Mechanisms of switching on p53: a role for covalent modification?

TP53 in Acute Myeloid Leukemia: Molecular Aspects and Patterns of Mutation

Mutation of the tumor suppressor gene, TP53, is associated with abysmal survival outcomes in acute myeloid leukemia (AML). Although it is the most commonly mutated gene in cancer, its occurrence is observed in only 5–10% of de novo AML, and in 30% of therapy related AML (t-AML). TP53 mutation serves as a prognostic marker of poor response to standard-of-care chemotherapy, particularly in t-AML and AML with complex cytogenetics. In light of a poor response to traditional chemotherapy and only a modest improvement in outcome with hypomethylation-based interventions, allogenic stem cell transplant is routinely recommended in these cases, albeit with a response that is often short lived. Despite being frequently mutated across the cancer spectrum, progress and enthusiasm for the development of p53 targeted therapeutic interventions is lacking and to date there is no approved drug that mitigates the effects of TP53 mutation. There is a mounting body of evidence indicating that p53 mutants differ in functionality and form from typical AML cases and subsequently display inconsistent responses to therapy at the cellular level. Understanding this pathobiological activity is imperative to the development of effective therapeutic strategies. This review aims to provide a comprehensive understanding of the effects of TP53 on the hematopoietic system, to describe its varying degree of functionality in tumor suppression, and to illustrate the need for the adoption of personalized therapeutic strategies to target distinct classes of the p53 mutation in AML management.

A Review of Natural Therapies Potentially Relevant in Triple Negative Breast Cancer Aimed at Targeting Cancer Cell Vulnerabilities

We reviewed the research into the mechanisms of growth of triple negative breast cancer (TNBC) based on laboratory pre-clinical studies that have shaped understanding of the disease over the past decade. In response to these findings, we propose an approach to potentially prevent cancer metabolic adaptation and recurrence. This paper collates pre-clinical results, first to determine the tumor’s mechanisms of growth and then to source natural substances that could potentially suppress those mechanisms. The results from in vivo and in vitro studies of TNBC were combined first to select 10 primary mechanisms (Hypoxia-inducible factor 1α, Hedgehog, MAPK, MTAP, NF-κ B, Notch, P13K, STAT3, and Wnt signaling pathways plus p53 and POL2A gene expression) that promote TNBC growth, and second to propose a treatment array of 21 natural compounds that suppress laboratory models of TNBC via these mechanisms. We included BRCA mutations in the review process, but only pathways with the most preclinical studies utilizing natural products were included. Then we outlined potential biomarkers to assess the changes in the micro-environment and monitor biochemical pathway suppression. This suppression-centric aim targets these mechanisms of growth with the goal of potentially halting tumor growth and preventing cancer cell metabolic adaptation. We chose TNBC to demonstrate this 5-step strategy of supplementary therapy, which may be replicated for other tumor types.

p53-PHLDA3-Akt Network: The Key Regulators of Neuroendocrine Tumorigenesis

p53 is a well-known tumor suppressor gene and one of the most extensively studied genes in cancer research. p53 functions largely as a transcription factor and can trigger a variety of antiproliferative programs via induction of its target genes. We identified PHLDA3 as a p53 target gene and found that its protein product is a suppressor of pancreatic neuroendocrine tumors (PanNETs) and a repressor of Akt function. PHLDA3 is frequently inactivated by loss of heterozygosity (LOH) and methylation in human PanNETs, and LOH at the PHLDA3 gene locus correlates with PanNET progression and poor prognosis. In addition, in PHLDA3-deficient mice, pancreatic islet cells proliferate abnormally and acquire resistance to apoptosis. In this article, we briefly review the roles of p53 and Akt in human neuroendocrine tumors (NETs) and describe the relationship between the p53-PHLDA3 and Akt pathways. We also discuss the role of PHLDA3 as a tumor suppressor in various NETs and speculate on the possibility that loss of PHLDA3 function may be a useful prognostic marker for NET patients indicating particular drug therapies. These results suggest that targeting the downstream PHLDA3-Akt pathway might provide new therapies to treat NETs.

Mutant p53 as an Antigen in Cancer Immunotherapy

The p53 tumor suppressor plays a pivotal role in cancer and infectious disease. Many oncology treatments are now calling on immunotherapy approaches, and scores of studies have investigated the role of p53 antibodies in cancer diagnosis and therapy. This review summarizes the current knowledge from the preliminary evidence that suggests a potential role of p53 as an antigen in the adaptive immune response and as a key monitor of the innate immune system, thereby speculating on the idea that mutant p53 antigens serve as a druggable targets in immunotherapy. Except in a few cases, the vast majority of published work on p53 antibodies in cancer patients use wild-type p53 as the antigen to detect these antibodies and it is unclear whether they can recognize p53 mutants carried by cancer patients at all. We envision that an antibody targeting a specific mutant p53 will be effective therapeutically against a cancer carrying the exact same mutant p53. To corroborate such a possibility, a recent study showed that a T cell receptor-like (TCLR) antibody, initially made for a wild-type antigen, was capable of discriminating between mutant p53 and wild-type p53, specifically killing more cancer cells expressing mutant p53 than wild-type p53 in vitro and inhibiting the tumour growth of mice injected with mutant p53 cancer cells than mice with wild-type p53 cancer cells. Thus, novel antibodies targeting mutant p53, but not the wild-type isoform, should be pursued in preclinical and clinical studies.

Dephosphorylation of p53 Ser 392 Enhances Trimethylation of Histone H3 Lys 9 via SUV39h1 Stabilization in CK2 Downregulation-Mediated Senescence

Cellular senescence is an irreversible form of cell cycle arrest. Senescent cells have a unique gene expression profile that is frequently accompanied by senescence-associated heterochromatic foci (SAHFs). Protein kinase CK2 (CK2) downregulation can induce trimethylation of histone H3 Lys 9 (H3K9me3) and SAHFs formation by activating SUV39h1. Here, we present evidence that the PI3K-AKTmTOR-reactive oxygen species-p53 pathway is necessary for CK2 downregulation-mediated H3K9me3 and SAHFs formation. CK2 downregulation promotes SUV39h1 stability by inhibiting its proteasomal degradation in a p53dependent manner. Moreover, the dephosphorylation status of Ser 392 on p53, a possible CK2 target site, enhances the nuclear import and subsequent stabilization of SUV39h1 by inhibiting the interactions between p53, MDM2, and SUV39h1. Furthermore, p21Cip1/WAF1 is required for CK2 downregulation-mediated H3K9me3, and dephosphorylation of Ser 392 on p53 is important for efficient transcription of p21Cip1/WAF1. Taken together, these results suggest that CK2 downregulation induces dephosphorylation of Ser 392 on p53, which subsequently increases the stability of SUV39h1 and the expression of p21Cip1/WAF1, leading to H3K9me3 and SAHFs formation.

