Cell biological studies with monoclonal and polyclonal antibodies against human casein kinase II subunit beta demonstrate participation of the kinase in mitogenic signaling

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.

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.

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.

DEK over-expression promotes mitotic defects and micronucleus formation

The DEK gene encodes a nuclear protein that binds chromatin and is involved in various fundamental nuclear processes including transcription, RNA splicing, DNA replication and DNA repair. Several cancer types characteristically over-express DEK at the earliest stages of transformation. In order to explore relevant mechanisms whereby DEK supports oncogenicity, we utilized cancer databases to identify gene transcripts whose expression patterns are tightly correlated with that of DEK. We identified an enrichment of genes involved in mitosis and thus investigated the regulation and possible function of DEK in cell division. Immunofluorescence analyses revealed that DEK dissociates from DNA in early prophase and re-associates with DNA during telophase in human keratinocytes. Mitotic cell populations displayed a sharp reduction in DEK protein levels compared to the corresponding interphase population, suggesting DEK may be degraded or otherwise removed from the cell prior to mitosis. Interestingly, DEK overexpression stimulated its own aberrant association with chromatin throughout mitosis. Furthermore, DEK co-localized with anaphase bridges, chromosome fragments, and micronuclei, suggesting a specific association with mitotically defective chromosomes. We found that DEK over-expression in both non-transformed and transformed cells is sufficient to stimulate micronucleus formation. These data support a model wherein normal chromosomal clearance of DEK is required for maintenance of high fidelity cell division and chromosomal integrity. Therefore, the overexpression of DEK and its incomplete removal from mitotic chromosomes promotes genomic instability through the generation of genetically abnormal daughter cells. Consequently, DEK over-expression may be involved in the initial steps of developing oncogenic mutations in cells leading to cancer initiation.

PD-1 increases PTEN phosphatase activity while decreasing PTEN protein stability by inhibiting casein kinase 2

Programmed death 1 (PD-1) is a potent inhibitor of T cell responses. PD-1 abrogates activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, but the mechanism remains unclear. We determined that during T cell receptor (TCR)/CD3- and CD28-mediated stimulation, PTEN is phosphorylated by casein kinase 2 (CK2) in the Ser380-Thr382-Thr383 cluster within the C-terminal regulatory domain, which stabilizes PTEN, resulting in increased protein abundance but suppressed PTEN phosphatase activity. PD-1 inhibited the stabilizing phosphorylation of the Ser380-Thr382-Thr383 cluster within the C-terminal domain of PTEN, thereby resulting in ubiquitin-dependent degradation and diminished abundance of PTEN protein but increased PTEN phosphatase activity. These effects on PTEN were secondary to PD-1-mediated inhibition of CK2 and were recapitulated by pharmacologic inhibition of CK2 during TCR/CD3- and CD28-mediated stimulation without PD-1. Furthermore, PD-1-mediated diminished abundance of PTEN was reversed by inhibition of ubiquitin-dependent proteasomal degradation. Our results identify CK2 as a new target of PD-1 and reveal an unexpected mechanism by which PD-1 decreases PTEN protein expression while increasing PTEN activity, thereby inhibiting the PI3K/Akt signaling axis.

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.

Disruption of CK2beta in embryonic neural stem cells compromises proliferation and oligodendrogenesis in the mouse telencephalon

Genetic programs that govern neural stem/progenitor cell (NSC) proliferation and differentiation are dependent on extracellular cues and a network of transcription factors, which can be regulated posttranslationally by phosphorylation. However, little is known about the kinase-dependent pathways regulating NSC maintenance and oligodendrocyte development. We used a conditional knockout approach to target the murine regulatory subunit (beta) of protein kinase casein kinase 2 (CK2beta) in embryonic neural progenitors. Loss of CK2beta leads to defects in proliferation and differentiation of embryonic NSCs. We establish CK2beta as a key positive regulator for the development of oligodendrocyte precursor cells (OPCs), both in vivo and in vitro. We show that CK2beta directly interacts with the basic helix-loop-helix (bHLH) transcription factor Olig2, a critical modulator of OPC development, and activates the CK2-dependent phosphorylation of its serine-threonine-rich (STR) domain. Finally, we reveal that the CK2-targeted STR domain is required for the oligodendroglial function of Olig2. These findings suggest that CK2 may control oligodendrogenesis, in part, by regulating the activity of the lineage-specific transcription factor Olig2. Thus, CK2beta appears to play an essential and uncompensated role in central nervous system development.

