The research presented at ENDO 2018 ran the gamut in terms of breakthroughs in endocrine science. Of particular interest were studies that took a closer look at how offspring could be harmed by endocrine disruptors — specifically BPA and DEHP — while still in the womb.
Three studies presented at ENDO 2018 in March, the Endocrine Society’s 100th annual convention, provide further evidence that in utero exposures to chemicals and other substances can have profound effects for the individual’s life span and even beyond the directly exposed individual to future generations.
Neurogenesis and BPA: Wires Tangled
In “Exposure to low levels of BPA during pregnancy can lead to altered brain development,” lead researcher Deborah Kurrasch, PhD, at the University of Calgary in Canada and team exposed pregnant mice to environmentally relevant levels of bisphenol A (BPA) to investigate whether exposure to BPA during gestation would affect brain development and, if so, what lasting effects the exposure might have on behavior from birth onward.
Environmental exposure to the xenoestrogen BPA and other endocrine-disrupting chemicals (EDCs) is known to increase the risk for many health problems, including such endocrine disorders as impaired fertility, diabetes, and obesity; cardiovascular disorders; several cancers; and behavioral changes in children. BPA is also known to cross the placenta, thereby exposing fetuses.
“It is tempting to think that DEHP may be a contributing factor to the decreased sperm counts and qualities in modern men compared to their previous generations. This study therefore finds the importance of educating the public to try their best effort to reduce their exposure to this chemical and also of the need to substitute this chemical with a safer one.” – Radwa Barakat, BVSC, MSc, PhD student, University of Illinois at Urbana-Champaign, Urbana, Ill.
The research team fed the dams either a diet lacking BPA, a low-dose BPA diet, or a high-dose BPA diet and measured fetal serum concentrations. For context, the low dose fed to the mouse dams resulted in a fetal serum concentration 350-fold lower than what has been reported in human placenta. They then looked at how the fetal brains were developing and found a precocious shift in the timing of when neurons were born in the BPA-exposed mice. “The first step in forming a brain is neurogenesis, the process of a progenitor cell becoming a neuron,” says Kurrasch. “These neurons then move to a particular place and form a proper connection. With more neurons developing too early, they probably will not migrate to the correct place to form the needed connections.”
Disrupted neurogenesis is concerning enough, but the researchers went on to show that the BPA exposure had lasting effects on behavior. As has been demonstrated in prior studies, BPA can cause hyperexciteability/hyperactivity. Kurrasch and team further validated that fetal exposure caused the mice to wander out into the open more than unexposed mice, which preferred to remain close to the walls of the testing chamber, suggesting that the BPA-exposed mice display a decrease awareness of self preservation. Using the elevated plus maze, a measure of anxiety, they again showed that the mice did not restrict themselves to covered areas but moved about freely, consistent with the notion that mice exposed to BPA during gestation are less cautious.
The team plans to follow up this research by delving more fully into the neurogenesis —specifically what neurons are being affected and what is happening to the potentially premature neuronal connections.
There is a message to heed in the meantime, however: Err on the side of caution where plastics are concerned, especially those likely to contain BPA. “Even though this was an animal study,” Kurrasch says, “I generally advise that pregnant women should be careful with their use of plastics, and they should follow ‘good plastic guidelines,’ such as avoiding putting plastics in the dishwasher or the microwave. If the plastics are old and showing signs of distress, throw them out. Use glass when you can.” There is still education to be done concerning plastics and what they expose us to, including BPA. At least in mice, its harmful effects on the brain might be lasting.
Transgenerational Infertility and DEHP
In “Prenatal exposure to DEHP leads to premature reproductive senescence in future generations,” lead author Radwa Barakat, B.V.S.C., MSc, a PhD student at the University of Illinois at Urbana-Champaign, Urbana, Ill., from the faculty of Veterinary Medicine, Benha University, in Egypt, and team investigated another widely used EDC, the plasticizer di-(2-ethylhexyl) phthalate (DEHP). Knowing that DHEP has been demonstrated to induce infertility in male mice, possibly by interference in fetal testosterone biosynthesis, these researchers wondered whether this effect is transmitted to following generations.
