Distribution of MUC1 in the normal human oral cavity is localized to the ducts of minor salivary glands

Internal reference genes with the potential for normalizing quantitative PCR results for oral fluid specimens

In basic research, testing of oral fluid specimens by real-time quantitative polymerase chain reaction (qPCR) has been used to evaluate changes in gene expression levels following experimental treatments. In diagnostic medicine, qPCR has been used to detect DNA/RNA transcripts indicative of bacterial or viral infections. Normalization of qPCR using endogenous and exogenous reference genes is a well-established strategy for ensuring result comparability by controlling sample-to-sample variation introduced during sampling, storage, and qPCR testing. In this review, the majority of recent publications in human (n = 136) and veterinary (n = 179) medicine did not describe the use of internal reference genes in qPCRs for oral fluid specimens (52.9% animal studies; 57.0% human studies). However, the use of endogenous reference genes has not been fully explored or validated for oral fluid specimens. The lack of valid internal reference genes inherent to the oral fluid matrix will continue to hamper the reliability, reproducibility, and generalizability of oral fluid qPCR assays until this issue is addressed.

Characterization of tumor-associated MUC1 and its glycans expressed in mucoepidermoid carcinoma

Mucoepidermoid carcinoma (MEC) is one of the most frequently misdiagnosed tumors. Glycans are modulated by malignant transformation. Mucin 1 (MUC1) is a mucin whose expression is upregulated in various tumors, including MEC, and it has previously been investigated as a diagnostic and prognostic tumor marker. The present study aimed to reveal the differences in the mucin glycans between MEC and normal salivary glands (NSGs) to discover novel diagnostic markers. Soluble fractions of salivary gland homogenate prepared from three MEC salivary glands and 7 NSGs were evaluated. Mucins in MEC and NSGs were separated using supported molecular matrix electrophoresis, and stained with Alcian blue and monoclonal antibodies. The glycans of the separated mucins were analyzed by mass spectrometry. MUC1 was found in MEC but not in NSGs, and almost all glycans of MUC1 in MEC were sialylated, whereas the glycans of mucins in NSGs were less sialylated. The core 2 type glycans, (Hex)₂(HexNAc)₂(NeuAc)₁ and (Hex)₂(HexNAc)₂(NeuAc)₂, were found to be significantly abundant glycans of MUC1 in MEC. MEC markedly produced MUC1 modified with sialylated core 2 glycans. These data were obtained from the soluble fractions of salivary gland homogenates. These findings provide a basis for the utilization of MUC1 as a serum diagnostic marker for the preoperative diagnosis of MEC.

Regulation of mucin 1 expression and its relationship with oral diseases

OBJECTIVE

The aim of this study is to describe the polymorphic mucin 1 (MUC1), and to provide an overview of the known complex and multiple functions of MUC1 in normal oral mucosa and oral mucosal lesions in compromised situations as well as exploring the challenges associated with the heterogeneous nature of MUC1. We will review the current knowledge and provide insights into the future management possibilities of using MUC1 as a therapeutic agent.

METHODS

A literature search of the electronic databases included MEDLINE (1966 -December 2019) and hand searches of cross-references were undertaken using terms related to mucins, MUC1.

RESULTS

MUC1 is a large transmembrane glycoprotein expressed on the apical surface of most of epithelial cell surfaces. Not only is it involved in lubrication, cell surface hydration, and protection against degrading enzymes, MUC1 also promotes abnormal cellular signalling, angiogenesis, anti-adhesion and tumorigenesis. Aberrant glycosylation, overexpression, loss of apical constraint are characteristics of the transformation of a normal cell to a cancerous cell. This review summarizes studies of MUC1 expression and function with a special emphasis on oral epithelial cells in normal and abnormal conditions. In addition, current knowledge of MUC1 and unexplored areas of MUC1 are presented.

CONCLUSION

MUC1 is an archetypical transmembrane protein, the presence of MUC1 in ectopic regions may lead to dysregulation of certain enzymes and activation of various pathways, favouring the development of inflammatory responses and tumour formation. This review examines the potential of MUC1 in the development of future therapeutics.

