Targeted expression of MDM2 uncouples S phase from mitosis and inhibits mammary gland development independent of p53

MDM2 Inhibitors for Cancer Therapy: The Past, Present, and Future

Since its discovery over 35 years ago, MDM2 has emerged as an attractive target for the development of cancer therapy. MDM2's activities extend from carcinogenesis to immunity to the response to various cancer therapies. Since the report of the first MDM2 inhibitor more than 30 years ago, various approaches to inhibit MDM2 have been attempted, with hundreds of small-molecule inhibitors evaluated in preclinical studies and numerous molecules tested in clinical trials. Although many MDM2 inhibitors and degraders have been evaluated in clinical trials, there is currently no Food and Drug Administration (FDA)-approved MDM2 inhibitor on the market. Nevertheless, there are several current clinical trials of promising agents that may overcome the past failures, including agents granted FDA orphan drug or fast-track status. We herein summarize the research efforts to discover and develop MDM2 inhibitors, focusing on those that induce MDM2 degradation and exert anticancer activity, regardless of the p53 status of the cancer. We also describe how preclinical and clinical investigations have moved toward combining MDM2 inhibitors with other agents, including immune checkpoint inhibitors. Finally, we discuss the current challenges and future directions to accelerate the clinical application of MDM2 inhibitors. In conclusion, targeting MDM2 remains a promising treatment approach, and targeting MDM2 for protein degradation represents a novel strategy to downregulate MDM2 without the side effects of the existing agents blocking p53-MDM2 binding. Additional preclinical and clinical investigations are needed to finally realize the full potential of MDM2 inhibition in treating cancer and other chronic diseases where MDM2 has been implicated. SIGNIFICANCE STATEMENT: Overexpression/amplification of the MDM2 oncogene has been detected in various human cancers and is associated with disease progression, treatment resistance, and poor patient outcomes. This article reviews the previous, current, and emerging MDM2-targeted therapies and summarizes the preclinical and clinical studies combining MDM2 inhibitors with chemotherapy and immunotherapy regimens. The findings of these contemporary studies may lead to safer and more effective treatments for patients with cancers overexpressing MDM2.

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.

Fibronectin Modulates the Expression of miRNAs in Prostate Cancer Cell Lines

Prostate cancer (PCa) is a significant cause of cancer-related deaths among men and companion animals, such as dogs. However, despite its high mortality and incidence rates, the molecular mechanisms underlying this disease remain to be fully elucidated. Among the many factors involved in prostate carcinogenesis, the extracellular matrix (ECM) plays a crucial role. This ECM in the prostate is composed mainly of collagen fibers, reticular fibers, elastic fibers, proteoglycans and glycoproteins, such as fibronectin. Fibronectin is a glycoprotein whose dysregulation has been implicated in the development of multiple types of cancer, and it has been associated with cell migration, invasion, and metastasis. Furthermore, our research group has previously shown that fibronectin induces transcriptional changes by modulating the expression of protein coding genes in LNCaP cells. However, potential changes at the post-transcriptional level are still not well understood. This study investigated the impact of exposure to fibronectin on the expression of a key class of regulatory RNAs, the microRNAs (miRNAs), in prostate cancer cell lines LNCaP and PC-3. Five mammalian miRNAs (miR-21, miR-29b, miR-125b, miR-221, and miR-222) were differentially expressed after fibronectin exposure in prostate cell lines. The expression profile of hundreds of mRNAs predicted to be targeted by these miRNAs was analyzed using publicly available RNA-Sequencing data (GSE64025, GSE68645, GSE29155). Also, protein-protein interaction networks and enrichment analysis were performed to gain insights into miRNA biological functions. Altogether, these functional analyzes revealed that fibronectin exposure impacts the expression of miRNAs potentially involved in PCa causing changes in critical signaling pathways such as PI3K-AKT, and response to cell division, death, proliferation, and migration. The relationship here demonstrated between fibronectin exposure and altered miRNA expression improves the comprehension of PCa in both men and other animals, such as dogs, which naturally develop prostate cancer.

Polyploid Giant Cancer Cells, a Hallmark of Oncoviruses and a New Therapeutic Challenge

Tumors are renowned as intricate systems that harbor heterogeneous cancer cells with distinctly diverse molecular signatures, sizes and genomic contents. Among those various genomic clonal populations within the complex tumoral architecture are the polyploid giant cancer cells (PGCC). Although described for over a century, PGCC are increasingly being recognized for their prominent role in tumorigenesis, metastasis, therapy resistance and tumor repopulation after therapy. A shared characteristic among all tumors triggered by oncoviruses is the presence of polyploidy. Those include Human Papillomaviruses (HPV), Epstein Barr Virus (EBV), Hepatitis B and C viruses (HBV and HCV, respectively), Human T-cell lymphotropic virus-1 (HTLV-1), Kaposi's sarcoma herpesvirus (KSHV) and Merkel polyomavirus (MCPyV). Distinct viral proteins, for instance Tax for HTLV-1 or HBx for HBV have demonstrated their etiologic role in favoring the appearance of PGCC. Different intriguing biological mechanisms employed by oncogenic viruses, in addition to viruses with high oncogenic potential such as human cytomegalovirus, could support the generation of PGCC, including induction of endoreplication, inactivation of tumor suppressors, development of hypoxia, activation of cellular senescence and others. Interestingly, chemoresistance and radioresistance have been reported in the context of oncovirus-induced cancers, for example KSHV and EBV-associated lymphomas and high-risk HPV-related cervical cancer. This points toward a potential linkage between the previously mentioned players and highlights PGCC as keystone cancer cells in virally-induced tumors. Subsequently, although new therapeutic approaches are actively needed to fight PGCC, attention should also be drawn to reveal the relationship between PGCC and oncoviruses, with the ultimate goal of establishing effective therapeutic platforms for treatment of virus-associated cancers. This review discusses the presence of PGCCs in tumors induced by oncoviruses, biological mechanisms potentially favoring their appearance, as well as their consequent implication at the clinical and therapeutic level.

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.

