Regulation of Absorption and ABC1-Mediated Efflux of Cholesterol by RXR Heterodimers
Tóm tắt
Several nuclear hormone receptors involved in lipid metabolism form obligate heterodimers with retinoid X receptors (RXRs) and are activated by RXR agonists such as rexinoids. Animals treated with rexinoids exhibited marked changes in cholesterol balance, including inhibition of cholesterol absorption and repressed bile acid synthesis. Studies with receptor-selective agonists revealed that oxysterol receptors (LXRs) and the bile acid receptor (FXR) are the RXR heterodimeric partners that mediate these effects by regulating expression of the reverse cholesterol transporter, ABC1, and the rate-limiting enzyme of bile acid synthesis, CYP7A1, respectively. Thus, these RXR heterodimers serve as key regulators of cholesterol homeostasis by governing reverse cholesterol transport from peripheral tissues, bile acid synthesis in liver, and cholesterol absorption in intestine.
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Tài liệu tham khảo
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Mice were fed ad libitum Teklad 7001 rodent diet supplemented with 0.2% cholesterol and LG268 or vehicle. Animals were housed in temperature-controlled rooms (22°C) with 12-hour light/dark cycles. For studies involving the measurement of bile acids and enzymes/RNA of the bile acid biosynthetic enzymes mice were killed 2 hours before the end of the dark cycle to minimize effects due to the diurnal rhythm of this system. Animal experiments were approved by the Institution Animal Care and Research Advisory Committee of the University of Texas Southwestern Medical Center. A 30 mg/kg of body weight (mpk) dose was achieved by the addition of LG268 at 0.015% of the diet. LG268 was solubilized at 4.5 mg/ml in a vehicle containing 0.9% carboxymethylcellulose 9% PEG-400 and 0.05% Tween 80. Control animals received diet supplemented with vehicle.
Northern blots were hybridized with [ 32 P]-labeled cDNA probes for cholesterol 7α-hydroxylase (CYP7A1) (46) oxysterol 7α-hydroxylase (CYP7B1) (47) and sterol 12α-hydroxylase [CYP8B1 generated by reverse transcription polymerase chain reaction (RT-PCR) with primers 5′-GGGTACCAGTCTGTAGATGG and 5′-AGTCTCTGGTGGAAGAGACG based on the mouse sequence detailed in GenBank accession number AF090317].
[ 32 P]-labeled cDNA probes were apolipoprotein A-I (Apo A-I) (48) scavenger receptor-B1 (SR-B1) from H. Hobbs; acyl CoA:cholesterol acyltransferase-2 (ACAT-2) from R. Farese Jr.; mouse ABC1 generated by RT-PCR with primers 5′-TCTCGCCTGTTCTCAGACGC and 5′-CTCCAGGTATACACAGAGCC to generate a probe that spans nucleotides 390 to 1537 (GenBank accession number ); and mouse microsomal triglyceride transfer protein (MTP) created by RT-PCR with primers 5′-CGTTGTGTTACTGTGGAGG and 5′-TCTTCTCTCCTCGAAGTCC to amplify a product spanning nucleotides 216 to 1051 (GenBank accession number ).
We thank C. Fievet and J. Auwerx for determination of the lipoprotein profiles; D. W. Russell M. S. Brown J. L. Goldstein H. H. Hobbs and R. V. Farese Jr. for reagents and critically reading the manuscript; A. Venkateswaran and P. A. Edwards for the reagents for the macrophage experiments; H. Lawrence E. Moore J. Graven B. Jefferson and A. Fletcher for excellent technical assistance; and M. Schwarz and the members of the Mango lab for their helpful comments and assistance in animal dissections. J.J.R. and J.M.L. are research associates and D.J.M. is an investigator of the Howard Hughes Medical Institute (HHMI). This work was funded by HHMI and the Robert Welch Foundation and the Human Frontiers Science Program (D.J.M.). Funding to J.M.D. and S.D.T. was provided by the U.S. Public Health Service Grant R37 HL 09610 and the Moss Heart Fund.