Estramustine-phosphate binds to a tubulin binding domain on microtubule-associated proteins MAP-2 and tau

Selective interaction of lansoprazole and astemizole with tau polymers: potential new clinical use in diagnosis of Alzheimer's disease

We describe the interactions of two benzimidazole derivatives, astemizole (AST) and lansoprazole (LNS), with anomalous aggregates of tau protein (neurofibrillary tangles). Interestingly, these compounds, with important medical applications in the treatment of allergies and gastrointestinal disorders respectively, specifically bind to aggregated variants of tau protein and to paired helical filaments isolated from brains of Alzheimer's disease (AD) patients. These ligands appear to be a powerful tool to tag brain-isolated tau-aggregates and heparin-induced polymers of recombinant tau. The interactions of AST and LNS with tau aggregates were assessed by classical radioligand assays, surface plasmon resonance, and bioinformatic approaches. The affinity of AST and LNS for tau aggregates was comparatively higher than that for amyloid-beta polymers according to our data. This is relevant since senile plaques are also abundant but are not pathognomonic in AD patients. Immunochemical studies on paired helical filaments from brains of AD patients and surface plasmon resonance studies confirm these findings. The capacity of these drugs to penetrate the blood-brain barrier was evaluated: i) in vitro by parallel artificial membrane permeability assay followed by experimental Log P determinations; and ii) in vivo by pharmacokinetic studies comparing distribution profiles in blood and brain of mice using HPLC/UV. Importantly, our studies indicate that the brain/blood concentration ratios for these compounds were suitable for their use as PET radiotracers. Since neurofibrillary tangles are positively correlated with cognitive impairment, we concluded that LNS and AST have a great potential in PET neuroimaing for in vivo early detection of AD and in reducing the formation of neurofibrillary tangles.

Designing effective hybrid toxins

A study of a well-designed androgen-mustard conjugate provides evidence supporting a novel mechanism for its selective toxicity in androgen-receptor-positive cancer cells. This represents a solid step forward on the path toward effective hybrid toxins for targeted cancer therapy.

Oxaline, a fungal alkaloid, arrests the cell cycle in M phase by inhibition of tubulin polymerization

Oxaline and neoxaline, fungal alkaloids, were found to inhibit cell proliferation and to induce cell cycle arrest at the G(2)/M phase in Jurkat cells. CBP501 (a peptide corresponding to amino acids 211-221 of Cdc25C phosphatase), which inhibits the G(2) checkpoint, did not affect the G(2)/M arrest caused by oxaline, suggesting that oxaline causes M phase arrest but not G(2) phase arrest. The Cdc2 phosphorylation level of oxaline-treated cell lysate was lower than that of the control cells, indicating that oxaline arrests the M phase. Oxaline disrupted cytoplasmic microtubule assembly in 3T3 cells. Furthermore, oxaline inhibited polymerization of microtubule protein and purified tubulin dose-dependently in vitro. In a binding competition assay, oxaline inhibited the binding of [(3)H]colchicine to tubulin, but not that of [(3)H]vinblastine. These results indicate that oxaline inhibits tubulin polymerization, resulting in cell cycle arrest at the M phase.

In vitro and in vivo activity of LS 4477 and LS 4559, novel analogues of the tubulin binder estramustine

LS 4477 and LS 4559, two of a series of N-acyl-aminoalkyl phenyl ethers, are rationally designed compounds based on the tubulin binder estramustine. This study investigated their mechanism of action and compared their effectiveness in relation to estramustine in vitro against a panel of human and murine cell lines and in vivo against two murine colon tumour models (MAC). At biologically relevant concentrations, LS 4477 and LS 4559 caused a 59.9 and 56% reduction in tubulin assembly, respectively, compared with a 28.4% reduction in tubulin assembly by estramustine. The analogues were approximately 100 times more potent in chemosensitivity tests in vitro than the parent compound. Both analogues were orally active against the MAC 15A murine tumour model, to a greater extent than estramustine, producing significant growth delays (P<0.01). Significant activity was also shown against the slower growing MAC 26 tumour for LS 4577 (the soluble pro-drug of LS 4559). The results presented in this study suggest these compounds warrant further development with a view to assessing their clinical activity.

