Page 855 - The Toxicology of Fishes

Reproductive Impairment of Great Lakes Lake Trout by Dioxin-Like Chemicals 835

It is true that interactions between the zfAHR2 and hypoxia signaling pathways were observed in
zebrafish (Prasch et al., 2004a), but there was no evidence to suggest this as a mechanism of TCDD
toxicity. In addition, the zfahr2 and zfarnt1 morpholino studies demonstrated that both zfAHR2 and
zfARNT1 are essential for the generation of developmental toxicity in zebrafish. The requirement for
both proteins indicates that interactions of zfAHR2 with other signaling pathways or removal of
zfARNT1 from other signaling pathways cannot be mechanisms mediating TCDD toxicity and suggests
that the toxicity is more likely dependent on AhRE binding and alterations in gene expression caused
by the zfAHR2 and zfARNT1 heterodimer. Nevertheless until specific genes and signaling pathways
and their roles in mediating specific endpoints of TCDD developmental toxicity in zebrafish have been
clearly identified, mechanisms of cross-talk with other signaling pathways cannot be ruled out.

Structure–Activity Relationships for PCB,
PCDD, and PCDF Congeners as AhR Agonists
The AhR-mediated toxicity of PCBs, PCDDs, and PCDFs varies with the degrees of chlorination and
patterns of chlorine substitution on the aromatic rings. There are 75 possible PCDD, 135 PCDF, and
209 PCB congeners, and the toxic potencies of these congeners vary by several orders of magnitude.
The congeners are most potent when they have lateral chlorine substitutions that make them approximate
isostereomers of 2,3,7,8-TCDD (Safe, 1990). PCDD and PCDF congeners with additional chlorination
beyond the 2, 3, 7, and 8 positions or the loss of chlorine at either of these four positions decreases
potency of the congener. The PCB congeners that are most potent have chlorine substitutions in the meta
and para positions (positions 3, 4, or 5) that allow them to maintain a planar conformation. These non-
ortho-substituted, planar PCB congeners include PCB 77, 81, 126, and 169. Chlorine substitutions in
the ortho positions (positions 2 and 6) limit the planar conformation of the congener, and consequently
mono-ortho-substituted PCB congeners are less potent in producing TCDD-like effects and di-ortho-
substituted PCB congeners are inactive as AhR agonists. The mono-ortho substituted PCBs are far weaker
AhR agonists in fish than in mammals and birds (Gooch et al., 1989). Mono-ortho-substituted PCBs
are essentially inactive in causing early-life-stage mortality in rainbow trout (Walker and Peterson, 1991;
Zabel et al., 1995a). This is probably due to reduced intrinsic efficacy of the mono-ortho PCBs as AhR
agonists (Hestermann et al., 2000) in causing sac fry mortality rather than a pharmacokinetic difference,
because mono-ortho PCBs are eliminated from rainbow trout larvae at essentially the same rate as planar
PCBs which cause early-life-stage mortality (Zabel et al., 1995a).

TCDD Toxicity Equivalency Factor Approach

The TCDD toxicity equivalency factor (TEF) approach is used to characterize the risk of toxicity for
organisms exposed to mixtures of PCBs, PCDDs, and PCDFs. TCDD is recognized as the most potent
AhR agonist in vertebrates and therefore has been adapted as the prototype for estimation or calculation
of relative toxicological potency (Safe, 1994). Congener-specific TEFs describe the potency of individual
PCB, PCDD, and PCDF congeners relative to TCDD in causing AhR-related toxicity. The TEF approach
assumes that congeners act by the same AhR-mediated mechanism and interact additively to produce
toxicity. Because there are significant differences in the relative potency (REP) of certain AhR agonists,
such as the mono-ortho-substituted PCBs which are essentially inactive in causing AhR-mediated early-
life-stage toxicity in fish compared to birds and mammals (Walker and Peterson, 1991; Zabel et al.,
1995a), it has been necessary for the World Health Organization (WHO) to derive fish-specific, bird-
specific, and mammal-specific TEFs for individual PCB, PCDD, and PCDF congeners for use in
ecological risk assessment (Table 21.1) (van den Berg et al., 1998).
Toxicity equivalency factors permit calculation of the AhR-related potency resulting from a mixture
of PCB, PCDD, and PCDF congeners in an organism, or organ, based on chemical concentrations and
the vertebrate class-specific potency values (TEFs). More specifically, REPs describe the potencies of
individual PCB, PCDD, and PCDF congeners that are AhR agonists relative to TCDD. REPs are relative