Modulation of sodium/iodide symporter expression and function in thyroid.

摘要

Na+/I- Symporter (NIS) is an intrinsic membrane glycoprotein expressed on the baso-lateral membrane of thyroid follicular cells that mediates active iodide uptake into the thyroid gland. NIS-mediated iodide uptake and iodide organification is the basis for the post-operative use of radioiodide in detection and targeted ablation of differentiated thyroid cancer. However, about 20-30% patients with metastatic thyroid cancer do not benefit from radioiodine therapy due to reduced or absent NIS expression/function. Thus, it is of clinical importance to investigate the underlying mechanism of NIS modulation such that strategies to selectively upregulate NIS expression and/or functional activity can be devised.;Much effort has been made to identify reagents that increase NIS-mediated radioiodine accumulation in cultured thyroid cells, i.e. our lab has previously reported that 17-allyamino-17-demethoxygeldanamycin (17-AAG) increases NIS-mediated radioiodine accumulation by decreasing iodide efflux in NIS-expressing immortalized non-transformed and RET/PTC1 oncogene transformed cultured thyroid cells. However, the finding has not been validated using immune-competent genetically engineered mice carrying thyroid tumors surrounded by physiologically/pathologically relevant microenvironment. Non-invasive radionuclide imaging with micro-single photon emission computed tomography (micro-SPECT) was used to examine and quantify temporal radioiodine accumulation in both thyroid and salivary glands modulated by triiodothyronine (T3), bovine TSH (bTSH) and/or 17-AAG. The extent of thyroidal radioiodine accumulation stimulated by a single dose of exogenous bTSH in T3-supplemented endogenous TSH suppressed mice was much less than that in non-treated mice, justifying the clinical use of multiple doses of recombinant human TSH or the merit of thyroxine (T4) withdrawal in thyroid cancer patients prior to radioiodine therapy. In addition, the extent and duration of radioiodine accumulation stimulated by bTSH was reduced in thyroid tumor-bearing thyroid glands of thyroid-targeted RET/PTC1 (Tg-PTC1) thyroid cancer mouse model compared to the thyroids in wild type (WT) mice. Lastly, the effect of 17-AAG on increasing thyroidal, but not salivary, radioiodine accumulation was validated in WT mice and in Tg-PTC1 thyroid cancer mouse model. This encourages the study of 17-AAG derivatives not only as novel chemotherapeutic agents to impede tumor progression but also to improve radioiodine therapy in patients with thyroid cancer.;We aimed understand the mechanism underlying regulation of NIS expression/function to identify potential pharmacological inhibitors targeting signaling nodes that would increase NIS mRNA/protein levels and/or radioiodine accumulation in cultured thyroid cells. In particular, the crosstalk between PI3K-Akt-mTORC1 and MEK signaling nodes on NIS modulation was examined. Inhibition of PI3K by LY294002 increased NIS-mediated RAIU activity through upregulation of NIS expression. In contrast, inhibition of mTORC1 by Rapamycin decreased NIS-mediated RAIU activity yet increased NIS protein levels. The differential regulation of NIS-mediated RAIU activity by LY294002 and Rapamycin is likely due to their differential effect on pERK and pAkt levels. The effect of Akt inhibition on NIS expression/function was further examined showing that Akti-1/2 markedly increased NIS-mediated RAIU activity despite that NIS mRNA and protein levels were not increased. The possible effect of Akt inhibition on cell surface NIS levels was excluded. Akt inhibition decreased iodide efflux rate and increased iodide transport rate and iodide affinity of NIS, suggesting that Akt inhibition increased NIS functional activity. Furthermore, treatment of Akti-1/2 further increased NIS-mediated RAIU activity and decreased total NIS protein levels in LY294002, Rapamycin or PD98059 treated PC Cl3 cells suggesting that NIS modulation by Akt at the protein level as well as RAIU activity appears to prevail over PI3K, mTORC1, and MEK signaling nodes. Interestingly, Akti-1/2 had little effect on acute TSH-stimulated RAIU activity in PC Cl3 cells. The discrepancy of Akti-1/2 effect on RAIU activity between PC Cl3 cells chronically and acutely stimulated by TSH indicates that Akti-1/2 effect on increasing RAIU activity is less prominent in cells acutely stimulated by TSH. Lastly, the effect of LY294002 and Akti-1/2 on NIS protein levels and RAIU activity is not restricted to PC Cl3 cells, but also seen in FRTL-5 cells. However, LY294002 and Akti-1/2 had little effect on cell surface or total NIS protein levels yet decreased NIS-mediated RAIU activity in HEK293 human kidney cells stably expressing exogenous rat NIS. This suggests that NIS-mediated RAIU activity may be modulated by LY294002 and Akti-1/2 in a cell context dependent manner. Consequently, PI3K and Akt inhibitors may serve as potential pharmacological reagents to selectively increase thyroidal radioiodine accumulation.