RB regulates the stability and the apoptotic function of p53 via MDM2

Discovery of Novel Antitumor Small-Molecule Agent with Dual Action of CDK2/p-RB and MDM2/p53

Cell cycle-dependent kinase 2 (CDK2) is located downstream of CDK4/6 in the cell cycle and regulates cell entry into S-phase by binding to Cyclin E and hyper-phosphorylating Rb. Proto-oncogene murine double minute 2 (MDM2) is a key negative regulator of p53, which is highly expressed in tumors and plays an important role in tumorigenesis and progression. In this study, we identified a dual inhibitor of CDK2 and MDM2, III-13, which had good selectivity for inhibiting CDK2 activity and significantly reduced MDM2 expression. In vitro results showed that III-13 inhibited proliferation of a wide range of tumor cells, regardless of whether Cyclin E1 (CCNE1) was overexpressed or not. The results of in vivo experiments showed that III-13 significantly inhibited proliferation of tumor cells and did not affect body weight of mice. The results of the druggability evaluation showed that III-13 was characterized by low bioavailability and poor membrane permeability when orally administered, suggesting the necessity of further structural modifications. Therefore, this study provided a lead compound for antitumor drugs, especially those against CCNE1-amplified tumor proliferation.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Genome homeostasis defects drive enlarged cells into senescence

Cellular senescence refers to an irreversible state of cell-cycle arrest and plays important roles in aging and cancer biology. Because senescence is associated with increased cell size, we used reversible cell-cycle arrests combined with growth rate modulation to study how excessive growth affects proliferation. We find that enlarged cells upregulate p21, which limits cell-cycle progression. Cells that re-enter the cell cycle encounter replication stress that is well tolerated in physiologically sized cells but causes severe DNA damage in enlarged cells, ultimately resulting in mitotic failure and permanent cell-cycle withdrawal. We demonstrate that enlarged cells fail to recruit 53BP1 and other non-homologous end joining (NHEJ) machinery to DNA damage sites and fail to robustly initiate DNA damage-dependent p53 signaling, rendering them highly sensitive to genotoxic stress. We propose that an impaired DNA damage response primes enlarged cells for persistent replication-acquired damage, ultimately leading to cell division failure and permanent cell-cycle exit.

Identification of MDM2 as a prognostic and immunotherapeutic biomarker in a comprehensive pan-cancer analysis: A promising target for breast cancer, bladder cancer and ovarian cancer immunotherapy

BACKGROUND

The murine double minute 2 (MDM2) gene is a crucial factor in the development and progression of various cancer types. Multiple rigorous scientific studies have consistently shown its involvement in tumorigenesis and cancer progression in a wide range of cancer types. However, a comprehensive analysis of the role of MDM2 in human cancer has yet to be conducted.

METHODS

We used various databases, including TIMER2.0, TCGA, GTEx and STRING, to analyze MDM2 expression and its correlation with clinical outcomes, interacting genes and immune cell infiltration. We also investigated the association of MDM2 with immune checkpoints and performed gene enrichment analysis using DAVID tools.

RESULTS

The pan-cancer MDM2 analysis found that MDM2 expression and mutation status were observably different in 25 types of cancer tissue compared with healthy tissues, and prognosis analysis showed that there was a significant correlation between MDM2 expression and patient prognosis. Furthermore, correlation analysis showed that MDM2 expression was correlated with tumor mutational burden, microsatellite instability and drug sensitivity in certain cancer types. We found that there was an association between MDM2 expression and immune cell infiltration across cancer types, and MDM2 inhibitors might enhance the effect of immunotherapy on breast cancer, bladder cancer and ovarian cancer.

CONCLUSIONS

The first systematic pan-cancer analysis of MDM2 was conducted, and it demonstrated that MDM2 was a reliable prognostic biomarker and was closely related to cancer immunity, providing a potential immunotherapeutic target for breast cancer, bladder cancer and ovarian cancer.

MDM2 Implications for Potential Molecular Pathogenic Therapies of Soft-Tissue Tumors

Murine double minute 2 (MDM2, gene name MDM2) is an oncogene that mainly codes for a protein that acts as an E3 ubiquitin ligase, which targets the tumor suppressor protein p53 for degradation. Overexpression of MDM2 regulates the p53 protein levels by binding to it and promoting its degradation by the 26S proteasome. This leads to the inhibition of p53's ability to regulate cell cycle progression and apoptosis, allowing for uncontrolled cell growth, and can contribute to the development of soft-tissue tumors. The application of cellular stress leads to changes in the binding of MDM2 to p53, which prevents MDM2 from degrading p53. This results in an increase in p53 levels, which triggers either cell cycle arrest or apoptosis. Inhibiting the function of MDM2 has been identified as a potential therapeutic strategy for treating these types of tumors. By blocking the activity of MDM2, p53 function can be restored, potentially leading to tumor cell death and inhibiting the growth of tumors. However, further research is needed to fully understand the implications of MDM2 inhibition for the treatment of soft-tissue tumors and to determine the safety and efficacy of these therapies in clinical trials. An overview of key milestones and potential uses of MDM2 research is presented in this review.

Dynamic tumor microenvironment, molecular heterogeneity, and distinct immunologic portrait of triple-negative breast cancer: an impact on classification and treatment approaches

Heterogeneity of the tumor microenvironment (TME) and the lack of a definite targetable receptor in triple-negative breast cancer (TNBC) has carved a niche for this cancer as a particularly therapeutically challenging form of breast cancer. However, recent advances in high-throughput genomic analysis have provided new insights into the unique microenvironment and defining characteristics of various subsets of TNBC. This improved understanding has contributed to the development of novel therapeutic strategies including targeted therapies such as PARP inhibitors and CDK inhibitors. Moreover, the recent FDA approval of the immune checkpoint inhibitor against programmed cell death protein 1 (PD-1), pembrolizumab and atezolizumab, holds the promise of improving the quality of life and increasing the overall survival of TNBC patients. This recent approval is one of the many therapeutically novel strategies that are currently being exploited in clinical trials toward eventual contribution to the oncologist's toolbox against TNBC. In this review, we comprehensively discuss TNBC's distinct TME and its immunophenotype. Furthermore, we highlight the histological and molecular classification of this cancer. More importantly, we describe how these characteristics and classifications contribute to the current standards of care and how they steer the development of newer and more targeted therapies toward achieving peak therapeutic goals in the treatment of TNBC.

Almost famous: Human adenoviruses (and what they have taught us about cancer)

Papillomaviruses, polyomaviruses and adenoviruses are collectively categorized as the small DNA tumour viruses. Notably, human adenoviruses were the first human viruses demonstrated to be able to cause cancer, albeit in non-human animal models. Despite their long history, no human adenovirus is a known causative agent of human cancers, unlike a subset of their more famous cousins, including human papillomaviruses and human Merkel cell polyomavirus. Nevertheless, seminal research using human adenoviruses has been highly informative in understanding the basics of cell cycle control, gene expression, apoptosis and cell differentiation. This review highlights the contributions of human adenovirus research in advancing our knowledge of the molecular basis of cancer.

