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Yanping Zhang - One of the best experts on this subject based on the ideXlab platform.
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protection against high fat diet induced obesity in Mdm2c305f mice due to reduced p53 activity and enhanced energy expenditure
2017Co-Authors: Derek A Franklin, Yanping ZhangAbstract:Summary The RPL11-Mdm2 interaction constitutes a p53 signaling pathway activated by deregulated ribosomal biosynthesis in response to stress. Mice bearing an Mdm2 C305F mutation that disrupts RPL11-Mdm2 binding were analyzed on a high-fat diet (HFD). The Mdm2 C305F/C305F mice, although phenotypically indistinguishable from wild-type (WT) mice when fed normal chow, demonstrated decreased fat accumulation along with improved insulin sensitivity and glucose tolerance after prolonged HFD feeding. We found that HFD increases expression of c-MYC and RPL11 in both WT and Mdm2 C305F/C305F mice; however, p53 was induced in WT but not in Mdm2 C305F/C305F mice. Reduced p53 activity in HFD-fed Mdm2 C305F/C305F mice resulted in higher levels of p53 downregulated targets GLUT4 and SIRT1, leading to increased biosynthesis of NAD + , and increased energy expenditure. Our study reveals a role for the RPL11-Mdm2-p53 pathway in fat storage during nutrient excess and suggests that targeting this pathway may be a potential treatment for obesity.
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the Mdm2 ring domain and central acidic domain play distinct roles in Mdm2 protein homodimerization and Mdm2 mdmx protein heterodimerization
2015Co-Authors: Patrick L Leslie, Yanping ZhangAbstract:The oncoprotein murine double minute 2 (Mdm2) is an E3 ligase that plays a prominent role in p53 suppression by promoting its polyubiquitination and proteasomal degradation. In its active form, Mdm2 forms homodimers as well as heterodimers with the homologous protein murine double minute 4 (MDMX), both of which are thought to occur through their respective C-terminal RING (really interesting new gene) domains. In this study, using multiple Mdm2 mutants, we show evidence suggesting that Mdm2 homo- and heterodimerization occur through distinct mechanisms because Mdm2 RING domain mutations that inhibit Mdm2 interaction with MDMX do not affect Mdm2 interaction with WT Mdm2. Intriguingly, deletion of a portion of the Mdm2 central acidic domain selectively inhibits interaction with Mdm2 while leaving intact the ability of Mdm2 to interact with MDMX and to ubiquitinate p53. Further analysis of an Mdm2 C-terminal deletion mutant reveals that the C-terminal residues of Mdm2 are required for both Mdm2 and MDMX interaction. Collectively, our results suggest a model in which Mdm2-MDMX heterodimerization requires the extreme C terminus and proper RING domain structure of Mdm2, whereas Mdm2 homodimerization requires the extreme C terminus and the central acidic domain of Mdm2, suggesting that Mdm2 homo- and heterodimers utilize distinct Mdm2 domains. Our study is the first to report mutations capable of separating Mdm2 homo- and heterodimerization.
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cancer associated mutations in the Mdm2 zinc finger domain disrupt ribosomal protein interaction and attenuate Mdm2 induced p53 degradation
2007Co-Authors: Mikael S Lindstrom, Gabrielle White Wolf, Aiwen Jin, Chad Deisenroth, Yanping ZhangAbstract:The p53-inhibitory function of the oncoprotein Mdm2 is regulated by a number of Mdm2-binding proteins, including ARF and ribosomal proteins L5, L11, and L23, which bind the central acidic domain of Mdm2 and inhibit its E3 ubiquitin ligase activity. Various human cancer-associated Mdm2 alterations targeting the central acidic domain have been reported, yet the functional significance of these mutations in tumor development has remained unclear. Here, we show that cancer-associated missense mutations targeting Mdm2's central zinc finger disrupt the interaction of Mdm2 with L5 and L11. We found that the zinc finger mutant Mdm2 is impaired in undergoing nuclear export and proteasomal degradation as well as in promoting p53 degradation, yet retains the function of suppressing p53 transcriptional activity. Unlike the wild-type Mdm2, whose p53-suppressive activity can be inhibited by L11, the Mdm2 zinc finger mutant escapes L11 inhibition. Hence, the Mdm2 central zinc finger plays a critical role in mediating Mdm2's interaction with ribosomal proteins and its ability to degrade p53, and these roles are disrupted by human cancer-associated Mdm2 mutations.
