Mutational inactivation of adenomatous polyposis coli (APC) is an early event in colorectal cancer (CRC) progression that affects the stability and increases the activity of β-catenin, a mediator of Wnt signaling. Progression of CRC also involves inactivation of signaling via transforming growth factor β and bone morphogenetic protein (BMP), which are tumor suppressors. However, the interactions between these pathways are not clear. We investigated the effects of loss of the transcription factor Smad4 on levels of β-catenin messenger RNA (mRNA) and Wnt signaling.

We used microarray analysis to associate levels of Smad4 and β-catenin mRNA in colorectal tumor samples from 250 patients. We performed oligonucleotide-mediated knockdown of Smad4 in human embryonic kidney (HEK293T) and in HCT116 colon cancer cells and transgenically expressed Smad4 in SW480 colon cancer cells. We analyzed adenomas from (APC^Δ1638/+) and (APC^Δ1638/+) × (K19Cre^ERT2Smad4^lox/lox) mice by using laser capture microdissection.

In human CRC samples, reduced levels of Smad4 correlated with increased levels of β-catenin mRNA. In Smad4-depleted cell lines, levels of β-catenin mRNA and Wnt signaling increased. Inhibition of BMP or depletion of Smad4 in HEK293T cells increased binding of RNA polymerase II to the β-catenin gene. Expression of Smad4 in SW480 cells reduced Wnt signaling and levels of β-catenin mRNA. In mice with heterozygous disruption of Apc(APC^Δ1638/+), Smad4-deficient intestinal adenomas had increased levels of β-catenin mRNA and expression of Wnt target genes compared with adenomas from APC^Δ1638/+ mice that expressed Smad4.

Transcription of β-catenin is inhibited by BMP signaling to Smad4. These findings provide important information about the interaction among transforming growth factor β, BMP, and Wnt signaling pathways in progression of CRC.