Mdm2 facilitates the association of p53 with the proteasome

Structure and function of MDM2 and MDM4 in health and disease

Both mouse double-minute 2 (MDM2), an E3 ubiquitin ligase, and its closely related paralog, MDM4, which lacks E3 activity, play central roles in cellular homeostasis. MDM-linked dysfunction is associated with an increased risk of oncogenesis, primarily through targeting the tumor suppressor protein p53 for ubiquitination and degradation. Recent studies have revealed multifaceted roles of MDM proteins that are p53 independent with implications for their oncogenic properties. This review aims to provide an overview of MDM2 and MDM4, by assessing gene and protein structure and implications for protein-protein interactions and functions in cell and animal physiology. We also explore MDM2 and MDM4 role(s) in angiogenesis, a critical feature of solid tumor growth and progression. Finally, we discuss the current landscape in the development of MDM2 and MDM4 inhibitors for cancer therapy.

Establishment and application of a zebrafish model of Werner syndrome identifies sapanisertib as a potential antiaging drug

Aging is a complex process that affects multiple organs, and the discovery of a pharmacological approach to ameliorate aging is considered the Holy Grail of medicine. Here, we performed an N-ethyl-N-nitrosourea forward genetic screening in zebrafish and identified an accelerated aging mutant named meteor (met), harboring a mutation in the Werner syndrome RecQ-like helicase (wrn) gene. Loss of wrn leads to a short lifespan and age-related characteristics in the intestine of zebrafish embryos, such as cellular senescence, genomic instability, and epigenetic alteration. Therefore, we conducted a screening of antiaging drugs using the met mutant and revealed that sapanisertib effectively ameliorated most of the aging phenotypes of the mutant. Mechanistically, the geroprotective effects of sapanisertib may be attributed to inhibition of mTORC1/2. Furthermore, sapanisertib also attenuated chronological aging in wild-type aged zebrafish and replicative-senescence in human foreskin fibroblasts. Taken together, our study introduces a unique and efficient model for large-scale antiaging drug screening in vertebrates and suggests sapanisertib as a potential therapeutic option for treating premature aging and promoting healthy aging.

High-Throughput Discovery of Substrate Peptide Sequences for E3 Ubiquitin Ligases Using a cDNA Display Method

Cells utilize ubiquitin as a posttranslational protein modifier to convey various signals such as proteasomal degradation. The dysfunction of ubiquitylation or following proteasomal degradation can give rise to the accumulation and aggregation of improperly ubiquitylated proteins, which is known to be a general causation of many neurodegenerative diseases. Thus, the characterization of substrate peptide sequences of E3 ligases is crucial in biological and pharmaceutical sciences. In this study, we developed a novel high-throughput screening system for substrate peptide sequences of E3 ligases using a cDNA display method, which enables covalent conjugation between peptide sequences and their corresponding cDNA sequences. First, we focused on the MDM2 E3 ligase and its known peptide substrate as a model to establish the screening method, and confirmed that cDNA display method was compatible with in vitro ubiquitylation. Then, we demonstrated identification of MDM2 substrate sequences from random libraries to identify a novel motif (VKFTGGQLA). Bioinformatics analysis of the hit sequences was performed to gain insight about endogenous substrate proteins.

Molecular signaling and clinical implications in the human aging-cancer cycle

It is well documented that aging is associated with cancer, and likewise, cancer survivors display accelerated aging. As the number of aging individuals and cancer survivors continues to grow, it raises additional concerns across society. Therefore, unraveling the molecular mechanisms of aging in tissues is essential to developing effective therapies to fight the aging and cancer diseases in cancer survivors and cancer patients. Indeed, cellular senescence is a critical response, or a natural barrier to suppress the transition of normal cells into cancer cells, however, hypoxia which is physiologically required to maintain the stem cell niche, is increased by aging and inhibits senescence in tissues. Interestingly, oxygen restriction or hypoxia increases longevity and slows the aging process in humans, but hypoxia can also drive angiogenesis to facilitate cancer progression. In addition, cancer treatment is considered as one of the major reasons that drive cellular senescence, subsequently followed by accelerated aging. Several clinical trials have recently evaluated inhibitors to eliminate senescent cells. However, some mechanisms of aging typically can also retard cancer cell growth and progression, which might require careful strategy for better clinical outcomes. Here we describe the molecular regulation of aging and cancer in crosstalk with DNA damage and hypoxia signaling pathways in cancer patients and cancer survivors. We also update several therapeutic strategies that might be critical in reversing the cancer treatment-associated aging process.

Alternative polyadenylation determines the functional landscape of inverted Alu repeats

Inverted Alu repeats (IRAlus) are abundantly found in the transcriptome, especially in introns and 3' untranslated regions (UTRs). Yet, the biological significance of IRAlus embedded in 3' UTRs remains largely unknown. Here, we find that 3' UTR IRAlus silences genes involved in essential signaling pathways. We utilize J2 antibody to directly capture and map the double-stranded RNA structure of 3' UTR IRAlus in the transcriptome. Bioinformatic analysis reveals alternative polyadenylation as a major axis of IRAlus-mediated gene regulation. Notably, the expression of mouse double minute 2 (MDM2), an inhibitor of p53, is upregulated by the exclusion of IRAlus during UTR shortening, which is exploited to silence p53 during tumorigenesis. Moreover, the transcriptome-wide UTR lengthening in neural progenitor cells results in the global downregulation of genes associated with neurodegenerative diseases, including amyotrophic lateral sclerosis, via IRAlus inclusion. Our study establishes the functional landscape of 3' UTR IRAlus and its role in human pathophysiology.

Targeting p53 misfolding conundrum by stabilizing agents and their analogs in breast cancer therapy: a comprehensive computational analysis

Cancer continues to be a major global public health concern and one of the foremost causes of death. Delays in the diagnosis and cure may cause an increase in advanced stage disease and mortality. The most common cancer found in women currently is breast carcinoma. Breast carcinoma has surpassed lung carcinoma and currently represents the chief type of cancer diagnosed (2.3 million new cases, which amount to 11.7% of all cancer cases). In addition, by 2040, the incidence will increase by more than 46% as per the estimates of GLOBOCAN. Triple-negative breast cancer (TNBC) represents a highly aggressive and invasive subtype of breast cancer, characterized by rapid progression, short response time to the available treatment, and poor clinical results. Thus, it is very crucial to develop novel diagnostic tools and therapeutics with good efficacy. A majority of cancers display malfunction along the p53 pathway. Moreover, p53 not only loses its function but is also prone to misfolding and aggregation, leading to formation of amyloid aggregates as well. Research is being carried out to find ways to restore the normal action and expression of p53. Here, we have explored PhiKan-083 for its possible stabilizing effect on p53 in order to address the problem with its misfolding. Thus, examining the analogs of PhiKan-083 that have a role in p53 stability will help update our understanding of cancer progression and may expedite the progress of new anticancer treatments. We anticipate that the drug molecules and their analogs targeting p53 aggregation may be used in combination with other anticancer compounds to solve the problem with p53 aggregation. In this study, by employing ADMET analysis, the compounds were screened, and we further examined the chosen compounds with the help of molecular docking. By using databases like UALCAN, TIMER, GEPIA, and PredictProtein, we investigated TP53's expression pattern and prognostic relevance in various cancer settings.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

