Selectivity of 4,5,6,7-tetrabromobenzotriazole, an ATP site-directed inhibitor of protein kinase CK2 ('casein kinase-2')

The specificity of 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), an ATP/GTP competitive inhibitor of protein kinase casein kinase-2 (CK2), has been examined against a panel of 33 protein kinases, either Ser/Thr- or Tyr-specific. In the presence of 10 microM TBB (and 100 microM ATP) only CK2 was drastically inhibited (>85%) whereas three kinases (phosphorylase kinase, glycogen synthase kinase 3 beta and cyclin-dependent kinase 2/cyclin A) underwent moderate inhibition, with IC(50) values one--two orders of magnitude higher than CK2 (IC(50)=0.9 microM). TBB also inhibits endogenous CK2 in cultured Jurkat cells. A CK2 mutant in which Val66 has been replaced by alanine is much less susceptible to inhibition by TBB as well as by another ATP competitive inhibitor, emodin. These data show that TBB is a quite selective inhibitor of CK2, that can be used in cell-based assays.

The formation of human populations in South and Central Asia

By sequencing 523 ancient humans, we show that the primary source of ancestry in modern South Asians is a prehistoric genetic gradient between people related to early hunter-gatherers of Iran and Southeast Asia. After the Indus Valley Civilization's decline, its people mixed with individuals in the southeast to form one of the two main ancestral populations of South Asia, whose direct descendants live in southern India. Simultaneously, they mixed with descendants of Steppe pastoralists who, starting around 4000 years ago, spread via Central Asia to form the other main ancestral population. The Steppe ancestry in South Asia has the same profile as that in Bronze Age Eastern Europe, tracking a movement of people that affected both regions and that likely spread the distinctive features shared between Indo-Iranian and Balto-Slavic languages.

Protein kinase CK2 phosphorylates and upregulates Akt/PKB

Treatment of Jurkat cells with specific inhibitors of protein kinase CK2 induces apoptosis. Here we provide evidence that the anti-apoptotic effect of CK2 can be at least partially mediated by upregulation of the Akt/PKB pathway. Such a conclusion is based on the following observations: (1) inhibition of CK2 by cell treatment with two structurally unrelated CK2 inhibitors induces downregulation of Akt/PKB, as judged from decreased phosphorylation of its physiological targets, and immunoprecipitate kinase assay; (2) similar results are observed upon reduction of CK2 catalytic subunit by the RNA-interference technique; (3) Akt/PKB Ser129 is phosphorylated by CK2 in vitro and in vivo; (4) such a phosphorylation of activated Akt/PKB correlates with a further increase in catalytic activity. These data disclose an unanticipated mechanism by which constitutive phosphorylation by CK2 may be required for maximal activation of Akt/PKB.

The Protein Kinase CK2 Facilitates Repair of Chromosomal DNA Single-Strand Breaks

CK2 was the first protein kinase identified and is required for the proliferation and survival of mammalian cells. Here, we have identified an unanticipated role for CK2. We show that this essential protein kinase phosphorylates the scaffold protein XRCC1 and thereby enables the assembly and activity of DNA single-strand break repair protein complexes in vitro and at sites of chromosomal breakage. Moreover, we show that inhibiting XRCC1 phosphorylation by mutation of the CK2 phosphorylation sites or preventing CK2 activity using a highly specific inhibitor ablates the rapid repair of cellular DNA single-strand breaks by XRCC1. These data identify a direct role for CK2 in the repair of chromosomal DNA strand breaks and in maintaining genetic integrity.

Optimization of protein kinase CK2 inhibitors derived from 4,5,6,7-tetrabromobenzimidazole

Casein kinase 2 (CK2) is a ubiquitous, essential, and highly pleiotropic protein kinase whose abnormally high constitutive activity is suspected to underlie its pathogenic potential in neoplasia and infective diseases. Thus, CK2 inhibitors designed to dissect the signaling pathways affected by this kinase, in perspective, may give rise to pharmacological tools. One of the most successful CK2 inhibitors is TBB (4,5,6,7-tetrabromobenzotriazole). Here we show that its inhibitory properties can be markedly improved by generating adducts in which N(2) is replaced by a carbon atom bound to a variety of polar functions. The most efficient inhibitor is 4,5,6,7-tetrabromo-2-(dimethylamino)benzimidazole (2c) followed by the methylsulfanyl (8), isopropylamino (2e), and amino (2a) congeners. All these compounds display K(i) values <100 nM (40 nM in the case of 2c). 2c induces apoptosis of Jurkat cells more readily than TBB (DC(50) value 2.7 vs 17 microM) and, unlike TBB, it does not display any side effect on mitochondria polarization up to 10 microM concentration. Molecular modeling of the CK2-2c complex, based on the crystal structure of the CK2-TBB complex suggests that a number of additional apolar contacts between its two methyl groups and hydrophobic residues nearby could account for its superior inhibitory properties. Consequently, 2c is even more susceptible than TBB to mutations of the unique hydrophobic residues V66 and/or I174 to alanine. We propose to adopt 2c as first choice CK2 inhibitor instead of TBB, especially for in cell studies.

