BMP-induced growth suppression in colon cancer cells is mediated by p21WAF1 stabilization and modulated by RAS/ERK.

Bone morphogenetic proteins (BMPs) regulate cell differentiation, proliferation, and apoptosis through a canonical SMAD signaling cascade. Absence of BMP signaling causes the formation of intestinal juvenile polyps in the colon cancer-prone syndrome familial juvenile polyposis. As sporadic colon cancers appear to have intact BMP signaling, we evaluated if K-RAS, driving a mitogenic pathway frequently activated in colon cancer, negatively affects BMP growth suppression. We treated non-tumorigenic but activated RAS/ERK FET cells with BMP2, and in combination with pharmacological or genetic inhibition of RAS/ERK, examined BMP-SMAD signaling, transcriptional activity, and cell growth, and also assessed p21(WAF1) mRNA, transcriptional activation, and protein levels. BMP2 increased nuclear phospho-SMAD1 2-fold, which increased another 2-3 fold when RAS/ERK was inhibited. BMP2 increased BMP-specific SMAD transcriptional activity 2-fold over control and decreased cell growth, but inhibition of RAS/ERK further enhanced BMP-specific transcriptional activity by an additional 1.5-2 fold and enhanced growth suppression by 20%. BMP-induced growth suppression is mediated in part by p21(WAF1), not by transcriptional upregulation but by improved p21 protein stability, which is inhibited by RAS/ERK. In colon cancer cells, BMP-SMAD signaling and growth suppression is facilitated by p21(WAF1) but modulated by oncogenic K-RAS to reduce the growth suppression directed by this pathway.