Overexpression of an epitope-tagged beta-tubulin in Chinese hamster ovary cells causes an increase in endogenous alpha-tubulin synthesis

Expanding the genetic spectrum of TUBB1-related thrombocytopenia

β1-Tubulin plays a major role in proplatelet formation and platelet shape maintenance, and pathogenic variants in TUBB1 lead to thrombocytopenia and platelet anisocytosis (TUBB1-RT). To date, the reported number of pedigrees with TUBB1-RT and of rare TUBB1 variants with experimental demonstration of pathogenicity is limited. Here, we report 9 unrelated families presenting with thrombocytopenia carrying 6 β1-tubulin variants, p.Cys12LeufsTer12, p.Thr107Pro, p.Gln423*, p.Arg359Trp, p.Gly109Glu, and p.Gly269Asp, the last of which novel. Segregation studies showed incomplete penetrance of these variants for platelet traits. Indeed, most carriers showed macrothrombocytopenia, some only increased platelet size, and a minority had no abnormalities. Moreover, only homozygous carriers of the p.Gly109Glu variant displayed macrothrombocytopenia, highlighting the importance of allele burden in the phenotypic expression of TUBB1-RT. The p.Arg359Trp, p.Gly269Asp, and p.Gly109Glu variants deranged β1-tubulin incorporation into the microtubular marginal ring in platelets but had a negligible effect on platelet activation, secretion, or spreading, suggesting that β1-tubulin is dispensable for these processes. Transfection of TUBB1 missense variants in CHO cells altered β1-tubulin incorporation into the microtubular network. In addition, TUBB1 variants markedly impaired proplatelet formation from peripheral blood CD34+ cell-derived megakaryocytes. Our study, using in vitro modeling, molecular characterization, and clinical investigations provides a deeper insight into the pathogenicity of rare TUBB1 variants. These novel data expand the genetic spectrum of TUBB1-RT and highlight a remarkable heterogeneity in its clinical presentation, indicating that allelic burden or combination with other genetic or environmental factors modulate the phenotypic impact of rare TUBB1 variants.

Exposure to low intensity ultrasound removes paclitaxel cytotoxicity in breast and ovarian cancer cells

BACKGROUND

Paclitaxel (Taxol) is a microtubule-stabilizing drug used to treat several solid tumors, including ovarian, breast, non-small cell lung, and pancreatic cancers. The current treatment of ovarian cancer is chemotherapy using paclitaxel in combination with carboplatin as a frontline agent, and paclitaxel is also used in salvage treatment as a second line drug with a dose intensive regimen following recurrence. More recently, a dose dense approach for paclitaxel has been used to treat metastatic breast cancer with success. Paclitaxel binds to beta tubulin with high affinity and stabilizes microtubule bundles. As a consequence of targeting microtubules, paclitaxel kills cancer cells through inhibition of mitosis, causing mitotic catastrophes, and by additional, not yet well defined non-mitotic mechanism(s).

RESULTS

In exploring methods to modulate activity of paclitaxel in causing cancer cell death, we unexpectedly found that a brief exposure of paclitaxel-treated cells in culture to low intensity ultrasound waves prevented the paclitaxel-induced cytotoxicity and death of the cancer cells. The treatment with ultrasound shock waves was found to transiently disrupt the microtubule cytoskeleton and to eliminate paclitaxel-induced rigid microtubule bundles. When cellular microtubules were labelled with a fluorescent paclitaxel analog, exposure to ultrasound waves led to the disassembly of the labeled microtubules and localization of the signals to perinuclear compartments, which were determined to be lysosomes.

CONCLUSIONS

We suggest that ultrasound disrupts the paclitaxel-induced rigid microtubule cytoskeleton, generating paclitaxel bound fragments that undergo degradation. A new microtubule network forms from tubulins that are not bound by paclitaxel. Hence, ultrasound shock waves are able to abolish paclitaxel impact on microtubules. Thus, our results demonstrate that a brief exposure to low intensity ultrasound can reduce and/or eliminate cytotoxicity associated with paclitaxel treatment of cancer cells in cultures.

