MUC1

Mucin 1, cell surface associated (MUC1) or polymorphic epithelial mucin (PEM) is a is_associated_with::mucin encoded by the MUC1 is_associated_with::gene in humans. MUC1 is a is_associated_with::glycoprotein with extensive is_associated_with::O-linked glycosylation of its extracellular domain. Mucins line the apical surface of epithelial cells in the lungs, stomach, intestines, eyes and several other organs. Mucins protect the body from infection by is_associated_with::pathogen binding to is_associated_with::oligosaccharides in the extracellular domain, preventing the pathogen from reaching the cell surface. Overexpression of MUC1 is often associated with colon, breast, ovarian, lung and pancreatic cancers. is_associated_with::Joyce Taylor-Papadimitriou identified and characterised the antigen during her work with breast and ovarian tumors.

Structure
MUC1 is a member of the mucin family and encodes a membrane bound, glycosylated is_associated_with::phosphoprotein. MUC1 has a core protein mass of 120-225 kDa which increases to 250-500 kDa with glycosylation. It extends 200-500 nm beyond the surface of the cell.

The protein is anchored to the apical surface of many epithelia by a transmembrane domain. Beyond the transmembrane domain is a SEA domain that contains a cleavage site for release of the large extracellular domain. The release of mucins is performed by is_associated_with::sheddases. The extracellular domain includes a 20 amino acid variable number tandem repeat (VNTR) domain, with the number of repeats varying from 20 to 120 in different individuals. These repeats are rich in serine, threonine and proline residues which permits heavy o-glycosylation.

Multiple alternatively spliced transcript variants that encode different isoforms of this gene have been reported, but the full-length nature of only some has been determined.

MUC1 is cleaved in the is_associated_with::endoplasmic reticulum into two pieces, the cytoplasmic tail including the transmembrane domain and the extracellular domain. These domain tightly associate in a non-covalent fashion. This tight, non-covalent association is not broken by treatment with is_associated_with::urea, low pH, high salt or boiling. Treatment with is_associated_with::sodium dodecyl sulfate triggers dissociation of the subunits. The cytoplasmic tail of MUC1 is 72 amino acids long and contains several phosphorylation sites.

Function
The protein serves a protective function by binding to pathogens and also functions in a cell signaling capacity.

Overexpression, aberrant intracellular localization, and changes in is_associated_with::glycosylation of this protein have been associated with is_associated_with::carcinomas. e.g. The is_associated_with::CanAg is_associated_with::tumour antigen is a novel glycoform of MUC1.

Diagnostic utility
In is_associated_with::histopathology, the product of the MUC-1 gene is commonly referred to as epithelial membrane antigen or EMA. Using is_associated_with::immunohistochemistry, it can be identified in a wide range of secretory epithelia and their neoplastic equivalents. It can also be used for the identification of mesenchymal tumours, such as is_associated_with::synovial sarcoma and is_associated_with::ovarian granulosa cell tumours.

Although other antibodies, such as is_associated_with::cytokeratins, are more commonly used for the identification of metastatic carcinoma deposits, EMA can be used to distinguish is_associated_with::mesothelioma, in which it is restricted to the cell membranes and associated micovilli, from is_associated_with::adenocarcinoma, in which it is diffusely spread through the is_associated_with::cytoplasm.

Interactions
MUC1 has been shown to interact with:
 * is_associated_with::CTNND1,
 * ERBB2,
 * is_associated_with::GRB2,
 * JUP, and
 * is_associated_with::SOS1.

Role in Cancer
The ability of chemotherapeutic drugs to access the cancer cells is inhibited by the heavy glycosylation in the extracellular domain of MUC1. The glycosylation creates a highly hydrophilic region which prevents hydrophobic chemotherapeutic drugs from passing through. This prevents the drugs from reaching their targets which usually reside within the cell. Similarly, the glycosylation has been shown to bind to growth factors. This allows cancer cells which produce a large amount of MUC1 to concentrate growth factors near their receptors, increasing receptor activity and the growth of cancer cells. MUC1 also prevents the interaction of immune cells with receptors on the cancer cell surface through steric hindrance. This inhibits an anti-tumor immune response. Using MUC1, vaccines are being tested against a type of blood cancer called is_associated_with::multiple myeloma. The technology could in theory be applied to 90 percent of all known cancers, including prostate and breast cancer, solid and non-solid tumors. This method would activate the is_associated_with::immune system by training is_associated_with::T-cells to search out and destroy cells that display a specific molecule (or marker) of MUC1. MUC1 is found on nearly all epithelial cells, but it is over expressed in cancer cells, and its associated glycans are shorter than those of non-tumor-associated MUC1.

Preventing Cell Death
MUC1 cytoplasmic tail has been shown to bind to is_associated_with::p53. This interaction is increased by genotoxic stress. MUC1 and p53 were found to be associated with the p53 response element of the is_associated_with::p21 gene promoter. This results in activation of p21 which results in cell cycle arrest. Association of MUC1 with p53 in cancer results in inhibition of p53-mediated apoptosis and promotion of p53-mediated cell cycle arrest.

