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

Role of increased guanosine triphosphate cyclohydrolase-1 expression and tetrahydrobiopterin levels upon T cell activation

Tetrahydrobiopterin (BH(4)) is an essential co-factor for the nitric-oxide (NO) synthases, and in its absence these enzymes produce superoxide (O(2)(·-)) rather than NO. The rate-limiting enzyme for BH(4) production is guanosine triphosphate cyclohydrolase-1 (GTPCH-1). Because endogenously produced NO affects T cell function, we sought to determine whether antigen stimulation affected T cell GTPCH-1 expression and ultimately BH(4) levels. Resting T cells had minimal expression of inducible NOS (NOS2), endothelial NOS (NOS3), and GTPCH-1 protein and nearly undetectable levels of BH(4). Anti-CD3 stimulation of T cells robustly stimulated the coordinated expression of NOS2, NOS3, and GTPCH-1 and markedly increased both GTPCH-1 activity and T cell BH(4) levels. The newly expressed GTPCH-1 was phosphorylated on serine 72 and pharmacological inhibition of casein kinase II reduced GTPCH-1 phosphorylation and blunted the increase in T cell BH(4). Inhibition of GTPCH-1 with diaminohydroxypyrimidine (1 mmol/liter) prevented T cell BH(4) accumulation, reduced NO production, and increased T cell O(2)(·-) production, due to both NOS2 and NOS3 uncoupling. GTPCH-1 inhibition also promoted TH(2) polarization in memory CD4 cells. Ovalbumin immunization of mice transgenic for an ovalbumin receptor (OT-II mice) confirmed a marked increase in T cell BH(4) in vivo. These studies identify a previously unidentified consequence of T cell activation, promoting BH(4) levels, NO production, and modulating T cell cytokine production.

Treatment of cultured human astrocytes and vascular endothelial cells with protein kinase CK2 inhibitors induces early changes in cell shape and cytoskeleton

Ubiquitous protein kinase CK2 is a key regulator of cell migration, proliferation and tumor growth. CK2 is abundant in retinal astrocytes, and its inhibition suppresses retinal neovascularization in a mouse retinopathy model. In human astrocytes, CK2 co-distributes with GFAP-containing intermediate filaments, which implies its association with cytoskeleton. Contrary to astrocytes, CK2 is co-localized in microvascular endothelial cells (HBMVEC) with microtubules and actin stress fibers, but not with vimentin-containing intermediate filaments. Specific CK2 inhibitors (TBB, TBI, TBCA and DMAT) and nine novel CK2 inhibiting compounds (TID43, TID46, Quinolone-7, Quinolone-39, FNH28, FNH62, FNH64, FNH68 and FNH74) were tested at 10-200 μM for their ability to induce morphological alterations in cultured human astrocytes (HAST-40), and HBMVEC (For explanation of the inhibitor names, see "Methods" section). CK2 inhibitors caused dramatic changes in shape of cultured cells with effective inhibitor concentrations between 50 and 100 μM. Attached cells retracted, acquired shortened processes, and eventually rounded up and detached. CK2 inhibitor-induced morphological alterations were completely reversible and were not blocked by caspase inhibition. However, longer treatment or higher inhibitor concentration did cause apoptosis. The speed and potency of the CK2 inhibitors effects on cell shape and adhesion were inversely correlated with serum concentration. Western analyses showed that TBB and TBCA elicited a significant (about twofold) increase in the activation of p38 and ERK1/2 MAP kinases that may be involved in cytoskeleton regulation. This novel early biological cell response to CK2 inhibition may underlie the anti-angiogenic effect of CK2 suppression in the retina.

Protein kinase CK2 is a central regulator of topoisomerase I hyperphosphorylation and camptothecin sensitivity in cancer cell lines

Topoisomerase I (topo I) is required to unwind DNA during synthesis and provides the unique target for camptothecin-derived chemotherapeutic agents, including Irinotecan and Topotecan. While these agents are highly effective anticancer agents, some tumors do not respond due to intrinsic or acquired resistance, a process that remains poorly understood. Because of treatment toxicity, there is interest in identifying cellular factors that regulate tumor sensitivity and might serve as predictive biomarkers of therapy sensitivity. Here we identify the serine kinase, protein kinase CK2, as a central regulator of topo I hyperphosphorylation and activity and cellular sensitivity to camptothecin. In nine cancer cell lines and three normal tissue-derived cell lines we observe a consistent correlation between CK2 levels and camptothecin responsiveness. Two other topo I-targeted serine kinases, protein kinase C and cyclin-dependent kinase 1, do not show this correlation. Camptothecin-sensitive cancer cell lines display high CK2 activity, hyperphosphorylation of topo I, elevated topo I activity, and elevated phosphorylation-dependent complex formation between topo I and p14ARF, a topo I activator. Camptothecin-resistant cancer cell lines and normal cell lines display lower CK2 activity, lower topo I phosphorylation, lower topo I activity, and undetectable topo I/p14ARF complex formation. Experimental inhibition or activation of CK2 demonstrates that CK2 is necessary and sufficient for regulating these topo I properties and altering cellular responses to camptothecin. The results establish a cause and effect relationship between CK2 activity and camptothecin sensitivity and suggest that CK2, topo I phosphorylation, or topo I/p14ARF complex formation could provide biomarkers of therapy-responsive tumors.

Threonine 48 in the BIR domain of survivin is critical to its mitotic and anti-apoptotic activities and can be phosphorylated by CK2 in vitro

In this study we report that the protein kinase CK2 phosphorylates survivin specifically on threonine 48 (T48) within its BIR domain, and that T48 is critical to both the mitotic and anti-apoptotic roles of survivin. Interestingly, during mitosis T48 mutants localise normally, but are unable to support cell growth when endogenous survivin is removed by siRNA. In addition, while overexpression of survivin normally confers inhibition of TRAIL-mediated apoptosis, this protection is abolished by mutation of T48. Furthermore in interphase cells depletion of endogenous survivin causes redistribution of T48 mutants from the cytoplasm to the nucleus and treatment of cells expressing survivin-GFP with the CK2 inhibitor TBB phenocopies this nuclear redistribution. Finally, we show T48 mutants have increased affinity for borealin, and that this association and cell proliferation can be restored by introduction of a second mutation at T97. To our knowledge these data are the first to identify T48 as a key regulatory site on survivin, and CK2 as a mediator of its mitotic and anti-apoptotic functions.

