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Despite the fact that many new active drugs recently became available for phase II studies of the different sarcoma types, only soft tissue sarcoma has been included in these phase II testing programs. The groups active in the treatment of osteosarcomas and Ewing's sarcomas should be encouraged to investigate the new active drugs in the relapsing patients. For soft tissue sarcoma, many new drugs have been tested by different groups. The results to date have been disappointing, because only docetaxel has been identified as a new active drug in one phase II study. Hints of activity have also been observed for edatrexate and topotecan. Further studies with these three drugs are necessary to identify their proper role.
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Recent development of new anticancer drugs in Japan was reviewed. Since there are a high incidence of cancer of the digestive organs, lung cancer and breast cancer in Japan, we have been looking for novel anticancer drugs effective against these solid cancers. Currently, some of fluoropyrimidine compounds, methotrexate analogues, tubulin inhibitors from plant origins have shown their clinical activities against human non-small cell lung cancer and/or breast cancer. As the end-point on these drug development, we have put an emphasis to find drugs to contribute for the survival prolongation and improvement of quality of life of the treated patients.
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Microtubules are stiff polymers that extend throughout the cytoplasm of eukaryotic cells and regulate the location of intracellular compartments. Microtubules are formed by the polymerization of tubulin molecules, each of which is a heterodimer consisting of two closely related globular polypeptides, alpha-tubulin and beta tubulin. Many of the microtubule arrays are labile and are turning over rapidly; they depolymerize and repolimerize continually in living cells. One of the most striking examples is the mitotic spindle, which forms after the cytoplasmic microtubules disassemble at the onset of mitosis. The mitotic spindle is the target of a variety of agents that act by interfering with the exchange of tubulin subunits between the microtubules and the free tubulin pool; some of these agents can be used as antimitotic or anticancer drugs. We recently have developed a simple and sensitive method of screening for tubulin inhibitors, to which we have successfully applied the nerve growth factor/PC12 cell system. In this report, we describe the details of this method for screening tubulin inhibitors, by which we have analyzed several well-known and new compounds a model experiments.
Drug sensitivity was studied for the tubulin inhibitors taxol, taxotere, rhizoxin and for doxorubucin and cisplatin, in human lung and breast cancer cell lines, including drug-selected cell lines, overexpressing the membrane transporter P-glycoprotein (Pgp) or the multidrug resistance protein (MRP). All tubulin-inhibiting agents were more potent than doxorubicin and cisplatin in all cell lines. In the drug resistance-selected cell lines (doxorubicin or mitoxantrone resistant) there was cross-resistance between the tubulin inhibitors and the selecting agent; however, MRP overexpressing cells were relatively less resistant to taxanes than the Pgp overexpressing cells. Polymerization of microtubules after exposure to taxol was observed in drug sensitive cell lines, but not in resistant cell lines, even at high taxol concentrations and after long exposure times. In the Pgp overexpressing cell lines, steady accumulation of 14C-taxol was defective and could be reverted by verapamil. MRP overexpressing cells did not have a significant accumulation defect of taxol, compared to the parental cell lines, and verapamil did not have any effect. These data confirm that the Pgp overexpression is an important mechanism of resistance to taxanes and rhizoxin in human lung and breast tumor cells. However, the presence of mechanisms other than transport defects may play an important role in non-Pgp expressing cells, and these may include an altered function of tubulins.
Beta(beta)-tubulin isotype variation has recently been implicated in the modulation of resistance to paclitaxel in human lung cancer cells and in primary human ovarian tumour samples. Whether alpha-tubulin is involved in drug resistance has not been reported. We have generated a paclitaxel-resistant cell line (H460/T800) from the sensitive human lung carcinoma parental cell line NCI-H460. The resistant cells are more than 1000-fold resistant to taxol and overexpress P-glycoprotein. Interestingly, H460/T800 cells also overexpress alpha- and beta-tubulin as detected by Western blot analysis. From Northern blot analysis, the mechanism of tubulin overexpression appears to be post-transcriptional. To understand whether alpha-tubulin plays a role in drug resistance, we transfected antisense human kalpha1 cDNA construct into the H460/T800 paclitaxel-resistant cells. The antisense clones displayed a reduced alpha-tubulin expression, and the cells were 45-51% more sensitive to paclitaxel and other known antimitotic drugs, compared with vector transfected controls. Complementary experiments of transfecting the sense kalpha1 cDNA into H460 cells conferred a 1.8- to 3.3-fold increase in the IC(50) of several antimitotic agents. Our study suggests that alpha-tubulin is one of the factors that contributes to drug resistance.
