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. 2013 Jun 10;23(6):839-52.
doi: 10.1016/j.ccr.2013.04.008. Epub 2013 May 16.

Phosphorylation of EZH2 activates STAT3 signaling via STAT3 methylation and promotes tumorigenicity of glioblastoma stem-like cells

Eunhee Kim  1 , Misuk Kim, Dong-Hun Woo, Yongjae Shin, Jihye Shin, Nakho Chang, Young Taek Oh, Hong Kim, Jingeun Rheey, Ichiro Nakano, Cheolju Lee, Kyeung Min Joo, Jeremy N Rich, Do-Hyun Nam, Jeongwu Lee
Affiliations

Phosphorylation of EZH2 activates STAT3 signaling via STAT3 methylation and promotes tumorigenicity of glioblastoma stem-like cells

Eunhee Kim et al. Cancer Cell. .

Abstract

Glioblastoma multiforme (GBM) displays cellular hierarchies harboring a subpopulation of stem-like cells (GSCs). Enhancer of Zeste Homolog 2 (EZH2), the lysine methyltransferase of Polycomb repressive complex 2, mediates transcriptional repression of prodifferentiation genes in both normal and neoplastic stem cells. An oncogenic role of EZH2 as a transcriptional silencer is well established; however, additional functions of EZH2 are incompletely understood. Here, we show that EZH2 binds to and methylates STAT3, leading to enhanced STAT3 activity by increased tyrosine phosphorylation of STAT3. The EZH2-STAT3 interaction preferentially occurs in GSCs relative to non-stem bulk tumor cells, and it requires a specific phosphorylation of EZH2. Inhibition of EZH2 reverses the silencing of Polycomb target genes and diminishes STAT3 activity, suggesting therapeutic strategies.

