suggest the possible conservation of biological function for FWL proteins.

Thiazolidinediones prevent PDGF-BB-induced CREB depletion in pulmonary artery smooth muscle cells by preventing upregulation of casein kinase 2 alpha' catalytic subunit

BACKGROUND

The transcription factor CREB is diminished in smooth muscle cells (SMCs) in remodeled, hypertensive pulmonary arteries (PAs) in animals exposed to chronic hypoxia. Forced depletion of cyclic adenosine monophosphate response element binding protein (CREB) in PA SMCs stimulates their proliferation and migration in vitro. Platelet-derived growth factor (PDGF) produced in the hypoxic PA wall promotes CREB proteasomal degradation in SMCs via phosphatidylinositol-3-kinase/Akt signaling, which promotes phosphorylation of CREB at 2 casein kinase 2 (CK2) sites. Here we tested whether thiazolidinediones, agents that inhibit hypoxia-induced PA remodeling, attenuate SMC CREB loss.

METHODS

Depletion of CREB and changes in casein kinase 2 catalytic subunit expression and activity were measured in PA SMC treated with PDGF. PA remodeling and changes in medial PA CREB and casein kinase 2 levels were evaluated in lung sections from rats exposed to hypoxia for 21 days.

RESULTS

We found that the thiazolidinedione rosiglitazone prevented PA remodeling and SMC CREB loss in rats exposed to chronic hypoxia. Likewise, the thiazolidinedione troglitazone blocked PA SMC proliferation and CREB depletion induced by PDGF in vitro. Thiazolidinediones did not repress Akt activation by hypoxia in vivo or by PDGF in vitro. However, PDGF-induced CK2 alpha' catalytic subunit expression and activity in PA SMCs, and depletion of CK2 alpha' subunit prevented PDGF-stimulated CREB loss. Troglitazone inhibited PDGF-induced CK2 alpha' subunit expression in vitro and rosiglitazone blocked induction of CK2 catalytic subunit expression by hypoxia in PA SMCs in vivo.

CONCLUSION

We conclude that thiazolidinediones prevent PA remodeling in part by suppressing upregulation of CK2 and loss of CREB in PA SMCs.

A member of the highly conserved FWL (tomato FW2.2-like) gene family is essential for soybean nodule organogenesis

A soybean homolog of the tomato FW2.2 gene, here named GmFWL1 (Glycine max FW2.2-like 1), was found to respond strongly to inoculation with the nitrogen-fixing symbiotic bacterium Bradyrhizobium japonicum. In tomato, the FW2.2 gene is hypothesized to control 30% of the variance in fruit weight by negatively regulating cell division. In the present study, the induction of GmFWL1 expression in root hair cells and nodules in response to B. japonicum inoculation was documented using quantitative RT-PCR and transcriptional fusions to both beta-glucuronidase (GUS) and green fluorescent protein (GFP). RNAi-mediated silencing of GmFWL1 expression resulted in a significant reduction in nodule number, with a concomitant reduction in nuclear size and changes in chromatin structure. The reduction in nuclear size is probably due to a change in DNA heterochromatinization, as the ploidy level of wild-type and RNAi-silenced nodule cells was similar. GmFWL1 was localized to the plasma membrane. The data suggest that GmFWL1 probably acts indirectly, perhaps through a cellular cascade, to affect chromatin structure/nuclei architecture. As previously proposed in tomato, this function may be a result of effects on plant cell division.

Halogenated imidazole derivatives block RNA polymerase II elongation along mitogen inducible genes

Background

Aberrant activation of protein kinases is one of the essential oncogenic driving forces inherent to the process of tumorigenesis. The protein kinase CK2 plays an important role in diverse biological processes, including cell growth and proliferation as well as in the governing and transduction of prosurvival signals. Increased expression of CK2 is a hallmark of some cancers, hence its antiapoptotic properties may be relevant to cancer onset. Thus, the designing and synthesis of the CK2 inhibitors has become an important pursuit in the search for cancer therapies.

