One-thousand-and-one substrates of protein kinase CK2?

Oral administration of the casein kinase 2 inhibitor TBB leads to persistent KCa2.2 channel up-regulation in the epileptic CA1 area and cortex, but lacks anti-seizure efficacy in the pilocarpine epilepsy model

Temporal lobe epilepsy (TLE) is the most common epileptic syndrome in adults and often presents with seizures that prove intractable with currently available anticonvulsants. Thus, there is still a need for new anti-seizure drugs in this condition. Recently, we found that the casein kinase 2 inhibitor 4,5,6,7-tetrabromotriazole (TBB) prevented the emergence of spontaneous epileptic discharges in an acute in vitro epilepsy model. This prompted us to study the anti-seizure effects of TBB in the pilocarpine model of chronic epilepsy in vivo. To this end, we performed long-term video-EEG monitoring lasting 78-167 days of nine chronically epileptic rats and obtained a baseline seizure rate of 3.3 ± 1.3 per day (baseline of 27-80 days). We found a significant age effect with more pronounced seizure rates in older animals as compared to younger ones. However, the seizure rate increased to 6.3 ± 2.2 per day during the oral TBB administration (treatment period of 21-50 days), and following discontinuation of TBB, this rate remained stable with 5.2 ± 1.4 seizures per day (follow-up of 30-55 days). After completing the video-EEG during the follow-up the hippocampal tissue was prepared and studied for the expression of the Ca²⁺-activated K⁺ channel K_Ca2.2. We found a significant up-regulation of K_Ca2.2 in the epileptic CA1 region and in the neocortex, but in no other hippocampal subfield. Hence, our findings indicate that oral administration of TBB leads to persistent up-regulation of K_Ca2.2 in the epileptic CA1 subfield and in the neocortex, but lacks anti-seizure efficacy in the pilocarpine epilepsy model.

CK2 inhibition protects white matter from ischemic injury

Strokes occur predominantly in the elderly and white matter (WM) is injured in most strokes, contributing to the disability associated with clinical deficits. Casein kinase 2 (CK2) is expressed in neuronal cells and was reported to be neuroprotective during cerebral ischemia. Recently, we reported that CK2 is abundantly expressed by glial cells and myelin. However, in contrast to its role in cerebral (gray matter) ischemia, CK2 activation during ischemia mediated WM injury via the CDK5 and AKT/GSK3β signaling pathways (Bastian et al., 2018). Subsequently, CK2 inhibition using the small molecule inhibitor CX-4945 correlated with preservation of oligodendrocytes as well as conservation of axon structure and axonal mitochondria, leading to improved functional recovery. Notably, CK2 inhibition promoted WM function when applied before or after ischemic injury by differentially regulating the CDK5 and AKT/GSK3β pathways. Specifically, blockade of the active conformation of AKT conferred post-ischemic protection to young, aging, and old WM, suggesting a common therapeutic target across age groups. CK2 inhibitors are currently being used in clinical trials for cancer patients; therefore, it is important to consider the potential benefits of CK2 inhibitors during an ischemic attack.

Multisite dependency of an E3 ligase controls monoubiquitylation-dependent cell fate decisions

Metazoan development depends on tightly regulated gene expression programs that instruct progenitor cells to adopt specialized fates. Recent work found that posttranslational modifications, such as monoubiquitylation, can determine cell fate also independently of effects on transcription, yet how monoubiquitylation is implemented during development is poorly understood. Here, we have identified a regulatory circuit that controls monoubiquitylation-dependent neural crest specification by the E3 ligase CUL3 and its substrate adaptor KBTBD8. We found that CUL3^KBTBD8 monoubiquitylates its essential targets only after these have been phosphorylated in multiple motifs by CK2, a kinase whose levels gradually increase during embryogenesis. Its dependency on multisite phosphorylation allows CUL3^KBTBD8 to convert the slow rise in embryonic CK2 into decisive recognition of ubiquitylation substrates, which in turn is essential for neural crest specification. We conclude that multisite dependency of an E3 ligase provides a powerful mechanism for switch-like cell fate transitions controlled by monoubiquitylation.

Genome-wide identification and characterization of the CKII gene family in the cultivated banana cultivar (Musa spp. cv Tianbaojiao) and the wild banana (Musa itinerans)

Plant casein kinase II (CKII) plays an essential role in regulating plant growth and development, and responses to biotic and abiotic stresses. Here, we report the identification and characterization of the CKII family genes in Musa spp. cv. ‘Tianbaojiao’ (AAA group) and the wild banana (Musa itinerans). The 13 cDNA sequences of the CKII family members were identified both in ‘Tianbaojiao’ and wild banana, respectively. The differences between CKII α and CKII β members are corroborated through the subcellular localizations, phosphorylation sites and gene structures. The cloning of CKII β-like-2 gDNA sequences in wild banana and ‘Tianbaojiao’ and the analysis of gene structures showed MiCKIIβ-like-2b and MaCKIIβ-like-2 are likely alternatively spliced transcripts, which were derived from the alternative splicing events that involved exon deletion. The qPCR validation showed differential expression CKII family members in response to cold stress and also in all tested tissues (leaf, pseudostem and root) of wild banana. In particular, the normal transcript MiCKIIβ-like-2a was highly expressed in response to cold stress in wild banana; oppositely, the alternatively spliced transcript MiCKIIβ-like-2b was quite lowly expressed. The complex origin and long-term evolution of Musa lineage might explain the alternative splicing events of CKII β-like-2.

PIP30/FAM192A is a novel regulator of the nuclear proteasome activator PA28γ

PA28γ is a nuclear activator of the 20S proteasome involved in the regulation of several essential cellular processes, such as cell proliferation, apoptosis, nuclear dynamics, and cellular stress response. Unlike the 19S regulator of the proteasome, which specifically recognizes ubiquitylated proteins, PA28γ promotes the degradation of several substrates by the proteasome in an ATP- and ubiquitin-independent manner. However, its exact mechanisms of action are unclear and likely involve additional partners that remain to be identified. Here we report the identification of a cofactor of PA28γ, PIP30/FAM192A. PIP30 binds directly and specifically via its C-terminal end and in an interaction stabilized by casein kinase 2 phosphorylation to both free and 20S proteasome-associated PA28γ. Its recruitment to proteasome-containing complexes depends on PA28γ and its expression increases the association of PA28γ with the 20S proteasome in cells. Further dissection of its possible roles shows that PIP30 alters PA28γ-dependent activation of peptide degradation by the 20S proteasome in vitro and negatively controls in cells the presence of PA28γ in Cajal bodies by inhibition of its association with the key Cajal body component coilin. Taken together, our data show that PIP30 deeply affects PA28γ interactions with cellular proteins, including the 20S proteasome, demonstrating that it is an important regulator of PA28γ in cells and thus a new player in the control of the multiple functions of the proteasome within the nucleus.

