Mechanism of action of the antimitotic drug 2,4-dichlorobenzyl thiocyanate: alkylation of sulfhydryl group(s) of beta-tubulin

Characterization of the covalent binding of N-phenyl-N'-(2-chloroethyl)ureas to {beta}-tubulin: importance of Glu198 in microtubule stability

N-Phenyl-N'-(2-chloroethyl)ureas (CEUs) are antimicrotubule agents interacting covalently with β-tubulin near the colchicine-binding site (C-BS). Glutamyl 198 residue in β-tubulin (Glu198), which is adjacent to the C-BS behind the two potent nucleophilic residues, Cys239 and Cys354, has been shown to covalently react with 1-(2-chloroethyl)-3-(4-iodophenyl)urea (ICEU). By use of mass spectrometry, we have now identified residues in β-tubulin that have become modified irreversibly by 1-(2-chloroethyl)-3-[3-(5-hydroxypentyl)phenyl]urea (HPCEU), 1-[4-(3-hydroxy-4-methoxystyryl)phenyl]-3-(2-chloroethyl)urea (4ZCombCEU), and N,N'-ethylenebis(iodoacetamide) (EBI). The binding of HPCEU and 4ZCombCEU to β-tubulin resulted in the acylation of Glu198, a protein modification of uncommon occurrence in living cells. Prototypical CEUs then were used as molecular probes to assess, in mouse B16F0 and human MDA-MB-231 cells, the role of Glu198 in microtubule stability. For that purpose, we studied the effect of Glu198 modification by ICEU, HPCEU, and 4ZCombCEU on the acetylation of Lys40 on α-tubulin, a key indicator of microtubule stability. We show that modification of Glu198 by prototypical CEUs correlates with a decrease in Lys40 acetylation, as observed also with other microtubule depolymerizing agents. Therefore, CEU affects the stability and the dynamics of microtubule, likewise a E198G mutation, which is unusual for xenobiotics. We demonstrate for the first time that EBI forms an intramolecular cross-link between Cys239 and Cys354 of β-tubulin in living cells. This work establishes a novel basis for the development of future chemotherapeutic agents and provides a framework for the design of molecules useful for studying the role of Asp and Glu residues in the structure/function and the biological activity of several cellular proteins under physiological conditions.

2-Cyano-3,12-dioxooleana-1,9(11)-diene-28-oic Acid Disrupts Microtubule Polymerization: A Possible Mechanism Contributing to Apoptosis

The semisynthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO) has several biological activities, including the induction of apoptosis in many cancer cell lines. To identify potential protein targets, immobilized biotinylated CDDO was used to screen the proteome of a human lymphoma cell line (U937) sensitive to CDDO-induced apoptosis. Tubulin was identified as one of several putative targets of CDDO. CDDO was shown to selectively bind to tubulin, with a dissociation constant of approximately 7 microM, and to disrupt microtubules both in vivo and in vitro. CDDO inhibits tubulin polymerization in vitro, possibly through interactions with a hydrophobic site on beta-tubulin. The CDDO-tubulin interaction may also involve a reversible 1,4-addition with a protein sulfhydryl group. Unlike other known spindle poisons, CDDO does not result in a temporal increase in the mitotic index. Rather, CDDO seems to initiate apoptosis early in M phase.

A Radioligand Binding Assay for Antitubulin Activity in Tumor Cells

The benzamide RH-5854 is shown to be highly potent toward tumor cells and to arrest nuclear division by a highly specific covalent binding to the β-subunit of tubulin in the colchicine binding region. Binding of 3H-RH-5854 to β-tubulin in HCT-116 colon cancer cells is saturable and has been exploited in the development of a cell-based competitive binding assay, which allows antitubulin effects to be detected inwhole cells. 3H-RH-5854 binding is strongly inhibited by preincubating the cells with compounds that bind to the colchicine site andwith paclitaxel. Binding of 3H-RH-5854 is enhanced by preincubating the cells with vinblastine but not by other agents that bind at or near the vinblastine site (ansamitocin P-3 and phomopsin A). Various cytotoxic agents that do not act on tubulin do not affect binding of 3H-RH-5854 in HCT-116 cells, demonstrating specificity of the assay for detection of antitubulin activity. As an alternative to traditional assays that employ isolated brain tubulin, the 3HRH-5854 binding assay enables screening for antitubulin effects directly in tumor cells, providing an assay that accounts for cell-specific criteria that influence sensitivity such as different tubulin isotypes, tubulin mutations, drug metabolism, and efflux mechanisms.

