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Advances in Brief
Role of the p53 Tumor Suppressor Gene in Cell Cycle Arrest and Radiosensitivity
of Burkitt's Lymphoma Cell Lines
Patrick M. O'Connor,1 Joany Jackman, Daniel Jondle, Kishor Bhatia, Ian Magrath, and Kurt W. Kohn
Laboratory of Molecular Pharmacology,
Developmental
Thcra[>eulic.ÃP̄rogram, Division of Cancer Treatment ¡P.M. ()., J. J., D. J., K. W. A'./, and Lymphoiil Biology Section,
Pediatrics Branch /K. B., I. M./, National Cancer Institute, N1H, Rethesda, Maryland 20892
Abstract
ization. Several lines of evidence suggest that p53 functions by bind
ing to DNA as an oligomer. Also, cells heterozygous for p53 form
heterooligomeric complexes (mutant/wild-type p53 complexes), in
which the ability of the wild-type protein to function is suppressed
We have assessed the role of the p5Ì
tumor suppressor gene in cell cycle
arrest and cytotoxicity of ionizing radiation in 17 Burkitt's lymphoma and
Ivmphoblastoid cell lines. Cell cycle arrest was assessed by flow cytometry
of cells 16 h following irradiation. In addition to the usual G2 arrest, the
cell lines exhibited three types of responses in I.,: Class I, strong arrest in
G i following radiation; Class II, minimal arrest; and Class III, an inter
mediate response. All Class I cells contained normal p53 genes. Of the ten
lines that showed minimal (., arrest, eight had mutant p53 alÃ-eles,and two
lines were heterozygous for p53 mutations. Both of the lines showing an
intermediate response contained wild-type p53. Our results are consistent
(11). It appears that the p53 protein is not required for normal mouse
development since transgenic mice lacking both p53 genes are born
normal (12). Nonetheless, knockout mice and mice expressing mutant
p53 alÃ-eleshave a much higher frequency of developing tumors than
their wild-type counterparts (12, 13). These findings are reminiscent
of the predisposition of patients with Li-Fraumeni syndrome to mul
tiple neoplasms (14, 15). These patients suffer germ line mutations in
with the view that mutations abrogate the ability ofp53 to induce (-, arrest
one of the p53 alÃ-elessuch that each cell expresses one wild-type and
following radiation. Studies with the hétérozygotes
showed that the mu
one mutant p53 protein.
tant protein can have a dominant negative influence upon wild-type ¡>5.1.
Overexpression of wild-type p53 causes cells to arrest in G, of the
and the reduced ability of two normal p53 lines to arrest in t., indicated
cell cycle, in accordance with inhibition by p53 of the initiation of
that p53 function can be impaired by other mechanisms. The radiosensitivity of most of the lines appeared to depend on the ability ofp53 to induce
a G, arrest. The mean radiation dose that inhibited proliferation of the
Class I lines by 50% was 0.98 Gy. Of the eight pS3 mutant cell lines tested,
five lines required approximately 2.9 Gy to cause a 50% inhibition of cell
proliferation. The two hétérozygotes
were also more resistant to radiation
than the Class I cells (50% inhibitory dose, 2.1 and 2.9 Gy). Our results
suggest that radioresistance is afforded by a loss of function of wild-type
p53, which would normally induce a G| arrest and promote cell death in
the presence of DNA damage.
Introduction
The p53 tumor suppressor gene is the most commonly mutated gene
in human cancer (1-3). The normal gene product exerts antiproliferative and antitransforming activity and in some cases promotes cell
death via apoptosis. The precise mechanism by which p53 exerts its
actions is still unclear; however, p53 binds to specific DNA sequences
and can act both as a transcriptional activator and repressor (4-6).
