Transcript FACTEURS DE COMPLEXITE DANS L’ETUDE DES …

Chemical pollutants of the food
chain.
Catherine Viguié CR INRA
Type of contaminations
ENVIRONNEMENT
(water/soil/air)
Végétal
Animal
Human
Pollutants evolution in the
environnement
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Different pathways for molecule chemical
transformations
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Abiotic (oxidation – light-unduced)
Biotic (through alive organism from bacterias to
and vegetal organisms)
Consequences:
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From one initial molecule to numerous metabolites
Inactivation (liver metabolism)
Bioactivation (metaboliste more toxic than the first
molecule)
Modulation of toxicity (1)
Transport mechanisms through the
Physiological barriers
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Passive diffusion
Active transporters (specifics)
Efflux pumps
Physical barriers (tight junctions)
ABSORPTION
(Digestive tract)
Determining step for blood
concentrations:
global exposure
Exposure of target tissues
Brain
Placenta-Foetus
Potential for toxicity
Competition for
transporters
Modulation of toxicity (2)
Plasma transportation
In the blood the molecule can be free or
bound
Binding can occur with specific or non
specific transporters
Limiting factor for clearance
mechanisms
highly bound molecules to specific
transporters (binding proteins) :
high potential for bioaccumulation
Potential Toxicity
Competition with specific binding
protein of endogenous molecule
such as hormones will be
associated to an increase in
hormone clearance
Modulation of toxicity (3)
METABOLISM
Phase I (cyt P450): enzymes
Phase II
Elimination (kidney – liver)
BIOACTIVATION vs.
DETOXIFICATION
Limiting factor for the
elimination of the
xenobiotic
•Bioaccumulation
Potential toxicity
•Competition/inhibition of
enzymes
•Induction of enzymes
Mechanisms and sites of
action
Endocrine disruptors
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Metabolism of hormones
Transportation
Receptors
Hormone synthesis
Pathogens (bacterias- parasites)
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Resistance to therapeutic agents
Central nervous systems
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Neurodegenerative diseases
Alteration of the development of the central nervous system
Cancers
Effects are dose and time
dependent
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Oral contamination
very low doses
+
Long period exposure
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Mecanisms of action
Critical period
The relevance of animal model for
the risk analysis of food contaminant
for human health

Transport mechanisms
through the Physiological
barriers

placenta
 efflux pumps
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Metabolism of the toxic
Physiology of the altered
function:
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Plasma binding
Neuroregulation
Hormone metabolism
=
All these phenomenon =
causes for interspecies
differences in the
sensitivity to toxic
effects of xenobiotics =>
Need for relevant model
for human from the
standpoints of:
 the metabolism of the
xenobiotic
 the regulatory scheme
of the function
The thyroid function
HOT spot 2
Hypothalamus
TRH
Pituitary TSH
Thyroïd
TPO
TG
NIS
Clearance
-
bound TH
(T3 T4)
blood
free T3, T4
HOT spot 1
Hot spots: debates on the relevance of animal models
HOT spot 3
Whole body / all life effects
The relevance of animal model: analysis
of the case of the evaluation of fipronil as
a thyroid disruptor
Fipronil and thyroid disruption in the rat
Hypothalamus
Fipronil
TRH
Clearance
Anterior pituitary
Thyroid
Increased T4
clearance
// hepatic
enzyme
induction
TSH
-
0.8
*
T3, T4 bound
PL
T3, T4 free
(mL/min/kg)
0.6
0.4
0.2
0
Solvant
Fipronil
The relevance of animal model: analysis
of the case of the evaluation of fipronil as
a thyroid disruptor
With is the pathophysiological scheme of action of fipronil as a thyroid
disruptor considered as non relevant to human?
Fipronil
T3, T4 bound
Bound T4:
TBG
TTR
Albumine
73%
19%
8%
PL
53%
36%
11%
T3, T4 free
0% in adult
85%
15%
Clearance
TBG expression:
•protects TH from
peripheral
elimination
•Pool of TH
The sheep as a good model to study thyroid disruptors?
To
be
OR
?
Not
to
be
The question is :
What is the relevance of
animal models for an
endocrine system that exhibits
multiple interspecies
particularities in its regulatory
scheme ?
The relevance of animal model: analysis
of the case of the evaluation of fipronil
as a thyroid disruptor
The protective role of specific thyroid hormone binding protein TBG
•Free T4 fraction (%)
•T4 half-life (Days)
•TBG
T4 Dissociation constant (nM)
T4 Maximal binding capacity(nm/l)
•TTR
T4 Dissociation constant (nM)
T4 Maximal binding capacity(nm/l)
0.02
2
0.04
5-9
0.07
0.5
0.112
0.105
NA/adult
7.14
4494
6.25
3230
2.78
3968
160
266
The relevance of animal model: analysis
of the case of the evaluation of fipronil
as a thyroid disruptor
Does the effect of fipronil on thyroid function differ between rat en
sheep accordingly to the assumes protective role of TBG?
THX+ T3
Fipronil
0.6
40
20
0
0
10
20
30
*
0.4
0.2
0.04
240
0.03
160
(mL/min/kg)
0.8
Total T4
(ng/ml)
60
Vehicle
(mL/min/kg)
Total T4 (ng/ml)
80
80
0
0
Vehicle
0
Fipronil
25
50
75
Temps (h)
YES
100
125
0.02
0.01
0.00
before after
The relevance of animal model: analysis
of the case of the evaluation of fipronil as
a thyroid disruptor
Is the interspecies difference in TBG expression the only explanation
for the diffrential effect of fipronil on thyroid function between rat
and sheep?
The role of fipronil metabolic pathways
Plasma concentrations (ng/mL)
10000
Fipronil administrations
Fipronil
sulfone
1000
Sulfone
1000
100
100
Fipronil
Fipronil
10
0
5
10
Time (days)
15
20
10
0
5
10
15
20
Temps (j)
25
30
35
The relevance of animal model: analysis
of the case of the evaluation of fipronil
as a thyroid disruptor
Is the interspecies difference in TBG expression the only explanation for the
diffrential effect of fipronil on thyroid function between rat and sheep?
The role of fipronil metabolic pathways
Sulfone/FIP>100
1000
100
Fipronil
Sulfone
10000
Plasma concentrations
(ng/ml)
Plasma concentration
(ng/ml)
10000
Sulfone/FIP= 4
1000
100
Fipronil
Sulfone
Hypothesis: transformation of fipronil in fipronil sulfone (hapatic cytochromes) =
bioactivation relative to potential thyroid toxicity. Sensitivity to fipronil as a thyroid
disruptor is modulated by hepatic metabolism of fipronil (‡ between species)
Conclusion
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Never forget the physiology (function &
metabolism)
The relevance of experimental animal model
should always be addressed carefully
Necessity to develop and adapt these models
to allow long term low dose exposure studies
relevant to human exposure
The need for physiologically-based models
allowing a global assessment of endocrine
function with a predictive value and
mechanistical outlets.