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Introduction

Esophageal cancer is among the most common malignancies worldwide, and in the United States has a 5-year survival rate following diagnosis of only ~19% (1). Squamous cell carcinoma and adenocarcinoma are the two main subtypes of esophageal cancer. The incidence of esophageal adenocarcinoma (EAC) has increased substantially in the United States, Western Europe, Australia and other developed countries over the past four decades (2). It is generally accepted that gastroesophageal reflux disease and obesity are explanations for the increased incidence of EAC (3). However, the underlying mechanism remains unclear.

Several genes have been reported to serve important roles in the development of EAC. The P53 gene has been found to be dysregulated in most cancer types (4). Furthermore, it is considered that P53 may be involved in the development of different cancers. For instance, a cohort study of chemoradiotherapy-naive surgically treated EAC reported that p53 expression was significantly correlated with disease-free survival and overall survival, independent of tumor stage (5). Meanwhile, a genome-wide association study of 2,515 EAC cases and 3,207 controls provided data to suggest that germline variations at the cyclin-dependent kinase inhibitor 2A locus may influence susceptibility to EAC (6). In addition, Gli and epithelial-mesenchymal transition-related protein expression was previously examined by western blot analysis in paired EAC patient tissues and cell lines. The results suggested that Gli may be critical for the metastasis and recurrence of esophageal adenocarcinomas (7). Osteopontin (OPN) isoforms have also been investigated in EAC, where results indicated that all OPN isoforms were frequently co-overexpressed in primary EACs, and that isoforms OPNb and OPNc enhanced invasion and dissemination through collective yet distinct mechanisms (8). However, despite these in-depth studies to identify novel targets for the treatment of EAC, there lacks a comprehensive presentation of the key genes and pathways implicated in EAC.

Gene expression profile analysis is a high-throughput method for detecting messenger RNA expression in tissue or cell samples. By analyzing the different gene expression between cancer patients and normal controls, an improved understanding of the molecular pathogenesis of a tumor can be obtained, facilitating the identification of the potential target genes and pathways for therapy (9,10).

The present study aimed to investigate the pathogenesis of EAC by a computational bioinformatics analysis of gene expression. Data from the Gene Expression Omnibus (GEO) database was extracted, and differentially expressed genes (DEGs) between EAC and normal samples were identified. The possible functions of the DEGs were predicted using enrichment analysis. Furthermore, protein-protein interaction (PPI) networks were visualized and module analysis was conducted using Cytoscape software to search for key genes that may be involved in the development of EAC.

Materials and methods

Affymetrix microarray data

The gene expression profiles of GSE92396, contributed by Peng et al (11), were downloaded from the Gene Expression Omnibus (GEO) database (http://www.ncbi.nlm.nih.gov/geo/). The platform was GPL6244, HuGene-1_0-st Affymetrix Human Gene 1.0 ST Array. The dataset included 12 esophageal adenocarcinoma samples and 9 normal esophageal samples; 9 were tumor-normal pairs.

Identification of DEGs

The data were pre-processed in R language (version 3.4.3; http://www.r-project.org/) using the oligo package (version 1.32.0; http://www.bioconductor.org/packages/release/bioc/html/oligo.html) (12,13). Probe levels were calculated and converted into the gene expression levels according to the annotation files in the GEO database. The DEGs of GSE92396 between the normal tissues and the tumor samples were analyzed with limma package (version 3.34.8) in R language (14). Fold-changes (FCs) in the gene expression values were calculated. |log2 FC|>2 and adjusted P-values <0.05 were considered to be the cut-off criteria for the identification of DEGs. A volcano plot was drawn using the gplots package (version 3.0.1).

Gene ontology (GO) and pathway enrichment analysis of the DEGs

The online analysis tool, the Database for Annotation, Visualization and Integrated Discovery (DAVID; version 6.8; http://david.abcc.ncifcrf.gov/) was used to analyse the DEGs for GO term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment. Enriched terms with >2 genes and a P-value <0.05 were considered to be statistically significant.