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regulation of the Mdm2 p53 pathway by ribosomal protein l11 involves a post ubiquitination mechanism
2006Co-Authors: Mu Shui Dai, Yanping Zhang, Yetao Jin, Dingding Shi, Xiao Xin Sun, Steven R GrossmanAbstract:Inhibition of the Mdm2-p53 feedback loop is critical for p53 activation in response to cellular stresses. The ribosomal proteins L5, L11, and L23 can block this loop by inhibiting Mdm2-mediated p53 ubiquitination and degradation in response to ribosomal stress. Here, we show that L11, but not L5 and L23, leads to a drastic accumulation of ubiquitinated and native Mdm2. This effect is dependent on the ubiquitin ligase activity of Mdm2, but not p53, and requires the central Mdm2 binding domain (residues 51-108) of L11. We further show that L11 inhibited 26 S proteasome-mediated degradation of ubiquitinated Mdm2 in vitro and consistently prolonged the half-life of Mdm2 in cells. These results suggest that L11, unlike L5 and L23, differentially regulates the levels of ubiquitinated p53 and Mdm2 and inhibits the turnover and activity of Mdm2 through a post-ubiquitination mechanism.
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ribosomal protein l11 negatively regulates oncoprotein Mdm2 and mediates a p53 dependent ribosomal stress checkpoint pathway
2003Co-Authors: Yanping Zhang, Gabrielle White Wolf, Krishna P Bhat, Aiwen Jin, Theresa Allio, William Burkhart, Yue XiongAbstract:The gene encoding p53 mediates a major tumor suppression pathway that is frequently altered in human cancers. p53 function is kept at a low level during normal cell growth and is activated in response to various cellular stresses. The Mdm2 oncoprotein plays a key role in negatively regulating p53 activity by either direct repression of p53 transactivation activity in the nucleus or promotion of p53 degradation in the cytoplasm. DNA damage and oncogenic insults, the two best-characterized p53-dependent checkpoint pathways, both activate p53 through inhibition of Mdm2. Here we report that the human homologue of Mdm2, HDM2, binds to ribosomal protein L11. L11 binds a central region in HDM2 that is distinct from the ARF binding site. We show that the functional consequence of L11-HDM2 association, like that with ARF, results in the prevention of HDM2-mediated p53 ubiquitination and degradation, subsequently restoring p53-mediated transactivation, accumulating p21 protein levels, and inducing a p53-dependent cell cycle arrest by canceling the inhibitory function of HDM2. Interference with ribosomal biogenesis by a low concentration of actinomycin D is associated with an increased L11-HDM2 interaction and subsequent p53 stabilization. We suggest that L11 functions as a negative regulator of HDM2 and that there might exist in vivo an L11-HDM2-p53 pathway for monitoring ribosomal integrity.
Ruiwen Zhang - One of the best experts on this subject based on the ideXlab platform.