Diversity of mucins in labial glands of infants

Mucins as highly glycosylated proteins comprise multiple functions like protection, homeostasis, immune defense, cell signaling. Various epithelial tissues including glandular structures express different specific mucin types. We investigated labial salivary glands in infants for the occurrence of MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, and MUC7 by immunohistochemistry. MUC1 and MUC4 were detected in serous and ductal glandular cells, partially intensified at the apical plasma membrane. MUC3 was found in ductal glandular cells and in myoepithelial cells. MUC5B exhibited a mosaic expression pattern in mucous glandular endpieces. MUC2 and MUC7 were abundant in serous acini. Glandular structures were negative for MUC5AC. A comprehensive study of specific mucins in labial salivary glands of infants was presented for the first time. As a representative of the minor salivary glands, labial glands are, due to their localization, directly exposed to environmental influences. The distribution of a broad spectrum of mucins in infantile labial glands indicates their importance early in human development to sustain oral health.

Importance of digestive mucus and mucins for designing new functional food ingredients

The mucus, mainly composed of the glycoproteins mucins, is a rheological substance that covers the intestinal epithelium and acts as a protective barrier against a variety of harmful molecules, microbial infection and varying lumen environment conditions. Alterations in the composition or structure of the mucus could lead to various diseases such as inflammatory bowel disease or colorectal cancer. Recent studies revealed that an exogenous intake of probiotic bacteria or other dietary components (such as bioactive peptides and probiotics) derived from food influence mucus layer properties as well as modulate gene expression and secretion of mucins. Therefore, the use of such components for designing new functional ingredients and then foods, could constitute a novel approach to preserve the properties of mucus. After presenting some aspects of the mucus and mucins in the gastrointestinal tract as well as mucus role in the gut health, this review will address role of dietary ingredients in improving mucus/mucin production and provides new suggestions for further investigations of how dietary ingredients/probiotics based functional foods can be developed to maintain or improve the gut health.

The physico-chemistry of adhesions of protein resistant and weak polyelectrolyte brushes to cells and tissues

The non-specific adhesion of polymers and soft tissues is of great interest to the field of biomedical engineering, as it will shed light on some of the processes that regulate interactions between scaffolds, implants and nanoparticles with surrounding tissues after implantation or delivery. In order to promote adhesion to soft tissues, a greater understanding of the relationship between polymer chemistry and nanoscale adhesion mechanisms is required. In this work, we grew poly(dimethylaminoethyl methacrylate) (PDMAEMA), poly(acrylic acid) (PAA) and poly(oligoethylene glycol methacrylate) (POEGMA) brushes from the surface of silica beads, and investigated their adhesion to a variety of substrates via colloidal probe-based atomic force microscopy (AFM). We first characterised adhesion to a range of substrates with defined surface chemistry (self-assembled monolayers (SAMs) with a range of hydrophilicities, charge and hydrogen bonding), before studying the adhesion of brushes to epithelial cell monolayers (primary keratinocytes and HaCaT cells) and soft tissues (porcine epicardium and keratinized gingiva). Adhesion assays to SAMs reveal the complex balance of interactions (electrostatic, van der Waals interactions and hydrogen bonding) regulating the adhesion of weak polyelectrolyte brushes. This resulted in particularly strong adhesion of PAA brushes to a wide range of surface chemistries. In turn, colloidal probe microscopy on cell monolayers highlighted the importance of the glycocalyx in regulating non-specific adhesions. This was also reflected by the adhesive properties of soft tissues, in combination with their mechanical properties. Overall, this work clearly demonstrates the complex nature of interactions between polymeric biomaterials and biological samples and highlights the need for relatively elaborate models to predict these interactions.

Oral epithelial MUC1 and oral health

OBJECTIVES

To provide information about MUC1, epithelial membrane-bound mucin, in terms of its role in oral health.

