EDCs Flashcards
What is the EPA definition of EDCs?
exogenous agents that interfere with the synthesis, secretion, transport, binding, action or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, repro, development, and/or behaviour
major concerns with EDC?
may have effects on developmental processes in the embryos/neonates resulting in subtle, long-term, irreversible effects;
effects may not manifest until long after exposure has stopped
what are the hypothesized effects of EDCs?
- impaired fertility
- delyaed development/malformation of repro organs
- cancer in hormone responsive organs (breast, ovary, etc)
- altered sex ratios/phenotypes
- neurobiological/behavioural alteration
- “ambiguous gender”
- epigenetic changes (mods to DNA)
what biological processes are hormones involved in?
- growth
- development
- energy homeostasis
- reproduction
- water/electrolyte homeostasis
types of hormones
- peptide hormones
- amino acid derivatives (catecholamines, thyroid hormones)
- cholesterol derivatives (steroid hormones, vit D)
- eicosanoids (prostaglandins)
what is the HPG axis?
hypothalamus-pituitary-gland axis; hypothal releases GnRH which acts on ant pit, which releases LH and FSH, which acts on ovaries and testes; ovaies produce estrogen and progesterone–either can act +/- on AP or hypothal; in males, testes release testosterone–>act negatively on AP or hypothal
general mechanisms of endocrine modulation?
- hormone synthesis
- hormone transport
- hormone degradation
- hormone action
what is StAR?
steroidogenic acute regulatory protein; moves cholesterol into mitochondria during steroidogenesis–> critical process
what are the targets in hormone synthesis?
availability of precursors– StAR, cholesterol
Altered enzyme activities in synthesis– P450scc, aromatase, 5a-reductase (all enzymes potential targets)
possible transcriptional and translational control
what are the targets for hormone transport?
CBG, SSBG, and transthyretin– bind corticosteroid, sex steroid, and thyroxines; if these are bound by a EDCs, then more hormone is able to diffuse into cells and bind to receptors
what are the targets for hormone degradation?
biotransformative enzymes– eg phase I hydroxylation, demethylation, and deamination enzymes; phase II glucuronidation, sulfation enzymes; inhibition/induction of these enzymes can alter levels of circulating hormones`
what are the targets for hormone action?
- -nuclear receptor super family: ligand-activated transcription factors (alters gene expression)
- -EDCs can mimic endogenous hormone–bind to receptors (agonist, partial agonist, antagonist)
what are the physio roles of endogenous estrogens?
repro; CV system, bone, thyroid hormone actions
why are XEs problematic in the environment?
stable in the enviro, potential to bioaccumulate;
what are the difficulties in risk assessment of XEs?
relative potentcy–> ER-binding affinity;
nonmonotonic DR relationships
additivity assumed in mixtures (i.e. we are exposed to mixtures of XEs)
what is relative potency?
biological activity of a test compound compared to the standard compound, estradiol (E2)–>assigns estrogen equivalency factor
– E2 - Eq = sum(EEF*[XE])
describe Tier I assays, and what they find
Use Yeast reporter gene (YES) assay–> transcriptionally activate a reporter gene construct via an ERE; E screen assay– induce prolif of E2-responsive human MCF-7 breast cancer cells;
Classical ER competitive binding assay–tests for ER affinity
limitations of Tier I assay?
most assays focus on ER binding and direct receptor-mediated effects; but not all effects are receptor-mediated
- -e.g. altered enzyme activity - -changes in signal transduction pathways
describe the H295R steroidogenesis assay
adapted tier I assay; use a human adrenocortical cancer cell line–>expresses entire steroid biosynth pathway, therefore you can detect non-R mediated effects;
also no animal use–>big advantage
describe tier II assays
- mouse uterotropic assay–>measure uterine weight before and after exposure to XE
- full life-cycle test
- fish, invertebrate
- two generation reproductive tox tests
- fish, frog, bird, mammal
- exposure animal before its mature–>let it repro–>follow offspring through life
what is sex determination vs sex differentiation
sex determination
–dependent upon genetics or temps; signals to initiate male or female development
sex diff
–embryonic process of DEVELOPING into a male or female (sex already decided)
what is teh Jost paradigm of sex diff?
