Mucin short variant S1, also called polymorphic epithelial mucin (PEM) or epithelial membrane antigen (EMA), is a mucin encoded by the MUC1 gene in humans. [3] Mucin short variant S1 is a glycoprotein with extensive 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. [4] Mucins protect the body from infection by pathogen binding to oligosaccharides in the extracellular domain, preventing the pathogen from reaching the cell surface. [5] Overexpression of MUC1 is often associated with colon, breast, ovarian, lung and pancreatic cancers. [6] Joyce Taylor-Papadimitriou identified and characterised the antigen during her work with breast and ovarian tumors.
MUC1 is a member of the mucin family and encodes a membrane bound, glycosylated 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. [7]
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 sheddases. [8] 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. [7]
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. [9]
MUC1 is cleaved in the endoplasmic reticulum into two pieces, the cytoplasmic tail including the transmembrane domain and the extracellular domain. These domains tightly associate in a non-covalent fashion. [10] This tight, non-covalent association is not broken by treatment with urea, low pH, high salt or boiling. Treatment with sodium dodecyl sulfate triggers dissociation of the subunits. [11] The cytoplasmic tail of MUC1 is 72 amino acids long and contains several phosphorylation sites. [12]
The protein serves a protective function by binding to pathogens [13] and also functions in a cell signaling capacity. [12]
Overexpression, aberrant intracellular localization, and changes in glycosylation of this protein have been associated with carcinomas. e.g. The CanAg tumour antigen is a novel glycoform of MUC1. [14] In the cell nucleus, the protein MUC1 regulates the activity of transcription factor complexes that have a documented role in tumor-induced changes of host immunity. [15]
MUC1 has been shown to interact with:
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. [4]
MUC1 cytoplasmic tail has been shown to bind to p53. This interaction is increased by genotoxic stress. MUC1 and p53 were found to be associated with the p53 response element of the 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. [20]
Overexpression of MUC1 in fibroblasts increased the phosphorylation of Akt. Phosphorylation of Akt results in phosphorylation of Bcl-2-associated death promoter. This results in dissociation of Bcl-2-associated death promoter with Bcl-2 and Bcl-xL. Activation was shown to be dependent on the upstream activation of 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 cytochrome c from mitochondria, thereby preventing apoptosis. [21] MUC1 cytoplasmic tail is shuttled to the mitochondria through interaction with hsp90. This interaction is induced through phosphorylation of the MUC1 cytoplasmic tail by 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. [22]
MUC1 cytoplasmic tail was shown to interact 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. [23] Studies have demonstrated that MUC1 is phosphorylated on a YEKV motif. Phosphorylation of this site has been demonstrated by LYN through mediation of interleukin 7, [24] Src through mediation of EGFR, [25] [26] and PRKCD. [27] This interaction is antagonized by degradation of beta-catenin by GSK3B. MUC1 blocks the phosphorylation-dependent degradation of beta-catenin by GSK3B. [28] [29] The result is that increased expression of MUC1 in cancer increases stabilized beta-catenin. This promotes the expression of 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 epithelial-mesenchymal transition which promotes the formation of metastases. [30] [31]
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.
Using immunohistochemistry, MUC1 can be identified in a wide range of secretory epithelia and their neoplastic equivalents: [32]
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Using MUC1, vaccines are being tested against a type of blood cancer called 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 immune system by training 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. [34]
Because MUC1 is overexpressed (and differently glycosylated) in many cancers it has been investigated as a drug target, e.g. for the MUC1 vaccine ONT-10, which has had a phase 1 clinical study. [35]
Mucin-16(MUC-16) also known as Ovarian cancer-related tumor marker CA125 is a protein that in humans is encoded by the MUC16 gene. MUC-16 is a member of the mucin family glycoproteins. MUC-16 has found application as a tumor marker or biomarker that may be elevated in the blood of some patients with specific types of cancers, most notably ovarian cancer, or other conditions that are benign.
Mucins are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. They often take an inhibitory role. Some mucins are associated with controlling mineralization, including nacre formation in mollusks, calcification in echinoderms and bone formation in vertebrates. They bind to pathogens as part of the immune system. Overexpression of the mucin proteins, especially MUC1, is associated with many types of cancer.
