Breast cancer metastasis suppressor 1 (BRMS1) is decreased in non–small cell lung cancer (NSCLC) and other solid tumors, and its loss correlates with increased metastases. We show that BRMS1 is posttranslationally regulated by TNF-induced casein kinase 2 catalytic subunit (CK2α') phosphorylation of nuclear BRMS1 on serine 30 (S30), resulting in 14-3-3ϵ–mediated nuclear exportation, increased BRMS1 cytosolic expression, and ubiquitin-proteasome–induced BRMS1 degradation. Using our in vivo orthotopic mouse model of lung cancer metastases, we found that mutation of S30 in BRMS1 or the use of the CK2-specific small-molecule inhibitor CX4945 abrogates CK2α'-induced cell migration and invasion and decreases NSCLC metastasis by 60-fold. Analysis of 160 human NSCLC specimens confirmed that tumor CK2α' and cytoplasmic BRMS1 expression levels are associated with increased tumor recurrence, metastatic foci, and reduced disease-free survival. Collectively, we identify a therapeutically exploitable posttranslational mechanism by which CK2α-mediated degradation of BRMS1 promotes metastases in lung cancer. Cancer Res; 76(9); 2675–86. ©2016 AACR.
Breast cancer metastasis suppressor 1 (BRMS1) has been implicated in the suppression of breast, lung, and bladder cancer metastases without significantly affecting primary tumor growth (1, 2). Specific to non–small cell lung cancer (NSCLC), BRMS1 protein and transcript are differentially expressed in noncancerous lung tissue (high) and tumors (low; refs. 2, 3). We previously demonstrated that transcriptional repression of BRMS1 occurs via RelA/p65-DNMT-1–mediated promoter methylation (4). Other groups have indicated that BRMS1 is also regulated via the Cul3–SPOP complex (5).
Casein kinase 2 (CK2) is a pleiotropic, highly conserved serine/threonine kinase that consists of two α (α or α') catalytic and two β regulatory subunits (6). CK2 controls the stability of both IκB (7) and the tumor suppressor PML (8) and regulates Snail 1–induced epithelial–mesenchymal transition (EMT; ref. 9) and promotion of NCoR-mediated cancer cell invasion (10). CK2 is a driver of malignant progression and a classic example of nononcogene addiction in tumors with high levels of CK2 (11). More recently, CK2 has been added to the human “druggable kinome,” as evidence exists that selective CK2 inhibitors enhance apoptosis in drug-resistant cancer cells, and clinical trials have demonstrated the antitumor activity of these compounds (12).
We (2) and others (13) have observed that although BRMS1 is primarily a nuclear protein, it is also present in the cytosol in cancer cells and primary human tumors. This suggests that the function of BRMS1 may be modulated by intracellular compartmentalization. Although Rivera and colleagues identified a conserved nuclear exportation motif in BRMS1 (14), the mechanisms through which BRMS1 undergoes nuclear export and the biologic significance of this process to the development of metastases are unknown.
Herein, we report that BRMS1 protein is regulated by TNF-induced activation of CK2α'. CK2α'-mediated phosphorylation of BRMS1 at serine 30 (S30) promotes nuclear exportation of BRMS1 via a 14-3-3–dependent mechanism. Using an orthotopic lung cancer model and CT visualization and quantification of tumor metastatic deposits, we show that CK2 phosphorylation of S30 on BRMS1 results in a significant increase in metastatic cells. Moreover, we observed abnormally elevated levels of CK2α' in human NSCLC specimens, compared with adjacent noncancerous tissues. The increased expression of CK2α' was significantly associated with nuclear exportation of BRMS1 and increased tumor recurrence. Collectively, our observations demonstrate that CK2-mediated phosphorylation of BRMS1 is an important posttranslational modification that regulates BRMS1 nuclear export and protein stability. We also identify BRMS1 as a new target of CK2 activity and provide mechanistic support and, thus, a strong rationale for the use of CK2-specific inhibitors in the treatment of lung cancer.
