Keeping p53 in check: essential and synergistic functions of Mdm2 and Mdm4.
TL;DR: This work presents a novel and scalable approach to gene expression engineering that allows for real-time annotation of gene expression changes in response to cancerigenicity and shows promise in finding novel and efficient treatments for cancer.
Abstract: 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
Citations
More filters
TL;DR: The results suggest that USP2a binds to and stabilizes MDM4; thus in turn, it enhances the mitochondrial localization of p53 and promotes apoptosis in glioma cells.
Abstract: The mouse double minute 4 (MDM4) oncoprotein may inhibit tumorigenesis by regulating the apoptotic mediator p53. Ubiquitin-specific protease 2a (USP2a) is a deubiquitinating enzyme that protects MDM4 against degradation, so USP2-MDM4 interaction may be a key determinant of the malignant potential of human cancers. MDM4 and USP2a, as well as the MDM4-USP2a complex, were more highly expressed in glioblastoma multiforme tissue samples from patients with good prognosis compared with patients with poor prognosis. Analysis of the prognostic parameters indicated that MDM4 expression was positively correlated with an increased likelihood for survival. Compared with the poor prognosis patients, mitochondria from good prognosis glioma patients contained higher levels of both MDM4 and the proapoptotic protein p53Ser46(P). In U87MG glioma cell line, the overexpression of MDM4 enhanced ultraviolet (UV)-induced cytochrome c release and apoptosis. In contrast, MDM4 knockdown decreased mitochondrial p53Ser46(P) levels and rescued cells from UV-induced apoptosis. The expression of MDM4 and USP2a were positively correlated with each other. MDM4-USP2a complexes were found only in the cytoplasmic fraction, whereas the mitochondrial fraction contained MDM4-p53Ser46(P) and MDM4-Bcl-2 complexes. Overexpression of USP2a increased p53 and p53Ser46(P) levels in the mitochondria, whereas simultaneous MDM4 knockdown completely reversed this effect. UV-induced apoptosis was reduced by USP2a knockdown but restored by the simultaneous overexpression of MDM4. This apoptotic response was reduced by knockdown of p53 but not p21. Our results suggest that USP2a binds to and stabilizes MDM4; thus in turn, it enhances the mitochondrial localization of p53 and promotes apoptosis in glioma cells.
48 citations
Cites background from "Keeping p53 in check: essential and..."
...Both fulllength MDM4 and MDM2 demonstrated oncogenic potential via inhibition of tumor suppressor p53 (4,5)....
[...]
...Both MDM4 and MDM2 were shown to be crucial inhibitors of the tumor suppressor p53 protein (4,5), and MDM4 is considered to be a negative regulator of the growth arrest mediated by p53 (27)....
[...]
TL;DR: It is worthy to note that the significant downregulation of full-length MDM4 in PTC reveals a novel status of this factor in human cancer that counsels careful evaluation of its role in human tumorigenesis and of its potential as therapeutic target.
Abstract: A wild-type (wt) p53 gene characterizes thyroid tumors, except for the rare anaplastic histotype. Because p53 inactivation is a prerequisite for tumor development, alterations of p53 regulators represent an alternative way to impair p53 function. Indeed, murine double minute 2 (MDM2), the main p53 negative regulator, is overexpressed in many tumor histotypes including those of the thyroid. A new p53 regulator, MDM4 (a.k.a. MDMX or HDMX) an analog of MDM2, represents a new oncogene although its impact on tumor properties remains largely unexplored. We estimated levels of MDM2, MDM4, and its variants, MDM4-S (originally HDMX-S) and MDM4-211 (originally HDMX211), in a group of 57 papillary thyroid carcinomas (PTC), characterized by wt tumor protein 53, in comparison to matched contra-lateral lobe normal tissue. Further, we evaluated the association between expression levels of these genes and the histopathological features of tumors. Quantitative real-time polymerase chain reaction revealed a highly significant downregulation of MDM4 mRNA in tumor tissue compared to control tissue (P < 0.0001), a finding confirmed by western blot on a subset of 20 tissue pairs. Moreover, the tumor-to-normal ratio of MDM4 levels for each individual was significantly lower in late tumor stages, suggesting a specific downregulation of MDM4 expression with tumor progression. In comparison, MDM2 messenger RNA (mRNA) and protein levels were frequently upregulated with no correlation with MDM4 levels. Lastly, we frequently detected overexpression of MDM4-S mRNA and presence of the aberrant form, MDM4-211 in this tumor group. These findings indicate that MDM4 alterations are a frequent event in PTC. It is worthy to note that the significant downregulation of full-length MDM4 in PTC reveals a novel status of this factor in human cancer that counsels careful evaluation of its role in human tumorigenesis and of its potential as therapeutic target.
