THIAZOLIDINEDIONES PRODUCE A CONFORMATIONAL CHANGE IN PEROXISOMAL PROLIFERATOR-ACTIVATED RECEPTOR-GAMMA - BINDING AND ACTIVATION CORRELATE WITH ANTIDIABETIC ACTIONS IN DB DB MICE/

The thiazolidinediones are novel insulin sensitizers that serve as ora lly active antidiabetic agents in rodents, nonhuman primates, and man. We have examined the effects of 4-week oral administration of three t hiazolidinediones (AD-5075, BRL 49653, and CS-045) on plasma glucose a nd triglyceride concentrations in obese hyperglycemic db/db mice. All three agents lower plasma glucose and triglyceride concentrations. Nor mal levels of glucose are achieved after treatment with AD-5075 (>1.7 mg/kg) or BRL 49653 (greater than or equal to 30 mg/kg), whereas CS-04 5 (100 or 300 mg/kg) produces only modest reductions in either paramet er. Although the thiazolidinediones have demonstrated insulin-sensitiz ing activities both in vivo and in vitro, their primary molecular targ et has been unclear. We have compared the in vivo antidiabetic actions described above with the in vitro activities on peroxisomal prolifera tor-activated receptor-gamma (PPAR gamma). Hamster PPAR gamma 1 was tr ansiently expressed in COS-1 cells to study the binding of [H-3]AD-507 5. The concentrations of compounds needed to displace radiolabeled AD- 5075 from PPAR gamma correlate with their in vivo potency; the K-i val ues for displacement by cold AD-5075, BRL 49653, and CS-045 are 22, 68 , and 1600 nM, respectively. To examine activation of the receptor, it was transiently cotransfected into COS-1 cells with a reporter plasmi d containing two copies of a peroxisome proliferator response element. The EC(50) values for activation are 2, 6, and 140 nM for AD-5075, BR L 49653, and CS-045, respectively. We have also analyzed limited prote olytic digests of in vitro translated hamster PPAR gamma. The thiazoli dinediones produce a conformational change in PPAR gamma analogous to those produced by agonists of other nuclear hormone receptors. In the presence of saturating concentrations of either AD-5075 or BRL 49653, a receptor fragment of 27 kDa is protected from proteolysis by trypsin . These data support the conclusion that the antidiabetic actions of t he thiazolidinediones are directly mediated through binding to PPAR ga mma and the resulting active conformation of the receptor. Therefore, binding and transactivation assays using PPAR gamma should serve to id entify other novel therapeutic agents with potential antidiabetic acti vities.