Nutlin-3, the small-molecule inhibitor of MDM2, promotes senescence and radiosensitises laryngeal carcinoma cells harbouring wild-type p53

Potential Alternative Therapeutic Modalities for Management Head and Neck Squamous Cell Carcinoma: A Review

Head and neck squamous cell carcinoma (HNSCC) includes malignancies of the lip and oral cavity, oropharynx, nasopharynx, larynx, and hypopharynx. It is among the most common malignancy worldwide, affecting nearly 1 million people annually. The traditional treatment options for HNSCC include surgery, radiotherapy, and conventional chemotherapy. However, these treatment options have their specific sequelae, which produce high rates of recurrence and severe treatment-related disabilities. Recent technological advancements have led to tremendous progress in understanding tumor biology, and hence the emergence of several alternative therapeutic modalities for managing cancers (including HNSCC). These treatment options are stem cell targeted therapy, gene therapy, and immunotherapy. Therefore, this review article aims to provide an overview of these alternative treatments of HNSCC.

p53 Modulates Radiosensitivity in Head and Neck Cancers-From Classic to Future Horizons

p53, initially considered a tumor suppressor, has been the subject of research related to cancer treatment resistance in the last 30 years. The unfavorable response to multimodal therapy and the higher recurrence rate, despite an aggressive approach, make HNSCC a research topic of interest for improving therapeutic outcomes, even if it is only the sixth most common malignancy worldwide. New advances in molecular biology and genetics include the involvement of miRNA in the control of the p53 pathway, the understanding of mechanisms such as gain/loss of function, and the development of different methods to restore p53 function, especially for HPV-negative cases. The different ratio between mutant p53 status in the primary tumor and distant metastasis originating HNSCC may serve to select the best therapeutic target for activating an abscopal effect by radiotherapy as a "booster" of the immune system. P53 may also be a key player in choosing radiotherapy fractionation regimens. Targeting any pathway involving p53, including tumor metabolism, in particular the Warburg effect, could modulate the radiosensitivity and chemo-sensitivity of head and neck cancers.

HMG-CoA Reductase Inhibitor Statins Activate the Transcriptional Activity of p53 by Regulating the Expression of TAZ

Transcriptional coactivator with PDZ-binding motif (TAZ) is a downstream transcriptional regulator of the Hippo pathway that controls cell growth and differentiation. The aberrant activation of TAZ correlates with a poor prognosis in human cancers, such as breast and colon cancers. We previously demonstrated that TAZ inhibited the tumor suppressor functions of p53 and enhanced cell proliferation. Statins, which are used to treat dyslipidemia, have been reported to suppress the activity of TAZ and exert anti-tumor effects. In the present study, we focused on the regulation of p53 functions by TAZ and investigated whether statins modulate these functions via TAZ. The results obtained suggest that statins, such as simvastatin and fluvastatin, activated the transcriptional function of p53 by suppressing TAZ protein expression. Furthermore, co-treatment with simvastatin and anti-tumor agents that cooperatively activate p53 suppressed cancer cell survival. These results indicate a useful mechanism by which statins enhance the effects of anti-tumor agents through the activation of p53 and may represent a novel approach to cancer therapy.

Role of p53 in Regulating Radiation Responses

p53 is known as the guardian of the genome and plays various roles in DNA damage and cancer suppression. The p53 gene was found to express multiple p53 splice variants (isoforms) in a physiological, tissue-dependent manner. The various genes that up- and down-regulated p53 are involved in cell viability, senescence, inflammation, and carcinogenesis. Moreover, p53 affects the radioadaptive response. Given that several studies have already been published on p53, this review presents its role in the response to gamma irradiation by interacting with MDM2, NF-κB, and miRNA, as well as in the inflammation processes, senescence, carcinogenesis, and radiation adaptive responses. Finally, the potential of p53 as a biomarker is discussed.

KDM6B interacts with TFDP1 to activate P53 signalling in regulating mouse palatogenesis

Epigenetic regulation plays extensive roles in diseases and development. Disruption of epigenetic regulation not only increases the risk of cancer, but can also cause various developmental defects. However, the question of how epigenetic changes lead to tissue-specific responses during neural crest fate determination and differentiation remains understudied. Using palatogenesis as a model, we reveal the functional significance of Kdm6b, an H3K27me3 demethylase, in regulating mouse embryonic development. Our study shows that Kdm6b plays an essential role in cranial neural crest development, and loss of Kdm6b disturbs P53 pathway-mediated activity, leading to complete cleft palate along with cell proliferation and differentiation defects in mice. Furthermore, activity of H3K27me3 on the promoter of Trp53 is antagonistically controlled by Kdm6b, and Ezh2 in cranial neural crest cells. More importantly, without Kdm6b, the transcription factor TFDP1, which normally binds to the promoter of Trp53, cannot activate Trp53 expression in palatal mesenchymal cells. Furthermore, the function of Kdm6b in activating Trp53 in these cells cannot be compensated for by the closely related histone demethylase Kdm6a. Collectively, our results highlight the important role of the epigenetic regulator KDM6B and how it specifically interacts with TFDP1 to achieve its functional specificity in regulating Trp53 expression, and further provide mechanistic insights into the epigenetic regulatory network during organogenesis.

CCPE: cell cycle pseudotime estimation for single cell RNA-seq data

Pseudotime analysis from scRNA-seq data enables to characterize the continuous progression of various biological processes, such as the cell cycle. Cell cycle plays an important role in cell fate decisions and differentiation and is often regarded as a confounder in scRNA-seq data analysis when analyzing the role of other factors. Therefore, accurate prediction of cell cycle pseudotime and identification of cell cycle stages are important steps for characterizing the development-related biological processes. Here, we develop CCPE, a novel cell cycle pseudotime estimation method to characterize cell cycle timing and identify cell cycle phases from scRNA-seq data. CCPE uses a discriminative helix to characterize the circular process of the cell cycle and estimates each cell's pseudotime along the cell cycle. We evaluated the performance of CCPE based on a variety of simulated and real scRNA-seq datasets. Our results indicate that CCPE is an effective method for cell cycle estimation and competitive in various applications compared with other existing methods. CCPE successfully identified cell cycle marker genes and is robust to dropout events in scRNA-seq data. Accurate prediction of the cell cycle using CCPE can also effectively facilitate the removal of cell cycle effects across cell types or conditions.

