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Cancer Research 67, 11147, December 1, 2007. doi: 10.1158/0008-5472.CAN-07-1371
© 2007 American Association for Cancer Research
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Molecular Biology, Pathobiology, and Genetics

Epithelial to Mesenchymal Transition in Human Breast Epithelial Cells Transformed by 17β-Estradiol

Yong Huang1, Sandra V. Fernandez2, Shirlean Goodwin1, Patricia A. Russo2, Irma H. Russo2, Thomas R. Sutter1 and Jose Russo2

1 W. Harry Feinstone Center for Genomic Research, University of Memphis, Memphis, Tennessee and 2 Breast Cancer Research Laboratory, Fox Chase Cancer Center, Philadelphia, Pennsylvania

Requests for reprints: Jose Russo, Breast Cancer Research Laboratory, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111-2497. Phone: 215-728-4782; Fax: 215-728-2180; E-mail: Jose.Russo{at}fccc.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The estrogen dependence of breast cancer has long been recognized; however, the role of 17β-estradiol (E2) in cancer initiation was not known until we showed that it induces complete neoplastic transformation of the human breast epithelial cells MCF-10F. E2 treatment of MCF-10F cells progressively induced high colony efficiency and loss of ductulogenesis in early transformed (trMCF) cells and invasiveness in Matrigel invasion chambers. The cells that crossed the chamber membrane were collected and identified as bsMCF; their subclones were designated bcMCF; and the cells harvested from carcinoma formation in severe combined immunodeficient mice were designated caMCF. These phenotypes correlated with gene dysregulation during the progression of the transformation. The highest number of dysregulated genes was observed in caMCF, being slightly lower in bcMCF, and lowest in trMCF. This order was consistent with the extent of chromosome aberrations (caMCF > bcMCF >>> trMCF). Chromosomal amplifications were found in 1p36.12-pter, 5q21.1-qter, and 13q21.31-qter. Losses of the complete chromosome 4 and 8p11.21-23.1 were found only in tumorigenic cells. In tumor-derived cell lines, additional losses were found in 3p12.1-14.1, 9p22.1-pter, and 18q11.21-qter. Functional profiling of dysregulated genes revealed progressive changes in the integrin signaling pathway, inhibition of apoptosis, acquisition of tumorigenic cell surface markers, and epithelial-mesenchymal transition. In tumorigenic cells, the levels of E-cadherin, epithelial membrane antigen, and various keratins were low and CD44E/CD24 were negative, whereas SNAI2, vimentin, S100A4, FN1, HRAS, transforming growth factor β1, and CD44H were high. The phenotypic and genomic changes triggered by estrogen exposure that lead normal cells to tumorigenesis confirm the role of this steroid hormone in cancer initiation. [Cancer Res 2007;67(23):11147–57]


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Breast cancer is a malignancy whose dependence on ovarian function was shown by Beatson (1), who induced regression of advanced cancer in premenopausal women by surgically removing the ovaries. Thereafter, the same procedure was also proven to control the progression of metastatic disease (2). The identification of estrogen (E2) production by the ovaries, the isolation of the estrogen receptor (ER) protein, and the greater incidence of ER{alpha}-positive tumors observed in postmenopausal women led to the identification of a strong association between estrogen exposure with increased breast cancer risk (3). Despite the epidemiologic and clinical evidence linking cumulative and sustained exposure to estrogens with increased risk of developing breast cancer (3), the ultimate mechanisms by which estrogens induce cancer and the specific cells they act upon for initiating malignant transformation have not been fully identified. Among the mechanisms of estrogen action, the most widely acknowledged is the binding of the hormone to its specific nuclear ER{alpha}, initiating a signal that is potently mitogenic (4). However, the fact that ER{alpha} knockout mice expressing the Wnt-1 oncogene (ERKO/Wnt-1) develop mammary tumors in response to treatment with estrogen provides direct evidence that E2 may cause breast cancer through a genotoxic, non-ER{alpha}–mediated mechanism (5). This postulate is further supported by the observation that when ovariectomized mice are supplemented with estrogen, they develop a higher tumor incidence with shorter latency time than control animals, even in the presence of the pure antiestrogen ICI-182,780. Experimental studies on estrogen metabolism (6), formation of DNA adducts (7), carcinogenicity (8, 9), mutagenicity (10), and cell transformation (11, 12) have supported the hypothesis that reaction of specific estrogen metabolites, namely catechol estrogen-3,4-quinones (CE-3,4-Q) and, to a much lesser extent, CE-2,3-Q, can generate critical DNA mutations that initiate breast, prostate, and other cancers (13–15).

Our observations that ductal carcinomas originate in lobules type 1 (Lob.1) of the immature breast (16), which are the structures with the highest proliferative activity and highest percentage of ER{alpha} and progesterone receptor–positive cells, provide a mechanistic explanation for the higher susceptibility of these structures to undergo neoplastic transformation when exposed to chemical carcinogens, as shown by in vitro experiments (17). However, the role of ER{alpha}-positive and ER{alpha}-negative cells in the initiation of breast cancer is not clear. The fact that the cells that do proliferate in culture are ER{alpha}-negative suggests that the stem cells that originate cancer are the ER{alpha}-negative proliferating cells. This idea is further supported by our observations that MCF-10F, a spontaneously immortalized ER{alpha}-negative human breast epithelial cell line derived from breast tissues containing Lob.1 and Lob.2 (18), becomes malignant after exposure to the chemical carcinogen benz(a)pyrene (17) and 17β-estradiol (E2; refs. 12, 19).

