Secondary interaction between MDMX and p53 core domain inhibits p53 DNA binding

Identification and validation of the biomarkers related to ferroptosis in calcium oxalate nephrolithiasis

Ferroptosis is a specific type of programmed cell death characterized by iron-dependent lipid peroxidation. Understanding the involvement of ferroptosis in calcium oxalate (CaOx) stone formation may reveal potential targets for this condition. The publicly available dataset GSE73680 was used to identify 61 differentially expressed ferroptosis-related genes (DEFERGs) between normal kidney tissues and Randall's plaques (RPs) from patients with nephrolithiasis through employing weighted gene co-expression network analysis (WGCNA). The findings were validated through in vitro and in vivo experiments using CaOx nephrolithiasis rat models induced by 1% ethylene glycol administration and HK-2 cell models treated with 1 mM oxalate. Through WGCNA and the machine learning algorithm, we identified LAMP2 and MDM4 as the hub DEFERGs. Subsequently, nephrolithiasis samples were classified into cluster 1 and cluster 2 based on the expression of the hub DEFERGs. Validation experiments demonstrated decreased expression of LAMP2 and MDM4 in CaOx nephrolithiasis animal models and cells. Treatment with ferrostatin-1 (Fer-1), a ferroptosis inhibitor, partially reversed oxidative stress and lipid peroxidation in CaOx nephrolithiasis models. Moreover, Fer-1 also reversed the expression changes of LAMP2 and MDM4 in CaOx nephrolithiasis models. Our findings suggest that ferroptosis may be involved in the formation of CaOx kidney stones through the regulation of LAMP2 and MDM4.

MDMX in Cancer: A Partner of p53 and a p53-Independent Effector

The p53 tumor suppressor protein plays an important role in physiological and pathological processes. MDM2 and its homolog MDMX are the most important negative regulators of p53. Many studies have shown that MDMX promotes the growth of cancer cells by influencing the regulation of the downstream target gene of tumor suppressor p53. Studies have found that inhibiting the MDMX-p53 interaction can effectively restore the tumor suppressor activity of p53. MDMX has growth-promoting activities without p53 or in the presence of mutant p53. Therefore, it is extremely important to study the function of MDMX in tumorigenesis, progression and prognosis. This article mainly reviews the current research progress and mechanism on MDMX function, summarizes known MDMX inhibitors and provides new ideas for the development of more specific and effective MDMX inhibitors for cancer treatment.

MDM4 was associated with poor prognosis and tumor-immune infiltration of cancers

MDM4 is one of the MDM protein family and is generally recognized as the key negative regulator of p53. As a cancer-promoting factor, it plays a non-negligible role in tumorigenesis and development. In this article, we analyzed the expression levels of MDM4 in pan-cancer through multiple databases. We also investigated the correlations between MDM4 expression and prognostic value, immune features, genetic mutation, and tumor-related pathways. We found that MDM4 overexpression is often accompanied by adverse clinical features, poor prognosis, oncogenic mutations, tumor-immune infiltration and aberrant activation of oncogenic signaling pathways. We also conducted transcriptomic sequencing to investigate the effect of MDM4 on transcript levels in colon cancer and performed qPCR to verify this. Finally, we carried out some in vitro experiments including colony formation assay, chemoresistance and senescence-associated β-galactosidase activity assay to study the anti-tumor treatment effect of small molecule MDM4 inhibitor, NSC146109. Our research confirmed that MDM4 is a prognostic biomarker and potential therapeutic target for a variety of malignancies.

Protein disorder and autoinhibition: The role of multivalency and effective concentration

Regulation of protein binding through autoinhibition commonly occurs via interactions involving intrinsically disordered regions (IDRs). These intramolecular interactions can directly or allosterically inhibit intermolecular protein or DNA binding, regulate enzymatic activity, and control the assembly of large macromolecular complexes. Autoinhibitory interactions mediated by protein disorder are inherently transient, making their identification and characterization challenging. In this review, we explore the structural and functional diversity of disorder-mediated autoinhibition for a variety of biological mechanisms, with a focus on the role of multivalency and effective concentration. We also discuss the evolution of disordered motifs that participate in autoinhibition using examples where sequence conservation varies from high to low. In some cases, identifiable motifs that are essential for autoinhibition remain intact within a rapidly evolving sequence, over long evolutionary distances. Finally, we examine the potential of AlphaFold2 to predict autoinhibitory intramolecular interactions involving IDRs.

FAM193A is a positive regulator of p53 activity

Inactivation of the p53 tumor suppressor, either by mutations or through hyperactivation of repressors such as MDM2 and MDM4, is a hallmark of cancer. Although many inhibitors of the p53-MDM2/4 interaction have been developed, such as Nutlin, their therapeutic value is limited by highly heterogeneous cellular responses. We report here a multi-omics investigation of the cellular response to MDM2/4 inhibitors, leading to identification of FAM193A as a widespread regulator of p53 function. CRISPR screening identified FAM193A as necessary for the response to Nutlin. FAM193A expression correlates with Nutlin sensitivity across hundreds of cell lines. Furthermore, genetic codependency data highlight FAM193A as a component of the p53 pathway across diverse tumor types. Mechanistically, FAM193A interacts with MDM4, and FAM193A depletion stabilizes MDM4 and inhibits the p53 transcriptional program. Last, FAM193A expression is associated with better prognosis in multiple malignancies. Altogether, these results identify FAM193A as a positive regulator of p53.