Restoring p53 Function in Head and Neck Squamous Cell Carcinoma to Improve Treatments

TP53 mutation is one of the most frequent genetic alterations in head and neck squamous cell carcinoma (HNSCC) and results in an accumulation of p53 protein in tumor cells. This makes p53 an attractive target to improve HNSCC therapy by restoring the tumor suppressor activity of this protein. Therapeutic strategies targeting p53 in HNSCC can be divided into three categories related to three subtypes encompassing WT p53, mutated p53 and HPV-positive HNSCC. First, compounds targeting degradation or direct inhibition of WT p53, such as PM2, RITA, nutlin-3 and CH1iB, achieve p53 reactivation by affecting p53 inhibitors such as MDM2 and MDMX/4 or by preventing the breakdown of p53 by inhibiting the proteasomal complex. Second, compounds that directly affect mutated p53 by binding it and restoring the WT conformation and transcriptional activity (PRIMA-1, APR-246, COTI-2, CP-31398). Third, treatments that specifically affect HPV+ cancer cells by targeting the viral enzymes E6/E7 which are responsible for the breakdown of p53 such as Ad-E6/E7-As and bortezomib. In this review, we describe and discuss p53 regulation and its targeting in combination with existing therapies for HNSCC through a new classification of such cancers based on p53 mutation status and HPV infection.

The Role of p53 Protein in the Realization of the Exogenous Heat Shock Protein 70 Anti-Apoptotic Effect during Axotomy

The search for effective neuroprotective agents for the treatment of neurotrauma has always been of great interest to researchers around the world. Extracellular heat shock protein 70 (eHsp70) is considered a promising agent to study, as it has been demonstrated to exert a significant neuroprotective activity against various neurodegenerative diseases. We showed that eHsp70 can penetrate neurons and glial cells when added to the incubation medium, and can accumulate in the nuclei of neurons and satellite glial cells after axotomy. eHsp70 reduces apoptosis and necrosis of the glial cells, but not the neurons. At the same time, co-localization of eHsp70 with p53 protein, one of the key regulators of apoptosis, was noted. eHsp70 reduces the level of the p53 protein apoptosis promoter both in glial cells and in the nuclei and cytoplasm of neurons, which indicates its neuroprotective effect. The ability of eHsp70 to reverse the proapoptotic effect of the p53 activator WR1065 may indicate its ability to regulate p53 activity or its proteosome-dependent degradation.

Combination Strategies to Improve Targeted Radionuclide Therapy

In recent years, targeted radionuclide therapy (TRT) has emerged as a promising strategy for cancer treatment. In contrast to conventional radiotherapy, TRT delivers ionizing radiation to tumors in a targeted manner, reducing the dose that healthy tissues are exposed to. Existing TRT strategies include the use of 177Lu-DOTATATE, 131I-metaiodobenzylguanidine, Bexxar, and Zevalin, clinically approved agents for the treatment of neuroendocrine tumors, neuroblastoma, and non-Hodgkin lymphoma, respectively. Although promising results have been obtained with these agents, clinical evidence acquired to date suggests that only a small percentage of patients achieves complete response. Consequently, there have been attempts to improve TRT outcomes through combinations with other therapeutic agents; such strategies include administering concurrent TRT and chemotherapy, and the use of TRT with known or putative radiosensitizers such as poly(adenosine diphosphate ribose) polymerase and mammalian-target-of-rapamycin inhibitors. In addition to potentially achieving greater therapeutic effects than the respective monotherapies, these strategies may lead to lower dosages or numbers of cycles required and, in turn, reduce unwanted toxicities. As of now, several clinical trials have been conducted to assess the benefits of TRT-based combination therapies, sometimes despite limited preclinical evidence being available in the public domain to support their use. Although some clinical trials have yielded promising results, others have shown no clear survival benefit from particular combination treatments. Here, we present a comprehensive review of combination strategies with TRT reported in the literature to date and evaluate their therapeutic potential.

Pinning Control for the p53-Mdm2 Network Dynamics Regulated by p14ARF

p53 regulates the cellular response to genotoxic damage and prevents carcinogenic events. Theoretical and experimental studies state that the p53-Mdm2 network constitutes the core module of regulatory interactions activated by cellular stress induced by a variety of signaling pathways. In this paper, a strategy to control the p53-Mdm2 network regulated by p14ARF is developed, based on the pinning control technique, which consists into applying local feedback controllers to a small number of nodes (pinned ones) in the network. Pinned nodes are selected on the basis of their importance level in a topological hierarchy, their degree of connectivity within the network, and the biological role they perform. In this paper, two cases are considered. For the first case, the oscillatory pattern under gamma-radiation is recovered; afterward, as the second case, increased expression of p53 level is taken into account. For both cases, the control law is applied to p14ARF (pinned node based on a virtual leader methodology), and overexpressed Mdm2-mediated p53 degradation condition is considered as carcinogenic initial behavior. The approach in this paper uses a computational algorithm, which opens an alternative path to understand the cellular responses to stress, doing it possible to model and control the gene regulatory network dynamics in two different biological contexts. As the main result of the proposed control technique, the two mentioned desired behaviors are obtained.

