A DR4:tBID axis drives the p53 apoptotic response by promoting oligomerization of poised BAX

Immunomodulatory molecules in colorectal cancer liver metastasis

Colorectal cancer (CRC) ranks as the third most common cancer and the second leading cause of cancer-related deaths. According to clinical diagnosis and treatment, liver metastasis occurs in approximately 50 % of CRC patients, indicating a poor prognosis. The unique immune tolerance of the liver fosters an immunosuppressive tumor microenvironment (TME). In the context of tumors, numerous membrane and secreted proteins have been linked to tumor immune evasion as immunomodulatory molecules, but much remains unknown about how these proteins contribute to immune evasion in colorectal cancer liver metastasis (CRLM). This article reviews recently discovered membrane and secreted proteins with roles as both immunostimulatory and immunosuppressive molecules within the TME that influence immune evasion in CRC primary and metastatic lesions, particularly their mechanisms in promoting CRLM. This article also addresses screening strategies for identifying proteins involved in immune evasion in CRLM and provides insights into potential protein targets for treating CRLM.

FAM193A is a positive regulator of p53 activity

Inactivation of the p53 tumor suppressor, either by mutations or through hyperactivation of repressors such as MDM2 and MDM4, is a hallmark of cancer. Although many inhibitors of the p53-MDM2/4 interaction have been developed, such as Nutlin, their therapeutic value is limited by highly heterogeneous cellular responses. We report here a multi-omics investigation of the cellular response to MDM2/4 inhibitors, leading to identification of FAM193A as a widespread regulator of p53 function. CRISPR screening identified FAM193A as necessary for the response to Nutlin. FAM193A expression correlates with Nutlin sensitivity across hundreds of cell lines. Furthermore, genetic codependency data highlight FAM193A as a component of the p53 pathway across diverse tumor types. Mechanistically, FAM193A interacts with MDM4, and FAM193A depletion stabilizes MDM4 and inhibits the p53 transcriptional program. Last, FAM193A expression is associated with better prognosis in multiple malignancies. Altogether, these results identify FAM193A as a positive regulator of p53.

P-TEFb promotes cell survival upon p53 activation by suppressing intrinsic apoptosis pathway

Positive transcription elongation factor b (P-TEFb) is the crucial player in RNA polymerase II (Pol II) pause release that has emerged as a promising target in cancer. Because single-agent therapy may fail to deliver durable clinical response, targeting of P-TEFb shall benefit when deployed as a combination therapy. We screened a comprehensive oncology library and identified clinically relevant antimetabolites and Mouse double minute 2 homolog (MDM2) inhibitors as top compounds eliciting p53-dependent death of colorectal cancer cells in synergy with selective inhibitors of P-TEFb. While the targeting of P-TEFb augments apoptosis by anti-metabolite 5-fluorouracil, it switches the fate of cancer cells by the non-genotoxic MDM2 inhibitor Nutlin-3a from cell-cycle arrest to apoptosis. Mechanistically, the fate switching is enabled by the induction of p53-dependent pro-apoptotic genes and repression of P-TEFb-dependent pro-survival genes of the PI3K-AKT signaling cascade, which stimulates caspase 9 and intrinsic apoptosis pathway in BAX/BAK-dependent manner. Finally, combination treatments trigger apoptosis of cancer cell spheroids. Together, co-targeting of P-TEFb and suppressors of intrinsic apoptosis could become a viable strategy to eliminate cancer cells.

Cordycepin Induces Apoptosis through JNK-Mediated Caspase Activation in Human OEC-M1 Oral Cancer Cells

