The role of the B-ring of colchicine in the stability of the colchicine-tubulin complex

Interaction of pseudolaric acid B with the colchicine site of tubulin

We purified pseudolaric acid B (PAB) from the root and stem bark of Pseudolarix kaempferi (Lindl.) Gorden. Confirming previous findings, we found that the compound had high nanomolar IC₅₀ antiproliferative effects in several cultured cell lines, causing mitotic arrest and the disappearance of intracellular microtubules. PAB strongly inhibited tubulin assembly (IC₅₀, 1.1 μM) but weakly inhibited the binding of colchicine to tubulin, as demonstrated by fluorescence and with [³H]colchicine. Kinetic analysis demonstrated that the mechanism of inhibition was competitive, with an apparent K(i) of 12-15 μM. Indirect studies demonstrated that PAB bound rapidly to tubulin and dissociated more rapidly from tubulin than the colchicine analog 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone, whose complex with tubulin is known to have a half-life of 17s at 37 °C. We modeled PAB into the colchicine site of tubulin, using the crystal structure 1SA0 that contains two αβ-tubulin heterodimers, both bound to a colchicinoid and to a stathmin fragment. The binding model of PAB revealed common pharmacophoric features between PAB and colchicinoids, not readily apparent from their chemical structures.

Drugs that target dynamic microtubules: a new molecular perspective

Microtubules have long been considered an ideal target for anticancer drugs because of the essential role they play in mitosis, forming the dynamic spindle apparatus. As such, there is a wide variety of compounds currently in clinical use and in development that act as antimitotic agents by altering microtubule dynamics. Although these diverse molecules are known to affect microtubule dynamics upon binding to one of the three established drug domains (taxane, vinca alkaloid, or colchicine site), the exact mechanism by which each drug works is still an area of intense speculation and research. In this study, we review the effects of microtubule-binding chemotherapeutic agents from a new perspective, considering how their mode of binding induces conformational changes and alters biological function relative to the molecular vectors of microtubule assembly or disassembly. These "biological vectors" can thus be used as a spatiotemporal context to describe molecular mechanisms by which microtubule-targeting drugs work.

Synthesis, anticancer activity, and inhibition of tubulin polymerization by conformationally restricted analogues of lavendustin A

Compounds in the lavendustin A series have been shown to inhibit both protein-tyrosine kinases (PTKs) and tubulin polymerization. Since certain lavendustin A derivatives can exist in conformations that resemble both the trans-stilbene structure of the PTK inhibitor piceatannol and the cis-stilbene structure of the tubulin polymerization inhibitor combretastatin A-4, the possibility exists that the ratio of the two types of activities of the lavendustins could be influenced through the synthesis of conformationally restricted analogues. Accordingly, the benzylaniline structure of a series of pharmacologically active lavendustin A fragments was replaced by either their cis- or their trans-stilbene relatives, and effects on both inhibition of tubulin polymerization and cytotoxicity in cancer cell cultures were monitored. Both dihydrostilbene and 1,2-diphenylalkyne congeners were also prepared and evaluated biologically. Surprisingly, conformational restriction of the bridge between the two aromatic rings of the lavendustins had no significant effect on biological activity. On the other hand, conversion of the three phenolic hydroxyl groups of the lavendustin A derivatives to their corresponding methyl ethers consistently abolished their ability to inhibit tubulin polymerization and usually decreased cytotoxicity in cancer cell cultures as well, indicating the importance of at least one of the phenolic hydroxyl groups. Further investigation suggested that the phenolic hydroxyl group in the salicylamide ring was required for activity, while the two phenol moieties in the hydroquinone ring could be methylated with retention of activity. Two of the lavendustin A derivatives displayed IC(50) values of 1.4 microM for inhibition of tubulin polymerization, which ranks them among the most potent of the known tubulin polymerization inhibitors.

