Volume 16 Number 10
October 2014
pp. 789–800 789
www.neoplasia.com
CK2 Phosphorylates and Inhibits
TAp73 Tumor Suppressor Function
to Promote Expression of Cancer
Stem Cell Genes and Phenotype in
Head and Neck Cancer1,2
Hai Lu*, †, 3, Carol Yan*, ‡, 3 , Xin Xin Quan* ,
Xinping Yang* , Jialing Zhang* , Yansong Bian* ,
Zhong Chen*, 4 and Carter Van Waes*, 4
* Tumor Biology Section, Head and Neck Surgery Branch,
National Institute on Deafness and Other Communication
Disorders, National Institutes of Health, Bethesda, MD
20892; † Orthopaedic Center, Zhujiang Hospital, Southern
Medical University, Guangzhou, China; ‡ Howard Hughes
Medical Institute-NIH Research Scholars Program,
Bethesda, MD 20892, USA
Abstract
Cancer stem cells (CSC) and genes have been linked to cancer development and therapeutic resistance, but the signaling
mechanisms regulating CSC genes and phenotype are incompletely understood. CK2 has emerged as a key signal serine/
threonine kinase that modulates diverse signal cascades regulating cell fate and growth. We previously showed that CK2 is
often aberrantly expressed and activated in head and neck squamous cell carcinomas (HNSCC), concomitantly with mutant
(mt) tumor suppressor TP53, and inactivation of its family member, TAp73. Unexpectedly, we observed that classical stem cell
genes Nanog, Sox2, and Oct4, are overexpressed in HNSCC with inactivated TAp73 and mtTP53. However, the potential
relationship between CK2, TAp73 inactivation, and CSC phenotype is unknown. We reveal that inhibition of CK2 by
pharmacologic inhibitors or siRNA inhibits the expression of CSC genes and side population (SP), while enhancing TAp73 mRNA
and protein expression. Conversely, CK2 inhibitor attenuation of CSC protein expression and the SP by was abrogated by TAp73
siRNA. Bioinformatic analysis uncovered a single predicted CK2 threonine phosphorylation site (T27) within the N-terminal
transactivation domain of TAp73. Nuclear CK2 and TAp73 interaction, confirmed by co-immunoprecipitation, was attenuated by
CK2 inhibitor, or a T27A point-mutation of this predicted CK2 threonine phospho-acceptor site of TAp73. Further, T27A mutation
attenuated phosphorylation, while enhancing TAp73 function in repressing CSC gene expression and SP cells. A new CK2
inhibitor, CX-4945, inhibited CSC related SP cells, clonogenic survival, and spheroid formation. Our study unveils a novel
regulatory mechanism whereby aberrant CK2 signaling inhibits TAp73 to promote the expression of CSC genes and phenotype.
Neoplasia (2014) 16, 789–800
Introduction
The development of cancers has recently been linked to a small subset
of cells capable of reproducing the cancer cell population and forming
tumors, designated as tumor-initiating or cancer stem cells (CSC)
[1,2]. In head and neck squamous cell carcinomas (HNSCC), cells
with CSC-like phenotype and tumor forming properties have been
identified in tumors and cell lines [2–6]. Recently, HNSCC CSC
were found to be enriched within the “side population” (SP) of cells
Abbreviations: CK2, Casein Kinase 2; CSC, Cancer Stem Cells; DMAT, 2-Dimethylamino4,5,6,7-tetrabromo-1H-benzimidazole; HNSCC, Head and neck squamous cell carcinoma;
HEKA, Human epidermal keratinocytes; HOK, Human oral keratinocytes; mt, Mutant; SP,
Side population; TAp73, Transactivating p73; TP53, Transforming Protein p53; UM-SCC,
University of Michigan Squamous Cell Carcinoma; wt, Wild-type
Address all correspondence to: Carter Van Waes, MD, PhD, Head and Neck Surgery Branch or
Zhong Chen, MD, PhD, Head and Neck Surgery Branch, NIDCD/NIH, 10/5D55, MSC1419, Bethesda, MD 20892-1419. E-mail: vanwaesc@nidcd.nih.gov, chenz@nidcd.nih.gov
1
Grant support: HL, XXQ, XY, JZ, YB, ZC and CVW, supported by NIDCD Intramural Research
Projects Z01-DC-000016 and DC-000073. CY, supported by the HHMI-NIH Scholars Program.
2
Conflicts of Interest: CX-4945 was obtained by Materials Transfer Agreement from
Cylene Pharmaceuticals.
3
Contributed equally as first authors.
4
Contributed equally as senior authors.
Received 2 March 2014; Revised 21 August 2014; Accepted 22 August 2014
Published by Elsevier Inc. on behalf of Neoplasia Press, Inc. This is an open access article
under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1476-5586/14
http://dx.doi.org/10.1016/j.neo.2014.08.014
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Lu et al.
excluding Hoechst dye 33342 by fluorescence activated cell sorter
analysis [6], a phenotype also associated with export and resistance to
chemotherapy. Such isolated SP cells, when compared to non-SP
cells, differentially expressed stem cell gene markers BMI-1 and
ABCG2 transporter, formed self-replicating spheroids in vitro, and
initiated tumors, characteristic of CSCs. Genes encoding key stem cell
factors that promote the developmental stem cell phenotype,
including Sox2, Oct4 and Nanog, are also increased within tumors
and CSC in HNSCC [7]. Sox2, Oct4, and Nanog activation, target
gene regulation, and the CSC phenotype are inducible, supporting
their functional importance in HNSCC CSCs. However, the signal
and transcription factors orchestrating expression of these genes and
the CSC phenotype in HNSCC are incompletely understood.