Phosphorylation of ARC is a critical element in the antiapoptotic effect of anesthetic preconditioning

BACKGROUND

Transient exposure to volatile anesthetics before cardiac ischemia/reperfusion (I/R), termed anesthetic preconditioning, limits myocardial injury and inhibits apoptosis. Apoptosis repressor with caspase recruitment domain (ARC) is a novel protein that has been demonstrated to protect cardiomyocytes from apoptosis induced by I/R and is regulated by phosphorylation. We therefore hypothesized that the antiapoptotic effect of anesthetic preconditioning is, in part, mediated by phosphorylation of ARC.

METHODS

In the experiments we used a perfused rat heart model of sevoflurane anesthetic preconditioning and I/R. In addition to measures of left ventricular function, phosphorylation of ARC was measured with and without anesthetic preconditioning. Because the phosphorylation status of ARC is determined by calcineurin and protein kinase CK2, the role of ARC was defined by measuring calcineurin activity and using the calcineurin inhibitor FK506 and the ARC phosphorylation inhibitor 4,5,6,7-tetrabromobenzotrizole (TBB).

RESULTS

I/R without anesthetic preconditioning increased calcineurin and reduced ARC phosphorylation levels, whereas anesthetic preconditioning significantly improved functional recovery, decreased ischemic injury, limited the increase in calcineurin activity, increased the phosphorylation level of ARC, reduced cytochrome c release, and blocked the increase in caspase-8 after I/R. The effects of anesthetic preconditioning were mirrored by FK506 and abolished by TBB.

CONCLUSION

This study has identified a novel cardiac pathway in which anesthetic preconditioning prevents the increase in calcineurin after I/R, resulting in increased phosphorylated ARC and decreased markers of apoptosis.

Activation, regulation, and inhibition of DYRK1A

Dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) is a protein kinase with diverse functions in neuronal development and adult brain physiology. Higher than normal levels of DYRK1A are associated with the pathology of neurodegenerative diseases and have been implicated in some neurobiological alterations of Down syndrome, such as mental retardation. It is therefore important to understand the molecular mechanisms that control the activity of DYRK1A. Here we review the current knowledge about the initial self-activation of DYRK1A by tyrosine autophosphorylation and propose that this mechanism presents an ancestral feature of the CMGC group of kinases. However, tyrosine phosphorylation does not appear to regulate the enzymatic activity of DYRK1A. Control of DYRK1A may take place on the level of gene expression, interaction with regulatory proteins and regulated nuclear translocation. Finally, we compare the properties of small molecule inhibitors that target DYRK1A and evaluate their potential application and limitations. The β-carboline alkaloid harmine is currently the most selective and potent inhibitor of DYRK1A and has proven very useful in cellular assays.

Relative role of halogen bonds and hydrophobic interactions in inhibition of human protein kinase CK2α by tetrabromobenzotriazole and some C5-substituted analogues

To examine the relative role of halogen bonding and hydrophobic interactions in the inhibition of human CK2alpha by 4,5,6,7-tetrabromobenzotriazole (TBBt), we have synthesized a series of 5-substituted benzotriazoles (Bt) and the corresponding 5-substituted 4,6,7-tribromobenzotriazoles (Br3Bt) and examined their inhibition of human CK2alpha relative to that of TBBt. The various C(5) substituents differ in size (H and CH3), electronegativity (NH2 and NO2), and hydrophobicity (COOH and Cl). Some substituents were halogen bond donors (Cl, Br), while others were fluorine bond donors (F and CF3). Most of the 5-substituted analogues of Br3Bt (with the exception of COOH and NH2) exhibited inhibitory activity comparable to that of TBBt, whereas the 5-substituted analogues of the parent Bt were only weakly active (Br, Cl, NO2, CF3) or inactive. The observed effect of the volume of a ligand molecule pointed to its predominant role in inhibitory activity, indicating that presumed halogen bonding, identified in crystal structures and by molecular modeling, is dominated by hydrophobic interactions. Extended QSAR analysis additionally pointed to the monoanion and a preference for the N(1)-H protomer of the neutral ligand as parameters crucial for prediction of inhibitory activity. This suggests that the monoanions of TBBt and its congeners are the active forms that efficiently bind to CK2alpha, and the binding affinity is coupled with protomeric equilibrium of the neutral ligand.

Proteomic profile regulated by the anticancer peptide CIGB-300 in non-small cell lung cancer (NSCLC) cells

CIGB-300 is a proapoptotic peptide-based drug that abrogates the CK2-mediated phosphorylation. This peptide has antineoplastic effect on lung cancer cells in vitro and in vivo. To understand the mechanisms involved on such anticancer activity, the NCI-H125 cell line proteomic profile after short-term incubation (45 min) with CIGB-300 was investigated. As determined by 2-DE or 2D-LC-MS/MS, 137 proteins changed their abundances more than 2-fold in response to the CIGB-300 treatment. The expression levels of proteins related to ribosome biogenesis, metastasis, cell survival and proliferation, apoptosis, and drug resistance were significantly modulated by the presence of CIGB-300. The protein translation process was the most affected (23% of the identified proteins). From the proteome analysis of the NCI-H125 cell line, novel potentialities for CIGB-300 as anticancer agent were evidenced.

Discovery of 1H-benzo[d][1,2,3]triazol-1-yl 3,4,5-trimethoxybenzoate as a potential antiproliferative agent by inhibiting histone deacetylase

Twenty-one benzotriazoles (3-16 and 18-24) were synthesized and half of them (5, 8-16, 20, and 21) were reported for the first time. Their antiproliferative activities against three human cancer cells were assayed. It revealed that 1H-benzo[d][1,2,3]triazol-1-yl 3,4,5-trimethoxybenzoate (9) showed considerable activity against three human cancer cell lines with the half maximal inhibitory concentration (IC(50)) values of 1.2-2.4 nM, which were close to the value of the positive control, doxorubicin. Further investigation indicated compound 9 was a potential histone deacetylase inhibitor (IC(50)=9.4 μM) and its binding mode was simulated using docking method.

The 14-3-3ζ protein binds to the cell adhesion molecule L1, promotes L1 phosphorylation by CKII and influences L1-dependent neurite outgrowth

BACKGROUND

The cell adhesion molecule L1 is crucial for mammalian nervous system development. L1 acts as a mediator of signaling events through its intracellular domain, which comprises a putative binding site for 14-3-3 proteins. These regulators of diverse cellular processes are abundant in the brain and preferentially expressed by neurons. In this study, we investigated whether L1 interacts with 14-3-3 proteins, how this interaction is mediated, and whether 14-3-3 proteins influence the function of L1.

