Glaziovianin A prevents endosome maturation via inhibiting microtubule dynamics

Crystal structure of tubulin-barbigerone complex enables rational design of potent anticancer agents with isoflavone skeleton

BACKGROUND

Isoflavones possess many biological activities, including anti-inflammatory and anticancer effects. Microtubules (composed of αβ-tubulin heterodimers) are described as one possible cellular target of some of these isoflavones. However, the binding of tubulin to isoflavones has not been extensively studied, and until now, no crystal structure of the tubulin-isoflavone complex has been solved, and details of the isoflavone-tubulin interaction remain elusive.

PURPOSE

Barbigerone is an isoflavone mainly found in the genus Milletti, such as the edible leguminous plant Millettia ferruginea, with anticancer activity. This study aims to confirm the cellular target of barbigerone and to study its anticancer mechanism.

METHOD

Surface plasmon resonance assays and X-ray crystallography were used to study the interaction of barbigerone with tubulin protein. Immunofluorescence, in-cell and in vitro tubulin polymerization assays were employed to investigate the mechanism. MTT assays, cell clonal formation assays, wound healing assays, tube formation assays and H460 xenograft models were conducted to evaluate the in vitro and in vivo anticancer activities of barbigerone and one of its derivatives, 0412.

RESULTS

Here, we found that barbigerone binds to tubulin to inhibit tubulin polymerization. Moreover, we solved the X-ray crystal structure of the tubulin-barbigerone complex at 2.33 Å resolution, which unambiguously determined the orientation and position of barbigerone in the colchicine-binding site. Illuminated by the X-ray data, we synthetized and obtained a more active isoflavone, 0412. Both barbigerone and 0412 inhibit cancer cell proliferation, tubulin polymerization, migration of HeLa cells and capillary-like tube formation of HUVECs, induce G2/M phase cell cycle arrest and apoptosis, and exhibit anticancer activity in an H460 xenograft model.

CONCLUSION

In all, through biochemical and X-ray crystal structure results, we identified tubulin as the cellular target of one isoflavone, barbigerone, and proved that the tubulin-barbigerone complex plays a guiding role in obtaining a more active compound, 0412. These studies provide a crucial research basis for the development of isoflavones as anticancer candidate compounds.

Dual Inhibition of γ-Tubulin and Plk1 Induces Mitotic Cell Death

α/β-Tubulin inhibitors that alter microtubule (MT) dynamics are commonly used in cancer therapy, however, these inhibitors also cause severe side effects such as peripheral neuropathy. γ-Tubulin is a possible target as antitumor drugs with low side effects, but the antitumor effect of γ-tubulin inhibitors has not been reported yet. In this study, we verified the antitumor activity of gatastatin, a γ-tubulin specific inhibitor. The cytotoxicity of gatastatin was relatively weak compared with that of the conventional MT inhibitors, paclitaxel and vinblastine. To improve the cytotoxicity, we screened the chemicals that improve the effects of gatastatin and found that BI 2536, a Plk1 inhibitor, greatly increases the cytotoxicity of gatastatin. Co-treatment with gatastatin and BI 2536 arrested cell cycle progression at mitosis with abnormal spindles. Moreover, mitotic cell death induced by the combined treatment was suppressed by the Mps1 inhibitor, reversine. These findings suggest that co-treatment with Plk1 and γ-tubulin inhibitors causes spindle assembly checkpoint-dependent mitotic cell death by impairing centrosome functions. These results raise the possibility of Plk1 and γ-tubulin inhibitor co-treatment as a novel cancer chemotherapy.

Structure Optimization of Gatastatin for the Development of γ-Tubulin-Specific Inhibitor

Gatastatin (O ⁷-benzyl glaziovianin A) is a γ-tubulin-specific inhibitor that is used to investigate γ-tubulin function in cells. We have previously reported that the unsubstituted phenyl ring of the O ⁷-benzyl group in gatastatin is important for γ-tubulin inhibition. To obtain further structural information regarding γ-tubulin inhibition, we synthesized several gatastatin derivatives containing a fixed O ⁷-benzyl moiety. Modifications of the B-ring resulted in drastic decrease in cytotoxicity, abnormal spindle formation activity, and inhibition of microtubule (MT) nucleation. In contrast, various O ⁶-alkylated gatastatin derivatives showed potent cytotoxicity, induced abnormal spindle formation, and inhibited MT nucleation. We had previously reported that O ⁶-benzyl glaziovianin A is a potent α/β-tubulin inhibitor; thus, these new results suggest that the O ⁶-position restricts affinity for α/β- and γ-tubulin. Considering that an O ⁷-benzyl group increases specificity for γ-tubulin, more potent and specific γ-tubulin inhibitors can be generated through O ⁶-modifications of gatastatin.

