(Received for publication, November 7, 1996, and in revised form, January 19, 1997)
From the Department of Population Dynamics, Division
of Reproductive Biology, Johns Hopkins University School of Hygiene and
Public Health, Baltimore, Maryland 21205 and the
§ Department of Molecular Biology and Genetics and Johns
Hopkins University School of Medicine, Baltimore, Maryland 21205
Transcriptional regulation by the androgen receptor (AR) requires its binding to hormone response element nucleotide sequences in DNA. A consensus glucocorticoid response element (GRE) can mediate transactivation by AR and other members of the AR/glucocorticoid (GR)/progesterone (PR)/mineralocorticoid (MR) receptor subfamily. We identified putative androgen response element (ARE) sequences by binding of a human AR DNA-binding domain fusion protein to DNA in a random sequence selection assay. A 17-base pair consensus nucleotide sequence, termed IDR17, containing three potential GRE-like core binding sites organized as both inverted and direct repeats, was determined from a pool of degenerate oligonucleotides. IDR17 was active in mediating androgen-dependent induction of reporter gene expression in transient transfection assays. Dissection of the IDR17 sequence revealed an 11-base pair sequence (DR-1), consisting of two potential core binding sites oriented as an overlapping direct repeat, as the most potent ARE. DR-1 demonstrated a strong preference for AR binding and transactivation when compared with GR. To our knowledge, this is the first observation that a direct repeat of GRE-like core motifs functions as a preferred hormone response element within the AR/GR/PR/MR subfamily of nuclear receptors.
Nuclear hormone receptors constitute a family of transcription factors that function by binding to specific DNA sequences in regulatory regions of target genes known as hormone response elements (HREs)1 (1-5). The general structure of HREs has been well characterized (6-8). They are generally composed of six base pair receptor binding sites, oriented as inverted or direct sequence repeats and separated by a variable number of spacing nucleotides. Specificity of an HRE is a property of the primary sequence of the individual binding sites as well as the spacing and orientation of the binding motifs (7-12). Specificity of hormone action may also be conferred by interaction between the receptor and other transcription factors (6, 7). In this case, the HRE is a more complex element that includes multiple simple HREs (partial palindromes or a single binding site) together with adjacent binding sites for other factors (13-16).
HREs comprise several subgroups corresponding to evolutionary conservation among subfamilies of the receptors. Androgen receptors (ARs) share a high degree of homology with glucocorticoid receptors (GRs), progesterone receptors (PRs), and mineralocorticoid receptors (MRs) in their DNA-binding domains (17-22). A well characterized HRE for this subfamily of receptors is the glucocorticoid response element (GRE), which is composed of inverted nonidentical hexamer binding sites separated by a spacing of 3 nucleotides, GGTACAnnnTGTTCT (6-8, 23). This sequence mediates androgen-induced, as well as glucocorticoid- and progesterone-induced, gene expression (23-26). Another subfamily of nuclear receptors consists of the thyroid hormone (TR), retinoic acid (RAR), and vitamin D (VDR) receptors (7-9). A striking feature of this latter subfamily of receptors is their recognition of a similar consensus nucleotide sequence binding site, with specificity of hormone response being determined by different spacing and orientation of the binding site motifs. Spacing of the consensus binding half-sites by 3, 4, or 5 nucleotides creates HREs for VDR, TR, and RAR, respectively (7, 10-12). In addition to direct repeats, both TR and RAR can recognize HREs consisting of inverted receptor binding sites (11, 12).
Naturally occurring androgen response elements (AREs) fall into two
categories. Some AREs, such as those identified in the C3 subunit gene
of rat prostatic binding protein and mouse sex-limited protein (13, 27,
28), consist of GRE-like sequences with inverted binding sites
separated by a 3-bp nucleotide spacer. In other AREs, like those in the
probasin (29) and 20-kDa cystatin-like (16) genes, only single binding
site sequences are apparent. However, a recent report showed that the
5-subsequence, 5
-GGTTCT-3
, within the partial palindrome of the
probasin ARE-2 binding site, excludes GR binding but permits binding of
AR (30). Another possibility for receptor-specific binding is that
sequences adjacent to receptor binding sites create composite
activation elements that also involve the binding of other factors
(13-16). An additional hypothetical means of conferring hormone
specificity is the spacing and orientation of binding sites within the
DNA sequences that bind members of the AR, GR, PR, and MR subfamily. GR
and AR were recently shown to bind to direct repeat DNA elements (30,
31) in addition to the more typical inverted repeat sequences. In this
report, we provide evidence that a member of this subfamily, namely AR,
can activate transcription of a reporter gene through an HRE consisting
of a novel direct repeat nucleotide sequence that preferentially binds
AR.
