RNF8 promotes epithelial-mesenchymal transition of breast cancer cells
© The Author(s). 2016
Received: 22 March 2016
Accepted: 20 May 2016
Published: 4 June 2016
Epithelial-mesenchymal transition (EMT) is a crucial step for solid tumor progression and plays an important role in cancer invasion and metastasis. RNF8 is an ubiquitin E3 ligase with RING domain, and plays essential roles in DNA damage response and cell cycle regulation. However the role of RNF8 in the pathogenesis of breast cancer is still unclear.
The expression of RNF8 was examined in different types of breast cell lines by Western Blotting. EMT associated markers were examined by Immunofluorescence and Western Blotting in MCF-7 when RNF8 was ectopically overexpressed, or in MDA-MB-231 when RNF8 was depleted. Transwell and wound healing assays were performed to assess the effect of RNF8 on cell mobility. The xenograft model was done with nude mice to investigate the role of RNF8 in tumor metastasis in vivo. Breast tissue arrays were used to examine the expression of RNF8 by immunohistochemistry. Kaplan-Meier survival analysis for the relationship between survival time and RNF8 signature in breast cancer was done with an online tool (http://kmplot.com/analysis/).
RNF8 is overexpressed in highly metastatic breast cancer cell lines. Overexpression of RNF8 in MCF-7 significantly promoted EMT phenotypes and facilitated cell migration. On the contrary, silencing of RNF8 in MDA-MB-231 induced MET phenotypes and inhibited cell migration. Furthermore, we proved that these metastatic behavior promoting effects of RNF8 in breast cancer was associated with the inactivation of GSK-3β and activation of β-catenin signaling. With nude mice xenograft model, we found that shRNA mediated-downregulation of RNF8 reduced tumor metastasis in vivo. In addition, we found that RNF8 expression was higher in malignant breast cancer than that of the paired normal breast tissues, and was positively correlated with lymph node metastases and poor survival time.
RNF8 induces EMT in the breast cancer cells and promotes breast cancer metastasis, suggesting that RNF8 could be used as a potential therapeutic target for the prevention and treatment of breast cancer.
KeywordsBreast cancer Metastasis EMT RNF8
Breast cancer (BC) is the most invasive form of cancer in women and is the second leading cause of cancer death in industrial nations ; the incidence rate of the disease has been rising in many countries . Advanced breast cancer is associated with significant mortality because it metastasizes to vital organs. Tumor metastasis is a multistage process, among which epithelial-mesenchymal transition (EMT) is believed to be an initial step, during which non-motile, polarized epithelial cells lost their cell-cell junctions and converted into individual, non-polarized, motile and invasive mesenchymal cells [3, 4]. EMT can be induced or regulated by various growth and differentiation factors, including TGF-β, Wnt and Notch, as well as the tyrosine kinase receptor pathway [5, 6]. Activation of these pathways leads to transcriptional repression of a series of target genes that are involved in epithelial maintenance, including CDH1 gene, leading to the functional loss of E-Cadherin, one of the well-known hallmarks of EMT. The disappearance of E-Cadherin from adherent junctions results in the release of β-catenin into cytosol, and subsequent translocation to the nucleus where it can activate LEF/TCF (lymphoid enhancer factor/T cell factor)-mediated transcription, induce the expression of Snail, Slug and Twist , thereby contributing to the EMT program. Therefore, EMT is a cellular reprogrammed process that involves multiple layers of regulation for the gene transcription, including epigenetic regulation such as the post-transcriptional modification mediated by histone methylation and acetylation or deacetylation, as well as protein ubiquitination. It is assumed that many enzymes participated these processes are also important EMT regulators and play essential role in tumor metastasis [8, 9].
