Protein Kinase CK2α’, More than a Backup of CK2α

The serine/threonine protein kinase CK2 is implicated in the regulation of fundamental processes in eukaryotic cells. CK2 consists of two catalytic α or α’ isoforms and two regulatory CK2β subunits. These three proteins exist in a free form, bound to other cellular proteins, as tetrameric holoenzymes composed of CK2α2/β2 , CK2αα’/β2 , or CK2α’2/β2 as well as in higher molecular forms of the tetramers. The catalytic domains of CK2α and CK2α’ share a 90% identity. As CK2α contains a unique C-terminal sequence. Both proteins function as protein kinases. These properties raised the question of whether both isoforms are just backups of each other or whether they are regulated differently and may then function in an isoform-specific manner. The present review provides observations that the regulation of both CK2α isoforms is partly different concerning the subcellular localization, post-translational modifications, and aggregation. Up to now, there are only a few isoform-specific cellular binding partners. The expression of both CK2α isoforms seems to vary in different cell lines, in tissues, in the cell cycle, and with differentiation. There are different reports about the expression and the functions of the CK2α isoforms in tumor cells and tissues. In many cases, a cell-type-specific expression and function is known, which raises the question about cell-specific regulators of both isoforms. Another future challenge is the identification or design of CK2α’-specific inhibitors

Inhibition of Protein Kinase CK2 Prevents Adipogenic Differentiation of Mesenchymal Stem Cells Like C3H/10T1/2 Cells

Protein kinase CK2 as a holoenzyme is composed of two catalytic α- or α’-subunits and two non-catalytic β-subunits. Knock-out experiments revealed that CK2α and CK2β are required for embryonic development. Little is known about the role of CK2 during differentiation of stem cells. Mesenchymal stem cells (MSCs) are multipotent cells which can be differentiated into adipocytes in vitro. Thus, MSCs and in particular C3H/10T1/2 cells are excellent tools to study a possible role of CK2 in adipogenesis. We found downregulation of the CK2 catalytic subunits as well as a decrease in CK2 kinase activity with progression of differentiation. Inhibition of CK2 using the potent inhibitor CX-4945 impeded differentiation of C3H/10T1/2 cells into adipocytes. The inhibited cells lacked the observed decrease in CK2 expression, but showed a constant expression of all three CK2 subunits. Furthermore, inhibition of CK2 resulted in decreased cell proliferation in the early differentiation phase. Analysis of the main signaling cascade revealed an elevated expression of C/EBPβ and C/EBPδ and reduced expression of the adipogenic master regulators C/EBPα and PPARγ2. Thus, CK2 seems to be implicated in the regulation of different steps early in the adipogenic differentiation of MSC

Protein Kinase CK2 and Epstein–Barr Virus

Protein kinase CK2 is a pleiotropic protein kinase, which phosphorylates a number of cellular and viral proteins. Thereby, this kinase is implicated in the regulation of cellular signaling, controlling of cell proliferation, apoptosis, angiogenesis, immune response, migration and invasion. In general, viruses use host signaling mechanisms for the replication of their genome as well as for cell transformation leading to cancer. Therefore, it is not surprising that CK2 also plays a role in controlling viral infection and the generation of cancer cells. Epstein–Barr virus (EBV) lytically infects epithelial cells of the oropharynx and B cells. These latently infected B cells subsequently become resting memory B cells when passing the germinal center. Importantly, EBV is responsible for the generation of tumors such as Burkitt’s lymphoma. EBV was one of the first human viruses, which was connected to CK2 in the early nineties of the last century. The present review shows that protein kinase CK2 phosphorylates EBV encoded proteins as well as cellular proteins, which are implicated in the lytic and persistent infection and in EBV-induced neoplastic transformation. EBV-encoded and CK2-phosphorylated proteins together with CK2-phosphorylated cellular signaling proteins have the potential to provide efficient virus replication and cell transformation. Since there are powerful inhibitors known for CK2 kinase activity, CK2 might become an attractive target for the inhibition of EBV replication and cell transformation

The Phosphorylation of PDX-1 by Protein Kinase CK2 Is Crucial for Its Stability

The homeodomain protein PDX-1 is a critical regulator of pancreatic development and insulin production in pancreatic β-cells. We have recently shown that PDX-1 is a substrate of protein kinase CK2; a multifunctional protein kinase which is implicated in the regulation of various cellular aspects, such as differentiation, proliferation, and survival. The CK2 phosphorylation site of PDX-1 is located within the binding region of the E3 ubiquitin ligase adaptor protein PCIF1. To study the interaction between PDX-1 and PCIF1 we used immunofluorescence analysis, co-immunoprecipitation, GST-pull-down studies, and proximity ligation assay (PLA). For the analysis of the stability of PDX-1 we performed a cycloheximide chase. We used PDX-1 in its wild-type form as well as phosphomutants of the CK2 phosphorylation site. In pancreatic β-cells PDX-1 binds to PCIF1. The phosphorylation of PDX-1 by CK2 increases the ratio of PCIF1 bound to PDX-1. The stability of PDX-1 is extended in the absence of CK2 phosphorylation. Our results identified protein kinase CK2 as new important modulator of the stability of PDX-1

SGC-CK2-1 Is an Efficient Inducer of Insulin Production and Secretion in Pancreatic β-Cells

