Taxol differentially modulates the dynamics of microtubules assembled from unfractionated and purified beta-tubulin isotypes

Drug resistance associated with loss of p53 involves extensive alterations in microtubule composition and dynamics

In the present study, we compared the dynamics and composition of microtubules in cell lines derived from the human breast adenocarcinoma MCF-7 containing either the wild-type p53 (wt-p53; MN1) or a dominant-negative variant of p53 gene (mut-p53; MDD2). Mut-p53 cells were significantly resistant to the cytotoxicity of the microtubule-targeted drugs (vinca alkaloids and taxanes), as compared with wt-p53 cells. Studies by high-resolution time-lapse fluorescence microscopy in living cells indicated that the dynamics of microtubules of mut-p53 cells were altered in complex ways and were significantly increased as compared with microtubules in wt-p53 cells. The percentage of time microtubules spent in growing and shortening phases increased significantly, their catastrophe frequency increased, and their overall dynamicity increased by 33%. In contrast, their shortening rate and the mean length shortened decreased. Cells containing mut-p53 displayed increased polymerisation of tubulin, increased protein levels of the class IV beta-tubulin isotype, STOP and survivin, and reduced protein levels of class II beta-tubulin isotype, MAP4 and FHIT. We conclude that p53 protein may contribute to the regulation of microtubule composition and function, and that alterations in p53 function may generate complex microtubule-associated mechanisms of resistance to tubulin-binding agents.

Class III beta-tubulin in human development and cancer

The differential cellular expression of class III beta-tubulin isotype (betaIII) is reviewed in the context of human embryological development and neoplasia. As compared to somatic organs and tissues, betaIII is abundant in the central and peripheral nervous systems (CNS and PNS) where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar and sympathoadrenal neurogenesis, the distribution of betaIII is neuron-associated, exhibiting distinct temporospatial gradients according to the regional neuroepithelia of origin. However, transient expression of this protein is also present in the subventricular zones of the CNS comprising putative neuronal- and/or glial precursor cells, as well as in Kulchitsky neuroendocrine cells of the fetal respiratory epithelium. This temporally restricted, potentially non-neuronal expression may have implications in the identification of presumptive neurons derived from embryonic stem cells. In adult tissues, the distribution of betaIII is almost exclusively neuron-specific. Altered patterns of expression are noted in cancer. In "embryonal"- and "adult-type" neuronal tumors of the CNS and PNS, betaIII is associated with neuronal differentiation and decreased cell proliferation. In contrast, the presence of betaIII in gliomas and lung cancer is associated with an ascending histological grade of malignancy. Thus, betaIII expression in neuronal tumors is differentiation-dependent, while in non-neuronal tumors it is aberrant and/or represents "dedifferentiation" associated with the acquisition of progenitor-like phenotypic properties. Increased expression in various epithelial cancer cell lines is associated with chemoresistance to taxanes. Because betaIII is present in subpopulations of neoplastic, but not in normal differentiated glial or somatic epithelial cells, the elucidation of mechanisms responsible for the altered expression of this isotype may provide insights into the role of the microtubule cytoskeleton in tumorigenesis and tumor progression.

IDN 5390: a new concept in taxane development

IDN 5390 is a seco-derivative cytostatic taxane. Originally selected for its ability to affect endothelial cell motility, the anti-angiogenic properties of IDN 5390 have been documented in experimental models, in vivo and in vitro. Preclinical studies indicate that, in vivo, oral IDN 5390 has a favorable bioavailability, is well tolerated and shows a significant anti-neoplastic activity on a panel of different tumor models, including paclitaxel-resistant tumors. According to its cytostatic rather than cytotoxic nature, frequent administrations of non-toxic doses have proven to be the optimal schedule for IDN 5390 treatment. Preliminary findings suggest the use of this compound in combination with conventional anti-neoplastic therapy. IDN 5390 can be considered the prototype of a new class of well-tolerated, orally available anti-angiogenic taxane derivatives with cytostatic properties.

Fast kinetics of Taxol binding to microtubules. Effects of solution variables and microtubule-associated proteins

The kinetics of Taxol association to and dissociation from stabilized microtubules has been measured by competition with the reference fluorescent derivative Flutax-1 (Diaz, J. F., Strobe, R., Engelborghs, Y., Souto, A. A., and Andreu, J. M. (2000) J. Biol. Chem. 275, 26265-26276). The association rate constant at 37 degrees C is k(+) = (3.6 +/- 0.1) x 10(6) m(-1) s(-1). The reaction profile is similar to that of the first step of Flutax-1 binding, which probably corresponds to the binding of the Taxol moiety. The rate constant of the initial binding of Flutax-1 is inversely proportional to the viscosity of the solution, which is compatible with a diffusion-controlled reaction. Microtubule-associated proteins bound to the microtubule outer surface slow down the binding of Flutax-1 and Flutax-2 10-fold. The binding site is fully accessible to Flutax-2 in native cytoskeletons of PtK2 cells; the observed kinetic rates of Flutax-2 microtubule staining and de-staining are similar to the reaction rates with microtubule associated proteins-containing microtubules. The kinetic data prove that taxoids bind directly from the bulk solution to an exposed microtubule site. Several hypotheses have been analyzed to potentially reconcile these data with the location of a Taxol-binding site at the model microtubule lumen, including dynamic opening of the microtubule wall and transport from an initial Taxol-binding site at the microtubule pores.

Different effects of vinblastine on the polymerization of isotypically purified tubulins from bovine brain

Vinblastine, a highly successful antitumor drug, targets the tubulin molecule. Tubulin, the subunit protein of microtubules, consists of an alpha- and a beta-subunit, both of which consist of isotypes encoded by different genes. We have purified three isotypes of bovine brain tubulin, namely, alpha(beta)II, alpha(beta)III and alpha(beta)IV. Microtubule associated protein-2 (MAP2) and Tau-induced assembly of these isotypes were compared in the presence and absence of vinblastine. MAP2-induced assembly of unfractionated tubulin and all the isotypes except alpha(beta)II tubulin was resistant to 1 microM vinblastine. Vinblastine at low concentrations (< 10 microM) progressively inhibited the assembly of all of the isotypes but the vinblastine concentration required for inhibition of MAP2-induced microtubule assembly was minimal for alpha(beta)II. The tau-induced assembly of unfractionated tubulin and alpha(beta)III were equally sensitive to 1 microM vinblastine whereas alpha(beta)II and alpha(beta)IV were much more sensitive to vinblastine. The microtubules obtained in the presence of tau from unfractionated tubulin, alpha(beta)II and alpha(beta)IV could be easily aggregated by 20 microM vinblastine whereas such as aggregation of microtubules obtained from alpha(beta)III and tau required approximatedly 40 microM vinblastine. Our results suggest that among the tubulin isotypes, alpha(beta)II is the most sensitive to vinblastine in the presence of MAPs while alpha(beta)III is the most resistant and this intrinsic resistance of alpha(beta)III dimers persists in the polymeric form of alpha(beta)III tubulin as well. These results may be relevant to the therapeutic and toxic actions of vinblastine.

