CK2 phosphorylates and inhibits TAp73 tumor suppressor function to promote expression of cancer stem cell genes and phenotype in head and neck cancer

Cancer stem cells (CSC) and genes have been linked to cancer development and therapeutic resistance, but the signaling mechanisms regulating CSC genes and phenotype are incompletely understood. CK2 has emerged as a key signal serine/threonine kinase that modulates diverse signal cascades regulating cell fate and growth. We previously showed that CK2 is often aberrantly expressed and activated in head and neck squamous cell carcinomas (HNSCC), concomitantly with mutant (mt) tumor suppressor TP53, and inactivation of its family member, TAp73. Unexpectedly, we observed that classical stem cell genes Nanog, Sox2, and Oct4, are overexpressed in HNSCC with inactivated TAp73 and mtTP53. However, the potential relationship between CK2, TAp73 inactivation, and CSC phenotype is unknown. We reveal that inhibition of CK2 by pharmacologic inhibitors or siRNA inhibits the expression of CSC genes and side population (SP), while enhancing TAp73 mRNA and protein expression. Conversely, CK2 inhibitor attenuation of CSC protein expression and the SP by was abrogated by TAp73 siRNA. Bioinformatic analysis uncovered a single predicted CK2 threonine phosphorylation site (T27) within the N-terminal transactivation domain of TAp73. Nuclear CK2 and TAp73 interaction, confirmed by co-immunoprecipitation, was attenuated by CK2 inhibitor, or a T27A point-mutation of this predicted CK2 threonine phospho-acceptor site of TAp73. Further, T27A mutation attenuated phosphorylation, while enhancing TAp73 function in repressing CSC gene expression and SP cells. A new CK2 inhibitor, CX-4945, inhibited CSC related SP cells, clonogenic survival, and spheroid formation. Our study unveils a novel regulatory mechanism whereby aberrant CK2 signaling inhibits TAp73 to promote the expression of CSC genes and phenotype.

A critical role for CK2 in cytokine-induced activation of NFκB in pancreatic β cell death

This study aimed to assess the role of constitutive protein kinase CK2 in cytokine-induced activation of NFκB in pancreatic β cell death. The CK2 inhibitors DRB (5,6-dichloro-1-β-D-ribofuranosylbenzimidazole) (50 μM) and DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole) (5 μM), which decreased CK2 activity by approx. 65 %, rescued INS-1E β cells and mouse islets from cytokine (IL-1β, TNF-α plus IFN-γ)-induced β cell death without affecting H2O2- or palmitate-induced β cell death. Western blot analysis revealed that while DRB or DMAT did not influence cytokine-induced IκBα degradation, they inhibited NFκB-dependent IκBα resynthesis, demonstrating that cytokine-induced NFκB activity is dependent on CK2. Both DRB and DMAT inhibited the constitutive phosphorylation of NFκB p65 at serine 529, while leaving cytokine-induced phosphorylations of NFκB p65 at serines 276 and 536 unaltered. In comparison, putative phosphorylation sites for CK2 on HDACs 1, 2, and 3 at serines 421/423, 394, and 424, respectively, which may stimulate NFκB transcriptional activity, were unchanged by cytokines and CK2 inhibitors. Whereas IL-1β and TNF-α stimulate IκBα degradation and NFκB activation, IFN-γ potentiates cytokine-induced β cell death through activation of STAT1. DRB and DMAT inhibited IFN-γ-stimulated phosphorylation of STAT1 at serine 727, while leaving IFN-γ-induced phosphorylation of STAT1 at tyrosine 701 unaffected. Inhibition of cytokine-induced β cell death by CK2 inhibitors was, however, not dependent on IFN-γ, and IFN-γ did not affect CK2-dependent IκBα turnover. In conclusion, it is suggested that cytokine-induced activation of NFκB in β cells is dependent on CK2 activity, which phosphorylates NFκB p65 at serine 529.

