Involvement of caspase activation and mitochondrial stress in taxol-induced apoptosis of Epstein-Barr virus-infected Akata cells

Taxol (paclitaxel) is one of the most potent antimicrotubule agents currently used in cancer chemoprevention and treatment. However, the effects of taxol on the induction of apoptosis in Epstein-Barr virus (EBV)-infected cells are unknown. This study investigated the mechanisms of taxol on cell cycle arrest and apoptosis induction using the EBV-infected cell line, Akata. Taxol treatment sensitively and dose-independently induced growth inhibition, cytotoxicity, and apoptosis in the cells, which was demonstrated by the decreased level of tritium incorporation and cell viability, the increased number of positively stained cells in the trypan blue staining and TUNEL assay, the increased population of cells in the sub-G(0)/G(1) phase in flow cytometric analysis, and ladder formation of the genomic DNA. Treatment with z-VAD-fmk almost completely protected the cells from taxol-induced apoptosis indicating that the taxol-induced apoptosis of Akata cells is caspase-dependent. In addition, taxol-induced apoptosis is proposed to be associated with a lower mitochondrial membrane potential and G(2)/M arrest. However, the tubulin expression level doses not appear to be a direct mediator of taxol-induced apoptosis in cells. The presence of EBV in these cells was not related to the sensitivity of the cells to the induction of apoptosis by taxol. Overall, these results demonstrate that taxol induces apoptosis in EBV-infected Akata cells in a dose-independent manner, and that caspase activation and mitochondrial stress are involved in the induction.     

Taxol-induced mitotic block triggers rapid onset of a p53-independent apoptotic pathway.

BACKGROUND: At therapeutic concentrations, the antineoplastic agent taxol selectively perturbs mitotic spindle microtubules. Taxol has recently been shown to induce apoptosis, similar to the mechanism of cell death induced by other antineoplastic agents. However, taxol has shown efficacy against drug-refractory cancers, raising the possibility that this pharmacological agent may trigger an alternative apoptotic pathway. MATERIALS AND METHODS: The kinetics and IC50 of mitotic (M) block, aberrant mitosis, and cytotoxicity following taxol treatment were analyzed in human cell lines as well as normal mouse embryo fibroblasts (MEFs) and MEFs derived from p53-null mice. Apoptosis was followed by DNA gel electrophoresis and by in situ DNA end-labeling (TUNEL). RESULTS: Taxol induced two forms of cell cycle arrest: either directly in early M at prophase or, for those cells progressing through aberrant mitosis, arrest in G1 as multimininucleated cells. TUNEL labeling revealed that DNA nicking occurred within 30 min of the arrest in prophase. In contrast, G1-arrested, multimininucleated cells became TUNEL positive only after several days. In the subset of cells that became blocked directly in prophase, both wt p53-expressing and p53-null MEFs responded similarly to taxol, showing rapid onset of DNA nicking and apoptosis. However, p53-null MEFs progressing through aberrant mitosis failed to arrest in the subsequent G1 phase or to become TUNEL positive, and remained viable. CONCLUSIONS: Taxol induces two forms of cell cycle arrest, which in turn induce two independent apoptotic pathways. Arrest in prophase induces rapid onset of a p53-independent pathway, whereas G1-block and the resulting slow (3-5 days) apoptotic pathway are p53 dependent.     

CDK1 siRNA干扰在Taxol诱导细胞凋亡中的作用

目的研究转染细胞周期依赖性蛋白激酶1（cyclin．dependent kinase1,CDK1）siRNA、以及转染后进行凋亡刺激对细胞周期和凋亡的影响,探讨CDK1在细胞凋亡中的确切作用,揭示细胞周期与细胞凋亡协调的分子机制。方法以人宫颈癌细胞株HeLa细胞为研究对象,脂质体转染CDK1siRNA,转染后48h加紫杉醇（Tax01）（20μg／m1）刺激凋亡,Western印迹检测CDK1和抗凋亡蛋白BCL2表达,AnnexinV／PI法检测细胞的凋亡,流式细胞仪分析DNA含量检测细胞周期。结果转染CDK1 siRNA后,CDK1蛋白的表达下降,细胞周期G2／M期比例增加,细胞凋亡率与对照相比没有明显升高。只加Taxol刺激12h后细胞凋亡率增加并伴有S期和G2／M期比例增加。转染CDKlsiRNA后再用Taxol刺激,其细胞凋亡率没有明显改变,G2／M期阻滞效应也没有叠加。BCL2蛋白只在加Taxol刺激组表达下降,与CDK1表达减少没有相关性。结论siRNA沉默导致的CDK1表达降低只导致细胞周期G2／M期阻滞,没有引起细胞凋亡;CDK1的表达降低对紫杉醇所诱导的细胞周期阻滞和细胞凋亡效应没有明显影响。     

