Endocrinology Vol. 141, No. 10 3587-3594
Copyright © 2000 by The Endocrine Society
Transcriptional Regulation of Human 11ß-Hydroxylase (hCYP11B1)
Xiao-Li Wang,
Mary Bassett,
Yin Zhang,
Su Yin,
Colin Clyne,
Perrin C. White and
William E. Rainey
Departments of Obstetrics and Gynecology and Pediatrics, University
of Texas Southwestern Medical Center, Dallas, Texas 75390-9032
Address all correspondence and requests for reprints to: William E. Rainey, Ph.D., Division of Reproductive Endocrinology, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9032. E-mail:
braine{at}mednet.swmed.edu
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Abstract
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Steroid 11ß-hydroxylase is a mitochondrial enzyme that catalyzes the
conversion of deoxycortisol to cortisol. The gene encoding human
11ß-hydroxylase (hCYP11B1) is expressed in the adrenal
cortex under the control of circulating levels of ACTH. The current
study was undertaken to define the cis-regulatory
elements and transacting factors that regulate hCYP11B1
transcription. The hCYP11B1 5'-flanking DNA was studied
using transient transfection of luciferase reporter constructs in
NCI-H295R human adrenocortical cells. A cAMP analogue
((Bu)2cAMP) increased expression of a construct
containing -1102 bp of hCYP11B1 5'-flanking DNA
(pB11102). An element at position -71/-64 (TGACGTGA, previously
termed Ad1) resembling a consensus cAMP response element (CRE) was
required for maximal induction by cAMP. The Ad1 element bound several
transcriptional factors in electrophoretic mobility shift assays,
including CRE-binding protein, activating transcription factor-1
(ATF-1), and ATF-2, but only the ATF-2 complex migrated similarly to a
complex seen using H295R nuclear extract. In addition, Western analysis
of H295R and adrenal lysates demonstrated expression of high levels of
ATF-2 and ATF-1. CRE-binding protein levels varied among the strains of
H295R cells tested. Transcription of CYP11B1 also
appeared to be regulated by steroidogenic factor-1 (SF-1). Luciferase
reporter gene activity was increased after cotransfection with
expression vectors containing SF-1. An element in
hCYP11B1 at positions -242/-234 (CCAAGGCTC),
previously termed Ad4, was required for maximal induction by SF-1 and
was found to bind SF-1 in electrophoretic mobility shift assays. The
key role for SF-1 in hCYP11B1 transcription is in
contrast to its lack of an effect on expression of the
hCYP11B2 (aldosterone synthase) isozyme. The
differential effects of SF-1 on transcription of
hCYP11B1 and hCYP11B2 may be one of the
mechanisms controlling differential expression of these isozymes within
the zonae fasciculata and glomerulosa of the human adrenal cortex.
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Introduction
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THE ADRENAL cortex is responsible for
the production of both mineralocorticoids and glucocorticoids.
Mineralocorticoids are synthesized in the zona glomerulosa, whereas
glucocorticoids are produced within the zona fasciculata. The zonal
production of these steroids is in part the result of tightly
controlled transcription of the CYP11B genes (1, 2).
Aldosterone synthase is responsible for the conversion of
deoxycorticosterone to aldosterone, is expressed only within the zona
glomerulosa, and is mainly under the control of circulating angiotensin
II and potassium levels. Steroid 11ß-hydroxylase is responsible for
the conversion of deoxycortisol to cortisol, is localized to the zona
fasciculata, at least in rodents (2, 3), and is under the
control of circulating ACTH levels.
The expression of each isozyme appears to be controlled at the level of
gene transcription. The 5'-flanking region of CYP11B2, the
gene encoding aldosterone synthase, has been analyzed in detail for the
mouse (4, 5, 6, 7), hamster (8, 9), and human genes
(10, 11). However, the mechanisms regulating transcription
of CYP11B1, the gene encoding 11ß-hydroxylase, are less
completely understood. Although the bovine CYP11B (bCYP11B)
gene has been studied in some detail (12, 13, 14, 15, 16), cattle have
only one active bCYP11B gene that is expressed in both the
glomerulosa and fasciculata of the adrenal (17).
Therefore, the regulatory elements in bovine bCYP11B
5'-flanking DNA may differ from those that control the zone-specific
expression of the multiple CYP11B genes observed in rodents and
humans.