Genes that wild-type p53 /raws-activates include the MDM2 gene, the
function of which appears to antagonize the activity of p53 (7), and the
GADD45 gene, which was originally identified by its coordinate
induction following growth arrest and DNA damage (8, 9). The spe
cific DNA binding domain ofp53 resides within a central region of the
protein that contains putative metal binding sites that are important for
maintenance of the wild-type p53 conformation (10). Mutations in
p53 cluster predominantly within this DNA binding region and lead to
a loss of function of both the DNA binding and biological activity of
the protein (1-5, 9-10). The half-life of wild-type p53 is on the order
of 20 min. However, mutant forms of p53 frequently have longer
half-lives, leading to constitutively elevated levels of mutant p53 in
tumor cells (1-3). The COOH-terminal region of p53 contains nuclear
localization sequences and a domain that is important for oligomer-
DNA replication ( 16). It is now clear that wild-type p53 is required for
G, arrest following ionizing radiation; cells having mutant or no p53
genes fail to demonstrate this response (9, 17, 18). The above findings
suggest that p53 acts as a checkpoint control protein that halts the cell
cycle in G, while DNA damage is present. This would presumably
allow more time for DNA repair to be completed before progression
into S phase. The role of p53 is in this sense analogous to that of the
RAD9 gene, which in yeast inhibits progression of cells from G2 into
mitosis following DNA damage (19). The participation of p53 and
RAD9 in checkpoint controls that ensure fidelity in the transmission of
genetic material from one cell generation to the next is supported by
findings that cells lacking p53 or RAD9 activity exhibit a greater
frequency of gene amplification/mutations than do wild-type cells (19,
20). The actions of p53 and RAD9 might also be expected to protect
cells from the cytotoxic effects of DNA damaging agents. Abrogation
of G2 arrest, either by genetic inactivation of RAD9 or with methylxanthines, increases the sensitivity of cells to DNA damaging agents
(19, 21, 22), indicating that at least the G2 checkpoint plays a protec
tive role against DNA damage induced cytotoxicity.
In the present study we investigated whether activation of the p53
dependent checkpoint in GÌ
would afford protection to ionizing ra
diation. For this purpose we assayed 17 Burkitt's lymphoma and
Rcccivcd 8/13/9.1; accepted 9/2/93.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore he hereby marked advertisement in accordance with
18 U.S.C. Section 1734 solely to indicate Ihis fact.
1To whom requests for reprints should be addressed, at Room 5C-25. Bldg. 37.
National Cancer Institute. Bcthesda. MD 20892.
lymphoblastoid cell lines for their ability to arrest in G| following
7-irradiation and correlated this response to the status of the p53 gene
and radiosensitivity. The results suggest that contrary to expectation,
normal p53 function in Burkitt's lymphoma and lymphoblastoid cells
enhances radiosensitivity. A possible reason for this is discussed.
Materials and Methods
Cell Culture.
Burkitt's lymphoma and lymphoblastoid cell lines were de
rived either at the National Cancer Institute from biopsies or normal periph
eral lymphocytes, or from the American Type Culture Collection (Rockville,
MD) or National Institute of General Medical Sciences cell repositories
(Camden, NJ). Cells were grown at 37°Cin 95% air/5% CO2 in RPMI 1640
containing
15% heat-inactivated
fetal bovine serum, 2 HIMt.-glutamine, 50
units penicillin, and 50 fig/ml streptomycin. All tissue culture products were
obtained from Advanced Biotechnologies (Columbia, MD) and routinely
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ROLE OF pS3 IN RADIOSENSITIVITY
Table 1 Characteristics
of the Burkitt's lymphoma and ¡ymphoblastoid cell line panel
Shown is the p53 status of each cell line as confirmed by single-strand conformation polymorphism analysis and DNA sequencing of exons 5 through 8 (23) and the status of each
cell line with regard to the presence of the EBV genome.
time
(h)242525213022242120212
statusWTAVTWTAVTWTAVTWTAVTWTAVTWTAVTWTAVTWT/mutantWT/mutantMutantMutantMutantMutantMutantMutantMutantMutantExon
mutation75775768786AlÃ-elemutation248158248254175,
lineWMNFWLNL2AG876SHOJLP119EW36AKUAST486CA46RamosSG568NamalwaP3HR1MCI
Cell
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeWild
typeArg
typeWild
typeWild
GinArg
to
HisArg
to
typeWild
typeDeletedDeletedArg
GinHe
to
AspRepeatedArg
to
176248163287238234ChangeWild
to TrpEBVNegativePositivePositivePositivePositiveNegativeNegativeP
GinTyr
to
HisGlu
to
EndCys
to
TyrTyr
to
16HWLJD38CharacterizationBurkitt'sLymphoblastoidLymphoblastoidBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitl'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sBurkitt'sp53
to CysChangeWild
" EBV, Epstein-Barr virus.
monitored for the presence of Mycoplasma contamination. The status of the
p53 gene in the cell lines used was previously assessed by single-strand con
formation polymorphism analysis of "hot spot" exons 5 through 8. This tech
nique was used to identify the exon(s) harboring the mutations and then polymerase chain reaction products of exons showing abnormal migration were
subjected to direct sequencing (23). The results of these studies are shown
in Table 1.