“Sperm counts among men have dropped substantially over the last few decades,” Barakat says. “The reason for such an alarming phenomenon is not known, but chemicals introduced to consumer products have been suspected to be contributing factors. In particular, chemicals that disrupt hormonal action in our body are prime suspects.”
The team fed mouse dams (generation 0) a diet containing 20 μg, 200 μg, 500 mg, or 750 mg/kg/day of DEHP or a diet of tocopherol-stripped corn oil from around the time of fetal gonadal development (gestational day 11) until birth. The adult male offspring (generation 1) were bred with unexposed females, producing generation 2. These adult male offspring were, in turn, bred with unexposed females, producing generation 3. At 15 months, a time when male mice are normally reproductively mature, the researchers measured serum androgen levels, assessed sperm concentration and motility in the cauda epididymis, and examined testicular histology.
Results from generation 1 were previously published and showed age- and dose-dependent gonadal dysfunction. As early as seven months, the 750 mg/kg/day group had significantly reduced fertility. At 19 months of age, 86% of the 750 mg/kg/day mice were infertile, compared with 25% of control mice. At this age, all of the DEHP-exposed mice had lower serum testosterone levels, higher serum estradiol levels, and higher luteinizing hormone levels than control mice.
In generation 2, only the offspring of the highest-dose (750 mg/kg/day) group originally exposed to DEHP had abnormal results, including lower serum testosterone concentration; fewer and less motile sperm in the cauda epididymis; and histological evidence of testicular degeneration in the form of germ cell sloughing, which is normally seen in aged mice.
In generation 3, DEHP males continued to exhibit these reproductive abnormalities. However, the researchers were surprised to find that, despite the exposure having happened two generations prior, offspring of all dose exposures were affected, not just the group exposed to the highest dose. Perhaps even more surprisingly, in generation 3, reproductive indices were least promising of all among the lowest-dose (20 μg/kg/day) group exposed to DEHP.
“Even though this was an animal study,” Kurrasch says, “I generally advise that pregnant women should be careful with their use of plastics, and they should follow ‘good plastic guidelines,’ such as avoiding putting plastics in the dishwasher or the microwave. If the plastics are old and showing signs of distress, throw them out. Use glass when you can.” – Deborah Kurrasch, PhD, University of Calgary, Calgary, Canada
“Thus, the shape of the non-monotonic dose–response curve (NMDRC) reverses as the level of contamination goes up,” Barakat says. “Some NMDRCs are shaped like U’s, with high responses at low and at high levels of contamination as we found in DEHP-exposed mice.” The difference in effect between low- and high-dose exposures could be caused by anything from saturation of biotransformation pathways or protein-binding sites, to depletion of intracellular cofactors, to differences in ligand affinity and in efficacy of signal transduction.
Says Barakat of these findings: “It is tempting to think that DEHP may be a contributing factor to the decreased sperm counts and qualities in modern men compared to their previous generations. This study therefore finds the importance of educating the public to try their best effort to reduce their exposure to this chemical and also of the need to substitute this chemical with a safer one.”
-Horvath is a freelance writer based in Baltimore, Md. She wrote about the T Trials in the June issue.
- Because EDCs interfere with endogenous endocrine signaling, perinatal exposure increases the likelihood of lifelong endocrine disruption.
- Exposure to BPA caused earlier differentiation of progenitor cells into neurons during neurogenesis in mouse fetuses, disrupting formation of appropriate neuronal connections and adversely affecting behavior throughout the lifespan.
- Male mice prenatally exposed to DEHP as well as their non-exposed offspring for two subsequent generations exhibited abnormal findings on reproductive indices and consequently lost fertility prematurely.