TRIM25 targets p300 for degradation

p300 is an important transcriptional co-factor. By stimulating the transfer of acetyl residues onto histones and several key transcription factors, p300 enhances transcriptional initiation and impacts cellular processes including cell proliferation and cell division. Despite its importance for cellular homeostasis, its regulation is poorly understood. We show that TRIM25, a member of the TRIM protein family, targets p300 for proteasomal degradation. However, despite TRIM25's RING domain and E3 activity, degradation of p300 by TRIM25 is independent of TRIM25-mediated p300 ubiquitination. Instead, TRIM25 promotes the interaction of p300 with dynein, which ensures a microtubule-dependent transport of p300 to cellular proteasomes. Through mediating p300 degradation, TRIM25 affects p300-dependent gene expression.

Extracellular and intracellular functions of coiled-coil domain containing 3

Coiled-coil domain containing 3 (CCDC3, also called Favine) is a highly conserved protein initially identified as a protein secreted from adipocytes and endothelial cells in the vascular system with endocrine-like functions. Recently, CCDC3 was also found to function as a nuclear tumor suppressor in breast cancers. Although it is still understudied, CCDC3, since its discovery, has been shown to play multiple roles in lipid metabolism, fatty liver, abdominal obesity, anti-inflammation, atherosclerosis, and cancer. This essay is thus composed to offer an overview of these extracellular endocrine-like and intracellular (nuclear) functions of CCDC3. We also discuss the possible underlying cellular and molecular mechanisms of CCDC3, the implications for clinical translation, and the remaining puzzles about this special molecule.

Cell cycle-specific phosphorylation of p53 protein in A549 cells exposed to cisplatin and standardized air pollutants

Exposure to particulate matter is associated with DNA damage and the risk of lung cancer. Protein p53 is activated by multi-site phosphorylation in the early stages of DNA damage and affects cell outcome. Our study aimed to assess the effect of (100 µg/mL-1/24 h) standardized air pollutants: carbon black (CB), urban dust (UD), and nanoparticle carbon black (NPCB) on cell cycle, DNA damage and 53 phosphorylation at Ser 9, Ser 20, Ser 46, and Ser 392 in proliferating and quiescent A549 cells and in cells that survived cisplatin (CisPT) exposure. Phosphorylated p53 was quantified in cell subpopulations by flow cytometry using specific fluorochrome-tagged monoclonal antibodies and analysis of bivariate fluorescence distribution scatterplots. CisPT, UD and NPCB increased site-specific p53 phosphorylation producing unique patterns. NPCB activated all sites irrespectively on the cell cycle, while the UD was more selective. p53 Ser 9-P and p53 Ser 20-P positively correlated with the numbers of CisPT-treated cells at G0/G1, and NPCB and NPCB + CisPT produced a similar effect. A positive correlation and integrated response were also found between Ser 20-P and Ser 392-P in resting A549 cells treated with NPCB and CisPT but not UD. Interdependence between the expression of p53 phosphorylated at Ser 20, and Ser 392 and cell cycle arrest show that posttranslational alterations are related to functional activation. Our data suggest that p53 protein phosphorylation in response to specific DNA damage is driven by multiple independent and integrated pathways to produce functional activation critical in cancer prevention and treatment.

Targeted Protein Degradation: Principles and Applications of the Proteasome

The proteasome is a multi-catalytic protease complex that is involved in protein quality control via three proteolytic activities (i.e., caspase-, trypsin-, and chymotrypsin-like activities). Most cellular proteins are selectively degraded by the proteasome via ubiquitination. Moreover, the ubiquitin-proteasome system is a critical process for maintaining protein homeostasis. Here, we briefly summarize the structure of the proteasome, its regulatory mechanisms, proteins that regulate proteasome activity, and alterations to proteasome activity found in diverse diseases, chemoresistant cells, and cancer stem cells. Finally, we describe potential therapeutic modalities that use the ubiquitin-proteasome system.

Tau protein binds to the P53 E3 ubiquitin ligase MDM2

Tau gene mutations cause a progressive dementia and neurotoxic Tau forms deposited in neurofibrillary tangles are hallmarks of neurodegenerative tauopathies. Loss of non-canonical Tau functions may contribute to disease. In fact, Tau depletion affects the cellular response to DNA damage and tauopathies exhibit the accumulation of DNA lesions. Moreover, Tau modulates P53 activity and cell fate. Considering that MDM2 is the main antagonist of P53, we investigated, using orthogonal assays, if Tau interacts with MDM2. We report the existence in cells and brain of a Tau-MDM2 complex that, in vitro, exhibits reduced P53 ubiquitination activity in a manner sensitive to a Tau mutation. The Tau-MDM2 interaction involves the microtubule-binding domain of Tau and the acidic domain of MDM2, reminiscent of the binding of Tau to negatively charged microtubules. Notably, MDM2 accumulates aberrantly in neurofibrillary tangles. Aging-associated insults may expose a novel loss-of-function of Tau in neurodegeneration and cancer.

Decoding the Role of MDM2 as a Potential Ubiquitin E3 Ligase and Identifying the Therapeutic Efficiency of Alkaloids against MDM2 in Combating Glioblastoma

Glioblastomas (GBMs) represent the most aggressive form of brain tumor arising from the malignant transformation of astrocytes. Despite various advancements, treatment options remain limited to chemotherapy and radiotherapy followed by surgery giving an overall survival of 14-15 months. These therapies are somewhere restricted in giving a better survival and cure. There is a need for new therapeutics that could potentially target GBM based on molecular pathways and pathology. Here, ubiquitin E3 ligases can be used as targets as they bind a wide array of substrates and therefore can be attractive targets for new inhibitors. Through this study, we have tried to sort various ubiquitin E3 ligases based on their expression, pathways to which these ligases are associated, and mutational frequencies, and then we tried to screen potent inhibitors against the most favorable E3 ligase as very few studies are available concerning inhibition of E3 ligase in GBM. Our study found MDM2 to be the most ideal E3 ligase and further we tried to target MDM2 against various compounds under the alkaloid class. Molecular Docking and MD simulations combined with ADMET properties and BBB scores revealed that only evodiamine and sanguinarine were effective in inhibiting MDM2. We also tried to give a proposed mechanism of how these inhibitors mediate the p53 signaling in GBM. Therefore, the new scaffolds predicted by the computational approach could help in designing promising therapeutic agents targeting MDM2 in glioblastoma.

