Protein kinase CK2 in hematologic malignancies: reliance on ... - Nature

Leukemia (2012) 26, 1174 - 1179 & 2012 Macmillan Publishers Limited All rights reserved 0887-6924/12 www.nature.com/leu

CONCISE REVIEW

Protein kinase CK2 in hematologic malignancies: reliance on a pivotal cell survival regulator by oncogenic signaling pathways F Piazza1,3, S Manni1,3, M Ruzzene2,3, LA Pinna2,3, C Gurrieri1,3 and G Semenzato1,3 CK2 is a multitask kinase whose role is essential for a countless number of cellular processes, many of which are critical for blood cell development. A prevailing task for this kinase rests on counteracting programmed cell death triggered by multiple stimuli. CK2 is overexpressed in many solid tumors and in vivo mouse models have proven its tumorigenic potential. Recent data have suggested that CK2 may also have a significant role in the pathogenesis of hematopoietic tumors, such as multiple myeloma, chronic lymphocytic leukemia, acute myelogenous leukemia, acute lymphoblastic leukemia and chronic myeloproliferative neoplasms. CK2 regulates hematopoiesis-associated signaling pathways and seems to reinforce biochemical cascades indispensable for tumor growth, proliferation and resistance to conventional and novel cytotoxic agents. Although its activity is multifold, recent evidence supports the rationale of CK2 inhibition as a therapeutic strategy in solid and hematological tumors and phase-I clinical trials are in progress to test the efficacy of this innovative therapeutic approach. In this review, we will summarize the data supporting CK2 as an oncogenic kinase in blood tumors and we will describe some critical signaling pathways, whose regulation by this protein kinase may be implicated in tumorigenesis. Leukemia (2012) 26, 1174 -- 1179; doi:10.1038/leu.2011.385; published online 13 January 2012 Keywords: hematopoiesis; blood tumors; protein kinase CK2; kinase inhibitors INTRODUCTION Protein kinase CK2 is a highly conserved pleiotropic acidophilic serine-threonine kinase whose essential role in several cellular processes has been increasingly appreciated over the last decade.1 Structurally, CK2 is a tetrameric enzyme, composed by the assembly of two catalytic (a and/or a‘) and two regulatory b subunits. The catalytic and regulatory subunits may also be present in the cell as unassembled molecules and it seems that this state is associated with distinct and specific functions. The two catalytic subunits a and a‘ share 90% sequence homology in their N-terminal region. The regulatory subunit b instead does not have any similarity to the other two subunits.2 CK2 recognizes motifs characterized by a S/T N-terminal to acidic amino acids, its minimum consensus being S/T-X-X-Ac, where X is any amino acid and Ac is a residue with a carboxylic or phosphorylated side chain (E, D, pS, pY). The spectrum of actions of this kinase is impressive as it can phosphorylate hundreds of substrate proteins, which are involved in disparate cellular processes, such as regulation of cell structure, division, proliferation, programmed death, DNA damage repair, protein translation, gene transcription, organelle function and so forth.3 It has been estimated that a substantial proportion of the human phosphoproteome (up to 20%) is generated by CK2 alone. For this reason, CK2 has long been viewed as a ‘non-targetable’ kinase for therapeutic purposes. The essential role of CK2 for cell and organism life is underscored by the lethal phenotype of its inactivation in the mouse: both CK2b and CK2a knockout results in in utero death and multiple embryonic alterations.4,5 CK2a‘ knockout mice, however, are viable but show profound changes in spermatogenesis.6

Among the striking number of cellular processes regulated by this protein kinase, a central task of CK2 is the promotion of cell growth and sustenance of cell viability. Indeed, accumulating evidence has demonstrated that CK2 controls critical pathways involved in the triggering of cell proliferation, as well as in the modulation of cell sensitivity to different cytotoxic noxae.1,7 Given the importance of this protein kinase for cell survival, it is not surprising that it has been found overexpressed, both in terms of protein levels and of catalytic activity, in several solid tumors. In particular, over the last years CK2 has been shown to have a critical pro-survival role in prostate, breast, colon, head and neck and lung cancer (reviewed in Trembley et al. 8). Importantly, in some of these tumors, CK2 protein levels were also shown to correlate with prognosis. While CK2 expression and function have been extensively studied in solid tumors, only in the last few years basic and translational research has investigated the role of this kinase in hematological malignancies. Nevertheless, significant reports have clearly shown that CK2 could have a central role also in the growth of blood tumors by impinging on crucial signaling pathways. In this review, we will briefly summarize the role of CK2 in the regulation of hematopoiesis-associated pathways, the knowledge obtained on CK2 function in normal and malignant blood cells and the implication of these findings for the development of novel therapies. CK2: NOT A LAZY PROTEIN KINASE! The involvement of CK2 in a multitude of cellular processes has marked this enzyme as ‘multitasking’ or ‘multifaceted’. Indeed, the regulation by CK2 of several of cellular functions deems this