Cell-permeable dual inhibitors of protein kinases CK2 and PIM-1: structural features and pharmacological potential

It has been proposed that dual inhibitors of protein kinases CK2 and PIM-1 are tools particularly valuable to induce apoptosis of cancer cells, a property, however, implying cell permeability, which is lacking in the case of selective CK2/PIM-1 inhibitors developed so far. To fill this gap, we have derivatized the scaffold of the promiscuous CK2 inhibitor TBI with a deoxyribose moiety, generating TDB, a selective, cell-permeable inhibitor of CK2 and PIM-1. Here, we shed light on the structural features underlying the potency and narrow selectivity of TDB by exploiting a number of TDB analogs and by solving the 3D structure of the TDB/CK2 complex at 1.25 Å resolution, one of the highest reported so far for this kinase. We also show that the cytotoxic efficacy of TDB is almost entirely due to apoptosis, is accompanied by parallel inhibition of cellular CK2 and PIM-1, and is superior to both those observed combining individual inhibitors of CK2 and PIM-1 and by treating cells with the CK2 inhibitor CX4945. These data, in conjunction with the observations that cancer cells are more susceptible than non-cancer cells to TDB and that such a sensitivity is maintained in a multi-drug resistance background, highlight the pharmacological potential of this compound.

TNFα inhibits IGFBP-3 through activation of p38α and casein kinase 2 in human retinal endothelial cells

We recently reported a reciprocal relationship between tumor necrosis factor alpha (TNFα) and insulin-like receptor growth factor binding protein 3 (IGFBP-3) in whole retina of normal and IGFBP-3 knockout mice. A similar relationship was also observed in cultured retinal endothelial cells (REC). We found that TNFα significantly reduced IGFBP-3 levels and vice-versa, IGFBP-3 can lower TNFα and TNFα receptor expression. Since IGFBP-3 is protective to the diabetic retina and TNFα is causative in the development of diabetic retinopathy, we wanted to better understand the cellular mechanisms by which TNFα can reduce IGFBP-3 levels. For these studies, primary human retinal endothelial cells (REC) were used since these cells undergo TNFα-mediated apoptosis under conditions of high glucose conditions and contribute to diabetic retinopathy. We first cultured REC in normal or high glucose, treated with exogenous TNFα, then measured changes in potential signaling pathways, with a focus on P38 mitogen-activated protein kinase alpha (P38α) and casein kinase 2 (CK2) as these pathways have been linked to both TNFα and IGFBP-3. We found that TNFα significantly increased phosphorylation of P38α and CK2. Furthermore, specific inhibitors of P38α or CK2 blocked TNFα inhibition of IGFBP-3 expression, demonstrating that TNFα reduces IGFBP-3 through activation of P38α and CK2. Since TNFα and IGFBP-3 are key mediators of retinal damage and protection respectively in diabetic retinopathy, increased understanding of the relationship between these two proteins will offer new therapeutic options for treatment.

Protein kinase CK2 and angiogenesis

CK2 is an ubiquitously expressed protein kinase, which is composed of two catalytic a- and a'- and two noncatalytic b-subunits. CK2 protein levels and kinase activity is elevated in rapidly proliferating cells including cancer cells. There is increasing evidence that CK2 also plays an essential role in angiogenesis, either by interaction or phosphorylation of growth factors or by phosphorylation or binding to proteins in signalling cascades, which are implicated in angiogenesis. Over the last ten years a great number of inhibitors for CK2 were detected, two of them are now in clinical phase II trials for the treatment of cancer patients. Some of these inhibitors were also found to be active in the inhibition of angiogenesis. Thus, CK2 inhibitors probably together with inhibitors of other signalling molecules involved in angiogenesis might be powerful tools for the treatment of cancer and cancer connected angiogenesis.

Synergistic anti-leukemic effects of CK2 inhibitors and pentabromobenzylisothioureas in vitro

BACKGROUND

Casein kinase-2 (CK2) inhibitors and pentabromobenzylisothioureas are promising anti-leukemic agents for treatment, both alone and in combination. In this study, we examined pro-apoptotic and cytostatic effects of three CK2 inhibitors: one known, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT) and two new: 2-(4-methylpiperazin-1-yl)-4,5,6,7-tetrabromo-1H-benzimidazole (MPT) and 2-aminoethyleneamino-4,5,6,7-tetrabromo-1H-benzimidazole (AEAT), as well as of certain S-2,3,4,5,6-pentabromobenzylisothiouronium bromides: ZKK-3, ZKK-9, ZKK-13, against the human acute myelogenous leukemia cell line (KG-1). Cells were treated with CK2 inhibitors alone and in combination with the pentabromobenzylisothioureas.

