A PPARγ mutant serves as a dominant negative inhibitor of PPAR signaling and is localized in the nucleus

The peroxisomal proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily that act as ligand-activated transcription factors. PPAR gamma plays a critical role in regulating adipocyte differentiation and lipid metabolism. Recently, thiazolidinedione (TZD) and select non-TZD antidiabetic agents have been identified as PPAR gamma agonists. To further characterize this receptor subclass, a mutant hPPAR gamma lacking five carboxyl-terminal amino acids was produced (hPPAR gamma 2 Delta 500). In COS-I cells transfected with PPAR-responsive reporter constructs, the mutant receptor could not be activated by a potent PPAR gamma agonist. When cotransfected with hPPAR gamma 2 or hPPAR alpha, hPPAR gamma 2 Delta 500 abrogated wild-type receptor activity in a dose-responsive manner. hPPAR gamma 2 Delta 500 was also impaired with respect to binding of a high-affinity radioligand. In addition, its conformation was unaffected by normally saturating concentrations of PPAR gamma agonist as determined by protease protection experiments. Electrophoretic mobility shift assays demonstrated that hPPAR gamma 2 Delta 500 and hPPAR gamma 2 both formed heterodimeric complexes with human retinoid x receptor alpha (hRXR alpha) and could bind a peroxisome proliferator-responsive element (PPRE) with similar affinity. Therefore, hPPAR gamma 2 Delta 500 appears to repress PPAR activity by competing with wild type receptor to dimerize with RXR and bind the PPRE. In addition, the mutant receptor may titrate out factors required for PPAR-regulated transcriptional activation. Both hPPAR gamma 2 and hPPAR gamma 2 Delta 500 localized to the nucleus of transiently transfected COS-1 cells as determined by immunofluorescence using a PPAR gamma-specific antibody. Thus, nuclear localization of PPAR gamma occurs independently of its activation state. The dominant negative mutant, hPPAR gamma 2 Delta 500, may prove useful in further studies to characterize PPAR functions both in vitro and in vivo (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.