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Previous studies have suggested that there is an interrelationship between responses mediated by retinoic acid (RA) and those to thyroid hormone (T3). These experiments have used transfected gene constructs, often in receptor-negative cells. To study the relationship between RA- and T3-mediated responses in intact human cells, we incubated HepG2 cells for 4 days in serum-free medium with T3 and/or RA or 9-cis-RA. Measured responses were stimulation of secreted sex hormone-binding globulin (SHBG) or inhibition of secreted T4-binding globulin (TBG). T3 induced a dose-responsive increase in SHBG secretion that was maximal at 10nM (206 +/- 24% of untreated value) and half-maximal at 0.36 +/- 0.16 nM T3. RA and 9-cis-RA, up to 100 nM, induced a slight fall in SHBG secretion to 79 +/- 9% and 88 +/- 9%, respectively. T3 induction of SHBG secretion was significantly attenuated in cells coincubated with T3(0-10nM) and RA. With T3 (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 T3 and 9-cis-RA (100 nM), maximal stimulation was 169 +/- 20%. Importantly, the effective half-maximal stimulatory concentration of T3 in the presence of either retinoid (3-100 nM) was unchanged at 0.3 nM T3. In addition, the inhibitory effect of 9-cis RA could not be overcome even with 300 nM T3. The threshold for the RA effect was between 0.3-1 nM, with half-maximal inhibition at 30 nM. 9-cis-RA was approximately 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 T3. Conversely, T3 reduced TBG secretion, with maximal suppression to 74 +/- 5% of the control value at a T3 concentration of 10 nM. RA alone reduced TBG secretion to 76% of the control value. RA did not attenuate the effect of T3, and the two agents combined showed no synergism. Neither T3 nor RA, alone or in combination, influenced secreted total protein or albumin. RA did not alter the concentration of nuclear T3-binding sites. These data suggest that retinoids act via a gene-dependent mechanism to modulate maximal, but not half-maximal, responses to T3 in HepG2 cells with the specificity of RA greater than that of 9-cis-RA.

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