Ubiquitination of p53 and p21 is differentially affected by ionizing and UV radiation

Regulation of eukaryotic transcription initiation in response to cellular stress

Transcription initiation is a vital step in the regulation of eukaryotic gene expression. It can be dysregulated in response to various cellular stressors which is associated with numerous human diseases including cancer. Transcription initiation is facilitated via many gene-specific trans-regulatory elements such as transcription factors, activators, and coactivators through their interactions with transcription pre-initiation complex (PIC). These trans-regulatory elements can uniquely facilitate PIC formation (hence, transcription initiation) in response to cellular nutrient stress. Cellular nutrient stress also regulates the activity of other pathways such as target of rapamycin (TOR) pathway. TOR pathway exhibits distinct regulatory mechanisms of transcriptional activation in response to stress. Like TOR pathway, the cell cycle regulatory pathway is also found to be linked to transcriptional regulation in response to cellular stress. Several transcription factors such as p53, C/EBP Homologous Protein (CHOP), activating transcription factor 6 (ATF6α), E2F, transforming growth factor (TGF)-β, Adenomatous polyposis coli (APC), SMAD, and MYC have been implicated in regulation of transcription of target genes involved in cell cycle progression, apoptosis, and DNA damage repair pathways. Additionally, cellular metabolic and oxidative stressors have been found to regulate the activity of long non-coding RNAs (lncRNA). LncRNA regulates transcription by upregulating or downregulating the transcription regulatory proteins involved in metabolic and cell signaling pathways. Numerous human diseases, triggered by chronic cellular stressors, are associated with abnormal regulation of transcription. Hence, understanding these mechanisms would help unravel the molecular regulatory insights with potential therapeutic interventions. Therefore, here we emphasize the recent advances of regulation of eukaryotic transcription initiation in response to cellular stress.

Biphasic JNK-Erk signaling separates the induction and maintenance of cell senescence after DNA damage induced by topoisomerase II inhibition

Genotoxic stress in mammalian cells, including those caused by anti-cancer chemotherapy, can induce temporary cell-cycle arrest, DNA damage-induced senescence (DDIS), or apoptotic cell death. Despite obvious clinical importance, it is unclear how the signals emerging from DNA damage are integrated together with other cellular signaling pathways monitoring the cell's environment and/or internal state to control different cell fates. Using single-cell-based signaling measurements combined with tensor partial least square regression (t-PLSR)/principal component analysis (PCA) analysis, we show that JNK and Erk MAPK signaling regulates the initiation of cell senescence through the transcription factor AP-1 at early times after doxorubicin-induced DNA damage and the senescence-associated secretory phenotype (SASP) at late times after damage. These results identify temporally distinct roles for signaling pathways beyond the classic DNA damage response (DDR) that control the cell senescence decision and modulate the tumor microenvironment and reveal fundamental similarities between signaling pathways responsible for oncogene-induced senescence (OIS) and senescence caused by topoisomerase II inhibition. A record of this paper's transparent peer review process is included in the supplemental information.

Isolation and Characterization of Novel Canine Osteosarcoma Cell Lines from Chemotherapy-Naïve Patients

The present study aimed to establish novel canine osteosarcoma cell lines (COS3600, COS3600B, COS4074) and characterize the recently described COS4288 cells. The established D-17 cell line served as a reference. Analyzed cell lines differed notably in their biological characteristics. Calculated doubling times were between 22 h for COS3600B and 426 h for COS4074 cells. COS3600B and COS4288 cells produced visible colonies after anchorage-independent growth in soft agar. COS4288 cells were identified as cells with the highest migratory capacity. All cells displayed the ability to invade through an artificial basement membrane matrix. Immunohistochemical analyses revealed the mesenchymal origin of all COS cell lines as well as positive staining for the osteosarcoma-relevant proteins alkaline phosphatase and karyopherin α2. Expression of p53 was confirmed in all tested cell lines. Gene expression analyses of selected genes linked to cellular immune checkpoints (CD270, CD274, CD276), kinase activity (MET, ERBB2), and metastatic potential (MMP-2, MMP-9) as well as selected long non-coding RNA (MALAT1) and microRNAs (miR-9, miR-34a, miR-93) are provided. All tested cell lines were able to grow as multicellular spheroids. In all spheroids except COS4288, calcium deposition was detected by von Kossa staining. We believe that these new cell lines serve as useful biological models for future studies.

Combined Transcriptomic and Proteomic Analyses Reveal the Different Responses to UVA and UVB Radiation in Human Keratinocytes

Ultraviolet (UV) radiation from sunlight is a major risk factor for many cutaneous pathologies including skin aging and cancers. Despite decades of research, the different responses to UVA and UVB in human keratinocytes have not been systemically investigated. Here, we performed multi-omics to characterize the common and different changes in gene transcription and protein expression after exposure to UVB and UVA, respectively. Keratinocyte cells, treated with or without UV, were analyzed by TMT-labeled MS/MS spectra and RNA-sequencing. A common set of genes/proteins was found to be impacted by both UVA and UVB and the other differential genes/proteins showed wavelength specificity. The common set of genes/proteins were mainly involved in keratinization, lipid metabolic processes and stimulus response. The UVB specifically responsive genes/proteins were mainly related to RNA processing, gene silencing regulation and cytoskeleton organization. The UVA specifically responsive genes/proteins were mainly involved in vesicle-mediated transport and oxygen-containing compound response. Meanwhile, the hub differential genes/proteins in each set were identified by protein-protein interaction networks and cluster analysis. This work provided a global view of the similar and differential molecular mechanisms of UVB- and UVA-induced cell damage in keratinocytes, which would be beneficial for further studies in the prevention or treatment of UV-related pathologies.

Alternative transcribed 3' isoform of long non-coding RNA Malat1 inhibits mouse retinal oxidative stress

The function of the cancer-associated lncRNA Malat1 during aging is as-of-yet uncharacterized. Here, we show that Malat1 interacts with Nucleophosmin (NPM) in young mouse brain, and with Lamin A/C, hnRNP C, and KAP1 with age. RNA-seq and RT-qPCR reveal a persistent expression of Malat1_2 (the 3'isoform of Malat1) in Malat1Δ1 (5'-1.5 kb deletion) mouse retinas and brains at 1/4^th level of the full-length Malat1, while Malat1_1 (the 5'isoform) in Malat1Δ2 (deletion of 3'-conserved 5.7 kb) at a much lower level, suggesting an internal promoter driving the 3' isoform. The 1774 and 496 differentially expressed genes in Malat1Δ2 and Malat1Δ1 brains, respectively, suggest the 3' isoform regulates gene expression in trans and the 5' isoform in cis. Consistently, Malat1Δ2 mice show increased age-dependent retinal oxidative stress and corneal opacity, while Malat1Δ1 mice show no obvious phenotype. Collectively, this study reveals a physiological function of the lncRNA Malat1 3'-isoform during the aging process.

