RNA silencing of checkpoint regulators sensitizes p53-defective prostate cancer cells to chemotherapy while sparing normal cells

The POU-Domain Transcription Factor Oct-6/POU3F1 as a Regulator of Cellular Response to Genotoxic Stress

DNA damage and the generation of reactive oxygen species (ROS) are key mechanisms of apoptotic cell death by commonly used genotoxic drugs. However, the complex cellular response to these pharmacologic agents remains yet to be fully characterized. Several studies have described the role of transcription factor octamer-1 (Oct-1)/Pit-1, Oct-1/2, and Unc-86 shared domain class 2 homeobox 1 (POU2F1) in the regulation of the genes important for cellular response to genotoxic stress. Evaluating the possible involvement of other POU family transcription factors in these pathways, we revealed the inducible expression of Oct-6/POU3F1, a regulator of neural morphogenesis and epidermal differentiation, in cancer cells by genotoxic drugs. The induction of Oct-6 occurs at the transcriptional level via reactive oxygen species (ROS) and ataxia telangiectasia mutated- and Rad3-related (ATR)-dependent mechanisms, but in a p53 independent manner. Moreover, we provide evidence that Oct-6 may play a role in the regulation of cellular response to DNA damaging agents. Indeed, by using the shRNA approach, we demonstrate that in doxorubicin-treated H460 non-small-cell lung carcinoma (NSCLC) cells, Oct-6 depletion leads to a reduced G2-cell cycle arrest and senescence, but also to increased levels of intracellular ROS and DNA damage. In addition, we could identify p21 and catalase as Oct-6 target genes possibly mediating these effects. These results demonstrate that Oct-6 is expressed in cancer cells after genotoxic stress, and suggests its possible role in the control of ROS, DNA damage response (DDR), and senescence.

Involvement of AMP-activated protein kinase and Death Receptor 5 in TRAIL-Berberine-induced apoptosis of cancer cells

Our previous studies indicated that combination of Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and PPARγ ligand Troglitazone (TZD), can induce significant apoptosis in various TRAIL-resistant prostate and hepatocellular carcinoma (HCC) cells. These also suggested serine/threonine kinase AMP-activated protein kinase (AMPK) to be a mediator of TRAIL-TZD-induced apoptosis. To further validate AMPK’s role in TRAIL sensitization, we determined the apoptotic potential of TRAIL in combination with the natural compound Berberine (BBR), the latter being a potent activator of AMPK. These demonstrated a significant reduction of cell viability and induction of apoptosis (increased cleavage of caspase 3, 8, 9) when treated with TRAIL-BBR combination. This apoptosis is attenuated in cells overexpressing AMPKα-dominant negative (DN) or following AMPKα knockdown, confirming involvement of AMPK. To identify potential downstream mediators involved, an apoptosis RT² PCR array analysis was performed. These showed induction of several genes including TNFRSF10B (expresses DR5) and Harakiri following BBR treatment, which were further validated by qPCR analysis. Furthermore, knocking down DR5 expression significantly attenuated TRAIL-BBR-induced apoptosis, suggesting DR5 to be a mediator of this apoptosis. Our studies indicate that combination of TRAIL and AMPK activator BBR might be an effective means of ameliorating TRAIL-resistance involving DR5 in advanced cancer.

Cdc6 contributes to cisplatin-resistance by activation of ATR-Chk1 pathway in bladder cancer cells

High activation of DNA damage response is implicated in cisplatin (CDDP) resistance which presents as a serious obstacle for bladder cancer treatment. Cdc6 plays an important role in the malignant progression of tumor. Here, we reported that Cdc6 expression is up-regulated in bladder cancer tissues and is positively correlated to high tumor grade. Cdc6 depletion can attenuate the malignant properties of bladder cancer cells, including DNA replication, migration and invasion. Furthermore, higher levels of chromatin-binding Cdc6 and ATR were detected in CDDP-resistant bladder cancer cells than in the parent bladder cancer cells. Intriguingly, down-regulation of Cdc6 can enhance sensitivity to CDDP both in bladder cancer cells and CDDP-resistant bladder cancer cells. Cdc6 depletion abrogates S phase arrest caused by CDDP, leading to aberrant mitosis by inactivating ATR-Chk1-Cdc25C pathway. Our results indicate that Cdc6 may be a promising target for overcoming CDDP resistance in bladder cancer.

Inhibition of ATR potentiates the cytotoxic effect of gemcitabine on pancreatic cancer cells through enhancement of DNA damage and abrogation of ribonucleotide reductase induction by gemcitabine

Pancreatic cancer is a highly malignant disease with a dismal prognosis. Gemcitabine (GEM)-based chemotherapy is the first-line treatment for patients with advanced disease, although its efficacy is very limited, mainly due to drug resistance. Ataxia telangiectasia and Rad3-related (ATR) plays a critical role in the DNA damage response (DDR) which has been implicated in GEM resistance. Thus, targeting ATR represents a promising approach to enhance GEM antitumor activity. In the present study, we tested the antitumor activity of AZ20, a novel ATR-selective inhibitor, alone or combined with GEM in 5 pancreatic cancer cell lines. AZ20 treatment of the pancreatic cancer cell lines resulted in growth inhibition, with IC50 values ranging from 0.84 to 2.4 µM, but limited cell death. As expected, treatment of pancreatic cancer cell lines with AZ20 caused decreased phosphorylation of CHK1 (S-345). However, this was accompanied by DNA damage and S and G2/M cell cycle arrest, independent of TP53 gene mutational status. Importantly, combination of AZ20 with GEM resulted in synergistic inhibition of cell growth and cooperative induction of cell death in the pancreatic cancer cell lines. AZ20 significantly increased GEM-induced DNA damage and almost completely abrogated GEM-induced expression of the M2 subunit of ribonucleotide reductase. These findings suggest that inhibition of ATR is a promising strategy to enhance the antitumor activity of GEM for treating pancreatic cancer.

