Human mutations that confer paclitaxel resistance

MicroRNAs: key players of taxane resistance and their therapeutic potential in human cancers

The successful long-term use of taxane for cancer therapy is often prevented by the development of drug resistance in clinic. Thus, exploring the mechanisms involved is a first step towards rational strategies to overcome taxane resistance. Taxane resistance-related microRNA (miRNAs) are under investigation and miRNAs could induce the taxane resistance of tumour cells by regulating cell cycle distribution, survival and/or apoptosis pathways, drug transports, epithelial–mesenchymal transition and cancer stem cell. This article summarizes current research involving miRNAs as regulators of key target genes for tanxanxe chemoresistance and discusses the complex regulatory networks of miRNAs. Also, the authors will envisage future developments towards the potential use of targeting miRNAs as a novel strategy for improving response of tumour patients to taxane. miRNAs play critical roles in taxane chemoresistance and the miRNA-based therapies will be helpful for overcoming drug resistance and developing more effective personalized anti-cancer treatment strategies. Further research studies should be performed to promote therapeutic–clinical use of taxane resistance-related miRNAs in cancer patients, especially in those patients with taxane-resistant cancers.

Knockdown of cyclophilin A reverses paclitaxel resistance in human endometrial cancer cells via suppression of MAPK kinase pathways

PURPOSE

Paclitaxel resistance remains to be a major obstacle to the chemotherapy of endometrial cancer. Using proteomic-based approach, we used to identify cyclophilin A (CypA) as a potential therapeutic target for endometrial cancer. As a natural continuation, this study aimed to reveal the correlation between CypA and paclitaxel resistance and evaluate the possibility of CypA as a therapeutic target for reversal of resistance.

METHODS

Two paclitaxel-resistant endometrial cancer cell sublines HEC-1-B/TAX and AN3CA/TAX were generated, and expressions of CypA, P-gp, MRP-2 and survivin were demonstrated by Western blotting. CypA was knocked down by RNA interference, and the subsequent effects on the alteration of paclitaxel resistance were examined by MTT, flow cytometry and migratory/invasive transwell assays. MAPK kinases activities were examined by Western blotting.

RESULTS

CypA knockdown led to significant inhibition of cell proliferation, induction of apoptosis and suppression of migratory/invasive capacity in HEC-1-B/TAX and AN3CA/TAX cells when exposed to paclitaxel. CypA knockdown led to reductions in total and phosphorylated MAPK kinases, including Akt, ERK1/2, p38 MAPK and JNK, in HEC-1-B/TAX cells. Furthermore, pretreatment with MAPK kinase inhibitors exhibited a synergistic effect in combination with CypA knockdown.

CONCLUSIONS

These results demonstrated that CypA expression was up-regulated in paclitaxel-resistant cancer cells, and knockdown of CypA could reverse the paclitaxel resistance through, at least partly, suppression of MAPK kinase pathways, presenting a possibility of CypA serving as a therapeutic target to overcome paclitaxel resistance.

Sensitivity and mechanisms of taxol-resistant prostate adenocarcinoma cells to Vernonia amygdalina extract

Prostate cancer (PC) patients once Paclitaxel (TAX) treatment responsive later develop hormone refractory PC, thus becoming TAX-insensitive. This underscores the urgent need to develop novel anti-PC therapies. Vernonia amygdalina (VA) could be one such candidate agent. We have shown that androgen-independent PC-3 cells are sensitive to VA treatment in vitro. VA extract (0.01, 0.1 and 1 mg/ml) inhibited DNA synthesis by 12%, 45% (p<0.05), and 73% (p<0.01) respectively. In contrast, TAX (0.01, 0.1, and 1 μM) failed to significantly affect cell growth, suggesting TAX resistance. We tested molecular mechanisms which may lend to the observed PC-3 cell VA sensitivity/TAX resistance. Though both VA and TAX stimulated MAPK activity, VA's induction was more intense, but transient, compared to TAX's sustained action. NF-κB activation was inhibited on average by 50% by either 1 mg/ml VA or 1 μM TAX. VA extract caused 35% and 45% increases in c-Myc activity at 10 and 60 min intervals respectively, with the highest stimulation attained 1h after treatment. In contrast, similar levels were attained by TAX rapidly (within 5 min) and were sustained compared to the slow/multi-phasic action of VA. VA extract treatments had no effect on AKT gene expression, while TAX treatments yielded a four-fold (P<0.01) increase; and P-glycoprotein (P-gp) activity was inhibited by VA and stimulated by TAX, compared to control (basal ATPase activity). This study shows that TAX-resistant PC-3 cells are sensitive to VA, perhaps explained by differential regulatory patterns of MAPK, c-Myc, AKT, and Pgp activities/expressions.

