American Academy of Ophthalmology Web Site: www.aao.org
Interview With Judah Folkman
The Man Who Made Blood Flow Backward
Dr. Folkman will present the Keynote Address during the Annual Meeting in New Orleans. In preparation for that event, EyeNet talked to him about his angio-genesis research and its potential clinical applications.
Sea-green light from a hundred aquariums glimmered around Judah Folkman, MD, as he led a visitor through a lab lined with Danio rerio breeding tanks. "First a drop of anesthetic is added to their water to make them sleepy," Dr Folkman said, offering his regard for the creatures whose tails would soon be volunteered to angiogenesis research. "Zebrafish can regenerate many body parts, so after the tips of the tails are removed we can administer a new agent and see if it permits or inhibits the regrowth of blood vessels and the tail itself."
These zebrafish are in caring hands, and famous hands as well: Dr. Folkman is widely credited as the founder of angiogenesis theory and research. More than 30 years ago, Dr Folkman proposed that solid tumors are dependent on neovascularization for their growth, a notion that would one day have implications for many other disease processes. He is now the director of the vascular biology program at Children's Hospital in Boston, as well as a professor of cell biology and pediatric surgery at Harvard Medical School.
Born in 1933 and having graduated magna cum laude from Harvard Medical School in 1957, Dr. Folkman has already filled several careers with his accomplishments. And yet he continues to direct his lab in some of the most closely watched research in the world today.
Several years ago your work was the subject of a well-regarded feature on Nova.1 In one wonderful moment in that program you describe how the tumor that pathologists and radiologists usually work with is not anything like the tumor seen directly at the time of surgery; as a surgeon, you could observe firsthand that tumors are highly vascularized.
Yes, whenever I was operating on a tumor, such as a sarcoma in the pelvis, it was hot. You could feel it just as a mother could feel if a child has a fever. Your hands are very sensitive to that. Second of all, it sometimes took hours to tie and cut the blood vessels from the tumor. They were coming in from 360 degrees, unlike the two or three vessels you’d see feeding a normal organ. The tumor was clearly attracting new vessels from long distances, and an entirely new blood supply had been recruited by the tumor.
And by the time it got to pathology it was just a pale, cold exsanguinated mass.
That’s right. And I also noticed that if, during the surgery, you lost blood faster than the anesthetist was putting it in, the surrounding organs, like the liver and kidneys, would turn white and cold, reflexively clamping down their own blood supply to save blood for the heart and brain. But the tumor stayed red and warm because it was so generously supplied with new vessels. Today these surgeries are often done endoscopically, so you can’t feel the warmth from them, but you can still see the tumor’s vessel supply.
Many oncologists were disbelieving of your angiogenesis theory, but one would think other surgeons were supportive of it.
Yes, most surgeons were very supportive. They didn’t necessarily know the science that would get you to the next step: How do you find the mechanisms, the right molecules, etc., for understanding angiogenesis? But surgeons, including ophthalmologists, have an advantage in that they can visualize pathology in their patients and directly see evidence of something like neovascularization.
Tell us a bit about your early work on rabbit corneas. Did you have any idea that someday angiogenesis would have a bearing on the eye in nononcologic terms—such as for retinal neovascularization?
In 1976, we were absolutely stymied in our attempts to find a mechanism of neovascular growth. We had implanted tiny rabbit tumors into the cornea of the rabbit eye. New blood vessels grew through empty cornea and reached the tumor. This showed that tumors released some unknown diffusible substance or substances that recruited new capillary blood vessels. However, when we tried to isolate the responsible molecule, tumor extracts implanted into the cornea just diffused away and did not stay put long enough to stimulate new blood vessels.
Then Robert Langer from MIT came to work in our lab and he figured out a polymer that was noninflammatory in the eye. He chose the powdered polymer used to make soft contact lenses, which he mixed with alcohol to form a rubbery pellet. Then he added the protein extract that we had isolated from highly vascularized tumors. We, in turn, embedded the pellet into a rabbit cornea while the animals were anesthetized. After approximately eight to 10 days, you could clearly see new blood vessels streaming from the periphery of the cornea toward the pellet and its protein, which later became the first known angiogenic factor.
So you had evidence that something in the tumor was able to override the normal avascularity of the cornea.
Yes, but when we removed the pellet, none of us knew what would happen. We made bets, and some thought the new vessels would remain in the cornea indefinitely. But in a few days, in the absence of the protein, they started to disappear. That’s when we wrote the paper called “The Sequence of Events and the Regression of Corneal Capillaries.”2 The vascularization clearly has to have constant angiogenic stimulation to work. If you take away the stimulus, the vessels go away completely. This experiment gave us confidence that it might be possible to discover an angiogenesis inhibitor that could make new blood vessels go away. We surmised that if you could make a drug that thwarted angiogenesis, you could reverse tumor growth.