Results

Using a high-throughput microarray approach, we demonstrate that two potent inhibitors of CK2, 4,5,6,7-tetrabromo-benzimidazole (TBBz) and 2-Dimethyloamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), blocked mitogen induced mRNA expression of immediate early genes. Given the impact of these inhibitors on the process of transcription, we investigated their effects on RNA Polymerase II (RNAPII) elongation along the mitogen inducible gene, EGR1 (early growth response 1), using chromatin immunoprecipitation (ChIP) assay. ChIP analysis demonstrated that both drugs arrest RNAPII elongation. Finally, we show that CDK9 kinase activity, essential for the triggering of RNAPII elongation, was blocked by TBBz and to lesser degree by DMAT.

Conclusions

Our approach revealed that small molecules derived from halogenated imidazole compounds may decrease cell proliferation, in part, by inhibiting pathways that regulate transcription elongation.

The p53-p21(Cip1/WAF1) pathway is necessary for cellular senescence induced by the inhibition of protein kinase CKII in human colon cancer cells

We have previously shown that the down-regulation of protein kinase CKII activity is tightly associated with cellular senescence of human fibroblast IMR-90 cells. Here, we examined the roles of p53 and p21(Cip1/WAF1) in senescence development induced by CKII inhibition using wild-type, isogenic p53-/- and isogenic p21-/- HCT116 human colon cancer cell lines. A senescent marker appeared after staining for senescence-associated beta-galactosidase activity in wild-type HCT116 cells treated with CKII inhibitor or CKIIalpha siRNA, but this response was almost abolished in p53- or p21(Cip1/WAF1)-null cells. Increased cellular levels of p53 and p21(Cip1/WAF1) protein occurred with the inhibition of CKII. CKII inhibition upregulated p53 and p21(Cip1/WAF1) expression at post-transcriptional level and transcription level, respectively. RB phosphorylation significantly decreased in cells treated with CKII inhibitor. Taken together, this study shows that the activation of the p53-p21(Cip1/WAF1) pathway acts as a major mediator of cellular senescence induced by CKII inhibition.

Evidence for regulation of mitotic progression through temporal phosphorylation and dephosphorylation of CK2alpha

Proper mitotic progression is crucial for maintenance of genomic integrity in proliferating cells and is regulated through an intricate series of events, including protein phosphorylation governed by a complex network of protein kinases. One kinase family implicated in the regulation of mitotic progression is protein kinase CK2, a small family of enzymes that is overexpressed in cancer and induces transformation in mice and cultured fibroblasts. CK2alpha, one isoform of the catalytic subunits of CK2, is maximally phosphorylated at four sites in nocodazole-treated cells. To investigate the effects of CK2alpha phosphorylation on mitotic progression, we generated phosphospecific antibodies against its mitotic phosphorylation sites. In U2OS cells released from S-phase arrest, these antibodies reveal that CK2alpha is most highly phosphorylated in prophase and metaphase. Phosphorylation gradually decreases during anaphase and becomes undetectable during telophase and cytokinesis. Stable expression of phosphomimetic CK2alpha (CK2alpha-4D, CK2alpha-4E) results in aberrant centrosome amplification and chromosomal segregation defects and loss of mitotic cells through mitotic catastrophe. Conversely, cells expressing nonphosphorylatable CK2alpha (CK2alpha-4A) show a decreased ability to arrest in mitosis following nocodazole treatment, suggesting involvement in the spindle assembly checkpoint. Collectively, these studies indicate that reversible phosphorylation of CK2alpha requires precise regulation to allow proper mitotic progression.

