Sterol-dependent Transactivation of theABC1 Promoter by the Liver X Receptor/Retinoid X Receptor

Abstract

Tangier disease, a condition characterized by low levels of high density lipoprotein and cholesterol accumulation in macrophages, is caused by mutations in the ATP-binding cassette transporter ABC1. In cultured macrophages, ABC1 mRNA was induced in an additive fashion by 22(R)-hydroxycholesterol and 9-cis-retinoic acid (9CRA), suggesting induction by nuclear hormone receptors of the liver X receptor (LXR) and retinoid X receptor (RXR) family. We cloned the 5′-end of the human ABC1 transcript from cholesterol-loaded THP1 macrophages. When transfected into RAW macrophages, the upstream promoter was induced 7-fold by 22(R)-hydroxycholesterol, 8-fold by 9CRA, and 37-fold by 9CRA and 22(R)-hydroxycholesterol. Furthermore, promoter activity was increased in a sterol-responsive fashion when cotransfected with LXRα/RXR or LXRβ/RXR. Further experiments identified a direct repeat spaced by four nucleotides (from −70 to −55 base pairs) as a binding site for LXRα/RXR or LXRβ/RXR. Mutations in this element abolished the sterol-mediated activation of the promoter. The results show sterol-dependent transactivation of the ABC1 promoter by LXR/RXR and suggest that small molecule agonists of LXR could be useful drugs to reverse foam cell formation and atherogenesis. Tangier disease, a condition characterized by low levels of high density lipoprotein and cholesterol accumulation in macrophages, is caused by mutations in the ATP-binding cassette transporter ABC1. In cultured macrophages, ABC1 mRNA was induced in an additive fashion by 22(R)-hydroxycholesterol and 9-cis-retinoic acid (9CRA), suggesting induction by nuclear hormone receptors of the liver X receptor (LXR) and retinoid X receptor (RXR) family. We cloned the 5′-end of the human ABC1 transcript from cholesterol-loaded THP1 macrophages. When transfected into RAW macrophages, the upstream promoter was induced 7-fold by 22(R)-hydroxycholesterol, 8-fold by 9CRA, and 37-fold by 9CRA and 22(R)-hydroxycholesterol. Furthermore, promoter activity was increased in a sterol-responsive fashion when cotransfected with LXRα/RXR or LXRβ/RXR. Further experiments identified a direct repeat spaced by four nucleotides (from −70 to −55 base pairs) as a binding site for LXRα/RXR or LXRβ/RXR. Mutations in this element abolished the sterol-mediated activation of the promoter. The results show sterol-dependent transactivation of the ABC1 promoter by LXR/RXR and suggest that small molecule agonists of LXR could be useful drugs to reverse foam cell formation and atherogenesis. high density lipoprotein 9-cis-retinoic acid 22(R)-hydroxycholesterol 25-hydroxycholesterol 7-ketocholesterol liver X receptor retinoid X receptor cotransfected mixture of two receptors putative heterodimer complex steroidogenic factor 1 cholesterol ester transfer protein direct repeat rapid amplification of cDNA ends base pair(s) polymerase chain reaction low density lipoprotein Plasma HDL1-cholesterol levels are inversely related to the incidence of coronary artery disease (1Castelli W.P. Garrison R.J. Wilson P.W. Abbott R.D. Kalousdian S. Kannel W.B. J. Am. Med. Assoc. 1986; 256: 2835-2838Crossref PubMed Scopus (2091) Google Scholar). Two genetic diseases illustrate this phenomenon, the rare Tangier disease and the more common familial HDL deficiency. Tangier disease is characterized by an extremely low concentration of circulating HDL and the accumulation of cholesteryl esters in tonsils, liver, spleen, and intestinal mucosa, mostly in macrophage foam cells (2Serfaty-Lacrosniere C. Civeira F. Lanzberg A. Isaia P. Berg J. Janus E.D. Smith Jr., M.P. Pritchard P.H. Frohlich J. Lees R.S. Barnard G.F. Ordovas J.M. Schaefer E.J. Atherosclerosis. 1994; 107: 85-98Abstract Full Text PDF PubMed Scopus (216) Google Scholar). Patients with familial HDL deficiency exhibit a low concentration of HDL particles and an increased risk of coronary artery disease (3Marcil M. Brooks-Wilson A. Clee S.M. Roomp K. Zhang L.H., Yu, L. Collins J.A. van Dam M. Molhuizen H.O. Loubster O. Ouellette B.F. Sensen C.W. Fichter K. Mott S. Denis M. Boucher B. Pimstone S. Genest Jr., J. Kastelein J.J. Hayden M.R. Lancet. 1999; 354: 1341-1346Abstract Full Text Full Text PDF PubMed Scopus (322) Google Scholar). A common explanation for the cardioprotective effect of HDL is the major role it plays in reverse cholesterol transport (4Bruce C. Chouinard Jr., R.A. Tall A.R. Annu. Rev. Nutr. 1998; 18: 297-330Crossref PubMed Scopus (229) Google Scholar). It is commonly accepted that the efflux of cholesterol from cells is caused by two different pathways: the first is passive and promotes efflux from the cell membrane to HDL and the second is energy-dependent and apolipoprotein-mediated (5Rothblat G.H. de la Llera-Moya M. Atger V. Kellner-Weibel G. Williams D.L. Phillips M.C. J. Lipid Res. 1999; 40: 781-796Abstract Full Text Full Text PDF PubMed Google Scholar). The latter was characterized in fibroblasts and macrophages and involves lipid-poor or -free apolip