Endocrine Disrupt Cpds.ppt
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Endocrine Disrupt Cpds.ppt Endocrine Disrupt Cpds.ppt Presentation Transcript

  • Endocrine Disrupting Compounds Any exogenous agent that causes adverse health effects in an intact organism, or its progeny, consequent to changes in endocrine function.
  • Specifically….
    • Any exogenous chemical that interferes with the production, release, transport, binding, action, or elimination of natural hormones responsible for the maintenance of homeostasis and regulation of developmental processes.
  • Chemicals: Wide Variety -
    • Pesticides
    • Herbicides
    • Fungicides
    • Plasticizers
    • Surfactants
    • Organometals
    • Halogenated PAHs
    • Phytoestrogens
    • At Least 4 Modes of Action
    • Serving as steroid receptor ligands.
    • Modifying steroid hormone-metabolizing enzymes.
    • Perturbing hypothalamic pituitary release of trophic hormones.
    • Miscellaneous or unknown.
    • Interactions with the functions of estrogens, androgens, and thyroid hormones have been the most highly studied.
  • Prototypical Exoestrogens: Note the Diverse Chemical Structures
  •  
  • Nuclear Receptor Super Family
    • Steroid Receptors
    • Androgen R.
    • Progesterone R.
    • Glucocorticoid R.
    • Mineralocorticoid R.
    • Estrogen Receptors
    • Thyroid Receptors
    • Thyroid Hormone R.
    • Retinoic Acid R.
    • RXR
    • Vitamin D R.
  •  
  • Mechanisms of Endocrine Disruption
    • Binding and activating the estrogen receptor (thereby acting as an estrogen)
    •  
    • Binding but not activating the estrogen receptor (thereby acting as an anti-estrogen
  • Mechanisms (Continued)
    • Binding other receptors ( e.g ., androgen receptors in males)
      • activating the receptor thereby acting like an androgen
      • not activating the receptor thereby acting like an anti-androgen
    • Modifying the metabolism of natural hormones
  • Mechanisms (Continued)
    • Modifying the number of hormone receptors in a cell (reduce or increase the number)
    • Modify the production of natural hormones
    • Interactions with steroid binding proteins
  • Individual Mechanisms
    • Binding and activating the estrogen receptor (thereby acting as an estrogen)
      • Exogenous estrogens can bind to the estrogen receptor but the affinity with which these exogenous estrogens bind to the receptor is usually only a fraction of the binding affinity of 17 B-estradiol.
    • Xenobiotics may affect transcription and signal transduction.
    • Can act through both i) receptor-mediated and non-receptor-mediated mechanisms.
    • e.g ., genistein is a weak est receptor agonist, but can also modulate the activity of tyr kinases and DNA topoisomerases.
  • Estrogen Receptor (Continued)
    • Compounds may act as either estrogens or anti-estrogens depending on the cellular environment.
      • Certain hydroxylated PCBs are able to bind the estrogen receptor and activiate gene transcription at high concentrations.
      • However, these PCB metabolites are weak agonists at appropriate concentrations, and they may have the potential to interfere by competing with endogenous estrogens for binding sites.
  •  
  • Estrogen Receptor (cont’d)
    • We have additive effects: several chemicals binding and activating the estrogen receptor – their combined effects will be additive.
    • e.g. , butylbenzyl phthalate and di- n -butyl phthalate can add their effects to any natural estrogen present.
    • Synergism, however, has not yet been demonstrated.
  • Estrogen Receptor (Continued)
    • Hydroxylated PCBs: altered temperature dependent sex determination in turtles.
      • It has been suggested that PCBs and other combinations of weak environmental estrogens such as the insecticides dieldrin and toxaphene, have synergistic activities that are mediated by the estrogen receptor.
      • 160 to 1600 x more potent than either of the individual chemicals alone
      • other studies just suggest additive interactions / Still a controversy on this subject.
  • Estrogen Receptor (cont’d)
    • PCBs: learning and locomotor deficits in juvenile rodents and monkeys.