Interdependent effect of P-glycoprotein-mediated drug efflux and intracellular drug binding on intracellular paclitaxel pharmacokinetics: application of computational modeling

Intracellular concentration of paclitaxel is determined by the extracellular drug concentration, the level of the mdr1 P-glycoprotein (Pgp), and binding to intracellular proteins including tubulins/microtubules. The present study used a computational method to examine the effects of these factors, singly and in combination, on intracellular paclitaxel pharmacokinetics. The study was performed using our previously described intracellular pharmacokinetic model. The parameters representing Pgp-mediated drug efflux and intracellular drug binding (i.e., number of Pgp and binding sites and binding affinity) were altered systematically and used to generate computer simulations depicting the intracellular paclitaxel pharmacokinetics at clinically relevant extracellular (e.g., plasma) drug concentrations. The simulation results indicate that all four factors played a role in determining the intracellular drug accumulation. The rank order of the importance of these parameters was extracellular drug concentration >> intracellular binding capacity > intracellular binding affinity > Pgp expression. The results further showed that omission of one or more of these factors in the experimental design would lead to erroneous conclusions on the importance of other factors, as simultaneous changes in more than one parameter altered the relative importance and offset the effects of other parameters. In summary, results of the present study demonstrate the use of computational modeling to depict the effects of biological parameters such as drug efflux transporters, drug binding sites, and binding affinity on intracellular accumulation and retention of drugs that bind to cellular components.

The two alpha-tubulin isotypes in budding yeast have opposing effects on microtubule dynamics in vitro

The yeast Saccharomyces cerevisiae has two genes for alpha-tubulin, TUB1 and TUB3, and one beta-tubulin gene, TUB2. The gene product of TUB3, Tub3, represents approximately 10% of alpha-tubulin in the cell. We determined the effects of the two alpha-tubulin isotypes on microtubule dynamics in vitro. Tubulin was purified from wild-type and deletion strains lacking either Tub1 or Tub3, and parameters of microtubule dynamics were examined. Microtubules containing Tub3 as the only alpha-tubulin isotype were less dynamic than wild-type microtubules, as shown by a shrinkage rate and catastrophe frequency that were about one-third of that for wild-type microtubules. Conversely, microtubules containing Tub1 as the only alpha-tubulin isotype were more dynamic than wild-type microtubules, as shown by a shrinkage rate that was 50% higher and a catastrophe frequency that was 30% higher than those of wild-type microtubules. The results suggest that a role of Tub3 in budding yeast is to control microtubule dynamics.

Coordination of posttranslational modifications of bovine brain alpha-tubulin. Polyglycylation of delta2 tubulin

Microtubules participate in a large number of intracellular events including cell division, intracellular transport and secretion, axonal transport, and maintenance of cell morphology. They are composed of tubulin, a heterodimeric protein, consisting of two similar polypeptides alpha and beta. In mammalian cells, both alpha- and beta-tubulin occur as seven to eight different genetic variants, which also undergo numerous posttranslational modifications that include tyrosination-detyrosination and deglutamylation, phosphorylation, acetylation, polyglutamylation, and polyglycylation. Tyrosination-detyrosination is one of the major posttranslational modifications in which the C-terminal tyrosine residue in alpha-tubulin is added or removed reversibly. Although this modification does not alter the assembly activity of tubulin in vitro, these two forms of tubulin have been found to be distributed differently in vivo and are also correlated with microtubule stability (Gunderson, G. G., Kalnoski, M. H., and Bulinski, J. C. (1984) Cell 38, 779-789). Thus, the question arises as to whether these two forms of tubulin differ in any other modifications. In an effort to answer this question, the tyrosinated and the nontyrosinated forms of the alpha1/2 isoform have been purified from brain tubulin by immunoaffinity chromatography. matrix-assisted laser desorption/ionization-time of flight mass spectrometric analysis of the C-terminal peptide revealed that the tyrosinated form is polyglutamylated with one to four Glu residues, while the Delta2 tubulin is polyglycylated with one to three Gly residues. These results indicate that posttranslational modifications of tubulin are correlated with each other and that polyglutamylation and polyglycylation of tubulin may have important roles in regulating microtubule assembly, stability, and function in vivo.

Microtubule structure at 8 A resolution

We have obtained a 3D reconstruction of intact microtubules, using cryoelectron microscopy and image processing, at a resolution of about 8 A, sufficient to resolve much of the secondary structure. The level of detail in the map allows docking of the tubulin structure previously determined by electron crystallography, with very strong constraints, providing several important insights not previously available through docking tubulin into lower-resolution maps. This work provides an improved picture of the interactions between adjacent protofilaments, which are responsible for microtubule stability, and also suggests that some structural features are different in microtubules from those in the zinc sheets with which the tubulin structure was determined.

Concentration dependence of variability in growth rates of microtubules

Growth and shortening of microtubules in the course of their polymerization and depolymerization have previously been observed to occur at variable rates. To gain insight into the meaning of this prominent variability, we studied the way in which its magnitude depends on the growth rate of experimentally observed and computer-simulated microtubules. The dynamic properties of plus-ended microtubules nucleated by pieces of Chlamydomonas flagellar axonemes were observed in real time by video-enhanced differential interference contrast light microscopy at differing tubulin concentrations. By means of a Monte Carlo algorithm, populations of microtubules were simulated that had similar growth and dynamic properties to the experimentally observed microtubules. By comparison of the experimentally observed and computer-simulated populations of microtubules, we found that 1) individual microtubules displayed an intrinsic variability that did not change as the rate of growth for a population increased, and 2) the variability was approximately fivefold greater than predicted by a simple model of subunit addition and loss. The model used to simulate microtubule growth has no provision for incorporation of lattice defects of any type, nor sophisticated geometry of the growing end. Thus, these as well as uncontrolled experimental variables were eliminated as causes for the prominent variability.