Breast cancer has been subdivided into five major subtypes: basal-like, Her2 (ERBB2)–overexpressing, normal breast tissue-like, and two subtypes of luminal-like, luminal A and luminal B (20). The luminal-like subtypes display moderate to high expression of ER{alpha} and luminal cytokeratins, whereas the basal-like subtype is negative for both ER{alpha} and ERBB2, with high expression of basal cytokeratins 5 and 17. The ERBB2-overexpressing subtype is also ER{alpha} negative and, like the basal-like tumors, is associated with poorer prognosis as measured as time to development of distal metastasis (20–22). Altogether, these data support the concept that ER-positive and ER-negative tumors may originate from two different cell populations, as postulated earlier (14). In addition to differences inherent to the type of cell in which cancer originates, neoplastically initiated cells lose specific characteristics of epithelial differentiation as the result of their progression toward malignancy. As the epithelial cells lose their polarity and cell-to-cell junctions, regulated in part by the expression of E-cadherin, they acquire characteristics of mesenchymal cells, which lack stable intercellular junctions (21). This epithelial to mesenchymal transition (EMT) leads to exacerbation of motility and invasiveness in many cell types and is often considered a prerequisite for tumor infiltration and metastasis (21).

To outline the pathways through which estrogen acts as carcinogen in the human breast (i.e., either through the receptor pathway or through a genotoxic effect in a specific cell type of the breast), we used an in vitro-in vivo system in which the spontaneously immortalized ER{alpha}-negative human breast epithelial cell (HBEC) line MCF-10F was transformed by treatment with E2 (23). E2-transformed cells progressively express phenotypes of in vitro cell transformation, including colony formation in agar methocel, decreased ductulogenesis, increased invasiveness in a Matrigel invasion system, and tumorigenesis in a heterologous host. Tumors formed in severe combined immunodeficient (SCID) mice by invasive cells and by cell lines derived from those tumors were poorly differentiated ER{alpha}-, progesterone receptor–, and ERBB2-negative adenocarcinomas (24). These characteristics are similar to those of basal cell type primary carcinomas previously described (25). To better understand the molecular events associated with the progressive phenotypic changes that were observed during estrogen-mediated malignant cell transformation, we performed Affymetrix 100k single nucleotide polymorphism (SNP) arrays to measure chromosomal copy number and loss of heterozygosity (LOH), and HG-U133_Plus_2 array for analyzing mRNA expression in MCF-10F cells at different stages of cell transformation. By integrating these data, we were able to identify associations between copy number changes, LOH, and tumorigenic phenotype, as well as the related changes in transcript expression. Functional analyses of these data identified several dysregulated pathways associated with progressive tumorigenic and invasive capacity.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Experimental model of transformation of MCF-10F cells by E2 treatment. These studies were performed using the spontaneously immortalized ER{alpha}-negative human breast epithelial cell line MCF-10F. As it was described in a previous publication, cells cultured and treated with 70 nmol/L E2 were collected 24 h after the last treatment and maintained in culture for 10 additional passages (24). Thereafter, control and E2-treated MCF-10F cells were evaluated for colony formation in agar-methocel, or colony efficiency, ductulogenic capacity in collagen-matrix, invasiveness in Matrigel invasion chambers, and tumorigenic assay in SCID mice, as previously described (24). E2-treated MCF-10F cells formed colonies in agar methocel, exhibiting a high colony efficiency, and lost their ductulogenic capacity in collagen-matrix, forming instead spherical masses; cells exhibiting these characteristics were classified as transformed (trMCF). Briefly, as it was described, trMCF cells were seeded onto Matrigel invasion chambers and at the end of a 22-h incubation period, cells that had crossed the membrane were collected and identified as bsMCF cells (24). The tumorigenic ability of control MCF-10F, trMCF, and bsMCF was tested by injecting them into the mammary fat pad of 45-day-old female SCID mice (24). MCF-10F and trMCF cells did not induce tumors, whereas bsMCF formed tumors. From the tumors formed by the bsMCF cells, four cancer cell lines were derived and identified as caMCF1, caMCF2, caMCF3, and caMCF4 (these cells were previously called E2-70-C5-A1-T1, E2-70-C5-A4-T4, E2-70-C5-A8-T8, and E2-70-C5-A6-T6, respectively; ref. 24). The caMCF1, caMCF2, caMCF3, and caMCF4 were isolated from four tumors from four different animals (24).

Ring cloning and tumorigenesis in mice. The invasive bsMCF cells were plated at low density and cell colonies were isolated using cloning rings. The cells were cultured in DMEM:F12 medium containing 1.05 mmol/L calcium, antimycotics, hormones, growth factors, and equine serum (24). After trypsinization and plating, the clones obtained were identified as bcMCF-1, bcMCF-2, bcMCF-3, bcMCF-4, bcMCF-5, bcMCF-6, and bcMCF-7. The tumorigenic ability of the bcMCF subclones was tested by injecting them into the mammary fat pad of 45-day-old female SCID mice (see Supplementary Table S1).

