Importance of glucose-6-phosphate dehydrogenase activity for cell growth

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 microg/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 microM) did not alter the LPS-mediated enhancement of G6PD mRNA expression or PPP activity. Blockade of nuclear factor-kappaB (NF-kappaB) activation by N-benzyloxycarbonyl-Ile-Glu-(O-tert-butyl)-Ala-leucinal (1 microM) 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-kappaB 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 microM), 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.

Modulation of type M2 pyruvate kinase activity by the human papillomavirus type 16 E7 oncoprotein

We report here that the E7 oncoprotein encoded by the oncogenic human papillomavirus (HPV) type 16 binds to the glycolytic enzyme type M2 pyruvate kinase (M2-PK). M2-PK occurs in a tetrameric form with a high affinity to its substrate phosphoenolpyruvate and a dimeric form with a low affinity to phosphoenolpyruvate, and the transition between both conformations regulates the glycolytic flux in tumor cells. The glycolytic intermediate fructose 1, 6-bisphosphate induces the reassociation of the dimeric to the tetrameric form of M2-PK. The expression of E7 in an experimental cell line shifts the equilibrium to the dimeric state despite a significant increase in the fructose 1,6-bisphosphate levels. Investigations of HPV-16 E7 mutants and the nononcogenic HPV-11 subtype suggest that the interaction of HPV-16 E7 with M2-PK may be linked to the transforming potential of the viral oncoprotein.

Regulation of tumor necrosis factor-induced, mitochondria- and reactive oxygen species-dependent cell death by the electron flux through the electron transport chain complex I

Tumor necrosis factor (TNF) induces a caspase-independent but mitochondria-dependent cell death process in the mouse fibrosarcoma cell line L929. Mitochondria actively participate in this TNF-induced necrotic cell death by the generation of mitochondrial reactive oxygen species (ROS). The aim of this study was to identify the mitochondrial components involved in TNF-induced production of ROS and their regulation by bioenergetic pathways. Therefore, we analyzed the bioenergetic characteristics in two metabolic L929 variants that exhibit different sensitivities to TNF. L929gln cells use glutamine as respiratory substrate and are far more susceptible to TNF-induced ROS generation and cell death as L929glc cells that use glucose as respiratory substrate. We show that the higher levels of reducing NAD(P)H equivalents, detected in the desensitized L929glc cells, do not cause diminished ROS generation. To the contrary, TNF increases the levels of NAD(P)H, probably altering complex I activity. A multiparameter analysis of electron flux through the mitochondrial electron transport chain, TNF-induced ROS levels, and cell death convincingly demonstrates a dependence of TNF signaling on complex I activity. Also, the sensitizing effect of glutamine metabolism correlates with an enhanced contribution of complex I to the overall electron flux. This participation of complex I activity in TNF-induced cell death is regulated by substrate availability rather than by a direct modification of complex I proteins. From the results presented in this paper we conclude that TNF-induced ROS generation and cell death are strongly regulated by bioenergetic pathways that define electron flux through complex I of the electron transport chain.