Awakening guardian angels: drugging the p53 pathway

Reactive Oxygen Species Induced p53 Activation: DNA Damage, Redox Signaling, or Both?

Significance: The p53 tumor suppressor has been dubbed the "guardian of genome" because of its various roles in the response to DNA damage such as DNA damage repair, cell cycle arrest, senescence, and apoptosis, all of which are in place to prevent mutations from being passed on down the lineage. Recent Advances: Reactive oxygen species (ROS), for instance hydrogen peroxide derived from mitochondrial respiration, have long been regarded mainly as a major source of cellular damage to DNA and other macromolecules. Critical Issues: More recently, ROS have been shown to also play important physiological roles as second messengers in so-called redox signaling. It is, therefore, not clear whether the observed activation of p53 by ROS is mediated through the DNA damage response, redox signaling, or both. In this review, we will discuss the similarities and differences between p53 activation in response to DNA damage and redox signaling in terms of upstream signaling and downstream transcriptional program activation. Future Directions: Understanding whether and how DNA damage and redox signaling-dependent p53 activation can be dissected could be useful to develop anti-cancer therapeutic p53-reactivation strategies that do not depend on the induction of DNA damage and the resulting additional mutational load.

Targeting p53 in chronic lymphocytic leukemia

INTRODUCTION

Genomic studies have allowed to identify molecular predictors for chronic lymphocytic leukemia (CLL) treatment tailoring. TP53 disruption is the strongest predictor of chemo-refractoriness and its assessment is the first decisional node in the disease treatment algorithm.

AREAS COVERED

The review covers the p53 biological pathway, its genetic alterations and clinical implications in CLL, and its druggable targets. The potential therapeutic options for TP53 disrupted patients are described, including: i) agents circumventing TP53 disruption; ii) targeted therapies restoring the physiological function of mutant p53; and iii) medicines potentiating p53 function.

EXPERT OPINION

The key approach to improve CLL outcome is treatment tailoring in individual patients. BCR and BCL2 inhibitors have significantly improved CLL survival, however TP53 disrupted patients still have a less favorable outcome than wild type cases, possibly because these novel drugs do not directly target p53 and do not restore the function of the disrupted p53 pathway. Emerging innovative molecules in cancer are able to restore the p53 mutant protein and/or potentiate the activity of the p53 wild type protein. If these compounds were confirmed as efficacious also for CLL, they would represent another step forward in the care of high risk CLL patients with TP53 abnormalities.

Multifaceted Roles of TRIM Proteins in Colorectal Carcinoma

Colorectal cancer (CRC) is one of the most frequently diagnosed tumor in humans and one of the most common causes of cancer-related death worldwide. The pathogenesis of CRC follows a multistage process which together with somatic gene mutations is mainly attributed to the dysregulation of signaling pathways critically involved in the maintenance of homeostasis of epithelial integrity in the intestine. A growing number of studies has highlighted the critical impact of members of the tripartite motif (TRIM) protein family on most types of human malignancies including CRC. In accordance, abundant expression of many TRIM proteins has been observed in CRC tissues and is frequently correlating with poor survival of patients. Notably, some TRIM members can act as tumor suppressors depending on the context and the type of cancer which has been assessed. Mechanistically, most cancer-related TRIMs have a critical impact on cell cycle control, apoptosis, epithelial–mesenchymal transition (EMT), metastasis, and inflammation mainly through directly interfering with diverse oncogenic signaling pathways. In addition, some recent publications have emphasized the emerging role of some TRIM members to act as transcription factors and RNA-stabilizing factors thus adding a further level of complexity to the pleiotropic biological activities of TRIM proteins. The current review focuses on oncogenic signaling processes targeted by different TRIMs and their particular role in the development of CRC. A better understanding of the crosstalk of TRIMs with these signaling pathways relevant for CRC development is an important prerequisite for the validation of TRIM proteins as novel biomarkers and as potential targets of future therapies for CRC.

Advances in synthetic lethality for cancer therapy: cellular mechanism and clinical translation

Synthetic lethality is a lethal phenomenon in which the occurrence of a single genetic event is tolerable for cell survival, whereas the co-occurrence of multiple genetic events results in cell death. The main obstacle for synthetic lethality lies in the tumor biology heterogeneity and complexity, the inadequate understanding of synthetic lethal interactions, drug resistance, and the challenges regarding screening and clinical translation. Recently, DNA damage response inhibitors are being tested in various trials with promising results. This review will describe the current challenges, development, and opportunities for synthetic lethality in cancer therapy. The characterization of potential synthetic lethal interactions and novel technologies to develop a more effective targeted drug for cancer patients will be explored. Furthermore, this review will discuss the clinical development and drug resistance mechanisms of synthetic lethality in cancer therapy. The ultimate goal of this review is to guide clinicians at selecting patients that will receive the maximum benefits of DNA damage response inhibitors for cancer therapy.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

Multifaceted behavior of PEST sequence enriched nuclear proteins in cancer biology and role in gene therapy

The amino acid sequence enriched with proline (P), glutamic acid (E), serine (S), and threonine (T) (PEST) is a signal-transducing agent providing unique features to its substrate nuclear proteins (PEST-NPs). The PEST motif is responsible for particular posttranslational modifications (PTMs). These PTMs impart distinct properties to PEST-NPs that are responsible for their activation/inhibition, intracellular localization, and stability/degradation. PEST-NPs participate in cancer metabolism, immunity, and protein transcription as oncogenes or as tumor suppressors. Gene-based therapeutics are getting the attention of researchers because of their cell specificity. PEST-NPs are good targets to explore as cancer therapeutics. Insights into PTMs of PEST-NPs demonstrate that these proteins not only interact with each other but also recruit other proteins to/from their active site to promote/inhibit tumors. Thus, the role of PEST-NPs in cancer biology is multivariate. It is hard to obtain therapeutic objectives with single gene therapy. An especially designed combination gene therapy might be a promising strategy in cancer treatment. This review highlights the multifaceted behavior of PEST-NPs in cancer biology. We have summarized a number of studies to address the influence of structure and PEST-mediated PTMs on activation, localization, stability, and protein-protein interactions of PEST-NPs. We also recommend researchers to adopt a pragmatic approach in gene-based cancer therapy.

Schiff Bases and their Metal Complexes as Potential Anticancer Candidates: A Review of Recent Works

BACKGROUND

Schiff bases and their metal complexes are emerging as key classes of medicinal compounds, possessing an enormous potential of biological activities like anticancer, anticonvulsant and antioxidant etc. The aim of this review is to examine the anticancer activity of different classes of Schiff bases and their metal complexes.