The Notch signal pathway plays important roles in proliferation, apoptosis, and differentiation. Abnormalities in Notch signaling are linked to many human diseases. After ligand binding, Notch signaling is activated through the cleavage of Notch receptors to release and translocate the Notch intracellular domain into the nucleus. The Notch1 receptor intracellular domain (N1IC), the activated form of the Notch1 receptor, can modulate downstream target genes via C promoter-binding factor 1-dependent and -independent pathways. To further dissect the Notch1 signaling pathway, we screened the N1IC-associated proteins using a yeast two-hybrid system and identified nuclear beta(II)-tubulin as a candidate for the N1IC-associated proteins. It was suggested that the presence of beta(II)-tubulin in nuclei might be correlated with the cancerous state of cells. However, the function of beta(II)-tubulin locating in the nucleus still is unknown. Herein, we show that the complex of alpha- and beta(II)-tubulin is associated with N1IC in cancer cells by a coimmunoprecipitation analysis. The ankyrin domain of the Notch1 receptor alone was sufficient to associate with beta(II)-tubulin. Furthermore, alpha- and beta(II)-tubulin were localized in the nucleus and formed a complex with N1IC. Treatment with Taxol increased the amounts of nuclear alpha- and beta(II)-tubulin in K562 and HeLa cells and promoted the C promoter-binding factor 1-dependent transactivation activity of N1IC. We also show that nuclear beta(II)-tubulin was bound on the C promoter-binding factor 1 response elements via the association with N1IC. These results suggest that nuclear beta(II)-tubulin can modulate Notch signaling through interaction with N1IC in cancer cells.
Many ATP-dependent molecular chaperones, including Hsp70, Hsp90, and the chaperonins GroEL/Hsp60, require cofactor proteins to regulate their ATPase activities and thus folding functions in vivo. One conspicuous exception has been the eukaryotic chaperonin CCT, for which no regulator of its ATPase activity, other than non-native substrate proteins, is known. We identify the evolutionarily conserved PhLP3 (phosducin-like protein 3) as a modulator of CCT function in vitro and in vivo. PhLP3 binds CCT, spanning the cylindrical chaperonin cavity and contacting at least two subunits. When present in a ternary complex with CCT and an actin or tubulin substrate, PhLP3 significantly diminishes the chaperonin ATPase activity, and accordingly, excess PhLP3 perturbs actin or tubulin folding in vitro. Most interestingly, however, the Saccharomyces cerevisiae PhLP3 homologue is required for proper actin and tubulin function. This cellular role of PhLP3 is most apparent in a strain that also lacks prefoldin, a chaperone that facilitates CCT-mediated actin and tubulin folding. We propose that the antagonistic actions of PhLP3 and prefoldin serve to modulate CCT activity and play a key role in establishing a functional cytoskeleton in vivo.
Substoichiometric binding of taxol to tubulin in microtubules potently suppresses microtubule dynamics, which appears to be the most sensitive antiproliferative mechanism of taxol. To determine whether the beta-tubulin isotype composition of a microtubule can modulate sensitivity to taxol, we measured the effects of substoichiometric ratios of taxol bound to tubulin in microtubules on the dynamics of microtubules composed of purified alphabeta(II)-, alphabeta(III)-, or alphabeta(IV)-tubulin isotypes and compared the results with the effects of taxol on microtubules assembled from unfractionated tubulin. Substoichiometric ratios of bound taxol in microtubules assembled from purified beta-tubulin isotypes or unfractionated tubulin potently suppressed the shortening rates and the lengths shortened per shortening event. Correlation of the suppression of the shortening rate with the stoichiometry of bound taxol revealed that microtubules composed of purified alphabeta(II)-, alphabeta(III)-, and alphabeta(IV)-tubulin were, respectively, 1.6-, 7.4-, and 7.2-fold less sensitive to the effects of bound taxol than microtubules assembled from unfractionated tubulin. These results indicate that taxol differentially modulates microtubule dynamics depending upon the beta-tubulin isotype composition. The results are consistent with recent studies correlating taxol resistance in tumor cells with increased levels of beta(III0- and beta(IV)-tubulin expression and suggest that altered cellular expression of beta-tubulin isotypes can be an important mechanism by which tumor cells develop resistance to taxol.
