Regulation of sodium iodide symporter expression/function and tissue-targeted gene transfer of sodium iodide symporter

The Sodium/Iodide Symporter (NIS) is an intrinsic membrane glycoprotein that mediates active iodide transport into thyroid follicular cells. Active iodide uptake is the basis for the use of radioiodine in diagnosis and treatment of follicular-cell-derived differentiated thyroid cancers (FDTC). However, approximately one third of patients with FDTC have decreased NIS expression on cell surface and cannot benefit from radioiodine therapy. Thus, it is clinically important to study the regulatory mechanisms underlying NIS expression and cell surface trafficking. In Chapter 2, I demonstrate that untranslated regions (UTRs) of human NIS (hNIS) regulate hNIS expression and/or cell surface trafficking. The 5′UTR of hNIS, 362 nt in length, increases hNIS function by facilitating cell surface trafficking, despite decreasing total protein expression levels. Deletion studies reveal that the proximal 9 nt of the 5 ′UTR are sufficient to increase hNIS function by facilitating cell surface trafficking, with no effect on hNIS total protein expression levels. Moreover, the proximal 58 nt of the 3′UTR greatly increase hNIS mRNA expression, yet only modestly increase total hNIS protein levels. These studies provide new insight into the roles of UTRs in the regulation of hNIS expression and function. In Chapter 3, I clone and characterize a 3.2 kb 5′-flanking region of mouse NIS gene (mNIS). I have identified the mNIS Upstream Enhancer (mNUE), which enhances mNIS transcription in a thyroid-specific and thyroid stimulating hormone (TSH)-responsive manner. Based on the degree of homology, I propose that mouse and rat NIS share similar mechanisms for transcriptional regulation. Finally, in Chapter 4, I investigate the efficacy of Cre/loxP system to enhance thyroid-targeted hNIS expression driven by thyroglobulin (Tg) promoter in a variety of thyroid cells. Three recombinant adenoviruses were constructed: rAd-Tg-hNIS (hNIS expression driven by Tg promoter), rAd-Tg-Cre (Cre expression driven by Tg promoter), and rAd-CMV-IoxP-hNIS (CMV promoter and hNIS cDNA separated by loxP-GFP-Zeo-loxP). In cells with weak Tg promoter activity, co-infection of rAd-Tg-Cre and rAd-CMV-IoxP-hNIS induce higher hNIS expression than single infection of rAd-Tg-hNIS. Thus, the relative promoter activity of Tg versus CMV is critical for the efficacy of Cre/loxP system.