Mucin1 (MUC1) is a transmembrane glycoprotein that plays a key role as an oncogene in the tumorigenesis of many human adenocarcinomas. However, the role of MUC1 in human hepatocellular carcinoma (HCC) progression remains unclear. In the present study, we silenced MUC1 to investigate its effect on the human HCC cell line SMMC-7721 and found that knockdown of MUC1 significantly inhibited cell proliferation, enhanced cell-cell aggregation and induced apoptosis. No significant differences were found in in vitro migration or invasion. We also observed that knockdown of MUC1 decreased the translocation of β‑catenin to the nucleus, reduced the activity of T cell factor and blocked the expression of cyclin D1 and c-Myc. In addition, MUC1 knockdown enhanced the expression of E-cadherin, a molecular chaperone of β‑catenin that plays an important role in cell-cell aggregation. In vivo assays demonstrated that there was no tumor growth in mice injected with MUC1-silenced cells. Global gene expression analysis showed that a series of genes encoding molecules in the Wnt/β‑catenin, nuclear factor-κB (NF-κB), mitogen-activated protein kinase (MAPK), insulin, transforming growth factor β (TGF-β) and vascular endothelial growth factor (VEGF) signaling pathways were all influenced by the knockdown of MUC1, and these may contribute to the phenotypic alterations observed. Collectively, our results indicate that MUC1 plays a key role in HCC tumorigenesis.

Mucin 1 (MUC1) is an oncogene that has a crucial role in the pathogenesis and progression of the majority of epithelial malignant tumors. Our previous study demonstrated that MUC1 gene silencing inhibited the growth of SMMC-7721 cells in vitro and in vivo, however, whether this growth inhibition is associated with apoptotic cell death remains to be elucidated. In the present study, it was found that MUC1 gene silencing not only resulted in the inhibition of SMMC-7721 cell growth, determined using a clone formation assay in vitro and a tumor xenograft mouse model with an in vivo imaging system, but also induced apoptotic alterations in SMMC-7721 cells, determined using Hoechst 33342 staining, flow cytometry with an Annexin V-PE staining and a DNA ladder assay. Further investigation using western blotting revealed that cytochrome c was released from the mitochondria into the cytoplasm, and caspase-8 and caspase-9 were activated in MUC1 gene-silenced SMMC-7721 cells. The pro-apoptotic protein Bcl-2-associated X protein (Bax) and the tumor suppressor p53 were increased, while the anti-apoptotic protein B-cell lymphoma 2 was decreased in MUC1 gene-silenced cells. In addition, results from the co-immunoprecipitation experiments demonstrated that the MUC1 cytoplasmic tail can bind directly to Bax or caspase-8 and these interactions were reduced upon MUC1 gene silencing in SMMC-7721 cells. The above results indicate that MUC1 gene silencing induces growth inhibition in SMMC-7721 cells through Bax-mediated mitochondrial and caspase-8-mediated death receptor apoptotic pathways.