CK2beta is expressed in endometrial carcinoma and has a role in apoptosis resistance and cell proliferation

Protein kinase CK2 (CK2) is a serine/threonine kinase that participates in important cellular processes. We have recently demonstrated that CK2 plays a role in resistance to TRAIL/Fas-induced apoptosis in endometrial carcinoma (EC) by regulating FLIP. Here, we assessed the immunohistochemical expression of CK2beta in EC and checked its role in cell proliferation and anchorage-independent cell growth. CK2beta immunostaining was assessed in two tissue microarrays, one constructed from paraffin-embedded blocks of 95 ECs and another from 70 samples of normal endometrium. CK2beta expression was correlated with histological type; grade and stage; cell proliferation (Ki-67) and apoptotic index; immunostaining for cyclin D1, PTEN, AKT, beta-catenin, and FLIP. Moreover, the Ishikawa EC cell line was subjected to down-regulation of CK2 by shRNA. CK2beta expression was frequent in EC (nuclear, 100%; cytoplasmic, 87.5%). The staining was more intense in EC than in normal endometrium (P = 0.000), and statistically correlated with AKT, PTEN, beta-catenin, and FLIP. In EC, CK2beta expression correlated with cell proliferation. Knock-down of CK2beta blocked colony formation of EC in soft agar, and also resulted in decreased expression of cyclin D1 and ERK phosphorylation. The results confirm that CK2beta is widely expressed in EC, and suggest a role in cell proliferation and anchorage-independent cell growth.

Drosophila protein kinase CK2 is rendered temperature-sensitive by mutations of highly conserved residues flanking the activation segment

CK2 is a Ser/Thr protein kinase essential for animal development. Although null alleles for CK2 are available in the mouse and Drosophila models, they are lethal when homozygous, thus necessitating conditional alleles for analysis of its developmental roles. We describe the isolation of temperature-sensitive (ts) alleles of Drosophila CK2alpha (dCK2alpha). These alleles efficiently rescue lethality of yeast lacking endogenous CK2 at 29 degrees C, but this ability is lost at higher temperatures in an allele-specific manner. These ts-variants exhibit properties akin to the wild type protein, and interact robustly with dCK2beta. Modeling of these ts-variants using the crystal structure of human CK2alpha indicates that the affected residues are in close proximity to the active site. We find that substitution of Asp(212) elicits potent ts-behavior, an important finding because this residue contributes to stability of the activation segment and is invariant in other Ser/Thr protein kinases.

A role for protein kinase CK2 in plant development: evidence obtained using a dominant-negative mutant

Protein kinase CK2 is an evolutionary conserved Ser/Thr phosphotransferase composed of two distinct subunits, alpha (catalytic) and beta (regulatory), that combine to form a tetrameric complex. Plant genomes contain multiple genes for each subunit, the expression of which gives rise to different active holoenzymes. In order to study the effects of loss of function of CK2 on plant development, we have undertaken a dominant-negative mutant approach. We generated an inactive catalytic subunit by site-directed mutagenesis of an essential lysine residue. The mutated open reading frame was cloned downstream of an inducible promoter, and stably transformed Arabidopsis thaliana plants and tobacco BY2 cells were isolated. Continuous expression of the CK2 kinase-inactive subunit did not prevent seed germination, but seedlings exhibited a strong phenotype, affecting chloroplast development, cotyledon expansion, and root and shoot growth. Prolonged induction of the transgene was lethal. Moreover, dark-germinated seedlings exhibited an apparent de-etiolated phenotype that was not caused by disruption of the light-signalling pathways. Short-term induction of the CK2 kinase-inactive subunit allowed plant survival, but root growth and lateral root formation were significantly affected. The expression pattern of CYCB1;1::GFP in the root meristems of mutant plants demonstrated an important decrease of mitotic activity, and expression of the CK2 kinase-inactive subunit in stably transformed BY2 cells provoked perturbation of the G1/S and G2 phases of the cell cycle. Our results are consistent with a model in which CK2 plays a key role in cell division and cell expansion, with compelling effects on Arabidopsis development.

