Taking Advantage of the TGFB1 Biology in Differentiated Thyroid Cancer to Stimulate Sodium Iodide Symporter (NIS)-Mediated Iodide Uptake in Engineered Mesenchymal Stem Cells

The sodium iodide symporter (NIS) mediates the active transport of iodide into thyroid follicular cells, providing the basis for the use of radioiodide for diagnostic imaging and therapy of differentiated thyroid cancer and also non-thyroidal tumors after tumor-selective NIS gene transfer. Based on their excellent tumor-homing capacity, mesenchymal stem cells (MSCs) can be employed as tumor-selective NIS gene delivery vehicles. Transgenic expression of NIS in genetically engineered MSCs allows noninvasive imaging of functional NIS expression as well as therapeutic application of (131)I. The use of promoters activated by tumor micromilieu-derived signals to drive NIS expression enhances selectivity and effectiveness, while limiting potential off-target effects. In this study we aimed to exploit the central role of transforming growth factor B1 (TGFB1) in tumor milieu-associated signaling using a TGFB1-inducible synthetic SMAD-responsive promoter to selectively drive NIS-transgene expression in engineered MSCs (SMAD-NIS-MSC) in the context of differentiated thyroid cancer based on the critical role of TGFB1 in the pathogenesis of radioiodine refractory differentiated thyroid cancer. To evaluate the TGFB1 expression in thyroid cancer cell lines, the TGFB1 concentration in conditioned medium (CM) from an array of established human papillary thyroid cancer (PTC) cell lines (BCPAP and K1) was measured by ELISA. BCPAP-CM showed a higher concentration of TGFB1, while a lower concentration was measured in K1-CM. Stimulation of SMAD-NIS-MSCs with PTC-CM showed a significant increase of NIS-mediated radioiodide-125 uptake in these MSCs in vitro. In addition, iodide uptake in SMAD-NIS-MSCs was significantly stimulated by co-culture with thyroid cancer cells. Cell migration assay was performed to validate the effect of PTC-CM in MSC recruitment. MSCs subjected to a gradient between tumor CM and serum free medium showed a directed chemotaxis towards CM with increased forward migration index (FMI) and center of mass (CoM). In a next step, based on the in vitro studies, SMAD-NIS-MSCs will be systemically applied via the tail vein to mice harboring subcutaneous PTC tumors and tumoral iodide uptake will be monitored by (123)I-scintigraphy. Taken together, these data indicate the feasibility of commandeering TGF-β/SMAD signaling in the TGFB1-rich tumor environments of radioiodine refractory differentiated thyroid carcinomas to re-establish functional NIS expression using engineered mesenchymal stem cells as therapy vehicles.