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

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

TUBB1 mutation disrupting microtubule assembly impairs proplatelet formation and results in congenital macrothrombocytopenia

This report describes a family with TUBB1-associated macrothrombocytopenia diagnosed based on abnormal platelet β1-tubulin distribution. A circumferential marginal microtubule band was undetectable, whereas microtubules were frayed and disorganized in every platelet from the affected individuals. Patients were heterozygous for novel TUBB1 p.F260S that locates at the α- and β-tubulin intradimer interface. Mutant β1-tubulin was not incorporated into microtubules with endogenous α-tubulin, and α-tubulin expression was decreased in transfected Chinese hamster ovary cells. Transduction of mutant β1-tubulin into mouse fetal liver-derived megakaryocytes demonstrated no incorporation of mutant β1-tubulin into microtubules with endogenous α-tubulin and diminished proplatelet formation, leading to the production of fewer, but larger, proplatelet tips. Furthermore, mutant β1-tubulin was not associated with endogenous α-tubulin in the proplatelets. Deficient functional microtubules might lead to defective proplatelet formation and abnormal protrusion-like platelet release, resulting in congenital macrothrombocytopenia.

Disruption of microtubule integrity initiates mitosis during CNS repair

Mechanisms of CNS repair have vital medical implications. We show that traumatic injury to the ventral midline of the embryonic Drosophila CNS activates cell divisions to replace lost cells. A pilot screen analyzing transcriptomes of single cells during repair pointed to downregulation of the microtubule-stabilizing GTPase mitochondrial Rho (Miro) and upregulation of the Jun transcription factor Jun-related antigen (Jra). Ectopic Miro expression can prevent midline divisions after damage, whereas Miro depletion destabilizes cortical β-tubulin and increases divisions. Disruption of cortical microtubules, either by chemical depolymerization or by overexpression of monomeric tubulin, triggers ectopic mitosis in the midline and induces Jra expression. Conversely, loss of Jra renders midline cells unable to replace damaged siblings. Our data indicate that upon injury, the integrity of the microtubule cytoskeleton controls cell division in the CNS midline, triggering extra mitosis to replace lost cells. The conservation of the identified molecules suggests that similar mechanisms may operate in vertebrates.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Quality control of cytoskeletal proteins and human disease

Actins and tubulins are abundant cytoskeletal proteins that support diverse cellular processes. Owing to the unique properties of these filament-forming proteins, an intricate cellular machinery consisting minimally of the chaperonin CCT, prefoldin, phosducin-like proteins, and tubulin cofactors has evolved to facilitate their biogenesis. More recent evidence also suggests that regulated degradation pathways exist for actin (via TRIM32) and tubulin (via parkin or cofactor E-like). Collectively, these pathways maintain the quality control of cytoskeletal proteins ('proteostasis'), ensuring the appropriate function of microfilaments and microtubules. Here, we focus on the molecular mechanisms of the quality control of actin and tubulin, and discuss emerging links between cytoskeletal proteostasis and human diseases.

Dystrophin is a microtubule-associated protein

Cytolinkers are giant proteins that can stabilize cells by linking actin filaments, intermediate filaments, and microtubules (MTs) to transmembrane complexes. Dystrophin is functionally similar to cytolinkers, as it links the multiple components of the cellular cytoskeleton to the transmembrane dystroglycan complex. Although no direct link between dystrophin and MTs has been documented, costamere-associated MTs are disrupted when dystrophin is absent. Using tissue-based cosedimentation assays on mice expressing endogenous dystrophin or truncated transgene products, we find that constructs harboring spectrinlike repeat 24 through the first third of the WW domain cosediment with MTs. Purified Dp260, a truncated isoform of dystrophin, bound MTs with a K_d of 0.66 µM, a stoichiometry of 1 Dp260/1.4 tubulin heterodimer at saturation, and stabilizes MTs from cold-induced depolymerization. Finally, α- and β-tubulin expression is increased ∼2.5-fold in mdx skeletal muscle without altering the tubulin–MT equilibrium. Collectively, these data suggest dystrophin directly organizes and/or stabilizes costameric MTs and classifies dystrophin as a cytolinker in skeletal muscle.