Involvement of caspase activation and mitochondrial stress in taxol-induced apoptosis of Epstein–Barr virus-infected Akata cells

Taxol (paclitaxel) is one of the most potent antimicrotubule agents currently used in cancer chemoprevention and treatment. However, the effects of taxol on the induction of apoptosis in Epstein–Barr virus (EBV)-infected cells are unknown. This study investigated the mechanisms of taxol on cell cycle arrest and apoptosis induction using the EBV-infected cell line, Akata. Taxol treatment sensitively and dose-independently induced growth inhibition, cytotoxicity, and apoptosis in the cells, which was demonstrated by the decreased level of tritium incorporation and cell viability, the increased number of positively stained cells in the trypan blue staining and TUNEL assay, the increased population of cells in the sub-G₀/G₁ phase in flow cytometric analysis, and ladder formation of the genomic DNA. Treatment with z-VAD-fmk almost completely protected the cells from taxol-induced apoptosis indicating that the taxol-induced apoptosis of Akata cells is caspase-dependent. In addition, taxol-induced apoptosis is proposed to be associated with a lower mitochondrial membrane potential and G₂/M arrest. However, the tubulin expression level doses not appear to be a direct mediator of taxol-induced apoptosis in cells. The presence of EBV in these cells was not related to the sensitivity of the cells to the induction of apoptosis by taxol. Overall, these results demonstrate that taxol induces apoptosis in EBV-infected Akata cells in a dose-independent manner, and that caspase activation and mitochondrial stress are involved in the induction.     

Paclitaxel sensitization of multidrug-resistant cells to chemotherapy is independent of the cell cycle

BACKGROUND: Recent studies have shown that paclitaxel (Taxol) is an active chemotherapeutic in the treatment of small cell lung cancer. Paclitaxel binds to tubulin and prevents depolymerization. This causes cells to arrest in the G(2)/M phase of the cell cycle, resulting in sensitization of cells to drug or radiation treatment. METHODS: A drug-resistant H69 small cell lung cancer subline was established. Cytotoxicity of cisplatin and chlorambucil was determined using the MTT cell viability assay and distribution of DNA in the cell cycle. DNA distribution was analyzed by flow cytometry after treatment with paclitaxel or the other tubulin-binding drugs, vinblastine and navelbine. RESULTS: The H69-EPR drug-resistant subline was resistant to epirubicin (sixfold) and was cross-resistant to cisplatin (7.5-fold) and chlorambucil (7.5-fold). Pretreatment with paclitaxel or vinblastine, but not navelbine, sensitized the subline to cisplatin and chlorambucil (P < 0.05), with no effect on parental H69 cells. Sensitization was dose dependent and occurred at doses below those that caused a G(2)/M block in the cell cycle. CONCLUSION: Sensitization of drug-resistant cells by paclitaxel was not associated with its ability to cause a G(2)/M block in the cell cycle. Sensitization by paclitaxel and vinblastine, but not navelbine, which preferentially targets mitotic tubulin, suggests that sensitization may involve changes in the tubulin-dependent intracellular transport processes rather than changes in mitotic tubulin and the G(2)/M block.     