The interaction of the B-ring of colchicine with alpha-tubulin: a novel footprinting approach

Tubulin, the major structural component of the microtubules, participates actively in mitotic spindle formation and chromosomal organization during cell division. Tubulin is the major target for a variety of anti-mitotic drugs. Some of the drugs, such as Vinca alkaloids and taxol, are routinely used for cancer chemotherapy. It is unfortunate that our knowledge of the binding sites on tubulin of these drugs is limited because of lack of a useful and appropriate tool. The photoaffinity labeling approach is the major technique available at present to detect the binding sites of drugs on tubulin. This method, however, has several limitations. First, only part of the binding site can be identified, namely, the residues which react with the photoaffinity label. Second, there are regions of tubulin which are not at the binding site but are affected by the binding of the drug; these regions can not be detected by the photoaffinity labeling approach. The third, and perhaps most serious, limitation is that the traditional approach can detect areas which have nothing to do with the binding of the ligand but which are within a certain distance of the binding site, that distance being less than the length of the photoreactive moiety attached to the ligand. There has been a great deal of controversy on the localization of the binding site of colchicine on tubulin, with some reports suggesting that the binding site is on alpha and some supporting a binding site on beta. Colchicine also has significant effects on tubulin conformation, but the regions which are affected have not been identified. We have attempted here to address these questions by a novel "footprinting" method by which the drug-binding sites and as well as the domain of tubulin affected by drug-induced conformational changes could be determined. Here, we report for the first time that the interaction of the B-ring of colchicine with the alpha-subunit affects a domain of tubulin which appears to be far from its binding site. This domain includes the cysteine residues at positions 295, 305, 315 and 316 on alpha-tubulin; these residues are located well away from the alpha/beta interface where colchicine appears to bind. This is correlated with the stabilizing effect of colchicine on the tubulin molecule. Furthermore, we also found that the B-ring of colchicine plays a major role in the stability of tubulin while the A and the C-rings have little effect on it. Our results therefore, support a model whereby colchicine binds at the alpha/beta interface of tubulin with the B-ring on the alpha-subunit and the A and the C-rings on the beta-subunit.

The use of structure-activity relationships and markers of cell toxicity to detect non-genotoxic carcinogens

In contrast to the situation for genotoxic carcinogens, few in vitro tests exist that can detect early markers of the events thought to be associated with non-genotoxic carcinogenesis. Also, comparatively little is known about the quantitative structure-activity relationships (Q)SARs of these agents. This review discusses published SAR studies conducted on non-genotoxic carcinogens, in relation to the use of several markers of in vitro cell toxicity (inhibition of gap-junctional intercellular communication, inhibition of tubulin polymerization, modulation of apoptosis and induction of cell proliferation), which are used as endpoints for screening this class of carcinogen. Much of the work has involved the identification of new biophores (substructural features of molecules associated with toxicity), as well as other structural features, which are thought to predispose the chemicals to ligand binding with specific target molecules acting as possible receptors (e.g. protein kinase C, the oestrogen, peroxisome-proliferator and tubulin protein receptors), implicated in the mechanism of toxicity involved. It is concluded that (a) there is an urgent need for more information on (Q)SARs for non-genotoxic carcinogens; (b) this information should be acquired by using several different approaches in a variety of laboratories; and (c) such research should proceed together with more studies on the mechanisms of cell toxicity caused by these chemicals, including the identification and characterisation of further specific receptors involved in mediating the various types of cell toxicity associated with this type of carcinogenesis.

The influence of microtubule integrity on plasma membrane fluidity in L929 cells

The aim of this work was to examine the possible influence of the integrity of the microtubule network on the plasma membrane fluidity of L929 mouse fibroblasts. The L929 cell line was selected for the ease of culture and the stability of its characteristics. The cells were treated with colchicine, nocodazole and vinblastine, three microtubule-depolymerizing drugs, at various concentrations and for various times. Membrane fluidity was assessed from fluorescence depolarization measurements with the plasma membrane probe TMA-DPH. Each of the drugs induced a significant, dose-dependent decrease in fluorescence anisotropy. The effect levelled off (5-7% decrease) after approximately 90 min of treatment, and could be unambiguously interpreted as resulting from an increase in membrane fluidity. The cumulative action of the drugs did not significantly increase the effect. The effects of colchicine and nocodazole could be reversed by incubation in drug-free medium, but not that of vinblastine. The results are discussed in correlation with the kinetics of the three drugs interaction with tubulin or microtubules. It is concluded that the microtubule integrity contributed to the high plasma membrane lipidic order, but less than other factors, like the lipid composition and the cholesterol content.

