A Study On The Epigenetic Mechanisms Mediating Effects Of Developmental Exposure To BPA On Rat Pancreatic Development And Learning/Memory Function

In 1995, David J. P. Baker first introduced the Fetal Origins of Adult Disease hypothesis (FOAD), which developed later into the Developmental Origins of Health and Disease hypothesis (DOHaD), stating that human exposure to adverse factors during early development, including fetal, infant, and childhood, can induce permanent or programmed changes in organ organization and function, which in turn can have effects on the development of adult non-communicable disease, including diabetes, metabolic syndrome, cardiovascular diseases, and behavioral anomaly. As a widespread environmental pollutant, endocrine disrupting chemicals (EDCs) has drawn more and more attention in terms of its effects on the organ development and the susceptibility to adult diseases when exposed during development.Bisphenol A (BPA) is one of the most widely used and highest production-volume EDCs around the world. BPA is used as monomer to manufacture polycarbonate plastics and epoxy resin to be used in food and beverage containers. Human are mainly exposed to BPA, which is leached from food and beverage packages, through ingesting. Others like dermal contact and inhalation are also important exposure pathways. Furthermore, previous studies indicate that BPA can be transferred to fetuses and neonates through placenta and human milk, possessing huge threats to human health. Both animal studies and human epidemiology studies have suggested that developmental exposure to BPA lead to adult neurobehavior impairments including learning/memory and emotional regulation. However, the underlying mechanisms remain elusive. Previous studies also revealed that developmental exposure to BPA can increase the susceptibility to diabetes in adulthood, and the mechanisms are still unclear.This study first focuses on the development of pancreatic ? cell to explore the possible mediating events between developmental exposure to BPA and disrupted glucose homeostasis in adulthood, as well as the potential epigenetic mechanisms. The second part of this study concentrates on the effects of perinatal exposure to BPA on learning/memory and emotional regulation in adulthood, and the expression of responsible genes in related brain region, as well as the potential epigenetic mechanisms.Part I:Epigenetics Mediates Effects of Developmental exposure to BPA on pancreatic development and glucose homeostasis in adulthoodObjectives:Explore the effects of developmental exposure to low-dose BPA on the development of pancreatic ? cells, and the expression of related key regulator in this process. Based on these data, explore the effects of BPA exposure on adult glucose metabolism and the potential epigenetic mechanisms.Methods:Weight matched pregnant Wistar rats were randomly assigned into control or BPA exposure group. From gestational day 0 on, rats were treated with 10 ?g/kg bw/d BPA or the vehicle control corn oil through gavage. After birth, immunohistochemistry and quantitative PCR were used to evaluate the pancreatic ? cell mass and the expression of key regulator genes; then, fetal pancreas at gestational day (GD) 15.5 were isolated to evaluate the expression of key regulator genes in this early life stage. From birth to adulthood, glucose homeostasis of rats was evaluated using intraperitoneal glucose tolerance test and insulin tolerance test. Chromatin immunoprecipitation and mass spectrometry were used to continuously monitor the histone modifications and DNA methylation of key gene from fetal stage to adulthood.Results:Fetal exposure to 10 ?g/kg/d BPA decreased pancreatic ? cell mass at birth, and the expression of key regulator, pancreatic duodenal homeobox 1(Pdx1), in neonatal pancreas. Further study at GD 15.5 found that BPA exposure could reduce the PDX1 positive cell fraction at this life stage. Fetal exposure to BPA significantly changed histone modifications at the promotor of Pdxl to an inactive trend, manifested by reduced acetylation of histone H3 and H4, and trimethylation of histone H3 lysine 4 (H3K4me3), while elevated dimethylation of histone H3 lysine 9 (H3K9me2). Furthermore, this inactive trend progressively strengthened until adulthood, when disrupted glucose homeostasis occurred. However, the DNA methylation of Pdxl gene promotor did not change significantly after BPA exposure in this study.Conclusions:Developmental exposure to BPA affects pancreatic ? cell development by significantly alters the histone modifications of key regulator gene Pdxl in the development process towards an inactive status, and ultimately induces disruption in glucose homeostasis in adulthood.Part ?:Developmental exposure to BPA impairs learning/memory and DNA methylation of estrogen receptor a in hippocampusObjectives:Explore the effects of developmental exposure to BPA on the learning/memory and emotional control and exploratory behaviors, as well as the underlying epigenetic mechanisms.Methods:Pregnant Sprague-Dawley (SD) rats were weight matched and randomly assigned into two treatment groups. From gestational day 0 to weaning at postnatal day (PND) 21, rats were treated with 40?g/kg body weight (bw)/day BPA or vehicle control corn oil orally. At PND 60, rats were subjected to Morris water maze (MWM) to evaluate the learning/memory function. At PND 85, offspring were subjected to elevated plus maze (EPM) to evaluate the anxiety-like behaviors and exploratory behavior. Serum corticosterone was determined by ELISA kit. The expression of estrogen receptor ? and ? in rat hippocampus were determined by quantitative PCR. At last, the DNA methylation status of the ERa gene promotor in hippocampus was determined to explore the epigenetic mechanisms underlying the effects of developmental BPA exposure on the learning/memory.Results:Developmental exposure to BPA first prolonged the escape latency independent of gender during the successive training test, and decreased the percentage of time spent in the target quadrant in the probe test when examined in MWM task. Although no substantial alteration was observed between two treatment groups in the EPM test, serum corticosterone level was altered in a gender-specific manner. Compared to the control, BPA exposure significantly decreased the expression of mRNA for ERa in the hippocampus, while no significant changes were observed in ER? mRNA. Developmental exposure to BPA also significantly elevated the DNA methylation of ERa gene promoter in hippocampus.Conclusions:These results suggest that developmental exposure to BPA impairs learning/memory function and elevated DNA methylation of ERa gene in hippocampus may be involved.