The MDMX Acidic Domain Uses Allovalency to Bind Both p53 and MDMX

Autoinhibition of p53 binding to MDMX requires two short-linear motifs (SLiMs) containing adjacent tryptophan (WW) and tryptophan-phenylalanine (WF) residues. NMR spectroscopy was used to show the WW and WF motifs directly compete for the p53 binding site on MDMX and circular dichroism spectroscopy was used to show the WW motif becomes helical when it is bound to the p53 binding domain (p53BD) of MDMX. Binding studies using isothermal titration calorimetry showed the WW motif is a stronger inhibitor of p53 binding than the WF motif when they are both tethered to p53BD by the natural disordered linker. We also investigated how the WW and WF motifs interact with the DNA binding domain (DBD) of p53. Both motifs bind independently to similar sites on DBD that overlap the DNA binding site. Taken together our work defines a model for complex formation between MDMX and p53 where a pair of disordered SLiMs bind overlapping sites on both proteins.

The MDMX acidic domain competes with the p53 transactivation domain for MDM2 N-terminal domain binding

The tumor suppressor protein p53 governs many cellular pathways to control genome integrity, metabolic homeostasis, and cell viability. The critical roles of p53 highlight the importance of proper control over p53 in maintaining normal cellular function, with the negative regulators MDM2 and MDMX playing central roles in regulating p53 activity. The interaction between p53 and either MDM2 or MDMX involves the p53 transactivation domain (p53TD) and the N-terminal domains (NTD) of MDM2 or MDMX. Recently, the acidic domain (AD) of MDMX was found to bind to its own NTD, inhibiting the p53-MDMX interaction. Given the established structural and functional similarity between the MDM2 and MDMX NTDs, we hypothesized that the MDMX AD would also directly bind to MDM2 NTD to inhibit p53-MDM2 interaction. Through solution-state nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC), we show that the MDMX AD can indeed directly interact with the MDM2 NTD and, as a result, can compete for p53 binding. The MDMX AD is thus able to serve as a regulatory domain to inhibit the MDM2-p53 interaction and may also play a direct role in p53 activation.

Hesperidin Inhibits the p53-MDMXInteraction-Induced Apoptosis of Non-Small-Cell Lung Cancer and Enhances the Antitumor Effect of Carboplatin

Objective

This study aimed to observe the effect of hesperidin on the apoptosis, proliferation, and invasion of non-small-cell lung cancer, as well as to explore the possible mechanism. The inhibitory effect of hesperidin combined with carboplatin on non-small-cell lung cancer was also investigated.

Methods

A549 and NCI-H460 cells were treated with different concentrations of hesperidin (10, 50, and 100 μM). The effect of siRNA knockdown on MDMX on the antitumor effect of hesperidin was observed. CCK-8 was used to detect cell activity. The apoptosis rate was determined by TUNEL. The transwell assay detects the ability of cell migration and invasion. The expression levels of the apoptosis-related proteins p53, MDM2, MDMX, p21, PUMA, Bcl-2, and Bax were detected by qRT-PCR. Cell-proliferation and transwell assays were used to detect the effects of the combined use of hesperidin and carboplatin on lung cancer cells.

Results

Hesperidin significantly inhibited the activity and invasion of A549 and NCI-H460 cells in a dose-dependent manner. Hesperidin also induced the apoptosis of A549 and NCI-H460 cells. Hesperidin further inhibited the interaction between p53 and MDMX, increased the expression of p53, and played an anticancer role. The combination of hesperidin and carboplatin showed the most obvious antitumor effect.

Conclusion

Hesperidin can inhibit lung cancer by inhibiting the interaction between p53 and MDMX. Moreover, the combination of hesperidin and carboplatin can inhibit the migration and invasion of lung cancer cell lines through p53 upregulation, thereby increasing the antitumor effect of carboplatin.

Decreased Expressions of CK1α and PTEN in Sinonasal Inverted Papilloma

To investigate the diagnostic value of casein kinase 1α (CK1α) and phosphatase and tensin homolog (PTEN) in sinonasal inverted papilloma (SNIP), 42 control subjects and 56 SNIP patients were recruited in this study. Demographic and clinical characteristics, computerized tomography scans and endoscopic examinations were analyzed according to the Krouse staging system. Real-time quantitative-polymerase chain reaction and Western blotting were performed to detect CK1α and PTEN expression levels in different subgroups. Receiver operating characteristic and correlation analyses were conducted to assess their clinical significance in SNIP diagnosis. The expression levels of CK1α and PTEN were decreased in SNIP patients. Interestingly, the declined mRNA levels were consistent with the elevated Krouse staging and closely associated with the pathophysiological characteristics. Their expression levels also negatively correlated with neutrophil counts and positively correlated with lymphocyte counts in the blood of SNIP patients. This study suggests that CK1α and PTEN might be useful biomarkers for the occurrence and recurrence diagnosis of SNIP.

