Regulation of DNA-dependent protein kinase by protein kinase CK2 in human glioblastoma cells

DNA-PK is activated by SIRT2 deacetylation to promote DNA double-strand break repair by non-homologous end joining

DNA-dependent protein kinase (DNA-PK) plays a critical role in non-homologous end joining (NHEJ), the predominant pathway that repairs DNA double-strand breaks (DSB) in response to ionizing radiation (IR) to govern genome integrity. The interaction of the catalytic subunit of DNA-PK (DNA-PKcs) with the Ku70/Ku80 heterodimer on DSBs leads to DNA-PK activation; however, it is not known if upstream signaling events govern this activation. Here, we reveal a regulatory step governing DNA-PK activation by SIRT2 deacetylation, which facilitates DNA-PKcs localization to DSBs and interaction with Ku, thereby promoting DSB repair by NHEJ. SIRT2 deacetylase activity governs cellular resistance to DSB-inducing agents and promotes NHEJ. SIRT2 furthermore interacts with and deacetylates DNA-PKcs in response to IR. SIRT2 deacetylase activity facilitates DNA-PKcs interaction with Ku and localization to DSBs and promotes DNA-PK activation and phosphorylation of downstream NHEJ substrates. Moreover, targeting SIRT2 with AGK2, a SIRT2-specific inhibitor, augments the efficacy of IR in cancer cells and tumors. Our findings define a regulatory step for DNA-PK activation by SIRT2-mediated deacetylation, elucidating a critical upstream signaling event initiating the repair of DSBs by NHEJ. Furthermore, our data suggest that SIRT2 inhibition may be a promising rationale-driven therapeutic strategy for increasing the effectiveness of radiation therapy.

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Essential role of CK2α for the interaction and stability of replication fork factors during DNA synthesis and activation of the S-phase checkpoint

The ataxia telangiectasia mutated and Rad3-related (ATR)-CHK1 pathway is the major signalling cascade activated in response to DNA replication stress. This pathway is associated with the core of the DNA replication machinery comprising CDC45, the replicative MCM2-7 hexamer, GINS (altogether forming the CMG complex), primase-polymerase (POLε, -α, and -δ) complex, and additional fork protection factors such as AND-1, CLASPIN (CLSPN), and TIMELESS/TIPIN. In this study, we report that functional protein kinase CK2α is critical for preserving replisome integrity and for mounting S-phase checkpoint signalling. We find that CDC45, CLSPN and MCM7 are novel CK2α interacting partners and these interactions are particularly important for maintenance of stable MCM7-CDC45, ATRIP-ATR-MCM7, and ATR-CLSPN protein complexes. Consistently, cells depleted of CK2α and treated with hydroxyurea display compromised replisome integrity, reduced chromatin binding of checkpoint mediator CLSPN, attenuated ATR-mediated S-phase checkpoint and delayed recovery of stalled forks. In further support of this, differential gene expression analysis by RNA-sequencing revealed that down-regulation of CK2α accompanies global shutdown of genes that are implicated in the S-phase checkpoint. These findings add to our understanding of the molecular mechanisms involved in DNA replication by showing that the protein kinase CK2α is essential for maintaining the stability of the replisome machinery and for optimizing ATR-CHK1 signalling activation upon replication stress.

DNA-PKcs: A Targetable Protumorigenic Protein Kinase

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a pleiotropic protein kinase that plays critical roles in cellular processes fundamental to cancer. DNA-PKcs expression and activity are frequently deregulated in multiple hematologic and solid tumors and have been tightly linked to poor outcome. Given the potentially influential role of DNA-PKcs in cancer development and progression, therapeutic targeting of this kinase is being tested in preclinical and clinical settings. This review summarizes the latest advances in the field, providing a comprehensive discussion of DNA-PKcs functions in cancer and an update on the clinical assessment of DNA-PK inhibitors in cancer therapy.

DNA-PK in human malignant disorders: Mechanisms and implications for pharmacological interventions

The DNA-PK holoenzyme is a fundamental element of the DNA damage response machinery (DDR), which is responsible for cellular genomic stability. Consequently, and predictably, over the last decades since its identification and characterization, numerous pre-clinical and clinical studies reported observations correlating aberrant DNA-PK status and activity with cancer onset, progression and responses to therapeutic modalities. Notably, various studies have established in recent years the role of DNA-PK outside the DDR network, corroborating its role as a pleiotropic complex involved in transcriptional programs that operate biologic processes as epithelial to mesenchymal transition (EMT), hypoxia, metabolism, nuclear receptors signaling and inflammatory responses. In particular tumor entities as prostate cancer, immense research efforts assisted mapping and describing the overall signaling networks regulated by DNA-PK that control metastasis and tumor progression. Correspondingly, DNA-PK emerges as an obvious therapeutic target in cancer and data pertaining to various pharmacological approaches have been published, largely in context of combination with DNA-damaging agents (DDAs) that act by inflicting DNA double strand breaks (DSBs). Currently, new generation inhibitors are tested in clinical trials. Several excellent reviews have been published in recent years covering the biology of DNA-PK and its role in cancer. In the current article we are aiming to systematically describe the main findings on DNA-PK signaling in major cancer types, focusing on both preclinical and clinical reports and present a detailed current status of the DNA-PK inhibitors repertoire.

Down-regulation of CK2α correlates with decreased expression levels of DNA replication minichromosome maintenance protein complex (MCM) genes

Protein kinase CK2 is a serine/threonine kinase composed of two catalytic subunits (CK2α and/or CK2α’) and two regulatory subunits (CK2β). It is implicated in every stage of the cell cycle and in the regulation of various intracellular pathways associated with health and disease states. The catalytic subunits have similar biochemical activity, however, their functions may differ significantly in cells and in vivo. In this regard, homozygous deletion of CK2α leads to embryonic lethality in mid-gestation potentially due to severely impaired cell proliferation. To determine the CK2α-dependent molecular mechanisms that control cell proliferation, we established a myoblast-derived cell line with inducible silencing of CK2α and carried out a comprehensive RNA-Seq analysis of gene expression. We report evidence that CK2α depletion causes delayed cell cycle progression through the S-phase and defective response to replication stress. Differential gene expression analysis revealed that the down-regulated genes were enriched in pathways implicated in cell cycle regulation, DNA replication and DNA damage repair. Interestingly, the genes coding for the minichromosome maintenance proteins (MCMs), which constitute the core of the replication origin recognition complex, were among the most significantly down-regulated genes. These findings were validated in cells and whole mouse embryos. Taken together, our study provides new evidence for a critical role of protein kinase CK2 in controlling DNA replication initiation and the expression levels of replicative DNA helicases, which ensure maintenance of proliferative potential and genome integrity in eukaryotic cells.

