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).

Multiple myeloma cell survival relies on high activity of protein kinase CK2

Casein kinase 2 (CK2) is a ubiquitous cellular serine-threonine kinase that regulates relevant biologic processes, many of which are dysregulated in malignant plasma cells. Here we investigated its role in multiple myeloma (MM). Analysis of MM cell lines and highly purified malignant plasma cells in patients with MM revealed higher protein and CK2 activity levels than in controls (normal in vitro-generated polyclonal plasma cells and B lymphocytes). The inhibition of CK2 with specific synthetic compounds or by means of RNA interference caused a cytotoxic effect on MM plasma cells that could not be overcome by IL-6 or IGF-I and that was associated with the activation of extrinsic and intrinsic caspase cascades. CK2 blockage lowered the sensitivity threshold of MM plasma cells to the cytotoxic effect of melphalan. CK2 inhibition also resulted in impaired IL-6-dependent STAT3 activation and in decreased basal and TNF-alpha-dependent I kappaB alpha degradation and NF-kappaB-driven transcription. Our data show that CK2 was involved in the pathophysiology of MM, suggesting that it might play a crucial role in controlling survival and sensitivity to chemotherapeutics of malignant plasma cells.

Cross talk between protein kinase CK2 and eukaryotic translation initiation factor eIF2beta subunit

The beta-subunit of eukaryotic translation initiation factor eIF2 is a substrate and a partner for protein kinase CK2. Surface plasmon resonance analysis shows that the truncated form corresponding to residues 138-333 of eIF2beta (eIF2beta-CT) interacts with CK2beta as efficiently as full length eIF2beta, whereas the form corresponding to residues 1-137, which contains the CK2 phosphorylation sites, (eIF2beta-NT) does not bind. The use of different mutants and truncated forms of CK2alpha allowed us to map the basic segment K74-K83 at the beginning of helix alphaC and residues R191R195K198 in the p + 1 loop as the main determinants for the binding to eIF2beta-CT of either the isolated CK2alpha subunit or the CK2 holoenzyme. The presence of eIF2beta-CT stimulated the activity of CK2alpha towards the RRRAADSDDDDD peptide substrate; effect that was not observed with the CK2a K74-77A whose ability to bind to eIF2beta-CT is severely impaired. Gel filtration analysis confirmed the ability of CK2alpha to form complexes with eIF2beta-CT, and the contribution of the basic cluster in CK2alpha (K74-K77) in this association.

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.

A protein kinase CK2 site flanking the nuclear targeting signal enhances nuclear transport of human cytomegalovirus ppUL44

The processivity factor of the human cytomegalovirus (HCMV) DNA polymerase phosphoprotein ppUL44 plays an essential role in viral replication, showing nuclear localization in infected cells. The present study examines ppUL44's nuclear import pathway for the first time, ectopic expression of ppUL44 revealing a strong nuclear localization in transfected COS-7 and other cell types, implying that no other HCMV proteins are required for nuclear transportation and retention. We show that of the two potential nuclear localization signals (NLSs) located at amino acids 162-168 (NLS1) and 425-431 (NLS2), NLS2 is necessary and sufficient to confer nuclear localization. Moreover, using enzyme-linked immunosorbent assays and gel mobility shift assays, we show that NLS2 is recognized with high affinity by the importin (IMP) alpha/beta heterodimer. Using gel mobility shift and transient transfection assays, we find that flanking sequences containing a cluster of potential phosphorylation sites, including a consensus site for protein kinase CK2 (CK2) at Ser413 upstream of the NLS, increase NLS2-dependent IMP binding and nuclear localization, suggesting a role for these sites in enhancing UL44 nuclear transport. Results from site-directed mutagenic analysis and live-cell imaging of green fluorescent protein (GFP)-UL44 fusion protein-expressing cells treated with the CK2-specific inhibitor 4,5,6,7-tetrabromobenzotriazole are consistent with phosphorylation of Ser413 enhancing ppUL44 nuclear transport.

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.