METHODS

Anticancer activity of the already synthesized as well as the novel Schiff bases and their metal complexes was studied using different assays such as 3- [4,5-dimethyltiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT), PI staining, Sulforhodamine, Allium cepa, Sulfo- Rhodamine-B-stain(SRB), viability and potato disc against various human and animal cancer cell lines.

RESULTS

The test results indicated significant differences in anticancer activity between subclasses of Schiff base compounds as well as between the Schiff base ligands and their metal complexes. Quinazolines showed a very high activity against HepG2 and MCF-7 cell lines. Pyrazole-naphthalene derivatives exhibited high activity against numerous carcinoma cells while [Ni(HL1)2(OAc)2] showed the highest. Azosal and its tin(IV) complexes displayed high activity against U-1242 MG and excellent activity against HCT-116 cell lines. 2-thiouracil sulfonamides displayed high activity against MCF7, CaCo-2 carcinoma cells. Vitamin-B6 and its oxovanadium complex showed good activity against MCF-7, 3T3 and cervical cancer HeLa cancer cell lines in the presence of visible light. Indoles displayed high activity against AMJ13. Porphyrines derivatives exhibited good activity while its binuclear(Y and K) complexes displayed high activity against several carcinoma cells. Chitosan complexes of [Pd(II) and Pt(II)] showed a very high anticancer activity against MCF-7 carcinoma cell.

CONCLUSION

Schiff bases possess a high potential to inhibit carcinoma cells which enhanced with complexation, but the mechanism of their antitumor activity is still doubt.

The pseudo-caspase FLIP(L) regulates cell fate following p53 activation

p53 is the most frequently mutated, well-studied tumor-suppressor gene, yet the molecular basis of the switch from p53-induced cell-cycle arrest to apoptosis remains poorly understood. Using a combination of transcriptomics and functional genomics, we unexpectedly identified a nodal role for the caspase-8 paralog and only human pseudo-caspase, FLIP(L), in regulating this switch. Moreover, we identify FLIP(L) as a direct p53 transcriptional target gene that is rapidly up-regulated in response to Nutlin-3A, an MDM2 inhibitor that potently activates p53. Genetically or pharmacologically inhibiting expression of FLIP(L) using siRNA or entinostat (a clinically relevant class-I HDAC inhibitor) efficiently promoted apoptosis in colorectal cancer cells in response to Nutlin-3A, which otherwise predominantly induced cell-cycle arrest. Enhanced apoptosis was also observed when entinostat was combined with clinically relevant, p53-activating chemotherapy in vitro, and this translated into enhanced in vivo efficacy. Mechanistically, FLIP(L) inhibited p53-induced apoptosis by blocking activation of caspase-8 by the TRAIL-R2/DR5 death receptor; notably, this activation was not dependent on receptor engagement by its ligand, TRAIL. In the absence of caspase-8, another of its paralogs, caspase-10 (also transcriptionally up-regulated by p53), induced apoptosis in Nutlin-3A-treated, FLIP(L)-depleted cells, albeit to a lesser extent than in caspase-8-proficient cells. FLIP(L) depletion also modulated transcription of canonical p53 target genes, suppressing p53-induced expression of the cell-cycle regulator p21 and enhancing p53-induced up-regulation of proapoptotic PUMA. Thus, even in the absence of caspase-8/10, FLIP(L) silencing promoted p53-induced apoptosis by enhancing PUMA expression. Thus, we report unexpected, therapeutically relevant roles for FLIP(L) in determining cell fate following p53 activation.

Roles of computational modelling in understanding p53 structure, biology, and its therapeutic targeting

The transcription factor p53 plays pivotal roles in numerous biological processes, including the suppression of tumours. The rich availability of biophysical data aimed at understanding its structure–function relationships since the 1990s has enabled the application of a variety of computational modelling techniques towards the establishment of mechanistic models. Together they have provided deep insights into the structure, mechanics, energetics, and dynamics of p53. In parallel, the observation that mutations in p53 or changes in its associated pathways characterize several human cancers has resulted in a race to develop therapeutic modulators of p53, some of which have entered clinical trials. This review describes how computational modelling has played key roles in understanding structural-dynamic aspects of p53, formulating hypotheses about domains that are beyond current experimental investigations, and the development of therapeutic molecules that target the p53 pathway.

Resurrecting a p53 peptide activator - An enabling nanoengineering strategy for peptide therapeutics

Many high-affinity peptide antagonists of MDM2 and MDMX have been reported as activators of the tumor suppressor protein p53 with therapeutic potential. Unfortunately, peptide activators of p53 generally suffer poor proteolytic stability and low membrane permeability, posing a major pharmacological challenge to anticancer peptide drug development. We previously obtained several potent dodecameric peptide antagonists of MDM2 and MDMX termed PMIs, one of which, TSFAEYWALLSP, bound to MDM2 and MDMX at respective affinities of 0.49 and 2.4 nM. Here we report the development of gold nanoparticles (Np) as a membrane-traversing delivery vehicle to carry PMI for anticancer therapy. Np-PMI was substantially more active in vitro than Nutlin-3 in killing tumor cells bearing wild-type p53, and effectively inhibited tumor growth in metastasis in a mouse homograft mode of melanoma and a patient-derived xenograft model of colon cancer with a favorable safety profile. This clinically viable drug delivery strategy can be applied not only to peptide activators of p53 for cancer therapy, but also to peptide therapeutics in general aimed at targeting intracellular protein-protein interactions for disease intervention.

Competition NMR for Detection of Hit/Lead Inhibitors of Protein-Protein Interactions

Screening for small-molecule fragments that can lead to potent inhibitors of protein–protein interactions (PPIs) is often a laborious step as the fragments cannot dissociate the targeted PPI due to their low μM–mM affinities. Here, we describe an NMR competition assay called w-AIDA-NMR (weak-antagonist induced dissociation assay-NMR), which is sensitive to weak μM–mM ligand–protein interactions and which can be used in initial fragment screening campaigns. By introducing point mutations in the complex’s protein that is not targeted by the inhibitor, we lower the effective affinity of the complex, allowing for short fragments to dissociate the complex. We illustrate the method with the compounds that block the Mdm2/X-p53 and PD-1/PD-L1 oncogenic interactions. Targeting the PD-/PD-L1 PPI has profoundly advanced the treatment of different types of cancers.

