CHO mutants resistant to colchicine, colcemid or griseofulvin have an altered beta-tubulin

Design, synthesis and biological evaluation of new 2,6-difluorinated phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates as new antimicrotubule agents

We previously discovered a novel family of antimicrotubule agents designated as phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates (PIB-SOs). In this study, we evaluated the effect of the difluorination of the aromatic ring bearing the imidazolidin-2-one moiety (ring A) at positions 3, 5 and 2, 6 on their antiproliferative activity on four cancer cell lines, their ability to disrupt the microtubules and their toxicity toward chick embryos. We thus synthesized, characterized and biologically evaluated 24 new difluorinated PIB-SO derivatives designated as phenyl 3,5-difluoro-4-(2-oxoimidazolidin-1-yl)benzenesulfonates (3,5-PFB-SOs, 4-15) and phenyl 2,6-difluoro-4-(2-oxoimidazolidin-1-yl)benzenesulfonates (2,6-PFB-SOs, 16-27). The concentration of the drug required to inhibit cell growth by 50% (IC50) of 3,5-PFB-SOs is over 1000 nM while most of 2,6-PFB-SOs exhibit IC50 in the nanomolar range (23-900 nM). Furthermore, the most potent 2,6-PFB-SOs 19, 26 and 27 arrest the cell cycle progression in G2/M phase, induce cytoskeleton disruption and impair microtubule polymerization. Docking studies also show that the most potent 2,6-PFB-SOs 19, 21, 24, 26 and 27 have binding affinity toward the colchicine-binding site (C-BS). Moreover, their antiproliferative activity is not affected by antimicrotubule- and multidrug-resistant cell lines. Besides, they exhibit improved in vitro hepatic stability in the mouse, rat and human microsomes compared to their non-fluorinated counterparts. They also showed theoretical pharmacokinetic, physicochemical and drug-like properties suited for further in vivo assays. In addition, they exhibit low to no systemic toxicity toward chick embryos. Finally, our study evidences that PIB-SOs must be fluorinated in specific positions on ring A to maintain both their antiproliferative activity and their biological activity toward microtubules.

The tubulin database: Linking mutations, modifications, ligands and local interactions

Microtubules are polymeric filaments, constructed of α-β tubulin heterodimers that underlie critical subcellular structures in eukaryotic organisms. Four homologous proteins (γ-, δ-, ε- and ζ-tubulin) additionally contribute to specialized microtubule functions. Although there is an immense volume of publicly available data pertaining to tubulins, it is difficult to assimilate all potentially relevant information across diverse organisms, isotypes, and categories of data. We previously assembled an extensive web-based catalogue of published missense mutations to tubulins with >1,500 entries that each document a specific substitution to a discrete tubulin, the species where the mutation was described and the associated phenotype with hyperlinks to the amino acid sequence and citation(s) for research. This report describes a significant update and expansion of our online resource (TubulinDB.bio.uci.edu) to nearly 18,000 entries. It now encompasses a cross-referenced catalog of post-translational modifications (PTMs) to tubulin drawn from public datasets, primary literature, and predictive algorithms. In addition, tubulin protein structures were used to define local interactions with bound ligands (GTP, GDP and diverse microtubule-targeting agents) and amino acids at the intradimer interface, within the microtubule lattice and with associated proteins. To effectively cross-reference these datasets, we established a universal tubulin numbering system to map entries into a common framework that accommodates specific insertions and deletions to tubulins. Indexing and cross-referencing permitted us to discern previously unappreciated patterns. We describe previously unlinked observations of loss of PTM sites in the context of cancer cells and tubulinopathies. Similarly, we expanded the set of clinical substitutions that may compromise MAP or microtubule-motor interactions by collecting tubulin missense mutations that alter amino acids at the interface with dynein and doublecortin. By expanding the database as a curated resource, we hope to relate model organism data to clinical findings of pathogenic tubulin variants. Ultimately, we aim to aid researchers in hypothesis generation and design of studies to dissect tubulin function.

Cytospin-A Regulates Colorectal Cancer Cell Division and Migration by Modulating Stability of Microtubules and Actin Filaments

Simple Summary

In this study, we report the effects of depleting cytospin-A (CYTSA), also known as the sperm antigen with calponin homology and coiled-coil domain (SPECC1L) protein, on the proliferation and migration of colorectal cancer (CRC) cells. Mutations in this protein have been previously linked to different developmental disorders. In our studies, depletion of CYTSA in various CRC cells led to significant decreases in proliferation, increases in cell death, and increased formation of multinucleated cells. Knocking down CYTSA also led to severe inhibition of CRC cell migration and invasion. These effects could be related to a significant decrease in the stability of microtubules and alterations in polymerized actin filaments in CYTSA depleted CRC cells. Our studies, for the first time, provide evidence suggesting that targeting CYTSA may be a novel therapeutic strategy for patients with CRC.

Abstract

Proteins that interact with cytoskeletal elements play important roles in cell division and are potentially important targets for therapy in cancer. Cytospin-A (CYTSA), a protein known to interact with actin and microtubules, has been previously described to be important in various developmental disorders, including oblique facial clefting. We hypothesized that CYTSA plays an important role in colorectal cancer (CRC) cell division. The effects of CYTSA depletion on CRC cell proliferation were analyzed using cell growth assays, microscopic analyses of live and fixed cells, and time-lapse imaging. CYTSA depletion led to inhibition of cell proliferation, significant increases in CRC cell death, and accumulation of doublet cells during and following cell division. Depletion of CYTSA also resulted in strong inhibition of CRC cell migration and invasion. Mechanistically, CYTSA depletion resulted in significant decreases in the stability of microtubules and altered polymerization of actin filaments in CRC cells. Finally, bioinformatic analyses were performed to determine the correlation between CYTSA expression and survival of patients with CRC. Interestingly, a strong correlation between high CYTSA expression and poor survival was observed in the TCGA adenocarcinoma data set but not in an independent data set. Since inhibiting CYTSA significantly reduces CRC cell proliferation, migration, and invasion, targeting CYTSA may be a potential novel therapeutic option for patients with metastatic CRC.

