Role of protein kinase CK2 in the regulation of tumor necrosis factor-related apoptosis inducing ligand-induced apoptosis in prostate cancer cells

Protein kinase CK2 – diverse roles in cancer cell biology and therapeutic promise

The association of protein kinase CK2 (formerly casein kinase II or 2) with cell growth and proliferation in cells was apparent at early stages of its investigation. A cancer-specific role for CK2 remained unclear until it was determined that CK2 was also a potent suppressor of cell death (apoptosis); the latter characteristic differentiated its function in normal versus malignant cells because dysregulation of both cell growth and cell death is a universal feature of cancer cells. Over time, it became evident that CK2 exerts its influence on a diverse range of cell functions in normal as well as in transformed cells. As such, CK2 and its substrates are localized in various compartments of the cell. The dysregulation of CK2 is documented in a wide range of malignancies; notably, by increased CK2 protein and activity levels with relatively moderate change in its RNA abundance. High levels of CK2 are associated with poor prognosis in multiple cancer types, and CK2 is a target for active research and testing for cancer therapy. Aspects of CK2 cellular roles and targeting in cancer are discussed in the present review, with focus on nuclear and mitochondrial functions and prostate, breast and head and neck malignancies.

Anti-Growth, Anti-Angiogenic, and Pro-Apoptotic Effects by CX-4945, an Inhibitor of Casein Kinase 2, on HuCCT-1 Human Cholangiocarcinoma Cells via Control of Caspase-9/3, DR-4, STAT-3/STAT-5, Mcl-1, eIF-2α, and HIF-1α

Overexpression of casein kinase 2 (CK2) has an oncogenic and pro-survival role in many cancers. CX-4945 (Silmitasertib) is a CK2 inhibitor with anti-cancerous and anti-angiogenic effects. Up to date, the anti-cancer effect and mechanism of CX-4945 on human cholangiocarcinoma (CCA) remain unclear. This study investigated whether CX-4945 inhibits growth and induces apoptosis of HuCCT-1 cells, a human CCA cell line. Of note, treatment with CX-4945 at 20 μM markedly reduced survival and induced apoptosis of HuCCT-1 cells, as evidenced by nuclear DNA fragmentation, PARP cleavage, activation of caspase-9/3, and up-regulation of DR-4. Although CX-4945 did not affect the phosphorylation and expression of CK2, it vastly inhibited the phosphorylation of CK2 substrates, supporting the drug’s efficacy in inhibiting CK2 and its downstream pathway. Importantly, knockdown of CK2 that partially suppressed the phosphorylation of CK2 substrates resulted in a significant reduction of HuCCT-1 cell survival. In addition, CX-4945 reduced the phosphorylation and expression of STAT-3 and STAT-5 in HuCCT-1 cells, and pharmacological inhibition or respective knockdown of these proteins resulted in significant growth suppression of HuCCT-1 cells. CX-4945 also had abilities to decrease Mcl-1 expression while increasing eIF-2α phosphorylation in HuCCT-1 cells. Furthermore, there was a time-differential negative regulation of HIF-1α expression by CX-4945 in HuCCT-1 cells, and knockdown of HIF-1α caused a significant reduction of the cell survival. In summary, these results demonstrated that CX-4945 has anti-growth, anti-angiogenic, and pro-apoptotic effects on HuCCT-1 cells, which are mediated through control of CK2, caspase-9/3, DR-4, STAT-3/5, Mcl-1, eIF-2α, and HIF-1α.

Inhibition of CK2 Reduces NG2 Expression in Juvenile Angiofibroma

Juvenile angiofibroma (JA) is a rare fibrovascular neoplasm predominately found within the posterior nasal cavity of adolescent males. JA expresses the proteoglycan nerve–glial antigen (NG)2, which crucially determines the migratory capacity of distinct cancer cells. Moreover, it is known that the protein kinase CK2 regulates NG2 gene expression. Therefore, in the present study, we analyzed whether the inhibition of CK2 suppresses NG2-dependent JA cell proliferation and migration. For this purpose, we assessed the expression of NG2 and CK2 in patient-derived JA tissue samples, as well as in patient-derived JA cell cultures by Western blot, immunohistochemistry, flow cytometry and quantitative real-time PCR. The mitochondrial activity, proliferation and migratory capacity of the JA cells were determined by water-soluble tetrazolium (WST)-1, 5-bromo-2′-deoxyuridine (BrdU) and collagen sprouting assays. We found that NG2 and CK2 were expressed in both the JA tissue samples and cell cultures. The treatment of the JA cells with the two CK2 inhibitors, CX-4945 and SGC-CK2-1, significantly reduced NG2 gene and protein expression when compared to the vehicle-treated cells. In addition, the loss of CK2 activity suppressed the JA cell proliferation and migration. These findings indicate that the inhibition of CK2 may represent a promising therapeutic approach for the treatment of NG2-expressing JA.

Protein kinase CK2 impact on intracellular calcium homeostasis in prostate cancer

Protein kinase CK2 plays multiple roles in cell function in normal and disease states. CK2 is elevated in numerous types of cancer cells, and CK2 suppression of apoptosis represents a key link to the cancer cell phenotype. CK2 regulation of cell survival and death involves diverse processes, and our previous work suggested that mitochondrial machinery is a key locus of this function. One of the earliest responses of prostate cells to inhibition of CK2 is a change in mitochondrial membrane potential, possibly associated with Ca²⁺ signaling. Thus, in the present work, we investigated early impact of CK2 on intracellular Ca²⁺ dynamics. Three prostate cancer (PCa) cell lines, PC3-LN4, C4-2B, and 22Rv1, were studied. PCa cells were treated with the CK2 small molecule inhibitors 4,5,6,7-tetrabrombenzotriazole and CX-4945 followed by analysis of Ca²⁺ levels in various cellular compartments over time. The results showed dose-dependent loss in cytosolic Ca²⁺ levels starting within 2 min and reaching maximal loss within 5-10 min. There was a concomitant increase in Ca²⁺ in the endoplasmic reticulum (ER) and mitochondrial compartments. The results suggest that inhibition of CK2 activity results in a rapid movement of Ca²⁺ out of the cytosol and into the ER and mitochondria, which may be among the earliest contributory factors for induction of apoptosis in cells subjected to inhibition of CK2. In cells with death-inducing levels of CK2 inhibition, total cellular Ca²⁺ levels dropped at 2 h post-treatment. These novel observations represent a potential mechanism underlying regulation of cell survival and death by CK2 activity.