Mechanisms of Cellular Internalization of Quantum Dot® Conjugated Bone Formation Mimetic Peptide CK2.3

Osteoporosis is a debilitating skeletal disorder that is characterized by loss of bone density over time. It affects one in two women and one in four men, age 50 and older. New treatments that specifically drive bone formation are desperately needed. We developed a peptide, CK2.3, that acts downstream of the bone morphogenetic protein receptor type Ia and it induces osteogenesis in-vitro and in-vivo. However, its mechanism of action, especially its mode of uptake by cells remains unknown. To demonstrate CK2.3 internalization within a cell, we conjugated CK2.3 to Quantum Dot®s (Qdot®s), semiconductor nanoparticles. We purified CK2.3-Qdot®s by size exclusion chromatography and verified the conjugation and stability using UV/VIS and Fourier transform infrared spectroscopy. Our results show that CK2.3 was conjugated to the Qdot®s and the conjugate was stable for at least 4 days at 37 °C. Moreover, CK2.3-Qdot®s exerted biological response similar to CK2.3. Addition of CK2.3-Qdot®s to cells followed by confocal imaging revealed that CK2.3-Qdot®s were internalized at 6 h post stimulation. Furthermore, using pharmacological inhibitors against endocytic pathways, we demonstrated that CK2.3-Qdot®s were internalized by caveolae. These results show for the first time that the novel peptide CK2.3 is taken up by the cell through caveolae mediated endocytosis.

CK2 inhibition confers functional protection to young and aging axons against ischemia by differentially regulating the CDK5 and AKT signaling pathways

White matter (WM) is injured in most strokes, which contributes to functional deficits during recovery. Casein kinase 2 (CK2) is a protein kinase that is expressed in brain, including WM. To assess the impact of CK2 inhibition on axon recovery following oxygen glucose deprivation (OGD), mouse optic nerves (MONs), which are pure WM tracts, were subjected to OGD with or without the selective CK2 inhibitor CX-4945. CX-4945 application preserved axon function during OGD and promoted axon function recovery when applied before or after OGD. This protective effect of CK2 inhibition correlated with preservation of oligodendrocytes and conservation of axon structure and axonal mitochondria. To investigate the pertinent downstream signaling pathways, siRNA targeting the CK2α subunit identified CDK5 and AKT as downstream molecules. Consequently, MK-2206 and roscovitine, which are selective AKT and CDK5 inhibitors, respectively, protected young and aging WM function only when applied before OGD. However, a novel pan-AKT allosteric inhibitor, ARQ-092, which targets both the inactive and active conformations of AKT, conferred protection to young and aging axons when applied before or after OGD. These results suggest that AKT and CDK5 signaling contribute to the WM functional protection conferred by CK2 inhibition during ischemia, while inhibition of activated AKT signaling plays the primary role in post-ischemic protection conferred by CK2 inhibition in WM independent of age. CK2 inhibitors are currently being used in clinical trials for cancer patients; therefore, our results will provide rationale for repurposing these drugs as therapeutic options for stroke patients by adding novel targets.

Casein kinase 2 promotes interaction between Rad17 and the 9-1-1 complex through constitutive phosphorylation of the C-terminal tail of human Rad17

An interaction between the Rad17-RFC2-5 and 9-1-1 complexes is essential for ATR-Chk1 signaling, which is one of the major DNA damage checkpoints. Recently, we showed that the polyanionic C-terminal tail of human Rad17 and the embedded conserved sequence iVERGE are important for the interaction with 9-1-1 complex. Here, we show that Rad17-S667 in the C-terminal tail is constitutively phosphorylated in vivo in a casein kinase 2-dependent manner, and the phosphorylation is important for 9-1-1 interaction. The serine phosphorylation of Rad17 could be seen in the absence of exogenous genotoxic stress, and was mostly abolished by S667A substitution. Rad17-S667 was also phosphorylated when the C-terminal tail was fused with EGFP, but the phosphorylation was inhibited by two casein kinase 2 inhibitors. Furthermore, interaction between Rad17 and the 9-1-1 complex was inhibited by the casein kinase 2 inhibitor CX-4945/Silmitasertib, and the effect was dependent on the Rad17-S667 residue, indicating that S667 phosphorylation is the only role of casein kinase 2 in the 9-1-1 interaction. Our data raise the possibility that the C-terminal tail of vertebrate Rad17 regulates ATR-Chk1 signaling through multi-site phosphorylation in the iVERGE.

CK2 Controls Th17 and Regulatory T Cell Differentiation Through Inhibition of FoxO1

Growing evidence demonstrates that the highly conserved serine/threonine kinase CK2 promotes Th17 cell differentiation while suppressing the generation of Foxp3⁺ regulatory T cells (Tregs); however, the exact mechanism by which CK2 regulates the Th17/Treg axis remains unclear. CK2 can be composed of three distinct subunits: two catalytic subunits, CK2α and CK2α', and the regulatory subunit CK2β. We generated mice that lack the major catalytic subunit of CK2, CK2α, specifically in mature T cells using the distal Lck-Cre (CK2α^-/-). Importantly, CK2α deficiency resulted in a significant decrease in the overall kinase activity of CK2. Further, CK2α deficiency resulted in a significant defect in Th17 cell polarization and a reciprocal increase in Tregs both in vitro and in vivo in the context of autoimmune neuroinflammation. The transcription factor forkhead box protein O1 (FoxO1) directly inhibits Th17 cell differentiation and is essential for the generation of Tregs. CK2α^-/- CD4⁺ T cells exhibit less phosphorylated FoxO1 and a corresponding increase in the transcription of FoxO1-regulated genes. Treatment of CK2α^-/- CD4⁺ T cells with the FoxO1 inhibitor AS1842856 or short hairpin RNA knockdown of FoxO1 is sufficient to rescue Th17 cell polarization. Through use of a genetic approach to target CK2 kinase activity, the current study provides evidence of a major mechanism by which CK2 regulates the Th17/Treg axis through the inhibition of FoxO1.

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

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

The protein kinase CK2 catalytic domain from Plasmodium falciparum: crystal structure, tyrosine kinase activity and inhibition

Malaria causes every year over half-a-million deaths. The emergence of parasites resistant to available treatments makes the identification of new targets and their inhibitors an urgent task for the development of novel anti-malaria drugs. Protein kinase CK2 is an evolutionary-conserved eukaryotic serine/threonine protein kinase that in Plasmodium falciparum (PfCK2) has been characterized as a promising target for chemotherapeutic intervention against malaria. Here we report a crystallographic structure of the catalytic domain of PfCK2α (D179S inactive single mutant) in complex with ATP at a resolution of 3.0 Å. Compared to the human enzyme, the structure reveals a subtly altered ATP binding pocket comprising five substitutions in the vicinity of the adenine base, that together with potential allosteric sites, could be exploited to design novel inhibitors specifically targeting the Plasmodium enzyme. We provide evidence for the dual autophosphorylation of residues Thr⁶³ and Tyr³⁰ of PfCK2. We also show that CX4945, a human CK2 inhibitor in clinical trials against solid tumor cancers, is effective against PfCK2 with an IC₅₀ of 13.2 nM.

CIGB-300: A peptide-based drug that impairs the Protein Kinase CK2-mediated phosphorylation

Protein kinase CK2, formerly referred to as casein kinase II, is a serine/threonine kinase often found overexpressed in solid tumors and hematologic malignancies that phosphorylates many substrates integral to the hallmarks of cancer. CK2 has emerged as a viable oncology target having been experimentally validated with different kinase inhibitors, including small molecule ATP-competitors, synthetic peptides, and antisense oligonucleotides. To date only two CK2 inhibitors, CIGB-300 and CX-4945, have entered the clinic in phase 1-2 trials. This review provides information on CIGB-300, a cell-permeable cyclic peptide that inhibits CK2-mediated phosphorylation by targeting the substrate phosphoacceptor domain. We review data that support the concept of CK2 as an anticancer target, address the mechanism of action, and summarize preclinical studies showing antiangiogenic and antimetastatic effects as well as synergism with anticancer drugs in preclinical models. We also summarize early clinical research (phase 1/2 trials) of CIGB-300 in cervical cancer, including data in combination with chemoradiotherapy. The clinical data demonstrate the safety, tolerability, and clinical effects of intratumoral injections of CIGB-300 and provide the foundation for future phase 3 clinical trials in locally advanced cervical cancer in combination with standard chemoradiotherapy.

