MDM2: life without p53

The MDM2 protein suppresses the ability of p53 to inhibit cellular proliferation or to induce cell death. This property underlies the oncogenic potential of MDM2, which is overexpressed in various human tumours. However, MDM2 also has p53-independent activities, which we focus on here. Similar to other oncogenes, surveillance pathways might counteract the deleterious effects of deregulated MDM2 expression. These pathways need to be inactivated for MDM2 oncogenic activity, which targets p53 but also other proteins.     

RYBP stabilizes p53 by modulating MDM2

The mouse double minute 2 (MDM2)–p53 interaction regulates the activity of p53 and is a potential target for human cancer therapy. Here, we report that RYBP (RING1‐ and YY1‐binding protein), a member of the polycomb group (PcG), interacts with MDM2 and decreases MDM2‐mediated p53 ubiquitination, leading to stabilization of p53 and an increase in p53 activity. RYBP induces cell‐cycle arrest and is involved in the p53 response to DNA damage. Expression of RYBP is decreased in human cancer tissues compared with adjacent normal tissues. These results show that RYBP is a new regulator of the MDM2–p53 loop and that it has tumour suppressor activity.     

A role for Numb in p53 stabilization

The cell-fate determinant Numb has recently been shown to help activate the tumor suppressor protein p53. Loss of Numb in breast cancers would result, therefore, in both the activation of the potential oncogene Notch and the diminution of tumor suppression by p53.     

The Regulation of the p53-mediated Stress Response by MDM2 and MDM4

Exquisite control of the activity of p53 is necessary for mammalian survival. Too much p53 is lethal, whereas too little permits tumorigenesis. MDM2 and MDM4 are structurally related proteins critical for the control of p53 activity during development, homeostasis, and the response to stress. These two essential proteins regulate both the activation of p53 in response to stress and the recovery of cells following resolution of the damage, yet both are oncogenic when overexpressed. Thus, multiple regulatory circuits ensure that their activities are fine-tuned to promote tumor-free survival. Numerous diverse stressors activate p53, and much research has gone into trying to find commonalities between them that would explain the mechanism by which p53 becomes active. It is now clear that although these diverse stressors activate p53 by different biochemical pathways, one common feature is the effort they direct, through a variety of means, toward disrupting the functions of both MDM2 and MDM4. This article provides an overview of the relationship between MDM2 and MDM4, features the various biochemical mechanisms by which p53 is activated through inhibition of their functions, and proposes some emerging areas for investigation of the p53-mediated stress response.Regulation of the p53-mediated stress response by the essential inhibitory proteins MDM2 and MDM4 is critical for survival. In response to stressors such as ionizing radiation, p53 induces a number of potentially lethal but tumor-suppressive processes, including cell cycle arrest, senescence, and apoptosis (reviewed by Horn and Vousden 2007). Both MDM2 and MDM4 are critical to surviving the p53-mediated stress response to whole body ionizing irradiation as mice with reduced levels of either protein undergo p53-dependent death after exposure to doses of radiation that are sublethal to wild-type mice (Mendrysa et al. 2003; Terzian et al. 2007). MDM2 and MDM4 are also required to control p53 function during development, as shown by the early embryonic death of mice lacking either MDM2 or MDM4, unless they also lack p53 (Jones et al. 1995; Montes de Oca Luna et al. 1995; Parant et al. 2001; Migliorini et al. 2002).Although both MDM2 and MDM4 are essential for development, they are detrimental to long-term survival when in excess, because both are oncogenic. Both MDM2 and MDM4 confer the tumorigenic phenotype on cultured cells when experimentally overexpressed (Fakharzadeh et al. 1991; Danovi et al. 2004). In addition, targeted expression of MDM2 in the mammary gland results in tumorigenesis (Lundgren et al. 1997). In people, single nucleotide polymorphisms that reduce expression of either of the orthologs of MDM2 or MDM4 (also referred to as Hdm2 and Hdm4) correlate with increased risk for breast cancer (Bond et al. 2004; Atwal et al. 2009). Approximately 10% of human tumors have been found to overexpress either MDM2 or MDM4 and many of these express wild-type p53 (reviewed in Toledo and Wahl 2006). Because the majority of human cancers express mutant forms of p53, overexpression of MDM2 and MDM4 in the subset of tumors expressing wild-type p53 supports the notion that excessive MDM2 and MDM4 promote tumorigenesis, at least in part, by blocking p53 function. Thus, limiting the activities of MDM2 and MDM4 is important to prevent cancer.     

