Mitotic catastrophe constitutes a special case of apoptosis whose suppression entails aneuploidy

Directly targeting the mitochondrial pathway of apoptosis for cancer therapy using BH3 mimetics - recent successes, current challenges and future promise

Apoptosis within cancer cells is controlled by the BCL-2 family of proteins, making them powerful arbiters of cell fate in response to stress induced by neoplastic transformation as well as exposure to anti-cancer therapies. Many cancers evade pro-apoptotic stress signals by up-regulating anti-apoptotic proteins such as BCL-2, BCL-X_L or MCL-1 to maintain their survival. However, this may come at a cost, as these cancers may also become dependent on these anti-apoptotic proteins for survival. The development and deployment of BCL-2 family inhibitors (drugs that mimic the activity of pro-apoptotic BH3-only proteins or 'BH3 mimetics') is based on this paradigm, and the first potent and specific molecules are now being evaluated in clinical trials. We review the recent successes in this field, the challenges currently being faced, and the promising future ahead.

Caspase-2: an orphan enzyme out of the shadows

Caspase-2 has been embodied as an initiator or executioner protease in diverse apoptotic scenarios. However, accumulating evidence is challenging this view, pertaining to its true role. The enzyme's catalytic activity is currently implicated in various functions required for correct cell proliferation, such as counteracting genomic instability, as well as suppressing tumorigenesis. Here, apart from summarizing the latest observations in caspase-2-related research, we make an attempt to reconcile these findings and discuss their implications for future directions.

BZML, a novel colchicine binding site inhibitor, overcomes multidrug resistance in A549/Taxol cells by inhibiting P-gp function and inducing mitotic catastrophe

Multidrug resistance (MDR) interferes with the efficiency of chemotherapy. Therefore, developing novel anti-cancer agents that can overcome MDR is necessary. Here, we screened a series of colchicine binding site inhibitors (CBSIs) and found that 5-(3, 4, 5-trimethoxybenzoyl)-4-methyl-2-(p-tolyl) imidazol (BZML) displayed potent cytotoxic activity against both A549 and A549/Taxol cells. We further explored the underlying mechanisms and found that BZML caused mitosis phase arrest by inhibiting tubulin polymerization in A549 and A549/Taxol cells. Importantly, BZML was a poor substrate for P-glycoprotein (P-gp) and inhibited P-gp function by decreasing P-gp expression at the protein and mRNA levels. Cell morphology changes and the expression of cycle- or apoptosis-related proteins indicated that BZML mainly drove A549/Taxol cells to die by mitotic catastrophe (MC), a p53-independent apoptotic-like cell death, whereas induced A549 cells to die by apoptosis. Taken together, our data suggest that BZML is a novel colchicine binding site inhibitor and overcomes MDR in A549/Taxol cells by inhibiting P-gp function and inducing MC. Our study also offers a new strategy to solve the problem of apoptosis-resistance.

Mitotic Catastrophe in BC3H1 Cells following Yessotoxin Exposure

The marine toxin yessotoxin (YTX) can cause various cytotoxic effects depending on cell type and cell line. It is well known to trigger distinct mechanisms for programmed cell death which may overlap or cross-talk. The present contribution provides the first evidence that YTX can cause genotoxicity and induce mitotic catastrophe which can lead to different types of cell death. This work also demonstrates potential information gain from non-intrusive computer-based tracking of many individual cells during long time. Treatment of BC3H1 cells at their exponential growth phase causes atypical nuclear alterations and formation of giant cells with multiple nuclei. These are the most prominent morphological features of mitotic catastrophe. Giant cells undergo slow cell death in a necrosis-like manner. However, apoptotic-like cell death is also observed in these cells. Electron microscopy of treated BC3H1 cells reveal uncondensed chromatin and cells with double nuclei. Activation of p-p53, p-H2AX, p-Chk1, p-ATM, and p-ATR and down-regulation of p-Chk2 indicate DNA damage response and cell cycle deregulation. Micronuclei formation further support this evidence. Data from tracking single cells reveal that YTX treatment suppresses a second round of cell division in BC3H1 cells. These findings suggest that YTX can induce genomic alterations or imperfections in chromosomal segregation leading to permanent mitotic failure. This understanding extends the list of effects from YTX and which are of interest to control cancer and tumor progression.

Caspase 2 in mitotic catastrophe: The terminator of aneuploid and tetraploid cells

Mitotic catastrophe is an oncosuppressive mechanism that targets cells experiencing defective mitoses via the activation of specific cell cycle checkpoints, regulated cell death pathways and/or cell senescence. This prevents the accumulation of karyotypic aberrations, which otherwise may drive oncogenesis and tumor progression. Here, we summarize experimental evidence confirming the role of caspase 2 (CASP2) as the main executor of mitotic catastrophe, and we discuss the signals that activate CASP2 in the presence of mitotic aberrations. In addition, we summarize the main p53-dependent and -independent effector pathways through which CASP2 limits chromosomal instability and non-diploidy, hence mediating robust oncosuppressive functions.

Regulation of Cancer Cell Responsiveness to Ionizing Radiation Treatment by Cyclic AMP Response Element Binding Nuclear Transcription Factor

Cyclic AMP response element binding (CREB) protein is a member of the CREB/activating transcription factor (ATF) family of transcription factors that play an important role in the cell response to different environmental stimuli leading to proliferation, differentiation, apoptosis, and survival. A number of studies highlight the involvement of CREB in the resistance to ionizing radiation (IR) therapy, demonstrating a relationship between IR-induced CREB family members' activation and cell survival. Consistent with these observations, we have recently demonstrated that CREB and ATF-1 are expressed in leukemia cell lines and that low-dose radiation treatment can trigger CREB activation, leading to survival of erythro-leukemia cells (K562). On the other hand, a number of evidences highlight a proapoptotic role of CREB following IR treatment of cancer cells. Since the development of multiple mechanisms of resistance is one key problem of most malignancies, including those of hematological origin, it is highly desirable to identify biological markers of responsiveness/unresponsiveness useful to follow-up the individual response and to adjust anticancer treatments. Taking into account all these considerations, this mini-review will be focused on the involvement of CREB/ATF family members in response to IR therapy, to deepen our knowledge of this topic, and to pave the way to translation into a therapeutic context.

Caspase-2-mediated cell death is required for deleting aneuploid cells

Caspase-2, one of the most evolutionarily conserved of the caspase family, has been implicated in maintenance of chromosomal stability and tumour suppression. Caspase-2 deficient (Casp2^−/−) mice develop normally but show premature ageing-related traits and when challenged by certain stressors, succumb to enhanced tumour development and aneuploidy. To test how caspase-2 protects against chromosomal instability, we utilized an ex vivo system for aneuploidy where primary splenocytes from Casp2^−/− mice were exposed to anti-mitotic drugs and followed up by live cell imaging. Our data show that caspase-2 is required for deleting mitotically aberrant cells. Acute silencing of caspase-2 in cultured human cells recapitulated these results. We further generated Casp2^C320S mutant mice to demonstrate that caspase-2 catalytic activity is essential for its function in limiting aneuploidy. Our results provide direct evidence that the apoptotic activity of caspase-2 is necessary for deleting cells with mitotic aberrations to limit aneuploidy.

