Glutathione-Dependent Thyroxine 5′-Monodeiodination Modulates Growth Hormone Production by Cultured Nonthyrotropic Rat Pituitary Cells*

Publisher Website
Google Scholar
Abstract
Conversion of L-T4 (T4) to L-T3 (T3) was assessed in GH3 rat pituitary cells which secrete GH and PRL. We tested the hypothesis that 5′-monodeiodination of T4 by these cells was dependent on intracellular glutathione (GSH), as previously suggested for liver and kidney. The GSH content of sonicated pituitary cell homogenates was 40 nmol/mg protein. This sonicate catalyzed the 5′-monodeiodination, generating 0.92 ± 0.08 pmol T3/mg • h in the presence of 50 nM T4 and 1 mM dithiothreitol (DTT). After depletion of GSH by dialysis of the cell sonicate, no T3 was generated. The addition of GSH (10 μM) to the dialysate restored 5′-monodeiodinase activity. This showed the thiol dependence of the reaction. When intact cells were incubated overnight with intracellular GSH-depleting agents (diethylmaleate or diamide), the 5′-monodeiodination of T4 was abolished. Propylthiouracil (PTU; 5 μM) did not inhibit T4 to T3 conversion in the intact cells in monolayer culture or in the cell sonicate. In contrast, sodium ipodate (10 μM) inhibited generation of T3 from T4. To test the functional significance of thiol-dependent intracellular generation of T3, GH3 cells in monolayer culture were incubated in medium enriched with thyroidectomized calf serum (15%), and GH production was measured after the addition of 10 μM T4, 10 nM T4 plus 10 μM sodium ipodate, 10 nM T4 plus 20 μM diamide, or 0.1 μM T3. GH production induced by T4 alone was enhanced by the addition of 1 μM DTT (dithiothreitol, a sulfhydryl protector). GH production was inhibited when T4 (10 nM) was added to the medium in the presence of ipodate or diamide compared to that when T4 was added alone. The conversion of T4 to T3 by these nonthyrotropic pituitary tumor cells depends directly on intracellular GSH. Thiol-dependent cellular T3 generation is accompanied by GH production; stimulation of GH production occurs with enhanced cellular T3 generation by DTT, while corresponding inhibition of GH production results from the inhibition of cellular T3 generation by ipodate and GSH antagonists. Thus, the ability of T4 to induce GH synthesis is at least in part related to GSH-dependent intracellular 5′-monodeiodination of T4 to T3.