Toward Discovery of Novel Microtubule Targeting Agents: A SNAP-tag-Based High-Content Screening Assay for the Analysis of Microtubule Dynamics and Cell Cycle Progression.

Microtubule targeting agents (MTAs) are used for the treatment of cancer. Novel MTAs could provide additional and beneficial therapeutic options. To improve the sensitivity and throughput of standard immunofluorescence assays for the characterization of MTAs, we used SNAP-tag technology to produce recombinant tubulin monomers. To visualize microtubule filaments, A549 cells transfected with SNAP-tubulin were stained with a membrane-permeable, SNAP-reactive dye. The treatment of SNAP-tubulin cells with stabilizing MTAs such as paclitaxel resulted in the formation of coarsely structured microtubule filaments, whereas depolymerizing MTAs such as nocodazole resulted in diffuse staining patterns in which the tubulin filaments were no longer distinguishable. By combining these components with automated microscopy and image analysis algorithms, we established a robust high-content screening assay for MTAs with a Z' factor of 0.7. Proof of principle was achieved by testing a panel of 10 substances, allowing us to identify MTAs and to distinguish between stabilizing and destabilizing modes of action. By extending the treatment of the cells from 2 to 20 h, our assay also detected abnormalities in cell cycle progression and in the formation of microtubule spindles, providing additional readouts for the discovery of new MTAs and facilitating their early identification during drug-screening campaigns.

All-trans retinoic acid impairs the vasculogenic mimicry formation ability of U87 stem-like cells through promoting differentiation.

The poor therapeutic effect of traditional antiangiogenic therapy on glioblastoma multiforme (GBM) may be attributed to vasculogenic mimicry (VM), which was previously reported to be promoted by cancer stem-like cells (SLCs). All-trans retinoic acid (ATRA), a potent reagent which drives differentiation, was reported to be able to eradicate cancer SLCs in certain malignancies. The aim of the present study was to investigate the effects of ATRA on the VM formation ability of U87 glioblastoma SLCs. The expression of cancer SLC markers CD133 and nestin was detected using immunocytochemistry in order to identify U87 SLCs. In addition, the differentiation of these SLCs was observed through detecting the expression of glial fibrillary acidic protein (GFAP), ß-tubulin III and galactosylceramidase (Galc) using immunofluorescent staining. The results showed that the expression levels of GFAP, ß-tubulin III and Galc were upregulated following treatment with ATRA in a dose-dependent manner. Furthermore, ATRA significantly reduced the proliferation, invasiveness, tube formation and vascular endothelial growth factor (VEGF) secretion of U87 SLCs. In conclusion, the VM formation ability of SLCs was found to be negatively correlated with differentiation. These results therefore suggested that ATRA may serve as a promising novel agent for the treatment of GBM due to its role in reducing VM formation.

Synthesis and bioevaluation of diphyllin glycosides as novel anticancer agents.

A series of diphyllin glycosides were synthesized from diphyllin by phase transfer catalysis glycosylation, deprotection, and etherification, and the structures were established by (1) H NMR, (13) C NMR, and HRMS. These glycosides were evaluated for their in vitro cytotoxicity against HCT-116, A549, and A549T cancer cell lines by MTT assay, and most of them were cytotoxic at submicromolar concentrations. They were also effective against the paclitaxel-resistant cell line A549T. The kDNA decatenation assay indicated that most of these compounds inhibited topoisomerase IIα-mediated kDNA decatenation. In addition, the in vitro tubulin polymerization study showed that compounds 5 and 6 had antimicrotubule activity with a paclitaxel-like mode of action. Taken together, these results suggest that these diphyllin glycosides act on both TopoII and tubulin.

Inhibitors and promoters of tubulin polymerization: synthesis and biological evaluation of chalcones and related dienones as potential anticancer agents.

A series of dihalogenated chalcones and structurally related dienones were synthesized and evaluated for their antiproliferative activity in 10 different cancer cell lines and for their effect on microtubule assembly. All compounds showed cytotoxic activity, with IC(50) values in the 5-280 µM range depending on the chalcone structure and the cell line. Five of the compounds were found to be tubulin polymerization inhibitors. In contrast, one of the compounds was found to stabilize tubulin to the same extent as the anticancer drug docetaxel. Molecular modeling suggested that the tubulin inhibitors bind to the colchicine binding site of ß-tubulin while the novel tubulin stabilization agent seems to interact with the paclitaxel binding site.