Characterization of nuclear betaII-tubulin in tumor cells: a possible novel target for taxol

As the subunits of microtubules, alpha- and beta-tubulins have been thought to only exist in the cytoplasm where they are incorporated into microtubules. However, the beta(II) isotype of tubulin has recently been observed in the nuclei of rat kidney mesangial cells [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284]. In this study, we detected nuclear beta(II)-tubulin in rat C6 glioma cells, human T98G glioma cells, human MCF-7 breast carcinoma cells, human MDA-MB-435 breast carcinoma cells, and human Hela cervix carcinoma cells. In addition, nuclear beta(II)-tubulin in these cells was found to exist as alphabeta(II) dimers instead of assembled microtubules and appeared to be particularly concentrated in the nucleoli. Several anti-tubulin drugs were used to treat C6 cells to determine their influence on nuclear beta(II)-tubulin. Taxol, a tubulin drug with higher specificity for beta(II)-tubulin than for other beta-tubulin isotypes, irreversibly decreased nuclear beta(II) content in a concentration-dependent manner in C6 cells. Meanwhile, cells were found to be apoptotic as was suggested by the presence of multiple micronuclei and DNA fragmentation. On the other hand, no depletion of nuclear beta(II)-tubulin was observed when C6 cells were incubated with colchicine or nocodazole, two anti-tubulin drugs with higher specificity for the alphabeta(IV) isotype, supporting the hypothesis that drugs with higher specificity for beta(II)-tubulin deplete nuclear beta(II)-tubulin.

Noncovalent dimerization of paclitaxel in solution: evidence from electrospray ionization mass spectrometry

Paclitaxel, a unique antimitotic chemotherapy agent that inhibits cell division by binding to microtubules and prevents them from "depolymerizing," has received widespread interest because of its efficacy in fighting certain types of cancer, including breast and ovarian cancer. Paclitaxel undergoes aggregation at millimolar concentrations in both aqueous media and solvents of low polarity (mimicking hydrophobic environments). Its aggregation may have impact on its aqueous stability and its ability to stabilize microtubules. Here, we investigated the dimerization phenomenon of paclitaxel by electrospray ionization mass spectrometry (ESI-MS). Paclitaxel dimers were stable in solutions of acetonitrile/aqueous ammonium acetate (80/20) and aqueous sodium acetate/acetonitrile (92/8 or 95/5) at various pH values. Additional experiments using solution-phase hydrogen/deuterium exchange were employed to ascertain whether or not the observed dimers were formed in solution or as an artifact of the ESI process by ion-molecule reaction. The evidence supports formation of the dimer in solution, and the approach used can be extended to investigation of other types of drug-drug interactions.

Mutational analysis of the class I beta-tubulin gene in human breast cancer

Non-small cell lung cancers have a high incidence of somatic mutations of the beta-tubulin (class I) gene, suggesting involvement in the acquisition of resistance to taxanes, which exert their effects through binding to beta-tubulin. Since taxanes are often used in the treatment of breast cancer, we carried out a mutational analysis of the class I beta-tubulin (GenBank accession AF070600) gene in breast cancer. We paid special attention to the primer design so as not to amplify the pseudogenes. We identified 1 somatic mutation, codon 306 [Arg (CGC) to Cys (TGC)], and 2 genetic polymorphisms, codon 217 [Leu (CTG) to Leu (CTA)] and (C to T) at 57 bases downstream from exon 4. Our results suggest that acquisition of resistance to taxanes is unlikely to be explained by somatic mutations of the class I beta-tubulin gene in most breast cancers. In addition, the overestimation of the incidence of somatic mutations of the class I beta-tubulin gene due to the pseudogenes is discussed.

Increased levels of tyrosinated alpha-, beta(III)-, and beta(IV)-tubulin isotypes in paclitaxel-resistant MCF-7 breast cancer cells

Paclitaxel (PTX), the diterpene alkaloid, is a potent anti-cancer drug and is routinely used for the treatment of breast and ovarian cancers. The cellular targets of PTX are microtubules, which are composed of alpha- and beta-tubulin. Development of PTX resistance in patients has been a major problem associated with cancer chemotherapy. In an effort to get insight into this phenomenon of drug resistance, a PTX-resistant cell line from MCF-7 breast cancer cells has been generated. Western analysis of the cell extracts revealed that the resistant cells contain 2-fold higher amount of tyrosinated alpha-tubulin than those of the wild-type MCF-7 cells. Similar analyses of beta-tubulin with the isotype-specific monoclonal antibodies demonstrated that the PTX-resistant cells contain 2.5-fold higher amounts of beta(III) and 1.5-fold higher amount of beta(IV)-tubulin, while no difference was observed in the level of beta(I) isotype. These results demonstrate for the first time that PTX resistance is associated with an increase in the level of tyrosinated alpha-tubulin.

Low HER2/neu gene expression is associated with pathological response to concurrent paclitaxel and radiation therapy in locally advanced breast cancer

PURPOSE

The objective of this study was twofold: first, to identify patients with locally advanced breast cancer (LABC) who will achieve a pathological response to a preoperative regimen of concurrent paclitaxel and radiation; and second, to explore associations between molecular markers from the original tumors and pathological response.

METHODS AND MATERIALS

Patients with previously untreated LABC were eligible to receive a regimen of preoperative concurrent paclitaxel, 30 mg/m(2) twice a week for a total of 8 weeks, and radiation delivered Weeks 2--6, 45 Gy at 1.8 Gy per fraction to the breast, ipsilateral axilla, and supraclavicular nodes. At mastectomy, pathologic findings were classified as pathological complete response (pCR) = no residual invasive cells in the breast and axillary contents; pathological partial response (pPR) = presence of < or = 10 microscopic foci of invasive cells; no pathological response (pNR) = pathological persistence of tumor. For each patient, pretreatment breast cancer biopsies were prospectively analyzed by immunohistochemistry (IHC) for estrogen and progesterone (ER/PR) hormonal receptors, HER2/neu and p53 overexpression. Estrogen receptor (ER), HER2/neu, metablastin, beta-tubulin III and IV, microtubule-associated protein-4 (MAP-4), bcl-2, bax, and cyclooxygenase-2 (COX-2) gene expression were measured using real-time quantitative polymerase chain reaction (PCR).

RESULTS

A total of 36 patients had pretreatment biopsies and were evaluable for the analysis of the association of molecular markers with pathological response. Pathological response in the mastectomy specimen was achieved in 12 of these 36 patients (33%). Only HER2/neu and ER gene expression were found to be significantly associated with the extent of pathological response to the regimen, i.e., tumors with low HER2/neu gene expression and negative estrogen receptors were more likely to respond to the tested regimen (p = 0.009 and p = 0.006, respectively). Conversely, p53 protein expression measured by IHC did not appear to be associated with pathological response (p = 0.67).

CONCLUSION

Further studies in LABC should assess whether patient selection for treatment based on the original tumor molecular characteristics could affect their chance to achieve a pathological response.

