New way to study proteins in live cells

Mar 9 2006

For the first time, Johns Hopkins researchers were able to easily jumpstart the activity of a well-known cancer protein in live cells with a small molecule, a strategy that pinpointed key players in the cancer process and can be used to determine new therapeutic targets.

What's more, the scientists' study, published in Science, identifies a simple method to further understand the complex mechanisms that underlie cancer as well as other diseases and may provide an easy model to screen for new cancer drugs.

"Our study reveals a new way to study proteins in live cells, in this case, a tyrosine kinase implicated in causing cancer," says the study's lead author, Philip A. Cole, M.D., Ph.D., director of the Department of Pharmacology and Molecular Sciences at The Johns Hopkins University School of Medicine. "This approach helped identify potentially important therapeutic targets and in the future may provide a method to easily screen cancer treatments."

In the study, Cole and his colleagues examined the tyrosine kinase Src (pronounced SARK), a clinically important cancer protein that scientists have heavily studied but do not completely understand. The Johns Hopkins researchers developed a special mutated version of the Src protein and incorporated it into live animal cells. The mutated version was inactive but contained an "ignition switch" that would turn it back on. They determined that the small molecule, imidazole, could act as the key. Imidazole fit into a pocket in the mutated structure of the Src protein, which mended the structure and reinstated Src's activity. Removal of imidazole quickly shut the protein off again.

"This strategy provided a controlled environment to study Src," says Cole. "This helped us uncover some new and unexpected insights into how the cancer protein creates its havoc, as well as new treatment leads." For one, the model provided evidence that Src interacts with CrkL, a signaling protein not previously known to be targeted by Src's actions. The study also found direct evidence that Src activates MAP kinase pathways, which help transfer information from growth factors, molecules that aid in the development of cancer cells. Previously the role of Src in these pathways was controversial.

"Understanding the functions of different proteins in normal states and disease states is crucial for treatment development because it can help identify new therapeutic targets," says Cole. Insights into tyrosine kinases could be particularly important for determining new cancer treatments, since scientists think that many different types are involved. "For example, Gleevec, which is used to treat gastrointestinal stromal tumors and chronic myeloid leukemia, is the most successful magic bullet against cancer in many years and works by blocking tyrosine kinase activity," Cole says.

As a next step, Cole and his colleagues plan to further examine the role of Src in cancer using their new model. They also plan to adapt the approach to develop a drug screen.

In the future, it also may be possible to use their chemical technique to mend mutated proteins found in people with certain genetic diseases, according to Cole. For example, the immune system disorder agammaglobulinemia involves mutated tyrosine kinases. Possibly researchers could identify a small molecule that rescues the activity of the mutated tyrosine kinases in the same way that imidazole corrected the structure of the mutated Src and jumpstarted its activity.

Other contributors to this study include Akhilesh Pandey, M.D., Ph.D., an assistant professor in the Institute of Genetic Medicine; Jin Zhang, Ph.D., an assistant professor in the Department of Pharmacology and Molecular Science, and Henrik Molina, M.S., a lab manager in the Institute of Genetic Medicine.