A comparative study of human and zebrafish glucocorticoid receptor activities of natural and pharmaceutical steroids

Introduction The action of environmental steroids on the human glucocorticoid receptor (hGR) has been pointed out with the risk to impair physiological immune and metabolic processes regulated by this nuclear receptor. However, there is still a lack of mechanistic information regarding their ability to interact with GR in aquatic species. Methods To investigate ligand activation differences between hGR and zebrafish GR (zfGR), we tested several natural and synthetic steroids using reporter cell lines expressing hGR or zfGR. Results and discussion Almost all the glucocorticoids tested (dexamethasone, cortisol, bimedrazol, medrol, cortivazol and fluticasone) are agonists of the two receptors with similar potencies. The dissociated glucocorticoids, RU24782 and RU24858 are agonists of both zfGR and hGR but with a better potency for the latter. On the other hand, the synthetic glucocorticoid forbimenol and the mineralocorticoid aldosterone are agonist on hGR but antagonist on zfGR. The other steroids tested, androgens and progestins, are all antagonists of both GRs with equal or lower potency on zfGR than on hGR. Surprisingly, the lower efficacy and potency on zfGR of aldosterone, forbimenol and the dissociated glucocorticoids is not related to their affinity for the receptors which would suggest that it could be related to less efficacious recruitment of coactivators by zfGR compared to hGR.


Introduction
The glucocorticoid receptor (NR3C1, GR) is a ligand-activated transcription factor belonging to the family of the nuclear receptors (NRs) (1).GR is composed of three major domains: i) an N-terminal transactivation domain (NTD); ii) a small central DNA-binding domain (DBD) and iii) a C-terminal ligand binding domain (LBD) which hosts the ligand-dependent transcriptional activation function 2 (AF-2) (2).In the absence of ligand, GR is located in the cytosol and is affiliated with a large multiprotein complex that includes heat shock protein (HSP) 90, HSP70 and immunophilins (3,4).Upon ligand binding with the ligand, the HSP complex disassociates and the receptor translocates into the nucleus to exert its transactivating effects, where it binds as a homodimer to glucocorticoid response elements (GREs) in the promoter regions of target genes.GR can also exert transrepressing effects by binding to negative GRE (nGREs) and probably by interfering with the binding of other transcription factors (5).Moreover, GR can also exert its transrepression activity independently of the DNA binding by interacting with transcription factors such as NF-kB and AP-1, which control the genes of many mediators of inflammation and immunity (6).
Glucocorticoids (GCs) are cholesterol-derived lipophilic steroid hormones produced by adrenal glands in response to external and internal signals.The main endogenous glucocorticoid hormone produced in human is cortisol, while synthetic GCs like dexamethasone and prednisolone are used extensively both in the treatment of chronic inflammatory diseases, such as rheumatoid arthritis and asthma, and for their immunosuppressant action in preventing organ rejection post transplantation (7,8).As beneficial effects of GCs are limited by their undesirable side effects like diabetes, osteoporosis, hypertension and skin thinnings, synthetic GCs called "dissociated glucocorticoids" have been synthetized (9).These chemicals less able to maintain hGR in a conformation able to recruit coactivators than full agonists like dexamethasone displayed limited transactivation potency but strong transrepression activity (10).
Recent studies have reported that xenobiotic substances such as metals, bisphenols, vinclozolin metabolites, organotins, polybrominated diphenyl ethers and polychlorinated biphenyls can interfere with hGR (11)(12)(13)(14)(15).Moreover, due to the wide use of synthetic glucocorticoids and other steroids as medicaments and their incomplete removal in discharged water systems, they have been detected in the aquatic environment (16-18).
In the last decades, the presence in the environment of substances potentially interfering with NRs has been widely investigated using different in vitro assays expressing human NRs (19)(20)(21).However, it has been recently underlined that extrapolation of data from mammalian pharmacology and toxicology into fish species could not be appropriate due to interspecies differences between receptors (21-26) On the other hand, the use of zebrafish as model organism for aquatic toxicology is a reliable tool due to its specific features that made its use considerably grown in the last decades in scientific research.In this regard, we have established two reporter cell lines expressing respectively hGR and zfGR with the aim of improve Endocrine disrupting chemicals (EDCs) risk assessment in water quality monitoring.In this work, we tested several natural and pharmaceutical steroids on these established cell lines with the aim of identifying possible human and zebrafish differences in the capacity of these chemicals to transactivate GR, as long as we have recently pointed out species-specificity differences in the activation or inhibition of hNRs and zfNRs in water extracts (21).The findings of the current study provide new information on the activities of the chemicals on hGR and zfGR and allow the development of new biological tools to evaluate EDC in environmental samples.
Stock solutions of chemicals were prepared in dimethyl sulfoxide (DMSO) and stored at -20°C.Fresh solution of test chemicals in test medium were prepared before each experiment.The final DMSO concentrations during treatment did not exceed 0.1% (v/v) of the test medium.

