Limited role of N-terminal phosphoserine residues in the activation of transcription by p53

Real-time surface plasmon resonance monitoring of site-specific phosphorylation of p53 protein and its interaction with MDM2 protein

Real-time monitoring of site-specific phosphorylation of p53 protein and its binding to MDM2 is conducted using dual-channel surface plasmon resonance (SPR).

Conserved oncogenic behavior of the FAM83 family regulates MAPK signaling in human cancer

FAM83B (family with sequence similarity 83, member B) was recently identified as a novel oncogene involved in activating CRAF/MAPK signaling and driving epithelial cell transformation. FAM83B is one of eight members of a protein family (FAM83) characterized by a highly conserved domain of unknown function (DUF1669), which is necessary and sufficient to drive transformation. Here, it is demonstrated that additional FAM83 members also exhibit oncogenic properties and have significantly elevated levels of expression in multiple human tumor types using a TissueScan Cancer Survey Panel PCR array and database mining. Furthermore, modeling the observed tumor expression of FAM83A, FAM83C, FAM83D, or FAM83E promoted human mammary epithelial cell (HMEC) transformation, which correlated with the ability of each FAM83 member to bind CRAF (RAF1) and promote CRAF membrane localization. Conversely, ablation of FAM83A or FAM83D from breast cancer cells resulted in diminished MAPK signaling with marked suppression of growth in vitro and tumorigenicity in vivo. Importantly, each FAM83 member was determined to be elevated in at least one of 17 distinct tumor types examined, with FAM83A, FAM83B, and FAM83D most frequently overexpressed in several diverse tissue types. Finally, evidence suggests that elevated expression of FAM83 members is associated with elevated tumor grade and decreased overall survival.

IMPLICATIONS

FAM83 proteins represent a novel family of oncogenes suitable for the development of cancer therapies aimed at suppressing MAPK signaling.

Hyperactivation of EGFR and downstream effector phospholipase D1 by oncogenic FAM83B

Despite the progress made in targeted anticancer therapies in recent years, challenges remain. The identification of new potential targets will ensure that the arsenal of cancer therapies continues to expand. FAM83B was recently discovered in a forward genetic screen for novel oncogenes that drive human mammary epithelial cell (HMEC) transformation. We report here that elevated FAM83B expression increases Phospholipase D (PLD) activity, and that suppression of PLD1 activity prevents FAM83B-mediated transformation. The increased PLD activity is engaged by hyperactivation of epidermal growth factor receptor (EGFR), which is regulated by an interaction involving FAM83B and EGFR. Preventing the FAM83B/EGFR interaction by site-directed mutation of lysine 230 of FAM83B suppressed PLD activity and MAPK signaling. Furthermore, ablation of FAM83B expression from breast cancer cells inhibited EGFR phosphorylation and suppressed cell proliferation. We propose that understanding the mechanism of FAM83B-mediated transformation will provide a foundation for future therapies aimed at targeting its function as an intermediary in EGFR, MAPK, and mTOR activation.

Hypoxia-induced cytotoxic drug resistance in osteosarcoma is independent of HIF-1Alpha

Survival rates from childhood cancer have improved dramatically in the last 40 years, such that over 80% of children are now cured. However in certain subgroups, including metastatic osteosarcoma, survival has remained stubbornly poor, despite dose intensive multi-agent chemotherapy regimens, and new therapeutic approaches are needed. Hypoxia is common in adult solid tumours and is associated with treatment resistance and poorer outcome. Hypoxia induces chemotherapy resistance in paediatric tumours including neuroblastoma, rhabdomyosarcoma and Ewing’s sarcoma, in vitro, and this drug resistance is dependent on the oxygen-regulated transcription factor hypoxia inducible factor-1 (HIF-1). In this study the effects of hypoxia on the response of the osteosarcoma cell lines 791T, HOS and U2OS to the clinically relevant cytotoxics cisplatin, doxorubicin and etoposide were evaluated. Significant hypoxia-induced resistance to all three agents was seen in all three cell lines and hypoxia significantly reduced drug-induced apoptosis. Hypoxia also attenuated drug-induced activation of p53 in the p53 wild-type U2OS osteosarcoma cells. Drug resistance was not induced by HIF-1α stabilisation in normoxia by cobalt chloride nor reversed by the suppression of HIF-1α in hypoxia by shRNAi, siRNA, dominant negative HIF or inhibition with the small molecule NSC-134754, strongly suggesting that hypoxia-induced drug resistance in osteosarcoma cells is independent of HIF-1α. Inhibition of the phosphoinositide 3-kinase (PI3K) pathway using the inhibitor PI-103 did not reverse hypoxia-induced drug resistance, suggesting the hypoxic activation of Akt in osteosarcoma cells does not play a significant role in hypoxia-induced drug resistance. Targeting hypoxia is an exciting prospect to improve current anti-cancer therapy and combat drug resistance. Significant hypoxia-induced drug resistance in osteosarcoma cells highlights the potential importance of hypoxia as a target to reverse drug resistance in paediatric osteosarcoma. The novel finding of HIF-1α independent drug resistance suggests however other hypoxia related targets may be more relevant in paediatric osteosarcoma.

