Kinetics of micronucleus induction by 125I-labelled thyroid hormone in hormone-responsive cells

Two cell lines, CHO and GC, different in their tissue origin, were investigated with the aim of discovering the correlation between the level of 125I-T3 binding and chromosomal damage induced by 125I decay. Incubation of cells with 125I-T3 has been performed in two exposure schedules: continuous incubation for one to six cell cycles and a pulse-chase schedule involving exposure for one cell cycle. The cellular uptake of 125I-T3, its compartmentization and kinetics were different in the two cell lines. GC cells contained about 7 times more 125I-T3 than CHO cells when incubated with the same external 125I activity concentration (74 kBq of 125I-T3 ml-1 medium). Approximately 70% of the cellular 125I-T3 was found in nuclei of GC cells and only 5% in the nuclei of CHO cells. During the long-term incubation of GC cells with 74 kBq of 125I-T3 ml-1 medium, the 125I activity concentration in cells and their nuclei initially decreased by a half, and thereafter reached a plateau after the third doubling time. In CHO cells and nuclei a very slow linear increase of 125I activity was observed. In GC cells, micronucleus frequency was found to be correlated with nuclear 125I activity. One cell cycle pulse labelling with 74 kBq of 125I-T3 ml-1 medium caused a significant enhancement of micronucleus frequency above the control level during six doubling times, with a maximum at the first post-labelling doubling time. In GC cells continuously incubated with 74 kBq of 125I-T3 ml-1 medium, the micronucleus frequency increased with the incubation time. A model of T3 receptor-dependent dose delivery to nuclei of GC cells continuously incubated with 125I-T3 is proposed. The frequency of micronuclei in the CHO cell line continuously incubated with 125I-T3 did not differ significantly from the control, whereas in the pulse-chase schedule the mean frequency of micronucleated binuclear cells was lower during 4 post-labelling doubling times (significantly at the first and second post-labelling doubling time and insignificantly at the later doubling times) than in the control. Incubation of GC cells with various activity concentrations in medium for four cell cycles resulted in a linear increase of 125I activity in cells and nuclei; however, with a saturation in the region of highest 125I-T3 concentrations used. The frequency of binuclear cells bearing micronuclei was linearly dependent on the nuclear 125I-T3 concentration.