Inactivation of the p53-homologue p73 by the mdm2-oncoprotein

Targeted MDM2 Degradation Reveals a New Vulnerability for p53-Inactivated Triple-Negative Breast Cancer

Triple-negative breast cancers (TNBC) frequently inactivate p53, increasing their aggressiveness and therapy resistance. We identified an unexpected protein vulnerability in p53-inactivated TNBC and designed a new PROteolysis TArgeting Chimera (PROTAC) to target it. Our PROTAC selectively targets MDM2 for proteasome-mediated degradation with high-affinity binding and VHL recruitment. MDM2 loss in p53 mutant/deleted TNBC cells in two-dimensional/three-dimensional culture and TNBC patient explants, including relapsed tumors, causes apoptosis while sparing normal cells. Our MDM2-PROTAC is stable in vivo, and treatment of TNBC xenograft-bearing mice demonstrates tumor on-target efficacy with no toxicity to normal cells, significantly extending survival. Transcriptomic analyses revealed upregulation of p53 family target genes. Investigations showed activation and a required role for TAp73 to mediate MDM2-PROTAC-induced apoptosis. Our data, challenging the current MDM2/p53 paradigm, show MDM2 is required for p53-inactivated TNBC cell survival, and PROTAC-targeted MDM2 degradation is an innovative potential therapeutic strategy for TNBC and superior to existing MDM2 inhibitors.

SIGNIFICANCE

p53-inactivated TNBC is an aggressive, therapy-resistant, and lethal breast cancer subtype. We designed a new compound targeting an unexpected vulnerability we identified in TNBC. Our MDM2-targeted degrader kills p53-inactivated TNBC cells, highlighting the requirement for MDM2 in TNBC cell survival and as a new therapeutic target for this disease. See related commentary by Peuget and Selivanova, p. 1043. This article is highlighted in the In This Issue feature, p. 1027.

p53 Family in Resistance to Targeted Therapy of Melanoma

Metastatic melanoma is one of the most aggressive tumors, with frequent mutations affecting components of the MAPK pathway, mainly protein kinase BRAF. Despite promising initial response to BRAF inhibitors, melanoma progresses due to development of resistance. In addition to frequent reactivation of MAPK or activation of PI3K/AKT signaling pathways, recently, the p53 pathway has been shown to contribute to acquired resistance to targeted MAPK inhibitor therapy. Canonical tumor suppressor p53 is inactivated in melanoma by diverse mechanisms. The TP53 gene and two other family members, TP63 and TP73, encode numerous protein isoforms that exhibit diverse functions during tumorigenesis. The p53 family isoforms can be produced by usage of alternative promoters and/or splicing on the C- and N-terminus. Various p53 family isoforms are expressed in melanoma cell lines and tumor samples, and several of them have already shown to have specific functions in melanoma, affecting proliferation, survival, metastatic potential, invasion, migration, and response to therapy. Of special interest are p53 family isoforms with increased expression and direct involvement in acquired resistance to MAPK inhibitors in melanoma cells, implying that modulating their expression or targeting their functional pathways could be a potential therapeutic strategy to overcome resistance to MAPK inhibitors in melanoma.

Deciphering the Role of p53 and TAp73 in Neuroblastoma: From Pathogenesis to Treatment

Neuroblastoma (NB) is an embryonic cancer that develops from neural crest stem cells, being one of the most common malignancies in children. The clinical manifestation of this disease is highly variable, ranging from spontaneous regression to increased aggressiveness, which makes it a major therapeutic challenge in pediatric oncology. The p53 family proteins p53 and TAp73 play a key role in protecting cells against genomic instability and malignant transformation. However, in NB, their activities are commonly inhibited by interacting proteins such as murine double minute (MDM)2 and MDMX, mutant p53, ΔNp73, Itch, and Aurora kinase A. The interplay between the p53/TAp73 pathway and N-MYC, a known biomarker of poor prognosis and drug resistance in NB, also proves to be decisive in the pathogenesis of this tumor. More recently, a strong crosstalk between microRNAs (miRNAs) and p53/TAp73 has been established, which has been the focused of great attention because of its potential for developing new therapeutic strategies. Collectively, this review provides an updated overview about the critical role of the p53/TAp73 pathway in the pathogenesis of NB, highlighting encouraging clues for the advance of alternative NB targeted therapies.

Delineating functional mechanisms of the p53/p63/p73 family of transcription factors through identification of protein-protein interactions using interface mimicry

Members of the p53 family of transcription factors-p53, p63, and p73-share a high degree of homology; however, members can be activated in response to different stimuli, perform distinct (sometimes opposing) roles and are expressed in different tissues. The level of complexity is increased further by the transcription of multiple isoforms of each homolog, which may interact or interfere with each other and can impact cellular outcome. Proteins perform their functions through interacting with other proteins (and/or with nucleic acids). Therefore, identification of the interactors of a protein and how they interact in 3D is essential to fully comprehend their roles. By utilizing an in silico protein-protein interaction prediction method-HMI-PRED-we predicted interaction partners of p53 family members and modeled 3D structures of these protein interaction complexes. This method recovered experimentally known interactions while identifying many novel candidate partners. We analyzed the similarities and differences observed among the interaction partners to elucidate distinct functions of p53 family members and provide examples of how this information may yield mechanistic insight to explain their overlapping versus distinct/opposing outcomes in certain contexts. While some interaction partners are common to p53, p63, and p73, the majority are unique to each member. Nevertheless, most of the enriched pathways associated with these partners are common to all members, indicating that the members target the same biological pathways but through unique mediators. p63 and p73 have more common enriched pathways compared to p53, supporting their similar developmental roles in different tissues.

MDM2 binds and ubiquitinates PARP1 to enhance DNA replication fork progression

The MDM2 oncoprotein antagonizes the tumor suppressor p53 by physical interaction and ubiquitination. However, it also sustains the progression of DNA replication forks, even in the absence of functional p53. Here, we show that MDM2 binds, inhibits, ubiquitinates, and destabilizes poly(ADP-ribose) polymerase 1 (PARP1). When cellular MDM2 levels are increased, this leads to accelerated progression of DNA replication forks, much like pharmacological inhibition of PARP1. Conversely, overexpressed PARP1 restores normal fork progression despite elevated MDM2. Strikingly, MDM2 profoundly reduces the frequency of fork reversal, revealed as four-way junctions through electron microscopy. Depletion of RECQ1 or the primase/polymerase (PRIMPOL) reverses the MDM2-mediated acceleration of the nascent DNA elongation rate. MDM2 also increases the occurrence of micronuclei, and it exacerbates camptothecin-induced cell death. In conclusion, high MDM2 levels phenocopy PARP inhibition in modulation of fork restart, representing a potential vulnerability of cancer cells.

