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Department of Molecular Endocrinology (F.C., K.K., E.B., J.F., H.W., A.S., S.H., L.P.F., A.A.R.), Merck Research Laboratories, West Point, Pennsylvania 19486; and Centro de Investigación Básica (M.M., C.T.M.), Merck, Sharp & Dohme de España, Madrid 28027, Spain
Address all correspondence and requests for reprints to: Fang Chen, Ph.D., Department of Molecular Endocrinology, Merck Research Laboratories, WP26A-1000, Sumneytown Pike, West Point, Pennsylvania 19486. E-mail: fang_chen{at}merck.com.
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and ERß, with Ki values of 1.1 and 0.5 µM, respectively. Despite the similar binding affinities, DHEA showed preferential agonism of ERß with an EC50 of approximately 200 nM and maximal activation at 1 µM. With ER we found 30–70% agonism at 5 µM, depending on the assay. Physiological levels of DHEA are approximately 30 nM and up to 90 nM in the prostate. DHEA at 30 nM is actually sufficient to activate ERß transcription to the same degree as estrogen at its circulating concentration, and additive effects are seen when the two were combined. Taken together, DHEA has the potential for physiologically relevant direct activation of ERß. With peak levels at term and age 30 yr, there is also a potential for antagonist effects on AR and partial agonism of ER. |
Introduction |
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DHEA-S serves as reservoir for DHEA and is converted via a cellular sulfatase (3, 5). DHEA can then serve as a precursor for androstenediol, androstenedione, testosterone, estrone, testosterone (T), dihydrotestosterone (DHT), and 17ß-estradiol (E2) (2, 7). Physiological roles for DHEA and DHEA-S have been assigned whereby these serve as precursors for peripheral synthesis of DHT, T, and E2. Levels of these precursors determine the DHT/T and E2 concentrations in peripheral tissues, although they have little effect on circulating levels of the sex hormones (1, 5). Results from animal studies have suggested that DHEA may be beneficial in preventing age-related obesity, diabetes, cancer, heart disease and immune dysfunction (8, 9, 10, 11). Dietary supplementation of DHEA has also been used to counter age-related declines in sexual function and bone mass, among others (12).
The discovery of peripheral synthesis of androgens/estrogens from DHEA and DHEA-S (2, 5) led to the discovery of intracrinology, which holds that androgens and estrogens can, within the same cell, be synthesized from DHEA and/or androstenedione and exert their biological function (2). Biological function is mediated through the action of these sex hormones on androgen (AR) or estrogen receptor (ER), which belong to the nuclear receptor superfamily/steroid receptor subfamily (13). To date, only one AR gene has been identified with affinity to DHT and T in the subnanomolar range (14). Meanwhile, E2 binds with subnanomolar affinity to either of two ER isoforms, ER and ERß (15). All three receptors function as hormone-regulated transcription factors. Recently interest has arisen in identifying possible direct effects of hormone metabolites or precursors that may also act via these receptors (16, 17). For instance, androstenediol binds to ER and ERß with equilibrium binding constants (Kis) of 3.6 and 0.9 nM, respectively (18). Meanwhile, androstenedione showed no apparent affinity to either receptor. Furthermore, we found that both androstenedione and androstenediol bind and activate the endogenous AR with either a transfected reporter or an endogenous gene readout [prostate-specific antigen (PSA)] (Ref.17 and our unpublished data). The available information indicates a clear role for certain intermediate metabolites or precursors of DHEA to exhibit direct biological functions in addition to serving as precursors of sex hormones.
In the present study, we investigated whether DHEA might possess direct biological functions through interaction with AR and ER using various in vitro and cell-based assays. Our results clearly indicated that DHEA could function directly via ER, especially ERß, as an agonist and via AR as an antagonist.