The crucial role of DNA-dependent protein kinase and myelin transcription factor 1-like protein in the miR-141 tumor suppressor network

Glioblastoma is the most aggressive brain tumor. Although miR-141 has been demonstrated to primarily function as a tumor suppressor in numerous malignancies, including glioblastoma, the mechanisms involved remain poorly understood. Here, it is shown that miR-141 is downregulated in glioblastoma cell lines and tissues and may exert its biological function via directly targeting myelin transcription factor 1-like (MYT1L). Using two glioblastoma cell lines that differ from each other by the functionality of DNA-dependent protein kinase (DNAPK), a functional involvement of DNAPK in the miR-141 tumor suppression network was observed. In M059K cells with a normal function of DNAPK, the enforced expression of miR-141 attenuated MYT1L expression and suppressed cell proliferation. Conversely, the inhibition of miR-141 expression promoted cell proliferation; however, in M059J cells with a loss-of-function DNAPK, miR-141 constitutively inhibited cell proliferation upon ectopic overexpression or inhibition. An overexpression of miR-141 suppressed M059J cell migration, while it had no effect on M059K. Furthermore, the ectopic expression of miR-141 induced an S-phase arrest in both cell lines, whereas the inhibition of miR-141 caused a G1 arrest in M059J and accelerated the S phase in M059K. An overexpression and suppression of miR-141 resulted in an aberrant expression of cell-cycle proteins, including p21. Moreover, MYT1L may be a transcription factor of p21 in p53-mutant cells, whereas DNAPK may function as a repressor of MYT1L. The findings revealed the crucial role of DNAPK in miR-141-mediated suppression of gliomagenesis and demonstrated that it may be a target molecule in miR-141-associated therapeutic interventions for glioblastoma.

Role of protein kinase CK2 in antitumor drug resistance

Drug resistance represents the major reason of pharmacological treatment failure. It is supported by a broad spectrum of mechanisms, whose molecular bases have been frequently correlated to aberrant protein phosphorylation. CK2 is a constitutively active protein kinase which phosphorylates hundreds of substrates; it is expressed in all cells, but its level is commonly found higher in cancer cells, where it plays anti-apoptotic, pro-migration and pro-proliferation functions. Several evidences support a role for CK2 in processes directly responsible of drug resistance, such as drug efflux and DNA repair; moreover, CK2 intervenes in signaling pathways which are crucial to evade drug response (as PI3K/AKT/PTEN, NF-κB, β-catenin, hedgehog signaling, p53), and controls the activity of chaperone machineries fundamental in resistant cells. Interestingly, a panel of specific and effective inhibitors of CK2 is available, and several examples are known of their efficacy in resistant cells, with synergistic effect when used in combination with conventional drugs, also in vivo. Here we analyze and discuss evidences supporting the hypothesis that CK2 targeting represents a valuable strategy to overcome drug resistance.

Protein Kinase CK2, a Potential Therapeutic Target in Carcinoma Management

The Protein kinase CK2 (formerly known as casein kinase 2) is a highly conserved serine/ threonine kinase overexpressed in various human carcinomas and its high expression often correlates with poor prognosis. CK2 protein is localized in the nucleus of many tumor cells and correlates with clinical features in many cases. Increased expression of CK2 in mice results in the development of various types of carcinomas (both solids and blood related tumors, such as (breast carcinoma, lymphoma, etc), which reveals its carcinogenic properties. CK2 plays essential roles in many key biological processes related to carcinoma, including cell apoptosis, DNA damage responses and cell cycle regulation. CK2 has become a potential anti-carcinoma target. Various CK2 inhibitors have been developed with anti-neoplastic properties against a variety of carcinomas. Some CK2 inhibitors have showed good results in in vitro and pre-clinical models, and have even entered in clinical trials. This article will review effects of CK2 and its inhibitors on common carcinomas in in vitro and pre-clinical studies.

CD163, a novel therapeutic target, regulates the proliferation and stemness of glioma cells via casein kinase 2

Glioma is a devastating cancer with a dismal prognosis and there is an urgent need to discover novel glioma-specific antigens for glioma therapy. Previous studies have identified CD163-positive tumour cells in certain solid tumours, but CD163 expression in glioma remains unknown. In this study, via an analysis of public datasets, we demonstrated that CD163 overexpression in glioma specimens correlated with an unfavourable patient prognosis. CD163 expression was increased in glioma cells, especially primary glioma cells. The loss of CD163 expression inhibited both cell cycle progression and the proliferation of glioblastoma multiforme (GBM) cell lines and primary glioma cells. CD163 interacted directly with casein kinase 2 (CK2) and CD163 silencing reduced AKT/GSK3β/β-catenin/cyclin D1 pathway activity via CK2. Moreover, CD163 was upregulated in CD133-positive glioma stem cells (GSCs), and CD163 downregulation decreased the expression of GSC markers, including CD133, ALDH1A1, NANOG and OCT4. The knockdown of CD163 impaired GSC stemness by inhibiting the CK2/AKT/GSK3β/β-catenin pathway. Finally, a CD163 antibody successfully induced complement-dependent cytotoxicity against glioma cells. Our findings indicate that CD163 contributes to gliomagenesis via CK2 and provides preclinical evidence that CD163 and the CD163 pathway might serve as a therapeutic target for glioma.

CX-4945 Induces Methuosis in Cholangiocarcinoma Cell Lines by a CK2-Independent Mechanism

Cholangiocarcinoma is a disease with a poor prognosis and increasing incidence and hence there is a pressing unmet clinical need for new adjuvant treatments. Protein kinase CK2 (previously casein kinase II) is a ubiquitously expressed protein kinase that is up-regulated in multiple cancer cell types. The inhibition of CK2 activity using CX-4945 (Silmitasertib) has been proposed as a novel treatment in multiple disease settings including cholangiocarcinoma. Here, we show that CX-4945 inhibited the proliferation of cholangiocarcinoma cell lines in vitro. Moreover, CX-4945 treatment induced the formation of cytosolic vacuoles in cholangiocarcinoma cell lines and other cancer cell lines. The vacuoles contained extracellular fluid and had neutral pH, features characteristic of methuosis. In contrast, simultaneous knockdown of both the α and α' catalytic subunits of protein kinase CK2 using small interfering RNA (siRNA) had little or no effect on the proliferation of cholangiocarcinoma cell lines and failed to induce the vacuole formation. Surprisingly, low doses of CX-4945 increased the invasive properties of cholangiocarcinoma cells due to an upregulation of matrix metallopeptidase 7 (MMP-7), while the knockdown of CK2 inhibited cell invasion. Our data suggest that CX-4945 inhibits cell proliferation and induces cell death via CK2-independent pathways. Moreover, the increase in cell invasion brought about by CX-4945 treatment suggests that this drug might increase tumor invasion in clinical settings.

