Mutations in TP53, the gene that encodes the tumour suppressor p53, are found in 50% of human cancers, and increased levels of its negative regulators MDM2 and MDM4 (also known as MDMX) downregulate p53 function in many of the rest. Understanding p53 regulation remains a crucial goal to design broadly applicable anticancer strategies based on this pathway. This Review of in vitro studies, human tumour data and recent mouse models shows that p53 post-translational modifications have modulatory roles, and MDM2 and MDM4 have more profound roles for regulating p53. Importantly, MDM4 emerges as an independent target for drug development, as its inactivation is crucial for full p53 activation.

Regulating the p53 pathway: in vitro hypotheses, in vivo veritas

The MDM2 and MDMX (also known as HDMX and MDM4) proteins are deregulated in many human cancers and exert their oncogenic activity predominantly by inhibiting the p53 tumour suppressor. However, the MDM proteins modulate and respond to many other signalling networks in which they are embedded. Recent mechanistic studies and animal models have demonstrated how functional interactions in these networks are crucial for maintaining normal tissue homeostasis, and for determining responses to oncogenic and therapeutic challenges. This Review highlights the progress made and pitfalls encountered as the field continues to search for MDM-targeted antitumour agents.

/pdf/mdm2-mdmx-and-p53-in-oncogenesis-and-cancer-therapy-fq03b3bj72.pdf

MDM2, MDMX and p53 in oncogenesis and cancer therapy.

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

Tumour suppression by p53: a role for the DNA damage response?

https://www.cell.com/trends/molecular-medicine/pdf/S1471-4914(06)00259-0.pdf

MDM2 inhibitors for cancer therapy

Over the years, p53 has been shown to sit at the centre of an increasingly complex web of incoming stress signals and outgoing effector pathways The number and diversity of stress signals that lead to p53 activation illustrates the breadth of p53's remit - responding to a wide variety of potentially oncogenic insults to prevent tumour development Interestingly, different stress signals can use different and independent pathways to activate p53, and there is some evidence that different stress signals can mediate different responses How each of the responses to p53 contributes to inhibition of malignant progression is beginning to be clarified, with the hope that identification of responses that are key to tumour suppression will allow a more focused and effective search for new therapeutic targets In this review, we will highlight some recently identified roles for p53 in tumour suppression, and discuss some of the numerous mechanisms through which p53 can be regulated and activated

Coping with stress: multiple ways to activate p53

The Mdm2 and Mdm4 oncoproteins are key negative regulators of the p53 tumor suppressor. However, their physiological contributions to the regulation of p53 stability and activity remain highly controversial. Here, we combined a p53 knock-in allele, in which p53 is silenced by a transcriptional stop element flanked by loxP sites, with the mdm2- and mdm4-null alleles. This approach allows Cre-mediated conditional p53 expression in tissues in vivo and cells in vitro lacking Mdm2, Mdm4, or both. Using this strategy, we show that Mdm2 and Mdm4 are essential in a nonredundant manner for preventing p53 activity in the same cell type, irrespective of the proliferation/differentiation status of the cells. Although Mdm2 prevents accumulation of the p53 protein, Mdm4 contributes to the overall inhibition of p53 activity independent of Mdm2. We propose a model in which Mdm2 is critical for the regulation of p53 levels and Mdm4 is critical for the fine-tuning of p53 transcriptional activity, both proteins acting synergistically to keep p53 in check.

Pascal Froment

Papers

Mdm4 and Mdm2 cooperate to inhibit p53 activity in proliferating and quiescent cells in vivo

Distinct roles of Mdm2 and Mdm4 in red cell production