1Departments of Medicine and Cell Biology, University of Alabama at Birmingham, and Veterans Administration Medical Center, and 2Department of Surgery, University of Alabama at Birmingham, Birmingham, Alabama 35294; 3Department of Medicine, 4Genomics Institute, Bioinformatics Core, and 5Department of Medicine and Cell and Molecular Biology Graduate Group, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6144
Submitted 30 July 2003 ; accepted in final form 8 December 2003
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ABSTRACT |
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hepatocyte growth factor; Madin-Darby canine kidney cells
The Madin-Darby canine kidney (MDCK) cell line, derived from the kidney tubules of a normal cocker spaniel in 1958 (16), is one of the best-characterized and most widely used epithelial cell lines (38). In vitro, treatment of previously well-polarized MDCK cells with HGF leads to a loss of cell polarity (dedifferentiation) (2). In addition, HGF treatment of well-polarized monolayers results in detachment of E-cadherin from the actin cytoskeleton (3), loss of contact inhibition of mitosis (1), and increased susceptibility to bacterial pathogens such as Escherichia coli and Pseudomonas aeruginosa (7, 44). When MDCK cells are placed in a three-dimensional collagen matrix and allowed to form cysts, HGF-induced tubulogenesis involves the eight-protein exocyst complex and the Rho family of small GTPases (17, 31). Elucidation of the genomic pathways activated by HGF would provide important clues about the mechanisms of epithelial transformation to carcinoma and epithelial organ regeneration. Some of the signaling pathways initiated by HGF activation of the c-met receptor tyrosine kinase have been identified (32). However, the picture is not complete.
Recent advancements in microarray technology have revolutionized genomics by allowing researchers to study the expression of thousands of genes simultaneously during highly complex processes (13). A microarray analysis of early gene expression changes induced by HGF treatment of polarized MDCK cell monolayers would provide important information regarding the signaling processes that are modulated during epithelial dedifferentiation. Until recently, the only mammalian DNA chips available for analysis have been derived from humans, mice, and rats. Because MDCK cells are derived from Canis familiaris, the genomic responses to various stimuli by MDCK cells have not previously been assayed using DNA microarray technology, as DNA chips spotted with thousands of C. familiaris gene sequences did not exist. Here, we describe and validate the response of MDCK cells grown on Transwell filters for 4 days and stimulated with recombinant HGF for 0, 3, or 24 h using a novel canine DNA microarray designed to query 12,473 gene sequences from C. familiaris.
This newly developed canine DNA microarray chip should be extremely useful not only for canine research but also for the many epithelial cell biologists who utilize the MDCK epithelial cell model (a PubMed search using "MDCK" as a keyword identified 3,384 published manuscripts). For example, Boccaccio and colleagues (4) describe multiple signaling pathways from HGF. Some of these HGF signaling pathways can be inhibited; i.e., use of a tyrosine-phosphorylated peptide interferes with both STAT dimerization and the association of STAT to the receptor, and hence it is possible to see which pathways regulate these genes. An additional application, which we are pursuing, compares HGF signaling in MDCK cells grown in a two-dimensional monolayer with a three-dimensional matrix.
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MATERIALS AND METHODS |
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Microarray analysis. For our experiments, all protocols were conducted as described in the Affymetrix GeneChip Expression Analysis Technical Manual. Fifteen micrograms of total RNA, collected from MDCK cells grown for 4 days on Transwell filters and exposed to 0, 3, or 24 h of HGF, were converted to first-strand cDNA. Second-strand cDNA synthesis was followed by in vitro transcription for linear amplification of each transcript and incorporation of biotinylated CTP and UTP. The cRNA products were fragmented to 200 nucleotides or less, heated at 99°C for 5 min, and hybridized for 16 h at 45°C to the ToxExpress GeneChip Canine Array, which was developed as a custom array by Gene Logic and Pfizer, using Affymetrix standard design parameters and manufactured by Affymetrix. The ToxExpress GeneChip Canine Array contains probe sets corresponding to 12,473 gene sequences from C. familiaris. In the ToxExpress GeneChip Canine Array, oligonucleotide probes are synthesized in situ complimentary to each corresponding sequence. Sixteen pairs of oligonucleotide probes are used to measure the expression level of each gene sequence represented on the array.
The ToxExpress GeneChip Canine Array chips were then washed at low and high stringency and stained with streptavidin-phycoerythrin. Fluorescence was amplified by adding biotinylated anti-streptavidin and an additional aliquot of streptavidin-phycoerythrin stain. A confocal scanner was used to collect fluorescence signal at 3-nm resolution after excitation at 570 nm. The average signal from two sequential scans was calculated for each microarray feature. A complete microarray expression data set has been submitted to the Gene Expression Omnibus (GEO) database at the National Center for Biotechnology Information's (NCBI; GEO submissions GPL374 [NCBI GEO] , GSM8880 [NCBI GEO] -GSM8888, GSE578 [NCBI GEO] ; NCBI tracking system no.15020840).