Retinoblastoma: from discovery to clinical management

The retinoblastoma gene (RB1) was the first tumour suppressor cloned; the role of its protein product (RB) as the principal driver of the G1 checkpoint in cell cycle control has been extensively studied. However, many other RB functions are continuously reported. Its role in senescence, DNA repair and apoptosis, among others, is indications of the significance of RB in a vast network of cellular interactions, explaining why RB loss or its malfunction is one of the leading causes of a large number of paediatric and adult cancers. RB was first reported in retinoblastoma, a common intraocular malignancy in the paediatric population worldwide. Currently, its diagnosis is clinical, and in nondeveloped countries, where the incidence is higher, it is performed in advanced stages of the disease, compromising the integrity of the eye and the patient's life. Even though new treatments are being continuously developed, enucleation is still a major choice due to the late disease stage diagnosis and treatments costs. Research into biomarkers is our best option to improve the chances of good results in the treatment and hopes of patients' good quality of life. Here, we recapitulated the history of the disease and the first treatments to put the advances in its clinical management into perspective. We also review the different functions of the protein and the progress in the search for biomarkers. It is clear that there is still a long way to go, but we should offer these children and their families a better way to deal with the disease with the community's effort.

miR-10a as a therapeutic target and predictive biomarker for MDM2 inhibition in acute myeloid leukemia

Pharmacological inhibition of MDM2/4, which activates the critical tumor suppressor p53, has been gaining increasing interest as a strategy for the treatment of acute myeloid leukemia (AML). While clinical trials of MDM2 inhibitors have shown promise, responses have been confined to largely molecularly undefined patients, indicating that new biomarkers and optimized treatment strategies are needed. We previously reported that the microRNA miR-10a is strongly overexpressed in some AML, and demonstrate here that it modulates several key members of the p53/Rb network, including p53 regulator MDM4, Rb regulator RB1CC1, p21 regulator TFAP2C, and p53 itself. The expression of both miR-10a and its downstream targets were strongly predictive of MDM2 inhibitor sensitivity in cell lines, primary AML specimens, and correlated to response in patients treated with both MDM2 inhibitors and cytarabine. Furthermore, miR-10a inhibition induced synergy between MDM2 inhibitor Nutlin-3a and cytarabine in both in vitro and in vivo AML models. Mechanistically this synergism primarily occurs via the p53-mediated activation of cytotoxic apoptosis at the expense of cytoprotective autophagy. Together these findings demonstrate that miR-10a may be useful as both a biomarker to identify patients most likely to respond to cytarabine+MDM2 inhibition and also a druggable target to increase their efficacy.

Circular RNA CDR1as disrupts the p53/MDM2 complex to inhibit Gliomagenesis

Background

Inactivation of the tumor suppressor p53 is critical for pathogenesis of glioma, in particular glioblastoma multiforme (GBM). MDM2, the main negative regulator of p53, binds to and forms a stable complex with p53 to regulate its activity. Hitherto, it is unclear whether the stability of the p53/MDM2 complex is affected by lncRNAs, in particular circular RNAs that are usually abundant and conserved, and frequently implicated in different oncogenic processes.

Methods

RIP-seq and RIP-qPCR assays were performed to determine the most enriched lncRNAs (including circular RNAs) bound by p53, followed by bioinformatic assays to estimate the relevance of their expression with p53 signaling and gliomagenesis. Subsequently, the clinical significance of CDR1as was evaluated in the largest cohort of Chinese glioma patients from CGGA (n = 325), and its expression in human glioma tissues was further evaluated by RNA FISH and RT-qPCR, respectively. Assays combining RNA FISH with protein immunofluorescence were performed to determine co-localization of CDR1as and p53, followed by CHIRP assays to confirm RNA-protein interaction. Immunoblot assays were carried out to evaluate protein expression, p53/MDM2 interaction and p53 ubiquitination in cells in which CDR1as expression was manipulated. After AGO2 or Dicer was knocked-down to inhibit miRNA biogenesis, effects of CDR1as on p53 expression, stability and activity were determined by immunoblot, RT-qPCR and luciferase reporter assays. Meanwhile, impacts of CDR1as on DNA damage were evaluated by flow cytometric assays and immunohistochemistry. Tumorigenicity assays were performed to determine the effects of CDR1as on colony formation, cell proliferation, the cell cycle and apoptosis (in vitro), and on tumor volume/weight and survival of nude mice xenografted with GBM cells (in vivo).

Results

CDR1as is found to bind to p53 protein. CDR1as expression decreases with increasing glioma grade and it is a reliable independent predictor of overall survival in glioma, particularly in GBM. Through a mechanism independent of acting as a miRNA sponge, CDR1as stabilizes p53 protein by preventing it from ubiquitination. CDR1as directly interacts with the p53 DBD domain that is essential for MDM2 binding, thus disrupting the p53/MDM2 complex formation. Induced upon DNA damage, CDR1as may preserve p53 function and protect cells from DNA damage. Significantly, CDR1as inhibits tumor growth in vitro and in vivo, but has little impact in cells where p53 is absent or mutated.

Conclusions

Rather than acting as a miRNA sponge, CDR1as functions as a tumor suppressor through binding directly to p53 at its DBD region to restrict MDM2 interaction. Thus, CDR1as binding disrupts the p53/MDM2 complex to prevent p53 from ubiquitination and degradation. CDR1as may also sense DNA damage signals and form a protective complex with p53 to preserve p53 function. Therefore, CDR1as depletion may play a potent role in promoting tumorigenesis through down-regulating p53 expression in glioma. Our results broaden further our understanding of the roles and mechanism of action of circular RNAs in general and CDR1as in particular, and can potentially open up novel therapeutic avenues for effective glioma treatment.

Acute alcohol consumption-induced let-7a inhibition exacerbates hepatic apoptosis by regulating Rb1 in mice

Alcohol consumption is a critical risk factor for hepatic pathogenesis, including alcoholic liver diseases (ALD), but implications of alcohol-induced dysregulation of microRNA (miRNA) in ALD pathogenesis are not completely understood. In the present study, C57BL/6J male mice were treated with saline (CON; oral gavage; n = 8) or alcohol (EtOH; 3 g/kg body weight; oral gavage; n = 8) for 7 days. A total of 599 miRNAs and 158 key mRNAs related to fatty liver and hepatotoxicity pathways were assessed in mice liver tissues. The mRNA expression datasets were then utilized to predict interactions with miRNAs that were changed by alcohol consumption. Predicted miRNA-mRNA interactions were validated using in vitro miRNA transfection experiments. The results showed that let-7a was significantly decreased in the EtOH group and Rb1 mRNA was predicted as a target gene. This was further supported by an inverse correlation of RB1 and let-7a expression in mice liver tissue. Additionally, key protein expressions involved in RB1-apoptosis axis [i.e., p73, cleaved CASP-3 (cCASP-3), and cCASP-7] showed a trend of increase in the EtOH mice; this was also confirmed by capase-3 enzyme activity and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay in livers of mice that had consumed alcohol. In line with our in vivo observations, alcohol treatment suppressed the let-7a expression and subsequently upregulated p73, cCASP-3, and cCASP-7 protein expressions in mice hepatocytes. Additional proteins in the apoptosis regulatory pathway (i.e., MDM2-p53 axis) were significantly changed in response to let-7a suppression in the cells. Taken together, the current study provides mechanistic evidence that alcohol consumption-induced let-7a suppression results in the upregulation of RB1, thereby promoting hepatic apoptosis through induction of pro-apoptotic proteins (e.g., p73), and by, at least in part, preventing MDM2-mediated p53 degradation.

Cardiac-specific ablation of the E3 ubiquitin ligase Mdm2 leads to oxidative stress, broad mitochondrial deficiency and early death

The maintenance of normal heart function requires proper control of protein turnover. The ubiquitin-proteasome system is a principal regulator of protein degradation. Mdm2 is the main E3 ubiquitin ligase for p53 in mitotic cells thereby regulating cellular growth, DNA repair, oxidative stress and apoptosis. However, which of these Mdm2-related activities are preserved in differentiated cardiomyocytes has yet to be determined. We sought to elucidate the role of Mdm2 in the control of normal heart function. We observed markedly reduced Mdm2 mRNA levels accompanied by highly elevated p53 protein expression in the hearts of wild type mice subjected to myocardial infarction or trans-aortic banding. Accordingly, we generated conditional cardiac-specific Mdm2 gene knockout (Mdm2^f/f;mcm) mice. In adulthood, Mdm2^f/f;mcm mice developed spontaneous cardiac hypertrophy, left ventricular dysfunction with early mortality post-tamoxifen. A decreased polyubiquitination of myocardial p53 was observed, leading to its stabilization and activation, in the absence of acute stress. In addition, transcriptomic analysis of Mdm2-deficient hearts revealed that there is an induction of E2f1 and c-Myc mRNA levels with reduced expression of the Pgc-1a/Ppara/Esrrb/g axis and Pink1. This was associated with a significant degree of cardiomyocyte apoptosis, and an inhibition of redox homeostasis and mitochondrial bioenergetics. All these processes are early, Mdm2-associated events and contribute to the development of pathological hypertrophy. Our genetic and biochemical data support a role for Mdm2 in cardiac growth control through the regulation of p53, the Pgc-1 family of transcriptional coactivators and the pivotal antioxidant Pink1.