Model-based optimization of combination protocols for irradiation-insensitive cancers

Alternations in the p53 regulatory network may render cancer cells resistant to the radiation-induced apoptosis. In this theoretical study we search for the best protocols combining targeted therapy with radiation to treat cancers with wild-type p53, but having downregulated expression of PTEN or overexpression of Wip1 resulting in resistance to radiation monotherapy. Instead of using the maximum tolerated dose paradigm, we exploit stochastic computational model of the p53 regulatory network to calculate apoptotic fractions for both normal and cancer cells. We consider combination protocols, with irradiations repeated every 12, 18, 24, or 36 h to find that timing between Mdm2 inhibitor delivery and irradiation significantly influences the apoptotic cell fractions. We assume that uptake of the inhibitor is higher by cancer than by normal cells and that cancer cells receive higher irradiation doses from intersecting beams. These two assumptions were found necessary for the existence of protocols inducing massive apoptosis in cancer cells without killing large fraction of normal cells neighboring tumor. The best found protocols have irradiations repeated every 24 or 36 h with two inhibitor doses per irradiation cycle, and allow to induce apoptosis in more than 95% of cancer cells, killing less than 10% of normal cells.

TP53 mutations in head and neck cancer cells determine the Warburg phenotypic switch creating metabolic vulnerabilities and therapeutic opportunities for stratified therapies

Patients with mutated TP53 have been identified as having comparatively poor outcomes compared to those retaining wild-type p53 in many cancers, including squamous cell carcinomas of the head and neck (SCCHN). We have examined the role of p53 in regulation of metabolism in SCCHN cells and find that loss of p53 function determines the Warburg effect in these cells. Moreover, this metabolic adaptation to loss of p53 function creates an Achilles' heel for tumour cells that can be exploited for potential therapeutic benefit. Specifically, cells lacking normal wild-type p53 function, whether through mutation or RNAi-mediated downregulation, display a lack of metabolic flexibility, becoming more dependent on glycolysis and losing the ability to increase energy production from oxidative phosphorylation. Thus, cells that have compromised p53 function can be sensitised to ionizing radiation by pre-treatment with a glycolytic inhibitor. These results demonstrate the deterministic role of p53 in regulating energy metabolism and provide proof of principle evidence for an opportunity for patient stratification based on p53 status that can be exploited therapeutically using current standard of care treatment with ionising radiation.

BH3 mimetics selectively eliminate chemotherapy-induced senescent cells and improve response in TP53 wild-type breast cancer

TP53 wild-type breast tumors rarely undergo a complete pathological response after chemotherapy treatment. These patients have an extremely poor survival rate and studies show these tumors preferentially undergo senescence instead of apoptosis. These senescent cells persist after chemotherapy and secrete cytokines and chemokines comprising the senescence associated secretory phenotype, which promotes survival, proliferation, and metastasis. We hypothesized that eliminating senescent tumor cells would improve chemotherapy response and extend survival. Previous studies have shown "senolytic" agents selectively kill senescent normal cells, but their efficacy in killing chemotherapy-induced senescent cancer cells is unknown. We show that ABT-263, a BH3 mimetic that targets antiapoptotic proteins BCL2/BCL-XL/BCL-W, had no effect on proliferating cells, but rapidly and selectively induced apoptosis in a subset of chemotherapy-treated cancer cells, though sensitivity required days to develop. Low NOXA expression conferred resistance to ABT-263 in some cells, necessitating additional MCL1 inhibition. Gene editing confirmed breast cancer cells relied on BCL-XL or BCL-XL/MCL1 for survival in senescence. In a mouse model of breast cancer, ABT-263 treatment following chemotherapy led to apoptosis, greater tumor regression, and longer survival. Our results reveal cancer cells that have survived chemotherapy by entering senescence can be eliminated using BH3 mimetic drugs that target BCL-XL or BCL-XL/MCL1. These drugs could help minimize residual disease and extend survival in breast cancer patients that otherwise have a poor prognosis and are most in need of improved therapies.

Oncogene-induced senescence: From biology to therapy

Oncogene-induced senescence (OIS) is a powerful intrinsic tumor-suppressive mechanism, arresting cell cycle progression upon oncogene-activating genomic alterations. The discovery and characterization of the senescence-associated secretome unveiled a rich additional complexity to the senescence phenotype, including extrinsic impacts on the microenvironment and engagement of the immune response. Emerging evidence suggests that senescence phenotypes vary depending on the oncogenic stimulus. Therefore, understanding the mechanisms underlying OIS and how they are subverted in cancer will provide invaluable opportunities to identify alternative strategies for treating oncogene-driven cancers. In this review, we primarily discuss the key mechanisms governing OIS driven by the RAS/MAPK and PI3K/AKT pathways and how understanding the biology of senescent cells has uncovered new therapeutic possibilities to target cancer.

Pifithrin-α alters p53 post-translational modifications pattern and differentially inhibits p53 target genes

Pifithrin-α (PFT-α) is a small molecule which has been widely used as a specific inhibitor of p53 transcription activity. However, its molecular mechanism of action remains unclear. PFT-α has also been described to display potent p53-independent activity in cells. In this study, we addressed the mechanism of action of PFT-α. We found that PFT-α failed to prevent the effects of Mdm2 inhibitor Nutlin-3 on cell cycle and apoptosis in several cancer cell lines. However, PFT-α rescued normal primary fibroblasts from growth inhibition by Nutlin-3. PFT-α displayed a very limited effect on p53-dependent transcription upon its activation by Nutlin-3. Moreover, PFT-α inhibitory effect on transcription was highly dependent on the nature of the p53 target gene. PFT-α attenuated post-translational modifications of p53 without affecting total p53 protein level. Finally, we found that PFT-α can decrease the level of intracellular reactive oxygen species through activation of an aryl hydrocarbon receptor (AHR)-Nrf2 axis in a p53-independent manner. In conclusion, PFT-α inhibits only some aspects of p53 function, therefore it should be used with extreme caution to study p53-dependent processes.

