Thyroid Mediated C S Dysfunction. Outline

Transcription

1 Mediated C S Dysfunction How to use what we know about the structure and function of the thyroid system to generate data using in vitro methods that can populate QSAR models Kevin M. Crofton, PhD US Environmental Protection Agency McKim Conference Duluth MN September 17, 2008 Outline hormones and homeostatic mechanisms A mode-of-action for thyroid disruption and adverse outcomes on nervous system development Targets for disruption Targets for screening Summary 1

2 Hormones- Structure and Function T3 and T4 are the principle hormones synthesized and released by the thyroid gland Development - Critical for differentiation and growth Transient disruption = permanent effects Adult mportant for energy and thermoregulation Transient disruption = transient effects Thyroxin (T4) HO O CH2 CH H 2 C O H O Triiodothyronine (T3) HO O CH2 CH C O H H 2 O Regulation of Hormones TRβ Hypothalamus TRH Elimination from the body Catabolic Enzymes Liver TRβ TH binding proteins + Pit TSH Blood T3/T4 Acts as a ligand for nuclear thyroid hormone receptors (TRs) T 4 T 3 5 -deiodinases Target Tissues 2

3 Cellular Acton of TH Corepressors Activator TR TR T3 X AAAAA Transporter T4 D T3 Zoeller, 2003 MOAs Targets Effects Cancer & Non-Cancer al Thyroperoxidase odine Symporter Early Biological Effect Tissue Specific Effect Altered Structure/ Function Clinical Disease Exposure Extra-al Hepatic UDPGTs Deiodinases Serum T3 & T4 Changes TSH Tissue T3 Changes Hyperplasia Altered Development Tumors Birth Defects Cellular Transporters T4 TTR Binding Receptors 3

4 Adverse Outcome Pathways Toxicity Targets Pathways Effects Cancer & Non-Cancer al Thyroperoxidase odine Symporter Early Biological Effect Tissue Specific Effect Altered Structure/ Function Clinical Disease Exposure Extra-al TSH Hyperplasia Tumors Hepatic UDPGTs Serum T3 & T4 Changes Deiodinases Tissue Altered Birth What do we know and not know T3 about Changes these Development pathways? Defects Cellular Transporters T4 TTR Binding Receptors Major Sequelae of Disruption Adult Exposure tumors in laboratory animals Not a relevant mechanism for human cancer May increase incidence of cardiovascular disease Neurodevelopment Lack of THs result in adverse neurological development (sensory, motor, cognitive) Species independent (fact) Rat is appropriate animal model for neurodevelopmental effects These are two different outcomes that can result from the same molecular targets One is relevant for human health and one is not 4

5 Neurodevelopment and Dysfunction FACT: without adequate TH the nervous fails to properly develop odine deficiency Congenital hypothyroidism Dose-Response and Critical Window Dioxins, Furans and PCBs - Hearing Loss Exposure Hepatic Parent or Metabolite Binding to PXR Binding to AhR Hepatic Phase Enzymes Serum T4 & T3 Tissue T3 Alter TR Mediated Proteins Loss of cochlear hair cells Hearing Loss 5

6 Response PXR Binding UGTs D tissue CNS Protein (1) CNS malformation T4 serum T3 serum T3 tissue TR activation CNS Protein (2) Functional Loss (eg. Q) ncreasing Dose and/or Time PHAHs and Ototoxicity Fetal/Postnatal PCB Exposure Critical-Period Model for Chemical-nduced Postnatal Hypothyroxinemia and Ototoxicity Low-frequency Hearing loss High -- Frequency -- Low Cochlear Development Hormone Hearing Loss, db SPL (difference from control) Hearing Loss, db SPL (difference from control) Log Thyroxine Concentration, % Control y = *logX r ² = Conception Birth Weaning Log Thyroxine Concentration, % Control 6

7 Dose-Response Perchlorate, Propylthiouracil and Hippocampal Physiology Exposure PTU Perchlorate Binding to TPO nhibition Of TPO Serum T4 & T3 Hippocampal T3 Alter TR Mediated Proteins Synaptic Malformation Altered Synaptic Function Learning mpariment Normalized Population Spike Dose-Response Perchlorate, Propylthiouracil and Hippocampal Physiology Latency in Seconds Hippocampal physiology Normalized Spike vs EPSP ppm 3 ppm 10 ppm Normalized EPSP Water maze learning Cue Day 0ppm (n=11) 3ppm (n=15) 10ppm (n=10)* BL EPSP Max % Control Dam T4 vs BL EPSP Max - % Control M0805 PTU M0703 PERC M0102 PTU 40 r2= Dam T4 Percent of Control 7

8 What do we not know? Dose-response relationships Critical windows Sensitive biomarkers (T4?) However, we can t get lost in the need to understand everything in the pathway Causative and predictive is minimum Quantitative models are the holy grail What can we do to inform QSAR models Develop in vitro test methods for known targets 8

9 n Vitro Models for Disruptors odine Symporter (NS) Thyroperoxidase (TPO) Deiodinases Transporters Blood Transporters Cellular Receptors Hepatic Nuclear Receptors 9

10 n Vitro Models Receptor Beta GeneBLAzer TRβ-UAS-bla HEK293 Cell Line Ligand Substrate T3 Stimulation of TRβ T3 (M) Cell line contains a beta-lactamse reporter gene under the control of an UAS response element stably integrated in Hek293 cells. This line also stably expresses a fusion protein consisting of the GAL4 DNA binding domain and the TRβ ligand binding domain. Courtesy of Keith Houck, NCCT Human TRβ Reporter Gene Assay Heat Map of AC50 s 1456 chemicals; 14 concentrations; agonist and antagonist modes T3 Levothyroxine ACTVES 65 SELECTVE 2 ACTVES 62 SELECTVE 0 Courtesy of Keith Houck, NCCT 10

11 TH Action Assay T-Screen (Gutleb et al. EnvToxPharm 2005) - measures TH dependent cell proliferation in GH3 cells - 96 well plate assay Summary pathways are known Multiple targets involved n vitro models are available for many of the targets Need to begin testing chemicals 11

12 Thank You 12