The role of intracellular glutathione in methylmercury-induced toxicity inembryonic neuronal cells

Previous studies indicate that the ability of cells to up-regulate levels o f intracellular glutathione (GSH) synthesis may determine their sensitivity to MeHg exposure. The purpose of the current study is two-fold. First, we determined whether the vulnerability of the developing central nervous syst em (CNS) to MeHg lies in its intracellular GSH content. The intracellular G SH content and the activity of gamma-glutamyl cysteine synthetase (GCS) wer e determined with and without MeHg exposure in primary cultures ei rat embr yonic CNS cells. In addition, the effect of GSH modulation on MeHg-induced cytotoxicity was determined. Second, we characterized the mechanism of GCS regulation, initially by studying the GCS heavy chain subunit (GCS-HC). Pri mary embryonic limb bud cells were used as a reference cell type for compar ing the response of CNS cells. The results indicate that constitutive intra cellular GSH content, GCS activity, and GCS-HC mRNA and protein levels of C NS cells were approximately ten-, two-, five-, and ten-fold higher, respect ively, than those in limb bud cells. A dose-dependent increase in GSH level s and GCS activity was observed in CNS and limb bud cells following 1 and 2 mu M MeHg exposure for 20 hr. Further characterization of GCS up-regulatio n in CNS cells showed that the increase in GCS activity following MeHg expo sure, unlike limb bud cells, was not accompanied by an elevation of GCS-HC mRNA and protein levels. Pretreatment with N-acetylcysteine led to a signif icant increase in intracellular GSH, while L-buthionine-(S,R)-sulfoximine ( BSO) resulted in decreased GSH levels, however neither pretreatment had a s ignificant impact on MeHg-induced cytotoxicity in either cell type. Our res ults suggest that although oxidative stress may mediate aspects of MeHg tox icity, disruption of GSH homeostasis alone is not responsible for the sensi tivity of embryonic CNS cells to MeHg. (C) 1999 Inter Press, Inc.