The human TIGAR gene is composed of 6 exons spanning genomic region about 50,4 kb (GenBank NC_000012.11). The transcript mRNA is 8,2 kb (GenBank NM_020375.2) and it is composed by the exon regions 5827..5937, 15914..15951, 21673..21794, 34453..34530, 35901..36011 and 36894..44662.

The human TIGAR coding sequence consists of 813 bp from the start codon to the stop codon. There are no splicing variants reported.

A pseudogene of the ribosomal protein S15 has been located in the region 8981..9473 by computational analysis using a gene prediction method, but there is not experimental data proving it.

Human TIGAR protein consists of 270 aminoacids, with a molecular weight of 30063 Da. It is composed of a bisphosphatase active site in which two histidines, His-11 and His-198, and one glutamic acid, Glu-102, are key residues for its function. TIGAR catalytic domain belongs to the histidine phosphatase superfamily of proteins, a conserved group of proteins which contain a domain with a histidine forming a phosphoenzyme transiently during the catalysis (Rigden, 2008). This domain shares similarity with enzymes of the phosphoglycerate mutase family (PGAM) and with the bisphosphatase domain of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), in which the three aminoacids in the catalytic domain are conserved.
More information about TIGAR protein can be found in Uniprot Q9NQ88.
Human TIGAR structure contains different motifs as represented in the image below (PDB reference 3DCY).
The crystallized structure of Danio rerio TIGAR is available in PDB (3E9D reference) and was published by Li and Jogl, 2009.

TIGAR mRNA is expressed in all the tissues in which it has been analysed and it is overexpressed in several cancer cell lines such as T-lymph Jurkat cancer, kidney HEK-293, liver HuH-7 and HepG2, lung A549, colon RKO, bone U2OS, brain GAMG, prostate LnCap, cervix HeLa and breast MCF7.
All this information can be found in GeneCards (sections proteins and expression).

TIGAR is mainly localized in the cytoplasm. Under hypoxic conditions, a relocalization of the protein linked to HK-2 (hexokinase 2) in the outer mitochondrial membrane has been described (Mathupala et al., 2009; Cheung et al., 2012).

