Nuclear 3,5,3'-Triiodothyronine Receptor Concentration Increases during Deoxyribonucleic Acid Synthesis in Partially Synchronized GC Cell Cultures*

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Abstract
The relationship between DNA synthesis and the appearance of nuclear T3-binding sites was examined in cultured GC cells which were partially synchronized by incubation with 2 mM thymidine for 25 h. After removal of thymidine, 75–85% of the cells actively synthesized DNA, and mitoses were absent until 4–6 h later. Nuclear T3-binding capacity (BC) was measured in paired asynchronous control cultures (C) and thymidine- blocked cultures 0–3 h after removal of thymidine (early S phase), after 3–6 h (mid-S phase), and after 18 h when all of the thymidine-blocked cells had divided. Compared to asynchronous controls, cells in the early S phase had a significant increase in the mean DNA content from 13.53 ± 0.42 to 16.55 ± 0.79 (±SEM) μg/106 cells (P < 0.007), and this was associated with an increase in the mean concentration of nuclear T3-binding sites from 46.8 ± 2.8 to 61.1 ± 3.8 fmol/106 cells (P < 0.001). There was, however, no significant change in the concentration of T3-binding sites per 100 μg DNA (C, 344 ± 19 fmol/100 μg DNA; thymidine-blocked, 367 ± 20 fmol/100 ±g DNA; P > 0.05). In the mid-S phase, there was an increase in both the mean BC per cell from 47.2 ± 2.5 to 67.8 ± 3.9 fmol/106 cells (P < 0.013) and the mean T3 BC per 100 μg DNA from 331 ± 17 to 407 ± 14 fmol/100 μg DNA (P < 0.002). After 18 h when blocked cells had divided, the T3 BC per 100 μg DNA or per 106 cells was similar to the control value. In summary, new nuclear T3-binding sites appeared rapidly in the S phase of the cell cycle and increased in parallel to DNA within the first 3 h of the S phase. Later in the S phase, nuclear T3-binding sites appeared to accumulate at a rate somewhat greater than that of DNA. Thus, the appearance of nuclear T3-binding sites seems well coordinated with DNA synthesis in the cell cycle, so that newly divided cells have a full complement of nuclear T3 receptor sites.