Interaction between the retinoblastoma protein and the oncoprotein MDM2

Arterial-Lymphatic-Like Endothelial Cells Appear in Hereditary Hemorrhagic Telangiectasia 2 and Contribute to Vascular Leakage and Arteriovenous Malformations

BACKGROUND

Arteriovenous malformations (AVMs) are characteristic of hereditary hemorrhagic telangiectasia. Loss-of-function mutations in the activin receptor-like kinase 1 (Alk1) are linked to hemorrhagic telangiectasia type 2.

METHODS

Endothelial-specific deletion of Alk1, endothelial lineage tracing, transcriptomics of single-cell analysis, and electron microscopy were performed to examine the vascular phenotype and characteristics of ALK1-deficient endothelial cells (ECs) after EC-specific Alk1 deletion. Ischemia assays were used to examine the cell capacity for vascular malformation. Connectivity Map with transcriptomic analysis was applied to identify chemical compounds. Specific methods for arteriovenous malformations, such as micro-computed tomography, with other molecular and cell biological tools were also performed.

RESULTS

We performed endothelial-specific deletion of Alk1 in mice and found severe arteriovenous malformations and vascular leakage. The transcriptomics of single-cell analysis revealed a new distinctive cell cluster formed after Alk1 deletion where the cells coexpressed arterial and lymphatic endothelial markers. The analysis projected that these cells potentially originated from arterial ECs after Alk1 deletion. This new population was referred to as arterial-lymphatic-like ECs according to its cellular markers, and its appearance was validated in the pulmonary small arteries after Alk1 deletion. Transplantation of these cells caused vascular malformations. Endothelial lineage tracing confirmed that these new arterial-lymphatic-like ECs were derived from ALK1 depleted ECs, potentially arterial ECs. We discovered that SOX17 (SRY-box transcription factor 17) induction was responsible for the derivation of these arterial-lymphatic-like ECs. We showed that direct binding of MDM2 (mouse double minute 2) was required for Sox17 to execute this activity. Inhibition of MDM2 reduced the arteriovenous malformations in the mouse model.

CONCLUSIONS

Together, our studies revealed the mechanistic underpinnings of ALK1 signaling in regulating the endothelial phenotype and provided possibilities for new therapeutic strategies in hemorrhagic telangiectasia type 2.

Activation of the Snail transcription factor induces Mdm2 gene expression

Epithelial-like tumor cells can become metastatic by undergoing molecular and phenotypic reprogramming in a process referred to as epithelial-to-mesenchymal transition (EMT). In response to EMT genes that promote migration and condition the tumor microenvironment to permit intravasation into the bloodstream, dissemination and extravasation into new organs are induced. While the mutant p53 has been implicated in extravasation, one negative regulator of p53, the oncogene murine double minute-2 gene (Mdm2), is required in the early stages of metastasis and the driver of EMT. This activity is independent of Mdm2's role in the p53-Mdm2 autoregulatory feedback loop. Herein, we examine the EMT transcription factor Snail as a downstream effector of kinase signaling pathways. We show that the activation of upstream receptors and KRas signaling increase Snail levels. Snail binds to Ebox DNA motifs, and Mdm2 has two Ebox DNA-binding domains in the second promoter. Snail binds to the second Ebox and induces Mdm2 gene expression. Knockdown of endogenous Snail by shRNA shows a decrease in Mdm2 and is associated with reduced migration. The reintroduction of Mdm2 in shSnail cells restores cellular migration. These data integrate upstream pathways that induce Snail-Mdm2 to promote the metastasis of tumor cells.

Cell fate regulation governed by p53: Friends or reversible foes in cancer therapy

Cancer is a leading cause of death worldwide. Targeted therapies aimed at key oncogenic driver mutations in combination with chemotherapy and radiotherapy as well as immunotherapy have benefited cancer patients considerably. Tumor protein p53 (TP53), a crucial tumor suppressor gene encoding p53, regulates numerous downstream genes and cellular phenotypes in response to various stressors. The affected genes are involved in diverse processes, including cell cycle arrest, DNA repair, cellular senescence, metabolic homeostasis, apoptosis, and autophagy. However, accumulating recent studies have continued to reveal novel and unexpected functions of p53 in governing the fate of tumors, for example, functions in ferroptosis, immunity, the tumor microenvironment and microbiome metabolism. Among the possibilities, the evolutionary plasticity of p53 is the most controversial, partially due to the dizzying array of biological functions that have been attributed to different regulatory mechanisms of p53 signaling. Nearly 40 years after its discovery, this key tumor suppressor remains somewhat enigmatic. The intricate and diverse functions of p53 in regulating cell fate during cancer treatment are only the tip of the iceberg with respect to its equally complicated structural biology, which has been painstakingly revealed. Additionally, TP53 mutation is one of the most significant genetic alterations in cancer, contributing to rapid cancer cell growth and tumor progression. Here, we summarized recent advances that implicate altered p53 in modulating the response to various cancer therapies, including chemotherapy, radiotherapy, and immunotherapy. Furthermore, we also discussed potential strategies for targeting p53 as a therapeutic option for cancer.

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.

FRMD8 targets both CDK4 activation and RB degradation to suppress colon cancer growth

Cyclin-dependent kinase 4 (CDK4) and retinoblastoma protein (RB) are both important cell-cycle regulators that function in different scenarios. Here, we report that FERM domain-containing 8 (FRMD8) inhibits CDK4 activation and stabilizes RB, thereby causing cell-cycle arrest and inhibiting colorectal cancer (CRC) cell growth. FRMD8 interacts separately with CDK7 and CDK4, and it disrupts the interaction of CDK7 with CDK4, subsequently inhibiting CDK4 activation. FRMD8 competes with MDM2 to bind RB and attenuates MDM2-mediated RB degradation. Frmd8 deficiency in mice accelerates azoxymethane/dextran-sodium-sulfate-induced colorectal adenoma formation. The FRMD8 promoter is hypermethylated, and low expression of FRMD8 predicts poor prognosis in CRC patients. Further, we identify an LKCHE-containing FRMD8 peptide that blocks MDM2 binding to RB and stabilizes RB. Combined application of the CDK4 inhibitor and FRMD8 peptide leads to marked suppression of CRC cell growth. Therefore, using an LKCHE-containing peptide to interfere with the MDM2-RB interaction may have therapeutic value in CDK4/6 inhibitor-resistant patients.