Among possible candidates, CK2 (formerly casein kinase II) has
emerged as a key signal serine/threonine kinase that modulates diverse
proteins and target cascades to regulate cell fate and growth [8]. CK2
is dysregulated in most cancers examined, including HNSCC, where
it is aberrantly expressed and activated [8–10]. CK2 is detected as a
tetrameric complex comprised of catalytic α and/or α′ and regulatory
β subunits in the cytoplasm that mediate cell signaling. Additionally,
catalytic CK2α subunits have also been found to be localized to the
nucleus and complexed with chromatin, suggesting a potential role
for CK2α in regulating gene transcription and expression [10].
Supporting this possibility, we demonstrated that CK2α is a key
mediator repressing expression and function of the critical transcription factor and tumor suppressor TP53, in a subset of HNSCC with
wild type TP53 genotype [11]. Knockdown of CK2 by siRNA,
particularly CK2α, increased TP53 mRNA and protein expression,
inducing TP53-mediated growth arrest and apoptosis in vitro, and
inhibiting tumorigenesis of wtTP53 HNSCC xenografts in vivo [11].
Intriguingly, TP53 activated by ultraviolet light-induced DNA damage
has also been previously implicated in terminating embryonic stem cell
renewal, by suppressing Nanog transcription and expression [12].
Unfortunately, TP53 is directly mutated in the majority of epithelial
malignancies, and N 70% of HNSCC [13], compromising its potential to
suppress CSC gene expression and tumorigenesis. However, the TP53
family also includes p63 and p73, which are implicated in regulation of
self-renewal and programmed cell death and differentiation of squamous
epithelia [14,15]. These observations raise the question whether these
TP53 homologues that control physiological epithelial self-renewal and
differentiation may also be dysregulated by CK2 to unleash the expression
of stem cell genes and phenotype in cancer.
We recently showed that HNSCC with mtTP53 often retain and
overexpress related family member, TAp73, which has the potential
to replace TP53 function [16]. TAp73 has an N-terminal
transactivation (TA) domain which shares homology, transactivating,
and tumor suppressor function with TP53. In HNSCC with
mtTP53, our studies revealed that TAp73 is capable of repressing
expression of key TP53 target growth arrest and apoptotic genes
including p21, NOXA and PUMA. However, although overexpressed,
TAp73 is inactivated by a reversible mechanism involving inflammatory signaling and displacement from p53 promoter response
elements by ΔNp63α, a p63 isoform lacking the full N-terminal TA
domain. Whether and how CK2 signaling may contribute to TAp73
inactivation, and CSC gene expression and phenotype, is unknown,
but could provide a potential mechanism to target for prevention of
malignant progression in cells after mutation of TP53.
In the present study, we noted from gene expression profiling that
Sox2, Oct4 and Nanog gene expression is increased in HNSCC lines
in which TAp73 was increased but inactivated, and in the side
population previously demonstrated to contain CSC [6]. Thus, we
hypothesized that CK2 signaling may inactivate TAp73 to promote
CSC gene expression and phenotype in HNSCC with mtTP53.
Here, we examined whether CK2 mediates inactivation of TAp73,
to orchestrate expression of key CSC-related transcription factor
genes Nanog, Sox2 and Oct4, the side population, clonogenic
survival, and sphere forming CSC phenotypes in HNSCC
expressing TAp73 with mtTP53.
Materials and Methods
Cell Lines
The UM-SCC cell lines were obtained from Dr. Thomas E. Carey,
University of Michigan, and re-genotyped and origin confirmed in
2010 [17]. Genotyped stocks were frozen and used within 3 months
of thawing. Expression of TP53, p63, and p73 isoforms and TP53
sequence for exons 4 to 9 was confirmed in our laboratory as
previously reported [16,18]. Primary human epidermal keratinocytes
(HEKA) or Oral Keratinocytes (HOK) were cultured in accordance
with the supplier's protocol (Invitrogen) and used within 5 passages.
Reagents, siRNA and Plasmid Transfection
CK2 inhibitor 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole
(DMAT) was from Calbiochem and used as described previously [11].
CX-4945 is a novel selective CK2 inhibitor [19] obtained from Cylene
Pharmaceuticals under a Materials Transfer Agreement with NIDCD.
The oligonucleotide sequences for TAp73 specific siRNA inhibition
were: 5′r(CGGAUUCCAGCAUGGACGU)d(TT)3′and 5′r
(ACGUCCAUGCUGGAAUCCG).
d(TT)3′ (Integrated DNA Technologies, IDT). The CK2 specific
siRNAs were from Dharmacon/Thermo Scientific, CK2A1, siGENOME
SMARTpool (Cat# M-003475-03); CK2A2 ON-TARGET
plus SMARTpool (Cat# L-004752-00); CK2B, ON-TARGETplus
SMARTpool (Cat# L-007679-00); Control siRNA, ON-TARGETplus
Non-targeting Pool (Cat# D-001810-10-05). The p53/p73 specific
response element pG13-luc, PUMA-luc, and p21/WAF1-luc luciferase
reporter genes were kindly provided by Dr. Alex Zaika, Vanderbilt
University [20]. The expression vector containing a human Flag-pcDNA3TAp73 was kindly provided by Dr. Zhi-Min Yuan, Harvard University
[21]. The TAp73-T27A mutant, in which Thr-27 was substituted to Ala
(T27A), was synthesized by GENEWIZ, Inc, and sequence verified. All
transfections were performed using Lipofectamine 2000 according to the
manufacturer's instructions (Invitrogen/Life Technology). Each sample was
assayed in triplicate and data were presented as mean ± SD.
