Li Q L, Jansen E, Brent G A, Naqvi S, Wilber J F, Friedman T C
Department of Medicine, Cedars-Sinai Research Institute, University of California School of Medicine, Los Angeles 90048, USA.
Endocrinology. 2000 Sep;141(9):3256-66. doi: 10.1210/endo.141.9.7674.
The majority of prohormones are cleaved at paired basic residues to generate bioactive hormones by prohormone convertases (PCs). As PC1 and PC2, two neuroendocrine-specific PCs, appear to be the key enzymes capable of processing a variety of prohormones, alterations of PC2 and/or PC1 levels will probably have a profound effect on hormonal homeostasis. We investigated the regulation of PC2 messenger RNA (mRNA) by thyroid hormone using GH3 cells to demonstrate that T3 negatively regulated PC2 mRNA levels in a dose- and time-dependent fashion. Functional analysis of progressive 5'-deletions of the human (h) PC2 promoter luciferase constructs in GH3 cells demonstrated that the regulation probably occurs at the transcriptional level, and that putative negative thyroid hormone response elements were located within the region from -44 to + 137 bp relative to the transcriptional start site. Transient transfections in JEG-3 cells and COS-1 cells showed that the suppressive effect of T3 was equally mediated by the thyroid hormone receptor (TR) isoforms TRalpha1 and TRbeta1. Electrophoretic mobility shift assays using purified TRal and retinoid X receptor-beta protein as well as GH3 nuclear extracts showed that regions from +51 to +71 bp and from +118 to +137 bp of the hPC2 promoter bind to TRalpha1 as both a monomer and a homodimer and with TRalpha1/retinoid X receptor-beta as a heterodimer. Finally, the in vivo regulation of pituitary PC2 mRNA by thyroid status was demonstrated in rats. These results demonstrate that T3 negatively regulates PC2 expression at the transcriptional level and that functional negative thyroid hormone response elements exist in the hPC2 promoter. We postulate that the alterations of PC2 activity may mediate some of the pathophysiological consequences of hypo- or hyperthyroidism.
大多数激素原在成对的碱性残基处被激素原转化酶（PCs）切割，从而生成生物活性激素。由于PC1和PC2这两种神经内分泌特异性的PCs似乎是能够加工多种激素原的关键酶，因此PC2和/或PC1水平的改变可能会对激素稳态产生深远影响。我们利用GH3细胞研究了甲状腺激素对PC2信使核糖核酸（mRNA）的调节作用，结果表明，三碘甲状腺原氨酸（T3）以剂量和时间依赖性方式对PC2 mRNA水平产生负调节作用。对人（h）PC2启动子荧光素酶构建体进行逐步5'端缺失的功能分析表明，这种调节可能发生在转录水平，并且假定的负性甲状腺激素反应元件位于相对于转录起始位点-44至+137 bp的区域内。在JEG-3细胞和COS-1细胞中进行的瞬时转染实验表明，T3的抑制作用同样由甲状腺激素受体（TR）异构体TRα1和TRβ1介导。使用纯化的TRα1和视黄酸X受体-β蛋白以及GH3细胞核提取物进行的电泳迁移率变动分析表明，hPC2启动子的+51至+71 bp区域以及+118至+137 bp区域可作为单体、同二聚体以及与TRα1/视黄酸X受体-β形成异二聚体的形式与TRα1结合。最后，在大鼠体内证实了甲状腺状态对垂体PC2 mRNA的调节作用。这些结果表明，T3在转录水平对PC2表达产生负调节作用，并且hPC2启动子中存在功能性负性甲状腺激素反应元件。我们推测，PC2活性的改变可能介导了甲状腺功能减退或亢进的一些病理生理后果。
