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Endocrinology Vol. 145, No. 2 508-518
Copyright © 2004 by The Endocrine Society

Adrenocorticotropin/3',5'-Cyclic AMP-Mediated Transcription of the Scavenger akr1-b7 Gene in Adrenocortical Cells Is Dependent on Three Functionally Distinct Steroidogenic Factor-1-Responsive Elements

Pierre Val, Christelle Aigueperse, Bruno Ragazzon, Georges Veyssière, Anne-Marie Lefrançois-Martinez and Antoine Martinez

Unité Mixte de Recherche Centre National de la Recherche Scientifique, Physiologie Comparée et Endocrinologie Moléculaire, Université Blaise Pascal, 63177 Aubière, France

Address all correspondence and requests for reprints to: Prof. Anne-marie Lefrançois-Martinez, Unité Mixte de Recherche Centre National de la Recherche Scientifique, 6547 Physiologie Comparée et Endocrinologie Moléculaire, Université Blaise Pascal, Clermont II, Complexe Universitaire des Cézeaux, 24 avenue des Landais, 63177 Aubière cedex, France. E-mail: a-marie.lefrancois-martinez{at}geem.univ-bpclermont.fr.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The akr1-b7 gene encodes a scavenger enzyme expressed in steroidogenic glands under pituitary control. In the zona fasciculata of the adrenal cortex where its expression is controlled by ACTH, AKR1-B7 detoxifies isocaproaldehyde produced during the first step of steroidogenesis. Three steroidogenic factor-1 (SF-1)-responsive elements (SFREs) are contained within the -510/+41 promoter region, which was previously demonstrated to drive gene expression in transgenic mice adrenal cortex. All these sequences bind at least SF-1 in Y1 adrenocortical cell nuclear extracts and can be activated by overexpression of this factor in HeLa cells. However, the three SFREs show distinct properties regarding akr1-b7 promoter activity in Y1 cells. Whereas the proximal -102 SFRE supports basal promoter activity, the -458 bona fide SFRE is essential for both basal promoter activity and cAMP responsiveness, although it is unresponsive to cAMP when isolated from its promoter context. This suggests that SF-1 is not a cAMP-responsive factor per se. The neighboring SFRE at -503 is a palindromic sequence that binds monomeric and heteromeric SF-1 as well as an adrenal-specific complex. Using MA-10 Leydig cells and Y1–10r9 mutant cells, we provide evidence that its activity in adrenocortical cells depends on the binding of the adrenal-specific factor, which is required for basal and cAMP-induced promoter activity. Furthermore, the -503 site has intrinsic cAMP-sensing ability in Y1 cells, which is correlated with increased adrenal-specific complex binding. Collectively, our results suggest that cAMP responsiveness of the akr1-b7 promoter is achieved through cooperation between the adrenal-specific factor bound to the -503 site and SF-1 bound to the -458 site.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
IN THE ZONA FASCICULATA of the adrenal cortex, steroidogenesis is controlled by the pituitary hormone ACTH through the cAMP pathway, which triggers coordinate transcription of all the steroidogenic enzymes (1, 2, 3, 4, 5) as well as cholesterol transporters (6, 7, 8) and detoxifying enzymes (9, 10), allowing increased production of corticosterone, the active glucocorticoid in rodents. This transcriptional activation implies complex arrays of transcription factors, some of which are ubiquitous like selective promoter factor 1 (Sp1) (11, 12, 13, 14) and activator proteins 1 (15) and 2 (16), whereas others are tissue-enriched like GATA-4 (17, 18) and CCAAT/enhancer-binding protein-ß (C/EBPß) (11, 18) or nuclear receptors like Nur77 (19) and steroidogenic factor-1 (SF-1).

SF-1/Ftz-F1 encodes a tissue-specific orphan nuclear receptor (20, 21) whose expression is restricted to steroidogenic tissues (20), pituitary gonadotropes, ventromedial hypothalamus (22, 23, 24), spleen (25, 26), and skin (27). It activates gene transcription by binding to core nuclear receptor sequences (AAGGTCA) as a monomer. Its extensive role in the development and differentiation of steroidogenic tissues is illustrated by the complete lack of adrenals and gonads in homozygous knockout mice and by the effects of mutations in humans (for review, see Ref.28). Moreover, SF-1 controls the tissue-restricted expression of all steroidogenic enzymes (28) and also participates to the cAMP-induced expression of steroidogenic acute regulatory protein (StAR) (29), CYP11A1 (30), CYP11B1 (31), and CYP17 (32, 33), although the underlying mechanism is still controversial (34, 35, 36, 37).

Some steroidogenic enzymes are specifically expressed in the gonads (CYP17 and CYP19), in the adrenals (CYP21), or even in certain adrenocortical zones (CYP11B1 in the fasciculata/reticularis; CYP11B2 in the glomerulosa). Because SF-1 is expressed in the gonads and throughout the entire adrenal cortex, it cannot account by itself for such a restricted expression pattern. This may rather rely on specific transcription factor combinations or on as yet unidentified cell-specific factors.

