“BPA-free” labels on consumer products imply that these products are safer than versions containing the plasticizer bisphenol A (BPA). But a growing body of evidence suggests that chemical replacements for BPA, such as bisphenol AF (BPAF), may pose a greater risk to human health than the original compound. A recent study published in Environmental Health Perspectives adds to this evidence.1 Using a mouse model of endometriosis, investigators at the University of Cincinnati College of Medicine found that BPAF exhibited more potent endocrine activity than BPA.

Endometriosis affects an estimated 10 million American women and girls of reproductive age.2 Its most debilitating symptoms include chronic pelvic pain and fertility problems. The disease appears to arise when tissue from the uterine lining (endometrium) escapes from the uterus and attaches itself to sites in the peritoneal cavity. This is thought to occur by “retrograde menstruation,” in which menstrual tissue flows back into the cavity3; however, retrograde menstruation occurs in about 90% of women,3 whereas only 10% develop endometriosis.4 Therefore, other factors likely come into play.

Endometriosis affects an estimated 10 million American women and girls of reproductive age. The incidence of endometriosis has risen since the 1970s, but the reasons why are unclear. By one estimate, endometriosis cost more than $12,000 in direct and indirect medical costs per patient in the year after diagnosis.2 Image: © iStockphoto/nensuria.

Little is known about environmental factors that may contribute to endometriosis, but epidemiologic studies have reported higher levels of BPA in urine5 and serum6 samples of women with the condition. This, and the growing use of BPA analogs in manufacturing, led the Cincinnati team to perform their study.

“Our earlier in vitro studies7 of several BPA substitutes found BPAF to be the most estrogenic compound,” says senior author Katherine Burns, an assistant professor in the Department of Environmental Health. “That is why we focused on BPA and BPAF, its chemically more stable analog, in our mouse models.”

The researchers induced endometriosis in both hormonally intact and ovariectomized (i.e., lacking endogenous estrogen) mice by transferring minced uterine tissue from donor mice into the peritoneal cavity of host females. Prior to the transfer, the mice were fed one of three chemicals: BPA, BPAF, or ethinyl estradiol (EE), a synthetic form of estrogen. Three different doses included the no- and lowest-observed-adverse-effect levels (determined by risk assessors) plus a concentration calculated by the authors to represent documented human exposures. A fourth group of animals served as controls. Six weeks later, the investigators evaluated the number and relative size of lesions at the endometrial implant sites, while in the intact animals, they examined the ovaries.

Earlier animal studies by Burns8 and others9 found that the initiation of endometriosis involves inflammatory processes, while disease progression is hormonally driven. Consistent with those findings, the current study showed that the chemicals did not affect the number of initial lesions. However, all bisphenol doses increased the number of dead oocytes in the ovaries (a phenomenon that, in women, could lead to oocyte depletion and possibly premature menopause). In this regard, the ovary was more sensitive to low-dose BPAF than low-dose BPA.

In ovariectomized mice, the bisphenols stimulated lesion growth and increased the expression levels of genes targeted by the estrogen receptor. High-dose BPAF had a stronger effect on lesion weight and volume (measures of disease progression) than high-dose BPA. In intact mice, the highest dose of BPAF, but not EE or BPA, arrested the estrous cycle, which is the equivalent of the menstrual cycle in women.

A strength of this design was the use of both intact and ovariectomized mice. “Most animal models of endometriosis focus on mice whose ovaries were removed,” says Romana Nowak, a professor of animal sciences at the University of Illinois at Urbana-Champaign, who was not involved in the study. “What makes this study unique is its use of intact mice who still produce their own estrogen. This is a physiologically more relevant model for human endometriosis.”

The study also addresses the importance of considering life stage when studying endocrine disruptors. “Many of our [endocrine disruptor] animal models have focused on exposures during critical developmental windows, such as the prenatal period, so people may assume that adult animals are less likely to be affected because they make their own hormones,” says Laura Vandenberg, an associate professor of environmental health science at the University of Massachusetts Amherst, who was not involved in the study. “But the mice in this study were only exposed during adulthood, which shows that this assumption is [sometimes] incorrect.”

For Burns, one of the most striking findings was that the highest BPAF levels eliminated the estrous cycle in intact mice. This suggests a disruption of the normal hormonal feedback loop at doses comparable to BPA levels measured in human samples. She says, “Now that we have demonstrated a stronger estrogenic effect of BPAF [compared with BPA], an important next step will be to study exactly how this [endocrine disruptor] competes with endogenous hormones in intact mice.”