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identification of a new class of natural product Mdm2 inhibitor in vitro and in vivo anti breast cancer activities and target validation
2015Co-Authors: Wei Wang, Hui Wang, Sukesh Voruganti, Weidong Zhang, Ruiwen ZhangAbstract:// Jiang-Jiang Qin 1,* , Wei Wang 1,2,* , Sukesh Voruganti 1 , Hui Wang 3 , Wei-Dong Zhang 4 and Ruiwen Zhang 1,2 1 Department of Pharmaceutical Sciences, School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, TX, USA 2 Cancer Biology Center, School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, TX, USA 3 Institute for Nutritional Sciences, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, PR China 4 School of Pharmacy, Shanghai Jiao Tong University, Shanghai, PR China * These authors contributed equally to this work Correspondence: Ruiwen Zhang, email: // Keywords : Mdm2 inhibitor, p53-independent, breast cancer, lung metastasis Received : November 20, 2014 Accepted : December 25, 2014 Published : December 30, 2014 Abstract The Mdm2 oncogene has been suggested as a molecular target for treating human cancers, including breast cancer. Most Mdm2 inhibitors under development are targeting the Mdm2-p53 binding, and have little or no effects on cancers without functional p53, such as advanced breast cancer. The present study was designed to develop a new class of Mdm2 inhibitors that exhibit anticancer activity in Mdm2-dependent and p53-independent manners. The selective Mdm2 inhibitors were discovered by a computational structure-based screening, yielding a lead compound, termed JapA. We further found that JapA inhibited cell growth, decreased cell proliferation, and induced G2/M phase arrest and apoptosis in breast cancer cells through an Mdm2-dependent mechanism, regardless of p53 status. It also inhibited the tumor growth and lung metastasis in breast cancer xenograft models without causing any host toxicity. Furthermore, JapA directly bound to Mdm2 protein and reduced Mdm2 levels in cancer cells in vitro and in vivo by promoting Mdm2 protein degradation and inhibiting Mdm2 transcription, which is distinct from the existing Mdm2 inhibitors. In conclusion, JapA represents a new class of Mdm2 inhibitor that exerts its anticancer activity through directly down-regulating Mdm2, and might be developed as a novel cancer therapeutic agent.
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identification of ribosomal protein s25 rps25 Mdm2 p53 regulatory feedback loop
2013Co-Authors: Wei Wang, Hui Wang, Xu Zhang, Minghai Wang, Ruiwen ZhangAbstract:There is an increasing interest in determining the role of ribosomal proteins (RPs) in the regulation of Mdm2-p53 pathway in coordinating cellular response to stress. Herein, we report a novel regulatory role of ribosomal protein S25 (RPS25) in Mdm2-mediated p53 degradation and a feedback regulation of S25 by p53. We demonstrated that S25 interacted with Mdm2 and inhibited its E3 ligase activity, resulting in the reduction of Mdm2-mediated p53 ubiquitination and the stabilization and activation of p53. S25, Mdm2 and p53 formed a ternary complex following ribosomal stress. The nucleolar localization and Mdm2-binding domains of S25 were critical for its role in Mdm2-mediated p53 regulation. Knockdown of S25 by siRNA attenuated the induction and activation of p53 following ribosomal stress. S25 stabilized and cooperated with MDMX to regulate Mdm2 E3 ligase activity. Furthermore, S25 was identified to be a transcriptional target of p53; p53 directly bound to S25 promoter and suppressed S25 expression. Our results suggest that there is a S25-Mdm2-p53 regulatory feedback loop, which may have an important role in cancer development and progression.
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ribosomal protein s7 as a novel modulator of p53 Mdm2 interaction binding to Mdm2 stabilization of p53 protein and activation of p53 function
2007Co-Authors: Dung Tsa Chen, Wei Wang, Zhuo Zhang, Elizabeth R Rayburn, Donald L Hill, Hui Wang, Ruiwen ZhangAbstract:As a major negative regulator of p53, the Mdm2 oncogene plays an important role in carcinogenesis and tumor progression. Mdm2 promotes p53 proteasomal degradation and negatively regulates p53 function. The mechanisms by which the Mdm2-p53 interaction is regulated are not fully understood, although several Mdm2-interacting molecules have recently been identified. To search for novel Mdm2-binding partners, we screened a human prostate cDNA library by the yeast two-hybrid assay using full-length Mdm2 protein as the bait. Among the candidate proteins, ribosomal protein S7 was identified and confirmed as a novel Mdm2-interacting protein. Herein, we demonstrate that S7 binds to Mdm2, in vitro and in vivo, and that the interaction between Mdm2 and S7 leads to modulation of Mdm2-p53 binding by forming a ternary complex among Mdm2, p53 and S7. This results in the stabilization of p53 protein through abrogation of Mdm2-mediated p53 ubiquitination. Consequently, S7 overexpression increases p53 transactivational activities, induces apoptosis, and inhibits cell proliferation. The identification of S7 as a novel Mdm2-interacting partner contributes to elucidation of the complex regulation of the Mdm2-p53 interaction and has implications in cancer prevention and therapy.