Design, synthesis, and evaluation of 4,5,6,7-tetrahydrobenzo[d]thiazole-based novel dual kinase inhibitors of CK2 and GSK3β

Casein kinase 2 (CK2) and glycogen synthase kinase-3beta (GSK3β) are responsible for the phosphorylation of a tumor suppressor protein (PTEN) in a cooperative manner which causes its deactivation. Thus, it is essential to inhibit both kinases simultaneously to prevent PTEN deactivation more efficiently. In this study, we have designed a novel lead from Hit15 which was identified in silico as a dual kinase inhibitor against CK2 and GSK3β through our previous study. The dataset of structural analogs of the lead was designed and confirmed by pharmacophore mapping and molecular docking. The screened analogs were considered further and a series of "tetrahydrobenzo[d]thiazoles" were synthesized. Compound 1g has shown highest dual kinase inhibitory activity at a concentration of 1.9 μM against CK2 and 0.67 μM against GSK3β. Our results suggest that the presence of a carboxyl group at the meta position of the phenyl ring plays a vital role in dual kinase inhibition.

Pathogenesis of cardiac ischemia reperfusion injury is associated with CK2α-disturbed mitochondrial homeostasis via suppression of FUNDC1-related mitophagy

Disturbed mitochondrial homeostasis contributes to the pathogenesis of cardiac ischemia reperfusion (IR) injury, although the underlying mechanism remains elusive. Here, we demonstrated that casein kinase 2α (CK2α) was upregulated following acute cardiac IR injury. Increased CK2α was shown to be instrumental to mitochondrial damage, cardiomyocyte death, infarction area expansion and cardiac dysfunction, whereas cardiac-specific CK2α knockout (CK2α^CKO) mice were protected against IR injury and mitochondrial damage. Functional assay indicated that CK2α enhanced the phosphorylation (inactivation) of FUN14 domain containing 1 (FUNDC1) via post-transcriptional modification at Ser13, thus effectively inhibiting mitophagy. Defective mitophagy failed to remove damaged mitochondria induced by IR injury, resulting in mitochondrial genome collapse, electron transport chain complex (ETC) inhibition, mitochondrial biogenesis arrest, cardiolipin oxidation, oxidative stress, mPTP opening, mitochondrial debris accumulation and eventually mitochondrial apoptosis. In contrast, loss of CK2α reversed the FUNDC1-mediated mitophagy, providing a survival advantage to myocardial tissue following IR stress. Interestingly, mice deficient in both CK2α and FUNDC1 failed to show protection against IR injury and mitochondrial damage through a mechanism possible attributed to lack of mitophagy. Taken together, our results confirmed that CK2α serves as a negative regulator of mitochondrial homeostasis via suppression of FUNDC1-required mitophagy, favoring the development of cardiac IR injury.

Sensitization of glycoengineered interferon-β1a-resistant cancer cells by cFLIP inhibition for enhanced anti-cancer therapy

In this study, we examined the molecular mechanism underlying the resistance of cancer cells to R27T, a glycoengineered version of recombinant human interferon (IFN)-β1a, and sought to overcome R27T resistance through combination therapy. R27T has been shown to induce anti-proliferation and apoptosis in human OVCAR-3 and MCF-7 cells, but not in HeLa cells. R27T treatment increased caspase-8 activity and the consequent cleavage of caspase-8 and -3 in R27T-sensitive OVCAR-3 cells, but not in R27T-resistant HeLa cells. Conversely, R27T increased the expression of cellular FLICE-like inhibitory protein (cFLIP) in HeLa cells, but not in OVCAR-3 cells. The sensitization of HeLa cells with cFLIP small interfering RNA or 4,5,6,7-tetrabromobenzotriazole (TBB, an inhibitor of casein kinase-2) facilitated R27T-induced caspase activation, and consequently apoptosis. In OVCAR-3-xenografted mice, intraperitoneal administration of R27T showed 2.1-fold higher anti-tumor efficacy than did the control vehicle. The combined administration of R27T and TBB showed the greatest anti-tumor effect in HeLa tumor-bearing mice, reducing the relative tumor volume by 35.7% compared to that in R27T-treated mice. Taken together, our results suggest that R27T has potential as an anti-cancer drug, and combination therapy with cFLIP inhibitors may be an effective strategy for overcoming R27T resistance.

Casein Kinase II (CK2), Glycogen Synthase Kinase-3 (GSK-3) and Ikaros mediated regulation of leukemia

Signaling networks that regulate cellular proliferation often involve complex interactions between several signaling pathways. In this manuscript we review the crosstalk between the Casein Kinase II (CK2) and Glycogen Synthase Kinase-3 (GSK-3) pathways that plays a critical role in the regulation of cellular proliferation in leukemia. Both CK2 and GSK-3 are potential targets for anti-leukemia treatment. Previously published data suggest that CK2 and GSK-3 act synergistically to promote the phosphatidylinositol-3 kinase (PI3K) pathway via phosphorylation of PTEN. More recent data demonstrate another mechanism through which CK2 promotes the PI3K pathway - via transcriptional regulation of PI3K pathway genes by the newly-discovered CK2-Ikaros axis. Together, these data suggest that the CK2 and GSK-3 pathways regulate AKT/PI3K signaling in leukemia via two complementary mechanisms: a) direct phosphorylation of PTEN and b) transcriptional regulation of PI3K-promoting genes. Functional interactions between CK2, Ikaros and GSK3 define a novel signaling network that regulates proliferation of leukemia cells. This regulatory network involves both direct posttranslational modifications (by CK and GSK-3) and transcriptional regulation (via CK2-mediated phosphorylation of Ikaros). This information provides a basis for the development of targeted therapy for leukemia.