Mutation of the beta1-tubulin gene associated with congenital macrothrombocytopenia affecting microtubule assembly

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. We identified the first TUBB1 mutation, R318W, in a patient with congenital macrothrombocytopenia. The patient was heterozygous for Q43P, but this single-nucleotide polymorphism (SNP) did not relate to macrothrombocytopenia. Although no abnormal platelet beta1-tubulin localization/marginal band organization was observed, the level of beta1-tubulin was decreased by approximately 50% compared with healthy controls. Large and irregular bleb protrusions observed in megakaryocytes derived from the patient's peripheral blood CD34(+) cells suggested impaired megakaryocyte fragmentation and release of large platelets. In vitro transfection experiments in Chinese hamster ovary (CHO) cells demonstrated no incorporation of mutant beta1-tubulin into microtubules, but the formation of punctuated insoluble aggregates. These results suggested that mutant protein is prone to aggregation but is unstable within megakaryocytes/platelets. Alternatively, mutant beta1-tubulin may not be transported from the megakaryocytes into platelets. W318 beta1-tubulin may interfere with normal platelet production, resulting in macrothrombocytopenia.

Regulation of protein translation through mRNA structure influences MHC class I loading and T cell recognition

Many viruses avoid immune surveillance during latent infection through reduction in the synthesis of virally encoded proteins. Although antigen presentation critically depends on the level of viral protein synthesis, the precise mechanism used to regulate the generation of antigenic peptide precursors remains elusive. Here, we demonstrate that a purine overloaded virally encoded mRNA lacking secondary structure significantly impacts the efficiency of protein translation and prevents endogenous antigen presentation. Reducing this purine bias through the generation of constructs expressing codon-modified sequences, while maintaining the encoded protein sequence, increased the stem-loop structure of the corresponding mRNA and dramatically enhanced self-synthesis of the viral protein. As a consequence, a higher number of HLA-peptide complexes were detected on the surface of cells expressing this viral protein. Furthermore, these cells were more efficiently recognized by virus-specific T cells compared with those expressing the same antigen expressed by a purine-biased mRNA. These findings delineate a mechanism by which viruses regulate self-synthesis of proteins and offer an effective strategy to evade CD8(+) T cell-mediated immune regulation.

A direct test of the hypothesis that increased microtubule network density contributes to contractile dysfunction of the hypertrophied heart

Contractile dysfunction in pressure overload-hypertrophied myocardium has been attributed in part to the increased density of a stabilized cardiocyte microtubule network. The present study, the first to employ wild-type and mutant tubulin transgenes in a living animal, directly addresses this microtubule hypothesis by defining the contractile mechanics of the normal and hypertrophied left ventricle (LV) and its constituent cardiocytes from transgenic mice having cardiac-restricted replacement of native β4-tubulin with β1-tubulin mutants that had been selected for their effects on microtubule stability and thus microtubule network density. In each case, the replacement of cardiac β4-tubulin with mutant hemagglutinin-tagged β1-tubulin was well tolerated in vivo. When LVs in intact mice and cardiocytes from these same LVs were examined in terms of contractile mechanics, baseline function was reduced in mice with genetically hyperstabilized microtubules, and hypertrophy-related contractile dysfunction was exacerbated. However, in mice with genetically hypostabilized cardiac microtubules, hypertrophy-related contractile dysfunction was ameliorated. Thus, in direct support of the microtubule hypothesis, we show here that cardiocyte microtubule network density, as an isolated variable, is inversely related to contractile function in vivo and in vitro, and microtubule instability rescues most of the contractile dysfunction seen in pressure overload-hypertrophied myocardium.