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.

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.

Cdc34 C-terminal tail phosphorylation regulates Skp1/cullin/F-box (SCF)-mediated ubiquitination and cell cycle progression

The ubiquitin-conjugating enzyme Cdc34 (cell division cycle 34) plays an essential role in promoting the G1-S-phase transition of the eukaryotic cell cycle and is phosphorylated in vivo. In the present study, we investigated if phosphorylation regulates Cdc34 function. We mapped the in vivo phosphorylation sites on budding yeast Cdc34 (yCdc34; Ser207 and Ser216) and human Cdc34 (hCdc34 Ser203, Ser222 and Ser231) to serine residues in the acidic tail domain, a region that is critical for Cdc34's cell cycle function. CK2 (protein kinase CK2) phosphorylates both yCdc34 and hCdc34 on these sites in vitro. CK2-mediated phosphorylation increased yCdc34 ubiquitination activity towards the yeast Saccharomyces cerevisiae Sic1 in vitro, when assayed in the presence of its cognate SCFCdc4 E3 ligase [where SCF is Skp1 (S-phase kinase-associated protein 1)/cullin/F-box]. Similarly, mutation of the yCdc34 phosphorylation sites to alanine, aspartate or glutamate residues altered Cdc34-SCFCdc4-mediated Sic1 ubiquitination activity. Similar results were obtained when yCdc34's ubiquitination activity was assayed in the absence of SCFCdc4, indicating that phosphorylation regulates the intrinsic catalytic activity of Cdc34. To evaluate the in vivo consequences of altered Cdc34 activity, wild-type yCdc34 and the phosphosite mutants were introduced into an S. cerevisiae cdc34 deletion strain and, following synchronization in G1-phase, progression through the cell cycle was monitored. Consistent with the increased ubiquitination activity in vitro, cells expressing the phosphosite mutants with higher catalytic activity exhibited accelerated cell cycle progression and Sic1 degradation. These studies demonstrate that CK2-mediated phosphorylation of Cdc34 on the acidic tail domain stimulates Cdc34-SCFCdc4 ubiquitination activity and cell cycle progression.

Induction of alkaline phosphatase activity by L-ascorbic acid in human osteoblastic cells: a potential role for CK2 and Ikaros

OBJECTIVE

To investigate the effect of L-ascorbic acid (AsA) on osteoblast differentiation, we examined the effects of AsA on in vitro osteoblastic differentiation markers such as collagen synthesis, alkaline phosphatase (ALP) activity, and receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) expression. The role of Ikaros and casein kinase 2 (CK2) in regulating osteoblast differentiation was also determined.

METHODS

This study examined the expression of RANKL and OPG, collagen synthesis, and ALP activity in AsA-treated osteoblast-like cells (MG63) using reverse transcription-polymerase chain reaction and biochemical assays. In addition, Ikaros activity and CK2 expression were assessed by electrophoretic mobility shift assays and western blot assays, respectively.

RESULTS

The results showed that AsA treatment slightly downregulated OPG mRNA expression, whereas the mRNA expression of RANKL and collagen was unaffected. AsA significantly increased ALP activity after 4 d, and this activation was inhibited by the CK2 inhibitors, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazimidazole. Small interfering RNA-mediated depletion of CK2-alpha also decreased ALP activity in AsA-stimulated cells. Moreover, western blot analysis showed that AsA induced the activation of CK2. AsA dose-dependently decreased the DNA binding affinity of the transcription factor Ikaros, which is a bifunctional differentiation factor. Moreover, cells treated with AsA and CK2 inhibitor exhibited increased Ikaros activity compared with those treated with AsA alone.

CONCLUSION

These results suggest that AsA stimulates osteoblastic differentiation by enhancing ALP activity and suppressing Ikaros activity. Moreover, this process might be related to CK2 regulation.

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.

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.

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.

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.

