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Jun 1, 2012 (Vol. 32, No. 11)

Kinase Assays Find Niche in Cancer Drug R&D

  • Click Image To Enlarge +
    A graduate student at UNMC analyzes a Western blot from an immunodepletion assay that was used to examine the regulation of Aurora B kinase activity by TPX2. [Jintana Saowapa]

    Kinases are a class of highly regulated enzymes that play a central role in the regulation of numerous cellular processes. Due to the prevalence of aberrant kinase expression in many types of cancer, kinases have become a popular drug target in the war against cancer. But kinases also happen to be extremely difficult to target. Specifically targeting a single kinase in a pool of up to 500 different kinases in a human cell is no small feat, especially since kinases share a significant amount of structural similarity with one another.

    To help assist in the development of anticancer therapies that target specific kinases, researchers have developed kinase assays for a variety of applications, ranging from fundamental biological studies to kinase inhibitor discovery and characterization.

    At the recent American Association for Cancer Research conference in Chicago, a number of scientists discussed how they utilize various kinase assays in their investigations on cancer.

    Potential drug targets are often identified by looking for proteins that are overexpressed in cancer cells. However, if researchers do not understand how a particular protein works in concert with others, it can be difficult to develop inhibitors that effectively downregulate the overexpressed protein in diseased cells.

    Aurora B kinase is overexpressed in many types of cancer. To lay the groundwork to develop more effective therapeutics that target Aurora B, researchers need to understand how the kinase is regulated by other proteins in the cell. With this goal in mind, Ming-Ying Tsai, Ph.D., assistant professor at the University of Nebraska Medical Center, and graduate student Jyoti Iyer performed a research study that helps shed light on the cellular mechanism of Aurora B kinase.

    The team studied the role of mitotic protein TPX2 (targeting protein for Xenopus kinesin-like protein 2) in Aurora B regulation. In the cell, Aurora B is part of an enzyme complex known as the chromosomal passenger complex (CPC), along with three other proteins: INCENP, Survivin, and Borealin. Iyer first performed in vitro binding assays to determine which residues of TPX2 are sufficient to enhance the interactions between Aurora B and Survivin, a known activator. Using this fragment of TPX2, composed of residues 138 through 328, she performed an in vitro kinase assay with Aurora B and found that the TPX2 peptide fragment was able to increase Aurora B kinase activity.

    In a subsequent in vitro kinase assay, Iyer incubated recombinant, bacterially expressed CPC proteins and histone H3, a known substrate of Aurora B, in the presence or absence of the TPX2 fragment. She performed a Western blot with an antibody specific to phosphorylated histone H3 and found an increase in histone H3 phosophorylation in the presence of the TPX2 fragment compared to the control. The results suggest that TPX2 functions as a coactivator of Aurora B and serves as a scaffold protein for the assembly of the CPC.

    “I anticipate these findings will open lots of doors for future research” on the role of TPX2 in regulating Aurora B and the CPC, Iyer says.

  • Discovery and Development

    Beyond fundamental research studies, scientists in both academic and industrial labs are looking to kinase assays to discover and develop new anticancer treatments.

    Oncolytic viruses are a promising approach to treating cancer due to their potential to selectively kill cancer cells in the midst of healthy tissue. In a collaboration spearheaded by Bruce Bejcek, Ph.D., and Karim Essani, Ph.D., both professors at Western Michigan University, researchers are currently exploring the potential of tanopox virus (TPV) as an additional tool in the arsenal of oncolytic therapies.

    Tumor suppressor protein p53 is a promising target for virotherapy since it is mutated in more than 50% of cancers. To determine the potential of TPV as an oncolytic therapy, Dr. Bejcek’s team performed in vitro assays with the 142R protein of TPV, which has a serine-threonine kinase domain that is homologous to the p53-phosphorylating B1R gene in the more well-known vaccinia virus. The team found that 142R interacts with p53 in vitro and is thus a good candidate for creating a conditionally replicating virus that can selectively target cancer cells and leave healthy cells unharmed.

    The kinase assay involved mixing recombinant, bacterially expressed 142R and p53 in the presence of radiolabeled ATP. The researchers ran the proteins out on a gel and observed that radioactive phosphate was added onto p53 only in the presence of 142R, suggesting that 142R phosphorylates p53. The team plans to determine if 142R is able to phosphorylate p53 in cells, and also hope to determine whether knocking out 142R can cause TPV viruses to selectively replicate in, and ultimately kill, cancer cells lacking wild-type p53, Dr. Bejcek says.

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