Heightened Sensitivity to Paclitaxel in Class IVa β-Tubulin-transfected Cells Is Lost as Expression Increases

Stably transfected Chinese hamster ovary cell lines expressing increasing levels of beta4a, a class IV neuronal-specific beta-tubulin, were compared for effects on microtubule organization, assembly, and sensitivity to antimitotic drugs. It was found that beta4a reduced microtubule assembly in proportion to its abundance and thereby caused supersensitivity to microtubule disruptive drugs such as colcemid, vinblastine, and nocodazole. However, the response to paclitaxel was more complex. Low expression of beta4a caused supersensitivity to paclitaxel, whereas higher expression resulted in the loss of supersensitivity. The results suggest that beta4a may possess an enhanced ability to bind paclitaxel that increases sensitivity to the drug and acts substoichiometrically. At high levels of beta4a expression, however, microtubule disruptive effects counteract the assembly promoting pressure exerted by paclitaxel binding, and drug supersensitivity is lost. beta4a-Tubulin differs from the more ubiquitous beta4b isotype at relatively few amino acid residues, yet beta4b expression has little effect on microtubule assembly or drug response. To determine which amino acids mediate the effects of beta4a expression, beta4a and beta4b were altered by site-directed mutagenesis and expressed in Chinese hamster ovary cells. The introduction of N332S or N335S mutations into beta4b-tubulin was sufficient to confer microtubule disruption and increased colcemid sensitivity. On the other hand, mutation of Ala(115) to serine in beta4a-tubulin almost completely reversed heightened sensitivity to paclitaxel, but introduction of an S115A mutation into beta4b had no effect, suggesting that a complex interaction of multiple amino acids are necessary to produce this phenotype.

Influence of translation efficiency of homologous viral proteins on the endogenous presentation of CD8+ T cell epitopes

A significant proportion of endogenously processed CD8(+) T cell epitopes are derived from newly synthesized proteins and rapidly degrading polypeptides (RDPs). It has been hypothesized that the generation of rapidly degrading polypeptides and CD8(+) T cell epitopes from these RDP precursors may be influenced by the efficiency of protein translation. Here we address this hypothesis by using the Epstein-Barr virus-encoded nuclear antigen 1 protein (EBNA1), with or without its internal glycine-alanine repeat sequence (EBNA1 and EBNA1DeltaGA, respectively), which display distinct differences in translation efficiency. We demonstrate that RDPs constitute a significant proportion of newly synthesized EBNA1 and EBNA1DeltaGA and that the levels of RDPs produced by each of these proteins directly correlate with the translation efficiency of either EBNA1 or EBNA1DeltaGA. As a consequence, a higher number of major histocompatibility complex-peptide complexes can be detected on the surface of cells expressing EBNA1DeltaGA, and these cells are more efficiently recognized by virus-specific cytotoxic T lymphocytes compared to the full-length EBNA1. More importantly, we also demonstrate that the endogenous processing of these CD8(+) T cell epitopes is predominantly determined by the rate at which the RDPs are generated rather than the intracellular turnover of these proteins.

Paclitaxel resistance in cells with reduced β-tubulin

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

BetaIII-tubulin induces paclitaxel resistance in association with reduced effects on microtubule dynamic instability

The development of resistance to paclitaxel in tumors is one of the most significant obstacles to successful therapy. Overexpression of the betaIII-tubulin isotype has been associated with paclitaxel resistance in a number of cancer cell lines and in tumors, but the mechanism of resistance has remained unclear. Paclitaxel inhibits cancer cell proliferation by binding to the beta-subunit of tubulin in microtubules and suppressing microtubule dynamic instability, leading to mitotic arrest and cell death. We hypothesized that betaIII-tubulin overexpression induces resistance to paclitaxel either by constitutively enhancing microtubule dynamic instability in resistant cells or by rendering the microtubules less sensitive to the suppression of dynamics by paclitaxel. Using Chinese hamster ovary cells that inducibly overexpress either betaI- or betaIII-tubulin, we analyzed microtubule dynamic instability during interphase by microinjection of rhodamine-labeled tubulin and time-lapse fluorescence microscopy. In the absence of paclitaxel, there were no differences in any aspect of dynamic instability between the two beta-tubulin-overexpressing cell types. However, in the presence of 150 nm paclitaxel, dynamic instability was suppressed to a significantly lesser extent (suppressed only 12%) in cells overexpressing betaIII-tubulin than in cells overexpressing similar levels of betaI-tubulin (suppressed 47%). The results suggest that overexpression of betaIII-tubulin induces paclitaxel resistance by reducing the ability of paclitaxel to suppress microtubule dynamics. The results also suggest that endogenous regulators of microtubule dynamics may differentially interact with individual tubulin isotypes, supporting the idea that differential expression of tubulin isotypes has functional consequences in cells.