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Materials and Methods |
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and HEK293/hERß containing estrogen response element (ERE)2-alkaline phosphatase (ALP) were from CIBE (19). LipofectAmine 2000, phenol red-free DMEM, MEM, RPMI 1640, Gentamicin, L-glutamine, HEPES, trypsin, regular and charcoal-stripped fetal bovine serum (FBS) were all from Gibco BRL/Invitrogen (Carlsbad, CA). Hydroxyapatite was from Calbiochem (La Jolla, CA). Human insulin, polyethylimine, sodium molybdate, triamcinolone acetonide, DHEA, DHEA-S, T, DHT, and E2 were all from Sigma (St. Louis, MO). R1881, [3H]R1881, and [3H]E2 were from NEN Life Science Products (Boston, MA). Proteinase inhibitor and FuGENE6 transfection reagent were from Roche Molecular Biochemicals (Indianapolis, IN). The Dual-Glo luciferase assay system and phRL-TK vector were from Promega (Madison, WI). Unifilter-96 GF/B and MICROSCINT were from Packard (Boston, MA). The PSA ELISA and DHEA enzyme immunoassay kits were from Diagnostic Systems Laboratories (Webster, TX). Cytostar 96-well scintillating microplates and [methyl-14C] thymidine CFA532 were from Amersham (Piscataway, NJ).
Preparation of nuclear receptors
Endogenous receptors for AR and glucocorticoid receptor (GR) were from the MDA-MB453 human breast cell line and were prepared as previously described (17). Endogenous AR from LNCaP cells and progesterone receptor (PR) from T47D cells were also prepared using the same method, although LNCaP and T47D cells were cultured in RPMI 1640 medium without insulin.
Both hER (accession no. X03635) and ERß (accession no. af051427) were expressed in COS-1 cells. COS-1 cells were cultured in DMEM containing 2 mM L-glutamine, 10% FBS, and 20 µg/ml gentamicin. Cells were transfected with 15 µg DNA per 5 x 106 cells in 150-mm plates using LipofectAmine 2000 (Gibco BRL/Invitrogen) as per the manufacturer’s protocol. Two to three days after transfection, receptors were harvested as described above.
Cells were suspended at approximately 107 cells/ml in TEGM/PI [10 mM Tris-HCl, 1 mM EDTA, 10% glycerol, 1 mM ß-mercaptoethanol, 10 mM sodium molybdate (pH 7.2), and one pellet of protease inhibitors in 50 ml of buffer] and then snap frozen in ethanol/dry ice and stored at –80 C. Immediately before performing binding assays, frozen cells were thawed on ice. Cell lysates were collected by centrifugation at 10,000 x g at 4 C for 20 min after thawing the cell suspension. The lysate containing receptor was titrated before use for radioligand displacement assays.
Hydroxyapatite-based nuclear receptor binding assays
The nuclear receptor binding assays were performed as previously described (17). IC50 values for each compound were generated using MRLCalc (Merck & Co., West Point, PA). IC50s were converted to equilibrium binding constants (Kis) using the equation: Ki = Kd x IC50/(Kd + L), where L is the concentration of radioligand. For AR, Kd was calculated as the equilibrium dissociation constant for [3H]R1881 binding to either MDA-MB453 (MDA) hAR (0.45 nM) or LNCaP hAR (0.56 nM). For ER, the Kd for binding of [3H]estradiol to hER was 0.12 nM, whereas for ERß, the Kd was 0.35 nM.
MDA-MB453 endogenous hAR and transfected mouse mammary tumor virus (MMTV) luciferase (luc) reporter assays
For each 96-well assay plate, 2 x 106 MDA cells were transfected with 6 µg MMTVluc (17), which was divided equally into each of the wells. The phRL-TK Renilla luciferase reporter, included in each transfection for signal normalization, was used at 60 ng per 96-well plate. On the day after transfection, cell media were replaced by fresh OptiMEM medium (Invitrogen) containing the test ligands. Cells were treated with compounds either at a single dose or across a concentration range, as described in Results. Luciferase and Renilla luciferase activities were measured 24 h after initiation of treatment as per manufacturer’s protocols and using a 1450 MicrobetaJet (PerkinElmer, Shelton, CT).
PSA induction and cell proliferation of LNCaP cells
Freshly prepared LNCaP cells were suspended in RPMI 1640 medium containing 10% charcoal-stripped FBS at 2 x 105 cells/ml for PSA assays and 1 x 105 cells/ml for proliferation assays. Cells in 100 µl of medium were seeded into each well of 96-well plates. For plates used in the proliferation study, 0.5 µCi/ml of [methyl-14C]thymidine were added to the medium. Two to four hours after seeding, cells were treated with compounds, which were added in a 25 µl volume of medium in either the presence (antagonist mode) or absence (agonist mode) of 0.5 nM DHT or R1881, as indicated. PSA levels in the medium were measured 24 h after initiation of compound treatment using a PSA ELISA kit. Cell proliferation was measured using a 1450 MicrobetaJet (Wallac) at 24, 48, 72, or 120 h.