Materials and Methods
Cell culture, human NSCLC specimens, antibodies, and reagents
Normal human bronchial epithelial (NHBE) cells and NSCLC H1299, H1993, A549, and H157 cells were purchased from ATCC and grown as described in ref. 3. NHBE, H1299, H1993, and A549 cells were used within 6 months after purchase. H157 cells were authenticated in December 2015 by PCR and genomic DNA sequencing. Mycoplasma was tested routinely. Low-passage (<6) cell cultures were used for the experiments. Human NSCLC specimens were obtained following written, informed consent and approval by the Human Investigations Committee at Memorial Sloan Kettering Cancer Center (MSKCC). The antibodies used were BRMS1, phospho-serine/threonine, Myc-tag, and His-tag (Abcam); RNA Pol II, α-tubulin, CK2α', 14-3-3, and β-actin (Santa Cruz Biotechnology); and HA-epitope tag (BD Biosciences). The reagents used were human recombinant TNF, cycloheximide, and 4,5,6,7-tetrabromobenzotriazole (TBB; Sigma Aldrich); CX4945 (Silmitasertib, Selleck Chemicals); MG132 (EMD Biosciences); human recombinant CK2 and substrate (Promega); 35S-labeled methionine/cysteine and [γ-33P]-ATP (PerkinElmer); and antibiotics (puromycin, geneticin, blasticidin, zeocin, and tetracycline; Thermo Fisher Scientific). siRNA control and CK2α' pool were purchased from Santa Cruz Biotechnology.
HA-tagged BRMS1, GST fusion BRMS1, His(6)-tagged ubiquitin, and shRNA BRMS1 were described previously (3, 15, 16). Site-directed mutagenesis (S→A) was performed using the QuikChange Mutagenesis Kit (Agilent). For pBabe-puro FLAG-tagged BRMS1, BRMS1 was amplified from pCMV HA-tagged BRMS1 by PCR and inserted into BamHI/EcoRI sites. To construct shRNA-resistant BRMS1, the shRNA-targeting sequence was synonymously mutated. Plasmids encoding subunits of CK2 were provided by Professor D. Litchfield (University of Western Ontario, Ontario, Canada). For construction of pcDNA 4/TO Myc/His(6)-tagged CK2α' (Thermo Fisher Scientific), CK2α' from pRC/CMV-HA- CK2α' was amplified by PCR and inserted into BamHI/EcoRI sites. pcDNA3-luciferase was purchased from Addgene.
Preparation of cellular fractions
Nuclear and cytoplasmic extracts were isolated as described previously (17).
Cultured cells were transfected using PolyFect Reagent for plasmid transfection and Oligofectamine for siRNA transfection as described previously (3).
Virus production and stable cell generation
Viruses were generated and H157 BRMS1 knockdown (KD) cells were established as described previously (16). These BRMS1KD cells were cotransfected with pcDNA3-luciferase and Tet-on CK2α'. Stable clones were selected by geneticin (400 μg/mL), blasticidin (5 μg/mL), and zeocin (150 μg/mL). Myc/His(6)- CK2α' expression was confirmed by immunoblot after treatment with tetracycline (1 μg/mL) for 48 hours. Then, the H157 BRMS1KD/luciferase/Tet-on CK2α' cells were infected with pBabe-shRNA–resistant FLAG-BRMS1 wild-type, S30A mutant, or empty vector and selected with puromycin (1 μg/mL). Flag-BRMS1 expression was confirmed by Western blot analysis.
Generation of phosphospecific BRMS1 antibody
BRMS1 (pS30) antibody was generated by Open Biosystems. In brief, two rabbits were immunized with KHL-conjugated BRMS1 peptide centered on phospho-Ser30, and the phosphospecific antibody affinity was purified with phosphopeptide.
Protein half-life analysis
Cycloheximide blocking analysis was performed to determine the half-life of endogenous BRMS1. Cells were incubated with cycloheximide (100 μmol/L) for various times, and endogenous BRMS1 was detected by Western blot analysis. The densitometry of immunoblots for BRMS1 and tubulin was measured using the ChemiDoc MP System (Bio-Rad). The level of BRMS1 was quantified by normalization with tubulin, the percentage of remaining BRMS1 was plotted on a logarithmic scale over time, and half-life was determined using Prism 6.0 (GraphPad Software).
For detection of the turnover rate of HA-BRMS1, pulse-chase assays were performed (18). In brief, cells transfected with HA-BRMS1 were pulsed with 100 μCi of 35S-labeled methionine/cysteine for 30 minutes and chased for various times. Immunoprecipitations were performed using antibody against HA-tag (5 μg). Proteins were resolved by SDS-PAGE gel and visualized by autoradiography.
