It is well known that triiodothyronine (T3) activates the growth hormone (GH)-IGF-I axis by increasing the synthesis and secretion of pituitary GH, as well as liver IGF-I. In turn, IGF-I is able to inhibit GH in vitro and in vivo. We have studied the impact of IGF-I on thyroid hormone (TH) action in different tissues and presented evidence that IGF-I feeds back to limit some specific metabolic actions of T3 in rat liver and pituitary through a down-regulation of nuclear TR protein and mRNA by a reduction in the transcriptional rate of TR gene. The goal of this study was to further study the molecular mechanism of the IGF-I-induced reduction of TR gene transcription. For this purpose we studied: 1) the effect of IGF-I on TR gene promoter transcriptional activity in vitro: GH3 cells were transfected with a Luc-plasmid containing TRbeta1 promoter (-1325 to +44 bp); cells cultured in medium with T3-stripped FBS and treated with rh-IGF-I (10-1000 ng/ml) for 24 h in presence or absence of T3 (100 nM). 2) differentially expressed mRNAs from liver of IGF-I treated rats to search for mediators of IGF-I action on TR gene transcription: male Wistar rats were treated with vehicle or rh-IGF-I (240 ìg/100 g bw for 24 h). Total RNA was extracted from liver and differential display analysis conducted. Results: 1) T3 significantly increased Luc activity of GH3 cells transfected with the TR promoter plasmid as previously reported. Cells treated with IGF-I and T3 showed a reduced Luc activity (100 ng/ml and 1000 ng/ml: 67% (p<0.05) and 63% (p<0.05) compared to T3-treated cells, respectively). IGF-I did not induce a significant difference in Luc activity in GH3 cells cultured in the absence of T3. 2) Differential display analysis revealed the presence of several differentially expressed genes by IGF-I treatment, most of them down-regulated. CONCLUSION: Results indicate that IGF-I reduced the transcriptional activity of the promoter of TR gene. Sequence analysis of differentially expressed mRNAs, presently under study, will allow the identification of IGF-I-modulated protein/s linked to TR gene expression.
