Distribution of triiodothyronine nuclear receptors during the cell cycle in mouse leukemia cells

The process of the appearance of thyroid hormone receptors in the cell cycle was studied in mouse leukemia cells L1210. After cell synchronization by 2 mM thymidine for 12 h followed by 80 nM Colcemid for 4 h, the specific binding of triiodothyronine (T3) to its nuclear receptors was determined 3, 6, 9 and 12 h after release from the thymidine-Colcemid block. Three h after release from the block, [125I]T3 specific binding was 11.4 +/- 2.5% of a control value measured for an asynchronous population. An upward slope in progression of T3 nuclear receptors was found 6 h (32.3% +/- 4.5%), 9 h (47.8% +/- 5.2%) and 12 h (83% +/- 4.3%) after release from thymidine-Colcemid block. The data suggest that processes involving T3 receptor promotion in cell nuclei are operative within the late G1 and S phases of the cell cycle, and thus the increase in T3 receptor concentration in the nuclei is in a positive correlation with the number of cells in the S compartment of the cell cycle.