Associations between an MDM2 gene polymorphism and ulcerative colitis by ARMS-PCR

Ulcerative colitis is a form of inflammatory bowel disease characterized by chronic inflammation of the colon and rectum. The abnormal lesions in the digestive system caused by ulcerative colitis and intermittent colitis are of major clinical importance. MDM2 is a phospho-protein that functions as a ubiquitin ligase for p53. Recently, a T>G substitution in the promoter of the MDM2 gene (rs309) has been identified. In this case-control study, 174 ulcerative colitis biopsy samples and 82 control samples were collected from colonoscopy centers, hospitals, and clinics in Mazandaran and Gilan Provinces in Iran from October 2014 to May 2015. This MDM2 polymorphism was investigated in DNA samples (extracted from biopsy samples) by amplification-refractory mutation system polymerase chain reaction. The mean age of patients with ulcerative colitis was 46.5 years (range, 28 to 69 years) and that of control individuals was 45.3 years (range, 26 to 71 years). Seventy-eight patients (44.82%) were men and 96 (55.18%) were women. The distribution of the TT, TG, and GG genotypes was 17.93%, 27.59%, and 34.48%, respectively, in the ulcerative colitis patients and 31.70%, 24.40%, and 43.90%, respectively, in the control individuals (odds ratio of GG for ulcerative colitis, 7.142; 95% confidence interval, 2.400 to 9.542; p = 0.001). It was found that a single-nucleotide polymorphism at rs309 in the MDM2 gene was associated with ulcerative colitis. A direct relationship was found between age and ulcerative colitis, while no relationship was found with sex. This finding is of note because the occurrence of intestinal inflammation and subsequent ulcers can precede the development of cancer.

Regulation of cell cycle by MDM2 in prostate cancer cells through Aurora Kinase-B and p21WAF1/CIP1 mediated pathways

Overexpression of MDM2 oncoprotein has been detected in a large number of diverse human malignancies and has been shown to play both p53-dependent and p53-independent roles in oncogenesis. Our study was designed to explore the impact of MDM2 overexpression on the levels of various cell cycle regulatory proteins including Aurora kinase-B (AURK-B), CDC25C and CDK1, which are known to promote tumor progression and increase metastatic potential. Our data from human cell cycle RT² profiler PCR array experiments revealed significant changes in the expression profile of genes that are involved in different phases of cell cycle regulation in LNCaP-MST (MDM2 transfected) prostate cancer cells. Our current study has demonstrated a significant increase in the expression level of AURK-B, CDC25C, Cyclin A2, Cyclin B and CDK1 in LNCaP-MST cells as compared with wild type LNCaP cells that were modulated by MDM2 specific inhibitor Nutlin-3. In fact, the expression levels of the above- mentioned proteins were significantly altered at both mRNA and protein levels after treating the cells with 20 μM Nutlin-3 for 24h. Additionally, the pro-apoptotic proteins including p53, p21, and Bax were elevated with the concomitant decrease in the key anti-apoptotic proteins following MDM2 inhibitor treatment. Also, Nutlin-3 treated cells demonstrated caspase-3 activation was observed with an in-vitro caspase-3 fluorescent assay performed with caspase 3/7 specific DEVD-amc substrate. Our results offer significant evidence towards the effectiveness of MDM2 inhibition in causing cell cycle arrest via blocking the transmission of signals through AURKB-CDK1 axis and inducing apoptosis in LNCaP-MST cancer cells. It is evident from our data that MDM2 overexpression probably is the primary cause for CDK1 up-regulation in the LNCaP-MST cells, which might have occurred possibly through activation of AURK-B. However, further studies in this direction should shed more light on the intracellular mechanisms involved in the regulation of Aurora kinase-B and CDK1 axis in MDM2 positive cancers.

DNA replication in progenitor cells and epithelial regeneration after lung injury requires the oncoprotein MDM2

Depletion of epithelial cells after lung injury prompts proliferation and epithelial mesenchymal transition (EMT) of progenitor cells, and this repopulates the lost epithelial layer. To investigate the cell proliferative function of human oncoprotein MDM2, we generated mouse models targeting human MDM2 expression in either lung Club or alveolar cells after doxycycline treatment. We report that MDM2 expression in lung Club or alveolar cells activates DNA replication specifically in lung progenitor cells only after chemical- or radiation-induced lung injury, irrespective of their p53 status. Activation of DNA replication by MDM2 triggered by injury leads to proliferation of lung progenitor cells and restoration of the lost epithelial layers. Mouse lung with no Mdm2 allele loses its ability to replicate DNA, whereas loss of 1 Mdm2 allele compromises this function, demonstrating the requirement of endogenous MDM2. We show that the p53-independent ability of MDM2 to activate Akt signaling is essential for initiating DNA replication in lung progenitor cells. Furthermore, MDM2 activates the Notch signaling pathway and expression of EMT markers, indicative of epithelial regeneration. This is the first report to our knowledge demonstrating a direct p53-independent participation of MDM2 in progenitor cell proliferation and epithelial repair after lung injury, distinct from a p53-degrading antiapoptotic effect preventing injury.

A DHX9-lncRNA-MDM2 interaction regulates cell invasion and angiogenesis of cervical cancer

Cervical cancer (CC) is the third most common carcinoma and the fourth leading cause of cancer-associated mortality in women. Here, we report that MDM2-DHX9 interaction mediates CC motility and angiogenesis in a long noncoding RNA-dependent fashion. A long noncoding RNA, named lnc-CCDST, is significantly downregulated in CC tissues, and binds to pro-oncogenic DHX9. DHX9 is upregulated in CC tissue, and promotes CC cell motility and angiogenesis. The lnc-CCDST and DHX9 interaction promotes DHX9 degradation through the ubiquitin proteasome pathway. Furthermore, DHX9 bound to E3 ubiquitin ligase MDM2, and this interaction is enhanced by lnc-CCDST. Thus, lnc-CCDST promotes DHX9 degradation by serving as a scaffold to facilitate the formation of MDM2 and DHX9 complexes. Moreover, HPV oncogenes E6 and E7 abolish the expression of lnc-CCDST resulting in the increase of DHX9. Our results have revealed a novel mechanism by which high-risk HPVs promote motility and angiogenesis of CC by inhibiting expression of lnc-CCDST to disrupt MDM2 and DHX9 interaction, and DHX9 degradation, and identified a potential therapeutic target for CC.

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.

MDM2 antagonists as a novel treatment option for acute myeloid leukemia: perspectives on the therapeutic potential of idasanutlin (RG7388)

Acute myeloid leukemia (AML) is a clonal heterogenous malignancy of the myeloid cells with a poor prognosis lending itself to novel treatment strategies. TP53 is a critical tumor suppressor and plays an essential role in leukemogenesis. Although TP53 is relatively unusual in de novo AML, inactivation of wild-type p53 (WT-p53) is a common event. Murine double minute 2 (MDM2) is a key negative regulator of p53 and its expression; inhibition of MDM2 is postulated to reactivate WT-p53 and its tumor suppressor functions. Nutlins were the first small molecule inhibitors that bind to MDM2 and target its interaction with p53. RG7388 (idasanutlin), a second-generation nutlin, was developed to improve upon the potency and toxicity profile of earlier nutlins. Preliminary data from early phase trials and ongoing studies suggest clinical response with RG7388 (idasanutlin) both in monotherapy and combination strategies in AML. We herein briefly discuss currently approved therapies in AML and review the clinical data for RG7388 (idasanutlin) and MDM2 inhibition as novel treatment strategies in AML. We further describe efficacy and toxicity profile data from completed and ongoing trials of RG7388 (idasanutlin) and other MDM2-p53 inhibitors in development. Many targeted therapies have been approved recently in AML, with a focus on the older and unfit population for intensive induction therapy and in relapsed/refractory disease. The "nutlins", including RG7388 (idasanutlin), merit continued investigation in such settings.