Biosynthesis of radiolabeled curacin A and its rapid and apparently irreversible binding to the colchicine site of tubulin

Curacin A is a potent competitive inhibitor of colchicine binding to tubulin, and it inhibits the growth of tumor cells. We prepared [(14)C]curacin A biosynthetically to investigate its interaction with tubulin. Binding was rapid, even at 0 degrees C, with a minimum k(f) of 4.4 x 10(3) M(-1) s(-1). We were unable to demonstrate any dissociation of the [(14)C]curacin A from tubulin. Consistent with these observations, the K(a) value was so high that an accurate determination by Scatchard analysis was not possible. The [(14)C]curacin A was released from tubulin following urea treatment, indicating that covalent bond formation does not occur. We concluded that curacin A binds more tightly to tubulin than does colchicine. Besides high-affinity binding to the colchicine site, we observed significant superstoichiometric amounts of the [(14)C]curacin A bound to tubulin, and Scatchard analysis confirmed the presence of two binding sites of relatively low affinity with a K(a) of 3.2 x 10(-5) M(-1).

Structure-activity analysis of the interaction of curacin A, the potent colchicine site antimitotic agent, with tubulin and effects of analogs on the growth of MCF-7 breast cancer cells

Originally purified as a major lipid component of a strain of the cyanobacterium Lyngbya majuscula isolated in Curaçao, curacin A is a potent inhibitor of cell growth and mitosis, binding rapidly and tightly at the colchicine site of tubulin. Because its molecular structure differs so greatly from that of colchicine and other colchicine site inhibitors, we prepared a series of curacin A analogs to determine the important structural features of the molecule. These modifications include reduction and E-to-Z transitions of the olefinic bonds in the 14-carbon side chain of the molecule; disruption of and configurational changes in the cyclopropyl moiety; disruption, oxidation, and configurational reversal in the thiazoline moiety; configurational reversal and substituent modifications at C13; and demethylation at C10. Inhibitory effects on tubulin assembly, the binding of colchicine to tubulin, and the growth of MCF-7 human breast carcinoma cells were examined. The most important portions of curacin A required for its interaction with tubulin seem to be the thiazoline ring and the side chain at least through C4, the portion of the side chain including the C9-C10 olefinic bond, and the C10 methyl group. Only two modifications totally eliminated the tubulin-drug interaction. The inactive compounds were a segment containing most of the side chain, including its two substituents, and analogs in which the methyl group at the C13 oxygen atom was replaced by a benzoate residue. Antiproliferative activity comparable with that observed with curacin A was only reproduced in compounds that were potent inhibitors of the binding of colchicine to tubulin. Molecular modeling and quantitative structure-activity relationship studies demonstrated that most active analogs overlapped extensively with curacin A but failed to provide an explanation for the apparent structural analogy between curacin A and colchicine.

Interactions of a bicyclic analog of colchicine with beta-tubulin isoforms alphabeta(II), alphabeta(III) and alphabeta(IV)

Tubulin exists as various isoforms, which differ in their assembly, drug-binding properties, and the dynamic properties of the microtubules they compose. One of the most striking differences in drug binding among the isoforms is observed with colchicine, which binds much better to the alphabeta(II) and alphabeta(IV) isoforms than to the alphabeta(III) isoform. Here we have studied the interaction of these isoforms with 2-methoxy-5-(2',3',4'-trimethoxyphenyl) tropone (MTPT), an analog of colchicine that lacks the B-ring. The kinetics of association and dissociation were studied fluorometrically, and the kinetic parameters for the two-step binding were determined for different beta-tubulin isoforms. The apparent on-rate constants for alphabeta(II), alphabeta(III) and alphabeta(IV) were 13358, 4558 and 10828 M(-1) s(-1), the off-rate constants (k(-2)) were 0.04, 0.03 and 0.02 s(-1), and the affinity constants are 3.33 x 10(5), 1.56 x 10(5) and 5.44 x 10(5) M(-1), respectively. The differences in kinetic parameters among different beta-tubulin isoforms are greatly reduced when the B-ring is removed. Our results indicate that the B-ring plays a major role in determining the isoform differences, and the results might be of importance for designing tissue-specific analogs of colchicine for cancer chemotherapy.

