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B and cAMP Response Element Binding Protein Mediate Opposite Transcriptional Effects on the Flk-1/KDR Gene Promoter
From the Laboratorio di Patologia Vascolare, Istituto Dermopatico dellImmacolata (B.I., L.M., M.C.C., C.G.), Rome, Italy; Department of Biology, Center for Molecular Genetics (P.L.P.), University of California San Diego, La Jolla, Calif.
Correspondence to Dr Carlo Gaetano, Laboratorio di Patologia Vascolare, Istituto Dermopatico dellImmacolata, Via dei Monti di Creta 104, 00167 Rome, Italy. E-mail gaetano{at}idi.it
| Abstract |
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B (NF-
B)related antigens bind specific sequences in the
Flk-1/KDR promoter. Functional studies demonstrate that cAMP represses
whereas tumor necrosis factor-
, an activator of NF-
B,
stimulates promoter activity. Histone acetyltransferases (HATs) P/CAF
and CBP/p300 together with p65/RelA, the catalytic subunit of NF-
B,
increase Flk-1/KDR promoter activity 10- to 20-fold.
Consistently, inhibition by cAMP is reverted by increasing
intracellular HATs and is completely abolished by site-specific
mutagenesis of the cAMP response element. In contrast, specific
mutations in the NF-
B response element abolish responsiveness to
p65/RelA and HATs without affecting cAMP-dependent repression. These
results suggest that opposing signaling pathways, activating NF-
B or
CREB and requiring HAT molecules, control Flk-1/KDR promoter activity.
The full text of this article is available at
http://www.circresaha.org.
Key Words: vascular endothelial growth factor receptor promoter nuclear factor-
B transcription angiogenesis
| Introduction |
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(TNF-
) and cAMP seem
to retain opposite angiogenic effects.12 13 These
molecules exert their action via nuclear factor-
B (NF-
B) and cAMP
response element binding protein (CREB) and may operate on opposing
signaling pathways, via a competitive interaction with histone
acetyltransferases (HATs),14 15 16 17 such as
p30018 CBP,19 and P/CAF.20 The
present study examines the role of putative NF-
B response
element (
B-RE) and cAMP response element (CRE) as positive and
negative regulatory elements of Flk-1/KDR promoter transcriptional
activity. | Materials and Methods |
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Cell Culture, Transfections, and Luciferase Assays
Bovine aortic endothelial cells (BAECs) were
prepared as previously described.21 BAECs and murine
fibroblast STO cells were cultured in complete DMEM with 10% FBS;
NIH-3T3 cells were cultured in complete DMEM with 5% FBS, and human
umbilical vein endothelial cells (HUVECs) were cultured
in EGM-2 complete medium (Clonetics). Transient transfections were
performed with LipofeCTAMINE-Plus Reagent (Life Technologies) according
to the manufacturers instructions; 9x105 cells
(passages 4 through 6) were seeded in 60-mm culture plates 24 hours
before transfection. In each experiment, the total amount of DNA was
always kept constant (20 µg), filling in with empty backbone vectors
or pBluescript plasmid. Luciferase assays were performed using a
luciferase assay system (Promega), according to the manufacturers
instructions. Luciferase activity, expressed as single photon
countsxs-1 (cps), was
determined with an A9904V Top Count (Packard), and it was normalized
for protein concentration and transfection efficiency, which was
determined by ß-galactosidase production. All transfection
data are represented as the mean±SD of 3 or 4 independent
experiments performed in duplicate.
Northern Blot
Total RNA was isolated from 107 HUVECs
treated for 24 hours with 20 ng/mL TNF-
, 1 mmol/L cAMP (Sigma)
or control solvent, using TriZOL reagent (Life Technologies). RNA (25
µg) was electrophoresed on a 1% formaldehyde-agarose gel,
capillary-blotted on Hybond-N membrane (Amersham Pharmacia Biotech),
and hybridized with a 700-bp cDNA probe, which was specific for the
human KDR gene.22
Stable Transfection and FACS Analysis
A -442/+297 promoter fragment was subcloned in pEGFP reporter
plasmid (Clontech), generating a Flk-1GFP reporter vector.
Linearized Flk-1/GFP and pEGFP plasmids (20 µg) were
transfected in BAECs (107 cells, passages 3
through 5) by electroporation with a Gene Pulser II (Bio-Rad), at 200 V
(capacitance 250 µF). Transfected cells were selected by adding 0.5
mg/mL of G418 (Gibco BRL). 48 hours after electroporation. Cells were
analyzed for fluorescence using a FACScan (Becton
Dickinson).
