Mdm2 enhances ligase activity of parkin and facilitates mitophagy

Loss-of-function mutations in the E3 ubiquitin ligase parkin have been implicated in the death of dopaminergic neurons in the substantia nigra, which is the root cause of dopamine deficit in the striatum in Parkinson's disease. Parkin ubiquitinates proteins on mitochondria that lost membrane potential, promoting the elimination of damaged mitochondria. Neuroprotective activity of parkin has been linked to its critical role in the mitochondria maintenance. Here we report a novel regulatory mechanism: another E3 ubiquitin ligase Mdm2 directly binds parkin and enhances its enzymatic activity in vitro and in intact cells. Mdm2 translocates to damaged mitochondria independently of parkin, enhances parkin-dependent ubiquitination of the outer mitochondria membrane protein mitofusin1. Mdm2 facilitates and its knockdown reduces parkin-dependent mitophagy. Thus, ubiquitously expressed Mdm2 might enhance cytoprotective parkin activity. The data suggest that parkin activation by Mdm2 could be targeted to increase its neuroprotective functions, which has implications for anti-parkinsonian therapy.

Novel roles of ER stress in repressing neural activity and seizures through Mdm2- and p53-dependent protein translation

Seizures can induce endoplasmic reticulum (ER) stress, and sustained ER stress contributes to neuronal death after epileptic seizures. Despite the recent debate on whether inhibiting ER stress can reduce neuronal death after seizures, whether and how ER stress impacts neural activity and seizures remain unclear. In this study, we discovered that the acute ER stress response functions to repress neural activity through a protein translation-dependent mechanism. We found that inducing ER stress promotes the expression and distribution of murine double minute-2 (Mdm2) in the nucleus, leading to ubiquitination and down-regulation of the tumor suppressor p53. Reduction of p53 subsequently maintains protein translation, before the onset of translational repression seen during the latter phase of the ER stress response. Disruption of Mdm2 in an Mdm2 conditional knockdown (cKD) mouse model impairs ER stress-induced p53 down-regulation, protein translation, and reduction of neural activity and seizure severity. Importantly, these defects in Mdm2 cKD mice were restored by both pharmacological and genetic inhibition of p53 to mimic the inactivation of p53 seen during ER stress. Altogether, our study uncovered a novel mechanism by which neurons respond to acute ER stress. Further, this mechanism plays a beneficial role in reducing neural activity and seizure severity. These findings caution against inhibition of ER stress as a neuroprotective strategy for seizures, epilepsies, and other pathological conditions associated with excessive neural activity.

One-third of epilepsy patients respond poorly to current anti-epileptic drugs. Thus, there is an urgent need to characterize cellular behavior during seizures, and the corresponding molecular mechanisms in order to develop better therapies. Seizures are known to induce ER stress but how the ER stress response functions to modulate seizure activity is unknown. Our study provides evidence to demonstrate a novel and beneficial role for the ER stress response in reducing neural activity and seizure severity. Mechanistically, we found that these beneficial effects are mediated by elevated protein translation, which is triggered by the activation of Mdm2-p53 signaling, during the early ER stress response. Our findings suggest that therapeutic attempts to reduce ER stress in epilepsies may result in worsening seizure activity and therefore caution against inhibition of ER stress as a neuroprotective strategy for epilepsies.

Regulatory Network of ARF in Cancer Development

ARF is a tumor suppressor protein that has a pivotal role in the prevention of cancer development through regulating cell proliferation, senescence, and apoptosis. As a factor that induces senescence, the role of ARF as a tumor suppressor is closely linked to the p53-MDM2 axis, which is a key process that restrains tumor formation. Thus, many cancer cells either lack a functional ARF or p53, which enables them to evade cell oncogenic stress-mediated cycle arrest, senescence, or apoptosis. In particular, the ARF gene is a frequent target of genetic and epigenetic alterations including promoter hyper-methylation or gene deletion. However, as many cancer cells still express ARF, pathways that negatively modulate transcriptional or post-translational regulation of ARF could be potentially important means for cancer cells to induce cellular proliferation. These recent findings of regulators affecting ARF protein stability along with its low levels in numerous human cancers indicate the significance of an ARF post-translational mechanism in cancers. Novel findings of regulators stimulating or suppressing ARF function would provide new therapeutic targets to manage cancer- and senescence-related diseases. In this review, we present the current knowledge on the regulation and alterations of ARF expression in human cancers, and indicate the importance of regulators of ARF as a prognostic marker and in potential therapeutic strategies.

Caveolin-1 is involved in DNA damage and repair signaling in X-irradiated Chang liver cells

Caveolin-1 (Cav-1), as an important structural protein of caveolae, has been proven to be correlated with several signal transduction pathways. Recent studies have shown that Cav-1 may play a critical role in response to DNA damage in irradiated pancreatic cancer cells. However, it is not known whether down-regulation of Cav-1 is required to enhance the damage of other kinds of human cells exposed to X-radiation. In this study, the role of Cav-1 in Chang liver cell line (CHL) exposed to X-radiation was investigated. Cav-1 knockdown cell line (CHL-CAV7) was stably established by the siRNA plasmids transfection, and Cav-1 expression was suppressed by 60%, compared with that of control group (CHL-C) which was transfected with non-targeting plasmids. Cellular survival ability and the expressions of proteins related to DNA damage and repair were examined by colony formation assay and Western blot, respectively. Down-regulation of Cav-1 expression induced a significant decrease of the survival rate in CHL-CAV7 cells exposed to 8 and 10 Gy X-radiation. Compared with CHL-C cells, CHL-CAV7 cells showed increased γH2AX expression, as well as decreased p-ATM, DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) and p53 protein expressions when treated with X-radiation. Meanwhile, the colocalization of Mdm2 and Cav-1 was decreased in CHL-CAV7 cells compared with that in CHL-C cells. These results suggest that the down-regulation of Cav-1 may aggravate DNA damage of CHL cells through reducing the interaction of Cav-1 and Mdm2, which results in the promotion of p53 degradation.

BCL9L Dysfunction Impairs Caspase-2 Expression Permitting Aneuploidy Tolerance in Colorectal Cancer

Chromosomal instability (CIN) contributes to cancer evolution, intratumor heterogeneity, and drug resistance. CIN is driven by chromosome segregation errors and a tolerance phenotype that permits the propagation of aneuploid genomes. Through genomic analysis of colorectal cancers and cell lines, we find frequent loss of heterozygosity and mutations in BCL9L in aneuploid tumors. BCL9L deficiency promoted tolerance of chromosome missegregation events, propagation of aneuploidy, and genetic heterogeneity in xenograft models likely through modulation of Wnt signaling. We find that BCL9L dysfunction contributes to aneuploidy tolerance in both TP53-WT and mutant cells by reducing basal caspase-2 levels and preventing cleavage of MDM2 and BID. Efforts to exploit aneuploidy tolerance mechanisms and the BCL9L/caspase-2/BID axis may limit cancer diversity and evolution.