Affymetrix microarray expression and genotyping assay. MCF-10F cells at three different passages, designated MCF-10F1, MCF-10F2, and MCF-10F3 (passages 135, 137, and 138, respectively); trMCF cells at three different passages, designated trMCF1, trMCF2, and trMCF3 (passages 20, 22, and 23, respectively); bcMCF clones 1, 2, and 3; and caMCF 1, 2, and 3 cells were used for microarray expression and genotyping assay (Fig. 1A ). Total RNA and high molecular weight genomic DNA were isolated using Stat-60 (Tel-Test, Inc.) and the DNA isolation protocol previously described (24). After hybridization, the chips were scanned using GeneChip Scanner 3000. The SNP genotype calls (heterozygous or homozygous) were determined by GTYPE v 4.0. MCF-10F served as the diploid reference for detection of copy number changes by dChip. The human genome release v17 was used to generate the genome information for SNP data analysis. Hierarchical clustering of samples based on copy number was performed using the default parameters in dChip.


Figure 1
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  		Figure 1. A, transformation of MCF-10F cells by E2 treatment. Experimental protocol: MCF-10F cells treated with 70nmol/L E2 that expressed high colony efficiency and loss of ductulogenic capacity in collagen-matrix were classified as transformed (trMCF). Transformed cells that were invasive in a Matrigel Boyden-type invasion chambers were selected (bsMCF) and plated at low density for cloning (bcMCF). MCF-10F, trMCF, bsMCF, and bcMCF were tested for carcinogenicity by injecting them into the mammary fat pad of 45-d-old female SCID mice. MCF-10F and trMCF cells did not induce tumors (canceled arrow); bsMCF and bcMCF formed solid tumors from which four cell lines, identified as caMCF, were derived and proven to be tumorigenic in SCID mice. B and C, chromosome copy number analysis using Affymetrix 100k SNP chips and dChip software. B, display of inferred copy number. MCF-10F at three different passages serves as diploid reference. Pink shade, diploidy; darker red and lighter pink, regions of copy number amplification and deletion, respectively. Gray box, range from 0 to 4 copies (blue curve); red line, baseline for diploidy. C, complete genome view of LOH: yellow, retention of heterozygosity; blue, LOH; white, no information due to lack of SNPs. Each column in B and C represent the different chromosomes; the chromosome number, from 1 to 22 and chromosome X are at the top of B and, at the left of these panels, the different cells are indicated.

 
The intensities of probe sets were calculated by dChip with Perfect-match/Mismatch difference model after invariant-set normalization. Three lists of differentially expressed genes (dysregulated genes) were generated by pairwise comparison using MCF-10F cells as reference. The significance of differential expression was determined by the following combined criteria: (a) the gene in all three samples of at least one group in pairwise comparison is expressed (determined by GCOS "Present" call); (b) difference between the average value of the gene in the two compared groups ≥50; (c) fold change ≥1.7; (d) unpaired t test using log-transformed intensities, followed by Benjamini-Hochberg procedure controlling false discovery rate (q ≤ 0.08). For probe sets representing the same Entrez Gene or UniGene ID, only the one with lowest q value was included in the gene list.

Identification of chromosomes and Gene Ontology categories enriched with differentially expressed genes. The functional profiles were represented by the biological processes in the Gene Ontology (GO) database (26). The number of dysregulated genes in each chromosome or GO category was compared with that of all genes in the HG-U133_Plus_2 chip to determine the significance of the chromosome or GO category. The analysis was performed using Onto-Express,3 with the default selection of statistical method (hypergeometric distribution followed by false discovery rate correction). The three lists of genes dysregulated in trMCF, bcMCF, or caMCF were uploaded into Onto-Express to identify significant GO categories (q ≤ 0.05 with five or more genes). The up- or down-regulated genes were uploaded into Onto-Express separately to identify the individual chromosomes enriched (q ≤ 0.0001) with these genes.

Ingenuity pathway analysis. The differentially expressed genes were uploaded into the Ingenuity Pathway Analysis software.4

Immunocytochemical analysis. The cells were scraped into the medium, centrifuged, and the cell pellets were suspended in 10% phosphate-buffered formalin. After overnight fixation at 4°C, the pellets were embedded in 2% agarose, postfixed in 10% phosphate-buffered formalin, and embedded in paraffin. The following mouse monoclonal antibodies were used: E-cadherin, clone 36 (BD Transduction Laboratories), epithelial membrane antigen (EMA) clone E29 and vimentin, clone V9, both from DakoCytomation, Inc. All sections were lightly counterstained with hematoxylin.

