Molecule Links Nerve, Blood Vessel, and Tumor Cells

By William J. Cromie

Gazette Staff

A small protein that guides the assembly of a new brain has been found to be involved in the movement of blood vessels and, possibly, the spreading of cancer cells.

"The finding links together two of life's most basic processes by showing that they share one crucial protein," says Michael Klagsbrun, professor of surgery at Harvard Medical School, in whose laboratory the discovery was made. "It also has important implications for understanding how new blood vessels are guided around blocked arteries and veins, and how they penetrate and aid the growth of cancer tumors."

The critical protein, called neuropilin, prevents nerve cells from taking a wrong turn as they wire up a brain and nervous system growing in the womb. Finding neuropilin on the surface of blood vessels indicates it also may guide their growth.

"People never thought of blood vessels growing this way," says Klagsbrun. "We tended to think of them just sprouting in all directions."

Neuropilin was found on the blood vessels by Shay Soker, an instructor in surgery who works with Klagsbrun. The protein attracts and binds a potent blood-vessel growth promoter known as VEGF, or vascular endothelial growth factor. Other researchers are trying to use VEGF to grow new vessels that can carry blood around blocked arteries and veins in the heart and legs.

Conversely, VEGF can enhance the growth of blood vessels that nourish cancer tumors and it may accelerate their spread.

"It might be possible to augment both activities by manipulating neuropilin, aiding its activity on the one hand, blocking it on the other," Klagsbrun speculates.

Neuropilin, first discovered in 1989, was later found to attract another molecule called semaphorin, a member of a family of proteins that guides nerve cells to locations where they must connect with other cells for the brain to develop properly. Also known as collapsins, these proteins work by collapsing a nerve ending when it veers in the wrong direction.

A human brain, developing in the womb, eventually will contain a trillion brain cells, each of which may connect to as many as a thousand other cells. How these cells sort themselves out to assemble a thinking, feeling, instinctual brain and accompanying nervous system remained a deep mystery until about 20 years ago. Today, labs all over the world are working out the details of how chemical traffic lights, such as neuropilin, guide nerve cell fibers to an address that makes sense.

"It was an eye-opener to find that this molecule is the same one that gives blood vessels a sense of direction," Klagsbrun comments.

Isolating the Traffic Signals

Klagsbrun has studied growth factors for the past 20 years. In 1983, his lab was the first to purify a powerful protein known as fibroblast growth factor (FGF). As things turned out, FGF also does double duty as a nerve-cell traffic director and an agent to guide growth of blood vessels.

Both FGF and VEGF are now being tested in patients to see if they will speed the growth of new blood vessels around arteries that have been blocked in the heart.

Other Harvard researchers are looking at administering FGF after a stroke to limit the death of brain cells. A team led by Seth Finklestein at Massachusetts General Hospital in Boston observed 60-70 percent survival of cells in animals who received FGF within four hours after a stroke. The hope is that the same thing can be accomplished in humans.

FGF and VEGF are also being investigated as a way to bypass blocked blood vessels in legs. Jeffrey Isner of St. Elizabeth's Medical Center in Brighton, Mass., recently reported the successful treatment of about 20 patients.

In 1992, Shay Soker came from Israel to Klagsbrun's lab at Children's Hospital in Boston to study how VEGF binds with blood vessels. Two VEGF receptors, called KDR and FLT-1, were already known. Working with Seiji Takashima, a Japanese researcher, Soker purified a third receptor, which turned out to be neuropilin.

Yin and Yang

Klagsbrun's Harvard colleague, Judah Folkman, discovered that cancer tumors will not grow to a dangerous size without blood vessels to nourish them. As the researchers see it, tumor cells release VEGF which makes its way to neuropilin receptors on nearby capillaries, the smallest of blood vessels. That causes the vessels to grow toward, then into the tumor, providing nourishment the tumor needs to grow and spread.

That process seemed biologically logical enough. But Klagsbrun and crew then found that breast and prostate tumors make large quantities of neuropilin themselves. Why?

Klagsbrun doesn't know yet, but he thinks that by binding to tumors VEGF somehow aids the survival of tumor cells. It also may play a part in their spreading to other parts of the body.

"We're taking a yin and yang approach to VEGF," Klagsbrun explains. "For growing new blood vessels, we're studying how to enhance the union between neuropilin, VEGF, and other growth factors. To fight cancer, we want to learn how to block such unions."

In one experiment, for example, Soker engineered blood-vessel cells to contain both neuropilin and KDR. The combination more than doubled the response to KDR alone; that is, the growth and directed movement of blood vessels far exceeded what it would have been with KDR alone.

The result has started Klagsbrun looking into the question of whether malignant and spreading cancer cells contain more neuropilin than less aggressive cells.

Along with researchers at the University of Pennsylvania, Klagsbrun is also testing whether the VEGF-neuropilin combination works on brain cells and the semaphorin/neuropilin combination acts on blood vessels.

All of these experiments have to do with finding better ways to stimulate the growth of blood vessels on the one hand, or to inhibit damage from strokes and cancer on the other.

However, the most intriguing question, from the biological point of view, involves the fundamental link between the growth of nerves and blood vessels as an embryo develops. Do they both operate on the same principles and, if so, is there chemical communication between them?

"Now that we have evidence of synchronization between two basic systems in biology," says Klagsbrun, "we are in the position to learn how they may influence each other."