BMP3 suppresses colon tumorigenesis via ActRIIB/SMAD2-dependent and TAK1/JNK signaling pathways

BMP3 gene is often found hypermethylated and hence inactivated in several types of cancers including colorectal cancer (CRC), indicating that it has a suppressor role in carcinogenesis. Though BMP3 is a reliable biomarker for screening CRC, the molecular mechanism of BMP3 in carcinogenesis remains largely unknown. The expression level of BMP3 was examined by immunohistochemistry staining and western blot. Methylation-specific PCR (MSP) and real-time quantitative MSP were used to test the hypermethylation status of BMP3 gene. Analyses of BMP3 function in colon cancer cell proliferation, migration, invasion, and apoptosis were performed using HCT116 and KM12 cells. BMP3 was further knocked down or overexpressed in CRC cells, and the effects on cell growth of xenograft tumors in nude mice were assessed. Co-immunoprecipitation and immunofluorescence staining were used to analyze the association between BMP3 and BMPR2 or BMP3 and ActRIIB. Microarray analysis was performed to identify most differentially expressed genes and pathways regulated by BMP3. The BMP3-regulated SMAD2-dependent signaling pathway and TAK1/JNK signal axes were further investigated by quantitative PCR and western blot. BMP3 gene was hypermethylated and its expression was downregulated in both CRC tissues and cell lines. Expressing exogenous BMP3 in HCT116 inhibited cell growth, migration, and invasion and increased rate of apoptosis both in vitro and in vivo. However, shRNA-mediated attenuation of endogenous BMP3 in KM12 reversed such inhibitory and apoptotic effects. Furthermore, BMP3 could bind to ActRIIB, an activin type II receptor at the cellular membrane, thereby activating SMAD2-dependent pathway and TAK1/JNK signal axes to regulate downstream targets including caspase-7, p21, and SMAD4 that play crucial roles in cell cycle control and apoptosis. Our study reveals a previously unknown mechanism of BMP3 tumor suppression in CRC and provides a rationale for future investigation of BMP3 as a potential target for the development of novel therapeutic agents to fight CRC.