Protein that stimulates blood vessel growth also helps repair broken bones
Bones that refuse to heal may one day be set straight by a drug that stimulates
the growth of new blood vessels, according to new research from the University
of California, San Francisco. So far, however, the growth factor drug has been
tested only in mice, and it could be years before it is used in hospitals.
These results were presented at this week’s annual meeting of the Orthopaedic
Research Society, in Orlando, Florida.
As cancer and cardiology researchers already know, VEGF, or vascular
endothelial growth factor, promotes blood vessel growth. Oncologists at a few
biotech companies are running clinical trials of anti-VEGF drugs to reduce the
flow of blood to tumors. Cardiologists are studying whether VEGF can sprout
new blood vessels to bypass blocked arteries in patients with inoperable heart
disease.
Blood vessel growth, known as angiogenesis, is also thought to help deliver the
chemicals needed for bone cells to rebuild after a fracture, says Jill Helms,
PhD, DDS, assistant professor of orthopedic surgery at UCSF. “Vascular
invasion is one of the critical steps of bone repair,” said Helms, who
collaborated with Zena Werb, PhD, a professor of anatomy at UCSF.
While most broken bones will repair to their original strength within a matter
of weeks, some breaks stubbornly refuse to heal for months, years, or longer,
she said. Scientists have suggested a few factors that may interfere with bone
healing, such as nutrition, illnesses such as diabetes, and damage to soft
tissue surrounding the bone. Helms and her colleagues suspected that soft
tissue damage interrupts proper blood flow to the fracture, and that proteins
that encourage angiogenesis, such as VEGF, might help to heal these stubborn
breaks.
To test VEGF as a possible treatment, Helms’ team worked with 20 mice with
broken limbs that were being treated with pain-relieving drugs. After
splinting the legs of these mice, the researchers shifted the splint each day
to a different position, a procedure that prevents healing growth of new bone,
presumably by destroying the newly forming networks of blood vessels.
Manipulating these tiny splints consistently and accurately was a challenging
technical feat, Helms said. “It requires people with great hands,” she said,
such as Diane Hu, MD, the staff specialist who perfected the technique.
After 10 days of this treatment, x-rays showed that only cartilage and other
fibrous cells grew in the gap, or interzone, between the pieces of broken
bone.
Helms and her colleagues then injected doses of VEGF into the interzones of 10
broken mouse legs, and made sham injections to 10 others. After ten days of
splint-shifting, the researchers could see osteoblasts, or bone growth cells,
developing in interzones of the mice injected with VEGF. The sham-injected
mice still had only fibrous tissue growth. The VEGF-injected mice also had
much higher expression of the gene Cbfa1, which is thought to help stimulate
formation of new osteoblasts.
Although more research is necessary, VEGF treatment could help to give a happy
ending to the tragic stories of patients who suffer from non-bony healing, said
Ted Miclau, MD, an orthopedic surgeon who works in Helms’ lab. “The fractures
that tend not to heal are open fractures and those from high energy accidents,
such as high speed auto, or pedestrian vs. auto accidents,’ he said.
In addition to studying VEGF’s role in bone healing, Helms’ lab is examining
whether it might also be important in embryonic bone growth. They have begun
injecting either VEGF or inhibitors of VEGF into the limbs of embryonic
chickens as they develop in the egg. They will then examine the effects of too
little or too much VEGF on skeletal development.
Other researchers collaborating on the project were UCSF orthopaedic surgery
residents Mark Lee, MD, and Christian Oglivie, MD, post-doctoral fellow Celine
Colnot, PhD, and Thiennu Vu, MD, PhD, a UCSF assistant professor of medicine.
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