Impact of Metformin and Compound C on NIS Expression and Iodine Uptake In Vitro and In Vivo: A Role for CRE in AMPK Modulation of Thyroid Function

Abstract

Although adenosine monophosphate activated protein kinase (AMPK) plays a crucial role in energy metabolism, a direct effect of AMPK modulation on thyroid function has only recently been reported, and much of its function in the thyroid is currently unknown. The aim of this study was to investigate the mechanism of AMPK modulation in iodide uptake. Furthermore, we wanted to investigate the potential of the AMPK inhibitor compound C as an enhancer of iodide uptake by thyrocytes. The in vitro and in vivo effects of AMPK modulation on sodium-iodide symporter (NIS) protein levels and iodide uptake were examined in follicular rat thyroid cell-line cells and C57Bl6/J mice. Activation of AMPK by metformin resulted in a strong reduction of iodide uptake (up to sixfold with 5 mM metformin after 96 h) and NIS protein levels in vitro, whereas AMPK inhibition by compound C not only stimulated iodide uptake but also enhanced NIS protein levels both in vitro (up to sevenfold with 1 μM compound C after 96 h) and in vivo (1.5-fold after daily injections with 20 mg/kg for 4 days). We investigated the regulation of NIS expression by AMPK using a range of promoter constructs consisting of either the NIS promoter or isolated CRE (cAMP response element) and NF-κB elements, which are present within the NIS promoter. Metformin reduced NIS promoter activity (0.6-fold of control), whereas compound C stimulated its activity (3.4-fold) after 4 days. This largely coincides with CRE activation (0.6- and 3.0-fold). These experiments show that AMPK exerts its effects on iodide uptake, at least partly, through the CRE element in the NIS promoter. Furthermore, we have used AMPK-alpha1 knockout mice to determine the long-term effects of AMPK inhibition without chemical compounds. These mice have a less active thyroid, as shown by reduced colloid volume and reduced responsiveness to thyrotropin. NIS expression and iodine uptake in thyrocytes can be modulated by metformin and compound C. These compounds exert their effect by modulation of AMPK, which, in turn, regulates the activation of the CRE element in the NIS promoter. Overall, this suggests that the use of AMPK modulating compounds may be useful for the enhancement of iodide uptake by thyrocytes, which could be useful for the treatment of thyroid cancer patients with radioactive iodine.