Tunicamycin enhances the sensitivity to TRAIL in gastric cancer cells by up-regulation of TRAIL-R2

Objective: To investigate the promoting effect of TM (tunicamycin) on apoptosis of gastric cancer cells induced by TRAIL [TNF (tumor necrosis factor)-related apoptosis-inducing ligand], and to explore its possible mechanism. Methods: The effect of treatment with TM (1 μmol/L)/TRAIL (100 μg/L) alone or in combination for 3, 6, 16, 24 and 36 h on the apoptotic rate of SGC-7901 cells was detected by FCM (flow cytometry) using propidium iodide DNA staining. The cell surface expression levels of different types of TRAIL receptors including TRAIL-R1, TRAIL-R2, TRAIL-R3 and TRAIL-R4 before and after TM treatment were detected by FCM, and the expression level of TRAIL-R2 mRNA was detected by RFQ-PCR (real-time fluoregenic quantitative-PCR). The expression levels of GRP78 (78-kDa glucose-regulated protein) and CHOP (CCAAT/enhancer-binding protein homologous protein) proteins were detected by Western blotting. The splicing of XBP1 (X-box binding protein 1) mRNA was detected by RT-PCR. Results: Treatment with TM alone induced minimal level of apoptosis of SGC-7901 cells. The apoptosis rate of SGC-7901 cells was increased significantly after treatment with TM in combination with TRAIL. TM could markedly up-regulate cell surface expression level of TRAIL-R2 on cell surface. In contrast, TM did not induce any changes in the expressions of TRAIL-R1, TRAIL-R3 and TRAIL-R4.The expression level of TRAIL-R2 mRNA of SGC-7901 cells induced by TM treatment was up-regulated in a time-independent manner. The results of up-regulation of GRP78 and the splicing of XBP1 mRNA demonstrated the activation of  UPR (unfolded protein response) induced by TM. Treatment with TM also resulted in a remarkable increase in the expression of CHOP protein. Conclusion: TM enhances TRAIL-induced apoptosis in gastric cancer cells by up-regulation of TRAIL-R2 expression via UPR. CHOP may be responsible for this effect involved in up-regulation of TRAIL-R2. DOI:10.3781/j.issn.1000-7431.2013.01.003