SMURF1 facilitates estrogen receptor ɑ signaling in breast cancer cells
Estrogen receptor alpha (ER alpha) is expressed in the majority of breast cancers and promotes estrogen-dependent cancer progression. ER alpha positive breast cancer can be well controlled by ER alpha modulators, such as tamoxifen. However, tamoxifen resistance is commonly observed by altered ER alpha signaling. Thus, further understanding of the molecular mechanisms, which regulates ER alpha signaling, is important to improve breast cancer therapy.
SMURF1 and ER alpha protein expression levels were measured by western blot, while the ER alpha target genes were measured by real-time PCR. WST-1 assay was used to measure cell viability; the xeno-graft tumor model were used for in vivo study. RNA sequencing was analyzed by Ingenuity Pathway Analysis. Identification of ER alpha signaling was accomplished with luciferase assays, real-time RT-PCR and Western blotting. Protein stability assay and ubiquitin assay was used to detect ER alpha protein degradation. Immuno-precipitation based assays were used to detect the interaction domain between ER alpha and SMURF1. The ubiquitin-based Immuno-precipitation based assays were used to detect the specific ubiquitination manner happened on ER alpha.
Here, we identify the E3 ligase SMURF1 facilitates ER alpha signaling. We show that depletion SMURF1 decreases ER alpha positive cell proliferation in vitro and in vivo. SMURF1 depletion based RNA-sequence data shows SMURF1 is necessary for ER alpha target gene expression in the transcriptomic scale. Immunoprecipitation indicates that SMURF1 associates with the N-terminal of ER alpha in the cytoplasm via its HECT domain. SMURF1 increases ER alpha stability, possibly by inhibiting K48-specific poly-ubiquitination process on ER alpha protein. Interestingly, SMURF1 expression could be induced via estradiol treatment.
Our study reveals a novel positive feedback between SMURF1 and ER alpha signaling in supporting breast cancer growth. Targeting SMURF1 could be one promising strategy for ER alpha positive breast cancer treatment.
KeywordsSMURF1 ER alpha Breast cancer Ubiquitination Protein stability
Breast cancer ranks number one in woman malignancy worldwide and causes the second most frequent cancer mortality in females . About 60–70% breast cancers are estrogen receptor alpha (ER alpha) positive, where ER alpha transcription program is required for breast cancer progression . ER alpha belongs to the ligand-dependent subfamily of the nuclear receptor or transcription factors, and its activity is mainly regulated by estrogen . Like other nuclear receptors, ERα has several distinct domains: Activator Function 1 (AF1) domain at the N-terminus that recruit co-factors, DNA-binding domain (DBD) that binds to the estrogen response elements (EREs), and Activator Function 2 (AF2) domain that is the ligand-dependent transactivation domain . As part of ER alpha transactivation function, the AF2 domain recruits several co-activators and co-repressors to control ERα activity . Upon estrogen stimulation, the ER alpha protein can shuttle into the nuclear and bind to cis-regulatory DNA regions of target genes, which subsequently trans-activates certain gene expression and promotes cancer progression.
Since ER alpha signaling is necessary for the progression of luminal type of breast cancers, hormone depletion and ERα antagonists have been widely used to treat ER+ breast cancer patients, such as tamoxifen . Although tamoxifen largely improves breast cancer patient survival, the development of tamoxifen resistance is common. Thus the understanding of dys-regulation of ER alpha that trigger inappropriate estrogen signaling and drug resistance is of utmost importance. A number of confirmed and possible mechanisms, which triggers inappropriate estrogen signaling, are shown to relate to the transcriptional and epigenetic control of ER alpha protein . Others that account for the dys-regulation of estrogen signaling involve the controls of ER alpha activities by the ligands, cofactors and post-translational modifications [7, 8].
Although the understanding of how ER alpha protein are controlled in breast cancer is largely unclear, the regulation of ER alpha protein stability is evident from that recent studies showing that several post-translational modifications are involved in ER alpha protein stability, such as phosphorylation, ubiquitination and SUMOylation, which link to the ubiquitin-proteasome system . A few E3 ubiquitin ligases have been shown to play roles in modulating ER alpha signaling. For example, BRCA1 and MDM2 could ubiquitinate ER alpha and trigger proteasomal degradation [6, 7, 9]. Interestingly, recent studies show that a few E3 ligases could modulates ER alpha signaling through stabilizing ER alpha protein, such as RNF31 and RNF8 [10, 11, 12].