DESIGN

The expression and functional roles of MUC1 in the oral mucosa and salivary glands were reviewed. Information on the modulation of oral mucosal epithelial MUC1 expression compared with that of endometrial epithelial MUC1 expression was also reviewed. In addition, the possible associations between oral mucosal epithelial MUC1 and oral diseases were explored.

RESULTS

MUC1 is expressed in the oral mucosa and major and minor salivary glands. Protection of oral mucosal surfaces is believed to be the main function of oral mucosal epithelial MUC1. Its role in the salivary glands is thought to facilitate salivary flow through the ductal system. Information on the role of MUC1 in signal transduction and modulation of immune function in the oral cavity is sparse. However, the possible roles of MUC1 in dry mouth conditions, burning mouth syndrome, dental caries, and oral candidiasis have been suggested.

CONCLUSIONS

Despite limited information, it is clear that oral epithelial MUC1 plays an important role in oral health. Further research evaluating the physiological and pathological roles of MUC1 in the oral cavity is warranted.

Salivary mucins in host defense and disease prevention

Mucus forms a protective coating on wet epithelial surfaces throughout the body that houses the microbiota and plays a key role in host defense. Mucins, the primary structural components of mucus that creates its viscoelastic properties, are critical components of the gel layer that protect against invading pathogens. Altered mucin production has been implicated in diseases such as ulcerative colitis, asthma, and cystic fibrosis, which highlights the importance of mucins in maintaining homeostasis. Different types of mucins exist throughout the body in various locations such as the gastrointestinal tract, lungs, and female genital tract, but this review will focus on mucins in the oral cavity. Salivary mucin structure, localization within the oral cavity, and defense mechanisms will be discussed. These concepts will then be applied to present what is known about the protective function of mucins in oral diseases such as HIV/AIDS, oral candidiasis, and dental caries.

Growth Factor Independence-1 (Gfi1) Is Required for Pancreatic Acinar Unit Formation and Centroacinar Cell Differentiation

BACKGROUND & AIMS

The genetic specification of the compartmentalized pancreatic acinar/centroacinar unit is poorly understood. Growth factor independence-1 (Gfi1) is a zinc finger transcriptional repressor that regulates hematopoietic stem cell maintenance, pre-T-cell differentiation, formation of granulocytes, inner ear hair cells, and the development of secretory cell types in the intestine. As GFI1/Gfi1 is expressed in human and rodent pancreas, we characterized the potential function of Gfi1 in mouse pancreatic development.

METHODS

Gfi1 knockout mice were analyzed at histological and molecular levels, including qRT-PCR, in situ hybridization, immunohistochemistry, and electron microscopy.

RESULTS

Loss of Gfi1 impacted formation and structure of the pancreatic acinar/centroacinar unit. Histologic and ultrastructural analysis of Gfi1-null pancreas revealed specific defects at the level of pancreatic acinar cells as well as the centroacinar cells (CACs) in Gfi1^-/- mice when compared with wild-type littermates. Pancreatic endocrine differentiation, islet architecture, and function were unaffected. Organ domain patterning and the formation of ductal cells occurred normally during the murine secondary transition (E13.5-E14.5) in the Gfi1^-/- pancreas. However, at later gestational time points (E18.5), expression of cellular markers for CACs was substantially reduced in Gfi1^-/- mice, corroborated by electron microscopy imaging of the acinar/centroacinar unit. The reduction in CACs was correlated with an exocrine organ defect. Postnatally, Gfi1 deficiency resulted in severe pancreatic acinar dysplasia, including loss of granulation, autolytic vacuolation, and a proliferative and apoptotic response.

CONCLUSIONS

Gfi1 plays an important role in regulating the development of pancreatic CACs and the function of pancreatic acinar cells.

Mucin levels in saliva of adolescents with dental caries

Background

Human saliva, a complex secretion that contains a mixture of inorganic and organic molecules, plays an essential role in the maintenance of oral health. Mucins are the major macromolecular component of the secretion and are considered the first line of defense for epithelial tissues. The aim of this study was to compare levels of mucins (MUC5B, MUC7, and MUC1) in saliva of young subjects with dental caries.