MDM2 promotes genome instability by ubiquitinating the transcription factor HBP1

Genome instability is a common feature of tumor cells, and the persistent presence of genome instability is a potential mechanism of tumorigenesis. The E3 ubiquitin ligase MDM2 is intimately involved in genome instability, but its mechanisms are unclear. Our data demonstrated that the transcription factor HBP1 is a target of MDM2. MDM2 facilitates HBP1 proteasomal degradation by ubiquitinating HBP1, regardless of p53 status, thus attenuating the transcriptional inhibition of HBP1 in the expression of its target genes, such as the DNA methyltransferase DNMT1 and histone methyltransferase EZH2, which results in global DNA hypermethylation and histone hypermethylation and ultimately genome instability. The repression of HBP1 by MDM2 finally promotes cell growth and tumorigenesis. Next, we thoroughly explored the regulatory mechanism of the MDM2/HBP1 axis in DNA damage repair following ionizing radiation. Our data indicated that MDM2 overexpression-mediated repression of HBP1 delays DNA damage repair and causes cell death in a p53-independent manner. This investigation elucidated the mechanism of how MDM2 promotes genome instability and enhances tumorigenesis in the absence of p53, thus providing a theoretical and experimental basis for targeting MDM2 as a cancer therapy.

Double amplifications of CDK4 and MDM2 in a gastric inflammatory myofibroblastic tumor mimicking cancer with local invasion of the spleen and diaphragm

Inflammatory myofibroblastic tumor (IMT) is currently recognized as an intermediate mesenchymal neoplasm. It can arise anywhere in the body, but it is particularly common in the lungs. Gastric IMT is very rare in adults. In this study, we report a case of a 68-year-old woman with IMT in the gastric cardia, with invasion into the spleen and diaphragm. Because of its location and aggressive clinical features, it was first mistaken for gastric cancer. However, pathology and immunohistochemistry were used to finally confirm the diagnosis of IMT after total resection of the tumor and spleen and partial resection of the diaphragm. In order to provide better understanding of this rare tumor, targeted next-generation sequencing (NGS) and IHC were performed to assess genetic and protein abnormalities of the tumor. Both IHC and NGS were found to be negative for ALK or other gene fusions. However, double amplification of CDK4 and MDM2 were found by NGS, and IHC also found CDK4 and MDM2 to be positive. To the best of our knowledge, this is the first gastric IMT report to show double invasion of the spleen and the diaphragm, and double amplification of CDK4 and MDM2 in IMT are also reported for the first time. This genomic aberration with protein overexpression is the most likely tumorigenic driver of this rare and aggressive tumor.

Inhibition of Noncanonical Murine Double Minute 2 Homolog Abrogates Ocular Inflammation through NF-κB Suppression

Uveitis is estimated to account for 10% of all cases of blindness in the United States, including 30,000 new cases of legal blindness each year. Intraocular and oral corticosteroids are the effective mainstay treatment, but they carry the risk of serious long-term ocular and systemic morbidity. New noncorticosteroid therapies with a favorable side effect profile are necessary for the treatment of chronic uveitis, given the paucity of existing treatment choices. We have previously demonstrated that Nutlin-3, a small-molecule inhibitor of murine double minute 2 (MDM2) homolog, suppresses pathologic retinal angiogenesis through a p53-dependent mechanism, but the noncanonical p53-independent functions have not been adequately elucidated. Herein, we demonstrate an unanticipated function of MDM2 inhibition, where Nutlin-3 potently abrogates lipopolysaccharide-induced ocular inflammation. Furthermore, we identified a mechanism by which transcription and translation of NF-κB is mediated by MDM2, independent of p53, in ocular inflammation. Small-molecule MDM2 inhibition is a novel noncorticosteroid strategy for inhibiting ocular inflammation, which may potentially benefit patients with chronic uveitis.

Estrogen-activated MDM2 disrupts mammary tissue architecture through a p53-independent pathway

The Cancer Genome Atlas (TCGA) data indicate that high MDM2 expression correlates with all subtypes of breast cancer. Overexpression of MDM2 drives breast oncogenesis in the presence of wild-type or mutant p53 (mtp53). Importantly, estrogen-receptor positive (ER+) breast cancers overexpress MDM2 and estrogen mediates this expression. We previously demonstrated that this estrogen-MDM2 axis activates the proliferation of breast cancer cell lines T47D (mtp53 L194F) and MCF7 (wild-type p53) in a manner independent of increased degradation of wild-type p53 (ie, p53-independently). Herein we present data supporting the role of the estrogen-MDM2 axis in regulating cell proliferation and mammary tissue architecture of MCF7 and T47D cells in a p53-independent manner. Inducible shRNA mediated MDM2 knockdown inhibited colony formation in soft agar, decreased mass size and induced lumen formation in matrigel and also significantly reduced mitosis as seen by decreased phospho-histone H3 positive cells. The knockdown of MDM2 in both cell lines decreased Rb phosphorylation and the level of E2F1 protein. This signaling was through the estrogen receptor because fulvestrant (a selective estrogen receptor degrader) decreased MDM2 protein levels and decreased phosphorylation of Rb. Taken together these data indicate that in some ER+ breast cancers the estrogen-MDM2-Rb-E2F1 axis is a central hub for estrogen-mediated p53-independent signal transduction. This is the first indication that estrogen signaling utilizes the estrogen-MDM2 axis to provoke phosphorylation of Rb and increase E2F1 while promoting abnormal mammary architecture.

MDM2 and CDK4 amplifications are rare events in salivary duct carcinomas

Salivary duct carcinoma (SDC) is an aggressive adenocarcinoma of the salivary glands associated with poor clinical outcome. SDCs are known to carry TP53 mutations in about 50%, however, only little is known about alternative pathogenic mechanisms within the p53 regulatory network. Particularly, data on alterations of the oncogenes MDM2 and CDK4 located in the chromosomal region 12q13-15 are limited in SDC, while genomic rearrangements of the adjacent HMGA2 gene locus are well documented in subsets of SDCs. We here analyzed the mutational status of the TP53 gene, genomic amplification of MDM2, CDK4 and HMGA2 rearrangement/amplification as well as protein expression of TP53 (p53), MDM2 and CDK4 in 51 de novo and ex pleomorphic adenoma SDCs.

25 of 51 cases were found to carry TP53 mutations, associated with extreme positive immunohistochemical p53 staining levels in 13 cases. Three out of 51 tumors had an MDM2 amplification, one of them coinciding with a CDK4 amplification and two with a HMGA2 rearrangement/amplification. Two of the MDM2 amplifications occurred in the setting of a TP53 mutation. Two out of 51 cases showed a CDK4 amplification, one synchronously being MDM2 amplified and the other one displaying concurrent low copy number increases of both, MDM2 and HMGA2.

