A Genetic "Gang of
Four" drives spread of breast cancer
Studies of human tumor cells implanted in mice
have shown that the abnormal activation of four
genes drives the spread of breast cancer to the
lungs. The new studies by Howard Hughes Medical
Institute researchers reveal that the aberrant
genes work together to promote the growth of
primary breast tumors. Cooperation among the
four genes also enables cancerous cells to
escape into the bloodstream and penetrate
through blood vessels into lung tissues.
Although shutting off these genes individually
can slow cancer growth and metastasis, the
researchers found that turning off all four
together had a far more dramatic effect on
halting cancer growth and metastasis.
Metastasis occurs when cells from a primary
tumor break off and invade another organ. It is
the deadliest transformation that a cancer can
undergo, and therefore researchers have been
looking for specific genes that propel
metastasis.
“While
silencing these genes individually was
effective, silencing the quartet nearly
completely eliminated tumor growth and spread.”Joan
Massagué
In the newly published experiments, the
researchers also found that they could
reduce the growth and spread of human breast
tumors in mice by simultaneously targeting
two of the proteins produced by these genes,
using drugs already on the market. The
researchers are exploring clinical testing
of combination therapy with the drugs—cetuximab
(trade name Erbitux) and celecoxib (Celebrex)—to
treat breast cancer metastasis.
The research team, led by Howard Hughes Medical
Institute investigator Joan Massagué at the
Memorial Sloan-Kettering Cancer Center,
published its findings in articles in the April
12, 2007, issue of the journal Nature and in the
online early edition of the Proceedings of the
National Academy of Sciences on April 9, 2007.
In an earlier study, Massagué and his colleagues had
identified 18 genes whose abnormal activity is
associated with breast cancer's ability to
spread to the lungs. In the new study published
in Nature, Massagué and his colleagues at
Sloan-Kettering, along with researchers from
Hospital Clinic de Barcelona and the Institute
for Research in Biomedecine in Spain, focused on
four of these genes. These genes, which code for
proteins called epiregulin, COX2, and matrix
metalloproteinases 1 and 2, were already known
to help regulate growth and remodeling of blood
vessels, said Massagué.
“Our understanding of the genes for these four proteins and
their behavior in metastasis led us to
hypothesize that they might be cooperating with
each other in a way that would give an advantage
to cells in the primary tumor,” said Massagué.
“These same genes, we believed, might also be
used for some related purpose in the target
organ, the lung.”
To test this idea, the researchers silenced various
combinations of the four genes in human breast
cancer cells that had metastasized to the lung,
and then tested these cells in mice. To silence
the four genes, the scientists used a technique
called RNA interference, in which RNA molecules
are tailored to suppress expression of target
genes.
“We found that depriving aggressive metastatic tumor cells of
these genes decreased both their ability to grow
large aggressive tumors in the mouse mammary
gland and also the ability to release cells from
these tumors into the circulation,” said
Massagué. “The remarkable thing was that while
silencing these genes individually was
effective, silencing the quartet nearly
completely eliminated tumor growth and spread.”
Microscopic analysis of blood vessel structure in the tumors
revealed that knocking down all four genes
greatly reduced growth of the tangle of blood
vessels typically seen in tumors. Further
experiments revealed that the tumor blood
vessels that did form allowed fewer cancer cells
to escape into circulation.
The researchers next explored how loss of the four abnormal
genes affected the metastatic capability of the
cells in the lung. They injected cells deficient
in the four genes directly into the circulatory
system of mice. “When these cells reached the
lung capillaries, they just got stuck there,”
said Massagué. “We concluded that metastatic
cells use these same genes to loosen up cells in
capillaries, so that the cells can penetrate the
lung tissue to grow there.
“These findings provide a beautiful explanation for how the
genes that we identified in breast cancer
patients as being associated with lung
metastasis manipulate blood vessels to give them
an advantage both in the primary tumors and in
the lung,” he said.
Two drugs already on the market act directly on proteins
produced by the genes Massagué's group had been
studying. Cetuximab is an antibody that blocks
the action of epiregulin and is used to treat
advanced colorectal cancer. Celecoxib is an
inhibitor of COX2 that is used as an
anti-inflammatory, and is being tested in
clinical trials against many types of cancer.
The researchers also tested whether cetuximab
and celecoxib would work effectively in concert
to reduce metastasis in mice.
“We found that the combination of these two inhibitory drugs
was effective, even though the drugs
individually were not very effective,” said
Massagué. “This really nailed the case that if
we can inactivate these genes in concert, it
will affect metastasis,” he said.
Massagué said that while clinical trials of the drug
combination are being discussed, “there are
already treatments to diminish the chance of
metastasis in breast cancer, so such trials
would have to be designed very carefully to
understand how and whether the new drug
combination would be of additional benefit.”
In the article published in the Proceedings of the
National Academy of Sciences, Massagué and
his colleagues explored how the entire group of
18 genes, called the `lung metastasis
gene-expression signature' (LMS) influenced both
breast tumor growth and spread to the lungs.
Co-authors on the paper were from the University
of Chicago, The Netherlands Cancer Institute,
Veridex L.L.C., The Cleveland Clinic and the
Erasmus Medical Center in The Netherlands.
“There has been an undeniable link between tumor size and
growth and metastatic risk, but the molecules
and mechanisms underlying this link have
remained unresolved,” said Massagué. “The
hypothesis we wanted to test was that these
signature genes play a role in both primary
tumor growth and metastasis to the lung.”
After analyzing 738 human breast cancer tumors, the
researchers concluded that those in which the
LMS genes were abnormally active were, indeed,
more likely to develop lung metastases. They
also found that the activity of these LMS genes
gave cancer cells a growth advantage by allowing
tumors to develop a rich network of blood
vessels to deliver oxygen and nutrients, said
Massagué.
Although large tumors are more likely to metastasize,
Massagué said his group's findings indicated
that the activity of the LMS genes was also
critical to the metastasis process. “As the
tumors grow and become enriched with LMS-positive
cells, because the genes give them an advantage,
they reach a point where the tumor becomes
richly vascularized,” said Massagué. “Then, they
can massively execute the advantage the LMS
genes provide them to metastasize to the lung.”
Massagué said he and his colleagues will explore in more
detail the function of other LMS genes, in
addition to the four reported in the Nature
paper. They plan to investigate whether shutting
down other LMS genes will affect metastasis of
breast cancer to the lung, and whether the LMS
genes influence breast cancer metastasis to
other sites, such as the bone and brain.
Finally, they will explore whether the LMS genes
play a corresponding role in metastasis of other
cancers — such as sarcoma, melanoma and colon
cancer — to the lung, said Massagué.
HHMI
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