1 Hormone-Associated Cancer Research Group, Laboratory of Cancer Diagnosis and Therapy, Saitama Cancer Center Research Institute and
2 Department of Breast Surgery, Saitama Cancer Center Hospital, 818 Komuro, Ina, Saitama 362-0806, Japan,
3 Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institute, S-171 77, Stockholm, Sweden and
4 Department of Critical Care Medicine and
5 Department of Second Surgery, Gunma University School of Medicine, 3-39-22 Showa, Maebashi, Gunma 371-8511, Japan
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Abstract |
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Abbreviations: 5-aza-dC, 5-aza-2'-deoxycytidine; ER, estrogen receptor; ERBF-1, estrogen receptor promoter B associated factor 1; ERF-1, estrogen receptor factor; GAPDH, glyceraldehyde phosphate dehydrogenase; RTPCR, reverse transcriptionpolymerase chain reaction; TK, thymidine kinase.
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Introduction |
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Transcription of the human ER gene occurs from at least two different promoters (46), with the distal promoter (promoter B) located 2 kb upstream of the proximal one (promoter A). The resultant transcripts from the two promoters differ only in the non-coding region at the 5' end, and both types of ER
mRNA encode the same protein. We have previously shown that the levels of expression of total ER
mRNA and of transcript from promoter B correlated well with the amount of ER
protein in human primary breast cancer, indicating that promoter B plays an essential role in the regulation of ER
gene expression in human breast cancer (7). We have also identified a novel trans-acting factor, estrogen receptor promoter B associated factor 1 (ERBF-1), which is critical for the transcription activity of promoter B in ER
-positive breast cancer cell lines (8).
Loss of ER gene expression may be explained by either epigenetic modifications of the gene or loss of trans-acting factors which are essential for expression of the ER
gene. One mechanism that may block transcription is methylation of cytosine/guanine-rich areas, termed CpG islands, which are located in the 5' regulatory regions of housekeeping genes and numerous tissue-specific genes (9). Cancer cells often display anomalous patterns of DNA methylation, with site-specific hypermethylation in CpG islands and hypomethylation of bulk genomic DNA (10). In breast cancer, aberrant hypermethylation has been reported with respect to some genes, such as the E-cadherin (11), BRCA1 (12,13) and HIC-1 genes (14).
The ER gene has a typical CpG island within its promoters and first exon. The CpG island is unmethylated in both normal breast tissue (15) and non-tumorigenic breast epithelium cell lines that do not express ER
(16). Recent studies have reported that methylation of the CpG island in ER
gene promoter A and exon 1 was associated with loss of ER
gene expression in human breast cancer tissue and cell lines (15,1719) and that ER
-negative breast cancer cells MDA-MB-231 treated with the demethylating reagent, 5-aza-2'-deoxycytidine (5-aza-dC) induced expression of ER
mRNA, resulting in production of functional ER
protein (20). However, until now there have been no reports on the methylation status of the promoter B region, which is responsible for overexpression of ER
protein: Direct evidence of suppression of ER
gene expression by methylation, and the molecular mechanism of this suppression, have not been addressed so far.
In this report, we show that the methylation status of promoter B of the ER gene, along with the methylation status of promoter A, was inversely associated with expression of this gene in human breast cancer and in breast cancer cell lines. Specific methylation of ER
gene promoters directly decreased the transcription of this gene in a transient transfection experiment. However, demethylating treatment of ER
-negative breast cancer cells induced ER
gene expression in BT-20 cells but not in MDA-MB-231 cells, and MDA-MB-231 cells showed little activity in transfection of reporter constructs containing unmethylated promoter B. Our results indicated that methylation of the two promoter regions of the ER
gene can directly modulate expression of this gene, though some other mechanism, such as loss of critical transcriptional factors, may also be involved in some cases of ER
-negative breast cancer.
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Materials and methods |
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Cells and culture
Human breast cancer cells (MCF-7, T-47-D, ZR-75-1, MDA-MB-231 and BT-20) were cultured in RPMI 1640 medium supplemented with 5% or 10% fetal calf serum (FCS), 2 mM L-glutamine and 2 µg/ml gentamicin at 37°C in a humidified atmosphere of 5% CO2 in air.
