Manna P R, Kero J, Tena-Sempere M, Pakarinen P, Stocco D M, Huhtaniemi I T
Department of Physiology, Institute of Biomedicine, University of Turku, FIN-20520 Turku, Finland.
Endocrinology. 2001 Jan;142(1):319-31. doi: 10.1210/endo.142.1.7900.
Recently, we demonstrated that triiodothyronine (T(3)) stimulated steroid hormone biosynthesis and steroidogenic acute regulatory (StAR) protein expression in mLTC-1 mouse Leydig tumor cells through the mediation of steroidogenic factor 1 (SF-1). We now report a dual response mechanism of T(3) on steroidogenesis and StAR expression, and on LH receptor (LHR) expression and binding in mLTC-1 cells. T(3) acutely (8 h), induced a 260% increase in StAR messenger RNA (mRNA) expression over the basal level which was coincident with an increase in progesterone (P) production. In contrast, chronic stimulation with T(3) (beyond 8 h), resulted in an attenuation of StAR expression and P production. This attenuation was most likely caused by a decrease in cholesterol delivery to the inner mitochondrial membrane as demonstrated by incubations with the hydrophilic steroid precursors, 22R hydroxycholesterol and pregnenolone, which restored P synthesis. In similar studies, chronic treatment with T(3) increased the levels of cytochrome P450scc mRNA by 83%, whereas those of cytochrome P450 17alpha-hydroxylase and 3ss-hydroxysteroid dehydrogenase decreased. The diminished response in steroidogenesis following chronic T(3) exposure was not a result of alterations in StAR mRNA stability, but rather was due to inhibition of transcription of the StAR gene. Similar acute stimulatory and chronic inhibitory responses to T(3) were found when LHR mRNA expression and LHR ligand binding were examined. Transfections with an LHR or StAR promoter/luciferase reporter construct demonstrated that a 173-bp fragment of the LHR promoter containing an SF-1 binding motif was involved in T(3) response, as was the SF-1 recognition site at -135 bp in the StAR promoter. Furthermore, the importance of SF-1 in T(3) function was also verified employing mutation in the bases of SF-1 sequences using electrophoretic mobility shift assays. The potential physiological relevance of these findings was demonstrated when similar responses were obtained in mice rendered hypo and hyperthyroid. Collectively, these observations further characterize the thyroid-gonadal connection and provide insights into the mechanisms for a dual regulatory role of thyroid hormone in Leydig cell functions.
最近，我们证明了三碘甲状腺原氨酸（T(3)）通过类固醇生成因子1（SF-1）的介导，刺激了mLTC-1小鼠睾丸间质细胞瘤细胞中的类固醇激素生物合成和类固醇生成急性调节（StAR）蛋白表达。我们现在报告T(3)对mLTC-1细胞中类固醇生成、StAR表达以及促黄体生成素受体（LHR）表达和结合的双重反应机制。T(3)急性（8小时）诱导StAR信使核糖核酸（mRNA）表达比基础水平增加260%，这与孕酮（P）生成增加相一致。相比之下，T(3)慢性刺激（超过8小时）导致StAR表达和P生成减弱。这种减弱很可能是由于胆固醇向线粒体内膜的转运减少所致，用亲水性类固醇前体22R羟基胆固醇和孕烯醇酮孵育证明了这一点，它们可恢复P合成。在类似研究中，T(3)慢性处理使细胞色素P450scc mRNA水平增加83%，而细胞色素P450 17α-羟化酶和3β-羟基类固醇脱氢酶的水平则降低。T(3)慢性暴露后类固醇生成反应减弱不是StAR mRNA稳定性改变的结果，而是由于StAR基因转录受到抑制。当检测LHR mRNA表达和LHR配体结合时，发现了对T(3)类似的急性刺激和慢性抑制反应。用LHR或StAR启动子/荧光素酶报告构建体转染表明，含有SF-1结合基序的LHR启动子173碱基对片段参与了T(3)反应，StAR启动子中-135碱基对处的SF-1识别位点也参与其中。此外，使用电泳迁移率变动分析对SF-1序列碱基进行突变也证实了SF-1在T(3)功能中的重要性。当在甲状腺功能减退和甲状腺功能亢进的小鼠中获得类似反应时，证明了这些发现的潜在生理相关性。总的来说，这些观察结果进一步描述了甲状腺与性腺的联系，并为甲状腺激素在睾丸间质细胞功能中的双重调节作用机制提供了见解。
