Association between mitotic spindle checkpoint impairment and susceptibility to the induction of apoptosis by anti-microtubule agents in human lung cancers

Nek4 status differentially alters sensitivity to distinct microtubule poisons

Microtubule poisons are widely used in cancer treatment, but the factors determining the relative efficacy of different drugs in this class remain obscure. In this study, we identified the NIMA kinase Nek4 in a genetic screen for mediators of the response to Taxol, a chemotherapeutic agent that stabilizes microtubules. After Taxol treatment, Nek4 promoted microtubule outgrowth, whereas Nek4 deficiency impaired G(2)-M arrest and decreased formation of mitotic-like asters. In contrast, Nek4 deficiency sensitized cells to vincristine, which destabilizes microtubules. Therefore, Nek4 deficiency may either antagonize or agonize the effects of microtubule poisons, depending on how they affect microtubule polymerization. Of note, Nek4 gene maps to a commonly deleted locus in non-small cell lung cancer. Thus, Nek4 deletion in this disease may rationalize the use of particular types of microtubule poisons for lung cancer therapy.

Nek4 status differentially alters sensitivity to distinct microtubule poisons

Microtubule poisons are widely used in cancer treatment, but the factors determining the relative efficacy of different drugs in this class remain obscure. In this study, we identified the NIMA kinase Nek4 in a genetic screen for mediators of the response to Taxol, a chemotherapeutic agent that stabilizes microtubules. After Taxol treatment, Nek4 promoted microtubule outgrowth, whereas Nek4 deficiency impaired G(2)-M arrest and decreased formation of mitotic-like asters. In contrast, Nek4 deficiency sensitized cells to vincristine, which destabilizes microtubules. Therefore, Nek4 deficiency may either antagonize or agonize the effects of microtubule poisons, depending on how they affect microtubule polymerization. Of note, Nek4 gene maps to a commonly deleted locus in non-small cell lung cancer. Thus, Nek4 deletion in this disease may rationalize the use of particular types of microtubule poisons for lung cancer therapy.

The ability to survive mitosis in the presence of microtubule poisons differs significantly between human nontransformed (RPE-1) and cancer (U2OS, HeLa) cells

We used live cell imaging to compare the fate of human nontransformed (RPE-1) and cancer (HeLa, U2OS) cells as they entered mitosis in nocodazole or taxol. In the same field, and in either drug, a cell in all lines could die in mitosis, exit mitosis and die within 10 h, or exit mitosis and survive > or =10 h. Relative to RPE-1 cells, significantly fewer HeLa or U2OS cells survived mitosis or remained viable after mitosis: in nocodazole concentrations that inhibit spindle microtubule assembly, or in 500 nM taxol, 30% and 27% of RPE-1 cells, respectively, died in or within 10 h of exiting mitosis while 90% and 49% of U2OS and 78% and 81% of HeLa died. This was even true for clinically relevant taxol concentrations (5 nM) which killed 93% and 46%, respectively, of HeLa and U2OS cells in mitosis or within 10 h of escaping mitosis, compared to 1% of RPE-1 cells. Together these data imply that studies using HeLa or U2OS cells, harvested after a prolonged block in mitosis with nocodazole or taxol, are significantly contaminated with dead or dying cells. We also found that the relationship between the duration of mitosis and survival is drug and cell type specific and that lethality is related to the cell type and drug used to prevent satisfaction of the kinetochore attachment checkpoint. Finally, work with a pan-caspase inhibitor suggests that the primary apoptotic pathway triggered by nocodazole during mitosis in RPE-1 cells is not active in U2OS cells. Cell Motil. Cytoskeleton 2008. (c) 2008 Wiley-Liss, Inc.

Nuclear translocation of dihydrofolate reductase is not a pre-requisite for DNA damage induced apoptosis

Dihydrofolate reductase (DHFR) is a key enzyme for the synthesis of thymidylate, and therefore, of DNA. By applying subcellular proteomic analysis, we identified that the DHFR protein was translocated from cytoplasm into the nucleus when apoptosis was induced by NSC606985, a camptothecin analogue. The nuclear translocation of DHFR protein during apoptosis was independent of the cellular context, but it was more sensitive in cell death induction by DNA damaging agents such as doxorubicin, etoposide and ultraviolent radiation than endoplasmic reticulum stressors (brefeldin-A and tunicamycin) and anti-microtubule agents (paclitaxel and nocodozole). The addition of methotrexate almost completely blocked the nuclear translocation of DHFR protein. Further investigations showed that the nuclear translocation of DHFR was not a pre-requisite for DNA damage induced apoptosis. Therefore, its potential biological significance remains to be further explored.