MDM2 Inhibition in a Subset of Sarcoma Cell Lines Increases Susceptibility to Radiation Therapy by Inducing Senescence in the Polyploid Cells

Purpose

Inhibition of p53 by amplification of MDM2 is one of the key contributors in the oncogenesis of a subset of soft tissue sarcoma (STS) known as well-/dedifferentiated liposarcoma. A small molecule MDM2 antagonist, nutlin-3, induces the p53 pathway by disrupting the interaction between MDM2 and p53. Radiation therapy is an integral component for treating liposarcoma and induces p53. Based on wild-type TP53 status in liposarcoma, it was investigated whether MDM2 inhibition with irradiation led to enhanced reactivation of p53 and subsequent p53-mediated effects in liposarcoma.

Methods and Materials

Clonogenic assays, immunoblotting, flow cytometry/fluorescence-activated cell sorting, and senescence assays were employed in liposarcoma cell lines after co-treatment with nutlin-3 and radiation.

Results

Upon treatment with nutlin-3, 2 of the well-/dedifferentiated liposarcoma (MDM2^Amp/TP53^WT) cell lines displayed radiosensitivity with sensitization enhancement ratio values of >1. In contrast, the cell line with mutant TP53 showed sensitization enhancement ratio values of ∼1. Immunoblotting revealed induced reactivation of the p53-MDM2-p21 signaling axis in response to combination therapy in all cell lines with wild-type TP53. Removal of MDM2 inhibitor (with or without radiation therapy) led to the emergence of ploidy-based heterogeneous subpopulations (4N and >4N) in TP53 wild-type cells and not in TP53 mutant cells. Immunoblotting of cell cycle markers (G1, G2/M) revealed the generation of 4N G1 cells. Sorting and long-term fate analysis of different populations (2N, 4N, and >4N) by colony assay displayed attenuated colony-forming potential and augmented senescence of the 4N and >4N cells contributing to the radiosensitization effect.

Conclusions

Nutlin-3 increases the vulnerability of liposarcoma cell lines to radiation by augmented activation of p53. The cells underwent senescence. Presence and activation of p53 are required for exertion of the radiosensitizing effect by nutlin-3, but this is not the sole determinant of the effect. This study opens avenues for the clinical translation in a stratified group of patients with liposarcoma.

The Benefit of Reactivating p53 under MAPK Inhibition on the Efficacy of Radiotherapy in Melanoma

Radiotherapy (RT) in patients with melanoma historically showed suboptimal results, because the disease is often radioresistant due to various mechanisms such as scavenging free radicals by thiols, pigmentary machinery, or enhanced DNA repair. However, radiotherapy has been utilized as adjuvant therapy after the complete excision of primary melanoma and lymph nodes to reduce the rate of nodal recurrences in high-risk patients. The resistance of melanoma cells to radiotherapy may also be in relation with the constitutive activation of the MAPK pathway and/or with the inactivation of p53 observed in about 90% of melanomas. In this study, we aimed to assess the potential benefit of adding RT to BRAF-mutated melanoma cells under a combined p53 reactivation and MAPK inhibition in vitro and in a preclinical animal model. We found that the combination of BRAF inhibition (vemurafenib, which completely shuts down the MAPK pathway), together with p53 reactivation (PRIMA-1Met) significantly enhanced the radiosensitivity of BRAF-mutant melanoma cells. This was accompanied by an increase in both p53 expression and activity. Of note, we found that radiation alone markedly promoted both ERK and AKT phosphorylation, thus contributing to radioresistance. The combination of vemurafenib and PRIMA-1Met caused the inactivation of both MAPK kinase and PI3K/AKT pathways. Furthermore, when combined with radiotherapy, it was able to significantly enhance melanoma cell radiosensitivity. Interestingly, in nude mice bearing melanoma xenografts, the latter triple combination had not only a synergistic effect on tumor growth inhibition, but also a potent control on tumor regrowth in all animals after finishing the triple combination therapy. RT alone had only a weak effect. In conclusion, we provide a basis for a strategy that may overcome the radioresistance of BRAF-mutated melanoma cells to radiotherapy. Whether this will translate into a rational to use radiotherapy in the curative setting in BRAF-mutated melanoma patients deserves consideration.

Helping the Released Guardian: Drug Combinations for Supporting the Anticancer Activity of HDM2 (MDM2) Antagonists

The protein p53, known as the “Guardian of the Genome”, plays an important role in maintaining DNA integrity, providing protection against cancer-promoting mutations. Dysfunction of p53 is observed in almost every cancer, with 50% of cases bearing loss-of-function mutations/deletions in the TP53 gene. In the remaining 50% of cases the overexpression of HDM2 (mouse double minute 2, human homolog) protein, which is a natural inhibitor of p53, is the most common way of keeping p53 inactive. Disruption of HDM2-p53 interaction with the use of HDM2 antagonists leads to the release of p53 and expression of its target genes, engaged in the induction of cell cycle arrest, DNA repair, senescence, and apoptosis. The induction of apoptosis, however, is restricted to only a handful of p53^wt cells, and, generally, cancer cells treated with HDM2 antagonists are not efficiently eliminated. For this reason, HDM2 antagonists were tested in combinations with multiple other therapeutics in a search for synergy that would enhance the cancer eradication. This manuscript aims at reviewing the recent progress in developing strategies of combined cancer treatment with the use of HDM2 antagonists.