1 Department of Clinical and Experimental Medicine, Hematology and Clinical Immunology Branch, University of Padua School of Medicine, Padua, Italy; 2Department of Biological Chemistry, University of Padua, Padua, Italy and 3Venetian Institute of Molecular Medicine, Hematological Malignancies Unit, Padua, Italy. Correspondence: Dr F Piazza, Department of Clinical and Experimental Medicine, University of Padua, Via Giustiniani 2, Padua 35128, Italy. E-mail: [email protected] Received 23 September 2011; revised 23 November 2011; accepted 25 November 2011; published online 13 January 2012

Protein kinase CK2 in hematologic malignancies F Piazza et al

protein kinase as a master ‘housekeeping’ molecule, which the cell cannot give over. CK2 is involved in numerous physiological processes, including cell structure regulation, cell motility, division and cycling, DNA damage repair, protein synthesis, gene transcription. Given its multitask role in cell biology, it was not surprising to observe that CK2 is constitutively active inside the cell and hardly susceptible to external/internal stimuli. As a general rule, the level of CK2 is high; for example, in yeast it is the fifth most abundant protein kinase.9 However, it is quite variable depending on the nature of the tissue and the metabolic conditions of the cell: in particular, it has been shown to increase in response to stimuli that cause cellular stress, such as oxidative stress, protein misfolding and DNA damage (reviewed in Filhol et al 10), and whenever this kind of comparison has been done, its level is higher in cancer cells than in its ‘normal’ counterparts.11

A POSSIBLE ROLE FOR CK2 IN HEMATOPOIESIS? The function of CK2 in normal hematopoieis is up to now largely unknown. However, it is likely that this protein kinase might profoundly affect several signaling pathways, which are involved in blood and immune cell physiology. To restrict the exemplification to a few of them, particularly relevant is the role of CK2 in the regulation of signaling cascades involved in hematopoietic stem cell (HSC) biology. The main evidence of wingless (Wnt), Hedgehog (Hh) and PTEN pathways involvement in normal and malignant hematopoiesis and CK2 intersection with these signaling cascades are briefly outlined below. The Wnt pathway in normal and malignant hematopoiesis and its regulation by CK2 Canonical Wnt signals are initiated by soluble Wnt engagement of Frizzled receptors and LRP5/6 co-receptors on the cell surface. This leads to Dvl (dishevelled) stabilization and consequent inhibition of the b-catenin destruction complex. b-catenin escapes proteasome degradation and accumulates inside the cell, enters the nucleus and binds to TCF/LEF transcription factors initiating the transcription of target genes (reviewed in McNeill et al 12). Several studies have investigated the role of the canonical Wnt pathway in the context of hematopoieis (reviewed in Staal and Luis13). Wnt proteins were found at sites of fetal hematopoieis and Wnt5a was involved in self-renewal and proliferation of HSC. Wnt3a also increases HSC self-renewal in vivo in adult mice. Human CD34 þ lineage HSCs also expand under Wnt stimulation. Constitutively active b-catenin was able to sustain HSC self-renewal and longterm repopulation ability and expansion in cooperation with Notch. However, other studies reported no gross abnormalities in hematopoiesis of b-catenin -/- or b-catenin/g-catenin double -/cells. Two recent works have demonstrated that the degree and duration of b-catenin activity may influence Wnt-dependent HSC self-renewal and expansion. Constitutive b-catenin in hematopoietic cells caused a depletion of HSC and a multilineage differentiation block, whereas Wnt signaling was shown to be essential for HSC long-term repopulating activity and for the leukemia stem cell (LSC) repopulating activity of chronic myeloid leukemia (CML)-like but not acute lymphoblastic leukemia (ALL)like leukemias in a mouse model and in CML cells from patients. These data indicate the importance of Wnt/b-catenin signaling in normal HSC biology and LSC originating from HSC but not from early progenitors.13 The role of CK2 in Wnt signaling is straightforward: it activates Wnt signaling through different mechanisms, that is, phosphorylation and activation of Dvl, direct phosphorylation and stabilization of b-catenin and recruitment of Wnt regulators to target genes.14 - 16 CK2 has been also demonstrated to upregulate b-catenin transcriptional activity by means of Ser129 Akt phosphorylation.17 CK2-activated AKT in turns phosphorylates b-catenin & 2012 Macmillan Publishers Limited

