The IUCN/SSC Shark Specialist Group
Shark News 2: October 1994
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Sharks and cancer the real story
Carl A. Luer, Manager, Biomedical Program, Mote Marine laboratory
Sharks and their relatives, the skates and rays,
have enjoyed tremendous success during
their nearly 400 million vears of
existence on earth. One reason
for this certainly is their
uncanny ability to resist disease.
Sharks do get sick, but their incidence of
disease is much lower than among the other fishes.
The low incidence of tumors found among the
sharks and their relatives has prompted biochemists and
immunologists at Mote Marine Laboratory (MML) to explore the
mechanisms that may explain the unusual disease resistance of these
animals. We designed experiments to see whether tumors could be
induced in the sharks and skates by exposing them to potent carcinogenic
(cancer-causing) chemicals. While there were simiiarities and differences
in the responses when compared with mammals, no changes in the
target tissues or their genetic material ever resulted in cancerous tumor
formation in the sharks or skates.
These chemical exposure studies, encompassing about ten years
of research effort, have led us to more recent investigations of the
shark immune system. Compared to the mammalian system
which is quite specialised, the shark immune system appears
primitive but remarkably effective. Sharks apparently
possess immune cells with the same functions as those of
mammals, but the shark cells appear to be produced and
stimulated differently. Furthermore, in contrast to the variety of
immunoglobulins produced in the mammalian immune system,
sharks have only one class of immunoglobulin (termed lgM). This
normally circulates in shark blood at very high levels and appears to
be ready to attack invading substances at all times.
Another difference lies in the fact that sharks, skates, and ray lack
a bony skeleton, and so do not have bone marrow, where the immune
cells in mammals are produced. In sharks, the immune cells are
produced in the spleen, thymus, and unique tissues associated with
the gonads and oesophagus. Our studies at MML in collaboration
with researchers at Clemson University have determined that a
significant number of immune cells, which mature and circulate in
the shark's blood, may be available to respond without a lag period
resulting in a more efficient immune response.
Our future plans include further investigations of the differences
between mammals and sharks in the regulation of immune cells. This
information may someday lead to improved methods of immune cell
regulation in humans, especially cancer patients.
But human health applications from our research are many years in
the future, and will rely on continued funding and active collaboration
with the drug industry and medical community. Meanwhile, there are
many public misconceptions about the clinical potential of shark-derived
products for treatment of disease. For example, freeze-dried shark cartilage
pills, marketed as food supplements through health
food stores and mail order houses, are being touted as miracle cures for
cancer as well as arthritis.
The claims are based on research conducted during the 1980s at
the Massachusetts Institute of Technology (MIT) and at Mote Marine
Laboratory. These studies of cow and shark cartilage were designed
to understand how cartilage is naturally able to resist penetration by
blood capillaries. If the basis for this inhibition could be identified, it
was reasoned, then it might lead to the development of a new drug
therapy. Such a drug could control the spread of blood vessels feeding
a cancerous tumor, or the inflammation associated with arthritis.
The research showed that only very small amounts of active material,
with limited ability to control blood vessel growth, can be obtained from
large amounts of raw cartilage. The cartilage must be subjected to several
weeks of harsh chemical procedures to extract and concentrate the active
ingredients. Once this is done, the resulting material is able to inhibit
blood vessel growth in laboratory tests on animal models, when the
concentrated extract is directly applied near the growing blood vessels.
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 in a manner
similar to the laboratory tests. Also, there is no reason to think that shark
cartilage contains anything which is not found in other animal cartilage.
Sharks are used because their entire skeleton is cartilaginous.
Finally, since in laboratory tests it must be chemically extracted,
there is no reason to assume that the active material in the cartilage is
passively released to inhibit blood vessel growth elsewhere in the body.
This means that cartilage, no matter whether in a shark, a cow, a dog,
or a human, probably plays no active role in disease resistance in the
living animal. The statements made by shark cartilage pill promoters
that it is cartilage that give sharks their immunity to cancer, then, are
inaccurate and irresponsible.
This is not to say that the basic research to understand the chemical
basis for cartilage resistance to blood vessels shouldn't continue. The
potential for this research to lead to new drug therapies is very real. But
what should be halted is the use of sharks as a source of cartilage to be
ground up and marketed as a panacea, with little or no support for
effective results in fighting disease when taken as a food supplement.
The above story was excerpted with permission from an article
that first appeared in Mote Marine Labs' Shark Line, Summer 1993.
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