Editors' ChoiceCellular Metabolism

PPP to the rescue

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Sci. Signal.  11 Aug 2015:
Vol. 8, Issue 389, pp. ec225
DOI: 10.1126/scisignal.aad2050

Skin cells are the body’s first line of defense and are exposed to ultraviolet (UV) radiation and various damaging chemicals. UV and some chemicals, such as hydrogen peroxide (H2O2), result in the production of reactive oxygen species (ROS) that can cause cellular damage. Kuehne et al. found that skin keratinocytes and fibroblasts in culture responded to UV exposure or H2O2 with a rapid change in metabolism that enabled the production of reducing equivalents in the form of NADPH to prevent the accumulation of damaging ROS. Untargeted metabolic profiling (to monitor all detectable changes) and targeted metabolic profiling (to monitor flux through specific biochemical pathways) revealed that within minutes of exposure to UV or H2O2, skin cells rerouted carbon through the oxidative branch of the pentose phosphate pathway (PPP), which produces NADPH that is necessary for the regeneration of reduced glutathione, a major ROS scavenger in cells. An increase in the precursor of purine biosynthesis occurred on the same timescale as the increased flux through the PPP. The change in carbon flux through the intermediates in the glycolysis pathway indicated that the increased PPP flux did not involve a block in the glycolytic pathway or inhibition of PKM2 (pyruvate kinase M2) or GAPDH (glyceraldehyde 3-phosphate dehydrogenase). “Futile” cycling of carbon through the oxidative and nonoxidative branches of the PPP produces NADPH, and metabolic flux analysis by carbon labeling indicated that this cycling occurred in skin cells exposed to a high concentration of H2O2. Impairment of the PPP by knockdown of enzymes in this metabolic pathway resulted in increased ROS and a reduction in the ratio of reduced to oxidized glutathione in skin fibroblasts exposed to UV irradiation, confirming the importance of this rapid metabolic rewiring in enabling cells to cope with oxidative stress. In vitro analysis of the activity of two enzymes [glucose-6-phosphate dehydrogenase (G6PD) and phosphoglucoseisomerase (PGI)] that can control flux through the oxidative PPP showed that NADPH inhibited G6PD activity and that 6-phosphogluconate (6PG) inhibited PGI in lysates of skin keratinocytes or fibroblasts. Thus, the authors propose that as ROS increase, NADPH concentration drops, which alleviates the inhibition of G6PD thereby increasing the production of 6PG, which in turn reduces the flux of carbon into the lower glycolytic pathway by inhibiting PGI.

A. Kuehne, H. Emmert, J. Soehle, M. Winnefeld, F. Fischer, H. Wenck, S. Gallinat, L. Terstegen, R. Lucius, J. Hildebrand, N. Zamboni, Acute activation of oxidative pentose phosphate pathway as first-line response to oxidative stress in human skin cells. Mol. Cell 59, 359–371 (2015). [PubMed]