The protein p53 is a tumor suppressor that is mutated or turned off in a variety of cancers. David Lane and co-workers at the A*STAR p53 Laboratory, the Institute of Medical Biology and the Institute of Molecular and Cell Biology have now created two lines of mice in which they can track where and when p53 is activated.
It is well known that p53 battles tumor induction by binding to the DNA within cells and driving the expression of genes that prevent cells from becoming cancerous. One subset of p53-induced genes causes cell death, including p53 up-regulated modulator of apoptosis (Puma). Another subset of genes upregulated by p53 blocks cell division, including p21. However, it is as yet unclear which stimuli steer p53 to genes that affect one process or the other.
It is also widely recognized that a possible strategy for cancer therapy is to restore p53 activity within the tumor, so that the cancer cells either stop growing or die. However, inducing p53 in normal tissue could result in toxic side effects. Therefore, it is important to study the effects of potential cancer therapies in the context of a whole animal.
To investigate p53 activity further, the researchers isolated the sections of the DNA from the Puma and p21 genes to which p53 binds to induce their expression. They then attached green fluorescent protein to these DNA sections and inserted the DNA into mice. In this way, cells in one line of mice would glow green when p53 induced Puma expression and cell death, and cells in the other line of mice would glow green when p53 induced p21 expression and prevented cell division.
The researchers showed that a factor that inhibits p53 degradation and increases p53 levels could induce green fluorescence within cells from both lines of mice. They then treated mice with radiation (see image), which is often used to fight cancer, or with a chemotherapeutic drug called doxorubicin, which injures cancer cells by damaging their DNA. They observed increases in green fluorescence in the mice, but in different tissues, at different times, and in a manner dependent on the mouse strain and on how the mouse was treated.
“Our results illustrate the potential use of these mice for preclinical drug studies,” says Lane. “By studying p53 activity and its effects in an animal model, one can optimize cancer treatments to ensure that the tumor is targeted as effectively as possible while minimizing harmful side effects.”