(CANCER RESEARCH 50, 339-344. Januar) 15. 1990)
Modulation of Gene Expression in Syrian Hamster Embryo Cells following
Ionizing Radiation1
Gayle E. Woloschak,2 Chin-Mei Chang-Liu, Pocahontas Slit-arm Jones,3 and Carol A. Jones
Biological and Medical Research Division, Argonne National Laboratory, Argonne, Illinois 60439-4833
ABSTRACT
We examined the modulation of gene expression in Syrian hamster
embryo (SHE) cells at various times following exposure to low doses of
ionizing radiation. Early passage SHE cells were irradiated in plateau
phase (>95% G0-G| cells) with 21-cGy fission-spectrum neutrons, 75cGy X-rays, or 90-cGy 7-rays, none of which induced more than 10%
loss in cell viability. RNA harvested at various times after exposure was
examined for levels of particular RNA species by dot blot and Northern
blot hybridizations. Levels of /9-actin-specific RNA decreased within 15
min after exposure of the cells. The kinetics of repression of /8-actin
niKNA were similar for all qualities of radiation (X-rays, 7-rays, and
neutrons) for 12 h post-irradiation. Within l h after neutron exposure
(21 cGy), we observed a decrease in accumulation of RNA species
(relative to RNA from nonirradiated cells) encoding the enzyme ornithine
decarboxylase; this decrease continued for up to 12 h. Similar results
were obtained with ->-and X-rays. RNA encoding interleukin 1, however,
was induced by 3 h after neutron irradiation but reduced to background
levels by 7 h. Amounts of rRNA remained constant in all experiments,
although total transcription on a per cell basis was reduced within 15
min following irradiation and did not return to normal until 7 h postirradiation. No alterations, relative to untreated control cells, in overall
cell viability or the rate of cell cycle progression were observed in cells
either immediately or within 24 h post-irradiation. Our results demon
strate modulation of specific genes following low-dose irradiation. In
addition, our findings suggest that some molecular responses to different
qualities of ionizing radiation (X-rays, 7-rays, and neutrons) may be
similar.
control transcription of induced or repressed genes (8, 9).
Recent work has even characterized DNA-binding proteins
capable of enhancing or inhibiting transcription of specific
genes (8). In addition, many recent studies have shown that
neutrons are significantly more biologically effective than low
linear-energy-transfer radiations (such as 7-rays or X-rays) at
inducing transformation, mutation, and cell death (2, 3). How
ever, few differences between the radiation qualities have been
observed when the overall relative biological effectiveness for
early DNA lesions caused by neutrons or low linear-energytransfer radiations has been examined (10-12). The work re
ported here addresses the effect of qualities of radiation on the
kinetics of transcriptional events.
We report the modulation of selected gene expression as
early as 15 min following irradiation. Genes induced by tumor
promoters and growth factors (such as ß-actin,ornithine decar
boxylase) were all repressed within 1 h following radiation
exposure. Furthermore, whereas other work has shown that the
quality of DNA lesions induced by neutrons and X-rays/7-rays
may be different (10-12), our work suggests that at least part
of the transcriptional response (up to 7 h post-exposure) is
similar. The specificity of our observed transcriptional re
sponses for radiation (as compared to other stresses such as
heat-shock) is unknown, although genes we have observed to
be modulated following exposure to ionizing radiation have
been reported to be unaffected by heat-shock and other similar
stresses (6, 7).
INTRODUCTION
The cellular target most frequently implicated in deleterious
effects of ionizing radiations is genomic DNA. Damage can
result in mutation induction, neoplastic transformation, or cell
death; alternatively, the cells can recover from the radiation
insult (1-3). Although the outcome depends on the quality,
dose, and dose rate of the radiation, it is clearly influenced by
cellular processes as well (4). The molecular mechanisms in
volved in these processes are not well understood.