A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

In developing retina, the nucleus of the elongated neuroepithelial cells undergoes interkinetic nuclear migration (INM), that is it migrates back and forth across the proliferative layer during the cell cycle. S-phase occurs at the basal side, while M-phase occurs at the apical margin of the retinal progenitors. G1 and G2-phases occur along the nuclear migration pathway. We tested whether this feature of the retinal cell cycle is controlled by CK2, which, among its many substrates, phosphorylates both molecular motors and cytoskeletal components. Double immunolabeling showed that CK2 is contained in BrdU-labeled retinal progenitors. INM was examined after pulse labeling the retina of newborn rats with BrdU, by plotting nuclear movement from basal to apical sides of the retinal progenitors during G2. The CK2 specific inhibitor 4,5,6,7-tetrabromobenzotriazole inhibited the activity of rat retinal CK2, and blocked nuclear movement proper in a dose-dependent way. No apoptosis was detected, and total numbers of BrdU-labeled nuclei remained constant following treatment. Immunohistochemistry showed that, following inhibition of CK2, the tubulin cytoskeleton is disorganized, with reduced acetylated and increased tyrosinated tubulin. This indicates a reduction in stable microtubules, with accumulation of free tubulin dimers. The results show that CK2 activity is required for INM in retinal progenitor cells.

Phosphorylation of threonine 61 by cyclin a/Cdk1 triggers degradation of stem-loop binding protein at the end of S phase

Histone mRNA levels are cell cycle regulated, and a major regulatory mechanism is restriction of stem-loop binding protein (SLBP) to S phase. Degradation of SLBP at the end of S phase results in cessation of histone mRNA biosynthesis, preventing accumulation of histone mRNA until SLBP is synthesized just before entry into the next S phase. Degradation of SLBP requires an SFTTP (58 to 62) and KRKL (95 to 98) sequence, which is a putative cyclin binding site. A fusion protein with the 58-amino-acid sequence of SLBP (amino acids 51 to 108) fused to glutathione S-transferase (GST) is sufficient to mimic SLBP degradation at late S phase. Using GST-SLBP fusion proteins as a substrate, we show that cyclin A/Cdk1 phosphorylates Thr61. Furthermore, knockdown of Cdk1 by RNA interference stabilizes SLBP at the end of S phase. Phosphorylation of Thr61 is necessary for subsequent phosphorylation of Thr60 by CK2 in vitro. Inhibitors of CK2 also prevent degradation of SLBP at the end of S phase. Thus, phosphorylation of Thr61 by cyclin A/Cdk1 primes phosphorylation of Thr60 by CK2 and is responsible for initiating SLBP degradation. We conclude that the increase in cyclin A/Cdk1 activity at the end of S phase triggers degradation of SLBP at S/G(2).

The tumor suppressor gene DLEC1 is frequently silenced by DNA methylation in hepatocellular carcinoma and induces G1 arrest in cell cycle

BACKGROUND/AIMS

The chromosome locus 3p21.3 is a "hot-spot" for chromosomal aberrations and loss of heterozygosity in cancers. The 35 genes mapped to the AP20 subregion of this locus were screened for their expression to identify candidate tumor suppressor genes. DLEC1 was selected for further characterization in primary hepatocellular carcinomas and cell lines.

METHODS

RT-PCR and methylation-specific PCR were performed to examine the expression and methylation. Stable clones with DLEC1 overexpression were established to analyze cell proliferation and cell cycle.

RESULTS

DLEC1 was silenced and hypermethylated in 9 of 11 cell lines examined. Treatment with 5-aza-2'-deoxycytidine reversed the methylation and restored DLEC1 expression. The correlation between hypermethylation and expression was also demonstrated in 10 pairs of hepatocellular carcinoma and adjacent normal tissues (t-test, p<0.05). Hypermethylation of DLEC1 was detected in 70.6% of tumors, compared to 10.3% in normal tissues (n=68, p<0.001, chi(2)). Of interest, DLEC1 methylation was associated with the AJCC staging of the tumors (p=0.036, chi(2)). DLEC1 over-expression in cell lines decreased colony formation, cell growth and cell size, and induced a G1 arrest in cell cycle.

CONCLUSIONS

Our data indicate that DLEC1 is a candidate tumor suppressor gene that plays an important role in the development and progression of hepatocellular carcinoma.

