Page 636 - The Toxicology of Fishes

616 The Toxicology of Fishes

is inherited. Recent studies indicate genetic differences between AhR1 in New Bedford Harbor and
reference mummichog. AhR1 in New Bedford Harbor vs. reference mummichog differed genetically
but not functionally with respect to binding capacity and affinities for dioxin, suggesting that these
differences cannot account for differences in dioxin sensitivity between populations (Hahn et al., 2004).
In addition to AhR1, there appear to be differences in the regulation of AhHR in New Bedford Harbor
and reference fish. AhHR is believed to negatively regulate transcriptional activity of the AhR (Karchner
et al., 2002; Mimura et al., 1999). DLCs appear to induce AhHR in reference fish but not in New Bedford
Harbor fish (Karchner et al., 2002), suggesting that upregulation of AhHR is not involved in resistance.
Recently developed molecular tools are being used to explore mechanisms by which mummichog
populations have adapted to DLCs; for example, the promoter region of CYP1A was cloned and
characterized functionally as containing xenobiotic or dioxin response elements (XREs) and glucocor-
ticoid-response elements (GREs) (Powell et al., 2004). Sensitive, specific polyclonal antibodies were
developed for AhR1, AhR2, and AhR repressor protein for mummichog (Merson et al., 2006). Merson
and coauthors (2006) described the application of these antibodies to demonstrate the occurrence of
AhR pathway proteins in heart, brain, ovary, and other tissues of mummichog. Recent evidence suggests
that AhR2 plays an important role in DLC toxicity in zebrafish (Prasch, 2003) and may also do so in
mummichog. Taken together, the body of work by Hahn and colleagues has provided a great deal of
information on components of the AhR pathway in mummichog and other species, including important
clues to the molecular basis for inherited tolerance (genetic adaptation) to DLCs.
Interesting parallels in mechanisms of tolerance have been revealed in studies of another estuarine
fish species resident to estuaries contaminated with DLCs. Populations of Atlantic tomcod (Microgadus
tomcod) inhabiting the Hudson River differ from populations inhabiting less contaminated sites in their
responsiveness to toxic pollutants (Wirgin and Waldman, 1998). Like mummichog, tomcod resides in
6
or close to natal estuaries throughout its life cycle and are found in large (10 ) populations (Yuan et al.,
2006). Tomcod residing in the Hudson are highly contaminated with DLCs and PAHs and (in some
years) exhibit a high prevalence of tumors (Cormier and Racine, 1990). Tomcod collected from the
Hudson River and depurated in the laboratory exhibit reduced responsiveness to AhR agonists (Courtenay
et al., 1999). In one study, responsiveness was regained in progeny from these fish (Roy et al., 2001);
however, recent evidence indicated that Hudson River progeny (F and F ) were resistant to waterborne
2
1
PCBs at concentrations that are environmentally relevant to the field site and toxic to reference fish
(Chambers et al., 2003). In addition, early developmental gene expression differed in Hudson River
tomcod vs. reference populations when similarly exposed to DLCs (Carlson et al., 2003). Specifically,
CYP1A and AhRR were not induced in F and F Hudson River tomcod embryos exposed to DLCs
2
1
(Carlson et al., 2003). Taken together these findings suggest that Atlantic tomcod of the Hudson River
estuary have developed resistance to DLCs but not to PAHs, as demonstrated by refraction to CYP1A
inducibility (Yuan et al., 2006). This resistance is at least in part heritable (Roy et al., 2001; Yuan et al.,
2006) and likely reflective of resistance to the early-life-stage effects of DLCs (Yuan et al., 2001).
Furthermore, these tolerant populations occupy a large spatial region, much of which is highly contam-
inated (200 miles of the Hudson is a Superfund (Yuan et al., 2006).

Other Evidence Involving CYP1A Suppression

From the examples given above, suppression of CYP1A is a common feature of mummichog inhabiting
DLC contaminated sites; however, the role of CYP1A and its contribution to toxicity are still unclear
and likely system dependent (Schlezinger et al., 2006). Concurrent inhibition of CYP1A, for example,
reduces DLC toxicity in mummichog embryos (Wassenburg and Di Giulio, 2004) but not in zebrafish
morpholino knock-downs for CYP1A (Carney et al., 2004). Downregulation of CYP1A has been
observed in isolated hepatocytes during exposure to oxyradicals (Barker et al., 1994). Oxyradicals can
be produced during CYP-dependent biotransformation of contaminants and through redox cycling (see
Chapter 6). Reactive oxygen species are produced in marine fish exposed to dioxin-like PCB congeners
and are associated with AhR-mediated CYP1A uncoupling and subsequent inhibition (Schlezinger et
al., 2006). This uncoupling is characteristic of substrates such as DLCs but unlike some PAH AhR
agonists that bind tightly to CYP1A and that are slowly metabolized (Schlezinger et al., 2006). In fish