Cleavage of cholesterol by CYP11A1 during the first step of steroidogenesis delivers isocaproaldehyde, a toxic compound that is degraded by the product of the aldose-reductase akr1-b7 gene (38). As expected from its function, akr1-b7 is expressed in the gonads (39, 40, 41) and in the zona fasciculata of the adrenal cortex, where its expression is transcriptionally controlled by ACTH through activation of the cAMP pathway (9, 42). Previous experiments by our group have shown that a small proximal promoter fragment (–120/+41) was active in Y1 cells where it was responsive to cAMP. These experiments have shown the role of a cryptic SF-1-responsive element (SFRE) (–102) and of a nuclear factor-1 binding site (–76) in the basal activity of this promoter, whereas a C/EBP (–61) and a Sp1 binding site (–52) conveyed cAMP responsiveness (11). However, evidence from a number of experiments suggests that in the adrenal cortex, akr1-b7 expression relies on a larger promoter fragment encompassing regions from -510 to +41. Indeed, this region is able to recapitulate all of the endogenous gene features in transgenic mice (43, 44) and shows the highest activity in transient transfections in Y1 cells (11). Last, the previously described proximal binding sites for C/EBP (–61), Sp1 (–52), nuclear factor 1 (–76), and SF-1 (–102) are dispensable for the expression of the ACTH-sensitive rakr1-b7 rat orthologous gene in Y1 adrenocortical cells (41).

In the present paper, we describe the distinct functions of three SFREs implicated in the activity of the -510/+41 akr1-b7 promoter in adrenocortical cells.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Cell transfections
Y1 cells were maintained in DMEM-Ham’s F-12 medium supplemented with 10% fetal calf serum, 2 mM L-glutamine, 100 U/ml penicillin, and 100 µg streptomycin. HeLa cells were maintained in DMEM with Glutamax supplemented with 5% fetal calf serum, 100 U/ml penicillin, and 100 µg streptomycin. MA-10 cells were maintained in Waymouth medium supplemented with 15% horse serum, 1% insulin transferrin selenium, 2 mM L-glutamine, and antibiotics as above. Y1 10r9 cells (a kind gift of Dr. Schimmer, University of Toronto, Toronto, Canada) were maintained in Ham-F10 medium supplemented with 15% horse serum, 2.5% fetal calf serum, 2 mM L-glutamine, and antibiotics. Y1, HeLa, MA-10, and Y1 10r9 cells were transfected in six-well plates (Falcon 35–3502, Merck Eurolab, Strasbourg, France) 24 h after seeding at a density of 300 x 103 cells per well in complete medium. Y1 and Y1 10r9 cells were transfected with 1 µg reporter gene and 2 ng pRLSV40 (Promega, Charbonnières, France) using FuGene:6 (Roche, Meylan, France) following the manufacturer’s instructions. The effect of 10-5 M forskolin (in dimethylsulfoxide) was assayed for 24 h, 12 h after transfection. HeLa cells were transfected for 29 h with Exgen500 reagent (Euromedex, Mundolsheim, France), according to the manufacturer’s instructions. In the experiments described in Fig. 4Go, A and B, HeLa cells were transfected with 1 µg reporter gene, 2 ng pRLSV40 and 5 ng of pSG5-SF-1, encoding human SF-1 (11) or 5 ng of empty pSG5 (Stratagene Europe). MA-10 cells were transfected with 1 µg reporter gene using Exgen500 reagent. Effect of 10-5 M forskolin (in dimethylsulfoxide) was assayed for 24 h, 12 h after transfection. Luciferase and Renilla assays were performed in an automated luminometer with GenofaxA and GenofaxB reagents (Yelen, France), respectively. Each experiment was performed in triplicate and repeated at least three times. All data are expressed as means ± SD.



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  		FIG. 4. SF-1 activates SFREs of the akr1-b7 promoter in heterologous HeLa cells. A, SF-1 differently activates the isolated SFREs. Heterologous HeLa cells were transfected with 1 µg of the dimerized SFRE constructs driving the expression of the luciferase reporter gene through the minimal Tk promoter. The cells were cotransfected for 29 h with 5 ng pSG5-SF-1 expression vector or the corresponding amount of pSG5 empty vector. Results are expressed as induction over the empty vector ± SD, reflecting SF-1 sensitivity. The data are the mean of at least three independent experiments performed in triplicate. Relative luciferase units were corrected to the Renilla activity of a cotransfected expression vector. B, Effect of SFREs mutations on SF-1 induction of the -510/+41 promoter. One microgram of the 0.5-kb wild-type or mutant promoter was transfected in HeLa cells, cotransfected with 5 ng pSG5-SF-1 expression vector or the corresponding amount of pSG5 empty vector for 29 h. Results are expressed as above. Statistical analysis was performed using the Student’s t test. *, P < 0.05 for induction being significantly different from induction of the wild-type construct.

 
Gel shift assays
Gel shift experiments were performed as previously described (11). Where not stated, competitors were added as a 100-fold molar excess. Anti-SF-1 antibody was a kind gift of Dr. P. Berta (Montpellier, France). It was used at a concentration of 1 µl/20 µl reaction. Competitor oligonucleotides were COUP-TF, 5'-ACCCTTGACCCCTGCCCTGCAGC-3' (45); estrogen-responsive element (ERE), 5'-CGTCAGGTCACAGTGACCTGATG-3' (46); Nur77/NGFI-B-responsive element (NBRE), 5'-TCGAGAGATAGAAAGGTCAGACGAC-3' (47); 21-OH, 5'-CCACAGATTCTCCAAGGCTGATGG-3' (48); and polyoma enhancer activator protein 3 (PEA3), 5'-GGCTAGTGAGCAGGAAGTAGGGAGAG-3' (49).