Development of a Stabilized Form of the Regulatory CK2β Subunit That Inhibits Cell Proliferation

A number of cancers are characterized by elevated expression of CK2 (formerly casein kinase II), which has been implicated as a key component in cell proliferation and transformation. Two lines of evidence, (a) deregulated expression of CK2 and (b) CK2beta ubiquitination and degradation of these in a proteasome-dependent manner prompted further investigation of the regulation of CK2beta protein stability. We demonstrate that mutating six surface-exposed lysine residues to arginine (6KR) to interfere with ubiquitin attachment can stabilize CK2beta. Examination of 6KR expression in cells revealed increased stability over time and increased its steady-state expression level compared with CK2beta. In cells, 6KR was no longer sensitive to proteasome inhibition but maintained an elevated expression level. In our studies, 6KR functioned as a normal CK2 regulatory subunit, because it participated in CK2beta dimerization, associated with catalytic subunits, was autophosphorylated, and formed active, stable CK2 tetramers. The physiological role of CK2beta stabilization was investigated in cell proliferation assays, which showed a significant decrease in proliferation in cells expressing 6KR compared with CK2beta. Overall, our results indicate that a stabilized form of CK2beta can be used to inhibit cell proliferation.

Synthesis and biological evaluation of 3-(substituted-benzylidene)-1,3-dihydro-indolin derivatives as human protein kinase CK2 and p60(c-Src) tyrosine kinase inhibitors

Human protein kinase CK2 is an ubiquitous serine/threonine kinase that is typically found in tetrameric complexes consisting of two catalytic (alpha and/or alpha') and two regulatory beta subunits. Although there is growing evidence that besides the participation of CK2 in a complex series of cellular functions, this protein kinase is involved in cell viability, cell proliferation, and neoplastic transformation. In the present study, a series of 3-(substituted-benzylidene)-1,3-dihydro-indolin-2-thione derivatives and the corresponding indolin-2-one congeners were tested for their inhibition of human recombinant protein kinase CK2 in vitro. The efficacy of these compounds was compared with their inhibitory results of p60(c-Src) tyrosine kinase. It was found that 3-(substituted-benzylidene)-1,3-dihydro-indolin-2-thione derivatives are more effective than indolin-2-one congeners for the inhibition of CK2 and p60(c-Src) tyrosine kinase.

The fibroblast growth factor-2 antisense gene inhibits nuclear accumulation of FGF-2 and delays cell cycle progression in C6 glioma cells

Fibroblast growth factor-2 (FGF-2) is a potent heparin-binding protein with growth-promoting and anti-apoptotic activity. Transcription of the GFG/NUDT6 gene on the opposite DNA strand generates an overlapping antisense RNA (FGF-AS) implicated in the post-transcriptional regulation of FGF-2. C6 glioma cells coordinately express FGF-2 and FGF-AS mRNA in a cell cycle-dependent manner. Cellular FGF-2 immunoreactivity was also cell cycle-dependent, with marked nuclear accumulation during S-phase. Stable transfection and overexpression of the FGF-AS RNA resulted in suppression of total cellular FGF-2, and a reduction in nuclear accumulation of FGF-2 isoforms. Serum stimulation of growth-arrested wild-type cells evoked a rapid nuclear translocation of FGF-2, and cell cycle re-entry. FGF-AS transfectants, in contrast, showed a significant delay in recovery of both nuclear FGF-2 staining and S-phase re-entry. Similar results were observed when cells were released from aphidicolin-induced G1 arrest or subjected to heat shock. These findings indicate that FGF-AS RNA inhibits expression and cell cycle-dependent nuclear accumulation of FGF-2, and this is associated with a marked delay in S-phase progression. The results suggest that the endogenous FGF antisense RNA may play a significant functional role in the regulation of FGF-2 dependent cell proliferation in FGF-2 expressing cells.

Inhibition of protein kinase CK2 leads to a modulation of androgen receptor dependent transcription in prostate cancer cells

BACKGROUND

The androgen receptor (AR) mediates the biological responses of androgens in the prostate gland. In prostate cancer, this pathway is often deregulated and causes an uncontrolled proliferation.

