Hepatocellular response to chemical stress in CD-1 mice: Induction of early genes and γ-glutamylcysteine synthetase

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Abstract
Exposure of cells to toxic chemical species can result in reduced glutathione (GSH) depletion, generation of free radicals, and/or binding to critical cell determinants. Chemical stress is usually followed by a concerted cellular response aimed at restoring homeostasis, although the precise initial stimulus for the response is unclear. We have focused on one component of this stress response, the up-regulation of γ-glutamylcysteine synthetase (γ-GCS) and the preceding molecular events involved in its regulation in an in vivo mouse model. Male CD-1 mice received buthionine sulphoximine (BSO; 7.2 mmol/kg), diethyl maleate (DEM; 4.2 mmol/kg), paracetamol (APAP; 3.5 and 1.0 mmol/kg), or carbon tetrachloride (CCl4; 1.0 and 0.2 mmol/kg). Biochemical (serum transaminase and hepatic GSH levels) and molecular (c-jun and c-fos messenger RNA [mRNA] levels and activator protein 1 [AP-1] DNA binding activity) parameters were measured, as well as the consequent effects on γ-GCS levels and activity. All compounds produced GSH depletion, but only the higher doses of APAP and CCl4 caused liver damage. DEM, APAP, and CCl4 increased c-jun and c-fos mRNA levels, together with an increase in AP-1 binding; BSO failed to induce AP-1 despite an increase in c-fos. Interestingly, the effects on γ-GCS varied markedly according to the compound: BSO and DEM increased γ-GCS enzyme activity, although only DEM, but not BSO, resulted in an increase in γ-GCSh mRNA and protein. In contrast, APAP and CCl4 both increased γ-GCSh mRNA and protein; however, there was a marked dose-dependent decrease in γ-GCS activity. These data indicate that the effect of chemical stress on the liver is compound specific and is not merely dependent on depletion of GSH.