Decreased radioiodine uptake of FRTL-5 cells after (131)I incubation in vitro: molecular biological investigations indicate a cell cycle-dependent pathway.

Purpose: In radioiodine therapy the "stunning phenomenon" is defined as a reduction of radioiodine uptake after diagnostic application of (131)I. In the current study, we established an in vitro model based on the "Fisher rat thyrocyte cell line no. 5" (FRTL-5) to investigate the stunning.

Methods: TSH-stimulated FRTL-5 cells were incubated with (131)I. Time-dependent (131)I uptake and the viability of FRTL-5 cells were evaluated at 4-144 h after radioiodine application. All data was corrected for number of viable cells, half life and (131)I concentration. Sodium iodide symporter (NIS) and the housekeeping gene (beta-actin, GAPDH) levels were quantified by quantitative polymerase chain reaction (qPCR). Additionally, immunohistochemical staining (IHC) of NIS on the cell membrane was carried out.

Results: FRTL-5 monolayer cell cultures showed a specific maximum uptake of (131)I 24-48 h after application. Significantly decreased (131)I uptake values were observed after 72-144 h. The decrease in radioiodine uptake was correlated with decreasing mRNA levels of NIS and housekeeping genes. In parallel, unlike in controls, IHC staining of NIS on FRTL-5 cells declined significantly after (131)I long-term incubation.

Conclusions: It could be demonstrated that during (131)I incubation of FRTL-5 cells, radioiodine uptake decreased significantly. Simultaneously decreasing levels of NIS mRNA and protein expression suggest a NIS-associated mechanism. Since mRNA levels of housekeeping genes decreased, too, the reduced NIS expression might be provoked by a cell cycle arrest. Our investigations recommend the FRTL-5 model as a valuable tool for further molecular biological investigations of the stunning phenomenon.