Expression of CD24 and CD44. The microarray expression levels of CD24 and CD44 were validated and further explored by reverse transcription-PCR (RT-PCR) and flow cytometry analysis [fluorescence-activated cell sorting (FACS)] that was performed at the Cell Sorting Facility of the Fox Chase Cancer Center. For RT-PCR, 40 to 80 ng of cDNA were used for PCR amplification in GeneAmp PCR System 9700 (Applied Biosystems). PCR primers were designed to span exons 2 to 18 of CD44, to characterize the expression of alternative splicing variants. The sequence of forward (F) and reverse (R) primers are as follows: CD24, F-GCCAGTCTCTTCGTGGTCTC; R-CTCCATTCCACAATCCCATC; CD44, F-GAGCATCGGATTTGAGACCTG; R-AGCTCCATTGCCACTGTTGAT; β-actin, F-ACCCACACTGTGCCCATCTACGA; R-AGCTGGAAGCAGCCGTGGCCAT. The density of the bands was quantified using a scanner (Expression 836XL, EPSON) and measurement software (Image J, W.S. Rasband, NIH, MD).5 FACS was performed using FITC mouse anti-human CD24 and phycoerythrin mouse anti-human CD44 antibodies from BD Biosciences. Cells (106) were stained and analyzed by FACS using a LSR II system (BD Biosciences). These experiments were repeated twice.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Analysis of chromosome copy number and LOH in neoplastically transformed MCF-10F cells. MCF-10F cells that after treatment with E2 expressed high colony efficiency and loss of ductulogenic capacity in collagen-matrix represented the first level of in vitro transformation. Cells expressing these two variables were classified as transformed (trMCF), which after further selection for invasiveness in a Matrigel invasion chamber originated the second level of transformation: the invasive (bsMCF) and the cloned (bcMCF) cells (Fig. 1A). The bsMCF cells formed tumors in SCID mice from which four cell lines, caMCF, were derived (Fig. 1A). By ring cloning, seven subclones were isolated from the invasive bsMCF cells: bcMCF-1, bcMCF-2, bcMCF-3, bcMCF-4, bcMCF-5, bcMCF-6, and bcMCF-7. All the bcMCF subclones produced invasive poorly differentiated tumors in SCID mice with different morphologic phenotypes: spindle cell type (bcMCF-1 and bcMCF-4), epithelial cell type (bcMCF-2, bcMCF-6, and bcMCF-7), and with mix features of spindle and epithelial type (bcMCF-3 and bcMCF-5; Supplementary Table S1). As it was previously reported, MCF-10F cells were seeded on Boyden chamber as control; cells that passed through the membrane were selected, expanded, and injected in SCID mice; these cells did not produce tumors (24).

Using the 100k SNP GeneChip Mapping Array set, the DNA from MCF-10F, trMCF, bcMCF, and caMCF were analyzed for the structure of chromosomes 1 to 22 and X at very high resolution (Fig. 1B and C). Changes in copy number (Fig. 1B) and LOH (Fig. 1C) were progressive in these cells. The three different passages of MCF-10F exhibited identical copy number, and these data served as the reference for copy number analysis. Only small and scattered chromosome gains were observed in trMCF cells and most alterations were observed in the bcMCF and caMCF (Fig. 1B). Large fragments of chromosome gain were observed only in the telomeric ends at 1p36.12-pter and 5q21.1-qter in both bcMCF and caMCF, and 13q21.31-qter in bcMCF (Fig. 1B, dark pink areas). Almost no chromosome loss was detected in the trMCF cells and very few in bcMCF1 clone, whereas bcMCF2 and bcMCF3 clones and the three caMCF cell lines exhibited loss of the whole chromosome 4 and loss of 8p11.21-23.1 (Fig. 1B, pale pink areas). Loss of 3p12.1-14.1, 9p22.1-pter, and 18q11.2-qter was observed in the three lines of caMCF (Fig. 1B). Deletion was also observed in 7pter and 13pter of bcMCF2, but not in the rest of tumorigenic cells, indicating that such deletion is not required for the expression of tumorigenic capacity.

The subclone bcMCF1 displayed higher frequency of chromosome 8 amplification (Supplementary Fig. S1A) and lower frequency of chromosome 4 deletion when compared with the subclones bcMCF2 and bcMCF3 (Supplementary Fig. S1B). Also, subclones bcMCF2 and bcMCF3 showed a small deletion on chromosome 8 that bcMCF1 did not have (Fig. 1B, pale pink area in bcMCF2 and bcMCF3). However, sample clustering based on copy number profile grouped the bcMCF1 in the tumorigenic class together with bcMCF2; bcMCF3; and caMCF1, caMCF2, and caMCF3 (Supplementary Fig. S1C), and this was confirmed by sample clustering based on gene expression profile (see below EMT phenotype). The cells trMCF1, trMCF2, and trMCF3 were clustered as nontumorigenic (Supplementary Fig. S1C). Furthermore, all bcMCF clones were shown to form tumors in SCID mice (Supplementary Table S1). Therefore, the isolation of subclones may afford the possibility to detect minimal regions of copy number change associated with neoplastic cell transformation.

This cell transformation model involves treatment of immortal MCF-10F cells and selection for a transformed phenotype followed by further selection for invasiveness and then tumorigenesis (Fig. 1A). As major copy number changes and LOH were observed in chromosome 4 (Fig. 1B and C), we present this chromosome at higher resolution. Interestingly, the copy number loss in chromosome 4 detected in bcMCF can be observed in trMCF cells (Supplementary Fig. S2, arrows), albeit at a level that does not reach statistical significance.