Cytotoxicity, genotoxicity, and immunohistochemical expression of p53 in the oral mucosal epithelium of adults following cone-beam computed tomography

This study sought to assess the cytotoxicity, genotoxicity, and immunohistochemical (IHC) expression of p53 in the oral mucosal epithelium of adults following cone-beam computed tomography (CBCT). This before-and-after observational study evaluated 30 patients (15 males and 15 females); mean age 35 years, who required CBCT. Exfoliating epithelial cells of the buccal mucosa were collected with a cotton swab immediately before and 10 days after CBCT. Following Papanicolaou staining, genotoxicity was evaluated by the micronucleus assay. The frequencies of pyknosis, karyorrhexis, karyolysis, budding and binucleation as cytotoxicity factors were also recorded. Expression of p53 was evaluated by IHC staining. Data were analyzed by paired samples t-test. Micronucleated cells, pyknosis, karyorrhexis, karyolysis, budding, cytotoxicity, and expression of p53 increased significantly after CBCT. CBCT exerts genotoxic and cytotoxic effects and leads to overexpression of p53 protein in the oral buccal mucosal cells.

Killing by Degradation: Regulation of Apoptosis by the Ubiquitin-Proteasome-System

Apoptosis is a cell suicide process that is essential for development, tissue homeostasis and human health. Impaired apoptosis is associated with a variety of human diseases, including neurodegenerative disorders, autoimmunity and cancer. As the levels of pro- and anti-apoptotic proteins can determine the life or death of cells, tight regulation of these proteins is critical. The ubiquitin proteasome system (UPS) is essential for maintaining protein turnover, which can either trigger or inhibit apoptosis. In this review, we will describe the E3 ligases that regulate the levels of pro- and anti-apoptotic proteins and assisting proteins that regulate the levels of these E3 ligases. We will provide examples of apoptotic cell death modulations using the UPS, determined by positive and negative feedback loop reactions. Specifically, we will review how the stability of p53, Bcl-2 family members and IAPs (Inhibitor of Apoptosis proteins) are regulated upon initiation of apoptosis. As increased levels of oncogenes and decreased levels of tumor suppressor proteins can promote tumorigenesis, targeting these pathways offers opportunities to develop novel anti-cancer therapies, which act by recruiting the UPS for the effective and selective killing of cancer cells.

Kaempferol protects against cadmium chloride-induced hippocampal damage and memory deficits by activation of silent information regulator 1 and inhibition of poly (ADP-Ribose) polymerase-1

The neuroprotective effect of Kaempferol against cadmium chloride (CdCl₂) -induced neurotoxicity is well reported. The silent information regulator 1 (SIRT1) and poly (ADP-Ribose) polymerase-1 (PARP1) are two related cellular molecules that can negatively affect the activity of each other to promote or inhibit cell survival, respectively. It is still largely unknown if the neurotoxicity of CdCl₂ or the neuroprotection of Kaempferol are mediated by modulating SIRT1 and/or PAPR1 activities. In this study, we tested the hypothesis that CdCl₂-induced memory deficit and hippocampal damage are associated with downregulation/inhibition of SIRT1 and activation of PAPR1, an effect that can be reversed by co-treatment with Kaempferol. Rats (n = 12/group) were divided into 4 groups as control, control + Kaempferol (50 mg//kg), CdCl₂ (0.5 mg/kg), and CdCl₂ + Kaempferol. All treatments were administered orally for 30 days daily. As compared to control rats, CdCl2 reduced rat's final body weights (21.8%) and their food intake (30%), induced oxidative stress and apoptosis in their hippocampi, and impaired their short and long-term recognition memory functions. Besides, the hippocampi of CdCl₂-treated rats had higher levels of TNF-α (197%), and IL-6 (190%) with a concomitant increase in nuclear activity and levels of NF-κB p65 (721% & 554%). Besides, they showed reduced nuclear activity (53%) and levels (74%) of SIRT1, higher nuclear activity and levels of PARP1 (292% & 138%), increased nuclear levels of p53 (870%), and higher acetylated levels of NF-κB p65 (513%), p53 (644%), PARP1 (696%), and FOXO-2 (149%). All these events were significantly reversed in the CdCl₂ + Kaempferol-treated rats. Of note, Kaempferol also increased levels of MnSOD (73.5%), and GSH (40%), protein levels of Bcl-2 (350%), and nuclear activity (67%) and levels (46%) of SIRT1 in the hippocampi of the control rats. In conclusion, Kaempferol ameliorates CdCl₂-induced memory deficits and hippocampal oxidative stress, inflammation, and apoptosis by increasing SIRT1 activity and inhibiting PARP1 activity.

Regulation of the p53 Family Proteins by the Ubiquitin Proteasomal Pathway

The tumor suppressor p53 and its homologues, p63 and p73, play a pivotal role in the regulation of the DNA damage response, cellular homeostasis, development, aging, and metabolism. A number of mouse studies have shown that a genetic defect in the p53 family could lead to spontaneous tumor development, embryonic lethality, or severe tissue abnormality, indicating that the activity of the p53 family must be tightly regulated to maintain normal cellular functions. While the p53 family members are regulated at the level of gene expression as well as post-translational modification, they are also controlled at the level of protein stability through the ubiquitin proteasomal pathway. Over the last 20 years, many ubiquitin E3 ligases have been discovered that directly promote protein degradation of p53, p63, and p73 in vitro and in vivo. Here, we provide an overview of such E3 ligases and discuss their roles and functions.

Competing memories of mitogen and p53 signalling control cell-cycle entry

Regulation of cell proliferation is necessary for immune responses, tissue repair, and upkeep of organ function to maintain human health. When proliferating cells complete mitosis, a fraction of newly born daughter cells immediately enter the next cell cycle, while the remaining cells in the same population exit to a transient or persistent quiescent state. Whether this choice between two cell-cycle pathways is due to natural variability in mitogen signalling or other underlying causes is unknown. Here we show that human cells make this fundamental cell-cycle entry or exit decision based on competing memories of variable mitogen and stress signals. Rather than erasing their signalling history at cell-cycle checkpoints before mitosis, mother cells transmit DNA damage-induced p53 protein and mitogen-induced cyclin D1 (CCND1) mRNA to newly born daughter cells. After mitosis, the transferred CCND1 mRNA and p53 protein induce variable expression of cyclin D1 and the CDK inhibitor p21 that almost exclusively determines cell-cycle commitment in daughter cells. We find that stoichiometric inhibition of cyclin D1-CDK4 activity by p21 controls the retinoblastoma (Rb) and E2F transcription program in an ultrasensitive manner. Thus, daughter cells control the proliferation-quiescence decision by converting the memories of variable mitogen and stress signals into a competition between cyclin D1 and p21 expression. We propose a cell-cycle control principle based on natural variation, memory and competition that maximizes the health of growing cell populations.