    • e.g. , Arochlor 1254  hypothyroidism and  testes weight and sperm production in rats 
    • Stunted growth
    • Cognitive deficits
    • Delayed eye opening
    • Hyperactivity
    • Auditory deficits
  • Individual Mechanisms
    • Binding but not activating the estrogen receptor (thereby acting as an anti-estrogen = ______________?
    • E.g. , Est receptor: tamoxifen is an anti-breast cancer drug, but is an agonist in preservation of bone mineral density.
    • E.g ., Androgen receptor antagonists (some insecticides) cause demasculinizing traits.
  • Individual Mechanisms
    • E.g ., Glucocorticoid receptor: antagonists adversely affect growth and development.
    • Mixed agonists/antagonists.
    • Binding and activation or lack of activation depends on the:
    • Affinity for the receptor
    • Receptor number
    • [xenoagonist]
    • [endogenous hormone]
    • The effects are predicted to be summed .
  • Individual Mechanisms
    • Binding other receptors ( e.g ., androgen receptors in males)
      • DDT: has been reported to induce reproductive abnormalities, particularly in wildlife species such as the American alligator, birds, and laboratory rodents.
      • The breakdown product of DDT, DDE, is able to act as an anti-androgen by blocking the testosterone receptor and producing effects that are phenotypically similar to those caused by estrogens.
  • Other Receptors (Continued)
    • Retinoid X receptor forms heterodimers with either the thyroid hormone receptor or with the peroxisome proliferator activated receptor and these heterodimers can bind to the estrogen response elements on DNA.
    • While these heterodimers are capable of binding to the estrogen response elements on DNA they are not capable of increasing gene expression.
  • Other Receptors (Continued)
    • Instead, binding of the heterodimers to the estrogen response elements results in decreased transcription due to competition with the estrogen receptor for the estrogen response elements.
    • The heterodimers may act as a specific inhibitor of estrogen receptor mediated gene transcription and ER-mediated signal transduction.
  • Other Receptors (cont’d)
    • All of this suggests that there is receptor cross-reactivity of the appropriate response elements.
    • Exogenous compounds have great broad receptor specificity across the nuclear receptor superfamily.
  • Individual Mechanisms
    • Modifying the metabolism of natural hormones
      • Endocrine disrupters may affect homeostasis through alterations in steroid synthesis or metabolism. Therefore, any substance that modifies either the enzymes involved in steroid biosynthesis and/or metabolism of steroids could be classified as an endocrine disrupter.
      • A number of compounds acting through the Ah receptor have demonstrated anti-estrogenic activities. TCDD is one of these compounds.
  • Individual Mechanisms: Modifying the metabolism of natural hormones
    • Some chemicals, e.g ., lindane and atrazine, can affect the metabolic pathway of estradiol  producing more estrogenic metabolites.
  • Individual Mechanisms: Modifying the metabolism of natural hormones
    • Effects of EDCs on Hormone Synthesis and Metabolism:
    • A cpd may adversely affect the levels of critical endogenous hormones by inducing or inhibiting biosynthetic or metabolic enzyme activity.
    • e.g ., phytoestrogens can interact with 17-β-DH, which regulates levels of
    • 17-β-estradiol or estrone.
  • Individual Mechanisms: Modifying the metabolism of natural hormones
    •  phytoestrogens can modulate overall estrogen levels in addition to acting as a ligand for the estrogen receptor.
    • E.g. , perchlorate competitively inhibits thyroid I uptake  disrupts thyroid hormone synthesis.
  • Timing of Exposure
    • Sensitivity of an individual to gonadal steroids depends on where (s)he is temporally in life.
    • Thus, a chemical may have little-to-no impact on a young/older adult, but may have profound development-disrupting effects if exposure occurs in utero or during puberty.
  • Timing of Exposure (cont’d)
    • E.g ., PCBs and dioxin affect development more during gestational than during lactational exposure.
    • Generally, sensitivity to EDCs is greater during fetal and perinatal exposure than during adulthood.
