2,3,7,8-Tetrachlorodibenzo-p-dioxin Decreases Responsiveness of the Hypothalamus to Estradiol as a Feedback Inducer of Preovulatory Gonadotropin Secretion in the Immature Gonadotropin-Primed Rat

doi:10.1006/taap.2000.9099

Toxicology and Applied Pharmacology

Volume 170, Issue 3, 1 February 2001, Pages 181-190

https://doi.org/10.1006/taap.2000.9099 Get rights and content

Abstract

Sprague–Dawley rats (23-day-old) were dosed with TCDD (32 μg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected (5 IU, sc) 24 h later to induce follicular development. Another 24 h later, half of TCDD- or corn oil-treated rats were injected (sc) with 17β-estradiol-cypionate (ECP, at 0.004 to 0.5 mg/kg). Blood and ovaries were collected on expected proestrous (preovulatory period) at 51, 54, and 58 h after eCG injection as well as in the morning after ovulation (72 h after eCG). Serum concentrations of 17β-estradiol (E), progesterone (P), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of ova shed was measured at 72 h after injection of eCG by irrigating ova from oviducts. During the preovulatory period (∼58 h after eCG injection), a circulating level of 70–100 pg E/ml coincided with LH and FSH surges and later normal ovulation of 10 to 12 ova/rat was observed in controls. However, the same concentration of E was not associated with LH and FSH surges in rats treated with TCDD (32 μg/kg), resulting in reduced ovarian weight gain and reduction of ovulation by 70 to 80% (2–3 ova/rat). Blockage of the gonadotropin surge, reduced ovarian weight gain, and ovulation were all reversed completely by the lowest effective dose of ECP (0.1 mg/kg). At 72 h after eCG, serum P secretion was reduced and serum E levels were significantly increased compared to those of corn oil-treated controls. ECP alone had no effect on serum P levels at any time point, but in rats treated with TCDD and ECP, both the reduction of P (at 58 and 72 h) and the increase in E secretion (72 h) were completely reversed. Further studies confirmed that restoration by ECP of gonadotropin surges and associated ovulation could not be attained until circulating levels of E rose sufficiently high to trigger the LH and FSH surges. The new action threshold of E for inducing gonadotropin surges in rats treated with TCDD (32 μg/kg) was determined to be eight- to 10-fold higher than that in controls. Thus, it is apparent that TCDD decreased the responsiveness of the hypothalamus to E as a feedback inducer of preovulatory gonadotropin secretion.

References (29)

R.C. Bookstaff et al.
Altered regulation of pituitary gonadotropin-releasing hormone (GnRH) receptor number and pituitary responsiveness to GnRH in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated male rats
Toxicol. Appl. Pharmacol.
(1990)
R.C. Bookstaff et al.
2,3,7,8-Tetrachlorodibenzo-p-dioxin increases the potency of androgens and estrogens as feedback inhibitors of luteinizing hormone secretion in male rats
Toxicol. Appl. Pharmacol.
(1990)
C.L. Chaffin et al.
Estrous cycle-dependent changes in the expression of aromatic hydrocarbon receptor (AHR) and AHR-nuclear translocator (ARNT) mRNAs in the rat ovary and liver
Chem.–Biol. Interact
(2000)
X. Gao et al.
Gonadotropin releasing hormone (GnRH) partially reverses the inhibitory effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on ovulation in the immature gonadotropin-treated rat
Toxicology
(2000)
X. Gao et al.
Toxic equivalency factors of polychlorinated dibenzo-p-dioxins in an ovulation model: Validation of the toxic equivalency concept for one aspect of endocrine disruption
Toxicol. Appl. Pharmacol.
(1999)
I. Heimler et al.
Modulation of ovarian follicle maturation and effects on apoptotic cell death in Holtzman rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in utero and lactationally
Biol. Reprod.
(1998)
I. Kharat et al.
Antiestrogenic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin are mediated by direct transcriptional interference with liganded estrogen receptor. Cross-talk between aryl hydrocarbon- and estrogen-mediated signaling
J. Biol. Chem.
(1996)
X. Li et al.
Effects of 2,3,7,8-tetrachlorobibenzo-p-dioxin (TCDD) on estrous cyclicity and ovulation in female Sprague–Dawley rats
Toxicol. Lett.
(1995)
X. Li et al.
Reproductive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female rats: Ovulation, hormonal regulation, and possible mechanism(s)
Toxicol. Appl. Pharmacol.
(1995)
B.K. Petroff et al.
Interaction of estradiol and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in an ovulation model: Evidence for systemic potentiation and local ovarian effects
Reprod. Toxicol.
(2000)

R. Pohjanvirta et al.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on plasma and tissue beta-endorphin-like immunoreactivity in the most TCDD-susceptible and the most TCDD-resistant rat strain

Life Sci.

