The catalytic subunit of human protein kinase CK2 structurally deviates from its maize homologue in complex with the nucleotide competitive inhibitor emodin

Discovery of a novel, selective CK2 inhibitor class with an unusual basic scaffold

CK2 is a Ser/Thr-protein kinase playing a crucial role in promoting cell growth and survival, hence it is considered a promising target for anti-cancer drugs. However, many previously reported CK2 inhibitors lack selectivity. In search of novel scaffolds for selective CK2 inhibition, we identified a dihydropyrido-thieno[2,3-d]pyrimidine derivative displaying submicromolar inhibitory activity against CK2α. This scaffold captured our interest because of the basic secondary amine, a rather unusual motif for CK2 inhibitors. Our optimization strategy comprised the incorporation of a 4-piperazinyl moiety as a linker group and introduction of varying substituents on the pendant phenyl ring. All resulting compounds exhibited potent CK2α inhibition, with IC50 values in the nanomolar range. Compound 10b demonstrated the most balanced activity profile with a cell-free IC50 value of 36.7 nM and a notable cellular activity with a GI50 of 7.3 μM and 7.5 μM against 786-O renal cell carcinoma and U937 lymphoma cells, respectively. 10b displayed excellent selectivity when screened against a challenging kinase selectivity profiling panel. Moreover, 10b inhibited CK2 in the cells, albeit less potently than CX-4945, but induced cell death more strongly than CX-4945. Altogether, we have identified a novel CK2 inhibitory scaffold with drug-like physicochemical properties in a favorable basic pKa range.

Synthesis and evaluation of chemical linchpins for highly selective CK2α targeting

Casein kinase-2 (CK2) are serine/threonine kinases with dual co-factor (ATP and GTP) specificity, that are involved in the regulation of a wide variety of cellular functions. Small molecules targeting CK2 have been described in the literature targeting different binding pockets of the kinase with a focus on type I inhibitors such as the recently published chemical probe SGC-CK2-1. In this study, we investigated whether known allosteric inhibitors binding to a pocket adjacent to helix αD could be combined with ATP mimetic moieties defining a novel class of ATP competitive compounds with a unique binding mode. Linking both binding sites requires a chemical linking moiety that would introduce a 90-degree angle between the ATP mimetic ring system and the αD targeting moiety, which was realized using a sulfonamide. The synthesized inhibitors were highly selective for CK2 with binding constants in the nM range and low micromolar activity. While these inhibitors need to be further improved, the present work provides a structure-based design strategy for highly selective CK2 inhibitors.

Structure-guided discovery of adenosine triphosphate-competitive casein kinase 2 inhibitors

Casein kinase 2 (CK2) is a ubiquitous, highly pleiotropic serine-threonine kinase. CK2 has been identified as a potential drug target for the treatment of cancer and related disorders. Several adenosine triphosphate-competitive CK2 inhibitors have been identified and have progressed at different levels of clinical trials. This review presents details of CK2 protein, structural insights into adenosine triphosphate binding pocket, current clinical trial candidates and their analogues. Further, it includes the emerging structure-based drug design approaches, chemistry, structure-activity relationship and biological screening of potent and selective CK2 inhibitors. The authors tabulated the details of CK2 co-crystal structures because these co-crystal structures facilitated the structure-guided discovery of CK2 inhibitors. The narrow hinge pocket compared with related kinases provides useful insights into the discovery of CK2 inhibitors.

Analyzing the interactome of human CK2β in prostate carcinoma cells reveals HSP70-1 and Rho guanin nucleotide exchange factor 12 as novel interaction partners

CK2β is the non-catalytic modulating part of the S/T-protein kinase CK2. However, the overall function of CK2β is poorly understood. Here, we report on the identification of 38 new interaction partners of the human CK2β from lysates of DU145 prostate cancer cells using photo-crosslinking and mass spectrometry, whereby HSP70-1 was identified with high abundance. The KD value of its interaction with CK2β was determined as 0.57 μM by microscale thermophoresis, this being the first time, to our knowledge, that a KD value of CK2β with another protein than CK2α or CK2α' was quantified. Phosphorylation studies excluded HSP70-1 as a substrate or activity modulator of CK2, suggesting a CK2 activity independent interaction of HSP70-1 with CK2β. Co-immunoprecipitation experiments in three different cancer cell lines confirmed the interaction of HSP70-1 with CK2β in vivo. A second identified CK2β interaction partner was Rho guanin nucleotide exchange factor 12, indicating an involvement of CK2β in the Rho-GTPase signal pathway, described here for the first time to our knowledge. This points to a role of CK2β in the interaction network affecting the organization of the cytoskeleton.

Strategies of Targeting CK2 in Drug Discovery: Challenges, Opportunities, and Emerging Prospects

CK2 (casein kinase 2) is a serine/threonine protein kinase that is ubiquitous in eukaryotic cells and plays important roles in a variety of cellular functions, including cell growth, apoptosis, circadian rhythms, DNA damage repair, transcription, and translation. CK2 is involved in cancer pathogenesis and the occurrence of many diseases. Therefore, targeting CK2 is a promising therapeutic strategy. Although many CK2-specific small-molecule inhibitors have been developed, only CX-4945 has progressed to clinical trials. In recent years, novel CK2 inhibitors have gradually become a research hotspot, which is expected to overcome the limitations of traditional inhibitors. Herein, we summarize the structure, biological functions, and disease relevance of CK2 and emphatically analyze the structure-activity relationship (SAR) and binding modes of small-molecule CK2 inhibitors. We also discuss the latest progress of novel strategies, providing insights into new drugs targeting CK2 for clinical practice.