Targeting hyperactivated DNA-PKcs by KU0060648 inhibits glioma progression and enhances temozolomide therapy via suppression of AKT signaling

The overall survival remains undesirable in clinical glioma treatment. Inhibition of DNA-PKcs activity by its inhibitors suppresses tumor growth and enhances chemosensitivity of several tumors to chemotherapy. However, whether DNA-PKcs could be a potential target in glioma therapy remains unknown. In this study, we reported that the hyperactivated DNA-PKcs was profoundly correlated with glioma malignancy and observe a significant association between DNA-PKcs activation and survival of the glioma patients. Our data also found that inhibition of DNA-PKcs by its inhibitor KU0060648 sensitized glioma cells to TMZ in vitro. Specifically, we demonstrated that KU0060648 interrupted the formation of DNA-PKcs/AKT complex, leading to suppression of AKT signaling and resultantly enhanced TMZ efficacy. Combination of KU0060648 and TMZ substantially inhibited downstream effectors of AKT. The in vivo results were similar to those obtained in vitro. In conclusion, this study indicated that inhibition of DNA-PKcs activity could suppress glioma malignancies and increase TMZ efficacy, which was mainly through regulation of the of AKT signaling. Therefore, DNA-PKcs/AKT axis may be a promising target for improving current glioma therapy.

TGF-β signaling is an effective target to impair survival and induce apoptosis of human cholangiocarcinoma cells: A study on human primary cell cultures

Cholangiocarcinoma (CCA) and its subtypes (mucin- and mixed-CCA) arise from the neoplastic transformation of cholangiocytes, the epithelial cells lining the biliary tree. CCA has a high mortality rate owing to its aggressiveness, late diagnosis and high resistance to radiotherapy and chemotherapeutics. We have demonstrated that CCA is enriched for cancer stem cells which express epithelial to mesenchymal transition (EMT) traits, with these features being associated with aggressiveness and drug resistance. TGF-β signaling is upregulated in CCA and involved in EMT. We have recently established primary cell cultures from human mucin- and mixed-intrahepatic CCA. In human CCA primary cultures with different levels of EMT trait expression, we evaluated the anticancer effects of: (i) CX-4945, a casein kinase-2 (CK2) inhibitor that blocks TGF-β1-induced EMT; and (ii) LY2157299, a TGF-β receptor I kinase inhibitor. We tested primary cell lines expressing EMT trait markers (vimentin, N-cadherin and nuclear catenin) but negative for epithelial markers, and cell lines expressing epithelial markers (CK19-positive) in association with EMT traits. Cell viability was evaluated by MTS assays, apoptosis by Annexin V FITC and cell migration by wound-healing assay. Results: at a dose of 10 μM, CX4945 significantly decreased cell viability of primary human cell cultures from both mucin and mixed CCA, whereas in CK19-positive cell cultures, the effect of CX4945 on cell viability required higher concentrations (>30μM). At the same concentrations, CX4945 also induced apoptosis (3- fold increase vs controls) which correlated with the expression level of CK2 in the different CCA cell lines (mucin- and mixed-CCA). Indeed, no apoptotic effects were observed in CK19-positive cells expressing lower CK2 levels. The effects of CX4945 on viability and apoptosis were associated with an increased number of γ-H2ax (biomarker for DNA double-strand breaks) foci, suggesting the active role of CK2 as a repair mechanism in CCAs. LY2157299 failed to influence cell proliferation or apoptosis but significantly inhibited cell migration. At a 50 μM concentration, in fact, LY2157299 significantly impaired (at 24, 48 and 120 hrs) the wound-healing of primary cell cultures from both mucin-and mixed-CCA. In conclusion, we demonstrated that CX4945 and LY2157299 exert relevant but distinct anticancer effects against human CCA cells, with CX4945 acting on cell viability and apoptosis, and LY2157299 impairing cell migration. These results suggest that targeting the TGF-β signaling with a combination of CX-4945 and LY2157299 could have potential benefits in the treatment of human CCA.

Protein kinase CK2 regulates redox homeostasis through NF-κB and Bcl-xL in cardiomyoblasts

Oxygen consumption is particularly elevated in cardiac cells as they are equipped with a large number of mitochondria and high levels of respiratory chain components. Consequently, production of reactive oxygen species (ROS) is tightly controlled as an imbalance in redox reactions can lead to irreversible cellular damage. siRNA-mediated down-regulation of protein kinase CK2 has been implicated in the accumulation of ROS in cells. The present study was undertaken in order to investigate the role of CK2 in redox homeostasis in cardiomyoblasts. We found that inhibition or silencing of CK2 causes elevated levels of ROS, notably superoxide radical, and this is accompanied by suppression of NF-κB transcriptional activity and mitochondrial dysfunction. We show that CK2 regulates the expression of manganese superoxide dismutase, the enzyme catalyzing the dismutation of superoxide, in cancer cells but not in cardiomyoblasts. Furthermore, we report evidence that impaired expression of CK2 results in destabilization of the Bcl-2 mammalian homolog Bcl-xL, which is known to stabilize the mitochondrial membrane potential, through a mechanism involving disruption of the chaperone function of heat shock protein 90. Analysis of differential mRNA expression related to oxidative stress revealed that CK2 silencing caused a statistically significant deregulation of four genes associated with the oxidative damage, i.e., Fmo2, Ptgs1, Dhcr24, and Ptgs2. Overall, the results reported here are consistent with the notion that CK2 plays a role in conferring protection against oxidative stress by positively regulating pro-survival signaling molecules and the protein folding machinery in cardiomyoblasts.

