Critical role of histone demethylase RBP2 in human gastric cancer angiogenesis
© Li et al.; licensee BioMed Central Ltd. 2014
Received: 31 January 2014
Accepted: 4 April 2014
Published: 9 April 2014
The molecular mechanisms responsible for angiogenesis and abnormal expression of angiogenic factors in gastric cancer, including vascular endothelial growth factor (VEGF), remain unclear. The histone demethylase retinoblastoma binding protein 2 (RBP2) is involved in gastric tumorgenesis by inhibiting the expression of cyclin-dependent kinase inhibitors (CDKIs).
The expression of RBP2, VEGF, CD31, CD34 and Ki67 was assessed in 30 human gastric cancer samples and normal control samples. We used quantitative RT-PCR, western blot analysis, ELISA, tube-formation assay and colony-formation assay to characterize the change in VEGF expression and associated biological activities induced by RBP2 silencing or overexpression. Luciferase assay and ChIP were used to explore the direct regulation of RBP2 on the promoter activity of VEGF. Nude mice and RBP2-targeted mutant mice were used to detect the role of RBP2 in VEGF expression and angiogenesis in vivo.
RBP2 and VEGF were both overexpressed in human gastric cancer tissue, with greater microvessel density (MVD) and cell proliferation as compared with normal tissue. In gastric epithelial cell lines, RBP2 overexpression significantly promoted the expression of VEGF and the growth and angiogenesis of the cells, while RBP2 knockdown had the reverse effect. RBP2 directly bound to the promoter of VEGF to regulate its expression by histone H3K4 demethylation. The subcutis of nude mice transfected with BGC-823 cells with RBP2 knockdown showed reduced VEGF expression and MVD, with reduced carcinogenesis and cell proliferation. In addition, the gastric epithelia of RBP2 mutant mice with increased H3K4 trimethylation showed reduced VEGF expression and MVD.
The promotion of gastric tumorigenesis by RBP2 was significantly associated with transactivation of VEGF expression and elevated angiogenesis. Overexpression of RBP2 and activation of VEGF might play important roles in human gastric cancer development and progression.
KeywordsRBP2 VEGF Gastric cancer Angiogenesis
Gastric cancer is an important health problem around the world and the cause of 12% of all cancer-related deaths each year [1, 2], especially in China, which accounts for 42% of the global total cases . Carcinogenesis is a multistep process involving the transformation, survival, proliferation, invasion, angiogenesis, and metastasis of tumor. During this process, the accumulation of genetic and epigenetic alterations leads to progressive transformation . Determining the molecular mechanism can benefit diagnosis and treatment.
Angiogenesis is a process of neovascular formation from pre-existing blood vessels, which consists of sequential steps for vascular destabilization, angiogenic sprouting, lumen formation and vascular stabilization. Induction of angiogenesis represents one of the major hallmarks of cancer . Vascular endothelial growth factor (VEGF) is a key mediator in the neovascularization of cancers . Through a VEGF-induced signaling pathway, such as PI3K-AKT, JAK2-STAT5 and ERK1/2, VEGF is involved in angiogenesis and proliferation . VEGF overexpression involves abnormal expression of growth factors and their receptors . Epigenetic regulation is important for VEGF expression. VEGF expression is suppressed by promoter hypermethylation . MiR-125a and miR-126 can inhibit its expression [10, 11]. To better understand the mechanism of angiogenesis, the regulation of VEGF expression is a key point.
Histone modification plays an important role in carcinogenesis, including angiogenesis [5, 12]. For example, histone methylase MLL1 has critical roles in tumor growth and angiogenesis . HDAC3 acts as a negative regulator of angiogenesis . Retinoblastoma binding protein 2 (RBP2), a newly found histone demethylase for H3 lysine 4 (H3K4) trimethylation and dimethylation, is involved in gastric tumorgenesis with its epigenetic inhibition of cyclin-dependent kinase inhibitors (CDKIs) . We also found a high expression of RBP2 in hepatocellular carcinoma and its inhibition triggered cell senescence . Others also suggested that loss of RBP2 suppresses tumorigenesis in mice lacking Rb1 or Men1 .