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Figures

Figure 1
Figure 1. EZH2 protein interacts with STAT3 in GSCs
A and B) Co-immunoprecipitation (Co-IP) of EZH2 and STAT3 in GSCs (827 and 387). IgG represents a control antibody used for IPs. For IP-immunoblotting data, antibodies used for IP and Western blotting (WB) were labeled as red and blue, respectively. Two hundred μg of lysates were used for each IP reaction and total lysates (20 μg) were used as input controls. (C) Co-IP and immunoblot analysis of EZH2 and STAT3 in GSCs and NPCs vs. differentiated progenies. Differentiation was induced by culturing these cells in serum (10%)-containing media for three days. The EZH2-STAT3 complex was analyzed by co-IP, followed by immunoblot analysis. Protein levels of EZH2, SOX2 (a GSC-specific transcription factor), and GFAP (an astroglial differentiation marker) were examined. β-actin was used as a loading control. (D) Co-IP and immunoblots of the EZH2-STAT3 complexes in various cells. A 293T cell line (derived from human embryonic kidney cells) was used as a reference. See also Figure S1.
Figure 2
Figure 2. EZH2 targeting decreases STAT3 activation in GSCs
(A) Immunoblots of EZH2, H3K27me3, pY-STAT3 (Y705) and total STAT3 in GSCs transduced with shEZH2-I, shEZH2-II, or non-targeting (NT) control shRNA. (B) Quantification of protein intensities in immunoblots including the one shown in (A) was performed by densitometry. (C) Immunoblots of pY-STAT3 and total STAT3 in GSCs treated with EZH2 inhibitors DZNep (2 μM) or GSK-126 (2 μM) for the indicated times (0 hr to 4 hr). Representative blots using 827 cells were shown. (D) Semi-quantitative real time RT-PCR analysis to determine mRNA expression of STAT3 target genes in GSCs (827 and 387) treated with DZNep for 1 day. Expression levels of these genes were normalized and compared to those of the untreated control cells. Data are means ± SD (n = 3). (E) Determination of STAT3 transcription activity by STAT3-responsive luciferase reporter assays. Luciferase activities in GSCs treated with DZNep were compared to those of the untreated control. Cells treated with cucurbitacin (a STAT3 inhibitor) were used as a positive control. Error bar represents SD. *p < 0.01.
Figure 3
Figure 3. EZH2 methylates STAT3 protein in GSCs
(A and B) Co-IP and immunoblots of the methylated STAT3 in GSCs. Either EZH2 knockdown (A) or treatment with DZNep (B) decreased methylated STAT3 in GSCs. Methyl K represents an antibody recognizing the methylated lysine. (C) Co-IP and immunoblots of the methylated STAT3 in GSCs that express GFP, wild-type EZH2, and EZH2 H689A. Arrows indicate protein bands of endogenous EZH2 (endo) and exogenous EZH2 transgenes (exo). (D) Expression levels of DKK1, a PRC2 target gene, in GSCs expressing EZH2 transgenes were determined by real time RT-PCR analysis. Data are means ± SD (n = 3). * p < 0.01. (E) IP and immunoblots of methylated STAT3 in various GSC-derived GBM tumors. (F) Kaplan-Meier survival curves of mice orthotopically implanted with 211 GSCs transduced with either control or EZH2 shRNA expressing lentivirus (n = 7). p < 0.01. (G) Immunoblots of pY-STAT3 and H3K27me3 in xenograft tumors (F). Human tumor cells were harvested three months after intracranial implantation and processed for immunoblot analysis. (H) Measurement of subcutaneous xenograft tumor size after treatment with DZNep. Error bar represents SD (n = 5). ** p <0.01. GSC-derived xenograft tumors were treated with DZNep (1 mg/kg body weight twice a week via intraperitoneal injection) for two weeks, harvested, and processed for immunoblots and co-IP analysis (I and J) and IF staining (K). (K) IF staining of pY-STAT3 (green) on the frozen sections of GBM xenografts treated with vehicle or DZNep. Nuclei were stained with DAPI. Bar represents 10 microns.
Figure 4
Figure 4. Effects of STAT3 methylation on STAT3 activity and GSC self-renewal
(A) Co-IP analyses of methylated STAT3, pY-STAT3 and the EZH2-STAT3 complex in 131 GSCs expressing STAT3 K180 mutants. V5 is a protein tag fused with STAT3. Methyl K denotes an antibody recognizing either di- or tri-methylated lysine. (B) STAT3 transcriptional activity in response to IL-6 in 131 GSCs expressing the empty vector control, wild-type STAT3, or the STAT3 K180 mutants. * p < 0.01. (C) Real time RT-PCR analysis to determine mRNA levels of SOCS3 and c-MYC in GSCs (827 and 131) expressing either wild-type or STAT3 K180A mutant. Messenger RNA levels of SOCS3 and c-MYC in STAT3 K180 expressing cells were compared to those in wild-type STAT3 expressing cells (set to 1). Data are means ± SD (n = 3). * p < 0.01. (D) IF