Localization of human TACC3 to mitotic spindles is mediated by phosphorylation on Ser558 by Aurora A: a novel pharmacodynamic method for measuring Aurora A activity

Aurora kinase inhibitors: identification and preclinical validation of their biomarkers

Aurora kinases are key regulators of mitosis and inhibitors being developed by a wide range of pharmaceutical and biotechnology companies for the treatment of cancer. Tumor cells respond differentially on inhibition of different Aurora kinase family members and these differences have to be considered in the clinical development of small-molecule inhibitors with respect to the chosen indications, the schedules or the selection of appropriate end points and they should also guide the development of biomarkers. Preclinical validation of potential biomarkers for Aurora kinase inhibitors led to a first application in clinical trials, as exemplified for the phosphorylation of histone H3 to follow Aurora-B inhibition. This review discusses the criteria for translation into the clinic and the value of pharmacodynamic biomarkers and their potential, but also their limitations to be used as surrogate markers for clinical end points.

Aurora-A: the maker and breaker of spindle poles

The gene encoding the Aurora-A protein kinase is located in the 20q13 breast cancer amplicon and is also overexpressed in colorectal, pancreatic and gastric tumours. Although Aurora-A may not be a bona fide oncoprotein in humans, it is a promising drug target in cancer therapy. Thus, it is surprising that so little is known of its role in normal cells. The primary function of Aurora-A is to promote bipolar spindle assembly, but the molecular details of this process remained obscure until recently. The discovery of several novel Aurora-A-binding proteins and substrates has implicated Aurora-A in centrosome maturation and separation, acentrosomal and centrosomal spindle assembly, kinetochore function, cytokinesis and in cell fate determination. Here we discuss recent advances in determining the early mitotic role of Aurora-A, with a strong emphasis on its function at the mitotic spindle poles.

Integrin-linked kinase localizes to the centrosome and regulates mitotic spindle organization

Integrin-linked kinase (ILK) is a serine-threonine kinase and scaffold protein with well defined roles in focal adhesions in integrin-mediated cell adhesion, spreading, migration, and signaling. Using mass spectrometry–based proteomic approaches, we identify centrosomal and mitotic spindle proteins as interactors of ILK. α- and β-tubulin, ch-TOG (XMAP215), and RUVBL1 associate with ILK and colocalize with it to mitotic centrosomes. Inhibition of ILK activity or expression induces profound apoptosis-independent defects in the organization of the mitotic spindle and DNA segregation. ILK fails to localize to the centrosomes of abnormal spindles in RUVBL1-depleted cells. Additionally, depletion of ILK expression or inhibition of its activity inhibits Aurora A–TACC3/ch-TOG interactions, which are essential for spindle pole organization and mitosis. These data demonstrate a critical and unexpected function for ILK in the organization of centrosomal protein complexes during mitotic spindle assembly and DNA segregation.

Polo and Aurora kinases: lessons derived from chemical biology

During the cell division cycle, mitotic entry, spindle assembly, chromosome segregation, and cytokinesis must all be carefully coordinated to ensure that the two daughter cells inherit all the genetic material required for further growth and development. Central to this coordination are several protein kinases including Aurora A, Aurora B, and the Polo-like kinase, Plk1. A number of small-molecule Aurora and Plk1 inhibitors have been developed because these kinases are seen as attractive anticancer drug targets. These inhibitors are now being widely used as chemical biology tools to understand how these kinases ensure faithful genome transmission.

Human Cep192 is required for mitotic centrosome and spindle assembly

As cells enter mitosis, centrosomes dramatically increase in size and ability to nucleate microtubules. This process, termed centrosome maturation, is driven by the accumulation and activation of gamma-tubulin and other proteins that form the pericentriolar material on centrosomes during G2/prophase. Here, we show that the human centrosomal protein, Cep192 (centrosomal protein of 192 kDa), is an essential component of the maturation machinery. Specifically, we have found that siRNA depletion of Cep192 results in a complete loss of functional centrosomes in mitotic but not interphase cells. In mitotic cells lacking Cep192, microtubules become organized around chromosomes but rarely acquire stable bipolar configurations. These cells contain normal numbers of centrioles but cannot assemble gamma-tubulin, pericentrin, or other pericentriolar proteins into an organized PCM. Alternatively, overexpression of Cep192 results in the formation of multiple, extracentriolar foci of gamma-tubulin and pericentrin. Together, our findings support the hypothesis that Cep192 stimulates the formation of the scaffolding upon which gamma-tubulin ring complexes and other proteins involved in microtubule nucleation and spindle assembly become functional during mitosis.