Trp53 regulates platelets in bone marrow via the PI3K pathway

The p53 gene is well known as a key tumor suppressor gene; it is vital for hematopoietic stem cell differentiation and growth. In the present study, the change of platelets (PLTs) in p53 knockout mice (p53^-/- mice) was investigated. The peripheral blood cell subsets and PLT parameters in p53^-/-mice were compared with those in age-matched p53^+/+ mice. Bleeding time as well as the alteration of PLT levels, were analyzed with the PLT marker CD41 antibody using flow cytometry. The results revealed that the number of PLTs in p53^-/- mice was significantly lower than that in p53^+/+ mice. Bleeding time was prolonged in the peripheral blood of p53^-/- mice compared with that of p53^+/+ mice. Furthermore, the related gene expression of the PI3K signaling pathway in the bone marrow of p53^-/- mice was shown to be associated with plateletogenesis. PI3K inhibitor (LY294002) was also used to treat p53^-/- mice, and the results demonstrated that LY294002 revert the change of PLTs in these mice. In summary, PLTs were altered in p53^-/- mice, and the PI3K signaling pathway was involved in that process, suggesting that the p53-dependent PI3K signaling pathway is involved in thrombocytopenia or PLT diseases. PLT number is reduced in p53 deficiency; however, this reduction could be reverted by inhibiting the PI3K pathway.

Tetra-substituted pyrrole derivatives act as potent activators of p53 in melanoma cells

Cutaneous melanoma, the most aggressive form of skin cancer, is characterized by activating BRAF mutations. Despite the initial success of selective BRAF inhibitors, only few patients exhibited complete responses, whereas many showed disease progression. Melanoma is one of the few types of cancer in which p53 is not frequently mutated, but p53 inactivation can be indirectly achieved by a stable activation of MDM2 induced by a deletion in CDKN2A (Cyclin Dependent Kinase Inhibitor 2A) locus, encoding for p16INK4A and p14ARF, two tumor suppressor genes. In this study, we tested the efficacy of the previously synthesized tetra-substituted pyrrole derivatives, 8 g, 8 h and 8i, in melanoma cell lines, and we compared the effects of the most active of these, the 8i compound, with that exerted by Nutlin 3, a well-known inhibitor of p53-MDM2 interaction. The obtained results showed that 8i potentiates the inhibitory effect of Nutlin 3 and the combined use of 8i and Nutlin 3 triggers apoptosis and significantly impairs melanoma viability. Finally, the 8i compound reduces p53-MDM2 interaction and induces p53-HSP90 complex formation, suggesting that the observed raise in p53 transcriptional activity could be mediated by HSP90. Because the main feature of melanoma is the resistance to most chemotherapeutics, our studies suggest that the 8i tetra-substituted pyrrole derivative, restoring p53 functions and its transcriptional activities, may have potential application, at least as adjuvant, in the treatment of human melanoma.

Cisplatin binds to the MDM2 RING finger domain and inhibits the ubiquitination activity

Cisplatin can directly bind to the RING finger domain of MDM2, leading to the zinc-release and protein unfolding. Consequently, cisplatin inhibits the MDM2-mediated ubiquitination, which is the molecular basis of p53 activation. This work provides insight into the cisplatin-induced p53-elevation that is involved in cell apoptosis.

Subversion of Host Innate Immunity by Human Papillomavirus Oncoproteins

The growth of human papillomavirus (HPV)-transformed cells depends on the ability of the viral oncoproteins E6 and E7, especially those from high-risk HPV16/18, to manipulate the signaling pathways involved in cell proliferation, cell death, and innate immunity. Emerging evidence indicates that E6/E7 inhibition reactivates the host innate immune response, reversing what until then was an unresponsive cellular state suitable for viral persistence and tumorigenesis. Given that the disruption of distinct mechanisms of immune evasion is an attractive strategy for cancer therapy, the race is on to gain a better understanding of E6/E7-induced immune escape and cancer progression. Here, we review recent literature on the interplay between E6/E7 and the innate immune signaling pathways cGAS/STING/TBK1, RIG-I/MAVS/TBK1, and Toll-like receptors (TLRs). The overall emerging picture is that E6 and E7 have evolved broad-spectrum mechanisms allowing for the simultaneous depletion of multiple rather than single innate immunity effectors. The cGAS/STING/TBK1 pathway appears to be the most heavily impacted, whereas the RIG-I/MAVS/TBK1, still partially functional in HPV-transformed cells, can be activated by the powerful RIG-I agonist M8, triggering the massive production of type I and III interferons (IFNs), which potentiates chemotherapy-mediated cell killing. Overall, the identification of novel therapeutic targets to restore the innate immune response in HPV-transformed cells could transform the way HPV-associated cancers are treated.

Prognostic value of the combined expression of tumor-associated trypsin inhibitor (TATI) and p53 in patients with bladder cancer undergoing radical cystectomy

BACKGROUND

Urothelial carcinoma of the bladder is a heterogeneous disease for which reliable prognostic molecular biomarkers have not been established.

OBJECTIVE

To investigate the prognostic value of tumor-associated trypsin inhibitor (TATI) expression combined with p53 expression in bladder cancer patients who have undergone radical cystectomy.

METHODS

Tissue microarrays from 110 patients were analyzed immunohistochemically for TATI and p53 protein expression. Complete clinical-pathological information and follow-up data were collected. Univariable Kaplan-Meier analysis and log-rank test were performed to assess the association between TATI and p53 expression patterns with clinical outcomes. Cox's proportional hazard analysis was performed to identify potential independent risk factors for predicting disease progression and evaluate the prognostic value of combining the expression of TATI and p53 on progression-free survival (PFS) and overall survival (OS).

RESULTS

TATI expression was positively correlated with favorable differentiation of bladder cancer, and lower tumor stage. p53 expression was positively related to tumor stage, tumor grade, and lymph-node invasion. Univariate Kaplan-Meier analysis revealed significant differences between TATI-positive vs. TATI-negative and p53-positive vs. p53-negative patients, regarding PFS. Multivariate analysis showed that both TATI and p53 expression were independent factors for predicting disease progression.

CONCLUSION

TATI expression patterns could enhance the prognostic value of p53 overexpression on progression.