staining of pY-STAT3 (green) on GSCs expressing the wild-type STAT3 and STAT3 K180 mutants in response to IL-6 (50ng/ml). V5 (red) was used to stain exogenous STAT3 and DAPI (blue) was used to stain nuclei. (E) Real time RT-PCR analysis to determine mRNA expression of stem cell associated transcription factors (OLIG2, SOX2, and NANOG) in GSCs (827 and 131) expressing STAT3 mutants. Data are means ± SD (n = 3). * p < 0.01. (F to I) Effects of STAT3 K180 mutant overexpression on proliferation and clonogenic growth of GSCs. 5′-ethynyl-2′-deoxyuridine (EdU)-positive cells (stained green) were counted in three random fields and plotted (in F and G). *p < 0.01. Cell counts of STAT3 mutant-expressing GSCs after 6 days of culture (H) and limiting dilution assay results (I) were shown. Error bars represent SD. Bars (in D and F) represent 10 microns. See also Figure S2.
Figure 5
Figure 5. AKT signaling is essential for the EZH2-STAT3 interaction in GSCs and AKT inhibition decreases STAT3 activity in GBM xenografts
(A) Immunoblot analysis of AKT, pS473 AKT, EZH2, pS21 EZH2, and trimethylated H3K27 in GSCs treated with a PI3K/AKT pathway inhibitor LY294002. (B and C) Co-IP analysis of methylated STAT3 and the EZH2-STAT3 complex in GSCs treated with LY294002 for one day. (D) Tumor growth of 827 GSC-derived xenografts treated with an AKT inhibitor perifosine (30 mg/kg body weight). Twenty-three days after tumor implantation in mice, perifosine was administered by intraperitoneal injection (daily for 5 days). Error bars represent SD. Five mice per group, * p < 0.01. (E) Immunoblot analysis of pY-STAT3, EZH2, AKT, and trimethylated H3K27 in lysates from xenograft tumors in (D). (F) Co-IP analysis of methylated STAT3 in GBM xenograft tumors treated with perifosine. (G) IF staining of pS21 EZH2 and pY-STAT3 on the frozen sections of GBM xenografts treated with vehicle or perifosine. Nuclei were stained with DAPI. Bar represents 10 microns.
Figure 6
Figure 6. Phosphorylation of EZH2 at Serine 21 is critical for STAT3 methylation and STAT3 activity in GSCs
(A) Co-IP and Immunoblot analysis of pS21 EZH2 and methylated STAT3. (B) Immunoblot analysis of pS21 EZH2 in GSC-enriched or GSC-depleted GBM cells (CD133+ or − and CD15+ or −). (C) Co-IP and immunoblot analyses of methylated STAT3 in GSCs transduced with various EZH2 mutants. EZH2 transgenes were detected by Flag tag. (D) Quantification of methylated STAT3 shown in (C). Protein amounts were estimated by densitometry of immunoblots. Error bars represent SD. * p < 0.01. (E and F) Real time RT-PCR analysis to determine mRNA expression of STAT3 and SOCS3 in 131 GSCs expressing various EZH2 mutants. Data are means ± SD (n = 3). *p < 0.01. (G) Determination of STAT3 transcription activity by reporter promoter assays. Luciferase activity in GSCs expressing various EZH2 constructs was compared to that of the GFP-expressing control cells. Error bars represent SD. *p < 0.01. (H) Effects of ectopic expression of various EZH2 constructs on GSC tumorsphere formation. GSC cells (131) transduced with lentivirus expressing various EZH2 transgenes were cultured for limiting dilution assays. Frequency of stem-like clonogenic cells was determined by a web-based tool “ELDA” (extreme limiting dilution analysis, available on http://bioinf.wehi.edu.au/software/elda/) and represented as a box plot. Boxes indicate the upper and lower confidence intervals, and the lines denote the estimated values. *p < 0.01. See also Figure S3.
Figure 7
Figure 7. EZH2 contributes to GBM tumor growth by both H3K27 trimethylation-dependent and STAT3 dependent pathways
(A and B) Immunoblot and Co-IP analysis of methylated STAT3 and H3K27me3 in GSCs that express GFP, wild-type EZH2, or EZH2 S21A. (C) Immunoblots of exogenous EZH2 S21A and STAT3-C proteins in 131 GSCs. (D) Representative images of 131 GSCs expressing EZH2 S21A and STAT3-C protein. Bar represent 50 microns. (E) Effects of ectopic expression of EZH2 S21A and STAT3-C protein on GSC tumorsphere formation. From limiting dilution assays, frequencies of stem-like clonogenic cells were calculated using ELDA and presented as a box plot. *p < 0.01. (F) Kaplan-Meier survival curves of mice (n = 10 for each group) intracranially implanted with 131 GSCs expressing control, EZH2 689A or EZH2 S21A proteins (control vs. EZH2 H689A, p <0.0001; control vs. EZH2 S21A, p <0.0001; EZH2 H689A vs. EZH2 S21A, p = 0.015 by log ranked analysis). (G) A schematic model that illustrates two functional arms of EZH2. P and Me represent phosphorylation and methylation, respectively. See also Figure S4.
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