Transfer of the human NaI symporter gene enhances iodide uptake in hepatoma cells.

UNLABELLED: The characteristic feature of thyroid cells of taking up iodide enables benign thyroid diseases and differentiated thyroid carcinoma to be successfully treated with radioiodide therapy. The transport of iodide across the cell membrane is mediated by the human NaI symporter (hNIS). We therefore investigated whether the accumulation of iodide may be induced by the retroviral transfer of the hNIS gene in nonthyroid tumor cells.
METHODS: With use of a bicistronic retroviral vector for the transfer of the hNIS coding sequence and the hygromycin resistance gene, rat Morris hepatoma (MH3924A) cells were infected with retroviral particles and 32 hNIS-expressing cell lines were generated by hygromycin selection. After incubation of the genetically modified and wild-type hepatoma cells and the rat thyroid cell line FRTL5 with Na125I, the uptake and efflux of iodide were determined. In addition, the iodide distribution in rats bearing wild-type and genetically modified hepatomas was monitored.
RESULTS: Genetically modified MH3924A cell lines accumulated up to 235 times more iodide than did noninfected hepatoma cells. The maximal iodide uptake in the cells was observed after 60 min incubation time. Competition experiments in the presence of sodium perchlorate revealed a dose-dependent decrease of iodide uptake (87%-92%). Moreover, carbonyl cyanide p-trifluoromethoxyphenylhydrazone led to a loss of accumulated I- (32%), whereas 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene increased the I- uptake into the cells (22%). However, a rapid efflux of the radioactivity (80%) was observed during the first 10 min after 125I(-)-containing medium had been replaced by nonradioactive medium. In rats, the hNIS-expressing tumors accumulated six times more iodide than did the contralateral wild-type tumor as monitored by scintigraphy. The ex vivo quantitation of the iodide content performed 1 h after tracer administration in 1 g of tumor tissue revealed a 17-fold higher iodide accumulation in the genetically modified tumors. In accordance with the in vitro data, we also observed a rapid efflux of radioactivity from the tumor in vivo.
CONCLUSION: The transduction of the hNIS gene per se is sufficient to induce 125I transport in Morris hepatoma cells in vitro and in vivo. With regard to a therapeutic application, however, additional conditions need to be defined that inhibit the iodide efflux from the tumor cells.