MATERIALS AND METHODS

Evaluation of synergistic and pro-apoptotic effects, mitochondrial membrane potential (ΔΨm) assay, poly(ADP-ribose) polymerase (PARP) cleavage assay, and cell-cycle progression of KG-1 cells were carried out using the flow cytometric technique and fluorescent microscopic analysis. Western blots were used for analysis of B-cell lymphoma-2 (BCL-2) family proteins in whole-cell extracts.

RESULTS

The tested CK2 inhibitors DMAT, MPT, AEAT exhibited synergistic proapoptotic effect in combination with ZKK-3, ZKK-9 and ZKK-13. The agents revealed different pro-apoptotic efficacies against leukemia cell line KG-1. The highest apoptotic activity of the tested compounds was exhibited by AEAT.

CONCLUSION

Combination of CK2 inhibitors and pentabromobenzylisothioureas-induced synergistic anti-leukemic effects against KG-1 acute myelogenous leukemia cells in vitro.

Coumestrol induces senescence through protein kinase CKII inhibition-mediated reactive oxygen species production in human breast cancer and colon cancer cells

An inhibitor of the protein kinase CKII (CKII) was purified from leaves of Glycine max (L.) Merrill and was identified as coumestrol by structural analysis. Coumestrol inhibited the phosphotransferase activity of CKII toward β-casein, with an IC50 of about 5 μM. It acted as a competitive inhibitor with respect to ATP as a substrate, with an apparent Ki value of 7.67 μM. Coumestrol at 50μM resulted in 50% and 30% growth inhibition of human breast cancer MCF-7 and colorectal cancer HCT116 cells, respectively. Coumestrol promoted senescence through the p53-p21(Cip1/WAF1) pathway by inducing reactive oxygen species (ROS) production in MCF-7 and HCT116 cells. The ROS scavenger N-acetyl-l-cysteine (NAC), NADPH oxidase inhibitor apocynin and p22(phox) siRNA almost completely abolished this event. Overexpression of CKIIα antagonised cellular senescence mediated by coumestrol, indicating that this compound induced senescence via a CKII-dependent pathway. Since senescence is an important tumour suppression process in vivo, these results suggest that coumestrol can function by inhibiting oncogenic disease, at least in part, through CKII inhibition-mediated cellular senescence.

Endothelin-1 receptor antagonists regulate cell surface-associated protein disulfide isomerase in sickle cell disease

Increased endothelin-1 (ET-1) levels, disordered thiol protein status, and erythrocyte hydration status play important roles in sickle cell disease (SCD) through unresolved mechanisms. Protein disulfide isomerase (PDI) is an oxidoreductase that mediates thiol/disulfide interchange reactions. We provide evidence that PDI is present in human and mouse erythrocyte membranes and that selective blockade with monoclonal antibodies against PDI leads to reduced Gardos channel activity (1.6±0.03 to 0.56±0.02 mmol·10(13) cell(-1)·min(-1), P<0.001) and density of sickle erythrocytes (D50: 1.115±0.001 to 1.104±0.001 g/ml, P=0.012) with an IC50 of 4 ng/ml. We observed that erythrocyte associated-PDI activity was increased in the presence of ET-1 (3.1±0.2 to 5.6±0.4%, P<0.0001) through a mechanism that includes casein kinase II. Consistent with these results, in vivo treatment of BERK sickle transgenic mice with ET-1 receptor antagonists lowered circulating and erythrocyte associated-PDI activity (7.1±0.3 to 5.2±0.2%, P<0.0001) while improving hematological parameters and Gardos channel activity. Thus, our results suggest that PDI is a novel target in SCD that regulates erythrocyte volume and oxidative stress and may contribute to cellular adhesion and endothelial activation leading to vasoocclusion as observed in SCD.

The protein kinase 2 inhibitor CX-4945 regulates osteoclast and osteoblast differentiation in vitro

Drug repositioning can identify new therapeutic applications for existing drugs, thus mitigating high R&D costs. The Protein kinase 2 (CK2) inhibitor CX-4945 regulates human cancer cell survival and angiogenesis. Here we found that CX-4945 significantly inhibited the RANKL-induced osteoclast differentiation, but enhanced the BMP2-induced osteoblast differentiation in a cell culture model. CX-4945 inhibited the RANKL-induced activation of TRAP and NFATc1 expression accompanied with suppression of Akt phosphorylation, but in contrast, it enhanced the BMP2-mediated ALP induction and MAPK ERK1/2 phosphorylation. CX-4945 is thus a novel drug candidate for bone-related disorders such as osteoporosis.

