The ongoing revolution in tumor, cellular, molecular, and structural
biology is having a profound impact on the approaches that will
be taken in cancer therapy. The current rate of generation of
new knowledge is extraordinarily rapid, and the steady elucidation
of the processes involved in tumor development and progression
is revealing a wide range of new possibilities for the prevention
and treatment of cancer. Advances in the understanding of the
principles underlying radiation biology have paralleled those
in other basic science fields. Furthermore, many concepts that
previously had been investigated primarily in radiation biology
laboratories, such as DNA repair, cell-cycle perturbation, cellular
stress response, and the tumor microenvironment, including tumor
and stromal interaction, are of major interest to a wide range
This paper will focus on emerging biologic concepts that are of
interest to the oncology practitioner. Conceptually, it will focus
on translational research--the bridge between the basic science
laboratory and the clinic. Given the limits of space and the rapid
growth of new knowledge, it is recognized that this paper provides
only a small sample of the ongoing developments in radiation oncology
and biology. These developments are taking place in parallel with
new technical advances, such as conformal radiation therapy, improved
brachytherapy techniques, new hyperthermia treatment capabilities,
and particle-based radiotherapy .
Figure 1 is a schematic illustrating the current scope of radiation
biology research. The biotechnical revolution of the 1980s brought
new complexity and understanding to the field of radiation biology,
as compared with the previous "classical" models. These
classical models, however, are still of interest and importance
to the clinician and scientist.
Classical chemotherapy models have established the importance
of dose and the use of non-cross-resistant agents. Likewise, the
classical radiation models are of use in clinical investigation
and practice. These include:
- the understanding of the shape of the radiation cell-survival
curves of tumors and normal tissues, which serves as the basis
for altered fractionation (eg, hyperfractionation);
- in vitro assays of clonogenic tumor cell survival and tumor
growth kinetics as possible predictors of treatment outcome;
- tumor hypoxia as a predictor of overall tumor response to
- cell-cycle effects and DNA damage repair, which have led to
the development of combined-modality regimens with radiation and
Therefore, even though the mechanisms behind the models are not
fully understood, they serve as useful scientific and clinical
The processes illustrated in Figure 1 will serve as the basis
for the discussions that follow. The potential clinical importance
of each of these processes also will be reviewed.
DNA is felt to be the primary target and mediator of radiation-induced
cell killing [2,3]. Ionizing radiation deposits energy in a distribution
that results in closely spaced damage, termed "locally multiply
damaged sites."  Many varieties of DNA base damage and
cross-links are produced [2,3], which lead to both single- and
double-strand breaks. The accumulation of unrepaired double-strand
breaks correlates closely with the loss of cell survival .
Within a double-strand break, the etiology of the damage on each
of the strands of DNA may be different, including a deletion on
one strand, and a repair-induced break produced by an endonuclease
on the other . Radiation-induced DNA lesions can undergo enzymatic
repair involving base excision repair [2,3] or recombination [4,5].
The specific enzymes involved are being identified, and include
enzymes that recognize, excise, and ligate damaged regions; those
with DNA helicase activity ; and others involved in DNA recombination
DNA Repair Deficiencies
DNA repair deficiencies are often identified from mutant cell
lines or from patients with clinical syndromes characterized by
sensitivity to x-ray-induced DNA damage, such as ataxia-telangiectasia
[6,7] and severe combined immunodeficiency (SCID) [4,5]. Patients
with defective DNA repair have an increased sensitivity to radiation-induced
damage, a phenomenon that can be observed in vitro using clonogenic
assays on irradiated fibroblasts derived from affected individuals
Another type of repair recently determined to be important in
genetic hypermutability and cancer progression is called mismatch
repair . This surge in new knowledge of the various types of
DNA repair should lead to a better understanding of tumor progression,
and the cellular response to radiation.
Of equal significance to the clinician and biologist is the shape
of the radiation survival curve for each cell type of interest;
with sensitive and resistant cells showing more or less cell killing,
respectively, for a given dose of radiation. The terms commonly
used to define cell survival are D0, the slope of the survival
curve, or SF 2 Gy, the surviving fraction of cells in culture
after a clinically relevant dose of 2 Gy.
