Revealing A Novel Ligand For Peroxisome Proliferator Activated Receptorγ With A Unique Binding Mode

Peroxisome Proliferator Activated Receptor y (PPARy) is a member of nuclear receptor family, which plays a critical role in the physiological and pathological regulation of various biological activities, including metabolism, adipocyte differentiation, inflammation and atherosclerosis. The most prominent role of PPARy is its contribution in improving insulin sensitivty upon binding to specific ligands. Therefore, PPARy is one of the most popular molecular targets for anti-diabetic drugs research of the world. The first kind of insulin sensitisers is Thiazolidinedione (TZDs), which can significantly improve insulin sensitivity. However, TZDs also display severe side effects, giving rise to fluid retention, weight gain, liver toxicity and cardiovascular disease. Therefore, it is imperative to develop alternative PPARy ligands that retain the benefits of TZDs in improving insulin resistance but that have reduced side effects.In this study, we found that ionomycin is a novel modulating ligand for PPARy by high-throughput screening. Biochemical binding affinity assay and cell-based reporter assay were used to validate ionomycin as a partial agonist of PPARy. To determine the binding characteristics of ionomycin to PPARy, we solved the crystal structure of PPARy complexed with ionomycin and SRC1-2LXXLL motif. This structure revealed the unique binding mode of PPARy/ionomycin complex at an atomic level and elucidated the conformational changes of PPARy LBD induced by ionomycin binding. As known to all, rosiglitazone is a typical PPARy agonist. Comparing the structures of PPARy/ionomycin and PPARy/rosiglitazone, we found some obvious differences. Especially, our structural observations indicated the loss of interactions between ionomycin and several residues on PPARy, including one critical epitope on the activation function2(AF-2) helix that is used by both natural PPARy ligands and synthetic TZDs. Therefore, ionomycin is a partial agonist of PPARy. Our research showed that ionomycin can serve as a unique template. Based on the optimisation of its chemical structure, we can design a new type drug that is different from traditional TZDs while not only maintaining the efficacy, but also overcoming the adverse side effects of TZDs. In a nutshell, our findings together indicated that ionomycin is a significantly potential template for drug designing, and this research makes some contributions in the future application, structural and biological significance of ionomycin.