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Ras induces experimental lung metastasis through up-regulation of RbAp46 to suppress RECK promoter activity
© Yeh et al.; licensee BioMed Central. 2015
Received: 13 August 2014
Accepted: 2 March 2015
Published: 25 March 2015
Mutant Ras plays multiple functions in tumorigenesis including tumor formation and metastasis. Reversion-inducing cysteine-rich protein with Kazal motifs (RECK), a metastasis inhibitor gene, suppresses matrix metalloproteinase (MMP) activity in the metastatic cascade. Clarifying the relationship between Ras and RECK and understanding the underlying molecular mechanism may lead to the development of better treatment for Ras-related tumors.
Suppression subtractive hybridization PCR (SSH PCR) was conducted to identify Ha-rasval12 up-regulated genes in bladder cancer cells. Stable cell lines of human breast cancer (MCF-7-ras) and mouse NIH3T3 fibroblasts (7–4) harboring the inducible Ha-rasval12 oncogene, which could be induced by isopropylthio-β-D-galactoside (IPTG), were used to clarify the relationship between Ras and the up-regulated genes. Chromatin immunoprecipitation (ChIP) assay, DNA affinity precipitation assay (DAPA) and RECK reporter gene assay were utilized to confirm the complex formation and binding with promoters.
Retinoblastoma binding protein-7 (RbAp46) was identified and confirmed as a Ha-rasval12 up-regulated gene. RbAp46 could bind with histone deacetylase (HDAC1) and Sp1, followed by binding to RECK promoter at the Sp1 site resulting in repression of RECK expression. High expression of Ras protein accompanied with high RbAp46 and low RECK expression were detected in 75% (3/4) of the clinical bladder cancer tumor tissues compared to the adjacent normal parts. Ras induced RbAp46 expression increases invasion of the bladder cancer T24 cells and MMP-9 activity was increased, which was confirmed by specific lentiviral shRNAs inhibitors against Ras and RbAp46. Similarly, knockdown of RbAp46 expression in the stable NIH3T3 cells “7-4” by shRNA decreased Ras-related lung metastasis using a xenograft nude mice model.
We confirmed that RbAp46 is a Ha-rasval12 up-regulated gene and binds with HDAC1 and Sp1. Furthermore, RbAp46 binds to the RECK promoter at the Sp1 site via recruitment by Sp1. RECK is subsequently activated, leading to increased MMP9 activity, which may lead to increased metastasis in vivo. Our findings of Ras upregulation of RbAp46 may lead to revealing a novel mechanism of Ras-related tumor cell metastasis.
H-ras, K-ras, and N-ras proteins are members of a large superfamily of low-molecular-weight GTP-binding proteins which control signaling pathways that are key regulators of numerous aspects of normal cell growth and malignant transformation . About 30% of human tumors express Ras point mutations . At least three major effectors of Ras are responsible for downstream signal transduction, including the Raf/mitogen-activated protein kinase (MAPK) pathway, the phosphatidylinositol 3-kinases (PI3Ks) pathway and the Ral guanine nucleotide exchange factors (RalGEFs) pathway . Contributions of these pathways are mainly observed in tumor initiation, such as cell survival, proliferation and transformation. However, little is known about their involvement in Ras-induced cell invasion and metastasis. Moreover, the roles of mediators in Ras induction of invasion and metastasis are not fully understood . Therefore, the precise effects of Ras-related factors and their functions in tumorigenesis warrant further investigation.
