Back in the fifth century b.c. Hippocrates once received a woman complaining of a sore breast. Upon examination he noticed large lumps from which radiated long, swollen veins. He called the affliction karkinos, Greek for crab. In Latin the word is cancer.
The invading, spreading tendrils of cancer appeared to Hippocrates like the arms of a crab, explains pathologist Lance Liotta. And he was right on the mark. The most life-threatening aspect of cancer involves invasion and spread.
Liotta, chief of the pathology laboratory at the National Cancer Institute in Bethesda, Maryland, has devoted his career to investigating the mechanism of cancer spread, or metastasis. Now several recent discoveries in his lab are moving us closer to preventing this deadly occurrence. We’re finally looking at the molecular mechanisms of metastasis, Liotta says. It’s very exciting because it is metastasis that kills.
Whereas some tumors tend to stay in one place, where they may be easily detected and removed, metastasizing tumors are restless, sending out cells to other parts of the body, where they may be impossible to root out. If there were some way to slow or stop metastasis, doctors could go a long way toward stopping cancer.
We know that when cancer patients are first diagnosed, almost two-thirds of them have metastasis somewhere in their body, says Liotta. But only half those metastases are visible. The rest haven’t yet grown large enough.
Liotta’s priorities, then, are clear. First, we’d like to be able to know for sure if a patient has a ‘silent’ metastasis somewhere. Next, if the metastasis does exist, we’d like to be able to eradicate it or at least inhibit its growth.
One technique now under study in Liotta’s lab might help that first goal be realized. It involves a gene dubbed nonmetastatic 23, or nm23, whose function in the body appears to be to halt the spread of malignant cells. The gene was discovered in 1988 by molecular biologist Patricia Steeg, who had been studying malignant tumors in mice, hunting for a single gene that might be responsible for giving cancer its ability to spread. She analyzing different samples of cancerous tissue, looking for a messenger RNA common to all of them. This would indicate that a gene common to all was at work. As it turned out, she found just the opposite.
At first, she says, the question I asked was, ‘What genes are turned on during metastasis?’ I looked at forty thousand genes from mouse melanoma tumors, some of which metastasized very little, some of which formed cancers all over the body. And I didn’t find any that turned on during metastasis.
To her surprise, what she did find was a gene that turned off. The only common messenger RNA Steeg did find was in tumors that were not yet metastatic--that is, she found there was a gene active in these nonmetastatic tumors that for some reason was not active in tumors that had begun to spread. Steeg traced this marker back to the gene that appeared to be producing it, cloned the gene, and inserted fresh, active copies of it into the mouse tumor cells. The result was exactly what she had hoped for: almost immediately, the gene-treated cancer stopped spreading. Although Steeg has no idea how the gene works, the halt of malignant spread was proof positive that it is the absence of nm23 that fuels metastasis, at least in the mouse melanoma cells.
Steeg and others have now analyzed metastatic breast cancer tissue in humans and found that it too is accompanied by a lack of nm23 activity. While it could be decades before therapeutic treatment with nm23 could be used to stop metastatic tumors, the gene may at least help diagnose them. By measuring levels of nm23 in breast cancer tissue, researchers might be able to determine if the malignancy has begun to migrate to other parts of the body, forming tumors still too small to see.
If left untreated, these metastases will ultimately kill the patient, says Liotta. Measurement of nm23 loss is a potential way to identify these patients so they can be treated immediately.
For most cancer patients, of course, even such early treatment can be excruciating. Conventional chemotherapy and radiation work by killing fast-growing cells, but these can include stomach cells, hair cells, and the cells that make up other healthy tissue. What would be ideal is a treatment that attacks the cancer alone.
Pathologist William Stetler-Stevenson of Liotta’s team has discovered a protein, called TIMP-2 (for tissue inhibitor metallo proteinase, type 2), that may lead to such a treatment. The protein stops tumor spread by preventing cancer cells from boring into blood and lymphatic vessels. Without access to these bodily thoroughfares, the cells can’t take off for remote destinations.
