Microarray Analyses during Adipogenesis: Understanding the Effects of Wnt Signaling on Adipogenesis and the Roles of Liver X Receptor α in Adipocyte Metabolism

Abstract

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.

Divergent Effects of Peroxisome Proliferator-activated Receptor γ Agonists and Tumor Necrosis Factor α on Adipocyte ApoE Expression

“…Interestingly, this enhancer contains a conserved functional LXR response element. Because the expression of LXR␣ is positively regulated by PPAR␥ agonists in adipocytes (42)(43)(44)(45), this LXR element, in the presence of an endogenously generated ligand, could account for increased apoE gene expression in response to ciglitazone. On the other hand, a functional PPAR␥ response element has also been identified for transducing the apoE gene response to ciglitazone in U87MG astrocytoma cells (31).…”

Fat poetry: a kingdom for PPARγ

“…Next, 48 h after the initiation of differentiation, the cells start to acquire the "early adipocyte phenotype", which represents the third step. Fourth, in the "differentiated adipocytes", genes already expressed at low levels in the early adipocyte phenotype, are now at their maximal expression levels, especially genes involved in energy storage and fat metabolism, such as C/EBPβ and PPARg [157,158]. 3T3L1 cells, frequently used as adipocyte differentiation models, which have been manipulated to express small interference RNA (siRNA) against PPARg or embryonic stem cells (ES cells) deficient for PPARg (PPARg-/-), are unable to differentiate into adipocytes.…”

Wnt10b Inhibits Development of White and Brown Adipose Tissues

“…Wnt signaling inhibits differentiation of preadipocytes in vitro (7,8,11,12). To determine whether signaling by Wnt10b blocks adipogenesis in vivo, we examined adiposity of transgenic mice that express Wnt10b from the well characterized mouse FABP4 promoter (Fig.…”

“…Although the mechanism is still under investigation, alterations in cell cycle regulatory proteins may mediate some of the actions of Wnt (12). Spontaneous adipogenesis occurs when Wnt signaling is inhibited in preadipocytes, indicating that preadipocytes produce an endogenous Wnt that represses differentiation.…”