DNA damage signaling induced by the G-quadruplex ligand 12459 is modulated by PPM1D/WIP1 phosphatase

The triazine derivative 12459 is a potent G-quadruplex ligand that triggers apoptosis or delayed growth arrest, telomere shortening and G-overhang degradation, as a function of its concentration and time exposure to the cells. We have investigated here the DNA damage response induced by 12459 in A549 cells. Submicromolar concentrations of 12459 triggers a delayed Chk1-ATR–mediated DNA damage response associated with a telomeric dysfunction and a G2/M arrest. Surprisingly, increasing concentrations of 12459 leading to cell apoptosis induced a mechanism that bypasses the DNA damage signaling and leads to the dephosphorylation of Chk1 and γ-H2AX. We identified the phosphatase Protein Phosphatase Magnesium dependent 1D/Wild-type P53-Induced Phosphatase (PPM1D/WIP1) as a factor responsible for this dephosphorylation. SiRNA-mediated depletion of PPM1D/WIP1 reactivates the DNA damage signaling by 12459. In addition, PPM1D/WIP1 is activated by reactive oxygen species (ROS) induced by 12459. ROS generated by 12459 are sufficient to trigger an early DNA damage in A549 cells when PPM1D/WIP1 is depleted. However, ROS inactivation by N-acetyl cysteine (NAC) treatment does not change the apoptotic response induced by 12459. Because PPM1D expression was recently reported to modulate the recruitment of DNA repair molecules, our data would suggest a cycle of futile protection against 12459, thus leading to a delayed mechanism of cell death.

Inorganic phosphate enhances sensitivity of human osteosarcoma U2OS cells to doxorubicin via a p53-dependent pathway

Osteosarcoma is the most common malignant primary bone tumor in children and adolescents. The clinical outcome for osteosarcoma remains discouraging despite aggressive surgery and intensive radiotherapy and chemotherapy regimens. Thus, novel therapeutic approaches are needed. Previously, we have shown that inorganic phosphate (Pi) inhibits proliferation and aggressiveness of human osteosarcoma U2OS cells identifying adenylate cyclase, beta3 integrin, Rap1, ERK1/2 as proteins whose expression and function are relevantly affected in response to Pi. In this study, we investigated whether Pi could affect chemosensitivity of osteosarcoma cells and the underlying molecular mechanisms. Here, we report that Pi inhibits proliferation of p53-wild type U2OS cells (and not of p53-null Saos and p53-mutant MG63 cells) by slowing-down cell cycle progression, without apoptosis occurrence. Interestingly, we found that Pi strongly enhances doxorubicin-induced cytotoxicity in U2OS, and not in Saos and MG63 cells, by apoptosis induction, as revealed by a marked increase of sub-G1 population, Bcl-2 downregulation, caspase-3 activation, and PARP cleavage. Remarkably, Pi/doxorubicin combination-induced cytotoxicity was accompanied by an increase of p53 protein levels and of p53 target genes mdm2, p21 and Bax, and was significantly reduced by the p53 inhibitor pifithrine-alpha. Moreover, the doxorubicin-induced cytotoxicity was associated with ERK1/2 pathway inhibition in response to Pi. Altogether, our data enforce the evidence of Pi as a novel signaling molecule capable of inhibiting ERK pathway and inducing sensitization to doxorubicin of osteosarcoma cells by p53-dependent apoptosis, implying that targeting Pi levels might represent a rational strategy for improving osteosarcoma therapy.