Role of tumor necrosis factor alpha-induced protein 1 in paclitaxel resistance

Paclitaxel has been extensively used as an antitumor drug to treat a broad range of epithelial cancers, including breast and cervical cancers. However, the efficacy of this drug is greatly limited by the development of acquired resistance. Identification of the underlying resistance mechanisms may inform the development of new therapies that elicit long-term response of tumors to paclitaxel treatment. Here we report that increased expression of TNFAIP1 (tumor necrosis factor alpha-induced protein 1) confers acquired resistance to paclitaxel. TNFAIP1 is shown to compete with paclitaxel for binding to β-tubulin, thereby preventing paclitaxel-induced tubulin polymerization, cell cycle arrest and ultimate cell death. We also show that expression of TNFAIP1 is regulated by the transcriptional factor Sp1. In a xenograft mouse model, increased expression of TNFAIP1 decreases, whereas knockdown of TNFAIP1 increases tumor response to paclitaxel. Therefore, these results reveal tnfaip1 as a novel paclitaxel-resistance associated gene and suggest that TNFAIP1 may represent a valuable therapeutic target for the treatment of cancer.

Identification of interconnected markers for T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a complex disease, resulting from proliferation of differentially arrested immature T cells. The molecular mechanisms and the genes involved in the generation of T-ALL remain largely undefined. In this study, we propose a set of genes to differentiate individuals with T-ALL from the nonleukemia/healthy ones and genes that are not differential themselves but interconnected with highly differentially expressed ones. We provide new suggestions for pathways involved in the cause of T-ALL and show that network-based classification techniques produce fewer genes with more meaningful and successful results than expression-based approaches. We have identified 19 significant subnetworks, containing 102 genes. The classification/prediction accuracies of subnetworks are considerably high, as high as 98%. Subnetworks contain 6 nondifferentially expressed genes, which could potentially participate in pathogenesis of T-ALL. Although these genes are not differential, they may serve as biomarkers if their loss/gain of function contributes to generation of T-ALL via SNPs. We conclude that transcription factors, zinc-ion-binding proteins, and tyrosine kinases are the important protein families to trigger T-ALL. These potential disease-causing genes in our subnetworks may serve as biomarkers, alternative to the traditional ones used for the diagnosis of T-ALL, and help understand the pathogenesis of the disease.

Impact of thymidylate synthase protein expression on efficacy of chemotherapy in advanced lung cancer patients

Advanced lung cancer is primarily treated with platinum combination chemotherapy, however, its prognosis remains poor. The aim of this study was to examine the correlation between the expression of thymidylate synthase (TS) in cancer tissues and the efficacy of chemotherapy in patients with advanced lung cancer in order to clarify the role of TS in the overall response. In total, 120 patients diagnosed with lung cancer between June 2004 and December 2010 at Nihon University Hospital, Tokyo, Japan, were included in this study. Cancer tissue specimens were obtained from the included patients by surgery or bronchofiberscopy prior to treatment. The expression of TS protein was evaluated using specimens immunostained with anti-TS antibody and H-scoring. TS protein expression tended to be higher in smokers compared with non-smokers. Overall survival (OS) (median value) was significantly prolonged in the low TS expression group compared with the high TS expression group. More favorable therapeutic effects were observed in the high TS expression group compared with the low TS expression group, when carboplatin + paclitaxel combined chemotherapy (CbPac therapy) was used. When the therapeutic effects were compared between CbPac therapy and carboplatin + pemetrexed combined chemotherapy (CbPem therapy) in the high TS expression group, prolongation of OS (median value) was observed with CbPac therapy. The present study suggests that TS protein expression is a critical factor in determining the efficacy of CbPac therapy in lung cancer. CbPac therapy is more effective when TS protein is highly expressed in lung cancer tissue.