METHODOLOGY/PRINCIPAL FINDINGS

By immunoprecipitation, we demonstrated that 14-3-3 proteins are associated with L1 in mouse brain. The site of 14-3-3 interaction in the L1 intracellular domain (L1ICD), which was identified by site-directed mutagenesis and direct binding assays, is phosphorylated by casein kinase II (CKII), and CKII phosphorylation of the L1ICD enhances binding of the 14-3-3 zeta isoform (14-3-3ζ). Interestingly, in an in vitro phosphorylation assay, 14-3-3ζ promoted CKII-dependent phosphorylation of the L1ICD. Given that L1 phosphorylation by CKII has been implicated in L1-triggered axonal elongation, we investigated the influence of 14-3-3ζ on L1-dependent neurite outgrowth. We found that expression of a mutated form of 14-3-3ζ, which impairs interactions of 14-3-3ζ with its binding partners, stimulated neurite elongation from cultured rat hippocampal neurons, supporting a functional connection between L1 and 14-3-3ζ.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that 14-3-3ζ, a novel direct binding partner of the L1ICD, promotes L1 phosphorylation by CKII in the central nervous system, and regulates neurite outgrowth, an important biological process triggered by L1.

Kinome profiling of myxoid liposarcoma reveals NF-kappaB-pathway kinase activity and casein kinase II inhibition as a potential treatment option

BACKGROUND

Myxoid liposarcoma is a relatively common malignant soft tissue tumor, characterized by a (12;16) translocation resulting in a FUS-DDIT3 fusion gene playing a pivotal role in its tumorigenesis. Treatment options in patients with inoperable or metastatic myxoid liposarcoma are relatively poor though being developed and new hope is growing.

RESULTS

Using kinome profiling and subsequent pathway analysis in two cell lines and four primary cultures of myxoid liposarcomas, all of which demonstrated a FUS-DDIT3 fusion gene including one new fusion type, we aimed at identifying new molecular targets for systemic treatment. Protein phosphorylation by activated kinases was verified by Western Blot and cell viability was measured before and after treatment of the myxoid liposarcoma cells with kinase inhibitors. We found kinases associated with the atypical nuclear factor-kappaB and Src pathways to be the most active in myxoid liposarcoma. Inhibition of Src by the small molecule tyrosine kinase inhibitor dasatinib showed only a mild effect on cell viability of myxoid liposarcoma cells. In contrast, inhibition of the nuclear factor-kappaB pathway, which is regulated by the FUS-DDIT3 fusion product, in myxoid liposarcoma cells using casein kinase 2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) showed a significant decrease in cell viability, decreased phosphorylation of nuclear factor-kappaB pathway proteins, and caspase 3 mediated apoptosis. Combination of dasatinib and TBB showed an enhanced effect.

CONCLUSION

Kinases associated with activation of the atypical nuclear factor-kappaB and the Src pathways are the most active in myxoid liposarcoma in vitro and inhibition of nuclear factor-kappaB pathway activation by inhibiting casein kinase 2 using TBB, of which the effect is enhanced by Src inhibition using dasatinib, offers new potential therapeutic strategies for myxoid liposarcoma patients with advanced disease.

Casein kinase 2 regulates the NR2 subunit composition of synaptic NMDA receptors

N-methyl-D-aspartate (NMDA) receptors (NMDARs) play a central role in development, synaptic plasticity, and neurological disease. NMDAR subunit composition defines their biophysical properties and downstream signaling. Casein kinase 2 (CK2) phosphorylates the NR2B subunit within its PDZ-binding domain; however, the consequences for NMDAR localization and function are unclear. Here we show that CK2 phosphorylation of NR2B regulates synaptic NR2B and NR2A in response to activity. We find that CK2 phosphorylates NR2B, but not NR2A, to drive NR2B-endocytosis and remove NR2B from synapses resulting in an increase in synaptic NR2A expression. During development there is an activity-dependent switch from NR2B to NR2A at cortical synapses. We observe an increase in CK2 expression and NR2B phosphorylation over this same critical period and show that the acute activity-dependent switch in NR2 subunit composition at developing hippocampal synapses requires CK2 activity. Thus, CK2 plays a central role in determining the NR2 subunit content of synaptic NMDARs.

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.

CK2 phosphorylation of Pdx-1 regulates its transcription factor activity

The duodenal homeobox-1 protein Pdx-1 is one of the regulators for the transcription of the insulin gene. Pdx-1 is a phosphoprotein, and there is increasing evidence for the regulation of some of its functions by phosphorylation. Here, we asked whether protein kinase CK2 might phosphorylate Pdx-1 and how this phosphorylation could be implicated in the functional regulation of Pdx-1. We used fragments of Pdx-1 as well as phosphorylation mutants for experiments with protein kinase CK2. Transactivation was measured by reporter assays using the insulin promoter. Our data showed that Pdx-1 is phosphorylated by protein kinase CK2 at amino acids thr(231) and ser(232), and this phosphorylation was implicated in the regulation of the transcription factor activity of Pdx-1. Furthermore, inhibition of protein kinase CK2 by specific inhibitors led to an elevated release of insulin from pancreatic beta-cells. Thus, these findings identify CK2 as a novel mediator of the insulin metabolism.

Reconstitution of PTEN activity by CK2 inhibitors and interference with the PI3-K/Akt cascade counteract the antiapoptotic effect of human stromal cells in chronic lymphocytic leukemia

Evidence suggests that tumor microenvironment is critically involved in supporting survival of chronic lymphocytic leukemia (CLL) cells. However, the molecular mechanisms of this effect and the clinical significance are not fully understood. We applied a microenvironment model to explore the interaction between CLL cells and stromal cells and to elucidate the role of phosphatidylinositol 3 kinase (PI3-K)/Akt/phosphatase and tensin homolog detected on chromosome 10 (PTEN) cascade in this process and its in vivo relevance. Primary human stromal cells from bone marrow, lymph nodes, and spleen significantly inhibited spontaneous apoptosis of CLL cells. Pan-PI3-K inhibitors (LY294002, wortmannin, PI-103), isotype-specific inhibitors of p110α, p110β, p110γ, and small interfering RNA against PI3-K and Akt1 counteracted the antiapoptotic effect of the stromal cells. Induction of apoptosis was associated with a decrease in phosphatidylinositol-3,4,5-triphosphate, PI3-K-p85, and dephosphorylation of phosphatidylinositol-dependent kinase-1 (PDK-1), Akt1, and PTEN. Freshly isolated peripheral blood mononuclear cells from patients with CLL (n = 44) showed significantly higher levels of phosphorylated Akt1, PDK-1, PTEN, and CK2 than healthy persons (n = 8). CK2 inhibitors (4,5,6,7-tetrabromo-1H-benzotriazole, apigenin, and 5,6-dichloro-1-β-D-ribofuranosylbenzimidazol) decreased phosphorylation of PTEN and Akt, induced apoptosis in CLL cells, and enhanced the response to fludarabine. In conclusion, bone marrow microenvironment modulates the PI3-K/Akt/PTEN cascade and prevents apoptosis of CLL cells. Combined inhibition of PI3-K/Akt and recovery of PTEN activity may represent a novel therapeutic concept for CLL.

