Nucleotide binding by the Mdm2 RING domain facilitates Arf-independent Mdm2 nucleolar localization

Treatment of Sulfur Mustard Corneal Injury by Augmenting the DNA Damage Response (DDR): A Novel Approach

Sulfur mustard (SM) is a highly reactive organic chemical has been used as a chemical warfare agent and terrorist threat since World War I. The cornea is highly sensitive to SM toxicity and exposure to low vapor doses can cause incapacitating acute injuries. Exposure to higher doses can elicit persistent secondary keratopathies that cause reduced quality of life and impaired or lost vision. Despite a century of research, there are no specific treatments for acute or persistent ocular SM injuries. SM cytotoxicity emerges, in part, through DNA alkylation and double-strand breaks (DSBs). Because DSBs can naturally be repaired by DNA damage response pathways with low efficiency, we hypothesized that enhancing the homologous recombination pathway could pose a novel approach to mitigate SM injury. Here, we demonstrate that a dilithium salt of adenosine diphosphoribose (INV-102) increases protein levels of p53 and Sirtuin 6, upregulates transcription of BRCA1/2, enhances γH2AX focus formation, and promotes assembly of repair complexes at DSBs. Based on in vitro evidence showing INV-102 enhancement of DNA damage response through both p53-dependent and p53-independent pathways, we next tested INV-102 in a rabbit preclinical model of corneal injury. In vivo studies demonstrate a marked reduction in the incidence and severity of secondary keratopathies in INV-102-treated eyes compared with vehicle-treated eyes when treatment was started 24 hours after SM vapor exposure. These results suggest DNA repair mechanisms are a viable therapeutic target for SM injury and suggest topical treatment with INV-102 is a promising approach for SM as well as other conditions associated with DSBs. SIGNIFICANCE STATEMENT: Sulfur mustard gas corneal injury currently has no therapeutic treatment. This study aims to show the therapeutic potential of activating the body's natural DNA damage response to activate tissue repair.

Vitamin B6 regulates IL-33 homeostasis to alleviate type 2 inflammation

Interleukin-33 (IL-33) is a crucial nuclear cytokine that induces the type 2 immune response and maintains immune homeostasis. The fine-tuned regulation of IL-33 in tissue cells is critical to control of the type 2 immune response in airway inflammation, but the mechanism is still unclear. Here, we found that healthy individuals had higher phosphate-pyridoxal (PLP, an active form of vitamin B6) concentrations in the serum than asthma patients. Lower serum PLP concentrations in asthma patients were strongly associated with worse lung function and inflammation. In a mouse model of lung inflammation, we revealed that PLP alleviated the type 2 immune response and that this inhibitory effect relied on the activity of IL-33. A mechanistic study showed that in vivo, pyridoxal (PL) needed to be converted into PLP, which inhibited the type 2 response by regulating IL-33 stability. In mice heterozygous for pyridoxal kinase (PDXK), the conversion of PL to PLP was limited, and IL-33 levels were increased in the lungs, aggravating type 2 inflammation. Furthermore, we found that the mouse double minute 2 homolog (MDM2) protein, an E3 ubiquitin-protein ligase, could ubiquitinate the N-terminus of IL-33 and sustain IL-33 stability in epithelial cells. PLP reduced MDM2-mediated IL-33 polyubiquitination and decreased the level of IL-33 through the proteasome pathway. In addition, inhalation of PLP alleviated asthma-related effects in mouse models. In summary, our data indicate that vitamin B6 regulates MDM2-mediated IL-33 stability to constrain the type 2 response, which might help develop a potential preventive and therapeutic agent for allergy-related diseases.

Altered Expression of TRIM Proteins - Inimical Outcome and Inimitable Oncogenic Function in Breast Cancer with Diverse Carcinogenic Hallmarks

Deregulation of ubiquitin-mediated degradation of oncogene products or tumor suppressors appears to be implicated in the genesis of carcinomas, according to new clinical findings. Conferring to recent research, some members of the tripartite motif (TRIM) proteins (a subfamily of the RING type E3 ubiquitin ligases) act as significant carcinogenesis regulators. Intracellular signaling, development, apoptosis, protein quality control, innate immunity, autophagy, and carcinogenesis are all regulated by TRIM family proteins, the majority of which have E3 ubiquitin ligase activity. The expression of TRIMs in tumors is likely to be related to the formation and/or progression of the disease, and TRIM expression could be used to predict cancer prognosis. Breast cancer is the most common malignancy in women and also the leading cause of death. TRIM family proteins have unique, vital activities, and their dysregulation, such as TRIM 21, promotes breast cancer, according to growing evidence. Many TRIM proteins have been identified as important cancer biomarkers, with decreased or elevated levels of expression. TRIM29 functions as a hypoxia-induced tumor suppressor gene, revealing a new molecular mechanism for ATM-dependent breast cancer suppression. In breast cancer cells, the TRIM28-TWIST1-EMT axis exists, and TRIM28 enhances breast cancer metastasis by stabilizing TWIST1, and thereby increasing epithelial-tomesenchymal transition. Interestingly, many TRIM proteins are involved in the control of p53, and many TRIM proteins are likewise regulated by p53, according to current research. Furthermore, TRIMs linked to specific tumors may aid in the creation of innovative TRIM-targeted cancer treatments. This review focuses on TRIM proteins that are involved in tumor development, progression, and are of clinical significance in breast cancer.