The small-molecule kinase inhibitor D11 counteracts 17-AAG-mediated up-regulation of HSP70 in brain cancer cells

Many types of cancer express high levels of heat shock proteins (HSPs) that are molecular chaperones regulating protein folding and stability ensuring protection of cells from potentially lethal stress. HSPs in cancer cells promote survival, growth and spreading even in situations of growth factors deprivation by associating with oncogenic proteins responsible for cell transformation. Hence, it is not surprising that the identification of potent inhibitors of HSPs, notably HSP90, has been the primary research focus, in recent years. Exposure of cancer cells to HSP90 inhibitors, including 17-AAG, has been shown to cause resistance to chemotherapeutic treatment mostly attributable to induction of the heat shock response and increased cellular levels of pro-survival chaperones. In this study, we show that treatment of glioblastoma cells with 17-AAG leads to HSP90 inhibition indicated by loss of stability of the EGFR client protein, and significant increase in HSP70 expression. Conversely, co-treatment with the small-molecule kinase inhibitor D11 leads to suppression of the heat shock response and inhibition of HSF1 transcriptional activity. Beside HSP70, Western blot and differential mRNA expression analysis reveal that combination treatment causes strong down-regulation of the small chaperone protein HSP27. Finally, we demonstrate that incubation of cells with both agents leads to enhanced cytotoxicity and significantly high levels of LC3-II suggesting autophagy induction. Taken together, results reported here support the notion that including D11 in future treatment regimens based on HSP90 inhibition can potentially overcome acquired resistance induced by the heat shock response in brain cancer cells.

CK2 in Cancer: Cellular and Biochemical Mechanisms and Potential Therapeutic Target

CK2 genes are overexpressed in many human cancers, and most often overexpression is associated with worse prognosis. Site-specific expression in mice leads to cancer development (e.g., breast, lymphoma) indicating the oncogenic nature of CK2. CK2 is involved in many key aspects of cancer including inhibition of apoptosis, modulation of signaling pathways, DNA damage response, and cell cycle regulation. A number of CK2 inhibitors are now available and have been shown to have activity against various cancers in vitro and in pre-clinical models. Some of these inhibitors are now undergoing exploration in clinical trials as well. In this review, we will examine some of the major cancers in which CK2 inhibition has promise based on in vitro and pre-clinical studies, the proposed cellular and signaling mechanisms of anti-cancer activity by CK2 inhibitors, and the current or recent clinical trials using CK2 inhibitors.

Contribution of classical end-joining to PTEN inactivation in p53-mediated glioblastoma formation and drug-resistant survival

DNA repair gene defects are found in virtually all human glioblastomas, but the genetic evidence for a direct role remains lacking. Here we demonstrate that combined inactivation of the XRCC4 non-homologous end-joining (NHEJ) DNA repair gene and p53 efficiently induces brain tumours with hallmark characteristics of human proneural/classical glioblastoma. The murine tumours exhibit PTEN loss of function instigated by reduced PTEN mRNA, and increased phosphorylated inactivation and stability as a consequence of aberrantly elevated CK2 provoked by p53 ablation and irrevocably deregulated by NHEJ inactivation. This results in DNA damage-resistant cytoplasmic PTEN and CK2 expression, and the attenuation of DNA repair genes. CK2 inhibition restores PTEN nuclear distribution and DNA repair activities and impairs tumour but not normal cell survival. These observations demonstrate that NHEJ contributes to p53-mediated glioblastoma suppression, and reveal a crucial role for PTEN in the early DNA damage signalling cascade, the inhibition of which promotes tumorigenicity and drug-resistant survival.

We know that defects in DNA repair genes are associated with cancer development. Here the authors eliminate XRCC4, a non-homologous end-joining protein, and p53 in the developing brain and find that this causes glioblastoma development as a consequence of reduced PTEN function.

Targeting CK2-driven non-oncogene addiction in B-cell tumors

Genetic mutations of oncogenes often underlie deranged cell growth and altered differentiation pathways leading to malignant transformation of B-lymphocytes. However, addiction to oncogenes is not the only drive to lymphoid tumor pathogenesis. Dependence on non-oncogenes, which act by propelling basic mechanisms of cell proliferation and survival, has also been recognized in the pathobiology of lymphoid leukemias, lymphomas and multiple myeloma. Among the growing number of molecules that may uphold non-oncogene addiction, a key place is increasingly being recognized to the serine-threonine kinase CK2. This enzyme is overexpressed and overactive in B-acute lymphoblastic leukemia, multiple myeloma, chronic lymphocytic leukemia and non-Hodgkin lymphomas, such as mantle cell, follicular, Burkitt's and diffuse large B-cell lymphomas. In these tumors, CK2 may serve the activity of oncogenes, similar to BCR-ABL and c-MYC, control the activation of critical signaling cascades, such as NF-κB (nuclear factor-κB), STAT3 (signal transducer and activator of transcription 3) and PTEN/PI3K/AKT (phosphatase and tensin homolog protein/phosphoinositide 3-kinase/AKR thymoma), and sustain multiple cellular stress-elicited pathways, such as the proteotoxic stress, unfolded protein and DNA-damage responses. CK2 has also been shown to have an essential role in tuning signals derived from the stromal tumor microenvironment. Not surprisingly, targeting CK2 in lymphoid tumor cell lines or mouse xenograft models can boost the cytotoxic effects of both conventional chemotherapeutics and novel agents, similar to heat-shock protein 90, proteasome and tyrosine kinases inhibitors. In this review, we summarize the evidence indicating how CK2 embodies most of the features of a cancer growth-promoting non-oncogene, focusing on lymphoid tumors. We further discuss the preclinical data of the use of small ATP-competitive CK2 inhibitors, which hold the promise to be additional options in novel drug combinations for the therapy of lymphoid and plasmacellular malignancies.

The kinase inhibitor D11 induces caspase-mediated cell death in cancer cells resistant to chemotherapeutic treatment

Background

Multi-drug resistance and predisposition to metastasize are major clinical problems in cancer treatment. Malignant primary brain tumor and pancreatic cancer are two well-known examples of malignant tumors resistant to conventional therapies where aberrant EGFR-mediated and NF-κB signal transduction pathways are likely to play an important role. We have recently identified 1,3-Dichloro-6-[(E)-((4-methoxyphenyl)imino)methyl] diben-zo(b,d) furan-2,7-diol (D11) as a potent and selective inhibitor of CK2 a serine/threonine protein kinase that modulates the aforementioned signaling cascades.

Methods

Human cancer cell lines (glioblastoma and pancreatic adenocarcinoma) resistant to conventional chemotherapeutic agents were incubated with increasing concentrations of D11 for variable amounts of time. Cell viability, cell death and effects on major signal transduction pathways deregulated in cancer cells were analyzed by ELISA, FACS and Western blot-based assays, respectively. Moreover, effects on cell migration and in cell protein-protein association were investigated by wound-healing and in situ proximity ligation assays, respectively.

Results

We show here, that D11 treatment leads to i) significant caspase-mediated apoptotic cell death, ii) down-regulation of EGFR expression and iii) inhibition of NF-κB transcriptional activity. Furthermore, cell exposure to D11 results in impaired cell migration and correlates with reduced expression of the ion co-transporter and cell volume regulator Na⁺-K⁺-2Cl⁻ (NKCC1).

