Cadmium-induced earthworm metallothionein-2 is associated with metal accumulation and counteracts oxidative stress

Metallothionein-2, a ubiquitous metal-inducible protein, is involved in direct metal sequestration and is associated with detoxification and metal accumulation in eukaryotic organisms. In this study, higher levels of cadmium (Cd) accumulation were detected in earthworms exposed to higher Cd concentrations or longer exposure times. However, the bio-concentration factor (BCF), derived from the ratio of the Cd levels in the earthworms to that in soil, indicated that earthworms could relatively efficiently remove Cd from soil with low Cd contamination levels. In addition, earthworm recombinant metallothionein-2 effectively eliminated oxidative DNA damage in vitro and was probably able to counteract DNA damage caused by Cd, as shown by the comet assay in earthworm coelomocytes. Because earthworm metallothionein-2 proteins were evaluated as possible biomarkers for heavy-metal pollution in soil, the metallothionein-2 genes of six Megascolecidae earthworms and one Glossoscolecidae earthworm were cloned and sequenced using mRNA differential display. Based on the resolved earthworm metallothionein-2 nucleotide sequences, quantitative PCR was used to investigate the metallothionein-2 transcription levels from three earthworm species (Metaphire posthuma, Polypheretima elongata and Pontoscolex corethrurus) in response to different Cd concentrations or exposure times. The result demonstrated that Cd treatments elevated metallothionein-2 transcription in a dose- and time-dependent manner. In conclusion, the induction of earthworm metallothionein-2 would not only provide cellular protection against Cd-induced oxidative stress but also increase Cd accumulation consistent with a potential role in bioremediation.