Protein kinase CK2-mediated phosphorylation of HDAC2 regulates co-repressor formation, deacetylase activity and acetylation of HDAC2 by cigarette smoke and aldehydes

Histone deacetylase 2 (HDAC2) mediates the repression of pro-inflammatory genes by deacetylating core histones, RelA/p65 and the glucocorticoid receptor. Reduced level of HDAC2 is associated with steroid resistant inflammation caused by cigarette smoke (CS)-derived oxidants and aldehydes. However, the molecular mechanisms regulating HDAC2 in response to CS and aldehydes is not known. Here, we report that CS extract, and aldehyde acrolein induced phosphorylation of HDAC2 which was abolished by mutations at serine sites S(394), S(411), S(422) and S(424). HDAC2 phosphorylation required direct interaction with serine-phosphorylated protein kinase CK2alpha and involved reduced HDAC2 deacetylase activity. Furthermore, HDAC2 phosphorylation was required for HDAC2 interaction with transcription factors, co-repressor complex formation, CBP recruitment, acetylation on lysine residues and modulates transrepression activity. Thus, phospho-acetylation of HDAC2 negatively regulates its deacetylase activity which has implications in steroid resistance in chronic inflammatory conditions.

A CE-based assay for human protein kinase CK2 activity measurement and inhibitor screening

A new assay for protein kinase CK2 activity determination based on the quantification of a phosphorylated substrate was developed. The common CK2 substrate peptide RRRDDDSDDD, conjugated with the fluorophore 5-[(2-aminoethyl)amino]naphthalene-1-sulfonic acid at the C-terminus served as the analyte. By means of CZE using 2 mol/L acetic acid as electrolyte and UV detection at 214 nm, the non-phosphorylated and the phosphorylated peptide variants could be resolved within 6 min from a complex assay mixture. By this means, activity of human CK2 could be monitored by a kinetic, as well as an endpoint, method. Inhibition of human recombinant CK2 holoenzyme by 6-methyl-1,3,8-trihydroxyanthraquinone and 4,5,6,7-tetrabromobenzotriazole resulted in IC(50) values of 1.33 and 0.27 microM, respectively, which were similar to those obtained with the standard radiometric assay. These results suggest that the CE/UV strategy described here is a straightforward assay for CK2 inhibitor testing.

DYRK1A and glycogen synthase kinase 3beta, a dual-kinase mechanism directing proteasomal degradation of CRY2 for circadian timekeeping

Circadian molecular oscillation is generated by a transcription/translation-based feedback loop in which CRY proteins play critical roles as potent inhibitors for E-box-dependent clock gene expression. Although CRY2 undergoes rhythmic phosphorylation in its C-terminal tail, structurally distinct from the CRY1 tail, little is understood about how protein kinase(s) controls the CRY2-specific phosphorylation and contributes to the molecular clockwork. Here we found that Ser557 in the C-terminal tail of CRY2 is phosphorylated by DYRK1A as a priming kinase for subsequent GSK-3beta (glycogen synthase kinase 3beta)-mediated phosphorylation of Ser553, which leads to proteasomal degradation of CRY2. In the mouse liver, DYRK1A kinase activity toward Ser557 of CRY2 showed circadian variation, with its peak in the accumulating phase of CRY2 protein. Knockdown of Dyrk1a caused abnormal accumulation of cytosolic CRY2, advancing the timing of a nuclear increase of CRY2, and shortened the period length of the cellular circadian rhythm. Expression of an S557A/S553A mutant of CRY2 phenocopied the effect of Dyrk1a knockdown in terms of the circadian period length of the cellular clock. DYRK1A is a novel clock component cooperating with GSK-3beta and governs the Ser557 phosphorylation-triggered degradation of CRY2.

Phosphorylation regulates removal of synaptic N-methyl-D-aspartate receptors after withdrawal from chronic ethanol exposure

Alterations in N-methyl-d-aspartate receptor (NMDAR) protein levels or subcellular localization in brain after chronic ethanol exposure may contribute to withdrawal-associated seizures and neurotoxicity. We have investigated synaptic localization of NMDARs in cultured hippocampal pyramidal neurons after prolonged (7 days) exposure to, and acute withdrawal from, 80 mM ethanol using fluorescence immunocytochemistry techniques. After chronic ethanol exposure, there was a significant increase in the clustering of NR1 and NR2B subunits and their colocalization with the synaptic proteins synaptophysin and postsynaptic density protein 95, respectively. There was also increased expression of NR1 variants containing the C2' cassette after chronic ethanol exposure. The ethanol-induced synaptic clustering and colocalization were rapidly reversed within 4 h after ethanol withdrawal. Surface labeling of NR2B subunits suggested that this rapid reversal involved lateral receptor movement to extrasynaptic sites rather than internalization of receptors. Receptor removal from the synapse during ethanol withdrawal was associated with changes in the phosphorylation state of NR2B Ser1480, controlled by the protein kinase CK2. The redistribution of NMDAR to synapses produced by long-term ethanol exposure, as well as the rapid removal during withdrawal, may not only affect neuronal withdrawal hyperexcitability but also may sensitize the system to subsequent synaptic plasticity.

p53 is dispensable for the induction of apoptosis after inhibition of protein kinase CK2

BACKGROUND

Protein kinase CK2 is a ubiquitously expressed heterotetramer consisting of two catalytic alpha/alpha' and two regulatory beta subunits. Expression of CK2 is highly elevated in tumor cells where it protects cells from apoptosis. A variety of different compounds were tested as inhibitors of protein kinase CK2 in order to find new therapy strategies. To analyze the role of p53 in the response to CK2 inhibition we used one of the most specific CK2 inhibitors available, TBB, in different prostate cancer cell lines.

