Endocrine Disrupting Compounds

1
Endocrine Disrupting Compounds

• Any exogenous agent that causes adverse health
  effects in an intact organism, or its progeny,
  consequent to changes in endocrine function.

2
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

3
Mechanisms (Continued)

• Binding other receptors (ex. 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

4
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

5
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.

6
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.

7
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.

8
Individual Mechanisms

• Binding but not activating the estrogen receptor
  (thereby acting as an anti-estrogen

9
Individual Mechanisms

• Binding other receptors (ex. 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.

10
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.

11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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.

18
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.

19
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.

20
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
  17B-estradiol
• It is hypothesized that TBT causes imposex and
  intersex in gastropods by interfering with
  steroid biosynthesis.

21
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 17B-estradiol.

22
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 17B-estradiol and
  more testosterone remains causing the imposex
  condition.
• Using a known inhibitor of CYP19A1 also causes
  imposex in tested species.

23
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.

24
Effects of PCBs on sex determination in Rainbow
Trout

• Hatchling fish were exposed to Aroclor 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 ug/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.

25
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 ug/g tissue
  are typically found in nature and concentrations
  can reach 10 ug/g tissue near point sources of
  PCB exposure.

26
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.

27
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.

28
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.

29
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.