Western Blot and Coimmunopreciptiations
Western blot analysis and co-immunoprecipitation was performed as
previously [16] with antibodies indicated, CK2α (Santa Cruz, sc-6479),
CK2α′ (Santa Cruz, sc-6481), Nanog (Cell Signaling, 4903), Oct4 (Cell
Signaling, 4286), Sox2 (Cell Signaling, 2748), beta-actin (Cell Signaling,
4967), TAp73 (IMGENEX,IMG-246), p73 (IMGENEX,IMG-259A),
Oct-1 (Santa Cruz, sc-53830), Flag antibody(Sigma, M2), PUMA (Cell
Signaling, 4976).
Real time RT-PCR
RNA isolation and cDNA synthesis were performed as previously [16].
PCR primers for TAp73(GGCTGCGACGGCTGCAGAGC;
GCTCAGCAGATTGAACTGGGCCAT)were synthesized by
Neoplasia Vol. 16, No. 10, 2014
Invitrogen, and other primers used were purchased (Applied Biosystems).
Amplification conditions were: 2 minutes at 50°C and 10 minutes at
95°C, followed by 40 cycles of 15 seconds at 95°C and 1 minute at
60°C, carried out using an ABI Prism 7700 Sequence Detection System
(Applied Biosystems). Relative gene expression values were calculated
after normalization to 18S rRNA.
Flow cytometric analysis
Flow cytometric assay for SP cells in HNSCC was adapted from
Tabor et al. [6]. We cultured the lines with control diluent culture
medium, DMAT, CX-4945 and/or transfected them with different
siRNAs where indicated. Both floating and adherent cells detached
using trypsin-EDTA (In vitrogen) were collected, centrifuged, washed
and resuspended in DMEM containing 2% FCS (staining medium)
and preincubated in a 1.5-ml Eppendorf tube at 37°C for 10 minutes.
Cells were labeled in the same medium at 37°C for 90 minutes with
2.5 μg/ml Hoechst 33342 dye (Sigma-Aldrich, St. Louis, MO), either
alone or in combination with 50 mM verapamil (Sigma-Aldrich), as
negative control. Cells were centrifuged and resuspended in cold
DMEM and filtered through 40 μm mesh. Propidium iodine (BD
Biosciences, San Diego, CA) was added at 2 μg/ml for detection of dead
cells. Cells were washed twice with cold PBS, then fixed with cold 70%
ethanol and kept overnight at 4°C. Then, 3 to 5 × 10 4 cells were
analyzed by a FACSVantage fluorescence-activated cell sorter (BD
Biosciences) using dual-wavelength analysis (blue, 424 nm; red,
630 nm) after excitation with 350-nm UV light. Propidium iodidepositive dead cells (b 15%) were excluded from the analysis.
In vitro kinase assays
The Flag-cDNA3-TAp73 and Flag-cDNA3-TAp73-T27A fusion
proteins were expressed in UM-SCC-46 cells and immobilized on antiFlag-agarose beads. Anti-Flag-agarose beads were incubated with 10 mg
of protein lysates of UM-SCC-46 cell 48°C for 2 hours, then washed
three times with buffer (20 mM Tris–HCl at pH 7.5, 200 mM NaCl,
1.5 mM MgCl2, 0.2 mM EDTA, 1% Triton X-100, 0.1 mM
dithiothreitol, 1 mM PMSF protease inhibitor). The samples were
then incubated in 400 ml buffer (100 mM Tris, pH 8.0 20 mM
MgCl2, 100 mM NaCl, 50 mM KCl and 100 mM ATP containing
5 μCi of γ-[ 32P]-ATP) with 300 U recombinant CK2 (α2β2, New
England Biolabs, P6010S) at 30°C for 30 minutes. The kinase reactions
were terminated by washing the samples twice and re-suspending the
samples in SDS sample buffer. The samples were boiled for 5 minutes
and the proteins resolved by SDS-PAGE. Phosphorylation of FlagTAp73 was assessed by SDS-PAGE and autoradiography of the dried
gels. Loading of the recombination TAp73 protein was compared by
Coomassie BlueTM-stained SDS-polyacrylamide gels.
Identification of CK2 motif in TAp73
CK2 phosphorylation sites on TAp73 were predicted using Scansite.
T27 was identified as a CK2 phorphorylation site on the human TAp73
gene. Coincidence with human T27, a similar CK2 phosphorylation site
T31 was predicted on mouse TAp73 gene using the Scansite program.
Clonogenic Assay
Human UM-SCC-1 and UM-SCC-46 were plated as 200cell/well
in 6-well plates. Each cell line was plated in triplicate and incubated
for 4 hours in CO2 incubator at 37°C to allow cells attach to the dish.
Then cells were immediately treated with 0.5, 1 and 5 μM CK2
inhibitor CX4945 with DMSO as negative control. The culture
medium is same as for sphere formation assay, below. After 14 days
CK2 suppresses TAp73 in cancer stem cells
Lu et al.
791
the cells were washed, fixed and stained with 0.5% crystal violet. The
colonies with ≥ 50 cells were counted.
Sphere formation assay
Human UM-SCC-1 and UM-SCC-46 cells were plated as
500cell/well in 6-well ultra-low adherent dish and treated with 0.5,
1 and 5 μM CK2 inhibitor CX4945 with DMSO as negative control.
The culture medium is modified as serum free Keratinocyte-SFM
medium (GIBCO) containing EGF (10 ng/ml) (StemCell) and FGF
(5 ng/ml) (StemCell). Spheres ≥ 50 μm were counted under the
microscopy after 14 days. SCC13 cells form monolayer colonies instead
of sphere and only colonies with ≥ 100 cells were counted.