and, as a consequence, the transcription of the b-catenindependent anti-apoptotic survivin gene is enhanced, conferring resistance to apoptosis.18 Overall, although CK2 activity fully supports Wnt/b-catenin signaling, a formal--- in vivo--- demonstration of Wnt signaling modulation by CK2 is still lacking. The Hh pathway in normal and malignant hematopoiesis and its regulation by CK2 The Hh signaling pathway is another fundamental cascade involved in development, cell proliferation and differentiation. In the absence of soluble Hh ligands, the surface membrane molecule Patched blocks Smo (Smoothened) activity in the cell membrane and cytosolic full-length Gli proteins are proteolytically processed to generate truncated forms that act as transcriptional repressors of Hh target genes. When Hh is present, Patched block on Smo is released and Gli proteins remain in their full-length, activated state to empower Hh-dependent transcription (reviewed in Jiang and Hui19). The Hh is also a crucial signaling pathway involved in the formation and maintenance of adult HSC (reviewed in Mar et al 20). In mice, Indian Hedgehog expression in stromal cells promotes HSC regeneration after bone marrow transplantation. Hh pathway activation regulates cell cycle genes and expansion of primitive hematopoietic cells. Moreover, the growth of a range of hematopoietic cancer has been shown to rely on an active Hh signaling--- including Non-Hodgkin Lymphomas, Multiple Myeloma (MM), CML--- and Hh through its component Smo has been shown to specifically regulate HSC and LSC renewal, proliferation and repopulating ability in CML.20 As much as for Wnt signaling, CK2 also regulates Hh signaling in a positive manner. In Drosophila, CK2 phosphorylates Smo, favoring the Hh-transduced signal and the downstream molecule Ci (the homolog of mammalian Gli), stabilizing it and overall causing a positive regulation.21 The same group more recently reported that the antagonistic actions of CK2 and B56-containing PP2A phosphatase might regulate the stability of Gli proteins through phosphorylating/dephosphorylating Dazinteracting protein 1 (Dzip1), which has been shown pivotal for Gli protein degradation. This work extended in mammalians and vertebrate (Xenopus) cells the description of a central role of CK2 in Hh-dependent signaling.22 The PI3K/PTEN pathway in normal and malignant hematopoiesis and its modulation by CK2 PIP3 (phosphatidylinositol 3,4,5 -- triphosphate) is generated by PI3K at the plasma membrane and triggers the activation of several downstream protein kinases, of which AKT/PKB critically regulates cell survival, proliferation and oncogenesis. PTEN is a lipid and protein phosphatase able to dephosphorylate PIP3 and Tyr as well as Ser/Thr residues and is an essential tumor suppressor mutated or altered in a wide variety of cancers, including leukemia (reviewed in Salmena et al. 23). The PI3K/PTEN pathway has recently been definitely involved in normal and malignant hematopoiesis. It was already known that the PI3K/AKT pathway resulted activated and PTEN inactivated in a number of leukemias. Two independent studies have shown that PTEN deletion in the hematopoietic system in mice resulted in HSC entry into the cell cycle and fast depletion of the HSC pool. PTEN -/- HSCs displayed an abrogated long-term repopulating activity. Mice developed spontaneous myeloproliferative disease and ALL. PTEN-/- LSCs were transplantable and exhibited an inexhaustible proliferative potential. Thus, PTEN loss discriminates between normal HSC and malignant LSC.24,25 A number of studies have shown that CK2 phosphorylates PTEN in the C-terminal region, stabilizes the protein against ubiquitin-mediated proteasomal degradation and regulates PTEN activity in a negative manner.26 - 28 In addition to its direct regulation of PTEN, CK2 is able to activate AKT through direct phosphorylation29 and through an indirect mechanism preventing the dephosphorylation of its active form.30 Leukemia (2012) 1174 - 1179