Preparation and biological evaluation of new antimicrotubule agents: Modification of the imidazolidin-2-one moiety of phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates

We prepared and biologically evaluated 32 novel molecules named phenyl 4-(dioxoimidazolidin-1-yl)benzenesulfonates (PID-SOs) and ethyl 2-(3-(4-(phenoxysulfonyl)phenyl)ureido)acetates (EPA-SOs). The antiproliferative activity of PID-SOs and EPA-SOs was assessed on four cancer cell lines (HT-1080, HT-29, M21, and MCF7). The most potent PID-SOs bearing an imidazolidin-2,4-dione group show antiproliferative activity in the nanomolar to low micromolar range (0.066 - 6 µM) while EPA-SOs and PID-SOs bearing an imidazolidin-2,5-dione moiety are mostly not active, exhibiting antiproliferative activity over 100 µM. The most potent PID-SOs (16-18) arrest the cell cycle progression in G2/M phase and interact with the colchicine-binding site leading to the microtubule and cytoskeleton disruption. Moreover, their antiproliferative activity is not impaired in vinblastine-, paclitaxel-, and multidrug-resistant cell lines. Finally, our study confirms that PID-SOs bearing the imidazolidin-2,4-dione moiety are a new family of promising antimitotics.

An internal amino-terminal FLAG-tag octapeptide alters oligomerization of expressed surfactant protein-A

Pulmonary surfactant protein-A (SP-A) is expressed by lung alveolar and bronchiolar epithelial cells and plays a critical role in innate immunity of the lung. Exposure of the lung to various environmental insults alters SP-A homeostasis. To investigate the cellular mechanisms involved in these alterations, we added the FLAG octapeptide (DYKDDDDK) to the carboxy-terminus (SP-A/C-FLAG) or near the amino-terminus (SP-A/N-FLAG) of mouse SP-A (WT-SP-A) to tag specific pools of protein. We hypothesized that addition of FLAG would have negligible effects on SP-A expression, oligomerization and secretion. Analysis of Chinese hamster ovary cells expressing these proteins indicated that tagged SP-A mRNA could be distinguished from WT-SP-A by northern analysis and RT-PCR using sequence-specific oligonucleotides. Tagged SP-A protein could be differentiated from WT-SP-A by western analysis using antibodies specific for the FLAG epitope. Subcellular fractionation and immunocytochemistry indicated the majority of each protein was present in punctuate (presumably endocytic) vesicles, and all forms of SP-A protein were secreted. These results suggest that a FLAG epitope added to the carboxy-terminus or inserted into the amino-terminus of the mature SP-A protein has little effect on its expression and cellular processing. However, disruptions of the amino-terminal end of SP-A prevents proper oligomerization, suggesting that this region of mature SP-A is critical in proper oligomeric assembly and is not useful for studies intended to define mechanisms underlying SP-A homeostasis.

The Roles of β-Tubulin Mutations and Isotype Expression in Acquired Drug Resistance

The antitumor drug paclitaxel stabilizes microtubules and reduces their dynamicity, promoting mitotic arrest and eventually apoptosis. Upon assembly of the α/β-tubulin heterodimer, GTP becomes bound to both the α and β-tubulin monomers. During microtubule assembly, the GTP bound to β-tubulin is hydrolyzed to GDP, eventually reaching steady-state equilibrium between free tubulin dimers and those polymerized into microtubules. Tubulin-binding drugs such as paclitaxel interact with β-tubulin, resulting in the disruption of this equilibrium. In spite of several crystal structures of tubulin, there is little biochemical insight into the mechanism by which anti-tubulin drugs target microtubules and alter their normal behavior. The mechanism of drug action is further complicated, as the description of altered β-tubulin isotype expression and/or mutations in tubulin genes may lead to drug resistance as has been described in the literature. Because of the relationship between β-tubulin isotype expression and mutations within β-tubulin, both leading to resistance, we examined the properties of altered residues within the taxane, colchicine and Vinca binding sites. The amount of data now available, allows us to investigate common patterns that lead to microtubule disruption and may provide a guide to the rational design of novel compounds that can inhibit microtubule dynamics for specific tubulin isotypes or, indeed resistant cell lines. Because of the vast amount of data published to date, we will only provide a broad overview of the mutational results and how these correlate with differences between tubulin isotypes. We also note that clinical studies describe a number of predictive factors for the response to anti-tubulin drugs and attempt to develop an understanding of the features within tubulin that may help explain how they may affect both microtubule assembly and stability.

Enhanced stability of microtubules contributes in the development of colchicine resistance in MCF-7 cells

Understanding the mechanism of resistance to tubulin-targeted anticancer drugs is important for improved chemotherapy. In this work, a colchicine-resistant MCF-7 cell line (MCF-7_Col30) was generated by the gradual increment of colchicine treatment and the MCF-7_Col30 showed ∼8-fold resistance towards colchicine. MCF-7_Col30 cells showed ∼2.5-fold resistance against microtubule depolymerizing agents, vinblastine, and nocodazole. In contrast, it displayed more sensitivity towards paclitaxel, a microtubule-polymerizing agent. MCF-7 and MCF-7_Col30 cells showed similar sensitivity towards cisplatin. Further, the level of P-glycoprotein did not increase in MCF-7_Col30 cells. MCF-7_Col30 cells resisted the microtubule depolymerizing effects of colchicine. The time-lapse imaging of individual microtubules in live cells showed that the dynamics of microtubules in MCF-7_Col30 cells was suppressed as compared to the parent MCF-7 cells. The levels of tubulin acetylation and glutamylation increased in MCF-7_Col30 cells than the parent MCF-7 cells suggesting that microtubules are stabilized in MCF-7_Col30 cells. Interestingly, the level of βIII tubulin was increased by 2.3 folds whereas that of βII and βIV tubulin was decreased by 55 and 150%, respectively in MCF-7_Col30 cells. The results suggested that the changes in the level of β-tubulin isoforms and the post-translational modifications of microtubules altered the stability and dynamics of microtubules and contributed to the development of colchicine-resistance in MCF-7 cells.

Antibody-targeted drugs and drug resistance--challenges and solutions

Antibody-based therapy of various human malignancies has shown efficacy in the past 30 years and is now one of the most successful and leading strategies for targeted treatment of patients harboring hematological malignancies and solid tumors. Antibody-drug conjugates (ADCs) aim to take advantage of the affinity and specificity of monoclonal antibodies (mAbs) to selectively deliver potent cytotoxic drugs to antigen-expressing tumor cells. Key parameters for ADC include choosing the optimal components of the ADC (the antibody, the linker and the cytotoxic drug) and selecting the suitable cell-surface target antigen. Building on the success of recent FDA approval of brentuximab vedotin (Adcetris) and ado-trastuzumab emtansine (Kadcyla), ADCs are currently a class of drugs with a robust pipeline with clinical applications that are rapidly expanding. The more ADCs are being evaluated in preclinical models and clinical trials, the clearer are becoming the parameters and the challenges required for their therapeutic success. This rapidly growing knowledge and clinical experience are revealing novel modalities and mechanisms of resistance to ADCs, hence offering plausible solutions to such challenges. Here, we review the key parameters for designing a powerful ADC, focusing on how ADCs are addressing the challenge of multiple drug resistance (MDR) and its rational overcoming.