Visualizing and quantifying molecular and cellular processes in C. elegans using light microscopy

Light microscopes are the cell and developmental biologists' "best friend", providing a means to see structures and follow dynamics from the protein to the organism level. A huge advantage of C. elegans as a model organism is its transparency, which coupled with its small size means that nearly every biological process can be observed and measured with the appropriate probe and light microscope. Continuous improvement in microscope technologies along with novel genome editing techniques to create transgenic probes have facilitated the development and implementation of a dizzying array of methods for imaging worm embryos, larvae and adults. In this review we provide an overview of the molecular and cellular processes that can be visualized in living worms using light microscopy. A partial inventory of fluorescent probes and techniques successfully used in worms to image the dynamics of cells, organelles, DNA, and protein localization and activity is followed by a practical guide to choosing between various imaging modalities, including widefield, confocal, lightsheet, and structured illumination microscopy. Finally, we discuss the available tools and approaches, including machine learning, for quantitative image analysis tasks, such as colocalization, segmentation, object tracking, and lineage tracing. Hopefully, this review will inspire worm researchers who have not yet imaged their worms to begin, and push those who are imaging to go faster, finer, and longer.

Phase-Separated Subcellular Compartmentation and Related Human Diseases

In live cells, proteins and nucleic acids can associate together through multivalent interactions, and form relatively isolated phases that undertake designated biological functions and activities. In the past decade, liquid–liquid phase separation (LLPS) has gradually been recognized as a general mechanism for the intracellular organization of biomolecules. LLPS regulates the assembly and composition of dozens of membraneless organelles and condensates in cells. Due to the altered physiological conditions or genetic mutations, phase-separated condensates may undergo aberrant formation, maturation or gelation that contributes to the onset and progression of various diseases, including neurodegenerative disorders and cancers. In this review, we summarize the properties of different membraneless organelles and condensates, and discuss multiple phase separation-regulated biological processes. Based on the dysregulation and mutations of several key regulatory proteins and signaling pathways, we also exemplify how aberrantly regulated LLPS may contribute to human diseases.

The p53 Pathway and Metabolism: The Tree That Hides the Forest

Simple Summary

The p53 pathway is a major tumor suppressor pathway that prevents the propagation of abnormal cells by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism, and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development. Converging lines of evidence support the notion that, in addition to p53, other key components of this molecular cascade are also important regulators of metabolism. Here, we illustrate the underestimated complexity of the metabolic network controlled by the p53 pathway and show how its perturbation contributes to human diseases including cancer, aging, and metabolic diseases.

Abstract

The p53 pathway is functionally inactivated in most, if not all, human cancers. The p53 protein is a central effector of numerous stress-related molecular cascades. p53 controls a safeguard mechanism that prevents accumulation of abnormal cells and their transformation by regulating DNA repair, cell cycle progression, cell death, or senescence. The multiple cellular processes regulated by p53 were more recently extended to the control of metabolism and many studies support the notion that perturbations of p53-associated metabolic activities are linked to cancer development, as well as to other pathophysiological conditions including aging, type II diabetes, and liver disease. Although much less documented than p53 metabolic activities, converging lines of evidence indicate that other key components of this tumor suppressor pathway are also involved in cellular metabolism through p53-dependent as well as p53-independent mechanisms. Thus, at least from a metabolic standpoint, the p53 pathway must be considered as a non-linear pathway, but the complex metabolic network controlled by these p53 regulators and the mechanisms by which their activities are coordinated with p53 metabolic functions remain poorly understood. In this review, we highlight some of the metabolic pathways controlled by several central components of the p53 pathway and their role in tissue homeostasis, metabolic diseases, and cancer.

Phosphoglycerate Mutase Cooperates with Chk1 Kinase to Regulate Glycolysis

Dysregulated glycolysis, including the cancerous Warburg effect, is closely involved in pathological mechanisms of diseased states. Among glycolytic enzymes, phosphoglycerate mutase (PGAM) has been known to exert certain physiological impact in vitro, whereas its regulatory role on glycolysis remains unclear. Here, we identified that PGAM plays a key role in regulating glycolysis in cancer cells but not in standard cells. Cancer-prone phenotype by PGAM overexpression in vivo was associated with upregulated glycolytic features. PGAM interacts and cooperates with Chk1 to regulate the enhanced glycolysis in cancer cells, especially under oncogenic Ras expressing conditions. Genetic or chemical interference of the PGAM-Chk1 interaction, with intact PGAM activity, abrogated the maintenance of cancerous enhanced glycolysis. Thus, the nonenzymatic function of PGAM is essential for the Warburg effect that accompanies cancerous proliferation.

The nucleolus: a central response hub for the stressors that drive cancer progression

The nucleolus is a sub-nuclear body known primarily for its role in ribosome biogenesis. Increased number and/or size of nucleoli have historically been used by pathologists as a prognostic indicator of cancerous lesions. This increase in nucleolar number and/or size is classically attributed to the increased need for protein synthesis in cancer cells. However, evidences suggest that the nucleolus plays critical roles in many cellular functions in both normal cell biology and disease pathologies, including cancer. As new functions of the nucleolus are elucidated, there is mounting evidence to support the role of the nucleolus in regulating additional cellular functions, particularly response to cellular stressors, maintenance of genome stability, and DNA damage repair, as well as the regulation of gene expression and biogenesis of several ribonucleoproteins. This review highlights the central role of the nucleolus in carcinogenesis and cancer progression and discusses how cancer cells may become "addicted" to nucleolar functions.

Nucleolar Sequestration: Remodeling Nucleoli Into Amyloid Bodies

This year marks the 20th anniversary of the discovery that the nucleolus can temporarily immobilize proteins, a process known as nucleolar sequestration. This review reflects on the progress made to understand the physiological roles of nucleolar sequestration and the mechanisms involved in the immobilization of proteins. We discuss how protein immobilization can occur through a highly choreographed amyloidogenic program that converts the nucleolus into a large fibrous organelle with amyloid-like characteristics called the amyloid body (A-body). We propose a working model of A-body biogenesis that includes a role for low-complexity ribosomal intergenic spacer RNA (rIGSRNA) and a discrete peptide sequence, the amyloid-converting motif (ACM), found in many proteins that undergo immobilization. Amyloid bodies provide a unique model to study the multistep assembly of a membraneless compartment and may provide alternative insights into the pathological amyloidogenesis involved in neurological disorders.