Cordycepin, a bioactive compound extracted from Cordyceps sinensis, can induce apoptosis in human OEC-M1 oral cancer cells. However, the exact mechanism is still unclear. The present study aimed to investigate the underlying mechanism of cordycepin-induced apoptosis in OEC-M1 cells. Following treatment with cordycepin, apoptosis was examined and quantified using a DNA laddering assay and a cytokeratin 18 fragment enzyme-linked immunosorbent assay, respectively. Expressions of mitogen-activated protein kinases (MAPKs) and apoptosis-related proteins were detected by the western blot analysis. Our results show that a pan-caspase inhibitor, Z-VAD-FMK, could significantly inhibit cordycepin-induced apoptosis in OEC-M1 cells. In addition, treatment with cordycepin not only activated caspase-8, caspase-9, and caspase-3 but also induced Bid and poly ADP-ribose polymerase cleavages. Furthermore, cordycepin also induced the activation of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase, and p38 MAPKs. Among MAPKs, activation of JNK solely contributed to cordycepin-induced apoptosis with the activation of caspase-8, caspase-9, and caspase-3 and cleavage of PARP. Taken together, the present study demonstrated that cordycepin activated JNK and caspase pathways to induce apoptosis in OEC-M1 cells.

Role of JNK activation in paclitaxel-induced apoptosis in human head and neck squamous cell carcinoma

It has been reported that paclitaxel activates cell cycle arrest and increases caspase protein expression to induce apoptosis in head and neck squamous cell carcinoma (HNSCC) cell lines. However, the potential signaling pathway regulating this apoptotic phenomenon remains unclear. The present study used OEC-M1 cells to investigate the underlying molecular mechanism of paclitaxel-induced apoptosis. Following treatment with paclitaxel, cell viability was assessed via the MTT assay. Necrosis, apoptosis, cell cycle and mitochondrial membrane potential (∆Ψm) were analyzed via flow cytometric analyses, respectively. Western blot analysis was performed to detect the expression levels of proteins associated with the MAPK and caspase signaling pathways. The results demonstrated that low-dose paclitaxel (50 nM) induced apoptosis but not necrosis in HNSCC cells. In addition, paclitaxel activated the c-Jun N-terminal kinase (JNK), but not extracellular signal-regulated kinase or p38 mitogen-activated protein kinase. The paclitaxel-activated JNK contributed to paclitaxel-induced apoptosis, activation of caspase-3, -6, -7, -8 and -9, and reduction of ∆Ψm. In addition, caspase-8 and -9 inhibitors, respectively, significantly decreased paclitaxel-induced apoptosis. Notably, Bid was truncated following treatment with paclitaxel. Taken together, the results of the present study suggest that paclitaxel-activated JNK is required for caspase activation and loss of ∆Ψm, which results in apoptosis of HNSCC cells. These results may provide mechanistic basis for designing more effective paclitaxel-combining regimens to treat HNSCC.

Gasdermin E-mediated programmed cell death: An unpaved path to tumor suppression

Hearing loss-associated protein gasdermin E (GSDME), an effector of secondary necrosis, has been identified in a new pathway of programmed cell death (PCD). GSDME epigenetic silencing and mutations resulting in loss-of-function have been reported in cancer tissues. Additionally, GSDME upregulation inhibits tumor proliferation as well as colony forming ability, and reduces the incidence of lymphatic metastasis, demonstrating that GSDME may act as a tumor suppressor. Here, we have focused on the molecular mechanisms of GSDME-mediated PCD, and tried to reveal the crosstalk between this cell death pathway and apoptosis, autophagy, GSDMD-mediated pyroptosis. Moreover, we concluded the anti-cancer activity of GSDME include forming permeable membranes, and triggering anti-cancer immunity. Thus, GSDME was potential to be a novel target for cancer prevention and treatment.

The NKL-code for innate lymphoid cells reveals deregulated expression of NKL homeobox genes HHEX and HLX in anaplastic large cell lymphoma (ALCL)