The reversion-inducing cysteine-rich protein with Kazal motifs (RECK) gene is a membrane-anchored glycoprotein that negatively regulates matrix metalloproteinases (MMPs) and inhibits tumor metastasis and angiogenesis [5,6]. The RECK gene was first isolated as a transformation suppressor gene to induce flat reversion in a v-Ki-ras-transformed NIH3T3 cell line . Further studies showed that RECK is down-regulated in many human cancers including pancreatic, breast, non-small cell lung cancer and osteosarcoma  and in cells that ectopically express active oncogenes . It is known that histone deacetylation and promoter methylation are the mechanisms by which cancer cells act to suppress RECK expression [9,10]. However, the underlying mechanism of decreased RECK expression in Ras-related tumorigenesis is still unclear. Chang HC et al. showed that silencing RECK by Ras oncogene is mediated by DNA methyltransferase 3b-induced promoter methylation . Recently, Loayza-Puch F et al. reported that hypoxia and Ras-signaling pathways cooperatively down-regulate RECK through miR-372/373 and miR-21 . Interestingly, RECK promoter activity suppressed by Ras through Sp1 protein binding at Sp1 binding motif has been reported . Chang CH et al. further demonstrated that induction of the Ha-rasval12 oncogene increases the binding between the HDACs-Sp1 complex and the Sp1 site on the RECK promoter to suppress RECK expression . Therefore, a better understanding of the underlying mechanism that enables Ras expression to induce RECK suppression may lead to the development of better treatments for Ras-related tumors.
The retinoblastoma binding protein 46 (RbAp46) was initially identified as a Rb binding protein, as shown by Rb affinity column studies . RbAp46 is expressed in the nucleus as a core component of histone deacetylase (HDAC) complexes: mSIN3, which is involved in global transcriptional repression, and NuRD, a multi-subunit complex exhibiting chromosome remodeling activity . RbAp46 is also known as histone acetyltransferase (HAT) type B subunit 2, formed when RbAp46 binds selectively to histones H2A and H4; it greatly stimulates HAT activity by recruiting HDAC enzymes to the target promoter . These studies indicate that RbAp46 plays important roles in modification and remodeling of chromatin during cell growth and differentiation. In addition, studies also revealed that RbAp46 acts as a tumor suppressor which inhibits the malignant phenotype of adenovirus-transformed human embryonic kidney (HEK) 293 cells in tumor formation and that RbAp46 arrests the cell cycle at the G2/M phase . Furthermore, overexpression of RbAp46 suppresses tumorigenicity of neoplastigenic breast epithelial cells . Constitutive expression of RbAp46 induces the epithelial-mesenchymal transition (EMT) in mammary epithelial cells . Moreover, RbAp46 overexpression correlates with migration of lung cancer cells and serves as a prognostic marker for distant metastasis of non-small cell lung cancer . All together, these studies indicate that RbAp46 plays an important role in the tumorigenesis of various cancers. We have identified RbAp46 as a Ras up-regulated gene using the IPTG inducible system in a bladder cancer cell line and a NIH3T3 fibroblast variant. It is known that RbAp46 can form a complex with HDAC to regulate gene suppression. Ras expression induces HDAC1 to bind with Sp1 and then suppresses RECK promoter activation via the Sp1 site. Therefore we were interested in investigating the roles of RbAp46 in Ras-induced RECK suppression and Ras-related tumorigenesis.
In this study, we demonstrated that Ras up-regulates RbAp46 expression and then down-regulates the metastasis suppressor RECK through association of RbAp46 with HDAC1 and Sp1. The RbAp46 could bind at the Sp1 site via association with Sp1 protein and subsequently repress RECK promoter activation. Importantly, we also found RbAp46 increases Ras-induced MMP-9 activity and metastasis. Our findings also showed clinical evidence of a relationship among Ras, RbAp46 and RECK in bladder cancer.
T24 (human bladder cancer cells), MCF-7 (human breast cancer cells) and the 7–4 cells (mouse NIH3T3 fibroblast derive) were maintained in Dulbecco’s modified Eagle’s medium (DMEM; Invitrogen Life Technologies, GIBCO, Gaithersburg, MD, USA) supplemented with 10% fetal bovine serum (GIBCO) at 37°C in a 5% CO2 incubator. T24 and MCF-7 cells were obtained from American Type Culture Collection (ATCC; Rockville, MD). The MCF-7-ras cells derived from MCF-7 contain an inducible Ha-ras val12 oncogene . The 7–4 cells derived from mouse fibroblast NIH 3T3 cells contain the same inducible Ha-ras val12 oncogene as that in MCF-7-ras cells .