β-Sitosterol Reverses Multidrug Resistance via BCRP Suppression by Inhibiting the p53-MDM2 Interaction in Colorectal Cancer

Phytosterols are widely present in vegetable oils, nuts, cereal products, fruits, and berries. Phytosterol-induced treatment sensitivity has recently shed light on alleviating multidrug resistance in cancer therapy. Here, we demonstrated that β-sitosterol, the most common dietary phytosterol, recovers oxaliplatin (OXA) sensitivity in drug-resistant colorectal cancer (CRC) cells by inhibiting breast cancer resistance protein (BCRP) expression. We further showed evidence that β-sitosterol could activate p53 by disrupting the p53-MDM2 interaction, leading to an increase in p53 translocation to the nucleus and silencing the nuclear factor-κB (NF-κB) pathway, which is necessary for BCRP expression. Finally, we suggested that the combination of OXA and β-sitosterol has a synergistic tumor suppression effect in vivo using a xenograft mouse model. These results revealed that β-sitosterol is able to mediate the p53/NF-κB/BCRP signaling axis to regulate the response of CRC to chemotherapy. The combined application of β-sitosterol and OXA can be a potential way to improve CRC treatment.

Design, Synthesis, and Biological Evaluation of Aromatic Amide-Substituted Benzimidazole-Derived Chalcones. The Effect of Upregulating TP53 Protein Expression

A series of benzimidazole-derived chalcones containing aromatic amide substituent were designed and synthesized. All of the chalcone compounds were tested for their in vitro antitumor activity against human cancer cell lines (HCT116, HepG2, A549, and CRL-5908). The antiproliferative activity of compounds 3, 6, 9, 14, 15, 16 against HCT116 cells was significantly better than that that of 5-Fluorouracil (IC50: 94.63 µM). The antitumor activity of these compounds showed obvious differences between the wild type HCT116 and mutant HCT116 (TP53^−/−) cells. A preliminary mechanistic study suggested that these compounds act by upregulating the expression of TP53 protein in tumor cells without inhibiting the MDM2-TP53 interaction.

Biomimetic Synthesis of Rhytidenone A and Mode of Action of Cytotoxic Rhytidenone F

The rhytidenone family comprises spirobisnaphthalene natural products isolated from the mangrove endophytic fungus Rhytidhysteron rufulum AS21B. The biomimetic synthesis of rhytidenone A was achieved by a Michael reaction/aldol/lactonization cascade in a single step from the proposed biosynthetic precursor rhytidenone F. Moreover, the mode of action of the highly cytotoxic rhytidenone F was investigated. The pulldown assay coupled with mass spectrometry analysis revealed the target protein PA28γ is covalently attached to rhytidenone F at the Cys92 residue. The interactions of rhytidenone F with PA28γ lead to the accumulation of p53, which is an essential tumor suppressor in humans. Consequently, the Fas-dependent signaling pathway is activated to initiate cellular apoptosis. These studies have identified the first small-molecule inhibitor targeting PA28γ, suggesting rhytidenone F may serve as a promising natural product lead for future anticancer drug development.

Endogenous 3-Iodothyronamine (T1AM) and Synthetic Thyronamine-like Analog SG-2 Act as Novel Pleiotropic Neuroprotective Agents Through the Modulation of SIRT6

3-iodothyronamine (T1AM) and the recently developed analog SG-2 are rapidly emerging as promising multi-target neuroprotective ligands able to reprogram lipid metabolism and to produce memory enhancement in mice. To elucidate the molecular mechanisms underlying the multi-target effects of these novel drug candidates, here we investigated whether the modulation of SIRT6, known to play a key role in reprogramming energy metabolism, might also drive the activation of clearing pathways, such as autophagy and ubiquitine-proteasome (UP), as further mechanisms against neurodegeneration. We show that both T1AM and SG-2 increase autophagy in U87MG cells by inducing the expression of SIRT6, which suppresses Akt activity thus leading to mTOR inhibition. This effect was concomitant with down-regulation of autophagy-related genes, including Hif1α, p53 and mTOR. Remarkably, when mTOR was inhibited a concomitant activation of autophagy and UP took place in U87MG cells. Since both compounds activate autophagy, which is known to sustain long term potentiation (LTP) in the entorhinal cortex (EC) and counteracting AD pathology, further electrophysiological studies were carried out in a transgenic mouse model of AD. We found that SG-2 was able to rescue LTP with an efficacy comparable to T1AM, further underlying its potential as a novel pleiotropic agent for neurodegenerative disorders treatment.

Optimized Inhibitors of MDM2 via an Attempted Protein-Templated Reductive Amination

Innovative and efficient hit-identification techniques are required to accelerate drug discovery. Protein-templated fragment ligations represent a promising strategy in early drug discovery, enabling the target to assemble and select its binders from a pool of building blocks. Development of new protein-templated reactions to access a larger structural diversity and expansion of the variety of targets to demonstrate the scope of the technique are of prime interest for medicinal chemists. Herein, we present our attempts to use a protein-templated reductive amination to target protein-protein interactions (PPIs), a challenging class of drug targets. We address a flexible pocket, which is difficult to achieve by structure-based drug design. After careful analysis we did not find one of the possible products in the kinetic target-guided synthesis (KTGS) approach, however subsequent synthesis and biochemical evaluation of each library member demonstrated that all the obtained molecules inhibit MDM2. The most potent library member (Ki =0.095 μm) identified is almost as active as Nutlin-3, a potent inhibitor of the p53-MDM2 PPI.

The Roles of Ubiquitination Factor E4B (UBE4B) in the Postoperative Prognosis of Patients with Renal Cell Carcinoma and in Renal Tumor Cells Growth and Metastasis

Object

This study aimed at investigating the clinical significance and biological function of ubiquitination factor E4B (UBE4B) in human renal cell carcinoma (RCC).

Methods

19 paired clear cell renal cell carcinoma (ccRCC) tumor samples and the matched neighboring non-tumor samples were used to detect the expression of UBE4B in RCC tumor by Western blotting and RT-qPCR. UBE4B expression was also detected in 151 ccRCC paraffin-embedded tumor samples by using immunohistochemistry. Overall survival (OS) in different UBE4B expression groups were compared with Log rank test. The prognostic value of UBE4B expression in OS was evaluated with the univariate and multivariate Cox regression models. UBE4B was knocked down by small interfering RNA (siRNA) technology, and the effect of UBE4B on cell proliferation, colony formation, metastasis, apoptosis and cell cycle of RCC cells were examined in vitro.

