Xenobiotic Regulation of Estrogen and Progesterone Receptor - Mediated Gene Expression

By Dr. Lawanda Schief

Have you ever wondered how chemicals in the environment affect cancer? Well, this book can give you some scientific insight on how common pesticide chemicals and industrial waste can affect the growth of breast cancer cells.

• Language: English
• Publisher: Page Publishing, Inc.
• Release date: Sep 11, 2018
• ISBN: 9781642148909

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Xenobiotic Regulation of Estrogen and Progesterone Receptor - Mediated Gene Expression

Dr. Lawanda Schief

Copyright © 2018 Dr. Lawanda Schief

All rights reserved

First Edition

Page Publishing, Inc

New York, NY

First originally published by Page Publishing, Inc 2018

ISBN 978-1-64214-889-3 (Paperback)

ISBN 978-1-64214-890-9 (Digital)

Printed in the United States of America

The primary mechanism of steroid hormone action in vertebrates involves hormone binding to its receptor followed by receptor phosphorylation and dimerization. This activated receptor then produces enhancer actions through hormone response elements (HREs) in the promoter region of hormone-regulated genes and results in subsequent gene transactivation or inactivation. It is important to know which chemicals in the environment mimic or inhibit hormone activity, what structural properties of these chemicals play a role in this activity, and what effects these compounds may have on the hormone-responsive processes in human development and disease. While environmental chemicals have been implicated in the etiology of many hormone-regulated human diseases, the many possible nuclear receptor–mediated mechanisms involved remain to be characterized.

It is known that certain xenobiotics can alter normal estrogen activity through interactions with the estrogen receptor (hER). This process involves receptor binding with specific estrogen response elements (EREs) in the promoter of genes. All EREs are derivatives of the 13-mer palindromic consensus ERE originally identified from the Xenopus and chicken vitellogenin genes. It has been proposed that xenobiotics may activate estrogen-regulated genes selectively, depending on the sequence of the ERE in the promoter.

While the estrogen activity of many environmental chemicals has been evaluated in a wide range of in vivo and in vitro assays, the capacity for xenobiotics to differentially activate genes regulated by nonconsensus EREs has been poorly characterized. Since progesterone receptor (hPR) is an ER-dependent gene, it is also very important to determine if xenobiotics with estrogen activity can alter transcriptional transactivation of genes via hPR interactions and the progesterone response element (PRE). While there have been some reports of environmental chemicals, such as metabolites and isomers of the pesticide dichlorodiphenyltrichloroethane (DDT), interacting with the hPR, none of these studies evaluate the effects of other types of environmentally relevant xenobiotics with more diverse structure on hPR-regulated gene activation. In addition, the concept of tissue-selective gene activation by endocrine-disrupting chemicals has not been well examined. This dissertation tests the hypothesis that xenobiotic chemicals may selectively regulate hormone-responsive genes depending on promoter content (hormone response element sequence) and that this activity may be mediated differently by multiple steroid receptors (hER and hPR) in a tissue- or cell-specific manner.

Acknowledgments

First and foremost, I thank God for guiding and directing my path over the course of my studies at Tulane University. Without God’s help, this dissertation would not have been possible. I thank my husband, Miguel A. Schief, for his endless support, encouragement, patience, and help as I worked on my degree and dissertation. I also thank my parents for the many sacrifices they have made to ensure that I was able to attend college and pursue my dreams in life. I thank my late mother, Helen I. Miller, who always taught me to reach for the stars but assured her love and support no matter what path I took in life. I thank my father, Joe A. Miller, for his never-ending encouragement, love, and support.

I also want to acknowledge those who have helped me in some way to complete the process leading to this dissertation including Thomas E. Wiese, PhD (Tulane Department of Environmental Health Sciences, School of Public Health and Tropical Medicine, and Xavier University of Louisiana School of Pharmacy); Huiming Li, Suzanne Nehls, and Marc Welt, PhD (Xavier University of Louisiana School of Pharmacy); and Kathryn B. Horwitz, PhD (University of Colorado Health Sciences Center). Thank you all for your help. The following provided funding without which this dissertation would not have been completed: EPA STAR Fellowship Program, Center for Bioenvironmental Research at Tulane and Xavier University, and Louisiana Board of Regents Joint Faculty Appointment Program.

Chapter I

Introduction

Endocrine-Disrupting Chemicals

Endocrine-disrupting chemicals (EDCs) are natural or synthetic xenobiotic compounds that can induce or inhibit hormone activity in the hormone responsive tissues of exposed wildlife or humans (1). Exposure to EDCs can occur from many sources that include three main routes. The first route of exposure is dietary, which is the direct ingestion of the substance (2, 3). This ingestion may be inadvertent, due to the contamination of food, food additives, and food packaging, or intentional, due to various medical treatments (2, 5). The other two routes of exposure are inhalation or dermal absorption of contaminants from items such as paints, cosmetics, clothing, and others that may be found in the home, workplace, or the environment (i.e., contaminated air and water) (2, 4). Many endocrine-active compounds are no longer used in the United States, but due to their lipophilic/hydrophobic chemical properties, are resistant to metabolism and may bioaccumulate (5) in the reproductive tissues, other organs, and adipose deposits (5). The adverse effects caused by EDCs are not always obvious and sometimes are not seen until later in life (e.g., puberty or midlife) or until large numbers of individuals in a population have been affected (6–8).