The following plasmid DNAs were generous gifts: OB7 (hGR) from Dr. R. Evans (Salk Institute for Biological Studies, La Jolla, CA); pCMV-hGR from Drs. F. French and J. Tan (University of North Carolina, Chapel Hill, NC); pBLCAT2 from Dr. C. Young (Mayo Clinic, Rochester, MN). MMTV-CAT and pCMV-hAR were used in our previous studies (32). Plasmid pGEX-2T was purchased from Pharmacia Biotech Inc., and pBluescript was obtained from Stratagene (La Jolla, CA).
Construction of hAR and hGR Prokaryotic Expression VectorsFragments of human AR and GR DNA-binding domains were
prepared by PCR amplification under standard conditions from hAR
(pCMV-hAR) and hGR (OB7) cDNAs. For hAR, the sense primer was
5-CCCGGAATTCCCTGCCTGATCTGTGGAGATGAA-3
, which hybridizes to nucleotide
positions 2035-2054, and the antisense primer was
5
-CCCGGGAATTCCCTCTCCTTCCTCCTGTAGTTT-3
, which hybridizes to
nucleotides 2274-2295 (17). For hGR, the sense primer was 5
-CCCGGAATTCCCTGCCTGGTGTGCTCTGATGAA-3
, which hybridizes to
nucleotides 1392-1413, and the antisense primer was
5
-CCCGGGAATTCCTGTAGTGGCCTGCTGAATTCC-3
, which hybridizes to
nucleotides 1627-1650 (33). The PCR products were digested with
EcoRI and ligated into pGEX-2T plasmid. Plasmid DNAs were
subjected to restriction and sequence analysis to verify the size,
orientation, and reading frame of the cloned inserts.
Freshly diluted cultures of pGEX-hAR or pGEX-hGR were
grown for 1 h prior to induction of protein expression by growth
in 1 mM isopropyl--D-thiogalactopyranoside
for an additional 3.5 h. Cells were collected by centrifugation
(5000 × g, 10 min, 4 °C) and lysed by sonication in
5 ml of ice-cold phosphate-buffered saline containing 1 mM
phenylmethylsulfonyl fluoride. After the addition of 1% Triton X-100
and centrifugation (10,000 × g, 5 min, 4 °C), the
supernatant was incubated (25 °C, 4 min) with 1 ml of 50% (w/v)
glutathione-agarose beads with gentle mixing. The beads were washed
three times with 10 ml of ice-cold phosphate-buffered saline. The
fusion proteins were eluted from the beads by incubating (25 °C, 4 min) with 0.5 ml of 50 mM Tris-HCl (pH 8.0) containing 15 mM reduced glutathione. The beads were recovered by
centrifugation, and the elution step was repeated four or five
times.
For random sequence selection
assays, a pool of degenerate 55-bp oligonucleotides was synthesized.
The 5 and 3
termini contained restriction endonuclease sites for
BamHI and EcoR I, respectively, for subcloning
purposes and were utilized for primer annealing to convert
oligonucleotides from single- to double-stranded DNA and for subsequent
PCR amplification. The sequence of Oligo 1 was
5
-AGACGGATCCATTGCAATAN18ATCCTGTAGGAATTCGGA-3
. The
sequence of Oligo 2 was
5
-AGACGGATCCATTGCAAN13TGTTCTGATCCTGTAGGAATTCGGA-3
. The pair of primers for both Oligos 1 and 2 were as follows: primer A
(sense), 5
-AGACGGATCCATTGCA; primer B (antisense), 5
-TCCGAATCCCTACAG. Double-stranded Oligo 1 and Oligo 2 were generated by annealing each
oligonucleotide to a 10-fold molar excess of primer B and extending the
complementary strand with Klenow fragment of E. coli DNA
polymerase in the presence of dNTPs. The double-stranded Oligo 1 and
Oligo 2 were purified on 10% polyacrylamide gels and eluted from the
gel by agitation in 0.5 M ammonium acetate, 1 mM EDTA, 0.1% SDS at 37 °C for 12 h.
