Although BPA has been proven to cause many
Although BPA has been proven to cause many health issues such as obesity and neurological toxicity (Rubin, 2011), the effects and mechanisms of BPA on the human tumorigenesis and cancer development are still not well illustrated. In the present study, we found that 10 to 10M BPA significantly promoted the in vitro proliferation of both human breast cancer MCF-7 and SkBr3 orexin a after exposure for 48h. The stimulation effects of 10M BPA were greater than other doses, suggesting that nanomolar BPA can significantly induce the cell proliferation. That low doses of BPA have significant effects on cell proliferation and other biological processes had also been found in numerous studies (Bouskine et al., 2009, vom Saal and Hughes, 2005). BPA has been detected in 100% of urine samples from Chinese children with a mean concentration of 3.00ng/ml, which is equivalent to approximately 1.3×10M (Li et al., 2013), and in 96% of urine samples from American college students with a mean concentration of 1.3ng/ml, which is equal to approximately 5.7×10M (Carwile et al., 2009). The mean BPA concentrations in plasma collected from Hong Kong were 0.95±0.56ng/ml, which was equivalent to approximately 4.2×10M (Wan et al., 2013). Therefore, the dose 10M used for the next studies is equivalent to the concentrations of BPA found in human body. Traditional views suggested that the estrogenic effects of EDCs were mediated by ERα/β that act as transcription factors to modulate activities of target genes by interacting with several DNA response elements. Recent studies also indicated that GPER can potentially mediate the endocrine disrupting effects via directly or indirectly modulating cellular responses such as calcium mobilization and nitric oxide production by no-genomic pathways (Albanito et al., 2007, Albanito et al., 2008, Prossnitz et al., 2008). However, both ERα/β and GPER were not involved in BPA induced cell proliferation in the present study, due to either ICI 182,780 or G15 can attenuate the stimulation effects of BPA on the proliferation of MCF-7 and SkBr3 cells. Recent studies indicated that nuclear hormone receptor super family has been linked directly or indirectly to oncogenesis (Marino et al., 2006). ERR family, a nuclear receptor super family which acts as transcription factor, has been determined in various cancer types at the RNA or protein level (Bianco et al., 2012). This is also confirmed in our study that both ERRα and ERRγ were greatly detected in both MCF-7 and SkBr3 cells. The expression of ERRα is positively related to the poor prognosis in tumors originating from the colon, endometrium, ovary, prostate and breast (Cavallini et al., 2005, Cheung et al., 2005, Fujimoto and Sato, 2009, Suzuki et al., 2004). The ERRγ, which had very high bind affinity to BPA (Okada et al., 2008), can also stimulate the proliferation of breast cancer cells (Ijichi et al., 2011). We found that both ERRα and ERRγ were greatly expressed in both ER positive and negative breast cancer cells. Then, we silenced both ERRα and ERRγ by their corresponding siRNA and found that si-ERRγ, while not si-ERRα, successfully abolished the BPA induced proliferation of breast cancer cells. To our knowledge, this is the first study which revealed that the estrogenic effects of BPA were mediated by ERRγ. Further, our study revealed that 10nM BPA can up regulate the mRNA and protein levels of ERRγ via a time dependent manner and then trigger its nuclear localization. A previous study also revealed that the expression of ERRγ mRNA was induced by estrogen in MCF-7 cells (Cheung et al., 2005). ERRγ was also noted to be significantly associated with an increased risk of recurrence and adverse clinical outcome in breast cancer (Ijichi et al., 2011). The ER negative endometrial cancer cells with stable ERRγ over expression showed faster proliferation than their parental cells (Yamamoto et al., 2012). Further, BPA treatment can trigger the nuclear translocation of ERRγ in both ER negative and positive breast cancer cells, where ERRγ can regulate the energy homeostasis and then promote cell proliferation by binding to a variety of ERRE-related sequences (Razzaque et al., 2004).