To date, the clinical syndromes associated with abnormal DNA repair
and the mutant cell lines are characterized by decreased repair
capacity and not by excessively proficient repair. This is of
importance in the search for the causes of clinical resistance
to radiation and in the development of predictive assays for use
in the clinic. In addition to the shape of the radiation cell-survival
curve, other possible determinants of clinical resistance include:
the number of clonogenic cells in the tumor, the ability to undergo
apoptosis, and microenvironmental factors. These factors and clinical
predictive assays are discussed below.
Given that radiation induces changes in cellular phenotype, such
as growth and cell-cycle arrest, it is logical to assume that
it would also lead to alterations in gene expression. The expression
of a gene can be altered in a number of ways, including: gene
induction (production of new mRNA), stabilization of preexisting
mRNA, or post-translational modification (stabilization) of a
A range of genes and gene products have been shown to be induced
following x-irradiation to a cell. These include inflammatory
response molecules, such as tumor necrosis factor (TNF) and interleukin-1
(IL-1); growth factors, such as epidermal growth factor (EGF);
cell-cycle-related genes (gadd genes); and cell-surface
receptor genes (integrins), to name a few examples [10-13].
Studies demonstrating gene induction have been carried out at
a wide range of radiation doses. High radiation doses are often
used to highlight an effect in the laboratory; however, these
doses are often in a range at which few, if any, cells remain
viable. Therefore, the gene induction observed in these experiments
may not necessarily be clinically relevant to surviving cells
within a tumor. The importance of defining the sequence of genes
induced by irradiation at clinically relevant conditions is obvious,
as it will lead not only to the understanding of how a cell responds
to x-rays, but also to novel therapeutic strategies to alter that
One way to study the effects of x-rays on gene expression is to
study the control regions of the genes, looking for radiation
responsive elements (RRE). Genes contain regulatory domains that
are proximal or upstream to the transcription site, which is the
starting site for mRNA synthesis. Whether a gene is induced or
repressed following cellular radiation depends on which elements
in the control region of the gene are bound by specific regulatory
Genes are activated when proteins called transcription factors
bind to their proper sequence in the control region of the gene.
Similarly, other bound factors can repress or inhibit gene expression.
Several transcription factors have been shown to be activated
by ionizing radiation, including: AP-1 factors (fos, jun), EGR-1,
and the c-rel-family (NF kappa beta). These transcription factors,
in turn, can act on numerous downstream target genes.
Activated transcription factors characteristically form oligomers
prior to binding to the promoter region of the gene. The AP-1
group of transcription factors form two component complexes termed
"dimers." Homodimers are from the same subgroup (jun-jun),
and heterodimers contain factors from two subgroups (fos-jun).
The specific oligomers that are induced may determine whether
or not a transcription factor will activate one gene or another.
Furthermore, genes contain many transcription-factor binding sites,
so that activation often requires multiple factors from a range
of transcription factor groups.
Genes may be activated in sequence. Early response genes, such
as the AP-1 regulated genes, produce a protein that is itself
a transcription factor for a secondary gene, which, in turn, may
produce a cytokine, structural protein, or enzyme. Since radiation
is just one of a whole range of stimuli to which a cell is exposed,
it is not likely that there will be a specific RRE on either the
primary or secondary genes; however, some combination of response
elements may be more readily induced by x-rays than are others--a
concept that has led to the development of genetic radiotherapy.
In genetic radiotherapy, a promoter with a radiation-activated
sequence is attached to a gene that can make a product that would
enhance the efficacy of radiation. One sequence of interest has
been the serum response element (SRE), which is in the promoter
region of the EGR1 gene, an "immediate early response
gene. " This promoter region has been attached to the
TNF-alpha gene, so that when a cell containing the EGR1-TNF-alpha
construct is irradiated, TNF is produced. When xenograft murine
tumors containing the transfected EGR1-TNF-alpha construct
were irradiated with a single large dose of 40 Gy, an improved
cure rate was demonstrated .