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.

A CK2-targeted Pt(IV) prodrug to disrupt DNA damage response

A Pt(IV) prodrug, Cx-platin, containing CX-4945 (a CK2 inhibitor) as an axial ligand was designed and prepared by targeting CK2 to disrupt DNA damage response. In vitro study indicated that Cx-platin had superior cytotoxicity to cisplatin against a number of cancer cell lines with distinct CK2-expressed levels, caused CK2-overexpressed cancer cells death via suppressing CK2-mediated DNA damage repair and reversed cisplatin resistance. Mechanistic investigation suggested that the potent antitumor activity of Cx-platin resulted from its major suppression of CK2-phosphorylated MDC1 to combine FHA domain of aprataxin to DNA double strand breaks (DSBs) caused by improved cellular uptakes of Pt and ATM deactivation. Further in vivo tests exhibited that Cx-platin displayed high tumor inhibition rates, increased weight gain, and hardly toxicity effects in contrast to cisplatin.

Regulation of cellular proliferation in acute lymphoblastic leukemia by Casein Kinase II (CK2) and Ikaros

The IKZF1 gene encodes the Ikaros protein, a zinc finger transcriptional factor that acts as a master regulator of hematopoiesis and a tumor suppressor in leukemia. Impaired activity of Ikaros is associated with the development of high-risk acute lymphoblastic leukemia (ALL) with a poor prognosis. The molecular mechanisms that regulate Ikaros' function as a tumor suppressor and regulator of cellular proliferation are not well understood. We demonstrated that Ikaros is a substrate for Casein Kinase II (CK2), an oncogenic kinase that is overexpressed in ALL. Phosphorylation of Ikaros by CK2 impairs Ikaros' DNA-binding ability, as well as Ikaros' ability to regulate gene expression and function as a tumor suppressor in leukemia. Targeting CK2 with specific inhibitors restores Ikaros' function as a transcriptional regulator and tumor suppressor resulting in a therapeutic, anti-leukemia effect in a preclinical model of ALL. Here, we review the genes and pathways that are regulated by Ikaros and the molecular mechanisms through which Ikaros and CK2 regulate cellular proliferation in leukemia.

Phosphorylation of apoptosis repressor with caspase recruitment domain by protein kinase CK2 contributes to chemotherapy resistance by inhibiting doxorubicin induced apoptosis

The development of cancer resistance to chemotherapy is the major obstacle to cancer therapy. Here, we identified that the phosphorylation of apoptosis repressor with caspase recruitment domain (ARC) at threonine 149 was essential to inhibit doxorubicin (DOX) induced apoptosis and mitochondrial fission in cancer cells. Our further study showed that casein kinase II (CK2) inhibitors could decrease the phosphorylation levels of ARC and make cancer cells sensitive to undergoing apoptosis. Furthermore, CK2α and CK2α', catalytic subunits of CK2, were observed to translocate into nuclear in cancer cells with the treatment of DOX. Finally, the synergistically therapeutic effect by combining DOX and CK2 inhibitor was confirmed in tumor xenograft model. Taken together, our results revealed that CK2-mediated phosphorylation of ARC contributed to chemotherapy resistance by inhibiting DOX induced apoptosis and combining DOX with CK2 inhibitor could induce apoptosis of cancer cells synergistically by down-regulating the phosphorylation of ARC. Therefore, development of new therapeutic strategies based on ARC and CK2, is promising for overcoming cancer resistance to chemotherapy.

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.

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.

Protein kinase CK2 inhibition induces cell death via early impact on mitochondrial function

CK2 (official acronym for casein kinase 2 or II) is a potent suppressor of apoptosis in response to diverse apoptotic stimuli-thus its molecular downregulation or activity inhibition results in potent induction of cell death. CK2 downregulation is known to impact mitochondrial apoptotic circuitry but the underlying mechanism(s) remain unclear. Utilizing prostate cancer cell lines subjected to CK2-specific inhibitors which cause loss of cell viability, we have found that CK2 inhibition in cells causes rapid early decrease in mitochondrial membrane potential (Δψm). Cells treated with the CK2 inhibitors TBB (4,5,6,7-tetrabromobenzotriazole) or TBCA (tetrabromocinnamic acid) demonstrate changes in Δψm which become apparent within 2 h, that is, significantly prior to evidence of activation of other mitochondrial apoptotic signals whose temporal expression ensues subsequent to loss of Δψm. Further, we have demonstrated the presence of CK2 in purified mitochondria and it appears that the effect on Δψm evoked by inhibition of CK2 may involve mitochondrial localized CK2. Results also suggest that alterations in Ca(2+) signaling may be involved in the CK2 mediated regulation of Δψm and mitochondrial permeability. Thus, we propose that a key mechanism of CK2 impact on mitochondrial apoptotic circuitry and cell death involves early loss of Δψm which may be a primary trigger for apoptotic signaling and cell death resulting from CK2 inhibition.

