Thyroid hormone (T3) inhibits ciprofibrate-induced transcription of genes encoding beta-oxidation enzymes: cross talk between peroxisome proliferator and T3 signaling pathways.
Peroxisome proliferators cause rapid and coordinated transcriptional activation of genes encoding peroxisomal beta-oxidation system enzymes by activating peroxisome proliferator-activated receptor ( PPAR) isoform(s). Since the thyroid hormone (T3; 3,3',5-triiodothyronine) receptor (TR), another member of the nuclear hormone receptor superfamily, regulates a subset of fatty acid metabolism genes shared with PPAR, we examined the possibility of interplay between peroxisome proliferator and T3 signaling pathways. T3 inhibited ciprofibrate-induced luciferase activity as well as the endogenous peroxisomal beta-oxidation enzymes in transgenic mice carrying a 3.2-kb 5'-flanking region of the rat peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase gene fused to the coding region of luciferase. Transfection assays in hepatoma H4-II-E-C3 and CV-1 cells indicated that this inhibition is mediated by TR in a ligand-dependent fashion. Gel shift assays revealed that modulation of PPAR action by TR occurs through titration of limiting amounts of retinoid X receptor (RXR) required for PPAR activation. Increasing amounts of RXR partially reversed the inhibition in a reciprocal manner; PPAR also inhibited TR activation. Results with heterodimerization-deficient TR and PPAR mutants further confirmed that interaction between PPAR and TR signaling systems is indirect. These results suggest that a convergence of the peroxisome proliferator and T3 signaling pathways occurs through their common interaction with the heterodimeric partner RXR.[1]References
- Thyroid hormone (T3) inhibits ciprofibrate-induced transcription of genes encoding beta-oxidation enzymes: cross talk between peroxisome proliferator and T3 signaling pathways. Chu, R., Madison, L.D., Lin, Y., Kopp, P., Rao, M.S., Jameson, J.L., Reddy, J.K. Proc. Natl. Acad. Sci. U.S.A. (1995) [Pubmed]
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