When it comes to cancer, one molecule stands out as being among the most extensively studied: the p53 tumor suppressor protein. p53 can exist in cells in several different forms. When p53 is in its so-called wild-type form, it is capable of activating various responses that contribute to tumor suppression. In their recent review, Columbia postdoc Rafaela Muniz de Quieroz and colleagues summarize the vast scientific literature on two key regulators of p53: MDM2 and MDMX. Both MDM2 and MDMX are known to interact with p53 and disrupt its function. Their absence has been linked not only to increased cancer development, but also to a number of dysfunctions, including embryonic lethality in mice. MDM2 has been shown to negatively regulate p53 by diverse mechanisms spanning from expression of the p53 gene to degradation of the p53 protein or its expulsion from the cellular nucleus, where the protein accomplishes its function. Although very similar to MDM2, MDMX is less well studied. We do know, however, that MDMX is a protein that can work together with the MDM2 in p53 degradation.
While many reviews and studies have pointed to the roles of MDM2, and to a lesser extent of MDMX, in p53 regulation, the current review by Quieroz and her colleagues puts a larger focus on the myriad of p53-independent activities of MDM2 and MDMX. The authors provide important details about the p53-independent functions of both MDMX alone and as part of a MDM2–MDMX complex. The review discusses some key features in the structure and function of the proteins, including key parts that are relevant for their function, for some associated abnormalities, or for the formation of MDM2 and MDMX complexes.
MDM2 and MDMX are regulated on multiple levels within cells. These include regulation on the DNA level, including usage of several alternative promoters (DNA sequences needed to turn a gene on or off). One of the promoters of MDM2 and MDMX is regulated by their target p53, but there are also p53-independent promoters capable of switching on the genes of MDM2 and MDMS regardless of p53. In addition, numerous variations in the DNA sequences, the so-called single nucleotide polymorphisms (SNPs), affect the expression of the two genes and are relevant to different pathologies. Regulation on the RNA level includes co-transcriptional regulation like alternative splicing, as well as post-transcriptional regulation by microRNAs, long non-coding RNAs, circular RNAs, or RNA binding proteins. The review also presents a detailed characterization of the regulation of MDM2 and MDMX at the protein level, by summarizing data on numerous post-translational modifications or interacting partners of the two proteins, with regards to the different p53 contexts of the cited studies. Amongst the presented binding partners are some of the more recently identified interactors of the MDMs, which include proteins involved in the defense against several viruses. Overall, both MDM2 and MDMX stand out as extensively regulated at virtually every known level which according to the authors “attests to their relevance not only as inhibitors of p53 but of myriad other cellular activities and outcomes on their own”.
Since MDM2 and MDMX have majorly been studied in their relation to inhibit wild-type p53, of a particular interest stands a section of the review summarizing numerous processes in which the two proteins have been shown to be involved in cells lacking wild-type p53 (Figure 1).
As shown in Figure 1, the p53-independent roles of MDM2 and MDMX in cancer and in other pathologies are versatile. That hints to the importance of uncovering molecules that can modulate the deleterious effects associated with dysfunctions of the two MDMs. A summary of numerous molecules that were shown to regulate the two proteins and thus consist of potential therapeutic targets, are also discussed in the review. Again the authors put an emphasis on how such small molecules might be useful in cells that lack wild-type p53. This is important not only because the two proteins have multiple functions other than regulating wild-type p53 which can be studied in such cells, but also because an important percentage of tumors is characterized by absence of wild-type p53.
The last section of the review points out some outstanding questions and directions for future research. If the fascinating questions of the versatile p53-independent roles MDM2 and MDMX have sparked your interest, find out more from the original paper.