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Endocrinology, doi:10.1210/en.2005-1143
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Endocrinology Vol. 147, No. 4 1895-1903
Copyright © 2006 by The Endocrine Society

Aryl Hydrocarbon Receptor Activation Impairs Cortisol Response to Stress in Rainbow Trout by Disrupting the Rate-Limiting Steps in Steroidogenesis

Neelakanteswar Aluru and Mathilakath M. Vijayan

Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Address all correspondence and requests for reprints to: Dr. M. M. Vijayan, Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. E-mail: mvijayan{at}uwaterloo.ca


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Anthropogenic stressors activating aryl hydrocarbon (Ah) receptor signaling, including polychlorinated biphenyls, impair the adaptive corticosteroid response to stress, but the mechanisms involved are far from clear. Using Ah receptor agonist (ß-naphthoflavone; BNF) and antagonist (resveratrol; RVT), we tested the hypothesis that steroidogenic pathway is a target for endocrine disruption by xenobiotics activating Ah receptor signaling. Trout (Oncorhynchus mykiss) were fed BNF (10 mg/kg·d), RVT (20 mg/kg·d) or a combination of both for 5 d, and subjected to a handling disturbance. BNF induced cytochrome P4501A1 expression in the interrenal tissue and liver, whereas this response was abolished by RVT, confirming Ah receptor activation. In control fish, handling disturbance transiently elevated plasma cortisol and glucose levels and transcript levels of interrenal steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol side chain cleavage (P450scc) and 11ß-hydroxylase over a 24-h period. BNF treatment attenuated this stressor-induced plasma and interrenal responses; these BNF-mediated responses were reverted back to the control levels in the presence of RVT. We further examined whether these in vivo impacts of BNF on steroidogenesis can be mimicked in vitro using interrenal tissue preparations. BNF depressed ACTH-mediated cortisol production, and this decrease corresponded with lower StAR and P450scc, but not 11ß-hydroxylase mRNA abundance. RVT eliminated this BNF-mediated depression of interrenal corticosteroidogenesis in vitro. Altogether, xenobiotics activating Ah receptor signaling are steroidogenic disruptors, and the mode of action includes inhibition of StAR and P450scc, the rate-limiting steps in steroidogenesis.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
CORTICOSTEROIDS, SYNTHESIZED by the adrenal cortex, are involved in the regulation of numerous physiological processes including intermediary metabolism, ion regulation, and immune responses (1, 2). Their secretion is tightly regulated by hormones released from the hypothalamus and pituitary, including CRH and ACTH, respectively. As in mammals, corticosteroid biosynthesis and secretion in teleostean fishes are also under the control of the hypothalamus-pituitary axis, but unlike mammals they lack a discrete adrenal gland. Instead, the corticosteroidogenic cells (interrenal cells) are distributed around the postcardinal vein in the anterior part of the kidney and secrete cortisol, the primary corticosteroid in bony fishes, in response to stressors (3, 4).

Cortisol biosynthesis involves a series of steps commencing with the stimulation of interrenal tissues by ACTH and the subsequent conversion of cholesterol, through a series of enzymatic steps, including cytochrome P450 family proteins, dehydrogenases and hydroxylases, to steroids (5). Steroidogenic acute regulatory protein (StAR), a rate-limiting step in steroidogenesis, plays a key role in the transport of cholesterol from outer to inner mitochondrial membrane (6). This provides the substrate for the primary rate-limiting step in the steroidogenic pathway, catalyzed by cytochrome P450 cholesterol side chain cleavage (P450scc) enzyme, converting cholesterol to pregnenolone. Cortisol is synthesized from pregnenolone through a series of isomerizations and hydroxylations, including the final step catalyzed by 11ß-hydroxylase (5, 7). Key players in the corticosteroidogenic response to stressors includes StAR and P450scc, both acutely regulated by trophic hormones (8, 9, 10, 11, 12), whereas enzymes downstream of P450scc, including 11ß-hydroxylase, are constitutively expressed and elevated levels are observed even in the absence of trophic hormone stimulation (13, 14).

Elevation of circulating cortisol levels in response to stressors is considered an adaptive response and important for homeostatic adjustments to cope with stress (1, 2). This is also true in teleost fishes (3, 4), but persistent organic pollutants, including polychlorinated biphenyls (PCBs), impair this adaptive response by decreasing the interrenal capacity for cortisol production (15, 16). Recent studies have proposed that PCBs act at multiple sites along the hypothalamus-pituitary-interrenal axis to weaken the stressor-mediated cortisol response, including decreased sensitivity of interrenals to trophic hormones (15, 16, 17), abnormal negative feedback regulation (18), altered cortisol clearance (4) and inhibition of steroidogenesis (19). Indeed, the resistance to 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD)-induced toxicity in aryl hydrocarbon (Ah) receptor null mice, including lack of changes in steroidogenic enzymes, suggests a link between Ah receptor signaling and impaired steroidogenesis (20, 21, 22). However, the mode of action of xenobiotics, acting via Ah receptor signaling, in impairing corticosteroid response to stress is far from clear. Against this backdrop, we tested the hypothesis that the rate-limiting steps in steroidogenesis are key targets for endocrine disruptors acting via Ah receptor activation in rainbow trout. Fish were administered Ah receptor agonist (ß-naphthoflavone, BNF) and antagonist (resveratrol, RVT; Refs.23 and 24) either alone or in combination to tease out the role of Ah receptor activation on plasma cortisol response and interrenal corticosteroid biosynthesis.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Chemicals
BNF, RVT (99% purity), protease inhibitor cocktail, bicinchoninic acid reagent, L15 medium, porcine ACTH (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39) and 2-phenoxyethanol were obtained from Sigma-Aldrich (St. Louis, MO). All electrophoresis reagents and molecular weight markers were from Bio-Rad (Hercules, CA). Cytochrome P4501A (CYP1A) antibody (mouse monoclonal anti-cod CYP1A) was from Biosense Laboratories (Bergen, Norway) and the secondary antibody, alkaline phosphatase-conjugated goat antimouse IgG, was from Sigma. Nitroblue tetrazolium and 5-bromo-4-chloro-3-indolyl phosphate p-toluidine salt were from Fisher Scientific (Nepean, Ontario, Canada).

