Involvement of TP53 in apoptosis induced in human lymphoblastoid cells by fast neutrons

We investigated the involvement of TP53 in apoptosis induced by fast neutrons in cells of three human B-lymphoblast cell lines derived from the same donor and differing in TP53 status: TK6 (wild-type TP53), WTK1 (mutant TP53) and NH32 (knockout TP53). Cells were exposed to X rays or to fast neutrons at doses ranging from 0.5 to 8 Gy. Apoptosis was determined by measurements of the sub-G0 /G1-phase DNA content and by the externalization of phosphatidylserine. Fast neutrons induced extensive apoptosis in TK6 cells, as shown by the formation of hypodiploid particles, the externalization of phosphatidylserine, and the activation of caspases. In contrast, cell death was triggered at a significantly lower rate in cells lacking functional TP53. However, TP53-independent cell death also expressed the morphological and biochemical hallmarks of apoptosis. Proliferation tests and clonogenic assays showed that fast neutrons can nevertheless kill WTK1 and NH32 cells efficiently. The absence of functional TP53 only delays radiation-induced cell death, which is also mediated by caspases. These results indicate that fast-neutron irradiation activates two pathways to apoptosis and that the greater relative biological effectiveness of fast neutrons reflects mainly an increase in clonogenic cell death.