Characterization of isotype-specific regions of five classes of canine beta-tubulin and their expression in several tissues and cell culture

The structure of isotype-specific regions of classes 1, II, III, IVa and IVb of canine beta-tubulin was characterized by 3'-RACE and the expression of these isotypes in canine tissues was examined by ribonuclease protection assay (RPA). Furthermore, a malignant mammary tumor-derived osteosarcoma-like cell line was established and the altered expression of beta-tubulin isotypes in taxol-resistant sublines was analyzed. The deduced amino acid sequences in isotype-specific regions corresponding to classes I, II and IVb were identical to those of humans and mice, but those in classes III and IVa showed slight differences among species. RPA revealed that classes I and IVb were widely distributed, but classes II, III and IVa were restricted to the brain. Because RPA could clearly distinguish the expression of class IVa from that of class IVb, it was thought to be more useful than northern blot for analysis of beta-tubulin isotype expression. In vitro, taxol-resistant sublines displayed a significant increase in class IVa as compared with taxol-sensitive cells, suggesting that altered expression of class IVa was associated with taxol resistance in these cell lines.

Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents that stabilize microtubules. Although 12,13-epoxide derivatives are potent antiproliferative agents, the activities of the corresponding 12,13-olefin analogs are significantly decreased. These data were confirmed for two new analogs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the natural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpressing NCI/Adr cells. By using tritiated pEB/pED, compound uptake, release, and nuclear accumulation were investigated in A431 and NCI/Adr cells. In these cells, epothilones can principally be recognized and exported by Verapamil-sensitive efflux pumps, which are not identical to MDR1. The degree of export depends on the structure, olefin vs. epoxide-analog, and also on the intracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 microM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCI/Adr (2-fold) and not in A431 cells. In addition, strong nuclear accumulation was observed for pEB (40-50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study suggests that differences in growth inhibitory efficacy between epoxide and olefin analogs may be based on different mechanisms of drug accumulation and subcellular distribution.

Resistance to Taxol in lung cancer cells associated with increased microtubule dynamics

Microtubule dynamics are crucial for mitotic spindle assembly and chromosome movement. Suppression of dynamics by Taxol appears responsible for the drug's potent ability to inhibit mitosis and cell proliferation. Although Taxol is an important chemotherapeutic agent, development of resistance limits its efficacy. To examine the role of microtubule dynamics in Taxol resistance, we measured the dynamic instability of individual rhodamine-labeled microtubules in Taxol-sensitive and -resistant living human cancer cells. Taxol-resistant A549-T12 and -T24 cell lines were selected from a human lung carcinoma cell line, A549. They are, respectively, 9- and 17-fold resistant to Taxol and require low concentrations of Taxol for proliferation. We found that microtubule dynamic instability was significantly increased in the Taxol-resistant cells. For example, with A549-T12 cells in the absence of added Taxol, microtubule dynamicity increased 57% as compared with A549 cells. The length and rate of shortening excursions increased 75 and 59%, respectively. These parameters were further increased in A549-T24 cells, with overall dynamicity increasing by 167% compared with parental cells. Thus, the decreased Taxol-sensitivity of these cells can be explained by their increased microtubule dynamics. When grown without Taxol, A549-T12 cells were blocked at the metaphase/anaphase transition and displayed abnormal mitotic spindles with uncongressed chromosomes. In the presence of 2-12 nM Taxol, the cells grew normally, suggesting that mitotic block resulted from excessive microtubule dynamics. These results indicate that microtubule dynamics play an important role in Taxol resistance, and that both excessively rapid dynamics and suppressed dynamics impair mitotic spindle function and inhibit proliferation.

Modulation of endogenous beta-tubulin isotype expression as a result of human beta(III)cDNA transfection into prostate carcinoma cells

Increases of individual beta tubulin isotypes in antimicrotubule drug resistant cell lines have been reported by several laboratories. We have previously described elevations in beta(III)and beta(IVa)isotypes in estramustine and paclitaxel resistant human prostate carcinoma cells. To investigate further the function of beta tubulin isotypes in antimicrotubule drug response, human prostate carcinoma cells that normally have very low to undetectable levels of beta(III)were stably transfected with beta(III)cDNA in pZeoSV system. An 18 bp haemagglutinin (HA) epitope tag was added at the 3' end prior to cloning into the vector. Cells were transfected with pZeoSV or pZeoSV-beta(III)plasmids and selected in the presence of Zeocin. Immunofluorescent staining of the transfectant cells have shown significant expression and incorporation of HA-tagged beta(III)tubulin into cellular microtubules. Quantitation of Western blots revealed the HA-tagged beta(III)levels to be approximately 7-fold higher than the vector control cells. RT-PCR analysis confirmed the increase at the transcript level and also revealed a collateral increase of beta(II)and beta(IVb)transcripts. Cell viability assays indicated that sensitivity of beta(III)transfected cells to various antimicrotubule agents was similar to vector transfected cells: IC50 values for estramustine, paclitaxel, colchicine and vinblastine were 4 microM, 4 nM, 22 nM and 2 nM, respectively for both cell lines. Thus, overexpression of beta(III)isotype in human prostate carcinoma cells by stable transfection failed to confer antimicrotubule drug resistance to these cells. Counterregulatory increases of endogenous beta(II)and beta(IVb)tubulin isotypes in these beta(III)transfected cells may be a compensatory mechanism used by the cells to overcome the effects of elevated beta(III)levels on the cellular microtubules. These results highlight the difficulty in isolating the contribution of single tubulin isotypes in drug response studies.

Structural insight into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of alpha/beta-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Tubulins in the primate retina: evidence that xanthophylls may be endogenous ligands for the paclitaxel-binding site

The xanthophylls-lutein, zeaxanthin, and meso-zeaxanthin (L&Z)-are found in the central region of the primate retina, which is called the macula lutea (yellow spot). How they are anchored there and what their function is has been debated for over 50 years. Here, we present evidence that they may be bound to the paclitaxel (Taxol) binding site of the beta-tubulin subunit of microtubules and that a major function may be to modulate the dynamic instability of microtubules in the macula. Also, we compare nucleic acid and amino acid sequences of tubulins that are in human brain with those we have isolated from human-retina and monkey-macula cDNA libraries. In so doing, we suggest that in primates, class I beta-tubulin consists of at least two subtypes (beta(Ia) and beta(Ib)). Alignment analysis of the sequences of the genes for beta(Ia) and beta(Ib) indicates that the corresponding mRNAs may have other functions in addition to that of coding for proteins. Furthermore, we show that there are at least five different types of beta-tubulin in the macula lutea of rhesus monkey.