Synergism Through WEE1 and CHK1 Inhibition in Acute Lymphoblastic Leukemia

Introduction: Screening for synthetic lethality markers has demonstrated that the inhibition of the cell cycle checkpoint kinases WEE1 together with CHK1 drastically affects stability of the cell cycle and induces cell death in rapidly proliferating cells. Exploiting this finding for a possible therapeutic approach has showed efficacy in various solid and hematologic tumors, though not specifically tested in acute lymphoblastic leukemia. Methods: The efficacy of the combination between WEE1 and CHK1 inhibitors in B and T cell precursor acute lymphoblastic leukemia (B/T-ALL) was evaluated in vitro and ex vivo studies. The efficacy of the therapeutic strategy was tested in terms of cytotoxicity, induction of apoptosis, and changes in cell cycle profile and protein expression using B/T-ALL cell lines. In addition, the efficacy of the drug combination was studied in primary B-ALL blasts using clonogenic assays. Results: This study reports, for the first time, the efficacy of the concomitant inhibition of CHK1/CHK2 and WEE1 in ALL cell lines and primary leukemic B-ALL cells using two selective inhibitors: PF-0047736 (CHK1/CHK2 inhibitor) and AZD-1775 (WEE1 inhibitor). We showed strong synergism in the reduction of cell viability, proliferation and induction of apoptosis. The efficacy of the combination was related to the induction of early S-phase arrest and to the induction of DNA damage, ultimately triggering cell death. We reported evidence that the efficacy of the combination treatment is independent from the activation of the p53-p21 pathway. Moreover, gene expression analysis on B-ALL primary samples showed that Chek1 and Wee1 are significantly co-expressed in samples at diagnosis (Pearson r = 0.5770, p = 0.0001) and relapse (Pearson r= 0.8919; p = 0.0001). Finally, the efficacy of the combination was confirmed by the reduction in clonogenic survival of primary leukemic B-ALL cells. Conclusion: Our findings suggest that the combination of CHK1 and WEE1 inhibitors may be a promising therapeutic strategy to be tested in clinical trials for adult ALL.

Role of curcumin in regulating p53 in breast cancer: an overview of the mechanism of action

p53 is a tumor suppressor gene involved in various cellular mechanisms including DNA repair, apoptosis, and cell cycle arrest. More than 50% of human cancers have a mutated nonfunctional p53. Breast cancer (BC) is one of the main causes of cancer-related deaths among females. p53 mutations in BC are associated with low survival rates and more resistance to the conventional therapies. Thus, targeting p53 activity was suggested as an important strategy in cancer therapy. During the past decades, cancer research was focused on the development of monotargeted anticancer therapies. However, the development of drug resistance by modulation of genes, proteins, and pathways was the main hindrance to the success of such therapies. Curcumin is a natural product, extracted from the roots of Curcuma longa, and possesses various biological effects including anticancer activity. Previous studies proved the ability of curcumin to modulate several signaling pathways and biomolecules in cancer. Safety and cost-effectiveness are additional inevitable advantages of curcumin. This review summarizes the effects of curcumin as a regulator of p53 in BC and the key molecular mechanisms of this regulation.

Selenocysteine inhibits human osteosarcoma cells growth through triggering mitochondrial dysfunction and ROS-mediated p53 phosphorylation

Osteosarcoma represents the most common primary malignant bone tumor in children and adolescents, which shows severe resistance toward standard chemotherapy because of high invasive capacity and growing incidence. Selenocysteine (SeC) is a naturally available Se-containing amino acid that displays splendid anticancer activities against several human tumors. However, little information about SeC-induced growth inhibition against human osteosarcoma is available. Herein, the anticancer efficiency and underlying mechanism of SeC against human osteosarcoma were evaluated in vitro and in vivo. The results revealed that SeC significantly inhibited MG-63 human osteosarcoma cells growth in vitro through induction of S-phase arrest and apoptosis, as reflected by the decrease of cyclin A and CDK-2, PARP cleavage, and caspases activation. SeC treatment also resulted in mitochondrial dysfunction through affecting Bcl-2 family expression. Moreover, SeC triggered p53 phosphorylation by inducing reactive oxygen species (ROS) overproduction. ROS inhibition effectively blocked SeC-induced cytotoxicity and p53 phosphorylation. Importantly, MG-63 human osteosarcoma xenograft growth in nude mice was significantly suppressed in vivo through triggering apoptosis and p53 phosphorylation. These results indicated that SeC had the potential to inhibit human osteosarcoma cells growth in vitro and in vivo through triggering mitochondrial dysfunction and ROS-mediated p53 phosphorylation, which validated the potential application of Se-containing compounds in treatment of human osteosarcoma.

Testosterone modulates FoxO3a and p53-related genes to protect C2C12 skeletal muscle cells against apoptosis

The loss of muscle mass and strength with aging, sarcopenia, is a prevalent condition among the elderly, associated with skeletal muscle dysfunction and enhanced muscle cell apoptosis. We have previously demonstrated that testosterone protects against H₂O₂-induced apoptosis in C2C12 muscle cells, at different levels: morphological, biochemical and molecular. Since we have observed that testosterone reduces p-p53 and maintains the inactive state of FoxO3a transcription factor, induced by H₂O₂, we analyzed if the hormone was exerting its antiapoptotic effect at transcriptional level, by modulating pro and antiapoptotic genes associated to them. We detected the upregulation of the proapoptotic genes Puma, PERP and Bim, and MDM2 in response to H₂O₂ at different periods of the apoptotic process, and the downregulation of the antiapoptotic gene Bcl-2, whereas testosterone was able to modulate and counteract H₂O₂ effects. Furthermore, ERK and JNK kinases have been demonstrated to be linked to FoxO3a phosphorylation and thus its subcellular distribution. This work show some transcription level components, upstream of the classical apoptotic pathway, that are activated during oxidative stress and that are points where testosterone exerts its protective action against apoptosis, exposing some of the puzzle pieces of the intricate network that aged skeletal muscle apoptosis represents.

Partial p53-dependence of anisomycin-induced apoptosis in PC12 cells

The bacterial antibiotic anisomycin is known to induce apoptosis by activating several mitogen-activated protein kinases and by inhibiting protein synthesis. In this study, the influence of p53 protein on the apoptosis-inducing effect of anisomycin was investigated. The effect of protein synthesis-inhibiting concentration of anisomycin on apoptotic events was analyzed using Western blot, DNA fragmentation, and cell viability assays in wild-type PC12 and in mutant p53 protein expressing p143p53PC12 cells. Anisomycin stimulated the main apoptotic pathways in both cell lines, but p143p53PC12 cells showed lower sensitivity to the drug than their wild-type counterparts. Anisomycin caused the activation of the main stress kinases, phosphorylation of the p53 protein and the eukaryotic initiation factor eIF2α, proteolytic cleavage of protein kinase R, Bid, caspase-9 and -3. Furthermore, anisomycin treatment led to the activation of TRAIL and caspase-8, two proteins involved in the extrinsic apoptotic pathway. All these changes were stronger and more sustained in wtPC12 cells. In the presence of the dominant inhibitory p53 protein, p53- dependent genes involved in the regulation of apoptosis may be less transcribed and this can lead to the decrease of apoptotic processes in p143p53PC12 cells.

