The research proposed in this application aims to leverage basic discoveries in obesity and diabetes to develop novel approaches for the therapy of these conditions. PPAR? is the master regulator of adipocyte biology and also plays crucial roles in regulating lipid metabolism, glucose homeostasis, inflammation, and other responses. It represents a key target for insulin sensitization. The PI has discovered that deacetylation of PPAR? on lysine K268 and K293 promotes the transformation of energy-storing white adipocytes into energy-dissipating brown-like adipocytes, a process called ?browning? or ?beiging?. To establish the physiological significance of PPAR? deacetylation, the PI generated deacetylation-mimetic PPAR? K268R/K293R mutant knock-in mice (2KR). As reported in the preliminary data, 2KR mice have increased energy expenditure and are protected from obesity, particularly visceral obesity. Strikingly, when 2KR mice are treated with thiazolidinediones (TZDs), they show a robust insulin-sensitizing response but fail to develop the cardiac and bone loss effects that have limited the clinical utilization of TZD. In this application, he proposes to define the mechanisms underlying the two most compelling effects of PPAR? deacetylation, increased beiging, and prevention of TZD-induced bone loss. In Aim 1, he will determine whether PPAR? deacetylation affects adipocyte beiging in an adipocyte cell-autonomous fashion by ex vivo adipocyte differentiation studies. Next, he will dissect the fat-specific functions of 2KR by employing an adipocyte conditional knock-in model. In Aim 2, using the prototypical white adipocyte gene Adipsin as a target, he will employ ChIP, reporter, and promoter pull-down assays to identify acetylation-responsive co-repressor that mediate the effects of 2KR. To demonstrate the feasibility of the approach, the PI shows that they identified transcription factor Osr1 as an acetylation-dependent PPAR? cofactor. He will test whether the 2KR mutant represses the white adipose program by preventing Osr1-mediated Adipsin expression. In Aim 3, the PI hypothesizes that the 2KR mutant regulates bone turnover by repressing Adipsin. He will first establish the role of Adipsin in osteogenesis and bone remodeling using ex vivo differentiation of bone marrow mesenchymal progenitor cells and Adipsin knockout mice. Next, he will determine whether repression of Adipsin is the mechanism by which the 2KR mutant is protected against TZD-induced bone loss using Adipsin gain- or loss-of-function studies in 2KR mice. The proposed studies will advance the field by elucidating the selective regulation of PPAR? through deacetylation and provide mechanistic insight to dissociate the insulin-sensitizing function of PPAR? activation from its adverse effects.