Toward the rational design of protein kinase casein kinase-2 inhibitors

Casein kinase-2 (CK2) probably is the most pleiotropic member of the protein kinase family, with more than 200 substrates known to date. Unlike the great majority of protein kinases, which are tightly regulated enzymes, CK2 is endowed with high constitutive activity, a feature that is suspected to underlie its oncogenic potential and possible implication in viral infections. This makes CK2 an attractive target for anti-neoplastic and antiviral drugs. Here, we present an overview of our present knowledge about CK2 inhibitors, with special reference to the information drawn from two recently solved crystal structures of CK2alpha in complex with emodin and with 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), this latter being the most specific CK2 inhibitor known to date. A comparison with a series of anthraquinone and xanthenone derivatives highlights the crucial relevance of the hydroxyl group at position 3 for inhibition by emodin, and discloses the possibility of increasing the inhibitory potency by placing an electron withdrawing group at position 5. We also present mutational data corroborating the relevance of two hydrophobic residues unique to CK2, Val66 and Ile174, for the interactions with emodin and TBB, but not with the flavonoid inhibitors quercetin and fisetin. In particular, the CK2alpha mutant V66A displays 27- and 11-fold higher IC(50) values with emodin and TBB, respectively, as compared with the wild-type, while the IC(50) value with quercetin is unchanged. The data presented pave the road toward the rational design of more potent and selective inhibitors of CK2 and the generation of CK2 mutants refractory to inhibition, useful to probe the implication of CK2 in specific cellular functions.

The replacement of ATP by the competitive inhibitor emodin induces conformational modifications in the catalytic site of protein kinase CK2

The structure of a complex between the catalytic subunit of Zea mays CK2 and the nucleotide binding site-directed inhibitor emodin (3-methyl-1,6,8-trihydroxyanthraquinone) was solved at 2.6-A resolution. Emodin enters the nucleotide binding site of the enzyme, filling a hydrophobic pocket between the N-terminal and the C-terminal lobes, in the proximity of the site occupied by the base rings of the natural co-substrates. The interactions between the inhibitor and CK2 alpha are mainly hydrophobic. Although the C-terminal domain of the enzyme is essentially identical to the ATP-bound form, the beta-sheet in the N-terminal domain is altered by the presence of emodin. The structural data presented here highlight the flexibility of the kinase domain structure and provide information for the design of selective ATP competitive inhibitors of protein kinase CK2.

Structural features underlying selective inhibition of protein kinase CK2 by ATP site-directed tetrabromo-2-benzotriazole

Two novel crystal structures of Zea mays protein kinase CK2alpha catalytic subunit, one in complex with the specific inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) and another in the apo-form, were solved at 2.2 A resolution. These structures were compared with those of the enzyme in presence of ATP and GTP (the natural cosubstrates) and the inhibitor emodin. Interaction of TBB with the active site of CK2alpha is mainly due to van der Waals contacts, with the ligand fitting almost perfectly the cavity. One nitrogen of the five-membered ring interacts with two charged residues, Glu 81 and Lys 68, in the depth of the cavity, through two water molecules. These are buried in the active site and are also generally found in the structures of CK2alpha enzyme analyzed so far, with the exception of the complex with emodin. In the N-terminal lobe, the position of helix alphaC is particularly well preserved in all the structures examined; the Gly-rich loop is displaced from the intermediate position it has in the apo-form and in the presence of the natural cosubstrates (ATP/GTP) to either an upper (with TBB) or a lower position (with emodin). The selectivity of TBB for CK2 appears to be mainly dictated by the reduced size of the active site which in most other protein kinases is too large for making stable interactions with this inhibitor.