Regulation of transcriptional activators by DNA-binding domain ubiquitination

Ubiquitin is a key component of the regulatory network that maintains gene expression in eukaryotes, yet the molecular mechanism(s) by which non-degradative ubiquitination modulates transcriptional activator (TA) function is unknown. Here endogenous p53, a stress-activated transcription factor required to maintain health, is stably monoubiquitinated, following pathway activation by IR or Nutlin-3 and localized to the nucleus where it becomes tightly associated with chromatin. Comparative structure-function analysis and in silico modelling demonstrate a direct role for DNA-binding domain (DBD) monoubiquitination in TA activation. When attached to the DBD of either p53, or a second TA IRF-1, ubiquitin is orientated towards, and makes contact with, the DNA. The contact is made between a predominantly cationic surface on ubiquitin and the anionic DNA. Our data demonstrate an unexpected role for ubiquitin in the mechanism of TA-activity enhancement and provides insight into a new level of transcriptional regulation.

p53: Multiple Facets of a Rubik's Cube

The p53 tumor suppressor has been studied for decades, and still there are many questions left unanswered. In this review, we first describe the current understanding of the wild-type p53 functions that determine cell survival or death, and regulation of the protein, with a particular focus on the negative regulators, the murine double minute family of proteins. We also summarize tissue-, stress-, and age-specific p53 activities and the potential underlying mechanisms. Among all p53 gene alterations identified in human cancers, p53 missense mutations predominate, suggesting an inherent biological advantage. Numerous gain-of-function activities of mutant p53 in different model systems and contexts have been identified. The emerging theme is that mutant p53, which retains a potent transcriptional activation domain, also retains the ability to modify gene transcription, albeit indirectly. Lastly, because mutant p53 stability is necessary for its gain of function, we summarize the mechanisms through which mutant p53 is specifically stabilized. A deeper understanding of the multiple pathways that impinge upon wild-type and mutant p53 activities and how these, in turn, regulate cell behavior will help identify vulnerabilities and therapeutic opportunities.

Tyropeptins, proteasome inhibitors produced by Kitasatospora sp. MK993-dF2

Tyropeptins are new proteasome inhibitors isolated from the culture broth of Kitasatospora sp. MK993-dF2. Tyropeptins permeate cell membranes, inhibit intracellular proteasomes and reduce the degradation of ubiquitinated proteins in mammalian cells. We performed structure-based drug design and structure-activity relationship studies on tyropeptin derivatives to obtain valuable information of derivatives. Among the synthesized tyropeptin derivatives, some boronic acid derivatives exhibited potent antitumor effects against human multiple myeloma. In this review, we summarize the discovery of tyropeptins and the development of tyropeptin derivatives.

The IGF-1R/AKT pathway determines cell fate in response to p53

p53 that is activated in response to DNA-damaging stress can induce apoptosis or either transient or permanent cell cycle arrests. Apoptosis and permanent cell cycle arrest (senescence) are bona-fide tumor suppressor mechanisms through which p53 inhibits cancer cell survival. In contrast, transient cell cycle arrests induced by p53 can increase survival by allowing cells time to repair their DNA before proceeding with cell division. Mechanisms that control the choice of response to p53 (apoptosis vs arrest) are not fully understood. There is abundant crosstalk between p53 and the IGF-1R/AKT/mTORC1 signaling pathway. Recent studies indicate this crosstalk can determine the choice of response to p53. These findings have clear implications for the potential use of IGF-1R pathway inhibitors against p53 wild-type or p53-null or mutant cancers.

Posttranscriptional Upregulation of p53 by Reactive Oxygen Species in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) cells multiply and become more resistant to immunochemotherapy in "proliferation centers" within tissues, whereas apoptosis occurs in the periphery. Various models recapitulate these microenvironments in vitro, such as stimulation with CD154 and IL4. Using this system, we observed a 30- to 40-fold induction of wild-type p53 protein in 50 distinct human CLL specimens tested, without the induction of either cell-cycle arrest or apoptosis. In contrast, the mRNA levels for p53 did not increase, indicating that its elevation occurred posttranscriptionally. Mechanistic investigations revealed that under the conditions studied, p53 was phosphorylated on residues associated with p53 activation and increased half-life. However, p53 protein induced in this manner could transcriptionally activate only a subset of target genes. The addition of a DNA-damaging agent further upregulated p53 protein levels, which led to apoptosis. p53 induction relied on the increase in intracellular reactive oxygen species observed after CD154 and IL4 stimulation. We propose that chronic oxidative stress is a characteristic of the microenvironment in B-cell "proliferation centers" in CLL that are capable of elevating the basal expression of p53, but to levels below the threshold needed to induce arrest or apoptosis. Our findings suggest that reactivation of the full transcriptional activities of p53 in proliferating CLL cells may offer a possible therapeutic strategy. Cancer Res; 76(21); 6311-9. ©2016 AACR.

Nuclear Localization Signal and p53 Binding Site in MAP/ERK Kinase Kinase 1 (MEKK1)

Previously, we showed that Mekk1 translocates to the nucleus, interacts with tumor suppressor protein p53, and co-represses PKD1 transcription via an atypical p53 binding site on the minimal PKD1 promoter (JBC 285:38,818-38,831, 2010). In this study, we report the mechanisms of Mekk1 nuclear transport and p53 binding. Using GFP-linked constitutively active-Mekk1 (CA-Mekk1) and a deletion strategy, we identified a nuclear localization signal (HRDVK) located at amino acid (aa) residues 1,349-1,353 in the C-terminal Mekk1 catalytic domain. Deletion of this sequence in CA-Mekk1 and full-length Mekk1 significantly reduced their nuclear translocation in both HEK293T and COS-1 cells. Using co-immunoprecipitation, we identified an adjacent sequence (GANLID, aa 1,354-1,360) in Mekk1 responsible for p53 binding. Deletion of this sequence markedly reduced the interaction of Mekk1 with p53. Mekk1 does not appear to affect phosphorylation of Ser15, located in the Mdm2 interaction site, or other Ser residues in p53. However, Mekk1 mediates p53 protein stability in the presence of Mdm2 and reduces p53 ubiquitination, suggesting an interference with Mdm2-mediated degradation of p53 by the ubiquitin-proteasome pathway.

