Inhibition of deregulated
protein tyrosine kinases represents an attractive strategy for controlling
cancer growth. However, target specificity is an essential aim of this strategy. In this report,
pp60(c-Src) kinase and
beta-catenin were found physically associated and constitutively activated on
tyrosine residues in human
colorectal cancer cells. The use of specific small-interfering RNAs (
siRNA) validated
pp60(c-Src) as the major
kinase responsible for
beta-catenin tyrosine phosphorylation in
colorectal cancer. Src-dependent activation of
beta-catenin was prevented by
SKI-606, a novel
Src family kinase inhibitor, which also abrogated
beta-catenin nuclear function by impairing its binding to the
TCF4 transcription factor and its trans-activating ability in
colorectal cancer cells. These effects were seemingly specific, as
cyclin D1, a crucial
beta-catenin/TCF4 target gene, was also down-regulated by
SKI-606 in a dose-dependent manner accounting, at least in part, for the reduced growth (IC50, 1.5-2.4 micromol/L) and clonogenic potential of
colorectal cancer cells.
Protein levels of
beta-catenin remained substantially unchanged by
SKI-606, which promoted instead a cytosolic/membranous retention of
beta-catenin as judged by immunoblotting analysis of cytosolic/nuclear extracts and cell immunofluorescence staining. The SKI-606-mediated relocalization of
beta-catenin increased its binding affinity to
E-cadherin and adhesion of
colorectal cancer cells, with ensuing reduced motility in a wound healing assay. Interestingly, the
siRNA-driven knockdown of
beta-catenin removed the effect of
SKI-606 on cell-to-cell adhesion, which was associated with prolonged stability of
E-cadherin protein in a pulse-chase experiment. Thus, our results show that
SKI-606 operates a switch between the transcriptional and adhesive function of
beta-catenin by inhibiting its pp60(c-Src)-dependent
tyrosine phosphorylation; this could constitute a new therapeutic target in
colorectal cancer.