Flow Cytometry. Cells were washed in ice-cold PBS,2 (pH 7.4, 5 ml),
fixed in 70% ethanol (5 ml), and stored at 4°C.Cells were then washed once
with ice-cold PBS (5 ml), treated with RNase (l h at 37°C,500 units/ml, Sigma
Chemical Co., St. Louis, MO), and DNA was stained with propidium iodide
(50 fig/ml). Cell cycle determination was performed using a Becton-Dickinson
fluorescence-activated
cell analyzer in which DNA content, as assayed by
propidium iodide staining, was used to distinguish each cell cycle phase.
Quantitation was performed using the sum-of-broadened-rectangle
model pro
gram provided by the manufacturer: 3—5S-phase peaks were used to fit the
tion according to the manufacturer's
recommendations
(Amersham).
Mono
clonal antibodies PAb 1801 and PAb 240 (Oncogene Science, Inc., Manhasset,
NY) recognize epitopes that reside between amino acids 32 and 79 and amino
acids 212 and 217 of p53, respectively.
Survival Studies. Cytotoxicity was determined from %-h growth inhibi
tion assays as described previously (24). Briefly, exponentially growing cells
(2 X laVml) were irradiated at room temperature (0.79-12.6 Gy) using a 137Cs
source delivering 5.25 Gy/min (1 Gy = 100 rads). Cells were postincubated
and cell counts and cell size were determined every 24 h using a Coulter
Counter and channelyzer (Coulter Electronics, Hialeah, FL). Growth fraction
was quantitated at the time the control untreated population had reached 8
times the initial inoculum (3 cell doublings). Duplicate determinations were
made within each of two independent experiments.
model. DNA synthesis was also monitored at 16 h following irradiation by
labeling the cells for 30 min with 10 /J.Mbromodeoxyuridine (Sigma). Follow
ing bromodeoxyuridine labeling, cells were washed and fixed as described
above. Cells were then resuspended in 1 ml 0.1 MHC1 containing 0.25% Triton
X-100 at 4°Cand left on ice for 10 min. Cells were then diluted with 5 ml of
distilled water and centrifuged and resuspended in 2 ml of water; DNA was
denatured by boiling for 10 min. Afterwards cells were cooled on an ice slurry
for 10 min, washed in 5 ml of PBS containing 0.25% Triton X-100, and then
resuspended in 0.1 ml of PBS/Triton containing 5 ng/ml of an anti-bromodeoxyuridine-fluorescein conjugate (Boehringer Mannheim Biochemicals, India
napolis, IN). Results were collected for a minimum of 15,000 cells for each
determination.
Gel Electrophoresis and Western Blotting. Cells were lysed on ice for 30
min in 1% Nonidet P-40 prepared in PBS that contained leupeptin (10 /j-g/ml),
aprotinin (10 (¿g/ml),2 IHM4-(2-aminoethyl)benzenesulfonyl
fluoride, 1 mm
sodium o-vanadate, 10 mM sodium fluoride, and 5 mm sodium pyrophosphate.
Protein determination was performed using the BCA protein assay kit accord
ing to the manufacturer's instructions (Pierce, Rockford, IL). Seventy-five fig
of total cell protein were loaded onto sodium dodecyl sulfate-polyacrylamide
gels and electrophoresed as described previously (22). Proteins were trans
ferred to Immobilon membranes (Millipore, Bedford, MA) using semidry
blotting techniques. Membranes were blocked for 30 min in 5% skim milk,
probed for l h with primary antibodies, and then probed with sheep anti-mouse
horseradish peroxidase second antibodies (Amersham Corporation, Arlington
Heights, IL). Antibody reaction was revealed using chemiluminescence detec
2 The abbreviations
used are: PBS, phosphate buffered saline; ID5(I radiation dose
required to cause 50% inhibition of cell proliferation.
mu
un un
Fig. 1. Measurement of Gt arrest following y-irradiation in Burkitt's lymphoma and
lymphoblastoid cell lines. Shown is the percentage of the control G¡population that re
mained in G, for up to 16 h following 6.3 Gy radiation. Cell cycle distribution was
quantitated using flow cytometry as described in "Materials and Methods." Measure
ments shown were made in the presence of nocodazole (0.4 ng/ml) to ensure that cells
from the previous cell cycle would not reenter Gìduring the course of the experiment.
Samples were grouped into three classes: Class I, strong arrest in GI following radiation
(>60% of the original population); Class II, minimal arrest in G, following radiation
(<25% of the original population); and Class III. an intermediate response. Abscissa,
status of the p53 gene.