Construction of PPI network and screening of modules

The online analysis tool, the Search Tool for the Retrieval of Interacting Genes (STRING version 10.0; http://string-db.org/) was used to assess the PPI network of the DEGs, with the required confidence (combined score) >0.4. Visualization of the network and module analysis were performed with Cytoscape software (version 3.6.0; http://www.cytoscape.org/) and the MCODE plug-in (version 1.5.1) (15). The degree was statistically analysed using the CentiScaPe plug-in (version 2.2) to obtain hub nodes or genes in the PPI network (16). An MCODE computed node score >4 and node number >4 were considered as the cut-off criteria. Subsequent GO function and KEGG pathway enrichment analyses of the DEGs in the modules were performed using DAVID.

Results

Identification of DEGs

To identify DEGs between EAC samples and normal controls, the microarray dataset GSE92396, obtained from the GEO database, was screened. DEGs with |log2 FC|>2 and a P-value <0.05 were determined. A total of 386 DEGs were identified in EAC samples compared with in the normal controls, including 150 upregulated and 236 downregulated DEGs. The volcano plot is presented in Fig. 1.

Figure 1. Volcano plot of differentially expressed genes in esophageal adenocarcinoma. Black, non-differentially expressed genes; red, significantly upregulated genes; green, significantly downregulated genes (based on |log2 FC|>2 and adjusted P<0.05). FC, fold-change; adj.P.Val, adjusted P-value.

GO and pathway enrichment analysis of DEGs

To categorize the representation of DEGs and the involved pathways, GO and KEGG pathway enrichment analyses were performed using the online tool DAVID. The upregulated DEGs were enriched in 60 GO terms and 3 KEGG pathways. The GO functions enriched for the upregulated DEGs were mainly within the extracellular exosome (P=1.09×10−14), extracellular space (P=5.44×10−15) and extracellular region (P=1.61×10−10). KEGG pathways enriched for the upregulated DEGs were mainly in complement and coagulation cascades (P=6.84×10−5), maturity onset diabetes of the young (P=0.002282) and protein digestion and absorption (P=0.012483).

The downregulated DEGs were enriched in 67 GO terms and 5 KEGG pathways. The GO functions enriched for the downregulated DEGs were mainly within the extracellular exosome (P=8.36×10−12), epidermis development (P=3.26×10−22) and keratinocyte differentiation (P=2.62×10−20). KEGG pathways enriched for the downregulated DEGs were mainly in amoebiasis (P=3.46×10−4), retinol metabolism (P=0.020258) and drug metabolism-cytochrome P450 (P=0.022809). The top 10 terms of the GO enrichment analysis for up- and downregulated genes are presented respectively in Table I. The results of KEGG enrichment analysis for up- and downregulated genes are presented respectively in Table II.

Table I. GO enrichment analysis of the differentially expressed genes.

Table I.

GO enrichment analysis of the differentially expressed genes.

Category	Term	Count	P-value
Upregulated genes
GOTERM_CC_DIRECT	GO:0005615 extracellular space	43	5.44×10−15
GOTERM_CC_DIRECT	GO:0070062 extracellular exosome	62	1.09×10−14
GOTERM_CC_DIRECT	GO:0005576 extracellular region	40	1.61×10−10
GOTERM_CC_DIRECT	GO:0005578 proteinaceous extracellular matrix	15	3.09×10−8
GOTERM_BP_DIRECT	GO:0022617 extracellular matrix disassembly	9	2.36×10−7
GOTERM_CC_DIRECT	GO:0031012 extracellular matrix	14	7.17×10−7
GOTERM_CC_DIRECT	GO:0016324 apical plasma membrane	13	3.74×10−6
GOTERM_BP_DIRECT	GO:0030198 extracellular matrix organization	11	6.13×10−6
GOTERM_CC_DIRECT	GO:0005796 Golgi lumen	8	1.15×10−5
GOTERM_BP_DIRECT	GO:0030574 collagen catabolic process	7	1.58×10−5
Downregulated genes
GOTERM_BP_DIRECT	GO:0008544 epidermis development	22	3.26×10−22
GOTERM_BP_DIRECT	GO:0030216 keratinocyte differentiation	20	2.62×10−20
GOTERM_CC_DIRECT	GO:0001533 cornified envelope	15	1.04×10−16
GOTERM_BP_DIRECT	GO:0018149 peptide cross-linking	14	1.69×10−14
GOTERM_BP_DIRECT	GO:0031424 keratinization	13	2.97×10−13
GOTERM_MF_DIRECT	GO:0005198 structural molecule activity	21	7.55×10−12
GOTERM_CC_DIRECT	GO:0070062 extracellular exosome	74	8.36×10−12
GOTERM_CC_DIRECT	GO:0030057 desmosome	8	8.15×10−9
GOTERM_MF_DIRECT	GO:0004867 serine-type endopeptidase inhibitor activity	10	1.66×10−6
GOTERM_BP_DIRECT	GO:0061436 establishment of skin barrier	6	1.76×10−6

[i] GO, gene ontology; _BP, biological process; _CC, cellular component; _MF, molecular function.

