Mucins in the pathogenesis of breast cancer: implications in diagnosis, prognosis and therapy

Mucins are high molecular weight, multifunctional glycoproteins comprised of two structural classes-the large transmembrane mucins and the gel-forming or secreted mucins. The primary function of mucins is to protect and lubricate the luminal surfaces of epithelium-lined ducts in the human body. Recent studies have identified a differential expression of both membrane bound (MUC1, MUC4 and MUC16) and secreted mucins (MUC2, MUC5AC, MUC5B and MUC6) in breast cancer tissues when compared with the non-neoplastic breast tissues. Functional studies have also uncovered many unique roles of mucins during the progression of breast cancer, which include modulation in proliferative, invasive and metastatic potential of tumor cells. Mucins function through many unique domains that can form complex association with various signaling molecules including growth factor receptors and intercellular adhesion molecules. While there is growing information about mucins in various malignancies including breast cancer, no focused review is there on the expression and functional roles of mucins in breast cancer. In this present review, we have discussed the differential expression and functional roles of mucins in breast cancer. The potential of mucins as diagnostic and prognostic markers and as therapeutic targets in breast cancer have also been discussed.     

Identification of MUC1 proteolytic cleavage sites in vivo

Mucins are high molecular weight glycoproteins that provide a protective layer on epithelial surfaces and are involved in cell-cell interactions, signaling, and metastasis. The identification of several membrane-tethered mucins, including MUC1, MUC3, MUC4, and MUC12, has incited interest in the processing of these mucins and the mechanisms that govern their release from the cell surface. MUC1 consists of an extracellular subunit and a membrane-associated subunit. The two moieties are produced from a single precursor polypeptide by an early proteolytic cleavage event but remain associated throughout intracellular processing and transport to the cell surface. We identified the MUC1 proteolytic cleavage site and showed it to be identical in pancreas and colon cell lines and not to be influenced by the presence of heavily glycosylated tandem repeats. The MUC1 cleavage site shows homology with sequences in other cell-surface-associated proteins and may represent a common mechanism for processing of these molecules.     

Mucin (Muc) expression during pancreatic cancer progression in spontaneous mouse model: potential implications for diagnosis and therapy

Pancreatic cancer (PC) is a lethal malignancy primarily driven by activated Kras mutations and characterized by the deregulation of several genes including mucins. Previous studies on mucins have identified their significant role in both benign and malignant human diseases including PC progression and metastasis. However, the initiation of MUC expression during PC remains unknown because of lack of early stage tumor tissues from PC patients. In the present study, we have evaluated stage specific expression patterns of mucins during mouse PC progression in (KrasG12D;Pdx1-Cre (KC)) murine PC model from pancreatic intraepithelial neoplasia (PanIN) to pancreatic ductal adenocarcinoma (PDAC) by immunohistochemistry and quantitative real-time PCR. In agreement with previous studies on human PC, we observed a progressive increase in the expression of mucins particularly Muc1, Muc4 and Muc5AC in the pancreas of KC (as early as PanIN I) mice with advancement of PanIN lesions and PDAC both at mRNA and protein levels. Additionally, mucin expression correlated with the increased expression of inflammatory cytokines IFN-γ (p?相似文献   

The membrane-bound mucins: From cell signalling to transcriptional regulation and expression in epithelial cancers

The membrane-bound mucins belong to an ever-increasing family of O-glycoproteins. Based on their structure and localization at the cell surface they are thought to play important biological roles in cell–cell and cell–matrix interactions, in cell signalling and in modulating biological properties of cancer cells. Among them, MUC1 and MUC4 mucins are best characterized. Their altered expression in cancer (overexpression in the respiratory, gastro-intestinal, urogenital and hepato-biliary tracts) indicates an important role for these membrane-bound mucins in tumour progression, metastasis, cancer cell resistance to chemotherapeutics drugs and as specific markers of epithelial cancer cells. Some mechanisms responsible for MUC1 and MUC4 role in tumour cell properties have been deciphered recently. However, much remains to be done in order to understand the molecular mechanisms and signalling pathways that control the expression of membrane-bound mucins during the different steps of tumour progression toward adenocarcinoma and evaluate their potential as prognostic/diagnostic markers and as therapeutic tools. In this review we focus on the molecular mechanisms and signalling pathways known to control the expression of membrane-bound mucins in cancer. We will discuss the mechanisms of regulation at the promoter level (including genetic and epigenetic modifications) that may be responsible for the mucin altered pattern of expression in epithelial cancers.     

Mucins: A biologically relevant glycan barrier in mucosal protection

Background

The mucins found as components of mucus gel layers at mucosal surfaces throughout the body play roles in protection as part of the defensive barrier on an organ and tissue specific basis.