Non-Canonical Cell Death Induced by p53

Programmed cell death is a vital biological process for multicellular organisms to maintain cellular homeostasis, which is regulated in a complex manner. Over the past several years, apart from apoptosis, which is the principal mechanism of caspase-dependent cell death, research on non-apoptotic forms of programmed cell death has gained momentum. p53 is a well characterized tumor suppressor that controls cell proliferation and apoptosis and has also been linked to non-apoptotic, non-canonical cell death mechanisms. p53 impacts these non-canonical forms of cell death through transcriptional regulation of its downstream targets, as well as direct interactions with key players involved in these mechanisms, in a cell type- or tissue context-dependent manner. In this review article, we summarize and discuss the involvement of p53 in several non-canonical modes of cell death, including caspase-independent apoptosis (CIA), ferroptosis, necroptosis, autophagic cell death, mitotic catastrophe, paraptosis, and pyroptosis, as well as its role in efferocytosis which is the process of clearing dead or dying cells.

Discovery of a Series of Acridinones as Mechanism-Based Tubulin Assembly Inhibitors with Anticancer Activity

Microtubules play critical roles in vital cell processes, including cell growth, division, and migration. Microtubule-targeting small molecules are chemotherapeutic agents that are widely used in the treatment of cancer. Many of these compounds are structurally complex natural products (e.g., paclitaxel, vinblastine, and vincristine) with multiple stereogenic centers. Because of the scarcity of their natural sources and the difficulty of their partial or total synthesis, as well as problems related to their bioavailability, toxicity, and resistance, there is an urgent need for novel microtubule binding agents that are effective for treating cancer but do not have these disadvantages. In the present work, our lead discovery effort toward less structurally complex synthetic compounds led to the discovery of a series of acridinones inspired by the structure of podophyllotoxin, a natural product with important microtubule assembly inhibitory activity, as novel mechanism-based tubulin assembly inhibitors with potent anticancer properties and low toxicity. The compounds were evaluated in vitro by wound healing assays employing the metastatic and triple negative breast cancer cell line MDA-MB-231. Four compounds with IC₅₀ values between 0.294 and 1.7 μM were identified. These compounds showed selective cytotoxicity against MDA-MB-231 and DU-145 cancer cell lines and promoted cell cycle arrest in G₂/M phase and apoptosis. Consistent with molecular modeling results, the acridinones inhibited tubulin assembly in in vitro polymerization assays with IC₅₀ values between 0.9 and 13 μM. Their binding to the colchicine-binding site of tubulin was confirmed through competitive assays.

Metabolic Regulation of Apoptosis in Cancer

Apoptosis is a cellular suicide program that plays a critical role in development and human diseases, including cancer. Cancer cells evade apoptosis, thereby enabling excessive proliferation, survival under hypoxic conditions, and acquired resistance to therapeutic agents. Among various mechanisms that contribute to the evasion of apoptosis in cancer, metabolism is emerging as one of the key factors. Cellular metabolites can regulate functions of pro- and antiapoptotic proteins. In turn, p53, a regulator of apoptosis, also controls metabolism by limiting glycolysis and facilitating mitochondrial respiration. Consequently, with dysregulated metabolism and p53 inactivation, cancer cells are well-equipped to disable the apoptotic machinery. In this article, we review how cellular apoptosis is regulated and how metabolism can influence the signaling pathways leading to apoptosis, especially focusing on how glucose and lipid metabolism are altered in cancer cells and how these alterations can impact the apoptotic pathways.

Cell-cycle involvement in autophagy and apoptosis in yeast

Regulation of the cell cycle and apoptosis are two eukaryotic processes required to ensure maintenance of genomic integrity, especially in response to DNA damage. The ease with which yeast, amongst other eukaryotes, can switch from cellular proliferation to cell death may be the result of a common set of biochemical factors which play dual roles depending on the cell's physiological state. A wide variety of homologues are shared between different yeasts and metazoans and this conservation confirms their importance. This review gives an overview of key molecular players involved in yeast cell-cycle regulation, and those involved in mechanisms which are induced by cell-cycle dysregulation. One such mechanism is autophagy which, depending on the severity and type of DNA damage, may either contribute to the cell's survival or death. Cell-cycle dysregulation due to checkpoint deficiency leads to mitotic catastrophe which in turn leads to programmed cell death. Molecular players implicated in the yeast apoptotic pathway were shown to play important roles in the cell cycle. These include the metacaspase Yca1p, the caspase-like protein Esp1p, the cohesin subunit Mcd1p, as well as the inhibitor of apoptosis protein Bir1p. The roles of these molecular players are discussed.

A mitosis-sensing caspase activation platform? New insights into the PIDDosome

In contrast to the apoptosome and death-inducing signaling complex, the PIDDosome remains an orphan caspase activation platform unassigned to a specific apoptotic pathway. We found that DNA damage-induced PIDDosome formation is blocked by the mitotic checkpoint factor BUBR1 (budding uninhibited by benzimidazole-related 1), via a direct interaction that disrupts the PIDDosome core scaffold. This inhibition occurs at the kinetochore, thus physically connecting the mitotic and apoptotic machineries.

Cytotoxic amounts of cisplatin induce either checkpoint adaptation or apoptosis in a concentration-dependent manner in cancer cells

BACKGROUND INFORMATION

Checkpoint adaptation (entry into mitosis with damaged DNA) is a process that links arrest at the G2/M cell cycle checkpoint and cell death in cancer cells. It is not known, however, whether cells treated with the genotoxic agent, cisplatin, undergo checkpoint adaptation or if checkpoint adaptation is a major pathway leading to cell death or not. Therefore, we investigated the relationship between treatment with cisplatin and cytotoxicity in cancer cells.

RESULTS

Treatment of HT-29 human colorectal adenocarcinoma cells with cisplatin can induce cell death by one of two different mechanisms. Cells treated with a cytotoxic 30 μM amount of cisplatin died after undergoing checkpoint adaptation. Before dying, however, almost all treated cells were positive for histone γH2AX staining and contained high levels of cyclin B1. Rounded cells appeared that were positive for phospho-Ser10 histone H3, with low levels of phospho-Tyr15 cyclin-dependent kinase 1, high levels of cyclin-dependent kinase 1 activity, and checkpoint kinase 1 that was not phosphorylated on Ser345. These cells were in mitosis with damaged DNA. Strikingly, with 30 μM cisplatin, 81% of cells had entered mitosis before dying. By contrast, after treatment with 100 μM cisplatin, nearly all cells died but only 7% of cells had entered mitosis. Instead, these cells died by apoptosis; they were positive for annexin-V staining, contained cleaved caspase 3, cleaved caspase 9 and cleaved PARP and did not contain Mcl-1.