Reporter gene cell lines
HMLN-hGR and UMLN-zfGR clonal cell lines were already described (14,21).Briefly, HMLN-hGR cells were obtained by stable co-transfection of GR positive human HeLa cells with a glucocorticoid responsive gene (MMTV-Luciferase) and a hGR (pSG5-hGRpuromycin) expressing plasmid.To obtain UMLN-zfGR cells, human U2OS cells that mildly expressed hGR (28,29) were stably co-transfected with the MMTV-Luciferase and a zfGR expressing plasmids (pSG5-zfGR-puromycin).48 h after the transfection, cells were treated with G418 (1 mg/ml) and puromycin (0.5 mg/ml).Within 21 days, G418 and puromycin resistant clones appeared.For each cell line, 10 clones were chosen for their ligand-induced luciferase expression.The clones were amplified, and luciferase expression was checked at several passages.For each cell line, the clone with the best induction of luciferase activity was selected and used for the screening of the different steroids.The basal expression of luciferase is 1% and 11% for HMLN-hGR and UMLN-zfGR cell lines, respectively, of the maximal luciferase expression obtained in presence of dexamethasone 100 nM.The stability and the inducibility of luciferase expression were checked during at least 20 passages (20 weeks).The stability of the hGR and zfGR expression by ligand binding assay was also checked at different passages.
To obtain UMLN-hGR pool cells, U2OS cells were stably cotransfected with the MMTV-Luciferase and the hGR expressing plasmids.ZFL-zfGR pool cells were obtained by stable cotransfection of zebrafish ZFL cells with the MMTV-Luciferase and the zfGR expressing plasmids.The transfected cells were treated 7 days with G418 and puromycin but not cloned.

In vitro transactivation assays
In vitro transactivation assays were performed in 96-wells white opaque clear bottom culture plates (Greiner CellStar, Dutscher, Brumath, France).GR reporter cell lines were seeded at density of 5 x 10 4 cells per well in 150 µL culture medium and incubated at 37°C and 5% CO 2 for 24 h.Then, medium was removed and cells were exposed to increasing dilutions of tested compounds in test medium (DMSO; final concentration 0.1% v/v).Cells were incubated for 16 h at 37°C and 5% CO 2 .
Results of transactivation activity were expressed as percentage of the maximum luciferase activity induced by dexamethasone at 10 -7 M.
For antagonistic activity assessment, cells were exposed to different concentrations of the tested compounds and 3 nM dexamethasone.At this concentration, dexamethasone yields 60-80% of the maximal response.After the incubation period, medium was removed and replaced with 50 µL/well of test medium containing 0.3 mM luciferin.Luminescence signal was monitored in intact living cells for 2 s per well using a MicroBeta Trilux microplate scintillation and luminescence counter (PerkinElmer, Courtaboeuf, France).
The effect of the tested chemicals on cell viability was assessed with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.Briefly, after luminescence detection, medium containing luciferin was removed and replaced with 100 µL/well of test medium containing 0,4 mg/ml MTT for 4 h.Colorimetric signal was monitored at 570 nM using a Pherastar microplate reader (BMG Labtech, Champigny s/Marne, france).Experiments were performed in quadruplicate and repeated three times.

Ligand binding assays
For measurement of GR expression, HMLN-hGR, U2OS and UMLN-zfGR cells were seeded in 24-wells transparent plates at a density of 400,000 cells per well in culture medium.Cells were incubated for 24 h in a 5% CO 2 humidified atmosphere at 37°C.Then, medium was then removed and replaced with test medium containing 10 nM [3H]-dexamethasone (84 Cu/mmol, Perkin Elmer), in the absence or presence of 10 mM of non-radioactive For ligand competition assays, HMLN-hGR and UMLN-zfGR cells were seeded in 96-wells white opaque clear bottom culture plates (Greiner CellStar, Dutscher) at density of 10 5 cells per well in 0.2 ml of culture medium and incubated at 37°C and 5% CO 2 for 24 h.Then, medium was removed and cells were exposed to with 1 nM [3H]dexamethasone in the absence or presence of increasing concentrations of non-radioactive competitive compounds.After 3 h, unbound material was aspirated, and cells were washed three times with cold PBS in order to remove additional unbound material.Then, 150 ml of lysis buffer (250 mM Tris phosphate pH 7.8, 0.1% triton X-100) was added and plates were shaked for 5 min.50 ml of the total cell lysate was mixed with of 50 ml of LSC-cocktail (Emulsifier-Safe, Perkin Elmer) and [3H] bound radioactivity was liquid scintillation counted (MicroBeta trilux, Perkin Elmer).Protein concentration of 50 ml of the total cell lysate was measured by Bio-Rad protein assay and used to normalize bound radioactivity values expressed in dpm.Results were plotted as measured dpm versus concentration of tested compound.IC50 values were defined as compound concentration required to decrease maximum [3H]-dexamethasone binding by 50%.All the experiments were performed in quadruplicates and in at least three independent experiments.