In summary, we here show that subgroups of SDCs display genomic amplifications of MDM2 and/or CDK4, partly in association with TP53 mutations and rearrangement/amplification of HMGA2. Further research is necessary to clarify the role of chromosomal region 12q13-15 alterations in SDC tumorigenesis and their potential prognostic and therapeutic relevance.

Mdm2 promotes Cdc25C protein degradation and delays cell cycle progression through the G2/M phase

Upon different types of stress, the gene encoding the mitosis-promoting phosphatase Cdc25C is transcriptionally repressed by p53, contributing to p53's enforcement of a G2 cell cycle arrest. In addition, Cdc25C protein stability is also decreased following DNA damage. Mdm2, another p53 target gene, encodes a ubiquitin ligase that negatively regulates p53 levels by ubiquitination. Ablation of Mdm2 by siRNA led to an increase in p53 protein and repression of Cdc25C gene expression. However, Cdc25C protein levels were actually increased following Mdm2 depletion. Mdm2 is shown to negatively regulate Cdc25C protein levels by reducing its half-life independently of the presence of p53. Further, Mdm2 physically interacts with Cdc25C and promotes its degradation through the proteasome in a ubiquitin-independent manner. Either Mdm2 overexpression or Cdc25C downregulation delays cell cycle progression through the G2/M phase. Thus, the repression of the Cdc25C promoter by p53, together with p53-dependent induction of Mdm2 and subsequent degradation of Cdc25C, could provide a dual mechanism by which p53 can enforce and maintain a G2/M cell cycle arrest.

Zinc-finger proteins in health and disease

Zinc-finger proteins (ZNFs) are one of the most abundant groups of proteins and have a wide range of molecular functions. Given the wide variety of zinc-finger domains, ZNFs are able to interact with DNA, RNA, PAR (poly-ADP-ribose) and other proteins. Thus, ZNFs are involved in the regulation of several cellular processes. In fact, ZNFs are implicated in transcriptional regulation, ubiquitin-mediated protein degradation, signal transduction, actin targeting, DNA repair, cell migration, and numerous other processes. The aim of this review is to provide a comprehensive summary of the current state of knowledge of this class of proteins. Firstly, we describe the actual classification of ZNFs, their structure and functions. Secondly, we focus on the biological role of ZNFs in the development of organisms under normal physiological and pathological conditions.

Mdm2 as a chromatin modifier

Mdm2 is the key negative regulator of the tumour suppressor p53, making it an attractive target for anti-cancer drug design. We recently identified a new role of Mdm2 in gene repression through its direct interaction with several proteins of the polycomb group (PcG) family. PcG proteins form polycomb repressive complexes PRC1 and PRC2. PRC2 (via EZH2) mediates histone 3 lysine 27 (H3K27) trimethylation, and PRC1 (via RING1B) mediates histone 2A lysine 119 (H2AK119) monoubiquitination. Both PRCs mostly support a compact and transcriptionally silent chromatin structure. We found that Mdm2 regulates a gene expression profile similar to that of PRC2 independent of p53. Moreover, Mdm2 promotes the stemness of murine induced pluripotent stem cells and human mesenchymal stem cells, and supports the survival of tumour cells. Mdm2 is recruited to target gene promoters by the PRC2 member and histone methyltransferase EZH2, and enhances PRC-dependent repressive chromatin modifications, specifically H3K27me3 and H2AK119ub1. Mdm2 also cooperates in gene repression with the PRC1 protein RING1B, a H2AK119 ubiquitin ligase. Here we discuss the possible implications of these p53-independent functions of Mdm2 in chromatin dynamics and in the stem cell phenotype. We propose that the p53-independent functions of Mdm2 should be taken into account for cancer drug design. So far, the majority of clinically tested Mdm2 inhibitors target its binding to p53 but do not affect the new functions of Mdm2 described here. However, when targeting the E3 ligase activity of Mdm2, a broader spectrum of its oncogenic activities might become druggable.

The role of MDM2 and MDM4 in breast cancer development and prevention

The major cause of death from breast cancer is not the primary tumour, but relapsing, drug-resistant, metastatic disease. Identifying factors that contribute to aggressive cancer offers important leads for therapy. Inherent defence against carcinogens depends on the individual molecular make-up of each person. Important molecular determinants of these responses are under the control of the mouse double minute (MDM) family: comprised of the proteins MDM2 and MDM4. In normal, healthy adult cells, the MDM family functions to critically regulate measured, cellular responses to stress and subsequent recovery. Proper function of the MDM family is vital for normal breast development, but also for preserving genomic fidelity. The MDM family members are best characterized for their negative regulation of the major tumour suppressor p53 to modulate stress responses. Their impact on other cellular regulators is emerging. Inappropriately elevated protein levels of the MDM family are highly associated with an increased risk of cancer incidence. Exploration of the MDM family members as cancer therapeutic targets is relevant for designing tailored anti-cancer treatments, but successful approaches must strategically consider the impact on both the target cancer and adjacent healthy cells and tissues. This review focuses on recent findings pertaining to the role of the MDM family in normal and malignant breast cells.

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.

Association of p73 gene G4C14-A4T14 polymorphism and MDM2 gene SNP309 with non-small cell lung cancer risk in a Chinese population

The present study aimed to investigate the association of p73 G4C14-A4T14 polymorphism and murine double minute 2 (MDM2) 309 T/G single nucleotide polymorphisms (SNPs) with the risk of developing non-small cell lung cancer (NSCLC) in Sothern China. The p73 and MDM2 genotypes of peripheral blood DNA from 186 patients with NSCLC and 196 normal controls were detected by polymerase chain reaction (PCR) with confronting two-pair primers (CTPP) and high resolution melting (HRM), respectively. The results of genotyping were consistent with those of direct sequencing. The p73 AT/AT [odds ratio (OR)=0.46; 95% confidence interval (CI)=0.22-0.97] and MDM2 TT (OR=0.48; 95% CI=0.26-0.86) genotypes were associated with a decreased risk of developing NSCLC compared with that of the p73 GC/GC and MDM2 GG genotypes, respectively. In addition, the interaction between the p73 and MDM2 polymorphisms reduced the risk of developing NSCLC in multiple ways (OR=0.13; 95% CI=0.03-0.59) for subjects carrying both the p73 AT/AT and MDM2 TT genotypes. Therefore, the SNP in p73 G4C14-A4T14 and the MDM2 309 polymorphism may be markers of genetic susceptibility to NSCLC in a Chinese population, and there is a possible gene-gene interaction involved in the incidence of NSCLC.

