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their metabolization capacities with a decrease in EROD and GST activities and an increase
in UDP-glucuronosyl transferase activity (Bloom et al. 2000). These decreases in enzyme
activities are considered to be a strategy of the fish that lowers energy costs to deal with
stress-induced energy demands.
3.3.4 Stress Proteins
In response to cellular stress, so far the only known universal system is the induction
of a protein family called stress proteins (HSP 90 or stress 90, HSP 70 or stress 70, chap-
eronin 60, stress proteins with low molecular weight: 16-24 kDa), which has been highly
conserved through evolution (Feige et al. 1996; Sonna et al . 2002; Gross 2004). These stress
proteins are able to repair those proteins damaged by stress, or eliminate them when they
cannot be further repaired. They act as molecular “chaperones,” supporting, monitoring,
and protecting other proteins (see reviews by Frydman 2001; Hartl and Hayer-Hartl 2002;
Wang et al. 2004). Moreover, the induction of stress proteins is maintained over time, mak-
ing them relevant for use as biomarkers (Bierkens 2000).
Initially, HSPs were given this name as their synthesis is induced when cultured cells
or whole organisms are exposed to elevated temperature. Among HSPs, the HSP70 family
members are the most investigated for their characterization and induction in response
to numerous environmental stressors in a range of species (Morimoto et al. 1992; Clark
and Peck 2009). Currently, literature data provide numerous examples of stress protein
induction in various animal, plant, and bacteria species, in response to an exposure to
environmental or chemical stress, although a few counterexamples have been reported
(see reviews by De Pomerai 1996; Bierkens 2000; Mukhopadhyay et al. 2003). Assuming
that stress proteins play a protective role against a wide variety of stress agents, is their
induction in response to a specific stress linked to the development of tolerance to any
subsequent stress? The first example demonstrated both in vivo and in vitro was that of
“thermo-tolerance,” defined as the ability of a cell or an organism to resist heat stress
after exposure to a sublethal heat shock. It has been clearly established that the induction
threshold of HSP is correlated with the stress level experienced by species in their natural
habitat; reflecting the significance of the “thermal history” of a particular species through-
out its evolution, and suggesting that HSPs are ecologically relevant for use by a species to
improve its tolerance to heat stress (Fangue et al. 2006 and literature cited therein).
In addition, examples of “cross tolerance” to various stresses acquired after a heat shock
have been observed. For example, this happens to be the case in daphnia ( Daphnia magna ),
which exhibit tolerance after exposure to a usually lethal dose of malathion following
heat pretreatment (Bond and Bradley 1997). In mussels ( Mytilus edulis ), heat pretreatment
involves an induction in HSP 70 concentrations and increased resistance to cadmium
(Tedengren et al . 2000). In organisms living in environments subjected to natural or chemi-
cal stress, the role played by stress proteins in the acquisition of tolerance to an additional
stress may vary according to the species and/or population. In oysters Crassostrea virginica
originating from three sites differing in their thermal regimes, overall HSP and MT pat-
terns were similar in oysters from the three geographically distant populations (Ivanina
et al. 2009). HSP levels were lower in Cd-exposed organisms than in their control counter-
parts during heat stress, suggesting that both stressors may have partially suppressed the
cytoprotection up-regulation of molecular chaperones. Synergistic interactions between
the effects of metals and heat could lead to a reduced tolerance to heat in metal-exposed
organisms (Sokolova and Lannig 2008). However, mussels ( M. edulis ) adapted to low salin-
ity levels in the Baltic Sea—at the limits of their geographical distribution—had lower
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