Real-time PCR. Fifteen micrograms of total RNA, collected from MDCK cells grown for 4 days on Transwell filters and exposed to 0, 3, or 24 h of HGF, were converted to first-strand cDNA. cDNA and the TaqMan primer/probe system, individualized for each mRNA, were used in conjunction with the 7700 PRISM Sequence Detection Instrument (both Applied Biosystems) as described in the Applied Biosystems Technical Manual. When the reaction product amplification exceeded the threshold value (denoted by the solid line in Fig. 4), the corresponding cycle number was termed CT (arrows). Fold-change between conditions was calculated through an exponential function of the observed difference in CT as previously described (19). The values were normalized to a control mRNA, the 18S ribosome, and all real-time PCR studies were performed in at least triplicate.
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Cell lysate preparation and Western blotting. MDCK II cells grown on 24-mm-diameter Costar filters with 0.4-µm-size pores were used for preparation of cell lysates for Western blot analysis. Control and HGF-treated cells on filters were exposed to 0.2 ml of 20 mM Tris·HCl (pH 7.4), 150 mM NaCl, 0.1% SDS, 1% Triton X-100, 1% deoxycholic acid, and 5 mM EDTA (RIPA buffer) containing inhibitors of proteases (2 mM phenylmethylsulfonyl fluoride, 50 µg/ml pepstatin, 50 µg/ml chymostatin, and 10 µg/ml antipain) for 15 min on ice. The protein concentration of each cell lysate was determined by using the bicinchoninic acid determination assay (Pierce Chemical, Rockford, IL). Western blot analysis was performed as previously described (2, 3). Briefly, equal amounts of protein in lysates of MDCK cells were run on 7.5% acrylamide gels at 160 V for 45 min. Proteins were transferred to Immobilon P membranes (Millipore, Bedford, MA). Membranes were blocked with for 30 min with PBS- (PBS without Ca2+ and Mg2+), 5% milk, and 0.1% Tween 20 (block solution). Filters were probed with antibodies against claudin-2 (rabbit polyclonal IgG, 1:250, Zymed), cyclooxygenase-2 (COX-2; mouse monoclonal IgG, 1:250, Transduction Laboratories), or Sec61 (a gift from V. Lingappa, UCSF, rabbit polyclonal IgG, 1:500) for 1 h and then washed with PBS-, 0.1% Tween 20 (4 times, 5 min). Filters were then probed with either horseradish peroxidase-labeled goat anti-mouse (1:2,500 for COX-2) or goat anti-rabbit (1:5,000 for claudin-2 and Sec61
) secondary antibodies diluted in block solution for 1 h. Filters were again washed (4 times, 5 min) with PBS-0.1% Tween 20. Filters were developed using enhanced chemiluminescence kit (Amersham) and visualized on Kodak X-OMAT film.
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RESULTS AND DISCUSSION |
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To test for early changes in gene expression levels after exposure of filter-grown MDCK cells to 0, 3, and 24 h of HGF, we harvested RNA, which was converted to cDNA and used to query the 12,473 gene sequences on the DNA microarray chip. Gene intensity values, corresponding to the mRNA levels, were calculated by Affymetrix MAS5 software. GeneSpring (Silicon Genetics) was used to scale the gene intensity values so that the median of each chip was equivalent, and then a per gene normalization to the median of each gene's expression across the nine samples (each of the 3 conditions in triplicate) was performed to facilitate comparison of the data. We then generated a list of genes that were flagged as present (P) by Affymetrix MAS5 software in all three replicates of at least one of our conditions. There were 5,573 genes in this list. We used this gene list to hierarchically cluster all nine of our individual samples. The resulting dendogram (tree) is shown with branches for each of our three conditions similarly colored (Fig. 1A).
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We then exported the normalized data for all genes to SAM (Statistical Analysis of Microarrays, Stanford University) and performed a multiclass analysis to test for significantly changed genes in any of three conditions. The three conditions were MDCK cells grown for 4 days on Transwell filters and exposed to recombinant HGF for 0, 3, or 24 h. We chose to accept a 10% false-positive rate (termed the False Discovery Rate or FDR). Of the 12,473 genes on the array, there were 1,672 that met our criteria. These 1,672 genes were imported into GeneSpring, and all six possible pairwise comparisons were performed: HGF 3 h>control, control>HGF 3 h, HGF 24 h>control, control>HGF 24 h, HGF 3 h>HGF 24 h, and HGF 24 h>HGF 3 h. For instance, HGF 3 h>control means MDCK cells exposed to HGF for 3 h that are upregulated compared with control MDCK cells (no HGF exposure). We defined "significantly regulated" to be a twofold or greater change (based on an average of three normalized replicates) and present in all replicates of the condition in which higher mRNA expression occurred. We then combined the pairwise lists to generate a final list of 667 unique genes (Fig. 1B).
