Aberrant activation of p53 due to loss of MDM2 or MDMX causes early lens dysmorphogenesis

The roles of mouse double minute 2 (MDM2) oncoprotein in ocular diseases: A review

Mouse double minute 2 (MDM2), an E3 ubiquitin ligase and the primary negative regulator of the tumor suppressor p53, cooperates with its structural homolog MDM4/MDMX to control intracellular p53 level. In turn, overexpression of p53 upregulates and forms an autoregulatory feedback loop with MDM2. The MDM2-p53 axis plays a pivotal role in modulating cell cycle control and apoptosis. MDM2 itself is regulated by the PI3K-AKT and RB-E2F-ARF pathways. While amplification of the MDM2 gene or overexpression of MDM2 (due to MDM2 SNP T309G, for instance) is associated with various malignancies, numerous studies have shown that MDM2/p53 alterations may also play a part in the pathogenetic process of certain ocular disorders (Fig. 1). These include cancers (retinoblastoma, uveal melanoma), fibrocellular proliferative diseases (proliferative vitreoretinopathy, pterygium), neovascular diseases, degenerative diseases (cataract, primary open-angle glaucoma, age-related macular degeneration) and infectious/inflammatory diseases (trachoma, uveitis). In addition, MDM2 is implicated in retinogenesis and regeneration after optic nerve injury. Anti-MDM2 therapy has shown potential as a novel approach to treating these diseases. Despite major safety concerns, there are high expectations for the clinical value of reformative MDM2 inhibitors. This review summarizes important findings about the role of MDM2 in ocular pathologies and provides an overview of recent advances in treating these diseases with anti-MDM2 therapies.

MDM2 phosphorylation mediates H2O2-induced lens epithelial cells apoptosis and age-related cataract

Lens epithelial cells (LECs) apoptosis induced by oxidative stress is a major factor in age-related-cataract (ARC) pathogenesis, but there are still many blind nodes in this progress. This study aimed to investigate the effects of MDM2 phosphorylation in ARC and H₂O₂-induced lens epithelial cells apoptosis. Our results confirmed that the levels of p-MDM2 (Ser166) and p-MDM2 (Ser186) in the anterior lens capsules of human cataracts were reduced compared to that in normal capsules. Similarly, in naturally aging cataract mice, the level of MDM2 phosphorylation also decreased. Oxidative stress-induced apoptosis model was constructed by cultivating HLE-B3 cells with 200 μM H₂O₂. It was confirmed that MDM2 could regulate lens epithelial cell apoptosis, and MDM2 inhibitors could partly inhibited AKT's role in suppressing apoptosis induced by H₂O₂. Besides, we examed the decreased level of p-AKT(Ser473) in apoptosis of lens epithelial cells and ARC. Our study revealed that MDM2 phosphorylation mediated H₂O₂-induced lens epithelial cells apoptosis and ARC, which could provide new ideas for the clinical treatment of ARC.

Mdm4 controls ureteric bud branching via regulation of p53 activity

The antagonism between Mdm2 and its close homolog Mdm4 (also known as MdmX) and p53 is vital for embryogenesis and organogenesis. Previously, we demonstrated that targeted disruption of Mdm2 in the Hoxb7+ ureteric bud (Ub) lineage, which gives rise to the renal collecting system, causes renal hypodysplasia culminating in perinatal lethality. In this study, we examine the unique role of Mdm4 in establishing the collecting duct system of the murine kidney. Hoxb7Cre driven loss of Mdm4 in the Ub lineage (Ub^Mdm4-/-) disrupts branching morphogenesis and triggers UB cell apoptosis. Ub^Mdm4-/- kidneys exhibit abnormally dilated Ub tips while the medulla is hypoplastic. These structural alterations result in secondary depletion of nephron progenitors and nascent nephrons. As a result, newborn Ub^Mdm4-/- mice have hypo-dysplastic kidneys. Transcriptional profiling revealed downregulation of the Ret-tyrosine kinase pathway components, Gdnf, Wnt11, Sox8, Etv4 and Cxcr4 in the Ub^Mdm4-/- mice relative to controls. Moreover, the expression levels of the canonical Wnt signaling members Axin2 and Wnt9b are downregulated. Mdm4 deletion upregulated p53 activity and p53-target gene expression including Cdkn1a (p21), Gdf15, Ccng1, PERP, and Fas. Germline loss of p53 in Ub^Mdm4-/- mice largely rescues kidney development and terminal differentiation of the collecting duct. We conclude that Mdm4 plays a unique and vital role in Ub branching morphogenesis and collecting system development.

Mouse modelling of the MDM2/MDMX-p53 signalling axis

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.

