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Targeting a Kinetochore-Associated Motor Protein to Kill Cancer Cells

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Targeting a Kinetochore-Associated Motor Protein to Kill Cancer Cells COMMENTARY Targeting a kinetochore-associated motor protein to kill cancer cells Sarah A. Wacker and Tarun M. Kapoor1 Laboratory of Chemistry and Cell Biology, The Rockefeller University, New York, NY 10065 umors are characterized by excessive proliferation and, T therefore, chemotherapeutics that target proteins involved in tumor cell division can be effective anti- cancer agents. However, as many of these proteins are involved in core mechanisms in dividing and nondividing cells, these therapeutic agents do not selectively kill tumor cells and their efficacy is limited by general toxicity. As new therapeutics that target proteins involved in these core processes are developed, the likelihood increases of identifying a protein that cancer cells depend on more than normal cells. Wood et al. report the discovery and characterization of an inhibitor of a mitotic motor protein, centromere-asso- ciated protein-E (CENP-E, kinesin-7) (1). This inhibitor for a protein whose func- tions are limited to cell division has the potential to lead to improved cancer che- motherapies. Currently used antimitotic therapeutics target the cytoskeletal protein tubulin, which polymerizes to form microtubules (2). During cell division, segregation of chromosomes requires a microtubule- based bipolar spindle. Only after all chro- mosome pairs have been attached to the Fig. 1. GSK923295, an inhibitor of the mitotic kinesin CENP-E. (A) Normal cell division requires alignment opposite ends of the bipolar spindle of chromosomes at the equator of the bipolar spindle (the metaphase plate). (B) Loss of CENP-E function through microtubules, the spindle results in a few chromosomes that remain stuck at spindle poles. (C) Chemical structures of GSK923295, the fi CENP-E inhibitor, and monastrol, the kinesin-5 inhibitor. These inhibitors bind at an allosteric site (yellow), assembly checkpoint is satis ed, chromo- which is ∼10 Å from the ADP-binding site (red), in the kinesin ATPase domain, as depicted on the CENP-E somes are separated, and the cell cycle motor domain (PDB: 1T5C) (D). progresses. When normal tubulin poly- merization dynamics are disrupted, proper chromosome-spindle attachments are not cludes 14 different families and more than It has been proposed that CENP-E con- established, and the cell cycle is blocked 40 individual kinesin genes in humans (5). tributes to a cooperative chromosome by the checkpoint. Through poorly un- Different mitotic kinesins have crucial roles alignment mechanism in which the motor derstood mechanisms, this cell-cycle in distinct aspects of spindle assembly and protein drives the movement of chromo- arrest can lead to cell death (3). However, function, including microtubule organ- somes from spindle poles to the meta- microtubules have essential roles in ization, chromosome movement, and cyto- phase plate alongside microtubules that other cellular processes, such as neuronal kinesis (6). Although more work is needed are properly oriented, as these filaments transport. Therefore, the use of tubulin- to determine whether kinesin-5 inhibitors link other aligned chromosomes to spindle targeting antimitotic agents is associated will be more effective than currently used poles (10, 11). Recently, Wood et al. with side effects, including neurotoxicity. tubulin-targeting chemotherapeutics, the conducted an in vitro high-throughput A decade ago, a small-molecule inhib- development of multiple kinesin-5-targeted screen to identify inhibitors of CENP-E itor, monastrol, was reported for kinesin-5 chemicals suggests that members of the ATPase activity. Optimization of initial (also called KSP or Eg5) (4). Kinesins are kinesin superfamily are “druggable” (7). “hits” resulted in GSK923295, a potent motor proteins that can use ATP hydrolysis CENP-E is a kinetochore-associated inhibitor of CENP-E (1). In dividing cells to drive transport of cellular cargoes along kinesin with an essential role in metaphase treated with GSK923295, chromosomes microtubules. Monastrol was the first chromosome alignment that does not are clustered at the spindle poles as would chemical inhibitor that targeted a protein, function in nondividing cells (8). Deple- other than tubulin, needed for mitotic tion of CENP-E from cultured human spindle assembly. This initial “hit” helped cells is characterized by a bipolar mitotic Author contributions: S.A.W. and T.M.K. wrote the paper. catalyze the development of drugs against spindle with chromosomes clustered close The authors declare no conflict of interest. kinesin-5, a protein that was not known to to either end of the bipolar spindle (i.e., See companion article on page 5839. have key functions in nondividing cells such the spindle pole), rather than aligning at 1To whom correspondence should be addressed. E-mail: as neurons. The kinesin superfamily in- the metaphase plate (Fig. 1 A and B) (9). [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1001277107 PNAS Early