Page 276 - The Toxicology of Fishes

256 The Toxicology of Fishes

appears to be the case in zebrafish, in which AhR2 has been implicated as the form responsible for
nearly all of the developmental toxicity of TCDD (see below). Thus, although in mammals the single
AhR (AhR1 ortholog) is required for TCDD toxicity during development (Mimura et al., 1997), in
zebrafish it is the AhR paralog (AhR2) that plays this role. Whether this is specific to zebrafish or is
true generally in fish remains to be determined. In addition to AhR1 and AhR2, cartilaginous fishes
possess a novel form of AhR designated AhR3 (Merson and Hahn, 2002). The functional characteristics
of elasmobranch AhRs have not yet been established.
Some fish possess multiple forms of ARNT. Zebrafish share with mammals the presence of two ARNT
genes—ARNT1 and ARNT2—and in both cases ARNT1 appears to be the toxicologically most relevant
partner for AhR (Prasch et al., 2004, 2006; Sekine et al., 2006; Walisser et al., 2004). In other species
of fish, only ARNT2 has been identified (Pollenz et al., 1996; Powell et al., 1999).
Recent studies using targeted knock-down of gene expression to study the toxicity of AhR ligands
illustrate the power of the zebrafish model in mechanistic research. Using morpholino antisense oligo-
nucleotides (MOs), AhR2 was shown to play a key role in the developmental toxicity of TCDD
(Antkiewicz et al., 2006; Bello et al., 2004; Dong et al., 2004; Prasch et al., 2003; Teraoka et al., 2003)
and in the ability of TCDD to inhibit fin regeneration (Mathew et al., 2006). The role of AhRs in the
toxicity of PAHs and other nonhalogenated ligands is more complex, with both AhR2-dependent and
AhR2-independent effects (Billiard et al., 2006; Incardona et al., 2004, 2005, 2006). MO-based knock-
down has also helped illuminate the role of CYP1A in the toxicity of PHAHs and PAHs. Because CYP1A
induction is the most well-known and most striking (in terms of magnitude) result of AhR activation,
an important role for CYP1A in TCDD toxicity has been hypothesized, possibly involving the generation
of reactive oxygen species (ROS) (Schlezinger et al., 1999; Teraoka et al., 2003); however, zebrafish
embryos in which CYP1A expression and induction are prevented or reduced by injection of a CYP1A-
MO are just as sensitive as uninjected fish to the developmental effects of TCDD (Carney et al., 2004).
In contrast, CYP1A knock-down enhances the developmental toxicity of the PAH-like AhR agonist
β-naphthoflavone (BNF) and PAH-containing mixtures (weathered crude oil) but provides partial pro-
tection against the toxicity of the PAH pyrene (Billiard et al., 2006; Incardona et al., 2005). CYP1A,
then, appears to have a protective role with respect to some nonhalogenated compounds but is involved
in the bioactivation of others.
The possible involvement of CYP induction in the toxicity of AhR ligands is complicated by the
existence of other AhR-regulated CYP1 enzymes in fishes: CYP1B1, CYP1B2, CYP1C1, and CYP1C2
(Godard et al., 2005; Leaver and George, 2000; Wang et al., 2006). The role, if any, of these enzymes
in PHAH and PAH toxicity has not yet been investigated.

Other Receptors and Ligand-Activated Transcription Factors
Aryl hydrocarbon receptors have been studied extensively in fishes and serve as an example of how
receptors—and the genes they regulate—participate in mechanisms of toxicity and how this may be
investigated using the approaches and tools available in fish biology. A variety of other receptors and other
types of transcription factors activated by xenobiotics are also involved in fish toxicology (Table 5.1),
although in many cases a molecular understanding is just beginning to emerge. We briefly mention a few
of these other transcription factors and some key features with regard to their presence and function in fishes.

Nuclear Receptors
Aryl hydrocarbon receptors are not unique among receptors in displaying diversity among taxa. Other
receptors that are targets of endocrine-disrupting compounds, such as estrogen receptors and other members
of the nuclear receptor (NR) superfamily, exhibit such diversity (Baker, 2005; Bardet et al., 2002; Hawkins
et al., 2000, 2005; Thornton, 2001). Although mammals possess two estrogen receptors (ERα and ERβ),
fish possess an ERα and two ERβ forms (ERβa and ERβb), the result of the fish-specific genome
duplication mentioned above (Hawkins et al., 2000; Menuet et al., 2002). Similarly, although mammals