Crystal structures of human CK2α2 in new crystal forms arising from a subtle difference in salt concentration

The catalytic subunits of protein kinase CK2 are classified into two subtypes: CK2α1 and CK2α2. CK2α1 is an attractive drug-discovery target for various diseases such as cancers and nephritis. CK2α2 is defined as an off-target of CK2α1 and is a potential target in the development of male contraceptive drugs. High-resolution crystal structures of both isozymes are likely to provide crucial clues for the design of selective inhibitors of CK2α1 and/or CK2α2. To date, several crystal structures of CK2α1 have been solved at high resolutions of beyond 1.5 Å. However, crystal structures of CK2α2 have barely achieved a low resolution of around 3 Å because of the formation of needle-shaped crystals. In this study, new crystal forms were exploited and one provided a crystal structure of CK2α2 at 1.89 Å resolution. This result, together with the structure of CK2α1, will assist in the development of highly selective inhibitors for both isozymes.

Casein kinase 2 inhibition impairs spontaneous and oxytocin-induced contractions in late pregnant mouse uterus

NEW FINDINGS

What is the central question of this study? Does the inhibition of the protein kinase casein kinase 2 (CK2) alter the uterine contractility? What is the main finding and its importance? Inhibition of CK2 impaired the spontaneous and oxytocin-induced contractility in late pregnant mouse uterus. This finding suggests that CK2 is a novel pathway mediating oxytocin-induced contractility in the uterus and thus opens up the possibility for this class of drugs to be developed as a new class of tocolytics.

ABSTRACT

The protein kinase casein kinase 2 (CK2) is a ubiquitously expressed serine or threonine kinase known to phosphorylate a number of substrates. The aim of this study was to assess the effect of CK2 inhibition on spontaneous and oxytocin-induced uterine contractions in 19 day pregnant mice. The CK2 inhibitor CX-4945 elicited a concentration-dependent relaxation in late pregnant mouse uterus. CX-4945 and another selective CK2 inhibitor, apigenin, also inhibited the oxytocin-induced contractile response in late pregnant uterine tissue. Apigenin also blunted the prostaglandin F_2α response, but CX-4945 did not. Casein kinase 2 was located in the lipid raft fractions of the cell membrane, and disruption of lipid rafts was found to reverse its effect. The results of the present study suggest that CK2, located in lipid rafts of the cell membrane, is an active regulator of spontaneous and oxytocin-induced uterine contractions in the late pregnant mouse.

A CK2-RNF4 interplay coordinates non-canonical SUMOylation and degradation of nuclear receptor FXR

Farnesoid X receptor (FXR) is a ligand-activated nuclear receptor that plays a central role in regulating genes involved in bile acid homeostasis, and fat and glucose metabolism. Here, we demonstrate a post-translational interplay between FXR phosphorylation, SUMOylation, and ubiquitination that directs the receptor into an activation-degradation pathway in hepatocytes. We identify a non-canonical SUMOylation motif termed pSuM that conjugates SUMO2 at Lys-325 of FXR under the direct control of casein kinase 2 (CK2), which provides the required negative charge for Ubc9 and PIAS1 to perform SUMOylation, by phosphorylating Ser-327. Lys-325 SUMOylation is indispensable to the promotion of efficient ligand activation and transcriptional coactivation of FXR. Constitutive pSuM activation using a phospho-mimic Ser-327 mutant or catalytic CK2 expression strongly induces SUMO2 conjugation, which directs FXR ubiquitination and proteasome-dependent degradation. We also determine that such SUMOylation-dependent ubiquitination of FXR is mediated by the E3 ubiquitin ligase RNF4, which is required to achieve maximal induction of FXR and optimal up- or downregulation of responsive genes involved in bile acid homeostasis and liver regeneration. Our findings identify a highly regulated atypical SUMO conjugation motif that serves to coordinate FXR transcriptional competence, thereby expanding the intricate dynamics of the SUMOylation process used by incoming signals to govern metabolic gene regulation.

Second-generation CK2α inhibitors targeting the αD pocket

CK2 is a critical cell cycle regulator that also promotes various anti-apoptotic mechanisms. Development of ATP-non-competitive inhibitors of CK2 is a very attractive strategy considering that the ATP binding site is highly conserved among other kinases. We have previously utilised a pocket outside the active site to develop a novel CK2 inhibitor, CAM4066. Whilst CAM4066 bound to this new pocket it was also interacting with the ATP site: herein, we describe an example of a CK2α inhibitor that binds completely outside the active site. This second generation αD-site binding inhibitor, compound CAM4712 (IC50 = 7 μM, GI50 = 10.0 ± 3.6 μM), has numerous advantages over the previously reported CAM4066, including a reduction in the number of rotatable bonds, the absence of amide groups susceptible to the action of proteases and improved cellular permeability. Unlike with CAM4066, there was no need to facilitate cellular uptake by making a prodrug. Moreover, CAM4712 displayed no drop off between its ability to inhibit the kinase in vitro (IC50) and the ability to inhibit cell proliferation (GI50).

Phosphorylation of an intrinsically disordered region of Ets1 shifts a multi-modal interaction ensemble to an auto-inhibitory state

Multi-modal interactions are frequently observed in intrinsically disordered regions (IDRs) of proteins upon binding to their partners. In many cases, post-translational modifications in IDRs are accompanied by coupled folding and binding. From both molecular simulations and biochemical experiments with mutational studies, we show that the IDR including a Ser rich region (SRR) of the transcription factor Ets1, just before the DNA-binding core domain, undergoes multi-modal interactions when the SRR is not phosphorylated. In the phosphorylated state, the SRR forms a few specific complex structures with the Ets1 core, covering the recognition helix in the core and drastically reducing the DNA binding affinities as the auto-inhibitory state. The binding kinetics of mutated Ets1 indicates that aromatic residues in the SRR can be substituted with other hydrophobic residues for the interactions with the Ets1 core.

The importance of negative determinants as modulators of CK2 targeting. The lesson of Akt2 S131

CK2 is a pleiotropic S/T protein kinase (formerly known as casein kinase 2) which is attracting increasing interest as therapeutic target, and the identification of its substrates is a crucial step in determining its involvement in different pathological conditions. We recently found that S131 of Akt2 (homologous to the well established CK2 target S129 of Akt1) is not phosphorylated by CK2 either in vitro or in vivo, although the consensus sequence recognized by CK2 (S/T-x-x-E/D/pS/pT) is conserved in it. Here, by exploiting synthetic peptides, in cell transfection experiments, and computational analysis, we show that a single sequence element, a T at position n+1, hampers phosphorylation, causing an α-helix structure organization which prevents the recognition of its own consensus by CK2. Our results highlight the role of negative determinants as crucial modulators of CK2 targeting and corroborate the concept that Akt1 and Akt2 display isoform specific features. Experiments with synthetic peptides suggest that Akt2 S131 could be phosphorylated by kinases of the Plk (Polo-like kinase) family, which are insensitive to the presence of the n+1 T. The low phylogenetic conservation of the Akt2 sequence around S131, as opposed to the extremely well-conserved Akt1 homologous sequence, would indicate a dominant positive role in the selective pressure only for the Akt1 phosphoacceptor site committed to undergo phosphorylation by CK2. By contrast, Akt2 S131 may mediate the response to specific physio/pathological conditions, being consequently shielded against basal CK2 targeting.