The mouse RECK promoter-luciferase plasmid pGL3-RECK and Sp1 mutant plasmid, originally isolated by Dr. Noda M. (Kyoto University, Japan) , were kindly provided by Dr. Hung WC . (National Sun Yat-Sen University, Taiwan). The full-length human RbAp46 gene (1278 base pairs) was amplified by RT-PCR. The primers used were RbAp46 forward 5′-ATGGCGAGTAAAGAGATGTT-3′ and RbAp46 reverse 5′-TTAAGATCCTTGTCCCTCCA-3′. The NotI site was used to clone the RbAp46 cDNA fragment into the pcDNA3.1 vector (Invitrogen, Carlsbad, CA, USA) to create the plasmid pcDNA-RbAp46. Genomic DNA extracted from transitional cell papilloma RT4 cells and HPV E7 immortalized uroepithelial cells were used to amplify RbAp46 promoter (Additional file 1).
Cell lysis and Western blot analysis
The cell lysates were prepared as previously described . Briefly, cells were harvested in lysis buffer (50 mM Tris–HCl, pH 7.4, 100 mM NaCl, 2 mM EDTA, 1% NP-40, 10 mM sodium orthovanadate, 0.1% SDS, 100 mM phenylmethylsulfonyl fluoride, 2 mg/ml aprotinin, 2 mg/ml pepstatin A, and 10 mg/ml leupeptin). Protein concentration was determined using the Bradford assay (Bio-Rad, Richmond, CA, USA). Equal amounts of cellular protein were loaded onto 10-12% sodium dodecyl sulfate (SDS) polyacrylamide gels (SDS-PAGE). Proteins were then transferred onto nitrocellulose membranes and the blots were probed with the following antibodies: anti-Ras antibody (Oncogene Science, La Jolla, CA, USA), anti-RbAp46 antibody (Calbiochem, Gibbstown, NJ, USA), anti-RECK antibody (MBL, Nagoya, Japan), anti-Sp1 and HDAC antibodies (Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA) and anti-β-actin antibody (Sigma, St. Louis, MO, USA). The bands were detected using enhanced chemiluminescence kit (ECL) according to the manufacturer’s instructions (PerkinElmer, Boston, MA, USA).
RNA extraction and RT-PCR
Total RNA of the cells was extracted using Trizol reagent (Invitrogen, Carlsbad, CA, USA). First-strand cDNA was synthesized from 0.2 μg to 1 μg of total RNA with the oligo-dT primer and the Moloney murine leukemia virus (MMLV) reverse transcriptase (Promega, Madison, WI, USA). The conditions for PCR were conducted with the RbAp46 or RECK primers for 30 cycles of denaturation (94°C/1 min), annealing (60°C/1 min) and extension (72°C/1 min), followed by 1 cycle of final extension (72°C/10 min). The conditions for β-actin were 25 cycles for denaturation (94°C/30 sec), annealing (55°C/30 sec) and extension (72°C/1 min), followed by 1 cycle of final extension (72°C/10 min). The primers used for RT-PCR were as follows: RbAp46 forward 5′-ATGGCGAGTAAAGAGATGTT-3′; RbAp46 reverse 5′- TGGAGGGACAAGGA-3′; RECK forward 5′-CCTCAGTGAGCACAGTTCAGA-3′; RECK reverse 5′-GCAGCACACACACTGCTGTA-3′; β-actin forward 5′-CCTGTGGCAT CCACGAAACT-3′; β-actin reverse 5′-GAAGCATTTGCGGTGGACGAT- 3′. After the reaction, PCR products were separated on a 2% agarose gel and detected under UV light.
