Regulation of p53 nuclear export through sequential changes in conformation and ubiquitination

Decoding the ubiquitin language: Orchestrating transcription initiation and gene expression through chromatin remodelers and histones

Eukaryotic transcription is a finely orchestrated process and it is controlled by transcription factors as well as epigenetic regulators. Transcription factors and epigenetic regulators undergo different types of posttranslational modifications including ubiquitination to control transcription process. Ubiquitination, traditionally associated with protein degradation, has emerged as a crucial contributor to the regulation of chromatin structure through ubiquitination of histone and chromatin remodelers. Ubiquitination introduces new layers of intricacy to the regulation of transcription initiation through controlling the equilibrium between euchromatin and heterochromatin states. Nucleosome, the fundamental units of chromatin, spacing in euchromatin and heterochromatin states are regulated by histone modification and chromatin remodeling complexes. Chromatin remodeling complexes actively sculpt the chromatin architecture and thereby influence the transcriptional states of genes. Therefore, understanding the dynamic behavior of nucleosome spacing is critical as it impacts various cellular functions through controlling gene expression profiles. In this comprehensive review, we discussed the intricate interplay between ubiquitination and transcription initiation, and illuminated the underlying molecular mechanisms that occur in a variety of biological contexts. This exploration sheds light on the complex regulatory networks that govern eukaryotic transcription, providing important insights into the fine orchestration of gene expression and chromatin dynamics.

Selective CK1α degraders exert antiproliferative activity against a broad range of human cancer cell lines

Molecular-glue degraders are small molecules that induce a specific interaction between an E3 ligase and a target protein, resulting in the target proteolysis. The discovery of molecular glue degraders currently relies mostly on screening approaches. Here, we describe screening of a library of cereblon (CRBN) ligands against a panel of patient-derived cancer cell lines, leading to the discovery of SJ7095, a potent degrader of CK1α, IKZF1 and IKZF3 proteins. Through a structure-informed exploration of structure activity relationship (SAR) around this small molecule we develop SJ3149, a selective and potent degrader of CK1α protein in vitro and in vivo. The structure of SJ3149 co-crystalized in complex with CK1α + CRBN + DDB1 provides a rationale for the improved degradation properties of this compound. In a panel of 115 cancer cell lines SJ3149 displays a broad antiproliferative activity profile, which shows statistically significant correlation with MDM2 inhibitor Nutlin-3a. These findings suggest potential utility of selective CK1α degraders for treatment of hematological cancers and solid tumors.

Transcription Factor TAL1 in Erythropoiesis

Lineage-specific transcription factors (TFs) regulate differentiation of hematopoietic stem cells (HSCs). They are decisive for the establishment and maintenance of lineage-specific gene expression programs during hematopoiesis. For this they create a regulatory network between TFs, epigenetic cofactors, and microRNAs. They activate cell-type specific genes and repress competing gene expression programs. Disturbance of this process leads to impaired lineage fidelity and diseases of the blood system. The TF T-cell acute leukemia 1 (TAL1) is central for erythroid differentiation and contributes to the formation of distinct gene regulatory complexes in progenitor cells and erythroid cells. A TAL1/E47 heterodimer binds to DNA with the TFs GATA-binding factor 1 and 2 (GATA1/2), the cofactors LIM domain only 1 and 2 (LMO1/2), and LIM domain-binding protein 1 (LDB1) to form a core TAL1 complex. Furthermore, cell-type-dependent interactions of TAL1 with other TFs such as with runt-related transcription factor 1 (RUNX1) and Kruppel-like factor 1 (KLF1) are established. Moreover, TAL1 activity is regulated by the formation of TAL1 isoforms, posttranslational modifications (PTMs), and microRNAs. Here, we describe the function of TAL1 in normal hematopoiesis with a focus on erythropoiesis.

ACA-28, an anticancer compound, induces Pap1 nuclear accumulation via ROS-dependent and -independent mechanisms in fission yeast

The nucleocytoplasmic transport of proteins is an important mechanism to control cell fate. Pap1 is a fission yeast nucleocytoplasmic shuttling transcription factor of which localization is redox regulated. The nuclear export factor Crm1/exportin negatively regulates Pap1 by exporting it from the nucleus to the cytoplasm. Here, we describe the effect of an anti-cancer compound ACA-28, an improved derivative of 1'-acetoxychavicol acetate (ACA), on the subcellular distribution of Pap1. ACA-28 induced nuclear accumulation of Pap1 more strongly than did ACA. ROS inhibitor N-acetyl-L-cysteine (NAC) partly antagonized the Pap1 nuclear accumulation induced by ACA-28. NAC almost abolished Pap1 nuclear localization upon H 2 O 2 , whereas leptomycin B (LMB)-mediated inhibition of Pap1 nuclear export was resistant to NAC. Collectively, ACA-28-mediated apoptosis in cancer cells may involve ROS-dependent and -independent mechanisms.

The crosstalk between ubiquitin-conjugating enzyme E2Q1 and p53 in colorectal cancer: An in vitro analysis

Colorectal cancer (CRC) is a prevalent gastrointestinal neoplasm that ranks fourth in terms of cancer-related deaths worldwide. In the process of CRC progression, multiple ubiquitin-conjugating enzymes (E2s) are involved; UBE2Q1 is one of those newly identified E2s that is markedly expressed in human colorectal tumors. Since p53 is a well-known tumor suppressor and defined as a key factor to be targeted by the ubiquitin-proteasome system, we hypothesized that UBE2Q1 might contribute to CRC progression through the modulation of p53. Using the lipofection method, the cultured SW480 and LS180 cells were transfected with the UBE2Q1 ORF-containing pCMV6-AN-GFP vector. Then, quantitative RT-PCR was used to assay the mRNA expression levels of p53's target genes, i.e., Mdm2, Bcl2, and Cyclin E. Moreover, Western blot analysis was performed to confirm the cellular overexpression of UBE2Q1 and assess the protein levels of p53, pre- and post-transfection. The expression of p53's target genes were cell line-dependent except for Mdm2 that was consistent with the findings of p53. The results of Western blotting demonstrated that the protein levels of p53 were greatly lower in UBE2Q1-transfected SW480 cells compared to the control SW480 cells. However, the reduced levels of p53 protein were not remarkable in the transfected LS180 cells compared to the control cells. The suppression of p53 is believed to be the result of UBE2Q1-dependent ubiquitination and its subsequent proteasomal degradation. Furthermore, the ubiquitination of p53 can act as a signal for degradation-independent functions, such as nuclear export and suppressing the p53's transcriptional activities. In this context, the decreased Mdm2 levels can moderate the proteasome-independent mono-ubiquitination of p53. The ubiquitinated p53 modulates the transcriptional levels of target genes. Therefore, the up-modulation of UBE2Q1 may influence the transcriptional activities depending on p53, and thereby contributes to CRC progression through regulating the p53.

Characterization of the interaction between the tumour suppressor p53 and heme and its role in the protein conformational dynamics studied by various spectroscopic techniques and hydrogen/deuterium exchange coupled with mass spectrometry

The tumour suppressor p53 regulates the expression of a myriad of proteins that are important for numerous cellular processes, including apoptosis, cell cycle arrest, DNA repair, metabolism, and even autophagy and ferroptosis. Aside from DNA, p53 can interact with many types of partners including proteins and small organic molecules. The ability of p53 to interact with heme has been reported so far. In this study, we used various spectroscopic studies to conduct a thorough biophysical characterization of the interaction between p53 and heme concerning the oxidation, spin, coordination, and ligand state of heme iron. We found that the p53 oligomeric state and zinc biding ability are preserved upon the interaction with heme. Moreover, we described the effect of heme binding on the conformational dynamics of p53 by hydrogen/deuterium exchange coupled with mass spectrometry. Specifically, the conformational flexibility of p53 is significantly increased upon interaction with heme, while its affinity to a specific DNA sequence is reduced by heme. The inhibitory effect of DNA binding by heme is partially reversible. We discuss the potential heme binding sites in p53 with respect to the observed conformational dynamics changes and perturbed DNA-binding ability of p53 upon interaction with heme.