Northern blot
MA-10 and Y1 cells were cultured for 1, 3, 6, 12 (or 19), and 24 h in the presence of 10-5 M forskolin. Untreated cells were included as a control. RNAs were extracted using the Trizol reagent (Invitrogen Life Technologies, Cergy Pontoise, France). Twenty micrograms of total RNAs were electrophoresed and blotted to a nylon Hybond N+ membrane (Amersham, Les Ulis, France). The akr1-b7, ß-actin, or 18S rRNAs were then revealed using complementary 32P-labeled probes.

DNA reporter constructs
The wild-type 0.5 kb akr1-b7 promoter (–510/+41) was subcloned into the SmaI and BglII restriction sites of the pGL3 vector (pGL3 0.5). Mutagenesis was performed using the Gene Editor Kit (Promega), following the manufacturer’s instructions. Oligonucleotides used were -503Bm, 5'-TTCTAGAAAGGTGTCCATTCCCTACATTTA-3'; -503M8, 5'-TTCTAGAAAGGTGTCCATGACCGGCATTTA-3'; -458m, 5'-AAAATCTTTCCACAATTTCACCTGC-3'; and -102m, 5'-TTGACATGAAGTTCCTTTTCTCATG-3'. Boldface indicates mutated nucleotides. Deletions were constructed by inverse PCR using Herculase DNA polymerase (Stratagene, Amsterdam, The Netherlands), starting with 15 ng pGL3 0.5 as a template. Common primer was XmaI-pGL3R, 5'-CTAGCACGCGTAAGAGCTCGGTACCTAT-3'; construct-specific primers were {Delta}503F, 5'-ACATTTAAAAAAAAAAAAAATCTTTCCACAAGGTCA-3' (leading to pGL3 503{Delta}AB), and {Delta}503AF, 5'-TGTCCATGACCTACATTTAAAAAAAAAAAAAATCTT-3' (leading to pGL3 503{Delta}A). The PCR product was gel purified and phosphorylated. Twenty nanograms of the linear product were then self-ligated overnight and transformed into the DH5{alpha} bacterial strain. Two copies of each putative SFRE were tandemly inserted in the XhoI site of a pGL3Tk reporter plasmid (a kind gift of Dr. A. Bruhat, Institut National de la Recherche Agonomique, Clermont-Ferrand, France), where luciferase expression is driven by the minimal Tk promoter. Briefly, tandem double-stranded oligonucleotides with 5' and 3' XhoI overhangs were phosphorylated by T4PNK; sequences are as follows (all are times two): SFRE LHßD, 5'-TCGACTTCCCTGACCTTGTCTGTGACTTCCCTGACCTTGTCTGTGA-3'; SFRE LHßR, 5'-TCGATCACAGACAAGGTCAGGGAAGTCACAGACAAGGTCAGGGAAG-3'; 503D, 5'-TCGATTCTAGAAAGGTGTCCATGACCTACATTTATTCTAGAAAGGT-GTCCATGACCTACATTTA-3'; 503R, 5'-TCGATAAATGTAGGTCAT-GGACACCTTTCTAGAATAAATGTAGGTCATGGACACCTTTCTAGAA-3'; 458D, 5'-TCGAAAAATCTTTCCACAAGGTCACCTGAAAATCT TTCCACAAGGTCACCTG-3'; 458R, 5'-TCGACAGGTGACCTTGTGGAAAGATTTTCAGGTGACCTTGTGGAAAGATTTT-3'; 503Ad, 5'-TCGATTCTAGAAAGGTGTCCATGTTCTAGAAAGG-TGTCCATG-3'; 503Ar, 5'-TCGACATGGACACCTTTCTAGAACATGGACACCTTTCTAGAA-3'; 503Bd, 5'-TCGATGTCCATGACCTACATTTATGTCCATGACCTACATTTA-3'; 503Br, 5'-TCGATAAATGTAGGTCATGGACATAAATGTAGGTCATGGACA-3'. After puri-fication (QiaQuick nucleotide removal kit, QIAGEN, Courtaboeuf, France), these were ligated in the open pGL3Tk vector. Inserted copy number was verified by DNA sequencing of the inserts.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The search for elements that could account for adrenal activity of the -510/+41 promoter identified two new putative binding sites for SF-1 (Fig. 1AGo). The site at -503 is an imperfect palindrome of the AGGTCA core binding sequence for nuclear receptors, resembling an ERE. We will refer to the complete site as -503A+B, the 5' half-site as -503A, and the 3' half-site as -503B. The site at -458 is a perfectly canonical SFRE (AAGGTCA). The atypical site that resides in the proximal promoter (–102) was previously described (11). It plays an important role for akr1-b7 promoter activity in vivo (44). Binding activities of the -458 and -503 sites were evaluated by EMSA experiments (Fig. 1BGo). When incubated with nuclear extracts from Y1 cells, the -458 site bound one specific complex that was displaced by the probe itself or by a consensus SFRE probe from the CYP21 promoter (48). Incubation of the DNA/protein complex with an antibody raised against SF-1 displaced and supershifted the complex. The identity of the retarded complex was further supported by incubation with in vitro-translated SF-1, which formed a complex comigrating with the one obtained with Y1 nuclear extracts. Collectively, these data indicate that the -458 site is a bona fide SFRE.