In vitro protein expression, purification, and kinase activity assays
GST-fusion proteins were expressed and purified as described previously (3). For in vivo kinase activity assays, endogenous CK2α' was immunoprecipitated by CK2α' antibody (5 μg), followed by incubation with GST-fusion BRMS1 (20 μg) or CK2-specific substrate peptide (100 nmol) in the presence of [γ-33P]-ATP or regular ATP, respectively. For experiments using GST-BRMS1 as substrate, phospho-GST-BRMS1 was resolved by SDS-PAGE gel and visualized by autoradiography. For experiments using CK2-specific substrate peptide as substrate, the CK2α' kinase activity was measured using the ADP-Glo Kinase Assay (Promega) according to the manufacturer's instructions.
For in vitro kinase assays, GST-BRMS1 (5 μg) was incubated with recombinant CK2 with [γ-33P]-ATP for 30 minutes at 30°C.
Immunoprecipitation, Western blotting, and immunofluorescence
Immunoprecipitation, Western blotting, and immunofluorescence were conducted as described previously (3).
NSCLC cells were transfected with HA-CK2α', and ubiquitination assays were conducted as described previously (15).
Invasion chamber assays
H157 stable cells were pretreated with or without tetracycline (1 μg/mL) for 48 hours. Invasion chamber assays were performed as described previously (16).
Orthotopic NSCLC xenograft model
All animal experiments were approved by the Animal Care and Use Committee at MSKCC (New York, NY; protocol #13-10-016). H157 stable cells (1 × 106) suspended in 100 μL of DPBS were injected into the left lungs of forty 5-week-old female athymic nude mice (nu/nu, Taconic), including BRMS1KD/Tet-on CK2α'/control (control; N = 10); BRMS1KD/Tet-on CK2α'/FLAG-BRMS1 S30A (BRMS1 S30A; N = 10); and BRMS1KD/Tet-on CK2α'/FLAG-BRMS1 wild-type (BRMS1 WT; N = 20). The BRMS1 S30A group and 10 mice from the BRMS1 WT group were administrated a doxycycline diet (0.625 g/kg; Harlan TEKLAD) on the day of injection.
In vivo imaging and quantification
Mice were anesthetized with 2.5% isoflurane and imaged after intraperitoneal injection of luciferin (150 mg/kg; Promega). Imaging was performed with an IVIS Spectrum-CT System (PerkinElmer) on day 8 after injection and repeated weekly (19). To determine the best time for imaging, a kinetic study was performed by continuously imaging at 5-minute intervals for 40 minutes after luciferin injection. Three-dimensional reconstruction was accomplished by the use of Living Image Software (version 4.2; Caliper).
For quantification, H157 BRMS1KD/luciferase cells were serially diluted into 96-well plates. After the addition of luciferin (15 mg/mL), the cells were continuously imaged at 2-minute intervals for 30 minutes to catch the peak luciferase signal. The standard curve of regions of interest (unit = radiance) versus cell numbers was drawn, and the cell numbers in primary tumors and metastatic sites were calculated using Living Image Software.
Tissue microarray and IHC
We created a tissue microarray (TMA) containing 160 NSCLCs and matched adjacent noncancerous tissue (Supplementary Table S1). The immunohistochemical techniques used were described previously (2, 20). BRMS1 staining in the nucleus and cytoplasm were evaluated separately. The BRMS1 and CK2α' antibodies were used at a 1:200 dilution for 30 minutes.
The results of all experiments represent the mean ± SEM of three separate experiments performed in triplicate. Statistical analysis was performed using Prism. Student t test, one-way ANOVA, Wilcoxon matched pairs signed rank test, Mann–Whitney test, and Spearman correlation were used. Progression-free survival (PFS) was defined as the time from surgery to the development of metastasis and was assessed using the Kaplan–Meier method and compared using the log-rank test. The CK2α' immunoreactivity score cut-off value (3.3; P = 0.002) was determined by ROC curve analysis using maximum sum of specificity and sensitivity. A two-sided P < 0.05 was considered to indicate statistical significance for all calculations.
