Application of High-Density DNA Microarray to Study Smoke- and Hydrogen PeroxideInduced Injury and Repair in Human Bronchial Epithelial Cells
Ken Yoneda,
Mary Mann-Jong Chang,
Ken Chmiel,
Yin Chen and
Reen Wu
Center for Comparative Respiratory Biology and Medicine, Department of Internal Medicine, University of California at Davis, Davis, California.
Correspondence to Dr. Reen Wu, Center for Comparative Respiratory Biology and Medicine, Surge 1 Annex, Room 1121, University of California at Davis, One Shields Avenue, Davis, CA 95616; Phone: 530-752-2648; Fax: 530-752-8623;
ABSTRACT. Recent advances in high-density DNA microarray techniqueallow the possibility to analyze thousands of genes simultaneouslyfor their differential gene expression patterns in various biologicprocesses. Through clustering analysis and pattern recognition,the significance of these differentially expressed genes canbe recognized and correlated with the biologic events that maytake place inside the cell and tissue. High-density DNA microarraynylon membranes were used to explore gene expression and regulationassociated with smoke- and hydrogen peroxideinduced injuryand repair in differentiated human bronchial epithelial cellsin vitro. At least three phases of change in gene expressioncould be recognized. The first phase seems to be an immediateevent in response to oxidant injury. This phase includes theinduction of bcl-2 and mdm2 genes that are involved in the regulationof apoptosis, and the mitogen-activated protein kinase phosphatase1 that functions as a regulator for various mitogen-activatedprotein kinase activities. The second phase, usually 5 h later,includes the induction of various stress proteins and ubiquitin,which are important in providing the chaperone mechanism andthe turnover of damaged macromolecules. The third phase, whichis 5 to 10 h later, includes the induction of genes that seemto be involved in reducing oxidative stress by metabolizingthe cellular level of reactive oxygen species. In this phase,enzymes associated with tissue and cell remodeling are alsoelevated. These results demonstrated a complex gene expressionarray by bronchial epithelial cells in response to a singleinsult of oxidants that are relevant to environmental pollutants.E-mail: rwu@ucdavis.edu
Conducting airway is one of the primary targeted tissues thatare constantly exposed to various air pollutants, includingthose of environmental oxidant pollutants, tobacco smoke, andozone. To exert the first line of pulmonary defense, airwayepithelium is responsible for the maintenance of mucociliaryclearance that traps and removes various inhaled air particulateand infectious agents (13). However, this function isfrequently impaired in airways after an exposure to these environmentaloxidant pollutants. This impairment may lead to more damageon airway epithelium by environmental air pollutants and thedevelopment of various pulmonary diseases (46). The effectsof these oxidant pollutants on airway cells are not completelyunderstood. Morphologic assessments of the injury on experimentalanimals reveal a wide range of cell injury and dysfunction ofthe epithelium, inflammation, and various tissue remodelingevidences (7). In addition, there is strong evidence to supportthe notion that the response by airway epithelial cells to theseinsults is complicated and varied among different individuals,different airway regions, and the chemical nature of the exposures.The difference in the responsiveness of these targeted epithelialcells to environmental oxidants is likely a reflection of differentialgene expression by these cells. A functional genomic approachat the expression level allows the examination of the relationshipbetween the change in gene expression pattern and the processof injury and response. Such a study will provide advanced knowledgeat the molecular level on how to contest various respiratorydiseases that seemed to be caused by environmental oxidant pollutants.
Large-scale study of thousands of gene expressions is a hallmarkof the transition from "structural" to "functional" genomics,in which knowing the complete DNA sequence of the genome isonly the first step in understanding how specific genes function.The next step, probably more challenging, is to sort out thebiologic function of these genes, and the manner of these genesthat are expressed is critical to various biologic processes.Central to the advancement of "functional" genomics is the developmentof high-density DNA microarray technology (816) thatis able to profile simultaneously thousands of gene expressionsand is also able to sort out which pattern of differential geneexpression is associated with a specific biologic event andwhat cellular process is carried out by a certain set of genes.
