PRKAA1

TP53 stimulates transcription of TIGAR, a D-fructose 2,6-bisphosphatase. TIGAR activity decreases glycolytic rate and lowers ROS (reactive oxygen species) levels in cells (Bensaad et al. 2006). TP53 may also negatively regulate the rate of glycolysis by inhibiting the expression of glucose transporters GLUT1, GLUT3 and GLUT4 (Kondoh et al. 2005, Schwartzenberg-Bar-Yoseph et al. 2004, Kawauchi et al. 2008).

TP53 negatively regulates several key points in PI3K/AKT signaling and downstream mTOR signaling, decreasing the rate of protein synthesis and, hence, cellular growth. TP53 directly stimulates transcription of the tumor suppressor PTEN, which acts to inhibit PI3K-mediated activation of AKT (Stambolic et al. 2001). TP53 stimulates transcription of sestrin genes, SESN1, SESN2, and SESN3 (Velasco-Miguel et al. 1999, Budanov et al. 2002, Brynczka et al. 2007). One of sestrin functions may be to reduce and reactivate overoxidized peroxiredoxin PRDX1, thereby reducing ROS levels (Budanov et al. 2004, Papadia et al. 2008, Essler et al. 2009). Another function of sestrins is to bind the activated AMPK complex and protect it from AKT-mediated inactivation. By enhancing AMPK activity, sestrins negatively regulate mTOR signaling (Budanov and Karin 2008, Cam et al. 2014). The expression of DDIT4 (REDD1), another negative regulator of mTOR signaling, is directly stimulated by TP63 and TP53. DDIT4 prevents AKT-mediated inactivation of TSC1:TSC2 complex, thus inhibiting mTOR cascade (Cam et al. 2014, Ellisen et al. 2002, DeYoung et al. 2008). TP53 may also be involved, directly or indirectly, in regulation of expression of other participants of PI3K/AKT/mTOR signaling, such as PIK3CA (Singh et al. 2002), TSC2 and AMPKB (Feng et al. 2007).

TP53 regulates mitochondrial metabolism through several routes. TP53 stimulates transcription of SCO2 gene, which encodes a mitochondrial cytochrome c oxidase assembly protein (Matoba et al. 2006). TP53 stimulates transcription of RRM2B gene, which encodes a subunit of the ribonucleotide reductase complex, responsible for the conversion of ribonucleotides to deoxyribonucleotides and essential for the maintenance of mitochondrial DNA content in the cell (Tanaka et al. 2000, Bourdon et al. 2007, Kulawiec et al. 2009). TP53 also transactivates mitochondrial transcription factor A (TFAM), a nuclear-encoded gene important for mitochondrial DNA (mtDNA) transcription and maintenance (Park et al. 2009). Finally, TP53 stimulates transcription of the mitochondrial glutaminase GLS2, leading to increased mitochondrial respiration rate and reduced ROS levels (Hu et al. 2010).

The great majority of tumor cells generate energy through aerobic glycolysis, rather than the much more efficient aerobic mitochondrial respiration, and this metabolic change is known as the Warburg effect (Warburg 1956). Since the majority of tumor cells have impaired TP53 function, and TP53 regulates a number of genes involved in glycolysis and mitochondrial respiration, it is likely that TP53 inactivation plays an important role in the metabolic derangement of cancer cells such as the Warburg effect and the concomitant increased tumorigenicity (reviewed by Feng and Levine 2010). On the other hand, some mutations of TP53 in Li-Fraumeni syndrome may result in the retention of its wild-type metabolic activities while losing cell cycle and apoptosis functions (Wang et al. 2013). Consistent with such human data, some mutations of p53, unlike p53 null state, retain the ability to regulate energy metabolism while being inactive in regulating its classic gene targets involved in cell cycle, apoptosis and senescence. Retention of metabolic and antioxidant functions of p53 protects p53 mutant mice from early onset tumorigenesis (Li et al. 2012)

Phosphorylation of TP53 at serine residue S46 promotes transcription of TP53-regulated apoptotic genes rather than cell cycle arrest genes. Several kinases can phosphorylate S46 of TP53, including ATM-activated DYRK2, which, like TP53, is targeted for degradation by MDM2 (Taira et al. 2007, Taira et al. 2010). TP53 is also phosphorylated at S46 by HIPK2 in the presence of the TP53 transcriptional target TP53INP1 (D'Orazi et al. 2002, Hofmann et al. 2002, Tomasini et al. 2003). CDK5, in addition to phosphorylating TP53 at S15, also phosphorylates it at S33 and S46, which promotes neuronal cell death (Lee et al. 2007).

MAPKAPK5 (PRAK) phosphorylates TP53 at serine residue S37, promoting cell cycle arrest and cellular senescence in response to oncogenic RAS signaling (Sun et al. 2007).

NUAK1 phosphorylates TP53 at S15 and S392, and phosphorylation at S392 may contribute to TP53-mediated transcriptional activation of cell cycle arrest genes (Hou et al. 2011). S392 of TP53 is also phosphorylated by the complex of casein kinase II (CK2) bound to the FACT complex, enhancing transcriptional activity of TP53 in response to UV irradiation (Keller et al. 2001, Keller and Lu 2002).

The activity of TP53 is inhibited by phosphorylation at serine residue S315, which enhances MDM2 binding and degradation of TP53. S315 of TP53 is phosphorylated by Aurora kinase A (AURKA) (Katayama et al. 2004) and CDK2 (Luciani et al. 2000). Interaction with MDM2 and the consequent TP53 degradation is also increased by phosphorylation of TP53 threonine residue T55 by the transcription initiation factor complex TFIID (Li et al. 2004).

Aurora kinase B (AURKB) has been shown to phosphorylate TP53 at serine residue S269 and threonine residue T284, which is possibly facilitated by the binding of the NIR co-repressor. AURKB-mediated phosphorylation was reported to inhibit TP53 transcriptional activity through an unknown mechanism (Wu et al. 2011). A putative direct interaction between TP53 and AURKB has also been described and linked to TP53 phosphorylation and S183, T211 and S215 and TP53 degradation (Gully et al. 2012)