Dihydroxy Bile Acids Activate the Transcription of
Cyclooxygenase-2*
Fan
Zhang
§,
Kotha
Subbaramaiah
¶,
Nasser
Altorki
§, and
Andrew J.
Dannenberg
¶
From the ¶ Departments of Medicine and Surgery and the
§ Department of Cardiothoracic Surgery, New York
Hospital-Cornell Medical Center and the
Strang Cancer
Prevention Center, New York, New York 10021
 |
ABSTRACT |
Bile acids, endogenous promoters of
gastrointestinal cancer, activate protein kinase C (PKC) and the
activator protein-1 (AP-1) transcription factor. Because other
activators of PKC and AP-1 induce cyclooxygenase-2 (COX-2), we
determined the effects of bile acids on the expression of COX-2 in
human esophageal adenocarcinoma cells. Treatment with the dihydroxy
bile acids chenodeoxycholate and deoxycholate resulted in an ~10-fold
increase in the production of prostaglandin E2
(PGE2). Enhanced synthesis of PGE2 was
associated with a marked increase in the levels of COX-2 mRNA and
protein, with maximal effects at 8-12 and 12-24 h, respectively. In
contrast, neither cholic acid nor conjugated bile acids affected the
levels of COX-2 or the synthesis of PGE2. Nuclear run-off
assays and transient transfections with a human COX-2
promoter construct showed that induction of COX-2 mRNA by
chenodeoxycholate and deoxycholate was due to increased transcription.
Bile acid-mediated induction of COX-2 was blocked by inhibitors of PKC
activity, including calphostin C and staurosporine. Treatment with bile
acid enhanced the phosphorylation of c-Jun and increased binding of
AP-1 to DNA. These data are important because dihydroxy bile
acid-mediated induction of COX-2 may explain, at least in part, the
tumor-promoting effects of bile acids.
 |
INTRODUCTION |
Recent studies have established that there are two distinct
isoforms of cyclooxygenase
(COX),1 both of which
catalyze the formation of prostaglandins from arachidonic acid.
COX-1 is thought to be a housekeeping gene with essentially constant levels of expression (1). In contrast, the gene for COX-2 is an early-response gene that, like c-fos
and c-jun, is induced rapidly following stimulation of
quiescent cells. The expression of COX-2 is stimulated by growth
factors, cytokines, oncogenes, phorbol esters, and carcinogens and by
overexpression of PKC (2-9). The different responses of the genes for
COX-1 and COX-2 reflect, in part, differences in
the regulatory elements in the 5
-flanking regions of the two genes
(10).
A large body of data from a variety of experimental systems suggest
that COX-2 is important for tumor formation. Oshima et al.
(11) have reported, for example, that a null mutation of COX-2 is
associated with a marked reduction in the number and size of intestinal
polyps in Apc
716 (adenomatous
polyposis coli) knockout mice, which are a
model for human familial adenomatous polyposis. Treatment with a
selective inhibitor of COX-2 also reduced the number of polyps in
Apc
716 mice (11). COX-2 deficiency also appears to
protect against the formation of tumors in other epithelia because
COX-2 knockout mice develop fewer chemically induced skin
papillomas than control mice (12). Although the exact causal link(s)
between the activity of COX-2 and carcinogenesis remain uncertain,
there are several different mechanisms that can account for this
linkage (13). Prostaglandins, e.g. PGE2, affect
cell proliferation and inhibit immune surveillance; thus,
overproduction of prostaglandins could favor malignant growth (14).
Overexpression of COX-2 inhibits apoptosis and increases the
invasiveness of malignant cells (15, 16). Amounts of COX-2 are
increased in transformed cells and tumors (7, 17-19), which results in
enhanced synthesis of prostaglandins in malignant tissue (20, 21).
Given these data, a reasonable strategy for inhibiting carcinogenesis
is to suppress the expression of COX-2 (22, 23). It therefore becomes
important to identify the factors that modulate the expression of
COX-2.
