MYBBP1A suppresses breast cancer tumorigenesis by enhancing the p53 dependent anoikis
© Akaogi et al.; licensee BioMed Central Ltd. 2013
Received: 15 October 2012
Accepted: 30 January 2013
Published: 7 February 2013
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© Akaogi et al.; licensee BioMed Central Ltd. 2013
Received: 15 October 2012
Accepted: 30 January 2013
Published: 7 February 2013
Tumor suppressor p53 is mutated in a wide variety of human cancers and plays a critical role in anoikis, which is essential for preventing tumorigenesis. Recently, we found that a nucleolar protein, Myb-binding protein 1a (MYBBP1A), was involved in p53 activation. However, the function of MYBBP1A in cancer prevention has not been elucidated.
Relationships between MYBBP1A expression levels and breast cancer progression were examined using patient microarray databases and tissue microarrays. Colony formation, xenograft, and anoikis assays were conducted using cells in which MYBBP1A was either knocked down or overexpressed. p53 activation and interactions between p53 and MYBBP1A were assessed by immunoprecipitation and western blot.
MYBBP1A expression was negatively correlated with breast cancer tumorigenesis. In vivo and in vitro experiments using the breast cancer cell lines MCF-7 and ZR-75-1, which expresses wild type p53, showed that tumorigenesis, colony formation, and anoikis resistance were significantly enhanced by MYBBP1A knockdown. We also found that MYBBP1A binds to p53 and enhances p53 target gene transcription under anoikis conditions.
These results suggest that MYBBP1A is required for p53 activation during anoikis; therefore, it is involved in suppressing colony formation and the tumorigenesis of breast cancer cells. Collectively, our results suggest that MYBBP1A plays a role in tumor prevention in the context of p53 activation.
Breast cancer is the most commonly occurring cancer in women worldwide. It has been estimated that more than 1.6 million new cases of breast cancer occurred in 2010 . Cancer cells develop features that are fundamentally different from those of normal cells. One hallmark of cancer cells is their ability to survive and proliferate in the absence of extracellular matrix (ECM)-derived signals .
The tumor suppressor p53 plays a central role in coordinating the responses to stresses induced by a wide array of stimuli. Under normal conditions, cellular p53 protein levels are maintained at basal levels. However, in response to genotoxic stresses, such as exposure to ultraviolet light or γ-irradiation, p53 protein levels increase and trigger either cell cycle arrest or apoptosis . p53 plays a critical role in cancer prevention, because p53 can suppress tumorigenesis by inducing cell cycle arrest and apoptosis through its transcriptional activity. p53 is one of the tumor suppressor genes that is most frequently found to be inactivated in cancer .
p53 also plays a critical role in anoikis. Anoikis, defined as detachment-induced apoptosis , reflects the essential requirement of most normal epithelial cells for ECM-derived survival signals . When these signals are denied, for example, upon detachment and continued culture in suspension or in soft agar, cells will rapidly undergo cell cycle arrest and apoptosis. The capacity of cancer cells for anchorage-independent growth under conditions of detachment and suspension in soft agar correlates well with their tumorigenic potential .
p53 is required for anoikis in many cell types , including epithelial cells [8–12]. Under detached conditions, p53 enhances p21, Bax, and PUMA transcription and induces cell cycle arrest and apoptosis [9, 11]. However, the signaling pathways that regulate p53-dependent anoikis are largely unknown.
Recently, we found that a nucleolar protein, Myb-binding protein 1a (MYBBP1A), was involved in p53 activation. When cells were exposed to cellular stresses, MYBBP1A translocated from the nucleolus to the nucleoplasm. The translocated MYBBP1A promoted p53 acetylation and accumulation by facilitating the interaction between p53 and histone acetyltransferase p300; thus, MYBBP1A could enhance p53 target gene transcription . Previous studies revealed that MYBBP1A was involved in regulating intracellular energy status, inflammation, and myogenesis [14–16]. In addition, Sanhueza et al. recently reported that MYBBP1A regulates the proliferation and migration of head and neck squamous cell carcinoma cells . However, the role of MYBBP1A in breast cancer prevention and the detailed mechanisms underlying these activities have not been determined.
In this study, we show that MYBBP1A expression is associated with breast cancer tumorigenesis through an extensive analysis of the Oncomine database. In vitro and in vivo experiments using the breast cancer cell lines, which expresses wild type p53, revealed that tumorigenesis, colony formation, and anoikis resistance were significantly enhanced by MYBBP1A knockdown. MYBBP1A binds to p53 under detached conditions and enhances p53 target gene transcription, as evidenced by co-immunoprecipitation experiments and RT-qPCR. These results suggest the physiological significance of MYBBP1A in p53 activation and cancer prevention.