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  		FIG. 1. SFREs of the -510/+41 akr1-b7 promoter. A, Drawing of the two putative (–503 and -458) and the previously described SFREs. The -503 site is divided into two nuclear receptor core sequences in inverted orientation and separated by three nucleotides, a structure reminiscent of the ERE. The 5' core sequence will be referred to as the -503A half-site, the 3' core sequence as the -503B half-site. The sequences of the oligonucleotides used in gel-shift experiments, as well as of the consensus SFRE, are shown. Core sequences are shaded. B, Binding activities of the -458 or the -503 sites. The binding properties of the putative SFREs were evaluated by gel-shift experiments. The 32P-labeled double-stranded oligonucleotide probes were incubated with 5 µg of adrenocortical Y1 cells (Y1) or mice adrenal nuclear extracts (Adr) as well as with 1 µl of in vitro-translated SF-1 protein (SF-1 or S) or control reticulocyte lysate (L). When indicated, cold oligonucleotide competitors were added to the binding reaction as a 50-fold molar excess. One microliter anti-SF-1 antibody was added to the reactions as indicated. Asterisk shows the supershifted complex. COUP-TF, binding site for COUP-TF; 21-OH, SF-1-responsive element of the CYP21 promoter; NBRE, Nur77/NGFI-B consensus responsive element. C, Complex II is adrenal specific. Gel-shift assays were performed as in B, except that nuclear extracts were either from Y1 cells, steroidogenic MA-10 Leydig cells, human choriocarcinoma steroidogenic JEG-3 cells, monkey kidney CV-1 cells, or Chinese hamster ovary CHO cells. Arrowhead shows complex II binding from control Y1 cells.

 
When incubated with nuclear extracts from Y1 cells, the -503A+B probe retained three specific complexes that were displaced by an excess of the cold probe itself. Competition with a consensus ERE did not displace any of the specific complexes, indicating that despite its high similarity with an ERE, the -503A+B site does not bind estrogen receptor in Y1 cells. Competition with the -458 SFRE or the CYP21 SFRE displaced the slowest (complex III) and the fastest (complex I) retarded complexes, indicating that these probably contained SF-1. This was confirmed by incubation with an antibody raised against SF-1. In vitro-translated SF-1 formed a complex comigrating with complex I, indicating that it was composed of monomeric SF-1. It must be noted, albeit not obvious from the data shown here, that complex I binding is hardly reproducible in repeated gel-shift experiments, probably reflecting low SF-1 affinity for the site. As complex III was not formed with in vitro-translated SF-1, we concluded that it may be composed of SF-1 associated with at least one other protein (SF-1/X complex). Retinoid X receptor (RXR) is a known partner of multiple nuclear receptors (50). Preincubation of EMSA reactions with an antibody raised against all RXR isoforms did not prevent the formation of the complexes, showing that RXR is not associated with the proteins bound to the -503A+B site (data not shown). COUP-TF and Nur77 have been shown to respectively inhibit (51) or activate (52, 53) steroidogenic genes promoter by binding to AGGTCA sequences. Competition with consensus oligonucleotides for these two factors failed to displace any of the specific complexes, indicating that these factors were not part of the binding activities observed on the -503A+B site (data not shown). The presence of the three protein complexes in nontumor cells was confirmed by incubation of the -503A+B probe with nuclear extracts prepared from mice adrenals. From these experiments, we can conclude that the -503A+B site retains three specific complexes in adrenal nuclear extracts. Low-affinity complex I is monomeric SF-1, whereas complex III is composed of SF-1 and at least an unidentified protein partner (SF-1/X complex). In an attempt to refine our knowledge on complex II properties, we performed EMSA experiments with nuclear extracts from steroidogenic MA-10 and JEG-3 cells as well as nonsteroidogenic CV-1 and CHO cells (Fig. 1CGo). Interestingly, these experiments demonstrated that complex II was adrenal specific, whereas complex I and III were also observed with MA-10 Leydig cells that express SF-1 but not with JEG-3, CV-1, or CHO cells. In addition, because complex III is formed with both nuclear extracts from Y1 and MA-10 cells, we can rule out the adrenal-specific protein as a likely partner in the formation of complex III by interaction with SF-1.

We then sought to define the precise requirements for binding of the three specific complexes. These were determined by using the -503A or -503B probes in EMSA as well as by sequential mutagenesis of competitor -503A+B oligonucleotides (Fig. 2AGo). Interestingly, when incubated with Y1 nuclear extracts, the -503A site failed to bind any of the three complexes, whereas the -503B site strongly bound complex II (efficiently competed by an excess of the probe itself or the -503A+B site but not by the consensus CYP21 SFRE) and poorly retained complex I (Fig. 2BGo). Nonetheless, the -503B site alone was unable to bind complex III. This indicates that the -503A site is required to stabilize complex III interaction. Oligonucleotides mutated in the -503A site or -503A/–503B intervening region (M0 to M4) competed efficiently for the binding of the three complexes whereas a 50-fold molar excess of -503B mutants (M6 to M8) did not. However, when used in increasing doses, mutants M6, M7, and M8 showed slightly differing binding properties. Indeed, whereas a 100-fold molar excess of M6 oligonucleotide was still inefficient as a competitor, mutants M7 and M8 to a greater extent efficiently prevented complex I and III binding at a 100-fold excess, although they were still unable to compete for complex II formation.