Utilizing mixture design response surface methodology to determine effective combinations of plant derived compounds as prostate cancer treatments

BACKGROUND

Prostate cancer (PC) is estimated to cause 13.1% of all new cancer cases in the United States in 2021. Natural bioactive compounds have drawn the interest of researchers worldwide in their efforts to find novel treatments for PC. Many of these bioactive compounds have been identified from traditional Chinese medicine (TCM) remedies often containing multiple bioactive compounds. However, in vitro studies frequently focus on the compounds in isolation.

AIM

We used mixture design response surface methodology (MDRSM) to assess changes in PC cell viability after 48 h of treatment to identify the optimal mixture of all 35 three-compound combinations of seven bioactive compounds from TCM.

METHODS AND RESULTS

We used berberine, wogonin, shikonin, curcumin, triptolide, emodin, and silybin to treat PC3 and LNCaP human PC cells at their IC50 concentrations that we calculated. These compounds modulate many chemotherapeutic pathways including intrinsic and extrinsic apoptosis, increasing reactive oxygen species, decreasing metastatic pathways, inhibiting cell cycle progression. We hypothesize that because these compounds bind to unique molecular targets to activate different chemotherapeutic pathways, they will act synergistically to decrease tumor cell viability. Results from MDRSM showed that two-way combinations were more effective than three-way or single compounds. Most notably wogonin, silybin, emodin and berberine responded well in two-compound combinations with each other in PC3 and LNCaP cells. We then conducted cell viability tests combining two bioactive compound ratios with docetaxel (Doc) and found significant results within the LNCaP cell line. In particular, mixtures of berberine and wogonin, berberine and silybin, emodin and berberine, and emodin and silybin reduced LNCaP cell viability up to an average of 90.02%. The two-compound combinations were significantly better than docetaxel treatment of LNCaP cells.

CONCLUSION

Within the PC3 cells, we show that a combination of berberine, wogonin and docetaxel is just as effective as docetaxel alone. Thus, we provide new combination treatments that are highly effective in vitro for treating androgen-dependent and androgen-independent PC.

Molecular Plasticity of Crystalline CK2α' Leads to KN2, a Bivalent Inhibitor of Protein Kinase CK2 with Extraordinary Selectivity

CK2α and CK2α' are paralogous catalytic subunits of CK2, which belongs to the eukaryotic protein kinases. CK2 promotes tumorigenesis and the spread of pathogenic viruses like SARS-CoV-2 and is thus an attractive drug target. Efforts to develop selective CK2 inhibitors binding offside the ATP site had disclosed the αD pocket in CK2α; its occupation requires large conformational adaptations of the helix αD. As shown here, the αD pocket is accessible also in CK2α', where the necessary structural plasticity can be triggered with suitable ligands even in the crystalline state. A CK2α' structure with an ATP site and an αD pocket ligand guided the design of the bivalent CK2 inhibitor KN2. It binds to CK2 with low nanomolar affinity, is cell-permeable, and suppresses the intracellular phosphorylation of typical CK2 substrates. Kinase profiling revealed a high selectivity of KN2 for CK2 and emphasizes the selectivity-promoting potential of the αD pocket.

Structural basis for the design of bisubstrate inhibitors of protein kinase CK2 provided by complex structures with the substrate-competitive inhibitor heparin

The Ser/Thr kinase CK2, a member of the superfamily of eukaryotic protein kinases, has an acidophilic substrate profile with the substrate recognition sequence S/T-D/E-X-D/E, and it is inhibited by polyanionic substances like heparin. The latter, a highly sulphated glucosamino glycan composed mainly of repeating 2-O-sulpho-α-l-idopyranuronic acid/N,O6-disulpho-α-d-glucosamine disaccharide units, is the longest known substrate-competitive CK2 inhibitor. The structural basis of CK2's preference for anionic substrates and substrate-competitive inhibitors is only vaguely known which limits the value of the substrate-binding region for the structure-based development of CK2 bisubstrate inhibitors. Here, a tetragonal and a monoclinic co-crystal structure of CK2α, the catalytic subunit of CK2, with a decameric heparin fragment are described. In the tetragonal structure, the heparin molecule binds to the polybasic stretch at the beginning of CK2α's helix αC, whereas in the monoclinic structure it occupies the central substrate-recognition region around the P+1 loop. Together, the structures rationalize the inhibitory efficacy of heparin fragments as a function of chain length. The monoclinic CK2α/heparin structure, in which the heparin fragment is particularly well defined, is the first CK2 structure with an anionic inhibitor of considerable size at the central part of the substrate-recognition site. The bound heparin fragment is so close to the binding site of ATP-competitive inhibitors that it can guide the design of linkers and pave the way to efficient CK2 bisubstrate inhibitors in the future.

Structural and thermodynamic analyses of interactions between death-associated protein kinase 1 and anthraquinones

Death-associated protein kinase 1 (DAPK1) is a serine/threonine protein kinase that regulates apoptosis and autophagy. DAPK1 is considered to be a therapeutic target for amyloid-β deposition, endometrial adenocarcinomas and acute ischemic stroke. Here, the potent inhibitory activity of the natural anthraquinone purpurin against DAPK1 phosphorylation is shown. Thermodynamic analysis revealed that while the binding affinity of purpurin is similar to that of CPR005231, which is a DAPK1 inhibitor with an imidazopyridazine moiety, the binding of purpurin was more enthalpically favorable. In addition, the inhibition potencies were correlated with the enthalpic changes but not with the binding affinities. Crystallographic analysis of the DAPK1-purpurin complex revealed that the formation of a hydrogen-bond network is likely to contribute to the favorable enthalpic changes and that stabilization of the glycine-rich loop may cause less favorable entropic changes. The present findings indicate that purpurin may be a good lead compound for the discovery of inhibitors of DAPK1, and the observation of enthalpic changes could provide important clues for drug development.