CONCLUSIONS

Our data demonstrate that cancer cells treated with cisplatin can undergo one of two modes of cell death depending upon concentration used. These findings suggest that checkpoint adaptation is likely a primary pathway in genotoxic cell death at pharmacological concentrations of cisplatin.

SIGNIFICANCE

Checkpoint adaptation might be a common biochemical pathway taken by human cancer cells in response to pharmacologically relevant, cytotoxic amounts of damaged DNA.

Oncogenic Herpesvirus Utilizes Stress-Induced Cell Cycle Checkpoints for Efficient Lytic Replication

Kaposi’s sarcoma herpesvirus (KSHV) causes Kaposi’s sarcoma and certain lymphoproliferative malignancies. Latent infection is established in the majority of tumor cells, whereas lytic replication is reactivated in a small fraction of cells, which is important for both virus spread and disease progression. A siRNA screen for novel regulators of KSHV reactivation identified the E3 ubiquitin ligase MDM2 as a negative regulator of viral reactivation. Depletion of MDM2, a repressor of p53, favored efficient activation of the viral lytic transcription program and viral reactivation. During lytic replication cells activated a p53 response, accumulated DNA damage and arrested at G2-phase. Depletion of p21, a p53 target gene, restored cell cycle progression and thereby impaired the virus reactivation cascade delaying the onset of virus replication induced cytopathic effect. Herpesviruses are known to reactivate in response to different kinds of stress, and our study now highlights the molecular events in the stressed host cell that KSHV has evolved to utilize to ensure efficient viral lytic replication.

Herpesviruses are known to wake up and reactivate in response to different kinds of stress. Our study now highlights the key molecular host cell events that KSHV has evolved to utilize for efficient viral lytic replication: the activation of p53 and upregulation of p21, which slows down the cell cycle, but promotes viral replication and transcription of viral lytic genes. Mutations in TP53 gene are rarely found in KSHV-associated malignancies. Therefore, our work now provides a mechanistic explanation as to why the virus has evolved to retain p53.

Cellular Pathways in Response to Ionizing Radiation and Their Targetability for Tumor Radiosensitization

During the last few decades, improvements in the planning and application of radiotherapy in combination with surgery and chemotherapy resulted in increased survival rates of tumor patients. However, the success of radiotherapy is impaired by two reasons: firstly, the radioresistance of tumor cells and, secondly, the radiation-induced damage of normal tissue cells located in the field of ionizing radiation. These limitations demand the development of drugs for either radiosensitization of tumor cells or radioprotection of normal tissue cells. In order to identify potential targets, a detailed understanding of the cellular pathways involved in radiation response is an absolute requirement. This review describes the most important pathways of radioresponse and several key target proteins for radiosensitization.

NSCLC cells demonstrate differential mode of cell death in response to the combined treatment of radiation and a DNA-PKcs inhibitor

The current standard of care for lung cancer consists of concurrent chemotherapy and radiation. Several studies have shown that the DNA-PKcs inhibitor NU7441 is a highly potent radiosensitizer, however, the mechanism of NU7441's anti-proliferation effect has not been fully elucidated. In this study, the combined effect of NU7441 and ionizing radiation (IR) in a panel of non-small cell lung cancer cell lines (A549, H460 and H1299) has been investigated. We found that NU7441 significantly enhances the effect of IR in all cell lines. The notable findings in response to this combined treatment are (i) prolonged delay in IR-induced DNA DSB repair, (ii) induced robust G2/M checkpoint, (iii) increased aberrant mitosis followed by mitotic catastrophe specifically in H1299, (iv) dramatically induced autophagy in A549 and (v) IR-induced senescence specifically in H460. H1299 cells show greater G2 checkpoint adaptation after combined treatment, which can be attributed to higher expression level of Plk1 compared to A549 and H460. The enhanced autophagy after NU7441 treatment in A549 is possibly due to the higher endogenous expression of pS6K compared to H1299 and H460 cells. In conclusion, choice of cell death pathway is dependent on the mutation status and other genetic factors of the cells treated.

Flubendazole induces mitotic catastrophe and senescence in colon cancer cells in vitro

Objectives

Flubendazole (FLU), a member of benzimidazole family of anthelmintic drugs, is able to inhibit proliferation of various cancer cells. The aim of present study was to elucidate the mechanisms of antiproliferative effect of FLU on colorectal cancer cells in vitro.

Methods

The effect of FLU on proliferation, microtubular network, DNA content, caspase activation and senescence induction was studied in SW480 and SW620 cell lines.

Key findings

Flubendazole significantly affected cell proliferation in a pattern typical for mitotic inhibitor. This was accompanied by decrease in cyclin D1 levels, increase in cyclin B1 levels, activation of caspase 2 and caspase 3/7 and PARP cleavage. Morphological observations revealed disruption of microtubular network, irregular mitotic spindles, formation of giant multinucleated cells and increase in nuclear area and DNA content. In SW620 cell line, 37.5% giant multinucleated cells induced by FLU treatment showed positivity for SA-β-galactosidase staining. Cell lines were able to recover from the treatment and this process was faster in SW480 cells.

Conclusion

Flubendazole in low concentration temporarily inhibits cell proliferation and induces mitotic catastrophe and premature senescence in human colon cancer cells in vitro.

Chelidonine induces mitotic slippage and apoptotic-like death in SGC-7901 human gastric carcinoma cells

The aim of the present study was to investigate the effect of chelidonine on mitotic slippage and apoptotic-like death in SGC-7901 human gastric cancer cells. The MTT assay was performed to detect the antiproliferative effect of chelidonine. Following treatment with chelidonine (10 µmol/l), the ultrastructure changes in SGC-7901, MCF-7 and HepG2 cells were observed by transmission electron microscopy. The effects of chelidonine on G2/M phase arrest and apoptosis of SGC-7901 cells were determined by flow cytometry. Indirect immunofluorescence assay and laser scanning confocal microscopy (LSCM) were used to detect the phosphorylation level of histone H3 (Ser10) and microtubule formation was detected using LSCM following immunofluorescent labeling. Subsequent to treatment with chelidonine (10 µmol/l), expression levels of mitotic slippage-associated proteins, including BUB1 mitotic checkpoint serine/threonine kinase B (BubR1), cyclin-dependent kinase 1 (Cdk1) and cyclin B1, and apoptosis-associated protein, caspase-3 were examined by western blotting at 24, 48 and 72 h. The half maximal inhibitory concentration of chelidonine was 23.13 µmol/l over 48 h and chelidonine induced G2/M phase arrest of cells. The phosphorylation of histone H3 at Ser10 was significantly increased following treatment with chelidonine for 24 h, indicating that chelidonine arrested the SGC-7901 cells in the M phase. Chelidonine inhibited microtubule polymerization, destroyed microtubule structures and induced cell cycle arrest in the M phase. Giant cells were observed with multiple micronuclei of varying sizes, which indicated that following a prolonged arrest in the M phase, the cells underwent mitotic catastrophe. Western blotting demonstrated that the protein expression levels of BubR1, cyclin B1 and Cdk1 decreased significantly between 48 and 72 h. Low expression levels of BubR1 and inactivation of the cyclin B1-Cdk1 complex results in the cells being arrested at mitosis and leads to mitotic slippage. In addition, apoptotic morphological changes in multinucleated cells were observed, the apoptosis rates increased gradually with administration of chelidonine in a time-dependent manner and the protein levels of caspase-3 increased significantly between 24 and 72 h. Thus, chelidonine induces mitotic slippage, and apoptotic-like death occurs in SGC-7901 cells undergoing mitotic catastrophe. Gastric cancer is a common malignancy, and ranks second in overall cancer-associated mortalities worldwide. The present study demonstrated that chelidonine induces M phase arrest and mitotic slippage of SGC-7901 human gastric carcinoma cells via downregulating the expression of BubR1, Cdk1 and cyclin B1 proteins. With the prolongation of chelidonine treatment, the giant cells with multiple micronuclei underwent mitotic slippage and were maintained in the G1 phase and did not survive. A number of multinucleated cells underwent apoptosis via a caspase-dependent signaling pathway. The current study proposes that chelidonine induces mitotic slippage and apoptotic-like death of SGC-7901 cells.

