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1.  Functions of MDMX in the Modulation of the p53-Response 
The MDM family proteins MDM2 and MDMX are two critical regulators of the p53 tumor suppressor protein. Expression of both proteins is necessary for allowing the embryonal development by keeping the activity of p53 in check. Upon stresses that need to activate p53 to perform its function as guardian of the genome, p53 has to be liberated from these two inhibitors. In this review, we will discuss the various mechanisms by which MDMX protein levels are downregulated upon various types of stress, including posttranslational modifications of the MDMX protein and the regulation of mdmx mRNA expression, including alternative splicing. In addition, the putative function(s) of the described MDMX splice variants, particularly in tumor development, will be discussed. Lastly, in contrast to common belief, we have recently shown the existence of a p53-MDMX feedback loop, which is important for dampening the p53-response at later phases after genotoxic stress.
PMCID: PMC3085504  PMID: 21541195
2.  High levels of Hdmx promote cell growth in a subset of uveal melanomas 
The p53 tumor suppressor pathway is inactivated in cancer either via direct mutation or via deregulation of upstream regulators or downstream effectors. P53 mutations are rare in uveal melanoma. Here we investigated the role of the p53 inhibitor Hdmx in uveal melanoma. We found Hdmx over-expression in a subset of uveal melanoma cell lines and fresh-frozen tumor samples. Hdmx depletion resulted in cell-line dependent growth inhibition, apparently correlating with differential Hdm2 levels. Surprisingly, p53 knockdown hardly rescued cell cycle arrest and apoptosis induction upon Hdmx knockdown, whereas it effectively prevented growth suppression induced by the potent p53 activator Nutlin-3. In addition, two compounds inhibiting Hdmx function or expression, SAH-p53-8 and XI-011, also elicited a growth inhibitory effect in a partly p53-independent manner. These findings suggest a novel, growth-promoting function of Hdmx that does not rely on its ability to inhibit p53. We provide evidence for a contribution of p27 protein induction to the observed p53-independent G1 arrest in response to Hdmx knockdown. In conclusion, our study establishes the importance of Hdmx as an oncogene in a subset of uveal melanomas and widens the spectrum of its function beyond p53 inhibition.
PMCID: PMC3433101  PMID: 22957303
Uveal melanoma; Hdmx; p53; Nutlin-3; p27; SAH-p53-8; XI-011; retinoblastoma
3.  Activation of cAMP Signaling Interferes with Stress-Induced p53 Accumulation in ALL-Derived Cells by Promoting the Interaction between p53 and HDM212 
Neoplasia (New York, N.Y.)  2011;13(7):653-663.
The tumor suppressor p53 provides an important barrier to the initiation and maintenance of cancers. As a consequence, p53 function must be inactivated for a tumor to develop. This is achieved by mutation in approximately 50% of cases and probably by functional inactivation in the remaining cases. We have previously shown that the second messenger cAMP can inhibit DNA damage-induced wild-type p53 accumulation in acute lymphoblastic leukemia cells, leading to a profound reduction of their apoptotic response. In the present article, we provide a mechanistic insight into the regulation of p53 levels by cAMP. We show that increased levels of cAMP augment the binding of p53 to its negative regulator HDM2, overriding the DNA damage-induced dissociation of p53 from HDM2. This results in maintained levels of p53 ubiquitination and proteasomal degradation, which in turn counteracts the DNA damage-induced stabilization of the p53 protein. The apoptosis inhibitory effect of cAMP is further shown to depend on this effect on p53 levels. These findings potentially implicate deregulation of cAMP signaling as a candidate mechanism used by transformed cells to quench the p53 response while retaining wild-type p53.
PMCID: PMC3132851  PMID: 21750659
4.  Increased radio-resistance and accelerated B-cell lymphomas in mice with Mdmx mutations that prevent modifications by DNA damage-activated kinases 
Cancer cell  2009;16(1):33-43.
