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.

The nuclear receptor liver X receptor (LXR) ␣, an important regulator of cholesterol and bile acid metabolism, was analyzed after insulin stimulation in liver in vitro and in vivo. A time-and dose-dependent increase in LXR␣ steady-state mRNA level was seen after insulin stimulation of primary rat hepatocytes in culture. A maximal induction of 10-fold was obtained when hepatocytes were exposed to 400 nM insulin for 24 h. Cycloheximide, a potent inhibitor of protein synthesis, prevented induction of LXR␣ mRNA expression by insulin, indicating that the induction is dependent on de novo synthesis of proteins. Stabilization studies using actinomycin D indicated that insulin stimulation increased the half-life of LXR␣ transcripts in cultured primary hepatocytes. Complementary studies where rats and mice were injected with insulin induced LXR␣ mRNA levels and confirmed our in vitro studies. Furthermore, deletion of both the LXR␣ and LXR␤ genes (double knockout) in mice markedly suppressed insulin-mediated induction of an entire class of enzymes involved in both fatty acid and cholesterol metabolism. The discovery of insulin regulation of LXR in hepatic tissue as well as gene targeting studies in mice provide strong evidence that LXRs plays a central role not only in cholesterol homeostasis, but also in fatty acid metabolism. Furthermore, LXRs appear to be important insulin-mediating factors in regulation of lipogenesis.Insulin plays a major role in the regulation of carbohydrate and lipid metabolism in the liver, adipose tissue, and muscle. Hepatic fatty acid oxidation, lipogenesis, and glycerolipid synthesis are subject to regulation by insulin (for review, see Ref.