Dual inhibition of nuclear factor kappa-B and Mdm2 enhance the antitumor effect of radiation therapy for pancreatic cancer

INTRODUCTION

Radiation therapy, alone or in combination with chemotherapy, is effective for patients with locally advanced and recurrent pancreatic cancer. Ionizing radiation induces cell cycle arrest and cell apoptosis through enhancement several signals such as p53, p21(Waf1/Cip1), and caspase. However, the therapeutic efficacy is attenuated by radiation-induced activation of NF-κB. Nafamostat mesilate, a synthetic serine protease inhibitor, inhibits NF-κB activation in pancreatic cancer. Therefore, we hypothesized that nafamostat mesilate inhibited radiation-induced activation of NF-κB and improves therapeutic outcome.

RESULTS

In combination group, NF-κB activation was significantly inhibited in comparison with that of radiation group. Nafamostat mesilate obviously down-regulated the expression levels of Mdm2 compared with control cells or irradiated cells. Consequently, p53 expression was stabilized inversely in correlation with Mdm2 protein expression level. The expression levels of p53, p21(Waf1/Cip1), cleaved caspase-3 and -8 were the highest in the combination group. Nafamostat mesilate enhanced ionizing radiation-induced cell apoptosis and G2/M cell cycle arrest. In combination group, cell proliferation and tumor growth were significantly slower than those in other groups.

CONCLUSION

Combination therapy of radiation with nafamostat mesilate exerts enhanced anti-tumor effect against human pancreatic cancer.

Isolation, characterization and functional analysis of full length p53 cDNA from Bubalus bubalis

p53 plays a pivotal role in maintaining the genomic integrity of the cell and has an important role in cellular transformation. We isolated and cloned a full length p53 cDNA (Bp53) from water buffalo in expression vectors designed to generate tagged proteins with FLAG or GFP. Bp53 was found to be 1161 nucleotide long and codes for 386 amino acid residues with 79% homology with human p53 containing 393 amino acids. Although Bp53 has some inherent differences in amino acid composition in different functional domains as compared to human p53 but the total electrostatic charge of amino acids has been maintained. Bp53 cDNA was transiently transfected in a p53 null human NSCLC cell line and as expected, it was predominantly localized in the nucleus. Besides, Bp53 effectively transactivates a number of target genes similar to human p53 and exerts most of its anti-tumorigenic potential in culture as observed in clonogenic and cell viability assays. Like human p53 mutants, core domain mutant version of Bp53 was found to be mis-localized to cytoplasm with diminished tumor suppressor activity. However, Bp53 appeared to be more sensitive to mdm2 mediated degradation and as a result, this protein was less stable as compared to human p53. For the first time we have characterized a functionally efficient wild-type p53 from buffalo having lower stability than human p53 and thus, buffalo p53 could be used as a model system for further insight to the molecular basis of wild-type p53 instability.

BAI1 regulates spatial learning and synaptic plasticity in the hippocampus

Synaptic plasticity is the ability of synapses to modulate the strength of neuronal connections; however, the molecular factors that regulate this feature are incompletely understood. Here, we demonstrated that mice lacking brain-specific angiogenesis inhibitor 1 (BAI1) have severe deficits in hippocampus-dependent spatial learning and memory that are accompanied by enhanced long-term potentiation (LTP), impaired long-term depression (LTD), and a thinning of the postsynaptic density (PSD) at hippocampal synapses. We showed that compared with WT animals, mice lacking Bai1 exhibit reduced protein levels of the canonical PSD component PSD-95 in the brain, which stems from protein destabilization. We determined that BAI1 prevents PSD-95 polyubiquitination and degradation through an interaction with murine double minute 2 (MDM2), the E3 ubiquitin ligase that regulates PSD-95 stability. Restoration of PSD-95 expression in hippocampal neurons in BAI1-deficient mice by viral gene therapy was sufficient to compensate for Bai1 loss and rescued deficits in synaptic plasticity. Together, our results reveal that interaction of BAI1 with MDM2 in the brain modulates PSD-95 levels and thereby regulates synaptic plasticity. Moreover, these results suggest that targeting this pathway has therapeutic potential for a variety of neurological disorders.

[Abnormal p53-HDM2 interaction in hematological malignancy]

The tumor suppressor protein p53 is a multifunctional transcription factor involved in the control of cell survival and death. p53 is inactivated by mutation of the p53 gene in approximately 50% of human cancers. While the rest (including hematological malignancies) encode wild-type p53, p53 is frequently inhibited by its negative regulator human double minute 2 (HDM2). HDM2 is a p53-specific E3 ubiquitin ligase. Therefore, there has been considerable interest in identifying compounds for disrupting the p53-HDM2 interaction. Small-molecule antagonist of HDM2, which binds HDM2 in the p53-binding pocket, negatively controls the activity of HDM2 and prevents p53 degradation. This stabilization of p53 results in its activation, leading to cell cycle arrest, growth inhibition, and apoptosis in wild type p53-haboring hematological malignant cells. Biology of p53-HDM2 interaction and anti-tumor effects of the HDM2 inhibitor in hematological malignant cells are described in this review.

The MDM2-p53 pathway: multiple roles in kidney development

The molecular basis of nephron progenitor cell renewal and differentiation into nascent epithelial nephrons is an area of intense investigation. Defects in these early stages of nephrogenesis lead to renal hypoplasia, and eventually hypertension and chronic kidney disease. Terminal nephron differentiation, the process by which renal epithelial precursor cells exit the cell cycle and acquire physiological functions is equally important. Failure of terminal epithelial cell differentiation results in renal dysplasia and cystogenesis. Thus, a better understanding of the transcriptional frameworks that regulate early and late renal cell differentiation is of great clinical significance. In this review, we will discuss evidence implicating the MDM2-p53 pathway in cell fate determination during development. The emerging central theme from loss- and gain-of-function studies is that tight regulation of p53 levels and transcriptional activity is absolutely required for nephrogenesis. We will also discuss how post-translational modifications of p53 (e.g., acetylation and phosphorylation) alter the spatiotemporal and functional properties of p53 and thus cell fate during kidney development. Mutations and polymorphisms in the MDM2-p53 pathway are present in more than 50 % of cancers in humans. This raises the question of whether sequence variants in the MDM2-p53 pathway increase the susceptibility to renal dysgenesis, hypertension or chronic kidney disease. With the advent of whole exome sequencing and other high throughput technologies, this hypothesis is testable in cohorts of children with renal dysgenesis.

Non-degradative ubiquitination of the Notch1 receptor by the E3 ligase MDM2 activates the Notch signalling pathway

The Notch receptor is necessary for modulating cell fate decisions throughout development, and aberrant activation of Notch signalling has been associated with many diseases, including tumorigenesis. The E3 ligase MDM2 (murine double minute 2) plays a role in regulating the Notch signalling pathway through its interaction with NUMB. In the present study we report that MDM2 can also exert its oncogenic effects on the Notch signalling pathway by directly interacting with the Notch 1 receptor through dual-site binding. This involves both the N-terminal and acidic domains of MDM2 and the RAM [RBP-Jκ (recombination signal-binding protein 1 for Jκ)-associated molecule] and ANK (ankyrin) domains of Notch 1. Although the interaction between Notch1 and MDM2 results in ubiquitination of Notch1, this does not result in degradation of Notch1, but instead leads to activation of the intracellular domain of Notch1. Furthermore, MDM2 can synergize with Notch1 to inhibit apoptosis and promote proliferation. This highlights yet another target for MDM2-mediated ubiquitination that results in activation of the protein rather than degradation and makes MDM2 an attractive target for drug discovery for both the p53 and Notch signalling pathways.