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stabilization of e2f1 protein by Mdm2 through the e2f1 ubiquitination pathway
2005Co-Authors: Zhuo Zhang, Elizabeth R Rayburn, Hui Wang, Sudhir Agrawal, Ruiwen ZhangAbstract:Although previous studies suggested that the tumorigenicity of mouse double minute 2 (Mdm2) was due to its negative regulation of p53, the p53-independent interactions may be equally as important. During recent studies utilizing Mdm2 inhibitors, we noted that E2F transcription factor 1 (E2F1) was downregulated upon inhibition of Mdm2, regardless of the p53 status of the cancer. The present study investigated the mechanisms responsible for the Mdm2-mediated increase in E2F1 expression. Mdm2 prolongs the half-life of the E2F1 protein by inhibiting its ubiquitination. Mdm2 displaces SCFSKP2, the E2F1 E3 ligase. Direct binding between Mdm2 and E2F1 is necessary for the negative effects of Mdm2 on E2F1 ubiquitination, and deletion of the Mdm2 nuclear localization signal does not result in loss of the ability to increase the E2F1 protein level. The downregulation of E2F1 upon Mdm2 inhibition was not due to either pRB or p14Arf. In addition, E2F1 was responsible for at least part of the inhibition of cell proliferation induced by Mdm2 knockdown. In conclusion, the present study provides evidence that stabilization of the E2F1 protein is likely another p53-independent component of Mdm2-mediated tumorigenesis. More knowledge about the Mdm2–E2F1 interaction may be helpful in developing novel anticancer therapies.
Guillermina Lozano - One of the best experts on this subject based on the ideXlab platform.
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genotoxic stress induces coordinately regulated alternative splicing of the p53 modulators Mdm2 and mdm4
2006Co-Authors: Dawn S Chandler, Ravi K Singh, Lisa C Caldwell, Jaquelyn L Bitler, Guillermina LozanoAbstract:The tumor suppressor protein p53 is a transcription factor that induces G(1) arrest of the cell cycle and/or apoptosis. The murine double-minute protein Mdm2 and its homologue MDM4 (also known as MDMX) are critical regulators of p53. Altered transcripts of the human homologue of Mdm2, Mdm2, have been identified in human tumors, such as invasive carcinoma of the breast, lung carcinoma, and liposarcoma. Mdm2 alternate forms act to negatively regulate the normal Mdm2 gene product, thus activating p53. Although many reports have documented a plethora of tumor types characterized by Mdm2 alternative transcripts, few have investigated the signals that might initiate alternative splicing. We have identified a novel role of these alternative Mdm2 transcripts in the normal surveillance mechanism of the cell and in DNA damage response. We report that alternate forms of Mdm2 are detected after UV irradiation. Furthermore, we show that mouse cells treated with UV are also characterized by alternative transcripts of Mdm2, suggesting that this is an important and evolutionarily conserved mechanism for regulating the expression of Mdm2/Mdm2. An additional p53 regulator and Mdm2 family member, MDM4, is likewise alternatively spliced following UV irradiation. By activating alternative splicing of both Mdm2 and MDM4, yet another layer of p53 regulation is initiated by the cells in response to damage. A stepwise model for malignant conversion by which alternate forms of Mdm2 and MDM4 place selective pressure on the cells to acquire additional alterations in the p53 pathway is herein proposed.
