Transcriptional Regulation of ATP-binding Cassette Transporter A1 Expression by a Novel Signaling Pathway

Xinping Chen,Yanfeng Zhao,Zhongmao Guo,Lichun Zhou,Emmanuel U Okoro,Hong Yang

doi:10.1074/jbc.m110.214429

Abstract

ATP-binding cassette transporter A1 (ABCA1) is a membrane-bound protein that regulates the efflux of cholesterol derived from internalized lipoproteins. Using a mouse macrophage cell line, this report studied the impact of low-density lipoproteins (LDL) on ABCA1 expression and the signaling pathway responsible for lipoprotein-induced ABCA1 expression. Our data demonstrated that treatment of macrophages with LDL increased ABCA1 mRNA and protein levels 4.3- and 3.5-fold, respectively. LDL also induced an ∼2-fold increase in macrophage surface expression of ABCA1 and a 14-fold-increase in apolipoprotein AI-mediated cholesterol efflux. In addition, LDL significantly increased the level of phosphorylated specificity protein 1 (Sp1) and the amount of Sp1 bound to the ABCA1 promoter without alteration in total Sp1 protein level. Mutation of the Sp1 binding site in the ABCA1 promoter and inhibition of Sp1 DNA binding with mithramycin A suppressed the ABCA1 promoter activity and reduced the ABCA1 expression level induced by LDL. LDL treatment also elevated protein kinase C-ζ (PKC-ζ) phosphorylation and induced PKC-ζ binding with Sp1. Inhibition of PKC-ζ with kinase inhibitors or overexpression of kinase-dead PKC-ζ attenuated Sp1 phosphorylation and ABCA1 expression induced by LDL. These results demonstrate for the first time that activation of the PKCζ-Sp1 signaling cascade is a mechanism for regulation of LDL-induced ABCA1 expression.

Highlights

ATP-binding cassette transporter A1 (ABCA1) in humans has been shown to induce cholesterol accumulation in cells of the reticuloendothelial system and increase susceptibility to atherosclerosis [7]
Mithramycin A Inhibits ABCA1 Expression—To address if specificity protein 1 (Sp1) binding to the ABCA1 promoter is necessary for low-density lipoproteins (LDL)-induced expression of ABCA1, we studied the effect of mithramycin A on ABCA1 expression
In agreement with previous reports [13, 14], data from this study clearly indicate that native LDL increases macrophage ABCA1 expression and induces ApoAI-mediated cholesterol efflux

Summary

Introduction

ABCA1 in humans has been shown to induce cholesterol accumulation in cells of the reticuloendothelial system and increase susceptibility to atherosclerosis [7]. Macrophages obtained from transgenic mice that overexpress ABCA1 have an elevated cholesterol efflux compared with normal macrophages [5]. The oxysterolactivated LXRs form heterodimers with retinoid X receptors (RXR) and bind to the ABCA1 promoter, inducing transcription [9, 11]. In addition to LXR and RXR, transcription factor specificity protein 1 (Sp1) has been suggested to be a regulator in ABCA1 expression. A recent study showed that treatment of cells with oxysterols induced physical interaction of Sp1 with the LXR and RXR heterodimer in the ABCA1 promoter [12]. Inhibition of Sp1 DNA binding attenuated oxysterol-induced ABCA1 expression [12]. PKC-␨-Sp1 Signaling Cascade Regulates ABCA1 Expression uates LDL-induced Sp1 phosphorylation and ABCA1 expression in macrophages. Our data suggest a novel pathway, except for LXR␣, in regulating macrophage ABCA1 expression

Methods

Results

Conclusion