Thyroid hormone stimulates de novo growth hormone synthesis in cultured GH1 cells: evidence for the accumulation of a rate limiting RNA species in the induction process.

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Abstract
We have previously demonstrated with radioimmunoassay techniques that physiologic concentrations of thyroid hormone stimulate an increase in growth hormone production in GH1 cells in culture. In this study, with radiolabel techniques and selective immunoprecipitation, we demonstrate that L-triiodothyronine stimulates growth hormonw production solely by inducing an increase in the de novo synthesis of the polypeptide hormone. L-Triiodothyronine stimulated synthetic rates of growth hormone by 1.5-fold in 1.25 hr, 2-fold in 2.5 hr, to a maximal of 3- to 4-fold after 8.5 hr of incubation. The time interval between significant L-triiodothyronine binding to putative nuclear receptors and a detectable increase in growth hormone synthesis was 45-60 min. Studies on the effect of actinomycin D, 3'-deoxyadenosine, and cycloheximide support the thesis that L-triiodothyronine induces the accumulation of an RNA species which is rate limiting for growth hormone synthesis and lends further support for a primary action of thyroid hormone at the nuclear level.