TNF promotes CK2-mediated BRMS1 phosphorylation at S30
We observed that BRMS1 protein levels were decreased 5 to 10 times more than transcript in NSCLC, compared with NHBE cells or adjacent noncancerous tissues (3). This suggests that BRMS1 is posttranscriptionally regulated. To assess the stability of endogenous BRMS1 protein in NSCLC cells, we performed cycloheximide blocking assays. As shown in Fig. 1A, BRMS1 protein had a significantly shorter half-life in A549 and H1299 cells than in NHBE cells (P < 0.0001). To exclude the possibility that a reduction of BRMS1 was secondary to transcriptional repression (4), we evaluated the half-life of ectopic HA-BRMS1. The half-life of HA-BRMS1 was also shorter in H1299 cells than in NHBE cells (P < 0.0001; Fig. 1B). Given the contributions of the tumor microenvironment and the associated proinflammatory cytokines to tumorigenesis and metastases (21, 22), we asked whether TNF accelerates BRMS1 protein degradation. TNF treatment resulted in a nearly 50% reduction in the half-life of BRMS1 in both NHBE and H1299 cells (Fig. 1B), confirming the importance of TNF in the posttranslational regulation of BRMS1. Using H157 cells, which stably express SR-IκB, a dominant negative inhibitor of NF-κB (H157I), or a vector control (H157V; ref. 4), subsequent experiments showed that TNF-induced BRMS1 protein degradation is independent of NF-κB activation (Supplementary Fig. S1A).
Analysis of BRMS1 protein (http://scansite.mit.edu) indicated the presence of three evolutionarily conserved CK2 consensus sites (Fig. 1C). We therefore hypothesized that BRMS1 may be a substrate of CK2. As shown in Fig. 1D, GST-fusion BRMS1 was phosphorylated by recombinant CK2 in vitro. Of importance, introducing an S30A mutation in BRMS1 completely inhibited CK2-induced phosphorylation (Fig. 1D). These data indicate BRMS1 S30 is the primary site of CK2 phosphorylation. This is consistent with mass spectroscopy data identifying phosphorylation at S30, but not S45 or 46 (http://www.Phosphosite.org).
To elucidate whether BRMS1 S30 is the primary phosphorylation site in response to TNF, we developed a BRMS1 phosphospecific S30 (pS30) antibody. Stimulation with TNF significantly increased BRMS1 (pS30) in a time-dependent manner (Fig. 1E). Moreover, preincubation of lung cancer cells with a CK2-specific pharmacologic inhibitor TBB blocked TNF-induced BRMS1 phosphorylation (Supplementary Fig. S1B). To confirm the specificity of our BRMS1 (pS30) antibody, we expressed either HA-tagged BRMS1 wild-type or S30A mutant in NSCLC cells. The level of BRMS1 (pS30) protein was robustly increased in cells expressing wild-type HA-BRMS1 following TNF treatment. However, no (pS30) protein was detected in cells expressing mutant BRMS1 (S30A; Fig. 1F). These data confirm that TNF induces phosphorylation of BRMS1 at S30 through a CK2-dependent pathway. In addition, similar to TNF, TGFβ and IL6 (but not EGF) are able to induce BRMS1 (pS30) (Supplementary Fig. S1C), suggesting that multiple inflammation-related cytokines in the tumor microenvironment are involved in CK2-induced BRMS1 phosphorylation.
CK2α' is responsible for TNF-induced degradation of BRMS1
To address whether CK2 participates in BRMS1 degradation, we treated NSCLC cells with TNF and/or CX4945, a specific inhibitor of CK2 (23) currently in clinical trials. Treatment with TNF alone significantly reduced levels of BRMS1 protein. However, pretreatment with CX4945 abrogated TNF-induced phosphorylation and degradation of BRMS1 (Fig. 2A). These data indicate that CK2 activity is required for TNF-induced degradation of BRMS1.
