p53 promotes transcriptional activation or repression of target genes by interacting with general transcriptional factors such as TFIID (Transcription Factor II D) [17
]. Genes activated by wild-type p53 are functionally diverse and constitute downstream effectors of signaling pathways that elicit diverse responses such as cell-cycle checkpoints, cell survival, apoptosis and senescence. These genes include genes involved in cell cycle arrest, DNA repair, apoptosis and senescence-related genes such as genes p21/Cip1 (Cyclin dependent kinase interacting protein 1), Gadd45 (growth arrest and DNA-damage inducible protein 45) and genes of the Bcl-2 family [13
p53 activation basically depends on cell type, environmental milieu and the nature of the stress. In response to sustained or severe stress signals, p53 leads to irreversible apoptosis or senescence. p53 triggered apoptosis involves the transcriptional induction of both the extrinsic and intrinsic death pathways including BAX, FAS, NOXA and PUMA [18
Numerous p53 target proteins function to inhibit apoptosis including p21, decoy death receptors such as DcR1 and DcR2, the transcription factor SLUG (which represses the expression of PUMA) and several activators of the AKT/PKB (protein kinase B) survival pathways [20
]. In some cases, p53 responds to potent stress by inducing cellular senescence through transcriptional activation of target genes such as p21, PAI1 and PML [21
Under conditions of lower levels of stress, when repair is possible, p53 causes cell-cycle arrest and DNA repair to allow cells to pause and repair any damage, thereby limiting the propagation of oncogenic mutations. Another protective, pro-survival mechanism is the capacity of p53 to up regulate the expression of antioxidant genes, such as sestrins 1 and 2 (SESN1 and SESN2, respectively), GPX1 and TIGAR which suppress the accumulation of reactive oxygen species, thereby maintaining genomic integrity [22
] (Figure 1).
p53 can also limit tumorigenesis through autophagy or ‘self- eating’, which can provoke cell death through the activation of genes such as AMPK, DRAM, SESN1 and SESN2. p53 can also exert non-cell-autonomous effects that are pivotal to tumor suppression; the ability to impede angiogenesis through induction of gene products such as thrombospondin-1 (TSP-1) and inhibition of tumor growth and metastasis by stimulating signaling from the fibroblast compartment of tumors [24
] (Figure 2).
The G1/S checkpoint prevents initiation of DNA replication in cells that have damaged DNA. Expression of p53 following DNA damage, arrests cells at the G1/S transition. Cell cycle progression is driven by phosphorylation events mediated by cyclin/CDK complexes. Cyclin D/CDK4, Cyclin E/CDK2 and Cyclin A/CDK2 complexes sequentially phosphorylate the tumour suppressor pRb and its family members resulting in release of the E2F family of transcription factors and transactivation of genes involved in DNA replication. These cyclin/CDK complexes are thus the main targets of the effectors of the G1/S checkpoint. Following DNA damage, p53 induces the expression of the CDK inhibitor p21. p21 once induced, localizes to the nucleus in wild-type p53 cells undergoing G1 arrest, but not in cells expressing mutant p53. Mouse embryo fibroblasts derived from p21 deficient mice have an impaired DNA damage induced G1 arrest [26
The critical cyclin B1/CDC2 complex is the main target of the G2 checkpoint and involves the activation of the ATM (Ataxia telangiectasia mutated protein) and ATR (Ataxia telangiectasia and Rad3-related protein) and their downstream substrates Chk1 and Chk2 (Checkpoint kinase 1 and 2). p53 is involved in the maintenance rather than the initiation of the G2 arrest. Several p53 target genes are shown to play a role in the p53 induced G2 arrest. Cdc25C, the phosphatase that promotes mitosis is inhibited after DNA damage through phosphorylation at Ser216 by Chk1, Chk2 and other kinases. This modification creates a binding site for 14-3-3δ regulatory protein and this association sequesters Cdc25C in the cytoplasm and/or inhibits its phosphatase activity. Cdc25C has been shown to be a target of repression by p53 following DNA damage [27
] (Figure 3).
In addition, a particular 14-3-3δ isoform, 14-3-3s, is also a p53 target gene and is upregulated following DNA damage. 14-3-3s prevents proper nuclear localization of cyclin B1/CDC2 after DNA damage. Although it has been suggested that p21 is a poor inhibitor of CDC2 in vitro
compared to other cyclin dependent kinases, p21 has also been implicated in the sustained G2 arrest, through inhibition of the cyclin B1/CDC2 complex activity [28
Another p53 target gene involved in the G2 arrest is GADD45. GADD45 also interacts with CDC2 and inhibits its kinase activity, presumably by causing dissociation of the cyclin B1/CDC2 complex. Further, induction of GADD45 results in G2 arrest associated with increased cytoplasmic cyclin B1. Of note, induction of G2 arrest by GADD45 requires the presence of wild-type p53 and depends on the type of DNA damage [26
]. The functions of p53 are summarized in Table 1.
Sequence-specific DNA binding of p53 is a prerequisite for the transactivation of target genes. Typically, p53 response elements (p53REs) are located within a few thousand nucleotides upstream or downstream from the transcription start site. Frequently, p53 targets contain at least two widely spaced p53REs. However, not all target genes are equally responsive to p53, suggesting additional layers of regulation. The interaction between p53 and its DNA target sequences is highly influenced by the cellular context [30
A plethora of partner proteins have been implicated in modulating the selection of p53 targets. Some of those proteins are transcription factors themselves, which presumably bind to promoter sites adjacent to p53REs to selectively induce specific response genes. Others influence the ability of p53 itself to bind preferentially to particular DNA target sequences and not to others. The cellular environment as well as the relative abundance of these potential partners under different conditions could obviously tip the life-or-death balance of p53 activity [30
] (Figure 4).
Additional transcriptional programs controlled by p53 in regulating metabolic pathways are being unraveled. For example, modulating glucose uptake [31
], enhancing mitochondrial respiration [32
], inhibition of glycolysis (through TIGAR) and promoting oxidative phosphorylation (through SCO2) to protect cells from metabolic reprogramming-known as the ‘Warburg effect’-which is considered significant in malignant transformation [33