In-Tether Chiral Center Induced Helical Peptide Modulators Target p53-MDM2/MDMX and Inhibit Tumor Growth in Stem-Like Cancer Cell

Inhibition of the interaction between p53 and MDM2/MDMX has attracted significant attention in anticancer therapy development. We designed a series of in-tether chiral center-induced helical stabilized peptides, among which MeR/PhR effectively reactivated p53. The activation of p53 inhibits cell proliferation and induces apoptosis in both the MCF-7 normal tumor cell line and the PA-1 pluripotent cancer cell line with only minimal cellular toxicity towards normal cells or cancer cell lines with p53 mutations. The in vivo bioactivity study of the peptide in the ovarian teratocarcinoma (PA-1) xenograft model showed a tumor growth rate inhibition of 70% with a dosage of 10 mg/kg (one injection every other day). This is the first application of a stabilized peptide modulator targeting stem-like cancer cell both in vitro and in vivo and provides references to cancer stem cell therapy.

Potential applications of MEG3 in cancer diagnosis and prognosis

LncRNAs are emerging as integral functional and regulatory components of normal biological activities and are now considered as critically involved in the development of different diseases including cancer. In this review, we summarized recent findings on maternally expressed gene 3 (MEG3), a noncoding lncRNA, locates in the imprinted DLK1–MEG3 locus on human chromosome 14q32.3 region. MEG3 is expressed in normal tissues but is either lost or decreased in many human tumors and tumor derived cell lines. Studies have demonstrated that MEG3 is associated with cancer initiation, progression, metastasis and chemo-resistance. MEG3 may affect the activities of TP53, MDM2, GDF15, RB1 and some other key cell cycle regulators. In addition, the level of MEG3 showed good correlation with cancer clinicopathological grade. In summary, MEGs is an RNA-based tumor suppressor and is involved in the etiology, progression, and chemosensitivity of cancers. The alteration of MEG3 levels in various cancers suggested the possibility of using MEG3 level for cancer diagnosis and prognosis.

The radioprotector ortho-phospho-L-tyrosine (pTyr) attenuates the side effects of fractionated irradiation in retinoblastoma mouse models but also decreases the local tumour control

BACKGROUND

Radiotherapy (RT) is used to treat retinoblastoma (Rb), the most frequent ocular tumour in children. Besides eradicating the tumour, RT can cause severe side effects including secondary malignancies. This study aimed to define whether the radioprotector ortho-phospho-L-tyrosine (pTyr) prevents RT-induced side effects and affects local tumour control in a xenograft and a genetic orthotopic Rb mouse model.

METHODS

B6;129-Rb1tm3Tyj/J (Rb^+/-) and Y79-Rb cell-xenografted nude mice were fractionated external beam irradiated (15 fractions of 5Gy 6MV photons during 3weeks) with or without pTyr pre-treatment (100mg/kg BW, 16h prior to each irradiation). One, three, six and nine months after RT, tumour control and RT toxicity were evaluated using in vivo imaging and histology. We also analysed pTyr dependant post irradiation cell survival and p53 activity in vitro.

RESULTS

In vitro pTyr pre-treatment showed no radioprotection on Y79 cells, but led to p53 stabilisation in unirradiated Y79 cells and to a facilitation of radiation-induced p21 up-regulation, confirming a modulation of p53 activity by pTyr. In both mouse models, secondary tumours were undetectable. In Rb^+/- mice, pTyr significantly lowered RT-induced greying of the fur, retinal thickness reduction and photoreceptor loss. However, in the xenografted Rb model, pTyr considerably decreased RT-mediated tumour control, which was observed in 16 out of 22 control eyes but in none of the 24 pTyr treated eyes.

CONCLUSIONS

In Rb^+/- mice pTyr significantly prevents RT-induced greying of the fur as well as retinal degeneration. However, since non-irradiated control mice were not used in our study, a formal possibility exists that the effect shown in the retina of Rb^+/- mice may be due to ageing of the animals and/or actions of pTyr alone. Unfortunately, as tested in a xenograft model, pTyr treatment reduced the control of Rb tumours.

MDMX under stress: the MDMX-MDM2 complex as stress signals hub

The tumor suppressor p53 plays a central role in safeguarding cellular homeostasis. Upon various types of stress signals such as DNA damage or oncogenic stress, p53 is promptly activated to prevent and repair damages that can threaten the genome stability. The two major negative regulators of p53 are MDM2 and MDMX, two homolog proteins that control p53 activity and turnover, hence keeping it in check during normal cell cycling. In the event of cellular stress, they have to be inhibited in order to relieve p53 from their suppression and allow its activation. As the essential upstream modulator of p53, the MDMX-MDM2 complex integrates multiple signaling pathways regulating p53 response to perturbations of cellular homeostasis. Given its predominantly cytoplasmic localization in normal conditions, we hypothesize that MDMX, rather than MDM2, is the first recipient of signaling cues directed towards the MDMX-MDM2 complex and aimed at modulating p53. In this review we give a synthetic overview of the phosphorylation sites of MDMX that are known to affect its degradation, ubiquitination, intracellular localization and interaction with MDM2 and p53, ultimately modulating the stability and activity of p53. The role of MDMX in response to the main types of cellular stress is also briefly discussed, along with the potential of the MDMX-MDM2 complex as therapeutic target to restore p53 activity.

The Double Role of p53 in Cancer and Autoimmunity and Its Potential as Therapeutic Target

p53 is a sequence-specific short-lived transcription factor expressed at low concentrations in various tissues while it is upregulated in damaged, tumoral or inflamed tissue. In normally proliferating cells, p53 protein levels and function are tightly controlled by main regulators, i.e., MDM2 (mouse double minute 2) and MDM4 proteins. p53 plays an important role due to its ability to mediate tumor suppression. In addition to its importance as a tumor suppressor, p53 coordinates diverse cellular responses to stress and damage and plays an emerging role in various physiological processes, including fertility, cell metabolism, mitochondrial respiration, autophagy, cell adhesion, stem cell maintenance and development. Interestingly, it has been recently implicated in the suppression of autoimmune and inflammatory diseases in both mice and humans. In this review based on current knowledge on the functional properties of p53 and its regulatory pathways, we discuss the potential utility of p53 reactivation from a therapeutic perspective in oncology and chronic inflammatory disorders leading to autoimmunity.