TIGAR was identified by microarray analysis of gene expression after TP53 induction (Bensaad et al., 2006). TIGAR plays a role in the TP53 tumor suppressor program by reducing reactive oxygen species (ROS) levels and preventing DNA damage-induced apoptosis. Their functions are based on glycolysis inhibition and the enhancement of pentose phosphate pathway (PPP).
TIGAR is a bisphosphatase that hydrolyzes Fructose-2,6-bisphosphate to Fructose-6-phosphate, which can enter the PPP to generate NADPH and ribose-5-phosphate, thus, reducing oxidative stress and generating nucleotide precursors (Bensaad et al., 2006).
Beta-D-fructose 2,6-bisphosphate + H2O ⇔ D-fructose 6-phosphate + phosphate
The switch from TP53-induced cell-cycle arrest to apoptosis following maintained stress is associated with downregulation of TIGAR and therefore the loss of P53-dependent survival agents could be the cause of the apoptotic response. It has been proposed that TP53-dependent metabolism regulation could be orchestrated by different mechanisms other than the ones that regulate TP53-induced apoptosis. TIGAR antiapoptotic effects are oxidative-stress dependent. The apoptosis induction by other mechanisms is not affected by TIGAR, as it was shown in IL-3-dependent apoptosis FL5.12 cell line, and in anti-Fas induced apoptosis in U2OS cells, suggesting that TIGAR can modulate apoptosis in a cell-dependent manner (Bensaad et al., 2006).
As described previously, under hypoxic conditions TIGAR is localized in outer mitochondrial membrane. In this situation, it binds together with HK2 to mitochondria (Mathupala et al., 2009) in a HIF1α (hypoxia-inducible factor 1α) dependent manner, and limits ROS levels. If HK2 is not present, oxidative stress reduction by TIGAR is lower. It has been shown that mutant TIGAR TM (Triple Mutant without enzymatic activity: H11A/E102A/H198A) is able to relocalize to mitochondria in response to hypoxia and, indeed, is able to bind to HK2 in the outer mitochondrial membrane. Therefore, the maintenance of mitochondrial membrane potential by TIGAR is independent of its bisphoshatase activity. However, the single deletion of four aminoacids in the C-terminus (258-261) avoided TIGAR mobilization to the mitochondria after hypoxia, although its bisphosphatase activity remained unaltered. The double mutant TM TIGAR/258-261 was not able to reduce ROS levels (Bensaad et al., 2006; Cheung et al., 2012). In conclusion, TIGAR lowers ROS levels both by inhibiting glycolysis and by enhancing an adequate mitochondrial function coupling to HK2.
TIGAR effects have also been related to autophagy as an inhibition of this cell process was described when cells were exposed to stress conditions such as nutrient starvation or metabolic stress, in parallel with an overexpression of TIGAR and a decrease in ROS cell levels. After TIGAR suppression, autophagy was induced to moderate apoptotic response by restraining ROS levels (Bensaad et al., 2009).
The relation between autophagy and apoptosis can be modulated differently depending on the stimulus and cell type. D-galactose (D-gal) treatment of neuroblastoma cells induced necroptosis and autophagy, as shown by upregulation of Bmf, Bnip3, Atg5 and TIGAR, but there were no changes in expression in genes related to apoptosis (Li et al., 2011).
Recently, it has been described a decrease in steady-state mRNA levels of TIGAR when the human hepatocellular carcinoma HepG2 cell line was exposed to high oxidative stress conditions induced by the superoxide radical-generating menadione, hydrogen peroxide (H2O2) or nutrient starvation, in parallel with a down-regulation of the damage-regulated autophagy modulator (DRAM). mRNA levels of both genes were recovered when cells were treated with antioxidants such as GSH or N-acetylcysteine, suggesting a complex regulation of tumor suppressor genes by ROS levels (Kim et al., 2013). In front of a disruption in the homeostasis balance of the cell, TIGAR would provide cytoprotection by its antioxidant properties rather than its ability to inhibit autophagy. After a threatening stimulus, a rapid increase in the autophagic flux occurs mostly regulated by post-translational modifications, followed by a transcriptionally-mediated delayed autophagyc response phase, in which TP53 would be activated. In this context, TIGAR would be rapidly transactivated and would mediate its antioxidant response, while autophagy is enhanced by other transcriptionally activated targets, such as DRAM (Pietrocola et al., 2013). Some authors have proposed a TP53-orchestrated mechanism by which this protein would regulate stress-induced autophagy by balancing two proteins with opposite effects: TIGAR and DRAM (Dewaele et al., 2010; Zhang et al., 2010).
In TP53 knock-out mice (TP53^-/-) cardioprotection against ischemic injury and resistance to cardiac remodeling were observed, and a significant TIGAR overexpression was described. The TIGAR knock-out (TIGAR^-/-) had the same effects and a TP53-dependent mechanism of autophagy inhibition through the mitophagy enhancer Bnip3 was described. Double knock-out of TP53 and TIGAR mice exposed to ischemia responded with an increase in ROS levels, followed by an overexpression of Bnip3 that lead to mitophagy and, thus, cardioprotection. The activation of Bnip3 and mitophagy was recovered by NAC, confirming that TIGAR-mediated mitophagy inhibition is mediated by ROS. Ventricular remodeling after myocardial infarction is a consequence of both impaired mitochondrial integrity and enhanced apoptosis, whereas mitophagy helps cells to undergo mitochondrial damage and avoid apoptosis, resulting in a diminished initial infarct size, less ventricular remodeling and restored homeostasis in ischemic myocardium (Kimata et al., 2010; Hoshino et al., 2012).
The analysis of lung tissues from idiopathic pulmonary fibrosis patients revealed decreased autophagyc activity, evidenced by less LC3 and p62 expression. When these cells were treated with TGFβ in vitro, impairment in autophagy was observed, in parallel with an increase in TIGAR expression, although the possible mechanism connecting TGFβ and TIGAR was not described (Patel et al., 2012).
In a co-culture system, Martinez-Outschoorn et al. showed that oxidative stress-induced autophagy correlated with Cav-1 downregulation in cancer associated fibroblasts, and with overexpression of TIGAR in adjacent cancer cells. Consequently, autophagy cancer associated fibroblasts provide recycled nutrients for cancer cell metabolism and, moreover, prevent cancer cells death by upregulating TIGAR and thus conferring resistance to apoptosis and autophagy (Martinez-Outschoorn et al., 2010). Further studies by the same group demonstrated that the metabolic coupling between cancer cells and fibroblasts can explain tamoxifen resistance as cancer associated fibroblasts enhance the activity of TIGAR in cancer cells, providing protection against tamoxifen-induced apoptosis, which was much higher in monocultures of cancer cells alone (Martinez-Outschoorn et al., 2011). In another study, glutamine was described as a needed factor for Cav-1 downregulation in cancer associated fibroblasts and for the decrease in autophagy mediators and markers in cancer cells, establishing a model in which autophagy fibroblasts may serve as a source of glutamine to fuel cancer cell mitochondrial activity. Therefore, a cycle between catabolic tumor stroma cells and anabolic tumor cells has been proposed to explain the relations between cells in tumor environment (Ko et al., 2011).
Besides, TIGAR has also been proposed to be an anticancer therapy target gene considering that autophagy inhibition in cancer cells would probably increase cell death (Dodson et al., 2013).
All these studies confirm that TIGAR functions are not only restricted to glycolysis regulation, as this protein plays key roles in different cell processes involving oxidative stress restriction.

