Effect of phomopsin A on the alkylation of tubulin

Marine Antitumor Peptide Dolastatin 10: Biological Activity, Structural Modification and Synthetic Chemistry

Dolastatin 10 (Dol-10), a leading marine pentapeptide isolated from the Indian Ocean mollusk Dolabella auricularia, contains three unique amino acid residues. Dol-10 can effectively induce apoptosis of lung cancer cells and other tumor cells at nanomolar concentration, and it has been developed into commercial drugs for treating some specific lymphomas, so it has received wide attention in recent years. In vitro experiments showed that Dol-10 and its derivatives were highly lethal to common tumor cells, such as L1210 leukemia cells (IC50 = 0.03 nM), small cell lung cancer NCI-H69 cells (IC50 = 0.059 nM), and human prostate cancer DU-145 cells (IC50 = 0.5 nM), etc. With the rise of antibody-drug conjugates (ADCs), milestone progress was made in clinical research based on Dol-10. A variety of ADCs constructed by combining MMAE or MMAF (Dol-10 derivatives) with a specific antibody not only ensured the antitumor activity of the drugs themself but also improved their tumor targeting and reduced the systemic toxicity. They are currently undergoing clinical trials or have been approved for marketing, such as Adcetris®, which had been approved for the treatment of anaplastic large T-cell systemic malignant lymphoma and Hodgkin lymphoma. Dol-10, as one of the most medically valuable natural compounds discovered up to now, has brought unprecedented hope for tumor treatment. It is particularly noteworthy that, by modifying the chemical structure of Dol-10 and combining with the application of ADCs technology, Dol-10 as a new drug candidate still has great potential for development. In this review, the biological activity and chemical work of Dol-10 in the advance of antitumor drugs in the last 35 years will be summarized, which will provide the support for pharmaceutical researchers interested in leading exploration of antitumor marine peptides.

Molecular dynamics modeling of tubulin C-terminal tail interactions with the microtubule surface

Tubulin, an α/β heterodimer, has had most of its 3D structure analyzed; however, the carboxy (C)-termini remain elusive. Importantly, the C-termini play critical roles in regulating microtubule structure and function. They are sites of most of the post-translational modifications of tubulin and interaction sites with molecular motors and microtubule-associated proteins. Simulated annealing was used in our molecular dynamics modeling to predict the interactions of the C-terminal tails with the tubulin dimer. We examined differences in their flexibility, interactions with the body of tubulin, and the existence of structural motifs. We found that the α-tubulin tail interacts with the H11 helix of β-tubulin, and the β-tubulin tail interacts with the H11 helix of α-tubulin. Tail domains and H10/B9 loops interact with each other and compete for interactions with positively-charged residues of the H11 helix on the neighboring monomer. In a simulation in which α-tubulin's H10/B9 loop switches on sub-nanosecond intervals between interactions with the C-terminal tail of α-tubulin and the H11 helix of β-tubulin, the intermediate domain of α-tubulin showed more fluctuations compared to those in the other simulations, indicating that tail domains may cause shifts in the position of this domain. This suggests that C-termini may affect the conformation of the tubulin dimer which may explain their essential function in microtubule formation and effects on ligand binding to microtubules. Our modeling also provides evidence for a disordered-helical/helical double-state system of the T3/H3 region of the microtubule, which could be linked to depolymerization following GTP hydrolysis.

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.

Interactions of antimitotic peptides and depsipeptides with tubulin

Tubulin is the target for an ever increasing number of structurally unusual peptides and depsipeptides isolated from a wide range of organisms. Since tubulin is the subunit protein of microtubules, the compounds are usually potently toxic to mammalian cells. Without exception, these (depsi)peptides disrupt cellular microtubules and prevent spindle formation. This causes cells to accumulate at the G2/M phase of the cell cycle through inhibition of mitosis. In biochemical assays, the compounds inhibit microtubule assembly from tubulin and suppress microtubule dynamics at low concentrations. Most of the (depsi)peptides inhibit the binding of Catharanthus alkaloids to tubulin in a noncompetitive manner, GTP hydrolysis by tubulin, and nucleotide turnover at the exchangeable GTP site on beta-tubulin. In general, the (depsi)peptides induce the formation of tubulin oligomers of aberrant morphology. In all cases tubulin rings appear to be formed, but these rings differ in diameter, depending on the (depsi)peptide present during their formation.