Chromosomal sex–> gonadal sex–> phenotypic sex
describe sex diff in mammals
early embryo development is identical– bipotential gonads
–having a Y chrom drives male development–> SRY gene in “tue mammals”
Males secret AMH, anti-mullerian hormone; testosterone, which causes the mullerian duct to refree and the wolfian duct to form; testosterone is tehn converted to DHT (dihydrotestosterone) by 5alpha-reductase–>causes masculinization of the bipot gonads
what are specific concerns to XE exposure?
- cryptorchidism–undescended testes
- hypospadias– occurs in both sexes; urethra opens in the wrong place
- decreases anogenital distance (AGD)– happens in mice and humans; sensitive marker of XE exposure
- altered sex ratio–
some effects may actually be due to anti-androgenic compounds (DDE, vinclozolin)
what was hte issue with DES exposure?
thought natural, so they didn’t not think there would be deleterious effects–> but exposure during organogenesis disrupted differentiation of estrogen responsive organs; also has effects of bones and immune system
what were the effects of DES sons?
increased incidence of testicular cancer; repro tract abnormalities–> cryptorchidism, hypospadias, cysts of epididymis; decreased fertility
DES mech of action?
some effects estrogen-receptor-mediated (especially through alpha isoform of ER);
gene imprinting effects–>epigenetic response; involves DNA methylation and histone modifications–> XEs effect these processes
what is the difference between DES and endogenous estros?
DES does not strongly bind to sex steroid binding glob–>therefore it crosses the placental barriers; has a greater binding affinity to ER than E2; there are also non-ER-mediated effects (effects on other pathways)
what is EE2, and main concerns?
17alpha-ethynylestradiol; potent synthetic estro; present as oral contraceptive
– released into enviro and wastewater effluents; main concern is effects of low conc in aquatic wildlife;
what is VTG and its effects?
vitellogenin; yolk precursor protien in oviparous (yolk-bearing) vertebrates;
–VTG is an estro-dependent gene product; usually only expressed in females; however,males will express VTG after XE exposure; sensitive biomarker
what is BPA, and how does it affect organisms?
monomer of plastics; found in epoxy resins and polycarbonate;
has structural similarities to DES and is a weak ER agonist; is about 2000-10000X weaker than E2;
effects of BPA in lab animals?
Female repro tract:
–decreased uterine weight
–increased expression of Era and progesterone receptor;
–vaginal cell proliferation
–earlier time of vaginal opening
mammary glands
–major cell proliferation occurs normally during increasing E2 levels; in immature mice exposed to BPA, mammary glands resemble those of pregnant mice–>may cause breast cancer?
what are phthalates used for, and what are the concerns?
used as plasticizers or “softeners”; ubiquitous in environments; the major phthalate is DEHP (di/bis2-ethylhexylphthalate)
what is DEHP’s mechanism of action?
weak ER agonist; 50 000X less potent than E2; but estrogenic effects in humans not considered relevant;
however, DEHP is an agonist of PPARa; PPARa involved in cell diff and growth, abundant in the liver–> causes hepatocarcinoma in rats and mice (not humans), because we have 10X fewer PPARa receptors;
activation of PPARa leads to inhibition of apoptosis, increases production of ROS
what are alkylphenols and what is the concern?
detergents and emulsifiers; present in soaps an ppc; degrade to nonylphenol ethoxylates–>sewage bacteria remove the ethoxylate groups, creating nonylphenol
what is the concern with nonylphenol?
stable and lipophilic; much more soluble in fat than water, BCF = 100 (bioaccumulates in fish)
nonylphenol effects and concerns
weak ER agonist, 30 000x less potent than E2; but present at very high levels; has effects in fish–> males: induction of VTG expression, intersex (eggs in testes); increases cell death in testes
females: fewer mature eggs
both sexes: delayed maturity, reduced hatching success, reduced success at swim up
what is p,p’-DDE?
dichlorodiphenyldichloroethylene; metabolite of DDT; most common DDT metabolite in enviro samples bc it’s stable and resistant to enviro degradation;
DDE mech of ac as an EDC?
weak ER agonist; 10 000X less potent than E2; similar to DDT; some other OCs are also estrogenic;
also an antagonist of androgen receptor;
causes eggshell thinning–> repro failure in bird pops, linked to reduced prostaglandin synthesis
