Biliary anionic peptide fraction/calcium binding protein inhibits apolipoprotein A-I-mediated cholesterol efflux from cultured cells

The sterol transporting heterodimer ABCG5/ABCG8 requires bile salts to mediate cholesterol efflux

The ATP binding cassette transporters ABCG5 and ABCG8 are indispensable for hepatobiliary cholesterol transport. In this study, we investigated the specificity of the heterodimer for cholesterol acceptors. Dog gallbladder epithelial cells were mono- or double-transfected with lentiviral mouse Abcg5 and Abcg8 vectors. Double-transfected cells showed increased efflux to different bile salt (BS) species, while mono-transfected cells did not show enhanced efflux. The efflux was initiated at micellar concentrations and addition of phosphatidylcholine increased efflux. Cholesterol secretion was highly BS dependent, whereas other cholesterol acceptors such as ApoAI, HDL or methyl-beta-cyclodextrin did not elicit Abcg5/g8 dependent cholesterol secretion.

ABCA1-dependent but apoA-I-independent cholesterol efflux mediated by fatty acid-bile acid conjugates (FABACs)

FABACs (fatty acid-bile acid conjugates) are synthetic molecules that are designed to treat a range of lipid disorders. The compounds prevent cholesterol gallstone formation and diet-induced fatty liver, and increase reverse cholesterol transport in rodents. The aim of the present study was to investigate the effect of FABACs on cholesterol efflux in human cells. Aramchol (3beta-arachidylamido-7alpha,12alpha,5beta-cholan-24-oic acid) increased cholesterol efflux from human skin fibroblasts in a dose-dependent manner in the absence of known efflux mediators such as apoA-I (apolipoprotein A-I), but had little effect on phospholipid efflux. An LXR (liver X receptor) agonist strongly increased Aramchol-induced cholesterol efflux; however, in ABCA1 (ATP-binding cassette transporter A1)-deficient cells from Tangier disease patients, the Aramchol effect was absent, indicating that activity of ABCA1 was required. Aramchol did not affect ABCA1 expression, but plasma membrane levels of the transporter increased 2-fold. Aramchol is the first small molecule that induces ABCA1-dependent cholesterol efflux without affecting transcriptional control. These findings may explain the beneficial effect of the compound on atherosclerosis.

The endothelial cholesterol efflux is promoted by the high-density lipoprotein anionic peptide factor

The prevention of atherosclerosis depends on the high-density lipoprotein (HDL) capacity to stimulate the efflux of unesterified cholesterol (UC). We tested here the effects of 2 HDL apolipoproteins, apo A-I and the 7-kd anionic peptide factor (APF), on the UC efflux by human endothelial ECV 304 cells in culture. Apolipoprotein A-I (10 micromol/L) or APF (3.5 micromol/L) in lipid-free forms or small particles (13 nm with apo A-I or 19 nm with APF) were incubated in the presence of [4-14C]UC. The phosphatidylcholines (PCs) were present either at a low level (0.35 mmol/L with apo A-I or 0.20 mmol/L with APF) or at a high level (1 mmol/L with apo A-I). We also tested either large 53-nm bile lipoprotein complex-like particles (3.5 micromol/L APF [13 microg/500 microL]) with a high PC level (0.65 mmol/L) or a 9-residue synthetic peptide (13 microg/500 microL), derived from the NH2-terminal domain of HDL3-APF, in a lipid-free or low-lipidated (0.20 mmol/L PCs) form. A control was developed in absence of the added compounds. A rapid [4-14C]UC efflux mediated by APF added in free form or in 19-nm complexes was 2.2- to 2.3-fold higher than that mediated by apo A-I in free form or in 13-nm particles (P < .05). The level of this high APF-related efflux was comparable with that obtained with the 12-nm native HDLs (10 micromol/L apo A-I) or free PCs (1 mmol/L). The increase in the UC efflux was much more limited (1.4-fold) in the presence of the 53-nm APF/high-PC particles, but it was higher than that mediated by apo A-I. In addition, the efflux mediated by the synthetic peptide, in lipid-free or low-lipidated form, constituted the major part of that related to the full-length APF. Thus, all these particles are very active HDL components, able to act as cholesterol acceptors. Interestingly, we further showed a new anti-atherogenic property of APF as well as its metabolic importance and clinical relevance. By its involvement in the first step of the reverse cholesterol transport, APF could reduce the risk of cardiovascular disease.