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keeping p53 in check essential and synergistic functions of Mdm2 and mdm4
2006Co-Authors: Jeanchristophe Marine, Geoffrey M Wahl, Sarah Francoz, Marion M Maetens, Franck Toledo, Guillermina LozanoAbstract:1 Laboratory For Molecular Cancer Biology, Flanders Interuniversity Institute for Biotechnology (VIB), University of Ghent, Technologiepark, 927, Ghent B9052, Belgium 2 Salk Institute for Biological Studies, Gene Expression Laboratory, La Jolla, CA 92037, USA 3 Gene Expression and Diseases Unit, Institut Pasteur, Paris, France 4 The University of Texas Graduate School of Biomedical Sciences and department of Molecular Genetics, Section of Cancer Genetics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA * Corresponding author: J-C Marine, Laboratory For Molecular Cancer Biology, VIB, Technologiepark, 927, Ghent B-9052, Belgium. Tel: þ 32-93-313-640; Fax: þ 32-93-313-516; E-mail: chris.marine@dmbr.ugent.be
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Mdm2 an introduction
2003Co-Authors: Tomoo Iwakuma, Guillermina LozanoAbstract:The murine double minute 2 (Mdm2) gene encodes a negative regulator of the p53 tumor suppressor. Amplification of Mdm2 or increased expression by unknown mechanisms occurs in many tumors. Thus, increased levels of Mdm2 would inactivate the apoptotic and cell cycle arrest functions of p53, as do deletion or mutation of p53, common events in the genesis of many kinds of tumors. Mdm2 functions as an E3 ubiquitin ligase to degrade p53. Mdm2 also binds another tumor suppressor, ARF. This interaction sequesters Mdm2 in the nucleolus away from p53, thus activating p53. Many additional Mdm2 interacting proteins have been identified. Functions of Mdm2 independent of p53 have also been identified. This article is an introduction to Mdm2, its structure and biological functions, as well as its relationship to its binding partners.
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rescue of early embryonic lethality in Mdm2 deficient mice by deletion of p53
1995Co-Authors: Roberta Montes De Oca Luna, Daniel S Wagner, Guillermina LozanoAbstract:The gene p53 encodes a transcriptional activator of genes involved in growth arrest, DNA repair and apoptosis. Loss of p53 function contributes to tumour development in vivo. The transcriptional activation function of p53 is inactivated by interaction with the Mdm2 gene product. Amplification of Mdm2 has been observed in 36% of human sarcomas, indicating that it may represent an alternative mechanism of preventing p53 function in tumour development. To study Mdm2 function in vivo, we generated an Mdm2 null allele by homologous recombination. Mdm2 null mice are not viable, and further analysis revealed embryonic lethality around implantation. To examine the importance of the interaction of Mdm2 with p53 in vivo, we crossed mice heterozygous for Mdm2 and p53 and obtained progeny homozygous for both p53 and Mdm2 null alleles. Rescue of the Mdm2-/- lethality in a p53 null background suggests that a critical in vivo function of Mdm2 is the negative regulation of p53 activity.
Carol Prives - One of the best experts on this subject based on the ideXlab platform.
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the roles of Mdm2 and mdmx in cancer
2016Co-Authors: Orit Karnischmidt, Maria Lokshin, Carol PrivesAbstract: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.
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ribosomal protein s7 is both a regulator and a substrate of Mdm2
2009Co-Authors: Yan Zhu, Masha V Poyurovsky, Lynn Biderman, Joachim Stahl, Xavier Jacq, Carol PrivesAbstract:Mdm2 associates with ribosomal protein S7, and this interaction is required to inhibit Mdm2's E3 ligase activity, leading to stabilization of Mdm2 and p53. Notably, the Mdm2 homolog MDMX facilitates the inhibition of Mdm2 E3 ligase activity by S7. Further, ablation of S7 inhibits Mdm2 and p53 accumulation induced by different stress signals in some cell types. Thus, ribosomal/nucleolar stress is likely a key integrating event in DNA damage signaling to p53. Interestingly, S7 is itself a substrate for Mdm2 E3 ligase activity both in vitro and in vivo. An S7-ubiquitin fusion protein (S7-Ub) selectively inhibits Mdm2 degradation of p53 and is unaffected by MDMX. S7-Ub promotes apoptosis to a greater extent than S7 alone. This indicates that Mdm2 ubiquitination of S7 is involved in sustaining the p53 response. Thus, S7 functions as both effector and affector of Mdm2 to ensure a proper cellular response to different stress signals.
