Localization of critical histidyl residues required for vinblastine-induced tubulin polymerization and for microtubule assembly

2,3-Dihydroquinazolin-4(1H)-ones and quinazolin-4(3H)-ones as broad-spectrum cytotoxic agents and their impact on tubulin polymerisation

Tubulin plays a central role in mitosis and has been the target of multiple anticancer drugs, including paclitaxel. Herein two separate families of 2,3-dihydroquinazoline-4(1H)-ones and quinazoline-4(3H) ones, comprising 57 compounds in total, were synthesised. Screening against a broad panel of human cancer cell lines (HT29 colon, U87 and SJ-G2 glioblastoma, MCF-7 breast, A2780 ovarian, H460 lung, A431 skin, Du145 prostate, BE2-C neuroblastoma, and MIA pancreas) reveals these analogues to be broad spectrum cytotoxic compounds. Of particular note, 2-styrylquinazolin-4(3H)-one 51, 2-(4-hydroxystyryl)quinazolin-4(3H)-one 63, 2-(2-methoxystyryl)quinazolin-4(3H)-one 64 and 2-(3-methoxystyryl)quinazolin-4(3H)-one 65 and 2-(naphthalen-1-yl)-2,3-dihydroquinazolin-4(1H)-one 39 exhibited sub-μM potency growth inhibition values. Of these 1-naphthyl 39 has activity <50 nM against the HT29, U87, A2780, H460 and BE2-C cell lines. Molecular modelling of these compounds, e.g. 2-(naphthalen-1-yl)-2,3-dihydroquinazolin-4(1H)-one 39, 2-(2-methoxystyryl)quinazolin-4(3H)-one 64, 2-(3-methoxystyryl)quinazolin-4(3H)-one 65, and 2-(4-methoxystyryl)quinazolin-4(3H)-one 50 docked to the known tubulin polymerisation inhibitor sites highlighted well conserved interactions within the colchicine binding pocket. These compounds were examined in a tubulin polymerisation assay alongside the known tubulin polymerisation promotor, paclitaxel (69), and tubulin inhibitor, nocodazole (68). Of the analogues examined, indoles 43 and 47 were modest promotors of tubulin polymerisation, but less effective than paclitaxel. Analogues 39, 64, and 65 showed reduced microtubule formation consistent with tubulin inhibition. The variation in ring methoxy substituent with 50, 64 and 65, from o- to m- to p-, results in a concomitant reduction in cytotoxicity and a reduction in tubulin polymerisation, with p-OCH350 being the least active in this series of analogues. This presents 64 as a tubulin polymerisation inhibitor possessing novel chemotype and sub micromolar cytotoxicity. Naphthyl 39, with complete inhibition of tubulin polymerisation, gave rise to a sub 0.2 μM cell line cytotoxicity. Compounds 39 and 64 induced G2 + M cell cycle arrest indicative of inhibition of tubulin polymerisation, with 39 inducing an equivalent effect on cell cycle arrest as nocodazole (68).

Application of proteomic profiling based on 2D-DIGE for classification of compounds according to the mechanism of action

The development of new anticancer agents derived from natural resources requires a rapid identification of their molecular mechanism of action. To make this step short, we have initiated the proteomic profiling of HeLa cells treated with anticancer drugs representing a wide spectrum of mechanisms of action using two-dimensional difference gel electrophoresis (2D-DIGE). Unique proteome patterns were observed in HeLa cells treated with the HSP90 inhibitor geldanamycin, and were similar to the patterns induced by radicicol, a structurally different HSP90 inhibitor. On the other hand, etoposide and ICRF-193, compounds claimed to be topoisomerase II inhibitors, showed different proteomic profiles, which reflect their different biological activities as revealed by cell-cycle analysis. Thus far, combined data from 19 compounds have allowed their successful classification by cluster analysis according to the mechanism of action.