Southern blot analysis
Ten to 15 µg of genomic DNA was digested first with 10 units/µg of SacI (methylation-insensitive; TaKaRa, Tokyo, Japan) in an appropriate buffer at 37°C for 8 h, followed by either 15 units/µg of HhaI or HpaII (methylation-sensitive; TaKaRa) at 37°C overnight. The DNA fragments were separated in 1% agarose gels, blotted on to Hybond-N nylon membrane (Amersham, Little Chalfont, UK), and fixed with UV radiation. Either a 1.4 kb fragment of promoter B (3284 to 1864) or a 1.3 kb fragment of promoter A (1168 to +190) of the ER gene, which was synthesized from genomic DNA using PCR, was labeled with [
-32P]dCTP (3000 Ci/mmol) using a Rediprime II DNA labeling kit (Amersham). Hybridization was carried out in ExpressHyb Hybridization Solution (Clontech, Palo Alto, CA) at 65°C for 1 h. The hybridized bands were visualized by autoradiography with a Fuji Bio-Image Analyzer BAS2000 (Fuji Film, Tokyo, Japan).
Reverse transcriptionpolymerase chain reaction (RTPCR)
Total RNAs were prepared from cultured cells (15 x 106 cells) and ~0.1 g of human breast cancer tissue according to the method of Chomczynski and Sacchi (21). RTPCR, used to detect total ER mRNA, mRNA transcribed from promoter A (type A) and mRNA from promoter B (type B), together with internal control GAPDH mRNA, was performed as previously reported (7). For ZR-75-1 cells, three more PCR cycles were used to detect ER
mRNAs than for other cells.
ER protein assay
ER enzyme immunoassay for tumor samples was performed with ER
enzyme immunoassay kits (Abbott, Abbott Park, IL). ER
values of <5 fmol/mg protein were considered negative.
Reporter plasmid constructions
The 1.3 kb fragment (1168 to +190) of promoter A and the 1.4 kb fragment (3284 to 1864) of promoter B were ligated to pGL2-Basic vector (Promega, Madison, WI) at the XhoI restriction site to give plasmids pGL2-ProA and pGL2-ProB, as described previously (8). Next, pGL2-ProB was digested with KpnI and HindIII and the promoter B region was ligated to pGL3-Basic vector (Promega); the resultant plasmid was named pGL3-ProB.
Methylation of promoter regions of the ER gene in reporter plasmid constructs
The promoter region in reporter plasmid constructs was methylated essentially as described (22). pGL2-ProA and pGL2-ProB were digested with KpnI and HindIII. The fragments of the promoter A and B regions were purified using a Geneclean III kit (Bio 101, Vista, CA), and methylated in vitro with 3 units of SssI methylase (New England Biolabs, Beverly, MA) per µg of DNA in the presence of 160 µM S-adenosyl-methionine at 37°C for 3 h. For the unmethylated control, the same fragments were incubated in the same conditions without the methylase. Complete methylation of the fragments was confirmed by HpaII and HhaI digestion. pGL3-Enhancer vector (Promega) was digested with KpnI and HindIII, and ligated with an equimolar concentration of methylated or unmethylated promoter fragments at 16°C for 30 min at a DNA concentration of 20 µg/ml. The resultant plasmids were ethanol-precipitated and named pGL3-E.-ProA and pGL3-E.-ProB. Four micrograms of ligated DNA was used for transfection experiments.