p53, MDM2, bcl-2 staining in follicular neoplasms of the thyroid gland

Sixty-two follicular adenomas of the thyroid were investigated by immunohistochemistry for the expression of p53, MDM2 and bcl-2 proteins. The wild type of 393 aminoacid nuclear p53 phosphoprotein is the product of a gene located on the short arm of chromosome 17. The p53 protein controls the growth of transformed cells in a culture and thus termed a suppressor gene product. Mouse double minute 2 (MDM2) gene product has been described to occur in malignant epithelial tissue, the protein product of this gene binds to and presumably inactivates the growth suppressive effect of wild type p53 protein. Bcl-2 is an oncogene whose product inhibits apoptosis in many cells types. Some scattered nuclei in two adenomas (3.2%) stained positively for p53. The adenomas with positive staining for p53 were subserially sectioned, but no signs of invasion were found, both patients are alive and well. In 12 adenomas (19%) there was positive reaction for MDM2 protein, whereas none of them where p53 positive. All cases were strongly positive for bcl-2 staining. We conclude that p53 protein expression is not confined to follicular adenomas, while MDM2 and bcl-2 genes products are.     

Molecular mechanism of the interaction between MDM2 and p53

We have investigated the kinetic and thermodynamic basis of the p53-MDM2 interaction using a set of peptides based on residues 15-29 of p53. Wild-type p53 peptide bound MDM2 with a dissociation constant of 580nM. Phosphorylation of S15 and S20 did not affect binding, but T18 phosphorylation weakened binding tenfold, indicating that phosphorylation of only T18 is responsible for abrogating p53-MDM2 binding. Truncation to residues 17-26 increased affinity 13-fold, but further truncation to 19-26 abolished binding. NMR studies of the binding of the p53-derived peptides revealed global conformational changes of the overall structure of MDM2, stretching far beyond the binding cleft, indicating significant changes in the domain dynamics of MDM2 upon ligand binding.     

Stabilization of the MDM2 oncoprotein by mutant p53

MDM2 is a short-lived protein that regulates p53 degradation. We report here that transient coexpression of MDM2 and several p53 hotspot mutants resulted in stabilization and increased expression of MDM2. Ectopic expression of the mutant p53(175H) allele by recombinant adenovirus infection or stable transfection also stabilized endogenous MDM2 in p53-null cells. A panel of human tumor cell lines expressing different endogenous mutant p53 alleles also contained stabilized nuclear MDM2 at elevated levels when compared with p53-null cells. MDM2 was present in complexes with mutant p53 in tumor cells, and stabilization of MDM2 required direct binding to mutant p53. These results reveal a novel property of mutant p53 and a unique feature of tumors with p53 missense mutations. Accumulation of stable MDM2 may contribute to tumorigenesis through its p53-independent transforming functions.     

MDM2反义寡核苷酸对血管平滑肌细胞MDM2和p53表达的影响

目的:研究小鼠双微体扩增基因(mouse double minute 2;MDM2)反义寡核苷酸(antisense oligonucleotide;ASON)对血管平滑肌细胞MDM2和p53表达的影响,探讨MDM2反义寡核苷酸包埋支架防治支架内再狭窄的可行性。方法:人工合成一段针对MDM2 mRNA的反义寡核苷酸,脂质体包裹不同浓度ASON转染兔血管平滑肌细胞,RT-PCR和Western-blotting检测MDM2反义寡核苷酸对兔血管平滑肌细胞MDM2和p53表达的影响。结果:不同浓度MDM2反义寡核苷酸作用于兔血管平滑肌细胞后,MDM2和p53 mRNA表达量各浓度组之间有显著性差异(P<0.01),MDM2和p53蛋白表达量各浓度组之间有显著性差异(P<0.01)。结论:MDM2反义寡核苷酸体外能够特异性抑制兔血管平滑肌细胞MDM2表达,提高细胞内p53基因表达量,MDM2反义寡核苷酸有望被进一步应用于药洗脱支架研究。     

