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Advances in Brief |
Cancer Research Campaign Colorectal Tumour Biology Research Group, Department of Pathology and Microbiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD [A. C. W., T. J. C., M. M., C. P.], and Department of Surgery, Bristol Royal Infirmary, Bristol BS2 8HW [C. M. P., P. N., J. M. P. H.], United Kingdom
| ABSTRACT |
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-irradiation. These results suggest that
IGFBP-3 enhances the p53-dependent apoptotic response of colorectal
cells to DNA damage. | Introduction |
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| Materials and Methods |
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Cell Lines and Culture Conditions.
AN/C1 and BH/C1 are clonogenic, nontumorigenic human adenoma-derived
cell lines that express wild-type p53 and were grown in conditioned
medium. RG/C2 is a clonogenic, nontumorigenic human adenoma-derived
cell line hemizygous for the p53 gene, with the
remaining allele having a 282 (Arg
Trp) mutation and grown in 20%
FBS DMEM. PC/JW is a human carcinoma derived cell line that is null for
p53 protein expression, routinely cultured in 10% FBS DMEM.
Treatment with 5 Gy of
-Irradiation ± IGFBP-3.
Cells were seeded at 2 x 106 per
flask in duplicate flasks and grown under standard growth conditions
until
60% confluent. Preliminary investigations showed that all of
the cell lines investigated were able to tolerate serum-free growth
conditions (SFM) well for up to 96 h (SFM is standard,
nonconditioned growth medium without the addition of FBS). Therefore,
cell lines were grown for 24 h in SFM to remove IGFBP-3 present
from the serum and then grown for 24 h in SFM supplemented with or
without 100 ng/ml IGFBP-3 (effective reported dose; Ref.
9
). The cells were then exposed to 5 Gy of
rays from a
137Cs source at a dose rate of 0.33 Gy/min in SFM
supplemented with 20 mM HEPES (Sigma) for buffering
(±IGFBP-3), and attached and floating (apoptotic, refer below) cell
yields were determined up to 48 h after irradiation.
Assessment of Apoptosis.
The level of apoptosis in cultured epithelial cell lines was assessed
by measuring the proportion of cells that detached from the monolayer
and were floating in the medium and by determining the fraction of
these floating cells that were apoptotic. The attached and floating
cell populations were stained with 5 µg/ml acridine orange in PBS and
analyzed by fluorescent microscopy for morphological features of
apoptosis [most obviously the characteristically condensed chromatin,
an example of which is shown in Fig. 8 and has been described
previously by Gregory et al. (13)
and Hague
et al. (14)
]. Analysis was carried out by an
experienced observer unaware of the cell type or treatment. The
fraction of floating cells that were apoptotic did not significantly
vary between treated and control untreated cell populations (between 85
and 95% for RG/C2 cells, for example), and therefore the number of
floating cells could be used as a measure of the induction of
apoptosis.
Treatment with 5 Gy of
-Irradiation ±
IR3
Antibody.
To verify that any effect of IGFBP-3 was not attributable to
sequestration of IGF-1, cells were incubated in SFM for 24 h,
followed by SFM plus the anti-IGF-1R antibody
IR3 (Oncogene Research
Products) at concentrations of 1 and 5 µg/ml (15)
for
24 h and then irradiated (5 Gy). The cell yield and floating cell
population in the presence of the
IR3 antibody were determined 24
and 48 h after irradiation.
| Results and Discussion |
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Trp)
mutation. The carcinoma-derived PC/JW cell line is null for p53 protein
expression. Cell lines were grown for 24 h in SFM to remove
IGFBP-3 present from the serum and then grown for up to 3 days in SFM
supplemented with 100 ng/ml IGFBP-3 [as reported by Gill et
al. (9)
]. Preliminary investigations had shown that
all of the cell lines tolerated serum-free growth conditions well for
up to 96 h. For all cell lines tested, the addition of IGFBP-3 to
the culture medium had no effect on cell growth (either cell yield or
number of detached floating cells, which were confirmed to be
apoptotic; refer below) as compared with the control cultures grown in
SFM alone (results not shown). Therefore, the addition of IGFBP-3 to
the tissue culture medium had no effect on cell growth or rate of
apoptosis in any of the adenoma- and carcinoma-derived cell lines
tested, regardless of p53 status. This finding was similar to that
reported by Gill et al. (9)
, where neither
significant growth arrest nor induction of apoptosis by IGFBP-3 was
seen in the Hs578T breast cancer cell line. In contrast, the addition
of IGFBP-3 was sufficient to induce apoptosis in the GBM cell line
(18)
and in the PC-3 prostate cancer cell line
(3)
, although both used 500 ng/ml recombinant IGFBP-3.
Therefore, the lack of response of the colonic epithelial-derived cell
lines may be attributable to the lower concentration of recombinant
IGFBP-3 used in this study (100 ng/ml), although it should be pointed
out that Gill et al. (9)
showed no response at
500 ng/ml IGFBP-3 in the Hs578T breast carcinoma cell line
(9)
. It is possible that the variation in cellular
response to IGFBP-3 may reflect tissue-specific differences in the
regulation of apoptosis.