Ecto-protein kinase CK2, the neglected form of CK2

Ecto-protein kinases, including protein kinase CK2 (former name, casein kinase 2), have been the focus of research for more than 30 years. At the beginning of the ecto-kinase research their identification was performed with substrates and inhibitors whose specificity under the current knowledge was rather limited. Since all currently known ecto-kinases, including ecto-CK2, have intracellular counterparts, one has to exclude that an ecto-localization originates from intracellular counterparts after cell damage. Protein kinase CK2 is involved in cellular key processes such as cell cycle progression, inhibition of apoptosis, DNA damage repair, differentiation and many other processes. CK2 is composed of two catalytic CK2α or CK2α' subunits and two non-catalytic CK2β subunits. Progress in the ecto-kinase and in particular ecto-CK2 studies was made with the use of transfected tagged CK2 subunits, which allowed to follow their individual transport and localization on the cell surface after transfection. Furthermore, immunofluorescence studies with antibodies against CK2 subunits as well as affinity chromatography with a binding partner of CK2 subunits have improved ecto-kinase research. The use of new and more specific inhibitors as well as of substrates, which do not cross the plasma membrane, have further improved the specificity for ecto-CK2. From the various substrates of ecto-CK2, it can be concluded that ecto-CK2 plays a role in Alzheimer disease, cell adhesion, platelet aggregation, immune response and cellular signalling. New tools and techniques, to study ecto-CK2 activity, are required to identify new substrates and thereby new functional implications for ecto-CK2.

A π-Halogen Bond of Dibenzofuranones with the Gatekeeper Phe113 in Human Protein Kinase CK2 Leads to Potent Tight Binding Inhibitors

Human protein kinase CK2 is an emerging target for neoplastic diseases. Potent lead structures for human CK2 inhibitors are derived from dibenzofuranones. Two new derivatives, 7,9-dichloro-1,2-dihydro-8-hydroxy-4-[(4-methoxyphenylamino)-methylene]dibenzo[b,d]furan-3(2H)-one (4a) and (E)-1,3-dichloro-6-[(4-methoxyphenylimino)-methyl]dibenzo[b,d]furan-2,7-diol (5) were tested for inhibition of CK2 and induction of apoptosis in LNCaP cells. Both turned out to be tight binding inhibitors, with IC₅₀ values of 7 nM (4a) and 5 nM (5) and an apparent K_i value of 0.4 nM for both. Compounds 4a and 5 reduced cellular CK2 activity, indicating cell permeability. Cell viability was substantially impaired in LNCaP cells, as well as apoptosis was induced, which was not appearing in non-neoplastic ARPE-19 cells. Co-crystallization of 4a and 5 revealed an unexpected π-halogen bond of the chloro substituent at C9 with the gatekeeper amino acid Phe113, leading to an inverted binding mode in comparison to parent compound 4b, with the Cl at C6 instead, which was co-crystallized as a control. This indicates that the position of the chloro substituent on ring A of the dibenzofuran scaffold is responsible for an inversion of the binding mode that enhances potency.

In mammalian skeletal muscle, phosphorylation of TOMM22 by protein kinase CSNK2/CK2 controls mitophagy

In yeast, Tom22, the central component of the TOMM (translocase of outer mitochondrial membrane) receptor complex, is responsible for the recognition and translocation of synthesized mitochondrial precursor proteins, and its protein kinase CK2-dependent phosphorylation is mandatory for TOMM complex biogenesis and proper mitochondrial protein import. In mammals, the biological function of protein kinase CSNK2/CK2 remains vastly elusive and it is unknown whether CSNK2-dependent phosphorylation of TOMM protein subunits has a similar role as that in yeast. To address this issue, we used a skeletal muscle-specific Csnk2b/Ck2β-conditional knockout (cKO) mouse model. Phenotypically, these skeletal muscle Csnk2b cKO mice showed reduced muscle strength and abnormal metabolic activity of mainly oxidative muscle fibers, which point towards mitochondrial dysfunction. Enzymatically, active muscle lysates from skeletal muscle Csnk2b cKO mice phosphorylate murine TOMM22, the mammalian ortholog of yeast Tom22, to a lower extent than lysates prepared from controls. Mechanistically, CSNK2-mediated phosphorylation of TOMM22 changes its binding affinity for mitochondrial precursor proteins. However, in contrast to yeast, mitochondrial protein import seems not to be affected in vitro using mitochondria isolated from muscles of skeletal muscle Csnk2b cKO mice. PINK1, a mitochondrial health sensor that undergoes constitutive import under physiological conditions, accumulates within skeletal muscle Csnk2b cKO fibers and labels abnormal mitochondria for removal by mitophagy as demonstrated by the appearance of mitochondria-containing autophagosomes through electron microscopy. Mitophagy can be normalized by either introduction of a phosphomimetic TOMM22 mutant in cultured myotubes, or by in vivo electroporation of phosphomimetic Tomm22 into muscles of mice. Importantly, transfection of the phosphomimetic Tomm22 mutant in muscle cells with ablated Csnk2b restored their oxygen consumption rate comparable to wild-type levels. In sum, our data show that mammalian CSNK2-dependent phosphorylation of TOMM22 is a critical switch for mitophagy and reveal CSNK2-dependent physiological implications on metabolism, muscle integrity and behavior.

Extending the phenotype associated with the CSNK2A1-related Okur-Chung syndrome-A clinical study of 11 individuals

Variants in the Protein Kinase CK2 alpha subunit, encoding the CSNK2A1 gene, have previously been reported in children with an intellectual disability and dysmorphic facial features syndrome: now termed the Okur-Chung neurodevelopmental syndrome. More recently, through trio-based exome sequencing undertaken by the Deciphering Developmental Disorders Study (DDD study), a further 11 children with de novo CSNK2A1 variants have been identified. We have undertaken detailed phenotyping of these patients. Consistent with previously reported patients, patients in this series had apparent intellectual disability, swallowing difficulties, and hypotonia. While there are some shared facial characteristics, the gestalt is neither consistent nor readily recognized. Congenital heart abnormalities were identified in nearly 30% of the patients, representing a newly recognized CSNK2A1 clinical association. Based upon the clinical findings from this study and the previously reported patients, we suggest an initial approach to the management of patients with this recently described intellectual disability syndrome.

PGRMC1 regulation by phosphorylation: potential new insights in controlling biological activity

Progesterone receptor membrane component 1 (PGRMC1) is a multifunctional protein implicated in multiple pathologies, including cancer and Alzheimer's disease. The recently published structure of PGRMC1 revealed heme-mediated dimerization that directed the PGRMC1-dependent cytochrome P450-mediated detoxification of doxorubicin. We describe here how the PGRMC1 structure also enables important new insights into the possible regulation of PGRMC1 function by phosphorylation. Predicted regulatory interaction sites for SH2- and SH3-domain proteins are in non-structured regions that could be available to cytoplasmic enzymes. Further to the published interpretation, we suggest that phosphorylation of PGRMC1 at position Y113 may promote the attested membrane trafficking function of PGRMC1. To stimulate further experimentation, we also discuss that heme-mediated dimerization of PGRMC1 and membrane trafficking may be mutually exclusive functions. These roles could potentially be reciprocally regulated by phosphorylation/dephosphorylation at Y113. It follows that the phosphorylation status of PGRMC1 should be further explored in order to better understand many of its proposed biological functions.

