By David N. Leff
Just as ocean-going ships and boats have home ports, cancerous tumor cells target remote organs and tissues, on which they home — that is, metastasize.
Prostate cancers, for instance, metastasize to bone; colon cancers to the liver; kidney cancers to the lung. By that token, when a tumor invades a twinned organ, such as lung, kidney or breast, it's likely to jump from its initial target to the opposite number.
In some cases, "jump" may be too fast-paced a word. A tumor removed from one breast may leave behind the invisible seeds of malignancy which many years, even decades, later will metastasize to the other breast.
This concept of cancer cells as seeds seeking a hospitable soil in which to germinate goes back to 1889. In that year, The Lancet published a paper by the celebrated British surgeon and pathologist Sir James Paget (1814-1899) which made precisely that point: Malignancies metastasize to the specific tissue in which they can best grow.
Just how a single malignant cell can set out to colonize a distant soil bed remains to this day a mysterious mechanism. But this mystery is a matter of life and death, inasmuch as the majority of cancer deaths result from metastatic disease.
A paper in today's Proceedings of the National Academy of Sciences (PNAS), reports two novel tools in elucidating — literally — just how cancer spreads. Its title: "Governing step of metastasis visualized in vitro."
The article's senior author is cell biologist Robert Hoffmann, chairman, president, CEO and R&D director of AntiCancer Inc. (ACI), in San Diego. (See BioWorld Today, Jan. 8, 1996, p. 1)
Hoffmann cites Paget's discovery over a century ago as sparking his company's dual forays into the mechanism of metastasis. "In autopsies of breast cancer deaths," Hoffmann told BioWorld Today, "Paget noticed that the tumor dissemination was very non-random — kind of a seed looking for fertile and compatible soil, the specific metastatic sites to which it should go."
Hoffmann continued: "Lung is a very good soil. It has a great propensity for lung-to-lung spread. Somebody has a lung tumor; you think you got the thing out, and it appears in the opposite lung — a death sentence."
He and his co-authors transfected pulmonary non-small cell carcinoma cells with the gene for green fluorescent protein (GFP), which lights up under the microscope. They injected this deadly package into the tail veins of nude mice, which can't reject foreign antigens. A week later, the cells had formed brilliantly shining tumor metastases on the lungs of those rodents.
To follow the progression of these colonies, the team transferred them onto tiny cubes — one to two millimeters on a side — of gel foam, a proprietary version of surgical-grade collagen sponge derived from gelatinized pig skin.
Cubic Cell Cultures Mimic Mouse Lung Tissue
"We cultured them for up to 52 days," Hoffmann went on, "which allowed time for the colonies of human lung cancer to form. Now, we could visualize all of the colonization that occurs over 52 days at least, because of these two technologies. The tumor cells were expressing high levels of GFP, while the histoculture system allowed for long-term culture of the tumor-seeded mouse lungs, in three dimensions in vitro."
Hoffman pointed out that "at this stage, we are perhaps the only ones that have such stably, high-GFP-expressing human tumor cell types. I've gotten requests to send them around the world. What makes them extremely valuable," he explained, "not only in vitro but in vivo, is that people will be able to trace metastases in real time and in live tissue. Right now, in a live, tumor-bearing mouse, they have to open it up, take a look, take the fluorescence photomicroscopy, then close."
By comparison, he went on, "here, all you do is take the plate out of the incubator, put it under the microscope, get your photomicroscopy, put it back in the incubator. And do this repeatedly as long as you want."
ACI is now building a library of various human tumor cells that have the GFP bright light, beginning with metastatic colon, kidney and prostate cancers. "So for example," Hoffmann said, "we would like to see prostate cancer cells all lit up and invading bone."
In actual therapeutic application, the ACI CEO sees his system "as a discovery assay for agents that would interrupt, prevent or reverse metastasis. That would be the route to human trials. You could imagine perhaps in a somewhat futuristic study that we could introduce the GFP gene into the patient's tumor, in order to follow its metastasis at the micro level.
"Or," he continued, "you could imagine the GFP given as a therapeutic fusion protein in a gene therapy context." By way of example, he cited "promotors or inhibitors of angiogenesis."
Seeing Your Enemy Is Half The Battle
This week, Hoffmann is in Mainz, Germany, reporting on his metastasis-monitoring techniques to the European Tissue Culture Society's biennial international meeting. Another attendee at that event is National Institutes of Health cell biologist Leonid Margolis, who also researches three-dimensional and cell-to-cell interactions.
Commenting on the therapeutic potential of ACI's new approach, Margolis told BioWorld Today: "Their cells are fluorescent, not by vital dyes, as people have done before, but by genetic manipulation. That means that the cells can be visualized a long time. So the essence of this paper, in my mind, is that you can follow the course of metastasis and colonization in a very in vivo situation, and then continue to follow it ex vivo for weeks and weeks."
Margolis foresees that "you can use the system for testing new antimetastasis drugs as they become available."
On a more popular note, he went on: "What Dr. Hoffmann has invented is really a very important breakthrough. If you compare it with a battle, he didn't invent any weapon; you can still be defeated. But you see better the disposition of your enemy, which is an important part of any battle."
Margolis concluded: "It doesn't guarantee, of course, that tomorrow we'll treat metastases, and get rid of them. But now we can see them with open eyes — literally." *