The pyrazolopyrimidine based compound SGC-CK2-1 is a potent and highly specific CK2 inhibitor and a new tool to study the biological functions of protein kinase CK2 irrespective from off-target effects. We used this compound in comparison with the well-established CK2 inhibitor CX-4945 to analyze the importance of CK2 for insulin production and secretion from pancreatic β-cells. Both inhibitors affected the proliferation and viability of MIN6 cells only marginally and downregulated the endogenous CK2 activity to a similar level. Furthermore, both inhibitors increased the message for insulin and boosted the secretion of insulin from storage vesicles. Thus, regarding the high specificity of SGC-CK2-1, we can clearly attribute the observed effects to biological functions of protein kinase CK2

Control of TRPM3 Ion Channels by Protein Kinase CK2-Mediated Phosphorylation in Pancreatic β-Cells of the Line INS-1

In pancreatic β-cells of the line INS-1, glucose uptake and metabolism induce the openings of Ca2+-permeable TRPM3 channels that contribute to the elevation of the intracellular Ca2+ concen tration and the fusion of insulin granules with the plasma membrane. Conversely, glucose-induced Ca2+ signals and insulin release are reduced by the activity of the serine/threonine kinase CK2. There fore, we hypothesized that TRPM3 channels might be regulated by CK2 phosphorylation. We used recombinant TRPM3α2 proteins, native TRPM3 proteins from INS-1 β-cells, and TRPM3-derived oligopeptides to analyze and localize CK2-dependent phosphorylation of TRPM3 channels. The func tional consequences of CK2 phosphorylation upon TRPM3-mediated Ca2+ entry were investigated in Fura-2 Ca2+-imaging experiments. Recombinant TRPM3α2 channels expressed in HEK293 cells displayed enhanced Ca2+ entry in the presence of the CK2 inhibitor CX-4945 and their activity was strongly reduced after CK2 overexpression. TRPM3α2 channels were phosphorylated by CK2 in vitro at serine residue 1172. Accordingly, a TRPM3α2 S1172A mutant displayed enhanced Ca2+ entry. The TRPM3-mediated Ca2+ entry in INS-1 β-cells was also strongly increased in the presence of CX-4945 and reduced after overexpression of CK2. Our study shows that CK2-mediated phosphorylation controls TRPM3 channel activity in INS-1 β-cells

CK2 kinase activity but not its binding to CK2 promoter regions is implicated in the regulation of CK2α and CK2β gene expressions

Protein kinase CK2, a ubiquitous serine/threonine kinase in control of a variety of crucial cellular functions, is composed of catalytic a- and a0-subunits and non-catalytic b-subunits which form holoenzymes such as CK2(ab)2, CK2aa\u27b2, or CK2(a\u27b)2. In addition, there is sample evidence for the occurrence of the individual subunits beside the holoenzyme. While the CK2 subunits are well analyzed on the protein level, only little is known about the regulation of their transcription. The existence of multiple forms of CK2 subunits raised the question about a mutual regulation of their expression. Here we defined two 50-upstream regions of the CK2alpha and the CK2beta genes, respectively, as sequences with promoter activities. We found that CK2alpah and CK2alpha\u27 stimulated the expression of the reporter constructs whereas, CK2beta was inactive. Using chromatin immunoprecipitation assays, we were unable to detect binding of endogenous CK2 subunits to these promoter sequences in vivo. However, it turned out that inhibition of the kinase activity of CK2 attenuated the promoter activity indicating that CK2alpha and CK2alpha\u27 might regulate their gene expression indirectly by phosphorylation reactions. Thus, we have shown here (i) that under normal physiological conditions CK2 does not bind to CK2 promoter regions and (ii) that the CK2 kinase activity is implicated in the regulation of its own expression

An Updated View on an Emerging Target: Selected Papers from the 8th International Conference on Protein Kinase CK2

The 8th International Conference on Protein Kinase CK2 took place in Homburg, Germany, from 6 September to 9 September 2016. Over 80 scientists from Australia, China, Japan, USA, Canada, Denmark, France, Italy, Spain, Poland and Germany participated. After the opening lecture by Lorenzo A. Pinna, Padova, Italy, entitled Exploring the CK2 Paradox: Restless, Dangerous, Dispensable, the scientists reported their latest research on the structural characterization of CK2, hence leading directly to the development of CK2 inhibitors. The driving force behind the development of inhibitors is their use in the treatment of various diseases, which was the next topic of the conference. New findings on protein kinase CK2 were addressed in the following session. The final topic of the conference addressed the role of CK2 in differentiation and development

Protein Kinase CK2—A Putative Target for the Therapy of Diabetes Mellitus?

Since diabetes is a global epidemic, the development of novel therapeutic strategies for the treatment of this disease is of major clinical interest. Diabetes is differentiated in two types: type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). T1DM arises from an autoimmune destruction of insulin-producing β-cells whereas T2DM is characterized by an insulin resistance, an impaired insulin reaction of the target cells, and/or dysregulated insulin secretion. In the past, a growing number of studies have reported on the important role of the protein kinase CK2 in the regulation of the survival and endocrine function of pancreatic β-cells. In fact, inhibition of CK2 is capable of reducing cytokine-induced loss of β-cells and increases insulin expression as well as secretion by various pathways that are regulated by reversible phosphorylation of proteins. Moreover, CK2 inhibition modulates pathways that are involved in the development of diabetes and prevents signal transduction, leading to late complications such as diabetic retinopathy. Hence, targeting CK2 may represent a novel therapeutic strategy for the treatment of diabetes