Trastuzumab emtansine: mechanisms of action and drug resistance

Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate that is effective and generally well tolerated when administered as a single agent to treat advanced breast cancer. Efficacy has now been demonstrated in randomized trials as first line, second line, and later than the second line treatment of advanced breast cancer. T-DM1 is currently being evaluated as adjuvant treatment for early breast cancer. It has several mechanisms of action consisting of the anti-tumor effects of trastuzumab and those of DM1, a cytotoxic anti-microtubule agent released within the target cells upon degradation of the human epidermal growth factor receptor-2 (HER2)-T-DM1 complex in lysosomes. The cytotoxic effect of T-DM1 likely varies depending on the intracellular concentration of DM1 accumulated in cancer cells, high intracellular levels resulting in rapid apoptosis, somewhat lower levels in impaired cellular trafficking and mitotic catastrophe, while the lowest levels lead to poor response to T-DM1. Primary resistance of HER2-positive metastatic breast cancer to T-DM1 appears to be relatively infrequent, but most patients treated with T-DM1 develop acquired drug resistance. The mechanisms of resistance are incompletely understood, but mechanisms limiting the binding of trastuzumab to cancer cells may be involved. The cytotoxic effect of T-DM1 may be impaired by inefficient internalization or enhanced recycling of the HER2-T-DM1 complex in cancer cells, or impaired lysosomal degradation of trastuzumab or intracellular trafficking of HER2. The effect of T-DM1 may also be compromised by multidrug resistance proteins that pump DM1 out of cancer cells. In this review we discuss the mechanism of action of T-DM1 and the key clinical results obtained with it, the combinations of T-DM1 with other cytotoxic agents and anti-HER drugs, and the potential resistance mechanisms and the strategies to overcome resistance to T-DM1.

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

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

The role of microtubules and their dynamics in cell migration

Although microtubules have long been implicated in cell locomotion, the mechanism of their involvement remains controversial. Most studies have concluded that microtubules play a positive role by regulating actin polymerization, transporting membrane vesicles to the leading edge, and/or facilitating the turnover of adhesion plaques. Here we used wild-type and mutant CHO cell lines with alterations in tubulin to demonstrate that microtubules can also act to restrain cell motility. Tubulin mutations or low concentrations of drugs that suppress microtubule dynamics without affecting the amount of microtubule polymer inhibited the rate of migration by preventing microtubule reorganization in the trailing portion of the cells where the more dynamic microtubules are normally found. Under these conditions, cells along the edge of a wound still extended lamellipodia and elongated toward the wound but were inhibited in their ability to retract their tails, thus retarding forward progress. The idea that microtubules normally act to restrain cell locomotion was confirmed by treating cells with high concentrations of nocodazole to depolymerize the microtubule network. In the absence of microtubules, wild-type CHO and HeLa cells could still move at near normal speeds, but the movement became more random. We conclude that microtubules act both to restrain cell movement and to establish directionality.

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.

Control of MCAK degradation and removal from centromeres

Mitotic centromere associated kinesin (MCAK) is a kinesin related protein with the ability to stimulate microtubule depolymerization. It is found at spindle poles, where it may be involved in poleward microtubule flux, and at kinetochores and centromeres where it plays a role in correcting chromosome alignment errors. Its microtubule depolymerase activity and recruitment to centromeres is regulated by phosphorylation, but little is known about how MCAK is maintained at appropriate levels. We previously reported that MCAK accumulates during the cell cycle and is then degraded during mitosis. Using proteomic analysis, we have now identified a new phosphorylation site on MCAK that is responsible for its degradation. Mutation of the site to prevent phosphorylation prolonged the stability of the protein beyond the metaphase to anaphase transition and into the subsequent cell cycle whereas a phosphomimetic mutation accelerated degradation. Unexpectedly, the mutation that prevented phosphorylation also inhibited the removal of MCAK from centromeres causing it to remain attached throughout the cell cycle. Even low expression of phosphorylation-resistant MCAK delayed mitosis and interfered with cell division. Mitotic defects were also observed by overexpressing a green fluorescent protein-tagged version of wild-type MCAK that similarly escaped degradation and accumulated to toxic levels, but did not remain associated with kinetochores during interphase. The results demonstrate that degradation is an important mechanism for controlling the activity of MCAK.

Substituted phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonamides as antimitotics. Antiproliferative, antiangiogenic and antitumoral activity, and quantitative structure-activity relationships

The importance of the bridge linking the two phenyl moieties of substituted phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates (PIB-SOs) was assessed using a sulfonamide group, which is a bioisostere of sulfonate and ethenyl groups. Forty one phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonamide (PIB-SA) derivatives were prepared and biologically evaluated. PIB-SAs exhibit antiproliferative activities at the nanomolar level against sixteen cancer cell lines, block the cell cycle progression in G(2)/M phase, leading to cytoskeleton disruption and anoikis. These results were subjected to CoMFA and CoMSIA analyses to establish quantitative structure-activity relationships. These results evidence that the sulfonate and sulfonamide moieties are reciprocal bioisosteres and that phenylimidazolidin-2-one could mimic the trimethoxyphenyl moiety found in the structure of numerous potent antimicrotubule agents. Finally, compounds 16 and 17 exhibited potent antitumor and antiangiogenic activities on HT-1080 fibrosarcoma cells grafted onto chick chorioallantoic membrane similar to CA-4 without significant toxicity for the chick embryos, making this class of compounds a promising class of anticancer agents.