Transcription assays using the Gal4 chimeric system
Gal4 DNA binding domain (DBD) derived from pM vector of CLONTECH was inserted downstream of the cytomegalovirus promoter of pcDNA3.1(+) (Invitrogen). Individual ligand binding domains (LBDs) of recombinant hAR, hER, or hERß were fused in frame with the Gal4 DBD. The Gal4 luciferase reporter was constructed by inserting five copies of consensus Gal4 binding sites from pG5CAT (Clontech) upstream of the luciferase reporter gene in pGL3 basic (Promega). COS-1 cells at 1 x 106 per plate were transfected with 2.0 µg receptor, 6.0 µg reporter, and 50 ng phRL/96-well plate using FuGene 6. For the Gal4AR system, 1.5 µg of the glucocorticoid interacting protein 1 coactivator (20) were included.
Measurement of DHEA concentrations
For transcription assays, serially diluted DHEA samples were collected after 24 h treatment of assay cells. The same set of DHEA samples was also collected from parallel plates that were treated in a similar fashion, except the wells lacked cells. DHEA concentrations were measured using a DHEA enzyme immunoassay kit as per the manufacturer’s protocol.
Statistic analysis
All data are presented as mean ± SD. Statistical significance was calculated using PRISM4 (GraphPad Inc., San Diego, CA) by one-way ANOVA. Significance is indicated (*, P < 0.05; **, P < 0.01; or ***, P < 0.001).
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and data not shown). We thus focused on a possible role for DHEA as a ligand for the sex hormone receptors, beginning with AR. Binding affinity of DHEA for AR was performed using endogenous receptor from either MDA or LNCaP cells (Table 1). As a counterscreen, we also examined endogenous GR from MDA and endogenous PR from T47D cells (data not shown). With tested concentrations as high as 5 µM, there was not even minimal detectable DHEA binding to either GR or PR. However, DHEA did competitively inhibit R1881 binding to AR from both cell lines with an average Ki of 1.2 µM. For comparison, binding constants for T and DHT displacement of R1881 binding to AR averaged 0.5 and approximately 0.2 nM, respectively (Table 1).
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). Unlike T and DHT, DHEA showed little or no transcriptional activation of the AR, even when dosed into the mircomolar range. Like DHEA, the AR antagonist, bicalutamide, also showed a complete lack of AR agonism in this setting. Owing to the relatively high affinity of E2 for AR (Table 1
), we also tested this ligand for putative agonist behavior. Interestingly, despite its low nanomolar affinity, E2 displayed no agonist behavior until dosing was increased to near 1 µM. Within this high concentration range, E2 displayed a full agonist profile with transcriptional activation 100% that of T or DHT. Therefore, there was a separation between the low nanomolar binding affinity and the transcriptional activation by E2.
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). This was achieved by coincubating the reporter-transfected MDA cells with 0.5 nM DHT along with either 2 or 5 µM DHEA. In addition, bicalutamide was used as a positive control for AR antagonism at its most efficacious dose in these cells, 1 µM (17). All data were normalized to the transcriptional activity of DHT at 10 nM, which was included as an independent, maximally efficacious control (100% activity). At 0.5 nM, DHT elicited a approximately 80% maximal activation. As expected, bicalutamide reduced transcriptional activity to approximately 20% that of maximum, which represented a 75% inhibitory effect (in comparison with the original 80% activation). Whereas there was slight but significant inhibitory effect of DHEA at 2 µM, there was a more profound 75% inhibition at 5 µM. Therefore, DHEA was as efficacious as bicalutamide in inhibiting the transcriptional activity induced by 0.5 nM DHT, although its potency was around 5-fold lower.