Results
Expression of CSC-related markers is increased in a HNSCC
subset overexpressing inactivated TAp73 and mtTP53, and
their CSC-like side population
Nanog, Oct4 and Sox2 are established stem cell markers, for which
expression has been studied in only a limited number of HNSCC
lines, and the mechanism(s) regulating their overexpression has not
been fully determined [7]. We surveyed expression of these CSC
markers in a panel of 9 UM-SCC lines. We observed increased
mRNA and/or protein levels of these CSC markers in a subset of cell
lines compared with human epidermal keratinocytes (HEKA) or oral
keratinocytes (HOK) as controls (Figure 1A and B). Interestingly,
Nanog, Oct4 and Sox2 protein expression was increased in four cell
lines (UM-SCC-22A, -B, -38, and -46), we previously found to
exhibit increased expression but attenuated function of tumor
suppressor TAp73 and mtTP53 [16]. Higher relative mRNA
expression of Sox2 detected by qRT-PCR in UM-SCC-22A, -22B,
-38, -46 cells was due to low signal in control HEKA. The results were
verified in independent experiments, and consistent with low Sox2
protein in control cells in Figure 1B by Western blot.
The side population (SP), which exhibits low Hoechst dye 33342
uptake by UV fluorescence activated cell sorter (FACS) analysis, has
previously been shown to be enriched for CSC markers, anchorage
independent sphere formation, and tumorigenic capacity in a variety
of cancers, including HNSCC [6]. We identified the SP in UM-SCC46 from the subset expressing the CSC markers with TAp73 and
mtTP53 (Figure 1C). The SP gated in UM-SCC-46 was significantly
reduced by treatment with verapamil, a blocker of calcium-dependent
Hoechst dye exclusion, previously shown to be a feature of the SP
containing CSC in HNSCC and other cancers that express ABC
transporters [6]. The SP displayed significantly increased expression
of Nanog, Oct4, and Sox2 (Figure 1D), which are key transcription
factors implicated in epithelial SP and CSC [1,2,7]. Further, we
confirmed that the SP isolated by FACS exhibited significantly
increased expression of mRNA for independent CSC markers BMI-1,
and transporter ABCG2, compared with non-SP cells (Figure 1D,
P b .05), as previously reported for HNSCC [6]. By contrast,
expression of pro-apoptotic gene PUMA, a TP53/TAp73 target
which exhibits reduced expression in HNSCC [16], was also slightly
reduced in SP cells (Figure 1D, P b .05). Thus, increased expression
of multiple established CSC transcription factors and markers is
detected in a subset of HNSCC lines, and enriched in the SP of UMSCC-46, one of the lines with increased expression of inactivated
TAp73 with mtTP53.
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CK2 suppresses TAp73 in cancer stem cells
Lu et al.
Neoplasia Vol. 16, No. 10, 2014
Figure 1. A subset of HNSCC cell lines and side population (SP) cells exhibit increased expression of CSC-related markers. A. Basal mRNA
levels for Nanog, Oct4, and Sox2 were quantified by qRT-PCR in HEKA and nine UM-SCC cell lines, including 3 lines with deficient
expression of wt TP 53 (UM-SCC-1, -6, -9), 2 lines lacking mtTP53 (UM-SCC-11A, -11B) and 4 lines overexpressing TAp73 with mt TP53
(UM-SCC-22A, -22B, -38 and -46)(16). Higher relative expression of Sox2 in UM-SCC was due to low signal in control HEKA, verified in an
independent experiment and consistent with low Sox2 protein in control in panel B by Western blot. B. Basal expression of Nanog, Oct4,
and Sox2 proteins was analyzed by Western blot in whole cell lysates of control HOK and 9 UM-SCC cell lines, with β-actin used as control
for loading. C. UM-SCC-46 cell lines were labeled with Hoechst 33342 dye and analyzed by flow cytometry without and with treatment
with verapamil. Hoechst Low SP cells highlighted within the window indicated are quantified. D. The mRNA expression of CSC related gene
markers Nanog, Oct4, Sox2, BMI1, ABCG2, and pro-apoptotic gene PUMA were assessed in non-SP and SP sub-populations sorted by
FACS, using QRT-PCR. The expression level in non-SP cells is normalized to 1.0. Columns, mean between triplicate samples; bars,
SD. *, Student t test, P b .05.
CK2 inhibitor or siRNA reduce CSC gene and protein
expression and the side population
As we previously observed that inhibition of protein kinase CK2
inhibits HNSCC tumorigenesis [11], we examined if established CK2
inhibitor (DMAT) or silencing RNAs (siRNA) could inhibit the CSC
markers, utilizing two independent cell lines from the subset
overexpressing these CSC markers and TAp73. CK2 inhibitor
DMAT significantly reduced Nanog, Oct4, and Sox2 mRNA
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CK2 suppresses TAp73 in cancer stem cells
Lu et al.
793
Figure 2. CK2 inhibition by DMAT or CK2α/α′ siRNA suppresses CSC-related SP cells and markers in two independent HNSCC lines. A.
CK2 inhibitor DMAT attenuates CSC marker gene Nanog, Oct4, and Sox2 mRNA expression in UM-SCC-22A (Upper panel) and UM-SCC46 (Lower panel). Quantitation of target genes was assessed using QRT-PCR with the treatment of increasing concentration of DMAT.