FAM83B mediates EGFR- and RAS-driven oncogenic transformation

Aberrant regulation of growth signaling is a hallmark of cancer development that often occurs through the constitutive activation of growth factor receptors or their downstream effectors. Using validation-based insertional mutagenesis (VBIM), we identified family with sequence similarity 83, member B (FAM83B), based on its ability to substitute for RAS in the transformation of immortalized human mammary epithelial cells (HMECs). We found that FAM83B coprecipitated with a downstream effector of RAS, CRAF. Binding of FAM83B with CRAF disrupted CRAF/14-3-3 interactions and increased CRAF membrane localization, resulting in elevated MAPK and mammalian target of rapamycin (mTOR) signaling. Ablation of FAM83B inhibited the proliferation and malignant phenotype of tumor-derived cells or RAS-transformed HMECs, implicating FAM83B as a key intermediary in EGFR/RAS/MAPK signaling. Analysis of human tumor specimens revealed that FAM83B expression was significantly elevated in cancer and was associated with specific cancer subtypes, increased tumor grade, and decreased overall survival. Cumulatively, these results suggest that FAM83B is an oncogene and potentially represents a new target for therapeutic intervention.

Simulating molecular mechanisms of the MDM2-mediated regulatory interactions: a conformational selection model of the MDM2 lid dynamics

Diversity and complexity of MDM2 mechanisms govern its principal function as the cellular antagonist of the p53 tumor suppressor. Structural and biophysical studies have demonstrated that MDM2 binding could be regulated by the dynamics of a pseudo-substrate lid motif. However, these experiments and subsequent computational studies have produced conflicting mechanistic models of MDM2 function and dynamics. We propose a unifying conformational selection model that can reconcile experimental findings and reveal a fundamental role of the lid as a dynamic regulator of MDM2-mediated binding. In this work, structure, dynamics and energetics of apo-MDM2 are studied as a function of posttranslational modifications and length of the lid. We found that the dynamic equilibrium between "closed" and "semi-closed" lid forms may be a fundamental characteristic of MDM2 regulatory interactions, which can be modulated by phosphorylation, phosphomimetic mutation as well as by the lid size. Our results revealed that these factors may regulate p53-MDM2 binding by fine-tuning the thermodynamic equilibrium between preexisting conformational states of apo-MDM2. In agreement with NMR studies, the effect of phosphorylation on MDM2 interactions was more pronounced with the truncated lid variant that favored the thermodynamically dominant closed form. The phosphomimetic mutation S17D may alter the lid dynamics by shifting the thermodynamic equilibrium towards the ensemble of "semi-closed" conformations. The dominant "semi-closed" lid form and weakened dependence on the phosphorylation seen in simulations with the complete lid can provide a rationale for binding of small p53-based mimetics and inhibitors without a direct competition with the lid dynamics. The results suggested that a conformational selection model of preexisting MDM2 states may provide a robust theoretical framework for understanding MDM2 dynamics. Probing biological functions and mechanisms of MDM2 regulation would require further integration of computational and experimental studies and may help to guide drug design of novel anti-cancer therapeutics.

A p53-independent role for the MDM2 antagonist Nutlin-3 in DNA damage response initiation

BACKGROUND

The mammalian DNA-damage response (DDR) has evolved to protect genome stability and maximize cell survival following DNA-damage. One of the key regulators of the DDR is p53, itself tightly regulated by MDM2. Following double-strand DNA breaks (DSBs), mediators including ATM are recruited to the site of DNA-damage. Subsequent phosphorylation of p53 by ATM and ATM-induced CHK2 results in p53 stabilization, ultimately intensifying transcription of p53-responsive genes involved in DNA repair, cell-cycle checkpoint control and apoptosis.

METHODS

In the current study, we investigated the stabilization and activation of p53 and associated DDR proteins in response to treatment of human colorectal cancer cells (HCT116p53+/+) with the MDM2 antagonist, Nutlin-3.

RESULTS

Using immunoblotting, Nutlin-3 was observed to stabilize p53, and activate p53 target proteins. Unexpectedly, Nutlin-3 also mediated phosphorylation of p53 at key DNA-damage-specific serine residues (Ser15, 20 and 37). Furthermore, Nutlin-3 induced activation of CHK2 and ATM - proteins required for DNA-damage-dependent phosphorylation and activation of p53, and the phosphorylation of BRCA1 and H2AX - proteins known to be activated specifically in response to DNA damage. Indeed, using immunofluorescent labeling, Nutlin-3 was seen to induce formation of γH2AX foci, an early hallmark of the DDR. Moreover, Nutlin-3 induced phosphorylation of key DDR proteins, initiated cell cycle arrest and led to formation of γH2AX foci in cells lacking p53, whilst γH2AX foci were also noted in MDM2-deficient cells.

CONCLUSION

To our knowledge, this is the first solid evidence showing a secondary role for Nutlin-3 as a DDR triggering agent, independent of p53 status, and unrelated to its role as an MDM2 antagonist.