Data analysis
Results of transactivation activity were expressed as percentage of the maximum luciferase activity induced by dexamethasone at 10 -7 M. Dose-response curves were fitted using the sigmoidal doseresponse function of a graphics and statistics software program (GraphPad Prism 8, GraphPad Software Inc.).We assumed a compound being agonist when an effect above 10% was noted as compared to the control cells, and antagonist when it decreased luciferase activity by more than 20% in the presence of the reference chemical at the concentration inducing 80% of the maximal response.Effective concentrations and inhibitory concentrations were derived from the Hill equation.For a given chemical, EC 50 was defined as the concentration inducing 50% of its maximal effect and IC 50 represented the concentration required for 50% inhibition.Lower and upper 95% confidence limits of EC 50 s of IC 50 s were calculated.
For ligand binding assays, dose-response curves were also fitted using GraphPad Prism.Inhibitory concentrations were derived from the Hill equation.Lower and upper 95% confidence limits of EC 50 s of IC 50 s were calculated.

GR expression of HMLN-hGR and UMLN-zfGR reporter cell lines
The HMLN-hGR and UMLN-zfGR clonal cell lines were previously established in our laboratory (21).To obtain HMLN-hGR cells, we transfected HeLa cells which express endogenously hGR by a hGR expressing plasmid to overexpress hGR and the MMTV-Luc GR-responsive gene.The UMLN-zfGR cell line was established in U2OS cells which slightly express hGR (28,29) by cotransfection of a zfGR expressing plasmid and the MMTV-Luc plasmid.As shown in Figure 1, receptor protein level (hGR or zfGR) expressed in each cell line was estimated by saturation ligandbinding assay (LBA) with 10 nM [3H]-DEX in a ''whole-cell'' experiment.These whole-cell LBAs confirmed the endogenous expression of GR in HeLa cells (217 fentomoles/mg protein) whereas in U2OS this receptor is very slightly expressed (17 fentomoles/mg protein).Stable transfection of an hGR expression plasmid enabled to increase the expression of hGR in HMLN-hGR cells (535 fentomoles/mg protein).Finally, transfection of an zfGR expression in UMLN-zfGR cells enabled to express 324 fentomoles of zfGR per mg of protein (Figure 1; Supplementary Table 2).