Material/Methods

All patients had DMF (decay/missing/filled) higher than value 0. Eight subjects with DMF=3 (control group) and 27 adolescents with DMF >11 (research group) were recruited for this study. Clinical evaluation procedures were oral examination, including tooth, periodontal, oral mucosal status, and collection of saliva samples. Saliva was collected for mucin assay. Enzyme-linked immunosorbent assay was used to quantitate MUC5B, MUC7, and MUC1.

Results

Our results indicate that adolescents with very high intensity of dental caries disease had increased levels of MUC1 and MUC5B. The membrane mucin MUC1 protein levels in the group with DMF>11 (research group) were higher compared to the group with DMF=3 (control group), and the increase was statistically significant (p=0.011). Similarly, secreted mucin MUC5B protein levels were higher (p=0.06) in the group with DMF>11 (research group). Although MUC7 protein levels were slightly reduced in symptomatic subjects, the decrease was statistically insignificant (p=0.918).

Conclusions

Our data suggest links between the production of mucins, especially MUC1 and MUC5B in saliva, and dental caries disease.

The oral cavity as a biological barrier system: design of an advanced buccal in vitro permeability model

An important area for future research lies in finding a drug delivery system across or into the oral mucosa. However, to design such systems, simplified biological models are necessary so that the mechanisms and/or interactions of interest can readily be studied. The oral epithelium is covered by a complex mucus layer, which enables exchange of nutrients and provides lubrication. However, it has been demonstrated that mucus has an impact on the mobility of nanoparticles and drug molecules. Thus, we aimed to develop an advanced buccal in vitro model for studying transport of nanoparticles, taking the mucus layer into account. First, animal mucins (porcine gastric, bovine submaxillary) were compared with natural human mucin regarding chemical and morphological structure. Second, an "external" mucus layer was prepared by a film method and deposited onto an oral cell line (TR 146), cultured on transwells®. Adherence of the mucin fibers was evaluated and the viability of the model was assessed. Nanoparticle transport studies were performed with this advanced in vitro model and an ex vivo diffusion system. The results revealed that porcine mucin is most similar to human natural mucin in chemical structure and morphology. Both the bovine and porcine mucin fibers adhered onto the oral cells: Due to the different morphology of bovine mucin, the viability of the oral cells decreased, whereas porcine mucin maintained the viability of the model for more than 48 h. Comparison of in vitro data with ex vivo data suggested reliability of the advanced buccal in vitro model. Additionally, it was demonstrated that the mucus layer in the oral cavity also acts as a strong barrier for the mobility of nanoparticles.

Recent advances in mucin immunohistochemistry in salivary gland tumors and head and neck squamous cell carcinoma

This review focuses on the immunohistochemical expression of members of the MUC-type mucin family in salivary gland tumors and head and neck squamous cell carcinomas (HNSCC). Information is available on changes in the expression levels and distribution profiles of MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7 in tumors of the salivary glands; and of MUC1, MUC2 and MUC4 in HNSCC. In salivary gland tumors the expression patterns of MUC2, MUC3, MUC5AC and MUC6 appear to be very closely correlated with the histopathological tumor type indicating their potential use to improve diagnostic accuracy in salivary gland neoplasia. Some MUC-type mucins have emerged as valuable prognostic indicators in pleomorphic adenoma, mucoepidermoid carcinoma and HNSCC. Nine antibodies directed against different MUC1 antigens have thus far been examined in HNSCC of which monoclonal antibodies DF3, HMFG-1 and Ma695 have shown significant correlations with disease outcome. The importance of taking the specific anti-MUC antibody into consideration when comparing the results of different studies on MUC expression in salivary gland tumors and HNSCC is also highlighted in this review.

MUC1 expression in the oral mucosal epithelial cells of the elderly

OBJECTIVE

MUC1 is primarily involved in the protection of epithelial surfaces. Decreases in oral mucosal defence can be a predisposing factor for the development of oral mucosal diseases in the elderly. The aim of this study was to compare MUC1 expression level in oral mucosal epithelial cells of the elderly with that of young adults.