Spatial Conformation and Topography of the Tyrosine Aromatic Ring in Substrate Recognition by Protein Tyrosine Kinases

The side chain orientation of the tyrosine residue included in a peptide, which is an excellent substrate of Syk tyrosine kinase, was fixed in different conformations by either incorporating the tyrosine in cyclic structures (6-OH-Tic, 5-OH-Aic, and Hat derivatives) or adding a sterically bulky substituent in the tyrosine side chain moiety (beta-MeTyr). Synthetic peptides containing tyrosine analogues displaying different side chain orientations were analyzed by NMR techniques and tested as potential substrates of the nonreceptor tyrosine kinases Syk, Csk, Lyn, and Fyn. The "rotamer scan" of the phosphorylatable residue generated optimal substrates in terms of both phosphorylation efficiency and selectivity for Syk tyrosine kinase, while the peptidomimetics were not recognized by the other tyrosine kinases. In particular, l-beta-MeTyr and d-Hat containing peptides resulted to be both suitable substrates for the specific monitoring of Syk and consensus sequence scaffolds for the design of potential inhibitors highly selective for this tyrosine kinase.

Chemical derivatization of phosphoserine and phosphothreonine containing peptides to increase sensitivity for MALDI-based analysis and for selectivity of MS/MS analysis

Protein phosphorylation is one of the most important and common ways of regulating protein function in cells. However, phosphopeptides are difficult to analyse, ionising poorly under standard MALDI conditions. Several methods have been developed to deal with the low sensitivity and specificity of phosphopeptide analysis. Here, we show an approach using a simple one-step beta-elimination/Michael addition reaction for the derivatization of phosphoserine and phosphothreonine. The substitution of the negatively charged phosphate group by a positively charged S-ethylpyridyl group greatly improves the ionisation of the modified peptides, especially in MALDI MS, increasing the sensitivity of the analysis. The modification allows the formation of a unique fragment ion at m/z 106 under mild collisional activation conditions, which can be used for parent (precursor) ion scanning in order to improve both the sensitivity and the selectivity of the analysis. The optimisation of the approach is described for a standard model peptide and protein and then applied to phosphorylation analysis in two biologically derived proteins purified from different experimental systems.

4-Fluoroproline derivative peptides: effect on PPII conformation and SH3 affinity

Eukaryotic signal transduction involves the assembly of transient protein-protein complexes mediated by modular interaction domains. Specific Pro-rich sequences with the consensus core motif PxxP adopt the PPII helix conformation upon binding to SH3 domains. For short Pro-rich peptides, little or no ordered secondary structure is usually observed before binding interactions. The association of a Pro-rich peptide with the SH3 domain involves unfavorable binding entropy due to the loss of rotational freedom on forming the PPII helix. With the aim of stabilizing the PPII helix conformation in the Pro-rich HPK1 decapeptide PPPLPPKPKF (P2), a series of P2 analogues was prepared, in which specific Pro positions were alternatively occupied by 4(S)- or 4(R)-4-fluoro-L-proline. The interactions of these peptides with the SH3 domain of the HPK1-binding partner HS1 were quantitatively analyzed by the NILIA-CD approach. A CD thermal analysis of the P2 analogues was performed to assess their propensity to adopt the PPII helix conformation. Contrary to our expectations, the K(d) values of the analogues were lower than that of the parent peptide P2. These results clearly show that the induction of a stable PPII helix conformation in short Pro-rich peptides is not sufficient to increase their affinity toward the SH3 domain and that the effect of 4-fluoroproline strongly depends on the position of this residue in the sequence and the chirality of the substituent in the pyrrolidine ring.

CK2 regulates in vitro the activity of the yeast cyclin-dependent kinase inhibitor Sic1

We have previously demonstrated that the cyclin-dependent kinase inhibitor (Cki) Sic1 of Saccharomyces cerevisiae is phosphorylated in vitro by the CK2 kinase on Ser(201) residue. Moreover, we have collected evidence showing that Sic1 is functionally and structurally related to mammalian Cki p27(Kip1) and binds to the mammalian Cdk2/cyclin A complex with a similar mode of inhibition. In this paper, we use SPR analysis to investigate the binding of Sic1 to the catatytic and regulatory subunits of CK2. Evidence is presented showing that phosphorylation of Sic1 at the CK2 consensus site QES(201)EDEED increases the binding of a Sic1-derived peptide to the Cdk2/cyclin A complex, a functional homologue of the yeast Cdk1/Clb5,6. Moreover, Sic1 fully phosphorylated in vitro on Ser(201) by CK2 is shown to be a stronger inhibitor of the Cdk/cyclin complexes than the unphosphorylated protein. Taken together, these data disclose the possibility that CK2 plays a role in the regulation of Sic1 activity.

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.