Real-time PCR reaction was performed in a total volume of 20 μl containing 2 μl cDNA, 10 μM each of forward and reverse primers, and 10 μl SYBR® Green PCR Master Mix (Applied Biosystems, Foster City, CA, USA). The cDNA in this reaction solution was denatured for 10 min at 95°C followed by 40 cycles of PCR, which included denaturing at 95°C for 15 sec and annealing/elongation at 60°C for 60 sec in the wells. Real-time PCR was performed on the StepOne real-time PCR System (Applied Biosystems, Foster City, CA, USA). The primers used for real-time PCR were as follows: RbAp46 forward 5′-GTAGGCGAGTAAAGAGATGT-3′; RbAp46 reverse 5′-TTAAGATCCTTGTCCCTCCA-3′; PPIA forward 5′-GTTTGCAGACAAGGTCCCA-3′; PPIA reverse 5′-ACCCGTATGCTTTAGGATG-3′.
Lentiviral shRNA infection
The Ha-rasval12 (TRCN0000040092) and RbAp46 (TRCN0000038885)-specific shRNA lentiviruses were designed and produced by the National RNAi Core Facility at the Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan. The cells (3 × 105) were seeded in a 10 cm cell culture dish and incubated in DMEM medium with 10% FBS overnight in a 5% CO2 incubator. Before viral infection, the cells were replaced with 5 ml culture medium containing 8 μg/ml polybrene (Sigma, St. Louis, MO, USA), and then the virus was added to cells. After 24 hr, the medium was removed from cells, and 5 ml fresh normal medium with 2 μg/ml puromycin (Sigma, St. Louis, MO, USA) was added to select cells for 48 hr.
Promoter activity assay
The cells were plated onto 24-well plates (5 × 104 cells/well) and grown overnight. The mouse RECK-luciferase plasmid pGL3-RECK (0.2 μg) and the internal control plasmid pRL-TK (0.025 μg) were co-transfected into the cells using Lipofectamine 2000 reagent (Invitrogen, Life Technologies, Inc., Grand Island, NY, USA). Luciferase activity was determined using the Dual-light Luciferase Assay System (Promega, Madison, WI, USA) according to the procedure suggested by the manufacturer and was normalized for Renilla luciferase activity. Ha-ras- and RbAp46-specific siRNA duplexes (Ha-ras 5′-TGGCTGCACGCACTGTGGAAT-3′; RbAp46 5′-CAAUCAGCAGA AGAUGCAU-3′), designed to target human Ha-ras and RbAp46 were synthesized from Qiagen (Carlsbad, CA, USA). Specific siRNAs were transfected into the cells using Lipofectamine 2000 reagent. Luciferase activity was determined 48 hr after transfection.
After various treatments, the cells were harvested in lysis buffer and cellular protein extracts (200 μg) were incubated with anti-RbAp46, anti-HDAC1 or anti-Sp1 antibodies at 4°C for 16 hr. Immuno-complexes were collected by adding 20 μl of protein A agarose beads (Amersham, Piscataway, NJ, USA). Samples were electrophoresed on 10% SDS polyacrylamide gels and transferred to poly- vinylidene fluoride (PVDF) membranes (Millipore, Billerica, MA, USA). Membranes were then reacted individually with anti-HDAC1 monoclonal antibody, anti-RbAp46 monoclonal antibody and anti-Sp1 polyclonal antibody.
DNA affinity precipitation assay (DAPA)
DAPA was performed using streptavidin-coated beads to bind a biotinylated DNA probe, which was used to interact with nuclear extract proteins. The sequence of the DNA probe was 5′- GCGCCGGGGGCGGGGCCTGGTGCC-3′corresponding to the Sp1 site, originally designated as Sp1(B) in the mouse RECK promoter . Nuclear extract proteins (200 μg) were incubated with 6 μg of biotinylated DNA probe and 45 μl of 4% streptavidin-coated beads at room temperature for 1 hr with constant shaking. After centrifugation, the beads were collected and washed three times with cold phosphate-buffered saline. Proteins bound to the beads were eluted with SDS-PAGE sample buffer and the binding proteins were resolved by 10% SDS-PAGE. Immunoblotting was performed as described above to examine the proteins bound to the DNA probe.