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and 4). We first measured PSA production in LNCaP cells (Fig. 3), whose AR encodes a T877A mutation known to broaden ligand specificity (21). Overexpression of this mutant in CV1 cells was reported to allow a 15-fold agonist response to DHEA (vs. 60- to 100-fold for T and DHT, respectively), whereas there was no agonist response to DHEA when wild-type AR was overexpressed (22). The possibility that DHEA acts as an antagonist on LNCaP or any other form of AR has not yet been established. We began by treating LNCaP cells with a single maximally efficacious dose of each ligand for 24 h, and the PSA levels in the medium were then measured by ELISA. Consistent with the semiendogenous transcriptional responses seen in MDA cells, DHT and T were fully efficacious in LNCaP cells at 10 and 100 nM, respectively. E2 also elicited a strong agonist response at 2 µM, although the maximal effect was only 70% that of the androgens. This E2 response is consistent with that described in a previous report (21). Consistent with the mild agonist response of the T877A mutant AR in CV1 cells, DHEA at 5 µM elicited an agonist response at about 12% that of 10 nM DHT. Meanwhile, 1 µM bicalutamide showed absolutely no agonism, and it even slightly repressed the basal activity (Fig. 3). The modest agonist effect of DHEA, in comparison with the pure antagonist response of bicalutamide, is reminiscent of the modest agonism elicited by the otherwise pure antagonist, cyproterone acetate, in these cells (21). Hydroxyflutamide has also been shown to exhibit a mild agonist response on the T877A mutant, but not wild-type AR, when overexpressed in CV1 cells (22).
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). LNCaP cells were treated with either DHT or R1881 (both 0.5 nM) in combination with DHEA (5 µM) or other reference compounds, as described above. Again, because the initial concentrations of DHT or R1881 were set at submaximum doses, we observed an additive effect when 10 nM DHT (10.5 nM final concentration) or 100 nM T were added in combination with the initial 0.5 nM dose. Consistent with its 80% maximal activation at 2 µM, E2 actually showed a small, 20–25% antagonism in these experiments. This suggested that E2 was actually a mixed agonist/antagonist in these cells. As expected, bicalutamide showed substantial 30 and 60% inhibition of DHT- and R1881-induced transcription, respectively. In further support of its potential for AR antagonism, DHEA elicited a 60–70% inhibition of either agonist on AR-mediated transcription in the LNCaP cells. This suggests that, although the T877A mutant allows some modest agonist responses to DHEA, this ligand acts primarily as an antagonist in the LNCaP background.
Collectively the data suggest that DHEA may be a naturally occurring antagonist of AR, albeit when used in the low micromolar range. However, because the reported circulation level for DHEA is at 7–31 nM and higher in specific tissues, it may be difficult to attribute this effect any physiologically meaningful effect under most conditions. Under these circumstances, peripheral conversion of DHEA to T or DHT could result in the generation of sufficient concentrations to elicit an agonist response that could not be overcome by the relatively low concentration of DHEA. Meanwhile, although the DHEA reservoir molecule, DHEA-S, does circulate at 1.3–6.8 µM, it remained transcriptionally inactive in all assays tested (see Fig. 6A and data not shown).
Proliferative effects of DHEA on LNCaP cells
In consideration of the antagonistic behavior of DHEA on AR in both MDA and LNCaP cells alone, one might hypothesize that it may also exhibit an antimitogenic effect on cell growth. Consistent with our earlier transcription approaches, we tested this by measuring DHEA effects on LNCaP cell proliferation in agonist mode in comparison with DHT, T, E2, and bicalutamide, each dosed at their transcriptionally active concentrations (Fig. 5A). As expected, both DHT (10 nM) and T (100 nM) enhanced the growth rate vs. the vehicle-treated control, with approximately 5 and 25% more cells seen at 48 and 72 h, respectively. It was previously reported that E2 stimulates LNCaP cell proliferation (24). Indeed, E2 was even more effective in stimulating cell proliferation than the androgens, with cell number reaching 45% above vehicle effects by 72 h. As expected, bicalutamide had no effect on cell growth, and cell number remained identical with that seen with the vehicle control. Unexpectedly, and contrary to its antagonist effects on PSA production in these cells, DHEA was a full agonist for cell growth, and stimulation was identical with that of DHT or T by 72 h. This response distinguished DHEA from bicalutamide, which otherwise shared a common ability to antagonize AR-mediated transcription, as discussed above.