Biochemical and three-dimensional-structural study of the specific inhibition of protein kinase CK2 by [5-oxo-5,6-dihydroindolo-(1,2-a)quinazolin-7-yl]acetic acid (IQA)

IQA [[5-oxo-5,6-dihydro-indolo(1,2-a)quinazolin-7-yl]acetic acid] is a novel ATP/GTP site-directed inhibitor of CK2 ('casein kinase 2'), a pleiotropic and constitutively active protein kinase whose activity is abnormally high in transformed cells. The K (i) value of IQA (0.17 microM) is lower than those of other CK2 inhibitors reported so far. Tested at 10 microM concentration in the presence of 100 microM ATP, IQA almost suppresses CK2 activity in vitro, whereas it is ineffective or weakly effective on a panel of 44 protein kinases and on phosphoinositide 3-kinase. In comparison, other CK2 inhibitors, notably apigenin and quercetin, are more promiscuous. The in vivo efficacy of IQA has been assessed by using the fact that treatment of Jurkat cells with IQA inhibits endogenous CK2 in a dose-dependent manner. IQA has been co-crystallized with maize CK2alpha, which is >70% identical with its human homologue, and the structure of the complex has been determined at 1.68 A (1 A=0.1 nm) resolution. The inhibitor lies in the same plane occupied by the purine moiety of ATP with its more hydrophobic side facing the hinge region. Major contributions to the interaction are provided by hydrophobic forces and non-polar interactions involving the aromatic portion of the inhibitor and the hydrophobic residues surrounding the ATP-binding pocket, with special reference to the side chains of V53 (Val53), I66, M163 and I174. Consequently, mutants of human CK2alpha in which either V66 (the homologue of maize CK2alpha I66) or I174 is replaced by alanine are considerably less sensitive to IQA inhibition when compared with wild-type. These results provide new tools for deciphering the enigmatic role of CK2 in living cells and may pave the way for the development of drugs depending on CK2 activity.

Inhibition of protein kinase CK2 by flavonoids and tyrphostins. A structural insight

Sixteen flavonoids and related compounds have been tested for their ability to inhibit three acidophilic Ser/Thr protein kinases: the Golgi apparatus casein kinase (G-CK) recently identified with protein FAM20C, protein kinase CK1, and protein kinase CK2. While G-CK is entirely insensitive to all compounds up to 40 μM concentration, consistent with the view that it is not a member of the kinome, and CK1 is variably inhibited in an isoform-dependent manner by fisetin and luteolin, and to a lesser extent by myricetin and quercetin, CK2 is susceptible to drastic inhibition by many flavonoids, displaying with six of them IC(50) values < 1 μM. A common denominator of these compounds (myricetin, quercetin, fisetin, kaempferol, luteolin, and apigenin) is a flavone scaffold with at least two hydroxyl groups at positions 7 and 4'. Inhibition is competitive with respect to the phospho-donor substrate ATP. The crystal structure of apigenin and luteolin in complex with the catalytic subunit of Zea mays CK2 has been solved, revealing their ability to interact with both the hinge region (Val116) and the positive area near Lys68 and the conserved water W1, the two main polar ligand anchoring points in the CK2 active site. Modeling experiments account for the observation that luteolin but not apigenin inhibits also CK1. The observation that luteolin shares its pyrocatechol moiety with tyrphostin AG99 prompted us to solve also the structure of this compound in complex with CK2. AG99 was found inside the ATP pocket, consistent with its mode of inhibition competitive with respect to ATP. As in the case of luteolin, the pyrocatechol group of AG99 is critical for binding, interacting with the positive area in the deepest part of the CK2 active site.

Identification of ellagic acid as potent inhibitor of protein kinase CK2: a successful example of a virtual screening application

Casein kinase 2 (CK2) is a ubiquitous, essential, and highly pleiotropic protein kinase whose abnormally high constitutive activity is suspected to underlie its pathogenic potential in neoplasia and other diseases. Using a virtual screening approach, we have identified the ellagic acid, a naturally occurring tannic acid derivative, as a novel potent CK2 inhibitor. At present, ellagic acid represents the most potent known CK2 inhibitor (K(i) = 20 nM).