NKL homeobox genes encode developmental transcription factors and display an NKL-code according to their physiological expression pattern in hematopoiesis. Here, we analyzed public transcriptome data from primary innate lymphoid cells (ILCs) for NKL homeobox gene activities and found that ILC3 expressed exclusively HHEX while in ILC1 and ILC2 these genes were silenced. Deregulation of the NKL-code promotes hematopoietic malignancies, including anaplastic large cell lymphoma (ALCL) which reportedly may derive from ILC3. Accordingly, we analyzed NKL homeobox gene activities in ALCL cell lines and investigated their role in this malignancy. Transcriptome analyses demonstrated low expression levels of HHEX but powerfully activated HLX. Forced expression of HHEX in ALCL cell lines induced genes involved in apoptosis and ILC3 differentiation, indicating tumor suppressor activity. ALCL associated NPM1-ALK and JAK-STAT3-signalling drove enhanced expression of HLX while discounting HHEX. Genomic profiling revealed copy number gains at the loci of HLX and STAT3 in addition to genes encoding both STAT3 regulators (AURKA, BCL3, JAK3, KPNB1, NAMPT, NFAT5, PIM3, ROCK1, SIX1, TPX2, WWOX) and targets (BATF3, IRF4, miR135b, miR21, RORC). Transcriptome data of ALCL cell lines showed absence of STAT3 mutations while MGA was mutated and downregulated, encoding a novel potential STAT3 repressor. Furthermore, enhanced IL17F-signalling activated HLX while TGFbeta-signalling inhibited HHEX expression. Taken together, our data extend the scope of the NKL-code for ILCs and spotlight aberrant expression of NKL homeobox gene HLX in ALCL. HLX represents a direct target of ALCL hallmark factor STAT3 and deregulates cell survival and differentiation in this malignancy.

Remote Ischemic Postconditioning Inhibits Hippocampal Neuronal Apoptosis and Mitophagy After Cardiopulmonary Resuscitation in Rats

BACKGROUND

Studies have shown that remote ischemic post-conditioning can improve brain damage caused by ischemia and hypoxia. However, the specific mechanism underlying this phenomenon is still unclear. The purpose of this study was to investigate the effects of remote ischemic post-conditioning on neuronal apoptosis and mitophagy after cardiopulmonary resuscitation (CPR) in rats.

METHODS

Male Sprague-Dawley rats were used to establish an asphyxia cardiac arrest model by clamping the tracheal duct. First, the expression levels of P53, Cytochrome c (Cytc), and Parkin in the cytoplasm and mitochondria were observed at 3, 6, 24, and 72 h after the restoration of spontaneous circulation (ROSC). Then neurological deficit scores, hippocampal neuron apoptosis, mitochondrial P53 and Parkin, cytoplasmic Cytc, and neuron ultrastructure were evaluated 24 h after ROSC.

RESULTS

P53 and Parkin can translocate from the cytoplasm to the mitochondria, promoting the translocation of cytoplasmic Cytc to mitochondria after CPR, reaching a peak at 24 h after the ROSC. The P53 inhibitor Pifithrin-μ reduced apoptosis induced by P53 mitochondrial translocation. Apoptosis was induced after cardiac arrest and attenuated by remote ischemic postconditioning via inhibiting P53 mitochondrial translocation and the release of Cytc to the cytoplasm. In addition, remote ischemic postconditioning could inhibit Parkin-mediated mitophagy.

CONCLUSION

Taken together, our results show that remote ischemic post-conditioning improves neural function after CPR by inhibiting P53 mitochondrial translocation-induced apoptosis and Parkin-mediated mitophagy.

Involvement of ERK1/2-mediated ELK1/CHOP/DR5 pathway in 6-(methylsulfinyl)hexyl isothiocyanate-induced apoptosis of colorectal cancer cells

6-(Methylsulfinyl)hexyl isothiocyanate (6-MSITC) is a major bioactive compound in Wasabi. Although 6-MSITC is reported to have cancer chemopreventive activities in rat model, the molecular mechanism is unclear. In this study, we investigated the anticancer mechanisms using two types of human colorectal cancer cells (HCT116 p53^+/+ and p53^-/-). 6-MSITC caused cell cycle arrest in G₂/M phase and induced apoptosis in both types of cells in the same fashion. Signaling data revealed that the activation of ERK1/2, rather than p53, is recruited for 6-MSITC-induced apoptosis. 6-MSITC stimulated ERK1/2 phosphorylation, and then activated ERK1/2 signaling including ELK1 phosphorylation, and upregulation of C/EBP homologous protein (CHOP) and death receptor 5 (DR5). The MEK1/2 inhibitor U0126 blocked all of these molecular events induced by 6-MSITC, and enhanced the cell viability in both types of cells in the same manner. These results indicated that ERK1/2-mediated ELK1/CHOP/DR5 pathway is involved in 6-MSITC-induced apoptosis in colorectal cancer cells. Abbreviations: CHOP: C/EBP homologous protein; DR5: death receptor 5; ELK1: ETS transcription factor; ERK1/2: extracellular signal-regulated kinase 1/2; JNK: Jun-N-terminal kinase; MAPK: mitogen-activated protein kinase; MEK1/2: MAP/ERK kinase 1/2; 6-MSITC: 6-(methylsulfinyl)hexyl isothiocyanate; MTT: 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; PARP: poly(ADP-ribose) polymerase.

