Adipogenesis is differentially impaired by thyroid hormone receptor mutant isoforms.
ABSTRACT To understand the roles of thyroid hormone receptors (TRs) in adipogenesis, we adopted a loss-of-function approach. We generated 3T3-L1 cells stably expressing either TRalpha1 mutant (TRalpha1PV) or TRbeta1 mutant (TRbeta1PV). TRalpha1PV and TRbeta1PV are dominant negative mutations with a frameshift in the C-terminal amino acids. In control cells, the thyroid hormone, tri-iodothyronine (T(3)), induced a 2.5-fold increase in adipogenesis in 3T3-L1 cells, as demonstrated by increased lipid droplets. This increase was mediated by T(3)-induced expression of the peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha), which are master regulators of adipogenesis at both the mRNA and protein levels. In 3T3-L1 cells stably expressing TRalpha1PV (L1-alpha1PV cells) or TRbeta1PV (L1-beta1PV cells), adipogenesis was reduced 94 or 54% respectively, indicative of differential inhibitory activity of mutant TR isoforms. Concordantly, the expression of PPARgamma and C/EBPalpha at the mRNA and protein levels was more repressed in L1-alpha1PV cells than in L1-beta1PV cells. In addition, the expression of PPARgamma downstream target genes involved in fatty acid synthesis - the lipoprotein lipase (Lpl) and aP2 involved in adipogenesis - was more inhibited by TRalpha1PV than by TRbeta1PV. Chromatin immunoprecipitation assays showed that TRalpha1PV was more avidly recruited than TRbeta1PV to the promoter to preferentially block the expression of the C/ebpalpha gene. Taken together, these data indicate that impaired adipogenesis by mutant TR is isoform dependent. The finding that induction of adipogenesis is differentially regulated by TR isoforms suggests that TR isoform-specific ligands could be designed for therapeutic intervention for lipid abnormalities.
[show abstract] [hide abstract]
ABSTRACT: Improved knowledge of all aspects of adipose biology will be required to counter the burgeoning epidemic of obesity. Interest in adipogenesis has increased markedly over the past few years with emphasis on the intersection between extracellular signals and the transcriptional cascade that regulates adipocyte differentiation. Many different events contribute to the commitment of a mesenchymal stem cell to the adipocyte lineage including the coordination of a complex network of transcription factors, cofactors and signalling intermediates from numerous pathways.Nature Reviews Molecular Cell Biology 01/2007; 7(12):885-96. · 39.12 Impact Factor
Article: Retardation of post-natal development caused by a negatively acting thyroid hormone receptor alpha1.[show abstract] [hide abstract]
ABSTRACT: Most patients with the syndrome resistance to thyroid hormone (RTH) express a mutant thyroid hormone receptor beta (TRbeta) with transdominant negative transcriptional effects. Since no patient with a mutant TRalpha has been identified, we introduced a point mutation into the mouse thyroid hormone receptor (TRalpha1) locus originally found in the TRbeta gene, that reduces ligand binding 10-fold. Heterozygous 2- to 3-week- old mice exhibit a severe retardation of post-natal development and growth, but only a minor reduction in serum thyroxine levels. Homozygous mice died before 3 weeks of age. Adult heterozygotes overcome most of these defects except for cardiac function abnormalities, suggesting that other factors compensate for the receptor defect. However, the additional deletion of the TRbeta gene in this mouse strain caused a 10-fold increase in serum thyroxine, restored hormonal regulation of target genes for TRs, and rescued the growth retardation. The data demonstrate a novel array of effects mediated by a dominant negative TRalpha1, and may provide important clues for identification of a potentially unrecognized human disorder and its treatment.The EMBO Journal 11/2002; 21(19):5079-87. · 9.20 Impact Factor