Effects of g-Tocotrienol on ApoB Synthesis, Degradation, and Secretion in HepG2 Cells

g-Tocotrienol (g-T3), a naturally occurring analog of tocopherol (vitamin E), has been shown to have a hypocholesterolemic effect in animals and humans. Unlike tocopherol, it has also been shown to reduce plasma apoB levels in hypercholesterolemic subjects. The aim of this study was to define the mechanism of action of g-T3 on hepatic modulation of apoB production using cultured HepG2 cells as the model system. HepG2 cells preincubated with g-T3 were initially shown to inhibit the rate of incorporation of [C]acetate into cholesterol in a concentrationand time-dependent manner, with a maximum 8663% inhibition at 50 mmol/L observed within 6 hours. g-T3, on the other hand, had no significant effect on the uptake of [C]glycerol into pools of cellular triacylglycerol and phospholipid relative to untreated control. The rate of apoB synthesis and secretion was then studied by an [S]methionine pulse-labeling experiment and quantified by immunoprecipitating apoB on chasing up to 3 hours. An average reduction of 2463% in labeled apoB in the media was apparent with g-T3 despite a 6062% increase in apoB synthesis. Fractionation of secreted apoB revealed a relatively denser lipoprotein particle, suggesting a less stable particle. Using a digitonin-permeabilized HepG2 cell system, the effects of g-T3 on apoB translocation and degradation in the endoplasmic reticulum were further investigated. The generation of a specific N-terminal 70-kDa proteolytic fragment proved to be a sensitive measure of the rate of apoB translocation and degradation. The abundance of this fragment increased significantly in g-T3-treated cells relative to untreated control cells (50621%) after 2 hours of chase. In addition, the presence of g-T3 resulted in an average decrease of 6468% in intact apoB. Taken together, the data suggest that g-T3 stimulates apoB degradation possibly as the result of decreased apoB translocation into the endoplasmic reticulum lumen. It is speculated that the lack of cholesterol availability reduces the number of secreted apoB-containing lipoprotein particles by limiting translocation of apoB into the endoplasmic reticulum lumen. (Arterioscler Thromb Vasc Biol. 1999;19:704-712.)