Of the 667 genes, 125 were annotated to known C. familiaris genes or had significant (>90%) homology to other known mammalian genes (Fig. 2). We found 8 genes among the 125 annotated genes that had previously been implicated in the HGF signaling pathway, thereby demonstrating that the system was responsive to HGF (Table 1). This group of regulated genes has already expanded the list of genes known to be involved in the various classes of HGF/c-met signaling "families." The remaining 542 regulated genes are not currently annotated. Nevertheless, many important genes were assayed. The number of genes that are "annotatable" on this DNA microarray chip will undoubtedly increase over time as a partial canine sequence (1.5x of the genome) was completed last year by The Institute for Genomic Research (Rockville, MD) (14), and a more complete, National Institutes of Health-funded project (6x sequence) has just started and has been predicted, by those involved, to take
1 yr to complete (Dr. Ewen Kirkness, TIGR, personal communication).
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The microarray gene expression results were confirmed by testing mRNA levels using real-time PCR for all 35 genes in which we had complete mRNA sequence information (Fig. 3). Complete sequence information was necessary to design the primers and probes for use with the TaqMan System and the 7700 PRISM Sequence Detection Instrument (both Applied Biosystems). For example, several extracellular matrix proteins, including matrix metalloproteinase-13 (MMP-13), were dramatically upregulated after HGF exposure. By real-time PCR, the calculated fold-increase for MMP-13, normalized to the 18S ribosome (a control mRNA), was 27.86 ± 0.19 and 15.18 ± 0.52 for 3 and 24 h of HGF exposure, respectively, compared with 0 h of HGF exposure (control) (Fig. 4). This was very similar to the fold-increase determined by microarray analysis (Fig. 2). With respect to the other 34 genes tested by real-time PCR, in all cases the direction of fold-change in both real-time PCR and microarray analysis was the same. The absolute fold-change values were also remarkably consistent, thereby validating the microarray results, although there were a few differences. For example, the Na-K-ATPase showed a 2.34-fold change by microarray analysis, although almost no change by real-time PCR (1.1 fold) (Fig. 3). This may be due to the fact that, as noted above, we chose to accept a 10% false-positive rate in our microarray analysis, and therefore real-time PCR may be more specific. Other extracellular matrix proteins that were dramatically upregulated were integrin -6 and fibronectin ED-A. Fibronectin is particularly interesting in that it was very recently shown to be required for branching morphogenesis (34) and, in a three-dimensional matrix, HGF induces tubulogenesis in MDCK cells (17, 26, 31). Another gene involved in branching is sprouty, which is transiently upregulated and is an antagonist of fibroblast growth factor (FGF) signaling in Drosophila airways (9). In our array, sprouty homolog 2 was also transiently upregulated.
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Another way to confirm the array results is by examining the protein expression levels by Western blot analysis. For example, claudin-2 is a component of the tight junction (41) and claudin-2 gene expression was downregulated 23.8-fold after 24 h of HGF exposure compared with controls. We confirmed this dramatic decrease in claudin-2 expression by HGF at the protein level (Fig. 5A). Interestingly, two other components of the tight junction, claudin-3 and occludin 1B, were upregulated by 24 h of HGF exposure compared with 3 h of exposure. We hypothesize that differential expression of claudin-2 plays a key role in mediating the cellular response of MDCK cells to HGF. Recently, HGF has been shown to modulate transepithelial resistance (TER) in MDCK cells (28). Our microarray data suggest that HGF-induced changes in claudin-2 expression contribute to these changes in MDCK cell TER. We have completed a detailed analysis of the effects of HGF on tight junction associated proteins to help understand how HGF alters MDCK cell TER, and this analysis supports our hypothesis regarding the importance of claudin-2 regulation (unpublished observations). Other cytoskeletal proteins were also upregulated, including actinin -1 and microtubule-associated proteins.