Fish
Juvenile rainbow trout (Oncorhynchus mykiss, average body mass 227 g) were obtained from Humber Springs trout hatchery; Mono Mills, Ontario, Canada. Fish were acclimated in 2000-liter tanks with continuous running water at 13 C and 12-h light, 12-h dark photoperiod for 3 wk before the experiment. The fish were fed to satiety (3 point sinking food; Martin Mills Inc., Elmira, Ontario, Canada) once daily 5 d a week.

Protocols
Role of Ah receptor in the stress response.
Groups of eight fish each were randomly distributed in 12 tanks (100 liter) and acclimated for 2 wk before the experiment. Treatments were assigned to the tanks (three tanks per treatment for four treatments) randomly and the fish were fed with feed laced with either vehicle (ethanol laced feed) or vehicle containing RVT (20 mg/kg body mass·d), BNF (10 mg/kg body mass·d) or a combination of BNF and RVT (RBNF; 20 mg RVT and 10 mg BNF/kg body mass·d) for 5 d. The duration of feeding was determined based on our previous study (19). Briefly, the feed was laced by evenly submerging in 95% ethanol alone (control) or ethanol containing RVT, BNF, or RBNF to ensure adequate coating of the pellets. The ethanol was allowed to evaporate by air-drying, and the feed was stored in a cool and dry place. This method provides an easy means of administering the drugs without stressing the fish unlike ip injections or implants (25).

After 5 d of feeding, six fish from each treatment (two fish each from three tanks) were sampled quickly and killed with an overdose of 2-phenoxyethanol (1:1000), and these were the unstressed (0 h) control fish. The remaining fish (six fish per tank from three tanks per treatment) were subjected to a standardized stress protocol of netting and chasing the fish for 5 min and were left undisturbed until they were sampled. Sampling consisted of netting all fish from one tank at 1, 4, and 24 h after stressor exposure. Also, an additional four tanks (five fish per tank from four tanks = a total of 20 fish) were maintained exactly as mentioned above as unstressed controls and sampled at 0, 1, 4, and 24 h to take into consideration the effect of sampling time on the measured parameters. Sampling consisted of quickly netting all fish from each tank, bleeding by caudal puncture into heparinized tubes, and the plasma was collected after centrifugation (6000 x g for 10 min) and stored frozen (–70 C) for plasma cortisol and glucose determination. Pieces of liver and head kidney were frozen immediately on dry ice for protein and mRNA determinations respectively (see below).

Temporal profile of StAR, P450scc, and 11ß-hydroxylase in head kidney.
Time course profiles of ACTH-stimulated increase in StAR, P450scc, and 11ß-hydroxylase mRNA abundance were determined by in vitro incubation of head kidney slices following previously standardized procedure (19). Briefly, fish were killed with an overdose of 2-phenoxyethanol (1:1000) and head kidney tissue was excised immediately and sliced into 1 mm3 pieces in ice-cold L15 media. The head kidney slices were distributed equally into 12 wells (six each for control and ACTH) in 24-well tissue culture plate (Corning Inc., Corning, NY). Tissue pieces were allowed to incubate with gentle shaking for 2 h at 13 C (equilibration period), after which the L15 medium was replaced with fresh medium containing either no ACTH (control wells) or ACTH (0.5 IU/ml; ACTH wells) and incubated for 0, 0.5, 1, 2, 3, and 4 h, exactly as mentioned before in trout (19). The tissue slices were collected at the end of each time point and frozen for later determination of StAR, P450scc, and 11ß-hydroxylase mRNA levels. For each experiment, head kidney tissue from two to three fish were pooled and this was repeated at least four times using fresh head kidney tissues from different fish (n = 4). The concentration of ACTH was determined previously by dose-response studies and 0.5 IU/ml ACTH elicited maximal cortisol production in trout head kidney tissues (17).

Role of Ah receptor on steroidogenesis in vitro.
The role of Ah receptor on acute regulation of interrenal steroidogenesis was assessed by in vitro exposure of head kidney tissue to BNF and RVT either alone or in combination and determining the steroidogenic capacity in response to ACTH stimulation. The incubation consisted of slicing and distributing the tissues (as described above) from each fish and equally splitting them into eight wells (two wells (one each for control and ACTH) per treatment x 4 treatments). The tissue pieces were allowed to incubate with gentle shaking for 2 h at 13 C (equilibration period), after which the supernatant was replaced with fresh medium and treated with either control (0.01% dimethylsulfoxide) or RVT (10–5 M), BNF (10–6 M) and RBNF (10–5 M RVT and 10–6 M BNF). In RBNF group, RVT was added 30 min before the addition of BNF. The tissues were incubated for 1 h and the media collected for later determination of cortisol concentration (basal cortisol production rate). The media were replaced with fresh media along with treatments and were stimulated either with or without ACTH (0.5 IU/ml) for 3 h. After 3 h, the supernatant was collected and frozen for later determination of cortisol concentration (stimulated cortisol production rate). Wet mass of the tissue in each well was recorded and cortisol production rate was expressed as ng/h·mg wet weight. The head kidney tissue was frozen for later determination of steroidogenic enzyme mRNA abundance. This was repeated at least four times with fresh head kidney tissue from different fish.

Cortisol and glucose
Plasma cortisol concentrations were measured using a commercially available ImmuChem 125I RIA kit (ICN Biomedicals, Costa Mesa, CA) according to established protocols (17). Plasma glucose levels were determined colorimetrically (Trinder method; Sigma).