Bars, SD. *Reduced versus non-treated, student t test, P b .05. B. Western blot showing Nanog, Oct4, and Sox2 protein expression are
decreased in whole cell extracts from UM-SCC-46 48 hours after treatment with increasing concentrations of DMAT. β-Actin was used as
loading control. Similar results for UM-SCC-22A are shown in Suppl Figure 1A. C. Effect of individual and combined CK2α and α′ siRNA
knockdown on CSC marker Nanog, Oct4, and Sox2 mRNA expression in UM-SCC-22A (Upper panel) and UM-SCC-46 (Lower panel). The
expression level in control siRNA-treated sample is normalized to 1.0. Columns, mean between triplicate samples; bars, SD. *Student
t test, P b 0.05. D. Effect of individual and combined CK2α and α′ siRNA knockdown on CSC marker protein expression in UM-SCC-46,
48 hours after transfection with siRNA of CK2α, CK2α′, CK2α + α′, and scramble siRNA control. Nanog, Sox2, Oct4, CK2α, CK2α′ were
detected, with β-actin used as loading control. Similar results for UM-SCC-22A are shown in Suppl Figure 1B. E. CK2 inhibitor DMAT and
F. CK2α/α′ siRNAs inhibit SP cell detection in HNSCC. UM-SCC-46 cells pre-treated with 10 or 20 μM DMAT or DMSO diluent containing
medium alone, or 100 nM scrambled control or CK2α/α′ siRNAs were labeled with Hoechst 33342 dye, and analyzed by flow cytometry.
The percent Hoechst Low SP cells highlighted within the window indicated were reduced by CK2 inhibitors.
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Lu et al.
expression in both UM-SCC-22A and -46 (Figure 2A). DMAT had
corresponding inhibitory effects on expression of these proteins in
UM-SCC-46 (Figure 2B), and -22A (Suppl. Figure 1A). Combining
siRNAs targeting both CK2α/α′ catalytic subunits inhibited most of
the CSC marker mRNAs in the 2 cell lines except Oct4 in UM-SCC22A (Figure 2C), but did inhibit all three CSC markers at the protein
level in UM-SCC-46 (Figure 2D) and -22A (Suppl. Figure1B). The
individual CK2α and α′ subunit siRNAs had a variable effect on
expression of the different CSC marker mRNA and proteins (Figure 2,
C and D, Suppl. Figure 1, A and B), similar to that observed
previously for multiple genes in other cell lines [11]. Corresponding
to the effects of DMAT on the CSC markers, is a marked reduction
in SP cells (Figure 2E). Normalized to control (0.97 = 100%),
DMAT decreased SP by 65% and 96% at 10 and 20 μM,
respectively (Figure 2E). Similarly, siRNA targeting both CK2α/α′
catalytic subunits potently reduced the SP, when compared with
transfection with scrambled control siRNA (Figure 2F). These
inhibitory effects of CK2 inhibitor DMAT or siRNA suggest that
CK2 may be important in regulation of CSC genes and the side
population phenotype in HNSCC.
CK2 inhibitor and siRNA increase expression and function of
TP53 family member TAp73 which acts as a suppressor of CSC
genes and the side population
We previously observed that CK2α suppresses TP53 and TAp63
family member gene expression [11]. Thus, we explored if CK2α
modulates the homologous TAp73 tumor suppressor isoform, and
whether CSC gene signatures and the SP cell phenotype are regulated by
a mechanism involving TAp73. CK2 inhibition by DMAT or CK2α
siRNA similarly enhanced expression of TAp73 mRNA in UM-SCC-46
(Figure 3A), supporting a role for CK2α in repression of TAp73 gene
expression. DMAT also increased TAp73 but not faster migrating
ΔNp73 isoforms as detected by TAp73 or p73 antibodies when
compared to constitutive Oct1 as a loading control in nuclear extracts
(Figure 3B), a requisite for possible tumor suppressor function in gene
regulation. Conversely, TAp73 siRNA knockdown resulted in dose
dependent enhancement of Oct4 and Nanog mRNA expression
(Figure 3C). TAp73 siRNA knockdown in UM-SCC-22A cells
enhanced Sox2 mRNA expression (Suppl. Figure 2A), but not in UMSCC-46 cells (Suppl. Figure 2B). Furthermore, a decrease of Sox2, Oct4
and Nanog protein expression with CK2 inhibitor DMAT treatment
was attenuated by TAp73 siRNA knockdown (Figure 3D). Together,
these findings suggest CK2 inhibitor modulation of TAp73 expression
and/or function may contribute to repression of these proteins. To
further confirm the potential of CK2 inhibition to enhance TAp73
function as a tumor suppressor, we examined the effects of DMAT and
CK2α siRNA on classical TP53/TAp73 inducible genes. CK2 inhibition
by DMAT or CK2α siRNA had a reciprocal effect, enhancing TAp73
inducible TP53/TAp73 response element specific reporter pG13, as well
as growth arrest and apoptotic genes CDKN2A(p21), and PUMA (Suppl
Figure 3A, B) [16,20]. This effect of CK2 inhibition was also confirmed
to be TAp73 dependent, requiring co-expression of TAp73-Flag (Suppl
Figure 3C-E), in cell line UM-SCC-1, which is deficient for TAp73 [16].
Furthermore, DMAT inhibited SP cells, while TAp73 knockdown by
siRNA strongly increased the number of SP cells, in the absence or
presence of DMAT (Figure 3E). Together, these results support the
hypothesis that CK2-mediated inactivation of TAp73 promotes CSC
gene expression and the SP phenotype, while inhibiting growth arrest
and apoptotic genes, and that this is reversible by CK2 inhibition.