The efficacy of genetic radiation therapy, like all forms of gene
therapy, will depend on sufficient gene product being made to
achieve the desired effect. If used as a radiation modifier, sufficient
gene product (eg, TNF) must be induced by a clinically relevant
radiation dose (~2 Gy), the timing of gene activation must be
in proper relationship to the x-ray treatment to provide sensitization,
and the gene construct would need some extended stability to last
throughout a course of ionizing radiation. Since most promoters
can be activated by many different stimuli, the release of the
radiation-induced gene product may not be tightly controllable.
This may not necessarily be a disadvantage, as other aspects of
the tumor microenvironment may help activate the RRE. Despite
the many hurdles between a laboratory concept and a clinically
useful therapy, these early studies are important steps in assessing
the ultimate utility of this approach.
1. Lichter AS, Lawrence TS: Recent advances in radiation oncology.
N Engl J Med 332:371-379, 1995.
2. Oleinick NL: Ionizing radiation damage to DNA: Molecular aspects
(symposium summary). Radiat Res 124:1-6, 1990.
3. Powell S, McMillan TJ: DNA damage and repair following treatment
with ionizing radiation. Radiother Oncol 19:95-108, 1990.
4. Kirchgessner CU, Patil CK, Evans JW, et al: DNA-dependent kinase
(p350) as a candidate gene for the murine SCID defect. Science
5. Lees-Miller SP, Godbout R, Chan DW, et al: Absense of p350
subunit of DNA-activated protein kinase from a radiosensitive
human cell line. Science 267:1183-1185, 1995.
6. Savitsky K, Bar-Shira A, Gilad S, et al: A single ataxia telangiectasis
gene with a product similar to PI-3 kinase. Science 268:1749-1753,
7. Shafman TD, Saleem A, Kyriakis J, et al: Defective induction
of stress-activated kinase activity in ataxia-telangiectasia cells
exposed to ionizing radiation. Cancer Res 55:3242-3245, 1995.
8. Badie C, Iliakis G, Foray N, et al: Defective repair of DNA
double-strand breaks and chromosome damage in fibroblasts from
a radiosensitive leukemia patient. Cancer Res 55:1232-1234, 1995.
9. Parsons R, Li GM, Longley M, et al: Mismatch repair deficiency
in phenotypically normal human cells. Science 268:738-740, 1995.
10. Weichselbaum RR, Hallahan D, Fuks Z, et al: Radiation induction
of immediate early genes: Effectors of the radiation-stress response.
Int J Radiat Oncol Biol Phys 30:229-234, 1994.
11. Schmidt-Ullrich RK, Valerie KC, Cahn W, et al: Altered expression
of epidermal growth factor receptor and estrogen receptor in MCF-7
cells after single and repeated radiation exposures. Int J Radiat
Oncol Biol Phys 29:813-819, 1994.
12. Onoda JM, Piechocki MP, Honn KV: Radiation-induced increase
in expression of the alpha I-IB beta 3 integrin in melanoma cells:
Effects on metastatic potential. Radiat Res 130:281-288, 1992.
13. Todd DG, Mikkelsen RB: Ionizing radiation induces a transient
increase in cytosolic free [Ca2+] in human epithelial tumor cells.
Cancer Res 54:5224-5230, 1994.
14. Datta R, Rubin E, Sukhatme V, et al: Ionizing radiation activates
transcription of the EGR1 gene via CArG elements. Proc Natl Acad
Sci (USA) 89:10149-10153, 1995.
15. Weichselbaum RR, Hallahan DE, Beckett MA, et al: Gene therapy
targeted by radiation preferentially radiosensitizes tumor cells.
Cancer Res 54:4266-4269, 1994.
16. Maity A, McKenna WG, Muschel RJ: The molecular basis for cell-cycle
delays following ionizing radiation: A review. Radiother Oncol
17. Khaarbanda S, Saleen A, Datta R, et al: Ionizing radiation
induces rapid tyrosine phosphorylation of p34cdc2. Cancer
Res 54:1412-1414, 1994.