Dual function of MDM2 and MDMX toward the tumor suppressors p53 and RB

The orchestrated crosstalk between the retinoblastoma (RB) and p53 pathways contributes to preserving proper homeostasis within the cell. The deregulation of one or both pathways is a common factor in the development of most types of human cancer. The proto-oncoproteins MDMX and MDM2 are the main regulators of the well- known tumor suppressor p53 protein. Under normal conditions, MDM2 and MDMX inhibit p53, either via repression of its transcriptional activity by protein-protein interaction, or via polyubiquitination as a result of MDM2-E3 ubiquitin ligase activity, for which MDM2 needs to dimerize with MDMX. Under genotoxic stress conditions, both become positive regulators of p53. The ATM-dependent phosphorylation of MDM2 and MDMX allow them to bind p53 mRNA, these interactions promote p53 translation. MDM2 and MDMX are also being revealed as effective regulators of the RB protein. MDM2 is able to degrade RB by two different mechanisms, that is, by ubiquitin dependent and independent pathways. MDMX enhances the ability of MDM2 to bind and degrade RB protein. However, MDMX also seems to stabilize RB through interaction and competition with MDM2. Here, we will contextualize the findings that suggest that the MDM2 and MDMX proteins have a dual function on both p53 and RB.

Association between positive murine double minute 2 expression and clinicopathological characteristics of esophageal squamous cell carcinoma: a meta-analysis

The correlations of murine double minute 2 (MDM2) T309G and esophageal cancer were elucidated because the association between MDM2 expression states and clinicopathological parameters of esophageal squamous cell carcinoma (ESCC) is controversial. We conducted a meta-analysis on studies screened from PubMed, Web of Science, Embase, the Cochrane Library, and the Chinese Biomedical Literature Databases that were published before October 2014. All studies describing the association between MDM2 and ESCC were traced. Meta-analysis was performed using the STATA software (Stata Corp., College Station, TX, USA). A total of 9 studies with 707 cases and 324 controls were included. MDM2 expression was higher in ESCC than in normal esophageal epithelium (odds ratio [OR] 10.38, 95% confidence interval [CI] 6.42-16.78, P < 0.001). High MDM2 expression was associated with early primary tumor stage (T1/T2 vs. T3/T4, OR 0.59, 95% CI 0.38-0.92, P = 0.018) and increased risk of regional lymph node metastasis (N0 vs. N1, OR 1.66, 95% CI 1.03-2.67, P = 0.039). However, no relationship was observed between MDM2 expression and the risk of distant metastasis (OR = 2.09, 95% CI 1.00-4.36, P = 0.050), and MDM2 was not significantly correlated with TP53 expression (OR 1.22, 95% CI 0.53-2.77, P = 0.643). Our analysis suggests that MDM2 acts as a potent marker of early primary tumor stage but higher risk of regional lymph node metastasis in ESCC. However, because of the limited number of studies included, the result should be further clarified by well-designed prospective studies.

MDM2 knockdown mediated by a triazine-modified dendrimer in the treatment of non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is the most common type of lung cancer and the five-year survival rate is lower in advanced NSCLC patients. Chemotherapy is a widely used strategy in NSCLC treatment, but is usually limited by poor therapeutic efficacy and adverse effects. Therefore, a new therapeutic regimen is needed for NSCLC treatment. Gene therapy is a new strategy in the treatment of NSCLC. However, the lack of efficient and low toxic vectors remains the major obstacle. Here, we developed a biocompatible dendrimer as a non-viral vector for the delivery of mouse double minute2 (MDM2) siRNA in vitro and in vivo to treat NSCLC. The triazine-modified dendrimer efficiently stimulates the down-regulation of MDM2 gene in NSCLC PC9 cells, which induces significant cell apoptosis through the activation of apoptosis markers such as caspase-8 and poly(ADP-ribose) polymerase (PARP) cleavage. Furthermore, the dendrimer/MDM2 siRNA polyplexes showed excellent activity in the inhibition of tumor growth in a PC9 xenograft tumor model. These results suggested that inhibition the expression of MDM2 might be a potential target in NSCLC treatment.

MDMX (MDM4), a Promising Target for p53 Reactivation Therapy and Beyond

The MDMX protein was identified as a p53-interacting protein with a strong similarity to MDM2. Like Mdm2, Mdmx expression is essential for curbing p53 activity during embryonic development, indicating nonredundant functions of Mdmx and Mdm2. There is now a large body of evidence indicating that cancers frequently up-regulate MDMX expression as a means to dampen p53 tumor-suppressor function. Importantly, MDMX also shows p53-independent oncogenic functions. These data make MDMX an attractive therapeutic target for cancer therapy. Here, we summarize the mechanisms used by cancer cells to increase MDMX expression and promising pharmacological strategies to target MDMX in cancer-in particular, the recent findings that antisense oligonucleotides (ASOs) can be used to efficiently modulate MDMX messenger RNA (mRNA) splicing.

Signaling pathways involved in megakaryocyte-mediated proliferation of osteoblast lineage cells

Recent studies suggest that megakaryocytes (MKs) may play a significant role in skeletal homeostasis, as evident by the occurrence of osteosclerosis in multiple MK related diseases (Lennert et al., 1975; Thiele et al., 1999; Chagraoui et al., 2006). We previously reported a novel interaction whereby MKs enhanced proliferation of osteoblast lineage/osteoprogenitor cells (OBs) by a mechanism requiring direct cell-cell contact. However, the signal transduction pathways and the downstream effector molecules involved in this process have not been characterized. Here we show that MKs contact with OBs, via beta1 integrin, activate the p38/MAPKAPK2/p90RSK kinase cascade in the bone cells, which causes Mdm2 to neutralizes p53/Rb-mediated check point and allows progression through the G1/S. Interestingly, activation of MAPK (ERK1/2) and AKT, collateral pathways that regulate the cell cycle, remained unchanged with MK stimulation of OBs. The MK-to-OB signaling ultimately results in significant increases in the expression of c-fos and cyclin A, necessary for sustaining the OB proliferation. Overall, our findings show that OBs respond to the presence of MKs, in part, via an integrin-mediated signaling mechanism, activating a novel response axis that de-represses cell cycle activity. Understanding the mechanisms by which MKs enhance OB proliferation will facilitate the development of novel anabolic therapies to treat bone loss associated with osteoporosis and other bone-related diseases.

Association of genetic variants in the retinoblastoma binding protein 6 gene with the risk of glioma: a case-control study in a Chinese Han population

OBJECT

The retinoblastoma binding protein 6 (RBBP6) gene plays an important role in the induction of apoptosis and regulation of the cell cycle, and interacts with both p53 and retinoblastoma protein in carcinogenesis. Recently, many studies investigating the function of the RBBP6 gene, including its roles in lung cancer and breast cancer, have been reported. However, the association between RBBP6 variants and glioma was unknown. Therefore, to uncover the association between single nucleotide polymorphisms (SNPs) of RBBP6 and glioma, a hospital-based case-control study was performed in a Chinese Han population.

METHODS

Ten common tagging SNPs of the RBBP6 gene (covering 100% of all SNPs) were genotyped with the Sequenom MassARRY iPLEX platform, including 992 cases and 1008 controls, according to the HapMap database based on a pairwise linkage disequilibrium r(2) threshold of 0.8, minor allele frequency of 0.05, and Hardy-Weinberg equilibrium of 0.05.

RESULTS

The authors found that 4 SNPs were significantly associated with glioma (rs2033214, p = 0.013, adjusted OR 2.46, 95% CI 1.18-5.14; rs11860248, p = 8.64 × 10-(6), adjusted OR 1.59, 95% CI 1.23-2.05; rs9933544, p = 3.65 × 10(-4), adjusted OR 1.39, 95% CI 1.13-1.87; rs13332653, p = 0.004, adjusted OR 1.49, 95% CI 1.14-1.95). Stratification analyses revealed that rs2033214 was only significantly associated with low-grade gliomas; rs9933544 and rs13332653 were only significantly associated with glioblastoma multiforme; and rs11860248 was significantly associated with both low-grade gliomas and glioblastoma multiforme, compared with the common wild-type homozygous genotype. Further stratified analysis revealed that rs11860248 was more pronounced in certain subgroups: adults, males, histological types, and family history of cancer. What's more, the haplotype and diplotype analyses consistently revealed that the subjects carrying 1 copy of haplotype CCGCC had a 53% increased glioma risk compared with their corresponding noncarriers (p = 0.018, adjusted OR 1.53, 95% CI 1.08-2.17).