Furano-1,2-naphthoquinone inhibits EGFR signaling associated with G2/M cell cycle arrest and apoptosis in A549 cells

Furano-1,2-naphthoquinone (FNQ), prepared from 2-hydroxy-1,4-naphthoquinone and chloroacetaldehyde in an efficient one-pot reaction, exhibits an anti-carcinogenic effect. FNQ exerted anti-proliferative activity with the G(2)/M cell cycle arrest and apoptosis in A549 cells. FNQ-induced G(2)/M arrest was correlated with a marked decrease in the expression levels of cyclin A and cyclin B, and their activating partner cyclin-dependent kinases (Cdk) 1 and 2 with concomitant induction of p53, p21, and p27. FNQ-induced apoptosis was accompanied with Bax up-regulation and the down-regulation of Bcl-2, X-linked inhibitor of apoptosis (XIAP), and survivin, resulting in cytochrome c release and sequential activation of caspase-9 and caspase-3. Western blot analysis revealed that FNQ suppressed EGFR phosphorylation and JAK2, STAT3, and STAT5 activation, but increased in activation of p38 MAPK and c-Jun NH2-terminal kinase (JNK) stress signal. The combined treatment of FNQ with AG1478 (a specific EGFR inhibitor) significantly enhanced the G(2)/M arrest and apoptosis, and also led to up-regulation in Bax, p53, p21, p27, release of mitochondrial cytochrome c, and down-regulation of Bcl-2, XIAP, survivin, cyclin A, cyclin B, Cdk1, and Cdk2 in A549 cells. These findings suggest that FNQ-mediated cytotoxicity of A549 cell related with the G(2)/M cell cycle arrest and apoptosis via inactivation of EGFR-mediated signaling pathway.     

Raf-1/MEK/MAPK pathway is necessary for the G2/M transition induced by nocodazole

The dynamic balance between polymerization and depolymerization of microtubules is critical for cells to enter and exit mitosis, and drugs that disrupt this balance, such as taxol, colchicine, and nocodazole, arrest the cell cycle in mitosis. Although the Raf/MEK/MAPK pathway can be activated by these drugs, its role in mitosis has not been addressed. Here, we characterize activation of Raf/MEK/MAPK by nocodazole when mitosis is induced. We find that at early time points (up to 3 h) in nocodazole induction, Raf/MEK/MAPK is activated, and inhibition of MAPK activation by a MEK inhibitor, PD98059 or U0126, reduces the number of cells entering mitosis by creating a block at G(2). At later time points and in mitosis, activation of MEK/MAPK is severely inhibited, even though Raf-1 activity remains high and can be further increased by growth factor. This inhibition is reversed when cells are released from metaphase and enter G(0)/G(1) phase. In addition, we find that binding of Raf-1 to 14-3-3 is progressively induced by nocodazole, reaching a maximum in mitosis, and that this binding is necessary to maintain mitotic Raf-1 activity. Our present study indicates that activation of the Raf/MEK/MAPK pathway is necessary for the G(2)/M progression.     

Vpr cytopathicity independent of G2/M cell cycle arrest in human immunodeficiency virus type 1-infected CD4+ T cells

The mechanism of CD4(+) T-cell depletion in human immunodeficiency virus type 1 (HIV-1)-infected individuals remains unknown, although mounting evidence suggests that direct viral cytopathicity contributes to this loss. The HIV-1 Vpr accessory protein causes cell death and arrests cells in the G(2)/M phase; however, the molecular mechanism underlying these properties is not clear. Mutation of hydrophobic residues on the surface of its third alpha-helix disrupted Vpr toxicity, G(2)/M arrest induction, nuclear localization, and self-association, implicating this region in multiple Vpr functions. Cytopathicity by virion-delivered mutant Vpr protein correlated with G(2)/M arrest induction but not nuclear localization or self-association. However, infection with whole virus encoding these Vpr mutants did not abrogate HIV-1-induced cell killing. Rather, mutant Vpr proteins that are impaired for G(2)/M block still prevented infected cell proliferation, and this property correlated with the death of infected cells. Chemical agents that inhibit infected cells from entering G(2)/M also did not reduce HIV-1 cytopathicity. Combined, these data implicate Vpr in HIV-1 killing through a mechanism involving inhibiting cell division but not necessarily in G(2)/M. Thus, the hydrophobic region of the third alpha-helix of Vpr is crucial for mediating G(2)/M arrest, nuclear localization, and self-association but dispensable for HIV-1 cytopathicity due to residual cell proliferation blockade mediated by a separate region of the protein.     

