Abstract
Peroxisome proliferator-activator receptor (PPAR) γ is a nuclear hormone receptor that regulates glucose homeostasis, lipid metabolism, and adipocyte function. PPARγ is a target for thiazolidinedione (TZD) class of drugs which are widely used for the treatment of type 2 diabetes. Recently, lobeglitazone was developed as a highly effective TZD with reduced side effects by Chong Kun Dang Pharmaceuticals. To identify the structural determinants for the high potency of lobeglitazone as a PPARγ agonist, we determined the crystal structures of the PPARγ ligand binding domain (LBD) in complex with lobeglitazone and pioglitazone at 1.7 and 1.8 Å resolutions, respectively. Comparison of ligand-bound PPARγ structures revealed that the binding modes of TZDs are well conserved. The TZD head group forms hydrogen bonds with the polar residues in the AF-2 pocket and helix 12, stabilizing the active conformation of the LBD. The unique p-methoxyphenoxy group of lobeglitazone makes additional hydrophobic contacts with the Ω-pocket. Docking analysis using the structures of TZD-bound PPARγ suggested that lobeglitazone displays 12 times higher affinity to PPARγ compared to rosiglitazone and pioglitazone. This structural difference correlates with the enhanced affinity and the low effective dose of lobeglitazone compared to the other TZDs.
Introduction
Type 2 diabetes mellitus is a progressive metabolic disorder, characterized by hyperglycemia and insulin resistance in peripheral tissue. Insulin resistance is a condition in which cells fail to respond to insulin properly, causing the impaired uptake and utilization of glucose in adipose tissue and skeletal muscle1. Type 2 diabetes can be treated by several types of medications that increase insulin secretion by the pancreas, increase the sensitivity of target organs to insulin, reduce excessive hepatic glucose production, increase glucose utilization in the peripheral tissues, and reduce the carbohydrate absorption in the intestines2.
Peroxisome proliferator-activated receptors (PPARs) include three subtypes: PPARα, PPARδ, and PPARγ; The PPARs are ligand-activated transcription factors that belong to the superfamily of the nuclear hormone receptors3. PPARs heterodimerize with the retinoid X receptor (RXR) and bind to peroxisome proliferator-response elements (PPREs), altering the transcription of target genes4. PPARγ which is mainly expressed in adipose tissue, liver, and skeletal muscle, regulates the genes involved in adipocyte differentiation and lipid metabolism. The endogenous ligands for PPARγ are polyunsaturated fatty acids, oxidized fatty acids, and prostaglandins4. Upon agonist binding, the conformation of the ligand binding domain (LBD) is altered to expose the binding cleft for the recruitment of transcriptional coactivators, which initiate the transcription of target genes5. Activation of PPARγ increases glucose uptake and utilization in the peripheral organs, stimulates fatty acid storage in adipocytes, enhances insulin signaling, and decreases gluconeogenesis in the liver, thereby improving insulin sensitivity6.
Thiazolidinediones (TZDs) including pioglitazone and rosiglitazone are synthetic antihyperglycemic agents that act as PPARγ agonists6. TZDs enhance insulin action and improve hyperglycemia in patients with type 2 diabetes3. All TZDs have similar effects on glycemic control, and a range of adverse effects, such as weight gain, fluid retention, and increased risk of heart failure, which seem to be PPARγ-mediated7. Some adverse effects of TZDs are considered clinically significant. Rosiglitazone is no longer widely used owing to increased risks of myocardial infarction and cardiovascular mortality, though the FDA removed the restrictions on rosiglitazone after reviewing clinical data8,9. Use of pioglitazone has been suspended in some European countries due to its possible association with bladder cancer10.
Lobeglitazone (trade name Duvie; Chong Kun Dang Pharmaceutical Corporation) was developed as a more effective and safe antidiabetic TZD drug. Lobeglitazone was conceptually designed by modification of the rosiglitazone structure with a substituted pyrimidine. Lobeglitazone has a p-methoxyphenoxy group at the 4-position of the pyrimidine moiety11 (Fig. 1A). Lobeglitazone showed more potent activity than the reference compounds (pioglitazone and rosiglitazone) with an EC50 value of 0.018 μM in a type 2 diabetes animal model, which is 16 times lower than pioglitazone (EC50 0.30 μM)11,12. Lobeglitazone exhibited similar efficacy profiles in glycemic control and lipid modulation to pioglitazone, but with a 30 times smaller dose in clinical studies13,14. The molar amount of lobeglitazone (DuvieR, 0.415 mg/tab.) contained in a single tablet is 14 times less than that of rosiglitazone (AvandiaR, 6.04 mg/tab.) and 32 times less than that of pioglitazone (ActosR, 13.6 mg/tab.), indicating the high potency of lobeglitazone. In addition, lobeglitazone displayed significantly reduced side effects regarding cardiovascular disease and bladder cancer15,16. Currently, three TZDs (rosiglitazone, pioglitazone, and lobeglitazone) are available for the treatment of type 2 diabetes. The structures of rosiglitazone-bound PPARγ were previous determined revealing a binding mode of the TZD drug5,17,18,19. Despite the effectiveness of TZD drugs for treating insulin resistance, the crystal structures of pioglitazone and lobeglitazone-bound PPARγ have not been reported. How the small modification introduced in lobeglitazone leads to the dramatic increase in potency and reduction of side effects is not known clearly.
