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* Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York 10021; and Mutations in the adenomatous polyposis coli
(APC) tumor suppressor gene are linked to both familial and sporadic human colon cancer. So far, a clear biological function for the APC gene product has not been
determined. We assayed the activity of APC in the early Xenopus embryo, which has been established as a
good model for the analysis of the signaling activity of
the APC-associated protein
Onyx
Pharmaceuticals, Richmond, California 94806
-catenin. When expressed in the future ventral side of a four-cell embryo,
full-length APC induced a secondary dorsoanterior axis
and the induction of the homeobox gene Siamois. This
is similar to the phenotype previously observed for ectopic
-catenin expression. In fact, axis induction by
APC required the availability of cytosolic
-catenin.
These results indicate that APC has signaling activity in
the early Xenopus embryo. Signaling activity resides in
the central domain of the protein, a part of the molecule that is missing in most of the truncating APC mutations in colon cancer. Signaling by APC in Xenopus
embryos is not accompanied by detectable changes in
expression levels of
-catenin, indicating that it has direct positive signaling activity in addition to its role in
-catenin turnover. From these results we propose a
model in which APC acts as part of the Wnt/
-catenin
signaling pathway, either upstream of, or in conjunction
with,
-catenin.
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