CONCLUSIONS

The authors' results suggested that RBBP6 gene variants are associated with glioma and contribute to glioma susceptibility, which was first reported elsewhere. Individuals with the so-called risk alleles might have an increased risk of glioma. These results might provide new insight into the occurrence of glioma.

Mutating RBF can enhance its pro-apoptotic activity and uncovers a new role in tissue homeostasis

The tumor suppressor retinoblastoma protein (pRb) is inactivated in a wide variety of cancers. While its role during cell cycle is well characterized, little is known about its properties on apoptosis regulation and apoptosis-induced cell responses. pRb shorter forms that can modulate pRB apoptotic properties, resulting from cleavages at caspase specific sites are observed in several cellular contexts. A bioinformatics analysis showed that a putative caspase cleavage site (TELD) is found in the Drosophila homologue of pRb(RBF) at a position similar to the site generating the p76Rb form in mammals. Thus, we generated a punctual mutant form of RBF in which the aspartate of the TELD site is replaced by an alanine. This mutant form, RBFD253A, conserved the JNK-dependent pro-apoptotic properties of RBF but gained the ability of inducing overgrowth phenotypes in adult wings. We show that this overgrowth is a consequence of an abnormal proliferation in wing imaginal discs, which depends on the JNK pathway activation but not on wingless (wg) ectopic expression. These results show for the first time that the TELD site of RBF could be important to control the function of RBF in tissue homeostasis in vivo.

MDM2 overexpression, activation of signaling networks, and cell proliferation

Frequent overexpression of MDM2 in human cancers suggests that the protein confers a survival advantage to cancer cells. However, overexpression of MDM2 in normal cells seems to restrict cell proliferation. This review discusses the cell growth regulatory functions of MDM2 in normal and genetically defective cells to assess how cancer cells evade the growth-restricting consequence of MDM2 overexpression. Similar to oncoproteins that induce a DNA damage response and oncogene induced senescence in non-transformed cells, MDM2 induces G1-arrest and intra-S phase checkpoint responses that control untimely DNA replication in the face of genetic challenges.

p53-independent effects of Mdm2

Mdm2 is best known as the primary negative regulator of p53, but a growing body of evidence suggests that Mdm2 also has a number of functions independent of its role in regulating p53. Although these functions are not yet well-characterized, they have been implicated in regulating of a number of cellular processes, including cell-cycle control, apoptosis, differentiation, genome stability, and transcription, among others. It appears that Mdm2 exerts these functions through a surprisingly wide variety of mechanisms. For example, it has been shown that Mdm2 can ubiquitinate alternative targets, can stimulate the activity of transcription factors, and can directly bind to mRNA to regulate its stability. Dysregulation of p53-independent functions could be responsible for the oncogenic properties of Mdm2 seen even in the absence of p53, and may explain why approximately 10 % of human tumors overexpress Mdm2 instead of inactivating p53 through other mechanisms. As the p53-independent functions of Mdm2 present novel targets for potential therapeutic interventions, fully characterizing these cellular and pathogenic roles of Mdm2 will be important in the study of tumor biology and the treatment of cancer.

Long Non-Coding RNAs Embedded in the Rb and p53 Pathways

In recent years, long non-coding RNAs (lncRNAs) have gained significant attention as a novel class of gene regulators. Although a small number of lncRNAs have been shown to regulate gene expression through diverse mechanisms including transcriptional regulation, mRNA splicing and translation, the physiological function and mechanism of action of the vast majority are not known. Profiling studies in cell lines and tumor samples have suggested a potential role of lncRNAs in cancer. Indeed, distinct lncRNAs have been shown to be embedded in the p53 and Rb networks, two of the major tumor suppressor pathways that control cell cycle progression and survival. Given the fact that inactivation of Rb and p53 is a hallmark of human cancer, in this review we discuss recent evidence on the function of lncRNAs in the Rb and p53 signaling pathways.

MDM2's social network

MDM2 is considered a hub protein due to its capacity to interact with a large number of different partners of which p53 is most well described. MDM2 is an E3 ubiquitin ligase, and many, but not all, of its interactions relate directly to this activity, such as substrates, adaptors or bridges, promoters, inhibitors or complementary factors. Some interactions serve regulatory functions that in response to cellular stresses control the localisation and functions of MDM2 including protein kinases, ribosomal proteins and proteases. Moreover, interactions with nucleotides serve other functions such as mRNA to regulate protein synthesis and DNA to control transcription. To perform such a pleiotropic panorama of different functions, MDM2 is subjected to a multitude of post-translational modifications and is expressed in different isoforms. The large and diverse interactome is made possible due to the plasticity of MDM2 and in this review we have listed the MDM2 interactions until now and we will discuss how this multifaceted protein can interact with such a variety of substrates to provide a key intermediary role in different signalling pathways.

Function of death-associated protein 1 in proliferation, differentiation, and apoptosis of chicken satellite cells

INTRODUCTION

Muscle growth and regeneration are processes closely associated with proliferation, differentiation, and apoptosis of muscle cells. Death-associated protein 1 (DAP1) has been identified as a negative regulator of autophagy. Little is known about the function of DAP1 in the regulation of myogenesis and satellite cells.

METHODS

Chicken satellite cells were transfected with DAP1 cloned into the pCMS-enhanced green fluorescent protein vector or pcDNA3.1 vector, or a small interference RNA against the endogenous DAP1 gene. The cells were assayed for proliferation, differentiation, and apoptosis.

RESULTS

The overexpression of DAP1 increased proliferation, differentiation, and myotube diameter, but it had no effect on satellite cell apoptosis. In contrast, knockdown of DAP1 significantly decreased proliferation, differentiation, and number of nuclei per myotube, and it increased apoptosis of the cells.

CONCLUSION

DAP1 is required for regulating myogenesis and apoptosis of satellite cells, which may affect muscle mass accretion and regeneration, and ameliorate muscle sarcopenia.

TXNIP maintains the hematopoietic cell pool by switching the function of p53 under oxidative stress

Reactive oxygen species (ROS) are critical determinants of the fate of hematopoietic stem cells (HSCs) and hematopoiesis. Thioredoxin-interacting protein (TXNIP), which is induced by oxidative stress, is a known regulator of intracellular ROS. Txnip(-/-) old mice exhibited elevated ROS levels in hematopoietic cells and showed a reduction in hematopoietic cell population. Loss of TXNIP led to a dramatic reduction of mouse survival under oxidative stress. TXNIP directly regulated p53 protein by interfering with p53- mouse double minute 2 (MDM2) interactions and increasing p53 transcriptional activity. Txnip(-/-) mice showed downregulation of the antioxidant genes induced by p53. Introduction of TXNIP or p53 into Txnip(-/-) bone marrow cells rescued the HSC frequency and greatly increased survival in mice following oxidative stress. Overall, these data indicate that TXNIP is a regulator of p53 and plays a pivotal role in the maintenance of the hematopoietic cells by regulating intracellular ROS during oxidative stress.

The MDM2-p53 pathway revisited

The p53 tumor suppressor is a key transcription factor regulating cellular pathways such as DNA repair, cell cycle, apoptosis, angiogenesis, and senescence. It acts as an important defense mechanism against cancer onset and progression, and is negatively regulated by interaction with the oncoprotein MDM2. In human cancers, the TP53 gene is frequently mutated or deleted, or the wild-type p53 function is inhibited by high levels of MDM2, leading to downregulation of tumor suppressive p53 pathways. Thus, the inhibition of MDM2-p53 interaction presents an appealing therapeutic strategy for the treatment of cancer. However, recent studies have revealed the MDM2-p53 interaction to be more complex involving multiple levels of regulation by numerous cellular proteins and epigenetic mechanisms, making it imperative to reexamine this intricate interplay from a holistic viewpoint. This review aims to highlight the multifaceted network of molecules regulating the MDM2-p53 axis to better understand the pathway and exploit it for anticancer therapy.

