Bisphenol A (BPA) is a high volume (> 3 million tons/year) synthetic monomer used in the production of polycarbonate plastics, epoxy resins that line food and beverage cans, and innumerous common household and consumer products [http:www.bisphenol-a.org
2007]. Although polymeric forms are relatively stable, water-soluble BPA monomers are released when exposed to heat, acidic pH, after repeated use and over time, leaching into consumable liquids and foods and bioaccumulating in environments worldwide [1
]. Consequently, the majority of humans tested (>93%) have detectable levels of BPA in their system [5
] with highest levels found in infants and children [7
]. Since BPA is rapidly metabolized and excreted with a half-life < 6 hours, this indicates that humans are being continuously exposed to BPA [11
Health concerns regarding human exposures to BPA stem from its estrogenic properties. While affinity for nuclear estrogen receptors (ERα and ERβ) is low relative to natural estradiol [12
], BPA has activational capacity equivalent to estradiol for membrane associated ERs [13
] and can rapidly activate membrane-initiated ER signaling at low doses [15
]. While the topic remains highly controversial, there is a growing body of evidence that BPA has adverse effects on multiple hormone responsive tissues at environmentally relevant doses [16
]. It is of particular note that the majority of low-dose BPA effects have been observed during the developmental period when sensitive organs are most susceptible to reprogramming by steroidal exposures.
Estrogens play a physiologic role during normal prostate gland development and inappropriate levels or timing of estrogenic exposures during early life can reprogram the gland and predispose to prostate neoplasia with aging [17
]. In 2006, our laboratory was the first to demonstrate that transient, early-life exposure to BPA at low-doses increased susceptibility to adult-onset precancerous lesions and hormonal carcinogenesis. Specifically, subcutaneous (s.c.) injection of BPA at 10 μg/kg BW to neonatal Sprague-Dawley (SD) rats on postnatal days (PND) 1, 3 and 5 significantly increased the incidence and score of adult estrogen-induced prostate intraepithelial neoplasia (PIN), the precursor lesion for prostate cancer, as to compared to control rats [20
]. This model of heightened sensitivity to hormonal carcinogenesis is highly relevant to humans in that relative estradiol levels increase in the aging male and may contribute to prostate disease risk [21
]. Furthermore, our study identified an epigenetic underpinning for early life reprogramming by BPA with multiple genes exhibiting life-long alterations in DNA methylation patterns and gene transcription. In 2008, the National Toxicology Program (NTP) released a report that concluded there is “some concern for BPA exposure in fetuses, infants and children at current human exposures based on effects in the prostate gland” [16
], based in part on this work with our rat model. More recently, the FDA adopted these concerns by the NTP and called for further studies to provide additional information and clarify present uncertainties about the risks of BPA.
A number of valid issues were raised by the NTP with regards to our initial studies on BPA effects on the prostate gland. Among them was the s. c. route of exposure that was employed and the inherent differences in BPA metabolism in rodents and humans, both of which may affect the internal dosimetry of the free, biologically active form of BPA. It is currently believed that the majority of BPA intake in humans occurs through oral ingestion and is thus subjected to hepatic first-pass metabolism, also referred to as presystemic Phase II metabolism [22
]. In adults, free BPA is rapidly metabolized through glucuronidation to an inactive form by the liver detoxifying enzyme UDP-glucuronosyltransferase isoform, UGT2B1 [23
]. Since UGT2B1 is highly efficient, ~99% of orally ingested BPA by adults is metabolized to BPA-glucuronide prior to entry into the general circulation [22
]. In contrast, BPA delivery via s.c. injection will enter the circulation prior to liver glucuronidation, thus initial exposure levels of free BPA by this route may be markedly higher than current human circulating levels. This must be taken into consideration when evaluating BPA toxicity and its relevance to human health. However, it is also known that liver UGT2B1 expression is absent during fetal life in rats and is low during the immediate neonatal period [24
], thus it is possible that BPA exposures through the oral versus s.c. injection route may result in circulating free BPA levels that are not dissimilar from one another.
Another issue that has been raised is the species difference in BPA metabolism between rodents and humans with rodents having a slower clearance rate as well as deconjugation of bound BPA by gut β-glucuronidases and enterohepatic recirculation [16
]. Based on this fact, it has been argued that adverse effects observed in rodent models may not be relevant for human diseases [16
]. Ideally, animal studies used to evaluate the potential health effects of BPA should be designed so that internal levels of free BPA are similar between the model system employed and that observed in human circulation.
In this context, the objectives of the present study were threefold. First, we sought to determine the internal dosimetry of total and free BPA levels in neonatal male SD rat sera immediately following s.c. injection of 10 μg BPA/kg BW as used in our prior studies so that they can be directly compared to values currently observed in the human population. Second, we aimed to compare the internal BPA levels after s.c. injection to those following oral exposure at the same applied dosage in matched littermates. Finally, we wanted to directly compare the prostate lesions observed at 7 months of age after a 16 week exposure to androgen-supported, elevated estradiol in rats neonatally treated with vehicle, oral or s.c. injection of 10 μg BPA/kg BW. Our findings reveal significant differences in internal free BPA levels between the two routes of exposure; however, the s.c. injection of a BPA depot delivers free BPA levels similar to those reported in multiple studies for the human fetus and newborn males [11
]. Furthermore, elevations in estrogen-driven PIN incidence and score are observed in the ventral and lateral rat prostate lobes following neonatal BPA treatments irrespective of route of exposure. Together, these findings support the relevance of our rat model for developmental BPA exposures to human potential prostate health risks as a function of early-life BPA exposures.