Newly synthesized podophyllotoxin derivative, LJ12, induces apoptosis and mitotic catastrophe in non-small cell lung cancer cells in vitro

Deoxypodophyllotoxin (DPT), an active compound isolated from a number of herbs and used in traditional medicine, has been reported to exhibit promising anti‑tumor activity. A newly synthesized derivative, N-(1-oxyl‑4'-demethyl-4-deoxyp odophyllic)-L‑methine-4'-piperazine carbamate (LJ12) may have improved antitumor activity and fewer side effects. The present study assessed the effect of LJ12 on cell viability, apoptosis, cell cycle distribution and mitotic catastrophe in A549 human lung cancer cells in vitro. The molecular mechanisms underlying the antitumor activity of LJ12 were also examined. The results demonstrated that LJ12 reduced A549 cell viability in a time‑ and dose‑dependent manner, with a lower half maximal inhibitory concentration of ~0.1 µM, compared with another known DPT derivative, etoposide (10 µM). Flow cytometric analysis showed that LJ12 induced tumor cell arrest at the G2/M phase of the cell cycle. The present study also observed an expected concomitant decrease in the numbers of cells cells in the G0/G1 and S phases. LJ12 was found to upregulate the protein expression levels of Cdc2 and Cyclin B1. Furthermore, LJ12 induced tumor cell apoptosis and the protein expression of B cell lymphoma‑2‑associated X protein, caspase‑3 and p53. The present study also observed the formation of giant, multinucleated cells, indicating that LJ12 induced mitotic catastrophe in the tumor cells. These results indicated that LJ12 has anti‑non‑small cell lung cancer activity in vitro. Further investigations aim to develop LJ12 as a therapeutic agent for the treatment of lung cancer.

Inhibition of the mitochondrial fission protein dynamin-related protein 1 (Drp1) impairs mitochondrial fission and mitotic catastrophe after x-irradiation

The role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. This study demonstrates that IR triggers Drp1-dependent mitochondrial fission and that Drp1 inhibition attenuates radiation-induced mitotic catastrophe, suggesting that Drp1 is involved in determining the fate of cells after irradiation.

Accumulating evidence suggests that mitochondrial dynamics is crucial for the maintenance of cellular quality control and function in response to various stresses. However, the role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. In this study, we provide evidence that IR triggers mitochondrial fission mediated by the mitochondrial fission protein dynamin-related protein 1 (Drp1). We also show IR-induced mitotic catastrophe (MC), which is a type of cell death associated with defective mitosis, and aberrant centrosome amplification in mouse embryonic fibroblasts (MEFs). These are attenuated by genetic or pharmacological inhibition of Drp1. Whereas radiation-induced aberrant centrosome amplification and MC are suppressed by the inhibition of Plk1 and CDK2 in wild-type MEFs, the inhibition of these kinases is ineffective in Drp1-deficient MEFs. Furthermore, the cyclin B1 level after irradiation is significantly higher throughout the time course in Drp1-deficient MEFs than in wild-type MEFs, implying that Drp1 is involved in the regulation of cyclin B1 level. These findings strongly suggest that Drp1 plays an important role in determining the fate of cells after irradiation via the regulation of mitochondrial dynamics.

The mechanisms of genetically modified vaccinia viruses for the treatment of cancer

The use of oncolytic viruses for the treatment of cancer is an emerging field of cancer research and therapy. Oncolytic viruses are designed to induce tumor specific immunity while replicating selectively within cancer cells to cause lysis of the tumor cells. While there are several forms of oncolytic viruses, the use of vaccinia viruses for oncolysis may be more beneficial than other forms of oncolytic viruses. For example, vaccinia viruses have been shown to exert their anti-tumor effects through genetic engineering strategies which enhance their therapeutic efficacy. This paper will address some of the most common forms of genetically modified vaccinia viruses and will explore the mechanisms whereby they selectively target, enter and destroy cancer cells. Furthermore, this review will highlight how vaccinia viruses activate host immune responses against cancer cells and will address clinical trials evaluating the tumor-directed and killing efficacy of these viruses against solid tumors.

Telomerase abrogates aneuploidy-induced telomere replication stress, senescence and cell depletion

The causal role of aneuploidy in cancer initiation remains under debate since mutations of euploidy-controlling genes reduce cell fitness but aneuploidy strongly associates with human cancers. Telomerase activation allows immortal growth by stabilizing telomere length, but its role in aneuploidy survival has not been characterized. Here, we analyze the response of primary human cells and murine hematopoietic stem cells (HSCs) to aneuploidy induction and the role of telomeres and the telomerase in this process. The study shows that aneuploidy induces replication stress at telomeres leading to telomeric DNA damage and p53 activation. This results in p53/Rb-dependent, premature senescence of human fibroblast, and in the depletion of hematopoietic cells in telomerase-deficient mice. Endogenous telomerase expression in HSCs and enforced expression of telomerase in human fibroblasts are sufficient to abrogate aneuploidy-induced replication stress at telomeres and the consequent induction of premature senescence and hematopoietic cell depletion. Together, these results identify telomerase as an aneuploidy survival factor in mammalian cells based on its capacity to alleviate telomere replication stress in response to aneuploidy induction.

Selective killing of G2 decatenation checkpoint defective colon cancer cells by catalytic topoisomerase II inhibitor

Cancer cells with defective DNA decatenation checkpoint can be selectively targeted by the catalytic inhibitors of DNA topoisomerase IIα (topo IIα) enzyme. Upon treatment with catalytic topo IIα inhibitors, cells with defective decatenation checkpoint fail to arrest their cell cycle in G2 phase and enter into M phase with catenated and under-condensed chromosomes resulting into impaired mitosis and eventually cell death. In the present work we analyzed decatenation checkpoint in five different colon cancer cell lines (HCT116, HT-29, Caco2, COLO 205 and SW480) and in one non-cancerous cell line (HEK293T). Four out of the five colon cancer cell lines i.e. HCT116, HT-29, Caco2, and COLO 205 were found to be compromised for the decatenation checkpoint function at different extents, whereas SW480 and HEK293T cell lines were found to be proficient for the checkpoint function. Upon treatment with ICRF193, decatenation checkpoint defective cell lines failed to arrest the cell cycle in G2 phase and entered into M phase without proper chromosomal decatenation, resulting into the formation of tangled mass of catenated and under-condensed chromosomes. Such cells underwent mitotic catastrophe and rapid apoptosis like cell death and showed higher sensitivity for ICRF193. Our study suggests that catalytic inhibitors of topoisomerase IIα are promising therapeutic agents for the treatment of colon cancers with defective DNA decatenation checkpoint.