Structural diversity of p63 and p73 isoforms

Abstract

The p53 protein family is the most studied protein family of all. Sequence analysis and structure determination have revealed a high similarity of crucial domains between p53, p63 and p73. Functional studies, however, have shown a wide variety of different tasks in tumor suppression, quality control and development. Here we review the structure and organization of the individual domains of p63 and p73, the interaction of these domains in the context of full-length proteins and discuss the evolutionary origin of this protein family.

Facts

Open questions

MDM2, MDMX, and p73 regulate cell-cycle progression in the absence of wild-type p53

The p53 tumor suppressor protein, known to be critically important in several processes including cell-cycle arrest and apoptosis, is highly regulated by multiple mechanisms, most certifiably the Murine Double Minute 2-Murine Double Minute X (MDM2-MDMX) heterodimer. The role of MDM2-MDMX in cell-cycle regulation through inhibition of p53 has been well established. Here we report that in cells either lacking p53 or expressing certain tumor-derived mutant forms of p53, loss of endogenous MDM2 or MDMX, or inhibition of E3 ligase activity of the heterocomplex, causes cell-cycle arrest. This arrest is correlated with a reduction in E2F1, E2F3, and p73 levels. Remarkably, direct ablation of endogenous p73 produces a similar effect on the cell cycle and the expression of certain E2F family members at both protein and messenger RNA levels. These data suggest that MDM2 and MDMX, working at least in part as a heterocomplex, may play a p53-independent role in maintaining cell-cycle progression by promoting the activity of E2F family members as well as p73, making them a potential target of interest in cancers lacking wild-type p53.

p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies

Simple Summary

The p53 family of proteins comprises p53, p63, and p73, which share high structural and functional similarity. The two distinct promoters of each locus, the alternative splicing, and the alternative translation initiation sites enable the generation of numerous isoforms with different protein-interacting domains and distinct activities. The co-expressed p53/p73 isoforms have significant but distinct roles in carcinogenesis. Their activity is frequently impaired in human tumors including colorectal carcinoma due to dysregulated expression and a dominant-negative effect accomplished by some isoforms and p53 mutants. The interactions between isoforms are particularly important to understand the onset of tumor formation, progression, and therapeutic response. The understanding of the p53/p73 network can contribute to the development of new targeted therapies.

Abstract

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

Dalhat M, Abdullah O, Kaleem M, Hosawi S, A Al-Abbasi F, Wu W, Choudhry H, Alhosin M. Targeting Post-Translational Modifications of the p73 Protein: A Promising Therapeutic Strategy for Tumors

The tumor suppressor p73 is a member of the p53 family and is expressed as different isoforms with opposing properties. The TAp73 isoforms act as tumor suppressors and have pro-apoptotic effects, whereas the ΔNp73 isoforms lack the N-terminus transactivation domain and behave as oncogenes. The TAp73 protein has a high degree of similarity with both p53 function and structure, and it induces the regulation of various genes involved in the cell cycle and apoptosis. Unlike those of the p53 gene, the mutations in the p73 gene are very rare in tumors. Cancer cells have developed several mechanisms to inhibit the activity and/or expression of p73, from the hypermethylation of its promoter to the modulation of the ratio between its pro- and anti-apoptotic isoforms. The p73 protein is also decorated by a panel of post-translational modifications, including phosphorylation, acetylation, ubiquitin proteasomal pathway modifications, and small ubiquitin-related modifier (SUMO)ylation, that regulate its transcriptional activity, subcellular localization, and stability. These modifications orchestrate the multiple anti-proliferative and pro-apoptotic functions of TAp73, thereby offering multiple promising candidates for targeted anti-cancer therapies. In this review, we summarize the current knowledge of the different pathways implicated in the regulation of TAp73 at the post-translational level. This review also highlights the growing importance of targeting the post-translational modifications of TAp73 as a promising antitumor strategy, regardless of p53 status.

A FRET-Based Fluorescent Probe to Screen Anticancer Drugs, Inhibiting p73 Binding to MDM2

The protein p73 acts as a transcription factor, resulting in tumour suppression. MDM2, an oncogenic protein, can negatively influence p73-mediated apoptosis by binding to p73 transactivation domains (TAD). Inhibition of the protein-protein interaction between p73 and oncogenic proteins is an attractive strategy for promoting p73-mediated apoptosis. Herein, we describe the use of a modified p73-TAD peptide for the FRET-based assay of the binding of p73-TAD to MDM2. The FRET probe, equipped with 1-naphthylamine (λ_ex =330 nm, λ_em =445 nm), serves as a FRET acceptor, and the tryptophan of the protein acts as FRET donor (λ_ex =280 nm, λ_em =340 nm). Sensitized emission from the FRET probe was observed upon excitation of the protein-FRET-probe complex at the excitation wavelength of Trp. Furthermore, addition of the MDM2 inhibitor Nutiln-3 drastically reduced the FRET signal, thus indicating that the FRET probe competes with Nutiln-3 for MDM2 binding. The developed FRET binding assay might be applicable in high-throughput screening of novel drugs that inhibit interactions between p73 and MDM2.

p73: From the p53 shadow to a major pharmacological target in anticancer therapy

p73, along with p53 and p63, belongs to the p53 family of transcription factors. Besides the p53-like tumor suppressive activities, p73 has unique roles, namely in neuronal development and differentiation. In addition, the TP73 gene is rarely mutated in tumors. This makes p73 a highly appealing therapeutic target, particularly towards cancers with a null or disrupted p53 pathway. Distinct isoforms are transcribed from the TP73 locus either with (TAp73) and without (ΔNp73) the N-terminal transactivation domain. Conversely to TA tumor suppressors, ΔN proteins exhibit oncogenic properties by inhibiting p53 and TA protein functions. As such, p73 isoforms compose a puzzled and challenging regulatory pathway. This state-of-the-art review affords an update overview on p73 structure, biological functions and pharmacological regulation. Importantly, it addresses the relevance of p73 isoforms in carcinogenesis, highlighting their potential as drug targets in anticancer therapy. A critical discussion of major pharmacological approaches to promote p73 tumor suppressive activities, with relevant survival outcomes for cancer patients, is also provided.

The Undervalued Avenue to Reinstate Tumor Suppressor Functionality of the p53 Protein Family for Improved Cancer Therapy-Drug Repurposing

p53 and p73 are critical tumor suppressors that are often inactivated in human cancers through various mechanisms. Owing to their high structural homology, the proteins have many joined functions and recognize the same set of genes involved in apoptosis and cell cycle regulation. p53 is known as the 'guardian of the genome' and together with p73 forms a barrier against cancer development and progression. The TP53 is mutated in more than 50% of all human cancers and the germline mutations in TP53 predispose to the early onset of multiple tumors in Li-Fraumeni syndrome (LFS), the inherited cancer predisposition. In cancers where TP53 gene is intact, p53 is degraded. Despite the ongoing efforts, the treatment of cancers remains challenging. This is due to late diagnoses, the toxicity of the current standard of care and marginal benefit of newly approved therapies. Presently, the endeavors focus on reactivating p53 exclusively, neglecting the potential of the restoration of p73 protein for cancer eradication. Taken that several small molecules reactivating p53 failed in clinical trials, there is a need to develop new treatments targeting p53 proteins in cancer. This review outlines the most advanced strategies to reactivate p53 and p73 and describes drug repurposing approaches for the efficient reinstatement of the p53 proteins for cancer therapy.

