Studies on radiation-induced apoptosis in G0 human lymphocytes

Purpose: To determine the relationships between the frequencies of radiation-induced chromosomal alterations and the extent of apoptosis in G(0) human lymphocytes. Material and methods: G(0) human peripheral blood lymphocytes (HPBL) were X or gamma-irradiated, in the presence or absence of the repair inhibitor cytosine arabinoside (Ara-C). Directly after irradiation, a part of the lymphocytes were stimulated to grow while the rest were stimulated 48 h after irradiation. These lymphocyte cultures were analysed for induction of chromosomal aberrations. A subset of lymphocytes was kept in G(0) and analysed for cell viability, apoptosis and p53 expression. Results: The fraction of cells bearing dicentrics was reduced in lymphocytes stimulated to grow 48 h post irradiation as compared to lymphocytes stimulated immediately after irradiation. The decrease in the frequency of dicentrics correlated with the increase in the number of apoptotic cells. The operative apoptotic pathway in irradiated G(0) lymphocytes was dependent on the expression of p53. Conclusions: The radiation-induced apoptotic response of G(0) lymphocytes is p53 dependent and increases with the time they are held in G(0). When mitogen was added 48 h after irradiation, cells with dicentrics were either preferentially eliminated or did not enter mitosis. Thus the radiation-induced damage can be underevaluated depending on the time between radiation exposure and the induction of proliferation. These results may have relevance for biodosimetry studies or for evaluations of the efficacy of radiotherapy which are based on the frequencies of chromosomal aberrations.