Casein kinase 1α regulates murine spermatogenesis via p53-Sox3 signaling

Casein kinase 1α (CK1α), acting as one member of the β-catenin degradation complex, negatively regulates the Wnt/β-catenin signaling pathway. CK1α knockout usually causes both Wnt/β-catenin and p53 activation. Our results demonstrated that conditional disruption of CK1α in spermatogonia impaired spermatogenesis and resulted in male mouse infertility. The progenitor cell population was dramatically decreased in CK1α conditional knockout (cKO) mice, while the proliferation of spermatogonial stem cells (SSCs) was not affected. Furthermore, our molecular analyses identified that CK1α loss was accompanied by nuclear stability of p53 protein in mouse spermatogonia, and dual-luciferase reporter and chromatin immunoprecipitation assays revealed that p53 directly targeted the Sox3 gene. In addition, the p53 inhibitor pifithrin α (PFTα) partially rescued the phenotype observed in cKO mice. Collectively, our data suggest that CK1α regulates spermatogenesis and male fertility through p53-Sox3 signaling, and they deepen our understanding of the regulatory mechanism underlying the male reproductive system.

Cryptic in vitro ubiquitin ligase activity of HDMX towards p53 is likely regulated by an induced fit mechanism

HDMX and its homologue HDM2 are two essential proteins for the cell; after genotoxic stress, both are phosphorylated near to their RING domain, specifically at serine 403 and 395, respectively. Once phosphorylated, both can bind the p53 mRNA and enhance its translation; however, both recognise p53 protein and provoke its degradation under normal conditions. HDM2 has been well-recognised as an E3 ubiquitin ligase, whereas it has been reported that even with the high similarity between the RING domains of the two homologs, HDMX does not have the E3 ligase activity. Despite this, HDMX is needed for the proper p53 poly-ubiquitination. Phosphorylation at serine 395 changes the conformation of HDM2, helping to explain the switch in its activity, but no information on HDMX has been published. Here we study the conformation of HDMX and its phospho-mimetic mutant S403D, investigate its E3 ligase activity and dissect its binding with p53. We show that phospho-mutation does not change the conformation of the protein, but HDMX is indeed an E3 ubiquitin ligase in vitro; however, in vivo, no activity was found. We speculated that HDMX is regulated by induced fit, being able to switch activity accordingly to the specific partner as p53 protein, p53 mRNA or HDM2. Our results aim to contribute to the elucidation of the contribution of the HDMX to p53 regulation.

1H, 15N and 13C backbone resonance assignments of the acidic domain of the human MDMX protein

The human MDMX protein, also known as MDM4, plays a pivotal role in regulating the activity of the tumor suppressor protein p53 by restricting p53 transcriptional activity and stimulating the E3 ubiquitin ligase activity of another key regulatory protein, MDM2, to promote p53 degradation. MDMX is ubiquitously expressed in most tissue types and overexpression of MDMX has been implicated in many forms of cancer. MDMX has been shown to require an intact N-terminal p53-binding domain and C-terminal RING domain to exert inhibitory effects on p53. The presence of a tryptophan-rich sequence in the central acidic domain of MDMX has also been implicated in regulating the interaction between MDMX and p53, directly interacting with the p53 DNA-binding domain. To date, little structural information has been obtained for this acidic region of MDMX that encompasses the Trp-rich sequence. In order to gain insight into the structure and function of this region, we have carried out solution-state NMR spectroscopy studies utilizing the segment of MDMX spanning residues 181-300-with bounds specifically chosen through multiple sequence alignment-which encompasses nearly 25% of MDMX. Here, we report the ¹H, ¹⁵N and ¹³C backbone chemical shift assignments of the acidic domain of MDMX and show that it exhibits hallmarks of intrinsic disorder and localized variation in inferred secondary structure propensity.

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.

Direct Interaction of miRNA and circRNA with the Oncosuppressor p53: An Intriguing Perspective in Cancer Research

Simple Summary

MicroRNAs and circular RNAs, which are single-stranded non-coding RNAs, play a key role as regulators at post-transcriptional level. Abnormal levels or dysregulation of miRNA or circRNA are linked to several cancerous pathologies. Starting from the evidence that some miRNAs and circRNAs are involved in the regulatory networks of the tumor suppressor protein p53, the possibility that a functional inhibition of p53 could arise from a direct interaction between p53 and oncogenic miRNAs or circRNAs was explored. Along this direction, the experimental evidence of the interaction between p53 and miRNAs and/or circRNAs is reviewed and discussed in connection with the development of new anticancer strategies.

