3. BPA and Its Exposure
BPA was first synthesized in 1891 [
11]. The formula is (CH
3)
2C(C
6H
4OH)
2 or C
15H
16O
2 and the chemical name is 4,4’-dihydroxy-2,2-diphenyl propane [
7]. It has been widely used in the manufacture of resins such as polycarbonate and epoxy. These chemicals are mainly used for the interior lacquer coating of food and beverage cans, thermal receipt paper, some dental sealants and fillings, detergents, soaps, lotions, shampoos, conditioners, and nail polishes [
12,
13].Over 3.5 million tons of BPA are produced a year worldwide and more than 100 tons are released into the atmosphere [
14].
BPA is ubiquitous in the environment. Ingestion, inhalation and dermal absorption are the main routes of daily exposure [
15]. The acceptable dosage of BPA, representing the safe exposure level in human, is ≤50 ug/kg/day [
16]. In the general population, BPA has been detected in various tissues, including adult sera (0.2–20 ng/mL), placental tissues (11.2 ng/g), human breast milk (0.28–0.97 ng/mL), human colostrums (1–7 ng/mL), urine (1.12 ng/mL in women) [
17], umbilical cord blood [
18], saliva [
19], follicular fluid (~1–2 ng/mL ), and amniotic fluid [
20]. In adults BPA is mainly metabolized by the hepatic glucuronidation pathway. The biologic half-life of BPA is approximately 6 h, with almost complete excretion via urine in 24 h [
21]. However, despite the rapid metabolism, BPA can accumulate in tissues for an extremely long time and experience a conjugation-deconjugation cycling. Thus, the excretion of partial BPA is delayed [
22].
BPA can accumulate in reproductive organs [
17] and act as an endocrine disruptor owing to its structural similarity to estrogen. It is a mixed agonist-antagonist to affect estrogens and other steroid hormones [
23] such as antagonizing the hippocampal synaptogenesis induced by estrogens [
24]. BPA could exert its impact at a very low dose. For example, BPA had estrogenic effects at 2 μg/kg [
25] and may be more estrogenic
in vivo [
26]. Nevertheless, the estrogenic potency of BPA is 1000–100,000-fold less than that of estradiol [
27].
Woodruff
et al. [
28] showed that the sensitive windows of human exposure to BPA include progestation (before, during and shortly after the formation of fertilized eggs), pregnancy, infancy, childhood and puberty. Fetuses are more vulnerable to the adverse effects of BPA due to an immature drug-metabolizing system [
3].
5. BPA-Gene Interaction
In the post genome era, gene-environment interactions have become a hot research topic [
71]. Exposure to environmental factors, mainly nutritional, chemical and physical factors, potentially alters gene expression and changes the epigenome that can modify adult disease susceptibility and lead to disease phenotype [
72,
73].
While most previous animal and human studies examined the direct effect of BPA exposure on adverse pregnancy outcomes and infertility, little research has explored the role of BPA-gene interactions. Even the limited studies mostly focused on male rather than female infertility. Available evidence suggests that gene-environment interaction may be one of the major contributors to female infertility, and have lifelong [
74] and transgenerational impact [
75]. However, the mechanisms of gene-environment interactions are not yet fully clarified [
72].
BPA was originally thought to work via binding to estrogen receptors (ERs) and triggering agonistic effects by mimicking hormonal action [
76]. For instance, BPA selectively binds with classic ERs (ESR1 and ESR2) and acts as an ER modulator [
77,
78]. ESR1 and ESR2 play a significant role in the events of steroidogenesis, the growth of follicle, ovulation, and endometrial cycle [
79]. In an
in vivo mice study, BPA up-regulated mRNA expression of
ESR1 gene by 2.85-fold while the expression of
ESR2 gene showed no significant difference [
80]. However, another
in vitro study that exposed human fetal oocytes to BPA showed that the expression of
ESR2 gene was as up-regulated as
ESR1 and
ERRΓ [
81].