Noscapine induces mitochondria-mediated apoptosis in human colon cancer cells in vivo and in vitro

Noscapine, a phthalide isoquinoline alkaloid derived from opium, has been widely used as a cough suppressant for decades. Noscapine has recently been shown to potentiate the anti-cancer effects of several therapies by inducing apoptosis in various malignant cells without any detectable toxicity in cells or tissues. However, the mechanism by which noscapine induces apoptosis in colon cancer cells remains unclear. The signaling pathways by which noscapine induces apoptosis were investigated in colon cancer cell lines treated with various noscapine concentrations for 72 h, and a dose-dependent inhibition of cell viability was observed. Noscapine effectively inhibited the proliferation of LoVo cells in vitro (IC(50)=75 μM). This cytotoxicity was reflected by cell cycle arrest at G(2)/M and subsequent apoptosis, as indicated by increased chromatin condensation and fragmentation, the upregulation of Bax and cytochrome c (Cyt-c), the downregulation of survivin and Bcl-2, and the activation of caspase-3 and caspase-9. Moreover, in a xenograft tumor model in mice, noscapine injection clearly inhibited tumor growth via the induction of apoptosis, which was demonstrated using a TUNEL assay. These results suggest that noscapine induces apoptosis in colon cancer cells via mitochondrial pathways. Noscapine may be a safe and effective chemotherapeutic agent for the treatment of human colon cancer.     

Genome‐wide siRNA screen reveals coupling between mitotic apoptosis and adaptation

The antimitotic anti‐cancer drugs, including taxol, perturb spindle dynamics, and induce prolonged, spindle checkpoint‐dependent mitotic arrest in cancer cells. These cells then either undergo apoptosis triggered by the intrinsic mitochondrial pathway or exit mitosis without proper cell division in an adaptation pathway. Using a genome‐wide small interfering RNA (siRNA) screen in taxol‐treated HeLa cells, we systematically identify components of the mitotic apoptosis and adaptation pathways. We show that the Mad2 inhibitor p31^comet actively promotes mitotic adaptation through cyclin B1 degradation and has a minor separate function in suppressing apoptosis. Conversely, the pro‐apoptotic Bcl2 family member, Noxa, is a critical initiator of mitotic cell death. Unexpectedly, the upstream components of the mitochondrial apoptosis pathway and the mitochondrial fission protein Drp1 contribute to mitotic adaption. Our results reveal crosstalk between the apoptosis and adaptation pathways during mitotic arrest.     

The mitotic spindle checkpoint is a critical determinant for topoisomerase-based chemotherapy

A novel strategy in cancer therapy is the induction of mitotic cell death by the pharmacological abrogation of cell cycle checkpoints. UCN-01 is such a compound that overrides the G2 cell cycle arrest induced by DNA damage and forces cells into a deleterious mitosis. The molecular pathways leading to mitotic cell death are largely unknown although recent evidence indicates that mitotic cell death represents a special case of apoptosis. Here, we demonstrate that the mitotic spindle checkpoint is activated upon chemotherapeutic treatment with topoisomerase II poisons and UCN-01. Cells that are forced to enter mitosis in the presence of topoisomerase inhibition arrest transiently in a prometaphase like state. By using a novel pharmacological inhibitor of the spindle checkpoint and spindle checkpoint-deficient cells we show that the spindle checkpoint function is required for the mitotic arrest and, most importantly, for efficient induction of mitotic cell death. Thus, our results demonstrate that the mitotic spindle checkpoint is an important determinant for the outcome of a chemotherapy based on the induction of mitotic cell death. Its frequent inactivation in human cancer might contribute to the observed resistance of tumor cells to these chemotherapeutic drugs.     

