Stabilization of Mdm2 via decreased ubiquitination is mediated by protein kinase B/Akt-dependent phosphorylation

The tumor suppressor p53 is commonly inhibited under conditions in which the phosphatidylinositide 3'-OH kinase/protein kinase B (PKB)Akt pathway is activated. Intracellular levels of p53 are controlled by the E3 ubiquitin ligase Mdm2. Here we show that PKB inhibits Mdm2 self-ubiquitination via phosphorylation of Mdm2 on Ser(166) and Ser(188). Stimulation of human embryonic kidney 293 cells with insulin-like growth factor-1 increased Mdm2 phosphorylation on Ser(166) and Ser(188) in a phosphatidylinositide 3'-OH kinase-dependent manner, and the treatment of both human embryonic kidney 293 and COS-1 cells with phosphatidylinositide 3'-OH kinase inhibitor LY-294002 led to proteasome-mediated Mdm2 degradation. Introduction of a constitutively active form of PKB together with Mdm2 into cells induced phosphorylation of Mdm2 at Ser(166) and Ser(188) and stabilized Mdm2 protein. Moreover, mouse embryonic fibroblasts lacking PKBalpha displayed reduced Mdm2 protein levels with a concomitant increase of p53 and p21(Cip1), resulting in strongly elevated apoptosis after UV irradiation. In addition, activation of PKB correlated with Mdm2 phosphorylation and stability in a variety of human tumor cells. These findings suggest that PKB plays a critical role in controlling of the Mdm2.p53 signaling pathway by regulating Mdm2 stability.     

Phosphorylation of serine 18 regulates distinct p53 functions in mice

The p53 protein acts a tumor suppressor by inducing cell cycle arrest and apoptosis in response to DNA damage or oncogene activation. Recently, it has been proposed that phosphorylation of serine 15 in human p53 by ATM (mutated in ataxia telangiectasia) kinase induces p53 activity by interfering with the Mdm2-p53 complex formation and inhibiting Mdm2-mediated destabilization of p53. Serine 18 in murine p53 has been implicated in mediating an ATM- and ataxia telangiectasia-related kinase-dependent growth arrest. To explore further the physiological significance of phosphorylation of p53 on Ser18, we generated mice bearing a serine-to-alanine mutation in p53. Analysis of apoptosis in thymocytes and splenocytes following DNA damage revealed that phosphorylation of serine 18 was required for robust p53-mediated apoptosis. Surprisingly, p53Ser18 phosphorylation did not alter the proliferation rate of embryonic fibroblasts or the p53-mediated G(1) arrest induced by DNA damage. In addition, endogenous basal levels and DNA damage-induced levels of p53 were not affected by p53Ser18 phosphorylation. p53Ala18 mice developed normally and were not susceptible to spontaneous tumorigenesis, and the reduced apoptotic function of p53Ala18 did not rescue the embryo-lethal phenotype of Mdm2-null mice. These results indicate that phosphorylation of the ATM target site on p53 specifically regulates p53 apoptotic function and further reveal that phosphorylation of p53 serine 18 is not required for p53-mediated tumor suppression.     

Damage-mediated phosphorylation of human p53 threonine 18 through a cascade mediated by a casein 1-like kinase. Effect on Mdm2 binding

The p53 tumor suppressor protein is stabilized in response to ionizing radiation and accumulates in the nucleus. Stabilization is thought to involve disruption of the interaction between the p53 protein and Mdm2, which targets p53 for degradation. Here we show that the direct association between a p53 N-terminal peptide and Mdm2 is disrupted by phosphorylation of the peptide on Thr(18) but not by phosphorylation at other N-terminal sites, including Ser(15) and Ser(37). Thr(18) was phosphorylated in vitro by casein kinase (CK1); this process required the prior phosphorylation of Ser(15). Thr(18) was phosphorylated in vivo in response to DNA damage, and such phosphorylation required Ser(15). Our results suggest that stabilization of p53 after ionizing radiation may result, in part, from an inhibition of Mdm2 binding through a phosphorylation-phosphorylation cascade involving DNA damage-activated phosphorylation of p53 Ser(15) followed by phosphorylation of Thr(18).     