To determine which subunit of CK2 is involved in BRMS1 degradation, wild-type and kinase-dead forms (CK2α K68/m and CK2α' K69/m; ref. 24) of CK2 subunits were individually expressed in H1299 cells. Endogenous BRMS1 was clearly decreased in cells with overexpression of CK2α' wild-type, but not in cells expressing other isoforms (Fig. 2B). Coimmunoprecipitation assays revealed that BRMS1 physically interacts with ectopic and endogenous CK2α', but not with CK2α (Fig. 2C and Supplementary Fig. S2A). TNF failed to induce BRMS1 degradation following siRNA knockdown of CK2α' (Fig. 2D). Although CK2α and α' share an 86% similarity in their catalytic domain (25), Western blots indicated that only α', not α, was specifically knocked down by siRNA (Fig. 2D). Collectively, these results indicate that CK2α' is the key catalytic subunit mediating BRMS1 degradation.
To determine whether the ubiquitination–proteasome pathway is involved in CK2α'-mediated degradation of BRMS1, in vivo ubiquitination assays were performed. The proteasome inhibitor MG132 was used to block ectopic CK2α'–induced BRMS1 degradation. Ectopic CK2α' significantly enhanced BRMS1 polyubiquitination in NSCLC cells (Fig. 2E). In addition, treatment with MG132 rescued CK2α'-induced BRMS1 degradation (Supplementary Fig. S2B). The introduction of mutant BRMS1 (S30A) abrogated CK2α'-induced BRMS1 degradation (Fig. 2F). Thus, phosphorylation of BRMS1 at S30 is required for CK2α'-induced degradation.
TNF activates CK2α' to phosphorylate BRMS1
To confirm that TNF induces CK2α' kinase activity in NSCLC, we demonstrated that TNF induces the phosphotransferase activity of CK2α' in a time-dependent manner (Fig. 3A and B). Importantly, pretreatment with CX4945 significantly decreased both basal and TNF-induced CK2α' kinase activity (Fig. 3B). In addition, TNF-induced activation of CK2α' resulted in robust phosphorylation of BRMS1 in vitro and in vivo (Fig. 3C and D) and was completely blocked following siRNA knockdown (Fig. 3D). To confirm that TNF induces phosphorylation of BRMS1 at S30 through activation of CK2α' at a single-cell level, immunofluorescence analysis was performed. Stimulation with TNF significantly increased levels of BRMS1 (pS30) in NSCLC cells (Fig. 3E). Interestingly, unlike BRMS1, the majority of BRMS1 (pS30) is located in the cytoplasm, suggesting that phosphorylation alters subcellular localization of BRMS1. Preincubation with CX4945 abrogated TNF-induced BRMS1 (pS30) but did not affect the expression of CK2α', suggesting that CX4945 inhibits CK2α' kinase activity, not expression. Similar to a recent study (26), we observed that TNF increased CK2α' protein levels (Fig. 3C–E). TNF increases the phosphotransferase activity of CK2α', resulting in phosphorylation of BRMS1 at S30.
TNF-induced phosphorylation promotes 14-3-3ϵ–dependent BRMS1 nuclear export
BRMS1 is primarily a nuclear protein, whereas ubiquitin-dependent degradation occurs within the cytoplasm (27). To explore whether TNF alters the subcellular localization of BRMS1, we isolated nuclear and cytosolic extracts from NSCLC cells after stimulation with TNF. In the absence of stimulus, BRMS1 was primarily nuclear (Fig. 4A). Following TNF stimulation, there was a significant shift of BRMS1 protein from the nucleus to the cytosol of cells in a time-dependent manner (Fig. 4A). However, preincubation with CX4945 completely blocked TNF-induced nuclear exportation of BRMS1 (Fig. 4A). Subsequent immunofluorescence assays showed that stimulation with TNF dramatically increased cytosolic BRMS1, compared with vehicle treatment (Supplementary Fig. S3A). Moreover, the BRMS1 S30 mutant was primarily intranuclear and did not undergo nuclear exportation, even with TNF stimulation (Fig. 4B). We therefore conclude that CK2-induced phosphorylation is required for the nuclear exportation of BRMS1.