Distinct and overlapping binding sites for IKP104 and vinblastine on tubulin

IKP104 is one of a group of tubulin-binding drugs whose interaction with tubulin suggests that it may bind to the protein at or close to the region where vinblastine binds. By itself IKP104 is a potent enhancer of tubulin decay as evidenced by the fact that it induces the exposure of the sulfhydryl groups and hydrophobic areas on tubulin. In this respect, IKP104 differs from vinblastine and other drugs such as phomopsin A, dolastatin 10, rhizoxin, and maytansine which are competitive or noncompetitive inhibitors of vinblastine binding. In contrast, however, in the presence of colchicine, IKP104 behaves differently and strongly stabilizes tubulin, to an extent much greater than does colchicine alone. IKP104 appears to have two classes of binding site on tubulin, differing in affinity; the acceleration of decay appears to be mediated by the low-affinity site (Chaudhuri et al., 1998, J. Protein Chem., in press). We investigated the relationship of the binding of IKP104 and vinblastine. We found that the high-affinity site or sites of IKP104 overlap with or interact with the vinblastine-binding sites, but that the low-affinity site is distinctly different.

Interaction of phomopsin A with normal and subtilisin-treated bovine brain tubulin

Tubulin, the major component of microtubules, has a tendency to lose its ability to assemble or to bind to ligands in a time-dependent process known as "decay." The decay process also causes tubulin to expose sulfhydryl groups and hydrophobic areas. The antimitotic drug phomopsin A strongly protects the tubulin molecule from decay. Here we have studied the interaction of phomopsin A with alpha beta tubulin and tubulin which has been treated with subtilisin to remove selectively the C-termini of the alpha and beta chains (alpha(s) beta(s)). The binding of phomopsin A to alpha beta tubulin decreases the sulfhydryl titer by approximately 1.0 mol/mol. Selective removal of the peptides from the C-terminal ends does not affect phomopsin A's interaction with tubulin. Moreover, the alpha(s) beta(s) tubulin-phomopsin A complex appears to be more stable than the alpha bet tubulin-phomopsin A complex as determined by the time-dependent increase in exposure of sulfhydryl groups and hydrophobic areas on tubulin. In fact, phomopsin A inhibits the decay process of alpha(s) beta(s) tubulin completely. This observation raises the possibility of determining the conformation of this configuration of tubulin.

Interaction of ustiloxin A with bovine brain tubulin

Ustiloxin A is a modified peptide derived from false smut balls on rice panicles, caused by the fungus Ustilaginoidea virens; structurally, it resembles phomopsin A. Ustiloxin A is cytotoxic and is an inhibitor of microtubule assembly in vitro. Because of its resemblance to phomopsin A, we examined its interaction with tubulin and compared the results with those obtained with phomopsin A and dolastatin 10, both of which were found previously to have very similar effects. We determined that ustiloxin A inhibited the formation of a particular intra-chain cross-link in beta-tubulin, as do vinblastine, maytansine, rhizoxin, phomopsin A, dolastatin 10, halichondrin B and homohalichondrin B; this is in contrast to colchicine and podophyllotoxin which do not inhibit formation of this cross-link. Ustiloxin A also inhibited the alkylation of tubulin by iodo[14C]acetamide, as do phomopsin A and dolastatin 10; vinblastine was almost as potent as inhibitor of alkylation as ustiloxin A, whereas maytansine, halichondrin B and homohalichondrin B have little or no effect. In addition, ustiloxin A inhibited exposure of hydrophobic areas on the surface of the tubulin molecule. In this respect, ustiloxin A was indistinguishable from phomopsin A but slightly more effective than dolastatin 10 and considerably more effective than vinblastine; this provides a strong contrast to maytansine, rhizoxin, and homohalichondrin B which have no effect on exposure of hydrophobic areas and to halichondrin B which enhances exposure. Lastly, ustiloxin A strongly stabilized the binding of [3H]colchicine to tubulin. The combination of ustiloxin A with cholchicine stabilized tubulin with a half-life of over 8 days, comparable with results obtained with phomopsin A and colchicine. A comparison of the structures of ustiloxin A, phomopsin A and dolastatin 10 raised the possibility that the strong stabilization of the tubulin structure may require a short segment of hydrophobic amino acids such as the modified valine-isoleucine sequence present in all three compounds. The rest of the structure, specifically the large ring of ustiloxin A and phomopsin A, may serve to place this sequence in an appropriate conformation to interact with tubulin.