Genomic analysis of exceptional responders to radiotherapy reveals somatic mutations in ATM

Radiation therapy is a mainstay of cancer treatment, yet the molecular determinants of clinical response are poorly understood. We identified exceptional responders to radiotherapy based on clinical response, and investigated the associated tumor sequencing data in order to identify additional patients with similar mutations. Among head and neck squamous cell cancer patients receiving palliative radiotherapy at our institution, we identified one patient with documented complete metabolic response. Targeted sequencing analysis of the tumor identified a somatic frame-shift mutation in ATM, a gene known to be associated with radio-sensitivity in the germline. To validate the association of somatic ATM mutation with radiotherapy response, we identified eight patients with ATM truncating mutations who received radiotherapy, all of whom demonstrated excellent responses with a median local control period of 4.62 years. Analysis of 22 DNA repair genes in The Cancer Genome Atlas (TCGA) data revealed mutations in 15.9% of 9064 tumors across 24 cancer types, with ATM mutations being the most prevalent. This is the first study to suggest that exceptional responses to radiotherapy may be determined by mutations in DNA repair genes. Sequencing of DNA repair genes merits attention in larger cohorts and may have significant implications for the personalization of radiotherapy.

From breaking bad to worse: exploiting homologous DNA repair deficiency in cancer

DNA repair deficiencies are common among cancer cells and represent a potential vulnerability that might be exploited by targeting compensatory repair pathways. However, the identification of synthetically lethal combinations of DNA repair defects, although of significant clinical relevance, has been somewhat anecdotal. Although numerous models have been proposed to explain synthetic lethality among DNA repair mutations, we have only a limited understanding of why a given mutation should render cells sensitive to another. In this issue of Cancer Discovery, Dietlein and colleagues define a general connection between mutations in genes involved in homologous recombination and sensitivity to inhibitors of non-homologous end joining. In doing so, they provide a mechanism to demarcate a set of seemingly diverse tumors that may be highly responsive to established DNA repair-targeted therapeutics.

DNA Damage Signalling and Repair Inhibitors: The Long-Sought-After Achilles' Heel of Cancer

For decades, radiotherapy and chemotherapy were the two only approaches exploiting DNA repair processes to fight against cancer. Nowadays, cancer therapeutics can be a major challenge when it comes to seeking personalized targeted medicine that is both effective and selective to the malignancy. Over the last decade, the discovery of new targeted therapies against DNA damage signalling and repair has offered the possibility of therapeutic improvements in oncology. In this review, we summarize the current knowledge of DNA damage signalling and repair inhibitors, their molecular and cellular effects, and future therapeutic use.

Trial Watch: Targeting ATM-CHK2 and ATR-CHK1 pathways for anticancer therapy

The ataxia telangiectasia mutated serine/threonine kinase (ATM)/checkpoint kinase 2 (CHEK2, best known as CHK2) and the ATM and Rad3-related serine/threonine kinase (ATR)/CHEK1 (best known as CHK1) cascades are the 2 major signaling pathways driving the DNA damage response (DDR), a network of processes crucial for the preservation of genomic stability that act as a barrier against tumorigenesis and tumor progression. Mutations and/or deletions of ATM and/or CHK2 are frequently found in tumors and predispose to cancer development. In contrast, the ATR-CHK1 pathway is often upregulated in neoplasms and is believed to promote tumor growth, although some evidence indicates that ATR and CHK1 may also behave as haploinsufficient oncosuppressors, at least in a specific genetic background. Inactivation of the ATM-CHK2 and ATR-CHK1 pathways efficiently sensitizes malignant cells to radiotherapy and chemotherapy. Moreover, ATR and CHK1 inhibitors selectively kill tumor cells that present high levels of replication stress, have a deficiency in p53 (or other DDR players), or upregulate the ATR-CHK1 module. Despite promising preclinical results, the clinical activity of ATM, ATR, CHK1, and CHK2 inhibitors, alone or in combination with other therapeutics, has not yet been fully demonstrated. In this Trial Watch, we give an overview of the roles of the ATM-CHK2 and ATR-CHK1 pathways in cancer initiation and progression, and summarize the results of clinical studies aimed at assessing the safety and therapeutic profile of regimens based on inhibitors of ATR and CHK1, the only 2 classes of compounds that have so far entered clinics.

Chk1 and Wee1 control genotoxic-stress induced G2-M arrest in melanoma cells

In the present report, the role of ATR-Chk1-Wee1 and ATM-Chk2-p53-p21 pathways in stress-induced cell cycle control is analysed in melanoma cells. Treatment of p53 wild-type melanoma cells with the genotoxic agent doxorubicin induces G2-M arrest, inhibitory phosphorylation of cell cycle kinase Cdc2 (CDK1) and enhanced expression of p53/p21. Wee1 inhibition under doxorubicin pulse-treatment reduces G2-M arrest and induces apoptosis. Inhibition of upstream kinase Chk1 under doxorubicin treatment almost completely abolishes stress-induced G2-M arrest and induces enhanced apoptosis. Interestingly, Chk1 inhibition alone even further increases apoptosis. While Chk1 inhibition alone almost completely abolishes G0-G1 arrest, combined treatment with doxorubicin re-establishes G0-G1 arrest. Moreover, Chk1 inhibition alone induces only a slight p53/p21 induction, while a strong induction of both proteins is observed by the combination with doxorubicin. These findings are suggestive for a particular role of p53/p21 in G0-G1, and Chk1 in G0-G1 and G2-M arrest. In line with this, the p53-mutant SK-Mel-28 melanoma cells do not mount a significant G0-G1 arrest under combined doxorubicin and Chk1 inhibitor treatment but rather show extensive apoptosis. Moreover, knockdown of p21 dramatically reduces stress-induced G0-G1 arrest under doxorubicin and Chk1 inhibitor treatment accompanied by massive DNA damage and apoptosis induction. Treatment of melanoma cells with an inhibitor of Chk2 upstream kinase ATM and doxorubicin almost completely abolishes G0-G1 arrest. Taken together, both Chk1 and Wee1 are mediators of G2-M arrest, while p53, p21 and Chk1 are mediators of G0-G1 arrest in melanoma cells. Combined treatment with chemotherapeutic agents such as doxorubicin and Chk1 inhibitors may help to overcome apoptosis resistance of p53-proficient melanoma cells. But treatment with Chk1 inhibitor alone may even be more efficient.

