MAPK/c-Jun signaling pathway contributes to the upregulation of the anti-apoptotic proteins Bcl-2 and Bcl-xL induced by Epstein-Barr virus-encoded BARF1 in gastric carcinoma cells

BARF1, encoded by Epstein-Barr virus (EBV), has been hypothesized to function as an oncogene, which was expressed in gastric carcinoma cells. Additionally, it has been reported that the anti-apoptotic function is closely associated with the expression of the B-cell lymphoma-2 (Bcl-2) protein. In addition, the signaling pathway has been reported to be involved in numerous diseases, including the mitogen-activated protein kinase (MAPK) cascade. In order to study the specific mechanism of anti-apoptotic function, BARF1-stably-expressing immortalized normal human embryo gastric epithelial cell line GES1 (GES-BARF1), and well-, moderately- and poorly-differentiated gastric carcinoma cell lines, MKN28 (which has been reported to be contaminated with the moderately-differentiated MKN74 gastric carcinoma cell line), SGC7901 and BGC823 (MKN-BARF1, SGC-BARF1 and BGC-BARF1, respectively) (GCC-BARF1) were constructed, with transfection of cells with the empty vector pSG5 acting as controls. Western blot analysis was performed to analyze the protein expression and the phosphorylation levels. Compared with the controls, it was found that the protein expression levels of c-Jun, Bcl-2 and B-cell lymphoma-extra large (Bcl-xL), as well as the phosphorylation levels of c-Jun, c-Jun N-terminal kinase (JNK) 1/2/3, p38 and extracellular signal-regulated kinase (ERK) 1/2 proteins were upregulated in 3 GCC-BARF1 but not significantly changed in GES-BARF1. The expression levels of the c-Jun, Bcl-2 and Bcl-xL proteins, and levels of c-Jun protein phosphorylation were significantly decreased in SGC-BARF1 cells subsequent to treatment with SP600125, SB203580, and U0126, which were the specific inhibitors of JNK1/2/3, p38 and ERK1/2 respectively. In addition, there was a gradual increase in the protein expression and phosphorylation levels between normal gastric epithelial cells, and well-differentiated, moderately-differentiated and poorly-differentiated gastric carcinoma cells, but this was not statistically significant. Therefore, the present study hypothesized that JNK1/2/3-, p38- and ERK1/2-MAPK/c-Jun cascade signaling pathways may contribute to the upregulation of the expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL induced by BARF1 in gastric carcinoma cells. This mechanism may mainly work in the progressive phase of the development in EBV-associated gastric carcinoma.