St Germain D L
Endocrinology. 1986 Aug;119(2):840-6. doi: 10.1210/endo-119-2-840.
The central nervous system manifests complex homeostatic mechanisms for the maintenance of thyroid hormone economy. The present studies used the NB41A3 mouse neuroblastoma cell line as a model system to study the hormonal regulation of the enzymatic conversion of T4 to T3 in neural tissue. NB41A3 cells manifested a thiol-dependent 6-n-propyl-2-thiouracil-insensitive iodothyronine 5'-deiodinase (I5'D) with a Km for T4 of approximately 10 nM. I5'D activity was increased 2- to 4-fold in cells grown in thyroid hormone-depleted medium. Exposure of cells in situ to various thyroid hormones resulted in a rapid dose-dependent inhibition of enzyme activity with the following order of potency: rT3 = T4 greater than T3. The potent inhibitory effect of rT3 on I5'D activity could not be attributed to substrate competition with T4 in the reaction assay. The addition of dexamethasone (2 X 10(-7) M) to the culture medium also inhibited I5'D activity by 46 +/- 6% (+/- SE; n = 4 experiments; P less than 0.02), whereas insulin and epinephrine were without effect. In other experiments, saturation analysis using a purified preparation of isolated nuclei from NB41A3 cells demonstrated the presence of saturable, high affinity nuclear binding sites which had a Kd value for T3 of 0.13 +/- 0.05 nM and a maximum binding capacity of 0.13 +/- 0.01 pmol T3/mg DNA. These studies demonstrate that NB41A3 cells have a low Km (type II) I5'D process and nuclear T3-binding sites very similar to those previously described in the rat central nervous system. I5'D activity in this cell line appears to be regulated by multiple serum factors, including thyroid hormones and glucocorticoids. The potent regulatory effect of rT3 and T4 suggests that T3 formation by thyroid hormones in neural tissue is controlled by a unique cellular mechanism independent of the nuclear T3 receptor. Since tissue and plasma concentrations of T4 are considerably higher than those of rT3, the former hormone is likely to be the principal thyroid hormone regulating this enzymatic process.
中枢神经系统表现出复杂的稳态机制以维持甲状腺激素的代谢平衡。本研究使用NB41A3小鼠神经母细胞瘤细胞系作为模型系统，来研究神经组织中甲状腺激素对T4向T3酶促转化的调节作用。NB41A3细胞表现出一种硫醇依赖性、对6-正丙基-2-硫氧嘧啶不敏感的碘甲腺原氨酸5'-脱碘酶（I5'D），其对T4的Km值约为10 nM。在甲状腺激素缺乏的培养基中生长的细胞中，I5'D活性增加了2至4倍。将细胞原位暴露于各种甲状腺激素中会导致酶活性迅速出现剂量依赖性抑制，其效力顺序如下：反式T3 = T4大于T3。反式T3对I5'D活性的强大抑制作用不能归因于反应测定中与T4的底物竞争。向培养基中添加地塞米松（2×10^(-7) M）也会使I5'D活性抑制46±6%（±标准误；n = 4次实验；P < 0.02），而胰岛素和肾上腺素则无作用。在其他实验中，使用从NB41A3细胞中分离出的纯化细胞核制剂进行饱和分析，结果表明存在可饱和的高亲和力核结合位点，其对T3的Kd值为0.13±0.05 nM，最大结合容量为0.13±0.01 pmol T3/mg DNA。这些研究表明，NB41A3细胞具有低Km（II型）I5'D过程和与先前在大鼠中枢神经系统中描述的非常相似的核T3结合位点。该细胞系中的I5'D活性似乎受多种血清因子调节，包括甲状腺激素和糖皮质激素。反式T3和T4的强大调节作用表明，神经组织中甲状腺激素形成T3的过程受一种独立于核T3受体的独特细胞机制控制。由于组织和血浆中T4的浓度远高于反式T3，前一种激素可能是调节这一酶促过程的主要甲状腺激素。