Chromosomes enriched with the dysregulated genes. Three gene lists were generated by pairwise comparison of trMCF, bcMCF, or caMCF with the reference MCF-10F.6 The highest number of total dysregulated genes was observed in caMCF cells (1,306 genes, 340 up-regulated versus 966 down-regulated), slightly lower in bcMCF (1,236 genes, 160 up-regulated versus 1,076 down-regulated), and the lowest in trMCF cells (260 genes, 45 up-regulated versus 215 down-regulated). This order was consistent with the extent of chromosome aberrations: number of regions and size in caMCF > bcMCF >>> trMCF (Fig. 1B).

To further investigate the relationship between transcript levels and copy number, we analyzed the list of up- and down-regulated genes using Onto-Express software, to identify chromosomes significantly enriched (or overrepresented) with dysregulated genes. The chromosomes enriched with up-regulated (Fig. 2A ) or down-regulated (Fig. 2B) genes also showed large regions of amplification or deletion (Fig. 1B), respectively. For example, chromosomes 18, 4, and 8 were significantly overrepresented by down-regulated genes in caMCF cells (Fig. 2B, caMCF), and, correspondently, showed large regions of deletion in caMCF (Fig. 1B). Similarly, chromosome 4 was overrepresented by down-regulated genes in bcMCF cells (Fig. 2B, bcMCF), and showed the largest copy number loss in these cells (Fig. 1B). Chromosomes 1 and 5 were significantly overrepresented by up-regulated genes in both bcMCF and caMCF cells (Fig. 2A, bcMCF and caMCF), and, correspondently, showed largest copy number amplification (Fig. 1B). However, some exceptions were noteworthy. Chromosome 13 showed a high frequency of SNP amplification (50%) in bcMCF (Supplementary Fig. S1A), but was not enriched with up-regulated genes. In fact, there were only six genes up-regulated in this region. Moreover, this amplification was not maintained in caMCF (Fig. 1B), indicating that this aberration was not required for tumorigenesis. Also, chromosome 11 was overrepresented by down-regulated genes in trMCF (Fig. 2B, trMCF), although trMCF cells did not show any copy number deletion (Fig. 1B). This might imply that certain epigenetic modifications, instead of copy number loss, are related to the observed decrease in the number of genes expressed on chromosome 11 of trMCF cells.


Figure 2
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  		Figure 2. A and B, chromosomes enriched with differentially expressed genes. The top three chromosomes (ranked by q value) in trMCF, bcMCF, and caMCF cell lines enriched with up-regulated (A) or down-regulated (B) genes. *, the chromosomes overrepresented (q ≤ 0.0001) by up- or down-regulated genes. A q value of 0.0 indicates a value smaller than 10–5. C, canonical pathways altered in progressive malignant cell transformation. The pathways significantly enriched with the dysregulated genes are identified by Ingenuity Pathway Analysis. The number of differentially expressed genes in each pathway is labeled above the corresponding cell. The genes identifying the integrin signaling pathway are displayed in Table 2; those in the other five canonical pathways are displayed in Supplementary Table S3. ERK/MAPK, extracellular signal-regulated kinase/mitogen-activated protein kinase.

 

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  		Table 2. Dysregulated genes involved in integrin signaling pathway

 
Biological processes enriched with the dysregulated genes. GO analysis identified eight biological processes enriched with dysregulated genes in trMCF, bcMCF, and caMCF cells, suggesting that changes in these processes were involved in the in vitro transformation phenotype (Table 1 ). Two additional processes, DNA replication and nucleosome assembly, were found only in the trMCF cells. The DNA replication process in trMCF cells contained six genes. Each of these genes was up-regulated, including replication factor C, 2.1-fold; SET translocation, 1.8-fold; DNA directed polymerase epsilon 2, 2.7-fold; ribonucleotide reductase M2 polypeptide, 1.7-fold; minichromosome maintenance deficient 4, 1.7-fold; and topoisomerase (DNA) II{alpha}, 1.8-fold. Moreover, 11 biological processes were found uniquely in the tumorigenic cells bcMCF and caMCF, suggesting that they were involved in the process of neoplastic transformation (Table 1). Most of the genes in the 11 GO categories showed decreased expression relative to MCF-10F. For example, the fraction of down-regulated genes was 24 of 28 in "apoptosis" (Supplementary Table S2), 13 of 16 in "positive regulation of IKB/NFKB cascade," and 19 of 21 in "lipid metabolism" (data not shown).


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  		Table 1. Biological processes enriched with differentially expressed genes in tumorigenic and nontumorigenic cells

 
Identification of significant canonical pathways. Ingenuity Pathway Analysis revealed six canonical pathways significantly dysregulated in one or more groups (Fig. 2C). The genes associated with each of these pathways, integrin signaling, pyrimidine metabolism, transforming growth factor β (TGF-β) signaling, glutathione metabolism, extracellular signal-regulated kinase/mitogen-activated protein kinase signaling, and amyloid processing are listed in Table 2 and Supplementary Table S3.

