Liver X receptors (LXRs) are members of the nuclear receptor superfamily defined as ligand-activated transcription factors. LXR ␣ (NR1H3) and LXR ␤ (NR1H2) (1-3) belong to a subclass that form obligate heterodimers with retinoid X receptors (RXRs). They share significant amino acid identity in their DNA and ligand binding domains and are highly conserved between human and rodent species (3). The elucidation of their role as key regulators of cholesterol homeostasis followed from the recent identification of oxysterols as physiological ligands for LXR (4-7).LXR nuclear receptors act as a transcriptional master switch for the coordinated regulation of genes involved in cellular cholesterol homeostasis, cholesterol transport, catabolism, and absorption (8, 9). Several LXR-responsive target genes, ATP binding cassette proteins, ABCA1 (8, 10-12) and ABCG1 (11, 13), cholesteryl ester transfer protein (CETP) (14), and apolipoprotein E (apoE) (15), with defined roles in the reverse cholesterol transport (RCT) process, govern the transport of excess cholesterol for eventual uptake and elimination by the liver. The ultimate disposal of this excess cholesterol returning to liver, its conversion to bile acids for secretion into bile, and its final elimination in feces are also under LXR control in some species. LXR-mediated upregulation of cholesterol 7 ␣ -hydroxylase (cyp7a), the rate-limiting enzyme for bile acid synthesis, correlates with resistance to dietary cholesterol and atherosclerosis (5, 16) in mice. The proximal promoter of the human gene does not contain an LXRE and consequently is not regulated by oxysterols or LXR. Thus, the regulation of murine and human CYP7A1 genes differs. A role for LXR target genes in the modulation of cholesterol absorption has also been implicated for ABCA1, ABCG5, and ABCG8 (8,17,18).The identification of LXR nuclear receptors as direct regulators of ABC transporter gene expression focused attention on their potentially critical role in peripheral macrophages (8, 10-13). Here, the membrane-bound ATP binding cassette protein (ABCA1) initiates the process of cholesterol elimination from peripheral tissues. The ABCA1 Abbreviations: ACC, acetyl-CoA carboxylase; acLDL, acetylated low density lipoprotein; apoE, apolipoprotein E; CETP, cholesteryl ester transfer protein; cyp7a, cholesterol 7 ␣ -hydroxylase; DMHCA, N,Ndimethyl-3 ␤ -hydroxycholenamide; LXR, liver X receptor; RCT, reverse cholesterol transport; RXR, retinoid X receptor; SREBP-1c, sterolresponse element binding protein 1c.

The nuclear receptors liver X receptor (LXR) LXR␣ and LXR␤ are differentially expressed ligand-activated transcription factors that induce genes controlling cholesterol homeostasis and lipogenesis. Synthetic ligands for both receptor subtypes activate ATP binding cassette transporter A1 (ABCA1)-mediated cholesterol metabolism, increase reverse cholesterol transport, and provide atheroprotection in mice. However, these ligands may also increase hepatic triglyceride (TG) synthesis via a sterol response element binding protein 1c (SREBP-1c)-dependent mechanism through a process reportedly regulated by LXR␣. We studied pan-LXR␣/␤ agonists in LXR␣ knockout mice to assess the contribution of LXR␤ to the regulation of selected target genes. In vitro dose-response studies with macrophages from LXR␣Ϫ/Ϫ and ␤Ϫ/Ϫ mice confirm an equivalent role for LXR␣ and LXR␤ in the regulation of ABCA1 and SREBP-1c gene expression. Cholesterol-efflux studies verify that LXR␤ can drive apoA1-dependent cholesterol mobilization from macrophages. The in vivo role of LXR␤ in liver was further evaluated by treating LXR␣Ϫ/Ϫ mice with a pan-LXR␣/␤ agonist. Highdensity lipoprotein (HDL) cholesterol increased without significant changes in plasma TG or very low density lipoprotein. Analysis of hepatic gene expression consistently revealed less activation of ABCA1 and SREBP-1c genes in the liver of LXR␣ null animals than in treated wild-type controls. In addition, hepatic CYP7A1 and several genes involved in fatty acid/TG biosynthesis were not induced. In peripheral tissues from these LXR␣-null mice, LXR␤ activation increases ABCA1 and SREBP-1c gene expression in a parallel manner. However, putative elevation of SREBP-1c activity in these tissues did not cause hypertriglyceridemia. In summary, selective LXR␤ activation is expected to stimulate ABCA1 gene expression in macrophages, contribute to favorable HDL increases, but circumvent hepatic LXR␣-dominated lipogenesis.There is great interest in targeting LXR nuclear receptors and their modulation for the treatment of atherosclerosis. These transcription factors play a critical role in the control of cholesterol homeostasis and have been the topic of several recent reviews (Jaye,