The E3 Ligases in Cervical Cancer and Endometrial Cancer

Endometrial (EC) and cervical (CC) cancers are the most prevalent malignancies of the female reproductive system. There is a global trend towards increasing incidence and mortality, with a decreasing age trend. E3 ligases label substrates with ubiquitin to regulate their activity and stability and are involved in various cellular functions. Studies have confirmed abnormal expression or mutations of E3 ligases in EC and CC, indicating their vital roles in the occurrence and progression of EC and CC. This paper provides an overview of the E3 ligases implicated in EC and CC and discusses their underlying mechanism. In addition, this review provides research advances in the target of ubiquitination processes in EC and CC.

DNA damage response revisited: the p53 family and its regulators provide endless cancer therapy opportunities

Antitumor therapeutic strategies that fundamentally rely on the induction of DNA damage to eradicate and inhibit the growth of cancer cells are integral approaches to cancer therapy. Although DNA-damaging therapies advance the battle with cancer, resistance, and recurrence following treatment are common. Thus, searching for vulnerabilities that facilitate the action of DNA-damaging agents by sensitizing cancer cells is an active research area. Therefore, it is crucial to decipher the detailed molecular events involved in DNA damage responses (DDRs) to DNA-damaging agents in cancer. The tumor suppressor p53 is active at the hub of the DDR. Researchers have identified an increasing number of genes regulated by p53 transcriptional functions that have been shown to be critical direct or indirect mediators of cell fate, cell cycle regulation, and DNA repair. Posttranslational modifications (PTMs) primarily orchestrate and direct the activity of p53 in response to DNA damage. Many molecules mediating PTMs on p53 have been identified. The anticancer potential realized by targeting these molecules has been shown through experiments and clinical trials to sensitize cancer cells to DNA-damaging agents. This review briefly acknowledges the complexity of DDR pathways/networks. We specifically focus on p53 regulators, protein kinases, and E3/E4 ubiquitin ligases and their anticancer potential.

Selinexor in Advanced, Metastatic Dedifferentiated Liposarcoma: A Multinational, Randomized, Double-Blind, Placebo-Controlled Trial

PURPOSE

Antitumor activity in preclinical models and a phase I study of patients with dedifferentiated liposarcoma (DD-LPS) was observed with selinexor. We evaluated the clinical benefit of selinexor in patients with previously treated DD-LPS whose sarcoma progressed on approved agents.

METHODS

SEAL was a phase II-III, multicenter, randomized, double-blind, placebo-controlled study. Patients age 12 years or older with advanced DD-LPS who had received two-five lines of therapy were randomly assigned (2:1) to selinexor (60 mg) or placebo twice weekly in 6-week cycles (crossover permitted). The primary end point was progression-free survival (PFS). Patients who received at least one dose of study treatment were included for safety analysis (ClinicalTrials.gov identifier: NCT02606461).

RESULTS

Two hundred eighty-five patients were enrolled (selinexor, n = 188; placebo, n = 97). PFS was significantly longer with selinexor versus placebo: hazard ratio (HR) 0.70 (95% CI, 0.52 to 0.95; one-sided P = .011; medians 2.8 v 2.1 months), as was time to next treatment: HR 0.50 (95% CI, 0.37 to 0.66; one-sided P < .0001; medians 5.8 v 3.2 months). With crossover, no difference was observed in overall survival. The most common treatment-emergent adverse events of any grade versus grade 3 or 4 with selinexor were nausea (151 [80.7%] v 11 [5.9]), decreased appetite (113 [60.4%] v 14 [7.5%]), and fatigue (96 [51.3%] v 12 [6.4%]). Four (2.1%) and three (3.1%) patients died in the selinexor and placebo arms, respectively. Exploratory RNA sequencing analysis identified that the absence of CALB1 expression was associated with longer PFS with selinexor compared with placebo (median 6.9 v 2.2 months; HR, 0.19; P = .001).

CONCLUSION

Patients with advanced, refractory DD-LPS showed improved PFS and time to next treatment with selinexor compared with placebo. Supportive care and dose reductions mitigated side effects of selinexor. Prospective validation of CALB1 expression as a predictive biomarker for selinexor in DD-LPS is warranted.

Generation and characterization of site-specifically mono-ubiquitylated p53

The tumor suppressor p53 is regulated by various posttranslational modifications including different types of ubiquitylation, which exert distinct effects on p53. While modification by ubiquitin chains targets p53 for degradation, attachment of single ubiquitin moieties (mono‐ubiquitylation) affects the intracellular location of p53 and/or its interaction with chromatin. However, how this is achieved at the molecular level remains largely unknown. Similarly, since p53 can be ubiquitylated at different lysine residues, it remains unclear if the eventual effect depends on the position of the lysine modified. Here, we combined genetic code expansion with oxime ligation to generate p53 site‐specifically mono‐ubiquitylated at position 120. We found that mono‐ubiquitylation at this position neither interferes with p53 ubiquitylation by the E3 ligases HDM2 and E6AP in complex with the viral E6 oncoprotein nor affects p53 binding to a cognate DNA sequence. Thus, ubiquitylation per se does not affect physiologically relevant properties of p53.

Targeting Post-Translational Regulation of p53 in Colorectal Cancer by Exploiting Vulnerabilities in the p53-MDM2 Axis

The role played by the key tumor suppressor gene p53 and the implications of p53 mutations for the development and progression of neoplasia continue to expand. This review focuses on colorectal cancer and the regulators of p53 expression and activity identified over the past decade. These newly recognized regulatory mechanisms include (1) direct regulation of mouse double minute 2 homolog (MDM2), an E3 ubiquitin-protein ligase; (2) modulation of the MDM2-p53 interaction; (3) MDM2-independent p53 degradation; and (4) inhibition of p53 nuclear translocation. We positioned these regulatory mechanisms in the context of p53 missense mutations, which not only evade canonical p53 degradation machinery but also exhibit gain-of-function phenotypes that enhance tumor survival and metastasis. Lastly, we discuss current and potential therapeutic strategies directed against p53 mutant-bearing tumors.

Mutated p53 in HGSC-From a Common Mutation to a Target for Therapy

Simple Summary

Ovarian high-grade serous cancer (HGSC), the most common and the deadliest subtype of epithelial ovarian cancer, is characterized by frequent mutations in the TP53 tumor suppressor gene, encoding for the p53 protein in nearly 100% of cases. This makes p53 the focus of many studies trying to understand its role in HGSC. The aim of our review paper is to provide updates on the latest findings related to the role of mutant p53 in HGSC. This includes the clinical outcomes of TP53 mutations in HGSC, upstream regulators and downstream effectors of p53, its function in the earliest stages of HGSC development and in the interplay between the tumor cells and their microenvironment. We summarize with the likelihood of p53 mutants to serve as biomarkers for early diagnosis and as targets for therapy in HGSC.

Abstract

Mutations in tumor suppressor gene TP53, encoding for the p53 protein, are the most ubiquitous genetic variation in human ovarian HGSC, the most prevalent and lethal histologic subtype of epithelial ovarian cancer (EOC). The majority of TP53 mutations are missense mutations, leading to loss of tumor suppressive function of p53 and gain of new oncogenic functions. This review presents the clinical relevance of TP53 mutations in HGSC, elaborating on several recently identified upstream regulators of mutant p53 that control its expression and downstream target genes that mediate its roles in the disease. TP53 mutations are the earliest genetic alterations during HGSC pathogenesis, and we summarize current information related to p53 function in the pathogenesis of HGSC. The role of p53 is cell autonomous, and in the interaction between cancer cells and its microenvironment. We discuss the reduction in p53 expression levels in tumor associated fibroblasts that promotes cancer progression, and the role of mutated p53 in the interaction between the tumor and its microenvironment. Lastly, we discuss the potential of TP53 mutations to serve as diagnostic biomarkers and detail some more advanced efforts to use mutated p53 as a therapeutic target in HGSC.