In this regard, bile acids are known promoters of gastrointestinal
cancer in vivo and enhance cell transformation in
vitro (24-28). The precise mechanism(s) by which bile acids
promote tumor formation is not known. But like tumor-promoting phorbol
esters (29), bile acids activate PKC (30-32) and induce AP-1 activity (33). The latter effect may be especially important because inhibitors
of AP-1-mediated transcription have anti-cancer properties (34). To
further examine the mechanism by which bile acids promote tumor
formation, we have examined their effects as inducers of COX-2. The
rationale for this approach to defining the bile acid-cancer connection
is that phorbol esters are prototypic inducers of COX-2 (2, 4, 5). Our
data show that selected bile acids enhance the transcription of
COX-2 and that inhibitors of PKC activity block this
effect.
 |
EXPERIMENTAL PROCEDURES |
Materials--
DMEM, Opti-MEM, FCS, and Lipofectin were from
Life Technologies, Inc. Enzyme immunoassay reagents for
PGE2 assays were from Cayman Chemical Co., Inc. (Ann Arbor,
MI). Bile acids, sodium arachidonate, staurosporine,
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT;
thiazolyl blue) and phorbol 12-myristate 13-acetate (PMA) were from
Sigma. Calphostin C was from BIOMOL Research Labs Inc. (Plymouth
Meeting, PA). [32P]CTP and [32P]UTP were
from NEN Life Science Products. The AP-1 consensus oligonucleotide was
obtained from Promega (Madison, WI). Random-priming kits were from
Boehringer Mannheim. Reagents for the luciferase assay were from
Analytical Luminescence Laboratory (San Diego, CA). Nitrocellulose
membranes were from Schleicher & Schuell. The 18 S rRNA cDNA was
from Ambion Inc. (Austin, TX). Rabbit polyclonal anti-human COX-2
antiserum was from Oxford Biomedical Research, Inc. (Oxford, MI). Goat
polyclonal anti-human COX-1 and monoclonal anti-c-Jun (specific for the
phosphorylated form of c-Jun) antisera were from Santa Cruz
Biotechnology, Inc. (Santa Cruz, CA). Western blot detection reagents
(ECL) were from Amersham Corp. Plasmid DNA was prepared using a kit
from QIAGEN Inc. (Chatsworth, CA).
Tissue Culture--
The SK-GT-4 cell line was established from a
well differentiated adenocarcinoma arising in Barrett's epithelium of
the distal esophagus (35). Cells were maintained in DMEM supplemented
with 10% FCS, 100 units/ml penicillin, and 100 µg/ml streptomycin in a 5% CO2/water-saturated incubator at 37 °C. Treatments
with vehicle (0.1% ethanol) or bile acid were carried out in 1% FCS.
Cellular cytotoxicity was assessed by measurements of cell number,
release of lactate dehydrogenase, trypan blue exclusion, and MTT assay. Levels of lactate dehydrogenase release were measured in the
supernatants used for PGE2 analyses. MTT assay was
performed according to the method of Denizot and Lang (36). Lactate
dehydrogenase assays were performed according to the manufacturer's
instructions. For trypan blue analysis, following treatment with bile
acid for 12 h, cells were combined 1:1 with 0.4% trypan blue and
examined for dye exclusion.
PGE2 Production--
Cells (1 × 104/well) were plated in 24-well dishes and grown to 60%
confluence in DMEM containing 10% FCS. The medium was then replaced
with DMEM containing 1% FCS and vehicle (0.1% ethanol) or bile acids
(400 µM) for 12 h. At the end of the treatment
period, the culture medium was collected to determine the amounts of
PGE2 secreted spontaneously by these cells. Fresh DMEM
containing 1% FCS and 10 µM sodium arachidonate was then
added. After 30 min, the medium was collected for analysis of
PGE2. The levels of PGE2 released by the cells
were measured by enzyme immunoassay (7). Rates of production of
PGE2 were normalized to protein concentrations.