Regulation of p53--insights into a complex process

The p53 protein is one of the most important tumor suppressor proteins. Normally, the p53 protein is in a latent state. However, when its activity is required, e.g. upon DNA damage, nucleotide depletion or hypoxia, p53 becomes rapidly activated and initiates transcription of pro-apoptotic and cell cycle arrest-inducing target genes. The activity of p53 is regulated both by protein abundance and by post-translational modifications of pre-existing p53 molecules. In the 30 years of p53 research, a plethora of modifications and interaction partners that modulate p53's abundance and activity have been identified and new ones are continuously discovered. This review will summarize our current knowledge on the regulation of p53 abundance and activity.

Dissecting the role of p53 phosphorylation in homologous recombination provides new clues for gain-of-function mutants

Regulation of homologous recombination (HR) represents the best-characterized DNA repair function of p53. The role of p53 phosphorylation in DNA repair is largely unknown. Here, we show that wild-type p53 repressed repair of DNA double-strand breaks (DSBs) by HR in a manner partially requiring the ATM/ATR phosphorylation site, serine 15. Cdk-mediated phosphorylation of serine 315 was dispensable for this anti-recombinogenic effect. However, without targeted cleavage of the HR substrate, serine 315 phosphorylation was necessary for the activation of topoisomerase I-dependent HR by p53. Moreover, overexpression of cyclin A1, which mimics the situation in tumors, inappropriately stimulated DSB-induced HR in the presence of oncogenic p53 mutants (not Wtp53). This effect required cyclin A1/cdk-mediated phosphorylation for stable complex formation with topoisomerase I. We conclude that p53 mutants have lost the balance between activation and repression of HR, which results in a net increase of potentially mutagenic DNA rearrangements. Our data provide new insight into the mechanism underlying gain-of-function of mutant p53 in genomic instability.

Regulation of Chk2 ubiquitination and signaling through autophosphorylation of serine 379

The Chk2 protein kinase protects genome integrity by promoting cell cycle arrest or apoptosis in response to DNA double-strand breaks, and Chk2 mutations are found in both familial and sporadic cancers. Exposure of cells to ionizing radiation (IR) or radiomimetic drugs induces Chk2 phosphorylation by ATM, followed by Chk2 oligomerization, auto-/transphosphorylation, and activation. Here we demonstrate that Chk2 is ubiquitinated upon activation and that this requires Chk2 kinase activity. Serine 379 (S379) was identified as a novel IR-inducible autophosphorylation site required for ubiquitination of Chk2 by a Cullin 1-containing E3 ligase complex. Importantly, S379 was required for Chk2 to induce apoptosis in cells with DNA double-strand breaks. Thus, auto-/transphosphorylation of S379 is required for Chk2 ubiquitination and effector function.

The C/H3 domain of p300 is required to protect VRK1 and VRK2 from their downregulation induced by p53

Background

The vaccinia-related kinase 1 (VRK1) protein, an activator of p53, can be proteolytically downregulated by an indirect mechanism, which requires p53-dependent transcription.

Principal Findings

In this work we have biochemically characterized the contribution of several p53 transcriptional cofactors with acetyl transferase activity to the induction of VRK1 downregulation that was used as a functional assay. Downregulation of VRK1 induced by p53 is prevented in a dose dependent manner by either p300 or CBP, but not by PCAF, used as transcriptional co-activators, suggesting that p53 has a different specificity depending on the relative level of these transcriptional cofactors. This inhibition does not require p53 acetylation, since a p53 acetylation mutant also induces VRK1 downregulation. PCAF can not revert the VRK1 protection effect of p300, indicating that these two proteins do not compete for a common factor needed to induce VRK1 downregulation. The protective effect is also induced by the C/H3 domain of p300, a region implicated in binding to several transcription factors and SV40 large T antigen; but the protective effect is lost when a mutant C/H3Del33 is used. The protective effect is a consequence of direct binding of the C/H3 domain to the transactivation domain of p53. A similar downregulatory effect can also be detected with VRK2 protein.

Conclusions/Significance

Specific p53-dependent effects are determined by the availability and ratios of its transcriptional cofactors. Specifically, the downregulation of VRK1/VRK2 protein levels, as a consequence of p53 accumulation, is thus dependent on the levels of the p300/CBP protein available for transcriptional complexes, since in this context this cofactor functions as a repressor of the effect. These observations point to the relevance of knowing the cofactor levels in order to determine one effect or another.