Features and potentials of ATP-site directed CK2 inhibitors

A panel of quite specific, fairly potent and cell-permeable inhibitors of protein kinase CK2 belonging to the classes of condensed polyphenolic compounds, tetrabromobenzimidazole/triazole derivatives and indoloquinazolines have been developed, with K(i) values in the submicromolar range. Nine structures have been solved to date of complexes between the catalytic alpha subunit of CK2 and a number of these compounds, many of which display a remarkable specificity toward CK2 as compared to a panel of >30 kinases tested. The structural basis for such selectivity appears to reside in the shape and size of a hydrophobic pocket adjacent to the ATP binding site where these ATP competitive ligands are entrapped mainly by van der Waals interactions and by an energy contribution derived from the hydrophobic effect. In CK2, this cavity is smaller than in the majority of other protein kinases due to a number of unique bulky apolar residues. Consequently, the replacement of two of these residues (V66 and I174) in human CK2 alpha with alanines gives rise to mutants, which are markedly less susceptible than wild type to these classes of inhibitors. Cell-permeable CK2 inhibitors have been successfully employed, either alone or in combination with CK2 mutants refractory to inhibition, to dissect signalling pathways affected by CK2 and/or to validate the identification of in vivo targets of this pleiotropic kinase. Moreover, the remarkable pro-apoptotic efficacy of these compounds toward cell lines derived from a wide spectrum of tumors, disclose the possibility that in perspective CK2 inhibitors might become leads for the development of anti-cancer drugs.

Extracellular phosphorylation of C9 by protein kinase CK2 regulates complement-mediated lysis

Ecto-protein kinases (ecto-PK) are expressed on many cell types, both normal and malignant, yet their functions are largely unknown. An ecto-PK capable of phosphorylating the C9 component of the complement system is described. This C9 ecto-PK could be inhibited by TBB, Emodin and DRB, selective inhibitors of protein kinase CK2. Treatment of Raji human B lymphoma cells with these CK2 inhibitors augmented cell killing by Rituximab (anti-CD20 antibodies) and human complement. Analysis of C5b-7-bearing Raji cells showed that extracellular inhibition of the ecto-CK2 enhanced cell lysis by C8 and C9. Blocking of the membrane complement regulator CD59 with monoclonal antibodies further enhanced the effect of the CK2 inhibitors on Raji cell death by complement. C9 ecto-CK2 activity was increased on cancer cells relative to normal fibroblasts and blood cells. Therefore, ecto-CK2 appears to be an additional factor protecting cells from complement-mediated lysis, probably by phosphorylation/inhibition of complement C9.

Roscovitine targets, protein kinases and pyridoxal kinase

(R)-Roscovitine (CYC202) is often referred to as a "selective inhibitor of cyclin-dependent kinases." Besides its use as a biological tool in cell cycle, neuronal functions, and apoptosis studies, it is currently evaluated as a potential drug to treat cancers, neurodegenerative diseases, viral infections, and glomerulonephritis. We have investigated the selectivity of (R)-roscovitine using three different methods: 1) testing on a wide panel of purified kinases that, along with previously published data, now reaches 151 kinases; 2) identifying roscovitine-binding proteins from various tissue and cell types following their affinity chromatography purification on immobilized roscovitine; 3) investigating the effects of roscovitine on cells deprived of one of its targets, CDK2. Altogether, the results show that (R)-roscovitine is rather selective for CDKs, in fact most kinases are not affected. However, it binds an unexpected, non-protein kinase target, pyridoxal kinase, the enzyme responsible for phosphorylation and activation of vitamin B6. These results could help in interpreting the cellular actions of (R)-roscovitine but also in guiding the synthesis of more selective roscovitine analogs.

Autophosphorylation at the regulatory β subunit reflects the supramolecular organization of protein kinase CK2

Among the features of protein kinase CK2, autophosphorylation at its beta-subunit(s) upon incubation with ATP/Mg++ was early detected as a rapid and stoichiometric event occurring through an intramolecular mechanism as judged from kinetic analyses. The autophosphorylation site was mapped to Ser2 and, to a lesser extent, Ser3 both fulfilling the CK2 consensus sequence (MSSSEEV). The crystal structure of the heterotetrameric holoenzyme, however, is not compatible with an intramolecular autophosphorylation of the N-terminal stretch of either of the two beta subunits. Here we show that efficient "intramolecular" autophosphorylation of the beta subunit is crucially dependent on the formation of oligomers composed by several holoenzyme heterotetrameric protomers. Increasing ionic strength of the incubation medium promoting dissociation of the supramolecular oligomers abrogates beta subunit autophosphorylation, although CK2 catalytic activity, as judged from the phosphorylation of exogenous substrates, is still quite evident. These findings, in conjunction with graphic modelization, support the view that CK2 autophosphorylation at its beta subunits takes place through an "intraoligomeric" mechanism where the beta subunits of a protomer are phosphorylated by the catalytic subunits of another adjacent protomer. It appears therefore that in vivo beta autophosphorylation is symptomatic of supramolecular CK2 oligomers.