Coiled-coil domain containing 3 suppresses breast cancer growth by protecting p53 from proteasome-mediated degradation

Coiled-coil domain containing 3 (CCDC3) was previously shown to regulate liver lipid metabolism as a secretory protein. Here, we report an unexpected intracellular role of CCDC3 as a tumor suppressor in breast cancer (BrC). Bioinformatics datasets analysis showed that CCDC3 is under-expressed in BrCs, while its higher levels are correlated with higher overall survival and lower relapse of cancer patients, and CCDC3 is positively correlated with p53 and its target genes. Ectopic CCDC3 markedly suppressed proliferation, colony formation, and xenograft tumor growth by augmenting p53 activity in BrC cells. Depletion of endogenous CCDC3 by CRISPR-Cas9 increased proliferation and drug resistance of BrC cells by alleviating 5-Fluorouracil (5-FU)-induced p53 level and activity. Mechanistically, CCDC3 bound to the C-termini of p53 and MDM2, consequently stabilizing p53 in the nucleus and impairing MDM2 recruitment of p53 to the 26S proteosome without inhibiting p53 ubiquitination. p53 induced CCDC3 expression by binding to its promoter in BrC cells. Our results unveil a unique mechanism underlying CCDC3 activation of p53 in a positive feedback fashion to suppress BrC growth.

Ubiquitin receptors play redundant roles in the proteasomal degradation of the p53 repressor MDM2

Much remains to be determined about the participation of ubiquitin receptors in proteasomal degradation and their potential as therapeutic targets. Suppression of the ubiquitin receptor S5A/PSMD4/hRpn10 alone stabilises p53/TP53 but not the key p53 repressor MDM2. Here, we observed S5A and the ubiquitin receptors ADRM1/PSMD16/hRpn13 and RAD23A and B functionally overlap in MDM2 degradation. We provide further evidence that degradation of only a subset of ubiquitinated proteins is sensitive to S5A knockdown because ubiquitin receptor redundancy is commonplace. p53 can be upregulated by S5A modulation while degradation of substrates with redundant receptors is maintained. Our observations and analysis of Cancer Dependency Map (DepMap) screens show S5A depletion/loss substantially reduces cancer cell line viability. This and selective S5A dependency of proteasomal substrates make S5A a target of interest for cancer therapy.

USP49-mediated histone H2B deubiquitination regulates HCT116 cell proliferation through MDM2-p53 axis

Post-translational histone modifications play important roles in regulating chromatin structure and transcriptional regulation. Histone H2B monoubiquitination (H2Bub) is an essential regulator for transcriptional elongation and ongoing transcription. Here we reported that USP49, as a histone H2B deubiquitinase, is involved in HCT116 cell proliferation through modulating MDM2-p53 pathway genes. USP49 knockout contributes to increased HCT116 cell proliferation and migration. Importantly, USP49 knockout stimulated MDM2 transcriptional level and then inhibited the mRNA levels of TP53 target genes. Conversely, overexpression of USP49 suppressed MDM2 gene expression and then promoted TP53 target genes. Moreover, chromatin immunoprecipitation revealed that USP49 directly bound to the promoter of MDM2 gene. USP49 knockout increased the H2Bub enrichment at MDM2 gene whereas USP49 overexpression downregulated the H2Bub level at MDM2 gene. Therefore, our findings indicated that USP49-mediated H2B deubiquitination controls the transcription of MDM2-p53 axis genes in the process of HCT116 cell proliferation.

The ion channel TRPM7 regulates zinc-depletion-induced MDMX degradation

Zinc deficiency has been linked to human diseases, including cancer. MDMX, a crucial zinc-containing negative regulator of p53, has been found to be amplified or overexpressed in various cancers and implicated in the cancer initiation and progression. We report here that zinc depletion by the ion chelator TPEN or Chelex resin results in MDMX protein degradation in a ubiquitination-independent and 20S proteasome-dependent manner. Restoration of zinc led to recovery of cellular levels of MDMX. Further, TPEN treatment inhibits growth of the MCF-7 breast cancer cell line, which is partially rescued by overexpression of MDMX. Moreover, in a mass-spectrometry-based proteomics analysis, we identified TRPM7, a zinc-permeable ion channel, as a novel MDMX-interacting protein. TRPM7 stabilizes and induces the appearance of faster migrating species of MDMX on SDS-PAGE. Depletion of TRPM7 attenuates, while TRPM7 overexpression facilitates, the recovery of MDMX levels upon adding back zinc to TPEN-treated cells. Importantly, we found that TRPM7 inhibition, like TPEN treatment, decreases breast cancer cell MCF-7 proliferation and migration. The inhibitory effect on cell migration upon TRPM7 inhibition is also partially rescued by overexpression of MDMX. Together, our data indicate that TRPM7 regulates cellular levels of MDMX in part by modulating the intracellular Zn²⁺ concentration to promote tumorigenesis.

Mechanisms of TP53 Pathway Inactivation in Embryonic and Somatic Cells-Relevance for Understanding (Germ Cell) Tumorigenesis

The P53 pathway is the most important cellular pathway to maintain genomic and cellular integrity, both in embryonic and non-embryonic cells. Stress signals induce its activation, initiating autophagy or cell cycle arrest to enable DNA repair. The persistence of these signals causes either senescence or apoptosis. Over 50% of all solid tumors harbor mutations in TP53 that inactivate the pathway. The remaining cancers are suggested to harbor mutations in genes that regulate the P53 pathway such as its inhibitors Mouse Double Minute 2 and 4 (MDM2 and MDM4, respectively). Many reviews have already been dedicated to P53, MDM2, and MDM4, while this review additionally focuses on the other factors that can deregulate P53 signaling. We discuss that P14^ARF (ARF) functions as a negative regulator of MDM2, explaining the frequent loss of ARF detected in cancers. The long non-coding RNA Antisense Non-coding RNA in the INK4 Locus (ANRIL) is encoded on the same locus as ARF, inhibiting ARF expression, thus contributing to the process of tumorigenesis. Mutations in tripartite motif (TRIM) proteins deregulate P53 signaling through their ubiquitin ligase activity. Several microRNAs (miRNAs) inactivate the P53 pathway through inhibition of translation. CCCTC-binding factor (CTCF) maintains an open chromatin structure at the TP53 locus, explaining its inactivation of CTCF during tumorigenesis. P21, a downstream effector of P53, has been found to be deregulated in different tumor types. This review provides a comprehensive overview of these factors that are known to deregulate the P53 pathway in both somatic and embryonic cells, as well as their malignant counterparts (i.e., somatic and germ cell tumors). It provides insights into which aspects still need to be unraveled to grasp their contribution to tumorigenesis, putatively leading to novel targets for effective cancer therapies.

