Page 624 - The Toxicology of Fishes

604 The Toxicology of Fishes

Biochemical Mechanisms and Models That
Contribute to Our Understanding of Tolerance

Biological resistance to drugs and toxic chemicals has been studied in diverse organisms because of
issues relating to drug resistance in the medical field (e.g., cancer, infectious diseases) and issues related
to insecticide resistance in the fields of agriculture, forestry, and public health. To our knowledge, direct
cause (molecular alterations) and effect (development of resistance) relationships have not been estab-
lished in the less widely studied fish systems; however, similarities likely exist in mechanisms underlying
resistance across phyla (Bunger et al., 2003; Daborn et al., 2002; Doehmer et al., 1993; Gottesman,
2002; Oakeshott et al., 2003; Shaw, 1999; Taylor and Feyereisen, 1996). Studies in the more highly
explored mammalian and insect systems have provided the basis for hypothesizing and testing associa-
tions between molecular responses and chemical tolerance in fish. The toxicity of all chemicals (drugs
and toxicants) is dependent on their interaction with cell targets and receptors. General mechanisms by
which tolerance is achieved reflect alterations in these targets or receptors or activation of pathways that
reduce the amount of chemical reaching the target/receptor (Table 13.3). Modification of a toxicity target
can result in reduced damage to the target by the chemical. DDT resistance in insects appears in some
cases to involve modification of ion channels that represent toxicity targets for this pesticide. Tolerance
might also be achieved by modification of a cellular receptor that normally recognizes and interacts with
the chemical, triggering a signal transduction cascade that leads to a toxic response. In this case,
modification of the receptor can result in reduced toxicant-receptor interaction, producing a concomitant
reduction in toxicity. Alterations in one or more of the signal transduction components that become
activated by toxicant–receptor interaction could also contribute to tolerance. Resistance can also result
from upregulation (induction) of proteins involved in toxicant sequestration or binding. Other commonly
studied resistance mechanisms involve altered regulation of proteins and enzymes involved in toxicant
biotransformation (detoxification or activation) or efflux (elimination). Several widely studied molecular
responses addressed in studies of pollution-tolerant fish fall into the framework provided in Table 13.3.
Examples of underlying mechanisms that have been addressed in resistant fish include altered toxicant–
receptor interaction (e.g., aryl hydrocarbon receptor and related signal transduction pathway), seques-
tration (e.g., metallothionein), biotransformation (e.g., cytochrome P450, glutathione transferases), and
efflux (e.g., P-glycoprotein). A brief overview of these systems will provide a better understanding of
the role they might play in toxicity resistance.

Aryl Hydrocarbon Receptor

Most of the toxic effects of DLCs in vertebrates are believed to be mediated by their binding to the aryl
hydrocarbon receptor (AhR) and activation of the AhR signal transduction pathway (Birnbaum, 1994;
Bunger et al., 2003; Kirkvliet et al., 1995; Thurmond et al., 1999) (see Chapter 5). AhR signaling and
toxicity involve passage of the DLC–AhR complex into the nucleus (Hahn, 2001; Karchner et al., 2002;
Mimura et al., 1999). In the nucleus, DLC–AhR becomes associated with the AhR nuclear translocator
(ARNT) to form a DLC–AhR–ARNT complex. This complex has a high affinity for xenobiotic responsive
elements (XREs) located in the promotor region of DLC responsive genes. AhR-mediated toxicities are
believed to result from altered or inappropriate transcriptional regulation of proteins involved in cell
growth, regulation, development, and immune function (Birnbaum, 1994; Kerkvliet, 1995, Toomey et
al., 2001; Van den Heuvel and Lucier, 1993).
Strong evidence indicates a close relationship between DLC–AhR strength of binding and degree of
toxicity of individual DLCs; for example, the most toxic DLC known, TCDD, binds with higher affinity
than other DLCs. PCB 126 (3,3′,4,4′,5-pentachlorobiphenyl) and PCB 77 (3,3′,4,4′-tetrachlorobiphenyl)
are among the most potent AhR agonists and most toxic PCB congeners. Invertebrates lacking an AhR
that binds DLCs (Hahn, 1998) or strains of mice that carry a defective AhR (Fernandez-Salguero et al.,
1996) are relatively insensitive to DLCs. Two forms of AhR (AhR1 and AhR2) have been identified in
some fish species, including a species for which DLC-adapted populations have been identified, Fundulus
heteroclitus (Hahn et al., 2000).