Here, we identify the E3 ubiquitin ligase SMURF1 (SMAD Specific E3 Ubiquitin Protein Ligase 1) as one such kind of factor. SMURF1 was firstly identified as a negative regulator of SMADs (Mothers Against DDP homolog), which mediated SMADs ubiquitination and degradation . In this study, we characterize a novel non-genomic regulatory mechanism that SMURF1 controls ER alpha ubiquitination and stability, which subsequently regulates estrogen-dependent gene expression and cancer cell proliferation.
SMURF1 depletion inhibits ER alpha positive breast cancer cell proliferation in vitro and in vivo
SMURF1 depletion decreases the expression of ER alpha target genes in breast cancer cells
SMURF1 depletion or inhibition in breast cancer cells decreases ER alpha signaling activity
SMURF1 associates with ER alpha and increases its stability
SMURF1 interacts with ER alpha AF1 domain through its HECT domain and prohibits ER alpha K48 specific poly-ubiquitination
ER alpha belongs to the nuclear receptor superfamily of transcription factors, and specifically to the ligand-dependent subfamily . The basal and ligand-induced ER alpha protein levels are under tight control by ubiquitin-proteasome system . Several ubiquitin ligases were shown to promote ER alpha protein degradation via facilitating ER alpha polyubiquitination, including CHIP and MDM2 [6, 9]. For example, CHIP protein was shown to associate with unliganded ER alpha and promotes its degradation. Interestingly, recently studies showed a group of E3 ligases exerted their stabilization effect on ER alpha protein through nonproteolytic ubiquitin manner . For example, RNF8 was shown to interact with ER alpha in the nuclear and promote ER alpha mono-ubiquitination. Our previous studies also showed RNF31 could stabilize ER alpha via inducing ER alpha mono-ubiquitination [10, 11]. In our current study, we show that SMURF1 is involved in a positive feedback and possibly functions to maintain ER alpha stability. SMURF1 associates with the N terminal of ER alpha in the cytoplasm and prohibits ER alpha K48 poly-ubiquitination. There are two possible models to explain this stabilization effect. One is that SMURF1 exerts its function in an ubiquitin modification dependent manner. Another is that SMURF1 binds to ER alpha and prohibits its binding to other proteolytic E3 ubiquitin ligase. Based on our current assay, we do not found the known ubiquitination manner on ER alpha, which is significantly induced by SMURF1, including K63, K48 and Monoubiquitination (data not shown). However, our data indicate SMURF1 interacts with ER alpha AF1 domain, which account for a possible explanation for the stabilization effect. Previous studies showed that a few ER alpha co-activators could bind to AF1 domain and exert its stabilization effects, such as PIN1, MUC1 and ABL [19, 20, 21].
SMURF1 belongs to the HECT-type E3 ligases family proteins, which was firstly discovered as the antagonist of transforming growth factor beta (TGF beta) signaling . SMURF1 interacts with several SMADs and promotes their poly-ubiqutination and degradation. Besides, SMURF1 is also shown to participate in Wnt signaling by promoting Prickle protein poly-ubiquitination and degradation . However, other studies in SMURF1 also indicate that SMURF1 could also modulate certain protein in a nonproteolytic manner. For example, SMURF1 could stabilize MDM2, which subsequently promotes P53 degradation . Besides, SMURF1 could also mediate K29-linked nonproteolytic polyubiquitination of axin in Wnt signaling . In our study, we identify SMURF1 exerts its stabilization effect on ER alpha protein, which effect is dependent on HECT domain. Thus it is a bit contradictory that HECT domain is known to be a catalytic domain for E3 ubiquitin ligase. It remains to be determined that why SMURF1 exerts different protein stability consequences on its target proteins. Our study also indicates SMURF1 is one of ER alpha signaling target genes, which could be transcriptional increased by estradiol. However, previous study showed that ER alpha could promote SMURF1 protein degradation in MCF-7 and HEK293 cells . This might indicate ER alpha could regulate SMURF1 mRNA and protein level in opposite directions.