Nuclear Extract and Electrophoretic Mobility Shift Assays
(EMSAs)
Nuclear extracts were obtained as previously
described.23 Binding reactions were performed on ice; 5
µg of nuclear extract was incubated for 10 minutes in the presence or
absence of cold competitor oligonucleotides or
anti-CREB, anti p65 antibodies (Santa Cruz), or normal immunoglobulins
used as a negative control, and then with a
32P-labeled oligoprobe
(4x104 cpm/sample) as described.23
Electrophoresis was performed for 2 hours at 180 V, at 4°C. Gels were
dried in a gel dryer apparatus (Bio-Rad) and exposed for 2
to 3 hours with Biomax film (Kodak).
Site-Directed Mutagenesis
Site-directed mutagenesis was performed using the QUICK CHANGE
Site Directed Mutagenesis Kit (Stratagene), according to the
manufacturers instructions. Mutagenic
oligonucleotides were designed as follows; wild-type
CRE and NF-
B consensus sequences are underlined and mutated
nucleotides are shown in lowercase
letters.
B-RE
B Mutagenized regions were sequenced to verify the accuracy of mutagenesis before use.
Statistical Analysis
Results were analyzed by one-way ANOVA. Post hoc tests
according to the Student-Newman-Keuls method were used to assess
statistically significant differences among different groups. A value
of P<0.05 was considered statistically significant.
| Results |
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B-REs
phage
genomic DNA library. Specific oligonucleotides were
designed to amplify by PCR portions of the Flk-1/KDR promoter to
subclone in pGL2 basic vector. The promoter reporter vectors
-1500/+297Flkluc, -1000/+297Flkluc,
-442/+297Flkluc, and -442/+79Flkluc were tested
for their transcriptional activity in BAECs and STO cells (Figure 1A
|
The constructs -1000/+297Flkluc,
-442/+297Flkluc, and -442+79Flkluc showed a
significant level of transcription in BAECs; construct
-442/+297Flkluc was the most active (Figure 1A
) and
was used in all subsequent experiments. In STO cells, transcriptional
activity of constructs -1000/+297Flkluc,
-442/+297Flkluc, and -442/+79Flkluc were above
control but considerably lower than in BAECs. Sequence analysis
revealed the presence of a CRE, TGAGTCCT, at nucleotides
+11/+18, and recent data7 revealed the presence of a
putative
B-RE at position -62/-55 in the murine promoter (Figure 1B
). Comparison analysis showed that the CRE in the
Flk-1/KDR promoter retained a high level of similarity to the CRE of
other genes such as somatostatin24 and
chromogranin A25 26 (Figure 2A
, top) and that the
B-RE was highly
homologous (>80%) with functional NF-
B elements present in the
promoter region of other genes (Figure 2A
, bottom). CRE and
B-RE probes and mutant oligonucleotides (Figure 2B
) were designed to perform EMSAs. By this approach, specific
complexes bound to the CRE were detected in nuclear extracts from both
BAECs (Figure 2C
, left) and STO cells (not shown). CREB-related
proteins were bound to this DNA element in BAECs, as suggested by a
supershift of the complex bound to CRE (Figure 2C
) but not in
those from fibroblasts (not shown). Competition analysis
(Figure 2C
, right) showed that oligonucleotides
M1 through M6 could efficiently compete the nuclear protein binding to
the wild-type sequence. In contrast, the M7 oligoprobe did not
interfere with DNA/nuclear protein interactions (Figure 2C
),
indicating the conserved G residue at position +12 as important for
nuclear protein binding to this sequence. In other promoters, mutation
in the CRE region encompassing the same G residue also reduced
competition capacity or cAMP responsiveness.27 28
|
In Figure 2D
, it is shown that nuclear complexes containing
NF-
B immunologically related proteins were bound to an oligoprobe
designed across the -62/-55 region. An antip65 antibody (lane 5,
left), raised against the DNA binding and dimerization domain of
p65,29 reduced protein binding, whereas no band depletion
was observed with a control IgG (lane 6); a cold
B-RE
oligonucleotide (see Figure 2B
for sequence)
efficiently competed nuclear complex binding (Figure 2D
, left,
lanes 2 through 4; right, lanes 2 and 3), whereas an
oligonucleotide mutated in the
B-RE did not
interfere with nuclear protein binding (m
B
oligonucleotide, Figure 2D
, right, lanes 4
through 7). The M7 and the m
B mutant regions (Figure 2B
) were
used to generate the mutated promoter constructs mCREluc and
m
Bluc used in transient transfections (see below).