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  		FIG. 2. Binding properties of the -503 site. A, Sequence alignment of the oligonucleotides used in gel-shift assays. Core nuclear receptor binding sequences are shaded. B, Binding properties of the mutant oligonucleotides. Gel-shift experiments were performed under the same conditions as described in Fig. 1BGo. Increasing doses of mutant oligonucleotides competitors were 50- and 100-fold molar excess. C, Summary of the binding activities. Complex II requires an intact -503B site to bind. Its binding is specifically abolished by mutation M8. Complex III forms only in the presence of an intact -503A site. Complex I and III binding is disrupted by a 5' mutation (M6) in the -503B site but not by the most 3' mutations (M8 and M9).

 
Collectively, these data allow us to propose that complex I (monomeric SF-1) and complex II build on the -503B site alone, whereas complex III (SF-1/X) forms only with the complete -503A+B region, presumably by binding of the SF-1 partner on -503A (Fig. 2CGo). Whereas complex II formation requires the integrity of the whole -503B site (M6 to M8), complex I and III formation seems to be essentially affected by mutation in the 5' part of the -503B site (M6). Based on these observations, binding properties of the mutants that will be used in transfection experiments are summarized in Fig. 2CGo, lower panel.

Effect of a two-nucleotide mutation on SF-1 binding to the -458 site was also analyzed by competition in EMSA. As shown in Fig. 2BGo, right panel, the mutant oligonucleotide failed to compete for SF-1 binding on the -458 wild-type probe, indicating that the mutation was indeed efficient.

To evaluate the relative SF-1 binding efficiencies of the different SFREs, we have compared the ability of increasing amounts of the -102, -458, and -503 oligonucleotides to compete for SF-1 binding on the -458 probe (Fig. 3Go). Quantitative analyses of the remaining binding activities after competition has demonstrated that the -458 site has the highest affinity for SF-1, followed by the -503 and -102 sites. Similar results were obtained with the CYP21 SFRE as a probe (data not shown).



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  		FIG. 3. Relative affinity of the SFREs for SF-1. Relative affinity for SF-1 was determined by competition in EMSA. Five microgams of Y1 nuclear extracts were incubated with 80 x 103 cpm of the labeled -458 probe in the presence of increasing amounts (10-, 20-, and 50-fold molar excess) of the different competitors. 21-OH SFRE and PEA3 oligonucleotides were included as positive and negative controls, respectively. Intensity of the SF-1 retarded complex was quantitated by phosphorimager analysis and normalized to the amount of free probe (in excess). Results are indicated as the mean percentage of binding activity (two independent experiments) relative to the experiments conducted in the absence of competitor DNA.

 
Although our EMSA experiments have shown that the -503B, the -458, and the -102 sites were all able to bind SF-1 albeit with different affinities, only the -102 SFRE has been shown to mediate SF-1 transactivation in cell transfections (11). The ability of SF-1 to transactivate dimers of the isolated putative -503 and -458 SFREs driving the expression of the luciferase reporter gene through a minimal Tk promoter was evaluated by cotransfection experiments in the heterologous HeLa cell line (Fig. 4AGo). Cotransfection of the SF-1 expression plasmid led to a marked increase in the activities of the dimerized -503A+B (x4.9), -503B (x7.5), -458 (x4.1), and of the control LHß SFRE (x3.0), when compared with the empty vector transfection. The -503A dimer, which showed no SF-1 binding in gel-shift assays, displayed only slight nonsignificant activation upon SF-1 overexpression (x1.9). Collectively, this shows that the isolated -503B and -458 sites are responsive to SF-1 in heterologous cells.

We then sought to analyze the effect of the SFRE mutations in the context of the -510/+41 promoter in response to SF-1 in HeLa cells (Fig. 4BGo). The wild-type -510/+41 promoter was markedly induced upon SF-1 transfection (x4.5), but this induction was markedly reduced by a single mutation of the -458 SFRE. Mutation of the -503B site alone (M6) or combined with mutation of the -102 site significantly, albeit slightly, affected promoter responsiveness to SF-1 overexpression. At last, deletion of the -503A site or mutation of the -102 site did not significantly alter promoter responsiveness to SF-1. Altogether, these data indicate a central role of the -458 SFRE for SF-1 responsiveness. These observations also indicate that the -503B site is not a highly SF-1-responsive sequence in the context of the -510/+41 promoter, suggesting that other protein complexes binding to this site (Figs. 1BGo and 2BGo) may have a role for its activity in the adrenals.