Conclusions

Data reported here underline the therapeutic potential of D11 with respect to certain types of cancer that carry aberrant intracellular signaling cascades and/or exhibit sustained cell migration and suggest a new therapeutic strategy against chemotherapy resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-015-0234-6) contains supplementary material, which is available to authorized users.

Protein kinase CK2 in breast cancer: the CK2β regulatory subunit takes center stage in epithelial plasticity

Structurally, protein kinase CK2 consists of two catalytic subunits (α and α') and two regulatory subunits (β), which play a critical role in targeting specific CK2 substrates. Compelling evidence shows the complexity of the CK2 cellular signaling network and supports the view that this enzyme is a key component of regulatory protein kinase networks that are involved in several aspects of cancer. CK2 both activates and suppresses the expression of a number of essential oncogenes and tumor suppressors, and its expression and activity are upregulated in blood tumors and virtually all solid tumors. The prognostic significance of CK2α expression in association with various clinicopathological parameters highlighted this kinase as an adverse prognostic marker in breast cancer. In addition, several recent studies reported its implication in the regulation of the epithelial-to-mesenchymal transition (EMT), an early step in cancer invasion and metastasis. In this review, we briefly overview the contribution of CK2 to several aspects of cancer and discuss how in mammary epithelial cells, the expression of its CK2β regulatory subunit plays a critical role in maintaining an epithelial phenotype through CK2-mediated control of key EMT-related transcription factors. Importantly, decreased CK2β expression in breast tumors is correlated with inefficient phosphorylation and nuclear translocation of Snail1 and Foxc2, ultimately leading to EMT induction. This review highlights the pivotal role played by CK2β in the mammary epithelial phenotype and discusses how a modest alteration in its expression may be sufficient to induce dramatic effects facilitating the early steps in tumor cell dissemination through the coordinated regulation of two key transcription factors.

Involvement of the insulin-like growth factor binding proteins in the cancer cell response to DNA damage

The complex mechanisms that cells have evolved to meet the challenge of constant exposure to DNA-damaging stimuli, also serve to protect cancer cells from the cytotoxic effects of chemo- and radiotherapy. IGFBPs appear to be involved, directly or indirectly, in some of these protective mechanisms. Activation of p53 is an early response to genotoxic stress, and all six human IGFBP genes have predicted p53 response elements in their promoter and/or intronic regions, at least some of which are functional. IGFBP3 has been extensively characterized as a p53-inducible gene, but in some cases it is suppressed by mutant p53 forms. DNA double-strand breaks (DSBs), induced by radiotherapy and some chemotherapies, potentially lead to apoptotic cell death, senescence, or repair and recovery. DSB damage can be repaired by homologous recombination or non-homologous end-joining (NHEJ), depending on the cell cycle stage, availability of key repair proteins, and other factors. The epidermal growth factor receptor (EGFR) has been implicated in the NHEJ pathway, and EGFR inhibition may inhibit repair, promoting apoptosis and thus improving sensitivity to chemotherapy or radiotherapy. Both IGFBP-3 and IGFBP-6 interact with components of the NHEJ pathway, and IGFBP-3 can facilitate this process through direct interaction with both EGFR and the catalytic subunit of DNA-PK. Cell fate after DNA damage may in part be regulated by the balance between the sphingolipids ceramide and sphingosine-1-phosphate, and IGFBPs can influence the production of both lipids. A better understanding of the involvement of IGFBPs in the DNA damage response in cancer cells may lead to improved methods of sensitizing cancers to DNA-damaging therapies.

Cytotoxic effects exerted by pentachlorophenol by targeting nodal pro-survival signaling pathways in human pancreatic cancer cells

Pancreatic adenocarcinoma is one of the deadliest human solid tumors in the developed countries characterized by high resistance toward chemotherapeutic treatment. We have previously shown that silencing of the pro-survival protein kinase CK2 by RNA interference contributes to enhance the cytotoxicity of the chemotherapeutic agent 2′,2′-difluoro 2′-deoxycytidine (gemcitabine). Initial experiments showed that pentachlorophenol (PCP) inhibits CK2 and induces cell death in human pancreatic cancer cell lines. We report here evidence that exposure of this type of cells to PCP induces caspase-mediated apoptosis, inhibition of the lysosome cysteine protease cathepsin B and mitochondrial membrane depolarization. Beside cellular inhibition of CK2, the analysis of signaling pathways deregulated in pancreatic cancer cells revealed that PCP causes decreased phosphorylation levels of NF-κB/p65, suppresses its nuclear translocation and leads to activation of JNK-mediated stress response. Surprisingly, exposure to PCP results in increased phosphorylation levels of AKT at the canonical S473 and T308 activation sites supporting previous data showing that AKT phosphorylation is not predictive of tumor cell response to treatment. Taken together, our study provides novel insights into the effects induced by the exposure of pancreatic cancer cells to chlorinated aromatic compounds posing the basis for more advanced studies in vivo.

Casein kinase 2α regulates glioblastoma brain tumor-initiating cell growth through the β-catenin pathway

Glioblastoma (GBM) is the most common and fatal primary brain tumor in humans and it is essential that new and better therapies are developed to treat this disease. Previous research suggests that casein kinase 2 (CK2), may be a promising therapeutic target for GBMs. CK2 has enhanced expression or activity in numerous cancers, including GBM and it has been demonstrated that inhibitors of CK2 regressed tumor growth in GBM xenograft mouse models. Our studies demonstrate that the CK2 subunit, CK2α, is overexpressed in and plays an important role in regulating brain tumor initiating cells (BTIC) in GBM. Initial studies showed that two GBM cell lines (U87-MG and U138) transduced with CK2α had enhanced proliferation and anchorage-independent growth. Inhibition of CK2α using siRNA or small molecule inhibitors (TBBz, CX-4945) reduced cell growth and decreased tumor size and increased the survival rate in GBM xenograft mouse models. We also verified that inhibition of CK2α decreased the activity of a well-known GBM initiating cell regulator, β-catenin. Loss of CK2α decreased two β-catenin-regulated genes that are involved in GBM initiating cell growth, OCT4 and NANOG. To determine the importance of CK2α in GBM stem cell maintenance, we reduced CK2α activity in primary GBM samples and tumor spheres derived from GBM patients. We discovered that loss of CK2α activity reduced the sphere forming capacity of BTIC and decreased numerous GBM stem cell markers including CD133, CD90, CD49f, and A2B5. Our study suggests that CK2α is involved in GBM tumorigenesis by maintaining BTIC through the regulation of β-catenin.