Antikinetoplastid activity of 3-aryl-5-thiocyanatomethyl-1,2,4-oxadiazoles

A series of 5-thiocyanatomethyl- and 5-alkyl-3-aryl-1,2,4-oxadiazoles were synthesized and evaluated for their activity against kinetoplastid parasites. Formation of the oxadiazole ring was accomplished through the reaction of benzamidoximes with acyl chlorides, while the thiocyanate group was inserted by reacting the appropriate 5-halomethyl oxadiazole with ammonium thiocyanate. The thiocyanate-containing compounds possessed low micromolar activity against Leishmania donovani and Trypanosoma brucei, while the 5-alkyl oxadiazoles were less active against these parasites. 3-(4-Chlorophenyl)-5-(thiocyanatomethyl)-1,2,4-oxadiazole (compound 4b) displayed modest selectivity for L. donovani axenic amastigote-like parasites over J774 macrophages, PC3 prostate cancer cells, and Vero cells (6.4-fold, 3.8-fold, and 9.1-fold, respectively), while 3-(3,4-dichlorophenyl)-5-(thiocyanatomethyl)-1,2,4-oxadiazole (compound 4 h) showed 30-fold selectivity against Vero cells but was not selective against PC3 cells. In a murine model of visceral leishmaniasis, compound 4b decreased liver parasitemia caused by L. donovani by 48% when given in five daily i.v. doses at 5mg/kg and by 61% when administered orally for 5 days at 50 mg/kg. These results indicate that aromatic thiocyanates hold promise for the treatment of leishmanial infections if the selectivity of these compounds can be improved.

Cellular effects of leishmanial tubulin inhibitors on L. donovani

To aid our investigation of tubulin as an antileishmanial drug target, the effects of the mammalian antimicrotubule agents ansamitocin P3, taxol, and hemiasterlin on Leishmania donovani promastigotes were described. These drugs affected the assembly of purified leishmanial tubulin and inhibited the growth of L. donovani promastigotes at micromolar concentrations. When promastigotes were treated with these agents, mitotic partitioning of nuclear DNA and cytokinesis were usually inhibited. The spatial orientation of kinetoplasts was often disturbed, suggesting a role for microtubules in the segregation of these organelles during mitosis. Aberrant cell types produced in drug-treated cultures included parasites with one nucleus and two geometrically distinct kinetoplasts, parasites with multiple kinetoplasts, and cytoplasts containing a kinetoplast but no nucleus. A subset of unique cell types, parasites containing two nuclei, a spindle fiber, and two geometrically distinct kinetoplasts, were observed in hemiasterlin-treated cultures. Flow cytometric analysis of L. donovani promastigotes treated with these three drugs indicated a dramatic shift toward the G2 + M phase of the cell cycle, with some cells containing four times the amount of DNA present in G1. These results were used to evaluate the cellular effects of WR85915, an aromatic thiocyanate with in vitro antileishmanial and anti-tubulin activity, on L. donovani. Treatment of parasites with WR85915 did not produce the unusual cell types described above and did not cause the accumulation of parasites in G2 + M, suggesting that WR85915 acts on target(s) in Leishmania in addition to tubulin. These studies validate tubulin as a suitable antileishmanial drug target and provide criteria to assess the cellular mechanism of action of new candidate antileishmanial agents.

RPR112378 and RPR115781: two representatives of a new family of microtubule assembly inhibitors

A screening program aimed at the discovery of new antimicrotubule agents yielded RPR112378 and RPR115781, two natural compounds extracted from the Indian plant Ottelia alismoides. We report their isolation, structural determination, and mechanisms of action. RPR112378 is an efficient inhibitor of tubulin polymerization (IC(50) = 1.2 microM) and is able to disassemble preformed microtubules. Regarding tubulin activity, RPR115781 is 5-fold less active than RPR112378. Tubulin-RPR112378 complexes, when isolated by gel filtration, were able to block further tubulin addition to growing microtubules, a mechanism that accounts for the substoichiometric effect of the drug. RPR112378 was found to prevent colchicine binding but not vinblastine binding to tubulin. Although colchicine binding is known to induce an increase of tubulin GTPase activity, no such increase was observed with RPR112378. We show that RPR112378 is a highly cytotoxic compound and that RPR115781 is 10, 000-fold less active as an inhibitor of KB cell growth. Part of the cytotoxicity of RPR112378 is probably caused by a reaction of addition with sulfhydryl groups, an observation that has not been made with RPR115781. In conclusion, these molecules represent a new class of inhibitors of microtubule assembly with potential therapeutic value.