The 14-3-3 family of proteins regulates the subcellular localization of multiple proteins via direct binding. Isoforms ϵ and ζ are the most abundant in lung tissue (28). As shown in Supplementary Fig. S3B, the 14-3-3 isoform ϵ, and not ζ, endogenously associates with BRMS1. TNF significantly enhanced the interaction of BRMS1 and 14-3-3ϵ (Fig. 4C), whereas the BRMS1 S30 mutant failed to bind to 14-3-3ϵ (Fig. 4D). Furthermore, knockdown of 14-3-3ϵ not only blocked cytoplasmic translocation of BRMS1 but also increased BRMS1 protein within the cell, suggesting that 14-3-3ϵ is required for BRMS1 nuclear export (Fig. 4E). In addition, we observed that 14-3-3ϵ knockdown blocked TNF-induced nuclear export of BRMS1 (Fig. 4F). Collectively, these data suggest that phosphorylation of BRMS1 by CK2 enhances 14-3-3ϵ interaction, which promotes BRMS1 nuclear export and degradation.
BRMS1 S30A mutant tumors have a lower rate of metastases
To determine the significance of CK2α'-mediated degradation of BRMS1 in NSCLC, we created H157 cell lines in which endogenous BRMS1 was stably knocked down. These cells were then used to establish H157 lines that ectopically expressed shRNA-resistant BRMS1 wild-type or S30A mutant. In addition, these cells stably expressed Myc–CK2α' construct (Tet-on-Myc-CK2α') under the control of the tetracycline-responsive operator. Treatment with tetracycline induced expression of Myc-tagged CK2α' and reduced the level of BRMS1 wild-type (Supplementary Fig. S4A). Cells expressing BRMS1 wild-type had significantly less invasion of NSCLC cells, compared with BRMS1 knockdown (control) cells. As expected, the S30A mutation did not affect the capacity of BRMS1 to repress invasion of NSCLC. Tetracycline-induced CK2 expression significantly increased the NSCLC invasion in cells expressing BRMS1 wild-type, but not the S30A mutant (Supplementary Fig. S4B).
To examine the contribution of CK2α'-mediated degradation of BRMS1 to the progression of NSCLC in vivo, we used an orthotopic xenograft lung cancer model. The position of primary tumors in the lung tissue was confirmed by bioluminescence IVIS Spectrum-CT scan (Supplementary Fig. S5A), and ectopic expression of BRMS1 and CK2α' was verified by Western blot analysis (Supplementary Fig. S5B). No significant difference was observed for growth of the primary tumors among groups (Supplementary Fig. S5C). As expected, animals that received H157 cells expressing BRMS1 wild-type had significantly fewer metastases compared with the H157 control group in which endogenous BRMS1 was reduced (Fig. 5A–C). Administration of doxycycline induced Myc-CK2α' expression, which abrogated BRMS1 wild-type–mediated suppression of metastasis but was unable to drive metastases in the BRMS1 S30A–mutant group (Fig. 5A–C). Mutation of S30 in BRMS1 resulted in a 60-fold reduction of CK2-driven metastatic tumor burden (Fig. 5B and C). Collectively, these data show that BRMS1 suppresses NSCLC metastasis in vitro and in vivo and that CK2α'-mediated phosphorylation of S30 results in degradation of BRMS1 and a robust increase in metastasis.
CK2α' correlates with intracellular BRMS1 localization and metastases in human NSCLC
Having demonstrated the importance of BRMS1 in suppressing CK2α'-driven metastases in our in vivo model, we wanted to determine whether similar correlations exist in human lung cancer. Immunohistochemical analysis was performed on a TMA of 160 human NSCLC and matched adjacent noncancerous tissues (Supplementary Table S1). As shown in Fig. 6A, CK2α' is overexpressed in human NSCLCs compared with adjacent noncancerous tissues (mean ± SD, 4.07 ± 3.4 vs. 2.99 ± 2.92; P = 0.004). Importantly, CK2α' expression levels were significantly higher in patients who developed distant metastases compared with patients who remained progression free (P = 0.022; Fig. 6B). We also observed that high expression of CK2α' in NSCLC is an independent factor associated with decreased PFS (P = 0.04; Fig. 6C).
To explore the relationship between CK2α' activity and cytosolic localization of BRMS1 in vivo, we examined the protein levels of BRMS1 in the same TMA. IHC scores for cytosolic BRMS1 were lower than those for nuclear BRMS1 (1.37 ± 1.94 vs. 3.61 ± 3.83; P < 0.0001), confirming that BRMS1 was primarily nuclear in human tumors. Cytosolic BRMS1 was commonly found in tumors with higher levels of CK2α' (P = 0.0001; Fig. 6D). Strikingly, protein levels of cytosolic BRMS1 positively correlated with increased levels of CK2α' protein in NSCLC [Spearman R = 0.31 (95% confidence interval, 0.145–0.452) P = 0.0002]. These data provide important correlative confirmation that CK2α' promotes nuclear exportation of BRMS1 in NSCLC and begin to lay the foundation for the use of CK2α' inhibitors in selected individuals with lung cancer.