The roles of mouse double minute 2 (MDM2) oncoprotein in ocular diseases: A review

Mouse double minute 2 (MDM2), an E3 ubiquitin ligase and the primary negative regulator of the tumor suppressor p53, cooperates with its structural homolog MDM4/MDMX to control intracellular p53 level. In turn, overexpression of p53 upregulates and forms an autoregulatory feedback loop with MDM2. The MDM2-p53 axis plays a pivotal role in modulating cell cycle control and apoptosis. MDM2 itself is regulated by the PI3K-AKT and RB-E2F-ARF pathways. While amplification of the MDM2 gene or overexpression of MDM2 (due to MDM2 SNP T309G, for instance) is associated with various malignancies, numerous studies have shown that MDM2/p53 alterations may also play a part in the pathogenetic process of certain ocular disorders (Fig. 1). These include cancers (retinoblastoma, uveal melanoma), fibrocellular proliferative diseases (proliferative vitreoretinopathy, pterygium), neovascular diseases, degenerative diseases (cataract, primary open-angle glaucoma, age-related macular degeneration) and infectious/inflammatory diseases (trachoma, uveitis). In addition, MDM2 is implicated in retinogenesis and regeneration after optic nerve injury. Anti-MDM2 therapy has shown potential as a novel approach to treating these diseases. Despite major safety concerns, there are high expectations for the clinical value of reformative MDM2 inhibitors. This review summarizes important findings about the role of MDM2 in ocular pathologies and provides an overview of recent advances in treating these diseases with anti-MDM2 therapies.

MDM2 as a Rational Target for Intervention in CDK4/6 Inhibitor Resistant, Hormone Receptor Positive Breast Cancer

With the adoption of inhibitors of cyclin dependent kinases 4 and 6 (CDK4/6i) in combination with endocrine therapy as standard of care for the treatment of advanced and metastatic estrogen receptor positive (ER+) breast cancer, the search is now on for novel therapeutic options to manage the disease after the inevitable development of resistance to CDK4/6i. In this review we will consider the integral role that the p53/MDM2 axis plays in the interactions between CDK4/6, ERα, and inhibitors of these molecules, the current preclinical evidence for the efficacy of MDM2 inhibitors in ER+ breast cancer, and discuss the possibility of targeting the p53/MDM2 via inhibition of MDM2 in the CDK4/6i resistance setting.

MDM2, MDMX, and p73 regulate cell-cycle progression in the absence of wild-type p53

The p53 tumor suppressor protein, known to be critically important in several processes including cell-cycle arrest and apoptosis, is highly regulated by multiple mechanisms, most certifiably the Murine Double Minute 2-Murine Double Minute X (MDM2-MDMX) heterodimer. The role of MDM2-MDMX in cell-cycle regulation through inhibition of p53 has been well established. Here we report that in cells either lacking p53 or expressing certain tumor-derived mutant forms of p53, loss of endogenous MDM2 or MDMX, or inhibition of E3 ligase activity of the heterocomplex, causes cell-cycle arrest. This arrest is correlated with a reduction in E2F1, E2F3, and p73 levels. Remarkably, direct ablation of endogenous p73 produces a similar effect on the cell cycle and the expression of certain E2F family members at both protein and messenger RNA levels. These data suggest that MDM2 and MDMX, working at least in part as a heterocomplex, may play a p53-independent role in maintaining cell-cycle progression by promoting the activity of E2F family members as well as p73, making them a potential target of interest in cancers lacking wild-type p53.

Association of Murine Double Minute 2 Genotypes and Lung Cancer Risk

AIM

The aim of this study was to evaluate the contribution of human mouse double minute 2 (MDM2) gene polymorphisms to the risk of Taiwan lung cancer.

MATERIALS AND METHODS

In this case-control study, the association of MDM2 rs2279744 genotypes with lung cancer risk was investigated among 358 lung cancer patients and 716 age-, gender- and smoking status-matched controls in Taiwan.

RESULTS

The percentages of MDM2 rs2279744 GT and GG genotypes were 50.0% and 27.4% in lung cancer group and 50.0% and 26.5% in control group, respectively [odds ratio (OR)=1.03 and 1.07, 95% confidence interval (CI)=0.75-1.43 and 0.75-1.53, respectively]. The analysis about allelic frequency showed that G allele at MDM2 rs2279744 conferred a non-significant increased cancer risk (OR=1.03, 95%CI=0.86-1.24).

CONCLUSION

Polymorphisms of MDM2 rs2279744 may play a role in lung carcinogenesis. However, the studied genotypes were not shown as predictors of lung cancer susceptibility.

Rescuing lethal phenotypes induced by disruption of genes in mice: a review of novel strategies

Approximately 35 % of the mouse genes are indispensable for life, thus, global knock-out (KO) of those genes may result in embryonic or early postnatal lethality due to developmental abnormalities. Several KO mouse lines are valuable human disease models, but viable homozygous mutant mice are frequently required to mirror most symptoms of a human disease. The site-specific gene editing systems, the transcription activator-like effector nucleases (TALENs), Zinc-finger nucleases (ZFNs) and the clustered regularly interspaced short palindrome repeat-associated Cas9 nuclease (CRISPR/Cas9) made the generation of KO mice more efficient than before, but the homozygous lethality is still an undesired side-effect in case of many genes. The literature search was conducted using PubMed and Web of Science databases until June 30th, 2020. The following terms were combined to find relevant studies: "lethality", "mice", "knock-out", "deficient", "embryonic", "perinatal", "rescue". Additional manual search was also performed to find the related human diseases in the Online Mendelian Inheritance in Man (OMIM) database and to check the citations of the selected studies for rescuing methods. In this review, the possible solutions for rescuing human disease-relevant homozygous KO mice lethal phenotypes were summarized.

MDM2 regulates RB levels during genotoxic stress

The retinoblastoma tumour suppressor protein (RB) regulates a number of diverse cellular functions including differentiation, angiogenesis, chromatin remodelling, senescence and apoptosis. The best-characterised function of RB is cell cycle regulation, and it has been considered a phosphoprotein regulated by cyclin-dependent kinases. In its hypophosphorylated form, RB binds the transcription factor E2F1, arresting the cell cycle in the G1 phase. Here, we show that MDM2 controls the cell cycle through synthesis and degradation of RB protein in a cell cycle condition-dependent fashion. MDM2 induces G1 cell cycle arrest by enhancing the translation of the RB mRNA under genotoxic stress. Translation requires direct interaction between the RB mRNA and the MDM2 protein that accompanies the RB mRNA to the polysomes. However, MDM2 ubiquitinates and degrades RB protein at the G2/M phase under genotoxic stress. The ATM phosphomimetic mutant MDM2(S395D) corroborates that the effect on the RB levels is dependent on the DNA damage. These results provide the basis of a dual regulatory mechanism by which MDM2 controls cell cycle progression during DNA damage.

Current developments of targeting the p53 signaling pathway for cancer treatment

p53 is one of the most well-studied tumor suppressors. It is mutated or deleted in half of all cancers. In the other half carrying wild type p53, the p53 signaling pathway is disrupted by abnormalities of other components in the pathway. Due to its paramount role in tumor suppression, p53 has attracted great interest in drug development as any clinically successful therapeutic agent to target the p53 pathway will save millions of lives. However, designing therapeutics targeting the pathway has been extremely challenging, despite more than forty years of research. This review will summarize past and current efforts of developing p53-based gene therapy and targeted therapies for cancer treatment. In addition, the current efforts of exploiting the immunogenicity of p53 protein for cancer immunotherapy will be reviewed. Challenges and future directions for targeting the p53 pathway will be discussed.