Securin enhances the anti-cancer effects of 6-methoxy-3-(3',4',5'-trimethoxy-benzoyl)-1H-indole (BPR0L075) in human colorectal cancer cells

BPR0L075 [6-methoxy-3-(3',4',5'-trimethoxy-benzoyl)-1H-indole] is a novel anti-microtubule drug with anti-tumor and anti-angiogenic activities in vitro and in vivo. Securin is required for genome stability, and is expressed abundantly in most cancer cells, promoting cell proliferation and tumorigenesis. In this study, we found that BPR0L075 efficiently induced cell death of HCT116 human colorectal cancer cells that have higher expression levels of securin. The cytotoxicity of BPR0L075 was attenuated in isogenic securin-null HCT116 cells. BPR0L075 induced DNA damage response, G(2)/M arrest, and activation of the spindle assembly checkpoint in HCT116 cells. Interestingly, BPR0L075 induced phosphorylation of securin. BPR0L075 withdrawal resulted in degradation of securin, mitotic exit, and mitotic catastrophe, which were attenuated in securin-null cells. Inhibition of cdc2 decreased securin phosphorylation, G(2)/M arrest and cell death induced by BPR0L075. Moreover, BPR0L075 caused cell death through a caspase-independent mechanism and activation of JNK and p38 MAPK pathways. These findings provided evidence for the first time that BPR0L075 treatment is beneficial for the treatment of human colorectal tumors with higher levels of securin. Thus, we suggest that the expression levels of securin may be a predictive factor for application in anti-cancer therapy with BPR0L075 in human cancer cells.     

The antidiabetic drug ciglitazone induces high grade bladder cancer cells apoptosis through the up-regulation of TRAIL

Background

Ciglitazone belongs to the thiazolidinediones class of antidiabetic drug family and is a high-affinity ligand for the Peroxisome Proliferator-Activated Receptor γ (PPARγ). Apart from its antidiabetic activity, this molecule shows antineoplastic effectiveness in numerous cancer cell lines.

Methodology/Principal Findings

Using RT4 (derived from a well differentiated grade I papillary tumor) and T24 (derived from an undifferentiated grade III carcinoma) bladder cancer cells, we investigated the potential of ciglitazone to induce apoptotic cell death and characterized the molecular mechanisms involved. In RT4 cells, the drug induced G2/M cell cycle arrest characterized by an overexpression of p53, p21^waf1/CIP1 and p27^Kip1 in concomitance with a decrease of cyclin B1. On the contrary, in T24 cells, it triggered apoptosis via extrinsic and intrinsic pathways. Cell cycle arrest and induction of apoptosis occurred at high concentrations through PPARγ activation-independent pathways. We show that in vivo treatment of nude mice by ciglitazone inhibits high grade bladder cancer xenograft development. We identified a novel mechanism by which ciglitazone kills cancer cells. Ciglitazone up-regulated soluble and membrane-bound TRAIL and let TRAIL-resistant T24 cells to respond to TRAIL through caspase activation, death receptor signalling pathway and Bid cleavage. We provided evidence that TRAIL-induced apoptosis is partially driven by ciglitazone-mediated down-regulation of c-FLIP and survivin protein levels through a proteasome-dependent degradation mechanism.

Conclusions/Significance

Therefore, ciglitazone could be clinically relevant as chemopreventive or therapeutic agent for the treatment of TRAIL-refractory high grade urothelial cancers.     

Differential Taxol-dependent arrest of transformed and nontransformed cells in the G1 phase of the cell cycle, and specific-related mortality of transformed cells

Taxol (paclitaxel) induces a microtubule hyperassembled state, and effectively blocks cells in mitosis. Here we report that Taxol also induces a stable late-G1 block in nontransformed REF-52 and WI-38 mammalian fibroblast cells, but not in T antigen-transformed cells of the same parental lineage. G1 arrest is characterized by partially dephosphorylated pRb, and inactive cdk2 kinase. Nontransformed cells recover normally from Taxol arrest. In contrast, T antigen transformed cells continue inappropriately past both G1 and G2-M in the presence of Taxol, and undergo a rapid death upon release. These results demonstrate a microtubule sensitive step in G1 regulation of nontransformed fibroblast cells. Also, Taxol selectively induces death of transformed cells, possibly because they slip the Taxol-dependent G1 arrest, as well as G2/M arrest, which are both specific to nontransformed cells.     

