Molecular Pathways Clinical Cancer Research Molecular Pathways: Emergence of Protein Kinase CK2 (CSNK2) as a Potential Target to Inhibit Survival and DNA Damage Response and Repair Pathways in Cancer Cells Adam J. Rabalski1, Laszlo Gyenis1, and David W. Litchfield1,2 Abstract Protein kinase CK2 (designated CSNK2) is a constitutively pant in many processes related to cancer, its potential to active protein kinase with a vast repertoire of putative sub- modulate caspase action may be particularly pertinent to its strates that has been implicated in several human cancers, emergence as a therapeutic target. Because the substrate rec- including cancer of the breast, lung, colon, and prostate, as ognition motifs for CSNK2 and caspases are remarkably sim- well as hematologic malignancies. On the basis of these ilar, CSNK2 can block the cleavage of many caspase substrates observations, CSNK2 has emerged as a candidate for targeted through the phosphorylation of sites adjacent to cleavage sites. therapy, with two CSNK2 inhibitors in ongoing clinical trials. Phosphoproteomic strategies have also revealed previously CX-4945 is a bioavailable small-molecule ATP-competitive underappreciated roles for CSNK2 in the phosphorylation of inhibitor targeting its active site, and CIGB-300 is a cell-per- several key constituents of DNA damage and DNA repair meable cyclic peptide that prevents phosphorylation of the E7 pathways. Going forward, applications of proteomic strategies protein of HPV16 by CSNK2. In preclinical models, either of to interrogate responses to CSNK2 inhibitors are expected to these inhibitors exhibit antitumor efficacy.Furthermore,in reveal signatures for CSNK2 inhibition and molecular insights combinations with chemotherapeutics such as cisplatin or to guide new strategies to interfere with its potential to inhibit gemcitabine, either CX-4945 or CIGB-300 promote synergistic caspase action or enhance the susceptibility of cancer cells to induction of apoptosis. While CSNK2 is a regulatory partici- DNA damage. Clin Cancer Res; 22(12); 2840–7. Ó2016 AACR. Disclosure of Potential Conflicts of Interest No potential conflicts of interest were disclosed. Editor's Disclosures The following editor(s) reported relevant financial relationships: P.S. Steeg reports receiving commercial research grants from Genentech. CME Staff Planners' Disclosures The members of the planning committee have no real or apparent conflicts of interest to disclose. Learning Objectives Upon completion of this activity, the participant should have a better understanding of the role of protein kinase CK2 (CSNK2) in cellular processes that underlie malignancy, including its capacity to interfere with caspase action and involvement in DNA repair pathways, and the biological rationale of CSNK2 inhibitors as promising candidates for novel therapeutic strategies. Acknowledgment of Financial or Other Support This activity does not receive commercial support. 1Department of Biochemistry, Schulich School of Medicine & Dentis- Background try, University of Western Ontario, London, Ontario, Canada. 2Depart- ment of Oncology, Schulich School of Medicine & Dentistry, University Protein kinase CK2 (designated CSNK2) represents one small of Western Ontario, London, Ontario, Canada. protein kinase family that has recently emerged as a potential Note: Supplementary data for this article are available at Clinical Cancer therapeutic target based on alterations in its expression or activity in Research Online (http://clincancerres.aacrjournals.org/). a number of cancers (Fig. 1; ref. 1). CSNK2 is composed of Corresponding Author: David W. Litchfield, Department of Biochemistry, two closely related catalytic isoforms (CSNK2A1 or CSNK2A2) Schulich School of Medicine & Dentistry, University of Western Ontario, London, that both display catalytic activity in the presence or absence of its Ontario N6A 5C1, Canada. Phone: 519-661-4186; Fax: 519-661-3175; E-mail: regulatory CSNK2B; subunit (see Supplementary Table S1 for litchfi@uwo.ca definitions of the acronyms that appear throughout this article). doi: 10.1158/1078-0432.CCR-15-1314 The regulatory CSNK2B subunit is not essential for activity but can Ó2016 American Association for Cancer Research. affect the ability of the catalytic subunits to phosphorylate certain 2840 Clin Cancer Res; 22(12) June 15, 2016 Downloaded from clincancerres.aacrjournals.org on October 2, 2021. © 2016 American Association for Cancer Research. Targeting Protein Kinase CK2 (CSNK2) in Cancer DNA damage Circadian rhythm response ARNTL MDC1 DBP TCOF1 XRCC4 PER2 Apoptosis Chaperone-mediatedprotein folding CDC37 XRCC1 CRY2 RAD51 OTUB1 HSPA HSP90 TP53BP1 HSPH1 CASP3 FKBP4 PTEN CANX AKT1 NOL3 Figure 1. EIF5 MAX BID CSNK2 has been implicated in a diverse