Page 710 - The Toxicology of Fishes

690 The Toxicology of Fishes

Olsson et al., 1995). Regarding the latter, sexually mature female fish again may be poor choices to
conduct biomarker studies with MT because they possess enhanced hepatic and serum concentrations
of vitellogenin, the zinc-containing egg yolk precursor protein. Consequently, in certain species such as
trout, MT levels tend to be repressed by estradiol in sexually mature females (Olsson et al., 1995). This,
however, does not appear to be the case in channel catfish, which did not show any statistical differences
in MT levels between sexes following chronic (10-week) copper exposures (Perkins et al., 1996).
As mentioned above, other factors also tend to induce MT, especially physiological processes that
induce cytokine production or oxidative damage, such as inflammation or general stress. Many of these
processes related to general stress also involve the redistribution of metals as well; for example, induction
of cortisol tends to cause a redistribution of zinc and copper to the liver which has been shown to induce
MT expression (Schlenk et al., 1998). Because MT can be an indicator of acute as well as chronic stress,
it is imperative that other acute-phase proteins be measured to verify an acute stress condition (see later
discussion on heat shock proteins). Moreover, handling of animals should be minimized to enhance the
signal-to-noise ratio between exposed and control samples. This is especially important when conducting
cage studies, where animals should be maintained for periods to allow acclimation to occur.
Several investigators have indicated that simple tissue residue measurements of metals would provide
the same information as MT and be a better indicator of exposure and possibly effect. A study of channel
catfish by Perkins et al. (1996) compared various whole-animal endpoints such as body length, weight,
liver weight, and condition factors to hepatic MT expression and copper content after a 10-week exposure
to copper. They found that MT protein expression had a significantly stronger correlation to each endpoint
than hepatic copper content; in addition, strong correlations were observed between lipid peroxidation
and MT in trout chronically exposed to zinc and copper (Farag et al., 1995). These data suggest that
MT may also be useful as a biomarker of effect, but only effects mediated by MT-binding metals. Studies
examining the effects of low-level arsenical exposure demonstrated dose-dependent increases in MT
expression in channel catfish but failed to deplete hepatic glutathione or induce lipid peroxidation
(Schlenk et al., 1997). Conversely, studies performed in PLHC-1 cells showed a direct correlation
between glutathione depletion and MT induction following cadmium exposure (Schlenk and Rice, 1998).
Reasons for this discrepancy are unclear, but future studies with nonbinding inducers may help us to
better understand the functional roles of MT and promote its use as an indicator of effect. For discussion
about the potential uses of MT as a biomarker of exposure and effect, see Stegeman et al. (1992). For
discussions regarding measurement methods, see Schlenk and Di Giulio (2002).
Few studies have examined the role of MT as a biomarker of susceptibility. Several groups have
examined species differences in MT expression and have shown that fish with higher basal concentrations
or who are more efficient inducers tend to be protected from metal toxicity (Kille and Olsson, 1994;
Kille et al., 1991). Benson and Birge (1985) demonstrated that fish residing within metal-contaminated
environments had higher MT levels than the same species in a reference pond and that the animals from
the metal-contaminated site were more resistant to cadmium and copper. Genetic knockout (MT-null)
mice not only are more susceptible to metal toxicity and oxidative stress (Masters et al., 1994) but are
also obese, indicating involvement in energetics or basal metabolism (Kling and Olsson, 2005). Clearly,
more studies are necessary to determine whether individuals within populations may be more or less
susceptible to toxicity based on their ability to express MTs.

Stress Proteins
Stress proteins, originally known as heat shock/stress proteins (HSPs), are a nonspecific group of positive
acute-phase proteins that serve several protective and homeostatic functions within the cell. This dis-
cussion here focuses primarily on HSP70, HSP30, HSP60, and HSP90. The number and exact size of
heat shock proteins are specific to both tissue and species. Except for the highly inducible proteins of
the HSP70 family (specifically, HSP72), all of these proteins are present in low concentrations under
normal conditions in most organisms studied (Iwama et al., 1998; Sanders, 1990, 1993). Proteins of the
HSP70 and HSP60 families are primarily chaperone proteins that are upregulated during proteolysis and
aid cells in the folding and transport of newly synthesized proteins. They have been characterized in
several fish species and consistently respond to numerous organic and inorganic chemicals (Iwama et