Regulation of STAT signaling by acetylation

Signal transducers and activators of transcription (STAT) belong to a family of latent cytoplasmic factors that can be activated by tyrosine phosphorylation by the members of the Jak tyrosine kinase family in response to a variety of cytokines and growth factors. Activated STATs form dimers and translocate into nucleus to induce expression of critical genes essential for normal cellular events. In the past several years, significant progress has been made in the characterization of STAT acetylation, which is dependent on the balance between histone deacetylases (HDACs) and histone acetyltransferases (HATs) such as CBP/p300. Acetylation of STAT1, STAT2, STAT3, STAT5b and STAT6 has been identified. This review will highlight acetylation on the modulation of STAT activation.

Natural product MDM2 inhibitors: anticancer activity and mechanisms of action

The mdm2 oncogene has recently been suggested to be a valuable target for cancer therapy and prevention. Overexpression of mdm2 is often seen in various human cancers and correlates with high-grade, late-stage, and more treatment-resistant tumors. The MDM2-p53 auto-regulatory loop has been extensively investigated and is an attractive cancer target, which indeed has been the main focus of anti-MDM2 drug discovery. Much effort has been expended in the development of small molecule MDM2 antagonists targeting the MDM2-p53 interaction, and a few of these have advanced into clinical trials. However, MDM2 exerts its oncogenic activity through both p53-dependent and -independent mechanisms. Recently, there is an increasing interest in identifying natural MDM2 inhibitors; some of them have been shown to decrease MDM2 expression and activity in vitro and in vivo. These identified natural MDM2 inhibitors include a plethora of diverse chemical frameworks, ranging from flavonoids, steroids, and sesquiterpenes to alkaloids. In addition to a brief review of synthetic MDM2 inhibitors, this review focuses on natural product MDM2 inhibitors, summarizing their biological activities in vitro and in vivo and the underlying molecular mechanisms of action, targeting MDM2 itself, regulators of MDM2, and/or the MDM2-p53 interaction. These MDM2 inhibitors can be used alone or in combination with conventional treatments, improving the prospects for cancer therapy and prevention. Their complex and unique molecular architectures may provide a stimulus for developing synthetic analogs in the future.

Rapamycin inhibits IGF-1-mediated up-regulation of MDM2 and sensitizes cancer cells to chemotherapy

The Murine Double Minute 2 (MDM2) protein is a key regulator of cell proliferation and apoptosis that acts primarily by inhibiting the p53 tumor suppressor. Similarly, the PI3-Kinase (PI3K)/AKT pathway is critical for growth factor-mediated cell survival. Additionally, it has been reported that AKT can directly phosphorylate and activate MDM2. In this study, we show that IGF-1 up-regulates MDM2 protein levels in a PI3K/AKT-dependent manner. Inhibition of mTOR by rapamycin or expression of a dominant negative eukaryotic initiation factor 4E binding protein 1 (4EBP1) mutant protein, as well as ablation of eukaryotic initiation factor 4E (eIF4E), efficiently abolishes IGF-1-mediated up-regulation of MDM2. In addition, we show that rapamycin effectively inhibits MDM2 expression and sensitizes cancer cells to chemotherapy. Taken together, this study reveals a novel mechanism by which IGF-1 activates MDM2 via the mTOR pathway, and that pharmacologic inhibition of mTOR combined with chemotherapy may be more effective in treatment of a subset of cancers harboring increased MDM2 activation.

RNA polymerase III repression by the retinoblastoma tumor suppressor protein

The retinoblastoma (RB) tumor suppressor protein regulates multiple pathways that influence cell growth, and as a key regulatory node, its function is inactivated in most cancer cells. In addition to its canonical roles in cell cycle control, RB functions as a global repressor of RNA polymerase (Pol) III transcription. Indeed, Pol III transcripts accumulate in cancer cells and their heightened levels are implicated in accelerated growth associated with RB dysfunction. Herein we review the mechanisms of RB repression for the different types of Pol III genes. For type 1 and type 2 genes, RB represses transcription through direct contacts with the core transcription machinery, notably Brf1-TFIIIB, and inhibits preinitiation complex formation and Pol III recruitment. A contrasting model for type 3 gene repression indicates that RB regulation involves stable and simultaneous promoter association by RB, the general transcription machinery including SNAPc, and Pol III, suggesting that RB may impede Pol III promoter escape or elongation. Interestingly, analysis of published genomic association data for RB and Pol III revealed added regulatory complexity for Pol III genes both during active growth and during arrested growth associated with quiescence and senescence. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Emerging roles of RB family: new defense mechanisms against tumor progression

The retinoblastoma (RB) family of proteins, including RB1/p105, retinoblastoma-like 1 (RBL1/p107), and retinoblastoma-like 2 (RBL2/p130), is principally known for its central role on cell cycle regulation. The inactivation of RB proteins confers a growth advantage and underlies multiple types of tumors. Recently, it has been shown that RB proteins have other important roles, such as preservation of chromosomal stability, induction and maintenance of senescence and regulation of apoptosis, cellular differentiation, and angiogenesis. RB proteins are involved in many cellular pathways and act as transcriptional regulators able to bind several transcription factors, thus antagonizing or potentiating their functions. Furthermore, RB proteins might control the expression of specific target genes by recruiting chromatin remodeling enzymes. Although many efforts have been made to dissect the different functions of RB proteins, it remains still unclear which are necessary for cancer suppression and the role they play at distinct steps of carcinogenesis. Moreover, RB proteins can behave differently in various cell types or cell states. Elucidating the intricate RB protein network in regulating cell fate might provide the knowledge necessary to explain their potent tumor suppressor activity and to design novel therapeutic strategies.

Kaposi's sarcoma-associated herpesvirus lana2 protein interacts with the pocket proteins and inhibits their sumoylation

The pocket proteins retinoblastoma protein (pRb), p107 and p130 are the key targets of oncoproteins expressed by DNA tumor viruses. Some of these viral proteins contain an LXCXE motif that mediates the interaction with the three pocket proteins and the inhibition of the pRb SUMOylation. Kaposi's sarcoma herpesvirus (KSHV) contains at least two proteins that can regulate pRb function but, so far, a KSHV-encoded protein targeting p107 and p130 has not been identified. Here, we show that the KSHV latent protein LANA2 binds to pRb, p107 and p130. LANA2 contains an LXCXE motif that is required for bypassing pRb-mediated cell-cycle arrest and for inhibiting pRb SUMOylation. Finally, we demonstrate that, in addition to pRb, both p107 and p130 can be SUMOylated, and this modification is also inhibited by LANA2 in an LXCXE-dependent manner. These results demonstrate, for the first time, the SUMOylation of p107 or p130 and, so far, they represent the first example of a KSHV protein able to interact with the three pocket proteins and to inhibit their conjugation to SUMO.

Loss of CDKN2B promotes p53-dependent smooth muscle cell apoptosis and aneurysm formation

OBJECTIVE

Genomewide association studies have implicated allelic variation at 9p21.3 in multiple forms of vascular disease, including atherosclerotic coronary heart disease and abdominal aortic aneurysm. As for other genes at 9p21.3, human expression quantitative trait locus studies have associated expression of the tumor suppressor gene CDKN2B with the risk haplotype, but its potential role in vascular pathobiology remains unclear.