Edition | 1of2 Downloaded by guest on September 27, 2021 be expected for CENP-E inhibition. This silencing, such as the transport of check- Determining how these changes in the finding goes beyond previous studies by point proteins away from kinetochores mechanism of inhibition by a small organic showing that CENP-E’s motor activity is (12, 14). Additional experiments are nee- molecule occur will likely require high- needed for chromosome alignment. ded to explain these findings, and it is resolution structural studies. As kinesins CENP-E also has an important role in likely that GSK923295 will be a powerful structurally resemble GTPases, such stud- the spindle assembly checkpoint (12). tool for dissecting these complex and ies may provide insight into how drugs may Specifically, CENP-E has been hypothe- dynamic cellular mechanisms. be designed for this important class of an- sized to be the receptor protein respon- ticancer targets for which good chemical sible for silencing the spindle assembly inhibitors have been difficult to develop. checkpoint after proper microtubule- Wood et al. report the Recent studies have shown that mice kinetochore attachment. It has been re- with only one functional CENP-E allele ported that microtubule capture by the discovery and character- have decreased tumor incidence, sug- CENP-E motor domain is sufficient to si- gesting inhibition of CENP-E may pres- lence activity of a kinase required for the ization of an inhibitor of ent a viable strategy to treat cancer (17). spindle assembly checkpoint (13). Based a mitotic motor protein. In fact, GSK923295 displayed dose- on these data, and the fact that dependent antitumor activity in vivo GSK923295 forces CENP-E to tightly bind against mice bearing xenografts of human microtubules (a “rigor” state) (1), cells To clarify the mechanism of CENP-E tumor cell lines, including the induction treated with this inhibitor should progress inhibition by GSK923295, crosslinking of partial and complete regressions (1). through mitosis, as the checkpoint would and mutagenesis studies were conducted. However, the responses of tumor cell be satisfied. Therefore, it is difficult to GSK923295 binding was mapped to a lines to treatment with GSK923295 are explain the observation that cells treated region in CENP-E that is ∼10 Å away variable and there were no obvious fea- with GSK923295 are arrested in mitosis. It from the nucleotide-binding pocket, be- tures common to the resistant tumor lines is possible that the mitotic arrest could be tween helices α2 and α3, adjacent to loop (1). This likely reflects a gap in our un- due to off-target effects of the chemical L5 (1). Remarkably, this site is similar to derstanding of the link between mitotic inhibitor (e.g., inhibition of proteins other that bound by many of the known kinesin-5 perturbation and cell death (3). Encour- than the kinesins tested). Alternatively, inhibitors (15), suggesting that this site agingly, the novel CENP-E inhibitors this inconsistency could simply be reveal- may be a “hotspot” targeted by kinesin in- offer an exciting tool to bridge this ing our incomplete understanding of the hibitors (Fig. 1 C and D). These findings knowledge gap. pathways that lead to a spindle assembly also raise the intriguing possibility that The mitotic spindle has proven to be an checkpoint-dependent block. Another in- native cellular ligands may exist that bind important target for cancer chemotherapy triguing possibility is that inhibition of at this site and regulate kinesins. Another (2, 18). The new generation of drugs that CENP-E by GSK923295 keeps the protein notable point is that this class of CENP-E target proteins whose functions are limited and, therefore, the chromosomes bound inhibitors can switch between ATP- to cell division offers the promise of im- to microtubule tracks, effectively generat- uncompetitive and ATP-competitive proved efficacy with reduced side effects. ing a “roadblock” for other transport inhibition with chemical modifications as We await the findings from clinical studies processes that are needed for checkpoint small as a single carbon extension (16). with GSK923295. 1. Wood KW, et al. (2010) Antitumor activity of an allosteric 7. Bergnes G, Brejc K, Belmont L (2005) Mitotic kinesins: 13. Mao Y, Desai A, Cleveland DW (2005) Microtubule cap- inhibitor of centromere-associated protein-E (CENP-E). Prospects for antimitotic drug discovery. Curr Top Med ture by CENP-E silences BubR1-dependent mitotic Proc Natl Acad Sci USA 107:5839–5844. Chem 5:127–145. checkpoint signaling. J Cell Biol 170:873–880. 2. Jordan MA, Wilson L (2004) Microtubules as a target 8. Wood KW, Chua P, Sutton D, Jackson JR (2008) 14. Howell BJ, et al. (2001) Cytoplasmic dynein/dynactin for anticancer drugs. Nat Rev Cancer 4:253–265. Centromere-associated protein E: A motor that puts drives kinetochore protein transport to the spindle 3. Rieder CL, Maiato H (2004) Stuck in division or pass- the brakes on the mitotic checkpoint. Clin Cancer Res poles and has a role in mitotic spindle checkpoint in- – – ing through: What happens when cells cannot sat- 14:7588 7592.
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