A single-stranded 55-bp oligonucleotide containing a consensus GRE,
5-AGACGGATCCATTGCAAAGTCCAGGATCCTGTAGGAATTCGGA-3
was synthesized and made double-stranded by annealing primer B and extending it with Klenow fragment of DNA polymerase as described above.
Other double-stranded oligonucleotides, IR0
(5-CTAGATCCTGAAGACTGAGT),
IR5
(5
-CTAGACTGAATCGAGGTTCTGACTT) and DR-1
(5
-CTAGATCCTGAAGACTGAGCT)
were made by annealing equal amounts of their complementary
oligonucleotides.
DNA was
5-end labeled with [
-32P]ATP using T4 polynucleotide
kinase under standard conditions, purified on 5-10% polyacrylamide gels, and recovered by elution from the gel as described above. Approximately 0.5 pmol (105 cpm) of 32P-labeled
double-stranded DNA were incubated with 200 ng of the AR-DBD fusion
protein in 20 µl of 20 mM HEPES (pH 7.6), 50 mM KCl, 3 mM MgCl2, 1 mM EDTA, 0.5% Nonidet P-40, and 8% glycerol at 25 °C
for 15 min. Poly(dI-dC) (500 ng) was added to each reaction to reduce
nonspecific binding. The incubation mixtures were loaded on 5%
nondenaturing polyacrylamide gels, and EMSAs were run at 10 V/cm for
4 h in 100 mM Tris-HCl (pH 8.0), 100 mM
boric acid, 1 mM EDTA buffer at 4 °C. For analytical
purposes, the gels were dried before autoradiography. For preparative
purposes, gels were exposed directly without drying for 1-3 h at
4 °C to X-Omat film for localization of the DNA and DNA-protein
complex(es). Regions of the gel containing the shifted bands of DNA
were excised and recovered by elution. Half of the recovered DNAs were
amplified by PCR, radiolabeled, and used for successive rounds of
selection. For EMSA in other experiments, oligonucleotides (GRE, IDR17,
and DR-1) with defined sequences were used as probes. The DR-1 (2x) probe represented a dimer of annealed and ligated DR-1 oligonucleotides derived by digesting the DR-1-pBLCAT plasmid (described below) with
HindIII and BamHI. A 90-bp DNA fragment was
isolated and radiolabeled by Klenow fragment in the presence of
[
-32P]dCTP. For these experiments, 50-500 ng of
either AR-DBD or GR-DBD fusion proteins were incubated with
32P-labeled DNAs in binding reactions. For competition
assays, the proteins were first incubated with 10-1000-fold excess of
unlabeled double-stranded oligonucleotide competitor at 25 °C for 10 min before the addition of the radiolabeled DNAs.
Oligonucleotide DNAs recovered from EMSA were dissolved in 10 µl of 0.1 × TE buffer. For PCR amplification, 5 µl of the DNAs were used as templates in a standard 50-µl reaction containing 50 pmol each of primers A and B. The amplified DNAs were purified on 10% polyacrylamide gels and recovered by elution as described above.
The DNAs amplified by PCR after the last round of selection were digested with BamHI and EcoRI to create cohesive ends and ligated into the plasmid, pBluescript KS+. Individual clones were sequenced by the dideoxynucleotide termination method (Sequenase 2.0 kit, U.S. Biochemical Corp.).
Construction of Chimeric Reporter GenesSingle-stranded oligonucleotides IDR17, IR0, IR5, and DR-1 were phosphorylated, and equal amounts of the complementary strand of each oligonucleotide were annealed to generate cohesive ends with overhangs corresponding to XbaI recognition sites. The double-stranded oligonucleotides were concatamerized with T4 DNA ligase, and the ladder of concatamerized DNAs was separated on 5% polyacrylamide gels. Dimers of IDR17, IR0, IR5, and DR-1 were excised from the gel, eluted, recovered by ethanol precipitation, and ligated into the XbaI site of the pBLCAT2 plasmid. Positive clones were screened by restriction mapping and confirmed by sequencing.
Cell Culture and DNA TransfectionCV-1 cells were
transfected with plasmid DNAs by the calcium phosphate/DNA
co-precipitation method, as described previously (32). The cells were
subsequently cultured in Dulbecco's modified Eagle's medium
containing 5% charcoal-stripped fetal bovine serum in the absence or
presence of 0.01-10 nM R1881 (methyltrienolone) or 1-1000
nM dexamethasone for 16-24 h. The CAT reporter gene plasmids IDR17-, IR0-, IR5-, and DR-1-pBLCAT2 or MMTV-CAT were cotransfected with the receptor expression vector pCMV-hAR or pCMV-hGR.