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.     

Mutation of mouse p53 Ser23 and the response to DNA damage

Recent studies have suggested that phosphorylation of human p53 at Ser20 is important for stabilizing p53 in response to DNA damage through disruption of the interaction between MDM2 and p53. To examine the requirement for this DNA damage-induced phosphorylation event in a more physiological setting, we introduced a missense mutation into the endogenous p53 gene of mouse embryonic stem (ES) cells that changes serine 23 (S23), the murine equivalent of human serine 20, to alanine (A). Murine embryonic fibroblasts harboring the p53(S23A) mutation accumulate p53 as well as p21 and Mdm2 proteins to normal levels after DNA damage. Furthermore, ES cells and thymocytes harboring the p53(S23A) mutation also accumulate p53 protein to wild-type levels and undergo p53-dependent apoptosis similarly to wild-type cells after DNA damage. Therefore, phosphorylation of murine p53 at Ser23 is not required for p53 responses to DNA damage induced by UV and ionizing radiation treatment.     

Concomitant Inhibition of MDM2 and Bcl-2 Protein Function Synergistically Induce Mitochondrial Apoptosis in AML

Disruption of Mdm2-p53 interaction activates p53 signaling, disrupts the balance ofantiapoptotic and proapoptotic Bcl-2 family proteins and induces apoptosis in acutemyeloid leukemia (AML). Overexpression of Bcl-2 may inhibit this effect. Thus,functional inactivation of antiapoptotic Bcl-2 proteins may enhance apoptogenic effects ofMdm2 inhibition. We here investigate the potential therapeutic utility of combinedtargeting of Mdm2 by Nutlin-3a and Bcl-2 by ABT-737, recently developed inhibitors ofprotein-protein interactions. Nutlin-3a and ABT-737 induced Bax conformational changeand mitochondrial apoptosis in AML cells in a strikingly synergistic fashion. Nutlin-3ainduced p53-mediated apoptosis predominantly in S and G₂/M cells, while cells in G₁ were protected through induction of p21. In contrast, ABT-737 induced apoptosis predominantly in G₁ , the cell cycle phase with the lowest Bcl-2 protein levels and Bcl-2/Bax ratios. In addition, Bcl-2 phosphorylation on Ser⁷⁰ was absent in G₁ but detectable in G₂/M, thus lower Bcl-2 levels and absence of Bcl-2 phosphorylation appeared to facilitate ABT-737-induced apoptosis of G₁ cells. The complementary effects of Nutlin-3a and ABT-737 in different cell cycle phases could, in part, account for their synergistic activity. Our data suggest that combined targeting of Mdm2 and Bcl-2 proteins could offer considerable therapeutic promise in AML.     

Silibinin up-regulates DNA-protein kinase-dependent p53 activation to enhance UVB-induced apoptosis in mouse epithelial JB6 cells

In the present study, we employed a well established JB6 mouse epithelial cell model to define the molecular mechanism of efficacy of a naturally occurring flavonoid silibinin against ultraviolet B (UVB)-induced skin tumorigenesis. UVB exposure of cells caused a moderate phosphorylation of ERK1/2 and Akt and a stronger phosphorylation of p53 at Ser(15), which was enhanced markedly by silibinin pretreatment. Kinase activity of ERK1/2 for Elk-1 and Akt for glycogen synthase kinase-3beta was also potently enhanced by silibinin pretreatment. Furthermore, silibinin increased the UVB-induced level of cleaved caspase 3 as well as apoptotic cells. Based on these observations, next we investigated the role of upstream kinases, ATM/ATR and DNA-PK, which act as sensors for UVB-induced DNA damage and transduce signals leading to DNA repair or apoptosis. Whereas UVB strongly activated ATM as observed by Ser(1981) phosphorylation, it was not affected by silibinin pretreatment. However, pretreatment of cells with the DNA-protein kinase (PK) inhibitor LY294002 strongly reversed silibinin-enhanced Akt-Ser(473) and p53-Ser(15) as well as ERK1/2 phosphorylation together with a dose-dependent decrease in cleaved caspase 3 and apoptosis (p < 0.05). In addition, silibinin pretreatment strongly enhanced H2A.X-Ser(139) phosphorylation and DNA-PK-associated kinase activity as well as the physical interaction of p53 with DNA-PK; pretreatment of cells with LY294002 but not caffeine abolished the silibinin-caused increase in both DNA-PK activation and p53-Ser(15) phosphorylations. Together, these findings suggest that silibinin preferentially activates the DNA-PK-p53 pathway for apoptosis in response to UVB-induced DNA damage, and that this could be a predominant mechanism of silibinin efficacy against UVB-induced skin cancer.     