Oncoprotein 18 overexpression increases the sensitivity to vindesine in the human lung carcinoma cells

BACKGROUND

Oncoprotein 18 (op18) was first isolated as a molecule overexpressed in several malignant cells, suggesting a function of op18 in malignant processes, such as differentiation in hematologic malignancies, op18 also was found to enhance microtubule deassembly in the cells. Antimitotic agents that bind to tubulin have been used for chemotherapy to treat solid tumors, such as lung carcinoma. Vinca alkaloids, such as vindesine and vincristine, have commonly been used for chemotherapy of nonsmall cell lung carcinoma. The authors examined the role of op18 in the sensitivity of human lung carcinoma cells to antimitotic agents.

METHODS

Expression of op18 mRNA was detected in all 17 lung carcinoma cell lines tested by Northern blotting. Oncoprotein 18 cDNA was transfected to SBC-3 human lung carcinoma cells, and the stable transfectants, SBC-3/op1-3, were isolated. The sensitivity of these transfectants against antimitotic agents were examined by the MTT assay in vitro. Cell cycle distribution of the transfectants on DNA histogram was analyzed by flow cytometry.

RESULTS

Oncoprotein 18-transfected cells showed higher sensitivity to vindesine and vincristine, but not to taxanes. Vindesine-exposure increased the G2/M population of the cell cycle in the Mock transfectants, but not in SBC-3/op1, suggesting that the cell cycle dynamics were altered by op18 expression in SBC-3/op1.

CONCLUSION

Oncoprotein 18 expression is associated with lung carcinoma cell sensitivity to vindesine and may be able to serve as a surrogate marker for the chemosensitivity to Vinca alkaloids in human lung carcinomas.

Drug-induced apoptosis by anti-microtubule agent, estramustine phosphate on human malignant glioma cell line, U87MG; in vitro study

The drug effect of estramustine phosphate (EMP), an anti-microtubule agent on human glioma cells has been studied with the focus being mainly its cytotoxity or its targeting of organelles. However, the pharmacological knowledge of estramustine with respect to its cytotoxity and mechanism is limited. To acquire such knowledge, the present study investigates the ability of EMP to induce apoptosis in a human malignant glioma cell line. Transmission electron microscope (TEM) images were examined to monitor periodic changes. Agarose gel electrophoresis was also examined. Cellular DNA fragmentation ELISA was performed to investigate the DNA fragmentation rates and an MTT assay was studied to evaluate the ID50. A TEM study revealed condensing and fragmentation of the chromatin. Laddering of the bands was observed in all EMP exposure groups in agarose gel electrophoresis. DNA fragmentation in all EMP groups began at 0.5 h following an exposure with EMP and increased in a dose- and time-dependent manner as revealed by DNA ELISA fragmentation. ID50 at 24 h was 5.0 microM according to the MTT assay, a value close to 4.8 microM of ID50 was revealed by the DNA fragmentation assay. None of the above mentioned changes was observed in the control group. These results indicated that EMP caused a drug-induced apoptosis in the human malignant glioma cell line, U87MG.

Estramustine resistance

Estramustine (EM), a conjugate of nornitrogen mustard and estradiol, is a antimicrotubule drug currently in use for the treatment of advanced prostatic carcinoma. Experimental data are accumulating concerning the antitumor effect of EM in other malignancies, and clinical studies in other malignancies are ongoing. This review summarizes the information available to date concerning the effects of EM and the development of drug resistance. EM depolymerizes microtubules by binding to microtubule-associated proteins (MAPs) as well as tubulin. Because of the radiosensitizing effect of this drug there has been a recent increase in interest concerning estramustine and its clinical use. Recently, it was proposed that EM induces an apoptotic cell death in glioma cells in vitro and in a rat model. EM resistance is distinct from MDR phenotype; it has been used in combination with antimitotic agents which are part of the MDR phenotype. Observations made with estramustine-resistant cell lines show the acquisition of estramustine resistance is a function of multiple adaptation by changes at tubulin expression pattern, and is also associated with changes in tau expression and phosphorylation.