The role of miRNAs in MDMX-p53 interplay

MicroRNAs (miRNAs) are endogenous noncoding RNAs of 19-24 nucleotides in length and are tightly related to tumorigenesis and progression. Recent studies have demonstrated that the tumor suppressor p53 and its negative controller MDMX are regulated by miRNAs in different ways. Some miRNAs directly target p53 and regulate its expression and function, whereas some miRNAs target MDMX and regulate p53's activity indirectly. The overexpression of several miRNAs can restore the activity of p53 by negatively regulating MDMX in cancer cells. Therefore, a better understanding of the miRNAs-MDMX-p53 network will put forward potential research directions for developing anticancer therapeutics. In the present review, we mainly focus on the regulatory effects of miRNAs on the MDMX-p53 interplay as well as the role of the miRNAs-MDMX-p53 network in human cancer.

p53 tetramerization: at the center of the dominant-negative effect of mutant p53

In this review, Gencel-Augusto and Lozano summarize the data on p53 mutants with a functional tetramerization domain that form mixed tetramers and in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. They conclude that the DNE is mostly observed after DNA damage but fails in other contexts.

The p53 tumor suppressor functions as a tetrameric transcription factor to regulate hundreds of genes—many in a tissue-specific manner. Missense mutations in cancers in the p53 DNA-binding and tetramerization domains cement the importance of these domains in tumor suppression. p53 mutants with a functional tetramerization domain form mixed tetramers, which in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. DNA damage appears necessary but not sufficient for DNE, indicating that upstream signals impact DNE. Posttranslational modifications and protein–protein interactions alter p53 tetramerization affecting transcription, stability, and localization. These regulatory components limit the dominant-negative effects of mutant p53 on wild-type p53 activity. A deeper understanding of the molecular basis for DNE may drive development of drugs that release WT p53 and allow tumor suppression.

HOPS and p53: thick as thieves in life and death

The oncosuppressor protein p53 plays a major role in transcriptionally controlling the expression of a number of genes, which in turn regulates many functions in response to DNA damage, oncogene triggering, oxidative, and additional cell stresses. A developing area of interest in p53 is the studies related to its cytoplasmic function(s). Many investigations revealed the significant role of p53 in the cytoplasm, acting in a transcriptional-independent manner in important processes related to cell homeostasis such as; apoptosis, autophagy, metabolism control, drug, and oxidative stress response. The studies on cytoplasmic p53 have shown intricate mechanisms by which posttranslational modifications allow p53 to perform its cytoplasmic functions. A number of ubiquitins, deubiquitins, and small ubiquitin-like proteins, have a pivotal role in controlling cytoplasmic stability and localization. Recently, HOPS/TMUB1 a novel small ubiquitin-like protein has been described as a vital molecule stabilizing p53 half-life, directing it to the mitochondria and favoring p53-mediated apoptosis. Furthermore, HOPS/TMUB1 competing with importin-α lessens p53 nuclear localization, thereby increasing cytoplasmic concentration. HOPS/TMUB1 as p53 modifiers could be attractive candidates to elucidate apoptosis or other important transcriptional-independent functions which are key in cancer research in order to develop new therapeutic approaches.

Functional Gene Clusters in Global Pathogenesis of Clear Cell Carcinoma of the Ovary Discovered by Integrated Analysis of Transcriptomes

Clear cell carcinoma of the ovary (ovarian clear cell carcinoma (OCCC)) is one epithelial ovarian carcinoma that is known to have a poor prognosis and a tendency for being refractory to treatment due to unclear pathogenesis. Published investigations of OCCC have mainly focused only on individual genes and lack of systematic integrated research to analyze the pathogenesis of OCCC in a genome-wide perspective. Thus, we conducted an integrated analysis using transcriptome datasets from a public domain database to determine genes that may be implicated in the pathogenesis involved in OCCC carcinogenesis. We used the data obtained from the National Center for Biotechnology Information (NCBI) Gene Expression Omnibus (GEO) DataSets. We found six interactive functional gene clusters in the pathogenesis network of OCCC, including ribosomal protein, eukaryotic translation initiation factors, lactate, prostaglandin, proteasome, and insulin-like growth factor. This finding from our integrated analysis affords us a global understanding of the interactive network of OCCC pathogenesis.

Fam83F induces p53 stabilisation and promotes its activity

p53 is one of the most important tumour suppressor proteins currently known. It is activated in response to DNA damage and this activation leads to proliferation arrest and cell death. The abundance and activity of p53 are tightly controlled and reductions in p53's activity can contribute to the development of cancer. Here, we show that Fam83F increases p53 protein levels by protein stabilisation. Fam83F interacts with p53 and decreases its ubiquitination and degradation. Fam83F is induced in response to DNA damage and its overexpression also increases p53 activity in cell culture experiments and in zebrafish embryos. Downregulation of Fam83F decreases transcription of p53 target genes in response to DNA damage and increases cell proliferation, identifying Fam83F as an important regulator of the DNA damage response. Overexpression of Fam83F also enhances migration of cells harbouring mutant p53 demonstrating that it can also activate mutant forms of p53.

Regulation of Protein Degradation by Proteasomes in Cancer

Imbalance of protein homeostasis (proteostasis) is known to cause cellular malfunction, cell death, and diseases. Elaborate regulation of protein synthesis and degradation is one of the important processes in maintaining normal cellular functions. Protein degradation pathways in eukaryotes are largely divided into proteasome-mediated degradation and lysosome-mediated degradation. Proteasome is a multisubunit complex that selectively degrades 80% to 90% of cellular proteins. Proteasome-mediated degradation can be divided into 26S proteasome (20S proteasome + 19S regulatory particle) and free 20S proteasome degradation. In 1980, it was discovered that during ubiquitination process, wherein ubiquitin binds to a substrate protein in an ATP-dependent manner, ubiquitin acts as a degrading signal to degrade the substrate protein via proteasome. Conversely, 20S proteasome degrades the substrate protein without using ATP or ubiquitin because it recognizes the oxidized and structurally modified hydrophobic patch of the substrate protein. To date, most studies have focused on protein degradation via 26S proteasome. This review describes the 26S/20S proteasomal pathway of protein degradation and discusses the potential of proteasome as therapeutic targets for cancer treatment as well as against diseases caused by abnormalities in the proteolytic system.

Natural products targeting the p53-MDM2 pathway and mutant p53: Recent advances and implications in cancer medicine

The p53 tumor suppressor plays a major role in controlling the initiation and development of cancer by regulating cell cycle arrest, apoptosis, senescence, and DNA repair. The MDM2 oncogene is a major negative regulator of p53 that inhibits the activity of p53 and reduces its protein stability. MDM2, p53, and the p53-MDM2 pathway represent well-documented targets for preventing and/or treating cancer. Natural products, especially those from medicinal and food plants, are a rich source for the discovery and development of novel therapeutic and preventive agents against human cancers. Many natural product-derived MDM2 inhibitors have shown potent efficacy against various human cancers. In contrast to synthetic small-molecule MDM2 inhibitors, the majority of which have been designed to inhibit MDM2-p53 binding and activate p53, many natural product inhibitors directly decrease MDM2 expression and/or MDM2 stability, exerting their anticancer activity in both p53-dependent and p53-independent manners. More recently, several natural products have been reported to target mutant p53 in cancer. Therefore, identification of natural products targeting MDM2, mutant p53, and the p53-MDM2 pathway can provide a promising strategy for the development of novel cancer chemopreventive and chemotherapeutic agents. In this review, we focus our discussion on the recent advances in the discovery and development of anticancer natural products that target the p53-MDM2 pathway, emphasizing several emerging issues, such as the efficacy, mechanism of action, and specificity of these natural products.