Mdm2: Open questions

The Mdm2 oncoprotein and its association with p53 were discovered 30 years ago, and a cornucopia of activities and regulatory pathways have been associated with it. In this review, we will raise questions about Mdm2 and its cousin Mdm4 that we consider worth pursuing in future research, reaching from molecular structures and intracellular activities all the way to development, evolution, and cancer therapy. We anticipate that such research will not only close a few gaps in our knowledge but could add new dimensions to our current view. This compilation of questions contributes to the preparation for the 10th Mdm2 Workshop in Tokyo.

Regulation of the p53 Family Proteins by the Ubiquitin Proteasomal Pathway

The tumor suppressor p53 and its homologues, p63 and p73, play a pivotal role in the regulation of the DNA damage response, cellular homeostasis, development, aging, and metabolism. A number of mouse studies have shown that a genetic defect in the p53 family could lead to spontaneous tumor development, embryonic lethality, or severe tissue abnormality, indicating that the activity of the p53 family must be tightly regulated to maintain normal cellular functions. While the p53 family members are regulated at the level of gene expression as well as post-translational modification, they are also controlled at the level of protein stability through the ubiquitin proteasomal pathway. Over the last 20 years, many ubiquitin E3 ligases have been discovered that directly promote protein degradation of p53, p63, and p73 in vitro and in vivo. Here, we provide an overview of such E3 ligases and discuss their roles and functions.

Downregulation of AKT and MDM2, Melatonin Induces Apoptosis in AGS and MGC803 Cells

Melatonin, a neurohormone secreted by the pineal gland, has a variety of biological functions, such as circadian rhythms regulation, anti-oxidative activity, immunomodulatory effects, and anittumor, etc. At present, its antitumor effect has attracted people's attention due to its extensive tissue distribution, good tissue compatibility, and low toxic and side effects. In the gastrointestinal tract, there is high level of melatonin and many studies showed melatonin has effects of anti-gastric cancer. In this experiment, human gastric cancer cell lines AGS and MGC803 were used to investigate the intracellular molecular mechanism of melatonin against gastric cancer. After AGS and MGC803 have been treated with melatonin, the changes of cell morphology and cellular structure were observed under electron microscope. Flow cytometer and apoptosis detection kits were used to analyze the effect of apoptosis on AGS and MGC803. The alterations of apoptosis-related proteins Caspase 9, Caspase 3, and upstream regulators AKT, MDM2 including expression, phosphorylation, and activation were detected to analyze the intracellular molecular mechanism of melatonin inhibiting gastric cancer. In AGS and MGC803 cells with melatonin exposure, cleaved Caspase 9 was upregulated and Caspase 3 was activated; moreover, MDM2 and AKT expression and phosphorylation were downregulated. Melatonin promoted apoptosis of AGS and MGC803 cells by the downregulation of AKT and MDM2. Anat Rec, 302:1544-1551, 2019. © 2019 American Association for Anatomy.

The roles of moonlight ribosomal proteins in the development of human cancers

"Moonlighting protein" is a term used to define a single protein with multiple functions and different activities that are not derived from gene fusions, multiple RNA splicing, or the proteolytic activity of promiscuous enzymes. Different proteinous constituents of ribosomes have been shown to have important moonlighting extra-ribosomal functions. In this review, we introduce the impact of key moonlight ribosomal proteins and dependent signal transduction in the initiation and progression of various cancers. As a future perspective, the potential role of these moonlight ribosomal proteins in the diagnosis, prognosis, and development of novel strategies to improve the efficacy of therapies for human cancers has been suggested.

Pleckstrin homology domain-containing protein PHLDB3 supports cancer growth via a negative feedback loop involving p53

The tumour suppressor p53 transactivates the expression of its target genes to exert its functions. Here, we identify a pleckstrin homology domain-containing protein (PHLDB3)-encoding gene as a p53 target. PHLDB3 overexpression increases proliferation and restrains apoptosis of wild-type p53-harboring cancer cells by reducing p53 protein levels. PHLDB3 binds to MDM2 (mouse double minute 2 homolog) and facilitates MDM2-mediated ubiquitination and degradation of p53. Knockdown of PHLDB3 more efficiently inhibits the growth of mouse xenograft tumours derived from human colon cancer HCT116 cells that contain wild type p53 compared with p53-deficient HCT116 cells, and also sensitizes tumour cells to doxorubicin and 5-Fluorouracil. Analysis of cancer genomic databases reveals that PHLDB3 is amplified and/or highly expressed in numerous human cancers. Altogether, these results demonstrate that PHLDB3 promotes tumour growth by inactivating p53 in a negative feedback fashion and suggest PHLDB3 as a potential therapeutic target in various human cancers.

p53 is an oncosuppressor regulating several genes at the transcriptional level. Here, the authors identify a negative feedback loop between PHLDB3 and p53; PHLDB3 is a transcriptional target of p53 which facilitates MDM2-mediated p53 ubiquitination and degradation, impacting on tumorigenesis.

Matrine‑induced apoptosis in Hep3B cells via the inhibition of MDM2

Matrine, an alkaloid component derived from the Sophora root, can inhibit cancer cell proliferation and induce autophagy via p53 associated pathways. However, numerous tumor cells lack functional p53 and little is known about the effect of matrine on the p53‑deficient/mutant cancer cells. The present study aimed to assess anticancer effects of matrine in p53‑deficient human Hep3B hepatoma cells. The present results demonstrated that matrine caused Hep3B cell apoptosis by suppressing gene expression of minute double‑mutant (MDM)2. Notably, it was revealed that matrine inhibited MDM2 at the transcriptional level in a time‑ and dose‑dependent manner. This MDM2 inhibition resulted in induction of the p53 family member, p73; however, the functions of p73 were not induced since matrine‑induced p73 failed to activate its target genes, p21 and p53 upregulated modulator of apoptosis. The matrine‑induced downregulation of MDM2 led to an inhibition of inhibitor of apoptosis protein 3, which might serve a critical role in matrine‑induced apoptosis in MDM2‑overexpressing Hep3B cells. Finally, combination therapy of matrine with 100 µM epotoside successfully killed more Hep3B cells, suggesting that matrine can sensitize p53‑deficient Hep3B cells to epotoside‑induced apoptosis.

