NOVEL BIOLOGICAL INTERACTION FOUND TO EXPLAIN BLOOD VESSEL GROWTH TO
TUMORS
DURHAM, N.C. -- Duke University Medical Center researchers believe
they have answered one of cancer's central enigmas: why some blood vessels are
able to grow to, and feed, tumors, while other vessels are not.
In the
March 16 issue of the
Proceedings of the National
Academy of Science, the scientists report the blood protein angiostatin,
which is known to stop the growth of new blood vessels to tumors, works by
depleting the chemical energy that blood vessel cells need to grow.
To do
this, angiostatin latches on to and inhibits ATPsynthase, an enzyme that
provides chemical energy for the cell. Without that energy, blood vessels cannot
grow to the site of a tumor, and without the nutrient supply in blood, tumors
cannot grow larger than a pinhead.
Conversely, when unchecked by
angiostatin, ATPsynthase provides a generator of sorts to blood vessels so that
they can survive in the atmosphere of cell death caused by cancer. Cancer
researchers have long wondered how these vessels stay vigorous enough to
continue to grow to and feed tumors.
Lead scientists Dr. Sal Pizzo and
Tammy Moser said the discovery is "startling" because the ATPsynthase
enzyme has never before been found on the surface of endothelial cells, which
are the cells that line blood vessels.
In fact, ATPsynthase has not been
known to exist outside of a cell body. It has only been found within a
mitochondria, a sac-like structure which acts as a cell's chemical
powerhouse.
"It was surprising to see ATPsynthase on blood vessel
cells because it was never expected there," said Pizzo in an interview.
"But it makes perfect biological sense, and it's terribly
exciting."
The finding offers both theoretical as well as practical
implications, Pizzo said. It offers a novel insight into the body's use of
ATPsynthase as "power packs" in situations where energy is depleted.
And it suggests a new route to developing drugs that block
angiostatin.
"Instead of using the whole angiostatin protein as a
drug to stop tumor angiogenesis, as efforts are now underway to do, it may be
possible to design a small molecule that will do what angiostatin does - turn
off ATPsynthase," Moser said.
Pizzo added that, in the future, it
may be possible to find a molecule that turns on ATPsynthase "in cases
where you want new blood vessel growth, such as in heart
disease."
Because she led the laboratory investigation, Moser
checked, rechecked and expanded her findings over a span of several years to be
sure of the conclusions. "A lot of labs might have pulled out and said it
was a mistake, but it seemed a very rational explanation worthy of
replication," she said.
The discovery is very important because
"the ATPsynthase-binding protein may be used as a target to find small
molecules, which could mimic angiostatin, but could be taken orally, and perhaps
would be easier to manufacture," according to Dr. Judah Folkman, a cancer
researcher at Children's Hospital in Boston who pioneered angiostatin research.
"If such small molecules are developed, they may also enhance the activity
of angiostatin."
Folkman, a member of the National Academy of
Sciences, was the scientist who submitted the paper to PNAS, the academy's
journal, although he didn't participate in the research.
Working with
Pizzo and Moser on the study were Duke researchers Iain Asplin, Jan Enghild and
Lorraine Everitt. Their collaborators include Danish researcher Peter Hojrup,
and, from Northwestern University Medical School, Sharon Stack, Susan Hubchak
and William Schnaper. The study was supported by a research grant from Glaxo
Wellcome Inc. Duke University holds the patent rights to the
discovery.
The field of angiostatin research grew from the observation
that some, but not all, tumors seem to be able to control the spread, or
metastasis, of cancer elsewhere in the body.
Cancer physicians have long
known that sometimes, when a single large tumor is removed from a patient,
subsequent tumors will come back quickly and spread, seeding the body with
deadly fast-growing tumors.
A decade ago, Folkman proposed that tumors
themselves regulate the growth of blood vessels. One notion was that tumors
could produce proteins that would travel through the bloodstream, preventing
vessels from growing to any new cancer metastasis. This idea explains what has
been observed, although no one understands why a tumor would exert such control,
or why only some tumors work in this way.
Folkman's lab later found the
substance that inhibits blood vessel growth. It was a small piece of a large and
common blood protein called plasminogen, which is involved in blood clotting. He
called this protein "angiostatin," and demonstrated it was involved in
"anti-angiogenesis" - stopping the process of new blood vessel
growth.
Folkman then showed in animal experiments that injections of the
angiostatin protein stopped tumors from growing, and efforts have been underway
in the last year to produce a drug for human cancer therapy based on the
angiostatin molecule.
Pizzo and Moser set out to study what happened when
angiostatin "bound" on the surface of endothelial cells. They looked
for where angiostatin's "key" fit into the cellular "lock"
that then shut down vessel growth.
Since plasminogen is known to bind to
a protein called annexin II on endothelial cells, they expected to find that
angiostatin bound to same site.
After several years of gathering material
and analyzing data, Moser found a different molecule, which she identified by
mass spectrometry as ATPsynthase.
Moser then tested her findings by
introducing an antibody to ATPsynthase that would block the action between
angiostatin and endothelial cells. Indeed, she found the antibody blocked
angiostatin's ability to inhibit proliferation by 90 percent.
The
researchers stressed that while there is still much to learn about how
angiostatin regulates blood vessel growth, there are now many new interesting
theories to explore.
For example, Pizzo and Moser speculate that blood
vessels grow into tumors by capitalizing on the death of cancer cells in the
core of tumors due to lack of oxygen.
A tumor is an ever expanding knot
of cancer cells, and its central core is often composed of cells that are dying
from a lack of oxygen. When cells die, they release a depleted form of chemical
fuel called ADP. And it is ADP that the ATPsynthase molecule uses to produce
ATP, the body's high energy fuel. So it may be that "vessels are feeding on
cell death to grow," Moser said. "This is a very novel
strategy."
Folkman added: "Because ATP is necessary for cells
to resist conditions of low oxygen, and because the enzyme to produce ATP is
situated on the outer surface of endothelial cells, angiostatin, by binding to
this enzyme could preferentially inhibit endothelial cells that are in the
vascular bed of a tumor, where oxygen would be lower than elsewhere in the
body."
