Selva David M, Hammond Geoffrey L
Child and Family Research Institute, British Columbia, Canada.
J Mol Endocrinol. 2009 Jul;43(1):19-27. doi: 10.1677/JME-09-0025. Epub 2009 Mar 31.
Thyroid hormones increase hepatic sex hormone-binding globulin (SHBG) production, which is also regulated by hepatocyte nuclear factor-4alpha (HNF-4alpha) in response to changes in the metabolic state of the liver. Since the human SHBG promoter lacks a typical thyroid hormone response element, and because thyroid hormones influence metabolic state, we set out to determine whether thyroid hormones mediate SHBG expression indirectly via changes in HNF-4alpha levels in HepG2 human hepatoblastoma cells, and in the livers of transgenic mice that express a 4.3 kb human SHBG transgene under the control of its own 0.8 kb promoter sequence. Thyroid hormones (triiodothyronine (T(3)) and thyroxine (T(4))) increase SHBG accumulation in HepG2 cell culture medium over 5 days, and increase cellular SHBG mRNA levels. In addition, T(4) treatment of HepG2 cells for 5 days increased HNF-4alpha mRNA and HNF-4alpha levels in concert with decreased cellular palmitate levels. Plasma SHBG levels were also increased in mice expressing a human SHBG transgene after 5 days treatment with T(3) along with increased hepatic HNF-4alpha levels. In HepG2 cells, the human SHBG promoter failed to respond acutely (within 24 h) to T(4) treatment, but a 4-day pre-treatment with T(4) resulted in a robust response that was prevented by co-treatment with HNF-4alpha siRNA, or by blocking the beta-oxidation of palmitate through co-treatment with the carnitine palmitoyltransferase I inhibitor, etomoxir. These data lead us to conclude that thyroid hormones increase SHBG production indirectly by increasing HNF-4alpha gene expression, and by reducing cellular palmitate levels that further contribute to increased HNF-4alpha levels in hepatocytes.
甲状腺激素可增加肝脏性激素结合球蛋白（SHBG）的产生，肝脏代谢状态的变化也会通过肝细胞核因子-4α（HNF-4α）对其产生调节作用。由于人类SHBG启动子缺乏典型的甲状腺激素反应元件，且甲状腺激素会影响代谢状态，因此我们着手研究甲状腺激素是否通过改变HepG2人肝癌细胞以及在自身0.8 kb启动子序列控制下表达4.3 kb人类SHBG转基因的转基因小鼠肝脏中HNF-4α的水平，间接介导SHBG的表达。甲状腺激素（三碘甲状腺原氨酸（T(3)）和甲状腺素（T(4)））在5天内可增加HepG2细胞培养基中SHBG的积累，并提高细胞内SHBG mRNA水平。此外，用T(4)处理HepG2细胞5天可使HNF-4α mRNA和HNF-4α水平升高，同时细胞内棕榈酸水平降低。用T(3)处理5天后，表达人类SHBG转基因的小鼠血浆SHBG水平也会升高，同时肝脏HNF-4α水平也会增加。在HepG2细胞中，人类SHBG启动子对T(4)处理无急性（24小时内）反应，但用T(4)预处理4天会产生强烈反应，而与HNF-4α siRNA共同处理或通过与肉碱棕榈酰转移酶I抑制剂依托莫西共同处理来阻断棕榈酸的β氧化可阻止这种反应。这些数据使我们得出结论，甲状腺激素通过增加HNF-4α基因表达以及降低细胞内棕榈酸水平来间接增加SHBG的产生，而棕榈酸水平的降低进一步导致肝细胞中HNF-4α水平升高。