Tumor suppressor OTUD3 induces growth inhibition and apoptosis by directly deubiquitinating and stabilizing p53 in invasive breast carcinoma cells

Background

P53 pathway inactivation plays an important role in the process of breast cancer tumorigenesis. Post-translational protein modification abnormalities have been confirmed to be an important mechanism underlying inactivation of p53. Numerous deubiquitinating enzymes are aberrantly expressed in breast cancer, and a few deubiquitination enzymes can deubiquitinate and stabilize p53. Here, we report that ovarian tumor (OTU) deubiquitinase 3 (OTUD3) is a deubiquitylase of p53 in breast carcinoma (BC).

Methods

Correlations between the mRNA expression levels of OTUD3, TP53 and PTEN and the prognosis of BC were assessed with the Kaplan-Meier Plotter tool. OTUD3 protein expression in 80 pairs of specimens in our cohort was examined by immunohistochemistry and western blotting. The relationship among OTUD3, p53, and p21 proteins was analyzed. Half-life analysis and ubiquitylation assay were performed to elucidate the molecular mechanism by which OTUD3 stabilizes p53. The interaction between OTUD3 and p53 in BC cells was verified by a co-immunoprecipitation assay and GST pulldown experiments. MTS assay for proliferation detection, detection of apoptosis induced by cisplatin and colony formation assay were employed to investigate the functional effects of OTUD3 on breast cancer cells.

Results

OTUD3 downregulation is correlated with a poor prognosis in BC patients. OTUD3 expression is decreased in breast cancer tissues and not associated with the histological grade. OTUD3 also inhibits cell proliferation and clone formation and increases the sensitivity of BC cells to apoptosis induced by chemotherapy drugs. Reduced OTUD3 expression accompanied by decreased p53 abundance is correlated with human breast cancer progression. Ectopic expression of wild-type OTUD3, but not its catalytically inactive mutant, stabilizes and activates p53. Mechanistically, OTUD3 interacts directly with p53 through the amino-terminal OTU region. Finally, OTUD3 protects p53 from murine double minute 2 (Mdm2)-mediated ubiquitination and degradation, enabling the deubiquitination of p53 in BC cells.

Conclusions

In summary, we found that OTUD3 may be a potential therapeutic target for restoring p53 function in breast cancer cells and suggest that the OTUD3-p53 signaling axis may play a critical role in tumor suppression.

The Impact of the Ubiquitin System in the Pathogenesis of Squamous Cell Carcinomas

The ubiquitin system is a dynamic regulatory pathway controlling the activity, subcellular localization and stability of a myriad of cellular proteins, which in turn affects cellular homeostasis through the regulation of a variety of signaling cascades. Aberrant activity of key components of the ubiquitin system has been functionally linked with numerous human diseases including the initiation and progression of human tumors. In this review, we will contextualize the importance of the two main components of the ubiquitin system, the E3 ubiquitin ligases (E3s) and deubiquitinating enzymes (DUBs), in the etiology of squamous cell carcinomas (SCCs). We will discuss the signaling pathways regulated by these enzymes, emphasizing the genetic and molecular determinants underlying their deregulation in SCCs.

Targeting Mouse Double Minute 2: Current Concepts in DNA Damage Repair and Therapeutic Approaches in Cancer

Defects in DNA damage repair may cause genome instability and cancer development. The tumor suppressor gene p53 regulates cell cycle arrest to allow time for DNA repair. The oncoprotein mouse double minute 2 (MDM2) promotes cell survival, proliferation, invasion, and therapeutic resistance in many types of cancer. The major role of MDM2 is to inhibit p53 activity and promote its degradation. In this review, we describe the influence of MDM2 on genomic instability, the role of MDM2 on releasing p53 and binding DNA repair proteins to inhibit repair, and the regulation network of MDM2 including its transcriptional modifications, protein stability, and localization following DNA damage in genome integrity maintenance and in MDM2-p53 axis control. We also discuss p53-dependent and p53 independent oncogenic function of MDM2 and the outcomes of clinical trials that have been used with clinical inhibitors targeting p53-MDM2 to treat certain cancers.

A cohort study on the association of MDM2 SNP309 with lung cancer risk in Bangladeshi population

BACKGROUND/AIMS

Bangladesh is a densely populated country with an increased incidence of lung cancer, mostly due to smoking. Therefore, elucidating the association of mouse double minute 2 homolog (MDM2) single nucleotide polymorphism (SNP) 309 (rs2279744) with lung cancer risk from smoking in Bangladeshi population has become necessary.

METHODS

DNA was extracted from blood samples of 126 lung cancer patient and 133 healthy controls. The MDM2 SNP309 was genotyped by polymerase chain reaction- restriction fragment length polymorphism (PCR-RFLP), using the restriction enzymes MspA1I. Logistic regression was then carried out to calculate odds ratios (ORs) and 95% confidence intervals (CIs) to estimate the risk of lung cancer. A meta-analysis of SNP309 was also carried out on 12,758 control subjects and 11,638 patient subjects.

RESULTS

In multivariate logistic regression, significantly increased risk of lung cancer was observed for MDM2 SNP309 in the dominant model (TG + GG vs. TT: OR, 2.13; 95% CI, 1.29 to 3.53). Stratification analysis revealed that age, sex, obesity, and smoking also increases the risk of lung cancer when carrying the MDM2 SNP309. Our meta-analysis revealed that MDM2 SNP309 was considerably associated with lung cancer in Asian populations (TG + GG vs. TT: OR, 1.32; 95% CI , 1.12 to 1.56; p = 0.019 for heterogeneity).

CONCLUSION

The MDM2 SNP309 was associated with high risk of lung cancer in Bangladeshi and Asian population, particularly with increased age, smoking, and body mass index.