Pterygium-The Good, the Bad, and the Ugly

Pterygium is a multifaceted pathology that displays apparent conflicting characteristics: benign (e.g., self-limiting and superficial), bad (e.g., proliferative and potentially recurrent) and ugly (e.g., signs of preneoplastic transformation). The natural successive question is: why are we lacking reports showing that pterygium lesions become life-threatening through metastasis, especially since pterygium has considerable similarities with UV-related malignancies on the molecular level? In this review, we consider how our pathophysiological understanding of the benign pterygium pathology overlaps with ocular surface squamous neoplasia and skin cancer. The three UV-related disorders share the same initial insult (i.e., UV radiation) and responsive repair mechanisms to the ensuing (in)direct DNA damage. Their downstream apoptotic regulators and other cellular adaptations are remarkably alike. However, a complicating factor in understanding the fine line between the self-limiting nature of pterygium and the malignant transformation in other UV-related diseases is the prominent ambiguity in the pathological evaluation of pterygium biopsies. Features of preneoplastic transformation (i.e., dysplasia) are used to define normal cellular reactions (i.e., atypia and metaplasia) and vice versa. A uniform grading system could help in unraveling the true nature of this ancient disease and potentially help in identifying the earliest intervention point possible regarding the cellular switch that drives a cell’s fate towards cancer.

Selinexor versus doxorubicin in dedifferentiated liposarcoma PDXs: evidence of greater activity and apoptotic response dependent on p53 nuclear accumulation and survivin down-regulation

BACKGROUND

Dedifferentiated liposarcoma (DDLPS), a tumor that lacks effective treatment strategies and is associated with poor outcomes, expresses amplified MDM2 in the presence of wild-type p53. MDM2 ubiquitination of p53 facilitates its XPO1-mediated nuclear export, thus limiting p53 tumor suppressor functions. Consequently, nuclear export is a rational target in DDLPS. We directly compared the antitumor activity of the first-in class XPO1 inhibitor selinexor and doxorubicin, the standard front-line therapy in sarcomas, in DDLPS patient-derived xenografts (PDXs) and primary cell lines.

METHODS

Drug activity was assessed in three PDXs (and two corresponding cell lines) established from the dedifferentiated component of primary untreated retroperitoneal DDLPS with myogenic (N = 2) and rhabdomyoblastic (N = 1) differentiation from patients who underwent surgery. These models were marked by amplification of MDM2, CDK4 and HMGA2 genes.

RESULTS

Selinexor was moderately active in the three PDXs but achieved greater tumor response compared to doxorubicin (maximum tumor volume inhibition: 46-80 % vs. 37-60 %). The PDX harboring rhabdomyoblastic dedifferentiation showed the highest sensitivity to both agents. PDX response to selinexor and doxorubicin was not associated with the extent of MDM2 and CDK4 gene amplification. Interestingly, the most chemosensitive PDX model showed the lowest extent of HMGA2 amplification. Selinexor was also more efficient than doxorubicinin in inducing an apoptotic response in PDXs and cell lines. Consistently, an increased nuclear accumulation of p53 was seen in all selinexor-treated models. In addition, a time-dependent decrease of survivin expression, with an almost complete abrogation of the cytoplasmic anti-apoptotic pool of this protein, was observed as a consequence of the decreased acetylation/activation of STAT3 and the increased ubiquitination of nuclear survivin.

CONCLUSIONS

Selinexor showed a moderate antitumor activity in three DDLPS PDXs, which was, however, consistently higher than doxorubicin across all different models regardless the extent of MDM2 amplification and the histological differentiation. The depletion of survivin protein seems to significantly contribute to the induction of apoptosis through which selinexor exerts its antitumor activity.

Flow Cytometric Analyses of p53-Mediated Cell Cycle Arrest and Apoptosis in Cancer Cells

Phenotypic analysis of the effects of a gene of interest may be limited because stable expression of some genes leads to adverse consequences in cell survival, such as disturbance of cell cycle progression, senescence, autophagy, and programmed cell death. One of the best examples is tumor suppressor p53. p53 functions as a tumor suppressor by inducing cell cycle arrest and apoptosis in response to genotoxic and environmental insults. The choice and timing of either pathways induced by p53 depend on cellular context, cell types, and the degree of cellular/genomic damage (For review, see (Chen J, Cold Spring Harb Perspect Med 6:a026104, 2016)). The uncertainty makes the studies on the long-term effects of p53 in cells challenging. This chapter describes a method of flow cytometric analysis of ectopic expression of p53 to better quantify cell cycle distribution and apoptosis in cells treated with DNA damaging agents. The method can be easily adapted to other genes of interest to study their contributions to the fate of variety of cell types in response to endogenous or exogenous stresses.

Pterygium: new insights

Pterygia are common conjunctival degenerations with well-documented risk factors but an unclear pathogenesis. Better understanding of the pathogenesis of pterygium could lead to improved surgical outcomes and decreased postoperative recurrence. Currently, pterygium excision with conjunctival autograft remains the preferred surgical technique to decrease pterygium recurrence. Many adjuvant therapies have been used in pterygium surgery to varying degrees of success. Topical cyclosporine, an immunosuppressive medication, in conjunction with conjunctival autograft was found to be most successful in decreasing pterygium recurrence according to a recent meta-analysis. Other adjuvant therapies such as mitomycin-C (MMC), 5-fluorouracil (5-FU), and beta-irradiation have also been used, though usage of these may cause multiple adverse effects. Recent research indicates that interactions between mouse double minute 2 (MDM2) and p53 could play a role in the occurrence of pterygium. Nutlin, an MDM2 antagonist, was found to have significantly less toxicity in conjunctival cells when compared with MMC on laboratory analysis of pterygium samples.

Dedifferentiated Liposarcoma: Systemic Therapy Options

OPINION STATEMENT

Over the last several years, the systemic treatment landscape for dedifferentiated liposarcoma (DDLPS) has notably expanded. Historically, systemic therapy options have been limited to cytotoxic chemotherapy agents, including doxorubicin, ifosfamide, gemcitabine, and docetaxel, that were shown to have efficacy in unselected populations of patients with soft tissue sarcomas. More recently, however, there have been phase II and III trials establishing clinical benefit of the cytotoxic agents trabectedin and eribulin along with the tyrosine kinase inhibitor pazopanib in patients with advanced liposarcoma and DDLPS. Additionally, there are several investigational targeted therapies that have incorporated advances in the understanding of DDLPS disease biology, exploiting the fact that nearly all such tumors include highly amplified expression of MDM2 and CDK4. Recent clinical trials have supported the benefit of the CDK4 inhibitor abemaciclib and the nuclear export inhibitor selinexor and support continued development of anti-MDM2 therapies, with particular attention to the bone marrow toxicity and resultant thrombocytopenia that has thus far limited their use. In contrast, the checkpoint inhibitors pembrolizumab and nivolumab remain of questionable benefit, although these immunotherapy drugs may have a role when combined with other therapeutic agents. Ongoing phase III trials will clarify the role of these novel agents. Future directions include directly comparing current standard-of-care options and newer therapies, developing synergistic combinations of novel agents, and evaluating their role in patients with localized DDLPS.

The Emerging Landscape of p53 Isoforms in Physiology, Cancer and Degenerative Diseases

p53, first described four decades ago, is now established as a master regulator of cellular stress response, the “guardian of the genome”. p53 contributes to biological robustness by behaving in a cellular-context dependent manner, influenced by several factors (e.g., cell type, active signalling pathways, the type, extent and intensity of cellular damage, cell cycle stage, nutrient availability, immune function). The p53 isoforms regulate gene transcription and protein expression in response to the stimuli so that the cell response is precisely tuned to the cell signals and cell context. Twelve isoforms of p53 have been described in humans. In this review, we explore the interactions between p53 isoforms and other proteins contributing to their established cellular functions, which can be both tumour-suppressive and oncogenic in nature. Evidence of p53 isoform in human cancers is largely based on RT-qPCR expression studies, usually investigating a particular type of isoform. Beyond p53 isoform functions in cancer, it is implicated in neurodegeneration, embryological development, progeroid phenotype, inflammatory pathology, infections and tissue regeneration, which are described in this review.