SDS-PAGE and Western blotting
Protein concentrations were determined using the bicinchoninic acid method with BSA as the standard. The procedure for SDS-PAGE and Western blotting were according to established protocols (17). Briefly, samples (40 µg protein/sample) were separated on 8% polyacrylamide gels using a discontinuous buffer system (26). The proteins were transferred onto a nitrocellulose membrane (20 V for 20 min) with a semidry transfer unit (Bio-Rad) using transfer buffer [25 mM Tris (pH 8.3), 192 mM glycine, and 20% (vol/vol) methanol]. The membrane was blocked with 5% skimmed milk in TBS-t [20 mM Tris (pH 7.5), 300 mM NaCl and 0.1% (vol/vol) Tween 20 with 0.02% sodium azide] for 60 min. The mouse monoclonal anti-cod CYP1A primary antibody (1:3000) and alkaline phosphatase-conjugated goat antimouse secondary antibody (1:1000 dilution) were diluted in the blocking solution. The membranes were incubated in primary antibody for 60 min at room temperature, washed with TBS-t (twice for 5 min each), incubated with secondary antibody for 60 min, washed with TBS-t (twice for 5 min each), and finally washed with TBS (once for 15 min). Visualization of bands was carried out with nitroblue tetrazolium (0.033% wt/vol) and 5-bromo-4-chloro-3-indolyl phosphate p-toluidine salt (0.017% wt/vol) and the molecular mass confirmed using prestained low molecular weight marker (Bio-Rad). Quantification of bands was carried out with Chemi imager using the AlphaEase software (Alpha Innotech, San Leandro, CA).

Quantitative real-time PCR
RNA isolation and first strand cDNA synthesis.
Total RNA was isolated from head kidney using RNeasy mini-kit (QIAGEN, Mississauga, Ontario, Canada) following the manufacturer’s protocol and quantified spectrophotometrically at 260 nm. RNA was deoxyribonuclease treated to avoid genomic contamination following the manufacturer’s instructions. The first strand cDNA was synthesized from 1 µg of total RNA using First Strand cDNA synthesis kit (MBI Fermentas, Burlington, Ontario, Canada). Briefly, total RNA was heat denatured (70 C) and cooled on ice. The sample was used in a 20-µl reverse transcriptase reaction using 0.5 µg oligo deoxythymidine primers and 1 mM each deoxynucleotide triphosphate, 20 U ribonuclease inhibitors, and 40 U Moloney murine leukemia virus reverse transcriptase. The reaction was incubated at 37 C for 1 h and stopped by heating at 70 C for 10 min.

Relative standard curve.
Primers were designed using rainbow trout CYP1A1, StAR, P450scc, 11ß-hydroxylase, and ß-actin cDNA sequences and the size of the amplified product was 100 bp for CYP1A1 and ß-actin and 500 bp for StAR, P450scc and 11ß-hydroxylase (Table 1Go). A relative standard curve was constructed for target genes (CYP1A1, StAR, P450scc, and 11ß-hydroxylase) and housekeeping gene (ß-actin) using either cDNA stock or plasmid vectors with inserted target sequences according to established protocols (27). Briefly, the concentration of cDNA or plasmid vector stock was assumed to be 500 pg µl–1 and reactions were set up with different concentrations ranging from 10–3000 pg per reaction for the standard curve (27). Platinum Quantitative PCR SuperMix-uracil DNA glycosylase (Invitrogen, Carlsbad, CA) used was 2x concentrated, and every 25-µl reaction had 1.5 U Platinum Taq DNA polymerase, 20 mM Tris-HCl (pH 8.4), 50 mM KCl, 3 mM MgCl2, 200 µM deoxy (d) GTP, 200 µM dATP, 200 µM dCTP, 400 µM deoxyuridine triphosphate, and 1 U uracil DNA glycosylase; the reaction also contained 0.2 µM forward and reverse primers, fluoroscein calibration dye (1:2000; Bio-Rad) and SYBR green I nucleic acid gel stain (1:100,000; Roche, Laval, Quebec, Canada). Master mixes, to reduce pipetting errors, were prepared at every stage for triplicate reactions (3 x 25 µl) for each standard.


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  		TABLE 1. Oligonucleotide primers used in qPCR

 
Quantification
One microliter of cDNA sample was used as a template for every 25-µl reaction of target genes, and 0.5 µl cDNA was used as a template for ß-actin. Each sample was set up in triplicates for the quantitative real-time PCR (qPCR). The reaction components were exactly the same as the previous section and for every single test sample, a qPCR for both the target (CYP1A1, StAR, P450scc, and 11ß-hydroxylase) and the housekeeping gene (ß-actin) was performed. The following PCR cycle was used for gene amplification: 95 C for 3 min; 40 cycles: denaturing 95 C for 30 sec, annealing (Table 1Go) for 30 sec, extend 72 C for 30 sec, followed by 4 C hold at the end of the PCR. The transcript abundance was obtained from their respective standard curves after normalization with ß-actin; threshold cycles for ß-actin showed little change with treatments and were used for the normalization.

Statistical analyses
Data were expressed as mean ± SEM and log-transformed, wherever necessary, for homogeneity of variance, but nontransformed values are shown in the figures. Two-way ANOVA was used to determine the significant time, treatment and interaction effects on plasma cortisol and glucose concentrations. One-way ANOVA was used for all other comparisons. A post hoc (Bonferroni test) test was used for pair-wise comparison wherever significant differences were observed. A probability level of P ≤ 0.05 was considered significant. All statistical analyses were performed with SPSS version 12.0.1 (SPSS Inc., Chicago, IL).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Ah receptor activation in vivo
CYP1A expression in vivo.
BNF significantly elevated liver CYP1A protein expression compared with all other treatments (Fig. 1AGo). This BNF-induced CYP1A response was abolished by RVT (Fig. 1AGo). In head kidney, CYP1A protein expression was undetectable and, therefore, CYP1A1 mRNA abundance was ascertained by qPCR. As expected, CYP1A1 mRNA abundance was significantly higher with BNF compared with other treatments and this response was also eliminated by RVT (Fig. 1BGo).


