Inhibitory effect of PCSK9 on Abca1 protein expression and cholesterol efflux in macrophages

Background and aims: Proprotein convertase subtilisin/kexin type 9 (PCSK9) may have extra-hepatic effects on cholesterol homeostasis of vascular macrophages. In this study, we aimed to investigate PCSK9 role on the anti-atherogenic process of ATP binding cassette transporter A1 (Abca1)-mediated cholesterol efflux. Methods: Abca1-mediated cholesterol efflux was evaluated by a radioisotopic technique in mouse peritoneal macrophages (MPM) from wild-type (WT) or LDL receptor knock-out (Ldlr(-/-)) mice exposed to human recombinant PCSK9, in the presence of liver X receptor/retinoid X receptor (LXR/RXR) ligands or acetylated LDL (AcLDL) to stimulate Abca1 expression. Protein and gene expression was evaluated by Western blot and quantitative real time PCR, respectively. Results: PCSK9 inhibited Abca1-mediated cholesterol efflux induced by LXR/RXR agonists in WT MPM (-55%, p < 0.05) but not in Ldlr(-/-) MPM. This effect was fully abrogated by the co-incubation with an anti-PCSK9 antibody. The inhibition of Abca1-dependent efflux induced by PCSK9 was associated with a reduction of Abca1 protein expression only in WT cells. Abca1 gene expression was significantly downregulated by PCSK9 in WT macrophages (-64%, p < 0.001) and, to a lesser extent, in MPM lacking Ldlr (-35%, p < 0.001). The inhibitory effect on Abca1-mediated efflux was also confirmed in AcLDL-treated macrophages. PCSK9 had a marginal or no effect on the expression of the lipid transporters Srb1 and Abcg1. Conclusions: PCSK9 plays a direct role on Abca1-mediated cholesterol efflux through a downregulation of Abca1 gene and Abca1 protein expression. This extrahepatic effect may influence relevant steps in the pathogenesis of atherosclerosis, such as foam cell formation. (C) 2016 Elsevier Ireland Ltd. All rights reserved.