Plasma HDL particles are highly heterogeneous in structure, metabolism and antiatherogenic properties . Their key rote in cellular cholesterol efflux and reverse cholesterol transport underlie their capacity to attenuate progression of atherosclerotic disease . Multiple mechanisms for efflux of cell cholesterol exist. ABCA1 transporter, has been reported to play a major role in cholesterol efflux from macrophages and foam cells to lipid poor or lipid free apoAl . Scavenger receptor class B type I (SR-BI) has been reported to mediate cellular cholesterol efflux preferentially to large phospholipids-rich HDL particles . More recently ABCG1 has been identified to stimulate cholesterol export to phospholipid-containing acceptons such as HDL . Low circulating levels of HDL (<40 mg/dl) represents a strong and independent risk factor for premature atherosclerosis and coronary heart disease . Thus it has been suggested that raising plasma HDL-C levels might afford clinicat benefit.
In the present study we evaluated the relationship between variation in plasma HDLC levels and both the HDL particle profile and fonction. We analyzed the quantitative and qualitative features of plasma HDL particles in a normolipidemic population which has been divided into two subgroups according their plasma HDL-C levels. Our data demonstrate that elevated plasma HDL-cholesterol levels is associated with increased levels of large HDL2 particles which preferentially stimulate cellular cholesterol efflux via SRBI pathway.
PLASMA LIPIDS AND LIPOPROTEINS DISTRIBUTION
The normolipidemic population has been divided into two subgroups according to their plasma HDL cholesterol levels. No significant difference in age, BMI, plasma lipid, apolipoproteinB levels and CETP activity was observed between males and females. Plasma ApoB or ApoAl containing lipoprotein particles were separated by density gradient ultracentrifugation yelding to multiple LDL and HDL subfractions. Within the density range of HDL, a significant increase (+24% ; P<0.00005) in the mean total HDL mass was observed in subjects with high plasma HDL-C levels as compared to those with low plasma HDL-C levels (347±96 mgldl and 455±120 mg/dl in low and high HDL-C groups respectively). The elevation in plasma HDL concentration was observed in both male and female with high HDL-C levels and resulted from a significant and specific increase in plasma HDL2 levels subfractions (+42% ; P<0.001 and +30% ;P<0.005 in male and female respectively with high HDLC levels vs male and female displaying low HDL-C levels). Moreover, female displayed significant higher plasma HDL2 levels as compared to males whatever the HDL-C levels (+34% ; P<0.001 and +20% ; P< 0.01 in women vs male in the low HDLC group and in the high HDL-C group respectively). A strong positive correlation between plasma HDL-C levels and plasma HDL2 concentrations (r= 0.84 ; P< 0.0001) was observed whereas no significant relationship was noted between plasma HDL-C levels and plasma HDL3 concentrations.
EFFECTS OF HDL-C LEVELS ON CELLULAR FREE CHOLESTEROL EFFLUX
Experiments were yeld on cultured rat Fu5AH cells expressing high levels of SRBI, on mouse macrophages RAW 267.4 expressing ABCA1 and on CHO cells overexpressing hABCG1. Using 40 fold diluted serum, a significant elevation (+25% ; P< 0.005) in the capacity of serum from normolipidemic women with high HDL-C levels to mediate cellular cholesterol efflux in a SRBI dependent pathway was observed as compared to both women with low HDL-C levels and males whatever their plasma HDL-C concentrations, We observed that in women displaying high HDL-C levels, the SRBI mediated cellular free cholesterol efflux was significantly correlated with plasma HDL2 levels (r=0.39 ; P< 0.02) whereas such relationship was not observed within the male population. The ABCA1 dependent cellular free cholesterol efflux to total serum was not related to HDL-C levels. Indeed we observed a similar capacity of serum to mediate cholesterol efflux via ABCA1 for both male and female with low or high HDL cholesterol levels with serum from men dysplaing increased capacity of efflux as compared to women (+24% ; P<0.001). Interestingly the ABCG1 mediated cellular efflux was not related to HDL-C levels or HDL2 levels. In parallel we determined the capacity of isolated HDL2 and HDL3 subfractions to mediate cellular free cholesterol efflux via SRBI, ABCA1 and ABCG1 pathways. HDL2 isolated from the plasma of normolipidemic subjects with low HDL-C levels displayed a similar capacity to mediate cellular cholesterol efflux via SRBI or ABCA1 or ABCA1 pathways than those isolated from high HDL-C levels subjects and no significant difference was noted between male and female HDL subfraction capacity of efflux. Within the total normolipidemic population the HDL2 subfraction displayed a significant higher capacity (+20% ; P<0.001) of efflux via the SRBI pathway than the HDL3 subfraction. By contrast the HDL3 subfraction represented the free cholesterol preferential acceptor via ABCA1 pathway as compared to HDL2 (+30% ; P<0.001). When matched according to PL content an equivalent capacity of efflux for HDL2 and HDL3 was observed via the ABCG1 pathway.
We conclude that in a normolipidemic healthy subjects increasing HDL cholesterol levels are only correlated with an increase in the number of large HDL2 particles and not with a qualitative difference in both lipid or protein composition of HDL particles. Our data indicate that plasma from healthy subjects displaying a high HDL-C phenotype possess a significant higher capacity to mediate cellular cholesterol efflux via both SRBI receptor.
Cellular SR-BI and ABCA1-mediated cholesterol efflux are gender specific in healthy subjects. Giovanna CATALANO, Emilie DUCHENE, Zélie JULIA, Wilfried LE GOFF, Eric BRUCKERT, M John CHAPMAN and Maryse GUERIN. J Lipid Res. 2007 Dec 5. PMID : 18057374
 von Eckardstein A, Nofer JR, Assmann G. High density lipoproteins and arteriosclerosis. Role of cholesterol efflux and reverse cholesterol transport. Arterioscler Thromb Vasc Biol. 2001 ;21(1):13-27. Review.
 Wang N, Lan D, Chen W, Matsuura F, Tall AR. « ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol eff lux to high-density lipoproteins. » Proc Natl Acad Sci U S A. 2004 ;101(26):9774-9.
 de la Llera-Moya M, Rothblat GH, Connelly MA, Kellner-Weibel G, Sakr SW, Phillips MC, Williams DL. Scavenger receptor BI (SR-BI) mediates free cholesterol flux independently of HDL tethering to the cell surface. J Lipid Res. 1999 ;40(3):575-80.
 Gelissen IC, Harris M, Rye KA, Quinn C, Brown AJ, Kockx M, Cartland S, Packianathan M, Kritharides L, Jessup W. ABCA1 and ABCG1 synergize to mediate cholesterol export to apoA-I. Arterioscler Thromb Vasc Biol. 2006 ;26(3):534-40.
 Chapman MJ, Assmann G, Fruchart JC, Shepherd J, Sirtori C ; European Consensus Panel on HDL-C. Raising high-density lipoprotein cholesterol with reduction of cardiovascular risk : the role of nicotinic acid-a position paper developed by the European Consensus Panel on HDL-C. Curr Med Res Opin. 2004 ;20(8):1253-68.