cAMP and TNF-
Mediate Opposite Effects on Flk-1/KDR Gene
Expression and Promoter Activity
The effects of TNF-
and cAMP on Flk-1/KDR mRNA and promoter
activity were evaluated in HUVECs and BAECs, respectively. Dibutyryl
cAMP repressed Flk-1/KDR promoter activity
2-fold, whereas TNF-
enhanced it
3-fold. Northern blot analysis performed in
TNF-
treated HUVECs showed an increase in Flk-1/KDR mRNA
(
3-fold), as previously demonstrated,12 whereas cAMP
reduced Flk-1/KDR expression in parallel with the inhibition of the
promoter activity (Figure 3A
).
Consistently, FACS analysis (Figure 3B
)
performed on BAECs stably transfected with a Flk-1GFP
reporter vector showed that TNF-
increased (panel 2) whereas cAMP
decreased (panel 3) the number of green fluorescent
protein (GFP)positive cells. TNF-
(panel 4) and the histone
deacetylase inhibitor trichostatin A (TSA) (panel 5)
treatments counteracted cAMP inhibition whereas TSA enhanced TNF-
effects (panel 6). Expression was tissue-specific, because no
fluorescence was detected in NIH-3T3 cells transfected with the
same reporter vector (not shown). These results indicate that signals
activated by TNF-
and cAMP mediate opposite effects on
Flk-1/KDR gene expression and promoter activity and suggest a role for
histone acetylation in this process. The molecular
mechanism of this process was examined in the following
experiments.
|
NF-
B Requires HATs to Stimulate Transcription From the
Flk-1/KDR Promoter
NF-
B is one of the main effectors of TNF-
intracellular functions, and NF-
B cooperation with HAT molecules
could be important for the optimal induction of IL-6 promoter
activity.16 Therefore, it was examined whether the NF-
B
active subunit p65/RelA modulated Flk-1/KDR promoter activity and,
eventually, whether this effect required the contribution of HATs. The
direct transfection of the NF-
B active subunit p65/RelA did not
stimulate promoter activity above basal level (Figure 4A
). In contrast, p65/RelA
overexpression, in the presence of cotransfected P/CAF, p300, or CBP,
produced a 6- to 10-fold increase in promoter activity, which was
increased to 20-fold when p65/RelA, P/CAF, and p300 were cotransfected
(Figure 4A
). Transfection of backbone vectors alone had no
effect (not shown), and the protein level of transfected p65/RelA was
not modulated by the presence of HATs, as determined by Western blot
analysis (not shown). In addition, it was found that a
functional HAT domain was required, because p300 mutants
1472 to
1522 and
1603 to 165330 failed to cooperate with
p65/RelA (Figure 4B
), and cotransfection of p300 mutants with
the wild-type plasmid rescued p65 promoter induction whereas
transfection of backbone vectors had no effect (not shown). Notably,
HAT molecules directly stimulated transcription from the
-442/+297Flkluc reporter, a phenomenon that may be ascribed
to their interaction with other trans-acting factors as well
as members of the basal transcription
machinery.14
|
cAMP and CREB Are Negative Regulators of Flk-1/KDR Promoter
Activity
In BAECs, treatment with cAMP produced a 2-fold repression
of Flk-1/KDR transcription (Figures 3A
and 4C
); this was
further increased to 5-fold by CREB, whereas CREB overexpression in the
absence of cAMP did not modulate basal transcriptional activity (Figure 4C
). In a similar experiment, cAMP stimulated the activity of a
control reporter plasmid, bearing a CRE upstream of luciferase gene
(not shown). P/CAF and p300, alone or in combination, stimulated
Flk-1/KDR transcription from 2- to 7-fold; however, cAMP and CREB
inhibited this effect and their action was additive (Figure 4C
).
These findings were further investigated by using a dominant-negative
CREB (
CREB).31 The expression of
CREB per se
produced a 2-fold increase in promoter basal activity whereas cAMP
treatment still exerted its negative effect (Figure 4C
).
CREB
cotransfected with HATs enhanced Flk-1/KDR promoter activity up to
50-fold. Remarkably, a cAMP inhibitory effect was still
detectable despite HATs overexpression (Figure 4C
), indicating
that DNA binding independent mechanisms could also be operating.