Blockade of MDM2 with inactive Cas9 prevents epithelial to mesenchymal transition in retinal pigment epithelial cells

Epithelial to mesenchymal transition (EMT) plays an important role in the pathogenesis of proliferative vitreoretinopathy (PVR). We aimed to demonstrate the role of mouse double minute 2 (MDM2) in transforming growth factor-beta 2 (TGF-β2)-induced EMT in human retinal pigment epithelial cells (RPEs). Immunofluorescence was used to assess MDM2 expression in epiretinal membranes (ERMs) from patients with PVR. A single guide (sg)RNA targeting the second promoter of MDM2 was cloned into a mutant lentiviral Clustered Regularly Interspaced Short Palindromic Repeats (lentiCRISPR) v2 (D10A and H840A) vector for expressing nuclease dead Cas9 (dCas9)/MDM2-sgRNA in RPEs. In addition, MDM2-sgRNA was also cloned into a pLV-sgRNA-dCas9-Kruppel associated box (KRAB) vector for expressing dCas9 fused with a transcriptional repressor KRAB/MDM2-sgRNA. TGF-β2-induced expression of MDM2 and EMT biomarkers were assessed by quantitative polymerase chain reaction (q-PCR), western blot, or immunofluorescence. Wound-healing and proliferation assays were used to evaluate the role of MDM2 in TGF-β2-induced responses in RPEs. As a result, we found that MDM2 was expressed obviously in ERMs, and that TGF-β2-induced expression of MDM2 and EMT biomarkers Fibronectin, N-cadherin and Vimentin in RPEs. Importantly, we discovered that the dCas9/MDM2-sgRNA blocked TGF-β2-induced expression of MDM2 and the EMT biomarkers without affecting their basal expression, whereas the dCas9-KRAB/MDM2-sgRNA suppressed basal MDM2 expression in RPEs. These cells could not be maintained continuously because their viability was greatly reduced. Next, we found that Nutlin-3, a small molecule blocking the interaction of MDM2 with p53, inhibited TGF-β2-induced expression of Fibronectin and N-cadherin but not Vimentin in RPEs, indicating that MDM2 functions in both p53-dependent and -independent pathways. Finally, our experimental data demonstrated that dCas9/MDM2-sgRNA suppressed TGF-β2-dependent cell proliferation and migration without disturbing the unstimulated basal activity. In conclusion, the CRISPR/dCas9 capability for blocking TGF-β2-induced expression of MDM2 and EMT biomarkers can be exploited for a therapeutic approach to PVR.

A Propensity Score-adjusted Analysis of the Effects of Ubiquitin E3 Ligase Copy Number Variation in Peripheral Blood Leukocytes on Colorectal Cancer Risk

Background: The ubiquitin ligases E3 (E3s) plays a key role in the specific protein degradation in many carcinogenic biological processes. Colorectal cancer (CRC) development may be affected by the copy number variation (CNV) of E3s. Prior studies may have underestimated the impact of potential confounding factors' effects on the association between gene CNV and CRC risk, and CRC risk predictive model integrating gene CNV patterns is lacking. Our research sought to assess the genes CNVs of MDM2, SKP2, FBXW7, β-TRCP, and NEDD4-1 and CRC risk by using propensity score (PS) adjustment and developing models that integrate CNV patterns for CRC risk predictions.

Methods: This study comprising 1036 participants used traditional regression and different PS techniques to adjust the confounding factors to evaluate the relationships between five gene CNVs and CRC risk, and to establish a CRC risk predictive model. The AUC was applied to evaluate the effect of the model. The categorical net reclassification improvement (NRI) and the integrated discrimination improvement (IDI) were analyzed to evaluate the discriminatory accuracy improvement among the models.

Results: Compared to variable adjustment, the odds ratios (ORs) tended to be conservative and accurate with narrow confidence intervals (CIs) after PS adjustment. After PS adjustment, MDM2 amplification was related to increased CRC risk (Amp-pattern: OR = 8.684, 95% CI: 1.213-62.155, P = 0.031), whereas SKP2 deletion and the (del+amp) genotype were associated with reduced CRC risk (Del-pattern: OR = 0.323, 95% CI: 0.106-0.979, P = 0.046; Var-pattern: OR = 0.339, 95% CI: 0.135-0.854, P = 0.024). The predictive model integrating the gene CNV pattern could correctly reclassify 1.7% of the subjects.

Conclusions: MDM2 amplification and SKP2 CNVs are associated with increased and decreased CRC risk, respectively; abnormal CNV-integrated model is more precise for predicting CRC risk. Further studies are needed to verify these encouraging outcomes.

MDM2 copy number increase: a poor prognostic, molecular event in esophageal squamous cell carcinoma

The present study aimed to elucidate the clinicopathological significance of molecular alterations in MDM2 in esophageal squamous cell carcinoma (ESCC). A total of 399 resected cases of ESCC were examined by dual-color in situ hybridization for MDM2 and immunohistochemistry for p53 using tissue microarrays. Clinicopathological features were correlated with the MDM2 status. Among 362 cases with a successful dual-color in situ hybridization analysis, 19 (5%) and 13 (4%) had MDM2 amplification and chromosome 12 polysomy, respectively, and these were examined as an MDM2-positive group. A comparison between amplified and polysomic cases revealed that the latter were more strongly associated with preoperative chemotherapy than the former. Sixteen (50%) of 32 MDM2-positive cases had positive results in all tissue cores examined, indicating diffuse MDM2 alterations. Cases with the diffuse alteration of MDM2 were characterized by an advanced pT stage and extensive vascular infiltration. The relationship between MDM2 copy number increases and p53 mutations was weak, with the overexpression of p53 being similarly detected in MDM2-positive and MDM2-negative cases (59% versus 49%; P = .267). Overall survival was shorter in patients with MDM2-positive ESCC than in those without MDM2 alterations (P = .033). The poor prognostic value of MDM2 alterations became more obvious when only diffusely altered cases were counted (P = .005). In conclusion, the present study revealed that MDM2 copy number increases occurred in 9% of ESCC cases, and MDM2 alterations, particularly diffuse abnormalities, were associated with a poor prognosis. MDM2-altered ESCC may achieve beneficial effects from MDM2-targeted therapy.

Downregulation of AKT and MDM2, Melatonin Induces Apoptosis in AGS and MGC803 Cells

Melatonin, a neurohormone secreted by the pineal gland, has a variety of biological functions, such as circadian rhythms regulation, anti-oxidative activity, immunomodulatory effects, and anittumor, etc. At present, its antitumor effect has attracted people's attention due to its extensive tissue distribution, good tissue compatibility, and low toxic and side effects. In the gastrointestinal tract, there is high level of melatonin and many studies showed melatonin has effects of anti-gastric cancer. In this experiment, human gastric cancer cell lines AGS and MGC803 were used to investigate the intracellular molecular mechanism of melatonin against gastric cancer. After AGS and MGC803 have been treated with melatonin, the changes of cell morphology and cellular structure were observed under electron microscope. Flow cytometer and apoptosis detection kits were used to analyze the effect of apoptosis on AGS and MGC803. The alterations of apoptosis-related proteins Caspase 9, Caspase 3, and upstream regulators AKT, MDM2 including expression, phosphorylation, and activation were detected to analyze the intracellular molecular mechanism of melatonin inhibiting gastric cancer. In AGS and MGC803 cells with melatonin exposure, cleaved Caspase 9 was upregulated and Caspase 3 was activated; moreover, MDM2 and AKT expression and phosphorylation were downregulated. Melatonin promoted apoptosis of AGS and MGC803 cells by the downregulation of AKT and MDM2. Anat Rec, 302:1544-1551, 2019. © 2019 American Association for Anatomy.

