Inability of p53-reactivating compounds Nutlin-3 and RITA to overcome p53 resistance in tumor cells deficient in p53Ser46 phosphorylation

The p53 tumor suppressor protein plays key roles in protecting cells from tumorigenesis. Phosphorylation of p53 at Ser46 (p53Ser46) is considered to be a crucial modification regulating p53-mediated apoptosis. Because the activity of p53 is impaired in most human cancers, restoration of wild-type p53 (wt-p53) function by its gene transfer or by p53-reactivating small molecules has been extensively investigated. The p53-reactivating compounds Nutlin-3 and RITA activate p53 in the absence of genotoxic stress by antagonizing the action of its negative regulator Mdm2. Although controversial, Nutlin-3 was shown to induce p53-mediated apoptosis in a manner independent of p53 phosphorylation. Recently, RITA was shown to induce apoptosis by promoting p53Ser46 phosphorylation. Here we examined whether Nutlin-3 or RITA can overcome resistance to p53-mediated apoptosis in p53-resistant tumor cell lines lacking the ability to phosphorylate p53Ser46. We show that Nutlin-3 did not rescue the apoptotic defect of a Ser46 phosphorylation-defective p53 mutant in p53-sensitive tumor cells, and that RITA neither restored p53Ser46 phosphorylation nor induced apoptosis in p53Ser46 phosphorylation-deficient cells retaining wt-p53. Furthermore, treatment with Nutlin-3 or RITA together with adenoviral p53 gene transfer also failed to induce apoptosis in p53Ser46 phosphorylation-deficient cells either expressing or lacking wt-p53. These results indicate that neither Nutlin-3 nor RITA in able to induce p53-mediated apoptosis in the absence of p53Ser46 phosphorylation. Thus, the dysregulation of this phosphorylation in tumor cells may be a critical factor that limits the efficacy of these p53-based cancer therapies.     

Hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites

Hyperoxia has been shown to cause DNA damage resulting in growth arrest of cells in p53-dependent, as well as p53-independent, pathways. Although H2O2 and other peroxides have been shown to induce ataxia telangiectasia-mutated (ATM)-dependent p53 phosphorylation in response to DNA damage, the signal transduction mechanisms in response to hyperoxia are currently unknown. Here we demonstrate that hyperoxia phosphorylates the Ser15 residue of p53 independently of ATM. Hyperoxia phosphorylated p53 (Ser15) in DNA-dependent protein kinase null (DNA-PK-/-) cells, indicating that it may not depend on DNA-PK for phosphorylation of p53 (Ser15). We show that Ser37 and Ser392 residues of p53 are also phosphorylated in an ATM-independent manner in hyperoxia. In contrast, H2O2 did not phosphorylate Ser37 in either ATM+/+ or ATM-/- cells. Furthermore, H2O2 failed to phosphorylate Ser15 in ATM-/- cells. Additionally, overexpression of kinase-inactive ATM-and-Rad3-related (ATR) in HEK293T cells diminished Ser15, Ser37, and Ser392 phosphorylation compared with vector-only transfected cells. In contrast, wild-type ATR overexpression did not diminish Ser15, Ser37, or Ser392 phosphorylation. We also show that checkpoint kinase 1 (Chk1) is phosphorylated on Ser345 in response to hyperoxia, which could be inhibited by caffeine or wortmannin, potent inhibitors of phosphoinositide 3-kinase-related kinases. Hyperoxia also phosphorylated Chk1 in ATM+/+ as well as in ATM-/- cells, demonstrating an ATM-independent mechanism in Chk1 phosphorylation. Together, our data suggest that hyperoxia activates the ATR-Chk1 pathway and phosphorylates p53 at multiple sites in an ATM-independent manner, which is different from other forms of oxidative stress such as H2O2 or UV light.     

