The colR4 and colR15 beta-tubulin mutations in Chlamydomonas reinhardtii confer altered sensitivities to microtubule inhibitors and herbicides by enhancing microtubule stability

The flagellar length control system: exploring the physical biology of organelle size

How cells build and maintain dynamic structures of defined size is currently an important unsolved problem in quantitative cell biology. The flagella of the unicellular green algaChlamydomonasprovide a highly tractable model system to investigate this general question, but while the powerful genetics of this organism have revealed numerous genes required for proper flagellar length, in most cases we do not understand their mechanistic role in length control. Flagellar length can be viewed as the steady state solution of a dynamical system involving assembly and disassembly of axonemal microtubules, with assembly depending on an active transport process known as intraflagellar transport (IFT). The inherent length dependence of IFT gives rise to a family of simple models for length regulation that can account for many previously described phenomena such as the ability of flagella to maintain equal lengths. But these models requires that the cell has a way to measure flagellar length in order to adjust IFT rates accordingly. Several models for length sensing have been modeled theoretically and evaluated experimentally, allowing them to be ruled out. Current data support a model in which the diffusive return of the kinesin motor driving IFT provides a length dependence that ultimately is the basis for length regulation. By combining models of length sensing with a more detailed representation of cargo transport and availability, it is now becoming possible to formulate concrete hypotheses to explain length altering mutants.

TK216 targets microtubules in Ewing sarcoma cells

Ewing sarcoma (EWS) is a pediatric malignancy driven by the EWSR1-FLI1 fusion protein formed by the chromosomal translocation t(11; 22). The small molecule TK216 was developed as a first-in-class direct EWSR1-FLI1 inhibitor and is in phase II clinical trials in combination with vincristine for patients with EWS. However, TK216 exhibits anti-cancer activity against cancer cell lines and xenografts that do not express EWSR1-FLI1, and the mechanism underlying cytotoxicity remains unresolved. We apply a forward-genetics screening platform utilizing engineered hypermutation in EWS cell lines and identify recurrent mutations in TUBA1B, encoding ⍺-tubulin, that prove sufficient to drive resistance to TK216. Using reconstituted microtubule (MT) polymerization in vitro and cell-based chemical probe competition assays, we demonstrate that TK216 acts as an MT destabilizing agent. This work defines the mechanism of cytotoxicity of TK216, explains the synergy observed with vincristine, and calls for a reexamination of ongoing clinical trials with TK216.

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Recurrent TUBA1B mutations were identified in TK216-resistant Ewing sarcoma cells

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TUBA1B mutations independently drove resistance to TK216

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TK216 suppressed tubulin polymerization in vitro in a stereo-selective manner

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TK216 and vincristine act on microtubules through distinct binding mechanisms

Chlamydomonas reinhardtii tubulin-gene disruptants for efficient isolation of strains bearing tubulin mutations

The single-cell green alga Chlamydomonas reinhardtii possesses two α-tubulin genes (tua1 and tua2) and two β-tubulin genes (tub1 and tub2), with the two genes in each pair encoding identical amino acid sequences. Here, we screened an insertional library to establish eight disruptants with defective tua2, tub1, or tub2 expression. Most of the disruptants did not exhibit major defects in cell growth, flagellar length, or flagellar regeneration after amputation. Because few tubulin mutants of C. reinhardtii have been reported to date, we then used our disruptants, together with a tua1 disruptant obtained from the Chlamydomonas Library Project (CLiP), to isolate tubulin-mutants resistant to the anti-tubulin agents propyzamide (pronamide) or oryzalin. As a result of several trials, we obtained 8 strains bearing 7 different α-tubulin mutations and 12 strains bearing 7 different β-tubulin mutations. One of the mutations is at a residue similar to that of a mutation site known to confer drug resistance in human cancer cells. Some strains had the same amino acid substitutions as those reported previously in C. reinhardtii; however, the mutants with single tubulin genes showed slightly stronger drug-resistance than the previous mutants that express the mutated tubulin in addition to the wild-type tubulin. Such increased drug-resistance may have facilitated sensitive detection of tubulin mutation. Single-tubulin-gene disruptants are thus an efficient background of generating tubulin mutants for the study of the structure–function relationship of tubulin.

The elusive actin cytoskeleton of a green alga expressing both conventional and divergent actins

The green alga Chlamydomonas reinhardtii is a leading model system to study photosynthesis, cilia, and the generation of biological products. The cytoskeleton plays important roles in all of these cellular processes, but to date, the filamentous actin network within Chlamydomonas has remained elusive. By optimizing labeling conditions, we can now visualize distinct linear actin filaments at the posterior of the nucleus in both live and fixed vegetative cells. Using in situ cryo-electron tomography, we confirmed this localization by directly imaging actin filaments within the native cellular environment. The fluorescently labeled structures are sensitive to the depolymerizing agent latrunculin B (Lat B), demonstrating the specificity of our optimized labeling method. Interestingly, Lat B treatment resulted in the formation of a transient ring-like filamentous actin structure around the nucleus. The assembly of this perinuclear ring is dependent upon a second actin isoform, NAP1, which is strongly up-regulated upon Lat B treatment and is insensitive to Lat B-induced depolymerization. Our study combines orthogonal strategies to provide the first detailed visual characterization of filamentous actins in Chlamydomonas, allowing insights into the coordinated functions of two actin isoforms expressed within the same cell.

Transient Internalization and Microtubule-Dependent Trafficking of a Ciliary Signaling Receptor from the Plasma Membrane to the Cilium

Cilia are ancient organelles used by unicellular and multicellular organisms not only for motility but also to receive and respond to multiple environmental cues, including light, odorants, morphogens, growth factors, and contact with cilia of other cells. Much is known about the cellular mechanisms that deliver membrane proteins to cilia during ciliogenesis. Execution of a ciliary signaling pathway, however, can critically depend on rapid alterations in the receptor composition of the cilium itself, and our understanding of the mechanisms that underlie these rapid, regulated alterations remains limited [1-6]. In the bi-ciliated, unicellular alga Chlamydomonas reinhardtii, interactions between cilia of mating type plus and mating type minus gametes mediated by adhesion receptors SAG1 and SAD1 activate a ciliary signaling pathway [7]. In response, a large, inactive pool of SAG1 on the plasma membrane of plus gametes rapidly becomes enriched in the peri-ciliary membrane and enters the cilia to become active and maintain and enhance ciliary adhesion and signaling [8-14]. Ciliary entry per se of SAG1 is independent of anterograde intraflagellar transport (IFT) [13], but the rapid apical enrichment requires cytoplasmic microtubules and the retrograde IFT motor, dynein 1b [14]. Whether the receptors move laterally within the plasma membrane or transit internally during redistribution is unknown. Here, in coupled immunolocalization/biochemical studies on SAG1, we show that, within minutes after gamete activation is initiated, cell-surface SAG1 is internalized, associates with an apico-basally polarized array of cytoplasmic microtubules, and returns to the cell surface at a peri-ciliary staging area for entry into cilia.