Contribution of Murine Double Minute 2 Genotypes to Colorectal Cancer Risk in Taiwan

BACKGROUND/AIM

The genomic role of human mouse double minute 2 (MDM2) in colorectal cancer (CRC) is unclear, therefore, our study aimed to evaluate the contribution of MDM2 genotype to the risk of CRC in Taiwan.

MATERIALS AND METHODS

In this case-control study, MDM2 SNP309 T to G (rs2279744) genotypes were determined and their association with CRC risk were investigated among 362 patients with CRC and 362 age- and gender-matched healthy controls in central Taiwan. In addition, the interaction of MDM2 SNP309 genotypes with personal behaviors and clinicopathological features were also examined.

RESULTS

The percentage of variant GG for the MDM2 SNP309 genotype was 30.9% in the CRC group and 24.0% in the control group, respectively (odds ratio (OR)=1.78, 95% confidence interval (CI)=1.25-2.86, p=0.0057). The allelic frequency distribution analysis showed that the variant G allele of MDM2 SNP309 conferred a significantly increased susceptibility to CRC compared with the wild-type T allele (OR=1.32, 95% CI=1.14-1.69, p=0.0062). As for the gene-lifestyle interaction, there was an obvious joint effect of MDM2 SNP309 GG genotype on the risk of CRC among ever-smokers and non-alcohol drinkers, but not non-smoker or alcohol drinker subgroups. No statistically significant correlation was observed between MDM2 SNP309 genotypic distributions and age, gender, tumor size, location or metastasis status.

CONCLUSION

The genotypes of MDM2 SNP309 may allow forr early detection of and predictor for CRC risk, especially among smokers and non-alcohol drinkers, but not for prognosis. The combined effects of MDM2 SNP309 and other genes (such as matrix metalloproteinases) on CRC susceptibility and prognosis, should also be taken into consideration in the era of precision medicine.

GPRC5A deficiency leads to dysregulated MDM2 via activated EGFR signaling for lung tumor development

GPRC5A, a retinoic acid induced gene, is preferentially expressed in lung tissue. Gprc5a gene deletion leads to spontaneous lung tumor development. However, the mechanism of Gprc5a-mediated lung tumor suppression is not fully understood. Here we showed that MDM2, a p53-negative regulator, was dysregulated in Gprc5a-knockout (ko) mouse tracheal epithelial cells (KO-MTEC) compared to wild type ones. Targeting MDM2 in 1601-a Gprc5a-ko mouse derived lung tumor cell line-and A549-human lung cancer cells, by MDM2 inhibitor Nutlin-3a or small hairpin RNA (sh-RNA)-restored p53 signaling pathway, reduced cancer stem cell markers, and inhibited tumorigenicity. This suggests that dysregulated MDM2 pathway is essential for the oncogenic activities of these cells. MDM2 was found to be stabilized mainly by activated EGFR signaling as targeting EGFR by Erlotinib or sh-RNA repressed MDM2 in a transcription-independent manner. Importantly, overexpression of MDM2 and reduced GPRC5A expression at both protein and mRNA levels were frequently found in clinical human lung cancer tissues. Taken together, GPRC5A deficiency contributes to dysregulated MDM2 via activated EGFR signaling, which promotes lung tumor development.

Modeling the role of microRNA-449a in the regulation of the G2/M cell cycle checkpoint in prostate LNCaP cells under ionizing radiation

Recent studies showed that induced microRNA-449a (miR-449a) enhances a G2/M cell cycle checkpoint arrest in prostate cancer (LNCaP) and lung adenocarcinoma cell lines. In the case of LNCaP cells, upregulated miR-449a directly downregulates c-Myc that is required to induce the cell cycle regulators Cdc25A and Cdc2/CyclinB whose inactivation blocks G2 to M phase transition. However, the molecular mechanisms involved are yet unclear, although in other prostate cancer cells the interactions among p53, miR-449a and Sirt-1 can affect the induction of the G2/M arrest. In order to clarify these molecular mechanisms, in this work we propose a boolean model of the G2/M checkpoint arrest regulation contemplating the influence of miR-449a. The model shows that the cell fate determination between two cellular phenotypes: G2/M-Arrest for DNA repair and G2/M-induced apoptosis is stochastic and influenced by miR-449a state of activation. The results were compared with experimental data available presenting agreement. We also found that several feedback loops are involved in this cell fate regulation and we indicate, through in silico gain or loss of function perturbations of genes, which of these feedback loops are more efficient to favor a specific phenotype.

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.

Regulation of the cell cycle and centrosome biology by deubiquitylases

Post-translational modification of proteins by ubiquitylation is increasingly recognised as a highly complex code that contributes to the regulation of diverse cellular processes. In humans, a family of almost 100 deubiquitylase enzymes (DUBs) are assigned to six subfamilies and many of these DUBs can remove ubiquitin from proteins to reverse signals. Roles for individual DUBs have been delineated within specific cellular processes, including many that are dysregulated in diseases, particularly cancer. As potentially druggable enzymes, disease-associated DUBs are of increasing interest as pharmaceutical targets. The biology, structure and regulation of DUBs have been extensively reviewed elsewhere, so here we focus specifically on roles of DUBs in regulating cell cycle processes in mammalian cells. Over a quarter of all DUBs, representing four different families, have been shown to play roles either in the unidirectional progression of the cell cycle through specific checkpoints, or in the DNA damage response and repair pathways. We catalogue these roles and discuss specific examples. Centrosomes are the major microtubule nucleating centres within a cell and play a key role in forming the bipolar mitotic spindle required to accurately divide genetic material between daughter cells during cell division. To enable this mitotic role, centrosomes undergo a complex replication cycle that is intimately linked to the cell division cycle. Here, we also catalogue and discuss DUBs that have been linked to centrosome replication or function, including centrosome clustering, a mitotic survival strategy unique to cancer cells with supernumerary centrosomes.

A novel mouse model of rhabdomyosarcoma underscores the dichotomy of MDM2-ALT1 function in vivo

Alternative splicing of the oncogene murine double minute 2 (MDM2) is induced in response to genotoxic stress. MDM2-ALT1, the major splice variant generated, is known to activate the p53 pathway and impede full-length MDM2's negative regulation of p53. Despite this perceptible tumor-suppressive role, MDM2-ALT1 is also associated with several cancers. Furthermore, expression of MDM2-ALT1 has been observed in aggressive metastatic disease in pediatric rhabdomyosarcoma (RMS), irrespective of histological subtype. Therefore, we generated a transgenic MDM2-ALT1 mouse model that would allow us to investigate the effects of this splice variant on the progression of tumorigenesis. Here we show that when MDM2-ALT1 is ubiquitously expressed in p53 null mice it leads to increased incidence of spindle cell sarcomas, including RMS. Our data provide evidence that constitutive MDM2-ALT1 expression is itself an oncogenic lesion that aggravates the tumorigenesis induced by p53 loss. On the contrary, when MDM2-ALT1 is expressed solely in B-cells in the presence of homozygous wild-type p53 it leads to significantly increased lymphomagenesis (56%) when compared with control mice (27%). However, this phenotype is observable only at later stages in life (⩾18 months). Moreover, flow cytometric analyses for B-cell markers revealed an MDM2-ALT1-associated decrease in the B-cell population of the spleens of these animals. Our data suggest that the B-cell loss is p53 dependent and is a response mounted to persistent MDM2-ALT1 expression in a wild-type p53 background. Overall, our findings highlight the importance of an MDM2 splice variant as a critical modifier of both p53-dependent and -independent tumorigenesis, underscoring the complexity of MDM2 posttranscriptional regulation in cancer. Furthermore, MDM2-ALT1-expressing p53 null mice represent a novel mouse model of fusion-negative RMS.