Although ER alpha has been well documented to have a critical in cancer progression in breast cancer, SMURF1 emerges to be a new component of ER alpha signaling in breast cancer. Beside, previous studies showed SMURF1 was recognized as an oncogene, since it was found to suppress several tumor suppressor pathways, such as TGF beta signaling and P53 signaling [24, 27]. Several clinical studies also showed that SMURF1 was amplified in the chromatin in pancreatic and esophageal carcinomas [28, 29]. Here, we identify the oncogenic role of SMURF1 in supporting estrogen signaling and breast cancer progression. Thus targeting SMURF1 could be a promising strategy or drug target for several kinds of human carcinomas, including ER alpha positive breast cancer.
MCF-7 and HEK293 cells were cultured in DMEM (Invitrogen, Carlsbad, CA) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin (Invitrogen) at 37 °C in a humidified atmosphere of 5% CO2 in air. T47D cells were cultured in RPMI 1640 (Invitrogen) supplemented with 10% FBS (GIBCO) and 1% penicillin/streptomycin.
The pRK-Myc-SMURF1 plasmid was acquired from Addgene (Plasmid #13676). The SMURF delta-HECT construct and delta-WW + HECT construct were sub-cloned from the original plasmid. The PcDNA3-HA-ER alpha plasmid was acquired from Addgene (Plasmid #49498). The ER alpha delta-AF1 construct, delta-AF1 + DBD construct, delta-AF2 construct and delta-AF2 + DBD construct were sub-cloned from ER alpha original plasmid. The HA-K48 and HA-K63 Ubi plasmids were gifted from Dr. Bo Yang and Jie Wang . The flag-ER alpha plasmid was described from previous paper . The ERE-TK-luc reporter and the pRL-TK control were described in previous study .
siRNA and plasmids transfection
Cells were transfected with 50 nM siRNA. SMURF1 siRNAs sequences were shown here: SMURF1 siRNA #1: CCAGUAUUCUACGGACAAUdTdT; siRNA #2: CAUGAAAUGCUGAAUCCUUdTdT. Control siRNA sequences were shown: UUCUCCGAACGUGUCACGUTT. INTERFERin transfection reagent (Polyplus Transfection, 409–10) was used according to the manufacturer’s protocol. Plasmids were transfected by Lipofectamin 2000 (1,662,298, Invitrogen).
RNA extraction and qPCR analysis
RNeasy kits were used to extract total RNA (Qiagen). qPCR was performed as previously described. 36B4 was used as internal control. Primer sequences for qPCR are provided in Additional file 1: Table S1.
Quantification of cell viability
MCF-7 and T47D cells were transfected with siSMURF1 or siControl in 24-well plates. After 24 h, the cells were seeded into 96-well plates. Estrogen and vehicle were added in each group. Cell numbers were determined using the WST-1 cell proliferation reagent as previously described .
Xenograft tumor model
MCF-7 cells were infected with shControl virus or shSMURF1 virus (SC108080 and SC41673V). After 48 h of infection, cells were treated with 2μg/ml puromycin for 3 days. Female nonobese diabetic-SCID mice were implanted with slow-relase 17 beta-estradiol pellets (0.72 mg/90-day, Innovative Research of America). After 1 day, each mouse was injected with 1X106 MCF-7 cells together with matrigel solution into the mammary fat pad. The tumor sizes were measured each 3–5 days. After 2 mouths, the mice were sacrificed and the tumors were fixed with Formalin for measuring weight and photograph. The experiments were performed under the protocols approved by ethnic committee of Xinxiang Medical University.
Wound healing assay
MCF-7 and T47D cells were seeded and transfected with 50 nM SMURF1 siRNA or control siRNA. 24 h after transfection, cells were seeded into 6-well plates with 1% FBS with 100% confluence. One yellow pipette tip was used to make a straight scratch. The wound distance was measured at indicated time points and normalized with starting time point. Percentage wound recovery was expressed as: [1-(Width of the wound at a given time/width of the wound at t = 0)] × 100%.
Clone formation assay
MCF-7 and T47D cells were plated in six-well plates overnight and treated with 50 nM SMURF1 siRNA or 50 nM siControl. After 24 h, the cells were washed with PBS, trypsinized and plated at low density (5000 cell/well in six-well plate). The cells were cultivated for 7 days and the medium was refreshed every two days. The colonies were stained with crystal violet. The number of the clones in a given area was counted for each condition.