MDMX (MDM4), a Promising Target for p53 Reactivation Therapy and Beyond

The MDMX protein was identified as a p53-interacting protein with a strong similarity to MDM2. Like Mdm2, Mdmx expression is essential for curbing p53 activity during embryonic development, indicating nonredundant functions of Mdmx and Mdm2. There is now a large body of evidence indicating that cancers frequently up-regulate MDMX expression as a means to dampen p53 tumor-suppressor function. Importantly, MDMX also shows p53-independent oncogenic functions. These data make MDMX an attractive therapeutic target for cancer therapy. Here, we summarize the mechanisms used by cancer cells to increase MDMX expression and promising pharmacological strategies to target MDMX in cancer-in particular, the recent findings that antisense oligonucleotides (ASOs) can be used to efficiently modulate MDMX messenger RNA (mRNA) splicing.

MDM2 Associates with Polycomb Repressor Complex 2 and Enhances Stemness-Promoting Chromatin Modifications Independent of p53

The MDM2 oncoprotein ubiquitinates and antagonizes p53 but may also carry out p53-independent functions. Here we report that MDM2 is required for the efficient generation of induced pluripotent stem cells (iPSCs) from murine embryonic fibroblasts, in the absence of p53. Similarly, MDM2 depletion in the context of p53 deficiency also promoted the differentiation of human mesenchymal stem cells and diminished clonogenic survival of cancer cells. Most of the MDM2-controlled genes also responded to the inactivation of the Polycomb Repressor Complex 2 (PRC2) and its catalytic component EZH2. MDM2 physically associated with EZH2 on chromatin, enhancing the trimethylation of histone 3 at lysine 27 and the ubiquitination of histone 2A at lysine 119 (H2AK119) at its target genes. Removing MDM2 simultaneously with the H2AK119 E3 ligase Ring1B/RNF2 further induced these genes and synthetically arrested cell proliferation. In conclusion, MDM2 supports the Polycomb-mediated repression of lineage-specific genes, independent of p53.

p53 Activity Dominates That of p73 upon Mdm4 Loss in Development and Tumorigenesis

Mdm4 negatively regulates the p53 tumor suppressor. Mdm4 loss in mice leads to an embryonic lethal phenotype that is p53-dependent. Biochemical studies indicate that Mdm4 also binds p73, a member of the p53 family, with higher affinity than p53. In this study, the significance of the Mdm4 and p73 interaction in vivo during embryogenesis and tumorigenesis was examined. The data revealed that p73 loss did not rescue either the early Mdm4-deficient embryonic lethality or the runted phenotype of Mdm4(Δ2/Δ2) p53(+/-) embryos. Furthermore, studies in the developing central nervous system wherein both genes have prominent roles indicated that loss of p73 also did not rescue the Mdm4-null brain phenotype as did p53 loss. This p53 dependency occurred despite evidence for p73-specific transcriptional activity. In tumor studies, the combination of Mdm4 overexpression and p73 loss did not alter survival of mice or the tumor spectrum as compared with Mdm4 overexpression alone. In summary, these data demonstrate that the Mdm4-p73 axis cannot override the dominant role of p53 in development and tumorigenesis.

IMPLICATIONS

Genetic characterization of the Mdm4 and p73 interaction during development and tumorigenesis suggests new insight into the role of p53 family members, which may influence treatment options for patients.

Novel Implications of DNA Damage Response in Drug Resistance of Malignant Cancers Obtained from the Functional Interaction between p53 Family and RUNX2

During the lifespan of cells, their genomic DNA is continuously exposed to the endogenous and exogenous DNA insults. Thus, the appropriate cellular response to DNA damage plays a pivotal role in maintaining genomic integrity and also acts as a molecular barrier towards DNA legion-mediated carcinogenesis. The tumor suppressor p53 participates in an integral part of proper regulation of DNA damage response (DDR). p53 is frequently mutated in a variety of human cancers. Since mutant p53 displays a dominant-negative behavior against wild-type p53, cancers expressing mutant p53 sometimes acquire drug-resistant phenotype, suggesting that mutant p53 prohibits the p53-dependent cell death pathway following DNA damage, and thereby contributing to the acquisition and/or maintenance of drug resistance of malignant cancers. Intriguingly, we have recently found that silencing of pro-oncogenic RUNX2 enhances drug sensitivity of aggressive cancer cells regardless of p53 status. Meanwhile, cancer stem cells (CSCs) have stem cell properties such as drug resistance. Therefore, the precise understanding of the biology of CSCs is quite important to overcome their drug resistance. In this review, we focus on molecular mechanisms behind DDR as well as the serious drug resistance of malignant cancers and discuss some attractive approaches to improving the outcomes of patients bearing drug-resistant cancers.

Potentiation of Carboplatin-Mediated DNA Damage by the Mdm2 Modulator Nutlin-3a in a Humanized Orthotopic Breast-to-Lung Metastatic Model

Triple-negative breast cancers (TNBC) are typically resistant to treatment, and strategies that build upon frontline therapy are needed. Targeting the murine double minute 2 (Mdm2) protein is an attractive approach, as Mdm2 levels are elevated in many therapy-refractive breast cancers. The Mdm2 protein-protein interaction inhibitor Nutlin-3a blocks the binding of Mdm2 to key signaling molecules such as p53 and p73α and can result in activation of cell death signaling pathways. In the present study, the therapeutic potential of carboplatin and Nutlin-3a to treat TNBC was investigated, as carboplatin is under evaluation in clinical trials for TNBC. In mutant p53 TMD231 TNBC cells, carboplatin and Nutlin-3a led to increased Mdm2 and was strongly synergistic in promoting cell death in vitro. Furthermore, sensitivity of TNBC cells to combination treatment was dependent on p73α. Following combination treatment, γH2AX increased and Mdm2 localized to a larger degree to chromatin compared with single-agent treatment, consistent with previous observations that Mdm2 binds to the Mre11/Rad50/Nbs1 complex associated with DNA and inhibits the DNA damage response. In vivo efficacy studies were conducted in the TMD231 orthotopic mammary fat pad model in NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (NSG) mice. Using an intermittent dosing schedule of combined carboplatin and Nutlin-3a, there was a significant reduction in primary tumor growth and lung metastases compared with vehicle and single-agent treatments. In addition, there was minimal toxicity to the bone marrow and normal tissues. These studies demonstrate that Mdm2 holds promise as a therapeutic target in combination with conventional therapy and may lead to new clinical therapies for TNBC.

