Page 377 - The Toxicology of Fishes

Liver Toxicity 357

heart, ovary, testis, and gut (Suzuki et al., 2000). Flounder VDRβ expression was found to be weak in
liver, whereas VDRα was absent. In both species, the strong expression in the intestine and gut signifies
a putative role in regulation of intestinal metabolic activities.
The presence of a high-affinity VDR in the sea lamprey suggests that regulation of calcium is not a
critical function of ancestral VDR (Whitfield et al., 2003). Supporting this claim is the observation that
VDR expression precedes bone formation during Xenopus development (Li et al., 1997). Furthermore,
in mammals, VDRs mediate critical functions other than calcium and phosphate regulation, including
immune function, skin development, cell proliferation, and catabolism of endogenous and exogenous
substrates. The latter two are of particular importance in this consideration of liver toxicology. The fact
that lamprey VDR is capable of binding and transcriptional activation of mammalian CYP3A4 promoter
demonstrates conservation in the DNA binding behavior of an early form of this receptor and a possible
role in endobiotic and xenobiotic metabolism in the livers of fish (Whitfield et al., 2003). More functional
data are needed regarding the role of VDR in endobioitc and xenobiotic metabolism. Interesting, like
PXR, VDR contains a large insert between helices 1 and 3. While significantly different in sequence
than PXR, this insert may be associated with novel low-affinity ligand binding/transactivation activities
of VDR, specifically given the differing physical and nutritional environments between aquatic and
terrestrial species.

Peroxisome Proliferator-Activated Receptor (PPAR) in Fish
Peroxisome proliferator-activated receptors are ligand-dependent transcription factors belonging to the
NR1C subfamily of nuclear receptors. PPARs typically bind natural and synthetic fatty acids and certain
pharmaceutical ligands. More recently, however, it has been demonstrated that select members of the
PPAR subfamily bind environmental contaminants such as phthalate monoesters and organotins, making
them putative mechanistic targets for endocrine disruption (Grun and Blumberg, 2006; Hurst and Wax-
man, 2004). PPAR nuclear receptors exist in three forms: PPARα, PPARβ, and PPARγ. The ligand-
binding affinities for each form differ, as does the tissue-specific expression (Lee et al., 1995, 2003;
Schoonjans et al., 1996). In teleosts, all homologs of all three mammalian-defined PPAR isotypes (α, β,
and γ) have been identified in both marine and freshwater fish, although differences in total number of
PPAR genes (variants of the three isotypes) and their tissue distribution exist in comparison to mammals
(Andersen et al., 2000; Batista-Pinto et al., 2005; Ibabe et al., 2002; Leaver et al., 2005; Maglich et al.,
2003; Robinson-Rechavi and Laudet, 2001). From the molecular perspective, teleost PPARs heterodimer-
ize with RXR and bind to peroxisomal proliferator response element (PPRE)-like direct repeat elements
in promoter/enhancer regions, resulting in altered expression of target genes similar to that observed in
mammals (Leaver et al., 2005). Structurally, the N-terminal A/B domains of the PPARs are the least
conserved regions between fish and mammals; however, DNA-binding domains and ligand-binding
domains demonstrate a high degree of sequence identities among fish, amphibians, and mammals,
although the LBDs are generally longer in fish (Andersen et al., 2000; Leaver et al., 2005). PPARγ from
pufferfish, salmon, flounder, and flatfish are unique in that they contain key amino acid differences within
the ligand-binding domains, thus preventing the binding of acidic ligands. It has been suggested that
fatty acids are not likely to activate fish PPARγ (Leaver et al., 2005; Maglich et al., 2003). This represents
a significant difference between teleostean and mammalian forms of this receptor and is perhaps reflective
of early adaptations of the receptor to specific demands present in aquatic environments.
In mammals (rat and mouse), PPARα activation leads to peroxisome proliferation (number and volume)
and concurrent transcriptional activation of genes involved in lipid homeostasis. Genes involved in fatty
acid uptake and binding (fatty acid binding protein [FABP]), fatty acid ω-oxidation (acyl-CoA oxidase
[AOX]), lipoprotein assembly and transport (various apolipoproteins), and others are all modulated by
PPARα (Desvergne and Wahli, 1999; Hamadeh et al., 2002; Martin et al., 1997; Schoonjans et al., 1996;
Yadetie et al., 2003) and thus are excellent targets of investigation in fish model systems. Although some
studies have suggested that fish are refractory to peroxisome proliferation (Baldwin et al., 1993; Pretti
et al., 1999), in vivo and in vitro experiments show that many fish species are responsive to PPARα
ligands such as the fibrate-based pharmaceuticals and other natural peroxisome proliferators (Donohue
et al., 1993; Haasch et al., 1998; Ruyter et al., 1997; Scarano et al., 1994; Yang et al., 1990). More