Thus, the cornea became the basis of rapid progress in the field of angiogenesis research. We borrowed the ophthalmologists “workshop.” And after we established other angiogenic proteins and started looking for angiogenic inhibitors, we turned again to test them in the cornea. In 25 years we identified 12 inhibitors. Eight of these were in the body, including endostatin, which would turn out to be very important because it regulated other angiogenesis inhibitor machinery in the body.
How did angiogenesis inhibitors come to benefit eye medicine?
The capacity to reverse vascularization was an obvious boost to ophthalmology all along. But more than that, Evangelos Gragoudas, Joan Miller, Björn Olsen and colleagues have recently discovered that neovascular macular degeneration can actually be considered analogous to a deficiency disease of endostatin.3 Endostatin normally suppresses VEGF receptors. So they made a neovascular model in mice and successfully gave endostatin as replacement therapy. Endostatin replacement therapy may prevent neovascularization in the retina, and it has virtually no side effects.
With what might be called “messaging battles” between growth factors and antigrowth factors, it seems that a lot of your work parallels the study of cell-signaling in immune cascades.
That’s a good analogy. Let’s say that a tumor makes a protein that goes to an endothelial cell that is lining blood vessels. The protein eventually stimulates these endothelial cells to make new vascular sprouts and so forth. But before that, when the angiogenic protein binds to the endothelial cell’s receptors, there’s a complex pathway of steps on the way to the nucleus. You can block that cell’s receptor or block the pathway at a number of steps. But the cell may confound the situation by presenting a new and amenable receptor to a different angiogenic protein produced by the tumor. Those various cascades are similar to an immune reaction.
What do you imagine might be the very deep origins of angiogenesis and oncogenesis? In the model of malaria, for instance, in which certain alleles of hemoglobin genes confer protection against malaria but can also cause sickle cell disease. Is it likewise possible that cancer pathogenesis is actually a sad artifact of some ancient beneficial adaptation?
It’s an interesting thought. You could certainly wonder if the animals that could heal their wounds quickly could also be more prone to cancer. A lion takes a bite out a wildebeest, for example, and the injured wildebeest that heals quickly will avoid infection and gain a survival advantage. But then, over millions of years, the fast healers could also become those with rapidly dividing cells, which are then occasionally more susceptible to uncontrolled cell division. This of course, is pure speculation and nearly impossible to prove.
There is, however, a more tractable but contrasting example of genetics in cancer. Individuals with Down syndrome—about one in 700 births—are the most protected against cancer of all humans. This has been known for about 20 years or more, with 30 countries reporting the same thing. There are approximately 200 types of human cancer, yet individuals with trisomy 21 are only susceptible to two of them—leukemia and testicular cancer. Individuals with Down syndrome have a very low probability, less than 0.1 percent, of developing the other cancers. Years ago the simplest explanation was that they didn’t live long enough to develop cancers. But that idea doesn’t hold up now—they can live quite long—and still their cancer incidence is almost nil.
In 2001, it was reported in the literature that individuals with Down syndrome have almost twofold more endostatin because they have an extra copy of the gene for collagen 18 on chromosome 21.4 Endostatin is an internal fragment that is released from collagen 18. Raghu Kalluri of the Beth Israel-Deaconess Hospital in Boston has engineered mice to produce more endostatin, and tumors grow 300 percent more slowly in them. And there may be a second antiangiogenic factor in humans that has since been found to originate on the same chromosome. It has been found that in people with Down syndrome there is almost no macular degeneration or diabetic retinopathy because they appear to be protected against certain types of abnormal neovascularization.
Mind you, these are correlations, and correlations cannot be assumed to be causal. We always remind our postdocs that all doors have doorknobs but that doesn’t make them causal! Nevertheless, anyone who doesn’t pay attention to something like this correlation of low cancer rates in Down syndrome individuals could really miss an important discovery.
Do you imagine a future where people are given endostatin or a similar agent prophylactically?
You could imagine that in high-risk patients, for example women with the breast cancer gene or individuals with a high family risk for colon cancer, it may someday be possible to measure the blood levels of endostatin or of other angiogenesis inhibitors in the body. Twenty-nine such endogenous angiogenesis inhibitors are known to date. If endostatin or other angiogenesis inhibitors were deficient, it may eventually become an option to provide an individual with “replacement” or “prophylactic” therapy.
In patients with colon cancer, 50 to 60 percent will be cured by surgery, and the others may have recurrent cancer in five or six years. In the latter group, you could possibly watch the biomarkers and give antiangiogenic agents as needed. Until they cause symptoms or until they can be detected by various imaging methods, these tumors may not be amenable to conventional therapies such as surgery, ionizing radiation or cytotoxic chemotherapy. But, someday, it may be possible to treat recurrent cancer by antiangiogenic therapy and by other modalities such as immunotherapy and telomerase inhibitors at an ultra-early stage, guided by angiogenesis-based biomarkers in the blood or urine.
