Mouse modelling of the MDM2/MDMX−p53 signalling axis

34 j Journal of Molecular Cell Biology (2017), 9(1), 34–44 doi:10.1093/jmcb/mjx006 Published online February 8, 2017 Review Mouse modelling of the MDM2/MDMX−p53 signalling axis Nicole R. Tackmann1,2 and Yanping Zhang1,3,* 1 Department of Radiation Oncology, Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, USA Curriculum in Genetics and Molecular Biology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, USA 3 Jiangsu Center for the Collaboration and Innovation of Cancer Biotherapy, Cancer Institute, Xuzhou Medical College, Xuzhou 221002, China * Correspondence to: Yanping Zhang, Tel: +1-919-966-7713, Fax: +1-919-966-7681, E-mail: 2 It is evident that p53 activity is critical for tumour prevention and stress response through its transcriptional activation of genes affecting cellular senescence, apoptosis, cellular metabolism, and DNA repair. The regulation of p53 is highly complex, and MDM2 and MDMX are thought to be critical for deciding the fate of p53, both through inhibitory binding and post-translational modification. Many mouse models have been generated to study the regulation of p53 in vivo, and they have altered our interpretations of how p53 is regulated by MDM2 and MDMX. Although MDM2 is absolutely required for p53 regulation, certain functions are dispensable under unstressed conditions, including the ability of MDM2 to degrade p53. MDMX, on the other hand, may only be required in select situations, like embryogenesis. These models have also clarified how cellular stress signals modify the p53-inhibiting activities of MDM2 and MDMX in vivo. It is clear that more work will need to be performed to further understand the contexts for each of these signals and the requirements of various MDM2 and MDMX functions. Here, we will discuss what we have learned from mouse modelling of MDM2 and MDMX and underscore the ways in which these models could inform future therapies. Keywords: p53, MDM2, MDMX, E3 ubiquitin ligase, cancer Introduction The role of p53 as a tumour-suppressing transcription factor is abundantly clear, and it is well known that p53 is frequently mutated or inactivated in various cancers (Muller and Vousden, 2013). It is also apparent that p53 regulation is highly complex, but two proteins are critically important for proper control of p53: MDM2 and MDMX (also known as MDM4) (Wade et al., 2010). p53 transcription and translation are thought to occur ubiquitously, while MDM2 and MDMX cooperate to control both the posttranslational stability and activity of p53 (Hu et al., 2007; Wade et al., 2010). MDM2 is also a transcriptional target of p53 (Barak et al., 1993), which contributes to a feedback loop of regulation. MDM2, but not MDMX, harbours E3 ubiquitin ligase activity towards p53 (Haupt et al., 1997; Honda et al., 1997; Kubbutat Received November 23, 2016. Revised December 19, 2016. Accepted January 12, 2017. © The Author (2017). Published by Oxford University Press on behalf of Journal of Molecular Cell Biology, IBCB, SIBS, CAS. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs licence (http://creativecommons. org/licenses/by-nc-nd/4.0/), which permits non-commercial reproduction and distribution of the work, in any medium, provided the original work is not altered or transformed in any way, and that the work is properly cited. For commercial re-use, please contact et al., 1997; Jackson and Berberich, 2000), and both proteins can directly bind to the p53 transactivation domain and inhibit transcription (Chen et al., 1993; Shvarts et al., 1996). MDM2 and MDMX interact to form a heterodimer (Tanimura et al., 1999), which is thought to promote more efficient p53 inhibition. Although these activities have been clearly demonstrated in vitro, the relative importance of MDM2−p53 and/or MDMX−p53 binding, MDM2–MDMX heterodimer formation, or MDM2 E3 ligase activity towards in vivo p53 activity has been incompletely understood. For instance, it was previously thought that MDM2 E3 ligase activity was essential for basal p53 regulation, but evidence from mouse models suggests that MDM2 E3 ligase activity is dispensable under normal conditions (Tollini et al., 2014). The mechanisms of p53 regulation are still being elucidated. Studies in mouse models have both confirmed existing hypotheses and often challenged widely held beliefs about how MDM2 and MDMX function together to regulate p53. This review will address how in vitro and in vivo evidences have conflicted. We will first discuss what MDM2 and MDMX knockout mouse models have told us about how p53 is differentially regulated during embryogenesis and adulthood. Then, we will explore how knockin mouse models have clarified the mechanistic Mouse modelling of the MDM2/MDMX−p53 signalling axis cooperation of MDM2 and MDMX and the upstream signals that regulate their inhibition of p53. Last, we will comment on how mouse models could inform the discovery of novel drug targets or treatment strategies to fight cancer. Temporal and tissue-specific roles for MDM2 and MDMX: lessons from knockout mice Mdm2 knockout mice In the following section, we will review work from whole body MDM2 and MDMX knockout studies. For a more comprehensive discussion of tissue-specific deletion studies, please refer to an accompanying review by Guillermina Lozano and her colleagues (Moyer et al., 2017) in this special issue. Early in vitro work demonstrated that MDM2 could bind to p53 and mask p53 transactivation activity (Chen et al., 1993; Oliner et al., 1993). However, the degree of MDM2 importance to p53 regulation was not fully appreciated until the creation of Mdm2 deletion alleles in the mouse (Montes de Oca Luna et al., 1995; Jones et al., 1995). Interestingly, mice deficient for p53 are viable, but tend to develop tumours (typically lymphomas) and die by 6 months of age (Donehower et al., 1992). Surprisingly, mice deficient for MDM2 die between embryonic days 4.5–6.5, with pronounced levels of apoptosis. This embryonic lethality caused by loss of MDM2 is rescued by concomitant loss of p53, suggesting that the primary function of MDM2 during embryogenesis is to inhibit undue p53 activation or accumulation. These studies also established that MDM2 and p53 are expressed ubiquitously during embryonic development. It is also apparent that although MDM2 expression is found throughout the embryo and required during embryogenesis in the presence of p53, MDM2-mediated p53 regulation remains essential in the adult mouse as a whole. The p53-dependent embryonic lethality caused by MDM2 deficiency renders the study of MDM2 in p53 regulation difficult in vivo. To address this, Christophorou et al. (2005) developed a mouse model expressing the hormonebinding domain of a modified oestrogen receptor placed at the 3′ end of the p53 coding sequence, therefore generating a (...truncated)