Metallothionein gene expression in fish cell lines: its activation in embryonic cells by 5-azacytidine.

We have investigated the regulation of metallothionein gene expression in two fish cell lines. Rainbow trout hepatoma (RTH) cells synthesized metallothionein in response to heavy metal exposure. The maximum level of metallothionein synthesis detected during zinc exposure was much greater than during cadmium exposure. The time-courses of metallothionein synthesis were different for the different metal inducers, suggesting that metallothionein may be differentially regulated by cadmium and zinc in these cells. The metal-induced synthesis of metallothionein was correlated with increased translational activity and accumulation of metallothionein-mRNA, suggesting that metallothionein may be regulated at the transcriptional and post-transcriptional levels in RTH cells. Chinook salmon embryo (CHSE) cells, unlike RTH cells, did not synthesize metallothionein or metallothionein-mRNA in response to heavy metal exposure. However, when these cells were treated with 5-azacytidine prior to heavy metal exposure, the synthesis of metallothionein was induced, suggesting that DNA methylation may play a role in metallothionein gene expression in fish.