Mdmx is a critical negative regulator of the p53 pathway that is stoichiometrically limiting in some tissues. Post-translational modification and degradation of Mdmx after DNA damage have been proposed to be essential for p53 activation. We tested this model in vivo, where critical stoichiometric relationships are preserved. We generated an Mdmx mutant mouse in which three conserved serines (S341, S367, S402) targeted by DNA damage-activated kinases were replaced by alanines to investigate whether modifications of these residues are important for Mdmx degradation and p53 activation. The mutant mice were remarkably resistant to radiation, and very susceptible to Myc-induced lymphomagenesis. These data demonstrate that Mdmx down-regulation is crucial for effective p53-mediated radiation responses and tumor suppression in vivo.
P53 function is sensitive to the levels of its negative regulators, Mdm2 and Mdmx. Cell culture studies have suggested the importance of post-translational modifications in Mdm2 and Mdmx for p53 activation, but this has not been rigorously tested in vivo. This work shows that DNA damage and activated c-Myc both require phosphorylation of Mdmx in residues targeted by the damage kinases ATM and Chk2 for robust p53 activation. Preventing Mdmx post-translational modification stabilizes this negative regulator, which mitigates p53 activation, and presumably enables c-Myc to drive tumor cells with defective genomes into cycle in vivo. The data also stress the relevance of Mdmx as a potential therapeutic target.
PMCID: PMC2758524  PMID: 19573810
5.  Differential Roles of ATM- and Chk2-Mediated Phosphorylations of Hdmx in Response to DNA Damage†  
Molecular and Cellular Biology  2006;26(18):6819-6831.
The p53 tumor suppressor plays a major role in maintaining genomic stability. Its activation and stabilization in response to double strand breaks (DSBs) in DNA are regulated primarily by the ATM protein kinase. ATM mediates several posttranslational modifications on p53 itself, as well as phosphorylation of p53's essential inhibitors, Hdm2 and Hdmx. Recently we showed that ATM- and Hdm2-dependent ubiquitination and subsequent degradation of Hdmx following DSB induction are mediated by phosphorylation of Hdmx on S403, S367, and S342, with S403 being targeted directly by ATM. Here we show that S367 phosphorylation is mediated by the Chk2 protein kinase, a downstream kinase of ATM. This phosphorylation, which is important for subsequent Hdmx ubiquitination and degradation, creates a binding site for 14-3-3 proteins which controls nuclear accumulation of Hdmx following DSBs. Phosphorylation of S342 also contributed to optimal 14-3-3 interaction and nuclear accumulation of Hdmx, but phosphorylation of S403 did not. Our data indicate that binding of a 14-3-3 dimer and subsequent nuclear accumulation are essential steps toward degradation of p53's inhibitor, Hdmx, in response to DNA damage. These results demonstrate a sophisticated control by ATM of a target protein, Hdmx, which itself is one of several ATM targets in the ATM-p53 axis of the DNA damage response.
PMCID: PMC1592859  PMID: 16943424
6.  DNA Damage-Induced Phosphorylation of MdmX at Serine 367 Activates p53 by Targeting MdmX for Mdm2-Dependent Degradation†  
Molecular and Cellular Biology  2005;25(21):9608-9620.
Understanding how p53 activity is regulated is crucial in elucidating mechanisms of cellular defense against cancer. Genetic data indicate that Mdmx as well as Mdm2 plays a major role in maintaining p53 activity at low levels in nonstressed cells. However, biochemical mechanisms of how Mdmx regulates p53 activity are not well understood. Through identification of Mdmx-binding proteins, we found that 14-3-3 proteins are associated with Mdmx. Mdmx harbors a consensus sequence for binding of 14-3-3. Serine 367 (S367) is located within the putative binding sequence for 14-3-3, and its substitution with alanine (S367A) abolishes binding of Mdmx to 14-3-3. Transfection assays indicated that the S367A mutation, in cooperation with Mdm2, enhances the ability of Mdmx to repress the transcriptional activity of p53. The S367A mutant is more resistant to Mdm2-dependent ubiquitination and degradation than wild-type Mdmx, and Mdmx phosphorylated at S367 is preferentially degraded by Mdm2. Several types of DNA damage markedly enhance S367 phosphorylation, coinciding with increased binding of Mdmx to 14-3-3 and accelerated Mdmx degradation. Furthermore, promotion of growth of normal human fibroblasts after introduction of Mdmx is enhanced by the S367 mutation. We propose that Mdmx phosphorylation at S367 plays an important role in p53 activation after DNA damage by triggering Mdm2-dependent degradation of Mdmx.