H+- and Na+- elicited rapid changes of the microtubule cytoskeleton in the biflagellated green alga Chlamydomonas

Although microtubules are known for dynamic instability, the dynamicity is considered to be tightly controlled to support a variety of cellular processes. Yet diverse evidence suggests that this is not applicable to Chlamydomonas, a biflagellate fresh water green alga, but intense autofluorescence from photosynthesis pigments has hindered the investigation. By expressing a bright fluorescent reporter protein at the endogenous level, we demonstrate in real time discreet sweeping changes in algal microtubules elicited by rises of intracellular H⁺ and Na⁺. These results from this model organism with characteristics of animal and plant cells provide novel explanations regarding how pH may drive cellular processes; how plants may respond to, and perhaps sense stresses; and how organisms with a similar sensitive cytoskeleton may be susceptible to environmental changes.

Flexural Rigidity and Shear Stiffness of Flagella Estimated from Induced Bends and Counterbends

Motile cilia and flagella are whiplike cellular organelles that bend actively to propel cells or move fluid in passages such as airways, brain ventricles, and the oviduct. Efficient motile function of cilia and flagella depends on coordinated interactions between active forces from an array of motor proteins and passive mechanical resistance from the complex cytoskeletal structure (the axoneme). However, details of this coordination, including axonemal mechanics, remain unclear. We investigated two major mechanical parameters, flexural rigidity and interdoublet shear stiffness, of the flagellar axoneme in the unicellular alga Chlamydomonas reinhardtii. Combining experiment, theory, and finite element models, we demonstrate that the apparent flexural rigidity of the axoneme depends on both the intrinsic flexural rigidity (EI) and the elastic resistance to interdoublet sliding (shear stiffness, ks). We estimated the average intrinsic flexural rigidity and interdoublet shear stiffness of wild-type Chlamydomonas flagella in vivo, rendered immotile by vanadate, to be EI = 840 ± 280 pN⋅μm(2) and ks = 79.6 ± 10.5 pN/rad, respectively. The corresponding values for the pf3; cnk11-6 double mutant, which lacks the nexin-dynein regulatory complex (N-DRC), were EI = 1011 ± 183 pN·μm(2) and ks = 39.3 ± 6.0 pN/rad under the same conditions. Finally, in the pf13A mutant, which lacks outer dynein arms and inner dynein arm c, the estimates were EI = 777 ± 184 pN·μm(2) and ks = 43.3 ± 7.7 pN/rad. In the two mutant strains, the flexural rigidity is not significantly different from wild-type (p > 0.05), but the lack of N-DRC (in pf3; cnk11-6) or dynein arms (in pf13A) significantly reduces interdoublet shear stiffness. These differences may represent the contributions of the N-DRCs (∼40 pN/rad) and residual dynein interactions (∼35 pN/rad) to interdoublet sliding resistance in these immobilized Chlamydomonas flagella.

Quantitative analysis and modeling of katanin function in flagellar length control

A mutation in a microtubule-severing enzyme, katanin, causes flagella to become short due to a reduced cytoplasmic precursor pool. These results suggest that competition between flagella and cytoplasmic microtubules for a limited tubulin pool is facilitated by katanin, which is confirmed by stochastic models.

Flagellar length control in Chlamydomonas reinhardtii provides a simple model system in which to investigate the general question of how cells regulate organelle size. Previous work demonstrated that Chlamydomonas cytoplasm contains a pool of flagellar precursor proteins sufficient to assemble a half-length flagellum and that assembly of full-length flagella requires synthesis of additional precursors to augment the preexisting pool. The regulatory systems that control the synthesis and regeneration of this pool are not known, although transcriptional regulation clearly plays a role. We used quantitative analysis of length distributions to identify candidate genes controlling pool regeneration and found that a mutation in the p80 regulatory subunit of katanin, encoded by the PF15 gene in Chlamydomonas, alters flagellar length by changing the kinetics of precursor pool utilization. This finding suggests a model in which flagella compete with cytoplasmic microtubules for a fixed pool of tubulin, with katanin-mediated severing allowing easier access to this pool during flagellar assembly. We tested this model using a stochastic simulation that confirms that cytoplasmic microtubules can compete with flagella for a limited tubulin pool, showing that alteration of cytoplasmic microtubule severing could be sufficient to explain the effect of the pf15 mutations on flagellar length.

Katanin localization requires triplet microtubules in Chlamydomonas reinhardtii

Centrioles and basal bodies are essential for a variety of cellular processes that include the recruitment of proteins to these structures for both centrosomal and ciliary function. This recruitment is compromised when centriole/basal body assembly is defective. Mutations that cause basal body assembly defects confer supersensitivity to Taxol. These include bld2, bld10, bld12, uni3, vfl1, vfl2, and vfl3. Flagellar motility mutants do not confer sensitivity with the exception of mutations in the p60 (pf19) and p80 (pf15) subunits of the microtubule severing protein katanin. We have identified additional pf15 and bld2 (ε-tubulin) alleles in screens for Taxol sensitivity. Null pf15 and bld2 alleles are viable and are not essential genes in Chlamydomonas. Analysis of double mutant strains with the pf15-3 and bld2-6 null alleles suggests that basal bodies in Chlamydomonas may recruit additional proteins beyond katanin that affect spindle microtubule stability. The bld2-5 allele is a hypomorphic allele and its phenotype is modulated by nutritional cues. Basal bodies in bld2-5 cells are missing proximal ends. The basal body mutants show aberrant localization of an epitope-tagged p80 subunit of katanin. Unlike IFT proteins, katanin p80 does not localize to the transition fibers of the basal bodies based on an analysis of the uni1 mutant as well as the lack of colocalization of katanin p80 with IFT74. We suggest that the triplet microtubules are likely to play a key role in katanin p80 recruitment to the basal body of Chlamydomonas rather than the transition fibers that are needed for IFT localization.

The Hsp70 and Hsp40 chaperones influence microtubule stability in Chlamydomonas

Mutations at the APM1 and APM2 loci in the green alga Chlamydomonas reinhardtii confer resistance to phosphorothioamidate and dinitroaniline herbicides. Genetic interactions between apm1 and apm2 mutations suggest an interaction between the gene products. We identified the APM1 and APM2 genes using a map-based cloning strategy. Genomic DNA fragments containing only the DNJ1 gene encoding a type I Hsp40 protein rescue apm1 mutant phenotypes, conferring sensitivity to the herbicides and rescuing a temperature-sensitive growth defect. Lesions at five apm1 alleles include missense mutations and nucleotide insertions and deletions that result in altered proteins or very low levels of gene expression. The HSP70A gene, encoding a cytosolic Hsp70 protein known to interact with Hsp40 proteins, maps near the APM2 locus. Missense mutations found in three apm2 alleles predict altered Hsp70 proteins. Genomic fragments containing the HSP70A gene rescue apm2 mutant phenotypes. The results suggest that a client of the Hsp70-Hsp40 chaperone complex may function to increase microtubule dynamics in Chlamydomonas cells. Failure of the chaperone system to recognize or fold the client protein(s) results in increased microtubule stability and resistance to the microtubule-destabilizing effect of the herbicides. The lack of redundancy of genes encoding cytosolic Hsp70 and Hsp40 type I proteins in Chlamydomonas makes it a uniquely valuable system for genetic analysis of the function of the Hsp70 chaperone complex.