Differential effects of active isomers, segments, and analogs of dolastatin 10 on ligand interactions with tubulin. Correlation with cytotoxicity

Dolastatin 10 is a potent antimitotic peptide isolated from the marine mollusk Dolabella auricularia. Four of its five residues are modified amino acids (in sequence, dolavaline, valine, dolaisoleuine, dolaproine, dolaphenine). Besides inhibiting tubulin polymerization, dolastatin 10 non-competitively inhibits vinca alkaloid binding to tubulin, inhibits nucleotide exchange and formation of the beta s cross-link, and stabilizes the colchicine binding activity of tubulin. To examine the mechanism of action of dolastatin 10 we prepared six chiral isomers, one tri- and one tetrapeptide segment, and one pentapeptide analog of dolastatin 10, all of which differ little from dolastatin 10 as inhibitors of tubulin polymerization. However, only two of the chiral isomers were similar to dolastatin 10 in their cytotoxicity for L1210 murine leukemia cells and in their effects on vinblastine binding, nucleotide exchange, beta s cross-link formation, and colchicine binding. These were isomer 2, with reversal of configuration at position C(19a) in the dolaisoleuine moiety, and isomer 19, with reversal of configuration at position C(6) in the dolaphenine moiety. The pentapeptides with reduced cytotoxicity and reduced effects on tubulin interactions with other ligands were all modified in the dolaproine moiety at positions C(9) and/or C(10). The tripeptide and tetrapeptide segments which inhibited polymerization but not ligand interactions were the amino terminal tripeptide (lacking dolaproine and dolaphenine) and the carboxyl terminal tetrapeptide (lacking dolavaline). We speculate that strong inhibition of other ligand interactions with tubulin requires stable peptide binding to tubulin (i.e. slow dissociation), but that inhibition of polymerization requires only rapid binding to tubulin.

Interaction of halichondrin B and homohalichondrin B with bovine brain tubulin

Halichondrin B is a polyether macrolide of marine origin which binds to tubulin and inhibits microtubule assembly in vitro and in vivo. As is the case with phomopsin A and dolastatin 10, halichondrin B is a non-competitive inhibitor of vinblastine binding to tubulin. Analogous to maytansine, which by contrast is a competitive inhibitor of vinblastine binding, halichondrin B has no effect on colchicine binding, which is greatly stabilized by phomopsin A and dolastatin 10, but not by maytansine. We have previously developed assays which allow sensitive discrimination among the interactions of various ligands with tubulin, and examined the effects of ligands on the reactivity of tubulin sulfhydryl groups and the exposure of hydrophobic areas on the surface of the tubulin molecule. To classify the nature of the interaction between halichondrin B and tubulin, in this study we examined the effects of halichondrin B and its closely related analogue, homohalichondrin B, by these assays. We found that: (1) halichondrin B and homohalichondrin B both inhibited formation of an intra-chain cross-link between two sulfhydryl groups in beta-tubulin, as do phomopsin A, dolastatin 10, maytansine, and vinblastine; (2) halichondrin B resembles maytansine in that it had no effect on alkylation of tubulin sulfhydryl groups by iodoacetamide, unlike phomopsin A, dolastatin 10 and vinblastine, all of which inhibit alkylation; (3) halichondrin B differs from other anti-mitotic drugs in that it enhanced exposure of hydrophobic areas on tubulin; (4) homohalichondrin B, like maytansine and in contrast to phomopsin A, dolastatin 10 and vinblastine, had no effect on exposure of hydrophobic areas; and (5) homohalichondrin B, contrary to maytansine, inhibited alkylation of tubulin sulfhydryl groups in the presence of GTP and MgCl2. In their interactions with the tubulin molecule, halichondrin B and homohalichondrin B appear to have unique conformational effects which differ from those of other drugs and also from the effects of each other as well.

Interaction of dolastatin 10 with bovine brain tubulin

Dolastatin 10 is an unusual peptide of marine origin which binds to tubulin in the vinblastine/maytansine/phomopsin-binding region and potently inhibits mitosis. Using N,N'-ethylenebis(iodoacetamide) (EBI) and iodo[14C]acetamide as probes for the effects of ligands on the thiol groups of tubulin, we found that dolastatin 10 has effects on the sulfhydryls indistinguishable from those of phomopsin A but quite different from those of vinblastine and maytansine. Using the binding of bis-5,5'-[8-(N-phenyl)aminonaphthalene-1-sulfonic acid] (BisANS) as a measure of tubulin decay, we found that dolastatin 10 resembled phomopsin A in that decay was not detectable by this assay in its presence. Interestingly, both otherwise very different peptides are among the most effective inhibitors of tubulin decay yet discovered.

