Impairments of Endocrine Functions: Causes and Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 193

numerous surfactants, plasticizers, pesticides, and pharmaceuticals still widely used at
present.
Estrogenic activities are elicited through several mechanisms of action. Xenoestrogens
can bind to mammalian intracellular estrogen receptors (ERs). After binding and nuclear
translocation, they can directly act as ligand-inducible transcription factors, and specifi-
cally regulate the expression of target genes. Such a mechanism is generally referred to
as the genomic or “classical” pathway of estrogen action (Porte et al. 2006). Some can pro-
duce effects similar to those of estrogens without being mediated by the ER. They bind
other receptors able to interact with estrogen responsive elements, or can go through
other receptor and/or signal transduction pathways, including interactions with a vari-
ety of binding globulins, growth factors, different receptor systems, and/or steroidogenic
enzymes (Gillesby and Zacharewski 1998). One example of such a pathway is the binding
of xenoestrogens (as PCBs or dioxins) with the aryl hydrocarbon receptor (AhR), which is
involved in Cyp P450 activity, and 17β-estradiol metabolism (Gillesby and Zacharewski
1998; Navas and Segner 1998).
The effect of xenoestrogens is dependent on both the dose and the developmental stage at
which exposure occurred. Numerous studies have demonstrated that exposure to estrogens
at critical times during fetal development may irreversibly influence responsive tissues as
well as subsequent growth. These effects may be manifested as structural, functional, or
long-term pathologic changes that are not limited to one sex (McLachlan and Arnold 1996).
Hormonal-mimic activities (estrogenic, androgenic) of pure chemicals and of environ-
mental mixtures such as wastewater, river water, or sediment pore water can be searched
for by using in vitro cellular tests. Several tools exist and are implemented to detect mul-
tiple hormonal activities in environmental samples (Van der Linden et al. 2008; Swart et al.
2011). They are based on steroid-responsive reporter cell lines developed for human cells
[E-Screen, CALUX (Chemically Activated Luciferase EXpression)], or yeast. Briefly, these
tests are based on tumoral cells proliferation (MCF-7, CEF) (White et al. 1994) or by mak-
ing use of the fact that steroid receptors are transcription factors that induce transcription
of target genes after binding to specific DNA sequences in their promoter. These DNA
sequences are linked to the gene of a readily measurable protein, such as β-galactosidase
in yeast assay (McLachlan and Arnold 1996; Routledge and Sumpter 1996) or firefly lucif-
erase in ER and AR CALUX assays (Sonneveld et al. 2005).
One of the most commonly used in vivo biomarkers of steroidogenesis impairment is
the quantification of VTG. VTG is the major precursor of egg yolk proteins, which provide
energy reserves for embryonic development in oviparous organisms. In mature females,
VTG is generally synthesized in response to endogenous estrogens, whereas in males, the
VTG gene, although present, is normally silent. However, it may be activated by (xeno)
estrogens (Matozzo et al. 2008). EDC-induced changes in VTG may be examined either at
the level of the gene by using polymerase chain reaction (PCR) (Biales et al. 2007; Nadzialek
et al. 2010) or at the level of the protein by using the enzyme-linked immunosorbent assay
(ELISA) test or by liquid chromatography tandem mass spectrometry (LC-MS/MS) as
recently proposed by Simon et al. (2010).
8.3.2 Antiestrogenic Activities
Xenobiotics can exert antiestrogenic activities on aquatic vertebrates, as has been reported
in several fish species exposed to AhR agonists. AhR is a ligand activating transcription fac-
tor that controls the activities of several genes encoding phase I and II biotransformation
enzymes such as cytochrome P450 1A1, and mediates sex steroid hormone-related actions.