(1993)

S. Safe

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

Pharmacol. Ther.

(1995)

H.M. Theobald et al.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: Effects on development of the male and female reproductive system of the mouse

Toxicol. Appl. Pharmacol.

(1997)

M. Viluksela et al.

Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxin in rats. I. Design, general observations, hematology, and liver concentrations

Toxicol. Appl. Pharmacol.

(1998)

Cited by (29)

Dioxin exposure reduces the steroidogenic capacity of mouse antral follicles mainly at the level of HSD17B1 without altering atresia
2012, Toxicology and Applied Pharmacology
Citation Excerpt :
TCDD is an endocrine disruptor and potent ovarian toxicant. Numerous studies show that besides its actions on the hypothalamus–pituitary axis to disrupt reproductive processes such as ovulation in rats, TCDD acts directly on the ovary (Cao et al., 2011; Gao et al., 2001; Mizuyachi et al., 2002; Petroff et al., 2000, 2001, 2003). Studies also indicate that TCDD acts directly on antral follicles and inhibits steroidogenesis (Karman et al., 2012).
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent ovarian toxicant. Previously, we demonstrated that in vitro TCDD (1 nM) exposure decreases production/secretion of the sex steroid hormones progesterone (P4), androstenedione (A4), testosterone (T), and 17β-estradiol (E2) in mouse antral follicles. The purpose of this study was to determine the mechanism by which TCDD inhibits steroidogenesis. Specifically, we examined the effects of TCDD on the steroidogenic enzymes, atresia, and the aryl hydrocarbon receptor (AHR) protein. TCDD exposure for 48 h increased levels of A4, without changing HSD3B1 protein, HSD17B1 protein, estrone (E1), T or E2 levels. Further, TCDD did not alter atresia ratings compared to vehicle at 48 h. TCDD, however, did down regulate the AHR protein at 48 h. TCDD exposure for 96 h decreased transcript levels for Cyp11a1, Cyp17a1, Hsd17b1, and Cyp19a1, but increased Hsd3b1 transcript. TCDD exposure particularly lowered both Hsd17b1 transcript and HSD17B1 protein. However, TCDD exposure did not affect levels of E1 in the media nor atresia ratings at 96 h. TCDD, however, decreased levels of the proapoptotic factor Bax. Collectively, these data suggest that TCDD exposure causes a major block in the steroidogenic enzyme conversion of A4 to T and E1 to E2 and that it regulates apoptotic pathways, favoring survival over death in antral follicles. Finally, the down‐regulation of the AHR protein in TCDD exposed follicles persisted at 96 h, indicating that the activation and proteasomal degradation of this receptor likely plays a central role in the impaired steroidogenic capacity and altered apoptotic pathway of exposed antral follicles.
2,3,7,8-Tetrachlorodibenzo-p-dioxin activates the aryl hydrocarbon receptor and alters sex steroid hormone secretion without affecting growth of mouse antral follicles in vitro
2012, Toxicology and Applied Pharmacology
Citation Excerpt :
Similarly, TCDD exposures lead to early puberty, irregular estrous cycles, reduced or blocked ovulation, decreased circulating estradiol levels (E2), and early reproductive senescence in female rodents (Chaffin et al., 1996; Franczak et al., 2006; Gray and Ostby, 1995; Jablonska et al., 2010; Li et al., 1995; Myllymäki et al., 2005; Shi et al., 2007). Though it has been shown that TCDD can affect the development and functioning of the rat hypothalamus and pituitary, there is convincing evidence that it has direct effects on the ovary as well (Cao et al., 2011; Gao et al., 2001; Mizuyachi et al., 2002; Petroff et al., 2001, 2003). Further, it has been demonstrated that TCDD can accumulate in ovarian follicles following controlled dosing experiments in animals and in culture (Baldridge and Hutz, 2007; Grochowalski et al., 2001).
The persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an ovarian toxicant. These studies were designed to characterize the actions of TCDD on steroidogenesis and growth of intact mouse antral follicles in vitro. Specifically, these studies tested the hypothesis that TCDD exposure leads to decreased sex hormone production/secretion by antral follicles as well as decreased growth of antral follicles in vitro. Since TCDD acts through binding to the aryl hydrocarbon receptor (AHR), and the AHR has been identified as an important factor in ovarian function, we also conducted experiments to confirm the presence and activation of the AHR in our tissue culture system. To do so, we exposed mouse antral follicles for 96 h to a series of TCDD doses previously shown to have effects on ovarian tissues and cells in culture, which also encompass environmentally relevant and pharmacological exposures (0.1–100 nM), to determine a dose response for TCDD in our culture system for growth, hormone production, and expression of the Ahr and Cyp1b1. The results indicate that TCDD decreases progesterone, androstenedione, testosterone, and estradiol levels in a non-monotonic dose response manner without altering growth of antral follicles. The addition of pregnenolone substrate (10 μM) restores hormone levels to control levels. Additionally, Cyp1b1 levels were increased by 3–4 fold regardless of the dose of TCDD exposure, evidence of AHR activation. Overall, these data indicate that TCDD may act prior to pregnenolone formation and through AHR transcriptional control of Cyp1b1, leading to decreased hormone levels without affecting growth of antral follicles.