Interaction of phomopsin A with porcine brain tubulin. Inhibition of tubulin polymerization and binding at a rhizoxin binding site

Phomopsin A is an antimitotic cyclic peptide containing a 13-member ring including an ether linkage. It was isolated from the fungus Phomopsis leptostromiformis as the causal agent of lupinosis. Phomopsin A strongly inhibited microtubule assembly (IC50: 2.4 microM). Our study using radiolabeled phomopsin A, prepared biosynthetically by feeding L-[U-14C]isoleucine to the culture of P. leptostromiformis, indicated that at least two binding sites of phomopsin A exist on tubulin on the basis of a Scatchard analysis; i.e. the dissociation constants of a high affinity site (Kd1) and a low affinity site (Kd2) at 37 degrees were determined to be 1 x 10(-8) and 3 x 10(-7) M, respectively. Phomopsin A inhibited the binding of radiolabeled rhizoxin to tubulin with an inhibition constant (Ki) of 0.8 x 10(-8) M. This showed that the high affinity site of phomopsin A is identical to the rhizoxin binding site. The binding of the radiolabeled phomopsin A was also inhibited by rhizoxin and ansamitocin P-3, with an inhibition constant of 10(-7) M, and to a lesser extent by vinblastine. Phomopsin A had no inhibitory effect on colchicine binding to tubulin.

Natural products which interact with tubulin in the vinca domain: maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B

This paper summarizes published data on the interactions of tubulin with antimitotic compounds that inhibit the binding of vinca alkaloids to the protein. These are all relatively complex natural products isolated from higher plants, fungi and marine invertebrate animals. These agents are maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B and their congeners. Effects on tubulin polymerization, ligand binding interactions and structure-activity relationships are emphasized.

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.

Interactions of colchicine with tubulin

Colchicine exerts its biological effects through binding to the soluble tubulin heterodimer, the major component of the microtubule. The colchicine-binding abilities of tubulins from a variety of sources are summarized, and the mechanism of colchicine binding to brain tubulin is explored in depth. The relationship between colchicinoid structure and tubulin binding activity provides insight into the structural features of colchicine responsible for high affinity binding to tubulin and is reviewed for analogs in the colchicine series. The thermodynamic and kinetic aspects of the association are described and evaluated in terms of the binding mechanism. Colchicine binding to tubulin results in unusual alterations in the low energy electronic spectra of colchicine. The spectroscopic features of colchicine bound to tubulin are discussed in terms of the nature of the colchicine-tubulin complex. Attempts to locate the high affinity colchicine binding site on tubulin are presented.

Alpha-, beta-, and gamma-tubulins: sequence comparisons and structural constraints

Comparison of congruent to 160 alpha-, beta-, and gamma-tubulins, and excluding the highly divergent C-terminal peptide, indicates that the three subclasses have similar tertiary structures. Conserved sequences within or between the subclasses have been identified, together with the locations of known epitopes, chemical modifications, and mutations. Evidence is also reviewed concerning the identity of the GTP-binding sites, about which residues are exposed in the assembled microtubule and at subunit:subunit interfaces. These characteristics constrain the possible tertiary structure of the tubulin subunit.

Tubulin sulfhydryl groups as probes and targets for antimitotic and antimicrotubule agents

The sulfhydryl groups of tubulin are highly reactive entities. The reactivity of the sulfhydryl groups is sensitive to the presence of tubulin ligands, making these groups excellent probes for the interaction of tubulin with ligands. When tubulin is reacted with N,N'-ethylenebis-(iodoacetamide), two intrachain cross-links form in the beta subunit. Formation of one of these cross-links is completely blocked by colchicine, podophyllotoxin, and nocodazole; formation of the other is blocked completely by maytansine, phomopsin A and GTP and partly by Vinca alkaloids. Different ligands also differ in their effect on the rate of alkylation of tubulin with iodo[14C]acetamide, with vinblastine and phomopsin A being strong inhibitors and maytansine having very little effect. Oxidation of certain key sulfhydryl groups can inhibit microtubule assembly. One of these sulfhydryl groups appears to be cys239, but there are others not yet identified. Sulfhydryl-oxidizing agents also interfere with microtubule-mediated processes in vivo, raising the question of the existence of a physiological regulator of microtubule assembly. Potential physiological regulators have been examined to see if they can control microtubule assembly in vitro at their physiological concentrations. Of the ones that have been examined, thioredoxin and thioredoxin reductase are much better candidates for being physiological regulators than are either cystamine or glutathione.