γH2AX, 53BP1 and Rad51 protein foci changes in mesenchymal stem cells during prolonged X-ray irradiation

Abstract

At high exposure levels ionizing radiation is a carcinogen. Little is known about how human stem cells, which are known to contribute to tumorigenesis, respond to prolonged radiation exposures. We studied formation of DNA double strand breaks, accessed as γH2AX and 53BP1 foci, in human mesenchymal stem cells (MSCs) exposed to either acute (5400 mGy/h) or prolonged (270 mGy/h) X-irradiation. We show a linear γH2AX and 53BP1 dose response for acute exposures. In contrast, prolonged exposure resulted in a dose-response curve that had an initial linear portion followed by a plateau. Analysis of Rad51 foci, as a marker of homologous recombination, in cells exposed to prolonged irradiation revealed a threshold in a dose response. Using Ki67 as a marker of proliferating cells, we show no difference in the γH2AX distribution in proliferating vs. quiescent cells. However, Rad51 foci were found almost exclusively in proliferating cells. Concurrent increases in the fraction of S/G2 cells were detected in cells exposed to prolonged irradiation by scoring CENPF-positive cells. Our data suggest that prolonged exposure of MSCs to ionizing radiation leads to cell cycle redistribution and associated activation of homologous recombination. Also, proliferation status may significantly affect the biological outcome, since homologous repair is not activated in resting MSCs.

Formation of γH2AX and pATM Foci in Human Mesenchymal Stem Cells Exposed to Low Dose-Rate Gamma-Radiation

“…The studies of the effects of low-dose and low dose-rate IR exposures on DSB repair in multipotent mesenchymal stem/stromal cells (MSCs) are of particular importance because the high proliferative potential of MSCs creates a risk of a transmission of accumulated DNA damage and mutations to both the self-renewed stem cell pool and the differentiated progeny of exposed cells [10,11]. Under normal conditions, MSCs exert suppressive effects on cancer cells [12].…”

Residual γH2AX foci induced by low dose x-ray radiation in bone marrow mesenchymal stem cells do not cause accelerated senescence in the progeny of irradiated cells

“…Each DNA repair focus represents a single DNA DSB [ 17 ], allowing very accurate quantitative measurements even at low-dose radiation exposures (10-100 mGy) [ 18 ]. Histone variant H2AX phosphorylated at DSB sites by several protein kinases (ATM, ATR and DNA-PK) and called γH2AX is the most widely used marker of DNA DSBs [ 19 – 23 ]. A substantial number of reports have been published that examine various aspects of the dynamics of γН2АХ foci formation in mammalian cells exposed to low-dose radiation [ 24 – 26 ], with a focus being the unusually long retention of these foci in low-dose irradiated cells [ 27 – 30 ].…”

Ionizing Radiation and Translation Control: A Link to Radiation Hormesis?

“…Furthermore, BRCA1 was shown to effect its regulatory function by binding to mRNA in a PABP1-dependent manner [151,152]. Shifting a balance from the error-prone NHEJ towards the error-free HR DSB repair pathways that can be regulated by 53BP1, including via eIF4G1-mediated translation control [125], has been proposed as one of the potential responses to chronic LDR exposure in primary human mesenchymal stem cells [153], human fibroblasts [75] and myoblasts [154]. Therefore, it is feasible to suggest that a translational reprogramming of DSB signaling and repair is involved in LDR responses that lead to enhanced ability to cope with either exogenous (e.g., genotoxic environmental stresses) or endogenous (e.g., oxidative metabolism and replication errors) DNA damage.…”