Blocking of N-acetylglucosaminyltransferase V induces cellular endoplasmic reticulum stress in human hepatocarcinoma 7,721 cells

N-acetylglucosaminyltransferase V (GnT-V) is an important tumorigenesis and metastasis-associated enzyme. To study its biofunction, the GnT-V stably suppressed cell line (GnT-V-AS/7,721) was constructed from 7,721 hepatocarcinoma cells in previous study. In this study, cDNA array gene expression profiles were compared between GnT-V-AS/7,721 and parental 7,721 cells. The data indicated that GnT-V-AS/7,721 showed a characteristic expression pattern consistent with the ER stress. The molecular mechanism of the ER stress was explored in GnT-V-AS/7,721 by the analysis on key molecules in both two unfolded protein response (UPR) pathways. For ATF6 and Ire1/XBP-1 pathway, it was evidenced by the up-regulation of BIP at mRNA and protein level, and the appearance of the spliced form of XBP-1. As for PERK/eIF2alpha pathway, the activation of ER eIF2alpha kinase PERK was observed. To confirm the results from GnT-V-AS/7,721 cells, the key molecules in the UPR were examined again in 7,721 cells interfered with the GnT-V by the specific RNAi treatment. The results were similar with those from GnT-V-AS/7721, indicating that blocking of GnT-V can specifically activate ER stress in 7,721 cells. Rate of (3)H-Man incorporation corrected with rate of (3)H-Leu incorporation in GnT-V-AS/7,721 was down-regulated greatly compared with the control, which demonstrated the deficient function of the enzyme synthesizing N-glycans after GnT-V blocking. Moreover, the faster migrating form of chaperone GRP94 associated with the underglycosylation, and the extensively changed N-glycans structures of intracellular glycoproteins were also detected in GnT-V-AS/7,721. These results supported the mechanism that blocking of GnT-V expression impaired functions of chaperones and N-glycan-synthesizing enzymes, which caused UPR in vivo.