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cyclin a cdk phosphorylation regulates Mdm2 protein interactions
2001Co-Authors: Tingting Zhang, Carol PrivesAbstract:The product of the Mdm2 gene interacts with and regulates a number of proteins, in particular the tumor suppressor p53. The Mdm2 protein is likely to be extensively modified in vivo, and such modification may regulate its functions in cells. We identified a potential cyclin-dependent kinase (CDK) site in murine Mdm2, and found the protein to be efficiently phosphorylated in vitro by cyclin A-containing complexes (cyclin A-CDK2 and cyclin A-CDK1), but Mdm2 was either weakly or not phosphorylated by other cyclin-containing complexes. Moreover, a peptide containing a putative Mdm2 cyclin recognition motif specifically inhibited phosphorylation by cyclin A-CDK2. The site of cyclin A-CDK2 phosphorylation was identified as Thr-216 by two-dimensional phosphopeptide mapping and mutational analysis. Phosphorylation of Mdm2 at Thr-216 both weakens its interaction with p53 and modestly augments its binding to p19(ARF). Interestingly, an Mdm2-specific monoclonal antibody, SMP14, cannot recognize Mdm2 phosphorylated at Thr-216. Changes in SMP14 reactivity of Mdm2 in staged cell extracts indicate that phosphorylation of Mdm2 at Thr-216 in vivo is most prevalent at the onset of S phase when cyclin A first becomes detectable.
Arnold J Levine - One of the best experts on this subject based on the ideXlab platform.
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a single nucleotide polymorphism in the Mdm2 gene disrupts the oscillation of p53 and Mdm2 levels in cells
2007Co-Authors: Zhaohui Feng, John Wagner, John Jeremy Rice, Gustavo Stolovitzky, Arnold J LevineAbstract:Oscillations of both p53 and Mdm2 proteins have been observed in cells after exposure to stress. A mathematical model describing these oscillations predicted that oscillations occur only at selected levels of p53 and Mdm2 proteins. This model prediction suggests that oscillations will disappear in cells containing high levels of Mdm2 as observed with a single nucleotide polymorphism in the Mdm2 gene (SNP309). The effect of SNP309 upon the p53-Mdm2 oscillation was examined in various human cell lines and the oscillations were observed in the cells with at least one wild-type allele for SNP309 (T/T or T/G) but not in cells homozygous for SNP309 (G/G). Furthermore, estrogen preferentially stimulated the transcription of Mdm2 from SNP309 G allele and increased the levels of Mdm2 protein in estrogen-responsive cells homozygous for SNP309 (G/G). These results suggest the possibility that SNP309 G allele may contribute to gender-specific tumorigenesis through further elevating the Mdm2 levels and disrupting the p53-Mdm2 oscillation. Furthermore, using the H1299-HW24 cells expressing wild-type p53 under a tetracycline-regulated promoter, the p53-Mdm2 oscillation was observed only when p53 levels were in a specific range, and DNA damage was found to be necessary for triggering the p53-Mdm2 oscillation. This study shows that higher levels of Mdm2 in cells homozygous for SNP309 (G/G) do not permit coordinated p53-Mdm2 oscillation after stress, which might contribute to decreased efficiency of the p53 pathway and correlates with a clinical phenotype (i.e., the development of cancers at earlier age of onset in female).
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p19arf stabilizes p53 by blocking nucleo cytoplasmic shuttling of Mdm2
1999Co-Authors: Weikang Tao, Arnold J LevineAbstract:The INK4a-ARF locus encodes two distinct tumor suppressors, p16INK4a and p19ARF. Whereas p16INK4a restrains cell growth through preventing phosphorylation of the retinoblastoma protein, p19ARF acts by attenuating Mdm2-mediated degradation of p53, thereby stabilizing p53. Recent data indicate that Mdm2 shuttles between the nucleus and the cytoplasm and that nucleo-cytoplasmic shuttling of Mdm2 is essential for Mdm2’s ability to promote p53 degradation. Therefore, Mdm2 must export p53 from the nucleus to the cytoplasm where it targets p53 for degradation. We show here that coexpression of p19ARF blocks the nucleo-cytoplasmic shuttling of Mdm2. Moreover, subnuclear localization of Mdm2 changes from the nucleoplasm to the nucleolus in a shuttling time-dependent manner, whereas p19ARF is exclusively located in the nucleolus. In heterokaryons containing Mdm2 and p19ARF, the longer the Mdm2 shuttling is allowed, the more Mdm2 protein colocalizes with p19ARF in the nucleolus, implying that Mdm2 moves from the nucleoplasm to the nucleolus and then associates with p19ARF there. Furthermore, whether or not Mdm2 colocalizes with p19ARF in the nucleolus, p19ARF prevents Mdm2 shuttling. This observation suggests that Mdm2 might be exported through the nucleolus and p19ARF could inhibit the nuclear export of Mdm2 by tethering Mdm2 in the nucleolus. Taken together, p19ARF could stabilize p53 by inhibiting the nuclear export of Mdm2.