TRIM28 multi-domain protein regulates cancer stem cell population in breast tumor development

The expression of Tripartite motif-containing protein 28 (TRIM28)/Krüppel-associated box (KRAB)-associated protein 1 (KAP1), is elevated in at least 14 tumor types, including solid and hematopoietic tumors. High level of TRIM28 is associated with triple-negative subtype of breast cancer (TNBC), which shows higher aggressiveness and lower survival rates. Interestingly, TRIM28 is essential for maintaining the pluripotent phenotype in embryonic stem cells. Following on that finding, we evaluated the role of TRIM28 protein in the regulation of breast cancer stem cells (CSC) populations and tumorigenesis in vitro and in vivo. Downregulation of TRIM28 expression in xenografts led to deceased expression of pluripotency and mesenchymal markers, as well as inhibition of signaling pathways involved in the complex mechanism of CSC maintenance. Moreover, TRIM28 depletion reduced the ability of cancer cells to induce tumor growth when subcutaneously injected in limiting dilutions. Our data demonstrate that the downregulation of TRIM28 gene expression reduced the ability of CSCs to self-renew that resulted in significant reduction of tumor growth. Loss of function of TRIM28 leads to dysregulation of cell cycle, cellular response to stress, cancer cell metabolism, and inhibition of oxidative phosphorylation. All these mechanisms directly regulate maintenance of CSC population. Our original results revealed the role of the TRIM28 in regulating the CSC population in breast cancer. These findings may pave the way to novel and more effective therapies targeting cancer stem cells in breast tumors.

Activation of p53 and destabilization of androgen receptor by combinatorial inhibition of MDM2 and MDMX in prostate cancer cells

Castration-resistant prostate cancer (CRPC) frequently develops after initial standard radiation and androgen deprivation therapy, leaving patients with limited further treatment options. Androgen receptor (AR) is a transcription factor that plays a key role in the initiation and progression of prostate cancer. p53, a major tumor suppressor that is rarely mutated in early-stages of prostate cancer, is often deregulated during prostate cancer progression. Here, we report an unusual co-amplification of MDM2 and MDMX, two crucial negative regulators of p53, in CRPC datasets. We demonstrate that combinatorial inhibition of MDM2 and MDMX, with nutlin-3 and NSC207895 respectively, has a profound inhibitory effect on cell proliferation of androgen-responsive, wild-type TP53 gene carrying prostate cancer cells LNCaP and 22Rv1. We further show that the combinatorial inhibition of MDM2 and MDMX not only activates p53, but also decreases cellular levels of AR and represses its function. Additionally, co-expression of MDM2 and MDMX stabilizes AR. Together, our results indicate that combinatorial inhibition of MDM2 and MDMX may offer a novel compelling strategy for prostate cancer therapy.

Tripartite motif containing 28 (TRIM28) promotes breast cancer metastasis by stabilizing TWIST1 protein

TRIM28 regulates its target genes at both transcriptional and posttranscriptional levels. Here we report that a TRIM28-TWIST1-EMT axis exists in breast cancer cells and TRIM28 promotes breast cancer metastasis by stabilizing TWIST1 and subsequently enhancing EMT. We find that TRIM28 is highly expressed in both cancer cell lines and advanced breast cancer tissues, and the levels of TRIM28 and TWIST1 are positively correlated with the aggressiveness of breast carcinomas. Overexpression and depletion of TRIM28 up- and down-regulates the protein, but not the mRNA levels of TWIST1, respectively, suggesting that TRIM28 upregulates TWIST1 post-transcriptionally. Overexpression of TRIM28 in breast cancer cell line promotes cell migration and invasion. Knockdown of TRIM28 reduces the protein level of TWIST1 with concurrent upregulation of E-cadherin and downregulation of N-cadherin and consequently inhibits cell migration and invasion. Furthermore, Immunoprecipitation and GST pull-down assays demonstrated that TRIM28 interacts with TWIST1 directly and this interaction is presumed to protect TWIST1 from degradation. Our study revealed a novel mechanism in breast cancer cells that TRIM28 enhances metastasis by stabilizing TWIST1, suggesting that targeting TRIM28 could be an efficacious strategy in breast cancer treatment.

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.

Characterization of cancer-associated missense mutations in MDM2

MDM2 is an E3 ubiquitin ligase that binds the N-terminus of p53 and promotes its ubiquitin-dependent degradation. Elevated levels of MDM2 due to overexpression or gene amplification can contribute to tumor development by suppressing p53 activity. Since MDM2 is an oncogene, we explored the possibility that other genetic lesions, namely missense mutations, might alter its activities. We selected mutations in MDM2 that reside in one of the 4 key regions of the protein: p53 binding domain, acidic domain, zinc finger domain, and the RING domain. Unexpectedly, we observed that individual mutations in several of these domains compromised the ability of MDM2 to degrade p53. Mutations in the N-terminal p53 binding domain prevented the formation of a p53-MDM2 complex, thereby protecting p53 from degradation. Additionally, as would be predicted, several cancer-associated mutations in the RING finger domain disrupted the ubiquitin ligase activity of MDM2 and prevented p53 degradation. Interestingly, we observed that amino acid substitutions at the same codon differentially affected MDM2 activity. Our data reveal that mutations in this oncogene can have the paradoxical effect of suppressing its activity. Further understanding of how these mutations perturb MDM2 function may yield novel approaches to inhibiting its activity.

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.

Senescence-inducing stress promotes proteolysis of phosphoglycerate mutase via ubiquitin ligase Mdm2

Despite the well-documented clinical significance of the Warburg effect, it remains unclear how the aggressive glycolytic rates of tumor cells might contribute to other hallmarks of cancer, such as bypass of senescence. Here, we report that, during oncogene- or DNA damage-induced senescence, Pak1-mediated phosphorylation of phosphoglycerate mutase (PGAM) predisposes the glycolytic enzyme to ubiquitin-mediated degradation. We identify Mdm2 as a direct binding partner and ubiquitin ligase for PGAM in cultured cells and in vitro. Mutations in PGAM and Mdm2 that abrogate ubiquitination of PGAM restored the proliferative potential of primary cells under stress conditions and promoted neoplastic transformation. We propose that Mdm2, a downstream effector of p53, attenuates the Warburg effect via ubiquitination and degradation of PGAM.