Allosteric effects mediate CHK2 phosphorylation of the p53 transactivation domain

The tumour suppressor p53 is a tetrameric protein that is phosphorylated in its BOX-I transactivation domain by checkpoint kinase 2 (CHK2) in response to DNA damage. CHK2 cannot phosphorylate small peptide fragments of p53 containing the BOX-I motif, indicating that undefined determinants in the p53 tetramer mediate CHK2 recognition. Two peptides derived from the DNA-binding domain of p53 bind to CHK2 and stimulate phosphorylation of full-length p53 at Thr 18 and Ser 20, thus identifying CHK2-docking sites. CHK2 can be fully activated in trans by the two p53 DNA-binding-domain peptides, and can phosphorylate BOX-I transactivation-domain fragments of p53 at Thr 18 and Ser 20. Although CHK2 has a basal Ser 20 kinase activity that is predominantly activated towards Thr 18, CHK1 has constitutive Thr 18 kinase activity that is predominantly activated in trans towards Ser 20. Cell division cycle 25C (CDC25C) phosphorylation by CHK2 is unaffected by the p53 DNA-binding-domain peptides. The CHK2-docking site in the BOX-V motif is the smallest of the two CHK2 binding sites, and mutating certain amino acids in the BOX-V peptide prevents CHK2 activation. A database search identified a p53 BOX-I-homology motif in p21^WAF1 and although CHK2 is inactive towards this protein, the p53 DNA-binding-domain peptides induce phosphorylation of p21^WAF1 at Ser 146. This provides evidence that CHK2 can be activated allosterically towards some substrates by a novel docking interaction, and identify a potential regulatory switch that may channel CHK2 into distinct signalling pathways in vivo.     

The C-terminal regulatory domain of p53 contains a functional docking site for cyclin A

Radiation injury to cells enhances C-terminal phosphorylation of p53 at both Ser315 and Ser392 in vivo, suggesting the existence of two cooperating DNA damage-responsive pathways that play a role in stimulating p53-dependent gene expression. Our previous data has shown that cyclin A-cdk2 is the major enzyme responsible for modifying p53 at Ser315 in vivo after irradiation damage and in this report we dissect the mechanism of cyclinA-cdk2 binding to and phosphorylation of p53. Although cyclin B(1)-dependent protein kinases can phosphorylate small peptides containing the Ser315 site, cyclin A-cdk2 does not phosphorylate such small peptides suggesting that additional determinants are required for cyclin A-cdk2 interaction with p53. Peptide competition studies have localized a cyclin A interaction site to a Lys381Lys382Leu383Met384Phe385 sequence within C-terminal negative regulatory domain of human p53. An alanine mutation at any one of four key positions abrogates the efficacy of a synthetic peptide containing this motif as an inhibitor of cyclin A-cdk2 phosphorylation of p53 protein. Single amino acid mutations of full-length p53 protein at Lys382, Leu383, or Phe385 decreases cyclin A-cdk2 dependent phosphorylation at Ser315. Cyclin B(1)-cdk2 complexes are not inhibited by KKLMF motif-containing peptides nor is p53 phosphorylation by cyclin B-cdk2 reduced by mutation of the cyclin A interaction site. These data identifying a KKLMF cyclin A docking site on p53 protein highlight a common cyclin A interaction motif that is shared between the tumour suppressor proteins pRb and p53.     

JNK1 Phosphorylates SIRT1 and Promotes Its Enzymatic Activity

SIRT1 is a NAD-dependent deacetylase that regulates a variety of pathways including the stress protection pathway. SIRT1 deacetylates a number of protein substrates, including histones, FOXOs, PGC-1α, and p53, leading to cellular protection. We identified a functional interaction between cJUN N-terminal kinase (JNK1) and SIRT1 by coimmunoprecipitation of endogenous proteins. The interaction between JNK1 and SIRT1 was identified under conditions of oxidative stress and required activation of JNK1 via phosphorylation. Modulation of SIRT1 activity or protein levels using nicotinamide or RNAi did not modify JNK1 activity as measured by its ability to phosphorylate cJUN. In contrast, human SIRT1 was phosphorylated by JNK1 on three sites: Ser27, Ser47, and Thr530 and this phosphorylation of SIRT1 increased its nuclear localization and enzymatic activity. Surprisingly, JNK1 phosphorylation of SIRT1 showed substrate specificity resulting in deacetylation of histone H3, but not p53. These findings identify a mechanism for regulation of SIRT1 enzymatic activity in response to oxidative stress and shed new light on its role in the stress protection pathway.     