METHODS AND RESULTS

Here we used vascular injury models and found that Cdkn2b knockout mice displayed the expected increase in proliferation after injury, but developed reduced neointimal lesions and larger aortic aneurysms. In situ and in vitro studies suggested that these effects were attributable to increased smooth muscle cell apoptosis. Adoptive bone marrow transplant studies confirmed that the observed effects of Cdkn2b were mediated through intrinsic vascular cells and were not dependent on bone marrow-derived inflammatory cells. Mechanistic studies suggested that the observed increase in apoptosis was attributable to a reduction in MDM2 and an increase in p53 signaling, possibly due in part to compensation by other genes at the 9p21.3 locus. Dual inhibition of both Cdkn2b and p53 led to a reversal of the vascular phenotype in each model.

CONCLUSIONS

These results suggest that reduced CDKN2B expression and increased smooth muscle cell apoptosis may be one mechanism underlying the 9p21.3 association with aneurysmal disease.

The Many Faces of MDM2 Binding Partners

Mdm2 is an essential regulator of the p53 tumor suppressor. Mdm2 is modified at transcriptional, post-transcriptional, and post-translational levels to control p53 activity in normal versus stressed cells. Importantly, errors in these regulatory mechanisms can result in aberrant Mdm2 expression and failure to initiate programmed cell death in response to DNA damage. Such errors can have severe consequences as evidenced by tumor phenotypes resulting from amplification at the Mdm2 locus and changes in post-transcriptional and post-translational regulation of Mdm2. Although Mdm2 mediated inhibition of p53 is well characterized, Mdm2 interacts with many additional proteins and also targets many of these for proteosomal degradation. Mdm2 also has E3-ligase independent functions and p53-independent functions that have important implications for genome stability and cancer.

DNA damage-induced RORα is crucial for p53 stabilization and increased apoptosis

A critical component of the DNA damage response is the p53 tumor suppressor, and aberrant p53 function leads to uncontrolled cell proliferation and malignancy. Several molecules have been shown to regulate p53 stability; however, genome-wide systemic approaches for determining the affected, specific downstream target genes have not been extensively studied. Here, we first identified an orphan nuclear receptor, RORα, as a direct target gene of p53, which contains functional p53 response elements. The functional consequences of DNA damage-induced RORα are to stabilize p53 and activate p53 transcription in a HAUSP/Usp7-dependent manner. Interestingly, microarray analysis revealed that RORα-mediated p53 stabilization leads to the activation of a subset of p53 target genes that are specifically involved in apoptosis. We further confirmed that RORα enhances p53-dependent, in vivo apoptotic function in the Drosophila model system. Together, we determined that RORα is a p53 regulator that exerts its role in increased apoptosis via p53.

Transcriptional repressor NIR functions in the ribosome RNA processing of both 40S and 60S subunits

BACKGROUND

NIR was identified as an inhibitor of histone acetyltransferase and it represses transcriptional activation of p53. NIR is predominantly localized in the nucleolus and known as Noc2p, which is involved in the maturation of the 60S ribosomal subunit. However, how NIR functions in the nucleolus remains undetermined. In the nucleolus, a 47S ribosomal RNA precursor (pre-rRNA) is transcribed and processed to produce 18S, 5.8S and 28S rRNAs. The 18S rRNA is incorporated into the 40S ribosomal subunit, whereas the 28S and 5.8S rRNAs are incorporated into the 60S subunit. U3 small nucleolar RNA (snoRNA) directs 18S rRNA processing and U8 snoRNA mediates processing of 28S and 5.8 S rRNAs. Functional disruption of nucleolus often causes p53 activation to inhibit cell proliferation.

METHODOLOGY/PRINCIPAL FINDINGS

Western blotting showed that NIR is ubiquitously expressed in different human cell lines. Knock-down of NIR by siRNA led to inhibition of the 18S, 28S and 5.8S rRNAs evaluated by pulse-chase experiment. Pre-rRNA particles (pre-rRNPs) were fractionated from the nucleus by sucrose gradient centrifugation and analysis of the pre-RNPs components showed that NIR existed in the pre-RNPs of both the 60S and 40S subunits and co-fractionated with 32S and 12S pre-rRNAs in the 60S pre-rRNP. Protein-RNA binding experiments demonstrated that NIR is associated with the 32S pre-rRNA and U8 snoRNA. In addition, NIR bound U3 snoRNA. It is a novel finding that depletion of NIR did not affect p53 protein level but de-repressed acetylation of p53 and activated p21.

CONCLUSIONS

We provide the first evidence for a transcriptional repressor to function in the rRNA biogenesis of both the 40S and 60S subunits. Our findings also suggested that a nucleolar protein may alternatively signal to p53 by affecting the p53 modification rather than affecting p53 protein level.

MDM2 regulates estrogen receptor α and estrogen responsiveness in breast cancer cells

Murine double minute clone 2 (MDM2) is a multifunctional protein, which modulates nuclear receptor-mediated transactivation. In this study, we show that MDM2 significantly enhanced estrogen receptor α (ERα) and ERα/specificity protein-mediated transactivation in MCF-7 and ZR-75 breast cancer cells. This was demonstrated by both MDM2 overexpression and knockdown experiments by RNA interference. ERα interacted with wild-type MDM2 and deletion mutants of MDM2 containing amino acids 1-342 (C-terminal deletion) and 134-490 (N-terminal deletion), but not 134-342. In contrast, only wild-type but not mutant MDM2 enhanced ERα-mediated transactivation. Protein-protein interactions in vitro were 17β-estradiol (E(2)) independent, whereas fluorescent resonance energy transfer experiments in living cells showed that E(2) enhanced ERα-MDM2 interactions. Subsequent RNA interference and mammalian two-hybrid experiments suggested that MDM2 did not directly interact with endogenous coactivators such as the steroid receptor coactivators but played a role in enhancing ERα-mediating gene expression and estrogen responsiveness through interactions with ERα.

Apoptotic caspases regulate induction of iPSCs from human fibroblasts

The molecular mechanisms involved in the derivation of induced pluripotent stem cells (iPSCs) from differentiated cells are poorly understood. Here we report that caspases 3 and 8, two proteases associated with apoptotic cell death, play critical roles in induction of iPSCs from human fibroblasts. Activation of caspases 3 and 8 occurs soon after transduction of iPSC-inducing transcription factors. Oct-4, a key iPSC transcription factor, is responsible for the activation. Inhibition of caspase 3 or 8 in human fibroblast cells partially or completely (respectively) prevents the induction of iPSCs. Furthermore, retinoblastoma susceptibility (Rb) protein appears to be one of the factors that act downstream of the caspases. We propose that caspases are key facilitators of nuclear reprogramming in iPSC induction.

Evaluation of the association studies of single nucleotide polymorphisms and hepatocellular carcinoma: a systematic review

PURPOSE

For decades of years, hundreds of candidate gene-based association studies explored the relationship between single nucleotide polymorphisms (SNPs) and hepatocellular carcinoma (HCC). There was no systematic review summarized the results of these association studies of candidate SNPs and HCC to date. In order to summarize the results of the association studies, we conducted a concise systematic review.

METHODS

By searching Pubmed database before October 2010, we reviewed all the association studies about candidate SNPs and HCC. If the eligible study number on a given SNP was more than three, we conducted a meta-analysis. We reported here only the overall positive-association results with statistical significance and evaluated the reliability of the associations by using false-positive report probability (FPRP) analysis and the Venice guidelines on genetic epidemiology studies.

RESULTS

Six SNPs of five genes (rs1800562 of HFE, rs17868323 and rs11692021 of UGT1A7, rs2279744 of MDM2, rs1143627 of IL-1B, and rs4880 of MnSOD) showed overall significant associations with HCC. The eligible number of the studies varied from three to nine. Two SNPs (rs1800562 of HFE and rs2279744 of MDM2) passed the FPRP threshold (FPRP < 0.20). According to the Venice guidelines, the associations between the two SNPs (rs1800562 and rs2279744) and HCC were of moderate evidence.