Nutlin-3a activates p53 to both down-regulate inhibitor of growth 2 and up-regulate mir-34a, mir-34b, and mir-34c expression, and induce senescence

Nutlin-3, an MDM2 inhibitor, activates p53, resulting in several types of cancer cells undergoing apoptosis. Although p53 is mutated or deleted in approximately 50% of all cancers, p53 is still functionally active in the other 50%. Consequently, nutlin-3 and similar drugs could be candidates for neoadjuvant therapy in cancers with a functional p53. Cellular senescence is also a phenotype induced by p53 activation and plays a critical role in protecting against tumor development. In this report, we found that nutlin-3a can induce senescence in normal human fibroblasts. Nutlin-3a activated and repressed a large number of p53-dependent genes, including those encoding microRNAs. mir-34a, mir-34b, and mir-34c, which have recently been shown to be downstream effectors of p53-mediated senescence, were up-regulated, and inhibitor of growth 2 (ING2) expression was suppressed by nutlin-3a treatment. Two candidates for a p53-DNA binding consensus sequence were found in the ING2 promoter regulatory region; thus, we performed chromatin immunoprecipitation and electrophoretic mobility shift assays and confirmed p53 binding directly to those sites. In addition, the luciferase activity of a construct containing the ING2 regulatory region was repressed after p53 activation. Antisense knockdown of ING2 induces p53-independent senescence, whereas overexpression of ING2 induces p53-dependent senescence. Taken together, we conclude that nutlin-3a induces senescence through p53 activation in normal human fibroblasts, and p53-mediated mir34a, mir34b, and mir34c up-regulation and ING2 down-regulation may be involved in the senescence pathway.

The absence of Ser389 phosphorylation in p53 affects the basal gene expression level of many p53-dependent genes and alters the biphasic response to UV exposure in mouse embryonic fibroblasts

Phosphorylation is important in p53-mediated DNA damage responses. After UV irradiation, p53 is phosphorylated specifically at murine residue Ser389. Phosphorylation mutant p53.S389A cells and mice show reduced apoptosis and compromised tumor suppression after UV irradiation. We investigated the underlying cellular processes by time-series analysis of UV-induced gene expression responses in wild-type, p53.S389A, and p53(-/-) mouse embryonic fibroblasts. The absence of p53.S389 phosphorylation already causes small endogenous gene expression changes for 2,253, mostly p53-dependent, genes. These genes showed basal gene expression levels intermediate to the wild type and p53(-/-), possibly to readjust the p53 network. Overall, the p53.S389A mutation lifts p53-dependent gene repression to a level similar to that of p53(-/-) but has lesser effect on p53-dependently induced genes. In the wild type, the response of 6,058 genes to UV irradiation was strictly biphasic. The early stress response, from 0 to 3 h, results in the activation of processes to prevent the accumulation of DNA damage in cells, whereas the late response, from 12 to 24 h, relates more to reentering the cell cycle. Although the p53.S389A UV gene response was only subtly changed, many cellular processes were significantly affected. The early response was affected the most, and many cellular processes were phase-specifically lost, gained, or altered, e.g., induction of apoptosis, cell division, and DNA repair, respectively. Altogether, p53.S389 phosphorylation seems essential for many p53 target genes and p53-dependent processes.

Analysis of a minimal model for p53 oscillations

Oscillatory behaviours in genetic networks are important examples for studying the principles underlying the dynamics of cellular regulation. Recently the team of Alon has reported a surprisingly rich oscillatory response of the p53 tumor suppressor to irradiation stress et al. [Lahav, G., Rosenfeld, N., Sigal, A., Geva-Zatorsky, N., Levine, A.J., Elowitz, M.B., Alon, U., 2004. Dynamics of the p53-Mdm2 feedback loop in individual cells. Nat. Genet. 36 (2), 147-150; Geva-Zatorsky, N., Rosenfeld, N., Itzkovitz, S., Milo, R., Sigal, A., Dekel, E., Yarnitzky, T., Liron, Y., Polak, P., Lahav, G., Alon, U., 2006. Oscillations and variability in the p53 system. Mol. Syst. Biol. 2, 2006.0033]. Several models for this system have been proposed by different groups, based essentially on negative feedback loops. In this paper we investigate in detail oscillations and stability in a deterministic time delayed differential model of the core circuit for p53 expression. This model is representative of a class of modelling approaches of this system, based on a "minimal" set of well-established biomolecular regulations. Depending on the protein degradation rates we show the existence of bifurcations between a stable steady state and oscillations both in presence and absence of stress.

