Divergent consequences arise from metallothionein overexpression in astrocytes: zinc buffering and oxidant-induced zinc release.

Excessive accumulation of the heavy metal zinc is cytotoxic. As a consequence, cellular vulnerability to zinc-induced injury may be regulated by the abundance of proteins that maintain intracellular free zinc concentrations ([Zn2+]i). In this study, we overexpressed the zinc-binding protein metallothionein-II (MT) in astrocytes to assess its impact as (1) an acute zinc buffering mechanism, and (2) an oxidant-releasable zinc pool. Overexpression of MT in primary astrocyte cultures was accomplished using an adenoviral vector. Using the zinc-sensitive fluorescent indicator mag-fura-2, we monitored [Zn2+]i after stimulating zinc influx or oxidant treatment. With MT overexpression, we observed an acute buffering effect manifested as a dampening of stimulus-induced increases in [Zn2+]i. In contrast, we also saw enhanced zinc release with application of the sulfhydryl oxidizing agent 2,2'-dithiodipyridine. These results indicate that overexpression of a zinc-binding protein can quickly diminish [Zn2+]i following zinc influx, but elevate [Zn2+]i under conditions of oxidative stress, providing protective yet potentially endangering effects. Copyright 2003 Wiley-Liss, Inc.