Order or chaos? An evaluation of the regulation of protein kinase CK2

CK2 is a highly conserved, ubiquitously expressed protein serine/threonine kinase present in all eukaryotes. Circumscribed as having a vast array of substrates located in a number of cellular compartments, CK2 has been implicated in critical cellular processes such as proliferation, apoptosis, differentiation, and transformation. Despite advances in elucidating its substrates and involvement in cellular regulation, its precise mode of regulation remains poorly defined. In this respect, there are currently conflicting views as to whether CK2 is constitutively active or modulated in response to specific stimuli. Perhaps an important consideration in resolving these apparent discrepancies is recognition of the existence of many discrete CK2 subpopulations that are distinguished from one another by localization or association with distinct cellular components. The existence of these subpopulations brings to light the possibility of each population being regulated independently rather than the entire cellular CK2 content being regulated globally. Logically, each local population may then be regulated in a distinct manner to carry out its precise function(s). This review will examine those mechanisms including regulated expression and assembly of CK2 subunits, phosphorylation of CK2, and interactions with small molecules or cellular proteins that could contribute to the local regulation of distinct CK2 populations.

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.

The Pleckstrin Homology Domain of CK2 Interacting Protein-1 Is Required for Interactions and Recruitment of Protein Kinase CK2 to the Plasma Membrane

CKIP-1 is a recently identified interaction partner of protein kinase CK2 with a number of protein-protein interaction motifs, including an N-terminal pleckstrin homology domain. To test the hypothesis that CKIP-1 has a role in targeting CK2 to specific locations, we examined the effects of CKIP-1 on the localization of CK2. These studies demonstrated that CKIP-1 can recruit CK2 to the plasma membrane. Furthermore, the pleckstrin homology domain of CKIP-1 was found to be required for interactions with CK2 and for the recruitment of CK2 to the plasma membrane. In this regard, point mutations in this domain abolish membrane localization and compromise interactions with CK2. In addition, replacement of the pleckstrin homology domain with a myristoylation signal was insufficient to elicit any interaction with CK2. An investigation of the lipid binding of CKIP-1 reveals that it has broad specificity. A comparison with other pleckstrin homology domains revealed that the pleckstrin homology domain of CKIP-1 is distinct from other defined classes of pleckstrin homology domains. Finally, examination of CK2alpha for a region that mediates interactions with CKIP-1 revealed a putative HIKE domain, a complex motif found exclusively in proteins that bind pleckstrin homology domains. However, mutations within this motif were not able to abolish CKIP-1-CK2 interactions suggesting that this motif by itself may not be sufficient to mediate interactions. Overall, these results provide novel insights into how CK2, a predominantly nuclear enzyme, is targeted to the plasma membrane, and perhaps more importantly how it may be regulated.

Human herpesvirus 6 immediate-early 2 protein interacts with heterogeneous ribonucleoprotein K and casein kinase 2

Human herpesvirus 6 (HHV-6) immediate-early (IE) 2 protein (IE2) may play important but incompletely defined roles during infection. We used yeast two-hybrid screening to detect proteins interacting with HHV-6 IE2, and found heterogeneous nuclear ribonucleoprotein K (hnRNP K) and the beta subunit of casein kinase 2 (CK2beta) specifically interacted with HHV-6 IE2. The interactions were confirmed by GST pull-down assay, coimmunoprecipitation, and colocalization studies. These findings indicate that the HHV-6 IE2 protein interacts with hnRNP K and CK2, and these interactions may affect viral and cellular RNA transcription and translation in viral replication.

Phosphorylation of threonine 10 on CKBBP1/SAG/ROC2/Rbx2 by protein kinase CKII promotes the degradation of IkappaBalpha and p27Kip1

In eukaryotic cells, protein kinase CKII is required for progression through the cell division cycle. We recently reported that CKBBP1/SAG/ROC2/Rbx2 associates with the beta-subunit of CKII and is phosphorylated by purified CKII in the presence of ATP in vitro. In this report, we demonstrate that CKBBP1 is efficiently phosphorylated in vitro by purified CKII in the presence of GTP and by heparin-sensitive protein kinase in HeLa cell extract. Mutational analysis indicates that CKII phosphorylates threonine at residue 10 within CKBBP1. Furthermore, CKBBP1 is phosphorylated in vivo and threonine to alanine mutation at residue 10 abrogates the phosphorylation of CKBBP1 observed in vivo, indicating that CKII is a major kinase that is responsible for in vivo phosphorylation of CKBBP1. As compared with the wild-type CKBBP1 or CKBBP1T10E (in which threonine 10 is replaced by glutamate), overexpression of nonphosphorylatable CKBBP1 (CKBBP1T10A) results in accumulation of IkappaBalpha and p27Kip1. Experiments using proteasome inhibitor MG132 and CKII inhibitor 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole suggest that the accumulation of IkappaBalpha and p27Kip1 results primarily from the reduction of proteasomal degradation in cells expressing CKBBP1T10A, and that CKII-mediated CKBBP1 phosphorylation is required for efficient degradation of IkappaBalpha and p27Kip1. Overexpression of CKBBP1T10A in HeLa cells suppresses cell proliferation and causes accumulation of G1/G0 peak of the cell cycle. Taken together, our results indicate that CKII may control IkappaBalpha and p27Kip1 degradation and thereby G1/S phase transition through the phosphorylation of threonine 10 within CKBBP1.

