TIGAR activity can have multiple cellular effects. TIGAR acts as a direct<unk>  of fructose-2,6-bisphosphate levels and hexokinase 2 activity, and this can lead indirectly to many changes within the cell in a chain of biochemical events. TIGAR is a fructose bisphosphatase which activates p53, in results of inhibiting the expression of glucose transporter and also regulating the expression of hexokinase and phosphoglycerate mutase. TIGAR also inhibit the Phosphofructokinase (PFK) by lowering the level of fructose-2,6,bisphosphate, therefore,<unk>  is inhibited and pentose phosphate pathway is promoted. Fructose-2,6-bisphosphate regulation TIGAR decreases cellular fructose-2,6-bisphosphate levels. It catalyses the removal of a phosphate group from fructose-2,6-bisphosphate (F-2,6-BP): Fructose-2,6-Bisphosphate->Fructose-6-phosphate (F-6-P) + phosphate F-2,6-BP is an allosteric<unk>  of cellular glucose metabolism pathways. Ordinarily F-2,6-BP binds to and increases the activity of phosphofructokinase 1. Phosphofructokinase-1 catalyses the addition of a phosphate to F-6-P to form Fructose-1,6-bisphosphate (F-1,6-BP). This is an essential step in the<unk>  pathway, which forms the first part of aerobic respiration in mammals. F-2,6-BP also binds to and decreases the activity of fructose-1,6-bisphosphatase. Fructose-1,6-bisphosphatase catalyses the removal of phosphate from F-1,6-BP to form F-6-P. This reaction is part of the gluconeogenesis pathway, which synthesizes glucose, and is the reverse of<unk> . When TIGAR decreases F-2,6-BP levels, phosphofructokinase becomes less active whilst fructose-1,6-bisphosphatase activity increases. Fructose-6-phosphate levels build up, which has multiple effects inside the cell: *The rate of<unk>  decreases *The rate of gluconeogenesis increases *Excess fructose-6-phosphate is converted to glucose-6-phosphate in an isomerization reaction *Excess glucose-6-phosphate enters the pentose phosphate pathway. This ultimately leads to the removal of reactive oxygen species (ROS) in the cell * The removal of ROS helps to prevent<unk>  (cell suicide), and may also reduce build-up of DNA damage over time. DNA damage response and cell cycle arrest TIGAR can act to prevent a cell progressing through the stages of its growth and division cycle by decreasing cellular ATP levels. This is known as cell cycle arrest. This function of TIGAR forms part of the p53 mediated DNA damage response where, under low levels of cellular stress, p53 initiates cell cycle arrest to allow the cell time for repair. Under high levels of cellular stress, p53 initiates<unk>  instead. In non-resting cells, the cell cycle consists of G0 -> G1 -> S -> G2 -> M phases, and is tightly regulated at checkpoints between the phases. If the cell has undergone stress, certain proteins are expressed that will prevent the specific sequence of macromolecular interactions at the checkpoint required for progression to the next phase. TIGAR activity can prevent cells progressing into S phase through a checkpoint known in humans as the restriction point. At the very start of G1 phase, a protein called retinoblastoma (Rb) exists in an un-phosphorylated state. In this state, Rb binds to a protein transcription factor E2F and prevents E2F from activating transcription of proteins essential for S-phase. During a normal cell cycle, as G1 progresses, Rb will become phosphorylated in a specific set of sequential steps by proteins called cyclin dependent kinases (cdks) bound to cyclin proteins. The specific complexes that phosphorylate Rb are cyclin D-cdk4 and cyclin E-cdk2. When Rb has been phosphorylated many times, it dissociates from E2F. E2F is free to activate expression of S-phase genes. TIGAR can indirectly prevent a cell passing through the Restriction Point by keeping Rb unphosphorylated. When expressed, TIGAR decreases cellular ATP levels through its phosphatase activity. Less ATP is available for Rb phosphorylation, so Rb remains un-phosphorylated and bound to E2F, which cannot activate S phase genes. Expression of cyclin D, ckd4, cyclin E and cdk2 decreases when TIGAR is active, due to a lack of ATP essential for their transcription and translation. This TIGAR activity