To assess the potential role of the proteins binding to -503A+B and -458 sites for akr1-b7 expression in adrenals, constructs of the -510/+41 promoter bearing either individual or combined mutations and deletions of each of the SFREs were transfected in the Y1 adrenocortical cell line, and their ability to respond to cAMP increases was evaluated (Fig. 5AGo). The wild-type promoter construct drove a robust expression of the luciferase reporter gene that was further increased by forskolin treatment (5.6-fold induction). Deletion of the -503A site, which specifically prevents complex III binding (SF-1/X), resulted in a slight but significant increase in basal activity and forskolin sensitivity of the akr1-b7 promoter, indicating that this complex might exhibit some repressive potential. However, complete deletion of the -503A+B site markedly decreased basal activity and forskolin sensitivity demonstrating that in Y1 cells, the whole site mainly retains activating rather than inhibiting complexes. Mutations of the -503B site that either prevented binding of complex II (M8) or of the three complexes (M6) also decreased basal promoter activity and forskolin sensitivity to a similar extent as -503A+B removal. As mutations that prevent the binding of complex II alone (M8) or the binding of the three complexes together (M6) have the same effect on promoter activity, it can be speculated that this is adrenal-specific complex II rather than SF-1 that acts as a stimulator of basal and forskolin-induced akr1-b7 promoter activity. This will be demonstrated later in this paper.



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  		FIG. 5. Role of the SFREs in akr1-b7 promoter activity in Y1 adrenocortical cells. A, Effect of individual or combined mutations or deletions. The role of the three SFREs in akr1-b7 promoter activity in Y1 cells that endogenously express AKR1-B7 was evaluated by mutating (white rectangle) or deleting the different SFREs in the context of the 0.5-kb promoter (–510/+41), driving the expression of the luciferase reporter gene. Two different mutations were performed in the -503B site (M6 and M8). The ability of the constructs to sense cAMP increases was evaluated by incubating the cells with 10-5 M forskolin (Fsk) for 24 h. Results are expressed as the mean of at least three independent experiments performed in triplicate ± SD. Firefly luciferase units were corrected by the activity of a cotransfected Renilla expression vector. Statistical analysis was performed with Student’s t test. *, Significantly different from 0.5-kb wild-type construct basal level (P < 0.05); **, significantly different from 0.5-kb wild-type construct-induced level (P < 0.05); when not stated, induction factors are significantly different from wild-type construct factor (P < 0.05); n.s., not significantly different from the forskolin induction factor of the wild-type construct (P > 0.05). R.L.U., Relative luciferase units. B, cAMP sensitivity of the isolated SFREs. The ability of the different SFREs to mediate cAMP responsiveness when isolated from their promoter context was assessed. The SFREs were dimerized in front of the minimal Tk promoter driving the expression of the luciferase reporter, and the resulting constructs were transfected in Y1 cells, treated or not by 10-5 M forskolin for 24 h. Results are expressed as fold induction by forskolin ± SD. Data are the mean of at least three independent experiments performed in triplicate. Relative luciferase units were corrected to the Renilla activity of a cotransfected expression vector.

 
Mutation of the -458 bona fide SFRE was the most potent in reducing overall activity as well as forskolin sensitivity of the promoter. Interestingly, combination of the -458 mutation with any other mutation did not show further decrease in promoter activity when compared with -458 mutation alone, suggesting that this SFRE plays a central role in akr1-b7 promoter function in Y1 cells.

As already described (11, 44), mutation of the -102 site resulted only in a marked decrease in the basal activity of the promoter, whereas forskolin sensitivity was not affected.

Given that mutations of the -503B (or removal of the -503A+B site) and the -458 sites resulted in a decrease in cAMP responsiveness, the ability of these sites to sense cAMP increases when isolated from their promoter context was evaluated. The constructs of the dimerized SFREs were transfected into Y1 cells that were untreated or treated with 10-5 M forskolin (Fig. 5BGo). Despite the major effect of -458 mutation on forskolin responsiveness, the -458 site was completely unresponsive to forskolin when isolated from the promoter, suggesting that SF-1 bound to this site was not intrinsically responsive to cAMP. In fact, the -503B site was the minimally forskolin-responsive region in these conditions. As expected from the effect of their mutations, the -102 and -503A sites were not responsive to forskolin induction. Collectively, these experiments show that despite its central role in the basal and forskolin-induced promoter activity, the -458 site probably needs to cooperate with the nearby -503B site to achieve cAMP responsiveness in Y1 cells.

Our results suggest that -503A+B site activity relies on the -503B region, which binds complex II with high affinity and SF-1 (complex I) with low affinity. We thus undertook a number of experiments to understand which of these two complexes was responsible for transcription activation on the -503B site.