Mechanism of destruction of microtubule structures by 4-hydroxy-2-nonenal

A major end product of lipid peroxidation, 4-hydroxy-2-nonenal (HNE), is an electrophilic alkenal and produces Michael adducts with cellular proteins. It is known that exposure of cultured cells to HNE causes rapid disappearance of microtubule networks. In this study we addressed the mechanism. Immunochemical studies revealed that HNE preferentially modified alpha-tubulin in rat primary neuronal cells, PC12 cells, and rat fibroblast cell line 3Y1 cells. This was morphologically associated with the disappearance of microtubule structures in those cells. In a purified rat brain microtubule fraction, HNE modified unpolymerized tubulin and impaired its polymerizability, with a concomitant increase in insolubilized tubulin. Nevertheless, HNE had a marginal effect on the stability of pre-polymerized microtubules. These results suggest that disruption of microtubule assembly as a result of HNE modification of unpolymerized tubulin, rather than destruction of assembled microtubules, is responsible for the disappearance of microtubule structures in cells exposed to HNE.

Identification of the N-terminal peptide binding site of glucose-regulated protein 94

Because the stress protein GRP94 can augment presentation of peptides to T cells, it is important to define how it, as well as all other HSP90 family members, binds peptides. Having previously shown that the N-terminal half of GRP94 can account for the peptide binding activity of the full-length protein, we now locate this binding site by testing predictions of a molecular docking model. The best predicted site was on the opposite face of the beta sheet from the pan-HSP90 radicicol-binding pocket, in close proximity to a deep hydrophobic pocket. The peptide and radicicol-binding sites are distinct, as shown by the ability of a radicicol-refractive mutant to bind peptide. When the fluorophore acrylodan is attached to Cys117 within the hydrophobic pocket, its fluorescence is reduced upon peptide binding, consistent with proximity of the two ligands. Substitution of His125, which contacts the bound peptide, compromises peptide-binding activity. We conclude that peptide binds to the concave face of the beta sheet of the N-terminal domain, where binding is regulated during the action cycle of the chaperone.

Hemichannel and junctional properties of connexin 50

Lens fiber connexins, cx50 and cx46 (alpha3 and alpha8), belong to a small subset of connexins that can form functional hemichannels in nonjunctional membranes. Knockout of either cx50 or cx46 results in a cataract, so the properties of both connexins are likely essential for proper physiological functioning of the lens. Although portions of the sequences of these two connexins are nearly identical, their hemichannel properties are quite different. Cx50 hemichannels are much more sensitive to extracellular acidification than cx46 hemichannels and differ from cx46 hemichannels both in steady-state and kinetic properties. Comparison of the two branches of the cx50 hemichannel G-V curve with the junctional G-V curve suggests that cx50 gap junctions gate with positive relative polarity. The histidine-modifying reagent, diethyl pyrocarbonate, reversibly blocks cx50 hemichannel currents but not cx46 hemichannel currents. Because cx46 and cx50 have very similar amino acid sequences, one might expect that replacing the two histidines unique to the third transmembrane region of cx50 with the corresponding cx46 residues would produce mutants more closely resembling cx46. In fact this does not happen. Instead the mutant cx50H161N does not form detectable hemichannels but forms gap junctions indistinguishable from wild type. Cx50H176Q is oocyte lethal, and the double mutant, cx50H61N/H176Q, neither forms hemichannels nor kills oocytes.

Conformational change in the vinculin C-terminal depends on a critical histidine residue (His-906)

A phospholipid-controlled interaction between the N-terminal and C-terminal domains of vinculin is thought to be a major mechanism that regulates binding activities of the protein. To probe the mechanisms underlying these interactions we used chemical modification and site-directed mutagenesis directed at histidine residues. Diethylpyrocarbonate (DEPC) modification of the C-terminal, but not the N-terminal, domain greatly decreased affinity of the N-terminal-C-terminal binding, implicating histidine residues in the C-terminal. Mutation of either or both C-terminal histidines (at positions 906 and 1026), however, did not affect N-C binding at neutral pH. The H906A mutation did prevent DEPC effects and also prevented the normal decrease in binding affinity for the N-terminal at lower pH. We found that the wild type C-terminal domain, but not the H906A mutant, underwent a conformational change at pH 6.5, reflected in an altered circular dichroism spectrum and apparent oligomerization. Phospholipid also induced conformational changes in the wild type C-terminal domain but not in the H906A mutant, even though the mutant protein did bind to the phospholipid. Finally, the sensitivity of the N-C interaction to phospholipid was much reduced by the H906A mutation. These results show that H906 plays a key role in the conformational dynamics of the C-terminal domain and thus the regulation of vinculin.