The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligandregulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. T he liver plays a central role in the control of whole-body lipid homeostasis, and hepatic lipid metabolism is continuously adjusted to fit the needs of the organism. This adaptation requires major adjustments in the hepatic metabolic gene program, including a strong upregulation of lipogenic gene expression in the fed state, whereas in the fasting state, the expression of genes involved in fatty acid oxidation as well as ketogenesis and hepatic glucose production is highly induced. Class II nuclear receptors (NRs), i.e., NRs forming heterodimers with retinoid X receptor (RXR), play a key role in coordinating these changes. They include the liver X receptor (LXR) (29, 57) and peroxisome proliferatoractivated receptor (PPAR) (32, 34, 41) families as well as farnesoid X receptor (FXR) (44, 55, 88), pregnane X receptor (PXR) (5, 33), vitamin D receptor (VDR) (43), constitutive androstane receptor (CAR) (3, 9, 23), and retinoic acid receptors (RARs) (13).The LXR family consists of the two subtypes, LXR␣ (NR1H3) and LXR␤ (NR1H2), both of which form obligate heterodimers with RXR. LXR-RXR heterodimers are reported to bind to LXR response elements (LXREs) that consist of a direct repeat of the core sequence 5=-AGGTCA-3= spaced by 4 nucleotides (DR4) (2,72,76,79,92). LXRs are activated by oxidized cholesterol derivatives and play an important role in the regulation of cholesterol homeostasis in the liver. Thus, pharmacological activation of LXR leads to the induction of several genes implicated in reverse cholesterol transport and mobilization of cholesterol, such as the ATP binding cassette (ABC) transporter genes Abca1, Abcg1, Abcg5, and Abcg8 and the apolipoprotein E gene (ApoE) (12,31,37,60,63,84). Furthermore, a recent genomewide study of LXR in human hepatoma cells showed that LXR also downregulates expression of the cholesterologenic genes for lanosterol 14␣-demethylase (Cyp51A1) and squalene synthase (Fdst1) (89). Moreover, LXR activation induces triglyceride synthesis partly through induction of the lipogenic transcription factors sterol regulatory element-binding protein 1c (SREBP-1c) (42,61,95) and carbohydrate response element-binding protein (ChREBP) (8) but also by direct activation of genes encoding lipogenic enzymes such as fatty acid synthase (Fasn), stearoyl coenzyme A (CoA) desaturase (Scd1), ...

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Wnt signaling maintains preadipocytes in an undifferentiated state. When Wnt signaling is enforced, 3T3-L1 preadipocytes no longer undergo adipocyte conversion in response to adipogenic medium. Here we used microarray analyses to identify subsets of genes whose expression is aberrant when differentiation is blocked through enforced Wnt signaling. Furthermore, we used the microarray data to identify potentially important adipocyte genes and chose one of these, the liver X receptor ␣ (LXR␣), for further analyses. Our studies indicate that enforced Wnt signaling blunts the changes in gene expression that correspond to mitotic clonal expansion, suggesting that Wnt signaling inhibits adipogenesis in part through dysregulation of the cell cycle. Experiments designed to uncover the potential role of LXR␣ in adipogenesis revealed that this transcription factor, unlike CCAAT/enhancer binding protein ␣ and peroxisome proliferator-activated receptor gamma, is not adipogenic but rather inhibits adipogenesis if inappropriately expressed and activated. However, LXR␣ has several important roles in adipocyte function. Our studies show that this nuclear receptor increases basal glucose uptake and glycogen synthesis in 3T3-L1 adipocytes. In addition, LXR␣ increases cholesterol synthesis and release of nonesterified fatty acids. Finally, treatment of mice with an LXR␣ agonist results in increased serum levels of glycerol and nonesterified fatty acids, consistent with increased lipolysis within adipose tissue. These findings demonstrate new metabolic roles for LXR␣ and increase our understanding of adipogenesis.Adipocytes play a central role in energy balance, both as a reservoir, storing and releasing fuel, and as endocrine cells, secreting factors that regulate whole-body energy metabolism (49). Understanding this cell type is becoming increasingly important because of the rising incidence of obesity and its associated disorder, type II diabetes. One widely used model for studying the adipocyte is the 3T3-L1 cell line, which was derived from disaggregated mouse embryos and selected based on the propensity of these cells to differentiate into adipocytes in culture (14). Over the last 30 years, this cell line has proven to be a faithful model for studying adipocyte biology, particularly adipogenesis and energy metabolism.