Scope of the review

The human MUC gene family codes up to 20 known proteins, which can be divided into secreted and membrane-associated forms each with a typical protein domain structure. The secreted mucins are adapted to cross link in order to allow formation of the extended mucin networks found in the secreted mucus gels. The membrane-associated mucins possess membrane specific domains which enable their various biological functions as part of the glycocalyx. All mucins are highly O-glycosylated and this is tissue specific and linked with specific biological functions at these locations. Mucin biology is dynamic and the processes of degradation and turnover are well integrated with biosynthesis to maintain a continuous mucosal protection against all external aggressive forces. Interaction of mucins with microflora plays an important role in normal function. Mucins are modified in a variety of diseases and this may be due to abberant mucin peptide or glycosylation.

Major conclusions

Mucins represent a family of glycoprotein having fundamental roles in mucosal protection and communication with external environment.

General significance

The review emphasises the nature of mucins as glycoproteins and their role in presenting an array of glycan structures at the mucosal cell surface.     

MUC genes: mucin or not mucin? That is the question

Mucins are macromolecules lying the cells in contact with external environment and protect the epithelium against constant attacks such as digestive fluids, microorganisms, pollutants, and toxins. Mucins are the main components of mucus and are synthesized and secreted by specialized cells of the epithelium (goblet cells, cells of mucous glands) or non mucin-secreting cells. Human mucin genes show common features: large size of their mRNAs, large nucleotide tandem repeat domains, complex expression both at tissular and cellular level. Since 1987, 21 MUC symbols have been used to designate genes encoding O-glycoproteins containing tandem repeat domains rich in serine, threonine and proline. Some of these genes encode true mucins while others encode non mucin adhesion O-glycoproteins. In this paper, we propose a classification based on sequence similarities and expression areas. Two main families can be distinguished: secreted mucins or gel-forming mucins (MUC2, MUC5AC, MUC5B, MUC6), and membrane-bound mucins (MUC1, MUC3, MUC4, MUC12, MUC17). Muc-deficient mice will provide important models in the study of functional relationships between these two mucin families.     

Four modes of adhesion are used during Helicobacter pylori binding to human mucins in the oral and gastric niches

Background:  Helicobacter pylori causes peptic ulcer disease and gastric cancer, and the oral cavity is likely to serve as a reservoir for this pathogen. We investigated the binding of H. pylori to the mucins covering the mucosal surfaces in the niches along the oral to gastric infection route and during gastric disease and modeled the outcome of these interactions.
Materials and Methods:  A panel of seven H. pylori strains with defined binding properties was used to identify binding to human mucins from saliva, gastric juice, cardia, corpus, and antrum of healthy stomachs and of stomachs affected by gastritis at pH 7.4 and 3.0 using a microtiter-based method.
Results:  H. pylori binding to mucins differed substantially with the anatomic site, mucin type, pH, gastritis status, and H. pylori strain all having effect on binding. Mucins from saliva and gastric juice displayed the most diverse binding patterns, involving four modes of H. pylori adhesion and the MUC5B, MUC7, and MUC5AC mucins as well as the salivary agglutinin. Binding occurred via the blood-group antigen-binding adhesin (BabA), the sialic acid-binding adhesin (SabA), a charge/low pH-dependent mechanism, and a novel saliva-binding adhesin. In the healthy gastric mucus layer only BabA and acid/charge affect binding to the mucins, whereas in gastritis, the BabA/Le^b-dependent binding to MUC5AC remained, and SabA and low pH binding increased.
Conclusions:  The four H. pylori adhesion modes binding to mucins are likely to play different roles during colonization of the oral to gastric niches and during long-term infection.     

Expression profiles of MUC1, MUC2, and MUC4 mucins in human neoplasms and their relationship with biological behavior

Mucins are high molecular weight glycoproteins that play important roles in carcinogenesis or tumor invasion. To clarify the relationship of the expression patterns of mucins in human neoplasms with their biological behavior, we examined the expression profiles of MUC1, MUC2, and MUC4 mucins in various human neoplasms using immunohistochemistry and in situ hybridization, and compared them with clinicopathologic factors including outcome of the patients. MUC1 or MUC4 expression is related with the aggressive behavior of human neoplasms and a poor outcome of the patients. In contrast, MUC2 expression tends to be related with the indolent behavior of human neoplasms and a favorable outcome of the patients, although indolent pancreatobiliary neoplasms sometimes show invasive growth with MUC1 expression in the invasive areas. The expression of MUC2 mucin in indolent pancreatobiliary neoplasms coincided with expression of MUC2 mRNA. Our recent studies to clarify the MUC2 gene regulation mechanism disclosed that DNA methylation and histone modification in the 5' flanking region of the MUC2 promoter may play an important role. Further studies of the epigenetics also in MUC1 and MUC4 gene expression may be needed to understand the relationship between the expression of mucins in human neoplasms with their biological behavior.     

