Wnts are secreted signaling proteins that regulate developmental processes. Here we show that Wnt signaling, likely mediated by Wnt-10b, is a molecular switch that governs adipogenesis. Wnt signaling maintains preadipocytes in an undifferentiated state through inhibition of the adipogenic transcription factors CCAAT/enhancer binding protein alpha (C/EBPalpha) and peroxisome proliferator- activated receptor gamma (PPARgamma). When Wnt signaling in preadipocytes is prevented by overexpression of Axin or dominant-negative TCF4, these cells differentiate into adipocytes. Disruption of Wnt signaling also causes transdifferentiation of myoblasts into adipocytes in vitro, highlighting the importance of this pathway not only in adipocyte differentiation but also in mesodermal cell fate determination.

Wnts comprise a family of secreted signaling proteins that regulate diverse developmental processes. Activation of Wnt signaling by Wnt10b inhibits differentiation of preadipocytes and blocks adipose tissue development; however, the effect of Wnt10b on other mesenchymal lineages has not been defined. To explore the physiological role of Wnt signaling in bone development, we analyzed FABP4-Wnt10b mice, which express the Wnt10b transgene in marrow. Femurs from FABP4-Wnt10b mice have almost four times as much bone in the distal metaphyses and are mechanically stronger. These mice maintain elevated bone mass at least through 23 months of age. In addition, FABP4-Wnt10b mice are protected from the bone loss characteristic of estrogen deficiency. We used pharmacological and genetic approaches to demonstrate that canonical Wnt signaling stimulates osteoblastogenesis and inhibits adipogenesis of bipotential mesenchymal precursors. Wnt10b shifts cell fate toward the osteoblast lineage by induction of the osteoblastogenic transcription factors Runx2, Dlx5, and osterix and suppression of the adipogenic transcription factors C͞EBP␣ and PPAR␥. One mechanism whereby Wnt10b promotes osteoblastogenesis is suppression of PPAR␥ expression. Finally, Wnt10b؊͞؊ mice have decreased trabecular bone and serum osteocalcin, confirming that Wnt10b is an endogenous regulator of bone formation.adipogenesis ͉ development ͉ stem cells M esenchymal progenitors can differentiate into a number of cell types, including adipocytes and osteoblasts (1). One factor that regulates the reciprocal relationship between these lineages is Wnt signaling, which inhibits adipogenesis and stimulates osteoblastogenesis. Activation of Wnt signaling blocks preadipocyte differentiation by inhibiting expression of the adipogenic transcription factors C͞EBP␣ and PPAR␥ (2-5). The endogenous inhibitory Wnt signal may be initiated by Wnt10b, which is expressed in preadipocytes and stromal vascular cells but not in adipocytes (2, 3). Expression of Wnt10b from the FABP4 promoter decreases accumulation of white adipose tissue by Ϸ50% and completely blocks the development of brown fat (6, 7). Furthermore, FABP4-Wnt10b mice resist diet-induced obesity and the associated glucose intolerance.The first indication that Wnt signaling plays a critical role in bone formation came from human studies where inactivating mutations in the Wnt coreceptor LRP5 were shown to cause osteoporosis (8). These findings were supported by the observation that LRP5Ϫ͞Ϫ mice also have low bone mass (9). Furthermore, gain-of-function mutations in LRP5 that increase Wnt signaling result in higher bone density in humans and mice (10,11). Consistent with the effects of LRP5 on bone mass being mediated through canonical Wnt signaling, activation of this pathway in vitro results in the expression of alkaline phosphatase, an early osteoblast marker (12)(13)(14). Although these and other studies suggest that endogenous Wnt signaling regulates osteoblastogenesis and bone formation (15), a specific Wnt or Wnts...

We have identified Wnt10b as a potent inhibitor of adipogenesis that must be suppressed for preadipocytes to differentiate in vitro. Here, we demonstrate that a specific inhibitor of glycogen synthase kinase 3, CHIR 99021, mimics Wnt signaling in preadipocytes. CHIR 99021 stabilizes free cytosolic ␤-catenin and inhibits adipogenesis by blocking induction of CCAAT/enhancer-binding protein ␣ and peroxisome proliferatoractivated receptor ␥. Preadipocyte differentiation is inhibited when 3T3-L1 cells are exposed to CHIR 99021 for any 24 h period during the first 3 days of adipogenesis. Consistent with this time frame of inhibition, expression of Wnt10b mRNA is suppressed upon induction of differentiation, with a 50% decline by 6 h and complete inhibition by 36 h. Of the agents used to induce differentiation, exposure of 3T3-L1 cells to methyl-isobutylxanthine or cAMP is sufficient to suppress expression of Wnt10b mRNA. Inhibition of adipogenesis by Wnt10b is likely mediated by Wnt receptors, Frizzled 1, 2, and/or 5, and co-receptors low density lipoprotein receptor-related proteins 5 and 6. These receptors, like Wnt10b, are highly expressed in preadipocytes and stromal vascular cells. Finally, we demonstrate that disruption of extracellular Wnt signaling by expression of secreted Frizzled related proteins causes spontaneous adipocyte conversion.White adipose tissue is an important depot for short and long term energy storage. In addition, adipose tissue is an endocrine organ that regulates energy homeostasis through secretion of leptin, Acrp30/adiponectin, resistin, and other factors (1). The study of adipocyte biology has been greatly facilitated by development of immortalized preadipocyte lines (e.g. 3T3-L1) (2) that differentiate into adipocytes and recapitulate many of the metabolic and endocrine functions of adipocytes in vivo. Analysis of differentiation in preadipocyte lines and in animals has led to a paradigmatic model of the genetic program of adipogenesis (3-5). In response to stimulators of adipogenesis, two transcription factors, CCAAT/enhancer-binding protein ␤ (C/ EBP␤) 1 and C/EBP␦ are rapidly and transiently induced. These proteins then stimulate expression of the key adipogenic transcription factors, C/EBP␣ and peroxisome proliferator-activated receptor ␥ (PPAR␥), which together induce expression of the complement of genes necessary to create the adipocyte phenotype. Investigations of factors that regulate progression through this adipogenic program are important for our understanding of adipocyte differentiation. Differentiation of preadipocytes into adipocytes is regulated by a balance of local and endocrine factors that either stimulate or inhibit differentiation (6). Well known factors that stimulate differentiation of preadipocyte lines include glucocorticoid agonists, high concentrations of insulin to stimulate the insulinlike growth factor 1 receptor, PPAR␥ agonists, and agents that elevate cAMP (3). Factors that counter these positive stimuli include Wnt10b, tumor necrosis factor ␣ (TNF␣), ...