One difficulty in studying the CYP11B genes has been the availability
of an appropriate in vitro model system. Previous studies
have used the mouse Y-1 adrenal cell line for a preliminary analysis of
hCYP11B1 5'-flanking DNA (18, 19). More
recently, we and others have characterized a human adrenocortical
carcinoma cell line (H295R) that produces both aldosterone and cortisol
(20, 21). These cells also express both
hCYP11B1 and hCYP11B2 transcripts, which are
under the differential control of cAMP and angiotensin II signaling
pathways, respectively (22, 23, 24). The current study was
undertaken to analyze the 5'-flanking DNA of the hCYP11B1
gene and to define the cis-regulatory elements and
transacting factors that are necessary for cAMP induction of
transcription. The results suggest that ACTH induction of
hCYP11B1 transcription requires primarily two
cis-regulatory elements, one (termed Ad1) resembling a
consensus ccAMP response element (CRE), and the other (termed Ad4) that
binds steroidogenic factor-1 (SF-1).
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Materials and Methods
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Preparation of reporter constructs with serial deletions of
hCYP11B1 5'-flanking DNA
A transient expression system using the luciferase reporter gene
was used to characterize the hCYP11B1 promoter. A 1102-bp
fragment (pB11102) extending from position -2 (relative to the
transcriptional start site) to -1102 was cloned into the promoterless
pGL3-Basic luciferase reporter plasmid (Promega Corp.,
Madison, WI). Several 5'-deletion plasmids were constructed using
available restriction endonuclease sites (XbaI, pB1924;
NheI, pB1777; XmnI, pB1514; DraI,
pB1330; ApaI, pB1214). The pB165 construct was
prepared using PCR to introduce a unique KpnI restriction
site.
Preparation of mutant constructs
Deoxyribonuclease I footprint analysis of the bCYP11B
5'-flanking DNA demonstrated several regions that bind nuclear proteins
(12). These protected regions were termed Ad-1 to Ad-6.
Similar sequences were observed in the hCYP11B1 gene;
therefore, this terminology will be used to discuss the regulatory
elements. For the Ad1/CRE mutants, the sequence 5'-TGACGTGA-3'
(-71/-64) was changed to 5'-gGtaccGA-3', which included a
KpnI site (underlined). To do so, PCR was
performed on pB11102 using a set of primers, 1 and 2 (Table 1
). The Ad4 sequence 5'-ATCCAAGGCTCT-3'
(-244/-233) was changed to 5'-ATCgAAttCTCT-3', and the
Ad5 sequence 5'-CCTGACCTCT-3' (-121/-112) was changed to
5'-CCTGAatTCT-3', each of which included an
EcoRI site (underlined). The pB11102 construct
was used as the template for PCR reactions using primers 3 and 4 for
the Ad4 mutant and primers 5 and 6 for the Ad5 mutation (Table 1
).
Cell culture and transfections
H295R adrenocortical cells (25) were cultured in
DMEM/F-12 (Life Technologies, Inc./BRL, Gaithersburg, MD)
supplemented with 2% Ultroser G (BioSepra SA, Villeneuve la Garenne,
France), 1% ITS Plus (6.25 µg/ml insulin, 6.25 µg/ml transferrin,
6.25 ng/ml selenium, and 5.35 µg/ml linoleic acid;
Collaborative Research, Bedford, MA) and antibiotics.
Mouse Y-1 adrenal cells were obtained from the American Type Culture Collection (Manassas, VA) and grown under the same
conditions as the H295R cells. For transfection experiments, cells were
subcultured onto 12-well culture dishes (150,000 cells/well) and were
used 24 h later. Transfection was carried out using 2.0 µl
Fugene (Roche Molecular Biochemicals, Indianapolis, IN)
and 1.0 µg reporter plasmid DNA in DMEM/F12 medium (1.1 ml) for
6 h at 37 C. For cotransfection experiments, various amounts of
expression plasmids were included in the transfection reaction, and the
total amount of DNA was kept constant by the addition of carrier DNA
(empty expression vector). After transfection, cells were incubated
with 2.0 ml low serum medium (DMEM/F12 medium containing 0.1% Ultroser
G) for 24 h before being treated with agonists for 6 h. Cells
were then lysed and assayed for activity using a luciferase assay
(Promega Corp.).