Novel 4-(3-phenylpropionamido), 4-(2-phenoxyacetamido) and 4-(cinnamamido) substituted benzamides bearing the pyrazole or indazole nucleus: synthesis, biological evaluation and mechanism of action

Based on some common structural features of known compounds interfering with p53 pathways and our previously synthesized benzamides, we synthesized new ethyl 5-(4-substituted benzamido)-1-phenyl-1H-pyrazole-4-carboxylates 26a-c, ethyl 5-(4-substituted benzamido)-1-(pyridin-2-yl)-1H-pyrazole-4-carboxylates 27a-c and N-(1H-indazol-6-yl)-4-substituted benzamides 31a,b bearing in the 4 position of the benzamido moiety the 2-phenylpropanamido or 2-phenoxyacetamido or cinnamamido groups. A preliminary test to evaluate the antiproliferative activity against human lung carcinoma H292 cells highlighted how compound 26c showed the best activity. This last was therefore selected for further studies with the aim to find the mechanism of action. Compound 26c induces intrinsic apoptotic pathway by activating p53 and is also able to activate TRAIL-inducing death pathway by promoting increase of DR4 and DR5 death receptors, downregulation of c-FLIP_L and caspase-8 activation.

CDK8 Kinase Activity Promotes Glycolysis

Aerobic glycolysis, also known as the Warburg effect, is a hallmark of cancerous tissues. Despite its importance in cancer development, our understanding of mechanisms driving this form of metabolic reprogramming is incomplete. We report here an analysis of colorectal cancer cells engineered to carry a single point mutation in the active site of the Mediator-associated kinase CDK8, creating hypomorphic alleles sensitive to bulky ATP analogs. Transcriptome analysis revealed that CDK8 kinase activity is required for the expression of many components of the glycolytic cascade. CDK8 inhibition impairs glucose transporter expression, glucose uptake, glycolytic capacity and reserve, as well as cell proliferation and anchorage-independent growth, both in normoxia and hypoxia. Importantly, CDK8 impairment sensitizes cells to pharmacological glycolysis inhibition, a result reproduced with Senexin A, a dual inhibitor of CDK8/CDK19. Altogether, these results contribute to our understanding of CDK8 as an oncogene, and they justify investigations to target CDK8 in highly glycolytic tumors.

Activation of the p53 Transcriptional Program Sensitizes Cancer Cells to Cdk7 Inhibitors

Cdk7, the CDK-activating kinase and transcription factor IIH component, is a target of inhibitors that kill cancer cells by exploiting tumor-specific transcriptional dependencies. However, whereas selective inhibition of analog-sensitive (AS) Cdk7 in colon cancer-derived cells arrests division and disrupts transcription, it does not by itself trigger apoptosis efficiently. Here, we show that p53 activation by 5-fluorouracil or nutlin-3 synergizes with a reversible Cdk7^as inhibitor to induce cell death. Synthetic lethality was recapitulated with covalent inhibitors of wild-type Cdk7, THZ1, or the more selective YKL-1-116. The effects were allele specific; a CDK7^as mutation conferred both sensitivity to bulky adenine analogs and resistance to covalent inhibitors. Non-transformed colon epithelial cells were resistant to these combinations, as were cancer-derived cells with p53-inactivating mutations. Apoptosis was dependent on death receptor DR5, a p53 transcriptional target whose expression was refractory to Cdk7 inhibition. Therefore, p53 activation induces transcriptional dependency to sensitize cancer cells to Cdk7 inhibition.

