Divergent consequences arise from metallothionein overexpression in astrocytes: Zinc buffering and oxidant‐induced zinc release

Abstract

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.