Down-regulation Of N-acetylglucosaminyltransferase V Induces ER Stress By Changing Glycosylation And Function Of GLUT1

N-acetylglucosaminyltransferaseV (GnT-V, EC 2. 4. 1. 155), which localizes in Golgi apparatus, catalyzes the transfer of GlcNAc residue from UDP-GlcNAc to anα-1, 6 mannoside arm in the acceptor glycans to synthesize the GlcNAc-β1, 6-Man branching (GlcNAcβ1, 6 Manα1, 6), then forms tri-or tetra-antennary N-linked oligosaccharide chains. GnT-V is a key enzyme in the processing of N-glycans during synthesis of glycoproteins. It is well known that GnT-V is associated with tumorigensis of normal cells and metastasis of tumor cells. In our laboratory, using antisense oligonucleotide and RNAi techniques, it was found that down-regulating GnT-V could activate endoplasmic reticulum stress ( ER stress ) responses in 7721 cells, a human hepatocarcinoma cell line ( the expression of GnT-V was decreased about 56% ). There was an up-regulation of Bip at mRNA and protein levels, and appearance of the spliced form of XBP1mRNA in antisense GnT-V transfectant. In this study, we further investigated the molecular mechanism of ER stress induced by down-regulating GnT-V.We selected glucose transporter (GLUT) as the object of study, since GLUT was a glycoprotein containing an N-linked glycan chain and responsible for basic supply of cells with glucose. Glucose starvation has been shown to induce ER stress in tumor ells. Therefore we hypothesized that GnT-V change glycosylation of GLUT and further affect the function of GLUT. Furthermore, inspired by a literature, we observed the effect of GnT-V on ER stress existed in HuH7 hepatocarcinoma cell line. The results obtained from this study are as follows: (1) GLUT1 has become the predominant GLUT isoform of 7721 cells. (2) The content oftri-or tetra-antennary sugar chain of GLUT1 decreased in antisense GnT-V transfectant. (3) Glucose transport activity of GLUT1 in antisense GnT-V transfectant is at least 2-fold lower than that in mock tansfectant. (4) Modulation of GLUT1 glycosylation does not interfere with the distribution of GLUT1 protein within cells. (5) Glucose deprivation can activate ER stress responses in 7721 cells. (6) GnT-V overexpression attenuates the ER stress in HuH7 cells. In summary, the glucose transport activity and glycosylation of GLUT1 decreased in antisense GnT-V transfectant, which may be one possible mechanism of ER stress induced by down-regulating GnT-V, and GnT-V may contribute to the regulation of glucose uptake by modifying glycosylation of GLUT1 in some tumor cells.