Characterization of accelerated iron-ion-induced damage in gap junction-competent and -incompetent thyroid follicular cells

Early- and late-passage cultures of Fischer rat thyroid cells differ in their growth properties and gap junction competency. Previous studies comparing early- and late-passage cultures exposed to γ rays and proton beams revealed that differences in growth rate did not influence their responses; however, the presence of connexin 32 gap junctions conferred resistance to γ radiation. To further assess differences in radiation quality, suspension cultures of early- and late-passage cells were exposed to accelerated iron ions, and their comparative biological responses were measured. The iron-ion-irradiated cells displayed sustained levels of incorporated dUTP, reflecting persistent DNA damage. These results were supported by the frequency of chromosomal damage measured by micronucleus formation. Iron-ion irradiation induced micronuclei at a rate of eight per gray per 100 binucleated cells scored in early-passage cells and nine per gray per 100 binucleated cells scored in late-passage cells. Relative to photons, the calculated radiobiological effectiveness for frequency of micronuclei was 5.7 and 6.4 for the early- and late-passage cultures, respectively (P > 0.05). Levels of apoptosis fluctuated as a function of dose, and modest increases above basal levels persisted throughout the 48-h period. The comparison of retained follicular structures revealed differences in the α components of the linear-quadratic dose-response curves (0.60 Gy^-1 for early-passage and 0.71 Gy^-1 for late-passage cultures, P < 0.014). Cell cycle phase redistribution resulted in a G₂ arrest (P < 0.001) for both early- and late-passage cultures. In conclusion, the response of thyroid follicular cells to high-LET radiation was not influenced by the presence of gap junctions or the proliferative status of the target cells.