Anthraquinone: a promising scaffold for the discovery and development of therapeutic agents in cancer therapy

Cancer, characterized by uncontrolled malignant neoplasm, is a leading cause of death in both advanced and emerging countries. Although, ample drugs are accessible in the market to intervene with tumor progression, none are totally effective and safe. Natural anthraquinone (AQ) equivalents such as emodin, aloe-emodin, alchemix and many synthetic analogs extend their antitumor activity on different targets including telomerase, topoisomerases, kinases, matrix metalloproteinases, DNA and different phases of cell lines. Nano drug delivery strategies are advanced tools which deliver drugs into tumor cells with minimum drug leakage to normal cells. This review delineates the way AQ derivatives are binding on these targets by abolishing tumor cells to produce anticancer activity and purview of nanoformulations related to AQ analogs.

Structural analysis of fungal pathogenicity-related casein kinase α subunit, Cka1, in the human fungal pathogen Cryptococcus neoformans

CK2α is a constitutively active and highly conserved serine/threonine protein kinase that is involved in the regulation of key cellular metabolic pathways and associated with a variety of tumours and cancers. The most well-known CK2α inhibitor is the human clinical trial candidate CX-4945, which has recently shown to exhibit not only anti-cancer, but also anti-fungal properties. This prompted us to work on the CK2α orthologue, Cka1, from the pathogenic fungus Cryptococcus neoformans, which causes life-threatening systemic cryptococcosis and meningoencephalitis mainly in immunocompromised individuals. At present, treatment of cryptococcosis remains a challenge due to limited anti-cryptococcal therapeutic strategies. Hence, expanding therapeutic options for the treatment of the disease is highly clinically relevant. Herein, we report the structures of Cka1-AMPPNP-Mg²⁺ (2.40 Å) and Cka1-CX-4945 (2.09 Å). Structural comparisons of Cka1-AMPPNP-Mg²⁺ with other orthologues revealed the dynamic architecture of the N-lobe across species. This may explain for the difference in binding affinities and deviations in protein-inhibitor interactions between Cka1-CX-4945 and human CK2α-CX-4945. Supporting it, in vitro kinase assay demonstrated that CX-4945 inhibited human CK2α much more efficiently than Cka1. Our results provide structural insights into the design of more selective inhibitors against Cka1.

Rhein inhibits ATP-triggered inflammatory responses in rheumatoid rat fibroblast-like synoviocytes

Rheumatoid arthritis (RA) is a chronic and systemic inflammatory disorder, which may lead to joint disabilities. So far the pathogenesis of RA remains largely undetermined, and there are still no potent drugs for clinical treatment. Rhein, a natural bioactive anthraquinone derivative, exhibited significant anti-inflammatory activities demonstrated by previous studies. Here we aimed to investigate the effects of rhein on ATP-induced inflammation responses in fibroblast-like synoviocytes isolated from a rat model of collagen induced arthritis (CIA). Our results showed that ATP triggered rapid cytosolic calcium concentration ([Ca²⁺]_c) increase depending on extracellular Ca²⁺ entry. Given the major P2 subtypes expressed in rat synoviocytes were P2X₄ and P2Y₂ receptors, ATP-elicited calcium entry should be mainly resulted from activating P2X₄. Interestingly, rhein could effectively block the ATP-induced [Ca²⁺]_c increases in a dose-dependent manner. Besides, rhein also suppressed the production of intracellular reactive oxygen species (ROS) induced by ATP in synoviocytes that was resulted from P2X₄-mediated Ca²⁺ entry. Brilliant blue G (BBG), which can block P2X₄ receptor at high concentration, showed similar suppressive effects on above responses. Furthermore, in lipopolysaccharide-primed cells, application of ATP synergistically promoted the gene expression of cyclooxygenase-2, interleukin-6 and matrix metalloproteinase-9. Both rhein and BBG attenuated these inflammatory gene expressions enhanced by ATP. Above data together suggested a potential anti-arthritic role of rhein by inhibiting ATP-induced [Ca²⁺]_c increase, ROS production and inflammatory gene expression targeting P2X₄ in CIA rat synoviocytes, which will provide a novel insight in the therapy of RA.

Diacritic Binding of an Indenoindole Inhibitor by CK2α Paralogs Explored by a Reliable Path to Atomic Resolution CK2α' Structures

CK2α and CK2α' are the two isoforms of the catalytic subunit of human protein kinase CK2, an important target for cancer therapy. They have similar, albeit not identical functional and structural properties, and were occasionally reported to be inhibited with distinct efficacies by certain ATP-competitive ligands. Here, we present THN27, an indeno[1,2-b]indole derivative, as a further inhibitor with basal isoform selectivity. The selectivity disappears when measured using CK2α/CK2α' complexes with CK2β, the regulatory CK2 subunit. Co-crystal structures of THN27 with CK2α and CK2α' reveal that subtle differences in the conformational variability of the interdomain hinge region are correlated with the observed effect. In the case of CK2α', a crystallographically problematic protein so far, this comparative structural analysis required the development of an experimental strategy that finally enables atomic resolution structure determinations with ab initio phasing of potentially any ATP-competitive CK2 inhibitor and possibly many non-ATP-competitive ligands as well bound to CK2α'.