The therapeutic potential of microbial proteasome inhibitors

The proteasome influences cellular homeostasis through the degradation of regulatory proteins, many of which are also involved in disease pathogenesis. In particular, numerous regulatory proteins associated with tumor growth, such as cyclins, cyclin-dependent kinase inhibitors, tumor suppressors, and NF-κB inhibitors are degraded by the proteasome. Proteasome inhibitors can stabilize these regulatory proteins, resulting in the suppression of tumor development and the regulation of immune responses. Thus, proteasome inhibitors are promising candidate antitumor agents and immune-regulatory agents. Bortezomib is the first-in-class proteasome inhibitor approved for the treatment of multiple myeloma. Despite its high efficiency, however, a large proportion of patients do not attain sufficient clinical response due to toxicity and drug resistance. Therefore, the development of new proteasome inhibitors with improved pharmacological properties is needed. Natural products produced by microorganisms are a promising source of such compounds. This review provides an overview of proteasome inhibitors produced by microorganisms, with special focus on inhibitors isolated from actinomycetes.

The novel tubulin polymerization inhibitor MHPT exhibits selective anti-tumor activity against rhabdomyosarcoma in vitro and in vivo

The dose-limiting toxicity caused by standard chemotherapy has become a major roadblock to successful rhabdomyosarcoma chemotherapy. By screening a thiazolidinone library including 372 compounds, a novel synthetic compound, 2-((4-hydroxyphenyl)imino)-5-(3-methoxybenzylidene)thiazolidin-4-one (MHPT), was identified as a potent and selective anti-rhabdomyosarcoma agent. MHPT inhibited 50% of the growth of the rhabdomyosarcoma cell lines RD and SJ-RH30 at 0.44 μM and 1.35 μM, respectively, while displaying no obvious toxicity against normal human fibroblast cells at 100 μM. Further investigation revealed that MHPT suppressed the polymerization of tubulin, leading to rhabdomyosarcoma cell growth arrest at the G2/M phase followed by apoptosis. In vivo, MHPT inhibited tumor growth by 48.6% relative to the vehicle control after 5 intraperitoneal injections of 40 mg/kg without appreciable toxicity to normal tissues and systems in an RD xenograft mouse model, while vincristine caused lethal toxicity when similar growth inhibition was achieved. As a moderate tubulin polymerization inhibitor compared with vincristine, MHPT requires a more dynamic tubulin to exert its cytotoxicity, which is a situation that only exists in cancer cells. This attribute may account for the low toxicity of MHPT in normal cells. Our data suggest that MHPT has the potential to be further developed into a selective anti-rhabdomyosarcoma drug with low toxicity.

Phosphomimetic mutation of the N-terminal lid of MDM2 enhances the polyubiquitination of p53 through stimulation of E2-ubiquitin thioester hydrolysis

Mouse double minute 2 (MDM2) has a phosphorylation site within a lid motif at Ser17 whose phosphomimetic mutation to Asp17 stimulates MDM2-mediated polyubiquitination of p53. MDM2 lid deletion, but not Asp17 mutation, induced a blue shift in the λ(max) of intrinsic fluorescence derived from residues in the central domain including Trp235, Trp303, Trp323, and Trp329. This indicates that the Asp17 mutation does not alter the conformation of MDM2 surrounding the tryptophan residues. In addition, Phe235 mutation enhanced MDM2 binding to p53 but did not stimulate its ubiquitination function, thus uncoupling increases in p53 binding from its E3 ubiquitin ligase function. However, the Asp17 mutation in MDM2 stimulated its discharge of the UBCH5a-ubiquitin thioester adduct (UBCH5a is a ubiquitin-conjugating enzyme E2D 1 UBC4/5 homolog yeast). This stimulation of ubiquitin discharge from E2 was independent of the p53 substrate. There are now four known effects of the Asp17 mutation on MDM2: (i) it alters the conformation of the isolated N-terminus as defined by NMR; (ii) it induces increased thermostability of the isolated N-terminal domain; (iii) it stimulates the allosteric interaction of MDM2 with the DNA-binding domain of p53; and (iv) it stimulates a novel protein-protein interaction with the E2-ubiquitin complex in the absence of substrate p53 that, in turn, increases hydrolysis of the E2-ubiquitin thioester bond. These data also suggest a new strategy to disrupt MDM2 function by targeting the E2-ubiquitin discharge reaction.

Antitumor effects of tyropeptin-boronic acid derivatives: New proteasome inhibitors

The proteasome degrades numerous regulatory proteins that are critical for tumor growth. Thus, proteasome inhibitors are promising antitumor agents. New proteasome inhibitors, such as tyropeptins and tyropeptin-boronic acid derivatives, have a potent inhibitory activity. Here we report the antitumor effects of two new tyropeptin-boronic acid derivatives, AS-06 and AS-29. AS-06 and AS-29 significantly suppress the degradation of the proteasome-sensitive fluorescent proteins in HEK293PS cells, and induce the accumulation of ubiquitinated proteins in human multiple myeloma cells. We show that these derivatives also suppress the degradation of the NF-κB inhibitor IκB-α and the nuclear translocation of NF-κB p65 in multiple myeloma cells, resulting in the inhibition of NF-κB activation. Furthermore, we demonstrate that AS-06 and AS-29 induce apoptosis through the caspase-8 and caspase-9 cascades. In a xenograft mouse model, i.v. administration of tyropeptin-boronic acid derivatives inhibits proteasome in tumors and clearly suppresses tumor growth in mice bearing human multiple myeloma. Our results indicate that tyropeptin-boronic acid derivatives could be lead therapeutic agents against human multiple myeloma.

Synthesis and Activity of Tyropeptin A Derivatives as Potent and Selective Inhibitors of Mammalian 20S Proteasome

Tyropeptin A, a potent proteasome inhibitor, was isolated from the culture broth of Kitasatospora sp. MK993-dF2. We synthesized the derivatives of tyropeptin A to enhance its inhibitory potency. Among the synthesized derivatives, the most potent compound, TP-104, exhibited a 20-fold inhibitory potency enhancement for chymotrypsin-like activity of 20S proteasome compared to tyropeptin A. Additionally, TP-110 specifically inhibited the chymotrypsin-like activity, but did not inhibit the post-glutamyl-peptide hydrolyzing (PGPH) and the trypsin-like activities of 20S proteasome. In vitro TP-110 strongly inhibited the growth of various cell lines.