    • However, sometimes, fetal serum-binding proteins may protect the fetus from harmful EDCs (lower sensitivity).
    • e.g ., α-fetoprotein binding 17-β-estradiol protects the fetal male rat from maternal estrogens.
  • Timing of Exposure (cont’d)
    • IMPORTANT: Estrogen levels are NOT feedback regulated in a typical homeostatic mechanism (- feedback).
    • RATHER, there is a feed-forward mechanism (+ feedback) 
    •      [estradiol] throughout most of pregnancy in rodents and humans.
  • Timing of Exposure (cont’d)
    • Thus, an exogenous dose of any estrogen agonist will be additive to the endogenous level, be cause there is NO (-) feedback to  endogenous hormone production in a compensatory way.
  • Timing of Exposure (cont’d)
    • Also, in rodents and humans, the specific estradiol binding proteins, α-fetoprotein, test/estradiol binding protein also steadily  during pregnancy to protect the fetus from the high levels of circulating estrogens;
    • C.f ., rodent littermates influenced by neighbors’ gender.
  • Timing of Exposure (cont’d)
    • Xenoestrogens that fail to bind to these proteins have increased bioavailability and therefore, increased receptor-binding ability. But…,
    • E.g ., DES has ~100-fold lower affinity for est receptor than does E2.
    • E.g ., Fungal estrogen, zearalenone, is ~0.66% as potent as E2 for the uterotrophic responses in humans;
      • 5% in rats.
  • Individual Mechanisms: Modifying the metabolism of natural hormones
    • Other chemicals activate enzymes, which accelerate the metabolism of hormones, thereby disrupting their natural state.
    • E.g ., the testes contain specific enzymes to metabolize estrogens to a metabolite, which can no longer bind the estrogen receptor.
  • Individual Mechanisms: Modifying the metabolism of natural hormones
    • However, these enzymes are compromised by a xenoestrogen, metabolism  ,   exposure of the testes to estrogen  problems during fetal development.
  • Metabolism (Continued)
      • A number of genes have been shown to be induced by the Ah receptor including CYP1A1, CYP1A2, and CYP1B1, all of which are involved in the oxidative metabolism of 17B-estradiol.
      • Therefore, decreased estrogen bio-availability due to increased estrogen metabolism may explain some of the anti-estrogenic activities observed with Ah receptor ligands.
  • Individual Mechanisms
    • Modifying the number of hormone receptors in a cell (reduce or increase the number)
      • Following TCDD pretreatment, cells in culture display decreased estrogen receptor levels. Retinoids have also been shown to down-regulate estrogen receptor levels.
      • Therefore, any mechanism that significantly decreases estrogen receptor levels within the cell may limit the magnitude or duration of exposure.
  • Modifying the production of natural hormones
    • Chemicals can interfere with this process in the thyroid, immune, and nervous systems.
  • Individual Mechanisms
    • Interactions with steroid binding proteins
      • Proteins such as serum albumin, sex hormone binding globulin, and alpha-fetoprotein bind estrogens.
      • These proteins could play a role by decreasing the bio-availability of endocrine disrupters or, alternatively, by increasing the bio-availability of more potent estrogens through displacement of the endogenous hormones by the endocrine disrupter.
  • Specific Examples of Endocrine Disruption
    • Tributyltin:
      • Causes imposex and intersex in gastropod mollusks
      • Neogastropods have separate sexes but it was observed that many female dogwhelk from certain areas of the United Kingdom had a penis-like structure behind the right tentacle.
  • Tributyltin (Continued)
      • This was also seen later in other gastropod species in the eastern United States. These gastropods also had a vas deferens (sperm duct) and a convoluted gonoduct. The term “imposex” was coined to describe the superimposition of male characters onto females.
      • It was demonstrated that levels of imposex were elevated close to marinas, a feature attributed to the presence of anti-fouling paints.
  • Tributyltin
    • Results of experimental exposure of gastropods to TBT confirmed this idea.
      • The initial stages of imposex appear at a TBT [ ] of 1.0 ng/L and the syndrome is irreversible.
