Association and assembly of triglyceride and phospholipid with glycosylated and unglycosylated apoproteins of very low density lipoprotein in the intact liver cell

Using estrogen-induced chick liver cells which synthesize and secrete large amounts of very low density lipoprotein (VLDL), we have previously shown (Siuta-Mangano, P., Howard, S., Lennarz, W. J., and Lane, M. D. (1982) J. Biol. Chem. 257, 4292-4300) that the major protein constituent of VLDL, the 350,000 molecular weight apoprotein (apoprotein B), is synthesized as a single polypeptide to which core oligosaccharides are added co-translationally. This system has now been employed to study the assembly of the apoproteins of VLDL with their glycerolipid (triglyceride and phospholipid) components and the secretion of the VLDL glycerolipids. In the presence of cycloheximide such that VLDL apoprotein synthesis is inhibited 98%, the secretion of lipids labeled from a [3H]palmitate pulse by hepatocyte monolayers was halted only after completed apoprotein chains had cleared the cell. Under conditions whereby tunicamycin inhibited [3H]glucosamine incorporation into apoprotein B by 98% and [3H]leucine incorporation into the VLDL apoproteins minimally, the unglycosylated form of apoprotein B assembled with the usual complement of triglyceride and phospholipid as did glycosylated apoprotein B to form a VLDL which was readily secreted by the hepatocyte. Taken together, these findings demonstrate that whereas apoprotein synthesis is necessary for the secretion of the major lipid components of VLDL, glycosylation of apoprotein B is not required for either the assembly of VLDL glycerolipids or for the secretion of the VLDL particle.