Pharmacologic abrogation of the mitotic spindle checkpoint by an indolocarbazole discovered by cellular screening efficiently kills cancer cells

The mitotic spindle checkpoint represents a signal transduction pathway that prevents the onset of anaphase until all chromosomes are properly aligned on a metaphase plate. Partial inactivation of this checkpoint allows premature separation of sister chromatids and results in aneuploidy, which might contribute to tumorigenesis. Unlike other cell cycle checkpoints, the spindle checkpoint is essential for cell viability, giving rise to the idea that the spindle checkpoint itself might represent a valuable target for anticancer therapy. We used a cell-based screen and identified the indolocarbazole compound Gö6976 as a pharmacologic inhibitor of the spindle checkpoint. Gö6976 potently overrides a spindle checkpoint-mediated mitotic arrest by abrogating the phosphorylation and kinetochore localization of several spindle checkpoint proteins. We identified the Aurora-A and Aurora-B kinases, which have been previously implicated in proper mitotic progression and spindle checkpoint function, as targets for Gö6976. Accordingly, Gö6976 treatment causes severe mitotic abnormalities and chromosome alignment defects, which are not properly detected by the drug-inactivated spindle checkpoint. This results in an aberrant progression of mitosis, leading to apoptosis in various human cancer cell lines, including spindle checkpoint-compromised cancer cells. Thus, our work describes a novel and promising strategy for anticancer treatment that targets the mitotic spindle checkpoint.

K858, a novel inhibitor of mitotic kinesin Eg5 and antitumor agent, induces cell death in cancer cells

The aim of this study was to investigate the mechanism of inhibition of Eg5 (kinesin spindle protein), a mitotic kinesin that plays an essential role in establishing mitotic spindle bipolarity, by the novel small molecule inhibitor K858. K858 was selected in a phenotype-based forward chemical genetics screen as an antimitotic agent, and subsequently characterized as an inhibitor of Eg5. K858 blocked centrosome separation, activated the spindle checkpoint, and induced mitotic arrest in cells accompanied by the formation of monopolar spindles. Long-term continuous treatment of cancer cells with K858 resulted in antiproliferative effects through the induction of mitotic cell death, and polyploidization followed by senescence. In contrast, treatment of nontransformed cells with K858 resulted in mitotic slippage without cell death, and cell cycle arrest in G(1) phase in a tetraploid state. In contrast to paclitaxel, K858 did not induce the formation of micronuclei in either cancer or nontransformed cells, suggesting that K858 has minimal effects on abnormalities in the number and structure of chromosomes. K858 exhibited potent antitumor activity in xenograft models of cancer, and induced the accumulation of mitotic cells with monopolar spindles in tumor tissues. Importantly, K858, unlike antimicrotubule agents, had no effect on microtubule polymerization in cell-free and cell-based assays, and was not neurotoxic in a motor coordination test in mice. Taken together, the Eg5 inhibitor K858 represents an important compound for further investigation as a novel anticancer therapeutic.

Heat shock protein inhibitors, 17-DMAG and KNK437, enhance arsenic trioxide-induced mitotic apoptosis

Arsenic trioxide (ATO) has recently emerged as a promising therapeutic agent in leukemia because of its ability to induce apoptosis. However, there is no sufficient evidence to support its therapeutic use for other types of cancers. In this study, we investigated if, and how, 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG), an antagonist of heat shock protein 90 (HSP90), and KNK437, a HSP synthesis inhibitor, potentiated the cytotoxic effect of ATO. Our results showed that cotreatment with ATO and either 17-DMAG or KNK437 significantly increased ATO-induced cell death and apoptosis. siRNA-mediated attenuation of the expression of the inducible isoform of HSP70 (HSP70i) or HSP90alpha/beta also enhanced ATO-induced apoptosis. In addition, cotreatment with ATO and 17-DMAG or KNK437 significantly increased ATO-induced mitotic arrest and ATO-induced BUBR1 phosphorylation and PDS1 accumulation. Cotreatment also significantly increased the percentage of mitotic cells with abnormal mitotic spindles and promoted metaphase arrest as compared to ATO treatment alone. These results indicated that 17-DMAG or KNK437 may enhance ATO cytotoxicity by potentiating mitotic arrest and mitotic apoptosis possibly through increased activation of the spindle checkpoint.

Effects of Eg5 knockdown on human prostate cancer xenograft growth and chemosensitivity

OBJECTIVES

Microtubular inhibitors, including docetaxel, are active cytotoxics in many cancers, including prostate cancer (CaP). The Eg5 gene, a member of the kinesin-5 family, plays critical roles in proper mitotic spindle function, and is a potential microtubule-related target for proliferating cancer cells. To investigate the functional activities of Eg5 in CaP, we used an antisense oligonucleotide (ASO) targeting Eg5 to assess the potency and anti-cancer activity of Eg5 ASO treatment for androgen-independent CaP cells in vitro and in vivo.

RESULTS

PC3 cells express higher Eg5 protein and mRNA levels compared to LNCaP cells. In both cell lines, Eg5 ASO treatment reduced mRNA and protein levels in a dose-dependent manner and a complete reduction of Eg5 protein levels was observed at 100 nM. Dose-dependent inhibition in cell growth, potent G2/M phase arrest, and increases in apoptotic sub-G1 fraction were also observed using Eg5 ASO. Surprisingly, low dose Eg5 ASO significantly antagonized cytotoxic effects of paclitaxel. In vivo, Eg5 ASO monotherapy significantly reduced both LNCaP and PC-3 tumor growth but combination treatment with paclitaxel did not yield additive benefits.