      • At [ ]s exceeding 5 ng/L vas deferens formation progresses further, overgrowing the genital papilla blocking the vulva and invading the oviduct. Females in this condition are sterile because egg capsules cannot be expelled.
  • Tributyltin
    • In some species, the appearance of a penis and sperm duct appears to cause little interference with reproductive activity of the affected female.
    • In other species the structure of the oviduct may be so modified that breeding is inhibited resulting in population decline and extinction.
  • Mechanism of TBT Action
    • Mollusks contain vertebrate-like steroids, and molluskan steroidogenesis is also similar to the vertebrate plan.
      • Cholesterol ----------- Androstenedione------------ testosterone or estrone
      • Testosterone can also be converted to 17-β-estradiol
      • It is hypothesized that TBT causes imposex and intersex in gastropods by interfering with steroid biosynthesis.
  • Mechanism (Continued)
    • Inhibition of CYP19A1 hypothesis:
      • In a study of gastropods exposed to TBT where the steroids were measured in the body, estrogen levels were not affected but testosterone levels were greatly increased. Inhibition of conversion of testosterone to 17-β-estradiol.
      • NET EFFECT:  Testosterone
  • Mechanism (Continued)
    • Probably a competitive type of inhibition and not a total inhibition of the CYP enzyme.
      • TBT and testosterone compete for the same CYP. With increased levels of TBT in the body, less testosterone is converted to 17-β-estradiol and more testosterone remains causing the imposex condition.
      • Using a known inhibitor of CYP19A1 also causes imposex in tested species.
      • NET EFFECT:  Testosterone
  • Mechanisms (Continued)
    • Second Hypothesis
      • Another hypothesis says that TBT metabolites compete with testosterone for sulfur conjugation (phase II reaction) and therefore less testosterone is excreted. Testosterone builds up in the body because of less sulphate conjugation and less excretion.
      • NET EFFECT:  Testosterone
  • Effects of PCBs on sex determination in Rainbow Trout:
    • Hatchling fish were exposed to Arochlor 1260 in 1, 5, and 20 mg/L solutions.
      • Animals in the 1 and 20 mg/L treatments accumulated the same amount of PCBs in the body (~ 2.1 to 2.5 μg/g tissue).
      • No effect of treatment on survival was observed and although more females were produced in the treatment groups compared to the control groups, the difference was not statistically significant.
  • PCBs (Continued)
    • Treatment groups did have a significantly greater proportion of grossly visible gonad abnormalities in females compared to controls (Lack of oocyte development).
    • Tissue concentrations of 2.1 to 2.5 μg/g tissue are typically found in nature and concentrations can reach 10 μg/g tissue near point sources of PCB exposure.
  • General Sexual Disruption in Fish:
    • Rivers and estuaries throughout the world are repositories for enormous amounts of industrial and domestic waste containing thousands of chemicals, both natural and human-made.
  • Sex Disruption (Continued)
    • In fish, exposure to estrogens and their mimics has been shown to cause the synthesis and secretion of vitellogenin, a female-specific protein, in male fish.
    • Male fish, which do not usually express the vitellogenin gene and hence have no vitellogenin in their plasma, were held in cages placed in effluent channels of sewage treatment works close to where effluent entered the river, and at various distances downstream, including sites where water was extracted for domestic use.
  • Sex Disruption (Continued)
    • After only one week in effluent, plasma vitellogenin concentrations had risen one thousand-fold or more, and by three weeks the concentrations were in the tens of mg/ml. At this time vitellogenin was the major blood protein comprising more than 50% of total plasma protein.
  • Second Study on Fish
    • Jobling et al. 1998 used wild populations of roach, a river fish found in the UK. Sampled fish both upstream and downstream of sewage treatment works on each of eight rivers. Control sites were lakes or canals that did not receive treated sewage.
      • Histological examination of the fish showed a large proportion of the males were in fact intersex, as defined by the simultaneous presence of both male and female gonad characteristics.
      • Incidence ranged from 4% in a laboratory group and at one control site to 100% at two effluent downstream sites.