Title: Ionizing Radiation Induced
Adaptive Responses in Normal Human Cells Maintained in
Three-Dimensional Architecture: the Role of Oxidative
Metabolism
Authors: Sonia M. de
Toledo1, Nesrin Asaad1, Venkatachalam Perumal1, Ling Li2,
Douglas R. Spitz2 and Edouard I.
Azzam1
Institutions: 1Department of
Radiology, UMDNJ - New Jersey Medical School, Newark,
NJ 07103 2 Free Radical and Radiation Biology Program,
University of Iowa, Iowa City, IA 52242
We are testing the hypothesis
that “endogenous
oxidative metabolism modulates molecular signaling pathways,
residual DNA damage and proliferation potential in mammalian
cells exposed to low dose, low dose-rate γ-radiation”.
To this end, we have exposed normal human diploid fibroblasts
adapted to grow in three-dimensional architecture, which mimics
cell growth in vivo, to γ-rays delivered at various dose
rates. AG1522 cells were irradiated with a single dose of 10
cGy (from a 137Cs source) delivered acutely or protracted over
various periods up to 48 h. Chromosomal damage and changes
in the expression of stress related proteins were measured
following the exposure. Compared to sham-manipulated control
cells, a significant increase in micronucleus formation (a
surrogate form of DNA damage) was observed in the acutely irradiated
cells. Protraction of the dose over 24 h resulted in significantly
reduced effects; its protraction over 48 h reduced the micronucleus
frequency to a level below the spontaneous rate. This pattern
of chromosomal damage correlated with that of changes in the
phosphorylation of serine15 in the p53 protein, a marker we
found to be highly sensitive to ionizing radiation, being increased
in cells exposed to an acute dose as low as 1 cGy. Ectopic
overexpression of either of the antioxidant enzymes CuZn-SOD,
Mn-SOD, catalase or glutathione peroxidase, or the inhibition
of oxidant generation by flavin-containing oxidases, significantly
attenuates stressful effects induced in cells exposed to 10
cGy acutely or at moderate dose rates. The above studies with
a single low dose were extended to examine the effect of the
biological responses induced by the low dose on chromosomal
damage caused by a subsequent acute challenge dose of γ-rays.
Pre-exposure to 1, 5, or 10 cGy (0.2 cGy/h) significantly protected
against damage from a subsequent dose of 1 Gy (0.8 Gy/min).
The induced mitigating effects were transient and disappeared
by 48 h. Our data suggest that oxidative metabolism, DNA repair
as well as cell cycle checkpoints are involved in the induced
adaptive responses. In investigating the role of flavin oxidases
in the control of normal cell cycle progression, we have found
that reactive oxygen species induced by moderate γ-ray
doses delivered at low dose rate are biologically similar
to those generated as a result of endogenous oxidative
metabolism.
Supported by Research Grant FG02-02ER63447
from the U.S. Department of Energy