Association of low tumor RNA integrity with response to chemotherapy in breast cancer patients

The CAN-NCIC-MA22 phase I/II clinical trial evaluated women with locally advanced or inflammatory breast cancer treated with epirubicin and docetaxel at 2 or 3 weekly intervals in sequential cohorts. The relationship between various biomarkers and treatment response was assessed. Breast biopsy cores were obtained from 50 patients pre-, mid-, and post-treatment. Immunohistochemical staining was performed to determine baseline levels of estrogen receptor (ER), progesterone receptor (PR), Her2/Neu protein (HER2), and topoisomerase II (Topo 2),expressed as percent positive stain. Tumor RNA integrity(RIN) and tumor cellularity were measured pre-, mid- and post-treatment by capillary electrophoresis and light microscopy after hematoxylin/eosin staining, respectively.Associations between 1) maximum RIN and 2) tumor cellularity at the three time points with baseline levels of ER,PR, Her2, and topo II were assessed using Spearman and Pearson correlation coefficients. Associations between RIN and tumor cellularity with chemotherapy dose level orpathologic response were assessed using one-way ANOVA.In this study, we observed that low mid-treatment maximum RIN (but not tumor cellularity) was associated with high chemotherapy drug dose level (P = 0.05) and eventual pathologic complete response (pCR) (P = 0.01). Posttreatment,low maximum RIN was found to be associated with low tumor cellularity (P = 0.004), and low tumor cellularity with pCR (P = 0.01). Post-treatment tumor cellularity was lowest in patients with tumors having high baseline PR levels (P = 0.05). The association of midtreatment RIN with drug dose level and with pCR suggests that tumor RIN may represent an important new biomarker for measuring response to chemotherapy in breast cancer patients.

A randomized phase II study of ixabepilone (BMS-247550) given daily x 5 days every 3 weeks or weekly in patients with metastatic or recurrent squamous cell cancer of the head and neck: an Eastern Cooperative Oncology Group study

Ixabepilone is a tubulin-polymerizing agent with potential activity in squamous cell carcinoma of the head and neck (SCCHN). Patients were eligible who had incurable, measurable SCCHN and less than two prior regimens for metastatic/recurrent disease. Eastern Cooperative Oncology Group performance status of less than or equal to one and adequate renal/hepatic/hematological function were required. Patients were randomly assigned to receive ixabepilone 6 mg/m(2)/day x 5 days every 21 days (arm A) or 20 mg/m(2) on days 1, 8, and 15 of a 28-day cycle (arm B). Each arm accrued taxane-naive and -exposed strata in a two-stage design. The primary end point was response. Eighty-five eligible patients entered; there was one response in a taxane-exposed patient among 32 patients on arm A. Five of 35 taxane-naive patients on arm B had partial responses (14%). No taxane-exposed patient on arm B responded. Common grades 3 and 4 toxic effects were fatigue, neutropenia, and sensory/motor neuropathy. Median survival for arm A taxane-naive and taxane-exposed patients is 5.6 and 6.5 months; for arm B, taxane-naive and taxane-exposed patients is 7.8 and 6.5 months. Weekly ixabepilone 20 mg/m(2) is active in taxane-naive patients with SCCHN. A high incidence of motor and sensory grade 3 neuropathy resulted at this dose and schedule. Further development of ixabepilone in previously treated head and neck cancer is not warranted on the basis of these data.

Mutations in alpha-tubulin confer dinitroaniline resistance at a cost to microtubule function

Protozoan microtubules are sensitive to disruption by dinitroanilines, compounds that kill intracellular Toxoplasma gondii parasites without affecting microtubules in vertebrate host cells. We previously isolated a number of resistant Toxoplasma lines that harbor mutations to the alpha1-tubulin gene. Some of the mutations are localized in or near the M and N loops, domains that coordinate lateral interactions between protofilaments. Other resistance mutations map to a computationally identified binding site beneath the N loop. Allelic replacement of wild-type alpha1-tubulin with the individual mutations is sufficient to confer dinitroaniline resistance. Some mutations seem to increase microtubule length, suggesting that they increase subunit affinity. All mutations are associated with replication defects that decrease parasite viability. When parasites bearing the N loop mutation Phe52Tyr are grown without dinitroaniline selection, they spontaneously acquired secondary mutations in the M loop (Ala273Val) or in an alpha-tubulin-specific insert that stabilizes the M loop (Asp367Val). Parasites with the double mutations have both reduced resistance and diminished incidence of replication defects, suggesting that the secondary mutations decrease protofilament affinity to increase parasite fitness.

Gene expression profiles as biomarkers for the prediction of chemotherapy drug response in human tumour cells

Genome profiling approaches such as cDNA microarray analysis and quantitative reverse transcription polymerase chain reaction are playing ever-increasing roles in the classification of human cancers and in the discovery of biomarkers for the prediction of prognosis in cancer patients. Increasing research efforts are also being directed at identifying set of genes whose expression can be correlated with response to specific drugs or drug combinations. Such genes hold the prospect of tailoring chemotherapy regimens to the individual patient, based on tumour or host gene expression profiles. This review outlines recent advances and challenges in using genome profiling for the identification of tumour or host genes whose expression correlates with response to chemotherapy drugs both in vitro and in clinical studies. Genetic predictors of response to a variety of anticancer agents are discussed, including the anthracyclines, taxanes, topoisomerase I and II inhibitors, nucleoside analogs, alkylating agents, and vinca alkaloids.

beta-Tubulin mutations are associated with resistance to 2-methoxyestradiol in MDA-MB-435 cancer cells

