Temporal and anatomical sensitivities to the aryl hydrocarbon receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin leading to premature acyclicity with age in rats

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates diverse dioxin toxicities. While the acute effects of activation of the AhR pathway by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been a focus of past study, the role of this pathway in normal physiology and ageing is unclear. The purpose of this study was to identify the portion of the reproductive axis [ovary vs. hypothalamus and pituitary gland (H-H axis)] and the stages of the reproductive lifespan (foetal and early post-natal life vs. adolescence and adulthood) that are particularly sensitive to the effects of TCDD during female reproductive ageing. Adult pregnant Lewis rat dams were dosed with corn oil vehicle or TCDD (50 ng/kg-week by gavage) on days 14 and 21 of gestation and post-natal days 7 and 14 to provide in utero and lactational (IUL) exposure to pups. Female pups (n = 96) were weaned on post-natal day 21 and dosed with TCDD or vehicle weekly. Half of the pups were used as donors for ovary transplantation while the remainder were recipients. Following ovary transplantation, rats (n = 6-8 per group) received weekly TCDD or vehicle again until sacrifice at 8 months of age. Beginning at vaginal opening, reproductive cycles were monitored by vaginal cytology for 10 days each month. Blood samples were collected at 22.00 h on proestrus to measure concentration of 17beta-oestradiol in serum. Real-time PCR was used to determine differences in Cyp1a1, Cyp19a1, Cyp17a1, LH receptor (LHR), FoxA2 and FoxJ1 genes expression between control and remaining groups. IUL exposure of the H-H axis plus adult exposure of the whole body to TCDD significantly delayed puberty in females rats. Data analysis revealed an accelerated onset of acyclicity by 5 months in all groups involving IUL exposure of the developing ovary to TCDD. 17beta-oestradiol was significantly decreased in animals receiving TCDD during IUL exposure of the H-H axis. CYP1a1 expression was markedly greater in the liver than in ovarian tissue and correlated with ongoing TCDD exposure. Aromatase, 17alpha-hydroxylase and LHR gene expressions were largely unchanged (or occasionally elevated) by TCDD. FoxA2 and FoxJ1 mRNAs were similarly of limited value mechanistically, although FoxJ1 was much higher in TTT females (receiving TCDD as donor, recipient and adult). This study reveals a particular sensitivity of the developing ovary to TCDD leading to early loss of reproductive function with age.

Keywords: TCDD; aging; aryl hydrocarbon receptor; ovary; rat.

Fig. 1

Experimental scheme for ovary transfer experiment. Adult pregnant dams were dosed with corn oil vehicle (V) or TCDD (50 ng/kg-week by gavage; T) on days 14 and 21 of gestation and postnatal days 7 and 14 to provide in utero and lactational (IUL) exposure to pups. Female pups (n=96) were weaned on postnatal day 21 and dosed with TCDD or vehicle weekly. Half of the pups were used as donors for ovary transplantation while the remainder were recipients. Following ovary transplantation, rats (n=6-8 per group) received weekly TCDD or vehicle again until sacrifice at 8 months of age. Animal groups were named by three-letter designations, with the first letter indicating IUL exposure for the donor animal, the second letter indicating IUL recipient treatment, and the third letter denoting post-transplantation treatment.

Fig. 2

Female age (days) at vaginal opening in Lewis Furth rats exposed to vehicle (V) or TCDD (50 ng/kg/week; T). The experimental design, depicted in Fig. 1, included ovarian transplantation performed after weaning and involved intrauterine and lactational (IUL) as well as postnatal exposure to vehicle or TCDD. Animal groups were named by three-letter designations, with the first letter indicating IUL exposure for the donor animal, the second letter indicating IUL recipient treatment, and the third letter denoting post-transplantation (adult) treatment. Bars with different letters denote a significant differences (p<0.05) among groups.

Fig. 3

Percentage of females showing estrous cyclicity (%) following exposure to vehicle (V) or TCDD (50 ng/kg/week; T) performed according to experimental design depicted in Fig. 1. Control group constituted of animals prenatally and postnatally treated with vehicle (VVV). Females in utero and lactationally (IUL) treated with vehicle (VVT, VTV, VTT) were pooled into “no IUL TCDD” group and females IUL treated with TCDD (TVV, TVT, TTV, TTT) produced “IUL TCDD” group. Asterisks represent significant differences (p<0.05) among groups.

Fig. 4

Proestrus serum concentration of 17β-estradiol (mean±SEM) in Lewis Furth females following exposure to vehicle (V) or TCDD (50 ng/kg/week; T) performed according to experimental design depicted in Fig. 1. Control group constituted of animals prenatally and postnatally treated with vehicle (VVV). Females in utero and lactationally (IUL) treated with vehicle (VVT, VTV, VTT) were pooled into “no IUL TCDD” group and females IUL treated with TCDD (TVV, TVT, TTV, TTT) produced “IUL TCDD” group. Bars with different letters denote a significant differences (p<0.05).

Fig. 5

Hepatic (A) and ovarian (B) cytochrome P450, subfamily 1A, polypeptide 1 (Cyp1a1) gene expression measured by real-time PCR in female rats exposed to vehicle (V) or TCDD (50 ng/kg/week; T). The experimental design, depicted in Fig. 1, included ovarian transplantation performed after weaning and involved intrauterine and lactational (IUL) as well as postnatal exposure to vehicle or TCDD. Animal groups were named by three letter designations with the first letter indicating IUL exposure for the donor animal, the second letter indicating IUL recipient treatment and the third letter denoting post-transplantation (adult) treatment. Data were normalized by relative quantification with β-actin as housekeeping gene. Bars with different letters denote a significant differences (p<0.05) among groups.

Fig. 6

Ovarian (A) aromatase (Cyp19a1), (B) 17-α hydroxylase (Cyp17a1), and (C) LH receptor (LHR) gene expression measured by real-time PCR in female rats exposed to vehicle (V) or TCDD (50 ng/kg/week; T). The experimental design, depicted in Fig. 1, included ovarian transplantation performed after weaning and involved intrauterine and lactational (IUL) as well as postnatal exposure to vehicle or TCDD. Animal groups were named by three letter designations with the first letter indicating IUL exposure for the donor animal, the second letter indicating IUL recipient treatment and the third letter denoting post-transplantation (adult) treatment. Data were normalized by relative quantification with β-actin as housekeeping gene. Asterisk denotes a significant difference (p<0.05) between control (VVV) and treated group.

Fig. 7

Ovarian (A) forkhead box A2 (FoxA2) and (B) forkhead box J1 (FoxJ1) gene expression measured by real-time PCR in female rats exposed to vehicle (V) or TCDD (50 ng/kg/week; T). The experimental design, depicted in Fig. 1, included ovarian transplantation performed after weaning and involved intrauterine and lactational (IUL) as well as postnatal exposure to vehicle or TCDD. Animal groups were named by three letter designations with the first letter indicating IUL exposure for the donor animal, the second letter indicating IUL recipient treatment and the third letter denoting post-transplantation (adult) treatment. Data were normalized by relative quantification with β-actin as housekeeping gene. Bars with different letters denote a significant difference (p<0.05) among groups. Asterisk designates a significant difference (p<0.05) between control (VVV) and TTT group.