Molecular and genetic properties of tumors associated with local immune cytolytic activity

How the genomic landscape of a tumor shapes and is shaped by anti-tumor immunity has not been systematically explored. Using large-scale genomic data sets of solid tissue tumor biopsies, we quantified the cytolytic activity of the local immune infiltrate and identified associated properties across 18 tumor types. The number of predicted MHC Class I-associated neoantigens was correlated with cytolytic activity and was lower than expected in colorectal and other tumors, suggesting immune-mediated elimination. We identified recurrently mutated genes that showed positive association with cytolytic activity, including beta-2-microglobulin (B2M), HLA-A, -B and -C and Caspase 8 (CASP8), highlighting loss of antigen presentation and blockade of extrinsic apoptosis as key strategies of resistance to cytolytic activity. Genetic amplifications were also associated with high cytolytic activity, including immunosuppressive factors such as PDL1/2 and ALOX12B/15B. Our genetic findings thus provide evidence for immunoediting in tumors and uncover mechanisms of tumor-intrinsic resistance to cytolytic activity.

Regulation of the proapoptotic functions of prostate apoptosis response-4 (Par-4) by casein kinase 2 in prostate cancer cells

The proapoptotic protein, prostate apoptosis response-4 (Par-4), acts as a tumor suppressor in prostate cancer cells. The serine/threonine kinase casein kinase 2 (CK2) has a well-reported role in prostate cancer resistance to apoptotic agents or anticancer drugs. However, the mechanistic understanding on how CK2 supports survival is far from complete. In this work, we demonstrate both in rat and humans that (i) Par-4 is a new substrate of the survival kinase CK2 and (ii) phosphorylation by CK2 impairs Par-4 proapoptotic functions. We also unravel different levels of CK2-dependent regulation of Par-4 between species. In rats, the phosphorylation by CK2 at the major site, S124, prevents caspase-mediated Par-4 cleavage (D123) and consequently impairs the proapoptotic function of Par-4. In humans, CK2 strongly impairs the apoptotic properties of Par-4, independently of the caspase-mediated cleavage of Par-4 (D131), by triggering the phosphorylation at residue S231. Furthermore, we show that human Par-4 residue S231 is highly phosphorylated in prostate cancer cells as compared with their normal counterparts. Finally, the sensitivity of prostate cancer cells to apoptosis by CK2 knockdown is significantly reversed by parallel knockdown of Par-4. Thus, Par-4 seems a critical target of CK2 that could be exploited for the development of new anticancer drugs.

Inhibition of protein kinase CK2 with the clinical-grade small ATP-competitive compound CX-4945 or by RNA interference unveils its role in acute myeloid leukemia cell survival, p53-dependent apoptosis and daunorubicin-induced cytotoxicity

Background

The involvement of protein kinase CK2 in sustaining cancer cell survival could have implications also in the resistance to conventional and unconventional therapies. Moreover, CK2 role in blood tumors is rapidly emerging and this kinase has been recognized as a potential therapeutic target. Phase I clinical trials with the oral small ATP-competitive CK2 inhibitor CX-4945 are currently ongoing in solid tumors and multiple myeloma.

Methods

We have analyzed the expression of CK2 in acute myeloid leukemia and its function in cell growth and in the response to the chemotherapeutic agent daunorubicin We employed acute myeloid leukemia cell lines and primary blasts from patients grouped according to the European LeukemiaNet risk classification. Cell survival, apoptosis and sensitivity to daunorubicin were assessed by different means. p53-dependent CK2-inhibition-induced apoptosis was investigated in p53 wild-type and mutant cells.

Results

CK2α was found highly expressed in the majority of samples across the different acute myeloid leukemia prognostic subgroups as compared to normal CD34⁺ hematopoietic and bone marrow cells. Inhibition of CK2 with CX-4945, K27 or siRNAs caused a p53-dependent acute myeloid leukemia cell apoptosis. CK2 inhibition was associated with a synergistic increase of the cytotoxic effects of daunorubicin. Baseline and daunorubicin-induced STAT3 activation was hampered upon CK2 blockade.

Conclusions

These results suggest that CK2 is over expressed across the different acute myeloid leukemia subsets and acts as an important regulator of acute myeloid leukemia cell survival. CK2 negative regulation of the protein levels of tumor suppressor p53 and activation of the STAT3 anti-apoptotic pathway might antagonize apoptosis and could be involved in acute myeloid leukemia cell resistance to daunorubicin.

Overexpressions of CK2β and XIAP are associated with poor prognosis of patients with cholangiocarcinoma

To investigate the expressions of casein kinase II β (CK2β) and X-Linked inhibitor of apoptosis protein (XIAP) in cholangiocarcinoma (CCA) and evaluated their correlations with major clinicopathologic features and patients' survival. Fifty CCA specimens and 20 normal liver tissues were included in the study. Immunohistochemical staining was used to determine the expression levels of CK2β, XIAP in normal and CCA tissues. The relationships of CK2β and XIAP expressions with clinicopathologic parameters and clinical outcome were evaluated. High immunostaining of CK2β and XIAP were observed in 66% (33/50) and 68% (34/50) of CCA tissues, which were significantly higher than that of normal liver tissues 0% (0/20) and 25% (5/20). The high expression of CK2β was significantly associated with TNM stage (P = 0.036), histological grade (P = 0.020) and high serum CEA level (P = 0.010), while high expression of XIAP was only associated with TNM stage (P = 0.014) and high serum CEA level (P = 0.001). By univariant analysis, patients with high expression of CK2β and XIAP demonstrate significantly poorer overall survival (P = 0.003 vs P = 0.018). Cox regression model showed that positive expression of CK2βis an independent factor of prognosis (P = 0.004). The expressions of CK2β and XIAP in CCA tissues showed strong correlations with the tumor progression, CK2β may be applied as a potential prognostic marker for CCA.

