Department of Anatomy and Physiology, Kansas State University College of Veterinary Medicine, 1600 Denison Avenue, Manhattan, KS 66506, USA.
Exp Physiol. 2010 Dec;95(12):1132-44. doi: 10.1113/expphysiol.2010.054700. Epub 2010 Aug 20.
Subclinical hypothyroidism has been linked to cystic fibrosis, and the cystic fibrosis transmembrane conductance regulator (CFTR) shown to be expressed in the thyroid. The thyroid epithelium secretes Cl⁻ and absorbs Na(+) in response to cAMP. Chloride secretion may provide a counter-ion for the SLC26A4 (pendrin)-mediated I⁻ secretion which is required for the first step of thyroid hormonogenesis, thyroglobulin iodination. In contrast, few models exist to explain a role for Na(+) absorption. Whether CFTR mediates the secretory Cl⁻ current in thyroid epithelium has not been directly addressed. We used thyroids from a novel pig CFTR(-/-) model, generated primary pig thyroid epithelial cell cultures (pThECs), analysed these cultures for preservation of thyroid-specific transcripts and proteins, and monitored the following parameters: (1) the Cl⁻ secretory response to the cAMP agonist, isoprenaline; and (2) the amiloride-sensitive Na(+) current. Baseline short-circuit current (I(sc)) did not differ between CFTR(+/+) and CFTR(-/-) cultures. Serosal isoprenaline increased I(sc) in CFTR(+/+), but not CFTR(-/-), monolayers. Compared with CFTR(+/+) thyroid cultures, amiloride-sensitive Na(+) absorption measured in CFTR(-/-) pThECs represented a greater fraction of the resting I(sc). However, levels of transcripts encoding epithelial sodium channel (ENaC) subunits did not differ between CFTR(+/+) and CFTR(-/-) pThECs. Immunoblot analysis verified ENaC subunit protein expression, but quantification indicated no difference in expression levels. Our studies definitively demonstrate that CFTR mediates cAMP-stimulated Cl⁻ secretion in a well-differentiated thyroid culture model and that knockout of CFTR promotes increased Na(+) absorption by a mechanism other than increased ENaC expression. These findings suggest several models for the mechanism of cystic fibrosis-associated hypothyroidism.
亚临床甲状腺功能减退症与囊性纤维化有关，囊性纤维化跨膜电导调节因子（CFTR）已被证明在甲状腺中表达。甲状腺上皮细胞通过 cAMP 分泌 Cl⁻并吸收 Na(+)。氯离子分泌可能为 SLC26A4（pendrin）介导的 I⁻分泌提供反离子，这是甲状腺激素生成的第一步，即甲状腺球蛋白碘化所必需的。相比之下，很少有模型可以解释 Na(+)吸收的作用。CFTR 是否介导甲状腺上皮细胞中的分泌性 Cl⁻电流尚未得到直接解决。我们使用新型猪 CFTR(-/-)模型的甲状腺，生成原代猪甲状腺上皮细胞培养物（pThECs），分析这些培养物中甲状腺特异性转录本和蛋白质的保留情况，并监测以下参数：（1）cAMP 激动剂异丙肾上腺素刺激的 Cl⁻分泌反应；和（2）阿米洛利敏感的 Na(+)电流。CFTR(+/+)和 CFTR(-/-)培养物之间的基础短路电流（I(sc)）没有差异。在 CFTR(+/+)单层细胞中，腔侧异丙肾上腺素增加了 I(sc)，但在 CFTR(-/-)单层细胞中没有增加。与 CFTR(+/+)甲状腺培养物相比，在 CFTR(-/-) pThECs 中测量的阿米洛利敏感的 Na(+)吸收占静息 I(sc)的更大比例。然而，CFTR(+/+)和 CFTR(-/-) pThECs 之间编码上皮钠通道（ENaC）亚基的转录本水平没有差异。免疫印迹分析证实了 ENaC 亚基蛋白的表达，但定量分析表明表达水平没有差异。我们的研究明确证明 CFTR 介导了在分化良好的甲状腺培养模型中 cAMP 刺激的 Cl⁻分泌，并且 CFTR 敲除通过除 ENaC 表达增加以外的机制促进了 Na(+)吸收的增加。这些发现为囊性纤维化相关甲状腺功能减退症的机制提出了几种模型。