serves to arrest cells in G1. Activity of hexokinase 2 Under low oxygen conditions known as hypoxia, a small amount of TIGAR travels to the mitochondria and increases the activity of Hexokinase 2 (HK2) by binding to it During hypoxia, a protein called Hif1α is activated and causes TIGAR to re-localise from the cytoplasm to the outer mitochondrial membrane. Here, HK2 is bound to an anion channel in the outer mitochondrial membrane called VDAC. TIGAR binds hexokinase 2 and increases its activity by an as yet unknown mechanism. Hexokinase 2 (HK2) carries out the following reaction: Glucose + ATP -> Glucose-6-phosphate + ADP HK2 is believed to maintain the mitochondrial membrane potential by keeping ADP levels high. It also prevents<unk>  in several ways: it reduces mitochondrial ROS levels, and it prevents<unk> -causing protein Bax from creating a channel with VDAC. This stops cytochrome C protein passing out through VDAC into the cytoplasm where it triggers<unk>  via a caspase protein cascade. TIGAR does not re-localise to the mitochondria and bind HK2 under normal cellular conditions, or if the cell is starved of glucose. Re-localisation to the mitochondria does not require TIGAR's phosphatase domain. Instead 4 amino acids at the C-terminal end of TIGAR are essential. Protection from<unk>  Increased expression of TIGAR protects cells from oxidative-stress induced<unk>  by decreasing the levels of ROS. TIGAR can indirectly reduce ROS in two distinctive ways. The intracellular environment of the cell will determine which of these two modes of TIGAR action is more prevalent in the cell at any one time. The fructose-2,6-bisphosphatase activity of TIGAR reduces ROS by increasing the activity of the Pentose Phosphate Pathway (PPP). Glucose-6-phosphate builds up due to de-phosphorylation of F-2,6-BP by TIGAR and enters the PPP. This causes the PPP to generate more nicotinamide adenine dinucleotide (NADPH). NADPH is a carrier of electrons that is used by the cell as a reducing agent in many anabolic reactions. NADPH produced by the PPP passes electrons to an oxidized glutathione molecule (GSSG) to form reduced glutathione (GSH). GSH becomes the reducing agent, and passes electrons on to the ROS hydrogen peroxide to form harmless water in the reaction: GSH + H2O2 -> H2O + GSSG The decrease in H2O2 as a result of TIGAR activity protects against<unk> . TIGAR also reduces ROS by increasing the activity of HK2. HK2 reduces ROS levels indirectly by keeping ADP levels at the outer mitochondrial membrane high. If ADP levels fall, the rate of respiration decreases and causes the electron transport chain to become over-reduced with excess electrons. These excess electrons pass to oxygen and form ROS. The action of the TIGAR/HK2 complex only protects cells from<unk>  under low oxygen conditions. Under normal or glucose starved conditions, TIGAR mediated protection from<unk>  comes from its bis-phosphatase activity alone. TIGAR cannot prevent<unk>  via death pathways that are independent from ROS and p53. In some cells, TIGAR expression can push cells further towards<unk> . Interleukin 3 (IL-3) is a growth factor that can bind to receptors on a cell's surface and tells the cell to survive and grow. When IL-3 dependent cell lines are deprived of IL-3 they die due to decreased uptake and metabolism of glucose. When TIGAR is overexpressed in IL-3 deprived cells the rate of<unk>  decreases further which enhances the<unk>  rate. Autophagy Autophagy is when a cell digests some of its own organelles by lysosomal degradation. Autophagy is employed to remove damaged organelles, or under starvation conditions to provide additional nutrients. Normally, autophagy occurs by the TSC-Mtor pathway, but can be induced by ROS. TIGAR, even at very low levels, inhibits autophagy by decreasing ROS levels. The mechanism by which TIGAR does this is independent from the Mtor pathway, but the exact details are unknown.
TP53-inducible glycolysis and apoptosis regulator
Insulin signal transduction pathway
Winged-helix transcription factors
Metastasis suppressor
Hedgehog signaling pathway
G protein-coupled receptor kinase 3
Mir-3180 microRNA precursor family
Mir-590 microRNA precursor family