Electrophoretic mobility shift assay (EMSA)
Nuclear extracts were prepared from H295R cells by the method of
Schreiber et al. (26). Double stranded
oligonucleotides (25 pmol) were labeled with 30 µCi
[
-32P]ATP and 10 U T4 polynucleotide kinase
at 37 C for 30 min. Nuclear extract and each radiolabeled probe (40,000
dpm) were incubated at room temperature for 20 min in 20 µl reaction
mixture [20 mM HEPES (pH 8.0), 80
mM KCl, 1 mM EDTA, 10%
glycerol, 1 mM dithiothreitol, 0.5 mg/ml BSA, and
0.025 mg/ml poly(dI-dC) as nonspecific competitor]. When antibodies
were included in the reaction, nuclear extract and antibody were
preincubated on ice for 20 min before the addition of probe and
reaction mixture. For competition analysis, reaction mixtures
containing various amounts of nonradiolabeled oligonucleotide were
added simultaneously with probe. The resulting DNA/protein complexes
were separated from free probe by electrophoresis using a 4% high
ionic strength native polyacrylamide gel with 1 x Tris-glycine
running buffer (2). The gel was dried and visualized after
autoradiography at -70 C for 24 h. The following double stranded
DNA probes used were Ad4 (-250/-227; sequence 7 in Table 1
),
Ad4-mutant (sequence 8 in Table 1
), Ad1 (-79/-56; sequence 9 in Table 1
), and Ad1-mutant (sequence 10 in Table 1
). Rabbit polyclonal
anti-SF-1 (Ad4BP) was provided by Dr. Ken-ichirou Morohashi, Kyushu
University (Fukuoka, Japan). Human SF-1, CRE-binding protein
(CREB), activating transcription factor-1 (ATF-1), and ATF-2 were
prepared using a Promega Corp. in vitro
transcription/translation system.
Protein immunoblotting analysis
Cultured H295R cells were used to prepare nuclear extract as
previously described (26). Two groups of H295R cells were
used to isolate nuclear extracts that were used for the Western
analysis. One group of H295R cells was representative of the cells used
for the transfection EMSA and studies (Fig. 3
, H295 NE-1). The second
population of H295R cells was maintained in culture for extended
periods (>1 yr). Adult adrenal glands were obtained and homogenized in
lysis buffer (27). HeLa cell nuclear extract, which was
used as a positive control, was obtained from Upstate Biotechnology, Inc. (Lake Placid, NY). In addition, CREB, ATF-1,
and ATF-2 were prepared using a Promega Corp. in
vitro transcription/translation system. PAGE was carried out on
the samples using a precast Novex gel electrophoresis
system with 412% bis-Tris NuPage gels (Novex, San
Diego, CA). Proteins were electrophoretically transferred onto
polyvinylidene difluoride membranes by wet transfer for 1 h at 25
V. After transfer the membranes were incubated with ATF-2 (1:1500
dilution), CREB (1:2000 dilution), or ATF-1 (1:800 dilution) antibodies
overnight at 4 C. The ATF-2 and CREB antibodies were obtained from
New England Biolabs, Inc. (Beverly, MA). The ATF-1
antibody was obtained from Santa Cruz Biotechnology, Inc.
(Santa Cruz, CA). In addition to the manufacturers characterization,
antibodies were tested for specificity using purified or in
vitro translated CREB, ATF-1, or ATF-2. Membranes were incubated
with horseradish peroxidase-conjugated secondary antibodies, and
immunoreactive bands were visualized using enhanced chemiluminescence
Western blotting detection reagents from Amersham Pharmacia Biotech (Piscataway, NJ).

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Figure 3. Adrenal expression of CRE-binding proteins.
Protein immunoblot analysis was used to examine expression of CREB,
ATF-1, and ATF-2 in adrenal lysates and H295R nuclear extracts.
Homogenates of adrenal gland (20 µg) and aldosterone-producing
adrenal adenoma (20 µg) were compared with nuclear extracts from two
strains of H295R cells (4.5 µg). The H295R NE-1 was isolated from
cells that were similar to those used for the transfection and EMSA.
H295R NE-2 was isolated from cells that had been in culture longer than
1 yr. These nuclear extracts were representative of three extracts
tested. HeLa cell nuclear extract (5 µg) and recombinant ATF-1, CREB,
or ATF-1 were used as a positive control for members of the CREB/ATF
family of proteins. The antibody used to visualize CREB has previously
been shown to also recognize ATF-1, as can be seen by the presence of a
band of approximately 35 kDa.