CONCLUSIONS

These findings suggest that while Eg5 is a potential target to delay androgen-independent CaP growth, combination treatment with paclitaxel may not be desirable.

Cancer cells display profound intra- and interline variation following prolonged exposure to antimitotic drugs

Drugs targeting the mitotic spindle are used extensively during chemotherapy, but surprisingly, little is known about how they kill tumor cells. This is largely because many of the population-based approaches are indirect and lead to vague and confusing interpretations. Here, we use a high-throughput automated time-lapse light microscopy approach to systematically analyze over 10,000 single cells from 15 cell lines in response to three different classes of antimitotic drug. We show that the variation in cell behavior is far greater than previously recognized, with cells within any given line exhibiting multiple fates. We present data supporting a model wherein cell fate is dictated by two competing networks, one involving caspase activation, the other protecting cyclin B1 from degradation.

Cell type variation in responses to antimitotic drugs that target microtubules and kinesin-5

To improve cancer chemotherapy, we need to understand the mechanisms that determine drug sensitivity in cancer and normal cells. Here, we investigate this question across a panel of 11 cell lines at a phenotypic and molecular level for three antimitotic drugs: paclitaxel, nocodazole, and an inhibitor of kinesin-5 (also known as KSP, Eg5, Kif11). Using automated microscopy with markers for mitosis and apoptosis (high content screening), we find that the mitotic arrest response shows relatively little variation between cell types, whereas the tendency to undergo apoptosis shows large variation. We found no correlation between levels of mitotic arrest and apoptosis. Apoptosis depended on entry into mitosis and occurred both from within mitosis and after exit. Response to the three drugs strongly correlated, although paclitaxel caused more apoptosis in some cell lines at similar levels of mitotic arrest. Molecular investigations showed that sensitivity to apoptosis correlated with loss of an antiapoptotic protein, XIAP, during the drug response, but not its preresponse levels, and to some extent also correlated with activation of the p38 and c-Jun NH(2) kinase pathways. We conclude that variation in sensitivity to antimitotic drugs in drug-naive cell lines is governed more by differences in apoptotic signaling than by differences in mitotic spindle or spindle assembly checkpoint proteins and that antimitotics with different mechanisms trigger very similar, but not identical, responses.

Epothilones and new analogues of the microtubule modulators in taxane-resistant disease

BACKGROUND

Microtubule-stabilising agents typified by the epothilone class of drug have demonstrated promising activity in Phase II and III clinical trials.

OBJECTIVE

Data supporting the efficacy of these agents are reviewed and their potential use in taxane-refractory disease assessed.

METHODS

Preclinical evidence assessing the role of the spindle assembly checkpoint in determining the cellular response to microtubule stabilization are presented together with clinical data documenting the efficacy of non-taxane microtubule modulators.

RESULTS/CONCLUSIONS

Evidence suggests that microtubule-stabilising agents prolong activation of the spindle assembly checkpoint which may promote cancer cell death in mitosis or following mitotic exit. A weakened spindle assembly checkpoint is associated with altered sensitivity to agents targeting the microtubule and therefore pathways of drug resistance may be shared by these cytotoxic therapies. Preliminary clinical trial data do suggest modest activity of epothilones in truly taxane-resistant patient cohorts, indicating the potential niche for these agents in a molecularly undefined patient group, potentially implicating the role of P-glycoprotein in the acquisition of taxane-resistant disease. Trial data of these antimitotic agents will be presented together with their potential role in taxane-resistant disease and the implications for future clinical trial design.

Caspase-independent apoptosis is activated by diazepam-induced mitotic failure in HeLa cells, but not in human primary fibroblasts

DZ, a benzodiazepine known to affect centrosome separation at prophase, leads to a higher degree of mitotic arrest in HeLa cells than in primary human fibroblasts. In fact, differently from fibroblasts, which undergo a transient block in prophase-to-prometaphase transition, a high proportion of tumor cells attempt to escape from the DZ-imposed mitotic block, fail to undergo complete mitosis and die by mitotic failure. DZ-treated samples showed certain biochemical hallmarks of apoptosis, such as induction of the proapototic Bax protein, mitochondrial alterations assessed by JC-1 staining and TEM analysis, PARP cleavage, and DNA fragmentation. However, in DZ-treated cells, we observed a very low or absent caspase activation as shown by immunofluorescence and immunoblot experiments with antibodies directed to activated caspases and by staining with the pancaspase inhibitor FITC-VAD-FMK. Experiments on mitochondrial depolymerization and apoptosis induction carried out in the presence of specific inhibitors of caspase-2 and caspase-3/7 indicated a caspase-independent apoptotic process induced by DZ. Accordingly, TEM analysis of treated cells revealed ultrastructural features resembling those reported for caspase-independent apoptosis. In conclusion, we hypothesize that HeLa cells override the prophase block imposed by DZ, producing a high rate of aberrant pro-metaphases, which, in turn, activates caspase-independent, apoptosis-like mitotic catastrophe.