The effect of caffeine on cisplatin-induced apoptosis of lung cancer cells

Background

Cisplatin is an important DNA-damaging anticancer drug that has been used to treat many cancer types. However, the effectiveness of cisplatin treatment diminishes quickly as cancer cells develop resistance to the drug, which eventually results in treatment failure. Caffeine is an ingredient contained in many food sources. Caffeine can inhibit activities of both ATM and ATR, two important protein kinases involved in DNA damage-induced cell cycle arrest and apoptosis. The effect of caffeine on cisplatin-based cancer treatment is not well known.

Methods

Caspase-3 activation and cell growth inhibition assays were used to determine the effect of caffeine on cisplatin-induced apoptosis and cell growth in lung cancer cells. Real time PCR, immunoblotting, and flow cytometry assays were used determine a mechanism through which the presence of caffeine increased cisplatin-induced apoptosis of the lung cancer cells.

Results

Our caspase-3 activation studies demonstrated that the presence of caffeine increased the cisplatin-induced apoptosis in both HTB182 and CRL5985 lung cancer cells. Our cell growth inhibition studies indicated that the presence of caffeine caused a more increase for cisplatin-induced cell growth inhibition. The results obtained from our real time PCR and western blot studies revealed that the presence of caffeine increased cisplatin-induced expression of the PUMA pro-apoptotic protein in these lung cancer cells. The results of our protein phosphorylation studies indicated that the presence of caffeine caused a decrease in CHK1 phosphorylation at Ser³¹⁷/Ser³⁴⁵ but an increase in ATM phosphorylation at Ser¹⁹⁸¹ in the lung cancer cells treated with cisplatin. In addition, our flow cytometry studies also revealed that the presence of caffeine caused an increase in G1 cell population but a decrease for cisplatin-induced cell cycle arrests at the S and the G2 checkpoints in HTB182 and CRL5985 cells respectively.

Conclusion

Our results suggest that the presence of caffeine increases the cisplatin-induced lung cancer cell killings by inhibiting ATR but inducing ATM activation, resulting in an increase in expression of PUMA protein and an increase in apoptosis.

Phosphoprotein phosphatase 1-interacting proteins as therapeutic targets in prostate cancer

Prostate cancer is a major public health concern worldwide, being one of the most prevalent cancers in men. Great improvements have been made both in terms of early diagnosis and therapeutics. However, there is still an urgent need for reliable biomarkers that could overcome the lack of cancer-specificity of prostate-specific antigen, as well as alternative therapeutic targets for advanced metastatic cases. Reversible phosphorylation of proteins is a post-translational modification critical to the regulation of numerous cellular processes. Phosphoprotein phosphatase 1 (PPP1) is a major serine/threonine phosphatase, whose specificity is determined by its interacting proteins. These interactors can be PPP1 substrates, regulators, or even both. Deregulation of this protein-protein interaction network alters cell dynamics and underlies the development of several cancer hallmarks. Therefore, the identification of PPP1 interactome in specific cellular context is of crucial importance. The knowledge on PPP1 complexes in prostate cancer remains scarce, with only 4 holoenzymes characterized in human prostate cancer models. However, an increasing number of PPP1 interactors have been identified as expressed in human prostate tissue, including the tumor suppressors TP53 and RB1. Efforts should be made in order to identify the role of such proteins in prostate carcinogenesis, since only 26 have yet well-recognized roles. Here, we revise literature and human protein databases to provide an in-depth knowledge on the biological significance of PPP1 complexes in human prostate carcinogenesis and their potential use as therapeutic targets for the development of new therapies for prostate cancer.

Targeting ATR in DNA damage response and cancer therapeutics

The ataxia telangiectasia and Rad3-related (ATR) plays an important role in maintaining genome integrity during DNA replication through the phosphorylation and activation of Chk1 and regulation of the DNA damage response. Preclinical studies have shown that disruption of ATR pathway can exacerbate the levels of replication stress in oncogene-driven murine tumors to promote cell killing. Additionally, inhibition of ATR can sensitise tumor cells to radiation or chemotherapy. Accumulating evidence suggests that targeting ATR can selectively sensitize cancer cells but not normal cells to DNA damage. Furthermore, in hypoxic conditions, ATR blockade results in overloading replication stress and DNA damage response causing cell death. Despite the attractiveness of ATR inhibition in the treatment of cancer, specific ATR inhibitors have remained elusive. In the last two years however, selective ATR inhibitors suitable for in vitro and - most recently - in vivo studies have been identified. In this article, we will review the literature on ATR function, its role in DDR and the potential of ATR inhibition to enhance the efficacy of radiation and chemotherapy.

A synthetic interaction screen identifies factors selectively required for proliferation and TERT transcription in p53-deficient human cancer cells

Numerous genetic and epigenetic alterations render cancer cells selectively dependent on specific genes and regulatory pathways, and represent potential vulnerabilities that can be therapeutically exploited. Here we describe an RNA interference (RNAi)–based synthetic interaction screen to identify genes preferentially required for proliferation of p53-deficient (p53−) human cancer cells. We find that compared to p53-competent (p53+) human cancer cell lines, diverse p53− human cancer cell lines are preferentially sensitive to loss of the transcription factor ETV1 and the DNA damage kinase ATR. In p53− cells, RNAi–mediated knockdown of ETV1 or ATR results in decreased expression of the telomerase catalytic subunit TERT leading to growth arrest, which can be reversed by ectopic TERT expression. Chromatin immunoprecipitation analysis reveals that ETV1 binds to a region downstream of the TERT transcriptional start-site in p53− but not p53+ cells. We find that the role of ATR is to phosphorylate and thereby stabilize ETV1. Our collective results identify a regulatory pathway involving ETV1, ATR, and TERT that is preferentially important for proliferation of diverse p53− cancer cells.

The conversion of a normal cell into a cancer cell involves activating genes that promote cancer growth (oncogenes) and/or inactivating genes that normally act to inhibit cancer growth (tumor suppressor genes). The tumor suppressor gene p53 is the most frequently mutated gene in human cancers, being inactivated in approximately half of all tumors. In addition, loss of p53 function is often associated with increased resistance to chemotherapy and/or poor survival. For these reasons, the selective destruction of p53-deficient (p53−) tumors has remained one of the most important goals and challenges of cancer therapy. One strategy for destroying p53− tumors is to inactivate genes that are preferentially required for the growth or survival of p53− cells. Here we carry out a large-scale genetic screen to identify a cellular pathway that is preferentially required for growth of p53− cancer cells.