Mitotic centromere-associated kinesin (MCAK) mediates paclitaxel resistance

Paclitaxel has powerful anticancer activity, but some tumors are inherently resistant to the drug, whereas others are initially sensitive but acquire resistance during treatment. To deal with this problem, it will be necessary to understand the mechanisms of drug action and resistance. Recent studies indicate that paclitaxel blocks cell division by inhibiting the detachment of microtubules from centrosomes. Here, we demonstrate that mitotic centromere-associated kinesin (MCAK), a kinesin-related protein that destabilizes microtubules, plays an important role in microtubule detachment. Depletion of MCAK altered mitotic spindle morphology, increased the frequency of lagging chromosomes, and inhibited the proliferation of WT CHO cells, confirming that it is an essential protein for cell division. In contrast, MCAK depletion rescued the proliferation of mutant paclitaxel-dependent cell lines that are unable to divide because of defective spindle function resulting from altered α-tubulin or class III β-tubulin overexpression. In concert with the correction of mitotic defects, loss of MCAK reversed an aberrantly high frequency of microtubule detachment in the mutant cells and increased their sensitivity to paclitaxel. The results indicate that MCAK affects cell sensitivity to mitotic inhibitors by modulating the frequency of microtubule detachment, and they demonstrate that changes in a microtubule-interacting protein can reverse the effects of mutant tubulin expression.

Collateral sensitivity of multidrug-resistant cells to the orphan drug tiopronin

A major challenge in the treatment of cancer is multidrug resistance (MDR) that develops during chemotherapy. Here we demonstrate that tiopronin (1), a thiol-substituted N-propanoylglycine derivative, was selectively toxic to a series of cell lines expressing the drug efflux pump P-glycoprotein (P-gp, ABCB1) and MRP1 (ABCC1). Treatment of MDR cells with 1 led to instability of the ABCB1 mRNA and consequently a reduction in P-gp protein, despite functional assays demonstrating that tiopronin does not interact with P-gp. Long-term exposure of P-gp-expressing cells to 1 sensitized them to doxorubicin and paclitaxel, both P-gp substrates. Treatment of MRP1-overexpressing cells with tiopronin led to a significant reduction in MRP1 protein. Synthesis and screening of analogues of tiopronin demonstrated that the thiol functional group was essential for collateral sensitivity while substitution of the amino acid backbone altered but did not destroy specificity, pointing to future development of targeted analogues.

Design, synthesis, biological evaluation, and structure-activity relationships of substituted phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates as new tubulin inhibitors mimicking combretastatin A-4

Sixty-one phenyl 4-(2-oxoimidazolidin-1-yl)benzenesulfonates (PIB-SOs) and 13 of their tetrahydro-2-oxopyrimidin-1(2H)-yl analogues (PPB-SOs) were prepared and biologically evaluated. The antiproliferative activities of PIB-SOs on 16 cancer cell lines are in the nanomolar range and unaffected in cancer cells resistant to colchicine, paclitaxel, and vinblastine or overexpressing the P-glycoprotein. None of the PPB-SOs exhibit significant antiproliferative activity. PIB-SOs block the cell cycle progression in the G₂/M phase and bind to the colchicine-binding site on β-tubulin leading to cytoskeleton disruption and cell death. Chick chorioallantoic membrane tumor assays show that compounds 36, 44, and 45 efficiently block angiogenesis and tumor growth at least at similar levels as combretastatin A-4 (CA-4) and exhibit low to very low toxicity on the chick embryos. PIB-SOs were subjected to CoMFA and CoMSIA analyses to establish quantitative structure–activity relationships.

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.

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.

Human mutations that confer paclitaxel resistance

The involvement of tubulin mutations as a cause of clinical drug resistance has been intensely debated in recent years. In the studies described here, we used transfection to test whether beta1-tubulin mutations and polymorphisms found in cancer patients are able to confer resistance to drugs that target microtubules. Three of four mutations (A185T, A248V, R306C, but not G437S) that we tested caused paclitaxel resistance, as indicated by the following observations: (a) essentially 100% of cells selected in paclitaxel contained transfected mutant tubulin; (b) paclitaxel resistance could be turned off using tetracycline to turn off transgene expression; (c) paclitaxel resistance increased as mutant tubulin production increased. All the paclitaxel resistance mutations disrupted microtubule assembly, conferred increased sensitivity to microtubule-disruptive drugs, and produced defects in mitosis. The results are consistent with a mechanism in which tubulin mutations alter microtubule stability in a way that counteracts drug action. These studies show that human tumor cells can acquire spontaneous mutations in beta1-tubulin that cause resistance to paclitaxel, and suggest that patients with some polymorphisms in beta1-tubulin may require higher drug concentrations for effective therapy.

Molecular basis for class V beta-tubulin effects on microtubule assembly and paclitaxel resistance

Vertebrates produce at least seven distinct beta-tubulin isotypes that coassemble into all cellular microtubules. The functional differences among these tubulin isoforms are largely unknown, but recent studies indicate that tubulin composition can affect microtubule properties and cellular microtubule-dependent behavior. One of the isotypes whose incorporation causes the largest change in microtubule assembly is beta5-tubulin. Overexpression of this isotype can almost completely destroy the microtubule network, yet it appears to be required in smaller amounts for normal mitotic progression. Moderate levels of overexpression can also confer paclitaxel resistance. Experiments using chimeric constructs and site-directed mutagenesis now indicate that the hypervariable C-terminal region of beta5 plays no role in these phenotypes. Instead, we demonstrate that two residues found in beta5 (Ser-239 and Ser-365) are each sufficient to inhibit microtubule assembly and confer paclitaxel resistance when introduced into beta1-tubulin; yet the single mutation of residue Ser-239 in beta5 eliminates its ability to confer these phenotypes. Despite the high degree of conservation among beta-tubulin isotypes, mutations affecting residue 365 demonstrate that amino acid substitutions can be context sensitive; i.e. an amino acid change in one isotype will not necessarily produce the same phenotype when introduced into a different isotype. Modeling studies indicate that residue Cys-239 of beta1-tubulin is close to a highly conserved Cys-354 residue suggesting the possibility that disulfide formation could play a significant role in the stability of microtubules formed with beta1- but not with beta5-tubulin.

A minor beta-tubulin essential for mammalian cell proliferation

Mammals use tubulin from multiple genes to construct microtubules. Some genes are expressed in a tissue specific manner, while others are expressed in almost all cell types. beta5-Tubulin is a minor, ubiquitous isoform whose overexpression was recently shown to disrupt microtubules. Using inhibitory RNA, we now report that suppression of beta5 production in both human and hamster cells blocks cell proliferation. Cells depleted of beta5 either trigger the mitotic checkpoint and undergo apoptosis; or they experience a transient mitotic block, a high incidence of lagging chromosomes, and progression into G1 without cytokinesis to become large, flat cells with elevated DNA content. Microtubules appear to be normally organized in cells depleted of beta5, but they are rich in acetylated alpha-tubulin indicating that they may be more stable than normal. The results provide the first evidence that a specific isoform of beta-tubulin is required for mitosis.