Chromatin immunoprecipitation (ChIP) assay
The cells (2×106 cells/10 cm plate) were treated with IPTG (5 mM, Invitrogen, Boston, MA, USA) for 24 hr, and ChIP assay was conducted as previously described . Briefly, cells were crosslinked at 37°C for 5 min using 1% formaldehyde. After sonication, the resulting soluble chromatin was diluted 1:10 with ChIP dilution buffer and immunoprecipitated by anti-Sp1 antibody (Millipore, Billerica, MA, USA), anti-RbAp46 antibody (Abcam, Cambridge, MA, USA) or control IgG. The chromatin-antibody complexes were incubated with salmon sperm DNA/Protein A Agarose-50% (Millipore Corp., Billerica, MA, USA) overnight at 4°C with rotation. The DNA was eluted from the beads using ChIP elution buffer and purified by spin column. The primers used for detection of RECK promoter were as follows: forward: 5′-CAGCTGGCCCATAACAAAGA- 3′ and reverse: 5′-CGGCCAGCA GAAGTA GCA- 3′.
TranswellTM invasion assay
Cell invasion assay was performed in a 24-well Transwell™ (Costar, Cambridge, MA, USA). The upper chamber surface of the filter was coated with Matrigel (R&D systems, Minneapolis, MN, USA) before the experiment. The cells were prepared (3 × 105/100 μl) with serum-free DMEM and loaded into the upper chamber. DMEM medium containing 10% FBS was added to the bottom chamber as the chemoattractant. After 18 hr incubation, wet cotton was used to remove the non-invaded cells from the wells. The cells were fixed with 1% formaldehyde for 15 min at room temperature, stained with 0.1% crystal violet for 15 min and quantified by counting the total number of cells in four independent areas under the light-field microscope.
MMP-9 activity assay
The T24 cells with different treatments (5 × 105/well) were seeded onto 12-well plates and filled with 500 μl medium for 48 hr. The supernatants were collected after centrifugation at 2500 rpm (4°C) for 10 min. MMP-9 activity was assessed by SensoLyte® 520 MMP-9 assay kit (ANASPEC, San Jose, CA, USA) according to the manufacturer’s instructions. The MMP-9 activity was monitored at excitation/emission = 490 nm/520 nm by fluorescence spectrometer (Thermo Scientific, Waltham, MA, USA).
Experimental metastasis assay
Eight-week-old male BALB/c nude mice were obtained from the Laboratory Animal Center, National Cheng Kung University, Tainan, Taiwan, where they were maintained in a pathogen-free environment throughout the study. All animal experiment protocols were approved by the Institutional Animal Care and Use Committee (IACUC), National Cheng Kung University. The 7–4 cells, 7–4 cells treated with IPTG, or 7–4 cells carrying RbAp46 shRNA and treated with IPTG were injected through the tail vein into the nude mice (3 × 105 cells/mice). There were three groups, each containing five mice. Two groups were feed with IPTG (12.5 mM) in their drinking water during the experimental period. After three weeks, the mice were sacrificed to evaluate the lung nodules, weight and protein expression.
Cancer patient specimens
Four pairs of human primary bladder cancer and matched adjacent normal tissues were collected from Chiayi Christian Hospital, Chiayi City, Taiwan. All procedures were approved by the Institutional Review Board of Chiayi Christian Hospital (The approved No. IRB099058), with written informed consent from all donors. In addition, the specimens were coded and deleted of all of personal information.
All data were expressed as means ± S.D. Student’s t test was used to analyze the significance. Spearman correlation was performed using GraphPad Prism 6 to show correlations among Ras, RbAp46 and RECK.
Ras up-regulates RbAp46 expression in human cancer and mouse NIH 3T3 derivative cells
RbAp46 participates in the inhibitory action of Ras on RECK expression in T24 and 7–4 cells
To determine how Ras and RbAp46 regulate RECK expression, mouse RECK promoter reporter plasmid pGL3-RECK was transfected into 7–4 cells. The results showed that the decrease of RECK promoter activity inversely correlates with the gradual increase of Ras protein in a time-dependent manner (Figure 2D, lower panel vs. upper bar diagram). We further showed that the inhibition of RECK promoter activity was increased in a dose-dependent fashion in 7–4 cells by transfection of RbAp46 plasmid DNA (pcDNARbAp46) (Figure 2E). To further clarify the inhibitory effect of RbAp46 on RECK activity, RbAp46 siRNA was used. Consistently, RbAp46 siRNA significantly counteracted suppression of RECK promoter activity induced by endogenous RbAp46 (pGLC3-RECK only), Ras (pGLC3-RECK + Ras), and exogenous RbAp46 (pGLC3-RECK + RbAp46) (Figure 2F). The aforementioned findings demonstrate that Ras is an upstream regulator of RbAp46 which negatively regulates RECK expression at the transcriptional level.
