Cigarette smoke induces epithelial to mesenchymal transition and increases the metastatic ability of breast cancer cells
© Di Cello et al.; licensee BioMed Central Ltd. 2013
Received: 8 March 2013
Accepted: 25 July 2013
Published: 6 August 2013
Recent epidemiological studies demonstrate that both active and involuntary exposure to tobacco smoke increase the risk of breast cancer. Little is known, however, about the molecular mechanisms by which continuous, long term exposure to tobacco smoke contributes to breast carcinogenesis because most previous studies have focused on short term treatment models. In this work we have set out to investigate the progressive transforming effects of tobacco smoke on non-tumorigenic mammary epithelial cells and breast cancer cells using in vitro and in vivo models of chronic cigarette smoke exposure.
We show that both non-tumorigenic (MCF 10A, MCF-12A) and tumorigenic (MCF7) breast epithelial cells exposed to cigarette smoke acquire mesenchymal properties such as fibroblastoid morphology, increased anchorage-independent growth, and increased motility and invasiveness. Moreover, transplantation experiments in mice demonstrate that treatment with cigarette smoke extract renders MCF 10A cells more capable to survive and colonize the mammary ducts and MCF7 cells more prone to metastasize from a subcutaneous injection site, independent of cigarette smoke effects on the host and stromal environment. The extent of transformation and the resulting phenotype thus appear to be associated with the differentiation state of the cells at the time of exposure. Analysis by flow cytometry showed that treatment with CSE leads to the emergence of a CD44hi/CD24low population in MCF 10A cells and of CD44+ and CD49f + MCF7 cells, indicating that cigarette smoke causes the emergence of cell populations bearing markers of self-renewing stem-like cells. The phenotypical alterations induced by cigarette smoke are accompanied by numerous changes in gene expression that are associated with epithelial to mesenchymal transition and tumorigenesis.
Our results indicate that exposure to cigarette smoke leads to a more aggressive and transformed phenotype in human mammary epithelial cells and that the differentiation state of the cell at the time of exposure may be an important determinant in the phenotype of the final transformed state.
Multiple epidemiological studies have established the association between active and involuntary exposure to tobacco smoke and increased risk of breast cancer. The link, which has been a controversial topic for many years, was initially demonstrated in younger, primarily premenopausal women [1, 2], and subsequently in postmenopausal women [2–4]. The epidemiological evidence is backed up by several studies showing that tobacco carcinogens are present and active in the breast tissue of smokers [1, 5–7]. Except for the documented formation of mutagenic DNA adducts [6, 8], it is unclear how these compounds affect cell behavior in the breast contributing to cancer development, progression, and metastasis. Emerging evidence suggests that cigarette smoke condensate (CSC), or aqueous cigarette smoke extract (CSE) can induce changes in morphology and gene expression indicative of epithelial to mesenchymal transition (EMT) in immortalized human bronchial epithelial cells  and in lung carcinoma cells [9, 10]. This implies the acquisition of mesenchymal properties, including traits that are associated with malignancy such as increased motility and invasiveness . Although these studies provide some mechanistic data on tobacco smoke tumorigenesis in lung, data for breast cancer are limited. In this work we have set out to investigate the progressive transforming effects of tobacco smoke on non-tumorigenic mammary epithelial cells and breast cancer cells using in vitro and in vivo models. Our results indicate that exposure to cigarette smoke leads to a more aggressive and transformed phenotype in human mammary epithelial cells, and that the differentiation state of the cell at the time of exposure may be an important determinant in the phenotype of the final transformed state.
Cigarette smoke induces anchorage-independent cell growth, migration, invasion and morphological changes in mammary epithelial cells and breast cancer cells
CSE confers the ability to colonize mammary ducts and metastasize to mammary epithelial cells and breast cancer cells, respectively
CSE causes changes in stem cell markers in MCF 10A and MCF7 cells
Exposure of mammary epithelial cells to CSE affects the expression of genes associated with EMT and tumorigenesis
After many years of debate, there is now ample proof that tobacco smoke increases the risk of breast cancer. Multiple studies, including some published after the last Surgeon General Report  and IARC monographs on the subject  show that active and passive exposure to tobacco smoke increases the risk of breast cancer in both premenopausal and postmenopausal women [1–5]. With the epidemiological evidence now conclusive, the task remains to investigate the molecular mechanism by which exposure to tobacco smoke, either voluntary or involuntary, leads to increased breast cancer risk .