A protein folding molecular imaging biosensor monitors the effects of drugs that restore mutant p53 structure and its downstream function in glioblastoma cells

Misfolding mutations in the DNA-binding domain of p53 alter its conformation, affecting the efficiency with which it binds to chromatin to regulate target gene expression and cell cycle checkpoint functions in many cancers, including glioblastoma. Small molecule drugs that recover misfolded p53 structure and function may improve chemotherapy by activating p53-mediated senescence. We constructed and optimized a split Renilla luciferase (RLUC) complementation molecular biosensor (NRLUC-p53-CRLUC) to determine small molecule-meditated folding changes in p53 protein. After initial evaluation of the biosensor in three different cells lines, we engineered endogenously p53^P98L mutant (i.e. not affecting the DNA-binding domain) Ln229 glioblastoma cells, to express the biosensor containing one of four different p53 proteins: p53^wt, p53^Y220C, p53^G245S and p53^R282W. We evaluated the consequent phenotypic changes in these four variant cells as well as the parental cells after exposure to PhiKan083 and SCH529074, drugs previously reported to activate mutant p53 folding. Specifically, we measured induced RLUC complementation and consequent therapeutic response. Upon stable transduction with the p53 biosensors, we demonstrated that these originally p53^P98L Ln229 cells had acquired p53 cellular phenotypes representative of each p53 protein expressed within the biosensor fusion protein. In these engineered variants we found a differential drug response when treated with doxorubicin and temozolomide, either independently or in combination with PhiKan083 or SCH529074. We thus developed a molecular imaging complementation biosensor that mimics endogenous p53 function for use in future applications to screen novel or repurposed drugs that counter the effects of misfolding mutations responsible for oncogenic structural changes in p53.

Pre-clinical efficacy and synergistic potential of the MDM2-p53 antagonists, Nutlin-3 and RG7388, as single agents and in combined treatment with cisplatin in ovarian cancer

Ovarian cancer is the fifth leading cause of cancer-related female deaths. Due to serious side effects, relapse and resistance to standard chemotherapy, better and more targeted approaches are required. Mutation of the TP53 gene accounts for 50% of all human cancers. In the remaining malignancies, non-genotoxic activation of wild-type p53 by small molecule inhibition of the MDM2-p53 binding interaction is a promising therapeutic strategy. Proof of concept was established with the cis-imidazoline Nutlin-3, leading to the development of RG7388 and other compounds currently in early phase clinical trials. This preclinical study evaluated the effect of Nutlin-3 and RG7388 as single agents and in combination with cisplatin in a panel of ovarian cancer cell lines. Median-drug-effect analysis showed Nutlin-3 or RG7388 combination with cisplatin was additive to, or synergistic in a p53-dependent manner, resulting in increased p53 activation, cell cycle arrest and apoptosis, associated with increased p21^WAF1 protein and/or caspase-3/7 activity compared to cisplatin alone. Although MDM2 inhibition activated the expression of p53-dependent DNA repair genes, the growth inhibitory and pro-apoptotic effects of p53 dominated the response. These data indicate that combination treatment with MDM2 inhibitors and cisplatin has synergistic potential for the treatment of ovarian cancer, dependent on cell genotype.

Significance of Wild-Type p53 Signaling in Suppressing Apoptosis in Response to Chemical Genotoxic Agents: Impact on Chemotherapy Outcome

Our genomes are subject to potentially deleterious alterations resulting from endogenous sources (e.g., cellular metabolism, routine errors in DNA replication and recombination), exogenous sources (e.g., radiation, chemical agents), and medical diagnostic and treatment applications. Genome integrity and cellular homeostasis are maintained through an intricate network of pathways that serve to recognize the DNA damage, activate cell cycle checkpoints and facilitate DNA repair, or eliminate highly injured cells from the proliferating population. The wild-type p53 tumor suppressor and its downstream effector p21^WAF1 (p21) are key regulators of these responses. Although extensively studied for its ability to control cell cycle progression, p21 has emerged as a multifunctional protein capable of downregulating p53, suppressing apoptosis, and orchestrating prolonged growth arrest through stress-induced premature senescence. Studies with solid tumors and solid tumor-derived cell lines have revealed that such growth-arrested cancer cells remain viable, secrete growth-promoting factors, and can give rise to progeny with stem-cell-like properties. This article provides an overview of the mechanisms by which p53 signaling suppresses apoptosis following genotoxic stress, facilitating repair of genomic injury under physiological conditions but having the potential to promote tumor regrowth in response to cancer chemotherapy.

LC-MS/MS-based targeted proteomics quantitatively detects the interaction between p53 and MDM2 in breast cancer

In breast cancer, p53 could be functionally compromised by interaction with several proteins. Among those proteins, MDM2 serves as a pivotal negative regulator and counteracts p53 activation. Thus, the ability to quantitatively and accurately monitor the changes in level of p53-MDM2 interaction with disease state can enable an improved understanding of this protein-protein interaction (PPI), provide a better insight into cancer development and allow the emergence of advanced treatments. However, rare studies have evaluated the quantitative extent of PPI including p53-MDM2 interaction so far. In this study, a LC-MS/MS-based targeted proteomics assay was developed and coupled with co-immunoprecipitation (Co-IP) for the quantification of p53-MDM2 complex. A p53 antibody with the epitope residing at 156-214 residues achieved the greatest IP efficiency. 321KPLDGEYFTLQIR333 (p53) and 327ENWLPEDK334 (MDM2) were selected as surrogate peptides in the targeted analysis. Stable isotope-labeled synthetic peptides were used as internal standards. An LOQ (limit of quantification) of 2ng/mL was obtained. Then, the assay was applied to quantitatively detect total p53, total MDM2 and p53-MDM2 in breast cells and tissue samples. Western blotting was performed for a comparison. Finally, a quantitative time-course analysis in MCF-7 cells with the treatment of nutlin-3 as a PPI inhibitor was also monitored.

BIOLOGICAL SIGNIFICANCE

Proteins do not function as single entities but rather as a team player that has to communicate. Protein-protein interaction (PPI), normally by means of non-covalent contact among binary or large protein complex, is essential for many cellular processes including cancer progression. Thus, the ability to quantitatively and accurately monitor the changes in level of PPI with disease state can enable an improved understanding of PPI, provide a better insight into cancer development and allow the emergence of advanced treatments. However, rare studies have evaluated the quantitative extent of PPI so far. The major issue of current available approaches is the trade-off between sensitivity and specificity. Thus, techniques with the ability to quantify PPIs with both high sensitivity (low false-negative rate) and high specificity (low false-positive rate) are eagerly desired. Liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based targeted proteomics has shown its potential to study biomolecules because of its high sensitivity, high selectivity and wide dynamic range. In this study, we made an effort to develop a LC-MS/MS-based targeted proteomics assay for the quantitative detection of p53-MDM2 interaction in breast cells and tissue samples.