Sodium bisulfite genomic sequencing
Sodium bisulfite genomic sequencing was performed essentially as described (2326). Ten micrograms of genomic DNA from cell lines was digested with XbaI in 90 µl of reaction mixture, denatured with 10 µl of 3 M NaOH at 37°C for 15 min and then incubated with 1040 µl of freshly prepared 3.6 M sodium bisulfite, pH 5.0 (Wako, Osaka, Japan) and 50 µl of freshly prepared 10 mM hydroquinone (Wako) for 16 h at 50°C. After bisulfite treatment, the DNA was desalted using a Geneclean III kit, denatured with 22 µl of 3 M NaOH at 37°C for 15 min, precipitated with 140 µl of 5 M ammonium acetate pH 7.0 and 900 µl of ethanol, and resusupended in 100 µl of MilliQ water. First-round PCR amplification was performed in 50 µl of reaction mixture containing 2 µl of bisulfite-treated genomic DNA, 0.5 µM each of primers, 200 µM dNTPs, 1.5 mM MgCl2 and 1.25 units of AmpliTaq Gold polymerase (PerkinElmer, Foster City, CA) under the following conditions: 95°C for 30 s, 58°C for 1 min, 72°C for 1 min for 40 cycles. To obtain products for sequencing, a second round of PCR was performed with nested primers in 50 µl of reaction mixture containing 10 ng of the PCR product, 0.5 µM each of primers, 200 µM dNTPs, and 2.5 units of Pyrobest DNA polymerase (Takara) under the following conditions: 98°C for 10 s, 52°C for 1 min, 72°C for 1 min for 30 cycles. Modified primers were as follows: ERBM1U (5'-TTAAGGGTAGGGGTAAAGGGGTTGGGGTTT-3') and ERBM1D (5'-ATACCCCCATAAAAAACAACAATCCTCATC-3') for first-round PCR, ERBF-b (5'-TGGTTGTGTTATATTGTTTT-3') and ERBM2D (5'-TCTCCCTACTAAAATACAAACACTAACCAA-3') for second-round PCR. The resultant PCR products were purified using a Geneclean III KIT (BIO 101) and subcloned into pGEM-7Zf(+) vector (Promega). Sequencing was performed using an ABI PRISM 310 Genetic Analyzer (PerkinElmer).
5-Aza-dC treatment of cell lines
The demethylating reagent, 5-aza-dC (Sigma, St Louis, MO), was freshly prepared in MilliQ water. Human breast cancer cells (510 x 105 cells) were plated on to 100 mm plastic culture dishes in RPMI 1640 with 10% FCS. Twenty-four hours later, cells were treated with 106 M 5-aza-dC. The medium was changed 24 h after treatment and then every 3 days. DNA and RNA were prepared 7 days after treatment.
Transient transfection and luciferase assay
MCF-7 cells were transiently transfected using SuperFect (Qiagen, Hilden, Germany) and ZR-75-1 and MDA-MB-231 cells using Lipofectamine (GibcoBRL), according to the manufacturers' instructions. All the transfection experiments were done in triplicate. Transient transfection was performed essentially as described previously, with a slight modification (27). Briefly, culture cells were grown on 35 mm plastic culture dishes in RPMI medium with 5% or 10% FCS to 3050% confluence. A plasmid cocktail with an internal control vector, pRL-TK (Promega), was mixed with the transfection reagent and added to the culture. After 48 h of incubation, the cells were lysed in a lysis buffer and the luciferase activity of plasmid constructs and pRL-TK, which encodes a renilla luciferase reporter gene driven by a thymidine kinase (TK) promoter, was measured using a dual luciferase assay kit (Promega). All the measured luciferase activity of plasmid constructs was normalized against the renilla luciferase activity of pRL-TK.
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Results |
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These results indicate that methylation of the promoter B region as well as the promoter A region of the ER gene is inversely associated with expression of this gene.
Methylation status of the ER gene promoter regions in human breast cancer cell lines
We next analyzed the methylation status of ER gene promoter regions in human breast cancer cell lines. The methylation status of the promoter B region was evaluated in terms of SacI+HhaI digestion and of SacI+HpaII digestion (Figure 2A
). Among three ER
-positive cell lines, HhaI site (a) in T-47-D cells was completely unmethylated (lane 6). In MCF-7 cells, together with a 1.0 kb band, additional longer bands were observed (lane 3), indicating that HhaI site (a) was partially methylated. Importantly, ZR-75-1 cells did not show a 1.0 kb band (lane 9), which seems to signify complete methylation of HhaI site (a). Likewise, HhaI site (a) of ER
-negative BT-20 cells was completely methylated (lane 15). Surprisingly, other ER
-negative MDA-MB-231 cells showed a 1.0 kb band with a 3.1 kb band (lane 12), so apparently HhaI site (a) was partially methylated.
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We also measured the amounts of total, type A and type B ER mRNAs for these cell lines by RTPCR (Figure 2C
): MCF-7 (lane 1) and T-47-D (lane 2) showed high expression of these mRNAs. ZR-75-1 cells use only promoter A in the transcription of ER
mRNA (7); in accordance with this, we found that ZR-75-1 cells (lane 3) showed low expression of total and type A mRNA, and no expression of type B mRNA. MDA-MB-231 (lane 4) and BT-20 (lane 5) cells did not express total, type B or type A mRNA.