The compound 14-keto-stypodiol diacetate from the algae Stypopodium flabelliforme inhibits microtubules and cell proliferation in DU-145 human prostatic cells

We investigated the effects of the drug 14-keto-stypodiol diacetate (SDA) extracted from the seaweed product Stypopodium flabelliforme, in inhibiting the cell growth and tumor invasive behavior of DU-145 human prostate cells. In addition, the molecular action of the drug on microtubule assembly was analyzed. The effects of this diterpenoid drug in cell proliferation of DU-145 tumor cells in culture revealed that SDA at concentrations of 5 microM decreased cell growth by 14%, while at 45 microM a 61% decrease was found, as compared with control cells incubated with the solvent but in the absence of the drug. To study their effects on the cell cycle, DU-145 cells were incubated with increasing concentrations of SDA and the distribution of cell-cycle stages was analyzed by flow cytometry. Interestingly, the data showed that 14-keto-stypodiol diacetate dramatically increased the proportion of cells in the G2/M phases, and decreased the number of cells at the S phase of mitosis, as compared with appropriate controls. Studies on their action on the in vitro assembly of microtubules using purified brain tubulin, showed that SDA delayed the lag period associated to nucleation events during assembly, and decreased significantly the extent of polymerization. The studies suggest that this novel derivative from a marine natural product induces mitotic arrest of tumor cells, an effect that could be associated to alterations in the normal microtubule assembly process. On the other hand, a salient feature of this compound is that it affected protease secretion and the in vitro invasive capacity, both properties of cells from metastases. The secretion of plasminogen activator (u-PA) and the capacity of DU-145 cells to migrate through a Matrigel-coated membrane were significantly inhibited in the presence of micromolar concentrations of SDA. These results provide new keys to analyze the functional relationships between protease secretion, invasive behavior of tumor cells and the microtubule network.

Estramustine resistance correlates with tau over-expression in human prostatic carcinoma cells

Estramustine (EM) is an anti-microtubule drug used in the treatment of hormone-refractory advanced prostate cancer. Since microtubules are the targets for EM cytotoxicity, we investigated the effects of EM on the microtubule-associated protein tau to determine what role it may play in drug resistance. We have compared tau expression in human prostate cancer cells (DU145) and an EM-resistant derived cell line (E4). Reverse transcriptase polymerase chain reaction has established that tau is expressed in both cell lines but increased 1.9-fold in E4 compared with DU145 cells. This result was confirmed at the protein level by Western blotting. Tau is a phosphoprotein, most of its reported phosphorylation sites being serine or threonine residues. We have shown, however, that tau is also phosphorylated at tyrosine residues in DU145 cells and that the phosphotyrosine level of tau is significantly increased in E4 cells. Moreover, DU145 cells exposed to short term micromolar drug concentrations enter a phase of microtubule depolymerization, display an increased level of tau phosphorylation and follow a pattern similar to that observed in EM-resistant E4 cells. EM is therefore able to induce a very rapid change in the posttranslational state of tau. Our results show that the acquisition of EM resistance in E4 cells, which is accompanied by changes at the tubulin level, is also associated with important changes in tau expression and phosphorylation.