Protein kinase CK2 inhibition down modulates the NF-κB and STAT3 survival pathways, enhances the cellular proteotoxic stress and synergistically boosts the cytotoxic effect of bortezomib on multiple myeloma and mantle cell lymphoma cells

CK2 is a pivotal pro-survival protein kinase in multiple myeloma that may likely impinge on bortezomib-regulated cellular pathways. In the present study, we investigated CK2 expression in multiple myeloma and mantle cell lymphoma, two bortezomib-responsive B cell tumors, as well as its involvement in bortezomib-induced cytotoxicity and signaling cascades potentially mediating bortezomib resistance. In both tumors, CK2 expression correlated with that of its activated targets NF-κB and STAT3 transcription factors. Bortezomib-induced proliferation arrest and apoptosis were significantly amplified by the simultaneous inhibition of CK2 with two inhibitors (CX-4945 and K27) in multiple myeloma and mantle cell lymphoma cell lines, in a model of multiple myeloma bone marrow microenvironment and in cells isolated from patients. CK2 inhibition empowered bortezomib-triggered mitochondrial-dependent cell death. Phosphorylation of NF-κB p65 on Ser529 (a CK2 target site) and rise of the levels of the endoplasmic reticulum stress kinase/endoribonuclease Ire1α were markedly reduced upon CK2 inhibition, as were STAT3 phospho Ser727 levels. On the contrary, CK2 inhibition increased phospho Ser51 eIF2α levels and enhanced the bortezomib-dependent accumulation of poly-ubiquitylated proteins and of the proteotoxic stress-associated chaperone Hsp70. Our data suggest that CK2 over expression in multiple myeloma and mantle cell lymphoma cells might sustain survival signaling cascades and can antagonize bortezomib-induced apoptosis at different levels. CK2 inhibitors could be useful in bortezomib-based combination therapies.

CK2 inhibitor CX-4945 blocks TGF-β1-induced epithelial-to-mesenchymal transition in A549 human lung adenocarcinoma cells

BACKGROUND

The epithelial-to-mesenchymal transition (EMT) is a major phenotype of cancer metastasis and invasion. As a druggable cancer target, the inhibition of protein kinase CK2 (formally named to casein kinase 2) has been suggested as a promising therapeutic strategy to treat EMT-controlled cancer metastasis. This study aimed to evaluate the effect of the CK2 inhibitor CX-4945 on the processes of cancer migration and invasion during the EMT in A549 human lung adenocarcinoma cells.

MATERIALS AND METHODS

The effect of CX-4945 on TGF-β1-induced EMT was evaluated in A549 cells treated with TGF-β1 (5 ng/ml) and CX-4945. The effect of CX-4945 on TGF-β1-induced cadherin switch and activation of key signaling molecules involved in Smad, non-Smad, Wnt and focal adhesion signaling pathways were investigated by Western blot analysis, immunocytochemistry and reporter assay. Additionally, the effect of CX-4945 on TGF-β1-induced migration and invasion was investigated by wound healing assay, Boyden chamber assay, gelatin zymography, and the quantitative real-time PCR.

RESULTS

CX-4945 inhibits the TGF-β1-induced cadherin switch and the activation of key signaling molecules involved in Smad (Smad2/3, Twist and Snail), non-Smad (Akt and Erk), Wnt (β-catenin) and focal adhesion signaling pathways (FAK, Src and paxillin) that cooperatively regulate the overall process of EMT. As a result, CX-4945 inhibits the migration and invasion of A549 cells accompanied with the downregulation of MMP-2 and 9.

CONCLUSIONS

Clinical evaluation of CX-4945 in humans as a single agent in solid tumors and multiple myeloma has established its promising pharmacokinetic, pharmacodynamic, and safety profiles. Beyond regression of tumor mass, CX-4945 may be advanced as a new therapy for cancer metastasis and EMT-related disorders.