2-Methoxyestradiol is an estradiol metabolite with significant antiproliferative and antiangiogenic activity independent of estrogen receptor status. To identify a molecular basis for acquired 2-methoxyestradiol resistance, we generated a stable 2-methoxyestradiol-resistant (2ME2R) MDA-MB-435 human cancer cell line by stepwise exposure to increasing 2-methoxyestradiol concentrations. 2ME2R cells maintained in the presence of the drug and W435 cells maintained in the absence of the drug showed 32.34- to 40.07-fold resistance to 2-methoxyestradiol. Cross-resistance was observed to Vinca alkaloids, including vincristine, vinorelbine, and vinblastine (4.29- to 6.40-fold), but minimal resistance was seen to colchicine-binding agents including colchicine, colcemid, and AVE8062A (1.72- to 2.86-fold). No resistance was observed to paclitaxel and epothilone B, polymerizing agents (0.89- to 1.14-fold). Genomic sequencing identified two different heterozygous point mutations in the class I (M40) isotype of beta-tubulin at amino acids 197 (Dbeta197N) and 350 (Kbeta350N) in 2ME2R cells. Tandem mass spectrometry confirmed the presence of both wild-type and the mutant beta-tubulin in 2ME2R cells at the protein level. Consistently, treatment of parental P435 cells with 2-methoxyestradiol resulted in a dose-dependent depolymerization of microtubules, whereas 2ME2R cells remained unaffected. In contrast, paclitaxel affected both cell lines. In the absence of 2-methoxyestradiol, 2ME2R cells were characterized by an elevated level of detyrosination. Upon 2-methoxyestradiol treatment, levels of acetylated and detyrosinated tubulins decreased in P435 cells, while remaining constant in 2ME2R cells. These results, together with our structure-based modeling, show a tight correlation between the antitubulin and antiproliferative effects of 2-methoxyestradiol, consistent with acquired tubulin mutations contributing to 2-methoxyestradiol resistance.

Molecular bases for sensitivity to tubulin-binding herbicides in green foxtail

We investigated the molecular bases for resistance to several classes of herbicides that bind tubulins in green foxtail (Setaria viridis L. Beauv.). We identified two alpha- and two beta-tubulin genes in green foxtail. Sequence comparison between resistant and sensitive plants revealed two mutations, a leucine-to-phenylalanine change at position 136 and a threonine-to-isoleucine change at position 239, in the gene encoding alpha2-tubulin. Association of mutation at position 239 with herbicide resistance was demonstrated using near-isogenic lines derived from interspecific pairings between green foxtail and foxtail millet (Setaria italica L. Beauv.), and herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Association of mutation at position 136 with herbicide resistance was demonstrated using herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Both mutations were associated with recessive cross resistance to dinitroanilines and benzoic acids, no change in sensitivity to benzamides, and hypersensitivity to carbamates. Using three-dimensional modeling, we found that the two mutations are adjacent and located into a region involved in tubulin dimer-dimer contact. Comparison of three-dimensional alpha-tubulin models for organisms with contrasted sensitivity to tubulin-binding herbicides enabled us to propose that residue 253 and the vicinity of the side chain of residue 251 are critical determinants for the differences in herbicide sensitivity observed between organisms, and that positions 16, 24, 136, 239, 252, and 268 are involved in modulating sensitivity to these herbicides.

A beta-tubulin mutation selectively uncouples nuclear division and cytokinesis in Tetrahymena thermophila

The ciliated protozoan Tetrahymena thermophila contains two distinct nuclei within a single cell-the mitotic micronucleus and the amitotic macronucleus. Although microtubules are required for proper division of both nuclei, macronuclear chromosomes lack centromeres and the role of microtubules in macronuclear division has not been established. Here we describe nuclear division defects in cells expressing a mutant beta-tubulin allele that confers hypersensitivity to the microtubule-stabilizing drug paclitaxel. Macronuclear division is profoundly affected by the btu1-1 (K350M) mutation, producing cells with widely variable DNA contents, including cells that lack macronuclei entirely. Protein expressed by the btu1-1 allele is dominant over wild-type protein expressed by the BTU2 locus. Normal macronuclear division is restored when the btu1-1 allele is inactivated by targeted disruption or expressed as a truncated protein. Immunofluorescence studies reveal elongated microtubular structures that surround macronuclei that fail to migrate to the cleavage furrows. In contrast, other cytoplasmic microtubule-dependent processes, such as cytokinesis, cortical patterning, and oral apparatus assembly, appear to be unaffected in the mutant. Micronuclear division is also perturbed in the K350M mutant, producing nuclei with elongated early-anaphase spindle configurations that persist well after the initiation of cytokinesis. The K350M mutation affects tubulin dynamics, as the macronuclear division defect is exacerbated by three treatments that promote microtubule polymerization: (i) elevated temperatures, (ii) sublethal concentrations of paclitaxel, and (iii) high concentrations of dimethyl sulfoxide. Inhibition of phosphatidylinositol 3-kinase (PI 3-kinase) with 3-methyladenine or wortmannin also induces amacronucleate cell formation in a btu1-1-dependent manner. Conversely, the myosin light chain kinase inhibitor ML-7 has no effect on nuclear division in the btu1-1 mutant strain. These findings provide new insights into microtubule dynamics and link the evolutionarily conserved PI 3-kinase signaling pathway to nuclear migration and/or division in Tetrahymena.

Cross-resistance studies of isogenic drug-resistant breast tumor cell lines support recent clinical evidence suggesting that sensitivity to paclitaxel may be strongly compromised by prior doxorubicin exposure

Less than half of breast cancer patients respond to second-line chemotherapy with paclitaxel after failing treatment with anthracyclines such as doxorubicin. A recent clinical trial by Paridaens et al. [J. Clin. Oncol. 18 : 724-733, 2000] examined whether patients may derive a better clinical benefit if paclitaxel was administered before doxorubicin. While overall survival was similar regardless of the order of drug administration, a >4-fold reduction in the response rate to paclitaxel was observed after late crossover from doxorubicin, compared to the response rate to doxorubicin after late crossover from paclitaxel. This may be related to differences in the ability of the drugs to induce cross-resistance to each other. To test this hypothesis, we examined whether isogenic breast tumor cells selected for resistance to doxorubicin exhibit greater cross-resistance to paclitaxel and other drugs than identical cells selected for resistance to paclitaxel. We found that cells selected for resistance to paclitaxel showed strong resistance (>/=40-fold) to paclitaxel and docetaxel, with little cross-resistance (4-fold) to doxorubicin. In contrast, cells selected for resistance to doxorubicin exhibited 50-fold resistance to doxorubicin and a dramatic 4700-fold and 14,600-fold cross-resistance to paclitaxel and docetaxel, respectively. Doxorubicin-resistant cells exhibited higher P-glycoprotein and breast cancer resistance protein (BCRP) levels than paclitaxel-resistant cells. In addition, procaspase-9 was strongly downregulated in doxorubicin-resistant cells but not in paclitaxel-resistant cells. These differences may account for the contrasting cross-resistance profiles observed for the two cell lines and may help to explain why treatment of breast cancer patients with paclitaxel appears to be compromized by prior doxorubicin exposure.