Table II. KEGG pathway enrichment analysis of the differentially expressed genes.

Table II.

KEGG pathway enrichment analysis of the differentially expressed genes.

Category	Term	Count	P-value
Upregulated genes
KEGG_PATHWAY	hsa04610 Complement and coagulation cascades	7	6.84×10−5
KEGG_PATHWAY	hsa04950 Maturity onset diabetes of the young	4	0.002282
KEGG_PATHWAY	hsa04974 Protein digestion and absorption	5	0.012483
Downregulated genes
KEGG_PATHWAY	hsa05146 Amoebiasis	7	3.46×10−4
KEGG_PATHWAY	hsa00830 Retinol metabolism	4	0.020258
KEGG_PATHWAY	hsa00982 Drug metabolism - cytochrome P450	4	0.022809
KEGG_PATHWAY	hsa05204 Chemical carcinogenesis	4	0.034682
KEGG_PATHWAY	hsa00350 Tyrosine metabolism	3	0.038991

[i] KEGG, Kyoto Encyclopedia of Genes and Genomes; hsa, homo sapiens.

Construction of PPI network and screening of modules

Based on information from the STRING database, a PPI network comprising of 369 nodes and 534 edges was constructed using the Cytoscape software (Fig. 2). The top 10 hub nodes with the highest degrees were interleukin (IL)-8, involucrin (IVL), tissue inhibitor of metalloproteinase 1 (TIMP1), fibronectin 1 (FN1), serpin family E member 1 (SERPINE1), serpin family A member 1 (SERPINA1), cystic fibrosis transmembrane conductance regulator (CFTR), secreted phosphoprotein 1 (SPP1), collagen type I alpha 1 chain (COL1A1) and angiotensinogen (AGT). A total of 6 modules from the PPI network satisfied the criteria of an MCODE computed node score >4 and number of nodes >4. The results are presented in Fig. 3. The functional annotation of the DEGs involved in the modules was determined using DAVID. The results showed that the module DEGs were enriched in 66 GO terms and 9 KEGG pathways. The GO functions enriched for the module DEGs were mainly within the extracellular exosome (P=1.27×10−8), extracellular region (P=8.63×10−11) and extracellular space (P=1.21×10−7). KEGG pathways enriched for the module DEGs were mainly within arachidonic acid metabolism (P=1.02×10−4), complement and coagulation cascades (P=1.56×10−4), and rheumatoid arthritis (P=3.98×10−4). The top 10 terms of the GO and KEGG enrichment analyses for module DEGs are presented in Table III.

Figure 2. Protein-protein interaction network of differentially expressed genes. Red, significantly upregulated genes; green, significantly downregulated genes. Node size is negatively related to P-value; edge color and width are positively related to combined score.

Figure 3. (A-F) Modules of PPI network determined using the MCODE plugin of Cytoscape. Red, significantly upregulated genes; green, significantly downregulated genes. Node size is negatively related to P-value; edge color and width are positively related to combined score.

Table III. GO and KEGG enrichment analyses of the differentially expressed genes in the modules.

Table III.

GO and KEGG enrichment analyses of the differentially expressed genes in the modules.