TRRAP is essential for regulating the accumulation of mutant and wild-type p53 in lymphoma

Tumors accumulate high levels of mutant p53 (mutp53), which contributes to mutp53 gain-of-function properties. The mechanisms that underlie such excessive accumulation are not fully understood. To discover regulators of mutp53 protein accumulation, we performed a large-scale RNA interference screen in a Burkitt lymphoma cell line model. We identified transformation/transcription domain-associated protein (TRRAP), a constituent of several histone acetyltransferase complexes, as a critical positive regulator of both mutp53 and wild-type p53 levels. TRRAP silencing attenuated p53 accumulation in lymphoma and colon cancer models, whereas TRRAP overexpression increased mutp53 levels, suggesting a role for TRRAP across cancer entities and p53 mutations. Through clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 screening, we identified a 109-amino-acid region in the N-terminal HEAT repeat region of TRRAP that was crucial for mutp53 stabilization and cell proliferation. Mass spectrometric analysis of the mutp53 interactome indicated that TRRAP silencing caused degradation of mutp53 via the MDM2-proteasome axis. This suggests that TRRAP is vital for maintaining mutp53 levels by shielding it against the natural p53 degradation machinery. To identify drugs that alleviated p53 accumulation similarly to TRRAP silencing, we performed a small-molecule drug screen and found that inhibition of histone deacetylases (HDACs), specifically HDAC1/2/3, decreased p53 levels to a comparable extent. In summary, here we identify TRRAP as a key regulator of p53 levels and link acetylation-modifying complexes to p53 protein stability. Our findings may provide clues for therapeutic targeting of mutp53 in lymphoma and other cancers.

Experimental Therapy of Advanced Breast Cancer: Targeting NFAT1-MDM2-p53 Pathway

Advanced breast cancer, especially advanced triple-negative breast cancer, is typically more aggressive and more difficult to treat than other breast cancer phenotypes. There is currently no curable option for breast cancer patients with advanced diseases, highlighting the urgent need for novel treatment strategies. We have recently discovered that the nuclear factor of activated T cells 1 (NFAT1) activates the murine double minute 2 (MDM2) oncogene. Both MDM2 and NFAT1 are overexpressed and constitutively activated in breast cancer, particularly in advanced breast cancer, and contribute to its initiation, progression, and metastasis. MDM2 regulates cancer cell proliferation, cell cycle progression, apoptosis, migration, and invasion through both p53-dependent and -independent mechanisms. We have proposed to target the NFAT1-MDM2-p53 pathway for the treatment of human cancers, especially breast cancer. We have recently identified NFAT1 and MDM2 dual inhibitors that have shown excellent in vitro and in vivo activities against breast cancer, including triple-negative breast cancer. Herein, we summarize recent advances made in the understanding of the oncogenic functions of MDM2 and NFAT1 in breast cancer, as well as current targeting strategies and representative inhibitors. We also propose several strategies for inhibiting the NFAT1-MDM2-p53 pathway, which could be useful for developing more specific and effective inhibitors for breast cancer therapy.

Ubiquitin-specific peptidase 48 regulates Mdm2 protein levels independent of its deubiquitinase activity

The overexpression of Mdm2 has been linked to the loss of p53 tumour suppressor activity in several human cancers. Here, we present results suggesting that ubiquitin-specific peptidase 48 (USP48), a deubiquitinase that has been linked in previous reports to the NF-κB signaling pathway, is a novel Mdm2 binding partner that promotes Mdm2 stability and enhances Mdm2-mediated p53 ubiquitination and degradation. In contrast to other deubiquitinating enzymes (DUBs) that have been previously implicated in the regulation of Mdm2 protein stability, USP48 did not induce Mdm2 stabilization by significantly reducing Mdm2 ubiquitination levels. Moreover, two previously characterized USP48 mutants lacking deubiquitinase activity were also capable of efficiently stabilizing Mdm2, indicating that USP48 utilizes a non-canonical, deubiquitination-independent mechanism to promote Mdm2 oncoprotein stability. This study represents, to the best of our knowledge, the first report suggesting DUB-mediated target protein stabilization that is independent of its deubiquitinase activity. In addition, our results suggest that USP48 might represent a new mechanism of crosstalk between the NF-κB and p53 stress response pathways.

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.

MDM2 E3 ligase-mediated ubiquitination and degradation of HDAC1 in vascular calcification

Vascular calcification (VC) is often associated with cardiovascular and metabolic diseases. However, the molecular mechanisms linking VC to these diseases have yet to be elucidated. Here we report that MDM2-induced ubiquitination of histone deacetylase 1 (HDAC1) mediates VC. Loss of HDAC1 activity via either chemical inhibitor or genetic ablation enhances VC. HDAC1 protein, but not mRNA, is reduced in cell and animal calcification models and in human calcified coronary artery. Under calcification-inducing conditions, proteasomal degradation of HDAC1 precedes VC and it is mediated by MDM2 E3 ubiquitin ligase that initiates HDAC1 K74 ubiquitination. Overexpression of MDM2 enhances VC, whereas loss of MDM2 blunts it. Decoy peptide spanning HDAC1 K74 and RG 7112, an MDM2 inhibitor, prevent VC in vivo and in vitro. These results uncover a previously unappreciated ubiquitination pathway and suggest MDM2-mediated HDAC1 ubiquitination as a new therapeutic target in VC.

Vascular calcification (VC) increases morbidity and mortality in cardiovascular and metabolic diseases. Here, Kwon et al. show that calcification stimuli induce MDM2- mediated ubiquitination and proteasomal degradation of HDAC1, suggesting a possible therapeutic strategy for treatment of VC patients.

The alternative translated MDMX(p60) isoform regulates MDM2 activity

Isoforms derived from alternative splicing, mRNA translation initiation or promoter usage extend the functional repertoire of the p53, p63 and p73 genes family and of their regulators MDM2 and MDMX. Here we show cap-independent translation of an N-terminal truncated isoform of hMDMX, hMDMX^p60, which is initiated at the 7th AUG codon downstream of the initiation site for full length hMDMX^FL at position +384. hMDMX^p60 lacks the p53 binding motif but retains the RING domain and interacts with hMDM2 and hMDMX^FL. hMDMX^p60 shows higher affinity for hMDM2, as compared to hMDMX^FL. In vitro data reveal a positive cooperative interaction between hMDMX^p60 and hMDM2 and in cellulo data show that low levels of hMDMX^p60 promote degradation of hMDM2 whereas higher levels stabilize hMDM2 and prevent hMDM2-mediated degradation of hMDMX^FL. These results describe a novel alternatively translated hMDMX isoform that exhibits unique regulatory activity toward hMDM2 autoubiquitination. The data illustrate how the N-terminus of hMDMX regulates its C-terminal RING domain and the hMDM2 activity.

