Differential expression of Cu/Zn superoxide dismutase mRNA during exposures to heavy metals in rockbream (Oplegnathus fasciatus)

A full-length complementary DNA clone (789 bp) encoding Cu/Zn superoxide dismutase (Cu/Zn SOD) was isolated from the liver cDNA library of a marine fish, rockbream (Oplegnathus fasciatus). The rockbream SOD cDNA contained a 462 bp open reading frame (ORF) encoding 154 amino acids. Deduced amino acid sequence of rockbream SOD shared relatively high identity with teleost (78–91%) and mammalian (64–70%) orthologues. The SOD transcripts were ubiquitously detected in all the tissues examined including brain, gill, intestine, kidney, liver, muscle, skin and spleen determined by RT-PCR and northern dot blot hybridization. Based on semi-quantitative RT-PCR, real-time PCR and/or scanning densitometry of RNA dot blot hybridization, the transcriptional level of SOD gene was differentially affected by various experimental exposures to heavy metals. Intraperitoneal injections (0, 1, 2, 5 or 10 mg CdCl2 per kg body weight for 48 h) and immersion treatments (0, 25, 50 or 100 μM of cadmium, copper or zinc for 96 h) revealed that (1) the expression of SOD at mRNA level was generally dependent upon the exposure doses of heavy metals, (2) the stimulation of SOD transcripts was more apparent in liver than kidney and gill, and (3) that cadmium was more potent to induce the expression of SOD than copper and zinc. Immersion exposures using high (25 μM Cd) and low (2 or 5 μM) doses of cadmium for prolonged periods up to 14 (high dose) or 28 (low doses) days also showed that the levels of SOD transcripts were higher with longer durations and higher concentrations. Results from the present study indicate that assessment of transcriptional activity of fish SOD gene at mRNA level might be useful as a sensitive molecular biomarker system to address the oxidative stress caused by heavy metal pollutants in fish.