METHODS

We treated prostate cancer cells with the CK2 inhibitor TBB and determined its effect on CK2 activity by an in vitro phosphorylation assay and its effect on viability by an MTT assay. Furthermore, we analyzed changes in the expression of p53 and PARP cleavage by Western Blot analysis.

RESULTS

Inhibition of CK2 by TBB led to a decrease in cell viability and apoptosis in two cell lines which express wild-type p53 whereas two other cell lines expressing mutant or no p53 failed to show signs of apoptosis. Moreover, cell lines expressing wild-type p53 showed an increase of the amount of p53 and of its transactivation efficiency. However, down-regulation of p53 by RNAi showed that p53 is not necessary for the induction of apoptosis.

CONCLUSIONS

Wild-type p53 is not necessary for the induction of apoptosis by TBB in prostate cancer cells.

Assessment of CK2 constitutive activity in cancer cells

At variance with the great majority of protein kinases that become active only in response to specific stimuli and whose implication in tumors is caused by genetic alterations conferring to them unscheduled activity, the highly pleiotropic Ser/Thr-specific protein kinase CK2 is constitutively active even under normal conditions and no gain-of-function CK2 mutants are known. Nevertheless, CK2 level is abnormally high in cancer cells where it is believed to generate an environment favorable to the development of malignancy, through a mechanism denoted as "non-oncogene addiction." This makes CK2 not only an appealing target to counteract different kinds of tumors but also a valuable marker of cells predisposed to undergo neoplastic transformation owing to the presence in them of CK2 level exceeding a critical threshold. Such a prognostic exploitation of CK2 would imply the availability of methods suitable for the reliable, sensitive, and specific quantification of its activity in biological samples and in living cells. The aim of this chapter is to describe a number of procedures applicable to the quantitative determination of CK2 activity and to provide experimental details designed for rendering these assays as sensitive and selective as possible even in the presence of many other protein kinases. The procedures described roughly fall in three categories: (i) in vitro quantification of CK2 activity in crude biological samples and cell lysates; (ii) in-cell assay of endogenous CK2 activity based on the phosphorylation of reporter substrates; (iii) identification of CK2 targets in malignant and normal cells.

Casein kinase activity in etiolated Cucumis sativus cotyledons

Two calcium- and light-dependent protein kinases have been reported in etiolated Cucumis sativus cotyledons (Vidal et al. 2007). In the present work, we studied casein kinase (CK) activity in etiolated cucumber cotyledons of in-gel and in vitro kinase assays, using specific CK inhibitors, and ATP and GTP as phosphate donors. Two proteins with CK activity were detected in both casein gels and autophosphorylation assays. One of them, with a molecular mass of approximately 36 kDa, showed biochemical CK1 characteristics: it was inhibited by specific CK1 inhibitors and only used ATP as phosphate donor. The second, with a molecular mass of approximately 38 kDa, had CK2 characteristics; it used both ATP and GTP as phosphate donors, was inhibited by all specific CK2 inhibitors, and was recognized by a polyclonal antibody directed against the alpha catalytic subunit of a CK2 from tobacco. The kinase activity of the CK2 detected in etiolated cucumber cotyledons showed circadian rhythmicity in both in vitro and in-gel casein phosphorylation and in autophosphorylation assays. Thus, our results suggest that the CK2 of approximately 38 kDa could be related to the circadian oscillator of C. sativus cotyledons.

Inhibition of protein kinase CK2 closes the CFTR Cl channel, but has no effect on the cystic fibrosis mutant deltaF508-CFTR

BACKGROUND

Deletion of phenylalanine-508 (DeltaF508) from the first nucleotide-binding domain (NBD1) in the wild-type cystic fibrosis (CF) transmembrane-conductance regulator (wtCFTR) causes CF. However, the mechanistic relationship between DeltaF508-CFTR and the diversity of CF disease is unexplained. The surface location of F508 on NBD1 creates the potential for protein-protein interactions and nearby, lies a consensus sequence (SYDE) reported to control the pleiotropic protein kinase CK2.

METHODS

Electrophysiology, immunofluorescence and biochemistry applied to CFTR-expressing cells, Xenopus oocytes, pancreatic ducts and patient biopsies.

RESULTS

Irrespective of PKA activation, CK2 inhibition (ducts, oocytes, cells) attenuates CFTR-dependent Cl(-) transport, closing wtCFTR in cell-attached membrane patches. CK2 and wtCFTR co-precipitate and CK2 co-localized with wtCFTR (but not DeltaF508-CFTR) in apical membranes of human airway biopsies. Comparing wild-type and DeltaF508CFTR expressing oocytes, only DeltaF508-CFTR Cl(-) currents were insensitive to two CK2 inhibitors. Furthermore, wtCFTR was inhibited by injecting a peptide mimicking the F508 region, whereas the DeltaF508-equivalent peptide had no effect.

CONCLUSIONS

CK2 controls wtCFTR, but not DeltaF508-CFTR. Others find that peptides from the F508 region of NBD1 allosterically control CK2, acting through F508. Hence, disruption of CK2-CFTR interaction by DeltaF508-CFTR might disrupt multiple, membrane-associated, CK2-dependent pathways, creating a new molecular disease paradigm for deleted F508 in CFTR.

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.

Dephosphorylation and inactivation of Akt/PKB is counteracted by protein kinase CK2 in HEK 293T cells

Akt (PKB) is a critical kinase in cell-survival pathways. Its activity depends on the phosphorylation of Thr308 and Ser473, by PDK1 and mTORC2, respectively. We found that Akt can be further stimulated through phosphorylation of Ser129 by another kinase, CK2. Here we show that phosphorylation of Akt at Ser129 also facilitates its association with Hsp90 chaperone, thus preventing Thr308 dephosphorylation. This is supported by the following observations: (1) phospho-Thr308 decreases when Ser129 is mutated to alanine, (2) this decrease is abolished by cell treatment with okadaic acid (to inactivate PP2A) or geldanamycin (to inactivate Hsp90), (3) phosphorylation of Ser129 neither enhances the activity of PDK1 nor hampers the in vitro activity of PP2A on Thr308, but increases the Hsp90 association to Akt. These data support the view that the antiapoptotic potential of CK2 is at least in part mediated by its ability to maintain Akt in its active form.