Tryprostatin A, a specific and novel inhibitor of microtubule assembly

We have investigated the cell cycle inhibition mechanism and primary target of tryprostatin A (TPS-A) purified from Aspergillus fumigatus. TPS-A inhibited cell cycle progression of asynchronously cultured 3Y1 cells in the M phase in a dose- and time-dependent manner. In contrast, TPS-B (the demethoxy analogue of TPS-A) showed cell-cycle non-specific inhibition on cell growth even though it inhibited cell growth at lower concentrations than TPS-A. TPS-A treatment induced the reversible disruption of the cytoplasmic microtubules of 3Y1 cells as observed by indirect immunofluorescence microscopy in the range of concentrations that specifically inhibited M-phase progression. TPS-A inhibited the assembly in vitro of microtubules purified from bovine brains (40% inhibition at 250 microM); however, there was little or no effect on the self-assembly of purified tubulin when polymerization was induced by glutamate even at 250 microM TPS-A. TPS-A did not inhibit assembly promoted by taxol or by digestion of the C-terminal domain of tubulin. However, TPS-A blocked the tubulin assembly induced by inducers interacting with the C-terminal domain, microtubule-associated protein 2 (MAP2), tau and poly-(l-lysine). These results indicate that TPS-A is a novel inhibitor of MAP-dependent microtubule assembly and, through the disruption of the microtubule spindle, specifically inhibits cell cycle progression at the M phase.

Suppression of matrix metalloproteinase-2-mediated cell invasion in U87MG, human glioma cells by anti-microtubule agent: in vitro study

Because microtubules are important components of cell motility and intracellular transport, it is reasonable to propose that the depolymerizing effect of an antimicrotubule agent, estramustine, on glioma microtubules would modulate cell invasiveness. To determine whether matrix metalloproteinases, key factors in cell invasion, are affected by exposure to estramustine, a cell proliferation assay, a zymogram, a collagenolysis assay and a haptoinvasion assay were used in this study. The zymogram revealed that an activated (62 kDa) form of matrix metalloproteinase-2 diminished with increasing estramustine concentrations. The collagenolysis assay demonstrated approximately 2.5- to 21-fold lower rates of enzymatic activity suppressed by estramustine in a dose-dependent manner at estramustine concentrations of 1, 5, and 10 microM, compared with the control group. On the haptoinvasion assay, no statistically significant difference was seen in the 0.5 microM estramustine group, whereas 1-10 microM estramustine groups revealed significant suppression of invasion from 6 to 24 h in a dose-dependent manner. The results suggest that estramustine suppresses the invasion of U87MG cells in vitro using the decreasing available matrix metalloproteinase-2, an effect caused by the disassembly of microtubules. Suppression of the infiltrative capacity of malignant glioma cells could be of significant value in the treatment of this disease.

Stabilization of microtubule dynamics by estramustine by binding to a novel site in tubulin: a possible mechanistic basis for its antitumor action

The cellular targets for estramustine, an antitumor drug used in the treatment of hormone-refractory prostate cancer, are believed to be the spindle microtubules responsible for chromosome separation at mitosis. Estramustine only weakly inhibits polymerization of purified tubulin into microtubules by binding to tubulin (Kd, approximately 30 microM) at a site distinct from the colchicine or the vinblastine binding sites. However, by video microscopy, we find that estramustine strongly stabilizes growing and shortening dynamics at plus ends of bovine brain microtubules devoid of microtubule-associated proteins at concentrations substantially below those required to inhibit polymerization of the microtubules. Estramustine strongly reduced the rate and extent both of shortening and growing, increased the percentage of time the microtubules spent in an attenuated state, neither growing nor shortening detectably, and reduced the overall dynamicity of the microtubules. Significantly, the combined suppressive effects of vinblastine and estramustine on the rate and extent of shortening and dynamicity were additive. Thus, like the antimitotic mechanisms of action of the antitumor drugs vinblastine and taxol, the antimitotic mechanism of action of estramustine may be due to kinetic stabilization of spindle microtubule dynamics. The results may explain the mechanistic basis for the benefit derived from combined use of estramustine with vinblastine or taxol, two other drugs that target microtubules, in the treatment of hormone-refractory prostate cancer.