Mutational analysis reveals a dual role of Mdm2 acidic domain in the regulation of p53 stability

The exact role of the central acidic domain of Mdm2 in p53 degradation remains unclear. We therefore performed a systematic and comprehensive analysis of the acidic domain using a series of short deletions and found that only a minor part of the domain was indispensable for Mdm2-mediated p53 ubiquitylation. Moreover, we identified a short stretch of acidic amino acids required for p53 degradation but not ubiquitylation, indicating that, in addition to p53 ubiquitylation, the acidic domain might be involved in a critical post-ubiquitylation step in p53 degradation. Rather than representing a single functional domain, different parts of the acidic region perform separate functions in p53 degradation, suggesting that it might be possible to therapeutically target them independently.

MDM2 regulates MYCN mRNA stabilization and translation in human neuroblastoma cells

The MYCN gene has a critical role in determining the clinical behavior of neuroblastoma. Although it is known that genomic amplification occurs in high-risk subsets, it remains unclear how MYCN expression is regulated in the pathogenesis of neuroblastomas. Here, we report that MYCN expression was regulated by the oncoprotein MDM2 at the post-transcriptional level and was associated with neuroblastoma cell growth. Increasing MDM2 by ectopic overexpression in the cytoplasm enhanced both mRNA and protein expression of MYCN. Mechanistic studies found that the C-terminal RING domain of the MDM2 protein bound to the MYCN mRNA's AREs within the 3'UTR and increased MYCN 3'UTR-mediated mRNA stability and translation. Conversely, MDM2 silencing by specific siRNA rendered the MYCN mRNA unstable and reduced the abundance of the MYCN protein in MYCN-amplified neuroblastoma cell lines. Importantly, this MDM2 silencing resulted in a remarkable inhibition of neuroblastoma cell growth and induction of cell death through a p53-independent pathway. Our results indicate that MDM2 has a p53-independent role in the regulation of both MYCN mRNA stabilization and its translation, suggesting that MDM2-mediated MYCN expression is one mechanism associated with growth of MYCN-associated neuroblastoma and disease progression.

JAK2(V617F) negatively regulates p53 stabilization by enhancing MDM2 via La expression in myeloproliferative neoplasms

JAK2(V617F) is a gain of function mutation that promotes cytokine-independent growth of myeloid cells and accounts for a majority of myeloproliferative neoplasms (MPN). Mutations in p53 are rarely found in these diseases before acute leukemia transformation, but this does not rule out a role for p53 deregulation in disease progression. Using Ba/F3-EPOR cells and ex vivo cultured CD34(+) cells from MPN patients, we demonstrate that expression of JAK2(V617F) affected the p53 response to DNA damage. We show that E3 ubiquitin ligase MDM2 accumulated in these cells, due to an increased translation of MDM2 mRNA. Accumulation of the La autoantigen, which interacts with MDM2 mRNA and promotes its translation, was responsible for the increase in MDM2 protein level and the subsequent degradation of p53 after DNA damage. Downregulation of La protein or cell treatment with nutlin-3, a MDM2 antagonist, restored the p53 response to DNA damage and the cytokine-dependence of Ba/F3-EPOR-JAK2(V617F) cells. Altogether, these data indicate that the JAK2(V617F) mutation affects p53 response to DNA damage through the upregulation of La antigen and accumulation of MDM2. They also suggest that p53 functional inactivation accounts for the cytokine hypersensitivity of JAK2(V617F) MPN and might have a role in disease progression.

Dmp1 physically interacts with p53 and positively regulates p53's stability, nuclear localization, and function

The transcription factor Dmp1 is a Ras/HER2-activated haplo-insufficient tumor suppressor that activates the Arf/p53 pathway of cell-cycle arrest. Recent evidence suggests that Dmp1 may activate p53 independently of Arf in certain cell types. Here, we report findings supporting this concept with the definition of an Arf-independent function for Dmp1 in tumor suppression. We found that Dmp1 and p53 can interact directly in mammalian cells via the carboxyl-terminus of p53 and the DNA-binding domain of Dmp1. Expression of Dmp1 antagonized ubiquitination of p53 by Mdm2 and promoted nuclear localization of p53. Dmp1-p53 binding significantly increased the level of p53, independent of the DNA-binding activity of Dmp1. Mechanistically, p53 target genes were activated synergistically by the coexpression of Dmp1 and p53 in p53(-/-);Arf(-/-) cells, and genotoxic responses of these genes were hampered more dramatically in Dmp1(-/-) and p53(-/-) cells than in Arf(-/-) cells. Together, our findings identify a robust new mechanism of p53 activation mediated by direct physical interaction between Dmp1 and p53.

The regulation of p53 synthesis

The regulation of p53 expression levels is critical in controlling p53 activity in normal and damaged cells. This is well illustrated by the E3 ubiquitin ligase MDM2 that targets p53 for proteasomal degradation under normal conditions and is essential for controlling p53 activity during development. MDM2 is over-expressed in human cancers and together with some other E3 ligases that have also been implicated in controlling p53 stability, which emphasises the importance of post-translational regulation of p53 expression. At the level of synthesis, TP53 mRNA levels do not change in response to stresses and it is instead its rate of translation initiation that provides the mechanism of choice for expression control. More recent work has shown that TP53 mRNA plays an important role in mediating the cellular regulation of p53 activity. We will discuss the regulation of p53 synthesis and its implications for controlling p53 activity under normal conditions and during different types of stress response.

Pax3 stimulates p53 ubiquitination and degradation independent of transcription

Background

Pax3 is a developmental transcription factor that is required for neural tube and neural crest development. We previously showed that inactivating the p53 tumor suppressor protein prevents neural tube and cardiac neural crest defects in Pax3-mutant mouse embryos. This demonstrates that Pax3 regulates these processes by blocking p53 function. Here we investigated the mechanism by which Pax3 blocks p53 function.

Methodology/Principal Findings

We employed murine embryonic stem cell (ESC)-derived neuronal precursors as a cell culture model of embryonic neuroepithelium or neural crest. Pax3 reduced p53 protein stability, but had no effect on p53 mRNA levels or the rate of p53 synthesis. Full length Pax3 as well as fragments that contained either the DNA-binding paired box or the homeodomain, expressed as GST or FLAG fusion proteins, physically associated with p53 and Mdm2 both in vitro and in vivo. In contrast, Splotch Pax3, which causes neural tube and neural crest defects in homozygous embryos, bound weakly, or not at all, to p53 or Mdm2. The paired domain and homeodomain each stimulated Mdm2-mediated ubiquitination of p53 and p53 degradation in the absence of the Pax3 transcription regulatory domains, whereas Splotch Pax3 did not stimulate p53 ubiquitination or degradation.

Conclusions/Significance

Pax3 inactivates p53 function by stimulating its ubiquitination and degradation. This process utilizes the Pax3 paired domain and homeodomain but is independent of DNA-binding and transcription regulation. Because inactivating p53 is the only required Pax3 function during neural tube closure and cardiac neural crest development, and inactivating p53 does not require Pax3-dependent transcription regulation, this indicates that Pax3 is not required to function as a transcription factor during neural tube closure and cardiac neural crest development. These findings further suggest novel explanations for PAX3 functions in human diseases, such as in neural crest-derived cancers and Waardenburg syndrome types 1 and 3.