The protein of a new gene, Tctex4, interacts with protein kinase CK2beta subunit and is highly expressed in mouse testis

Casein kinase 2 (CK2) is a ubiquitous, multifunctional eukaryotic serine/threonine kinase that phosphorylates an array of proteins. CK2 is a heterotetramer composed of two catalytic (alpha,alpha(')) and two regulatory (beta) subunits. CK2 plays an essential role in regulatory pathways in cell transformation and proliferation. But the role and function of the individual subunits of CK2, which are not in the holoenzyme, are not yet clear. Northern blot analysis reveals the highest CK2beta activity in mouse testicles and brain. By employing a yeast two-hybrid screen to identify the proteins that interact with CK2beta, we have isolated a cDNA clone encoding a 14-kDa protein with homology to dynein light chains and have designated it as Tctex4. CK2beta interacts specifically with Tctex4 both in a yeast two-hybrid system and in an in vitro interaction assay. Northern blot and in situ hybridization showed that Tctex4 is a novel gene that is expressed in mouse testis.

[Protein kinase CK2: an enzyme that likes to be different]

Protein kinase CK2 (formerly known as casein kinase 2) was among the first protein kinases to be identified and characterized. Surprisingly, in spite of intense efforts, the regulation and cellular functions of CK2 remain obscure. However, recent data on its molecular structure, its signal-mediated intracellular dynamic localization and its unexpected function in cell survival have raised new interest in this enzyme. These studies reveal unique features of CK2 and highlight its importance in the transduction of survival signals.

Genome-wide expression screens indicate a global role for protein kinase CK2 in chromatin remodeling

Protein kinase CK2, a vital, pleiotropic and highly conserved serine/threonine phosphotransferase is involved in transcription-directed signaling, gene control and cell cycle regulation and is suspected to play a role in global processes. Searching for these global roles, we analyzed the involvement of CK2 in gene expression at cell cycle entry by using genome-wide screens. Comparing expression profiles of Saccharomyces cerevisiae wild-type strains with strains with regulatory or catalytic subunits of CK2 deleted, we found significant alterations in the expression of genes at all cell cycle phases and often in a subunit- and isoform-specific manner. Roughly a quarter of the genes known to be regulated by the cell cycle are affected. Functionally, the genes are involved with cell cycle entry, progression and exit, including spindle pole body formation and dynamics. Strikingly, most CK2-affected genes exhibit no common transcriptional control features, and a considerable proportion of temporarily altered genes encodes proteins involved in chromatin remodeling and modification, including chromatin assembly, (anti-)silencing and histone (de-)acetylation. In addition, various metabolic pathway and nutritional supply genes are affected. Our data are compatible with the idea that CK2 acts at different levels of cellular organization and that CK2 has a global role in transcription-related chromatin remodeling.

Phosphorylation regulates the stability of the regulatory CK2beta subunit

Protein kinase CK2 is a protein serine/threonine kinase that exhibits elevated expression in a number of cancers and displays oncogenic activity in mice. The regulatory CK2beta subunit has a central role in assembly of functional tetrameric CK2 complexes where it participates in modulation of catalytic activity and substrate specificity. Since overexpression of CK2beta results in elevated levels of CK2 activity, we investigated the molecular mechanisms that control its degradation since perturbations in these pathways could contribute to elevated CK2 in cancer. In this study, we demonstrate that CK2beta is degraded by a proteasome-dependent pathway and that it is ubiquitinated. We have also investigated the role of phosphorylation and a putative destruction box in regulating its stability in cells. Importantly, replacement of three serine residues within the autophosphorylation site of CK2beta with glutamic acid residues resulted in a significant decrease in its degradation indicating that autophosphorylation is involved in regulating its stability. Notably, although the autophosphorylation site of CK2beta is remarkably conserved between species, this is the first functional role ascribed to this site. Furthermore, based on these results, we speculate that alterations in the phosphorylation or dephosphorylation of the regulatory CK2beta subunit could underlie the elevated expression of CK2 that is observed in cancer cells.

