Muscle or liver-specific Sirt3 deficiency induces hyperacetylation of mitochondrial proteins without affecting global metabolic homeostasis

Sirt3 is a mitochondrial sirtuin, predominantly expressed in highly metabolic tissues. Germline ablation of Sirt3 has major metabolic consequences, including increased susceptibility to metabolic damage and oxidative stress after high fat feeding. In order to determine the contribution of liver and skeletal muscle to these phenotypes, we generated muscle-specific Sirt3 (Sirt3skm−/−) and liver-specific Sirt3 (Sirt3hep−/−) knock-out mice. Despite a marked global hyperacetylation of mitochondrial proteins, Sirt3skm−/− and Sirt3hep−/− mice did not manifest any overt metabolic phenotype under either chow or high fat diet conditions. Similarly, there was no evidence for increased oxidative stress in muscle or liver when Sirt3 was ablated in a tissue-specific manner. These observations suggest that the mitochondrial hyperacetylation induced by Sirt3-deletion in a tissue specific manner is not necessarily linked to mitochondrial dysfunction and does not recapitulate the metabolic abnormalities observed in the germline Sirt3 knock-out mice

Reversible acetylation of PGC-1: connecting energy sensors and effectors to guarantee metabolic flexibility

rganisms adapt their metabolism to meet ever changing environmental conditions. This metabolic adaptation involves at a cellular level the fine tuning of mitochondrial function, which is mainly under the control of the transcriptional co-activator proliferator-activated receptor gamma co-activator (PGC)-1alpha. Changes in PGC-1alpha activity coordinate a transcriptional response, which boosts mitochondrial activity in times of energy needs and attenuates it when energy demands are low. Reversible acetylation has emerged as a key way to alter PGC-1alpha activity. Although it is well established that PGC-1alpha is deacetylated and activated by Sirt1 and acetylated and inhibited by GCN5, less is known regarding how these enzymes themselves are regulated. Recently, it became clear that the energy sensor, AMP-activated kinase (AMPK) translates the effects of energy stress into altered Sirt1 activity by regulating the intracellular level of its co-substrate nicotinamide adenine dinucleotide (NAD)(+). Conversely, the enzyme ATP citrate lyase (ACL), relates energy balance to GCN5, through the control of the nuclear production of acetyl-CoA, the substrate for GCN5's acetyltransferase activity. We review here how these metabolic signaling pathways, affecting GCN5 and Sirt1 activity, allow the reversible acetylation-deacetylation of PGC-1alpha and the adaptation of mitochondrial energy homeostasis to energy levels

Familial partial lipodystrophy phenotype resulting from a single-base mutation in deoxyribonucleic acid-binding domain of peroxisome proliferator-activated receptor-gamma

CONTEXT: Familial partial lipodystrophy (FPLD) results from coding sequence mutations either in LMNA, encoding nuclear lamin A/C, or in PPARG, encoding peroxisome proliferator-activated receptor-gamma (PPARgamma). The LMNA form is called FPLD2 (MIM 151660) and the PPARG form is called FPLD3 (MIM 604367). OBJECTIVE: Our objective was to investigate whether the clinical phenotype of this proband is due to mutation(s) in PPARgamma. DESIGN: This is a case report. Patient and Setting: A 31-yr-old female with the clinical phenotype of FPLD3, i.e. lipodystrophy and early childhood diabetes with extreme insulin resistance and hypertriglyceridemia leading to recurrent pancreatitis, was assessed at an academic medical center. RESULTS: The proband was heterozygous for a novel C-->T mutation in the PPARG gene that led to the substitution of arginine 194 in PPARgamma2 isoform, a conserved residue located in the zinc finger structure involved in DNA binding, by tryptophan (R194W). The mutation was absent from the genomes of 100 healthy Caucasians. In vitro analysis of the mutated protein showed that R194W (and R166W in PPARgamma1 isoform) could not bind to DNA and had no transcriptional activity. Furthermore, R194W had no dominant-negative activity. CONCLUSIONS: The R194W mutation in PPARG disrupts its DNA binding activity and through haploinsufficiency leads to clinical manifestation of FPLD3 and the associated metabolic disturbance

The Multiple Endocrine Neoplasia Type 1 (MEN1) Tumor Suppressor Regulates Peroxisome Proliferator-Activated Receptor γ-Dependent Adipocyte Differentiation▿

Menin, the product of the MEN1 (multiple endocrine neoplasia type 1) tumor suppressor gene, is involved in activation of gene transcription as part of an MLL1 (mixed-lineage leukemia 1)/MLL2 (KMT2A/B)-containing protein complex which harbors methyltransferase activity for lysine 4 of histone H3 (H3K4). As MEN1 patients frequently develop lipomas and peroxisome proliferator-activated receptor γ (PPARγ) is expressed in several MEN1-related tumor types, we investigated regulation of PPARγ activity by menin. We found that menin is required for adipocyte differentiation of murine 3T3-L1 cells and PPARγ-expressing mouse embryonic fibroblasts. Menin augments PPARγ target gene expression through recruitment of H3K4 methyltransferase activity. Menin interacts directly with the activation function 2 transcription activation domain of PPARγ in a ligand-independent fashion. Ligand-dependent coactivation, however, is dependent on the LXXLL motif of menin and the intact helix 12 of PPARγ. We propose that menin is an important factor in PPARγ-mediated adipogenesis and that loss of PPARγ function may contribute to lipoma development in MEN1 patients

Peroxisome Proliferator-activated Receptor γ Regulates Expression of the Anti-lipolytic G-protein-coupled Receptor 81 (GPR81/Gpr81)*

The ligand-inducible nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) plays a key role in the differentiation, maintenance, and function of adipocytes and is the molecular target for the insulin-sensitizing thiazoledinediones (TZDs). Although a number of PPARγ target genes that may contribute to the reduction of circulating free fatty acids after TZD treatment have been identified, the relevant PPARγ target genes that may exert the anti-lipolytic effect of TZDs are unknown. Here we identified the anti-lipolytic human G-protein-coupled receptor 81 (GPR81), GPR109A, and the (human-specific) GPR109B genes as well as the mouse Gpr81 and Gpr109A genes as novel TZD-induced genes in mature adipocytes. GPR81/Gpr81 is a direct PPARγ target gene, because mRNA expression of GPR81/Gpr81 (and GPR109A/Gpr109A) increased in mature human and murine adipocytes as well as in vivo in epididymal fat pads of mice upon rosiglitazone stimulation, whereas small interfering RNA-mediated knockdown of PPARγ in differentiated 3T3-L1 adipocytes showed a significant decrease in Gpr81 protein expression. In addition, chromatin immunoprecipitation sequencing analysis in differentiated 3T3-L1 cells revealed a conserved PPAR:retinoid X receptor-binding site in the proximal promoter of the Gpr81 gene, which was proven to be functional by electromobility shift assay and reporter assays. Importantly, small interfering RNA-mediated knockdown of Gpr81 partly reversed the inhibitory effect of TZDs on lipolysis in 3T3-L1 adipocytes. The coordinated PPARγ-mediated regulation of the GPR81/Gpr81 and GPR109A/Gpr109A genes (and GPR109B in humans) presents a novel mechanism by which TZDs may reduce circulating free fatty acid levels and perhaps ameliorate insulin resistance in obese patients