Farwell A P, Leonard J L
Molecular Endocrinology Laboratory, University of Massachusetts Medical School, Worcester 01655.
Endocrinology. 1992 Aug;131(2):721-8. doi: 10.1210/endo.131.2.1322280.
T4 dynamically regulates the levels of type II iodothyronine 5'-deiodinase in the brain. Using an astrocyte cell culture model, we have shown that thyroxine increases inactivation of this enzyme through a mechanism using the actin cytoskeleton. In the absence of T4, the filamentous actin (F-actin) stress fibers are absent, and deiodinase inactivation is relatively slow. T4 increases inactivation of type II 5'-deiodinase by 1) restoring the F-actin stress fibers, 2) promoting the binding of the enzyme to F-actin, and 3) stimulating enzyme internalization. To determine whether inactivation of the deiodinase is due solely to the restoration of stress fibers by T4 or also involves direct thyroxine-mediated enzyme-F-actin interactions, we examined the effects of retinoids on both actin polymerization and type II 5'-deiodinase activity in cultured astrocytes, as these hormones have been shown to alter cytoskeletal organization in other tissues. In thyroid hormone-deficient astrocytes, retinoic acid increased F-actin levels, with no change in total cell actin. The F-actin content increased approximately 40% within 30 min after the addition of retinoic acid. After a plateau of 6-8 h, the F-actin content increased further to approximately 90% of the total cell actin and was associated with the reappearance of stress fibers. Only this latter retinoid-stimulated increase in F-actin content was blocked by actinomycin-D. Restoration of the F-actin stress fibers by retinoids did not increase the turnover of the type II 5'-deiodinase (t1/2, 1.99 h-1) or promote binding of the enzyme to F-actin in the absence of T4. Similarly, retinoids did not affect the rapid T4-mediated turnover (t1/2, 0.18 h-1) of type II 5'-deiodinase. These data show that an intact F-actin cytoskeleton in the absence of T4 is inadequate to alter the inactivation of type II 5'-deiodinase and that specific T4-enzyme-F-actin interactions are necessary to initiate the rapid inactivation/internalization of this enzyme.
T4动态调节大脑中II型碘甲状腺原氨酸5'-脱碘酶的水平。利用星形胶质细胞培养模型，我们发现甲状腺素通过一种利用肌动蛋白细胞骨架的机制增加了该酶的失活。在没有T4的情况下，丝状肌动蛋白（F-肌动蛋白）应力纤维不存在，脱碘酶失活相对较慢。T4通过以下方式增加II型5'-脱碘酶的失活：1）恢复F-肌动蛋白应力纤维，2）促进该酶与F-肌动蛋白的结合，3）刺激酶的内化。为了确定脱碘酶的失活是否仅归因于T4对应力纤维的恢复，还是也涉及甲状腺素直接介导的酶-F-肌动蛋白相互作用，我们研究了视黄酸对培养星形胶质细胞中肌动蛋白聚合和II型5'-脱碘酶活性的影响，因为这些激素已被证明会改变其他组织中的细胞骨架组织。在甲状腺激素缺乏的星形胶质细胞中，视黄酸增加了F-肌动蛋白水平，而总细胞肌动蛋白没有变化。添加视黄酸后30分钟内，F-肌动蛋白含量增加了约40%。在6-8小时的平台期后，F-肌动蛋白含量进一步增加至总细胞肌动蛋白的约90%，并与应力纤维的重新出现相关。只有视黄酸刺激的F-肌动蛋白含量的这种后期增加被放线菌素-D阻断。在没有T4的情况下，视黄酸恢复F-肌动蛋白应力纤维并没有增加II型5'-脱碘酶的周转（半衰期，1.99小时-1），也没有促进该酶与F-肌动蛋白的结合。同样，视黄酸也不影响T4介导的II型5'-脱碘酶的快速周转（半衰期，0.18小时-1）。这些数据表明，在没有T4的情况下，完整的F-肌动蛋白细胞骨架不足以改变II型5'-脱碘酶的失活，并且特定的T4-酶-F-肌动蛋白相互作用对于启动该酶的快速失活/内化是必要的。