However, evidence from other systems now reveals that BPA can also affect gene expression directly and/or to impact epigenetic modification of fertility-related genes [
82,
83,
84]. For example, in adult men, the gene expression of
ESR2 and
ESR1 increased in men with higher urinary BPA concentrations [
85]. Caserta
et al. [
79] investigated 111 women aged 18–40 years, who were affected by primary infertility. The gene expression of nuclear receptors (
ESR1 and
ESR2), androgen receptor (
AR), pregnane X receptor (
PXR), aryl hydrocarbon receptor (
AhR), and peroxisome proliferator-activated receptor gamma (
PPARγ) was analyzed as biomarkers in peripheral blood mononuclear cell. A positive correlation was found between BPA levels and
ESR1, ESR2, AR, AhR, and
PXR expression, while
PPARγ expression did not show any meaningful difference (
Table 1). These findings were confirmed in another small study [
86] and supported the hypothesis that BPA acts on nuclear receptor (NR) through disturbing hormone response pathways and/or steroidogenesis [
79] and, therefore, affects female infertility.
Table 1.
BPA exposure and fertility related gene expression and genetic modification.
Table 1.
BPA exposure and fertility related gene expression and genetic modification.
Author and Year | Species | Treatment Period | Dose | Tested Tissue | Change of Gene Expression |
---|
Caserta, et al. 2013 [79] | Infertile women | - | - | peripheral blood mononuclear cell | ESR1, ESR2,AhR, PXR up-regulation; AhR, PPARγ no difference |
Chao, et al. 2012 [80] | Mice | Postnatal day 7–14 or 5–20 | 20–40ug/kg per day or per 5 days | Ovarian | ESR1 up-regulated; ESR2 no difference |
Oocytes | IGF 2Γ, PEG3 methylated sites decreased; H19 no difference |
Brieno-Enriquez, et al. 2012 [81] | Human (fetuses) | Cultured 7–21 days | 30umol/L | Cultured oocytes | Up-regulation: H2ax, Rpa, Spo11, ESR1, ESR2, ERRΓ, Blm at 14 day;No difference: Stra8, Nalp5, Smc1B, Sycp1 |
Cultured fibroblasts | Up-regulation: H2ax, Rpa, Blm, ESR1, ESR2, ERRΓ, MIh1 at 21 day |
Li, et al. 2014 [87] | Wistar rats | Postnatal day 28–35 day | 10 or 40 mg/kg/day | Ovarian | FIGLA, H1FOO and AMH no difference |
160 mg/kg/day | FIGLA, H1FOO down-regulation; AMH up-regulation |
Calhoun, et al. 2014 [88] | Rhesus Macaque | Gestational day 100–165 | 400 ug/kg/day deuterated BPA | Fetal uteri | Up-regulated: PDE11A, HOXC9, IGHMBP2, CSTL1, HOXC10, IL26, KLK3, ALX3, DOK6, ABHD1, HOXC6, HOXC8, HOXC9, HOXC10, HOXD1, HXOD3, HOXD9, WNT2, WNT4, WNT5A |
Down-regulated: CDH4, GDE1, GJB3, TFAP2C, RNF186, HOXA13, FGF10, CLIC6, CXCL14, SST, HOXA13 |
Susiarjo, et al. 2013 [89] | Mice | 2 weeks prior to mating and embryonic (E) day 0–9.5 | 10 ug or 10 mg /kg/day | Placenta | LOI: Snrpn, Kcnq1ot1 Average total RNA (placentas with or without LOI) expression up-regulated: Snrpn, Kcnq1ot1; Placentas with LOI had higher RNA expression at lower dose: Snrpn; Average total RNA expression down-regulated: Cdkn1c (upper dose and lower dose), Ube3a; Methylation levels reduced: Snrpn, Kcnq1ot1 |
Embryo | LOI: Igf2 Average total RNA expression up-regulated: Igf2; Methylation levels increased: Igf2 DMR1; Methylation levels reduced: H19/Igf2 ICR |
Brieno-Enriquezet
et al. [
81] found that the expression of genes involved in double-strand break generation (
Spo11), signaling (
H2ax) and repair (
Rpa, Blm) increased significantly in cultured fetal oocytes treated with BPA during meiotic prophase (
Table 1). This possible molecular mechanism may explain the effects of BPA on female germ cells [
81]. An
in vivo animal study [
90] focusing on the effect of BPA on the expression of meiosis-related genes treated pregnant mice with BPA (20 ng/g/day) beginning at 11 days postcoitus (dpc). Fetal ovaries were collected at 12-, 12.5-, 13.5-, and 14.5-day in pregnancy. Sixteen meiosis-specific genes were selected: meiotic entry gene
Stra8;
Spo11,
Sycp1,
Sycp2,
Sycp3,
Syce1,
Syce2 and
Tex12 (associated with the formation of synaptonemal complex);
Rec8,
Stag3,
Smc1b (associated with sister chromatid cohesion),
Dmc1,
Mei1,
Msh4,
and Msh5,
Prdm9 (meiotic recombination pathway genes). All of these genes were up-regulated after BPA treatment for 3.5 days, although only
Msh4,
Dmc1, and
Sycp2 reached statistical significance. A dramatic increase in expression of all of the above genes was observed from 12 to 14.5 dpc. It raises the possibility that fetal BPA exposure may limit expansion of the primordial germ cell population.