Amino acid substitutions at proline 220 of beta-tubulin confer resistance to paclitaxel and colcemid

Chinese hamster ovary cells selected for resistance to paclitaxel have a high incidence of mutations affecting L215, L217, and L228 in the H6/H7 loop region of beta1-tubulin. To determine whether other mutations in this loop are also capable of conferring resistance to drugs that affect microtubule assembly, saturation mutagenesis of the highly conserved P220 codon in beta1-tubulin cDNA was carried out. Transfection of a mixed pool of plasmids encoding all possible amino acid substitutions at P220 followed by selection in paclitaxel produced cell lines containing P220L and P220V substitutions. Similar selections in colcemid, on the other hand, yielded cell lines with P220C, P220S, and P220T substitutions. Site-directed mutagenesis and retransfection confirmed that these mutations were responsible for drug resistance. Expression of tubulin containing the P220L and P220V mutations reduced microtubule assembly, conferred resistance to paclitaxel and epothilone A, but increased sensitivity to colcemid and vinblastine. In contrast, tubulin with the P220C, P220S, and P220T mutations increased microtubule assembly, conferred resistance to colcemid and vinblastine, but increased sensitivity to paclitaxel and epothilone A. The results are consistent with molecular modeling studies and support a drug resistance mechanism based on changes in microtubule assembly that counteract the effects of drug treatment. These studies show for the first time that different substitutions at the same amino acid residue in beta1-tubulin can confer cellular resistance to either microtubule-stabilizing or microtubule-destabilizing drugs.

New Soft Alkylating Agents with Enhanced Cytotoxicity against Cancer Cells Resistant to Chemotherapeutics and Hypoxia

Chloroethylureas (CEU) are soft alkylating agents that covalently bind to beta-tubulin (betaTAC) and affect microtubule polymerization dynamics. Herein, we report the identification of a CEU subset and its corresponding oxazolines, which induce cell growth inhibition, apoptosis, and microtubule disruption without alkylating beta-tubulin (N-betaTAC). Both betaTAC and N-betaTAC trigger the collapse of mitochondrial potential (DeltaPsi(m)) and modulate reactive oxygen species levels, following activation of intrinsic caspase-8 and caspase-9. Experiments using human fibrosarcoma HT1080 respiratory-deficient cells (rho(0)) and uncoupler of the mitochondrial respiratory chain (MRC) showed that betaTAC and N-betaTAC impaired the MRC. rho(0) cells displayed an increased sensitivity toward N-betaTAC as compared with rho(+) cells but, in contrast, were resistant to betaTAC or classic chemotherapeutics, such as paclitaxel. Oxazoline-195 (OXA-195), an N-betaTAC derivative, triggered massive swelling of isolated mitochondria. This effect was insensitive to cyclosporin A and to Bcl-2 addition. In contrast, adenine nucleotide translocator (ANT) antagonists, bongkrekic acid or atractyloside, diminished swelling induced by OXA-195. The antiproliferative activities of the N-betaTACs CEU-025 and OXA-152 were markedly decreased in the presence of atractyloside. Conversely, pretreatment with cyclosporin A enhanced growth inhibition induced by betaTAC and N-betaTAC. One of the proteins alkylated by N-betaTAC was identified as the voltage-dependent anion channel isoform-1, an ANT partner. Our results suggest that betaTAC and N-betaTAC, despite their common ability to affect the microtubule network, trigger different cytotoxic mechanisms in cancer cells. The role of mitochondria in these mechanisms and the potential of N-betaTAC as a new therapeutic approach for targeting hypoxia-resistant cells are discussed.

Mutations affecting beta-tubulin folding and degradation

Revertants of a colcemid-resistant Chinese hamster ovary cell line with an altered (D45Y) beta-tubulin have allowed the identification of four cis-acting mutations (L187R, Y398C, a 12-amino acid in-frame deletion, and a C-terminal truncation) that act by destabilizing the mutant tubulin and preventing it from incorporating into microtubules. These unstable beta-tubulins fail to form heterodimers and are predominantly found in association with the chaperonin CCT, suggesting that they cannot undergo productive folding. In agreement with these in vivo observations, we show that the defective beta-tubulins do not stably interact with cofactors involved in the tubulin folding pathway and, hence, fail to exchange with beta-tubulin in purified alphabeta heterodimers. Treatment of cells with MG132 causes an accumulation of the aberrant tubulins, indicating that improperly folded beta-tubulin is degraded by the proteasome. Rapid degradation of the mutant tubulin does not elicit compensatory changes in wild-type tubulin synthesis or assembly. Instead, loss of beta-tubulin from the mutant allele causes a 30-40% decrease in cellular tubulin content with no obvious effect on cell growth or survival.

Mutations at Leucine 215 of β-Tubulin Affect Paclitaxel Sensitivity by Two Distinct Mechanisms†

Paclitaxel resistance mutations in Chinese hamster ovary cells frequently alter a cluster of leucine residues in the H6-H7 loop region of beta-tubulin. To gain further insight into the role of this region in microtubule assembly and drug resistance, site-directed mutagenesis was used to systematically change amino acid L215. The mutated genes were cloned into a tetracycline-regulated expression vector and transfected into wild-type cells. Most of the mutations destabilized microtubule assembly, causing a decreased fraction of tubulin to appear in the microtubule cytoskeleton. In each case, the decreased level of assembly was associated with paclitaxel resistance and increased colcemid sensitivity. In two cases, however, the alteration did not significantly perturb the level of assembled tubulin or confer resistance to paclitaxel. One of these, L215V, produced little or no detectable phenotype, while the other, L215I, conferred increased sensitivity to paclitaxel. The increased drug sensitivity did not extend to epothilone A, a drug that binds to the same site and has a mechanism of action similar to that of paclitaxel, or colcemid, a drug with an opposing mechanism of action and a distinct binding site. Moreover, L215I conferred enhanced paclitaxel sensitivity at very low levels of expression, and sensitivity was not further enhanced in cells with higher levels of expression, implying that paclitaxel acts substoichiometrically. These properties, along with the proximity of L215 to the drug binding site, suggests that the L215I substitution may enhance the binding or effectiveness of paclitaxel. Our studies confirm the importance of the H6-H7 loop of beta-tubulin in microtubule assembly and resistance to antimitotic drugs. They also identify the first mammalian mutation shown to specifically increase sensitivity to paclitaxel.