Exploiting the repertoire of CK2 inhibitors to target DYRK and PIM kinases

Advantage has been taken of the relative promiscuity of commonly used inhibitors of protein kinase CK2 to develop compounds that can be exploited for the selective inhibition of druggable kinases other than CK2 itself. Here we summarize data obtained by altering the scaffold of CK2 inhibitors to give rise to novel selective inhibitors of DYRK1A and to a powerful cell permeable dual inhibitor of PIM1 and CK2. In the former case one of the new compounds, C624 (naphto [1,2-b]benzofuran-5,9-diol) displays a potency comparable to that of the first-in-class DYRK1A inhibitor, harmine, lacking however the drawback of drastically inhibiting monoamine oxidase-A (MAO-A) as harmine does. On the other hand the promiscuous CK2 inhibitor 4,5,6,7-tetrabromo-1H-benzimidazole (TBI,TBBz) has been derivatized with a sugar moiety to generate a 1-(β-D-2'-deoxyribofuranosyl)-4,5,6,7-tetrabromo-1H-benzimidazole (TDB) compound which inhibits PIM1 and CK2 with comparably high efficacy (IC50 values<100nM) and remarkable selectivity. TDB, unlike other dual PIM1/CK2 inhibitors described in the literature is readily cell permeable and displays a cytotoxic effect on cancer cells consistent with concomitant inhibition of both its onco-kinase targets. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Diapause prevention effect of Bombyx mori by dimethyl sulfoxide

HCl treatment has been, for about 80 years, the primary method for the prevention of entry into embryonic diapauses of Bombyx mori. This is because no method is as effective as the HCl treatment. In this study, we discovered that dimethyl sulfoxide (DMSO) prevented entry into the diapause of the silkworm, Bombyx mori. The effect of diapause prevention was 78% as a result of treatment with 100% DMSO concentration, and the effect was comparable to that of the HCl treatment. In contrast, in the case of non-diapause eggs, hatchability was decreased by DMSO in a concentration-dependent manner. The effect of DMSO was restricted within 24 hours after oviposition of diapause eggs, and the critical period was slightly shorter than the effective period of the HCl treatment. DMSO analogs, such as dimethyl formamide (DMF) and dimethyl sulfide (DMS), did little preventive effect against the diapause. Furthermore, we also investigated the permeation effects of chemical compounds by DMSO. When treated with an inhibitor of protein kinase CK2 (CK2) dissolved in DMSO, the prevention rate of the diapause was less than 40%. This means that the inhibition effect by the CK2 inhibitor was the inhibition of embryonic development after diapause prevention by DMSO. These data suggest that DMSO has the effects of preventing from entering into the diapause and permeation of chemicals into diapause eggs.

N-substituted Indole-2 and 3-carboxamide derivatives as inhibitors of human protein kinase CK2: in vitro assay and molecular modelling study

Protein kinase CK2 (Casein Kinase 2) is involved in cell growth; proliferation and suppression of apoptosis. Hence, it strongly promotes cell survival and can be considered an important target for human cancers. In the present study, a series of N-substituted indole-2- and 3-carboxamide derivatives were tested for inhibitions of human recombinant protein kinase CK2 to evaluate their anticancer properties. The inhibition test revealed that the most active compound 4 (1-benzyl-N-(2,4-dichlorobenzyl)-1H-indole-2-carboxamide) showed an IC50 value of 14.6 µM towards human protein kinase CK2. A molecular docking study of the compounds with CK2 was performed and revealed the binding mode of the most active compound 4, underlying its inhibitory activity.

Proapoptotic effects of new pentabromobenzylisothiouronium salts in a human prostate adenocarcinoma cell line

Prostate cancer is the second most common cancer in elderly men worldwide and its incidence rate is rising continuously. Agents capable of inducing apoptosis in prostate cancer cells seem a promising approach to treat this malignancy. In this study we describe the synthesis of a number of novel N- and N,N'-substituted S-2,3,4,5,6-pentabromobenzylisothiouronium bromides and their activity against the human prostate adenocarcinoma PC3 cell line. All the compounds produced changes in mitochondrial transmembrane potential and cell cycle progression, showed a cytostatic effect and induced apoptosis in the tested cancer line in a concentration- and time-dependent manner. The most effective compounds ZKK-3, ZKK-9 and ZKK-13 produced, at 20 microM concentration, apoptosis in 42, 46, and 66% of the cells, respectively, after 48 h incubation. Two selected S-2,3,4,5,6-pentabromobenzylisothiouronium bromides (ZKK-3, ZKK-9) showed also a synergic proapoptotic effect with the new casein kinase II inhibitor 2-(4-methylpiperazin-1-yl)-4,5,6,7-tetrabromo-1H-benzimidazole (TBIPIP) in the PC3 cell line.