CK2 inhibitor CX-4945 suppresses DNA repair response triggered by DNA-targeted anticancer drugs and augments efficacy: mechanistic rationale for drug combination therapy

Drug combination therapies are commonly used for the treatment of cancers to increase therapeutic efficacy, reduce toxicity, and decrease the incidence of drug resistance. Although drug combination therapies were originally devised primarily by empirical methods, the increased understanding of drug mechanisms and the pathways they modulate provides a unique opportunity to design combinations that are based on mechanistic rationale. We have identified protein kinase CK2 as a promising therapeutic target for combination therapy, because CK2 regulates not just one but many oncogenic pathways and processes that play important roles in drug resistance, including DNA repair, epidermal growth factor receptor signaling, PI3K/AKT/mTOR signaling, Hsp90 machinery activity, hypoxia, and interleukin-6 expression. In this article, we show that CX-4945, a clinical stage selective small molecule inhibitor of CK2, blocks the DNA repair response induced by gemcitabine and cisplatin and synergizes with these agents in models of ovarian cancer. Mechanistic studies show that the enhanced activity is a result of inactivation of XRCC1 and MDC1, two mediator/adaptor proteins that are essential for DNA repair and that require phosphorylation by CK2 for their function. These data position CK2 as a valid pharmacologic target for intelligent drug combinations and support the evaluation of CX-4945 in combination with gemcitabine and platinum-based chemotherapeutics in the clinical setting.

Protein kinase CK2/PTEN pathway plays a key role in platelet-activating factor-mediated murine anaphylactic shock

Platelet-activating factor (PAF) is a major mediator in the induction of fatal hypovolemic shock in murine anaphylaxis. This PAF-mediated effect has been reported to be associated with PI3K/Akt-dependent eNOS-derived NO. The phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is phosphatidylinositol phosphate phosphatase, which negatively controls PI3K by dephosphorylating the signaling lipid, phosphatidylinositol 3,4,5-triphosphate. In this study, we examined the possible involvement of PTEN in PAF-mediated anaphylactic shock. Induction of anaphylaxis or PAF injection resulted in a rapid decrease in PTEN activity, followed by increases in PI3K activity and phosphorylation of Akt and eNOS. Systemic administration of adenoviruses carrying PTEN cDNA (adenoviral PTEN), but not the control AdLacZ, not only attenuated anaphylactic symptoms, but also reversed anaphylaxis- or PAF-induced changes in PTEN and PI3K activities, as well as phosphorylation of Akt and eNOS. We found that the decreased PTEN activity was associated with PTEN phosphorylation, the latter effect being prevented by the protein kinase CK2 inhibitor, DMAT. DMAT also inhibited anaphylactic symptoms as well as the anaphylaxis- or PAF-mediated PTEN/PI3K/Akt/eNOS signaling cascade. CK2 activity was increased by PAF. The present data provide, as the key mechanism underlying anaphylactic shock, PAF triggers the upstream pathway CK2/PTEN, which ultimately leads to the activation of PI3K/Akt/eNOS. Therefore, CK2/PTEN may be a potent target in the control of anaphylaxis and other many PAF-mediated pathologic conditions.

Protein kinase CK2α subunit over-expression correlates with metastatic risk in breast carcinomas: quantitative immunohistochemistry in tissue microarrays

BACKGROUND

CK2α is a signalling molecule that participates in major events in solid tumour progression. The aim of this study was to evaluate the prognostic significance of the immunohistochemical expression of CK2α in breast carcinomas.

METHODS

Quantitative measurements of immunohistochemical expression of 33 biomarkers using high-throughput densitometry, assessed on digitised microscopic tissue micro-array images were correlated with clinical outcome in 1000 breast carcinomas using univariate and multivariate analyses.

RESULTS

In univariate analysis, CK2α was a significant prognostic indicator (p<0.001). Moreover, a multivariable model allowed the selection of the best combination of the 33 biomarkers to predict patients' outcome through logistic regression. A nine-marker signature highly predictive of metastatic risk, associating SHARP-2, STAT1, eIF4E, pmapKAPk-2, pAKT, caveolin, VEGF, FGF-1 and CK2α permitted to classify well 82.32% of patients (specificity 81.59%, sensitivity 92.55%, area under ROC curve 0.939). Importantly, in a node negative subset of patients an even more (86%) clinically relevant association of eleven markers was found predictive of poor outcome.

CONCLUSION

A strong quantitative CK2α immunohistochemical expression in breast carcinomas is individually a significant indicator of poor prognosis. Moreover, an immunohistochemical signature of 11 markers including CK2α accurately (86%) well classifies node negative patients in good and poor outcome subsets. Our results suggest that CK2α evaluation together with key downstream CK2 targets might be a useful tool to identify patients at high risk of distant metastases and that CK2 can be considered as a relevant target for potential specific therapy.

How druggable is protein kinase CK2?

CK2 is a pleiotropic, ubiquitous, and constitutively active protein kinase (PK), with both cytosolic and nuclear localization in most mammalian cells. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. CK2 catalyzes the phosphorylation of more than 300 substrates characterized by multiple acidic residues surrounding the phosphor-acceptor amino acid, and, consequently, it plays a key role in several physiological and pathological processes. But how can one kinase orchestrate all these tasks faithfully? How is it possible that one kinase can, despite all pleiotropic characteristics of PKs in general, be involved in so many different biochemical events? Is CK2 a druggable target? Several questions are still to be clearly answered, and this review is an occasion for a fruitful discussion.

