Interaction of a cyclostreptin analogue with the microtubule taxoid site: the covalent reaction rapidly follows binding

Microtubule-stabilizing agents: New drug discovery and cancer therapy

Microtubule-stabilizing agents (MSAs) have been highly successful in the treatment of cancer in the past 20years. To date, three classes of MSAs have entered the clinical trial stage or have been approved for clinical anticancer chemotherapy, and more than 10 classes of novel structural MSAs have been derived from natural resources. The microtubule typically contains two MSA-binding sites: the taxoid site and the laulimalide/peloruside site. All defined MSAs are known to bind at either of these sites, with subtle but significant differences. MSAs with different binding sites may produce a synergistic effect. Although having been extensively applied in the clinical setting, paclitaxel and other approved MSAs still pose many challenges such as multidrug resistance, low bioavailability, poor solubility, high toxicity, and low passage through the blood-brain barrier. A variety of studies focus on the structure-activity relationship in order to improve the pharmaceutical properties of these agents. Here, the mechanisms of action, advancements in pharmacological research, and clinical developments of defined MSAs during the past decade are discussed. The latest discovered MSAs are also briefly introduced in this review. The increasing number of natural MSAs indicates the potential discovery of more novel, natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

Taccalonolide binding to tubulin imparts microtubule stability and potent in vivo activity

The taccalonolides are highly acetylated steroids that stabilize cellular microtubules and overcome multiple mechanisms of taxane resistance. Recently, two potent taccalonolides, AF and AJ, were identified that bind to tubulin directly and enhance microtubule polymerization. Extensive studies were conducted to characterize these new taccalonolides. AF and AJ caused aberrant mitotic spindles and bundling of interphase microtubules that differed from the effects of either paclitaxel or laulimalide. AJ also distinctly affected microtubule polymerization in that it enhanced the rate and extent of polymerization in the absence of any noticeable effect on microtubule nucleation. In addition, the resulting microtubules were found to be profoundly cold stable. These data, along with studies showing synergistic antiproliferative effects between AJ and either paclitaxel or laulimalide, suggest a distinct binding site. Direct binding studies demonstrated that AJ could not be displaced from microtubules by paclitaxel, laulimalide, or denaturing conditions, suggesting irreversible binding of AJ to microtubules. Mass spectrometry confirmed a covalent interaction of AJ with a peptide of β-tubulin containing the cyclostreptin-binding sites. Importantly, AJ imparts strong inter-protofilament stability in a manner different from other microtubule stabilizers that covalently bind to tubulin, consistent with the distinct effects of the taccalonolides as compared with other stabilizers. AF was found to be a potent and effective antitumor agent that caused tumor regression in the MDA-MB-231 breast cancer xenograft model. The antitumor efficacy of some taccalonolides, which stabilize microtubules in a manner different from other microtubule stabilizers, provides the impetus to explore the therapeutic potential of this site.

Cyclostreptin derivatives specifically target cellular tubulin and further map the paclitaxel site

Cyclostreptin is the first microtubule-stabilizing agent whose mechanism of action was discovered to involve formation of a covalent bond with tubulin. Treatment of cells with cyclostreptin irreversibly stabilizes their microtubules because cyclostreptin forms a covalent bond to β-tubulin at either the T220 or the N228 residue, located at the microtubule pore or luminal taxoid binding site, respectively. Because of its unique mechanism of action, cyclostreptin overcomes P-glycoprotein-mediated multidrug resistance in tumor cells. We used a series of reactive cyclostreptin analogues, 6-chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and [(14)C-acetyl]-8-acetyl-cyclostreptin, to characterize the cellular target of the compound and to map the binding site. The three analogues were cytotoxic and stabilized microtubules in both sensitive and multidrug resistant tumor cells. In both types of cells, we identified β-tubulin as the only or the predominantly labeled cellular protein, indicating that covalent binding to microtubules is sufficient to prevent drug efflux mediated by P-glycoprotein. 6-Chloroacetyl-cyclostreptin, 8-chloroacetyl-cyclostreptin, and 8-acetyl-cyclostreptin labeled both microtubules and unassembled tubulin at a single residue of the same tryptic peptide of β-tubulin as was labeled by cyclostreptin (219-LTTPTYGDLNHLVSATMSGVTTCLR-243), but labeling with the analogues occurred at different positions of the peptide. 8-Acetyl-cyclostreptin reacted with either T220 or N228, as did the natural product, while 8-chloroacetyl-cyclostreptin formed a cross-link to C241. Finally, 6-chloroacetyl-cyclostreptin reacted with any of the three residues, thus labeling the pathway for cyclostreptin-like compounds, leading from the pore where these compounds enter the microtubule to the luminal binding pocket.

What tubulin drugs tell us about microtubule structure and dynamics

A wide range of small molecules, including alkaloids, macrolides and peptides, bind to tubulin and disturb microtubule assembly dynamics. Some agents inhibit assembly, others inhibit disassembly. The binding sites of drugs that stabilize microtubules are discussed in relation to the properties of microtubule associated proteins. The activities of assembly inhibitors are discussed in relation to different nucleotide states of tubulin family protein structures.

Modulation of protein-protein interactions as a therapeutic strategy for the treatment of neurodegenerative tauopathies

The recognition that malfunction of the microtubule (MT) associated protein tau is likely to play a defining role in the onset and/or progression of a number of neurodegenerative diseases, including Alzheimer's disease, has resulted in the initiation of drug discovery programs that target this protein. Tau is an endogenous MT-stabilizing agent that is highly expressed in the axons of neurons. The MT-stabilizing function of tau is essential for the axonal transport of proteins, neurotransmitters and other cellular constituents. Under pathological conditions, tau misfolding and aggregation results in axonal transport deficits that appear to have deleterious consequences for the affected neurons, leading to synapse dysfunction and, ultimately, neuronal loss. This review focuses on both progress and unresolved issues surrounding the development of novel therapeutics for the treatment of neurodegenerative tauopathies, which are based on (A) MT-stabilizing agents to compensate for the loss of normal tau function, and (B) small molecule inhibitors of tau aggregation.

Cyclostreptin and microtubules: is a low-affinity binding site required?

Cyclostreptin (CS) is a recently discovered natural product with cytotoxic activity caused by microtubule stabilization. It is the only known microtubule-stabilizing agent (MSA) that covalently binds to tubulin. It also exhibits the fast-binding kinetics seen for other MSAs. Through careful peptide digestion and mass spectrometry analysis, Buey et al. found that two amino acids are labeled by CS: Asn228, near the known taxane-binding site, and Thr220, in the type I microtubule pore. This led Buey et al. to propose Thr220 resides at the site previously predicted to be a way station or low-affinity site. By using molecular dynamics simulations and structural considerations of the microtubule pore and tubulin dimer, we conclude that postulation of a low-affinity site is unnecessary to explain the available experimental data. An alternative explanation views the microtubule pore as a structural entity that presents a substantial kinetic barrier to ligand passage to the known taxane-binding site-an entry point to the microtubule lumen that becomes completely blocked if cyclostreptin is bound at Thr220. Simulations of the free dimer also suggest a common mechanism of microtubule stabilization for taxane site MSAs through their conformational effect on the M-loop. Such an effect explains the low tubulin polymerization caused by cyclostreptin in vitro despite its covalent attachment.