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Researchers
find chemistry behind skin's barrier
By Mary Beth Gardiner
September. 12, 2002
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Photo
by Anne Rayner |
| Alan
Brash, left, and Zheyong Yu |
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A group of researchers at Vanderbilt has discovered unusual chemical activity
behind an inherited skin disorder that causes fish-like scaling and flaking.
The findings give life to an enzyme thought to be inactive, and link its function
to a second enzyme, both of which appear to be essential to creating the normal
permeability barrier of the skin.
The group found that the two enzymes, both lipoxygenases, work together in
a coordinated and unexpected way that provides a logical explanation for why
disruption of one or other of the genes encoding the enzymes leads to skin
disease. A description of the work appears in the Aug. 5 issue of Proceedings
of the National Academy of Sciences.
There are six lipoxygenase (LOX) enzymes known to occur in people and mice,
according to Alan R. Brash, Ph.D., professor of Pharmacology and principal
investigator of the study. One of them — 5-LOX — is particularly well understood,
because it's involved in inflammation, activating substances called leukotrienes.
The enzyme works in a similar manner to the way cyclooxygenase, or COX, enzymes
activate prostaglandins.
“In the same way that you take aspirin and Celebrex, which are COX inhibitors,
to knock down prostaglandins for an anti-inflammatory effect, you can take
medications to combat these leukotrienes,” said Brash. “The drug Singular,
for example, is an asthma treatment that blocks the receptor for products of
5-LOX.”
The function of the other LOX enzymes is less certain, but they are known
to be specific to certain cell types and to produce distinct products.
“The two that are essential to this study — 12R-LOX and eLOX3 — are mainly
confined to the skin,” Brash said.
The 12R-LOX enzyme, which Brash's lab discovered in 1998,
is highly expressed in psoriasis. “The thinking was that this enzyme might have an inflammatory
role in psoriasis in the same way that 5-LOX has a role in inflammation of
the lungs in asthma,” he said. “But when Diane Keeney, Ph.D., assistant professor
of Biochemistry, looked to see where the enzyme was expressed, it wasn't deep
down in the skin where all the trouble was arising. It was expressed out near
the edge, just where the cells are changing from the last living cells to the
ones that help form a barrier.”
The other enzyme, eLOX3, was discovered at the German Cancer Institute in
Heidelberg. The enzyme is expressed in the epidermis, but until this latest
study, Brash said, no one knew what it did.
“We tried the human eLOX3, the mouse eLOX3, and we tried them under lots of
different conditions to get the enzyme to make products,” he said, “and nothing.
It didn't do anything. So it sat in the freezer waiting for a better idea.”
A better idea came along, Brash said, when a paper came
out linking mutations in these two LOX enzymes with a rare type of ichthyosis,
or scaly skin disease. “The
funny thing was, some of the families had mutations in 12R-LOX and some of
them had mutations in eLOX3, which we thought had no function, yet both had
the same skin problems.”
Up to this point, all LOX genes were considered to work
independently, but these results suggested that the enzymes might be in the
same pathway. “It
was an enlightening thought,” said Brash. “Neither we nor anybody else had
thought about that possibility, but it makes sense because you could block
the pathway by one way or the other.”
Zheyong Yu, a third year graduate student in Brash's lab, took on the task
of exploring this idea. Yu added eLOX3 to every lipoxygenase product available
to test for activity.
“It turned out that the products from 12R-LOX were the best substrate by far,” said
Brash. “And they were being converted by eLOX3 by a previously undescribed
enzymatic activity that appears to be unique.
“The reason this is such a nice finding is that the enzyme that seemed to
be dead really did have activity when the right substrate was provided,” he
said. “And it pointed to a very rational explanation for what was wrong in
these patients with the inherited skin disease. Furthermore, it suggests that
in normal people these two enzymes are helping in the final stages of sealing
off the barrier of the skin.”
Brash theorizes that in the disease, the skin is making a lot of extra cells
in an attempt to compensate for the imperfect process. The result is the scaling
and flaking that gives ichthyosis its name, which stems from a Greek root meaning
fish.
The lab's next steps will include testing the product of eLOX3 in cultured
skin cells to determine its biological activity; determining if the product
is being made in normal skin or in normal cultured skin cells; and looking
at where eLOX3 is expressed in skin and if it is expressed in the same cells
as 12R-LOX. Other Vanderbilt researchers listed as authors on the PNAS paper
include Claus Schneider, Ph.D., William E. Boeglin, and Lawrence J. Marnett,
Ph.D. The work was supported by grants from the National Institutes of Health
and the Core Laboratories of the Vanderbilt Skin Disease Research Center Grant.
| Originally
published in |
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Alan
Brash's home page
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