Endogenous expression of the sodium iodide symporter mediates uptake of iodide in murine models of colorectal carcinoma

Abstract

The sodium iodide symporter (NIS) mediates iodide uptake into the thyroid. Because of this mechanism, differentiated thyroid cancer is susceptible for radioiodine therapy. Functional NIS expression in extrathyroidal tumors has been reported mainly in breast cancer. We screened colorectal tumors for NIS expression and investigated the mechanisms regulating NIS activity. Cell lines were screened for iodide uptake in vitro and NIS expression was evaluated by real-time RT-PCR, immunocytochemistry and immunoblotting. Iodide and pertechnetate uptake were evaluated in allograft tumors by biodistribution studies and scintigraphy. Tumors of transgenic mouse models for colorectal cancer harboring mutations in the oncogenes KRAS, beta-catenin or the tumor-suppressor gene adenomatous-polyposis coli (APC) were screened for NIS expression by RT-PCR. In vitro, functional NIS activity was detected in murine CMT93 rectal carcinoma cells and NIS expression was verified on mRNA and protein level. Inhibition of tyrosine kinases increased iodide uptake. Inhibition of tyrosine phosphatases decreased iodide uptake. In vivo, functional NIS expression was preserved in CMT93 tumors and tumor uptake could be enhanced by treatment of mice with tyrosine kinase inhibitors. In transgenic murine models of colorectal cancer, 14% of endogenous tumors expressed elevated levels of NIS mRNA. We conclude that NIS is functionally expressed in a subset of murine colorectal tumors and its activity is regulated by tyrosine phosphorylation. Therefore, with specific tyrosine kinase inhibition, these tumors might be susceptible for radioiodine treatment. Further studies are justified to identify the specific pathways regulating NIS activity and to transfer these findings to human cell lines and tissues.