Catenins are a family of proteins found in complexes with cadherin cell adhesion molecules of animal cells. The first two catenins that were identified became known as α-catenin and β-catenin. α-Catenin can bind to β-catenin and can also bind filamentous actin (F-actin). β-Catenin binds directly to the cytoplasmic tail of classical cadherins. Additional catenins such as γ-catenin and δ-catenin have been identified. The name "catenin" was originally selected because it was suspected that catenins might link cadherins to the cytoskeleton.
Catenin beta-1, also known as beta-catenin (β-catenin), is a protein that in humans is encoded by the CTNNB1 gene.
Alpha-catenin functions as the primary protein link between cadherins and the actin cytoskeleton. It has been reported that the actin binding proteins vinculin and alpha-actinin can bind to alpha-catenin. It has been suggested that alpha-catenin does not bind with high affinity to both actin filaments and the E-cadherin-beta-catenin complex at the same time. It has been observed that when alpha-catenin is not in a molecular complex with beta-catenin, it dimerizes and functions to regulate actin filament assembly, possibly by competing with Arp2/3 protein. Alpha catenin exhibits significant protein dynamics. However, a protein complex including a cadherin, actin, beta-catenin and alpha-catenin has not been isolated.
Beta-catenin-interacting protein 1 is a protein that is encoded in humans by the CTNNBIP1 gene.
Protein kinase C delta type is an enzyme that in humans is encoded by the PRKCD gene.
Glycogen synthase kinase-3 beta, (GSK-3 beta), is an enzyme that in humans is encoded by the GSK3B gene. In mice, the enzyme is encoded by the Gsk3b gene. Abnormal regulation and expression of GSK-3 beta is associated with an increased susceptibility towards bipolar disorder.
Axin-1 is a protein that in humans is encoded by the AXIN1 gene.
Mucin-4 (MUC-4) is a mucin protein that in humans is encoded by the MUC4 gene. Like other mucins, MUC-4 is a high-molecular weight glycoprotein.
Disintegrin and metalloproteinase domain-containing protein 15 is an enzyme that in humans is encoded by the ADAM15 gene.
p120, and called catenin delta-1 is a protein that in humans is encoded by the CTNND1 gene.
Receptor-type tyrosine-protein phosphatase F is an enzyme that in humans is encoded by the PTPRF gene.
Cadherin-3, also known as P-Cadherin, is a protein that in humans is encoded by the CDH3 gene.
Epithelial cell adhesion molecule (EpCAM), also known as CD326 among other names, is a transmembrane glycoprotein mediating Ca2+-independent homotypic cell–cell adhesion in epithelia. EpCAM is also involved in cell signaling, migration, proliferation, and differentiation. Additionally, EpCAM has oncogenic potential via its capacity to upregulate c-myc, e-fabp, and cyclins A & E. Since EpCAM is expressed exclusively in epithelia and epithelial-derived neoplasms, EpCAM can be used as diagnostic marker for various cancers. It appears to play a role in tumorigenesis and metastasis of carcinomas, so it can also act as a potential prognostic marker and as a potential target for immunotherapeutic strategies.
Segment polarity protein dishevelled homolog DVL-1 is a protein that in humans is encoded by the DVL1 gene.
Receptor-type tyrosine-protein phosphatase kappa is an enzyme that in humans is encoded by the PTPRK gene. PTPRK is also known as PTPkappa and PTPκ.
αE-catenin, also known as Catenin alpha-1 is a protein that in humans is encoded by the CTNNA1 gene. αE-catenin is highly expressed in cardiac muscle and localizes to adherens junctions at intercalated disc structures where it functions to mediate the anchorage of actin filaments to the sarcolemma. αE-catenin also plays a role in tumor metastasis and skin cell function.
Mucin-17 is a protein that in humans is encoded by the MUC17 gene.
Mucin-1(MUC-1) is a heterodimer transmembrane protein of the mucin family encoded in humans by the MUC1 gene. It is cleaved into two chains: Mucin-1 subunit alpha and Mucin-1 subunit beta. These subunits differ in size due to proteolytic cleavage of the translated precursor protein in the Endoplasmic Reticulum. The larger subunit of MUC-1 is characterized by numerous O-glycosylation bonds and a terminal sialic acid, creating a net negative charge on MUC-1. The smaller subunit contains a juxtamembrane region of the extracellular area, a transmembrane domain, and the cytoplasmic tail. The extracellular domain of MUC-1 is composed of 20 identical amino acid tandem repeats (TR). Each tandem repeat contains two Serine and three Threonine amino acid residues, providing five sites for potential O-glycosylation. MUC-1 protein is estimated to weigh 120-225 kDA.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.