Page 376 - The Toxicology of Fishes

356 The Toxicology of Fishes

following exposure to the potent mammalian CYP2B/CYP2C inducers phenobarbital and 1,4-bis[2-(3,5-
dichloropyridyloxy)]benzene (TCPOBOP) (Elskus and Stegeman, 1989; Kleinow, 1990). In mammals,
induction of CYP2B proceeds through activation of CAR (Honkakoski and Negishi, 1997; Honkakoski
et al., 1998a,b). Curiously, efforts to clone CAR from numerous teleosts have proven unsuccessful, and
the mining of several teleost genomes suggests an obvious lack of CAR. In agreement with genome
sequencing, experimental evidence has demonstrated that in teleost species only a single PXR/CAR-like
gene is present. Iwata et al. (2002) identified a single immunoreactive protein from scup liver cytosol
with CAR-like antigenic properties; however, no induction of CYP2-like protein or translocation of such
protein was observed following treatment with TCPOBOB. Phylogenetic analysis suggests that NR genes
appear to be subject to strong purifying selection, particularly in the DNA-binding domains (Krasowski
et al., 2005b); however, estimates of the ratio of the nonsynonymous to synonymous nucleotide substi-
tutions for the NR1I subfamily indicate that only PXR and CAR have undergone recent positive selection.
Current convention suggests that CAR arose with duplication of a premammalian PXR (Krasowski et
al., 2005b). It is most likely that teleosts lack a CAR-like receptor. In fishes, however, other members
of the NR1I family (PXR and VDR) may demonstrate broader ligand profiles. An alternative hypothesis
is that CAR has been lost in some teleosts species.

Vitamin D Receptor (VDR) in Fish
The vitamin D receptor (VDR) has been cloned from a number of teleosts, including flounder, trout,
zebrafish, salmon, carp, sea bass, and medaka. Recently, VDR was also cloned from the jawless agnathan
lamprey, which represents the most ancient lineage among extant vertebrates (Whitfield et al., 2003).
Prior to the cloning of VDR in aquatic organisms, general convention held that the 1,25(OH) D –VDR
3
2
system originated in terrestrial animals. It is well known that vitamin D is one of the oldest hormones
and is present in the earliest life forms, including phytoplankton and zooplankton (Holick, 2003a,b).
Workers have questioned the significance of the capacity for vitamin D production in early eukaryotes,
and one hypothesis is that vitamin D photoproducts serve as a photon sink, protecting DNA, RNA, and
proteins from the damaging effects of high-energy ultraviolet radiation.
Certain fish are very rich sources of vitamin D as compared to higher vertebrates, which have
insignificant amounts of this vitamin. This high level of vitamin D is found both in teleosts and in the
cartilaginous elasmobranchs, signifying that high levels are not related to skeletal calcium. This finding
led to speculation that function of vitamin D and the VDR in fish may be different from its known
classical functions in terrestrial animals (Rao and Raghuramulu, 1999a,b). Moreover, endocrine physi-
ology in fish has adapted to an external aquatic environment that provides a constant source of calcium
ion; thus, the mechanisms regulating calcium mobilization may differ between aquatic and terrestrial
organisms and between marine and freshwater fish species (Power et al., 2002). To date, however, the
role of vitamin D in calcium regulation in teleosts remains equivocal. Studies in different fish species
are contradictory, with a majority of evidence suggesting that VDR does not classically mediate calcium
mobilization from fish enterocytes (Larsson et al., 2003; Nemere et al., 2000; Rao and Raghuramulu,
1999a). These results may reflect the enormous diversity in teleost physiology.
Recently, in mammals, VDR had been demonstrated to act as a low-affinity receptor for the secondary
bile acid lithocholic acid (Makishima et al., 2002). This action may act as a hepatoprotective mechanism
similar to that observed with FXR and PXR for the metabolism and export of toxic bile acids. To date,
VDR exhibits receptor cross-talk with PXR for the transcriptional regulation of the broad-specificity
hepatic and intestinal phase I enzymes CYP2B6, CYP2C9, and CYP3A. Thus, other than the high-
affinity endocrine effects, VDRs exhibit considerable overlap in metabolic transcriptional targets. The
role of teleost VDR in regulating hepatic and intestinal metabolizing enzymes has not been fully
investigated. Like many teleost genes, two VDR genes (VDRα and VDRβ) are observed in some fish
species. In concordance with subfunctional portioning theory (Postlethwait et al., 2004), the tissue
distributions of the VDRα and VDRβ forms are quite different. In pufferfish, VDRα exhibits a ubiquitous
tissue distribution, including strong expression in brain, gill, heart, and to a lesser extent liver (Maglich
et al., 2003). Pufferfish VDRβ, on the other hand, is localized solely in the gut. The distribution of both
VDRα and VDRβ in the Japanese flounder is widespread, with high expression levels in the gill, brain,