The second issue is that the core components of the Wnt pathway are "hard"
drug targets, according to Dr. Dale. "Drugs that inhibit easy targets,
such as enzymes, can be developed through standard screening
approaches using purified enzymes. The drugs usually fit into small
pockets on the enzyme like a broken key fitting into a lock and
preventing the normal key from getting access."
"Drugs that inhibit hard targets, such as protein-protein interactions,
are more difficult to develop since they usually don't have small
pockets. Most "core" targets of the Wnt pathway fall into this class",
Dr. Dale explains.
Two of the core targets of the Wnt pathway are the proteins
beta-catenin and TCF. In normal cells, the level of beta-catenin in
the cytoplasm is regulated by a complex of three proteins to which it
is bound and which label it for destruction in the waste basket of the
cell, the proteasome. One of the three proteins is the tumor
suppressor APC (adenomatous polyposis coli).
In the presence of Wnt signaling, beta-catenin is set free from this
complex of three proteins and moves into the cell nucleus. There,
beta-catenin activates genes by binding to the TCF/Lef factor.
This activation of genes in the cell's nucleus leads to the onset of
diseases such as cancer, brain and heart disorders, and others. Thus
researchers try to prevent beta-catenin from being translocated from
the cytoplasm to the cell's nucleus. Questions remain as to when, and
where to intervene.
According to Dr. Dale, much of the pathway outside the core pathway
has not been well studied. However, this allowed Dr. Dale and his team
to apply a broader approach for drug screening. They decided to use
the response of live cells to screen 68,000 compounds for activity
against the Wnt pathway. "Once the active compounds were identified,
we were then faced with the problem of identifying the targets of the
pathway."
They were able to show that two of the compounds block the pathway
near beta-catenin in the core pathway and two block the pathway near
the TCF protein. The compounds blocked tumor cell growth in the cell
culture and in zebrafish embyros. "However, we don't know as of yet
exactly what the compounds bind", Dr. Dale says.
Furthermore, some of the zebrafish embryos showed developmental
abnormalities that were similar to those previously described in
mutated Wnt proteins. "This suggests that the compounds were having
the same effect as blocking Wnt production." For example, one side
effect was a loss of part of the zebrafish brain. In addition, side
effects varied with the compounds. In some cases, there were no side
effects whereas for others , there were many additional developmental
changes which may - or may not - be related to blocking the Wnt
pathway. This is still being studied.
In future studies, Dr. Dale and his co-workers want to use variants of
their four compounds to block colon cancer in mice with de-regulated
Wnt signaling. Researchers believe that APC triggers this disease, as
it is mutated in 90 percent of all cases of colon cancer in humans.
It remains to be seen if, based on the results in mice, the
researchers will be able to move on to human trials in the near future.
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