The present studies were designed to examine alterations in
gene expression at early times following low doses of neutrons,
7-rays, or X-rays to help elucidate some of the transcriptional
responses of cells after exposure to ionizing radiation. In recent
years, it has been shown that specific genes are induced or
repressed by specific agents such as phorbol esters, growth
factors, etc. (5-7). These studies have led to a better understand
ing of the cellular response to specific chemicals, the identifi
cation of proteins required for tumor promotion and cell divi
sion, and the mapping of specific regulatory elements that
Received 3/20/89; revised 8/21/89; accepted 10/12/89
The costs of publication of this article were defrayed in part by the payment
of page charges. This article must therefore be hereby marked advertisement in
accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
' Supported in part by United States Department of Energy, Office of Health
and Environmental Research, under Contract W-31-109-Eng-38 and by Grant
R01-CA33974 from the NIH.
2 To whom requests for reprints should be addressed, at Biological and Medical
Research Division. Argonne National Laboratory. 9700 South Cass Avenue.
Argonne, IL 60439-4833.
3 Work accomplished as a visiting scientist from the Department of Science
and Mathematics, Saint Paul's College, Lawreneeville. VA.
MATERIALS
AND METHODS
Cells and Culture Conditions. In all experiments, we examined mod
ulation of gene expression by radiation in SHE4 fibroblasts, which are
normal diploid cells that can be neoplastically transformed by low doses
of ionizing radiation (13, 14).
All cell cultures were established in Dulbecco's modified Eagle's
medium containing 10% fetal calf serum, 2 HIMglutamine, 100 units/
ml penicillin, and 100 units/ml streptomycin. Cells were grown to
confluence; at 48 h prior to radiation, they were placed in medium
containing 1.0% fetal calf serum to maintain the cells in plateau phase.
Studies of preirradiated SHE cells grown under this protocol show
them to be a mixed population of fibroblasts with >90-95% of the cells
in Go-Gi stage of the cell cycle. Cells were from passage 1 or 4.
Preliminary studies showed no differences with regard to transcriptional
responses between these passages.
Radiation Treatment and DNA Analysis. Cells plated in 100-mm
Petri plates containing 10 ml medium were irradiated with ""Co 7-rays,
50 kVp X-rays, or fission-spectrum neutrons (0.85 MeV) from the
JANUS reactor. All irradiations were performed at room temperature
on cells in plateau phase (>95% G0-G, cells); equitoxic doses of
neutrons. X-rays, and 7-rays were selected on the basis of survival data
(i.e., 21 cGy of neutrons, 90 cGy 7-rays, and 75 cGy X-rays cause 10%
decreases in cell survival relative to nonirradiated cells, as measured by
cloning efficiencies and a significant frequency of morphological trans
formation, as determined in a 10-day colony assay) (14). At time points
'The abbreviations used are: SHE cells, Syrian hamster embryo cells; IL-1,
interleukin 1; NaOAC, sodium acetate: ODC, ornithine decarboxylase; TPA, 12O-tetradecanoyl-phorbol-13-acetate;
SDS, sodium dodecyl sulfate; poly(A)*
RNA, polyadenylated RNA.
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GENE EXPRESSION
FOLLOWING
used in these experiments, few if any transformed cells should be present
in the cultures.
Our studies focused on establishing the presence of a molecular
transcriptional response to ionizing radiation evident within 4 h after
exposure because it has already been well established that changes in
DNA synthetic rates, cell volume, cell shape, and DNA repair capabil
ities all occur within the first 4 h after cells have been exposed to
radiation (15-17). We wished to determine whether modulation of
expression of specific genes occurred reproducibly during this same
time interval in response to ionizing radiation. Control cells were taken
to the radiation chamber but not exposed to radiation. Plates of cells
were then incubated at 37°Cfor various times (15 min to 12 h) after
radiation exposure prior to harvest of the RNA.
Samples of the unirradiated cell population and cells at 0 and 12 h
after irradiation were analyzed by flow cytometry. We chose to study
plateau phase cells to minimize transcriptional differences that might
be attributed to cell cycle effects. The cells were fixed in 70% ethanol
and then stained with propidium iodide and the DNA content was
analyzed with a PARTEC PAS II particle-analyzing system. These
studies are routinely performed on all cell populations in our laboratory
to minimize cell cycle differences. In all cases, confluent SHE cells
prepared as described above have over 95% of cells in Go-G, phase.