Building immunity to cancer with radiation therapy

Over the last decade there has been a dramatic shift in the focus of cancer research toward understanding how the body's immune defenses can be harnessed to promote the effectiveness of cytotoxic anti-cancer therapies. The ability of ionizing radiation to elicit anti-cancer immune responses capable of controlling tumor growth has led to the emergence of promising combination-based radio-immunotherapeutic strategies for the treatment of cancer. Herein we review the immunoadjuvant properties of localized radiation therapy and discuss how technological advances in radio-oncology and developments in the field of tumor-immunotherapy have started to revolutionize the therapeutic application of radiotherapy.

Tumour suppressive effects of WEE1 gene silencing in breast cancer cells

BACKGROUND

WEE1 is a G2/M checkpoint regulator protein. Various studies have indicated that WEE1 could be a good target for cancer therapy. The main aim of this study was to asssess the tumor suppressive potential of WEE1 silencing in two different breast cancer cell lines, MCF7 which carries the wild-type p53 and MDA-MB468 which contains a mutant type.

MATERIALS AND METHODS

After WEE1 knockdown with specific shRNAs downstream effects on cell viability and cell cycle progression were determined using MTT and flow cytometry analyses, respectively. Real-time PCR and Western blotting were conducted to assess the effect of WEE1 inhibition on the expression of apoptotic (p53) and anti-apoptotic (Bcl2) factors and also a growth marker (VEGF).

RESULTS

The results showed that WEE1 inhibition could cause a significant decrease in the viability of both MCF7 and MDA-MB-468 breast cancer cell lines by more than 50%. Interestingly, DNA content assays showed a significant increase in apoptotic cells following WEE1 silencing. WEE1 inhibition also induced up- regulation of the apoptotic marker, p53, in breast cancer cells. A significant decrease in the expression of VEGF and Bcl-2 was observed following WEE1 inhibition in both cell lines.

CONCLUSIONS

In concordance with previous studies, our data showed that WEE1 inhibition could induce G2 arrest abrogation and consequent cell death in breast cancer cells. Moreover, in this study, the observed interactions between the pro- and anti-apoptotic proteins and decrease in the angiogenesis marker expression confirm the susceptibility to apoptosis and validate the tumor suppressive effect of WEE1 inhibition in breast cancer cells. Interestingly, the levels of the sensitivity to WEE1 silencing in breast cancer cells, MCF7 and MDA-MB468, seem to be in concordance with the level of p53 expression.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Histone deacetylase inhibitors promote glioma cell death by G2 checkpoint abrogation leading to mitotic catastrophe

Glioblastoma multiforme is resistant to conventional anti-tumoral treatments due to its infiltrative nature and capability of relapse; therefore, research efforts focus on characterizing gliomagenesis and identifying molecular targets useful on therapy. New therapeutic strategies are being tested in patients, such as Histone deacetylase inhibitors (HDACi) either alone or in combination with other therapies. Here two HDACi included in clinical trials have been tested, suberanilohydroxamic acid (SAHA) and valproic acid (VPA), to characterize their effects on glioma cell growth in vitro and to determine the molecular changes that promote cancer cell death. We found that both HDACi reduce glioma cell viability, proliferation and clonogenicity. They have multiple effects, such as inducing the production of reactive oxygen species (ROS) and activating the mitochondrial apoptotic pathway, nevertheless cell death is not prevented by the pan-caspase inhibitor Q-VD-OPh. Importantly, we found that HDACi alter cell cycle progression by decreasing the expression of G2 checkpoint kinases Wee1 and checkpoint kinase 1 (Chk1). In addition, HDACi reduce the expression of proteins involved in DNA repair (Rad51), mitotic spindle formation (TPX2) and chromosome segregation (Survivin) in glioma cells and in human glioblastoma multiforme primary cultures. Therefore, HDACi treatment causes glioma cell entry into mitosis before DNA damage could be repaired and to the formation of an aberrant mitotic spindle that results in glioma cell death through mitotic catastrophe-induced apoptosis.

Regulation of the metastasis suppressor Nm23-H1 by tumor viruses

Metastasis is the most common cause of cancer mortality. To increase the survival of patients, it is necessary to develop more effective methods for treating as well as preventing metastatic diseases. Recent advancement of knowledge in cancer metastasis provides the basis for development of targeted molecular therapeutics aimed at the tumor cell or its interaction with the host microenvironment. Metastasis suppressor genes (MSGs) are promising targets for inhibition of the metastasis process. During the past decade, functional significance of these genes, their regulatory pathways, and related downstream effector molecules have become a major focus of cancer research. Nm23-H1, first in the family of Nm23 human homologues, is a well-characterized, anti-metastatic factor linked with a large number of human malignancies. Mounting evidence to date suggests an important role for Nm23-H1 in reducing virus-induced tumor cell motility and migration. A detailed understanding of the molecular association between oncogenic viral antigens with Nm23-H1 may reveal the underlying mechanisms for tumor virus-associated malignancies. In this review, we will focus on the recent advances to our understanding of the molecular basis of oncogenic virus-induced progression of tumor metastasis by deregulation of Nm23-H1.

MJ-66 induces malignant glioma cells G2/M phase arrest and mitotic catastrophe through regulation of cyclin B1/Cdk1 complex

Malignant gliomas are among the most devastating cancers as they are resistant to many kinds of treatment. Despite recent advances in the diagnosis and treatment, the prognosis of patients remains very poor and the development of new drug is urgently needed. Here, we report that a synthetic quinazolinone analog 2-(naphthalene-1-yl)-6-pyrrolidinyl-4-quinazolinone (MJ-66) induced glioma cell death. Immunofluorescence staining showed that MJ-66-induced cell death was associated with multinucleated phenotype and multipolar spindles that were typical characteristics of mitotic catastrophe. Flow cytometry analysis revealed that MJ-66 caused glioma cell cycle arrest at G2/M phase and increased the proportion of polyploidy cells. Western blotting indicated that the expression of cyclin B1, Cdk1 pY15 and Cdk1 increased after treatment with MJ-66. MJ-66 effectively inhibited tumor growth and induced apoptosis in the xenograft animal model of U87 human glioma cells. Together, these results suggest that MJ-66 inhibited malignant gliomas growth through inducing mitotic catastrophe by interference with G2/M cell cycle checkpoint which may open a new avenue for the treatment of malignant gliomas.