Dinitroanilines bind alpha-tubulin to disrupt microtubules

Protozoan parasites are remarkably sensitive to dinitroanilines such as oryzalin, which disrupt plant but not animal microtubules. To explore the basis of dinitroaniline action, we isolated 49 independent resistant Toxoplasma gondii lines after chemical mutagenesis. All 23 of the lines that we examined harbored single point mutations in alpha-tubulin. These point mutations were sufficient to confer resistance when transfected into wild-type parasites. Several mutations were in the M or N loops, which coordinate protofilament interactions in the microtubule, but most of the mutations were in the core of alpha-tubulin. Docking studies predict that oryzalin binds with an average affinity of 23 nM to a site located beneath the N loop of Toxoplasma alpha-tubulin. This binding site included residues that were mutated in several resistant lines. Moreover, parallel analysis of Bos taurus alpha-tubulin indicated that oryzalin did not interact with this site and had a significantly decreased, nonspecific affinity for vertebrate alpha-tubulin. We propose that the dinitroanilines act through a novel mechanism, by disrupting M-N loop contacts. These compounds also represent the first class of drugs that act on alpha-tubulin function.

Mutations in α-tubulin promote basal body maturation and flagellar assembly in the absence of δ-tubulin

We have isolated suppressors of the deletion allele of δ-tubulin, uni3-1, in the biflagellate green alga Chlamydomonas reinhardtii. The deletion of δ-tubulin produces cells that assemble zero, one or two flagella and have basal bodies composed primarily of doublet rather than triplet microtubules. Flagellar number is completely restored in the suppressed strains. Most of the uni3-1 suppressors map to the TUA2 locus, which encodes α2-tubulin. Twelve independent tua2 mutations were sequenced. Amino acids D205 or A208, which are nearly invariant residues in α-tubulin, were altered. The tua2 mutations on their own have a second phenotype - they make the cells colchicine supersensitive. Colchicine supersensitivity itself is not needed for suppression and colchicine cannot phenocopy the suppression. The suppressors partially restore the assembly of triplet microtubules. These results suggest that the δ-tubulin plays two roles: it is needed for extension or stability of the triplet microtubule and also for early maturation of basal bodies. We suggest that the mutant α-tubulin promotes the early maturation of the basal body in the absence of δ-tubulin, perhaps through interactions with other partners, and this allows assembly of the flagella.

Dinitroaniline herbicides against protozoan parasites: the case of Trypanosoma cruzi

The drugs presently in use against Chagas disease are very toxic, inducing a great number of side effects. Alternative treatments are necessary, not only for Chagas disease but also for other diseases caused by protozoan parasites where current drugs pose toxicity problems. The plant microtubule inhibitor trifluralin has previously been tested with success against Leishmania, Trypanosoma brucei and several other protozoan parasites. Trypanosoma cruzi, the causative agent of Chagas disease, is also sensitive to the drug. This sensitivity has been correlated with the deduced amino acid sequences of alpha- and beta-tubulin of T. cruzi as compared with plant, mammal and other parasite sequences.

Extragenic bypass suppressors of mutations in the essential gene BLD2 promote assembly of basal bodies with abnormal microtubules in Chlamydomonas reinhardtii

bld2-1 mutant Chlamydomonas reinhardtii strains assemble basal bodies with singlet microtubules; bld2-1 cells display flagellar assembly defects as well as positioning defects of the mitotic spindle and cleavage furrow. To further understand the role of the BLD2 gene, we have isolated three new bld2 alleles and three partially dominant extragenic suppressors, rgn1-1, rgn1-2, and rgn1-3. bld2 rgn1-1 strains have phenotypes intermediate between those of bld2 and wild-type strains with respect to flagellar number, microtubule rootlet organization, cleavage furrow positioning, and basal body structural phenotypes. Instead of the triplet microtubules of wild-type cells, bld2 rgn1-1 basal bodies have mixtures of no, singlet, doublet, and triplet microtubules. The bld2-4 allele was made by insertional mutagenesis and identified in a noncomplementation screen in a diploid strain. The bld2-4 allele has a lethal phenotype based on mitotic segregation in diploid strains and in haploid strains generated by meiotic recombination. The lethal phenotype in haploid strains is suppressed by rgn1-1; these suppressed strains have similar phenotypes to other bld2 rgn1-1 double mutants. It is likely that BLD2 is an essential gene that is needed for basal body assembly and function.

Pharmacological and genetic evidence for a role of rootlet and phycoplast microtubules in the positioning and assembly of cleavage furrows in Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, specialized cytoskeletal structures known as rootlet microtubules are present throughout interphase and mitosis. During cytokinesis, an array of microtubules termed the phycoplast is nucleated from rootlet microtubules and forms coincidentally with the cleavage furrow [Johnson and Porter, 1968: J. Cell Biol. 38:403-425; Holmes and Dutcher, 1989: J. Cell Sci. 94:273-285; Gaffel and el-Gammel, 1990: Protoplasma 156:139-148; Schibler and Huang, 1991: J. Cell Biol. 113:605-614]. We have obtained two independent lines of evidence that support the hypothesis that the rootlet and phycoplast microtubules play a direct role in cleavage furrow placement and assembly. First, the destabilization of spindle and phycoplast microtubules by pharmacological agents was accompanied by the aberrant distribution of actin and a failure of cytokinesis. Second, we characterized mutant strains that failed to complete cytokinesis properly. Actin and myosin were mislocalized to additional rootlet microtubules in the cyt2-1 strain, and this mislocalization was correlated with the presence of additional cleavage furrows. This evidence suggests that microtubules are necessary for the correct positioning and assembly of functional cleavage furrows in C. reinhardtii.

An insertional mutant of Chlamydomonas reinhardtii with defective microtubule positioning

cmu1-1 is a new mutation of Chlamydomonas reinhardtii that causes a change in cell shape due to an alteration of cytoplasmic microtubule organization. cmu1 mutant cells were first identified based on their altered cell shape. Unlike wild-type cells, which are ellipsoid, cmu1 cells tend to be either round or egg-shaped with the flagella extending from the narrow end of the cell. Electron microscopic comparison of mutant and wild-type cells indicated that microtubule distribution was altered in the mutant cells. Immunofluorescence microscopy using anti-beta-tubulin antibodies revealed that, in wild-type cells, microtubules arise from the anterior end of the cell in the region of the basal bodies, pass posteriorly subjacent to the plasma membrane, and terminate near the posterior end of the cell. In mutant cells, the microtubules also arise from the basal body region but then become disarrayed. They frequently curl back anteriorly or wrap around the equator of the cell; some microtubules also extend completely to the posterior end of the cell, then turn back toward the anterior end. No changes in the basal body region were detected by electron microscopy. Some cmu1 cells had multiple nuclei or an aberrant number of flagella, both of which may be due to defects in cell division, a process dependent upon microtubules. Thus, cmu1-1, which was generated by insertional mutagenesis and is tagged, appears to encode a protein that plays an essential role in the spatial organization of cytoplasmic microtubules involved in both interphase and mitotic functions.