Results

Both protein and mRNA levels of UBE4B were up-regulated in ccRCC tumor tissues in contrast to the corresponding adjacent nontumor ones. UBE4B expression was positively associated with tumor-node-metastasis (TNM) stage and distant metastasis in ccRCC patients. Survival analyses indicated that low expression of UBE4B was associated with increased OS in ccRCC patients. Functional analyses demonstrated that siRNA silencing of UBE4B expression in SKRC39 and ACHN cells further reduced the growth, motility and invasiveness of RCC cells. Moreover, siRNA silencing of UBE4B in the RCC cell lines did not induce apoptosis, and an increase in the cell population was observed during the G₀/G₁ phase of the cell cycle.

Conclusion

UBE4B might act as an oncogene in regulating RCC development. Therefore it could be served as an effective indicator to predict OS and a potential biomarker for targeted therapy of RCC patients.

α-Helix-Mimicking Sulfono-γ-AApeptide Inhibitors for p53-MDM2/MDMX Protein-Protein Interactions

The use of peptidomimetic scaffolds is a promising strategy for the inhibition of protein-protein interactions (PPIs). Herein, we demonstrate that sulfono-γ-AApeptides can be rationally designed to mimic the p53 α-helix and inhibit p53-MDM2 PPIs. The best inhibitor, with K_d and IC₅₀ values of 26 nM and 0.891 μM toward MDM2, respectively, is among the most potent unnatural peptidomimetic inhibitors disrupting the p53-MDM2/MDMX interaction. Using fluorescence polarization assays, circular dichroism, nuclear magnetic resonance spectroscopy, and computational simulations, we demonstrate that sulfono-γ-AApeptides adopt helical structures resembling p53 and competitively inhibit the p53-MDM2 interaction by binding to the hydrophobic cleft of MDM2. Intriguingly, the stapled sulfono-γ-AApeptides showed promising cellular activity by enhancing p53 transcriptional activity and inducing expression of MDM2 and p21. Moreover, sulfono-γ-AApeptides exhibited remarkable resistance to proteolysis, augmenting their biological potential. Our results suggest that sulfono-γ-AApeptides are a new class of unnatural helical foldamers that disrupt PPIs.

Adoption of a Turn Conformation Drives the Binding Affinity of p53 C-Terminal Domain Peptides to 14-3-3σ

The interaction between the adapter protein 14-3-3σ and transcription factor p53 is important for preserving the tumor-suppressor functions of p53 in the cell. A phosphorylated motif within the C-terminal domain (CTD) of p53 is key for binding to the amphipathic groove of 14-3-3. This motif is unique among 14-3-3 binding partners, and the precise dynamics of the interaction is not yet fully understood. Here, we investigate this interaction at the molecular level by analyzing the binding of different length p53 CTD peptides to 14-3-3σ using ITC, SPR, NMR, and MD simulations. We observed that the propensity of the p53 peptide to adopt turn-like conformation plays an important role in the binding to the 14-3-3σ protein. Our study contributes to elucidate the molecular mechanism of the 14-3-3-p53 binding and provides useful insight into how conformation properties of a ligand influence protein binding.

Competitive ubiquitination activates the tumor suppressor p53

Blocking p53 ubiquitination through disrupting its interaction with MDM2 or inhibiting the MDM2 catalytic activity is the central mechanism by which the tumor suppressor p53 is activated in response to genotoxic challenges. Although MDM2 is first characterized as the major E3 ubiquitin ligase for p53, it can also catalyze the conjugation of ubiquitin moieties to other proteins (e.g., activating transcription factor 3, or ATF3). Here we report that ATF3 can act as an ubiquitin “trap” and competes with p53 for MDM2-mediated ubiquitination. While ATF3-mediated p53 stabilization required ATF3 binding to the MDM2 RING domain, we demonstrated that ATF3 ubiquitination catalyzed by MDM2 was indispensable for p53 activation in response to DNA damage. Moreover, a cancer-derived ATF3 mutant (R88G) devoid of ubiquitination failed to prevent p53 from MDM2-mediated degradation and thus was unable to activate the tumor suppressor. Therefore, we have identified a previously-unknown mechanism that can activate p53 in the genotoxic response.

Simultaneous measurement of p53:Mdm2 and p53:Mdm4 protein-protein interactions in whole cells using fluorescence labelled foci

In this report we describe the development of a Fluorescent Protein-Protein Interaction-visualization (FLUOPPI) to enable the simultaneous measurement of both Mdm2:p53 and Mdm4:p53 interactions in order to assess the relative efficiencies of mimetic molecules of the p53 peptide helix against both PPIs. Mdm2 and Mdm4 overexpression frequently leads to the inactivation of non-mutated p53 in human cancers, via inhibition of its transcriptional activity, enhancing its degradation by the proteasome or by preventing its nuclear import. Development of inhibitors to disrupt the binding of one or both of these protein interactions have been the subject of intensive pharmaceutical development for anti-cancer therapies. Using the bimodal FLUOPPI system we have characterised compounds that were either monospecific for Mdm2 or bispecific for both Mdm2 and Mdm4. We have also demonstrated that the FLUOPPI assay can reliably differentiate between specific and non-specific disruption of these protein complexes via accurate assessment and normalization to the cell population under measurement. We envision that this methodology will increase the efficiency of identifying compounds that are either specific against a single PPI from a closely related family of interactions or compounds that interact across multiple related PPI pairs, depending on which is more desirable.

The Dual Interactions of p53 with MDM2 and p300: Implications for the Design of MDM2 Inhibitors

Proteins that limit the activity of the tumour suppressor protein p53 are increasingly being targeted for inhibition in a variety of cancers. In addition to the development of small molecules, there has been interest in developing constrained (stapled) peptide inhibitors. A stapled peptide ALRN_6924 that activates p53 by preventing its interaction with its negative regulator Mdm2 has entered clinical trials. This stapled peptide mimics the interaction of p53 with Mdm2. The chances that this peptide could bind to other proteins that may also interact with the Mdm2-binding region of p53 are high; one such protein is the CREB binding protein (CBP)/p300. It has been established that phosphorylated p53 is released from Mdm2 and binds to p300, orchestrating the transcriptional program. We investigate whether molecules such as ALRN_6924 would bind to p300 and, to do so, we used molecular simulations to explore the binding of ATSP_7041, which is an analogue of ALRN_6924. Our study shows that ATSP_7041 preferentially binds to Mdm2 over p300; however, upon phosphorylation, it appears to have a higher affinity for p300. This could result in attenuation of the amount of free p300 available for interacting with p53, and hence reduce its transcriptional efficacy. Our study highlights the importance of assessing off-target effects of peptide inhibitors, particularly guided by the understanding of the networks of protein-protein interactions (PPIs) that are being targeted.