Paclitaxel and docetaxel in prostate cancer

Although their ultimate value in prostate cancer therapy remains to be defined in randomized trials, docetaxel and paclitaxel are active agents in HRPC. Combination therapies using either of these taxanes plus oral EMP show reproducible antitumor activity that appears to be greater and more durable than that of single-agent treatment. Although the optimal combination and schedule have not been determined, weekly paclitaxel and EMP and docetaxel given every 3 weeks or by weekly infusion with EMP are useful treatment options for patients with progressive HRPC. The gastrointestinal toxicity of EMP has been reduced by intermittent rather than continuous administration, and other toxicities may be reduced further by use of intravenous EMP. Although there has been progress, the median time to progression of 5 to 6 months for current taxane-based therapies suggests that they will not have major impact on overall survival for patients with HRPC. Greater benefit may be possible earlier in the course of prostate cancer, and the activity of the taxane-EMP combinations is sufficient to justify clinical trials of adjuvant or neoadjuvant chemotherapy for selected groups of patients with locally advanced and poor-prognosis tumors. Armed with many new molecularly targeted agents that may interact favorably with taxanes, it should be possible to build on current antimicrotubule regimens to improve activity in HRPC. Taxane-EMP combinations provide a platform on which to test additional agents that may enhance the apoptotic response or circumvent cellular stress adaptations that confer drug resistance. Further elucidation of signaling pathways that regulate microtubule dynamics and programmed cell death after exposure to microtubule inhibitors would provide a more rational guide for integrating specific inhibitors of signal transduction with current taxane-based therapies. Pharmacokinetic and pharmacodynamic studies will play a key role in the development of future taxane-based therapies for prostate cancer.

Aberrant localization of the neuronal class III beta-tubulin in astrocytomas

BACKGROUND

The class III beta-tubulin isotype (betaIII) is widely regarded as a neuronal marker in development and neoplasia. In previous work, we have shown that the expression of betaIII in neuronal/neuroblastic tumors is differentiation dependent. In contrast, the aberrant localization of this isotype in certain nonneuronal neoplasms, such as epithelial neuroendocrine lung tumors, is associated with anaplastic potential.

OBJECTIVE

To test the generality of this observation, we investigated the immunoreactivity profile of betaIII in astrocytomas.

DESIGN

Sixty archival, surgically excised astrocytomas (8 pilocytic astrocytomas, WHO grade 1; 18 diffuse fibrillary astrocytomas, WHO grade 2; 4 anaplastic astrocytomas, WHO grade 3; and 30 glioblastomas, WHO grade 4), were studied by immunohistochemistry using anti-betaIII monoclonal (TuJ1) and polyclonal antibodies. A monoclonal antibody to Ki-67 nuclear antigen (NC-MM1) was used as a marker for cell proliferation. Antibodies to glial fibrillary acidic protein (GFAP) and BM89 synaptic vesicle antigen/synaptophysin were used as glial and neuronal markers, respectively.

RESULTS

The betaIII immunoreactivity was significantly greater in high-grade astrocytomas (anaplastic astrocytomas and glioblastomas; median labeling index [MLI], 35%; interquartile range [IQR], 20%-47%) as compared with diffuse fibrillary astrocytomas (MLI, 4%; IQR, 0.2%-21%) (P <.0001) and was rarely detectable in pilocytic astrocytomas (MLI, 0%; IQR, 0%-0.5%) (P <.0001 vs high-grade astrocytomas; P <.01 vs diffuse fibrillary astrocytomas). A highly significant, grade-dependent relationship was observed between betaIII and Ki-67 labeling and malignancy, but this association was stronger for Ki-67 than for betaIII (betaIII, P <.006; Ki-67, P <.0001). There was co-localization of betaIII and GFAP in neoplastic astrocytes, but no BM89 synaptic vesicle antigen/synaptophysin staining was detected.

CONCLUSIONS

In the context of astrocytic gliomas, betaIII immunoreactivity is associated with an ascending gradient of malignancy and thus may be a useful ancillary diagnostic marker. However, the significance of betaIII-positive phenotypes in diffuse fibrillary astrocytomas with respect to prognostic and predictive value requires further evaluation. Under certain neoplastic conditions, betaIII expression is not neuron specific, calling for a cautious interpretation of betaIII-positive phenotypes in brain tumors.

Structural insights into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of /¿-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

HLA-B2702 (77-83/83-77) peptide binds to beta-tubulin on human NK cells and blocks their cytotoxic capacity

It has been described that peptides derived from a highly conserved region of the alpha1 helix of the first domain of HLA class I Ags exhibit immunomodulatory capacity blocking both T and NK cell cytotoxicity. In vivo treatment with these peptides prolongs survival of MHC-mismatched allografts. However, the molecular bases of these effects are still unclear. In this study, we further analyze the mechanisms by which the dimeric peptide HLA-B2702 (77-83/83-77) induces suppression of NK cell cytotoxicity. This peptide inhibits natural and redirected lysis mediated by NK cells without significantly affecting effector-target cell binding. We have also isolated and sequenced a protein that binds this inhibitory peptide, which structurally corresponds to beta-tubulin. Tubulin is the major protein of microtubules and is involved in target cell killing. Furthermore, B2702 peptide promotes GTP-independent tubulin assembly, producing aggregates that cannot be depolymerized by cold. Treatment of NK cells with Taxol or demecolcine, which interfere with microtubule organization, also prevents NK cell cytotoxicity. Taken together, these results support the hypothesis that the peptide B2702 (77-83/83-77) exerts its inhibitory effect on NK cell cytotoxicity by inducing polymerization of microtubules and interfering with their normal assembly/disassembly dynamics.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Energetics of vinca alkaloid interactions with tubulin isotypes: implications for drug efficacy and toxicity

A number of vinca alkaloids, including vincristine, vinblastine, and vinorelbine, are currently used in cancer chemotherapy. These three vinca alkaloids interact differently with a range of solid and hematologic tumors. To test the possibility that the tubulin isotype composition is an important determinant in antineoplastic efficacy, we determined thermodynamic parameters for vinca alkaloid interactions with purified beta-tubulin isotypes, alphabetaII or alphabetaIII, as well as mixtures of alphabetaII and alphabetaIII, alphabetaII and alphabetaI&IV, or alphabetaIII and alphabetaI&IV (referred to as isotype-depleted tubulin) by quantitative sedimentation velocity. Vincristine-, vinblastine-, or vinorelbine-induced isotype self-association was studied at 25 degrees C in 10 mM Pipes, pH 6.9, 1 mM MgSO4, and 2 mM EGTA in the presence of 50 microM GTP or GDP. For all three drugs, we observed no significant differences in overall affinities, K1K2, or in GDP enhancement of purified isotypes compared to unfractionated tubulin, suggesting that differential antitumor efficacy observed clinically for these vinca alkaloids is not determined by tissue isotype composition. Small, but significant differences in the individual binding parameters, K1 and K2, are found in the vincristine data. In the presence of vincristine and GTP, K1, the affinity of drug for tubulin heterodimers, tends to be larger for purified alphabetaII- or alphabetaIII-tubulin compared to unfractionated tubulin. Furthermore, the apparent dimerization constant, K2app, at physiologically significant drug concentrations is larger for these purified isotypes. When alphabetaII- and alphabetaIII-tubulin are combined, the cooperativity between drug binding and spiral formation approaches that of unfractionated PC-tubulin. These differences are not observed in the presence of vinblastine or vinorelbine. The differences found with vincristine may be implicated in the dose-limiting neurotoxicity found with this drug, but not found with vinblastine or vinorelbine.