In higher organisms, there is a large variety of tubulin isoforms, due to multiple tubulin genes and extensive post-translational modification. The properties of microtubules may be modulated by their tubulin isoform composition. Polyglutamylation is a post-translational modification that is thought to influence binding of both structural microtubule associated proteins (MAPs) and mechano-chemical motors to tubulin. The present study investigates the role of tubulin polyglutamylation in a vesicle transporting system, cod (Gadus morhua) melanophores. We did this by microinjecting an antibody against polyglutamylated tubulin into these cells. To put our results into perspective, and to be able to judge their universal application, we characterized cod tubulin polyglutamylation by Western blotting technique, and compared it to what is known from mammals. We found high levels of polyglutamylation in tissues and cell types whose functions are highly dependent on interactions between microtubules and motor proteins. Microinjection of the anti-polyglutamylation antibody GT335 into cultured melanophores interfered with pigment granule dispersion, while dynein-dependent aggregation was unaffected. Additional experiments showed that GT335-injected cells were able to aggregate pigment even when actin filaments were depolymerized, indicating that the maintained ability of pigment aggregation in these cells was indeed microtubule-based and did not depend upon actin filaments. The results indicate that dynein and the kinesin-like dispersing motor protein in cod melanophores bind to tubulin on slightly different sites, and perhaps depend differentially on polyglutamylation for their interaction with microtubules. The binding site of the dispersing motor may bind directly to the polyglutamate chain, or more closely than dynein.
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.
beta-Tubulin isoforms in brain tissues and in cell lines were analyzed by high-resolution isoelectric focusing in combination with monoclonal antibodies. Post-translational modifications of brain non-class-III beta-tubulin isoforms were found in phylogenetically distant species ranging from pig to carp. Less extensive modifications were also observed in Neuro-2a, HeLa and 3T3 cells, where most acidic isoforms were glutamylated, while the basic, most abundant isoforms were not. The data suggest post-translational modification of non-class-III beta-tubulin isoforms in neuronal as well as in non-neuronal cells. Such modification might modulate interaction of tubulin with microtubule-associated proteins.
Elongation factor-1alpha (EF-1alpha), a highly conserved protein named for its role in protein translation, is also a microtubule-associated protein (MAP). We used high-resolution differential interference contrast microscopy to quantify the effect of substoichiometric amounts of EF-1alpha (isolated from Daucus carota) on the dynamic instability of microtubules assembled in vitro from either animal or plant tubulin. EF-1alpha modulates the dynamic behavior of microtubules assembled from either tubulin source, resulting in longer and more persistent microtubules. EF-1alpha, at a 1:20 molar ratio to tubulin, significantly (P < 0.05) reduces the frequency of catastrophe threefold and decreases shortening velocities almost twofold for microtubules assembled from animal tubulin. For microtubules assembled from plant tubulin, substoichiometric amounts of EF-1alpha significantly (P < 0.05) suppress the frequency of catastrophe greater than twofold and causes an almost threefold reduction in shortening velocities. Elongation velocities increase almost twofold and rescues, which are not observed in the absence of EF-1alpha, occur. In addition, calcium/calmodulin (Ca2+/CaM), which regulates the ability of EF-1alpha to bundle taxol-stabilized microtubules in vitro, also modulates the effect of EF-1alpha on the dynamic behavior of microtubules assembled in vitro from animal tubulin. Microtubule severing in the presence of EF-1alpha was never observed. These data support the hypothesis that EF-1alpha modulates the dynamic behavior of microtubules assembled in vitro in a Ca2+/CaM-dependent manner.