MDM2/X inhibitors under clinical evaluation: perspectives for the management of hematological malignancies and pediatric cancer

The two murine double minute (MDM) family members MDM2 and MDMX are at the center of an intense clinical assessment as molecular target for the management of cancer. Indeed, the two proteins act as regulators of P53, a well-known key controller of the cell cycle regulation and cell proliferation that, when altered, plays a direct role on cancer development and progression. Several evidence demonstrated that functional aberrations of P53 in tumors are in most cases the consequence of alterations on the MDM2 and MDMX regulatory proteins, in particular in patients with hematological malignancies where TP53 shows a relatively low frequency of mutation while MDM2 and MDMX are frequently found amplified/overexpressed. The pharmacological targeting of these two P53-regulators in order to restore or increase P53 expression and activity represents therefore a strategy for cancer therapy. From the discovery of the Nutlins in 2004, several compounds have been developed and reported with the ability of targeting the P53-MDM2/X axis by inhibiting MDM2 and/or MDMX. From natural compounds up to small molecules and stapled peptides, these MDM2/X pharmacological inhibitors have been extensively studied, revealing different biological features and different rate of efficacy when tested in in vitro and in vivo experimental tumor models. The data/evidence coming from the preclinical experimentation have allowed the identification of the most promising molecules and the setting of clinical studies for their evaluation as monotherapy or in therapeutic combination with conventional chemotherapy or with innovative therapeutic protocols in different tumor settings. Preliminary results have been recently published reporting data about safety, tolerability, potential side effects, and efficacy of such therapeutic approaches. In this light, the aim of this review is to give an updated overview about the state of the art of the clinical evaluation of MDM2/X inhibitor compounds with a special attention to hematological malignancies and to the potential for the management of pediatric cancers.

MDM2 Inhibition Sensitizes Prostate Cancer Cells to Androgen Ablation and Radiotherapy in a p53-Dependent Manner

PURPOSE: Increased murine double minute 2 (MDM2) expression, independent of p53 status, is associated with increased cancer-specific mortality for men with prostate cancer treated with radiotherapy. We assessed MI-219, a small molecule inhibitor of MDM2 with improved pharmacokinetics over nutlin-3, for sensitization of prostate cancer cells to radiotherapy and androgen deprivation therapy, a standard treatment option for men with high-risk prostate cancer. EXPERIMENTAL DESIGN: The effect of MDM2 inhibition by MI-219 was assessed in vitro and in vivo with mouse xenograft models across multiple prostate cancer cell lines containing varying p53 functional status. RESULTS: MDM2 inhibition by MI-219 resulted in dose- and time-dependent p53 activation and decreased clonogenic cell survival after radiation in a p53-dependent manner. Mechanistically, radiosensitization following inhibition of MDM2 was largely the result of p53-dependent increases in apoptosis and DNA damage as evidenced by Annexin V flow cytometry and γ-H2AX foci immunofluorescence. Similarly, treatment with MI-219 enhanced response to antiandrogen therapy via a p53-dependent increase in apoptotic cell death. Lastly, triple therapy with radiation, androgen deprivation therapy, and MI-219 decreased xenograft tumor growth compared with any single- or double-agent treatment. CONCLUSION: MDM2 inhibition with MI-219 results in p53-dependent sensitization of prostate cancer cells to radiation, antiandrogen therapy, and the combination. These findings support MDM2 small molecule inhibitor therapy as a therapy intensification strategy to improve clinical outcomes in high-risk localized prostate cancer. TRANSLATIONAL RELEVANCE: The combination of radiotherapy and androgen deprivation therapy is a standard treatment option for men with high-risk prostate cancer. Despite improvements in outcomes when androgen deprivation therapy is added to radiation, men with high-risk prostate cancer have significant risk for disease recurrence, progression, and even death within the first 10 years following treatment. We demonstrate that treatment with MI-219 (an inhibitor of MDM2) results in prostate cancer cell sensitization to radiation and androgen deprivation therapy in vitro and in vivo. Triple therapy with MI-219, radiation, and androgen deprivation therapy dramatically decreased tumor growth compared with any single- or double-agent therapy. These findings provide evidence that inhibition of MDM2 is a viable means by which to enhance the efficacy of both radiation and androgen deprivation therapy and thereby improve outcomes in the treatment of prostate cancer. As such, further investigation is warranted to translate these findings to the clinical setting.

p53 regulates the cardiac transcriptome

The tumor suppressor Trp53 (p53) inhibits cell growth after acute stress by regulating gene transcription. The mammalian genome contains hundreds of p53-binding sites. However, whether p53 participates in the regulation of cardiac tissue homeostasis under normal conditions is not known. To examine the physiologic role of p53 in adult cardiomyocytes in vivo, Cre-loxP-mediated conditional gene targeting in adult mice was used. Genome-wide transcriptome analyses of conditional heart-specific p53 knockout mice were performed. Genome-wide annotation and pathway analyses of >5,000 differentially expressed transcripts identified many p53-regulated gene clusters. Correlative analyses identified >20 gene sets containing more than 1,000 genes relevant to cardiac architecture and function. These transcriptomic changes orchestrate cardiac architecture, excitation-contraction coupling, mitochondrial biogenesis, and oxidative phosphorylation capacity. Interestingly, the gene expression signature in p53-deficient hearts confers resistance to acute biomechanical stress. The data presented here demonstrate a role for p53, a previously unrecognized master regulator of the cardiac transcriptome. The complex contributions of p53 define a biological paradigm for the p53 regulator network in the heart under physiological conditions.

hTERT promotes the invasion of gastric cancer cells by enhancing FOXO3a ubiquitination and subsequent ITGB1 upregulation

BACKGROUND AND AIMS

Human telomerase reverse transcriptase (hTERT) plays an important role in cancer invasion, but the relevant mechanism is not well known. This study aims to investigate the role and mechanism of hTERT in gastric cancer metastasis.