Presence of the betaII isotype of tubulin in the nuclei of cultured mesangial cells from rat kidney

Tubulin has generally been considered to be a cytosolic protein whose only function is to form microtubules. This assumption is supported by a great deal of evidence derived from immunohistochemical studies using antibodies directed against whole tubulin or its component polypeptides alpha- and beta-tubulin. We have re-examined the intracellular distribution of tubulin using monoclonal antibodies specific for the betaI, betaII, betaIII, and betaIV isotypes of beta-tubulin. Our test system is the cultured rat kidney mesangial cell. We have found that betaIII is absent from these cells and that beta1 and betaIV are present in microtubules throughout the cytosol. In contrast, betaII is present largely in the nuclei. Immunoblotting of purified nuclear extracts shows that the betaII-reactive antigen co-migrates with beta-tubulin. Extraction of the cytosol and chromatin suggests that betaII is concentrated in the nucleoli and also in a reticulated network in the rest of the nucleoplasm. An antibody to tyrosinated alpha-tubulin shows that alpha is also present in the nucleoli. Treatment of the cells with fluorescent colchicine shows an accumulation of colchicine in the nucleoli. Finally, fluorescently labeled alphabetaII-tubulin dimers, when microinjected into the cells, enter the nuclei and are concentrated in the nucleoli. These results suggest that the betaII isotype of tubulin is present as an alphabetaII dimer in the nuclei of cultured mesangial cells and suggest the possibility that different tubulin isotypes may have specific functions within the cell.

Expression of oncogenic epidermal growth factor receptor family kinases induces paclitaxel resistance and alters beta-tubulin isotype expression

Oncogenic transformation confers resistance to chemotherapy through a variety of mechanisms, including suppression of apoptosis, increased drug metabolism, and modification of target proteins. Oncogenic epidermal growth factor receptor family members, including EGFRvIII and HER2, are expressed in a broad spectrum of human malignancies. Cell lines transfected with EGFRvIII and HER2 are more resistant to paclitaxel-mediated cytotoxicity, and tubulin polymerization induced by paclitaxel is suppressed compared with cells expressing wild type epidermal growth factor receptor. Because differential expression of beta-tubulin isotypes has been proposed to modulate paclitaxel resistance, we analyzed beta-tubulin isotypes expressed in cell lines transfected with different oncogenes. EGFRvIII- and HER2-expressing cells demonstrated equivalent total beta-tubulin protein compared with cells transfected with wild type receptor or untransfected controls. EGFRvIII-expressing cells demonstrated increases in class IVa (2.5-fold) and IVb (3.1-fold) mRNA, and HER2-expressing cells showed increases in class IVa (2. 95-fold) mRNA. Expression of oncogenic Ha-Ras did not change class IV RNA levels significantly. Inhibition of EGFRvIII kinase activity using a mutant allele with an inactivating mutation in the kinase domain decreased expression of class IVa by 50% and partially reversed resistance to paclitaxel. Expression of oncogenic epidermal growth factor receptor family members is associated with modulation of both beta-tubulin isotype expression and paclitaxel resistance in cells transformed by expression of the receptor. This effect on tubulin expression may modulate drug resistance in human malignancies that express these oncogenes.

Molecular mechanisms of resistance to taxanes and therapeutic implications

The mechanism of resistance to taxanes has not been fully elucidated. Since Taxol is a substrate for P-glycoprotein, overexpression of this transport system is recognized as a relevant mechanism of resistance. Additional mechanisms include changes of microtubule structure and/or composition resulting in reduced drug binding to the target. Current efforts are directed at clarifying the role of cellular response to drug-induced damage to cytoskeleton and mitotic spindle. Downstream events, such as control of cell cycle progression and regulation of cell death pathways, are likely to play a relevant role in cellular sensitivity to antimicrotubule agents. The identification of resistance factors and critical determinants of antitumor efficacy of microtubule-stabilizing agents is essential to (i) improve their therapeutic efficacy; and (ii) to design non-cross-resistant compounds. The present review discusses the possible therapeutic implications of the recent progress in the field of resistance to taxanes. Copyright 1999 Harcourt Publishers Ltd.

Overexpression of class I, II or IVb beta-tubulin isotypes in CHO cells is insufficient to confer resistance to paclitaxel

Recent studies have suggested a correlation between increased expression of specific beta-tubulin isotypes and paclitaxel resistance in drug-selected cell lines. In an attempt to establish a causal link, we have transfected Chinese hamster ovary cells with cDNAs encoding epitope-tagged class I, II, and IVb beta-tubulins, as well as a class I beta-tubulin with a mutation previously characterized in a paclitaxel resistant mutant. To eliminate possible toxicity that might be associated with overexpression of non-native tubulin, each of the cDNAs was placed under the control of a tetracycline regulated promoter. All transfected cDNAs produced assembly competent tubulin whose synthesis could be turned off or on by the presence or absence of tetracycline. Production of betaI, betaII, or betaIVb tubulin had no effect on the sensitivity of the cells to paclitaxel, but production of the mutant betaI-tubulin conferred clear resistance to the drug. We conclude from these experiments that simple overexpression of class I, II, or IVb isoforms of beta-tubulin is insufficient to confer resistance to paclitaxel.

Differentiation of human colon cancer cells changes the expression of beta-tubulin isotypes and MAPs

The human colon adenocarcinoma HT29-D4 cell line is an interesting model for studies on epithelial cell differentiation. Undifferentiated cells are malignant proliferating cells, whereas differentiated cells act like epithelial polarized cells. In the present study, we first characterized the action of taxoids on the microtubular network of HT29-D4 cells according to the state of differentiation. Microtubular bundles were found in undifferentiated cells but not in differentiated cells, even with 500-fold higher taxoid concentrations for 96 h. This finding led us to study changes in microtubules according to the polarity status of the cell. E-MAP-115 was expressed only in differentiated cells; expression of beta-tubulin isotypes was altered in them relative to undifferentiated cells. Classes I, II, III, IVa and IVb isotypes were expressed in both phenotypes; however, differentiated epithelial cells displayed a specific increase in class III beta-tubulin. Thus, the increase in expression of this beta-tubulin isotype in differentiated cells is not restricted to neuronal cells. Moreover, these expression changes may reflect a higher stability of microtubular network in differentiated cells, which may explain the lower activity of anti-microtubule agents, independently of the mitotic process. These results indicate that the composition of microtubules should be considered as one of the criteria involved in the response of tumour cells to chemotherapy with anti-microtubule agents.

