Zaballos Miguel A, Garcia Bibian, Santisteban Pilar
Instituto de Investigaciones Biomedicas Alberto Sols, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, 28029 Madrid, Spain.
Mol Endocrinol. 2008 May;22(5):1183-99. doi: 10.1210/me.2007-0093. Epub 2008 Jan 17.
Signaling by TSH through its receptor leads to the dissociation of trimeric G proteins into Galpha and Gbetagamma. Galphas activates adenylyl cyclase, which increases cAMP levels that induce several effects in the thyroid cell, including transcription of the sodium-iodide symporter (NIS) gene through a mechanism involving Pax8 binding to the NIS promoter. Much less is known about the function of Gbetagamma in thyroid differentiation, and therefore we studied their role in TSH signaling. Gbetagamma overexpression inhibits NIS promoter activation and reduces NIS protein accumulation in response to TSH and forskolin. Conversely, inhibition of Gbetagamma-dependent pathways increases NIS promoter activity elicited by TSH but does not modify forskolin-induced activation. Gbetagamma dimers are being released from the Gs subfamily of proteins, because cholera toxin mimics the effects elicited by TSH, whereas pertussis toxin has no effect on NIS promoter activity. We also found that TSH stimulates Akt phosphorylation in a phosphoinositide 3-kinase (PI3K)-dependent and cAMP-independent manner. This is mediated by Gbetagamma, because its overexpression or specific sequestration, respectively, increased or reduced phosphorylated Akt levels upon TSH stimulation. Gbetagamma sequestration increases NIS protein levels induced by TSH and Pax8 binding to the NIS promoter, which is also increased by PI3K inhibition. This is, at least in part, caused by Gbetagamma-mediated Pax8 exclusion from the nucleus that is attenuated when PI3K activity is blocked. These data unequivocally demonstrate that Gbetagamma released by TSH action stimulate PI3K, inhibiting NIS gene expression in a cAMP-independent manner due to a decrease in Pax8 binding to the NIS promoter.
促甲状腺激素（TSH）通过其受体发出的信号导致三聚体G蛋白解离为Gα和Gβγ。Gαs激活腺苷酸环化酶，从而增加环磷酸腺苷（cAMP）水平，这会在甲状腺细胞中引发多种效应，包括通过一种涉及配对盒蛋白8（Pax8）与钠-碘同向转运体（NIS）启动子结合的机制来转录NIS基因。关于Gβγ在甲状腺分化中的功能了解较少，因此我们研究了它们在TSH信号传导中的作用。Gβγ的过表达抑制NIS启动子的激活，并减少对TSH和福斯可林产生反应时NIS蛋白的积累。相反，抑制Gβγ依赖性途径会增加TSH引发的NIS启动子活性，但不会改变福斯可林诱导的激活。Gβγ二聚体从G蛋白的Gs亚家族中释放出来，因为霍乱毒素模拟TSH引发的效应，而百日咳毒素对NIS启动子活性没有影响。我们还发现，TSH以磷酸肌醇3-激酶（PI3K）依赖性且不依赖cAMP的方式刺激Akt磷酸化。这是由Gβγ介导的，因为其过表达或特异性隔离分别在TSH刺激后增加或降低了磷酸化Akt水平。Gβγ隔离增加了TSH诱导的NIS蛋白水平以及Pax8与NIS启动子的结合，PI3K抑制也会增加这种结合。这至少部分是由于Gβγ介导Pax8从细胞核中排除，而当PI3K活性被阻断时这种排除作用会减弱。这些数据明确表明，TSH作用释放的Gβγ刺激PI3K，由于Pax8与NIS启动子的结合减少，以不依赖cAMP的方式抑制NIS基因表达。