48 citations
Cites background from "Keeping p53 in check: essential and..."
...The murine double minute (MDM) family members are key regulators of levels and activity of the oncosuppressor p53 [1]....
[...]
...MDM4 has been reported as inhibitor of p53mediated growth arrest [1]; thus, it might be hypothesized that the presence of MDM4 may predispose to multifocality and tumor development, while a decrease in its expression may confer an advantage to tumor progression....
[...]
...In recent years, another member of the MDM family, MDM4 (also named MDMX) has come into the limelight and its function in the inactivation of p53 has been revealed by molecular and genetic approaches [1, 3]....
[...]
...In view of the suggested strategies for abrogation of MDM4 in human tumors [1, 3], our findings counsel acquisition of more information on the molecular functions of this protein in different human tumors in order to carefully evaluate the application of such therapies....
[...]
TL;DR: It is demonstrated that the association of p53 and Ubc13 on polysomes requires ongoing translation and results in p53 ubiquitination that interferes with its tetramerization.
Abstract: The p53 tumor suppressor protein is a key regulator of cellular proliferation and survival whose function is tightly regulated at the levels of transcription and protein stability. Here, we unveil the fine control of p53 on translationally active polysomes. We have previously reported that Ubc13, an E2 ubiquitin-conjugating enzyme, directly regulates p53 localization and transcriptional activity. We now demonstrate that the association of p53 and Ubc13 on polysomes requires ongoing translation and results in p53 ubiquitination that interferes with its tetramerization. JNK phosphorylation of p53 at Threonine 81 occurring on polysomes is required for the dissociation of Ubc13 from p53, leading to p53 multimerization and transcriptional activation. Inhibition of JNK activity or expression of a nonphosphorylatable mutant of p53 maintains an Ubc13-p53 complex that inhibits p53 multimerization. Our findings reveal a layer in the regulation of p53 multimerization that requires the concerted action of JNK and Ubc13 on polysome-bound p53.
48 citations
Cites background from "Keeping p53 in check: essential and..."
...We have recently shown that Ubc13, an E2 ubiquitin-conjugating enzyme, elicits K63-dependent ubiquitination of p53, which attenuates Hdm2-dependent polyubiquitination and subsequent degradation of p53 (7)....
[...]
...In particular, the ribosomal protein L26, whose stability is regulated by the p53 ubiquitin ligases Hdm2, has been shown to bind and regulate the translation of p53 mRNA (13, 14)....
[...]
...In nonstressed cells, p53 levels are relatively low because of its short half-life regulated by ubiquitin ligases, including Hdm2 (5, 6)....
[...]
...The likely candidates are Hdm2, which has been directly implicated in the regulation of p53 mRNA translation (13), PARC, which regulates p53 stability within large molecular weight complexes in the cytosol (15), and MSL2 and WWP1, which were shown to promote p53 ubiquitination without affecting its stability (16, 17)....
[...]
TL;DR: The role of components of the ubiquitin-proteasome system in the regulation of p53 are given and progress in targeting these proteins to activate wild-type p53 for the treatment of cancer is reviewed.
47 citations
TL;DR: It is shown that the relatively high amount of p53 protein correlates with a higher amount ofp53 RNA in ES cells compared to differentiated cells, and p53 RNA is more stable in embryonic stem cells and the p53protein is more often transcribed.
47 citations
Cites background from "Keeping p53 in check: essential and..."
