Down-Regulation of Type II L-Thyroxine, 5′-Monodeiodinase in Cultured GC Cells: Different Pathways of Regulation by L-Triiodothyronine and 3,3′,5′-Triiodo-L-Thyronine

The current consensus is that iodothyronines down-regulate type II T₄ monodeiodinase (5′-DII) by an extranuclear acceleration of enzyme inactivation. We have investigated 5′-DII regulation in cultured GC cells, in which thyroid hormone responses are mediated by nuclear thyroid receptor (TR). GC cells actively converted T₄ to T₃, independent of propylthiouracil and with a K_m of 1.4 nM, which are characteristics of 5′-DII. When GC cells were incubated with 10 nM T₃, the K_m, was not affected. However, the maximum velocity was significantly down-regulated by 10 nM T₃, from 0.15 to 0.018 pmol/mg protein · min. Dose-response studies showed that a 50% reduction in enzyme activity was achieved with either 0.25 nM T₃ or 12 nM rT₃. Time-course studies showed that a 50% reduction in enzyme activity occurred after 40 min of incubation with 100 nM rT₃ and after 160 min of incubation with 10 nM T₃. The down-regulation of 5′-DII by physiological concentrations of T₃ has the characteristics of an effect that is mediated by nuclear TR. Our studies, therefore, suggest that down-regulation of 5′-DII by these iodothyronines in GC cells may occur by different mechanisms: enzyme inactivation for rT₃, in agreement with the current consensus, and decreased enzyme production for T₃, probably mediated by TR.