Expression screening using a Medaka cDNA library identifies evolutionarily conserved regulators of the p53/Mdm2 pathway

BACKGROUND

The p53 tumor suppressor protein is mainly regulated by alterations in the half-life of the protein, resulting in significant differences in p53 protein levels in cells. The major regulator of this process is Mdm2, which ubiquitinates p53 and targets it for proteasomal degradation. This process can be enhanced or reduced by proteins that associate with p53 or Mdm2 and several proteins have been identified with such an activity. Furthermore, additional ubiquitin ligases for p53 have been identified in recent years. Nevertheless, our understanding of how p53 abundance and Mdm2 activity are regulated remains incomplete. Here we describe a cell culture based overexpression screen to identify evolutionarily conserved regulators of the p53/Mdm2 circuit. The results from this large-scale screening method will contribute to a better understanding of the regulation of these important proteins.

METHODS

Expression screening was based on co-transfection of H1299 cells with pools of cDNA's from a Medaka library together with p53, Mdm2 and, as internal control, Ror2. After cell lysis, SDS-PAGE/WB analysis was used to detect alterations in these proteins.

RESULTS

More than one hundred hits that altered the abundance of either p53, Mdm2, or both were identified in the primary screen. Subscreening of the library pools that were identified in the primary screen identified several potential novel regulators of p53 and/or Mdm2. We also tested whether the human orthologues of the Medaka genes regulate p53 and/or Mdm2 abundance. All human orthologues regulated p53 and/or Mdm2 abundance in the same manner as the proteins from Medaka, which underscores the suitability of this screening methodology for the identification of new modifiers of p53 and Mdm2.

CONCLUSIONS

Despite enormous efforts in the last two decades, many unknown regulators for p53 and Mdm2 abundance are predicted to exist. This cross-species approach to identify evolutionarily conserved regulators demonstrates that our Medaka unigene cDNA library represents a powerful tool to screen for these novel regulators of the p53/Mdm2 pathway.

MAGE-A Cancer/Testis Antigens Inhibit MDM2 Ubiquitylation Function and Promote Increased Levels of MDM4

Melanoma antigen A (MAGE-A) proteins comprise a structurally and biochemically similar sub-family of Cancer/Testis antigens that are expressed in many cancer types and are thought to contribute actively to malignancy. MAGE-A proteins are established regulators of certain cancer-associated transcription factors, including p53, and are activators of several RING finger-dependent ubiquitin E3 ligases. Here, we show that MAGE-A2 associates with MDM2, a ubiquitin E3 ligase that mediates ubiquitylation of more than 20 substrates including mainly p53, MDM2 itself, and MDM4, a potent p53 inhibitor and MDM2 partner that is structurally related to MDM2. We find that MAGE-A2 interacts with MDM2 via the N-terminal p53-binding pocket and the RING finger domain of MDM2 that is required for homo/hetero-dimerization and for E2 ligase interaction. Consistent with these data, we show that MAGE-A2 is a potent inhibitor of the E3 ubiquitin ligase activity of MDM2, yet it does not have any significant effect on p53 turnover mediated by MDM2. Strikingly, however, increased MAGE-A2 expression leads to reduced ubiquitylation and increased levels of MDM4. Similarly, silencing of endogenous MAGE-A expression diminishes MDM4 levels in a manner that can be rescued by the proteasomal inhibitor, bortezomid, and permits increased MDM2/MDM4 association. These data suggest that MAGE-A proteins can: (i) uncouple the ubiquitin ligase and degradation functions of MDM2; (ii) act as potent inhibitors of E3 ligase function; and (iii) regulate the turnover of MDM4. We also find an association between the presence of MAGE-A and increased MDM4 levels in primary breast cancer, suggesting that MAGE-A-dependent control of MDM4 levels has relevance to cancer clinically.

The MDM2 RING domain and central acidic domain play distinct roles in MDM2 protein homodimerization and MDM2-MDMX protein heterodimerization

The oncoprotein murine double minute 2 (MDM2) is an E3 ligase that plays a prominent role in p53 suppression by promoting its polyubiquitination and proteasomal degradation. In its active form, MDM2 forms homodimers as well as heterodimers with the homologous protein murine double minute 4 (MDMX), both of which are thought to occur through their respective C-terminal RING (really interesting new gene) domains. In this study, using multiple MDM2 mutants, we show evidence suggesting that MDM2 homo- and heterodimerization occur through distinct mechanisms because MDM2 RING domain mutations that inhibit MDM2 interaction with MDMX do not affect MDM2 interaction with WT MDM2. Intriguingly, deletion of a portion of the MDM2 central acidic domain selectively inhibits interaction with MDM2 while leaving intact the ability of MDM2 to interact with MDMX and to ubiquitinate p53. Further analysis of an MDM2 C-terminal deletion mutant reveals that the C-terminal residues of MDM2 are required for both MDM2 and MDMX interaction. Collectively, our results suggest a model in which MDM2-MDMX heterodimerization requires the extreme C terminus and proper RING domain structure of MDM2, whereas MDM2 homodimerization requires the extreme C terminus and the central acidic domain of MDM2, suggesting that MDM2 homo- and heterodimers utilize distinct MDM2 domains. Our study is the first to report mutations capable of separating MDM2 homo- and heterodimerization.

TRIM25 has a dual function in the p53/Mdm2 circuit

P53 is an important tumor suppressor that, upon activation, induces growth arrest and cell death. Control of p53 is thus of prime importance for proliferating cells, but also for cancer therapy, where p53 activity contributes to the eradication of tumors. Mdm2 functionally inhibits p53 and targets the tumor suppressor protein for degradation. In a genetic screen, we identified TRIM25 as a novel regulator of p53 and Mdm2. TRIM25 increased p53 and Mdm2 abundance by inhibiting their ubiquitination and degradation in 26 S proteasomes. TRIM25 co-precipitated with p53 and Mdm2 and interfered with the association of p300 and Mdm2, a critical step for p53 polyubiquitination. Despite the increase in p53 levels, p53 activity was inhibited in the presence of TRIM25. Downregulation of TRIM25 resulted in an increased acetylation of p53 and p53-dependent cell death in HCT116 cells. Upon genotoxic insults, TRIM25 dampened the p53-dependent DNA damage response. The downregulation of TRIM25 furthermore resulted in massive apoptosis during early embryogenesis of medaka, which was rescued by the concomitant downregulation of p53, demonstrating the functional relevance of the regulation of p53 by TRIM25 in an organismal context.

