© 1999 by Oxford University Press
Journal of the National Cancer Institute, Vol. 91, No. 22, 1910-1911,
November 17, 1999
© 1999 Oxford University Press
EDITORIALS |
Radiosensitivity and Transcription Factor NF-
B Inhibition—Progress and Pitfalls
Correspondence to: Anatoly Dritschilo, M.D., Department of Radiation Medicine, Georgetown University Medical Center, TRB E202A-B, 3970 Reservoir Rd., N.W., Washington, DC 20007 (e-mail: dritscha{at}gunet.georgetown.edu).
Cellular responses to ionizing radiation include activation of
signal transduction cascades that may originate at the plasma membrane,
cytoplasm, or nucleus. The cell's success in dealing with radiation
"stress" determines its survival or death. Activation of
transcription factor NF-
B, an immediate early response after
exposure to ionizing radiations, functions to protect cells from
apoptosis (programmed cell death), but the role of NF-B in mitotic
cell death has not been fully defined (1,2). NF-B is
constitutively activated in lymphoid cells, immortal ataxia
telangiectasia (AT) fibroblasts, and other cells (3-6).
Activation of NF-B after ionizing radiation requires intact ATM
gene function in AT fibroblasts (7). Because AT fibroblasts
exhibit dysregulated NF-B activation and extreme radiation
sensitivity, it is reasonable to ask if radiation responses of tumor
cells can be modified by inhibiting NF-B.
NF-B/Rel transcription factors are activated by a variety of different signals in pathways
that converge on the phosphorylation and degradation of IBs, inhibitors of NF-B. This
results in the unmasking of the nuclear localization signals that lead to translocation of
NF-B/Rel dimers into the nucleus. Five known mammalian NF-B/Rel proteins include
c-Rel, p65 (RelA), RelB, p50 (NF-B1), and p52 (NF-B2) that bind DNA as
homodimers or heterodimers (8). It should also be noted that
p50-p65/IB is not the only complex relevant to NF-B activation. The p50-p65
heterodimer can interact with other IB family members, and NF-B can also exist in
other homodimer or heterodimer complexes, such as p50-p50 and p65-p65 (8). Furthermore, the IB family consists of three members—IB
,
IBß, and IB
. In addition, the carboxyl-terminal regions of the precursors for
p50 and p52, p105 and p100, respectively, can also function as inhibitory proteins. These
inhibitors respond differentially to NF-B-inducing signals in a cell type-dependent and
stimulus-dependent manner in vivo (9).
In this issue of the Journal, Pajonk et al. (10) report that activation of
NF-B does not determine the intrinsic radiosensitivity of cancer cells. Cells were transduced
with an adenoviral vector expressing a mutant form of IB, an inhibitor of NF-B.
The resultant clonogenicity of cells transduced with mutant IB was markedly reduced
to 7.4% (compared with 29.5% for control cells), and the apoptotic index was
90%. Therefore, subsequent radiation survival experiments were done on relatively small
subsets of cells, whereas biochemical assays were done on mostly apoptotic cells.
It is unusual to see publication of essentially negative studies in the Journal. However, there is potential value if definitive data can provide direction for other scientists in the field. Clinically, intrinsic radiation sensitivity has been implicated in tumor curability (11). The ability to manipulate radiation sensitivity offers a potential strategy for improving the therapeutic ratio in cancer treatment. Therefore, an examination of the experimental results and data interpretation underlying the negative radiobiologic conclusions of Pajonk et al. (10) are appropriate.
- 1) Are PC3 and HD-MyZ cells resistant to radiation? Pajonk et al. (10) state that the two cell lines were chosen because they have high constitutive levels
of NF-
B that might confer relatively high resistance to radiation. Strictly speaking, radiation
resistance is an imprecise term that may include various aspects of cellular responses to ionizing
radiation (12). On the other hand, radiation sensitivity is defined by the
steepness of the terminal slope of the cellular radiation survival curve (single-hit multitarget
model). A Do value (the dose required to decrease survival by 1/e)
can be determined for relative quantitation of radiation sensitivities. Do
values for PC3 and HD-MyZ cells are estimated to be 1.0-1.1 Gy [Fig. 3 in (10)], which is quite sensitive for tumor cell lines and is comparable in sensitivity to
AT heterozygote fibroblasts (13). Disruption of signal transduction
pathways of resistant cells has been reported to result in radiosensitization (14), but it may be difficult to make already sensitive cells more sensitive. An alternative
interpretation of the constitutively activated NF-B levels observed in PC3 and HD-MyZ
cells may be related to apoptotic stresses rather than reflecting potential resistance. This is
consistent with subsequent observations of Pajonk et al. (10) that
90% of these cells die of apoptosis when mutant IB- is expressed, inhibiting
protective functions of activated NF-B. 2) Does the I
B superrepressor product inhibit radiation-induced NF-B
activation in the small subset of transduced cells that do not undergo apoptosis? Gel-shift
experiments are offered by Pajonk et al. (10) showing DNA binding by
NF-B to be present in cytosolic extracts from control cells but not from
Ad5-IB-transduced cells that were either given 30 Gy of ionizing radiation or not. These
experiments were performed 48 hours after transduction of cells and are interpreted to show that
the mutated IB gene product inhibits constitutive NF-B activity and radiation-induced
activation. An alternative interpretation of these data may be that cytosolic extracts obtained from
still viable control cells differ from those obtained from the Ad5-IB-transduced cells that
were mostly dead. The substantial disparity of surviving cell fractions makes the reported
biochemical comparisons in survivors unreliable. Furthermore, the use of cytosolic extracts from
cells undergoing apoptosis must include consideration of possible effects of caspases targeting
components of NF-B (15).