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Results
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Deletion analysis of the hCYP11B1 promoter
To identify the cis-elements that regulate basal and
cAMP-regulated transcription, a series of deletion constructs was
prepared containing progressively shorter fragments of
hCYP11B1 5'-flanking DNA (pB11102, pB1924, pB1777,
pB1514, pB1330, pB1214, and pB165). These constructs were
transiently transfected into H295R cells. Figure 1
illustrates the reporter activity of
constructs under basal conditions or after treatment with
(Bu)2cAMP (1 mM) for 6
h. Basal luciferase activities of the constructs containing 5'-flanking
sequences from -1102 to -214 bp were similar. Further deletion to
-65 bp decreased luciferase activity to that observed for the pGL3
Basic empty vector, indicating that DNA between -214 and -65 is
essential for basal gene expression. (Bu)2cAMP
treatment of H295R cells transfected with pB11102 construct resulted
in a 5-fold increase in luciferase activity. Deletion to -214 bp did
not significantly alter (Bu)2cAMP stimulation of
luciferase activity. Deletion to -65, however, abolished
(Bu)2cAMP induction of reporter gene expression,
indicating that DNA between -214 and -65 contains the minimally
essential regions for both basal and cAMP-regulated transcription of
hCYP11B1.

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Figure 1. Deletion analysis of the hCYP11B1
5'-flanking DNA to define basal and (Bu)2cAMP regulatory
regions. H295R cells were transiently transfected with luciferase
reporter constructs containing serial deletions of
hCYP11B1 5'-flanking DNA (1 µg/well). After recovery
for 24 h, cells were treated with or without (Bu)2cAMP
(dbcAmp) (1 mM) for 6 h. Cells were then lysed,
and luciferase activity was measured. Results are expressed as a
percentage of the basal reporter activity of pB11102 and represent
the mean ± SEM of data from four independent
experiments (*, P < 0.0001 compared with the basal
level).
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Role of the Ad1 cis-element in hCYP11B1 transcription
A near-consensus CRE, previously termed Ad1, was shown to play a
critical role in transcription of bCYP11B as well as mouse
and human CYP11B2 (5, 11). To determine whether
the corresponding sequence in hCYP11B1 (TGACGTGA) is
important for transcription, it was mutated (gGtaccGA) in pB11102.
Mutation of Ad1 decreased both basal and
(Bu)2cAMP induction, suggesting that this element
is needed for transcription of hCYP11B1 (Table 2
).
To begin characterization of nuclear proteins that bind the Ad1
element, a synthetic oligonucleotide probe encompassing this sequence
was prepared and used in EMSA. In the presence of H295R nuclear
extract, at least two specific protein-DNA complexes were formed (Fig. 2
). Formation of these complexes was
abolished by the addition of nonradiolabeled competitor. Because the
Ad1 element is almost identical to a consensus CRE, transcription
factors know to interact with this sequence were examined using EMSA.
In vitro prepared CREB, ATF-1, and ATF-2 formed complexes
with the Ad1 element, giving rise to complexes that migrated
differently. ATF-2, but not CREB or ATF-1, formed a radiolabeled
complex that comigrated with a complex observed using H295R cell
nuclear extract (complex 1). Antibody directed against ATF-2
supershifted complex 1, suggesting that it represents a complex
containing ATF-2. Antibodies directed against CREB or ATF-1 did not
significantly modify any of the three major complexes. Oligonucleotides
that included a mutated Ad1 sequence (gGtaaGA) did not form complexes
with in vitro prepared CREB, ATF-1, or ATF-2 (Fig. 2
).

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Figure 2. Binding of specific transcription factors to
hCYP11B1 Ad1/CRE (-71/-64). EMSA was performed using a
32P-labeled oligonucleotide probe corresponding to
hCYP11B1 -71/-64. Radiolabeled probe alone is shown in
lane 1 (FP), and probe with H295R nuclear extract (NE; 5.0 µg) is
shown in lane 2. Protein/DNA complexes (designated 1, 2, and 3) were
separated from free probe by electrophoresis. Nonradiolabeled
self-competitor DNA was added at a 100-fold molar excess (lane 3, NE +
100x) to identify specific protein/DNA interactions. Lanes 46 show
binding activity in the presence of in vitro prepared
CREB, ATF-1, or ATF-2, respectively. Lanes 79 show the effects of
incubation of probe with nuclear extract in the presence of antibodies
directed against CREB, ATF-1, or ATF-2, as indicated. Lanes 1015
examine the mutated Ad1/CRE oligonucleotide using EMSA. Radiolabeled
probe only is shown in lane 10 (FP), and probe with H295R nuclear
extract is shown in lane 11 (NE). Nonradiolabeled self-competitor DNA
was added at a 100-fold molar excess, as shown in lane 12 (NE + 100x).