In all experiments, the pCH110 plasmid expressing -galactosidase was
also cotransfected as a control for the efficiency of transfection. The
amount of each plasmid used in the transfection experiments was
equivalent to 5 µg of DNA/culture dish. The reporter gene plasmids
were used without cotransfection of the receptor expression vector or
with the "empty" expression vector as negative controls for the
specificity of steroid induction.
CAT and -galactosidase enzyme activities were assayed in 50-µl
aliquots of the cell extracts as described previously (32). CAT
activity was quantitated by cutting and counting the radioactive areas
from the thin layer chromatography plates by scintillation spectrophotometry. CAT activities were normalized for transfection efficiency based upon their corresponding
-galactosidase
activities.
The method of random sequence selection and amplification originally described by Blackwell and Weintraub (34) was adopted to isolate AR binding sites on DNA from a pool of oligonucleotides with a region of degenerate nucleotide sequence. The AR protein was a recombinant protein composed of amino acids 559-644 of the human AR-DBD fused to glutathione S-transferase (GST). This fusion protein was expressed in Escherichia coli and purified by affinity binding to glutathione agarose and elution in the presence of reduced glutathione. In preliminary studies, the AR-DBD fusion protein was incubated with an oligonucleotide containing a consensus GRE/ARE nucleotide sequence and was shown to form a protein-DNA complex by EMSA. For the random sequence selection assay, the AR-DBD fusion protein was incubated with a 55-bp oligonucleotide containing an 18-bp central region of totally degenerate nucleotide sequence (N18). The presence of an 18-bp degenerate region was based upon the assumption that a 15-bp consensus GRE sequence was sufficient for binding of an AR dimer and that a specific binding site for AR would resemble this model. On the initial round of selection, binding of the AR-DBD fusion protein to a small percentage of the degenerate oligonucleotides was detected by EMSA (data not shown). The DNA was isolated from the gel and amplified by PCR, to enrich the population of DNAs bound by the AR-DBD fusion protein, prior to the next round of selection. As expected, the proportion of selected and amplified DNA that bound to the AR-DBD fusion protein increased during successive rounds of selection by EMSA. Following six rounds of selection, DNA from the shifted protein-DNA complex was isolated and subcloned, and 39 individual clones were sequenced (Table I). Alignment revealed a single hexamer nucleotide consensus binding site, TGTTC(T/C), identical to a GRE half-site, within the degenerate region as a preferred sequence for AR binding.
|
Based upon these preliminary studies and the similarity of the
consensus hexamer binding site to that within known GREs that bind AR,
we synthesized a second pool of 55-bp oligonucleotides containing a
13-bp region of degenerate nucleotide sequence (N13) adjacent to a 6-bp (TGTTCT) consensus AR/GR/PR binding site,
i.e. 5-N13TGTTCT-3
. We predicted that the
consensus binding site sequence would accommodate binding of one AR
molecule and that the random sequence would allow identification of a
second binding site with optimal sequence, spacing, and orientation to
confer the cooperative binding of AR monomers residing on the DNA. By restricting the region of degenerate sequence to 13 bp, we
theoretically could examine the complete pool of 6.7 × 107 possible sequences.
Binding of the AR-DBD fusion protein to the 13-bp degenerate
oligonucleotide pool is shown in Fig. 1. In the first
round of selection, one shifted band was observed by EMSA. This band
presumably represents a DNA-AR monomer complex. The binding site for
the AR monomer could be the defined hexamer, TGTTCT, or another
half-site located within the 13-bp degenerate region. Alternatively,
some oligonucleotides containing two binding sites positioned
appropriately for dimer binding may be bound by an AR dimer. Additional
complexes with slower mobilities were not detected in the first round
of selection, presumably due to their very low abundance within the oligonucleotide pool. DNA was recovered from the single shifted band,
amplified by PCR, and incubated with the AR-DBD fusion protein for the
second round of selection by EMSA. Two shifted bands, one of slower
mobility than observed during the first round of selection, appeared on
the gel. Based upon the reduced mobility of this DNA-protein complex,
we presumed that this additional complex represented the binding of two
molecules of AR-DBD fusion protein. The DNA was recovered from this
less mobile complex and enriched during subsequent rounds of selection
and amplification.
