Identification of the antibiotic ionomycin as an unexpected peroxisome proliferator-activated receptor γ (PPARγ) ligand with a unique binding mode and effective glucose-lowering activity in a mouse model of diabetes

Existing thiazolidinedione (TZD) drugs for diabetes have severe side effects. The aim of this study is to develop alternative peroxisome proliferator-activated receptor gamma (PPAR gamma) ligands that retain the benefits in improving insulin resistance but with reduced side effects. We used AlphaScreen assay to screen for new PPAR gamma ligands from compound libraries. In vitro biochemical binding affinity assay and in vivo cell-based reporter assay were used to validate ionomycin as a partial ligand of PPAR gamma. A mouse model of diabetes was used to assess the effects of ionomycin in improving insulin sensitivity. Crystal structure of PPAR gamma complexed with ionomycin revealed the unique binding mode of ionomycin, which elucidated the molecular mechanisms allowing the discrimination of ionomycin from TZDs. We found that the antibiotic ionomycin is a novel modulating ligand for PPAR gamma. Both the transactivation and binding activity of PPAR gamma by ionomycin can be blocked by PPAR gamma specific antagonist GW9662. Ionomycin interacts with the PPAR gamma ligand-binding domain in a unique binding mode with properties and epitopes distinct from those of TZD drugs. Ionomycin treatment effectively improved hyperglycaemia and insulin resistance, but had reduced side effects compared with TZDs in the mouse model of diabetes. In addition, ionomycin effectively blocked the phosphorylation of PPAR gamma at Ser273 by cyclin-dependent kinase 5 both in vitro and in vivo. Our studies suggest that ionomycin may represent a unique template for designing novel PPAR gamma ligands with advantages over current TZD drugs.