Measurement of Transcription Levels. Total cumulative RNA syn
thesis in nuclei of SHE cells at various times post-irradiation was
determined in transcription run-on experiments. Nuclei were harvested
at 4°Cas previously described (18). In brief, SHE cells were lysed with
1% Triton detergent in hypotonie solution, and nuclei were obtained
by centrifugation through a 2 M sucrose pad. Newly synthesized RNA
was measured by pulse-labeling of isolated nuclei in vitro for 15 min
with [a-"P]-UTP (15). RNA was purified by digestion with DNase I (5
^g/ml) and proteinase K (50 Mg/ml), followed by precipitation at 4°C
with trichloroacetic acid. Trichloroacetic acid-insoluble counts were
measured from nuclei purified from cells at various times following
irradiation. Equal numbers of nuclei were used for each point, with
spectrofluorometric techniques used to standardize for DNA content
( 18). As a control, a-amanitin ( 10 Mg/m') was added to the transcription
mix as a specific inhibitor of RNA polymerase II activity (15). This
was done to determine in fact that isolated nuclei were capable of aamanitin-sensitive transcription. In all experiments, 5-10% suppres
sion of cpm was detectable in the presence of 10 ^g/ml a-amanitin. All
results are representative of experiments performed at least 3 times.
Purification of RNA and Northern Blots. RNA was prepared by
isolation in 6 M guanidine isothiocyanate, extraction with phenol, and
precipitation from 3 M NaOAc, pH 6.0 (18-20). Poly(A)* RNA was
isolated by oligodeoxthymidylate-cellulose column chromatography un
til no unbound material absorbing at 254 nm was detected. RNA was
stored as an ethanol precipitate at —¿20°C.
Routinely we found that
poly(A)* RNA represented 5-10% of the total RNA fraction.
RNA was separated by using formaldehyde-agarose gel electrophoresis, as described previously (21). Poly(A)+ RNA samples (microgram
amounts indicated in figure legends) were denatured in 50% formamide,
1.9 M formaldehyde, 0.2 M 3-[7V-morpholino]propanesulfonic acid, 50
mM NaOAc, 1 mivi Na2EDTA, pH 7.5, for 15 min at 55°Cand then
separated on 1.2% agarose gels in 0.2 M 3-[yV-morpholino]propanesulfonic acid, 50 mM NaOAc, 1 mM Na2EDTA, 2.2 M formaldehyde.
rRNA (Escherichia coli and mouse) and RNA ladder (BRL Laborato
ries, Bethesda, MD) markers were stained with ethidium bromide and
photographed under UV light for use in sizing.
Northern transfers were performed as described (21). Blots were
hybridized to 32P-nick-translated or oligo-labeled cDNA probes. Hy
IRRADIATION
mM sodium citrate (pH 7.4), 15 mM NaCI, 50 ¿ig/"1'herring sperm
DNA (sonicated, denatured), 0.1% SDS. The blots were then dried and
exposed to X-ray film at -20°C.
In some experiments, the same blot was washed and hybridized
successively to several different probes. Each probe was eluted by
washing for 24 h in distilled deionized water at 43°C,and blots were
checked (for total removal of the labeled probes) by 24-h exposure to
X-ray film. Blots were washed 3 times in hybridization buffer before
rehybridization to a different probe. All blots are representative of
results from three independent experiments.
Although equal amounts (based on weight) of RNA were loaded in
each well of a given gel, as determined by spectrophotometry, we found
sufficient variation from one preparation to another to make poly(A)*
analysis essential. mRNA analysis systems measuring the molar con
centration of RNA with 3' poly(A)+ tails (Molecular Genetics Re
sources, Tampa, FL) were used for all poly(A)+ RNA preparations, and
only RNA samples showing equimolar concentrations of poly(A)* RNA
were loaded onto the same gel.
cDNA Clones. We gratefully acknowledge the following people who
made clones available to us. Isotype-specific actin cDNA clones were
obtained from Dr. L. Kedes (Stanford University, Palo Alto, CA), and
a-tubulin cDNA was obtained from Dr. C. Veneziale (Mayo Clinic,
Rochester, MN). The American Type Culture Collection provided us
with the probe for c-raf. A cDNA clone specific for ODC was obtained
from Dr. P. Coffino (University of California, San Francisco, CA). The
clone for IL-1 was sent by Dr. Mizel (Pennsylvania State University,
University Park, PA).