Genistein abrogates G2 arrest induced by curcumin in p53 deficient T47D cells

BACKGROUND

The high cost and low level of cancer survival urge the finding of new drugs having better mechanisms. There is a high trend of patients to be "back to nature" and use natural products as an alternative way to cure cancer. The fact is that some of available anticancer drugs are originated from plants, such as taxane, vincristine, vinblastine, pacitaxel. Curcumin (diferuloylmethane), a dietary pigment present in Curcuma longa rizhome is reported to induce cell cycle arrest in some cell lines. Other study reported that genistein isolated from Glycine max seed inhibited phosphorylation of cdk1, gene involved during G2/M transition and thus could function as G2 checkpoint abrogator. The inhibition of cdk1 phosphorylation is one of alternative strategy which could selectively kill cancer cells and potentially be combined with DNA damaging agent such as curcumin.

METHODS

T47D cell line was treated with different concentrations of curcumin and genistein, alone or in combination; added together or with interval time. Flow Cytometry and MTT assay were used to evaluate cell cycle distribution and viability, respectively. The presence of apoptotic cells was determined using acridine orange-ethidium bromide staining.

RESULTS

In this study curcumin induced G2 arrest on p53 deficient T47D cells at the concentration of 10 μM. Increasing concentration up to 30 μM increased the number of cell death. Whilst genistein alone at low concentration (≤10 μM) induced cell proliferation, addition of genistein (20 μM) 16 h after curcumin resulted in more cell death (89%), 34% higher than that administered at the same time (56%). The combination treatment resulted in apoptotic cell death. Combining curcumin with high dose of genistein (50 μM) induced necrotic cells.

CONCLUSIONS

Genistein increased the death of curcumin treated T47D cells. Appropriate timing of administration and concentration of genistein determine the outcome of treatment and this method could potentially be developed as an alternative strategy for treatment of p53 defective cancer cells.

Interferon-gamma enhances radiation-induced cell death via downregulation of Chk1

Interferon-gamma (IFNγ) is a cytokine with roles in immune responses as well as in tumor control. Interferon is often used in cancer treatment together with other therapies. Here we report a novel approach to enhancement of cancer cell killing by combined treatment of IFNγ with ionizing radiation. We found that IFNγ treatment alone in HeLa cells induced phosphorylation of Chk1 in a time- and dose-dependent manner, and resulted in cell arrest. Moreover IFNγ treatment was correlated with attenuation of Chk1 as the treatment shortened protein half-life of Chk1. As Chk1 is an essential cell cycle regulator for viability after DNA damage, attenuation of Chk1 by IFNγ pre-treatment in HeLa cells resulted in increased cell death following ionizing radiation about 2-folds than ionizing radiation treatment alone whereas IFNγ treatment alone had little effect on cell death. X-linked inhibitor of apoptosis-associated factor 1 (XAF1), an IFN-induced gene, seems to partly regulate IFNγ-induced Chk1 destabilization and radiation sensitivity because transient depletion of XAF1 by siRNA prevented IFNγ-induced Chk1 attenuation and partly protected cells from IFNγ-enhanced radiation cell killing. Therefore the results provide a novel rationale to combine IFNγ pretreatment and DNA-damaging anti-cancer drugs such as ionizing radiation to enhance cancer cell killing.

Defective cell cycle checkpoints as targets for anti-cancer therapies

Conventional chemotherapeutics target the proliferating fraction of cells in the patient's body, which will include the tumor cells, but are also toxic to actively proliferating normal tissues. Cellular stresses, such as those imposed by chemotherapeutic drugs, induce cell cycle checkpoint arrest, and currently approaches targeting these checkpoints are being explored to increase the efficacy and selectivity of conventional chemotherapeutic treatments. Loss of a checkpoint may also make cancer cells more reliant on other mechanisms to compensate for the loss of this function, and these compensatory mechanisms may be targeted using synthetic lethal approaches. Here we will discuss the utility of targeting checkpoint defects as novel anti-cancer therapies.

Upon the tightrope in prostate cancer: two acrobats on the same tightrope to cross the finishline

Prostate cancer is a multifactorial, multistep progressive disorder that is undruggable to date because of stumbling blocks in the standardization of therapy. It is triggered by a broad range of proteins, signaling networks and DNA damage response modulators. It is becoming increasingly apparent that DNA repair mediators have split personalities, as they are instrumental in suppressing and promoting carcinogenesis. In this article, we discuss on post-transcriptional processing of regulators of DNA damage response, and how DNA repair proteins trigger shuttling of androgen receptor. Substantial fraction of information has been added into the existing literature of ATM biology; however, the particular area of post-transcriptional processing errors and gene therapy for reprogramming of ATM has been left unaddressed in prostate cancer. It is therefore noteworthy that the facet of targeting strategy, antisense morpholino oligonucleotides chemistry, and systematic delivery of AOs has promising outlook in splice-targeted antisense-mediated therapy.

Analysis of the specific pathways and networks of prostate cancer for gene expression profiles in the Chinese population

The global physiological function of specifically expressed genes of prostate cancer in Chinese patients is unclear. This study aims to determine the genome-wide expression of genes related to prostate cancer in the Chinese population. Genes that were differentially expressed in prostate cancer were identified using DNA microarray technology. Expressions were validated by using real-time PCR. The identified genes were analyzed using the ingenuity pathway analysis (IPA) to investigate the gene ontology, functional pathway and network. A total of 1,444 genes (Fold time ≥ 1.5; P ≤ 0.05) were differentially expressed in prostate primary tumor tissue compared with benign tissue. IPA revealed a unique landscape where inductions of certain pathways were involved in Cell Cycle Regulation and proliferation. Network analysis not only confirmed that protein interactions lead to the deregulation of DNA Replication, Recombination and Repair, Cellular Compromise and Cell Cycle, Genetic Disorders and Connective Tissue Disorders, but it was also observed that many of the genes regulated by Myc contributed to the modulation of lipid Metabolism and Nucleic Acid Metabolism. Both pathway and network analysis exhibited some remarkable characteristics of prostate cancer for Chinese patients, which showed profound differences from that of other non-Chinese populations. These differences may provide new insights into the molecular cascade of prostate cancer that occurs in Chinese patients.