However, because IGFBP-3 has been shown to be transcriptionally
regulated by p53 in EB1-transfected colon carcinoma cells
(19)
and to enhance ceramide-induced apoptosis in a breast
carcinoma cell line (9)
, we wished to address the question
as to whether secreted IGFBP-3 could affect the p53-dependent apoptotic
response of colonic epithelium-derived cell lines to cellular stress
such as that resulting from DNA damage. Therefore, for these
experiments, 100 ng/ml recombinant IGFBP-3 was added to two
adenoma-derived cell lines (BH/C1 and AN/C1), which secrete relatively
low levels of the IGFBP-3 protein (data not shown) and express
wild-type p53 protein. Previously, although induction of a
p53-dependent G1 arrest could be detected in cell
lines with wild-type p53 expression after exposure to 5 Gy of
-irradiation (20)
, significant rapid p53-dependent
apoptosis had not been detected in any of these cell
lines.4
In addition, the cellular response of the adenoma-derived cell line
RG/C2 (282 Arg
Trp p53 mutation) and the carcinoma-derived PC/JW2
cell line (which is null for p53 protein expression) was also
investigated. All cell lines were grown in SFM for 24 h, followed
by SFM ± 100 ng/ml IGFBP-3 for 24 h, and were then
irradiated with 5 Gy. The cultures were maintained in SFM ± IGFBP-3 for up to 48 h (prior to the onset of
p53-independent apoptosis), and attached and floating cell yields were
determined. As we and others have described previously, the level of
apoptosis in cultured epithelial cell lines can be assessed by
measuring the proportion of cells that have detached from the monolayer
and are floating in the medium and by determining the fraction of these
floating cells that are apoptotic using acridine orange staining to
confirm morphological features of apoptosis (14
, 21)
.
The results show that in the presence of exogenous IGFBP-3, there is a
>30% decrease in cell yield and increase in apoptosis in the
wild-type p53-expressing cell lines (BH/C1 and AN/C1) after exposure to
5 Gy of
-irradiation (Fig. 2)
. To verify that this induction of apoptosis was not attributable to
sequestration of IGF-1 (a potent survival factor) by the addition of
IGFBP-3, the anti-IGF-1R antibody,
IR3, was added to irradiated
cultures, and the cell yield and floating cell population was
determined at 24 and 48 h after irradiation (Fig. 3)
. In both the AN/C1 and BH/C1 cell lines, addition of the
IR3
antibody had no effect on cell yield or apoptosis of the irradiated
cultures. Therefore, the induction of apoptosis by DNA damage in the
presence of IGFBP-3 was independent of its effect on the IGF-1 pathway.
In addition, this induction of apoptosis appears to be dependent on the
expression of wild-type p53 because no change in cell yield or
induction of apoptosis was observed in either the mutant p53-expressing
RG/C2 or p53-null JW/2 cell lines (Fig. 4)
.
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2-fold increase in p53-dependent
apoptosis in the colorectal epithelium-derived cell lines after 48 h. This induction of cell death after DNA damage was similar to that
reported for the Hs578T breast carcinoma-derived cell line after
treatment with a ceramide analogue (9)
, although the
Hs578T cells have been shown to be mutant for p53
expression,5
suggesting that the potentiation of cell death was independent of p53
function in this breast carcinoma cell line. In contrast, in the
current investigation, induction of apoptosis was limited to those cell
lines that express wild-type p53 protein. The lack of apoptotic
response in the p53-null cell line showed that this was not related to
expression of a mutant p53 protein. Therefore, our findings suggest
that expression of wild-type p53 protein is necessary for
IGFBP-3-induced apoptosis after irradiation of colonic epithelial
cells. Finally, the question arises as to the significance of these findings for colonic tumor cell survival. Previous studies suggest that as IGFBP-3 appears to act as a proapoptotic protein in a number of cell systems, loss of p53 function would lead to a down-regulation of secreted IGFBP-3 protein, potentially resulting in loss of apoptotic signals, and ultimately leading to aberrant cell survival. However, in addition, a recent report by Zou et al. (22) suggests that the DNA sequence of IGFBP-3 may be altered in both the normal and tumor tissue of individuals with gastrointestinal tumors and suggest that such changes may lead to a predisposition for developing gastrointestinal malignancy (22) . Furthermore, Manousos et al. (23) suggest that the level of circulating IGFBP-3 may be inversely associated with risk for colorectal cancer. Our results suggest a potential mechanism by which such a predisposition may occur: Secretion of IGFBP-3 leads to growth inhibition (either through an autocrine or paracrine mechanism) via potentiation of p53-dependent apoptosis. Loss of IGFBP-3 function would therefore lead to loss of sensitivity to growth inhibition under conditions of cellular stress or DNA damage, resulting in the clonal expansion of cell populations that fail to express functional IGFBP-3 protein. The level of IGFBP-3 secretion may therefore contribute to the survival of adenoma cells, because loss of the IGFBP-3 response prior to acquisition of a p53 mutation could lead to clonal selection of the aberrant cell populations.
In summary, differential expression in the normal colonic crypt suggests an important role for IGFBP-3 in the regulation of colonic cell survival. In addition, IGFBP-3 has been shown to enhance p53-dependent apoptosis after DNA damage. Therefore, loss of IGFBP-3 could contribute to the development of colonic adenomas that retain wild-type p53 function through suppression of p53-dependent apo-ptotic signals, allowing aberrant cell survival and tumor formation. These results therefore suggest a possible role for IGFBP-3 in cancer prevention by sensitizing cells to p53-dependent damage/stress-induced apoptosis and hence inhibiting the development of adenomas while having no effect on surrounding, undamaged cells.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was funded by a program grant from the
United Kingdom Cancer Research Campaign. ![]()
2 To whom requests for reprints should be
addressed, at CRC Colorectal Tumour Biology Research Group, Department
of Pathology and Microbiology, School of Medical Sciences, University
of Bristol, University Walk, Bristol BS8 1TD, United Kingdom. Phone:
0117 9287892; Fax: 0117 9287896; E-mail: A.C.Williams{at}bristol.ac.uk ![]()
3 The abbreviations used are: IGFBP, insulin-like
growth factor binding protein; TGF, transforming growth factor; FBS,
fetal bovine serum; SFM, serum-free medium. ![]()
5 Z. P. Gill, personal communication. ![]()
Received 9/ 1/99. Accepted 11/10/99.
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