Random mutagenesis of β-tubulin defines a set of dispersed mutations that confer paclitaxel resistance

PURPOSE

Previous research showed that mutations in β1-tubulin are frequently involved in paclitaxel resistance but the question of whether the mutations are restricted by cell-type specific differences remains obscure.

METHODS

To circumvent cellular constraints, we randomly mutagenized β-tubulin cDNA, transfected it into CHO cells, and selected for paclitaxel resistance.

RESULTS

A total of 26 β1-tubulin mutations scattered throughout the sequence were identified and a randomly chosen subset were confirmed to confer paclitaxel resistance using site-directed mutagenesis of β-tubulin cDNA and transfection into wild-type cells. Immunofluorescence microscopy and biochemical fractionation studies indicated that cells expressing mutant tubulin had decreased microtubule polymer and frequently suffered mitotic defects that led to the formation of large multinucleated cells, suggesting a resistance mechanism that involves destabilization of the microtubule network. Consistent with this conclusion, the mutations were predominantly located in regions that are likely to be involved in lateral or longitudinal subunit interactions. Notably, fourteen of the new mutations overlapped previously reported mutations in drug resistant cells or in patients with developmental brain abnormalities.

CONCLUSIONS

A random mutagenesis approach allowed isolation of a wider array of drug resistance mutations and demonstrated that similar mutations can cause paclitaxel resistance and human neuronal abnormalities.

βII-Tubulin and βIII-Tubulin Mediate Sensitivity to Peloruside A and Laulimalide, but not Paclitaxel or Vinblastine, in Human Ovarian Carcinoma Cells

Increased abundance of βII- and βIII-tubulin isotypes in cancer cells confers resistance to vinca and taxoid site drugs; however, the role of these isotypes in the acquired resistance of cancer cells to non-vinca or non-taxoid site binding agents has not been described. Peloruside A (PLA) and laulimalide are the only known non-taxoid site microtubule-stabilizing agents. A human ovarian cancer cell line, 1A9-L4 (L4), previously selected in high concentrations of laulimalide, has both a single point mutation in βI-tubulin and overexpression of βII- and βIII-tubulin. The cells are highly resistant to PLA as well as laulimalide but show no cross-resistance to taxoid site drugs or drugs that bind to the vinca site on β-tubulin. To understand the functional significance of the βII- and βIII-tubulin changes in this resistant cell line, isotype-specific short interfering RNA was used to knock down the expression of the βII and βIII isotypes, and the cellular effects of PLA and laulimalide were examined before and after silencing. It was found that inhibition of βII- and βIII-tubulin partially sensitized L4 cells to PLA and laulimalide, as seen by increased potency of PLA and laulimalide for inducing growth inhibition, cellular tubulin polymerization, microtubule aberrations, and G2-M arrest in the resistant cells. The sensitivity to paclitaxel, vinblastine, ixabepilone, and cisplatin was unaffected by the inhibition of isotype expression. It was concluded that the increased βII- and βIII-tubulin contributed significantly to the resistance phenotype, along with the tubulin structural mutation, and that the altered isotype effect was binding site specific. Mol Cancer Ther; 11(2); 393–404. ©2011 AACR.