Therefore, it was attempted to rescue the promoter activity in the
presence of cAMP by increasing the total intracellular level of pCAF
and p300. It was found that increasing amounts of
coactivators rescued, at least up to 50%, cAMP-mediated
repression (Figure 4D
). These results suggest that the
cAMP-mediated negative transcription signals could be transduced via
CREB by its functional interaction with HATs.
Site-Directed Mutagenesis of CRE and
B-RE Alters Transcription
From the Flk-1/KDR Promoter
To investigate the direct role of CRE and
B-RE, specific
nucleotides were mutagenized inside their core regions
(Figure 5
), according to the results of
the oligonucleotide competition experiments (Figures 2C
and 2D
). The mCREluc and m
Bluc
reporter vectors were generated and used in transfection experiments.
mCREluc lost sensitivity to cAMP, gaining direct
inducibility by p65/RelA even in the absence of cotransfected HATs.
Remarkably, p300 and p65/RelA overexpression stimulated Flk-1/KDR
promoter activity
45- to 50-fold (Figure 5
), ie, at levels
similar to those achieved by
CREB overexpression, previously shown
(Figure 4C
). Nevertheless, cAMP still exerted its negative
effect in the presence of p300, likely because cAMP-activated
CREB was still able to bind p300 but not the mutated CRE. The
m
Bluc construct was still sensitive to cAMP-mediated
repression whereas the overexpression of p65/RelA, despite the presence
of p300, failed to stimulate transcription. These data indicate that
the CRE,
B-RE, and their cognate binding proteins could play
distinct and opposite regulatory roles on Flk-1/KDR promoter
activity.
|
| Discussion |
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activates
NF-
B function and enhances human KDR expression,12
whereas cAMP has been described as a negative regulator of
angiogenesis.13 However, their effects on Flk-1/KDR gene
transcription are poorly characterized. In the present study,
putative cis-acting CRE and
B-RE have been mapped at
position +11/+18 and -62/-55, respectively, in the Flk-1/KDR 5'
flanking region. These predicted binding sites for CREB and NF-
B are
indeed functional, acting as negative and positive regulators of
Flk-1/KDR promoter activity, respectively. In fact, impairing CREB or
p65/RelA binding to the DNA by site-directed mutagenesis reduces or
abolishes cAMP-mediated repression of Flk-1/KDR promoter transcription
as well as p65/RelA stimulatory effect without interfering with the
basal level of transcription. Because CREB and p65/RelA are both capable of HATs recruitment,19 16 we suggest that the positive/negative regulation of the Flk-1/KDR promoter relies on a competition for limited intracellular amounts of HATs. This interpretation is in agreement with a recent study in which the competitive association of CBP/p300 with CREB and p65/RelA has been shown to mediate distinct transcriptional effects on E-selectin and vascular cell adhesion molecule-1 promoters.15 A functional competition for HATs between p53 and p65/RelA has also been described.17 Those reports and the present study suggest that HATs recruitment may represent a general mechanism to control promoter responsiveness to opposing transcription signals.
It is noteworthy that the majority of the experiments reported in the present study were performed by transient transfection of supercoiled plasmid DNA. Under this condition, plasmids are expected to retain an episomal position in the cell nucleus, and this may make them less sensitive to remodeling signals than the cellular chromatin. However, transfected plasmids, which may be organized into nucleosomal-like structures,32 still seem sensitive to histone acetylation.33 Therefore, the biological relevance of our results, despite only being suggestive, is further substantiated by the in vivo regulation of the stable integrated and chromatinized Flk-1GFP reporter construct.
The hypothesis of a direct role for HAT molecules in the process of
vasculogenesis and/or angiogenesis, suggested by the present study,
is also supported by the analysis of transgenic mice bearing a
truncated dominant-negative form of CBP and showing abnormalities in
the production of endothelial
precursors.34 The development of transgenic animals
bearing wild-type or mutated Flk-1/ KDR promoter constructs
will be useful to better understand the role of NF-
B and CRE
elements in the adaptation of Flk-1/KDR gene expression to
proangiogenic or antiangiogenic stimuli in vivo.
Although the regulatory mechanism described in the present study has been reported for other genes in different cell types,14 it may be relevant in a variety of clinical applications aimed at enhancing blood flow to ischemic tissue or downregulating new blood vessel development to inhibit tumor growth.
| Acknowledgments |
|---|
Received May 8, 2000; accepted June 2, 2000.
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