Cell cycle perturbations and cytogenetic damage induced by low energy protons in human primary fibroblasts

Endpoints related to cell cycle perturbations were investigated in human primary fibroblasts irradiated with 1-4 Gy of either protons (7.7 and 28.5 keV.micron-1) or X rays. Regardless of the quality of radiation, 8 h after irradiation cells accumulated in the G2 phase, but such accumulation was maintained at 24 h from treatment only in 28.5 keV.micron-1 protons. A marked G1/S-transition block was observed in proton as well as in X ray-treated cultures up to 24 h, with doses as low as 1 Gy. On the other hand, the number of cells positive to p21 antibody reaction was higher after proton than after X ray treatment, indicating that the 'complexity' of lesions may play a critical role in the induction of p21.