History of Biomarkers - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 19

leading to translocation of AHR from the cytoplasm to the nucleus. Within the nucleus,
Hsp90s are released, and AHR heterodimerizes with another protein, the Aryl Receptor
Nuclear Translocator (ARNT). The AHR-ARNT complex then binds to multiple enhancer
elements in the promoter region of responsive genes in the AHR battery such as CYP1A.
The P450 enzymes, involved in the detoxification of xenobiotics, are slightly expressed
under normal physiological conditions, but are on the other hand strongly inducible: their
content or their activity is increased in response to one or more exogenic molecules. The
biological advantage of this induction process by xenobiotics is generally to amplify their
metabolic degradation. Nelson regularly publishes a review of P450 cytochromes accord-
ing to their families and subfamilies (drnelson.uthsc.edu/CytochromeP450.html). As of
February 2009, more than 8100 distinct CYP gene sequences have already been known.
The nomenclature used for cytochrome P450s is based on sequence homology (Nebert
and Nelson 1991): two cytochrome P450s belong to the same family when their peptide
sequence presents more than 45% amino acid homology and to the same subfamily if the
homology is higher than 55%. The abbreviation CYP (cytochrome P450 gene) is completed
with a number representing the family, then a letter indicating the subfamily (e.g., CYP4A),
and a last number when there are several genes within the same subfamily (e.g., CYP4A1,
CYP4A2). Conventionally, genes are written in italics CYP1A1 (Goksøyr and Förlin 1992),
whereas mRNA and proteins are in capitals. Nelson (1998) has developed a classification
scheme where CYP families are classified into CLANS, that is, clusters of higher order
groupings of P450 families.
They are ubiquitous proteins, the presence of which was demonstrated in plants and
animals, from bacteria to mammals. P4501A1 enzymes (in particular, EROD measured
in fish) may be induced by compounds sterically analogous to dioxin such as aromatic
hydrocarbons, polychlorinated biphenyls, and polychloroazobenzenes. The first work on
EROD and other P450 enzymes as biomarkers was completed on freshwater and marine
fish livers (Addison 1984; Addison and Payne 1987; Flammarion et al. 1998). Polycyclic
aromatic hydrocarbons (PAHs) induce P4501A1 in all fish considered by different authors
from agnathans to teleosts and selachians (Stegeman 1987; Andersson and Nilsson 1989).
CYP1As are induced by PAHs, coplanar PCBs, polychlorinated dibenzodioxins, and
polychlorinated dibenzofurans (Goksøyr and Förlin 1992), which are pollutants of the
3-methylcholanthrene type and are now considered AH receptor agonists. Three enzyme
activities, EROD, ethoxycoumarin O -deethylase, and arylhydrocarbon (B[ a ]P) hydroxylase
are largely specific in their response to these compounds. Many PAHs are both induc-
ers and substrates for CYP1A. In contrast, coplanar PCBs, although often good inducers,
are frequently poor substrates for CYP1A (Di Giulio et al. 1995). In their review, Goksøyr
and Förlin (1992) reported that CYP2B is induced by coplanar PCBs (phenobarbital type),
CYP3A by endogenous steroids, and CYP4A by endogenous fatty acids and xenobiotics
such as phthalates and peroxisome proliferators (Simpson 1997). Therefore, members of
the cytochrome P450 family of monoxygenases can metabolize and often produce more
toxic forms from (see below) a wide variety of endogenous molecules and xenobiotics.
In contrast to fish, the presence of the AH receptor is not confirmed in mollusks. The
cytochrome P450 pathway in PAH metabolism in mussels is low compared to the radical
manner which leads to the formation of quinones. However, the existence of a CYP1A-like
gene in mussels (Wootton et al. 1995) justifies research into the mechanisms of activation
and detoxification already identified in fish. The capacity to metabolize in vitro B[ a ]P into
derived diol, quinone, and phenol was demonstrated in the mussel Mytilus galloprovincialis
(Michel et al. 1993). The activity of B[ a ]P hydroxylase BPH, measured in the digestive gland
of this mussel (measurement based on the production of phenol metabolites resulting from