Structural thermal adaptation of β-tubulins from the Antarctic psychrophilic protozoan Euplotes focardii

Tubulin dimers of psychrophilic eukaryotes can polymerize into microtubules at 4°C, a temperature at which microtubules from mesophiles disassemble. This unique capability requires changes in the primary structure and/or in post-translational modifications of the tubulin subunits. To contribute to the understanding of mechanisms responsible for microtubule cold stability, here we present a computational structural analysis based on molecular dynamics (MD) and experimental data of three β-tubulin isotypes, named EFBT2, EFBT3, and EFBT4, from the Antarctic protozoon Euplotes focardii that optimal temperature for growth and reproduction is 4°C. In comparison to the β-tubulin from E. crassus, a mesophilic Euplotes species, EFBT2, EFBT3, and EFBT4 possess unique amino acid substitutions that confer different flexible properties of the polypeptide, as well as an increased hydrophobicity of the regions involved in microtubule interdimeric contacts that may overcome the microtubule destabilizing effect of cold temperatures. The structural analysis based on MD indicated that all isotypes display different flexibility properties in the regions involved in the formation of longitudinal and lateral contacts during microtubule polymerization. We also investigated the role of E. focardii β-tubulin isotypes during the process of cilia formation. The unique characteristics of the primary and tertiary structures of psychrophilic β-tubulin isotypes seem responsible for the formation of microtubules with distinct dynamic and functional properties.

Mutations in the β-tubulin binding site for peloruside A confer resistance by targeting a cleft significant in side chain binding

Peloruside A is a microtubule-stabilizing macrolide that binds to beta tubulin at a site distinct from the taxol site. The site was previously identified by H-D exchange mapping and molecular docking as a region close to the outer surface of the microtubule and confined in a cavity surrounded by a continuous loop of protein folded so as to center on Y340. We have isolated a series of peloruside A-resistant lines of the human ovarian carcinoma cell line A2780(1A9) to better characterize this binding site and the consequences of altering residues in it. Four resistant lines (Pel A-D) are described with single-base mutations in class I β-tubulin that result in the following substitutions: R306H, Y340S, N337D, and A296S in various combinations. The mutations are localized to peptides previously identified by Hydrogen-Deuterium exchange mapping, and center on a cleft in which the drug side chain appears to dock. The Pel lines are 10-15-fold resistant to peloruside A and show cross resistance to laulimalide but not to any other microtubule stabilizers. They show no cross-sensitivity to any microtubule destabilizers, nor to two drugs with targets unrelated to microtubules. Peloruside A induces G2/M arrest in the Pel cell lines at concentrations 10-15 times that required in the parental line. The cells show notable changes in morphology compared to the parental line.

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.

New insights into mechanisms of resistance to microtubule inhibitors

Mechanisms to explain tumor cell resistance to drugs that target the microtubule cytoskeleton have relied on the assumption that the drugs act either to suppress microtubule dynamics or to perturb the balance between assembled and nonassembled tubulin. Recently, however, it was found that these drugs also alter the stability of microtubule attachment to centrosomes, and do so at the same concentrations that are needed to inhibit cell division. Based on this new information, a new model is presented that explains resistance resulting from a variety of molecular changes that have been reported in the literature. The improved understanding of drug action and resistance has important implications for chemotherapy with these agents.

Peloruside- and laulimalide-resistant human ovarian carcinoma cells have βI-tubulin mutations and altered expression of βII- and βIII-tubulin isotypes

