Salvatore DeBonis

S-Trityl-L-cysteine Is a Reversible, Tight Binding Inhibitor of the Human Kinesin Eg5 That Specifically Blocks Mitotic Progression

Human Eg5, responsible for the formation of the bipolar mitotic spindle, has been identified recently as one of the targets of S-trityl-l-cysteine, a potent tumor growth inhibitor in the NCI 60 tumor cell line screen. Here we show that in cell-based assays S-trityl-l-cysteine does not prevent cell cycle progression at the S or G2 phases but inhibits both separation of the duplicated centrosomes and bipolar spindle formation, thereby blocking cells specifically in the M phase of the cell cycle with monoastral spindles. Following removal of S-trityl-l-cysteine, mitotically arrested cells exit mitosis normally. In vitro, S-trityl-l-cysteine targets the catalytic domain of Eg5 and inhibits Eg5 basal and microtubule-activated ATPase activity as well as mant-ADP release. S-Trityl-l-cysteine is a tight binding inhibitor (estimation of Ki,app <150 nm at 300 mm NaCl and 600 nm at 25 mm KCl). S-Trityl-l-cysteine binds more tightly than monastrol because it has both an ∼8-fold faster association rate and ∼4-fold slower release rate (6.1 μM–1 s–1 and 3.6 s–1 for S-trityl-l-cysteine versus 0.78 μM–1 s–1 and 15 s–1 for monastrol). S-Trityl-l-cysteine inhibits Eg5-driven microtubule sliding velocity in a reversible fashion with an IC50 of 500 nm. The S and d-enantiomers of S-tritylcysteine are nearly equally potent, indicating that there is no significant stereospecificity. Among nine different human kinesins tested, S-trityl-l-cysteine is specific for Eg5. The results presented here together with the proven effect on human tumor cell line growth make S-trityl-l-cysteine a very attractive starting point for the development of more potent mitotic inhibitors.

Structure of Human Eg5 in Complex with a New Monastrol-based Inhibitor Bound in the R Configuration

Drugs that target mitotic spindle proteins have been proven useful for tackling tumor growth. Eg5, a kinesin-5 family member, represents a potential target, since its inhibition leads to prolonged mitotic arrest through the activation of the mitotic checkpoint and apoptotic cell death. Monastrol, a specific dihydropyrimidine inhibitor of Eg5, shows stereo-specificity, since predominantly the (S)-, but not the (R)-, enantiomer has been shown to be the biologically active compound in vitro and in cell-based assays. Here, we solved the crystal structure (2.7Å) of the complex between human Eg5 and a new keto derivative of monastrol (named mon-97), a potent antimitotic inhibitor. Surprisingly, we identified the (R)-enantiomer bound in the active site, and not, as for monastrol, the (S)-enantiomer. The absolute configuration of this more active (R)-enantiomer has been unambiguously determined via chemical correlation and x-ray analysis. Unexpectedly, both the R- and the S-forms inhibit Eg5 ATPase activity with IC50 values of 110 and 520 nm (basal assays) and 150 nm and 650 nm (microtubule-stimulated assays), respectively. However, the difference was large enough for the protein to select the (R)- over the (S)-enantiomer. Taken together, these results show that in this new monastrol family, both (R)- and (S)-enantiomers can be active as Eg5 inhibitors. This considerably broadens the alternatives for rational drug design.

Structure-Activity Relationship of S-Trityl-L-Cysteine Analogues as Inhibitors of the Human Mitotic Kinesin Eg5

The human kinesin Eg5 is a potential drug target for cancer chemotherapy. Eg5 specific inhibitors cause cells to block in mitosis with a characteristic monoastral spindle phenotype. Prolonged metaphase block eventually leads to apoptotic cell death. S-trityl-L-cysteine (STLC) is a tight-binding inhibitor of Eg5 that prevents mitotic progression. It has proven antitumor activity as shown in the NCI 60 tumor cell line screen. It is of considerable interest to define the minimum chemical structure that is essential for Eg5 inhibition and to develop more potent STLC analogues. An initial structure-activity relationship study on a series of STLC analogues reveals the minimal skeleton necessary for Eg5 inhibition as well as indications of how to obtain more potent analogues. The most effective compounds investigated with substitutions at the para-position of one phenyl ring have an estimated K i (app) of 100 nM in vitro and induce mitotic arrest with an EC 50 of 200 nM.