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CK2α interaction with TAp73 is inhibited by DMAT and
T27A mutation of a predicted CK2 phospho-acceptor site in the
transactivation domain of TAp73
Bioinformatic analysis of TAp73 as a potential substrate for CK2
serine/threonine kinase uncovered a single high probability motif
containing threonine at amino acid position 27 (T27) within the
transactivation (TA) domain of human TAp73 (Figure 4A; Suppl
Figure 4). Supporting the importance of the predicted site, the highly
conserved homologous site is found in the TA domain of human (T27)
and mouse (T31) in a region of predicted surface accessibility of TAp73
protein (Suppl Figure 4). Reciprocal co-immunoprecipitations established that interaction occurs between CK2α and TAp73, and this
interaction is attenuated by CK2 inhibitor DMAT in a dose dependent
manner (Figure 4B). Immunoprecipitation of TAp73 with anti-TAp73,
or cells transfected with TAp73-Flag with anti-Flag antibodies, showed
enhanced CK2 interaction, whereas this interaction was markedly
reduced upon equivalent expression of a sequence-verified T27A pointmutant of TAp73-Flag (Figure 4, C and E, top panel). Increased
expression of TAp73-Flag was accompanied by increased phosphorylation, while equivalent overexpression of TAp73 with T27A point
mutation of the predicted CK2 phosphoacceptor site showed markedly
reduced phosphorylation when cell lysates were incubated with
recombinant CK2α2β2 in an in vitro kinase assay (Figure 4, D and E,
top panel). These results reveal that CK2α-TAp73 interaction and
phosphorylation of TAp73 is inhibited by DMAT or T27A mutation
of a predicted CK2 phosphoacceptor site in the transactivation domain
of TAp73.
Mutation of the predicted CK2 T27A phosphoacceptor site
enhances TAp73 inhibition of CSC marker expression and SP cells
Overexpression of TAp73, which exhibited increased CK2 interaction and phosphorylation (Figure 4, C and D), resulted in only slight
inhibition of CSC markers Nanog and Sox2, or reciprocal enhancement
of TAp73-inducible proapototic protein PUMA (Figure 4E). However,
similar expression of the CK2 phospho-acceptor mutant T27A-TAp73
that exhibited reduced interaction and phosphorylation (Figure 4C and
D), strongly repressed the CSC markers, and reciprocally enhanced
TAp73 inducible proapoptotic protein PUMA (Figure 4E). Consistent
with these effects, overexpression of TAp73 only partially reduced SP
cells, while T27A-TAp73 strongly reduced SP cells detected
(Figure 5A). Treatment with CK2 inhibitor DMAT resulted in
inhibition of SP cells detected in empty vector control and TAp73
transfected cells, but had little additional effect after near complete SP
inhibition observed with overexpression of T27A-TAp73 (Figure 5B).
These data support that pharmacologic CK2 inhibition or prevention of
T27 phosphorylation by T27A mutation enhances the repressive effect
of TAp73 on these CSC markers and SP cells.
CK2 promotes clonogenic survival and CSC spheres
To further examine if CK2 regulates CSC phenotypes, we
investigated its role in clonogenic colony and tumor spheroid
formation, two CSC features previously shown to correspond to SP
and enhanced tumorigenicity in HNSCC (6). We used a novel selective
CK2 inhibitor, CX4945, currently under investigation in clinical trials
[19]. CK2 inhibitor CX-4945 significantly reduced clonogenic survival
(Figure 6A) and sphere formation (Figure 6B) in UM-SCC-1 and -46,
in a dose dependent manner (*P b .05). The morphologic effects of
CK2 inhibitor on colony (Figure 6C) and sphere formation (Figure 6D)
are linked with the SP CSC phenotype [5]. Together, these data indicate
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Figure 3. CK2 inhibition enhanced TAp73 expression and TAp73 dependent suppression of CSC-related markers and SP cells. A. TAp73
mRNA expression was significantly increased 48 hours after treatment with CK2 inhibitor DMAT (left) or transfection with CK2α siRNA
(right) in UM-SCC46 cells. B. TAp73 and total p73 protein expression was increased in nuclear extracts 48 hours after UM-SCC-46 cells
were treated with increasing concentrations of CK2 inhibitor DMAT, as detected by Western blot. Nuclear Oct1 is shown as a constitutive
loading control. C. CSC-related Oct4 and Nanog mRNA expression was increased in UM-SCC-46 48 hours after transfection with
increasing concentration of 50, 100 and 200 nM TAp73 siRNA. D. CSC-related Sox2, Oct4, and Nanog proteins were decreased 48 hours
after DMAT treatment of UM-SCC-46, while TAp73 siRNA knockdown attenuated this effect. E. UM-SCC-46 cells were labeled with
Hoechst 33342 dye and analyzed by flow cytometry 48 hours after transfection with control scrambled siRNA or TAp73 siRNA −/+
DMAT. SP cell number decreased after DMAT treatment, while DMAT showed no significant effect on SP cells after TAp73 knockdown.
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Figure 4. CK2 and TAp73 interaction is inhibited by CK2 inhibitor DMAT or T27A point mutation within a predicted CK2 phospho-acceptor
motif in TAp73. A. CK2 interaction with TAp73 is predicted by presence of a high probability CK2 phosphorylation site at Threonine 27
(T27) within the TA domain of human TAp73 using Motifscan (Suppl Figure 4). B. Immunoprecipitation (IP) with anti-CK2α or TAp73
antibodies demonstrates reciprocal interaction between CK2α and TAp73 on immunoblotting (IB) in whole cell lysates of UM-SCC-46
cells. The interaction is decreased after treatment with increasing amount of CK2 specific inhibitor DMAT (10, 20 μM). C. Interaction
between CK2α and TAp73 is decreased 48 hours after transfection with Flag-T27A-TAp73 mutant when compared with Flag-TAp73
control. Whole cell lysates from UM-SCC-46 cells were immunoprecipitated (IP) with TAp73 or Flag antibodies, and then immunoblotted
(IB) with CK2α antibody. Physical interaction between CK2α and TAp73 was increased after over-expression of wild type TAp73, but
decreased between Flag-T27A and CK2α. D. In vitro kinase assay shows decreased phosphorylation of TAp73 after T27A mutation.