Overexpression of MUC1 in is_associated_with::fibroblasts increased the phosphorylation of is_associated_with::Akt. Phosphorylation of Akt results in phosphorylation of is_associated_with::Bcl-2-associated death promoter. This results in dissociation of Bcl-2-associated death promoter with is_associated_with::Bcl-2 and is_associated_with::Bcl-xL. Activation was shown to be dependent on the upstream activation of is_associated_with::PI3K. Additionally, MUC1 was shown to increase expression of Bcl-xL. Overexpression of MUC1 in cancer. The presence of free Bcl-2 and Bcl-xL prevents the release of is_associated_with::cytochrome c from mitochondria, thereby preventing apoptosis. MUC1 cytoplasmic tail is shuttled to the mitochondria through interaction with is_associated_with::hsp90. This interaction is induced through phosphorylation of the MUC1 cytoplasmic tail by is_associated_with::Src (gene). Src is activated by the EGF receptor family ligand Neuregulin. The cytoplasmic tail is then inserted into the mitochondrial outer membrane. Localization of MUC1 to the mitochondria prevents the activation of apoptotic mechanisms.

Promoting tumor invasion
MUC1 cytoplasmic tail was shown to interact with is_associated_with::Beta-catenin. A SXXXXXSSL motif was identified in MUC1 that is conserved with other beta-catenin binding partners. This interaction was shown to be dependent on cell adhesion. Studies have demonstrated that MUC1 is phosphorylated on a YEKV motif. Phosphorylation of this site has been demonstrated by is_associated_with::LYN through mediation of is_associated_with::interleukin 7, Src through mediation of EGFR, and is_associated_with::PRKCD. This interaction is antagonized by degradation of beta-catenin by is_associated_with::GSK3B. MUC1 blocks the phosphorylation-dependent degradation of beta-catenin by GSK3B. The end result is that increased expression of MUC1 in cancer increases stabilized beta-catenin. This promotes the expression of is_associated_with::vimentin and CDH2. These proteins are associated with a mesenchymal phenotype, characterized by increased motility and invasiveness. In cancer cells, increased expression of MUC1 promotes cancer cell invasion through beta-catenin, resulting in the initiation of is_associated_with::epithelial-mesenchymal transition which promotes the formation of metastases.

Cancer Antigens (CA) 27.29 and 15-3
CA 27.29 (aka BR 27.29) and CA 15-3 measure different epitopes of the same protein antigen product of the MUC1 gene seen in breast cancer. CA 27.29 has enhanced sensitivity and specificity compared to CA 15-3 and is elevated in 30% of patients with low-stage disease and 60 to 70% of patients with advanced-stage breast cancer.

CA 27.29 levels over 100 U/mL and CA 15-3 levels over 25 U/mL are rare in benign conditions and suggest malignancy.

Diagnostic use in clinical pathology

 * Epithelial marker
 * breast micropapillary carcinoma
 * bladder micropapillary carcinoma
 * Distinguish systemic anaplastic large-cell lymphoma (MUC1+) from cutaneous anaplastic large-cell lymphoma (usually MUC1-)

Positive tumoral cells

 * is_associated_with::Adenocarcinomas (breast, colorectal, pancreatic, other)
 * is_associated_with::Carcinoid tumor
 * is_associated_with::Chordoma
 * is_associated_with::Choriocarcinoma
 * is_associated_with::Desmoplastic small round cell tumor (DSRCT)
 * is_associated_with::Epithelioid sarcoma
 * is_associated_with::Follicular dendritic cell sarcoma, is_associated_with::interdigitating dendritic cell / is_associated_with::reticulum cell sarcoma
 * Lung: is_associated_with::type II pneumocyte lesions (is_associated_with::type II cell hyperplasia, is_associated_with::dysplastic type II cells, is_associated_with::apical alveolar hyperplasia)
 * is_associated_with::Anaplastic large-cell lymphoma, is_associated_with::diffuse large B cell lymphoma (variable), is_associated_with::plasmablastic lymphoma, is_associated_with::primary effusion lymphoma
 * is_associated_with::Epithelioid mesotheliomas
 * is_associated_with::Myeloma, is_associated_with::Plasmacytomas
 * is_associated_with::Perineurioma
 * is_associated_with::Renal cell carcinoma
 * is_associated_with::Synovial sarcoma (epithelial areas)
 * is_associated_with::Thymic carcinoma (often)
 * is_associated_with::Meningioma
 * Paget’s disease

Negative tumoral cells

 * is_associated_with::Adrenal cell carcinoma
 * is_associated_with::Hepatocellular carcinoma
 * is_associated_with::Germ cell tumors (except is_associated_with::choriocarcinoma)
 * is_associated_with::Acquired cystic disease-associated renal cell carcinoma
 * is_associated_with::Leiomyosarcomas (usually)
 * is_associated_with::Liposarcomas
 * is_associated_with::Melanoma
 * is_associated_with::Neuroblastoma
 * is_associated_with::Paraganglioma
 * is_associated_with::Solitary fibrous tumor (SFT)
 * is_associated_with::Myoepithelial cells