The p53 mRNA: an integral part of the cellular stress response

A large number of signalling pathways converge on p53 to induce different cellular stress responses that aim to promote cell cycle arrest and repair or, if the damage is too severe, to induce irreversible senescence or apoptosis. The differentiation of p53 activity towards specific cellular outcomes is tightly regulated via a hierarchical order of post-translational modifications and regulated protein-protein interactions. The mechanisms governing these processes provide a model for how cells optimize the genetic information for maximal diversity. The p53 mRNA also plays a role in this process and this review aims to illustrate how protein and RNA interactions throughout the p53 mRNA in response to different signalling pathways control RNA stability, translation efficiency or alternative initiation of translation. We also describe how a p53 mRNA platform shows riboswitch-like features and controls the rate of p53 synthesis, protein stability and modifications of the nascent p53 protein. A single cancer-derived synonymous mutation disrupts the folding of this platform and prevents p53 activation following DNA damage. The role of the p53 mRNA as a target for signalling pathways illustrates how mRNA sequences have co-evolved with the function of the encoded protein and sheds new light on the information hidden within mRNAs.

Actinomycin V Suppresses Human Non-Small-Cell Lung Carcinoma A549 Cells by Inducing G2/M Phase Arrest and Apoptosis via the p53-Dependent Pathway

Actinomycin V, extracted and separated from marine-derived actinomycete Streptomyces sp., as the superior potential replacement of actinomycin D (which showed defect for its hepatotoxicity) has revealed an ideal effect in the suppression of migration and invasion in human breast cancer cells as referred to in our previous study. In this study, the involvement of p53 in the cell cycle arrest and pro-apoptotic action of actinomycin V was investigated in human non-small-cell lung carcinoma A549 cells. Results from the 3-(4,5-dimethylthiazol)-2,5-diphenyltetrazolium bromide assay showed that cytotoxic activity of actinomycin V on A549 cells (with wild-type p53) was stronger than the NCI-H1299 cells (p53-deficient). Actinomycin V upregulated both of the protein and mRNA expression levels of p53, p21Waf1/Cip1 and Bax in A549 cells. For this situation, actinomycin V decreased the M-phase related proteins (Cdc2, Cdc25A and Cyclin B1) expression, arrested cells in G2/M phase and subsequently triggered apoptosis by mediating the Bcl-2 family proteins' expression (Bax and Bcl-2). Furthermore, the effects of cell cycle arrest and apoptosis in A549 cells which were induced by actinomycin V could be reversed by the pifithrin-α, a specific inhibitor of p53 transcriptional activity. Collectively, our results suggest that actinomycin V causes up-regulation of p53 by which the growth of A549 cells is suppressed for cell cycle arrest and apoptosis.

In Silico Design of MDM2-Targeting Peptides from a Naturally Occurring Constrained Peptide

Naturally occurring constrained peptides are frequently used as scaffolds for bioactive peptide grating due to their high stability. Here, we used in silico methods to design several constrained peptides comprising a scorpion toxin scaffold, a MDM2 binding epitope, and a cluster of positively charged residues. The designed peptides displayed varied binding affinity to MDM2 despite differing by only one or two residues. One of the peptides, SC426, had nanomolar binding affinity (K_D =6.6±2.6 nm) to MDM2, and exhibited stronger inhibitory activity on the proliferation of HCT116 cells (p53-wild type) and SW480 cells (p53-mutant) than that of nutlin-3a. Binding mode analysis of the designed peptide at MDM2 suggests that the conserved "FWL" epitope was buried in the hydrophobic binding pocket, and the residues located at the periphery of the binding site contributed to the high binding affinity of SC426. Overall, in silico design of miniproteins with therapeutic potential through epitope grafting to the naturally occurring constrained peptide is an effective strategy.

Identification of p53 Activators in a Human Microarray Compendium

Biomarkers predictive of molecular and toxicological effects are needed to interpret emerging high-throughput transcriptomic data streams. The previously characterized 63 gene TGx-DDI biomarker that includes 20 genes known to be regulated by p53 was previously shown to accurately predict DNA damage in chemically treated cells. We comprehensively evaluated whether the molecular basis of the DDI predictions was based on a p53-dependent response. The biomarker was compared to microarray data in a compendium derived from human cells using the Running Fisher test, a nonparametric correlation test. Using the biomarker, we identified conditions that led to p53 activation, including exposure to the chemical nutlin-3 which disrupts interactions between p53 and the negative regulator MDM2 or by knockdown of MDM2. The expression of most of the genes in the biomarker (75%) were found to depend on p53 activation status based on gene behavior after TP53 overexpression or knockdown. The biomarker identified DDI chemicals that were strong inducers of p53 in wild-type cells; these p53 responses were decreased or abolished in cells after p53 knockdown by siRNAs. Using the biomarker, we screened ∼1950 chemicals in ∼9800 human cell line chemical vs control comparisons and identified ∼100 chemicals that caused p53 activation. Among the positive chemicals were many that are known to activate p53 through direct and indirect DNA damaging mechanisms. These results contribute to the evidence that the TGx-DDI biomarker is useful for identifying chemicals that cause DDI and activate p53.

Simulations of mutant p53 DNA binding domains reveal a novel druggable pocket

The DNA binding domain (DBD) of the tumor suppressor p53 is the site of several oncogenic mutations. A subset of these mutations lowers the unfolding temperature of the DBD. Unfolding leads to the exposure of a hydrophobic β-strand and nucleates aggregation which results in pathologies through loss of function and dominant negative/gain of function effects. Inspired by the hypothesis that structural changes that are associated with events initiating unfolding in DBD are likely to present opportunities for inhibition, we investigate the dynamics of the wild type (WT) and some aggregating mutants through extensive all atom explicit solvent MD simulations. Simulations reveal differential conformational sampling between the WT and the mutants of a turn region (S6-S7) that is contiguous to a known aggregation-prone region (APR). The conformational properties of the S6-S7 turn appear to be modulated by a network of interacting residues. We speculate that changes that take place in this network as a result of the mutational stress result in the events that destabilize the DBD and initiate unfolding. These perturbations also result in the emergence of a novel pocket that appears to have druggable characteristics. FDA approved drugs are computationally screened against this pocket.