Purification, characterization, and drug susceptibility of tubulin from Leishmania

Past work suggests that tubulin from kinetoplastid parasites may present an excellent drug target. To explore this possibility, tubulin was purified on a milligram scale from Leishmania mexicana amazonensis promastigotes by sonication, DEAE-Sepharose chromatography, and one cycle of assembly-disassembly. Purified leishmanial tubulin is recognized by commercially available anti-tubulin antibodies and displays concentration dependent assembly in vitro. The vinca site agents vinblastine, maytansine, and rhizoxin bind to leishmanial tubulin as assessed by the quenching of intrinsic tubulin fluorescence and the alteration of the proteins reactivity with the sulfhydryl-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid). They also interfere with the assembly of leishmanial tubulin at low micromolar concentrations. Electrophilic compounds such as phenyl arsenoxide and 4-chloro-3,5-dinitro-alpha,alpha,alpha-trifluorotoluene (chloralin), which are of interest as traditional and experimental antiparasitic agents, respectively, inhibit the assembly of leishmanial tubulin in vitro as well. Colchicine-site agents and trifluralin, on the other hand, have little or no effect on leishmanial tubulin in these assays. Maytansine, taxol, and the electrophiles block the growth of Leishmania donovani amastigote-like forms in vitro at low ( <1 microM) concentrations, while colchicine site agents, trifluralin, vinblastine, and rhizoxin are at least two orders of magnitude less toxic to the parasite.

Tubulin as a target for anticancer drugs: agents which interact with the mitotic spindle

Tubulin is the biochemical target for several clinically used anticancer drugs, including paclitaxel and the vinca alkaloids vincristine and vinblastine. This review describes both the natural and synthetic agents which are known to interact with tubulin. Syntheses of the more complex agents are referenced and the potential clinical use of the compounds is discussed. This review describes the biochemistry of tubulin, microtubules, and the mitotic spindle. The agents are discussed in relation to the type of binding site on the protein with which they interact. These are the colchicine, vinca alkaloid, rhizoxin/maytansine, and tubulin sulfhydryl binding sites. Also included are the agents which either bind at other sites or unknown sites on tubulin. The literature is reviewed up to October 1997.

Identification of cysteine 354 of beta-tubulin as part of the binding site for the A ring of colchicine

The colchicine analog 3-chloroacetyl-3-demthylthio-colchicine (3CTC) is a competitive inhibitor of colchicine binding to tubulin, binds to tubulin at 37 degrees C, but not at 0 degree C, and covalently reacts with beta-tubulin at 37 degree C, but not at 0 degree C, in a reaction inhibited by colchicine site drugs. The approximate intramolecular distance between the oxygen at position C-3 in 3CTC and the chlorine atom of the 3-chloroacetyl group is 3 A. using decylagarose chromatography, we purified beta-tubulin that had reacted with 3-(chloromethyl-[14C] Carbonyl)-3- demethylthiocolchicine ([14C]3CTC). This beta-tubulin that had reacted with 3-(chloromethyl-[14C]carbonyl)- 3-demethythiocolchicine ([14C]3CTC). This beta-tubulin was digested with formic acid, cyanogen bromide, endoproteinase Glu-C, or endoproteinase Lys-C, and the radio-labeled peptide(s) were isolated. The sequences of these peptides indicated that as much as 90% of the covalent reaction between the [14C]3CTC and beta-tubulin occurred at cysteine 354. This finding indicates that the C-3 oxygen atom of colchicinoids is within 3 A of the sulfur atom of the Cys-354 residue, suggests that the colchicine A ring lies between Cys-354 and Cys-239, based on the known 9 A distance between these residues, and may indicate that the tropolone C ring lies between the peptide region containing Cys-239 and the amino-terminal beta-tubulin sequence, based on the labeling pattern observed following direct photoactivation of tubulin-bound colchicine.