Functional interplay between MDM2, p63/p73 and mutant p53

Many cancers express mutant p53 proteins that have lost wild-type tumor suppressor activity and, in many cases, have acquired oncogenic functions that can contribute to tumor progression. These activities of mutant p53 reflect interactions with several other proteins, including the p53 family members p63 and p73. Mutations in p53 that affect protein conformation (such as R175H) show strong binding to p63 and p73, whereas p53 mutants that only mildly affect the conformation (such as R273H) bind less well. A previously described aggregation domain of mutant p53 is not required for p63 or p73 binding; indeed, mutations within this region lead to the acquisition of a mutant p53 phenotype-including a conformational shift, p63/p73 binding and the ability to promote invasion. The activity of wild-type p53 is regulated by an interaction with MDM2 and we have investigated the potential role of MDM2 in the mutant p53/p63/p73 interactions. Both mutant p53 and p73 bind MDM2 well, whereas p63 binds much more weakly. We found that MDM2 can inhibit p63 binding to p53R175H but enhances the weaker p53R273H/p73 interaction. These effects on the interactions are reflected in an ability of MDM2 to relieve the inhibition of p63 by p53R175H, but enhance the inhibition of p73 activity by p53R175H and R273H. We propose a model in which MDM2 competes with p63 for binding to p53R175H to restore p63 activity, but forms a trimeric complex with p73 and p53R273H to more strongly inhibit p73 function.

Pathway crosstalk analysis of microarray gene expression profile in human hepatocellular carcinoma

Liver cancer is the third most common cause of cancer death in the world. Hepatocellular carcinoma (HCC) is the main pathological types in liver cancer, which amounts to 70-85 % of primary liver cancer in the world and 90 % in China. The aim of this study was to establish a PPI network and a pathway crosstalk network to isolate important dysfunctional pathways which play an important role in the pathogenesis of HCC. System biology approach was used in this research. A PPI network was firstly built and then a dysfunctional crosstalk network of HCC related pathways was constructed. Several important significant dysfunctional crosstalk pathways were identified. Basal transcription factors (hsa03022), Glycerophospholipid metabolism (hsa00564) and Metabolism of xenobiotics by cytochrome P450 (hsa00980) were significantly interact with Pathway in cancer (hsa05200). Besides, pathway Axon guidance (hsa04360) was also dysfunctional crosstalk with Pathway in cancer (hsa05200). The crosstalks among these pathways reveal some evidence that the pathways closely cooperated and play important tasks in HCC progression. Besides, the pathway hsa04360 dysfunctional crosstalk with the hsa05200 indicates there would be a same mechanism for HCC invasion and migration.

Ribosomal proteins L11 and L5 activate TAp73 by overcoming MDM2 inhibition

Over the past decade, a number of ribosomal proteins (RPs) have been found to have a role in activating the tumor suppressor p53 by directly binding to MDM2 and impeding its activity toward p53. Herein, we report that RPL5 and RPL11 can also enhance the transcriptional activity of a p53 homolog TAp73, but through a distinct mechanism. Interestingly, even though RPL5 and RPL11 were not shown to bind to p53, they were able to directly associate with the transactivation domain of TAp73 independently of MDM2 in response to RS. This association led to perturbation of the MDM2-TAp73 interaction, consequently preventing MDM2 from its association with TAp73 target gene promoters. Furthermore, ectopic expression of RPL5 or RPL11 markedly induced TAp73 transcriptional activity by antagonizing MDM2 suppression. Conversely, ablation of either of the RPs compromised TAp73 transcriptional activity, as evident by the reduction of p21 and Puma expression, in response to 5-fluorouracil (5-FU). Consistently, overexpression of RPL5 or RPL11 enhanced, but knockdown of either of them hampered, TAp73-mediated apoptosis. Intriguingly, simultaneous knockdown of TAp73 and either of the RPs was required for rescuing the 5-FU-triggered S-phase arrest of p53-null tumor cells. These results demonstrate a novel mechanism underlying the inhibition of tumor cell proliferation and growth by these two RPs via TAp73 activation.

TP63 and TP73 in cancer, an unresolved "family" puzzle of complexity, redundancy and hierarchy

TP53 belongs to a small gene family that includes, in mammals, two additional paralogs, TP63 and TP73. The p63 and p73 proteins are structurally and functionally similar to p53 and their activity as transcription factors is regulated by a wide repertoire of shared and unique post-translational modifications and interactions with regulatory cofactors. p63 and p73 have important functions in embryonic development and differentiation but are also involved in tumor suppression. The biology of p63 and p73 is complex since both TP63 and TP73 genes are transcribed into a variety of different isoforms that give rise to proteins with antagonistic properties, the TA-isoforms that act as tumor-suppressors and DN-isoforms that behave as proto-oncogenes. The p53 family as a whole behaves as a signaling "network" that integrates developmental, metabolic and stress signals to control cell metabolism, differentiation, longevity, proliferation and death. Despite the progress of our knowledge, the unresolved puzzle of complexity, redundancy and hierarchy in the p53 family continues to represent a formidable challenge.

Regulation of p63 protein stability via ubiquitin-proteasome pathway

The p53-related p63 gene encodes multiple protein isoforms, which are involved in a variety of biological activities. p63 protein stability is mainly regulated by the ubiquitin-dependent proteasomal degradation pathway. Several ubiquitin E3 ligases have been identified and some protein kinases as well as other kinds of proteins are involved in regulation of p63 protein stability. These regulators are responsive to diverse extracellular signaling, resulting in changes of the p63 protein levels and impacting different biological processes.

MDM2's social network

MDM2 is considered a hub protein due to its capacity to interact with a large number of different partners of which p53 is most well described. MDM2 is an E3 ubiquitin ligase, and many, but not all, of its interactions relate directly to this activity, such as substrates, adaptors or bridges, promoters, inhibitors or complementary factors. Some interactions serve regulatory functions that in response to cellular stresses control the localisation and functions of MDM2 including protein kinases, ribosomal proteins and proteases. Moreover, interactions with nucleotides serve other functions such as mRNA to regulate protein synthesis and DNA to control transcription. To perform such a pleiotropic panorama of different functions, MDM2 is subjected to a multitude of post-translational modifications and is expressed in different isoforms. The large and diverse interactome is made possible due to the plasticity of MDM2 and in this review we have listed the MDM2 interactions until now and we will discuss how this multifaceted protein can interact with such a variety of substrates to provide a key intermediary role in different signalling pathways.

Rapamycin inhibits IGF-1-mediated up-regulation of MDM2 and sensitizes cancer cells to chemotherapy

The Murine Double Minute 2 (MDM2) protein is a key regulator of cell proliferation and apoptosis that acts primarily by inhibiting the p53 tumor suppressor. Similarly, the PI3-Kinase (PI3K)/AKT pathway is critical for growth factor-mediated cell survival. Additionally, it has been reported that AKT can directly phosphorylate and activate MDM2. In this study, we show that IGF-1 up-regulates MDM2 protein levels in a PI3K/AKT-dependent manner. Inhibition of mTOR by rapamycin or expression of a dominant negative eukaryotic initiation factor 4E binding protein 1 (4EBP1) mutant protein, as well as ablation of eukaryotic initiation factor 4E (eIF4E), efficiently abolishes IGF-1-mediated up-regulation of MDM2. In addition, we show that rapamycin effectively inhibits MDM2 expression and sensitizes cancer cells to chemotherapy. Taken together, this study reveals a novel mechanism by which IGF-1 activates MDM2 via the mTOR pathway, and that pharmacologic inhibition of mTOR combined with chemotherapy may be more effective in treatment of a subset of cancers harboring increased MDM2 activation.

