Inhibition of acyl-CoA: cholesterol acyltransferase decreases apolipoprotein B-100-containing lipoprotein secretion from HepG2 cells.

217

The citrus flavonoids, naringenin and hesperetin, lower plasma cholesterol in vivo. However, the underlying mechanisms are not fully understood. The ability of these flavonoids to modulate apolipoprotein B (apoB) secretion and cellular cholesterol homeostasis was determined in the human hepatoma cell line, HepG2. apoB accumulation in the media decreased in a dose-dependent manner following 24-h incubations with naringenin (up to 82%, P Ͻ 0.00001) or hesperetin (up to 74%, P Ͻ 0.002). Decreased apoB secretion was associated with reduced cellular cholesteryl ester mass. Cholesterol esterification was decreased, dose-dependently, up to 84% ( P Ͻ 0.0001) at flavonoid concentrations of 200 M. Neither flavonoid demonstrated selective inhibition of 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 Ͻ 0.001) by both flavonoids, whereas ACAT1 mRNA was unaffected. In addition, naringenin and hesperetin decreased both the activity ( ؊ 20% to ؊ 40%, P Ͻ 0.00004) and expression ( ؊ 30% to ؊ 40%, P Ͻ 0.02) of microsomal triglyceride transfer protein (MTP). Both flavonoids caused a 5-to 7-fold increase ( P Ͻ 0.02) in low density lipoprotein (LDL) receptor mRNA, which resulted in a 1.5-to 2-fold increase in uptake and degradation of 125 I-LDL.We conclude that both naringenin and hesperetin decrease the availability of lipids for assembly of apoB-containing lipoproteins, an effect mediated by 1) reduced activities of ACAT1 and ACAT2, 2) a selective decrease in ACAT2 expression, and 3) reduced MTP activity. Together with an enhanced expression of the LDL receptor, these mechanisms may explain the hypocholesterolemic properties of the citrus flavonoids. -Wilcox, L. J., N. M. Borradaile, L. E. de Dreu, and M. W. Huff. Secretion of hepatocyte apoB is inhibited by the flavonoids, naringenin and hesperetin, via reduced activity and expression of ACAT2 and MTP.

Section: Discussionsupporting

confidence: 87%

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

Wilcox¹,

Borradaile²,

Dreu

et al. 2001

217

“…The concentration of free cholesterol substrate in the cell regulates enzymatic activity of ACAT (9). Furthermore, inhibition of newly synthesized cholesteryl ester catalyzed by ACAT has been shown to decrease apoB secretion in HepG2 cells (6,(10)(11)(12)(13)(14)(15)(16), primary hepatocytes (11,17,18), perfused monkey livers (19) as well as in several small animal models (8). ACAT inhibitors decrease plasma cholesterol and apoB concentrations in rats and rabbits fed cholesterol-supplemented diets (20)(21)(22)(23).…”

mentioning

confidence: 99%

Inhibition of ACAT by avasimibe decreases both VLDL and LDL apolipoprotein B production in miniature pigs