Beyond DNA repair: DNA-PK function in cancer

The DNA-dependent protein kinase (DNA-PK) is a pivotal component of the DNA repair machinery that governs the response to DNA damage, serving to maintain genome integrity. However, the DNA-PK kinase component was initially isolated with transcriptional complexes, and recent findings have illuminated the impact of DNA-PK-mediated transcriptional regulation on tumor progression and therapeutic response. DNA-PK expression has also been correlated with poor outcome in selected tumor types, further underscoring the importance of understanding its role in disease. Herein, the molecular and cellular consequences of DNA-PK are considered, with an eye toward discerning the rationale for therapeutic targeting of DNA-PK.

SIGNIFICANCE

Although DNA-PK is classically considered a component of damage response, recent findings illuminate damage-independent functions of DNA-PK that affect multiple tumor-associated pathways and provide a rationale for the development of novel therapeutic strategies.

Beyond DNA repair: DNA-PK function in cancer

The DNA-dependent protein kinase (DNA-PK) is a pivotal component of the DNA repair machinery that governs the response to DNA damage, serving to maintain genome integrity. However, the DNA-PK kinase component was initially isolated with transcriptional complexes, and recent findings have illuminated the impact of DNA-PK-mediated transcriptional regulation on tumor progression and therapeutic response. DNA-PK expression has also been correlated with poor outcome in selected tumor types, further underscoring the importance of understanding its role in disease. Herein, the molecular and cellular consequences of DNA-PK are considered, with an eye toward discerning the rationale for therapeutic targeting of DNA-PK.

SIGNIFICANCE

Although DNA-PK is classically considered a component of damage response, recent findings illuminate damage-independent functions of DNA-PK that affect multiple tumor-associated pathways and provide a rationale for the development of novel therapeutic strategies.

DNA-PK: a dynamic enzyme in a versatile DSB repair pathway

DNA double stranded breaks (DSBs) are the most cytoxic DNA lesion as the inability to properly repair them can lead to genomic instability and tumorigenesis. The prominent DSB repair pathway in humans is non-homologous end-joining (NHEJ). In the simplest sense, NHEJ mediates the direct re-ligation of the broken DNA molecule. However, NHEJ is a complex and versatile process that can repair DSBs with a variety of damages and ends via the utilization of a significant number of proteins. In this review we will describe the important factors and mechanisms modulating NHEJ with emphasis given to the versatility of this repair process and the DNA-PK complex.

HspA1A facilitates DNA repair in human bronchial epithelial cells exposed to Benzo[a]pyrene and interacts with casein kinase 2

Benzo[a]pyrene (BaP) is a ubiquitously distributed environmental pollutant that induces deoxyribonucleic acid (DNA) damage. The inducible heat shock protein (HspA1A) can function as a molecular chaperone; however, its role in DNA repair remains largely unknown. In the present study, human bronchial epithelial cells (16HBE) stably transfected with plasmids carrying HspA1A gene or shRNAs against HspA1A were treated with BaP. DNA damage levels of the cells were evaluated by comet assay. Results suggest that HspA1A could protect cells against DNA damage and facilitate the decrease of DNA damage levels during the first 2 h of DNA repair. DNA repair capacity (DRC) of Benzo(a)pyrene diol epoxide (BPDE)-DNA adducts was evaluated by host cell reactivation assay in the stable 16HBE cells transfected with luciferase reporter vector PCMVluc pretreated with BPDE. Compared with control cells, cells overexpressing HspA1A showed higher DRC (p < 0.01 at 10 μM BPDE and p < 0.05 at 20 μM BPDE, respectively), while knockdown of HspA1A inhibited DNA repair (p < 0.05 at 10 μM BPDE). Moreover, casein kinase 2 (CK2) was shown to interact with HspA1A by mass spectrometry and co-immunoprecipitation assays. The two proteins were co-localized in the cell nucleus and perinuclear region during DNA repair, and were identified by confocal laser scanning microscope. In addition, cells overexpressing HspA1A showed an increased CK2 activity after BaP treatment compared with control cells (p < 0.01). Our results suggest that HspA1A facilitates DNA repair after BaP treatment. HspA1A also interacts with CK2 and enhances the kinase activities of CK2 during DNA repair.

Casein Kinase 2: a novel player in glioblastoma therapy and cancer stem cells

Casein kinase 2 (CK2) is an oncogenic protein kinase which contributes to tumor development, proliferation, and suppression of apoptosis in multiple cancer types. The mechanism by which CK2 expression and activity leads to tumorigenesis in glioblastoma (GBM), a stage IV primary brain tumor, is being studied. Recent studies demonstrate that CK2 plays an important role in GBM formation and growth through the inhibition of tumor suppressors and activation of oncogenes. In addition, intriguing new reports indicate that CK2 may regulate GBM formation in a novel manner; CK2 may play a critical role in cancer stem cell (CSC) maintenance. Since glial CSCs have the ability to self-renew and initiate tumor growth, new treatments which target these CSCs are needed to treat this fatal disease. Inhibition of CK2 is potentially a novel method to inhibit GBM growth and reoccurrence by targeting the glial CSCs. A new, orally available, selective CK2 inhibitor, CX-4945 has had promising results when tested in cancer cell lines, in vivo xenograft models, and human clinical trials. The development of CK2 targeted inhibitors, starting with CX-4945, may lead to a new class of more effective cancer therapies.

Identification of a novel protein interaction motif in the regulatory subunit of casein kinase 2

Casein kinase 2 (CK2) regulates multiple cellular processes and can promote oncogenesis. Interactions with the CK2β regulatory subunit of the enzyme target its catalytic subunit (CK2α or CK2α') to specific substrates; however, little is known about the mechanisms by which these interactions occur. We previously showed that by binding CK2β, the Epstein-Barr virus (EBV) EBNA1 protein recruits CK2 to promyelocytic leukemia (PML) nuclear bodies, where increased CK2-mediated phosphorylation of PML proteins triggers their degradation. Here we have identified a KSSR motif near the dimerization interface of CK2β as forming part of a protein interaction pocket that mediates interaction with EBNA1. We show that the EBNA1-CK2β interaction is primed by phosphorylation of EBNA1 on S393 (within a polyserine region). This phosphoserine is critical for EBNA1-induced PML degradation but does not affect EBNA1 functions in EBV replication or segregation. Using comparative proteomics of wild-type (WT) and KSSR mutant CK2β, we identified an uncharacterized cellular protein, C18orf25/ARKL1, that also binds CK2β through the KSSR motif and show that this involves a polyserine sequence resembling the CK2β binding sequence in EBNA1. Therefore, we have identified a new mechanism of CK2 interaction used by viral and cellular proteins.

