Identification and characterization of the oncogenic MUC1-C subunit. The Kufe laboratory identified DF3/MUC1 in the 1980s as a glycoprotein that is aberrantly overexpressed in ~90% of human breast cancers. Cloning of the DF3/MUC1 gene further identified a unique physical structure comprised of 20 amino acid tandem repeats. Subsequent work showed that MUC1 is a complex consisting of an extracellular N-terminal subunit with glycosylated tandem repeats (MUC1-N; the mucin component) and a C-terminal transmembrane subunit (MUC1-C; the signaling component). In providing protection from the external environment, normal epithelial cells respond to stress with release of MUC1-N into the protective mucous barrier and MUC1-C functions in the transduction of signals to the interior of the cell to promote loss of polarity and induce wound healing and repair. Appropriation of this response by overexpression of MUC1-C, as found in carcinoma cells, was shown to be sufficient for inducing anchorage-independent growth and tumorigenicity, providing the first evidence that MUC1-C functions as an oncoprotein.
MUC1-C activates cell membrane and nuclear signaling pathways. Subsequent work demonstrated that MUC1-C drives loss of polarity, forms complexes with RTKs, such as EGFR, HER2 and others at the cell membrane, and contributes to their activation. These findings were extended by the demonstration that MUC1-C transduces signals from the cell membrane to the nucleus. As one example, MUC1-C was found to directly activate the proinflammatory IKKNF-B p65 pathway, form complexes with p65 on promoters of target genes and contribute to their regulation. Other work showed that MUC1-C promotes the inflammatory response through activation of nuclear STAT1/3 signaling pathways, supporting the notion that MUC1-C contributes to the association between inflammation and cancer.
MUC1-C induces EMT, self-renewal and epigenetic reprogramming. The Kufe laboratory first demonstrated that MUC1-C functions in the regulation of genes that drive EMT and self-renewal of cancer stem-like cells. Further work linked MUC1-C to epigenetic reprogramming by showing that MUC1-C regulates DNA methylation by activating the DNMT1 and DNMT3b genes. Advances also included unrecognized roles for MUC1-C in activating the PRC1/BMI1 and PRC2/EZH2 complexes in driving the repression of tumor suppressor genes, such as such as CHD1, CDKN2A, PTEN, BRCA1 and RASSF1A. Other studies showed that MUC1-C activates the highly conserved NuRD complex, which regulates chromatin assembly and reorganization and is of importance for EMT and invasion. These findings linked MUC1-C to integrating DNA methylation, histone regulation and chromatin remodeling in cancer cells.
MUC1-C drives genotoxic and targeted drug resistance. The early finding that MUC1-C confers resistance to genotoxic anti-cancer agents was attributed in part to suppression of the DNA damage-induced apoptotic response. Other work in this area extended involvement of MUC1-C in the repair of DNA damage and, more recently, the integration of chromatin remodeling in that response. Other studies showed that MUC1-C confers resistance to targeted agents, such as trastuzumab, tamoxifen, afatinib and others, and that targeting MUC1-C is synergistic with these agents in resistant cells.
MUC1-C inhibits immune recognition and destruction. The Kufe laboratory first demonstrated that MUC1-C activates the PD-L1 gene and represses expression of immune effectors in cancer cells. In concert with these functions, targeting MUC1-C with a novel inhibitor downregulates PD-L1 in mouse tumor models and activates CD8+ T-cells in the immune microenvironment. In addition, MUC1 expression in human cancers correlates negatively with that of the CD8 and IFNG genes and is associated with poor clinical outcomes. These findings provided the first evidence that MUC1-C plays a role in immune evasion.