18. Murray A: Cyclin ubiquitination: The destructive end of mitosis.
Cell 81:149-152, 1995.
19. Murray AW: Creative blocks: Cell-cycle checkpoints and feedback
controls. Nature 359:599-604, 1992.
20. Chang R, Syrjänen S, Syrjänen K: Implications of
the p53 tumor-suppressor gene in clinical oncology. J Clin Oncol
21. Lowe SW, Ruley HE, Jacks T, et al: p53-dependent apoptosis
modulates the cytotoxicity of anticancer agents. Cell 74:957-967,
22. McIlwrath AJ, Vasey PA, Ross GM, et al: Cell-cycle arrests
and radiosensitivity of tumor cell lines: Dependence on wild-type
p53 for radiosensitivity. Cancer Res 54:3718-3722, 1994.
23. Nagasawa H, Li CY, Maki CG, et al: Relationship between radiation-induced
G1 phase arrest and p53 function in human tumor cells. Cancer
Res 55:1842-1846, 1995.
24. Stewart BW: Mechanisms of apoptosis: Integration of genetic,
biochemical, and cellular indicators. J Natl Cancer Inst 86:1286-1296,
25. D'Amico AV, McKenna WG: Apoptosis and a reinvestigation of
the biologic basis for cancer therapy. Radiother Oncol 33:3-10,
26. Fisher DE: Apoptosis in cancer therapy: Crossing the threshold.
Cell 78:539-542, 1994.
27. Stellar H: Mechanisms and genes of cellular suicide. Science
28. Thompson CB: Apoptosis in the pathogenesis and treatment of
disease. Science 267:1456-1461, 1995.
29. Oltvai AN, Korsmeyer SJ: Checkpoints of dueling dimers foil
death wishes. Cell 79:189-192, 1994.
30. Shimizu S, Eguchi Y, Kosaka H, et al: Prevention of hypoxia-induced
cell-death by Bcl-2 and Bcl-xL. Nature 374:811-814, 1995.
31. Yao KS, Clayton M, O'Dwyer PJ: Apoptosis in human adenocarcinoma
HT29 cells induced by exposure to hypoxia. J Natl Cancer Inst
32. Jacobson MD, Raff MC: Programmed cell-death and Bcl-2 protection
in very low oxygen. Nature 374:814-816, 1995.
33. Chen M, Quintans J, Fuks Z, et al: Suppression of Bcl-2 messenger
RNA production may mediate apoptosis after ionizing radiation,
tumor necrosis factor alpha and ceramide. Cancer Res 55:991-994,
34. Langley RE, Palayoor ST, Coleman CN, et al: Modifiers of radiation-induced
apoptosis. Radiat Res 136:320-326, 1993.
35. Bump EA, Braunhut SJ, Palayoor ST, et al: Novel concepts in
modification of radiation sensitivity. Int J Radiat Oncol Biol
Phys 29:249-253, 1994.
36. Hill CS, Treisman R: Transcriptional regulation by extracellular
signals: Mechanisms and specificity. Cell 80:199-211, 1995.
37. Stevenson MA, Pollock SS, Coleman CN, et al: X-irradiation,
phorbol esters, and H2O2 stimulate mitogen-activated protein kinase
activity in NIH-3T3 cells through the formation of reactive oxygen
intermediates. Cancer Res 54:12-15, 1994.
38. Kharbanda S, Ruibao R, Pandey P, et al: Activation of the
c-Abl tyrosine kinase in the stress response to DNA-damaging agents.
Nature 376:785-788, 1995.
39. Hallahan DE, Virudachalam S, Sherman ML, et al: Tumor necrosis
factor gene expression is mediated by protein kinase C following
activation by ionizing radiation. Cancer Res 51:4565-4569, 1991.
40. Mohan N, Meltz ML: Induction of nuclear factor kappa beta
after low-dose ionizing radiation involves a reactive oxygen intermediate
signaling pathway. Radiat Res 140:97-104, 1994.
41. Hallahan DE, Virudachalan S, Schwartz JL, et al: Inhibition
of protein kinases sensitizes human tumor cells to ionizing radiation.