The response of breast epithelial cells and breast cancer cell to cigarette smoke has been previously examined [15, 16, 30, 31], but these studies focused on short-term treatment (up to one week) while we have analyzed the effect of continuous long-term exposure. We demonstrated that chronic exposure to tobacco smoke in the form of CSE or CSC can alter the phenotype of mammary epithelial cells, promoting the acquisition of mesenchymal traits such as increased anchorage-independent growth, motility, invasion, and the expression of markers associated with self-renewal and tumor initiation. Numerous groups have demonstrated the emergence of a CD44+/CD24-/low stem-like signature from CD44+/low/CD24+ cells upon the induction of an EMT phenotype characterized by loss of E-cadherin and gain of vimentin [11, 32]. The CD44+/CD24-/low phenotype has been consistently associated with self-renewing mammary epithelial cells, which are also more tumorigenic and basal-like than CD44+/CD24+ cells . Similarly, we showed that treatment of MCF 10A cells with CSE leads to the emergence of a CD44hi/CD24low population, and our in vivo experiments demonstrated that CSE-treated MCF 10A cells have increased survival and colonization ability. Although MCF 10A cells did not become malignant, treatment of the MCF7 cancer cell line led to increased metastatic potential, consistent with published evidence that the differentiation state of the cell of origin is a strong determinant of the cellular phenotype of the final transformed state . Other studies in animal models have previously shown that tobacco smoke can increase the risk of metastasis from breast cancer, but this has been attributed mainly to smoking-induced inhibition of host antitumor immune defenses, or to damage of the host tissue [35, 36]. In contrast, our data from ex vivo exposure followed by orthotopic or subcutaneous transplantation into mice indicate that tobacco smoke can directly affect the ability of breast epithelial cells to invade or metastasize, independent of other cigarette smoke effects on the host and stromal environment.
The phenotypical alterations induced by cigarette smoke were accompanied by multiple gene expression changes. We concentrated our analysis on genes associated with EMT, loss of tumor suppression and the acquisition of malignancy traits. Our data indicates that ERβ is epigenetically repressed by tobacco smoke, which is consistent with a recent study showing that methylation of ERβ is a frequent event in breast cancer . Contrary to the better known and structurally similar ERα, ERβ does not induce mitogenic response and can reduce basal, hormone-independent cell proliferation . ERβ is widely expressed in normal mammary epithelium, but frequently lost in breast cancer, where its presence generally correlates with better prognosis [28, 38]. Knock down of ERβ in MCF 10A or MCF7 cells was shown to cause a significant growth increase of both cell types in a ligand-independent manner , while expression of exogenous ERβ in the receptor negative breast cancer cell line MDA-MB-231 inhibited proliferation . Cigarette smoke also caused downregulation of claudin 1, 3, 4, 7, and 8. The claudins are integral components of tight junctions, and their expression in cancer appears to be tissue specific, with some claudins downregulated in certain tumors and upregulated in others . A small subgroup of breast cancer has been identified as expressing low levels of claudins, and is referred to as the “claudin low” group [41–43]. Claudin low tumors represent 12-13% of breast cancers, are generally basal like, and overexpress EMT markers [41–43]. Mouse claudin-low tumors generated in a p53-null animal model were found to be markedly enriched in tumor-initiating cells . Consistently, our claudin-low CSE-treated breast cells are more tumorigenic than untreated cells, and exhibit gene expression changes indicative of EMT, such as downregulation of E-cadherin and occludin, and upregulation of N-cadherin, fibronectin and vimentin. Downregulation of occludin can reduce cancer sensitivity to apoptogenic factors by modulating apoptosis-associated genes. In addition, occludin decreases cellular invasiveness and motility, thus its downregulation can potentially favor cancer metastasis . The downregulation of occludin and claudin 1  may also be the result of epigenetic regulation, since we have observed increased methylation at the promoter of these genes and in the case of claudin 1, the gene can be re-expressed with demethylating agents such as 5-azacytidine and decitabine . CSE treatment upregulated TGFBR3 and TGFB2 in MCF 10A cells, which is consistent with the reported observation that endothelial cells undergoing EMT express TGFBR3, and TGFBR3-specific antisera can inhibit mesenchyme formation and migration . Moreover, ectopic overexpression of TGFBR3 in non-transforming ventricular endothelial cells conferred transformation in response to TGFB2 . Since we observed upregulation of TGFBR3 and TGFB2 in MCF 10A cells that are undergoing EMT-like changes, but are not completely transformed by cigarette smoke, our results suggest that overexpression of these two genes by cigarette smoke may be a component of EMT that is not associated with transformation. Alternatively, this could be a very early event in transformation and cancer development. We also observed that the EMT-promoting transcription factors TWIST1, TWIST2, ZEB1, ZEB2 and FOXC2 were upregulated, while FOXC1 and SNAI1 (Snail) were downregulated by CSE. Except for the decreased SNAI1, these data are consistent with recent reports that the MDA-MB-231 and MDA-MB-435 basal B cell lines express higher levels of fibronectin, N-cadherin, SNAI1 and ZEB2, and lower E-cadherin and FOXC1 than the luminal epithelial cell line, MCF7 . The same study showed that overexpression of TWIST1, as well as the EMT-promoting factor TGF-β1, consistently upregulates ZEB1 and ZEB2 and FOXC2 in human mammary epithelial cells. Interestingly, TGF-β1 is up-regulated by TWIST1, but is not required for TWIST1-induced up-regulation of FOXC2, which occurs in mammary epithelial cells overexpressing TWIST1 even in the presence of a TGF-β signaling inhibitor . Taken together our observations in the MCF 10A breast epithelial cell line exposed to CSE are consistent with a model of EMT where TWIST drives the transition and upregulates FOXC2, ZEB1 and ZEB2, with potential involvement of TGFβ signaling.