Mdm2 is required for inhibition of Cdk2 activity by p21, thereby contributing to p53-dependent cell cycle arrest

p53 is extensively posttranslationally modified in response to various types of cellular stress. Such modifications have been implicated in the regulation of p53 protein levels as well as its DNA binding and transcriptional activities. Treatment of cells with doxorubicin causes phosphorylation and acetylation of p53, transcriptional upregulation of p21 and other target genes, and growth arrest. In contrast, downregulation of Mdm2 by a small interfering RNA (siRNA) approach led to increased levels of p53 lacking phosphorylation at serine 15 and acetylation at lysine 382. Levels of binding of p53 to the p21 promoter were comparable following treatment with doxorubicin or Mdm2 siRNA. Moreover, p53 was transcriptionally active and capable of inducing or repressing a variety of its target genes. Surprisingly, p53 upregulated by Mdm2 siRNA had no effect on cell cycle progression. Although comparable in level to that achieved by treatment with the p53 activators actinomycin D and nutlin-3, the increases in p53 and p21 after downregulation of Mdm2 were not sufficient to trigger cell cycle arrest. This version of p21 was capable of interacting with cyclin-dependent kinase 2 (Cdk2) but failed to inhibit its activity. Taken together, these results argue that Mdm2 is needed for full inhibition of Cdk2 activity by p21, thereby positively contributing to p53-dependent cell cycle arrest.

The MDM2 ubiquitination signal in the DNA-binding domain of p53 forms a docking site for calcium calmodulin kinase superfamily members

Genetic and biochemical studies have shown that Ser(20) phosphorylation in the transactivation domain of p53 mediates p300-catalyzed DNA-dependent p53 acetylation and B-cell tumor suppression. However, the protein kinases that mediate this modification are not well defined. A cell-free Ser(20) phosphorylation site assay was used to identify a broad range of calcium calmodulin kinase superfamily members, including CHK2, CHK1, DAPK-1, DAPK-3, DRAK-1, and AMPK, as Ser(20) kinases. Phosphorylation of a p53 transactivation domain fragment at Ser(20) by these enzymes in vitro can be mediated in trans by a docking site peptide derived from the BOX-V domain of p53, which also harbors the ubiquitin signal for MDM2. Evaluation of these calcium calmodulin kinase superfamily members as candidate Ser(20) kinases in vivo has shown that only CHK1 or DAPK-1 can stimulate p53 transactivation and induce Ser(20) phosphorylation of p53. Using CHK1 as a prototypical in vivo Ser(20) kinase, we demonstrate that (i) CHK1 protein depletion using small interfering RNA can attenuate p53 phosphorylation at Ser(20), (ii) an enhanced green fluorescent protein (EGFP)-BOX-V fusion peptide can attenuate Ser(20) phosphorylation of p53 in vivo, (iii) the EGFP-BOX-V fusion peptide can selectively bind to CHK1 in vivo, and (iv) the Deltap53 spliced variant lacking the BOX-V motif is refractory to Ser(20) phosphorylation by CHK1. These data indicate that the BOX-V motif of p53 has evolved the capacity to bind to enzymes that mediate either p53 phosphorylation or ubiquitination, thus controlling the specific activity of p53 as a transcription factor.

Macrophage inhibitory cytokine 1 mediates a p53-dependent protective arrest in S phase in response to starvation for DNA precursors

p53 is essential for the cellular responses to DNA damage that help to maintain genomic stability. Protective p53-dependent cell-cycle checkpoints are activated in response to a wide variety of stresses, including not only DNA damage but also arrest of DNA synthesis and of mitosis. In addition to its role in activating the G(1) and G(2) checkpoints, p53 also helps to protect cells in S phase when they are starved for DNA precursors by treatment with the specific aspartate transcarbamylase inhibitor N-phosphonacetyl-l-aspartate (PALA), which blocks the synthesis of pyrimidine nucleotides. Even though p53 is activated, PALA-treated cells expressing low levels of p53 or lacking expression of p21 do not arrest in G(1) or G(2) but are blocked in S phase instead. In the complete absence of p53, PALA-treated cells continue to synthesize DNA slowly and eventually progress through S phase, suffering severe DNA damage that in turn triggers apoptosis. Expression of the secreted protein macrophage inhibitory cytokine 1 (MIC-1), a member of the TGF-beta superfamily, increases substantially after PALA treatment, and application of exogenous MIC-1 or its constitutive expression from a cDNA provides remarkable protection of p53-null cells from PALA-mediated apoptosis, arguing that the p53-dependent secretion of MIC-1 provides a major part of such protection. Stimulation of MIC-1-dependent S phase arrest in normal gut epithelial cells might help to revitalize the clinical use of PALA, which has been limited by gut toxicity.