DESIGN

Thirty elderly (mean age, 71.1±4.6 years) and thirty young (mean age, 26.4±2.4 years) adults (15 men and 15 women in each group) were included. Oral examination, including tooth, periodontal, and oral mucosal status, was performed and whole saliva samples were collected along with flow rate measurements. Precipitates of stimulated whole saliva were used for the evaluation of MUC1 expression using real-time PCR. Clarified supernatants were used for the measurement of pro-inflammatory cytokines, IL-1β, IL-6, and TNF-α.

RESULTS

There was a significant decrease in the amounts of MUC1 transcripts in elderly subjects compared with those of young subjects, a result seen in both men (0.589-fold) and women (0.547-fold). The MUC1 expression level was not correlated with salivary cytokine level but did show a significant positive correlation with the level of periodontal inflammation (r(s)=0.505, P<0.01) in the elderly group.

CONCLUSIONS

Oral mucosal defence provided by MUC1 was decreased in the elderly; this decrease may play a role in the development of oral mucosal diseases in the aged population.

MUC1 and the simple mucin-type antigens: Tn and Sialyl-Tn are differently expressed in salivary gland acini and ducts from the submandibular gland, the vestibular folds, and the soft palate

OBJECTIVE AND DESIGN

Autopsies of the submandibular gland, the vestibular folds and the soft palate from 65-87 old humans were examined to record the immunohistochemical expression of MUC1 and the simple mucin-type antigens Tn and Sialyl-Tn.

RESULTS

(1) The serous acini in the submucosal glands from the larynx and the soft palate expressed MUC1-associated glycans that were not detectable in the serous acini from the submandibular gland. (2) Virtually all the submucosal acini at oral site of the soft palate are mucous, and in contrast to mucous acini in the vestibular folds and submandibular gland, the palatinal acini in the submucosa underneath the oral mucosa showed a well-defined cytoplasmic reaction with anti-MUC1 antibodies as wells as with anti-Tn. (3) Both the mucous acini and the ducts at the oral site of the soft palate showed reaction for Sialyl-Tn while in the vestibular folds and in the submandibular gland expression for this carbohydrate was observed only in the acini. (4) The staining obtained after incubation with the Tn antibodies showed no cross localization with the staining obtained after incubation with an anti-A blood group antibody. (5) All the autopsies showed reaction in the glands after incubation with the MUC1 antibodies while some autopsies reacted with the anti-Tn antibodies and/or with the anti-Sialyl-Tn antibodies and others did not.

CONCLUSION

The mucin expression in the acini and ducts from the upper human aerodigestive tract strongly depended on the location of the glandular tissue.

Ultrastructural localization of salivary mucins MUC5B and MUC7 in human labial glands

As a result of their presence throughout the mouth in the submucosa or between muscle fibers, minor salivary glands secrete directly and continuously into the oral cavity, providing mucosal surfaces with highly glycosylated proteins that are active in bacterial aggregation and in oral tissue lubrication. In this study, we investigated the ultrastructural localization of the MUC5B and MUC7 mucins in human labial glands by means of a postembedding immunogold technique. Thin sections of normal human labial glands, obtained during surgery, were incubated with polyclonal antibodies to human salivary mucins MUC5B and MUC7, and then with gold-labeled secondary antibodies. Specific MUC5B reactivity was found in the secretory granules of mucous cells of all glands examined, and was associated with the luminal membrane of duct cells. MUC7 labeling was observed in the granules of both mucous and seromucous secretory cells of the glandular parenchyma. Quantitative analyses demonstrated that seromucous granules have higher immunogold labeling densities for MUC7 than mucous granules. Our immunohistochemical data extend the results of previous light microscopic studies of MUC5B and MUC7 localizations, pointing out the significant contribution of human labial glands in the secretion process of these two mucins.

