Ongoing activation of p53 pathway responses is a long‐term consequence of radiation exposure in vivo and associates with altered macrophage activities

Abstract

The major adverse consequences of radiation exposure, including the initiation of leukaemia and other malignancies, are generally attributed to effects in the cell nucleus at the time of irradiation. However, genomic damage as a longer term consequence of radiation exposure has more recently been demonstrated due to untargeted radiation effects including delayed chromosomal instability and bystander effects. These processes, mainly studied in vitro, are characterized by un-irradiated cells demonstrating effects as though they themselves had been irradiated and have been associated with altered oxidative processes. To investigate the potential for these untargeted effects of radiation to produce delayed damaging events in vivo, we studied a well-characterized model of radiation-induced acute myeloid leukaemia in CBA/Ca mice. Haemopoietic tissues of irradiated CBA/Ca mice exhibit enhanced levels of p53 stabilization, increased levels of p21(waf1), and increased amounts of apoptosis, as expected, in the first few hours post-irradiation, but also at much later times: weeks and months after the initial exposure. Because these responses are seen in cells that were not themselves directly irradiated but are the descendants of irradiated cells, the data are consistent with an initial radiation exposure leading to persistently increased levels of ongoing DNA damage, analogous to radiation-induced chromosomal instability. To investigate the potential source of ongoing oxidative processes, we show increased levels of 3-nitrotyrosine, a marker of damaging nitrogen/oxygen species in macrophages. Not all animals show increased oxidative activity or p53 responses as long-term consequences of irradiation, but increased levels of p53, p21, and apoptosis are directly correlated with increased 3-nitrotyrosine in individual mice post-irradiation. The data implicate persistent activation of inflammatory-type responses in irradiated tissues as a contributory bystander mechanism for causing delayed DNA damage.

The effect of genetic background and dose on non-targeted effects of radiation

“…The role of apoptosis in radiation responses is linked to DNA damage and signalling events, providing a balance between DNA damage repair and removal of irreparable damaged cells (Takahashi et al 2010). We and others have shown a genetic dependence in apoptotic response following both in vitro and in vivo exposure in mice (Gridley et al 2011, Coates et al 2008a). …”

Differential Effects of Irradiation with Carbon Ions and X-Rays on Macrophage Function

“…Considering the possibility that different types of macrophages might respond differently to radiation with regard to NO and cytokine production, 19) we are presently investigating the responses of primary, human monocytederived macrophages to X-ray and carbon ion irradiation as a follow-up to this report. Our preliminary results show that these primary, non-proliferating cells are also not able to be induced to NO or cytokine production by radiation alone, but are responsive to LPS.…”

“…Such studies are decidedly relevant, as it has been suggested that ongoing inflammatory-type responses are also a contributory factor for delayed radiation-induced genomic damage. 19) Ion radiotherapy represents an ideal tool for treatment of deep-seated tumors. 20) In contrast to X-rays or gamma-rays, charged particles such as protons and heavier ions have an inverse depth-dose profile in which the dose increases with depth.…”

Chromosomal Instability in Unirradiated Hemaopoietic Cells Induced by Macrophages Exposed In vivo to Ionizing Radiation

“…However, delayed chromosome aberrations, characteristic of a chromosomal instability phenotype, have been shown in the bone marrow of irradiated CBA/Ca mice (9), and clastogenic factors, able to induce chromosome damage in unirradiated cells, have been shown in the blood of radiotherapy patients, Japanese atomic bomb survivors, and Chernobyl liquidators (4,5,10,11). The relationship of the production of clastogenic factors to the demonstration of delayed unstable chromosome aberrations in vivo has not been established, but clastogenic factors, delayed cellular damage responses, and delayed unstable chromosomal aberrations have all been associated with genotype-dependent inflammatory processes (10,12,13). Late-appearing adverse chronic side effects in unavoidably irradiated normal tissues of radiotherapy patients are also associated with such processes (14).…”