Estramustine in malignant glioma

Estramustine, a carbamate ester combining 17 beta-estradiol and nornitrogen mustard, has primarily been employed in the treatment of advanced prostatic carcinoma. However, a significant amount of preclinical investigation has been directed toward estramustine's activity against human malignant glioma. These studies have demonstrated that estramustine has potent antiproliferative effects against malignant glioma both in vitro and in vivo. Similar antimitotic effects also have been demonstrated for other carbamate esters. Estramustine does not impair proliferation of nonneoplastic astrocytes at concentrations that inhibit glioma cells. Although the reasons for this selective activity remain to be determined, it has been shown that malignant gliomas expresses an estramustine-specific binding site, estramustine-binding protein, more than brain tissue. In the clinical situation, an uptake and accumulation of estramustine in human glioma tissue have been demonstrated. Estramustine has been shown to enhance the cytotoxic effects of irradiation in relatively radioresistant glioma cells both in cell culture and in a rat glioma model. Estramustine has been regarded as mainly an anti-mitotic drug but recently other effects such as inhibition of DNA synthesis, induction of apoptosis, and membrane alterations have been shown. This report summarizes the preclinical observations concerning the effects of estramustine and related compounds on human malignant gliomas. These findings form the basis for proposing further laboratory and clinical investigation regarding estramustine and human malignant gliomas.

The secretion of urokinase-like plasminogen activator is inhibited by microtubule-interacting drugs

The secretion of proteinases into the extracellular matrix is one of the main features of tumour cells, as related to their invasive behaviour. Considering the role of the microtubule cytoskeleton, and particularly the action of microtubule-associated protein (MAPs) in mediating protein secretion, the effects of the anti-microtubule drugs estramustine and taxol, on the secretion of urokinase-type plasminogen activator (u-PA) and the 72 kDa gelatinase were investigated. Treatment of 5637 bladder carcinoma cells with estramustine and taxol inhibited u-PA secretion into the conditioned medium in a drug concentration-dependent fashion. This inhibition was confirmed by determinations of u-PA enzymatic activities and by measurements of the levels of immunoreactive activator. Studies using gelatin zymograms also showed an inhibition of another tumoural proteinase namely the 72 kDa gelatinase. Time-course uptake experiments showed that estramustine was incorporated into the cells, a process which depended on temperature. On the other hand, immunofluorescence studies indicated that the microtubule network was affected by taxol with the formation of bundles of microtubules at different cell domains. Minor effects were visualized after treatment of the cells with estramustine-phosphate, a drug that blocks primarily the action of microtubule-associated proteins. The studies provide a way to analyse the relationships between u-PA secretion and the integrity of the cytoskeletal network.

Estramustine induces disorganization of microtubules, perinuclear retraction of vimentin and endoplasmatic reticulum, and inhibits cell migration

The effects of the mitotic inhibitor estramustine on the cytoskeleton of DU 145 and AG 1518 cells were studied. Estramustine caused a partial disassembly of microtubules and withdrawal of microtubules from the cell periphery, disorganized microtubules and delayed regrowth of disassembled microtubules. It also induced a spheroid cellular morphology and affected cellular adhesion and survival. Sometimes microtubules seemed to be organized from several microtubule-organizing centers. The cytoskeleton-dependent cell migration was inhibited in the presence of estramustine and the microtubule-interacting vimentin and endoplasmatic reticulum retracted to the perinuclear area. Our results show that not only a complete disassembly of microtubules, but also disturbances of the microtubule network can have dramatic effects on microtubule-dependent processes and localization of cellular organelles. These effects could be of importance in the treatment of prostatic carcinoma with estramustine.