Several DNA microarray systems are available, which includethe oligonucleotide arrays based on the "gene chip" concept(14,17,18) and developed by Affymetrix Incorp, and high-densityDNA arrays on glass slide (811) or on nylon membrane(16). All of these systems take the advantage of the high-densityconcept that nucleotide hybridization kinetics can be efficientlycarried out. With computer-assisted software programs, imagesof the hybridization intensity on thousands of DNA dots on thesegene chips can be quantified and clustered into various geneexpression patterns (1921). We previously developed aDNA microarray system based on spotting DNA on "positively chargednylon membrane" (16). The reason for choosing nylon membraneover the glass surface is that the amount of DNA spotted onnylon membrane is much higher than what a glass surface canreceive. Thus, the hybridization kinetics is not limited bythe amount of target DNA on the membrane (22). Using this system,we initiate the study to profiling the gene expression patternsassociated with smoke- and hydrogen peroxide (H2O2)-inducedinjury and repair on an immortalized human bronchial epithelialcell line, HBE1 (23), and primary human bronchial epithelialcells.
Creating a High-Density DNA Microarray in Nylon Membrane
The procedure to generate target DNA inserts and array spottingon nylon membrane has been described (16). A similar procedureis used for DNA spotting on a glass slide surface. Most targetDNA are either from expression sequence tag (EST) clones ofthe IMAGE Consortium human cDNA libraries or from cDNA clonesderived from a cDNA library of a well-differentiated human tracheobronchialepithelial cell culture. Currently, there are approximately45,000 UniEST clones with sequence verification available fromResearch Genetics. We used 9600 EST clones from this pool forthis initial study. These amplified target DNA are then spottedhigh-density onto a nylon membrane with a robotic spotting machine(Arrayer). The one we used is the Arrayer-02 or -03 from theWITTECH Inc. (Taipei, Taiwan), which is capable of spottingDNA on an area smaller than 75 µm diameter and with 100to 150 µm apart from each spot. The size of the DNA microarraymembrane is 1.8 x 2.7 cm for 9600 DNA spots (cf. Figure 2).
Figure 2. Dual-color microarray analysis of smoke-induced gene expression. HBE1 cells were cultured in a condition as described in Figure 1. Fifteen hours after smoke exposure, total RNA were isolated from these cultures treated or untreated with smoke. These RNA were further purified for mRNA preparation. Approximately 1 µg of mRNA from each smoke-treated and -untreated culture was used to generate digoxigenin- and biotin-labeled cDNA probes, respectively. Hybridizations were carried out on a membrane of 1.8 x 2.7 cm size containing 9600 Uni-EST DNA spots. Hybridization condition and the color development were the same as those described previously (16).
Figure 1. Microarray analysis of gene expression pattern in airway epithelial cells after H2O2 treatment. Cells were cultured on plastic culture dishes in a serum-free, hormone-supplemented medium (32). At 80% confluence, cultures were exposed to 0.1 mM H2O2. Total RNA were isolated from these cultures at 0 (A), 1 (B), 3 (C), and 48 (D) h later. These RNA were used to produce digoxigenin-cDNA probes as described previously (16). Hybridization on microarray membranes and data imaging and analysis were the same as described before (16). Genes whose expressions were elevated by H2O2 were circled as shown.
DNA Microarray Imaging and Data Processing
The process of hybridization and detection of genes on filtermembrane is a standard molecular biology technique in Northernand Southern blot hybridization analyses. For the DNA microarraymembrane, a colorimetric detection method has been developed(16). This method is based on the well-established quantificationmethod traditionally used in quantitative protein measurements,such as enzyme immunoassays or ELISA (2426). cDNA probeslabeled by digoxigenin- or biotin-dUTP are hybridized with themembrane. The hybridization results are then developed witha single or dual color through the enzyme/substrate reactionof color-forming enzymes. We estimated that each 75-µm-diameterDNA spot on nylon membrane contains more than 10 ng of DNA,which corresponds to approximately 109 molecules per spot (assuming1000 bp DNA insert). This amount of target DNA is sufficientto carry out first-order kinetics to hybridize cDNA probes generatedfrom mRNA templates, which is approximately 106 molecules perreaction per template.