Targeting mitosis for anti-cancer therapy

Basic research that has focused on achieving a mechanistic understanding of mitosis has provided unprecedented molecular and biochemical insights into this highly complex phase of the cell cycle. The discovery process has uncovered an ever-expanding list of novel proteins that orchestrate and coordinate spindle formation and chromosome dynamics during mitosis. That many of these proteins appear to function solely in mitosis makes them ideal targets for the development of mitosis-specific cancer drugs. The clinical successes seen with anti-microtubule drugs such as taxanes and the vinca alkaloids have also encouraged the development of drugs that specifically target mitosis. Drugs that selectively inhibit mitotic kinesins involved in spindle and kinetochore functions, as well as kinases that regulate these activities, are currently in various stages of clinical trials. Our increased understanding of mitosis has also revealed that this process is targeted by inhibitors of farnesyl transferase, histone deacetylase, and Hsp90. Although these drugs were originally designed to block cell proliferation by inhibiting signaling pathways and altering gene expression, it is clear now that these drugs can also directly interfere with the mitotic process. The increased attention to mitosis as a chemotherapeutic target has also raised an important issue regarding the cellular determinants that specify drug sensitivity. One likely contribution is the mitotic checkpoint, a failsafe mechanism that delays mitotic exit so that cells whose chromosomes are not properly attached to the spindle have extra time to correct their errors. As the biochemical activity of the mitotic checkpoint is finite, cells cannot indefinitely sustain the delay, as in cases where cells are treated with anti-mitotic drugs. When the mitotic checkpoint activity is eventually lost, cells will exit mitosis and become aneuploid. While many of the aneuploid cells may die because of massive chromosome imbalance, survivors that continue to proliferate will no doubt be selected. This is clearly an undesirable outcome, thus efforts to obtain fundamental insights into why some cells that arrest in mitosis die without exiting mitosis will be exceedingly important in enhancing our understanding of the drug sensitivity of cancer cells.

Mitotic drug targets and the development of novel anti-mitotic anticancer drugs

Drugs that interfere with the normal progression of mitosis belong to the most successful chemotherapeutic compounds currently used for anti-cancer treatment. Classically, these drugs are represented by microtubule binding drugs that inhibit the function of the mitotic spindle in order to halt the cell cycle in mitosis and to induce apoptosis in tumor cells. However, these compounds act not only on proliferating tumor cells, but exhibit significant side effects on non-proliferating cells including neurons that are highly dependent on intracellular transport processes mediated by microtubules. Therefore, there is a particular interest in developing novel anti-mitotic drugs that target non-microtubule structures. In fact, recently several novel drugs that target mitotic kinesins or the Aurora and polo-like kinases have been developed and are currently tested in clinical trials. In addition, approaches of cell cycle checkpoint abrogation during mitosis and at the G2/M transition inducing mitosis-associated tumor cell death are promising new strategies for anti-cancer therapy. It is expected that this "next generation" of anti-mitotic drugs will be as successful as the classical anti-microtubule drugs, while avoiding some of the adverse side effects.

Combretastatin CA-4 and combretastatin derivative induce mitotic catastrophe dependent on spindle checkpoint and caspase-3 activation in non-small cell lung cancer cells

Combretastatin A-4 (CA-4), a natural stilbenoid isolated from Combretum caffrum, is a new vascular targeting agent (VTA) known for its antitumor activity due to its anti-tubulin properties. We investigated the molecular mechanisms leading to cell death in non-small cell lung cancer H460 cells induced by natural (CA-4) and synthetic stilbenoids (ST2151) structurally related to CA-4. We found that both compounds induced depolymerization and rearrangement of spindle microtubules, as well as an increasingly aberrant organization of metaphase chromosomes in a dose- and time-dependent manner. Prolonged exposition to ST2151 led cells to organize multiple sites of tubulin repolymerization, whereas tubulin repolymerization was observed only after CA-4 washout. H460 cells were arrested at a pro-metaphase stage, with condensed chromosomes and a triggered spindle assembly checkpoint, as evaluated by kinetochore localization of Bub1 and Mad1 antibodies. Persistent checkpoint activation led to mitochondrial membrane permeabilization (MMP) alterations, cytochrome c release, activation of caspase-9 and -3, PARP cleavage and DNA fragmentation. On the other hand, caspase-2, and -8 were not activated by the drug treatment. The ability of cells to reassemble tubulin in the presence of an activated checkpoint may be responsible for ST2151-induced multinucleation, a recognized sign of mitotic catastrophe. In conclusion, we believe that discovery of new agents able to trigger mitotic catastrophe cell death as a result of mitotic block and prolonged spindle checkpoint activation is particularly worthwhile, considering that tumor cells have a high proliferative rate and mitotic failure occurs irrespective of p53 status.

