Induction of glucose-6-phosphate dehydrogenase by lipopolysaccharide contributes to preventing nitric oxide-mediated glutathione depletion in cultured rat astrocytes

Treatment of cultured rat astrocytes with lipopolysaccharide (LPS; 1 mu g/m l) increased mRNA expression of glucose-6-phosphate dehydrogenase (G6PD), t he rate-limiting step in the pentose phosphate pathway (PPP), in a time-dep endent fashion (0-24 h). This effect was accompanied by an increase in G6PD activity (1.74-fold) and in the rate of glucose oxidation through the PPP (6.32-ford). Inhibition of inducible nitric oxide synthase (iNOS) activity by 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT; 50 mu M) did not alte r the LPS-mediated enhancement of G6PD mRNA expression or PPP activity. Blo ckade of nuclear factor kappa B (NF-kappa B) activation by N-benzyloxycarbo nyl- IIe-Glu-(O-tert-butyl)-Ala-leucinal (1 mu M) prevented the expression of both iNOS mRNA and G6PD mRNA, suggesting that iNOS and G6PD are cc-induc ed by LPS through a common transcriptional pathway involving NF-kappa B act ivation. Incubation of cells with LPS for 24 h increased intracellular NADP H concentrations (1.63-fold) as compared with untreated cells, but GSH conc entrations were not modified by LPS treatment up to 60 h of incubation. How ever. inhibition of G6PD activity by dehydroepiandrosterone (DHEA; 100 mu M ), which prevented LPS-mediated enhancements in PPP activity and NADPH conc entrations, caused a 50% decrease in the GSH/GSSG ratio after 24-36 h and i n GSH concentrations after 60 h of incubation. Furthermore, the changes in glutathione concentrations caused by DHEA were abolished by AMT, suggesting that nitric oxide and/or its reactive derivatives would be involved in thi s process. From these results, we conclude that LPS-mediated G6PD expressio n prevents GSH depletion due to nitric oxide and suggest that this phenomen on may be a contributing factor in the defense mechanisms that protect astr ocytes against nitric oxide-mediated cell injury.