Effect of glycosidase inhibitors on the biosynthesis of alpha 2-plasmin inhibitor and antithrombin III in Hep G2 cells.

We studied the effect of the glucosidase I inhibitor, N-methyl-1-deoxynojirimycin (MdN) and the mannosidase inhibitor, 1-deoxymannojirimycin (dMM) on the biosynthesis and secretion of alpha 2-plasmin inhibitor (alpha 2-PI) and antithrombin III (ATIII) in cultures of human hepatoma (Hep-G2) cells. Incubation with 1 mM MdN decreased secreted alpha 2-PI activity and antigen levels by about 40%, whereas those of ATIII were not affected. Neither inhibitor affected the messenger RNA levels as determined by Northern blotting. Pulse-chase studies using [35S]-methionine showed that MdN decreased alpha 2-PI and ATIII secretion rates. By the 18 h chase, MdN had decreased secreted alpha 2-PI to 50-60%, with little effect on ATIII. Intracellular forms of alpha 2-PI or ATIII synthesized by cells treated with 1 mM MdN were sensitive to endoglycosidase H (Endo H), whereas almost all the secreted forms were resistant, suggesting the presence of complex-type oligosaccharides. In the presence of 1 mM dMM, cells synthesized Endo H-sensitive alpha 2-PI and ATIII with similar secretion rates. These results suggest that retention of glucose on N-linked oligosaccharides not only retards the exit of alpha 2-PI and ATIII, but also changes the catabolic rate of alpha 2-PI in the endoplasmic reticulum.