Secretion of hepatocyte apoB is inhibited by the flavonoids, naringenin and hesperetin, via reduced activity and expression of ACAT2 and MTP

The citrus flavonoids, naringenin and hesperetin, lower plasma cholesterol in vivo. However, the underlying mechanisms are not fully understood, The a bility of these flavonoids to modulate apolipoprotein B (apoB) secretion an d cellular cholesterol homeostasis was determined in the human hepatoma cel l line, HepG2, apoB accumulation in the media decreased in a dose-dependent manner following 24-h incubations with naringenin (up to 82%, P < 0.00001) or hesperetin (up to 74%, P < 0.002). Decreased apoB secretion was associa ted with reduced cellular cholesteryl ester mass. Cholesterol esterificatio n was decreased, dose-dependently, up to 84% (P < 0.0001) at flavonoid conc entrations of 200 muM. Neither navonoid demonstrated selective inhibition o f either form of acyl CoA:cholesterol acyltransferase (ACAT) as determined using CHO cells stably transfected with either ACAT1 or ACAT2. However, in HepG2 cells, ACAT2 mRNA was selectively decreased (-50%, P < 0.001) by both flavonoids, whereas ACAT1 mRNA was unaffected. In addition, naringenin and hesperetin decreased both the activity (-20% to -40%, P <less than> 0.0000 4) and expression (-30% to -40%, P < 0.02) of microsomal triglyceride trans fer protein (MTP), Both flavonoids caused a 5- to 7-fold increase (P < 0.02 ) in low density lipoprotein (LDL) receptor mRNA, which resulted in a 1.5- to 8-fold increase in uptake and degradation of I-125-LDL. We conclude that both naringenin and hesperetin decrease the availability of lipids for ass embly of apoB-containing lipoproteins, an effect mediated by 1) reduced act ivities of ACAT1 and ACAT2, 2) a selective decrease in ACAT2 expression, an d 3) reduced MTP activity. Together with an enhanced expression of the LDL receptor, these mechanisms may explain the hypocholesterolemic properties o f the citrus flavonoids.