Effect of Actinomycin D on Iodide Transport in FRTL-5 Thyroid Cells

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Abstract
TSH-induced I- uptake in FRTL-5 thyroid cells requires new protein synthesis. During the induction of I- uptake, which takes nearly 60 h to reach its maximum, 2 waves of protein synthesis can be identified: one during the first 8 h and another after 24-30 h, each involving different proteins. Cycloheximide (CH) added during the first 10 h of a 48 h incubation with TSH completely inhibits the induction of I- uptake; 58% inhibition is observed with CH added at 24 h; no inhibition is observed when CH is added 36 h after TSH. Like CH, actinomycin D (ActD), added at the beginning of the 48 h period, inhibits TSH induction of I- uptake; partial inhibition (83%, 72% and 28%) is observed when ActD is added at 1, 5 and 10 h, respectively. Treatment with ActD at 24 h (ActD-treated cells) paradoxically increases I- uptake (1.8- to 3.5-fold over the control value). The characteristics of I- uptake in ActD-treated cells are the same as those in untreated cells; both are Na+ dependent and can be inhibited in a comparable manner by anions. Kinetic measurements of I- transport indicate that ActD increases the rate of I- influx (2-fold or greater increase in maximum velocity without a significant change in Km), with only minor changes in I- release. Enhanced I- uptake in ActD-treated cells is inhibited by the simultaneous (24 h) administration of CH, indicating that protein synthesis is required for the late ActD effect. Despite an overall 2-fold decrease in protein synthesis in cells treated with ActD at 24 h, the synthesis of individual proteins maximally induced by TSH during the first 8 h is increased, whereas that of some proteins maximally synthesized after 24-30 h is markedly reduced. TSH-induced I- uptake in FRTL-5 cells involves a regulatory action of TSH that operates at the mRNA level.