MCF-10A, human mammary epithelial cells, were maintained in Dulbecco’s modified Eagle’s medium-F12 (DMEM-F12) (Invitrogen, Carlsbad, CA) supplemented with 0.5 μg/ml hydrocortisone (Sigma-Aldrich, St Louis, MO), 10 μg/ml insulin (Sigma-Aldrich, St Louis, MO), and 20 ng/ml recombinant human EGF (Peprotech, Rocky Hill, NJ). MCF-7 human breast cancer cells were maintained in DMEM (Sigma-Aldrich, St Louis, MO). ZR-75-1 human breast cancer cells were maintained in RPMI 1640 (Nacalai Tesque, Kyoto, Japan). All media were supplemented with 10% fetal bovine serum (FBS) and 1% penicillin–streptomycin solution (Nacalai Tesque, Kyoto, Japan). Transfection was performed using Lipofectamine LTX (Invitrogen, Carlsbad, CA).
cDNAs encoding full-length p53 and MYBBP1A were amplified using PCR and subcloned into pcDNA3 plasmids (Invitrogen, Carlsbad, CA) and pQCXIP (Clontech, Mountain View, CA) containing sequences encoding FLAG sequences. Anti-β-Actin (Sigma-Aldrich, St Louis, MO) and anti-human-p53 (Santa Cruz, Santa Cruz, CA) monoclonal antibodies and rabbit anti-p53-K382Ac(Cell Signaling Technology, Danvers, MA) polyclonal antibody were used according to the manufacturers’ instructions. Rabbit anti-human MYBBP1A antibody was raised against a synthetic peptide corresponding to amino acids 1265–1328 of human MYBBP1A.
The Oncomine database and gene microarray analysis tool, a repository for published complementary DNA microarray data [18, 19], were explored (July 2012) for MYBBP1A mRNA expression in non-neoplastic and breast cancer tissues. Statistical analysis of the differences in MYBBP1A expression between these tissues used Oncomine algorithms, which provided multiple comparisons among different studies [20–22]. Data sets obtained from TCGA Breast, Finak Breast, and Richardson Breast 2 included various stage, and all cancer samples were invasive.
Human breast cancer tissue microarrays were purchased from SuperBioChips Laboratories (Seoul, Korea) included various stages, and all cancer samples were invasive. Formalin-fixed tissues were dewaxed in xylene and rehydrated in alcohol. For antigen retrieval, the sections were subsequently heated in EDTA buffer (1 mM, pH 8.0) in a microwave oven for 5 min. Endogenous peroxidase activity was suppressed using a solution of 3% hydrogen peroxide in methanol for 6 min. The samples were stained with avidin–biotin–peroxidase complexes using a Histofine SAB-PO Immunohistochemical Staining Kit (Nichirei, Tokyo, Japan) according to the manufacturer’s instructions. Rabbit anti-MYBBP1A antibody was used at a dilution of 1:50.
Methods for stable RNA interference followed those described by Kajiro et al.. To generate a shRNA retroviral supernatant, GP2-293 cells (Clontech, Mountain View, CA) were co-transfected with a p10A1 vector encoding an envelope protein and pRETRO-SUPER (Oligo Engine, Seattle, WA) vector containing either the MYBBP1A or luciferase (control) target sequence. MCF-7 cells were incubated with the retroviral supernatant in the presence of 8 μg/ml polybrene. Infected cells were selected using 1 μg/ml of puromycin. The target sequences were 5′- TTCAGGGCATATTTCATCTCGGACC-3′ for MYBB1A #1, 5′-TTCAGGGCATATTTCATCTCGGACC-3′ for MYBBP1A #2, and 5′-GAAGCTGCGCGGTGGTGTTGT-3′ for luciferase. For siRNA transfection, cells at 30%–50% confluency were transfected using Lipofectamine RNAi MAX (Invitrogen, Carlsbad, CA) according to the manufacturer’s protocol. All siRNAs were purchased from Invitrogen. The siRNA duplexes were MYBBP1A, 5′-UCUUUCAGUCAGGUCGGCUGGUGAA-3′ and p53, 5′-CCAGUGGUAAUCUACUGGGACGGAA-3′. Stealth RNAi negative control was used as a negative control.