Figure 1
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  		FIG. 1. Liver CYP1A protein expression (A) and head kidney CYP1A1 mRNA abundance (B) in rainbow trout. Fish were given control feed or feed containing RVT (20 mg/kg body mass·d), BNF (10 mg/kg body mass·d) or RBNF (10 mg BNF/kg body mass·d and 20 mg RVT/kg body mass·d) for 5 d. The samples were immunodetected for CYP1A protein using mouse monoclonal anti-cod antibody. All values represent mean + SEM (n = 5–6 fish). Different letters denote statistically significant differences between the treatments (one-way ANOVA; P < 0.05).

 
Cortisol and glucose.
In the unstressed control fish, plasma cortisol levels did not change with time in the present study (Fig. 2AGo). Handling disturbance significantly increased plasma cortisol levels at 1 h after stressor, but the levels declined thereafter and were not statistically different from the unstressed levels at 4 and 24 h after stressor exposure (Fig. 2AGo). However, BNF but not other treatments significantly attenuated this stressor-induced plasma cortisol elevation, whereas RVT (RBNF group) inhibited this response back to the control levels (Fig. 2AGo). The plasma cortisol levels before handling disturbance were not significantly different between treatments.


Figure 2
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  		FIG. 2. Handling disturbance induced elevation in plasma cortisol and glucose concentrations in rainbow trout. Plasma cortisol (A) and plasma glucose (B) concentrations in fish fed either control feed or feed containing RVT, BNF or RBNF for 5 d and subjected to a standardized handling stressor (5 min repeated netting and chasing). Plasma samples were collected at 0 (before stress), 1, 4 and 24 h after handling stressor. See Fig. 1Go legend for details. All values represent mean ± SEM (n = 5–6 fish); time points with different letters are statistically significant; significant treatment effects were observed only at 1 h after handling and BNF (–{triangledown}–) group had significantly lower plasma cortisol levels compared with Control (–{circ}–), RVT (–{blacktriangleup}–) and RBNF (–bullet–) groups (two-way ANOVA; P < 0.05). The unstressed group (–{blacksquare}–), for both plasma cortisol and glucose, is shown just for reference and there was no statistically significant difference among the time points in that group.

 
Plasma glucose concentration also showed no significant time effect in the control unstressed fish (Fig. 2BGo). There was a significant time and treatment effects on plasma glucose levels after stressor exposure (Fig. 2BGo). Glucose levels peaked at 4 h after stressor, and at 24 h the levels in all treatments were back to resting values. As seen with cortisol, BNF treatment attenuated the stressor-induced elevation of plasma glucose concentration, whereas this response was offset by RVT (Fig. 2BGo). There were no treatment effects on plasma glucose levels before handling disturbance in the present study.

StAR, P450scc, and 11ß-hydroxylase.
There was no significant effect of sampling time on StAR, P450scc, and 11ß-hydroxylase mRNA abundance in the unstressed groups (Fig. 3Go, A–C). StAR and 11ß-hydroxylase transcripts were significantly higher in response to handling at 1 h after stressor, and the levels remained significantly elevated even at 4 and 24 h after stressor compared with unstressed controls (Fig. 3Go, A and C). P450scc mRNA levels increased with handling disturbance with significantly higher levels observed at 4 h after stressor, but the levels returned to unstressed levels 24 h after handling disturbance (Fig. 3BGo).


Figure 3
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  		FIG. 3. Handling stressor-induced changes in StAR (A), P450scc (B) and 11ß-hydroxylase (C) transcript levels in trout head kidney. Fish were either unstressed or exposed to handling stressor and sampled at 0, 1, 4, and 24 h later and transcript levels determined using qPCR; values are shown as percent pre-stressor levels (0 h); all values represent mean ± SEM (n = 6 fish for handling group and n = 4 for unstressed group); different letters denote significant differences between time points (one-way ANOVA, P < 0.05).

 
No significant differences in the mRNA levels of StAR, P450scc, and 11ß-hydroxylase among treatments (control, BNF, RVT, RBNF) were observed before handling disturbance. However, BNF significantly attenuated both StAR and P450scc mRNA abundance at 1 h after stressor compared with all other groups (Fig. 4Go, A and B). This BNF-mediated steroidogenic response was eliminated in the presence of RVT (RBNF group) in the present study. No significant difference in the mRNA abundance of 11ß-hydroxylase was observed between treatments (Fig. 4CGo).


Figure 4
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  		FIG. 4. Ah receptor-mediated effects on StAR (A), P450scc (B) and 11ß-hydroxylase (C) transcript levels at 1 h after handling disturbance. Magnitude of change in mRNA levels to stressor exposure was calculated by deducting the pre-stressor (0 h) levels from 1 h after stressor levels for all treatments. Transcript levels were measured using qPCR; all values represent mean + SEM (n = 4 fish). Different letters denote statistically significant differences between the treatments (one-way ANOVA; P < 0.05).

 
Ah receptor activation in vitro
CYP1A1 expression.
In vitro administration of BNF to head kidney slices significantly elevated CYP1A1 mRNA abundance compared with other treatments. This BNF-induced elevation in the mRNA levels was blocked by RVT (RBNF group; Fig. 5AGo).