Tp53 and its potential therapeutic role as a target in bladder cancer

INTRODUCTION

Despite more than 30 years of research on p53 resulting in >50,000 publications, we are now beginning to figure out the complexity of the p53 pathway, gene ontology and conformational structure of the molecule. Recent years brought great advances in p53 related drugs and the potencial ways in which p53 is inactivated in cancer. Areas covered: We searched for related publications on Pubmed and ClinicalTrial.gov using the following keywords 'p53, Tp53, p53 and bladder cancer, p53 and therapeutic target'. Relevant articles improved the understanding on p53 pathways and their potential as candidate to targeted therapy in bladder cancer. Expert opinion: Novel strategies developed to restore the function of mutants with chemical chaperones or by using compounds to improved pharmacokinetic properties are in development with potential to be applied in the oncology clinic. Other strategies targeting aberrantly overexpressed p53 regulators with wild-type p53 are also an active area of research. In particular, studies inhibiting the interaction of p53 with its negative regulators MDMX and MDM2 are an important field in drug discovery. Small molecules for inhibition of MDM2 are now in clinical trials process. However, personalized anticancer therapy might eventually advance through analyses of p53 status in cancer patients.

p53 in the game of transposons

Throughout the animal kingdom, p53 genes function to restrain mobile elements and recent observations indicate that transposons become derepressed in human cancers. Together, these emerging lines of evidence suggest that cancers driven by p53 mutations could represent "transpospoathies," i.e. disease states linked to eruptions of mobile elements. The transposopathy hypothesis predicts that p53 acts through conserved mechanisms to contain transposon movement, and in this way, prevents tumor formation. How transposon eruptions provoke neoplasias is not well understood but, from a broader perspective, this hypothesis also provides an attractive framework to explore unrestrained mobile elements as inciters of late-onset idiopathic disease. Also see the video abstract here.

SCF(Fbxo22)-KDM4A targets methylated p53 for degradation and regulates senescence

Recent evidence has revealed that senescence induction requires fine-tuned activation of p53, however, mechanisms underlying the regulation of p53 activity during senescence have not as yet been clearly established. We demonstrate here that SCF^Fbxo22-KDM4A is a senescence-associated E3 ligase targeting methylated p53 for degradation. We find that Fbxo22 is highly expressed in senescent cells in a p53-dependent manner, and that SCF^Fbxo22 ubiquitylated p53 and formed a complex with a lysine demethylase, KDM4A. Ectopic expression of a catalytic mutant of KDM4A stabilizes p53 and enhances p53 interaction with PHF20 in the presence of Fbxo22. SCF^Fbxo22-KDM4A is required for the induction of p16 and senescence-associated secretory phenotypes during the late phase of senescence. Fbxo22^−/− mice are almost half the size of Fbxo22^+/− mice owing to the accumulation of p53. These results indicate that SCF^Fbxo22-KDM4A is an E3 ubiquitin ligase that targets methylated p53 and regulates key senescent processes.

Cellular senescence—the permanent cessation of cell proliferation—is a process that can be deregulated in cancer and other aging-related diseases. Here the authors demonstrate that the SCF^Fbxo22-KDM4A complex plays an essential role during senescence as an E3 ligase that targets methylated p53 for degradation.

The p53-Mdm2 interaction and the E3 ligase activity of Mdm2/Mdm4 are conserved from lampreys to humans

Here, Coffill et al. characterize Tp53, Tp63, and Tp73 in a jawless vertebrate, the Japanese lamprey, as well as the Mdm2 and Mdm4 genes using genome analysis. Functional analysis reveals conservation of p63 and p73 compared with p53, which shows substantial variability within the C-terminal and N-terminal domains, and that lamprey Mdm2 degrades human p53 with great efficiency; however, this interaction is not inhibited by currently available small molecule inhibitors of the human HDM2 protein.

The extant jawless vertebrates, represented by lampreys and hagfish, are the oldest group of vertebrates and provide an interesting genomic evolutionary pivot point between invertebrates and jawed vertebrates. Through genome analysis of one of these jawless vertebrates, the Japanese lamprey (Lethenteron japonicum), we identified all three members of the important p53 transcription factor family—Tp53, Tp63, and Tp73—as well as the Mdm2 and Mdm4 genes. These genes and their products are significant cellular regulators in human cancer, and further examination of their roles in this most distant vertebrate relative sheds light on their origin and coevolution. Their important role in response to DNA damage has been highlighted by the discovery of multiple copies of the Tp53 gene in elephants. Expression of lamprey p53, Mdm2, and Mdm4 proteins in mammalian cells reveals that the p53–Mdm2 interaction and the Mdm2/Mdm4 E3 ligase activity existed in the common ancestor of vertebrates and have been conserved for >500 million years of vertebrate evolution. Lamprey Mdm2 degrades human p53 with great efficiency, but this interaction is not blocked by currently available small molecule inhibitors of the human HDM2 protein, suggesting utility of lamprey Mdm2 in the study of the human p53 signaling pathway.

FV-429 Induced Apoptosis Through ROS-Mediated ERK2 Nuclear Translocation and p53 Activation in Gastric Cancer Cells

Following our previous finding which revealed that FV-429 induces apoptosis in human hepatocellular carcinoma HepG2 cells, in this study, we found that FV-429 could also induce apoptosis in human gastric cancer cells. Firstly, FV-429 inhibited the viability of BGC-823 and MGC-803 cells with IC50 values in the range of 38.10 ± 6.28 and 31.53 ± 6.84 µM for 24 h treatment by MTT-assay. Secondly, FV-429 induced apoptosis in BGC-823 and MGC-803 cells through the mitochondrial-mediated pathway, showing an increase in Bax/Bcl-2 ratios, and caspase-9 activation, without change in caspase-8. Further research revealed that the mitogen-activated protein kinases, including c-Jun N-terminal kinase, extracellular regulated kinase, and p38 mitogen-activated protein kinase, could be activated by FV-429-induced high level ROS. Moreover, FV-429 also promoted the ERK2 nuclear translocation, resulting in the co-translocation of p53 to the nucleus and increased transcription of p53-regulated proapoptotic genes. FV-429 significantly inhibited the nude mice xenograft tumors growth of BGC-823 or MGC-803 cells in vivo.

The depletion of securin enhances butein-induced apoptosis and tumor inhibition in human colorectal cancer

Butein (3,4,2',4'-tetrahydroxychalcone) is a promising natural polyphenolic compound that shows the growth inhibitory activity in human cancer cells; however, the precise mechanism is still unclear. Securin plays pivotal role in cancer cell proliferation and tumorigenesis. Here, we report the presence of securin that could modulate apoptosis and tumor growth ability in the butein-treated human colorectal cancer. Butein induced caspase-3 activation and PARP protein cleavage for apoptosis induction in human colorectal cancer cells. Interestingly, butein reduced the securin protein levels but conversely increased the phospho-histone H3 proteins, mitotic arrest and abnormal chromosomes segregation in cancer cells. The securin-null colorectal cancer cells were more sensitive on the reduction of cell viability than the securin-wild type cancer cells following butein treatment. The loss of securin in human colorectal cancer cells decreased tumor growth ability in nude mice. Moreover, butein reduced the tumor size of xenografted human colorectal tumors of nude mice. Taken together, this study demonstrates for the first time that the depletion of securin mediates the butein-induced apoptosis and colorectal tumor inhibition.