Mechanisms linked to differences in the mutagenic potential of 1,3-dinitropyrene and 1,8-dinitropyrene

This study explores and characterizes the toxicity of two closely related carcinogenic dinitro-pyrenes (DNPs), 1,3-DNP and 1,8-DNP, in human bronchial epithelial BEAS-2B cells and mouse hepatoma Hepa1c1c7 cells. Neither 1,3-DNP nor 1,8-DNP (3–30 μM) induced cell death in BEAS-2B cells. In Hepa1c1c7 cells only 1,3-DNP (10–30 μM) induced a mixture of apoptotic and necrotic cell death after 24 h. Both compounds increased the level of reactive oxygen species (ROS) in BEAS-2B as measured by CM-H₂DCFDA-fluorescence. A corresponding increase in oxidative damage to DNA was revealed by the formamidopyrimidine-DNA glycosylase (fpg)-modified comet assay. Without fpg, DNP-induced DNA damage detected by the comet assay was only found in Hepa1c1c7 cells. Only 1,8-DNP formed DNA adduct measured by ³²P-postlabelling. In Hepa1c1c cells, 1,8-DNP induced phosphorylation of H2AX (γH2AX) and p53 at a lower concentration than 1,3-DNP and there was no direct correlation between DNA damage/DNA damage response (DR) and induced cytotoxicity. On the other hand, 1,3-DNP-induced apoptosis was inhibited by pifithrin-α, an inhibitor of p53 transcriptional activity. Furthermore, 1,3-DNP triggered an unfolded protein response (UPR), as measured by an increased expression of CHOP, ATF4 and XBP1. Thus, other types of damage possibly linked to endoplasmic reticulum (ER)-stress and/or UPR could be involved in the induced apoptosis. Our results suggest that the stronger carcinogenic potency of 1,8-DNP compared to 1,3-DNP is linked to its higher genotoxic effects. This in combination with its lower potency to induce cell death may increase the probability of causing mutations.

Combination of Cl‑IB‑MECA with paclitaxel is a highly effective cytotoxic therapy causing mTOR‑dependent autophagy and mitotic catastrophe on human melanoma cells

PURPOSE

Metastatic melanoma is the deadliest form of skin cancer. It is highly resistant to conventional therapies,particularly to drugs that cause apoptosis as the main anticancer mechanism. Recently, induction of autophagic cell death is emerging as a novel therapeutic target for apoptotic-resistant cancers. We aimed to investigate the underlying mechanisms elicited by the cytotoxic combination of 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5′-N-methyluronamide(Cl-IB-MECA, a selective A(3) adenosine receptor agonist; 10 μM) and paclitaxel (10 ng/mL) on human C32 and A375 melanoma cell lines.

METHODS

Cytotoxicity was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reduction, neutral red uptake, and lactate dehydrogenase leakage assays, after 48-h incubation. Autophagosome and autolysosome formation was detected by fluorescence through monodansylcadaverine-staining and CellLight(®) Lysosomes-RFP-labelling, respectively. Cell nuclei were visualized by Hoechst staining, while levels of p62 were determined by an ELISA kit. Levels of mammalian target of rapamycin (mTOR) and the alterations of microtubule networks were evaluated by immunofluorescence.

RESULTS

We demonstrated, for the first time, that the combination of Cl-IB-MECA with paclitaxel significantly increases cytotoxicity, with apoptosis and autophagy the major mechanisms involved in cell death. Induction of autophagy, using clinically relevant doses,was confirmed by visualization of autophagosome and autolysosome formation, and downregulation of mTOR and p62 levels. Caspase-dependent and caspase-independent mitotic catastrophe evidencing micro- and multinucleation was also observed in cells exposed to our combination.

CONCLUSIONS

The combination of Cl-IB-MECA and paclitaxel causes significant cytotoxicity on two melanoma cell lines through multiple mechanisms of cell death. This multifactorial hit makes this therapy very promising as it will help to avoid melanoma multiresistance to chemotherapy and therefore potentially improve its treatment.

Death induced by CD95 or CD95 ligand elimination

CD95 (Fas/APO-1), when bound by its cognate ligand CD95L, induces cells to die by apoptosis. We now show that elimination of CD95 or CD95L results in a form of cell death that is independent of caspase-8, RIPK1/MLKL, and p53, is not inhibited by Bcl-xL expression, and preferentially affects cancer cells. All tumors that formed in mouse models of low-grade serous ovarian cancer or chemically induced liver cancer with tissue-specific deletion of CD95 still expressed CD95, suggesting that cancer cannot form in the absence of CD95. Death induced by CD95R/L elimination (DICE) is characterized by an increase in cell size, production of mitochondrial ROS, and DNA damage. It resembles a necrotic form of mitotic catastrophe. No single drug was found to completely block this form of cell death, and it could also not be blocked by the knockdown of a single gene, making it a promising way to kill cancer cells.

Visualizing Vpr-induced G2 arrest and apoptosis

Vpr is an accessory protein of human immunodeficiency virus type 1 (HIV-1) with multiple functions. The induction of G2 arrest by Vpr plays a particularly important role in efficient viral replication because the transcriptional activity of the HIV-1 long terminal repeat is most active in G2 phase. The regulation of apoptosis by Vpr is also important for immune suppression and pathogenesis during HIV infection. However, it is not known whether Vpr-induced apoptosis depends on the ability of Vpr to induce G2 arrest, and the dynamics of Vpr-induced G2 arrest and apoptosis have not been visualized. We performed time-lapse imaging to examine the temporal relationship between Vpr-induced G2 arrest and apoptosis using HeLa cells containing the fluorescent ubiquitination-based cell cycle indicator2 (Fucci2). The dynamics of G2 arrest and subsequent long-term mitotic cell rounding in cells transfected with the Vpr-expression vector were visualized. These cells underwent nuclear mis-segregation after prolonged mitotic processes and then entered G1 phase. Some cells subsequently displayed evidence of apoptosis after prolonged mitotic processes and nuclear mis-segregation. Interestingly, Vpr-induced apoptosis was seldom observed in S or G2 phase. Likewise, visualization of synchronized HeLa/Fucci2 cells infected with an adenoviral vector expressing Vpr clearly showed that Vpr arrests the cell cycle at G2 phase, but does not induce apoptosis at S or G2 phase. Furthermore, time-lapse imaging of HeLa/Fucci2 cells expressing SCAT3.1, a caspase-3-sensitive fusion protein, clearly demonstrated that Vpr induces caspase-3-dependent apoptosis. Finally, to examine whether the effects of Vpr on G2 arrest and apoptosis were reversible, we performed live-cell imaging of a destabilizing domain fusion Vpr, which enabled rapid stabilization and destabilization by Shield1. The effects of Vpr on G2 arrest and subsequent apoptosis were reversible. This study is the first to characterize the dynamics of the morphological changes that occur during Vpr-induced G2 arrest and apoptosis.

