Increase in nuclear thyroid and glucocorticoid receptors and growth hormone production during the deoxyribonucleic acid synthesis phase of the cell growth cycle

We have previously described a significant increase in cellular DNA and nuclear T₃ receptor concentration in S phase cultures of GC cells, a rat pituitary cell line that produces GH. We have now measured GH production and some aspects of GH regulation in asynchronous cells and in cultures that were synchronized in early S phase by 25-h treatment with 2 mM thymidine (dT). Cellular DNA and both cellular and medium GH increased significantly at the end of dT treatment and for 3-6 h after removal of dT (S phase). Pulse-labeling experiments with [³H]leucine followed by specific immunoprecipitation of rat GH suggested that an S phase-associated 87% increase in GH could be attributed predominantly to a 101% increase in the GH synthesis rate. The relative GH synthesis rate (GH synthesis ÷ total protein synthesis) increased from 0.85% to 1.70% during the S phase. Since cortisol (115 nM) augmented GH production in both asynchronous and S phase cultures, studies of glucocorticoid receptor concentration were also carried out. In comparison with asynchronous cultures, a significant increase in glucocorticoid receptor was identified in the nuclear fraction but not in the cytosol of cultures synchronized in the S phase. Studies with cells grown in media which were selectively depleted or repleted with T₃ and/or cortisol and experiments with the glucocorticoid antagonist 17α-methyltestosterone suggested that T₃ was primarily responsible for augmented GH during the S phase and that cortisol modulated the amplitude of this response. Our studies suggest that increases in DNA, nuclear receptors for T₃ and glucocorticoids, and GH synthesis occur in GC cell cultures synchronized in the S phase. We conclude that the GH production rate observed in asynchronous cultures may be an integrated value for all phases of the cell cycle, each of which may have a distinctive rate of hormone synthesis.