In the present study, we sought to determine the potential role of RBP2 in human gastric cancer angiogenesis via the regulation of VEGF and the underlying mechanisms in vivo and in vitro.
Association of RBP2 overexpression with increased VEGF expression and MVD in human gastric cancer
Regulation of VEGF expression by RBP2 in human gastric cancer cells and RBP2-targeted mutant mice
Transcriptional activation of VEGF expression in gastric cancer cells by RBP2
Direct effect of RBP2 on VEGF expression in the tumorigenicity of human gastric cancer cell lines
The transfection of RBP2 expression vector markedly increased the secretion of VEGF in BGC-823, SGC-7901 and GES-1 cell supernatant (Figure 5E) and RBP2 silencing with RBP2 siRNA suppressed the VEGF secretion (Additional file 3: Figure S2). Consistent with increased expression of VEGF, the supernatants of BGC-823 and GES-1 cells with RBP2 overexpression were more angiogenic than those of control vector–transfected BGC-823 and GES-1 cells as determined by endothelial-cell tube-formation assay (Figure 5F and G). These results suggest the direct effect of RBP2 on VEGF expression in tumorgenesis and angiogenesis.
Regulation of VEGF expression, tumorgenesis and angiogenesis by RBP2 in gastric tumors in nude mice
In this study, we focused on the critical role of the histone H3K4 demethylase RBP2 in angiogenesis of gastric cancer, especially the direct regulation of VEGF by RBP2. First, we determined the association of RBP2 expression, VEGF expression and MVD status in human gastric cancer tissue species. Second, we detected the direct regulation of VEGF by RBP2 both in human gastric cancer cell lines and RBP2-targeted mutant mice, which showed that H3K4 demethylation by RBP2 expression was important for VEGF expression and MVD status. RBP2 bound to the promoter of VEGF directly and activated the promoter of VEGF for regulation at the transcriptional level. Third, VEGF expression induced by RBP2 overexpression activated tumorigenesis and angiogenic potential in human gastric cancer cells and animal models, whereas silencing of RBP2 expression had the reverse effect. Therefore, abnormal RBP2 expression and VEGF activation might explain the poor prognosis with gastric cancer and contribute directly to gastric tumor angiogenesis and aggressive gastric cancer biology.
Angiogenesis is a critical process in the invasion, growth and metastasis of most solid tumors . Angiogenesis is complex and involves a large number of molecules. VEGF, fibroblast growth factor, angiopoietin, Notch, transforming growth factor β, Hedgehog and WNT signaling cascades orchestrate angiogenesis through the direct or indirect regulation of quiescence, migration and the proliferation of endothelial cells [19–23]. VEGF has critical roles in tumor angiogenesis . Small-molecule compounds and human/humanized monoclonal antibodies interrupting VEGF signaling have been developed as anti-angiogenic therapeutics for cancer [25, 26]. Epigenetic regulation in angiogenesis is a new pathway in carcinogenesis and metastasis. Lysine acetylation and cytosine methylation are important transcriptional regulators of angiogenic genes in endothelial cells. Lysine acetylation and cytosine methylation inhibitors idiosyncratically tune the transcriptome and affect expression of key modulators of angiogenesis such as VEGF and endothelial nitric oxide synthase .
We have found that RBP2, a new histone H3K4 demethylase, is involved in carcinogenesis by escaping cell senescene and inhibiting CDKIs [15, 16]. In this research, we first found RBP2 activation associated with VEGF expression and tumorgenesis and angiogenesis in human gastric cancer: RBP2 expression was positively associated with VEGF mRNA and protein expression and with MVD as identified by CD31 and CD34 antibody staining and cell proliferation by Ki67 overexpression, which suggests a link between RBP2 activation and VEGF overexpression. RBP2 overexpression significantly affected VEGF expression in gastric cancer cells via demethylation of H3K4. In RBP2-targeted mutant mice with H3K4 demethylation of RBP2 silenced and H3K4 trimethylation enhanced, the expression of VEGF and the aniogenic phenotype were also inhibited. As well, RBP2 expression was involved in VEGF promoter activity in gastric cancer cells. Specifically, we identified one potential RBP2 binding site on the VEGF promoter. Mutation of the site profoundly attenuated but did not completely eliminate RBP2-mediated transactivation of the VEGF promoter. Therefore, we confirmed by ChIP assay that this binding site was functional, which showed active recruitment of RBP2 to the binding site.