Targeting protein kinase CK2 suppresses prosurvival signaling pathways and growth of glioblastoma

PURPOSE

Gliomas are the most frequently occurring primary malignancies in the brain, and glioblastoma is the most aggressive of these tumors. Protein kinase CK2 is composed of two catalytic subunits (α and/or α') and two β regulatory subunits. CK2 suppresses apoptosis, promotes neoangiogenesis, and enhances activation of the JAK/STAT, NF-κB, PI3K/AKT, Hsp90, Wnt, and Hedgehog pathways. Aberrant activation of the NF-κB, PI3K/AKT, and JAK/STAT-3 pathways is implicated in glioblastoma progression. As CK2 is involved in their activation, the expression and function of CK2 in glioblastoma was evaluated.

EXPERIMENTAL DESIGN AND RESULTS

Analysis of 537 glioblastomas from The Cancer Genome Atlas Project demonstrates the CSNK2A1 gene, encoding CK2α, has gene dosage gains in glioblastoma (33.7%), and is significantly associated with the classical glioblastoma subtype. Inhibition of CK2 activity by CX-4945, a selective CK2 inhibitor, or CK2 knockdown by siRNA suppresses activation of the JAK/STAT, NF-κB, and AKT pathways and downstream gene expression in human glioblastoma xenografts. On a functional level, CX-4945 treatment decreases the adhesion and migration of glioblastoma cells, in part through inhibition of integrin β1 and α4 expression. In vivo, CX-4945 inhibits activation of STAT-3, NF-κB p65, and AKT, and promotes survival of mice with intracranial human glioblastoma xenografts.

CONCLUSIONS

CK2 inhibitors may be considered for treatment of patients with glioblastoma.

Carboxyl-terminal modulator protein positively regulates Akt phosphorylation and acts as an oncogenic driver in breast cancer

Akt activation has been implicated broadly in tumorigenesis, but the basis for its dysregulation in cancer cells is incompletely understood. In this study, we sought to clarify a regulatory role for the Akt-binding carboxy-terminal modulator protein (CTMP), which has been controversial. In evaluating CTMP expression in paired normal-tumor specimens of 198 patients with breast cancer, we found that CTMP was upregulated in breast tumors, where it was associated with poor patient survival. Notably, CTMP expression also correlated positively with Akt phosphorylation in breast cancer clinical specimens and cell lines. Furthermore, ectopic expression of CTMP promoted cell proliferation and enhanced the tumorigenic properties of estrogen-dependent breast cancer cells. This effect was correlated with increased sensitivity to insulin-induced Akt phosphorylation, which is mediated primarily by the phosphoinositide 3-kinase-Akt pathway. In contrast, short hairpin RNA-mediated silencing of endogenous CTMP decreased the proliferation of estrogen-dependent or estrogen-independent breast cancer cells. Mechanistic investigations defined the N-terminal domain of CTMP at amino acids 1 to 64 as responsible for Akt binding. Taken together, our results firmly corroborate the concept that CTMP promotes Akt phosphorylation and functions as an oncogenic molecule in breast cancer.

Cis-9,trans-11-conjugated linoleic acid affects lipid raft composition and sensitizes human colorectal adenocarcinoma HT-29 cells to X-radiation

BACKGROUND

Investigations concerned the mechanism of HT-29 cells radiosensitization by cis-9,trans-11-conjugated linoleic acid (c9,t11-CLA), a natural component of human diet with proven antitumor activity.

METHODS

The cells were incubated for 24h with 70μM c9,t11-CLA and then X-irradiated. The following methods were used: gas chromatography (incorporation of the CLA isomer), flow cytometry (cell cycle), cloning (survival), Western blotting (protein distribution in membrane fractions), and pulse-field gel electrophoresis (rejoining of DNA double-strand breaks). In parallel, DNA-PK activity, γ-H2AX foci numbers and chromatid fragmentation were estimated. Gene expression was analysed by RT-PCR and chromosomal aberrations by the mFISH method. Nuclear accumulation of the EGF receptor (EGFR) was monitored by ELISA.

RESULTS AND CONCLUSIONS

C9,t11-CLA sensitized HT-29 cells to X-radiation. This effect was not due to changes in cell cycle progression or DNA-repair-related gene expression. Post-irradiation DSB rejoining was delayed, corresponding with the insufficient DNA-PK activation, although chromosomal aberration frequencies did not increase. Distributions of cholesterol and caveolin-1 in cellular membrane fractions changed. The nuclear EGFR translocation, necessary to increase the DNA-PK activity in response to oxidative stress, was blocked. We suppose that c9,t11-CLA modified the membrane structure, thus disturbing the intracellular EGFR transport and the EGFR-dependent pro-survival signalling, both functionally associated with lipid raft properties.

GENERAL SIGNIFICANCE

The results point to the importance of the cell membrane interactions with the nucleus after injury inflicted by X -rays. Compounds like c9,t11-CLA, that specifically alter membrane properties, could be used to develop new anticancer strategies.

Downregulation of protein kinase CK2 induces autophagic cell death through modulation of the mTOR and MAPK signaling pathways in human glioblastoma cells

Glioblastoma multiforme is the most common primary brain tumor and one of the most aggressive types of cancer in adults. Survival signaling and apoptosis resistance are hallmarks of malignant glioma cells. However, recent studies have shown that other types of cell death such as autophagy can be induced in malignant glioma cells. This suggests that stimulation of this process may be explored in new therapeutic strategies against glioblastoma multiforme. Protein kinase CK2 is a highly conserved and constitutively active enzyme that promotes numerous cellular processes such as survival, proliferation and differentiation. CK2 has been found elevated in several malignancies including brain tumors, and to confer resistance against chemotherapeutic agents and apoptotic stimuli. Recently, we have shown that the siRNA-mediated downregulation of CK2 leads to cell death in DNA-PK-proficient human glioblastoma cells. We show, here, that lack of CK2 results in significant induction of autophagic cell death in two human glioblastoma cell lines, M059K and T98G, as indicated by the positive staining of cells with the acidotropic dye acridine orange, and the specific recruitment of microtubule-associated protein 1 light chain 3 (LC3) to autophagosome membranes. Induction of autophagy is accompanied by CK2-dependent decreased phosphorylation of p70 ribosomal S6 and AKT kinases and significantly reduced expression levels of Raptor. In contrast, phosphorylation and activity levels of ERK1/2 are enhanced suggesting an inhibition of the PI3K/AKT/mTORC1 and activation of the ERK1/2 pathways. Furthermore, siRNA-mediated silencing of CK2 results in increased mitochondrial superoxide production in both glioblastoma cell lines. However, mitochondrial reactive oxygen species release correlates with induction of autophagy only in T98G cells. Taken together, our findings identify CK2 as a novel component of the autophagic machinery and underline the potential of its downregulation to kill glioblastoma cells by overcoming the resistance to multiple anticancer agents.