Cardiac-specific ablation of the E3 ubiquitin ligase Mdm2 leads to oxidative stress, broad mitochondrial deficiency and early death

The maintenance of normal heart function requires proper control of protein turnover. The ubiquitin-proteasome system is a principal regulator of protein degradation. Mdm2 is the main E3 ubiquitin ligase for p53 in mitotic cells thereby regulating cellular growth, DNA repair, oxidative stress and apoptosis. However, which of these Mdm2-related activities are preserved in differentiated cardiomyocytes has yet to be determined. We sought to elucidate the role of Mdm2 in the control of normal heart function. We observed markedly reduced Mdm2 mRNA levels accompanied by highly elevated p53 protein expression in the hearts of wild type mice subjected to myocardial infarction or trans-aortic banding. Accordingly, we generated conditional cardiac-specific Mdm2 gene knockout (Mdm2^f/f;mcm) mice. In adulthood, Mdm2^f/f;mcm mice developed spontaneous cardiac hypertrophy, left ventricular dysfunction with early mortality post-tamoxifen. A decreased polyubiquitination of myocardial p53 was observed, leading to its stabilization and activation, in the absence of acute stress. In addition, transcriptomic analysis of Mdm2-deficient hearts revealed that there is an induction of E2f1 and c-Myc mRNA levels with reduced expression of the Pgc-1a/Ppara/Esrrb/g axis and Pink1. This was associated with a significant degree of cardiomyocyte apoptosis, and an inhibition of redox homeostasis and mitochondrial bioenergetics. All these processes are early, Mdm2-associated events and contribute to the development of pathological hypertrophy. Our genetic and biochemical data support a role for Mdm2 in cardiac growth control through the regulation of p53, the Pgc-1 family of transcriptional coactivators and the pivotal antioxidant Pink1.

Identification of a core TP53 transcriptional program with highly distributed tumor suppressive activity

The tumor suppressor TP53 is the most frequently mutated gene product in human cancer. Close to half of all solid tumors carry inactivating mutations in the TP53 gene, while in the remaining cases, TP53 activity is abrogated by other oncogenic events, such as hyperactivation of its endogenous repressors MDM2 or MDM4. Despite identification of hundreds of genes regulated by this transcription factor, it remains unclear which direct target genes and downstream pathways are essential for the tumor suppressive function of TP53. We set out to address this problem by generating multiple genomic data sets for three different cancer cell lines, allowing the identification of distinct sets of TP53-regulated genes, from early transcriptional targets through to late targets controlled at the translational level. We found that although TP53 elicits vastly divergent signaling cascades across cell lines, it directly activates a core transcriptional program of ∼100 genes with diverse biological functions, regardless of cell type or cellular response to TP53 activation. This core program is associated with high-occupancy TP53 enhancers, high levels of paused RNA polymerases, and accessible chromatin. Interestingly, two different shRNA screens failed to identify a single TP53 target gene required for the anti-proliferative effects of TP53 during pharmacological activation in vitro. Furthermore, bioinformatics analysis of thousands of cancer genomes revealed that none of these core target genes are frequently inactivated in tumors expressing wild-type TP53. These results support the hypothesis that TP53 activates a genetically robust transcriptional program with highly distributed tumor suppressive functions acting in diverse cellular contexts.

ATM regulates cell fate choice upon p53 activation by modulating mitochondrial turnover and ROS levels

Despite extensive study, the mechanisms of cell fate choice upon p53 activation remain poorly understood. Using genome-wide shRNA screening, we recently identified the ATM kinase as synthetic lethal with Nutlin-3, an MDM2 inhibitor that leads to non-genotoxic p53 activation. Here, we demonstrate that while this synthetic lethal interaction relies upon components of both the intrinsic and extrinsic apoptotic pathways (e.g., BAX and BID), it is not due to significant ATM effects on the expression of p53 target genes. Instead, loss of ATM activity results in increased mitochondria and reactive oxygen species that drive apoptosis. Finally, we provide evidence that pharmacologic inhibition of ATM blocks autophagy in direct opposition to p53, which activates this process, and that inhibition of autophagy is sufficient to elicit an apoptotic response when combined with Nutlin-3.