Double mutation in eleusine indica alpha-tubulin increases the resistance of transgenic maize calli to dinitroaniline and phosphorothioamidate herbicides

The repeated use of dinitroaniline herbicides on the cotton and soybean fields of the southern United States has resulted in the appearance of resistant biotypes of one of the world's worst weeds, Eleusine indica. Two biotypes have been characterized, a highly resistant (R) biotype and an intermediate resistant (I) biotype. In both cases the resistance has been attributed to a mutation in alpha-tubulin, a component of the alpha/beta tubulin dimer that is the major constituent of microtubules. We show here that the I-biotype mutation, like the R-biotype mutation shown in earlier work, can confer dinitroaniline resistance on transgenic maize calli. The level of resistance obtained is the same as that for E. indica I- or R-biotype seedlings. The combined I- and R-biotype mutations increase the herbicide tolerance of transgenic maize calli by a value close to the summation of the maximum herbicide tolerances of calli harbouring the single mutations. These data, taken together with the position of the two different mutations within the atomic structure of the alpha/beta tubulin dimer, imply that each mutation is likely to exert its effect by a different mechanism. These mechanisms may involve increasing the stability of microtubules against the depolymerizing effects of the herbicide or changing the conformation of the alpha/beta dimer so that herbicide binding is less effective, or a combination of both possibilities.

Dinitroaniline herbicide resistance and the microtubule cytoskeleton

Dinitroaniline herbicides have been used for pre-emergence weed control for the past 25 years in cotton, soybean, wheat and oilseed crops. Considering their long persistence and extensive use, resistance to dinitroanilines is fairly rare. However, the most widespread dinitroaniline-resistant weeds, the highly resistant (R) and the intermediate (I) biotypes of the invasive goosegrass Eleusine indica, are now infesting more than 1000 cotton fields in the southern states of the USA. The molecular basis of this resistance has been identified, and found to be a point mutation in a major microtubule cytoskeletal protein, alpha-tubulin. These studies have served both to explain the establishment of resistance and to reveal fundamental properties of tubulin gene expression and microtubule structure.

Molecular characterization of four beta-tubulin genes from dinitroaniline susceptible and resistant biotypes of Eleusine indica

Dinitroaniline herbicides are antimicrotubule drugs that bind to tubulins and inhibit polymerization. As a result of repeated application of dinitroaniline herbicides, resistant biotypes of goosegrass (Eleusine indica) developed in previously susceptible wild-type populations. We have previously reported that alpha-tubulin missense mutations correlate with dinitroaniline response phenotypes (Drp) (Plant Cell 10: 297-308, 1998). In order to ascertain associations of other tubulins with dinitroaniline resistance, four beta-tubulin cDNA classes (designated TUB1, TUB2, TUB3, and TUB4) were isolated from dinitroaniline-susceptible and -resistant biotypes. Sequence analysis of the four beta-tubulin cDNA classes identified no missense mutations. Identified nucleotide substitutions did not result in amino acid replacements. These results suggest that the molecular basis of dinitroaniline resistance in goosegrass differs from those of colchicine/dinitroaniline cross-resistant Chlamydomonas reinhardtii and benzimidazole-resistant fungi and yeast. Expression of the four beta-tubulins was highest in inflorescences. This is in contrast to alpha-tubulin TUA1 that is expressed predominantly in roots. Collectively, these results imply that beta-tubulin genes are not associated with dinitroaniline resistance in goosegrass. Phylogenetic analysis of the four beta-tubulins, together with three alpha-tubulins, suggests that the resistant biotype developed independently in multiple locations rather than spreading from one location.

Alpha-tubulin missense mutations correlate with antimicrotubule drug resistance in Eleusine indica

Dinitroaniline herbicides are antimicrotubule drugs that bind to tubulins and inhibit polymerization. As a result of repeated application of dinitroaniline herbicides, highly resistant and intermediately resistant biotypes of goosegrass (Eleusine indica) developed in previously wild-type populations. Three alpha-tubulin cDNA classes (designated TUA1, TUA2, and TUA3) were isolated from each biotype. Nucleotide differences between the susceptible and the resistant (R) alpha-tubulins were identified in TUA1 and TUA2. The most significant differences were missense mutations that occurred in TUA1 of the R and intermediately resistant (I) biotypes. Such mutations convert Thr-239 to Ile in the R biotype and Met-268 to Thr in the I biotype. These amino acid substitutions alter hydrophobicity; therefore, they may alter the dinitroaniline binding property of the protein. These mutations were correlated with the dinitroaniline response phenotypes (Drp). Plants homozygous for susceptibility possessed the wild-type TUA1 allele; plants homozygous for resistance possessed the mutant tua1 allele; and plants heterozygous for susceptibility possessed both wild-type and mutant alleles. Thus, we conclude that TUA1 is at the Drp locus. Using polymerase chain reaction primer-introduced restriction analysis, we demonstrated that goosegrass genomic DNA can be diagnosed for Drp alleles. Although not direct proof, these results suggest that a mutation in an alpha-tubulin gene confers resistance to dinitroanilines in goosegrass.

Alpha-tubulin missense mutations correlate with antimicrotubule drug resistance in Eleusine indica

Dinitroaniline herbicides are antimicrotubule drugs that bind to tubulins and inhibit polymerization. As a result of repeated application of dinitroaniline herbicides, highly resistant and intermediately resistant biotypes of goosegrass (Eleusine indica) developed in previously wild-type populations. Three alpha-tubulin cDNA classes (designated TUA1, TUA2, and TUA3) were isolated from each biotype. Nucleotide differences between the susceptible and the resistant (R) alpha-tubulins were identified in TUA1 and TUA2. The most significant differences were missense mutations that occurred in TUA1 of the R and intermediately resistant (I) biotypes. Such mutations convert Thr-239 to Ile in the R biotype and Met-268 to Thr in the I biotype. These amino acid substitutions alter hydrophobicity; therefore, they may alter the dinitroaniline binding property of the protein. These mutations were correlated with the dinitroaniline response phenotypes (Drp). Plants homozygous for susceptibility possessed the wild-type TUA1 allele; plants homozygous for resistance possessed the mutant tua1 allele; and plants heterozygous for susceptibility possessed both wild-type and mutant alleles. Thus, we conclude that TUA1 is at the Drp locus. Using polymerase chain reaction primer-introduced restriction analysis, we demonstrated that goosegrass genomic DNA can be diagnosed for Drp alleles. Although not direct proof, these results suggest that a mutation in an alpha-tubulin gene confers resistance to dinitroanilines in goosegrass.

Multiple centrosomal microtubule organising centres and increased microtubule stability are early features of VP-16-induced apoptosis in CCRF-CEM cells

Microtubular reorganisation contributing to apoptotic morphology occurs in normal and neoplastic cells undergoing apoptosis induced by cytotoxic drugs [1-3]. The aim of this study was to correlate the changes in the microtubules (MTs) with behavior of the centrosome in apoptotic cells, and to see whether post-translational changes in tubulin occurred with the emergence of apoptotic MT bands. Apoptosis was induced in the human T-cell leukaemia line (CCRF-CEM) by treatment with 17 microM etoposide over a 4 h period. The time course of changes was assessed using flow cytometry (FCM) and immunocytochemistry in cells labelled for a centrosomal antigen (CSP-alpha) or alpha-tubulins. One hour following treatment we observed multiple centrosomal microtubule organising centres (MTOCs) associated with the nucleus and the transient appearance of a subset of stable MTs detected with an antibody specific for acetylated alpha-tubulin, as the bands of MTs which lobulate the nucleus are formed. The altered properties of the MTs thus reflect changes in function as apoptosis progresses.