Cells were lysed with RIPA lysis buffer. Anti- ER alpha mouse (1D5, SC56833) was from Santa Cruz Biotechnology. Anti-ER alpha rabbit (D8H8, #8644) was from Cell Signaling Technology. Anti-SMURF1 (AB117552), anti-Myc mouse (AB32), anti-Myc rabbit (AB9106) and anti-FLAG (M2, ab48763) were acquired from Abcam. Anti-HA (MMS-101R) was acquired from COVANCE. Anti-actin (8H10D10) was acquired from Cell Signaling Technology.
Immunoprecipitation was performed as previous described . 100μg cell lysls were pre-cleared with ribbit IgG for 2 h and subsequently incubated with ER alpha antibody (D8H8, #8644) over night, while rabbit IgG was used as the negative control. The bounded protein was analyzed by Anti-SMURF1 antibody (AB117552). For the overexpression experiment, HEK293 cells were transfected with 5μg Myc-SMURF1 and ERɑ plasmid in 10 cm dish. Cell lysates were pre-cleared with IgG and subsequently incubate with Myc (AB9106) antibody or ERɑ (D8H8, #8644) antibody, while rabbit IgG was used as the negative control. The bound proteins were analyzed by western blotting.
Chromatin immuno-precipitation (ChIP) assay
ChIP assay was performed in our previous study. MCF-7 cells were fixed for crosslinking for 30 min. After that, the cells was mixed with 0.1375 M glycine, washed with cold PBS/1 mM PMSF and scratched into PBS/1 mM PMSF for centrifuge. Then cells were treated by SDS lysis buffer and sonicated for 10 min (30 s on/off). Then the ChIP assay kit (Millipore, 17–295) was used for following steps. The following antibodies were used in the ChIP experiments: anti-H3K27AC (Ab4729) and anti-ERɑ rabbit (D8H8, #8644). The primer sequences for ChIP assay were shown: Forward- CAACCTCCAGCCATTCTCACT; Reverse- TGTCTCCATATGCTGGTGGTG.
MCF-7 cells cultured on sterile glass cover slips were fixed with 4% formaldehyde for 10 min. Then the cells were incubated in permeabilization buffer (0.3% Triton X100, in PBS) for 10 min. Ten percent donkey serum was added to suppress nonspecific antibody binding at room temperature for 30 min and primary antibodies were incubated overnight at 4 degree. Fluorochrome-conjugated secondary antibodies were added after wash in a dark chamber at room temperature. The slides were washed with PBS and mounted using mounting solution containing DAPI. Finally, slides were visualized with a NIKON80i fluorescent microscope. The antibodies were used as follows: anti-ER alpha (SC-130072, santa cruz); anti-SMURF1 (AB57573,Abcam).
Protein stability assays
HEK293 cells (105) were seeded into 24 well plates and transfected with 0.5μg ERɑ plasmid together with 0.5μg Myc-SMURF1 or empty vector. After 48 h, cells were treated with 100uM cycloheximide (C7698, Sigma) for indicated time points. Samples were subject to western blot for ERɑ degradation.
Pull down assay
ERα fragments corresponding to amino acid (aa) 1–300 and aa 294–595 were individually expressed as his-fusion proteins using the His-CtsR system. His-tagged proteins were purified with nickel-nitrilotriacetic acide (Nii-NTA) resin. GST-fusion SMURF1 proteins were purified using glutathione-Sepharose beads (17–0756-01, GE Healthcare, Uppsala, Sweden) according to the Manufacturer’s protocol. The mixture was incubated at 4 °C with rotation for 30 min; the resin was washed twice with PBS containing 30 mM imidazole, followed by washing twice with PBS containing 0.01% Triton X-100. Bound proteins were eluted with elution buffer (50 mM sodium phosphate, 300 mM NaCl, 250 mM imidazole [pH 7.4]) and subjected to SDS-PAGE analysis.
Analysis of protein ubiquitination
HEK293 cells were transfected with 2 μg ERɑ plasmid together with 2 μg Myc-SMURF1 or empty vector. After 48 h, cells were treated with 10uM MG132 (474,787, Sigma) or ethanol for 6 h. Cells were directly harvested. The poly-ubiquitination of ERɑ was detected by western blotting analysis.