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.

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.

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.

Unexpected Binding Mode of a Potent Indeno[1,2-b]indole-Type Inhibitor of Protein Kinase CK2 Revealed by Complex Structures with the Catalytic Subunit CK2α and Its Paralog CK2α'

Protein kinase CK2, a member of the eukaryotic protein kinase superfamily, is associated with cancer and other human pathologies and thus an attractive drug target. The indeno[1,2-b]indole scaffold is a novel lead structure to develop ATP-competitive CK2 inhibitors. Some indeno[1,2-b]indole-based CK2 inhibitors additionally obstruct ABCG2, an ABC half transporter overexpressed in breast cancer and co-responsible for drug efflux and resistance. Comprehensive derivatization studies revealed substitutions of the indeno[1,2-b]indole framework that boost either the CK2 or the ABCG2 selectivity or even support the dual inhibition potential. The best indeno[1,2-b]indole-based CK2 inhibitor described yet (IC₅₀ = 25 nM) is 5-isopropyl-4-(3-methylbut-2-enyl-oxy)-5,6,7,8-tetrahydroindeno[1,2-b]indole-9,10-dione (4p). Herein, we demonstrate the membrane permeability of 4p and describe co-crystal structures of 4p with CK2α and CK2α′, the paralogs of human CK2 catalytic subunit. As expected, 4p occupies the narrow, hydrophobic ATP site of CK2α/CK2α′, but surprisingly with a unique orientation: its hydrophobic substituents point towards the solvent while its two oxo groups are hydrogen-bonded to a hidden water molecule. An equivalent water molecule was found in many CK2α structures, but never as a critical mediator of ligand binding. This unexpected binding mode is independent of the interdomain hinge/helix αD region conformation and of the salt content in the crystallization medium.

Structural Hypervariability of the Two Human Protein Kinase CK2 Catalytic Subunit Paralogs Revealed by Complex Structures with a Flavonol- and a Thieno[2,3-d]pyrimidine-Based Inhibitor

Protein kinase CK2 is associated with a number of human diseases, among them cancer, and is therefore a target for inhibitor development in industry and academia. Six crystal structures of either CK2α, the catalytic subunit of human protein kinase CK2, or its paralog CK2α′ in complex with two ATP-competitive inhibitors—based on either a flavonol or a thieno[2,3-d]pyrimidine framework—are presented. The structures show examples for extreme structural deformations of the ATP-binding loop and its neighbourhood and of the hinge/helix αD region, i.e., of two zones of the broader ATP site environment. Thus, they supplement our picture of the conformational space available for CK2α and CK2α′. Further, they document the potential of synthetic ligands to trap unusual conformations of the enzymes and allow to envision a new generation of inhibitors that stabilize such conformations.

Detection of Peptide-Binding Sites on Protein Surfaces Using the Peptimap Server

Peptide-mediated interactions are of primordial importance to the cell, and the structure of such interaction provides an important starting point for their further characterization. In many cases, the structure of the peptide-protein complex has not been solved by experiment, and modeling tools need to be applied to generate structural models of the interaction. PeptiMap is a protocol that identifies the peptide-binding site when only the structure of the receptor is known, but no information about where the peptide binds is available. This is achieved by mapping the surface for solvents to identify ligand-binding sites, similar in approach to ANCHORMAP in which amino acids are mapped. Peptimap is a free open access web-based server. It can be accessed at http://peptimap.cluspro.org .

A Note of Caution on the Role of Halogen Bonds for Protein Kinase/Inhibitor Recognition Suggested by High- And Low-Salt CK2α Complex Structures

CK2 is a Ser/Thr kinase recruited by tumor cells to avoid cell death. 4'-Carboxy-6,8-dibromo-flavonol (FLC26) is a nanomolar CK2 inhibitor reducing the physiological phosphorylation of CK2 biomarkers and inducing cell death. Its binding mode to the ATP site was predicted to depend primarily on noncovalent interactions not comprising halogen bonds. We confirm this by two independent cocrystal structures which additionally show that FLC26 is selective for an open, protein kinase-untypical conformation of the hinge/helix αD region. The structures suggest how the bromo substituents, found previously in lead optimization studies, contribute to the inhibitory efficacy. In this context, one of the complex structures, obtained by crystallization with the kosmotropic salt NaCl, revealed an unconventional π-halogen bond between the 8-bromo substituent of FLC26 and an aromatic side chain which is absent under low-salt conditions. The kosmotropic salt sensitivity of π-halogen bonds is a novel feature which requires attention in structural comparisons and halogen-bond-based explanations.

