PPARγ S273 Phosphorylation Modifies the Dynamics of Coregulator Proteins Recruitment

The nuclear receptor PPARγ is essential to maintain whole-body glucose homeostasis and insulin sensitivity, acting as a master regulator of adipogenesis, lipid, and glucose metabolism. Its activation through natural or synthetic ligands induces the recruitment of coactivators, leading to transcription of target genes such as cytokines and hormones. More recently, post translational modifications, such as PPARγ phosphorylation at Ser273 by CDK5 in adipose tissue, have been linked to insulin resistance trough the dysregulation of expression of a specific subset of genes. Here, we investigate how this phosphorylation may disturb the interaction between PPARγ and some coregulator proteins as a new mechanism that may leads to insulin resistance. Through cellular and in vitro assays, we show that PPARγ phosphorylation inhibition increased the activation of the receptor, therefore the increased recruitment of PGC1-α and TIF2 coactivators, whilst decreases the interaction with SMRT and NCoR corepressors. Moreover, our results show a shift in the coregulators interaction domains preferences, suggesting additional interaction interfaces formed between the phosphorylated PPARγ and some coregulator proteins. Also, we observed that the CDK5 presence disturb the PPARγ-coregulator's synergy, decreasing interaction with PGC1-α, TIF2, and NCoR, but increasing coupling of SMRT. Finally, we conclude that the insulin resistance provoked by PPARγ phosphorylation is linked to a differential coregulators recruitment, which may promote dysregulation in gene expression.

Screening for PPAR Non-Agonist Ligands Followed by Characterization of a Hit, AM-879, with Additional No-Adipogenic and cdk5-Mediated Phosphorylation Inhibition Properties

Peroxisome proliferator-activated receptor gamma (PPARγ) is a member of a nuclear receptor superfamily and acts as a ligand-dependent transcription factor, playing key roles in maintenance of adipose tissue and in regulation of glucose and lipid homeostasis. This receptor is the target of thiazolidinediones, a class of antidiabetic drugs, which improve insulin sensitization and regulate glycemia in type 2 diabetes. Despite the beneficial effects of drugs, such as rosiglitazone and pioglitazone, their use is associated with several side effects, including weight gain, heart failure, and liver disease, since these drugs induce full activation of the receptor. By contrast, a promising activation-independent mechanism that involves the inhibition of cyclin-dependent kinase 5 (CDK5)-mediated PPARγ phosphorylation has been related to the insulin-sensitizing effects induced by these drugs. Thus, we aimed to identify novel PPARγ ligands that do not possess agonist properties by conducting a mini-trial with 80 compounds using the sequential steps of thermal shift assay, 8-anilino-1-naphthalenesulfonic acid fluorescence quenching, and a cell-based transactivation assay. We identified two non-agonist PPARγ ligands, AM-879 and P11, and one partial-agonist, R32. Using fluorescence anisotropy, we show that AM-879 does not dissociate the NCOR corepressor in vitro, and it has only a small effect on TRAP coactivator recruitment. In cells, AM-879 could not induce adipocyte differentiation or positively regulate the expression of genes associated with adipogenesis. In addition, AM-879 inhibited CDK5-mediated phosphorylation of PPARγ in vitro. Taken together, these findings supported an interaction between AM-879 and PPARγ; this interaction was identified by the analysis of the crystal structure of the PPARγ:AM-879 complex and evidenced by AM-879's mechanism of action as a putative PPARγ non-agonist with antidiabetic properties. Moreover, we present an optimized assay pipeline capable of detecting ligands that physically bind to PPARγ but do not cause its activation as a new strategy to identify ligands for this nuclear receptor.

Molecular mechanisms of transcriptional control by Rev-erbα: An energetic foundation for reconciling structure and binding with biological function

Rev-erbα and β are nuclear receptors that function as transcriptional repressors of genes involved in regulating circadian rhythms, glucose, and cholesterol metabolism and the inflammatory response. Given these key functions, Rev-erbs are important drug targets for treatment of a number of human pathologies, including cancer, heart disease, and type II diabetes. Transcriptional repression by the Rev-erbs involves direct competition with transcriptional activators for target sites, but also recruitment by the Rev-erbs of the NCoR corepressor protein. Interestingly, Rev-erbs do not appear to interact functionally with a very similar corepressor, Smrt. Transcriptional repression by Rev-erbs is thought to occur in response to the binding of heme, although structural, and ligand binding studies in vitro show that heme and corepressor binding are antagonistic. We carried out systematic studies of the ligand and corepressor interactions to address the molecular basis for corepressor specificity and the energetic consequences of ligand binding using a variety of biophysical approaches. Highly quantitative fluorescence anisotropy assays in competition mode revealed that the Rev-erb specificity for the NCoR corepressor lies in the first two residues of the β-strand in Interaction Domain 1 of NCoR. Our studies confirmed and quantitated the strong antagonism of heme and corepressor binding and significant stabilization of the corepressor complex by a synthetic ligand in vitro. We propose a model which reconciles the contradictory observations concerning the effects of heme binding in vitro and in live cells.