Small molecules as pro-apoptotic anticancer agents

The quest for potent and selective targeted therapies in anticancer research is taking advantage of apoptosis-related mechanisms of action to identify a number of novel clinical candidates. This review is chemically focused on small molecules and deals with five target families that influence caspase-dependent apoptosis: caspase-3, Bcl-2 and IAP protein family members, p53 and the proteasome. Each target class is briefly described at first in terms of its involvement and relevance in tumor initiation and progression. Drug candidates currently undergoing clinical trials are then presented for each target class, followed by a quick summary of target-modulating chemotypes that have appeared in patent literature since 2006. Finally, future trends likely to become significant in apoptosis-targeted cancer therapies are presented and discussed.

Global analysis of p53-regulated transcription identifies its direct targets and unexpected regulatory mechanisms

The p53 transcription factor is a potent suppressor of tumor growth. We report here an analysis of its direct transcriptional program using Global Run-On sequencing (GRO-seq). Shortly after MDM2 inhibition by Nutlin-3, low levels of p53 rapidly activate ∼200 genes, most of them not previously established as direct targets. This immediate response involves all canonical p53 effector pathways, including apoptosis. Comparative global analysis of RNA synthesis vs steady state levels revealed that microarray profiling fails to identify low abundance transcripts directly activated by p53. Interestingly, p53 represses a subset of its activation targets before MDM2 inhibition. GRO-seq uncovered a plethora of gene-specific regulatory features affecting key survival and apoptotic genes within the p53 network. p53 regulates hundreds of enhancer-derived RNAs. Strikingly, direct p53 targets harbor pre-activated enhancers highly transcribed in p53 null cells. Altogether, these results enable the study of many uncharacterized p53 target genes and unexpected regulatory mechanisms.DOI: http://dx.doi.org/10.7554/eLife.02200.001.

Drugging the p53 pathway: understanding the route to clinical efficacy

The tumour suppressor p53 is the most frequently mutated gene in human cancer, with more than half of all human tumours carrying mutations in this particular gene. Intense efforts to develop drugs that could activate or restore the p53 pathway have now reached clinical trials. The first clinical results with inhibitors of MDM2, a negative regulator of p53, have shown efficacy but hint at on-target toxicities. Here, we describe the current state of the development of p53 pathway modulators and new pathway targets that have emerged. The challenge of targeting protein-protein interactions and a fragile mutant transcription factor has stimulated many exciting new approaches to drug discovery.

Radiation responses and resistance

The cytotoxic effect of ionizing radiation makes it a popular therapeutic tool against cancer. It is, however, a double-edged sword. Radiation exposure unleashes a plethora of protective and survival mechanisms that include increased proliferation, remodeling of the vasculature and altered cell motility. These mechanisms can play a pro-survival role in remaining cells, contributing to repopulation of tumors after radiation treatment. Understanding these mechanisms and finding ways to minimize their impact would improve the outcome of radiotherapy.

HIF1A employs CDK8-mediator to stimulate RNAPII elongation in response to hypoxia

The transcription factor HIF1A is a key mediator of the cellular response to hypoxia. Despite the importance of HIF1A in homeostasis and various pathologies, little is known about how it regulates RNA polymerase II (RNAPII). We report here that HIF1A employs a specific variant of the Mediator complex to stimulate RNAPII elongation. The Mediator-associated kinase CDK8, but not the paralog CDK19, is required for induction of many HIF1A target genes. HIF1A induces binding of CDK8-Mediator and the super elongation complex (SEC), containing AFF4 and CDK9, to alleviate RNAPII pausing. CDK8 is dispensable for HIF1A chromatin binding and histone acetylation, but it is essential for binding of SEC and RNAPII elongation. Global analysis of active RNAPII reveals that hypoxia-inducible genes are paused and active prior to their induction. Our results provide a mechanistic link between HIF1A and CDK8, two potent oncogenes, in the cellular response to hypoxia.