Thermodynamics parameters for binding of halogenated benzotriazole inhibitors of human protein kinase CK2α

The interaction of human CK2α (hCK2α) with nine halogenated benzotriazoles, TBBt and its analogues representing all possible patterns of halogenation on the benzene ring of benzotriazole, was studied by biophysical methods. Thermal stability of protein-ligand complexes, monitored by calorimetric (DSC) and optical (DSF) methods, showed that the increase in the mid-point temperature for unfolding of protein-ligand complexes (i.e. potency of ligand binding to hCK2α) follow the inhibitory activities determined by biochemical assays. The dissociation constant for the ATP-hCK2α complex was estimated with the aid of microscale thermophoresis (MST) as 4.3±1.8 μM, and MST-derived dissociation constants determined for halogenated benzotriazoles, when converted according to known ATP concentrations, perfectly reconstruct IC50 values determined by the biochemical assays. Ligand-dependent quenching of tyrosine fluorescence, together with molecular modeling and DSC-derived heats of unfolding, support the hypothesis that halogenated benzotriazoles bind in at least two alternative orientations, and those that are efficient hCK2α inhibitors bind in the orientation which TBBt adopts in its complex with maize CK2α. DSC-derived apparent heat for ligand binding (ΔΔHbind) is driven by intermolecular electrostatic interactions between Lys68 and the triazole ring of the ligand, as indicated by a good correlation between ΔΔHbind and ligand pKa. Overall results, additionally supported by molecular modeling, confirm that a balance of hydrophobic and electrostatic interactions contribute predominantly (~40 kJ/mol), relative to possible intermolecular halogen/hydrogen bonding (less than 10 kJ/mol), in binding of halogenated benzotriazoles to the ATP-binding site of hCK2α. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

Conformational flexibility of human casein kinase catalytic subunit explored by metadynamics

Casein kinase CK2 is an essential enzyme in higher organisms, catalyzing the transfer of the γ phosphate from ATP to serine and threonine residues on protein substrates. In a number of animal tumors, CK2 activity has been shown to escape normal cellular control, making it a potential target for cancer therapy. Several crystal structures of human CK2 have been published with different conformations for the CK2α catalytic subunit. This variability reflects a high flexibility for two regions of CK2α: the interdomain hinge region, and the glycine-rich loop (p-loop). Here, we present a computational study simulating the equilibrium between three conformations involving these regions. Simulations were performed using well-tempered metadynamics combined with a path collective variables approach. This provides a reference pathway describing the conformational changes being studied, based on analysis of free energy surfaces. The free energies of the three conformations were found to be close and the paths proposed had low activation barriers. Our results indicate that these conformations can exist in water. This information should be useful when designing inhibitors specific to one conformation.

Crystal structure of human CK2α at 1.06 Å resolution

The Ser/Thr kinase CK2 consists of two catalytic subunits (CK2α) and a dimer of the regulatory subunits (CK2β), and is a ubiquitous enzyme that regulates growth, proliferation and the survival of cells. CK2 is a remarkable drug target for potentially treating a wide variety of tumours and glomerulonephritis. The purified CK2α protein was crystallized using ethylene glycol as a precipitant. The crystal structure of CK2α with 21 loci of alternative conformations, including a niacin, 19 ethylene glycols and 346 waters, was determined at 1.06 Å resolution to an Rwork of 14.0% (Rfree = 16.5%). The alternative ensemble in the internal hydrophobic core underpins the plasticity of the αD-helix responsible for the regulation of ATP/GTP binding. The clear density map indicates that a niacin molecule, contained in the Escherichia coli culture medium, binds to the ATP binding site. An ethylene glycol molecule binds in the hydrophobic pocket lateral to the αD-helix forming the rim of the active site. The other ethylene glycol molecules occupy physiologically significant sites, including the CK2β binding interface and substrate binding site, as well as the gap in the crystal packing. Together with water molecules in the active site, these structural insights should facilitate drug discovery.

Discovery and design of tricyclic scaffolds as protein kinase CK2 (CK2) inhibitors through a combination of shape-based virtual screening and structure-based molecular modification

Protein kinase CK2 (CK2), a ubiquitous serine/threonine protein kinase for hundreds of endogenous substrates, serves as an attractive anticancer target. One of its most potent inhibitors, CX-4945, has entered a phase I clinical trial. Herein we present an integrated workflow combining shape-based virtual screening for the identification of novel CK2 inhibitors. A shape-based model derived from CX-4945 was built, and the subsequent virtual screening led to the identification of several novel scaffolds with high shape similarity to that of CX-4945. Among them two tricyclic scaffolds named [1,2,4]triazolo[4,3-c]quinazolin and [1,2,4]triazolo[4,3-a]quinoxalin attracted us the most. Combining strictly chemical similarity analysis, a second-round shape-based screening was performed based on the two tricyclic scaffolds, leading to 28 derivatives. These compounds not only targeted CK2 with potent and dose-dependent activities but also showed acceptable antiproliferative effects against a series of cancer cell lines. Our workflow supplies a high efficient strategy in the identification of novel CK2 inhibitors. Compounds reported here can serve as ideal leads for further modifications.

Low-density crystal packing of human protein kinase CK2 catalytic subunit in complex with resorufin or other ligands: a tool to study the unique hinge-region plasticity of the enzyme without packing bias

A low-resolution structure of the catalytic subunit CK2α of human protein kinase CK2 (formerly known as casein kinase 2) in complex with the ATP-competitive inhibitor resorufin is presented. The structure supplements previous human CK2α structures in which the interdomain hinge/helix αD region adopts a closed conformation correlating to a canonically established catalytic spine as is typical for eukaryotic protein kinases. In the corresponding crystal packing the hinge/helix αD region is nearly unaffected by crystal contacts, so that largely unbiased conformational adaptions are possible. This is documented by published human CK2α structures with the same crystal packing but with an open hinge/helix αD region, one of which has been redetermined here with a higher symmetry. An overview of all published human CK2α crystal packings serves as the basis for a discussion of the factors that determine whether the open or the closed hinge/helix αD conformation is adopted. Lyotropic salts in crystallization support the closed conformation, in which the Phe121 side chain complements the hydrophobic catalytic spine ensemble. Consequently, genuine ligand effects on the hinge/helix αD conformation can be best studied under moderate salt conditions. Ligands that stabilize either the open or the closed conformation by hydrogen bonds are known, but a general rule is not yet apparent.