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CK2 regulation of other hematopoietic-specific transcription factors and molecules CK2 has also been described as a regulative kinase of a multitude of hematopoietic transcription factors and molecules. For instance, CK2 phosphorylates and modulates c-Myb DNA binding and activity,31 which are instrumental for adult hematopoiesis and B-lymphopoiesis; CK2 stimulates PU.1 activity,32 another transcription factor involved in B-cell lineage specification; CK2 also regulates the Ets-family transcription factors Sp1 and Sp3,33 which are important for normal hematopoiesis and cancer cell proliferation.33 Recently, the negative regulation of the promyelocytic leukemia tumor suppressor by CK2-mediated phosphorylation has been demonstrated to have an important role in the stability and half-life of this molecule.34 CK2 IN LYMPHOID TUMORS Mouse models The first evidence of a pro-survival role for CK2 in lymphocytes came from in vivo mouse models of lymphomagenesis that were prompted by previous demonstration of CK2 involvement in lymphomas in the cattle. CK2a-overexpression favored T-cell lymphomagenesis in Tal1-transgenic mice.35 MRL-lpr/lpr mice bearing a Fas receptor mutation develop a lymphoproliferative disease and autoimmuity resembling systemic erythematous lupus.36 Double MRL-lpr/lpr CK2-transgenic mice displayed an accelerated tumor phenotype characterized by massive enlargement of lymphoid organs due to proliferation of aberrant, though polyclonal, T-lymphocytes.37 Parallel and subsequent work demonstrated in this model that CK2 cooperates both with loss of the tumor suppressor p53 and with overexpression of the oncogene c-myc in causing lymphocyte proliferation and clonal expansion.38,39 These initial studies established a proliferative and apoptosis-protective role for CK2 in T lymphocyte in the mouse. However, a more straightforward role of this kinase in lymphocyte oncogenesis has been recently suggested by studies in human lymphoid tumors, both arising from precursor cells and from mature lymphocytes. CK2 in acute lymphoblastic leukemia Mishra et al.40 demonstrated that CK2a is able to interact with the BCR moiety of the p190 or p210 BCR-ABL fusion oncogenes (namely within a fragment between residues 242 -- 413), which are involved in ALL and CML. CK2 function was shown to be essential for BCR-ABL-bearing tumor cells. Inhibition of CK2 with chemicals, such as 4,5,6,7 -- tetrabromobenzimidazole (TBB) or a newly developed CK2 inhibitor, 2 -- dimethylamino-4,5,6,7 -- tetrabromo1H-benzimidazole, caused a marked reduction of the proliferation of BCR-ABL-overexpressing cells, in in vitro as well as in in vivo experiments with p190 BCR-ABL1 transgenic mice.40,41 These initial reports implicating CK2 in lymphoid precursor tumors were followed by the demonstration that CK2 regulates the PI3K/AKT/ PTEN signaling cascade in T-ALL. Silva et al.42 showed that in primary T-ALL cells CK2 is overexpressed or hyperactivated. CK2-mediated phosphorylation of PTEN is known to result in its intracellular stabilization and--- in parallel--- in its functional inactivation. As a consequence, the PTEN phosphatase activity towards PIP3 is dramatically reduced leading to constitutive PI3K/ AKT signaling. Inhibition of CK2 restored PTEN activity and reduced PI3K/AKT activation, causing T-ALL cell death without affecting normal T-cell viability.42 These data showed that PTEN could be inactivated by a protein kinase-mediated post-transcriptional, post-translational mechanism in ALL. In a follow-up work, the same group demonstrated the potential therapeutic efficacy of targeting CK2 with TBB in combination with the inhibition of Notch-dependent signaling through the use of gamma-secretase inhibitors, in T-ALL samples.43 Leukemia (2012) 1174 - 1179

