Adipocyte SIRT1 knockout promotes PPARγ activity, adipogenesis and insulin sensitivity in chronic-HFD and obesity

Objective Adipose tissue is the primary site for lipid deposition that protects the organisms in cases of nutrient excess during obesogenic diets. The histone deacetylase Sirtuin 1 (SIRT1) inhibits adipocyte differentiation by targeting the transcription factor peroxisome proliferator activated-receptor gamma (PPARγ). Methods To assess the specific role of SIRT1 in adipocytes, we generated Sirt1 adipocyte-specific knockout mice (ATKO) driven by aP2 promoter onto C57BL/6 background. Sirt1flx/flxaP2Cre+ (ATKO) and Sirt1flx/flxaP2Cre- (WT) mice were fed high-fat diet for 5 weeks (short-term) or 15 weeks (chronic-term). Metabolic studies were combined with gene expression analysis and phosphorylation/acetylation patterns in adipose tissue. Results On standard chow, ATKO mice exhibit low-grade chronic inflammation in adipose tissue, along with glucose intolerance and insulin resistance compared with control fed mice. On short-term HFD, ATKO mice become more glucose intolerant, hyperinsulinemic, insulin resistant and display increased inflammation. During chronic HFD, WT mice developed a metabolic dysfunction, higher than ATKO mice, and thereby, knockout mice are more glucose tolerant, insulin sensitive and less inflamed relative to control mice. SIRT1 attenuates adipogenesis through PPARγ repressive acetylation and, in the ATKO mice adipocyte PPARγ was hyperacetylated. This high acetylation was associated with a decrease in Ser273-PPARγ phosphorylation. Dephosphorylated PPARγ is constitutively active and results in higher expression of genes associated with increased insulin sensitivity. Conclusion Together, these data establish that SIRT1 downregulation in adipose tissue plays a previously unknown role in long-term inflammation resolution mediated by PPARγ activation. Therefore, in the context of obesity, the development of new therapeutics that activate PPARγ by targeting SIRT1 may provide novel approaches to the treatment of T2DM.
Graphical abstract Dual role of SIRT1 in obesity and chronic HFD. A: PPARγ activity regulation. While p300 acetyltransferase enhances the transcriptional activation properties of PPARγ by increasing lipogenesis, SIRT1 deacetylase and CDK5 kinase promotes lipolysis by inhibiting PPARγ. Obesity and pro-inflammatory signals lead to increase pY15-CDK5 via a mechanism involving the cleavage of the p35 protein to p25 in the cytoplasm, then p25 translocate to the nucleus, where it binds to CDK5 and activates it. B: Involvement of SIRT1 and PPARγ in repression/expression of different target genes in adipocytes. C: ATKO SIRT1 mice exhibit an insulin sensitive phenotype over long-term HFD/obesity, showing a hyperplasic eWAT rather than the normal hypertrophic adipose tissue often related with inflammation, obesity and insulin resistance. This effect is is strengthened in ATKO eWAT by increasing PPAR activity, releasing of IL-10 and FGF21, leading to a reduction in inflammation and improved metabolic status. SIRT1, sirtuin 1. PPARγ, peroxisome proliferator activated receptor gamma. CDK5, cyclin-dependent kinase 5. p300, Ep300 E1A binding protein. p35/p25, Cdk5r1 cyclin-dependent kinase 5, regulatory subunit 1 (p35). NcoR, nuclear receptor co-repressor 1. SMRT, nuclear receptor co-repressor 2. FGF21, fibroblast growth factor 21. FOXO1, forkhead box O1. C/EBPα, CCAAT/enhancer binding protein alpha. TZD, thiazolidinedione. AC, Acetyl residue. P, Phosphate. AD, adipocyte. Mφ-Macrophage.