Temporal assessment of histone H3 phospho-acetylation and casein kinase 2 activation in dentate gyrus from ischemic rats

Hippocampal dentate gyrus possesses an exceptional capacity of adaptation to ischemic insults. Recently, using a transient global ischemic model in the adult rat, we identified a neuroprotective signalling cascade in the dentate gyrus involving calcium/calmodulin-dependent protein kinase IV (CaMKIV), cyclic AMP response element (CRE)-binding protein (CREB) and brain-derived neurotrophic factor (BDNF), a major regulator of survival. We have shown that intracerebroventricular injections of anti-BDNF and anti-CREB are sufficient to cause substantial tissular damages and apoptotic deaths in late periods (48-72 h) after ischemia. Herein, we provide immunohistochemical and biochemical evidence that antibody-induced impairment of the protective CaMKIV/CREB/BDNF pathway induces an apparent duality of response in the dentate gyrus. The experimental protocol is performed as follows: (a) rats are anesthetized and vertebral arteries are occluded by electrocauterization; (b) on the following day, transient global ischemia is produced by occlusion of carotid arteries for 25 min; (c) finally, rats are infused with the pharmacologic agents into the left cerebral ventricle and then perfusion-fixed at different time points after ischemia for immunohistochemical and immunoblotting analyses. After infusion with anti-CaMKIV, phosphorylation of mitogen-activated protein kinases (MAPK) MKK3, MKK6 and p38 and phospho-acetylation of histone H3 occur at 6 h after ischemia without presence of any caspase-9 activation and cellular injuries. In contrast, infusion of anti-BDNF or anti-CREB surprisingly results in a remarkable stimulation of casein kinase 2 (CK2) and caspase-9 activities at 48-72 h post-insult. This is accompanied by the disappearance of phosphorylation of MKK(3/6) and p38 and phospho-acetylation of histone H3. These results suggest that: (1) activation of a MKK(3/6)/p38/H3 cascade at early periods post-ischemia may be capable of causing a short transient protective effect in the dentate gyrus; (2) CK2 might be implicated in inhibition of activity of molecules such as MKK(3/6), p38 and deacetylases at late periods post-insult, thereby promoting injuries and cell deaths in the dentate cell layer.

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).

Progress in studies on the DEK protein and its involvement in cellular apoptosis

DEK protein is an ubiquitous phosphorylated nuclear protein. Specific binding of DEK to DNA could change the topology of DNA and then affect the gene activity of the underlying DNA sequences. It is speculated that there might be some potential relationship between the stress reaction of cells and DEK proteins. The phosphorylation status of DEK protein is altered during death-receptor-mediated cell apoptosis. Both phosphorylation and poly(ADP-ribosyl)ation could promote the release of DEK from apoptotic nuclei to extracellular environment, and in this case DEK becomes a potential autoantigen of some autoimmune diseases. The available evidence powerfully suggests that DEK protein is closely relevant to apoptosis. The overexpression of DEK protein has dual function in cell apoptosis, in terms of inhibiting or triggering cell apoptosis.

Exploring the binding of inhibitors derived from tetrabromobenzimidazole to the CK2 protein using a QM/MM-PB/SA approach

We present an adaptation of the MM-PB/SA method for the estimation of binding free energies in protein-ligand complexes simulated with QM/MM molecular dynamics. The method is applied to understand the binding of a set of tetrabromobenzimidazole inhibitors to the CK2 protein. We find that the QM/MM interaction energy alone cannot always be used as a predictor of the binding affinity, and the inclusion of solvation effects via the PB/SA method is essential in getting reliable results. In agreement with experimental observations, we show that the van der Waals interactions are the driving force for the binding, while the electrostatic interactions orient these inhibitors in the CK2 active site. Additionally a per-residue energy decomposition analysis was applied to determine the individual contributions to the protein-inhibitor interaction. Based on these results, we hypothesize that the inclusion of a sufficiently large polar group on the tetrabromobenzimidazole skeleton could increase the binding affinity. The results show that the QM/MM-PB/SA method can be successfully employed to understand complicated structure-activity relationships and to design new inhibitors.

CAPS activity in priming vesicle exocytosis requires CK2 phosphorylation

CAPS (Ca(2+)-dependent activator protein for secretion) functions in priming Ca(2+)-dependent vesicle exocytosis, but the regulation of CAPS activity has not been characterized. Here we show that phosphorylation by protein kinase CK2 is required for CAPS activity. Dephosphorylation eliminated CAPS activity in reconstituting Ca(2+)-dependent vesicle exocytosis in permeable and intact PC12 cells. Ser-5, -6, and -7 and Ser-1281 were identified by mass spectrometry as the major phosphorylation sites in the 1289 residue protein. Ser-5, -6, and -7 but not Ser-1281 to Ala substitutions abolished CAPS activity. Protein kinase CK2 phosphorylated CAPS in vitro at these sites and restored the activity of dephosphorylated CAPS. CK2 is the likely in vivo CAPS protein kinase based on inhibition of phosphorylation by tetrabromo-2-benzotriazole in PC12 cells and by the identity of in vivo and in vitro phosphorylation sites. CAPS phosphorylation by CK2 was constitutive, but the elevation of Ca(2+) in synaptosomes increased CAPS Ser-5 and -6 dephosphorylation, which terminates CAPS activity. These results identify a functionally important N-terminal phosphorylation site that regulates CAPS activity in priming vesicle exocytosis.

Structure-based discovery of triphenylmethane derivatives as inhibitors of hepatitis C virus helicase

Hepatitis C virus nonstructural protein 3 (HCV NS3) helicase is believed to be essential for viral replication and has become an attractive target for the development of antiviral drugs. A fluorescence resonant energy transfer helicase assay was established for fast screening of putative inhibitors selected from virtual screening using the program DOCK. Soluble blue HT (1) was first identified as a novel HCV helicase inhibitor. Crystal structure of the NS3 helicase in complex with soluble blue HT shows that the inhibitor bears a significantly higher binding affinity mainly through a 4-sulfonatophenylaminophenyl group, and this is consistent with the activity assay. Subsequently, fragment-based searches were utilized to identify triphenylmethane derivatives for more potent inhibitors. Lead optimization resulted in a 3-bromo-4-hydroxyl substituted derivative 12 with an EC(50) value of 2.72 microM to Ava.5/Huh-7 cells and a lower cytotoxicity to parental Huh-7 cells (CC(50) = 10.5 microM), and it indeed suppressed HCV replication in the HCV replicon cells. Therefore, these inhibitors with structural novelty may serve as a useful scaffold for the discovery of new HCV NS3 helicase inhibitors.