Radiat Res 129:345-350, 1992.
42. Fuks Z, Persaud RS, Alfieri A, et al: Basic fibroblast growth
factor protects endothelial cells against radiation-induced programmed
cell-death. Cancer Res 54:2582-2590, 1994.
43. Haimovitz-Friedman A, Balaban N, McLoughlin M, et al: Protein
kinase C mediates basic fibroblast growth factor protection of
endothelial cells against radiation-induced apoptosis. Cancer
Res 54:2591-2597, 1994.
44. Tee PG, Travis EL: Basic fibroblast growth factor does not
protect against classical radiation pneumonitis in two strains
of mice. Cancer Res 55:298-302, 1995.
45. Wollman R, Yahalom J, Maxy R, et al: Effect of epidermal growth
factor on the growth and radiation sensitivity of human breast
cancer cells in vitro. Int J Radiat Oncol Biol Phys 30:91-98,
46. Roberts CM, Foulcher E, Zaunders JJ, et al: Radiation pneumonitis:
A possible lymphocyte-mediated hypersensitivity reaction. Ann
Intern Med 118:696-700, 1993.
47. Rubin R, Johnston CJ, Williams JP, et al: A perpetual cascade
of cytokines postirradiation leads to pulmonary fibrosis. Int
J Radiat Oncol Biol Phys 33:99-109, 1995.
48. Coleman CN: Radiation and chemotherapy sensitizers and protectors,
in Chabner BA, Longo DL (eds): Cancer Chemotherapy. Philadelphia,
JB Lippincott, 1996 (in press).
49. Stone HB, Brown JM, Phillips TL, et al: Oxygen in human tumors:
Correlations between methods of measurement and response to therapy.
Radiat Res 136:422-434, 1993.
50. Hlatky L, Tsionou C, Hahnfeldt P, et al: Mammary fibroblasts
may influence breast tumor angiogenesis via hypoxia-induced vascular
endothelial growth factor up-regulation and protein expression.
Cancer Res 54:6083-6086, 1994.
51. Hall EJ: Time, dose, and fractionation in radiotherapy, in
Radiobiology for the Radiologist, pp 211-229. Philadelphia, JB
52. Okunieff P, Morgan DM, Niemierko A, et al: Radiation dose-response
of human tumors. Int J Radiat Oncol Biol Phys 32:1227-1237, 1995.
53. Girinsty T, Lubin R, Pignon JP, et al: Predictive value of
in vitro radiosensitivity parameters in head and neck cancers,
and cervical carcinomas: Preliminary correlations with local control
and overall survival. Int J Radiat Oncol Biol Phys 25:3-7, 1992.
54. Brock WA, Tucker SL, Geara FB, et al: Fibroblast radiosensitivity
versus acute and late normal skin responses in patients treated
for breast cancer. Int J Radiat Oncol Biol Phys 32:1371-1379,
55. Terry NHA, Peter LJ: The predictive value of tumor-cell kinetic
parameters in radiotherapy: Considerations regarding data production
and analysis (editorial). J Clin Oncol 13:1833-1836, 1995.
56. Begg AC: The clinical status of T pot as a predictor? Or why
no tempest in the T pot (editorial)! Int J Radiat Oncol Biol Phys
57. Horiot JC, LeFur R, N'Guyen T, et al: Hyperfractionation versus
conventional fractionation in oropharyngeal carcinoma: Final analysis
of a randomized trial of the EORTC cooperative group of radiotherapy.
Radiother Oncol 25:231-241, 1992.
58. Dobrowsky W, Dobrowsky E, Naudé J, et al: Conventional
versus accelerated fractionation in head and neck cancer. Br J
Cancer, 1996 (in press).
59. Glick J, Kemp G, Rose P, et al: A randomized trial of cyclophosphamide
and cisplatin ± amifostine in the treatment of advanced
epithelial ovarian cancer (abstract). Proc Am Soc Clin Oncol,
60. Overgaard J: Clinical evaluation of nitroimidazoles as modifiers
of hypoxia in solid tumors. Oncol Res 6:509-518, 1994.