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). Over this more extended time, E2 elicited a 100% greater number of cells than was seen in the control group with an EC50 of approximately 50 nM and a maximal induction at 100 nM or more. Thus, activation of proliferation by E2 is seen at concentrations well below those required for activation of AR-mediated transcription, which is consistent with an ER-mediated effect. The magnitude of the response to DHEA over the same 5-d period was comparable with that of E2, with a 100% increase in cell growth at the top-tested dose of 5 µM. However, the EC50 was approximately 100-fold higher than that for E2. In consideration of the transcriptional antagonism of the AR in LNCaP cells by DHEA in contrast to its mitogenic effects, we therefore hypothesized that the cell proliferation was mediated via a non-AR mechanism, possibly ER based.
Binding of DHEA to ER and ERß
As noted above, E2 treatment was previously shown to elicit a mitogenic affect on LNCaP cell growth. Interestingly, this effect was associated with its action on ERß (24) and not ER, which is the key driver for mitogenic effects in breast cancer cells. We therefore extended our initial receptor binding assays to include ER and ERß (Table 1). In these analyses, we examined the ability of DHEA to competitively inhibit E2 binding to receptor that was transiently expressed in COS-1 cells. This method was chosen over the use of endogenously expressed receptor, owing to the lack of readily available cells with selective and substantial expression of either ER or ERß alone. DHEA exhibited a similar binding affinity for ER to that for AR (1.1 µM), whereas it bound to ERß with a slightly lower Ki of 0.5 ± 0.3 µM (Table 1). Similar results were observed with stable transfectants (data not shown).
Transcriptional activity of DHEA on ER targets
Binding analyses indicated that DHEA can interact with both ER isoforms with affinities that extend into the midnanomolar range. We therefore investigated potential transcriptional activation, based on the proliferative effects on LNCaP cells, which were comparable with those of E2. For these analyses, we examined activation function 2-dependent transcription using Gal4-DBD/ER-LBD chimeras (Gal4ER) (Fig. 6A), which we transiently expressed in COS-1 cells. A Gal4-DBD/AR-LBD chimera (Gal4AR) was also tested for comparison. For these analyses, percent activation was compared with an independent control using 200 nM E2 (data not shown). A sample of E2 at the same concentration as the control was also assessed as an independent test sample, which yielded a value of approximately 115% vs. the reference control, which is nonstatistically different (Fig. 6A). Consistent with its mitogenic potential in the LNCaP system, DHEA, at both 2 and 5 µM, also exhibited a strong agonist activation of Gal4ERß, and activity was similar to or somewhat higher (150%) than that elicited by E2 in the control group. Although E2 exhibited an essentially equal stimulation of both ER and ERß in these analyses, DHEA exhibited a clear preference for ERß, with no activation on ER at the lower concentration of 2 µM. At 5 µM, the 55% stimulation of ER was roughly one third the level of activity seen with ERß at the same concentration. Using the activation function 2-dependent system, we observed no transcriptional activation of AR by either DHEA or E2 in COS-1 cells. DHEA-S was inactive against all receptors, whereas DHT and T activated only Gal4AR (Fig. 6A).
In separate analyses, the effects of DHEA on Gal4ERß-mediated transcription were tested in the absence or presence of the pure estrogen antagonist, ICI-182780 (Fig. 6B). Two concentrations of DHEA (1 and 5 µM) and E2 (200 nM and 1 µM) were tested. All test samples were compared with an independent control stimulated with E2 at 200 nM. In antagonist control experiments, ICI-182,780 (100 nM) was fully efficacious in blocking E2-mediated transcription at the lower tested dose. However, when the concentration of E2 was increased 5-fold, ICI-182,780 lost its efficacy. Meanwhile ICI-182780 fully blocked DHEA-mediated transcription at either dose. Together, these data suggest that DHEA-induced, ERß-mediated transcription is achieved through direct binding of DHEA to this receptor’s LBD.
