Proteomic alteration in gastic adenocarcinomas from Japanese patients

Gastric adenocarcinomas comprise one of the common types of cancers in Asian countries including Japan, being second only to lung cancer as to the number of deaths it causes. In spite of the recent development of diagnostic techniques, most gastric cancer patients are diagnosed at an advanced stage and have a very low five-year survival rate (less than 10%) [1]. This is partially due to a lack of specific and sensitive biomarkers for the diagnosis and monitoring of disease progress at an early stage, although some gastric tumor markers, including the carcinoembryonic antigen, have been used and are partly effective. As gastric carcinogenesis is a multistep process, comprehensive analysis is also required for individual cases, in which different molecular events occur in each carcinogenic process.

Recently, proteomic analysis was utilized to comprehensively examine protein expression in bodily fluids, tissues and cells [2‐4]. This approach, as clinical proteomics, is very useful for identifying disease-associated proteins that show changes in expression and modification corresponding to a disease condition [5, 6]. These disease-related proteins are expected to be biomarkers for diagnosis and putative targeted proteins for treatment [7‐9]. On the other hand, comprehensive analyses of transcriptomes in tumor tissues from various cancer patients using DNA microarrays and gene chips have been performed in recent years [10]. However, a lack of correlation between changes in mRNAs and carcinogenesis has been demonstrated, and quantitative and qualitative changes of post-translationally modified proteins as final gene products are considered to be more informative than those of mRNAs in tumor tissues for studying the molecular events in carcinogenesis. Proteomic studies for the identification of tumor-associated proteins in gastric cancer are increasing, and proteome databases for gastric tissues [11] and cell lines [12] have been constructed. Most of them concern specific proteins or antigens that reflect the chemo- and thermo-resistant properties of stomach cancer [13‐15], and that are associated with Helicobactor pylori [16, 17]. In the present study, we performed comprehensive proteome analysis of tumor and nontumor tissues in Japanese patients with gastric carcinomas, and identified several proteins of which the expression levels are commonly altered in clinical cases. In particular, the expression of gastrokine-1 (GKN-1) was suggested to be under both transcriptional and translational control.

In this study we performed proteomic analysis of tumor and nontumor tissues derived from Japanese gastric adenocarcinoma patients. Sixty-nine different proteins in the tumor tissue from a gastric cancer case were identified by 2D-EP and LC-ESI-MS/MS, which included stress proteins, Hsp70, Hsp90 and chaperonine-containing TCP1 (CCT), for self-protection; glycolytic enzymes, triose phosphate isomerase 1, α-enolase and PGK-1, for the growing energy requirement; cytoskeletal proteins, ezrin, gelsolin isoform b and vimentin; proteins involved in cell differentiation and proliferation, galectin-3 and transferrin; and proteins exhibiting putative tumor suppressor activity, FOV. Many of these proteins have been reported to be associated with the tumorigenesis of gastric adenocarcinomas involving multiple steps and factors [18, 19].

We also demonstrated that the expression levels of several proteins in tumor tissues were commonly altered in five Japanese patients with gastric adenocarcinomas, compared to those in nontumor tissues. A common increase in protein expression in tumor tissues occurred for MnSOD, HMG-1 and PGK-1, whereas a common decrease in tumor tissues occurred for FOV, GST and AST.

Superoxide (O₂^-), a free radical, is essential for the anti-microbial action of granulocytes and monocytes. Superoxide dismutase (SOD) rapidly removes the excess amount of superoxide produced in the stress and biological reactions in vivo through catalysis of the conversion of superoxide with H⁺ to H₂O₂ and O₂. MnSOD, one of the SODs, is located in mitochondria and contributes to the protection of mitochondrial DNA from damage. It has been reported that enhanced expression of MnSOD in progressive gastric cancer should be related to the 5-year survival rate after surgery [20] and sensitivity to chemotherapy [21]. In this study, protein expression of MnSOD was up-regulated in 4 of 5 gastric cancer patients, suggesting a self-protecting response. On the other hand, the up-regulation of MnSOD in tumor tissues has been considered to interfere with effective chemotherapy based on radical production, which may cause a decrease in sensitivity to anti-cancer drugs and determine the severity of the cancer.

HMG-1 regulates the transcription of various genes and the structural stabilization of chromosomes as a DNA-binding protein. HMG-1 has been reported to be associated with carcinogenesis and metastasis in colorectal and breast cancer [22].

Transcriptional up-regulation of HMG-1 in gastric cancer has also been demonstrated [23]. Expression of HMG-1 in tumor tissues was suggested to be related to resistance to cisplatin [24]. We demonstrated here an increase in this protein in three cases.

GST is a drug-metabolizing enzyme that catalyzes the conjugation of reduced glutathione to drugs and metabolites, and also contributes to the detoxification of carcinogens. Therefore, a decrease in the activity may be a risk factor for carcinogenesis. Infection by Helicobacter pylori has been reported to cause a decrease in GST expression [25], suggesting that a decrease in GST activity might be a cause of gastric carcinogenesis. In this study, a decrease in GST protein expression was observed in three cases. A decrease in GST protein expression may be utilized as a biomarker for the diagnosis of gastric tumors.