Western Blotting--
Lysates for the detection of COX-2 and
COX-1 were prepared by treating cells with lysis buffer (150 mM NaCl, 100 mM Tris (pH 8.0), 1% Tween 20, 50 mM diethyl dithiocarbamate, 1 mM EDTA, and 1 mM phenylmethylsulfonyl fluoride). Lysates for the
detection of the phosphorylated form of c-Jun were prepared by treating cells in radioimmune precipitation assay buffer (150 mM
NaCl, 50 mM Tris (pH 7.5), 1% Nonidet P-40, 0.5% sodium
deoxycholate, 0.1% SDS, and 1 mM phenylmethylsulfonyl
fluoride). Lysates were sonicated for 20 s on ice and centrifuged
at 10,000 × g for 10 min to sediment the particulate
material. The protein concentration of the supernatant was measured by
the method of Lowry et al. (37). SDS-polyacrylamide gel
electrophoresis was performed under reducing conditions on 10%
polyacrylamide gels as described by Laemmli (38). The resolved proteins
were transferred onto nitrocellulose sheets as detailed by Towbin
et al. (39). The nitrocellulose membrane was then incubated
with rabbit polyclonal anti-COX-2 antiserum, polyclonal anti-COX-1
antiserum, or a monoclonal antibody to phosphorylated c-Jun. The blots
were probed with the corresponding secondary antibodies to IgG
conjugated to horseradish peroxidase. Blots were probed with the ECL
Western blot detection system according to the manufacturer's
instructions.
Northern Blotting--
Total cellular RNA was isolated from cell
monolayers using an RNA isolation kit from QIAGEN Inc. 10 µg of total
cellular RNA/lane was electrophoresed on a formaldehyde-containing
1.2% agarose gel and transferred to nylon-supported membranes. After
baking, membranes were prehybridized overnight in a solution containing 50% formamide, 5 × sodium chloride/sodium phosphate/EDTA buffer (SSPE), 5 × Denhardt's solution, 0.1% SDS, and 100 µg/ml
single-stranded salmon sperm DNA and were then hybridized for 12 h
at 42 °C with a radiolabeled human COX-2 cDNA probe.
After hybridization, membranes were washed twice for 20 min in 2 × SSPE and 0.1% SDS at room temperature, twice for 20 min in the same
solution at 55 °C, and twice for 20 min in 0.1 × SSPE and
0.1% SDS at 55 °C. Washed membranes were then subjected to
autoradiography. To verify equivalency of RNA loading in the different
lanes, the blot was stripped of radioactivity and rehybridized to
determine the levels of 18 S rRNA. COX-2 and 18 S rRNA
probes were labeled with [32P]CTP by random priming.
Nuclear Run-off Assay--
Cells (1 × 105)
were plated in 100-mm dishes. Cells were grown in DMEM containing 10%
FCS until ~60% confluent. The medium was then replaced with fresh
DMEM containing 1% FCS and vehicle (0.1% ethanol), CD (400 µM), or DC (400 µM) for 8 h. Nuclei
were isolated and stored in liquid nitrogen. For the transcription assay, nuclei (1.0 × 107) were thawed and incubated
in reaction buffer (10 mM Tris (pH 8), 5 mM
MgCl2, and 0.3 M KCl) containing 100 µCi of
uridine 5
-[
-32P]triphosphate and 1 mM
unlabeled nucleotides. After 30 min, labeled nascent RNA transcripts
were isolated. The human COX-2 and 18 S rRNA cDNAs were
immobilized onto nitrocellulose and prehybridized overnight in
hybridization buffer (10 mM TES, 10 mM EDTA,
0.2% SDS, and 0.6 M NaCl). Hybridization was carried out
at 42 °C for 24 h using equal cpm/ml of labeled nascent RNA
transcripts for each treatment group. The membranes were washed twice
with 2 × SSC for 1 h at 55 °C and then treated with 10 mg/ml RNase A in 2 × SSC for 30 min at 37 °C, dried, and
autoradiographed.
Plasmids--
The COX-2 promoter construct
(
327/+59) was a gift of Dr. Tadashi Tanabe (National Cardiovascular
Center Research Institute, Osaka, Japan) (10). The human
COX-2 cDNA was generously provided by Dr. Stephen M. Prescott (University of Utah, Salt Lake City, UT). pSV-
gal was
obtained from Promega.