RESULTS
Effect of Irradiation on Total Transcription. Fig. 1 presents
the results of experiments examining total transcription rates
in nuclei that were pulse labeled in vitro for 15 min at various
harvest times post-exposure to JANUS fission-spectrum neu
trons (21 cGy), 7-rays (90 cGy), or X-rays (75 cGy). Within 15
to 30 min after neutron exposure, transcription rates dropped
but began to increase by l h post-exposure. Transcription rates
remained lower than that for untreated cells until at least 12 h
post-irradiation. Total transcription of X-ray- and -y-ray-treated
cells also dropped within 15 to 30 min post-exposure but began
to increase by 1 h post-irradiation, remaining at relatively high
TOTAL TRANSCRIPTION
30-
' Neutrons
bridization conditions were 50% deionized formamide, 0.75 M NaCI,
75 mM sodium citrate, 25-50 mM sodium phosphate, pH 6.5, 0.2%
SDS, 0.2% bovine serum albumin, 0.2% Ficoll, 0.2% polyvinylpyrrolidone, and 50 Mg/ml sonicated denatured herring sperm DNA at 43°C.
Prior to hybridization, all labeled probes were heat denatured at 90°C
for 5 min. After hybridization, nonspecific binding was reduced by
washing the hybridized blots 3 times for l h each at 43°Cin 45 mM
sodium citrate (pH 7.4), 0.45 M NaCI, 0.2% Ficoll, 0.2% polyvinylpyrrolidone, 0.2% bovine serum albumin, 50 ng/m' herring sperm DNA
(sonicated, denatured), 0.1% SDS, and then 3 times for l h each in 1.5
o
2
4
«
e
MRSAFTER IRRADIATION
io
12
Fig. 1. Total amount of RNA transcription in nuclei isolated from SHE cells
at various times after exposure to 21-cGy JANUS neutrons, 75-cGy X-rays, or
90-cGy -y-rays. Nuclear run-on assays were used to measure ["P]-UTP incorpo
ration into trichloroacetic acid-insoluble counts following a IS-min pulse label,
cpm are xlO3. Maximum variations in cpm are 5% or less of the reported values
for each time point.
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GENE EXPRESSION FOLLOWING IRRADIATION
levels for the duration of the assay. All data reported in Fig. 1
used pulse-labeling times of 15 min. Transcription for longer
time points (30 and 60 min) showed a depression in total
incorporated counts. «-Amanitin, a specific inhibitor of RNA
Polymerase II, uniformly depressed [32P]-UTP incorporation
by 5-10% in all nuclei preparations. This demonstrated that
isolated nuclei were capable of RNA polymerase II transcription
and suggested that radiation had effects on non-RNA polym
erase II transcripts as well. Controls of untreated cells are
reported in Fig. 1 as the 0-time point. Incubation of cells in
culture for up to 12 h has not shown detectable changes in the
relative transcription level (data not shown).
Identification of Genes Modulated by Irradiation. In prelimi
nary work, we screened mRNA populations isolated from un
treated SHE cells and irradiated SHE cells 1 and 3 h postirradiation (21 cGy neutrons) in dot-blot assays to detect dif
ferences in accumulation of specific mRNAs (resulting from
either induction or repression) following irradiation. In these
experiments, we examined expression of 36 different genes all
known to be transcriptionally affected by growth factors, tumor
promoters, or carcinogens. The majority of these genes were
either not expressed or not detectably affected by the radiation
within 3 h after exposure. Those that we found to be reproducibly increased or decreased following irradiation were further
assessed by means of Northern blots and kinetic experiments.
