Page 382 - The Toxicology of Fishes

362 The Toxicology of Fishes

hormones T and T . Sodium-independent transport is regulated by the adenosine triphosphate (ATP)-
3
4
dependent superfamily of transporters (e.g., MDR1/ABCB1, MDR3/ABCB4, MRP2/ABCC2, BSEP/
ABCB11) and the multispecific organic anion and cation transporters (e.g., OATPs/SLC21A, OCTs).
Of these, MRP2, which transports primarily glutathione and glucuronyl conjugates, including the phys-
iologically important conjugates of bilirubin, estradiol, and xenobiotic metabolites (Faber et al., 2003;
Keppler et al., 1997), is the major determinant of bile-salt-independent bile flow and the only known
MRP-related canalicular transporter.
Expression of transporter proteins is regulated by multiple and complex biofeedback mechanisms,
involving gastrointestinal hormones and peptides such as secretin, vasoactive intestinal peptide (VIP),
bile salts within the bile pool, intermediate bile acid metabolites, nuclear receptors, and the cholinergic
nervous system (Boyer 1996; Trauner and Boyer 2003; Trauner et al., 2000). Bile acids and salts
themselves function as specific ligands for nuclear hormone receptors where they regulate transporter
expression via transcriptional events (Chawla et al., 2001).

Fish Studies
Although we currently have only a nascent understanding of piscine hepatobiliary transport mechanisms,
it is becoming clear that certain transport mechanisms may be conserved in vertebrate evolution; for
example, basolateral uptake of bile acids by trout hepatocytes, as well as the disruption of these transport
processes by toxicants, has been demonstrated (Rabergh et al., 1992). Likewise, a sodium-independent
carrier system that mediates taurocholic acid and cholic acid transport has been identified in trout
(Rabergh et al., 1994). Hepatocellular uptake of bile acids in trout was found to resemble corresponding
systems in mammalian liver cells, although trout carriers were distinguished by high efficiency at low
temperatures. Below is a comparative summary of known transporters in fish.

MDR-Encoded Transporters
Several highly conserved members of the MDR1 (P-glycoprotein [Pgp]) transporter family have been
identified in rainbow trout (Sturm et al., 2001b) and winter flounder (Pleuronectes americanus) (Chan
et al., 1992). Immunohistochemical studies have also shown conserved distribution of Pgp epitopes in
the tissues of guppy (Poecilia reticulata), epitopes that resemble mammalian forms (e.g., react with
mammalian antibodies) (Hemmer et al., 1998). Other studies have identified MDR-like proteins. Local-
ization experiments using immunohistochemical techniques with an antibody prepared against human
Pgp revealed positive reaction at the bile canalicular pole in Fundulus heteroclitus hepatocyte cultures
(Albertus and Laine, 2001). Similarly, a Pgp-related protein of approximately 170 kDa was demonstrated
in the intestine and liver of catfish (Doi et al., 2001). Several studies using isolated fish hepatocytes have
demonstrated transport activities exhibiting characteristic Pgp-mediated transport (Albertus and Laine,
2001; Sturm et al., 2001b; Tutundjian et al., 2002). Pgp reversal agents, for example, have been shown
to preclude the transport of Pgp substrates in teleost hepatocytes. In rainbow trout hepatocytes, both the
accumulation and efflux of the fluorescent Pgp substrate (rhodamine 123) were modified by the reversal
agents verapamil, cyclosporin A, and vinblastine, as well as the ATPase inhibitor vanadate (Sturm et al.,
2001b). In contrast, tetraethylammonium chloride, a substrate for type I sinusoidal organic cation uptake
+
systems and electroneutral canalicular H /organic cation antiporter, had no effect on rhodamine 123
transport (Sturm et al., 2001b). In vitro preparations of isolated killifish and turbot hepatocytes and in
vivo studies in carp and catfish have demonstrated the uptake of fluorescent Pgp substrates that proved
to be verapamil and estradiol (E2) sensitive (Albertus and Laine, 2001; Kleinow et al., 2004; Smital and
Sauerborn, 2002).

Organic Anion-Transporting Polypeptides (OATPs)
Organic anion-transporting polypeptides have been cloned and functionally characterized in the skate
(Raja erinacea) (Cai et al., 2002). Phylogenetic and sequence comparisons with a liver-specific OATP
isolated from hepatocytes indicate that skate OATP is most closely related to OATP-F (SLC21A14) from
human brain (50.4% identity) and rat OATP4, which is also called Lst1 (Slc21a10; 41.2% identity) (Cai