A targeted proteomics approach to the quantitative analysis of ERK/Bcl-2-mediated anti-apoptosis and multi-drug resistance in breast cancer

Apoptosis suppression caused by overexpression of anti-apoptotic proteins is a central factor to the acquisition of multi-drug resistance (MDR) in breast cancer. As a highly conserved anti-apoptotic protein, Bcl-2 can initiate an anti-apoptosis response via an ERK1/2-mediated pathway. However, the details therein are still far from completely understood and a quantitative description of the associated proteins in the biological context may provide more insights into this process. Following our previous attempts in the quantitative analysis of MDR mechanisms, liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based targeted proteomics was continually employed here to describe ERK/Bcl-2-mediated anti-apoptosis. A targeted proteomics assay was developed and validated first for the simultaneous quantification of ERK1/2 and Bcl-2. In particular, ERK isoforms (i.e., ERK1 and ERK2) and their differential phosphorylated forms including isobaric ones were distinguished. Using this assay, differential protein levels and site-specific phosphorylation stoichiometry were observed in parental drug-sensitive MCF-7/WT cancer cells and drug-resistant MCF-7/ADR cancer cells and breast tissue samples from two groups of patients who were either suspected or diagnosed to have drug resistance. In addition, quantitative analysis of the time course of both ERK1/2 and Bcl-2 in doxorubicin (DOX)-treated MCF-7/WT cells confirmed these findings. Overall, we propose that targeted proteomics can be used generally to resolve more complex cellular events.

Novel Candidate Biomarkers of Chemoradiosensitivity in Esophageal Squamous Cell Carcinoma: A Systematic Review

There is no doubt that, along with surgery, chemoradiotherapy is an important treatment for esophageal squamous cell carcinoma (ESCC). Patients who respond well to chemoradiotherapy obtain great benefits toward overcoming their cancer, and so a more favorable prognosis. On the other hand, patients who do not respond well have wasted valuable time and experienced severe toxicity and seriously diminished quality of life, only to have their cancer recur with an unfavorable prognosis. For this reason, a reliable biomarker of chemoradiosensitivity in ESCC has long been sought. In this review, we will enumerate recently reported candidate biomarkers of chemoradiosensitivity in ESCC that have the potential for future clinical application.

XI-011 enhances cisplatin-induced apoptosis by functional restoration of p53 in head and neck cancer

Head and neck cancer (HNC), one of the most common cancers worldwide, frequently involves mutation of the TP53 gene and dysregulation of the p53 pathway. Overexpression of MDM2 or MDM4 inactivates the tumor-suppressive function of p53. Restoration of p53 function that counteracts these p53 repressors can lead to in vivo tumor regression. Therefore, the present study assessed the ability of the small molecule p53 activator XI-011 (NSC146109) to induce apoptosis in HNC by restoring p53 function. We tested the effects of XI-011 treatment in HNC cell lines, either individually or in combination with cisplatin and assessed growth suppression, cell cycle arrest, and apoptosis. The drug effects on in vivo growth of HNC cells were examined in mice xenograft model. XI-011 exerted the highest growth suppression in tumor cells that overexpress MDM4, in which p53 is degraded. XI-011 treatment downregulated MDM4 mRNA and protein levels, and upregulated expression of proapoptotic genes and promoted apoptosis, in a dose-dependent manner. The apoptotic response was blocked by inhibition of p53 or expression of MDM4, demonstrating that the effects of XI-011 depend on p53 and MDM4. In combination treatments, XI-011 acted synergistically with cisplatin to inhibit growth of HNC cells in vitro and in vivo. MDM4 inhibition and functional restoration of p53 by XI-011 effectively enhanced cisplatin-induced cytotoxicity in HNC cells, an activity that suggests a promising strategy for treating HNC.

FBXO44-Mediated Degradation of RGS2 Protein Uniquely Depends on a Cullin 4B/DDB1 Complex

The ubiquitin-proteasome system for protein degradation plays a major role in regulating cell function and many signaling proteins are tightly controlled by this mechanism. Among these, Regulator of G Protein Signaling 2 (RGS2) is a target for rapid proteasomal degradation, however, the specific enzymes involved are not known. Using a genomic siRNA screening approach, we identified a novel E3 ligase complex containing cullin 4B (CUL4B), DNA damage binding protein 1 (DDB1) and F-box protein 44 (FBXO44) that mediates RGS2 protein degradation. While the more typical F-box partners CUL1 and Skp1 can bind FBXO44, that E3 ligase complex does not bind RGS2 and is not involved in RGS2 degradation. These observations define an unexpected DDB1/CUL4B-containing FBXO44 E3 ligase complex. Pharmacological targeting of this mechanism provides a novel therapeutic approach to hypertension, anxiety, and other diseases associated with RGS2 dysregulation.

Murine double minute 2 predicts response of advanced esophageal squamous cell carcinoma to definitive chemoradiotherapy

Background

Definitive chemoradiotherapy (dCRT) has recently become one of the most effective therapies for the treatment of esophageal squamous cell carcinoma (ESCC). However, it is also true this treatment has not been effective in all patients. Therefore, it is very important to evaluate the surrogate marker of dCRT in order to improve clinical outcomes of patients with ESCC. On the other hand, our previous study had suggested that murine double minute 2 (MDM2) and p16 were associated with chemoradioresistance in ESCC.

Methods

We selected pretreatment biopsy specimens of ESCC patients from our prospective clinical study on dCRT. Seventy-nine cases histologically diagnosed as ESCC were used. We immunohistochemically investigated these specimens using antibodies against MDM2, p53, p16, and Ki-67.