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

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

Phenotypic consequences of beta1-tubulin expression and MAP4 decoration of microtubules in adult cardiocytes

In pressure-overload cardiac hypertrophy, microtubule network densification is one cause of contractile dysfunction. Cardiac transcriptional upregulation of beta1-tubulin rather than the constitutive beta4-tubulin and of microtubule-associated protein (MAP)4 accompanies hypertrophy, with extensive microtubule decoration by MAP4. Because MAP4 stabilizes microtubules, and because the isoform-variable carboxy terminus of beta-tubulin binds to MAP4, we wished to determine whether one or both of these proteins has etiologic significance for cardiac microtubule network densification. Recombinant adenoviruses encoding beta1-tubulin, beta4-tubulin, and MAP4 were used to infect isolated cardiocytes. Overexpressed MAP4 caused a shift of tubulin dimers to the polymerized fraction and formation of a dense, stable microtubule network. Overexpressed beta1- or beta4-tubulin had neither any independent effect on these variables nor any effect additive to that of simultaneously overexpressed MAP4. Results from transgenic mice with cardiac overexpression of beta1-tubulin or MAP4 were confirmatory, but unlike the effects of brief adenovirus-mediated MAP4 overexpression in isolated cardiocytes, MAP4 transgenic hearts showed a marked increase in total alpha- and beta-tubulin. Thus MAP4 overexpression caused increased tubulin expression, formation of stable microtubules, and altered microtubule network properties, such that MAP4 upregulation may be one cause for the dense, stable microtubule network characteristic of pressure-overloaded, hypertrophied cardiocytes.

Expression of class III beta-tubulin reduces microtubule assembly and confers resistance to paclitaxel

Human brain and testis specific betaIII-tubulin was amplified from a cDNA library, modified to encode a C-terminal hemagglutinin antigen epitope tag, and cloned into a vector that allows tetracycline regulated expression in mammalian cells. Immunofluorescence analysis of transfected Chinese hamster ovary cells demonstrated that expressed HA-tagged betaIII-tubulin is able to assemble with endogenous tubulin into microtubules even though betaIII-tubulin is not a normal constituent of these cells. A stable G418-resistant clone with moderate HAbetaIII-tubulin expression displayed weak (1.5-2-fold) resistance to paclitaxel. A second clone with higher HAbetaIII-tubulin expression could not grow unless tetracycline was present to repress transcription of the transfected cDNA. Analysis of cellular microtubules in each of these clones indicated that incorporation of HAbetaIII-tubulin led to a significant expression-dependent decrease in assembled tubulin. Paclitaxel resistant cells were also directly selected from the transfected cell population using a paclitaxel concentration 4 times higher than the minimum toxic dose. Few cells were able to survive the selection and they grew very slowly. Western blot analysis of these resistant cells revealed very high HAbetaIII-tubulin expression that led to almost complete replacement of endogenous beta-tubulin at steady state. Transfected betaIII-tubulin with no epitope tag behaved in a very similar fashion indicating that presence of the HA tag had no discernible functional effect. The results demonstrate that betaIII-tubulin diminishes microtubule assembly, is toxic when present at high levels, but is able to confer weak resistance to paclitaxel when expressed at moderate levels in mammalian cells.

Beta class II tubulin predominates in normal and tumor breast tissues

BACKGROUND

Antimitotic chemotherapeutic agents target tubulin, the major protein in mitotic spindles. Tubulin isotype composition is thought to be both diagnostic of tumor progression and a determinant of the cellular response to chemotherapy. This implies that there is a difference in isotype composition between normal and tumor tissues.

METHODS

To determine whether such a difference occurs in breast tissues, total tubulin was fractionated from lysates of paired normal and tumor breast tissues, and the amounts of beta-tubulin classes I + IV, II, and III were measured by competitive enzyme-linked immunosorbent assay (ELISA). Only primary tumor tissues, before chemotherapy, were examined. Her2/neu protein amplification occurs in about 30% of breast tumors and is considered a marker for poor prognosis. To gain insight into whether tubulin isotype levels might be correlated with prognosis, ELISAs were used to quantify Her2/neu protein levels in these tissues.

