Increased Aurora B expression reduces substrate phosphorylation and induces chromosomal instability.

Increased Aurora B protein expression, which is common in cancers, is expected to increase Aurora B kinase activity, yielding elevated phosphorylation of Aurora B substrates. In contrast, here we show that elevated expression of Aurora B reduces phosphorylation of six different Aurora B substrates across three species and causes defects consistent with Aurora B inhibition. Complexes of Aurora B and its binding partner INCENP autophosphorylate in trans to achieve full Aurora B activation. Increased expression of Aurora B mislocalizes INCENP, reducing the local concentration of Aurora B:INCENP complexes at the inner centromere/kinetochore. Co-expression of INCENP rescues Aurora B kinase activity and mitotic defects caused by elevated Aurora B. However, INCENP expression is not elevated in concert with Aurora B in breast cancer, and increased expression of Aurora B causes resistance rather than hypersensitivity to Aurora B inhibitors. Thus, increased Aurora B expression reduces, rather than increases, Aurora B kinase activity.

Chromosomal instability sensitizes patient breast tumors to multipolar divisions induced by paclitaxel.

Paclitaxel (Taxol) is a cornerstone of cancer treatment. However, its mechanism of cytotoxicity is incompletely understood and not all patients benefit from treatment. We show that patients with breast cancer did not accumulate sufficient intratumoral paclitaxel to induce mitotic arrest in tumor cells. Instead, clinically relevant concentrations induced multipolar mitotic spindle formation. However, the extent of early multipolarity did not predict patient response. Whereas multipolar divisions frequently led to cell death, multipolar spindles focused into bipolar spindles before division at variable frequency, and maintaining multipolarity throughout mitosis was critical to induce the high rates of chromosomal instability necessary for paclitaxel to elicit cell death. Increasing multipolar divisions in paclitaxel resulted in improved cytotoxicity. Conversely, decreasing paclitaxel-induced multipolar divisions reduced paclitaxel efficacy. Moreover, we found that preexisting chromosomal instability sensitized breast cancer cells to paclitaxel. Both genetic and pharmacological methods of inducing chromosomal instability were sufficient to increase paclitaxel efficacy. In patients, the amount of pretreatment chromosomal instability directly correlated with taxane response in metastatic breast cancer such that patients with a higher rate of preexisting chromosomal instability showed improved response to taxanes. Together, these results support the use of baseline rates of chromosomal instability as a predictive biomarker for paclitaxel response. Furthermore, they suggest that agents that increase chromosomal instability or maintain multipolar spindles throughout mitosis will improve the clinical utility of paclitaxel.

The Functional Significance of Posttranslational Modifications on Polo-Like Kinase 1 Revealed by Chemical Genetic Complementation.

Mitosis is coordinated by carefully controlled phosphorylation and ubiquitin-mediated proteolysis. Polo-like kinase 1 (Plk1) plays a central role in regulating mitosis and cytokinesis by phosphorylating target proteins. Yet, Plk1 is itself a target for posttranslational modification by phosphorylation and ubiquitination. We developed a chemical-genetic complementation assay to evaluate the functional significance of 34 posttranslational modifications (PTMs) on human Plk1. To do this, we used human cells that solely express a modified analog-sensitive Plk1 (Plk1AS) and complemented with wildtype Plk1. The wildtype Plk1 provides cells with a functional Plk1 allele in the presence of 3-MB-PP1, a bulky ATP-analog inhibitor that specifically inhibits Plk1AS. Using this approach, we evaluated the ability of 34 singly non-modifiable Plk1 mutants to complement Plk1AS in the presence of 3-MB-PP1. Mutation of the T-loop activating residue T210 and adjacent T214 are lethal, but surprisingly individual mutation of the remaining 32 posttranslational modification sites did not disrupt the essential functions of Plk1. To evaluate redundancy, we simultaneously mutated all phosphorylation sites in the kinase domain except for T210 and T214 or all sites in the C-terminal polo-box domain (PBD). We discovered that redundant phosphorylation events within the kinase domain are required for accurate chromosome segregation in anaphase but those in the PBD are dispensable. We conclude that PTMs within the T-loop of Plk1 are essential and nonredundant, additional modifications in the kinase domain provide redundant control of Plk1 function, and those in the PBD are dispensable for essential mitotic functions of Plk1. This comprehensive evaluation of Plk1 modifications demonstrates that although phosphorylation and ubiquitination are important for mitotic progression, many individual PTMs detected in human tissue may have redundant, subtle, or dispensable roles in gene function.

Identification of Selective Lead Compounds for Treatment of High-Ploidy Breast Cancer.

Increased ploidy is common in tumors but treatments for tumors with excess chromosome sets are not available. Here, we characterize high-ploidy breast cancers and identify potential anticancer compounds selective for the high-ploidy state. Among 354 human breast cancers, 10% have mean chromosome copy number exceeding 3, and this is most common in triple-negative and HER2-positive types. Women with high-ploidy breast cancers have higher risk of recurrence and death in two patient cohorts, demonstrating that it represents an important group for improved treatment. Because high-ploidy cancers are aneuploid, rather than triploid or tetraploid, we devised a two-step screen to identify selective compounds. The screen was designed to assure both external validity on diverse karyotypic backgrounds and specificity for high-ploidy cell types. This screen identified novel therapies specific to high-ploidy cells. First, we discovered 8-azaguanine, an antimetabolite that is activated by hypoxanthine phosphoribosyltransferase 1 (HPRT1), suggesting an elevated gene-dosage of HPRT1 in high-ploidy tumors can control sensitivity to this drug. Second, we discovered a novel compound, 2,3-diphenylbenzo[g]quinoxaline-5,10-dione (DPBQ). DPBQ activates p53 and triggers apoptosis in a polyploid-specific manner, but does not inhibit topoisomerase or bind DNA. Mechanistic analysis demonstrates that DPBQ elicits a hypoxia gene signature and its effect is replicated, in part, by enhancing oxidative stress. Structure-function analysis defines the core benzo[g]quinoxaline-5,10 dione as being necessary for the polyploid-specific effects of DPBQ. We conclude that polyploid breast cancers represent a high-risk subgroup and that DPBQ provides a functional core to develop polyploid-selective therapy. Mol Cancer Ther; 15(1); 48-59. ©2015 AACR.

Lipid domain-dependent regulation of single-cell wound repair.

After damage, cells reseal their plasma membrane and repair the underlying cortical cytoskeleton. Although many different proteins have been implicated in cell repair, the potential role of specific lipids has not been explored. Here we report that cell damage elicits rapid formation of spatially organized lipid domains around the damage site, with different lipids concentrated in different domains as a result of both de novo synthesis and transport. One of these lipids-diacylglycerol (DAG)-rapidly accumulates in a broad domain that overlaps the zones of active Rho and Cdc42, GTPases that regulate repair of the cortical cytoskeleton. Formation of the DAG domain is required for Cdc42 and Rho activation and healing. Two DAG targets, protein kinase C (PKC) ß and η, are recruited to cell wounds and play mutually antagonistic roles in the healing process: PKCß participates in Rho and Cdc42 activation, whereas PKCη inhibits Rho and Cdc42 activation. The results reveal an unexpected diversity in subcellular lipid domains and the importance of such domains for a basic cellular process.