September 01, 2000
New Approach to Thwarting Inflammation
Howard Hughes Medical Institute (HHMI) researchers have discovered a
way to shut down the inflammatory response in cells that spares related
mechanisms that cells need in order to function properly. Their
experiments have demonstrated that such treatment relieves inflammation
in mice with surprising effectiveness.
In an article published in the September 1, 2000, issue of
Science, HHMI investigator Sankar Ghosh and his colleagues at
Yale University report that they have found a way to short circuit
NF-kB, a central coordinator of the cell’s inflammatory response.
When NF-kB is triggered by an external chemical signal, NF-kB sets in
motion the gene expression machinery that drives inflammation.
“I think that the remarkable anti-inflammatory effect that we achieved validates the approach of relieving inflammation by interfering with the activation of NF-kB alone.”
Sankar Ghosh
"It has become clear over the years that NF-kB plays a crucial,
evolutionarily conserved role in the cell’s response for getting
rid of pathogens," said Ghosh. "As a byproduct of this immune response,
inflammation can get out of control and cause pathogenic states. We now
know that many diseases that at first glance don’t seem to have
much in common actually have inflammation as an underlying reason for
their pathology." Alzheimer’s disease, for example, might well
involve inflammatory responses that damage brain tissue, Ghosh
noted.
The most widely used anti-inflammatory drugs, including salicylates
such as aspirin, and steroids, inhibit NF-kB to some extent, but they
can also have severe side effects. The great need for more precise
inhibitors of inflammation prompted Ghosh and his colleagues to look at
ways to block inflammation by interfering with NF-kB.
The scientists knew that in unstimulated cells NF-kB remains in the
cell’s cytoplasm attached to inhibitory proteins known as IkBs.
When an external inflammatory signal affects the cell, the IkB-kinase
(IKK) complex—which consists of a pair of catalytic
enzymes—is activated and phosphorylates the IkB proteins. The
phosphorylated IkB is then rapidly degraded, thus freeing NF-kB to
trigger the inflammatory process.
While the majority of research on blocking the inflammatory process
has focused on blocking the catalytic activity of the IKK complex
itself, Ghosh and his colleagues chose another route. "By themselves,
the enzymes of the IKK complex don’t respond to signals,"
explained Ghosh. "Each enzyme needs a regulatory subunit protein called
NEMO, and it has been shown that when NEMO is knocked out in mice,
NF-kB becomes unresponsive to signaling."
In a series of biochemical experiments, Ghosh and his colleagues
sought to identify the minimal region of the IKK enzymes that
interacted with NEMO. "We were quite surprised to find that a very
small region of the IKK enzymes seemed to be completely responsible for
interacting with NEMO," said Ghosh. The scientists called the region
the NEMO-binding domain (NBD).
"We reasoned that if the NBD was such a small region, then maybe we
could use it as a way to disrupt the whole IKK complex and prevent it
from forming," said Ghosh. Sure enough, when the scientists synthesized
a small peptide that mimicked the NBD and put it into cells, they
discovered that NF-kB activation was blocked significantly.
"Then we raised the stakes," said Ghosh. "We decided to see if this
blocking method would work in vivo in animals." The scientists
used the NBD peptide to attempt to block inflammation in two mouse
models where chemicals are used to induce inflammation
artificially.
"In both models, when we injected our peptide, we observed quite a
dramatic amelioration of the inflammatory process," said Ghosh. "I
think that the remarkable anti-inflammatory effect that we achieved
validates the approach of relieving inflammation by interfering with
the activation of NF-kB alone. This approach should become a major
focus of future research."
Ghosh also said that blocking the activation of NF-kB by external
inflammatory factors quite likely spares the basal activity of the
NF-kB machinery that operates via a different pathway, and which cells
need for normal function.
"One of the concerns that has been raised about inhibiting NF-kB, is
that NF-kB also has some beneficial effects, such as helping cells
survive apoptosis, or programmed cell death," said Ghosh.
Ghosh and his colleagues are planning further studies to investigate
the NEMO-IKK interaction. They are going to test the NBD peptide on
mouse models of asthma and other diseases that involve inflammation.
They also hope to initiate collaborations with pharmaceutical companies
to develop mimics of the NBD peptide that can be used as precise
inhibitors of inflammation.
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