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.
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