Induction of apoptosis by monastrol, an inhibitor of the mitotic kinesin Eg5, is independent of the spindle checkpoint

Spindle poisons such as paclitaxel are widely used as cancer therapeutics. By interfering with microtubule dynamics, paclitaxel induces mitotic arrest and apoptosis. Targeting the kinesin Eg5, which is required for the formation of a bipolar spindle, is a promising therapeutic alternative to drugs that interfere with microtubule dynamics. Recent data suggest that the spindle checkpoint can determine the response of tumor cells to microtubule poisons. The relationship between checkpoint function and Eg5 inhibition, however, has not yet been fully investigated. Here, we used time-lapse video microscopy and biochemical analysis to study the effect of spindle checkpoint abrogation on the response of HeLa cells to monastrol, a selective Eg5 inhibitor. In HeLa cells, monastrol activated the spindle checkpoint, leading to mitotic arrest and apoptosis. Small interfering RNA-mediated depletion of the spindle checkpoint proteins BubR1 or Mad2 significantly shortened drug-induced arrest, causing premature mitotic exit without cell division. Time-lapse microscopy as well as analysis of caspase activation shows that these checkpoint-deficient cells initiate apoptosis after mitotic exit in response to monastrol. Checkpoint-deficient cells treated with paclitaxel, on the other hand, yielded a higher frequency of cells with >4N DNA content and a decreased incidence of apoptotic events, particularly in Mad2-depleted cells. These results indicate that the immediate fate of postmitotic cells is influenced by both the nature of the checkpoint defect and the type of drug used. Furthermore, these results show that inactivation of the kinesin Eg5 can induce apoptosis in tumor cells in the absence of critical spindle checkpoint components.

Bub1 up-regulation and hyperphosphorylation promote malignant transformation in SV40 tag-induced transgenic mouse models

Rodents do not naturally develop prostate cancer. Currently, most widely used genetically engineered mouse prostate cancer models use SV40 T/tag oncogene. To understand the mechanism underlying prostate cancer development in transgenic and knock-in SV40 Tag mouse models, we did cDNA microarray analyses, comparing gene expression profiles of prostate cancer tissues from early-, late-, and advance-stage androgen-independent prostate cancers. Of the 67 genes that were up-regulated by > or = 10-fold, 40 are known to be required for chromosome stability. In particular, the spindle checkpoint component Bub1 was persistently up-regulated from early to advanced androgen-independent prostate cancer lesions. Significantly, Bub1, which is required for accurate chromosome segregation during mitosis, has recently been reported to bind SV40 Tag. Consistent with a spindle checkpoint defect, flow cytometry experiments indicate that advanced androgen-independent prostate cancer tumors exhibit aneuploidy, along with up-regulation of levels of both Bub1 mRNA and Bub1 protein or hyperphosphorylation. Importantly, up-regulation and hyperphosphorylation of Bub1 were also observed in established human prostate cancer cell lines and in clinical studies. Furthermore, analysis of human prostate cancer lines showed impaired spindle checkpoint function and endoreduplication following exposure to spindle toxins. Small interfering RNA-mediated repression of Bub1 in the human prostate cancer line PC-3 restrained cell proliferation, an effect mimicked by inhibition of mitogen-activated protein kinase, an upstream activator of Bub1. Thus, by perturbing Bub1 function, our observations suggest a new mechanism whereby the SV40 Tag oncoprotein promotes chromosomal instability and aneuploidy in transgenic mouse prostate cancer models. Whereas the exact details of this mechanism remain unclear, our novel findings raise the possibility of exploiting Bub1 as a new therapeutic target in the treatment of prostate cancer, the most common cancer in adult men in North America.

In vitro and in vivo characterization of a combination chemotherapy formulation consisting of vinorelbine and phosphatidylserine

The purpose of these studies was to design an intravenous drug formulation consisting of two active agents having synergistic in vitro activity. Specifically, we describe a novel drug combination consisting of a cytotoxic agent (vinorelbine) with an apoptosis-inducing lipid (phosphatidylserine, PS). In vitro cytotoxicity screening of PS and vinorelbine, alone and in combination, against human MDA435/LCC6 breast cancer and H460 lung cancer cells was used to identify the molar ratio of these two agents required for synergistic activity. PS and vinorelbine were co-formulated in a lipid-based system at the synergistic molar ratio and the pharmacokinetic and antitumor characteristics of the combination assessed in mice bearing H460 tumors. The cytotoxicity of the lipid, and the synergy between the lipid and vinorelbine, were specific to PS; these effects were not observed using control lipids. A novel formulation of PS, incorporated as a membrane component in liposomes, and encapsulating vinorelbine using a pH gradient based loading method was developed. The PS to vinorelbine ratio in this formulation was 1/1, a ratio that produced synergistic in vitro cytotoxicity over a broad concentration range. The vinorelbine and PS dual-agent treatment significantly delayed the growth of subcutaneous human H460 xenograft tumors in Rag2M mice compared to the same dose of free vinorelbine given alone or given as a cocktail of the free vinorelbine simultaneously with empty PS-containing liposomes. These studies demonstrate the potential to develop clinically relevant drug combinations identified using in vitro drug-drug interactions combined with lipid-based delivery systems to co-formulate drugs at their synergistic ratios.