Phosphorylation of tau protein in tau-transfected 3T3 cells

The tau protein of Alzheimer paired helical filaments (PHFs) is aberrantly phosphorylated, as evidenced by its reactivity with several phosphate-dependent antibodies. We sought to identify whether this unusual phosphorylation state exists in tau expressed by transfected NIH 3T3 fibroblasts. Immunoblot analysis of cell clones transfected with constructs for either the 3-repeat or 4-repeat isoforms of tau revealed two tau bands, with the lower band migrating with unmodified tau in each case. Antibodies T3P and tau-1 were used to probe these bands, as they also react with PHF-tau in a phosphate-dependent manner. The epitopes for both antibodies were phosphorylated in both tau isoforms. Only the upper band was phosphorylated at the T3P site whereas phosphorylation at the tau-1 site was not always associated with a shift of tau mobility on gels. Tau in both bands was soluble, in contrast to PHF-tau, and was competent to bind to exogenously added bovine microtubules. Colchicine treatment of the cells resulted in an inhibition of phosphorylation at both sites, through an unknown mechanism. In conclusion human tau expressed in 3T3 cells was phosphorylated at the T3P and tau-1 sites as is PHF-tau, although no PHFs formed and the phosphorylated tau was competent to bind to microtubules.

A tau fragment containing a repetitive sequence induces bundling of actin filaments

Much indirect evidence suggests that the interconnections of actin microfilaments with the microtubule system are mediated by microtubule-associated proteins (MAPs). In this study we provide new data to support the interaction of a specific tubulin-binding domain on tau with actin in vitro. In actin polymerization assays, the synthetic peptide VRSKIGSTENLKHQPGGG, corresponding to the first repetitive sequence of tau protein, increased turbidity at 320 nm in a dose-dependent fashion. A salient feature of the tau peptide-induced assembly process is the formation of a large amount of actin filament bundles, as revealed by electron microscopic analysis. An increase in the tau peptide concentration resulted in a proportional increase in the bundling of actin filaments. It is interesting that a gradual decrease of pH within the range 7.6-4.7 resulted in a higher effect of tau peptide in promoting bundles of actin filaments. A similar pH-dependent effect was observed for tau protein-induced bundling. An analysis of the mechanisms that operate in the peptide induction of actin filament bundles suggests the involvement of electrostatic forces, because the neutralization of epsilon-aminolysyl residues by selective carbamoylation resulted in a complete loss of the peptide induction of actin bundles. The data suggest that a tau repetitive sequence (also found in MAP-2 and MAP-4) containing a common tubulin binding motif may constitute a functional domain on tau for the dynamics of the interconnections between actin filaments and microtubules.

The antineoplastic agent estramustine and the derivative estramustine-phosphate inhibit secretion of interleukin-3 in leukemic cells. Possible roles of MAPs

The antineoplastic drug estramustine is an adduct of estradiol and nor-nitrogen mustard. It has been shown that this drug interferes with microtubule assembly, an effect mediated by estramustine interaction with microtubule-associated proteins (MAPs). In the present report we demonstrate that estramustine and the phosphorylated derivative of the drug, estramustine-phosphate, inhibit the secretion of interleukin-3 by WEHI-3B cells. These studies also show that the estramustine derivative specifically interacts with a MAPs component found in these cells, which exhibited characteristics ressembling those of tau protein isoforms. Western blots using a unique monoclonal antibody MTB6.22 that recognizes microtubule-binding domains on MAPs, indicated that this WEHI protein factor contained the antigenic determinant that are functionally significant for microtubule assembly. ELISA assays using this antibody, also showed a decrease in the levels of the immunoreactive protein in WEHI cells after treatment with EMP. Interestingly, it has been recently described that the action of estramustine-phosphate is mediated by a direct interaction with MAP-binding sites on the microtubule surface, which are recognized by the site-specific monoclonal antibody. These findings together with immuno-precipitation experiments using anti-interleukin-3 antibodies and the inhibitory effect of the estramustine derivative on WEHI secretion process suggest that this anti-mitotic agent may block IL-3 secretion by a mechanism involving its interaction with a 'tau-like' MAPs component present in these cells.