Role of Mdm2 and Mdmx in DNA repair

Mdm2 and Mdmx are critical regulators of the p53 tumour suppressor and are overexpressed in many human malignancies. However, in recent years, their impact on genome instability was shown to be at least, in part, independent of p53. Both Mdm2 and Mdmx inhibit DNA break repair through their association with the Mre11/Rad50/Nbs1 DNA repair complex. Recent evidence indicates that harnessing Mdm2 and/or Mdmx-mediated inhibition of DNA break repair in cancer cells could provide a therapeutic opportunity, particularly for those malignancies that have lost functional p53.

MDM2 and HIF1alpha expression levels in different histologic subtypes of malignant pleural mesothelioma: correlation with pathological and clinical data

Malignant pleural mesothelioma (MPM) is an aggressive tumor with poor prognosis and limited treatment options. Sarcomatoid/biphasic mesotheliomas are characterized by more aggressive behaviour and a poorer prognosis compared with the epithelioid subtype. To date prognostic and tailored therapeutic biomarkers are lacking. The present study analyzed the expression levels of MDM2 and HIF1alpha in different histologic subtypes from chemonaive MPM patients. Diagnostic biopsies of MPM patients from four Italian cancer centers were centrally collected and analyzed. MDM2 and HIF1alpha expression levels were investigated through immunohistochemistry and RT-qPCR. Pathological assessment of necrosis, inflammation and proliferation index was also performed. Molecular markers, pathological features and clinical characteristics were correlated to overall survival (OS) and progression free survival (PFS). Sixty MPM patients were included in the study (32 epithelioid and 28 non-epithelioid). Higher levels of MDM2 (p < 0.001), HIF1alpha (p = 0.013), necrosis (p = 0.013) and proliferation index (p < 0.001) were seen mainly in sarcomatoid/biphasic subtypes. Higher levels of inflammation were significantly associated with epithelioid subtype (p = 0.044). MDM2 expression levels were correlated with HIF1alpha levels (p = 0.0001), necrosis (p = 0.008) and proliferation index (p = 0.009). Univariate analysis showed a significant correlation of non-epithelioid histology (p = 0.04), high levels of necrosis (p = 0.037) and proliferation index (p = 0.0002) with shorter PFS. Sarcomatoid/biphasic and epithelioid mesotheliomas showed different MDM2 and HIF1alpha expression levels and were characterized by different levels of necrosis, proliferation and inflammation. Further studies are warranted to confirm a prognostic and predictive role of such markers and features.

Relationship Between Murine Double Minute 2 (MDM2) T309G Polymorphism and Endometrial Cancer Risk: A Meta-Analysis

Background

Endometrial cancer is one of the most common cancers in female patients. Many studies have investigated the association between the MDM2 T309G genotype and endometrial cancer incidence, but the results have been inconclusive.

Material/Methods

We performed a systematic search in PubMed and Web of Science databases (update until October 21, 2015) for all English-language publications. The associations are indicated as pooled odds ratio (OR) and 95% confidence intervals (CI).

Results

We identified 8 relevant publications (9 case-control studies), including 2188 cases and 4654 controls, that assessed the relationship between MDM2 T309G polymorphism and endometrial cancer risk. There was a significant association between MDM2 T309G polymorphism and endometrial cancer risk in the overall population in the recessive model (OR=1.61; 95% CI: 1.19–2.19; P=0.002). In the subgroup of different ethnic populations, the subgroup analysis showed MDM2 T309G polymorphism was significantly associated with increased endometrial cancer risk in Caucasians (OR=1.75; 95% CI: 1.16–2.63; P=0.007). No similar result was found in Asians.

Conclusions

Our meta-analysis provides evidence that MDM2 T309G polymorphism is associated with endometrial cancer, especially in Caucasians.

Genome-scale functional analysis of the human genes modulating p53 activity by regulating MDM2 expression in a p53-independent manner

MDM2, a critical negative regulator of p53, is often overexpressed in leukemia, but few p53 mutations are found, suggesting that p53-independent MDM2 expression occurs due to alterations in MDM2 upstream regulators. In this study, a high MDM2 transcription level was observed (41.17%) regardless of p53 expression in patient with acute myeloid leukemia (AML). Therefore, we performed genome-scale functional screening of the human genes modulating MDM2 expression in a p53-independent manner. We searched co-expression profiles of genes showing a positive or negative pattern with MDM2 expression in a DNA microarray database, selected1089 links, and composed a screening library of 368 genes. Using MDM2 P1 and P2 promoter-reporter systems, we screened clones regulating MDM2 transcriptions in a p53-independent manner by overexpression. Nine clones from the screening library showed enhanced MDM2 promoter activity and MDM2 expression in p53-deficient HCT116 cells. Among them, six clones, including NTRK2, GNA15, SFRS2, EIF5A, ELAVL1, and YWHAB mediated MAPK signaling for expressing MDM2. These results indicate that p53-independent upregulation of MDM2 by increasing selected clones may lead to oncogenesis in AML and that MDM2-modulating genes are novel potential targets for AML treatment.

The Role of MDM2 Amplification and Overexpression in Tumorigenesis

Mouse double minute 2 (MDM2) is a critical negative regulator of the tumor suppressor p53, playing a key role in controlling its transcriptional activity, protein stability, and nuclear localization. MDM2 expression is up-regulated in numerous cancers, resulting in a loss of p53-dependent activities, such as apoptosis and cell-cycle arrest. Genetic amplification and inheritance of MDM2 promoter single-nucleotide polymorphisms (SNPs) are the two best-studied mechanisms for up-regulating MDM2 activity. This article provides an overview of these events in human cancer, highlighting the frequent occurrence of MDM2 amplification in sarcoma and the role of SNP309 and SNP285 in regulating MDM2 expression and cancer risk. The availability of large-scale genomic profiling datasets, like those from The Cancer Genome Atlas Research Network, have provided the opportunity to evaluate the consequences of MDM2 amplification and SNP inheritance across high-quality tumor samples from diverse cancer indications.

Genome Stability Requires p53

It is now clear that functional p53 is critical to protect the genome from alterations that lead to tumorigenesis. However, with the myriad of cellular stresses and pathways linked to p53 activation, much remains unknown about how p53 maintains genome stability and the proteins involved. The current understanding of the multiple ways p53 contributes to genome stability and how two of its negative regulators, Mdm2 and Mdmx, induce genome instability will be described.

