Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers
 
 

 

  Muñoz-de-Toro, M, C Markey, PR Wadia, EH Luque, BS Rubin, C Sonnenschein and AM Soto 2005. Perinatal Exposure to Bisphenol A alters peripubertal mammary gland development in mice. Endocrinology 146: 4138-4147.

International teleconference about these results

Bisphenol A in the news

 

 

In experiments with mice, Muñoz-de-Toro et al. show that perinatal exposure to environmentally-relevant doses of bisphenol A (BPA) causes changes in patterns of mammary gland development at the time of puberty. Comparable changes in women indicate a heightened risk to breast cancer.

The doses of BPA that were used in these experiments are the lowest yet shown to alter animal development: 25 parts per trillion. This level is 2 million times lower than the ‘lowest observed adverse effect level’ –-50 mg/kg— used in 1988 to establish EPA’s current reference dose for bisphenol A of 50 µg/kg (the level at which exposure is estimated to be safe). It is 2 thousand times lower than that reference dose. It is within the range, however, reported by physiologists to be capable of altering important cell signaling events. Research in Japan indicates that human exposure levels are comparable to those used in these experiments.

 

Human exposure to this endocrine-disrupting compound—used to make polycarbonate plastic and in resins that line food cans coat teeth—is virtually ubiquitous, according to CDC data.

What did they do? Muñoz-de-Toro et al. exposed pregnant mice to bisphenol A via surgically-implanted osmotic pumps at levels they calculate are within the range experienced commonly by people.

They used two doses: 25 nanograms per kilogram body weight and 250 ng/kg, comparing them to control animals that received no BPA. These doses are equivalent to 25 and 250 parts per trillion, or 0.025 and .25 parts per billion. The animals were dosed from day 9 of pregnancy through to day 4 after birth.

Animals were sacrificed for examination at various times after birth (days 20, 30, at the transition to sexual maturity [first proestrus] and 4 months), with one pup examined per litter (4-6 litters per treatment in each group at each time).

Multiple analyses were carried out on each sacrificied animal, allowing Muñoz-de-Toro to examine the impact of BPA on mammary gland development from a variety of perspectives. The relevant methods are described below in descriptions of major findings.

What did they find? As background bear in mind that mammary gland development typically reawakens around puberty in both mice and people. This process is under the control of estrogens like the natural hormone estradiol. The normal process involves the formation of 'terminal end buds' at the tips of the mammary gland ducts. These structures differentiate and mature into the adult form, at which time growth ends.

 

Thinking about dose

Muñoz-de-Toro et al. used doses-- 25 and 250 ng/kg-- in their experiments that they estimate are within the range that people experience, delivering them with surgically- implanted osmotic pumps.

How did they choose those doses? Consider these several measurements of BPA in people:

  • placental tissue: median of 12 parts per billion; range from 1 to 104 ppb
  • fetal plasma: median of 2.3 ppb; ranged from 0.2 to 9 ppb.
  • pregnant women's plasma: median of 3.1 ppb; range from 0.3 to 19 ppb
  • average daily intake of 0.23 μg/kg/day (ppb)

These observations of BPA in people and chronic exposure levels indicate that the levels in this experiment, equivalent to 0.025 and 0.25 ppb are at or beneath what typical people are experiencing.

The uncertainties in comparing these experimentally-delivered doses to human experience arise from incomplete knowledge about how BPA is metabolized in people. Two critical issues: (i) what percentage of BPA that is ingested is absorbed into the bloodstream, and of that, (ii) what percentage makes it to the developing fetus. Muñoz-de-Toro et al. take a conservative approach to this by using one exposure that is somewhat above what can be typically measured in serum and one that is significantly lower.

Muñoz-de-Toro et al. found that bisphenol A caused marked changes in mammary gland development. No differences could be seen by day 20, but by day 30, differences were evident between the control group and experimental animals in numbers of terminal buds. This difference was highly significant for those in the 250 ng/kg group (p= 0.008) and marginally so for those from the 25 ng/kg group (p=0.058). Comparing the increase in terminal bud area to ductal area, Muñoz-de-Toro et al. noted that ductal growth itself appeared to be impaired.

Notably, Muñoz-de-Toro et al. also found that early exposure to BPA increased subsequent sensitivity to estrogen.

  This can be seen to the left. In BPA-exposed animals (right), estrogen stimulation from the ages of 25 days to 35 days-old dramatically increased terminal bud formation, as measured by the number of buds at when measured.

For many of the measurements, the lower dose had no detectable effect, but for several it did, and in a few cases the scientists noted that the lower dose actually caused a larger effect than the higher dose (a non-monotonic dose-response curve).

For example, the lower dose of 25 ppt BPA caused a greater decrease in the percentage of apoptotic cells. According to the authors, the decrease in natural cell death (apoptosis) they measured may underlie the increases in terminal buds and alteration in ductal growth noted above.  
* p < 0.05
** p < 0.01

 

 

In another example, The density of mammary gland duct side branches is higher in the treatment group that received 25 ng/kg BPA than the group that was exposed to 250 ng/kg.

* p < 0.05; black dots are means while lavender are individual data points.

 

The effect on mammary gland duct density is readily evident to the right, which shows profound differences between controls and experimentals in the mammary glands of treated animals.  

 

What does it mean?

This study adds to the many now available demonstrating adverse effects by bisphenol A at exposure levels far beneath current health standards. The levels used in these experiments are the lowest yet shown to have impacts, 25 ng/kg or 25 parts per trillion. It should be noted, however, that because the experiment involved implanted osmotic pumps releasing BPA directly into the mouse, the levels are not directly comparable to those used in experiments when BPA is delivered in water, for example, demonstrating low-dose effects on prostate development. These later experiments, working at 10 ppb, may in fact involve comparable exposures to the fetus because some only a fraction of that 10 ppb will be ingested and create fetal exposures.

Likewise, in vitro experiments with cells that show physiological responses to BPA at even lower concentration levels may also involve comparable exposures, because in those experiments the exposure is delivered directly to the cell.

The changes in mammary gland development observed in this study are consistent with changes that, were they taking place in humans, could contribute to an increase in breast cancer risk:

  • Perinatal BPA increased pubertal breast tissue sensitivity to estrogen. Estrogen is a well-established risk factor for breast cancer. Exposure to BPA also increased the number of ‘terminal end buds’ in the mammary tissue.
  • These are the locations, in both people and rodents, where mammary cancer originates. In addition, BPA exposure increased the area of the mammary gland occupied by ductal structures (glandular density). This is equivalent to increased mammographic density, which is also a risk factor for breast cancer in people.

The authors conclude that their results "suggest that perinatal exposure to BPA in particular, and to estrogens in general, may increase susceptibility to breast cancer."

 
   
   

 

 

 


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