Diosgenin-induced biliary cholesterol secretion in mice requires Abcg8

The plant sterol diosgenin has been shown to stimulate biliary cholesterol secretion in mice without affecting the expression of the adenosine triphosphate-binding cassette transporter heterodimer Abcg5/g8. The aim of this study was to investigate the mechanism of diosgenin-induced cholesterol hypersecretion and to identify the genes involved. Surprisingly, despite its lack of effect on Abcg5/g8 expression in wild-type mice, diosgenin did not stimulate biliary cholesterol secretion in mice deficient for Abcg8. Analysis of the kinetics of cholesterol secretion suggested that diosgenin probably activates a step before Abcg5/g8. To identify potential diosgenin targets, gene expression profiling was performed in mice fed a diosgenin-supplemented diet. Diosgenin feeding increased hepatic expression of genes involved in cholesterol synthesis as well as genes encoding for several cytochrome P450s. No significant change in expression of known cholesterol transporters was found. Comparison with published expression-profiling data for Srebp2-overexpressing mice, another mouse model in which biliary cholesterol secretion is elevated, revealed a number of genes with unknown function that were upregulated in both diosgenin-fed mice and mice overexpressing Srebp2. In conclusion, we found that although Abcg8 is essential for most diosgenin-induced biliary cholesterol hypersecretion, diosgenin probably does not interact directly with Abcg5/Abcg8, but rather increases cholesterol delivery to the heterodimer. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Polarized cholesterol and phospholipid efflux in cultured gall-bladder epithelial cells: evidence for an ABCA1-mediated pathway

Gall-bladder epithelial cells (GBEC) are exposed to high concentrations of cholesterol in bile. Whereas cholesterol absorption by GBEC is established, the fate of this absorbed cholesterol is not known. The aim of this study was to determine whether ABCA1 (ATP-binding cassette transporter A1) mediates cholesterol efflux in GBEC. Polarized canine GBEC were cultured on porous membrane filters allowing separate access to apical (AP) and basolateral (BL) compartments. After AP loading of cells with model bile and [14C]cholesterol, cholesterol efflux was measured. Cholesterol loading together with 8-bromo-cAMP treatment, which increased ABCA1 expression, led to a significant increase in cholesterol efflux with apolipoprotein A-I (apoA-I) as the acceptor. Cholesterol efflux was observed predominantly into the BL compartment. Similar results were found for phospholipid efflux. Confocal immunofluorescence microscopy showed a predominantly BL ABCA1 localization. Interestingly, apoA-I added to either the AP or the BL compartments elicited BL lipid efflux with cAMP treatment. No paracellular or transcellular passage of 125I-apoA-I occurred. Ligands for the nuclear hormone receptors liver X receptor alpha (LXRalpha) and retinoid X receptor (RXR) elicited AP and BL cholesterol efflux, suggesting the involvement of both ABCA1- and non-ABCA1-mediated pathways. In summary, BL cholesterol/phospholipid efflux consistent with an ABCA1-mediated mechanism occurs in GBEC. This efflux pathway is stimulated by cAMP and by LXRalpha/RXR ligands, and in the case of the cAMP pathway appears to involve a role for biliary apoA-I.

Biliary anionic peptide fraction and apoA-I regulate intestinal cholesterol uptake

Evidence is now in favor of protein-facilitated mechanisms for the intestinal cholesterol absorption. Here we report that the unesterified cholesterol uptake by rat jejunal brush border membrane vesicles (BBMVs) is efficient, saturable, and protein-mediated. The human apolipoproteins biliary anionic peptide factor (APF) and A-I (apoA-I) up-regulate micellar cholesterol uptake in a dose-dependent manner, but for all tested concentrations (0.1-20 microM), the lipid-free APF was more efficient than apoA-I. This uptake stimulation was suppressed after addition of Pabs directed to the external lipid-binding domain of the CLA-1/SR-BI and reduced by Pabs directed to the external loop of CD36. Thus, CLA-1/SR-BI and to a lesser extent CD36 are involved in the regulation of intestinal cholesterol uptake. APF, the main protein bound to biliary lipids, is likely one of their physiological effectors. As APF is an unesterified cholesterol carrier, it could facilitate the intestinal absorption of biliary cholesterol.