After six rounds, DNA from the less mobile complex was isolated and subcloned, and the nucleotide sequence of individual clones was determined. As shown in Table II, a conserved region of 11 bp with the consensus sequence -GGAACGGAACA- was obtained. This selected sequence includes two potential AR binding sites immediately adjacent to the original site specified by the sequence, -TGTTCT (see Fig. 4A). One potential binding site is a 6-bp inverted repeat, -GGAACA-, which allows no intervening spacer between itself and the original binding site, -TGTTCT. In addition, there is another 5-bp sequence, GGAAC-, which forms a direct repeat, overlapping by 1 bp with the adjoining 6-bp sequence, -GGAACA. We named this putative 17-bp AR binding site sequence IDR17 due to its content of both inverted repeats (IRs) and direct repeats (DRs).
|
Functional Activity of IDR17
To examine the IDR17 consensus AR binding site sequence as a functional ARE, two copies of IDR17 were cloned into the pBLCAT2 plasmid upstream of the thymidine kinase promoter. The IDR17-pBLCAT reporter gene construct was tested for androgen-dependent expression of CAT activity following cotransfection with the human AR cDNA expression vector, pCMVhAR, into CV-1 cells. The synthetic androgen, R1881, induced a dose-dependent increase of CAT activity, which was not observed in the absence of androgen or in cells transfected with the empty pCMV5 expression vector (data not shown). These results indicate that the IDR17 sequence functions as an ARE in vivo.
To examine the specificity of hormone response, IDR17-pBLCAT was
compared with MMTV-LTR-CAT, which contains four GRE/AREs in its
5-flanking sequence. Expression plasmids containing either the human
AR or GR cDNAs were co-transfected with a reporter gene plasmid,
and cells were incubated with R1881 or the synthetic glucocorticoid,
dexamethasone. In contrast to MMTV-LTR-CAT, where CAT activity was
induced to a higher level by glucocorticoid than by androgen in the
presence of their respective receptors, IDR17-pBLCAT was much more
responsive in the presence of androgen and its receptor than in the
presence of glucocorticoid and its receptor (Fig. 2).
Nearly maximal induction of CAT activity by AR with IDR17 occurred at 1 nM R1881, whereas much lower stimulation of CAT activity by
GR was evident with IDR17 even at 100 nM dexamethasone. These results clearly demonstrate that IDR17 not only functions as an
efficient ARE but also that it is preferentially induced by androgens
when compared with the generic GRE/ARE-like hormone response elements
of MMTV-LTR that respond more favorably in the presence of GR rather
than AR.
The binding site specificity of IDR17 for AR was also tested by EMSA.
Binding of the AR-DBD fusion protein to IDR17 was compared with the
binding of a similar human GR recombinant protein containing an
analogous region of its DNA-binding domain amino acids (421-506) fused
to GST. An oligonucleotide containing a perfect 15-bp palindromic GRE
nucleotide sequence (-AGAACAcagTGTTCT-) with the same flanking sequence
as IDR17 was tested in parallel. As shown in Fig. 3, the
AR-DBD and GR-DBD fusion proteins bound quantitatively similarly to the
consensus GRE at each of several protein concentrations. By contrast,
the AR-DBD fusion protein bound with high affinity to IDR17 and
displayed a distinctive pattern of protein-DNA complexes, whereas
binding of the GR-DBD fusion protein to IDR17 was relatively weak. This
pattern of protein-DNA complexes was also characterized by reproducible
differences in the relative mobility of complexes formed by AR compared
with those formed by GR with the identical DNA fragments, suggesting
possible differences in their binding conformations. This experiment
further validates the specificity of IDR17 as an androgen receptor
binding site.
Dissection of IDR17 Binding Sites
Three potential dimeric
binding sites can be proposed among the three putative hexamer
sequences within IDR17. To determine which of these sequences acts as
an ARE, we created the oligonucleotides shown in Fig.