Nuclear-cytoplasmic Shuttling in Chronic Myeloid Leukemia: Implications in Leukemia Maintenance and Therapy

Nuclear-cytoplasmic shuttling is a highly regulated and complex process, which involves both proteins and nucleic acids. Changes in cellular compartmentalization of various proteins, including oncogenes and tumor suppressors, affect cellular behavior, promoting or inhibiting proliferation, apoptosis and sensitivity to therapies. In this review, we will recapitulate the role of various shuttling components in Chronic Myeloid Leukemia and we will provide insights on the potential role of shuttling proteins as therapeutic targets.

Abrogation of FBW7α-dependent p53 degradation enhances p53's function as a tumor suppressor

The gene encoding the tumor suppressor p53 is mutated in most cancers. p53 expression is known to be tightly controlled by several E3 ligases. Here, we show that F-box and WD repeat domain-containing 7α (FBW7α), the substrate-recognition component of the SCF^FBW7 multiprotein E3 ligase complex, targets both WT and tumor-derived mutants of p53 for proteasomal degradation in multiple human cancer cell lines (HCT116 and U2OS). We found that lack of FBW7α stabilizes p53 levels, thereby increasing its half-life. p53 ubiquitylation and subsequent degradation require the F-box and the C-terminal WD40 repeats in FBW7α. The polyubiquitylation of p53 occurred via Lys-48 linkage and involved phosphorylation on p53 at Ser-33 and Ser-37 by glycogen synthase kinase 3β (GSK3β) and DNA-dependent protein kinase (DNA-PK), respectively. These phosphorylation events created a phosphodegron that enhanced p53 binding to FBW7α, allowing for the attachment of polyubiquitin moieties at Lys-132 in p53. FBW7α-dependent p53 polyubiquitylation apparently occurred during and immediately after DNA double-strand breaks induced by either doxorubicin or ionizing radiation. Accordingly, in cells lacking FBW7α, p53 induction was enhanced after DNA damage. Phosphodegron-mediated polyubiquitylation of p53 on Lys-132 had functional consequences, with cells in which FBW7α-mediated p53 degradation was abrogated exhibiting enhancement of their tumorigenic potential. We conclude that p53, which previously has been reported to transactivate FBW7, is also targeted by the same E3 ligase for degradation, suggesting the presence of a regulatory feedback loop that controls p53 levels and functions during DNA damage.

Patrolling the nucleus: inner nuclear membrane-associated degradation

Protein quality control and transport are important for the integrity of organelles such as the endoplasmic reticulum, but it is largely unknown how protein homeostasis is regulated at the nuclear envelope (NE) despite the connection between NE protein function and human disease. Elucidating mechanisms that regulate the NE proteome is key to understanding nuclear processes such as gene expression, DNA replication and repair as NE components, particularly proteins at the inner nuclear membrane (INM), are involved in the maintenance of nuclear structure, nuclear positioning and chromosome organization. Nuclear pore complexes control the entry and exit of proteins in and out of the nucleus, restricting movement across the nuclear membrane based on protein size, or the size of the extraluminal-facing domain of a transmembrane protein, providing one level of INM proteome regulation. Research in budding yeast has identified a protein quality control system that targets mislocalized and misfolded proteins at the INM. Here, we review what is known about INM-associated degradation, including recent evidence suggesting that it not only targets mislocalized or misfolded proteins, but also contributes to homeostasis of resident INM proteins.

Distribution of Proteins at the Inner Nuclear Membrane Is Regulated by the Asi1 E3 Ligase in Saccharomyces cerevisiae

Inner nuclear membrane (INM) protein composition regulates nuclear function, affecting processes such as gene expression, chromosome organization, nuclear shape, and stability. Mechanisms that drive changes in the INM proteome are poorly understood, in part because it is difficult to definitively assay INM composition rigorously and systematically. Using a split-GFP complementation system to detect INM access, we examined the distribution of all C-terminally tagged Saccharomyces cerevisiae membrane proteins in wild-type cells and in mutants affecting protein quality control pathways, such as INM-associated degradation (INMAD), ER-associated degradation, and vacuolar proteolysis. Deletion of the E3 ligase Asi1 had the most specific effect on the INM compared to mutants in vacuolar or ER-associated degradation pathways, consistent with a role for Asi1 in the INMAD pathway. Our data suggest that Asi1 not only removes mistargeted proteins at the INM, but also controls the levels and distribution of native INM components, such as the membrane nucleoporin Pom33. Interestingly, loss of Asi1 does not affect Pom33 protein levels but instead alters Pom33 distribution in the nuclear envelope through Pom33 ubiquitination, which drives INM redistribution. Taken together, our data demonstrate that the Asi1 E3 ligase has a novel function in INM protein regulation in addition to protein turnover.

Regulators of Oncogenic Mutant TP53 Gain of Function

The tumor suppressor p53 (TP53) is the most frequently mutated human gene. Mutations in TP53 not only disrupt its tumor suppressor function, but also endow oncogenic gain-of-function (GOF) activities in a manner independent of wild-type TP53 (wtp53). Mutant TP53 (mutp53) GOF is mainly mediated by its binding with other tumor suppressive or oncogenic proteins. Increasing evidence indicates that stabilization of mutp53 is crucial for its GOF activity. However, little is known about factors that alter mutp53 stability and its oncogenic GOF activities. In this review article, we primarily summarize key regulators of mutp53 stability/activities, including genotoxic stress, post-translational modifications, ubiquitin ligases, and molecular chaperones, as well as a single nucleotide polymorphism (SNP) and dimer-forming mutations in mutp53.

p53 inhibition by MDM2 in human pterygium

AIMS

To confirm that mouse double minute 2 (MDM2) could inhibit p53 activity in human pterygium. And to show the disruption of MDM2-p53 interaction could reactive the functions of p53 in pterygium.

METHOD

Pterygium and corresponding conjunctiva tissues were collected for establishment of primary cell lines. Expression patterns of MDM2 and p53 were detected by immunofluorescence. Protein localization of p53 and MDM2, and transcriptional activity of p53 in both untreated and MDM2 antagonist (Nutlin) treated pterygium cells were quantified.

RESULTS

In pterygium, p53 was highly expressed in cytoplasm and slightly expressed in the nuclei. MDM2 was localized in the nuclei. A p53 transcriptional regulated target gene, p21, was not expressed in pterygium tissues, suggesting the p53 transcriptional activity was not active in pterygium. After treatment with Nutlin, increased nuclear localization of p53 (4.05%-80.56%) was observed in pterygium cells along with increasing Nutlin dosages (from 0 to 50 μM, p < 0.001). The expression of p21 was increased after Nutlin treatments in pterygium cells (2.49 folds in 20 μM Nutlin treated cells compared to control treated cells, p = 0.012).

CONCLUSION

We discovered a novel mechanism in pterygium whereby MDM2 suppresses p53 transcriptional activity despite abundant p53 in pterygium. Disruption of MDM2-p53 interaction by Nutlin could be a potential treatment for pterygium.