Lysates from cells transfected with empty vector, Flag-TAp73, or Flag-T27A were incubated with the recombinant CK2α2β2 protein in the
presence of [γ-32P]ATP. The reaction mixtures were separated by SDS-PAGE and subjected to autoradiography (top panel). Bottom panel
shows the Coomassie Brilliant Blue staining of the Flag-TAp73 fusion proteins as the loading control. E. Top panel, equivalent expression
of Flag-TAp73 and Flag-T27A TAp73 in lysates used for C, D, E, obtained 48 hours after UM-SCC-46 cells were transfected with empty
vector, wild type TAp73, or T27A mutant form of TAp73; lower panels, T27A TAp73 enhances inhibition of CSC markers and expression of
proapoptotic protein PUMA. The protein expression of TAp73, Nanog, Sox2 and PUMA in whole cell lysates were assessed by Western
blot, with β-actin as the loading control.
the potential of CK2 inhibition to inhibit SP, clonogenic and sphere
phenotypic features, that are characteristic of CSC in HNSCC [5].
Discussion
Here, we unveil a new mechanism by which kinase CK2mediated TAp73 phosphorylation and inactivation, enables enhanced
expression of stem cell genes Nanog, Sox2, Oct4 and phenotypes that are
implicated in CSC of HNSCC and other cancers (Figure 7) [1,7]. Recent
data available from The Cancer Genome Atlas provides evidence for
amplification of Sox2, Oct4 and/or Nanog genes in over 20% of tumors,
indicating that these genes may also be deregulated by direct genomic
alterations in HNSCC [13,22]. Unexpectedly, we observed that
expression of these CSC transcription factors is increased in a subset of
HNSCC lines with increased expression of inactivated TAp73 and
mtTP53 [16]. Pharmacologic or siRNA inhibition of CK2 further
enhanced TAp73 expression. CK2 inhibition or mutational inactivation
of a predicted CK2 phospho-acceptor motif in the transactivation domain
of TAp73 then restored p73 function, repressing mRNA and protein
expression of these CSC transcription factor genes (Figure 7). Conversely,
we found CK2 inhibition reciprocally induced TAp73-inducible growth
arrest and proapoptotic genes CDKN2A(p21) and PUMA in HNSCC
[16; Suppl Figure 1], indicating a pivotal role as a switch regulating genes
that determine cell fate. Concordantly, CK2 inhibition attenuated the SP
subset, clonogenicity, and sphere formation, linked to CSC phenotype
and tumorgenicity in HNSCC and other cancers [6].
We found that CK2 inhibition enhanced TAp73 expression and
dependent repression of several known CSC transcription factor genes
in two independent lines from an HNSCC subset overexpressing
TAp73 with mtTP53. We recently demonstrated that CK2 similarly
contributes to repression of TP53 and TAp63 mRNA and protein in a
subset of HNSCC with wtTP53 genotype, an effect also reversible by
CK2 inhibition [11]. Together, our current and prior studies indicate
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Figure 5. TAp73-T27A mutation of CK2 phosphoacceptor site and CK2 inhibitor DMAT suppress CSC SP cells. 48 hours after transfection
with empty vector control, Flag-TAp73, or Flag-T27A TAp73, UM-SCC-46 cells were labeled with Hoechst 33342 dye and analyzed by flow
cytometry. A. Without treatment. B. Treatment with DMAT 20 μM. UM-SCC-46 cells were examined for Hoechst dye uptake. Hoechst Low
SP cells are highlighted, and % total cells are shown.
an important role of CK2 in pan-repression of expression and function
of tumor suppressor TP53 and isoforms TAp63, and TAp73 in
HNSCC subsets with wt or mtTP53. With the prevalence of TP53
mutation or inactivation in nearly all HNSCC [13], identification of
key kinases reversibly regulating TAp73 inactivation is of potential
biologic and therapeutic importance. Because TAp73 is rarely mutated
and may function as a tumor suppressor in place of TP53 [15,16],
druggable kinase targets such as CK2 capable of reactivating TAp73
have potential for prevention or therapy.
CK2α/α′ kinase subunits are reported to be overexpressed and form
nuclear complexes associated with chromatin in HNSCC and other
cancers [8,10], but their nuclear function was heretofore obscure.
Previously, we observed predominantly nuclear localization of both
CK2α/α′ catalytic subunits and TAp73 in HNSCC [11,16]. We
establish here that nuclear CK2 and TAp73 interact by co-IP, and this
interaction is blocked by CK2 inhibitor DMAT, supporting their role in
a common mechanism reversible by CK2 inhibitors. The effect of CK2
inhibitor in suppressing CSC target protein expression and SP
phenotype was abrogated by TAp73 siRNA. Conversely, CK2
inhibition enhanced expression of growth arrest and pro-apoptotic
genes CDKN2A(p21) and PUMA required TAp73, together supporting
their dependence on the tumor suppressor function of TAp73. Further
defining the nature of this interaction, we identified a threonine site
(T27) within a single high probability and conserved CK2 motif within
the N-terminal domain critical for CK2 interaction and transactivation
function of TAp73. T27A substitution of this site attenuated CK2TAp73 interaction, TAp73 phosphorylation, reciprocal expression of
CSC marker and PUMA proteins, and the SP phenotype. CK2 inhibitor
CX-4945, which is currently in clinical trials, also inhibited SCC cell
clonogenicity and sphere formation in vitro. Examination of effects of
CK2α/α′ inhibition delivered by nanoparticles in specimens from our
previous study further indicated that CK2 inhibition can inhibit tumor
growth, and enhance p73 expression in tumor in vivo [11, Y. Bian and C.