Results

The patients included 68 males and 11 females with a mean age of 63.3 years. The number of patients in each clinical stage was as follows: 22 in c-Stage I; 17 in c-Stage II; and 40 in c-Stage III. cT, cN, and cStage were significantly more advanced in the Failure group (including patients with persistent and recurrent disease after dCRT) than in the complete response (CR) group (patients with persistent CR after dCRT). The clinical stage inversely correlated with the CR rate and the rescue rate after failure. The overall survival rate was significantly worse in the patients with advanced cT, cN, and cStage levels, and in the Failure group. MDM2 positivity was significantly higher in the Failure group than in the CR group in cStageIII (P = 0.014). The number of patients with an absence of p16 immunoreactivity was significantly higher in the Failure group than in the CR group in cStageIII (P = 0.010) but not in cStageI or cStageII. Moreover, the overall survival with a Ki-67 ≥ 33.7% was significantly better than that with <33.7% for patients in cStageIII (P = 0.024).

Conclusions

The results of this study suggested that MDM2 and p16 are predictive markers for chemoradioresistance in cStageIII ESCC and Ki-67 is a prognostic marker following dCRT in cStageIII ESCC. These issues could contribute to the formulation of treatment strategy for patients with advanced ESCC.

The Hsp90 inhibitor 17-(allylamino)-17-demethoxygeldanamycin increases cisplatin antitumor activity by inducing p53-mediated apoptosis in head and neck cancer

The tumor suppressor p53 is often inactivated in head and neck cancer (HNC) through TP53 mutations or overexpression of mouse double minute 2 or mouse double minute X. Restoration of p53 function by counteracting these p53 repressors is a promising strategy for cancer treatment. The present study assessed the ability of a heat shock protein 90 (Hsp90) inhibitor, 17-(Allylamino)-17-demethoxygeldanamycin (17AAG), to induce apoptosis in HNC by restoring p53 function. The effect of 17AAG, alone or in combination with Nutlin-3a or cisplatin, was assessed in HNC cells using growth and apoptosis, immunoblotting, quantitative reverse transcription-polymerase chain reaction, and preclinical tumor xenograft models. 17AAG activated and stabilized p53 in HNC cells bearing wild-type TP53 by disrupting the p53–MDMX interaction. 17AAG upregulated p21 and proapoptotic gene expression, and promoted apoptosis in a concentration-dependent manner. Growth inhibition by 17AAG was highest in tumor cells with MDMX overexpression. The apoptotic response was blocked by inhibition of p53 expression, demonstrating that the effect of 17AAG depended on p53 and MDMX. 17AAG synergized in vitro with Nutlin-3a and in vitro and in vivo with cisplatin to induce p53-mediated apoptosis. 17AAG effectively induced p53-mediated apoptosis in HNC cells through MDMX inhibition and increased the antitumor activity of cisplatin synergistically, suggesting a promising strategy for treating HNC.

Activation of p53 with Nutlin-3a radiosensitizes lung cancer cells via enhancing radiation-induced premature senescence

Radiotherapy is routinely used for the treatment of lung cancer. However, the mechanisms underlying ionizing radiation (IR)-induced senescence and its role in lung cancer treatment are poorly understood. Here, we show that IR suppresses the proliferation of human non-small cell lung cancer (NSCLC) cells via an apoptosis-independent mechanism. Further investigations reveal that the anticancer effect of irradiation correlates well with IR-induced premature senescence, as evidenced by increased senescence-associated β-glactosidase (SA-β-gal) staining, decreased BrdU incorporation and elevated expression of p16(INK4a) (p16) in irradiated NSCLC cells. Mechanistic studies indicate that the induction of senescence is associated with activation of the p53-p21 pathway, and that inhibition of p53 transcriptional activity by PFT-α attenuates IR-induced tumor cell killing and senescence. Gain-of-function assays demonstrate that restoration of p53 expression sensitizes H1299 cells to irradiation, whereas knockdown of p53 expression by siRNA inhibits IR-induced senescence in H460 cells. Furthermore, treatment with Nutlin-3a, a small molecule inhibitor of MDM2, enhances IR-induced tumor cell killing and senescence by stabilizing the activation of the p53-p21 signaling pathway. Taken together, these findings demonstrate for the first time that pharmacological activation of p53 by Nutlin-3a can sensitize lung cancer cells to radiation therapy via promoting IR-induced premature senescence.

Nutlin-3a, an MDM2 antagonist and p53 activator, helps to preserve the replicative potential of cancer cells treated with a genotoxic dose of resveratrol

Resveratrol is a natural compound that has been intensely studied due to its role in cancer prevention and potential as an anti-cancer therapy. Its effects include induction of apoptosis and senescence-like growth inhibition. Here, we report that two cancer cell lines (U-2 OS and A549) differ significantly in their molecular responses to resveratrol. Specifically, in U-2 OS cells, the activation of the p53 pathway is attenuated when compared to the activation in A549 cells. This attenuation is accompanied by a point mutation (458: CGA→TGA) in the PPM1D gene and overexpression of the encoded protein, which is a negative regulator of p53. Experimentally induced knockdown of PPM1D in U-2 OS cells resulted in slightly increased activation of the p53 pathway, most clearly visible as stronger phosphorylation of p53 Ser37. When treated with nutlin-3a, a non-genotoxic activator of p53, U-2 OS and A549 cells both responded with substantial activation of the p53 pathway. Nutlin-3a improved the clonogenic survival of both cell lines treated with resveratrol. This improvement was associated with lower activation of DNA-damage signaling (phosphorylation of ATM, CHK2, and histone H2AX) and higher accumulation of cells in the G1 phase of the cell cycle. Thus, the hyperactivation of p53 by nutlin-3a helps to preserve the replicative potential of cells exposed to resveratrol.