Mucin in cancer: a stealth cloak for cancer cells

Mucins are high molecular-weight epithelial glycoproteins and are implicated in many physiological processes, including epithelial cell protection, signaling transduction, and tissue homeostasis. Abnormality of mucus expression and structure contributes to biological properties related to human cancer progression. Tumor growth sites induce inhospitable conditions. Many kinds of research suggest that mucins provide a microenvironment to avoid hypoxia, acidic, and other biological conditions that promote cancer progression. Given that the mucus layer captures growth factors or cytokines, we propose that mucin helps to ameliorate inhospitable conditions in tumor-growing sites. Additionally, the composition and structure of mucins enable them to mimic the surface of normal epithelial cells, allowing tumor cells to escape from immune surveillance. Indeed, human cancers such as mucinous carcinoma, show a higher incidence of invasion to adjacent organs and lymph node metastasis than do non-mucinous carcinoma. In this mini-review, we discuss how mucin provides a tumor-friendly environment and contributes to increased cancer malignancy in mucinous carcinoma.     

MUC17, a novel membrane-tethered mucin

Membrane mucins have several functions in epithelial cells including cytoprotection, extravasation during metastases, maintenance of luminal structure, and signal transduction. In this paper we describe a large membrane mucin expressed in the normal intestine. This novel mucin, designated MUC17, contains an extended, repetitive extracellular glycosylation domain and a carboxyl terminus with two EGF-like domains, a SEA module domain, a transmembrane domain, and a cytoplasmic domain with potential serine and tyrosine phosphorylation sites. RNA blot analysis and in situ hybridization indicates that MUC17 is expressed in select pancreatic and colon cancer cell lines and in intestinal absorptive cells. Radiation hybrid mapping localized MUC17 to chromosome 7q22 where it resides in close proximity with three other membrane mucin genes, MUC3A, MUC3B, and MUC12. Thus, these membrane mucins reside together in a gene cluster, but are expressed in different tissues and are likely to have different functions as well.     

The MUC family: an obituary

Mucins are glycoproteins that are common on the surfaces of many epithelial cells; they are deemed to mediate many interactions between these cells and their milieu. Several of these mucins form the mucus layer that is found in many hollow organs. The biophysical properties of mucins are related to their extensive O-linked glycosylation rather than directly to their polypeptide sequences. Despite the frequent absence of sequence homology, many human genes encoding mucins have been named MUC followed by a number, unjustly suggesting the existence of one large gene family. In this article, it is suggested that the mucin genes be renamed according to their sequence homologies.     

MicroRNA-200c Modulates the Expression of MUC4 and MUC16 by Directly Targeting Their Coding Sequences in Human Pancreatic Cancer

Transmembrane mucins, MUC4 and MUC16 are associated with tumor progression and metastatic potential in human pancreatic adenocarcinoma. We discovered that miR-200c interacts with specific sequences within the coding sequence of MUC4 and MUC16 mRNAs, and evaluated the regulatory nature of this association. Pancreatic cancer cell lines S2.028 and T3M-4 transfected with miR-200c showed a 4.18 and 8.50 fold down regulation of MUC4 mRNA, and 4.68 and 4.82 fold down regulation of MUC16 mRNA compared to mock-transfected cells, respectively. A significant reduction of glycoprotein expression was also observed. These results indicate that miR-200c overexpression regulates MUC4 and MUC16 mucins in pancreatic cancer cells by directly targeting the mRNA coding sequence of each, resulting in reduced levels of MUC4 and MUC16 mRNA and protein. These data suggest that, in addition to regulating proteins that modulate EMT, miR-200c influences expression of cell surface mucins in pancreatic cancer.     

Mucin glycoproteins in neoplasia

Mucins are high molecular weight glycoproteins that are heavily glycosylated with many oligosaccharide side chains linked O-glycosidically to the protein backbone. With the recent application of molecular biological methods, the structures of apomucins and regulation of mucin genes are beginning to be understood. At least nine human mucin genes have been identified to date. Although a complete protein sequence is known for only three human mucins (MUC1, MUC2, and MUC7), common motifs have been identified in many mucins. The pattern of tissue and cell-specific expression of these mucin genes are emerging, suggesting a distinct role for each member of this diverse mucin gene family. In epithelial cancers, many of the phenotypic markers for pre-malignant and malignant cells have been found on the carbohydrate and peptide moieties of mucin glycoproteins. The expression of carbohydrate antigens appears to be due to modification of peripheral carbohydrate structures and the exposure of inner core region carbohydrates. The expression of some of the sialylated carbohydrate antigens appears to correlate with poor prognosis and increased metastatic potential in some cancers. The exposure of peptide backbone structures of mucin glycoproteins in malignancies appears to be due to abnormal glycosylation during biosynthesis. Dysregulation of tissue and cell-specific expression of mucin genes also occurs in epithelial cancers. At present, the role of mucin glycoproteins in various stages of epithelial cell carcinogenesis (including the preneoplastic state and metastasis), in cancer diagnosis and immunotherapy is under investigation.     

