Regulation of human growth hormone receptor gene transcription by triiodothyronine (T-3)

In this study the hypothesis that triiodothyronine (T-3) and growth hormone (GH) may have some direct or indirect effect on the regulation of GH-recep tor/GH-binding protein (GHR/GHBP) gene transcription was tested. Different concentrations of T-3 (0, 0.5, 2, 10 nmol/l) and GH (0, 10, 150 ng/ml) were added to human hepatoma (HuH7) cells cultured in serum-free hormonally-def ined medium for 0; 1 and 2 h. Thereafter GHR/GHBP mRNA expression was quant itatively assessed by using PCR amplification. GH at a concentration of 10 ng/ml resulted in a significant increase of GHR/GHBP gene expression wherea s a supraphysiological concentration of GH (150 ng/ml) caused a significant decrease of GHR/GHBP mRNA levels. The simultaneous addition of 0.5 nmol/l T-3 to the variable concentrations of GH did not modify GHR/GHBP mRNA level s whereas the addition of 2 nmol/l up-regulated GHR/GHBP gene expression al ready after 1 h, an increase which was even more marked when 10 nmol/l of T -3 was added. Interestingly, there was a positive correlation between the i ncrease of GHR/GHBP mRNA levels and the T-3 concentration used (r: 0.8). In addition, nuclear run-on experiments and GHBP determinations were performe d which confirmed the changes in GHR/GHBP mRNA levels. Cycloheximide (10 mu g/ml) did not alter transcription rate following GH addition but blocked G HR/GHBP gene transcription in T-3 treated cells indicating that up-regulati on of GHR/GHBP gene transcription caused by T-3 requires new protein synthe sis and is, therefore, dependent on indirect mechanisms. In conclusion, we present data showing that T-3 on its own has a stimulatory effect on GHR/GH BP gene transcription which is indirect and additive to the GH-induced chan ges. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.