The molecular basis of multidrug resistance in cancer: the early years of P-glycoprotein research

The discovery and characterization of P-glycoprotein, an energy-dependent multidrug efflux pump, as a mechanism of multidrug resistance in cancer is generally accepted as a significant contribution to the ongoing effort to end death and suffering from this disease. The historical reflections of Victor Ling and Michael Gottesman concerning the early years of this research highlight the important contributions of the multidisciplinary teams involved in these studies, and illustrate how technological developments in biochemistry and molecular and cell biology enabled this discovery.

Paclitaxel resistance in cells with reduced β-tubulin

We previously described the isolation of colcemid resistant Chinese hamster ovary cell lines containing alpha- and beta-tubulin mutations that increase microtubule assembly and stability. By analyzing colcemid sensitive revertants from one of the beta-tubulin mutants, we now find that loss or inactivation of the mutant allele represents the most common mechanism of reversion. Consistent with this loss, the revertants have 35% less tubulin at steady state, no evidence for the presence of a mutant polypeptide, and a normal extent of tubulin polymerization. In addition to the loss of colcemid resistance, the revertant cells exhibit increased resistance to paclitaxel relative to wild-type cells. This paclitaxel resistance can be suppressed by transfecting the revertant cells with a cDNA for wild-type beta-tubulin, indicating that the reduction in tubulin in the revertant cells is responsible for the resistance phenotype. We propose that reducing tubulin levels may represent a novel mechanism of paclitaxel resistance.

Cell structure and cytokinesis alterations in multidrug-resistant Leishmania (Leishmania) amazonensis

Multidrug-resistant Leishmania (Leishmania) amazonensis may be obtained by in vitro selection with vinblastine. In order to determine whether this phenotype is linked to structural alterations, we analyzed the cell architecture by electron microscopy. The vinblastine resistant CL2 clone of L. (L.) amazonensis, but not wild-type parasites, showed a cytokinesis dysfunction. The CL2 promastigotes had multiple nuclei, kinetoplasts and flagella, suggesting that vinblastine resistance may be associated with truncated cell division. The subpellicular microtubule plasma membrane connection was also affected. Wild-type parasites treated with vinblastine displayed similar alterations, presenting lobulated and multinucleated cells. Taken together, these data indicate that antimicrotubule drug-selected parasites may show evidence of the mutation of cytoskeleton proteins, impairing normal cell function.

The B-ring substituent at C-7 of colchicine and the α-C-terminus of tubulin communicate through the “tail-body” interaction

The carboxy terminals of alphabeta-tubulins are flexible regions rich in acidic amino acid residues that play an inhibitory role in the polymerization of tubulin to microtubules. We have shown that the binding of colchicine and its B-ring analogs (with C-7 substituents) to tubulin are pH sensitive and have high activation energies. Under identical conditions, the binding of analogs without C-7 substituents is pH independent and has lower activation energy. Beta-C-terminus-truncated tubulin (alphabeta(s)) shows similar pH sensitivity and activation energy to native tubulin (alphabeta). Removal of the C-termini of both subunits of tubulin (alpha(s)beta(s)) or the binding of a basic peptide P2 to the negatively charged alpha-C-terminus of tubulin causes a colchicine-tubulin interaction independent of pH with a low activation energy. Tubulin dimer structure shows that the C-terminal alpha-tail is too far from the colchicine binding site to interact directly with the bound colchicine. Therefore, it is likely that the interaction of the alpha-C-terminus with the main body of tubulin indirectly affects the colchicine-tubulin interaction via conformational changes in the main body. We therefore conclude that in the presence of tail-body interaction, a B-ring substituent makes contact with the alpha-tubulin and induces significant conformational changes in alpha-tubulin.

Intra-Allelic Suppression of a Mutation that Stabilizes Microtubules and Confers Resistance to Colcemid†

Cmd 4 is a colcemid resistant beta-tubulin mutant of Chinese hamster ovary cells that exhibits hypersensitivity to paclitaxel and temperature sensitivity for growth. The mutant beta-tubulin allele in this cell line encodes a D45Y amino acid substitution that produces colcemid resistance by making microtubules more stable. By selecting revertants of the temperature sensitive and paclitaxel hypersensitive phenotypes, we have identified three cis-acting suppressors of D45Y. One suppressor, V60A, maps to the same region as the D45Y alteration, and a second suppressor, Q292H, maps to a distant location. Both appear to produce compensatory changes in microtubule assembly that counteract the effects of the original D45Y substitution. Consistent with this view, expression of the V60A mutation in transfected wild-type cells produced paclitaxel resistance and greatly decreased microtubule assembly. Additionally, it produced a paclitaxel-dependent phenotype in which cells grew normally in the presence, but not the absence, of the drug. The Q292H mutation caused even greater disassembly of microtubules such that cells were unable to proliferate when the transgene was expressed; but, unlike the V60A mutation, cell growth could not be rescued by paclitaxel. A third suppressor, A254V, maps to a region near the interface between alpha- and beta-tubulin that contains the colchicine binding site. Although it made transfected wild-type cells hypersensitive to colcemid, it did not affect paclitaxel or vinblastine sensitivity, nor did it reduce microtubule assembly. We suggest that this mutation acts by increasing tubulin's affinity for colcemid.

A ubiquitous beta-tubulin disrupts microtubule assembly and inhibits cell proliferation

Vertebrate tubulin is encoded by a multigene family that produces distinct gene products, or isotypes, of both the alpha- and beta-tubulin subunits. The isotype sequences are conserved across species supporting the hypothesis that different isotypes subserve different functions. To date, however, most studies have demonstrated that tubulin isotypes are freely interchangeable and coassemble into all classes of microtubules. We now report that, in contrast to other isotypes, overexpression of a mouse class V beta-tubulin cDNA in mammalian cells produces a strong, dose-dependent disruption of microtubule organization, increased microtubule fragmentation, and a concomitant reduction in cellular microtubule polymer levels. These changes also disrupt mitotic spindle assembly and block cell proliferation. Consistent with diminished microtubule assembly, there is an increased tolerance for the microtubule stabilizing drug, paclitaxel, which is able to reverse many of the effects of class V beta-tubulin overexpression. Moreover, transfected cells selected in paclitaxel exhibit increased expression of class V beta-tubulin, indicating that this isotype is responsible for the drug resistance. The results show that class V beta-tubulin is functionally distinct from other tubulin isotypes and imparts unique properties on the microtubules into which it incorporates.