Regulation of p63 function by Mdm2 and MdmX.

p63, a p53-related protein, has been shown to activate p53-responsive genes and induce apoptosis in certain cell types. In this study, we examined the effects of Mdm2 and MdmX proteins on p63 transactivation, apoptosis, and protein levels. The isoforms of p63 most structurally similar to p53, p63gamma (p51A) and p63alpha (p51B), were chosen for study. Our results confirm earlier reports demonstrating that although both p63 isoforms can transactivate p53-responsive promoters and induce apoptosis, p63gamma has a stronger transactivation potential and is a more potent inducer of apoptosis than is p63alpha. In addition, both Mdm2 and MdmX were able to inhibit the transactivation induced by p63gamma and p63alpha. However, only Mdm2 overexpression led to a detectable decrease in p63-induced apoptosis. Although Mdm2 binding to p53 triggers ubiquitin-mediated proteosome degradation, p63 protein levels were unaltered by association with either Mdm2 or MdmX. Finally, immunofluorescence experiments showed that both p63 isoforms were localized in the nucleus and could be exported when coexpressed with Mdm2 but not with MdmX. These findings suggest that both Mdm2 and MdmX can downregulate p63 transactivation potential; however, only Mdm2 is capable of inhibiting the apoptotic function of p63 by removing it from the nucleus.     

Uteroplacental insufficiency increases p53 phosphorylation without triggering the p53-MDM2 functional circuit response in the IUGR rat kidney

Uteroplacental insufficiency (UPI) leads to intrauterine growth restriction (IUGR), which predisposes infants toward renal insufficiency early in life and increases the risk of kidney-related adult morbidities, such as hypertension. This compromised in utero environment has been demonstrated to impair nephrogenesis, as evidenced by a reduced nephron endowment in humans and in rats rendered IUGR by UPI. Concordantly, we have observed that IUGR rats have increased kidney p53 protein levels associated with increased apoptosis. Several factors can regulate p53 gene expression and activity, including posttranslational modifications and protein-protein interactions in the cell. Among these, two important mechanisms are 1) phosphorylation of the amino terminal serine 15 [phospho-p53 (Ser15)], which increases p53 stability and apoptotic activity, and 2) the murine double-minute (MDM2) functional circuit that limits further p53-induced apoptosis by promoting proteosomal degradation of p53. We hypothesize that UPI induces an increase in phospho-p53 (Ser15) in association with an absent MDM2 response, predisposing the kidney to increased apoptosis. To test our hypothesis, we induced IUGR through bilateral uterine artery ligation of the pregnant rat. UPI significantly increased phospho-p53 (Ser15), as well as ataxia teleangiectasia-mutated kinase/A-T-related kinase and dsDNA-activated protein kinase kinase levels, which induce phosphorylation of p53. In contrast, UPI induced no increase in kidney MDM2 mRNA and protein levels in IUGR pups. We conclude that among multiple mechanisms that affect nephrogenesis, UPI induces an increase in p53 phosphorylation without a corresponding increase in MDM2 expression, and we speculate that this response may contribute to the increased apoptosis previously described in the IUGR kidney.     