Abstract

MicroRNAs (miRNAs) are linear single-stranded non-coding RNAs oligonucleotides, widely distributed in cells, playing a key role as regulators of gene expression at post-transcriptional level. Circular RNAs (circRNAs) are single-stranded RNA oligonucleotides forming a covalently closed continuous loop, which confers them a high structural stability and which may code for proteins or act as gene regulators. Abnormal levels or dysregulation of miRNA or circRNA are linked to several cancerous pathologies, so that they are receiving a large attention as diagnostic and prognostic tools. Some miRNAs and circRNAs are strongly involved in the regulatory networks of the transcription factor p53, which plays a pivotal role as tumor suppressor. Overexpression of miRNAs and/or circRNAs, as registered in a number of cancers, is associated to a concomitant inhibition of the p53 onco-suppressive function. Among other mechanisms, it was recently suggested that a functional inhibition of p53 could arise from a direct interaction between p53 and oncogenic miRNAs or circRNAs; a mechanism that might be reminiscent of the p53 inhibition by some E3 ubiquitin ligase such as MDM2 and COP1. Such evidence might deserve important implications for restoring the p53 anticancer functionality, and pave the way to intriguing perspectives for novel therapeutic strategies. In the present paper, the experimental evidence of the interaction between p53 and miRNAs and/or circRNAs is reviewed and discussed in connection with the development of new anticancer approaches.

Polyphenols as Antitumor Agents Targeting Key Players in Cancer-Driving Signaling Pathways

Polyphenols constitute an important group of natural products that are traditionally associated with a wide range of bioactivities. These are usually found in low concentrations in natural products and are now available in nutraceuticals or dietary supplements. A group of polyphenols that include apigenin, quercetin, curcumin, resveratrol, EGCG, and kaempferol have been shown to regulate signaling pathways that are central for cancer development, progression, and metastasis. Here, we describe novel mechanistic insights on the effect of this group of polyphenols on key elements of the signaling pathways impacting cancer. We describe the protein modifications induced by these polyphenols and their effect on the central elements of several signaling pathways including PI3K, Akt, mTOR, RAS, and MAPK and particularly those affecting the tumor suppressor p53 protein. Modifications of p53 induced by these polyphenols regulate p53 gene expression and protein levels and posttranslational modifications such as phosphorylation, acetylation, and ubiquitination that influence stability, subcellular location, activation of new transcriptional targets, and the role of p53 in response to DNA damage, apoptosis control, cell- cycle regulation, senescence, and cell fate. Thus, deep understanding of the effects that polyphenols have on these key players in cancer-driving signaling pathways will certainly lead to better designed targeted therapies, with less toxicity for cancer treatment. The scope of this review centers on the regulation of key elements of cancer signaling pathways by the most studied polyphenols and highlights the importance of a profound understanding of these regulations in order to improve cancer treatment and control with natural products.

Recent Progress and Clinical Development of Inhibitors that Block MDM4/p53 Protein-Protein Interactions

MDM4 is a homologue of MDM2, serving cooperatively as the negative regulator of tumor suppressor p53. Under the shadow of MDM2 inhibitors, limited efforts had been put into the discovery of MDM4 modulators. Recent studies of the experimental drug ALRN-6924, a dual MDM4 and MDM2 inhibitor, suggest that concurrent inhibition of MDM4 and MDM2 might be beneficial over only MDM2 inhibition. In view of the present research progress, we summarized published inhibitors of MDM4/p53 interactions including both peptide-based compounds and small molecules. Cocrystal structures of ligand/MDM4 complexes have been examined, and their structural features were compiled and compared in order to show the molecular basis required for high MDM4 binding affinities. Representative examples of small-molecule MDM4 inhibitors were discussed, followed by clinical results of ALRN-6924, together, providing a consolidated reference for further development of MDM4 inhibitors, either dual or selective.

MDMX Recruits UbcH5c to Facilitate MDM2 E3 Ligase Activity and Subsequent p53 Degradation In Vivo

MDM2 regulates p53 degradation by functioning as an E3 ubiquitin ligase. The role of MDMX, an MDM2 homolog that lacks E3 ligase activity, in the regulation of p53 degradation remains incompletely understood and sometime controversial. This confusion is due at least in part to studies of p53 degradation mainly carried out in in vitro settings, as elimination of either MDM2 or MDMX from mice results in p53-dependent embryonic lethality, thus obfuscating in vivo studies of the individual roles of MDM2 and MDMX in p53 degradation. To overcome this problem, we generated mice expressing an inducible p53 allele under various MDM2 and MDMX deletion and mutation statuses and studied in vivo p53 degradation. Degradation of p53 in vivo was largely prevented in mice and mouse embryonic fibroblast retaining MDM2 but lacking MDMX. Although MDM2 and MDMX interacted with p53 in the absence of each other, they bound p53 more efficiently as a heterodimer. MDMX, but not MDM2, interacted with ubiquitin-conjugating enzyme UbcH5c, an interaction that was essential for MDMX to enable MDM2 E3 ligase activity for p53 degradation. Grafting the C-terminal residues of MDMX to the C-terminus of MDM2 allowed MDM2 to interact with UbcH5c and enhanced MDM2-mediated p53 degradation in the absence of MDMX. Together, these data indicate that MDMX plays an essential role for p53 degradation in vivo by recruiting UbcH5c to facilitate MDM2 E3 ligase function. SIGNIFICANCE: This study provides the first in vivo evidence of MDMX facilitating MDM2-mediated p53 degradation, clarifying its role in the regulation of this critical tumor suppressor.