Potentiality of DNA-dependent protein kinase to phosphorylate Ser46 of human p53

DNA damage induces accumulation and activation of p53 via various posttranslational modifications. Among them, several lines of evidence indicated the phosphorylation of Ser46 as an important mediator of DNA damage-induced apoptosis but the responsible kinase remains to be clarified, especially in the case of ionizing radiation (IR). Here we showed that DNA-dependent protein kinase (DNA-PK) could phosphorylate Ser46 of p53 in addition to reported phosphorylation sites Ser15 and Ser37. However, IR-induced phosphorylation of Ser46 was seen even in M059J, a human glioma cell line lacking DNA-PKcs, and it was, at most, only slightly less than in control M059K. On the other hand, a related kinase ataxia-telangiectasia mutated (ATM), which was shown to be essential for IR-induced phosphorylation of Ser46, could poorly phosphorylate Ser46 by itself. These results collectively suggested two pathways for IR-induced phosphorylation of Ser46, i.e., direct phosphorylation by DNA-PK and indirect phosphorylation via ATM.     

p53DINP1, a p53-inducible gene, regulates p53-dependent apoptosis

Using the differential display method combined with a cell line that carries a well-controlled expression system for wild-type p53, we isolated a p53-inducible gene, termed p53DINP1 (p53-dependent damage-inducible nuclear protein 1). Cell death induced by DNA double-strand breaks (DSBs), as well as Ser46 phosphorylation of p53 and induction of p53AIP1, were blocked when we inhibited expression of p53DINP1 by means of an antisense oligonucleotide. Overexpression of p53DINP1 and DNA damage by DSBs synergistically enhanced Ser46 phosphorylation of p53, induction of p53AIP1 expression, and apoptotic cell death. Furthermore, the protein complex interacting with p53DINP1 was shown to phosphorylate Ser46 of p53. Our results suggest that p53DINP1 may regulate p53-dependent apoptosis through phosphorylation of p53 at Ser46, serving as a cofactor for the putative p53-Ser46 kinase.     

Loss of p53 Ser18 and Atm results in embryonic lethality without cooperation in tumorigenesis

Phosphorylation at murine Serine 18 (human Serine 15) is a critical regulatory process for the tumor suppressor function of p53. p53Ser18 residue is a substrate for ataxia-telangiectasia mutated (ATM) and ATM-related (ATR) protein kinases. Studies of mice with a germ-line mutation that replaces Ser18 with Ala (p53(S18A) mice) have demonstrated that loss of phosphorylation of p53Ser18 leads to the development of tumors, including lymphomas, fibrosarcomas, leukemia and leiomyosarcomas. The predominant lymphoma is B-cell lymphoma, which is in contrast to the lymphomas observed in Atm(-/-) animals. This observation and the fact that multiple kinases phosphorylate p53Ser18 suggest Atm-independent tumor suppressive functions of p53Ser18. Therefore, in order to examine p53Ser18 function in relationship to ATM, we analyzed the lifespan and tumorigenesis of mice with combined mutations in p53Ser18 and Atm. Surprisingly, we observed no cooperation in survival and tumorigenesis in compound p53(S18A) and Atm(-/-) animals. However, we observed embryonic lethality in the compound mutant animals. In addition, the homozygous p53Ser18 mutant allele impacted the weight of Atm(-/-) animals. These studies examine the genetic interaction of p53Ser18 and Atm in vivo. Furthermore, these studies demonstrate a role of p53Ser18 in regulating embryonic survival and motor coordination.     

Elevated levels of oncogenic protein kinase Pim-1 induce the p53 pathway in cultured cells and correlate with increased Mdm2 in mantle cell lymphoma