METHODS

The current study focuses on the effects of an inhibition of protein kinase CK2 on the AR-mediated transcription in LNCaP prostate cancer cells. We used chemical inhibitors of CK2 as well as dominant-negative kinase mutants to downregulate the CK2 activity. We determined the effects of the inhibition by Western blot analysis of endogenous target genes of the AR as well as by reporter assays.

RESULTS

We found that inhibition of CK2 led to a downregulation of the AR-dependent transcription. Moreover, the amount of the AR protein decreased significantly.

CONCLUSION

According to the fact that AR pathways are involved in the development and progression of prostate cancer, the ability to modulate AR function should provide an alternative basis for the development of new cancer therapies.

Sic1 is phosphorylated by CK2 on Ser201 in budding yeast cells

We have previously identified Ser201 of Sic1, a yeast cyclin-dependent kinase inhibitor, as an in vitro target of protein kinase CK2. Here we present new evidence, by using specific anti-P-Ser201 antibodies and 2-D gel electrophoresis coupled to MALDI mass spectrometry analysis, that Sic1 is phosphorylated in vivo on Ser201 shortly after its de novo synthesis, during late anaphase in glucose-grown cells. This phosphorylation is also detected in Sic1 immunopurified from G1 cells. In agreement with these data we also show that the catalytic alpha' subunit of CK2, whose function is required for cell cycle progression, is detected in Sic1 immunopurified complexes, and that phosphorylation on Ser201 is reduced after CK2 inactivation at the non-permissive temperature in a cka1delta cka2(ts) yeast strain. These data strongly support the notion that CK2 phosphorylates Sic1 in vivo.

Drosophila CK2 regulates lateral-inhibition during eye and bristle development

Lateral inhibition is critical for cell fate determination and involves the functions of Notch (N) and its effectors, the Enhancer of Split Complex, E(spl)C repressors. Although E(spl) proteins mediate the repressive effects of N in diverse contexts, the role of phosphorylation was unclear. The studies we describe implicate a common role for the highly conserved Ser/Thr protein kinase CK2 during eye and bristle development. Compromising the functions of the catalytic (alpha) subunit of CK2 elicits a rough eye and defects in the interommatidial bristles (IOBs). These phenotypes are exacerbated by mutations in CK2 and suppressed by an increase in the dosage of this protein kinase. The appearance of the rough eye correlates, in time and space, to the specification and refinement of the 'founding' R8 photoreceptor. Consistent with this observation, compromising CK2 elicits supernumerary R8's at the posterior margin of the morphogenetic furrow (MF), a phenotype characteristic of loss of E(spl)C and impaired lateral inhibition. We also show that compromising CK2 elicits ectopic and split bristles. The former reflects the specification of excess bristle SOPs, while the latter suggests roles during asymmetric divisions that drive morphogenesis of this sensory organ. In addition, these phenotypes are exacerbated by mutations in CK2 or E(spl), indicating genetic interactions between these two loci. Given the centrality of E(spl) to the repressive effects of N, our studies suggest conserved roles for this protein kinase during lateral inhibition. Candidates for this regulation are the E(spl) repressors, the terminal effectors of this pathway.

B23 is a downstream target of polyamine-modulated CK2

Our previous studies have shown that the overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases the enzymatic activity of the polyamine-responsive enzyme casein kinase 2 (CK2). Because CK2 is known to preferentially associate with the nuclear matrix in response to other trophic stimuli, we investigated the effects of ODC overexpression on CK2 localisation and on the CK2-mediated phosphorylation of a known CK2 substrate, the nucleolar phosphoprotein B23. Immunofluorescence analysis of CK2 and B23 in primary keratinocytes revealed that ODC overexpression resulted in the colocalisation of CK2 with B23 at the nucleolar borders. ODC overexpression also increased CK2 kinase activity 2-fold at the nuclear matrix, a response which could be abrogated by treatment of K6/ODC transgenic keratinocytes with the ODC inhibitor alpha-difluoromethylornithine (DFMO). Levels of B23 protein were also elevated in ODC-overexpressing cells compared to normal cells or transgenic cells treated with DFMO. This increase in protein level was neither due to an increase in steady-state mRNA levels, nor was it due to increased stability of B23 protein. Phosphorylation of B23 was also increased in ODC-overexpressing cells, and this increased phosphorylation could be blocked by treatment of the cells with the CK2 kinase inhibitors apigenin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These data suggest that B23 may be a downstream effector of polyamines via phosphorylation by the protein kinase CK2.