Another
in vivo rat study [
87] showed a decrease in follicle number and an increase in constituent ratio of atretic follicles in relation to BPA levels. The alteration may be caused by the changed expression of follicle development-related genes such as
FIGLA, H1FOO and
AMH (
Table 1) with increasing BPA doses. For example, compared with the 0 (control) group, the expression of
FIGLA gene mRNA was significantly reduced in the 160mg/kg/day BPA group while the expression of
AMH gene mRNA was significantly increased. However,
H1FOO gene mRNA expression levels showed a significant decrease in all BPA groups. Whether these adverse effects and the potential mechanism are consistent with those in human needs further confirmation [
87] as the dose used in this study was much higher than that of human exposure.
The expression levels of key development-related and functional genes of the uterus also have attracted great attention. When pregnant rhesus macaques were exposed to BPA at gestation day (GD) 100–165, significant differences in genes expression in fetal uteri were observed between BPA-exposed and placebo groups at GD165 (
Table 1). It showed that BPA exposure in the third trimester could alter transcriptional signals and may, in turn, influence adult uterine function of the offspring. The article detailed the role of
HOXA13, WNT4 and
WNT5A, which are critical for development and function of human reproductive organ [
88]. The dose used in this study was 400 μg/kg/day, which resulted in unconjugated biologically active BPA of 0.3–0.5 ng/mL [
56], which is similar to serum levels in human adults and fetuses [
14].
Epigenetics may be another important mechanism of BPA-gene interaction in female infertility. The effects of epigenetic modifications generally include DNA methylation, histone modification (acetylation, methylation, phosphrylation, ubiquitination, sumoylation and ADP ribosylation), and expression of non-coding RNAs (including microRNA) [
91]. Environmental toxicants may alter epigenome rather than DNA sequence [
92]. Furthermore, the modification of the epigenome in the germ line might be transmitted to the progeny, therefore, promoting a transgenerational phenotype [
92]. Chao
et al. [
80] evaluated the effects of BPA on the reprogramming of imprinted genes and found that the increased concentration of BPA remarkably decreased the methylation pattern of maternal imprinted genes (
Table 1). Meanwhile, the expressions of four types of DNA methyltransferases (
Dnmt1,
Dnmt3a,
Dnmt3b and Dnmt3L) were all suppressed with increasing BPA treatment concentrations. Another
in vivo study [
89] found maternal BPA exposure reduced the imprinted genes (
Snrpn,
Ube3a,
Kcnq1ot1,
Cdkn1c,
and Ascl2) methylation levels in mice placentas with the dose of dietary BPA at 10 mg/kg/day. The RNA expression of these genes increased. While methylation levels of
Igf2 in embryos were up-regulated, the RNA expression was down-regulated at the dose of 10mg/kg/day BPA. Loss of imprinting (LOI) occurred at
Snrpn,
Kcnq1ot1 and
Igf2 (
Table 1). It was hypothesized that DNA methylation may regulate the imprinting, but perfect correlation was not found in this study [
89]. They suggested that abnormal development of the placenta and embryo disturbed fetal and postnatal health. Thus, the epigenome is vulnerable to environmental perturbations, particularly during embryogenesis, neonatal development, and adolescence via epigenetic mechanisms.