microRNA-145 downregulates SIP1-expression but differentially regulates proliferation, migration, invasion and Wnt signaling in SW480 and SW620 cells

microRNA-145 (miR-145) has been shown to act as a tumor suppressor in colorectal cancer but its role in the regulation of epithelial-mesenchymal transition (EMT) is unclear. Ectopic expression of miR-145 suppressed the proliferation, migration and invasion in SW480 but surprisingly enhanced these traits in its metastatic counterpart, SW620 cells, while, antimiR-145 reversed the effects of miR-145 in both of these human colorectal cancer cells. In SW480 and SW620 cells, SMAD-interacting protein 1 (SIP1), was identified as a target of miR-145, and its expression was suppressed both at mRNA and protein levels, and siRNA-SIP1 mimicked the effects of miR-145. Further, re-introduction of SIP1 alone or its co-expression with miR-145, rescued SW480 and SW620 cells from the effects of miR-145, indicating that the distinct functions of miR-145 might be mediated, in part, through SIP1. Since Wnt signaling plays an essential role in EMT in CRC progression, the effects of miR-145 on the expression of Wnt signaling intermediates and EMT markers were studied. Re-expression of miR-145 was found to downregulate the expression of CTNNB1, TCF4, CCND1, VIM, and SNAI, but, upregulated CDH1 expression in SW480 cells. On the other hand, miR-145 exhibited an oncogenic potential in SW620 cells by actuating Wnt signaling and the expression of EMT-relevant markers. These results strongly hint that the paradoxical functions of miR-145 in the regulation of proliferation, migration and invasion might be mediated through downregulation of SIP1, and differential tuning of Wnt signaling and EMT-mediators.