ATP stimulates MDM2-mediated inhibition of the DNA-binding function of E2F1

Taraxasterol regulates p53 transcriptional activity to inhibit pancreatic cancer by inducing MDM2 ubiquitination degradation

BACKGROUND

Pancreatic cancer (PC) is a malignant tumor with complex development mechanisms and a poor prognosis. Taraxasterol (TAX), a pentacyclic triterpenoid plant sterol derived from Taraxacum mongolicum, has multiple biological activities including an anti-tumor effect. However, the mechanism by which TAX exerts its anticancer effects in PC remains unclear.

PURPOSE

This study aimed to elucidate the molecular mechanism by which TAX suppresses the proliferation of PC.

METHODS

The intersection of TAX and PC targets was obtained through network pharmacology. RNA-seq was used to identify TAX-induced differentially expressed genes in PC. Molecular docking, CETSA, western blot analysis, and qRT-PCR were performed to confirm the effectiveness of targets. The influence of TAX on PC was assessed by analyzing proliferation, apoptosis, and the cell cycle via MTT assay, colony formation assay, and flow cytometry, respectively. Co-IP assay and immunofluorescence assay were used to evaluate the effect of TAX on targeted genes. A nude mouse xenograft model was constructed to determine the inhibitory effects of TAX on PC in vivo.

RESULTS

TAX suppressed PC cell proliferation by promoting apoptosis and inducing cell cycle arrest in vitro and in vivo. Mechanistically, TAX interacted with MDM2, a critical regulator of proliferation, and decreased its stability by inducing ubiquitin-mediated degradation, which facilitates the nuclear translocation of p53 and downregulation of CXCL5 transcription, ultimately suppressing PC cell proliferation.

CONCLUSION

MDM2/p53/CXCL5 is the key pathway of TAX inhibiting the proliferation of PC cells.

MDM2- an indispensable player in tumorigenesis

Murine double minute 2 (MDM2) is a well-recognized molecule for its oncogenic potential. Since its identification, various cancer-promoting roles of MDM2 such as growth stimulation, sustained angiogenesis, metabolic reprogramming, apoptosis evasion, metastasis, and immunosuppression have been established. Alterations in the expression levels of MDM2 occur in multiple types of cancers resulting in uncontrolled proliferation. The cellular processes are modulated by MDM2 through transcription, post-translational modifications, protein degradation, binding to cofactors, and subcellular localization. In this review, we discuss the precise role of deregulated MDM2 levels in modulating cellular functions to promote cancer growth. Moreover, we also briefly discuss the role of MDM2 in inducing resistance against anti-cancerous therapies thus limiting the benefits of cancerous treatment.

Chemotherapy of HER2- and MDM2-Enriched Breast Cancer Subtypes Induces Homologous Recombination DNA Repair and Chemoresistance

Simple Summary

MDM2 is a protein responsible for negative regulation of the p53 tumor suppressor. In addition, MDM2 exhibits chaperone-like properties similar to the HSP90 molecular chaperone. Multiple studies revealed that MDM2 is deeply involved in cancer development and progression. Some recently published results indicate that the role of MDM2 in DNA repair inhibition is more complex than previously thought. We show that MDM2 is directly involved in the homologous recombination DNA repair, and its chaperone-like activity is crucial for this function. The DNA repair inhibition is a result of inefficient MDM2 dissociation from the NBN protein complex. When cancer cells are treated with chemotherapy, MDM2 can be easily released from the interaction and degraded, resulting in effective homologous recombination DNA repair, which translates into the acquisition of a chemoresistant phenotype by the tumor. This knowledge may allow for identification of the patients that are at particular risk of tumor chemoresistance.

Abstract

Analyzing the TCGA breast cancer database, we discovered that patients with the HER2 cancer subtype and overexpression of MDM2 exhibited decreased post-treatment survival. Inhibition of MDM2 expression in the SKBR3 cell line (HER2 subtype) diminished the survival of cancer cells treated with doxorubicin, etoposide, and camptothecin. Moreover, we demonstrated that inhibition of MDM2 expression diminished DNA repair by homologous recombination (HR) and sensitized SKBR3 cells to a PARP inhibitor, olaparib. In H1299 (TP53^−/−) cells treated with neocarzinostatin (NCS), overexpression of MDM2 WT or E3-dead MDM2 C478S variant stimulated the NCS-dependent phosphorylation of ATM, NBN, and BRCA1, proteins involved in HR DNA repair. However, overexpression of chaperone-dead MDM2 K454A variant diminished phosphorylation of these proteins as well as the HR DNA repair. Moreover, we demonstrated that, upon NCS treatment, MDM2 K454A interacted with NBN more efficiently than MDM2 WT and that MDM2 WT was degraded more efficiently than MDM2 K454A. Using a proliferation assay, we showed that overexpression of MDM2 WT, but not MDM2 K454A, led to acquisition of resistance to NCS. The presented results indicate that, following chemotherapy, MDM2 WT was released from MDM2-NBN complex and efficiently degraded, hence allowing extensive HR DNA repair leading to the acquisition of chemoresistance by cancer cells.

