Opposite Effects of Plasma From Human Apolipoprotein A-II Transgenic Mice on Cholesterol Efflux From J774 Macrophages and Fu5AH Hepatoma Cells

Abstract

Abstract-Overexpression of human apolipoprotein A-II (hapo A-II) in transgenic mice (hAIItg mice) induced marked hypertriglyceridemia and low levels of plasma high density lipoprotein (HDL) with a high hapo A-II content. We sought to determine whether cholesterol efflux to plasma and HDL from these mice would be affected. In the Fu5AH cell system, plasma from hAIItg mice induced a markedly lower cholesterol efflux than did control plasma, in accordance with the dependence of efflux on HDL concentration. Moreover, HDLs from hAIItg mice were less effective acceptors than were control HDLs. In the J774 macrophage cell system, pretreatment with cAMP, which upregulates ATP binding cassette transporter 1, induced a marked increase in the efflux to hAIItg plasma as well as to purified hapo A-I and hapo A-II, whereas it had no effect on cholesterol efflux to control plasma. T he negative correlation between plasma HDL concentration and the risk of cardiovascular disease has been partly attributed to the ability of HDL to stimulate reverse cholesterol transport (RCT) from peripheral tissues to the liver for recycling. 1,2 The first step of RCT is efflux of free cholesterol from cell membranes to the extracellular medium. A major research effort these last decades has established that at least 3 mechanisms are involved: a nonspecific and relatively inefficient aqueous diffusion pathway, which operates in all cells, and 2 regulated pathways, which are modulated by phospholipid (PL)-containing acceptor particles (such as HDL) or lipid-poor apolipoproteins. 3 On the other hand, controversial results were obtained regarding the effect of the apolipoprotein composition of HDL, with some studies reporting a stimulation of cholesterol efflux by HDLs containing only apoA-I 4 and other studies reporting HDL carrying apoA-I and apoA-II. 5 Recently, the type of the acceptor particles was linked to the type of the receptors present in the various cells, 3 thus identifying the following 2 specific efflux mechanisms involving (1) the scavenger receptor class B type I (SR-BI) and (2) the ATP binding cassette transporter 1 (ABCA1). Cholesterol molecules that desorb from cell membranes rich in SR-BI diffuse through the aqueous phase and associate with PL-containing acceptor particles, in proportion to PL content. 6 Moreover, the ability of HDL and serum to stimulate cholesterol efflux from different cell types was correlated with the expression level of SR-BI. 7 SR-BI is equally efficient at mediating the import and export of cholesterol to and from cells to lipoproteins and other acceptors. 7,8 By contrast, the unidirectional efflux of membrane cholesterol and PL of cells expressing ABCA1 is promoted by apolipoproteins in lipid-poor form but is little affected by HDLs, small unilamellar vesicles, bile acid micelles, or cyclodextrin. 9 -11 ApoA-I is the major apolipoprotein component of HDL, and its role in RCT has been extensively studied. ApoA-II is the second most abundant HDL apolipoprotein, but its contribution to the function of H...

Measurement of cholesterol bidirectional flux between cells and lipoproteins

“…The bidirectional flux assay was developed and validated using Fu5AH rat hepatoma cells, which have been used in cholesterol efflux experiments in a large number of studies (26)(27)(28)(29)(30)(31). Fu5AH cells have high levels of SR-BI protein (32); thus, one of the major mechanisms for cholesterol flux is mediated by this receptor.…”

Moderate alcohol consumption increases cholesterol efflux mediated by ABCA1

“…Cholesterol efflux from J774 mouse macrophage cells was determined by the use of a previously validated modification (17,23,24) of the general technique described by Sakr et al (25). The current experiment is based on previous tests using normolipidemic human serum to obtain an optimal dilution for serum and was carried out strictly according to the experimental conditions described by Fournier et al (17).…”

In Vitro Transcriptional Induction of the Human Apolipoprotein A-II Gene by Glucose

“…Blood glucose was determined with the Glucotrend plus test (Roche Diagnostics, Mannheim, Germany). Human apoA-II was assayed in plasma by immunonephelometry, using a specific antibody (Dade Behring), as previously described (13). Livers were removed and immediately frozen in liquid N 2 for subsequent RNA analysis.…”