Peloruside A and laulimalide are potent microtubule-stabilizing natural products with a mechanism of action similar to that of paclitaxel. However, the binding site of peloruside A and laulimalide on tubulin remains poorly understood. Drug resistance in anticancer treatment is a serious problem. We developed peloruside A- and laulimalide-resistant cell lines by selecting 1A9 human ovarian carcinoma cells that were able to grow in the presence of one of these agents. The 1A9-laulimalide resistant cells (L4) were 39-fold resistant to the selecting agent and 39-fold cross-resistant to peloruside A, whereas the 1A9-peloruside A resistant cells (R1) were 6-fold resistant to the selecting agent while they remained sensitive to laulimalide. Neither cell line showed resistance to paclitaxel or other drugs that bind to the taxoid site on β-tubulin nor was there resistance to microtubule-destabilizing drugs. The resistant cells exhibited impaired peloruside A/laulimalide-induced tubulin polymerization and impaired mitotic arrest. Tubulin mutations were found in the βI-tubulin isotype, R306H or R306C for L4 and A296T for R1 cells. This is the first cell-based evidence to support a β-tubulin-binding site for peloruside A and laulimalide. To determine whether the different resistance phenotypes of the cells were attributable to any other tubulin alterations, the β-tubulin isotype composition of the cells was examined. Increased expression of βII- and βIII-tubulin was observed in L4 cells only. These results provide insight into how alterations in tubulin lead to unique resistance profiles for two drugs, peloruside A and laulimalide, that have a similar mode of action.

Overexpression of mitotic centromere-associated Kinesin stimulates microtubule detachment and confers resistance to paclitaxel

Numerous studies have implicated mutations in tubulin or the overexpression of specific tubulin genes in resistance to microtubule-targeted drugs. Much less is known about the role of accessory proteins that modulate microtubule behavior in the genesis of drug resistance. Here, we examine mitotic centromere-associated kinesin (MCAK), a member of the kinesin family of microtubule motor proteins that has the ability to stimulate microtubule depolymerization, and show that overexpressing the protein confers resistance to paclitaxel and epothilone A, but increases sensitivity to colcemid. Cells transfected with FLAG-tagged MCAK cDNA using a tet-off-regulated expression system had a disrupted microtubule cytoskeleton and were able to survive a toxic concentration of paclitaxel in the absence, but not in the presence of tetracycline, showing that drug resistance was caused by ectopic MCAK production. Moreover, a population that was heterogeneous with respect to FLAG-MCAK expression became enriched with cells that produced the ectopic protein when it was placed under paclitaxel selection. Similar to previously isolated mutants with altered tubulin, paclitaxel resistant cells resulting from MCAK overexpression were found to have decreased microtubule polymer and a seven-fold increase in the frequency of microtubule detachment from centrosomes. These data are consistent with a model for paclitaxel resistance that is based on stability of the attachment of microtubules to their nucleating centers, and they implicate MCAK in the mechanism of microtubule detachment.

Overcoming tumor multidrug resistance using drugs able to evade P-glycoprotein or to exploit its expression

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. Cellular overproduction of P-glycoprotein (P-gp), which acts as an efflux pump for various anticancer drugs (e.g. anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and some of the newer antitumor drugs) is one of the more relevant mechanisms underlying MDR. P-gp belongs to the superfamily of ATP-binding cassette transporters and is encoded by the ABCB1 gene. Its overexpression in cancer cells has become a therapeutic target for circumventing MDR. As an alternative to the classical pharmacological strategy of the coadministration of pump inhibitors and cytotoxic substrates of P-gp and to other approaches applied in experimental tumor models (e.g. P-gp-targeting antibodies, ABCB1 gene silencing strategies, and transcriptional modulators) and in the clinical setting (e.g. incapsulation of P-gp substrate anticancer drugs into liposomes or nanoparticles), a more intriguing strategy for circumventing MDR is represented by the development of new anticancer drugs which are not substrates of P-gp (e.g. epothilones, second- and third-generation taxanes and other microtubule modulators, topoisomerase inhibitors). Some of these drugs have already been tested in clinical trials and, in most of cases, show relevant activity in patients previously treated with anticancer agents which are substrates of P-gp. Of these drugs, ixabepilone, an epothilone, was approved in the United States for the treatment of breast cancer patients pretreated with an anthracycline and a taxane. Another innovative approach is the use of molecules whose activity takes advantage of the overexpression of P-gp. The possibility of overcoming MDR using the latter two approaches is reviewed herein. 