RNF8 is an ubiquitin E3 ligase with two conserved domains: the N-terminal FHA (Forkhead-Associated) and the C-terminal RING (Really Interesting New Gene) domain. The FHA can specifically bind phospho-peptides motif (pTXXF) in the target proteins [10, 11], while the RING is responsible for its E3 ligase activity. In response to DNA damage, gammaH2AX is phosphorylated, followed by the binding and subsequent phosphorylation of MDC1. This phosphorylation of MDC1 leads to the phosphorylation-dependent recruitment of RNF8 to the sites of DNA double strand breaks (DSBs), where RNF8 binds to the E2, Ubc13, and catalyses the formation of lysine 63-linked polyubiquitin chains (K63-Ubs) on histones [12–14]. The ubiquitination of histones at DSBs provides a structural chromatin platform for the subsequent recruitment of key DNA damage repair proteins such as 53BP1 and BRCA1, the latter of which is a well-known tumor suppressor, mutation of which strongly predisposes women to breast cancer [15–19]. In addition to the synthesis of K63Ubs, RNF8 can also catalyze the formation of K48-linked ubiquitin chains when coupled with other conjugating E2s such as UBCH8, UBE2E2, UbcH6, and UBE2E3 [20–24]. These RNF8-mediated K48-linked ubiquitin degradations play an important role in the control of protein abundance or turnover of many players, for example, JMJ2A and Ku80, which are involved in the DNA damage response (DDR) [22, 23].
In addition to its critical role in DDR, RNF8 is also implicated in many other biological processes, such as spermatogenesis, telomere end protection, mitosis and apoptosis [25, 26]. High-level amplification of RNF8 in lung cancer and leukaemia cells was revealed by the Cancer Genome Project (Sanger Institute) , but the role of RNF8 in tumorigenesis, especially in breast cancer metastasis remains poorly defined.
Here we report that RNF8 is overexpressed in highly metastatic breast cell lines and its overexpression can induce EMT in breast cancer cells. Furthermore, RNF8 is aberrantly expressed in invasive breast cancer and positively correlates with lymph node metastasis.
RNF8 is overexpressed in malignant breast cancer cell lines
RNF8 induces EMT of breast cancer cell line
Overexpression of RNF8 inactivates GSK-3β and increases the accumulation of β-catenin
RNF8 enhances the cell migration potential of breast cancer cells
RNF8 promotes breast cancer metastasis in vivo
It should be noted that the expression of RNF8 and E-cadherin, as well as the migration potential in MDA-MB-231-Luc-shRNF8 and MDA-MB-231-Luc- Control cells were detected to select the positive clones (Fig. 5d). Before injection, the bioluminescence signals of MDA-MB-231-Luc-shRNF8 and MDA-MB-231-Luc-Control cells were also measured to make sure the cells used for implantation had the same luminescence intensity and activities (Additional file 1: Figure S2).
RNF8 expression is associated with metastasis in human breast tumors
Association of RNF8 expression and clinical pathological features
RNF8-cytoplasmic N (%)
RNF8-Nuclear N (%)
II (A + B)
III (A + B + C)
T1 + T2
T3 + T4
N0 + N1
N2 + N3
Epithelial-mesenchymal transition plays an important role in cancer invasion and metastasis, triggered by a diverse set of stimuli that can lead to stable reprogramming of epithelial cells to mesenchymal states. Here, we identify RNF8 as a novel factor involved in EMT and breast cancer metastasis. We have found that RNF8 is overexpressed in highly metastatic breast cell lines. Overexpression of RNF8 induces EMT in breast cancer cells, promotes breast cell migration and tumor metastasis in mouse xenograft model. Importantly, the expression of RNF8 is up-regulated in breast cancers tissues and is positively correlates with lymph node metastasis, establishing an important role for RNF8 in breast cancer metastasis.