After the color development, the image on each DNA dot is digitizedby a scan on a high-resolution flat-bed scanner (Umax MagicScanat 3000 dpi). These digitized images are separated into cyan,magenta, and yellow colors. Based on the way the human eye perceivescolor, a color can be described by three components: hue, saturation,and brightness. A slight change in any of these three componentsresults in a perceivable difference. By using true-color signals,the digitized image can be reasonably quantified (16). For quantifyingthe expression levels of known genes in a cell, six plant genemRNA with different amounts are included in mammalian mRNA mixturein cDNA probe preparation during reverse transcription. Hybridizationintensities on these plant gene DNA spots in the microarrayDNA membrane served as internal references that are then usedto quantify the level of the gene expression. Based on thisapproach, we demonstrated a sensitivity at one to two copiesper cell level (at a hybridization condition with mRNA from1 million cells) quantified by this colorimetry detection approach.This sensitivity is comparable with those fluorescence probesused on glass slide surface (24).
Cell Culture Condition
In this study, primary cells derived from human bronchial epithelialtissues and an immortalized human bronchial epithelial cellline, HBE1 (23), were cultured under an air-liquid interfaceculture condition in a defined serum-free hormone-supplementedmedium as described before (24). Primary human bronchial tissueswere obtained from the local hospitals at UC Davis Medical Centerwith consent. HBE1 cell line is a clonal, papillomavirus-immortalizedhuman bronchial epithelial cell line (23). These cultured cellsat days 14 to 21 under the described culture condition expressedvarious mucociliary functions. It was under such a differentiatedcondition that these cells were exposed to H2O2 (20 to 200 µM)and smoke. At various times after these treatments, cultureswere harvested for RNA isolation (24).
These human bronchial epithelial cells grown under an air-liquidinterface cell culture system are able to express mucous celldifferentiation activity with an expression of mucin synthesisand secretion, conspicuous mucus-secreting granule formation,and mucin gene message expression (unpublished observations).Figure 1 shows a single-color approach on the hybridizationof cDNA probes of H2O2-treated and untreated control cultureson a microarray membrane. Arrows indicate genes whose expressionis significantly elevated after H2O2 treatment. The genes foundin this initial study were bcl-2 and nmn2 genes. The expressionof these genes was the earliest induced ones by H2O2. The timecourse study with mRNA samples from cultures after H2O2 treatmentfurther the inducibility of these genes by this treatment.
With the use of a dual-color approach, the effects of tobaccosmoke on HBE1 cells were investigated on a microarray membranecontaining 9600 EST cloned DNA (Figure 2). Both biotin- anddigoxigenin-labeled cDNA probes were prepared from mRNA of cellsuntreated and treated with smoke, respectively. After hybridizationand color development, the cDNA molecules labeled with biotinyielded a blue chromogen and the cDNA molecules labeled withdigoxigenin appeared red. The majority of spots appeared purple,indicating the level at which the expression of these geneswas not affected by smoke. However, some spots exhibited moredistinctive colors, more toward the red or the blue, which canbe used as an indication of differentially expressed genes aftersmoke exposure. After the image was digitized and these colorspots were quantified, DNA clones with more distinctive colortoward red were selected as smoke-induced genes. A similar approachwas also carried out in H2O2-treated cells. Twenty-two geneswere viewed as inducible genes (27), and 14 of these genes werefurther confirmed by Northern blot hybridization; all of themwere commonly elevated in smoke- and H2O2-exposed cells. A timecourse Northern blot hybridization study was carried out toelucidate the time course induction by smoke (Figure 3). Itseems that three phases of gene expression induction are recognizedin this study. The first phase occurs immediately after thesmoke and H2O2 treatment. These are genes of bcl-2, mdm-2 (seeFigure 1), and mitogen-activated protein kinase phosphatase1 (Figure 3). The second phase of genes induced by smoke andH2O2 are various stress proteins and ubiquitin genes, whichseemed to be induced in cells after 5 h of exposure. The thirdphase of gene induction is gene products related to oxidantmetabolism and cell tissue remodeling. The expression of thesegenes appeared 5 to 10 h later after smoke exposure. In additionto these 14 genes that are subsequently confirmed by Northernblot hybridization, other induced genes picked in this studyrequired further confirmation at the mRNA level.