CK2 in chronic lymphocytic leukemia In addition to having a role in immature lymphoid precursorderived blood tumors, CK2 has recently been implicated also in the pathogenesis of B-chronic lymphocytic leukemia (B-CLL). The group of Jaeger et al. found elevated CK2b phospho-Ser209 levels in primary samples from 44 B-CLL patients.44 CK2b phosphorylation on Ser209 has been involved in the regulation of CK2 activity and target binding.2 Remarkably, CK2 inhibition with apigenin or TBB caused the reduction in the phosphorylation of PTEN at Ser380 and of AKT at Ser473 and was associated to B-CLL cell apoptosis. Remarkably, the combination of CK2 and PI3K inhibitors (LY294002) was shown to produce a synergistic cytotoxic effect on B-CLL cells.44 Importantly, parallel work confirmed the pro-survival function of CK2 in the growth of B-CLL cells. Martins et al.45 showed that both CK2a and CK2b subunits are elevated in primary B-CLL from patients and CK2 inhibition in B-CLL cells is coupled with inactivation of protein kinase C, increased PTEN activity and apoptosis. Interestingly, the cytotoxic effect of CK2 inhibition was more pronounced on B-CLL cells isolated from advanced stage (Binet B or C) patients. Normal T and B lymphocytes were slightly affected by the treatment with CK2 inhibitors.45 Of note, in this work the authors used the novel compound CX-4945 as a CK2 inhibitor, which is currently under investigation in phase-I clinical trials in cancer patients.46 CK2 in MM CK2 has also been implicated in the pathogenesis of MM.47 Our group described that CK2 is crucial for malignant plasma cell survival demonstrating that this protein kinase positively regulates STAT3 and NF-kB-dependent signaling. CK2 activity and CK2a and b protein levels were found upregulated in MM cell lines and primary cells. CK2a knockdown or CK2 inhibition with TBB and TBB-derived agents caused MM cell apoptosis, which was not counteracted by the addition of growth factors, such as interleukin-6 and insulinlike growth factor-I. We also found that CK2 inhibitors made MM cells more sensitive to the cytotoxic effect of melphalan. Remarkably, CK2 silencing or inhibition was associated to IkBa stabilization and decreased NF-kB transcriptional activity. Similarly, we demonstrated that CK2 inhibition caused a reduction in STAT3 phospho-Tyr705 and phospho-Ser727 levels, implying this kinase as a positive regulator of two critical pro-survival pathways in MM.47 Moreover, CK2 could be instrumental for the modulation of MM cell sensitivity to novel therapeutic agents. A recent study has reported that CK2 could represent a central kinase downstream inhibition of the proteasome with bortezomib, thus influencing the cell phosphoproteome and many signaling pathways upon treatment with this central, widely employed therapeutic agent.48 CK2 IN MYELOID TUMORS As for lymphoid tumors, a pathogenic role for CK2 has been proposed for blood tumors derived from the myeloid lineage. CK2 expression and altered protein turnover in myeloid leukemia cells were described in early reports.49,50 These pioneering studies showed that CK2 was overexpressed in proliferating leukemic blasts of acute myeloid leukemia (AML) as well as CML in blast crisis.49 Years later, another study showed that CK2a could be a substrate of BCR-ABL and ABL protein tyrosin kinases. CK2 was demonstrated to be phosphorylated on Tyr residues and the associated Tyr kinases isolated in immunoprecipitation experiments were ABL in NIH3T3 murine fibroblasts and BCR-ABL in the CML cell line K562.51 This was the first evidence of CK2 involvement downstream of an oncogenic aberrant protein kinase. AML Only recently CK2 has been more robustly implicated in AML. In the study of Kim et al.,52 CK2a subunit expression was investigated & 2012 Macmillan Publishers Limited