The results given below document those genes confirmed by
Northern blots to be reproducibly modulated by radiation ex
posure.
Genes Repressed following Ionizing Radiation Exposure. Fig.
2 depicts results of Northern blot experiments examining ac
cumulation of poly(A)+ RNA specific for cytoskeletal elements
in second-passage SHE cells at 0, 1, 3, and 7 h following
exposure to 21-cGy neutrons. These blots demonstrated a
repression in 0-actin mRNA accumulation evident at 3 h postirradiation and slight but consistent induction of «-tubulin
mRNA within 3 h after neutron exposure, followed by repres
sion by 7 h post-exposure. Further experiments showed that 7actin mRNA was modulated, with kinetics similar to those for
0-actin mRNA expression, while mRNA specific for «-actin
and cardiac actin could not be detected in these cells (data not
shown).
To more closely examine the kinetics and mechanisms of the
mRNA response, we analyzed the accumulation of ß-actinspecific mRNA (which has a half-life of 15-30 min in untreated
fibroblasts) at earlier time points (15-60 min). SHE cells (7)
were exposed to different qualities of radiation (neutrons, 7rays, or X-rays). The Northern blot depicted in Fig. 3 demon
strates that, while levels of 0-actin mRNA were similar in
untreated cells and cells l h after exposure to X-rays, levels
between 0 time (untreated cells) and l h were strongly and
rapidly down-regulated during that interval. Clearly the cellular
response to low-dose radiation was rapid, resulting in downregulation of 0-actin mRNA within 15 min of exposure of the
cells. Microdensitometric analysis of the blot in Fig. 3 and other
similar blots derived from neutron- and X-ray-exposed cells is
depicted in Fig. 4 and clearly shows that the three qualities of
radiation had similar general effects on /i-actin mRNA accu
mulation until 3 h post-irradiation. Kinetics and total levels of
ß-actinmRNA were similar in each of these experiments.
Although many studies have shown that 7-rays and neutrons
may result in different DNA-specific lesions (11, 12), our results
suggest that at least some of the early transcriptional responses
following exposure to these radiations are similar.
In separate experiments, expression of another gene known
to be induced by tumor promoters such as ODC (Fig. 5) was
down-regulated by neutron irradiation. Densitometric analyses
of Northern blots (as in Fig. 4 for /3-actin) designed to measure
the expression of ODC mRNA demonstrated a rapid downregulation (Fig. 5). Very low levels of ODC-specific mRNA
were detected at 1 and 3 h after treatment, with undetectable
levels of the mRNA by 7 h post-irradiation. Although data in
Fig. 5 depict only results from neutron-exposed cultures, similar
results were obtained with X-rays and 7-rays.
Because general transcription was initially inhibited by radia
tion (Fig. 1), it was essential for us to demonstrate that results
of gene repression were not attributable to a dilution effect in
which all mRNAs decreased, thus preventing detection of any
specific mRNAs at 15 and 30 min post-irradiation. For that
reason, equivalent amounts of poly(A)+ RNA (rather than RNA
from equal numbers of cells) were loaded in each lane of all
Northern blots. In addition, mRNA content was initially esti
mated by absorbance but finally measured prior to electrophoresis by determination of the number of poly(A)*-bearing mol
ecules/unit volume, as described in "Materials and Methods."
Genes Induced or Not Changed following Ionizing Radiation
Exposure. The genes described above all demonstrate repression
of mRNA accumulation following irradiation; in contrast,
mRNA specific for IL-1 was shown to be induced by radiation
exposure. This induction was evident within 3 h after radiation
exposure (Fig. 5), with repression detected by 7 h post-irradia
tion. Similar patterns of induction were found with 75-cGy Xrays, 90-cGy 7-rays, and 21-cGy neutrons (data not shown).
Finally, we have shown that expression of other specific genes
is not affected by radiation. Among these was c-raf, a gene
whose transcription has been implicated by association with the
development of radioresistance in radiosensitive tumor-derived
cell lines (22). c-raf h thought to encode a kinase that may be
distantly related to the protein kinase C family. SHE cells
expressed high levels of two c-ra^specific transcripts, and
expression of neither of the RNAs was affected by radiation
(X-rays, 7-rays, or neutrons).