Murine double minute 2, a potential p53-independent regulator of liver cancer metastasis

Hepatocellular carcinoma (HCC) has emerged as one of the most commonly diagnosed forms of human cancer; yet, the mechanisms underlying HCC progression remain unclear. Unlike other cancers, systematic chemotherapy is not effective for HCC patients, while surgical resection and liver transplantation are the most viable treatment options. Thus, identifying factors or pathways that suppress HCC progression would be crucial for advancing treatment strategies for HCC. The murine double minute 2 (MDM2)-p53 pathway is impaired in most of the cancer types, including HCC, and MDM2 is overexpressed in approximately 30% of HCC. Overexpression of MDM2 is reported to be well correlated with metastasis, drug resistance, and poor prognosis of multiple cancer types, including HCC. Importantly, these correlations are observed even when p53 is mutated. Indeed, p53-independent functions of overexpressed MDM2 in cancer progression have been suitably demonstrated. In this review article, we summarize potential effectors of MDM2 that promote or suppress cancer metastasis and discuss the p53-independent roles of MDM2 in liver cancer metastasis from clinical as well as biological perspectives.

Estrogen receptor alpha (ERα/ESR1) mediates the p53-independent overexpression of MDM4/MDMX and MDM2 in human breast cancer

MDM2 and MDM4 are heterodimeric, non-redundant oncoproteins that potently inhibit the p53 tumor suppressor protein. MDM2 and MDM4 also enhance the tumorigenicity of breast cancer cells in in vitro and in vivo models and are overexpressed in primary human breast cancers. Prior studies have characterized Estrogen Receptor Alpha (ERα/ESR1) as a regulator of MDM2 expression and an MDM2- and p53-interacting protein. However, similar crosstalk between ERα and MDM4 has not been investigated. Moreover, signaling pathways that mediate the overexpression of MDM4 in human breast cancer remain to be elucidated. Using the Cancer Genome Atlas (TCGA) breast invasive carcinoma patient cohort, we have analyzed correlations between ERα status and MDM4 and MDM2 expression in primary, treatment-naïve, invasive breast carcinoma samples. We report that the expression of MDM4 and MDM2 is elevated in primary human breast cancers of luminal A/B subtypes and associates with ERα-positive disease, independently of p53 mutation status. Furthermore, in cell culture models, ERα positively regulates MDM4 and MDM2 expression via p53-independent mechanisms, and these effects can be blocked by the clinically-relevant endocrine therapies fulvestrant and tamoxifen. Additionally, ERα also positively regulates p53 expression. Lastly, we report that endogenous MDM4 negatively regulates ERα expression and forms a protein complex with ERα in breast cancer cell lines and primary human breast tumor tissue. This suggests direct signaling crosstalk and negative feedback loops between ERα and MDM4 expression in breast cancer cells. Collectively, these novel findings implicate ERα as a central component of the p53-MDM2-MDM4 signaling axis in human breast cancer.

The Roles of MDM2 and MDMX in Cancer

For more than 25 years, MDM2 and its homolog MDMX (also known as MDM4) have been shown to exert oncogenic activity. These two proteins are best understood as negative regulators of the p53 tumor suppressor, although they may have additional p53-independent roles. Understanding the dysregulation of MDM2 and MDMX in human cancers and how they function either together or separately in tumorigenesis may improve methods of diagnosis and for assessing prognosis. Targeting the proteins themselves, or their regulators, may be a promising therapeutic approach to treating some forms of cancer.

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.

A 40-bp insertion/deletion polymorphism of Murine Double Minute2 (MDM2) increased the risk of breast cancer in Zahedan, Southeast Iran

Background: MDM2 (Murine Double Minute2) is an oncoprotein that inhibits the P53 activity. Overexpression of MDM2 gene has been reported in several human tumors. In the present study, we aimed to evaluate the impact of 40-bp insertion/deletion (ins/del) polymorphism on the promoter of MDM2 and susceptibility to breast cancer in a sample of Iranian population. Methods: This case-control study was carried out on 236 patients with breast cancer and 203 healthy individuals. Genomic DNA was extracted from the whole blood by the salting-out method. The 40-bp ins/del polymorphism was determined by using polymerase chain reaction. Results: The findings indicated that MDM2 ins/del variant increased the risk of breast cancer in co-dominant- (odds ratio [OR] = 2.09, 95% CI = 1.14-3.85, P = 0.018, del/del vs. ins/ins), dominant- (OR = 1.49, 95% CI = 1.02-2.18, P = 0.038, ins/del + del/del vs. ins/ins), and recessive- (OR = 1.86, 95% CI = 1.03-3.34, P = 0.038, del/del vs. ins/ins + ins/del) tested inheritance models. The del allele increased the risk of breast cancer (OR = 1.48, 95% CI = 1.11-1.98, P = 0.008) compared with ins allele. Conclusions: Our result revealed that 40-bp ins/del polymorphism in the promoter of MDM2 increased the risk of breast cancer in an Iranian population. Further investigations with larger sample sizes and diverse ethnicities are needed to verify our findings.

Mdm2 overexpression and p73 loss exacerbate genomic instability and dampen apoptosis, resulting in B-cell lymphoma

Many human tumors express high levels of the p53 inhibitor Mdm2, resulting from amplification of the Mdm2 locus or aberrant post-translational regulation of the Mdm2 protein. While the importance of Mdm2 in regulating p53 is clear, Mdm2 also has p53-independent roles. For example, overexpression of Mdm2 results in genomic instability in a p53-independent manner. In addition, Mdm2 has many additional binding partners; some such as the tumor suppressor p73 have also been implicated in genomic instability. In this study, cells and tumors with Mdm2 overexpression and p73 loss exhibit increased genomic instability as compared to either alteration alone and cooperate in development of B-cell lymphomagenesis. Cytogenetic analyses of mouse embryonic fibroblasts and pre-malignant B-cells demonstrate that loss of p73 exacerbates the chromosome breaks and fusions observed in Mdm2^Tg cells. B-cell lymphomas from Mdm2^Tg;p73+/− mice retain the remaining p73 allele, and exhibit elevated levels of the anti-apoptotic protein Bcl2 and thus dampen apoptosis. In summary, Mdm2 overexpression and p73 loss cooperate in genomic instability and tumor development, indicating that the oncogenic functions of Mdm2 are a combined effect of inhibiting p53 and p73 functions. Given that p73 is lost or silenced in human B-cell lymphomas [1–4], the Mdm2^Tg;p73+/− mouse serves as a model for human disease and may provide additional insight into the pathways that contribute to B-cell lymphomagenesis.

Mdmx promotes genomic instability independent of p53 and Mdm2

The oncogene Mdmx is overexpressed in many human malignancies, and together with Mdm2, negatively regulates the p53 tumor suppressor. However, a p53-independent function of Mdmx that impacts genome stability has been described, but this function is not well understood. In the present study, we determined that of the 13 different cancer types evaluated, 6-90% of those that had elevated levels of Mdmx had concurrent inactivation (mutated or deleted) of p53. We show elevated levels of Mdmx-inhibited double-strand DNA break repair and induced chromosome and chromatid breaks independent of p53, leading to genome instability. Mdmx impaired early DNA damage-response signaling, such as phosphorylation of the serine/threonine-glutamine motif, mediated by the ATM kinase. Moreover, we identified Mdmx associated with Nbs1 of the Mre11-Rad50-Nbs1 (MRN) DNA repair complex, and this association increased upon DNA damage and was detected at chromatin. Elevated Mdmx levels also increased cellular transformation in a p53-independent manner. Unexpectedly, all Mdmx-mediated phenotypes also occurred in cells lacking Mdm2 and were independent of the Mdm2-binding domain (RING) of Mdmx. Therefore, Mdmx-mediated inhibition of the DNA damage response resulted in delayed DNA repair and increased genome instability and transformation independent of p53 and Mdm2. Our results reveal a novel p53- and Mdm2-independent oncogenic function of Mdmx that provides new insight into the many cancers that overexpress Mdmx.

