Differential modulation of thyroid hormone responsiveness by retinoids in a human cell line

Previous studies have suggested that there is an interrelationship between responses mediated by retinoic acid (RA) and those to thyroid hormone (T-3). These experiments have used transfected gene constructs, often in receptor-negative cells. To study the relationship between RA- and T-3-mediated responses in intact human cells, we incubated HepG2 cells for 4 days in serum-free medium with T-3 and/or RA or 9-cis-RA. Measured responses were stimulation of secreted sex hormone-binding globulin (SHBG) or inhibition of secreted T-4-binding globulin (TBG). T-3 induced a dose-responsive increase in SHBG secretion that was maximal at 10 nM (206 +/- 24% of untreated value) and half-maximal at 0.36 +/- 0.16 nM T-3. RA and 9-cis-RA, up to 100 nM, induced a slight fall in SHBG secretion to 79 +/- 9% and 88 +/- 9%, respectively. T-3 induction of SHBG secretion was significantly attenuated in cells coincubated with T-3 (0-10 nM) and RA. With T-3 (10 nM) together with RA (3, 10, or 100 nM), the maximal SHBG responses were reduced to 193 +/- 24%, 151 +/- 5%, and 132 +/- 30%, respectively. With T-3 and 9-cis-RA (100 nM), maximal Stimulation was 169 +/- 20%. Importantly, the effective half-maximal stimulatory concentration of T-3 in the presence of either retinoid (3-100 nM) was unchanged at 0.3 nM T-3. In addition, the inhibitory effect of 9-cis RA could not be overcome even with 300 nM T-3. The threshold for the RA effect was between 0.3-1 nM, with half-maximal inhibition at 30 nM. 9-cis-RA was similar to 10-fold less potent than RA. Preliminary studies suggested that changes in SHBG messenger RNA levels were similar to those in secreted SHBG. No effect was observed with vitamin D or clofibrate, either alone or combined with T-3. Conversely, T-3 reduced TBG secretion, with maximal suppression to 74 +/- 5% of the control value at a T-3 concentration of 10 nM. RA alone reduced TBG secretion to 76% of the control value. RA did not attenuate the effect of T-3, and the two agents combined showed no synergism. Neither T-3 nor RA, alone or in combination, influenced secreted total protein or albumin. RA did not alter the concentration of nuclear T-3-binding sites. These data suggest that retinoids act via a gene-dependent mechanism to modulate maximal, but not half-maximal, responses to T-3 in HepG2 cells with the specificity of RA greater than that of 9-cis-RA.