Naringenin, the principal
flavonoid in grapefruit, reduces plasma
lipids in vivo and inhibits
apoB secretion,
cholesterol esterification, and MTP activity in HepG2 human
hepatoma cells. Although
naringenin inhibits ACAT, we recently demonstrated that CE availability in the microsomal lumen does not regulate
apoB secretion in HepG2 cells. We therefore hypothesized that inhibition of TG accumulation in the ER lumen, secondary to MTP inhibition, is the primary mechanism whereby
naringenin blocks lipidation and subsequent secretion of
apoB. Multicompartmental modeling of pulse-chase studies was used to compare cellular
apoB kinetics in the presence of either
naringenin or the specific MTP inhibitor, BMS-197636. At concentrations that reduced
apoB secretion by 50%, both compounds selectively enhanced degradation via a kinetically defined, rapid, proteasomal pathway to the same extent. Subcellular fractionation experiments revealed that
naringenin and BMS-197636 reduced accumulation of newly synthesized TG in the microsomal lumen by 48% and 54%, respectively. Newly synthesized CE accumulation in the lumen was reduced by 80% and 33% with
naringenin and BMS-197636, respectively, demonstrating for the first time that MTP is involved in CE accumulation in the microsomal lumen. Reduced TG availability at this initial site of
lipoprotein assembly was associated with significant reductions in the secretion of
apoB-containing
lipoproteins. Both
naringenin and BMS-197636 were most effective in reducing secretion of IDL and
LDL, but also inhibited secretion of
apoB-containing HDL-sized particles. Furthermore, in McA-RH7777-derived cell lines,
naringenin reduced secretion of hapoB72 and hapoB100, which require significant assembly with
lipid to be secreted, but did not reduce secretion of hapoB17, hapoB23, and hapoB48, which require only minimal lipidation. Taken together, our results indicate that
naringenin inhibits the lipidation and subsequent secretion of
apoB-containing
lipoproteins primarily by limiting the accumulation of TG in the ER lumen, secondary to MTP inhibition.