Kinetics of Micronucleus Induction by ¹²⁵I-labelled Thyroid Hormone in Hormone-responsive Cells

Abstract

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.