Abstract 1722: RAR Agonists Enhance HDL-Mediated Cholesterol Efflux from Macrophages by Inducing ABCG1 Expression though RARα/RXR Acting on LXR Responsive Elements
ATP binding cassette transporter G1 (ABCG1) has been implicated in the efflux of cholesterol to HDL, and is transcriptionally regulated by liver receptor X (LXR)/retinoid X receptor (RXR) heterodimer through action on direct repeat 4 (DR4) in the promoter and introns. Retinoic acid receptors (RAR)/RXR reportedly bind not only DR5 but also non-canonical DR4. We observed that the RAR ligands, all-trans-retinoic acid and TTNPB, increased ABCG1 mRNA and protein levels in THP-1 macrophages and enhanced HDL-mediated cholesterol efflux from them in a dose-dependent manner. These effects were cancelled by treatment with an RAR antagonist. To clarify how the RAR ligands regulate ABCG1 expression, we investigated promoter activity using luciferase reporter constructs containing human ABCG1 promoter A and B (located upstream of exon 1 and 5, respectively), with or without mutated DR4. The RAR ligands activated promoter A in a DR4-dependent fashion. They also induced promoter B activity in the presence of LXR responsive elements located in intron 5 and 7 (LXRE-A and -B, respectively), which were inserted into the downstream site of the luciferase gene. However, these effects on promoter B were cancelled when DR4s in the LXREs were mutated. Moreover, overexpression of human RARα enhanced RAR ligand-induced elevation in promoter A and B with LXRE-B activity in a DR4-dependent manner, but in contrast, attenuated promoter B with LXRE-A activity. Quantitative real time RT-PCR analysis confirmed that the RAR agonists increased two major transcripts driven by promoter A and B using specific primers for each transcript. An electrophoretic mobility shift assay showed specific binding of the RARα/RXR heterodimer to DR4s in promoter A and LXRE-B, but not to mutated DR4. In conclusion, these results suggest that RARα transcriptionally inducedABCG1 expression in the macrophages by activating dual promoters in a LXRE-dependent manner, and RAR agonists may exert an anti-atherogenic effect by enhancing cholesterol efflux from the macrophages.