RESULTS

Beta-tubulin isotype distributions in normal and tumor breast tissues were similar. The most abundant beta-tubulin isotypes in these tissues were beta-tubulin classes II and I + IV. Her2/neu levels in tumor tissues were 5-30-fold those in normal tissues, although there was no correlation between the Her2/neu biomarker and tubulin isotype levels.

CONCLUSION

These results suggest that tubulin isotype levels, alone or in combination with Her2/neu protein levels, might not be diagnostic of tumorigenesis in breast cancer. However, the presence of a broad distribution of these tubulin isotypes (for example, 40-75% beta-tubulin class II) in breast tissue, in conjunction with other factors, might still be relevant to disease progression and cellular response to antimitotic drugs.

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

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

Thyroid hormone-dependent regulation of Talpha1 alpha-tubulin during brain development

Thyroid hormone (T3) is essential for brain development and most of its actions are exerted at the gene expression level after interaction with nuclear receptors. In particular, genes encoding cytoskeletal proteins are influenced by the thyroidal status. Thyroid hormone is involved in the normal downregulation of the Talpha1 alpha-tubulin gene during postnatal growth. The action of T3 on Talpha1 tubulin expression is complex and is exerted at least at two levels. In cultured cells, T3 induces a transient and fast decrease of Talpha1 mRNA concentration. This effect is enhanced when transcription is blocked by actinomycin D, suggesting that T3 increases mRNA degradation. In transgenic animals T3 affects the expression of beta-galactosidase under control of the Talpha1 promoter in the same way as the endogenous gene, supporting an effect mediated through the Talpha1 promoter. However, the Talpha1 promoter is not regulated by T3 in transfected cells and, therefore, the effects of the hormone in vivo are likely to be indirect. It is concluded that regulation of Talpha1 alpha-tubulin by thyroid hormone is the result of multiple influences including effects on mRNA half life and indirect effects at the promoter level.

Over-expression of betaI tubulin in MDCK cells and incorporation of exogenous betaI tubulin into microtubules interferes with adhesion and spreading

Little is known about the presence and distribution of tubulin isotypes in MDCK cells although essential epithelial functions in these monolayers are regulated by dynamic changes in the microtubule architecture. Using specific antibodies, we show here that the betaI, betaII, and betaIV isotypes are differentially distributed in the microtubules of these cells. Microtubules in subconfluent cells radiating from the perinuclear region contain betaI and betaII tubulins, while those extending to the cell edges are enriched in betaII. Confluent cells contain similar proportions of betaI and betaII along the entire microtubule length. betaIV is the less abundant isotype and shows a similar distribution to betaII. The effect of modifying tubulin isotype ratios in the microtubules that could affect their dynamics and function was analyzed by stably expressing in MDCK cells betaI tubulin from CHO cells. Three recombinant clones expressing different levels of the exogenous betaI tubulin were selected and subcloned. Clone 17-2 showed the highest expression of CHO beta1 tubulin. Total betaI tubulin levels (MDCK+CHO) in the clones were approximately 1.8 to 1.1-fold higher than in mock-transfected cells only expressing MDCK beta1 tubulin. In all the cells, betaII tubulin levels remained unchanged. The cells expressing CHO beta1 tubulin showed defective attachment, spreading, and delayed formation of adhesion sites at short times after plating, whereas mock-transfected cells attached and spread normally. Analysis of cytoskeletal fractions from clone 17-2 showed a MDCK betaI/CHO betaI ratio of 1.89 at 2 h that gradually decreased to 1.0 by 24 h. The ratio of the two isotypes in the soluble fraction remained unchanged, although with higher values than those found for the polymerized betaI tubulin. By 24 h, the transfected cells had regained normal spreading and formed a confluent monolayer. Our results show that excess levels of total betaI tubulin, resulting from the expression of the exogenous beta1 isotype, and incorporation of it into microtubules affect their stability and some cellular functions. As the levels return to normal, the cells recover their normal phenotype. Regulation of betaI tubulin levels implies the release of the MDCK betaI isotype from the microtubules into the soluble fraction where it would be degraded.