4777
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ROLE OF pS3 IN RADIOSENSITIVITY
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Fig. 2. Competence of G, arresi in Class I and lack of competence of G, arrest in Class II cells following 7-irradiation. Exponentially growing cells were irradiated with 6.3 Gy
and then postincuhated for 16 h in the absence of nocodazole. A typical Class I response showing strong arrest in G, and G2 following irradiation is illustrated for the case of WMN
cells. A typical Class II response showing minimal arrest in GÌ;
nonetheless arrest in GT following irradiation is illustrated in the case of CA46 cells. Flow cytometric determination
of cell cycle was performed hy staining the DNA with propidium iodide (PI, shown on the FL2 axis) and DNA synthesis was monitored using an fluorescein isothiocyanate labeled
anti-hromodeoxyuridine antibody (FITC shown on the FLÌ
axis).
Results and Discussion
In the present study we assessed the role of Ihe p53 tumor suppres
sor gene in cell cycle arrest and radiosensitivity of 17 Burkitt's lymphoma and lymphoblastoid cell lines. The purpose of these studies
was to determine whether the presence of a normal p53 gene was
essential for cell cycle arrest in G, following DNA damage and
whether the functional status of the p53 gene product would correlate
with the sensitivity of cells to ionizing radiation.
The ability of cells to arrest in G, following radiation was assessed
by flow cytometry of cells 16 h following irradiation of exponentially
growing cultures with 6.3 Gy. This experiment was performed with
coded samples so that the operators performing the analysis were
unaware of the p53 status of the cell lines until the results were
completed. Fig. 1 shows the results gathered from the 17 cell lines
tested. As a matter of convenience, the cell lines were divided into
three classes on the basis of their G! arrest responses: Class I, strong
arrest in G, following radiation (>60% of the original population);
Class II, minimal arrest in G, following radiation (<25% of the
original population); Class III, an intermediate response. Incubation of
cells with the mitotic inhibitor, nocodazole, following radiation (to
trap any cells that might leak from G2 into G, of the next cell cycle)
did not significantly alter the results, nor did increasing the radiation
dose by a factor of 2. These findings confirmed that cells observed in
G, at 16 h following radiation were arrested in G, and did not
represent cells leaking from G2 of the previous cell cycle. For illus
trative purposes Fig. 2 has been included to show the contrasting G,
responses of Class I and II cells following radiation. Both Class I and
II cells showed clearing of the S-phase population as evidenced by
decreased bromodeoxyuridine staining and arrest in G2 phase of the
cell cycle.
When the coded samples were indexed according to their p53 status
(Table 1), all of the five lines (29%) that showed strong G, arrest
contained only normal p53 genes. Of the ten lines (59%) that showed
minimal G, arrest, eight had only mutant p53 alÃ-eles,and two lines
were heterozygous, containing both mutant and normal p53 alÃ-eles.
The remaining two cell lines (12%) showed an intermediate response:
both of these lines by single-strand conformation polymorphism and
DNA sequencing contained only wild type p53 (23). Using this flow
cytometric approach we were able to predict correctly the status of the
p53 gene in 88% of the cases analyzed (15 correct of 17 tested). This
approach therefore provides a simple means to assess the functional
status of p53 in cultured cell lines.
We found that the presence of Epstein-Barr virus in several of the
wild type p53 lines tested did not significantly alter the ability of cells
to arrest in GI following irradiation (compare Fig. 1 with Table 1).
This is contrary to the inhibition of p53 function observed in cells
infected with SV40 and human papilloma virus (1-3, 25).
Our results are consistent with previous observations suggesting
that mutations in the p53 gene prevent cells from arresting in G]
following irradiation (17, 18). The behavior of the two lines that were
heterozygous for p53 suggests that the mutant protein has a dominant
negative influence upon the activity of the wild type p53 alÃ-ele.In the
seven cases where p53 was determined to be wild type, two lines,
JLP119 and EW36, showed a reduced ability to arrest in G! following
radiation. This suggested that JLP119 and EW36 cells have a defect in
another part of the pathway that controls G, arrest following ionizing
radiation. Inactivation of p53 in these lines is not due to overexpression of MDM2 (7), since MDM2 mRNA levels differed less than
2-fold among the lines tested.3 In an attempt to discover the cause of
this defect we analyzed the response of the p53 protein to ionizing
radiation. DNA damage induced by either ionizing radiation or UV
light leads to accumulation of the wild type p53 protein through a
mechanism that involves stabilization of the protein (17, 18, 26). Fig.