Category	Term	Count	P-value
GOTERM_CC_DIRECT	GO:0005576 extracellular region	22	8.63×10−11
GOTERM_BP_DIRECT	GO:0018149 peptide cross-linking	7	5.00×10−9
GOTERM_CC_DIRECT	GO:0070062 extracellular exosome	25	1.27×10−8
GOTERM_BP_DIRECT	GO:0030216 keratinocyte differentiation	7	6.52×10−8
GOTERM_CC_DIRECT	GO:0005615 extracellular space	17	1.21×10−7
GOTERM_BP_DIRECT	GO:0031424 keratinization	6	2.16×10−7
GOTERM_BP_DIRECT	GO:0016266 O-glycan processing	6	6.72×10−7
GOTERM_BP_DIRECT	GO:0022617 extracellular matrix disassembly	6	2.20×10−6
GOTERM_MF_DIRECT	GO:0005198 structural molecule activity	8	3.19×10−6
GOTERM_CC_DIRECT	GO:0005796 Golgi lumen	6	4.70×10−6
KEGG_PATHWAY	hsa00590 Arachidonic acid metabolism	5	1.02×10−4
KEGG_PATHWAY	hsa04610 Complement and coagulation cascades	5	1.56×10−4
KEGG_PATHWAY	hsa05323 Rheumatoid arthritis	5	3.98×10−4
KEGG_PATHWAY	hsa00982 Drug metabolism - cytochrome P450	4	0.002504
KEGG_PATHWAY	hsa00980 Metabolism of xenobiotics by cytochrome P450	4	0.003187
KEGG_PATHWAY	hsa00350 Tyrosine metabolism	3	0.008675
KEGG_PATHWAY	hsa00830 Retinol metabolism	3	0.028145
KEGG_PATHWAY	hsa00010 Glycolysis/Gluconeogenesis	3	0.02977
KEGG_PATHWAY	hsa05204 Chemical carcinogenesis	3	0.041201

[i] GO, gene ontology; KEGG, Kyoto Encyclopedia of Genes and Genomes; _BP, biological process; _CC, cellular component; _MF, molecular function; hsa, homo sapiens.

Discussion

EAC is one of the most common subtypes of esophageal cancer (17), and only ~19% of patients survive 5 year after diagnosis in the United States (1). Therefore, there is a need to screen for key genes and pathways that are associated with the progression of EAC, with the aim of improving its diagnosis and treatment.

The present study used bioinformatics analysis to identify the DEGs between EAC and normal tissue expression profiles. The results revealed that the expression of 386 genes was significantly altered in EAC samples (150 upregulated and 236 downregulated genes) compared with in the normal controls. A PPI network was constructed to reveal the associations between these genes. The top 10 genes with the highest degrees were identified. Furthermore, 6 modules were selected according to their respective MCODE computed node scores (>4), and their functions were determined by GO and KEGG pathway analyses.

The GO functions enriched for the upregulated DEGs were mainly within the extracellular exosome, extracellular space and extracellular region. KEGG pathways enriched for the upregulated DEGs were mainly within complement and coagulation cascades, maturity onset diabetes of the young and protein digestion and absorption. The GO functions enriched for the downregulated DEGs were mainly within the extracellular exosome, epidermis development and keratinocyte differentiation. KEGG pathways enriched for the downregulated DEGs were mainly within amoebiasis, retinol metabolism and drug metabolism-cytochrome P450. Previous study has demonstrated that activation of the coagulation cascade affected tumor development (18). The underlying mechanism through which coagulation cascade proteins promote tumorigenesis remains unclear. Therefore, investigating these identified signaling pathways may aid to elucidate the carcinogenic mechanism behind EAC.