Casein kinase 2 inhibition modulates the DNA damage response but fails to radiosensitize malignant glioma cells

Inhibitors of casein kinase 2 (CK2), a regulator of cell proliferation and mediator of the DNA damage response, are being evaluated in clinical trials for the treatment of cancers. Apigenin was capable of inhibiting the activation of CK2 following γ irradiation in LN18 and U87 malignant glioma cells. Apigenin and siRNA-mediated CK2 protein depletion further inhibited NF-κB activation and altered the Tyr68 phosphorylation of Chk2 kinase, a DNA damage response checkpoint kinase, following irradiation. However, CK2 inhibition did not decrease the ability of these glioma cells to repair double-strand DNA breaks, as assessed by COMET assays and γ-H2Ax staining. Likewise, apigenin and siRNA-induced depletion of CK2 failed to sensitize glioma cells to the cytotoxic effect of 2 to 10 G-rays of γ irradiation, as assessed by clonogenic assays. These results contrast with those found in other cancer types, and urge to prudence regarding the inclusion of malignant glioma patients in clinical trials that assess the radiosensitizing role of CK2 inhibitors in solid cancers.

Protein kinase CK2 localizes to sites of DNA double-strand break regulating the cellular response to DNA damage

Background

The DNA-dependent protein kinase (DNA-PK) is a nuclear complex composed of a large catalytic subunit (DNA-PKcs) and a heterodimeric DNA-targeting subunit Ku. DNA-PK is a major component of the non-homologous end-joining (NHEJ) repair mechanism, which is activated in the presence of DNA double-strand breaks induced by ionizing radiation, reactive oxygen species and radiomimetic drugs. We have recently reported that down-regulation of protein kinase CK2 by siRNA interference results in enhanced cell death specifically in DNA-PKcs-proficient human glioblastoma cells, and this event is accompanied by decreased autophosphorylation of DNA-PKcs at S2056 and delayed repair of DNA double-strand breaks.

Results

In the present study, we show that CK2 co-localizes with phosphorylated histone H2AX to sites of DNA damage and while CK2 gene knockdown is associated with delayed DNA damage repair, its overexpression accelerates this process. We report for the first time evidence that lack of CK2 destabilizes the interaction of DNA-PKcs with DNA and with Ku80 at sites of genetic lesions. Furthermore, we show that CK2 regulates the phosphorylation levels of DNA-PKcs only in response to direct induction of DNA double-strand breaks.

Conclusions

Taken together, these results strongly indicate that CK2 plays a prominent role in NHEJ by facilitating and/or stabilizing the binding of DNA-PKcs and, possibly other repair proteins, to the DNA ends contributing to efficient DNA damage repair in mammalian cells.

Inhibition of Casein kinase-2 induces p53-dependent cell cycle arrest and sensitizes glioblastoma cells to tumor necrosis factor (TNFα)-induced apoptosis through SIRT1 inhibition

Glioblastoma multiforme (GBM) are resistant to TNFα-induced apoptosis and blockade of TNFα-induced NF-κB activation sensitizes glioma cells to apoptosis. As Casein kinase-2 (CK2) induces aberrant NF-κB activation and as we observed elevated CK2 levels in GBM tumors, we investigated the potential of CK2 inhibitors (CK2-Is) - DRB and Apigenin in sensitizing glioma cells to TNFα-induced apoptosis. CK2-Is and CK2 small interfering RNA (siRNA) reduced glioma cell viability, inhibited TNFα-mediated NF-κB activation, and sensitized cell to TNFα-induced apoptosis. Importantly, CK2-Is activated p53 function in wild-type but not in p53 mutant cells. Activation of p53 function involved its increased transcriptional activation, DNA-binding ability, increased expression of p53 target genes associated with cell cycle progression and apoptosis. Moreover, CK2-Is decreased telomerase activity and increased senescence in a p53-dependent manner. Apoptotic gene profiling indicated that CK2-Is differentially affect p53 and TNFα targets in p53 wild-type and mutant glioma cells. CK2-I decreased MDM2-p53 association and p53 ubiquitination to enhance p53 levels. Interestingly, CK2-Is downregulated SIRT1 activity and over-expression of SIRT1 decreased p53 transcriptional activity and rescued cells from CK2-I-induced apoptosis. This ability of CK2-Is to sensitize glioma to TNFα-induced death via multiple mechanisms involving abrogation of NF-κB activation, reactivation of wild-type p53 function and SIRT1 inhibition warrants investigation.

Sublethal doses of β-amyloid peptide abrogate DNA-dependent protein kinase activity

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer disease (AD). In multicellular eukaryotes, double strand breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by the nonhomologous end joining pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains, but the molecular player causing DNA repair dysfunction is still undetermined. β-Amyloid (Aβ), a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by reactive oxygen species generation and oxidative stress induction, which may then cause DNA damage. Here, we show that in PC12 cells sublethal concentrations of aggregated Aβ(25-35) inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to nonlethal oxidative injury. The inhibition of DNA-PK activity is associated with down-regulation of the catalytic subunit DNA-PK (DNA-PKcs) protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of Aβ(1-42) oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species, and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall, these findings suggest that Aβ mediates inhibition of the DNA-PK-dependent nonhomologous end joining pathway contributing to the accumulation of DSBs that, if not efficiently repaired, may lead to the neuronal loss observed in AD.

Lack of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is accompanied by increased CK2α' levels

DNA-PKcs is the catalytic subunit of DNA-dependent protein kinase, an enzyme necessary for non-homologous end-joining (NHEJ) and hence repair of DNA double strand breaks. Characterization of two isogenic cell lines, M059K and M059J, which are DNA-PKcs-proficient and -deficient, respectively, revealed that lack of DNA-PKcs is accompanied by an increase in the protein level of one of the catalytic isozymes of protein kinase CK2, i.e., CK2α' and a concomitant increase in CK2 activity. The increase was also detectable at the mRNA level as measured by quantitative real time PCR. However, no increase at the DNA level was observed either by comparative PCR or fluorescent in situ hybridization indicating that gene amplification is not involved. Interestingly, only CK2α' was increased and not the other two subunits of CK2, i.e., CK2β or CK2α. In addition, the increase in CK2α' protein level was also observed in a DNA-PKcs-deficient mouse cell line.