Lanes 1315 examine complex formation with in vitro
prepared CREB, ATF-1, and ATF-2. Each oligonucleotide set was tested on
a minimum of three H295R nuclear extracts.
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As the adrenal expression of CRE-binding transcription factors has not
previously been completely characterized, we carried out Western
analysis using antibodies directed against CREB, ATF-1, and ATF-2 (Fig. 3
). Nuclear extracts from H295R and HeLa
cells as well as homogenates of human adrenal cortex expressed readily
detectable levels of ATF-2 and varying amounts of CREB and ATF-1. CREB
protein was detectable in HeLa nuclear extract, early passage H295R
nuclear extract (H295 NE-1), as well as adrenal lysate. ATF-1
expression appeared higher in the H295R adrenal cell than adrenal
lysate. Interestingly, CREB expression appeared to be lost in H295R
cells cultured for long periods (Fig. 3
, H295 NE-2).
Role of the Ad4 and Ad5 elements in hCYP11B1 transcription
Interestingly, as shown in Fig. 1
, deletion of the Ad4 site
(-242/-234) from hCYP11B1 in pB1214 did not affect basal
or cAMP induction of reporter gene expression. Because cotransfection
of expression vectors containing SF-1 enhanced reporter gene activity
in constructs using 5'-flanking DNA from the bovine CYP11A
(15), human CYP19 (28), mouse CYP21
(29), bovine CYP17 (30), and
bCYP11B (31) genes, we carried out similar
experiments in which H295R cells were transfected with the
hCYP11B1 serial deletion constructs and human SF-1
expression vector (Fig. 4
). Reporter gene
expression was similar using constructs containing -1102 to -330 bp
of hCYP11B1 5'-flanking DNA. However, in cells cotransfected
with SF-1 expression plasmid, deletion to -214 bp decreased luciferase
activity to 53% of that seen in cells transfected with the -330 bp
construct (pB1330). This observation suggests that the Ad4 site is
needed for maximal induction of transcription by SF-1.

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Figure 4. Deletion analysis of the hCYP11B1
5'-flanking DNA to determine SF-1 sensitive regions. H295R cells were
transiently transfected with luciferase reporter constructs containing
serial deletions of hCYP11B1 5'-flanking DNA.
Transfection of reporter constructs was performed with either empty
pcDNA3 expression vector (1 µg/well) or expression vector containing
the coding sequence for bovine SF-1 (1 µg/well). After recovery
for 24 h, cells were lysed, and luciferase activity was measured.
Results are expressed as a percentage of the basal reporter activity of
the pB11102 and represent the mean ± SEM of data
from five independent experiments (*, P < 0.0001
compared with the basal level).
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Sequence analysis revealed two potential SF-1-binding sites that have
previously been observed in the bCYP11B gene and were
designated Ad4 and Ad5 (12). Previous work from our
laboratory demonstrated an important role for the Ad5 element in
hCYP11B2 transcription (11), whereas the Ad4
element (through SF-1 interaction) plays a critical role in
bCYP11B transcription (31). To define the
relative contributions of the Ad4 (-242/-234) and Ad5 (-120/-112)
cis-elements in regulating hCYP11B1
transcription, they were each mutated using the pB11102 plasmid of
hCYP11B1 5'-flanking DNA (Fig. 5
). Mutation of the Ad4 site had little
effect on basal activity, but decreased the stimulatory effect of SF-1
coexpression to 27% of the stimulation seen using the native pB11102
construct. In contrast, mutation of the Ad5 did not significantly
affect luciferase activity. Constructs with mutations of both elements
had basal and SF-1-stimulated levels of reporter activity near that
observed with pGL3 Basic empty vector (not shown).

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Figure 5. Mutational analysis of the Ad4 and Ad5 elements.