Brain transcriptomics in response to β-naphthoflavone treatment in rainbow trout: The role of aryl hydrocarbon receptor signaling
2008, Aquatic Toxicology
Polychlorinated biphenyls (PCBs) exposure disrupts steroid production in teleostean fishes. While this suppression of plasma steroid levels is thought to involve aryl hydrocarbon receptor (AhR) signaling, the target tissues impacted and the molecular mechanisms involved have rarely been addressed. We tested the hypothesis that AhR activation downregulates genes involved in neuroendocrine function, including the control of brain-pituitary-interrenal (BPI) and -gonadal (BPG) axes in rainbow trout. To elucidate receptor-specific signaling, we utilized a pharmacological approach using β-naphthoflavone (BNF) and resveratrol (RVT) as AhR agonist and antagonist, respectively. The gene expression pattern in the brain was analysed using a low-density targeted trout cDNA array enriched with genes encoding proteins involved in endocrine signaling, stress response and metabolic adjustments. Upregulation of AhR and CYP1A1 gene expression with BNF and the inhibition of this response by RVT confirmed AhR-dependent signaling. RVT by itself impacted only a few genes, while BNF treatment significantly modulated the transcript level of 49 genes, many of which are involved in the neuroendocrine control of stress and reproduction. Of these, only 27% of the BNF-mediated transcriptional response was blocked by RVT, suggesting molecular regulation of neuroendocrine pathways that are also AhR-independent. Gene expression pattern for select genes seen with the microarray analysis was also confirmed using quantitative real-time PCR. Overall, our results reveal for the first time that BNF disrupts several key genes involved in the neuroendocrine control of stress and sex steroid biosynthesis, while the mode of action involves both AhR-dependent and -independent pathways in trout.
Effects of in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on rat follicular steroidogenesis
2006, Reproductive Toxicology
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant and causes adverse effects on female reproduction when administered to rats. Our aims were to study effects of gestational and lactational exposure to TCDD on ovarian steroidogenesis and steroidogenic enzyme expression of offspring on postnatal day (PND) 14 in the rat and sensitivity of enzymatically isolated ovarian follicles to TCDD in vitro. Synthetic estrogen diethylstilbestrol (DES) was used as a treatment control. Serum progesterone (P4) level in offspring increased significantly on PND 14 in the TCDD (1 μg/kg)-exposed group while body weight, FSH and E2 levels were not changed. In ovarian follicles of offspring on PND 14 in the TCDD-exposed groups, protein expression of cytochrome P-450 aromatase, cytochrome P-450 cholesterol side-chain cleavage, steroidogenic acute regulatory protein, 3β-hydroxy-steroid-dehydrogenase/Δ⁵-Δ⁴ isomerase type 1, or P4 receptor was not affected. TCDD decreased E2 and P4 production in ex vivo follicle culture. DES at a dose level of 0.1 mg/kg was dystocic while a dose 0.02 mg/kg increased ovarian ex vivo E2 and testosterone production without affecting P450arom activity indicating stimulation of early steps of steroidogenic pathway. Data suggests that TCDD has multiple targets in ovarian steroidogenesis, but the inhibitory action represented as decreased follicular steroid hormone production ex vivo is not apparent at the ovarian protein expression. Furthermore, TCDD had no direct effect on immature rat ovarian steroidogenesis in vitro suggesting that the follicle culture method is not a sensitive method to study the mechanisms of TCDD action.
Presence and functional activity of the aryl hydrocarbon receptor in isolated murine cerebral vascular endothelial cells and astrocytes
2004, NeuroToxicology
