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Regulation of peroxisome proliferator-activated receptor alpha-induced transactivation by the nuclear orphan receptor
Z H Yan1, W G Karam, J L Staudinger
1Cell Biology Section, Laboratory of Pulmonary Pathobiology, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709, USA.
Abstract:
Recently, we 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 proliferator-activated receptor alpha (PPARalpha) signaling pathway by TAK1. Site selection analysis showed that TAK1 has the greatest affinity for direct repeat-1 response elements (RE) containing AGGTCAAAGGTCA (TAK1-RE) to which it binds as a homodimer. TAK1 is a very weak inducer of TAK1-RE-dependent transcriptional activation. We observed that TAK1, as PPARalpha, is expressed within rat hepatocytes and is able to bind the peroxisome proliferator response elements (PPREs) present in the promoter of the PPARalpha target genes rat enoyl-CoA hydratase (HD) and peroxisomal fatty acyl-CoA oxidase (ACOX). TAK1 is unable to induce PPRE-dependent transcriptional activation and represses PPARalpha-mediated transactivation through these elements in a dose-dependent manner. Two-hybrid analysis showed that TAK1 does not form heterodimers with either PPARalpha or retinoid X receptor (RXRalpha), indicating that this repression does not involve a mechanism by which TAK1 titrates out PPARalpha or RXRalpha from PPAR.RXR complexes. Further studies demonstrated that the PPARalpha ligand 8(S)-hydroxyeicosatetraenoic acid strongly promotes the interaction of PPARalpha with the co-activator RIP-140 but decreases the interaction of PPARalpha with the co-repressor SMRT. In contrast, TAK1 interacts with RIP-140 but not with SMRT and competes with PPARalpha for RIP-140 binding. These observations indicated that the antagonistic effects of TAK1 on PPARalpha.RXRalpha transactivation act at least at two levels in the PPARalpha signaling pathway: competition of TAK1 with PPARalpha.RXR for binding to PPREs as well as to common co-activators, such as RIP-140. Our results suggest an important role for TAK1 in modulating PPARalpha-controlled gene expression in hepatocytes.
Insights
Thyroid-activating kinase 1 (TAK1) represses peroxisome proliferator-activated receptor alpha (PPARα) signaling in hepatocytes. TAK1 competes with PPARα for DNA binding and co-activator recruitment, modulating PPARα-controlled gene expression.
Area of Science:
- Molecular Endocrinology
- Nuclear Receptor Signaling
- Hepatocyte Gene Regulation
Background:
- Thyroid-activating kinase 1 (TAK1) is a nuclear orphan receptor.
- Peroxisome proliferator-activated receptor alpha (PPARα) is a key regulator of lipid metabolism and gene expression in hepatocytes.
- Understanding the interplay between nuclear receptors is crucial for metabolic disease research.
Purpose of the Study:
- To characterize the DNA binding requirements for TAK1.
- To investigate the mechanism by which TAK1 represses the PPARα signaling pathway.
- To elucidate the role of TAK1 in modulating PPARα-controlled gene expression in rat hepatocytes.
Main Methods:
- Site selection analysis to determine TAK1 DNA binding preferences.
- Reporter gene assays to assess transcriptional activation and repression.
- Two-hybrid analysis to study receptor-co-receptor interactions.
- Co-immunoprecipitation assays to analyze protein-protein interactions.
Main Results:
- TAK1 binds as a homodimer to direct repeat-1 response elements (TAK1-RE) with high affinity.
- TAK1 weakly induces transcription from TAK1-RE but represses PPARα-mediated transactivation of PPARα target genes (HD and ACOX) in a dose-dependent manner.
- TAK1 competes with PPARα for binding to peroxisome proliferator response elements (PPREs) and the co-activator RIP-140, without affecting PPARα/retinoid X receptor (RXRα) complex formation.
Conclusions:
- TAK1 antagonizes PPARα signaling by competing for PPREs and co-activators like RIP-140.
- TAK1 plays a significant role in modulating PPARα-controlled gene expression in hepatocytes.
- These findings reveal a novel regulatory mechanism in the PPARα signaling pathway.