Figure 5
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  		FIG. 5. Ah receptor activation impairs in vitro ACTH-stimulated corticosteroidogenesis in trout. CYP1A1 mRNA abundance (A) and ACTH-stimulated cortisol production (B) in head kidney tissue slices exposed to control [0.01% dimethylsulfoxide (DMSO)], RVT (10–5 M), BNF (10–6 M) or RBNF before stimulation with ACTH (0.5 IU/ml). The magnitude of change in cortisol production (normalized to wet tissue weight) was calculated by deducting the basal cortisol production from ACTH-stimulated increase and expressed as percent control; bars with different letters are statistically significant (one-way ANOVA, P < 0.05); all values represent mean + SEM (n = 4 fish).

 
Cortisol production.
BNF significantly attenuated in vitro ACTH-stimulated cortisol production compared with all other treatments (Fig. 5BGo). Administration of RVT before the addition of BNF overcame the BNF-mediated attenuation of ACTH-stimulated cortisol production (Fig. 5BGo). RVT alone did not significantly affect cortisol production.

StAR, P450scc, and 11ß-hydroxylase.
Unstimulated head kidney tissue (sham) showed no significant changes in the mRNA abundance of StAR, P450scc, and 11ß-hydroxylase with sampling time in vitro (Fig. 6Go, A–C). In vitro stimulation of head kidney tissue with ACTH temporally elevated all three transcript levels (Fig. 6Go). StAR mRNA abundance was significantly higher at 0.5 h and stayed elevated over the 4-h time period (Fig. 6AGo). P450scc mRNA levels were significantly higher at 1 h after ACTH stimulation and stayed elevated for the remaining sampling times (Fig. 6BGo). 11ß-Hydroxylase mRNA abundance also showed a significant elevation temporally, similar to that of P450scc; significantly higher at 1 h and remaining elevated over the 4-h period (Fig. 6CGo).


Figure 6
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  		FIG. 6. Temporal profiles of StAR (A), P450scc (B), and 11ß-hydroxylase (C) mRNA abundance in head kidney slices incubated either without (sham) or with ACTH (0.5 IU/ml) and sampled after 0.5, 1, 2, 3, and 4 h. All values represent mean ± SEM (n = 4 fish); *, significantly different from 0 h time control (one-way ANOVA; P < 0.05).

 
BNF administration significantly depressed ACTH-mediated increase in StAR and P450scc mRNA levels, but not 11ß-hydroxylase mRNA abundance (Fig. 7Go, A–C). Also, RVT blocked this BNF-mediated decrease in StAR and P450scc mRNA abundance in response to ACTH stimulation (Fig. 7Go, A and B). There was no effect of RVT either alone or in combination with BNF on 11ß-hydroxylase mRNA levels (Fig. 7CGo).


Figure 7
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  		FIG. 7. Ah receptor activation impairs in vitro ACTH-stimulated StAR (A), P450scc (B), and 11ß-hydroxylase (C) transcript levels in rainbow trout. Head kidney tissue slices were exposed to control (ACTH), RVT (10–5 M, BNF (10–6 M or RBNF and stimulated with ACTH (0.5 IU/ml) and mRNA abundance levels were measured using qPCR; the data are shown as % unstimulated (in the absence of ACTH) control; all values represent mean + SEM (n = 4 fish). Different letters denote statistically significant differences between the treatments (one-way ANOVA; P < 0.05).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Ah receptor activation impairs cortisol response to handling disturbance in rainbow trout. Specifically, attenuation of StAR and P450scc mRNA levels by BNF corresponded with a reduction in the stressor-induced cortisol response in vivo as well as cortisol production to ACTH stimulation in vitro. RVT, a recently characterized and effective AhR antagonist (23, 24) abolished this BNF-mediated response providing the first clear evidence that Ah receptor activation targets the rate-limiting steps in steroid biosynthesis, resulting in the attenuation of the adaptive cortisol response on exposure to any stressor.

Stress and corticosteroidogenesis
The initial step in corticosteroidogenesis involves the translocation of cholesterol from outer to the inner mitochondrial membrane by StAR, where it is converted to pregnenolone by P450scc (6). Both StAR protein and transcript levels are acutely regulated by trophic hormones in higher vertebrates (9, 28, 29, 30) and fish species (10, 11, 12). Our results are consistent with mammalian studies showing acute ACTH-stimulated increase in StAR mRNA levels within 30 min in interrenal cells in vitro. Also, in vivo handling disturbance elevated StAR mRNA levels at 1 h and remained elevated even at 24 h after stressor exposure. This prolonged elevation of the transcripts in vivo, even after plasma cortisol levels dropped to unstressed values, suggests increased mRNA stability, but if this has any functional significance remains to be elucidated. Further characterization of transcriptional, posttranscriptional (9), translational (31), and/or posttranslational regulation (29, 32) of StAR expression in lower vertebrates, as demonstrated in mammalian models, can shed some light on the mechanism of regulation of this important step in steroidogenesis.

Similar to the acute regulation of StAR, steroidogenic enzymes (P450scc and 11ß-hydroxylase) were also elevated with handling stressor and ACTH stimulation. Upon handling disturbance, P450scc mRNA transcripts achieved peak levels by 4 h, and at 24 h after stressor the levels were similar to unstressed levels, closely reflecting the plasma cortisol levels. Also, in vitro ACTH stimulated P450scc transcript levels in head kidney slices over a 4-h period showed similar profile suggesting that the stressor-induced elevation of this rate-limiting step in steroidogenesis may correspond to the transient plasma ACTH response seen with stress in fish (33). These results are in agreement with mammalian models clearly showing an elevation in P450scc transcript levels in response to ACTH stimulation (29, 30). Although the P450scc transcript profile reflected more closely with plasma cortisol levels in trout, 11ß-hydroxylase transcript levels remained elevated over the entire 24-h period after handling disturbance. This is in agreement with mammalian studies showing an up-regulation of 11ß-hydroxylase transcript levels after ACTH treatment (30), leading to the suggestion that this enzyme is not only up-regulated during the acute phase of stress. Considered together, handling disturbance and the associated cortisol response involve up-regulation of key steroidogenic enzymes in the interrenal tissue of rainbow trout. This concurs with a recent study that also showed an up-regulation of StAR and P450scc transcripts in response to handling disturbance in rainbow trout (12). Given the cumulative plasma cortisol response to multiple stressors in salmonids (34), it is reasonable to propose that the elevated StAR and steroidogenic enzyme transcript levels in response to initial stressor exposure may have adaptive significance for the rapid cortisol production in response to subsequent (multiple) stressor insults.

