Endocrine-disrupting chemicals (EDCs) are a group of exogenous substances which can interfere with the normal biological processes such as synthesis, secretion, transport and metabolism of natural hormones in an organism. EDCs can impact the functioning of nervous, immune and reproductive systems, resulting in endocrine diseases including polycystic ovary syndrome (PCOS). PCOS is a common endocrine disorder in women of childbearing age, characterized by sparse ovulation or anovulation, hyperandrogenism, and polycystic changes of the ovaries. In recent years, researchers have tried to explore the etiology of PCOS from genetic, metabolic, environmental, and inflammatory perspectives . Hyperandrogenism is one of the cardinal features of PCOS. Both clinical and laboratory studies have revealed that the over production of androgen is correlated to the expression of enzymes which involved in androgen synthesis (following called androgen synthetases) in the ovary [2, 3], such as StAR, CYP11, CYP17 , HSD3B , etc. It has been found that CYP11A1(tttta)n repeat polymorphism may be a potential molecular marker of PCOS risk [6, 7].
Being a very representative EDC on endocrine disorders, Bisphenol-A (BPA) was first exploited as a synthetic estrogen. This substance is also commonly used to harden plastics. BPA in the environment can enter the human body through the mouth, skin, respiratory tract, etc. The bound free BPA can be metabolized by glucuronic acid or sulfonic acid in the liver and excreted via the kidney . BPA exposure can affect the brain , heart , liver , kidney , adipose tissue, breast  and the reproductive system [14, 15] at lower relative concentrations, resulting in, for example, obesity, diabetes , PCOS . Furthermore, substantial BPA-induced reproductive toxicity has been observed. BPA is found to be able to change the phenotypes of progeny by stably altering the parental epigenome [18, 19]. Perinatal BPA exposure can alter the function of reproductive endocrine system by hypomethylating some imprinting genes during oocyte maturation or decreasing the expression of the estrogen receptor ER at both the mRNA and protein levels . The direct toxicity of BPA is manifested in the disruption of decidualization in vitro , enhanced expression of estrogen receptors-α (ER-α) and oxidative stress in embryonic stem cells , and promotion of apoptosis .
Clinical data demonstrate increased levels of BPA in patients with PCOS , and the levels can reach those in males . PCOS induced by BPA exposure is also associated with hyperandrogenism and hyperinsulinism. BPA not only stimulates ovarian production of testosterone but also inhibits the activity of testosterone hydroxylase which helps maintain the testosterone concentration ; BPA-treated mice develop ovarian polycystic changes and luteal loss [27, 28], unbalanced expression between androgen synthesis-related enzymes and androgen receptors in the ovary, and abnormal expression of steroidogenic genes . Following transient exposure to BPA before puberty, rats experience ovarian weight loss and follicle number decline . Direct cellular exposure to BPA induces an increase in the percentage of germ cells and a decrease in the percentage of primordial follicles . Several experiments have found that BPA exposure which exceeded the maximum safe dose would present direct toxicity by damaging structure or function on the reproductive system, but the dose below the maximum produced chronic toxicity . Whether the chronic toxicity is dominant depends on the factors such as species, sex, age, concentration and contact method, etc. Meanwhile, below the maximum safe dose, the interference of BPA exposure is more visible at a lower dose .
It is well established that epigenetic modification can regulate mRNA and protein expression without altering the DNA sequence. Several excellent reviews have alluded to the possible correlation of the hypomethylation status of gene promoter regions, such as EPHX1, to PCOS . N6-methyladenosine (m6A), formed with methyltransferase, demethylase and YTHDF proteins , is the most abundant regulator for the post-transcriptional modification of mammalian mRNA which is reversible. By using methylated RNA immunoprecipitation (MeRIP), the m6A residues are revealed to be located within a 100–200 nt-long transcript region. A strong correlation between the m6A and obesity, self-renewal and metastasis of stem cells , cell senescence , apoptosis  and metabolic regulation of cancer, for example, gastric cancer  and colorectal cancer , has been reported. The interaction of RNA and protein in vivo affected by m6A has been intensively investigated, affecting the structure, maturation and stability of mRNA . METTL3, highly conserved in most eukaryotes, is a major factor in abnormal m6A repair and cell self-renewal. Previous studies have indicated that increased levels of METTL3 and m6A are caused by the Wnt and PI3K-Akt pathway in gastric cancer , which is a detrimental factor in the prognosis of patients . Furthermore, negative correlation between wide distribution of m6A and gene expression has been confirmed in chicken embryos, which coincided with the increase in m6A methylation peaks and their transcript levels during follicle selection , suggesting that m6A is critical in the occurrence of life. It is noteworthy that METTL3 can also suppress cell apoptosis by promoting IL-1β production  and inhibit proliferation, migration and invasion of cancer cells through the p38/ERK pathway . However, the biological functions of METTL3 await more thorough investigation.
The effect of BPA on reproductive system has not been properly addressed in adolescent female rats. Moreover, RNA epigenetic modification has great impact in reproductive disorders . Therefore, we underwent to further determine the effect of BPA on reproduction from an epigenetic modification perspective using PCOS as a model system to explore the mechanism of BPA-induced hyperandrogenism in adolescent female rats.