Functional conservation between the human, nematode, and yeast CK2 cell cycle genes

Protein kinase CK2 (formerly casein kinase II) is a highly conserved serine/threonine protein kinase ubiquitous in eukaryotic organisms. Previously, we have shown that CK2 is required for cell cycle progression and essential for the viability of the yeast Saccharomyces cerevisiae. We now report that either the human or the nematode Caenorhabditis elegans CK2alpha catalytic subunit can substitute for the yeast catalytic subunits. Additionally, expression of the human CK2 regulatory subunit (CK2beta) can suppress the temperature sensitivity of either of the two yeast CK2 mutant catalytic subunits. Taken together, these observations reinforce the view that the CK2 cell cycle progression genes have been highly conserved during evolution from yeast to humans, not only in structure but also in function.

A role for protein kinase CK2 in cell proliferation: evidence using a kinase-inactive mutant of CK2 catalytic subunit alpha

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits generally combined to form alpha(2)beta(2), alphaalpha'beta(2), or alpha'(2)beta(2) heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits alpha, alpha', or beta was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2alpha catalytic subunit: CK2alpha-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (alpha, alpha' or beta) had no effect on cell proliferation. In contrast, overexpression of the CK2alpha kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2alpha-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive alpha subunit and as associated to the beta subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.

Transcriptional repression mediated by the KRAB domain of the human C2H2 zinc finger protein Kox1/ZNF10 does not require histone deacetylation

The KRAB domain of human Kox1, a member of the KRAB C2H2 zinc finger family, confers strong transcriptional repressor activities even to remote promoter positions. Here, HDAC inhibitors were used to demonstrate that histone deacetylation is not required for mediating transcriptional repression of KRAB zinc finger proteins. Two reporter systems with either stably integrated or transiently transfected templates, both under control of strong viral promoters, were analyzed. Under all circumstances, HDAC inhibition did not alter the repression potential of the KRAB domain. In case of the stably integrated luciferase reporter gene system, neither expression levels of the KRAB fusion protein nor complex formation with its putative co-repressor TIF1beta were significantly changed. Furthermore, the TIF1beta/KRAB complex was devoid of mSin3A and HDAC1. In the transient transfection system, the transcriptional repression induced by TIF1beta and HP1alpha was not diminished by HDAC inhibitors, whereas the repressory activity of TIF1alpha was significantly affected. Thus, KRAB, TIF1beta and HP1alpha are likely to be functionally linked. In conclusion, HDAC activity is not essential for the strong transcriptional repressor activity mediated by the KRAB domain of Kox1 in particular and, presumably, by KRAB domains in general. This feature might be helpful in identifying and characterizing target genes under the control of

On the physiological role of casein kinase II in Saccharomyces cerevisiae

Casein kinase II (CKII) is a highly conserved serine/threonine protein kinase that is ubiquitous in eukaryotic organisms. This review summarizes available data on CKII of the budding yeast Saccharomyces cerevisiae, with a view toward defining the possible physiological role of the enzyme. Saccharomyces cerevisiae CKII is composed of two catalytic and two regulatory subunits encoded by the CKA1, CKA2, CKB1, and CKB2 genes, respectively. Analysis of null and conditional alleles of these genes identifies a requirement for CKII in at least four biological processes: flocculation (which may reflect an effect on gene expression), cell cycle progression, cell polarity, and ion homeostasis. Consistent with this, isolation of multicopy suppressors of conditional cka mutations has identified three genes that have a known or potential role in either the cell cycle or cell polarity: CDC37, which is required for cell cycle progression in both G1 and G2/M; ZDS1 and 2, which appear to have a function in cell polarity; and SUN2, which encodes a protein of the regulatory component of the 26S protease. The identity and properties of known CKII substrates in S. cerevisiae are also reviewed, and advantage is taken of the complete genomic sequence to predict globally the substrates of CKII in this organism. Although the combined data do not yield a definitive picture of the physiological role of CKII, it is proposed that CKII serves a signal transduction function in sensing and/or communicating information about the ionic status of the cell to the cell cycle machinery.