Transient Transfection Assays--
SK-GT-4 cells were seeded at
a density of 5 × 104/well in 6-well dishes and grown
to 50-60% confluence in DMEM containing 10% FCS. For each well, 1.8 µg of COX-2-luciferase construct and 0.2 µg of
pSV-
gal were cotransfected into SK-GT-4 cells at a 1:4 ratio of DNA
to Lipofectin following the manufacturer's instructions. After
transfection, the medium was replaced with DMEM containing 10%
FCS for 24 h. The cells were then treated with DMEM containing 1%
FCS and vehicle (0.1% ethanol), CD (400 µM), or DC (400 µM). The activities of luciferase and
-galactosidase
were measured in cellular extract 8 h later (22). To adjust for
differences in transfection efficiencies, the luciferase values were
normalized using
-galactosidase.
Electrophoretic Mobility Shift Assay--
SK-GT-4 cells were
treated with DMEM containing 1% FCS and vehicle (0.1% ethanol) or CD
for 2 or 6 h, respectively. Cells were harvested and nuclear
extracts were prepared as described previously (40). For binding
studies, an AP-1 consensus oligonucleotide was used:
5
-CGCTTGATGAGTCAGCCGGAA-3
(sense) and 3
-GCGAACTACTCAGTCGGCCTT-5
(antisense). The complementary oligonucleotides were annealed in 20 mM Tris (pH 7.6), 50 mM NaCl, 10 mM
MgCl2, and 1 mM dithiothreitol. The annealed
oligonucleotide was phosphorylated at the 5
-end with
[
-32P]ATP and T4 polynucleotide kinase. The binding
reaction was performed by incubating 2 µg of nuclear protein in 20 mM HEPES (pH 7.9), 10% glycerol, 300 µg of bovine serum
albumin, and 1 µg of poly(dI·dC) in a final volume of 10 µl for
10 min at 25 °C. The labeled AP-1 consensus oligonucleotide was
added to the reaction mixture and allowed to incubate for an additional
20 min at 25 °C. The samples were electrophoresed on a 4%
nondenaturing polyacrylamide gel. The gel was then dried and subjected
to autoradiography at
80 °C.
Statistics--
Comparisons between groups were made by
Student's t test. A difference between groups of
p < 0.05 was considered significant.
 |
RESULTS |
Effect of Bile Acids on the Synthesis of
PGE2--
Fig. 1 shows the
effects of several bile acids and bile acid conjugates on the
production of PGE2. CD and DC caused ~3- and 10-fold
increases in the spontaneous production of PGE2,
respectively. Cholic acid and all conjugated bile acids, however, had
no effect on rates of spontaneous synthesis of PGE2. To
examine the synthesis of PGE2 in more detail, we also
measured the effect of bile acids on the production of PGE2
when an excess of arachidonic acid was added to the system. This
experiment was done because PGE2 production can be affected
by changes in the activity of phospholipase A2, which
provides the substrate for COX-catalyzed reactions. Adding excess
arachidonate minimizes any contribution of phospholipase A2
activity to the rate of production of PGE2. As for rates of spontaneous synthesis of PGE2, treatment with CD or DC
induced an ~10-fold increase in the synthesis of PGE2 in
the presence of excess arachidonate (Fig. 1). Neither cholic acid nor
the conjugated bile acids affected the production of PGE2
in the presence of excess arachidonic acid.

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Fig. 1.
CD and DC enhance the production of
PGE2. SK-GT-4 cells were treated with 400 µM cholic acid (CA), taurocholate
(TC), CD, glycochenodeoxycholate (GCD),
taurochenodeoxycholate (TCD), DC, or taurodeoxycholate
(TDC) or with vehicle (0.1% ethanol). Post-culture medium
was collected 12 h later to determine spontaneous release of
PGE2 (stippled columns). The medium was replaced
with DMEM containing 1% FCS and 10 µM sodium
arachidonate. 30 min later, the medium was collected to determine the
rate of synthesis of PGE2 following treatment with sodium
arachidonate (shaded columns). Production of
PGE2 was determined by enzyme immunoassay as described under "Experimental Procedures." The columns are the
means, and the bars are the S.D. (n = 4). *,
p < 0.001 compared with control.