All animal experiments were performed in accordance with institutional guidelines. The tumor xenograft models have been described previously . Each mouse was subcutaneously injected with 100 μl of cell suspension (5 × 106) in both flanks. At the time points indicated in the figures, the tumors were excised, weighed, and fixed or stored in liquid nitrogen.
For soft agar assays, 22,000 cells were mixed with 1 ml of 0.35% top agar and plated onto a 35-mm six-well plate containing bottom plugs (0.6% agar, 10% FBS, 1× DMEM). After the top agar had solidified (about 2 h at 37°C), 200 μl of DMEM was added into each well to prevent dehydration. This covering medium was changed every 2 or 3 days during culture. After allowing growth for 2 weeks at 37°C, colonies with a diameter of >100 μm were counted.
MCF-10A cells were plated at a density of 3 × 105 cells per well in six-well plates in an ultra-low attachment culture dish (Corning, Tewksbury, MA) and maintained in DMEM-F12 (Invitrogen, Carlsbad, CA) supplemented with 10% FBS, 0.5 μg/ml hydrocortisone (Sigma-Aldrich, St Louis, MO), 10 μg/ml insulin (Sigma-Aldrich, St Louis, MO), and 20 ng/ml recombinant human EGF (Peprotech, Rocky Hill, NJ). MCF-7 cells were plated at a density of 3 × 105 cells per well in DMEM with 5% FBS. ZR-75-1 cells were plated at a density of 3 × 105 cells per well in RPMI 1640 with 10% FBS. All media were supplemented with 1% penicillin–streptomycin solution (Nacalai Tesque, Kyoto, Japan). Before analysis, the samples were treated with trypsin to disperse the cells. Cell viability after detachment was determined by trypan blue dye exclusion. Viable cells were determined by MTT assay using an MTT cell counting kit (Nacalai Tesque, Kyoto, Japan). Apoptotic cell proportion was determined by fluorescence-activated cell sorting (FACS) analysis using PI and fluorescein isothiocyanate (FITC)-conjugated Annexin V (MBL, Tokyo, Japan) staining.
GAPDH fw primer: 5′- GTATGACTCCACTCACGGCAAA -3′
GAPDH rv primer: 5′-GGTCTCGCTCCTGGAAGATG-3′
p21 fw primer: 5′-GGAGACTCTCAGGGTCGAAA-3′
p21 rv primer: 5′-TTAGGGCTTCCTCTTGGAGA-3′
Bax fw primer: 5′-AGCAAACTGGTGCTCAAGG -3′
Bax rw primer: 5′-CTTGGATCCAGCCCAACAG -3′
PUMA fw primer: 5′-GGGCCCAGACTGTGAATCCT-3′
PUMA rv primer: 5′-ACGTGCTCTCTCTAAACCTATGCA-3′
Cells were lysed in TNE buffer [10 mM Tris–HCl (pH 7.8), 1% Nonidet P-40 (NP-40), 0.15 M NaCl, 1 mM ethylenediaminetetraacetic acid (EDTA), 1 M phenylmethylsulfonyl fluoride (PMSF), 1 g/ml aprotinin]. The extracted proteins were immunoprecipitated with antibody-coated protein G Sepharose beads (GE Healthcare Japan, Tokyo, Japan). Bound proteins were separated by SDS–PAGE, transferred to polyvinylidene difluoride membranes (Millipore, Temecula, CA), and detected with appropriate primary antibodies and horseradish peroxidase-conjugated secondary antibodies. Specific proteins were visualized using an enhanced chemiluminescence (ECL) western blot detection system (GE Healthcare Japan, Tokyo, Japan).
ChIP assay was performed according to the published procedures . The primers for real-time PCR were as follows: forward, TAATCCCAGCGCTTTGGAAG; reverse, TTGCTAGATCCAGGTCTCTGCA for the upstream region of the Bax gene.
Cells were fixed in 3.7% formaldehyde in PBS for 10 min. After rinsing twice with PBS, the cells were permeabilized in 0.1% Triton X-100 in PBS and later blocked with TBS-T buffer containing 0.5% bovine serum albumin and 10% goat serum for 1 h at room temperature. Subsequently, the cells were incubated with anti-MYBBP1A and anti-p53 antibodies for 1 h, stained with Alexa Fluor-conjugated secondary antibodies (Invitrogen, Carlsbad, CA) for 1 h, and mounted with Vectashield (Vector Laboratories, Burlingame, CA). Immunofluorescent images were obtained by Biozero immunofluorescence microscopy (Keyence, Osaka, Japan).