Mutation of the p53 gene is a common event during tumor pathogenesis. Other mechanisms, such as mdm2 amplification, provide alternative routes through which dysfunction of the p53 pathway is promoted. Here, we address the hypothesis that elevated expression of pim oncogenes might suppress p53 by regulating Mdm2. At a physiological level, we show that endogenous Pim-1 and Pim-2 interact with endogenous Mdm2. Additionally, the Pim kinases phosphorylate Mdm2 in vitro and in cultured cells at Ser(166) and Ser(186), two previously identified targets of other signaling pathways, including Akt. Surprisingly, at high levels of Pim expression, as would occur in tumors, active, but not inactive, Pim-1 or Pim-2 blocks the degradation of both p53 and Mdm2 in a manner that is independent of Mdm2 phosphorylation, leading to increased p53 levels and, proportionately, p53-dependent transactivation. Additionally, Pim-1 induces endogenous ARF, p53, Mdm2, and p21 in primary murine embryo fibroblasts and stimulates senescence-associated beta-galactosidase levels, consistent with the induction of senescence. Immunohistochemical analysis of a cohort of 33 human mantle cell lymphomas shows that elevated expression of Pim-1 occurs in 42% of cases, with elevated Pim-2 occurring in 9% of cases, all of which also express Pim-1. Notably, elevated Pim-1 correlates with elevated Mdm2 in MCL with a p value of 0.003. Taken together, our data are consistent with the idea that Pim normally interacts with the p53 pathway but, when expressed at pathological levels, behaves as a classic dominant oncogene that stimulates a protective response through induction of the p53 pathway.     

Defective apoptosis and B-cell lymphomas in mice with p53 point mutation at Ser 23

Phosphorylation of the p53 tumor suppressor at Ser20 (murine Ser23) has been proposed to be critical for disrupting p53 interaction with its negative regulator, MDM2, and allowing p53 stabilization. To determine the importance of Ser23 for the function of p53 in vivo, we generated a mouse in which the endogenous p53 locus was targeted to replace Ser23 with alanine. We show that, in mouse embryonic fibroblasts generated from Ser23 mutant mice, Ser23 mutation did not dramatically reduce IR-induced p53 protein stabilization or p53-dependent cell cycle arrest. However, in Ser23 mutant thymocytes and in the developing cerebellum, p53 stabilization following IR was decreased and resistance to apoptosis was observed. Homozygous Ser23 mutant animals had a reduced lifespan, but did not develop thymic lymphomas or sarcomas that are characteristic of p53-/- mice. Instead, Ser23 mutant animals died between 1 and 2 years with tumors that were most commonly of B-cell lineage. These data support an important role for Ser20/23 phosphorylation in p53 stabilization, apoptosis and tumor suppression.     

Cross talk between PML and p53 during poliovirus infection: implications for antiviral defense

PML nuclear bodies (NBs) are dynamic intranuclear structures harboring numerous transiently or permanently localized proteins. PML, the NBs' organizer, is directly induced by interferon, and its expression is critical for antiviral host defense. We describe herein the molecular events following poliovirus infection that lead to PML-dependent p53 activation and protection against virus infection. Poliovirus infection induces PML phosphorylation through the extracellular signal-regulated kinase pathway, increases PML SUMOylation, and induces its transfer from the nucleoplasm to the nuclear matrix. These events result in the recruitment of p53 to PML NBs, p53 phosphorylation on Ser15, and activation of p53 target genes leading to the induction of apoptosis. Moreover, the knock-down of p53 by small interfering RNA results in higher poliovirus replication, suggesting that p53 participates in antiviral defense. This effect, which requires the presence of PML, is transient since poliovirus targets p53 by inducing its degradation in a proteasome- and MDM2-dependent manner. Our results provide evidence of how poliovirus counteracts p53 antiviral activity by regulating PML and NBs, thus leading to p53 degradation.     

p53 localization at centrosomes during mitosis and postmitotic checkpoint are ATM-dependent and require serine 15 phosphorylation

We recently demonstrated that the p53 oncosuppressor associates to centrosomes in mitosis and this association is disrupted by treatments with microtubule-depolymerizing agents. Here, we show that ATM, an upstream activator of p53 after DNA damage, is essential for p53 centrosomal localization and is required for the activation of the postmitotic checkpoint after spindle disruption. In mitosis, p53 failed to associate with centrosomes in two ATM-deficient, ataxiatelangiectasia-derived cell lines. Wild-type ATM gene transfer reestablished the centrosomal localization of p53 in these cells. Furthermore, wild-type p53 protein, but not the p53-S15A mutant, not phosphorylatable by ATM, localized at centrosomes when expressed in p53-null K562 cells. Finally, Ser15 phosphorylation of endogenous p53 was detected at centrosomes upon treatment with phosphatase inhibitors, suggesting that a p53 dephosphorylation step at centrosome contributes to sustain the cell cycle program in cells with normal mitotic spindles. When dissociated from centrosomes by treatments with spindle inhibitors, p53 remained phosphorylated at Ser15. AT cells, which are unable to phosphorylate p53, did not undergo postmitotic proliferation arrest after nocodazole block and release. These data demonstrate that ATM is required for p53 localization at centrosome and support the existence of a surveillance mechanism for inhibiting DNA reduplication downstream of the spindle assembly checkpoint     

