Development of a dual membrane protein reporter system using sodium iodide symporter and mutant dopamine D2 receptor transgenes.

UNLABELLED: For noninvasive monitoring of cellular status by dual reporters, a dual membrane protein reporter system was developed and its in vivo applicability was examined. Human sodium iodide symporter (hNIS) and mutant dopamine D(2) receptor (D(2)R) transgenes were chosen considering their complementarity.
METHODS: pIRES-hNIS/D(2)R containing NIS and D(2)R linked with an internal ribosomal entry site (IRES) was constructed and transfected into human hepatoma SK-Hep1 and rat glioma C6 cells. The cell lines stably expressing hNIS and D(2)R (named SK-ND and C6-ND) were produced, which was confirmed by messenger RNA expression of reporter genes. The functional activities of hNIS and D(2)R were measured by (125)I uptake assay and (3)H-spiperone receptor-binding assays. A biodistribution study was performed on SK-ND tumor-bearing mice using (99m)Tc-pertechnetate and (3)H-spiperone. In vivo hNIS expression was examined using (99m)Tc-pertechnetate gamma-camera imaging and, D(2)R expression was examined using a (3)H-spiperone autoradiographic study.
RESULTS: (125)I uptake of SK-ND and C6-ND cell lines showed a maximum 97-fold and 43-fold increase, respectively, which were completely inhibited by KClO(4). Specific (3)H-spiperone binding to SK-ND and C6-ND cell homogenates was observed, which were completely inhibited by (+)-butaclamol. Among the dual reporter gene-expressing cell lines, the activities of both reporters were inversely correlated with each other. Competition assay of hNIS-expressing cells by D(2)R vector transfection and D(2)R-expressing cells by hNIS vector transfection showed a dose-dependent decrease of hNIS and D(2)R activities, respectively. In the biodistribution study, (99m)Tc-pertechnetate accumulated 10-fold and (3)H-spiperone accumulated 4-fold more in SK-ND tumors than that in parental SK tumors. In vivo imaging of (99m)Tc-pertechnetate persisted until 5 wk after the cell graft in SK-ND tumors. Autoradiographic study of brain tissues from these mice also revealed an accumulation of (3)H-spiperone in SK-ND tumors.
CONCLUSION: We developed a dual membrane-bound positron and gamma-imaging reporter system of hNIS and D(2)R. We observed its reporting capability in vitro and in vivo and elucidated that these 2 membrane protein reporters competed with each other in their expression. Although we expect that hNIS and D(2)R transgenes can complement each other as a dual reporter system, we suggest that one needs to validate the ratio of expression of the 2 membrane protein reporter transgenes for cellular status tracking.