Modulation of sodium iodide symporter expression and activity at post-translational levels

Na+/I- symporter (NIS) is an intrinsic membrane glycoprotein that mediates active uptake of iodide into the thyroid gland and several extra-thyroidal tissues. The iodide-concentrating activity of NIS is the basis for the post-operative use of radioiodide in detection and targeted ablation of differentiated thyroid carcinomas. However, NIS expression and functional activity is generally reduced or absent in advanced thyroid cancer such that radioiodide therapy is rendered ineffective. Thus, investigations of the underlying regulatory mechanisms that modulate NIS expression and functional activity are of clinical importance. Furthermore, NIS has been proposed to serve as an imaging reporter gene to monitor the co-expression of therapeutic genes in small animal and human gene therapy trials. For the application of NIS as an imaging reporter gene, it is important to determine whether a linear relationship exists between NIS expression and radioiodide uptake/imaging signal and whether post-translational processing and cell surface trafficking of exogenous NIS affect radioiodide uptake.; Nasolacrimal duct obstruction has been reported as a complication associated with radioiodide therapy. Expression of human NIS was examined in tissues of the lacrimal drainage system. NIS mRNA was expressed in the lacrimal sac, and NIS protein was detected in the columnar epithelial cells of normal lacrimal sac and nasolacrimal duct. In contrast, NIS-expressing columnar epithelial cells were absent and fibrosis was evident in the lacrimal sacs derived from I131-treated patients, suggesting that active uptake of radioiodide mediated by NIS may be responsible for radiation-induced injury of the lacrimal sac and nasolacrimal duct following radioiodide therapy. Strategies to differentially modulate NIS expression and functional activity in thyroid cancer versus nasolacrimal duct may minimize I131-associated nasolacrimal duct obstruction.; The extent of radioiodide uptake can vary considerably among different cell types expressing exogenous NIS. Post-translational processing, cell surface trafficking, and correlation of NIS protein levels and NIS-mediated radioiodide uptake activity were investigated and compared among different cell types expressing various levels of NIS. Cell surface NIS protein levels were proportional to total NIS protein levels in the investigated cell lines. In addition, a moderate increased in NIS protein levels conferred marked increases in radioiodide uptake. However, further increases in cell surface NIS proteins did not translate into further increases in radioiodide uptake beyond a certain range, suggesting that NIS cell surface expression is not the sole factor dictating the extent of radioiodide uptake.; RET/PTC oncoproteins mediate downregulation of NIS expression and functional activity in papillary thyroid cancers. As RET/PTC has been shown to reduce NIS expression through the MEK/MAPK signaling pathways, which is activated in about 70% of papillary thyroid cancers, the effects of MEK inhibition on NIS protein levels and functional activity in RET/PTC-expressing thyroid cells were examined. MEK inhibition increased NIS protein levels in thyroid cells, yet decreased radioiodide uptake in cells expressing either endogenous or exogenous NIS. The possible effects of MEK inhibition on cell surface NIS levels, NIS binding affinity for I-, and iodide efflux were excluded. MEK inhibition decreased NIS velocity of iodide transport without decreasing NIS cell surface levels, suggesting that MEK inhibition decreased NIS activity. Taken together, MEK signaling modulates not only NIS expression, but also NIS-mediated radioiodide uptake activity.; While NIS has been reported to be a phosphoprotein, neither the phosphorylation sites nor the significance of phosphorylation on NIS expression and activity have been explored. In vivo phosphorylation sites in NIS were identified by mass spectrometry analysis. Phospho-defective and phospho-m...