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Involvement of Insulin Signaling Disturbances in Bisphenol www.nature.com/scientificreports OPEN Involvement of Insulin Signaling Disturbances in Bisphenol A-Induced Alzheimer’s Disease-like Received: 10 November 2016 Accepted: 29 June 2017 Neurotoxicity Published: xx xx xxxx Tingwei Wang1, Cuiwei Xie1, Pengfei Yu1, Fangfang Fang1, Jingying Zhu2, Jie Cheng1, Aihua Gu1, Jun Wang1 & Hang Xiao1 Bisphenol A (BPA), a member of the environmental endocrine disruptors (EDCs), has recently received increased attention because of its efects on brain insulin resistance. Available data have indicated that brain insulin resistance may contribute to neurodegenerative diseases. However, the associated mechanisms that underlie BPA-induced brain-related outcomes remain largely unknown. In the present study, we identifed signifcant insulin signaling disturbances in the SH-SY5Y cell line that were mediated by BPA, including the inhibition of physiological p-IR Tyr1355 tyrosine, p-IRS1 tyrosine 896, p-AKT serine 473 and p-GSK3α/β serine 21/9 phosphorylation, as well as the enhancement of IRS1 Ser307 phosphorylation; these efects were clearly attenuated by insulin and rosiglitazone. Intriguingly, Alzheimer’s disease (AD)-associated pathological proteins, such as BACE-1, APP, β-CTF, α-CTF, Aβ 1–42 and phosphorylated tau proteins (S199, S396, T205, S214 and S404), were substantially increased after BPA exposure, and these efects were abrogated by insulin and rosiglitazone treatment; these fndings underscore the specifc roles of insulin signaling in BPA-mediated AD-like neurotoxicity. Thus, an understanding of the regulation of insulin signaling may provide novel insights into potential therapeutic targets for BPA-mediated AD-like neurotoxicity. Bisphenol A (BPA), a member of the environmental endocrine-disrupting chemicals (EDCs), is widely used in carbonated beverages and polyester food packing material, tooth solid sealing agents, baby bottles, infusion bags and other products with additives1. Widespread and continuous exposure to BPA in humans has been confrmed by biomonitoring studies in general populations, and individuals are at risk from internal exposure to unconju- gated BPA2, 3. According to a recent National Health and Nutrition Examination Survey, nearly all US citizens exhibit detectable amounts of BPA metabolites in urine and blood4, 5. Insulin is released from the pancreas into the bloodstream and can cross the blood-brain barrier (BBB) via a carrier-facilitated process; it is also secreted by the hippocampus, the prefrontal cortex and other regions in the brain4. Numerous studies have indicated that insulin binds to its receptors and plays an important role in the maintenance of brain neuronal survival, energy metabolism homeostasis, learning and memory5. Te insulin receptor (IR) is densely expressed in pyramidal cell axons in the hippocampal CAl region and is mainly distributed in the dominant learning, memory and cognitive function regions of the brain6. Under phys- iological conditions, insulin signaling is mediated by the IR tyrosine kinase receptor family. When insulin binds to its receptor, the IR tyrosine kinase is activated, which induces intracellular insulin receptor substrate (IRS) protein tyrosine phosphorylation7 and subsequently results in the activation of phosphatidylinositol 3-kinase (PI3K) and serine/threonine protein kinase B (protein kinase B, AKT)8, which have been shown to be involved in the regulation of insulin metabolism9. BPA exposure has recently been demonstrated to be a risk factor for insulin resistance and metabolic disor- ders10. Our previous fndings indicated that perinatal BPA exposure contributed to peripheral insulin resistance in ofspring during adulthood11. In parallel with the peripheral insulin resistance induced by BPA, intriguingly, 1Department of Toxicology, the Key Lab of Modern Toxicology (NJMU), Ministry of Education, School of Public Health, Nanjing Medical University, Nanjing, 211166, China. 2Wuxi Center for Disease Control and Prevention, 499 Jincheng Road, Liangxi District, Wuxi, Jiangsu, 214023, China. Correspondence and requests for materials should be addressed to J.W. (email: [email protected]) or H.X. (email: [email protected]) SCIENTIFIC REPORTS | 7: 7497 | DOI:10.1038/s41598-017-07544-7 1 www.nature.com/scientificreports/ Figure 1. BPA disturbed the insulin signaling pathway. Western blot expression analysis of (A) diferent concentrations of BPA (0, 2, 20, 200, and 2000 nM/L) on IR phosphorylation and IRS phosphorylation in SY5Y cells. (C) Efect of diferent concentrations of BPA (0, 2, 20, 200, and 2000 nM/L) on AKT and GSK3α/3β phosphorylation in SY5Y cells. (E) IR phosphorylation and IRS phosphorylation in SY5Y cells at various time points (0, 1, 3, 6, 12, and 24 h) of 20 nM/L BPA