Next, we performed xenograft experiments to test the effect of MYBBP1A expression on tumorigenicity in vivo. MYBBP1A knockdown cells formed tumors that were significantly larger than those of control cells. In contrast, MYBBP1A overexpressing cells formed smaller tumors than control cells (Figure 2D, 2E, and 2F). These results indicated that MYBBP1A could suppress breast cancer tumor growth.
Consistent with these results, p53 recruitment to the Bax promoter was significantly enhanced under detached conditions, while p53 recruitment was abrogated by MYBBP1A knockdown (Figure 7B).
A previous report showed that MYBBP1A activates p53 by facilitating its direct interaction with p53 in response to stress when MYBBP1A translocated from the nucleolus to the nucleoplasm . Therefore, we examined the localization of MYBBP1A and the interaction between MYBBP1A and p53 under detached conditions. Immunostaining revealed that MYBBP1A translocated from the nucleolus to the nucleoplasm under detached conditions (Figure 7C). Moreover, co-immunoprecipitation showed that endogenous MYBBP1A was bound to p53 in MCF-7 cells under detached conditions (Figure 7D). These results indicated that, under detached conditions, MYBBP1A translocates from the nucleolus to the nucleoplasm, and then binds to p53. Thus, MPBBP1A enhances p53 target gene transcription.
In this study, we revealed the physiological significance of MYBBP1A in p53 activation for prevention of cancer. MYBBP1A was originally identified as a protein that interacted with the negative regulatory domain of c-Myb . However, in studies done by a number of different groups, MYBBP1A was found to interact with and regulate several transcription factors. MYBBP1A binds to Prep1 or PGC-1α and inhibits their activity. Prep1 is involved in development and organogenesis, and PGC-1α is a key regulator of metabolic processes such as mitochondrial biogenesis, respiration, and gluco neogenesis in the liver [32, 33]. Correspondingly, MYBBBP1A also interacts with NF–κB and CRY1 and regulates their transcriptional activity [15, 34].
We previously reported that MYBBP1A interacts with p53 and activates its transcriptional capacity. MYBBP1A localizes predominantly in the nucleolus; however, it translocates from the nucleolus to the nucleoplasm in response to cellular stress and activates p53 [13, 14, 35]. Similar to other kinds of stress, detached conditions induce p53 acetylation and target gene transcription in an MYBBP1A-dependent manner (Figures 6 and 7A). MYBBP1A plays an important role in p53 activation in response to detached condition to induce anoikis.
The regulation of MYBBP1A localization under detached conditions is still to be elucidated. When cells are exposed to UV light, MYBBP1A translocation is accompanied by nucleolar segregation . Because the nucleolus appears to be intact after 2 h under detached conditions, there may be another signal that releases MYBBP1A from the nucleolus.
Numerous reports have shown that other nucleolar proteins can activate p53 similar to that by MYBBP1A. RPL11 directly bind to HDM2 and inhibit HDM2-mediated p53 ubiquitination [36–41]. Similarly, the nucleolar proteins NPM, NCL, NS, and ARF can also directly bind to HDM2 and inhibit p53 ubiquitination [42–46]. Unlike with these nucleolar factors, MYBBP1A promotes p53 activation by directly binding to p53 without affecting HDM2 function. With regard to the pronounced effect of MYBBP1A knockdown on p53 accumulation and acetylation under detached conditions (Figure 7A), MYBBP1A may have a unique and essential function in response to detached conditions.
In addition, Sanhueza et al. recently reported that MYBBP1A regulates the proliferation and migration of head and neck squamous cell carcinoma cells . However, the detailed mechanisms underlying these activities are unknown. In this study, we revealed a function for the nucleolar protein MYBBP1A in breast cancer. Therefore, our results provide a novel insight into the function of the nucleolar protein MYBBP1A in the biology of cancer cells.
Myb-binding protein 1a
Bcl-2–associated X protein
p53 upregulated modulator of apoptosis
Fluorescence-activated cell sorting
Reverse transcription quantitative polymerase chain reaction
Dulbecco’s modified Eagle’s medium
Fetal bovine serum
Polymerase chain reaction
Short hairpin RNA
Small interfering RNA
Reverse transcription polymerase chain reaction
Sodium dodecyl sulfate-polyaclylamidegel electrophoresis
Phosphate buffered saline
Tris-buffered saline and tween 20
Quantitativepolymerase chain reaction
Peroxisome proliferator-activated receptor gamma coactivator 1-alpha
nuclear factor kappa-light-chain-enhancer of activated B cells
This work was supported by Grant-in-Aid for Japan Society for the Promotion of Science (JSPS).
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