Gene expression changes associated with Barrett's esophagus and Barrett's-associated adenocarcinoma cell lines after acid or bile salt exposure
© Hao et al; licensee BioMed Central Ltd. 2007
Received: 19 January 2007
Accepted: 27 June 2007
Published: 27 June 2007
Esophageal reflux and Barrett's esophagus represent two major risk factors for the development of esophageal adenocarcinoma. Previous studies have shown that brief exposure of the Barrett's-associated adenocarcinoma cell line, SEG-1, or primary cultures of Barrett's esophageal tissues to acid or bile results in changes consistent with cell proliferation. In this study, we determined whether similar exposure to acid or bile salts results in gene expression changes that provide insights into malignant transformation.
Using previously published methods, Barrett's-associated esophageal adenocarcinoma cell lines and primary cultures of Barrett's esophageal tissue were exposed to short pulses of acid or bile salts followed by incubation in culture media at pH 7.4. A genome-wide assessment of gene expression was then determined for the samples using cDNA microarrays. Subsequent analysis evaluated for statistical differences in gene expression with and without treatment.
The SEG-1 cell line showed changes in gene expression that was dependent on the length of exposure to pH 3.5. Further analysis using the Gene Ontology, however, showed that representation by genes associated with cell proliferation is not enhanced by acid exposure. The changes in gene expression also did not involve genes known to be differentially expressed in esophageal adenocarcinoma. Similar experiments using short-term primary cultures of Barrett's esophagus also did not result in detectable changes in gene expression with either acid or bile salt exposure.
Short-term exposure of esophageal adenocarcinoma SEG-1 cells or primary cultures of Barrett's esophagus does not result in gene expression changes that are consistent with enhanced cell proliferation. Thus other model systems are needed that may reflect the impact of acid and bile salt exposure on the esophagus in vivo.
The incidence of esophageal adenocarcinoma has increased at a rate that is among the highest of all cancers. The major risk factor for esophageal adenocarcinoma is the presence of Barrett's esophagus, a premalignant neoplastic lesion that is characterized by intestinal metaplasia replacing the normal squamous esophageal epithelia. The presence of Barrett's esophagus increases the overall risk of adenocarcinoma by 40-fold. In addition, similarities in the gene expression profile of Barrett's esophagus to esophageal adenocarcinoma further support a close relationship between the two tissues.
Clinical studies have identified esophageal acid reflux as a risk factor for Barrett's esophagus and adenocarcinoma. In vitro experiments using adenocarcionoma and non-neoplastic Barrett's esophageal cell lines established that short-term exposure to an acidic environment results in increase cell proliferation [5–7]. In addition, activation of signal transduction pathways associated with cell proliferation is observed with acid exposure. Primary cultures of Barrett's esophageal tissues exposed to short pulses of acid or bile salts show increased immunocytochemical staining for PCNA or [3H]thymidine uptake, which serve as markers for cell proliferation [8–10].
Gene expression profiles based on a genome-wide assessment of gene expression represent a highly sensitive means of determining differences between cells or tissues. There are now numerous examples where gene expression profiles produce novel classifications of tissues in a manner that reflects differences in biological function or clinical outcomes[11, 12]. This study addresses whether exposure of Barrett's-associated adenocarcinoma cell lines or primary Barrett's esophageal tissues to acid or bile salts results in differences in gene expression patterns that support transformation to a malignant state. If present, changes in gene expression would provide an opportunity to determine how acid or bile salts contribute to malignant transformation, and may also identify disease-associated markers that are useful for assessing patient risk or as therapeutic targets.
Unselected patients scheduled for endoscopic evaluation for Barrett's esophagus were enrolled to participate in the study. Biopsies were obtained according to the Seattle protocol using a standard esophagogastroduodenoscope (Olympus GIF-XV10) and biopsy forceps (Radial Jaw 3, Boston Scientific Corp., Natick, MA). Four biopsies each were obtained from normal appearing esophagus (proximal to the Barrett's esophagus), salmon-colored Barrett's esophagus, and the duodenum. Twin biopsies were also obtained for each Barrett's esophagus sample and sent to pathology for analysis. All procedures were performed with patient consent and under approved human subjects protocols from Stanford University and the Palo Alto Veterans Affairs Health Care System.