3A shows that exponentially growing cells expressing wild-type p53
! K. Bhatia and I. Magrath, unpublished observations.
4778
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ROLE OF pS3 IN RADIOSENSITIVITY
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Fig. 3. Determination of p53 protein levels ¡nBurkitt's lymphoma and lymphoblastoid cell lines. A, exponentially growing cells were lysed and 75 /xg of total cell protein from each
cell line were electrophoresed on 10% sodium dodecyl sulfate-polyacrylamide gels, Western blotted, and probed with monoclonal antibodies raised against two independent epitope
sites on the p53 protein (Oncogene Science). Antibody reaction was disclosed using the Amersham enhanced chemilumincscence detection system as described previously (22). B, effect
of radiation on p53 protein levels. Exponentially growing cells were irradiated (6.3 Gy) and then incubaled for 4 h at 37°Cbefore cell lysis. Seventy-five (¿gof total cell protein from
each treatment was blotted and then probed with the monoclonal antibody PAblSOl (Oncogene Science). Along the top of ßis the p53 status of each cell line (data shown in Table
1) and the strength of G, arrest following radiation (data shown in Fig. 1).
produce barely detectable levels of the protein in Western blotting. By
4 h following radiation, however, the abundance of the p53 protein
increased markedly in three examples of Class I cells (Fig. 3fl). The
two cell lines that by our sequencing criteria had normal p53 alÃ-eles
but exhibited reduced ability to arrest in G, following radiation
showed either no increase (JLP119) or only a small increase (EW36)
in basal p53 protein levels following irradiation. The mechanism
responsible for p53 stabilization is therefore defective in these lines.
Analogous observations have been made in certain ataxia-telangiectasia cell lines (9). We are presently endeavoring to characterize this
defect in more detail.
Also shown in Fig. 3 is the response of four cell lines that are
representative of Class II cells that exhibited minimal arrest in G,
following irradiation (Fig. 1). Three of these cell lines (CA46, Ramos,
and MCI 16), like most mutant p53 lines we have tested, exhibited
constitutively elevated levels of the mutant p53 protein compared to
the wild type lines (Fig. 3). These mutant p53 lines did not show any
further accumulation of the p53 protein following radiation. Some cell
lines (AKUA, HWL, and P3HR1) which had mutant p53 alÃ-elesdid
not exhibit constitutively elevated p53 protein levels. It therefore
seems that not all mutations in lymphoid tumors prolong the half-life
of the protein (23). When we tested the p53 response of the AKUA and
HWL lines to radiation we observed a significant accumulation of the
mutant p53 protein (Fig. 3ß),indicating that the mechanism respon
sible for stabilization of p53 is normal in these cells, although G,
arrest is defective.
The cytotoxicity of ionizing radiation in the 17 lines was assessed
in 72-96-h growth inhibition assays as described in "Materials and
Methods." The radiation dose that inhibited proliferation of the Class
1 wild type p53 cell lines by 50% of the control clustered within a
range of 0.68-1.3 Gy (mean, 0.98 Gy; compare Figs. 1 and 4). Cell
size determination of cultures throughout the assay revealed a dose
and time dependent accumulation of cellular debris consistent with the
cytotoxic action of the radiation exposure (results not shown). Of the
eight p53 mutant cell lines tested, five lines required 2.35-3.73 Gy of
radiation (mean, 2.89 Gy) to cause a 50% inhibition of cell prolifera
tion. The two lines that were heterozygous for p53 mutations also
required higher radiation doses to kill an equivalent number of cells
compared to the Class I cells (ID5II = 2.1 and 2.9 Gy for AKUA and
ST486, respectively). The two cell lines that appeared to have normal
p53 alÃ-elesbut partiaily impaired G, delay, exhibited radiosensitivity
that was intermediate (JLP119 ID5() = 1.55 Gy, EW36 ID.,,, = 1.93
Gy) between Class I (mean, 0.98 Gy) and Class II (mean, 2.89 Gy)
cells. Thus, the majority of lines expressing either mutant or partially
active forms of p53 were more resistant to radiation (approximately
2-3-fold) compared to their Class I counterparts. Our studies suggest
a positive correlation between the ability of cells to escape G, arrest
and radioresistance (compare Figs. 1 and 4). Three of the 17 cell lines
tested did not comply with this relationship. MCI 16, HWL, and
P3HR1 all expressed mutant p53 and failed to arrest in G, following
radiation but were as radiosensitive as the Class I cells. We do not yet
know why these mutant p53 lines differed from the other Class II
cells. However, it is not unreasonable to suspect that factors other than
p53 will have a bearing on the outcome of radiation exposure.