The find grew out of Liotta’s discovery some 15 years ago that metastatic cancer cells contain an enzyme that can destroy the tissue surrounding them. This allows them to blast a hole in the walls of blood vessels and lymph vessels and squeeze inside.
While Liotta was studying this destructive enzyme, he encountered something odd. When we worked with the enzyme, we found that something kept sticking to it, he says. It was a protein that just wouldn’t let go.
When Stetler-Stevenson came to the lab in 1987, he decided to find out what the odd protein was. He soon discovered that far from being an inconsequential oddity, this sticky protein might be a potent antimetastasis therapy. We put tumor cells on top of the membranes that enclose blood vessels, he says. Normally the tumor cells would quickly force their way into the vessels. But when we added the protein to the tumors, Stetler-Stevenson says, the cells couldn’t invade the membranes. The protein stuck so tightly that it prevented the enzyme from encountering and destroying tissue.
Stetler-Stevenson concludes that the protein is a sort of cancer self-containment system: a substance produced by the malignancy that restricts invasion. It turns out that for every known enzyme that might be important in cancer, there’s an inhibitor, Liotta says. Nature doesn’t seem to allow any process that can’t be regulated by both sides. In the case of metastatic cancer, however, the balance is somehow tipped in favor of the malignancy.
Liotta’s team is now looking for ways to tip things the other way by introducing additional amounts of TIMP-2 to a metastasizing tumor. Stetler-Stevenson has just begun to test this application in animals and hopes to move on to humans within two years. Meanwhile medical oncologist Elise Kohn of Liotta’s team is beginning human trials of another encouraging drug, called CAI (for carboxi amino imidazole), which in effect paralyzes malignant cells.
A metastasizing tumor’s mobility is a complex process. Once in a vessel, cancer cells literally crawl to their destination, extending their leading edge, or pseudopod, grabbing hold of the vessel wall and tugging the rest of themselves along. This stepwise progress continues until the cells once again cut a hole in the vessel wall and escape to a new site in the body. But what causes the cell to fly the nest and migrate in the first place?
Kohn and others have found that the process can be triggered by common events within the body. For example, when a growth factor--a substance that initiates cell division--binds to a receptor on a cell’s surface, the contact activates a stimulation pathway, in which the growth factor’s instructions are conveyed into the cell’s nucleus by a series of enzymes, hormones, and other molecular messengers. In normal circumstances, once the signal is delivered, the pathway shuts down. But in a tumor cell, this pathway becomes dangerously intensified. Now, as though amplified to fever pitch, the growth signal continuously blasts into the cell, causing it to multiply wildly and wander to other parts of the body. The signal essentially gives the cells running shoes and the impetus to migrate, says Kohn.
The question of course is: Can something be done to interfere with this pathway? In 1985 during a meeting at the Merck Institute for Therapeutic Research in New Jersey, Liotta was giving a talk on the significance of the pathway in metastasis when somebody in the audience mentioned a drug that might be able to shut it down. The drug, CAI, was originally investigated as a treatment for fungal infections in chickens, but it was also thought to inhibit the workings of an enzyme similar to one at work in the stimulation pathway. Liotta began tinkering with CAI in his lab. Two years later, when Kohn arrived, she took over the project full- time.
Kohn has since found that CAI is indeed able to block the pathway in both healthy and malignant cells, although she continues to investigate just how. No matter what CAI’s anticancer mechanism is, however, it is apparently extremely effective. When the drug was later tested against a wide variety of human cancer cells, including breast cancer, ovarian cancer, colon cancer, prostate cancer, bladder cancer, and melanoma, it stopped every one from dividing. It was terribly exciting, Kohn says. You can see why we’re so thrilled.
The next step was to try CAI in actual growing tumors. By transplanting human tumors into laboratory mice, then giving the rodents oral doses of the compound, Kohn found that CAI was as effective as it was in the test tube. What’s more, there were virtually no side effects.
The Liotta team has scheduled human trials of CAI for this year and may continue them to 1993. People whose cancers have not responded to conventional treatment will be eligible for this trial, says Kohn. For them the drug just might make a big difference. Our overall feeling is one of tremendous hope, says Liotta. This is a brand new approach to cancer therapy.