Cytotoxicity of calotropin is through caspase activation and downregulation of anti-apoptotic proteins in K562 cells

Calotropin is one of cardenolides isolated from milkweed used for medicinal purposes in many Asian countries. Whereas calotropin possesses cytotoxicity against several cancer cells, the mechanisms of action remain unclear. We set out to evaluate the cytotoxic mechanism of calotropin on human chronic myeloid leukemia K562 cells. Calotropin inhibited the growth of K562 cells in a time- and dose-dependent manner by G₂/M phase arrest. It upregulated the expression of p27 leading to this arrest by downregulating the G2/M regulatory proteins, cyclins A and B, and by upregulating the cdk inhibitor, p27. Furthermore, it downregulated anti-apoptotic signaling (XIAP and survivin) and survival pathways (p-Akt and NFκB), leading to caspase-3 activation which resulted in the induction of apoptosis. In all, calotropin exerted its anticancer activity on K562 cells by modulating the pro-survival signaling that leads to induction of apoptosis.     

Cell death induction by isothiocyanates and their underlying molecular mechanisms

An important and promising group of compounds that have a chemopreventive property are organosulfur compounds, such as isothiocyanates (ITCs). In recent years, it has been shown that ITCs induce apoptosis in various cancer cell lines and experimental rodents. During the course of apoptosis induction by ITC, multiple signal-transduction pathways and apoptosis intermediates are modulated. We have also clarified the molecular mechanism underlying the relationship between cell cycle arrest and apoptosis induced by benzyl isothiocyanate (BITC), a major ITC compound isolated from papaya. The exposure of cells to BITC resulted in the inhibition of the G2/M progression that coincided with not only the up-regulated expression of the G2/M cell cycle arrest-regulating genes but also the apoptosis induction. The experiment using the phase-specific synchronized cells demonstrated that the G2/M phase-arrested cells are more sensitive to undergoing apoptotic stimulation by BITC than the cells in other phases. We identified the phosphorylated Bcl-2 as a key molecule linking the p38 MAPK-dependent cell cycle arrest with the JNK activation by BITC. We also found that BITC induced the cytotoxic effect more preferentially in the proliferating normal human colon epithelial cells than in the quiescent cells. Conversely, treatment with an excessive concentration of BITC resulted in necrotic cell death without DNA ladder formation. This review addresses the biological impact of cell death induction by BITC as well as other ITCs and the involved signal transduction pathways.     

Molecular mechanisms for inhibition of colon cancer cells by combined epigenetic-modulating epigallocatechin gallate and sodium butyrate

Bioactive compounds are considered safe and have been shown to alter genetic and epigenetic profiles of tumor cells. However, many of these changes have been reported at molecular concentrations higher than physiologically achievable levels. We investigated the role of the combinatorial effects of epigallocatechin gallate (EGCG), a predominant polyphenol in green tea, and sodium butyrate (NaB), a dietary microbial fermentation product of fiber, in the regulation of survivin, which is an overexpressed anti-apoptotic protein in colon cancer cells. For the first time, our study showed that the combination treatment induced apoptosis and cell cycle arrest in RKO, HCT-116 and HT-29 colorectal cancer cells. This was found to be regulated by the decrease in HDAC1, DNMT1, survivin and HDAC activity in all three cell lines. A G2/M arrest was observed for RKO and HCT-116 cells, and G1 arrest for HT-29 colorectal cancer cells for combinatorial treatment. Further experimentation of the molecular mechanisms in RKO colorectal cancer (CRC) cells revealed a p53-dependent induction of p21 and an increase in nuclear factor kappa B (NF-κB)-p65. An increase in double strand breaks as determined by gamma-H2A histone family member X (γ-H2AX) protein levels and induction of histone H3 hyperacetylation was also observed with the combination treatment. Further, we observed a decrease in global CpG methylation. Taken together, these findings suggest that at low and physiologically achievable concentrations, combinatorial EGCG and NaB are effective in promoting apoptosis, inducing cell cycle arrest and DNA-damage in CRC cells.     