Frequency domain analysis reveals external periodic fluctuations can generate sustained p53 oscillation

p53 is a well-known tumor suppressor protein that regulates many pathways, such as ones involved in cell cycle and apoptosis. The p53 levels are known to oscillate without damping after DNA damage, which has been a focus of many recent studies. A negative feedback loop involving p53 and MDM2 has been reported to be responsible for this oscillatory behavior, but questions remain as how the dynamics of this loop alter in order to initiate and maintain the sustained or undamped p53 oscillation. Our frequency domain analysis suggests that the sustained p53 oscillation is not completely dictated by the negative feedback loop; instead, it is likely to be also modulated by periodic DNA repair-related fluctuations that are triggered by DNA damage. According to our analysis, the p53-MDM2 feedback mechanism exhibits adaptability in different cellular contexts. It normally filters noise and fluctuations exerted on p53, but upon DNA damage, it stops performing the filtering function so that DNA repair-related oscillatory signals can modulate the p53 oscillation. Furthermore, it is shown that the p53-MDM2 feedback loop increases its damping ratio allowing p53 to oscillate at a frequency more synchronized with the other cellular efforts to repair the damaged DNA, while suppressing its inherent oscillation-generating capability. Our analysis suggests that the overexpression of MDM2, observed in many types of cancer, can disrupt the operation of this adaptive mechanism by making it less responsive to the modulating signals after DNA damage occurs.

Diosgenin suppresses hepatocyte growth factor (HGF)-induced epithelial-mesenchymal transition by down-regulation of Mdm2 and vimentin

Substantial activation of the hepatocyte growth factor (HGF)/c-Met pathway leads to cancer cell scattering and invasion and has been observed in several types of cancers, including prostate and colorectal cancers. The phosphorylation cascade downstream of HGF, particularly PI3K/Akt signaling, regulates epithelial-to-mesenchymal transition (EMT). How this signaling governs EMT and whether specific kinases respond to particular EMT effectors remain unclear. This study found specific increases in Mdm2 and vimentin rather than the coregulation of an array of EMT marker proteins in response to HGF-induced EMT in DU145 prostate cancer cells. Importantly, it was further found that diosgenin abrogated HGF-induced DU145 cell scattering and invasion. Moreover, diosgenin effectively inhibited the HGF-induced increases in Mdm2 and vimentin by down-regulating phosphorylated Akt and mTOR. In summary, the results suggest that diosgenin may be a potential compound for use in prostate cancer therapy to target the major HGF-induced EMT pathway.

Inhibition of MDM2 attenuates neointimal hyperplasia via suppression of vascular proliferation and inflammation

AIMS

Tumour protein p53 plays an important role in the vascular remodelling process as well as in oncogenesis. p53 is negatively regulated by murine double minute 2 (MDM2). A recently developed MDM2 inhibitor, nutlin-3, is a non-genotoxic activator of the p53 pathway. So far, the effect of MDM2 inhibition on vascular remodelling has not been elucidated. We therefore investigated the effect of nutlin-3 on neointima formation.

METHODS AND RESULTS

Nutlin-3 up-regulated p53 and its downstream target p21 in vascular smooth muscle cells (VSMCs). DNA synthesis assay and flow cytometric analysis revealed that nutlin-3 inhibited platelet-derived growth factor (PDGF)-induced VSMC proliferation by cell cycle arrest. This inhibitory effect was abrogated in p53-siRNA-transfected VSMCs. Furthermore, nutlin-3 inhibited PDGF-stimulated VSMC migration. Treatment with nutlin-3 attenuated neointimal hyperplasia at 28 days after vascular injury in mice, associated with up-regulation of p53 and p21. BrdU incorporation was decreased at 14 days after injury in nutlin-3-treated mice. TUNEL assay showed that nutlin-3 did not exaggerate apoptosis of the injured vessels. Infiltration of macrophages and T-lymphocytes and mRNA expression of chemokine (C-C motif) ligand-5, interleukin-6, and intercellular adhesion molecule-1 were decreased in the injured vessels of nutlin-3-treated mice. Nutlin-3 suppressed NF-κB activation in VSMCs, but not in p53-siRNA-transfected VSMCs.

CONCLUSIONS

The MDM2 antagonist nutlin-3 inhibits VSMC proliferation, migration, and NF-κB activation, and also attenuates neointimal hyperplasia after vascular injury in mice, which is associated with suppression of vascular cell proliferation and an inflammatory response. Targeting MDM2 might be a potential therapeutic strategy for the treatment of vascular proliferative diseases.

CHIP-dependent p53 regulation occurs specifically during cellular senescence

p53 regulates several biological processes, including senescence. Its protein stability is regulated by ubiquitination and proteasomal degradation, mainly mediated by Mdm2. However, other E3 ligases have been identified, such as the chaperone-associated ligase CHIP, although their precise function regarding p53 degradation remains elusive. Interestingly, CHIP deficiency has been recently shown to result in accelerated aging in mice, although the molecular basis of this phenotype was not completely understood. In this study, we explore the role of CHIP in regulating p53 in senescence. We demonstrate that in senescent human fibroblasts, CHIP is up-regulated concomitant with a significant down-regulation of p53. Moreover, CHIP partially translocates to the nucleus and acquires higher ubiquitination levels in senescent cells. Notably, CHIP overexpression in young cells, to levels similar to those recorded during senescence, leads to p53 degradation to below its basal levels. In addition, whereas CHIP silencing has no effect on p53 stability in young cells, a considerable p53 accumulation occurs in their senescent counterparts. Finally, we have observed an attenuation of the CHIP-associated molecular folding-refolding machinery during senescence, and supportively, inhibition of Hsp90 activity leads to rapid p53 degradation only in senescent cells. Taking these results together, we conclude that CHIP-dependent p53 regulation occurs specifically during senescence.

HDM2 impairs Noxa transcription and affects apoptotic cell death in a p53/p73-dependent manner in neuroblastoma

HDM2, a human homologue of MDM2, is a major negative regulator of p53 function, and increased expression of HDM2 by its promoter polymorphism SNP309 resulted in p53 inactivation and an increased risk of several tumours, including neuroblastoma (NB). Herein, we show that increased expression of HDM2 is related to a worse prognosis in MYCN-amplified NB patients. HDM2 plays an important role in the expression of Noxa, a pro-apoptotic molecule of the Bcl-2 family, which induces NB cell apoptotic death after doxorubcin (Doxo) treatment. Knockdown of HDM2 by siRNA resulted in the upregulation of Noxa at mRNA/protein levels and improved the sensitivity of Doxo-resistant NB cells, although these were not observed in p53-mutant NB cells. Noxa-knockdown abolished the recovered Doxo-induced cell death by HDM2 reduction. Intriguingly, resistance to Doxo was up-regulated by over-expression of HDM2 in Doxo-sensitive NB cells. By HDM2 expression, p53 was inactivated but its degradation was not accelerated, suggesting that p53 was degraded in a proteasome-independent manner in NB cells; downstream effectors of p53, p21(Cip1/Waf1) and Noxa were suppressed by HDM2. Noxa transcription was considerably regulated by both p53 and p73 in NB cells. Furthermore, in vivo binding of p53 and p73 to Noxa promoter was suppressed and Noxa promoter activation was inhibited by HDM2. Taken together, our results may indicate that the HDM2-related resistance to chemotherapeutic drugs of NB is regulated by p53/p73-dependent Noxa expression in NB.