AST is an aminotransferase that acts on amino acids and α-keto acid, that is distributed widely in human organs. AST is released into the blood stream due to enhanced permeability and destruction of tissues. A elevated concentration of AST protein has been reported in the blood of patients with hepatitis, malignant tumors including hepatomas, and leuchemia. Gene expression of GST has also been demonstrated to be up-regulated in colorectal tumor tissues [26]. However, a decrease in AST protein expression was commonly observed in all tumor tissues derived from the present five patients with gastric adenocarcinomas. Further study should be performed to elucidate whether a decrease in the AST protein is specifically observed in gastric tumor tissues or not.

Elevated expression of PGK-1, which is one of the enzymes in the glycolytic pathway and which catalyzes the dephosphorylation of 1,3-bisphosphoglycerate to produce ATP, has been observed in many malignant tumor tissues that are dependent on ATP as a major energy source. Solid tumors are thought to need the overproduction of ATP to maintain the enhanced proliferation. The up-regulation of glycolytic enzymes, including PGK-1 in lung cancer [27] and M2-type pyruvate kinase in colorectal cancer [28], has been reported and suggested to be useful for cancer screening. However, in the present study, a significant relation between PGK-1 protein expression and the phenotypes of gastric adenocarcinomas was hardly detected. Therefore, PGK-1 was considered to be not suitable for the diagnosis of gastric cancer.

Carbonic anhydrases (CAs) are zinc-containing enzymes that are distributed widely in various organs and that comprise a large family including CA-I to CA-IX. They catalyze the hydration of CO₂ for intermediate metabolism, and maintain the pH and ion equilibrium in the body. So far a direct relationship has been demonstrated between malignant transformation and protein expression for CAs I through VII [29]. Two earlier studies revealed that the expression of both CA-I and CA-II was significantly reduced in colorectal tumors compared to in normal colorectal epithelia or mucosae [30]. Another report presented results showing that reduced expression of CA-I and CA-II was correlated with the biological aggressiveness of colorectal cancer and synchronous distant metastasis [31]. As suggested previously [32], gastric and colorectal carcinomas may share a similar mechanism of cell proliferation and mucosal malignancy, and may become a biomarker for these carcinomas, because common decreases in the protein expression of CA-I and CA-II were also observed in this study.

FOV, which is identical to a stomach-specific 18-kDa antrum mucosa protein! (AMP-18) [33, 34], GKN1 [35], and trefoil factor interactions(z) (TFIZ) [36], was demonstrated to be dramatically down-regulated or absent in the gastric adenocarcinoma patients in this study. The human AMP-18 and human carcinoantigenic CA11 gene product, which encodes an amino acid sequence that differs from that of AMP-18 in only a single residue [37, 38], function as secreted growth factors partly responsible for maintaining a normal functional gastric epithelium [33]. Both AMP-18 and the CA11 gene product have been reported to be intensively expressed in normal stomach tissue but not in most gastric cancers [33, 37, 38]. Immunohistochemical studies demonstrated that AMP-18 appears to be present in mucosal epithelial cells of the normal human gastric antrum and duodenum [33]. Recently, TFIZ was demonstrated to act as a growth regulator of gastric epithelial cells through the formation of a heterodimer with trefoil factor 1 (TFF1) [36]. Our present data further confirmed the high expression of FOV in nontumor gastric tissues and the significant suppression of the protein in gastric adnocarcinomas, indicating that FOV play roles in the maintenance of normal differentiation of epithelial cells and tumor suppression but not in tumor tissues. Furthermore, we demonstrated that transcription of FOV mRNA was also commonly down-regulated in gastric cancer patients, indicating that a marked decrease in FOV protein was caused by suppression of FOV gene expression. Furthermore, in one patient the protein was not detected in nontumor tissues, suggesting that the expression level of FOV protein in individuals may determine the gastric adenocarcinoma phenotype. Accordingly, the expression level of FOV as a biomarker may be informative for the assessment of gastric cancer.

Protein profiling of tumor and nontumor tissues derived from Japanese patients with gastric adenocarcinomas was performed using 2D-EP and LC-ESI-MS/MS. The identified proteins included molecular chaperones, energy-producing enzymes, cytoskeletal proteins, and so on. Common protein alterations were detected in the gastric cancer patients. Protein expression of MnSOD and HMG-1 was up-regulated while that of GST, AST and FOV was down-regulated in gastric tumor tissues. A correlation between the alteration of these proteins and their transcriptional expression in gastric cancer was hardly observed in this study, except for in the case of FOV. Both the protein and gene expression of FOV, a stomach-specific secretory growth factor for normal gastric epithelial cells, was markedly down-regulated in tumor tissues derived from Japanese patients with gastric adenocarcinomas. Monitoring of the expression levels of this stomach-specific protein in clinical samples may provide useful information for the diagnosis of gastric cancer as a specific biomarker and for better understanding of gastric carcinogenesis.