Heterogeneity of high density lipoprotein generated by ABCA1 and ABCA7

Abstract

The assembly of HDL by helical apolipoprotein and cellular lipid was studied using HEK293 cells to which ecdysone-inducible human ABCA1 or human ABCA7 was transfected. Expression of both ABCA1 and ABCA7 was induced linearly proportional to ponasterone A concentration in the medium. In the experimental conditions used, the ABC protein expression levels limited the rate of lipid release when the apolipoprotein concentration was high, and the apolipoprotein concentration was rate-limiting when the ABC protein expression levels were high. When ABCA1 expression increased in conditions in which it was rate-limiting, relative cholesterol content to phospholipid increased in the HDL produced. In contrast, it was constant when ABCA7 expression increased. To investigate the background mechanism, the HDL particles were analyzed by density gradient ultracentrifugation and high performance lipid chromatography. The ABCA1-mediated reaction produced two distinct HDLs, large cholesterol-rich and small cholesterol-poor particles, and the ABCA7-mediated reaction generated mostly small cholesterol-poor particles. The increase of HDL assembly with the increase of ABCA1 expression was predominant in large cholesterol-rich particles, whereas only small cholesterol-poor HDL increased as ABCA7 expression increased. We conclude that ABCA1 generates cholesterol-rich and cholesterol-poor HDL and that the former is more prominently dependent on the increase of ABCA1 expression. ABCA7 produces this HDL subfraction only as a very minor component. ABCA1 mediates the assembly of HDL with extracellular helical apolipoprotein and cellular lipid (1). This reaction is the major source of plasma HDL (2-4) and one of the rate-liming reactions for the regulation of its level (5, 6). The reaction mediates the generation of HDL particles with apolipoprotein, primarily recruiting cellular phospholipid (7). Cholesterol content in these particles is independently regulated by various cellular factors, potentially including protein kinase C and related signaling machineries (8-10), caveolin-1 (11), acyl-CoA:cholesterol acyltransferase (10, 12), and also perhaps other factors relating to intracellular cholesterol trafficking pathways (13). When ABCA1 is transfected to HEK293 cells, which otherwise do not express ABCA1, phospholipid and cholesterol are both released and cholesterol-rich HDL is generated upon incubation of the cells with apolipoprotein A-I (apoA-I) (14-16). ABCA7 also mediates the generation of HDL with apolipoproteins when transfected to HEK293 cells, but the relative cholesterol content to phospholipid in the HDL was lower than that produced by the ABCA1-mediated reaction (16,17). The relative increase of cholesterol release seemed greater than that of phospholipid when ABCA1 protein level was upregulated by dibutyryl cAMP and phorbol ester (16), so we wondered whether the expression level of ABCA proteins is also a factor that regulates cholesterol content in the HDL. There are many reports that ABCA1 expression can be induced by ...

Characterization of nascent HDL particles and microparticles formed by ABCA1-mediated efflux of cellular lipids to apoA-I

“…Likewise, in our previous study, in which J774 macrophages expressing ABCA1 were incubated with human plasma apoA-I, the formation of more than one type of apoA-I-containing lipid particle was also observed (16). Similar sizes of HDL particles were observed when apoA-I was incubated with HEK293 cells (17) or CHO cells (18) transfected with ABCA1 and with several cell types, including human skin fibroblasts and CaCo-2 cells, in which ABCA1 expression was induced (18). The fact that the enhancement of ABCA1 activity in cells by either transfection or induction with agonists leads to nascent HDL particle heterogeneity is consistent with ABCA1 activity alone causing the formation of the various apoA-Icontaining particles.…”

The ABCA subfamily—gene and protein structures, functions and associated hereditary diseases

“…However, several studies [17,31] demonstrated that the role of ABCA7 in lipid transport is secondary in comparison to ABCA1, the major cellular lipid export protein.…”

“…Experiments by Hayashi et al [17] demonstrated that ABCA7 generates mostly small cholesterol-poor HDL particles, whereas ABCA1 forms predominantly big cholesterol-rich particles. Moreover, knockout of ABCA7 function in mouse macrophages did not seem to affect phospholipid or cholesterol efflux to apolipoprotein A-I, while inactivation of ABCA1 resulted in a complete absence of efflux [31].…”

ABCA1 and Biogenesis of HDL

“…However, it was recently reported that ABCA7 also mediates HDL assembly in vitro in a similar manner to ABCA1 (50)(51)(52). Although this reaction may not significantly contribute to the regulation of plasma HDL concentration (53), it may be important locally as a back-up system for ABCA1.…”