ABSTRACT
B-cell-specific Moloney murine leukemia virus integration site 1 (BMI1) is a component of the polycomb repressive complex 1 (PRC1) complex that is overexpressed in breast and other cancers, and promotes self-renewal of cancer stem-like cells. The oncogenic mucin 1 (MUC1) C-terminal (MUC1-C) subunit is similarly overexpressed in human carcinoma cells and has been linked to their self-renewal. There is no known relationship between MUC1-C and BMI1 in cancer. The present studies demonstrate that MUC1-C drives BMI1 transcription by a MYC-dependent mechanism in breast and other cancer cells. In addition, we show that MUC1-C blocks miR-200c-mediated downregulation of BMI1 expression. The functional significance of this MUC1-Câï¸BMI1 pathway is supported by the demonstration that targeting MUC1-C suppresses BMI1-induced ubiquitylation of H2A and thereby derepresses homeobox HOXC5 and HOXC13 gene expression. Notably, our results further show that MUC1-C binds directly to BMI1 and promotes occupancy of BMI1 on the CDKN2A promoter. In concert with BMI1-induced repression of the p16INK4a tumor suppressor, we found that targeting MUC1-C is associated with induction of p16INK4a expression. In support of these results, analysis of three gene expresssion data sets demonstrated highly significant correlations between MUC1-C and BMI1 in breast cancers. These findings uncover a previously unrecognized role for MUC1-C in driving BMI1 expression and in directly interacting with this stem cell factor, linking MUC1-C with function of the PRC1 in epigenetic gene silencing.
Subject(s)
Mucin-1/metabolism , Neoplasms/metabolism , Polycomb Repressive Complex 1/metabolism , Amino Acid Motifs , Amino Acid Sequence , Cell Line, Tumor , Cyclin-Dependent Kinase Inhibitor p16/genetics , Cyclin-Dependent Kinase Inhibitor p16/metabolism , Gene Expression Regulation, Neoplastic , Gene Silencing , Histones/metabolism , Homeodomain Proteins/genetics , Homeodomain Proteins/metabolism , Humans , Mucin-1/chemistry , Mucin-1/genetics , NF-kappa B/metabolism , Neoplasms/genetics , Promoter Regions, Genetic , Protein Binding , Protein Interaction Domains and Motifs , Proto-Oncogene Proteins c-myc/metabolism , Transcription, Genetic , UbiquitinationABSTRACT
BACKGROUND: Tumour-initiating cells (TICs) or cancer stem cells can exist as a small population in malignant tissues. The signalling pathways activated in TICs that contribute to tumourigenesis are not fully understood. METHODS: Several breast cancer cell lines were sorted with CD24 and CD44, known markers for enrichment of breast cancer TICs. Tumourigenesis was analysed using sorted cells and total RNA was subjected to gene expression profiling and gene set enrichment analysis (GSEA). RESULTS: We showed that several breast cancer cell lines have a small population of CD24(-/low)/CD44(+) cells in which TICs may be enriched, and confirmed the properties of TICs in a xenograft model. GSEA revealed that CD24(-/low)/CD44(+) cell populations are enriched for genes involved in transforming growth factor-beta, tumour necrosis factor, and interferon response pathways. Moreover, we found the presence of nuclear factor-kappaB (NF-kappaB) activity in CD24(-/low)/CD44(+) cells, which was previously unrecognised. In addition, NF-kappaB inhibitor dehydroxymethylepoxyquinomicin (DHMEQ) prevented tumourigenesis of CD24(-/low)/CD44(+) cells in vivo. CONCLUSION: Our findings suggest that signalling pathways identified using GSEA help to identify molecular targets and biomarkers for TIC-like cells.
