甲状腺激素类似物的研究进展

甲状腺激素类似物是一类新兴药物,通过与甲状腺激素受体选择性结合,在特定组织中发挥拟甲状腺激素作用,尤其在降血脂和能量代谢方面.本文主要介绍了靶向特异的甲状腺激素类似物在临床前动物实验和临床试验的研究进展及应用前景.     

甲状腺功能减退对新生早期大鼠各脑区甲状腺激素受体mRNA表达的影响

目的探讨甲状腺功能减退（甲减）对新生早期大鼠各脑区甲状腺激素受体（TR）mRNA表达的影响。方法建立甲减Wistar大鼠动物模型，分别于仔鼠0、14、21、45d，用实时荧光定量PCR方法检测大脑、小脑、脑干和海马TR mRNA的表达。结果与对照组相比，各时间点甲减仔鼠各脑区TRα1 mRNA的表达量呈总体下调趋势，而甲减0d仔鼠大脑、小脑、脑干TRα2 mRNA表达量明显增高（t=8．18、6．23、3．68，P〈0．01），且45d仔鼠各脑区TRα2 mRNA表达量仍高于对照组（t大脑=5．50、t小脑=5．46、t脑干=4．10、t海马=11．83，P〈0．01），TRα1、TRα2 mRNA表达峰（21d）均延迟于对照组（14d）出现。甲减仔鼠TRβ1 mRNA表达变化趋势与对照组相一致，但45d仔鼠各脑区TRβ1 mRNA表达量均低于对照组（t大脑=4．64、t小脑=2．73、t脑干=3．90、t海马=5．07，P〈0．01或〈0．05）。结论甲减时甲状腺激素受体mRNA表达峰值的延迟出现以及异常的表达变化与克汀病脑损伤机制密切相关。     

Characterization of thyroid hormone receptor alpha and beta in the metamorphosing Japanese conger eel,Conger myriaster

Two distinct TR alpha cDNA clones (TR alpha A and TR alpha B) were isolated from conger eel (Conger myriaster). The deduced amino acid sequences of the conger eel TR alphas showed higher homologies to the TR alphas of other vertebrates than to TR betas. Determination of TR mRNA in metamorphosing eels was performed using competitive RT-PCR. Of the two TR alpha mRNAs identified, TR alpha A mRNA expression was shown to be relatively higher than that of TR alpha B, and there was a peak in the expression of each during metamorphic climax. We hypothesize that both TR alphas play important roles in morphological differentiation during metamorphosis. The expression pattern of TR beta 1 mRNA was also higher during metamorphic climax and high levels of expression continued after metamorphosis. This suggests that TR beta 1 is the adult fish form which appears at high frequency after metamorphosis. It was also shown that TR beta 2 is highly expressed specifically in the brain and pituitary gland in larvae and juvenile forms including during metamorphosis and there was a peak in TR beta 2 mRNA in the elver after metamorphosis. Thus, we propose that TR beta 2 plays an important role in the regulation of the hypothalamic-pituitary-thyroid axis.     

Higher thyroid hormone receptor expression correlates with short larval periods in spadefoot toads and increases metamorphic rate

Spadefoot toad species display extreme variation in larval period duration, due in part to evolution of thyroid hormone (TH) physiology. Specifically, desert species with short larval periods have higher tail tissue content of TH and exhibit increased responsiveness to TH. To address the molecular basis of larval period differences, we examined TH receptor (TR) expression across species. Based on the dual function model for the role of TR in development, we hypothesized that desert spadefoot species with short larval periods would have (1) late onset of TR expression prior to the production of endogenous TH and (2) higher TR levels when endogenous TH becomes available. To test these hypotheses, we cloned fragments of TRα and TRβ genes from the desert spadefoot toads Scaphiopus couchii and Spea multiplicata and their non-desert relative Pelobates cultripes and measured their mRNA levels in tails using quantitative PCR in the absence (premetamorphosis) or presence (natural metamorphosis) of TH. All species express TRα and TRβ from the earliest stages measured (from just after hatching), but S. couchii, which has the shortest larval period, had more TRα throughout development compared to P. cultripes, which has the longest larval period. TRβ mRNA levels were similar across species. Exogenous T3 treatment induced faster TH-response gene expression kinetics in S. couchii compared to the other species, consistent with its higher TRα mRNA expression and indicative of a functional consequence of more TRα activity at the molecular level. To directly test whether higher TRα expression may contribute to shorter larval periods, we overexpressed TRα via plasmid injection into tail muscle cells of the model frog Xenopus laevis and found an increased rate of muscle cell death in response to TH. These results suggest that increased TRα expression evolved in S. couchii and contribute to its higher metamorphic rates.     

