Researchers Find that a Small Molecule Can
Activate an Important Cancer Suppressor Gene
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
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. Microarray analysis is a technique
that allows researchers to examine the expression of thousand of
genes simultaneously. 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
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."
For more information on Harris's research, please go to http://ccr.cancer.gov/staff/staff.asp?profileid=5761.
For more information about cancer, please visit the NCI Web site
or call NCI's Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
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and supporting basic, clinical and translational medical research,
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its programs, visit www.nih.gov.
Kumamoto K, Spillare EA, Fujita K, Horikawa I, Yamashita T, Appella
E, Nagashima M, Takenoshita, S, Yokota J, and Harris CC. Nutlin-3a
Activates p53 to Both Down-regulate Inhibitor of Growth 2 and Up-Regulate
mir-34a, mir-34b, and mir-34c Expression and Induce Senescence. Cancer
Research, Vol. 68, No. 9. May 1, 2008.