MDM2's dual mRNA binding domains co-ordinate its oncogenic and tumour suppressor activities

Cell growth requires a high level of protein synthesis and oncogenic pathways stimulate cell proliferation and ribosome biogenesis. Less is known about how cells respond to dysfunctional mRNA translation and how this feeds back into growth regulatory pathways. The Epstein-Barr virus (EBV)-encoded EBNA1 causes mRNA translation stress in cis that activates PI3Kδ. This leads to the stabilization of MDM2, induces MDM2's binding to the E2F1 mRNA and promotes E2F1 translation. The MDM2 serine 166 regulates the interaction with the E2F1 mRNA and deletion of MDM2 C-terminal RING domain results in a constitutive E2F1 mRNA binding. Phosphorylation on serine 395 following DNA damage instead regulates p53 mRNA binding to its RING domain and prevents the E2F1 mRNA interaction. The p14Arf tumour suppressor binds MDM2 and in addition to preventing degradation of the p53 protein it also prevents the E2F1 mRNA interaction. The data illustrate how two MDM2 domains selectively bind specific mRNAs in response to cellular conditions to promote, or suppress, cell growth and how p14Arf coordinates MDM2's activity towards p53 and E2F1. The data also show how EBV via EBNA1-induced mRNA translation stress targets the E2F1 and the MDM2 - p53 pathway.

The Roles of MDM2 and MDMX in Cancer

For more than 25 years, MDM2 and its homolog MDMX (also known as MDM4) have been shown to exert oncogenic activity. These two proteins are best understood as negative regulators of the p53 tumor suppressor, although they may have additional p53-independent roles. Understanding the dysregulation of MDM2 and MDMX in human cancers and how they function either together or separately in tumorigenesis may improve methods of diagnosis and for assessing prognosis. Targeting the proteins themselves, or their regulators, may be a promising therapeutic approach to treating some forms of cancer.

Identification of a second Nutlin-3 responsive interaction site in the N-terminal domain of MDM2 using hydrogen/deuterium exchange mass spectrometry

MDM2 is a multidomain protein that functions as an E3 ubiquitin ligase, transcription repressor, mRNA-binding protein, translation factor, and molecular chaperone. The small molecule Nutlin-3 has been engineered to bind to the N-terminal hydrophobic pocket domain of MDM2. This binding of Nutlin-3 has two consequences: (i) antagonistic effects through competitive disruption of the MDM2-p53 complex and (ii) agonist effects that allosterically stabilize MDM2 protein-protein interactions that increase p53 ubiquitination as well as nucleophosmin deoligomerization. We present a methodology using a hydrogen/deuterium (H/D) exchange platform that measures Nutlin-3 binding to the N-terminal domain of MDM2 (MDM2(1-126)) in order to begin to develop dynamic assays that evaluate MDM2 allostery. In order to localize the regions in MDM2 being suppressed by Nutlin-3, MDM2 was incubated with the ligand and H/D amide exchange was measured after pepsin digestion. One dynamic segment containing amino acids 55-60 exhibited slower deuterium exchange after Nutlin-3 binding, reflecting ligand binding within the hydrophobic pocket. However, another dominant suppression of H/D exchange was observed in a motif from amino acids 103-107 that reflects surface hydrophobic residues surrounding the hydrophobic pocket of MDM2. In order to explore the consequences of this latter Nutlin-3 interaction site on MDM2, the Y104G and L107G mutant series was constructed. The MDM2(Y104G) and MDM2(L107G) mutants were fully active in p53 binding. However, the authentic p53-derived peptide:MDM2(Y104G) complex exhibited partial resistance to Nutlin-3 inhibition, while the p53-mimetic 12.1 peptide:MDM2(Y104G) complex retained normal Nutlin-3 responsiveness. These data reveal the existence of a second functional Nutlin-3-binding site in a surface hydrophobic patch of MDM2, flanking the hydrophobic pocket. This reveals two modes of peptide binding by MDM2 and highlights the utility of H/D exchange as an assay for measuring allosteric effects in MDM2.