The tumor suppressor PTEN is phosphorylated by the protein kinase CK2 at its C terminus. Implications for PTEN stability to proteasome-mediated degradation

The tumor suppressor phosphatase PTEN regulates cell migration, growth, and survival by dephosphorylating phosphatidylinositol second messengers and signaling phosphoproteins. PTEN possesses a C-terminal noncatalytic regulatory domain that contains multiple putative phosphorylation sites, which could play an important role in the control of its biological activity. The protein kinase CK2 phosphorylated, in a constitutive manner, a cluster of Ser/Thr residues located at the PTEN C terminus. PTEN-phosphorylated defective mutants showed decreased stability in comparison with wild type PTEN and were more rapidly degraded by the proteasome. Inhibition of PTEN phosphorylation by the CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosyl-benzimidazole also diminished the PTEN protein content. Our results support the notion that proper phosphorylation of PTEN by CK2 is important for PTEN protein stability to proteasome-mediated degradation.

Phosphorylation of the vesicle-tethering protein p115 by a casein kinase II-like enzyme is required for Golgi reassembly from isolated mitotic fragments

Coat protein I (COPI) transport vesicles can be tethered to Golgi membranes by a complex of fibrous, coiled-coil proteins comprising p115, Giantin and GM130. p115 has been postulated to act as a bridge, linking Giantin on the vesicle to GM130 on the Golgi membrane. Here we show that the acidic COOH terminus of p115 mediates binding to both GM130 and Giantin as well as linking the two together. Phosphorylation of serine 941 within this acidic domain enhances the binding as well as the link between them. Phosphorylation is mediated by casein kinase II (CKII) or a CKII-like kinase. Surprisingly, the highly conserved NH(2)-terminal head domain of p115 is not required for the NSF (N-ethylmaleimide-sensitive fusion protein)-catalyzed reassembly of cisternae from mitotic Golgi fragments in a cell-free system. However, the ability of p115 to link GM130 to Giantin and the phosphorylation of p115 at serine 941 are required for NSF-catalyzed cisternal regrowth. p115 phosphorylation may be required for the transition from COPI vesicle tethering to COPI vesicle docking, an event that involves the formation of trans-SNARE [corrected] (trans-soluble NSF attachment protein [SNAP] receptor) complexes.

Intracellular distribution of mammalian protein kinase A catalytic subunit altered by conserved Asn2 deamidation

The catalytic (C) subunit of protein kinase A functions both in the cytoplasm and the nucleus. A major charge variant representing about one third of the enzyme in striated muscle results from deamidation in vivo of the Asn2 residue at the conserved NH(2)-terminal sequence myrGly-Asn-Ala (Jedrzejewski, P.T., A. Girod, A. Tholey, N. König, S. Thullner, V. Kinzel, and D. Bossemeyer. 1998. Protein Sci. 7:457-469). Because of the increase of electronegativity by generation of Asp2, it is reminiscent of a myristoyl-electrostatic switch. To compare the intracellular distribution of the enzymes, both forms of porcine or bovine heart enzyme were microinjected into the cytoplasm of mouse NIH 3T3 cells after conjugation with fluorescein, rhodamine, or in unlabeled form. The nuclear/cytoplasmic fluorescence ratio (N/C) was analyzed in the presence of cAMP (in the case of unlabeled enzyme by antibodies). Under all circumstances, the N/C ratio obtained with the encoded Asn2 form was significantly higher than that with the deamidated, Asp2 form; i.e., the Asn2 form reached a larger nuclear concentration than the Asp2 form. Comparable data were obtained with a human cell line. The differential intracellular distribution of both enzyme forms is also reflected by functional data. It correlates with the degree of phosphorylation of the key serine in CREB family transcription factors in the nucleus. Microinjection of myristoylated recombinant bovine Calpha and the Asn2 deletion mutant of it yielded N/C ratios in the same range as encoded native enzymes. Thus, Asn2 seems to serve as a potential site for modulating electronegativity. The data indicate that the NH(2)-terminal domain of the PKA C-subunit contributes to the intracellular distribution of free enzyme, which can be altered by site-specific in vivo deamidation. The model character for other signaling proteins starting with myrGly-Asn is discussed.