A detailed thermodynamic profile of cyclopentyl and isopropyl derivatives binding to CK2 kinase

The detailed understanding of the molecular features of a ligand binding to a target protein, facilitates the successful design of potent and selective inhibitors. We present a case study of ATP-competitive kinase inhibitors that include a pyradine moiety. These compounds have similar chemical structure, except for distinct terminal hydrophobic cyclopentyl or isopropyl groups, and block kinase activity of casein kinase 2 subunit α (CK2α), which is a target for several diseases, such as cancer and glomerulonephritis. Although these compounds display similar inhibitory potency against CK2α, the crystal structures reveal that the cyclopentyl derivative gains more favorable interactions compared with the isopropyl derivative, because of the additional ethylene moiety. The structural observations and biological data are consistent with the thermodynamic profiles of these inhibitors in binding to CK2α, revealing that the enthalpic advantage of the cyclopentyl derivative is accompanied with a lower entropic loss. Computational analyses indicated that the relative enthalpic gain of the cyclopentyl derivative arises from an enhancement of a wide range of van der Waals interactions from the whole complex. Conversely, the relative entropy loss of the cyclopentyl derivative arises from a decrease in the molecular fluctuation and higher conformational restriction in the active site of CK2α. These structural insights, in combination with thermodynamic and computational observations, should be helpful in developing potent and selective CK2α inhibitors.

Enzymatic activity with an incomplete catalytic spine: insights from a comparative structural analysis of human CK2α and its paralogous isoform CK2α'

Eukaryotic protein kinases are fundamental factors for cellular regulation and therefore subject of strict control mechanisms. For full activity a kinase molecule must be penetrated by two stacks of hydrophobic residues, the regulatory and the catalytic spine that are normally well conserved among active protein kinases. We apply this novel spine concept here on CK2α, the catalytic subunit of protein kinase CK2. Homo sapiens disposes of two paralog isoforms of CK2α (hsCK2α and hsCK2α'). We describe two new structures of hsCK2α constructs one of which in complex with the ATP-analog adenylyl imidodiphosphate and the other with the ATP-competitive inhibitor 3-(4,5,6,7-tetrabromo-1H-benzotriazol-1-yl)propan-1-ol. The former is the first hsCK2α structure with a well defined cosubstrate/magnesium complex and the second with an open β4/β5-loop. Comparisons of these structures with existing CK2α/CK2α' and cAMP-dependent protein kinase (PKA) structures reveal: in hsCK2α' an open conformation of the interdomain hinge/helix αD region that is critical for ATP-binding is found corresponding to an incomplete catalytic spine. In contrast hsCK2α often adopts the canonical, PKA-like version of the catalytic spine which correlates with a closed conformation of the hinge region. HsCK2α can switch to the incomplete, non-canonical, hsCK2α'-like state of the catalytic spine, but this transition apparently depends on binding of either ATP or of the regulatory subunit CK2β. Thus, ATP looks like an activator of hsCK2α rather than a pure cosubstrate.

Structure of the human protein kinase CK2 catalytic subunit CK2α' and interaction thermodynamics with the regulatory subunit CK2β

Protein kinase CK2 (formerly "casein kinase 2") is composed of a central dimer of noncatalytic subunits (CK2β) binding two catalytic subunits. In humans, there are two isoforms of the catalytic subunit (and an additional splicing variant), one of which (CK2α) is well characterized. To supplement the limited biochemical knowledge about the second paralog (CK2α'), we developed a well-soluble catalytically active full-length mutant of human CK2α', characterized it by Michaelis-Menten kinetics and isothermal titration calorimetry, and determined its crystal structure to a resolution of 2 Å. The affinity of CK2α' for CK2β is about 12 times lower than that of CK2α and is less driven by enthalpy. This result fits the observation that the β4/β5 loop, a key element of the CK2α/CK2β interface, adopts an open conformation in CK2α', while in CK2α, it opens only after assembly with CK2β. The open β4/β5 loop in CK2α' is stabilized by two elements that are absent in CK2α: (1) the extension of the N-terminal β-sheet by an additional β-strand, and (2) the filling of a conserved hydrophobic cavity between the β4/β5 loop and helix αC by a tryptophan residue. Moreover, the interdomain hinge region of CK2α' adopts a fully functional conformation, while unbound CK2α is often found with a nonproductive hinge conformation that is overcome only by CK2β binding. Taken together, CK2α' exhibits a significantly lower affinity for CK2β than CK2α; moreover, in functionally critical regions, it is less dependent on CK2β to obtain a fully functional conformation.

Interaction between CK2α and CK2β, the subunits of protein kinase CK2: thermodynamic contributions of key residues on the CK2α surface

The protein Ser/Thr kinase CK2 (former name: casein kinase II) exists predominantly as a heterotetrameric holoenzyme composed of two catalytic subunits (CK2α) bound to a dimer of noncatalytic subunits (CK2β). We undertook a study to further understand how these subunits interact to form the tetramer. To this end, we used recombinant, C-terminal truncated forms of human CK2 subunits that are able to form the holoenzyme. We analyzed the interaction thermodynamics between the binding of CK2α and CK2β as well as the impact of changes in temperature, pH, and the ionization enthalpy of the buffer using isothermal titration calorimetry (ITC). With structure-guided alanine scanning mutagenesis we truncated individual side chains in the hydrophobic amino acid cluster located within the CK2α interface to identify experimentally the amino acids that dominate affinity. The ITC results indicate that Leu41 or Phe54 single mutations were most disruptive to binding of CK2β. Additionally, these CK2α mutants retained their kinase activity. Furthermore, the substitution of Leu41 in combination with Phe54 showed that the individual mutations were not additive, suggesting that the cooperative action of both residues played a role. Interestingly, the replacement of Ile69, which has a central position in the interaction surface of CK2α, only had modest effects. The differences between Leu41, Phe54, and Ile69 in interaction relevance correlate with solvent accessibility changes during the transition from unbound to CK2β-bound CK2α. Identifying residues on CK2α that play a key role in CK2α/CK2β interactions is important for the future generation of small molecule drug design.