It is possible to imagine a future for cancer therapy where it may not be necessary to “see” microscopic recurrent cancer, or to anatomically locate it, before treating it. However, such a scenario would require very sensitive and specific biomarkers that could detect microscopic-sized cancer.
How would we identify and standardize such a biomarker?
Years ago the treatment for serious infections was surgical drainage of infected abscesses. Even into the 1930s, many chapters in a surgical text would have a road map for operating on a particular abscess, the goal being anatomic localization. By the early 1940s, with the introduction of penicillin and then other antibiotics, it became possible to treat infections guided by biomarkers such as the white blood cell count. Anatomical location of abscesses and their surgical drainage became less common.
An analogous future goal for cancer therapy, especially for antiangiogenic therapy, could be to convert it into a disease where we “treat the biomarkers.” One of those biomarkers might be platelets, which pick up angiogenesis regulatory proteins as they cruise by very tiny tumors (as Giannoula Klement, George Naumov and Joseph Italiano have shown in our laboratory). So changes in the platelet content of these angiogenesis-related proteins, called the “platelet angiogenesis proteome,” may eventually be used as a biomarker for ultra-early detection of recurrent cancer. These results with human tumors in mice remain to be validated in human clinical trials.
Could it also be possible to treat AMD or diabetic retinopathy early, cued by biomarkers, instead of waiting for frank disease?
As soon as we have a nontoxic drug shown to be effective, you can speculate that it would be possible to prevent retinal neovascularization. But it may be necessary to calibrate the antiangiogenic agents because if you suppress one vascular endothelial growth factor another one may replace it. We’re repeating exactly the lessons learned by physicians who tackled HIV infection: Viral resistance to the initial drug eventually necessitated the combination of two or three antiretrovirals. So, too, we may need combination, or broad-spectrum, anti-VEGF therapies.
Do you think there are other, truly fundamental biologic processes like angiogenesis that have not yet been elucidated in medicine?
Sure. Stem cell research, for example, is just now gaining momentum.
And yet that momentum may be stymied. We seem to be in a Golden Age of Medicine—so many wonderful things happening in science generally, and at the same time we live with very powerful forces that oppose stem cell research, that even oppose the teaching of evolution in schools. How do you reconcile this?
Well, it’s always been the case that as scientists approach one of history’s tipping points, people’s belief systems are threatened. When discoveries come fast, especially, people are pushed to change their minds. And the human species does not like to change its mind.
When Darwin introduced the theory of evolution, people couldn’t believe we were descended from animals. Endostatin, actually, is an interesting example of evolution. It is very old, at least 600 million years old. It is found in animals beginning from early worms onward, and its amino acids have been faithfully copied all these years. In primitive animals it was only used to guide nerve growth, but in more complex animals it is also used to guide vessel growth.
So over a half billion years, the conservation of a useful protein like endostatin is just as characteristic of natural selection as the diversification that led from worms to people.
Correct—yet even with such supportive evidence, still there are people who can’t accept evolution. Opposition to things like evolution and stem cell research can become quite powerful because peoples’ beliefs are shaken.
This seems like an opportune time for physicians and scientists to support each other.
Scientists make connections that physicians can use. Nobody ever thought the department of ophthalmology and the department of oncology would use each other’s therapies. They were different specialists who went to different meetings and they had patients in different parts of the hospital. Now they’re connected by a single molecule. That’s happening a lot in medicine, where things previously considered separate are now connected. There is a saying that if you drain the Pacific Ocean you shouldn’t be surprised to see that the islands are connected.
Your father was a rabbi. Do you think that heritage fostered in you an inquiring eye on life, or an ethical motivation to go into medicine?
Well, there are a few answers to that. First of all, there were three children in the family, and as a reward for good behavior each week we were allowed to go with our dad to a hospital as he visited the sick members of the congregation. We would sit quietly while he prayed. I once asked him why he let us come along, and he said that we were therapeutic for the patients, who thought that they couldn’t be dying if the rabbi saw fit to bring his own children to the hospital room. I also noticed that doctors could open oxygen tents to tend to the patients and my father was not allowed to do that. So I thought, I want to be a doctor.
Another memory is that every Friday night, before Shabbat dinner, Dad would say to each of us: “Have you asked a good question this week?” We just assumed every father expected that, so the notion of always asking questions became a habit: “Why is it that way?” “How did that happen?” and so on. My dad really nurtured questions. Once I had to tell him I really didn’t have a good question that week, and then I asked, “But could I still have dinner?”
2 Ausprunk, D. H., Falterman, K. and J. Folkman. J Lab Invest 1978;38:284–294.
3 Marneros, A. G. et al. FASEB J Published online May 25, 2007.
4 Zorick, T. S., Olsen, B., Passos-Bueno, M. R. et al. Eur J Hum Genet 2001;9(11):811–814.
Thanks to Joanne Chan, PhD, for the use of her zebrafish lab at the Folkman Laboratory.