Gentian violet induces wtp53 transactivation in cancer cells

Recent studies suggest that gentian violet (GV) may have anticancer activity by inhibiting for instance NADPH oxidases (Nox genes) whose overexpression is linked to tumor progression. Nox1 overexpression has been shown to inhibit transcriptional activity of the oncosuppressor p53, impairing tumor cell response to anticancer drugs. The tumor suppressor p53 is a transcription factor that, upon cellular stress, is activated to induce target genes involved in tumor cell growth inhibition and apoptosis. Thus, its activation is important for efficient tumor eradication. In this study, we examined the effect of GV on wild-type (wt) p53 activity in cancer cells. We found that GV was able to overcome the inhibitory effect of the NADPH oxidase Nox1 on p53 transcriptional activity. For the first time we show that GV was able to directly induce p53/DNA binding and transcriptional activity. In vitro, GV markedly induced cancer cell death and apoptotic marker PARP cleavage in wtp53-carrying cells. GV-induced cell death was partly inhibited in cells deprived of p53, suggesting that the anticancer activity of GV may partly depend on p53 activation. GV is US Food and Drug Administration approved for human use and may, therefore, have therapeutic potential in the management of cancer through p53 activation.

Interaction between transactivation domain of p53 and middle part of TBP-like protein (TLP) is involved in TLP-stimulated and p53-activated transcription from the p21 upstream promoter

TBP-like protein (TLP) is involved in transcriptional activation of an upstream promoter of the human p21 gene. TLP binds to p53 and facilitates p53-activated transcription from the upstream promoter. In this study, we clarified that in vitro affinity between TLP and p53 is about one-third of that between TBP and p53. Extensive mutation analyses revealed that the TLP-stimulated function resides in transcription activating domain 1 (TAD1) in the N-terminus of p53. Among the mutants, #22.23, which has two amino acid substitutions in TAD1, exhibited a typical mutant phenotype. Moreover, #22.23 exhibited the strongest mutant phenotype for TLP-binding ability. It is thus thought that TLP-stimulated and p53-dependent transcriptional activation is involved in TAD1 binding of TLP. #22.23 had a decreased transcriptional activation function, especially for the upstream promoter of the endogenous p21 gene, compared with wild-type p53. This mutant did not facilitate p53-dependent growth repression and etoposide-mediated cell-death as wild-type p53 does. Moreover, mutation analysis revealed that middle part of TLP, which is requited for p53 binding, is involved in TLP-stimulated and p53-dependent promoter activation and cell growth repression. These results suggest that activation of the p21 upstream promoter is mediated by interaction between specific regions of TLP and p53.

Verrucarin A enhances TRAIL-induced apoptosis via NF-κB-mediated Fas overexpression

We investigated whether verrucarin A (VA) sensitizes HepG2 hepatoma cells to tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-mediated apoptosis. We found that VA alone induces little apoptosis, but when combined with TRAIL (VA/TRAIL), it triggered significant apoptosis, causing little or no toxicity in normal mouse splenocytes. VA/TRAIL-induced cell death is involved in the loss of mitochondrial transmembrane potential and the consequent activation of caspases. Because nuclear factor (NF)-κB inhibition has been known as a critical target in TRAIL-mediated apoptosis, we also investigated the role of NF-κB in VA/TRAIL treatment. We found that VA upregulated the DNA binding activity of NF-κB, but that the antioxidants glutathione and N-acetyl-l-cysteine, as well as NF-κB inhibitor MG132, and mutant-IκB (m-IκB) transfection, significantly downregulated VA/TRAIL-induced cell death by inhibiting caspase-3 and NF-κB activities. Transfection of mutant-eIF2α also resulted in a decrease in VA/TRAIL-induced cell death by inhibiting of caspase-3, but not NF-κB activity. Although VA/TRAIL treatment led to an increase of DR5 expression, transfection of m-IκB had no influence on the DR5 expressional level. Finally, we showed that NF-κB-mediated Fas expression is critical to VA/TRAIL-induced apoptosis. Taken together, these results indicate that VA/TRAIL sensitizes HepG2 cells to apoptosis via NF-κB-mediated overexpression of Fas.

PDCD5-regulated cell fate decision after ultraviolet-irradiation-induced DNA damage

Programmed cell death 5 (PDCD5) is a human apoptosis-related molecule that is involved in both the cytoplasmic caspase-3 activity pathway (by regulating Bax translocation from cytoplasm to mitochondria) and the nuclear pathway (by interacting with Tip60). In this study, we developed a mathematical model of the PDCD5-regulated switching of the cell response from DNA repair to apoptosis after ultraviolet irradiation-induced DNA damage. We established the model by combining several hypotheses with experimental observations. Our simulations indicate that the ultimate cell response to DNA damage is dependent on a signal threshold mechanism, and the PDCD5 promotion of Bax translocation plays an essential role in PDCD5-regulated cell apoptosis. Furthermore, the model simulations revealed that PDCD5 nuclear translocation can attenuate cell apoptosis, and PDCD5 interactions with Tip60 can accelerate DNA damage-induced apoptosis, but the final cell fate decision is insensitive to the PDCD5-Tip60 interaction. These results are consistent with experimental observations. The effect of recombinant human PDCD5 was also investigated and shown to sensitize cells to DNA damage by promoting caspase-3 activity.

Murine protein serine-threonine kinase 38 activates p53 function through Ser15 phosphorylation

Murine protein serine-threonine kinase 38 (MPK38) is a member of the AMP-activated protein kinase-related serine/threonine kinase family. In this study, we show that MPK38 physically associates with p53 via the carboxyl-terminal domain of MPK38 and the central DNA-binding domain of p53. This interaction is increased by 5-fluorouracil or doxorubicin treatment and is responsible for Ser(15) phosphorylation of p53. Ectopic expression of wild-type Mpk38, but not kinase-dead Mpk38, stimulates p53-mediated transcription in a dose-dependent manner and up-regulates p53 targets, including p53, p21, MDM2, and BAX. Consistently, knockdown of MPK38 shows an opposite trend, inhibiting p53-mediated transcription. MPK38 functionally enhances p53-mediated apoptosis and cell cycle arrest in a kinase-dependent manner by stimulating p53 nuclear translocation. We also demonstrate that MPK38-mediated p53 activation is induced by removing MDM2, a negative regulator of p53, from the p53-MDM2 complex as well as by stabilization of interaction between p53 and its positive regulators, including NM23-H1, serine/threonine kinase receptor-associated protein, and 14-3-3. This leads to the enhancement of p53 stability. Together, these results suggest that MPK38 may act as a novel regulator for promoting p53 activity through direct phosphorylation of p53 at Ser(15).