Tetramerization-defects of p53 result in aberrant ubiquitylation and transcriptional activity

The tumor suppressor p53 regulates the expression of genes involved in cell cycle progression, senescence and apoptosis. Here, we investigated the effect of single point mutations in the oligomerization domain (OD) on tetramerization, transcription, ubiquitylation and stability of p53. As predicted by docking and molecular dynamics simulations, p53 OD mutants show functional defects on transcription, Mdm2-dependent ubiquitylation and 26S proteasome-mediated degradation. However, mutants unable to form tetramers are well degraded by the 20S proteasome. Unexpectedly, despite the lower structural stability compared to WT p53, p53 OD mutants form heterotetramers with WT p53 when expressed transiently or stably in cells wild type or null for p53. In consequence, p53 OD mutants interfere with the capacity of WT p53 tetramers to be properly ubiquitylated and result in changes of p53-dependent protein expression patterns, including the pro-apoptotic proteins Bax and PUMA under basal and adriamycin-induced conditions. Importantly, the patient derived p53 OD mutant L330R (OD1) showed the more severe changes in p53-dependent gene expression. Thus, in addition to the well-known effects on p53 stability, ubiquitylation defects promote changes in p53-dependent gene expression with implications on some of its functions.

Nucleolar control of p53: a cellular Achilles' heel and a target for cancer therapy

Nucleoli perform a crucial cell function, ribosome biogenesis, and of critical relevance to the subject of this review, they are also extremely sensitive to cellular stresses, which can cause loss of function and/or associated structural disruption. In recent years, we have learned that cells take advantage of this stress sensitivity of nucleoli, using them as stress sensors. One major protein regulated by this role of nucleoli is the tumor suppressor p53, which is activated in response to diverse cellular injuries in order to exert its onco-protective effects. Here we discuss a model of nucleolar regulation of p53, which proposes that key steps in the promotion of p53 degradation by the ubiquitin ligase MDM2 occur in nucleoli, thus providing an explanation for the observed link between nucleolar disruption and p53 stability. We review current evidence for this compartmentalization in p53 homeostasis and highlight current limitations of the model. Interestingly, a number of current chemotherapeutic agents capable of inducing a p53 response are likely to do so by targeting nucleolar functions and these compounds may serve to inform further improved therapeutic targeting of nucleoli.

Hip2 ubiquitin-conjugating enzyme overcomes radiation-induced G2/M arrest

Radiation induces cell cycle arrest and/or cell death in mammalian cells. In the present study, we show that Hip2, a ubiquitin-conjugating enzyme, can overcome radiation-induced G2/M cell cycle arrest and trigger the entry into mitosis. Ionizing radiation increased the levels of Hip2 by preventing its degradation but not its gene transcription. The stability of Hip2 in irradiated cells was further confirmed using live cell fluorescence imaging. Flow cytometric and molecular analyses revealed that Hip2 abrogated radiation-induced G2/M arrest, promoting entry into mitosis. Bimolecular fluorescence complementation assays and co-immunoprecipitation experiments showed that Hip2 interacted with and targeted p53 for degradation via the ubiquitin proteasome system, resulting in the activation of cdc2-cyclin B1 kinase to promote mitotic entry. These results contribute to our understanding of the mechanisms that regulate cell cycle progression and DNA damage-induced G2/M checkpoint cellular responses.

Two 4N cell-cycle arrests contribute to cisplatin-resistance

Cisplatin is a platinum-based drug that is used for the treatment of a wide-variety of primary human cancers. However, the therapeutic efficacy of cisplatin is often limited by intrinsic or acquired drug resistance. An important goal, therefore, is to identify mechanisms that lead to cisplatin resistance in cancer, and then use this information to more effectively target resistant cells. Cisplatin-resistant clones of the HCT116 cell line underwent a prolonged G2 arrest after cisplatin treatment while sensitive clones did not. The staurosporine analog UCN-01 abrogated this G2 arrest and sensitized the resistant clones to cisplatin. At later time points, 4N arrested cells assumed a tetraploid G1 state that was characterized by depletion of Cyclin A, Cyclin B, and CDC2, and increased expression of p53 and p21, in 4N cells. siRNA-mediated knockdown of p21 abrogated the tetraploid G1 arrest and induced killing that was dependent on p53. The results identify two targetable 4N arrests that can contribute to cisplatin resistance: First, a prolonged G2 arrest that can be targeted by UCN-01, and second, a tetraploid G1 arrest that can be targeted by siRNA against p21.

Effects of trifluralin on the mouse ovary

Trifluralin, a herbicide used to protect many arable and horticultural crops, was evaluated for its potential toxicity on the mammalian ovary. To this end, adult female mice were fed or not (control) with a trifluralin-enriched diet (150 mg/kg body weight/day) during gestation and lactation. After weaning, 3-week-old female mice from either trifluralin-treated or control groups were used to evaluate whether the exposure to this herbicide in utero and during lactation could induce stress responses in the ovary. It was found that trifluralin exposure caused a significantly higher level of p53, but not of pRb, in the whole ovary, and in particular in granulosa cells. TUNEL staining showed that herbicide treatment did not increase the apoptotic index of the somatic compartment. Also oocyte fertilizability was unaffected, as metaphase II oocytes retrieved from treated mice were capable of forming male and female pronuclei after in vitro fertilization as control mice. However, trifluralin determined a slightly higher number of oocytes with cytoplasmic degeneration compared with control animals. In conclusion, our results suggest that exposure to a low trifluralin dose during pregnancy and lactation does not impair oocyte quality, but can induce a stress response in ovarian somatic cells.

Human single-stranded DNA binding proteins are essential for maintaining genomic stability

The double-stranded conformation of cellular DNA is a central aspect of DNA stabilisation and protection. The helix preserves the genetic code against chemical and enzymatic degradation, metabolic activation, and formation of secondary structures. However, there are various instances where single-stranded DNA is exposed, such as during replication or transcription, in the synthesis of chromosome ends, and following DNA damage. In these instances, single-stranded DNA binding proteins are essential for the sequestration and processing of single-stranded DNA. In order to bind single-stranded DNA, these proteins utilise a characteristic and evolutionary conserved single-stranded DNA-binding domain, the oligonucleotide/oligosaccharide-binding (OB)-fold. In the current review we discuss a subset of these proteins involved in the direct maintenance of genomic stability, an important cellular process in the conservation of cellular viability and prevention of malignant transformation. We discuss the central roles of single-stranded DNA binding proteins from the OB-fold domain family in DNA replication, the restart of stalled replication forks, DNA damage repair, cell cycle-checkpoint activation, and telomere maintenance.

