Regulation of LDL receptor, apoB, and apoE protein and mRNA in Hep G2 cells

Modulation of hepG2 cell net apolipoprotein B secretion by the citrus polymethoxyflavone, tangeretin

The purpose of the present study was to examine the role of tangeretin, a polymethoxylated flavone from citrus fruits, on the regulation of apolipoprotein B (apoB) and lipid metabolism in the human hepatoma cell-line HepG2. The marked reduction in apoB secretion observed in cells incubated with 72.8 microM tangeretin was rapid, apoB-specific, and partly reversible. The reduction also was observed under lipid-rich conditions and found to be insensitive to proteasomal degradation of nascent apoB. We followed our study by examining lipid synthesis and mass. A 24-h exposure of cells to 72.8 microM tangeretin decreased intracellular synthesis of cholesteryl esters, free cholesterol, and TAG by 82, 45, and 64%, respectively; tangeretin also reduced the mass of cellular TAG by 37%. The tangeretin-induced suppression of TAG synthesis and mass were associated with decreased activities of DAG acyltransferase (up to -39.0 +/- 3.0% vs. control) and microsomal triglyceride transfer protein (up to -35.5 +/- 2.5% vs. control). Tangeretin was also found to activate the peroxisome proliferator-activated receptor, a transcription factor with a positive regulatory impact on FA oxidation and TAG availability (up to 36% increase vs. control). The data suggest that tangeretin modulates apoB-containing lipoprotein metabolism through multiple mechanisms.

Modulation of the LDL receptor and LRP levels by HIV protease inhibitors

Inhibitors of the human immunodeficiency virus (HIV)-1 protease have proven to be effective antiretroviral drugs. However, patients receiving these drugs develop serious metabolic abnormalities, including hypercholesterolemia. The objective of the present study was to identify mechanisms by which HIV protease inhibitors increase plasma cholesterol levels. We hypothesized that HIV protease inhibitors may affect gene regulation of certain LDL receptor (LDLR) family members, thereby altering the catabolism of cholesterol-containing lipoproteins. In this present study we investigated the effect of several HIV protease inhibitors (ABT-378, Amprenavir, Indinavir, Nelfinavir, Ritonavir, and Saquinavir) on mRNA, protein, and functional levels of LDLR family members. Our results demonstrate that one of these drugs, Nelfinavir, significantly decreases LDLR and LDLR-related protein (LRP) mRNA and protein levels, resulting in the reduced functional activity of these two receptors. Nelfinavir exerts its effect by reducing levels of active SREBP1 in the nucleus. The finding that Nelfinavir reduces the levels of two key receptors (LRP and LDLR) involved in lipoprotein catabolism and maintenance of vessel wall integrity identifies a mechanism that causes hypercholesterolemia complications in HIV patients treated with this drug and raises concerns about the atherogenic nature of Nelfinavir.

ACTH decreases the expression and secretion of apolipoprotein B in HepG2 cell cultures

Administration of adrenocorticotropic hormone (ACTH) has been shown to decrease plasma concentrations of apolipoprotein B (apoB) containing lipoproteins, including lipoprotein(a), in man. However, the mechanism behind this hypolipidemic effect is unknown. This study aimed at distinguishing between the main possibilities (increased elimination or decreased production of lipoproteins) using HepG2 cell cultures. Addition of ACTH to the cell culture medium selectively down-regulated apoB mRNA expression and apoB secretion in a dose-dependent manner. At 100 pmol/liter ACTH, the apoB mRNA level was about 40% lower than in the untreated cells, and the secretion of apoB into the medium was decreased to a similar extent. The expression and secretion of other apolipoproteins (apoA-I, apoE, and apoM), however, were not affected by ACTH. Under normal culture conditions the level of secretion of apoB from HepG2 cells is quite low. In the presence of 0.4 mmol/liter oleic acid secretion of apoB increased 3-fold, but this phenomenon was not seen in ACTH-treated cells. Binding and internalization of radiolabeled low density lipoprotein (LDL) by HepG2 cell, as well as LDL-receptor mRNA and scavenger receptor B-I mRNA levels, were not influenced by ACTH. In conclusion, ACTH directly and selectively down-regulated the production and secretion of apoB in HepG2 cell cultures, suggesting that a principal mechanism behind the cholesterol-lowering effect of ACTH in vivo may be a decreased production rate of apoB-containing lipoproteins from the liver.