Our results indicate that chronic, long-term exposure to cigarette smoke leads to a more aggressive and transformed phenotype in human mammary epithelial cells, and that the differentiation state of the cell at the time of exposure may be a critical determinant in the phenotype of the final transformed state. Non-malignant, human mammary epithelial cells (MCF 10A) exposed to cigarette smoke in the form of CSE survived intraductally in a mouse mammary gland many months beyond their normal capacity, and breast cancer cells which normally do not metastasize in mice (MCF7), formed metastatic colonies in the lung. All CSE-treated cell lines showed EMT-like behavior including increased anchorage-independent growth, increased motility and invasiveness, and we observed an increase in markers of self-renewing cells, along with accompanying gene expression changes indicative of EMT and malignancy.
Cell culture model of exposure to cigarette smoke
Cigarette smoke extract (CSE) was prepared weekly by burning 2 complete 1R3F cigarettes (Kentucky Tobacco Research and Development Center, Lexington, KY, USA) and drawing the smoke by vacuum into 10 ml of sterile PBS. CSE concentration was evaluated by measuring the optical density at 502.4 nm, and diluted to O.D. = 0.10±0.01 . This solution was considered 100% CSE. The concentration of nicotine was evaluated by mass spectrometry as previously described . The 100% CSE contained 253±22 μg/ml of nicotine which is equivalent to 0.2 cigarettes/ml. Cigarette smoke condensate (CSC) was purchased from Murty pharmaceuticals (Lexington, KY, USA) and is prepared by smoking a 1R3F cigarette on a smoking machine and collecting the particulate matter from the side stream smoke onto a filter for extraction with DMSO. Cell lines were purchased from ATCC (Manassas, VA, USA) and cultured with either CSE, or CSC refreshing the media and additives twice a week (see Additional file 3 for media formulation).
In vitro transformation assays
Anchorage-independent growth was assessed by seeding the cells on soft agar (0.4% top layer, 0.8% bottom layer); and counting the colonies after 14 days using an inverted microscope and 0.005% crystal violet for staining. Cell migration and invasion was assessed in Boyden chambers using 8μm-pore inserts, with or without matrigel coating (BD Bioscience, San Jose, CA, USA) according to the manufacturer’s instructions.
NOD-SCID and NSG mice were purchased from The Jackson Laboratory (Bar Harbor, ME, USA), and cared for in strict accordance with an approved Johns Hopkins ACUC protocol. Intraductal transplantation was performed as described previously . Briefly, 105 cells were injected in the mammary ducts of immunodeficient female NSG mice as 2 μL of single-cell suspension in PBS with 0.1% trypan blue, using a Hamilton syringe with a blunt-ended 1/2-inch 30-gauge needle. At the indicated times, mice were euthanized, and the mammary fat pads harvested and fixed in 10% neutral buffer formalin. For xenografts, CSE treated cells (106) were subcutaneously injected as a 50 μl single-cell suspension in a 1:1 solution of media and BD Matrigel Matrix (BD Bioscience). At the indicated times, the mice were euthanized, and fixed by perfusion with PBS followed by 10% neutral buffer formalin for necropsy. Female mice receiving MCF7 cells were implanted with beeswax pellets containing 20 μg of estrogen one day before injection . Paraffin embedded sections were analyzed by standard H&E staining, and by immunohistochemistry using a monoclonal antibody for human cytokeratin-18 (C1399, Sigma-Aldrich, St. Louis, MO, USA), and the Mouse on Mouse (M.O.M.) Fluorescein Kit (Vector Labs, Burlingame, CA, USA).
Fluorescence-activated cell sorting (FACS) was performed on a BD Bioscience SLRII instrument. Cells were labeled using the ALDEFLUOR® kit (Stem Cell Technologies, Vancouver, BC, Canada), or the antibody conjugates listed in the Additional file 3.
Analysis of gene expression and methylation
Microarray based gene expression and methylation analysis were performed at the microarray core of the SKCCC using the Agilent Human 44K expression array (Agilent Technologies, Santa Clara, CA, USA) and the Infinium Methylation27 Array (Illumina, Inc., San Diego, CA, USA) as previously described . The data is deposited in the GEO database under accession number GSE42668. Quantitative Real-Time PCR analysis (qRT-PCR) was performed using a 7500 Real-Time PCR System, the High Capacity cDNA Reverse Transcription Kit, TaqMan Gene Expression Master Mix, and the TaqMan Gene Expression Assays listed in the Additional file 3 (Applied Biosystems, Foster City, CA, US). Western blot analysis was performed as previously described  using the antibodies listed in the Additional file 3.
This work was supported by the Flight Attendant Medical Research Institute [062544_YCSA, 072156_CIA]; the Safeway Foundation; the Irving Hansen Foundation; and the National Institutes of Health [P30 CA006973].
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