Inhibidores PLK1, Aurora Kinasa A, Aurora Kinasa B, nuevos retos nuevas esperanzas

Co-inhibition of polo-like kinase 1 and aurora kinases promotes mitotic catastrophe
Jingjing Li, _ Myung Jin Hong, Jeremy P.H. Chow, Wing Yu Man, Joyce P.Y. Mak, Hoi Tang Ma, Randy Y.C. Poon
Jingjing Li, Myung Jin Hong, Jeremy P.H. Chow, Wing Yu Man, Joyce P.Y. Mak, Hoi Tang Ma, Randy Y.C. Poon
Division of Life Science, Center for Cancer Research, and State Key Laboratory of Molecular Neuroscience, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
Correspondence to: Randy Y.C. Poon, e-mail: rycpoon@ust.hk Keywords: anticancer drugs, antimitotic drugs, kinases, mitosis, mitotic slippage
Received: November 04, 2014 Accepted: February 08, 2015 Published: March 20, 2015

ABSTRACT
Mitosis is choreographed by a number of protein kinases including polo-like kinases and Aurora kinases. As these kinases are frequently dysregulated in cancers, small-molecule inhibitors have been developed for targeted anticancer therapies. Given that PLK1 and Aurora kinases possess both unique functions as well as co-regulate multiple mitotic events, whether pharmacological inhibition of these kinases together can enhance mitotic catastrophe remains an outstanding issue to be determined. Using concentrations of inhibitors that did not induce severe mitotic defects on their own, we found that both the metaphase arrest and mitotic slippage induced by inhibitors targeting Aurora A and Aurora B (MK-5108 and Barasertib respectively) were enhanced by a PLK1 inhibitor (BI 2536). We found that PLK1 is overexpressed in cells from nasopharyngeal carcinoma, a highly invasive cancer with poor prognosis, in comparison to normal nasopharyngeal epithelial cells. Nasopharyngeal carcinoma cells were more sensitive to BI 2536 as a single agent and co-inhibition with Aurora kinases than normal cells. These observations underscore the mechanism and potential benefits of targeting PLK1 and Aurora kinases to induce mitotic catastrophe in cancer cells.

INTRODUCTION

Accurate cell division relies on the actions of a well-balanced network of protein kinases and phosphatases [1]. Polo-like kinases and Aurora kinases are two of the most studied families of mitotic kinases. These kinases performs multiple functions in mitosis, including centrosome maturation, kinetochore-spindle attachment, chromosome segregation, and cytokinesis.

One of the critical functions of polo-like kinase 1(PLK1) in mitosis is for kick-starting the autocatalytic loop that controls the activity of cyclin B1–CDK1 [2]. Phosphorylation by PLK1 promotes the activation of CDC25 [3–5] and inactivation of WEE1 [6,7]. In addition to regulating cyclin B1–CDK1 activity through the CDC25/WEE1 axis, PLK1 also controls the localization of cyclin B1 by phosphorylating its nuclear export sequence. This inhibits the binding of the export receptor CRM1, thereby triggering nuclear accumulation and activation of cyclin B1 during prophase [8].

Similarly to many protein kinases, PLK1 activation requires phosphorylation of a residue on the T-loop (Thr210). PLK1Thr210 is phosphorylated by Aurora A (also called AURKA), an event that is assisted by Bora [9]. PLK1 in turn promotes the recruitment of AURKA to the centrosomes in late G2 [10]. PLK1 also phosphorylates Bora, generating a phosphodegron motif that is recognized by the ubiquitin ligase SCFβ–TrCP, thereby triggering Bora destruction [11]. Degradation of Bora is believed to be important for redistributing AURKA from a cytoplasmic Bora-containing complex to a TPX2-containing complex at the mitotic spindle. The TPX2–AURKA complex then promotes centrosome maturation and bipolar spindle formation in a Ran- GTP-dependent manner [12].

Although highly related to AURKA, Aurora B (also called AURKB) is a component of the chromosomal passenger complex (CPC), which is comprised of AURKB, INCENP, borealin, and survivin [13]. CPC localizes to chromosomes and kinetochores in early mitosis and functions in chromosome–microtubule interactions, sister chromatid cohesion, and the spindle-assembly checkpoint. In early mitosis, PLK1Thr210 is phosphorylated by AURKB at centromeres and kinetochores [14]. In anaphase, the CPC is relocated to the mid-zone to promote cytokinesis.

The activity of AURKA increases from late G2 phase onwards and peaks during prometaphase. On the other hand, the activity of AURKB peaks from metaphase to the end of mitosis. Activation of AURKA requires binding to specific cofactors including Ajuba, Bora, and TPX2, leading to the autophosphorylation of a residue in the T-loop (Thr288) [15]. Similarly, AURKB is activated by autophosphorylation of the T-loop (Thr232) after binding to members of the CPC [16]. At the end of mitosis, both AURKA and AURKB are degraded by APC/C-mediated ubiquitination.

Although PLK1, AURKA, and AURKB have their unique functions during mitosis, they also co-regulate multiple processes, including entry into mitosis, mitotic spindle formation, sister chromatid resolution, chromosome–spindle connections, and cytokinesis. Mechanistically, these kinases often phosphorylate different components involve in the same mitotic process [17].

As Aurora kinases are upregulated in several human cancers and correlated with poor prognosis, they are believed to be important anticancer drug targets [18]. More than 20 small-molecule Aurora kinase inhibitors have been developed and are at various stages of clinical trials [19]. While the early generations of Aurora kinase inhibitors inactivate both AURKA and AURKB indiscriminately, later generations of inhibitors are able to target AURKA or AURKB specifically. Similarly, scores of anticancer drug candidates targeting PLK1 have been developed [20].

Inhibition of PLK1 or the two Aurora kinases triggers a process generally termed mitotic catastrophe. More recent attempts to standardize the term mitotic catastrophe defined it to be associated with aberrant mitotic activity that ultimately triggers cell death or irreversible cell cycle arrest [21]. In this definition, cell death can occur either during or after the defective mitosis such as mitotic slippage.

Given the close relationship between PLK1 and the Aurora kinases, a salient question is the biological consequences when these kinases are targeted together. Will inhibitors of PLK1 and Aurora kinases synergistically enhance a particular mitotic defect? Or will they antagonize each other, causing different defects as if they act independently? Here we address these questions directly at single cells level using live-cell imaging, in particular comparing normal and cancer cells from nasopharyngeal carcinoma.