Predicted functions of MdmX in fine-tuning the response of p53 to DNA damage

Tumor suppressor protein p53 is regulated by two structurally homologous proteins, Mdm2 and MdmX. In contrast to Mdm2, MdmX lacks ubiquitin ligase activity. Although the essential interactions of MdmX are known, it is not clear how they function to regulate p53. The regulation of tumor suppressor p53 by Mdm2 and MdmX in response to DNA damage was investigated by mathematical modeling of a simplified network. The simplified network model was derived from a detailed molecular interaction map (MIM) that exhibited four coherent DNA damage response pathways. The results suggest that MdmX may amplify or stabilize DNA damage-induced p53 responses via non-enzymatic interactions. Transient effects of MdmX are mediated by reservoirs of p53∶MdmX and Mdm2∶MdmX heterodimers, with MdmX buffering the concentrations of p53 and/or Mdm2. A survey of kinetic parameter space disclosed regions of switch-like behavior stemming from such reservoir-based transients. During an early response to DNA damage, MdmX positively or negatively regulated p53 activity, depending on the level of Mdm2; this led to amplification of p53 activity and switch-like response. During a late response to DNA damage, MdmX could dampen oscillations of p53 activity. A possible role of MdmX may be to dampen such oscillations that otherwise could produce erratic cell behavior. Our study suggests how MdmX may participate in the response of p53 to DNA damage either by increasing dependency of p53 on Mdm2 or by dampening oscillations of p53 activity and presents a model for experimental investigation.

A Molecular Interaction Map (MIM) akin to a circuit diagram of an electric device can give a comprehensive view of cellular processes and help understand complex protein functions in cells. To this end, we generated a MIM focused on the p53-Mdm2-MdmX network proteins and performed computer simulations to help understand how Mdm2 and MdmX may regulate p53. Proper regulation of p53 is important for cell survival: elevated levels of p53 can lead to cell death, and decreased levels of p53 can lead to cancer. Mdm2 and MdmX are structurally homologous proteins that regulate p53. Mdm2 negatively regulates p53 by degradation, but MdmX regulation of p53 is not well understood. Recently, Mdm2 and MdmX have been recognized as potential cancer therapeutic targets. In an effort to better understand how MdmX can alter the p53 activity under various conditions, we used mathematical models based on the MIM network to generate hypotheses that can be tested by experiments. Our simulations suggest that MdmX may increase the dependency of p53 on Mdm2 or dampen p53 oscillations during DNA damage response.

Apoptosis in leukemias: regulation and therapeutic targeting

Nearly 25 years after the seminal publication of John Foxton Kerr that first described apoptosis, the process of regulated cell death, our understanding of this basic physiological phenomenon is far from complete [39]. From cardiovascular disease to cancer, apoptosis has assumed a central role with broad ranging therapeutic implications that depend on a complete understanding of this process, yet have also identified an incredibly complex regulatory system that is critical for development and is at the core of many diseases, challenging scientist and clinicians to step into its molecular realm and modulate its circuitry for therapeutic purposes. This chapter will review our understanding of the molecular circuitry that controls apoptosis in leukemia and the pharmacological manipulations of this pathway that may yield therapeutic benefit.

Phosphorylation and degradation of MdmX is inhibited by Wip1 phosphatase in the DNA damage response

MdmX and Mdm2 regulate p53 tumor suppressor functions by controlling p53 transcriptional activity and/or stability in cells exposed to DNA damage. Accumulating evidence indicates that ATM-mediated phosphorylation and degradation of Mdm2 and MdmX may be the initial driving force that induces p53 activity during the early phase of the DNA damage response. We have recently determined that a novel protein phosphatase, Wip1 (or PPM1D), contributes to p53 regulation by dephosphorylating Mdm2 to close the p53 activation loop initiated by the ATM/ATR kinases. In the present study, we determine that Wip1 directly dephosphorylates MdmX at the ATM-targeted Ser403 and indirectly suppresses phosphorylation of MdmX at Ser342 and Ser367. Wip1 inhibits the DNA damage-induced ubiquitination and degradation of MdmX, leading to the stabilization of MdmX and reduction of p53 activities. Our data suggest that Wip1 is an important component in the ATM-p53-MdmX regulatory loop.

Digital control circuitry of cancer cell and its apoptosis

This study, through a typical aerospace systems architecture, suggests an engineering design of a human cancer cell circuitry in which a digital optimal control matrix is assigned to repair the DNA damage level and/or to trigger its apoptosis. Here, the conceived machinery is proposed taking into account the state of the art in cancer investigation. However, it could be further generalized. The most recent studies on cancer pathologies give a predominant role to the oncosuppressor protein p53 and its antagonist, the oncogene Mdm2. Experimental and theoretical approaches are in agreement in deducing a "digital" response of the p53 when genomic integrity is damaged. Once DNA damage is present, the mutual influence of p53 and its antagonist, the Mdm2 oncogene, is closed in a feedback loop. In this work, starting from these current results, a novel molecular mechanism is proposed, based on a digital optimal control law, whereby p53 and Mdm2 proteins activities can be represented by appropriate circuitry and governed by the optimal control law of digital systems. This procedure obtains a real-time sequence evaluation of protein oscillations and an unexpected and relevant acceleration in the DNA repairing when suitable digital control matrix is implemented. Those effects suggest interesting perspectives for future scientific investigations. First of all, the proposed digital circuitry, receiving the p53 signal from a damaged cell, is able to repair the current level of genomic alteration. Moreover, the cell fate is newly conceived and bound by the modified pulsing mechanism of p53.

Determinants for the efficiency of anticancer drugs targeting either Aurora-A or Aurora-B kinases in human colon carcinoma cells

The mitotic Aurora kinases, including Aurora-A and Aurora- B, are attractive novel targets for anticancer therapy, and inhibitory drugs have been developed that are currently undergoing clinical trials. However, the molecular mechanisms how these drugs induce tumor cell death are poorly understood. We have addressed this question by comparing the requirements for an efficient induction of apoptosis in response to MLN8054, a selective inhibitor of Aurora-A, and the selective Aurora-B inhibitor ZM447439 in human colon carcinoma cells. By using various isogenic knockout as well as inducible colon carcinoma cell lines, we found that treatment with MLN8054 induces defects in mitotic spindle assembly, which causes a transient spindle checkpoint-dependent mitotic arrest. This cell cycle arrest is not maintained due to the activity of MLN8054 to override the spindle checkpoint. Subsequently, MLN8054-treated cells exit from mitosis and activate a p53-dependent postmitotic G(1) checkpoint, which subsequently induces p21 and Bax, leading to G(1) arrest followed by the induction of apoptosis. In contrast, inhibition of Aurora-B by ZM447439 also interferes with normal chromosome alignment during mitosis and overrides the mitotic spindle checkpoint but allows a subsequent endoreduplication, although ZM447439 potently activates the p53-dependent postmitotic G(1) checkpoint. Moreover, the ZM447439-induced endoreduplication is a prerequisite for the efficiency of the drug. Thus, our results obtained in human colon carcinoma cells indicate that although both Aurora kinase inhibitors are potent inducers of tumor cell death, the pathways leading to the induction of apoptosis in response to these drugs are distinct.