The subcellular localization of vaccinia-related kinase-2 (VRK2) isoforms determines their different effect on p53 stability in tumour cell lines

VRK is a new kinase family of unknown function. Endogenous human vacinia-related kinase 2 (VRK2) protein is present in both the nucleus and the cytosol, which is a consequence of alternative splicing of two VRK2 messages coding for proteins of 508 and 397 amino acids, respectively. VRK2A has a C-terminal hydrophobic region that anchors the protein to membranes in the endoplasmic reticulum (ER) and mitochondria, and it colocalizes with calreticulin, calnexin and mitotracker; whereas VRK2B is detected in both the cytoplasm and the nucleus. VRK2A is expressed in all cell types, whereas VRK2B is expressed in cell lines in which VRK1 is cytoplasmic. Both VRK2 isoforms have an identical catalytic N-terminal domain and phosphorylate p53 in vitro uniquely in Thr18. Phosphorylation of the p53 protein in response to cellular stresses results in its stabilization by modulating its binding to other proteins. However, p53 phosphorylation also occurs in the absence of stress. Only overexpression of the nuclear VRK2B isoform induces p53 stabilization by post-translational modification, largely due to Thr18 phosphorylation. VRK2B may play a role in controlling the binding specificity of the N-terminal transactivation domain of p53. Indeed, the p53 phosphorylated by VRK2B shows a reduction in ubiquitination by Mdm2 and an increase in acetylation by p300. Endogenous p53 is also phosphorylated in Thr18 by VRK2B, promoting its stabilization and transcriptional activation in A549 cells. The relative phosphorylation of Thr18 by VRK2B is similar in magnitude to that induced by taxol, which might use a different signalling pathway. In this context, VRK2B kinase might functionally replace nuclear VRK1. Therefore, these kinases might be components of a new signalling pathway that is likely to play a role in normal cell proliferation.

VRK1 signaling pathway in the context of the proliferation phenotype in head and neck squamous cell carcinoma

The vaccinia-related kinase (VRK) proteins are a new family with three members in the human kinome. The VRK1 protein phosphorylates several transcription factors and has been postulated to be involved in regulation of cell proliferation. In normal squamous epithelium, VRK1 is expressed in the proliferation area. Because VRK1 can stabilize p53, the expression of the VRK1 protein was analyzed in the context of the p53 pathway and the proliferation phenotype in a series of 73 head and neck squamous cell carcinomas. VRK1 protein level positively correlated with p53 response proteins, particularly hdm2 and p21. The VRK1 protein also correlated positively with several proteins associated with proliferation, such as cyclin-dependent kinase 2 (CDK2), CDK6, cdc2, cyclins B1 and A, topoisomerase II, survivin, and Ki67. The level of VRK1 protein behaves like a proliferation marker in this series of head and neck squamous cell carcinomas. To identify a possible regulatory role for VRK1 and because it regulates gene transcription, the promoters of two genes were studied, CDK2 and SURVIVIN, whose proteins correlated positively with VRK1. VRK1 increases the activity of both the CDK2 and SURVIVIN gene promoters. The expression of VRK1 was analyzed in the context of regulators of the G1-S transition. VRK1 protein levels increase in response to E2F1 and are reduced by retinoblastoma and p16. These data suggest that VRK1 might play a role in cell cycle regulation and is likely to represent the beginning of a new control mechanism of cell cycle, particularly late in the G1-S phase.

Defect in serine 46 phosphorylation of p53 contributes to acquisition of p53 resistance in oral squamous cell carcinoma cells

To investigate whether dysregulation of p53 phosphorylation confers tumor resistance to p53, we analysed the effects of wild-type p53 on oral squamous cell carcinoma (SCC) cell lines carrying various mutations of p53. Introduction of exogenous p53 neither induced apoptosis nor suppressed colony formation in HSC-3 cells lacking any detectable p53 and HSC-4 cells expressing mutant p53R248Q protein. Consistently, exogenous p53 did not induce proapoptotic p53-target genes in these p53-resistant cells. We found that phosphorylation of exogenous p53 on serine 46 (Ser46) was severely impaired in HSC-3 but not HSC-4 cells. A mutant mimicking Ser46-phosphorylation (p53S46D) enhanced proapoptotic Noxa promoter activity, and overcame the resistance to p53-mediated apoptosis and growth suppression in HSC-3 cells. Conversely, a mutant defective for Ser46-phosphorylation (p53S46A) failed to suppress the growth of p53-sensitive HSC-2 cells. In contrast to HSC-3 cells, p53S46D had no effect on HSC-4 cells, and inhibition of endogenous p53R248Q by siRNA restored p53-mediated apoptosis in HSC-4 cells, indicating a dominant-negative effect of p53R248Q protein on wild-type p53 function. These results demonstrate that the defect in Ser46 phosphorylation accounts for the p53 resistance of HSC-3 cells, and provide evidence for a mechanism underlying the acquisition of p53 resistance in oral SCC.

