REGULATION OF PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR ALPHA-INDUCED TRANSACTIVATION BY THE NUCLEAR ORPHAN RECEPTOR TAK1 TR4/

Recently, Re reported the cloning of the nuclear orphan receptor TAK1. In this study, we characterized the sequence requirements for optimal TAK1 binding and analyzed the repression of the peroxisome proliferat or-activated receptor alpha (PPAR alpha) signaling pathway by TAK1. Si te selection analysis showed that TAK1 has the greatest affinity for d irect repeat-1 response elements (RE) containing AGGTCAAAGGTCA (TAK1-R E) to which it binds as a homodimer, TAK1 is a very weak inducer of TA K1-RE-dependent transcriptional activation. We observed that TAK1, as PPAR alpha; is expressed within rat hepatocytes and is able to bind th e peroxisome proliferator response elements (PPREs) present in the pro moter of the PPAR alpha target genes rat enoyl-CoA hydratase (HD) and peroxisomal fatty acyl-CoA oxidase (ACOX). TAK1 is unable to induce PP RE-dependent transcriptional activation and represses PPAR alpha-media ted transactivation through these elements in a dose-dependent manner. Two-hybrid analysis showed that TAK1 does not form heterodimers with either PPAR alpha or retinoid X receptor (RXR alpha), indicating that this repression does not involve a mechanism by which TAK1 titrates ou t PPAR alpha or RXR alpha from PPAR.RXR complexes. Further studies dem onstrated that the PPAR alpha Ligand 8(S)-hydroxyeicosatetraenoic acid strongly promotes the interaction of PPAR alpha with the co-activator RIP-140 but decreases the interaction of PPAR alpha with the corepres sor SMRT. In contrast, TAK1 interacts with RIP-140 but not with SMRT a nd competes with PPAR alpha for RIP-140 binding. These observations in dicated that the antagonistic effects of TAK1 on PPAR alpha.RXR alpha transactivation act at least at two levels in the PPAR alpha signaling pathway: competition of TAK1 with PPAR alpha.RXR for binding to PPREs as well as to common co-activators, such as RIP-140. Our results sugg est an important role for TAK1 in modulating PPAR alpha-controlled gen e expression in hepatocytes.