Negative regulation of p53 by Ras superfamily protein RBEL1A

We had previously reported that RBEL1A, a novel Ras-like GTPase, was overexpressed in multiple human malignancies and that its depletion suppressed cell growth. However, the underlying molecular mechanism remained to be elucidated. Here we report that depletion of endogenous RBEL1A results in p53 accumulation due to increased p53 half-life whereas increased expression of RBEL1A reduces p53 levels under unstressed and genotoxic stress conditions. RBEL1A directly interacts with p53 and MDM2, and strongly enhances MDM2-dependent p53 ubiquitylation and degradation. We also found that RBEL1A modulation of p53 ubiquitylation by MDM2 does not depend on its GTPase activity. We have also defined the p53 oligomeric domain and RBEL1A GTPase domain to be the crucial regions for p53-RBEL1A interactions. Importantly, we have found that RBEL1A strongly interferes with p53 transactivation function; thus our results indicate that RBEL1A appears to function as a novel p53 negative regulator that facilitates MDM2-dependent p53 ubiquitylation and degradation.

Mdm2 and MdmX as Regulators of Gene Expression

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

Identification of functional DNA variants in the constitutive promoter region of MDM2

Although mutations in the oncoprotein murine double minute 2 (MDM2) are rare, MDM2 gene overexpression has been observed in several human tumors. Given that even modest changes in MDM2 levels might influence the p53 tumor suppressor signaling pathway, we postulated that sequence variation in the promoter region of MDM2 could lead to disregulated expression and variation in gene dosage. Two promoters have been reported for MDM2; an internal promoter (P2), which is located near the end of intron 1 and is p53-responsive, and an upstream constitutive promoter (P1), which is p53-independent. Both promoter regions contain DNA variants that could influence the expression levels of MDM2, including the well-studied single nucleotide polymorphism (SNP) SNP309, which is located in the promoter P2; i.e., upstream of exon 2. In this report, we screened the promoter P1 for DNA variants and assessed the functional impact of the corresponding SNPs. Using the dbSNP database and genotyping validation in individuals of European descent, we identified three common SNPs (-1494 G > A; indel 40 bp; and -182 C > G). Three major promoter haplotypes were inferred by using these three promoter SNPs together with rs2279744 (SNP309). Following subcloning into a gene reporter system, we found that two of the haplotypes significantly influenced MDM2 promoter activity in a haplotype-specific manner. Site-directed mutagenesis experiments indicated that the 40 bp insertion/deletion variation is causing the observed allelic promoter activity. This study suggests that part of the variability in the MDM2 expression levels could be explained by allelic p53-independent P1 promoter activity.

Regulation of p73 activity by post-translational modifications

The transcription factor p73 is a member of the p53 family that can be expressed as at least 24 different isoforms with pro- or anti-apoptotic attributes. The TAp73 isoforms are expressed from an upstream promoter and are regarded as bona fide tumor suppressors; they can induce cell cycle arrest/apoptosis and protect against genomic instability. On the other hand, ΔNp73 isoforms lack the N-terminus transactivation domain; hence, cannot induce the expression of pro-apoptotic genes, but still can oligomerize with TAp73 or p53 to block their transcriptional activities. Therefore, the ratio of TAp73 isoforms to ΔNp73 isoforms is critical for the quality of the response to a genomic insult and needs to be delicately regulated at both transcriptional and post-translational level. In this review, we will summarize the current knowledge on the post-translational regulatory pathways involved to keep p73 protein under control. A comprehensive understanding of p73 post-translational modifications will be extremely useful for the development of new strategies for treating and preventing cancer.

The Many Faces of MDM2 Binding Partners

Mdm2 is an essential regulator of the p53 tumor suppressor. Mdm2 is modified at transcriptional, post-transcriptional, and post-translational levels to control p53 activity in normal versus stressed cells. Importantly, errors in these regulatory mechanisms can result in aberrant Mdm2 expression and failure to initiate programmed cell death in response to DNA damage. Such errors can have severe consequences as evidenced by tumor phenotypes resulting from amplification at the Mdm2 locus and changes in post-transcriptional and post-translational regulation of Mdm2. Although Mdm2 mediated inhibition of p53 is well characterized, Mdm2 interacts with many additional proteins and also targets many of these for proteosomal degradation. Mdm2 also has E3-ligase independent functions and p53-independent functions that have important implications for genome stability and cancer.

p73 as a pharmaceutical target for cancer therapy

About half of all human tumors contain an inactivating mutation of p53, while in the remaining tumors, the p53 pathway is frequently abrogated by alterations of other components of its signaling pathway. In humans, the p53 tumor suppressor is part of a small gene family that includes two other members, p73 and p63, structurally and functionally related to p53. Accumulating evidences indicate that all p53-family proteins function as molecular hubs of a highly interconnected signaling network that coordinates cell proliferation, differentiation and death in response to physiological inputs and oncogenic stress. Therefore, not only the p53-pathway but the entire “p53-family pathway” is a primary target for cancer drug development. In particular, the p53-related protein p73 has a crucial role in determining cellular responses to chemotherapy, and can vicariate p53 functions in triggering cell death after DNA damage in multiple experimental models. The biology and regulation of p73 is complex, since the TP73 gene incorporates both tumor-suppressive and proto-oncogenic functions. However, the p73 gene is rarely mutated in tumors, so appropriate pharmacological manipulation of the p73 pathway is a very promising approach for cancer therapy. Here we provide an overview of the principal mechanism of p73 regulation, and describe several examples of pharmacological tools that can induce p73 accumulation and function by acting on upstream p73 modulators or displacing inhibitory p73 interactors. A better understanding of how the p73 pathway works is mandatory to discover additional players intervening in this pathway and has important implications for the improvement of cancer treatment with the development of new molecules or with the reposition of currently available drugs.

Targeting p73 in cancer

p73 is a member of the p53 family of tumor suppressors. Transactivating isoforms of p73 (TAp73) have p53-like, anti-proliferative and pro-apoptotic activities that are crucial for an efficient chemotherapy response. In line with this, genetic studies in mice have confirmed that TAp73 acts as a tumor suppressor. However, in contrast to p53, which is commonly inactivated in human cancer by point mutations, the TP73 gene is almost never mutated. Instead, the tumor suppressor activity of TAp73 is inhibited through a variety of mechanisms including epigenetic silencing and complex formation with inhibitory proteins. All these mechanisms have in common that they are in principle reversible and therefore amenable to therapeutic intervention. Here, we will review how tumor cells control the tumor suppressor activity of TAp73 and discuss possible strategies targeting p73 for reactivation.