Activations of Both Extrinsic and Intrinsic Pathways in HCT 116 Human Colorectal Cancer Cells Contribute to Apoptosis through p53-Mediated ATM/Fas Signaling by Emilia sonchifolia Extract, a Folklore Medicinal Plant

Emilia sonchifolia (L.) DC (Compositae), an herbaceous plant found in Taiwan and India, is used as folk medicine. The clinical applications include inflammation, rheumatism, cough, cuts fever, dysentery, analgesic, and antibacteria. The activities of Emilia sonchifolia extract (ESE) on colorectal cancer cell death have not been fully investigated. The purpose of this study explored the induction of apoptosis and its molecular mechanisms in ESE-treated HCT 116 human colorectal cancer cells in vitro. The methanolic ESE was characterized, and γ-humulene was formed as the major constituent (63.86%). ESE induced cell growth inhibition in a concentration- and time-dependent response by MTT assay. Apoptotic cells (DNA fragmentation, an apoptotic catachrestic) were found after ESE treatment by TUNEL assay and DNA gel electrophoresis. Alternatively, ESE stimulated the activities of caspase-3, -8, and -9 and their specific caspase inhibitors protected against ESE-induced cytotoxicity. ESE promoted the mitochondria-dependent and death-receptor-associated protein levels. Also, ESE increased ROS production and upregulated the levels of ATM, p53, and Fas in HCT 116 cells. Strikingly, p53 siRNA reversed ESE-reduced viability involved in p53-mediated ATM/Fas signaling in HCT 116 cells. In summary, our result is the first report suggesting that ESE may be potentially efficacious in the treatment of colorectal cancer.

Resistance and gain-of-resistance phenotypes in cancers harboring wild-type p53

Chemotherapy is the bedrock for the clinical management of cancer, and the tumor suppressor p53 has a central role in this therapeutic modality. This protein facilitates favorable antitumor drug response through a variety of key cellular functions, including cell cycle arrest, senescence, and apoptosis. These functions essentially cease once p53 becomes mutated, as occurs in ∼50% of cancers, and some p53 mutants even exhibit gain-of-function effects, which lead to greater drug resistance. However, it is becoming increasingly evident that resistance is also seen in cancers harboring wild-type p53. In this review, we discuss how wild-type p53 is inactivated to render cells resistant to antitumor drugs. This may occur through various mechanisms, including an increase in proteasomal degradation, defects in post-translational modification, and downstream defects in p53 target genes. We also consider evidence that the resistance seen in wild-type p53 cancers can be substantially greater than that seen in mutant p53 cancers, and this poses a far greater challenge for efforts to design strategies that increase drug response in resistant cancers already primed with wild-type p53. Because the mechanisms contributing to this wild-type p53 "gain-of-resistance" phenotype are largely unknown, a concerted research effort is needed to identify the underlying basis for the occurrence of this phenotype and, in parallel, to explore the possibility that the phenotype may be a product of wild-type p53 gain-of-function effects. Such studies are essential to lay the foundation for a rational therapeutic approach in the treatment of resistant wild-type p53 cancers.

COMPARING PEARSON, SPEARMAN AND HOEFFDING'S D MEASURE FOR GENE EXPRESSION ASSOCIATION ANALYSIS

DNA microarrays have become a powerful tool to describe gene expression profiles associated with different cellular states, various phenotypes and responses to drugs and other extra- or intra-cellular perturbations. In order to cluster co-expressed genes and/or to construct regulatory networks, definition of distance or similarity between measured gene expression data is usually required, the most common choices being Pearson's and Spearman's correlations. Here, we evaluate these two methods and also compare them with a third one, namely Hoeffding's D measure, which is used to infer nonlinear and non-monotonic associations, i.e. independence in a general sense. By comparing three different variable association approaches, namely Pearson's correlation, Spearman's correlation and Hoeffding's D measure, we aimed at assessing the most approppriate one for each purpose. Using simulations, we demonstrate that the Hoeffding's D measure outperforms Pearson's and Spearman's approaches in identifying nonlinear associations. Our results demonstrate that Hoeffding's D measure is less sensitive to outliers and is a more powerful tool to identify nonlinear and non-monotonic associations. We have also applied Hoeffding's D measure in order to identify new putative genes associated with tp53. Therefore, we propose the Hoeffding's D measure to identify nonlinear associations between gene expression profiles.

Novel quinazoline HMJ-30 induces U-2 OS human osteogenic sarcoma cell apoptosis through induction of oxidative stress and up-regulation of ATM/p53 signaling pathway

Human osteogenic sarcoma is the most common primary bone tumor. Despite of the success of frontline therapy, about 40% of patients have disease progression and further therapy is palliative and toxic. In this study, we developed a novel quinazoline HMJ-30 to investigate the cell growth inhibition and apoptotic responses in U-2 OS human osteogenic sarcoma cells. Our results demonstrated that HMJ-30 significantly reduced cell viabilities of U-2 OS, HOS, and 143B cells in a dose-dependent manner, but it exhibited low cytotoxicity in normal hFOB cells. HMJ-30 induced DNA damage and apoptosis in U-2 OS cells as revealed by morphologic changes, comet assay and DAPI staining. Immuno-staining, colorimetric assays, and Western blotting analyses indicated that activities of caspase-8, caspase-9, and caspase-3 and the levels of Bcl-2 family-related proteins (Bcl-2, Mcl-1, Bax, BAD, and t-Bid) were altered in HMJ-30-treated U-2 OS cells. Pretreatment of cells with caspase-8, -9, and -3 specific inhibitors significantly reduced the cell growth inhibition. HMJ-30-induced apoptosis was mediated through both death-receptor and mitochondria-dependent apoptotic pathways in U-2 OS cells. HMJ-30 induced early phosphorylation of p53(Ser18) was through the activation of ataxia telangiectasia mutated (ATM) in U-2 OS cells. The cell growth inhibition by HMJ-30 was substantially attenuated either by the pre-incubation of U-2 OS cells with N-acetylcysteine (NAC, an antioxidant) and caffeine (an ATM kinase inhibitor) or by p53 knockdown via RNAi. In conclusion, ROS dependent-ATM/p53 signaling pathway is involved in HMJ-30-induced apoptosis in U-2 OS cells.

Vesicular stomatitis virus expressing tumor suppressor p53 is a highly attenuated, potent oncolytic agent

Vesicular stomatitis virus (VSV), a negative-strand RNA rhabdovirus, preferentially replicates in and eradicates transformed versus nontransformed cells and is thus being considered for use as a potential anticancer treatment. The genetic malleability of VSV also affords an opportunity to develop more potent agents that exhibit increased therapeutic activity. The tumor suppressor p53 has been shown to exert potent antitumor properties, which may in part involve stimulating host innate immune responses to malignancies. To evaluate whether VSV expressing p53 exhibited enhanced oncolytic action, the murine p53 (mp53) gene was incorporated into recombinant VSVs with or without a functional viral M gene-encoded protein that could either block (VSV-mp53) or enable [VSV-M(mut)-mp53] host mRNA export following infection of susceptible cells. Our results indicated that VSV-mp53 and VSV-M(mut)-mp53 expressed high levels of functional p53 and retained the ability to lyse transformed versus normal cells. In addition, we observed that VSV-ΔM-mp53 was extremely attenuated in vivo due to p53 activating innate immune genes, such as type I interferon (IFN). Significantly, immunocompetent animals with metastatic mammary adenocarcinoma exhibited increased survival following treatment with a single inoculation of VSV-ΔM-mp53, the mechanisms of which involved enhanced CD49b+ NK and tumor-specific CD8+ T cell responses. Our data indicate that VSV incorporating p53 could provide a safe, effective strategy for the design of VSV oncolytic therapeutics and VSV-based vaccines.