Polycomb protein EZH2 regulates cancer cell fate decision in response to DNA damage

Polycomb protein histone methyltransferase enhancer of Zeste homologe 2 (EZH2) is frequently overexpressed in human malignancy and is implicated in cancer cell proliferation and invasion. However, it is largely unknown whether EZH2 has a role in modulating DNA damage response. Here, we show that EZH2 is an important determinant of cell fate decision in response to genotoxic stress. EZH2 depletion results in abrogation of both cell cycle G1 and G2/M checkpoints, directing DNA damage response toward predominant apoptosis in both p53-proficient and p53-deficient cancer cells, but not in normal cells. Mechanistically, EZH2 regulates DNA damage response in p53 wild-type cells mainly through transcriptional repression of FBXO32, which binds to and directs p21 for proteasome-mediated degradation, whereas it affects p53-deficient cells through regulating Chk1 activation by a distinct mechanism. Furthermore, pharmacological depletion of EZH2 phenocopies the effects of EZH2 knockdown on cell cycle checkpoints and apoptosis. These data unravel a crucial role of EZH2 in determining the cancer cell outcome following DNA damage and suggest that therapeutic targeting oncogenic EZH2 might serve as a strategy for improving conventional chemotherapy in a given malignancy.

Identification of aneuploidy-selective antiproliferation compounds

Aneuploidy, an incorrect chromosome number, is a hallmark of cancer. Compounds that cause lethality in aneuploid, but not euploid, cells could therefore provide new cancer therapies. We have identified the energy stress-inducing agent AICAR, the protein folding inhibitor 17-AAG, and the autophagy inhibitor chloroquine as exhibiting this property. AICAR induces p53-mediated apoptosis in primary mouse embryonic fibroblasts (MEFs) trisomic for chromosome 1, 13, 16, or 19. AICAR and 17-AAG, especially when combined, also show efficacy against aneuploid human cancer cell lines. Our results suggest that compounds that interfere with pathways that are essential for the survival of aneuploid cells could serve as a new treatment strategy against a broad spectrum of human tumors.

The ATM-Chk2 and ATR-Chk1 pathways in DNA damage signaling and cancer

DNA damage is a key factor both in the evolution and treatment of cancer. Genomic instability is a common feature of cancer cells, fuelling accumulation of oncogenic mutations, while radiation and diverse genotoxic agents remain important, if imperfect, therapeutic modalities. Cellular responses to DNA damage are coordinated primarily by two distinct kinase signaling cascades, the ATM-Chk2 and ATR-Chk1 pathways, which are activated by DNA double-strand breaks (DSBs) and single-stranded DNA respectively. Historically, these pathways were thought to act in parallel with overlapping functions; however, more recently it has become apparent that their relationship is more complex. In response to DSBs, ATM is required both for ATR-Chk1 activation and to initiate DNA repair via homologous recombination (HRR) by promoting formation of single-stranded DNA at sites of damage through nucleolytic resection. Interestingly, cells and organisms survive with mutations in ATM or other components required for HRR, such as BRCA1 and BRCA2, but at the cost of genomic instability and cancer predisposition. By contrast, the ATR-Chk1 pathway is the principal direct effector of the DNA damage and replication checkpoints and, as such, is essential for the survival of many, although not all, cell types. Remarkably, deficiency for HRR in BRCA1- and BRCA2-deficient tumors confers sensitivity to cisplatin and inhibitors of poly(ADP-ribose) polymerase (PARP), an enzyme required for repair of endogenous DNA damage. In addition, suppressing DNA damage and replication checkpoint responses by inhibiting Chk1 can enhance tumor cell killing by diverse genotoxic agents. Here, we review current understanding of the organization and functions of the ATM-Chk2 and ATR-Chk1 pathways and the prospects for targeting DNA damage signaling processes for therapeutic purposes.

DNA adducts of decarbamoyl mitomycin C efficiently kill cells without wild-type p53 resulting from proteasome-mediated degradation of checkpoint protein 1

The mitomycin derivative 10-decarbamoyl mitomycin C (DMC) more rapidly activates a p53-independent cell death pathway than mitomycin C (MC). We recently documented that an increased proportion of mitosene1-β-adduct formation occurs in human cells treated with DMC in comparison to those treated with MC. Here, we compare the cellular and molecular response of human cancer cells treated with MC and DMC. We find the increase in mitosene 1-β-adduct formation correlates with a condensed nuclear morphology and increased cytotoxicity in human cancer cells with or without p53. DMC caused more DNA damage than MC in the nuclear and mitochondrial genomes. Checkpoint 1 protein (Chk1) was depleted following DMC, and the depletion of Chk1 by DMC was achieved through the ubiquitin proteasome pathway since chemical inhibition of the proteasome protected against Chk1 depletion. Gene silencing of Chk1 by siRNA increased the cytotoxicity of MC. DMC treatment caused a decrease in the level of total ubiquitinated proteins without increasing proteasome activity, suggesting that DMC mediated DNA adducts facilitate signal transduction to a pathway targeting cellular proteins for proteolysis. Thus, the mitosene-1-β stereoisomeric DNA adducts produced by the DMC signal for a p53-independent mode of cell death correlated with reduced nuclear size, persistent DNA damage, increased ubiquitin proteolysis and reduced Chk1 protein.

DNA damage activates a spatially distinct late cytoplasmic cell-cycle checkpoint network controlled by MK2-mediated RNA stabilization

Following genotoxic stress, cells activate a complex kinase-based signaling network to arrest the cell cycle and initiate DNA repair. p53-defective tumor cells rewire their checkpoint response and become dependent on the p38/MK2 pathway for survival after DNA damage, despite a functional ATR-Chk1 pathway. We used functional genetics to dissect the contributions of Chk1 and MK2 to checkpoint control. We show that nuclear Chk1 activity is essential to establish a G(2)/M checkpoint, while cytoplasmic MK2 activity is critical for prolonged checkpoint maintenance through a process of posttranscriptional mRNA stabilization. Following DNA damage, the p38/MK2 complex relocalizes from nucleus to cytoplasm where MK2 phosphorylates hnRNPA0, to stabilize Gadd45α mRNA, while p38 phosphorylates and releases the translational inhibitor TIAR. In addition, MK2 phosphorylates PARN, blocking Gadd45α mRNA degradation. Gadd45α functions within a positive feedback loop, sustaining the MK2-dependent cytoplasmic sequestration of Cdc25B/C to block mitotic entry in the presence of unrepaired DNA damage. Our findings demonstrate a critical role for the MK2 pathway in the posttranscriptional regulation of gene expression as part of the DNA damage response in cancer cells.

