Mote Marine Laboratory

Killing Cancer?

By: Donna Self

A new NIH grant is helping shark researchers at Mote study a substance they've found that kill cancer cell lines in Petri dishes

Mote scientists Dr. Carl Luer and Dr. Cathy Walsh know something is killing the cancer cells they study in the lab.

But what? How does it kill the cancer cells in a Petri dish, and why? And what could it mean decades down the road to people with cancer?

Luer and Walsh have spent their professional careers in Mote Marine Laboratory’s Center for Shark Research trying to find out why sharks and rays rarely get cancer and what it might mean for humans who do. Now, thanks to new funding from the National Institutes of Health, their studies can continue.

The reward for every victory — large and small — has been a new set of questions, a new line of study and a new search for ways to pay for the scientific analysis needed to answer the questions.

“It’s not the kind of research where you all of a sudden find something. It’s more like a result of weeks and months and years of research that you realize you have something,” says Luer from behind a desk overrun with coffee mugs, piles of papers and dried skate purses.

The American Cancer Society estimates that in 2004, nearly 1.4 million people — roughly four times the population of Sarasota County, Fla. — will be diagnosed with cancer and that 563,700 people will die from the disease. Most people with cancer are treated with surgery, radiation and chemotherapy. But some oncologists are now looking at immunotherapy as a fourth mode of treatment.

That’s where Luer and Walsh have set their sights as well.

The long haul

Luer’s research into sharks started 25 years ago when the biochemist arrived at Mote in the summer of 1979 with the ink still damp on his University of Kansas Medical Center Ph.D. He joined a staff of about 20 and worked on a table in director emeritus Perry Gilbert’s office. His lab was in a trailer.

During graduate studies, he took and taught numerous comparative vertebrate biology courses, all of which included information on sharks. “I got to appreciate them on both a molecular level as well as a whole-animal level,” he says. “Too often, people who perform either molecular or behavioral studies don’t see the relationship there. I’m pleased that I can see both ends of the spectrum.”

Knowing about sharks led Luer to study the low incidence of disease, especially cancer, in elasmobranchs — the subclass of cartilaginous fishes that includes sharks, skates and rays. “We went to the literature and found that the impression that sharks didn’t get cancer was based primarily on anecdotal observations,” Luer says. “There had been little, if any, experimentation; it was just difficult to document any reasonable occurrence of sick sharks. The specimens used in prior studies were usually dead, and we wanted to be able to study live animals, so my first goal was to establish the use of elasmobranchs as lab animals.”

Who needs rats?

Luer picked nurse sharks and clearnose skates for several reasons.

Nurse sharks survive well in captivity and because of their sedentary nature, don’t require large holding spaces. They are also relatively docile and can be collected as newborn pups. They grow slowly, so it takes three or four years before they become too large to handle.

The downside is that it takes about 20 years for nurse sharks to become sexually mature, so animals old enough to reproduce are too large to be kept as experimental animals.

Clearnose skates, whose biochemistry and physiology closely resembles those of sharks, complement nurse shark studies. As sexually mature specimens, they’re relatively easy to maintain and breed in captivity. Since skates are egg-layers, mothers don’t have to be sacrificed for embryonic specimens. Luer could establish a controlled population, much like the lab rat population that can be bought with a complete genetic history — a sort of rat pedigree.

After picking his animals, Luer set out to explore whether sharks and skates really were resistant to cancer and other diseases.

He exposed sharks and skates to two potent chemical carcinogens to try to induce cancer. But the carcinogens — aflatoxin B1, a naturally occurring toxin produced by molds, and methylazoxymethanol-acetate (MAM) — didn’t succeed. No tumors developed.

While Luer did learn how those carcinogens are metabolized by the animals, nearly 10 years of experiments failed to produce evidence of a single tumor. “We felt we weren’t getting to the bottom line with this approach, so in 1989, with the field of immunology exploding due to the awareness of AIDS, we decided this might be a productive area of study. We began to investigate elasmobranch immunology and hoped that understanding how their immune system functioned might explain their apparent resistance to disease.”

Enter Cathy Walsh

Walsh came to Mote in 1991 for her first professional position after earning her Ph.D. from Clemson University. She hasn’t left.

With Walsh came new study goals: What kind of immune cells do elasmobranchs have and what is their function? Where are they produced? And, ultimately, how are these functions regulated?

The answer to the first question was that sharks and rays have essentially the same types of leukocytes, or immune cells, as humans: lymphocytes, granulocytes and macrophages.

Immune cells are produced and called to action in the face of injury or infection. In humans, they are made in the bone marrow, the spleen, the thymus and the lymph nodes. Sharks don’t have bones, so shark immune cells obviously aren’t produced in bone marrow. They also lack lymph nodes.

Elasmobranchs do have spleens that play an important role in their immune functions. They also have two other immune cell-producing organs unique to this group of fishes: the epigonal organ, located near the reproductive organs, and the Leydig organ, near the esophagus.

But what about the thymus?

The thymus is a sort of factory area where a specific type of immune cell is produced. T-cells, or thymus-derived lymphocytes, are made there. Evidence of the suspected location of a thymus in sharks had been published nearly 100 years before Luer and Walsh started their research. But a literature search turned up confusion as to whether sharks had a thymus beyond their embryonic stage.

“After five years of dissection and looking, finding the thymus was significant for us,” says Walsh. “I remember that the first time we were absolutely certain of its location was during the dissection of a near-term blacknose shark.”

Walsh and Luer found that one of the reasons the thymus had been especially difficult to locate was that it was easily destroyed during dissection, rendering it unrecognizable. It also diminishes gradually as the animal reaches sexual maturity. In humans, the thymus is prominent until puberty, then disintegrates and is replaced with connective tissue.

Another breakthrough

It remained extremely difficult, if not impossible, to explore the functions of shark immune cells in a live animal, and the cells didn’t live long enough in a test tube.

So Walsh came up with a method of putting shark immune cells in short-term culture, leading the way to the most recent area of study: the investigation of immune regulatory molecules that have the potential to inhibit the growth of tumor cell lines in the lab. When certain molecules secreted by the cultured shark cells are introduced to cancer cell lines, the consistent result is that about 80 percent of the cancer cells die.

With this finding comes still new questions. Luer and Walsh need to identify the secreted substance, determine if it contributes to the shark’s low incidence of cancer and explore what potential it holds for applications to human health.

The new NIH $335,000 two-year grant will provide partial support for the research that will hopefully allow them to characterize the compounds secreted by shark immune cells. So far, it looks like the material Drs. Luer and Walsh seek to identify appears to work using a mechanism that targets tumor cells rather than normal, healthy cells. “Receiving this new grant is exciting because it allows us to move to the next level of research – determining what this substance is and how it preferentially recognizes and inhibits tumor cells instead of normal cells,” said Dr. Luer.

The studies are complex, said Dr. Walsh. “We’re hoping to isolate and purify the active components. If they continue to show promise, that will be great but we still have a lot of work ahead of us.”

Learn more about: Dr. Luer's Program and Dr. Walsh's Program

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