Figure 4 | Scientific Reports

Figure 4

From: Phosphoenolpyruvate Carboxykinase Maintains Glycolysis-driven Growth in Drosophila Tumors

Figure 4

Model for the effect of PEPCK and NADH on glucose metabolism. (a) Proliferating cells require glucose for processes including the synthesis of nucleotides (via the pentose-phosphate pathway), membrane lipids (via pyruvate and citrate) and energy (via pyruvate or the glycerol-3-phosphate shuttle). Cytoplasmic NADH is produced in making phosphoenolpyruvate, which must be oxidised for glycolysis to continue. NADH can be oxidised by making lactate, but this prevents the use of pyruvate for lipid synthesis or energy. The glycerol-3-phosphate shuttle can oxidise NADH, but does so at the cost of generating elevated ROS levels. PEPCK mediates glyceroneogenesis (broad arrow), which uses citrate exported from mitochondria to generate glycerol-3-phosphate while oxidising NADH. Our model is that depletion of PEPCK decreases the levels of cytoplasmic NAD+ , which inhibits glycolysis unless the NAD+ can be regenerated by the glycerol-3-phosphate shuttle. This shuttle generates reactive oxygen species, so its use is limited in CIN cells which are already redox stressed. The inhibitors used to test this model are shown with their targets. Abbreviations: DHAP: dihydroxyacetone phosphate, Gpdh: Glycerol-3-phosphate Dehydrogenase, Gpo1: mitochondrial Glycerol-3-Phosphate Dehydrogenase/Glycero-phosphate oxidase, iGP1: inhibitor of Glycerophosphate oxidase 1, Met: Metformin, Nam: nicotinamide, FeCN: ferricyanide, PEPCK: Phosphoenolpyruvate Carboxykinase. (b) The effect of PEPCK depletion on glucose uptake in brat depleted brain tissue. The uptake of fluorescently labelled 2-NBD-glucose in 3rd instar larval brains was increased by the depletion of Brat (actin>Gal4; UAS-RFP; UAS-brat-RNAi; p < 0.001, n ≥ 25). Depleting PEPCK in these brat tumors (actin > Gal4; UAS-RFP; UAS-brat-RNAi; UAS-PEPCK-RNAi) led to significantly lower uptake of labelled glucose (p < 0.001, n ≥ 25), which was rescued by feeding the larvae with ferricyanide (FeCN, p < 0.05, n ≥ 23). Error bars show 95% confidence intervals. Comparisons were done by multiple t-tests using Tukey’s method.