Differential expression of MUC1 and carbohydrate antigens in primary and secondary head and neck squamous cell carcinoma

BACKGROUND

In head and neck squamous cell carcinoma (HNSCC), tumor markers may be helpful to evaluate prognosis accurately as well as to improve therapy selection. Detection of human MUC1 has been widely employed for the evaluation of carcinoma patients. This article aims to study MUC1, Tn, sTn, and Lewis antigenic expression in primary HNSCC, lymph node metastasis, and local recurrences.

METHODS

We used immunohistochemistry, tissue homogenization and differential centrifugation, isopycnic density gradient centrifugation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Western blot.

RESULTS

In primary tumors, MUC1 was detected in 80.0% of the samples; sLewis x in 23.2%, Lewis x in 45.6%, and Lewis y in 40.8%. Tn and sTn were found in 4.0% and 6.4% of samples, respectively. In metastatic lymph nodes, MUC1 showed a similar positive reaction as in primary tumors. Lewis y was detected in 20% lymph nodes whereas Lewis x, sLewis x, Tn, and sTn did not show differences. Some recurrences expressed MUC1 and only a few Lewis antigens, whereas Tn and sTn were not detected.

CONCLUSION

In primary HNSCC and metastatic nodes, a high expression of MUC1 and Lewis antigens was detected that diminished in local recurrences. We also found that differentiated tumors mainly expressed a linear pattern of MUC1CT and Lewis x.

Expression of Membrane-associated Mucins in Cultivated Human Oral Mucosal Epithelial Cells

PURPOSE

Membrane-associated mucins present in the cells of the ocular surface epithelium (MUC1, -4, and -16) are believed to contribute to the maintenance of the health and wet surface of epithelial cells. Recent studies have reported the use of cultivated oral mucosal epithelial sheet transplantation for reconstruction of the ocular surface. We studied the expression of membrane-associated mucins in cultivated human oral mucosal epithelial sheets and compared it with that in human ocular surface epithelial cells.

METHODS

Specimens (3 x 3 mm) of oral mucosal tissue were harvested from healthy volunteers. The oral mucosal epithelial cells obtained from the specimens, the corneal epithelial cells, or an oral mucosal cell line (KB cells) were cultured together with mitomycin C-treated 3T3 feeder cells on temperature-responsive culture surfaces for 2 weeks to produce stratified cell sheets. Reverse transcriptase-polymerase chain reaction was used to determine the expression of membrane-associated mucins (MUC1, -3, -4, -12, -13, -15, -16, and -17) in these cells.

RESULTS

MUC1, -4, and -16 but not -3, -12, -13, -15, or -17 mRNA was detected in the oral mucosal epithelial sheet and in the corneal epithelial sheet. KB cells, although unable to produce a stratified cell sheet, showed expression of MUC1, -12, -13, and -16 mRNA.

CONCLUSIONS

The membrane-associated mucins of the ocular surface, MUC1, -4, and -16, are expressed in human oral mucosal epithelial sheets and corneal epithelium. These membrane-associated mucins may therefore contribute to ocular surface reconstruction after oral mucosal epithelial sheet transplantation for patients with severe ocular surface disorders.

Lewis x is highly expressed in normal tissues: a comparative immunohistochemical study and literature revision

An immunohistochemical analysis was employed to determine the expression of carbohydrate antigens associated to mucins in normal epithelia. Tissue samples were obtained as biopsies from normal breast (18), colon (35) and oral cavity mucosa (8). The following carbohydrate epitopes were studied: sialyl-Lewis x, Lewis x, Lewis y, Tn hapten, sialyl-Tn and Thomsen-Friedenreich antigen. Mucins were also studied employing antibodies against MUC1, MUC2, MUC4, MUC5AC, MUC6 and also normal colonic glycolipid. Statistical analysis was performed and Kendall correlations were obtained. Lewis x showed an apical pattern mainly at plasma membrane, although cytoplasmic staining was also found in most samples. TF, Tn and sTn haptens were detected in few specimens, while sLewis x was found in oral mucosa and breast tissue. Also, normal breast expressed MUC1 at a high percentage, whereas MUC4 was observed in a small number of samples. Colon specimens mainly expressed MUC2 and MUC1, while most oral mucosa samples expressed MUC4 and MUC1. A positive correlation between MUC1VNTR and TF epitope (r=0.396) was found in breast samples, while in colon specimens MUC2 and colonic glycolipid versus Lewis x were statistically significantly correlated (r=0.28 and r=0.29, respectively). As a conclusion, a defined carbohydrate epitope expression is not exclusive of normal tissue or a determined localization, and it is possible to assume that different glycoproteins and glycolipids may be carriers of carbohydrate antigens depending on the tissue localization considered.

