Induction of Glucose-6-Phosphate Dehydrogenase by Lipopolysaccharide Contributes to Preventing Nitric Oxide-Mediated Glutathione Depletion in Cultured Rat Astrocytes

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Treatment of cultured rat astrocytes with lipopolysaccharide (LPS; 1 μg/ml) increased mRNA expression of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting step in the pentose phosphate pathway (PPP), in a time-dependent 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-fold). Inhibition of inducible nitric oxide synthase (iNOS) activity by 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT; 50 μM) did not alter the LPS-mediated enhancement of G6PD mRNA expression or PPP activity. Blockade of nuclear factor κB (NF-κB) activation by N-benzyloxycarbonyl-Ile-Glu-(O-tert-butyl)-Ala-leucinal (1 μM) prevented the expression of both iNOS mRNA and G6PD mRNA, suggesting that iNOS and G6PD are co-induced by LPS through a common transcriptional pathway involving NF-κB activation. Incubation of cells with LPS for 24 h increased intracellular NADPH concentrations (1.63-fold) as compared with untreated cells, but GSH concentrations were not modified by LPS treatment up to 60 h of incubation. However, inhibition of G6PD activity by dehydroepiandrosterone (DHEA; 100 μM), which prevented LPS-mediated enhancements in PPP activity and NADPH concentrations, caused a 50% decrease in the GSH/GSSG ratio after 24-36 h and in 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 this process. From these results, we conclude that LPS-mediated G6PD expression prevents GSH depletion due to nitric oxide and suggest that this phenomenon may be a contributing factor in the defense mechanisms that protect astrocytes against nitric oxide-mediated cell injury.

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