Altered time course of mRNA expression of alpha tubulin in the central nervous system of hens treated with diisopropyl phosphorofluoridate (DFP)

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester induced delayed neurotoxicity (OPIDN) in the hen, human and other sensitive species. We studied the effect of single dose of DFP (1.7 mg/kg/s.c.) on the expression of alpha tubulin which is one of the major sub-unit of tubulin polymers that constitute an important constituent of cellular architecture. The hens were sacrificed at different time points i.e. 1, 2, 5, 10, and 20 days. Total RNA was extracted from the following brain regions: cerebrum, cerebellum, and brainstem as well as spinal cord. Northern blots prepared using standard protocols were hybridized with alpha tubulin as well as with beta-actin and 28S RNA cDNA (controls) probes. The results indicate a differential/spatial/temporal regulation of alpha tubulin levels which may be the result of perturbed microtubule dynamics not only in the axons but also in perikarya of neurons in the CNS of DFP treated hens. In the highly susceptible tissues like brainstem and spinal cord the initial down-regulation of mRNA levels could be attributed to DFP induced stress response resulting in inhibited cell metabolism and or cell injury/cell death. Increase in levels of mRNA at 5 days and thereafter coincided with increased tubulin transport which may be due to increased phosphorylation of tubulins in both axons and perikarya and other intraaxonal changes resulting in impaired axonal transport. DFP induced decreased rate of tubulin polymerization resulting in increased levels of free tubulin monomers may be involved in the altered alpha tubulin mRNA expression at different time points by autoregulatory circuits. Cerebellum being the less susceptible tissue showed only a moderate decline at day 2, while the alpha tubulin remained at near control levels at day 1. Delayed down-regulation may be due to the co-ordinated up or down-regulation of different sub-types of alpha and beta tubulins as well as the differential response of specialised cell types in cerebellum. Continuous overexpression of alpha tubulin in cerebrum from the beginning may be its effective protective strategy to safeguard itself from neurotoxicity. Differential expression pattern observed could be due to the differential susceptibility and variability in the rate of axonal transport of different regions besides the tubulin heterogenity of CNS. Hence our results indicte differential expression of alpha tubulin is either one of the reasons for the development of OPIDN or the result of progressive changes taking place during OPIDN.

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.

Evidence for a role of the (alpha)-tubulin C terminus in the regulation of cyclin B synthesis in developing oocytes

Microinjected mAb YL1/2, an (alpha)-tubulin antibody specific for the tyrosinated form of the protein, blocks the cell cycle in developing oocytes. Here, we have investigated the mechanism involved in the mAb effect. Both developing starfish and Xenopus oocytes were injected with two different (alpha)-tubulin C terminus antibodies. The injected antibodies blocked cell entry into mitosis through specific inhibition of cyclin B synthesis. The antibody effect was independent of the presence or absence of polymerized microtubules and was mimicked by injected synthetic peptides corresponding to the tyrosinated (alpha)-tubulin C terminus, whereas peptides lacking the terminal tyrosine were ineffective. These results indicate that tyrosinated (alpha)-tubulin, or another protein sharing the same C-terminal epitope, is involved in specific regulation of cyclin B synthesis in developing oocytes.

A beta-tubulin leucine cluster involved in microtubule assembly and paclitaxel resistance

Analysis of beta-tubulin alleles from nine paclitaxel-resistant Chinese hamster ovary cell lines revealed an unexpected cluster of mutations affecting Leu-215, Leu-217, and Leu-228. Six of the mutant alleles encode a His, Arg, or Phe substitution at Leu-215; another mutant allele has an Arg substitution at Leu-217; and the final two mutant alleles have substitutions of His or Phe at Leu-228. Using plasmids that allow tetracycline regulated expression, the L215H, L217R, and L228F mutations were introduced into a hemagglutinin antigen-tagged beta-tubulin cDNA and transfected into wild-type Chinese hamster ovary cells. In all three cases, low to moderate expression of the transfected mutant gene conferred paclitaxel resistance. Higher levels of expression caused disruption of microtubule assembly, cell cycle arrest at mitosis, and failure to proliferate. Consistent with reduced microtubule stability, cells expressing mutant hemagglutinin beta-tubulin had fewer acetylated microtubules than nonexpressing cells in the same population. These data, together with previous studies showing that the paclitaxel-resistant mutant cell lines have less stable microtubules, indicate that the leucine cluster represents an important structural motif for microtubule assembly.