Histone methylation restrains the expression of subtype-specific genes during terminal neuronal differentiation in Caenorhabditis elegans

Although epigenetic control of stem cell fate choice is well established, little is known about epigenetic regulation of terminal neuronal differentiation. We found that some differences among the subtypes of Caenorhabditis elegans VC neurons, particularly the expression of the transcription factor gene unc-4, require histone modification, most likely H3K9 methylation. An EGF signal from the vulva alleviated the epigenetic repression of unc-4 in vulval VC neurons but not the more distant nonvulval VC cells, which kept unc-4 silenced. Loss of the H3K9 methyltransferase MET-2 or H3K9me2/3 binding proteins HPL-2 and LIN-61 or a novel chromodomain protein CEC-3 caused ectopic unc-4 expression in all VC neurons. Downstream of the EGF signaling in vulval VC neurons, the transcription factor LIN-11 and histone demethylases removed the suppressive histone marks and derepressed unc-4. Behaviorally, expression of UNC-4 in all the VC neurons caused an imbalance in the egg-laying circuit. Thus, epigenetic mechanisms help establish subtype-specific gene expression, which are needed for optimal activity of a neural circuit.

Cisplatin causes cell death via TAB1 regulation of p53/MDM2/MDMX circuitry

The interdependence of p53 and MDM2 is critical for proper cell survival and cell death. Zhu et al. find that TAB1, an activator of TAK1 and p38α, inhibits the E3 ligase activity of MDM2 toward p53 and its homolog, MDMX. Cisplatin-induced cell death is mitigated by TAB1 knockdown. TAB1 stabilizes MDMX and activates p38α to phosphorylate p53, allowing p53 target induction. TAB1 levels are relatively low in cisplatin-resistant clones of ovarian cells and in ovarian tumors, implicating TAB1 as a tumor suppressor.

The interdependence of p53 and MDM2 is critical for proper cell survival and cell death and, when altered, can lead to tumorigenesis. Mitogen-activated protein kinase (MAPK) signaling pathways function in a wide variety of cellular processes, including cell growth, migration, differentiation, and death. Here we discovered that transforming growth factor β-activated kinase 1 (TAK1)-binding protein 1 (TAB1), an activator of TAK1 and of p38α, associates with and inhibits the E3 ligase activity of MDM2 toward p53 and its homolog, MDMX. Depletion of TAB1 inhibits MDM2 siRNA-mediated p53 accumulation and p21 induction, partially rescuing cell cycle arrest induced by MDM2 ablation. Interestingly, of several agents commonly used as DNA-damaging therapeutics, only cell death caused by cisplatin is mitigated by knockdown of TAB1. Two mechanisms are required for TAB1 to regulate apoptosis in cisplatin-treated cells. First, p38α is activated by TAB1 to phosphorylate p53 N-terminal sites, leading to selective induction of p53 targets such as NOXA. Second, MDMX is stabilized in a TAB1-dependent manner and is required for cell death after cisplatin treatment. Interestingly TAB1 levels are relatively low in cisplatin-resistant clones of ovarian cells and in ovarian patient's tumors compared with normal ovarian tissue. Together, our results indicate that TAB1 is a potential tumor suppressor that serves as a functional link between p53–MDM2 circuitry and a key MAPK signaling pathway.

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.

MDM2 regulates MYCN mRNA stabilization and translation in human neuroblastoma cells

The MYCN gene has a critical role in determining the clinical behavior of neuroblastoma. Although it is known that genomic amplification occurs in high-risk subsets, it remains unclear how MYCN expression is regulated in the pathogenesis of neuroblastomas. Here, we report that MYCN expression was regulated by the oncoprotein MDM2 at the post-transcriptional level and was associated with neuroblastoma cell growth. Increasing MDM2 by ectopic overexpression in the cytoplasm enhanced both mRNA and protein expression of MYCN. Mechanistic studies found that the C-terminal RING domain of the MDM2 protein bound to the MYCN mRNA's AREs within the 3'UTR and increased MYCN 3'UTR-mediated mRNA stability and translation. Conversely, MDM2 silencing by specific siRNA rendered the MYCN mRNA unstable and reduced the abundance of the MYCN protein in MYCN-amplified neuroblastoma cell lines. Importantly, this MDM2 silencing resulted in a remarkable inhibition of neuroblastoma cell growth and induction of cell death through a p53-independent pathway. Our results indicate that MDM2 has a p53-independent role in the regulation of both MYCN mRNA stabilization and its translation, suggesting that MDM2-mediated MYCN expression is one mechanism associated with growth of MYCN-associated neuroblastoma and disease progression.

Mdm2's dilemma: to degrade or to translate p53?

In this issue of Cancer Cell, Gajjar et al. provide insight into how Mdm2 can both inhibit and enhance p53 activity. In the basal setting, Mdm2 binds p53 and promotes p53 degradation. Under stress conditions, ATM-dependent phosphorylation of Mdm2 results in its recruitment to p53 mRNA, thereby stimulating p53 translation.

The p53 mRNA-Mdm2 interaction controls Mdm2 nuclear trafficking and is required for p53 activation following DNA damage

The ATM kinase and p53 are key tumor suppressor factors that control the genotoxic stress response pathway. The ATM substrate Mdm2 controls p53 activity by either targeting p53 for degradation or promoting its synthesis by binding the p53 mRNA. The physiological role and regulation of Mdm2's dual function toward p53 is not known. Here we show that ATM-dependent phosphorylation of Mdm2 at Ser395 is required for the p53 mRNA-Mdm2 interaction. This event also promotes SUMO-conjugation of Mdm2 and its nucleoli accumulation. Interfering with the p53 mRNA-Mdm2 interaction prevents p53 stabilization and activation following DNA damage. These results demonstrate how ATM activity switches Mdm2 from a negative to a positive regulator of p53 via the p53 mRNA.

