Crowe T C, Loidl N M, Payne K L, Topliss D J, Stockigt J R, Barlow J W
Ewen Downie Metabolic Unit, Monash University, Alfred Hospital, Melbourne, Victoria, Australia.
Endocrinology. 1996 Aug;137(8):3187-92. doi: 10.1210/endo.137.8.8754738.
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.
以往的研究表明，视黄酸（RA）介导的反应与甲状腺激素（T3）介导的反应之间存在相互关系。这些实验通常使用转染的基因构建体，且多在受体阴性细胞中进行。为了研究完整人类细胞中RA和T3介导的反应之间的关系，我们将HepG2细胞在无血清培养基中与T3和/或RA或9-顺式RA一起孵育4天。所测量的反应为分泌的性激素结合球蛋白（SHBG）的刺激或分泌的甲状腺素结合球蛋白（TBG）的抑制。T3诱导SHBG分泌呈剂量依赖性增加，在10nM时达到最大值（为未处理值的206±24%），在0.36±0.16nM T3时达到半最大值。高达100nM的RA和9-顺式RA分别使SHBG分泌略有下降至79±9%和88±9%。在与T3（0 - 10nM）和RA共同孵育的细胞中，T3对SHBG分泌的诱导作用显著减弱。当T3（10nM）与RA（3、10或100nM）一起时，SHBG的最大反应分别降至193±24%、151±5%和132±30%。当T3与9-顺式RA（100nM）一起时，最大刺激为169±20%。重要的是，在存在任何一种类维生素A（3 - 100nM）的情况下，T3的有效半最大刺激浓度在0.3nM T3时保持不变。此外？即使使用300nM T3也无法克服9-顺式RA的抑制作用。RA作用的阈值在0.3 - 1nM之间，在30nM时达到半最大抑制。9-顺式RA的效力约为RA的十分之一。初步研究表明，SHBG信使RNA水平的变化与分泌的SHBG的变化相似。单独使用或与T3联合使用维生素D或氯贝丁酯均未观察到效果。相反，T3降低了TBG分泌，在T3浓度为10nM时最大抑制至对照值的74±5%。单独使用RA可将TBG分泌降至对照值的76%。RA并未减弱T3的作用，且两种药物联合使用未显示出协同作用。单独或联合使用T3和RA均未影响分泌的总蛋白或白蛋白。RA未改变核T3结合位点的浓度。这些数据表明，类维生素A通过基因依赖性机制调节HepG2细胞对T3的最大反应，但不调节半最大反应，且RA的特异性大于9-顺式RA。