MDM2-mediated degradation of p14ARF: a novel mechanism to control ARF levels in cancer cells

We here show a new relationship between the human p14ARF oncosuppressor and the MDM2 oncoprotein. MDM2 overexpression in various cancer cell lines causes p14ARF reduction inducing its degradation through the proteasome. The effect does not require the ubiquitin ligase activity of MDM2 and preferentially occurs in the cytoplasm. Interestingly, treatment with inhibitors of the PKC (Protein Kinase C) pathway and use of p14ARF phosphorylation mutants indicate that ARF phosphorylation could play a role in MDM2 mediated ARF degradation reinforcing our previous observations that ARF phosphorylation influences its stability and biological activity. Our study uncovers a new potentially important mechanism through which ARF and MDM2 can counterbalance each other during the tumorigenic process.

Autoactivation of the MDM2 E3 ligase by intramolecular interaction

The RING domain ubiquitin E3 ligase MDM2 is a key regulator of p53 degradation and a mediator of signals that stabilize p53. The current understanding of the mechanisms by which MDM2 posttranslational modifications and protein binding cause p53 stabilization remains incomplete. Here we present evidence that the MDM2 central acidic region is critical for activating RING domain E3 ligase activity. A 30-amino-acid minimal region of the acidic domain binds to the RING domain through intramolecular interactions and stimulates the catalytic function of the RING domain in promoting ubiquitin release from charged E2. The minimal activation sequence is also the binding site for the ARF tumor suppressor, which inhibits ubiquitination of p53. The acidic domain-RING domain intramolecular interaction is modulated by ATM-mediated phosphorylation near the RING domain or by binding of ARF. These results suggest that MDM2 phosphorylation and association with protein regulators share a mechanism in inhibiting the E3 ligase function and stabilizing p53 and suggest that targeting the MDM2 autoactivation mechanism may be useful for therapeutic modulation of p53 levels.

Transport of c-MYC by Kinesin-1 for proteasomal degradation in the cytoplasm

c-MYC is an oncogenic transcription factor that is degraded by the proteasome pathway. However, the mechanism that regulates delivery of c-MYC to the proteasome for degradation is not well characterized. Here, the results show that the motor protein complex Kinesin-1 transports c-MYC to the cytoplasm for proteasomal degradation. Inhibition of Kinesin-1 function enhanced ubiquitination of c-MYC and induced aggregation of c-MYC in the cytoplasm. Transport studies showed that the c-MYC aggregates moved from the nucleus to the cytoplasm and KIF5B is responsible for the transport in the cytoplasm. Furthermore, inhibition of the proteasomal degradation process also resulted in an accumulation of c-MYC aggregates in the cytoplasm. Moreover, Kinesin-1 was shown to interact with c-MYC and the proteasome subunit S6a. Inhibition of Kinesin-1 function also reduced c-MYC-dependent transformation activities. Taken together, the results strongly suggest that Kinesin-1 transports c-MYC for proteasomal degradation in the cytoplasm and the proper degradation of c-MYC mediated by Kinesin-1 transport is important for transformation activities of c-MYC. In addition, the results indicate that Kinesin-1 transport mechanism is important for degradation of a number of other proteins as well.

Dynamics matter: differences and similarities between alternatively designed mechanisms

Cells selectively respond to external stimuli to maintain cellular homeostasis by making use of different regulatory mechanisms. We studied two classes of signal-dependent regulatory inhibition and activation mechanisms in this study. Inhibition mechanisms assume that inhibition can occur in two different ways: either by increasing the degradation rate or decreasing the production rate. Similarly, it is assumed that signal-triggered activation can occur either through increasing production rate or decreasing degradation rate. We devised mathematical models (deterministic and stochastic) to compare and contrast responses of these activation and inhibition mechanisms to a time dependent discrete signal. Our simulation results show that the signal-dependent increased degradation mechanism is a more effective, noisier and quicker way to inhibit the protein abundance compared to the signal-dependent decreased activation mechanism. On the other hand, the signal-dependent increased production mechanism can produce a much stronger and faster response than the signal-dependent decreased degradation mechanism. However, our simulations predict that both of the activation mechanisms have roughly similar noise structures. Our analysis exemplifies the importance of mathematical modeling in the analysis of biological regulatory networks.

XPC promotes MDM2-mediated degradation of the p53 tumor suppressor

XPC binds MDM2 ubiquitin ligase and participates in the MDM2-mediated p53 degradation. Furthermore, XPC overexpression stimulates p53 degradation following UV irradiation. Combined, the results suggest a key role of XPC in p53 degradation.

Although ubiquitin receptor Rad23 has been implicated in bringing ubiquitylated p53 to the proteasome, how Rad23 recognizes p53 remains unclear. We demonstrate that XPC, a Rad23-binding protein, regulates p53 turnover. p53 protein in XPC-deficient cells remains ubiquitylated, but its association with the proteasome is drastically reduced, indicating that XPC regulates a postubiquitylation event. Furthermore, we found that XPC participates in the MDM2-mediated p53 degradation pathway via direct interaction with MDM2. XPC W690S pathogenic mutant is specifically defective for MDM2 binding and p53 degradation. p53 is known to become stabilized following UV irradiation but can be rendered unstable by XPC overexpression, underscoring a critical role of XPC in p53 regulation. Elucidation of the proteolytic role of XPC in cancer cells will help to unravel the detailed mechanisms underlying the coordination of DNA repair and proteolysis.

The degradation of p53 and its major E3 ligase Mdm2 is differentially dependent on the proteasomal ubiquitin receptor S5a

p53 and its major E3 ligase Mdm2 are both ubiquitinated and targeted to the proteasome for degradation. Despite the importance of this in regulating the p53 pathway, little is known about the mechanisms of proteasomal recognition of ubiquitinated p53 and Mdm2. In this study, we show that knockdown of the proteasomal ubiquitin receptor S5a/PSMD4/Rpn10 inhibits p53 protein degradation and results in the accumulation of ubiquitinated p53. Overexpression of a dominant-negative deletion of S5a lacking its ubiquitin-interacting motifs (UIM)s, but which can be incorporated into the proteasome, also causes the stabilization of p53. Furthermore, small-interferring RNA (siRNA) rescue experiments confirm that the UIMs of S5a are required for the maintenance of low p53 levels. These observations indicate that S5a participates in the recognition of ubiquitinated p53 by the proteasome. In contrast, targeting S5a has no effect on the rate of degradation of Mdm2, indicating that proteasomal recognition of Mdm2 can be mediated by an S5a-independent pathway. S5a knockdown results in an increase in the transcriptional activity of p53. The selective stabilization of p53 and not Mdm2 provides a mechanism for p53 activation. Depletion of S5a causes a p53-dependent decrease in cell proliferation, demonstrating that p53 can have a dominant role in the response to targeting S5a. This study provides evidence for alternative pathways of proteasomal recognition of p53 and Mdm2. Differences in recognition by the proteasome could provide a means to modulate the relative stability of p53 and Mdm2 in response to cellular signals. In addition, they could be exploited for p53-activating therapies. This work shows that the degradation of proteins by the proteasome can be selectively dependent on S5a in human cells, and that this selectivity can extend to an E3 ubiquitin ligase and its substrate.