Ochratoxin A induces cytotoxicity, DNA damage and apoptosis in rat hepatocyte primary cell culture at nanomolar concentration

Ochratoxin A (OTA), a mycotoxin produced by several species of Aspergillus and Penicillum, is widely found as a contaminant of food. OTA exhibits a wide range of toxic activities, including nephro- and hepatotoxicity. Although the mechanisms of its genotoxicity and carcinogenicity have been studied before, many controversial results have been published. In addition, the studies were mostly conducted with kidney cells. Therefore, the present study used a primary culture of Wistar rat hepatocytes incubated with increasing concentrations of OTA (2.0-6.0 nanomolar). OTA treatment showed dose-dependent cytotoxicity and DNA damage. Further, flow cytometric analysis of hepatocytes showed dose-dependent apoptosis, suggesting that OTA-induced hepatotoxicity is, may be partly, mediated by apoptosis. Vascular endothelial growth factor gene, a potent pro-angiogenic in hepatocellular carcinoma and responsible for hepatocyte regeneration, did not show any change with OTA treatment, as analysed by reverse transcription polymerase chain reaction. Thus, the present data indicated OTA-induced rat hepatotoxicity in vitro at nanomolar concentration, which inferred a major possible target other than kidney cells.

Synthesis and antiproliferative activity of conformationally restricted 1,2,3-triazole analogues of combretastatins in the sea urchin embryo model and against human cancer cell lines

A series of 1,5-diaryl- and 4,5-diaryl-1,2,3-triazole derivatives of combretastatin A4 were synthesized and evaluated as antimitotic microtubule destabilizing agents using the sea urchin embryo model. Structure-activity relationship studies identified compounds substituted with 3,4,5-trimethoxyphenyl and 3,4-methylenedioxy-5-methoxyphenyl ring A and 4-methoxyphenyl ring B as potent antiproliferative agents with high cytotoxicity against a panel of human cancer cell lines including multi-drug resistant cells. 4,5-Diaryl-1,2,3-triazoles (C-C geometry) were found to be considerably more active than the respective 1,5-diaryl-1,2,3-triazoles (N-C geometry). Compound 10ad' induced G2/M cell cycle arrest and apoptosis in human T-leukemia Jurkat cells via caspase 2/3/9 activation and downregulation of the antiapoptotic protein XIAP. A mitotic catastrophe has been evaluated as another possible cell death mode.

Hematopoietic stem cells are acutely sensitive to Acd shelterin gene inactivation

The shelterin complex plays dual functions in telomere homeostasis by recruiting telomerase and preventing the activation of a DNA damage response at telomeric ends. Somatic stem cells require telomerase activity, as evidenced by progressive stem cell loss leading to bone marrow failure in hereditary dyskeratosis congenita. Recent work demonstrates that dyskeratosis congenita can also arise from mutations in specific shelterin genes, although little is known about shelterin functions in somatic stem cells. We found that mouse hematopoietic stem cells (HSCs) are acutely sensitive to inactivation of the shelterin gene Acd, encoding TPP1. Homozygosity for a hypomorphic acd allele preserved the emergence and expansion of fetal HSCs but led to profoundly defective function in transplantation assays. Upon complete Acd inactivation, HSCs expressed p53 target genes, underwent cell cycle arrest, and were severely depleted within days, leading to hematopoietic failure. TPP1 loss induced increased telomeric fusion events in bone marrow progenitors. However, unlike in epidermal stem cells, p53 deficiency did not rescue TPP1-deficient HSCs, indicating that shelterin dysfunction has unique effects in different stem cell populations. Because the consequences of telomere shortening are progressive and unsynchronized, acute loss of shelterin function represents an attractive alternative for studying telomere crisis in hematopoietic progenitors.

Stilbene 5c, a microtubule poison with vascular disrupting properties that induces multiple modes of growth arrest and cell death

The stilbene derivative, cis-3,4',5-trimethoxy-3'-aminostilbene (stilbene 5c), is a potentially potent antitumor agent that acts via binding to the colchicine-binding site in tubulin. The current studies were designed to investigate the effectiveness of stilbene 5c against the HCT-116 human colon cancer cell line and B16/F10 melanoma cells as well as human endothelial cell tube formation and tumor perfusion. Stilbene 5c produced a time-dependent decrease in cell viability in both cell lines and the capacity of the cells to proliferate was not restored upon removal of the drug. Treatment with stilbene 5c also promoted both senescence and autophagy in both cell lines. TUNEL and annexin 5 staining indicated that apoptosis also occurs in stilbene 5c-treated HCT-116 cells, but not in B16/F10 melanoma cells. DAPI staining revealed morphological changes in the cell nuclei (binucleated and micronucleated cells) indicative of mitotic catastrophe in HCT-116 cells but not in the B16/F10 melanoma cells. p53-null HCT-116 cells demonstrated a similar growth arrest/cell death response to stilbene as p53-wild type HCT-116 cells. Stilbene 5c also completely inhibited human endothelial cell tube formation on Matrigel, consistent with potential anti-angiogenic actions. Using a new method developed for monitoring the pharmacodynamic effects of stilbene 5c in vivo, we found that a single injection of stilbene 5c reduced tumor perfusion by 65% at 4h, returning to baseline by 24h, while subsequent daily injections of stilbene 5c produced progressively larger reductions and smaller rebounds. This work indicates that stilbene 5c could potentially be effective against melanoma and colon cancer through the promotion of multiple modes of growth arrest and cell death coupled with anti-angiogenic and antivascular actions.

RhoA GTPase controls cytokinesis and programmed necrosis of hematopoietic progenitors

The GTPase RhoA is required for the appropriate division and survival of hematopoietic progenitor cells.

Hematopoietic progenitor cells (HPCs) are central to hematopoiesis as they provide large numbers of lineage-defined blood cells necessary to sustain blood homeostasis. They are one of the most actively cycling somatic cells, and their precise control is critical for hematopoietic homeostasis. The small GTPase RhoA is an intracellular molecular switch that integrates cytokine, chemokine, and adhesion signals to coordinate multiple context-dependent cellular processes. By using a RhoA conditional knockout mouse model, we show that RhoA deficiency causes a multilineage hematopoietic failure that is associated with defective multipotent HPCs. Interestingly, RhoA^−/− hematopoietic stem cells retained long-term engraftment potential but failed to produce multipotent HPCs and lineage-defined blood cells. This multilineage hematopoietic failure was rescued by reconstituting wild-type RhoA into the RhoA^−/− Lin⁻Sca-1⁺c-Kit⁺ compartment. Mechanistically, RhoA regulates actomyosin signaling, cytokinesis, and programmed necrosis of the HPCs, and loss of RhoA results in a cytokinesis failure of HPCs manifested by an accumulation of multinucleated cells caused by failed abscission of the cleavage furrow after telophase. Concomitantly, the HPCs show a drastically increased death associated with increased TNF–RIP-mediated necrosis. These results show that RhoA is a critical and specific regulator of multipotent HPCs during cytokinesis and thus essential for multilineage hematopoiesis.