Revertants of a Chinese hamster ovary cell mutant with an altered beta-tubulin: evidence that the altered tubulin confers both colcemid resistance and temperature sensitivity on the cell

We recently described the isolation of a mutant Chinese hamster ovary cell (Cmd 4) resistant to the cytotoxic effects of colcemid (Cabral et al., Cell 20:29-36, 1980). This mutant carries an altered beta-tubulin but still grows normally at 37 degrees C. In the present study we found that Cmd 4 is temperature sensitive for growth at 40.3 degrees C. A class of revertants selected for temperature resistance had simultaneously lost colcemid resistance and the altered beta-tubulin. In addition, we isolated a temperature-resistant revertant which carries a further alteration in the mutant beta-tubulin polypeptide. This second alteration appears to make the mutant beta-tubulin incompetent to assemble into microtubules, resulting in a strain which is again colcemid sensitive. These revertant cell lines provide strong evidence that a mutation in beta-tubulin can confer both colcemid resistance and temperature sensitivity on a mammalian cell line. Cellular microtubules studied by indirect immunofluorescence in both mutant and revertant cell lines had an apparently normal distribution at permissive and nonpermissive temperatures, yet mitosis appears to be abnormal in the mutant cell line. We conclude from these studies that incorporation of the altered beta-tubulin into microtubules does not affect their distribution but may affect their function during mitosis.

Revertants of a Chinese hamster ovary cell mutant with an altered beta-tubulin: evidence that the altered tubulin confers both colcemid resistance and temperature sensitivity on the cell

We recently described the isolation of a mutant Chinese hamster ovary cell (Cmd 4) resistant to the cytotoxic effects of colcemid (Cabral et al., Cell 20:29-36, 1980). This mutant carries an altered beta-tubulin but still grows normally at 37 degrees C. In the present study we found that Cmd 4 is temperature sensitive for growth at 40.3 degrees C. A class of revertants selected for temperature resistance had simultaneously lost colcemid resistance and the altered beta-tubulin. In addition, we isolated a temperature-resistant revertant which carries a further alteration in the mutant beta-tubulin polypeptide. This second alteration appears to make the mutant beta-tubulin incompetent to assemble into microtubules, resulting in a strain which is again colcemid sensitive. These revertant cell lines provide strong evidence that a mutation in beta-tubulin can confer both colcemid resistance and temperature sensitivity on a mammalian cell line. Cellular microtubules studied by indirect immunofluorescence in both mutant and revertant cell lines had an apparently normal distribution at permissive and nonpermissive temperatures, yet mitosis appears to be abnormal in the mutant cell line. We conclude from these studies that incorporation of the altered beta-tubulin into microtubules does not affect their distribution but may affect their function during mitosis.

A New Generation of N-Aryl-N‘-(1-alkyl-2-chloroethyl)ureas as Microtubule Disrupters: Synthesis, Antiproliferative Activity, and β-Tubulin Alkylation Kinetics

New N-aryl-N'-2-chloroethylureas (CEUs) with enhanced cytotoxicity were developed as antimitotic agents potentially useful in cancer chemotherapy. Highly potent CEUs were obtained by introduction of a branched alkylating chain, the N'-(1-methyl-2-chloro)ethyl group. Their cytotoxic activity was enantio-dependent and induced through specific alkylation of beta-tubulin, leading to microtubule disruption and mitotic arrest. Overall, the structural modifications of the CEUs described here significantly improved their kinetics of beta-tubulin alkylation. In this new series, CEUs 16a and 18a displayed particularly enhanced antiproliferative activity related to a faster reaction with beta-tubulin and merit further investigation as potential antitumor agents.

Microtubule disruption utilizes an NFkappa B-dependent pathway to stabilize HIF-1alpha protein

Hypoxia-inducible factor (HIF)-1alpha levels are elevated in normoxic cells undergoing physiological processes involving large scale microtubule reorganization, such as embryonic development, wound healing, and tumor cell metastasis. Although alterations in microtubules affect numerous cellular responses, no data have yet implicated microtubule dynamics in HIF-1alpha regulation. To investigate the effect of microtubule change upon HIF-1alpha regulation, we treated cells with the microtubule-depolymerizing agents (MDAs) colchicine, vinblastine or nocodazole. We demonstrate that these agents are able to induce transcriptionally active HIF-1. MDA-mediated induction of HIF-1alpha required microtubule depolymerization, since HIF-1alpha levels were unchanged in cells treated with either the microtubule-stabilizing agent paclitaxel, or an inactive form of colchicine, or in colchicine-resistant cells. HIF-1 induction was dependent upon cellular transcription, as transcription inhibitors abrogated HIF-1alpha protein up-regulation. The ability of transcriptional inhibitors to interfere with HIF-1alpha accumulation was specific to the MDA-initiated pathway, as they were ineffective in preventing hypoxia-mediated HIF-1 induction, which occurs by a distinct post-translational pathway. Moreover, we provide evidence implicating a requirement for NFkappaB transcription in the HIF-1 induction mediated by MDAs. The ability of MDAs to induce HIF-1alpha is dependent upon activation of NFkappaB, since inhibition of NFkappaB either pharmacologically or by transfection of an NFkappaB super-repressor plasmid abrogated this induction. Collectively, these data support a model in which NFkappaB is a focal point for the convergence of MDA-mediated signaling events leading to HIF-1 induction, thus revealing a novel aspect of HIF-1alpha regulation and function.

Microtubules: dynamics, drug interaction and drug resistance in Leishmania

Microtubules are cytoskeletal polymers essential for the survival of all eukaryotes. These proteins are the proposed cellular targets of many anticancerous, antifungal and antihelminthic drugs. Sufficient differences exist between the microtubules of kinetoplastid parasites like Leishmania and humans to explore the selective targeting of these proteins for therapeutic purposes. This review describes the basic structure of microtubules and its dynamics in general, with specific insights into leishmanial microtubules, the salient features of microtubule-drug interactions including the specificity of certain drugs for parasitic microtubules. Chemotherapy against leishmanial parasites is failing because of the emergence of drug resistant strains. The possible mechanisms of resistance to antimicrotubule agents along with insights into the role of microtubules in mediating drug resistance in Leishmania are discussed.