It Takes 15 to Tango: Making Sense of the Many Ubiquitin Ligases of p53

The transcription factor p53 regulates numerous cellular processes to guard against tumorigenesis. Cell-cycle inhibition, apoptosis, and autophagy are all regulated by p53 in a cell- and context-specific manner, underscoring the need for p53 activity to be kept low in most circumstances. p53 is kept in check primarily through its regulated ubiquitination and degradation by a number of different factors, whose contributions may reflect complex context-specific needs to restrain p53 activity. Chief among these E3 ubiquitin ligases in p53 homeostasis is the ubiquitously expressed proto-oncogene MDM2, whose loss renders vertebrates unable to limit p53 activity, resulting in early embryonic lethality. MDM2 has been validated as a critical, universal E3 ubiquitin ligase for p53 in numerous tissues and organisms to date, but additional E3 ligases have also been identified for p53 whose contribution to p53 activity is unclear. In this review, we summarize the recent advances in our knowledge regarding how p53 activity is apparently controlled by a multitude of ubiquitin ligases beyond MDM2.

Protective effects of Chlorella-derived peptide against UVC-induced cytotoxicity through inhibition of caspase-3 activity and reduction of the expression of phosphorylated FADD and cleaved PARP-1 in skin fibroblasts

UVC irradiation induces oxidative stress and leads to cell death through an apoptotic pathway. This apoptosis is caused by activation of caspase-3 and formation of poly(ADP-ribose) polymerase-1 (PARP-1). In this study, the underlying mechanisms of Chlorella derived peptide (CDP) activity against UVC-induced cytotoxicity were investigated. Human skin fibroblasts were treated with CDP, vitamin C, or vitamin E after UVC irradiation for a total energy of 15 J/cm². After the UVC exposure, cell proliferation and caspase-3 activity were measured at 12, 24, 48, and 72 h later. Expression of phosphorylated FADD and cleaved PARP-1 were measured 16 h later. DNA damage (expressed as pyrimidine (6-4) pyrimidone photoproducts DNA concentration) and fragmentation assay were performed 24 h after the UVC exposure. Results showed that UVC irradiation induced cytotoxicity in all groups except those treated with CDP. The caspase-3 activity in CDP-treated cells was inhibited from 12 h onward. Expression of phosphorylated FADD and cleaved PARP-1 were also reduced in CDP-treated cells. Moreover, UVC-induced DNA damage and fragmentation were also prevented by the CDP treatment. This study shows that treatment of CDP provides protective effects against UVC-induced cytotoxicity through the inhibition of caspase-3 activity and the reduction of phosphorylated FADD and cleaved PARP-1 expression.

Computational analysis of target hub gene repression regulated by multiple and cooperative miRNAs

MicroRNA (miRNA) target hubs are genes that can be simultaneously targeted by a comparatively large number of miRNAs, a class of non-coding RNAs that mediate post-transcriptional gene repression. Although the details of target hub regulation remain poorly understood, recent experiments suggest that pairs of miRNAs can cooperate if their binding sites reside in close proximity. To test this and other hypotheses, we established a novel approach to investigate mechanisms of collective miRNA repression. The approach presented here combines miRNA target prediction and transcription factor prediction with data from the literature and databases to generate a regulatory map for a chosen target hub. We then show how a kinetic model can be derived from the regulatory map. To validate our approach, we present a case study for p21, one of the first experimentally proved miRNA target hubs. Our analysis indicates that distinctive expression patterns for miRNAs, some of which interact cooperatively, fine-tune the features of transient and long-term regulation of target genes. With respect to p21, our model successfully predicts its protein levels for nine different cellular functions. In addition, we find that high abundance of miRNAs, in combination with cooperativity, can enhance noise buffering for the transcription of target hubs.

In vivo imaging of proteasome inhibition using a proteasome-sensitive fluorescent reporter

A proteasome degrades numerous regulatory proteins that are critical for tumor growth and is therefore recognized as a promising anticancer target. Determining proteasome activity in the tumors of mice bearing xenografts is essential for the development of novel proteasome inhibitors. We developed a system for in vivo imaging of proteasome inhibition in the tumors of living mice, using a proteasome-sensitive fluorescent reporter, ZsProSensor-1. This reporter consists of a green fluorescent protein, ZsGreen, fused to mouse ornithine decarboxylase, which is degraded by the proteasome without being ubiquitinated. In stably transfected cells expressing ZsProSensor-1, the fluorescent reporter was rapidly degraded under steady-state conditions, whereas it was stabilized in the presence of proteasome inhibitors. Subcutaneous inoculation of the transfected cells into nude mice resulted in tumor formation. When the proteasome inhibitor bortezomib was intravenously administered to mice bearing these tumors, the ZsProSensor-1 protein accumulated in the tumors and emitted a fluorescent signal in a dose-dependent manner. Robust fluorescence was sustained for 3 days and then gradually decreased to baseline levels within 15 days. Intravenous administration of bortezomib also showed potent antitumor activity. In contrast, oral administration of bortezomib did not result in fluorescent protein accumulation in tumors or exhibit any antitumor activity. These results indicate that in vivo imaging using the ZsProSensor-1 fluorescent protein can be used as an indicator of antitumor activity and will be a powerful tool for the development of novel proteasome inhibitors.

Regulation of MDM2 E3 ligase activity by phosphorylation after DNA damage

MDM2 is a major regulator of p53 by acting as a ubiquitin E3 ligase. The central acidic domain and C-terminal RING domain of MDM2 are both indispensable for ubiquitination of p53. Our previous study suggested that ATM phosphorylation of MDM2 near the C terminus inhibits RING domain oligomerization, resulting in p53 stabilization after DNA damage. We present here evidence that these modifications allosterically regulate the functions of both acidic domain and RING domain of MDM2. Using chemical cross-linking, we show that the MDM2 RING domain forms oligomers including dimer and higher-order complexes in vivo. RING domain dimerization efficiency is negatively regulated by upstream sequence. ATM-mediated phosphorylation of the upstream sequence further inhibits RING dimerization. Forced oligomerization of MDM2 partially overcomes the inhibitory effect of phosphorylation and stimulates p53 ubiquitination. Furthermore, the ability of MDM2 acidic domain to bind p53 core domain and induce p53 misfolding are also suppressed by the same C-terminal ATM sites after DNA damage. These results suggest that the acidic domain and RING domain of MDM2 are both allosterically coupled to the intervening ATM sites, which enables the same modification to regulate multiple MDM2 functions critical for p53 ubiquitination.