In initial binding analyses of ER and ERß (Table 1), DHEA appeared to exhibit a slight preference for ERß. To explore this further, dose-response effect on transcription was examined. For these analyses, COS-1 cells were transfected with GAL4ERs and GAL4-luc reporter (Fig. 7A) or HEK293 cells containing stably expressed intact receptors and ERE2-ALP reporter were used (Fig. 7B). Dose response curves compared DHEA with the native ligand E2. In the GAL4 chimeric system, DHEA maintained ERß specificity, with an EC50 of around 200 nM. With ER, approximately 40% activation was seen at the substantially higher concentration of 5 µM. Essentially full activation of the full-length ER receptor was observed in the HEK293 background with 90% activation seen at 5 µM, although there was little activation at 1 µM. The preference for ERß was also maintained in this background, with an EC50 of about 200 nM, as seen in the Gal4 chimeric system in COS-1 cells. These data support a model for DHEA activation of ER with an approximate 5- to 10-fold higher transcriptional specificity for the ERß receptor vs. ER.
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5 steroids, such as DHEA and its derivatives, could have served as the original ligands for the ancestral ER (7). Our data on DHEA binding to and activation of ER and ERß lend credence to this model. A biological role for DHEA as a precursor for sex hormones generated in peripheral tissues is already clearly established, and it is the sex hormones themselves that are believed to carry out the biological functions of this precursor (2). In this study, we found evidence to suggest that DHEA itself may also act as a sex hormone with specificity for the ERß receptor. In competition binding analyses, the data indicated that DHEA exhibited affinity to AR and ERs with some preference for ERß. Affinities for expressed ERs fell into the 0.2–1.5 µM range. There was no measurable affinity for endogenously expressed GR or PR. The affinity of DHEA to the ERs is generally consistent with a previous report (18), although transcriptional activities were not previously described. Our transcription analyses demonstrated that DHEA activates ERß with an maximum response similar to or slightly greater than that seen with E2.
Our initial analyses focused on DHEA as a potential androgen because it has long been considered an androgenic steroid precursor, exerting its action via peripheral conversion to more potent androgens, such as T and DHT (2, 25). DHEA did exhibit binding affinity to AR at micromolar concentration, but its affinity was 2- to 3-fold lower than that for ERß. DHEA transactivation function on AR was tested using a semiendogenous reporting system with endogenously expressed hAR in MDA-MB453 cells and minimal amounts of transfected MMTVluc promoter (17). Whereas this reporting system accurately measured DHT, T, and E2 transcriptional activation, it showed that DHEA, like bicalutamide, had undetectable agonist activity. Instead, DHEA, like bicalutamide, behaved more like an AR antagonist. This was seen in antagonist mode transcriptional profiling, which used DHT at levels yielding submaximal activity, whereby DHEA at 5 µM repressed DHT activity to the same extent as that seen with bicalutamide. That DHEA behaved more like an antagonist on endogenous AR was further established in its effects on LNCaP PSA production, in which this ligand exhibited minimal agonist activity and essentially maximal inhibitory activity in suppressing agonist-induced PSA expression. Indeed the antagonist effect was comparable with bicalutamide in both the MDA and LNCaP systems.
Our findings are in contrast to previous reports of DHEA androgenic activity on AR using transient cotransfection of AR along with an androgen responsive element-luciferase reporter (16, 21). The discrepancy between our current data and those from previous publications may stem from the difference between transiently expressed vs. endogenous AR, the latter being used here. Under normal levels of expression, AR resides in an unliganded state within the cytoplasm, in which it forms complexes with numerous chaperones, including heat shock proteins. When expressed in a transient transfection system, AR levels tend to be overexpressed, which can cause an imbalance between receptor and chaperone (26). Chaperones keep the receptor in a transcriptionally inactive state, and agonist binding induces the release of the chaperones in the process of inducing an active receptor conformation (27). Because the receptor lacking the accompanying chaperones may be more readily activated, substantial overexpression might better detect very weak agonist activity, which would not otherwise exist with normal receptor levels. In the present study, we used endogenously expressed AR in each of two different cell lines, and readouts included either a minimally expressed reporter (MMTV promoter) or an endogenous marker (PSA). The consistency in the results from the different reporter systems suggested predominantly antagonist effects of DHEA on AR-mediated transcription. It is notable, however, that there was a mild agonist response seen in the production of PSA in LNCaP cells in response to DHEA, possibly due to the mutant form of AR or, less likely, to a low level of metabolism of the ligand. There was a more profound DHEA-stimulated antagonism observed in both the PSA and MMTV readouts, suggesting that the primary role for the unmetabolized ligand is to serve as an AR antagonist. One should note, however, that because these effects required quite high concentrations, any physiological function(s) of DHEA through AR in vivo may be limited to certain stages of development.
