Samuels H H, Stanley F, Casanova J, Shao T C
J Biol Chem. 1980 Mar 25;255(6):2499-508.
The thyroid hormone receptor is a chromatin-associated protein which appears to mediate the actions of the thyroid hormones in mammalian cells. Unlike steroid hormone receptors, a cytoplasmic form of the receptor has not been identified, and the factors which govern the nuclear concentrations of the receptor are poorly understood. Using cultured GH1 cells, a rat pituitary cell line, we having previously demonstrated that thyroid hormones reduces the concentration of its receptor by a mechanism which involves the association of the ligand with the receptor binding site (Samuels, H.H., Stanley, F., and Shapiro, L.E. (1977) J. Biol. Chem. 252, 6052-6060). In this study, we demonstrate that n-butyrate and other aliphatic carboxylic acids elicit a reduction of thyroid hormone nuclear receptor levels without altering total cell protein synthetic rates. In contrast, the nuclear association and total cell level of the glucocorticoid receptor is not altered by n-butyrate. Evidence is presented that the aliphatic carboxylic acid-mediated reduction of thyroid hormone nuclear receptor levels is secondary to the inhibitory effect of these compounds on chromatin-associated deacetylases which is reflected as an increase in the acetylation of the nucleosome core histones. Isokinetic gradient centrifugation of chromatin solubilized from GH1 cell nuclei by micrococcal nuclease indicates that the receptor exists as a form associated with high molecular weight chromatin, as a 12.5 S form that sediments slightly faster than the bulk of the mononucleosomes, and as a 6.5 S form which appears to remain associated with low molecular weight chromatin components. Exclusive of the receptor associated with the high molecular weight chromatin, the 6.5 S form represents 80% and the 12.5 S form 10% of the receptor resolved in the gradient. n-Butyrate decreases both forms to the same degree suggesting that they are generated from the same "entity" of chromatin structure. Studies on the reappearance of receptor after restoration of the chromatin to the "normal" acetylated state are consistent with a model in which the affinity of chromatin for newly synthesized receptor is diminished in the "hyperacetylated" state.
甲状腺激素受体是一种与染色质相关的蛋白质，它似乎介导甲状腺激素在哺乳动物细胞中的作用。与类固醇激素受体不同，尚未鉴定出该受体的胞质形式，而且对控制该受体核浓度的因素了解甚少。利用培养的GH1细胞（一种大鼠垂体细胞系），我们先前已证明甲状腺激素通过一种涉及配体与受体结合位点结合的机制降低其受体的浓度（萨缪尔斯，H.H.，斯坦利，F.，以及夏皮罗，L.E.（1977年）《生物化学杂志》252，6052 - 6060）。在本研究中，我们证明正丁酸盐和其他脂肪族羧酸可引起甲状腺激素核受体水平降低，而不改变总细胞蛋白质合成速率。相反，正丁酸盐不会改变糖皮质激素受体的核结合及总细胞水平。有证据表明，脂肪族羧酸介导的甲状腺激素核受体水平降低是这些化合物对与染色质相关的去乙酰化酶的抑制作用的继发效应，这表现为核小体核心组蛋白乙酰化增加。用微球菌核酸酶从GH1细胞核中溶解染色质后进行等速梯度离心表明，该受体以与高分子量染色质相关的形式、沉降速度比大多数单核小体稍快的12.5 S形式以及似乎仍与低分子量染色质成分相关的6.5 S形式存在。不包括与高分子量染色质相关的受体，6.5 S形式占梯度中分离出的受体的80%，12.5 S形式占10%。正丁酸盐使这两种形式以相同程度降低，表明它们是由染色质结构的同一“实体”产生的。对染色质恢复到“正常”乙酰化状态后受体重新出现的研究与一个模型一致，在该模型中，染色质对新合成受体的亲和力在“高乙酰化”状态下降低。