Unique Substrate Specificity of Anaplastic Lymphoma Kinase (ALK): Development of Phosphoacceptor Peptides for the Assay of ALK Activity

The anaplastic lymphoma kinase (ALK), whose constitutively active fusion proteins are responsible for 5-10% of non-Hodgkin's lymphomas, shares with the other members of the insulin receptor kinase (IRK) subfamily an activation loop (A-loop) with the triple tyrosine motif Y-x-x-x-Y-Y. However, the amino acid sequence of the ALK A-loop differs significantly from the sequences of both the IRK A-loop and the consensus A-loop for this kinase subfamily. A major difference is the presence of a unique "RAS" triplet between the first and second tyrosines of the ALK A-loop, which in IRK is replaced by "ETD". Here we show that a peptide reproducing the A-loop of ALK is readily phosphorylated by ALK, while a homologous IRK A-loop peptide is not unless its "ETD" triplet is substituted by "RAS". Phosphorylation occurs almost exclusively at the first tyrosine of the Y-x-x-x-Y-Y motif, as judged by Edman analysis of the phosphoradiolabeled product. Consequently, a peptide in which the first tyrosine had been replaced by phenylalanine (FYY) was almost unaffected by ALK. In contrast, a peptide in which the second and third tyrosines had been replaced by phenylalanine (YFF) was phosphorylated more rapidly than the parent peptide (YYY). A number of substitutions in the YFF peptide outlined the importance of Ile and Arg at positions n - 1 and n + 6 in addition to the central triplet, to ensure efficient phosphorylation by ALK. Such a peculiar substrate specificity allows the specific monitoring of ALK activity in crude extracts of NPM-ALK positive cells, using the YFF peptide, which is only marginally phosphorylated by a number of other tyrosine kinases.

Thrombin-induced tyrosine phosphorylation of HS1 in human platelets is sequentially catalyzed by Syk and Lyn tyrosine kinases and associated with the cellular migration of the protein

Thrombin stimulation of platelets triggers Tyr phosphorylation of several signaling proteins, most of which remain unidentified. In this study, we demonstrate for the first time that hematopoietic lineage cell-specific protein 1 (HS1) undergoes a transient Tyr phosphorylation in human platelets stimulated with thrombin. The protein is synergistically phosphorylated by Syk and Lyn tyrosine kinases according to a sequential phosphorylation mechanism. By means of specific inhibitors (PP2, SU6656, and piceatannol) and phosphopeptide-specific antibodies, as well as by coimmunoprecipitation and binding competition experiments, we show that Syk acts as the primary kinase that phosphorylates HS1 at Tyr397 and that Syk phosphorylation is required for HS1 interaction with the Lyn SH2 domain. Upon docking to Syk-phosphorylated HS1, Lyn catalyzes the secondary phosphorylation of the protein at Tyr222. Once the secondary Tyr phosphorylation of HS1 is accomplished the protein dissociates from Lyn and undergoes a dephosphorylation process. HS1 Tyr phosphorylation does not occur when thrombin-induced actin assembly is inhibited by cytochalasin D even under conditions in which Syk and Lyn are still active. Immunofluorescence microscopic analysis shows that the agonist promotes HS1 migration to the plasma membrane and that the inhibition of Lyn-mediated secondary phosphorylation of HS1 abrogates the subcellular translocation of the protein. All together these results indicate that HS1 Tyr phosphorylation catalyzed by Syk and Lyn plays a crucial role in the translocation of the protein to the membrane and is involved in the cytoskeleton rearrangement triggered by thrombin in human platelets.