Targeting Casein Kinase 1 (CK1) in Hematological Cancers

The casein kinase 1 enzymes (CK1) form a family of serine/threonine kinases with seven CK1 isoforms identified in humans. The most important substrates of CK1 kinases are proteins that act in the regulatory nodes essential for tumorigenesis of hematological malignancies. Among those, the most important are the functions of CK1s in the regulation of Wnt pathways, cell proliferation, apoptosis and autophagy. In this review we summarize the recent developments in the understanding of biology and therapeutic potential of the inhibition of CK1 isoforms in the pathogenesis of chronic lymphocytic leukemia (CLL), other non-Hodgkin lymphomas (NHL), myelodysplastic syndrome (MDS), acute myeloid leukemia (AML) and multiple myeloma (MM). CK1δ/ε inhibitors block CLL development in preclinical models via inhibition of WNT-5A/ROR1-driven non-canonical Wnt pathway. While no selective CK1 inhibitors have reached clinical stage to date, one dual PI3Kδ and CK1ε inhibitor, umbralisib, is currently in clinical trials for CLL and NHL patients. In MDS, AML and MM, inhibition of CK1α, acting via activation of p53 pathway, showed promising preclinical activities and the first CK1α inhibitor has now entered the clinical trials.

USP7 Is a Master Regulator of Genome Stability

Genetic alterations, including DNA mutations and chromosomal abnormalities, are primary drivers of tumor formation and cancer progression. These alterations can endow cells with a selective growth advantage, enabling cancers to evade cell death, proliferation limits, and immune checkpoints, to metastasize throughout the body. Genetic alterations occur due to failures of the genome stability pathways. In many cancers, the rate of alteration is further accelerated by the deregulation of these processes. The deubiquitinating enzyme ubiquitin specific protease 7 (USP7) has recently emerged as a key regulator of ubiquitination in the genome stability pathways. USP7 is also deregulated in many cancer types, where deviances in USP7 protein levels are correlated with cancer progression. In this work, we review the increasingly evident role of USP7 in maintaining genome stability, the links between USP7 deregulation and cancer progression, as well as the rationale of targeting USP7 in cancer therapy.

MDMX inhibits casein kinase 1α activity and stimulates Wnt signaling

Casein kinase 1 alpha (CK1α) is a serine/threonine kinase with numerous functions, including regulating the Wnt/β-catenin and p53 pathways. CK1α has a well-established role in inhibiting the p53 tumor suppressor by binding to MDMX and stimulating MDMX-p53 interaction. MDMX purified from cells contains near-stoichiometric amounts of CK1α, suggesting that MDMX may in turn regulate CK1α function. We present evidence that MDMX is a potent competitive inhibitor of CK1α kinase activity (K_i  = 8 nM). Depletion of MDMX increases CK1α activity and β-catenin S45 phosphorylation, whereas ectopic MDMX expression inhibits CK1α activity and β-catenin phosphorylation. The MDMX acidic domain and zinc finger are necessary and sufficient for binding and inhibition of CK1α. P53 binding to MDMX disrupts an intramolecular auto-regulatory interaction and enhances its ability to inhibit CK1α. P53-null mice expressing the MDMX^{W 200S/W201G} mutant, defective in CK1α binding, exhibit reduced Wnt/β-catenin target gene expression and delayed tumor development. Therefore, MDMX is a physiological inhibitor of CK1α and has a role in modulating cellular response to Wnt signaling. The MDMX-CK1α interaction may account for certain p53-independent functions of MDMX.

The long and the short of it: the MDM4 tail so far

The mouse double minute 4 (MDM4) is emerging from the shadow of its more famous relative MDM2 and is starting to steal the limelight, largely due to its therapeutic possibilities. MDM4 is a vital regulator of the tumor suppressor p53. It restricts p53 transcriptional activity and also, at least in development, facilitates MDM2's E3 ligase activity toward p53. These functions of MDM4 are critical for normal cell function and a proper response to stress. Their importance for proper cell maintenance and proliferation identifies them as a risk for deregulation associated with the uncontrolled growth of cancer. MDM4 tails are vital for its function, where its N-terminus transactivation domain engages p53 and its C-terminus RING domain binds to MDM2. In this review, we highlight recently identified cellular functions of MDM4 and survey emerging therapies directed to correcting its dysregulation in disease.

Allosteric changes in HDM2 by the ATM phosphomimetic S395D mutation: implications on HDM2 function

Allosteric changes imposed by post-translational modifications regulate and differentiate the functions of proteins with intrinsic disorder regions. HDM2 is a hub protein with a large interactome and with different cellular functions. It is best known for its regulation of the p53 tumour suppressor. Under normal cellular conditions, HDM2 ubiquitinates and degrades p53 by the 26S proteasome but after DNA damage, HDM2 switches from a negative to a positive regulator of p53 by binding to p53 mRNA to promote translation of the p53 mRNA. This change in activity is governed by the ataxia telangiectasia mutated kinase via phosphorylation on serine 395 and is mimicked by the S395D phosphomimetic mutant. Here we have used different approaches to show that this event is accompanied by a specific change in the HDM2 structure that affects the HDM2 interactome, such as the N-termini HDM2–p53 protein–protein interaction. These data will give a better understanding of how HDM2 switches from a negative to a positive regulator of p53 and gain new insights into the control of the HDM2 structure and its interactome under different cellular conditions and help identify interphases as potential targets for new drug developments.