Radiosensitization of wildtype p53 cancer cells by the MDM2-inhibitor PXN727 is associated with altered heat shock protein 70 (Hsp70) levels

The oncoprotein MDM2 (murine double minute 2) is often overexpressed in human tumors and thereby attenuates the function of the tumor suppressor p53. In this study, we investigated the effects of the novel MDM2-inhibitor PXN727 on p53 activation, cell proliferation, cell cycle distribution and radiosensitivity. Since the localization of heat shock protein 70 (Hsp70) exerts different effects on radioresistance of tumor cells, we investigated the impact of PXN727 on intracellular, membrane, and secreted Hsp70 levels. We could show that PXN727 exerts its effects on wildtype p53 (HCT116 p53⁺/⁺, A549) but not p53 depleted (HCT116 p53⁻/⁻) or mutated (FaDu) tumor cells. PXN727 activates p53, induces the expression of p21, reduces the proportion of cells in the radioresistant S-phase and induces senescence. Radiosensitivity was significantly increased by PXN727 in HCT116 p53⁺/⁺ tumor cells. Furthermore, PXN727 causes a downregulation of Hsp70 membrane expression and an upregulated secretion of Hsp70 in wildtype p53 tumor cells. Our data suggest that re-activation of p53 by MDM2-inhibition modulates Hsp70 membrane expression and secretion which might contribute to the radiosensitizing effect of the MDM2-inhibitor PXN727.

The p53-reactivating small-molecule RITA enhances cisplatin-induced cytotoxicity and apoptosis in head and neck cancer

We evaluated whether the restoration of p53 function by the p53-reactivating small molecule RITA (reactivation of p53 and induction of tumor cell apoptosis enhances cisplatin-induced cytotoxicity and apoptosis in head-and-neck cancer (HNC). RITA induced prominent accumulation and reactivation of p53 in a wild-type TP53-bearing HNC cell line. RITA showed maximal growth suppression in tumor cells showing MDM2-dependent p53 degradation. RITA promoted apoptosis in association with upregulation of p21, BAX, and cleaved caspase-3; notably, the apoptotic response was blocked by pifithrin-α, demonstrating its p53 dependence. With increasing concentrations, RITA strongly induced apoptosis rather than G2-phase arrest. In combination therapy, RITA enhanced cisplatin-induced growth inhibition and apoptosis of HNC cells invitro and in vivo. Our data suggest that the restoration of p53 tumor-suppressive function by RITA enhances the cytotoxicity and apoptosis of cisplatin, an action that may offer an attractive strategy for treating HNC.

Integrin α5β1 plays a critical role in resistance to temozolomide by interfering with the p53 pathway in high-grade glioma

Integrins play a role in the resistance of advanced cancers to radiotherapy and chemotherapy. In this study, we show that high expression of the α5 integrin subunit compromises temozolomide-induced tumor suppressor p53 activity in human glioblastoma cells. We found that depletion of the α5 integrin subunit increased p53 activity and temozolomide sensitivity. However, when cells were treated with the p53 activator nutlin-3a, the protective effect of α5 integrin on p53 activation and cell survival was lost. In a functional p53 background, nutlin-3a downregulated the α5 integrin subunit, thereby increasing the cytotoxic effect of temozolomide. Clinically, α5β1 integrin expression was associated with a more aggressive phenotype in brain tumors, and high α5 integrin gene expression was associated with decreased survival of patients with high-grade glioma. Taken together, our findings indicate that negative cross-talk between α5β1 integrin and p53 supports glioma resistance to temozolomide, providing preclinical proof-of-concept that α5β1 integrin represents a therapeutic target for high-grade brain tumors. Direct activation of p53 may remain a therapeutic option in the subset of patients with high-grade gliomas that express both functional p53 and a high level of α5β1 integrin.

MDM2 interacts with NME2 (non-metastatic cells 2, protein) and suppresses the ability of NME2 to negatively regulate cell motility

MDM2 expression, combined with increased p53 expression, is associated with reduced survival in several cancers, but is particularly of interest in renal cell carcinoma (RCC) where evidence suggests the presence of tissue-specific p53/MDM2 pathway defects. We set out to identify MDM2-interacting proteins in renal cells that could act as mediators/targets of MDM2 oncogenic effects in renal cancers. We identified the non-metastatic cells 2, protein; NME2 (NDPK-B, NM23-B/-H2), a nucleoside diphosphate kinase, as an MDM2-interacting protein using both a proteomic-based strategy [affinity chromatography and tandem mass spectrometry [MS/MS] from HEK293 cells] and a yeast two-hybrid screen of a renal carcinoma cell-derived complementary DNA library. The MDM2-NME2 interaction is highly specific, as NME1 (87.5% amino acid identity) does not interact with MDM2 in yeast. Specific NME proteins display well-documented cell motility and metastasis-suppressing activity. We show that NME2 contributes to motility suppression under conditions where MDM2 is expressed at normal physiological/low levels. However, up-regulation of MDM2 in RCC cells abolishes the ability of NME2 to suppress motility. Significantly, when MDM2 expression is down-regulated in these cells using small interfering RNA, the motility-suppressing activity of NME2 is rescued, confirming that MDM2 expression causes the loss of NME2 cell motility regulatory function. Thus MDM2 up-regulation in renal cancer cells can act in a dominant manner to abrogate the function of a potent suppressor of motility and metastasis. Our studies identify a novel protein-protein interaction between MDM2 and NME2, which suggests a mechanism that could explain the link between MDM2 expression and poor patient survival in RCC.

Pharmacologic activation of p53 by small-molecule MDM2 antagonists

Restoring p53 activity by inhibiting the interaction between p53 and MDM2 represents an attractive approach for cancer therapy. To this end, a number of small-molecule p53-MDM2 binding inhibitors have been developed during the past several years. Nutlin-3 is a potent and selective small-molecule MDM2 antagonist that has shown considerable promise in pre-clinical studies. This review will highlight recent advances in the development of small-molecule MDM2 antagonists as potential cancer therapeutics, with special emphasis on Nutlin-3.