Transcriptional regulation of human thyroid hormone receptor β1 gene expression: effect of human retinoid X receptor and identification of a transcriptional silencer region

Effects of human retinoid X receptor (hRXR) and its ligand, 9-cis-retinoic acid, on T₃-mediated auto-regulation of hTRβ1 gene expression were examined using a chloramphenicol acetyltransferase (CAT) reporter system, and a deletional analysis of the promoter. hRXR enhanced T₃-dependent CAT induction mediated through the proximal (p) TRE in a ligand (9-cis-retinoic acid) independent manner. In a gel mobility shift assay, hRXR enhanced the binding of hTRβ1 to the pTRE by the formation of hRXR-hTRβ1 heterodimers. On the other hand, hRXR and 9-cis-retinoic acid did not show any effects on T₃-dependent CAT induction mediated through the distal (d) TRE or the binding of hTRβ1 to the dTRE. A four hundred-base pair (bp) fragment adjacent upstream of the dTRE showed a T₃ independent suppresser effect on the function of the pTRE and dTRE. Thus, this region may be an important regulator of the T₃ dependent up-regulation of the TRβ1 gene expression which is observed only under specific conditions.     

缺碘大鼠补碘对脑内甲状腺激素受体的影响

目的研究缺碘大鼠补充不同质量浓度碘后脑内甲状腺激素受体与激素结合动力学参数的变化。方法复制Wistar低碘大鼠,然后根据饮用不同质量浓度的碘水来分组。选用第2代20日龄仔鼠实验,测定甲状腺功能状态,利用放射配体结合分析法测定脑内T3受体。结果①血清T3浓度:低碘组(LI)、高碘组(HI)、适碘组(AI)与正常对照组(N)相比,差异无显著意义(P> 0.05)。血清FT3:LI组、HI组明显高于N组(P< 0.05);②血清T4和FT4:LI组和HI组均显著低于N组(P< 0.05);③受体最大结合容量(MBC):LI组明显高于N组(P< 0.05)。结论LI组血中TT4和FT4低于N组,而脑中T3核受体MBC高于N组,提示甲状腺功能低下(甲低)状态下,脑组织中T3核受体有代偿性升高。高碘状态下缺碘大鼠脑细胞核T3核受体、MBC虽略有升高,但血中TT4和FT4却低于正常,提示长期缺碘后过量补碘会出现碘性甲低。长期缺碘后,即使补充适量碘也会出现一过性碘性甲状腺功能亢进(甲亢)。     

Molecular mechanisms of corticosteroid synergy with thyroid hormone during tadpole metamorphosis

Corticosteroids (CS) act synergistically with thyroid hormone (TH) to accelerate amphibian metamorphosis. Earlier studies showed that CS increase nuclear 3,5,3′-triiodothyronine (T₃) binding capacity in tadpole tail, and 5′ deiodinase activity in tadpole tissues, increasing the generation of T₃ from thyroxine (T₄). In the present study we investigated CS synergy with TH by analyzing expression of key genes involved in TH and CS signaling using tadpole tail explant cultures, prometamorphic tadpoles, and frog tissue culture cells (XTC-2 and XLT-15). Treatment of tail explants with T₃ at 100 nM, but not at 10 nM caused tail regression. Corticosterone (CORT) at three doses (100, 500 and 3400 nM) had no effect or increased tail size. T₃ at 10 nM plus CORT caused tails to regress similar to 100 nM T₃. Thyroid hormone receptor beta (TRβ) mRNA was synergistically upregulated by T₃ plus CORT in tail explants, tail and brain in vivo, and tissue culture cells. The activating 5′ deiodinase type 2 (D2) mRNA was induced by T₃ and CORT in tail explants and tail in vivo. Thyroid hormone increased expression of glucocorticoid (GR) and mineralocorticoid receptor (MR) mRNAs. Our findings support that the synergistic actions of TH and CS in metamorphosis occur at the level of expression of genes for TRβ and D2, enhancing tissue sensitivity to TH. Concurrently, TH enhances tissue sensitivity to CS by upregulating GR and MR. Environmental stressors can modulate the timing of tadpole metamorphosis in part by CS enhancing the response of tadpole tissues to the actions of TH.     

甲状腺激素对心血管系统的影响

甲状腺激素在体内有广泛的生理作用,对心血管系统的功能状态有一定的调节和促进作用,体内甲状腺激素代谢紊乱可导致心血管系统疾病.甲状腺激素可以通过核内、核外等机制直接对心肌细胞产生影响,增加心脏收缩力和心肌耗氧量;另一方面,甲状腺激素也可以引起血液动力学变化,并通过对交感神经系统的影响增强儿茶酚胺对心肌的作用.     

甲状腺激素受体β激动剂的减重与调脂作用

甲状腺激素及甲状腺激素类似物的降脂及减轻体重的作用一直是目前的研究热点.近年的动物研究发现,甲状腺激素受体β(TRβ)激动剂如KB-141、GC-1能够选择性地结合TRβ1而发挥作用,可增加组织耗氧量、降低脂肪含量、减轻体重.此外,这类制剂还能降低低密度脂蛋白-胆固醇、脂蛋白a及甘油三酯等水平,有效降低发生动脉粥样硬化的风险.同时,这些药物能够加快新陈代谢但不增加食物摄入,亦不引起心率增快等心血管不良反应.因此,TRβ激动剂有望成为潜在的新型减肥、降脂药物.