The grapefruit
flavonoid,
naringenin, is hypocholesterolemic in vivo, and inhibits basal
apolipoprotein B (
apoB) secretion and the expression and activities of both ACAT and
microsomal triglyceride transfer protein (MTP) in human
hepatoma cells (HepG2). In this report, we examined the effects of
naringenin on
apoB kinetics in
oleate-stimulated HepG2 cells and determined the contribution of microsomal lumen
cholesteryl ester (CE) availability to
apoB secretion. Pulse-chase studies of
apoB secretion and intracellular degradation were analyzed by multicompartmental modeling. The model for
apoB metabolism in HepG2 cells includes an intracellular compartment from which
apoB can be either secreted or degraded by both rapid and slow pathways. In the presence of 0.1 mM
oleic acid,
naringenin (200 micro M) reduced the secretion of newly synthesized
apoB by 52%, due to a 56% reduction in the rate constant for secretion. Intracellular degradation was significantly increased due to a selective increase in rapid degradation, while slow degradation was unaffected. Incubation with either N-acetyl-leucinyl-leucinyl-norleucinal (ALLN) or
lactacystin showed that degradation via the rapid pathway was largely proteasomal. Although these changes in
apoB metabolism were accompanied by significant reductions in CE synthesis and mass, subcellular fractionation experiments comparing
naringenin to specific ACAT and
HMG-CoA reductase inhibitors revealed that reduced accumulation of newly synthesized CE in the microsomal lumen is not consistently associated with reduced
apoB secretion. However,
naringenin, unlike the ACAT and
HMG-CoA reductase inhibitors, significantly reduced lumenal TG accumulation. We conclude that
naringenin inhibits
apoB secretion in
oleate-stimulated HepG2 cells and selectively increases intracellular degradation via a largely proteasomal, rapid kinetic pathway. Although
naringenin inhibits ACAT, CE availability in the endoplasmic reticulum (ER) lumen does not appear to regulate
apoB secretion in HepG2 cells. Rather, inhibition of TG accumulation in the ER lumen via inhibition of MTP is the primary mechanism blocking
apoB secretion.