Protein B23/nucleophosmin/numatrin nuclear dynamics in relation to protein kinase CK2 and apoptotic activity in prostate cells

Protein B23/nucleophosmin/numatrin (B23) is a key nucleolar/nuclear matrix-associated protein required for cell growth-related functions, such as rRNA synthesis. Protein kinase CK2 (CK2) (formerly casein kinase 2, a protein Ser/Thr kinase signal that is involved in cell growth and cell death) mediates phosphorylation of B23, thereby influencing its functional activity. Here we have delineated the dynamics of B23 and its link to CK2 status in response to altered growth stimuli and induction of apoptosis in cultured prostate cells and in rat prostate cells in vivo. Our studies employing PC-3 and ALVA-41 prostate cancer cells demonstrated colocalization of CK2 and B23 in the nucleus. Further, CK2 and B23 underwent coordinate modulation in the nucleus related to their nucleocytoplasmic shuttling in response to induction of apoptotic activity in cells caused by downregulation of CK2 or by treatment with other apoptosis-inducing agents. These alterations in nuclear association of B23 occurred in the absence of a significant change in the level of cytoplasmic B23. Similar studies in the in vivo model of rat prostate epithelial cells subjected to androgen deprivation (that resulted in loss of nuclear CK2 and induction of apoptosis) demonstrated dynamic modulation of nuclear matrix-associated B23 without a significant change in its cytoplasmic level. These changes were reversed by androgen-mediated growth response in the prostate. Our results suggest that CK2-mediated phosphorylation of B23 is essential for its retention in the nucleus and that coordinated nuclear localization of B23 and CK2 is dynamically regulated in response to altered growth status in the cell.

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

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

Emergence of protein kinase CK2 as a key target in cancer therapy

Protein kinase CK2, a protein serine/threonine kinase, plays a global role in activities related to cell growth, cell death, and cell survival. CK2 has a large number of potential substrates localized in diverse locations in the cell including, for example, NF-kappaB as an important downstream target of the kinase. In addition to its involvement in cell growth and proliferation it is also a potent suppressor of apoptosis, raising its key importance in cancer cell phenotype. CK2 interacts with diverse pathways which illustrates the breadth of its impact on the cellular machinery of both cell growth and cell death giving it the status of a "master regulator" in the cell. With respect to cancer, CK2 has been found to be dysregulated in all cancers examined demonstrating increased protein expression levels and nuclear localization in cancer cells compared with their normal counterparts. We originally proposed CK2 as a potentially important target for cancer therapy. Given the ubiquitous and essential for cell survival nature of the kinase, an important consideration would be to target it specifically in cancer cells while sparing normal cells. Towards that end, our design of a tenascin based sub-50 nm (i.e., less than 50 nm size) nanocapsule in which an anti-CK2 therapeutic agent can be packaged is highly promising because this formulation can specifically deliver the cargo intracellularly to the cancer cells in vivo. Thus, appropriate strategies to target CK2 especially by molecular approaches may lead to a highly feasible and effective approach to eradication of a given cancer.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Regulation of cell proliferation and survival: convergence of protein kinases and caspases

Intricate networks of protein kinases are intimately involved in the regulation of cellular events related to cell proliferation and survival. In addition to protein kinases, cells also contain networks of proteases including aspartic-acid directed caspases organized in cascades that play a major role in the regulation of cell survival through their involvement in the initiation and execution phases of apoptosis. Perturbations in regulatory protein kinase and caspase networks induce alterations in cell survival and frequently accompany transformation and tumorigenesis. Furthermore, recent studies have documented that caspases or their substrates are subject to phosphorylation in cells illustrating a potential convergence of protein kinase and caspase signaling pathways. Interestingly, a number of caspase substrates are protected from cleavage when they are phosphorylated at sites that are adjacent to caspase cleavage sites. While it is theoretically possible that many distinct protein kinases could protect proteins from caspase-mediated cleavage, protein kinase CK2 is of particular interest because acidic amino acids, including aspartic acid residues that are recognized by caspases, are its dominant specificity determinants.

Involvement of reactive oxygen species in apoptosis induced by pharmacological inhibition of protein kinase CK2

It has been reported that inhibition of protein kinase CK2 (CK2) with antisense oligodeoxynucleotides (ODN) is a potent inducer of apoptosis in cancer cells but not in normal cells. In this regard, the apoptotic-inducing effect is attributed to the catalytic activity of the enzyme, which phosphorylates proapoptotic proteins to inhibit their functions. In this study we investigate the role of intracellular redox status in the proapoptotic activity of CK2 inhibition in human leukemia Cem cells. We provide evidence that inhibition of CK2 activity induces apoptotic cell death as evident by activation of caspase 3, DNA fragmentation, and phosphatidylserine externalization. Inhibition of CK2 resulted in a significant increase in intracellular hydrogen peroxide production, which we show as a critical mediator of apoptosis. To that end, apoptotic hallmarks, like DNA fragmentation and phosphatidylserine externalization, were blocked with the specific hydrogen peroxide scavenger catalase. We also show that inhibition of CK2 reduces cytosolic intracellular superoxide, a precursor of hydrogen peroxide. In summary, decreasing CK2 activity increases intracellular hydrogen peroxide, creating an intracellular environment conducive for death execution. Taken together, these data provide information on novel pathways involved in CK2 biology with implications for effective tools against drug-resistant tumors.

Enhancing the apoptotic potential of insulin-like growth factor-binding protein-3 in prostate cancer by modulation of CK2 phosphorylation

IGF-binding protein 3 (IGFBP-3) promotes apoptosis by both IGF-dependent and -independent mechanisms. We have previously reported that phosphorylation of IGFBP-3 (S156) by DNA-dependent protein kinase enhances its nuclear accumulation and is essential for its ability to interact with retinoid X receptor-alpha and induce apoptosis in cultured prostate cancer cells. Using specific chemical inhibitors and small interfering RNA, we demonstrate that preventing casein kinase 2 (CK2) activation enhanced the apoptotic potential of IGFBP-3. We mapped potential CK2 phosphosphorylation sites in IGFBP-3 to S167 and S175 and identified that wild-type IGFBP-3- and IGFBP-3-S175A-induced apoptosis to a comparable extent. In contrast, IGFBP-3-S167A was far more potently apoptosis inducing due to inability to undergo CK2 phosphorylation. Pretreatment of 22RV1 cells with IGFBP-3 small interfering RNA also limits the ability of high doses of CK2 inhibitor to induce apoptosis. These effects can be reversed by the addition of exogenous IGFBP-3 protein, suggesting reciprocal regulation of cell survival and apoptosis by IGFBP-3 and CK2. These studies reveal multisite phosphorylation of IGFBP-3 that both positively and negatively regulate its apoptotic potential. Understanding such intrinsic regulation of IGFBP-3 action may enhance the development of potential cancer therapies.