These results indicate that methylation of the promoter B region as well as the promoter A region is inversely associated with expression of the ER gene in human breast cancer cell lines.
Effects of methylation of ER gene promoters on its transcription
We investigated whether methylation of the ER gene promoters can directly modulate the transcriptional activity of this gene. First, we performed a luciferase reporter assay in MCF-7 cells by transient transfection using overall methylated pGL2-ProA or -ProB, in which all CpG sites present in these vectors were methylated in vitro. The luciferase activity of methylated pGL2-ProA and -ProB fell to 20.9% and 34.8% respectively, of that of the non-methylated ones (data not shown). Since methylation of the coding region of the luciferase gene has also been shown to influence the activity (28), we next performed transfection experiments using promoter region-specific methylated constructs. In vitro methylation of promoters A and B decreased luciferase activity to 49.7% and 60.7% respectively, of that of the unmethylated ones (Figure 3
). Thus, methylation of the ER
gene promoters directly suppresses transcription of the ER
gene.
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To examine the reliability of this method, we performed a control experiment using demethylated genomic DNA treated with SssI methylase. Both SssI-treated and untreated DNA were modified with sodium bisulfite and then amplified with PCR. The fragments were subcloned and the recombinants sequenced. Representative sequences are shown in Figure 4A. All cytosine residues outside the CpG sites were sequenced as thymines after treatment with sodium bisulfite, confirming that the method was working successfully. The frequency of conversion from cytosine to thymine at CpG sites was then compared between untreated and SssI-treated recombinants (Figure 4A
). SssI treatment resulted in low conversion (
20%) at CpG sites 1, 2, 3, 5 and 7 and modest conversion (4070%) at CpG sites 4, 6, 8 and 9. This result implies that sodium bisulfite genomic sequencing may underestimate the degree of methylation, specifically at CpG sites 4, 6, 8 and 9, possibly due to sequence-specific PCR bias as will be discussed later.
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Since ER-negative MDA-MB-231 cells showed partial methylation of CpGs in the analysed promoter region, which is similar to the methylation status in ER
-positive MCF-7 cells, methylation alone may not be the sole mechanism of the suppression of ER
gene expression.
Effects of 5-aza-dC on ER gene expression in human breast cancer cell lines
In order to determine whether methylation of the ER gene promoters suppresses expression of the ER
gene, we next examined expression of this gene in breast cancer cells treated with 1.0 µM 5-aza-dC for 7 days. This demethylating treatment has been reported to be sufficient to maintain a physiological level of ER
protein in MDA-MB-231 cells (20, 29). The methylation status of genomic DNA from treated cells was examined by Southern blot analysis; expression of ER
mRNAs was monitored by RTPCR. The promoter B region was partially demethylated in treated ZR-75-1 cells, and in treated MDA-MB-231 and BT-20 cells (data not shown).
ER-negative BT-20 cells treated with demethylating reagent induced expression of ER
mRNA, in particular type A mRNA (Figure 5
, lane 6). In ZR-75-1 cells, where promoter B was inactive, treatment of the cells (lane 2) enhanced expression of both type A and B ER
mRNA compared with untreated cells (lane 1). Thus, demethylation of these cells altered expression of the ER
gene. On the other hand, treatment of MDA-MB-231 cells did not affect expression of ER
mRNA (lane 4). Although we treated MDA-MB-231 cells with 5-aza-dC at different concentrations (0.1, 0.5, 1.0, 2.0 and 4.0 µM) for 7 days, and performed five more PCR cycles than used in Figure 5
for the cells, no expression of ER
mRNA was observed (data not shown). These data indicate that loss of ER
gene expression may be regulated by different mechanisms in two ER
-negative breast cancer cells.
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Discussion |
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We have previously shown that levels of transcription from promoter B correlated well with total ER mRNA levels and with ER
protein levels in human breast cancer, indicating that transcription from promoter B, but not from promoter A, is responsible for the enhanced ER
gene expression found in human breast cancer (7). Thus, we paid a great attention to the methylation of promoter B region which may be important in the process of loss of ER
gene expression in human breast cancer. In this paper, the methylation status of the promoter B region, as well as the promoter A region, correlated well with the expression of ER
in breast cancer tissue, indicating that methylation of both promoter regions may occur simultaneously during the loss of ER
gene expression.
