Role of RUNX3 in bone morphogenetic protein signaling in colorectal cancer.

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-beta (TGF-beta) superfamily, are multifunctional cytokines regulating a broad spectrum of biological functions. Recent studies show the presence of BMP receptor 1a mutations in juvenile polyposis and frequent Smad4 mutations in colon cancer, suggesting that aberrations in BMP signaling play an important role in intestinal cancer pathogenesis. However, the exact molecular mechanisms remain poorly understood. The Runt domain transcription factor RUNX3 is an integral component of signaling pathways mediated by TGF-beta and BMPs. RUNX3 is a gastric and colon tumor suppressor, functioning downstream of TGF-beta. Recently, we showed the tumor-suppressive effects of RUNX3 by its ability to attenuate beta-catenin/T-cell factors (TCFs) transactivation in intestinal tumorigenesis. Here, we explore the molecular basis of the tumor-suppressive function of the BMP pathway through RUNX3 in colorectal carcinogenesis. BMP exerted a growth-suppressive effect in HT-29, a human colorectal cancer cell line. c-Myc oncogene was found to be downregulated by BMP and/or RUNX3. We show that upregulation of RUNX3 by BMP reduces c-Myc expression. Evidence is presented suggesting that RUNX3 downregulates c-Myc expression by two parallel pathways-directly at the transcriptional level and through attenuation of beta-catenin/TCFs, downstream of BMPs in colorectal cancer cells.