Cooperation between p53 mutation and high telomerase transgenic expression in spontaneous cancer development

Telomerase reintroduction in adult somatic tissues is envisioned as a way to extend their proliferative capacity. It is still a question, however, whether constitutive telomerase expression in adult tissues impacts the normal aging and spontaneous cancer incidence of an organism. Here, we studied the aging and spontaneous cancer incidence of mice with transgenic telomerase expression in a wide range of adult tissues, K5-Tert mice. For this, we maintained large colonies of K5-Tert mice for more than 2 years. K5-Tert mice showed a decreased life span compared to wild-type cohorts associated with a higher incidence of preneoplastic and neoplastic lesions in various tissue types. Neoplasias in K5-Tert mice were coincident with transgene expression in the affected tissues. These observations suggest that high telomerase activity may cooperate with genetic alterations that occur with age to promote tumorigenesis. Indeed, we demonstrate here that increased cancer incidence and the reduced viability of K5-Tert mice are aggravated in a p53(+/-) genetic background, indicating that telomerase cooperates with loss of p53 function in inducing tumorigenesis. Altogether, these results demonstrate that constitutive high levels of telomerase activity result in a decreased life span associated with an increased incidence of neoplasias as the organism ages.     

FKBP25, a novel regulator of the p53 pathway, induces the degradation of MDM2 and activation of p53

The p53 tumour suppressor protein is tightly controlled by the E3 ubiquitin ligase, mouse double minute 2 (MDM2), but maintains MDM2 expression as part of a negative feedback loop. We have identified the immunophilin, 25 kDa FK506-binding protein (FKBP25), previously shown to be regulated by p53-mediated repression, as an MDM2-interacting partner. We show that FKBP25 stimulates auto-ubiquitylation and proteasomal degradation of MDM2, leading to the induction of p53. Depletion of FKBP25 by siRNA leads to increased levels of MDM2 and a corresponding reduction in p53 and p21 levels. These data are consistent with the idea that FKBP25 contributes to regulation of the p53-MDM2 negative feedback loop.

Structured summary

MINT-6823686:MDM2 (uniprotkb:Q00987) physically interacts (MI:0218) with FKBP25 (uniprotkb:Q00688) by anti bait coimmunoprecipitation (MI:0006)MINT-6823707, MINT-6823722:MDM2 (uniprotkb:Q00987) physically interacts (MI:0218) with FKBP25 (uniprotkb:Q62446) by pull down (MI:0096)MINT-6823775:P53 (uniprotkb:Q04637) physically interacts (MI:0218) with MDM2 (uniprotkb:Q00987) by anti bait coimmunoprecipitation (MI:0006)MINT-6823735, MINT-6823749:FKBP25 (uniprotkb:Q62446) binds (MI:0407) to MDM2 (uniprotkb:Q00987) by pull down (MI:0096)MINT-6823761:Ubiquitin (UNIPROTKB:62988)P physically interacts (MI:0218) with MDM2 (uniprotkb:Q00987) by pull down (MI:0096)MINT-6823669:MDM2 (uniprotkb:Q00987) physically interacts (MI:0218) with FKBP25 (uniprotkb:Q00688) by two hybrid (MI:0018)     

Study on the Spatial Architecture of p53, MDM2, and p14ARF Containing Complexes

We have developed a surface plasmon resonance (SPR)-based immunocapture approach to study multimeric protein–protein complexes. A composition and spatial architecture of protein complexes that contained GST-tagged p53, p14ARF, and MDM2 was examined by the developed approach. Obtained results verified that the p53 protein possesses two binding sites for MDM2. Ternary complexes containing p14ARF, MDM2, and p53 proteins could only be formed when MDM2 protein functions as a bridging molecule. That was confirmed by immunoprecipitation and immunostaining. Andrej Savchenko and Mariya Yurchenko have contributed equally to this article.