Antisense oligonucleotides to class III beta-tubulin sensitize drug-resistant cells to Taxol

A major impediment to the successful use of Taxol in the treatment of cancer is the development of drug resistance. The major cellular target of Taxol is the microtubule that is comprised of alpha- and beta-tubulin heterodimers. Binding sites for Taxol have been delineated on the beta-tubulin subunit that has six isotypes. We have recently described increased expression of the brain-specific human class III beta-tubulin isotype, encoded by the Hbeta4 gene, in both Taxol-resistant ovarian tumours and non-small-cell lung cancer cell lines. To evaluate directly the role of the class III beta-tubulin isotype in mediating Taxol resistance, antisense phosphorothioate oligodeoxynucleotides (ODN) targeted against various regions of the Hbeta4 gene have been designed and examined for their efficacy in reducing Hbeta4 gene and protein expression. Taxol-resistant lung cancer cells, A549-T24, which are 17-fold resistant to Taxol and display a fourfold increase in Hbeta4 expression compared to the parental A549 cells, were treated with 1 microM antisense ODNs. Two ODNs, AS1 and AS3, were found to reduce mRNA expression by 40-50%, as determined by reverse transcription polymerase chain reaction. A concentration-dependent reduction in Hbeta4 mRNA expression was demonstrated with AS1 ODN. Immunofluorescence staining of cells treated with AS1 ODN revealed a decrease in class III protein expression which corresponded to a 39% increase in sensitivity to Taxol (P < 0.005). These findings support an important role for Hbeta4 (class III) beta-tubulin expression in Taxol resistance and have potential implications for the treatment of Taxol-resistant tumours.

Taxol suppresses dynamics of individual microtubules in living human tumor cells

Microtubules are intrinsically dynamic polymers, and their dynamics play a crucial role in mitotic spindle assembly, the mitotic checkpoint, and chromosome movement. We hypothesized that, in living cells, suppression of microtubule dynamics is responsible for the ability of taxol to inhibit mitotic progression and cell proliferation. Using quantitative fluorescence video microscopy, we examined the effects of taxol (30-100 nM) on the dynamics of individual microtubules in two living human tumor cell lines: Caov-3 ovarian adenocarcinoma cells and A-498 kidney carcinoma cells. Taxol accumulated more in Caov-3 cells than in A-498 cells. At equivalent intracellular taxol concentrations, dynamic instability was inhibited similarly in the two cell lines. Microtubule shortening rates were inhibited in Caov-3 cells and in A-498 cells by 32 and 26%, growing rates were inhibited by 24 and 18%, and dynamicity was inhibited by 31 and 63%, respectively. All mitotic spindles were abnormal, and many interphase cells became multinucleate (Caov-3, 30%; A-498, 58%). Taxol blocked cell cycle progress at the metaphase/anaphase transition and inhibited cell proliferation. The results indicate that suppression of microtubule dynamics by taxol deleteriously affects the ability of cancer cells to properly assemble a mitotic spindle, pass the metaphase/anaphase checkpoint, and produce progeny.

Coalescence of Golgi fragments in microtubule-deprived living cells

The process of stack coalescence, an important mechanism of Golgi recovery from mitosis, was examined using novel experimental paradigms. In living cells with disrupted (by nocodazole) microtubules, galactosyl transferase-GFP-labelled Golgi fragments constantly appeared, grew, sometimes moved with a speed of 1-2 microns/min, coalesced or gradually diminished and disappeared. The rate of Golgi fragment turnover and coalescence was highly balanced to maintain a constant number of Golgi units per cell. Moreover some Golgi islands appear and some received new GalTase-GFP after photobleaching of cell cytoplasm. Short tubules extending from the rims of scattered Golgi fragments frequently formed bridges between ministacks, inducing their coalescence. The frequency of coalescence could also be inhibited by disruption of actin microfilaments. After the Golgi redistribution into endoplasmic reticulum induced by brefeldin A, either the growth of small Golgi fragments or their coalescence leads to compartmentalized stack formation without the participation of microtubules. These results demonstrate that this coalescence between isolated Golgi stacks is microtubule-independent and could thus be mediated by membranous tubules.

How Taxol stabilises microtubule structure

The structure of tubulin shows paclitaxel (Taxol(R)) binding to a pocket in beta tubulin on the microtubule's inner surface, which counteracts the effects of GTP hydrolysis occurring on the other side of the monomer.

Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death

PURPOSE

To analyze the available data concerning mechanisms of action of and mechanisms of resistance to the antitubulin agents, vinca alkaloids and taxanes, and more recently described compounds.

DESIGN

We conducted a review of the literature on classic and recent antitubulin agents, focusing particularly on the relationships between antitubulin agents and their intracellular target, the soluble tubulin/microtubule complex.

RESULTS AND CONCLUSION

Although it is widely accepted that antitubulin agents block cell division by inhibition of the mitotic spindle, the mechanism of action of antitubulin agents on microtubules remains to be determined. The classic approach is that vinca alkaloids depolymerize microtubules, thereby increasing the soluble tubulin pool, whereas taxanes stabilize microtubules and increase the microtubular mass. More recent data suggest that both classes of agents have a similar mechanism of action, involving the inhibition of microtubule dynamics. These data suggest that vinca alkaloids and taxanes may act synergistically as antitumor agents and may be administered as combination chemotherapy in the clinic. However, enhanced myeloid and neurologic toxicity, as well as a strong dependence on the sequence of administration, presently exclude these combinations outside the context of clinical trials. Although the multidrug resistance phenotype mediated by Pgp appears to be an important mechanism of resistance to these agents, alterations of microtubule structure resulting in altered microtubule dynamics and/or altered binding of antitubulin agents may constitute a significant mechanism of drug resistance.

Tubulin structure: insights into microtubule properties and functions

The structure of tubulin has recently been determined by electron crystallography, paving the way for a clearer understanding of the unique properties of tubulin that allow its varied functions within the cell. Some of the ongoing work on tubulin can be interpreted in terms of its structure, which can serve to guide future studies.