Structural basis of CX-4945 binding to human protein kinase CK2

Protein kinase CK2 (CK2), a constitutively active serine/threonine kinase, is involved in a variety of roles essential to the maintenance of cellular homeostasis. Elevated levels of CK2 expression results in the dysregulation of key signaling pathways that regulate transcription, and has been implicated in cancer. The adenosine-5'-triphosphate-competitive inhibitor CX-4945 has been reported to show broad spectrum anti-proliferative activity in multiple cancer cell lines. Although the enzymatic IC(50) of CX-4945 has been reported, the thermodynamics and structural basis of binding to CK2α remained elusive. Presented here are the crystal structures of human CK2α in complex with CX-4945 and adenylyl phosphoramidate at 2.7 and 1.3 Å, respectively. Biophysical analysis of CX-4945 binding is also described. This data provides the structural rationale for the design of more potent inhibitors against this emerging cancer target.

How druggable is protein kinase CK2?

CK2 is a pleiotropic, ubiquitous, and constitutively active protein kinase (PK), with both cytosolic and nuclear localization in most mammalian cells. 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. CK2 catalyzes the phosphorylation of more than 300 substrates characterized by multiple acidic residues surrounding the phosphor-acceptor amino acid, and, consequently, it plays a key role in several physiological and pathological processes. But how can one kinase orchestrate all these tasks faithfully? How is it possible that one kinase can, despite all pleiotropic characteristics of PKs in general, be involved in so many different biochemical events? Is CK2 a druggable target? Several questions are still to be clearly answered, and this review is an occasion for a fruitful discussion.

Docking and 3D-QSAR studies of 7-hydroxycoumarin derivatives as CK2 inhibitors

Protein kinase CK2 is involved in a broad range of physiological events. 3,8-dibromo-7-hydroxy-4-methylchromen-2-one (DBC) analogues show favorable inhibitory activity targeting CK2alpha. Two methods were used to build 3D-QSAR models for DBC derivatives. The ligand-based (LB) studies were performed based on the lower energy conformations employing atom fit alignment rule. The receptor-based (RB) models were also derived using bioactive conformations. Contour maps of RB CoMSIA model (q2=0.694, r2=0.916, N (no. of components)=7, r2(pred)=0.87) including the steric, electronic, hydrophobic and hydrogen bond acceptor fields were employed to explain factors affecting activities of inhibitors. The good consistency between the contour maps and the properties of CK2alpha amino acids provide useful hints for new inhibitors design.

Structural basis for decreased affinity of Emodin binding to Val66-mutated human CK2 alpha as determined by molecular dynamics

Protein kinase CK2 (casein kinase 2) is a multifunctional serine/threonine kinase that is involved in a broad range of physiological events. The decreased affinity of Emodin binding to human CK2 alpha resulting from single-point mutation of Val66 to Ala (V66A) has been demonstrated by experimental mutagenesis. Molecular dynamics (MD) simulations and energy analysis were performed on wild type (WT) and V66A mutant CK2 alpha-Emodin complexes to investigate the subtle influences of amino acid replacement on the structure of the complex. The structure of CK2 alpha and the orientation of Emodin undergo changes to different degrees in V66A mutant. The affected positions in CK2 alpha are mainly distributed over the glycine-rich loop (G-loop), the alpha-helix and the loop located at the portion between G-loop and alpha-helix (C-loop). Based on the coupling among these segments, an allosteric mechanism among the C-loop, the G-loop and the deviated Emodin is proposed. Additionally, an estimated energy calculation and residue-based energy decomposition also indicate the lower instability of V66A mutant in contrast to WT, as well as the unfavorable energetic influences on critical residue contributions.

Conformational plasticity of the catalytic subunit of protein kinase CK2 and its consequences for regulation and drug design

At the first glance CK2alpha, the catalytic subunit of protein kinase CK2, is a rigid molecule: in contrast to many eukaryotic protein kinases in CK2alpha the canonical regulatory key elements like the activation segment occur exclusively in their typical active conformations. This observation fits well to the constitutive activity of the enzyme, meaning, its independence from phosphorylation or other characteristic control factors. Most CK2alpha structures are based on the enzyme from Zea mays, supplemented by an increasing number of human CK2alpha structures. In the latter a surprising plasticity of important ATP-binding elements - the interdomain hinge region and the glycine-rich loop - was discovered. In fully active CK2alpha the hinge region is open and does not anchor the ATP ribose, but alternatively it can adopt a closed conformation, form hydrogen bonds to the ribose moiety and thus retract the gamma-phospho group from its functional position. In addition to this partially inactive state human CK2alpha was recently found in a fully inactive conformation. It is incompatible with ATP-binding due to a combination of a closed hinge and a collapse of the glycine-rich loop into the ATP cavity. These conformational transitions are apparently correlated with the occupation state of a remote docking site located at the interface to the non-catalytic subunit CK2beta: if CK2beta blocks this site, the fully active conformation of CK2alpha is stabilized, while the binding of certain small molecule seems to favour the partially and fully inactive states. This observation may be exploited to design effective and selective CK2 inhibitors.

