The mouse double minute 2 (MDM2) protein is an E3-ubiquitin ligase recognized for its role as a negative regulator of the tumor suppressor protein p53 [1]. Genetic studies showing that embryonic lethality of mice lacking MDM2 could only be rescued by the concomitant deletion of Tp53 suggested that the main physiological role of MDM2 is to inhibit p53. Furthermore, MDM2/p53 double knock-out (KO) mice displayed a cancer incidence similar to that observed in Tp53 KO animals, supporting the notion that the major role of MDM2 is to interfere with p53 activities [2, 3]. However, several reports have challenged this notion and convincingly showed that MDM2 functions extend beyond p53 regulation both in normal and cancer cells.

P53-independent functions of MDM2 in cancer

A first hint came from the analysis of transgenic mice overexpressing MDM2 in the absence of functional p53. These mice displayed higher incidence of sarcomas compared to p53-null animals, indicating a role for MDM2 in tumorigenesis independent of p53 [4]. This notion was further supported by the ability of MDM2 to transform p53-null cells in vitro [5] and by the identification of several MDM2 splice variants that are unable to bind p53 in certain tumor types [6]. Moreover, although rare, some soft tissue tumors exhibit both overexpression of MDM2 and mutations of Tp53, a feature that correlates with a higher tumor grade and a poorer prognosis compared to cancer patients harboring only one of these genetic alterations [7].

Different mechanisms through which MDM2 could affect the process of tumorigenesis independently of p53 have been proposed, including: (1) ubiquitination and consequent proteasomal degradation of other proteins involved in tumor suppression, (2) induction of genomic instability and DNA damage, and (3) stabilization of mRNAs encoding pro-tumorigenic/metastatic factors [1, 8]. Several reports have also highlighted a role for MDM2 in transcriptional regulation and epigenetics. It is noteworthy that, depending on the cellular context and upon specific stress conditions, chromatin-bound MDM2 can activate or repress the execution of both p53-dependent and p53-independent transcriptional programs. Already recognized as a repressor of p53-mediated transcription and as a post-translational regulator of histones at p53-responsive genes, MDM2 is also able to regulate gene expression in a p53-independent manner [9]. Thus, on the one hand, findings from Dobbelstein and colleagues indicated that chromatin-bound MDM2 transcriptionally represses the expression of lineage-specific genes through its physical interaction with components of the polycomb repressive complex 2 (PRC2) to promote stemness and cancer cell survival [10]. On the other hand, our group previously demonstrated that upon deprivation of exogenous serine and glycine, MDM2 is recruited to chromatin in a p53-independent manner to induce the expression of genes implicated in de novo serine synthesis, amino-acid metabolism and transport as well as in glutathione homeostasis. In this experimental context, MDM2 recruitment to its target genes requires the transcription factors ATF3/4 and is negatively regulated by an important metabolic enzyme, the M2-isoform of pyruvate kinase (PKM2) (Fig. 1). Consistent with this finding, interfering with MDM2 function when cancer cells face a limited availability of exogenous serine and glycine synergistically decreased tumor growth in xenograft assays, suggesting that MDM2-mediated control of serine/glycine metabolism represents an important driver of oncogenesis in tumors overexpressing MDM2 [11].

Fig. 1
figure 1

Schematic representation of p53-independent functions of MDM2 in transcription. Upon specific stress conditions, MDM2 can be recruited to chromatin and/or to the mitochondrial matrix. In basal conditions, MDM2 is sequestered in the cytosol by the pyruvate kinase M2 isoform (PKM2); following serine/glycine deprivation or upon oxidative stress, PKM2 inhibition promotes MDM2 translocation to chromatin at ATF3/4 responsive genes to induce the expression of genes involved in de novo serine synthesis, amino acid transport and glutathione homeostasis. Depending on the cellular context, chromatin-bound MDM2 is also able to inhibit gene expression, as demonstrated by its interaction with the polycomb repressor complex 2 (PRC2) which results in transcriptional repression of lineage-specific genes. Under hypoxic conditions or oxidative stress, a fraction of MDM2 is actively translocated  into the mitochondrial matrix. Mitochondrial MDM2 selectively binds to the mtDNA light strand promoter (LSP) and impairs transcription of MT-ND6, a gene encoding a subunit of mitochondrial electron transport chain complex I (CI). By reducing the expression of MT-ND6, mitochondrial MDM2 specifically impinges on CI activity and CI-driven respiration, resulting in accumulation of mitochondrial ROS

