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 T-4 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 T-4 to T-3, 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 T-3, the K-m was not affected. However, the maximum velocity was significantly down-regulated by 10 nM T-3 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 T-3 or 12 nM rT(3). Time-course studies showed that a 50% reduction in enzyme activity occurred after 40 min of incubation with 100 nM rT(3) and after 160 min of incubation with 10 nM T-3. The down-regulation of 5'-DII by physiological concentrations of T-3 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 rT(3), in agreement with the current consensus, and decreased enzyme production for T-3 probably mediated by TR.