Ets1 is a common element in directing transcription of the alpha and beta genes of human protein kinase CK2

How can a traffic light properly work if it is always green? The paradox of CK2 signaling

CK2 is a constitutively active protein kinase that assuring a constant level of phosphorylation to its numerous substrates supports many of the most important biological functions. Nevertheless, its activity has to be controlled and adjusted in order to cope with the varying needs of a cell, and several examples of a fine-tune regulation of its activity have been described. More importantly, aberrant regulation of this enzyme may have pathological consequences, e.g. in cancer, chronic inflammation, neurodegeneration, and viral infection. Our review aims at summarizing our current knowledge about CK2 regulation. In the first part, we have considered the most important stimuli shown to affect protein kinase CK2 activity/expression. In the second part, we focus on the molecular mechanisms by which CK2 can be regulated, discussing controversial aspects and future perspectives.

Estrogen receptor alpha transcriptionally activates casein kinase 2 alpha: A pivotal regulator of promyelocytic leukaemia protein (PML) and AKT in oncogenesis

Protein kinase CK2α is frequently upregulated in different cancers. Alteration of CK2α expression and its activity is sufficient to induce dramatic changes in cell fate. It has been established that CK2α induces oncogenesis through modulation of both AKT and PML. CK2α has been found to be overexpressed in breast cancer. In contrary, statistical reports have shown low level of PML. However, the regulation of CK2α gene expression is not fully understood. In the current study, we found that CK2α and activated AKT positively correlate with ERα, whereas PML follows an inverse correlation in human breast cancer tissues. Modulation of ERα signalling leads to recruitment of activated ERα on the ERE sites of CK2α promoter, resulting in CK2α transactivation. Furthermore, the DMBA induced tumours in rat showed elevated level of active CK2α. Consequently it mediates enhancement of AKT activity and PML degradation, resulting in increased cellular proliferation, migration and metastasis. Syngeneic ERα overexpressing stable mouse 4T1 cells produce larger primary tumours and metastatic lung nodules in mice, corroborating our in vitro findings. Hence, our study provides a novel route of ERα dependent CK2α mediated oncogenesis that causes upregulation and consequent AKT activation along with degradation of tumour suppressor PML.

Protein kinase CK2 is a critical regulator of epithelial homeostasis in chronic intestinal inflammation

The molecular mechanisms that restore intestinal epithelial homeostasis during colitis are incompletely understood. Here, we report that during intestinal inflammation, multiple inflammatory cytokines promote the activity of a master regulator of cell proliferation and apoptosis, serine/threonine kinase CK2. Enhanced mucosal CK2 protein expression and activity were observed in animal models of chronic colitis, particularly within intestinal epithelial cells (IECs). The in vitro treatment of intestinal epithelial cell lines with cytokines resulted in increased CK2 expression and nuclear translocation of its catalytic α subunit. Similarly, nuclear translocation of CK2α was a prominent feature observed in colonic crypts from individuals with ulcerative colitis and Crohn's disease. Further in vitro studies revealed that CK2 activity promotes epithelial restitution, and protects normal IECs from cytokine-induced apoptosis. These observations identify CK2 as a key regulator of homeostatic properties of the intestinal epithelium that serves to promote wound healing, in part through inhibition of apoptosis under conditions of inflammation.

Novel mechanism by which histone deacetylase inhibitors facilitate topoisomerase IIα degradation in hepatocellular carcinoma cells

Histone deacetylase (HDAC) inhibitors exhibit a unique ability to degrade topoisomerase (topo)IIα in hepatocellular carcinoma (HCC) cells, which contrasts with the effect of topoII-targeted drugs on topoIIβ degradation. This selective degradation might foster novel strategies for HCC treatment in light of the correlation of topoIIα overexpression with the aggressive tumor phenotype and chemoresistance. Here we report a novel pathway by which HDAC inhibitors mediate topoIIα proteolysis in HCC cells. Our data indicate that HDAC inhibitors transcriptionally activated casein kinase (CK)2α expression through increased association of acetylated histone H3 with the CK2α gene promoter. In turn, CK2 facilitated the binding of topoIIα to COP9 signalosome subunit (Csn)5 by way of topoIIα phosphorylation. Furthermore, we identified Fbw7, a Csn5-interacting F-box protein, as the E3 ligase that targeted topoIIα for degradation. Moreover, knockdown of CK2α, Csn5, or Fbw7 reversed HDAC inhibitor-induced topoIIα degradation. Mutational analysis indicates that the (1361) SPKLSNKE(1368) motif plays a crucial role in regulating topoIIα protein stability. This motif contains the consensus recognition sites for CK2 (SXXE), glycogen synthase kinase (GSK)3β (SXXXS), and Fbw7 (SPXXS). This study also reports the novel finding that topoIIα may be a target of GSK3β phosphorylation. Evidence suggests that CK2 serves as a priming kinase, through phosphorylation at Ser1365, for GSK3β-mediated phosphorylation at Ser1361. This double phosphorylation facilitated the recruitment of Fbw7 to the phospho-degron (1361) pSPKLpS(1365) of topoIIα, leading to its ubiquitin-dependent degradation.

