Objectives Several epidemiological and experimental evidences indicate that oestrogen is intimately involved in the pathogenesis of atherosclerosis. The anti-proliferatory, pro-apoptotic, anti-migratory, anti-oxidative, and anti-inflammatory effects of oestrogen on vascular smooth muscle cells (VSMCs) have been demonstrated in vitro. However the role of oestrogen and its underlying mechanism in lipid accumulation and reverse cholesterol transport in VSMCs are poorly understood. The aim of the study was to investigated whether 17β-estradiol (E2) attenuates lipid accumulation in VSMCs via an increased cholesterol efflux as well as to determine the involved molecular mechanisms.
Methods Mouse VSMCs were pretreated with indicated concentrations of E2 (10-9-10-7M) for 2h and then stimulated with oxLDL (50 μg/ml) for 72h. The intracellular lipid droplets were stained by oil red O staining and hematoxylin was used as a counterstain. Cellular free cholesterol and cholesteryl ester were quantified by an enzymatic method. The effect of E2 on cholesterol efflux was investigated using NBD-labelled cholesterol efflux. Real-time PCR and western blotting were used to detect the effect of E2 on ATP-binding cassette transporters (ABCA1), ABCG1 and liver X receptor (LXRα). Western blotting and NBD-labled cholesterol efflux were also used to examine the effects of oestrogen receptor (ER) and LXRα on E2-induced ABCA1 and ABCG1 expression and cholesterol efflux in VSMCs.
Results E2 treatment dose-dependently reduced lipid droplets and celluar cholesteryl ester accumulation in VSMCs. E2 markedly enhanced apoA-1- and HDL-mediated cholesterol efflux from VSMCs in a dose-dependent manner, which was associated with an increase in ABCA1 and ABCG1 mRNA and protein expression. Nonselective ER inhibitor ICI-182,780 abolished the upregulation of ABCA1 and ABCG1 expression by E2. Remarkably, ERβ agonist DPN dose dependently upregulated ABCA1 and ABCG1 protein expression, whereas ERα agonist PPT had no effect, indicating upregulation of ABCA1 and ABCG1 by E2 is selectively mediated by ERβ. Both E2 and DPN promoted LXRα mRNA and protein expression in VMSCs. Moreover, inhibition of LXRα with a pharmacological inhibitor GGPP or siRNA suppressed the stimulatory effects of E2 on ABCA1 and ABCG1 expression and cholesterol efflux.
Conclusions ERβ- and LXRα-dependent upregulation of ABCA1 and ABCG1 might mediate the atheroprotective effect of E2, which ameliorated the oxLDL-mediated lipid accumulation in VSMCs-derived foam cell formation.