Similar Extent of Apoptosis Induction at Doses of X-Rays and Neutrons Isoeffective for Cell Inactivation

Abstract

The relative biological effectiveness (RBE) of neutrons differs for various biological endpoints, and for various cell and tissue types. With respect to the apoptosis induction, a whole range of values can be found in the literature, but the decisive factors are not clear. Most previous studies have used apoptosis-prone hematopoietic cells, whereas tumor cells have received little attention. The authors therefore decided to investigate apoptosis induction caused by X-rays and neutrons in a line of human melanoma cells, at doses which are isoeffective for the loss of colony-forming ability. Human melanoma cells Be11, expressing p53 wild-type protein, were used throughout. Exponentially growing cells were exposed to two pairs of isoeffective doses (at surviving levels 10% and 1%) of 240-kV X-rays and 5.8-MeV neutrons. 1-8 days after irradiation, the frequency of apoptosis in adherent cells was assessed by two-parameter flow cytometric analysis with a DNA-dye-exclusion annexin-V-binding assay as well as by morphological examination with DAPI staining. Apoptosis was induced most significantly 6-7 days after irradiation. The time courses, as well as the magnitudes of apoptosis induction, after isoeffective doses of X-rays and neutrons with respect to loss of colony-forming ability appeared to be comparable. RBE values in the range of 4-5 were estimated for apoptosis 4-8 days after irradiation by both the annexin V assay and morphological examination. Radiation-induced apoptosis depends on ionization density in the same way as cell inactivation in general does, i.e., the RBE is similar, and the ratio of cells dying by apoptosis to cells dying otherwise does not depend on radiation quality.