The type 7 serotonin receptor, 5-HT 7 , is essential in the mammary gland for regulation of mammary epithelial structure and function

Autocrine-paracrine activity of serotonin (5-hydroxytryptamine, 5-HT) is a crucial homeostatic parameter in mammary gland development during lactation and involution. Published studies suggested that the 5-HT₇ receptor type was important for mediating several effects of 5-HT in the mammary epithelium. Here, using 5-HT₇ receptor-null (HT7KO) mice we attempt to understand the role of this receptor in mediating 5-HT actions within the mammary gland. We demonstrate for the first time that HT7KO dams are inefficient at sustaining their pups. Histologically, the HT7KO mammary epithelium shows a significant deviation from the normal secretory epithelium in morphological architecture, reduced secretory vesicles, and numerous multinucleated epithelial cells with atypically displaced nuclei, during lactation. Mammary epithelial cells in HT7KO dams also display an inability to transition from lactation to involution as normally seen by transition from a columnar to a squamous cell configuration, along with alveolar cell apoptosis and cell shedding. Our results show that 5-HT₇ is required for multiple actions of 5-HT in the mammary glands including core functions that contribute to changes in cell shape and cell turnover, as well as specialized secretory functions. Understanding these actions may provide new interventions to improve lactation performance and treat diseases such as mastitis and breast cancer.

Stress-induced alternative splice forms of MDM2 and MDMX modulate the p53-pathway in distinct ways

MDM2 and MDMX are the chief negative regulators of the tumor-suppressor protein p53 and are essential for maintaining homeostasis within the cell. In response to genotoxic stress and also in several cancer types, MDM2 and MDMX are alternatively spliced. The splice variants MDM2-ALT1 and MDMX-ALT2 lack the p53-binding domain and are incapable of negatively regulating p53. However, they retain the RING domain that facilitates dimerization of the full-length MDM proteins. Concordantly, MDM2-ALT1 has been shown to lead to the stabilization of p53 through its interaction with and inactivation of full-length MDM2. The impact of MDM2-ALT1 expression on the p53 pathway and the nature of its interaction with MDMX remain unclear. Also, the role of the architecturally similar MDMX-ALT2 and its influence of the MDM2-MDMX-p53 axis are yet to be elucidated. We show here that MDM2-ALT1 is capable of binding full-length MDMX as well as full-length MDM2. Additionally, we demonstrate that MDMX-ALT2 is able to dimerize with both full-length MDMX and MDM2 and that the expression of MDM2-ALT1 and MDMX-ALT2 leads to the upregulation of p53 protein, and also of its downstream target p21. Moreover, MDM2-ALT1 expression causes cell cycle arrest in the G1 phase in a p53 and p21 dependent manner, which is consistent with the increased levels of p21. Finally we present evidence that MDM2-ALT1 and MDMX-ALT2 expression can activate subtly distinct subsets of p53-transcriptional targets implying that these splice variants can modulate the p53 tumor suppressor pathway in unique ways. In summary, our study shows that the stress-inducible alternative splice forms MDM2-ALT1 and MDMX-ALT2 are important modifiers of the p53 pathway and present a potential mechanism to tailor the p53-mediated cellular stress response.

ROS and p53 in regulation of UVB-induced HDM2 alternative splicing

Alternative splicing plays an important role in proteasome diversity and gene expression regulation in eukaryotic cells. Hdm2, the human homolog of mdm2 (murine double minute oncogene 2), is known to be an oncogene as its role in suppression of p53. Hdm2 alternative splicing, occurs in both tumor and normal tissues, is believed to be a response of cells for cellular stress, and thus modulate p53 activity. Therefore, understanding the regulation of hdm2 splicing is critical in elucidating the mechanisms of tumor development and progression. In this study, we determined the effect of ultraviolet B light (UVB) on alternative splicing of hdm2. Our data indicated that UVB (50 mJ cm(-2)) alone is not a good inducer of alternative splicing of hdm2. The less effectiveness could be due to the induction of ROS and p53 by UVB because removing ROS by L-NAC (10 mm) in p53 null cells could lead to alternative splicing of hdm2 upon UVB irradiation.

Stress-induced isoforms of MDM2 and MDM4 correlate with high-grade disease and an altered splicing network in pediatric rhabdomyosarcoma

Pediatric rhabdomyosarcoma (RMS) is a morphologically and genetically heterogeneous malignancy commonly classified into three histologic subtypes, namely, alveolar, embryonal, and anaplastic. An issue that continues to challenge effective RMS patient prognosis is the dearth of molecular markers predictive of disease stage irrespective of tumor subtype. Our study involving a panel of 70 RMS tumors has identified specific alternative splice variants of the oncogenes Murine Double Minute 2 (MDM2) and MDM4 as potential biomarkers for RMS. Our results have demonstrated the strong association of genotoxic-stress inducible splice forms MDM2-ALT1 (91.6% Intergroup Rhabdomyosarcoma Study Group stage 4 tumors) and MDM4-ALT2 (90.9% MDM4-ALT2-positive T2 stage tumors) with high-risk metastatic RMS. Moreover, MDM2-ALT1-positive metastatic tumors belonged to both the alveolar (50%) and embryonal (41.6%) subtypes, making this the first known molecular marker for high-grade metastatic disease across the most common RMS subtypes. Furthermore, our results show that MDM2-ALT1 expression can function by directly contribute to metastatic behavior and promote the invasion of RMS cells through a matrigel-coated membrane. Additionally, expression of both MDM2-ALT1 and MDM4-ALT2 increased anchorage-independent cell-growth in soft agar assays. Intriguingly, we observed a unique coordination in the splicing of MDM2-ALT1 and MDM4-ALT2 in approximately 24% of tumor samples in a manner similar to genotoxic stress response in cell lines. To further explore splicing network alterations with possible relevance to RMS disease, we used an exon microarray approach to examine stress-inducible splicing in an RMS cell line (Rh30) and observed striking parallels between stress-responsive alternative splicing and constitutive splicing in RMS tumors.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

Mdm2 and MdmX involvement in human cancer

Discovered in 1987 and 1997 respectively, Mdm2 and MdmX represent two critical cellular regulators of the p53 tumor suppressor. This chapter reviews each from initial discovery to our current understanding of their deregulation in human cancer with a focus on how each regulator impacts p53 function. While p53 independent activities of Mdm2 and MdmX are noted the reader is directed to other reviews on this topic. The chapter concludes with an examination of the various mechanisms of Mdm-deregulation and an assessment of the current therapeutic approaches to target Mdm2 and MdmX overexpression.

