UCSF Researchers Challenge Paradigm of How a "Tumor Suppressor" Works

New cancer research reported online by UCSF Comprehensive Cancer Center members this week in Nature challenges conventional wisdom about how an archetypal, protective "tumor suppressor" protein works to prevent cancer. The p53 protein is a powerful tumor suppressor that prevents cells in our bodies from becoming cancerous. It does this by either killing or permanently shutting down cells exposed to stress or injury. Paradoxically, the UCSF study authors, led by Gerard Evan, an international leader among scientists who investigate the origins of cancer, propose that new drugs be developed and tested with the goal of shutting p53 down -- at least temporarily -- at the time of cancer treatment. This is because, while p53 blocks cancer, it also is responsible for side effects of conventional cancer therapies. By temporarily blocking p53, clinicians might be able to spare patients from many uncomfortable, harmful and dose-limiting side effects of chemotherapy and radiation therapy, the authors provocatively suggest. The researchers base their conclusions on experiments in mice exposed to DNA-damaging and mutation-causing radiation. P53 is a sensor of DNA damage within cells. It triggers suicide among cells in which the damage cannot be repaired. It has long been believed that p53 prevents cancer by causing damaged cells to commit suicide -- before mutations triggered within the cells can drive uncontrolled cell division and tumor growth. The scientific term for the distinctive biochemical chain of events leading to cellular suicide is "apoptosis." The Good and the Bad But just as environmental exposures can cause DNA damage -- such as UV radiation from the sun -- radiation therapy and chemotherapy for cancer can also cause DNA damage. This is especially true among cells that are dividing and replicating their DNA. These cells include not only cancer cells - the intended treatment target - but also healthy cells such as immune cells, blood cells and cells that line the gastrointestinal tract. The presence of therapy-induced mutations in sensitive cells is believed to be why survivors of childhood cancer are more susceptible to entirely new cancers later in life. In recent years, evidence has mounted that the loss of these normal tissues as a side effect of chemotherapy or radiation treatment is due to p53 triggering apoptosis in the wake of treatment-induced DNA damage. But the assumption has been that, when it comes to p53, one had to take the good with the bad - that the widespread apoptosis in sensitive tissues induced by cancer therapies served to wipe out the mutated cells that otherwise would eventually give rise to more cancers later. With the experiments reported this week, Evan asserts that his team has shown that the role of p53 in protecting against cancer is independent of its role of inducing apoptosis in cells with DNA damage. In other words, the good and the bad result from different processes within cells. Switched-On Mice Overcoming significant technical challenges, the UCSF researchers carried out strategic genetic manipulations on mice, creating strains in which p53 could be turned on or off at will. The genetically engineered p53 they developed was a chimera, in which functionally normal p53 was coupled with an altered form of an estrogen receptor that acts like a switch. P53 could function normally only when the mice were given the drug tamoxifen, which attaches to the estrogen receptor. Without the drug, there could be no p53 activity in the cells of the genetically altered mice. During the experiments the researchers gave the mice radiation doses comparable to exposures experienced by thousands of Japanese survivors of the Hiroshima and Nagasaki atomic bomb blasts. As expected from these experiments, the radiation caused widespread cell death in sensitive tissue in an unaltered strain of mice with normal p53. These mice survived and were normally protected by p53 against excessive cancers. Mice engineered in typical fashion to be "knockouts" completely lacking p53 were spared short-term tissue death. This result provides more evidence that p53 responses to DNA damage in normal mice -- or humans, for that matter -- are causing the cell death. But despite being protected from the immediate irradiation effects, the knockout mice all died from immune cell cancer within 24 weeks of irradiation. This again confirms a role for p53 in protecting against high cancer incidence. Experiments with the mice engineered so that p53 could be switched off at will replicated the findings obtained in experiments on normal and knockout mice, depending on whether tamoxifen was administered or withheld. But two additional experiments with the mice engineered so that p53 could be switched off at will were the most revealing. In one experiment the researchers first gave tamoxifen for a week prior to the radiation dose. This allowed for a normal p53 response to radiation. The researchers withdrew the drug at the time of radiation, turning p53 off completely. The mice had immediate tissue damage. Even so, they were not protected from subsequent cancer. In a second experiment the researchers did not give tamoxifen until a week following the radiation dose. By this time biochemical markers for DNA damage had already subsided, and the mice showed no ill effects from the radiation. When p53 activity was subsequently restored, the mice were significantly protected against later cancers. DNA Damage Response and Protection from Tumors Are Separate These experiments demonstrate that the protection against cancer provided by p53 is not due to its immediate response to DNA damage, according to Evan. So if the widespread cell death that accompanies DNA damage is not responsible for long-term protection against cancer, what is? Evan proposes that even after the bulk of the response to DNA-damage has largely disappeared - after most damaged cells have repaired themselves or undergone apoptosis -- p53-activating signals persist within a small group of cells that may be the progenitors for later cancers. "The same DNA damage that causes all the side effects triggers rare cancer-promoting mutations in a few surviving or misrepaired cells," he suggests. Abnormal cell proliferation that arises from these mutations frequently is accompanied by activation of a protein called p19ARF. Among the consequences of p19ARF activation is the triggering of p53 activation in turn. In most cases -- when signaled by p19ARF -- p53 activation can trigger the demise of abnormally proliferating cells before cancers arise, Evan explains. In an additional set of mouse irradiation experiments, the UCSF researchers showed that p19ARF activity was required for sustained p53 activation and measurable protection against cancer. "All that megadeath of cells in response to treatment, which is widely assumed to be an inevitable downside of p53 being a tumor suppressor, turns out to have nothing to do with tumor suppression," Evan concludes. "It's just bad." "Our data suggest that, by manipulating p53 function with drugs, we could retain the benefits of p53-mediated tumor suppression, but do away with radiation sickness and chemotherapy side effects."

"The pathological response to DNA damage does not contribute to p53-mediated tumour suppression" M.A. Christophorou, I. Ringshausen, A. J. Finch, L. Brown Swigart and G. I. Evan Nature advance online publication 6 September 2006 Abstract | Full Text | Full Text (PDF)

Photo/Kaz Tsuruta Related Links: "Tuning the body's defence to cancer: Turning off our natural killer could help to reduce chemotherapy side effects"
Nature, September 6, 2006 "Cell death unnecessary for tumor suppression: Study shows p53's pathological responses to DNA damage are irrelevant to tumor suppression"
The Scientist, September 6, 2006 "Protein p53 studied with surprising result"
United Press International, September 6, 2006 UCSF Comprehensive Cancer Center