Targeting the CK1α/CBX4 axis for metastasis in osteosarcoma

Osteosarcoma, an aggressive malignant cancer, has a high lung metastasis rate and lacks therapeutic target. Here, we reported that chromobox homolog 4 (CBX4) was overexpressed in osteosarcoma cell lines and tissues. CBX4 promoted metastasis by transcriptionally up-regulating Runx2 via the recruitment of GCN5 to the Runx2 promoter. The phosphorylation of CBX4 at T437 by casein kinase 1α (CK1α) facilitated its ubiquitination at both K178 and K280 and subsequent degradation by CHIP, and this phosphorylation of CBX4 could be reduced by TNFα. Consistently, CK1α suppressed cell migration and invasion through inhibition of CBX4. There was a reverse correlation between CK1α and CBX4 in osteosarcoma tissues, and CK1α was a valuable marker to predict clinical outcomes in osteosarcoma patients with metastasis. Pyrvinium pamoate (PP) as a selective activator of CK1α could inhibit osteosarcoma metastasis via the CK1α/CBX4 axis. Our findings indicate that targeting the CK1α/CBX4 axis may benefit osteosarcoma patients with metastasis.

Osteosarcoma is an aggressive tumour and little is known the mechanisms underpinning its highly metastatic nature. Here, the authors highlight a role for the CK1α/CBX4 axis in driving metastasis, suggesting that this pathway might be targeted for therapeutic benefit.

Tumor-derived CK1α mutations enhance MDMX inhibition of p53

Somatic missense mutations of the CSNK1A1 gene encoding casein kinase 1 alpha (CK1α) occur in a subset of myelodysplastic syndrome (MDS) with del(5q) karyotype. The chromosomal deletion causes CSNK1A1 haplo-insufficiency. CK1α mutations have also been observed in a variety of solid and hematopoietic tumors at low frequency. The functional consequence of CK1α mutation remains unknown. Here we show that tumor-associated CK1α mutations exclusively localize to the substrate-binding cleft. Functional analysis of recurrent mutants E98K and D140A revealed enhanced binding to the p53 inhibitor MDMX, increased ability to stimulate MDMX-p53 binding, and increased suppression of p21 expression. Furthermore, E98K and D140A mutants have reduced ability to promote phosphorylation of β-catenin, resulting in enhanced Wnt signaling. The results suggest that the CK1α mutations observed in tumors cause gain-of-function in cooperating with MDMX and inhibiting p53, and partial loss-of-function in suppressing Wnt signaling. These functional changes may promote expansion of abnormal myeloid progenitors in del(5q) MDS, and in rare cases drive the progression of other tumors.

Interaction between p53 N terminus and core domain regulates specific and nonspecific DNA binding

The p53 tumor suppressor is a sequence-specific DNA binding protein that activates gene transcription to regulate cell survival and proliferation. Dynamic control of p53 degradation and DNA binding in response to stress signals are critical for tumor suppression. The p53 N terminus (NT) contains two transactivation domains (TAD1 and TAD2), a proline-rich region (PRR), and multiple phosphorylation sites. Previous work revealed the p53 NT reduced DNA binding in vitro. Here, we show that TAD2 and the PRR inhibit DNA binding by directly interacting with the sequence-specific DNA binding domain (DBD). NMR spectroscopy revealed that TAD2 and the PRR interact with the DBD at or near the DNA binding surface, possibly acting as a nucleic acid mimetic to competitively block DNA binding. In vitro and in vivo DNA binding analyses showed that the NT reduced p53 DNA binding affinity but improved the ability of p53 to distinguish between specific and nonspecific sequences. MDMX inhibits p53 binding to specific target promoters but stimulates binding to nonspecific chromatin sites. The results suggest that the p53 NT regulates the affinity and specificity of DNA binding by the DBD. The p53 NT-interacting proteins and posttranslational modifications may regulate DNA binding, partly by modulating the NT-DBD interaction.

Mutant p53 Sequestration of the MDM2 Acidic Domain Inhibits E3 Ligase Activity

Missense p53 mutants often accumulate in tumors and drive progression through gain of function. MDM2 efficiently degrades wild-type p53 but fails to degrade mutant p53 in tumor cells. Previous studies revealed that mutant p53 inhibits MDM2 autoubiquitination, suggesting that the interaction inhibits MDM2 E3 activity. Recent work showed that MDM2 E3 activity is stimulated by intramolecular interaction between the RING and acidic domains. Here, we show that in the mutant p53-MDM2 complex, the mutant p53 core domain binds to the MDM2 acidic domain with significantly higher avidity than wild-type p53. The mutant p53-MDM2 complex is deficient in catalyzing ubiquitin release from the activated E2 conjugating enzyme. An MDM2 construct with extra copies of the acidic domain is resistant to inhibition by mutant p53 and efficiently promotes mutant p53 ubiquitination and degradation. The results suggest that mutant p53 interferes with the intramolecular autoactivation mechanism of MDM2, contributing to reduced ubiquitination and increased accumulation in tumor cells.