Casein kinase 2 inhibition attenuates androgen receptor function and cell proliferation in prostate cancer cells

BACKGROUND

Casein kinase 2 (CK2) is constitutively active with dual specificity and exists as a hetero-tetrameric complex of α, α', and β subunits. Its aberrant expression and elevated activity have been linked to many human cancers, including prostate cancer. As an effort to develop new chemotherapy for prostate cancers, in this study, we tested the effects of tetra-bromo-cinnamic acid (TBCA), a newly synthetic CK2-selective CK2 inhibitor, on androgen receptor (AR) transactivation, cell proliferation, and viability in multiple prostate cancer cell lines.

METHODS

We utilized a comprehensive approach of a newly synthetic CK2-selective inhibitor TBCA, plus gene-specific siRNAs in multiple cell-based assays to further understand the role of CK2 in AR signaling. Alamar-blue-based cell growth assay, flow cytometry for cell cycle distribution, Luciferase report gene assay for AR transactivation, and immuno-fluorescent approach for AR nuclear localization as well as quantitative PCR assay for AR-mediated gene expression were utilized. The significance of the differences between treatment and control was analyzed using the SPSS software (SPSS, Chicago, IL).

RESULTS

Our data revealed that TBCA reduced cell proliferation and caused G2/M cell cycle arrest in a dose-dependent manner. Further analysis demonstrated that TBCA blocked AR nuclear translocation and gene expression. To confirm the target specificity, we used gene-specific siRNAs for both CK2α and CK2α' subunits, and the results suggested that both CK2 catalytic subunits are involved in androgen-stimulated AR nuclear translocation and AR-mediated gene expression in prostate cancer cells.

CONCLUSIONS

CK2 subunits α and α' are likely involved in AR signaling, and TBCA might be useful in the management of prostate cancers as a chemo-preventive agent in the future.

[Participation of proteinkinase CK2 in regulation of human erythrocytes plasma membrane redox system activity: relative contribution of ca(2+)-dependent and ca(2+)-independent mechanisms of its activation]

Involvement of protein kinase CK2 (2.7.11.1) in modulation of live cells trans-plasma membrane electron transport was first discovered. Using human erythrocytes a decrease of plasma membrane redox system (PMRS) activity is shown under the action of specific protein kinase CK2 inhibitors. Using inhibitory analysis the activity regulation of human erythrocytes PMRS by Ca(2+)-dependent and Ca(2+)-independent mechanisms were investigated. It was shown that functional Ca(2+)-antagonists (nitrendipine and calmidazolium) significantly increased, and functional Ca(2+)-agonists to some extent reduced or did not affect the trans-plasma membrane electron transport in these cells.

Structural disorder and local order of hNopp140

Human nucleolar phosphoprotein p140 (hNopp 140) is a highly phosphorylated protein inhibitor of casein kinase 2 (CK2). As in the case of many kinase-inhibitor systems, the inhibitor has been described to belong to the family of intrinsically disordered proteins (IDPs), which often utilize transient structural elements to bind their cognate enzyme. Here we investigated the structural status of this protein both to provide distinct lines of evidence for its disorder and to point out its transient structure potentially involved in interactions and also its tendency to aggregate. Structural disorder of hNopp140 is apparent by its anomalous electrophoretic mobility, protease sensitivity, heat stability, hydrodynamic behavior on size-exclusion chromatography, (1)H NMR spectrum and differential scanning calorimetry scan. hNopp140 has a significant tendency to aggregate and the change of its circular dichroism spectrum in the presence of 0-80% TFE suggests a tendency to form local helical structures. Wide-line NMR measurements suggest the overall disordered character of the protein. In all, our data suggest that this protein falls into the pre-molten globule state of IDPs, with a significant tendency to become ordered in the presence of its partner as demonstrated in the presence of transcription factor IIB (TFIIB).