Antimicrotubular effect of calvatic acid and of some related compounds

A structure-activity relationship has been established between calvatic acid and some related synthetic compounds, and their ability to inhibit GTP-induced microtubular protein polymerization in vitro. These compounds were effective in a dose- and a time-dependent manner. The most active drug was the p-chloro substituted compound, which showed its inhibitory activity without any preincubation period, which the others needed. Since if cysteine was present, polymerization was no longer affected, an involvement of titratable -SH groups of tubulin could be suggested. In contrast, taxol-induced polymerization was only slightly inhibited by these compounds, and colchicine-binding activity was not generally impaired.

Studies on modulation of the effects of colchicine by L-cysteine using bone marrow of Swiss mice

Colchicine (COL) elevates the frequency of micronucleated polychromatic erythrocytes (PE), the ratio of normochromatic to polychromatic erythrocytes (N/PE) and the frequency of large PE due to spindle disruption. Simultaneous i.p. injection of L-cysteine (CYS) does not influence the effects of COL while if administered 1 h prior to COL, CYS suppresses the N/PE ratio and frequency of large PE but not the frequency of micronucleated PE elevated by COL. Preincubation of CYS with COL at 37 degrees C for 1 h results in a significant decrease in all the COL effects. The modulatory effect of exogenous CYS appears to be due to its competition with the endogenous tubulin cysteine residues for interacting with COL.

Possible regulation of the in vitro assembly of bovine brain tubulin by the bovine thioredoxin system

Microtubule assembly in vitro and in vivo is highly sensitive to a variety of sulfhydryl-reactive reagents, raising the question of the possible existence of a physiological sulfhydryl-mediated system for regulating microtubule assembly. However, the specific reagents which have previously been used to inhibit microtubule assembly in vitro are either nonphysiological or, if physiological, effective only at concentrations much higher than their physiological ones. Because of reports of association in vivo between microtubules and the sulfhydryl-reactive proteins thioredoxin and thioredoxin reductase, we decided to examine the interaction in vitro between microtubules and the thioredoxin system, comprising thioredoxin, thioredoxin reductase and NADPH. At pH 6.8, both the mammalian and the Escherichia coli thioredoxin systems inhibited microtubule assembly by 4-35% (19 +/- 9%) by reducing one intra-subunit disulfide bond in the tubulin dimer. The thioredoxin-reducible disulfide of the tubulin dimer remains protected from thioredoxin in the assembled microtubules. Thioredoxin or thioredoxin reductase alone, or together in the absence of NADPH, were incapable of either reducing tubulin or inhibiting microtubule assembly. Microtubules formed from reduced tubulin were found to be stable and morphologically identical to those obtained from native tubulin dimers. Since the components of the thioredoxin system were used at concentrations similar to their physiological ones, our results suggest a potential role of the thioredoxin system in regulation of microtubule assembly in vivo.

Sulfhydryl reagents alter epidermal growth factor receptor affinity and association with the cytoskeleton

Sulfhydryl (SH) reagents are known to influence the characteristics of many ligand-receptor systems. The SH reagent N-ethylmaleimide has been demonstrated to interact with EGF receptors, and to inhibit EGF receptor kinase activity. The data presented in this paper concern the effect of SH reagents on two intriguing features of the EGF receptor system, namely the presence of low and high affinity EGF binding sites, and the interaction of EGF receptors with the cytoskeleton. SH reagents were observed to induce a disappearance of high, but not low, affinity EGF receptors from the cell surface, and an increase in receptor-cytoskeleton interaction. Comparison of the effects of membrane-permeant and membrane-impermeant SH reagents on wild type and structurally modified EGF receptors suggested that sulfhydryl groups on the cytoplasmic, rather than the extracellular, receptor domain are involved. This indicates that the cytoplasmic domain of the EGF receptor plays a role in the high affinity binding of EGF, and in the interaction of EGF receptors with the cytoskeleton. Experiments with an anti-EGF receptor antibody that specifically blocks the binding of EGF to low affinity receptors indicated that EGF induces a shift in the EGF receptor from low to high affinity. SH reagents probably affect EGF binding by inhibiting this EGF-induced receptor conversion.