Tetrabromocinnamic acid (TBCA) and related compounds represent a new class of specific protein kinase CK2 inhibitors

Abnormally high constitutive activity of protein kinase CK2, levels of which are elevated in a variety of tumours, is suspected to underlie its pathogenic potential. The most widely employed CK2 inhibitor is 4,5,6,7-tetrabromobenzotriazole (TBB), which exhibits a comparable efficacy toward another kinase, DYRK1 a. Here we describe the development of a new class of CK2 inhibitors, conceptually derived from TBB, which have lost their potency toward DYRK1 a. In particular, tetrabromocinnamic acid (TBCA) inhibits CK2 five times more efficiently than TBB (IC50 values 0.11 and 0.56 microM, respectively), without having any comparable effect on DYRK1 a (IC50 24.5 microM) or on a panel of 28 protein kinases. The usefulness of TBCA for cellular studies has been validated by showing that it reduces the viability of Jurkat cells more efficiently than TBB through enhancement of apoptosis. Collectively taken, the reported data support the view that suitably derivatized tetrabromobenzene molecules may provide powerful reagents for dissecting the cellular functions of CK2 and counteracting its pathogenic potentials.

Protein kinase CK2 mutants defective in substrate recognition. Purification and kinetic analysis

Five mutants of protein kinase CK2 alpha subunit in which altogether 14 basic residues were singly to quadruply replaced by alanines (K74A,K75A,K76A,K77A; K79A, R80A,K83A; R191A,R195A,K198A; R228A; and R278A, K279A,R280A) have been purified to near homogeneity either as such or after addition of the recombinant beta subunit. By this latter procedure five mutated tetrameric holoenzymes were obtained as judged from their subunit composition, sedimentation coefficient on sucrose gradient ultracentrifugation, and increased activity toward a specific peptide substrate as compared with the isolated alpha subunits. The kinetic constants and the phosphorylation efficiencies (V(max)/Km) of all the mutants with the parent peptide RRRADDSDDDDD and a series of derivatives, in which individual aspartic acids were replaced by alanines, have been determined. Three mutants, namely K74A,K75A,K76A,K77A; K79A, R80A,K83A; and R191A,R195A, K198A, display dramatically lower phosphorylation efficiency and 8-50-fold higher Km values with the parent peptide, symptomatic of reduced attitude to bind the peptide substrate as compared with CK2 wild type. Such differences either disappear or are attentuated if the mutants R191A,R195A, K198A; K79A,R80A,K83A; and K74A,K75A, K76A,K77A are assayed with the peptides RRRADDSADDDD, RRRADDSDDADD, and RRRADDSDDDAA, respectively. In contrast, the phosphorylation efficiencies of the other substituted peptides decrease more markedly with these mutants than with CK2 wild type. These data show that one or more of the basic residues clustered in the 191-198, 79-83, and 74-77 sequences are implicated in the recognition of the acidic determinants at positions +1, +3, and +4/+5, respectively, and that if these residues are mutated, the relevance of the other acidic residues surrounding serine is increased. In contrast the other two mutants, namely R228A and R278A,K279A, R280A, display with all the peptides V(max) values higher than CK2 wild type, counterbalanced however by somewhat higher Km values. It can be concluded from these data that all five mutations performed are compatible with the reconstitution of tetrameric holoenzyme, but all of them influence the enzymatic efficiency of CK2 to different extents. Although the basic residues mutated in the 74-77, 79-83, and 191-198 sequences are clearly implicated in substrate recognition by interacting with acidic determinants at variable positions downstream from serine, the other basic residues seem to play a more elusive and/or indirect role in catalysis.

Inspecting the structure-activity relationship of protein kinase CK2 inhibitors derived from tetrabromo-benzimidazole

CK2 is a very pleiotropic protein kinase whose high constitutive activity is suspected to cooperate to neoplasia. Here, the crystal structure of the complexes between CK2 and three selective tetrabromo-benzimidazole derivatives inhibiting CK2 with Ki values between 40 and 400 nM are presented. The ligands bind to the CK2 active site in a different way with respect to the parent compound TBB. They enter more deeply into the cavity, establishing halogen bonds with the backbone of Glu114 and Val116 in the hinge region. A detailed analysis of the interactions highlights a major role of the hydrophobic effect in establishing the rank of potency within this class of inhibitors and shows that polar interactions are responsible for the different orientation of the molecules in the active site.