Inhibition of protein kinase CK2 expression and activity blocks tumor cell growth

Protein kinase CK2 (CK2) is a highly conserved and ubiquitous serine/threonine kinase. It is a multifunctional and pleiotropic protein kinase implicated in the regulation of cell proliferation, survival, and differentiation. Deregulation of CK2 is observed in a wide variety of tumors. It has been the focus of intensive research efforts to establish the cause-effect relationship between CK2 and neoplastic growth. Here, we further validate the role of CK2 in cancer cell growth using siRNA approach. We also screened a library of more than 200,000 compounds and identified several molecules, which inhibit CK2 with IC(50) < 1 microM. The binding mode of a representative compound with maize CK2 was determined. In addition, the cellular activity of the compounds was demonstrated by their inhibition of phosphorylation of PTEN Ser370 in HCT116 cells. Treatment of a variety of cancer cell lines with the newly identified CK2 inhibitor significantly blocked cell growth with IC(50)s as low as 300 nM.

Quinalizarin as a potent, selective and cell-permeable inhibitor of protein kinase CK2

Emodin (1,3,8-trihydroxy-6-methyl-anthraquinone) is a moderately potent and poorly selective inhibitor of protein kinase CK2, one of the most pleiotropic serine/threonine protein kinases, implicated in neoplasia and in other global diseases. By virtual screening of the MMS (Molecular Modeling Section) database, we have now identified quinalizarin (1,2,5,8-tetrahydroxyanthraquinone) as an inhibitor of CK2 that is more potent and selective than emodin. CK2 inhibition by quinalizarin is competitive with respect to ATP, with a Ki value of approx. 50 nM. Tested at 1 microM concentration on a panel of 75 protein kinases, quinalizarin drastically inhibits only CK2, with a promiscuity score (11.1), which is the lowest ever reported so far for a CK2 inhibitor. Especially remarkable is the ability of quinalizarin to discriminate between CK2 and a number of kinases, notably DYRK1a (dual-specificity tyrosine-phosphorylated and -regulated kinase), PIM (provirus integration site for Moloney murine leukaemia virus) 1, 2 and 3, HIPK2 (homeodomain-interacting protein kinase-2), MNK1 [MAPK (mitogen-activated protein kinase)-interacting kinase 1], ERK8 (extracellular-signal-regulated kinase 8) and PKD1 (protein kinase D 1), which conversely tend to be inhibited as drastically as CK2 by commercially available CK2 inhibitors. The determination of the crystal structure of a complex between quinalizarin and CK2alpha subunit highlights the relevance of polar interactions in stabilizing the binding, an unusual characteristic for a CK2 inhibitor, and disclose other structural features which may account for the narrow selectivity of this compound. Tested on Jurkat cells, quinalizarin proved able to inhibit endogenous CK2 and to induce apoptosis more efficiently than the commonly used CK2 inhibitors TBB (4,5,6,7-tetrabromo-1H-benzotriazole) and DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole).

Structural insight into human CK2alpha in complex with the potent inhibitor ellagic acid

We determined the 2.35-A crystal structure of a human CK2 catalytic subunit (referred to as CK2alpha complexed with the ATP-competitive, potent CK2 inhibitor ellagic acid. The inhibitor binds to CK2alpha with a novel binding mode, including water-mediated hydrogen bonds. This structural information may support discovery of potent CK2 inhibitors.

First inactive conformation of CK2 alpha, the catalytic subunit of protein kinase CK2

The Ser/Thr kinase casein kinase 2 (CK2) is a heterotetrameric enzyme composed of two catalytic chains (CK2alpha, catalytic subunit of CK2) attached to a dimer of two noncatalytic subunits (CK2beta, noncatalytic subunit of CK2). CK2alpha belongs to the superfamily of eukaryotic protein kinases (EPKs). To function as regulatory key components, EPKs normally exist in inactive ground states and are activated only upon specific signals. Typically, this activation is accompanied by large conformational changes in helix alpha C and in the activation segment, leading to a characteristic arrangement of catalytic key elements. For CK2alpha, however, no strict physiological control of activity is known. Accordingly, CK2alpha was found so far exclusively in the characteristic conformation of active EPKs, which is, in this case, additionally stabilized by a unique intramolecular contact between the N-terminal segment on one side, and helix alpha C and the activation segment on the other side. We report here the structure of a C-terminally truncated variant of human CK2alpha in which the enzyme adopts a decidedly inactive conformation for the first time. In this CK2alpha structure, those regulatory key regions still are in their active positions. Yet the glycine-rich ATP-binding loop, which is normally part of the canonical anti-parallel beta-sheet, has collapsed into the ATP-binding site so that ATP is excluded from binding; specifically, the side chain of Arg47 occupies the ribose region of the ATP site and Tyr50, the space required by the triphospho moiety. We discuss some factors that may support or disfavor this inactive conformation, among them coordination of small molecules at a remote cavity at the CK2alpha/CK2beta interaction region and binding of a CK2beta dimer. The latter stabilizes the glycine-rich loop in the extended active conformation known from the majority of CK2alpha structures. Thus, the novel inactive conformation for the first time provides a structural basis for the stimulatory impact of CK2beta on CK2alpha.