As previously described, the biopsies were immediately cultured in HEPES buffered Medium 199, pH 7.4, supplemented with 10% heat-inactivated fetal calf serum, 1 ug/ml insulin (Sigma-Aldrich, St. Louis, MO), 500 U/ml penicillin and 250 U/ml streptomycin. Samples were placed on a sterilized stainless grid in 12 well dishes in a sterile sealed jar (Torsion Balance, Clifton, NJ) that was previously perfused with 95% O2-5% CO2 and incubated at 37°C. Incubation times were derived from previous studies with an assessment of cell viability determined by cellular lactate dehydrogenase release and histology[8, 9].
Cell lines were derived from human esophageal adenocarcinomas associated with Barrett's metaplasia (SEG-1 and OE33), a poorly differentiated adenocarcinoma (TE7), and a squamous cell carcinoma (OE21). SEG-1 cells (a generous gift from Dr. David Beer, Univ. of Michigan) were grown at 5% CO2 in DMEM supplemented with 4.5 g/L glucose, and L-glutamine (Cellgro, Mediatech, Inc., Herndon, VA), penicillin (100 U/ml), streptomycin (100 U/ml), and 10% fetal bovine serum. Experiments were performed with SEG-1 cells within 15 passages after receipt from Dr. Beer. The OE21 and OE33 cell lines (European Collection of Cell Cultures, Wiltshire, United Kingdom) and TE7 cells (Dr. T.Nishihira, The Second Department of Surgery, Tohoku University School of Medicine, Japan) were grown in RPMI 1640 with 25 mM HEPES, 10% fetal bovine serum, and penicillin and streptomycin (100 U/ml). The passage number of the OE21, OE33, and TE7 cells were not known, but the gene expression profiles of the same cells have been determined and compared to Barrett's esophagus.
Acid and bile salt exposure
Cell lines were first cultured in serum-free media for 48 hours before exposure for various times to media made acidic with the addition of concentrated HCl to pH 3.5. Following acid exposure, normal tissue culture media at pH 7.4 containing 10% fetal bovine serum was added and incubated for an additional 24 hours. For the tissue explants in primary culture, samples were exposed to media at pH 3.5 for 60 minutes followed by incubation at pH 7.4 for 4 hours. A pH 3.5 was previously demonstrated to induce PCNA and villin expression.
Exposure to bile salts was achieved through incubation of the cell lines or tissue explants for 1 hour in a mixture of conjugated bile salts designed to resemble what is found in vivo, and consisted of 0.54 mM sodium glycocholate (C26H42NO6Na), 0.1 mM sodium taurocholate (C26H44NO7SNa), 0.2 mM glycocholic acid (C26H43NO6), and 0.1 mM sodium taurochenodeoxycholate (C26H44NNaO6S) at pH 7.4. A formulation of conjugated bile salts called "bile salt cocktail" that included conjugated deoxycholic and chenodeoxycholic acids was also applied to the cell lines and was composed of 0.25 mM sodium glycocholic acid, 0.10 mM sodium taurocholic acid, 0.30 mM sodium glycochenodeoxycholic acid, 0.10 mM sodium taurochenodeoxycholic acid, 0.15 mM sodium glycodeoxycholic acid, and 0.05 mM sodium taurodeoxycholic acid. After exposure to bile salts for 1 hour, the cell lines and tissue explants were incubated in regular culture media without bile salts for an additional 24 hours as previously described.
Total RNA (20–120 μg) was isolated using Trizol (Invitrogen, Carlsbad, CA). Total RNA from the primary tissue cultures were amplified once (MessageAmp™ II aRNA Amplification Kit Ambion Inc., Austin, TX). A common human reference RNA served as an internal standard (Universal Human Reference RNA, Stratagene Corp., La Jolla, CA).