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.

Dynamics of outer mitochondrial membrane permeabilization during apoptosis

Individual cells within a population undergo apoptosis at distinct, apparently random time points. By analyzing cellular mitotic history, we identified that sibling HeLa cell pairs, in contrast to random cell pairs, underwent apoptosis synchronously. This allowed us to use high-speed cellular imaging to investigate mitochondrial outer membrane permeabilization (MOMP), a highly coordinated, rapid process during apoptosis, at a temporal resolution approximately 100 times higher than possible previously. We obtained new functional and mechanistic insight into the process of MOMP: We were able to determine the kinetics of pore formation in the outer mitochondrial membrane from the initiation phase of cytochrome-c-GFP redistribution, and showed differential pore formation kinetics in response to intrinsic or extrinsic apoptotic stimuli (staurosporine, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)). We also detected that the onset of mitochondrial permeabilization frequently proceeded as a wave through the cytosol, and that the frequency of wave occurrence in response to TRAIL was reduced by inhibition of protein kinase CK2. Computational analysis by a partial differential equation model suggested that the spread of permeabilization signals could sufficiently be explained by diffusion-adsorption velocities of locally generated permeabilization inducers. Taken together, our study yielded the first comprehensive analysis of clonal cell-to-cell variability in apoptosis execution and allowed to visualize and explain the dynamics of MOMP in cells undergoing apoptosis.

Histone deacetylase 2 is phosphorylated, ubiquitinated, and degraded by cigarette smoke

Cigarette smoke (CS)-induced lung inflammation involves the reduction of histone deacetylase 2 (HDAC2) abundance, which is associated with steroid resistance in patients with chronic obstructive pulmonary disease and in individuals with severe asthma who smoke cigarettes. However, the molecular mechanism of CS-mediated reduction of HDAC2 is not clearly known. We hypothesized that HDAC2 is phosphorylated and subsequently degraded by the proteasome in vitro in macrophages (MonoMac6), human bronchial and primary small airway epithelial cells, and in vivo in mouse lungs in response to chronic CS exposure. Cigarette smoke extract (CSE) exposure in MonoMac6 and in bronchial and airway epithelial cells led to phosphorylation of HDAC2 on serine/threonine residues by a protein kinase CK2-mediated mechanism, decreased HDAC2 activity, and increased ubiquitin-proteasome-dependent HDAC2 degradation. CK2 and proteasome inhibitors reversed CSE-mediated HDAC2 degradation, whereas serine/threonine phosphatase inhibitor, okadaic acid, caused phosphorylation and subsequent ubiquitination of HDAC2. CS-induced HDAC2 phosphorylation was detected in mouse lungs from 2 weeks to 4 months of CS exposure, and mice showed significantly lower lung HDAC2 levels. Thus, CS-mediated down-regulation of HDAC2 in human macrophages and lung epithelial cells in vitro and in mouse lung in vivo involves the induction of serine/threonine phosphorylation and proteasomal degradation, which may have implications for steroid resistance and abnormal inflammation caused by cigarette smoke.

Phospho-regulated SUMO interaction modules connect the SUMO system to CK2 signaling

Attachment of SUMO to proteins regulates protein-protein interactions through noncovalent binding of the SUMO moiety to specialized SUMO interaction motifs (SIMs). A core of hydrophobic amino acids has been described as the major determinant of SIM function. Using the transcriptional coregulator and SUMO ligase PIAS1 as a model, we define an extended phospho-regulated SIM module. We show that serine residues adjacent to the hydrophobic core are phosphorylated by CK2 and demonstrate that this dictates binding of free SUMO and SUMO conjugates to PIAS1 in vivo. We provide evidence that the phosphorylated residues contact lysine 39 and 35 in SUMO1 and SUMO2, respectively. Phospho-dependent SUMO binding does not impair the ligase activity but affects the transcriptional coregulatory potential of PIAS1 and other PIAS family members. CK2-regulated phosphoSIM modules were also dissected in the tumor suppressor PML and the exosome component PMSCL1, indicating that these modules serve as general platforms that integrate CK2- and SUMO-regulated signaling networks.

FKBP25, a novel regulator of the p53 pathway, induces the degradation of MDM2 and activation of p53

The p53 tumour suppressor protein is tightly controlled by the E3 ubiquitin ligase, mouse double minute 2 (MDM2), but maintains MDM2 expression as part of a negative feedback loop. We have identified the immunophilin, 25kDa FK506-binding protein (FKBP25), previously shown to be regulated by p53-mediated repression, as an MDM2-interacting partner. We show that FKBP25 stimulates auto-ubiquitylation and proteasomal degradation of MDM2, leading to the induction of p53. Depletion of FKBP25 by siRNA leads to increased levels of MDM2 and a corresponding reduction in p53 and p21 levels. These data are consistent with the idea that FKBP25 contributes to regulation of the p53-MDM2 negative feedback loop.

Synthesis of new analogs of benzotriazole, benzimidazole and phthalimide--potential inhibitors of human protein kinase CK2

New derivatives of 4,5,6,7-tetrabromo-1H-1,2,3-benzotriazole (TBBt), 4,5,6,7-tetrabromo-1H-benzimidazole (TBBi), and N-substituted tetrabromophthalimides were synthesized and their effect on the activity of human protein kinase CK2 was examined. The most active were derivatives with N-hydroxypropyl substituents (IC(50) in 0.32-0.54 microM range) whereas derivatives of phthalimide were almost ineffective.

Phosphorylation of the HuR ligand APRIL by casein kinase 2 regulates CD83 expression

Fully mature DC and, to a lesser extent, activated T and B cells express CD83, a surface molecule that appears to fulfil an important role in efficient T-cell activation. Recently, it has been shown that CD83 mRNA is transported from the nucleus to the cytoplasm by an uncommon route, involving the cellular RNA-binding protein HuR and the nuclear export receptor CRM1. Moreover, the shuttle phosphoprotein APRIL (ANP32B) has been shown to be required for HuR-mediated nucleocytoplasmic translocation of the CD83 mRNA by acting as an adaptor that links HuR and CRM1. Here, we are able to report that casein kinase 2 (CK2) phosphorylates APRIL on residue threonine244 (Thr(244)) and demonstrate that the CK2-specific inhibitor 4,5,6,7-tetrabromo-2-azabenzimidazole abolishes CD83 expression in activated Jurkat T cells by interfering with the nucleocytoplasmic translocation of CD83 mRNA. Depletion and knockdown studies demonstrate that the CK2 alpha' subunit is necessary for this regulation, whereas the CK2 alpha subunit seems to be dispensable. Taken together, the data presented significantly extend our knowledge of the complex regulation of CD83 mRNA processing and provides a novel strategy to interfere with CD83 expression.