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functions of the Mdm2 oncoprotein
1999Co-Authors: D A Freedman, Arnold J LevineAbstract:The p53 protein is activated in response to physiological stress resulting in either a G1 arrest of cells or apoptosis. As such, p53 must be tightly regulated, and the Mdm2 oncoprotein plays a central role in that regulatory process. The transcription of the Mdm2 oncogene is induced by the p53 protein after DNA damage, and the Mdm2 protein then binds to p53 and blocks its activities as a tumour suppressor and promotes its degradation. These two proteins thus form an autoregulatory feedback loop in which p53 positively regulates Mdm2 levels and Mdm2 negatively regulates p53 levels and activity. Immediately after ultraviolet (UV) irradiation Mdm2 messenger RNA and protein levels fall in a p53-independent fashion, resulting in increased p53 levels. The p53 protein is then activated as a transcription factor by posttranslational modification permitting p53 to initiate its cell-cycle arrest or apoptotic (programmed cell death) functions. At later times, after the repair of DNA, Mdm2 levels increase in a p53-dependent fashion. This induction of Mdm2 results in the inhibition of p53 transcriptional activity and the degradation of p53 protein. Mdm2-p53 complexes in the nucleus are transported to the cytoplasm via signals present in the Mdm2 protein, where p53 is degraded in the proteasome. Thus Mdm2 acts as a nuclear-cytoplasmic shuttle for the p53 protein. There are many levels at which this process is regulated, and as such there are many places for chemotherapeutic interventions. The amino-terminal domain of the Mdm2 protein is all that is required to bind the p53 protein. The Mdm2 protein has additional domains and therefore may have additional functions. Any of these Mdm2 domains may contribute to Mdm2's activities as an oncogene independent of its inhibition of the tumour suppressor functions of p53. Thus Mdm2 itself could be a target for cancer therapeutic intervention.
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The Mdm2 oncoprotein binds specifically to RNA through its RING finger domain.
1996Co-Authors: Brian Elenbaas, Matthias Dobbelstein, Judith Roth, Thomas Shenk, Arnold J LevineAbstract:The cellular Mdm2 gene has transforming activity when overexpressed and is amplified in a variety of human tumors. At least part of the transforming ability of the Mdm2 protein is due to binding and inactivating the p53 tumor suppressor protein. Additionally, this protein forms a complex in vivo with the L5 ribosomal protein and its associated 5S ribosomal RNA and may be part of a ribosomal complex. A RNA homopolymer binding assay and a SELEX procedure have been used to characterize the RNA-binding activity of Mdm2. The Mdm2 protein binds efficiently to the homopolyribonucleotide poly(G) but not to other homopolyribonucleotides. This binding is independent of the interaction of Mdm2 with the L5 protein, which occurs through the central acidic domain of Mdm2. An RNA SELEX procedure was performed to identify specific RNA ligands that bind with high affinity to the human Mdm2 (HDM2) protein. After 10 rounds of selection and amplification, a subset of RNA molecules that bound efficiently to HDM2 was isolated from a randomized pool. Sequencing of these selected ligands revealed that a small number of sequence motifs were selected. The specific RNA binding occurs through the RING finger domain of the protein. Furthermore, a single amino acid substitution in the RING finger domain, G446S, completely abolishes the specific RNA binding. These observations, showing that Mdm2 binds the L5/5S ribosomal ribonucleoprotein particle and can also bind to specific RNA sequences or structures, suggest a role for Mdm2 in translational regulation in a cell.