Induction of apoptotic death by curcumin in human tongue squamous cell carcinoma SCC-4 cells is mediated through endoplasmic reticulum stress and mitochondria-dependent pathways

Curcumin from the rhizome of the Curcuma longa plant has been noted for its chemo-preventative and chemo-therapy activities, and it inhibits the growth of many types of human cancer cell lines. In this study, the mechanisms of cell death involved in curcumin-induced growth inhibition, including cell cycle arrest and induction of apoptosis in human tongue cancer SCC-4 cells, were investigated. Herein, we observed that curcumin inhibited cell growth of SCC-4 cells and induced cell death in a dose-dependent manner. Treatment of SCC-4 cells with curcumin caused a moderate and promoted the G(2) /M phase arrest, which was accompanied with decreases in cyclin B/CDK1 and CDC25C protein levels. Moreover, curcumin significantly induced apoptosis of SCC-4 cells with a decrease of the Bcl-2 level, reduction of mitochondrial membrane potential (ΔΨ(m) ), and promoted the active forms of caspase-3. Curcumin also promoted the releases of AIF and Endo G from the mitochondria in SCC-4 cells by using confocal laser microscope. Therefore, we suggest that curcumin induced apoptosis through a mitochondria-dependent pathway in SCC-4 cells. In addition, we also found that curcumin-induced apoptosis of SCC-4 cells was partly through endoplasmic reticulum stress. In conclusion, curcumin increased G(2) /M phase arrest and induced apoptosis through ER stress and mitochondria-dependent pathways in SCC-4 cells.     

The magnitude of methylmercury-induced cytotoxicity and cell cycle arrest is p53-dependent

BACKGROUND: Methylmercury (MeHg), a ubiquitous environmental contaminant, is a known potent teratogen selectively affecting the developing central nervous system. While a definitive mechanism for MeHg-induced developmental neurotoxicity remains elusive, in utero exposure has been associated with reduced brain weight and reduction in cell number. This suggests early toxicant interference with critical molecular signaling events controlling cell behavior, i.e., proliferation. METHODS: To examine the role of p53, a major regulator of the G(1)/S and G(2)/M cell cycle checkpoints, in MeHg toxicity, we isolated GD 14 primary embryonal fibroblasts from homozygous wild-type p53 (p53+/+) and homozygous null p53 (p53-/-) mice. Cells were treated at passages 4-7 for 24 or 48 hr with 0, 1.0, or 2.5 microM MeHg and analyzed for effects on viability, cell cycle progression (using BrdU-Hoechst flow cytometric analysis), and apoptosis via annexin V-FITC and propidium iodide (PI) staining. RESULTS: The p53+/+ cells are more sensitive than p53-/- cells to MeHg-induced cytotoxicity, cell cycle inhibition, and induction of apoptosis: at 24 hr, 2.5 microM MeHg reduced p53+/+ cell viability to 72.6% +/- 3.2%, while p53-/- viability was 94.6% +/- 0.4%. The p53-/- cells underwent less necrosis and less apoptosis following MeHg treatment. MeHg (2.5 microM) also halted all cycling in the p53+/+ cells, while 42.6% +/- 7.2% of p53-/- cells were able to reach a new G(0)/G(1) in 48 hr. Time- and dose-dependent accumulation of cells in G(2)/M phase (1.0 and 2.5 microM MeHg) was observed independent of the p53 genotype; however, the magnitude of change was p53-dependent. CONCLUSIONS: These studies suggest that MeHg-induced cell cycle arrest occurs via both p53-dependent and -independent pathways in our model system; however, cell death resulting from MeHg exposure is highly dependent on p53.     

A novel anticancer ribonucleoside, 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine, enhances radiation-induced cell death in tumor cells

1-(3-C-Ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd, TAS106) is a newly developed anti-tumor agent that targets RNA synthesis. We report here that a low dose of ECyd induces radiosensitization of caspase-dependent apoptosis and reproductive cell death in cells of the gastric tumor cell lines MKN45 and MKN28 and murine rectum adenocarcinoma Colon26. Flow cytometry demonstrated that TAS106 induced the abrogation of the X-ray-induced G(2)/M checkpoint. Western blot analysis showed that X rays increased the expression of cyclin B1, phospho-Cdc2 and Wee1, whereas co-treatment with X rays and TAS106 decreased the expression of these cell cycle proteins associated with the G(2)/M checkpoint. Furthermore, TAS106 was shown to decrease the radiation-induced expression of survivin but not Bcl2 and BclX(L) regardless of TP53 status and cell type. Overexpression of wild-type survivin in MKN45 cells inhibited the induction of apoptosis induced by co-treatment with X rays and TAS106. These results suggest that TAS106 enhances X-ray-induced cell death through down-regulation of survivin and abrogation of the cell cycle machinery.