in AML samples from patients. High CK2a levels were observed in a significant proportion of cases; however, CK2a was also expressed at lower levels in a subset of patients. Interestingly, CK2 high expression was shown to portend a worse prognosis in the normal karyotype AML subset, in terms of disease-free and overall survival. Moreover, CK2-high overexpressing AML blasts or cell lines were more susceptible to TBB or apigenin-induced apoptosis. CK2 high levels were associated to increased levels of a number of phospho-proteins, including phospho-AKT, phosphoBAD, phospho-FKHR, BCL-2, BCL-xL. Interestingly, overexpression of CK2a in AML cells was paralleled by a decrease of p53 protein levels. A follow-up paper recently published by the same group has demonstrated that CK2 inhibition might represent a suitable and useful strategy to specifically target the ‘stem cell’ component of AML--- that is, the LSC--- described as having the CD34 þ CD38surface immunophenotype. In this work, the combined use of CK2 inhibitor apigenin and of PI3K/AKT inhibitor LY294002 was shown to exert a synergistic cytotoxic effect on the CD34 þ CD38- AML cell fraction and to have minimal effects on the normal HSC counterpart.53 Although pointing to a potential critical role of CK2 in AML pathogenesis, these studies need to be substantiated by further observations, especially considering the relative promiscuity of apigenin as a multikinase inhibitor. Chronic myeloproliferative neoplasms Very recently, CK2 has also been shown to regulate the aberrant signal emanating from the JAK/STAT module in chronic myeloproliferative neoplasms.54 In this work, the authors show that both CK2a and CK2b are required for a proper cytokine-stimulated STAT3 activation, as their inhibition with TBB or silencing with siRNAs led to a marked impairment of oncostatin M, interferon-g and growth hormone-induced STAT1, STAT3 and STAT5 Tyr phosphorylation as well as transcription of their target gene SOCS-3 in mouse embryonic fibroblasts and human cancer cell lines. The mechanism through which CK2 regulates STATs signaling has been investigated. CK2 was found to control JAK1 and JAK2 activation and to constitutively associate with these molecules. Moreover, JAK2 was demonstrated to be a CK2 substrate. Remarkably, CK2a was found to associate with the JAK2 mutant JAK2V617F, which is present in myeloproliferative disorders, such as polycythemia vera, essential thrombocythemia and primitive myelofibrosis, and its inhibition caused a potent reduction of JAK2V617F and downstream STAT5 phosphorylation. Consequently, JAK2V617F-bearing cells (both the erythroid leukemia cell line HEL and primary mononuclear cells from polycythemia

Table 1.

vera patients) underwent apoptosis and displayed a downregulation of survival molecules, such as BCL-xL, and activation of caspase 8 and 3. This study suggests that inhibition of CK2 could represent a suitable therapeutic approach of tumors that display constitutive JAK-STAT signaling.54 THE THERAPEUTIC POTENTIAL OF CK2 INHIBITION IN BLOOD TUMORS The data reviewed above indicates that CK2 represents a pivotal molecule both regulating hematopoiesis-associated signaling cascades and driving the growth of different blood tumors (Table 1 and Figure 1). Accumulating and convincing evidence now indicates that CK2 targeting with selective inhibitors is feasible and highly effective in causing tumor cell death, whereas substantially sparing normal cells. Early reports describing the use of CK2 inhibitors in vivo in mouse models, in which no evident toxicity has been observed, supported this notion.55,56 Most importantly, very recently a phase-I clinical trial started in USA, sponsored by Cylene Pharmaceuticals, which is testing safety, tolerability and highest dose level of the CK2 inhibitor CX-4945 in patients with advanced solid cancers, Castleman’s disease or MM (NCT00891280). Preliminary results have displayed very encouraging results,46 and later a myeloma-dedicated phase-I trial has started (NCT01199718). The observation that inhibition of such an essential protein kinase does not significantly affect normal cell and tissues has been explained with the phenomenon of the ‘non-oncogene addiction’.57 This is an acquired status of transformed cells whereby pivotal molecules for survival and proliferation, which are not oncogenes per se, are upregulated in the absence of genetic lesions (mutations, amplifications) and are exploited by oncogenes to sustain their transformed program.58 Consequently, minimal oscillations in the level or activity of these molecules may dramatically impact on cancer cell biology, damaging decisive growth-supporting pathways. Thus, the manipulation of nononcogene cancer-promoting proteins, such as CK2, appears as a powerful mean to induce an irreversible ‘existential crisis’ in malignant cells without significantly impacting on normal tissues and cells. The hematological tumors, in which CK2 has been described to be upregulated and instrumental for growth, are particularly suitable for this approach. It is likely that the abovementioned phase-I clinical trials with CX-4945 will pave the way to extensive investigation of CK2 inhibitors in this setting. It is conceivable to foresee a scenario in which the abrogation of this kinase’s activity will be coupled with a number of established therapies. For instance, it is likely that CK2 inhibitors will be effective in AML, especially in those cases that are chemotherapy resistant with a bad prognosis or in B-cell neoplasms, such as

Potential and demonstrated roles of CK2 in the modulation of signaling molecules in hematopoiesis and blood tumors

