Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 233

To tal [14C] residue data over time in mussels

30

25
20

Exposure Clearance

15

10
5
0

0 5 10 15 20 25 30

To tal time (days)

FIGURE 9.10
Concentration of radiolabeled estradiol in tissues of mussels, Mytilus edulis . Mussels (both sexes) were exposed
to 10 ng L -1 E 2 14 C for 13 days (exposure period) and then placed in clean water for 12 days (depuration period).
Concentration of radiolabeled E 2 in tissues of mussels was determined by liquid scintillation.

days and the tissue concentration then remains stable, suggesting that these mussels, in
the environment, would accumulate and hold large amounts of hormones in their tissues;
they thus would allow the transfer of contaminants to potential consumers.
These experiments have also revealed that mollusks are able to rapidly metabolize estra-
diol into various estradiol esters, which are biologically inactive on the hormone receptor.
This esterification of steroids appears to be common among various mollusk species (De
Souza and De Oliveira 1976; Janer et al. 2005a,b, 2006) and is catalyzed from C16 fatty acids
in M. edulis (Labadie et al. 2007). Because the activity is lost, quantification performed
using the YES assay does not reflect the total amount of steroid hormones accumulated
in mussels. To be more accurate, saponification of the extracts that allows the steroids to
be released should be performed. Such measurements made using the YES assay after
saponification of mussel tissue extracts can multiply by 5 to 10 the amounts of steroids
obtained without saponification.
It should be noted that the esterification of estradiol is unlikely to be able to protect a
potential consumer of mussels since saponification takes place in the digestive tract, and
the steroids can probably be easily absorbed. The total concentrations measured in marine
mussels M. edulis sampled in the port of Le Havre (near the outlet of the sewage treat-
ment plant) can reach 100 ng estradiol equivalents per gram of tissue. This concentration is
among the highest concentrations currently reported in the scientific literature.
Steroid hormones are active at very low concentrations in vertebrates where their role
is well documented. In contrast, the role of sex steroids in bivalves is still a subject of
debate. The cloning of genes encoding the receptor for estradiol and vitellogenin (VTG)
in M. edulis and the study of their expression in the presence of steroid hormones did not
show a significant action of these compounds on the expression level of the cloned genes
(Puinean et al. 2006). Measurements of the levels of expression of these genes in mussels
collected at different sites more or less exposed to estrogenic contaminants (including
Le Havre) showed no clear relationship between these variables. This suggested that the
genes studied may not be (at least directly) under the control of sex steroids, in contrast
to what is known in vertebrates. Alternatively, mussel could only be sensitive to E 2 action
during a short period in their reproductive cycle. Indeed, under laboratory exposure, a
significant increase in VTG and estradiol receptor mRNA expression was observed in