H295R cells were transiently transfected with a luciferase reporter
vector driven by pB11102 containing the wild-type Ad4 and Ad5
sequences (-1102 wild-type), the mutated Ad4 sequence (-1102 Ad4
mutant), the mutated Ad5 sequence (-1102 Ad5 mutant), or the Ad4/Ad5
double mutant sequence (-1102 Ad4/Ad5 mutant). Cells were transfected
with reporter construct and either pcDNA3 empty expression (1
µg/well) vector or with expression vector containing coding sequence
for bovine SF-1 (1 µg/well). Where indicated, cells were treated with
(Bu)2cAMP (dbcAmp) (1 mM) for 6 h,
followed by lysis and luciferase assay. Results are expressed as a
percentage of the wild-type driven reporter activity and represent the
mean ± SEM of an average of determinations from two
to four independent experiments.
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To determine whether induction by cAMP could enhance the effects of
SF-1, H295R cells were transfected with native pB11102 or the Ad4,
Ad5, or Ad4/Ad5 mutant constructs, either alone or with the SF-1
expression vector. Transfected cells were subsequently treated with
(Bu)2cAMP for 6 h, whereas control cells
received no treatment (Fig. 5
). When cells were transfected with
pB11102 only and treated with (Bu)2cAMP,
activity increased 5-fold over basal levels. Additionally,
cotransfection with SF-1 in the absence of
(Bu)2cAMP caused a 6-fold increase in reporter
activity above basal levels. When cells were cotransfected with
pB11102 and the SF-1 expression plasmid and treated with
(Bu)2cAMP, there was a synergistic effect,
leading to a 40-fold increase above basal levels. Treatment with
(Bu)2cAMP also enhanced the effects of SF-1 on
the three mutant constructs tested. However, only the
(Bu)2cAMP-treated cells cotransfected with the
Ad5 mutant and SF-1 displayed levels of activity similar to that of the
native pB11102 construct.
To determine whether the Ad4 element could bind SF-1, a synthetic
oligonucleotide probe encompassing this sequence was prepared and used
in EMSA (Fig. 6
). In the presence of
H295R nuclear extracts, two major DNA/protein complexes were formed,
only one of which could be completely displaced by nonradiolabeled
oligonucleotide. In vitro prepared human SF-1 caused
formation of a complex that migrated in parallel to that observed using
H295R nuclear extract. In addition, antibody directed against SF-1
abolished the complex observed using H295R nuclear extract (data not
shown). Oligonucleotides that included a mutated Ad4 element did not
form complexes with SF-1 (Fig. 6
).

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Figure 6. Binding of SF-1 to hCYP11B1 Ad4
element (-244/-233). EMSA was performed using a
32P-labeled oligonucleotide probe that included the
hCYP11B1 Ad4 element -244/-233. Radiolabeled probe
alone is shown in lane 1 (FP), and probe with H295R nuclear extract
(NE; 5.0 µg) is shown in lane 2. Protein/DNA complexes (shown by
arrows) were separated from free probe by
electrophoresis. Nonradiolabeled competitor DNA was added at a
100-fold molar excess ((NE + 100x) to identify specific protein/DNA
interactions. Lane 4 corresponds to labeled probe with in
vitro translated human SF-1. Lanes 58 demonstrate that the
mutated Ad4/SF-1 oligonucleotide no longer binds in
vitro prepared SF-1 proteins. EMSA analysis was performed using
H295R nuclear extract from a minimum of three isolations.
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hCYP11B1 constructs in Y-1 adrenal cells
The only previous analysis of the 5'-flanking DNA from
hCYP11B1 was performed using the Y-1 mouse adrenal cell line
and a chloramphenicol acetyltransferase reporter system (18, 19). In these studies, deletion of the region between -1093 and
-505 bp of 5'-flanking DNA significantly increased reporter activity.
As this is different from what we observed in H295R cells, we tested
our pGL3 reporter constructs in Y-1 adrenal cells (Fig. 7
). Representative constructs (pB1924,
pB1330, pB1214, and pB165 bp) from hCYP11B1
5'-flanking DNA were transfected with empty pcDNA3 expression vector or
with the SF-1-containing expression vector. The results were very
similar to those obtained using H295R cells. The basal level of
reporter expression was not effected by deletion to -330 bp. Deletion
to -214 increased basal activity, whereas further deletion to -65 bp
decreased basal expression. Coexpression with SF-1 caused a 2.6-fold
increase in reporter gene activity. This effect was significantly
decreased by deletion to -214 bp and abolished when the pB165
construct was tested.