ATR controls the p21(WAF1/Cip1) protein up-regulation and apoptosis in response to low UV fluences

The universal cyclin-dependent kinase inhibitor p21(WAF1/Cip1) promotes cell cycle arrest and inhibits apoptosis in response to UV-induced DNA damage. Since the protein kinase ATR plays a major role in the cellular response to these carcinogenic lesions, we investigated the possible role of ATR in the modulation of p21(WAF1/Cip1) expression in response to UVC radiation. We have shown that p21(WAF1/Cip1) is up-regulated in human fibroblast and epithelial cells, but only in response to low UV fluences and low passage cells. Importantly, this up-regulation is ATR-dependent. In fact, in ATR-deficient or caffeine-treated cells UV light rather down-regulated the p21(WAF1/Cip1) protein through SKP2-dependent ubiquitination and degradation via the proteasomal pathway. Furthermore, we present evidence that ATR inhibits apoptosis in response to low fluences of UV light, through inhibiting the cleavage of caspase 3 and PARP as well as the repression of the proapoptotic proteins BAX and BAK. Interestingly, ATR is also required for the stability of the p21(WAF1/Cip1) protein in absence of genotoxic stress. Together, these results indicate that during the cellular response to low UVC fluences the ATR protein kinase up-regulates p21(WAF1/Cip1) and inhibits apoptosis. © 2011 Wiley Periodicals, Inc.

Histone deacetylase inhibitors demonstrate significant preclinical activity as single agents, and in combination with bortezomib in Waldenström's macroglobulinemia

We studied the role of histone deacetylase inhibitors in Waldenstrom's macroglobulinemia (WM). Gene expression profiling of bone marrow CD19+ cells from 30 patients and 10 healthy donors showed overexpression of HDAC4, HDAC9, and Sirt5, with validation of HDAC9 overexpression by q-PCR in primary and BCWM.1 cells. Suberoylanilide hydroxamic acid, trichostatin A, panobinostat, and sirtinol demonstrated dose-dependent killing of BCWM.1 cells. TSA showed the greatest potency with IC50 of 70 nM. Importantly, HDAC9 activity was decreased following TSA treatment suggesting an essential role for this HDAC in WM therapy. The combination of bortezomib plus HDAC inhibitors resulted in at least additive tumor cell killing in BCWM.1 cells. TSA and bortezomib-induced apoptosis depended on a similar set of caspase activation, whereas their effect on cell cycle regulators was distinctly different. These results provided a framework for examining HDAC inhibitors as monotherapy, as well as combination therapy with bortezomib in WM.

Acquisition of p53 mutations in response to the non-genotoxic p53 activator Nutlin-3

Wild-type p53 is a stress-responsive tumor suppressor and potent growth inhibitor. Genotoxic stresses (e.g. ionizing and UV radiation or chemotherapeutic drug treatment) can activate p53, but also induce mutations in the P53 gene and thus select for p53-mutated cells. Nutlin-3a (Nutlin) is pre-clinical drug that activates p53 in a non-genotoxic fashion. Nutlin occupies the p53-binding pocket of MDM2, activating p53 by blocking the p53-MDM2 interaction. Because Nutlin neither binds p53 directly nor introduces DNA damage, we hypothesized Nutlin would not induce P53 mutations and therefore not select for p53-mutated cells. To test this, populations of SJSA-1 (p53 wild-type) cancer cells were expanded that survived repeated Nutlin exposures, and individual clones were isolated. Group 1 clones were resistant to Nutlin-induced apoptosis, but still underwent growth-arrest. Surprisingly, while some Group 1 clones retained wild-type p53, others acquired a heterozygous p53 mutation. Apoptosis resistance in Group 1 clones was associated with decreased PUMA induction and decreased caspase 3/7 activation. Group 2 clones were resistant to both apoptosis and growth-arrest induced by Nutlin. Group 2 clones had acquired mutations in the p53 DNA-binding domain and expressed only mutant p53s that were induced by Nutlin treatment, but were unable to bind the P21 and PUMA gene promoters, and unable to activate transcription. These results demonstrate that non-genotoxic p53 activation (e.g. by Nutlin treatment) can lead to the acquisition of somatic mutations in p53 and select for p53-mutated cells. These findings have implications for the potential clinical use of Nutlin and other small molecule MDM2 antagonists.

p53 and p21(Waf1) are recruited to distinct PML-containing nuclear foci in irradiated and Nutlin-3a-treated U2OS cells

Promyelocytic leukemia nuclear bodies (PML-NBs) are multiprotein complexes that include PML protein and localize in nuclear foci. PML-NBs are implicated in multiple stress responses, including apoptosis, DNA repair, and p53-dependent growth inhibition. ALT-associated PML bodies (APBs) are specialized PML-NBs that include telomere-repeat binding-factor TRF1 and are exclusively in telomerase-negative tumors where telomere length is maintained through alternative (ALT) recombination mechanisms. We compared cell-cycle and p53 responses in ALT-positive cancer cells (U2OS) exposed to ionizing radiation (IR) or the p53 stabilizer Nutlin-3a. Both IR and Nutlin-3a caused growth arrest and comparable induction of p53. However, p21, whose gene p53 activates, displayed biphasic induction following IR and monophasic induction following Nutlin-3a. p53 was recruited to PML-NBs 3-4 days after IR, approximately coincident with the secondary p21 increase. These p53/PML-NBs marked sites of apparently unrepaired DNA double-strand breaks (DSBs), identified by colocalization with phosphorylated histone H2AX. Both Nutlin-3a and IR caused a large increase in APBs that was dependent on p53 and p21 expression. Moreover, p21, and to a lesser extent p53, was recruited to APBs in a fraction of Nutlin-3a-treated cells. These data indicate (1) p53 is recruited to PML-NBs after IR that likely mark unrepaired DSBs, suggesting p53 may either be further activated at these sites and/or function in their repair; (2) p53-p21 pathway activation increases the percentage of APB-positive cells, (3) p21 and p53 are recruited to ALT-associated PML-NBs after Nutlin-3a treatment, suggesting that they may play a previously unrecognized role in telomere maintenance.

