Niacin Promots Reverse Cholesterol Transport In Vivo And In Vitro Study

Background and ObjectiveMany epidemiological studies have indicated that the plasma level of high-density lipoprotein-cholesterol(HDL-C)is inversely correlated with the risk of cornary artery disease.It has been shown that HDL exerts its protective effect against atherosclerosis.Now,one of the key mechanisms is been linked that HKL takes part in the transport of execss cholesterol from macrophage to the liver(that is reverse cholesterol transport).RCT is the only pathway of transporting excess cholester and one of the key mechanisms of antiatherosclerosis.Niacin is a commonly used antidystipidemic agent that favorably affects all lipids and lipoproteins,and has a property of increasing HDL-C.Some studies have demonstrated that niacin significantly reduced the rate of carotid intima-media thickness progression,so we supposed promoting RCT was one of import mechanisms of niacin anti-atherosclerosis,but not exactly understood.Many proteins,receptors and transporters all play role in RCT,such as ATP binding cassette transporter family(ABCA1,ABCG1, ABCG5,ABCG8 eg.)and cholesterol 7alpha-hydroxylase(CYP7α),which are affected with liver X receptorα(LXRα)and peroxisome proliferator-activated receptorγ(PPARγ).Past studies show that niacin promoted cell effiux mediating by ABCA1.However,such studies were performed in vitro which can not exactly reflect the affection of niacin to RCT,and no research has been reported about the effect of niacin on other transporters of RCT.This study will use substituous method to evaluate the efficiency of RCT of niacin in vitro and in vivo respectively and explore the mechanisms.MethodsC57BL/6 mice were treated with niacin(0,1%,W/W)for 4 weeks, then these mice were injected intraperitoneally with ~3H-cholesterol-labeled and cholesterol-loaded macrophages,Samples of serum,liver and feces were analyzed for the appearance of ~3H-tracer.various concentration of niacin(0,0.2,1.0,5.0mM)incubated macrophage 264.7 cells,then cholesterol efflux rate was determined through measuring release of radioactivity from ~3H-cholesterol prelabled cells into medium containing HDL.Reverse transcription polymerase chain reaction (RT-PCR)was used to evaluate CYP7αof liver;ABCG5,LXRαof liver and intestine;SR-BI,ABCA1,ABCG1,LXRα,PPARγmRNA expression of macrophage.And using western-blot to explore ABCG5,LXRαof liver and ABCG1,LXRαof macrophage protein expression.Results1.Niacin greatly reduced both total cholesterol(TC)and low-density lipoprotein cholesterol(LDL-C)(- 40 and -63%,respcetively); triglyceride(TG)decreased slightly;while increase the level of HDL-C by 14%.2.In contrast to the control,niacin groups lead to lower ~3H-cholesterol in plasma by 20%(P＜0.05)and higher in liver and feces. The amount of ~3H-tracer in the liver and feces increased by 19%and 21%respectively.3.Niacin elevated the expression of ABCG5 in liver and intestine by 1.3-fold and 1.7-fold(P＜0.05);upregulated CYP7αby 125.9%(P＜0.05); and increased the expression of LXRαin liver and intestine by 1.2-fold and 2.1-fold respectively(P＜0.05).4.Few of ABCG5 and LXRαproteins expressed in control group,while both are markedly increased by 40-fold and 30-fold under the interference of niacin.5.Niacin could dose-dependently increase cholesterol effiux in macrophage mediating with HDL.Cholesterol efflux rate is 2.35±0.35, 4.58±0.16,7.24±0.29 and 13.26±0.43 exposuring to various concentration of niacin(0,0.2,1.0,5.0mM).6.SR-BI was highly expressed in macrophage,but niacin didn't influence its expression comparing with control(P＞0.05).7.Niacin could dose-dependently increase ABCA1,ABCG1,LXRαand PPARγmRNA expression.8.The protein expression of ABCG1 and LXRαin macrophage were also dose-dependently increased with niacin's concentration. Conclusions1.Niacin decreased the levels of TC,LDL-C and TG in serum,while increased the level of HDL-C.2.Niacin promoted reverse cholesterol transport of macrophage in mice in vivo.3.Niacin promted reverse cholesterol transport by increasing ABCG5 and CYP7αexpression which is stimulated by LXRα.4.Niacin could dose-dependently promot cholesterol efflux via ABCA1 and ABCG1 mediating,which might due to the up-regulation of LXRαand PPARγ.5.The mechanism of niacin promoting cholesterol effux in macrophage was unrelated with the expression of SR-BI.