The akr1-b7 gene was previously shown to be expressed in mouse testes and MA-10 Leydig cells (40, 41, 54). We thus asked whether akr1-b7 cAMP sensitivity would be conserved in MA-10 cells that do not express complex II but express SF-1. Interestingly, akr1-b7 mRNA accumulation was less sensitive to forskolin treatment in MA-10 when compared with Y1 cells (Fig. 6AGo). This was not due to low SF-1 binding activity as demonstrated by EMSA (data not shown). We then transfected the wild-type 0.5-kb promoter or -503B mutants (M6 and M8) in MA-10 cells and assessed their ability to sense cAMP increases (Fig. 6BGo). None of the two mutations significantly altered basal promoter activity or forskolin sensitivity, indicating that in MA-10 cells, the -503B site was simply not functional. Conversely, this demonstrates that -503B site activity in Y1 cells most probably relies on adrenal-specific complex II rather than on SF-1. This was further confirmed by analyzing the activities of the dimerized -503B and -503A+B sites in Y1 10r9 cells known to harbor an impaired SF-1 activity (55) (Fig. 6CGo). Indeed, the -503B half-site was even more sensitive to cAMP increases in Y1 10r9 cells than in wild-type Y1 cells, whereas the -503A+B site had an equivalent sensitivity. As a control, the -458 site did not show cAMP responsiveness. We finally asked whether adrenal-specific complex II was expressed in Y1 10r9 cells by performing EMSA experiments using the -503B nucleotide as a probe (Fig. 6DGo). As expected, this site retained a complex whose binding was efficiently competed by the -503B or -503A+B site but not by the -458 SFRE or by an antibody raised against SF-1. These binding properties are consistent with complex II being present in Y1 10r9 cells. Strongly arguing for its role in akr1-b7 promoter cAMP responsiveness, complex II binding is induced (2-fold) by treatment of Y1 10r9 cells with 10-5 M forskolin. A similar cAMP-dependent increase in complex II binding (2.1 ± 0.2-fold) was also observed in different nuclear extracts from wild-type Y1 cells (data not shown).



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  		FIG. 6. Adrenal-specific complex II is responsible for -503B half-site activity. A, cAMP sensitivity is markedly impaired in MA-10 cells. To evaluate endogenous akr1-b7 gene sensitivity to cAMP increases, MA-10 and Y1 cells were cultured for 1, 3, 6, 12 (or 19), and 24 h in the presence of 10-5 M forskolin (Fsk). Untreated cells were included as a control. RNAs were extracted using the Trizol reagent, electrophoresed, and blotted to a nylon membrane. The akr1-b7, ß-actin, or 18S rRNAs were then revealed using complementary 32P-labeled probes. B, In the absence of complex II, -503 half-site B is not functional. To differentiate between the activity of SF-1 and adrenal-specific complex II, MA-10 cells were transfected with either the 0.5-kb wild-type construct or two half-site B mutants (M6 and M8) and treated or not by 10-5 M forskolin for 24 h. The results are expressed as mean of three independent experiments performed in triplicate ± SD. Forskolin-activated 0.5-kb wild-type promoter activity was set as 100%. C, Absence of active SF-1 does not preclude -503 site forskolin sensitivity. Y1 10r9 cells that express a nonfunctional SF-1 protein were transfected with the dimerized SFRE constructs, and their ability to respond to forskolin stimulation (10-5 M for 24 h) was assayed (black bars). Results are expressed as fold induction by forskolin ± SD. Data are the means of at least three independent experiments performed in triplicate. Relative luciferase units were corrected to the Renilla activity of a cotransfected expression vector. Results of similar transfections conducted in Y1 cells (Fig. 5BGo) were included for comparison (gray bars). D, Adrenal-specific complex II binding is stimulated by forskolin in Y1 10r9 cells. Nuclear extracts from Y1 10r9 cells untreated or treated with 10-5 M forskolin for 24 h were submitted to EMSA experiments using the -503B half-site as a probe. Binding activity was measured by phosphorimager quantitation.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The present work was undertaken in an attempt to unravel the role of SFREs in controlling hormonal and cell-specific aspects of gene expression in the adrenals. Using the -510/+41 promoter region of the ACTH-regulated akr1-b7 scavenger gene as a model, we have identified three SFREs (–102, -458, and -503) and provide functional evidence that these sites have distinct functions to achieve akr1-b7 promoter activity in adrenocortical cells.

The cryptic SFRE at -102 was previously shown to be activated by SF-1 and is required for basal proximal promoter (–120/+41) activity in Y1 cells (11). The data presented here confirm that the -102 site is implicated only in basal activity, even in the context of the -510/+41 promoter. This is consistent with our recent paper showing that mutation of the -102 SFRE in the -1804/+41 promoter affects only promoter strength but not responsiveness to ACTH in the adrenals of transgenic mice (44). Interestingly, the study of the consequences of this mutation during development has suggested that this site influences promoter sensitivity to postnatal changes in SF-1 levels and thus is required to trigger akr1-b7 expression after 15 d of age but is dispensable for early expression (44).

Among the three SFREs contained within the -510/+41 promoter, the -458 sequence shows the greatest relative affinity for monomeric SF-1. Its mutation prevents promoter activation by SF-1 in HeLa cells and dominantly and dramatically impairs both basal and cAMP-induced promoter activity in Y1 cells. Indeed when the -458 site is mutated, mutation of any of the two remaining SFREs (–102 or -503) has no additional effect on promoter activity.

The palindromic -503A+B SFRE binds three different complexes: SF-1 as a monomer (complex I) with low affinity, a complex between SF-1 and another protein (SF-1/X, complex III), and an adrenal-specific complex with high affinity (complex II). Importantly, analyses of mutant -503A+B sites have shown that the three complexes recognize partly overlapping sequences, indicating that they interact in a mutually exclusive manner. Deletion of the -503A site, which specifically displaces heteromeric complex III, results in a slight but significant increase in overall akr1-b7 promoter activity, indicating that the SF-1/X complex might be acting as a repressor or as a transcriptionally silent complex that prevents activating factors binding. Recent papers have described the inhibitory association of SF-1 with DP103 (56) or P54nrb/NonO (32). Gel-shift experiments conducted with antibodies directed against DP103 or in vitro-translated DP103 (kind gifts of Dr. Grässer) rule out the participation of DP103 in the inhibitory complex (data not shown). A possible interaction between NonO and SF-1 on the akr1-b7 promoter would need further examination, even though it seems unlikely. Indeed, this complex was not formed with nuclear extracts from Y1 cells on the CYP17 promoter (32), whereas we showed that the SF-1 partner in complex III is present not only in Y1 and mouse adrenal cells but also in steroidogenic Leydig MA-10 cells.

