Transit and sorting of apolipoprotein B within the endoplasmic reticulum and Golgi compartments of isolated hepatocytes from normal and orotic acid-fed rats.

Apolipoprotein B (apoB) secretion by isolated rat hepatocytes was dependent on addition of oleate to the incubation medium and inhibited in hepatocytes isolated from livers of orotic acid-fed rats (OA hepatocytes). To investigate the intracellular transit of newly synthesized apoB under different conditions, normal hepatocytes (with or without oleate) or OA hepatocytes (with oleate) were incubated with [35S]methionine, and subcellular fractions (rough endoplasmic reticulum, smooth endoplasmic reticulum, cis-Golgi, and trans-Golgi and membrane and lumenal contents from these) were isolated at intervals. The specific activities and pool sizes of apoB100 and apoB48 were determined. The observations indicate that there are several points at which intracellular transit of apoB is regulated. Newly synthesized apoB is either translocated to the lumen of the rough endoplasmic reticulum or remains membrane bound and is degraded. The lumenal apoB is either retained and degraded or transferred to the Golgi lumen and secreted. In OA hepatocytes degradation of the membrane-bound form of apoB is inhibited, and the protein accumulates in the trans-Golgi membranes. Although apoB is translocated to the lumen of the rough endoplasmic reticulum in OA hepatocytes, it is not packaged with lipid and is transferred to the Golgi lumen only slowly.