RNF8 is an ubiquitin E3 ligase, localizes primarily to the nucleus during interphase; whereas under genotoxic stress (eg. IR treatment), RNF8 is localized to the sites of DNA damage, where it functions to ubiquitinate many chromatin substrates, including histone H2A, H2AX and H1 that are crucial for chromatin structure remodeling [11–14]. In addition, RNF8 also localizes at cytoplasm, especially upon some special stimuli (eg. stimulated by Human T lymphotropic virus type 1 (HTLV-1) trans-activator/oncoprotein, Tax), plays important roles in cell fate decision or genome stability through RNF8-mediated K-63 ubiquitination [34, 35]. In this study, we have found RNF8 protein expression is significantly higher in primary cancers than that of the matched noncancerous tissues. More importantly, a relatively larger proportion of either cytoplasmic or nuclear expression of RNF8 is higher in lymph node metastases than in the corresponding primary cancers. Overexpression of RNF8 led to the decrease of E-Cadherin, a hallmark of EMT, and RNF8 induced EMT is associated with the inactivation of GSK-3β and the accumulation of β-catenin. β-catenin is a critical player in the EMT signaling through shuttling from cytoplasm to nuclear, where it accumulates and complexes to TCF/LEF-1 to activate the downstream target genes involved in EMT [36–38]. These results reveal a strong association of RNF8 expression with EMT and breast cancer metastasis, raising the question on how RNF8 induces EMT signaling activation, and whether the increased expression in cancer cells is linked to an altered or disturbed RNF8 function, which in turn might contribute to the initiation and development of breast cancer. Further study will be needed to answer these questions.
Some metastasis-associated proteins are also important DNA damage response regulators, and hence contribute to IR/ anti-cancer drug resistance. One example is metastasis-associated protein 2 (MTA2), a subunit of the nucleosome-remodeling and histone deacetylation (NuRD) chromatin-remodeling complex. Depletion of MTA2 led to accumulation of spontaneous DNA damage and increased IR sensitivity . RNF8 is one of the most important DNA damage repair molecules, playing crucial roles in DDR. Loss of RNF8 function can sensitize cells to both IR and DNA damage-inducing agents including anti-cancer drugs [11, 40]. One of the most difficult problems in the treatment of cancer is cancer metastasis and anti-drug resistance. Since RNF8 is aberrantly expressed in many breast cancer patients and facilitates tumor metastasis, and is a key player in the DDR pathway, it is therefore a promising target for anti-cancer therapy. By targeting RNF8, scientists could kill two birds with one stone in the future: not only the metastatic potential of cancer cells could be suppressed or eliminated, but also the efficacy of anti-cancer drugs could be significantly improved due to the increased sensitivity of cancer cells to anti-cancer drugs upon RNF8 depletion.
In summary, RNF8 induces EMT in breast cancer cell line, and knock down of RNF8 reduces tumor metastasis in nude mice xenograft model. The expression of RNF8 is up-regulated in breast cancers tissues, positively correlates with lymph node metastasis and inversely correlates with survival time of the breast cancer patients. These findings highlight the potential of RNF8 as a therapeutic target of breast cancer.
Cell lines and culture
Human embryonic kidney HEK-293 T, Breast epithelial cell lines MCF-10A, MCF-10 F, MCF7, T47D and metastatic breast cancer cell lines MDA-MB-231, MDA-MB-435 and BT549 were purchased from American Type Culture Collection (ATCC) and preserved in our laboratory, MDA-MB-231-luc was a gift from Dr. Yongfeng Shang (Peking University Health Science Center). These cell lines were cultured in medium supplemented with 10 % FBS at 37 °C with 5 % CO2 in a humid atmosphere.