Cytotoxicity of Docetaxel (Taxotere®) Used as a Single Agent and in Combination with Radiation in Human Gastric, Cervical and Pancreatic Cancer Cells

BACKGROUND

Docetaxel (Taxotere) has gained increasing attention in clinical applications. We investigated the cytotoxic and radiosensitizing potential of docetaxel at nanomolar concentrations in six cell lines derived from tumors that rarely respond to radiation or chemotherapy, with special consideration of mechanisms of resistance, including the p53 mutational status.

METHODS

Cells derived from carcinomas of the human stomach (p53 mutant Hs746T, p53 wild type AGS), cervix (p53 wild type CaSki, p53 mutant HeLa) or pancreas (p53 mutant BxPC3 and Capan-1) were treated for 24 h with docetaxel at various concentrations (0.1-5 nM) to obtain drug doses for inhibiting clonogenicity by approximately 50% (IC(50)). Cells were X-irradiated without docetaxel or after 24 h of docetaxel treatment at IC(50). Radiation doses ranged from 0 up to 10 Gy. Mitotic index, multinucleation, apoptosis and necrosis after 24 h of drug exposure at 1 nM were quantified in representative gastric and cervical cell lines by fluorescence microscopy.

RESULTS

Docetaxel treatment for 24 h resulted in a dose-dependent loss of clonogenicity, with 1.0 or 0.3 nM producing approximately 50% survival of gastric or cervix and pancreatic cells, respectively. After correction for the drug toxicity, the combination of isoeffective concentrations of docetaxel with graded X-ray doses resulted either in a moderate synergy or additivity. The dose reduction factors at the 50 and 20% survival levels were statistically greater than those for Hs746T or AGS cells. For CaSki, HeLa, BxPC3 or Capan-1 cells, the dose reduction factors were statistically not different from unity.

CONCLUSION

Docetaxel was active against tumor cells of different origins. Combined effects of docetaxel and radiation were at least additive and depended on the intrinsic sensitivity to drug alone. There was no significant evidence of drug-induced mitotic arrest. Compared to drug-resistant gastric cells, exposure to the drug alone of drug-sensitive cervical cells resulted in more severe multinucleation. The p53 status did not contribute directly to the effect of drug alone or in combination with radiation.

Stable suppression of tumorigenicity by Pin1-targeted RNA interference in prostate cancer

PURPOSE

The peptidyl-prolyl isomrase Pin1 plays a catalytic role in oncogenesis in solid cancers, including prostate cancer. In the present study, we sought to determine the potential of Pin1-targeted gene silencing in inhibiting cellular growth and tumorigenicity in prostate cancer.

EXPERIMENTAL DESIGN

A retrovirus-mediated RNA interference targeting Pin1 was expressed in PC3 and LNCaP cells, and cell growth and several transformed properties were investigated.

RESULTS

The stable expression of Pin1-specific small interfering RNA constructs in PC3 and LNCaP cells significantly reduced cellular proliferation, colony formation, migration, and invasion but strongly enhanced the apoptotic response induced by serum depletion or treatment with anticancer agents. Furthermore, Pin1 depletion significantly suppressed tumorigenic potential in athymic mice, resulting in the inhibition of both tumor growth and angiogeneisis.

CONCLUSIONS

These results strongly suggest that Pin1 plays an important role not only in tumorigenesis but also in the maintenance of the transformed phenotype in prostate cancer cells. Hence, Pin1 may serve as a promising therapeutic target, particularly for recurrent prostate tumors.

Mad2β, an alternative variant of Mad2 reducing mitotic arrest and apoptosis induced by adriamycin in gastric cancer cells

Mad2beta is an alternative splicing variant of spindle checkpoint gene mad2, which was previously found by us and was related to the drug resistance in gastric cancer cells. In this paper, we explored the molecular mechanisms that Mad2beta variant promoted the formation of multidrug resistance in gastric cancer cells. We found that Mad2beta variant was detected only in the two human drug resistant gastric cancer cell sublines SGC7901/VCR and SGC7901/ADR, and it did not appear in its parental cell line SGC7901 and other detected gastric cancer cell lines. Expressions of Mad2 mRNA and protein in SGC7901 cells transfected with Mad2beta, SGC7901/VCR and SGC7901/ADR were significantly lower than that in SGC7901 cells. Moreover, SGC7901 cells overexpressing Mad2beta variant became more resistant to adriamycin, vincristine and mitomycin by abrogating mitotic arrest and apoptosis. This suggests that expression of Mad2beta variant decreases the relative expression of efficient MAD2, which may help gastric cancer cells to develop the phenotype of multidrug resistance.

Apoptosis of non-small-cell lung cancer cell lines after paclitaxel treatment involves the BH3-only proapoptotic protein Bim

A significant variation in susceptibility to paclitaxel-mediated killing was observed among a panel of short-term cultured non-small-cell lung cancer (NSCLC) cell lines. Susceptibility to killing by paclitaxel correlated with expression of the BH3-only protein, Bim, but not with other members of Bcl-2 family. NSCLC cell lines with the highest level of Bim expression are most susceptible to apoptosis induction after paclitaxel treatment. Forced expression of Bim increased paclitaxel-mediated killing of cells expressing an undetectable level of Bim. Conversely, knock down of Bim, but not Bcl-2 expression, decreased the susceptibility of tumor cells to paclitaxel-mediated killing. Similar observations were made using a panel of breast and prostate cancer cell lines. Paclitaxel impairs microtubule function, causes G2/M cell cycle blockade, mitochondria damage, and p53-independent apoptosis. These results established Bim as a critical molecular link between the microtubule poison, paclitaxel, and apoptosis.

