Expression and biological role of N-Acetylglucosaminyltransferase-III in human and murine liver disease

Abstract

N-Acetylglucosaminyltransferase-Ⅲ [EC 2. 4. 1. 144] (GnT-Ⅲ) catalyzes the addition of N-acetylglucosamine in β-1, 4- linkage to the β-linked mannose of the trimannosyl core of N-linked glycans to produce a bisecting glucosamine residue. There are several reports that GnT-Ⅲ expression is elevated in the tissues of hepatic disease and that the GnT-Ⅲ level in the serum of patients with hepatic disease is significantly increased compared to the normal value. The current investigation involves three aspects of human GnT-Ⅲ molecules: (1) Molecular characteristics of GnT-Ⅲ in hepatic cancer cells, (2) immunological/histochemical study of GnT-Ⅲ levels in the serum and tissues of normal vs. hepatic disease, and (3) development of a transgenic murine model which overexpresses human GnT-Ⅲ in the liver. The GnT-Ⅲ activity in human hepatoma cells (Hep3B and HepG2), normal primary hepatocytes and hepatoma tissues was studied, and observed that GnT-Ⅲ was primarily associated with the cell membranes of Hep3B and HepG2 cells. Using 2 substrates for GnT-Ⅲ, namely, GlcN,GlcN-biant-PA and UDP-GlcNAc, it was founded that the enzymatic activity of GnT-Ⅲ in Hep3B and HepG2 exhibits different characteristics as determined by their respective Km values. Judging from the GnT-Ⅲ activity of the cell extracts, hepatocarcinoma cells contain significantly higher levels of GnT-Ⅲ than that of normal liver cells or primary adult hepatocytes. The mechanism that induces this effect is presently unknown. To investigate the relationship between human GnT-Ⅲ and development of hepatic disease, the level of GnT-Ⅲ in hepatoma tissues and sera have been determined by both an enzyme-linked immunosorbent assay (ELISA) and the enzymatic activity measurement. To develop an ELISA, aglycosyl-recombinant human GnT-Ⅲ (AGR-GnT-Ⅲ) with molecular weight of about 59 kDa was produced and the protein to generate the monoclonal antibody (GT 273) that is specific for AGR-GnT-Ⅲ was used. The ELISA results showed elevate...