hGR and zfGR transactivation by natural and pharmaceutical steroids
The reference glucocorticoid agonist dexamethasone was tested in transactivation assays in both reporter cell models revealing close potency between the HMLN-hGR and UMLN-zfGR cell lines (EC 50 of 1.38 and 2 nM for hGR and zfGR, respectively) (Table 2; Figure 2A).Almost all the chemicals belonging to the group of GCs, presented in Table 1, showed full agonistic activity both on hGR and on zfGR in transactivation assays.Bimedrazole, cortivazol, fluticasone propionate and medrol exhibited similar potency on the two nuclear receptors (Table 2) whereas cortisol was slighly more potent on zfGR than on hGR with EC 50 of 40 nM and 15 nM for hGR and zfGR, respectively (Figure 2B; Table 2).On the contrary, the dissociated glucocorticoids RU24782 and RU24858 which were structurally designed to distinguish between the transrepression and transactivation (10) have showed better potency on hGR than zfGR (Table 2; Figures 2C, D).Interestingly, the synthetic glucocorticoid forbimenol only activated hGR (EC50 503 nM) (Table 2; Figure 2E).Similarly, the mineralocorticoid aldosterone, which is produced naturally in human but not in fish (30), was fully agonist on hGR but partial agonist on zfGR (maximal activity of 96 and 19% for hGR and zfGR respectively) (Table 2; Figure 2F).All the other chemicals tested lacked glucocorticoid agonist activity.
RU24782, RU24858, forbimenol and aldosterone were also assessed for antagonism in UMLN-zfGR cells.Interestingly, RU24782, RU24858 partially repress dexamethasone-induced activity at sub-micromolar concentrations (Figure 3A).As these GCs were described as less able to recruit coactivators than dexamethasone to hGR (10), their zfGR antagonism at submicromolar concentrations and agonism at micromolar concentrations is probably relied to their lower efficacy for recruiting coactivators.Forbimenol and aldosterone also antagonized zfGR (IC50 3160 and 13980 nM, respectively) (Table 3; Figure 3B).Finally, the anti-progestin mifepristone was partial agonist of hGR and zfGR with higher efficacy on the last one (Table 2; Figures 4A, B).In presence of dexamethasone, it acts as partial antagonist with IC50 in the 10 to 40 nanomolar range (Table 3; Figures 4A, B).The other chemicals lacking glucocorticoid agonist activity were also tested for their antagonist activity.They all exhibited antagonist activity on hGR and zfGR.IC50 varied Lower and upper 95% confidence limits of EC 50 s are indicated.Maximal activities (% max act) of the chemicals tested for their agonistic activity are expressed as a percentage of the maximal luciferase activity induced by 100 nM dexamethasone.They were determined at 10 -5 M excepted for bimedrazole, cortivazol and dexamethasone.For these chemicals, the maximal concentration tested was 10 -6 M. Relative potency (REP) of each competitor was calculated as ratio of concentrations of DEX or chemical required to induce the specific transactivation by 50% (ratio of EC50 values).REP value for DEX was arbitrarily set at 1. Half maximal effective concentration (EC 50 ), maximal activity (% max act) and relative potency (REP) of the chemicals on hGR and zfGR.between 772 (promegestone) and 6101 (dihydrostestosterone) nM for hGR and 1066 (17a-hydroxyprogesterone) and 7711 (spironolactone) nM for zfGR (Table 3).

Differences between hGR and zfGR transactivation are not due to differences of affinity
Whole-cell competitive binding assays were performed with HMLN-hGR and UMLN-zfGR cells to determine whether the different potencies observed in transactivation assays for some of the chemicals reflected their abilities to differently bind to hGR and zfGR.Table 4 summarized IC 50 and RBA values for hGR and zfGR.IC 50 of DEX of 5.8 and 3.1 nM for hGR and zfGR, respectively confirmed the similar potency of this chemical for the two receptors.By contrast, cortisol bound preferentially zfGR as shown by IC 50 s of 55.2 and 3.5 nM for hGR and zfGR, respectively (Table 4) and thus explained its higher potency on the fish receptor.RU24782 bound preferentially to hGR than to zfGR but these differences in affinity did not reflect the differences in potency reinforcing the hypothesis that that this chemical is less able to recruit coactivators for zfGR than for hGR.Finally, aldosterone and forbimenol which are agonists on hGR and antagonists on zfGR bound also slightly preferentially hGR than zfGR but these differences in affinity do not explain their differences in activities for the two receptors.Differences between hGR and zfGR transactivation are not dependant of the cellular context Since cellular context could influence the transcriptional activity of NRs and hGR and zfGR reporter cell lines are different (Hela for hGR and U2OS for zfGR), we established UMLN-hGR pool cells.We tested in these cells the activity of RU24782, RU24858, forbimenol and aldosterone.The response profiles obtained (Figure 5A) are very similar to those obtained with the HMLN-hGR cell line indicating that at least for hGR these chemicals had a similar potency.The four chemicals were agonists on these cells and  the EC 50 values for RU24782, RU24858, forbimenol and aldosterone were respectively of 3.1, 23.3, 54.5, 338 and 224 nM.We also tested in zebrafish liver ZFL-zfGR pool cells the activity of the three chemicals.Again, the response profiles obtained (Figure 5B) are very similar to those obtained with the UMLN-zfGR cell line.The EC 50 value for DEX, RU24782 and RU24858 were respectively of 6, 1938 and 1027 nM were respectively whereas forbimenol and aldosterone only slightly activated luciferase expression (Figure 5B).