Poly-ubiquitination detection assay
To directly detect the enriched K48-ubiquitinated or K63-ubiqutinated ERɑ from the cell extracts, HEK293 cells were transfected with 0.5 μg K48 Ubi or 4 μg K63 Ubi plasmid, 2 μg ERɑ together with 0.5 μg Flag-SMURF1 or Flag-vector. After 48 h, total protein was extracted and pre-cleared by 20ul protein A (santa cruz, SC-2001) for 2 h. The supernatant was corrected and immunoprecipitated by HA antibody. Western blot with rabbit anti- ERɑ antibody was performed to detect K48 or K63 poly-ubiquitinated ERɑ.
The luciferase activity was done using the Dual-Luciferase Reporter kit (Promega, Germany). The ERE luciferase reporter was transfect together with renila plasmid into the cells. The luciferase activity was measured after 24 h.
RNA sequence analysis
The global gene expression analysis was based on RNA sequencing platform from BGI (Beijing Genomic Institute). The RNA sequence data are deposited in the Gene Expression Omnibus (GEO) database (Assessing number: GSE102653). Analysis was performed for differentially expressed genes (P < 0.01 and fold change >2) by Ingenuity Pathway Analysis (IPA).
Student’s t-test and Pearson correlation coefficient were used for comparisons. A P-value of <0.05 was considered to be significant.
In conclusion, our study reveals a novel positive feedback between SMURF1 and ER alpha signaling in supporting breast cancer growth. Targeting SMURF1 could be one promising strategy for ER alpha positive breast cancer treatment.
The project was supported by the National Science Foundation for Young Scientists of China (No. 8170110153, Ting Zhuang), the Joint Fund of the National Natural Science Foundation of China (No.U1604190, Jian Zhu), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No.15IRTSTHN025, Hui Wang), the National High Technology Research and Development Program of China (2012AA02A201–1, Xiumin Li), the Foundation of Henan Educational Committee (No.16A310014 and No.17A310025, Jian Zhu and Ting Zhuang), and the Program for Ph.D. starting research funding from Xinxiang Medical University (Ting Zhuang and Jian Zhu). This study is funded by graduate innovative practice base for clinical medicine of Xinxiang Medical University, Yashijie medical laboratory institute,School of laboratory medicine, Xinxiang Medical University. We thank all the members of Xinxiang Medical University Immunology research center for sharing valuable material and research support.
Availability of data and materials
Additional data are available as Supplementary information.
HY, NY, and XL performed most of the bench work. JZ, WH, and TZ supervised the process of the study and performed the manuscript writing. WD and GW performed the xenograft mice study. XL, ZL, YZ, MX, SFY, XL and JZ participated in western blot, real time PCR work. XJ performed the RNA-sequence data analysis. GN is responsible for RNAseq data and public available CHIPseq data analysis. All authors read and approved the final manuscript.
Ethics approval and consent to participate
This study was reviewed and approved by the Ethical Board at Xinxiang Medical University.
Consent for publication
The authors’ declare that they have no competing of interest.
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- 5.Breast International Group 1-98 Collaborative G, Thurlimann B, Keshaviah A, Coates AS, Mouridsen H, Mauriac L, Forbes JF, Paridaens R, Castiglione-Gertsch M, Gelber RD, et al. A comparison of letrozole and tamoxifen in postmenopausal women with early breast cancer. N Engl J Med. 2005;353(26):2747–57.CrossRefGoogle Scholar
- 10.Zhu J, Zhao C, Kharman-Biz A, Zhuang T, Jonsson P, Liang N, Williams C, Lin CY, Qiao Y, Zendehdel K, et al. The atypical ubiquitin ligase RNF31 stabilizes estrogen receptor alpha and modulates estrogen-stimulated breast cancer cell proliferation. Oncogene. 2014;33(34):4340–51.CrossRefPubMedPubMedCentralGoogle Scholar
- 31.Zhuang T, Zhu J, Li Z, Lorent J, Zhao C, Dahlman-Wright K, Stromblad S. p21-activated kinase group II small compound inhibitor GNE-2861 perturbs estrogen receptor alpha signaling and restores tamoxifen-sensitivity in breast cancer cells. Oncotarget. 2015;6(41):43853–68.CrossRefPubMedPubMedCentralGoogle Scholar
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