miR-133b, a muscle-specific microRNA, is a novel prognostic marker that participates in the progression of human colorectal cancer via regulation of CXCR4 expression

Background: MicroRNA-133b (miR-133b), which is a muscle-specific microRNA, has been reported to be downregulated in human colorectal carcinoma (CRC) when compared to adjacent non-tumor tissue. However, its diagnostic value and role in CRC have yet to be described. CXC chemokine receptor-4 (CXCR4), which participates in multiple cell processes such as cell invasion-related signaling pathways, was predicted to be a potential target of miR-133b. The aim of this study was to investigate the associations and functions of miR-133b and CXCR4 in CRC initiation and invasion. Methods: Mature miR-133b and CXCR4 expression levels were detected in 31 tumor samples and their adjacent, non-tumor tissues from patients with CRC, as well as in 6 CRC cell lines, using real-time quantitative RT-PCR (qRT-PCR). Luciferase reporter assays and Western blots were used to validate CXCR4 as a putative target gene of miR-133b. Regulation of CXCR4 expression by miR-133b was assessed using qRT-PCR and Western blot analysis, and the effects of exogenous miR-133b and CXCR4 on cell invasion and migration were evaluated in vitro using the SW-480 and SW-620 CRC cell lines. Results: A significant downregulation of miR-133b was observed in 93.55% of CRC tissues, and the expression of miR-133b was much lower in metastatic tumors (stage C and D, stratified by the Modified Dukes Staging System) than in primary tumors (stage A and B). In contrast, CXCR4 protein expression significantly increased in 52.63% of CRC samples, and increased CXCR4 expression in CRC was associated with advanced tumor stage. CXCR4 was shown to be a direct target of miR-133b by luciferase reporter assays, and transfection of miR-133b mimics inhibited invasion and stimulated apoptosis of SW-480 and SW-620 CRC cells. Conclusions: Our study demonstrated that downregulated miR-133b contributed to increased cell invasion and migration in CRC by negatively regulating CXCR4. These findings may be significant for the development of therapy target for CRC.