5d, a novel analogue of 3-n-butylphthalide, decreases NADPH oxidase activity through the positive regulation of CK2 after ischemia/reperfusion injury

5d, a novel analogue of the racemic 3-n-butylphthalide (NBP), has been reported for its free radical scavenging activity in vitro and preventive neuroprotection in vivo. Nevertheless, the mechanism by which 5d attenuated ischemia/reperfusion (I/R) injury is still unknown. Our results showed that 5d significantly increased CK2 activity as well as CK2α and 2α' protein levels after I/R injury. Besides, 5d suppressed the translocation of cytosolic p47phox and Rac1 to the membrane, decreased NOX4 expression and ROS generation. Furthermore, 5d blocked the dissociation between CK2α and Rac1 so as to decrease NADPH oxidase activity. Based on these findings, we propose that the neuroprotective effect of 5d is due to an increase of CK2 activity, which blocks I/R-induced dissociation between CK2α and Rac1, decreases NADPH oxidase activity, inhibits ROS production and finally realizes the neuroprotection of I/R. These findings point to that 5d might be considered an attractive candidate for further studies in ischemic stroke.

The protein kinase CK2 substrate Jabba modulates lipid metabolism during Drosophila oogenesis

Lipid metabolism plays a critical role in female reproduction. During oogenesis, maturing oocytes accumulate high levels of neutral lipids that are essential for both energy production and the synthesis of other lipid molecules. Metabolic pathways within the ovary are partially regulated by protein kinases that link metabolic status to oocyte development. Although the functions of several kinases in this process are well established, the roles that many other kinases play in coordinating metabolic state with female germ cell development are unknown. Here, we demonstrate that the catalytic activity of casein kinase 2 (CK2) is essential for Drosophila oogenesis. Using an unbiased biochemical screen that leveraged an unusual catalytic property of the kinase, we identified a novel CK2 substrate in the Drosophila ovary, the lipid droplet-associated protein Jabba. We show that Jabba is essential for modulating ovarian lipid metabolism and for regulating female fertility in the fly. Our findings shed light on a CK2-dependent signaling pathway governing lipid metabolism in the ovary and provide insight into the long-recognized but poorly understood association between energy metabolism and female reproduction.

Circadian modification network of a core clock driver BMAL1 to harmonize physiology from brain to peripheral tissues

Circadian clocks dictate various physiological functions by brain SCN (a central clock) -orchestrating the temporal harmony of peripheral clocks of tissues/organs in the whole body, with adaptability to environments by resetting their timings. Dysfunction of this circadian adaptation system (CAS) occasionally causes/exacerbates diseases. CAS is based on cell-autonomous molecular clocks, which oscillate via a core transcriptional/translational feedback loop with clock genes/proteins, e.g., BMAL1: CLOCK circadian transcription driver and CRY1/2 and PER1/2 suppressors, and is modulated by various regulatory loops including clock protein modifications. Among mutants with a single clock gene, BMAL1-deficient mice exhibit the most drastic loss of circadian functions. Here, we highlight on numerous circadian protein modifications of mammalian BMAL1, e.g., multiple phosphorylations, SUMOylation, ubiquitination, acetylation, O-GlcNAcylation and S-nitrosylation, which mutually interplay to control molecular clocks and coordinate physiological functions from the brain to peripheral tissues through the input and output of the clocks.

Co-Localization of Insulin-Like Growth Factor Binding Protein-1, Casein Kinase-2β, and Mechanistic Target of Rapamycin in Human Hepatocellular Carcinoma Cells as Demonstrated by Dual Immunofluorescence and in Situ Proximity Ligation Assay

Insulin-like growth factor binding protein (IGFBP)-1 influences fetal growth by modifying insulin-like growth factor-I (IGF-I) bioavailability. IGFBP-1 phosphorylation, which markedly increases its affinity for IGF-I, is regulated by mechanistic target of rapamycin (mTOR) and casein kinase (CSNK)-2. However, the underlying molecular mechanisms remain unknown. We examined the cellular localization and potential interactions of IGFBP-1, CSNK-2β, and mTOR as a prerequisite for protein-protein interaction. Analysis of dual immunofluorescence images indicated a potential perinuclear co-localization between IGFBP-1 and CSNK-2β and a nuclear co-localization between CSNK-2β and mTOR. Proximity ligation assay (PLA) indicated proximity between IGFBP-1 and CSNK-2β as well as mTOR and CSNK-2β but not between mTOR and IGFBP-1. Three-dimensional rendering of the PLA images validated that IGFBP-1 and CSNK-2β interactions were in the perinuclear region and mTOR and CSNK-2β interactions were also predominantly perinuclear rather than nuclear as indicated by mTOR and CSNK-2β co-localization. Compared with control, hypoxia and rapamycin treatment showed markedly amplified PLA signals for IGFBP-1 and CSNK-2β (approximately 18-fold, P = 0.0002). Stable isotope labeling with multiple reaction monitoring-mass spectrometry demonstrated that hypoxia and rapamycin treatment increased IGFBP-1 phosphorylation at Ser98/Ser101/Ser119/Ser174 but most considerably (106-fold) at Ser169. We report interactions between CSNK-2β and IGFBP-1 as well as mTOR and CSNK-2β, providing strong evidence of a mechanistic link between mTOR and IGF-I signaling, two critical regulators of cell growth via CSNK-2.

Characterization of the APLF FHA-XRCC1 phosphopeptide interaction and its structural and functional implications

Aprataxin and PNKP-like factor (APLF) is a DNA repair factor containing a forkhead-associated (FHA) domain that supports binding to the phosphorylated FHA domain binding motifs (FBMs) in XRCC1 and XRCC4. We have characterized the interaction of the APLF FHA domain with phosphorylated XRCC1 peptides using crystallographic, NMR, and fluorescence polarization studies. The FHA–FBM interactions exhibit significant pH dependence in the physiological range as a consequence of the atypically high pK values of the phosphoserine and phosphothreonine residues and the preference for a dianionic charge state of FHA-bound pThr. These high pK values are characteristic of the polyanionic peptides typically produced by CK2 phosphorylation. Binding affinity is greatly enhanced by residues flanking the crystallographically-defined recognition motif, apparently as a consequence of non-specific electrostatic interactions, supporting the role of XRCC1 in nuclear cotransport of APLF. The FHA domain-dependent interaction of XRCC1 with APLF joins repair scaffolds that support single-strand break repair and non-homologous end joining (NHEJ). It is suggested that for double-strand DNA breaks that have initially formed a complex with PARP1 and its binding partner XRCC1, this interaction acts as a backup attempt to intercept the more error-prone alternative NHEJ repair pathway by recruiting Ku and associated NHEJ factors.