Identification of Novel Somatic TP53 Mutations in Patients with High-Grade Serous Ovarian Cancer (HGSOC) Using Next-Generation Sequencing (NGS)

Somatic mutations in TP53 are a hallmark of high-grade serous ovarian cancer (HGSOC), although their prognostic and predictive value as markers is not well defined. Next-generation sequencing (NGS) can identify novel mutations with high sensitivity, that may be repurposed as potential druggable anti-cancer targets and aid in therapeutic decisions. Here, a commercial NGS cancer panel comprising 26 genes, including TP53, was used to identify new genetic markers of platinum resistance and patient prognosis in a retrospective set of patients diagnosed with epithelial ovarian cancer. Six novel TP53 somatic mutations in untreated tumors from six distinct patients diagnosed with HGSOC were identified: TP53 c.728_739delTGGGCGGCATGA (p.Met243_Met247del, in-frame insertion or deletion (INDEL); TP53 c.795_809delGGGACGGAACAGCTT (p.Gly266_Phe270del, in-frame INDEL); TP53 c.826_827GC>AT (p.Ala276Ile, missense); TP53 c.1022insT (p.Arg342Profs*5, frameshift INDEL); TP53 c.1180delT (p.Ter394Aspfs*28, frameshift INDEL); and TP53 c.573insT (p.Gln192Serfs*17, frameshift INDEL). Novel TP53 variants were validated by classical sequencing methods and their impact on protein expression in tumors explored by immunohistochemistry. Further insights into the potential functional effect of the mutations were obtained by different in silico approaches, bioinformatics tools, and structural modeling. This discovery of previously unreported TP53 somatic mutations provides an opportunity to translate NGS technology into personalized medicine and identify new potential targets for therapeutic applications.

Wild-type and cancer-related p53 proteins are preferentially degraded by MDM2 as dimers rather than tetramers

The p53 tumor suppressor protein is the most well studied as a regulator of transcription in the nucleus, where it exists primarily as a tetramer. However, there are other oligomeric states of p53 that are relevant to its regulation and activities. In unstressed cells, p53 is normally held in check by MDM2 that targets p53 for transcriptional repression, proteasomal degradation, and cytoplasmic localization. Here we discovered a hydrophobic region within the MDM2 N-terminal domain that binds exclusively to the dimeric form of the p53 C-terminal domain in vitro. In cell-based assays, MDM2 exhibits superior binding to, hyperdegradation of, and increased nuclear exclusion of dimeric p53 when compared with tetrameric wild-type p53. Correspondingly, impairing the hydrophobicity of the newly identified N-terminal MDM2 region leads to p53 stabilization. Interestingly, we found that dimeric mutant p53 is partially unfolded and is a target for ubiquitin-independent degradation by the 20S proteasome. Finally, forcing certain tumor-derived mutant forms of p53 into dimer configuration results in hyperdegradation of mutant p53 and inhibition of p53-mediated cancer cell migration. Gaining insight into different oligomeric forms of p53 may provide novel approaches to cancer therapy.

The physical interaction of p53 and plakoglobin is necessary for their synergistic inhibition of migration and invasion

Plakoglobin (PG) is a paralog of β-catenin with similar adhesive, but contrasting signalling functions. Although β-catenin has well-known oncogenic function, PG generally acts as a tumor/metastasis suppressor by mechanisms that are just beginning to be deciphered. Previously, we showed that PG interacted with wild type (WT) and a number of mutant p53s, and that its tumor/metastasis suppressor activity may be mediated, at least partially, by this interaction. Here, carcinoma cell lines deficient in both p53 and PG (H1299), or expressing mutant p53 in the absence of PG (SCC9), were transfected with expression constructs encoding WT and different fragments and deletions of p53 and PG, individually or in pairs. Transfectants were characterized for their in vitro growth, migratory and invasive properties and for mapping the interacting domain of p53 and PG. We showed that when coexpressed, p53-WT and PG-WT cooperated to decrease growth, and acted synergistically to significantly reduce cell migration and invasion. The DNA-binding domain of p53 and C-terminal domain of PG mediated p53/PG interaction, and furthermore, the C-terminus of PG played a central role in the inhibition of invasion in association with p53.

The role of the NORE1A tumor suppressor in Oncogene-Induced Senescence

The Ras genes are the most frequently mutated oncogenes in human cancer. However, Ras biology is quite complex. While Ras promotes tumorigenesis by regulating numerous growth promoting pathways, activated Ras can paradoxically also lead to cell cycle arrest, death, and Oncogene-Induced Senescence (OIS). OIS is thought to be a critical pathway that serves to protect cells against aberrant Ras signaling. Multiple reports have highlighted the importance of the p53 and Rb tumor suppressors in Ras mediated OIS. However, until recently, the molecular mechanisms connecting Ras to these proteins remained unknown. The RASSF family of tumor suppressors has recently been identified as direct effectors of Ras. One of these members, NORE1A (RASSF5), may be the missing link between Ras-induced senescence and the regulation of p53 and Rb. This occurs both quantitatively, by promoting protein stability, as well as qualitatively via promoting critical pro-senescent post-translational modifications. Here we review the mechanisms by which NORE1A can activate OIS as a barrier against Ras-mediated transformation, and how this could lead to improved therapeutic strategies against cancers having lost NORE1A expression.

Identification of somatic and germ-line DICER1 mutations in pleuropulmonary blastoma, cystic nephroma and rhabdomyosarcoma tumors within a DICER1 syndrome pedigree

BACKGROUND

DICER1 syndrome is a pediatric cancer predisposition condition causing a variety of tumor types in children and young adults. In this report we studied a family with two relatives presenting a variety of neoplastic conditions at childhood.

METHODS

Germ-line mutation screening of the complete coding region of the DICER1 gene in genomic DNA from the proband was performed. The presence of somatic DICER1 mutation and further alterations in driver genes was investigated in genomic DNA obtained from available tumor samples.

RESULTS

A nonsense germ-line mutation in DICER1 causing a truncated protein at the IIIb domain level was identified segregating within a family including two affected relatives who developed in one case cystic nephroma and pleuropulmonary blastoma, and rhabdomyosarcoma and multinodular goiter in the other. Additional in trans DICER1 missense somatic mutations in the IIIb DICER1 domain were found both in the cystic nephroma and in the rhabdomyosarcoma, suggesting that neoplasms in this family might arise from the unusual two-hit mechanism for DICER-derived tumorigenesis in which after the presence of a truncated constitutive protein, a neomorphic DICER1 activity is somatically adquired. Additional genetic alterations, such as TP53 mutations, were identified in the rhabdomyosarcoma.

CONCLUSIONS

Besides DICER1 loss of standard activity, oncogenic cooperation of other genes, as mutated TP53, may involve developing higher grade tumors within this syndrome. Given the broad clinical spectrum that may arise, genetic counseling and close surveillance must be offered to all family members at risk of DICER1 syndrome.

Essential Roles of E3 Ubiquitin Ligases in p53 Regulation

The ubiquitination pathway and proteasomal degradation machinery dominantly regulate p53 tumor suppressor protein stability, localization, and functions in both normal and cancerous cells. Selective E3 ubiquitin ligases dominantly regulate protein levels and activities of p53 in a large range of physiological conditions and in response to cellular changes induced by exogenous and endogenous stresses. The regulation of p53’s functions by E3 ubiquitin ligases is a complex process that can lead to positive or negative regulation of p53 protein in a context- and cell type-dependent manner. Accessory proteins bind and modulate E3 ubiquitin ligases, adding yet another layer of regulatory control for p53 and its downstream functions. This review provides a comprehensive understanding of p53 regulation by selective E3 ubiquitin ligases and their potential to be considered as a new class of biomarkers and therapeutic targets in diverse types of cancers.

TBP-like Protein (TLP) Disrupts the p53-MDM2 Interaction and Induces Long-lasting p53 Activation

Stress-induced activation of p53 is an essential cellular response to prevent aberrant cell proliferation and cancer development. The ubiquitin ligase MDM2 promotes p53 degradation and limits the duration of p53 activation. It remains unclear, however, how p53 persistently escapes MDM2-mediated negative control for making appropriate cell fate decisions. Here we report that TBP-like protein (TLP), a member of the TBP family, is a new regulatory factor for the p53-MDM2 interplay and thus for p53 activation. We found that TLP acts to stabilize p53 protein to ensure long-lasting p53 activation, leading to potentiation of p53-induced apoptosis and senescence after genotoxic stress. Mechanistically, TLP interferes with MDM2 binding and ubiquitination of p53. Moreover, single cell imaging analysis shows that TLP depletion accelerates MDM2-mediated nuclear export of p53. We further show that a cervical cancer-derived TLP mutant has less p53 binding ability and lacks a proliferation-repressive function. Our findings uncover a role of TLP as a competitive MDM2 blocker, proposing a novel mechanism by which p53 escapes the p53-MDM2 negative feedback loop to modulate cell fate decisions.