Questioning the role of checkpoint kinase 2 in the p53 DNA damage response

Cdc25C and p53 have been reported to be physiological targets of checkpoint kinase 2 (Chk2). Surprisingly, although Chk2 purified from DNA damage sustaining cells has dramatically increased ability to phosphorylate Cdc25C when compared with untreated cells, its ability to phosphorylate p53 is weak before treatment, and there is no increase in its activity toward p53 after DNA damage by gamma irradiation or the radiomimetic agent neocarzinostatin. Furthermore, introduction of Chk2 short interfering RNA into three different human tumor cell lines leads to marked reduction of Chk2 protein, but p53 is still stabilized and active after DNA damage. The results with Chk1 short interfering RNA indicate as well that Chk1 does not play a role in human p53 stabilization after DNA damage. Thus, Chk1 and Chk2 are unlikely to be regulators of p53 in at least some human tumor cells. We discuss our results in the context of previous findings demonstrating a requirement for Chk2 in p53 stabilization and activity.     

Phosphorylation of serine 392 in p53 is a common and integral event during p53 induction by diverse stimuli

Post-translational modifications play important roles during the stabilisation and activation of p53 by various genotoxic and non-genotoxic stresses. Ser392 has been reported to be a major UV-stimulated phosphorylation site that is modified through the p38 MAPK pathway in a manner that may involve recruitment of CK2. Here we show that phosphorylation of Ser392 is an integral event that occurs not only in response to UV, but also during the induction of p53 by a range of stimuli including treatment of cells with the MDM2 inhibitor, Nutlin 3a. Strikingly, phosphorylation of Ser392 and Ser33 was also observed following induction of the p53 pathway by ARF which has previously been thought to induce p53 in a phosphorylation-independent manner. The induction of Ser392 phosphorylation by diverse stimuli can be explained by a common mechanism in which its phosphorylation at a low rate, coupled with the rapid turnover of p53, limits the accumulation of phosphorylated molecules until a stimulus stabilises p53 and allows the Ser392-phosphorylated p53 to accumulate. We also provide biological evidence that Ser392 phosphorylation is not mediated by a UV-associated route involving p38 MAPK, either directly or indirectly via CK2. These data suggest that, physiologically, Ser392 may be phosphorylated by an, as yet, unidentified protein kinase.     

SN2 DNA-alkylating agent-induced phosphorylation of p53 and activation of p21 gene expression

p53 is an important player in the cellular response to genotoxic stress whose functions are regulated by phosphorylation of a number of serine and threonine residues. Phosphorylation of p53 influences its DNA-binding and gene regulation activities. This study examines p53 phosphorylation in HCT-116 (MMR-deficient) and HCT-116+ch3 (MMR-proficient) human colon cancer cells treated with a S(N)2 DNA-alkylating agent, methylmethane sulfonate (MMS). MMS induces phosphorylation of p53 on Ser15 and Ser392 in a dose- and time-dependent manner. MMS-induced p53 phosphorylation is independent of DNA mismatch repair (MMR) activity. Nuclear extracts from MMS-treated HCT-116 cells had higher p21WAF1/Cip1 (p21) promoter DNA-binding activity in vitro opposed to untreated cells. After MMS treatment, the activation of the cloned p21 promoter in a transient transfection assay and endogenous p21 mRNA levels in HCT-116(p53+/+) versus HCT-116(p53-/-) cells increased, which correlates with an increased levels of phospho-p53(Ser15) and phospho-p53(Ser392). These results suggest that SN2 DNA-alkylating agent-induced phosphorylation of p53 on Ser15 and Ser392 increases its DNA-binding properties to cause an increased expression of p21 that may play a role in cell cycle arrest and/or apoptosis of HCT-116 cells.