Kinetic and thermodynamic effects of phosphorylation on p53 binding to MDM2

p53 is frequently mutated in human cancers. Its levels are tightly regulated by the E3 ubiquitin ligase MDM2. The complex between MDM2 and p53 is largely formed by the interaction between the N-terminal domain of MDM2 and the N-terminal transactivation (TA) domain of p53 (residues 15–29). We investigated the kinetic and thermodynamic basis of the MDM2/p53 interaction by using wild-type and mutant variants of the TA domain. We focus on the effects of phosphorylation at positions Thr18 and Ser20 including their substitution with phosphomimetics. Conformational propensities of the isolated peptides were investigated using in silico methods and experimentally by circular dichroism and ¹H-NMR in aqueous solution. Both experimental and computational analyses indicate that the p53 peptides are mainly disordered in aqueous solution, with evidence of nascent helix around the Ser20-Leu25 region. Both phosphorylation and the phosphomimetics at Thr18 result in a decrease in the binding affinity by ten- to twenty-fold when compared to the wild-type. Phosphorylation and phosphomimetics at Ser20 result in a smaller decrease in the affinity. Mutation of Lys24 and Leu25 also disrupts the interaction. Our results may be useful for further development of peptide-based drugs targeting the MDM2/p53 interaction.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

MDMX acidic domain inhibits p53 DNA binding in vivo and regulates tumorigenesis

The MDM2 homolog MDMX oncoprotein is indispensable for inhibition of p53 during normal embryonic development and malignant transformation, yet how MDMX harnesses p53 functions is unclear. In addition to a canonical N-terminal p53-binding domain, recent work suggests the central acidic domain of MDMX regulates p53 interaction through intramolecular mimicry and engages in second-site interaction with the p53 core domain in vitro. To test the physiological relevance of these interactions, we generated an MDMX knockin mouse having substitutions in a conserved WW motif necessary for these functions (W201S/W202G). Notably, MDMX^SG cells have normal p53 level but increased p53 DNA binding and target gene expression, and rapidly senesce. In vivo, MDMX^SG inhibits early-phase disease in Eµ-Myc transgenic mice but accelerates the onset of lethal lymphoma and shortens overall survival. Therefore, MDMX is an important regulator of p53 DNA binding, which complements the role of MDM2 in regulating p53 level. Furthermore, the results suggest that the WW motif has dual functions that regulate p53 and inhibit Myc-driven lymphomas independent of p53.

p53 promotes its own polyubiquitination by enhancing the HDM2 and HDMX interaction

HDM2 and HDMX are two homologs essential for controlling p53 tumor suppressor activity under normal conditions. Both proteins bind different sites on the p53 N-terminus, and while HDM2 has E3 ubiquitin ligase activity towards p53, HDMX does not. Nevertheless, HDMX is required for p53 polyubiquitination and degradation, but the underlying molecular mechanism remains unclear. Alone, HDMX and HDM2 interact via their respective C-terminal RING domains but here we show that the presence of p53 induces an N-terminal interface under normal cellular conditions. This results in an increase in HDM2-mediated p53 polyubiquitination and degradation. The HDM2 inhibitor Nutlin-3 binds the N-terminal p53 binding pocket and is sufficient to induce the HDM2-HDMX interaction, suggesting that the mechanism depends on allosteric changes that control the multiprotein complex formation. These results demonstrate an allosteric interchange between three different proteins (HDMX-HDM2-p53) and help to explain the molecular mechanisms of HDM2-inhibitory drugs.

Inhibition of Mdmx (Mdm4) in vivo induces anti-obesity effects

Although cell-cycle arrest, senescence and apoptosis remain as major canonical activities of p53 in tumor suppression, the emerging role of p53 in metabolism has been a topic of great interest. Nevertheless, it is not completely understood how p53-mediated metabolic activities are regulated in vivo and whether this part of the activities has an independent role beyond tumor suppression. Mdmx (also called Mdm4), like Mdm2, acts as a major suppressor of p53 but the embryonic lethality of mdmx-null mice creates difficulties to evaluate its physiological significance in metabolism. Here, we report that the embryonic lethality caused by the deficiency of mdmx, in contrast to the case for mdm2, is fully rescued in the background of p53^3KR/3KR, an acetylation-defective mutant unable to induce cell-cycle arrest, senescence and apoptosis. p53^3KR/3KR/mdmx^-/- mice are healthy but skinny without obvious developmental defects. p53^3KR/3KR/mdmx^-/- mice are resistant to fat accumulation in adipose tissues upon high fat diet. Notably, the levels of p53 protein are only slightly increased and can be further induced upon DNA damage in p53^3KR/3KR/mdmx^-/- mice, suggesting that Mdmx is only partially required for p53 degradation in vivo. Further analyses indicate that the anti-obesity phenotypes in p53^3KR/3KR/mdmx^-/- mice are caused by activation of lipid oxidation and thermogenic programs in adipose tissues. These results demonstrate the specific effects of the p53/Mdmx axis in lipid metabolism and adipose tissue remodeling and reveal a surprising role of Mdmx inhibition in anti-obesity effects beyond, commonly expected, tumor suppression. Thus, our study has significant implications regarding Mdmx inhibitors in the treatment of obesity related diseases.

