Genetically engineered mice convinced scientists at the Salk Institute for Biological Studies that it was time to overhaul widely held beliefs about how a powerful tumor suppressor called p53 is controlled in cells. Their new model of p53 regulation has important implications for the development of anticancer drugs.
This new model, published in the April issue of the journal Cancer Cell, emphasizes the independent role of two proteins, called Mdm2 and Mdm4. Both proteins are part of the tightly controlled system of checks and balances ensuring that p53 keeps a tight lid on unchecked cell growth but doesn't wreak havoc in healthy cells.
Up to this point, researchers thought Mdm2 and Mdm4 collaborated to halt the activities of p53. As a powerful tumor suppressor, p53 turns on genes that either halt cell division, to allow time for repair of damaged DNA, or, when all rescue attempts prove futile, order the cell to commit suicide. The mouse experiments revealed that, in fact, it is Mdm4 that renders p53 inactive, while Mdm2 mainly controls the stability of p53's structure.
The distinction is important, says the study's lead investigator, Professor Geoffrey M. Wahl, Ph.D., a professor in the Gene Expression Laboratory. "p53 is disarmed in more than half of all cancers, and Mdm2 and Mdm4 are over-expressed to act like cancer-causing oncogenes in much of the rest. We need to know how each of these p53 inhibitors work in normal cells before we can figure out the most effective therapeutic strategies to manipulate them in cancer cells," he says.
The new findings suggest that cancer drugs now being tested that inhibit Mdm2 may not work as hoped. Researchers thought that, since the functions of Mdm2 and Mdm4 were linked, it would suffice to inhibit Mdm2 to restore p53's tumor-suppressing activity in cancer cells.
"In fact, we observed that a partial decrease in Mdm2 or Mdm4 activity only marginally affects p53 function, but that a combined decrease of Mdm2 and Mdm4 dramatically increases p53 function to improve tumor suppression," says lead author Franck Toledo, Ph.D., a former Salk scientist now at the Pasteur Institute in Paris, France. "We also found that the complete ablation of Mdm4 activity leads to very efficient tumor suppression. The clinical implications of these findings are obvious: drugs that inhibit Mdm4 need to be actively searched for, as they should be powerful tools against cancer," Toledo adds.
Researchers already knew that Mdm2 and Mdm4 were important for controlling p53, but how these enzymes interacted with p53 has been the subject of controversy. The Salk researchers discovered that the primary role of Mdm2 is to flag p53 for destruction to keep p53 protein levels low, while Mdm4 prevents p53 from turning on genes when the tumor suppressor is not needed. For p53 to be activated, Mdm4 first needs to be eliminated.
Wahl and his team now believe that DNA damage triggers the release of specific enzymes that modify Mdm2 and Mdm4. This action flips a "switch," prompting Mdm2 to target both Mdm4 and itself for degradation. Then, as p53 activates genes to shut down the cell cycle, it also turns on the gene for Mdm2. Increased p53 activity leads to heightened expression of Mdm2, which increases degradation of Mdm4 - freeing p53 to function unhindered - at the same time keeping p53 from going overboard.
"This is a very elegant system, because it acts to titrate p53, giving the cell time to repair its DNA and to gauge how much damage there really is," Wahl says.
Authors who also contributed to this work include Kurt Krummel, Crystal Lee, Chung-Wen Liu, Luo-Wei Rodewald, and Mengjia Tang, all at the Salk Institute.