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Cytotoxicity in these experiments was assessed by cell number, lactate
dehydrogenase release, MTT assay, and trypan blue exclusion. No
evidence of cellular toxicity was detected (data not shown).
Dihydroxy Bile Acids Induce COX-2--
Western blotting was
carried out to determine whether the above differences in
PGE2 production were related to differences in amounts of
COX-2. We found that treatment with CD caused a dose-dependent induction of COX-2 protein with a peak
effect at 400 µM CD (Fig.
2). Moreover, CD and DC were the only
bile acids that induced COX-2 protein (Fig.
3), which is consistent with their
effects on the production of PGE2 (Fig. 1). We also
compared the time course for induction of COX-2 by CD versus
PMA. Maximal induction of COX-2 protein occurred after 12-24 h of
treatment with CD (Fig. 4). In contrast,
COX-2 was induced maximally by PMA after 8-12 h of treatment (Fig. 4).
COX-1 was not detectable in this cell line by Western blotting under
basal or stimulated conditions (data not shown).

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Fig. 2.
CD causes dose-dependent
induction of COX-2. SK-GT-4 cells were treated for 12 h with
vehicle (0.1% ethanol) or CD at concentrations ranging from 100 to 400 µM. Cellular lysate protein (30 µg/lane) was loaded
onto a 10% SDS-polyacrylamide gel, electrophoresed, and subsequently
transferred onto nitrocellulose. Immunoblots were probed with antibody
specific for COX-2. Ovine Cox-2 was used as a standard.
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Fig. 3.
Dihydroxy bile acids induce COX-2.
SK-GT-4 cells were treated for 12 h with vehicle (0.1% ethanol)
or with 400 µM cholic acid (CA), taurocholate
(TC), CD, glycochenodeoxycholate (GCD), taurochenodeoxycholate (TCD), DC, or taurodeoxycholate
(TDC). Cellular lysate protein (30 µg) was loaded onto a
10% SDS-polyacrylamide gel, electrophoresed, and subsequently
transferred onto nitrocellulose. Immunoblots were probed with
antibody specific for COX-2. Ovine Cox-2 was used as a standard.
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Fig. 4.
CD and PMA cause prolonged induction of
COX-2. SK-GT-4 cells were treated for 4-36 h with vehicle (0.1%
ethanol), CD (400 µM), or PMA (50 ng/ml). Cellular lysate
protein (30 µg) was loaded onto a 10% SDS-polyacrylamide gel,
electrophoresed, and subsequently transferred onto nitrocellulose.
Immunoblots were probed with antiserum specific for COX-2. Ovine Cox-2
was used as a standard.
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|
To further elucidate the mechanism responsible for the changes in
amounts of COX-2 protein, we measured the steady-state levels of COX-2
mRNA by Northern blotting. CD and DC stimulated the formation of
COX-2 mRNA; maximal induction of COX-2 mRNA was observed after treatment for ~8 h (Fig. 5). Bile
acid-mediated induction of COX-2 mRNA was still apparent after
24 h of treatment, which is consistent with the effects detected
by Western blotting.

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Fig. 5.
Time course of induction of COX-2 mRNA in
SK-GT-4 cells after treatment with bile acids. SK-GT-4 cells were
treated with 400 µM CD or DC or with vehicle (0.1%
ethanol) for the indicated time periods. Total cellular RNA was
isolated. Each lane contained 10 µg of RNA. The Northern blot was
probed sequentially with probes that recognized COX-2 and 18 S rRNAs.
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Transcription of COX-2 Is Enhanced by Dihydroxy Bile
Acids--
Differences in the levels of mRNA could reflect altered
rates of transcription or stability of mRNA. Nuclear run-off assays were performed to distinguish between these possibilities. As shown in
Fig. 6, we detected higher rates of
synthesis of nascent COX-2 mRNA after treatment with CD or DC,
consistent with the differences observed by Northern blotting. To
further investigate the importance of bile acids in modulating the
expression of COX-2, transient transfections were performed
with a human COX-2 promoter-luciferase construct. Treatment
with CD or DC led to approximately a doubling of COX-2
promoter activity (Fig. 7).