Based on the results of PPI network construction for the DEGs, a number of hub nodes were identified. The top 10 hub nodes with the highest degrees were IL8, IVL, TIMP1, FN1, SERPINE1, SERPINA1, CFTR, SPP1, COL1A1 and AGT. IL8, also named C-X-C motif (CXC) chemokine ligand 8, is a chemokine that mainly attracts inflammatory leukocyte infiltrate by acting on CXC chemokine receptor 1/2. Recent speculations propose that IL8 serves important roles in angiogenesis and survival signaling for cancer stem cells, and that the interleukin may stimulate the secretion of local growth factors in malignant tumors (19). IL8 stimulation on endothelial cells has been reported to begin angiogenic processes characterized by secretion of matrix metalloproteinases (MMPs), which can break down the extracellular matrix and stimulate the formation of new vessels (20). One study reported that IL8 was significantly upregulated in esophageal carcinogenesis, being detected in the serum of patients with esophageal adenocarcinoma (21). IVL is a squamous cell differentiation marker, and is associated with terminal differentiation of epithelial cells (22,23). Upon IL4 stimulation, the overall esophageal epithelia still contained stratified morphology. However, IVL was significantly decreased in esophageal basal and suprabasal layers, which was associated with a disorganized morphology of stratified layers on the basal side (24). TIMP1 is an inhibitor of matrix metalloproteinases, which has a key role in cancer cell dissemination and endothelial cell migration in angiogenesis (25). High serum levels of TIMP1 have been associated with tumor progression and poor prognosis in esophageal cancer patients (26). FN1, a mesenchymal marker (27), is an extracellular matrix glycoprotein that serves key roles in cell differentiation, growth and migration; through which it is associated with certain processes including wound healing, embryonic development and carcinogenesis (28). SERPINE1, also known as plasminogen activator inhibitor-1, is involved in the inhibition of urokinase-type plasminogen activator (29). It serves important roles in increasing tumor invasion and angiogenesis, and has been correlated with a poor prognosis (30). A high tumor level of plasminogen activator inhibitor-1 in patients with primary breast cancer is reportedly suggestive of poor prognosis (31), through this association requires verification in EAC. SERPINA1 encodes for α1-antitrypsin, which targets several proteases, including elastase, plasmin, thrombin, trypsin, chymotrypsin, and plasminogen activator (32). One study suggested that α1-antitrypsin may be involved in lung adenocarcinoma metastasis by targeting fibronectin (33). CFTR encodes an ATP-binding cassette membrane protein that functions as a chloride channel, and is mutated in cystic fibrosis (34,35). A previous large-scale meta-analysis suggested that the novel single nucleotide polymorphism (SNP) rs17451754, which is located within intron 21 of the CFTR gene, markedly associates with the risk of Barrett's esophagus and EAC (36). This region is reportedly involved with the enhancer histone modifications in the gastrointestinal tract mucosa and DNAse hypersensitivity (37). SPP1, also known as OPN, encodes a protein that binds hydroxyapatite, and is a cytokine that upregulates interferon-γ and IL12 (38). A previous study reported that SPP1 isoforms collectively enhanced tumor cell invasion and dissemination in EAC (8). COL1A1 encodes the pro-α1 chains of type I collagen. It has been reported COL1A1 is overexpressed in many tumours, including gastric cancer, hepatocellular carcinoma, non-small cell lung cancer, and colorectal cancer (39–42). One study suggested that COL1A1 may promote metastasis in colorectal cancer by regulating the Wnt/planar cell polarity pathway (39). AGT is involved in the renin-angiotensin system (RAS). Previous study reported that RAS participated in the physiological control of esophageal motor activity (43). Together with these previous findings, the present results are suggestive that RAS may be involved in the contraction disorder of esophageal adenocarcinoma.

A total of 6 modules from the PPI network satisfied the criteria of MCODE score >4 and number of nodes >4. The GO functions enriched for the module DEGs were mainly within the extracellular exosome, extracellular region and extracellular space. KEGG pathways enriched for the module DEGs were mainly in arachidonic acid metabolism, complement and coagulation cascades and rheumatoid arthritis. A previous study suggested that the activated arachidonic acid metabolism pathway serves an important role in tumorigenesis (44). The enzymes activated by this pathway and their products promote the inflammatory response and have been implicated in multiple cellular processes, including cell proliferation, invasion and metastasis, and thus may promote tumorigenesis. Additionally, previous study has demonstrated that activation of the coagulation cascade affected tumor development (18). Therefore, the arachidonic acid metabolism and complement and coagulation cascades pathways may be involved in the development of EAC.

In conclusion, the present study identified the genes differentially expressed between EAC and normal samples. The top most altered DEGs included IL8, IVL, TIMP1, FN1, SERPINE1, SERPINA1, CFTR, SPP1, COL1A1 and AGT, and the pathways of arachidonic acid metabolism, complement and coagulation cascades, and rheumatoid arthritis may potentially be used as diagnostic and therapeutic targets in EAC. However, the present study is limited to an extent due to the small sample size and lack of experimental validation. Further experimental confirmation of the expression profile in EAC by immunoblotting or immunohistochemical staining is therefore required to validate the current findings.

Acknowledgements

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Funding

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Availability of data and materials

The datasets used during the current study are available in the Gene Expression Omnibus database (accession no. GSE92396; http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE92396).

Authors' contributions

FH and BA designed the study. FH and LT analyzed and interpreted the data. FH was primarily responsible for writing the manuscript. All authors reviewed and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Glossary

Abbreviations

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References