Microtubule-dependent multilobular organization of the nucleus in sensitive and multidrug-resistant L0 leukemia cells

The relationship between the nuclear morphology and the microtubular organization of L100 and L1000 cells, two vincristine-induced multidrug resistant human acute lymphocytic leukemia cell lines, was examined and compared to that of L0 parental cells. The L0 parental cells contained a round nucleus and the microtubules were evenly distributed throughout the cytoplasm. In contrast, the microtubules of the L100 and L1000 cells were localized between the lobular structures of a multilobulated nucleus. Disassembly of microtubules in L100 and L1000 cells by colchicine resulted in the loss of the multilobulated morphology of the nucleus. While the total cellular content of tubulin of L0 and L100 cells was similar, the content of microtubules of L100 cells was only 55% of that observed in L0 cells. Two, 28 kDa (pI 6.9) and 31 kDa (pI 4.4), microtubule-associated proteins were found to be overexpressed in L100 and L1000 cells. The results indicate that the multilobulated nuclear morphology of L100 and L1000 cells is dependent upon the unique and intact organization of the microtubules; the distinct organization of the microtubules and the multilobular nuclear morphology of the two resistant cell lines may be due to the differential expression of specific microtubule-associated proteins.

Increased tubulin acetylation accompanies reversion to stable ploidy in vincristine-resistant CCRF-CEM cells

The T-cell leukemia line CCRF-CEM is unstable with respect to ploidy, whereas a vincristine-resistant subline, CEM/VCR R, maintains a stable pseudodiploid karyotype. Ploidy change in the parental cells requires the involvement of two cell cycle lesions. The first, in mitosis, prevents cell division after S-phase. The second, in G1, allows a cell with 4N DNA content to re-enter S-phase. We examined differences in expression of tubulin, a major component of the mitotic spindle and the cellular target for vincristine, between the two cell lines. Levels of the beta III isotype were decreased and levels of acetylated alpha-tubulin, a marker for microtubule stability, were increased in the CEM/VCR R cells relative to the parental line, which suggests that the CEM/VCR R cells have a more stable mitotic spindle. Both cell lines exhibit some level of constitutive expression of p53 and c-myc. Constitutive expression of and mutant p53 would contribute to the failure of these cells to recognise G1 checkpoints. Therefore, G1 checkpoint failure and the intrinsically less stable mitotic spindle in the CCRF-CEM cells may contribute to the observed ploidy instability. Conversely, the presence of markers of microtubule stability in the CEM/VCR R cells would predispose them to maintain their ploidy.

Loss of spatial control of the mitotic spindle apparatus in a Chlamydomonas reinhardtii mutant strain lacking basal bodies

The bld2-1 mutation in the green alga Chlamydomonas reinhardtii is the only known mutation that results in the loss of centrioles/basal bodies and the loss of coordination between spindle position and cleavage furrow position during cell division. Based on several different assays, bld2-1 cells lack basal bodies in > 99% of cells. The stereotypical cytoskeletal morphology and precise positioning of the cleavage furrow observed in wild-type cells is disrupted in bld2-1 cells. The positions of the mitotic spindle and of the cleavage furrow are not correlated with respect to each other or with a specific cellular landmark during cell division in bld2-1 cells. Actin has a variable distribution during mitosis in bld2-1 cells, but this aberrant distribution is not correlated with the spindle positioning defect. In both wild-type and bld2-1 cells, the position of the cleavage furrow is coincident with a specialized set of microtubules found in green algae known as the rootlet microtubules. We propose that the rootlet microtubules perform the functions of astral microtubules and that functional centrioles are necessary for the organization of the cytoskeletal superstructure critical for correct spindle and cleavage furrow placement in Chlamydomonas.

Gene-specific signal transduction between microtubules and tubulin genes in Tetrahymena thermophila

Mammalian cells regulate tubulin mRNA abundance by a posttranscriptional mechanism dependent on the concentration of tubulin monomer. Treatment of mammalian cells with microtubule-depolymerizing drugs and microtubule-polymerizing drugs causes decreases and increases in tubulin mRNA, respectively (D. W. Cleveland, Curr. Opin. Cell Biol. 1:10-14, 1989). In striking contrast to the case with mammalian cells, perturbation of microtubules in Tetrahymena thermophila by microtubule-depolymerizing or -polymerizing drugs increases the level of the single alpha-tubulin gene message by increasing transcription (L. A. Stargell, D. P. Heruth, J. Gaertig, and M. A. Gorovsky, Mol. Cell. Biol. 12:1443-1450, 1992). In this report we show that antimicrotubule drugs preferentially induce the expression of one of two beta-tubulin genes (BTU1) in T. thermophila. In contrast, deciliation induces expression of both beta-tubulin genes. Tubulin gene expression was examined in a mutant strain created by transformation with an in vitro-mutagenized beta-tubulin gene that conferred resistance to microtubule-depolymerizing drugs and sensitivity to the polymerizing drug taxol and in a strain containing a nitrosoguanidine-induced mutation in the single alpha-tubulin gene that conferred the same pattern of drug sensitivities. In both cases the levels of tubulin mRNA expression from the drug-inducible BTU1 gene in the mutant cells paralleled the altered growth sensitivities to microtubule drugs. These studies demonstrate that T. thermophila has distinct, gene-specific mechanisms for modulating tubulin gene expression depending on whether ciliary or cytoplasmic microtubules are involved. They also show that the cytoplasmic microtubule cytoskeleton itself participates in a signal transduction pathway that regulates specific tubulin gene transcription in T. thermophila.