Cabazitaxel: filling one of the gaps in the treatment of prostate cancer.

Prostate cancer is the second most frequently diagnosed cancer in men and the fifth most common cancer overall. Globally, more than 900,000 new cases of prostate cancer will be diagnosed in 2010 and more than 260,000 will, unfortunately, die from the disease. In the US, an estimated 217,000 new cases of prostate cancer and 32,000 deaths are expected this year. Definitive therapy (surgery or radiation) is highly effective, but if the tumor escapes the gland, treatment options are limited. For this population of patients, androgen suppression is the cornerstone of initial therapy. Furthermore, progression to castration resistant prostate cancer (CRPC) is inevitable. The current front-line treatment for patients with CRPC is the chemotherapeutic agent docetaxel (administered every 3 weeks). Until now, it is the only agent that has been shown to prolong survival in CRPC. The approval trial for docetaxel found a median overall survival of 19.2 months for patients receiving docetaxel plus prednisone compared to 16.3 months for patients receiving mitoxantrone plus prednisone (p=0.0094). Mitoxantrone plus prednisone is often utilized for its palliative benefits, but two randomized trials failed to demonstrate a survival advantage.

Stathmin and interfacial microtubule inhibitors recognize a naturally curved conformation of tubulin dimers.

Tubulin is able to switch between a straight microtubule-like structure and a curved structure in complex with the stathmin-like domain of the RB3 protein (T(2)RB3). GTP hydrolysis following microtubule assembly induces protofilament curvature and disassembly. The conformation of the labile tubulin heterodimers is unknown. One important question is whether free GDP-tubulin dimers are straightened by GTP binding or if GTP-tubulin is also curved and switches into a straight conformation upon assembly. We have obtained insight into the bending flexibility of tubulin by analyzing the interplay of tubulin-stathmin association with the binding of several small molecule inhibitors to the colchicine domain at the tubulin intradimer interface, combining structural and biochemical approaches. The crystal structures of T(2)RB3 complexes with the chiral R and S isomers of ethyl-5-amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl-carbamate, show that their binding site overlaps with colchicine ring A and that both complexes have the same curvature as unliganded T(2)RB3. The binding of these ligands is incompatible with a straight tubulin structure in microtubules. Analytical ultracentrifugation and binding measurements show that tubulin-stathmin associations (T(2)RB3, T(2)Stath) and binding of ligands (R, S, TN-16, or the colchicine analogue MTC) are thermodynamically independent from one another, irrespective of tubulin being bound to GTP or GDP. The fact that the interfacial ligands bind equally well to tubulin dimers or stathmin complexes supports a bent conformation of the free tubulin dimers. It is tempting to speculate that stathmin evolved to recognize curved structures in unassembled and disassembling tubulin, thus regulating microtubule assembly.

Beta3-tubulin is induced by estradiol in human breast carcinoma cells through an estrogen-receptor dependent pathway.

Microtubules are involved in a variety of essential cell functions. Their role during mitosis has made them a target for anti-cancer drugs. However development of resistance has limited their use. It has been established that enhanced beta3-tubulin expression is correlated with reduced response to antimicrotubule agent-based chemotherapy or worse outcome in a variety of tumor settings. However little is known regarding the regulation of beta3-tubulin expression. We investigated the regulatory mechanisms of expression of beta3-tubulin in the MCF-7 cell line, a model of hormone-dependent breast cancer. Exposure of MCF-7 cells to estradiol was found to induce beta3-tubulin mRNA as well as beta3-tubulin protein expression. Conversely, we did not observe induction of beta3-tubulin mRNA by estradiol in MDA-MB-231 cells which are negative for the estrogen receptor (ER). In order to determine whether beta3-tubulin up-regulation is mediated through the ER pathway, MCF-7 cells were exposed to two ER modulators. Exposure to tamoxifen, a selective estrogen receptor modulator, completely abolished the beta3-tubulin mRNA induction due to estradiol in MCF-7 cells. This result was confirmed with fulvestrant, a pure antagonist of ER. These results demonstrate that the effect of estradiol on beta3-tubulin transcription is mediated through an ER dependent pathway.