Glucocorticoids (GC) enhance T3-dependent specific metabolic actions. We demonstrated that Dex increased TRß1 protein, mRNA and the transcriptional rate of TR gene in rat liver. Herein, we further explored the mechanism of the Dex-induced increase of TRß1. We evaluated: a) the effect of Dex on in vitro TRß1 promoter gene transcriptional activity. COS-7 cells were transfected with a Luc-plasmid containing TRß1 promoter (-1325bp to +44bp), GR and TRß1 expression vectors. Cells were treated or not with T3 and Dex for 12 or 24 h. b) the interaction of proteins from liver nuclear extracts from control and Dex-treated rats to sequences in the TRß1 promoter by EMSA.
Results: a) Dex-treated COS-7 cells for 12 h increased TRß1 transcriptional activity only in the presence of T3. Dex for 24 h augmented TRß1 promoter activity in the absence or presence of T3. b) Sequence analysis of TRß1 promoter revealed a consensus sequence for GR-binding (GRE:-840/-835bp). EMSA using an oligonucleotide comprising the GRE conserving the flanking nucleotides of TRß1 promoter (-850/-824) formed two additional complexes with liver nuclear extracts from Dex-treated vs control rats. Both complexes decreased markedly after preincubation with anti-GR Ab. Protein extracts from COS-7 cells overexpressing GR with the oligonucleotide, formed the same complexes observed with nuclear extracts from Dex-treated rats, revealing the involvement of GR. TRß1-850/-824 and mutated GRE as competitors reduced such complexes; however mutations of GRE and the
The mechanism of the enhanced T3-dependent metabolic actions induced by Dex involves an increase of the transcriptional activity of the TRß1 promoter at least in part, through an interaction of GR and other transacting factors bound to sequences flanking the GRE in the TRß1 promoter.
The results are of importance considering the impact of GC effect on the peripheral thyroid hormone metabolic action.
