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Do Sharks Hold Secret to Human Cancer Fight?

Brian Handwerk
for National Geographic News
August 20, 2003
 
Sharks have survived some 400 million years on Earth. Could their longevity be due in part to an extraordinary resistance to cancer and other diseases? If so, humans might someday benefit from the shark's secrets—but leading researchers caution that today's popular shark cartilage "cancer cures" aren't part of the solution.


Carl Luer of the MOTE Marine Laboratory's Center for Shark Research in Sarasota, Florida, has been studying sharks' cancer resistance for some 25 years. He believes that MOTE research (in cooperation with institutions like Clemson University, The University of South Florida, and Tampa's H. Lee Moffitt Cancer Center) might someday lead to help for humans who suffer from cancer.

"Sharks have a low incidence of disease," he said, "but unfortunately many of the observations leading to this conclusion are anecdotal. It's been based on several things. One is that historically, going back to the late 1800s, sharks have been fished commercially and there have been few reports of anything out of the ordinary when removing internal organs or preparing meat for the marketplace.

"Scientists have also been dissecting sharks for years," he continued, "because many of the basic systems that humans have are found in sharks in simpler form. Most pre-medical students who took comparative anatomy probably dissected dogfish sharks, but you don't see reports of cancerous tumors."

Luer says that sharks aren't entirely cancer-free, a mistaken perception that has gained some public acceptance. "Lots of the media will overstate this [resistance] and say, 'sharks don't get cancer.' That's not the case, but their incidence is impressively low, the number of documented cases is low. We don't say that they don't get it, but we think that they must have some way to combat it."

But not all scientists are sure that sharks are even resistant to cancer. Gary Ostrander, professor of biology and comparative medicine at Johns Hopkins University in Baltimore, Maryland, says that just because not a lot of evidence exists doesn't mean that the animals aren't getting cancer.

"We don't know that they don't get cancer," he said. "Any suggestion that they get it at a lower rate than humans or other fish, is premature—because there haven't been any carefully conducted systematic studies.

"I have not seen anything in the scientific literature that gives any confidence, with certainty, that sharks get cancer at a lower rate than fish or other species," he said.

The incidence of shark disease is tough to ascertain. Systematic surveys of sharks are difficult to conduct, as capturing the animals in large numbers is time-consuming, and cancer tests would likely require the deaths of large numbers of sharks.

Still, Ostrander says that research like Luer's at MOTE is a positive step into understanding the relationship between sharks and disease.

"Carl and others are asking very good questions," he said. "There may be fundamental differences in shark immune systems so that they aren't as prone to cancer, or so that certain types of compounds might not be a problem for them."

Immune System Could Hold the Key

The major thrust of the Motes research focuses on the immunity of sharks and their relatives the skates and rays. While skates aren't as interesting to the public as their shark relatives, their similar biochemical immunology and their ability to breed in captivity make them perhaps more vital to Luer's lab work.

"We want to understand how the immune system functions, the differences and similarities to the higher animals, and what might possibly be the role of the immune system in their low incidence of disease," Luer said. "I'm not naïve enough to think that only the immune system is involved, but what role does it play?"

One role may stem from the shark's cartilaginous skeleton. The lack of bones means a lack of bone marrow—an important location where many animals produce immune cells.

"In mammals, a brief lag time follows exposure to foreign substances before immune cells are produced in the bone marrow and mobilized into the bloodstream to fight off the invaders," Luer explained. "In sharks, the immune cells are produced in the spleen, thymus, and in unique tissues associated with the gonads and esophagus. Our studies at MOTE in collaboration with researchers at Clemson University have determined that a significant number of immune cells in these animals actually replicate (divide and mature) as they circulate in the bloodstream. Immune cells already in the shark's blood may be available to respond without a lag period, resulting in a more efficient immune response."

Cartilage Cancer Cures?

Such research may someday lead to important medical breakthroughs. But what of the myriad of products already on store shelves, touting the cancer-fighting properties of shark cartilage?

There are factors within animal cartilage (not necessarily shark cartilage) which may stop new blood vessels from forming—a long-standing strategy for retarding the growth of cancerous tumors. Unfortunately, scientists say this doesn't mean that cartilage pills would have a similar effect. Luer describes the proliferation of pills as a "real problem" that taints the efforts of MOTE and others to do respectable research.

"You have people catching and killing millions of sharks, grinding up their cartilage, and making misstatements to try to convince people that eating it will make them cancer-free."

"Unfortunately, there is no logical reason to conclude that freeze-dried shark cartilage pills taken orally could "seek out" a malignant tumor in a cancer patient and inhibit the blood vessels feeding it," he continued. "Also, there is no reason to think that shark cartilage contains anything which is not found in other animal cartilage."

Though the FDA has limited the claims of shark cartilage products which had formerly marketed themselves as disease treatment, Ostrander believes that their use is not on the decline. "It hasn't crested at all," he said. "I have a colleague here at Johns Hopkins who thinks it's possible that maybe 50 percent of cancer patients are using them without even telling their doctors. The reason that there are four or five different size containers for sale, multiple brands, and lots of advertising, is because the stuff is selling—no question about it.

"People with cancer often have limited time left and limited resources," he continued, "and there is no good evidence at this point in time that they should be investing in shark-cartilage treatments. It just is not there, and from what we've seen it's not likely to ever be there. Studies of people taking the cartilage have shown that their remission rates are basically the same [as the overall cancer remission rate]."

While the cartilage pills haven't found scientific acceptance, there is still a chance that the shark will eventually provide an important clue in the fight against cancer. "We have to understand what's happening in the shark before we can make any applications," Luer cautioned. "But we're always open to the possibility that our findings might someday be developed into therapies. I do think there is potential down the road for that."
 

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