Discussion
Previous studies investigating progestin receptor (PR) and mineralocorticoid (MR) receptor have showed strong differences in binding affinity and transactivation properties of steroids (24,31).As few studies have been done on GR, in this work, we have evaluated the ability of 21 steroids to alter the transcriptional activity of hGR and zfGR.
Our data have showed that the majority of the steroids tested bind with a similar affinity to both GR.However, some of them presented marked differences in transactivation efficacies and potencies.The most different steroids are forbimenol and aldosterone.These chemicals have a full agonist profile on hGR while they do not induce luciferase activity in UMLN-zfGR cells.According to their ability to bind to zfGR, aldosterone and forbimenol were able to antagonize DEX-induced luciferase activity in a concentration-dependant manner in the zfGR assay.Strikingly, the dissociated glucocorticoids RU24782 and RU24858 which are synthetic GCs less able to recruit coactivators than full agonists (dexamethasone, bimedrazol) to hGR have similar affinities for hGR and zfGR but showed different potencies on the two receptors.Even more curiously, these compounds are zfGR antagonists at low concentrations (at which they bind to zfGR) and zfGR agonists at high concentrations.
As these four steroids have similar affinities for both GRs, their zfGR antagonism is probably relied to their lower efficacy for recruiting coactivators.Interestingly, we observed the same results in different cellular contexts (U2OS for hGR and ZFL for zfGR); indicating that these differences are probably not specific to the cellular context, but on the contrary reflected the intrinsic properties of steroids on zfGR.
Although we did not fully explain the differences of activity of the two GRs, we could point out that these differences could probably be due to different coactivators' recruitment by hGR and zfGR.This should be further confirmed by using site-directed mutagenesis and analyses of the structures of the hGR and zfGR ligand binding domain which are currently lacking.These techniques have made it possible to explain that progesterone and spironolactone act as antagonists on hMR and agonists on zfMR due to a substitution of threonine in hMR by a leucine on zfMR (32).These techniques would be probably also valuable to explain why several progestins are antagonist on hPR while they are agonist on hPR (24).
Although few of the steroids tested exhibited differences in activation between human and zebrafish receptor for GR than for MR and PR, our results showed that they exist and confirm that investigating inter-species differences is important and can improve EDC risk assessment.According to our results, toxicological data extrapolated from mammalian models could not be always suitable for predicting possible hazard in other species such as zebrafish.Regarding this latter aspect, since in the last decades numerous studies have detected glucocorticoid activity in the environment raising concern about their distribution and their harmful effects on wildlife (16; 33), we consider that the use of our established cell lines could improve the endocrine disrupting assessment and environmental contamination monitoring.

FIGURE 1
FIGURE 1 Expression of GR in HeLa, HMLN-hGR, U2OS and UMLN-zfGR cells.GR expression was measured by [3H]-DEX binding assay.Concentrations of GR are expressed in fentomoles/mg of protein.Values are means ± SD of 3 independent experiments.

FIGURE 3
FIGURE 2 Dose-response curves of dexamethasone (A), cortisol (B), RU24782 (C), RU24858 (D), forbimenol (E) and aldosterone (F) activity in HMLN-hGR and UMLN-zfGR cells.Results are expressed as the percentage of the maximum luciferase activity induced by 100 nM dexamethasone.Error bars represent standard deviation.

4
FIGURE 4 Dose-response curves of mifepristone activity in HMLN-hGR (A) and UMLN-zfGR (B) cells in absence or presence of dexamethasone 3 nM.Results are expressed as percentage of the maximum luciferase activity induced by 100 nM dexamethasone.Error bars represent standard deviation.

5
FIGURE 5 Dose-response curves of dexamethasone, RU24782, RU24858, aldosterone and forbimenol activity in UMLN-hGR (A) and ZFL-zfGR (B) cells.Results are expressed as percentage of the maximum luciferase activity induced by 100 nM dexamethasone.Error bars represent standard deviation.

TABLE 1
Classification of natural and synthetic steroids tested on HMLN-hGR cells and UMLN-zfGR cells.
EC 50 s are expressed in nM.Values of EC 50 are the mean from at least three separate experiments.

TABLE 3
Antagonistic activity assessment.

TABLE 3 Continued
50 s are expressed in nM.Values of IC 50 are the mean from at least three separate experiments.Lower and upper 95% confidence limits of IC 50 s are indicated.Minimal activities (% min act) of the chemicals tested for their antagonistic activity are expressed as a percentage of the maximal luciferase activity induced by 100 nM dexamethasone.They were determined at 10 -5 M. nc not calculated.Half maximal inhibitory concentration (IC 50 ) and minimal activity of the chemicals on hGR and zfGR. IC

TABLE 4
Whole-cell ligand competition assays.-cell ligand competition assays were performed using 1 nM [3H]-DEX as tracer.IC 50 s are expressed in nM.Values of IC 50 are the mean from at least three separate experiments.Lower and upper 95% confidence limits of IC 50 s are indicated.RBA of each competitor was calculated as ratio of DEX or chemical concentration required to reduce the specific radioligand binding by 50%.RBA is relative binding affinity where DEX = 100.Half minimal effective concentration (IC50) and relative binding activity (RBA) of DEX, cortisol, RU24782, forbimenol and aldosterone on hGR and zfGR. Whole