Transcript Endocrine Disrupting Compounds

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 At Least 4 Modes of Action
1.








Pesticides
Herbicides
Fungicides
Plasticizers
Surfactants
Organometals
Halogenated PAHs
Phytoestrogens
2.
3.
4.

Serving as steroid receptor
ligands.
Modifying steroid hormonemetabolizing 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 antiestrogen
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) receptormediated 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-nbutyl 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 antiestrogen = ______________?
E.g., Est receptor: tamoxifen is an antibreast 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, αfetoprotein, 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 receptorbinding 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 bioavailability due to increased estrogen
metabolism may explain some of the antiestrogenic 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-βestradiol
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 humanmade.
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.