Temporal and spatial profiling of nuclei-associated proteins upon TNF-alpha/NF-kappaB signaling

The tumor necrosis factor (TNF)-alpha/NF-kappaB-signaling pathway plays a pivotal role in various processes including apoptosis, cellular differentiation, host defense, inflammation, autoimmunity and organogenesis. The complexity of the TNF-alpha/NF-kappaB signaling is in part due to the dynamic protein behaviors of key players in this pathway. In this present work, a dynamic and global view of the signaling components in the nucleus at the early stages of TNF-alpha/NF-kappaB signaling was obtained in HEK293 cells, by a combination of subcellular fractionation and stable isotope labeling by amino acids in cell culture (SILAC). The dynamic profile patterns of 547 TNF-alpha-induced nuclei-associated proteins were quantified in our studies. The functional characters of all the profiles were further analyzed using that Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation. Additionally, many previously unknown effectors of TNF-alpha/NF-kappaB signaling were identified, quantified and clustered into differential activation profiles. Interestingly, levels of Fanconi anemia group D2 protein (FANCD2), one of the Fanconi anemia family proteins, was found to be increased in the nucleus by SILAC quantitation upon TNF-alpha stimulation, which was further verified by western blotting and immunofluorescence analysis. This indicates that FANCD2 might be involved in TNF-alpha/NF-kappaB signaling through its accumulation in the nucleus. In summary, the combination of subcellular proteomics with quantitative analysis not only allowed for a dissection of the nuclear TNF-alpha/NF-kappaB-signaling pathway, but also provided a systematic strategy for monitoring temporal and spatial changes in cell signaling.

Protein kinase CK2 in health and disease: CK2: a key player in cancer biology

Elevated levels of protein kinase CK2 (formerly casein kinase 2 or II) have long been associated with increased cell growth and proliferation both in normal and cancer cells. The ability of CK2 to also act as a potent suppressor of apoptosis offers an important link to its involvement in cancer since deregulation of both cell proliferation and apoptosis are among the key features of cancer cell biology. Dysregulated CK2 may impact both of these processes in cancer cells. All cancers that have been examined show increased CK2 expression, which may also relate to prognosis. The extensive involvement of CK2 in cancer derives from its impact on diverse molecular pathways controlling cell proliferation and cell death. Downregulation of CK2 by various approaches results in induction of apoptosis in cultured cell and xenograft cancer models suggesting its potential as a therapeutic target.

The phosphorylation of SEPT2 on Ser218 by casein kinase 2 is important to hepatoma carcinoma cell proliferation

SEPT2 plays an important role in cell division through its effect on cytoskeletons. It is a GTP-binding protein and can also form filament with SEPT6 and SEPT7. Knockdown of SEPT2, 6, and 7 causes stress fibers to disintegrate and then cells lose polarity and divide abnormally. Increasing evidence has shown that septins are related to the regulation of cell proliferation. In this study, the expression of SEPT2 was first identified to be up-regulated in human hepatoma carcinoma cells (HCC). In addition, SEPT2 was found to be phosphorylated on Ser218 by casein kinase 2 (CK2), which was also overexpressed in HCC. By overexpressing SEPT2 and its S218A mutant in SMMC7721 and L02 cell lines, we confirmed that the phosphorylation of SEPT2 on Ser218 by CK2 was crucial to the proliferation of HCC. These results suggest that SEPT2 might be a promising target for liver cancer therapy.

CK2beta is expressed in endometrial carcinoma and has a role in apoptosis resistance and cell proliferation

Protein kinase CK2 (CK2) is a serine/threonine kinase that participates in important cellular processes. We have recently demonstrated that CK2 plays a role in resistance to TRAIL/Fas-induced apoptosis in endometrial carcinoma (EC) by regulating FLIP. Here, we assessed the immunohistochemical expression of CK2beta in EC and checked its role in cell proliferation and anchorage-independent cell growth. CK2beta immunostaining was assessed in two tissue microarrays, one constructed from paraffin-embedded blocks of 95 ECs and another from 70 samples of normal endometrium. CK2beta expression was correlated with histological type; grade and stage; cell proliferation (Ki-67) and apoptotic index; immunostaining for cyclin D1, PTEN, AKT, beta-catenin, and FLIP. Moreover, the Ishikawa EC cell line was subjected to down-regulation of CK2 by shRNA. CK2beta expression was frequent in EC (nuclear, 100%; cytoplasmic, 87.5%). The staining was more intense in EC than in normal endometrium (P = 0.000), and statistically correlated with AKT, PTEN, beta-catenin, and FLIP. In EC, CK2beta expression correlated with cell proliferation. Knock-down of CK2beta blocked colony formation of EC in soft agar, and also resulted in decreased expression of cyclin D1 and ERK phosphorylation. The results confirm that CK2beta is widely expressed in EC, and suggest a role in cell proliferation and anchorage-independent cell growth.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Impact of protein kinase CK2 on inhibitor of apoptosis proteins in prostate cancer cells

We have previously demonstrated that protein kinase CK2 is a potent suppressor of apoptosis in cells subjected to diverse mediators of apoptosis. The process of apoptosis involves a complex series of molecules localized in various cellular compartments. Among the various proteins that modulate apoptotic activity are inhibitors of apoptosis proteins (IAPs) which are elevated in cancers and have been proposed to block caspase activity. We have examined the impact of CK2 signal on these proteins in prostate cancer cells. Cellular IAPs demonstrate distinct localization and responsiveness to altered CK2 expression or activity in the cytoplasmic and nuclear matrix fractions. Modulation of cellular CK2 by various approaches impacts on cellular IAPs such that inhibition or downregulation of CK2 results in reduction in these proteins. Further, IAPs are also reduced when cells are treated with sub-optimal concentrations of chemical inhibitors of CK2 combined with low or sub-optimal levels of apoptosis-inducing agents (such as etoposide) suggesting that downregulation of CK2 sensitizes cells to induction of apoptosis which may be related to attenuation of IAPs. Decreased IAP protein levels in response to apoptotic agents such as TNFalpha or TRAIL were potently blocked upon forced overexpression of CK2 in cells. Together, our results suggest that one of the modes of CK2-mediated modulation of apoptotic activity is via its impact on cellular IAPs.

