Role of metallothionein in carcinogenesis.

Metallothionein (MT) is a low-molecular-weight protein (6800 Da) and one-third of its amino acids are cysteine residues. The 20 cysteines coordinate 7 metal atoms (zinc, copper, and/or cadmium). This protein is extremely inducible by metals as well as a number of organic compounds. MT is though to be an important intracellular storage site for zinc and possibly other essential trace elements. In addition, tolerance to cadmium toxicity is often due to the induction of MT, which sequesters cadmium and lowers its concentration at critical intracellular sites. Recently it has been proposed that MT might play important roles in several aspects of the carcinogenic process. In this context a symposium was held recently on this topic at the 1993 Annual Society of Toxicology Meeting. At this symposium Dr. Cherian discussed the expression of MT in various human tumors and its use as a potential marker of tumor differentiation or cell proliferation. Dr. Imura provided data illustrating that induction of MT can be used as an adjunct in cancer chemotherapy, in preventing toxicity caused by gamma-irradiation or cisplatin (CDDP) and other chemotherapeutics. Induction of MT has been suggested to be an important mechanism of resistance of tumor cells to chemotherapeutic agents, such as CDDP. This is controversial, and various views on this topic were presented by Drs. Howell, Lazo, and Koropatnick. Dr. Waalkes then discussed the role of MT in the carcinogenic and anticarcinogenic effects of metals.