Vinblastine induces an interaction between FtsZ and tubulin in mammalian cells

The Escherichia coli cell division protein FtsZ was expressed in Chinese hamster ovary cells, where it formed a striking array of dots that were independent of the mammalian cytoskeleton. Although FtsZ appears to be a bacterial homolog of tubulin, its expression had no detectable effects on the microtubule network or cell growth. However, treatment of the cells with vinblastine at concentrations that caused microtubule disassembly rapidly induced a network of FtsZ filaments that grew from and connected the dots, suggesting that the dots are an active storage form of FtsZ. Cells producing FtsZ also exhibited vinblastine- and calcium-resistant tubulin polymers that colocalized with the FtsZ network. The FtsZ polymers could be selectively disassembled, indicating that the two proteins were not copolymerized. The vinblastine effects were readily reversible by washing out the drug or by treating the cells with the vinblastine competitor, maytansine. These results demonstrate that FtsZ assembly can occur in the absence of bacterial chaperones or cofactors, that FtsZ and tubulin do not copolymerize, and that tubulin-vinblastine complexes have an enhanced ability to interact with FtsZ.

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.

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.

Expression of Reticulomyxa filosa tubulins in Pichia pastoris: regulation of tubulin pools

We expressed the alpha2- and beta2-tubulin isoforms of the giant freshwater amoeba Reticulomyxa filosa in the methylotrophic yeast Pichia pastoris. Single expression lead to little or no detectable material. Coexpression of both tubulins, however, resulted in a significant increase of expressed proteins. At the same time, the detectable internal tubulins of the host yeast cell were downregulated. This finding indicates the functionality of the expressed amoeba tubulins. Further regulation phenomena were observed on the level of equilibrium between the two R. filosa tubulin isoforms and on the level of the total tubulin pool. The P. pastoris/R. filosa system therefore seems to be an accessible system for the simultaneous study of the various mechanisms involved in tubulin regulation.

alpha-Tubulin limits its own synthesis: evidence for a mechanism involving translational repression

A Chinese hamster alpha-tubulin cDNA was modified to encode an 11-amino acid carboxyl-terminal extension containing the immunodominant epitope from influenza hemagglutinin antigen (to create HA alpha 1-tubulin) and was cloned into a vector for expression in mammalian cells. 12 stable CHO cell lines expressing this HA alpha 1-tubulin were isolated and characterized. HA alpha 1-tubulin incorporated into all classes of microtubules, assembled to the same extent as the endogenous tubulin, and did not perturb the growth of the cells in which it was expressed. However, overexpression of HA alpha 1-tubulin strongly repressed the synthesis of endogenous alpha-tubulin while having little or no effect on the synthesis of beta-tubulin. Treatment of transfected cells with sodium butyrate to induce even greater expression of HA alpha 1-tubulin led to a further decrease in synthesis of endogenous alpha-tubulin that was fully reversible upon removal of the inducer. Decreased synthesis of alpha-tubulin in transfected cells did not result from decreased levels of alpha-tubulin mRNA, as demonstrated by ribonuclease protection assays. On the other hand, colchicine, a drug previously shown to destabilize the tubulin message, caused a clear reduction in both protein synthesis and mRNA levels for transfected HA alpha 1-tubulin and endogenous alpha-tubulin, thus indicating that the decreased alpha-tubulin synthesis observed as a result of HA alpha 1-tubulin overexpression is distinct from the previously described autoregulation of tubulin. The results are consistent with a mechanism in which free alpha-tubulin inhibits the translation of its own message as a way of ensuring stoichiometric synthesis of alpha- and beta-tubulin.