Mdm2 affects genome stability independent of p53

Mdm2 is a critical negative regulator of the p53 tumor suppressor and is frequently overexpressed in human cancers. However, reports, including our own studies, suggest that Mdm2 has both p53-dependent and p53-independent functions that contribute to genomic instability and transformation when deregulated. We recently elucidated a p53-independent role for Mdm2 in the regulation of the DNA double-strand break repair response, genomic stability, and transformation through interaction with Nbs1, a member of the Mre11/Rad50/Nbs1 DNA double-strand break repair complex. In light of these findings, targeting Mdm2 in human malignancies may have effects other than activating p53.

Induction of cytoplasmic accumulation of p53: a mechanism for low levels of arsenic exposure to predispose cells for malignant transformation

Although epidemiologic studies have linked arsenic exposure to the development of human cancer, the mechanisms underlying the tumorigenic role of arsenic remain largely undefined. We report here that treatment of cells with sodium arsenite at the concentrations close to environmental exposure is associated with the up-regulation of Hdm2 and the accumulation of p53 in the cytoplasm. Through the mitogen-activated protein kinase pathway, arsenite stimulates the P2 promoter-mediated expression of Hdm2, which then promotes p53 nuclear export. As a consequence, the p53 response to genotoxic stress is compromised, as evidenced by the impaired p53 activation and apoptosis in response to UV irradiation or 5FU treatment. The ability of arsenite to impede p53 activation is further demonstrated by a significantly blunted p53-dependent tissue response to 5FU treatment when mice were fed with arsenite-containing water. Together, our data suggests that arsenic compounds predispose cells to malignant transformation by up-regulation of Hdm2 and subsequent p53 inactivation.

CARF (collaborator of ARF) interacts with HDM2: evidence for a novel regulatory feedback regulation of CARF-p53-HDM2-p21WAF1 pathway

We initially cloned CARF (collaborator of ARF), as a novel ARF-binding protein by a yeast interaction screen. It also interacts with p53 directly leading to ARF-independent enhancement of p53 function and in turn undergoes a negative feedback regulation. Herein we report that i) CARF interacts with HDM2 and undergoes degradation by an HDM2-dependent proteasome pathway, and ii) it acts as a transcriptional repressor of HDM2. By overexpression and silencing studies, we demonstrated that CARF exerts a vital control on the p53-HDM2-p21WAF1 pathway that is frequently altered in cancer cells.

Fitting ordinary differential equations to short time course data

Ordinary differential equations (ODEs) are widely used to model many systems in physics, chemistry, engineering and biology. Often one wants to compare such equations with observed time course data, and use this to estimate parameters. Surprisingly, practical algorithms for doing this are relatively poorly developed, particularly in comparison with the sophistication of numerical methods for solving both initial and boundary value problems for differential equations, and for locating and analysing bifurcations. A lack of good numerical fitting methods is particularly problematic in the context of systems biology where only a handful of time points may be available. In this paper, we present a survey of existing algorithms and describe the main approaches. We also introduce and evaluate a new efficient technique for estimating ODEs linear in parameters particularly suited to situations where noise levels are high and the number of data points is low. It employs a spline-based collocation scheme and alternates linear least squares minimization steps with repeated estimates of the noise-free values of the variables. This is reminiscent of expectation-maximization methods widely used for problems with nuisance parameters or missing data.

The p53-Mdm2 network in progenitor cell expansion during mouse postnatal development

Mdm2, an E3 ubiquitin ligase, negatively regulates the tumour suppressor p53. Loss of Mdm2 in mice results in p53-dependent apoptosis and embryonic lethality. This phenotype was rescued by the p53(515C) allele, which encodes an apoptosis-deficient p53R172P protein. However, these mice died within 2 weeks of birth, due to a severe impairment of progenitor cell expansion during postnatal haematopoiesis and cerebellar development, leading to p53-dependent cell cycle arrest. Loss of Mdm2 led to phosphorylation of the p53R172P protein, p53R172P stability and activation of the cell cycle inhibitor p21 in proliferating cells, but not in differentiated cells, in multiple tissue compartments. Proliferating cells of epithelial origin were not affected. The haematopoietic and neural defects were alleviated in mice lacking Mdm2 and containing one p53(515C) and one p53-null allele, but spermatogenesis was arrested. These findings establish a crucial role for the p53-Mdm2 network in regulating proliferation and progenitor expansion in many cell lineages and have important implications for the use of drugs that aim to disrupt the p53-Mdm2 interaction.

Roles for CSN5 in control of p53/MDM2 activities

The 5th subunit of COP9 signalosome (CSN5, also known as Jab1 or COPS5) is implicated in regulating p53 activity and is overexpressed in various tumors. However, the precise roles of CSN5 in p53 network and tumorigenesis are not well characterized. Here we show that CSN5 is a critical regulator of both p53 and MDM2. We show that curcumin, an important inhibitor of CSN-associated kinases, can downregulate not only CSN5 but also MDM2, which results in p53 stabilization. Importantly, CSN5 interacts with p53. CSN5 expression leads to p53 degradation, facilitating MDM2-mediated p53 ubiquitination, and promoting p53 nuclear export. Additionally, CSN5 expression results in stabilization of MDM2 through reducing MDM2 self-ubiquitination and decelerating turnover rate of MDM2. Significantly, we further show that CSN5 antagonizes the transcriptional activity of p53. These results demonstrate that CSN5 is a pivotal regulator for both p53 and MDM2. Our studies may pave the way for targeting CSN5 for anti-cancer drug development.

Regulation of hdm2 by stress-induced hdm2alt1 in tumor and nontumorigenic cell lines correlating with p53 stability

Alternative and aberrant splicing of hdm2 occurs in tumor and normal tissues. However, the factors that induce these splice variants and whether they are translated to protein products in vivo is unknown, making it difficult to decipher which of these hdm2 transcripts have a normal physiologic function or contribute to carcinogenesis. We investigated the conditions that induce this post-transcriptional modification of hdm2 in tumor and nontumorigenic cell lines. We showed that UV and gamma radiation as well as cisplatin treatment induced alternative splicing of hdm2, which resulted in a single splice variant, hdm2(alt1), irrespective of the cell type. Interestingly, the mechanism of UV-induced splicing is independent of p53 status. Immunoanalysis revealed that, after UV radiation, HDM2(ALT1) protein was expressed and interacted with HDM2 that correlated to increased p53 protein levels and its accumulation in the nucleus, whereas HDM2 localized more to the cytoplasm with a decrease in its RNA and protein level. We propose that stress-induced HDM2(ALT1) regulates HDM2 at two levels, RNA and protein, further modulating the p53-HDM2 interaction or interactions of HDM2 with other cell cycle regulatory proteins. This kind of regulation may possibly restrict oncogenic functions of HDM2 and contribute to the many protective responses triggered by certain stress signals. Our data imply that HDM2(ALT1) possesses a normal physiologic function in damaged cells, perhaps facilitating cellular defense.