Ecto-phosphorylation of CD98 regulates cell-cell interactions

Ecto-phosphorylation plays an important role in many cellular functions. The transmembrane glycoprotein CD98 contains potential phosphorylation sites in its extracellular C-terminal tail. We hypothesized that extracellular signaling through ecto-protein kinases (ePKs) might lead to ecto-phosphorylation of CD98 and influence its multiple functions, including its role in cell-cell interactions. Our results show that recombinant CD98 was phosphorylated in vitro by ePKs from Jurkat cells and by the commercial casein kinase 2 (CK2). Alanine substitutions at serines-305/307/309 or serines-426/430 attenuated CK2-mediated CD98 phosphorylation, suggesting that these residues are the dominant phosphorylation sites for CK2. Furthermore, CD98 expressed in the basolateral membranes of intestinal epithelial Caco2-BBE cells was ecto-phosphorylated by Jurkat cell-derived ePKs and ecto-CK2 was involved in this process. Importantly, cell attachment studies showed that the ecto-phosphorylation of CD98 enhanced heterotypic cell-cell interactions and that the extracellular domain of CD98, which possesses the serine phosphorylation sites, was crucial for this effect. In addition, phosphorylation of recombinant CD98 increased its interactions with Jurkat and Caco2-BBE cells, and promoted cell attachment and spreading. In conclusion, here we demonstrated the ecto-phosphorylation of CD98 by ePKs and its functional importance in cell-cell interactions. Our findings reveal a novel mechanism involved in regulating the multiple functions of CD98 and raise CD98 as a promising target for therapeutic modulations of cell-cell interactions.

The selectivity of inhibitors of protein kinase CK2: an update

CK2 (casein kinase 2) is a very pleiotropic serine/threonine protein kinase whose abnormally high constitutive activity has often been correlated to pathological conditions with special reference to neoplasia. The two most widely used cell permeable CK2 inhibitors, TBB (4,5,6,7-tetrabromo-1H-benzotriazole) and DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole), are marketed as quite specific CK2 blockers. In the present study we show, by using a panel of approx. 80 protein kinases, that DMAT and its parent compound TBI (or TBBz; 4,5,6,7-tetrabromo-1H-benzimidazole) are potent inhibitors of several other kinases, with special reference to PIM (provirus integration site for Moloney murine leukaemia virus)1, PIM2, PIM3, PKD1 (protein kinase D1), HIPK2 (homeodomain-interacting protein kinase 2) and DYRK1a (dual-specificity tyrosine-phosphorylated and -regulated kinase 1a). In contrast, TBB is significantly more selective toward CK2, although it also inhibits PIM1 and PIM3. In an attempt to improve selectivity towards CK2 a library of 68 TBB/TBI-related compounds have been tested for their ability to discriminate between CK2, PIM1, HIPK2 and DYRK1a, ending up with seven compounds whose efficacy toward CK2 is markedly higher than that toward the second most inhibited kinase. Two of these, K64 (3,4,5,6,7-pentabromo-1H-indazole) and K66 (1-carboxymethyl-2-dimethylamino-4,5,6,7-tetrabromo-benzimidazole), display an overall selectivity much higher than TBB and DMAT when tested on a panel of 80 kinases and display similar efficacy as inducers of apoptosis.

Hemoglobin neurotoxicity is attenuated by inhibitors of the protein kinase CK2 independent of heme oxygenase activity

The heme oxygenase (HO) enzymes catalyze the rate-limiting step of heme breakdown, and may accelerate oxidative injury to neurons exposed to heme or hemoglobin. HO-1 and HO-2 are activated in vitro by the phos-phatidylinositol 3-kinase (PI3K)/Akt and protein kinase C (PKC)/CK2 pathways, respectively. The present study tested the hypotheses that CK2, PKC, and PI3K inhibitors would reduce both HO activity and neuronal vulnerability to hemoglobin in murine cortical cultures. Oxidative cell injury was quantified by LDH release and malondialdehyde assays. HO activity was assessed by carbon monoxide assay. Consistent with prior observations, treating primary cortical cultures with hemoglobin for 16h resulted in release of approximately half of neuronal LDH and a seven-fold increase in malondialdehyde. Both endpoints were significantly reduced by the CK2 inhibitors 4,5,6,7-tetrabromobenzotriazole (TBB) and 2-dimethyl-amino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), and by the PKC inhibitor GF109203X; the PI3K inhibitors LY294002 and wortmannin had no effect. None of these inhibitors altered basal HO activity. The 1.9-fold activity increase observed after hemoglobin treatment was largely prevented by LY294002 and LY303511, a structural analog of LY294002 that does not inhibit PI3K activity. It was not reduced by wortmannin, TBB or GF109203X. These results suggest that the protective effect of CK2 and PKC inhibitors in this model is not dependent on reduction in HO activity. In this culture system that expresses both HO-1 and HO-2, HO activity does not appear to be primarily regulated by the PKC/CK2 or PI3K pathways.

A structural insight into CK2 inhibition

The acidophilic Ser/Thr protein kinase CK2 displays some unique properties such as high pleiotropicity and constitutive activity. CK2 is involved in many fundamental aspects of the normal cell life, for instance it promotes cell survival and enhances the tumour phenotype under special circumstances. This makes CK2 an appealing target for the development of inhibitors with pharmacological potential. Here we present an overview of our recent studies on inhibitors directed to the CK2 ATP-binding site whose distinctive features are highlighted by the ability to use both ATP and GTP as co-substrates and by its low susceptibility to staurosporine inhibition. We discuss the effects of the binding of different chemical families of fairly selective inhibitors with potency in the nanomolar or low micromolar range. An important common energetic contribution to the binding is due to the hydrophobic interaction with the apolar surface region of the CK2 binding cleft. The analysis of the known CK2 crystal structures reveals the presence of some highly conserved water molecules in this region. 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 of two unique bulky residues, Ile66 and Ile174, responsible for the reduced dimension of the CK2 active site, play a critical role in determining ligand orientation and binding selectivity.