DISCUSSION
Our studies demonstrate that cellular changes in gene expres
sion occur very rapidly following exposure to ionizing radiation.
Repression of total transcription and specific mRNA accumu
lation was evident within 15 min post-irradiation. Many agents
(such as growth factors and tumor promoters) and cellular
insults (such as heat-shock) cause equally rapid changes in gene
expression; however, while the kinetics of the changes in gene
expression are similar between tumor promoters (especially
0137
Fig. 2. Northern blol of RNA (10 fig) de
rived from untreated (O) cells and cells at var
ious times (/, 2 h; 3, 3 h; 7, 7 h) post-exposure
to 21-cGy JANUS neutrons hybridized to ßactin or ,,-tubulin cDNA probes.
2.1kb
-Actin
2.5kb
a -Tubulin
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GENE EXPRESSION
0
- %--
FOLLOWING
15'
IRRADIATION
30'
1h
3h
7h
12h
' e
Fig. 3. Northern blot of RNA (10 ^g) de
rived from untreated SHE cells (O) or SHE
cells at various times (15 min. 30 min, l h, 3
h, 7 h, 12h) post-exposure lo 75-cGy X-rays
hybridized to /Ã--actin-specificcDNA.
2.1 kb
\
/i-Actin
X-rays
4.0
ODC (10x repression)
ILK 4x induction)
•¿
x-rays (75 cGy}
o 7-rays (90 cGy)
•¿
neutrons (21cGy)
o.o
0 15'
30'
1H
3H
7H
12H
TIME FOLLOWING IRRADIATION
Fig. 4. Microdensitometric plot of relative expression of fi-actin mRNA at
various times (in h) following exposure to neutrons (21 cGy), -y-rays (90 cGy), or
X-rays (75 cGy). Results are based on Northern blot hybridizations of 10 /¿g
poly(A)* RNA to J!P-labeled fj-actin cDNA (as in Fig. 3).
TPA) and ionizing radiation, the specific gene effects are op
posite. Ionizing radiation caused repression of ODC and ßactin mRNA accumulation, while tumor promoters induce in
creased accumulation of these same transcripts. Several phorbol
ester-responsive genes (such as collagenase, c-fos, human im
munodeficiency virus- 1, and metallothionein) are also induced
by UV radiation; studies by Herrlich and his group (23, 24)
suggest that transcriptional control elements for UV-responsive
genes and the recently identified TPA-responsive element may
overlap. Similarly, our data suggest that ionizing radiation may
induce a factor that prevents activation of TPA-responsive
elements. We do not yet know the combined effects of TPA
and radiation on radiation-induced gene repression, although
1
2
3
4
B
6
7
TIME FOLLOWING 21 cGy (no)
Fig. 5. Microdensitometric plot of relative expression of ODC and IL-1
mRNA at various times following exposure to 21-cGy JANUS neutrons. Results
are based on Northern blot hybridizations of 5 cu poly(A)* RNA to each of the
labeled cDNA clones. All mRNAs are compared relative to the amount of 0-actin
mRNA expressed in untreated cells. Error bars represent SE derived from three
independent experiments.
several groups (25, 26) have determined that addition of TPA
24 h post-radiation enhances -y-ray-induced transformation fre
quencies.
While total transcription on a per cell basis was depressed to
only 60% of control levels at 30 min post-irradiation (see Fig.
1), levels of 0-actin, ODC, and a-tubulin mRNA accumulation
were decreased relative to total mRNA content in the cell
(rather than on a per cell basis). The reasons for the general
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GENE EXPRESSION FOLLOWING
transcriptional decrease following irradiation are unknown;
possibly it may be associated with or required for specific DNA
repair mechanisms. The induction of some genes (such as IL1) during this time period suggests that the cell may have a
specific mechanism for selectively controlling transcription im
mediately following radiation exposure.