Signaling molecules

CK2 in hematopoiesis

CK2 in lymphoid tumors

CK2 in myeloid tumors

Wnt-bcatenin Hh AKT/PKB PTEN PML NF-kB Myc STATs

positive regulator positive regulator positive regulator negative regulator negative regulator positive regulator positive regulator positive regulator

positive regulator positive regulator positive regulator (CLL, ALL) negative regulator (CLL, ALL) negative regulator positive regulator (MM, CLL) positive regulator (mouse models) positive regulator (MM)

positive regulator positive regulator positive regulator (AML) negative regulator (AML) negative regulator positive regulator (AML) positive regulator positive regulator (CMD)

Abbreviations: AKT, AKR thymoma/protein kinase B; ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; CLL, chronic lymphocytic leukemia; CMD, chronic myeloproliferative disorders; Hh, hedgehog; MM, multiple myeloma; Myc, myelocytomatosis oncogene; NF-kB, nuclear factor-kB cell; PML, promyelocytic leukemia protein; PTEN, phosphatase and tensin homolog. Characters in italics denote potential and characters in bold denote demonstrated roles of CK2 in the modulation of signaling molecules in hematopoiesis and blood tumors, respectively.

& 2012 Macmillan Publishers Limited

Leukemia (2012) 1174 - 1179

1177


1178 CLL ALL

IL-6, CYTOKINES GROWTH FACTORS

AML

PIP3 PIP2 PDK

JAK

PTEN

PI3K

AKT

P

SRC P

P P P

P P

STAT3

GRB2

STAT3

RTK

STAT3

P

STAT3

GROWTH FACTORS

IKK complex

CK2 Ub

P

I p50

I p65

Proteasome

p65

P STAT3

p50

P P

STAT3

P

P NF-B pathway

STAT3 pathway

Nucleus

CLL MM

IL-6, TNF- , CYCLIN D1, BCL-2, BCL-XL, IAP2,...

CMD MM

Proliferation, survival

Figure 1. Involvement of protein kinase CK2 in blood tumor-associated signaling pathways. Among the several signaling pathways regulated by CK2, the modulation of the AKT/PKB/PI3K/PTEN cascade may be critical for the survival and proliferation of ALL, CLL and AML tumor cells; in CMD and in MM CK2 regulates the extent of JAK/STAT activation downstream of cytokine/growth factor signaling; CK2-mediated phosphorylation of IkBa in its PEST domain leads to IkBa inhibition through proteasome degradation and CK2 phosphorylation of NF-kB p65 on Ser529 causes the activation of the NF-kB pathway and this could be important for CLL and MM cell survival and resistance to cytotoxic therapies.

B-CLL and MM, which are characterized by a particular resistance to apoptosis, chiefly the cases carrying unfavorable genetic lesions. This could rely on the observation that CK2 has been shown to increase the sensitivity of these tumor cells to chemotherapeutics.47,59 Another scenario in which CK2 inhibition could be exploited is together with first and second generation BCR-ABL inhibitors, such as imatinib, dasatinib and nilotinib. Indeed, CK2 being an important target downstream of BCR-ABL, it likely contributes in sustaining the transformed phenotype promoted by this oncogenic fusion protein.41,51 Thus, CK2 inhibition could empower the BCR-ABL inhibition exerted by these agents; furthermore, it is possible that by blocking CK2, imatinib-resistant leukemia clones with mutated BCR-ABL could effectively be dampened down, allowing BCR-ABL-wild-typebearing clones to take over. In summary, CK2 can be considered an important kinase both for normal blood cell development and for driving blood cancer pathogenesis. Further experimental evidence will likely support this viewpoint in the next future. It will also be important to validate the concept that CK2 overexpression marks those ‘oncophilic’ cells,60 which are predisposed to develop into malignant blood cells. Finally, it will be essential to carry on parallel in vivo animal experimentation to fully elucidate CK2 function in normal and malignant hematopoietic cells. CONFLICT OF INTEREST The authors declare no conflict of interest.

ACKNOWLEDGEMENTS This work was supported by the Italian Ministry of University and Scientific Research (MIUR) grant no. RBFR086EW9 (FIRB - Futuro in Ricerca) to FP and by a grant from Associazione Italiana Ricerca sul Cancro (AIRC) to GS.

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