Regulation of metallothionein gene expression by oxidative stress and metal ions

The metallothioneins (MT) are small, cysteine-rich heavy metal binding prot eins which participate in an array of protective stress responses. Although a single essential function of MT has not been demonstrated, MT of higher eukaryotes evolved as a mechanism to regulate zinc levels and distribution within cells and organisms. These proteins can also protect against some to xic metals and oxidative stress-inducing agents. In mice, among the four kn own MT genes, the MT-I and -II genes are most widely expressed. Transcripti on of these genes is rapidly and dramatically up-regulated in response to z inc and cadmium, as well as in response to agents which cause oxidative str ess and/or inflammation. The six zinc-finger metal-responsive transcription factor MTF-1 plays a central role in transcriptional activation of the MT- I gene in response to metals and oxidative stress. Mutation of the MTF-1 ge ne abolishes these responses, and MTF-1 is induced to bind to the metal res ponse elements in proximal MT promoter in cells treated with zinc or during oxidative stress. The exact molecular mechanisms of action of MTF-1 are no t fully understood. Our studies suggest that the DNA-binding activity of MT F-1 in vivo and in vitro is reversibly activated by zinc interactions with the :inc finger domain. This reflects heterogeneity in the structure and fu nction of the six zinc fingers. We hypothesize that MTF-1 functions as a se nsor of free zinc pools in the cell. Changes in free zinc may occur in resp onse to chemically diverse inducers. MTF-1 also exerts effects on MT-I gene transcription which are independent of a large increase in MTF-1 DNA-bindi ng activity. For example, cadmium, which has little effect on the DNA-bindi ng activity of MTF-1 in vivo or in vitro, is a more potent inducer of MT ge ne expression than is zinc, The basic helix-loop-helix-leucine zipper prote in, USF (upstream stimulatory factor family), also plays a role in regulati ng transcription of the mouse MT- gene in response to cadmium or H2O2 Expre ssion of dominant negative USF-1 or deletion of its binding site from the p roximal promoter attenuates induction of the mouse MT-I gene. USF apparentl y functions in this context by interacting with as yet unidentified protein s which bind to an antioxidant response element which overlaps the USF bind ing site (USF/ARE). Interestingly, this composite element does not particip ate in the induction of MT-I gene transcription by zinc or redox-cycling qu inones. Thus, regulation of the mouse MT-I gene by metals and oxidative str ess involves multiple signaling pathways which depend on the species of met al ion and the nature of the oxidative stress. BIOCHEM PHARMACOL 59;1:95-10 4, 2000. (C) 1999 Elsevier Science Inc.