Integrin signaling was the most significantly altered pathway in each of the three cell lines, indicating a continuum change within this pathway throughout the progressive, malignant cell transformation of MCF-10F cells. In this pathway, eight genes were down-regulated in trMCF cells (Table 2). Of interest, expression levels of several of these decreased genes, such as ITGB6, LAMA3, and LAMC2, were inhibited to even a greater extent in the tumorigenic cells. In contrast, fibronectin 1, which was completely suppressed (–18.9 fold, "Absent") in trMCF, was strongly induced in bcMCF (4.9-fold) and caMCF (2.8-fold; Fig. 3A, FN1 ). In comparison with the trMCF, the number of dysregulated genes associated with integrin signaling was much higher in the bcMCF and caMCF cells, showing both increased and decreased levels of expression (Table 2).


Figure 3
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  		Figure 3. Expression profile of EMT markers and their regulators during malignant cell transformation. A, a list of EMT markers and promoting genes was generated a priori by literature search (Supplementary Table S5). Hierarchical clustering of cell lines and genes was performed using dChip software. Two sample clusters ({kappa} and {lambda}) and two gene clusters ({alpha} and β) were identified. Red, white, and blue, level above, at, and below mean expression, respectively. B, detection of epithelial and mesenchymal markers by immunocytochemistry. a, histologic sections of MCF-10F cells, reacted with preimmune mouse serum, were used as the negative control (x100); b, MCF-10F reacted for EMA (x100); c, MCF-10F reacted for E-cadherin (x100); d, MCF-10F reacted for vimentin (x100); e, trMCF cells reacted with preimmune mouse serum used as negative control (x100); f, g, and h, trMCF cells reacted for EMA, E-cadherin, and vimentin, respectively (x100); i, bsMCF cells reacted with preimmune mouse serum as a negative control (x100); j, k, l, bsMCF cells reacted for EMA, E-cadherin, and vimentin, respectively (x100); m, caMCF tumor cell line cells reacted with preimmune mouse serum used as negative control (x100); n, o, p, caMCF tumor cell lines reacted for EMA, E-cadherin, and vimentin, respectively (x100); q and r, invasive ductal carcinoma of the breast as positive control and immunoreacted for EMA and E-cadherin, respectively (x100); s, histologic section of an invasive adenocarcinoma immunoreacted for vimentin (x100).

 
Levels of expression of several genes involved in glutathione metabolism were decreased in the bcMCF and caMCF (Supplementary Table S3). Multiple glutathione S-transferase (GST) isoforms were down-regulated, including GST {kappa}1, {omega}2, pi, MGST 2, and MGST3. Moreover, glutamate cysteine ligase, the enzyme catalyzing the first rate-limiting reaction in glutathione biosynthesis, was also reduced in the tumorigenic cells. Glutathione is a cellular antioxidant and a substrate for GST, which catalyzes the conjugation reactions with electrophiles. Repressed antioxidant activity is associated with the genomic damage and cancer incidence induced by exposure to the cytotoxic free radical and reactive oxygen species (27).

EMT phenotype. Analysis of the GO cellular component also identified categories distinct between the nontumorigenic and tumorigenic cells. The "intermediate filament" component was found in the bcMCF (q = 2.0 x 10–5, 12 genes) and caMCF cells (q = 0.0, 13 genes), but not in trMCF cells (q = 0.4, 1 gene; Supplementary Table S4). Numerous cytokeratins were suppressed or absent, whereas vimentin was strongly induced in bcMCF (7.0-fold) and caMCF (8.1-fold). Because of these findings, we generated a gene list from published literature for epithelial and mesenchymal markers and their regulators (Supplementary Table S5). The 52 genes in the list were filtered by low stringency criteria of combined coefficient of variation (CV) >0.3 and "Present calls" in >30% of the samples. The 27 genes passing these criteria were used for sample and gene clustering (Fig. 3A). Two sample groups and two gene groups were identified. The nontumorigenic MCF-10F and trMCF cells were grouped into sample cluster {kappa}, whereas the tumorigenic bcMCF and caMCF cells were grouped into cluster {lambda}. On the other side, the genes were grouped into clusters {alpha} and β based on their expression pattern. The epithelial markers E-cadherin (CDH1), occludin (OCLN), desmoplakin (DSP), and cytokertins (KRT) were decreased, whereas the mesenchymal markers fibronectin (FN1), vimentin (VIM), and N-cadherin (CDH2) were increased in bcMCF and caMCF cells (Fig. 3). By real-time RT-PCR, it was confirmed that the expression of FN1, S100A4, SNAI2, HRAS, and TGFβ1 were increased, whereas CDH1 (E-cadherin) was decreased in bcMCF and caMCF cells (Supplementary Fig. S3). Immunocytochemical analysis using antibodies against EMA (also called MUC1) and E-cadherin displayed significant loss of these epithelial markers (Fig. 3B, j, k, n, o) and increased expression of the mesenchymal marker vimentin (Fig. 3B, l, p) in tumorigenic cells. These findings confirmed the EMT phenotype revealed by gene expression profile in Fig. 3A. Interestingly, whereas most trMCF cells were E-cadherin positive (+) and vimentin negative (–), a few cells were found to be E-cadherin (–) (Fig. 3B, g, arrow) and vimentin (+) (Fig. 3B, h, arrow).