Mechanisms of p53 Functional De-Regulation: Role of the IκB-α/p53 Complex

TP53 is one of the most frequently-mutated and deleted tumor suppressors in cancer, with a dramatic correlation with dismal prognoses. In addition to genetic inactivation, the p53 protein can be functionally inactivated in cancer, through post-transductional modifications, changes in cellular compartmentalization, and interactions with other proteins. Here, we review the mechanisms of p53 functional inactivation, with a particular emphasis on the interaction between p53 and IκB-α, the NFKBIA gene product.

Phase IB Study of Selinexor, a First-in-Class Inhibitor of Nuclear Export, in Patients With Advanced Refractory Bone or Soft Tissue Sarcoma

PURPOSE

We evaluated the pharmacokinetics (PKs), pharmacodynamics, safety, and efficacy of selinexor, an oral selective inhibitor of nuclear export compound, in patients with advanced soft tissue or bone sarcoma with progressive disease.

PATIENTS AND METHODS

Fifty-four patients were treated with oral selinexor twice per week (on days 1 and 3) at one of three doses (30 mg/m(2), 50 mg/m(2), or flat dose of 60 mg) either continuously or on a schedule of 3 weeks on, 1 week off. PK analysis was performed under fasting and fed states (low v high fat content) and using various formulations of selinexor (tablet, capsule, or suspension). Tumor biopsies before and during treatment were evaluated for pharmacodynamic changes.

RESULTS

The most commonly reported drug-related adverse events (grade 1 or 2) were nausea, vomiting, anorexia, and fatigue, which were well managed with supportive care. Commonly reported grade 3 or 4 toxicities were fatigue, thrombocytopenia, anemia, lymphopenia, and leukopenia. Selinexor was significantly better tolerated when administered as a flat dose on an intermittent schedule. PK analysis of selinexor revealed a clinically insignificant increase (approximately 15% to 20%) in drug exposure when taken with food. Immunohistochemical analysis of paired tumor biopsies revealed increased nuclear accumulation of tumor suppressor proteins, decreased cell proliferation, increased apoptosis, and stromal deposition. Of the 52 patients evaluable for response, none experienced an objective response by RECIST (version 1.1); however, 17 (33%) showed durable (≥ 4 months) stable disease, including seven (47%) of 15 evaluable patients with dedifferentiated liposarcoma.

CONCLUSION

Selinexor was well tolerated at a 60-mg flat dose on a 3-weeks-on, 1-week-off schedule. There was no clinically meaningful impact of food on PKs. Preliminary evidence of anticancer activity in sarcoma was demonstrated.

Identification of CASZ1 NES reveals potential mechanisms for loss of CASZ1 tumor suppressor activity in neuroblastoma

As a transcription factor, localization to the nucleus and the recruitment of cofactors to regulate gene transcription is essential. Nuclear localization and nucleosome remodeling and histone deacetylase (NuRD) complex binding are required for the zinc-finger transcription factor CASZ1 to function as a neuroblastoma (NB) tumor suppressor. However, the critical amino acids (AAs) that are required for CASZ1 interaction with NuRD complex and the regulation of CASZ1 subcellular localization have not been characterized. Through alanine scanning, immunofluorescence cell staining and co-immunoprecipitation, we define a critical region at the CASZ1 N terminus (AAs 23-40) that mediates the CASZ1b nuclear localization and NuRD interaction. Furthermore, we identified a nuclear export signal (NES) at the N terminus (AAs 176-192) that contributes to CASZ1 nuclear-cytoplasmic shuttling in a chromosomal maintenance 1-dependent manner. An analysis of CASZ1 protein expression in a primary NB tissue microarray shows that high nuclear CASZ1 staining is detected in tumor samples from NB patients with good prognosis. In contrast, cytoplasmic-restricted CASZ1 staining or low nuclear CASZ1 staining is found in tumor samples from patients with poor prognosis. These findings provide insight into mechanisms by which CASZ1 regulates transcription, and suggests that regulation of CASZ1 subcellular localization may impact its function in normal development and pathologic conditions such as NB tumorigenesis.

Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.

The p53 C terminus controls site-specific DNA binding and promotes structural changes within the central DNA binding domain

DNA binding by numerous transcription factors including the p53 tumor suppressor protein constitutes a vital early step in transcriptional activation. While the role of the central core DNA binding domain (DBD) of p53 in site-specific DNA binding has been established, the contribution of the sequence-independent C-terminal domain (CTD) is still not well understood. We investigated the DNA-binding properties of a series of p53 CTD variants using a combination of in vitro biochemical analyses and in vivo binding experiments. Our results provide several unanticipated and interconnected findings. First, the CTD enables DNA binding in a sequence-dependent manner that is drastically altered by either its modification or deletion. Second, dependence on the CTD correlates with the extent to which the p53 binding site deviates from the canonical consensus sequence. Third, the CTD enables stable formation of p53-DNA complexes to divergent binding sites via DNA-induced conformational changes within the DBD itself.

NORE1A Regulates MDM2 Via β-TrCP

Mouse Double Minute 2 Homolog (MDM2) is a key negative regulator of the master tumor suppressor p53. MDM2 regulates p53 on multiple levels, including acting as an ubiquitin ligase for the protein, thereby promoting its degradation by the proteasome. MDM2 is oncogenic and is frequently found to be over-expressed in human tumors, suggesting its dysregulation plays an important role in human cancers. We have recently found that the Ras effector and RASSF (Ras Association Domain Family) family member RASSF5/NORE1A enhances the levels of nuclear p53. We have also found that NORE1A (Novel Ras Effector 1A) binds the substrate recognition component of the SCF-ubiquitin ligase complex β-TrCP. Here, we now show that NORE1A regulates MDM2 protein levels by targeting it for ubiquitination by SCF-β-TrCP. We also show the suppression of NORE1A protein levels enhances MDM2 protein expression. Finally, we show that MDM2 can suppress the potent senescence phenotype induced by NORE1A over-expression. Thus, we identify a mechanism by which Ras/NORE1A can modulate p53 protein levels. As MDM2 has several important targets in addition to p53, this finding has broad implications for cancer biology in tumor cells that have lost expression of NORE1A due to promoter methylation.

MuRF2 regulates PPARγ1 activity to protect against diabetic cardiomyopathy and enhance weight gain induced by a high fat diet

BACKGROUND

In diabetes mellitus the morbidity and mortality of cardiovascular disease is increased and represents an important independent mechanism by which heart disease is exacerbated. The pathogenesis of diabetic cardiomyopathy involves the enhanced activation of PPAR transcription factors, including PPARα, and to a lesser degree PPARβ and PPARγ1. How these transcription factors are regulated in the heart is largely unknown. Recent studies have described post-translational ubiquitination of PPARs as ways in which PPAR activity is inhibited in cancer. However, specific mechanisms in the heart have not previously been described. Recent studies have implicated the muscle-specific ubiquitin ligase muscle ring finger-2 (MuRF2) in inhibiting the nuclear transcription factor SRF. Initial studies of MuRF2-/- hearts revealed enhanced PPAR activity, leading to the hypothesis that MuRF2 regulates PPAR activity by post-translational ubiquitination.

METHODS

MuRF2-/- mice were challenged with a 26-week 60% fat diet designed to simulate obesity-mediated insulin resistance and diabetic cardiomyopathy. Mice were followed by conscious echocardiography, blood glucose, tissue triglyceride, glycogen levels, immunoblot analysis of intracellular signaling, heart and skeletal muscle morphometrics, and PPARα, PPARβ, and PPARγ1-regulated mRNA expression.