Navitoclax enhances the efficacy of taxanes in non-small cell lung cancer models

PURPOSE

To explore the potential of navitoclax in combination with taxane-based chemotherapy in the treatment of non-small cell lung cancer (NSCLC) by defining mechanism of synergy and identifying correlative biomarkers.

EXPERIMENTAL DESIGN

We treated a panel of NSCLC lines with a dose matrix of paclitaxel and navitoclax (formerly ABT-263), an inhibitor of Bcl-2, Bcl-x(L), and Bcl-w (1), and evaluated synergy. We next used time-lapse microscopy to explore mechanism of synergy. Finally, we developed an immunohistochemical assay and assessed prevalence of Bcl-x(L) in NSCLC tumor tissues.

RESULTS

All cell lines exhibit greater than additive response to the combination of navitoclax and a taxane. These results were extended to mouse xenograft tumor models, in which the combination is more efficacious than either single-agent docetaxel or navitoclax. Addition of navitoclax to paclitaxel decreases the time from mitotic entry to cell death and changes cell fate from mitotic slippage to death during mitotic arrest. The relative levels of Bcl-x(L) and Mcl-1 correlate with the extent of synergy, suggesting that cancers with elevated levels of Bcl-x(L) will be relatively resistant to taxane-based therapy but could benefit from the addition of navitoclax to taxane treatment. Finally, a significant percentage of NSCLC patient samples exhibit relatively high Bcl-x(L) levels.

CONCLUSIONS

The addition of navitoclax to taxane-based chemotherapy in NSCLC has the potential to increase efficacy, particularly in patients whose tumors express high levels of Bcl-x(L).

Paclitaxel-dependent cell lines reveal a novel drug activity

We previously described the isolation of Tax 18 and Tax 11-6, two paclitaxel-dependent cell lines that assemble low amounts of microtubule polymer and require the drug for cell division. In the present studies, fluorescence time-lapse microscopy was used to measure microtubule dynamic instability behavior in these cells. The mutations were found to cause small decreases in microtubule growth and shortening, but the changes seemed unable to explain the defects in microtubule polymer levels or cell division. Moreover, paclitaxel further suppressed microtubule dynamics at low drug concentrations that were insufficient to rescue the mutant phenotype. Wild-type (WT) cells treated with similar low drug concentrations also had highly suppressed microtubules, yet experienced no problems with cell division. Thus, the effects of paclitaxel on microtubule dynamics seemed to be unrelated to cell division in both WT and mutant cell lines. The higher drug concentrations needed to rescue the mutant phenotype instead inhibited the formation of unstable microtubule fragments that appeared at high frequency in the drug-dependent, but not WT, cell lines. Live cell imaging revealed that the fragments were generated by microtubule detachment from centrosomes, a process that was reversed by paclitaxel. We conclude that paclitaxel rescues mutant cell division by inhibiting the detachment of microtubule minus ends from centrosomes rather than by altering plus-end microtubule dynamics.

Inhibition of cell migration and cell division correlates with distinct effects of microtubule inhibiting drugs

Drugs that target microtubules are thought to inhibit cell division and cell migration by suppressing dynamic instability, a "search and capture" behavior that allows microtubules to probe their environment. Here, we report that subtoxic drug concentrations are sufficient to inhibit plus-end microtubule dynamic instability and cell migration without affecting cell division or microtubule assembly. The higher drug concentrations needed to inhibit cell division act through a novel mechanism that generates microtubule fragments by stimulating microtubule minus-end detachment from their organizing centers. The frequency of microtubule detachment in untreated cells increases at prophase suggesting that it is a regulated cellular process important for spindle assembly and function. We conclude that drugs produce differential dose-dependent effects at microtubule plus and minus-ends to inhibit different microtubule-mediated functions.