PMCID: PMC1265801  PMID: 16227609
7.  Amplification of Mdmx (or Mdm4) Directly Contributes to Tumor Formation by Inhibiting p53 Tumor Suppressor Activity 
Molecular and Cellular Biology  2004;24(13):5835-5843.
Human tumors are believed to harbor a disabled p53 tumor suppressor pathway, either through direct mutation of the p53 gene or through aberrant expression of proteins acting in the p53 pathway, such as p14ARF or Mdm2. A role for Mdmx (or Mdm4) as a key negative regulator of p53 function in vivo has been established. However, a direct contribution of Mdmx to tumor formation remains to be demonstrated. Here we show that retrovirus-mediated Mdmx overexpression allows primary mouse embryonic fibroblast immortalization and leads to neoplastic transformation in combination with HRasV12. Furthermore, the human Mdmx ortholog, Hdmx, was found to be overexpressed in a significant percentage of various human tumors and amplified in 5% of primary breast tumors, all of which retained wild-type p53. Hdmx was also amplified and highly expressed in MCF-7, a breast cancer cell line harboring wild-type p53, and interfering RNA-mediated reduction of Hdmx markedly inhibited the growth potential of these cells in a p53-dependent manner. Together, these results make Hdmx a new putative drug target for cancer therapy.
PMCID: PMC480894  PMID: 15199139
8.  Critical Role for a Central Part of Mdm2 in the Ubiquitylation of p53 
Molecular and Cellular Biology  2003;23(14):4929-4938.
The stability of the p53 protein is regulated by Mdm2. By acting as an E3 ubiquitin ligase, Mdm2 directs the ubiquitylation of p53 and its subsequent degradation by the 26S proteasome. In contrast, the Mdmx protein, although structurally similar to Mdm2, cannot ubiquitylate or degrade p53 in vivo. To ascertain which domains determine this functional difference between Mdm2 and Mdmx and consequently are essential for p53 ubiquitylation and degradation, we generated Mdm2-Mdmx chimeric constructs. Here we show that, in addition to a fully functional Mdm2 RING finger, an internal domain of Mdm2 (residues 202 to 302) is essential for p53 ubiquitylation. Strikingly, the function of this domain can be fulfilled in trans, indicating that the RING domain and this internal region perform distinct activities in the ubiquitylation of p53.
PMCID: PMC162227  PMID: 12832478
9.  Mutational Analysis of Fibrillarin and Its Mobility in Living Human Cells 
The Journal of Cell Biology  2000;151(3):653-662.
Cajal bodies (CBs) are subnuclear organelles that contain components of a number of distinct pathways in RNA transcription and RNA processing. CBs have been linked to other subnuclear organelles such as nucleoli, but the reason for the presence of nucleolar proteins such as fibrillarin in CBs remains uncertain. Here, we use full-length fibrillarin and truncated fibrillarin mutants fused to green fluorescent protein (GFP) to demonstrate that specific structural domains of fibrillarin are required for correct intranuclear localization of fibrillarin to nucleoli and CBs. The second spacer domain and carboxy terminal alpha-helix domain in particular appear to target fibrillarin, respectively, to the nucleolar transcription centers and CBs. The presence of the RNP domain seems to be a prerequisite for correct targeting of fibrillarin. Time-lapse confocal microscopy of human cells that stably express fibrillarin-GFP shows that CBs fuse and split, albeit at low frequencies. Recovered fluorescence of fibrillarin-GFP in nucleoli and CBs after photobleaching indicates that it is highly mobile in both organelles (estimated diffusion constant ∼0.02 μm2 s−1), and has a significantly larger mobile fraction in CBs than in nucleoli.