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.

Inhibition of PI3K/mTOR overcomes nilotinib resistance in BCR-ABL1 positive leukemia cells through translational down-regulation of MDM2

Chronic myeloid leukemia (CML) is a cytogenetic disorder resulting from formation of the Philadelphia chromosome (Ph), that is, the t(9;22) chromosomal translocation and the formation of the BCR-ABL1 fusion protein. Tyrosine kinase inhibitors (TKI), such as imatinib and nilotinib, have emerged as leading compounds with which to treat CML. t(9;22) is not restricted to CML, 20-30% of acute lymphoblastic leukemia (ALL) cases also carry the Ph. However, TKIs are not as effective in the treatment of Ph+ ALL as in CML. In this study, the Ph+ cell lines JURL-MK2 and SUP-B15 were used to investigate TKI resistance mechanisms and the sensitization of Ph+ tumor cells to TKI treatment. The annexin V/PI (propidium iodide) assay revealed that nilotinib induced apoptosis in JURL-MK2 cells, but not in SUP-B15 cells. Since there was no mutation in the tyrosine kinase domain of BCR-ABL1 in cell line SUP-B15, the cells were not generally unresponsive to TKI, as evidenced by dephosphorylation of the BCR-ABL1 downstream targets, Crk-like protein (CrkL) and Grb-associated binder-2 (GAB2). Resistance to apoptosis after nilotinib treatment was accompanied by the constitutive and nilotinib unresponsive activation of the phosphoinositide 3-kinase (PI3K) pathway. Treatment of SUP-B15 cells with the dual PI3K/mammalian target of rapamycin (mTOR) inhibitor BEZ235 alone induced apoptosis in a low percentage of cells, while combining nilotinib and BEZ235 led to a synergistic effect. The main role of PI3K/mTOR inhibitor BEZ235 and the reason for apoptosis in the nilotinib-resistant cells was the block of the translational machinery, leading to the rapid downregulation of the anti-apoptotic protein MDM2 (human homolog of the murine double minute-2). These findings highlight MDM2 as a potential therapeutic target to increase TKI-mediated apoptosis and imply that the combination of PI3K/mTOR inhibitor and TKI might form a novel strategy to combat TKI-resistant BCR-ABL1 positive leukemia.

MDM2 binding protein, a novel metastasis suppressor

MDM2 binding protein (MTBP) is a protein that interacts with oncoprotein murine double minute (MDM2), a major inhibitor of the tumor suppressor p53. Overexpression of MTBP leads to p53-independent cell proliferation arrest, which is in turn blocked by simultaneous overexpression of MDM2. Importantly, reduced expression of MTBP in mice increases tumor metastasis and enhances migratory potential of mouse embryonic fibroblasts regardless of the presence of p53. Clinically, loss of MTBP expression in head and neck squamous cell carcinoma is associated with reduced patient survival, and is shown to serve as an independent prognostic factor when p53 is mutated in tumors. These results indicate the involvement of MTBP in suppressing tumor progression. Our recent findings demonstrate that overexpression of MTBP in human osteosarcoma cells lacking wild-type p53 inhibits metastasis, but not primary tumor growth, when cells are transplanted in femurs of immunocompromised mice. These data indicate that MTBP functions as a metastasis suppressor independent of p53 status. Furthermore, overexpression of MTBP suppresses cell migration and filopodia formation, in part, by inhibiting function of an actin crosslinking protein α-actinin-4. Thus, increasing evidence indicates the significance of MTBP in tumor progression. We summarize published results related to MTBP function and discuss caveats and future directions in this review article.

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.

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

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

MDM2 SNP309 rs2279744 polymorphism and gastric cancer risk: a meta-analysis

Background

MDM2 is a major negative regulator of p53, and a single nucleotide polymorphism (SNP) in the MDM2 promoter region SNP309 has been demonstrated to be associated with an increased MDM2 expression and a significantly earlier age of onset of several tumors, including gastric cancer. Several studies were published to evaluate the association between SNP309 and gastric cancer risk. However, the results remain conflicting rather than conclusive.

Objective

The aim of this study was to assess the association between the MDM2 SNP309 polymorphism and gastric risk.

Methods

We performed a meta-analysis to investigate this relationship. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to assess the strength of the association. The pooled ORs were performed for codominant model, dominant model, and recessive model, respectively.

Results

Five published case-control studies, including 1,621 gastric cancer cases and 2,639 controls were identified. We found that the MDM2 SNP309 polymorphism was associated with a significantly increased risk of gastric cancer risk when all studies were pooled into the meta-analysis (GG versus TT, OR = 1.54; 95%CI = 1.04–2.29, and GG versus GT/TT, OR = 1.49, 95%CI = 1.30–1.72). Furthermore, Egger's test did not show any evidence of publication bias (P = 0.799 for GG versus TT).

Conclusion

Our results suggest that the MDM2 SNP309 polymorphism may be a low-penetrant risk factor for the development of gastric cancer.

MDM2, MDMX and p53 in oncogenesis and cancer therapy

The MDM2 and MDMX (also known as HDMX and MDM4) proteins are deregulated in many human cancers and exert their oncogenic activity predominantly by inhibiting the p53 tumour suppressor. However, the MDM proteins modulate and respond to many other signalling networks in which they are embedded. Recent mechanistic studies and animal models have demonstrated how functional interactions in these networks are crucial for maintaining normal tissue homeostasis, and for determining responses to oncogenic and therapeutic challenges. This Review highlights the progress made and pitfalls encountered as the field continues to search for MDM-targeted antitumour agents.

Molecular pathways: targeting Mdm2 and Mdm4 in cancer therapy

The p53 tumor suppressor is activated in response to cellular stresses to induce cell-cycle arrest, cellular senescence, and apoptosis. The p53 gene is inactivated by mutations in more than 50% of human tumors. In addition, tumor cells dampen p53 activities via overexpression of p53-negative regulators, in particular 2 structurally related proteins, Mdm2 and Mdm4. And yet, Mdm2 and Mdm4 possess p53-independent activities, which also contribute to tumor formation and progression. Given that Mdm2 and Mdm4 inhibit p53 activities to promote tumor development, small molecules and peptides were developed to abrogate the inhibition of p53 by Mdm proteins. Antitumor activities of these molecules have already been confirmed in preclinical studies and early-phase clinical trials. These research endeavors and clinical advances constitute the main focus of this review.