Preclinical evaluation of AMG 900, a novel potent and highly selective pan-aurora kinase inhibitor with activity in taxane-resistant tumor cell lines

In mammalian cells, the aurora kinases (aurora-A, -B, and -C) play essential roles in regulating cell division. The expression of aurora-A and -B is elevated in a variety of human cancers and is associated with high proliferation rates and poor prognosis, making them attractive targets for anticancer therapy. AMG 900 is an orally bioavailable, potent, and highly selective pan-aurora kinase inhibitor that is active in taxane-resistant tumor cell lines. In tumor cells, AMG 900 inhibited autophosphorylation of aurora-A and -B as well as phosphorylation of histone H3 on Ser(10), a proximal substrate of aurora-B. The predominant cellular response of tumor cells to AMG 900 treatment was aborted cell division without a prolonged mitotic arrest, which ultimately resulted in cell death. AMG 900 inhibited the proliferation of 26 tumor cell lines, including cell lines resistant to the antimitotic drug paclitaxel and to other aurora kinase inhibitors (AZD1152, MK-0457, and PHA-739358), at low nanomolar concentrations. Furthermore, AMG 900 was active in an AZD1152-resistant HCT116 variant cell line that harbors an aurora-B mutation (W221L). Oral administration of AMG 900 blocked the phosphorylation of histone H3 in a dose-dependent manner and significantly inhibited the growth of HCT116 tumor xenografts. Importantly, AMG 900 was broadly active in multiple xenograft models, including 3 multidrug-resistant xenograft models, representing 5 tumor types. AMG 900 has entered clinical evaluation in adult patients with advanced cancers and has the potential to treat tumors refractory to anticancer drugs such as the taxanes.

Structure-based virtual screening of novel tubulin inhibitors and their characterization as anti-mitotic agents

Microtubule cytoskeletons are involved in many essential functions throughout the life cycle of cells, including transport of materials into cells, cell movement, and proper progression of cell division. Small compounds that can bind at the colchicine site of tubulin have drawn great attention because these agents can suppress or inhibit microtubule dynamics and tubulin polymerization. To find novel tubulin polymerization inhibitors as anti-mitotic agents, we performed a virtual screening study of the colchicine binding site on tubulin. Novel tubulin inhibitors were identified and characterized by their inhibitory activities on tubulin polymerization in vitro. The structural basis for the interaction of novel inhibitors with tubulin was investigated by molecular modeling, and we have proposed binding models for these hit compounds with tubulin. The proposed docking models were very similar to the binding pattern of colchicine or podophyllotoxin with tubulin. These new hit compound derivatives exerted growth inhibitory effects on the HL60 cell lines tested and exhibited strong cell cycle arrest at G2/M phase. Furthermore, these compounds induced apoptosis after cell cycle arrest. In this study, we show that the validated derivatives of compound 11 could serve as potent lead compounds for designing novel anti-cancer agents that target microtubules.

A chemosensitization screen identifies TP53RK, a kinase that restrains apoptosis after mitotic stress

Taxanes are very effective at causing mitotic arrest; however, there is variability among cancer cells in the apoptotic response to mitotic arrest. The variability in clinical efficacy of taxane-based therapy is likely a reflection of this variability in apoptotic response, thus elucidation of the molecular mechanism of the apoptotic response to mitotic stress could lead to improved clinical strategies. To identify genes whose expression influences the rate and extent of apoptosis after mitotic arrest, we screened a kinase-enriched small interfering RNA library for effects on caspase activation in response to maximally effective doses of paclitaxel, a PLK1 inhibitor, or cisplatin. Small interfering RNA oligonucleotides directed against an atypical protein kinase, TP53RK, caused the greatest increase in caspase-3/7 activation in response to antimitotic agents. Time-lapse microscopy revealed that cells entered mitosis with normal kinetics, but died after entry into mitosis in the presence of paclitaxel more rapidly when TP53RK was depleted. Because expression levels of TP53RK vary in cancers, TP53RK levels could provide a molecular marker to predict response to antimitotic agents. TP53RK inhibition may also sensitize cancers to taxanes.