Conformational dynamics of human protein kinase CK2α and its effect on function and inhibition

Protein kinase, casein kinase II (CK2), is ubiquitously expressed and highly conserved protein kinase that shows constitutive activity. It phosphorylates a diverse set of proteins and plays crucial role in several cellular processes. The catalytic subunit of this enzyme (CK2α) shows remarkable flexibility as evidenced in numerous crystal structures determined till now. Here, using analysis of multiple crystal structures and long timescale molecular dynamics simulations, we explore the conformational flexibility of CK2α. The enzyme shows considerably higher flexibility in the solution as compared to that observed in crystal structure ensemble. Multiple conformations of hinge region, located near the active site, were observed during the dynamics. We further observed that among these multiple conformations, the most populated conformational state was inadequately represented in the crystal structure ensemble. The catalytic spine, was found to be less dismantled in this state as compared to the "open" hinge/αD state crystal structures. The comparison of dynamics in unbound (Apo) state and inhibitor (CX4945) bound state exhibits inhibitor induced suppression in the overall dynamics of the enzyme. This is especially true for functionally important glycine-rich loop above the active site. Together, this work gives novel insights into the dynamics of CK2α in solution and relates it to the function. This work also explains the effect of inhibitor on the dynamics of CK2α and paves way for development of better inhibitors.

Prion protein inhibits fast axonal transport through a mechanism involving casein kinase 2

Prion diseases include a number of progressive neuropathies involving conformational changes in cellular prion protein (PrP^c) that may be fatal sporadic, familial or infectious. Pathological evidence indicated that neurons affected in prion diseases follow a dying-back pattern of degeneration. However, specific cellular processes affected by PrP^c that explain such a pattern have not yet been identified. Results from cell biological and pharmacological experiments in isolated squid axoplasm and primary cultured neurons reveal inhibition of fast axonal transport (FAT) as a novel toxic effect elicited by PrP^c. Pharmacological, biochemical and cell biological experiments further indicate this toxic effect involves casein kinase 2 (CK2) activation, providing a molecular basis for the toxic effect of PrP^c on FAT. CK2 was found to phosphorylate and inhibit light chain subunits of the major motor protein conventional kinesin. Collectively, these findings suggest CK2 as a novel therapeutic target to prevent the gradual loss of neuronal connectivity that characterizes prion diseases.

XRCC1-mediated repair of strand breaks independent of PNKP binding

Repair of DNA-protein crosslinks and oxidatively damaged DNA base lesions generates intermediates with nicks or gaps with abnormal and blocked 3'-phosphate and 5'-OH ends that prevent the activity of DNA polymerases and ligases. End cleaning in mammalian cells by Tdp1 and PNKP produces the conventional 3'-OH and 5'-phosphate DNA ends suitable for completion of repair. This repair function of PNKP is facilitated by its binding to the scaffold protein XRCC1, and phosphorylation of XRCC1 by CK2 at several consensus sites enables PNKP binding and recruitment to DNA damage. To evaluate this documented repair process, a phosphorylation mutant of XRCC1, designed to eliminate PNKP binding, was stably expressed in Xrcc1-/- mouse fibroblast cells. Analysis of PNKP-GFP accumulation at micro-irradiation induced damage confirmed that the XRCC1 phosphorylation mutant failed to support efficient PNKP recruitment, whereas there was rapid recruitment in cells expressing wild-type XRCC1. Recruitment of additional fluorescently-tagged repair factors PARP-1-YFP, GFF-XRCC1, PNKP-GFP and Tdp1-GFP to micro-irradiation induced damage was assessed in wild-type XRCC1-expressing cells. PARP-1-YFP recruitment was best fit to two exponentials, whereas kinetics for the other proteins were fit to a single exponential. The similar half-times of recruitment suggest that XRCC1 may be recruited with other proteins possibly as a pre-formed complex. Xrcc1-/- cells are hypersensitive to the DNA-protein cross-link inducing agent camptothecin (CPT) and the DNA oxidative agent H2O2 due in part to compromised PNKP-mediated repair. However, cells expressing the PNKP interaction mutant of XRCC1 demonstrated marked reversal of CPT hypersensitivity. This reversal represents XRCC1-dependent repair in the absence of the phosphorylation-dependent PNKP recruitment and suggests either an XRCC1-independent mechanism of PNKP recruitment or a functional back-up pathway for cleaning of blocked DNA ends.

A novel allosteric site in casein kinase 2α discovered using combining bioinformatics and biochemistry methods

Casein kinase 2 (CK2) is a highly pleiotropic serine-threonine kinase, which catalyzed phosphorylation of more than 300 proteins that are implicated in regulation of many cellular functions, such as signal transduction, transcriptional control, apoptosis and the cell cycle. On the other hand, CK2 is abnormally elevated in a variety of tumors, and is considered as a promising therapeutic target. The currently available ATP-competitive CK2 inhibitors, however, lack selectivity, which has impeded their development in cancer therapy. Because allosteric inhibitors can avoid the shortcomings of conventional kinase inhibitors, this study was aimed to discover a new allosteric site in CK2α and to investigate the effects of mutations in this site on the activity of CK2α. Using Allosite based on protein dynamics and structural alignment, we predicted a new allosteric site that was partly located in the αC helix of CK2α. Five residues exposed on the surface of this site were mutated to validate the prediction. Kinetic analyses were performed using a luminescent ADP detection assay by varying the concentrations of a peptide substrate, and the results showed that the mutations I78C and I78W decreased CK2α activity, whereas V31R, K75E, I82C and P109C increased CK2α activity. Potential allosteric pathways were identified using the Monte Carlo path generation approach, and the results of these predicted allosteric pathways were consistent with the mutation analysis. Multiple sequence alignments of CK2α with the other kinases in the family were conducted using the ClustalX method, which revealed the diversity of the residues in the site. In conclusion, we identified a new allosteric site in CK2α that can be altered to modulate the activity of the kinase. Because of the high diversity of the residues in the site, the site can be targeted using rational drug design of specific CK2α inhibitors for biological relevance.

Association of protein kinase CK2 inhibition with cellular radiosensitivity of non-small cell lung cancer

Protein kinase CK2 is a highly conserved protein Ser/Thr protein kinase and plays important roles in cell proliferation, protein translation and cell survival. This study investigated the possibility of using CK2 inhibition as a new approach for increasing the efficacy of radiotherapy in non-small cell lung cancer (NSCLC) and its underlying mechanisms. Kinase inhibition of CK2 was attempted either by using the specific CK2 inhibitor, Quinalizarin or by applying siRNA interference technology to silence the expression of the catalytic subunit of CK2 in A549 and H460 cells. The results showed that CK2α knockdown or Quinalizarin significantly enhanced the radiosensitivity of various NSCLC cells. The notable findings we observed after exposure to both CK2 inhibition and ionizing radiation (IR) were a prolonged delay in radiation-induced DNA double-strand breaks (DSB) repair, robust G2/M checkpoint arrest and increased apoptosis. In vivo studies further demonstrated that compared with each treatment alone, CK2 inhibition combined with IR reduced tumor growth in the H460 cell xenograft model. In conclusion, CK2 is a promising target for the enhancement of radiosensitivity in NSCLC.

A Sequentially Priming Phosphorylation Cascade Activates the Gliomagenic Transcription Factor Olig2

During development of the vertebrate CNS, the basic helix-loop-helix (bHLH) transcription factor Olig2 sustains replication competence of progenitor cells that give rise to neurons and oligodendrocytes. A pathological counterpart of this developmental function is seen in human glioma, wherein Olig2 is required for maintenance of stem-like cells that drive tumor growth. The mitogenic/gliomagenic functions of Olig2 are regulated by phosphorylation of a triple serine motif (S10, S13, and S14) in the amino terminus. Here, we identify a set of three serine/threonine protein kinases (glycogen synthase kinase 3α/β [GSK3α/β], casein kinase 2 [CK2], and cyclin-dependent kinases 1/2 [CDK1/2]) that are, collectively, both necessary and sufficient to phosphorylate the triple serine motif. We show that phosphorylation of the motif itself serves as a template to prime phosphorylation of additional serines and creates a highly charged "acid blob" in the amino terminus of Olig2. Finally, we show that small molecule inhibitors of this forward-feeding phosphorylation cascade have potential as glioma therapeutics.