Eukaryotic transcription factors: paradigms of protein intrinsic disorder

Gene-specific transcription factors (TFs) are key regulatory components of signaling pathways, controlling, for example, cell growth, development, and stress responses. Their biological functions are determined by their molecular structures, as exemplified by their structured DNA-binding domains targeting specific cis-acting elements in genes, and by the significant lack of fixed tertiary structure in their extensive intrinsically disordered regions. Recent research in protein intrinsic disorder (ID) has changed our understanding of transcriptional activation domains from ‘negative noodles’ to ID regions with function-related, short sequence motifs and molecular recognition features with structural propensities. This review focuses on molecular aspects of TFs, which represent paradigms of ID-related features. Through specific examples, we review how the ID-associated flexibility of TFs enables them to participate in large interactomes, how they use only a few hydrophobic residues, short sequence motifs, prestructured motifs, and coupled folding and binding for their interactions with co-activators, and how their accessibility to post-translational modification affects their interactions. It is furthermore emphasized how classic biochemical concepts like allostery, conformational selection, induced fit, and feedback regulation are undergoing a revival with the appreciation of ID. The review also describes the most recent advances based on computational simulations of ID-based interaction mechanisms and structural analysis of ID in the context of full-length TFs and suggests future directions for research in TF ID.

Intra molecular interactions in the regulation of p53 pathway

The p53 tumor suppressor is highly regulated at the level of protein degradation and transcriptional activity. The key players of the pathway, p53, MDM2, and MDMX are present at multiple conformational states that are responsive to regulation by post-translational modifications and protein-protein interactions. The structures of major functional domains of these proteins have been determined, but the mechanisms of several intrinsically disordered regions remain unclear despite their critical roles in signaling and regulation. Recent studies suggest that these disordered regions function in part by dynamic intra molecular interactions with the structured domains to regulate p53 DNA binding, MDM2 ubiquitin E3 ligase activity, and MDMX-p53 binding. These findings provide new insight on how p53 is controlled by various stress signals, and suggest potential targets for the search of allosteric regulators of the p53 pathway.

MDMX under stress: the MDMX-MDM2 complex as stress signals hub

The tumor suppressor p53 plays a central role in safeguarding cellular homeostasis. Upon various types of stress signals such as DNA damage or oncogenic stress, p53 is promptly activated to prevent and repair damages that can threaten the genome stability. The two major negative regulators of p53 are MDM2 and MDMX, two homolog proteins that control p53 activity and turnover, hence keeping it in check during normal cell cycling. In the event of cellular stress, they have to be inhibited in order to relieve p53 from their suppression and allow its activation. As the essential upstream modulator of p53, the MDMX-MDM2 complex integrates multiple signaling pathways regulating p53 response to perturbations of cellular homeostasis. Given its predominantly cytoplasmic localization in normal conditions, we hypothesize that MDMX, rather than MDM2, is the first recipient of signaling cues directed towards the MDMX-MDM2 complex and aimed at modulating p53. In this review we give a synthetic overview of the phosphorylation sites of MDMX that are known to affect its degradation, ubiquitination, intracellular localization and interaction with MDM2 and p53, ultimately modulating the stability and activity of p53. The role of MDMX in response to the main types of cellular stress is also briefly discussed, along with the potential of the MDMX-MDM2 complex as therapeutic target to restore p53 activity.

Dual function of MDM2 and MDMX toward the tumor suppressors p53 and RB

The orchestrated crosstalk between the retinoblastoma (RB) and p53 pathways contributes to preserving proper homeostasis within the cell. The deregulation of one or both pathways is a common factor in the development of most types of human cancer. The proto-oncoproteins MDMX and MDM2 are the main regulators of the well- known tumor suppressor p53 protein. Under normal conditions, MDM2 and MDMX inhibit p53, either via repression of its transcriptional activity by protein-protein interaction, or via polyubiquitination as a result of MDM2-E3 ubiquitin ligase activity, for which MDM2 needs to dimerize with MDMX. Under genotoxic stress conditions, both become positive regulators of p53. The ATM-dependent phosphorylation of MDM2 and MDMX allow them to bind p53 mRNA, these interactions promote p53 translation. MDM2 and MDMX are also being revealed as effective regulators of the RB protein. MDM2 is able to degrade RB by two different mechanisms, that is, by ubiquitin dependent and independent pathways. MDMX enhances the ability of MDM2 to bind and degrade RB protein. However, MDMX also seems to stabilize RB through interaction and competition with MDM2. Here, we will contextualize the findings that suggest that the MDM2 and MDMX proteins have a dual function on both p53 and RB.