H3-K4 methylation is associated with transcription activation [28–30]. As a histone demethylase specific for di- and tri-methylated H3-K4, RBP2 acts as a transcriptional repressor by inhibiting H3-K4 methylation at its target promoters [31, 32]. However, we found that VEGF was a direct target of RBP2. Not as the suppression of CDKIs promoters by RBP2 in gastric cancer, RBP2 induced the activation of VEGF promoter by directly binding to the CCGCCC DNA motif . Another important H3K4 demethylase, LSD1, which is overexpressed in numerous cancers, was found involved in inducing VEGF expression in prostate cancer . LSD1 was also identified to act through its demethylase activity to promote epigenetic modifications at Notch-target genes. Remarkably, LSD1 functions as a corepressor when associated with the CSL-repressor complex and as a NOTCH1 coactivator upon Notch activation . In addition, RBP2 is present in a number of chromatin-remodeling complexes [36, 37]. Polycomb-repressive complex 2 recruits RBP2 to its target genes via physical interaction . This situation may occur with the promoter of the VEGF gene. However, we need more evidence to determine whether RBP2 affects the VEGF promoter methylation status, which is involved in the regulation of VEGF expression  in human gastric cancer cells.
We detected the critical role of RBP2 in angiogenesis by the biological effect in human cancer cells and animal models. With RBP2 siRNA, RBP2 expression was silenced and VEGF expression inhibited. The clone formation of human gastric cancer cells was also suppressed in this process, which was partially reversed by the overexpression of VEGF in RBP2 siRNA-transfected cells. We found RBP2 involved in gastric carcinogenesis by the regulation of VEGF. In nude mice, tumors from transfected gastric cancer cells stably expressing RBP2 shRNA were smaller, with lower VEGF expression, and less MVD and cell proliferation than control cells. Recent genome-wide analyses of mouse embryonic stem cells and human leukemic cell lines revealed hundreds of RBP2 target genes and many of them implicated in development, proliferation and differentiation controls [32, 38, 39]. RBP2 was identified as a key molecule in drug tolerance of cancer cells and maintaining cancer stem cells [40, 41]. Our new findings that RBP2 is critical in constitutive and inducible VEGF expression might suggest its clinical implication in gastric tumor angiogenesis and progression.
Surgery plays a central role in the overall management of operable gastric cancer. Histone deacetylase inhibitors, based on epigenetic development, are being used clinically , and histone demethylase inhibitors are being addressed in clinical trials . Our study identified a novel molecular mechanism for RBP2 and provides better understanding of the molecular basis for angiogenetic signaling pathways, which might aid in the design of effective therapeutic modalities to control gastric cancer growth and metastasis.
In this study, we detected the regulation of VEGF by H3K4 demethylase RBP2 directly in vivo and in vitro. This regulation plays an important role in tumorgenesis and angiogenesis of human gastric cancer. RBP2 may be critical in cancer angiogenesis.
Materials and methods
Patients and tissue specimens
Resected pairs of human gastric cancer tissue and distal normal gastric tissue (>5 cm from the margin of the tumor) from 27 patients were harvested during surgery at Qilu Hospital of Shandong University in 2013. No patient had received adjuvant chemotherapy before surgery. The diagnosis of gastric cancer was confirmed histopathologically. The general information for patients is in Additional file 1: Table S1. The study was approved by the ethics committee of Shandong University School of Medicine.