A probable role for DHEA as a direct steroid receptor ligand at physiological concentrations would more likely be mediated through ER. With an EC50 of approximately 200 nM, there is a strong suggestion that ERß could be the most selective target. Based on the binding affinity of DHEA for this receptor and the ability of ICI-182780 to antagonize DHEA-induced transcription, this activity is most probably mediated through direct binding to ERß. ER could also be involved when concentrations rise high enough. Weak DHEA activity toward ER has also been reported previously in a study examining transcriptional effects but not receptor binding (28). Based on the physiological levels of DHEA, which reach to 30 nM or higher at certain developmental stages, it is unlikely that this ligand could exert a significant biological function via ER (EC50 > 1 µM) under most conditions. In contrast, because DHEA is a more potent transcriptional activator of ERß, there exists a real possibility for a physiologically meaningful interaction. Indeed, the concentrations of DHEA in circulation and local tissues are high enough to stimulate ERß to the same extent as that seen with circulating levels of E2 when tested at somewhat above its top normal level (outside ovulation). The ERß activity resulting from DHEA regulation could even rise higher during the fetal development, at adrenarche, and at around age 30 yr, because concentrations at these life stages are higher. The amount of estrogenic activity from DHEA could therefore contribute significantly to the total estrogenic milieu in the body.
Because of the perceived beneficial effects of DHEA, as outlined in the introductory text, it has been increasingly used as a dietary supplement, in an effort to retard the effects of aging (4, 12, 29). However, concerns over its safety, especially in breast or prostate cancer patients, suggests the need for caution (4, 5, 30, 31). In addition to being a direct regulator of ERß, DHEA can be metabolized into more potent estrogen and androgens in peripheral tissues, such as mammary gland and prostate, as has already been clearly established (2). Many research groups have reported stimulatory effects of androgens and estrogens on cancer cells (21, 31). We showed here that DHEA exhibited minimal agonist activity on the endogenous PSA promoter, and it actually antagonized subnanomolar DHT and R1881 activity. Nonetheless, our analyses of LNCaP cell proliferation demonstrated that at the indicated dose, DHEA was actually as effective as DHT and T in increasing LNCaP cell proliferation. Meanwhile, E2 appeared to be even more efficacious after three days of treatment, although by 5 d, the maximum effect of DHEA and E2 were comparable.
These observations are in general agreement with a previous report (4), although there were differences in potency that may be traced to methodology. In any event, our and the previous studies both indicate that DHEA, like androgens and estrogens, increase LNCaP cell proliferation, which is also the case for endothelial cell proliferation, as reported in an independent study (32). All three studies thus agree on the proliferative potential of DHEA. Whereas we suggest that the effect might be elicited via ERß, others have drawn no conclusion or suggested that neither ER nor AR is the responsible target (4, 32). As to whether the DHEA-ERß interaction is stimulatory for LNCaP growth should be the subject of further study. Such a study should include not only an analysis of how ER and AR antagonists might affect DHEA-mediated mitogenic stimulation but also potential metabolism of DHEA to sex steroids in these cells (23) as well as potential nongenomic effects, as have been suggested in other studies of endothelial cells (32, 33). Regardless of the mechanism of DHEA-mediated stimulation of LNCaP growth, our observation led us to test for possible effects of DHEA on ER, and our ultimate conclusions led us to ERß as a physiologically relevant target of this ligand.