Prospects for the Use of ATR Inhibitors to Treat Cancer

ATR is an apical kinase in one of the DNA-damage induced checkpoint pathways. Despite the development of inhibitors of kinases structurally related to ATR, as well as inhibitors of the ATR substrate Chk1, no ATR inhibitors have yet been developed. Here we review the effects of ATR downregulation in cancer cells and discuss the potential for development of ATR inhibitors for clinical use.

SOCS1 links cytokine signaling to p53 and senescence

SOCS1 is lost in many human tumors, but its tumor suppression activities are not well understood. We report that SOCS1 is required for transcriptional activity, DNA binding, and serine 15 phosphorylation of p53 in the context of STAT5 signaling. In agreement, inactivation of SOCS1 disabled p53-dependent senescence in response to oncogenic STAT5A and radiation-induced apoptosis in T cells. In addition, SOCS1 was sufficient to induce p53-dependent senescence in fibroblasts. The mechanism of activation of p53 by SOCS1 involved a direct interaction between the SH2 domain of SOCS1 and the N-terminal transactivation domain of p53, while the C-terminal domain of SOCS1 containing the SOCS Box mediated interaction with the DNA damage-regulated kinases ATM/ATR. Also, SOCS1 colocalized with ATM at DNA damage foci induced by oncogenic STAT5A. Collectively, these results add another component to the p53 and DNA damage networks and reveal a mechanism by which SOCS1 functions as a tumor suppressor.

Exploiting synthetic lethal interactions for targeted cancer therapy

Emerging data suggests that synthetic lethal interactions between mutated oncogenes/tumor suppressor genes and molecules involved in DNA damage signaling and repair can be therapeutically exploited to preferentially kill tumor cells. In this review, we discuss the concept of synthetic lethality, and describe several recent examples in which this concept was successfully implemented to target tumor cells in culture, in mouse models, and in human cancer patients.

The combined status of ATM and p53 link tumor development with therapeutic response

While the contribution of specific tumor suppressor networks to cancer development has been the subject of considerable recent study, it remains unclear how alterations in these networks are integrated to influence the response of tumors to anti-cancer treatments. Here, we show that mechanisms commonly used by tumors to bypass early neoplastic checkpoints ultimately determine chemotherapeutic response and generate tumor-specific vulnerabilities that can be exploited with targeted therapies. Specifically, evaluation of the combined status of ATM and p53, two commonly mutated tumor suppressor genes, can help to predict the clinical response to genotoxic chemotherapies. We show that in p53-deficient settings, suppression of ATM dramatically sensitizes tumors to DNA-damaging chemotherapy, whereas, conversely, in the presence of functional p53, suppression of ATM or its downstream target Chk2 actually protects tumors from being killed by genotoxic agents. Furthermore, ATM-deficient cancer cells display strong nononcogene addiction to DNA-PKcs for survival after DNA damage, such that suppression of DNA-PKcs in vivo resensitizes inherently chemoresistant ATM-deficient tumors to genotoxic chemotherapy. Thus, the specific set of alterations induced during tumor development plays a dominant role in determining both the tumor response to conventional chemotherapy and specific susceptibilities to targeted therapies in a given malignancy.

Dual regulation of Cdc25A by Chk1 and p53-ATF3 in DNA replication checkpoint control

Eukaryotic cells respond to DNA damage and stalled replication forks by activating signaling pathways that promote cell cycle arrest and DNA repair. A systematic screening of the protein kinase small interfering RNA library reveals that Chk1 and ataxia telangiectasia-mutated (ATM) and Rad3-related (ATR) are the main kinases responsible for intra-S-phase checkpoint upon topoisomerase I inhibitor camptothecin-induced DNA damage. It is well known that ATR-Chk1-mediated protein degradation of Cdc25A protein phosphatase is a crucial mechanism conferring this checkpoint activation. Here we describe another mechanism underlying Cdc25A down-regulation in response to DNA damage that occurs at the transcriptional level. We show that activation of tumor suppressor p53 by DNA damage results in inhibition of Cdc25A transcription as a result of activation of transcriptional repressor ATF3 that directly binds to the Cdc25A promoter. In cells deficient in both Chk1 and p53, Cdc25A down-regulation upon camptothecin-induced DNA damage is completely abolished, leading to severe defects in cell cycle checkpoints and remarkable cell death in mitosis. Our findings reveal two independent mechanisms acting in concert in regulation of Cdc25A in DNA damage response. Although Chk1 affects Cdc25A via rapid phosphorylation and protein turnover, inhibition of Cdc25A transcription by p53-ATF3 is required for the maintenance of cell cycle arrest.

Single cell cytometry of protein function in RNAi treated cells and in native populations

Background

High Content Screening has been shown to improve results of RNAi and other perturbations, however significant intra-sample heterogeneity is common and can complicate some analyses. Single cell cytometry can extract important information from subpopulations within these samples. Such approaches are important for immune cells analyzed by flow cytometry, but have not been broadly available for adherent cells that are critical to the study of solid-tumor cancers and other disease models.

Results

We have directly quantitated the effect of resolving RNAi treatments at the single cell level in experimental systems for both exogenous and endogenous targets. Analyzing the effect of an siRNA that targets GFP at the single cell level permits a stronger measure of the absolute function of the siRNA by gating to eliminate background levels of GFP intensities. Extending these methods to endogenous proteins, we have shown that well-level results of the knockdown of PTEN results in an increase in phospho-S6 levels, but at the single cell level, the correlation reveals the role of other inputs into the pathway. In a third example, reduction of STAT3 levels by siRNA causes an accumulation of cells in the G1 phase of the cell cycle, but does not induce apoptosis or necrosis when compared to control cells that express the same levels of STAT3. In a final example, the effect of reduced p53 levels on increased adriamycin sensitivity for colon carcinoma cells was demonstrated at the whole-well level using siRNA knockdown and in control and untreated cells at the single cell level.