Ah receptor impact on corticosteroidogenesis
Several studies reported the attenuation of interrenal steroidogenic capacity by Ah receptor agonists, including decreased plasma cortisol response to stressors and insensitivity to ACTH stimulation in both feral and hatchery reared teleostean fishes (15). Our results are in agreement with those studies clearly showing an attenuation of either stressor- or ACTH-induced cortisol production by BNF in rainbow trout (Figs. 1AGo and 6BGo). We did report a decrease in corticosteroid production with BNF recently and that corresponded with a depressed StAR and P450scc, but not 11ß-hydroxylase mRNA abundance clearly establishing a mechanistic link for steroid inhibition (19). Because this BNF-mediated depression of steroidogenesis was completely abolished with RVT, our results unequivocally demonstrate that Ah receptor signaling is involved in suppressing the rate-limiting steps in steroidogenesis.

In higher vertebrates, although StAR was impacted by xenobiotics (35, 36, 37, 38), a clear role for Ah receptor signaling in regulating StAR gene expression was lacking (22). Indeed studies have shown an attenuation of trophic hormone-induced P450scc activity/expression with TCDD (39, 40, 41, 42). Also, recently using Ah receptor-knockout mice, it was shown that TCDD impact steroidogenesis by affecting P450scc gene expression, but not StAR (22). Consequently, whereas Ah receptor signaling impacts P450scc in mammalian models, little is known about the role of this receptor on StAR regulation. Our study, both in vivo and in vitro, for the first time clearly establishes StAR as a target for endocrine disruption by xenobiotics acting via Ah receptor (Figs. 4AGo and 7AGo). The lack of any impact on the 11ß-hydroxylase mRNA accumulation by Ah receptor agonist in the present study also agrees with mammalian studies, and this could possibly be due to upstream regulation of StAR and P450scc by Ah receptor (39). It is clear from these studies that the attenuated cortisol response to stress seen in fish in vivo with Ah receptor agonists (15, 16, 43) involves impairment of the rate-limiting steps in interrenal steroidogenesis.

The mechanism(s) involved in the Ah receptor-mediated attenuation of StAR and P450scc is not known, but few studies have attributed a negative interaction of Ah receptor with the xenobiotic response elements (XREs) present on the steroidogenic enzymes as a possible mechanism of action (41). To our knowledge, only one steroidogenic enzyme (cyp19) promoter region was shown to have a XRE (44), but the possibility exists that XREs are present on the promoters of StAR and P450scc based on the responses observed with Ah receptor agonists (Ref.22 and this study). In addition to direct interaction, several other indirect mechanisms of interaction of Ah receptor with steroidogenic enzymes have also been proposed. For instance, Ah receptor competes with steroidogenic enzyme regulatory factors like orphan nuclear transcription factor SF-1, cAMP response element binding protein, and CCAAT/enhancer binding protein for coactivators and repressors like steroid receptor coactivator-1 and selective promoter factor 1, thereby modulating the genes involved in steroidogenesis (45, 46), but these interactions remain to be elucidated in fish. Taken together, it seems likely that the mode of action of Ah receptor in impairing steroidogenesis is similar in all vertebrates. This is not surprising given the fact that the genes encoding Ah receptor as well as the steroidogenic enzymes are highly conserved among vertebrates (47).

In conclusion, our results for the first time clearly define a role for Ah receptor in mediating the negative effects of PCBs on corticosteroidogenesis in vertebrates. Our results unequivocally demonstrate Ah receptor-mediated inhibition of StAR and P450scc gene expression in fish and suggest a mechanistic link for the impairment of cortisol response seen in teleostean fishes from polluted waters, including PCB contaminated sites (15). Because the rate-limiting steps in steroidogenesis are not just limited to corticosteroids, but also reproductive steroids, our results emphasize that endocrine disruption with Ah receptor activation may impact all aspects of animal performance, including growth and metabolism, iono and osmoregulation, immune function, and reproduction in fish. Consequently, xenobiotics activating Ah receptor signaling are more likely to be steroidogenic disruptors in animals.


   Footnotes
 
Support for this study was provided by the Natural Sciences and Engineering Research Council (NSERC), Canada, discovery grant, and Premier’s Research Excellence Award (to M.M.V.).

Disclosure: N.A. and M.M.V. have nothing to declare.

First Published Online January 12, 2006

Abbreviations: Ah, Aryl hydrocarbon; BNF, ß-naphthoflavone; CYP1A1, cytochrome P4501A1; P450scc, P450 cholesterol side chain cleavage; PCBs, polychlorinated biphenyls; qPCR, quantitative real-time PCR; RBNF, combination of BNF and RVT; RVT, resveratrol; StAR, steroidogenic acute regulatory protein; TCDD, 2,3,7,8-tetracholorodibenzo-p-dioxin; XRE, xenobiotic response element.

Received September 6, 2005.