SIRT6 Is Essential for Adipocyte Differentiation by Regulating Mitotic Clonal Expansion

Preadipocytes initiate differentiation into adipocytes through a cascade of events. Mitotic clonal expansion, as one of the earliest events, is essential for adipogenesis. However, the underlying mechanisms that regulate mitotic clonal expansion remain elusive. SIRT6 is a member of the evolutionarily conserved sirtuin family of nicotinamide adenine dinucleotide (NAD)+-dependent protein deacetylases. Here, we show that SIRT6 deficiency in preadipocytes blocks their adipogenesis. Analysis of gene expression during adipogenesis reveals that KIF5C, which belongs to the kinesin family, is negatively regulated by SIRT6. Furthermore, we show that KIF5C is a negative factor for adipogenesis through interacting with CK2α', a catalytic subunit of CK2. This interaction blocks CK2α' nuclear translocation and CK2 kinase activity and inhibits mitotic clonal expansion during adipogenesis. These findings reveal a crucial role of SIRT6 in adipogenesis and provide potential therapeutic targets for obesity.

Caspase signaling, a conserved inductive cue for metazoan cell differentiation

Caspase signaling pathways were originally discovered as conveyors of programmed cell death, yet a compendium of research over the past two decades have demonstrated that these same conduits have a plethora of physiologic functions. Arguably the most extensive non-death activity that has been attributed to this protease clade is the capacity to induce cell differentiation. Caspase control of differentiation is conserved across diverse metazoan organisms from flies to humans, suggesting an ancient origin for this form of cell fate control. Here we discuss the mechanisms by which caspase enzymes manage differentiation, the targeted substrates that may be common across cell lineages, and the countervailing signals that may be essential for these proteases to 'execute' this non-death cell fate.

Ubiquitous Nature of Fluoroquinolones: The Oscillation between Antibacterial and Anticancer Activities

Fluoroquinolones are synthetic antibacterial agents that stabilize the ternary complex of prokaryotic topoisomerase II enzymes (gyrase and Topo IV), leading to extensive DNA fragmentation and bacteria death. Despite the similar structural folds within the critical regions of prokaryotic and eukaryotic topoisomerases, clinically relevant fluoroquinolones display a remarkable selectivity for prokaryotic topoisomerase II, with excellent safety records in humans. Typical agents that target human topoisomerases (such as etoposide, doxorubicin and mitoxantrone) are associated with significant toxicities and secondary malignancies, whereas clinically relevant fluoroquinolones are not known to exhibit such propensities. Although many fluoroquinolones have been shown to display topoisomerase-independent antiproliferative effects against various human cancer cells, those that are significantly active against eukaryotic topoisomerase show the same DNA damaging properties as other topoisomerase poisons. Empirical models also show that fluoroquinolones mediate some unique immunomodulatory activities of suppressing pro-inflammatory cytokines and super-inducing interleukin-2. This article reviews the extended roles of fluoroquinolones and their prospects as lead for the unmet needs of "small and safe" multimodal-targeting drug scaffolds.

Role of protein kinases CK1α and CK2 in multiple myeloma: regulation of pivotal survival and stress-managing pathways

Multiple myeloma (MM) is a malignant tumor of transformed plasma cells. MM pathogenesis is a multistep process. This cancer can occur de novo (rarely) or it can develop from monoclonal gammopathy of undetermined significance (most of the cases). MM can be asymptomatic (smoldering myeloma) or clinically active. Malignant plasma cells exploit intrinsic and extrinsic bone marrow microenvironment-derived growth signals. Upregulation of stress-coping pathways is also instrumental to maintain MM cell growth. The phylogenetically related Ser/Thr kinases CSNK1A1 (CK1α) and CSNK2 (CK2) have recently gained a growing importance in hematologic malignancies arising both from precursors and from mature blood cells. In multiple myeloma, CK1α or CK2 sustain oncogenic cascades, such as the PI3K/AKT, JAK/STAT, and NF-κB, as well as propel stress-related signaling that help in coping with different noxae. Data also suggest that these kinases modulate the delivery of growth factors and cytokines from the bone marrow stroma. The “non-oncogene addiction” phenotype generated by the increased activity of CK1α and CK2 in multiple myeloma contributes to malignant plasma cell proliferation and survival and represents an Achilles’ heel for the activity of small ATP competitive CK1α or CK2 inhibitors.

Therapeutic targeting of CK2 in acute and chronic leukemias

CK2 is a ubiquitously expressed, constitutively active Ser/Thr protein kinase, which is considered the most pleiotropic protein kinase in the human kinome. Such a pleiotropy explains the involvement of CK2 in many cellular events. However, its predominant roles are stimulation of cell growth and prevention of apoptosis. High levels of CK2 messenger RNA and protein are associated with CK2 pathological functions in human cancers. Over the last decade, basic and translational studies have provided evidence of CK2 as a pivotal molecule driving the growth of different blood malignancies. CK2 overexpression has been demonstrated in nearly all the types of hematological cancers, including acute and chronic leukemias, where CK2 is a key regulator of signaling networks critical for cell proliferation, survival and drug resistance. The findings that emerged from these studies suggest that CK2 could be a valuable therapeutic target in leukemias and supported the initiation of clinical trials using CK2 antagonists. In this review, we summarize the recent advances on the understanding of the signaling pathways involved in CK2 inhibition-mediated effects with a particular emphasis on the combinatorial use of CK2 inhibitors as novel therapeutic strategies for treating both acute and chronic leukemia patients.

Regulating protein breakdown through proteasome phosphorylation

The ubiquitin proteasome system degrades the great majority of proteins in mammalian cells. Countless studies have described how ubiquitination promotes the selective degradation of different cell proteins. However, there is a small but the growing literature that protein half-lives can also be regulated by post-translational modifications of the 26S proteasome. The present study reviews the ability of several kinases to alter proteasome function through subunit phosphorylation. For example, PKA (protein kinase A) and DYRK2 (dual-specificity tyrosine-regulated kinase 2) stimulate the proteasome's ability to degrade ubiquitinated proteins, peptides, and adenosine triphosphate, while one kinase, ASK1 (apoptosis signal-regulating kinase 1), inhibits proteasome function during apoptosis. Proteasome phosphorylation is likely to be important in regulating protein degradation because it occurs downstream from many hormones and neurotransmitters, in conditions that raise cyclic adenosine monophosphate or cyclic guanosine monophosphate levels, after calcium influx following synaptic depolarization, and during phases of the cell cycle. Beyond its physiological importance, pharmacological manipulation of proteasome phosphorylation has the potential to combat various diseases. Inhibitors of phosphodiesterases by activating PKA or PKG (protein kinase G) can stimulate proteasomal degradation of misfolded proteins that cause neurodegenerative or myocardial diseases and even reduce the associated pathology in mouse models. These observations are promising since in many proteotoxic diseases, aggregation-prone proteins impair proteasome function, and disrupt protein homeostasis. Conversely, preventing subunit phosphorylation by DYRK2 slows cell cycle progression and tumor growth. However, further research is essential to determine how phosphorylation of different subunits by these (or other) kinases alters the properties of this complex molecular machine and thus influence protein degradation rates.