EEF1D EEF1B2 HCLS1 array of biologic processes. This figure EIF3C illustrates a selection of the biological EIF4G2 EIF3D HDAC1 processes in which CSNK2 has been Translation EEF1E1 CSNK2 EIF2S2 CDC34 CDK5 implicated with examples of putative CDK1 substrates or interacting proteins TP53 CDK2 CDC25C involved in each of these processes. SSRP1 FOXK1 EAPP CDK19 These proteins were identified and TFE3 HSF1 ETV6 analyzed using the following databases: Transcription BUB1B SUPT16H TCEB3 CENPF CDC16 GO: biological processes according to CTCF Cell cycle regulation UniProt (52), Reactome (53), PHOSIDA CTNNB1 ESPL1 TP53BP1 CDC34 UBE2 NEK9 (54), and PubMed (55). FOXM1 DVL2 TP53 DVL3 MEN1 IGF2R MYC APC MAP2K2 DCP1A WNT signaling pathway regulation Mitoticcheckpoint spindle wth and Cell gro proliferation © 2016 American Association for Cancer Research substrates (2). A recent analysis of transcript expression profiles the presence of second messengers to be catalytically active), for CSNK2A1, CSNK2A2, and CSNK2B in neoplastic tissues versus which is expressed at higher levels in many forms of human normal tissues deposited in the Oncomine database revealed cancer(1,3,7,8).CSNK2hasalsobeenshowntobeessential varying degrees of altered expression of individual CSNK2 sub- for viability and to enhance cancer cell survival associated units, suggesting that deregulation of CSNK2 subunit expression with inhibition of apoptosis. These observations are partic- profiles promotes cancer (3). To this point, the CSNK2-dependent ularly intriguing when considering the remarkable resem- phosphoproteome has not yet been fully elucidated, and the blance of the consensus recognition motif of CSNK2 with impact of alterations in CSNK2 on the phosphoproteome has not that of caspases (Fig. 2A) that cleave substrates at aspartic yet been systematically investigated. Nevertheless, it is evident both acid residues during the progression of apoptosis. In fact, from an extensive body of literature and computational predictions several published examples (Fig. 2A) show that caspase sub- of its substrates based on its consensus recognition motif (S/T-X-X- strates, including Bid, which promotes apoptotic progression D/E/pS/pY) that CSNK2 is involved in a vast landscape of biolog- when it is cleaved, can be phosphorylated by CSNK2 at sites ical processes (Fig. 1), with more than 2,000 putative phosphor- adjacent to caspase cleavage sites. Because phosphorylation ylation sites in the human proteome (4). While more comprehen- adjacent to caspase cleavage sites blocks cleavage, these sivediscussionsofCSNK2canbefoundelsewhere(5,6),wehigh- observations suggest that increased phosphorylation by light the involvement of CSNK2 in specific cellular processes, CSNK2 could promote cell survival by inhibiting caspase including its convergence with caspase pathways and its roles in action (9–14). DNA response and repair pathways, which have inspired ongoing Building on these observations, a combined computational efforts to exploit CSNK2 as a therapeutic target. and biochemical approach revealed an extensive repertoire of proteins with overlapping CSNK2 and caspase recognition Convergence of CSNK2 with caspase pathways motifs suggesting that CSNK2 could have widespread impact CSNK2 is generally considered to be a constitutively active on caspase action (15). In addition to the identification of enzyme (i.e., its catalytic subunits do not require activating many proteins known to be caspase substrates, these studies phosphorylation, association with its regulatory subunit, or also demonstrated that CSNK2 could phosphorylate CASP3 www.aacrjournals.org Clin Cancer Res; 22(12) June 15, 2016 2841 Downloaded from clincancerres.aacrjournals.org on October 2, 2021. © 2016 American Association for Cancer Research. Rabalski et al. A Caspase motif CSNK2 motif Overlapping motif –(S/T)–X–X–D/E– –E–X–D– –E–S/T–D–X–D/E– CSNK2 CASPASE P Phosphorylation blocks –E–S/T–D–X–D/E– –E–S/T–D X–D/E– cleavage CSNK2 active CSNK2 inhibited Figure 2. Protein kinase CSNK2 plays pivotal roles in the control of cell survival and Extrinsic Apoptosis Intrinsic Extrinsic Apoptosis Intrinsic responses to DNA damage. A, on the basis of the remarkable similarity of Chemicals UV Chemicals UV Fas Fas the consensus recognition motifs for phosphorylation by CSNK2 (S/T-D-X- X-D/E) and cleavage by caspases (E- X-D), it is evident that there are many P CASP 8 cellular proteins where CSNK2 FADD FADD BID phosphorylation could occur adjacent CSNK2 CASP 8 BID CASP 8 to caspase cleavage sites to block their cleavage by caspases (15). As SMAC/ SMAC/ described in the text, the proteins CYTOCHROME C CYTOCHROME C DIABLO DIABLO shown here represent caspase APAF-1 substrates including Bid, which CASP 9 promotes apoptotic progression when