Peroxisome proliferator-activated receptor gamma (PPARgamma) activity is regulated through association with ligands that include the thiazolidinedione class of antidiabetic drugs, as well as derivatives of polyunsaturated fatty acids. Induction of PPARgamma target gene expression involves ligand-dependent reconfiguration of the ligand-binding domain (LBD), followed by recruitment of specific transcriptional coactivators. In this study, we have identified an amino acid (F372) within helix 7 of the LBD that is required for the response of PPARgamma to endogenous ligands. Additionally, the data show that this amino acid is also required for expression of a novel subset of adipocyte genes (group 2), including fibroblast growth factor 21 (FGF21), and that the FGF21 gene is a direct target of PPARgamma. Expression of the group 2 genes is selectively repressed by the NAD-dependent deacetylase SIRT1 in mature 3T3-L1 adipocytes, since knockdown of SIRT1 through the constitutive expression of a corresponding RNA interference enhances their expression without affecting the expression of classic adipogenic genes, such as adiponectin and FABP4/aP2. It appears that many of the group 2 genes repressed by SIRT1 in mature adipocytes correspond to the same set of genes that are selectively activated by treatment of fat cells with the PPARgamma ligand, troglitazone. These data support a role for helix 7 of the LBD of PPARgamma in regulating adipocyte function and suggest that inhibition of SIRT1 in adipocytes induces the same insulin-sensitizing action as PPARgamma ligands.