Discovery of Mdm2-MdmX E3 ligase inhibitors using a cell-based ubiquitination assay

E3 ubiquitin ligases are of interest as drug targets for their ability to regulate protein stability and function. The oncogene Mdm2 is an attractive E3 ligase to target, as it is the key negative regulator of the tumor suppressor p53, which controls the transcription of genes involved in cell fate. Overexpression of Mdm2 facilitates tumorigenesis by inactivating p53, and through p53-independent oncogenic effects. We developed a high-throughput cellular Mdm2 auto-ubiquitination assay, which we used to discover a class of small-molecule Mdm2 ligase activity inhibitors. These compounds inhibit Mdm2 and p53 ubiquitination in cells, reduce viability of cells with wild-type p53, and synergize with DNA-damaging agents to cause cell death. We determined that these compounds effectively inhibit the E3 ligase activity of the Mdm2-MdmX hetero-complex. This mechanism may be exploitable to create a new class of anti-tumor agents.

Microtubule depolymerization in Caenorhabditis elegans touch receptor neurons reduces gene expression through a p38 MAPK pathway

Microtubules are integral to neuronal development and function. They endow cells with polarity, shape, and structure, and their extensive surface area provides substrates for intracellular trafficking and scaffolds for signaling molecules. Consequently, microtubule polymerization dynamics affect not only structural features of the cell but also the subcellular localization of proteins that can trigger intracellular signaling events. In the nematode Caenorhabditis elegans, the processes of touch receptor neurons are filled with a bundle of specialized large-diameter microtubules. We find that conditions that disrupt these microtubules (loss of either the MEC-7 β-tubulin or MEC-12 α-tubulin or growth in 1 mM colchicine) cause a general reduction in touch receptor neuron (TRN) protein levels. This reduction requires a p38 MAPK pathway (DLK-1, MKK-4, and PMK-3) and the transcription factor CEBP-1. Cells may use this feedback pathway that couples microtubule state and MAPK activation to regulate cellular functions.

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.

Enhanced neuronal RNAi in C. elegans using SID-1

We expressed SID-1, a transmembrane protein from Caenorhabditis elegans that is required for systemic RNAi, in C. elegans neurons. This expression increased the response of neurons to dsRNA delivered by feeding. Mutations in the lin-15b and lin-35 genes further enhanced this effect. Worms expressing neuronal SID-1 showed RNAi phenotypes for known neuronal genes and for uncharacterized genes with no previously known neuronal phenotypes. Neuronal expression of sid-1 decreased non-neuronal RNAi, suggesting that neurons expressing transgenic sid-1(+) served as a sink for dsRNA. This effect, or a sid-1(−) background, can be used to uncover neuronal defects for lethal genes. Expression of sid-1(+) from cell-specific promoters in sid-1 mutants results in cell-specific feeding RNAi. We used these strains to identify a role for integrin signaling genes in mechanosensation.

Mdm2 facilitates the association of p53 with the proteasome

The ubiquitin ligase Mdm2 targets the p53 tumor suppressor protein for proteasomal degradation. Mutating phosphorylation sites in the central domain of Mdm2 prevents p53 degradation, although it is still ubiquitylated, indicating that Mdm2 has a post-ubiquitylation function for p53 degradation. We show that Mdm2 associates with several subunits of the 19S proteasome regulatory particle in a ubiquitylation-independent manner. Mdm2 furthermore promotes the formation of a ternary complex of itself, p53, and the proteasome. Replacing phosphorylation sites within the central domain with alanines reduced the formation of the ternary complex. The C-terminus of Mdm2 was sufficient for interaction with the proteasome despite an additional proteasome binding site in the Mdm2 N-terminus. In addition to binding to the proteasome, the C-terminus of Mdm2 bound to the central domain, possibly competing with, and therefore blocking, Mdm2/proteasome interaction. We propose that Mdm2 facilitates, or at least enhances, the association of p53 with the proteasome and that phosphorylation of the central domain of Mdm2 regulates this process.

Conditional gene expression and RNAi using MEC-8-dependent splicing in C. elegans

We describe a method for conditional regulation of gene expression based on the processing of an intron cassette by a splicing factor. The RNA processing factor MEC-8 is necessary for the function of the Caenorhabditis elegans touch receptor neurons; mec-8 mutants are touch insensitive. We show here that this insensitivity involves the loss of MEC-8-dependent splicing of mec-2, which encodes a component of the mechanosensory transduction complex. MEC-8 is needed to remove the ninth intron in mec-2 pre-mRNA to form the longest of three mRNAs, mec-2a. Without MEC-8, splicing causes the termination of the transcript. Inclusion of mec-2 intron 9 is sufficient to convey mec-8-dependent regulation on other genes and, in mec-8(u218ts) mutants, resulted in their temperature-dependent expression. Because mec-8 is expressed ubiquitously in embryos and extensively in larvae, this system should produce temperature-sensitive expression for most genes. We report a strain that exhibits temperature-dependent RNA interference.

Ribosomal protein S7 is both a regulator and a substrate of MDM2

MDM2 associates with ribosomal protein S7, and this interaction is required to inhibit MDM2's E3 ligase activity, leading to stabilization of MDM2 and p53. Notably, the MDM2 homolog MDMX facilitates the inhibition of MDM2 E3 ligase activity by S7. Further, ablation of S7 inhibits MDM2 and p53 accumulation induced by different stress signals in some cell types. Thus, ribosomal/nucleolar stress is likely a key integrating event in DNA damage signaling to p53. Interestingly, S7 is itself a substrate for MDM2 E3 ligase activity both in vitro and in vivo. An S7-ubiquitin fusion protein (S7-Ub) selectively inhibits MDM2 degradation of p53 and is unaffected by MDMX. S7-Ub promotes apoptosis to a greater extent than S7 alone. This indicates that MDM2 ubiquitination of S7 is involved in sustaining the p53 response. Thus, S7 functions as both effector and affector of MDM2 to ensure a proper cellular response to different stress signals.