CONCLUSION

This study shows a novel pathway by which HDAC inhibitors facilitate the selective degradation of topoIIα, which underlies the complexity of the functional role of HDAC in regulating tumorigenesis and aggressive phenotype in HCC cells.

The catalytic subunit alpha' gene of human protein kinase CK2 (CSNK2A2): genomic organization, promoter identification and determination of Ets1 as a key regulator

The human genome contains four protein kinase CK2 loci, enclosing three active genes coding for the catalytic subunits alpha and alpha' and the regulatory subunit beta, and a processed alpha subunit pseudogene. Extensive structure and transcriptional control data of the genes are available, except for the CK2alpha' gene (CSNK2A2). Using in silico and experimental approaches, we find CSNK2A2 to be located on the long arm of chromosome 16 (in contrast to published data), to span 40kb and to consist of 12 exons, with the translational start in Exon 1 and the stop in Exon 11. Exon/intron boundaries conform to the gt/ag rule, and various potential polyadenylation signals determine transcript species with lengths of 1.7-5.7 kb. The upstream region of the gene displays housekeeping characteristics, lacking a TATA box and possessing several GC boxes as well as a CpG island around Exon 1. According to reporter gene assay results, the promoter activity ranges from -1308 to 197 with the highest activity in region -396 to -129. This region contains binding motifs for various transcription factors, including NFkappaB, Sp and Ets family members. Site-directed mutagenesis indicates that the Ets motifs play, in cooperation with Sp motif clusters, a central role in regulating CK2alpha' gene transcription. A similar control has been described for the transcription of the CK2alpha and CK2beta genes so that the presented data are compatible with the assumption of a coordinate transcriptional regulation of all three active human CK2 genes decisively determined by Ets family members.

Immunologically defined subclasses of the protein kinase CK2 beta-subunit in prostate carcinoma cell lines

Both, the activity as well as the expression of protein kinase CK2 is enhanced in various cancer types and in established tumour cell lines. This phenomenon is not due to an increase in the CK2 message but rather to posttranscriptional and posttranslational mechanisms. In order to get an insight into these posttranslational modifications we analyzed CK2 in prostate cancer cell lines, which differ by their hormone-sensitivity. We found that the CK2 activity is significantly higher in hormone-refractory than in hormone-sensitive cells although the amount of the catalytic alpha- and alpha'- subunits is comparable. In contrast, we detected seemingly lower amounts of the regulatory beta-subunit in the hormone-refractory cell lines, which later turned out to be an immunologically defined subclass. This subclass is realized by a phosphate group, which is attached to serine 209. The phosphorylation occurs in vivo during mitosis and is executed by the p34(cdc2)/cyclin B kinase. As this phosphorylation enhances the CK2 activity this change might well account for the higher activity of CK2 in prostate cancer cells.

The multiple personalities of the regulatory subunit of protein kinase CK2: CK2 dependent and CK2 independent roles reveal a secret identity for CK2beta

Protein kinase CK2 (formerly casein kinase II), an enzyme that participates in a wide variety of cellular processes, has traditionally been classified as a stable tetrameric complex consisting of two catalytic CK2α or CK2α' subunits and two regulatory CK2β subunits. While consideration of CK2 as a tetrameric complex remains relevant, significant evidence has emerged to challenge the view that its individual subunits exist exclusively within these complexes. This review will summarize biochemical and genetic evidence indicating that the regulatory CK2β subunit exists and performs functions independently of CK2 tetramers. For example, unbalanced expression of catalytic and regulatory CK2 subunits has been observed in a variety of tissues and tumors. Furthermore, localization studies including live cell imaging have demonstrated that while the catalytic and regulatory subunits of CK2 exhibit extensive co-localization, independent mobility of the individual CK2 subunits can also be observed within cells. Identification of proteins that interact with CK2β in the absence of catalytic CK2 subunits reinforces the notion that CK2β has functions distinct from CK2 and begins to offer insights into these CK2-independent functions. In this respect, the discovery that CK2β can interact with and modulate the activity of a number of other serine/threonine protein kinases including A-Raf, c-Mos and Chk1 is particularly striking. This review will discuss the interactions between CK2β and these protein kinases with special emphasis on the properties of CK2β that mediate these interactions and on the implications of these interactions in yielding new prospects for elucidation of the cellular functions of CK2β.