Downregulation of protein kinase CKII is associated with cellular senescence

Protein kinase CKII (CKII) plays a critical role in cell growth and proliferation. In this study, we examine how CKII activity is regulated during cellular senescence. Our results demonstrate that CKII activity apparently decreases during both replicative and H2O2-induced senescence in human diploid fibroblast IMR-90 cells. The mRNA and protein levels of CKIIalpha decreases significantly during replicative and H2O2-induced senescence, while only slight reduction in those of CKIIbeta is observed during replicative senescence. Treatment of IMR-90 cells with CKII inhibitors 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole and apigenin led cells to acquire a senescent phenotype as judged by the senescence-associated beta-galactosidase marker and overexpression of p53 and p21(Waf-1). Knockdown of CKIIalpha in IMR-90 cells by RNA interference also dramatically induced the senescent phenotype. In parallel, CKII activity was transcriptional downregulated in rat liver and testis with advancing age. Taken together, these results suggest that downregulation of CKII activity is tightly associated not only with cellular senescence but also with organism aging.

CK2 phosphorylation of eukaryotic translation initiation factor 5 potentiates cell cycle progression

Casein kinase 2 (CK2) is a ubiquitous eukaryotic Ser/Thr protein kinase that plays an important role in cell cycle progression. Although its function in this process remains unclear, it is known to be required for the G(1) and G(2)/M phase transitions in yeast. Here, we show that CK2 activity changes notably during cell cycle progression and is increased within 3 h of serum stimulation of quiescent cells. During the time period in which it exhibits high enzymatic activity, CK2 associates with and phosphorylates a key molecule for translation initiation, eukaryotic translation initiation factor (eIF) 5. Using MS, we show that Ser-389 and -390 of eIF5 are major sites of phosphorylation by CK2. This is confirmed using eIF5 mutants that lack CK2 sites; the phosphorylation levels of mutant eIF5 proteins are significantly reduced, relative to WT eIF5, both in vitro and in vivo. Expression of these mutants reveals that they have a dominant-negative effect on phosphorylation of endogenous eIF5, and that they perturb synchronous progression of cells through S to M phase, resulting in a significant reduction in growth rate. Furthermore, the formation of mature eIF5/eIF2/eIF3 complex is reduced in these cells, and, in fact, restricted diffusional motion of WT eIF5 was almost abolished in a GFP-tagged eIF5 mutant lacking CK2 phosphorylation sites, as measured by fluorescence correlation spectroscopy. These results suggest that CK2 may be involved in the regulation of cell cycle progression by associating with and phosphorylating a key molecule for translation initiation.

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.

Casein kinase 2 regulates both apoptosis and the cell cycle following DNA damage induced by 6-thioguanine

PURPOSE

The purine antimetabolite, 6-thioguanine (6-TG), is an effective drug in the management of acute leukemias. In this study, we analyze the mechanisms of apoptosis associated with 6-TG treatment and casein kinase 2 (CK2 or CKII) in human tumor cells.

EXPERIMENTAL DESIGN

Small interfering RNA and chemical CK2 inhibitors were used to reduce CK2 activity. Control and CK2 activity-reduced cells were cultured with 6-TG and assessed by flow cytometry to measure apoptosis and cell cycle profiles. Additionally, confocal microscopy was used to assess localization of CK2 catalytic units following 6-TG treatment.