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.

A function for the RING finger domain in the allosteric control of MDM2 conformation and activity

The MDM2 oncoprotein plays multiple regulatory roles in the control of p53-dependent gene expression. A picture of MDM2 is emerging where structurally discrete but interdependent functional domains are linked through changes in conformation. The domain structure includes: (i) a hydrophobic pocket at the N terminus of MDM2 that is involved in both its transrepressor and E3-ubiqutin ligase functions, (ii) a central acid domain that recognizes a ubiquitination signal in the core DNA binding domain of p53, and (iii) a C-terminal C2H2C4 RING finger domain that is required for E2 enzyme-binding and ATP-dependent molecular chaperone activity. Here we show that the binding affinity of MDM2s hydrophobic pocket can be regulated through the RING finger domain and that increases in pocket affinity are reflected by a gain in MDM2 transrepressor activity. Thus, mutations within the RING domain that affect zinc coordination, but not one that inhibits ATP binding, produce MDM2 proteins that have a higher affinity for the BOX-I transactivation domain of p53 and a reduced I(0.5) for p53 transrepression. An allosteric model for regulation of the hydrophobic pocket is supported by differences in protein conformation and pocket accessibility between wild-type and the RING domain mutant MDM2 proteins. Additionally the data demonstrate that the complex relationship between different domains of MDM2 can impact on the efficacy of anticancer drugs directed toward its hydrophobic pocket.

Study on the spatial architecture of p53, MDM2, and p14ARF containing complexes

We have developed a surface plasmon resonance (SPR)-based immunocapture approach to study multimeric protein-protein complexes. A composition and spatial architecture of protein complexes that contained GST-tagged p53, p14ARF, and MDM2 was examined by the developed approach. Obtained results verified that the p53 protein possesses two binding sites for MDM2. Ternary complexes containing p14ARF, MDM2, and p53 proteins could only be formed when MDM2 protein functions as a bridging molecule. That was confirmed by immunoprecipitation and immunostaining.

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

Murine double minute 2 (MDM2) protein exhibits many diverse biochemical functions on the tumour suppressor protein p53, including transcriptional suppression and E3 ubiquitin ligase activity. However, more recent data have shown that MDM2 can exhibit ATP-dependent molecular chaperone activity and directly mediate folding of the p53 tetramer. Analysing the ATP-dependent function of MDM2 will provide novel insights into the evolution and function of the protein. We have established a system to analyse the molecular chaperone function of MDM2 on another of its target proteins, the transcription factor E2F1. In the absence of ATP, MDM2 was able to catalyse inhibition of the DNA-binding function of E2F1. However, the inhibition of E2F1 by MDM2 was stimulated by ATP, and mutation of the ATP-binding domain of MDM2 (K454A) prevented the ATP-stimulated inhibition of E2F1. Further, ATP stabilized the binding of E2F1 to MDM2 using conditions under which ATP destabilized the MDM2:p53 complex. However, the ATP-binding mutant of MDM2 was as active as an E3 ubiquitin ligase on E2F1 and p53, highlighting a specific function for the ATP-binding domain of MDM2 in altering substrate protein folding. Antibodies to three distinct domains of MDM2 neutralized its activity, showing that inhibition of E2F1 is MDM2-dependent and that multiple domains of MDM2 are involved in E2F1 inhibition. Dimethylsulfoxide, which reduces protein unfolding, also prevented E2F1 inhibition by MDM2. These data support a role for the ATP-binding domain in altering the protein-protein interaction function of MDM2.

Targeted inactivation of Mdm2 RING finger E3 ubiquitin ligase activity in the mouse reveals mechanistic insights into p53 regulation

It is believed that Mdm2 suppresses p53 in two ways: transcriptional inhibition by direct binding, and degradation via its E3 ligase activity. To study these functions physiologically, we generated mice bearing a single-residue substitution (C462A) abolishing the E3 function without affecting p53 binding. Unexpectedly, homozygous mutant mice died before E7.5, and deletion of p53 rescued the lethality. Furthermore, reintroducing a switchable p53 by crossing with p53ER(TAM) mice surprisingly demonstrated that the mutant Mdm2(C462A) was rapidly degraded in a manner indistinguishable from that of the wild-type Mdm2. Hence, our data indicate that (1) the Mdm2-p53 physical interaction, without Mdm2-mediated p53 ubiquitination, cannot control p53 activity sufficiently to allow early mouse embryonic development, and (2) Mdm2's E3 function is not required for Mdm2 degradation.

MDM2 chaperones the p53 tumor suppressor

The murine double minute (mdm2) gene encodes an E3 ubiquitin ligase that plays a key role in the degradation of p53 tumor suppressor protein. Nevertheless recent data highlight other p53-independent functions of MDM2. Given that MDM2 protein binds ATP, can interact with the Hsp90 chaperone, plays a role in the modulation of transcription factors and protection and activation of DNA polymerases, and is involved in ribosome assembly and nascent p53 protein biosynthesis, we have evaluated and found MDM2 protein to possess an intrinsic molecular chaperone activity. MDM2 can substitute for the Hsp90 molecular chaperone in promoting binding of p53 to the p21-derived promoter sequence. This reaction is driven by recycling of MDM2 from the p53 complex, triggered by binding of ATP to MDM2. The ATP binding mutant MDM2 protein (K454A) lacks the chaperone activity both in vivo and in vitro. Mdm2 cotransfected in the H1299 cell line with wild-type p53 stimulates efficient p53 folding in vivo but at the same time accelerates the degradation of p53. MDM2 in which one of the Zn(2+) coordinating residues is mutated (C478S or C464A) blocks degradation but enhances folding of p53. This is the first demonstration that MDM2 possesses an intrinsic molecular chaperone activity, indicating that the ATP binding function of MDM2 can mediate its chaperone function toward the p53 tumor suppressor.

