Alterations of peroxisome proliferator-activated receptor delta activity affect fatty acid-controlled adipose differentiation

Fatty acids have been postulated to regulate adaptation of adipose mass to nutritional changes by controlling expression of genes implicated in lipid metabolism via activation of nuclear receptors. Ectopic expression of the nuclear receptors PPARgamma or PPARdelta promotes adipogenesis in fibroblastic cells exposed to thiazolidinediones or long-chain fatty acids. To investigate the role of PPARdelta in fatty acid regulation of gene expression and adipogenesis in a preadipose cellular context, we studied the effects of overexpressing the native receptor or the dominant-negative PPARdelta mutant in Ob1771 and 3T3-F442A cells. Overexpression of PPARdelta enhanced fatty acid induction of the adipose-related genes for fatty acid translocase, adipocyte lipid binding protein, and PPARgamma and fatty acid effects on terminal differentiation. A transactivation-deficient form of PPARdelta mutated in the AF2 domain severely reduced these effects. Findings are similar in Ob1771 or 3T3-F442A preadipose cells. These data demonstrate that PPARdelta plays a central role in fatty acid-controlled differentiation of preadipose cells. Furthermore, they suggest that modulation of PPARdelta expression or activity could affect adaptive responses of white adipose tissue to nutritional changes.     

Roles of peroxisome proliferator-activated receptors delta and gamma in myoblast transdifferentiation

Dietary long chain fatty acids and thiazolidinediones act as potent activators of adipogenesis in established preadipose cell lines. High concentrations of thiazolidinediones have also been shown to induce terminal differentiation of non-preadipose cells, such as fibroblasts and myoblasts, into adipose-like cells. This transdifferentiation was observed in both rodent and human myoblasts. In this report, we show that PPARdelta mediates some of the effects exerted by long chain fatty acids on myogenesis and adipogenesis. Activation of PPARdelta by long chain fatty acids impairs the expression of the determination factor MyoD1 and alpha-actin, abolishes the development of multinucleated myotubes, and in parallel induces the expression of PPARgamma gene, a master regulator of adipogenesis. Ectopic expression of PPARdelta in C2C12 myoblasts potentiated the fatty acid-induced expression of adipogenic markers, while expression of a dominant negative PPARdelta mutant exerted opposite effects. Furthermore, a sequential activation of first PPARdelta with long chain fatty acids and then PPARgamma with thiazolidinediones is required for adipogenesis in C2C12 myoblasts. This study demonstrates that PPARdelta, at least in part, is responsible for the dual effects of long chain fatty acids as inhibitors of myogenesis and inducers of transdifferentiation into preadipose-like cells.     

Peroxisome proliferator-activated receptor delta (PPARdelta )-mediated regulation of preadipocyte proliferation and gene expression is dependent on cAMP signaling

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a key regulator of terminal adipocyte differentiation. PPARdelta is expressed in preadipocytes, but the importance of this PPAR subtype in adipogenesis has been a matter of debate. Here we present a critical evaluation of the role of PPARdelta in adipocyte differentiation. We demonstrate that treatment of NIH-3T3 fibroblasts overexpressing PPARdelta with standard adipogenic inducers led to induction of PPARgamma2 expression and terminal adipocyte differentiation in a manner that was strictly dependent on simultaneous administration of a PPARdelta ligand and methylisobutylxanthine (MIX) or other cAMP elevating agents. We further show that ligands and MIX synergistically stimulated PPARdelta-mediated transactivation. In 3T3-L1 preadipocytes, simultaneous administration of a PPARdelta-selective ligand and MIX significantly enhanced the early expression of PPARgamma and ALBP/aP2, but only modestly promoted terminal differentiation as determined by lipid accumulation. Finally, we provide evidence that synergistic activation of PPARdelta promotes mitotic clonal expansion in 3T3-L1 cells with or without forced expression of PPARdelta. In conclusion, our results suggest that PPARdelta may play a role in the proliferation of adipocyte precursor cells, whereas activation of endogenous PPARdelta in 3T3-L1 cells appears to have only minor impact on the processes leading to terminal adipocyte differentiation.     

HRASLS3 is a PPARgamma-selective target gene that promotes adipocyte differentiation

The prevalence of obesity and its associated metabolic diseases worldwide has focused attention on understanding the mechanisms underlying adipogenesis. The nuclear receptor PPARgamma has emerged as a central regulator of adipose tissue function and formation. Despite the identification of numerous PPARgamma targets involved in a range of processes, from lipid droplet formation to adipokine secretion, information is still lacking on targets downstream of PPARgamma that directly affect fat cell differentiation. Here we identify HRASLS3 as a novel PPARgamma regulated gene with a role in adipogenesis. HRASLS3 expression increases during the differentiation of preadipocyte cell lines and is highly expressed in white and brown adipose tissue in mice. HRASLS3 expression is induced by PPARgamma ligands in preadipocyte cell lines as well in adipose tissue in vivo. We demonstrate that the HRASLS3 promoter contains a functional PPAR response element and is a direct target for regulation by PPARgamma/RXR heterodimers. Finally, we show that overexpression of HRASLS3 augments PPARgamma-driven lipid accumulation and adipogenesis, whereas siRNA-mediated knockdown of HRASLS3 expression decreases differentiation. Together, these results identify HRASLS3 as one of the downstream effectors of PPARgamma action in adipogenesis.     

SIRT2 regulates adipocyte differentiation through FoxO1 acetylation/deacetylation

The family of mammalian Sirtuin proteins comprises seven members homologous to yeast Sir2. Here we show that SIRT2, a cytoplasmic sirtuin, is the most abundant sirtuin in adipocytes. Sirt2 expression is downregulated during preadipocyte differentiation in 3T3-L1 cells. Overexpression of SIRT2 inhibits differentiation, whereas reducing SIRT2 expression promotes adipogenesis. Both effects are accompanied by corresponding changes in the expression of PPARgamma, C/EBPalpha, and genes marking terminal adipocyte differentiation, including Glut4, aP2, and fatty acid synthase. The mechanism underlying the effects of reduced SIRT2 in 3T3-L1 adipocytes includes increased acetylation of FOXO1, with direct interaction between SIRT2 and FOXO1. This interaction enhances insulin-stimulated phosphorylation of FOXO1, which in turn regulates FOXO1 nuclear and cytosolic localization. Thus, Sirt2 acts as an important regulator of adipocyte differentiation through modulation of FOXO1 acetylation/phosphorylation and activity and may play a role in controlling adipose tissue mass and function.