p130/p107/p105Rb-dependent transcriptional repression during DNA-damage-induced cell-cycle exit at G2

The progression of normal cells from G2 into mitosis is stably blocked when their DNA is damaged. Tumor cells lacking p53 arrest only transiently in G2, but eventually enter mitosis. We show that an important component of the stable G2 arrest in normal cells is the transcriptional repression of more than 20 genes encoding proteins needed to enter into and progress through mitosis. Studies from a number of labs including our own have shown that, by inducing p53 and p21/WAF1, DNA damage can trigger RB-family-dependent transcriptional repression. Our studies reported here show that p130 and p107 play a key role in transcriptional repression of genes required for G2 and M in response to DNA damage. For plk1, repression is partially abrogated by loss of p130 and p107, and is completely abrogated by loss of all three RB-family proteins. Mouse cells lacking RB-family proteins do not accumulate with a 4N content of DNA when exposed to adriamycin, suggesting that all three RB-family proteins contribute to G2 arrest in response to DNA damage. Stable arrest in the presence of functional p53-to-RB signaling is probably due to the ability of cells to exit the cell cycle from G2, a conclusion supported by our observation that KI67, a marker of cell-cycle entry, is downregulated in both G1 and G2 in a p53-dependent manner.

p53 Stabilization and accumulation induced by human vaccinia-related kinase 1

Variations in intracellular levels of p53 regulate many cellular functions and determine tumor susceptibility. Major mechanisms modulating p53 levels include phosphorylation and interaction of p53 with specific ubiquitin ligases that promote its degradation. N-terminal phosphorylation regulates the interaction of p53 with several regulatory molecules. Vaccinia-related kinase 1 (VRK1) is the prototype of a new Ser-Thr kinase family in the human kinome. VRK1 is located in the nucleus outside the nucleolus. Overexpression of VRK1 increases the stability of p53 by a posttranslational mechanism leading to its accumulation by a mechanism independent of the Chk2 kinase. Catalytically inactive VRK1 protein (a K179E mutant) does not induce p53 accumulation. VRK1 phosphorylates human p53 in Thr18 and disrupts p53-Mdm2 interaction in vitro, although a significant decrease in p53 ubiquitination by Mdm2 in vivo was not detected. VRK1 kinase does not phosphorylate Mdm2. VRK1-mediated p53 stabilization was also detected in Mdm2(-/-) cells. VRK1 also has an additive effect with MdmX or p300 to stabilize p53, and p300 coactivation and acetylation of p53 is enhanced by VRK1. The p53 stabilized by VRK1 is transcriptionally active. Suppression of VRK1 expression by specific small interfering RNA provokes several defects in proliferation, situating the protein in the regulation of this process. VRK1 might function as a switch controlling the proteins that interact with p53 and thus modifying its stability and activity. We propose VRK1 as the first step in a new pathway regulating p53 activity during cell proliferation.

Phosphorylation of p53 on key serines is dispensable for transcriptional activation and apoptosis

The p53 tumor suppressor is a key mediator of the cellular response to stress. Phosphorylation induced by multiple stress-activated kinases has been proposed to be essential for p53 stabilization, interaction with transcriptional co-activators, and activation of p53 target genes. However, genetic studies suggest that stress-activated phosphorylation may not be essential for p53 activation. We therefore investigated the role of p53 phosphorylation on six key serine residues (Ser(6), Ser(15), Ser(20), Ser(37), Ser(46), and Ser(392)) for p53 activation using nutlin-3, a recently developed small molecule MDM2 antagonist. We show here that nutlin does not induce the phosphorylation of p53. Comparison of the activity of unphosphorylated and phosphorylated p53 induced by the genotoxic drugs doxorubicin and etoposide in HCT116 and RKO cells revealed no difference in their sequence-specific DNA binding and ability to transactivate p53 target genes and to induce p53-dependent apoptosis. We conclude that p53 phosphorylation on six major serine sites is not required for activation of p53 target genes or biological responses in vivo.