Inhibition of CK2α down-regulates Hedgehog/Gli signaling leading to a reduction of a stem-like side population in human lung cancer cells

Protein kinase CK2 is frequently elevated in a variety of human cancers. The Hedgehog (Hh) signaling pathway has been implicated in stem cell maintenance, and its aberrant activation has been indicated in several types of cancer, including lung cancer. In this study, we show that CK2 is positively involved in Hh/Gli signaling in lung cancer cell lines A549 and H1299. First, we found a correlation between CK2α and Gli1 mRNA levels in 100 primary lung cancer tissues. Down-regulation of Gli1 expression and transcriptional activity were demonstrated after the silencing of CK2α in lung cancer cells. In addition, CK2α siRNA down-regulated the expression of Hh target genes. Furthermore, two small-molecule CK2α inhibitors led to a dose-dependent inhibition of Gli1 expression and transcriptional activity in lung cancer cells. Reversely, forced over-expression of CK2α resulted in an increase both in Gli1 expression and transcriptional activity in A549 cells. Finally, the inhibition of Hh/Gli by CK2α siRNA led to a reduction of a cancer stem cell-like side population that shows higher ABCG2 expression level. Thus, we report that the inhibition of CK2α down-regulates Hh/Gli signaling and subsequently reduces stem-like side population in human lung cancer cells.

Inhibition of casein kinase 2 modulates XBP1-GRP78 arm of unfolded protein responses in cultured glial cells

Stress signals cause abnormal proteins to accumulate in the endoplasmic reticulum (ER). Such stress is known as ER stress, which has been suggested to be involved in neurodegenerative diseases, diabetes, obesity and cancer. ER stress activates the unfolded protein response (UPR) to reduce levels of abnormal proteins by inducing the production of chaperon proteins such as GRP78, and to attenuate translation through the phosphorylation of eIF2α. However, excessive stress leads to apoptosis by generating transcription factors such as CHOP. Casein kinase 2 (CK2) is a serine/threonine kinase involved in regulating neoplasia, cell survival and viral infections. In the present study, we investigated a possible linkage between CK2 and ER stress using mouse primary cultured glial cells. 4,5,6,7-tetrabromobenzotriazole (TBB), a CK2-specific inhibitor, attenuated ER stress-induced XBP-1 splicing and subsequent induction of GRP78 expression, but was ineffective against ER stress-induced eIF2α phosphorylation and CHOP expression. Similar results were obtained when endogenous CK2 expression was knocked-down by siRNA. Immunohistochemical analysis suggested that CK2 was present at the ER. These results indicate CK2 to be linked with UPR and to resist ER stress by activating the XBP-1-GRP78 arm of UPR.

[Casein kinase 2, the versatile regulator of cell survival]

Casein kinase 2 (CK2), a highly conservative, multifunctional serine/threonine protein kinase, is critically important for the regulation of a plethora of processes in eukaryotes, such as cell proliferation, differentiation and death. CK2 is expressed in all tissues; in particular, its amount and activity are elevated in tumor cells. Unlike many regulatory proteins CK2 permanently adopts an active conformation. Of the utmost importance are the anti-apoptotic functions of CK2. This protein kinase is capable of regulating cell survival at multiple levels including DNA repair, NF-kappaB, Wnt, PI3K/Akt and JAK-STAT signaling cascades, chaperones, activation of anti-apoptotic proteins and down-regulation of pro-apoptotic counterparts, in particular, caspases. The versatility of CK2-mediated phosphorylation ensures the survival of tumor cells exposed to stimuli that differ in the origin and mechanisms of cytotoxicity. This manifold mode of CK2-dependent survival makes this enzyme an important target for antitumor therapy.

Release of mitochondrial apoptogenic factors and cell death are mediated by CK2 and NADPH oxidase

Activation of the NADPH oxidase subunit, NOX2, and increased oxidative stress are associated with neuronal death after cerebral ischemia and reperfusion. Inhibition of NOX2 by casein kinase 2 (CK2) leads to neuronal survival, but the mechanism is unknown. In this study, we show that in copper/zinc-superoxide dismutase transgenic (SOD1 Tg) mice, degradation of CK2α and CK2α' and dephosphorylation of CK2β against oxidative stress were markedly reduced compared with wild-type (WT) mice that underwent middle cerebral artery occlusion. Inhibition of CK2 pharmacologically or by ischemic reperfusion facilitated accumulation of poly(ADP-ribose) polymers, the translocation of apoptosis-inducing factor (AIF), and cytochrome c release from mitochondria after ischemic injury. The eventual enhancement of CK2 inhibition under ischemic injury strongly increased 8-hydroxy-2'-deoxyguanosine and phosphorylation of H2A.X. Furthermore, CK2 inhibition by tetrabromocinnamic acid (TBCA) in SOD1 Tg and gp91 knockout (KO) mice after ischemia reperfusion induced less release of AIF and cytochrome c than in TBCA-treated WT mice. Inhibition of CK2 in gp91 KO mice subjected to ischemia reperfusion did not increase brain infarction compared with TBCA-treated WT mice. These results strongly suggest that NOX2 activation releases reactive oxygen species after CK2 inhibition, triggering release of apoptogenic factors from mitochondria and inducing DNA damage after ischemic brain injury.