Activation and regulation
TIGAR belongs to that group of TP53 target genes that become rapidly activated by low levels of stress. There are two possible TP53 binding sites in TIGAR gene: one upstream of the first exon (BS1) and one within the 1^st intron (BS2), which is the most efficient one and has been validated by chromatin immunoprecipitation (ChIP) analysis (Bensaad et al., 2006).
In an experimental approach, TIGAR was induced by Actinomycin D and Adriamycin (Bensaad et al., 2006), two well-known activators of TP53, and also by Nutlin-3a, an antagonist of Mdm2 (Hasegawa et al., 2009). Other stimuli that have been described to trigger TIGAR expression are radiotherapy (Peña-Rico et al., 2011), glutamine (Ko et al., 2011), chemotherapy (Madan et al., 2012), UV light (Madan et al., 2012), TNFα and radiotherapy mimetics (Sinha et al., 2013).
When DNA damage occurs, TP53 is expressed either to repair DNA or lead the cell towards apoptosis. High ROS levels can compromise DNA stability which, in turn, could help cells to accumulate mutations and become tumorigenic. As TIGAR reduces ROS levels, it has been proposed as a tumor suppressor gene, although it should be taken into account that, as TIGAR can help tumor cells survival, it could act as an oncogene in some situations. TP53 is able to suppress tumor development when mechanisms of apoptosis, senescence and cell-cycle arrest are impeded. This supports the idea that metabolism, and thereby TIGAR, has a key role in cancer development (Li et al., 2012; Valente et al., 2013).
TIGAR expression can also be modulated in a TP53 independent manner, as results from studies with the TP53 null T98G and H1299 cell lines suggest (Bensaad et al., 2006; Peña-Rico et al., 2011). The mechanisms implicated in the regulation of TIGAR expression in the absence of TP53 are unclear. The CRE-binding protein (CREB) has been described to regulate TIGAR expression through a CRE-binding site at the TIGAR promoter, which was first annotated by bioinformatic analysis and then confirmed by electrophoretic mobility shift assay (EMSA) and ChIP. CREB knockdown reduced enhanced promoter activity and TIGAR expression; whereas CREB overexpression resulted in enhanced promoter activity and TIGAR expression levels (Zou et al., 2013). Another transcription factor, SP1, was found to bind to TIGAR promoter in a SP1-binding site located in a very short region (-56/-4) both in vitro and in vivo, and was considered a key factor for proper basal activity of TIGAR promoter (Zou et al., 2012). Recently, some authors have identified HIF-1α as a potential regulator of SCO2 and TIGAR gene expression suggesting the involvement of P300/CBP-associated factor (PCAF) in differential recruitment of HIF-1α and p53 to the promoter of TIGAR and/or SCO2 genes in response of hypoxia in tumoral cells (Rajendran et al., 2013).

Chromosomal rearrangements: copy number variants
Some alterations affecting TIGAR genome region have been described in patients, some of them showed phenotypic effects. Gain of 12:189578-34178209 resulted in hydronephrosis, micrognathia, edema, depressed nasal bridge, tricuspid regurgitation and diaphragmatic eventration. Patients with gain of 12:189561-41878937 suffered global developmental delay. The ones with gain of 12:230361-20643702 showed intellectual disability and electroencephalogram with localized low amplitude activity, whereas gain of 12:191619-8327369 resulted in macroencephaly, visual impairment, intellectual disability, muscular hipotonia, and seizures.
For more information, see DECIPHER.

Location in the mouse: chromosome 6, 61.92 cM, 127085116-127109550 bp, complement strand (MGI).
For a comparison of the gene among Homo sapiens, mouse, rat, chimpanzee, cattle, rhesus macaque, dog, chicken and zebrafish see MGI.
For all species known gene tree, see: Treefam database.

There have been observed 22 somatic mutations in patient tumor samples collected in the COSMIC database.
Nonsense substitutions: 1, located in c.466G>T (p.G156*).
Missense substitutions: 16, which represent 82% of the mutations described among all patients. Only two missense substitutions have been found in more than one patient: c.268A>G (p.R90G) was found in 5 patients and c.215G>T (p.R72I) was found in 2 patients.
Synonymous substitutions: 4.
No deletions, insertions or chromosomal fusions in TIGAR gene have been described in any tumor sample.

Genomic variants
There are 878 SNP variants described but none of them have clinical significance described (GeneCards).