Glutathione selectively inhibits Doxorubicin induced phosphorylation of p53Ser¹⁵, caspase dependent ceramide production and apoptosis in human leukemic cells

Glutathione (GSH) is the most abundant non-protein antioxidant in mammalian cells. It has been implicated in playing an important role in different signal transduction pathways, and its depletion is an early hallmark in the progression of apoptosis in response to a number of proapoptotic stimuli. We have selectively investigated the role of GSH in cytotoxic response of Jurkat and Molt-4 human leukemic cells to the anti-cancer drug Doxorubicin. In this study, we have shown that extracellular supplementation of GSH to human leukemic cells renders them a resistant phenotype to Doxorubicin treatment. Glutathione pre-treatment inhibits Doxorubicin-induced p53Ser(15) phosphorylation, caspase dependent ceramide (Cer) generation, Poly (ADP-ribose) polymerase (PARP) cleavage, and DNA fragmentation. Taken together, these results indicate that the major cellular antioxidant GSH influences the chemotherapeutic efficacy of Doxorubicin towards human leukemic cells.

FPipTB, a benzimidazole derivative, induces chondrosarcoma cell apoptosis via endoplasmic reticulum stress and apoptosis signal-regulating kinase 1

Chondrosarcoma is the second most common primary bone tumor and it responds poorly to both chemotherapy and radiation treatment. In this study, we investigated the anticancer effects of a new benzimidazole derivative, 2-(furanyl)-5-(piperidinyl)- (3,4,5-trimethoxybenzyl) benzimidazole (FPipTB) in human chondrosarcoma cells. FPipTB-induced apoptosis in human chondrosarcoma cell lines (JJ012 and SW1353) but not in primary chondrocytes. Furthermore, it triggered endoplasmic reticulum (ER) stress, which was characterized by changes in cytosolic calcium levels. Treatment of chondrosarcoma cells with FPipTB was associated with increased intracellular levels of ASK1, p38, p53, and Bax, followed by release of cytochrome c from mitochondria and activation of caspases. It is also known that ER stress activates apoptosis signal-regulating kinase 1 (ASK1), which mediates activation of JNK and p38 pathways. We also found that FPipTB-induced p38 and p53 phosphorylation and upregulated Bax expression. To study the mechanism of Bax upregulation, we determined that Bax promoter activity was increased in FPipTB-treated cells, leading to an increase in intracellular levels of Bax. In addition, cell treated with Ca(2+) chelator or p38 inhibitor showed reduced transcriptional activity. The results further suggest that FPipTB triggered ER stress, as indicated by changes in cytosolic calcium levels and activated the ASK1-MKK3/6-p38-p53-Bax pathway, causing chondrosarcoma cell death. Importantly, animal studies revealed a dramatic 40% reduction in tumor volume after 21 d of treatment. Thus, FPipTB may be a novel anticancer agent for the treatment of chondrosarcoma.

Pharmacological inhibition of Mdm2 triggers growth arrest and promotes DNA breakage in mouse colon tumors and human colon cancer cells

The p53 tumor suppressor protein performs a number of cellular functions, ranging from the induction of cell cycle arrest and apoptosis to effects on DNA repair. Modulating p53 activity with Mdm2 inhibitors is a promising approach for treating cancer; however, it is presently unclear how the in vivo application of Mdm2 inhibitors impact the myriad processes orchestrated by p53. Since approximately half of all colon cancers (predominately cancers with microsatellite instability) are p53-normal, we assessed the anticancer activity of the Mdm2 inhibitor Nutlin-3 in the mouse azoxymethane (AOM) colon cancer model, in which p53 remains wild type. Using a cell line derived from an AOM-induced tumor, we found that four daily exposures to Nutlin-3 induced persistent p53 stabilization and cell cycle arrest without significant apoptosis. A 4-day dosing schedule in vivo generated a similar response in colon tumors; growth arrest without significantly increased apoptosis. In adjacent normal colon tissue, Nutlin-3 treatment reduced both cell proliferation and apoptosis. Surprisingly, Nutlin-3 induced a transient DNA damage response in tumors but not in adjacent normal tissue. Nutlin-3 likewise induced a transient DNA damage response in human colon cancer cells in a p53-dependent manner, and enhanced DNA strand breakage and cell death induced by doxorubicin. Our findings indicate that Mdm2 inhibitors not only trigger growth arrest, but may also stimulate p53's reported ability to slow homologous recombination repair. The potential impact of Nutlin-3 on DNA repair in tumors suggests that Mdm2 inhibitors may significantly accentuate the tumoricidal actions of certain therapeutic modalities.

Naturally occurring, tumor-specific, therapeutic proteins

The emerging approach to cancer treatment known as targeted therapies offers hope in improving the treatment of therapy-resistant cancers. Recent understanding of the molecular pathogenesis of cancer has led to the development of targeted novel drugs such as monoclonal antibodies, small molecule inhibitors, mimetics, antisense and small interference RNA-based strategies, among others. These compounds act on specific targets that are believed to contribute to the development and progression of cancers and resistance of tumors to conventional therapies. Delivered individually or combined with chemo- and/or radiotherapy, such novel drugs have produced significant responses in certain types of cancer. Among the most successful novel compounds are those which target tyrosine kinases (imatinib, trastuzumab, sinutinib, cetuximab). However, these compounds can cause severe side-effects as they inhibit pathways such as epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor, which are also important for normal functions in non-transformed cells. Recently, a number of proteins have been identified which show a remarkable tumor-specific cytotoxic activity. This toxicity is independent of tumor type or specific genetic changes such as p53, pRB or EGFR aberrations. These tumor-specific killer proteins are either derived from common human and animal viruses such as E1A, E4ORF4 and VP3 (apoptin) or of cellular origin, such as TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and MDA-7 (melanoma differentiation associated-7). This review aims to present a current overview of a selection of these proteins with preferential toxicity among cancer cells and will provide an insight into the possible mechanism of action, tumor specificity and their potential as novel tumor-specific cancer therapeutics.

p53 Activates Either Survival or Apoptotic Signaling Responses in Lupulone-Treated Human Colon Adenocarcinoma Cells and Derived Metastatic Cells