DESIGN

Proteomics analysis, qPCR and western blotting were used to screen for hTERT-regulated candidate molecules in gastric cancer invasion. Chromatin immunoprecipitation (ChIP) qPCR was performed to identify the binding sites of hTERT at the regulatory region of the integrin β1 (ITGB1) gene. ChIP assays were further applied to elucidate the transcription factors that bound to the regulatory region. The interactions between hTERT and the transcription factors were tested by co-immunoprecipitation (Co-IP) and glutathione S-transferase (GST) pull-down experiments. Moreover, the revealed pathway was verified in tumour-bearing nude mice and human gastric cancer tissues.

RESULTS

ITGB1 was identified as a downstream gene of hTERT, and there were two hTERT-binding regions within this gene. hTERT alleviated the binding of forkhead box O3 (FOXO3a) to FOXO3a binding element (+9972∼+9978), but it enhanced the binding of forkhead box M1 (FOXM1) to FOXM1 binding element (-1104∼-1109) in ITGB1 gene. Importantly, FOXO3a played a major role in hTERT-induced ITGB1 expression, and the hTERT/murine double minute 2 (MDM2) complex promoted the ubiquitin-mediated degradation of FOXO3a. Moreover, hTERT increased ITGB1 expression in xenograft gastric cancer, and the level of hTERT was positively correlated with that of ITGB1 in human gastric cancer tissues.

CONCLUSIONS

The hTERT/MDM2-FOXO3a-ITGB1 pathway markedly contributes to hTERT-promoted gastric cancer invasion, suggesting that this pathway might be a novel target for the prevention and treatment of gastric cancer metastasis.

The human papillomavirus E7 oncoprotein as a regulator of transcription

High-risk human papillomaviruses (HPVs) encode oncoproteins which manipulate gene expression patterns in the host keratinocytes to facilitate viral replication, regulate viral transcription, and promote immune evasion and persistence. In some cases, oncoprotein-induced changes in host cell behavior can cause progression to cancer, but a complete picture of the functions of the viral oncoproteins in the productive HPV life cycle remains elusive. E7 is the HPV-encoded factor most responsible for maintaining cell cycle competence in differentiating keratinocytes. Through interactions with dozens of host factors, E7 has an enormous impact on host gene expression patterns. In this review, we will examine the role of E7 specifically as a regulator of transcription. We will discuss mechanisms of regulation of cell cycle-related genes by E7 as well as genes involved in immune regulation, growth factor signaling, DNA damage responses, microRNAs, and others pathways. We will also discuss some unanswered questions about how transcriptional regulation by E7 impacts the biology of HPV in both benign and malignant conditions.

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.

Genetic Polymorphism of MDM2 SNP309 in Patients with Helicobacter Pylori-Associated Gastritis

BACKGROUND

Helicobacter pylori plays an important role in gastric cancer, which has a relatively low inciduence in Thailand. MDM2 is a major negative regulator of p53, the key tumor suppressor involved in tumorigenesis of the majority of human cancers. Whether its expression might explain the relative lack of gastric cancer in Thailand was assessed here.

MATERIALS AND METHODS

This single-center study was conducted in the northeast region of Thailand. Gastric mucosa from 100 patients with Helicobacter pylori associated gastritis was analyzed for MDM2 SNP309 using real-time PCR hybridization (light-cycler) probes.

RESULTS

In the total 100 Helicobacter pylori associated gastritis cases the incidence of SNP 309 T/T homozygous was 78 % with SNP309 G/T heterozygous found in 19% and SNP309 G/G homozygous in 3%. The result show SNP 309 T/T and SNP 309 G/T to be rather common in the Thai population.

CONCLUSIONS

Our study indicates that the MDM2 SNP309 G/G homozygous genotype might be a risk factor for gastric cancer in Thailand and the fact that it is infrequent could explain to some extent the low incidence of gastric cancer in the Thai population.

A Functional Polymorphism (rs937283) in the MDM2 Promoter Region is Associated with Poor Prognosis of Retinoblastoma in Chinese Han Population

The effect of single nucleotide polymorphisms (SNPs) at MDM2 has been investigated in several cancer types. Three MDM2 SNPs(rs937283, rs2270744 and rs769412) have previously been suggested to be positively correlated with cancer. In this study, we aimed to explore the association of rs937283, rs2270744 and rs769412 polymorphisms with retinoblastoma (RB) risk, clinicopathological characteristics, and prognosis. Compared with wild-type genotype AA at rs937283, individuals carrying AG and GG genotype had a significantly increased risk for developing RB (OR = 1.86, 95% CI 1.13–3.08; OR = 2.48, 95% CI 1.10–5.62, respectively). RB patients with allele G at rs937283 were more susceptible to invasion and high tumor aggression (OR = 2.42, 95% CI 1.43–4.11; OR = 2.15, 95% CI 1.27–3.64, respectively). Kaplan-Meier curves and log-rank results revealed that RB patients harboring genotype GG and G allele at rs937283 had worse survival (P < 0.02 and P < 0.01, respectively). In addition, the A to G substitution at rs937283 significantly enhanced the transcription activity of the MDM2 gene in vitro. In vivo, we found that MDM2 mRNA and protein were overexpressed in individuals who carried the G allele at rs937283. This study suggested that the MDM2 rs937283 polymorphism is a novel functional SNP both in vitro and in vivo as well as a biomarker for poor prognosis in RB.

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.

Murine Double-Minute 2 Homolog Single Nucleotide Polymorphisms 285 and 309 in Cervical Carcinogenesis

Background and Objective

In Caucasians, the MDM2 single nucleotide polymorphism (SNP) 285 G>C (rs117039649) neutralizes the effect of 309 T>G (rs2279744), which increases MDM2 expression and impairs the p53 pathway. In this study, we examined the distribution of these two SNPs in Polish women with squamous cell carcinoma (SCC) (n = 379), adenocarcinoma (n = 59) and other cervical tumor types (n = 18).

Methods

The polymerase chain reaction-restriction fragment length polymorphism technique and DNA sequencing were employed in our study.

Results

The P trend value calculated for the MDM2 285 G>C polymorphism was statistically significant (P_trend = 0.016) for SCC. Using logistical regression analysis adjusted for the effect of age, pregnancy, oral contraceptive use, tobacco smoking, and menopausal status, we observed that the MDM2 285 G>C SNP protected against SCC, with an adjusted odd ratio (OR) for the C carriers versus G/G genotype of 0.536 (P = 0.019). Stratified analyses of MDM2 285 G>C revealed a protective role of the C allele against SCC in women with a positive history of oral contraceptive use (age-adjusted OR 0.413, P = 0.021) and in premenopausal women (age-adjusted OR 0.362, P = 0.022). We also found that the 285GG/309GG vs 285GG/309 TT genotype increased the risk of SCC (adjusted OR 1.890, P = 0.005). However, the 285CC/309GG + 285GC/309GG versus 285GG/309GG genotype reduced the risk of SCC (adjusted OR 0.311, P = 0.004).