RESULTS

Transfection of small interfering RNA against the CK2 alpha and/or alpha' catalytic subunits results in marked apoptosis of HeLa cells following treatment with 6-TG. Chemical inhibitors of CK2 also induce apoptosis following 6-TG treatment. Apoptosis induced by 6-TG is similarly observed in both mismatch repair-proficient and -deficient HCT116 and HeLa cells. Concomitant treatment with a pan-caspase inhibitor or transfection of apoptosis repressor with caspase recruitment domain markedly suppresses the apoptotic response to DNA damage by 6-TG in the CK2-reduced cells, indicating caspase regulation by CK2. CK2 alpha relocalizes to the endoplasmic reticulum after 6-TG treatment. Additionally, transfection of Cdc2 with a mutation at Ser(39) to Ala, which is the CK2 phosphorylation site, partially inhibits cell cycle progression in G(1) to G(2) phase following 6-TG treatment.

CONCLUSION

CK2 is essential for apoptosis inhibition following DNA damage induced by 6-TG, controlling caspase activity.

Protein kinase CK2 interacts with Chk2 and phosphorylates Mre11 on serine 649

The Mre11-Rad50-Nbs1 protein complex has been known to be involved in a variety of DNA metabolic events that involve DNA double-strand breaks (DSBs). The phosphorylation of Mre11 is increased in response to ionizing radiation, which suggests that phosphorylation of Mre11 may be an important regulatory mechanism of this complex. Mre11-phosphorylating kinase activities were observed in Chk2 immunoprecipitates and HeLa nuclear extracts. Through the tandem affinity tagging system and conventional chromatography, this kinase was purified and identified as protein kinase CK2. CK2 phosphorylates Mre11 in vitro. In vitro kinase assay with a series of truncated Mre11 proteins as substrates for CK2 and site-directed mutagenesis showed that serine 649 of Mre11 is mainly phosphorylated by CK2 in vitro. In vivo labeling and phosphopeptide mapping analysis revealed that this phosphorylation occurs in vivo. These data implicate CK2 as a potential upstream regulator of Mre11 function.

Protein kinase CK2 in gene control at cell cycle entry

Protein kinase CK2 has diverse links to gene control and cell cycle. Comparative genome-wide expression profiling of CK2 mutants of the budding yeast Saccharomyces cerevisiae at cell cycle entry has revealed that a significant proportion of cell-cycle genes are affected by CK2. Here, we examine how CK2 realizes this effect. We show that the CK2 action may be directed to gene promoters causing genes with promoter homologies to respond comparably to CK2 perturbation. Examples are metabolic pathway and nutrition supply genes such as the PHO and MET regulon genes, responsible for phosphate maintenance and methionine biosynthesis, respectively. CK2 perturbation affects both regulons permanently and both via repression of a central transcription factor, but with different mechanisms: In the PHO regulon, the gene encoding the central transcription factor Pho4 is repressed and, in addition, Pho4 and/or the cyclin-dependent kinase of the regulon's control complex may be affected by CK2 phosphorylation. In the MET regulon, the repression of the central transcription factor Met4 occurs not by expression inhibition, but rather by availability tuning via a CK2-mediated phosphorylation of a degradation complex. On the other hand, the CK2 action may be directed to the chromatin regulon, thus affecting globally the expression of genes, i.e., the CK2 perturbation results either in comparable responses of genes which have no promoter homologies or in deviating responses despite promoter homologies. The effect is rather transient and concerns aside various cell cycle control genes a notable number of genes encoding chromatin remodeling and modification proteins with functions in chromatin assembly and (anti-)silencing as well as in histone (de-)acetylation, and frequently are also substrates of CK2, suggesting additional tuning at protein level. In line with these findings, we observe in human cells sequence-independent but cell-cycle-dependent CK2 associations with promoters of cell-cycle-regulated genes at periods of extensive gene expression alterations, including cell cycle entry. Our observations are compatible with the idea that the gene control by CK2 is achieved via different mechanisms and at different levels of organization and includes a global role in transcription-related chromatin remodelling and modification.