Burnett

Wilcox

Telford

et al. 1999

An orally bioavailable acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor, avasimibe (CI-1011), was used to test the hypothesis that inhibition of cholesterol esterification, in vivo, would reduce hepatic very low density (VLDL) apolipoprotein (apo) B secretion into plasma. ApoB kinetic studies were carried out in 10 control miniature pigs, and in 10 animals treated with avasimibe (10 mg/kg/d, n ‫؍‬ 6; 25 mg/kg/d, n ‫؍‬ 4). Pigs were fed a diet containing fat (34% of calories) and cholesterol (400 mg/d; 0.1%). Avasimibe decreased the plasma concentrations of total triglyceride, VLDL triglyceride, and VLDL cholesterol by 31-40% 39-48%, and 31-35%, respectively. Significant reductions in plasma total cholesterol (35%) and low density lipoprotein (LDL) cholesterol (51%) concentrations were observed only with high dose avasimibe. Autologous 131 I-labeled VLDL, 125 Ilabeled LDL, and [ 3 H]leucine were injected simultaneously into each pig and apoB kinetic data were analyzed using multicompartmental analysis (SAAM II). Avasimibe decreased the VLDL apoB pool size by 40-43% and the hepatic secretion rate of VLDL apoB by 38-41%, but did not alter its fractional catabolism. Avasimibe decreased the LDL apoB pool size by 13-57%, largely due to a dose-dependent 25-63% in the LDL apoB production rate. Hepatic LDL receptor mRNA abundances were unchanged, consistent with a marginal decrease in LDL apoB FCRs. Hepatic ACAT activity was decreased by 51% ( P ‫؍‬ 0.050) and 68% ( P ‫؍‬ 0.087) by low and high dose avasimibe, respectively. The decrease in total apoB secretion correlated with the decrease in hepatic ACAT activity ( r ‫؍‬ 0.495; P ‫؍‬ 0.026). We conclude that inhibition of hepatic ACAT by avasimibe reduces both plasma VLDL and LDL apoB concentrations, primarily by decreasing apoB secretion. -Burnett,

“…The role of hepatic free and esterified cholesterol in the assembly and secretion of apoB-containing lipoproteins, however, remains controversial. The regulation of apoB secretion by the rate of cholesterol synthesis, esterification, and/or the mass of cholesteryl ester (CE) has been documented in vitro (11)(12)(13)(14)(15)(16) and in vivo (17)(18)(19)(20)(21)(22). In contrast, several studies in HepG2 cells argue against the regulation of apoB by free cholesterol (FC) and/or CE availability (5,(7)(8)(9)23).…”

mentioning

confidence: 99%

Differential regulation of apolipoprotein B secretion from HepG2 cells by two HMG-CoA reductase inhibitors, atorvastatin and simvastatin

Wilcox

Barrett

Huff

1999

The concept that hepatic cholesterol synthesis regulates hepatocyte assembly and secretion of apoB-containing lipoproteins remains controversial. The present study was carried out in HepG2 cells to examine the regulation of apoB secretion by the HMG-CoA reductase inhibitor atorvastatin. ApoB accumulation in the media was decreased by 24% and 36% at 10 M ( P Ͻ 0.02) and 20 M ( P Ͻ 0.01) of atorvastatin, respectively. Atorvastatin inhibited HepG2 cell cholesterol synthesis by up to 96% ( P Ͻ 0.001) and cellular cholesteryl ester (CE) mass by 54% ( P Ͻ 0.001). Another HMG-CoA reductase inhibitor, simvastatin, decreased cellular cholesterol synthesis and CE mass by up to 96% ( P Ͻ 0.001) and 52% ( P Ͻ 0.001), respectively. However, in contrast to atorvastatin, simvastatin had no effect on apoB secretion. To characterize the reduction in apoB secretion by atorvastatin (10 M ), pulse-chase experiments were performed and the kinetic data were analyzed by multicompartmental modeling using SAAM II. Atorvastatin had no affect on the synthesis of apoB, however, apoB secretion into the media was decreased by 44% ( P ‫؍‬ 0.048). Intracellular apoB degradation increased proportionately ( P ‫؍‬ 0.048). Simvastatin (10 M ) treatment did not significantly alter either the secretion or intracellular degradation of apoB, relative to control. The kinetics of apoB degradation were best described by a rapidly and a slowly turning over degradation compartment. The effect of atorvastatin on apoB degradation was largely confined to the rapid compartment. Neither inhibitor affected apoB mRNA concentrations, however, both significantly increased LDL receptor and HMG-CoA reductase mRNA levels. Atorvastatin treatment also decreased the mRNA for the microsomal triglyceride transfer protein (MTP) by 22% ( P Ͻ 0.02). These results show that atorvastatin decreases apoB secretion, by a mechanism that results in an enhanced intracellular degradation of apoB. -Wilcox, L. J., P. H. R. Barrett, and M. W. Huff. Differential regulation of apolipoprotein B secretion from HepG2 cells by two HMG-CoA reductase inhibitors, atorvastatin and simvastatin.