Delayed effects of exposure to a moderate radiation dose on transcription profiles in human primary fibroblasts

Ionizing radiation (IR) is used in a wide variety of medical and nonmedical applications and poses a potential threat to human health. Knowledge of changes in gene expression in irradiated cells may be helpful for the establishment of effective paradigms for radiation protection. IR-induced DNA damage triggers a complex cascade of signal transduction. Recently, genome-wide approaches have allowed the detection of alterations in gene expression across a wide range of radiation doses. However, the delayed or long-term biological effects of mild-doses of IR remain largely unknown. The main objective of the present study was to investigate the effects of a moderate dose of gamma-rays (50 cGy) on gene expression 6 days post-irradiation. Gene expression using cDNA microarrays revealed statistically significant changes in the expression of 59 genes (FDR < 0.07), whose functions are related to cell-cycle control, protein trafficking, ubiquitin cycle, Rho-GTPAse pathway, protein phosphatase signalization, oxidoreductase control, and stress response. A set of 464 genes was also selected by a less stringent approach, and we demonstrate that this broader set of genes can efficiently distinguish the irradiated samples from the unirradiated, defining a long-term IR signature in human primary fibroblasts. Our findings support the existence of persistent responses to mild doses of IR detectable by changes in gene expression profiles. These results provide insight into delayed effects observed in human primary cells as well as the role of long-term response in neoplastic transformation. Environ.

hSSB1 binds and protects p21 from ubiquitin-mediated degradation and positively correlates with p21 in human hepatocellular carcinomas

Downregulation of hSSB1, a single-stranded DNA-binding protein, causes increased radiosensitivity, defective checkpoint activation and genomic instability. However, the mechanisms of hSSB1 function in these responses remain to be uncovered. Here, we present evidence that hSSB1 directly binds p21 and this interaction may prevent p21 from ubiquitin-mediated degradation. Furthermore, both promotion of the G1/S transition and abrogation of the G2/M checkpoints induced by hSSB1 knockdown are partially dependent on p21. Most importantly, hSSB1 and p21 levels are positively correlated in human hepatocellular carcinomas (HCC), as determined by immunostaining. Therefore, hSSB1 may positively modulate p21 to regulate cell cycle progression and DNA damage response, implicating hSSB1 as a novel, promising therapeutic target for cancers such as HCC.

Decision making of the p53 network: death by integration

The tumor suppressor protein p53 plays a central role in the multiple response pathways activated by DNA damage. In particular, p53 is involved in both the pro-survival response of cell cycle arrest and DNA repair, and the pro-death response of apoptosis. How does the p53 network coordinate the different pathways that lead to the opposite cell fates and what is its strategy in making the life-death decisions? To address these questions, we develop an integrated mathematical model that embraces three key modules of the p53 network: p53 core regulation, p53-induced cell cycle arrest and p53-dependent apoptosis initiation. Our analyses reveal that different aspects of the nuclear p53 dynamic profile are being used to differentially regulate the pro-survival and the pro-death modules. While the activation of the pro-survival module is dependent on the current or recent status of the DNA damage, the activation of the pro-death module relies on the accumulation or integration of the damage level over time. Thus, the cell will take the death fate if it cannot recover from the damage within a time period that is inversely proportional to the damage level. This "adaptive timer" strategy is likely to be adopted in other stress response systems.

MAP/ERK kinase kinase 1 (MEKK1) mediates transcriptional repression by interacting with polycystic kidney disease-1 (PKD1) promoter-bound p53 tumor suppressor protein

Mitogen-activated protein kinase (MAPK) cascades regulate a wide variety of cellular processes that ultimately depend on changes in gene expression. We have found a novel mechanism whereby one of the key MAP3 kinases, Mekk1, regulates transcriptional activity through an interaction with p53. The tumor suppressor protein p53 down-regulates a number of genes, including the gene most frequently mutated in autosomal dominant polycystic kidney disease (PKD1). We have discovered that Mekk1 translocates to the nucleus and acts as a co-repressor with p53 to down-regulate PKD1 transcriptional activity. This repression does not require Mekk1 kinase activity, excluding the need for an Mekk1 phosphorylation cascade. However, this PKD1 repression can also be induced by the stress-pathway stimuli, including TNFα, suggesting that Mekk1 activation induces both JNK-dependent and JNK-independent pathways that target the PKD1 gene. An Mekk1-p53 interaction at the PKD1 promoter suggests a new mechanism by which abnormally elevated stress-pathway stimuli might directly down-regulate the PKD1 gene, possibly causing haploinsufficiency and cyst formation.

Mdm2 facilitates the association of p53 with the proteasome

The ubiquitin ligase Mdm2 targets the p53 tumor suppressor protein for proteasomal degradation. Mutating phosphorylation sites in the central domain of Mdm2 prevents p53 degradation, although it is still ubiquitylated, indicating that Mdm2 has a post-ubiquitylation function for p53 degradation. We show that Mdm2 associates with several subunits of the 19S proteasome regulatory particle in a ubiquitylation-independent manner. Mdm2 furthermore promotes the formation of a ternary complex of itself, p53, and the proteasome. Replacing phosphorylation sites within the central domain with alanines reduced the formation of the ternary complex. The C-terminus of Mdm2 was sufficient for interaction with the proteasome despite an additional proteasome binding site in the Mdm2 N-terminus. In addition to binding to the proteasome, the C-terminus of Mdm2 bound to the central domain, possibly competing with, and therefore blocking, Mdm2/proteasome interaction. We propose that Mdm2 facilitates, or at least enhances, the association of p53 with the proteasome and that phosphorylation of the central domain of Mdm2 regulates this process.

Mechanism of p53 stabilization by ATM after DNA damage

p53 suppresses tumor development by responding to unauthorized cell proliferation, growth factor or nutrient deprivation, and DNA damage. Distinct pathways have been identified that cause p53 activation, including ARF-dependent response to oncogene activation, ribosomal protein-mediated response to abnormal rRNA synthesis, and ATM-dependent response to DNA damage. Elucidating the mechanisms of these signaling events are critical for understanding tumor suppression by p53 and development of novel cancer therapeutics. More than a decade of research has established the ATM kinase as a key molecule that activates p53 after DNA damage. Our recent study revealed that ATM phosphorylation of MDM2 is likely to be the key step in causing p53 stabilization. Upon activation by ionizing irradiation, ATM phosphorylates MDM2 on multiple sites near its RING domain. These modifications inhibit the ability of MDM2 to poly-ubiquitinate p53, thus leading to its stabilization. MDM2 phosphorylation does not inactivate its E3 ligase activity per se, since MDM2 self-ubiquitination and MDMX ubiquitination functions are retained. The selective inhibition of p53 poly-ubiquitination is accomplished through disrupting MDM2 oligomerization that may provide a scaffold for processive elongation of poly ubiquitin chains. These findings suggest a novel model of p53 activation and a general mechanism of E3 ligase regulation by phosphorylation.