...the mouse double minute 2 (Mdm2) protein, which binds to p53 and inhibits several of p53's biochemical activities [9,10]....
[...]
References
More filters
TL;DR: A gene is identified, named WAF1, whose induction was associated with wild-type but not mutant p53 gene expression in a human brain tumor cell line and that could be an important mediator of p53-dependent tumor growth suppression.
8,339 citations
TL;DR: In this article, the authors identify potent and selective small-molecule antagonists of MDM2 and confirm their mode of action through the crystal structures of complexes, leading to cell cycle arrest, apoptosis, and growth inhibition of human tumor xenografts.
Abstract: MDM2 binds the p53 tumor suppressor protein with high affinity and negatively modulates its transcriptional activity and stability. Overexpression of MDM2, found in many human tumors, effectively impairs p53 function. Inhibition of MDM2-p53 interaction can stabilize p53 and may offer a novel strategy for cancer therapy. Here, we identify potent and selective small-molecule antagonists of MDM2 and confirm their mode of action through the crystal structures of complexes. These compounds bind MDM2 in the p53-binding pocket and activate the p53 pathway in cancer cells, leading to cell cycle arrest, apoptosis, and growth inhibition of human tumor xenografts in nude mice.
4,397 citations
TL;DR: It is proposed that the Mdm2-promoted degradation of p53 provides a new mechanism to ensure effective termination of the p53 signal.
Abstract: The p53 tumour-suppressor protein exerts antiproliferative effects, including growth arrest and apoptosis, in response to various types of stress. The activity of p53 is abrogated by mutations that occur frequently in tumours, as well as by several viral and cellular proteins. The Mdm2 oncoprotein is a potent inhibitor of p53. Mdm2 binds the transcriptional activation domain of p53 and blocks its ability to regulate target genes and to exert antiproliferative effects. On the other hand, p53 activates the expression of the mdm2 gene in an autoregulatory feedback loop. The interval between p53 activation and consequent Mdm2 accumulation defines a time window during which p53 exerts its effects. We now report that Mdm2 also promotes the rapid degradation of p53 under conditions in which p53 is otherwise stabilized. This effect of Mdm2 requires binding of p53; moreover, a small domain of p53, encompassing the Mdm2-binding site, confers Mdm2-dependent detstabilization upon heterologous proteins. Raised amounts of Mdm2 strongly repress mutant p53 accumulation in tumour-derived cells. During recovery from DNA damage, maximal Mdm2 induction coincides with rapid p53 loss. We propose that the Mdm2-promoted degradation of p53 provides a new mechanism to ensure effective termination of the p53 signal.
4,311 citations
TL;DR: It is shown that interaction with Mdm2 can also result in a large reduction in p53 protein levels through enhanced proteasome-dependent degradation, which may contribute to the maintenance of low p53 concentrations in normal cells.
Abstract: The tumour-suppressor p53 is a short-lived protein that is maintained at low, often undetectable, levels in normal cells. Stabilization of the protein in response to an activating signal, such as DNA damage, results in a rapid rise in p53 levels and subsequent inhibition of cell growth. Tight regulation of p53 function is critical for normal cell growth and development, and one mechanism by which p53 function is controlled is through interaction with the Mdm2 protein. Mdm2 inhibits p53 cell-cycle arrest and apoptic functions and we show here that interaction with Mdm2 can also result in a large reduction in p53 protein levels through enhanced proteasome-dependent degradation. Endogenous levels of Mdm2 are sufficient to regulate p53 stability, and overexpression of Mdm2 can reduce the amount of endogenous p53. Because mdm2 is transcriptionally activated by p53, this degradative pathway may contribute to the maintenance of low p53 concentrations in normal cells. Furthermore, mechanisms regulating the Mdm2-induced degradation of p53 may play a role in controlling the extent and duration of the p53 response.
3,298 citations
TL;DR: The data suggest that the MDM2 protein, which is induced by p53, functions as a ubiquitin ligase, E3, in human papillomavirus‐uninfected cells which do not have E6 protein.
1,962 citations