Paclitaxel-dependent mutants have severely reduced microtubule assembly and reduced tubulin synthesis

A subset of mutant cell lines selected for resistance to the antitumor drug paclitaxel are unable to progress normally through mitosis unless the drug is present in the growth medium. Without paclitaxel the cells form defective spindles, undergo aberrant mitoses, fail to complete cell division and eventually die. Analysis of these drug-dependent cells revealed a low amount of microtubule polymer and less tubulin production than wild-type cells. Ribonuclease protection experiments indicated that the decreased tubulin protein was due to decreased tubulin mRNA. Enhancing microtubule assembly by treating the cells with paclitaxel, restored tubulin to levels comparable with those of paclitaxel-treated wild-type cells, which demonstrated that the drug-dependent cells do not have a permanent impairment in their capacity to synthesize tubulin. Paclitaxel-resistant (but not dependent) cells have a smaller reduction in microtubule polymer with little or no decrease in tubulin production, whereas colcemidresistant cells have increased microtubule assembly but also exhibit little or no change in tubulin production. Finally,a mutant cell line producing an unstable β-tubulin protein has normal growth as well as normal synthesis and polymerization of tubulin, despite an approximately 30% decrease in steady state tubulin content. These studies establish a lower limit of tubulin assembly needed for cell survival and indicate that tubulin assembly must fall below this point to trigger a significant decrease in tubulin synthesis.

Disruption of microtubules uncouples budding and nuclear division inToxoplasma gondii

The tachyzoite stage of the protozoan parasite Toxoplasma gondiihas two populations of microtubules: spindle microtubules and subpellicular microtubules. To determine how these two microtubule populations are regulated, we investigated microtubule behavior during the cell cycle following treatment with microtubule-disrupting drugs. Previous work had established that the microtubule populations are individually nucleated by two distinct microtubule-organizing centers (MTOCs): the apical polar ring for the subpellicular microtubules and spindle pole plaques/centrioles for the spindle microtubules. When replicating tachyzoites were treated with 0.5 μM oryzalin or 1.0 mM colchicine they retained the capacity to form a spindle and undergo nuclear division. Although these parasites could complete budding,they lost the bulk of their subpellicular microtubules and the ability to reinvade host cells. Both nascent spindle and subpellicular microtubules were disrupted in 2.5 μM oryzalin or 5.0 mM colchicine. Under these conditions,parasites grew in size and replicated their genome but were incapable of nuclear division. After removal from 0.5 μM oryzalin, Toxoplasmatachyzoites were able to restore normal subpellicular microtubules and a fully invasive phenotype. When oryzalin was removed from Toxoplasmatachyzoites treated with 2.5 μM drug, the parasites attempted to bud as crescent-shaped tachyzoites. Because the polyploid nuclear mass could not be correctly segregated, many daughter parasites lacked nuclei altogether although budding and scission from the maternal mass was able to be completed. Multiple MTOCs permit Toxoplasma tachyzoites to control nuclear division independently from cell polarity and cytokinesis. This unusual situation grants greater cell cycle flexibility to these parasites but abolishes the checks for coregulation of nuclear division and cytokinesis found in other eukaryotes.

Mutational analysis of the beta-tubulin gene in lung cancer

Recently, several studies have suggested that a major mechanism of resistance to paclitaxel might involve mutations in the beta-tubulin gene in tumor cells. To investigate the frequency of beta-tubulin mutations in Japanese patients with small and non-small cell lung cancer, direct sequence analysis following reverse transcription-polymerase chain reaction (RT-PCR) of the beta-tubulin gene was performed using total RNA from 20 lung cancer cell lines and 22 specimens from lung cancer patients. First-strand cDNA sequence analysis of the 42 samples showed silent mutations at codon 180 of the beta-tubulin gene, which encodes the GTP-binding site of the protein, and codons 195 and 217. However, neither missense nor non-sense mutations affecting microtubule dynamics, within or near the GTP-binding site of the beta-tubulin gene, were detected. These results indicate that beta-tubulin gene mutations might not play a major role in the mechanism of resistance to paclitaxel in Japanese lung cancer patients. Further investigations are needed to clarify the mechanism of drug resistance.

Effects of orally active taxanes on P-glycoprotein modulation and colon and breast carcinoma drug resistance

BACKGROUND

The taxane paclitaxel (Taxol) is often of limited efficacy in chemotherapeutic regimens because some cancer cells express high levels of the efflux pump, P-glycoprotein (Pgp), which removes the drug from the cells. The orally active paclitaxel analog IDN-5109 has been reported to overcome Pgp-mediated drug resistance. We tested whether IDN-5109 acts by modulating Pgp activity.

METHODS

Human MDA435/LCC6mdr1 and MDA435/LCC6 breast carcinoma cells, which express and do not express Pgp, respectively, were incubated with [3H]IDN-5109 and paclitaxel to determine intracellular drug accumulation. Flow cytometry was used to analyze intracellular retention of two Pgp substrates, rhodamine 123 (Rh-123) and doxorubicin, in both breast carcinoma cell lines and in human colon carcinoma cells (SW-620, DLD1, and HCT-15, whose Pgp levels vary) treated with different taxanes. The effects of IDN-5109 and paclitaxel on tumor growth in vivo were studied with the use of tumors established through xenografts of Pgp-expressing SW-620 and DLD1 cells in severe combined immunodeficiency mice. All statistical tests were two-sided.

RESULTS

Pgp-expressing cells treated with IDN-5109 or with the taxane-based drug resistance reversal agent tRA96023, which blocks Pgp activity, retained 8.1- and 9.4-fold more Rh-123 (P =.0001), respectively, and 1.7- and 1.9-fold more doxorubicin (P =.001), respectively, than cells treated with paclitaxel. Non-Pgp-expressing cells treated similarly demonstrated no increased retention of either substrate. MDA435/LCC6mdr1 cells retained 5.3-fold more [3H]IDN-5109 than [3H]paclitaxel after 2 hours (P =.01). IDN-5109 showed statistically significantly higher tumor growth inhibition than paclitaxel against the SW-620 xenograft (P =.003).

CONCLUSIONS

IDN-5109 modulates Pgp activity, resulting in superior tumor growth inhibition against Pgp-expressing tumors as compared with paclitaxel. IDN-5109 may broaden the spectrum of taxane use to include colon tumors.

Mechanisms of action and resistance to tubulin-binding agents

Tubulin binding agents constitute an important class of antimitotics and are widely used for the treatment of solid tumours an haematopoietic malignancies. These compounds, currently represented by the vinca alkaloids and the taxanes, differ from most of the other clinically useful antimitotics in that their target is not nucleic acids, but the mitotic spindle, which is an essential component of the mitotic machinery. Recent data on the mechanisms of action of and mechanisms of resistance to tubulin binding agents are presented. The importance of microtubule dynamics is emphasised, in particular in relationship to the usefulness of drug combinations. Concerning the reported resistance mechanisms, an emerging body of data show that altered microtubule structure may be involved in reduced sensitivity to these compounds. Promising new molecules, including those derived from marine organisms are described.