The gene encoding TIGAR (Tp53-induced glycolysis and apoptosis regulator) was identified in a screen for genes whose expression was increased by p53. Bensaad et al. now show that TIGAR plays a role in protecting cells from apoptosis by directing cellular metabolism from the glycolytic pathway to the pentose phosphate pathway (PPP), which decreases reactive oxygen species (ROS) by producing increased NADPH, which is required to increase the abundance of reduced glutathione. Based on the time course of p53-induced expression and its activation by a p53 mutant that fails to stimulate some proapoptotic target genes, the gene encoding TIGAR appears to fall into the class of genes regulated by p53 that promote cell survival under conditions of moderate stress. TIGAR exhibits sequence similarity, especially with regard to conserved catalytic residues, with the bisphosphatase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), a bifunctional enzyme involved in regulation of glycolysis. Consistent with TIGAR having fructose-2,6-bisphosphatase activity, fructose-2,6-bisphophate (Fru-2,6-P2) concentration was decreased in cells overexpressing TIGAR and was increased in cells in which TIGAR was decreased by siRNA. Fru-2,6-P2 stimulates glycolysis, and cells overexpressing TIGAR showed decreased glycolytic rates and cells treated with TIGAR siRNA showed increased glycolytic rates. When glycolysis is inhibited, fructose-6-phosphate is converted to glucose-6-phosphate and shunted into the PPP, which can inhibit apoptosis by increasing the abundance of the ROS scavenger glutathione. In various apoptosis-stimulating conditions that proceeded through mechanisms involving ROS, TIGAR overexpression decreased cell death. TIGAR was not effective at preventing cell death induced by stimuli that did not increase ROS. TIGAR also did not protect cells from apoptosis if the PPP was blocked. Although enzymatic activity of TIGAR was not directly demonstrated, protection from ROS-sensitive apoptosis, decreased glycolytic rate, and lower intracellular Fru-2,6-P2 concentrations were not observed in cells overexpressing TIGAR with mutations in the putative catalytic conserved residues. These results connect p53 to cellular metabolism and provide insight into how p53 can contribute to DNA repair and cell survival by increasing flux through the PPP, which decreases ROS and increases the production of metabolites necessary for DNA repair (see Green and Chipuk).
K. Bensaad, A. Tsuruta, M. A. Selak, M. N. C. Vidal, K. Nakano, R. Bartrons, E. Gottlieb, K. H. Vousden, TIGAR, a p53-inducible regulator of glycolysis and apoptosis. Cell 126, 107-120 (2006). [Online Journal]
D. R. Green, J. E. Chipuk, p53 and metabolism: Inside the TIGAR. Cell 126, 30-32 (2006). [Online Journal]