Biochemistry, Cell Biology
University of Colorado Boulder
Dr. Ahn is also a professor of chemistry and biochemistry at the University of Colorado Boulder and an adjunct professor of biochemistry and a member of the University of Colorado Cancer Center at the University of Colorado Health Sciences Center, Denver.
Chemicals circulate through complex organisms to tell cells in organs and tissues how to act in response to hunger, stress, or desire. These external messages are translated into appropriate cellular conduct because cells have internal molecular circuits that react to the stimuli.
These ordered biochemical reactions are called signal transduction pathways. They rely on a stepwise sequence of enzymes (proteins that catalyze chemical reactions) and other substances to transmit the exterior communiqués to the cell's interior machinery. To activate a pathway, component enzymes and molecules usually undergo a chemical modification, such as the addition of a phosphate, which becomes attached by enzymes called protein kinases.
When Natalie Ahn was a postdoctoral student from 1988 to 1990 in the laboratory of Edwin Krebs at the University of Washington in Seattle, she was one of a group of scientists who discovered a signal transduction pathway (the mitogen-activated protein or MAP kinase pathway) that tells cells to divide and proliferate after exposure to growth factors. Her approach was painstaking, requiring more than 10,000 assays. She also found an enzyme called MAP kinase kinase, which adds a phosphate to the enzyme MAP kinase to make it active.
MAP kinase was among the early signal transduction pathways identified. The very first, the cyclic AMP-dependent kinase pathway, had been characterized by her mentor, who shared the Nobel Prize with Edmond Fischer in 1992 for that work. The work of Krebs and Fischer revealed how hormones like glucagon or epinephrine mobilize stored energy in the form of glycogen and break it down into metabolites of glucose for cellular use.
Since that time, scientists have described many more signal transduction pathways, and by understanding what happens during normal conditions have obtained new insights into causes of disease. It is now known that in many cancers, proteins in the MAP kinase pathway go awry, causing cells to divide and interact with other cells aberrantly. Indeed, steps in this pathway are now used as molecular targets by pharmaceutical companies developing new drugs to treat cancer and other diseases.
Today, as a chemistry professor at the University of Colorado at Boulder, Ahn continues researching the subtleties of the MAP kinase and other signaling pathways, and their possible role in cancer. She also studies the fundamental chemical behavior of proteins.
Recently, she found that the activities of two pathways change as a skin cancer, melanoma, evolves from early to advanced disease. In addition to the MAP kinase pathway, which becomes activated in an early stage of melanoma involving a precancerous mole, alterations in Rho GTPase pathways, which control the formation of F-actin, a protein fiber in the cell, also occur in melanoma. Rac, a Rho GTPase protein, is turned on during early- and late-stage cancer, while RhoA is selectively turned on during metastasis, the stage of the disease when cancer spreads throughout the body.
"The finding's implication is that the Rho GTPase proteins are important in events that control melanoma progression through different stages," Ahn said. What remains to be found, she said, are downstream proteins and other molecules affected by the aberrant activation of MAP kinase and Rho GTPase pathways.
To find these protein targets of signaling pathways, Ahn is devising new techniques to characterize and study proteins. Because much of a cell's work occurs at the protein level, her team studies protein behavior by measuring their changes in abundance and chemical modifications in response to signaling, using a technology referred to as proteomics.
When Ahn first arrived in Colorado in 1992, she introduced to her colleagues the use of an instrument called a mass spectrometer (MS) to analyze protein structure. Now, her team is using this technology to identify proteins inside cells and study how they control melanoma progression. Currently her approaches have enabled identification of approximately 40 percent of the proteins in cells. Her team is developing new strategies to expand these capabilities by observing more proteins and surveying their chemical modifications.
In another area of research, Ahn investigates how enzymatic activity is controlled in kinases. Although scientists believe that protein motions underlie many aspects of function, proving that this occurs and how it works is often very difficult, Ahn said. Using a method called hydrogen-deuterium exchange mass spectrometry, she has shown that MAP kinases undergo changes in protein mobility when they switch from inactive to active forms. She has also shown that these motional changes contribute to kinase activation, in a manner not predicted by other means of studying protein structure, such as x-ray crystallography. She is now trying to understand how they occur. In collaboration with her former doctoral advisor, Judith Klinman at the University of California, Berkeley, Ahn is also delving into other enzymes, which carry out catalysis through "hydrogen tunneling" events, and finding that protein motions also contribute significantly to their reactions.
Between running her laboratory, teaching, and university service, Ahn, like many senior scientists, often finds herself pulled in many directions. But being at the bench is what she enjoys most about science. Her renewed relationship with Klinman and new and current projects keep her motivated to personally get into the lab, she said. With her students and postdocs, Ahn continues to expand the frontiers of protein chemistry.