PMCID: PMC2185578  PMID: 11062265
nucleolus; Cajal (coiled) body; confocal microscopy; fibrillarin; transfection
10.  Distinct Regulation of p53 and p73 Activity by Adenovirus E1A, E1B, and E4orf6 Proteins 
Molecular and Cellular Biology  1999;19(5):3885-3894.
Multiple adenovirus (Ad) early proteins have been shown to inhibit transcription activation by p53 and thereby to alter its normal biological functioning. Since these Ad proteins affect the activity of p53 via different mechanisms, we examined whether this inhibition is target gene specific. In addition, we analyzed whether the same Ad early proteins have a comparable effect on transcription activation by the recently identified p53 homologue p73. Our results show that the large E1B proteins very efficiently inhibited the activity of p53 on the Bax, p21Waf1, cyclin G, and MDM2 reporter constructs but had no effect on the activation of the same reporter constructs by p73, with the exception of some inhibition of the Bax promoter by Ad12 E1B. The repressive effect of the E1A proteins on p53 activity is less than that seen with the large E1B proteins, but the E1A proteins inhibit the activity of both p53 and p73. We could not detect significant inhibition of p53 functions by E4orf6, but a clear repression of the transcription activation by p73 by this Ad early protein was observed. In addition, we found that stable expression of the Ad5 E1A and that of the E1B protein both caused increased p73 protein expression. The large E1B and the E4orf6 proteins together do not target the p73 protein for rapid degradation after adenoviral infection, as has previously been found for the p53 protein, probably because the large E1B protein does not interact with p73. Our results suggest that the p53 and p73 proteins are both inactivated after Ad infection and transformation but via distinct mechanisms.
PMCID: PMC84246  PMID: 10207112
11.  Oncogenic functions of hMDMX in in vitro transformation of primary human fibroblasts and embryonic retinoblasts 
Molecular Cancer  2011;10:111.
In around 50% of all human cancers the tumor suppressor p53 is mutated. It is generally assumed that in the remaining tumors the wild-type p53 protein is functionally impaired. The two main inhibitors of p53, hMDM2 (MDM2) and hMDMX (MDMX/MDM4) are frequently overexpressed in wild-type p53 tumors. Whereas the main activity of hMDM2 is to degrade p53 protein, its close homolog hMDMX does not degrade p53, but it represses its transcriptional activity. Here we study the role of hMDMX in the neoplastic transformation of human fibroblasts and embryonic retinoblasts, since a high number of retinoblastomas contain elevated hMDMX levels.
We made use of an in vitro transformation model using a retroviral system of RNA interference and gene overexpression in primary human fibroblasts and embryonic retinoblasts. Consecutive knockdown of RB and p53, overexpression of SV40-small t, oncogenic HRasV12 and HA-hMDMX resulted in a number of stable cell lines representing different stages of the transformation process, enabling a comparison between loss of p53 and hMDMX overexpression. The cell lines were tested in various assays to assess their oncogenic potential.
Both p53-knockdown and hMDMX overexpression accelerated proliferation and prevented growth suppression induced by introduction of oncogenic Ras, which was required for anchorage-independent growth and the ability to form tumors in vivo. Furthermore, we found that hMDMX overexpression represses basal p53 activity to some extent. Transformed fibroblasts with very high levels of hMDMX became largely resistant to the p53 reactivating drug Nutlin-3. The Nutlin-3 response of hMDMX transformed retinoblasts was intact and resembled that of retinoblastoma cell lines.
Our studies show that hMDMX has the essential properties of an oncogene. Its constitutive expression contributes to the oncogenic phenotype of transformed human cells. Its main function appears to be p53 inactivation. Therefore, developing new drugs targeting hMDMX is a valid approach to obtain new treatments for a subset of human tumors expressing wild-type p53.
PMCID: PMC3179748  PMID: 21910853
Transformation model; p53 pathway; tumorigenesis; hMDMX; hMDM2; retinoblastoma; Nutlin-3

Results 1-11 (11)