treatment. (G) AKT, AKT phosphorylation, GSK3α/3β, and GSK3α/3β phosphorylation in SY5Y cells at various time points (0, 1, 3, 6, 12, and 24 h) of 20 nM/L BPA treatment. (B,D,F,H) GAPDH levels detected in parallel served as controls. Mean values ± SEMs are representative of three independent isolations and three independent samples. Signifcant diferences between the treatment groups and the control group were determined via one-way ANOVA and the Dunnett multiple comparison procedure. (*P < 0.05, **P < 0.01, ***P < 0.001 compared with the control group). perinatal BPA exposure also resulted in brain insulin resistance in ofspring when they were 8 months of age12. Adult male mice treated with a subcutaneous injection of 100 μg/kg/d BPA for 30 days exhibited a signifcant decrease in insulin sensitivity and glucose transporter 1, 3 (GLUT1, 3) protein levels in the brain13. Tese fndings support disturbances in insulin signaling induced by BPA in both peripheral and central systems. AD is characterized by the deposition of two types of flamentous aggregates: the formation of senile plaques (SP) from amyloid-β (Aβ) and neurofbrillary tangles composed of phosphorylated tau14. Amyloid precursor protein (APP) forms Aβ polypeptide through a series of proteolytic reactions and exerts neurotoxic efects. Tese hydrolyzed polypeptides aggregate to form Aβ and amyloid protein and ultimately form SP. Te association between insulin resistance and Alzheimer’s disease (AD) has recently received substantial attention. Individuals with type 2 diabetes have approximately double the chances of developing AD15. Hoscheidt 16 et al. have suggested that an increased HOMA-IR was associated with increased sAβPPβ and Aβ42 . Moreover, impaired glucose metabolism in the brains of individuals with AD is a widely recognized early feature of the disease17. Considering the aforementioned studies and our previous work, we hypothesized that insulin signaling disturbances induced by BPA might induce APP and p-tau enhancement, thereby contributing to AD-like neu- rotoxicity. Considering the environmental estrogenic activity of BPA, the roles of estrogen receptors (ERs; ER-α, ER-β and GPR30) in the regulation of insulin signaling were also discussed. Results Effects of BPA on the expression of insulin signaling pathway components in SH-SY5Y cells. Te IR and IRS-1 play critical roles in insulin signaling activation and transduction18; thus, we investi- gated the phosphorylation of the IR and IRS-1. SH-SY5Y (SY5Y) cells were incubated with diferent concentra- tions of BPA (0, 2, 20, 200, or 2000 nM) for 12 hours; the phosphorylation site of the insulin receptor (IR) p-IR Tyr1355 was subsequently detected via western blotting. As indicated in Fig. 1A, BPA exposure signifcantly decreased the p-IR Tyr1355 level; moreover, it substantially enhanced the expression of IRS1 Ser307 (a phos- phorylation site that inhibits insulin signaling by antagonizing tyrosine phosphorylation), a key downstream signaling component of the IR, with the maximal level at the concentration of 20 nM (P < 0.05). As a downstream signaling mechanism of IRS-1, the alteration of Kinase B/AKT (AKT) activity is one key characteristic of insulin resistance. Figure 1C indicates a striking reduction in the phosphorylation level of AKT Ser473 afer treatment with various concentrations of BPA. In addition, the expression of pS9-GSK3β, a protein with a phosphorylation site that is key to the activity of GSK3β, was signifcantly decreased (P < 0.05). Similar results were also identifed for pS21-GSK3α. Twenty nM BPA signifcantly afected all insulin signaling components assessed; thus, this dose was selected for the evaluation of BPA-induced insulin signaling disturbances at various time points (0, 1, 3, 6, 12, and 24 h). Figure 1E indicates that BPA exposure clearly increased the expression of pY896-IRS1 at 1, 3 and 6 h; the expression then gradually decreased, with the maximal decrement occurring at 24 h. In addition, the AKT Ser473 phosphorylation was decreased from 6 h. Consistent with the expression of AKT Ser473 phosphorylation, the pS9-GSK3β and pS21-GSK3α phosphorylation was also substantially decreased, with the peak efect at 12 h (Fig. 1G). To further confrm the involvement of the insulin signaling pathways in BPA-induced neural insulin SCIENTIFIC REPORTS | 7: 7497 | DOI:10.1038/s41598-017-07544-7 2 www.nature.com/scientificreports/ resistance, mTOR and PP2Ac, which located downstream of AKT, were explored in the present work. As depicted in Fig. S1, BPA exposure signifcantly reduced the phosphorylation of mTOR and the methylation level of PP2Ac, both of which were linked to p-tau phosphorylation19, 20. 2+ BPA exposure contributes to intracellular [Ca ]i and ROS increase, and ATP and mitochondrial membrane potential decrease
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