DNA microarrays containing 41,125 cDNA clones, representing approximately 24,473 unique genes were printed on microscope slides and processed for hybridization as previously described [16–18]. The microarrays were hybridized with sample RNA labeled with Cy5-dUTP and a common human reference RNA labeled with Cy3-dUTP (Amersham Biosciences, Piscataway, NJ). The arrays were scanned using a GenePix 4000A scanner (Axon Instruments, Molecular Devices Corp., Palo Alto, CA). Detailed protocols can be found at these websites[16, 19].
Features containing artifacts or blemishes were flagged for exclusion from further analysis using GenePix Pro 5.0 (Axon Instruments). The raw data was deposited in the Stanford Microarray Database [20, 21], where analysis using unsupervised hierarchical clustering was performed. The expression data were visualized using Treeview .
Analysis included a hierarchical clustering algorithm that was applied to both the genes and arrays, and a supervised analysis by multi-class comparison using Significance Analysis of Microarrays (SAM) software [22, 23]. Interpretation of genes showing differential expression was facilitated by using GoMiner to classify differentially expressed genes in the context of the Gene Ontology (GO database May 2006)[24, 25]. Only genes that displayed at least a 3-fold difference in expression between samples were used in the GoMiner analysis. GO categories were considered significant if it contained more than 14 genes and a false discovery rate (FDR) of less than 0.1.
Cell proliferation was evaluated using three approaches: cell counting with a hemocytometer using trypan blue exclusion, [3H]thymidine incorporation, or the colorimetric MTT assay (American Type Culture Collection, Manassas, VA) . Thymidine incorporation was achieved using 1 uCi [3H]thymidine (Amersham Biosciences, Piscataway, NJ) added to the culture media in each well for the duration of the experiment. The samples were subjected to 3 freeze-thaw cycles followed by collection with a 96 Mach III harvester (Tomtec, Hamden, CT) and 90 × 120 mm glass fiber filters (Filtermat A, PerkinElmer, Wellesley, MA). The filters were dried and then counted with 5 ml of scintillation fluid (Betaplate Scint, PerkinElmer). The incorporated [3H]thymidine was assayed with a Wallac MicroBeta® TriLux scintillation reader (PerkinElmer). Immunoblotting was performed according to the method described by Towbin and used 0.2% Tween 20 (Sigma-Aldrich, St. Louis, MO) in the blotting buffer [28, 29].
Acid effects on Barrett's esophageal adenocarcinoma cell lines
With respect to bile salt exposure, both adenocarcinoma and squamous cell carcinoma cell lines showed increased cell proliferation with [3H]thymidine incorporation in response to bile salts (Figure 1), but not when evaluated using cell counting or the colorimetric MTT assay (data not shown).
Gene expression changes associated with acid exposure
Upregulated genes in acid treated SEG-1 cells in GO category: Spermatogenesis (GO:0007283)
Testis-specific Y-encoded protein 1
Synaptonemal complex protein 1
ADAM metallopeptidase domain 28
Tubulin, delta 1
Non-metastatic cells 5
Deleted in azoospermia-like
Testis-specific serine kinase 6
Bromodomain containing 2
Gene expression changes associated with bile exposure in cell lines and Barrett's tissues
Exposure of all cell lines to conjugated bile salts resulted in significant incorporation of [3H]thymidine (Figure 1). No changes in cell proliferation as determined by cell counting or the MTT colorimetric assay were observed.
Barrett's tissues from 3 patients were also exposed to a cocktail of conjugated bile salts for 1 hour followed by 24 hours incubation without bile salts. Gene expression analysis using unsupervised hierarchical clustering revealed no evidence of new classifications based on bile salt exposure. Supervised analysis using SAM also did not reveal any differences in gene expression in Barrett's tissues secondary to bile salt exposure (Figure 4).
