Induction of astrocyte metallothioneins (MTs) by zinc confers resistance against the acute cytotoxic effects of methylmercury on cell swelling, Na uptake, and K release

Metallothionein (MT) proteins play an important role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, we investigated the ability of the astrocyte-specific MT isoform, MT-I, to attenuate MeHg-induced cytotoxicity. Increased astrocytic MT expression was achieved by 24-h pretreatment of neonatal rat primary astrocyte cultures with 100 μM zinc (ZnSO₄). Subsequently, the astrocytes were treated with MeHg (10 μM), and its toxic effects on cell volume, Na⁺ uptake, and K⁺ release were investigated and compared to cells treated with or without MeHg, but in the absence of Zn pretreatment. Pretreatment of astrocytes with Zn was associated with a 2.9-fold increase in MT protein levels (P<0.02), and a 5.6-fold increase in MT mRNA levels (p<0.002) compared to control astrocytes. Astrocytes expressing increased MT protein levels were resistant to MeHg-induced swelling. In isotonic buffer the effect of MeHg on swelling was abolished (p<0.01) by 24-h Zn pretreatment, in such a way that volume profiles in these cells did not differ from controls. Zn-induced increased expression of MTs was also associated with significant attenuation of astrocytic Na⁺ uptake (p<0.01) and Rb⁺ (a marker for K⁺) release (p<0.001) in response to treatment with MeHg. These results demonstrate (1) that astrocytes can be induced to express high levels of MT proteins by pretreatment with Zn, and (2) that Zn confers resistance against the acute effect of MeHg on astrocytic swelling and the associated changes in ion (Na⁺ and K⁺) transport. Taken together, the data suggest that astrocytic MT induction offers effective cellular adaptation to MeHg cytotoxicity.