Specificity of the polycation-stimulated (type-2A) and ATP,Mg-dependent (type-1) protein phosphatases toward substrates phosphorylated by P34cdc2 kinase

p34cdc2 kinase, a critical regulator of the cell cycle, has been shown to recognize the consensus sequence S/TP in proteins such as histone H1, the retinoblastoma gene product RB and the carboxyl-terminal domain of eukaryotic RNA polymerase II. Using phosphorylated synthetic peptides, representing the p34cdc2 phosphorylation sites in these proteins and histone H1 protein as substrates, we investigated the substrate specificity of the different oligomeric forms of the polycation-stimulated (PCS/type-2A) protein phosphatase and the active catalytic subunit of the ATP,Mg-dependent (AMDc/type 1) protein phosphatase. The results show that the oligomeric structure of the PCS phosphatases is an important determinant for efficient dephosphorylation. The trimeric PCSH1 and PCSM phosphatases are about 10-20-fold-better histone H1 phosphatases than the dimeric PCSH2 and PCSL phosphatases and about 100-fold better than the catalytic subunit (PCSC), suggesting a regulatory role for the 72-kDa, 65-kDa and 55-kDa subunits. The RB peptide = INGS(P)PRT(P)PRRGQNR, is preferred over phosphorylase a (8-fold) by the PCSH1 phosphatase and is about a 40-fold and 95-fold-better substrate for the PCSH1 phosphatase than for the PCSM and PCSL phosphatases, respectively. The primary structure surrounding the S/T(P)P motif, by itself a strong negative determinant for dephosphorylation, can harbour positive features which relieve the constraint imposed by the carboxyl-terminal proline. Thus, the RB peptide INGS(P)PRT(P)PRRGQNR, in which the T(P)P configuration is preferred over the S(P)P sequence, is an extremely good and specific substrate for the PCSH1 phosphatase (Km = 10 microM, Vmax = 3882 nmol.min-1.mg-1). The AMDC phosphatase is a poor phosphatase for all the phosphopeptides tested, unless Mn2+ is added. Its histone H1 phosphatase activity is much less sensitive than its phosphorylase a and phosphopeptide phosphatase activity to inhibition by the modulator or inhibitor-1. The results strongly suggest a role for the trimeric PCSH1 phosphatase in reversing the p34cdc2 phosphorylations.

Protein kinase CK2 as a druggable target

CK2 is probably the most pleiotropic Ser/Thr protein kinase with hundreds of endogenous substrates already known, which are implicated in a variety of cellular functions. At variance with most protein kinases whose activity is turned on only in response to specific stimuli, and whose genetic alterations often underlie pathological situations, CK2 is not susceptible to tight regulation and there are no mutations known to affect its constitutive activity. Nevertheless an abnormally high level of CK2 is invariably found in tumours, and solid arguments have accumulated suggesting that CK2 plays a global pro-survival function, which under special circumstances creates a cellular environment particularly favourable to the development and potentiation of the tumour phenotype. Therefore any strategy aimed at attenuating CK2 activity may represent a "master key" for the treatment of different neoplastic diseases. Waiting for the clarification of the epigenetic mechanisms promoting the rise of CK2 in cells predisposed to develop a tumour phenotype, a useful pharmacological aid can come from the improvement of a number of fairly potent and selective CK2 inhibitors already available.

The ATP-binding site of protein kinase CK2 holds a positive electrostatic area and conserved water molecules

CK2 is a highly pleiotropic Ser/Thr protein kinase that is able to promote cell survival and enhance the tumour phenotype under specific circumstances. We have determined the crystal structure of three new complexes with tetrabromobenzimidazole derivatives that display K(i) values between 0.15 and 0.30 microM. A comparative analysis of these data with those of four other inhibitors of the same family revealed the presence of some highly conserved water molecules in the ATP-binding site. These waters reside near Lys68, in an area with a positive electrostatic potential that is able to attract and orient negatively charged ligands. The presence of this positive region and two unique bulky residues that are typical of CK2, Ile66 and Ile174, play a critical role in determining the ligand orientation and binding selectivity.