Human embryonic and induced pluripotent stem cells express TRAIL receptors and can be sensitized to TRAIL-induced apoptosis

Death ligands and their tumor necrosis factor receptor (TNFR) family receptors are the best-characterized and most efficient inducers of apoptotic signaling in somatic cells. In this study, we analyzed whether these prototypic activators of apoptosis are also expressed and able to be activated in human pluripotent stem cells. We examined human embryonic stem cells (hESC) and human-induced pluripotent stem cells (hiPSC) and found that both cell types express primarily TNF-related apoptosis-inducing ligand (TRAIL) receptors and TNFR1, but very low levels of Fas/CD95. We also found that although hESC and hiPSC contain all the proteins required for efficient induction and progression of extrinsic apoptotic signaling, they are resistant to TRAIL-induced apoptosis. However, both hESC and hiPSC can be sensitized to TRAIL-induced apoptosis by co-treatment with protein synthesis inhibitors such as the anti-leukemia drug homoharringtonine (HHT). HHT treatment led to suppression of cellular FLICE inhibitory protein (cFLIP) and Mcl-1 expression and, in combination with TRAIL, enhanced processing of caspase-8 and full activation of caspase-3. cFLIP likely represents an important regulatory node, as its shRNA-mediated down-regulation significantly sensitized hESC to TRAIL-induced apoptosis. Thus, we provide the first evidence that, irrespective of their origin, human pluripotent stem cells express canonical components of the extrinsic apoptotic system and on stress can activate death receptor-mediated apoptosis.

A genetic screen identifies TCF3/E2A and TRIAP1 as pathway-specific regulators of the cellular response to p53 activation

The p53 transcription factor participates in diverse cellular responses to stress, including cell-cycle arrest, apoptosis, senescence, and autophagy. The molecular mechanisms defining the ultimate outcome of p53 activation remain poorly characterized. We performed a genome-wide genetic screen in human cells to identify pathway-specific coregulators of the p53 target gene CDKN1A (p21), an inhibitor of cell-cycle progression, versus BBC3 (PUMA), a key mediator of apoptosis. Our screen identified numerous factors whose depletion creates an imbalance in the p21:PUMA ratio upon p53 activation. The transcription factor TCF3, also known as E2A, drives p21 expression while repressing PUMA across cancer cell types of multiple origins. Accordingly, TCF3/E2A depletion impairs the cell-cycle-arrest response and promotes apoptosis upon p53 activation by chemotherapeutic agents. In contrast, TRIAP1 is a specific repressor of p21 whose depletion slows down cell-cycle progression. Our results reveal strategies for driving cells toward specific p53-dependent responses.

The impact of post-transcriptional regulation in the p53 network

The p53 transcription factor regulates the synthesis of mRNAs encoding proteins involved in diverse cellular stress responses such as cell-cycle arrest, apoptosis, autophagy and senescence. In this review, we discuss how these mRNAs are concurrently regulated at the post-transcriptional level by microRNAs (miRNAs) and RNA-binding proteins (RBPs), which consequently modify the p53 transcriptional program in a cell type- and stimulus-specific manner. We also discuss the action of specific miRNAs and RBPs that are direct transcriptional targets of p53 and how they act coordinately with protein-coding p53 target genes to orchestrate p53-dependent cellular responses.

Type I interferons induce apoptosis by balancing cFLIP and caspase-8 independent of death ligands

Interferons induce a pleiotropy of responses through binding the same cell surface receptor. Here we investigated the molecular mechanism driving interferon-induced apoptosis. Using a nonbiased small interfering RNA (siRNA) screen, we show that silencing genes whose products are directly engaged in the initiation of interferon signaling completely abrogate the interferon antiproliferative response. Apoptosis-related genes such as the caspase-8, cFLIP, and DR5 genes specifically interfere with interferon-induced apoptosis, which we found to be independent of the activity of death ligands. The one gene for which silencing resulted in the strongest proapoptotic effect upon interferon signaling is the cFLIP gene, where silencing shortened the time of initiation of apoptosis from days to hours and increased dramatically the population of apoptotic cells. Thus, cFLIP serves as a regulator for interferon-induced apoptosis. A shift over time in the balance between cFLIP and caspase-8 results in downstream caspase activation and apoptosis. While gamma interferon (IFN-γ) also causes caspase-8 upregulation, we suggest that it follows a different path to apoptosis.

