P53 INDUCED BY IONIZING-RADIATION MEDIATES DNA END-JOINTING ACTIVITY,BUT NOT APOPTOSIS OF THYROID-CELLS

To understand the effects of ionizing radiation on thyroid cells, we i nvestigated the role of p53 in mediating apoptosis and in DNA repair f ollowing in vivo and in vitro irradiation of thyroid cells, In vitro e xposure of human thyroid cells to ionizing radiation of up to 5-8 Gy f ailed to induce apoptosis in primary cells, The same results were obta ined when the thyroid gland was irradiated in the intact rat, To explo re the mechanism of failure of the wild-type p53 in inducing apoptosis in thyroid cells, we investigated the expression of apoptosis-related genes, bar, bcl-2 and fas/APO-1 following irradiation or induction of temperature-sensitive p53. The expression of Bar, Bcl-2 and Fas/APO-1 in human primary cultured thyroid cells did not change after irradiat ion, To further confirm the results, we established a clonal cell line (tsFRO) in which a temperature sensitive p53 (Val138) expression vect or was stably transfected to a thyroid carcinoma cell line lacking end ogenous p53, Incubation of tsFRO cells at the permissive temperature f or three days, however, did not induce apoptosis although G(1) arrest was noted, Although enhanced expression of the bar mRNA level was obse rved, the expression of Bar, Bcl-2 and Fas/APO-1 protein did not chang e by shifting tsFRO cells to permissive temperature as well as irradia ted primary cells, Furthermore, DNA end-jointing ability was examined by transfection of linearized luciferase plasmid into tsFRO cells, Inc reased luciferase activity occurred when the cells were cultured at th e permissive temperature, indicating that the mild-type p53 enhances D NA end-jointing activity, Our results indicate that the wild-type p53 does not lead to apoptosis but facilitates DNA end-jointing in thyroid cells, These results may reflect specific responses in thyroid cells following irradiation.