Dexamethasone (Dex) increases the transcriptional rate and the in vitro activity of rat liver nuclear T3 receptor (T3R) gene

MONTESINOS, MM; PELLIZAS CG; SUSPERREGUY S; COLEONI AH

Villa Carlos Paz, Córdoba, Argentina

Congreso; X Congreso de la Sociedad Latinoamericana de Tiroides (SLAT); 2003

Sociedad Latinoamericana de Tiroides (SLAT)

Glucocorticoids enhance T₃-dependent specific metabolic actions. Previous results indicated that Dex-injected rats, increased the abundance of liver b T₃R-isoform and its mRNA. A similar effect on total protein level of retinoid acid receptor (RXR), the usual T₃R heterodimerization partner, was also observed. In this study we further explored the mechanisms of Dex effect on T₃R and RXR. We evaluated: a) the transcriptional rate of T₃R gene (nuclear run-on assay) in liver nuclei from adult male Wistar rats injected with Dex (250 ug/100 BW, every 12 h for 48 h) and RXR protein isoforms level (Western blot); b) the effect of Dex on in vitro T₃R promoter gene transcriptional activity. For this purpose COS-7 and GH3B6 cells were transfected with –1325 bp to + 44 bp T₃R promoter fused to LUC reporter gene and b-galactosidase (b-gal) reporter vector to normalize the transfection efficiency. In COS-7, glucocorticoid receptor and T₃R expression vectors were also cotransfected. Cells were exposed to 10^-8M Dex for 24 h. Activities of b-gal and luciferase were assayed by spectrofotometry and luminescence emission, respectively. Statistic: Student “t” test with p<0.05 were considered significant.

Results: Dex increased 209% T₃R gene transcription rate (arbitrary units; Mean±SEM); control: 0.122±0.05; Dex: 0.377±0.09; n=3; p<0.005. In vitro T₃R promoter gene transcription activity (LUC/b-gal ratio) was increased 166% by 10^-8 Dex in COS-7 (Mean±SD): control: 8.0±4.1; n=4; Dex: 21.3±4.1; n=3; p<0.01. In GH3B6 a 56% increase was registered under similar experimental conditions (Mean±SD): control: 3.2±1.0; n=6; Dex: 5.0±1.4; n=9; p<0.02.   Liver RXR isoforms increased after Dex administration to rats: aRXR by 30% (arbitrary units; Mean±SEM): control: 0.84±0.17; Dex: 1.08±0.16; n=6; p<0.05; and b RXR by 40%: control 0.65±0.14; n=5; Dex: 0.91±0.14; n=6; p<0.05.

Conclusion: These results indicate that the mechanism of the enhanced T₃-dependend metabolic actions induced by Dex involves an increase in the transcription rate of the T₃R gene as well as in the transcriptional activation of the T₃R promoter gene. The increment of liver RXR isoforms after Dex administration may play a role in some specific metabolic effect of T₃ ,provinding its common association with T₃R to form functional active heterodimers.