Tissues from human resected pairs and animal models were embedded with paraffin and sliced into 5-μm pieces, which were deparaffinized and dehydrated with xylene and a graded series of alcohol. Antigen retrieval involved heat treatment performed in 0.1 M citrate buffer at pH 6.0. Then 3% H2O2 was used to block endogenous peroxidase activity. The slides were further incubated with goat serum for 30 min, then with the antibodies monoclonal rabbit anti-human RBP2 (Sigma, USA); polyclonal rabbit anti-VEGF, anti-CD31, and anti-Ki67 (all Abcam, UK); and monoclonal mouse anti-CD34 (Santa Cruz Biotechnology, USA) overnight at 4°C. The results were detected with diaminobenzidine staining (Vector Laboratories, USA) under a microscope (Olympus BX60, Tokyo) and images were captured for analysis.
Cell lines and culture
The gastric epithelial-derived cancer cell lines BGC-823 and SGC-7901 were obtained from the cell repository for Academia Sinica (Shanghai). Human gastric epithelial immortalized GES-1 cells were maintained in our laboratory. All cell lines were grown in RPMI1640 medium (Gibco, USA) supplemented with 10% fetal bovine serum (Gibco, USA). Cell lines were incubated in a humidified atmosphere containing 5% CO2 at 37°C without antibiotics, then cultured on 6-well plates for 18 to 24 h before experiments.
RNA extraction and quantitative RT-PCR
The total RNA in human or animal samples and cell lines was extracted by use of Trizol (Invitrogen, Life technologies, USA). cDNA was synthesized with use of MMLV reverse transcriptase (Fermentas, Canada). The expression of genes was checked by PCR with the TaqMan gene expression assay kit (Life Technologies, USA) or SYBR Premix Ex Taq kit (Takara, Dalian) with β-actin as a control. Real-time PCR involved the ABI7500 sequence detector (Applied Biosystems, USA). The assay IDs for human resected tissues and cell lines were RBP2, Hs00188160_m1; and VEGF, Hs00900055_m1. The PCR primers for VEGF expression in mice were sense, 5'-CTGTACCTCCACCATGCCAAGT-3', and antisense, 5'-CTTCGCTGGTAGACATCCATGA-3'. RBP2 and VEGF mRNA expression was normalized to β-actin expression. Change in expression was calculated relative to the control (2-∆∆Ct).
Western blot analysis and Enzyme-linked immunosorbent assay (ELISA)
Total cellular proteins were extracted with RIPA lysis buffer. The membranes were probed with antibodies against RBP2 (Abcam, UK) and VEGF followed by anti- rabbit horseradish peroxidase-conjugated immunoglobulin G and developed by the enhanced chemiluminescent method (PIERCE, Thermo Fisher Scientific, USA). β-actin was a loading control. For immunoblotting of histone H3-K4 di- and trimethylation, we isolated a total histone fraction from nuclei using dilute acid extraction Zeng J et al., . Histone proteins were detected with antibodies against di- and trimethylated H3-K4 (Abcam, UK). H3 was the control. An ELISA kit (R&D Systems, USA) was used for quantification of human VEGF secretion in cell culture supernatants.
Promoter activity assay
The promoter sequence of VEGF was obtained from 2 databases, TRED and GENECARDS. The primer sequences for VEGF promoter amplification were sense, 5'-GGTACCTGTGAGCCTGGAGAAGTAGCC-3' (KpnI), and antisense, 5'-AAGCTTACAGTGATTTGGGGAAGTAGAGC-3' (HindIII). The result was detected by sequencing. The VEGF promoter sequence was cloned into pGL3-Basic Vector, named pGL3-VEGF. The mutation of the RBP2 binding site (CCGCCC) in the VEGF promoter sequence was preceded by PCR. The primers for mutation were sense, 5'- TGGTGGATTATAGTGGAGG -3', and antisense, 5'- CCTCCACTATAATCCACCAG -3'. The sequence was cloned into the plasmid, named pGL3-VEGF-mutant. The RBP2 expression vector and RBP2 siRNA were transfected into cells with the promoter reporter plasmids for VEGF. The Renilla luciferase-containing plasmid controlled by the thymidine kinase (TK) promoter was co-transfected as a control. Luciferase activity in the cell lysates was determined by dual luciferase reporter assay (Promega, USA) at 48 h after transfection, and the target promoter-driven firefly luciferase activity was normalized to that of TK renilla.