Accepted for publication December 28, 2005.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Sapolsky RM, Romero LM, Munck AU 2000 How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions. Endocr Rev 21:55–89[Abstract/Free Full Text]
  2. Charmandari E, Tsigos C, Chrousos G 2004 Endocrinology of the stress response. Annu Rev Physiol 67:259–284[CrossRef]
  3. Wendalaar Bonga SEW 1997 The stress response in fish. Physiol Rev 77:591–625[Abstract/Free Full Text]
  4. Mommsen TP, Vijayan MM, MoonTW 1999 Cortisol in teleosts: dynamics, mechanisms of action, and metabolic regulation. Rev Fish Biol Fish 9: 211–268
  5. Payne AH, Hales DB 2004 Overview of steroidogenic enzymes in the pathway from cholesterol to active steroid hormones. Endocr Rev 25:947–970[Abstract/Free Full Text]
  6. Stocco DM 2000 The role of the StAR in steroidogenesis: challenges for the future. J Endocrinol 164:247–253[Abstract]
  7. Sewer MB, Waterman MR 2003 ACTH modulation of transcription factors responsible for steroid hydroxylase gene expression in the adrenal cortex. Micro Res Tech 61:300–307
  8. Ariyoshi N, Kim YC, Artemenko I, Bhattacharyya KK, Jefcoate CR 1998 Characterization of the Rat StAR gene that encodes the predominant 3.5 kb pair mRNA. J Biol Chem 273:7610–7619[Abstract/Free Full Text]
  9. Fleury A, Ducharme L, Lehoux JG 1998 In vivo effects of adrenocorticotrophin on the expression of the hamster steroidogenic acute regulatory protein. J Mol Endocrinol 21:131–139[Abstract]
  10. Kusakabe M, Todo T, McQuillan HJ, Goetz FW, Young G 2002 Characterization and expression of steroidogenic acute regulatory protein and MLN64 cDNAs in trout. Endocrinology 143:2062–2070[Abstract/Free Full Text]
  11. Li YY, Inoue K, Takei Y 2003 Steroidogenic acute regulatory protein in eels: cDNA cloning and effects of ACTH and seawater transfer on its mRNA expression. Zoolog Sci 20:211–219[CrossRef][Medline]
  12. Geslin M, Auperin B 2004 Relationship between changes in mRNAs of the genes encoding steroidogenic acute regulatory protein and P450 cholesterol side chain cleavage in head kidney and plasma levels of cortisol in response to different kinds of acute stress in the rainbow trout (Oncorhynchus mykiss). Gen Comp Endocrinol 135:70–80[CrossRef][Medline]
  13. Payne AH, O’Shaughnessy PJ 1996 Structure, function and regulation of steroidogenic enzymes in the leydig cell. In: Payne AH, Hardy MP, Russsell LD, eds. The Leydig cell. Vienna, IL: Cache River Press; 260–285
  14. Jiang JQ, Young G, Kobayashi T, Nagahama Y 1998 Eel (Anguilla japonica) testis 11-ß hydroxylase gene is expressed in interrenal tissue and its product lacks aldosterone synthesizing activity. Mol Cell Endocrinol 146:207–211[CrossRef][Medline]
  15. Hontela A 2005 Stress and the hypothalamo-pituitary-interrenal axis: adrenal toxicology—effects of environmental pollutants on the structure and function of the HPI axis. In: Moon TW, Mommsen TP, eds. Biochemical and molecular biology of fishes. Vol. 6 Environmental toxicology. Amsterdam: Elsevier; 331–363
  16. Wilson JM, Vijayan MM, Kennedy CJ, Iwama GK, Moon TW 1998 ß-Naphthoflavone abolishes interrenal sensitivity to ACTH stimulation in rainbow trout. J Endocrinol 157:63–70[Abstract]
  17. Aluru N, Vijayan MM 2004 ß-Naphthoflavone disrupts cortisol production and liver glucocorticoid responsiveness in rainbow trout. Aquat Toxicol 67:273–285[CrossRef][Medline]
  18. Aluru N, Jorgensen EH, Maule AG, Vijayan MM 2004 PCB disruption of the hypothalamus-pituitary-interrenal axis involves brain glucocorticoid receptor downregulation in anadromous arctic charr. Am J Physiol Regul Integr Comp Physiol 287:R787–R793
  19. Aluru N, Renaud R, Leatherland JF, Vijayan MM 2005 Ah Receptor-mediated impairment of interrenal steroidogenesis involves StAR protein and P450scc gene attenuation in rainbow trout. Toxicol Sci 84:260–269[Abstract/Free Full Text]
  20. Fernando-Salguero P, Hilbert D, Rudikoff S, Ward J, Gonzalez F 1996 Aryl hydrocarbon receptor deficient mice are resistant to 2,3,7,8-tetrachlorodibenzo-p-dioxin induced toxicity. Toxicol Appl Pharm 140:173–179[CrossRef][Medline]
  21. Lin TM, Ko K, Moore RW, Simanainen U, Oberley TD, Peterson RE 2002 Effects of aryl hydrocarbon receptor null mutation and in utero and lactational 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure on prostate and seminal vesicle development in C57BL/6 mice. Toxicol Sci 68:479–487[Abstract/Free Full Text]
  22. Fukuzawa NH, Ohsako S, Wu Q, Sakaue M, Fujii-Kuriyama Y, Baba T, Tohyama C 2004 Testicular cytochrome P450scc and LHR as possible targets of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the mouse. Mol Cell Endocrinol 221:87–96[CrossRef][Medline]
  23. Ciolino HP, Daschner PJ, Yeh GC 1998 Resveratrol inhibits transcription of CYP1A1 in vitro by preventing activation of the aryl hydrocarbon receptor. Cancer Res 58:5707–5712[Abstract/Free Full Text]
  24. Casper RF, Quesne M, Rogers IM, Shirota T, Jolivet A, Milgrom E, Savouret JF 1999 Resveratrol has antagonist activity on the aryl hydrocarbon receptor: implications for prevention of dioxin activity. Mol Pharmacol 56:784–790[Abstract/Free Full Text]