A potential explanation for the radioresistance observed in our
studies might be afforded by loss of function of the wild type p53
protein, which would normally modulate the transcriptional activity of
genes to induce a G, arrest and/or promote cell death in the presence
of DNA damage. In agreement with this suggestion, the thymocytes of
transgenic mice having no p53 alÃ-elesare more resistant to apoptosis
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ROLE OF p53
IN RADIOSENSITIVITY
12: 2866-2871, 1992.
Mack, D. H., Vartikar, J., Pipas, J. M., and Laimins, L. A. Specific repression of
TATA-mediated but not initiator-mediated transcription by wild-type p53. Nature
(Lond.), 363: 281-283, 1993.
Xiangwei, Wu., Bayle, J. H., Olson, D., and Levine, A. J. The p53-mdm-2 auloregulatory feedback loop. Genes & Dev., 7: 1126-1132, 1993.
Fornace, A. J., Jr., Alamo, I., Jr., and Hollander, M. C. DNA damage-inducible
transcripts in mammalian cells. Proc. Nati. Acad. Sci. USA, 85: 8800-8804, 1988.
Kastan, M. B., Zhan, Q., El-Deiry, W. S., Carrier, F., Jacks, T., Walsh, W. V, Plunkett,
B., Vogelstein, B., and Fornace, A. J., Jr. A mammalian cell cycle checkpoint
utilizing p53 and GADD45 is defective in ataxia telangiectasia. Cell, 71: 587-597,
1992.
Hainaut, P., and Milner, J. A structural role for metal ions in the "wild-type" confor
•¿3.75—
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Fig. 4. Relationship between p53 status and radiosensitivity of a panel of Burkitt's
lymphoma cell lines. Exponentially growing cells (2—3X lOs/ml) were irradiated (0.7912.6 Gy) at a dose rate of 5.25 Gy/min at room temperature and then incubated for up to
96 h at 37°C.Cell counts and cell sizes were recorded every 24 h following irradiation as
described in "Materials and Methods." Values shown are the dose of radiation that caused
a 50% reduction in proliferation over the assay period. Each point represents the mean of
two independent experiments in which duplicate measurements were made within each
experiment. Along the x axis is shown the p53 status of each cell line (data shown in Table
1) and the strength of GÌ
arrest following radiation (data shown in Fig. I).
induced by radiation (27, 28). Wild type p53 has also been implicated
in triggering apoptosis in the presence of a constitutively active myc
gene (Ref. 29 and references therein). Since Burkitt's lymphomas
constitutively express c-myc due to a chromosomal translocation (24,
30), the connection between wild type p53, c-myc, and apoptosis
might be broken by loss of function mutations in the p53 gene. This
might in turn render cells more resistant to radiation. We are presently
exploring the relationship between p53 and apoptosis in this system.
In summary, our findings suggest that the ability of p53 to induce
G] arrest following ionizing radiation correlates, in most cases, with
the radiosensitivity of Burkitt's lymphoma or lymphoblastoid cell
lines. The p53 protein in these cells, therefore, operates in a role that
is contrary to the actions of RAD9 at the G2 checkpoint. Activation of
the G2 checkpoint tends to protect cells from the cytotoxicity of DNA
damaging agents (19, 21), while activation of the G, checkpoint
appears to increase the cytotoxicity of ionizing radiation, at least in
cells that have deregulated c-myc. Dissection of the downstream tar
gets of p53 action in G, arrest and apoptosis may provide useful
information for the development of new chemotherapeutic approaches
to kill cancer cells selectively.
References
1. Hollstein, M.. Sidransky. D., Vogelstein. B., and Harris, C. C. p53 mutations in human
cancers. Science (Washington DC), 253.' 49-52, 1991.
2. Vogelstein, B., and Kinzler, K. W. p53 function and dysfunction. Cell, 70: 523-526,
1992.
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Downloaded from cancerres.aacrjournals.org on June 16, 2017. © 1993 American Association for Cancer Research.
Role of the p53 Tumor Suppressor Gene in Cell Cycle Arrest
and Radiosensitivity of Burkitt's Lymphoma Cell Lines
Patrick M. O'Connor, Joany Jackman, Daniel Jondle, et al.
Cancer Res 1993;53:4776-4780.
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