Conclusion

We find that single cell analysis methods are generally applicable to a wide range of experiments in adherent cells using technology that is becoming increasingly available to most laboratories. It is well-suited to emerging models of signaling dysfunction, such as oncogene addition and oncogenic shock. Single cell cytometry can demonstrate effects on cell function for protein levels that differ by as little as 20%. Biological differences that result from changes in protein level or pathway activation state can be modulated directly by RNAi treatment or extracted from the natural variability intrinsic to cells grown under normal culture conditions.

Topoisomerase levels determine chemotherapy response in vitro and in vivo

Topoisomerase poisons are chemotherapeutic agents that are used extensively for treating human malignancies. These drugs can be highly effective, yet tumors are frequently refractory to treatment or become resistant upon tumor relapse. Using a pool-based RNAi screening approach and a well characterized mouse model of lymphoma, we explored the genetic basis for heterogeneous responses to topoisomerase poisons in vitro and in vivo. These experiments identified Top2A expression levels as major determinants of response to the topoisomerase 2 poison doxorubicin and showed that suppression of Top2A produces resistance to doxorubicin in vitro and in vivo. Analogously, using a targeted RNAi approach, we demonstrated that suppression of Top1 produces resistance to the topoisomerase 1 poison camptothecin yet hypersensitizes cancer cells to doxorubicin. Importantly, lymphomas relapsing after treatment display spontaneous changes in topoisomerase levels as predicted by in vitro gene knockdown studies. These results highlight the utility of pooled shRNA screens for identifying genetic determinants of chemotherapy response and suggest strategies for improving the effectiveness of topoisomerase poisons in the clinic.

Topoisomerase levels determine chemotherapy response in vitro and in vivo

Topoisomerase poisons are chemotherapeutic agents that are used extensively for treating human malignancies. These drugs can be highly effective, yet tumors are frequently refractory to treatment or become resistant upon tumor relapse. Using a pool-based RNAi screening approach and a well characterized mouse model of lymphoma, we explored the genetic basis for heterogeneous responses to topoisomerase poisons in vitro and in vivo. These experiments identified Top2A expression levels as major determinants of response to the topoisomerase 2 poison doxorubicin and showed that suppression of Top2A produces resistance to doxorubicin in vitro and in vivo. Analogously, using a targeted RNAi approach, we demonstrated that suppression of Top1 produces resistance to the topoisomerase 1 poison camptothecin yet hypersensitizes cancer cells to doxorubicin. Importantly, lymphomas relapsing after treatment display spontaneous changes in topoisomerase levels as predicted by in vitro gene knockdown studies. These results highlight the utility of pooled shRNA screens for identifying genetic determinants of chemotherapy response and suggest strategies for improving the effectiveness of topoisomerase poisons in the clinic.

Loss of ataxia telangiectasia mutated- and Rad3-related function potentiates the effects of chemotherapeutic drugs on cancer cell survival

The diverse responses of human cells to various forms of DNA damage are controlled by a complex network of signaling proteins. There has been considerable interest in the components of this signaling apparatus as potential targets for new forms of anticancer therapy. In this report, we examine the contributions of an upstream signaling molecule, the ataxia telangiectasia mutated- and Rad3-related (ATR) protein kinase, to the resistance of cancer cells to DNA-damaging agents that are commonly used as anticancer therapeutics. Loss of ATR function in knock-in cancer cells strikingly enhanced the effects of several of the most commonly used therapeutic compounds, impeding the progression of the cell cycle and reducing long-term cancer cell survival. Loss of ATR function potentiated the toxicity of alkylating agents most strikingly, antimetabolites moderately, and double-strand break-inducing agents to a lesser extent. These results suggest that specific inhibition of ATR activity will be a valid strategy to increase the effectiveness of currently used modes of therapy.

Silencing of endo-exonuclease expression sensitizes mouse B16F10 melanoma cells to DNA damaging agents

We previously identified an endo-exonuclease that is highly expressed in cancer cells and plays an important role in DSB repair mechanisms. A small molecular compound pentamidine, which specifically inhibited nuclease activity of the isolated endo-exonuclease from yeast as well as from mammalian cells, was capable of sensitizing tumor cells to DNA damaging agents. In this study, we investigated the effect of precisely silencing the endo-exonuclease expression by small interfering RNA (siRNA) upon treatment with a variety of DNA damaging agents in mouse B16F10 melanoma cells. A maximum of 3.6 to approximately 4-fold reduction in endo-exonuclease mRNA expression was achieved, over a period of 48-72 h of post transfection with a concomitant reduction in protein expression (approximately 4-5 fold), resulting in a substantial reduction (approximately 45-50%) of the corresponding nuclease activity. Suppressed endo-exonuclease expression conferred significant decrease in cell survival, ranging from approximately 30 to approximately 50% cell killing, in presence of DNA damaging drugs methyl methane sulfonate (MMS), cisplatin, 5-fluoro uracil (5-FU) and gamma-irradiation but not at varying dosages of ultra violet (UV) radiation. The data strongly support a role for the endo-exonuclease in repairing DNA damages, induced by MMS, cisplatin, 5-FU and gamma irradiation but not by UV radiation. The results presented in this study suggest that the endo-exonuclease siRNA could be useful as a therapeutic tool in targeting the endo-exonuclease in cancer therapy.

Downregulation of xIAP expression by small interfering RNA inhibits cellular viability and increases chemosensitivity to methotrexate in human hepatoma cell line HepG2

The aim of this study was to investigate whether downregulating the expression of xIAP by RNAi (RNA interference) technology can induce the apoptosis of HepG2 cells, inhibit cellular viability and increase chemosensitivity of cancer cells. HepG2 cells were transfected with U6 promoter plasmids coding for short interfering RNAs (siRNAs) targeting xIAP. RT-PCR and western blot analysis were used to assess the mRNA and protein levels of xIAP expression. T he suppression efficiency o f xIAPby RNAi was evaluated using the MTT assay for cellular viability and Annexin V-PI binding assay for apoptosis. These results showed that siRNAs reduced cellular viability and increased cellular apoptosis. Moreover, downregulation of xIAP expression enhanced the chemosensitivity of HepG2 cells to methotrexate. These results suggest that the downregulation of xIAP by RNAi could potentially be used as a therapeutic strategy for human hepatocellular carcinoma.