Numerous functions regulated by the central nervous system (CNS) are targeted by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); however, the cell specific targets and mechanisms of toxicity are unknown. Outside of the brain, the peripheral vascular endothelium has been identified as a significant cellular target of TCDD toxicity resulting in apoptosis, edema, hemorrhaging and vascular dysfunction. Possible effects of TCDD in the vascular endothelium of the CNS have not been examined. Cellular dysfunction in this endothelium may disrupt function of the blood–brain barrier (BBB), which could severely compromise neuronal homeostasis and potentiate neurotoxicity. TCDD toxicity is mediated primarily by the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor that modulates the expression of a large battery of genes. This study examined the presence and functional activity of the AhR in response to TCDD in endothelial cells and astrocytes, the two primary components of the BBB. Primary mouse cortical endothelial cells and astrocytes express the AhR, as shown by immunocytochemical and western blot analyses. AhR activity was assessed by time- and concentration-dependent analyses of CYP1A1 and CYP1B1 protein expression following TCDD treatment. Both CYP1A1 and CYP1B1 proteins were induced in endothelial cells after 4 and 8 h, respectively, while only CYP1B1 protein induction was detected in astrocytes after 16 h. The CYP450 protein induction was sustained for greater than 72 h in both cell types. These changes in protein expression were dependent on AhR activity as indicated by the inhibition of these responses by a receptor antagonist. Together these data indicate endothelial cells and astrocytes are responsive to TCDD through the AhR-mediated pathway and therefore could be targets of toxicity.
Alteration in ovarian gene expression in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin: Reduction of cyclooxygenase-2 in the blockage of ovulation
2002, Reproductive Toxicology
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a reproductive toxicant and endocrine disrupter that is known to block ovulation. This study was designed to investigate alterations in relevant ovarian genes that may be involved in the blockage of ovulation by TCDD in immature intact rats primed with equine chorionic gonadotropin (eCG). In this ovulation model, rats were given either 32 μg/kg TCDD or corn oil by gavage on 25 days of age. The next day, eCG (5 IU) was injected subcutaneously (s.c.) to stimulate follicular development. Ovulation occurs 72 h after administration of eCG in controls of this model. TCDD blocked ovulation at the expected time and also reduced both ovarian and body weights. At 72 h after eCG (the morning after expected ovulation), TCDD did not alter significantly serum concentrations of progesterone (P4) and androstenedione (A4). However, estradiol (E2) was significantly higher at 72 h after eCG in TCDD-treated rats when compared with controls. Western blots revealed that ovarian CYP1A1 was induced by TCDD. In addition, the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) were down- and up-regulated by TCDD, respectively, indicating that AhR-mediated signal transduction was altered in the ovary. Ovarian estrogen receptor (ER)α, ERβ and progesterone receptor (PR) were not altered significantly by TCDD, but ovarian glucocorticoid receptor (GR) was increased at 24 h after TCDD and decreased at 72 h after eCG when compared with controls. TCDD induced the early appearance of ovarian plasminogen activator inhibitor type-1 (PAI-1), plasminogen activator inhibitor type-2 (PAI-2), urokinase plasminogen activator (uPA), and tissue plasminogen activator (tPA) at 24 h after dosing when compared with controls. On the morning after ovulation (72 h after eCG), no significant differences between control and TCDD-treated rats were observed except that TCDD had still increased tPA and decreased PAI-2 when compared with controls. Interestingly, ovarian COX-2 was induced on the morning after ovulation (72 h after eCG) in controls, but was greatly inhibited in TCDD-treated rats at that time. On the other hand, COX-1 was constitutively expressed throughout the ovulatory period and remained unaffected by TCDD. Immunolocalization of COX-2 in the ovary revealed that TCDD inhibited COX-2 expression in the granulosa cell layer when assessed in the morning of expected ovulation. In conclusion, AhR signaling is activated in the ovary by TCDD and inhibition of COX-2 appeared to be a critical step in the TCDD blockage of ovulation because blockage or reduction of COX-2 expression is well known to be associated with failure of ovulation.

View all citing articles on Scopus

¹: To whom correspondence should be addressed at University of Kansas Medical Center, Department of Pharmacology, Toxicology, and Therapeutics, 3901 Rainbow Boulevard, Kansas, KS 66160-7417. Fax: (913) 588-7501. E-mail: [email protected].

View full text

Regular Article2,3,7,8-Tetrachlorodibenzo-p-dioxin Decreases Responsiveness of the Hypothalamus to Estradiol as a Feedback Inducer of Preovulatory Gonadotropin Secretion in the Immature Gonadotropin-Primed Rat

Abstract

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Chem.–Biol. Interact

Toxicology

Toxicol. Appl. Pharmacol.

Biol. Reprod.

J. Biol. Chem.

Toxicol. Lett.

Toxicol. Appl. Pharmacol.

Reprod. Toxicol.

Life Sci.

Pharmacol. Ther.

Toxicol. Appl. Pharmacol.

Toxicol. Appl. Pharmacol.

Regular Article
2,3,7,8-Tetrachlorodibenzo-p-dioxin Decreases Responsiveness of the Hypothalamus to Estradiol as a Feedback Inducer of Preovulatory Gonadotropin Secretion in the Immature Gonadotropin-Primed Rat