p53 is a tumour suppressor protein that regulates the expression of a wide variety of genes involved in Apoptosis, Growth arrest, Inhibition of cell cycle progression, Differentiation and accelerated DNA repair or Senescence in response to Genotoxic or Cellular Stress. Having a short half-life, p53 is normally maintained at low levels in unstressed mammalian cells by continuous ubiquitylation and subsequent degradation by the 26S Proteasome. Nonphosphorylated p53 is ubiquitylated by the MDM2(Mouse Double Minute-2) ubiquitin ligase. Thus MDM2 acts as a major regulator of the tumor suppressor p53 by targeting its destruction. When the cell is confronted with stress like DNA damage, Hypoxia, Cytokines, Metabolic changes, Viral infection, or Oncogenes, however, p53 ubiquitylation is suppressed and p53 accumulates in the nucleus, where it is activated and stabilized by undergoing multiple covalent modifications including Phosphorylation and Acetylation (Ref.1 and 2).
Phosphorylation of p53 mostly is mediated by several cellular kinases including Chks (Checkpoint Kinases), CSNK1-Delta (Casein Kinase-1-Delta), CSNK2B (Casein Kinase-2 Beta), PKA (Protein Kinase A), CDK7 (Cyclin-Dependent Kinase-7), DNA-PK (DNA-Activated- Protein Kinase), HIPK2 (Homeodomain-Interacting Protein Kinase-2), CAK (CDK-Activating Kinase), p38 and JNK (Jun NH2-terminal kinase). Notably, phosphorylation at Ser15 by ATM (Ataxia Telangiectasia Mutated Gene)/ATR (Ataxia-Telangiectasia and Rad3 Related), either directly or through Chk1 (Cell Cycle Checkpoint Kinase-1)/Chk2 (Cell Cycle Checkpoint Kinase-2), or at Ser20 by Chk1/Chk2 has been shown to alleviate the inhibition or degradation of p53, leading to p53 stabilization and activation. The phosphorylation-induced p53 stabilization and activation are mediated through multiple mechanisms and may vary according to the cellular context or microenvironment. PIAS1 and PIAS-Gamma function as SUMO (Small Ubiquitin Related Modifier-1) ligases for p53. Moreover, the RING finger domain of PIAS1 binds to the C-terminus of the tumor suppressor p53 and catalyzes its sumoylation, a modification which represses p53 activity on a reporter plasmid containing consensus p53 DNA binding sites. PML (Promyelocytic Leukemia) also activates p53 by recruiting it to multiprotein complexes termed PML-nuclear bodies. PML binds directly with p53 and recruits it to PML-NBs. Recruitment to PML-NBs activate p53 by bringing it in close proximity with CBP (CREB-Binding Protein) /p300. BRCA1(Breast Cancer-1 Gene) and p53 can also physically associate, both in vitro and in vivo and function in a common pathway of tumor suppression (Ref.3, 4 and 5).
Another important modification of p53 is acetylation. p53 is specifically acetylated at Lys370, Lys372, Lys373, Lys381, and Lys382 by p300/CBP and at Lys320 by PCAF(p300/CBP-associated factor). Acetylation has been shown to augment p53 DNA binding, and to stimulate p53-mediated transactivation of its downstream target genes through the recruitment of coactivators. Acetylation may also regulate the stability of p53 by inhibiting its ubiquitination by MDM2
. p53 can also be deacetylated by HDAC1 (Histone Deacetylase-1) and SIRT1. p53 deacetylation has been suggested to down-regulate the activation of genes such as Bax and p21WAF1. Phosphorylation and acetylation are interdependent. Indeed, phosphorylation at the p53 N-terminus has been shown to enhance its interaction with acetylase p300/CBP and to potentiate p53 acetylation. Activated p53 functions effectively as a transcription factor and induces transcription of several genes. Complete p53 is inactive for specific DNA binding unless activated by covalent and noncovalent modifications of the basic C-terminal domain. After p53 is activated it can be involved in cell-cycle inhibition, apoptosis, genetic repair, and inhibition of blood-vessel formation (Ref. 6 and 7).
Cell cycle inhibition takes place when there is a block in cell-cycle division. p53 does this by stimulating the expression of p21WAF1/CIP1 (Cyclin Dependent Kinase Inhibitor-p21). This protein is an inhibitor of CDKs (Cyclin-Dependent Kinases) that regulate the cell cycle via perturbation of their partner cyclin. Cyclins are involved to ensure successful transitions from S phase to G1. Additionally p53 can stimulate 14-3-3, a protein that sequesters Cyclin B1-CDK1 complexes out of the nucleus. This results in a G2 block. Activated p53 may also initiate apoptosis and stop cell proliferation. p53 stimulates a wide network of signals that act through two major apoptotic pathways: Extrinsic Pathways and Intrinsic Pathways. The extrinsic pathway involves engagement of particular `death' receptors that belong to the TNFR (Tumor Necrosis Factor Receptor) family and, through the formation of the DISC (Death-Inducing-Signaling-Complex), leads to a cascade of activation of Caspases, including Caspase8 and Caspase3, which in turn induce apoptosis. p53 may also have a role in maintaining genetic stability by 'nucleotide-excision' repair of DNA, chromosomal recombination and chromosome segmentation. GADD45 (Growth Arrest- and DNA Damage-Inducible Gene-45) is a multifunctional protein that is regulated by p53 and that may play a role in DNA repair and cell cycle checkpoints. p53 can play a role in the inhibition of blood-vessel formation. In order for tumours to reach a large size, they must initiate the growth of nutrient-bringing blood vessels in their vicinity, the process of angiogenesis. p53 activates the expression of the Tsp1 (Thrombospondin-1), an anti-angiogenic factor, along with other angiogenesis inhibitor BAI1 (Brain-specific Angiogenesis Inhibitor-1) (Ref. 8, 9 and 10).
In addition, p53 regulates MDM2 function in a negative feedback loop, because the MDM2 gene is a target for p53. Therefore, activation of p53 eventually leads to its own inactivation by switching on a pathway that leads to its destruction. PTEN
(Phosphatase and Tensin Homolog), on the other hand inhibits MDM2-mediated p53 degradation. p53 can transcriptionally activate PTEN
, which may further inhibit Akt activity. p53 represents an attractive target for the development of anti-cancer therapies. The complex biology and dual functions of p53 in cancer prevention and age-related cellular responses pose significant challenges to the development of p53-targeting cancer therapies. (Ref. 11 and 12).