Ras overexpression increases association of RbAp46 with HDAC1 on SP1 site of RECK promoter in 7–4 cells
Binding of RbAp46 complex to the RECK promoter was further confirmed by DNA affinity precipitation assay (DAPA) using a DNA probe of RECK promoter containing Sp1 binding site. The amount of RbAp46 and HDAC1 on the complex binding to the Sp1 site was increased when the RbAp46 expression level was increased either by increased Ras expression (Figure 3C, lane 2) or by increased RbAp46 expression (Figure 3C, lane 4). This phenomenon was abolished when RbAp46 siRNA was introduced into the cells in the presence of IPTG (Figure 3C, lane 3). Control cells without IPTG treatment showed basal levels of RbAp46, HDAC1 and Sp1 binding (Figure 3C, lane 1). To confirm that the RbAp46 complex indeed binds to the RECK promoter at the Sp1 binding site in vivo, a ChIP assay was conducted. The chromatin of 7–4 cells was extracted after IPTG treatment for 24 hr. The lysate was immunoprecipitated by Sp1, RbAp46, and HDAC1-specific antibodies. After elution, the precipitated DNA samples were screened by PCR for Sp1 binding sequence in RECK promoter region. Our data showed that the Sp1 sequence in the RECK promoter was amplified in RbAp46, HDAC1, and Sp1 precipitated DNA samples under Ras-overexpression conditions (Figure 3D). Taken together, the above results indicate that RbAp46 forms a complex with HDAC1 and Sp1, which binds to the Sp1 binding site in the RECK promoter to suppress RECK expression in the cell.
Suppression of RbAp46 decreases cell invasion and negatively regulates MMP-9 activity
Suppression of RbAp46 decreases metastasis of 7–4 cells in the xenograft BALB/c nude mice model
The above in vitro data showed that suppression of RbAp46 decreases cell invasion and inhibits MMP-9 activity. The role of RbAp46 in vivo in Ras-induced metastasis was then investigated. The 7–4 cells were infected with RbAp46 lentiviral shRNA and stable clones were selected. RbAp46 shRNA lentiviral clone #2 showed the best suppression of RbAp46 (Figure 4D). The cells with or without IPTG and/or RbAp46 shRNA were injected intravenously (i.v.) into male BALB/c nude mice. The mice were divided into three groups (n = 5). Two groups (7–4 and 7-4-shRbAp46 #2) were fed with IPTG water for three weeks. Tumor nodules were evaluated on day 21 after injection. When Ras expression was induced by IPTG, suppression of RbAp46 expression by shRNA showed fewer nodules in the lung compared with the number of nodules found in the other groups (Figure 4E). Similarly, inhibition of RbAp46 reduced lung weight compared with that in the other groups (Figure 4F). The lungs of five mice from each group were combined and protein extracts prepared to determine the expression levels of Ras and RbAp46 by Western blotting (Figure 4G). The results showed that the expression level of Ras was induced by IPTG (Figure 4G, lane 2) and RbAp46 was inhibited by RbAp46 shRNA (Figure 4G, lane 3) in mice compared with those in the control group (Figure 4G, lane 1). In summary, our data demonstrate that inhibition of RbAp46 by shRNA in 7–4 cells decreased the number of metastastic nodules induced by Ras in the nude mice model. Taken together, we reveal that Ras-related metastasis occurs through up-regulation of RbAp46 in the xenograft mouse model.
