By activating a cancer suppressor gene, a small molecule called nutlin-3a can block cancer cell division, according to researchers at the National Cancer Institute (NCI), part of the National Institutes of Health. This activation of the p53 gene leads to cellular senescence, a process by which cells lose their ability to grow and divide.
An opportunity for new genetic mutations occurs each time a cell divides, so limiting the number of cell divisions in a cancer cell inhibits tumor progression. This study is published in the May 1, 2008, issue of "Cancer Research".
Activation of p53 can suppress tumor growth through more than one mechanism. It can interfere with the cell cycle, prompting a cell with unrepaired DNA damage to commit suicide through a complex signaling pathway called apoptosis. Alternatively, p53 may trigger cellular senescence in response to DNA damage or cellular stress.
The expression of p53 is regulated by Mdm2, a protein that is overexpressed in several human cancers. Nutlins are small-molecule inhibitors that prevent the p53 protein from forming a complex with Mdm2, resulting in activation of p53. Previous studies have shown that nutlin can induce apoptosis in human cancer cells.
"Although p53 is mutated or deleted in about half of all cancers, it is still potentially functional in the other 50 percent," said Curtis C. Harris, M.D., chief of the Laboratory of Human Carcinogenesis at NCI's Center for Cancer Research and an author of the study. "A better understanding of molecules, such as nutlin-3a, that can activate p53 may lead to the development of new treatment options for certain cancers."
To examine the effects of nutlin-3a on cellular senescence, the Harris team exposed human skin cells and cancer cells to two different forms of nutlin-3: forms a and b. (Nutlin-3a has a 150-fold greater affinity for Mdm2 than nutlin-3b.)
After a seven-day exposure period, the scientists found that almost 100 percent of the cells treated with nutlin-3a had stopped proliferating. These cells did not regain the ability to proliferate even after being removed from nutlin-3a, indicating that they had undergone permanent senescence. By contrast, nutlin-3b had little effect on the cells.
Next, the researchers investigated whether the senescence induced by nutlin-3a is dependent on the presence of p53 protein. After exposure to nutlin-3a for seven or 14 days, more than 80 percent of the human cells containing a functional p53 gene exhibited signs of senescence.
The researchers also found that nutlin-3 treatment increased the expression of p53. However, the researchers did not observe any changes in p53-deficient cells.
Previous research by this team showed that the genes affected by p53 activation differed depending on the type of activator. To gain a better understanding of nutlin-3a-induced senescence, the researchers used microarray analysis to determine the effect of p53 activation on gene expression after cancer cells were treated with nutlin-3a.
Almost 3,000 genes were differentially expressed when cells with normal p53, cells with mutant p53, and p53-deficient cells were compared. Among the genes with increased expression after nutlin-3a-activation of p53 were several genes that play a role in cellular senescence and cell death.
The researchers also found that the inhibitor of growth 2 gene (ING2) was among those with decreased expression in response to nutlin-3a treatment. ING2 regulates gene activation or expression, and it may play a role in tumor development, cell proliferation, and senescence. The researchers found that p53 seemed to suppress ING2 expression by binding directly to two sites on the ING2 promoter.
"This study further characterizes the actions of nutlin-3a on genes that can play a role in the development of cancer," said Harris. "Our study reinforces the idea that using Mdm2 inhibitors, such as nutlin-3a, to promote the growth suppressive and cell-killing activity of p53 is a potentially valuable strategy to pursue in cancer treatment."