Sodium-iodine Symporter Gene Expression Controlled by the EGR-1 Promoter

Abstract

The objective of this study is to explore the feasibility of radioiodine treatment for cervical cancer using the early growth response (Egr-1) promoter to control sodium-iodine symporter (hNIS) gene expression. The hNIS gene was previously transfected into Hela cells under the control of either the cytomegalovirus (CMV) or Egr-1 promoters. Na125I uptake was measured in the presence or absence of NaClO4. Na125I efflux was measured. The effects of external beam radiation on iodine uptake and retention were studied. The cytotoxic effects of 131I were measured by clonogenic assay. The Na125I biodistribution was obtained using mice bearing control and transfected cells. The %ID/g of tumor and major organs were obtained for a range of times up to 48 hours post injection and the ratio of tumor to non-tumor activity (T/NT) was calculated. Tumors were imaged with Na131I and 99mTcO4−, and the ratio of tumor to background activity (T/B) was calculated. Na125I uptake in Hela cells was minimal in the absence of hNIS. Uptake in the transfected cells was strong, and could be blocked by NaClO4. The iodine uptake of Hela-Egr-1-hNIS cells increased after the irradiation, and the magnitude of this effect approximately matched the radiation dose delivered. The efflux of 125I was affected by neither the promoter sequence nor pre-irradiation. 131I reduced the clonogenic survival of symporter expressing cells, relative to the parental line. The effect was greatest in cells where hNIS was driven by the CMV promoter. Tumors formed from Hela-Egr-1-hNIS concentrated Na125I over a 12 hour period, in contrast to untransfected cells. These tumors could also be successfully imaged using either Na131I or 99mTcO4−. 131I uptake peaked at 4h, while 99mTcO4− accumulated over approximately 20 hours. In vivo uptake of 131I and 99mTcO4− was slightly higher in cells transfected with the Egr-1 promoter, compared to CMV. Hela-Egr-1-hNIS cells demonstrate highly enhanced iodine uptake, and this effect is further augmented by radiation, creating a positive feedback loop which may bolster radionuclide therapy in vivo.