The DNA damage response during mitosis

Cells are equipped with a cell-intrinsic signaling network called the DNA damage response (DDR). This signaling network recognizes DNA lesions and initiates various downstream pathways to coordinate a cell cycle arrest with the repair of the damaged DNA. Alternatively, the DDR can mediate clearance of affected cells that are beyond repair through apoptosis or senescence. The DDR can be activated in response to DNA damage throughout the cell cycle, although the extent of DDR signaling is different in each cell cycle phase. Especially in response to DNA double strand breaks, only a very marginal response was observed during mitosis. Early on it was recognized that cells which are irradiated during mitosis continued division without repairing broken chromosomes. Although these initial observations indicated diminished DNA repair and lack of an acute DNA damage-induced cell cycle arrest, insight into the mechanistic re-wiring of DDR signaling during mitosis was only recently provided. Different mechanisms appear to be at play to inactivate specific signaling axes of the DDR network in mitosis. Importantly, mitotic cells not simply inactivate the entire DDR, but appear to mark their DNA damage for repair after mitotic exit. Since the treatment of cancer frequently involves agents that induce DNA damage as well as agents that block mitotic progression, it is clinically relevant to obtain a better understanding of how cancer cells deal with DNA damage during interphase versus mitosis. In this review, the molecular details concerning DDR signaling during mitosis as well as the consequences of encountering DNA damage during mitosis for cellular fate are discussed.

Acquired expression of NFATc1 downregulates E-cadherin and promotes cancer cell invasion

NFATc1 is a transcription factor that regulates T-cell development, osteoclastogenesis, and macrophage function. Given that T cells, osteoclasts, and macrophages in the tumor microenvironment are thought to modulate tumor progression, tumor cells may acquire NFATc1 expression through fusion with these NFATc1-expressing normal cells. We here revealed that a small proportion of tumor cells in human carcinoma specimens expressed NFATc1. To investigate the consequences of NFATc1 acquisition by tumor cells, we established A549 and MCF7 cell lines expressing a constitutively active form of NFATc1 (NFATc1CA) in an inducible manner. The expression of NFATc1CA promoted cancer cell invasion in association with changes in cell morphology. Analysis of gene expression and RNA interference experiments revealed that NFATc1CA suppressed E-cadherin expression by upregulating the transcriptional repressors Snail and Zeb1 in a manner independent of TGF-β signaling. Induced expression of NFATc1CA also downregulated E-cadherin expression and increased invasive activity in tumor xenografts in vivo. Our results thus suggest that the acquisition of NFATc1 expression contributes to tumor progression.

Caspase-2 as a tumour suppressor

Ever since its discovery 20 years ago, caspase-2 has been enigmatic and its function somewhat controversial. Although many in vitro studies suggested that caspase-2 was important for apoptosis, demonstrating an in vivo cell death role for this caspase has been more problematic, with caspase-2-deficient mice showing limited, tissue-specific cell death defects. Recent results from different laboratories suggest that at least one of its physiological roles in animals is to protect against cellular stress and transformation. As such, loss of caspase-2 augments tumorigenesis in some mouse models of cancer, assigning a tumour suppressor function to this enigmatic caspase. This review focuses on this seemingly non-apoptotic function of caspase-2 as a tumour suppressor and reconciles some of the recent findings in the field.

The natural history of colorectal adenomas and early cancer

Adenomas represent the morphological precursors of the vast majority of colorectal cancers: although every adenoma has the capacity of malignant evolution, most adenomas stabilize their progression or even regress. Pathological factors are predictive of the natural history of adenomas in terms of potential and time interval for becoming malignant. Regression of adenomas is histologically well established, but it is thought to be a dynamic process, with cycling phases of regression and growth. Colorectal carcinoma invading the submucosa but not the muscular layer represents the earliest form of clinically relevant colorectal cancer. Grade of differentiation of carcinoma, lymphovascular invasion, and state of the resection margin predict the risk of metastasis. Microstaging of invasive cancer together with tumuor budding allow the metastatic risk to be further stratified into minimal, low, and high. Two distinct profiles are identifiable in the natural history of cancerous adenomas: blocking the growth of early cancer and allowing its progression towards advanced cancer. Thus, biomarkers to distinguish between progressive and non-progressive pT1 neoplasia are needed.

Depletion of RAD17 sensitizes pancreatic cancer cells to gemcitabine

Chemotherapy of advanced pancreatic cancer has mainly been gemcitabine-based for the past 15 years, with only limited effect. Recently, combination therapy that also targets checkpoint kinase 1 (CHK1) has become an attractive option. The central role of CHK1 in many DNA-damage response pathways, however, may result in undesired cytotoxicity in normal cells, causing side effects. We were searching for other target molecules of similar function that may be more specific and thus better suited for combination therapy. To this end a negative selection RNAi screen was performed in cell lines with small hairpin RNA molecules targeting over 10,000 genes. Genes that were found to be synthetically lethal with gemcitabine and whose proteins act upstream of CHK1 were characterised in more detail. In particular, the inhibition of RAD17 potentiated gemcitabine cytotoxicity in the pancreatic cancer cell lines BxPC-3 and MiaPaca-2 and in the primary cell line JoPaca-1 that closely resembles primary tumour tissue. Further analysis showed that the synergistic effect of RAD17 knockdown and gemcitabine leads to forced mitotic entry of cells arrested in S phase by gemcitabine treatment, resulting in asymmetric DNA distribution during anaphase followed by DNA fragmentation and finally cell death by mitotic catastrophe. Our data suggest RAD17 as a novel target protein for gemcitabine combination therapy supplementing or complementing inhibition of CHK1. In contrast to CHK1, RAD17 knockdown by itself does not lead to abnormal DNA segregation, suggesting a more specific action.

Bendamustine overcomes resistance to melphalan in myeloma cell lines by inducing cell death through mitotic catastrophe

Melphalan has been a mainstay of multiple myeloma (MM) therapy for many years. However, following treatment with this alkylator, malignant plasma cells usually escape both apoptosis and cell cycle control, and acquire drug-resistance resulting in tumor progression. Bendamustine is being used in MM patients refractory to conventional DNA-damaging agents, although the mechanisms driving this lack of cross-resistance are still undefined. Here, we investigated the molecular pathway of bendamustine-induced cell death in melphalan-sensitive and melphalan-resistant MM cell lines. Bendamustine affected cell survival resulting in secondary necrosis, and prompted cell death primarily through caspase-2 activation. Also, bendamustine blocked the cell cycle in the G2/M phase and induced micronucleation, erratic chromosome spreading and mitotic spindle perturbations in melphalan-resistant MM cells. In these cells, both Aurora kinase A (AURKA) and Polo-like kinase-1 (PLK-1), key components of the spindle-assembly checkpoint, were down-regulated following incubation with bendamustine, whereas levels of Cyclin B1 increased as a consequence of the prolonged mitotic arrest induced by the drug. These findings indicate that, at least in vitro, bendamustine drives cell death by promoting mitotic catastrophe in melphalan-resistant MM cells. Hence, activation of this alternative pathway of cell death may be a novel approach to the treatment of apoptosis-resistant myelomas.