Next, we examined the levels of CK2α' and BRMS1 (pS30) in selected human NSCLC tumors. CK2α' is overexpressed and BRMS1 is decreased in tumors compared with adjacent noncancerous tissues. BRMS1 (pS30) was also overexpressed in most tumors compared with adjacent tissues (9/12; Fig. 6E). In more than half of these tumors (7/12), levels of BRMS1 (pS30) were consistent with elevated levels of CK2α' (Fig. 6E). In addition, we observed significantly higher CK2α' kinase activity in primary human NSCLC tumors than in adjacent tissues (Supplementary Fig. S6A). Tumors with nodal metastasis had higher increased activity of CK2α' compared with tumors without nodal metastasis, indicating that elevated CK2α' activity is a strong predictor of the nononcogene addiction observed in NSCLC (Supplementary Fig. S6B). Collectively, our data demonstrate that both protein levels and kinase activity of CK2α' are increased in NSCLC compared with adjacent tissues. In addition, elevated levels of CK2α' strongly correlate with BRMS1 S30 phosphorylation, cytoplasmic localization, increased metastases, and poor clinical prognosis.
Herein, we have shown that BRMS1 is regulated posttranslationally via TNF-induced activation of CK2α', which phosphorylates BRMS1 on S30 and initiates 14-3-3–mediated nuclear export and proteasome-mediated degradation of BRMS1. Using an orthotopic NSCLC mouse model, we established the biologic significance of CK2α-mediated regulation of BRMS1 as a major driver of increased metastases (Fig. 7A).
Whereas CK2 kinase activity is constitutively active, CK2 substrate recognition is regulated by cytokines, inflammatory mediators, and growth factors, such as EGF, TNF, and TGFβ (29), as well as by p38 MAPK–mediated CK2 activation (12, 30). The role played by CK2 in cancer is illustrated in multiple myeloma, where CK2 phosphorylates the PIKK-regulatory proteins Tel2/Tti1, which facilitates their proteasomal degradation by Fbxo9 in the mTORC1 complex. This CK2-initiated, Fbxo9-mediated deregulated ubiquitinylation is associated with myelomagenesis (31). Other groups have shown that inositol pyrophosphates mediate DNA-PK/ATM-p53 cell death via CK2 phosphorylation (32) and that CK2-mediated phosphorylation of prostate apoptosis response-4 reduces its apoptotic function and increases cell survival (33). These reports, as well as our own observations regarding BRMS1, provide evidence that CK2 is a potential therapeutic target for tumors with high CK2 activity.
Protein expression of the CK2 catalytic subunit α or regulatory subunit β is elevated in various cancers, and overexpression of these subunits is correlated with poor prognosis (34, 35). In NSCLC, we found that CK2α' led to BRMS1 degradation, affirming the specificity of select CK2 catalytic subunits (36). Prior studies demonstrated that the free form of CK2α' phosphorylates NKX3.1 in prostate tumor cells (37). Recently, Turowec and colleagues revealed distinct functions of CK2 isoforms in promoting cancer cell survival by identifying isoform-specific phosphorylation of caspase-3 by CK2α', not CK2α (25). Confirming our observations that CK2α' is overexpressed in human NSCLC, a recent Oncomine (www.oncomine.org) analysis of lung cancer and normal lung tissue demonstrated that CK2α' was expressed at significantly higher levels in lung cancer tissue (38). Although the functions of CK2α and β are well studied, the functional role of CK2α' in tumor biology remains unclear. Herein, we show that CK2α' significantly enhances distant tumor metastasis in vivo and that expression of BRMS1 S30A mutant significantly diminishes CK2α'-enhanced metastasis in vitro and in vivo.