RAS-RAF-MEK-dependent oxidative cell death involving voltage-dependent anion channels

Therapeutics that discriminate between the genetic makeup of normal cells and tumour cells are valuable for treating and understanding cancer. Small molecules with oncogene-selective lethality may reveal novel functions of oncoproteins and enable the creation of more selective drugs. Here we describe the mechanism of action of the selective anti-tumour agent erastin, involving the RAS-RAF-MEK signalling pathway functioning in cell proliferation, differentiation and survival. Erastin exhibits greater lethality in human tumour cells harbouring mutations in the oncogenes HRAS, KRAS or BRAF. Using affinity purification and mass spectrometry, we discovered that erastin acts through mitochondrial voltage-dependent anion channels (VDACs)--a novel target for anti-cancer drugs. We show that erastin treatment of cells harbouring oncogenic RAS causes the appearance of oxidative species and subsequent death through an oxidative, non-apoptotic mechanism. RNA-interference-mediated knockdown of VDAC2 or VDAC3 caused resistance to erastin, implicating these two VDAC isoforms in the mechanism of action of erastin. Moreover, using purified mitochondria expressing a single VDAC isoform, we found that erastin alters the permeability of the outer mitochondrial membrane. Finally, using a radiolabelled analogue and a filter-binding assay, we show that erastin binds directly to VDAC2. These results demonstrate that ligands to VDAC proteins can induce non-apoptotic cell death selectively in some tumour cells harbouring activating mutations in the RAS-RAF-MEK pathway.

The RING finger domain of MDM2 is essential for MDM2-mediated TGF-beta resistance

In this study, we attempt to gain insights into the molecular mechanism underlying MDM2-mediated TGF-beta resistance. MDM2 renders cells refractory to TGF-beta by overcoming a TGF-beta-induced G1 cell cycle arrest. Because the TGF-beta resistant phenotype is reversible upon removal of MDM2, MDM2 likely confers TGF-beta resistance by directly targeting the cellular machinery involved in the growth inhibition by TGF-beta. Investigation of the structure-function relationship of MDM2 reveals three elements essential for MDM2 to confer TGF-beta resistance in both mink lung epithelial cells and human mammary epithelial cells. One of these elements is the C-terminal half of the p53-binding domain, which at least partially retained p53-binding and inhibitory activity. Second, the ability of MDM2 to mediate TGF-beta resistance is disrupted by mutation of the nuclear localization signal, but is restored upon coexpression of MDMX. Finally, mutations of the zinc coordination residues of the RING finger domain abrogates TGF-beta resistance, but not the ability of MDM2 to inhibit p53 activity or to bind MDMX. These data suggest that RING finger-mediated p53 inhibition and MDMX interaction are not sufficient to cause TGF-beta resistance and imply a crucial role of the E3 ubiquitin ligase activity of this domain in MDM2-mediated TGF-beta resistance.

Hetero-oligomerization with MdmX Rescues the Ubiquitin/Nedd8 Ligase Activity of RING Finger Mutants of Mdm2

Mdm2 is a member of the RING finger family of ubiquitin ligases and is best known for its role in targeting the tumor suppressor p53 for ubiquitination and degradation. Mdm2 can bind to itself and to the structurally related protein MdmX, and these interactions involve the RING finger domain of Mdm2 and MdmX, respectively. In this study, we performed a mutational analysis of the RING finger domain of Mdm2, and we identified several amino acid residues that are important for Mdm2 to exert its ubiquitin ligase function. Mutation of some of these residues interfered with the Mdm2-Mdm2 interaction indicating that a homomeric complex represents the active form of Mdm2. Mutation of other residues did not detectably affect the ability of Mdm2 to interact with itself but reduced the ability of Mdm2 to interact with UbcH5. Remarkably, MdmX efficiently rescued the ubiquitin ligase activity of the latter Mdm2 mutants in vitro and within cells. Because the interaction of Mdm2 with MdmX is more stable than the Mdm2-Mdm2 interaction, this suggests that Mdm2-MdmX complexes play a prominent role in p53 ubiquitination in vivo. Furthermore, we show that, similar to Mdm2, the Mdm2-MdmX complex has Nedd8 ligase activity and that all mutations that affect the ubiquitin ligase activity of Mdm2 also affect its Nedd8 ligase activity. From a mechanistic perspective, this suggests that the actual function of Mdm2 and Mdm2-MdmX, respectively, in p53 ubiquitination and in p53 neddylation is similar for both processes.

Energy-dependent nucleolar localization of p53 in vitro requires two discrete regions within the p53 carboxyl terminus

The p53 tumor suppressor is a nucleocytoplasmic shuttling protein that is found predominantly in the nucleus of cells. In addition to mutation, abnormal p53 cellular localization is one of the mechanisms that inactivate p53 function. To further understand features of p53 that contribute to the regulation of its trafficking within the cell, we analysed the subnuclear localization of wild-type and mutant p53 in human cells that were either permeabilized with detergent or treated with the proteasome inhibitor MG132. We, here, show that either endogenously expressed or exogenously added p53 protein localizes to the nucleolus in detergent-permeabilized cells in a concentration- and ATP hydrolysis-dependent manner. Two discrete regions within the carboxyl terminus of p53 are essential for nucleolar localization in permeabilized cells. Similarly, localization of p53 to the nucleolus after proteasome inhibition in unpermeabilized cells requires sequences within the carboxyl terminus of p53. Interestingly, genotoxic stress markedly decreases the association of p53 with the nucleolus, and phosphorylation of p53 at S392, a site that is modified by such stress, partially impairs its nucleolar localization. The possible significance of these findings is discussed.