4A. For oligonucleotide IR0, nucleotide bases
n1-5 of IDR17 were substituted with nonspecific
nucleotides, leaving only the hexamer IRs without a spacer
(n0) of nonspecific nucleotides. For IR5, nucleotides
n7-11 of IDR17 were substituted by five (n5)
nonspecific bases, leaving the two 6-bp binding sites at the 5 and 3
ends, respectively, as IRs. For DR-1, the 3
binding site specified by
-TGTTCT was substituted by nonspecific nucleotides, leaving only an
11-bp sequence containing a 1-bp overlap (n
1) of two
hexameric DRs. Two copies of each IDR17 derivative were cloned into
pBLCAT2 upstream of the thymidine kinase promoter and cotransfected
with AR to test the activities of IDR17 and its derivatives in
androgen-dependent induction of CAT reporter gene
expression (Fig. 4B). When transfected CV-1 cells were
incubated in the absence and presence of different concentrations of
R1881, IR0-pBLCAT showed constitutive CAT activity that was independent
of androgen induction. IR5-pBLCAT was not stimulated by androgen and
showed only basal expression. DR-1-pBLCAT, however, was highly
responsive to androgen induction, reaching a level 180-fold above base
line, and CAT activity exceeded that achieved by IDR17-pBLCAT
with equivalent concentrations of R1881. The transactivation function
of DR-1 was strictly androgen-dependent and required
cotransfection of the AR.
The finding that AR was able to
activate transcription from the direct repeat sequence, DR-1, was
unexpected in light of the structure of other ARE/GRE/PRE sequences
identified to date. To determine whether DR-1 was a binding site for
AR, the AR-DBD fusion protein was incubated with the DR-1
oligonucleotide and analyzed by EMSA. When a single DR-1 nucleotide
sequence was present in the DNA, a low level of binding was observed
for the shifted DNA-AR complex (Fig. 5A).
However, when a tandem repeat of the DR-1 nucleotide sequence (DR-1,
2x) was present in the DNA fragment used as the probe, the binding of
AR was greatly enhanced, and multiple shifted protein-DNA complexes
were seen (Fig. 5B). We conclude that a single copy of the
DR-1 sequence functions as a binding site for AR and that under
conditions favoring cooperative binding of AR as in the head-to-tail
tandem repeat (DR-1, 2x), this same nucleotide sequence functions as an
even higher affinity AR binding site.
DR-1 Is Specific for AR
The binding of AR-DBD and GR-DBD to
the tandem repeat of the DR-1 oligonucleotide sequence was compared by
EMSA and contrasted to the binding of each receptor to a consensus
15-bp palindromic GRE nucleotide sequence. As shown in Fig.
6, DR-1 had much higher affinity for AR than for GR,
whereas GR and AR bound to the GRE with similar affinity. These
findings suggest that the DR-1 sequence is an AR-selective binding
site.
DR-1 also mediates androgen-selective activation of gene transcription
in transfection assays. Fig. 7 shows a comparison of CAT
reporter gene expression mediated through binding of AR or GR to the
DR-1 nucleotide sequence in the presence of R1881 or dexamethasone,
respectively. R1881 induced a 10-12-fold increase in CAT activity by
AR compared with a minimal (0.8-fold) effect of dexamethasone on CAT
activity by GR, when a single copy of the DR-1 element was present.
Reversing the orientation of the DR-1 element had no effect on the
induction of CAT activity by AR or GR. As shown previously in Fig.
4B, tandem copies (2x) of DR-1 produced a
synergistic response in CAT activity by AR in the presence of R1881,
87-fold above base line, when compared with a single copy of DR-1
(11-fold above base line). In addition, stimulation of CAT activity by
AR was maximal at 1 nM R1881 and 5-fold greater than the
maximal CAT activity induced by GR at the highest concentration (100 nM dexamethasone) of glucocorticoid. In the same
experiment, GR and AR induced CAT activity maximally by 92- and
35-fold, respectively, from the MMTV-LTR promoter, in the presence of 1 nM R1881 and 1 µM dexamethasone (data not shown). These results allow us to conclude that DR-1 binds AR selectively and with high affinity, leading to the preferential androgen stimulation of gene expression.
How specificity of steroid hormone action is achieved remains an important question. In principle, specificity can be generated at any step in a signal transduction pathway. One obvious level of cellular control is determined by the differential expression of specific steroid receptors in target cells. The presence of specific HREs as genetic codes for selective regulation of gene transcription also plays a key role in hormone action (1-8). Although a consensus HRE has been associated with transcriptional regulation by the AR/GR/PR/MR subfamily of nuclear receptors (23-27), it is not known how the specificity of these individual receptors is determined. For instance, prostate cells express AR, GR, and PR, yet the actions of these hormones and their specificity for activating or repressing gene expression differ considerably (35, 36). Within the subfamily of receptors composed of TR, RAR, and VDR, both spacing and orientation of similar core binding motifs within the HREs determine receptor specificity (7-12). In an effort to understand how cells might distinguish among AR, GR, and PR actions, we hypothesized that a specific binding site sequence might differentiate AR action from other receptors of its subfamily, and we searched for an ARE in the present studies.