5-Fluorouracil signaling through a calcium-calmodulin-dependent pathway is required for p53 activation and apoptosis in colon carcinoma cells

5-Fluorouracil (5-FU) is an anti-metabolite that is in clinical use for treatment of several cancers. In cells, it is converted into three distinct fluoro-based nucleotide analogs, which interfere with DNA synthesis and repair, leading to genome impairment and, eventually, apoptotic cell death. Current knowledge states that in certain cell types, 5-FU-induced stress is signaling through a p53-dependent induction of tumor necrosis factor-receptor oligomerization required for death-inducing signaling complex formation and caspase-8 activation. Here we establish a role of calcium (Ca(2+)) as a messenger for p53 activation in response to 5-FU. Using a combination of pharmacological and genetic approaches, we show that treatment of colon carcinoma cells stimulates entry of extracellular Ca(2+) through long lasting-type plasma membrane channels, which further directs posttranslational phosphorylation of at least three p53 serine residues (S15, S33 and S37) by means of calmodulin (CaM) activity. Obstructing this pathway by the Ca(2+)-chelator BAPTA (1,2-bis(o-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid) or by inhibitors of CaM efficiently reduces 5-FU-induced caspase activities and subsequent cell death. Moreover, ectopic expression of p53 S15A in HCT116 p53(-/-) cells confirmed the importance of a Ca(2+)-CaM-p53 axis in 5-FU-induced extrinsic apoptosis. The fact that a widely used therapeutic drug, such as 5-FU, is operating via this pathway could provide new therapeutic intervention points, or specify new combinatorial treatment regimes.

Molecular targets of TRAIL-sensitizing agents in colorectal cancer

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily, interacts with its functional death receptors (DRs) and induces apoptosis in a wide range of cancer cell types. Therefore, TRAIL has been considered as an attractive agent for cancer therapy. However, many cancers are resistant to TRAIL-based therapies mainly due to the reduced expression of DRs and/or up-regulation of TRAIL pathway-related anti-apoptotic proteins. Compounds that revert such defects restore the sensitivity of cancer cells to TRAIL, suggesting that combined therapies could help manage neoplastic patients. In this article, we will focus on the TRAIL-sensitizing effects of natural products and synthetic compounds in colorectal cancer (CRC) cells and discuss the molecular mechanisms by which such agents enhance the response of CRC cells to TRAIL.

ATM and MET kinases are synthetic lethal with nongenotoxic activation of p53

The p53 tumor suppressor orchestrates alternative stress responses including cell cycle arrest and apoptosis, but the mechanisms defining cell fate upon p53 activation are poorly understood. Several small-molecule activators of p53 have been developed, including Nutlin-3, but their therapeutic potential is limited by the fact that they induce reversible cell cycle arrest in most cancer cell types. We report here the results of a genome-wide short hairpin RNA screen for genes that are lethal in combination with p53 activation by Nutlin-3, which showed that the ATM and MET kinases govern cell fate choice upon p53 activation. Genetic or pharmacological interference with ATM or MET activity converts the cellular response from cell cycle arrest into apoptosis in diverse cancer cell types without affecting expression of key p53 target genes. ATM and MET inhibitors also enable Nutlin-3 to kill tumor spheroids. These results identify new pathways controlling the cellular response to p53 activation and aid in the design of p53-based therapies.

The p53 circuit board

The p53 tumor suppressor is embedded in a large gene network controlling diverse cellular and organismal phenotypes. Multiple signaling pathways converge onto p53 activation, mostly by relieving the inhibitory effects of its repressors, MDM2 and MDM4. In turn, signals originating from increased p53 activity diverge into distinct effector pathways to deliver a specific cellular response to the activating stimuli. Much attention has been devoted to dissecting how the various input pathways trigger p53 activation and how the activity of the p53 protein itself can be modulated by a plethora of co-factors and post-translational modifications. In this review we will focus instead on the multiple configurations of the effector pathways. We will discuss how p53-generated signals are transmitted, amplified, resisted and eventually integrated by downstream gene circuits operating at the transcriptional, post-transcriptional and post-translational levels. We will also discuss how context-dependent variations in these gene circuits define the cellular response to p53 activation and how they may impact the clinical efficacy of p53-based targeted therapies.