Discovery of O6-benzyl glaziovianin A, a potent cytotoxic substance and a potent inhibitor of α,β-tubulin polymerization

We have discovered O⁶-benzyl glaziovianin A, which showed stronger inhibition of microtubule polymerization (IC₅₀=2.1μM) than known α,β-tubulin inhibitors, such as colchicine and glaziovianin A. Also, we performed competition binding experiments of O⁶-benzyl glaziovianin A and revealed that O⁶-benzyl glaziovianin A binds to the colchicine binding site with high affinity. It is interesting that glaziovianin A derivatives change their mode of action in benzylation at the O⁶ (α,β-tubulin inhibitor) or O⁷ (γ-tubulin-specific inhibitor) position.

Efficient Synthesis of Glaziovianin A Isoflavone Series from Dill and Parsley Extracts and Their in Vitro/in Vivo Antimitotic Activity

A concise six-step protocol for the synthesis of isoflavone glaziovianin A (GVA) and its alkoxyphenyl derivatives 9 starting with readily available plant metabolites from dill and parsley seeds was developed. The reaction sequence involved an efficient conversion of the key intermediate epoxides 7 into the respective β-ketoaldehydes 8 followed by their Cu(I)-mediated cyclization into the target series 9. The biological activity of GVA and its derivatives was evaluated using a panel of seven human cancer cell lines and an in vivo sea urchin embryo assay. Both screening platforms confirmed the antimitotic effect of the parent GVA (9cg) and its alkoxy derivatives. Structure-activity relationship studies suggested that compounds 9cd and 9cf substituted with trimethoxy- and dillapiol-derived B-rings, respectively, were less active than the parent 9cg. Of the evaluated human cancer cell lines, the A375 melanoma cell line was the most sensitive to the tested molecules. Notably, the target compounds were not cytotoxic against human peripheral blood mononuclear cells up to 10 μM concentration. Phenotypic readouts from the sea urchin assay unequivocally suggest a direct microtubule-destabilizing effect of isoflavones 9cg, 9cd, and 9cf.

The γ-tubulin-specific inhibitor gatastatin reveals temporal requirements of microtubule nucleation during the cell cycle

Inhibitors of microtubule (MT) assembly or dynamics that target α/β-tubulin are widely exploited in cancer therapy and biological research. However, specific inhibitors of the MT nucleator γ-tubulin that would allow testing temporal functions of γ-tubulin during the cell cycle are yet to be identified. By evolving β-tubulin-binding drugs we now find that the glaziovianin A derivative gatastatin is a γ-tubulin-specific inhibitor. Gatastatin decreased interphase MT dynamics of human cells without affecting MT number. Gatastatin inhibited assembly of the mitotic spindle in prometaphase. Addition of gatastatin to preformed metaphase spindles altered MT dynamics, reduced the number of growing MTs and shortened spindle length. Furthermore, gatastatin prolonged anaphase duration by affecting anaphase spindle structure, indicating the continuous requirement of MT nucleation during mitosis. Thus, gatastatin facilitates the dissection of the role of γ-tubulin during the cell cycle and reveals the sustained role of γ-tubulin.

Current microtubule inhibitors target α/β-tubulin, but no specific inhibitor of γ-tubulin has been developed. Here the authors present gatastatin as a γ-tubulin inhibitor and use it to probe the role of γ-tubulin during the cell cycle.

Synthesis, selective cytotoxicities and probable mechanism of action of 7-methoxy-3-arylflavone-8-acetic acids

Thirteen new analogues of flavone-8-acetic acid, that is, compounds 10a-m bearing a methoxy group at the 7-position and diverse subsitiuents on the benzene ring at the 2- and 3-positions of flavone nucleus, were synthesized and evaluated for their direct antiproliferative effects on two human tumor cell lines and for their indirect antiproliferative activities in the transwell co-culture system. The results indicated that most of compounds 10a-m showed moderate direct cytotoxicities. Among them, compound 10i exhibited higher direct cytotoxicity and selectivity for both cell lines over BJ human foreskin fibroblast cells than 5,6-dimethylxanthenone-4-acetic acid (DMXAA). Interestingly, compared with DMXAA, compound 10e showed comparable indirect cytotoxicity and higher selectivity. In addition, compound 10e was found to be able to induce tumor necrosis factor α (TNF-α) production in human peripheral blood mononuclear cells.