To identify a specific ARE with a relatively unbiased, sensitive, and
simple approach, we adopted the random sequence selection method (34)
and utilized a two-stage approach. In the first stage, binding of the
AR-DBD fusion protein to a pool of random oligonucleotides formed a
single DNA-protein complex with the consensus receptor binding
half-site, TGTTC(T/C), indicative of binding by a receptor monomer. We
also observed a single shifted protein-DNA complex when the consensus
GRE containing two half-sites as inverted repeats with a 3-bp
nucleotide spacer was bound to human AR-DBD and GR-DBD fusion proteins
until greater concentrations of either fusion protein led to the
formation of additional higher order complexes. Using
-galactosidase-human AR or protein A-rat AR fusion proteins, Young
et al. (37) and De Vos et al. (38), respectively,
also observed a single DNA-protein complex with the consensus GRE
sequence on gel shift assays.
The absence or weak ability for dimerization or cooperative binding of the AR-DBD fusion proteins may be due to absence of the receptor N and C termini, which contain strong dimerization functions that are active in solution (39). Although a weak dimerization domain is present in the DNA-binding region of the steroid receptors (7, 8, 40-42), this interaction may occur predominantly following binding to DNA, which places the receptor monomers in the preferred orientation (12). The presence of prokaryotic protein domains in the fusion proteins may also alter the cooperative interaction of the receptor-DBD dimerization domains, since glutathione S-transferase is known to form homodimers. In normal target cells, the native receptor proteins may also interact with other factors involved in transcriptional regulation, and such interaction may increase the specificity, as well as the affinity, of receptors for the cognate response elements (6, 12, 43).
To enhance the probability of finding the optimal half-site spacing for
an ARE, we synthesized a second pool of oligonucleotides containing the
previously defined half-site sequence, TGTTCT, adjacent to a 13-bp
random sequence. The appearance of a second lower mobility DNA-protein
complex in the second and subsequent rounds of binding and selection
suggests the involvement of more than one AR molecule in the binding to
individual oligonucleotides. By contrast, a previous study by Roche
et al. (44), using a random sequence selection method and a
protein A-rat AR DNA-binding domain fusion protein, selected a 15-bp
imperfect palindrome, 5-GG(A/T)ACAnnnTGTTCT-3
, which is the same as
the consensus GRE. Roche et al. (44) used oligonucleotides
with a longer 26-bp random sequence, such that all potential sequences
may not have been represented in the starting pool. Binding of AR
protein to the selected oligonucleotides resulted in two shifted
complexes in the second stage of our study, whereas Roche et
al. observed a single protein-DNA complex, as in our first stage
of selection. The presence of different fusion domains for the
recombinant AR proteins, such as GST in the present study and protein A
in Roche et al., may also create new specificities or
restrict potential binding to DNA sequences. The inclusion of GST did
not restrict binding, but perhaps protein A interfered with binding to
the DR-1 sequence. Binding of the GST-AR-DBD was not created by the fusion partner, since full-length AR activates transcription in vivo through DR-1.
The IDR17 sequence identified in our study features both fundamental similarities and striking differences to the consensus GRE. The primary half-site sequence, TGTTC(T/C) (or its inverted repeat GGAACA), which is almost identical to the GRE half-site sequence, was identified from both the totally random (n18) and partially random (n13) oligonucleotide pools. This similarity might be explained by the proximal (P) box located at the C-terminal stem of the first zinc finger of AR and GR, which share identical amino acid sequences, and is considered to be critical in specifying the primary nucleotide sequence of the HRE half-site (7, 8, 45-47). Despite agreement of the half-site sequences of IDR17 and GRE, the spacing and orientation of the binding sites are remarkably different. Differences in amino acid composition of the distal (D) box and the second zinc finger of the DNA-binding domain of AR and GR could contribute to the spacing and orientation differences of the half-sites (7, 9, 12), although AR has only one residue difference from GR among the five residues that compose the D box (7, 8). Whereas sequences outside the DBD have been implicated in dimer formation, the ability of homo- or heterodimers to bind properly spaced half-sites can be observed for DBDs alone (48-52). While these domains do not form dimers in solution in the absence of DNA, the target DNA serves as a scaffold that induces specific interactions between adjacent monomers, thus stabilizing the DNA-protein complex (53-55).
