A p53 autoregulatory feedback loop. Following accumulation, p53 can direct the inhibition of its own transcription by an indirect mechanism and inhibition of its own translation by binding to the 3′ UTR of the p53 mRNA.

The regulation of p53 by JNK. In addition to MDM2, JNK can direct ubiquitylation of p53. Following exposure to agents that induce JNK kinase activity, the JNK no longer can direct degradation of p53 but rather, phosphorylates p53 at the N-terminus. This phosphorylation leads to inhibition of MDM2 binding to p53. Thus, by activating the JNK, UV light knocks out two systems that normally direct degradation of p53.

Ubiquitin-dependent proteolysis of p53. In a sequential enzymatic reaction (E1–E3), ubiquitin (Ub) is activated and added onto the p53 protein. The ubiquitylated p53 protein is than targeted for proteolysis by the 26S proteasome. The proteins involved in directing ubiquitylation of p53 are the cellular proteins, MDM2 and JNK. The HPV 16 protein E6 can also target p53 for degradation by acting as a ubiquitin ligase. Following cellular stresses, proteasome-mediated degradation of p53 is stopped. The p53 protein can also accumulate by default following inhibition of proteasome activity with certain drugs.

Nucleocytoplasmic shuttling of p53. Following its synthesis, the cytoplasmic p53 protein may be directly degraded by the proteasome or imported into the nucleus. Phosphorylation of ser392 may stimulate nuclear localization by favoring tetramerization. p53 is also subject to nuclear export via the CRM1 nuclear export protein complex, a process that can be inhibited by the drug leptomycin B (LMB). The nuclear export process may be stimulated by CDK-mediated phosphorylation of ser315 and by the MDM2 protein. The phosphorylation of ser315, which can be reversed by the Cdc14 phosphatases, inhibits p53 tetramerization. The p14^ARF protein may interfere with the shuttling of both MDM2 and p53 by sequestering MDM2. Following export, the p53 protein is directed to the 26S proteasome by MDM2. Whether some of the exported p53 are allowed to re-enter the nucleus is not clear.

The p53 response is triggered by many different stresses involving both DNA-damaging and non-DNA-damaging agents. These include ribonucleotide synthesis inhibitors resulting in perturbations of nucleotide pools [270], thymidine dinucleotides [271], media depletion [272], hypoxia [273], antioxidants [274,275], inhibition of ubiquitylation or the proteasome proteolysis pathway [53,107,108], DNA strand breaks [95], bulky DNA lesions [10,11], DNA topoisomerase inhibitors [11,276], blockage of RNA polymerase II [71–75], heat shock [273,277], cold shock [278], viral infection [217,279], pRb deregulation [216,280] and oncogene expression [214,215].

Model describing some of the roles of p53 as a tumor suppressor. Cancer is a disease of multiple mutations most of which originate from unrepaired DNA damage induced by radiation or chemical carcinogens. Many defense mechanisms have evolved to safeguard the genetic material and to avoid the formation of mutations. In addition to antioxidants and DNA repair systems, mammalian cells can induce the p53 response to modify the behavior of a cell to avoid the formation of mutations. Following the induction of DNA damage, a “sensor” will trigger the induction of the p53 response that will manifest itself as cell cycle arrest, senescence, enhancement of nucleotide excision repair or apoptosis. All of these scenarios will reduce the probability of mutations arising in the exposed cells, and thus reduce the likelihood of the organism to develop cancer.

The p53 response is activated through multiple pathways. Top: UV light and cisplatin induce bulky lesions in DNA that if formed in the transcribed strand of an active gene will impede the elongation of RNA polymerases. Also, certain drugs that poison the activity of RNA polymerase II, such as actinomycin D, DRB and α-amanitin, trigger the p53 response. Right: DNA strand breaks induced by ionizing radiation may be recognized by the proteins ATM, ATR and NBS which directly or indirectly initiate p53 induction. Bottom: Inhibition of the proteasome by specific drugs, nitric oxide or the overexpression of β-catenin leads to accumulation of p53. Left: O⁶-methylguanine and cisplatin adducts are recognized by mismatch repair proteins resulting in the induction of p53 and p73 (see text for details).

The p53-MDM2-ARF regulatory feedback loops. Under non-stressed conditions, the MDM2 and p53 proteins balance each other and are found in low amounts. In order for p53 to accumulate in cells, It has to escape the inhibitory action of MDM2. This can be accomplished by (A) mutations in the p53 gene resulting in a transactivation-deficient p53 protein; (B) inhibition of general transcription by e.g., UV light; (C) modifications of the MDM2 protein so that it binds p53 less efficiently; (D) induction of the p14^ARF protein which reverses MDM2's inhibitory action on p53 or (E) modifications of the N-terminus of p53 leading to reduced binding to MDM2. DNA-damaging agents may cause p53 accumulation by stimulating the processes denoted with dashed lines, while solid lines denote processes repressing p53 accumulation. Following p53 accumulation, transactivation of the MDM2 gene ensures that the p53 response can be turned off soon after the signal that triggered p53 is removed.

Map of some of the modifications known to occur following ionizing radiation (top) and UV light (bottom). The p53 protein is divided in its three functional domains. The N-terminus contains the transactivation domain and binding sites for MDM2. The central core contains the sequence-specific binding domain. This is where most mutations are found in human tumors. The C-terminus harbors the NLS, NES and tetramerization domains. The p53-activating signal induced by IR is thought to be triggered by DNA strand breaks while UV light will induce p53 through a mechanism involving blocked RNA polymerase II at sites of DNA damage as well as from signals originating from the cell membrane (i.e., p38). The potential sensors are boxed in and mediators/effectors are listed. The dashed lines represents hypothetical pathways. Circled P represents phosphorylated residue while boxed Ac represents acetylated residue.