Johns Hopkins researchers have discovered that
plant-derived compounds known for their ability to protect
tissue also appear to block the activity of an enzyme that
triggers inflammation in joints. Their findings, based on
experiments with human cells in a lab, could lead to new
arthritis treatments and better methods of making
artificial cartilage.
The discovery was detailed in a paper published in the
Sept. 27 edition of Proceedings of the National Academy
of Sciences.
The findings came to light while the researchers were
studying the wildly different ways in which cells in human
blood vessels and joints respond to pressure gradients
generated from liquid moving along their surface, a force
called shear stress. In cells that line blood vessels, the
reaction to shear stress is beneficial: the boosting of
phase 2 enzymes that may protect the cells from
cancer-causing chemicals and other toxic agents. Yet in
joints, the response to high shear stress is potentially
harmful: an increase in the levels of COX-2 enzyme, which
triggers inflammation and pain and suppresses the activity
of phase 2 enzymes, ultimately causing the death of
chondrocytic cells. Healthy chondrocytes are responsible
for the smooth functioning of joints. When chondrocytes
stop functioning properly, the result can be arthritis.
The divergent responses to shear stress prompted a
series of experiments in a Johns Hopkins lab supervised by
Konstantinos Konstantopoulos,
associate professor of
chemical and biomolecular engineering and
Agarwal-Masson Faculty Scholar. His team knew that
strenuous exercise or heavy exertion of muscles can cause
joints to increase the levels of harmful COX-2 enzyme. What
would happen, the researchers wondered, if the vulnerable
chondrocyte cells in human joints were first exposed to the
beneficial phase 2 enzymes?
To find out, the researchers obtained compounds that
boost the activity of helpful phase 2 enzymes. They added
these phase 2 inducers to a dish containing the chondrocyte
cells that are crucial to maintaining healthy joints. After
24 hours, the cells were subjected to a stress test
designed to mimic aspects of strenuous exercise on a joint
as well as the hydrodynamic environment in a bioreactor
designed to generate artificial cartilage.
The results were surprising. "The beneficial phase 2
enzymes somehow seemed to prevent the activation of the
inflammatory COX-2 enzyme," Zachary R. Healy, a doctoral
student in Konstantopoulos' lab and lead author of the
journal paper, said. "The phase 2 enzymes inhibited the
inflammation and the apoptosis, the cellular suicide we'd
observed."
Some prescription drugs like Vioxx keep COX-2 enzyme
at bay by temporarily blocking its ability to send the
biochemical signals that set off pain and inflammation.
When the medication is stopped, however, the stockpiled
COX-2 enzyme can resume its damaging ways. Unlike these
traditional painkillers, Healy said, the phase 2 enzyme
inducers seemed to stop the increasing activity of COX-2
enzyme.
"That means these compounds could be useful as a
preventive measure, perhaps before strenuous exercise,"
Healy said. "This has the potential for stopping pain and
inflammation before they start."
Although these experiments appeared to be the first to
determine how phase 2 enzyme inducers affect chondrocytes,
these compounds have been studied extensively by
researchers at the Johns Hopkins School of Medicine.
Paul Talalay, the John
Jacob Abel Distinguished Service Professor of
Pharmacology, has shown that phase 2 enzymes can
detoxify certain cancer-causing agents and damaging free
radicals in tissue, including cells that line blood
vessels. He has isolated compounds in edible plants that
boost production of phase 2 enzymes. These phytochemicals
can be found in cruciferous plants, including broccoli.
Talalay provided one of the phase 2 inducers used in
Healy's experiments. "This was the first work done in
applying these phytochemicals to chondrocytes, which are
constantly under the influence of forces because of the way
we move our joints," Talalay said. "The phase 2 inducers
seemed to counteract the effects of that stress by
inhibiting the expression of COX-2 enzyme. It's interesting
to think that people may be able to obtain this benefit
through dietary components."
By showing a way to ward off inflammation and by
providing insights into the effects of shear stress, the
new chondrocyte research may also aid tissue engineers who
are trying to grow artificial cartilage or seeking to
revitalize human cartilage in the lab. This is important
because human bodies cannot make new cartilage to replace
tissue that's lost to injury or disease.
"More research is needed," said Konstantopoulos, who
directed and supervised the experiments, "but these
discoveries could provide guidelines for designing an ideal
hydrodynamic environment in bioreactors for generating
functional cartilage as well as for the treatment of
osteoarthritis."
Funding for the research was provided by a DuPont
Young Professor Award, a National Science Foundation
Graduate Research Fellowship and an Achievement Reward for
College Students Fellowship. Healy's co-authors on the PNAS
paper were Talalay; Konstantopoulos; Norman H. Lee, of the
Institute for Genomic Research; Xiangqun Gao, of the
Department of Pharmacology and Molecular Sciences at the
Johns Hopkins School of Medicine; Mary B. Goldring, of the
Harvard Institutes of Medicine; and Thomas W. Kensler, of
the Department of Environmental Health Sciences in the
Johns Hopkins Bloomberg School of Public Health.