"We now have a very important clue as to how Ski interferes with key proteins that prevent cells from becoming cancerous," says Yigong Shi, a molecular biologist at Princeton University and leader of one of the two teams that conducted the study. "Understanding how to stop Ski from disrupting the normal function of cells will probably be key to developing new anticancer drugs."
Ski prevents a protein called transforming growth factor-beta (TGF-b) from safeguarding cells against excessive growth. "TGF-b acts like a molecular traffic light, ordering certain cells to slow down and stop dividing," Shi says. "When TGF-b is blocked, for example by Ski, cells manage to speed through this checkpoint, triggering runaway cellular growth that eventually results in cancerous tumors."
TGF-b cannot enter cells, so it transmits its signal inside the cell by attaching to receptor proteins on the cell's outer surface. The signal generated by this interaction is carried across the cell membrane to proteins inside the cell. Some of these signaling proteins are triggered inside the cell cytoplasm and later bind to other proteins inside the nucleus. The combination of both types of signaling proteins activates genes necessary for the normal functioning of the cell.
Ski, which is already present in the human body, disrupts the signaling proteins when it is either overexpressed or introduced by a virus inside the body. The new study focused on the first of these two possible processes.
"Scientists have previously shown that Ski disrupts normal cell functioning by directly disrupting the expression of genes inside the cell's nucleus," Shi says. "But nobody has ever investigated whether Ski could disrupt the signaling proteins that activate the genes."
This later process was investigated by two teams of scientists, from Princeton and DOE's Lawrence Berkeley National Laboratory.
The Princeton team looked at the molecular details of a complex made of Ski and the nuclear signaling proteins by using a method called x-ray diffraction. The scientists first crystallized the complex, and then projected very bright x-rays produced by the NSLS onto the crystal. By looking at how the x-rays scattered off the crystal, the scientists measured a pattern of points with varying intensities, called a diffraction pattern, which represents a map of the atomic structure of the compound.
The researchers saw that, as they had suspected, Ski disrupts the cytoplasmic signaling proteins, so that when Ski binds to the nuclear signaling proteins, the cytoplasmic signaling proteins cannot attach to their nuclear counterparts. "This binding process is probably one of the major ways in which Ski disrupts the signaling proteins and, thus, suppresses the action of TGF-b," Shi says.
The Berkeley team performed various biochemical tests that confirmed these results by also showing that Ski binds to nuclear signaling proteins.
"These results force us to do some rethinking about the role of Ski in the development of cancerous tumors," Shi says. "Ski is not merely recruiting proteins that repress genes; it can also disrupt signaling proteins, which makes Ski a more effective tumor-inducing protein."
The scientists acknowledge that tumor development is very intricate and that a better understanding of the roles of many more proteins involved in cancer is still needed. "Cancer is not a simple disease, and there are many pathways through which it can develop," Shi says. "All these pathways need to be investigated to design effective anticancer drugs, and our results represent an important step in that direction."
This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields.
The U.S. Department of Energy's Brookhaven National Laboratory (http://www.