Scientists searching for molecular clues to cancer have produced a detailed picture showing how a key protein blocks a central promoter of cell growth involved in virtually all human cancers. The discovery sets the stage for developing drugs to mimic the protein, called p27, with the hope of halting the uncontrolled cell division that ultimately leads to the formation of tumors.
The three-dimensional image, produced in atom-by-atom detail using a combination of x-rays and crystals, was published in the July 25th issue of Nature by a team of researchers at Memorial Sloan-Kettering Cancer Center in New York City led by Nikola P. Pavletich, phd. The research is supported by the NIH and the Howard Hughes Medical Institute.
Cell division in many tissues is regulated by the p27 protein, which binds to and inhibits enzymes called cyclin-dependent kinases (CDKs). The crystal structure reveals in intimate detail how p27 paralyzes CDKs, thereby arresting the cell cycle.
Cyclin-dependent kinases are so crucial to cell division that cells--even those that are cancerous--will not multiply without them. Thus, the role of CDKs in tumor growth is a main focus of cancer research, says Joan Massagué, phd, Program Chairman for Cell Biology at Memorial Sloan-Kettering, and one of the paper's authors.
"Now that we understand exactly how p27 attaches to CDKs to halt the cell cycle, it may be possible to design or search for innovative cancer drugs that work in the same way," says Dr. Massagué, whose laboratory was the first to purify and clone p27 in 1994.
Research Has Broad Implications
The researchers' high-tech picture also is likely to have broad implications for other inhibitory proteins in the p27 family, such as p21 and p57. "We expect these proteins, which are structurally very similar to p27, to bind to CDKs in almost the same way as p27," says Dr. Massagué. "This work is important because it establishes a general principle by which an entire family of inhibitors will block cell growth."
Under normal, healthy conditions, these inhibitors keep cell growth in check as part of a universal pathway that, when altered by genetic mutations, leads to cancer. "What is remarkable is that this pathway, at one step or another, is crippled in many cancers," Dr. Massagué says.
Researchers have known for years that p27 exerts its inhibitory effect by binding to CDKs. Cyclin-dependent kinases are activated when they bind with cyclins. When a cell is ready to divide, cyclins latch onto CDKs, and the resulting complex activates other enzymes to copy cells' DNA.
Now, by using a sophisticated technique called x-ray crystallography, Drs. Pavletich and Massagué and their colleagues have shown exactly how p27 attaches to the cyclin-CDK complex. The researchers bombarded crystals containing millions of copies of the p27-cyclin-CDK complex with intense x-rays directed from many different angles. They then used an electronic detector to gather information from x-rays that ricocheted off the crystals. A powerful computer analyzed the data, and the researchers used the resulting information to piece together the molecular structure.
The crystal structure reveals that p27 interacts with both members of the cyclin-kinase complex, in this case cyclin A and CDK2 (see Figure 1). Specifically, p27 inhibits cell division by inserting itself into a pocket in the cyclin-CDK complex that normally binds the energy molecule adenosine(Drug information on adenosine) triphosphate (ATP). Without the phosphate from ATP, CDKs can't activate the enzymes that copy DNA.
In recent years, scientists at Memorial Sloan-Kettering have focused their efforts on elucidating the biologic and chemical controls that govern the reproduction of individual cells. In 1994, in Science, Dr. Pavletich reported using x-ray crystallography to identify the atomic structure of p53, a protein produced by a gene involved in triggering about half of all cancers. The p53 protein halts cell division by activating the p21 cell cycle-inhibitor protein.
Then in 1995, Drs. Pavletich and Massagué and their colleagues also used x-ray crystallography to identify one type of cyclin-CDK complex, which is one of the targets of the p2l and p27 inhibitors. This report was published in Nature.
The team now plans to use x-ray crystallography to reveal how another class of cell cycle-inhibitors, among them p15 and p16, binds to cyclin-CDK complexes to halt cell division.