The emerging CK2 interactome: insights into the regulation and functions of CK2

Protein kinase CK2 represents a small family of protein serine/threonine kinases implicated in a variety of biological processes including events relating to cell proliferation and survival. Notably, CK2 displays oncogenic activity in mice and exhibits altered expression in several types of cancer. Accordingly, a detailed understanding of the cellular functions of CK2 and elucidation of the mechanisms by which CK2 is regulated in cells is expected to contribute to understanding its role in tumorigenesis with the prospect of novel approaches to therapy. While CK2 has traditionally been viewed as a tetrameric complex composed of two catalytic and two regulatory subunits, mounting evidence suggests that its subunits may have functions independent of tetrameric CK2 complexes. In mammals, as is the case in the budding yeast Saccharomyces cerevisiae, there are two isozymic forms of CK2, adding additional heterogeneity to the CK2 family. Studies in yeast and in human cells demonstrate that the different forms of CK2 interact with a large number of cellular proteins. To reveal new insights regarding the regulation and functions of different forms of CK2, we have examined the emerging interactomes for each of the CK2 subunits. Analysis of these interactomes for both yeast and human CK2 reinforces the view that this family of enzymes participates in a broad spectrum of cellular events. Furthermore, while there is considerable overlap between the interactomes of the individual CK2 subunits, notable differences in each of the individual interactomes provides additional evidence for functional specialization for the individual forms of CK2.

Cilostazol protects rat chondrocytes against nitric oxide-induced apoptosis in vitro and prevents cartilage destruction in a rat model of osteoarthritis

OBJECTIVE

To examine whether cilostazol, a selective phosphodiesterase type III inhibitor, protects rat articular chondrocytes against nitric oxide (NO)-induced apoptosis and prevents cartilage destruction in mono-iodoacetate-induced osteoarthritis (OA) in a rat model in which inducible nitric oxide synthase (iNOS) is expressed.

METHODS

The NO donor sodium nitroprusside was administered to rat articular chondrocytes that had been pretreated with cilostazol. Induction of apoptosis was evaluated by DNA electrophoresis and pulsed-field gel electrophoresis. The expression level and the subcellular location of apoptosis-associated factors were examined by Western blot analysis and confocal microscopy, respectively. Protein kinase CK2 (PKCK2) activity was also assayed. To examine whether orally administered cilostazol prevents cartilage destruction in vivo, cartilage samples obtained from rats with experimentally induced OA were subjected to hematoxylin and eosin, Safranin O, and TUNEL staining and immunohistochemical analysis of iNOS expression.

RESULTS

Cilostazol prevented NO-induced reduction in viability, in a dose-dependent manner. It also prevented the up-regulation of phosphorylated p53 and p38, the down-regulation of heme oxygenase 1, the subcellular translocation of apoptosis-inducing factor and cytochrome c, and the activation of caspases 3, 7, and 8 induced by NO treatment, indicating that cilostazol prevented NO-induced cell death by blocking apoptosis. In addition, cilostazol prevented NO-induced translocation of cleaved Bid onto mitochondria, and caused phosphorylated Bid to accumulate in the nucleus and cytosol. Cilostazol prevented the down-regulation of PKCK2 and the reduction in PKCK2 activity induced by NO, indicating that its apoptosis-preventing activity was mediated via PKCK2. It also prevented chondrocyte apoptosis and cartilage destruction in a rat model of experimentally induced OA.

CONCLUSION

Our findings indicate that cilostazol prevents NO-induced apoptosis of chondrocytes via PKCK2 in vitro and prevents cartilage destruction in a rat model of OA.

Yeast surviving factor Svf1 as a new interacting partner, regulator and in vitro substrate of protein kinase CK2

Since Svf1 is phosphoprotein, we investigated whether it was a substrate for protein kinase CK2. According to the amino acid sequence Svf1 harbours 20 putative CK2 phosphorylation sites. Here, we have reported cloning, overexpression, purification and characterization of yeast Svf1 as a substrate for three forms of yeast CK2. Svf1 serves as a substrate for both the recombinant CK2alpha (Km 0.35 microM) and CK2alpha' (Km 0.18 microM) as well as CK2 holoenzyme (Km 1.1 microM). Different Km values argue that CK2beta(beta') subunit has an inhibitory effect on the activity of both CK2alpha and CK2alpha' towards surviving factor Svf1. Reconstitution of alpha'2betabeta' isoform of CK2 holoenzyme shows that beta/beta' subunits have regulatory effect depending on the kind of CK2 catalytic subunit. This effect was not observed in the case of alpha2betabeta' isoform, which may be due to interaction between Svf1 and regulatory CK2beta subunit (shown by co-immunoprecipitation experiments). Interactions between CK2 subunits and Svf1 protein may have influence on ATP as well as ATP-competitive inhibitors (TBBt and TBBz) binding. CK2 phosphorylates up to six serine residues in highly acidic peptide K199EVIPESDEEESSADEDDNEDEDEESGDSEEESGSEEESDSEEVEITYED248 of the Svf1 protein in vitro. Presented data may help to elucidate the role of protein kinase CK2 and Svf1 in the regulation of cell survival pathways.