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Fig. 6.
Bile acids induce COX-2
transcription. SK-GT-4 cells were treated with CD (400 µM), DC (400 µM), or vehicle (0.1%
ethanol) for 8 h. Nuclear run-off assays were performed as
described under "Experimental Methods." The COX-2 and 18 S rRNA cDNAs were immobilized onto nitrocellulose membranes and
hybridized with labeled nascent RNA transcripts.
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Fig. 7.
Bile acids increase COX-2
promoter activity. SK-GT-4 cells were cotransfected with 1.8 µg
of human COX-2 promoter construct ( 327/+59) and 0.2 µg
of pSV- gal. After transfection, cells were treated with 400 µM CD or DC or with vehicle (0.1% ethanol). Reporter activities were measured in cellular extract 8 h later. Luciferase activity represents data that have been normalized with
-galactosidase activity. Six wells were used for each of the three
conditions. The columns are the means, and the
bars are the S.D. *, p < 0.001 compared
with control.
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Mechanism of Bile Acid-mediated Induction of COX-2--
Bile acids
have been reported to activate PKC. Therefore, to elucidate the
signaling mechanism by which bile acids activate the transcription of
COX-2, we measured the induction of COX-2 by CD in the
presence of known inhibitors of PKC. Staurosporine and calphostin C
were used as inhibitors. Both agents blocked the induction of COX-2 by
bile acids (Fig. 8, A and
B). Dexamethasone also blocked the induction of COX-2 by
CD and PMA (Fig. 8C). Because activators of PKC modulate
gene expression via AP-1, we determined the effects of CD on the
phosphorylation state of c-Jun and AP-1 DNA-binding activity (Fig.
9). Treatment with CD increased the phosphorylation of c-Jun (Fig. 9A). Electrophoretic mobility
shift assay revealed increased binding of AP-1 to DNA following
treatment with CD (Fig. 9B).

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Fig. 8.
Inhibitors of protein kinase C suppress bile
acid-mediated induction of COX-2. SK-GT-4 cells were treated for
12 h with vehicle (0.1% ethanol), CD (400 µM), or
CD and calphostin C (A), CD and staurosporine
(B), or CD and dexamethasone (1 µM) (C). In C, cells were also treated with PMA (50 ng/ml) or PMA and dexamethasone (1 µM) for 12 h.
Lysate protein (30 µg/lane) was loaded onto a 10% SDS gel,
electrophoresed, and subsequently transferred onto nitrocellulose.
Immunoblots were probed with antibody specific for COX-2. Ovine Cox-2
was used as a standard.
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Fig. 9.
CD enhances the phosphorylation of c-Jun and
induces AP-1 DNA-binding activity. Cells were treated with CD (400 µM) or vehicle (0.1% ethanol) for 2 or 6 h. In
A, cellular lysate protein (30 µg/lane) was loaded onto a
10% SDS-polyacrylamide gel, electrophoresed, and subsequently
transferred onto nitrocellulose. The blot was probed with a monoclonal
antibody directed toward the phosphorylated form of c-Jun. In
B, 2 µg of nuclear protein was incubated with a
32P-labeled oligonucleotide containing an AP-1 consensus
site. The protein-DNA complex that formed was separated on a 4%
polyacrylamide gel as described under "Experimental
Procedures."
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 |
DISCUSSION |
Bile acids are known tumor promoters that activate PKC, which may
be the mechanism of bile acid-induced carcinogenesis (31-33). Dihydroxy bile acids are more effective in activating PKC than mono-
and trihydroxy bile acids (32). The exact mechanism by which bile acids
change the activity of PKC is uncertain. However, bile acids increase
the activity of PKC, in part, by inducing the translocation of selected
isoforms from cytosol to membrane fractions (31). It is also known, in
the context of the tumor-promoting effects of bile acids, that
dihydroxy bile acids stimulate the release of PGE2 by
colonic epithelium (41); but, as for the underlying specific
carcinogenic events modulated by PKC, the basis for the up-regulation
of PGE2 synthesis by bile acids remains uncertain. Our
prior work (7, 22, 23) and data from other laboratories (11-13,
15-19) provide evidence that COX-2, which is a
PKC-dependent gene (9), is important for the genesis of
cancer. Hence, there is evidence to link the activity of PKC to
carcinogenesis, in part, via the induction of COX-2 and overproduction
of prostaglandins. This work adds to this evidence by showing that bile
acids up-regulate the expression of COX-2. Moreover, we found that only
CD and DC induced COX-2, which is consistent with the greater effect of these two bile acids on activation of PKC versus cholic acid
or conjugated bile acids (32). The present data therefore suggest a
plausible mechanism to explain the promotion of esophageal (42-44) and
colon (24-27) cancer by bile acids: induction of COX-2.
