Developmental reprogramming of reproductive and metabolic dysfunction in sheep: native steroids vs. environmental steroid receptor modulators

Abstract

The inappropriate programming of developing organ systems by exposure to excess native or environmental steroids, particularly the contamination of our environment and our food sources with synthetic endocrine disrupting chemicals that can interact with steroid receptors, is a major concern. Studies with native steroids have found that in utero exposure of sheep to excess testosterone, an oestrogen precursor, results in low birth weight offspring and leads to an array of adult reproductive/metabolic deficits manifested as cycle defects, functional hyperandrogenism, neuroendocrine/ovarian defects, insulin resistance and hypertension. Furthermore, the severity of reproductive dysfunction is amplified by excess postnatal weight gain. The constellation of adult reproductive and metabolic dysfunction in prenatal testosterone-treated sheep is similar to features seen in women with polycystic ovary syndrome. Prenatal dihydrotestosterone treatment failed to result in similar phenotype suggesting that many effects of prenatal testosterone excess are likely facilitated via aromatization to oestradiol. Similarly, exposure to environmental steroid imposters such as bisphenol A (BPA) and methoxychlor (MXC) from days 30 to 90 of gestation had long-term but differential effects. Exposure of sheep to BPA, which resulted in maternal levels of 30-50 ng/mL BPA, culminated in low birth weight offspring. These female offspring were hypergonadotropic during early postnatal life and characterized by severely dampened preovulatory LH surges. Prenatal MXC-treated females had normal birth weight and manifested delayed but normal amplitude LH surges. Importantly, the effects of BPA were evident at levels, which approximated twice the highest levels found in human maternal circulation of industrialized nations. These findings provide evidence in support of developmental origin of adult reproductive and metabolic diseases and highlight the risk posed by exposure to environmental endocrine disrupting chemicals.

Fig. 1

Panel A: Schematic showing the time of appearance of different classes of follicles in sheep, timing of establishment of hypophyseal portal vasculature to pituitary and timing of appearance of LH and FSH in circulation and pituitary during fetal life in sheep. Panel B: Schematic showing the timing and duration of the various steroid / EDC treatments used in studies discussed in this review.

Fig. 2

Panel A: Plasma progesterone profiles from representative control, T60-90, and T30-90 females during the first and second breeding seasons are shown on the left. On the right are shown percentages of sheep cycling during the first and second breeding seasons. (modified from Birch et al. 2003. Panel B: Patterns of LH (closed circles) and E (open circles) from control (top), T30-90 (middle), and DHT30-90 (bottom) following estrous synchronization with PGF_2α (Veiga-Lopez et al. 2009). Panel C: Percentage of T60-90 females mated and becoming pregnant following estrous synchronization. Estrus was synchronized with two injections of PGF_2α administered 11 days apart. Mating was determined by heavy rump markings left by a fertility-proven raddled ram (modified from Steckler et al. 2007b).

Fig. 3

Panel A:Percent of control (C), over-fed control (OFC), T30-90 (T) and overfed T30-90 (OFT) females that showed a luteal progesterone increase following estrus synchronization with progesterone. Note that almost all of the overfed T30-90 females were anovulatory (modified from Steckler et al. 2009). Panel B: Schematic showing the two step model of programming severity of reproductive dysfunction with the first insult occurring from prenatal T excess during fetal life and the second metabolic insult stemming from overfeeding.

Fig. 4

Neuroendocrine feedback systems involved in the control of GnRH / LH secretion that are reprogrammed by prenatal T excess. GnRH / LH release is under the control of negative feedback action of estradiol (E) which is predominant during the prepubertal and anestrus period (feedback 1), stimulatory feedback action of E responsible for generation of the preovulatory LH surge (feedback 2) and negative feedback action of progesterone, operational during the luteal phase (feedback 3) (modified from Foster et al. 2007).

Fig. 5

Panel A: Follicular morphology of ovary from control and T30-90 females. Note the disrupted nature of follicular development in T30-90 sheep (from West et al. 2001). Panel B: Mean (± SEM) number of primordial and growing follicles on fetal days 90 and 140 and 10 months of age in control (open bars), T30-90 (closed bars) and DHT30-90 (gray bars) ovaries (from Smith et al. 2009). Panel C: Ovarian follicular dynamics determined by ultrasonography for 8 days in both ovaries control and T30-90 sheep during the first breeding season (from Manikkam et al. 2006). Each line represents only one follicle and follicles from both ovaries are shown in the same panel. Only follicles that reached a size of 3 mm and persisted for at least 2 days are shown. Note the increase in maximum size and duration of the largest follicles on the ovary in T30-90 sheep compared to controls.

Fig. 6

Panel A: Birth weight of control (open bars), prenatal MXC (gray bars) and BPA-treated (closed bars) female offspring (Savabieasfahani et al. 2006). Panel B: Mean circulating levels of LH in prepubertal control (open bars), prenatal MXC (gray bars) and BPA-treated (closed bars) female offspring (Savabieasfahani et al. 2006). Panel C: Circulating patterns of LH from 3 control, 3 prenatal MXC- and 3 BPA-treated females taken at 2 hourly intervals for 120 h, after induction of luteolysis with 2 injections of PGF_2α 11 days apart (Savabieasfahani et al. 2006). Panel D: Levels of circulating BPA achieved in control (open circles) and BPA treated (closed circles) pregnant sheep on day 50, 70 and 90 of gestation (days 20, 40 and 60 of treatment) following administration of 5 mg / kg / daily administration of BPA s.c. (Savabieasfahani et al. 2006). Panel E: Maternal levels of BPA (mean ± SEM) in Southeastern Michigan relative to maternal age (Padmanabhan et al. 2008).

Fig. 7

Schematic showing organizational palette of adult phenotype as influenced by genetic and epigenetic interactions.