A rapid separation and characterization of mucins from mouse submandibular glands by supported molecular matrix electrophoresis

Mucins are heavily glycosylated proteins with high molecular mass, and are involved in various diseases including infection, inflammation, and cancer. As easy separation method, such as gel electrophoresis, however, does not exist for mucins, due to their large molecular sizes and heterogeneities. In 2009, we published a supported molecular matrix electrophoresis (SMME) method that can be used to characterize mucins. For SMME analysis, mucins have been enriched by ultrafiltration of trypsin digests using a 100 KDa cutoff filter. However, this enrichment results in a loss of protein identification capability using proteomic approaches. In this study, we describe a simple enrichment of mucins without trypsinization for SMME analysis. The enrichment was developed using a porcine submandibular gland and then was applied to study and compare mouse submandibular glands between young and aged mice. From mouse submandibular glands, hyaluronic acid and some mucins were observed by SMME. One of the mucins was identified as MUC10 by proteomic analysis of the band on the SMME membrane and immunostaining using anti-MUC10 antibody. A major O-glycan of MUC10 was determined to be NeuAcα2-3Galβ1-3GalNAc. Furthermore, our experiments revealed that the concentrations of these molecules were lower in aged mice than in young mice, and that an unknown mucin-like molecule was detected only from the aged mouse submandibular gland.     

Diverse glycosylation of MUC1 and MUC2: potential significance in tumor immunity

Mucins are major epithelial luminal surface proteins and function as a physical and biological barrier protecting mucous epithelia. Diverse glycosylation of mucins potentially provides a basis for tissue-specific interaction with the milieu. When mucins are associated with malignant epithelial cells, they not only protect these cells from a host environment during metastatic dissemination but also generate immunogenic epitopes which are used by the host in the detection and immunological elimination of carcinoma cells potentially depending upon their status of glycosylation. Diverse mucin structures are generated by the combination of different core peptides, of which 10 have been reported so far, multiple types of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (pp-GalNAc-Ts), and the consequences of stepwise glycosylation events. For example, the mucin 1 (MUC1) associated with malignant cells was previously believed to exhibit unique features with a lower percentage of threonine and serine residues attached to N-acetylgalactosamine and/or without extension through core 2 structures. Some of MUC1-specific monoclonal antibodies and cytotoxic lymphocytes recognize the peptide sequences PDTR within the tandem repeat portion exposed by decreased degree of glycosylation. The specific arrangement of N-acetylgalactosamine residues is shown to be generated by a combination of pp-GalNAc-Ts with different specificities. The role of core 2 branching is somewhat confusing because well-known carcinoma-associated carbohydrate epitopes such as sialyl-Le(X), sialyl-Le(a), Le(Y), and others are often expressed when O-glycans are extended through core 2 branching. The other series of well-known carcinoma-associated carbohydrate structures are truncated O-glycans, conventionally called Tn and sialyl-Tn antigens. Interestingly, these are often found to be aligned on core polypeptides, resulting in three or more consecutive truncated O-glycans. MUC2 and other mucins, but not MUC1, have unique tandem repeats containing three or more consecutive serine or threonine residues, which potentially serve as a scaffold for trimeric Tn and sialyl-Tn epitopes. We recently found, using the MUC2 tandem repeat, that trimeric Tn is a high-affinity receptor for a calcium-type lectin expressed on the surface of histiocytic macrophages. The biosynthesis of trimeric Tn was strictly regulated by the acceptor specificity of pp-GalNAc-Ts. These results strongly suggest that variation in both glycan structures and distribution of glycans on the core polypeptides give mucins unique and diverse biological functions that play essential roles in carcinoma-host and other cellular interactions.     

唾液和泪液中主要黏蛋白MUC5B和MUC7协同互补功能

黏蛋白家族中MUC5B和MUC7是人体唾液和泪液中的两种主要黏蛋白成分,早期被认为在非特异性免疫应答、黏液构成以及组织功能的维护等方面发挥作用。近年来,它们的一些新功能如抗HIV、协助非黏蛋白功能的发挥等作用逐渐被发现,故其生理和病理意义也因传统观念的修正而得到扩充。由于MUC5B和MUC7在唾液和泪液中的存在方式并不完全相同,对它们在不同环境条件下功能的相似性、差异性和互补性的研究,具有重要的现实意义。