Splicing up mdm2 for cancer proteome diversity

Cancer cells often have high expression of Mdm2. However, in many cancers mdm2 is alternatively spliced, with more than 40 mRNA variants identified. Many of the alternative spliced mdm2 mRNAs have the potential to encode truncated Mdm2 isoforms. These putative Mdm2 isoforms can theoretically increase the diversity of the cancer proteome. The 3 best characterized are Mdm2-A, Mdm2-B, and Mdm2-C. As described in this review, the exogenous expression of these isoforms results in paradoxical phenotypes of transformation-associated growth as well as the inhibition of growth. Interestingly, these Mdm2 isoforms contribute tumor-promoting capacity in p53-null backgrounds. Herein we describe how alternative splicing of mdm2 may result in Mdm2 protein products that alter signal transduction to promote tumorigenesis. The tumor promoting capacity of Mdm2 isoforms is discussed in the context of functions that do not require the inhibition of p53. When N-terminal portions of Mdm2 are missing, the biochemical functions encoded by exon 12 are proposed to become more important. This may result in growth promoting functions when wild-type p53 is absent or compromised. The p53-independent tumor promoting activity of Mdm2 is proposed to result from C-terminal biochemical contributions of DNA binding, RNA binding, nucleolar localization, and nucleotide binding.

Mdm2 and MdmX as Regulators of Gene Expression

p53 is an important tumor suppressor, functioning as a transcriptional activator and repressor. Upon receiving signals from multiple stress related pathways, p53 regulates numerous activities such as cell cycle arrest, senescence, and cell death. When p53 activities are not required, the protein is held in check by interacting with 2 key homologous regulators, Mdm2 and MdmX, and a search for inhibitors of these interactions is well underway. However, it is now recognized that Mdm2 and MdmX function beyond simple inhibition of p53, and a complete understanding of Mdm2 and MdmX functions is ever more important. Indeed, increasing evidence suggests that Mdm2 and MdmX affect p53 target gene specificity and influence the activity of other transcription factors, and Mdm2 itself may even function as a transcription co-factor through post-translational modification of chromatin. Additionally, Mdm2 affects post-transcriptional activities such as mRNA stability and translation of a variety of transcripts. Thus, Mdm2 and MdmX influence the expression of many genes through a wide variety of mechanisms, which are discussed in this review.

Concepts in MDM2 Signaling: Allosteric Regulation and Feedback Loops

The function and regulation of MDM2 as a component of a p53-dependent negative feedback loop has formed a core paradigm in the p53 field. This concept, now 20 years old, has been solidified by fields of protein science, transgenic technology, and drug discovery in human cancer. However, it has been noted that a simple negative feedback loop between p53 and MDM2 lacks an intrinsic "activating" step that counteracts this inhibition and permits oscillation of the feedback to occur as p53 is switched on and off. More recent work has identified a solution to the missing piece of the picture that counters the negative feedback loop, which is MDM2 itself. Under conditions of genotoxic stress, MDM2 helps to activate p53 by increasing its rate of protein synthesis. This simple observation makes certain aspects of the p53 response more comprehensible such as why MDM2 is upregulated by p53 early on following DNA damage and how phosphorylation of MDM2 at the C-terminal Ser395 by ATM translates into p53 activation. The latter acts by inducing allosteric changes in the RING domain of MDM2 that expose its RNA binding pocket, support p53 synthesis, and suppress its degradation. This allosteric nature of MDM2 in the C-terminus mirrors the allosteric effects of the binding of small molecules to the p53 interacting pocket at the N-terminus of MDM2, which opens the core domain of MDM2 to central domains of p53, which controls p53 ubiquitination. Thus, the highly allosteric nature of MDM2 provides the basis for dynamic protein-protein interactions and protein-RNA interactions through which MDM2's activity is regulated in p53 protein destruction or in p53 protein synthesis. We discuss these mechanisms and how this information can be exploited for drug development programs aimed at activating p53 via targeting MDM2.

