Based on their excellent tumor-homing capacity, genetically engineered mesenchymal stem cells (MSCs) are under investigation as tumor-selective gene delivery vehicles. Transgenic expression of the sodium iodide symporter (NIS) in genetically engineered MSCs allows noninvasive tracking of MSC homing by imaging of functional NIS expression as well as therapeutic application of 131I. The use of tumor stroma-activated promoters can improve tumor-specific MSC-mediated transgene delivery. The essential role of transforming growth factor B1 (TGFB1) and the SMAD downstream target in the signaling between tumor and the surrounding stroma makes the biology of this pathway a potential option to better control NIS expression within the tumor milieu. Bone marrow-derived MSCs were stably transfected with a NIS-expressing plasmid driven by a synthetic SMAD-responsive promoter (SMAD-NIS-MSCs). Radioiodide uptake assays revealed a 4.9-fold increase in NIS-mediated perchlorate-sensitive iodide uptake in SMAD-NIS-MSCs after TGFB1 stimulation compared to unstimulated cells demonstrating the successful establishment of MSCs, which induce NIS expression in response to activation of TGFB1 signaling using a SMAD-responsive promoter. 123I-scintigraphy revealed significant tumor-specific radioiodide accumulation and thus NIS expression after systemic application of SMAD-NIS-MSCs into mice harboring subcutaneous tumors derived from the human hepatocellular carcinoma (HCC) cell line HuH7, which express TGFB1. 131I therapy in SMAD-NIS-MSCs-treated mice demonstrated a significant delay in tumor growth and prolonged survival. Making use of the tumoral TGFB1 signaling network in the context of MSC-mediated NIS gene delivery is a promising approach to foster tumor stroma-selectivity of NIS transgene expression and tailor NIS-based gene therapy to TGFB1-rich tumor environments.
Keywords: TGFB signaling; gene therapy; hepatocellular carcinoma; mesenchymal stem cells; sodium iodide symporter.