Transfection and overexpression of metallothionein-I in neonatal rat primary astrocyte cultures and in astrocytoma cells increases their resistance to methylmercury-induced cytotoxicity

Metallothionein-I (MT-I) was expressed in neonatal rat primary astrocyte cultures and an astrocytoma cell line by pGFAP-MT-I plasmid transfection under the control of the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter. Following transient transfection of the pGFAP-MT-I plasmid, MT-I mRNA and MT-I protein levels were determined by northern blot and immunoprecipitation analyses, respectively. The ability of cells over- expressing MT-I to withstand acute methylmercury (MeHg) treatment was measured by the release of preloaded Na₂ ⁵¹CrO₄, an indicator of membrane integrity. Transfection with the pGFAP-MT-I plasmid led to increased mRNA (2.5-fold in astrocytes and 7.4-fold in astrocytomas) and MT-I protein (2.4- fold in astrocytes and 4.0-fold in astrocytomas) levels compared with their respective controls. Increased expression of MT-I was associated with attenuated release of Na₂ ⁵¹CrO₄ upon MeHg (5 μM) treatment. These results demonstrate that MT-I can be highly expressed both in primary astrocyte cultures and astrocytomas by pGFAP-MT-I plasmid transfection, and lend credence to the hypothesis that increased expression of MT-I affords protection against the cytotoxic effects of MeHg. Taken together, the data suggest that MT offer effective cellular adaptation to MeHg cytotoxicity.