In the study, the cover story in the Feb. 17 issue of Molecular Cell, the researchers report they have discovered a new mechanism that regulates the action of K-Ras, a cellular protein that plays an important role in many human cancers. "The general feeling was that we had learned everything about ras that there is to know," says Mark Philips, M.D., Professor of Medicine, Cell Biology, and Pharmacology at NYU School of Medicine, who led the study. "But here is a completely new twist on the story."
The researchers "add a striking new dimension to our understanding of how Ras protein function can be regulated," writes Larry A. Feig, Ph.D., Professor of Biochemistry at Tufts University School of Medicine, Boston, in an editorial accompanying the study.
Ras proteins have captured the interest of cancer researchers since the late 1970s, when the first oncogenes -- genes that cause the transformation of normal cells into cancerous cells -- were discovered. One of those oncogenes was ras. There are three ras genes, and K-Ras is the most important in terms of its impact on human cancer.
K-Ras acts like as a molecular switch. In its normal form, the protein can be turned on and off to control pathways that regulate cell growth. The mutated form, however, is locked in the "on" position, causing cells to grow uncontrollably and, at the same time, turning off programmed cell death, or apoptosis, the process that tells a cell when it is time to die. The result is cancer.
Until recently, K-Ras was thought to function only at the cell membrane, where the protein is permanently anchored in place by lipid molecules and electrostatic forces. "We discovered that the position of K-Ras in membranes is not permanent, and its positioning can be regulated by a signaling enzyme called protein kinase C," says Dr. Philips. The inspiration for this study came from a separate experiment, in which cells were exposed to substances that stimulate protein kinase C (PKC). Unexpectedly, K-Ras began to appear in an unusual place. The current study was then launched to determine what was causing K-Ras to relocate and what it might be doing in its new home.
Dr. Philips and his colleagues discovered that PKC causes a phosphate molecule to be added to K-Ras. This "phosphorylation" process weakens the electrostatic bonds that anchor the protein, allowing it to dislodge from the plasma membrane.
Based on prior work, the researchers expected that the dislodged K-Ras would attach to the membrane of organelles in the cell, such as the endoplasmic reticulum and the Golgi body. "But we were surprised to find that dislodged K-Ras also goes to the surface of the mitochondria -- the powerhouse of the cell, and one of the organelles that is intimately involved in regulating apoptosis," says the researcher.
Next, Dr. Philips set out to determine what effect this loose K-Ras has on cellular function, if any. When phosphorylated K-Ras was introduced into cells grown in culture, it turned out to be highly toxic. "We found that the toxicity was due to the promotion of apoptosis," says Dr. Philips. "That was very surprising because ras is usually thought of as an oncogene. Oncogenes generally promote uncontrolled growth and block cell death. And here we were seeing a situation where ras was promoting cell death."
The most intriguing aspect of the study is that it identifies a potential new mode of suppressing tumors, says Dr. Philips.
"Our data suggested that if we could find a way to phosphorylate K-Ras, we might be able to promote programmed cell death in tumors driven by the ras oncogene."
To test this hypothesis, the researchers examined the effects in mouse tumor cells of byrostatin, a drug known to stimulate PKC. The drug was found to have some anti-tumor activity, both in test-tube and animal studies. When the drug was given to mice with tumors in which K-Ras was engineered so that it could not be phosphorylated, the drug had no effect. "That told us that the anti-tumor effect of this drug was indeed occurring through the mechanism we had just discovered," says Dr. Philips.
"This is a new way of thinking about treating tumors that are K-Ras dependent," he adds. "Bryostatin is not very effective, but one could look for other drugs that promote, in a more robust way, the phosphorylation of K-Ras. That might be a viable way of treating lung, pancreatic, colon, and other cancers."
"This is very early in the process," Dr. Philips cautions. "It is an interesting idea, but the concept has to be validated through a lot more work."
Journal
Molecular Cell