RNA interference technologies and their use in cancer research

PURPOSE OF REVIEW

Recently, RNA interference has evolved into a powerful research tool to functionally characterize genes. Genome-wide RNA interference reagents can study the loss-of-function phenotypes of candidate genes in the context of various disease model systems. In this review, we discuss the data from the most recent studies using RNA interference reagents with a focus on RNA interference-based genomic screening as a tool to expand our knowledge about the molecular basis of cancer.

RECENT FINDINGS

Tumorigenesis is the result of the progressive accumulation of mutations in genes controlling cell proliferation and death. Various genes carrying these alterations are known to be directly linked to tumor growth; however, how to translate this knowledge into effective chemotherapeutics, nontoxic to normal cells, is still a subject of intensive research.

SUMMARY

Loss-of-function studies offer a potential for validation of known and unrecognized tumor-associated targets. RNA interference-mediated gene knockdown can be exploited to study the reprogrammed circuitry of genes, discover gene interactions restricted to cancer cells and identify mechanisms of chemoresistance in cancer cells. In addition, the simultaneous use of cancer drugs and RNA interference also provides a paradigm to develop strategies to inactivate essential genes promoting neoplastic growth.

The DNA damage signaling pathway is a critical mediator of oncogene-induced senescence

Here we report that RNA interference against ATM inhibited p53 accumulation in cells expressing oncogenic STAT5 and cooperated with Rb inactivation to suppress STAT5A-induced senescence. Knocking down ATM was also effective to bypass E2F1-induced senescence and in combination with Rb inactivation, inhibited RasV12-induced senescence. Cells that senesced in response to ca-STAT5A or RasV12 accumulated DNA damage foci and activated ATM, ATR, Chk1, and Chk2, indicating that aberrant oncogene activation induces a DNA damage signaling response. Intriguingly, bypassing oncogene-induced senescence by inactivation of p53 and Rb did not eliminate the accumulation of oncogene-induced DNA damage foci (ODDI), suggesting a mechanism that may limit transformation in immortalized cells.

An E2F/miR-20a autoregulatory feedback loop

The E2F family of transcription factors is essential in the regulation of the cell cycle and apoptosis. While the activity of E2F1-3 is tightly controlled by the retinoblastoma family of proteins, the expression of these factors is also regulated at the level of transcription, post-translational modifications and protein stability. Recently, a new level of regulation of E2Fs has been identified, where micro-RNAs (miRNAs) from the mir-17-92 cluster influence the translation of the E2F1 mRNA. We now report that miR-20a, a member of the mir-17-92 cluster, modulates the translation of the E2F2 and E2F3 mRNAs via binding sites in their 3'-untranslated region. We also found that the endogenous E2F1, E2F2, and E2F3 directly bind the promoter of the mir-17-92 cluster activating its transcription, suggesting an autoregulatory feedback loop between E2F factors and miRNAs from the mir-17-92 cluster. Our data also point toward an anti-apoptotic role for miR-20a, since overexpression of this miRNA decreased apoptosis in a prostate cancer cell line, while inhibition of miR-20a by an antisense oligonucleotide resulted in increased cell death after doxorubicin treatment. This anti-apoptotic role of miR-20a may explain some of the oncogenic capacities of the mir-17-92 cluster. Altogether, these results suggest that the autoregulation between E2F1-3 and miR-20a is important for preventing an abnormal accumulation of E2F1-3 and may play a role in the regulation of cellular proliferation and apoptosis.

DNA damage signaling and p53-dependent senescence after prolonged beta-interferon stimulation

Interferons are cytokines with potent antiviral and antiproliferative activities. We report that although a transient exposure to beta-interferon induces a reversible cell cycle arrest, a sustained treatment triggers a p53-dependent senescence program. Beta-interferon switched on p53 in two steps. First, it induced the acetylation of p53 at lysine 320 and its dephosphorylation at serine 392 but not p53 activity. Later on, it triggered a DNA signaling pathway, the phosphorylation of p53 at serine 15 and its transcriptional activity. In agreement, beta-interferon-treated cells accumulated gamma-H2AX foci and phosphorylated forms of ATM and CHK2. The DNA damage signaling pathway was activated by an increase in reactive oxygen species (ROS) induced by interferon and was inhibited by the antioxidant N-acetyl cysteine. More important, RNA interference against ATM inhibited p53 phosphorylation at serine 15, p53 activity and senescence in response to beta-interferon. Beta-interferon-induced senescence was more efficient in cells expressing either, p53, or constitutive allele of ERK2 or RasV12. Hence, beta-interferon-induced senescence targets preferentially cells with premalignant changes.

Recent advances on multiple tumorigenic cascades involved in prostatic cancer progression and targeting therapies

Recent advances on differently-expressed gene products and their functions during the progression from localized androgen-dependent states into androgen-independent and metastatic forms of prostate cancer are reported. The expression levels of numerous oncogenes and tumor suppressor genes in distinct prostatic cancer epithelial cell lines and tissues relative to normal prostate cells are described. This is carried out to identify the signaling elements that are altered during the initiation, progression and metastatic process of prostate cancer. Additional information on the interactions between certain deregulated signaling pathways such as androgen receptor (AR), estrogen receptors, epidermal growth factor receptor (EGFR), hedgehog and Wnt/beta-catenin cascades in controlling the proliferation, survival and invasion of tumor prostate epithelial cells during the disease progression is described. The emphasis is on the critical functions of the AR and EGF-EGFR systems at all stages during prostate carcinogenesis. Of therapeutic interest, new strategies for the diagnosis and treatment of localized and metastatic forms of prostate cancer by targeting multiple tumorigenic signaling elements are also reported.

Silence of the transcripts: RNA interference in medicine

Silencing of gene expression by ribonucleic acid (RNA), known as RNA interference (RNAi), is now recognized as a major means of gene regulation in biology. In this mechanism, small noncoding double-stranded RNA molecules knock down gene expression through a variety of mechanisms that include messenger RNA (mRNA) degradation, inhibition of mRNA translation, or chromatin remodeling. The posttranscriptional mechanism of RNAi has been embraced by researchers as a powerful tool for generating deficient phenotypes without mutating the gene. In parallel, exciting recent results have promised its application in disease therapy. This review aims to summarize the current knowledge in this area and provide a roadmap that may eventually launch RNAi from the research bench to the medicine chest.