Regulation of kidney development by the Mdm2/Mdm4-p53 axis

While p53 activity is required for tumour suppression, unconstrained p53 activity on the other hand is detrimental to the organism, resulting in inappropriate cellular death or proliferation defects. Unimpeded p53 activity is lethal in the developing embryo, underlining the need for maintaining a tight control on p53 activity during this period. The critical role of the negative regulators of p53, Mdm2 and Mdm4, in vertebrate development came to light by fatal disruption of embryogenesis that was observed with Mdm2 and Mdm4 gene deletions in mice. Embryonic lethality was rescued only by superimposing p53 removal. Here we summarize the contribution of the Mdm2/Mdm4–p53 axis that occurs at multiple steps of kidney development. Conditional, cell type-specific deletions reveal distinct functions of these proteins in renal morphogenesis. The severe impact on the renal phenotype from targeted gene deletions underscores the critical role played by the Mdm2/Mdm4–p53 nexus on nephrogenesis, and emphasizes the need to monitor patients with aberrations in this pathway for kidney function defects and associated cardiovascular dysfunction.

Negative auto-regulators trap p53 in their web

The transcriptional factor p53 activates the expression of a myriad of target genes involving a complicated signalling network, resulting in various cellular outcomes, such as growth arrest, senescence, apoptosis, and metabolic changes, and leading to consequent suppression of tumour growth and progression. Because of the profoundly adverse effect of p53 on growth and proliferation of cancer cells, several feedback mechanisms have been employed by the cells to constrain p53 activity. Two major antagonists MDM2 and MDMX (the long forms) are transcriptionally induced by p53, but in return block p53 activity, forming a negative feedback circuit and rendering chemoresistance of several cancer cells. However, they are not alone, as cancer cells also employ other proteins encoded by p53 target genes to inhibit p53 activity at transcriptional, translational, and posttranslational levels. This essay is thus composed to review a recent progress in understanding the mechanisms for how cancer cells hijack the p53 autoregulation by these proteins for their growth advantage and to discuss the clinical implications of these autoregulatory loops.

Monitoring p53 by MDM2 and MDMX is required for endocrine pancreas development and function in a spatio-temporal manner

Although p53 is not essential for normal embryonic development, it plays a pivotal role in many biological and pathological processes, including cell fate determination-dependent and independent events and diseases. The expression and activity of p53 largely depend on its two biological inhibitors, MDM2 and MDMX, which have been shown to form a complex in order to tightly control p53 to an undetectable level during early stages of embryonic development. However, more delicate studies using conditional gene-modification mouse models show that MDM2 and MDMX may function separately or synergistically on p53 regulation during later stages of embryonic development and adulthood in a cell and tissue-specific manner. Here, we report the role of the MDM2/MDMX-p53 pathway in pancreatic islet morphogenesis and functional maintenance, using mouse lines with specific deletion of MDM2 or MDMX in pancreatic endocrine progenitor cells. Interestingly, deletion of MDM2 results in defects of embryonic endocrine pancreas development, followed by neonatal hyperglycemia and lethality, by inducing pancreatic progenitor cell apoptosis and inhibiting cell proliferation. However, unlike MDM2-knockout animals, mice lacking MDMX in endocrine progenitor cells develop normally. But, surprisingly, the survival rate of adult MDMX-knockout mice drastically declines compared to control mice, as blockage of neonatal development of endocrine pancreas by inhibition of cell proliferation and subsequent islet dysfunction and hyperglycemia eventually lead to type 1 diabetes-like disease with advanced diabetic nephropathy. As expected, both MDM2 and MDMX deletion-caused pancreatic defects are completely rescued by loss of p53, verifying the crucial role of the MDM2 and/or MDMX in regulating p53 in a spatio-temporal manner during the development, functional maintenance, and related disease progress of endocrine pancreas. Also, our study suggests a possible mouse model of advanced diabetic nephropathy, which is complementary to other established diabetic models and perhaps useful for the development of anti-diabetes therapies.

Mdm proteins: critical regulators of embry ogenesis and homeostasis

Mdm2 and Mdm4 are negative regulators of the tumor suppressor p53; hence, this relationship is the focus of many cancer related studies. A multitude of experiments across various developmental stages have been conducted to explore the tissue-specific roles of these proteins in the mouse. When Mdm2 or Mdm4 are deleted in the germline or specific tissues, they display different phenotypic defects, some of which lead to embryonic lethality. Mdm2 loss is often more deleterious than loss of its homolog Mdm4 All tissues experience activation of p53 target genes upon loss of Mdm2 or Mdm4; however, the degree to which the p53 pathway is perturbed is highly tissue-specific and does not correlate to the severity of the morphological phenotypes. Therefore, a need for further understanding of how these proteins regulate p53 activity is warranted, as therapeutic targeting of the p53 pathway is rapidly evolving and gaining attention in the field of cancer research. In this review, we discuss the tissue-specificity of Mdm proteins in regulating p53 and expose the need for investigation at the cell-specific level.