Characterization of molecular markers of tumorigenic cells. The cell surface molecule CD44+/CD24–/low phenotype has been identified as marker for tumor-initiating cells (28). Therefore, we examined the expression pattern of these markers in relation to the tumorigenic capacity of the cells in our model. Microarray data revealed that CD44 was slightly decreased in trMCF but increased in bcMCF and caMCF cells, whereas CD24 was completely lost in both bcMCF and caMCF (Figs. 4A and B ). The loss of CD24 in bcMCF and caMCF was independently shown by RT-PCR (Fig. 4C, middle). The alternative splicing variants of CD44 were identified by the size of their PCR products and further validated by DNA sequence analysis (data not shown). Only the variants of CD44H and CD44E were expressed (Fig. 4C, top). CD44H was significantly increased in both bcMCF and caMCF, whereas CD44E was completely lost in these cells. The ratio of CD44H/CD44E was significantly increased in trMCF, bcMCF, and caMCF (Fig. 4D). The CD44 and CD24 gene expression in the different cell lines correlated well with the cell surface protein expression studied by FACS analysis (Supplementary Fig. S4). In addition, the ESA (epithelial-specific antigen, also termed as Ep-CAM, or tumor-associated calcium signal transducer 1), was completely lost in bcMCF and caMCF, but not changed in trMCF cells (see gene list online6).


Figure 4
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  		Figure 4. Characterization of the expression of CD44 and CD24. A and B, microarray expression values of CD44 and CD24, respectively. Significance: P < 0.05 in unpaired t test compared with MCF-10F. C, RT-PCR validation for CD44 (top) and CD24 (middle) expression levels. D, densitometry ratio of CD44H to CD44E detected in RTPCR.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This study integrates structural and functional genomic data analyses to elucidate the progressive molecular events in the E2-mediated malignant transformation of ER (–) HBECs. Genomic aberrations progressively accumulated as the cells expressed more aggressive phenotypes (i.e., in the tumorigenic bcMCF and caMCF) in comparison with the nontumorigenic trMCF cells. Accordingly, the number of genes with altered levels of expression was greater in the tumorigenic cells, as where the chromosomes enriched with up- or down-regulated genes. Importantly, the 12 samples were correctly classified into tumorigenic or nontumorigenic groups based on the profile of copy number changes, indicating that in our model changes in copy number provide a genomic signature for the tumorigenic phenotype. Together, these findings revealed an intrinsic link between E2-induced copy number changes, gene expression alterations, and tumorigenesis in ER{alpha} (–) HBECs.

The integrin signaling pathway was the most significantly altered pathway in the progression of the neoplastic transformation. Integrins function as heterodimeric receptors for extracellular matrix proteins, mediating cell anchorage. The capacity of cells to survive and proliferate in the absence of integrin-mediated adhesion in vitro strongly correlates with tumorigenesis in vivo (29). The integrin signaling pathway was enriched with dysregulated genes in trMCF, bcMCF, and caMCF, indicating that this pathway was affected in early stages of cell transformation. In addition, GO analysis revealed enrichment of dysregulated genes in the apoptotic process in tumorigenic cells (Supplementary Table S2) and resistance to apoptosis is a hallmark of tumorigenesis. Therefore, suppression of apoptosis in the tumorigenic cells might be a potential mechanism that confers survival onto these cells with disrupted integrin signaling and anchorage-independent growth (24).

EMT involves dedifferentiation of polarized epithelial cells to a migratory fibroblastoid phenotype, a phenomenon that is increasingly considered to be an important event during cancer progression and metastasis (30, 31). Cells selected from MCF-10F by Boyden chamber are nontumorigenic (24), whereas all bcMCF cells displayed consistent chromosomal aberrations and formed tumors in SCID mice in our model. Although it cannot be totally ruled out, it is very unlikely that bcMCF cells are derived from preexisting mesenchymal cells.

EMT is accompanied by a profoundly altered mesenchymal gene expression program, which is characterized by loss of epithelial keratins and induction of mesenchymal vimentin (32). Induction of S100A4 is an important early event in the pathway toward EMT (33). The hallmark of EMT is the loss of expression of the cell adhesion molecule E-cadherin (34). E-cadherin is a cell-cell adhesion molecule that participates in homotypic, calcium-dependent interactions to form epithelial adherens junctions. This function is critical in the development and maintenance of a polar epithelium. GSK3β was down-regulated in bcMCF and caMCF (Fig. 3A). SNAI2 was down-regulated in trMCF but increased in bcMCF and caMCF (Fig. 3A). It was shown that inhibition of GSK3β resulted in the up-regulation of SNAI1 and down-regulation of E-cadherin in vivo. SNAI2 (or SLUG) and SNAI1 (or SNAIL) belong to the Snail family of proteins; both contain an NH2-terminal repression domain and a COOH-terminal zinc-finger DNA-binding domain. Snail proteins repress the transcription of E-cadherin (35–39). E-cadherin loss is believed to contribute to both cancer development and progression (35). HRAS and TGFβ1 were up-regulated in bcMCF and caMCF (Supplementary Table S3). It has been shown that HRAS cooperates with TGFβ1 to cause EMT and also it interacts with CD44 directly by increasing its expression (40, 41). TGFβ1 via HMGA2, which was also up-regulated in bcMCF and caMCF, regulates the expression of TWIST, SNAI1, and SNAI2 (42). EMT is associated with higher tumor grade, high motility index, and ER{alpha} (–) status (43). Therefore, these findings revealed an intrinsic link between EMT and tumorigenic capacity in our model, which, in part, may explain the poor prognosis of ER{alpha} (–) human breast carcinomas.