Although BRMS1 is a predominantly nuclear protein, it does shuttle between the nucleus and cytoplasm (14). Nuclear export of BRMS1 is not CRM-1 dependent (14), and 14-3-3 can transport proteins (e.g., HDAC7) independent of CRM1 (39). 14-3-3 proteins are capable of regulating intracellular localization of their binding partners, such as Raf, Cdc25, BAD, and FKHRL1 (40). We observed that CK2α'-mediated phosphorylation induced nuclear exportation of BRMS1 in a 14-3-3ϵ–dependent manner, which is enhanced in the presence of TNF. Simultaneously, we observed that CK2α'-mediated phosphorylation is required for nuclear export of BRMS1, as BRMS1 S30A mutant failed to interact with 14-3-3ϵ and undergo nuclear exportation, even in the presence of TNF. Importantly, our observation of a positive correlation between the protein levels of CK2α' and cytoplasmic BRMS1 expression in human NSCLC specimens confirms that phosphorylation by CK2α' is a prerequisite for nuclear exportation of BRMS1.
Our data indicate that the fate of phosphorylated BRMS1/14-3-3 complex, once it enters the cytosol, is to be degraded. Whereas the ubiquitin–proteasome components can be found in the nucleus, full degradation of targeted proteins occurs through cytoplasmic ubiquitin–proteasome machinery. This occurs for many proteins, including the nuclear proteins p53 (41), cyclin D1 (42), p27kip1(43), and IκBα (44). There have been conflicting observations regarding the clinical significance of cytosolic BRMS1, with some reports suggesting it is a favorable prognostic indicator (45) and others suggesting the opposite (13). We show that increased CK2α' is observed in tumors with BRMS1 localized to the cytoplasm; this was associated with increased metastasis, tumor recurrence, and reduced PFS in patients with NSCLC.
CX4945 is a potent and specific ATP-competitive inhibitor of both CK2α and α'. As the first orally bioavailable small-molecule inhibitor of CK2, CX4945 exhibits antitumor activity in multiple solid and hematopoietic tumor models, and its safety and efficacy have been established by a phase I/II clinical trial (23). A recent in vitro study demonstrated that CX4945 suppresses EMT and metastasis in A549 lung cancer cells through inhibition of multiple signaling pathways, including MAPK, AKT, FAK/Src, and MMPs (46). Our study demonstrates that CX4945 abrogates TNF-induced BRMS1 degradation through inhibition of CK2α'-mediated phosphorylation. This suggests that treatment with CX4945, potentially with the addition of a proteasome inhibitor, may result in a robust suppression of NSCLC metastasis via inhibition of BRMS1 degradation (Fig. 7B). Moreover, mutation of S30 in BRMS1 prevents CK2α'-mediated phosphorylation and degradation and leads to a 60-fold reduction in metastases; this illustrates the importance of BRMS1 as a primary regulator of metastases.
In summary, our work establishes a new, targetable mechanism through which BRMS1 is posttranslationally regulated. Given the increased expression of CK2α' and the correlative low levels of BRMS1 in a number of different highly metastatic tumors, these observations may have broad implications in the clinical management of metastatic cancer.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Conception and design: Y. Liu, M.W. Mayo, D.R. Jones
Development of methodology: Y. Liu, K. Kadota, D.R. Jones
Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): Y. Liu, E.B. Amin, K. Kadota, D.R. Jones
Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): Y. Liu, P.S. Adusumilli, D.R. Jones
Writing, review, and/or revision of the manuscript: Y. Liu, E.B. Amin, M.W. Mayo, N.P. Chudgar, P.R. Bucciarelli, K. Kadota, P.S. Adusumilli, D.R. Jones
Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): Y. Liu, P.S. Adusumilli, D.R. Jones
Study supervision: Y. Liu, D.R. Jones
The authors thank Professor D. Litchfield (University of Western Ontario, Ontario, Canada) for providing CK2 constructs and Dr. Neal Rosen (Memorial Sloan Kettering Cancer Center, New York, NY) for helpful comments during the manuscript preparation.
This work was supported by grants R01 CA136705 (D.R. Jones), R01 CA104397 (M.W. Mayo), R01 CA132580 (M.W. Mayo), U54 CA137788 (P.S. Adusumilli.), and 5 T32 CA 9501-27 (P.R. Bucciarelli) from the NIH/NCI. This work was also supported, in part, by NIH/NCI Cancer Center Support Grant P30 CA008748.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.