Physical and functional interactions between liver X receptor/retinoid X receptor and Sp1 modulate the transcriptional induction of the human ATP binding cassette transporter A1 gene by oxysterols and retinoids.
The lipid transporter ATP binding cassette transporter A1 (ABCA1) promotes the efflux of cellular phospholipids and cholesterol to lipid-free apolipoprotein A-I and thus initiates the biogenesis of high-density lipoprotein (HDL). The expression of the ABCA1 gene is controlled, coordinately with other genes of HDL metabolism, by liver X receptor/retinoid X receptor (LXR/RXR) heterodimers and their ligands oxysterols and retinoids. In the present study, we show that the oxysterol/retinoid-induced transcription of the ABCA1 gene is modulated by the ubiquitous transcription factor Sp1 that binds to the proximal ABCA1 promoter, adjacently to the LXR/RXR responsive element. The response of the ABCA1 gene to oxysterols/retinoids as well as the ligand-inducible recruitment of Sp1 and RXRalpha/LXRalpha heterodimers to the ABCA1 promoter was blocked by mithramycin A, a well-known Sp1 inhibitor. Using SL2 cells which lack endogenous Sp1, we showed that activation of the ABCA1 promoter by LXRalpha/RXRalpha heterodimers and their ligands requires Sp1. Functional interactions between these factors were demonstrated using the GAL4 transactivation system. Using both in vitro and in vivo assays, we show that physical interactions between Sp1 and LXRalpha require the N-terminal region of LXRalpha, which includes the AF1 and DNA binding domains and two different domains of Sp1: the transactivation domain B and the DNA binding domain. Overall, the present study revealed a novel mechanism of regulation of the human ABCA1 transporter which involves synergistic interactions between oxysterol/retinoid-inducible hormone nuclear receptors and the transcription factor Sp1.
Thymiakou
Efstathia
E
Laboratory of Biochemistry, Department of Basic Sciences, University of Crete Medical School, and Institute of Molecular Biology and Biotechnology, Foundation of Research and Technology of Hellas, Heraklion, Crete 71110, Greece.
Zannis
Vassilis I
VI
Kardassis
Dimitris
D
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019921:Biotinylation; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D005786:Gene Expression Regulation; D006801:Humans; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D010957:Plasmids; D011401:Promoter Regions, Genetic; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D047488:Retinoid X Receptors / Q000737:chemistry* / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D017931:DNA Primers; D004268:DNA-Binding Proteins; D000071518:Liver X Receptors; C000605213:NR1H3 protein, human; D057093:Orphan Nuclear Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D011994:Recombinant Proteins; D047488:Retinoid X Receptors