The Mdm2 RING domain C-terminus is required for supramolecular assembly and ubiquitin ligase activity

Mdm2, a key negative regulator of the p53 tumor suppressor, is a RING-type E3 ubiquitin ligase. The Mdm2 RING domain can be biochemically fractionated into two discrete species, one of which exists as higher order oligomers that are visible by electron microscopy, whereas the other is a monomer. Both fractions are ATP binding and E3 ligase activity competent, although the oligomeric fraction exhibits lower dependence on the E2 component of ubiquitin polymerization reactions. The extreme C-terminal five amino acids of Mdm2 are essential for E3 ligase activity in vivo and in vitro, as well as for oligomeric assembly of the protein. A single residue (phenylalanine 490) in that sequence is critical for both properties. Interestingly, the C-terminus of the Mdm2 homologue, MdmX (itself inert as an E3 ligase), can fully substitute for the equivalent segment of Mdm2 and restore its E3 activity. We further show that the Mdm2 C-terminus is involved in intramolecular interactions and can set up a platform for direct protein-protein interactions with the E2.

Solution Structure of the Hdm2 C2H2C4 RING, a Domain Critical for Ubiquitination of p53

Regulation of the transcriptional response to the tumor suppressor p53 occurs at many levels, including control of its transcriptional activity, and of its stability and concentration within the cell. p53 stability is regulated by the protein Hdm2, an E3 ubiquitin ligase that binds to p53 and promotes its ubiquitination and degradation. The C-terminal domain of Hdm2, which is critical for this activity, has been classified as a RING domain on the basis of sequence homology, although it lacks the canonical set of zinc ligands (RING domains typically have C3HC4 or C4C4 zinc coordination). Here, we report the solution structure of the C2H2C4 RING domain of Hdm2(429-491), which reveals a symmetrical dimer with a unique cross-brace zinc-binding scheme. Each subunit has one Cys4 Zn site and one His2Cys2 Zn site. The global fold of each subunit is similar to those reported for other RING domains, with a compact betabetaalphabeta fold, a small hydrophobic core, and two Zn ions, which are essential for maintaining the domain structure. The dimer structure is maintained by an extensive interface that buries a large hydrophobic area on each subunit. It has been proposed that Hdm2 and its homologue HdmX form a stable heterodimer through their RING domains, resulting in a synergistic increase in observed E3 activity. To test this proposal, we prepared an HdmX RING construct and showed by NMR titration that it forms a tight 1:1 complex with the Hdm2 RING. The resonances most perturbed by heterodimer formation are located within the subunit interface of the homodimer, far removed from the surface expected to form the docking site of the E2 ubiquitin-conjugating enzyme, providing a structure-based rationale for the function of the RING domains in p53 ubiquitination.

Excess HDM2 impacts cell cycle and apoptosis and has a selective effect on p53-dependent transcription

Mutational inactivation of p53 is only one of the ways that tumors lose p53 function. An alternate route is through overexpression of HDM2, the negative regulator of p53. To further understand how excess HDM2 regulates p53-mediated functions, we generated H1299 cell clones that constitutively express both ectopic HDM2 and tetracycline-regulated inducible p53. We found that over a range of p53 concentrations constitutively expressed HDM2 did not affect the levels of p53 protein. Nevertheless, cells with excess HDM2 displayed numerous changes in their response to p53. After DNA damage, such cells had both increased p53-mediated G2 arrest and reduced cell death. They also showed selective impairment of p53 target gene induction in that some p53 targets were unaffected whereas others were markedly less well induced in the presence of extra HDM2 protein. We also found that excess HDM2 was correlated with reduced p53 acetylation but did not affect p53 association with target promoters in vivo. Indeed, there was no significant difference in the amount of HDM2 associated with p53 at target promoters that differed in their expression depending on the presence of extra HDM2. Thus, HDM2 can selectively down-regulate the transcription function of p53 without either degrading p53 or affecting the interaction of p53 with target promoters.

Novel checkpoint response to genotoxic stress mediated by nucleolin-replication protein a complex formation

Human replication protein A (RPA), the primary single-stranded DNA-binding protein, was previously found to be inhibited after heat shock by complex formation with nucleolin. Here we show that nucleolin-RPA complex formation is stimulated after genotoxic stresses such as treatment with camptothecin or exposure to ionizing radiation. Complex formation in vitro and in vivo requires a 63-residue glycine-arginine-rich (GAR) domain located at the extreme C terminus of nucleolin, with this domain sufficient to inhibit DNA replication in vitro. Fluorescence resonance energy transfer studies demonstrate that the nucleolin-RPA interaction after stress occurs both in the nucleoplasm and in the nucleolus. Expression of the GAR domain or a nucleolin mutant (TM) with a constitutive interaction with RPA is sufficient to inhibit entry into S phase. Increasing cellular RPA levels by overexpression of the RPA2 subunit minimizes the inhibitory effects of nucleolin GAR or TM expression on chromosomal DNA replication. The arrest is independent of p53 activation by ATM or ATR and does not involve heightened expression of p21. Our data reveal a novel cellular mechanism that represses genomic replication in response to genotoxic stress by inhibition of an essential DNA replication factor.

Combinatorial Marking of Cells and Organelles with Reconstituted Fluorescent Proteins

Expression of GFP and other fluorescent proteins depends on cis-regulatory elements. Because these elements rarely direct expression to specific cell types, GFP production cannot always be sufficiently limited. Here we show that reconstitution of GFP, YFP, and CFP previously split into two polypeptides yields fluorescent products when coexpressed in C. elegans. Because this reconstitution involves two components, it can confirm cellular coexpression and identify cells expressing a previously uncharacterized promoter. By choosing promoters whose expression patterns overlap for a single cell type, we can produce animals with fluorescence only in those cells. Furthermore, when one partial GFP polypeptide is fused with a subcellularly localized protein or peptide, this restricted expression leads to the fluorescent marking of cellular components in a subset of cells.