Feasibility of a novel positive feedback effect of 131I-promoted Bac-Egr1-hNIS expression in malignant glioma via baculovirus

Abstract

As intracellular iodine is released rapidly, increased expression of sodium/iodide symporter (NIS) is required for effective radioiodine treatment of tumor. As Egr1 promoter is activated by ¹³¹I and may promote human NIS (hNIS) expression, hNIS also induces ¹³¹I uptake and activates Egr1, so the existence of a positive feedback effect of ¹³¹I-promoted Egr1-hNIS expression is possible. Our purpose was to investigate the possible existence of this positive feedback effect through a series of in vitro pioneer studies. Recombinant baculovirus (Bac-Egr1-hNIS) encoding the hNIS gene under the control of a radiation-inducible Egrl promoter was constructed. To test ¹³¹I-promoted hNIS expression, human malignant glioma U87 cells were transfected with Bac-Egr1-hNIS, stimulated with or without ¹³¹I; the expression of hNIS protein was detected by immunofluorescence and flow cytometry test. In addition, the uptake and efflux of ¹³¹I were determined after the incubation of Bac-Egr1-hNIS-transfected U87 cells with or without ¹³¹I. Immunocytochemical staining and flow cytometry test showed a higher hNIS protein expression in Bac-Egr1-hNIS-transfected U87 cells with ¹³¹I stimulation than in cells without stimulation. Bac-Egr1-hNIS-transfected U87 cells accumulated up to about 4.05 times of ¹³¹I after ¹³¹I stimulation. The amount of ¹³¹I uptake in both groups showed a baculovirus dose-dependent manner. However, rapid efflux of radioactivity was observed in both groups, with 50% lost during the first 2 min after the ¹³¹I-containing medium had been replaced by a nonradioactive medium. Our results indicated that an improved transgene expression of ¹³¹I-stimulated hNIS in U87 cells using a baculovirus vector containing the Egr1 promoter is possible, and the increased expression of hNIS is responsible for a higher ¹³¹I uptake. It might provide a reference for the existence of a positive feedback effect in ¹³¹I-promoted Bac-Egr1-hNIS expression in malignant glioma and is an interesting aspect of NIS-related studies.