RESULTS

MuRF2 protein levels increase ~20% during the development of diabetic cardiomyopathy induced by high fat diet. Compared to littermate wildtype hearts, MuRF2-/- hearts exhibit an exaggerated diabetic cardiomyopathy, characterized by an early onset systolic dysfunction, larger left ventricular mass, and higher heart weight. MuRF2-/- hearts had significantly increased PPARα- and PPARγ1-regulated gene expression by RT-qPCR, consistent with MuRF2's regulation of these transcription factors in vivo. Mechanistically, MuRF2 mono-ubiquitinated PPARα and PPARγ1 in vitro, consistent with its non-degradatory role in diabetic cardiomyopathy. However, increasing MuRF2:PPARγ1 (>5:1) beyond physiological levels drove poly-ubiquitin-mediated degradation of PPARγ1 in vitro, indicating large MuRF2 increases may lead to PPAR degradation if found in other disease states.

CONCLUSIONS

Mutations in MuRF2 have been described to contribute to the severity of familial hypertrophic cardiomyopathy. The present study suggests that the lack of MuRF2, as found in these patients, can result in an exaggerated diabetic cardiomyopathy. These studies also identify MuRF2 as the first ubiquitin ligase to regulate cardiac PPARα and PPARγ1 activities in vivo via post-translational modification without degradation.

Comparative assessment of the apoptotic potential of silver nanoparticles synthesized by Bacillus tequilensis and Calocybe indica in MDA-MB-231 human breast cancer cells: targeting p53 for anticancer therapy

Background

Recently, the use of nanotechnology has been expanding very rapidly in diverse areas of research, such as consumer products, energy, materials, and medicine. This is especially true in the area of nanomedicine, due to physicochemical properties, such as mechanical, chemical, magnetic, optical, and electrical properties, compared with bulk materials. The first goal of this study was to produce silver nanoparticles (AgNPs) using two different biological resources as reducing agents, Bacillus tequilensis and Calocybe indica. The second goal was to investigate the apoptotic potential of the as-prepared AgNPs in breast cancer cells. The final goal was to investigate the role of p53 in the cellular response elicited by AgNPs.

Methods

The synthesis and characterization of AgNPs were assessed by various analytical techniques, including ultraviolet-visible (UV-vis) spectroscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The apoptotic efficiency of AgNPs was confirmed using a series of assays, including cell viability, leakage of lactate dehydrogenase (LDH), production of reactive oxygen species (ROS), DNA fragmentation, mitochondrial membrane potential, and Western blot.

Results

The absorption spectrum of the yellow AgNPs showed the presence of nanoparticles. XRD and FTIR spectroscopy results confirmed the crystal structure and biomolecules involved in the synthesis of AgNPs. The AgNPs derived from bacteria and fungi showed distinguishable shapes, with an average size of 20 nm. Cell viability assays suggested a dose-dependent toxic effect of AgNPs, which was confirmed by leakage of LDH, activation of ROS, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells in MDA-MB-231 breast cancer cells. Western blot analyses revealed that AgNPs induce cellular apoptosis via activation of p53, p-Erk1/2, and caspase-3 signaling, and downregulation of Bcl-2. Cells pretreated with pifithrin-alpha were protected from p53-mediated AgNPs-induced toxicity.

Conclusion

We have demonstrated a simple approach for the synthesis of AgNPs using the novel strains B. tequilensis and C. indica, as well as their mechanism of cell death in a p53-dependent manner in MDA-MB-231 human breast cancer cells. The present findings could provide insight for the future development of a suitable anticancer drug, which may lead to the development of novel nanotherapeutic molecules for the treatment of cancers.

Protein monoubiquitylation: targets and diverse functions

Ubiquitin is a 76-amino acid protein whose conjugation to protein targets is a form of post-translational modification. Protein ubiquitylation is characterized by the covalent attachment of the COOH-terminal carboxyl group of ubiquitin to an amino group of the substrate protein. Given that the NH2 -terminal amino group is usually masked, internal lysine residues are most often targeted for ubiquitylation. Polyubiquitylation refers to the formation of a polyubiquitin chain on the substrate as a result of the ubiquitylation of conjugated ubiquitin. The structures of such polyubiquitin chains depend on the specific lysine residues of ubiquitin targeted for ubiquitylation. Most of the polyubiquitin chains other than those linked via lysine-63 and methionine-1 of ubiquitin are recognized by the proteasome and serve as a trigger for substrate degradation. In contrast, polyubiquitin chains linked via lysine-63 and methionine-1 serve as a binding platform for proteins that function in immune signal transduction or DNA repair. With the exception of a few targets such as histones, the functions of protein monoubiquitylation have remained less clear. However, recent proteomics analysis has shown that monoubiquitylation occurs more frequently than polyubiquitylation, and studies are beginning to provide insight into its biologically important functions. Here, we summarize recent findings on protein monoubiquitylation to provide an overview of the targets and molecular functions of this modification.

Transcription factors and cognate signalling cascades in the regulation of autophagy

Autophagy is a process that maintains the equilibrium between biosynthesis and the recycling of cellular constituents; it is critical for avoiding the pathophysiology that results from imbalance in cellular homeostasis. Recent reports indicate the need for the design of high-throughput screening assays to identify targets and small molecules for autophagy modulation. For such screening, however, a better understanding of the regulation of autophagy is essential. In addition to regulation by various signalling cascades, regulation of gene expression by transcription factors is also critical. This review focuses on the various transcription factors as well as the corresponding signalling molecules that act together to translate the stimuli to effector molecules that up- or downregulate autophagy. This review rationalizes the importance of these transcription factors functioning in tandem with cognate signalling molecules and their interfaces as possible therapeutic targets for more specific pharmacological interventions.

Raw and thermally treated cement asbestos exerts different cytotoxicity effects on A549 cells in vitro

Raw cement asbestos (RCA) undergoes a complete solid state transformation when heated at high temperatures. The secondary raw material produced, high temperatures-cement asbestos (HT-CA) is composed of newly-formed crystals in place of the asbestos fibers present in RCA. Our previous study showed that HT-CA exerts lower cytotoxic cell damage compared to RCA. Nevertheless further investigations are needed to deepen our understanding of pathogenic pathways involving oxidative and nitrative damage. Our aim is to deepen the understanding of the biological effects on A549 cells of these materials regarding DNA damage related proteins (p53, its isoform p73 and TRAIL) and nitric oxide (NO) production during inducible nitric oxide synthase (iNOS)-mediated inflammation. Increments of p53/p73 expression, iNOS positive cells and NO concentrations were found with RCA, compared to HT-CA and controls mainly at 48 h. Interestingly, ferrous iron causing reactive oxygen species (ROS)-mediated DNA damage was found in RCA as a contaminant. HT-CA thermal treatment induces a global recrystallization with iron in a crystal form poorly released in media. HT-CA slightly interferes with genome expression and exerts lower inflammatory potential compared to RCA on biological systems. It could represent a safe approach for storing or recycling asbestos and an environmentally friendly alternative to asbestos waste.

Pifithrin-α ameliorates resveratrol-induced two-cell block in mouse preimplantation embryos in vitro

Treatment with resveratrol at concentrations greater than 0.5 μmol/L resulted in the arrest of mouse embryo development at the two-cell stage. Resveratrol-induced cytotoxicity was investigated in embryos by evaluating morphologic features by using the bromodeoxyuridine assay and acridine orange and ethidium bromide double staining. Resveratrol was found to significantly increase the expressions of p53, p21, Atf3, smac/Diablo, Bax, Bak1, Bok, and Noxa mRNA in the embryos, whereas Cullin 3 and Cdk1 expressions were decreased. Furthermore, active p53 positive signal in embryos arrested at the two-cell stage was localized in the nucleus, whereas no active p53 signal was observed in control embryos. Pretreatment with pifithrin-α, a p53 inhibitor, downregulated active p53 in two-cell embryo nuclei and ameliorated approximately 50% of the embryonic developmental defect caused by resveratrol. The findings of the present study, therefore, suggest that pifithrin-α could be used as an effective cytoprotective agent against a reproductive toxin such as resveratrol.