c-MYC-induced sebaceous gland differentiation is controlled by an androgen receptor/p53 axis

Although the sebaceous gland (SG) plays an important role in skin function, the mechanisms regulating SG differentiation and carcinoma formation are poorly understood. We previously reported that c-MYC overexpression stimulates SG differentiation. We now demonstrate roles for the androgen receptor (AR) and p53. MYC-induced SG differentiation was reduced in mice lacking a functional AR. High levels of MYC triggered a p53-dependent DNA damage response, leading to accumulation of proliferative SG progenitors and inhibition of AR signaling. Conversely, testosterone treatment or p53 deletion activated AR signaling and restored MYC-induced differentiation. Poorly differentiated human sebaceous carcinomas exhibited high p53 and low AR expression. Thus, the consequences of overactivating MYC in the SG depend on whether AR or p53 is activated, as they form a regulatory axis controlling proliferation and differentiation.

Phosphorylated Hsp27 activates ATM-dependent p53 signaling and mediates the resistance of MCF-7 cells to doxorubicin-induced apoptosis

DNA damage activates p53 and its downstream target genes, which further leads to apoptosis or survival either by the cell cycle arrest or by DNA repair. In many tumors, the heat shock protein 27 (Hsp27) is expressed at high levels to provide protection against anticancer drugs. However, the roles of Hsp27 in p53-mediated cellular responses to DNA damage are controversial. Here, we investigated the interplay between the phosphorylation status of Hsp27 and p53 in kidney 293A (HEK293A) cells and found that over-expressing phosphorylated Hsp27 mimics (Hsp27-3D) activated p53/p21 in an ATM-dependent manner. In addition, incubation with doxorubicin (Dox), an anticancer drug, induced Hsp27 phosphorylation in human adenocarcinoma cells (MCF-7). In contrast, inhibition of Hsp27 phosphorylation retarded both p53 induction and p21 accumulation, and led to cell apoptosis. Furthermore, phosphorylated Hsp27 increased p53 nuclear importing and its downstream target gene expression such as p21 and MDM2, while de-phosphorylated Hsp27 impeded this procession. Taken together, our data suggest that Hsp27, in its phosphorylated or de-phosphorylated status, plays different roles in regulating p53 pathway and cell survival.

Localization of rRNA transcribed spacer domains in the nucleolinus and maternal procentrosomes of surf clam (Spisula) oocytes

The nucleolinus is a nuclear subcompartment long ago posited to play a role in cell division. In a recent study using surf clam oocytes, cytoplasmic foci containing a nucleolinar protein were shown to later recruit γ-tubulin, identifying them as centrosomal precursors. (1) We now demonstrate the presence of structural RNAs from the nucleolinus in these procentrosomes. They include the well-known but poorly understood rRNA-transcribed spacer regions. In situ hybridization revealed a specific and dynamic association of these structural RNAs with the cell division apparatus that extends through the early stages of meiosis. In addition to their bearing on the debate over the nature of centrosome- and spindle-associated RNAs, the observations also suggest that rRNA spacer regions are not simply waste products to be discarded immediately, but may be functional byproducts that play a role in formation of the cell division apparatus.

Integration of DNA damage and repair with murine double-minute 2 (Mdm2) in tumorigenesis

The alteration of tumorigenic pathways leading to cancer is a degenerative disease process typically involving inactivation of tumor suppressor proteins and hyperactivation of oncogenes. One such oncogenic protein product is the murine double-minute 2, or Mdm2. While, Mdm2 has been primarily associated as the negative regulator of the p53 tumor suppressor protein there are many p53-independent roles demonstrated for this oncogene. DNA damage and chemotherapeutic agents are known to activate Mdm2 and DNA repair pathways. There are five primary DNA repair pathways involved in the maintenance of genomic integrity: Nucleotide excision repair (NER), Base excision repair (BER), Mismatch repair (MMR), Non-homologous end joining (NHEJ) and homologous recombination (HR). In this review, we will briefly describe these pathways and also delineate the functional interaction of Mdm2 with multiple DNA repair proteins. We will illustrate the importance of these interactions with Mdm2 and discuss how this is important for tumor progression, cellular proliferation in cancer.

The Roles of MDM2 and MDMX Phosphorylation in Stress Signaling to p53

The p53 tumor suppressor is highly responsive to different physiological stresses such as abnormal cell proliferation, nutrient deprivation, and DNA damage. Distinct signaling mechanisms have evolved to activate p53, which in turn modulate numerous pathways to enhance fitness and survival of the organism. Elucidating the molecular mechanisms of these signaling events is critical for understanding tumor suppression by p53 and development of novel therapeutics. Studies in the past decade have established that MDM2 and MDMX are important targets of signaling input from different pathways. Here, we focus our discussion on MDM2 and MDMX phosphorylation, which is important for p53 activation by DNA damage. Investigations in this area have generated new insight into the inner workings of MDM2 and MDMX and underscore the importance of allosteric communication between different domains in achieving an efficient response to phosphorylation. It is likely that MDM2 and MDMX regulation by phosphorylation will share mechanistic similarities to other signaling hub molecules. Phosphorylation-independent p53 activators such as ARF and ribosomal proteins ultimately achieve the same outcome as phosphorylation, suggesting that they may induce similar changes in the structure and function of MDM2 and MDMX through protein-protein interactions.

Dual Roles of MDM2 in the Regulation of p53: Ubiquitination Dependent and Ubiquitination Independent Mechanisms of MDM2 Repression of p53 Activity

MDM2 oncogenic protein is the principal cellular antagonist of the p53 tumor suppresser gene. p53 activity needs exquisite control to elicit appropriate responses to differential cellular stress conditions. p53 becomes stabilized and active upon various types of stresses. However, too much p53 is not beneficial to cells and causes lethality. At the steady state, p53 activity needs to be leashed for cell survival. Early studies suggested that the MDM2 oncoprotein negatively regulates p53 activity through the induction of p53 protein degradation. MDM2 serves as an E3 ubiquitin ligase of p53; it catalyzes polyubiquitination and subsequently induces proteasome degradation to downregulate p53 protein level. However, the mechanism by which MDM2 represses p53 is not a single mode. Emerging evidence reveals another cellular location of MDM2-p53 interaction. MDM2 is recruited to chromatin, specifically the p53 responsive promoter regions, in a p53 dependent manner. MDM2 is proposed to directly inhibit p53 transactivity at chromatin. This article provides an overview of the mechanism by which p53 is repressed by MDM2 in both ubiquitination dependent and ubiquitination independent pathways.