Tubulin is able to switch between a straight microtubule-like structure and a curved structure in complex with the stathminlike 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 tubulinstathmin 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.Microtubules are essential for eukaryotic chromosome segregation, cellular architecture and intracellular trafficking, among other processes. Understanding microtubule dynamics, regulation, and organization requires knowledge of the nucleotide-regulated assembly switch of tubulin. Microtubules are hollow cylinders made of protofilaments of ␣-tubulin dimers in head to tail association, forming a pseudohelical lattice (1). The functional assembly-disassembly cycle of a ␣-tubulin molecule includes activation by GTP binding at the -subunit, polymerization into microtubules, GTP hydrolysis at the -␣ interdimer interface and depolymerization of GDP-tubulin, followed by replacement by GTP. Vectorial polymerization and GTP hydrolysis combine with tubulin structural plasticity in microtubule dynamics (2). Depolymerizing microtubule ends show characteristic curled protofilaments, whereas relatively straight sheets form at growing ends (3, 4). GDP-tubulin does not assemble into microtubules, but forms double rings (5), which also form upon microtubule depolymerization (6) and correspond to curved microtubule protofilaments (7,8). The tendency of GDP-tubulin to curve is thought to strain the microtubule lattice, causing disassembly when the terminal cap of GTP-bound tubulin...
BackgroundOver the past decades, in spite of intensive search, no significant increase in the survival of patients with glioblastoma has been obtained. The role of the blood-brain barrier (BBB) and especially the activity of efflux pumps belonging to the ATP Binding Cassette (ABC) family may, in part, explain this defect.MethodsThe in-vitro activities of JAI-51 on cell proliferation were assessed by various experimental approaches in four human and a murine glioblastoma cell lines. Using drug exclusion assays and flow-cytometry, potential inhibitory effects of JAI-51 on P-gp and BCRP were evaluated in sensitive or resistant cell lines. JAI-51 activity on in-vitro microtubule polymerization was assessed by tubulin polymerization assay and direct binding measurements by analytical ultracentrifugation. Finally, a model of C57BL/6 mice bearing subcutaneous GL26 glioblastoma xenografts was used to assess the activity of the title compound in vivo. An HPLC method was designed to detect JAI-51 in the brain and other target organs of the treated animals, as well as in the tumours.ResultsIn the four human and the murine glioblastoma cell lines tested, 10 μM JAI-51 inhibited proliferation and blocked cells in the M phase of the cell cycle, via its activity as a microtubule depolymerising agent. This ligand binds to tubulin with an association constant of 2 × 105 M-1, overlapping the colchicine binding site. JAI-51 also inhibited the activity of P-gp and BCRP, without being a substrate of these efflux pumps. These in vitro studies were reinforced by our in vivo investigations of C57BL/6 mice bearing GL26 glioblastoma xenografts, in which JAI-51 induced a delay in tumour onset and a tumour growth inhibition, following intraperitoneal administration of 96 mg/kg once a week. In accordance with these results, JAI-51 was detected by HPLC in the tumours of the treated animals. Moreover, JAI-51 was detected in the brain, showing that the molecule is also able to cross the BBB.ConclusionThese in vitro and in vivo data suggest that JAI-51 could be a good candidate for a new treatment of tumours of the CNS. Further investigations are in progress to associate the title compound chemotherapy to radiotherapy in a rat model.
A series of conformationally flexible furan-derived allocolchicinoids was prepared from commercially available colchicine in good to excellent yields using a three-step reaction sequence. Cytotoxicity studies indicated the potent activity of two compounds against human epithelial and lymphoid cell lines (AsPC-1, HEK293, and Jurkat) as well as against Wnt-1 related murine epithelial cell line W1308. The results of in vitro experiments demonstrated that the major effect of these compounds was the induction of cell cycle arrest in the G2/M phase as a direct consequence of effective tubulin binding. In vivo testing of the most potent furanoallocolchicinoid 10c using C57BL/6 mice inoculated with Wnt-1 tumor cells indicated significant inhibition of the tumor growth.
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