Bile acid-mediated induction of COX-2 can be important for
tumorigenesis in the gastrointestinal tract because the products of
COX-2 activity, e.g. prostaglandins, inhibit apoptosis,
diminish immune surveillance, and increase the invasiveness of
malignant cells (14-16). These conclusions are consistent with
evidence that inhibitors of COX-2 are chemopreventive against cancers
of the colon and upper gastrointestinal tract in experimental animals (45-48) and in humans (49, 50).
With regard to the mechanism by which bile acids induce COX-2, it is
known that the human COX-2 promoter contains a cyclic AMP
response element and sites for AP-2, NF-IL6 and NF-
B (10). Xie and
Herschman (51) showed that the AP-1 transcription factor complex is
important for activation of the murine Cox-2 promoter via a
cyclic AMP response element. This is consistent with other reports that
AP-1 transcription factors can modulate transcription via a cyclic AMP
response element (52). The current data show too that bile acids
increased the phosphorylation of c-Jun, an event known to be downstream
from activation of the PKC signaling pathway (53). This is important
because enhanced phosphorylation of c-Jun could potentiate its ability
to activate transcription of COX-2. Suppression of bile
acid-mediated induction of COX-2 by dexamethasone is consistent with
prior reports that AP-1 activity can be regulated negatively by
glucocorticoids (29).
AP-1 sites are only one of several cis-elements that mediate
the effects of PKC. Other PKC-responsive cis-elements
include the AP-2 recognition site (54) and the NF-
B-binding site
(55). Irrespective of the exact mechanism, however, the results
reported above suggest that inhibitors of PKC could be useful for
down-regulating COX-2 and thereby preventing and/or treating
cancer.
Although relatively high concentrations of bile acids were required to
induce COX-2, similar concentrations of bile acids are found in bile
(56) and the aqueous phase of the fecal stream (57). It will therefore
be worthwhile to investigate whether bile acids also induce COX-2
in vivo. Further studies are also needed to confirm that
induction of COX-2 by bile acids is mediated via AP-1 and to identify
the responsible COX-2 promoter element(s). Given the already
demonstrated effects of bile acids on the activity of PKC, it also
becomes important to identify the specific isoforms of PKC that mediate
the effects of bile acids on COX-2. A more detailed understanding of
the precise mechanism(s) by which bile acids induce COX-2 should
facilitate the development of chemopreventive strategies to diminish
the risk of carcinogenesis within regions of the gastrointestinal tract
exposed to bile acids.
 |
ACKNOWLEDGEMENTS |
We thank Drs. Babette B. Weksler and David
Zakim for reviewing this manuscript.
 |
FOOTNOTES |
*
This work was supported by Astra Merck Inc. (Wayne, PA),
U. S. Surgical Corp. (Norwalk, CT), and the Iris Cantor Research Unit.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: New York
Hospital-Cornell Medical Center, Div. of Digestive Diseases, Room
F-231, 1300 York Ave., New York, NY 10021. Tel.: 212-746-4403; Fax:
212-746-8447.
1
The abbreviations used are: COX, cyclooxygenase;
PKC, protein kinase C; PGE2, prostaglandin E2;
AP-1, activator protein-1; DMEM, Dulbecco's modified Eagle's medium;
FCS, fetal calf serum; MTT,
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PMA,
phorbol 12-myristate 13-acetate; CD, chenodeoxycholate; DC, deoxycholate.
 |
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