Plasmid construction, siRNA and antibodies
TG006 is a lentiviral empty vector with GFP. For construction of TG006-hRNF8, full-length cDNA encoding RNF8 was amplified by PCR with Phusion DNA polymerase and subcloned into TG006. siRNA oligonucleotides (Invitrogen) were synthesized: siRNF8-1 (5′-GGACAAUUAUGGACAACAA-3′); siRNF8-2 (5′UGCGGAGUAUGAAUAUGAA-3′). Lentiviral vector shRNF8-1 (the same sequence with siRNF8-1: 5′-GGACAAUUAUGGACAACAA-3′, named as 26620), shRNF8-2 (5′-ACATGAAGCCGTTATGAAT-3′) and empty vector CONO77 were purchased from GeneChem. The following antibodies were used: mouse anti-RNF8 (sc-271462; Santa Cruz Biotechnology, Inc.); rabbit anti-E-cadherin (#3195, Cell Signaling Technology); rabbit anti-phospho-β-catenin (Ser33/37/thr47) (#9561, Cell Signaling Technology); rabbit anti-β-catenin (#8480, Cell Signaling Technology); rabbit anti-snail (#3879, Cell Signaling Technology); Monoclonal anti-Vimentin (V6389, sigma); rabbit anti-actin (sc-1616-R; Santa Cruz Biotechnology, Inc.); rabbit anti-phospho-GSK3β (Ser9) (#9336, Cell Signaling Technology); rabbit anti-GSK3β (#9315, Cell Signaling Technology).
Transfection and infection
Lipofectamine RNAi MAX and Lipofectamine 2000 reagent (Invitrogen) were used for transient knockdown by siRNA or transient overexpression, respectively. All experiments were performed according to the manufacturer’s instructions. Lentiviral particles were produced by transfecting HEK293T cells with the TG006 vector or TG006-RNF8, as well as the psPAX2 and pMD2.G packaging vectors. 8 ug/ml polybrene was used for lentivirus infection of MCF7 cells and MDA-MB-231 cells.
Protein extraction and western blotting
Immunoblotting was carried out to analysis the protein expression. Cells were lysed in RIPA buffer containing protease inhibitor cocktail and phospho-stop (Roche, Basel, Switzerland). Protein concentrations were determined using Bradford protein assays. Whole cell lysates were then fractionated using 8–10 % SDS–PAGE and electrotransferred onto polyvinylidene difluoride membranes (Millipore Corp., Bedford, MA, USA). The membranes were then probed with the indicated antibodies, and the immunoreactive bands were developed using an ECL detection system (Millipore, Billerica, MA, USA). All experiments were repeated for three times. For quantification, the grey density of the target bands in the immunoblot was analyzed by Image J software (National Institutes of Health, Bethesda, Maryland, U.S.) and normalized to the grey density of β-actin.
Quantative realtime PCR
Total RNA was extracted using TRIzol reagent (Invitrogen) and cDNA was synthesized using SuperScript II Reverse Transcriptase (Invitrogen). Quantitative PCR was performed using an ABI PRISM 7000 Sequence Detection System (Applied Biosystems). E-cadherin amplification was performed using the following primers: 5′- TGGGCTGGACCGAGAGAGTTT-3′ and 5′-CGACGTTAGCCTCGTTCTCAG-3′. Samples were run in triplicates, and all samples were normalized against GAPDH using the comparative Ct method (△△Ct).
Cells grown on coverslips were fixed in 4 % paraformaldehyde at room temperature for 10 min, washed three times with PBS, blocked with 10 % goat serum at 37 °C for 30 min, incubated at 4 °C with the primary antibodies overnight, washed extensively and probed with the FITC- and Rhodamine red–conjugated goat anti–rabbit or anti–mouse IgG (Jackson ImmunoResearch Laboratories, Inc.) at 37 °C for 30 min. Coverslips were mounted in VECTASHIELD Mounting Medium with DAPI (Vector Laboratories).
Cell migration and wound-healing scratch assay
Cell migration was measured according to the ability of the cells to migrate across a transwell filter (8-μm pores, Costar, Cambridge, MA, USA).1 × 105 MDA-MB-231 cells or 2 × 105 MCF7 cells suspended in serum-free DMEM were added to the upper chamber, and DMEM medium containing 10 % fetal bovine serum was added to the lower chamber. After a 20 h (for the siRNA-transfected cells) or 24 h (for the lentivirus-infected cells) incubation at 37 °C in a 5 % CO2 humidified atmosphere, the non-migrated cells were scraped off of the filter using a cotton swab and the cells that migrated to the lower side of the upper chamber, were fixed with 4 % paraformaldehyde and stained with hematoxylin. The cells per microscopic field (MDA-MB-231 cells, 20×; MCF7 cells, 10×) were taken pictures and counted in 8 randomly chosen fields. Triplicate wells were performed in each assay, and the assay was repeated at least three times.