CD44+/CD24–/low is the cell surface marker of tumorigenic breast cancer cells in which the tumorigenic capacity is further increased by additional expression of ESA (28). These cells are characterized by a 186-gene "invasiveness" gene signature that is associated with risk of death and metastasis in breast cancer (44). CD24 encodes a small, heavily glycosylated cell-surface adhesion protein. CD44 undergoes extensive alternative splicing within its central region spanning exon 6a to 14, also termed as variable exon v1 to v10. The two variants expressed in our model are mRNA precursor variant 3 with exon 6a to 11 spliced out and variant 4 with exon 6a to 14 spliced out, corresponding to CD44E and CD44H, respectively (45). The CD44H is mainly expressed on cells of lymphohematopoietic origin; it plays an important role in cell adhesion and its expression promotes tumor cell migration (46). CD44E is preferentially expressed on epithelial cells and it is involved in the recognition of a common determinant in CD44H and CD44E promoting homotypic cellular aggregation (47). Microarray and RT-PCR analysis of CD44 and CD24 revealed an expression pattern of CD44Hhigh/CD44E–/CD24– in bcMCF and caMCF. The increased ratio of CD44H/CD44E in trMCF cells might represent an early marker for E2-transformed HBECs. The significant increase of CD44H and complete loss of CD44E might be a novel phenotype associated with the tumorigenic capacity. In addition, the loss of ESA in bcMCF and caMCF indicated that ESA expression is not required for the tumorigenic capacity in our model.

Two recent studies, using a GeneChip containing 1495 SNPs (48) or comparative genomic hybridization (49), have shown that LOH and allelic loss in 4p and 5q occur more frequently in subtypes of breast cancer characterized as ER{alpha} (–). SNP mapping reveals that LOH in 4p14-15.3, 5q11-32, and 18q22-23 are significantly associated with a gene expression profile in basal-like subtype (48). In our model, the tumor cells caMCF also showed LOH in all these three regions. Moreover, it has been shown that the CD44+/CD24– phenotype is associated with mesenchymal phenotype and invasion in breast cancer cell line, and may define breast cancers of basal/myoepithelial origin rather than luminal origin (50). These findings indicate a potential correlation of our model with the basal-like subtype. However, the bcMCF and caMCF cells displayed low or absent expression of both luminal and basal cytokeratins in microarray analysis and ERBB2 (–) in immunocytochemical staining (data not shown). Therefore, they cannot simply be classified into basal-like, ERBB2 (+), luminal A or B, or normal breast-like subtype (20). Carey et al. (51) has identified an "unclassified" subtype using immunohistochemical markers for the classification of breast cancer tissue. This subtype is characterized as ER{alpha} (–), ERBB2 (–), and progesterone receptor (–), which, for these markers, is the same phenotype as the basal-like subtype. Unlike the basal-like subtype, the unclassified subtype is cytokeratin 5 (–). It also displays a histologic grade and survival prognosis most close to that of basal-like subtype (51). Further comparison of the tumorigenic cells in our model to clinical basal-like or unclassified subtype is warranted.

Our results support the concept that E2-induced breast cancer is a polygenic disease having a large range of genomic instabilities. E2 and/or its metabolites can directly cause genomic aberrations without the mediation of ER{alpha}. The genomic aberrations lead to changes in gene expression, which result in disrupted integrin signaling and apoptotic pathways, and epithelial to mesenchymal transition. These functional changes lead to colony formation in agar-methocel, loss of ductulogenesis in collagen matrix, invasiveness in vitro, and tumor growth in SCID mice in vivo (24). However, MCF-10F is a spontaneously immortalized cell line harvested from a woman's breast that was free of malignancies, but had a diagnosis of benign fibrocystic disease (52). Hence, we cannot rule out its inherited susceptibility to estrogen and the disposition to tumorigenesis. Therefore, more normal primary human breast epithelial cell lines should be studied to validate and elaborate on the molecular mechanisms unveiled by our results.


   Acknowledgments
 
Grant support: Department of Defense grants DMAD17-00-1-0247 and DMAD17-03-1-0229 and the W. Harry Feinstone Center for Genomic Research.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

We thank Dr. Joseph Testa and the Research Cytogenetics and Genomics Facility of the Fox Chase Cancer Center for the performance and analysis of metaphase-CGH and karyotyping.

The microarray data have been deposited in National Center for Biotechnology Information Gene Expression Omnibus (series accession no. GSE5116) at http://www.ncbi.nlm.nih.gov/geo/.


   Footnotes
 
Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/), and http://www.fccc.edu/research/pid/russo-j/index.html.

Present address for Y. Huang: Section of Genetic Medicine, Department of Medicine, University of Chicago, Chicago, IL 60637.

3 http://www.geneontology.org/GO.tools.microarray.shtml#onto-e Back

4 http://www.ingenuity.com Back

5 http://rsb.info.nih.gov/ij Back

6 http://feinstone.memphis.edu/data/Estrogen Back

Received 4/12/07. Revised 9/14/07. Accepted 9/25/07.


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