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.

Rotavirus NSP1 Requires Casein Kinase II-Mediated Phosphorylation for Hijacking of Cullin-RING Ligases

The rotavirus nonstructural protein NSP1 repurposes cullin-RING E3 ubiquitin ligases (CRLs) to antagonize innate immune responses. By functioning as substrate adaptors of hijacked CRLs, NSP1 causes ubiquitination and proteasomal degradation of host proteins that are essential for expression of interferon (IFN) and IFN-stimulated gene products. The target of most human and porcine rotaviruses is the β-transducin repeat-containing protein (β-TrCP), a regulator of NF-κB activation. β-TrCP recognizes a phosphorylated degron (DSGΦXS) present in the inhibitor of NF-κB (IκB); phosphorylation of the IκB degron is mediated by IκB kinase (IKK). Because NSP1 contains a C-terminal IκB-like degron (ILD; DSGXS) that recruits β-TrCP, we investigated whether the NSP1 ILD is similarly activated by phosphorylation and whether this modification is required to trigger the incorporation of NSP1 into CRLs. Based on mutagenesis and phosphatase treatment studies, we found that both serine residues of the NSP1 ILD are phosphorylated, a pattern mimicking phosphorylation of IκB. A three-pronged approach using small-molecule inhibitors, small interfering RNAs, and mutagenesis demonstrated that NSP1 phosphorylation is mediated by the constitutively active casein kinase II (CKII), rather than IKK. In coimmunoprecipitation assays, we found that this modification was essential for NSP1 recruitment of β-TrCP and induced changes involving the NSP1 N-terminal RING motif that allowed formation of Cul3-NSP1 complexes. Taken together, our results indicate a highly regulated stepwise process in the formation of NSP1-Cul3 CRLs that is initiated by CKII phosphorylation of NSP1, followed by NSP1 recruitment of β-TrCP and ending with incorporation of the NSP1–β-TrCP complex into the CRL via interactions dependent on the highly conserved NSP1 RING motif.

Rotavirus is a segmented double-stranded RNA virus that causes severe diarrhea in young children. A primary mechanism used by the virus to inhibit host innate immune responses is to hijack cellular cullin-RING E3 ubiquitin ligases (CRLs) and redirect their targeting activity to the degradation of cellular proteins crucial for interferon expression. This task is accomplished through the rotavirus nonstructural protein NSP1, which incorporates itself into a CRL and serves as a substrate recognition subunit. The substrate recognized by the NSP1 of many human and porcine rotaviruses is β-TrCP, a protein that regulates the transcription factor NF-κB. In this study, we show that formation of NSP1 CRLs is a highly regulated stepwise process initiated by CKII phosphorylation of the β-TrCP recognition motif in NSP1. This modification triggers recruitment of the β-TrCP substrate and induces subsequent changes in a highly conserved NSP1 RING domain that allow anchoring of the NSP1–β-TrCP complex to a cullin scaffold.

Direct Monitoring of Protein O-GlcNAcylation by High-Resolution Native Mass Spectrometry

O-GlcNAcylation is one of the most abundant metazoan nuclear-cytoplasmic post-translational modifications. Proteins modified by O-GlcNAc play key cellular roles in signaling, transcription, metabolism, and cell division. Mechanistic studies on protein O-GlcNAcylation are hampered by the lack of methods that can simultaneously quantify O-GlcNAcylation, determine its stoichiometry, and monitor O-GlcNAcylation kinetics. Here, we demonstrate that high-resolution native mass spectrometry can be employed to monitor the small mass shifts induced by modification by O-GlcNAc on two known protein substrates, CK2α and TAB1, without the need for radioactive labeling or chemoenzymatic tagging using large mass tags. Limited proteolysis enabled further localization of the O-GlcNAc sites. In peptide-centric MS analysis, the O-GlcNAc moiety is known to be easily lost. In contrast, we demonstrate that the O-GlcNAc is retained under native MS conditions, enabling precise quantitative analysis of stoichiometry and O-GlcNAcylation kinetics. Together, the data highlight that high resolution native MS may provide an alternative tool to monitor kinetics on one of the most labile of protein post-translational modifications, in an efficient, reliable, and quantitative manner.

Effect of Phosphorylation on a Human-like Osteopontin Peptide

The last decade established that the dynamic properties of the phosphoproteome are central to function and its modulation. The temporal dimension of phosphorylation effects remains nonetheless poorly understood, particularly for intrinsically disordered proteins. Osteopontin, selected for this study due to its key role in biomineralization, is expressed in many species and tissues to play a range of distinct roles. A notable property of highly phosphorylated isoforms of osteopontin is their ability to sequester nanoclusters of calcium phosphate to form a core-shell structure, in a fluid that is supersaturated but stable. In Biology, this process enables soft and hard tissues to coexist in the same organism with relative ease. Here, we extend our understanding of the effect of phosphorylation on a disordered protein, the recombinant human-like osteopontin rOPN. The solution structures of the phosphorylated and unphosphorylated rOPN were investigated by small-angle x-ray scattering and no significant changes were detected on the radius of gyration or maximum interatomic distance. The picosecond-to-nanosecond dynamics of the hydrated powders of the two rOPN forms were further compared by elastic and quasi-elastic incoherent neutron scattering. Phosphorylation was found to block some nanosecond side-chain motions while increasing the flexibility of other side chains on the faster timescale. Phosphorylation can thus selectively change the dynamic behavior of even a highly disordered protein such as osteopontin. Through such an effect on rOPN, phosphorylation can direct allosteric mechanisms, interactions with substrates, cofactors and, in this case, amorphous or crystalline biominerals.

Progesterone receptor membrane component 1 is phosphorylated upon progestin treatment in breast cancer cells

Menopausal hormone therapy, using estrogen and synthetic progestins, is associated with an increased risk of developing breast cancer. The effect of progestins on breast cells is complex and not yet fully understood. In previous in vitro and in vivo studies, we found different progestins to increase the proliferation of Progesterone Receptor Membrane Component-1 (PGRMC1)-overexpressing MCF7 cells (MCF7/PGRMC1), suggesting a possible role of PGRMC1 in transducing membrane-initiated progestin signals.

Understanding the activation mechanism of PGRMC1 by progestins will provide deeper insights into the mode of action of progestins on breast cells and the often-reported phenomenon of elevated breast cancer rates upon progestin-based hormone therapy. In the present study, we aimed to further investigate the effect of progestins on receptor activation in MCF7 and T47D breast cancer cell lines. We report that treatment of both breast cancer cell lines with the progestin norethisterone (NET) induces phosphorylation of PGRMC1 at the Casein Kinase 2 (CK2) phosphorylation site Ser181, which can be decreased by treatment with CK2 inhibitor quinalizarin. Point mutation of the Ser181 phosphorylation site in MCF7/PGRMC1 cells impaired proliferation upon NET treatment. This study gives further insights into the mechanism of differential phosphorylation of the receptor and confirms our earlier hypothesis that phosphorylation of the CK2-binding site is essential for activation of PGRMC1. It further suggests an important role of PGRMC1 in the tumorigenesis and progression of breast cancer in progestin-based hormone replacement therapy.