The p53-induced factor Ei24 inhibits nuclear import through an importin β-binding-like domain

The etoposide-induced protein Ei24 was initially identified as a p53-responsive, proapoptotic factor, but no clear function has been described. Here, we use a nonbiased proteomics approach to identify members of the importin (IMP) family of nuclear transporters as interactors of Ei24 and characterize an IMPβ-binding-like (IBBL) domain within Ei24. We show that Ei24 can bind specifically to IMPβ1 and IMPα2, but not other IMPs, and use a mutated IMPβ1 derivative to show that Ei24 binds to the same site on IMPβ1 as the IMPα IBB. Ectopic expression of Ei24 reduced the extent of IMPβ1- or IMPα/β1-dependent nuclear protein import specifically, whereas specific alanine substitutions within the IBBL abrogated this activity. Induction of endogenous Ei24 expression through etoposide treatment similarly inhibited nuclear import in a mouse embryonic fibroblast model. Thus, Ei24 can bind specifically to IMPβ1 and IMPα2 to impede their normal role in nuclear import, shedding new light on the cellular functions of Ei24 and its tumor suppressor role.

The structural and functional signatures of proteins that undergo multiple events of post-translational modification

The structural, functional, and mechanistic characterization of several types of post-translational modifications (PTMs) is well-documented. PTMs, however, may interact or interfere with one another when regulating protein function. Yet, characterization of the structural and functional signatures of their crosstalk has been hindered by the scarcity of data. To this end, we developed a unified sequence-based predictor of 23 types of PTM sites that, we believe, is a useful tool in guiding biological experiments and data interpretation. We then used experimentally determined and predicted PTM sites to investigate two particular cases of potential PTM crosstalk in eukaryotes. First, we identified proteins statistically enriched in multiple types of PTM sites and found that they show preferences toward intrinsically disordered regions as well as functional roles in transcriptional, posttranscriptional, and developmental processes. Second, we observed that target sites modified by more than one type of PTM, referred to as shared PTM sites, show even stronger preferences toward disordered regions than their single-PTM counterparts; we explain this by the need for these regions to accommodate multiple partners. Finally, we investigated the influence of single and shared PTMs on differential regulation of protein-protein interactions. We provide evidence that molecular recognition features (MoRFs) show significant preferences for PTM sites, particularly shared PTM sites, implicating PTMs in the modulation of this specific type of macromolecular recognition. We conclude that intrinsic disorder is a strong structural prerequisite for complex PTM-based regulation, particularly in context-dependent protein-protein interactions related to transcriptional and developmental processes.

AVAILABILITY

www.modpred.org.

Zinc Finger 280B regulates sGCα1 and p53 in prostate cancer cells

The Zinc Finger (ZNF) 280B protein was identified as an unexpected target of an shRNA designed for sGCα1. Further analysis showed that these two proteins are connected in another way, with 280B up-regulation of sGCα1 expression. Knock-down and over-expression experiments showed that 280B serves pro-growth and pro-survival functions in prostate cancer. Surprisingly however, these pro-cancer functions of 280B are not mediated by sGCα1, which itself has similar functions in prostate cancer, but by down-regulated p53. The p53 protein is a second target of 280B in prostate cancer, but unlike sGCα1, p53 is down-regulated by 280B. 280B induces p53 nuclear export, leading to subsequent proteasomal degradation. The protein responsible for p53 regulation by 280B is Mdm2, the E3 ubiquitin ligase that promotes p53 degradation by inducing its nuclear export. We show here that 280B up-regulates expression of Mdm2 in prostate cancer cells, and this regulation is via the Mdm2 promoter. To demonstrate an in vivo relevance to this interaction, expression studies show that 280B protein levels are up-regulated in prostate cancer and these levels correspond to reduced levels of p53. Thus, by enhancing the expression of Mdm2, the uncharacterized 280B protein provides a novel mechanism of p53 suppression in prostate cancer.

Extensive post-translational modification of active and inactivated forms of endogenous p53

The p53 tumor suppressor protein accumulates to very high concentrations in normal human fibroblasts infected by adenovirus type 5 mutants that cannot direct assembly of the viral E1B 55-kDa protein-containing E3 ubiquitin ligase that targets p53 for degradation. Despite high concentrations of nuclear p53, the p53 transcriptional program is not induced in these infected cells. We exploited this system to examine select post-translational modifications (PTMs) present on a transcriptionally inert population of endogenous human p53, as well as on p53 activated in response to etoposide treatment of normal human fibroblasts. These forms of p53 were purified from whole cell lysates by means of immunoaffinity chromatography and SDS-PAGE, and peptides derived from them were subjected to nano-ultra-high-performance LC-MS and MS/MS analyses on a high-resolution accurate-mass MS platform (data available via ProteomeXchange, PXD000464). We identified an unexpectedly large number of PTMs, comprising phosphorylation of Ser and Thr residues, methylation of Arg residues, and acetylation, ubiquitinylation, and methylation of Lys residues-for example, some 150 previously undescribed modifications of p53 isolated from infected cells. These modifications were distributed across all functional domains of both forms of the endogenous human p53 protein, as well as those of an orthologous population of p53 isolated from COS-1 cells. Despite the differences in activity, including greater in vitro sequence-specific DNA binding activity exhibited by p53 isolated from etoposide-treated cells, few differences were observed in the location, nature, or relative frequencies of PTMs on the two populations of human p53. Indeed, the wealth of PTMs that we have identified is consistent with a far greater degree of complex, combinatorial regulation of p53 by PTM than previously anticipated.

MPK-09, a small molecule inspired from bioactive styryllactone restores the wild-type function of mutant p53

In the search for more efficacious and less toxic cancer drugs, the tumor suppressor p53 protein has long been a desirable therapeutic target. In the recent past, few independent studies have demonstrated that the antitumor activity of wild-type p53 can be restored in cancer cells harboring mutant form of p53 using small molecule activators. In this study, we describe a novel small molecule MPK-09, which is selective and highly potent against allele specific p53 mutations mainly, R175H, R249S, R273H, R273C, and E285K . Except E285K, all other mutations tested are among the six "hot spot" p53 mutations reported in majority of human cancer. Furthermore, our study conclusively demonstrates that the apoptotic activity of the small molecule MPK-09 against cancer cells harboring R273C and E285K mutations is due to restoration of the wild-type conformation to the corresponding mutant form of p53.

Controlled access of p53 to the nucleus regulates its proteasomal degradation by MDM2

The tumor suppressor p53 can be sent to the proteasome for degradation by placing its nucleo-cytoplasmic shuttling under ligand control. Endogenous p53 is ubiquitinated by MDM2 in the nucleus, and controlling the access of p53 to the nuclear compartment regulates its ubiquitination and proteasomal degradation. This was accomplished by the use of a protein switch that places nuclear translocation under the control of externally applied dexamethasone. Fluorescence microscopy revealed that sending protein switch p53 (PS-p53) to the nucleus produces a distinct punctate distribution in both the cytoplasm and nucleus. The nuclear role in accessing the proteasome was investigated by inhibiting classical nuclear export with leptomycin B. Trapping PS-p53 in the nucleus only allows this punctate staining in that compartment, suggesting that PS-p53 must translocate first to the nuclear compartment for cytoplasmic punctate staining to occur. The role of MDM2 binding was explored by inhibiting MDM2/p53 binding with nutlin-3. Inhibition of this interaction blocked both nuclear export and cytoplasmic and nuclear punctate staining, providing evidence that any change in localization after nuclear translocation is due to MDM2 binding. Further, blocking the proteolytic activity of the proteasome maintained the nuclear localization of the construct. Truncations of p53 were made to determine smaller constructs still capable of interacting with MDM2, and their subcellular localization and degradation potential was observed. PS-p53 and a smaller construct containing the two MDM2 binding regions of p53 (Box I + V) were indeed degraded by the proteasome as measured by loss of enhanced green fluorescent protein that was also fused to the construct. The influence of these constructs on p53 gene transactivation function was assessed and revealed that PS-p53 decreased gene transactivation, while PS-p53(Box I + V) did not significantly change baseline gene transactivation.

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.