Our goal was to determine whether acid or bile salt exposure results in gene expression patterns that reflect changes in biological processes within the cell. Previous work in cell lines demonstrated that short exposure to acid results in activation of mitogen-activated protein kinase pathways that contribute to cell proliferation. Work performed in primary Barrett's tissues also showed that only short and not continuous exposure to acid results in increased PCNA expression, a marker of DNA replication, as determined by the number of positive staining cells using immunohistochemistry. Despite using different periods of acid exposure, increased cell proliferation in esophageal adenocarcinoma cell lines was not detected using direct cell counting, colorimetric MTT assays, or [3H]thymidine uptake. Changes in PCNA levels as determined by protein immunoblotting also was not detected (data not shown). Changes in PCNA expression, however, may be difficult to detect in cancer cell lines like SEG-1 because they are already selected for optimal growth in culture. Bile salt exposure resulted in increase [3H]thymidine uptake in all adenocarcinoma and squamous cell cancer cell lines, suggesting that the effects may be non-specific with respect to cell type.
DNA microarrays provide a highly sensitive approach for detecting changes in gene expression on a genome-wide scale. Numerous studies have demonstrated how gene expression patterns are able to classify tissues in a manner that carries greater significance with respect to natural history and clinical outcomes than other modalities such as conventional pathology. Our original goal was to determine the gene expression profiles associated with acid and bile salt exposure. Subsequent analysis with the aid of clustering algorithms and other analytical tools would then provide an assessment of which biological pathways and processes are induced. SEG-1 cells showed changes in gene expression patterns that appeared to be related to the length of acid exposure. When the genes affected by acid exposure were analyzed with respect to the Gene Ontology, representation by genes in the GO category for spematogenesis did show enhanced expression in the acid treated SEG-1 cells. Using a recently published dataset, none of the 8 spermatogenesis genes with enhanced expression in acid-treated SEG-1 cells showed similarly enhanced expression in Barrett's esophagus tissues or esophageal adenocarcinomas. In addition, SEG-1 cells exposed to acid did not show enhanced expression for any gene or enhanced representation by any GO category that is similarly affected in esophageal adenocarinoma or Barrett's tissues. Thus the significance of the acid-induced gene expression changes observed in SEG-1 cells is unclear. Whether recently developed cell lines that represent non-neoplastic Barrett's cells will show different gene expression profiles in response to acid or bile salt exposure remains to be determined[30, 33, 34].
A potential problem with the experiments using SEG-1 cells is that they already represent cancer cells that have undergone malignant transformation and thus may not exhibit the same changes in gene expression with acid exposure as a cell derived from Barrett's epithelia. Thus Barrett's tissues were also examined for changes in gene expression in response to acid and bile salts. No changes in gene expression patterns, however, were observed for primary cultures of Barrett's tissues incubated in acid or bile. Supervised approaches for analysis also did not detect any significant changes in gene expression for any individual gene. As an internal control, each tissue examined (duodenum, normal esophagus, and Barrett's esophagus) displayed gene expression patterns that are consistent with previous work. A potential explanation for the absence of detectable differences in gene expression includes heterogeneity in Barrett's esophageal tissues. Previous studies showed an increase in PCNA staining after acid exposure for only a small percentage of cells. Thus if only a small subset of Barrett's esophageal cells are able to respond to acid or bile exposure, there may not be a significant impact on the overall gene expression profile for the tissue sample analyzed. As PCNA positive cells do not possess any distinct morphological features or known specific cell surface markers, their isolation using selective approaches such as laser capture microdissection is not feasible at the present time. A second potential explanation is that a single exposure to acid or bile salts may be insufficient to induce lasting gene expression changes that are detectable by current technologies. The limiting factor with respect to performing experiments of longer duration with greater exposure to acid or bile salts is the viability of the cell lines or primary cultures in such harsh conditions. For such studies to be successful, experimental models that can withstand conditions similar to those that occur in vivo need to be developed.
In summary, pulse acid or bile exposure of Barrett's esophagus or esophageal adenocarcinoma cell lines do not show expression changes in the same genes that are affected in the transformation to esophageal adenocarcinoma.
proliferating cell nuclear antigen
significance analysis of microarrays
false discovery rate
The study was supported by National Institutes of Health Award R01 DK063624 (A.W.L., G.T., and M.B.O.) and P30 DK56339 (Stanford University Digestive Disease Center).
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