Production and functional characterization of a soluble recombinant form of mouse CD59

This report describes the engineering, expression, purification and functional characterization of a soluble recombinant form of murine CD59 (srMoCD59). We report the expression in Chinese hamster ovary (CHO) cells of a modified mouse CD59 cDNA that had been truncated at D-74, resulting in the loss of the glycosylphosphatidyl inositol (GPI) anchor, and containing six additional C-terminal histidines. The expressed srMoCD59 was purified from tissue culture supernatant by means of its poly-histidine tag using immobilized metal affinity chromatography. In comparison with CD59 on mouse erythrocytes, the srMoCD59 had a reduced molecular weight (18-20 000 as compared with 20-28 000 for GPI-anchored srMoCD59). The terminal complement inhibitory capacity of this soluble recombinant protein was assessed using two methods: a cobra venom factor (CVF)-triggered 'reactive-lysis' system and a C5b-7 site assay. In both assays, srMoCD59 inhibited lysis by the sera from all three species tested in the rank order mouse > rat >> human. The amount of srMoCD59 required to produce 50% inhibition of lysis in the C5b-7 site assay, using purified terminal components to develop lysis, was 10-fold less than that required in the same assay when EDTA serum was used as a source of C8 and C9, or in the CVF reactive lysis system. These data indicate that the presence of serum markedly interfered with the activity of srMoCD59 and have important implications for the use of recombinant soluble CD59 analogues as therapeutic agents in complement-mediated diseases.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Specific binding of protein kinase CK2 catalytic subunits to tubulin

Protein kinase CK2 is composed of two regulatory beta-subunits and two catalytic alpha- or alpha'-subunits. To analyse these subunits individually we generated antibodies against unique peptides derived from the alpha-, alpha'- and beta-subunit. Immunofluorescence studies with these antibodies revealed the presence of all three CK2 subunits in the cytoplasm and weakly in the nucleus with strong signals around the nuclear membrane. Double staining experiments revealed a co-localisation of all three subunits with tubulin. A direct association between the CK2 alpha- and the alpha'-subunit and tubulin was confirmed by co-immunoprecipitation experiments as well as by Far Western analysis. There was no binding of the CK2 beta-subunit to tubulin. Thus, with tubulin we have identified a new binding partner specific for the catalytic subunits of CK2.

Expression of the casein kinase 2 subunits in Chinese hamster ovary and 3T3 L1 cells provides information on the role of the enzyme in cell proliferation and the cell cycle

In order to investigate the in vivo functions of protein kinase CK2 (CK2), the expression of Myc-tagged versions of the subunits, Myc-CK2alpha and Myc-CK2beta, was carried out in Chinese hamster ovary cells (CHO cells) and in 3T3 L1 fibroblasts. Cell proliferation in these cells was examined. CHO cells that transiently overexpressed the Myc-CK2beta subunit exhibited a severe growth defect, as shown by a much lower value of [(3)H]thymidine incorporation than the vector controls, and a rounded shrunken morphology. In contrast, cells overexpressing Myc-tagged CK2alpha showed a slightly but consistently higher value of [(3)H]thymidine incorporation than the controls. The defect in cell growth and changes in morphology caused by Myc-CK2beta overexpression were partially rescued by coexpression of Myc-tagged CK2alpha. In parallel to the studies in CHO cells, the stable transfection of Myc-CK2alpha and Myc-CK2beta subunits was achieved in 3T3 L1 fibroblast cells. Similarly, the ectopic expression of Myc-CK2beta, but not Myc-CK2alpha, caused a growth defect. By measuring [(3)H]thymidine incorporation, it was found that expression of Myc-CK2beta prolonged the G(1) phase and inhibited up-regulation of cyclin D1 expression during G(1). In addition, a lower mitotic index and lower mitotic cyclin-dependent kinase activities were detected in Myc-CK2beta-expressing cells. Detailed analysis of stable cells that were synchronously released into the cell cycle revealed that the expression of Myc-CK2beta inhibited cells entering into mitosis and prevented the activation of mitotic cyclin-dependent kinases. Taken together, results from both transient and stable expression of CK2 subunits strongly suggest that CK2 may be involved in the control of cell growth and progression of the cell cycle.

Tumour suppressor protein p53 released by nuclease digestion increases at the onset of rat liver regeneration

BACKGROUND/AIMS

Protein kinase CK2 (CK2) increases when cells are committed to proliferate, as in liver regeneration. This enzyme phosphorylates the tumour suppressor protein p53, whose expression controls the levels of many other cell cycle proteins. The aim of this study was to determine if CK2 was affected by p53.