CONCLUSIONS

Two SNPs (rs1800562 of HFE and rs2279744 of MDM2) were associated with HCC with moderate epidemiological evidence and deserve further study and additional biological and clinical assessment.

The E2F1/Rb and p53/MDM2 pathways in DNA repair and apoptosis: understanding the crosstalk to develop novel strategies for prostate cancer radiotherapy

Both the p53- and E2F1-signaling pathways are defective in almost all types of tumors, suggesting very important roles for their signaling networks in regulating the process of tumorigenesis and therapy response. Studies on Radiation Therapy Oncology Group tissue samples have identified aberrant expression of p53, MDM2 (an E3 ubiquitin ligase that targets p53 for proteosomal degradation), and p16 (an upstream regulator of retinoblastoma and hence E2F1 in prostate cancer); abnormal expression of these biomarkers has been associated with clinical outcome after radiotherapy ± androgen deprivation therapy. Although the proapoptotic properties of p53 are well documented, a relatively new aspect of p53 function as an active mediator of prosurvival signaling pathways is now emerging. E2F1 is a transcription factor that possesses both proapoptotic and prosurvival properties. Thus, the role of E2F1 in the process of tumorigenesis versus apoptosis is a contested issue that needs to be resolved. Furthermore, the role of E2F1 in DNA repair is being increasingly recognized. Thus, novel approaches to curb the prosurvival and DNA repair capability of E2F1 while promoting apoptotic function are of interest. In this review, we discuss the challenges involved in targeting the p53/E2F1 pathways and the crosstalk networks, and further propose potential therapeutic strategies for prostate cancer management.

1A6/DRIM, a novel t-UTP, activates RNA polymerase I transcription and promotes cell proliferation

BACKGROUND

Ribosome biogenesis is required for protein synthesis and cell proliferation. Ribosome subunits are assembled in the nucleolus following transcription of a 47S ribosome RNA precursor by RNA polymerase I and rRNA processing to produce mature 18S, 28S and 5.8S rRNAs. The 18S rRNA is incorporated into the ribosomal small subunit, whereas the 28S and 5.8S rRNAs are incorporated into the ribosomal large subunit. Pol I transcription and rRNA processing are coordinated processes and this coordination has been demonstrated to be mediated by a subset of U3 proteins known as t-UTPs. Up to date, five t-UTPs have been identified in humans but the mechanism(s) that function in the t-UTP(s) activation of Pol I remain unknown. In this study we have identified 1A6/DRIM, which was identified as UTP20 in our previous study, as a t-UTP. In the present study, we investigated the function and mechanism of 1A6/DRIM in Pol I transcription.

METHODOLOGY/PRINCIPAL FINDINGS

Knockdown of 1A6/DRIM by siRNA resulted in a decreased 47S pre-rRNA level as determined by Northern blotting. Ectopic expression of 1A6/DRIM activated and knockdown of 1A6/DRIM inhibited the human rDNA promoter as evaluated with luciferase reporter. Chromatin immunoprecipitation (ChIP) experiments showed that 1A6/DRIM bound UBF and the rDNA promoter. Re-ChIP assay showed that 1A6/DRIM interacts with UBF at the rDNA promoter. Immunoprecipitation confirmed the interaction between 1A6/DRIM and the nucleolar acetyl-transferase hALP. It is of note that knockdown of 1A6/DRIM dramatically inhibited UBF acetylation. A finding of significance was that 1A6/DRIM depletion, as a kind of nucleolar stress, caused an increase in p53 level and inhibited cell proliferation by arresting cells at G1.

CONCLUSIONS

We identify 1A6/DRIM as a novel t-UTP. Our results suggest that 1A6/DRIM activates Pol I transcription most likely by associating with both hALP and UBF and thereby affecting the acetylation of UBF.

HIV-1 viral infectivity factor interacts with TP53 to induce G2 cell cycle arrest and positively regulate viral replication

Viral infectivity factor, an accessory protein encoded in the HIV-1 genome, induces G2 cell cycle arrest; however, the biological significance and mechanism(s) remain totally unclear. Here we demonstrate that the TP53 pathway is involved in Vif-mediated G2 cell cycle arrest. Vif enhances the stability and transcriptional activity of TP53 by blocking the MDM2-mediated ubiquitination and nuclear export of TP53. Furthermore, Vif causes G2 cell cycle arrest in a TP53-dependent manner. HXB2 Vif lacks these activities toward TP53 and cannot induce G2 cell cycle arrest. Using mutagenesis, we demonstrate that the critical residues for this function are located in the N-terminal region of Vif. Finally, we construct a mutant NL4-3 virus with an NL4-3/HXB2 chimeric Vif defective for the ability to induce cell cycle arrest and show that the mutant virus replicates less effectively than the wild-type NL4-3 virus in T cells expressing TP53. These data imply that Vif induces G2 cell cycle arrest through functional interaction with the TP53/MDM2 axis and that the G2 cell cycle arrest induced by Vif has a positive effect on HIV-1 replication. This report demonstrates the molecular mechanisms and the biological significance of Vif-mediated G2 cell cycle arrest for HIV-1 infection.

p38 phosphorylates Rb on Ser567 by a novel, cell cycle-independent mechanism that triggers Rb-Hdm2 interaction and apoptosis

The retinoblastoma protein (Rb) inhibits both cell division and apoptosis, but the mechanism by which Rb alternatively regulates these divergent outcomes remains poorly understood. Cyclin dependent kinases (Cdks) promote cell division by phosphorylating and reversibly inactivating Rb by a hierarchical series of phosphorylation events and sequential conformational changes. The stress-regulated mitogen activated protein kinase (MAPK) p38 also phosphorylates Rb, but it does so in a cell cycle-independent manner that is associated with apoptosis rather than with cell division. Here, we show that p38 phosphorylates Rb by a novel mechanism that is distinct from that of Cdks. p38 bypasses the cell cycle-associated hierarchical phosphorylation and directly phosphorylates Rb on Ser567, which is not phosphorylated during the normal cell cycle. Phosphorylation by p38, but not Cdks, triggers an interaction between Rb and the human homologue of murine double minute 2 (Hdm2), leading to degradation of Rb, release of E2F1 and cell death. These findings provide a mechanistic explanation for how Rb regulates cell division and apoptosis through different kinases, and reveal how Hdm2 may functionally link the tumor suppressors Rb and p53.

The RNA helicase p68 modulates expression and function of the Δ133 isoform(s) of p53, and is inversely associated with Δ133p53 expression in breast cancer

The RNA helicase p68 is a potent co-activator of p53-dependent transcription in response to DNA damage. Previous independent studies have indicated that p68 and the Δ133p53 isoforms, which modulate the function of full-length p53, are aberrantly expressed in breast cancers. Here we identify a striking inverse association of p68 and Δ133p53 expression in primary breast cancers. Consistent with these findings, small interfering RNA depletion of p68 in cell lines results in a p53-dependant increase of Δ133p53 in response to DNA damage, suggesting that increased Δ133p53 expression could result from downregulation of p68 and provide a potential mechanistic explanation for our observations in breast cancer. Δ133p53α, which has been shown to negatively regulate the function of full-length p53, reciprocally inhibits the ability of p68 to stimulate p53-dependent transcription from the p21 promoter, suggesting that Δ133p53α may be competing with p68 to regulate p53 function. This hypothesis is underscored by our observations that p68 interacts with the C-terminal domain of p53, co-immunoprecipitates 133p53α from cell extracts and interacts only with p53 molecules that are able to form tetramers. These data suggest that p68, p53 and 133p53α may form part of a complex feedback mechanism to regulate the expression of Δ133p53, with consequent modification of p53-mediated transcription, and may modulate the function of p53 in breast and other cancers that harbour wild-type p53.