Page 193 - The Toxicology of Fishes

Biotransformation in Fishes 173

In teleosts, the effects of exposure to reproductive hormones on CYP3A expression differ depending
on species. Numerous studies with rainbow trout, brook trout, medaka, and other fish species have
demonstrated that exposure to 17β-estradiol results in a suppression of total microsomal CYP content.
This correlated with decreased CYP3A mRNA expression, CYP protein levels, or steroid hydroxylase
activities (Buhler et al., 2000; Celander et al., 1989; Pajor et al., 1990). Treatment of male Atlantic cod
with 17β-estradiol resulted in an increase in CYP3A protein expression (Hasselberg et al., 2004). The
mechanisms of hormonal regulation of CYP genes in teleosts have yet to be determined; however, it is
possible that circulating hormone levels (steroids or growth hormone) are associated with sexual dimor-
phic differences in CYP expression in fish.
Numerous pharmaceuticals and xenobiotics have been demonstrated to alter CYP3A gene transcription
in mammals via binding and transactivation of members of the nuclear receptor family NR1, including
the pregnane X receptor (PXR), the constitutive androstane receptor (CAR), and the vitamin D receptor
(VDR) (Pascussi et al., 2003; Plant and Gibson, 2003). As mentioned above, although Maglich et al.
(2003) did not find CAR in the pufferfish genome, they identified a single PXR/CAR gene that was
more PXR like, indicating CAR may have evolved after divergence of fish or may have been lost in
some or all teleosts. To date, PXR and VDR have been identified in few teleost species. Prototypical
mammalian NR1 (PXR) receptor agonists, including dexamethasone (DEX), rifampicin (RIF), and
pregnenolone-16α-carbonitrile (PCN), are seemingly less effective at altering teleost CYP3A gene
transcription (Celander et al., 1989, 1996a,b; Kullman, unpublished data). In tilapia, PCN treatment
resulted in twofold induction of CYP3A proteins (Pathiratne and George, 1996). The ligand-binding
region of PXR was isolated from zebrafish (Kliewer et al., 2002). This region was shown to be activated
by some prototypical mammalian PXR agonists, including nifedipine, phenobarbital, clotrimazole, and
some steroids, but PCN, DEX, and RIF did not activate the fish receptor (Moore et al., 2002). Induction
of hepatic CYP3A expression in vivo by ketoconazole was observed in juvenile rainbow trout and Atlantic
cod (Hasselberg et al., 2004; Hegelund et al., 2004). As mentioned above, response to 17β-estradiol
resulted in either enhanced or diminished CYP3A expression, depending on the species examined (Buhler
and Wang-Buhler, 1998; Hasselberg et al., 2004; Husoy et al., 1994; Pajor et al., 1990). Furthermore,
xenoestrogens also affect CYP3A expression. Male Atlantic cod exposed to alkylphenols showed
enhanced hepatic CYP3A expression, whereas treatment with ethinylestradiol and nonylphenol sup-
pressed CYP3A expression in Atlantic salmon and Atlantic cod (Arukwe et al., 1997; Hasselberg et al.,
2004). Slight increases in hepatic CYP3A expression have been demonstrated in rainbow trout exposed
to cortisol, whereas cortisol treatment had no effect on CYP3A protein levels in Arctic char (Salvelinus
alpinus) (Celander et al., 1989; Jorgensen et al., 2001).
Although gene sequences for PXR and VDR have been identified in fish (Kliewer et al., 2002), few
functional data exist regarding their role in transcriptional activation of CYP3 genes. Identification of
cognate hormone response elements upstream of pufferfish and medaka CYP3A genes suggest that
nuclear receptors may be involved in transcriptional regulation; however, this has yet to be determined
(Kullman, pers. commun.). Cloning and analysis of lamprey VDR suggest that this nuclear receptor is
capable of binding and transcriptional activation of the mammalian CYP3A4-DR3 hormone response
element. These results demonstrate conservation in the DNA binding behavior of an early form of this
receptor and a possible role in transcriptional activation of CYP genes in fish. (For more on nuclear
receptors in teleosts, see Chapter 7). Modulation of teleost CYP3A expression following xenobiotic
exposure is highly variable and species dependent.
Studies examining the binding behavior of mammalian PXR have demonstrated transactivation and
CYP3A induction by xenoestrogens, including bisphenol A, nonylphenol, DDT, and other organochlorine
pesticides (Courmoul et al., 2002; Masuyama et al., 2000; Takeshita et al., 2001; You, 2004). The response
of aquatic species to these and other PXR, CAR, and VDR ligands has been minimal; thus, the overall
induction of CYP3A in teleosts appears to be considerably weaker than that reported in mammals.
Compared to mammalian species, this suggests that teleost CYP3A expression may be governed by
alternative transcriptional mechanisms. Given the recent completion of several teleost genome projects,
examination of promoter sequences will shed some light on regulatory elements and transcriptional
control of theses genes. In several studies, discrepancies have been noted between changes in gene
expression and changes in steroid hydroxylase activity. In a study by Hasselberg et al. (2005), exposure