IDR17 contains three potential binding sites with both inverted and direct repeat configurations and functions as a preferred sequence for AR binding and androgen-dependent transactivation. When present in reporter gene constructs and cotransfected with the full-length AR into CV-1 cells, it is the truncated DR-1 sequence, however, instead of the full-length IDR17, that activates gene transcription most efficiently. Recent work has demonstrated that TR complexes can contact at least 20 nucleotides, including upstream and downstream flanking sequences, a spacer, and two half-sites (56, 57). In fact, random sequence selection assays determined that the optimum binding site for TR actually consisted of an octamer nucleotide sequence rather than a hexamer (57). Furthermore, direct repeats of the optimal binding site, separated by 1-5 bp, all functioned as equally strong TREs (12, 49). More recently, direct repeats of TGTTCT and RGGTCA motifs have been shown to function as response elements for GR and ER, respectively (31). In addition, binding of these receptors to direct repeats with different spacings between the half-sites suggested that binding to direct repeats was more flexible than binding to palindromic elements (31). However, binding of AR to a direct repeat, DR-1, with a one-base overlap remains unique. The asymmetrical head-to-tail arrangement of receptor dimers implied by the tandem of direct repeats suggests that the receptor subunits interact through a DNA-supported interface involving the carboxyl-terminal extension of the DNA-binding domain (12) or the ligand-binding domain (31) rather than the dimerization domain in the D box of the DNA-binding domain, which appears to be nonfunctional under such conditions. Studies specific to homodimerization of the ligand-activated AR have suggested that monomers were oriented antiparallel to each other, allowing interaction between N- and C-terminal domains of the opposing monomers (58).
A number of GRE-like sequences have been identified in the 5-flanking
or intron regions of some androgen-regulated genes, such as rat
prostatic binding protein C3 subunit (27, 28), rat probasin (29), human
prostate specific antigen (38), human glandular kallikrein (59), and
mouse sex-limited protein (Slp) (13). However, a more comprehensive
understanding of what composes an ARE may require a more broad-based
search, without bias for a prerequisite GRE-like sequence. A
nonconsensus androgen-responsive region, 5
-CAGGGATC AGGGAGTCTCAC-3
,
that binds AR and cooperates in androgen-regulated activity of the
prostate-specific antigen promoter has recently been reported (60). In
fact, most of the GRE-like sequences located in regulatory regions of
androgen-responsive genes contain only single receptor binding sites,
and the transcriptional activities of these AREs are relatively weak
unless assayed in the context of their surrounding nucleotide
sequences. Notably, a sequence located in the first intron (In-1) of
the androgen-regulated 20-kDa protein gene (16), 5
-TGTCCTGTTCC-3
,
resembles the direct repeat sequence identified in our study. The
227-bp fragment (D1) containing this sequence and several other GRE
half-site sequences functions as an androgen-specific enhancer. The
element, 5
-GGTTCTtggAGTACT-3
, in the promoter region of the probasin
gene selectively interacts with the DNA-binding domain of the rat AR
and not with that of the GR (60). Furthermore, it was concluded that
the left subsequence, 5
-GGTTCT-3
, was responsible for excluding the
binding of GR (60). Interestingly, direct repeats of GGTTCT separated
by a 3-bp spacer were able to specifically bind AR and not GR (60). The
observation that direct repeats of binding sites can function as AREs
provides the basis for a possible mechanism by which AR preferentially
interacts with specific DNA sequences.
In summary, a 17-bp androgen receptor-specific binding site was identified. This sequence shares the same receptor binding site core motif as the consensus GRE but differs in the context in which these core motifs are arranged. The identified sequence showed preferential binding to AR in gel mobility shift assays and specific response to androgen induction in transfection assays, indicating that an ARE sequence distinct from the consensus GRE may confer androgen specificity.
We thank Dr. R. Evans (Salk Institute) for the gift of the human GR cDNA, Drs. F. French and J. Tan (University of North Carolina, Chapel Hill) for the pCMV-hGR expression plasmid, and Dr. C. Young (Mayo Clinic) for the pBL-CAT2 reporter gene construct.