Van Waes, unpublished observations]. Together, these studies implicate
CK2-TAp73 interaction and T27 phosphorylation as a molecular switch
that promotes cancer stem cell genes, side population, and phenotype.
A novel finding of this study is the identification of a conserved CK2
T27 phospho-acceptor site in the transactivation domain of human and
murine TAp73, which is important in regulating the functional activity
of TAp73. T27 was also reported as a functional inhibitory
phosphorylation site for Polo-like Kinase-1 (Plk1) [23,24], another
key regulator promoting cell cycle progression and survival [25].
Interestingly, the TA domain of TP53 lacks this motif, but contains a
different T18 motif phosphorylated by kinases ATM/ATR, that
stabilize its expression and function, to promote growth arrest and
apoptosis in DNA damage responses [26]. Thus, while TP53 and
TAp73 can both function to suppress growth, the contrasting functions
of ATR/ATM and CK2 and respective phosphorylation sites in TP53
and TAp73 proteins could underlie the important functional
differences in their evolutionary roles in regulating growth arrest or
promotion in damaged and replicating epithelia.
Here we reveal that CK2 inhibition and TAp73 activation
mediates repression of Nanog, Oct4 and Sox2 mRNA and protein
expression, and CSC SP phenotype, while promoting expression of
known TAp73 inducible growth arrest and proapoptotic genes such
as p21WAF1 and PUMA [14–16]. TP53 was previously implicated
in binding the promoter and repressing Nanog mRNA and protein
expression, and promoting differentiation of embryonic stem cells in
response to DNA damage [12]. Recently, overexpression of the
ΔNp73 isoform, lacking the N-terminal region and full
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CK2 suppresses TAp73 in cancer stem cells
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Figure 6. Pharmacological CK2 inhibitor CX-4945 inhibits clonogenic survival and sphere formation by HNSCC. CK2 inhibitor CX4945
inhibits clonogenic survival (A) and sphere formation (B) of UM-SCC-1 and UM-SCC-46 lines in a dose dependent manner between 0.5 to
5 μM in 3 replicate experiments. *Students t test, P b .05. C. Photomicrographs showing CX-4945 inhibition of colony formation in UMSCC-46. D. Photomicrographs showing CX-4945 inhibition of tumor sphere formation in UM-SCC-1 and -46.
transactivating domain, was also shown to enhance Sox2, Oct4
expression and generation of human induced pluripotent stem cells
[27], supporting a broader overlap in function of TP53 and TAp73 in
repressing these stem cell genes. Preliminary analysis of the promoters
of Oct4, Nanog, and Sox2 reveal that they contain predicted sites for
TP53/TAp73 and other transcription factors, but it is also possible
that TAp73 regulated intermediary transcription factors or microRNAs co-modulate CSC genes.
Prior studies suggest that CK2 may modulate the CSC phenotype
through a variety of transcriptional mechanisms. A recent study
demonstrates that CK2α is associated with Hedgehog (Hh)-Gli1 and
Notch1 pathway transcription factors in lung cancers, where
knockdown of CK2α reduced Hh/Gli1 and Notch1 signaling and
the stem-like side population [28,29]. Knockout of the regulatory
CK2β subunit in mice inhibited transcription factor Olig2, embryonic
neural stem cell proliferation, and oligodendrocyte development [30].
Increased CK2α relative to CK2β is linked to upregulation of SNAIL1,
TWIST1, ZEB1/2, and epithelial mesenchymal transition of breast
cancer cells [31]. These observations suggest CK2 can enhance multiple
pathways important in CSC. Although it is not yet known if TAp73
directly or indirectly regulates CSC genes, preliminary chromatin
immunoprecipitation sequence analysis of the promoters of Oct4,
Nanog, and Sox2 reveal that they contain sites for TAp73 binding
(H Cheng and C Van Waes, unpublished observations). Future
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799
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.neo.2014.08.014.
Acknowledgments
Reading and helpful comments of Drs. James Battey and James
Mitchell are appreciated.
References
Figure 7. Model of the role of CK2 phosphorylation and inhibition of
TAp73 in CSC gene program and phenotype. CK2 theonine/serine
kinase mediates phosphorylation and inactivation of tumor suppressor TAp73, promoting expression of CSC genes and phenotype. CK2
inhibition by small molecule inhibitors DMAT or CX-4945, or T27A
mutation of the CK2 phosphoacceptor motif of TAp73 uncovers
TAp73 as a suppressor of CSC genes and phenotype.
chromatin immunoprecipitation sequencing studies for CK2α, TAp73
and other transcription factors in promoter regulation of these genes
may enhance understanding of the mechanisms and potential targets
involved in orchestrating this stem cell gene program.
Here, we find that small molecule CK2 inhibitors DMAT or CX4945 enhance TAp73 expression and function, and inhibit SP and
CSC phenotypes in HNSCC with mtTP53 in vitro. Interestingly,
CK2 inhibition was shown to inhibit PI3K/Akt axis signaling via
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involved in resistance may enhance the effects on tumor growth
and survival. Thus, CK2 and other inhibitors that enhance
reactivation of wtTP53 and/or TAp73 function merit further
investigation for prevention and therapy of SCC.
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