Elevated Mdm2 expression induces chromosomal instability and confers a survival and growth advantage to B cells

Mdm2, a regulator of the p53 tumor suppressor, is frequently overexpressed in lymphomas, including lymphomas that have inactivated p53. However, the biological consequences of Mdm2 overexpression in lymphocytes are not fully resolved. Here, we report that increased expression of Mdm2 in B cells augmented proliferation and reduced susceptibility to p53-dependent apoptosis, which was due to inhibition of p53 and suppression of p21 expression. Notably, developing and mature B cells from Mdm2 transgenic mice had an increased frequency of chromosomal/chromatid breaks and/or aneuploidy. This Mdm2-mediated genome instability occurred at a similar frequency as that in B cells overexpressing the oncogene c-Myc, but the chromosomal instability was not further enhanced when Mdm2 and c-Myc were overexpressed together. Elevated Mdm2 expression alone increased the occurrence of B-cell transformation in vivo and cooperated with c-Myc overexpression, resulting in an acceleration of B-cell lymphomagenesis. In addition, the frequency of p53 mutations was reduced, but not eliminated, in lymphomas arising in Mdm2/Emu-myc double transgenic mice. Therefore, increased Mdm2 expression facilitated B-cell lymphomagenesis, in part, through regulation of p53 by altering B-cell proliferation and susceptibility to apoptosis, and by inducing chromosomal instability.

Inhibition of p53-murine double minute 2 interaction by nutlin-3A stabilizes p53 and induces cell cycle arrest and apoptosis in Hodgkin lymphoma

PURPOSE

p53 is frequently expressed but rarely mutated in Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin's lymphoma (HL). p53 protein levels are regulated by murine double minute 2 (MDM2) through a well-established autoregulatory feedback loop. In this study, we investigated the effects of nutlin-3A, a recently developed small molecule that antagonizes MDM2 and disrupts the p53-MDM2 interaction, on p53-dependent cell cycle arrest and apoptosis in cultured HRS cells.

EXPERIMENTAL DESIGN

HL cell lines carrying wild-type (wt) or mutated p53 gene were treated with the potent MDM2 inhibitor nutlin-3A or a 150-fold less active enantiomer, nutlin-3B.

RESULTS

We show that nutlin-3A, but not nutlin-3B, stabilizes p53 in cultured HRS cells carrying wt p53 gene resulting in p53-dependent cell cycle arrest and apoptosis. Cell cycle arrest was associated with up-regulation of the cyclin-dependent kinase inhibitor p21. Nutlin-3A-induced apoptotic cell death was accompanied by Bax and Puma up-regulation and caspase-3 cleavage and was abrogated, in part, by inhibition of caspase-9 and caspase-3 activity. By contrast, no effects on cell cycle or apoptosis were found in HL cell lines harboring mutated p53 gene. Furthermore, combined treatment with nutlin-3A and doxorubicin revealed enhanced cytotoxicity in HRS cells with wt p53 gene. Blocking of nuclear export by leptomycin B, or inhibition of proteasome by MG132, stabilized p53 at a level comparable with that of nutlin-3A treatment in HRS cells with wt p53.

CONCLUSIONS

These data suggest that nutlin-3A stabilized p53 by preventing MDM2-mediated p53 degradation in HRS cells. wt p53 stabilization and activation by nutlin-3A may be a novel therapeutic approach for patients with HL.

Stabilization of p53 in human cytomegalovirus-initiated cells is associated with sequestration of HDM2 and decreased p53 ubiquitination

Human cytomegalovirus (HCMV) induces serum- or density-arrested human lung (LU) cells to traverse the cell cycle, providing it with a strategy to replicate in post-mitotic cells that are its cellular substrate in vivo. HCMV infection also induces high cellular levels of p53, seemingly in contradiction to the observed cell cycle progression. This study was undertaken to examine the mechanism(s) of the increased p53 abundance. HCMV infection caused a 4-fold increase in p53 that preceded a substantial increase in p53 transcripts by more than 24 h. p53 was stabilized in HCMV-infected cells (from a half-life of less than 30 min to about 8 h) and was less sensitive to proteasome-mediated degradation. Ubiquitination of p53 in mock-infected LU cells was sensitive to inhibition by trans-4-iodo, 4'-boranyl-chalcone, consistent with HDM2-catalyzing ubiquitination of p53. In HCMV-infected cells, ubiquitination of p53 was essentially undetectable. Although HDM2 had a nuclear distribution in mock-infected LU cells, in HCMV-infected cells HDM2 was translocated to the cytoplasm beginning at 12 h and demonstrated decreased cellular abundance thereafter. HDM2 was stabilized in the HCMV-infected cells by MG132, indicating a shift from p53 to HDM2 ubiquitination. p53 demonstrated a predominantly nuclear distribution in HCMV-infected cells through 48 h, resulting in p53 and HDM2 in distinct subcellular compartments. The principal mechanism responsible for increased p53 stabilization was nuclear export and degradation of HDM2. Thus, HCMV uses a shift from p53 to HDM2 ubiquitination and destabilization to obtain protracted high levels of p53, while promoting cell cycle traverse.

Increased MDM2 expression is associated with inferior survival in mantle-cell lymphoma, but not related to the MDM2 SNP309

We here show that increased expression of MDM2, a negative regulator of p53, correlates with inferior survival in a series of 43 mantle cell lymphomas. MDM2 overexpression is associated with copy number gains of the MDM2 locus in single tumors, but not with the recently reported MDM2 promoter SNP309.

The p53-MDM2 network: from oscillations to apoptosis

The p53 protein is well-known for its tumour suppressor function. The p53-MDM2 negative feedback loop constitutes the core module of a network of regulatory interactions activated under cellular stress. In normal cells, the level of p53 proteins is kept low by MDM2, i.e. MDM2 negatively regulates the activity of p53. In the case of DNA damage, the p53-mediated pathways are activated leading to cell cycle arrest and repair of the DNA. If repair is not possible due to excessive damage, the p53-mediated apoptotic pathway is activated bringing about cell death. In this paper, we give an overview of our studies on the p53-MDM2 module and the associated pathways from a systems biology perspective. We discuss a number of key predictions, related to some specific aspects of cell cycle arrest and cell death, which could be tested in experiments.

PKB and the mitochondria: AKTing on apoptosis

Cellular homeostasis depends upon the strict regulation of responses to external stimuli, such as signalling cascades triggered by nutrients and growth factors, and upon cellular metabolism. One of the major molecules coordinating complex signalling pathways is protein kinase B (PKB), a serine/threonine kinase also known as Akt. The number of substrates known to be phosphorylated by PKB and its interacting partners, as well as our broad understanding of how PKB is implicated in responses to growth factors, metabolic pathways, proliferation, and cell death via apoptosis is constantly increasing. Activated by the insulin/growth factor-phosphatidylinositol 3-kinase (PI3K) cascade, PKB triggers events that promote cell survival and prevent apoptosis. It is also now widely accepted that mitochondria are not just suppliers of ATP, but that they participate in regulatory and signalling events, responding to multiple physiological inputs and genetic stresses, and regulate both cell proliferation and death. Thus, mitochondria are recognized as important players in apoptotic events and it is logical to predict some form of interplay with PKB. In this review, we will summarize mechanisms by which PKB mediates its anti-apoptotic activities in cells and survey recent developments in understanding mitochondrial dynamics and their role during apoptosis.