Cytoskeleton and PCH-induced pigment aggregation in Macrobrachium potiuna erythrophores

Herein we report the effects of microtubule- and actin-like filament disrupting drugs, as well as the microtubule stabilizer taxol, on PCH-induced pigment granule aggregation within erythrophores of the freshwater crustacean Macrobrachium potiuna. Dose-response curves (DRCs) to the pigment-concentrating hormone PCH were determined under control and experimental conditions to evaluate the effects elicited by the cytoskeleton-affecting drugs. Colchicine, at temperatures 22 degrees C and 4 degrees C, and vinblastine significantly inhibited the aggregating response to PCH and affected the dynamics of the process, as shown by the change in the slope of the regression curve calculated from the DRCs. Lumicolchicine, a colchicine analogue with no affinity for tubulin, also inhibited pigment migration, though no change in the slope of the regression curve was observed. The inhibitory effects of lumicolchicine demonstrate that changes in sites other than cytoskeleton, such as membrane permeability, may also cause a decrease in the PCH-induced aggregating responses and that the colchicine effects may result from its action on cellular sites additional to the cytoskeleton. Taxol, a microtubule stabilizer, did not affect the DRC to PCH, and DMSO improved the PCH-evoked responses, pointing out to the maintenance of tubulin in the polymerized state as the appropriate condition for aggregation. Cytochalasin B, an actin-like filament disrupter, diminished the aggregating responses to the hormone, with no change in the slope of the regression curve, indicating that these elements take part in the process and that cytosolic calcium rise, sol/gel transformations and endoplasmic reticulum motility may well play an important role in granule migration. It is suggested that microtubules are steadily polymerized as a requirement for pigment aggregation and that process is biphasic, the initial phase being dependent on the microtubule integrity.

Mutations in the Caenorhabditis elegans beta-tubulin gene mec-7: effects on microtubule assembly and stability and on tubulin autoregulation

We have sequenced 45 mutations in mec-7, a beta-tubulin gene required for the production of 15-protofilament microtubules in the nematode Caenorhabditis elegans, and have correlated sequence alterations with mutant phenotypes. The expression patterns of most alleles have also been determined by in situ hybridization and immunocytochemistry. Most (12/16) complete loss-of-function alleles, which are recessive, result from nonsense mutations, insertions, or deletions; three others disrupt a putative GTP-binding domain. Three of the four loss-of-function, missense mutations result in elevated mec-7 message levels, suggesting a defect in tubulin autoregulation that may be attributable to a loss in the ability to form heterodimers. Most (8/9) mild alleles are caused by missense mutations. Two mild alleles appear to increase microtubule stability and lead to the elaboration of ectopic neuronal processes in mec-7-expressing cells. Most (15/23) mutations that cause severe dominant or semidominant phenotypes are clustered into three discrete domains; four others occur in putative GTP-binding regions. Many of these dominant mutations appear to completely disrupt microtubule assembly.

Radiation and taxol effects on synchronized human cervical carcinoma cells

PURPOSE

To evaluate the effectiveness of the plant derived chemotherapeutic agent taxol alone and in combination with ionizing radiation on synchronous and asynchronous human cervical carcinoma cells and to define the mechanistic basis for this cytotoxic response.

METHODS AND MATERIALS

Asynchronous and synchronous cells (obtained by modified mitotic shake-off) derived from carcinomas of the human uterine cervix were treated with a range of concentrations of taxol (0, 1.0, 2.5, 5.0, 10.0 and 20.0 nM) for either 8, 24 or 48 h. Synchronized cell cycling was evaluated by counting mitotic indices and by uptake of bromodeoxyuridine (BrdUrd). Cells were irradiated (137Cs gamma rays at 1.12 Gy/min) alone and after taxol treatment and plating efficiencies and radiosensitivity determined.

RESULTS

Taxol treatment resulted in a dose time dependent loss of colony forming ability with 10 nM for 24 h producing about 10% cell survival. Irradiating taxol treated cells resulted in a strictly additive response in contrast to previous supra-additive results with astrocytoma and melanoma cells. Mitotically synchronized cells rapidly moved into G1 phase with a second mitotic peak at 28 h (total cycle time). Taxol treatment resulted in a continued accumulation of mitoses, and a failure and/or delay of entry of a fraction of cells into S phase after a G1 phase of at least 10 h. That is, taxol effects cell cycling at a stage other than G2/M. Irradiating (3 Gy) synchronized cells showed a 10-fold variation in sensitivity, with mitosis as the most sensitive phase with taxol alone resulting in some cytotoxicity and combined effects additive or less than additive.

CONCLUSION

Taxol effects these cervical carcinoma cells at other stages of the cell cycle than G2/M. This may explain the failure to obtain taxol radiosensitization with these cells and it may indicate that taxol has a multiplicity of actions with differences in effectiveness likely between cells of different origins.

Electroporation-mediated replacement of a positively and negatively selectable beta-tubulin gene in Tetrahymena thermophila

Replacement of lysine-350 by methionine in the beta-tubulin gene of Chlamydomonas confers resistance to microtubule-depolymerizing drugs and increased sensitivity to the microtubule-stabilizing drug taxol. This mutation was created in cloned BTU1, one of two coexpressed beta-tublin genes of Tetrahymena thermophila. When introduced by electroporation, the mutated gene transformed Tetrahymena exclusively by gene replacement at the homologous locus. Taxol-sensitive transformants could be retransformed with a wild-type gene and selection for taxol resistance. Analyses of phenotypic assortment and of the mRNA in transformed cells suggest that complete replacement of the BTU1 gene in the polyploid macronucleus can be obtained. These studies demonstrate the utility of this marker for studying tublin gene function and show that electroporation allows facile gene replacement in Tetrahymena.

Competitive Inhibition of High-Affinity Oryzalin Binding to Plant Tubulin by the Phosphoric Amide Herbicide Amiprophos-Methyl

Amiprophos-methyl (APM), a phosphoric amide herbicide, was previously reported to inhibit the in vitro polymerization of isolated plant tubulin (L.C. Morejohn, D.E. Fosket [1984] Science 224: 874-876), yet little other biochemical information exists concerning this compound. To characterize further the mechanism of action of APM, its interactions with tubulin and microtubules purified from cultured cells of tobacco (Nicotiana tabacum cv Bright Yellow-2) were investigated. Low micromolar concentrations of APM depolymerized preformed, taxol-stabilized tobacco microtubules. Remarkably, at the lowest APM concentration examined, many short microtubules were redistributed into fewer but 2.7-fold longer microtubules without a substantial decrease in total polymer mass, a result consistent with an end-to-end annealing of microtubules with enhanced kinetic properties. Quasi-equilibrium binding measurements showed that tobacco tubulin binds [14C]oryzalin with high affinity to produce a tubulin-oryzalin complex having a dissociation constant (Kd) = 117 nM (pH 6.9; 23[deg]C). Also, an estimated maximum molar binding stoichiometry of 0.32 indicates pharamacological heterogeneity of tobacco dimers and may be related to structural heterogeneity of tobacco tubulin subunits. APM inhibits competitively the binding of [14C]oryzalin to tubulin with an inhibition constant (Ki) = 5 [mu]M, indicating the formation of a moderate affinity tubulin-APM complex that may interact with the ends of microtubules. APM concentrations inhibiting tobacco cell growth were within the threshold range of APM concentrations that depolymerized cellular microtubules, indicating that growth inhibition is caused by microtubules depolymerization. APM had no apparent effect on microtubules in mouse 3T3 fibroblasts. Because cellular microtubules were depolymerized at APM and oryzalin concentrations below their respective Ki and Kd values, both herbicides are proposed to depolymerize microtubules by a substoichiometric endwise mechanism.