Regulation of thyroid hormone nuclear receptor levels in GH1 cells by 3,5,3'-triiodo-L-thyronine. Use of dense amino acid labeling to determine the influence of hormone on the receptor half-life and the rate of appearance of newly synthesized receptor.

3,5,3'-Triiodo-~-thyronine (L-triiodothyronine) reduces the amount of its nuclear receptor protein in GHI cells in a time-and dose-dependent fashion.To define further the mechanism of receptor depletion, dense amino acid labeling has been used to study the influence of L-triiodothyronine on receptor turnover.When GH, cells are cultured in medium without thyroid hormone, receptor half-life is 4.7 h and the rate of appearance of newly synthesized receptor is 30 fmo1/100 pg of DNA/h.These rates account for the average steady state amount of receptor of 203 fmo1/100 pg of DNA.When these cells are cultured for 24 h or longer with 5 n~ L-triiodothyronine, receptor half-life decreases to 3.3 h and the receptor appearance rate decreases to 25 fm01/100 pg of DNA/h.These rates account for the lower steady state amount of receptor in the presence of L-triiodothyronine of about 119 fm01/100 pg of DNA.During the approach to the new steady state after addition of L-triiodothyronine to GHI cells, the initial decrease in the amount of receptor i s due to a rapid but partially transient inhibition of up to 56% in the receptor appearance rate.The change in receptor half-life occurs more slowly, and within 24 h the receptor appearance rate and half-life have attained the new steady state values described above.Following removal of hormone from receptor-depleted cells, the initial increase in amount of receptor is due to a rapid increase in the receptor appearance rate while receptor half-life remains characteristic of cells cultured with L-triiodothyronine for up to 9 h after hormone removal.Thus, during both the onset of and recovery from hormonemediated receptor depletion, the change in receptor appearance rate occurs rapidly and the change in receptor half-life occurs more slowly.Possible mechanisms for the observed receptor regulation by L-triiodothyronine are discussed.Previous studies in this laboratory have shown that GHI cells, a growth hormone-producing rat pituitary tumor cell line, are a physiologically relevant cell culture system for studying the action of thyroid hormones (1, 2).In these cells, 3,5,3"triiodo-~-thyronine causes a 3-to 10-fold increase in the rate of growth hormone synthesis (3) which is paralleled by