Identifying physiological roles for DHEA as a direct ligand for ERß is also complicated by the fact that DHEA can be converted to estrogen, among other metabolites, in cells. However, certain parallels in the actions of DHEA and ERß-selective ligands in in vivo models of inflammation suggest that direct action on this receptor is possible. ERß-selective ligands have been demonstrated to show efficacy in inflammatory bowel disease, adjuvant-induced arthritis, and the treatment of experimental endometriosis (34, 35). Thus, a role for ERß in suppressing inflammation is likely. Such a role for DHEA has also been suggested. Clinical studies have shown that DHEA (and DHEA-S) levels are reduced in subjects with rheumatoid arthritis and systemic lupus erythematosus, among other diseases (reviewed in Ref.11). Moreover, DHEA exhibits antiinflammatory effects in an animal model of antigen-induced arthritis, including an inhibition of the formation of autoantibodies (36). One possible reason for the beneficial effects of DHEA in arthritis could include its conversion to downstream metabolite(s). One can likely rule out 7-hydroxy-DHEA as a probable candidate because this metabolite has been correlated with increased severity of arthritis progression (37). This is important to note because others have reported that 7-hydroxy-DHEA is a partial agonist for ERß, albeit with greater than 10-fold lower affinity vs. that for DHEA (as reported here) (38). As to whether 7-hydroxy-DHEA can serve as an antagonist that could help to drive the inflammatory response remains to be established.
Based on the complexity of DHEA metabolism to various potential ligands for AR and the ERs vs. its direct effects on these receptors, it may be difficult to specifically attribute DHEA’s direct transcriptional actions to any specific beneficial effect. Nonetheless, there are several reasons to suggest DHEA as a direct ligand for ERß and to a lesser extent ER and AR. First, our receptor binding assays were performed using an in vitro cell-free system, in which metabolism of DHEA to other more potent derivatives is negligible. The affinity for AR and ERs therefore likely reflects an accurate measure of binding to these receptors. Second, the transcriptional effects of DHEA were observed in a concentration range consistent with affinity for each respective receptor. Third, we measured possible DHEA metabolism in the HEK293 system, whereby ER activation was clearly observed, by quantifying the concentration in cells 24 h post treatment against parallel DHEA samples incubated in the absence of any cells. There was no observable changes in DHEA concentrations in either sets of samples (data not shown), which suggests that DHEA was not significantly metabolized under our experimental conditions. Fourth, DHEA exhibited both agonist (ER and ERß) and antagonist (AR) behavior in the various transcription assays, whereas its major metabolites (T and E2) serve only as agonists. Fifth, in the case of the DHEA metabolite, 7-hydroxy-DHEA, which is ERß-selective in COS-1 cells, this metabolite displays only partial (50%) agonism of ERß and only at a substantially higher concentration of 20 µM (38). Sixth, with regard to ER vs. ERß, E2 showed equipotent activation of these receptors, whereas DHEA strictly favored ERß. Overall, the unique characteristics of DHEA-mediated transcriptional effects on ER, ERß, and AR cannot be replicated by any known downstream metabolite. Thus, the data suggest direct action of DHEA on these receptors with a preference for ERß vs. ER or AR.
In summary, the composite data suggest a role for DHEA as a direct ligand for ERß in addition to its role as a precursor for sex hormone production. Although the observed activity at physiologically relevant concentrations of DHEA is low and so is the activity of E2. Whereas DHEA has the potential to act as an agonist of ER or an antagonist of AR, this requires substantially higher concentrations than needed to activate ERß. Theoretically, suitable concentrations might be achieved for in vivo effects on ER and AR, although this would likely be limited to the fetus at term and perhaps one or two developmental stages after birth. The increased concentrations of DHEA at these stages would also make it a more efficacious agonist on ERß. Although metabolism of DHEA makes difficult the process of providing absolute proof of its direct action on ERß in vivo, the preponderance of the evidence leaves little reason to conclude otherwise.
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Abbreviations: ALP, Alkaline phosphatase; AR, androgen receptor; DBD, DNA binding domain; DHEA, dehydroepiandrosterone; DHEA-S, DHEA sulfate; DHT, dihydrotestosterone; E2, 17ß-estradiol; ER, estrogen receptor; ERE, estrogen response element; FBS, fetal bovine serum; GR, glucocorticoid receptor; h, human; HEK, human embryonic kidney; Kd, equilibrium dissociation constant; Ki, equilibrium binding constant; LBD, ligand binding domain; luc, luciferase; MDA, MDA-MB453; MMTV, mouse mammary tumor virus; PR, progesterone receptor; PSA, prostate-specific antigen; T, testosterone.
Received March 29, 2005.
Accepted for publication June 24, 2005.
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). A, COS-1 cells were transfected with Gal4-ER chimeras and assessed for ligand-dependent transcription across several doses. B, Stably transfected HEK293 cells expressing full-length ERs and an ERE2-ALP reporter (