| MUC1 is a type‑I transmembrane mucin glycoprotein of the epithelial mucin family that is normally expressed at the apical surface of many mucosal epithelia, where its heavily O‑glycosylated, tandem‑repeat‑rich extracellular domain forms a lubricating and protective barrier, but is frequently overexpressed and aberrantly glycosylated in carcinomas, converting it from a structural barrier molecule into a signaling platform that modulates oncogenic and immune pathways. The protein undergoes autoproteolytic cleavage in its SEA domain, yielding a large extracellular subunit non‑covalently associated with a smaller transmembrane subunit (MUC1‑C) that contains a single membrane‑spanning segment and a short cytoplasmic tail; the cytoplasmic tail comprises a compact linear sequence of tyrosine, serine and threonine motifs that serve as docking and phosphorylation sites for multiple kinases and effector proteins and is the principal signaling hub of MUC1. Mechanistically, the MUC1 cytoplasmic tail interacts with ErbB family receptors, including EGFR and ErbB2, and potentiates downstream MAPK/ERK1/2 signaling, while phospho‑motifs within the tail recruit adaptor proteins such as Grb2 and Shc, supporting sustained proliferative signaling; parallel interactions with β‑catenin integrate MUC1 into Wnt signaling, with phosphorylation of tail residues by GSK‑3β, PKCγ and Src regulating the stability and nuclear localization of MUC1–β‑catenin complexes that co‑activate transcription of proliferation and survival genes. A curated network map of MUC1 signaling collates hundreds of experimentally validated events and shows extensive crosstalk of MUC1‑regulated pathways with Wnt/β‑catenin, NF‑κB, PI3K–AKT and MAPK cascades, linking MUC1 activity to cell‑cycle progression, epithelial–mesenchymal transition, migration, invasion, angiogenesis and resistance to stress‑induced apoptosis in diverse tumors including breast, lung, pancreatic, colon, oral, prostate, ovarian and gastric cancers. The MUC1‑C cytoplasmic tail also couples to immune and inflammatory signaling: MUC1‑C drives activation of NF‑κB and STAT3, alters expression of chemokines and immune checkpoint molecules and has been shown to integrate IFN‑γ–JAK–STAT signaling with PD‑L1 upregulation in triple‑negative breast cancer, thereby promoting an immunosuppressive tumor microenvironment that limits responsiveness to immune checkpoint inhibitors. At mucosal surfaces, cell‑surface MUC1 acts as a barrier to infection and regulator of inflammation, serving as a binding site for pathogens such as Pseudomonas aeruginosa in the lung and modulating TLR and inflammasome signaling to dampen excessive inflammatory responses, and dysregulated MUC1 expression contributes to respiratory diseases including chronic obstructive pulmonary disease, asthma, lung fibrosis and lung cancer. Aberrant overexpression and hypoglycosylation of MUC1 in cancer expose truncated tandem‑repeat epitopes and increase availability of the signaling cytoplasmic tail, and clinical studies identify MUC1 as a pan‑cancer antigen and second‑ranked target for therapeutic development, with circulating MUC1‑derived markers (CA 15‑3/CA 27.29) used in breast cancer management and multiple MUC1‑targeted strategies—vaccines, monoclonal antibodies, antibody–drug conjugates, CAR‑T cells and radiopharmaceuticals—under evaluation. |