Partial downregulation of MAD1 causes spindle checkpoint inactivation and aneuploidy, but does not confer resistance towards taxol

The mitotic spindle assembly checkpoint ensures proper chromosome segregation during mitosis by inhibiting the onset of anaphase until all kinetochores are attached to the mitotic spindle and tension across the kinetochores is generated. Here, we report that the stable partial downregulation of the spindle checkpoint gene MAD1, which is observed in human cancer, leads to a functional inactivation of the spindle checkpoint resulting in gross aneuploidy. Interestingly, although Mad1 is thought to act as a kinetochore based activator of Mad2 during checkpoint activation, we show that normal levels of Mad2, but not of Mad1, are required for preventing premature sister chromatid separation and for maintaining the timing of an undisturbed mitosis, suggesting a Mad1 independent function of Mad2 that operates independent of its checkpoint function. Most significantly, a partial repression of either MAD1 or MAD2 confers resistance to nocodazole, a drug that inhibits microtubule attachment. In contrast, sensitivity to clinically relevant drugs like taxol or monastrol that inhibit the generation of tension across kinetochores is not modulated by partial downregulation of MAD1, suggesting a functional bifurcation of spindle checkpoint dependent apoptotic pathways.

Disruption of nuclear maturation and rearrangement of cytoskeletal elements in bovine oocytes exposed to heat shock during maturation

Meiotic maturation in mammalian oocytes is a complex process which involves extensive rearrangement of microtubules, actin filaments and chromosomes. Since cytoskeletal elements are sensitive to disruption by heat shock, a series of experiments were performed to determine whether physiologically relevant heat shock disrupts the progression of the oocyte through meiosis, fertilization and zygote formation. Cumulus–oocyte complexes were cultured at 38.5, 40.0 or 41.0 °C for the first 12 h of maturation. Incubation during the last 10 h of maturation and 18 h after fertilization was at 38.5 °C and in 5% (v/v) CO2for both treatments. Examination of the cytoskeleton and the chromosome organization in matured oocytes revealed that oocytes matured at 38.5°C were mostly at metaphase II (MII) stage, while the majority of heat-shocked oocytes were blocked at the first metaphase (MI), first anaphase or first telophase stages. A subset of heat-shocked oocytes possessed misshapen MI spindles with disorganized microtubules and unaligned chromosomes. A higher percentage of TUNEL-positive oocytes was noted for oocytes matured at 41.0 °C. Addition of 50 nmol/l sphingosine 1-phosphate to maturation medium blocked the effect of heat shock on progression through meiosis and apoptosis and increased the proportion of oocytes matured at 41.0 °C that were at MII. Following insemination, a high percentage of heat-shocked oocytes were unfertilized, while the majority of the control zygotes were fertilized and had two visible pronuclei. In conclusion, heat shock disrupts nuclear maturation and induces apoptosis. These alterations are likely to be involved in the mechanism underlying heat-shock-induced disruption of oocyte capacity for fertilization and subsequent development.

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.

Stuck in division or passing through: what happens when cells cannot satisfy the spindle assembly checkpoint

Cells that cannot satisfy the spindle assembly checkpoint (SAC) are delayed in mitosis (D-mitosis), a fact that has useful clinical ramifications. However, this delay is seldom permanent, and in the presence of an active SAC most cells ultimately escape mitosis and enter the next G1 as tetraploid cells. This review defines and discusses the various factors that determine how long a cell remains in mitosis when it cannot satisfy the SAC and also discusses the cell's subsequent fate.

The tubulin-binding agent combretastatin A-4-phosphate arrests endothelial cells in mitosis and induces mitotic cell death

The tubulin-binding agent combretastatin A-4-phosphate (CA-4-P), rapidly disrupts the vascular network of tumors leading to secondary tumor cell death. In vitro, CA-4-P destabilizes microtubules and causes endothelial cell death. In this study we analyze the mechanisms by which CA-4-P induces the death of proliferating endothelial cells. We demonstrate that at >/=7.5 nmol/L, CA-4-P damages mitotic spindles, arrests cells at metaphase, and leads to the death of mitotic cells with characteristic G(2)/M DNA content. Mitotic arrest was associated with elevated levels of cyclin B1 protein and p34(cdc2) activity. Inhibition of p34(cdc2) activity by purvalanol A caused mitotic-arrested cells to rapidly exit mitosis, suggesting that sustained p34(cdc2) activity was responsible for metaphase arrest. Pharmacological prevention of entry into mitosis protected cells from undergoing cell death, further establishing the link between mitosis and cell death induction by CA-4-P. CA-4-P-mediated cell death shared characteristics of apoptosis but was independent of caspase activation suggesting the involvement of a non-caspase pathway(s). These data suggest that induction of apoptosis in endothelial cells by CA-4-P is associated with prolonged mitotic arrest. Therefore, by activating cell death pathways, CA-4-P, in addition to being an effective anti-vascular agent, may also interfere with regrowth of blood vessels in the tumor.