Cloning and sequencing of an alpha-tubulin cDNA from Artemia franciscana: evidence for translational regulation of alpha-tubulin synthesis

The brine shrimp, Artemia franciscana, exhibits a limited number of tubulin isotypes which change little during early postgastrula growth. In order to better understand the synthesis of alpha-tubulins during Artemia development, a cDNA termed alphaAT1 was cloned and sequenced. Alignment analyses revealed that the polypeptide encoded by alphaAT1 is similar to alpha-tubulins from other species. Hybridization of alphaAT1 to restriction-digested DNA on Southern blots produced a simple banding pattern, indicating that Artemia have a small number of alpha-tubulin genes. Probing of Northern blots demonstrated an abundant supply of alpha-tubulin mRNA in dormant cysts, emerging nauplii and instar I larvae. However, it was not until instar I larvae were produced that the amount of polysomal alpha-tubulin mRNA increased, suggesting that synthesis of the tubulin corresponding to alphaAT1 is translationally controlled. This work provides one of the few examples where tubulin synthesis is thought to be translationally regulated. Moreover, when considered in the light of previous analyses, the findings imply that cell differentiation in postgastrula Artemia and the diversification of microtubule function certain to accompany this process occur with little or no change in alpha-tubulin composition.

Differential assembly kinetics of alpha-tubulin isoforms in the presence of paclitaxel

The antitumor drug paclitaxel (PTX) inhibits cell growth by binding to microtubules, the eukaryotic structures consisting of alpha- and beta-tubulin. PTX also promotes the assembly to tubulin in the absence of microtubule-associated proteins. Although recent studies have implicated beta-tubulin as the site of PTX binding, no information is available that relates alpha-tubulin to the binding site. In an effort to understand whether the alpha-tubulin is involved in the drug binding, we have studied the assembly of alpha-tubulin isoforms in the presence of PTX. The assembly results in the presence of 10 microM paclitaxel (PTX) show that the isoforms assemble at differential rates. The rate of assembly for tyrosinated M alpha 1/2 is about three-fold higher than that of the nontyrosinated M alpha 1/2 isoform. Such a strikingly different assembly behavior of the alpha-tubulin isoforms indicates that alpha-tubulin may be involved in the interaction of PTX with microtubules.

Altered beta-tubulin isotype expression in paclitaxel-resistant human prostate carcinoma cells

To investigate the role of beta-tubulin isotype composition in resistance to paclitaxel, an anti-microtubule agent, human prostate carcinoma (DU-145) cells were intermittently exposed to increasing concentrations of paclitaxel. Cells that were selected and maintained at 10 nM paclitaxel (Pac-10) were fivefold resistant to the drug. Pac-10 cells accumulated radiolabelled paclitaxel to the same extent as DU-145 cells and were negative for MDR-1. Analysis of Pac-10 and DU-145 cells by flow cytometry showed similar cell cycle patterns. Immunofluorescent staining revealed an overall increase of alpha- and beta-tubulin levels in Pac-10 cells compared with DU-145 cells. Examination of beta-tubulin isotype composition revealed a significant increase in betaIII isotype in the resistant cells, both by immunofluorescence and by western blot analysis. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the isotypes confirmed the increase observed for the betaIII by exhibiting ninefold higher betaIII mRNA levels and also showed fivefold increase of the betaIVa transcript. In addition, analysis of paclitaxel-resistant cells that were selected at increasing levels of the drug (Pac 2, 4, 6, 8 and 10) exhibited a positive correlation between increasing betaIII levels and increasing resistance to paclitaxel. Increased expression of specific beta-tubulin isotypes and subsequent incorporation into microtubules may alter cellular microtubule dynamics, providing a defence against the anti-microtubule effects of paclitaxel and other tubulin-binding drugs.

Microtubules and actin filaments: dynamic targets for cancer chemotherapy

Microtubules and actin filaments play important roles in mitosis, cell signaling, and motility. Thus these cytoskeletal filaments are the targets of a growing number of anti-cancer drugs. In this review we summarize the current understanding of the mechanisms of these drugs in relation to microtubule and actin filament polymerization and dynamics. In addition, we outline how, by targeting microtubules, drugs inhibit cell proliferation by blocking mitosis at the mitotic checkpoint and inducing apoptosis. The beta-tubulin isotype specificities of new anticancer drugs and the antitumor potential of agents that act on the actin cytoskeleton are also discussed.

Cloning and sequencing of human betaIII-tubulin cDNA: induction of betaIII isotype in human prostate carcinoma cells by acute exposure to antimicrotubule agents

Antimicrotubule drugs are used as chemotherapeutic agents due to their effects on essential cellular functions such as mitosis, organelle transport and maintenance of cell shape. When used in combination, paclitaxel with estramustine or vinblastine has demonstrated activity against hormone refractory prostate cancer. To understand the mechanism of resistance that develops in patients as a result of antimicrotubule drug therapy, we exposed human prostate carcinoma cells to IC20 and IC40 doses of estramustine, paclitaxel or vinblastine for 48 h and examined the beta-tubulin (the cellular target) isotype composition. The results revealed an increase in the betaIII-tubulin isotype as a result of drug treatment both at protein and message levels. In addition, examination of human brain cell lines with different intrinsic levels of betaIII showed that cell lines with higher betaIII levels were more resistant to paclitaxel. These results are in agreement with our previous findings in human prostate carcinoma cell lines that were made resistant to estramustine or paclitaxel and suggest an important function for betaIII in antimicrotubule drug resistance. Also, the complete coding sequence of human betaIII tubulin reported here will provide molecular tools for future investigations.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Taxol-resistant epithelial ovarian tumors are associated with altered expression of specific beta-tubulin isotypes

The treatment of advanced ovarian cancer with taxol is hindered by the development of drug resistance. The cellular target for taxol is the microtubule that is stabilized by the drug. Taxol preferentially binds to the beta subunit of tubulin of which there are six distinct isotypes in mammalian cells. We have used highly specific oligonucleotides and polymerase chain reaction to analyze expression of all six beta-tubulin genes. Human lung cancer cells (A549) were selected in 12 and 24 nM taxol resulting in cell lines that were 9- and 17-fold resistant, respectively. These cells displayed an altered ratio of classes I, II, III, and IVa beta-tubulin isotypes. Ovarian tumors, seven untreated primary and four taxol- resistant tumor-bearing ascites, displayed significant increases (P < 0.005) in classes I (3.6-fold), III (4.4-fold), and IVa (7.6-fold) isotypes in the taxol-resistant samples as compared with untreated primary ovarian tumors. The increased expression appears to be related to the resistance phenotype, as the basal levels of the class III and IVa isotypes in the untreated tumors were extremely low. This is the first report of altered expression of specific beta-tubulin genes in taxol-resistant ovarian tumors and we propose that the latter may play a role in clinical resistance to taxol.