Specific genes demonstrated by this study to be repressed
following exposure to ionizing radiation have been shown in
other studies to be associated with G0 to Gìtransition or
maintenance of the Gt state (27). This finding is particularly
relevant because we used established conditions such that >95%
of the cells used in these studies were in G0-G, state. ß-Actin,
•¿y-actin,
tt-tubulin, and other cytoskeletal elements have been
shown to be transcriptionally enhanced following exposure of
Go-arrested cells to growth factors (7, 28-30). ODC is expressed
at high levels in ( ¡,cells and may be required to maintain cells
in the GI stage (27). Down-regulation of these particular tran
scripts may be associated with the arrest in DNA synthesis
observed in fibroblasts and other cell types following exposure
to high doses of radiation (15-17). The levels of radiation
exposure used in these experiments have never been shown to
be sufficient in this or other cell systems to induce a detectable
arrest in DNA synthesis, but each cell may still respond molecularly to mobilize for an arrest in DNA synthesis should it be
necessary. Clearly, since SHE cells are a mixed cell population,
the Northern blot results reflect the average of the entire
population. It is possible that only a percentage of the cell
population is eliciting the measured molecular responses. In
situ methods are being developed to test this possibility.
Although most of the genes studied were transcriptionally
inhibited or unaffected by ionizing radiation, accumulation of
IL-1 was induced within 3 h following exposure. IL-1 has been
shown to be produced by and to act on many different cell types
(31), and it functions, among its many roles, as a radioprotector
both in vivo (32, 33) and in vitro (34). Induction of IL-1
expression following radiation may be one cellular radioprotective mechanism that allows the cells to handle radiation-induced
damage. Clearly, induction of IL-1 is not specific for ionizing
radiation because UV radiation has been reported to increase
levels of IL-1 protein release by cells in tissue culture (35, 36).
Much past work has shown that radiations of different qual
ities cause different qualities/quantities
of lesions in target
DNA from a variety of cell types (10-12). Our data suggest
that some molecular responses (0-actin mRNA expression) to
these different radiation qualities (neutrons, X-rays, 7-rays) are
similar; differences in this 0-actin response following different
radiation qualities were not evident until 3 h post-exposure.
Cells may cope with ionizing radiations of different qualities in
a similar manner, eliciting the same molecular response to the
radiation-mediated damage regardless of the type of radiation
initially inducing the damage. This down-regulation of ß-actin
mRNA accumulation, however, has not been detected in other
cell systems in response to heat-shock, UV radiation, or other
types of stress (6, 7, 28-30, 35, 36), thus suggesting that the
response may be peculiar to the effects of ionizing radiation
rather than to stress in general. Why molecular responses to
different qualities of radiation are different after the 3-h time
point is unknown, but the patterns we have obtained have been
remarkably reproducible from one independent experiment to
another.
Fornace's group (37-39) has studied modulation of many
genes in response to UV and ionizing radiation. Their work,
conducted generally with high doses of radiation, has shown
levels of actin mRNA accumulation to be relatively unchanged
IRRADIATION
by radiation exposure. Our results, when examining samples 1,
4, and 6 h post-irradiation as most of the studies by his group
have done, are similar. Our results demonstrating specific mod
ulation of 0-actin mRNA accumulation within 15 min following
low-dose radiation exposure, however, may be peculiar to low
doses of radiation or possibly confined to the very specific time
points we have measured with determinable kinetics. We are
currently examining dose-response effects on transcription of
/3-actin and other genes.
ACKNOWLEDGMENTS
The authors wish to thank Gordon Holmblad for his assistance in
all irradiations. Terri Harper for her excellent secretarial assistance,
and Drs. David Grdina, Meyrick Peak, Maryka Bhattacharyya, and
Thomas Fritz for their helpful comments on the manuscript.
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Modulation of Gene Expression in Syrian Hamster Embryo Cells
following Ionizing Radiation
Gayle E. Woloschak, Chin-Mei Chang-Liu, Pocahontas Shearin Jones, et al.
Cancer Res 1990;50:339-344.
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