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Figure 7. Deletion analysis of the hCYP11B1
5'-flanking DNA to determine SF-1-sensitive regions in Y-1 mouse
adrenocortical cells. Y-1 mouse adrenal cells were transiently
transfected with luciferase reporter constructs containing serial
deletions of hCYP11B1 5'-flanking DNA (1 µg/well).
Transfection of reporter constructs was performed with either empty
pcDNA3 expression vector (1 µg/well) or expression vector containing
the coding sequence for bovine SF-1 (1 µg/well). After recovery for
24 h, cells were lysed, and luciferase activity was measured.
Results are expressed as a percentage of the basal reporter activity of
pB1924 and represent the mean ± SEM of data from
three independent experiments [*, P < 0.0001; **,
P < 0.01 (compared with the basal level)].
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Discussion
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ACTH stimulation of steroid hormone production and expression of
steroid-metabolizing enzymes is controlled though the cAMP signaling
pathway. Interestingly, of the steroid-metabolizing genes to date
studied, only the hCYP11B1 and hCYP11B2 genes
have a near-consensus CRE, whereas different cis-elements
mediate cAMP responsiveness in other steroid-metabolizing genes
(12). A critical role for the Ad1/CRE has been shown for
mouse and human CYP11B2 using deletion and mutational
analysis, but previous studies of the 5'-flanking DNA of rat and human
CYP11B1 did not examine the role of the CRE in
transcriptional regulation (5, 11, 18, 32, 33).
However, there has been considerable work directed toward defining the
mechanisms that regulate transcription of the bCYP11B gene
(12, 14, 16, 31, 34). Cattle, unlike humans and rodents,
perform 11ß-hydroxylation, 18-hydroxylation, and 18-oxidation though
the action of a single 11ß-hydroxylase isozyme (CYP11B)
(35) that is expressed in both the glomerulosa and
fasciculata. Omura and colleagues using DNase I footprint analysis
defined protected regions in the bCYP11B proximal DNA termed
adrenal (Ad) 16 (12). Analyses of the role of each of
the putative cis-elements demonstrated a need for the Ad1
element in basal and cAMP-stimulated expression of reporter activity.
The Ad1 element closely resembled a consensus CRE and bound similar
proteins to the consensus CRE using nuclear extracts from steroidogenic
and nonsteroidogenic cells. However, the nature of these proteins was
not determined. Mutation of the Ad1 element blocked cAMP induction,
thus furthering the idea that the Ad1 site is most likely the key
element in ACTH-regulated induction of transcription of
bCYP11B. Our observations on the hCYP11B1 gene
are very similar to those observed for the bovine gene in that mutation
of the human CRE/Ad1 is critical for both basal and cAMP-dependent
transcription.
All CYP11B genes studied to date are positively regulated by the cAMP
signaling pathway. It was thought that both CYP11B genes would make use
of the Ad1/CRE site for regulation; however, there is a loss of
sequence identity in the mouse and rat CYP11B1 gene
(5'-AGTGACaTtATCA-3') (36) compared with the human Ad1
sequence (5'-CATGACGTGATCC-3'). The divergence in the rat
CYP11B1 sequence may explain why Ishimura and colleagues
observed that an AP-1 sequence in the rat CYP11B1 Ad1/CRE
flanking DNA was important in cAMP induction of transcription of this
gene (37, 38). In support of this premise, the researchers
went on to demonstrate that nuclei in the rat adrenal fasciculata
expressed much higher levels of AP-1-binding proteins (Jun and Fos)
than were observed in the glomerulosa. This contrasts with the
hCYP11B1 gene, in which sequence similarity at the Ad1 site
to a consensus CRE has been maintained while there is base substitution
(compared with the rat AP-1) decreasing the sequence similarly to a
consensus AP-1 site. The rationale for specific differences for the
differential regulation of CYP11B1 is not clear. However,
the CYP11B genes appear to have had considerable evolutionary
divergence, particularly when one considers that bovine, sheep, and pig
have only one CYP11B gene. Further, study of the rodent
CYP11B1 genes will be needed to confirm that the CRE (Ad1)
variations in sequence lead to transcriptional differences.