In wildlife, endocrine-disrupting xenobiotics have been linked to decreased fertility in birds, fish, and shellfish as well as to abnormal thyroid function and the masculinization or defeminization of females in the same animals (2, 6, 7). Exposure to these chemicals is also correlated with sex reversal of some fish species (2, 6, 7) and decreases in litter sizes of laboratory mice (9). Effects seen in wildlife are often caused by prenatal exposure before egg production or pregnancy (6). In humans, EDC exposure has been correlated to many health problems such as the increased incidence of breast, uterine, testicular, and prostatic cancers (2, 6, 10–12). Male reproductive development abnormalities, such as cryptorchidism and hypospadias, have also been associated with exposure to EDCs (2, 6). There is also an association between exposure to endocrine-disrupting xenobiotics and an increase in the number of recurrent pregnancy losses (13) and ectopic pregnancies (6). EDCs have also been correlated to decreases in sperm counts and increases in the ambiguity of secondary sex characteristics in some instances (2, 11, 12, 14, 15).

Some xenobiotics have demonstrated the ability to bind and antagonize androgen receptor (3, 23, 24) as well as the progesterone receptor in select animal models (25–27). However, there are only limited reports and information in regard to the progesterone receptor activity of xenobiotics. It has also been shown that some environmental endocrine-disrupting xenobiotics can increase or decrease normal estrogen activity through either receptor or nonreceptor-mediated mechanisms (16–19). The primary mechanism of estrogen action in vertebrates involves hormone binding to the estrogen receptor (hER) and the subsequent regulation of estrogen-regulated genes (14, 20). The subset of EDCs that affect estrogenic endpoints is called the xenoestrogens, which have molecular structures that range from very similar to very dissimilar from the natural steroid estrogen 17|3-estradiol. Examples include the plastic and resin component bisphenol A, the pesticide component o,p′-DDT, and the nonionic detergent degradation product 4-nonylphenol (fig. 1) (5, 14, 17, 19, 21, 22). Thus, the estrogenic activity of xenobiotics cannot be predicted based only on molecular structure.

Figure 1. Test compounds. Diagram depicts the chemical structures of the compounds tested. Compounds were selected based on previously determined agonist/antagonist activity, structural diversity, as well as environmental and biological relevance.

Test compounds

It is very important to know which chemicals in the environment have hormone activity, what chemical structure properties play a role in this hormone activity, and what ultimate effects these compounds may have on human development and disease. Several factors should be taken into consideration related to these questions: (a) Exposure to these compounds often cannot be avoided, (b) there may be beneficial as well as undesirable effects of exposure, (c) activity of these compounds may or may not be selective to certain target sites in an organism, and (d) beneficial or deleterious/toxic effects may be mediated through multiple mechanisms (5). It should also be recognized that both in vivo and in vitro studies will be needed to ultimately understand the role any particular xenobiotic may have on endocrine disruption (28).

This dissertation will utilize in vitro models to delineate the mechanistic basis of the endocrine-disrupting potential of xenobiotics with hormone activity. While xenoestrogens and other endocrine-disrupting xenobiotics have been implicated in the molecular etiology of hormone-controlled human diseases (29), the specific molecular mechanisms involved remain to be characterized. There is a particular need for more in-depth examination of the hormone activity of xenobiotics at the cellular and molecular level with regard to the promoter context of hormone-regulated genes, the tissue specificity of observed hormone activity, and specific receptor targets. All of these issues are important on a larger scale due to the apparent increased incidence of hormone-controlled diseases in people living in agricultural settings where endocrine-disrupting chemicals are used or near chemical manufacturing facilities (3). Thus, it is of high priority that the molecular mechanisms affected by environmental factors responsible for hormone-controlled diseases are clearly delineated so that better preventative measures and treatment methods may be developed and employed.

This dissertation seeks to provide new insight into the molecular mechanisms involved in endocrine disruption by elucidating the steroid receptor–specific, cell-specific, and promoter-specific effects of known endocrine-disrupting chemicals.

Steroid Hormones and Their Receptors

The steroid hormone receptors are a subgroup of the larger nuclear receptor superfamily (30, 31). There are six general types of the steroid hormone receptors in mammals that are each named for its activating ligand: (a) androgen, (b) estrogen, (c) glucocorticoid, (d) mineralocorticoid, (e) progesterone, and (f) vitamin D receptors (30–32). The natural hormones for most members of steroid hormone receptors are synthesized from cholesterol via multiple pathways utilizing several enzymes (32). Each of these steroid hormones acts through similar mechanisms to induce specific RNA transcription and subsequent protein synthesis in target tissues sensitive to the hormone (33, 34). The mechanism of each steroid hormone’s action involves binding to its respective receptor (SHR), induction of phosphorylation and dimerization of that receptor, receptor translocation into the nucleus, receptor binding to DNA at a specific steroid hormone response element within a promoter (SRE), and finally activation of hormone-responsive genes (30, 34–36). See figure 2 adapted from Wiese et al. (14).

Figure 2. General mechanism of steroid hormone action.