ATP site-directed inhibitors of protein kinase CK2: an update

CK2 denotes a pleiotropic, constitutively active protein kinase whose abnormally high level in many cancer cells is held as an example of "non oncogene addiction". A wide spectrum of cell permeable, fairly specific ATP site-directed CK2 inhibitors are currently available which are proving useful to dissect its biological functions and which share the property of inducing apoptosis of cancer cells with no comparable effect on their "normal" counterparts. One of these, CX-4945, has recently entered clinical trials for the treatment of advanced solid tumors, Castelman's disease and multiple myeloma. The solution of a wide range of 3D structures of inhibitors bound to the catalytic subunits of CK2 reveals that their efficacy substantially relies on hydrophobic interactions within a cavity which is smaller than in other protein kinases. Accordingly the potency of tetra-halogenated benzimidazoles increases upon replacement of chlorine by bromine and, even more, by iodine, and decreases if two unique bulky side chains on CK2 (Val66 and Ile174) are mutated to alanines. Many CK2 inhibitors have been tested on a panel of more than 60 kinases providing Promiscuity Scores useful to evaluate their selectivity, the lowest value (9.47), denoting highest selectivity, being displayed by quinalizarin. The observation that CK2 inhibitors with medium/high promiscuity scores share the ability to inhibit a group of protein kinases as effectively as CK2 discloses the possibility of using their scaffolds for the rational development of selective inhibitors of these kinases, with special reference to PIMs, DYRKs, HIPK2, PKD and ERK8.

Development and exploitation of CK2 inhibitors

A number of quite specific and fairly potent inhibitors of protein kinase CK2, belonging to the classes of condensed polyphenolic compounds, tetrabromobenzimidazole/triazole derivatives and indoloquinazolines are available to date. The structural basis for their selectivity is provided by a hydrophobic pocket adjacent to the ATP/GTP binding site, which in CK2 is smaller than in the majority of other protein kinases due to the presence of a number of residues whose bulky side chains are generally replaced by smaller ones. Consequently a doubly substituted CK2 mutant V66A,I174A is much less sensitive than CK2 wild type to these classes of inhibitors. The most efficient inhibitors both in terms of potency and selectivity are 4,5,6,7-tetrabromo-1H-benzotriazole, TBB (Ki = 0.4 microM), the TBB derivative 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole, DMAT (Ki = 0.040 microM), the emodin related coumarinic compound 8-hydroxy-4-methyl-9-nitrobenzo[g]chromen-2-one, NBC (Ki = 0.22 microM) and the indoloquinazoline derivative ([5-oxo-5,6-dihydroindolo-(1,2a)quinazolin-7-yl]acetic acid), IQA (Ki = 0.17 microM). These inhibitors are cell permeable as judged from ability to block CK2 in living cells and they have been successfully employed, either alone or in combination with CK2 mutants refractory to inhibition, to dissect signaling pathways affected by CK2 and to identify the endogenous substrates of this pleitropic kinase. By blocking CK2 these inhibitors display a remarkable pro-apoptotic efficacy on a number of tumor derived cell lines, a property which can be exploited in perspective to develop antineoplastic drugs.

Inhibition of Protein Kinase CK2 by Condensed Polyphenolic Derivatives. An in Vitro and in Vivo Study

ATP site-directed inhibitors that can target individual kinases are powerful tools for use in signal transduction research, all the more so in the case of a pleiotropic, constitutively active protein kinase such as CK2, which is not turned on in response to specific stimuli. By screening a library of more than 200 derivatives of natural polyphenolic compounds, we have identified 16 molecules which inhibit CK2 with IC(50) values of <or=1 microM. They belong to the classes of anthraquinones (six compounds), xanthenones (two compounds), fluorenones (one compound), and coumarins (seven compounds), and their inhibitory potency correlates with the presence of nitro, amino, or halogen substituents at specific positions. Three of the most potent inhibitors, MNX (1,8-dihydroxy-4-nitroxanthen-9-one), NBC (8-hydroxy-4-methyl-9-nitrobenzo[g]chromen-2-one), and DBC (3,8-dibromo-7-hydroxy-4-methylchromen-2-one), whose IC(50) values range between 0.13 and 0.36 microM, are quite specific toward CK2 within a panel of 33 protein kinases tested. Treatment of Jurkat cells with these compounds promotes inhibition of endogenous CK2 and induction of apoptosis. A correlation is observed between their efficacy as CK2 inhibitors (as judged from IC(50) values) and their capacity to induce cell death (DC(50) values). Mutations of the unique CK2alpha residues Val66 and/or Ile174 to alanine have a detrimental effect on inhibition by these compounds with 16-67-fold increases in IC(50) values. The combined usage of these reagents can be exploited to gain information about cellular functions mediated by CK2.

