Soft Tissue Sarcomas (STS) represent a heterogeneous group of mesenchymal tumours from various tissues of origin that display a spectrum of distributions across the age groups. These relatively rare tumours account for 1% of all cancers and have a poor prognosis, due to high recurrence rates and distant metastasis. The overall five year survival of STS is 50%. This has remained unchanged for the past 15 years .
Liposarcomas (LS) account for 20% of STS and are the most common type of STS in adult life. They are morphologically classified into five main subgroups: well-differentiated (WDLS); de-differentiated (DDLS); myxoid (MXLS); round cell (RCLS); and pleomorphic (PLLS). Cytogenetically, WDLS and DDLS characteristically show amplification of the MDM2 gene  and MXLS/RCLS usually have a specific chromosomal translocation t(12;16)(q13;p11) . The mainstay of treatment of LS is radical surgical excision with the use of adjuvant radiotherapy for intermediate and high grade tumours. Approximately 25% of liposarcomas present in the retroperitoneum. At this site, efforts to achieve wide clear surgical margins are more challenging, especially posteriorly, due to anatomical constraints. Conventional chemotherapies have an unproven role in the neo-adjuvant setting. They are mainly prescribed for advanced, inoperable and recurrent sarcomas, but with no significant evidence that they provide an improved survival rate . Therefore new effective, systemic, targeted therapies are clearly needed to improve the outcome for these tumours.
P53 is a key regulator of the cell cycle, apoptosis, DNA repair and cellular senescence . Mutations or deletions in P53 are seen in approximately 50% of all human cancers . However, the incidence of P53 mutations in STS had been reported to be significantly lower. Previous analyses have estimated that only 17% of liposarcomas have a P53 mutation [6, 7]. This observation emphasises the important role that other mechanisms probably play, which render wild type P53 inactive in the carcinogenic transformation of liposarcomas.
It is known that non-sarcomatous malignancies with wild type P53 usually demonstrate a clinical pattern that is more responsive to chemotherapy and radiotherapy . This response is not seen in liposarcomas due to the lack of targeted therapies against specific pathways of particular significance in STS formation. The best characterized pathway of this type is the interaction of wild type P53 with its main cellular inhibitors, the “murine double minute” 2 (MDM2) and the “murine double minute” X (MDMX) proteins .
MDM2 and P53 regulate each other’s functions through an auto-regulatory feedback loop. Upon activation, P53 promotes transcription of the MDM2 gene and, in turn, the MDM2 protein inhibits P53 activity. This inhibition is achieved mainly through MDM2 acting as a ubiquitin E3 ligase for P53, thus targeting P53 for proteasomal degradation . Although amplification of the MDM2 gene is seen in nearly 100% of WDLS and DDLS, over-expression of MDM2 protein is only observed in approximately 75% of these subtypes by immunohistochemistry . High levels of MDM2 mRNA have been reported as a negative prognostic factor in STS, including liposarcomas . It may be of prognostic significance that the phenomenon of MDM2-mediated P53 inactivation has a predilection to occur more often in retroperitoneal liposarcomas, compared to those that arise in the extremities .
MDMX (also known as MDM4) is an MDM2 homolog, which was described after MDM2 . The two proteins share striking structural similarities as both are comprised of an N-terminal hydrophobic pocket for P53 binding, a central acidic domain, a zinc-chelating structure and a RING (‘really interesting new gene’) domain of a rare C2H2C4 structural type, for potential binding to generate heterodimers . A significant body of evidence suggests that MDMX is, in addition, an independent negative regulator of P53 . However, in contrast to MDM2, MDMX lacks an intrinsic E3 ligase activity  due to structural differences in its RING and central acidic domains, compared to MDM2 . This particular feature of MDMX has provoked some controversy about its exact role in the STS transformation process .
Some studies have demonstrated that MDMX enhances the effects of MDM2 by inhibiting the latter’s self-ubiquitinylation and therefore increasing its relatively short cellular half-life. As a result, MDM2 is able to achieve increased P53 degradation [19–21]. MDMX forms heterodimers with MDM2, which also stimulates the ability of the latter to degrade P53 . Other studies, however, have suggested that MDMX may stabilise P53 and, in fact, antagonise the MDM2-targeted degradation of P53 [16, 23].
The mutual dependence model described by Gu et al., in modified cell lines, suggested that the two proteins rely on one another to sustain a potent P53 inhibition . The exact cellular functions of MDMX were noted to vary between activation and inhibition of MDM2 depending on the former’s relative expression levels in relation to MDM2 . This model provided an explanation for some of the controversies surrounding MDMX functions in cell lines, in a relatively coherent manner. However, it has previously lacked support from careful descriptive studies performed on actual human sarcoma tissue.
MDMX gene amplification had been detected in 17% of human LS . Recent studies have also reported MDMX co-amplification with MDM2 in some STS subtypes, particularly in LS [25, 26]. In addition, the over-expression of MDM2 and/or MDMX is generally accepted to correlate with retained wild type P53 . However, previous analysis of MDMX over-expression and of its relative co-expression with MDM2 in human liposarcomas is lacking. In this study, we aimed to characterize various subtypes of adult human liposarcomas in relation to their simultaneous expression levels of MDM2, MDMX and P53. Such a characterization has become a pertinent task due to the exponential growth of MDM2/MDMX single and dual affinity blocking compounds that have emerged in recent years as an attractive targeted therapeutic approach [28–34]. This characterization may also provide insights into the cellular function of MDMX in liposarcomas and may guide future functional studies to evaluate the utility of novel, dual MDM2/MDMX blocking compounds in the treatment of STS .