Specific expression of cytochrome P4501A1 gene in gill, intestine and liver of tilapia exposed to coastal sediments.

Toxicological effects of persistent organic pollutants (POPs) in aquatic ecosystems lead to the deterioration of water quality and adversely affect fish and human health. The highly lipophilic nature of these pollutants may enter fish through the diet or by water-borne exposure. In monitoring contamination in aquatic systems, induction of the cytochrome P450 1A1 (CYP1A1) gene of fish has been evaluated as a sensitive, "early warning" method. The objective of the present study was to characterize the induction of the gene in fish upon exposure to coastal sediments and to determine its specific expression in liver and extrahepatic organs (i.e. gill and intestine) in which the toxicological effects to the corresponding tissues could be addressed. Sediment samples were collected from different sites, including Victoria Harbour (VS), Ma Wan (MW), Tsim Bei Tsui (TBT) and Mai Po (MP). The samples were analyzed for polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). CYP1A1 mRNA expression was measured in juvenile tilapia exposed experimentally to coastal sediment for 3 and 7 days. A negative control group of fish maintained in seawater was used. Using the primer dropping polymerase chain reaction technique, gill, intestinal and hepatic CYP1A1 mRNAs were quantified. Chemical analysis shows that the samples from VS contaminated with the highest concentration of PCBs (45.24 p.p.b.) and PAHs (1663.7 p.p.b.), followed by MW (16.01 and 347.7 p.p.b.), TBT (14.48 and 235.2 p.p.b.) and MP (14.60 and 242.2 p.p.b.). Fish exposed to sediments were contaminated with various levels of PCBs (VS, 64.14-72.06 p.p.b.; MP, 27.06-31.62 p.p.b.; TBT, 27.29-33.92 p.p.b.; MW, 16.05-17.76 p.p.b.) and PAHs (VS, 124.7-304.9 p.p.b.; MP, 97.57-164.1 p.p.b.; TBT, 25.38-98 p.p.b.; MW, 24.07-68.13 p.p.b.). The control fish displayed moderate expression of CYP1A1 mRNA in liver (1.45 arbitrary units), gill (1.21 arbitrary units) and intestine (0.56 arbitrary units). Following sediment exposure, there was a large increase in CYP1A1 mRNA in intestine and liver but no significant changes in gill tissues. In the first 3 days, there was anywhere from 34-96 and 69-156% induction of the CYP1A1 transcripts in intestine and liver, respectively. Following 7 days of exposure, a continued induction of high level of CYP1A1 mRNA in intestine (73-157%) was observed. The induction of CYP1A1 in liver and intestine provided a defensive mechanism against POPs entering from the external environment.