Effect of the B ring and the C-7 substituent on the kinetics of colchicinoid-tubulin associations

The kinetics of four B-ring derivatives of colchicine binding to tubulin has been examined quantitatively. The bindings of deacetamidocolchicine, deacetylcolchicine, demecolcine, and N-methyl-demecolcine to tubulin were biphasic processes. The association rate constants were determined as a function of temperature, and the thermodynamic parameters for the transition states of the fast phase were calculated. The kinetic parameters for the formation of the deacetylcolchicine-tubulin, demecolcine-tubulin, and N-methyldemecolcine-tubulin complexes were very similar to each other, but different from the parameters for the colchicine-tubulin association. In particular, the global activation enthalpies for the formation of the three aminocolchicinoid-tubulin complexes were 3-5 kcal/mol greater than the global activation enthalpy of colchicine binding to tubulin. These results indicate that electronic rather than steric properties of the B-ring substituent are of greater importance in the activation enthalpy of colchicinoids binding to tubulin. In contrast, the global activation enthalpy for deacetamidocolchicine, which lacks a substituent on the C-7 carbon, binding to tubulin was virtually identical to the global activation enthalpy previously found for the colchicine analog that lacks the B ring, 2-methoxy-5-(2,3,4-trimethoxyphenyl)tropone, binding to tubulin (Bane, S., Puett, D., Macdonald, T. L., & Williams, R. C., Jr. (1984) J. Biol. Chem. 259, 7391-7398). This result demonstrates that the carbons of the B ring are not involved in the transition state for the formation of colchicinoid-tubulin complexes. The first-order dissociation rate constants of the colchicinoid-tubulin complexes were determined at 37 degrees C. The dissociation profiles of the colchicinoid-tubulin complexes also consisted of two phases.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.

Conformation of thiocolchicine and two B-ring-modified analogues bound to tubulin studied with optical spectroscopy

The interaction of tubulin with thiocolchicine and two thiocolchicine analogues, one lacking the B ring and one with a six-membered B ring, has been studied by using near-UV and CD spectroscopies. Rapid, reversible binding of the latter analogue to tubulin demonstrates the ability of the colchicine binding site to accommodate the phenyltropone system with a more coplanar conformation than is present in free colchicine. There is no evidence, however, that bound thiocolchicine should have a much less twisted conformation than free thiocolchicine. Thiocolchicine and the bicyclic analogue appear to have approximately the same conformation of the phenyltropone system, in both the free and the bound states, suggesting that this conformation has an optimal arrangement of the phenyl and tropone rings for binding to tubulin. In contrast to colchicine and related derivatives, the three thiocolchicine analogues show pronounced near-UV CD bands upon association to tubulin. No simple relation could be found between the sign pattern of the CD components in the near-UV band of the thiocolchicinoid chromophore and its axial chirality.

Antimitotic natural products combretastatin A-4 and combretastatin A-2: studies on the mechanism of their inhibition of the binding of colchicine to tubulin

Combretastatin A-4 (CS-A4), 3,4,5-trimethoxy-3'-hydroxy-4'-methoxy-(Z)-stilbene, and combretastatin A-2 (CS-A2), 3,4-(methylenedioxy)-5-methoxy-3'-hydroxy-4'-methoxy-(Z)-stilbene, are structurally simple natural products isolated from the South African tree Combretum caffrum. They inhibit mitosis and microtubule assembly and are competitive inhibitors of the binding of colchicine to tubulin [Lin et al. (1988) Mol. Pharmacol. 34, 200-208]. In contrast to colchicine, drug effects on tubulin were not enhanced by preincubating CS-A4 or CS-A2 with the protein. The mechanism of their binding to tubulin was examined indirectly by evaluating their effects on the binding of radiolabeled colchicine to the protein. These studies demonstrated rapid binding of both compounds to tubulin even at 0 degrees C (binding was complete at the earliest times examined), in contrast to the relatively slow and temperature-dependent binding of colchicine. Although the binding of the C. caffrum compounds to tubulin was quite tight, permitting ready isolation of near-stoichiometric amounts of drug-tubulin complex even in the absence of free drug, both CS-A4 and CS-A2 dissociated rapidly from tubulin in the presence of high concentrations of radiolabeled colchicine. Apparent rate constants for drug dissociation from tubulin at 37 degrees C were 3.2 x 10(-3) s-1 for CS-A4, 4.8 x 10(-3) s-1 for CS-A2, and 2.9 x 10(-5) s-1 for colchicine (half-lives of 3.6, 2.4, and 405 min, respectively). Thus, the effectiveness of the C. caffrum compounds as antimitotic agents appears to derive primarily from the rapidity of their binding to tubulin.(ABSTRACT TRUNCATED AT 250 WORDS)