Interrelationships among Ras, RbAp46 and RECK in clinical bladder cancer tissues
Using a TCC bladder cancer cell line harboring an inducible Ha-rasval12 gene , we identified and confirmed that RbAp46 is a Ras-induced gene by SSH PCR and Northern blotting, respectively (Additional file 2: Figure S1A and Table S1). This relationship between Ras and RbAp46 was further confirmed in the NIH3T3 derivative 7–4 cells, which also contain the inducible Ha-rasval12 gene [30-32] (Additional file 2: Figure S1B). In this study, we revealed that Ras-induced RbAp46 forms a complex with HDAC1 and Sp1, which further binds to the Sp1 site of the RECK promoter and suppresses its expression. The relationship among Ras, RbAp46 and RECK expression was also detected in 75% (3/4) of the bladder cancer specimens analyzed. This is the first report to show that RbAp46 as a Ras up-regulated gene plays a pivotal role in Ras-related lung metastasis through binding of the RbAp46 with HDAC1 and Sp1 to suppress RECK expression and activate MMP9. NIH3T3 fibroblasts were used in this study, because of their ease to be transfected and transformed by oncogenes.
Ahringer et al. reported that RbAp46 is a core component of the HDAC-SIN3-NuRD complex, which participates in gene regulation through interaction with specific co-repressors . Recruitment of HDACs to promoter regions is a general mechanism of transcriptional repression of the target genes in cells. For example, the recruitment of HDACs to specific promoters could be mediated through direct interactions with regulatory proteins such as retinoblastoma (Rb) and metastatic-associated protein 1 (MTA1) [33-36]. Chang et al., reported that Ras overexpression increases the binding of HDAC1 to the Sp1 site and suppresses RECK (metastasis inhibitor) expression . Here, we further revealed that RbAp46 utilizes Sp1 as a docking protein to recruit HDAC1 and RbAp46 and suppresses RECK expression via the Sp1 binding site. This result indicates that the increased binding of the RbAp46 with HDAC1 and Sp1 to the Sp1 site is dependent on the protein level of RbAp46, which is possibly responsible for the suppression of RECK expression. However, whether other processes of epigenetic regulation such as acetylation or methylation are involved along with complex formation requires further investigation. Jaejoon et al., reported that Sp1 and Sp3 form a complex with HDAC2 to repress human telomerase reverse transcriptase (hTERT) expression . Therefore, the possibility of the recruitment of HDAC2 by RbAp46 can not be excluded. Moreover, RbAp46 is an Rb-binding protein, so it is possible that RbAp46 could interact with Rb protein and recruit HDAC1 to the promoter.
RbAp46 overexpression induces epithelial-mesenchymal transition (EMT) and increases the migration and invasion of MCF10AT3B mammary epithelial cells . Wang et al., showed that elevated serum RbAp46 levels are highly correlated with non-small cell lung cancer (NSCLC) distant metastasis and knockdown of RbAp46 inhibits the migration ability of lung cancer cells . Those reports indicate that RbAp46 participates in EMT, cell migration, and invasion as well as tumor metastasis. However, there is no in vivo evidence to show the effect of RbAp46 on cancer metastasis. Our experimental metastatic mouse model provides the first evidence that RbAp46 participates in Ha-rasval12-induced lung metastasis. Our finding that RbAp46 suppresses RECK expression and decreases Ha-rasval12-induced lung metastasis is consistent with the results of other reports which showed that RECK protein reduces metastasis and that down-regulation of RECK is correlated with lymph node metastasis of lung cancer cells [29,38]. When RECK is down-regulated, the activity of MMP-9, a key enzyme involved in tumor invasion and metastasis, is increased . We also demonstrated that the RbAp46 expression level positively correlates with MMP-9 activity in T24 cells (Figure 4C). Our results provide a mechanistic link for RbAp46 regulated cell invasion and metastasis. Collectively, RbAp46 enhanced cell invasion and metastasis are possibly caused by increased activity of MMP-9 resulting from RECK suppression.
