BioWorld International Correspondent
LONDON - A new understanding of the signaling pathways that tell cells whether to divide or die could lead to new strategies for developing drugs to treat cancer.
The latest study in that field suggests that kinases - known to play a role in controlling the cell cycle - might be able to exert their effect without using their enzymatic function.
Walter Kolch, professor of molecular cell biology at the University of Glasgow in Scotland, told BioWorld International: "Until now, it has been thought that all the effects of a kinase were due to its catalytic activity. But we have shown that the kinase known as Raf-1 can work via a pathway that is completely independent of its function as a kinase. It is as though Raf-1 is acting like a hybrid between a kinase and a scaffolding protein. The latter acts to ensure that a protein complex is assembled in the correct way."
That finding could hold implications for work directed at developing kinase inhibitors as treatments for cancer, he added, because they so far have targeted the enzymatic function of those molecules.
"In this case, the function of Raf-1 is based on protein-protein interactions," Kolch said. "Such a target is more difficult to hit with a small-molecular drug, but it highlights that there is much more to regulatory activity within the cell than catalytic activity on its own."
Kolch and his colleagues report their study in the Dec. 23, 2004, issue of Science in a paper titled: "Role of the Kinase MST2 in Suppression of Apoptosis by the Proto-Oncogene Product Raf-1."
Kolch, group leader at the Beatson Institute for Cancer Research in Glasgow, together with colleagues in Glasgow and in Vienna, Austria, used a proteomics approach to study protein-protein interactions important to regulation of cell division and apoptosis.
That strategy involves purifying the protein of interest from cells and identifying which other proteins it is associated with.
The group started out studying Raf-1, which plays a role in passing signals from outside mammalian cells into the nucleus. When the cells were deprived of mitogens - substances that cause cells to start dividing - the researchers found that Raf-1 would interact with another protein called MST2.
At that time, all that was known about MST2 was that, when overexpressed in cells, it triggers apoptosis. Then, the group heard about several studies that had shown that MST2 was required to bring about apoptosis and cell-cycle arrest during eye development in Drosophila.
Another key piece of information came from a study of mice lacking a functional gene for Raf-1. The mice die as embryos, and the cause of death is failure to develop due to apoptosis.
"All these studies were pointing us toward the idea that these two proteins play a key role in controlling apoptosis," Kolch said.
The researchers knew that Raf-1's only known function was to regulate the ERK pathway, which acts to control cell proliferation, differentiation and survival, and which is frequently overactive in human tumors. Yet in the knockout mice lacking Raf-1, the ERK pathway operated normally. That's because another enzyme, B-Raf (a protein kinase related to Raf-1), is able to compensate for Raf-1 in that pathway.
Kolch explained: "Because the mice still die, this suggested to us that Raf-1 must have a new function not previously described, which protects against apoptosis but that cannot be compensated for by this other enzyme. It seems that this function is demonstrated by control of MST2."
Further investigations showed that Raf-1 suppresses MST2 in two ways, neither of which requires Raf-1's catalytic activity.
The MST2-Raf-1 complex, Kolch concluded, appears to direct the cell toward either division or death, according to the signals received.
"It is like a checkpoint that controls whether the cell has a license to divide or not," he said.
Future studies for the team include identifying which other proteins MST2 associates with in order to track down how MST2 induces apoptosis, and working out how MST2 is activated.
MST2 also is known to associate with a molecule called RASSF1, which is silenced in many common human tumors. The group also will be studying the interaction between MST2 and RASSF1.