Synthesis and antiproliferative evaluation of pyrazolo[1,5-a]-1,3,5-triazine myoseverin derivatives.

Pyrazolo[1,5-a]-1,3,5-triazine myoseverin derivatives 1a-c were prepared from 4-(N-methyl-N-phenylamino)-2-methylsulfanylpyrazolo[1,5-a]-1,3,5-triazine 2. Their cytotoxic activity, inhibition of tubulin polymerization, and cell cycle effects were evaluated. Compounds 1a and 1c are potent tubulin inhibitors and displayed specific antiproliferative activity in colorectal cancer cell lines at micromolar concentrations.

Proteome analysis of microtubule-associated proteins and their interacting partners from mammalian brain.

The microtubule (MT) cytoskeleton is essential for a variety of cellular processes. MTs are finely regulated by distinct classes of MT-associated proteins (MAPs), which themselves bind to and are regulated by a large number of additional proteins. We have carried out proteome analyses of tubulin-rich and tubulin-depleted MAPs and their interacting partners isolated from bovine brain. In total, 573 proteins were identified giving us unprecedented access to brain-specific MT-associated proteins from mammalian brain. Most of the standard MAPs were identified and at least 500 proteins have been reported as being associated with MTs. We identified protein complexes with a large number of subunits such as brain-specific motor/adaptor/cargo complexes for kinesins, dynein, and dynactin, and proteins of an RNA-transporting granule. About 25% of the identified proteins were also found in the synaptic vesicle proteome. Analysis of the MS/MS data revealed many posttranslational modifications, amino acid changes, and alternative splice variants, particularly in tau, a key protein implicated in Alzheimer’s disease. Bioinformatic analysis of known protein–protein interactions of the identified proteins indicated that the number of MAPs and their associated proteins is larger than previously anticipated and that our database will be a useful resource to identify novel binding partners.

STLC-resistant cell lines as tools to classify chemically divergent Eg5 targeting agents according to their mode of action and target specificity

Determining the mechanism of action of drugs and their target specificity in cells remains a major challenge. Here we describe the use of cell lines expressing two point mutations in the allosteric inhibitor binding pocket of the mitotic kinesin Eg5 (D130A, in the loop L5 region and L214A in helix α3), which following transfection, were selected for their ability to proliferate normally in the presence of STLC, a well known Eg5 inhibitor. The cell lines were used to discriminate the mechanism of action of other chemically distinct small molecule inhibitors of Eg5 that differ in their mode of action. The STLC resistant cells were capable of continuous proliferation in the presence of ATP uncompetitive inhibitors, such as K858 and dimethylenastron, but were still sensitive to ATP competitive inhibitors that are thought to bind to a distinct site on Eg5 than the allosteric binding pocket. The STLC resistant cell lines can therefore be used as a filter to distinguish Eg5 loop L5 binding drugs from drugs binding to other pockets without prior structural information. Additionally, the cells can be used to analyze whether inhibitors of Eg5 are specific to this potential drug target or whether they have additional targets in dividing cells.

Self protein-protein interactions are involved in TPPP/p25 mediated microtubule bundling.

TPPP/p25 is a microtubule-associated protein, detected in protein inclusions associated with various neurodegenerative diseases. Deletion analysis data show that TPPP/p25 has two microtubule binding sites, both located in intrinsically disordered domains, one at the N-terminal and the other in the C-terminal domain. In copolymerization assays the full-length protein exhibits microtubule stimulation and bundling activity. In contrast, at the same ratio relative to tubulin, truncated forms of TPPP/p25 exhibit either lower or no microtubule stimulation and no bundling activity, suggesting a cooperative phenomenon which is enhanced by the presence of the two binding sites. The binding characteristics of the N- and C-terminally truncated proteins to taxol-stabilized microtubules are similar to the full-length protein. However, the C-terminally truncated TPPP/p25 shows a lower Bmax for microtubule binding, suggesting that it may bind to a site of tubulin that is masked in microtubules. Bimolecular fluorescent complementation assays in cells expressing combinations of various TPPP/p25 fragments, but not that of the central folded domain, resulted in the generation of a fluorescence signal colocalized with perinuclear microtubule bundles insensitive to microtubule inhibitors. The data suggest that the central folded domain of TPPP/p25 following binding to microtubules can drive s homotypic protein-protein interactions leading to bundled microtubules.