Hepatic biotransformation responses in Atlantic salmon exposed to retinoic acids and 3,3′,4,4′-tetrachlorobiphenyl (PCB congener 77)

Active derivatives of vitamin A are essential in physiological processes such as cell growth, differentiation, morphogenesis and development. The biological functions of vitamin A are mediated through the retinoid acid receptors (RARs) and retinoid X receptors (RXRs). Aryl hydrocarbon receptor (AhR) agonists such as planar halogenated compounds are known to interfere with vitamin A homeostasis in both field and laboratory studies. In this study, we have investigated the molecular interactions between vitamin A and AhR signalling pathways using juvenile Atlantic salmon and agonists for both receptor pathways. Groups of juvenile salmon were treated with all-trans- and 9-cis-retinoic acid mixture (7:3 ratio) dissolved in DMSO (dimethyl sulfoxide) at 0.1, 1 and 10 mg/kg fish weight. The mixture was force fed singly or in combination with 0.1 mg 3,3′,4,4′-tetrachlorobiphenyl (co-planar congener 77)/kg fish weight dissolved in DMSO. Liver samples were collected 3 days after PCB-77 exposure. A separate group exposed to combined retinoic acid (1 mg/kg for 5 days) and PCB-77, was sampled at 3, 7 and 14 days after PCB-77 exposure. Liver samples collected from all exposure groups were analyzed for gene (RARα, AhR2α, AhR2β, CYP1A1, UGT1 and GSTπ) expression using real-time PCR and activity (7-ethoxyresorufin O-deethylase (EROD), UGT and GST) using biochemical methods with specific substrates. Our data showed that exposure to RA alone did not produce a significant increase of RARα mRNA levels, and the presence of PCB-77 attenuated the expression of RARα in RA dose- and time-specific manner. In addition, RA produced a dose-dependent increase of CYP1A1 mRNA and activity (EROD) levels without concomitant increase in AhR2 isoforms. When administered alone, PCB-77 produced increased CYP1A1, UGT1 and GSTπ mRNA and enzyme levels. The PCB-77-induced CYP1A1, UGT1 and GSTπ (mRNA and activity) levels were modulated by RA, in a parameter and dose-specific manner. In general, our data show an interaction between vitamin A and AhR signalling that may affect retinoid homeostasis in fish.