METHODS

Male Sprague-Dawley rats (200-250 g) were subjected to either partial hepatectomy or laparotomy and the levels and subcellular distribution of p53 were studied, following the approach used earlier for CK2. The levels of both proteins were also studied in the human cell lines HL-60 (devoid of p53) and HepG2 (with normal p53 levels) and in fibroblasts from transgenic p53-deficient mice (p53-/-) or homozygous for wild-type p53 (p53+/+). Computer-assisted search was used to detect p53 consensus sequences in genes for CK2 subunits Binding of p53 protein to some of these sequences was assayed by electrophoretic mobility shift assay.

RESULTS

Rat liver p53 protein was present mainly in the fraction extracted from intact nuclei by nucleases (S1) and showed a transient increase at 6 h post partial hepatectomy, as observed previously with nuclear CK2. The human CK2a gene presents the consensus sequence for trans-activation by p53 and specific binding of p53 protein to some of these sequences was detected in vitro. Total CK2a was higher in HepG2 than in HL-60 cells but total CK2 and its cytosolic/ nuclear distribution was similar in mice (p53+/+) fibroblasts and (p53-/-) fibroblasts.

CONCLUSIONS

p53 is present in the nuclease-extracted S1 fraction from liver cells, as described for CK2, and undergoes similar changes at the beginning of rat liver regeneration. However, the data on cultured cells suggest that the expression of CK2 and its subcellular localization are p53-independent events.

Inducible expression of protein kinase CK2 in mammalian cells. Evidence for functional specialization of CK2 isoforms

Protein kinase CK2 (formerly casein kinase II) exhibits elevated expression in a variety of cancers, induces lymphocyte transformation in transgenic mice, and collaborates with Ha-Ras in fibroblast transformation. To systematically examine the cellular functions of CK2, human osteosarcoma U2-OS cells constitutively expressing a tetracycline-regulated transactivator were stably transfected with a bidirectional plasmid encoding either catalytic isoform of CK2 (i.e. CK2alpha or CK2alpha') together with the regulatory CK2beta subunit in order to increase the cellular levels of either CK2 isoform. To interfere with either CK2 isoform, cells were also transfected with kinase-inactive CK2alpha or CK2alpha' (i. e. GK2alpha (K68M) or CK2alpha'(K69M)) together with CK2beta. In these cells, removal of tetracycline from the growth medium stimulated coordinate expression of catalytic and regulatory CK2 subunits. Increased expression of active forms of CK2alpha or CK2alpha' resulted in modest decreases in cell proliferation, suggesting that optimal levels of CK2 are required for optimal proliferation. By comparison, the effects of induced expression of kinase-inactive CK2alpha differed significantly from the effects of induced expression of kinase-inactive CK2alpha'. Of particular interest is the dramatic attenuation of proliferation that is observed following induction of CK2alpha'(K69M), but not following induction of CK2alpha(K68M). These results provide evidence for functional specialization of CK2 isoforms in mammalian cells. Moreover, cell lines exhibiting regulatable expression of CK2 will facilitate efforts to systematically elucidate its cellular functions.

Serum-stimulated cell cycle entry of fibroblasts requires undisturbed phosphorylation and non-phosphorylation interactions of the catalytic subunits of protein kinase CK2

Protein kinase CK2 is a pleiotropic Ser/Thr kinase occurring as alpha2beta2, alpha'2beta2, or alphaalpha'beta2 tetramers. A requirement in serum-stimulated cell cycle entry in both the cytoplasm and the nucleus of human fibroblasts for phosphorylation(s) by CK2 has been concluded from stimulation inhibition by microinjected antibodies against the regulatory subunit (beta). We have now examined this idea more directly by microinjection-mediated perturbation of phosphorylation and non-phosphorylation interactions of the catalytic subunits (alpha and alpha'), and by verifying the supposed matching of the cellular partition of CK2 subunits in the fibroblasts employed. While immunostaining and cell fractionation indicate that the partitions of subunits indeed match each other (with their predominant location in the nucleus in both quiescent and serum-stimulated cells), microinjection of substrate or pseudosubstrate peptides competing for the CK2-mediated phosphorylation in vitro resulted in significant inhibition of serum stimulation when placed into the nucleus but not when placed into the cytoplasm. Also inhibitory were nuclear but not cytoplasmic injections of antibodies against alpha and alpha' that affect neither their kinase activity in vitro nor their complexing to beta. The data indicate that the role played by CK2 in serum-stimulated cell cycle entry is predominantly nuclear and more complex than previously assumed, involving not only phosphorylation but also experimentally separable non-phosphorylation interactions by the catalytic subunits.