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Underlying the importance of MDM2 in the adaptation of cancer cells to their environmental conditions, we recently found that, under hypoxia, a fraction of cytoplasmic MDM2 also translocates in the mitochondrial matrix to control the transcription of the mitochondrial genome independently of p53. Mitochondrial MDM2 (mtMDM2) specifically binds the light strand promoter (LSP) of mitochondrial DNA (mtDNA) and repress the LSP-driven transcription of MT-ND6, a mitochondrial gene encoding an important structural subunit of complex I (CI) of the electron transport chain (ETC) (Fig. 1) [12]. mtMDM2-mediated inhibition of CI-driven respiration causes a significant increase in mitochondrial ROS production, an event previously associated with enhanced invasiveness and metastatic dissemination of cancer cells [13]. Consistent with this notion, lung cancer cells overexpressing a mitochondrial-targeted MDM2 mutant exhibited increased motility and pronounced migratory features. Thus, both chromatin-bound and mitochondrial MDM2 seem to contribute to carcinogenesis but further studies are required to better understand whether these 2 pools of MDM2 operate in parallel or are differentially activated during distinct phases of cancer progression. Some, but not all, types of oxidative stress can trigger the recruitment of MDM2 both in mitochondria and to chromatin, suggesting a coordinated action of these two pools of MDM2 protein that may contribute to the global metabolic reprogramming of cancer cells. However, it is also possible that the two MDM2 pools respond to distinct changes of the tumor microenvironment and play a complementary role at different steps of cancer progression in order: (1) to sustain cancer cell proliferation during amino acid starvation (chromatin-bound MDM2), and (2) to promote cancer cell migration and metastasis under conditions of low tumor oxygenation (mitochondrial MDM2). Notably, not only p53 but also the E3-ligase activity of MDM2 are dispensable for the transcriptional activities of both chromatin-bound and mtMDM2 on its metabolic and mitochondrial target genes, reinforcing the notion that MDM2 oncogenic activities extend beyond the degradation of p53.

Physiological roles of MDM2 independent of p53

Although the importance of serine and glycine metabolism regulation by chromatin-bound MDM2 in normal tissues remains to be determined, our data unambiguously show that accumulation of MDM2 in mitochondria under hypoxic conditions occurs not only in cancer cells but also in non-transformed cells. Thus, the amount of mtMDM2 increased in skeletal muscles of mice exposed to mild hypoxia, and correlated with decreased MT-ND6 mRNA and protein levels. Accordingly, mice lacking both MDM2 and p53 in their hind limbs displayed increased MT-ND6 levels and enhanced mitochondrial complex I activity relative to their corresponding p53 null control animals. As a consequence, MDM2-deficient mice exhibited increased endurance capacity in mild hypoxic conditions, a phenotype that was observed even in the absence of p53 [12]. Whether MDM2 recruitment to mitochondria occurs in other tissues than skeletal muscles and contributes to normal tissue homeostasis is currently under investigation. It is also tempting to speculate that deregulation of mtMDM2 activities plays an important role in pathologies other than cancer. One potential example is in Parkinson disease, a neurological disorder associated with ETC CI defects [14]. Our data also open new avenues to evaluate whether perturbations of mtMDM2 activities are causally involved in mitochondriopathies of unknown etiology. Moreover, the recent striking observation that chronic exposure to hypoxia markedly increased the survival and improved the phenotypic manifestations of a mouse model of Leigh syndrome [15], a pediatric mitochondrial disease commonly associated with ETC CI deficiency, raises interesting questions regarding the potential interest of pharmacological modulation of mtMDM2 activities for new therapies.