Structural features underlying the selectivity of the kinase inhibitors NBC and dNBC: role of a nitro group that discriminates between CK2 and DYRK1A

8-hydroxy-4-methyl-9-nitrobenzo(g)chromen-2-one (NBC) has been found to be a fairly potent ATP site-directed inhibitor of protein kinase CK2 (Ki = 0.22 μM). Here, we show that NBC also inhibits PIM kinases, especially PIM1 and PIM3, the latter as potently as CK2. Upon removal of the nitro group, to give 8-hydroxy-4-methyl-benzo(g)chromen-2-one (here referred to as "denitro NBC", dNBC), the inhibitory power toward CK2 is almost entirely lost (IC(50) > 30 μM) whereas that toward PIM1 and PIM3 is maintained; in addition, dNBC is a potent inhibitor of a number of other kinases that are weakly inhibited or unaffected by NBC, with special reference to DYRK1A whose IC(50) values with NBC and dNBC are 15 and 0.60 μM, respectively. Therefore, the observation that NBC, unlike dNBC, is a potent inducer of apoptosis is consistent with the notion that this effect is mediated by inhibition of endogenous CK2. The structural features underlying NBC selectivity have been revealed by inspecting its 3D structure in complex with the catalytic subunit of Z. mays CK2. The crucial role of the nitro group is exerted both through a direct electrostatic interaction with the side chain of Lys68 and, indirectly, by enhancing the acidic dissociation constant of the adjacent hydroxyl group which interacts with a conserved water molecule in the deepest part of the cavity. By contrast, the very same nitro group is deleterious for the binding to the active site of DYRK1A, as disclosed by molecular docking. This provides the rationale for preferential inhibition of DYRK1A by dNBC.

Antitumoral activity of allosteric inhibitors of protein kinase CK2

INTRODUCTION

Due to its physiological role into promoting cell survival and its dysregulation in most cancer cells, protein kinase CK2 is a relevant physiopathological target for development of chemical inhibitors. We report the discovery of azonaphthalene derivatives, as a new family of highly specific CK2 inhibitors. First, we demonstrated that CK2 inhibition (IC50= 0.4 µM) was highly specific, reversible and non ATP-competitive. Small Angle X-ray Scattering experiments showed that this inhibition was due to large conformational change of CK2α upon binding of these inhibitors. We showed that several compounds of the family were cell-potent CK2 inhibitors promoting cell cycle arrest of human glioblastoma U373 cells. Finally, in vitro and in vivo assays showed that these compounds could decrease U373 cell tumor mass by 83 % emphasizing their efficacy against these apoptosis-resistant tumors. In contrast, Azonaphthalene derivatives inactive on CK2 activity showed no effect in colony formation and tumor regression assays. These findings illustrate the emergence of nonclassical CK2 inhibitors and provide exciting opportunities for the development of novel allosteric CK2 inhibitors.

BACKGROUND

CK2 is an emerging therapeutic target and ATP-competitive inhibitors have been identified. CK2 is endowed with specific structural features providing alternative strategies for inhibition.

RESULTS

Azonaphthalene compounds are allosteric CK2 inhibitors showing antitumor activity.

CONCLUSION

CK2 may be targeted allosterically.

SIGNIFICANCE

These inhibitors provide a foundation for a new paradigm for specific CK2 inhibition.

Nanoencapsulated anti-CK2 small molecule drug or siRNA specifically targets malignant cancer but not benign cells

CK2, a pleiotropic Ser/Thr kinase, is an important target for cancer therapy. We tested our novel tenfibgen-based nanocapsule for delivery of the inhibitor 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT) and an siRNA directed against both CK2α and α' catalytic subunits to prostate cancer cells. We present data on the TBG nanocapsule itself and on CK2 inhibition or downregulation in treated cells, including effects on Nuclear Factor-kappa B (NF-κB) p65. By direct comparison of two CK2-directed cargos, our data provide proof that the TBG encapsulation design for delivery of drugs specifically to cancer cells has strong potential for small molecule- and nucleic acid-based cancer therapy.