The SW480 cell line is derived from a human colon adenocarcinoma, and SW620 cells are derived from a lymph node metastasis of the same patient. We have previously shown that lupulone induces apoptosis in SW480 cells, through a cross talk between the TRAIL-death receptor pathway and the mitochondrial apoptotic pathway. In SW620 cells, lupulone induced apoptosis only through TRAIL-death receptor activation. Both cell lines exhibit the same p53 mutations. Because p53 plays a central role in the response to cellular stresses by upregulating the transcription of several genes controlling apoptosis, we aimed to study the involvement of p53 on lupulone-triggered apoptosis. Our data show that in SW620 cells, lupulone upregulated p53 gene expression and caused a cloistering of p53 in the nucleus, allowing p53 to play a proapoptotic role by activating the TRAIL-death receptor pathway. In contrast, in lupulone-treated SW480 cells, p53 was translocated to the cytoplasm where it initiated a survival response associated with the up-regulation of antiapoptotic Bcl-2 and Mcl-1 proteins in an attempt to preserve mitochondrial integrity. These prosurvival effects of p53 in lupulone-treated SW480 cells were reversed by pifithrin-α, an inhibitor of p53 function, which caused a blocking of p53 in the nucleus leading to the down-regulation of Bcl-2 and Mcl-1, the up-regulation of proapoptotic Bax protein and TRAIL-death receptors leading to enhanced cell death. Our data support different functions of the same mutated p53 in colon adenocarcinoma and derived metastatic cells in response to the chemopreventive agent lupulone.

Glycolytic flux occurs in Drosophila melanogaster recovering from camptothecin treatment

Camptothecin (CPT) and CPT-derived drugs are widely used against gynaecological and colorectal cancers. On account of their mechanism of action these drugs target rapidly dividing cells and may have an adverse effect on normal tissues. We sought to investigate their impact on normal cells by using Drosophila as a model. We investigated the possible involvement of Drosophila homologue of p53 (Dmp53) and a member of the retinoblastoma binding protein 6 family, known as Snama. On account of its molecular features and experimental evidence gleaned from mammalian studies we propose Snama as a candidate in Dmp53 regulation. We have used proteomics and core molecular biology techniques on embryos and on adult flies. We found that flies that recover from CPT treatment display a metabolic programme characterized by glycolytic flux, depletion of Dmp53 and increase of Snama transcripts. When we introduced methyl pyruvate in the diet to bypass the glycolytic pathway, we noticed differential expression of Dmp53 and Snama and improvement in reproduction and embryonic development. The development of embryos into the pupal stage was significantly improved to 40% (P=0.02) when CPT was given to mothers in combination with methyl pyruvate. This investigation highlights the importance of energy production mechanisms in cells that recover from chemotherapy and differences between the metabolic programmes used by recovering cells and those adopted by cancer cells.

Oxaliplatin down-regulates survivin by p38 MAP kinase and proteasome in human colon cancer cells

Oxaliplatin, a platinum derivative cancer drug, has been used for treating human colorectal cancers. Survivin has been proposed as a cancer target, which highly expressed in most cancer cells but not normal adult cells. In this study, we investigated the regulation of survivin expression by exposure to oxaliplatin in human colon cancer cells. Oxaliplatin (3-9μM for 24h) markedly induced cytotoxicity, proliferation inhibition and apoptosis in the human RKO colon cancer cells. The survivin protein expression of RKO cells is dramatically reduced by oxaliplatin; however, the survivin gene expression is slightly altered. The survivin blockage of oxaliplatin elevated caspase-3 activation and apoptosis in RKO cells. Over-expression of survivin proteins by transfection with a survivin-expressed vector resisted the oxaliplatin-induced cancer cell death. Meantime, oxaliplatin elicited the phosphorylation of p38 mitogen-activated protein (MAP) kinase. SB202190, a specific p38 MAP kinase inhibitor, restored the survivin protein level and attenuated oxaliplatin-induced cancer cell death. In addition, oxaliplatin increased the levels of phospho-p53 (Ser-15) and total p53 proteins. Inhibition of p53 expression by a specific p53 inhibitor pifithrin-α reduced the phosphorylated p38 MAP kinase and active caspase-3 proteins in the oxaliplatin-exposed RKO cells. In contrast, SB202190 did not alter the oxaliplatin-induced p53 protein level. Furthermore, treatment with a specific proteasome inhibitor MG132 restored survivin protein level in the oxaliplatin-treated colon cancer cells. Taken together, our results demonstrate for the first time that survivin is down-regulated by p38 MAP kinase and proteasome degradation pathway after treatment with oxaliplatin in the human colon cancer cells.

HIV infection upregulates caveolin 1 expression to restrict virus production

Caveolin 1 (Cav-1) is a major protein of a specific membrane lipid raft known as caveolae. Cav-1 interacts with the gp41 of the human immunodeficiency virus (HIV) envelope, but the role of Cav-1 in HIV replication and pathogenesis is not known. In this report, we demonstrate that HIV infection in primary human monocyte-derived macrophages (MDMs), THP-1 macrophages, and U87-CD4 cells results in a dramatic upregulation of Cav-1 expression mediated by HIV Tat. The activity of p53 is essential for Tat-induced Cav-1 expression, as our findings show enhanced phosphorylation of serine residues at amino acid positions 15 and 46 in the presence of Tat with a resulting Cav-1 upregulation. Furthermore, inhibition of p38 mitogen-activated protein kinase (MAPK) blocked phosphorylation of p53 in the presence of Tat. Infection studies of Cav-1-overexpressing cells reveal a significant reduction of HIV production. Taken together, these results suggest that HIV infection enhances the expression of Cav-1, which subsequently causes virus reduction, suggesting that Cav-1 may contribute to persistent infection in macrophages.

6-dehydrogingerdione sensitizes human hepatoblastoma Hep G2 cells to TRAIL-induced apoptosis via reactive oxygen species-mediated increase of DR5

The anticancer effects of 6-dehydrogingerdione (6-DG), a compound isolated from the rhizomes of Zingiber officinale , and its mechanisms of sensitization to TRAIL-induced apoptosis were studied using human hepatoblastoma Hep G2 cells. This study demonstrates for the first time that 6-DG-induced apoptosis might be executed via mitochondrial- and Fas receptor-mediated pathways. Further studies also demonstrated that 6-DG could sensitize Hep G2 cells to TRAIL-induced apoptosis. 6-DG also up-regulated Ser-15 phosphorylation and evoked p53 nuclear translocation. Abrogation of p53 expression by p53 small interfering RNA significantly attenuated 6-DG-induced DR5 expression, thus rendering these cells resistant to TRAIL-induced apoptosis. DR5 expression after 6-DG treatment was accompanied by provoking intracellular reactive oxygen species (ROS) generation. Pretreatment with N-acetyl-l-cysteine (NAC) attenuated 6-DG-induced DR5 expression and inhibited TRAIL-induced apoptosis. In contrast to Hep G2 cells, DR5 up-regulation and sensitization to TRAIL-induced apoptosis instigated by 6-DG were not observed in normal MDCK cells. Taken together, these data suggested that in addition to the mitochondrial- and Fas receptor-mediated apoptotic pathways involved, ROS-dependent and p53-regulated DR5 expression was also demonstrated to play a pivotal role in the synergistic enhancement of TRAIL-induced apoptosis instigated by 6-DG in Hep G2 cells.

