ReviewMDM2 — master regulator of the p53 tumor suppressor protein
Introduction
Murine Double Minute Clone 2 (MDM2) was originally cloned from purified acentric chromosomes harbored within a spontaneously transformed Balb/c3T3 cell line called 3T3DM (Cahilly-Snyder et al., 1987). The rationale for cloning genes from these abnormal chromosomes, also known as double minutes, is that they often contain amplified genes that contribute to cellular proliferation and tumorigenesis. MDM2 was the second of two tandem genes cloned together from these amplified sequences. When a genomic clone of MDM2 was amplified in rodent cells, it conferred high tumorigenic potential in nude mice (Fakharzadeh et al., 1991). This observation gave the first suggestion that MDM2 is an oncogene. Independently, researchers in another laboratory were isolating proteins bound to the p53 tumor suppressor protein with the rationale that such proteins might regulate p53 activity (Momand et al., 1992). In a cultured transformed rat fibroblast line that overexpresses a temperature-sensitive mutant p53, a 90 kDa phosphoprotein was observed to co-immunoprecipitate with p53 at the permissive temperature. The p90 kDa protein turned out to be the product of the MDM2 gene. It was subsequently found that overexpression of the MDM2 gene blocked p53-mediated transactivation of a reporter gene bearing a p53-responsive element. Thus, a potential mechanism of MDM2-mediated oncogenicity was established-inactivation of the p53 tumor suppressor gene. In a third laboratory, the human homologue of MDM2 was mapped to chromosome 12q13–14 and was shown to be amplified in approximately 30% of osteosarcomas and soft tissue tumors (Oliner et al., 1992). Therefore, within a span of 2 years a new oncogene was discovered, a mechanism of its action was proposed, and its involvement in human cancers was established.
Since 1992, researchers have uncovered a clearer picture of how MDM2 modulates p53 activity and of the prevalence of MDM2 abnormalities in human cancers. Several reviews have covered MDM2 (Freedman et al., 1999, Haines, 1997, Juven-Gershon and Oren, 1999, Lane and Hall, 1997, Momand and Zambetti, 1997, Piette et al., 1997, Prives, 1998); however, the research pace has quickened and some major discoveries have been made quite recently. The purpose of this review is to point out fundamental milestones reached during the course of research into this interesting gene and to present an in-depth analysis of more recent progress in our understanding of MDM2's varied functions. Finally, we will chart potential areas of future research that will likely be important in furthering our understanding of this critical growth-control molecule. At the outset, we wish to apologize to the authors of those studies that could not be cited owing to space constraints.
Section snippets
Sequence analysis and species representation
To date, the MDM2 gene has been sequenced in human, hamster, mouse, zebrafish and frog (Fig. 1). Based on sequence similarity, a highly related gene, MDMX, was cloned from human and mouse. Alignment of the four MDM2 and two MDMX gene sequences highlights three regions of high identity, dubbed CR1, CR2 and CR3. Previously, these three conserved regions were identified using fewer MDM2 gene sequences (Piette et al., 1997). According to our analysis, the percent identity shared within CR1, CR2 and
Control of p53 stability
How does MDM2 control p53 function? Before answering this question it is necessary to briefly review the basic functions of p53. An overwhelming amount of evidence indicates that p53 acts as a checkpoint gene (Giaccia and Kastan, 1998, Kastan et al., 1991). In response to DNA damage and other types of stress, such as heat shock, hypoxia and hyperoxia, p53 is responsible for either blocking cell cycle progression or instigating programmed cell death (apoptosis). In response to most stressors,
Transactivation block
Increasing the proteolytic susceptibility of p53 is one mechanism by which MDM2 can turn off p53, but there is another mechanism as well. The p53 protein binds directly to DNA promoter sequences at a consensus sequence (El-Deiry et al., 1992) and transactivates a variety of genes to mediate cell cycle arrest and, in some cell types, to mediate apoptosis (Levine, 1997). Many transactivation factors have domains rich in acidic amino acid residues that appear to be required for increasing RNA
The p53 negative feedback loop
Early studies of MDM2 showed that p53 overexpression roughly correlates with MDM2 protein upregulation (Barak and Oren, 1992). On the surface, this observation seems at odds with studies showing that MDM2 leads to p53 proteolysis. It is now clear that MDM2 and p53 are involved in a negative feedback loop. In this loop, p53 activates MDM2, which, in turn, downregulates p53. p53 upregulates MDM2 at the transcription level (Barak et al., 1993, Wu et al., 1993). Sequence analysis of the MDM2
Regulation of p53–MDM2 complex formation
To date, three post-translational events have been demonstrated to affect p53–MDM2 complex formation: phosphorylation, oligomerization and binding to other proteins. Phosphorylation of sites within or near the p53–MDM2 interaction domains might be expected to modulate complex formation. Oligomerization of p53 might also affect binding of MDM2 if oligomerization alters the conformation of the MDM2 binding domain of p53. Finally, evidence suggests that the tumor suppressor protein p19Arf/p14Arf
p73
Proteins that bear an MDM2 binding motif similar to that of p53 are strong candidates for regulation by MDM2. Recently, proteins have been discovered that share significant sequence identity with p53 (Kaghad et al., 1997, Osada et al., 1998, Yang et al., 1998). These proteins, named p73, p63 and p51, encode N-terminal sequences that are very similar to the region of p53 that interacts with MDM2 (Fig. 2B). One of these p53-like proteins, p73, is capable of upregulating p53-responsive genes (
MDM2 in human tumors
When MDM2 gene amplifications were first detected in soft tissue tumors and osteosarcomas many investigators analyzed other tumors and malignancies for MDM2 gene amplification and MDM2 protein overexpression. To date, the overall gene amplification frequency is known to be 7%, with the highest frequency of amplification in soft tissue tumors (20%) (Momand et al., 1998). Because MDM2 plays an important role in controlling p53 activity it is predicted that those cancers with MDM2 amplification
Acknowledgements
The authors gratefully acknowledge the support of the University of California Breast Cancer Research Program (1KB-0102) and critical reading of the manuscript by Dr Susan Kane and Ms. Saori Furuta. The authors also thank Dr Yosef Shiloh and Dr Moshe Oren for sharing data prior to publication.
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2022, BiochimieCitation Excerpt :Murine double minute 2 or MDM2 is an oncogene that encodes MDM2 protein [51]. These proteins bind to p53 and inhibits this protein from functioning [51]. Peroietti et al. [52,53] conducted two studies on melatonin and indicated that this agent is able to decrease the amounts of MDM2 and thereby influence the activity of p53.
- 1
Present address: Department of Anatomy and Neurobiology, University of California at Irvine, 364 MedSurge II, Irvine, CA 92697, USA.
- 2
Present address: Children's Hospital at Orange County-Cancer Research, 455 S. Main Street, Orange, CA 92868, USA.