CK2: a protein kinase in need of control

Protein kinase CK2 is a heterotetrameric alpha2beta2 Ser/Thr protein kinase with some features unusual among the eukaryotic protein kinases: (1) CK2 recognizes phosphoacceptor sites specified by several acidic determinants; (2) CK2 can use both ATP and GTP as phosphoryl donors; and (3) the regulatory properties of CK2 are poorly understood; it is insensitive to any known second messenger and displays high basal activity. To gain insight into CK2 regulation and to understand its unusual properties, site-directed mutagenesis experiments on both subunits and X-ray crystallographic studies of the catalytic alpha-subunit were performed. The noncatalytic beta-subunit has at least three functions: (1) it protects the alpha-subunit against denaturing agents or conditions; (2) it alters the substrate specificity of the alpha-subunit; and (3) it modulates the activity of the enzyme, i.e., depending on the substrate, it increases or decreases the activity of the alpha-subunit. Mutagenesis experiments revealed that an acidic stretch between amino acids 55 and 64 has a down-regulatory and autoinhibitory function. Mutational analysis of the alpha-subunit has revealed a network of unique basic residues that are responsible for the recognition of phosphoacceptor substrates and for down-regulation by the beta-subunit and by polyanionic inhibitors. The resolution of the crystal structure of Zea mays CK2 alpha-subunit has disclosed the structural features that are responsible for high basal activity and for unusual response to nucleotide analogs. The increasing knowledge of CK2 structure-function relationships will allow the design of highly selective inhibitors of this pleiotropic kinase with oncogenic potential.

Unique activation mechanism of protein kinase CK2. The N-terminal segment is essential for constitutive activity of the catalytic subunit but not of the holoenzyme

CK2 is an essential, ubiquitous, and highly pleiotropic protein kinase whose catalytic subunits (alpha and alpha') and holoenzyme (composed by two catalytic and two regulatory beta-subunits) are both constitutively active, a property that is suspected to contribute to its pathogenic potential. Extensive interactions between the N-terminal segment and the activation loop are suspected to underlie the high constitutive activity of the isolated catalytic subunit. Here we show that a number of point mutations (Tyr(26) --> Phe, Glu(180) --> Ala, Tyr(182) --> Phe) and deletions (Delta 2-6, Delta 2-12, Delta 2-18, Delta 2-24, Delta 2-30) expected to affect these interactions are more or less detrimental to catalytic activity of the alpha-subunit of human CK2, the deleted mutants Delta 2-24 and Delta 2-30 being nearly inactive under normal assay conditions. Kinetic analyses showed that impaired catalytic activity of mutants Delta 2-12, Delta 2-18, Delta 2-24, and Y182F is mainly accounted for by dramatic increases in the K(m) values for ATP, whereas a drop in K(cat) with K(m) values almost unchanged was found with mutants Y26F and E180A. Holoenzyme reconstitution restored the activity of mutants Delta 2-12, Delta 2-18, Y26F, E180A, and Y182F to wild type level and also conferred catalytic activity to the intrinsically inactive mutants, Delta 2-24 and Delta 2-30. These data demonstrate that specific interactions between the N-terminal segment and the activation loop are essential to provide a fully active conformation to the catalytic subunits of CK2; they also show that these interactions become dispensable upon formation of the holoenzyme, whose constitutive activity is conferred by the beta-subunit through a different mechanism.

Role of CK2 inhibitor CX-4945 in anti-cancer combination therapy - potential clinical relevance

BACKGROUND

Protein kinase CK2 inhibition has long been considered as an attractive anti-cancer strategy based on the following considerations: CK2 is a pro-survival kinase, it is frequently over-expressed in human tumours and its over-expression correlates with a worse prognosis. Preclinical evidence strongly supports the feasibility of this target and, although dozens of CK2 inhibitors have been described in the literature so far, CX-4945 (silmitasertib) was the first that entered into clinical trials for the treatment of both human haematological and solid tumours. However, kinase inhibitor monotherapies turned out to be effective only in a limited number of malignancies, probably due to the multifaceted causes that underlie them, supporting the emerging view that multi-targeted approaches to treat human tumours could be more effective.

CONCLUSIONS

In this review, we will address combined anti-cancer therapeutic strategies described so far which involve the use of CX-4945. Data from preclinical studies clearly show the ability of CX-4945 to synergistically cooperate with different classes of anti-neoplastic agents, thereby contributing to an orchestrated anti-tumour action against multiple targets. Overall, these promising outcomes support the translation of CX-4945 combined therapies into clinical anti-cancer applications.