Restoration of p53 functions protects cells from concanavalin A-induced apoptosis

A great majority of human cancers encounter disruption of the p53 network. Identification and characterization of molecular components important in both p53-dependent and p53-independent apoptosis might be useful in developing novel therapies. Previously, we reported that concanavalin A (Con A) induced p73-dependent apoptosis of cells lacking functional p53. In the present study, we investigated the mechanism and role of p53 in protection from apoptosis induced by Con A. Treatment with Con A resulted in apoptosis of p53-null ovarian cancer, SKOV3, or Li-Fraumeni syndrome, MDAH041 (041), cells. However, their isogenic pairs, SKP53 and TR9-7, expressing wild-type p53 were much less sensitive and were protected by G(1) arrest. Inhibition of p53 function rendered these cells sensitive to Con A. Con A-induced apoptosis was accompanied by upregulation of forkhead box O1a (FOXO1a) and Bcl-2-interacting mediator (Bim), which were strongly inhibited after p53 expression and rescued after p53 ablation. Moreover, ablation of Bim by short hairpin RNA protected cells from apoptosis. Taken together, our study suggests that Con A induces apoptosis of cells lacking p53 by activating FOXO1a-Bim signaling and that expression of p53 protects these cells by inducing G(1) arrest and by downregulating the expression of both FOXO1a and Bim, identifying a novel cross-talk between FOXO1a and p53 transcription factors.

Regulation of cell proliferation and survival: convergence of protein kinases and caspases

Intricate networks of protein kinases are intimately involved in the regulation of cellular events related to cell proliferation and survival. In addition to protein kinases, cells also contain networks of proteases including aspartic-acid directed caspases organized in cascades that play a major role in the regulation of cell survival through their involvement in the initiation and execution phases of apoptosis. Perturbations in regulatory protein kinase and caspase networks induce alterations in cell survival and frequently accompany transformation and tumorigenesis. Furthermore, recent studies have documented that caspases or their substrates are subject to phosphorylation in cells illustrating a potential convergence of protein kinase and caspase signaling pathways. Interestingly, a number of caspase substrates are protected from cleavage when they are phosphorylated at sites that are adjacent to caspase cleavage sites. While it is theoretically possible that many distinct protein kinases could protect proteins from caspase-mediated cleavage, protein kinase CK2 is of particular interest because acidic amino acids, including aspartic acid residues that are recognized by caspases, are its dominant specificity determinants.

Posttranslational modification of p53: cooperative integrators of function

The p53 protein is modified by as many as 50 individual posttranslational modifications. Many of these occur in response to genotoxic or nongenotoxic stresses and show interdependence, such that one or more modifications can nucleate subsequent events. This interdependent nature suggests a pathway that operates through multiple cooperative events as opposed to distinct functions for individual, isolated modifications. This concept, supported by recent investigations, which provide exquisite detail as to how various modifications mediate precise protein-protein interactions in a cooperative manner, may explain why knockin mice expressing p53 proteins substituted at one or just a few sites of modification typically show only subtle effects on p53 function. The present article focuses on recent, exciting progress and develops the idea that the impact of modification on p53 function is achieved through collective and integrated events.

Regulation of p53 expression, phosphorylation and subcellular localization by a G-protein-coupled receptor

G-protein-coupled receptors (GPCRs) have been extremely successful drug targets for a multitude of diseases from heart failure to depression. This superfamily of cell surface receptors have not, however, been widely considered as a viable target in cancer treatment. In this study we show that a classical G(q/11)-coupled GPCR, the M(3)-muscarinic receptor, was able to regulate apoptosis through receptors that are endogenously expressed in the human neuroblastoma cell line, SH-SY5Y, and when ectopically expressed in Chinese hamster ovary (CHO) cells. Stimulation of the M(3)-muscarinic receptor was shown to inhibit the ability of the DNA-damaging chemotherapeutic agent, etoposide, from mediating apoptosis. This protective response in CHO cells correlated with the ability of the receptor to regulate the expression levels of p53. In contrast, stimulation of endogenous muscarinic receptors in SH-SY5Y cells did not regulate p53 expression but rather was able to inhibit p53 translocation to the mitochondria and p53 phosphorylation at serine 15 and 37. This study suggests the possibility that a GPCR can regulate the apoptotic properties of a chemotherapeutic DNA-damaging agent by regulating the expression, subcellular trafficking and modification of p53 in a manner that is, in part, dependent on the cell type.

Gene expression in mouse preimplantation embryos affected by apoptotic inductor actinomycin D

The aim of this study was to test the effect of actinomycin D on the expression of selected genes and to elucidate possible components of its apoptotic pathway in mouse embryos. Selected mRNAs and Trp53 protein were examined in blastocysts cultured for 24 h in vitro with or without the presence of a high concentration of actinomycin D. In all tested genes, the relative quantities of mRNA were significantly lower in treated blastocysts than in controls. The mRNA quantities of H2afz, Actb, Bax, Bad and Bcl2 were reduced at a similar rate, but the decreases in Bcl2l2 and Trp53 mRNA were significantly greater. Treatment with actinomycin D also changed the ratio between the mRNA levels of some pro-apoptotic and anti-apoptotic genes: the Bad/Bcl2l2 and the Bax/Bcl2l2 ratios were on average 4.39 and 2.66 times higher in the treated embryos than in the controls, respectively. Generally, treatment led to developmental arrest and significant increase in the incidence of cells with typical apoptotic features. However, its effect on Trp53 protein expression was not significant. The results suggest that mechanisms beyond the apoptotic effect of actinomycin D might include specific changes in the expression of pro-apoptotic and anti-apoptotic genes, shifting the expression ratio in favor of the pro-apoptotic ones. The results also show that the role of Trp53 is probably not crucial in this apoptotic pathway.

Cdh1 regulates cell cycle through modulating the claspin/Chk1 and the Rb/E2F1 pathways

APC/Cdh1 is a major cell cycle regulator and its function has been implicated in DNA damage repair; however, its exact role remains unclear. Using affinity purification coupled with mass spectrometry, we identified Claspin as a novel Cdh1-interacting protein and further demonstrated that Claspin is a novel Cdh1 ubiquitin substrate. As a result, inactivation of Cdh1 leads to activation of the Claspin/Chk1 pathway. Previously, we demonstrated that Rb interacts with Cdh1 to influence its ability to degrade Skp2. Here, we report that Cdh1 reciprocally regulates the Rb pathway through competing with E2F1 to bind the hypophosphorylated form of Rb. Although inactivation of Cdh1 in HeLa cells, with defective p53/Rb pathways, led to premature S phase entry, acute depletion of Cdh1 in primary human fibroblasts resulted in premature senescence. Acute loss of many other major tumor suppressors, including PTEN and VHL, also induces premature senescence in a p53- or Rb-dependent manner. Similarly, we showed that inactivation of the p53/Rb pathways by overexpression of SV40 LT-antigen partially reversed Cdh1 depletion-induced growth arrest. Therefore, loss of Cdh1 is only beneficial to cells with abnormal p53 and Rb pathways, which helps explain why Cdh1 loss is not frequently found in many tumors.