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Mesenchymal production of HGF has been described to be regulated by numerous factors including acidic and basic fibroblast growth factor, epidermal growth factor, and prostaglandins (8, 21, 22). COX-2 is overexpressed in multiple types of carcinoma including colon, lung, bladder, breast, melanocytes, head and neck, and pancreas (15). Several epidemiological studies have demonstrated that inhibition of COX-2 activity has a chemoprotective effect on carcinoma tumor growth and metastasis (15). Our analysis shows that COX-2 gene expression was upregulated 2.51-fold after 24 h of HGF exposure compared with controls. Western blot analysis confirmed a similar increase in COX-2 protein expression after 24 h of HGF treatment (Fig. 5B). A major metabolite of COX-2 activity is prostaglandin E2 (5). Interestingly, the prostaglandin E2 receptor, which increases intracellular cAMP after activation, was also upregulated by HGF treatment. In what may be an autocrine response to HGF, analysis of the array results showed that both epidermal growth factor and FGF receptor 2 were downregulated. It has also been reported that elevation of intracellular cAMP levels is an important signal for HGF production (24), and adrenomedullin precursor, which is involved in activation of cAMP, was dramatically downregulated after 3 h of HGF, but even more dramatically upregulated from 3 to 24 h of HGF. Conversely, transforming growth factor-1 and glucocorticoids have been shown to inhibit HGF (8, 23), and the activin A receptor was downregulated.
Another group of genes that changed in response to HGF were those encoding small GTPases. Most, like the Ras homolog Rac2, GDP dissociation inhibitor, and Rab10, were upregulated, as might be expected in the first phase (scattering) of the response to HGF (4). However, some, such as Rab3AIP and GEF1, were downregulated.
HGF induction of transformation, but not cell motility, requires stimulation of the Ras-MAP kinase cascade (29). HGF has also been recently shown to modulate invasiveness and motility of ovarian carcinomas by a Ras-mediated pathway (42). As noted, the Ras homolog was upregulated as was mitogen-activated protein kinase kinase kinase kinase 4; however, mitogen-activated protein kinase kinase kinase 5 was only transiently upregulated.
Stat-3 is also activated by HGF signaling during MDCK cell tubulogenesis (4); however, per a reverse BLAST search of the entire chip using Stat-3 cDNA, Stat-3 was not one of the 12,473 gene sequences on the chip. Other genes known to be in the HGF signaling pathway, including C-Fos, cyclin-D1, GAB1, phosphatidylinositol 3-kinase, PLC-, and Ron (2), were also not represented on the chip. In fact, of several dozen cDNAs used in a reverse BLAST search, we could only annotate an additional two,
-actin and the exocyst component Exo70, and neither of these genes met the criteria for a twofold change in expression. The c-met receptor was also not represented on the chip. While a canine c-met has now been deposited in GenBank (AB096697
[GenBank]
), it was submitted on November 22, 2002, after the ToxExpress GeneChip Canine Array, described here, was already constructed.
HGF has been identified in the endothelia (33), and the human serine protease (HGF-converting enzyme) that converts proHGF to the mature heterodimeric form exhibits structural similarity to blood coagulation factor XII (25). HGF has also been shown to initiate angiogenesis (43). HGF itself has 38% homology to the serine protease plasminogen (another kringle domain protein) (27). Several hematopoietic proteins were upregulated, including acetylhydrolase, which is involved in platelet activation (39), glycoprotein GPIIIA, and vascular endothelial growth factor receptor-1. Some were also downregulated, including endothelin-1 precursor and thrombospondin.
Not surprisingly for a growth factor, in response to HGF there were numerous transcription factors that were either up- or downregulated as well as nuclear factors. Interestingly, there were also a number of genes encoding proteins involved in translation and translocation of proteins across the endoplasmic reticulum (ER) that were all upregulated, including the Sec61 homologue (Fig. 5C), signal receptor protein -subunit, eukaryotic translation initiation factor 4E, and eukaryotic translation termination factor. We and others have recently shown that the exocyst complex is involved in protein synthesis, by interacting with the Sec61
component of the translational machinery of the ER (18, 40). As expected from the microarray results, no changes in Sec61
protein levels were detected in MDCK cell lysates after 3 h of HGF treatment (Fig. 5B). Of the eight exocyst components, only Exo70 was found on the chip (by a reverse BLAST search) and, as noted above, it did not quite meet the twofold criterion for change.
Conclusion. Using a novel canine DNA microarray designed to query 12,473 gene sequences from C. familiaris, we have evaluated HGF signaling in MDCK cells grown on two-dimensional Transwell filters and stimulated with recombinant HGF. We have identified known genes, as well as new genes, that are significantly up- or downregulated after exposure to HGF. In addition, we believe that this microarray will be of great interest to the general epithelial cell biology community who rely on the MDCK cell culture model.
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ACKNOWLEDGMENTS |
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This work was supported in part by grants from the Polycystic Kidney Disease Foundation (J. H. Lipschutz), National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-58090 and DK-02509 (J. H. Lipschutz), Cystic Fibrosis Pilot Grant CFF SORSCH98SO (D. F. Balkovetz), and a Veterans Administration Merit Award (D. F. Balkovetz).
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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REFERENCES |
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