Thyroid hormone receptor mutants: Dominant negative regulators of peroxisome proliferator-activated receptor action

Thyroid hormone (T3) and peroxisome proliferators have overlapping metabolic effects in the maintenance of lipid homeostasis. Their actions are mediated by their respective receptors: thyroid hormone receptors (TR) and peroxisome proliferator-activated receptors (PPAR). We recently found that a dominantly negative TR mutant (PV) that causes a genetic disease, resistance to thyroid hormone, acts to repress the ligand (troglitazone)-mediated transcriptional activity of PPAR in cultured thyroid cells. This finding suggests that TR mutants could crosstalk with PPAR -signaling pathways. The present study explored the molecular mechanisms by which PV represses the PPAR transcriptional activity. Gel-shift assays show that the PV, similar to wild-type TR , bound to the peroxisome proliferator response element (PPRE) as homodimers and heterodimers with PPAR or the retinoid X receptor (RXR), thereby competing with PPAR for binding to PPRE and for sequestering RXR. Association of PPRE-bound PV with corepressors [e.g., nuclear receptor corepressor (NCoR)] that led to transcriptional repression was independent of T3 and troglitazone. Chromatin immunoprecipitation assay further demonstrated that, despite the presence of ligands, NCoR was recruited to PPRE-bound PV on a PPAR -target gene, the lipoprotein lipase, in vivo, suggesting the dominant action of PV on PPAR -mediated transcriptional activity. Thus, the dominant negative action of PV is not limited on the wild-type TRs. The findings that TR mutants affect PPAR functions through dominant negative action provide insights into the molecular mechanisms by which TR regulates the PPAR -target genes involved in metabolic pathways, lipid homeostasis, and carcinogenesis.