Effects of ΔNp73β on cisplatin treatment in colon cancer cells

p73 can activate transcription of p53-responsive genes, thereby inhibiting cell growth. An alternative promoter in the TP73 gene gives rise to an N-terminally truncated isoform of p73, ΔNp73, which lacks the transactivation domain of the full length TAp73 protein. TAp73 is considered pro-apoptotic, and ΔNp73 anti-apoptotic. In this study, we overexpressed ΔNp73β in p53 wild type and p53 mutant colon cancer cell lines and further exposed the cells to cancer therapeutic drug cisplatin. The results showed that cisplatin decreased the protein expression levels of ΔNp73β in a dose-dependent manner, and both TAp73 and p53 were upregulated after cisplatin treatment. Further, clonogenic potential and cell viability were decreased, and apoptotic cells increased, in p53 mutant and in p53 wild type cells. Cellular viability was significantly higher in ΔNp73β-cells than mock-transfected cells. However, ΔNp73β overexpression did not affect the cellular susceptibility to cisplatin. In conclusion, the overexpression of ΔNp73β increases viability in p53 wild type and p53 mutant colon cancer cells, and cisplatin induces the degradation of ΔNp73β in a dose-dependent manner.

Mdm2 inhibition induces apoptosis in p53 deficient human colon cancer cells by activating p73- and E2F1-mediated expression of PUMA and Siva-1

Camptothecin (CPT) and Nutlin-3 caused apoptosis by increasing p53 protein and its activation in intestinal epithelial cells (IEC-6). We studied the effectiveness of these inducers on apoptosis in human colon cancer cells (Caco2) lacking p53 expression. CPT failed to activate caspase-3 and cause apoptosis in these cells. The absence of p53 expression, higher basal Bcl-xL and lower Bax proteins prevented CPT-induced apoptosis. However, the Mdm2 antagonist Nutlin-3 induced apoptosis in a dose dependent manner by activating caspases-9 and -3. Nutlin-3 prevented the activation of AKT via PTEN-mediated inhibition of the PI3K pathway. Nutlin-3 increased the phosphorylation of retinoblastoma protein causing E2F1 release leading to induction of Siva-1. Nutlin-3-mediated degradation of Mdm2 caused the accumulation of p73, which induced the expression of p53 up-regulated modulator of apoptosis (PUMA). E2F1 and p73 knockdown decreased the expression of Siva and PUMA, respectively and abolished Nutlin-3-induced caspase-3 activation. Cycloheximide (CHX) inhibited Nutlin-3-induced Siva, Noxa, and PUMA expression and inhibited apoptosis in IEC-6 and Caco2 cells. These results indicate that translation of mRNAs induced by Nutlin-3 is critical for apoptosis. In summary, apoptosis in Caco2 cells lacking functional p53 occurred following the disruption of Mdm2 binding with p73 and Rb leading to the expression of pro-apoptotic proteins, PUMA, Noxa, and Siva-1.

p73-Binding Partners and Their Functional Significance

p73 is one of the tumor-suppressor p53 family of nuclear transcription factor. As expected from the structural similarity between p53 and p73, p73 has a tumor-suppressive function. However, p73 was rarely mutated in human primary tumors. Under normal physiological conditions, p73 is kept at an extremely low level to allow cells normal growth. In response to a certain subset of DNA damages, p73 is induced dramatically and transactivates an overlapping set of p53-target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death. Cells undergo cell cycle arrest and/or apoptotic cell death depending on the type and strength of DNA damages. p73 is regulated largely through the posttranslational modifications such as phosphorylation and acetylation. These chemical modifications are tightly linked to direct protein-protein interactions. In the present paper, the authors describe the functional significance of the protein-protein interactions in the regulation of proapoptotic p73.

MDM2 promotes the proteasomal degradation of p73 through the interaction with Itch in HeLa cells

It has been shown that MDM2 inhibits the transcriptional and pro-apoptotic activities of p73 but does not promote its proteasomal degradation. In this study, we found that MDM2 indirectly induces the degradation of p73 through the interaction with Itch in HeLa cells. During adriamycin (ADR)-mediated apoptosis, p53 and p73 were induced to stabilize in association with a significant reduction of MDM2 and Itch, suggesting that, in addition to Itch, MDM2 could also be involved in the stability control of p73. As expected, forced expression of MDM2 resulted in a remarkable reduction of p73. MDM2-mediated degradation of p73 was inhibited by MG-132. Intriguingly, siRNA-mediated knockdown of Itch significantly attenuated the negative effect of MDM2 on p73. Additionally, MDM2 bound to Itch in HeLa cells but not in H1299 cells. Collectively, our present findings suggest that MDM2 promotes Itch-mediated degradation of p73 through the interaction with Itch in HeLa cells.

MDM2 promoter polymorphism is associated with increased susceptibility to hepatocellular carcinoma in Turkish population

BACKGROUND

The mouse double minute 2 (MDM2) gene represents one of the central nodes in the p53 pathway. A naturally occurring T/G single nucleotide polymorphism (SNP) in the intronic promoter of MDM2, SNP309 (rs2279744), was shown to influence MDM2 expression and p53 activity. SNP in the promoter region of MDM2 gene has recently been shown to be associated with accelerated tumor formation in both hereditary and sporadic cancers in humans. In this study, we aim to evaluate the association of SNP309 with the risk of hepatocellular carcinoma (HCC) development among Turkish population.

METHODS

MDM2 SNP309 polymorphism was investigated in 110 confirmed subjects with HCC and 110 cancer-free control subjects matched on age, gender, smoking and alcohol consumption by using a polymerase chain reaction-restriction fragment length polymorphism assay.

RESULTS

The allele frequencies of case subjects (T, 0.48; G, 0.52) were significantly different from those of control subjects (T, 0.65; G, 0.35) (p=0.003). The proportion of GG genotype of the SNP309 in patients with HCC (26%) was significantly higher than that in patients without HCC (14%). We observed that compared with the TT genotype, the genotypes containing G allele [TG (OR, 2.19; 95% CI, 1.18-4.07; p=0.013) or GG (OR, 3.63; 95% CI, 1.65-8.00; p=0.001)] were associated with significant increased susceptibility to HCC.