One day before scratch, MDA-MB-231/RNAi cells were trypsinized and seeded equally into six-well tissue culture plates and grew to reach almost total confluence in 24 h. An artificial homogenous wound was created onto the monolayer with a sterile 10-ul tip. After scratching, the cells were washed with serum-free medium. Images of the cells migrating into the wound were captured at time points of 0 h and 20 h by inverted microscope (10×), all the experiments were repeated for at least 3 times.
In vivo metastasis
MDA-MB-231 cells that stably express firefly luciferase (Xenogen) were infected with lentiviruses carrying control shRNA (CONO77), shRNF8-2, respectively. These cells were injected into the lateral tail vein (1 × 106 cells) of 6–7 week-old female BALB/c mice (n = 6). For bioluminescence, IVIS Spectrum Imaging System (Xenogen, Alameda, CA, USA) was used. Lungs were fixed in formalin and embedded in paraffin blocks for slicing into thin sections. The paraffinized sections were stained with hematoxylin and eosin (H&E) according to standard protocols. The stained sections were photographed using a Leica microscope (Leica, Wetzlar, Germany).
Immunohistochemistry (IHC) with breast tissue arrays
All breast tissue arrays were purchased from www.alenabio.com. BR804a with paired cancer adjacent normal breast tissue (40 cases/80cores, 4 cases missing), BR20837 included information on TNM (tumor, lymph node, metastasis) classification, clinical stage and histological grade (104 cases/208cores, 8 cases missing). The protocols of immunohistochemistry staining were also downloaded from www.alenabio.com. Antigen retrieval was performed by microwave oven method in Tris-EDTA-based solution, pH 9.0 to about 95 °C, 20 min. Blocking solution was used to prevent nonspecific binding of antibodies. The sections were incubated with mouse monoclonal anti-RNF8 antibody (Santa Cruz, sc-271462, 1:40 dilution) overnight at 4 °C. GTVision TM III Detection System/Mo&Rb (GK500705) was used for detection.
The histological diagnosis was made by two pathologists. SPSS version 17.0 was used for statistical analysis. Comparisons between cancers and adjacent normal tissues were performed using two-sample paired wilcoxon signed rank test. Association of RNF8 with clinic pathologic factors using K-independent samples Cruskal-Wallis test. The differences between two independent groups were analysed using Student’s t test. P-value < 0.05 was considered to be a statistically significant difference. Kaplan-Meier survival analysis for the relationship between survival time and RNF8 signature in breast cancer was performed using the online tool (http://kmplot.com/analysis/).
BC, Breast cancer; DAPI, diamidino-phenyl-indole; EMT, epithelial to mesenchymal transition; H&E, Hematoxylin and eosin; IF, immunofluorescence; IHC, Immunohistochemistry; IMC, image motion compensation; IVIS, In Vivo Imaging Systems; qPCR, Quantitative Realtime PCR; RNF8, ring finger protein 8; siRNAs, small interfering RNAs.
We thank Prof. Jose Russo at Fox Chase Cancer Center for his critics and comments on the manuscript. We also thank Prof. Yongfeng Shang and Dr. Luyang Sun for their generosity for providing MDA-MB-231-Luc. This work was supported by the National Natural Science Foundation of China , the Beijing Natural Science Foundation  and the Leading Academic Discipline Project of Beijing Education Bureau [BMU20110254].
JK performed the majority of the experiments, with contribution from LL, YX, and XW; LG and YS examined the IHC; and JK, LL and GS wrote the manuscript. GS directed the work. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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