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

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 similar to 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-zeta (PKC-zeta) phosphorylation and induced PKC-zeta binding with Sp1. Inhibition of PKC-zeta with kinase inhibitors or overexpression of kinase-dead PKC-zeta attenuated Sp1 phosphorylation and ABCA1 expression induced by LDL. These results demonstrate for the first time that activation of the PKC zeta-Sp1 signaling cascade is a mechanism for regulation of LDL-induced ABCA1 expression.