Modulation of Thyroid Hormone Nuclear Receptor Levels by 3,5,3’-Triiodo-L-thyronine in GH, Cells EVIDENCE FOR TWO FUNCTIONAL COMPONENTS OF NUCLEAR-BOUND RECEPTOR AND RELATIONSHIP TO THE INDUCTION OF GROWTH HORMONE SYNTHESIS*

We have previously reported that thyroid hormone decreases the concentration of its nuclear-bound receptor and stimulates growth hormone synthesis in cultured GH, cells, a rat pituitary cell line (Samuels, H. H., Stanley, F., and Shapiro, L. E. (1976) Proc. Natl. Acad. Sci. U. S. A. 73, 38773881). In that study, we could not determine whether the initiating event in thyroid hormone action resulted from hormone-mediated receptor depletion or the binding of 3,5,3’-triiodo+thyronine (L-triiodothyronine) to the remaining receptor population. In this study, we have further examined the kinetics of receptor depletion by thyroid hormone. Evidence is presented for two components of nuclearbound receptor having different half-lives in the presence of L-triiodothyronine; a hormone-depletable component which consists of 40 to 50% of the receptor population and a relatively nondepletable receptor component. The first order decay rate constant of the hormone-depletable component was directly related to the fraction of the receptor population occupied by hormone which suggests that receptor depletion is dependent on the binding of the hormone with the receptor. The hormone-depletable component of nuclearbound receptor was also reduced in the absence of hormone at high cell densities without any change in total cell protein synthetic rates. In contrast, the L-triiodothyronine increase in growth hormone synthetic rates decreased with inereasing cell density in an identical fashion with the reduction in the depletable receptor component. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis of the synthesized extranuclear and nonhistone nuclear protein supports a functional relationship between the reduction in depletable receptor levels and growth hormone induction. The distribution of the hormone-depletable and nondepletable receptor population in chromatin sheared by DNase II indicates that both populations are present and are enriched in the chromatin fraction active in transcription. An analysis of the dose-response characteristics and the kinetics of induction of growth hormone synthesis supports a model of thyroid hormone action in GH, cells which assumes a stable growth hormone mRNA in which the growth hormone response at