Figure 3. Northern hybridization analysis for smoke-induced genes. RNA samples were isolated from these smoke-exposed cultures at 0, 5, 10, and 15 h after smoke exposure. 32P-labeled probes were prepared from these selected genes.
We have developed a high-throughput microarray system on nylonmembrane that can be used to profile differential gene expressionpatterns. The sensitivity of the detection is at one to twocopies per cell, and the reactivity is linear to the level ofthe expression. The only limitation of the approach is the inherentnature of the color resolution, which is absorbance based at1 to 2 order resolution, whereas the resolution for a fluorescenceprobe is at 3 to 5 orders of magnitude (24). Despite this limitation,the system is useful for a simultaneous quantification of muchgene expression by a single hybridization step. The informationgenerated from this approach is superior to that obtained byNorthern blot hybridization, which is one gene per hybridization.What needs to be further improved in the system in the future,in addition to the improvement on the quantitative analysisand standardization and the cluster analysis, is the developmentof bioinformatics tools that can integrate various array dataand pattern recognition with various biologic processes. Sucha development will fulfill the theme of functional genomicsand provide integrated information regarding the various molecularevents associated with the cellular process.
Using the microarray membrane generated in our laboratory, weinitiated the study to profile different gene expression patternsassociated with H2O2- and smoke-induced injury and repair onhuman bronchial epithelial cell line HBE1. Northern blot andtime course study were used to verify the expression patternof these inducible genes. From this study, three phases of geneinduction can be assessed (27). The first one is an immediatephase, in which inducible genes could be seen within 1 h aftersmoke exposure. These genes include MKP-1, mdm-2, and bcl-2.In contrast to MKP-1, the expression of the rest of the genesis not transient, and their inductions are persistently seenafter 24 h after exposure. It is interesting that these genesare involved in the prevention of cell apoptosis. MKP-1 is animportant negative regulator of mitogen-activated protein kinasepathways, especially the JNK/SAPK and p38 kinase pathways (2830).The second phase of gene induction occurs later, and the inductionis transient and the expression level is back to normal at 15h after smoke exposure. The representative genes in this phaseare HSP40, HSP70, HSP90a, and ubiquitin p62. Most of the functionsof these genes are involved in repairing denatured proteins.Stress proteins are an important chaperone for the proper foldingof macromolecules, whereas ubiquitin protein can tag those moleculesthat are irreparable for protease degradation. The third phaseof gene induction occurs 10 h after smoke exposure. These genesare glutaredoxin, light chain of ferritin, dihydrodiol dehydrogenase,MMP-1, and SPRR1B. Except for MMP-1, most of the induced genesin this phase are involved in the metabolism of oxidants. MMP-1is responsible for a variety tissue remodeling, whereas SPRR1Bis a squamous cell differentiation marker (31) under AP-1 transcriptionfactors control (32). Thus, an early but transient expressionof MKP-1 may block the expression of this differentiated marker.Further study is needed to elucidate the significance of sucha complex array in gene expression by bronchial epithelial cellsafter oxidant injury.
Acknowledgments
The authors acknowledge the grant support from the NationalInstitutes of Health (ES09701, ES06230, HL35635, AI50496, ES04699,and ES05707) and the California Tobacco-Related Disease-ResearchProgram (10RT-026).
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