Spindle checkpoint function is required for mitotic catastrophe induced by DNA-damaging agents

Mitotic catastrophe is an important mechanism for the induction of cell death in cancer cells by antineoplastic agents that damage DNA. This process is facilitated by defects in the G1 and G2 checkpoints of the cell cycle that are apparent in most cancer cells and which allow the cells to enter mitosis with DNA damage. We have now characterized the dynamics of mitotic catastrophe induced by DNA-damaging agents in p53-deficient cancer cells. Cells that entered mitosis with DNA damage transiently arrested at metaphase for more than 10 h without segregation of chromosomes and subsequently died directly from metaphase. In those metaphase arrested precatastrophic cells, anaphase-promoting complex appeared to be inactivated and BubR1 was persistently localized at kinetochores, suggesting that spindle checkpoint is activated after the DNA damage. Furthermore, suppression of spindle checkpoint function by BubR1 or Mad2 RNA interference in the DNA damaged cells led to escape from catastrophic death and to subsequent abnormal mitosis. Dysfunction of the spindle checkpoint in p53-deficient cancer cells is thus likely a critical factor in resistance to DNA-damaging therapeutic agents.

Identification of Decatenation G2 Checkpoint Impairment Independently of DNA Damage G2 Checkpoint in Human Lung Cancer Cell Lines

It has been suggested that attenuation of the decatenation G(2) checkpoint function, which ensures sufficient chromatid decatenation by topoisomerase II before entering into mitosis, may contribute to the acquisition of genetic instability in cancer cells. To date, however, very little information is available on this type of checkpoint defect in human cancers. In this study, we report for the first time that a proportion of human lung cancer cell lines did not properly arrest before entering mitosis in the presence of a catalytic, circular cramp-forming topoisomerase II inhibitor ICRF-193, whereas the decatenation G(2) checkpoint impairment was present independently of the impaired DNA damage G(2) checkpoint. In addition, the presence of decatenation G(2) checkpoint dysfunction was found to be associated with diminished activation of ataxia-telangiectasia mutated in response to ICRF-193, suggesting the potential involvement of an upstream pathway sensing incompletely catenated chromatids. Interestingly, hypersensitivity to ICRF-193 was observed in cell lines with decatenation G(2) checkpoint impairment and negligible activation of ataxia-telangiectasia mutated. These findings suggest the possible involvement of decatenation G(2) checkpoint impairment in the development of human lung cancers, as well as the potential clinical implication of selective killing of lung cancer cells with such defects by this type of topoisomerase II inhibitor.

Inactivation of the mitotic checkpoint as a determinant of the efficacy of microtubule-targeted drugs in killing human cancer cells

Drugs that disrupt microtubule dynamics include some of the most important of cancer chemotherapies. While these drugs, which include paclitaxel (Taxol), are known to invoke the mitotic checkpoint, the factors that determine cancer cell killing remain incompletely characterized. Cells that are relatively resistant to killing by these drugs block robustly in mitosis, whereas cells sensitive to killing block only transiently in mitosis before undergoing nuclear fragmentation and death. Passage through mitosis was an absolute requirement of drug-induced death, because death was markedly reduced in cells blocked at both G1-S and G2. Cell killing was at least in part linked to the absence or inactivation of BubR1, a kinetochore-associated phosphoprotein that mediates the mitotic checkpoint. Sensitivity to paclitaxel correlated with decreased BubR1 protein expression in human cancer cell lines, including those derived from breast and ovarian cancers. Silencing of BubR1 via RNA interference inactivated the mitotic checkpoint in drug-resistant cells, and reversed resistance to paclitaxel and nocodazole. Together, these results suggest that the mitotic checkpoint is an important determinant of the efficacy of microtubule-targeting drugs in killing cancer cells, potentially providing novel targets for increasing treatment efficacy.

Paclitaxel and vinorelbine cause synergistic increases in apoptosis but not in microtubular disruption in human lung adenocarcinoma cells (A-549)

Concurrent administration of paclitaxel and vinorelbine results in cytotoxicity in vivo and in vitro in a number of tumor cell lines, yet the mechanisms of enhanced cell killing are undefined. In studies here, we show that low concentrations (1 nM) of paclitaxel and vinorelbine in combination result in enhanced cell killing by apoptosis ( P<0.05) in the human lung adenocarcinoma cell line, A-549. In contrast, necrotic cell death and formation of multinucleated cells, which were significantly increased by paclitaxel ( P<0.05) alone, but not vinorelbine, were not increased synergistically by both drugs. Paclitaxel also caused microtubular disruption which was not observed with vinorelbine. These data provide further rationale for the combined use of paclitaxel and vinorelbine in clinical trials, and suggest that the cooperative effects of drugs on apoptosis are not mediated through similar disruptional effects on microtubules.