Although deletion of the -503A site leads to a slight increase in basal promoter activity and forskolin sensitivity, deletion of the complete -503A+B site results in a marked decrease in basal activity and forskolin sensitivity, indicating that the major function of the site is to bind an activator of transcription, at least in adrenocortical cells. Interestingly, mutations of the -503B half-site perfectly mimic deletion of the complete -503A+B site, indicating that the activator complex binds to this region. As -503B mutations prevent the binding of both monomeric SF-1 and complex II in EMSA, one might conclude that SF-1 displacement is responsible for the decrease in promoter activity. However, several lines of evidence suggest a prominent role for adrenal-specific complex II. First of all, the -503B site binds adrenal-specific complex II with much more efficiency than monomeric SF-1. Second, in MA-10 cells that express great amounts of active SF-1 but fail to display complex II binding, mutations in the -503B half-site do not affect akr1-b7 promoter activity. Third, in mutant Y1 10r9 cells that show impaired SF-1 trans-acting properties (55) but retain complex II binding activity, the isolated -503B site is still sensitive to cAMP. We can thus conclude that an adrenal-specific factor is required for akr1-b7 promoter activity in adrenocortical cells. It is tempting to speculate that such a factor might also participate in the adrenal-restricted expression of other steroidogenic enzymes such as CYP21 or CYP11B1. Interestingly, an adrenal-specific factor [adrenal-specific protein (ASP)] has already been implicated in cAMP responsiveness of CYP21. In contrast to our observations, ASP bound Sp1-like GC-rich sequences (57, 58). Surprisingly, although the data presented at the time were rather convincing, ASP has never been isolated or cloned, thus precluding a comparison with our data.

A previous report from our lab has indicated that binding sites for C/EBP (–61) and Sp1 (–52) participate in the control of the proximal akr1-b7 promoter (–120/+41) by the cAMP pathway (11). Mutation of the -458 SFRE or, to a lesser extent, mutations of the -503B site markedly reduce forskolin sensitivity of the -510/+41 promoter, indicating that the proximal sites are not sufficient for full cAMP responsiveness. Numerous cell transfection experiments have shown that SF-1 could participate in the cAMP responsiveness of several steroidogenic genes promoters (30, 31, 51, 59, 60, 61, 62, 63) as well as the high-density lipoprotein receptor SR-BI (64, 65) and the ACTH receptor MC2R (66), although the underlying mechanisms are still contradictory (34, 36, 62, 67, 68, 69). Quite surprisingly, given the drastic effect of its mutation on cAMP responsiveness of the -510/+41 promoter, the isolated -458 SFRE is unable to confer forskolin sensitivity to the heterologous minimal Tk promoter in Y1 cells, indicating that SF-1 bound to this site is not intrinsically responsive to cAMP. On the contrary, the isolated -503B (or -503A+B) site is responsive to forskolin, suggesting that adrenal-specific complex II might act as the true sensor of cAMP increases. However, the dominant effect of the -458 SFRE mutation suggests that this complex needs to cooperate with SF-1 to achieve cAMP responsiveness of the akr1-b7 promoter in adrenocortical cells. This cooperation could involve chromatin remodeling evoked by the binding of SF-1 to the -458 SFRE, increasing accessibility to the adjacent -503B site. Another possibility is that the -458 SFRE recruits specific coactivators in response to -503B site activation by complex II binding. Likely candidates are p300/CBP/cointegrator-associated protein (p/CIP) and/or transcription intermediary factor 2 (TIF2), two recently described SF-1 coactivators that may participate in cAMP-mediated activation of the bovine CYP17 cAMP responsive sequence (36). Such a hypothesis will need further demonstration.

In summary, using a nonsteroidogenic gene encoding an ACTH-dependent detoxifying enzyme as an alternative model, we have shown that functionally specialized SFREs act in a coordinate fashion to integrate ACTH/cAMP and cell-specific trans-acting signals. Our results also suggest that, at least in adrenocortical cells, SF-1 has no intrinsic cAMP sensitivity but that it can integrate hormonal responsiveness by cooperating with a cis-acting element that binds a tissue-specific factor.


   Acknowledgments
 
We thank Alain Halère for excellent technical assistance, Dr. Enzo Lalli for critical reading of the manuscript, and Dr. Bernard Schimmer for kindly providing mutant Y1–10r9 mutant cells.


   Footnotes
 
Abbreviations: C/EBP, CCAAT/enhancer-binding protein; ERE, estrogen-responsive element; RXR, retinoid X receptor; SF-1, steroidogenic factor-1; SFRE, SF-1-responsive element; Sp1, selective promoter factor.

Received August 21, 2003.

Accepted for publication October 29, 2003.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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