CONCLUSION

Our findings suggest that the MDM2 promoter SNP309 G allele is associated with presence of HCC in Turkish population.

p53-family proteins and their regulators: hubs and spokes in tumor suppression

The tumor suppressor p53 is a central hub in a molecular network controlling cell proliferation and death in response to potentially oncogenic conditions, and a wide array of covalent modifications and protein interactions modulate the nuclear and cytoplasmic activities of p53. The p53 relatives, p73 and p63, are entangled in the same regulatory network, being subject at least in part to the same modifications and interactions that convey signals on p53, and actively contributing to the resulting cellular output. The emerging picture is that of an interconnected pathway, in which all p53-family proteins are involved in the response to oncogenic stress and physiological inputs. Therefore, common and specific interactors of p53-family proteins can have a wide effect on function and dysfunction of this pathway. Many years of research have uncovered an impressive number of p53-interacting proteins, but much less is known about protein interactions of p63 and p73. Yet, many interactors may be shared by multiple p53-family proteins, with similar or different effects. In this study we review shared interactors of p53-family proteins with the aim to encourage research into this field; this knowledge promises to unveil regulatory elements that could be targeted by a new generation of molecules, and allow more efficient use of currently available drugs for cancer treatment.

Nutlin-3 cooperates with doxorubicin to induce apoptosis of human hepatocellular carcinoma cells through p53 or p73 signaling pathways

PURPOSE

Despite recent advances in chemotherapeutic agents for Hepatocellular carcinoma (HCC) treatment, the results of chemotherapy remain unsatisfactory. Doxorubicin (DOX) still represents the cornerstone in HCC chemotherapy, but resistance and toxicity to normal cells are major obstacles to successful chemotherapy. Therefore, new active agents in HCC chemotherapy and agents that increase the chemosensitivity of HCC cells to DOX are still urgently required. Nutlin-3 is a small-molecule inhibitor that acts to inhibit murine double minute-2 (MDM2) binding to p53 or p73, and subsequently activates p53- or p73-dependent apoptosis signaling pathway. This study was designed to investigate whether Nutlin-3 alters cell toxicity to HCC cells following DNA damage and to assess the suitability of DOX/Nutlin-3 as a chemotherapeutic combination in HCC chemotherapy.

METHODS

Four human HCC cells were analyzed using cell proliferation assay, apoptosis assay, western blotting, co-immunoprecipitation and siRNA experiments. Anti-tumoral effects of Nutlin-3/DOX targeting the p53/MDM2 and p73/MDM2 pathways were evaluated in HCC cell lines.

RESULTS

Nutlin-3 enhances the growth inhibition by DOX and potentates the apoptotic effect in all HCC cell lines with different p53 types. Nutlin-3 acts through the disruption of p53-MDM2 binding in HepG2, and the disruption of p73-MDM2 in Huh-7 and Hep3B cell lines with subsequent activation of the apoptotic pathway, which leads to the increase in chemosensitivity to DOX in HCC cells.

CONCLUSIONS

Taken together, our findings suggest that Nutlin-3 will be active in the treatment of HCC and offers new prospects for overcoming DOX resistance.

Discovery of novel proteasome inhibitors using a high-content cell-based screening system

The regulated degradation of damaged or misfolded proteins, as well as down-regulation of key signaling proteins, within eukaryotic and bacterial cells is catalyzed primarily by large, ATP-dependent multimeric proteolytic complexes, termed proteasomes. Inhibition of proteasomal activity affects a wide variety of physiological and pathological processes, and was found to be particularly effective for cancer therapy. We report here on the development of a novel high throughput assay for proteasome inhibition using a unique, highly sensitive live-cell screening, based on the cytoplasm-to-nucleus translocation of a fluorescent proteasome inhibition reporter (PIR) protein, consisting of nuclear localization signal-deficient p53 derivative. We further show here that mdm2, a key negative regulator of p53 plays a key role in the accumulation of PIR in the nucleus upon proteasome inhibition. Using this assay, we have screened the NCI Diversity Set library, containing 1,992 low molecular weight synthetic compounds, and identified four proteasome inhibitors. The special features of the current screen, compared to those of other approaches are discussed.

Murine double minute 2: p53-independent roads lead to genome instability or death

The oncoprotein murine double minute 2 (Mdm2) is frequently overexpressed in many types of human malignancies. Although Mdm2 has an essential role in negatively regulating the p53 tumor suppressor, it also has less well characterized p53-independent functions that influence pathways that are crucial for controlling tumorigenesis. In addition to the impact Mdm2 has on p53-independent apoptosis, mounting evidence is linking increased Mdm2 levels to altered cell-cycle regulation, DNA replication and DNA repair leading to loss of genome stability. Mdm2 involvement in pathways that influence chromosome stability and cell death, distinct from its role in the p53 pathway, strengthens the position of Mdm2 as a desirable therapeutic target for the treatment of human cancers.

Activation of p73 and induction of Noxa by DNA damage requires NF-kappa B

Although the transcription factor NF-κB is most clearly linked to the inhibition of extrinsic apoptotic signals such as TNFα by upregulating known anti-apoptotic genes, NF-κB has also been proposed to be required for p53-induced apoptosis in transformed cells. However, the involvement of NF-κB in this process is poorly understood. Here we investigate this mechanism and show that in transformed MEFs lacking NF-κB (p65-null cells) genotoxin-induced cytochrome c release is compromised. To further address how NF-κB contributes to apoptosis, gene profiling by microarray analysis of MEFs was performed, revealing that NF-κB is required for expression of Noxa, a pro-apoptotic BH3-only protein that is induced by genotoxins and that triggers cytochrome c release. Moreover, we find that in the absence of NF-κB, genotoxin treatment cannot induce Noxa mRNA expression. Noxa expression had been shown to be regulated directly by genes of the p53 family, like p73 and p63, following genotoxin treatment. Here we show that p73 is activated after genotoxin treatment only in the presence of NF-κB and that p73 induces Noxa gene expression through the p53 element in the promoter. Together our data provides an explanation for how loss of NF-κB abrogates genotoxin-induced apoptosis.

Therapeutic considerations for Mdm2: not just a one trick pony

BACKGROUND: The mdm2 proto-oncogene is elevated in numerous late stage cancers. The Mdm2 protein manifests its oncogenic properties in part through inactivation of the tumor suppressor protein p53. Recent efforts in anti-cancer drug design have focused on the identification of small molecules that disrupt the Mdm2-p53 interaction, in hopes of re-engaging the p53 pathway. OBJECTIVE: In addition to binding p53, Mdm2 complexes with numerous proteins involved in DNA repair, translation, metabolic activities, tumor growth and apoptosis. Additional biochemical analysis is required to understand how Mdm2 integrates into all of these cellular processes. Post-translational modifications to Mdm2 can alter its ability to associate with numerous proteins. Changes in protein structure may also affect the ability of small molecule inhibitors to effectively antagonize Mdm2. CONCLUSION: The complexity of Mdm2 modification has been largely neglected during the development of previous Mdm2 inhibitors. Future high-throughput or in silico screening efforts will need to recognize the importance of post-translational modifications to Mdm2. Furthermore, the identification of molecules that target other domains in Mdm2 may provide a tool to prevent other pivotal p53-independent functions of Mdm2. These aims provide a useful roadmap for the discovery of new Mdm2 binding compounds with therapeutic potency that may exceed its predecessors.