Deconstructing nucleotide binding activity of the Mdm2 RING domain

Mdm2, a central negative regulator of the p53 tumor suppressor, possesses a Really Interesting New Gene (RING) domain within its C-terminus. In addition to E3 ubiquitin ligase activity, the Mdm2 RING preferentially binds adenine base nucleotides, and such binding leads to a conformational change in the Mdm2 C-terminus. Here, we present further biochemical analysis of the nucleotide–Mdm2 interaction. We have found that MdmX, an Mdm2 family member with high sequence homology, binds adenine nucleotides with similar affinity and specificity as Mdm2, suggesting that residues involved in nucleotide binding may be conserved between the two proteins and adenosine triphosphate (ATP) binding may have similar functional consequences for both Mdm family members. By generating and testing a series of proteins with deletions and substitution mutations within the Mdm2 RING, we mapped the specific adenine nucleotide binding region of Mdm2 to residues 429–484, encompassing the minimal RING domain. Using a series of ATP derivatives, we demonstrate that phosphate coordination by the Mdm2 P-loop contributes to, but is not primarily responsible for, ATP binding. Additionally, we have identified the 2′ and 3′ hydroxyls of the ribose and the C6 amino group of the adenine base moiety as being essential for binding.

CK1alpha plays a central role in mediating MDM2 control of p53 and E2F-1 protein stability

The ubiquitin ligase murine double minute clone 2 (MDM2) mediates ubiquitination and degradation of the tumor suppressor p53. The activation and stabilization of p53 by contrast is maintained by enzymes catalyzing p53 phosphorylation and acetylation. Casein kinase 1 (CK1) is one such enzyme; it stimulates p53 after transforming growth factor-beta treatment, irradiation, or DNA virus infection. We analyzed whether CK1 regulates p53 protein stability in unstressed conditions. Depletion of CK1 using small interfering RNA or inhibition of CK1 using the kinase inhibitor (D4476) activated p53 and destabilized E2F-1, indicating that steady-state levels of these proteins are controlled by CK1. Co-immunoprecipitation of endogenous CK1 with MDM2 occurred in undamaged cells, indicating the existence of a stable multiprotein complex, and as such, we evaluated whether the MDM2 Nutlin had similar pharmacological properties to the CK1 inhibitor D4476. Indeed, D4476 or Nutlin treatments resulted in the same p53 and E2F-1 steady-state protein level changes, indicating that the MDM2 x CK1 complex is both a negative regulator of p53 and a positive regulator of E2F-1 in undamaged cells. Although the treatment of cells with D4476 resulted in a partial p53-dependent growth arrest, the induction of p53-independent apoptosis by D4476 suggested a critical role for the MDM2 x CK1 complex in maintaining E2F-1 anti-apoptotic signaling. These data highlighting a pharmacological similarity between MDM2 and CK1 small molecule inhibitors and the fact that CK1 and MDM2 form a stable complex suggest that the MDM2 x CK1 complex is a component of a genetic pathway that co-regulates the stability of the p53 and E2F-1 transcription factors.

Regulation of the E3 ubiquitin ligase activity of MDM2 by an N-terminal pseudo-substrate motif

The tumor suppressor p53 has evolved a MDM2-dependent feedback loop that promotes p53 protein degradation through the ubiquitin-proteasome system. MDM2 is an E3-RING containing ubiquitin ligase that catalyzes p53 ubiquitination by a dual-site mechanism requiring ligand occupation of its N-terminal hydrophobic pocket, which then stabilizes MDM2 binding to the ubiquitination signal in the DNA-binding domain of p53. A unique pseudo-substrate motif or "lid" in MDM2 is adjacent to its N-terminal hydrophobic pocket, and we have evaluated the effects of the flexible lid on the dual-site ubiquitination reaction mechanism catalyzed by MDM2. Deletion of this pseudo-substrate motif promotes MDM2 protein thermoinstability, indicating that the site can function as a positive regulatory element. Phospho-mimetic mutation in the pseudo-substrate motif at codon 17 (MDM2(S17D)) stabilizes the binding of MDM2 towards two distinct peptide docking sites within the p53 tetramer and enhances p53 ubiquitination. Molecular modeling orientates the phospho-mimetic pseudo-substrate motif in equilibrium over a charged surface patch on the MDM2 at Arg(97)/Lys(98), and mutation of these residues to the MDM4 equivalent reverses the activating effect of the phospho-mimetic mutation on MDM2 function. These data highlight the ability of the pseudo-substrate motif to regulate the allosteric interaction between the N-terminal hydrophobic pocket of MDM2 and its central acidic domain, which stimulates the E3 ubiquitin ligase function of MDM2. This model of MDM2 regulation implicates an as yet undefined lid-kinase as a component of pro-oncogenic pathways that stimulate the E3 ubiquitin ligase function of MDM2 in cells.