Application of the Cre/loxP System to Enhance Thyroid-Targeted Expression of Sodium/Iodide Symporter

Radioiodide uptake activity mediated by the human Na(+)/I(-) symporter (hNIS) in thyroid follicular cells is the basis for effective (131)I therapy in thyroid cancer. However, radioiodide therapy is not effective in patients with thyroid cancer displaying low or absent hNIS expression. This study assessed the Cre/loxP system for enhancing thyroid-targeted hNIS expression driven by the thyroglobulin (Tg) promoter. The following three recombinant adenoviruses (rAd) were constructed: rAd-Tg-hNIS drives hNIS expression by the Tg promoter; rAd-Tg-Cre drives Cre expression by the Tg promoter; and rAd-CMV-loxP-hNIS drives hNIS expression by the cytomegalovirus (CMV) promoter after Cre-mediated excision of an intervening loxP-GFP-Zeo-loxP. Immortalized normal and malignant rat thyroid cell lines and primary cultures of normal human thyroid and human follicular adenoma cells were investigated. We found that the relative promoter activity of Tg vs. CMV is critical for the efficacy of the Cre/loxP system. In cells with weak Tg promoter activity, coinfection of rAd-Tg-Cre and rAd-CMV-loxP-hNIS induced higher hNIS expression than single infection of rAd-Tg-hNIS. Finally, Tg promoter activity was partially restored in malignant thyroid cells by forced expression of the paired domain-containing transcription factor (Pax-8), allowing the Cre/loxP system to mildly enhance radioiodide uptake.