In summary, the findings of Pajonk et al. (10)
should be interpreted with caution. These data show that mutant
IB expression in these cells results in cell death by
apoptosis. What they do not provide is insight into the role of
NF-B regulation in intrinsic radiation sensitivity.
REFERENCES
1 Fuks Z, Haimovitz-Friedman A, Hallahan DE, Kufe DW, Weichselbaum RR. Stress response genes induced in mammalian cells by ionizing radiation. Radiat Oncol Invest 1993;1:81-93.
2
Wang CY, Mayo MY, Baldwin AS Jr. TNF- and cancer
therapy-induced apoptosis: potentiation by inhibition of NF-B. Science 1996;274:784-7.
3
Baeuerle PA, Henkel T. Function and activation of NF-B in
the immune system. Annu Rev Immunol 1994:12:141-79.[ISI][Medline]
4
Jung M, Zhang Y, Lee SA, Dritschilo A. Correction of radiation
sensitivity in ataxia telangiectasia cells by truncated IB-. Science 1995;268:1619-21.
5 Krappmann D, Emmerich F, Kordes U, Scharschmidt E, Dorken B, Scheidereit C. Molecular mechanisms of constitutive NF-kappaB/Rel activation in Hodgkin/Reed-Sternberg cells. Oncogene 1999;18:943-53.[CrossRef][ISI][Medline]cancerlit;99146275
6
Wang W, Abbruzzese JL, Evans DB, Larry L, Cleary KR, Chiao
PJ. The nuclear factor-kappa B RelA transcription factor is constitutively activated in human
pancreatic adenocarcinoma cells. Clin Cancer Res 1999;5:119-27.
7 Lee SJ, Dimtchev A, Lavin MF, Dritschilo A, Jung M. A novel ionizing radiation-induced signaling pathway that activates the transcription factor NF-kappaB. Oncogene 1998;17:1821-6.[CrossRef][ISI][Medline]cancerlit;98449478
8
Thanos D, Maniatis J. NF-B: a lesson in family values. Cell 1995;80:529-32.[CrossRef][ISI][Medline]cancerlit;95171447
9
Simeonidis S, Stauber D, Chen G, Hendrickson WA, Thanos D.
Mechanisms by which IB proteins control NF-B activity. Proc Natl Acad Sci U
S A 1999;96:49-54.
10
Pajonk F, Pajonk K, McBride WH. Inhibition of NF-B,
clonogenicity, and radiosensitivity of human cancer cells. J Natl Cancer Inst 1999;91:1956-60.
11 Weichselbaum RR, Beckett MA, Schwartz JL, Dritschilo A. Radioresistant tumor cells are present in head and neck carcinomas that recur after radiotherapy. Int J Radiat Oncol Biol Phys 1988:15:575-9.[ISI][Medline]
12 Jung M, Dritschilo A. Signal transduction and cellular responses to ionizing radiation. Semin Radiat Oncol 1996;6:268-72.[CrossRef][Medline]
13 Dahlberg WK, Little JB. Response of dermal fibroblast cultures from patients with unusually severe responses to radiotherapy and from ataxia telangiectasia heterozygotes to fractionated radiation. Clin Cancer Res 1995;1:785-90.[Abstract]cancerlit;99034878
14
Kasid U, Pfeifer A, Brennan T, Beckett M, Weichselbaum RR,
Dritschilo A, et al. Effect of antisense c-raf-1 on tumorigenicity and radiation-sensitivity of a
human squamous carcinoma. Science 1989;243:1354-6.
15
Ravi R, Bedi A, Fuchs EJ, Bedi A. CD95 (Fas)-induced
caspase-mediated proteolysis of NF-B. Cancer Res 1998;58:882-6.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  A. Munshi, J. F. Kurland, T. Nishikawa, P. J. Chiao, M. Andreeff, and R. E. Meyn Inhibition of constitutively activated nuclear factor-{kappa}B radiosensitizes human melanoma cells Mol. Cancer Ther., August 1, 2004; 3(8): 985 - 992. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. M. Kim, Y. Zhang, B.-Y. Kim, S. J. Jeong, S. A. Lee, G.-D. Kim, A. Dritschilo, and M. Jung The p65 subunit of nuclear factor-{kappa}B is a molecular target for radiation sensitization of human squamous carcinoma cells Mol. Cancer Ther., June 1, 2004; 3(6): 693 - 698. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. G. Hillman, J. D. Forman, O. Kucuk, M. Yudelev, R. L. Maughan, J. Rubio, A. Layer, S. Tekyi-Mensah, J. Abrams, and F. H. Sarkar Genistein Potentiates the Radiation Effect on Prostate Carcinoma Cells Clin. Cancer Res., February 1, 2001; 7(2): 382 - 390. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||