In Vitro Analysis of Protein:Protein Interactions in the Human Cancer-Pertinent rp.eL42-p53-Mdm2 Pathway

Introduction:

We have recently demonstrated that the eukaryote-specific large subunit ribosomal protein (rp) eL42 assists catalysis of peptide bond formation at the peptidyl transferase center of 80S ribosomes in eukaryotic cells. Recently, several ribosomal proteins were shown to have extraribosomal functions independent of protein biosynthesis. Such functions include regulation of apoptosis, cell cycle arrest, cell proliferation, neoplastic transformation, cell migration and invasion, and tumorigenesis through both Mdm2-p53-dependent and p53-independent mechanisms. Our objective is to demonstrate that overexpression of eL42 in tumor may incapacitate cell anti-tumor mechanism through interaction with the tumor suppressor protein p53 and its partner Mdm2.

Methods:

Co-immunoprecipitation technique and the binding assays on Biacore were used to probe interactions between recombinant eL42, p53 and Mdm2 proteins in a so-called rp-p53-Mdm2 axis.

Results:

We demonstrate that the ribosomal protein eL42, the tumor suppressor protein p53 and the ubiquitin E3 ligase Mdm2 interact with each other in a ternary rp.eL42:p53:Mdm2 complex. Precisely, the interaction between eL42 and p53 is characterized by a strong binding affinity (KD value in the nanomolar range) that is likely to trigger the sequestration of p53 and the inhibition of its tumor suppressor activity. Furthermore, the p53:Mdm2 and eL42:Mdm2 complexes exhibit comparable binding affinities in the micromolar range compatible with Mdm2 being the enzyme which ubiquitinates both the p53 and eL42 substrates. Interestingly, pyridoxal 5'-phosphate (PLP), one of the active forms of vitamin B6, binds to eL42 and significantly inhibits the interaction between eL42 and p53, in accordance with the observation that vitamin B6 is associated with reduced risk of cancer.

Conclusion:

Our study emphasized one more major mechanism of p53 downregulation involving its sequestration by eL42 upon the overexpression of this ribosomal protein. The mechanism described in the present report complemented the well-known p53 downregulation triggered by proteasomal degradation mediated through its ubiquitination by Mdm2.

The Inability of the Choroid to Revascularize in Oxygen-Induced Retinopathy Results from Increased p53/miR-Let-7b Activity

Retinopathy of prematurity (ROP) is characterized by an initial retinal avascularization, followed by pathologic neovascularization. Recently, choroidal thinning has also been detected in children formerly diagnosed with ROP; a similar sustained choroidal thinning is observed in ROP models. But the mechanism underlying the lack of choroidal revascularization remains unclear and was investigated in an oxygen-induced retinopathy (OIR) model. In OIR, evidence of senescence was detected, preceded by oxidative stress in the choroid and the retinal pigment epithelium. This was associated with a global reduction of proangiogenic factors, including insulin-like growth factor 1 receptor (Igf1R). Coincidentally, tumor suppressor p53 was highly expressed in the OIR retinae. Curtailing p53 activity resulted in reversal of senescence, normalization of Igf1r expression, and preservation of choroidal integrity. OIR-induced down-regulation of Igf1r was mediated at least partly by miR-let-7b as i) let-7b expression was augmented throughout and beyond the period of oxygen exposure, ii) let-7b directly targeted Igf1r mRNA, and iii) p53 knock-down blunted let-7b expression, restored Igf1r expression, and elicited choroidal revascularization. Finally, restoration of Igf1r expression rescued choroid thickness. Altogether, this study uncovers a significant mechanism for defective choroidal revascularization in OIR, revealing a new role for p53/let-7b/IGF-1R axis in the retina. Future investigations on this (and connected) pathway could further our understanding of other degenerative choroidopathies, such as geographic atrophy.

Targeting RNA Polymerase I with Hernandonine Inhibits Ribosomal RNA Synthesis and Tumor Cell Growth

RNA polymerase I (RNA Pol. I) activity is consistently expanded in multiplying cells to continue the expanded interest for ribosome generation and protein synthesis, which are fundamental for cell development and division. Thus, selective inhibitors of RNA Pol. I may offer a general helpful intends to block cancer cell multiplication. Hernandonine, isolated from the root wood of Hernandia nymphaeifolia, causes rearrangement of nucleolar proteins consistent with segregation of the nucleolus, a hallmark of RNA Pol. I transcription stress. Furthermore, the compound destabilizes RPA194, the large catalytic protein of RNA Pol. I, in a proteasome-dependent manner and inhibits nascent rRNA synthesis and expression of the 45S rRNA precursor. Finally, hernandonine induces cellular apoptosis through a p53-dependent or p53-independent process in solid tumor cell lines. These outcomes feature the prevailing effect of RNA Pol. I transcription stress on apoptosis pathway initiation and present a synthetically novel and significant molecule that represses RNA Pol. I, making it a potential objective for malignancy treatment. IMPLICATIONS: Our findings position hernandonine as a potential, particular, and orally administered cancer treatment agent appropriate for use in investigational clinical trials.

Oridonin induces Mdm2-p60 to promote p53-mediated apoptosis and cell cycle arrest in neuroblastoma

Oridonin could induce NB (neuroblastoma) cells growth inhibition by inducing apoptosis and cell cycle arrest, and the molecular mechanisms behind the effects deserve to be further explored. Here, oridonin was confirmed to cause the reactivation of p53 (cellular tumor antigen p53) to promote the expression of a series of apoptosis‐ and cell cycle arrest‐related proteins for the biological effects. During the process, oridonin relied on the caspase activation to cleave p53‐induced Mdm2 (E3 ubiquitin‐protein ligase Mdm2) to generate Mdm2‐p60. The generation of Mdm2‐p60 stabilized p53, and resulted in p53 accumulation for p53 continuous activation. In our research, it was also found that the reactivation of p53 induced by oridonin was closely related with the generation of ROS (reactive oxygen species). Taken together, these findings explain that oridonin exerts its anticancer activity partially by targeting the Mdm2‐p53 axis in NB cells, which lay an experimental base for future research of exploring the effects and molecular mechanisms of oridonin.

