科研之友
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摘要
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 TR alpha 1 mutant (TR alpha 1PV) or TR beta 1 mutant (TR beta 1PV). TR alpha 1PV and TRb1PV 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 (PPAR gamma) and CCAAT/enhancer-binding protein alpha (C/EBP alpha), which are master regulators of adipogenesis at both the mRNA and protein levels. In 3T3-L1 cells stably expressing TR alpha 1PV (L1-alpha 1PV cells) or TR beta 1PV (L1-beta 1PV cells), adipogenesis was reduced 94 or 54% respectively, indicative of differential inhibitory activity of mutant TR isoforms. Concordantly, the expression of PPAR gamma and C/EBP alpha at the mRNA and protein levels was more repressed in L1-alpha 1PV cells than in L1-beta 1PV cells. In addition, the expression of PPAR gamma downstream target genes involved in fatty acid synthesis - the lipoprotein lipase (Lpl) and aP2 involved in adipogenesis - was more inhibited by TR alpha 1PV than by TR beta 1PV. Chromatin immunoprecipitation assays showed that TRa1PV was more avidly recruited than TRb1PV to the promoter to preferentially block the expression of the C/ebp alpha 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.
- 出版日期2010-4
- 单位NIH
