Inducers of gamma-glutamylcysteine synthetase and their effects on glutathione synthetase expression

Synthesis of GSH occurs via two enzymatic steps, the first is catalyzed by gamma-glutamylcysteine synthetase (GCS) and the second is catalyzed by GSH synthetase (GS). A heavy (HS) and light subunit (LS) make up GCS: regulatio n of both subunits have been well characterized, whereas regulation of GS i s largely unknown. In this study, we examined the effects of treatments kno wn to influence the gene expression of GCS subunits on GS expression. Insul in and hydrocortisone treatment of rat hepatocytes or ethanol-feeding of ra ts for 9 weeks, which increased the expression of GCS-HS only, had no influ ence on GS expression. However, two-thirds partial hepatectomy in rats whic h increased the expression of GCS-HS only, also increased GS expression. Tr eatment of hepatocytes or rats with diethyl maleate, buthionine sulfoximine , tert-butylhydroquinone, or thioacetamide, which increased the expression of both GCS subunits, increased the expression of GS. The GSH synthesis cap acity increased 50-100% by treatments that increased only the GCS-HS expres sion, whereas it increased 161-200% by treatments that increased both GCS-H S and GS expression Thioacetamide treatment of Chang cells increased cell G SH and GS expression by 50%, but had minimal influence on GCS subunits. Thu s, GS induction can further increase the cell's GSH synthetic capacity and in some cells may be as important as GCS in determining the rate of GSH syn thesis. (C) 2000 Elsevier Science B.V. All rights reserved.