RING Domain–Mediated Interaction Is a Requirement for MDM2's E3 Ligase Activity

The RING domain of MDM2 that is essential for its E3 ligase activity mediates binding to itself and its structural homologue MDMX. Whereas it has been reported that RING domain interactions are critical, it is not well understood how they affect the E3 ligase activity of MDM2. We report that the E3 ligase activity requires the RING domain-dependent complex formation. In vivo, MDM2 and MDMX hetero-RING complexes are the predominant form versus the MDM2 homo-RING complex. Importantly, the MDM2/MDMX hetero-RING complexes exhibit a greater E3 ligase activity than the MDM2 homo-RING complexes. Disruption of the binding between MDM2 and MDMX resulted in a marked increase in both abundance and activity of p53, emphasizing the functional importance of this heterocomplex in p53 control.

From mRNA metabolism to cancer therapy: chronic myelogenous leukemia shows the way

Altered mRNA metabolism is a feature of many cancers including blast crisis chronic myelogenous leukemia. Indeed, loss of function of many tumor suppressors regulating cell proliferation, survival, and differentiation results from aberrant mRNA processing, nuclear export, and/or translation. Here, we summarize the effects of increased BCR/ABL oncogenic activity on the expression and function of RNA binding proteins (e.g., FUS, hnRNP A1, hnRNP E2, hnRNP K, and La/SSB) with posttranscriptional and translational regulatory activities and their importance for the phenotype of BCR/ABL-transformed hematopoietic progenitors. We also provide evidence that these studies not only advance our understanding on the molecular mechanisms contributing to tumor/leukemia emergence, maintenance, and/or progression but they also serve for the identification of novel molecular targets useful for the development of alternative therapies for imatinib-resistant and blast crisis chronic myelogenous leukemia and, perhaps, for other cancers characterized by similar alterations in the mRNA metabolism.

The p53--Mdm2--HAUSP complex is involved in p53 stabilization by HAUSP

The ubiquitin-specific protease HAUSP is a critical component of the p53-Mdm2 pathway by acting as a specific deubiquitinase for both p53 and Mdm2. Recent structural studies have indicated that p53 and Mdm2 bind to the N-terminal TRAF-like domain of HAUSP in a mutually exclusive manner. To understand the mechanism of HAUSP-mediated effects, we have created a p53 mutant that lacks HAUSP binding based on the crystal structure analysis. Indeed, this mutant p53 protein can be degraded by Mdm2 but fails to interact with HAUSP both in vitro and in vivo. Surprisingly, however, we have found that direct interaction between HAUSP and p53 is not absolutely required for it to antagonize efficiently Mdm2-mediated ubiquitination of p53 and that HAUSP is capable of enzymatically functioning in trans on p53 by using Mdm2 as a bridge. Further, we show that a trimeric protein complex containing p53, Mdm2 and HAUSP can exist in vivo, despite mutually exclusive binding, with Mdm2 serving as a binding mediator for p53 and HAUSP. These findings reveal the complication of HAUSP-mediated effects in the p53-Mdm2 interplay. It also has important implications for the development of novel chemotherapeutic compounds aimed at blocking this protein-protein interaction.

Enhanced tumor cell kill by combined treatment with a small-molecule antagonist of mouse double minute 2 and adenoviruses encoding p53

Strategies to treat cancer by restoring p53 tumor suppressor functions are being actively investigated. These approaches range from expressing an exogenous p53 gene in p53 mutant cancers to antagonizing a p53 inhibitor in p53 wild-type (WT) cancer cells. In addition, exogenous p53 is used to strengthen the anticancer efficacy of oncolytic adenoviruses. Many cancers express high levels of the major negative regulator of p53, mouse double minute 2 (MDM2) protein. Recently, a novel class of highly potent and specific MDM2 antagonists, the Nutlins, was identified. We envisioned that Nutlins could protect both endogenous and exogenous p53 from MDM2-mediated inactivation. We therefore investigated treating human cancer cells with a combination of adenovirus-mediated p53 gene therapy and Nutlin. Combination treatment resulted in broadly effective cell kill of p53 WT and p53-negative cancer cells. Cytotoxicity was associated with profound cell cycle checkpoint activation and apoptosis induction. We also tested Nutlin in combination with oncolytic adenoviruses. Nutlin treatment accelerated viral progeny burst from oncolytic adenovirus-infected cancer cells and caused an estimated 10- to 1,000-fold augmented eradication of p53 WT cancer cells. These findings suggest that Nutlins are promising compounds to be combined with p53 gene therapy and oncolytic virotherapy for cancer.

Mechanistic studies of MDM2-mediated ubiquitination in p53 regulation

As a central regulator for cell cycle arrest, apoptosis, and cellular senescence, p53 requires multiple layers of regulatory control to ensure correct temporal and spatial functions. It is well accepted that Mdm2-mediated ubiquitination plays a crucial role in p53 regulation. In addition to proteasome-mediated degradation, ubiquitination of p53 by Mdm2 acts a key signal for its nuclear export. Nuclear export has previously been thought to require the disassociation of the p53 tetramer and exposure of the intrinsic nuclear export signal. To elucidate the molecular mechanism of degradation-independent repression on p53 by Mdm2, we have developed a two-step approach to purify ubiquitinated forms of p53 induced by Mdm2 from human cells. Surprisingly, however, we found that ubiquitination has no effect on the tetramerization/oligomerization of p53, arguing against this seemingly well accepted model. Moreover, nuclear export of p53 alone is not sufficient to completely abolish p53 activity. Ubiquitination-mediated repression of p53 by Mdm2 acts at least, in part, through inhibiting the sequence-specific DNA binding activity. Thus, our results have important implications regarding the mechanisms by which Mdm2 acts on p53.

PACT is a negative regulator of p53 and essential for cell growth and embryonic development

The tumor suppressor p53 regulates cell cycle progression and apoptosis in response to various types of stress, whereas excess p53 activity creates unwanted effects. Tight regulation of p53 is essential for maintaining normal cell growth. p53-associated cellular protein-testes derived (PACT, also known as P2P-R, RBBP6) is a 250-kDa Ring finger-containing protein that can directly bind to p53. PACT is highly up-regulated in esophageal cancer and may be a promising target for immunotherapy. However, the physiological role of the PACT-p53 interaction remains largely unclear. Here, we demonstrate that the disruption of PACT in mice leads to early embryonic lethality before embryonic day 7.5 (E7.5), accompanied by an accumulation of p53 and widespread apoptosis. p53-null mutation partially rescues the lethality phenotype and prolonged survival to E11.5. Endogenous PACT can interact with Hdm2 and enhance Hdm2-mediated ubiquitination and degradation of p53 as a result of the increase of the p53-Hdm2 affinity. Consequently, PACT represses p53-dependent gene transcription. Knockdown of PACT significantly attenuates the p53-Hdm2 interaction, reduces p53 polyubiquitination, and enhances p53 accumulation, leading to both apoptosis and cell growth retardation. Taken together, our data demonstrate that the PACT-p53 interaction plays a critical role in embryonic development and tumorigenesis and identify PACT as a member of negative regulators of p53.