The plasma cholesteryl ester transfer protein (CETP) catalyzes the transfer of phospholipids and neutral lipids between the lipoproteins. Thus, this protein may be important in modulating lipoprotein levels in the plasma. We have determined the primary structure and organization of the human CETP gene. Southern blotting of cellular DNA indicated a single copy of the CETP gene exists per haploid genome. Analysis of three overlapping genomic clones showed that the gene spans approximately 25 kbp and contains 16 exons (size range 32-250 bp). Overall, the sequence and organization of the CETP gene do not resemble those of other lipid-metabolizing enzymes or apolipoproteins. However, comparison of the CETP sequence, one exon at a time, with the sequences in the sequence databases revealed a striking identity of a pentapeptide sequence (ValLeuThrLeuAla) within the hydrophobic core of the signal sequences of human CETP, apolipoproteins A-IV and A-I, and lipoprotein lipase. This pentapeptide sequence was not found in the signal sequences of other proteins, suggesting that it may mediate a specialized function related to lipid metabolism or transport.

This article is available online at http://www.jlr.org Supplementary key words cynomolgus monkey • dyslipidemia • fi broblast growth factor 19 • hypertriglyceridemiaAtherosclerosis is the major cause of cardiovascular disease and its incidence is on the rise due to its tight relationship to obesity and diabetes. Therapeutic interventions targeted at reducing elevated plasma low-density lipoprotein cholesterol (LDLc), the primary risk factor for development of atherosclerosis, do not eliminate cardiovascular risk particularly in several high-risk subpopulations. The statin class of drugs achieve dramatic reductions in LDLc yet reduce heart attack risk only 33% per 1.5 mmol/L reduction in LDL ( 1 ). As statins primarily limit disease progression through the inhibition of endogenous cholesterol synthesis, newer treatment modalities directed at reversing established atherosclerotic plaque are likely to provide additional benefi t and can have important clinical implications for disease management. This is exemplifi ed by the exploratory clinical studies targeting the enhancement of high-density lipoprotein ( 2 ). In this study, intravenous

This study was undertaken to determine potential tissue sources of plasma cholesteryl ester transfer protein (CETP), and to assess the influence of CETP on lipoprotein concentrations and atherosclerosis. In a group of 28 cynomolgus monkeys fed high fat, high cholesterol diets, plasma CETP concentration was strongly correlated with the abundance of CETP mRNA in liver and in adipose tissue, and with the output of CETP in liver perfusates. Plasma CETP concentration showed a strong inverse correlation with HDL cholesterol concentrations (r = -0.62, P < 0.001) and a positive correlation with LDL cholesterol concentration (r = 0.54, P < 0.005) and molecular weight (r = 0.57, P < 0.001). The extent ofcoronary artery atherosclerosis was positively correlated with LDL cholesterol concentration and molecular weight, and with plasma CETP concentration. Thus, in monkeys fed an atherogenic diet, individual variation in CETP mRNA abundance in liver and adipose tissue probably plays a major role in the determination of plasma CETP levels. In plasma, CETP influences the distribution of cholesteryl esters between LDL and HDL, and CETP concentration appears to be a key determinant of the relative atherogenicity of the plasma lipoproteins. (J. Clin. Invest.