To better define the trans-acting factors that interact with
the Ad1/CRE we used in vitro prepared CRE-binding proteins
and H295R nuclear extract. CREB, ATF-1, and ATF-2 were able to form
complexes with the Ad1/CRE element. However, when compared with nuclear
proteins from H295R cells only the ATF-2/CRE complex migrated in a
similar manner (complex 1; Fig. 2
). The ability of an ATF-2 antibody to
supershift complex 1 further supports the idea that ATF-2, within these
cells, binds the hCYP11B1 Ad1/CRE site. The nature of
complex 2 remains to be determined. Although it did not migrate with
in vitro prepared ATF-1 or CREB, the possibility of
heterodimers of these proteins with other factors cannot be ruled out
at this time. To determine whether members of the CREB/ATF family of
proteins are expressed in H295R cells and adrenal cortex, we used
Western analysis. H295R nuclear extracts and adrenal lysates expressed
ATF-2, supporting the EMSA observations. Adrenal lysate also expressed
detectable levels of CREB and ATF-2 with very low levels of ATF-1. A
recent study of H295R cell expression of members of the CREB/ATF family
found no expression of CREB (39). Our data suggest that
certain strains (but not all) of the H295R cells have lost CREB
expression. The mechanism causing the loss of CREB expression is not
clear. However, Groussin and colleagues (39) demonstrate
an increase in the level CREM expression that they suggest compensates
for the loss of CREB. The expression of ATF-2 in adrenal cells and
tissue has not previously been described. Activation of ATF-2 can occur
through a number of signaling pathways used by certain hormones and
growth factors, including alterations in intracellular calcium. Further
experiments will be needed to determine which members of the
CRE-binding protein family can enhance the transcription of the
hCYP11B1 gene.
In addition to the Ad1/CRE, hCYP11B1 gene transcription
relied on the Ad4 element previously characterized in
bCYP11B. The trans-acting protein that binds the
Ad4 sequence was termed Ad4-binding protein (Ad4BP) and was
subsequently cloned from a bovine adrenal complementary DNA (cDNA)
library (31). Ad4BP is equivalent to SF-1 and is expressed
in other steroidogenic cells (29, 40). This protein is
considered a key transcription factor in the regulation of a number of
steroid hydroxylases as well as the steroidogenic acute regulatory
(StAR) protein and 3ß-hydroxysteroid dehydrogenase genes
(41, 42, 43). Herein, we demonstrated that the Ad4 site in the
hCYP11B1 gene was important for transcription; however, its
role was only apparent when cells (either Y-1 or H295R) were
cotransfected with an SF-1 expression vector. The rationale for a need
to overexpress SF-1 to induce hCYP11B1 reporter activity via
the Ad4 element is not clear, as by EMSA (Fig. 6
) and immunoblot
analysis (data not shown) the H295R and Y-1 cells express SF-1.
Nevertheless, overexpression of SF-1 has also been shown to greatly
increase reporter gene expression for constructs prepared from the
CYP17, 3ßHSD, CYP11A, and CYP19 genes (15, 28, 29, 30, 31). The
data thus support the requirement for SF-1 for maximal basal and
cAMP-stimulated transcription of hCYP11B1. This need for
SF-1 sharply differs from the effects of SF-1 on hCYP11B2
(aldosterone synthase) transcription (11).
CYP11B2 transcription is not increased by SF-1 coexpression
and mutation of the Ad4 element does not affect reporter gene
expression. In the case of hCYP11B2, we have previously
shown an important role for the Ad5 site in basal and stimulated
expression of reporter genes. Thus, these two isozymes have diverged in
the key cis-elements regulating gene transcription.
In summary, the regulation of the CYP11B genes appears to have evolved
species-specific cis-regulatory elements to control
transcription. In humans, hCYP11B1 and hCYP11B2
rely on the proximal Ad1/CRE site for transcription. This may differ in
mouse and rat CYP11B1, where the Ad1 site has lost identity
with the consensus CRE. Second, the requirement for SF-1 appears to
have diverged between hCYP11B1 and hCYP11B2. For
hCYP11B1 the Ad4 cis-element and expression of
SF-1 are both required for maximal transcription. However, this is not
the case for hCYP11B2, where the Ad4 element does not appear
necessary for transcription (11). Such differences between
the CYP11B1 and CYP11B2 genes may play an
important role in the differential expression of these isozymes within
the adrenocortical zones.
Received December 28, 1999.
 |
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