This week in our Topics in Biotechnology course, we discussed a paper from the laboratory of Karen Vousden of the Francis Crick Institute, TIGAR, a p53-inducible regulator of glycolysis and apoptosis. I wanted to take some time to summarize the conclusions of this paper here in order to prevent any misunderstanding that might have arisen following our in-class discussion of the place of TIGAR with respect to glucose metabolism.
The body is extraordinary in its ability to maintain itself. It keeps a temperature of 37 degrees. It keeps bad stuff out and good stuff in. It has an immune system that attacks and destroys invasive viruses, bacteria, fungi, and other parasites. Cells maintain a constant pH and balance the concentrations of salts, proteins, and nutrients. All these regulatory mechanisms work together to keep us healthy and functioning properly.
One of the ways our body does this is by having our cells monitor their own health and make difficult choices when they are unhealthy. When our cells suffer damage, they work hard to repair the damage, but they also balance this against the greater good of the body as a whole. If they can repair damage, they do so, but when they can’t, cells eliminate themselves by a process called apoptosis. A central protein that controls many of these processes is the ‘guardian of the genome,’ p53.
This paper explores the role of TIGAR (Tp53-induced glycolysis and apoptosis regulator) in modulating the pro-apoptotic effects of p53 and in reducing free radicals. Specifically, TIGAR exerts its effects by rebalancing the normal metabolism of glucose during glycolysis. This prevents further damage and also allows time for repair to occur before making a decision to terminate the cell if repair is unsuccessful.
Under normal conditions, cells take up and process glucose as a fuel for making ATP, which is used directly to power enzymes and carry out all the processes that keep us alive. As part of this process, glucose is broken down stepwise during glycolysis. Some fraction of the products of this reaction gets diverted by an enzyme called PFK-2, which makes Fructose-2,6,-bisphosphate. This sugar goes on to bind to, and activate PFK-1, which keeps the pathway flowing.
Under conditions following DNA damage, p53 will become activated and the cell will arrest glycolysis as well as any cell division while it initiates DNA repair mechanisms and acts to remediate the radical oxygen species (ROSs) that are often associated with this sort of damage.
Among the many genes that are turned on to carry out these operations is TIGAR. Bensaad et al. show that the gene for TIGAR is transcribed and translated into protein, and that protein goes on to act as an enzyme to regulate metabolism.
Specifically, TIGAR functions as an enzyme with a high degree of homology to FBPase-2, which converts Fructose-2,6,-bisphosphate to Fructose-6-phosphate. This has an important regulatory function because, as stated above, Fructose-2,6,-bisphosphate is required to activate the enzyme PFK-1, which is required for glycolysis. In the absence of Fructose-2,6,-bisphosphate, PFK-1 shuts down and the products of glycolysis start backing up.
At first, this results in a backlog of Fructose-6-phosphate. As this accumulates, it will result in the accumulation of Glucose-6-phophate. With nowhere also to go, this will be processed to 6-phosphogluconolactone, making NADPH. NADPH will then oxidize glutathione, which will break down the ROS, H2O2 to water.
With the reduction in the number of ROSs, DNA damage will cease and repair can take place, thus diverting the cell away from a pathway leading to apoptosis.
This paper represents an amazing amount of work and is nearly bulletproof in its findings. I highly recommend it to anyone interested in how DNA damage and p53 interact with metabolic pathways and how this interaction directly leads to a more complete understanding of how p53 does its job.