When oncogenic Ras is highly expressed, many down-stream signaling pathways are activated such as Raf-1, PI3K and Ral-GDS . Previous studies showed that oncogenic Ras and HER-2/Neu down-regulate RECK expression through MEK/ERK signaling [12,13]. We also revealed that Ha-rasval12 transcriptionally up-regulate RbAp46 (Additional file 2: Figure S2A) through the Raf-mediated MEK/ERK signaling pathway (Additional file 2: Figure S2B). A previous study showed that Raf-MEK-ERK1/2 pathway induced by Ha-rasval12 is necessary for experimental metastatic ability of NIH3T3 cells . Welch et al., further showed that MEK1 activation leads to the production of MMP-2 and MMP-9 which contribute to Ras-mediated metastatic potential . Therefore, Raf/MEK/ERK signaling may play a critical role in Ras-mediated RECK suppression and metastasis through up-regulation of RbAp46. However, the detailed mechanism of how RbAp46 is regulated remains to be determined. In T24 cells, RECK protein increased dramatically after Ras knockdown, however, there were no obvious changes in RbAp46 (Figure 2B). This finding suggests a parallel mechanism of Ras-induced RECK regulation. Chang et al. reported that Ras-induced down-regulation of RECK is mediated via a DNMT3b/promoter methylation mechanism . Loayza-Puch et al. reported that Ras-signaling pathways could downregulate RECK protein through microRNAs . Whether DNMT3b and microRNA regulate Ras-induced RECK downregulation independently of RbAp46 in T24 cells is worthy of investigation. In one bladder cancer specimen (case #358), Ras showed little difference between normal and tumor, and yet RbAp46 was strongly upregulated in the tumor. It is possible that endogenous Ras is regulated at the level of GDP-GTP exchange rather than altered protein expression.
Interestingly, RbAp46 is also a potent cell growth inhibitor that can suppress the transformed phenotype of tumor cells [17,18,41]. It is counterintuitive that a tumor suppressor promotes malignant behaviors. RbAp46 is an Rb-binding protein and has been proved to be a metastasis-associated protein 1 (MTA1) complex [42,43]. Expression of MTA1 correlates well with the metastasis potential of human cancers [44,45]. We observed that RbAp46 can directly interact with HDAC1. Therefore, it is possible that RbAp46 could promote tumor metastasis by regulating transcriptional suppression on RECK gene through histone deacetylation activity. However, the underlying mechanism needs to be further studied. It has been shown that RbAp46 expression inhibits the transformation of tumor cells by interfering with the normal cell cycle and/or enhancing apoptotic cell death . We also found that overexpression of exogenous RbAp46 suppressed cell proliferation and decreased Ras-related tumor formation (unpublished data), which is consistent with previous findings [17,18,41]. RbAp48, another member of a subfamily of nuclear WD-repeat proteins, could interact with HDAC1 and then be recruited by Rb/E2F1 to the promoters of E2F-regulated genes during the G1 phase of the cell cycle . It is possible that RbAp46 may utilize similar mechanisms to suppress cell proliferation and tumor formation.
In this study, we reveal that Ras up-regulates RbAp46 expression in human breast and bladder cancer cells as well as in mouse fibroblast 7–4 cells, and the binding of RbAp46 with HDAC and Sp1 represses RECK promoter activation via the Sp1 site. Taken together, RbAp46-induced down-regulation of RECK expression increases cell invasion and metastasis and is accompanied with promotion of MMP-9 activity. Finally, RbAp46 expression was found to be associated with mutant Ras expression and was inversely correlated with the expression of RECK in bladder cancer tissues. In conclusion, our findings provide a novel mechanism of Ras-related tumorigenesis in bladder cancer and suggest that RbAp46 could be a target in the treatment of Ras-related metastasis in bladder cancer.
This work was supported by grants from NCKU (Landmark Project Grant A25), funded by the Ministry of Education in Taiwan, and the National Science Council (NSC-96-2628-B-006-003-MY3, NSC-101-2320-B-006-025-MY3 and NSC-101-2314-B-705-003-MY3). We thank Dr. Robert Anderson for proofreading the manuscript and editing our English.
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