MDM2 inhibitor RG7388 potently inhibits tumors by activating p53 pathway in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a high-risk head and neck cancer with poor clinical outcomes and insufficient treatments. The mouse double minute 2 homolog (MDM2) is the main molecular target in the clinical treatment of cancer. Indeed, MDM2 negatively regulates p53 through ubiquitin-dependent degradation. Thus, inhibition of MDM2-p53 interaction is a potential strategy for treating NPC. The latest generation MDM2 inhibitor, RG7388, shows increased potency and improved bioavailability compared to previous treatments. In this study, we investigated the efficacy and specificity of this inhibitor in NPC cell lines, and tumor-bearing mice were used to examine the therapeutic efficacy and effects of RG7388 treatment. The results showed that RG7388 potently decreased cell proliferation and activated p53-dependent pathway, resulting in cell cycle arrest and apoptosis. RG7388 significantly inhibited tumors in tumor-bearing mice. Activation of the p53 pathway-inhibited cell proliferation, as observed by detecting Ki67-positive cells. Additionally, the activity of apoptotic caspase family proteins was induced in the cleaved caspase-3-positive cells in vivo. Our results demonstrate that the MDM2 small-molecule inhibitor RG7388 is effective for NPC tumors, supporting further clinical investigation as a potential therapy for NPC.

The facile and visualizable identification of broad-spectrum inhibitors of MDM2/p53 using co-expressed protein complexes

MDM2 is a well-known oncoprotein overexpressed in a variety of cancers, and the identification of inhibitors that disrupt the MDM2/p53 interaction is of great interest in anticancer drug development. Here we designed a platform for the facile and visualizable identification of inhibitors of MDM2 using co-expressed protein complexes of MDM2/p53. A hexahistidine-tag on MDM2 allows the binding of the protein complex to the Ni-NTA affinity resin, while the fluorescent protein fused to p53 enables the direct visualization of the interaction of p53 with MDM2. Hence, the inhibition of the MDM2/p53 interaction can be observed with the naked eye. The assay can be set up by directly loading cell lysate to the Ni-NTA affinity resin, and no chemical modification of proteins is needed. In addition to the qualitative analyses, the binding affinity of inhibitors to the MDM2 protein can be quantified by fluorescence titration. The applications of this system have been verified using small molecules and peptide inhibitors. As a proof of concept, we screened a small library using this platform. Interestingly, two types of novel inhibitors of MDM2, including cyclohexyl-triphenylamine derivatives and platinum complexes, were identified and their binding affinities were obtained. Quantitative measurements show that these new types of inhibitors demonstrate a high binding affinity (up to K_d = 51.9 nM) to MDM2.

Stereoisomerism of stapled peptide inhibitors of the p53-Mdm2 interaction: an assessment of synthetic strategies and activity profiles

Staple composition can influence target binding and bioactivity of peptides. We present strategies to modulate E/Z ratios and access saturated analogues.

All-hydrocarbon, i, i+7 stapled peptide inhibitors of the p53-Mdm2 interaction have emerged as promising new leads for cancer therapy. Typical chemical synthesis via olefin metathesis results in the formation of both E- and Z-isomers, an observation that is rarely disclosed but may be of importance in targeting PPI. In this study, we evaluated the effect of staple geometry on the biological activity of five p53-reactivating peptides. We also present strategies for the modulation of the E/Z ratio and attainment of the hydrogenated adduct through repurposing of the metathesis catalyst.

Analysis of Genetic Alterations in Tunisian Patients with Lung Adenocarcinoma

The identification of the mutations that drive lung cancer have furnished new targets for the treatment of non-small cell lung cancer (NSCLC) and led to the development of targeted therapies such as tyrosine kinase inhibitors that are used to combat the molecular changes promoting cancer progression. Furthermore, biomarkers identified from gene analysis can be used to detect early lung cancer, determine patient prognosis, and monitor response to therapy. In the present study we analyzed the molecular profile of seventy-three Tunisian patients with lung adenocarcinoma (LAD). Mutational analyses for EGFR and KRAS were performed using direct sequencing, immunohistochemistry or MassARRAY. Anaplastic lymphoma kinase (ALK) rearrangement was evaluated by immunohistochemistry using the D5F3 clone, and p53 expression was also assessed. The median age of patients at diagnosis was 61 years (range 23-82 years). Using different methodologies, EGFR mutations were found in 5.47% of patients and only exon 19 deletions "E746-A750 del" were detected. KRAS mutations were present in 9.58% of cases, while only one patient was ALK-positive. Moreover, abnormal immunostaining of p53 was detected in 56.16% of patients. In conclusion, the detected rates of EGFR and KRAS mutation and ALK rearrangement were lower than those found in European and Asian countries, whereas, abnormal p53 expression was slightly more frequent. Furthermore, given the small sample size of this study, a more comprehensive analysis of this patient set is warranted.

Mechanisms of oncogenic cell competition-Paths of victory

Cancer is a multistep process. In the early phases of this disease, mutations in oncogenes and tumor suppressors are thought to promote clonal expansion. These mutations can increase cell competitiveness, allowing tumor cells to grow within the tissue by eliminating wild type host cells. Recent studies have shown that cell competition can also function in later phases of cancer. Here, we examine the existing evidence linking cell competition and tumorigenesis. We focus on the mechanisms underlying cell competition and their contribution to disease pathogenesis.

Non-genotoxic MDM2 inhibition selectively induces a pro-apoptotic p53 gene signature in chronic lymphocytic leukemia cells

Chronic lymphocytic leukemia (CLL) is a clinically heterogeneous hematologic malignancy. In approximately 90% of cases the TP53 gene is in its wildtype state at diagnosis of this malignancy. As mouse double-minute-2 homolog (MDM2) is a primary repressor of p53, targeting this protein is an attractive therapeutic approach for non-genotoxic reactivation of p53. Since the discovery of the first MDM2 inhibitor, Nutlin-3a, newer potent and bioavailable compounds have been developed. In this study we tested the second-generation MDM2 inhibitor, RG7388, in patient-derived CLL cells and normal cells, examining its effect on the induction of p53-transcriptional targets. RG7388 potently decreased viability in p53-functional CLL cells, whereas p53-non-functional samples were more resistant to the drug. RG7388 induced a pro-apoptotic gene expression signature with upregulation of p53-target genes involved in the intrinsic (PUMA, BAX) and extrinsic (TNFRSF10B, FAS) pathways of apoptosis, as well as MDM2. Only a slight induction of CDKN1A was observed and upregulation of pro-apoptotic genes dominated, indicating that CLL cells are primed for p53-dependent apoptosis. Consequently, RG7388 led to a concentration-dependent increase in caspase-3/7 activity and cleaved poly (ADP-ribose) polymerase. Importantly, we observed a preferential pro-apoptotic signature in CLL cells but not in normal blood and bone marrow cells, including CD34⁺ hematopoietic cells. These data support the further evaluation of MDM2 inhibitors as a novel additional treatment option for patients with p53-functional CLL.