Mdm2 and Mdm4 loss regulates distinct p53 activities

Mutational inactivation of p53 is a hallmark of most human tumors. Loss of p53 function also occurs by overexpression of negative regulators such as MDM2 and MDM4. Deletion of Mdm2 or Mdm4 in mice results in p53-dependent embryo lethality due to constitutive p53 activity. However, Mdm2(-/-) and Mdm4(-/-) embryos display divergent phenotypes, suggesting that Mdm2 and Mdm4 exert distinct control over p53. To explore the interaction between Mdm2 and Mdm4 in p53 regulation, we first generated mice and cells that are triple null for p53, Mdm2, and Mdm4. These mice had identical survival curves and tumor spectrum as p53(-/-) mice, substantiating the principal role of Mdm2 and Mdm4 as negative p53 regulators. We next generated mouse embryo fibroblasts null for p53 with deletions of Mdm2, Mdm4, or both; introduced a retrovirus expressing a temperature-sensitive p53 mutant, p53A135V; and examined p53 stability and activity. In this system, p53 activated distinct target genes, leading to apoptosis in cells lacking Mdm2 and a cell cycle arrest in cells lacking Mdm4. Cells lacking both Mdm2 and Mdm4 had a stable p53 that initiated apoptosis similar to Mdm2-null cells. Additionally, stabilization of p53 in cells lacking Mdm4 with the Mdm2 antagonist nutlin-3 was sufficient to induce a cell death response. These data further differentiate the roles of Mdm2 and Mdm4 in the regulation of p53 activities.     

ATM mediates phosphorylation at multiple p53 sites, including Ser(46), in response to ionizing radiation

The p53 tumor suppressor protein preserves genome integrity by regulating growth arrest and apoptosis in response to DNA damage. In response to ionizing radiation (IR), ATM, the gene product mutated in ataxia telangiectasia, stabilizes and activates p53 through phosphorylation of Ser(15) and (indirectly) Ser(20). Here we show that phosphorylation of p53 on Ser(46), a residue important for p53 apoptotic activity, as well as on Ser(9), in response to IR also is dependent on the ATM protein kinase. IR-induced phosphorylation at Ser(46) was inhibited by wortmannin, a phosphatidylinositol 3-kinase inhibitor, but not PD169316, a p38 MAPK inhibitor. p53 C-terminal acetylation at Lys(320) and Lys(382), which may stabilize p53 and activate sequence-specific DNA binding, required Ser(15) phosphorylation by ATM and was enhanced by phosphorylation at nearby residues including Ser(6), Ser(9), and Thr(18). These observations, together with the proposed role of Ser(46) phosphorylation in mediating apoptosis, suggest that ATM is involved in the initiation of p53-dependent apoptosis after IR in human lymphoblastoid cells.     

Absence of point mutations at codon 17 of the mdm2 gene (serine 17) in human primary tumors

Mdm2 is a phosphoprotein that interacts with protein p53, inhibiting its activity. A serine located in position 17 of Mdm2, has been implicated in its phosphorylation process. We hypothesize that point mutations at serine 17 could block its phosphorylation and thereby increase the p53-Mdm2 interaction. This mechanism could increase the p53 degradation and cause a loss of the protective effect of p53 against tumorigenesis. This hypothesis was based on recent studies in vitro, demonstrating that when serine 17 is mutated, the DNA-dependent protein kinase, activated by genomic damage, is unable to phosphorylate it. Thus, we investigated whether structural point mutations at exon 3 of the Mdm2 gene, affecting codon 17, were present in 162 human primary tumors, 70 breast carcinomas, 14 bladder tumors, 18 colon adenocarcinomas and 60 testicular tumors. Direct sequencing of a fragment (204 bp) of exon 3 of the Mdm2 gene that contains the codon 17 showed no mutations at this position, independently of the presence or absence of p53 gene mutations in the same tumors. These results do not support the hypothesis that mutations in the Mdm2 gene at this level are involved in the tumorigenic process of human cancers.