  25. Gamperl AK, Vijayan MM, Boutilier RG 1994 Experimental control of stress hormone levels in fishes—techniques and applications. Rev Fish Biol Fish 4:215–255
  26. Laemmli UK 1970 Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680–685[CrossRef][Medline]
  27. Sathiyaa R, Vijayan MM 2003 Autoregulation of glucocorticoid receptor by cortisol in rainbow trout hepatocytes. Am J Physiol Cell Physiol 284:C1508–C1515
  28. Ivell R, Tillman G, Wang H, Nicol M, Stewart PM, Bartlick B, Walther N, Mason JL, Morley SD 2000 Acute regulation of the bovine gene for the steroidogenic acute regulatory protein in ovarian theca and adrenocortical cells. J Mol Endocrinol 24:109–118[Abstract]
  29. Lehoux JG, Fleury A, Ducharme L 1998 The acute and chronic effects of adrenocorticotropin on the levels of messenger ribonucleic acid and protein of steroidogenic enzymes in rat adrenal in vivo. Endocrinology 139:3913–3921[Abstract/Free Full Text]
  30. Le Roy C, Li JY, Stocco DM, Langlois D, Saez JM 2000 Regulation of adrenocorticotropin (ACTH), angiotensin II, transforming growth factor-b and insulin like growth factor I of bovine adrenal cell steroidogenic capacity and expression of ACTH receptor, steroidogenic acute regulatory protein, cytochrome P450c17, and 3b-hydroxysteroid dehydrogenase. Endocrinology 141:1599–1607[Abstract/Free Full Text]
  31. Kallen CB, Arakane F, Christenson LK, Watari H, Devoto L, Strauss III JF 1998 Unveiling the mechanism of action and regulation of the steroidogenic acute regulatory protein. Mol Cell Endocrinol 145:39–45[CrossRef][Medline]
  32. Artemenko IP, Zhao D, Hales DB, Hales KH, Jefcoate CR 2001 Mitochondrial processing of newly synthesized steroidogenic acute regulatory protein (StAR), but not total StAR, mediates cholesterol transfer to cytochrome P450 side chain cleavage enzyme in adrenal cells. J Biol Chem 276:46583–46596[Abstract/Free Full Text]
  33. Balm PHM, Pepels P, Helfrich S, Hovens MLM, Wendelaar Bonga SE 1994 Adrenocorticotropic hormone in relation to interrenal function during stress in tilapia (Oreochromis mossambicus). Gen Comp Endocrinol 96:347–360[CrossRef][Medline]
  34. Barton BA, Schreck CB, Sigismondi LA 1986 Multiple acute disturbances evoke cumulative physiological stress responses in juvenile chinook salmon. Trans Amer Fish Soc 115:245–251[CrossRef]
  35. Walsh LP, Stocco DM 2000 Effects of lindane on steroidogenesis and steroidogenic acute regulatory protein expression. Biol Reprod 63:1024–1033[Abstract/Free Full Text]
  36. Walsh LP, McCormick C, Martin C, Stocco DM 2000 Roundup inhibits steroidogenesis by disrupting steroidogenic acute regulatory (StAR) protein expression. Environ Health Perspect 108:769–776
  37. Walsh LP, Webster, DR, Stocco DM 2000 Dimethoate inhibits steroidogenesis by disrupting transcription of the steroidogenic acute regulatory (StAR) gene. J Endocrinol 167:253–263[Abstract]
  38. Walsh LP, Kuratko CN, Stocco DM 2000 Econazole and miconazole inhibit steroidogenesis and disrupt steroidogenic acute regulatory (StAR) protein expression post-transcriptionally. J Steroid Biochem Mol Biol 75:229–236[CrossRef][Medline]
  39. Moore RW, Jefcoate CR, Peterson RE 1991 2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits steroidogenesis in the rat testis by inhibiting the mobilization of cholesterol to cytochrome P450scc. Toxicol Appl Pharmacol 109:85–97[CrossRef][Medline]
  40. DiBartolomeis MJ, Moore RW, Peterson RE, Christian BJ, Jefcoate CR 1987 Altered regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats. Biochem Pharmacol 36:59–67[CrossRef][Medline]
  41. Dasmahapatra AK, Wimpee BA, Trewin AL, Wimpee CF, Ghorai JK, Hutz, RJ 2000 Demonstration of 2,3,7,8-tetrachlorodibenzo-p-dioxin attenuation of P450 steroidogenic enzyme mRNAs in rat granulosa cell in vitro by competitive reverse transcriptase-polymerase chain reaction assay. Mol Cell Endocrinol 164:5–18[CrossRef][Medline]
  42. Fukuzawa NH, Ohsako S, Nagano R, Sakaue M, Baba T, Aoki Y, Tohyama C 2003 Effects of 3,3',4,4',5-pentachlorobiphenyl, a coplanar polychlorinated biphenyl congener, on cultured neonatal mouse testis. Toxicol In Vitro 17:259–269[Medline]
  43. Quabias ES, Nolan DT, Allin CJ, Wendelaar Bonga SE 2000 Influence of dietary exposure to PCB 126 and nutritional state on stress response in tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss). Env Toxicol Chem 19:2892–2899[CrossRef]
  44. Tong S-K, Chung B-C 2003 Analysis of zebrafish cyp 19 promoters. J Steroid Biochem Mol Biol 86:381–386[CrossRef][Medline]
  45. Val P, Lefrancois-Martinez AM, Veyssiere G, Martinez A 2003 SF-1 a key player in the development and differentiation of steroidogenic tissues. Nucl Recept 1:8[CrossRef][Medline]
  46. Sugawara T, Nomura E, Sakuragi N, Fujimoto S 2001 The effect of the arylhydrocarbon receptor on the human steroidogenic acute regulatory gene promoter activity. J Steroid Biochem Mol Biol 78:253–260[CrossRef][Medline]
  47. Bauer MP, Bridgham JT, Langenau DM, Johnson AL, Goetz FW 2000 Conservation of steroidogenic acute regulatory (StAR) protein structure and expression in vertebrates. Mol Cell Endocrinol 168:119–125[CrossRef][Medline]



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