ITHACA, N.Y. -- Cancer researchers at Cornell University have learned how some proteins receive the marching orders that dispatch them to initiate signaling pathways and produce malignant cell transformation. The discovery offers new potential targets for anti-cancer drugs to block tumor growth.
Reporting in the June 15 issue of Nature (Vol. 405, No. 6788, pp. 800-804), a team led by Richard A. Cerione, professor of molecular medicine and chemical biology, describes what happens when normal protein traffic in cells runs wild: A hyperactive form of the molecular switch called Cdc42 increases the shuttling of other proteins throughout the cell, disrupting the orderly process of cell growth.
"We now believe that Cdc42 has to traffic proteins that are critical to cell growth toward the cell surface," Cerione said in a prepublication interview, describing the process by which protein-coated vesicles containing protein cargo are moved to the right place at the right time. "Cdc42 has to be switched on -- perhaps for only a minute -- to traffic proteins and stimulate cell growth. But if Cdc42 is mutated and remains switched on for an extended period of time, cellular activities are overstimulated," he said.
"The role of Cdc42 in protein traffic has long been suspected but never directly demonstrated," Cerione added," but now that we know about its trafficking target, we can try to design drugs to moderately inhibit the normal activity of Cdc42 --in effect to make it a little bit sick -- and tone down the cell-growth activities for therapeutic value."
The Cdc42 protein acts as a molecular "switch" with a key role in regulation of the cell cycle, the "program" that guides cell growth and cell division. It is believed to play a dual role, alternating as an essential protein for normal cell growth and as a switch that allows protein signals from a mutated Ras oncogene to cause cancer. (The Ras oncogene is implicated in 30 to 50 percent of all cancers.) Cdc42 was discovered and cloned in 1990 both from S. cerevisiae (budding yeast) by a group
at the University of Michigan and from a human cDNA library at Cornell by researchers in Cerione's laboratory.
In the February 2000 issue of the journal Cell, a research team led by Cerione announced the three-dimensional molecular structure of the protein complex of Cdc42 and GDI (for guanine nucleotide-dissociation inhibitor). The latest discovery of Cdc42's role in intracellular protein shuttling has prompted commentary articles by other cancer researchers in the same issue of Nature and in the next issue of Cell.
In addition to Cerione, who is a professor both in Cornell's College of Arts and Sciences and College of Veterinary Medicine, the finding was primarily the work of Wen Jin Wu, a Cornell graduate student in the field of pharmacology; other contributers to the study were Jon W. Erickson, a research associate in the Cerione laboratory; and Rui Lin, a Cornell graduate student at the time of the study and now a postdoctoral researcher at University of California--San Francisco. The study was supported by the National Institutes of Health.
While structural mapping of cellular switches, such as the Cdc42/GDI complex, is conducted by X-ray crystallography in Cornell's MacCHESS facility (macromolecular high-energy synchrotron source), the protein-traffic study was something of a fishing expedition, Cerione said.
"Wen Jin Wu baited the hook with Cdc42 that was switched on or activated and trolled through cell lysates to see what proteins he could catch and sequence," Cerione said, describing a commonly used technique by his laboratory to search for important Cdc42-targets. Wu fished in lysates prepared from mouse fibroblasts, a type of cell in connective tissue found in all mammals that is a standard cellular model for cancer studies.
Like a lure that attracts some fish but not others, the activated Cdc42 "caught" molecular binding sites on a specific protein that normally coats trafficking vesicles, the researchers reported.
"Cdc42 has to traffic specific proteins -- among the thousands of different proteins that are essential for cell growth -- and it now appears this all has to happen in synchrony with a number of other signaling events activated by Cdc42," Cerione explained. "This requires that Cdc42 talks to multiple targets, sending signals that influence cell shape as well as genetic activity in the nucleus. In effect, Cdc42 is like the conductor of an elaborate symphony, making certain that diverse cellular activities all occur with a precise timing and coordination.
"The problem is that when Cdc42 becomes hyperactive and gives the full-speed-ahead signal, you no longer have carefully regulated shape changes and there is a loss of cell growth control, thus yielding the hallmarks of cancer cells."
Cerione noted, "We're not saying that Cdc42 is the only or even primary perpetrator -- the problem often starts with mutated Ras genes -- but given the essential role of Cdc42 in Ras-induced malignant transformation, the identification of the critical cellular targets for Cdc42 will give us new possibilities for intervention."
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