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Comparison of CX-4945 and SCG-CK2-1 as Inhibitors of CSNK2 using Quantitative Phosphoproteomics: Triple SILAC in combination with Inhibitor-Resistant CSNK2

ABSTRACT: Specificity is a limiting factor when using small-molecule inhibitors to study protein kinase signalling. Since inhibitor-resistant kinase mutants (i.e., drug-resistant alleles) remain active in the presence of inhibitor, they facilitate validation of on-target effects. By combining an inhibitor-resistant kinase mutant with mass spectrometry-based phosphoproteomics, we previously devised a systematic strategy for reliable identification and validation of CSNK2 substrates. In this study, we use the same strategy to evaluate the selectivity of CX-4945, a clinical stage CSNK2 inhibitor, and SGC-CK2-1, a chemical probe selectively targeting CSNK2. Human osteosarcoma (U2OS) cells expressing exogenous wild-type CSNK2A1 (WT) or an inhibitor-resistant triple mutant (TM, V66A/H160D/I174A) were treated with CX-4945 or SGC-CK2-1 prior to analysis using triple SILAC (phospho)proteomics. The minority of phosphosites, 15% at 4 hours and 5% at 24 hours, that were significantly downregulated in response to CX-4945 treatment were determined to be CSNK2A1-dependent. By comparison, the majority of phosphosites, >55% at both 4 and 24 hours, that were significantly downregulated in response to SGC-CK2-1 were identified as CSNK2A1-dependent. This indicates that SGC-CK2-1 exhibits dramatically greater selectivity towards CSNK2A1 than CX-4945. Notably, utilization of SGC-CK2-1 in cells expressing TM-CSNK2A1 enabled the identification of 330 CSNK2A1-dependent phosphosites. Overall, this study highlights the utility of exploiting highly selective chemical probes together with inhibitor-resistant kinase mutants to identify bona fide kinase substrates.

INSTRUMENT(S): LTQ Orbitrap Elite, Q Exactive

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Epithelial Cell

DISEASE(S): Disease Free

SUBMITTER: Kristina Jurcic

LAB HEAD: David Litchfield

PROVIDER: PXD038050 | Pride | 2023-08-14

REPOSITORIES: Pride

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Project description:In recent years, several small molecule cytotoxic drugs have been identified as potential inhibitors of ribosome biogenesis (Drygin et al., 2011; Peltonen et al., 2014a; Peltonen et al., 2014b). CX-5461 is one such drug that has also demonstrated anticancer potential for a wide range of malignancies (Bywater et al., 2012; Cornelison et al., 2017; Devlin et al., 2015; Drygin et al., 2011; Hald et al., 2019; Hein et al., 2017; Ismael et al., 2019; Lawrence et al., 2018; Lee et al., 2017; Negi and Brown, 2015; Taylor et al., 2019; Xu et al., 2017; Yan et al., 2017) (Haddach et al., 2012), and is presently under phase I trials for the treatment of both hematological cancers and solid tumours (Group, 2016; Khot et al., 2019). CX-5461 was initially characterized as an inhibitor of RNA Polymerase I (RPI/PolR1/PolI) that is responsible for the synthesis of the major ribosomal RNAs and the initial step in ribosome biogenesis (Drygin et al., 2011). Since RPI and its corresponding core transcription factors are dedicated to this task alone, they present ideal molecular targets by which to modulate ribosome biogenesis. However, the specificity of CX-5461 has been questioned and it has been suggested that this drug may also act by stabilizing DNA G-quadruplexes or by “poisoning” topoisomerase II (Topo II). Thus, the primary target of this drug and its mode of action are still in doubt. Here we used Deconvolution-ChIP-Seq in NIH3T3 and HEK293T cells treated for different times with CX-5461. The data show that the primary target of CX5461 is the initiation of ribosomal RNA gene (rDNA) transcription. CX-5461 blocks transcription initiation in vitro and in vivo by arresting RNA polymerase I (RPI/Pol1) within the preinitiation complex. In contrast to previous suggestions, CX-5461 does not effect recruitment of the TBP-TAF complex SL1 to the rDNA promoter, the recruitment of the initiation competent RPI-Rrn3 complex or ongoing transcription elongation, arguing against a role for G-quadruplex stabilization or topoisomerase II poisoning. Inhibition of transcription by CX-5461 is not reversible, the RPI-Rrn3 complex remains arrested in the preinitiation complex even after drug removal. This leads to nucleolar stress, extensive DNA damage and cell senescence. Our data show that the cytotoxicity of CX-5461 is the downstream result of the highly specific inhibition of rDNA transcription. The observation that this inhibition is irreversible will be important for the future design of chemotherapeutic strategies and the avoidance of drug resistance.

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Comparison of CX-4945 and SCG-CK2-1 as Inhibitors of CSNK2 using Quantitative Phosphoproteomics: Triple SILAC in combination with Inhibitor-Resistant CSNK2

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