The pivotal role of liver X receptors (LXRs) in the metabolic conversion of cholesterol to bile acids in mice is well established. More recently, the LXRalpha promoter has been shown to be under tight regulation by peroxisome proliferator-activated receptors (PPARs), implying a role for LXRalpha in mediating the interplay between cholesterol and fatty acid metabolism. We have studied the role of LXR in fat cells and demonstrate that LXR is regulated during adipogenesis and augments fat accumulation in mature adipocytes. LXRalpha expression in murine 3T3-L1 adipocytes as well as in human adipocytes was up-regulated in response to PPARgamma agonists. Administration of a PPARgamma agonist to obese Zucker rats also led to increased LXRalpha mRNA expression in adipose tissue in vivo. LXR agonist treatment of differentiating adipocytes led to increased lipid accumulation. An increase of the expression of the LXR target genes, sterol regulatory binding protein-1 and fatty acid synthase, was observed both in vivo and in vitro after treatment with LXR agonists for 24 h. Finally, we demonstrate that fat depots in LXRalpha/beta-deficient mice are smaller than in age-matched wild-type littermates. These findings imply a role for LXR in controlling lipid storage capacity in mature adipocytes and point to an intriguing physiological interplay between LXR and PPARgamma in controlling pathways in lipid handling.

In a systematic search for peroxisome proliferatoractivated receptor-␥ (PPAR-␥) target genes, we identified S3-12 and perilipin as novel direct PPAR-␥ target genes. Together with adipophilin and tail-interacting protein of 47 kDa, these genes are lipid droplet-associating proteins with distinct expression pattern but overlapping expression in adipose tissue. The expression of S3-12 and perilipin is tightly correlated to the expression and activation of PPAR-␥ in adipocytes, and promoter characterization revealed that the S3-12 and the perilipin promoters contain three and one evolutionarily conserved PPAR response elements, respectively. We furthermore demonstrate that the expression of S3-12 and perilipin is reduced in obese compared with lean Zucker rats, whereas the expression of adipophilin is increased. Others have shown that perilipin is an essential factor in the hormonal regulation of lipolysis of stored triglycerides within adipose tissue. The direct regulation of perilipin and S3-12 by PPAR-␥ therefore is likely to be an important mediator of the in vivo effects of prolonged treatment with PPAR-␥ activators: insulin sensitization, fatty acid trapping in adipose tissue, reduced basal adipose lipolysis, and weight gain. Diabetes 53:1243-1252, 2004 T oday's western lifestyle, which involves a highcalorie diet and a lack of exercise, has led to an epidemic of obesity, which often is associated with type 2 diabetes, hypertension, hyperlipidemia, and cardiovascular disease. Increased energy intake results in an imbalance between fat synthesis and degradation, leading to an increase in circulating fatty acids (FAs) and accumulation of lipids in white adipose tissue (WAT). Whereas most tissues store triacylglycerol (TAG), cholesterol esters (CEs), or lipids in relatively small (Ͻ1 m diameter) droplets that can be used as an energy source or for membrane biogenesis, WAT stores most of the body's TAG reservoir in droplets that can exceed 50 m in diameter (1). Although the interior of those lipid droplets consists largely of neutral lipids, a number of proteins associate with the droplet surface. These include P 200 , caveolins, vimentin (1,2), mouse adipose differentiation-related protein (ADRP)/human adipophilin (hereafter referred to as adipophilin) (3), perilipin (4,5), S3-12 (6), and tail-interacting protein of 47 kDa (TIP-47) (7).Perilipin, adipophilin, and TIP-47 exhibit high sequence identity within an NH 2 -terminal motif termed PAT-1 (after perilipin, adipophilin, and TIP-47) and a more distally located PAT-2 domain (8,9). A fourth protein, S3-12, has been described along with these PAT family members. S3-12 contains a repeated 33-amino acid motif also found in adipophilin (10), and it shares protein sequence identity to both adipophilin and TIP-47 in the COOH terminus, but not to perilipin (8).At present, the lipid droplet-associating properties have been thoroughly studied only for adipophilin and perilipin (11,12). Adipophilin associates with smaller neutral lipid storage droplets located within most...