Chromatin immunoprecipitation (ChIP)
Control and RBP2 siRNA-treated BGC-823 cells were cross-linked by incubation in 1% formaldehyde-containing medium for 10 min at 37°C and then sonicated to make soluble chromatin with DNA fragments between 200 and 1000 bp. Antibodies against RBP2 and tri- or di- methylated H3-K4 were used to precipitate DNA fragments bound by their corresponding elements. The protein–DNA complex was collected with protein A Sepharose beads (Millipore), eluted, and reverse cross-linked. Following treatment with protease K (Sigma-Aldrich), samples were extracted with phenol-chloroform and precipitated with ethanol. The recovered DNA was resuspended in TE buffer and used for PCR amplification. The PCR primers were for the VEGF promoter, sense, 5'- GGCGGGTAGGTTTGAATC -3', and antisense, 5'- CGTATGCACTGTGGAGTC -3'; and GAPDH, sense, 5'- AAAGGGCCCTGACAACTCTT -3', and antisense, 5'- GGTGGTCCAGGGGTCTTACT -3', as a control.
Cells were incubated in 6-well plates for 18 to 24 h, then transfected with the corresponding vectors for 48 h or different siRNAs for 72 h. Single cells were seeded on 6-well plates (500 cells/well). After 10 to 14 days of incubation with 2% fetal bovine serum, plates were stained with Giemsa for 20 min. Colonies with more than 50 cells were counted.
Endothelial-cell tube-formation assay
The tube-formation assay was described previously Gong W et al., . Briefly, 250 μL of growth factor-reduced Matrigel (BD Biosciences, USA) was pipetted into each well of a 24-well plate and polymerized for 30 min at 37°C. Human umbilical vein endothelial cells were harvested after trypsin treatment and suspended in conditioned medium from 1 × 106 BGC-823 and GES-1-RBP2 overexpression cells or 1 × 106 BGC-823 and GES-1-control cells cultured for 48 h in Dulbecco modified Eagle medium containing 1% fetal bovine serum. Then 2 × 104 human umbilical vein endothelial cells in 300 μL conditioned medium were added to each well and incubated at 37°C, 5% CO2, for 20 h. The cultures were photographed by microscopy. The degree of tube formation was assessed as the percentage of cell surface area to total surface area. Control cell cultures values were set at 100%.
BGC-823 cells stably expressing RBP2 shRNA to inhibit RBP2 expression and matched controls were constructed. A total of 10 BALB/c nude mice 7 weeks old (Vital River Laboratories, P.R.China) were divided into 2 groups randomly. BGC-823 cells with RBP2 shRNA or the matched control were subcutaneously injected into mice (2 × 105 cells/mouse). 15 days later, tumors were harvested and tumor size measured. Tumor volume was calculated as greatest tumor diameter × (shortest tumor diameter)2/2. RBP2-targeted mutant mice were provided by the Jackson Laboratory [strain B6.129S6(FVB)-Kdm5atm1.1Kael/J]. The genotypes of mice were determined by protocols from the Jackson Laboratory. The primers used for PCR were for the wild type, forward, 5'- GTTTGAATTTCACTCTATGCTGGG -3'; mutant forward, 5'- GGTGCTGGGAACCATACTTG -3'; and common, 5'- TCCCAGCATGGATCTTGTCC -3'. The PCR results showed different genotypes: mutant = 530 bp, heterozygote = 216 and 530 bp, and wild type = 216 bp. All experimental protocols were approved by the local Animal Care and Use Committee.
Quantitative data are expressed as mean ± SEM. Statistical analysis involved use of SPSS 13.0 (SPSS Inc., USA) with two-tailed Student's t test or one-way ANOVA for more than 2 subgroups. Statistical significance was set at P < 0.05.
The study was supported by the National Natural Science Foundation of China (nos. 81272654, 81172354, 81171536, 81371781 and 81170514), the National Basic Research Program of China (973 Program, 2012CB911202) and the Independent Innovation Foundation of Shandong University (no. 2012TS106).
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