Chronic lymphocytic leukemia B cells contain anomalous Lyn tyrosine kinase, a putative contribution to defective apoptosis

B cell chronic lymphocytic leukemia (B-CLL) is a neoplastic disorder characterized by accumulation of B lymphocytes due to uncontrolled growth and resistance to apoptosis. Analysis of B cells freshly isolated from 40 patients with chronic lymphocytic leukemia demonstrated that the Src kinase Lyn, the switch molecule that couples the B cell receptor to downstream signaling, displays anomalous properties. Lyn is remarkably overexpressed at the protein level in leukemic cells as compared with normal B lymphocytes, with a substantial aliquot of the kinase anomalously present in the cytosol. Whereas in normal B lymphocytes Lyn activation is dependent on B cell-receptor stimulation, in resting malignant cells, the constitutive activity of the kinase accounts for high basal protein tyrosine phosphorylation and low responsiveness to IgM ligation. Addition of the Lyn inhibitors PP2 and SU6656 to leukemic cell cultures restores cell apoptosis, and treatment of malignant cells with drugs that induce cell apoptosis decreases both activity and amount of the tyrosine kinase. These findings suggest a direct correlation between high basal Lyn activity and defects in the induction of apoptosis in leukemic cells. They also support a critical role for Lyn in B-CLL pathogenesis and identify this tyrosine kinase as a potential therapeutic target.

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.

Involvement of protein kinase CK2 in angiogenesis and retinal neovascularization

PURPOSE

The purpose of the study was to characterize signaling intermediates involved in angiogenic responses of retinal endothelial cells (RECs) to the extracellular matrix and growth factors, by using specific inhibitors.

METHODS

Tubelike structure formation and the development of secondary sprouts on a basement membrane (BM) matrix, cell proliferation, and cell migration were studied in cultures of bovine and human RECs. Specific inhibitors were tested for inhibition of retinal neovascularization in a mouse model of oxygen-induced retinopathy (OIR).

RESULTS

In initial experiments, the broad-spectrum protein kinase inhibitors, H7 and H89, stabilized REC tubes on BM matrix and inhibited secondary sprouting, cell migration, and cell proliferation. Among more specific kinase inhibitors tested, only inhibitors of protein kinase CK2 (formerly, casein kinase II), such as emodin and DRB, were able to duplicate the effects of H7 and H89. Actinomycin D caused only minor changes in angiogenic assays, suggesting that CK2's effects on REC did not involve its known impact on transcription. The extent of retinal neovascularization in a mouse OIR model was reduced >70% (versus untreated or vehicle-treated groups) after treatment with emodin (6 days at 60 mg/kg per day) and by approximately 60% after treatment at the same dose with TBB, the most specific CK2 inhibitor known. In the treated retinas, the main vascular tree had minimal changes, but the neovascular tufts were greatly reduced in number or absent.

CONCLUSIONS

This is the first demonstration of the involvement of ubiquitous protein kinase CK2 in angiogenesis. Naturally derived CK2 inhibitors may be useful for treatment of proliferative retinopathies.

Phosphorylation of calmodulin fragments by protein kinase CK2. Mechanistic aspects and structural consequences

Calmodulin is phosphorylated in vivo and in vitro by protein kinase CK2 in a manner that is unique among CK2 substrates for being inhibited by the regulatory beta-subunit of the kinase and dramatically enhanced by polybasic peptides. Using synthetic fragments of calmodulin variably encompassing the CK2 phosphorylation sites here we show that individual phosphorylation of Thr79, Ser81, Ser101, and Thr117 is critically influenced by the size and composition of the peptides and that the C-terminal domain of calmodulin is implicated both in down-regulation of calmodulin phosphorylation by the beta-subunit and in its abnormal responsiveness to polylysine. A far-Western blot analysis discloses polylysine-dependent interaction between calmodulin and the N-terminal domain of the beta-subunit. We also show that phosphorylation of Ser81 hampers subsequent phosphorylation of Thr79 and by itself promotes the unfolding of the central helix, whose flexibility is instrumental to the interaction with calmodulin-dependent enzymes. Collectively taken, our data are consistent with a multifaceted regulation of calmodulin phosphorylation through the concerted action of distinct CaM domains, the catalytic and regulatory subunits of CK2, and polycationic effectors mimicking in vivo the effect of polylysine.

2-Dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole: a novel powerful and selective inhibitor of protein kinase CK2

Protein kinase CK2 is a highly pleiotropic enzyme whose high constitutive activity is suspected to be instrumental to the enhancement of the tumour phenotype and to the propagation of infectious diseases. Here we describe a novel compound, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), which is superior to the commonly used specific CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) in several respects. DMAT displays the lowest K(i) value ever reported for a CK2 inhibitor (40 nM); it is cell permeable and its efficacy on cultured cells, both in terms of endogenous CK2 inhibition and induction of apoptosis, is several fold higher than that of TBB. The selectivity of DMAT assayed on a panel of >30 protein kinases is comparable to that of TBB, with the additional advantage of being ineffective on protein kinase CK1 up to 200 microM. These properties make DMAT the first choice CK2 inhibitor for in vivo studies available to date.

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.