DOWN-REGULATION OF THYROXINE-BINDING GLOBULIN MESSENGER-RIBONUCLEIC-ACID BY 3,5,3'-TRIIODOTHYRONINE IN HUMAN HEPATOBLASTOMA CELLS

A sensitive [(125)]-T-4 binding assay was used to measure serum T-4-bi nding globulin (TBG) in 60 individuals selected on the basis of their total circulating T-3 concentrations, and a relationship between TBG a nd circulating thyroid hormone levels in humans was confirmed. There w as a significant correlation between serum TBG and T-3 or free T-4 ind ex. TBG secretion and TBG messenger ribonucleic acid (mRNB) production were studied with a continuous culture of the human hepatoblastoma ce ll line, HepG2. Cells were maintained in serum-free media For experime ntal manipulations. The addition of 100 nmol/L T-3 to the cell medium resulted in a time-dependent down-regulation of TBG mRNA to 33 +/- 6% (+/- SD, n = 4) of untreated control levels by 24 h. Suppression of TB G mRNA was first detectable at 8 h (57% of untreated control levels). The effect of T-3 was dose-responsive, with half-maximal suppression o f TBG mRNA occurring at a bioavailable T-3 concentration of approximat ely. 30 pmol/L. The effect of T-3 on TBG mRNA was not caused by a chan ge in mRNA stability. Proteins secreted by HepG2 cells bound T-4 with an affinity identical to that of normal circulating TBG. Cell secretio n of TBG was parallel to total protein secretion and consistent with a TBG secretion rate of 50 ng/10(6) cells per day. Variations in the co ncentration of secreted binding protein in the presence of T-3 corresp onded to the changes observed in TBG mRNA. These data show that circul ating TBG concentration is negatively correlated with total serum T-3 in vivo. The corresponding down-regulation observed between TBG mRNA a nd secreted protein in HepG2 cells suggests that this effect is the re sult of the action of T-3 on cellular TBG mRNA synthesis.