Conclusion

Our results demonstrate that the MDM2 285C gene variant and 285CC/309GG + 285GC/309GG genotypes protect against SCC, most likely by neutralizing the effect of the 309 T>G SNP. The 285GG/309GG genotype increases the risk of SCC possibly due to increased MDM2 expression.

Electronic supplementary material

The online version of this article (doi:10.1007/s40291-015-0153-4) contains supplementary material, which is available to authorized users.

Critical review about MDM2 in cancer: Possible role in malignant mesothelioma and implications for treatment

The tumor suppressor p53 regulates genes involved in DNA repair, metabolism, cell cycle arrest, apoptosis and senescence. p53 is mutated in about 50% of the human cancers, while in tumors with wild-type p53 gene, the protein function may be lost because of overexpression of Murine Double Minute 2 (MDM2). MDM2 targets p53 for ubiquitylation and proteasomal degradation. p53 reactivation through MDM2 inhibitors seems to be a promising strategy to sensitize p53 wild-type cancer cells to apoptosis. Moreover, additional p53-independent molecular functions of MDM2, such as neoangiogenesis promotion, have been suggested. Thus, MDM2 might be a target for anticancer treatment because of its antiapoptotic and proangiogenetic role. Malignant pleural mesothelioma (MPM) is an aggressive asbestos-related tumor where wild-type p53 might be present. The present review gives a complete landscape about the role of MDM2 in cancer pathogenesis, prognosis and treatment, with particular focus on Malignant Pleural Mesothelioma.

Nutlin-3 down-regulates retinoblastoma protein expression and inhibits muscle cell differentiation

The p53 tumor suppressor gene plays a critical role in regulation of proliferation, cell death and differentiation. The MDM2 oncoprotein is a major negative regulator for p53 by binding to and targeting p53 for proteasome-mediated degradation. The small molecule inhibitor, nutlin-3, disrupts MDM2-p53 interaction resulting in stabilization and activation of p53 protein. We have previously shown that nutlin-3 activates p53, leading to MDM2 accumulation as concomitant of reduced retinoblastoma (Rb) protein stability. It is well known that Rb is important in muscle development and myoblast differentiation and that rhabdomyosarcoma (RMS), or cancer of the skeletal muscle, typically harbors MDM2 amplification. In this study, we show that nutlin-3 inhibited myoblast proliferation and effectively prevented myoblast differentiation, as evidenced by lack of expression of muscle differentiation markers including myogenin and myosin heavy chain (MyHC), as well as a failure to form multinucleated myotubes, which were associated with dramatic increases in MDM2 expression and decrease in Rb protein levels. These results indicate that nutlin-3 can effectively inhibit muscle cell differentiation.

Mdm2 promotes myogenesis through the ubiquitination and degradation of CCAAT/enhancer-binding protein β

Myogenesis is a tightly regulated differentiation process during which precursor cells express in a coordinated fashion the myogenic regulatory factors, while down-regulating the satellite cell marker Pax7. CCAAT/Enhancer-binding protein β (C/EBPβ) is also expressed in satellite cells and acts to maintain the undifferentiated state by stimulating Pax7 expression and by triggering a decrease in MyoD protein expression. Herein, we show that C/EBPβ protein is rapidly down-regulated upon induction of myogenesis and this is not due to changes in Cebpb mRNA expression. Rather, loss of C/EBPβ protein is accompanied by an increase in Mdm2 expression, an E3 ubiquitin ligase. We demonstrate that Mdm2 interacts with, ubiquitinates and targets C/EBPβ for degradation by the 26 S proteasome, leading to increased MyoD expression. Knockdown of Mdm2 expression in myoblasts using a shRNA resulted in high C/EBPβ levels and a blockade of myogenesis, indicating that Mdm2 is necessary for myogenic differentiation. Primary myoblasts expressing the shMdm2 construct were unable to contribute to muscle regeneration when grafted into cardiotoxin-injured muscle. The differentiation defect imposed by loss of Mdm2 could be partially rescued by loss of C/EBPβ, suggesting that the regulation of C/EBPβ turnover is a major role for Mdm2 in myoblasts. Taken together, we provide evidence that Mdm2 regulates entry into myogenesis by targeting C/EBPβ for degradation by the 26 S proteasome.

MDMX exerts its oncogenic activity via suppression of retinoblastoma protein

Inactivation of the retinoblastoma protein (RB) has a major role in the development of human malignancies. We have previously shown that MDM2, an ubiquitin E3 ligase and major negative regulator of p53, binds to and promotes proteasome-mediated degradation of RB. MDMX, a homolog of MDM2, also binds to and inhibits p53 transactivation activity, yet it does not possess intrinsic ubiquitin ligase activity. Here, we show that MDMX binds to and promotes RB degradation in an MDM2-dependent manner. Specifically, the MDMX C-terminal ring domain binds to the RB C-pocket and enhances MDM2-RB interaction. Silencing MDMX induces RB accumulation, cell cycle arrest and senescence-like phenotypes, which are reverted by simultaneous RB knockdown. Furthermore, MDMX ablation leads to significant retardation of xenograft tumor growth, concomitant with RB accumulation. These results demonstrate that MDMX exerts oncogenic activity via suppression of RB, and suggest that both MDM2 and MDMX could be chemotherapeutic targets.

Pin1 inhibits PP2A-mediated Rb dephosphorylation in regulation of cell cycle and S-phase DNA damage

Inactivation of the retinoblastoma protein (Rb) has a key role in tumorigenesis. It is well established that Rb function is largely regulated by a dynamic balance of phosphorylation and dephosphorylation. Although much research has been done to understand the mechanisms and function of RB phosphorylation, the regulation of Rb dephosphorylation is still not well understood. In this study, we demonstrate that Pin1 has an important role in the regulation of Rb function in cell cycle progression and S-phase checkpoint upon DNA damage. We show that the Rb C-pocket directly binds to the Pin1 WW domain in vitro and in vivo, and that the phosphorylation of Rb C-pocket by G1/S Cyclin/Cyclin-dependent kinase complexes is critical for mediating this interaction. We further show that Rb-mediated cell cycle arrest and Rb-induced premature cellular senescence are effectively inhibited by Pin1 expression. In addition, DNA damage induces Rb dephosphorylation in a PP2A-dependent manner, and this process is inhibited by Pin1. Furthermore, the overexpression of Pin1 promotes Rb hyperphosphorylation upon S-phase DNA damage. Importantly, both the Pin1 WW domain and isomerase activity are required for its effect on S-phase checkpoint. Moreover, the overexpression of Pin1 is correlated with Rb hyperphosphorylation in breast cancer biopsies. These results indicate that Pin1 has a critical role in the modulation of Rb function by the regulation of Rb dephosphorylation, which may have an important pathological role in cancer development.

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.