DNA methylation has been attributed to increased activity of cytosine DNA methyltransferase during carcinogenesis (16,17,30,31). However, the molecular determinants of specific CpG sites, `hot spots' for methylation by DNA methyltransferase, are not known. Therefore, we examined the methylation status of nine CpG sites around the transcription start site of promoter B, using the sodium bisulfite genomic sequencing method. Our control experiment showed that cytosine residues within specific CpG sites, namely CpG 4, 6, 8, and 9, appeared to be converted to thymine (4070%), though the DNA had been treated with SssI methylase, implying that methylation of these CpG sites may be underestimated by this method. This may be due to the PCR bias observed for several genes: DNA templates containing certain thymines instead of cytosines at CpG sites are preferentially amplified in PCR (32). Methylation of CpG sites 1, 2, 3, 5 and 7 was, however, reliably detected by sodium bisulfite genomic sequencing. Results of sodium bisulfite sequencing agreed well with those of Southern blotting: CpG sites 1 and 3, which correspond to HhaI (a) and HpaII (y), respectively, were highly methylated in BT-20 cells, poorly methylated in T-47-D cells and partially methylated in MCF-7 and MDA-MB-231 cells.
Next, we examined ER gene expression in breast cancer cells treated with the demethylating reagent 5-aza-dC; MDA-MB-231 cells did not express ER
mRNA by this treatment. ERBF-1 is highly expressed in MCF-7 and T-47-D cells, much less so in ZR-75-1 cells and not at all in MDA-MB-231 and BT-20 cells (8). In fact, our MDA-MB-231 cells showed little luciferase activity in a reporter assay using unmethylated pGL3-ProB. Thus, loss of ERBF-1 in MDA-MB-231 cells may be critical for suppression of ER
gene. This result differed from that reported by Ferguson et al. (20). However, expression of ER
mRNA in MDA-MB-231 cells was not affected by demethylating treatment, even when the number of PCR cycles and the concentration of 5-aza-dC were increased. This discrepancy in MDA-MB-231 cells may be due to a change in the nature of cells during the wide distribution.
In ER-negative BT-20 cells, expression of ER
total and type A mRNA was induced. It has been reported that ER
mRNA is very faintly expressed in BT-20 cells and is transcribed from promoter A alone (33). It has also been reported that estrogen receptor factor 1 (ERF-1), a member of the AP2 transcription factor family, is important for transcriptional regulation from promoter A in T-47-D cells (3436); ERF-1 expression has been found in MCF-7, T-47-D and BT-20 cells, but not in MDA-MB-231 cells (34). Considering all of these results, promoter A-specific induction of ER
gene expression caused by the demethylating reagent may be explained by the fact that ERF-1 is present in BT-20 cells, while ERBF-1 is not.
In vitro methylation of promoter regions decreased promoter activity to ~5060% of that of unmethylated ones. Thus, we found that methylation of the promoters of the ER gene directly suppressed its transcription, although not completely. Recently, it has been shown that CpG methylation exerts its inhibitory effect indirectly through recruitment of methylated CpG-binding proteins such as MeCP1 and MeCP2 (37,38), with MeCP2 being more abundant than MeCP1 in cells and more tightly bound to DNA containing methyl-CpG pairs in the nucleus (39). Furthermore, it has been reported that MeCP2 forms a co-repressor complex containing a transcriptional suppressor, mSin3, and histone deacetylases, and that this complex indirectly suppresses transcription by modulating chromatin structure (40). Thus, not only methylation of the promoter region but also other mechanisms, such as alteration of the chromatin structure of the gene, would be required for complete suppression of transcription.
In conclusion, we have shown that that the methylation status of the ER gene promoters correlated well with the suppression of the expression of this gene in human breast cancer tissue as well as in human breast cancer cell lines. Methylation of ER
gene promoters directly affected ER
activity in breast cancer cells, though other mechanisms, such as loss of the critical transcriptional factors ERBF-1 and ERF-1, may also be at work in some cases.
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Notes |
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Acknowledgments |
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References |
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