The protein thiol metallothionein as an antioxidant and protectant against antineoplastic drugs

Metallothioneins (MTs) are major zinc-binding protein thiols that are readily inducible and whose functions remain unclear. Recent evidence supports a role for MT as an antioxidant. Mechanisms underlying this function may include direct interception of free radicals, complexation of redox sensitive transition metals, altered zinc homeostasis or interaction with glutathione (GSH). MT overexpression after direct gene transfer in cultured cells, decreases cytotoxicity, to partially reduce reactive oxygen and nitrogen species and markedly attenuates intracellular oxidation of reporter molecules including dichlorofluorescein and cis-parinaric acid. Conversely, enhanced intracellular oxidation is seen in cells derived from mice lacking both functional MTI and MTII genes. GSH levels are unaffected in MT null cells relative to wildtype, suggesting the antioxidant function of MT is independent of GSH. In tumor cells there is at least a 400-fold range in MT levels and a 10-fold difference in the ratio of nuclear to cytoplasmic distribution. No correlation exists between MT levels and GSH levels demonstrating the autonomous regulation of intracellular thiol pools. This may be important for cancer chemotherapies since MT overexpression is seen in human tumor cells with acquired drug resistance. The authors found no evidence for altered MT isoform profiles in drug resistant cells that overexpress MT. Recent evidence suggests MT subcellular location may dictate functionality and MT may help determine the threshold for apoptosis. Thus, MT is a stress-inducible protein with antioxidant attributes that may participate independently or in conjunction with GSH to protect cells against injurious agents.