Regulation of mucin expression: mechanistic aspects and implications for cancer and inflammatory diseases

Mucins are large multifunctional glycoproteins whose primary functions are to protect and lubricate the surfaces of epithelial tissues lining ducts and lumens within the human body. Several lines of evidence also support the involvement of mucins in more complex biological processes such as epithelial cell renewal and differentiation, cell signaling, and cell adhesion. Recent studies have uncovered the role of select mucins in the pathogenesis of cancer, underscoring the importance of a detailed knowledge about mucin biology. Under normal physiological conditions, the production of mucins is optimally maintained by a host of elaborate and coordinated regulatory mechanisms, thereby affording a well-defined pattern of tissue-, time-, and developmental state-specific distribution. However, mucin homeostasis may be disrupted by the action of environmental and/or intrinsic factors that affect cellular integrity. This results in an altered cell behavior that often culminates into a variety of pathological conditions. Deregulated mucin production has indeed been associated with numerous types of cancers and inflammatory disorders. It is, therefore, crucial to comprehend the underlying basis of molecular mechanisms controlling mucin production in order to design and implement adequate therapeutic strategies for combating these diseases. Herein, we discuss some physiologically relevant regulatory aspects of mucin production, with a particular emphasis on aberrations that pertain to pathological situations. Our views of the achievements, the conceptual and technical limitations, as well as the future challenges associated with studies of mucin regulation are exposed.

Unique expression of MUC3, MUC5AC and cytokeratins in salivary gland carcinomas

The differential diagnosis of salivary gland carcinoma is often difficult because of the confusing histopathological features of the different types of salivary gland carcinomas. The expression of MUC3, MUC5AC, MUC6, cytokeratin (CK)7 and CK20 was studied in 20 mucoepidermoid carcinomas (MEC), 20 adenoid cystic carcinomas (AdCC), and 11 acinic cell carcinomas (ACC). All the cases (51/51, 100%) were positive for CK7, but they were not positive for CK20. All the cases (100%) of the MEC were positive for MUC5AC, while all MEC (100%) were negative for MUC3. Only two cases (10%) were positive for MUC6. All cases (100%) of AdCC were negative for MUC3, MUC5AC and MUC6. Eight cases (73%) of ACC were positive for MUC3, but all the cases (100%) were negative for MUC5AC and MUC6. It is concluded that the positive expression of MUC5AC is very unique to MEC, and that the positive expression of MUC3 is very unique to ACC. These findings will be very useful for the differential diagnosis of the salivary gland carcinomas.

MUC1 and MUC2 expression in salivary gland tumors and in non-neoplastic salivary gland tissue

The expression of MUC1 and MUC2 was studied in salivary gland tumors and non-neoplastic salivary gland tissue. Formalin-fixed paraffin-embedded specimens from 101 patients (21 pleomorphic adenomas (PA), 22 Warthin's tumors (WT), 26 adenoid cystic carcinomas (ACC), 13 acinic cell adenocarcinomas (ACA), 9 mucoepidermoid carcinomas (MC), and 10 specimens of non-neoplastic parotid and submandibular gland tissue) were immunostained. All salivary gland tumors expressed MUC1. A strong immunoreactivity was noted in WT and MC, a moderate in ACC and ACA, and a weak in PA. Strong expression of MUC2 was noted in all WT, moderate expression in MC, and weak expression in PA and ACA. All cases of ACC except for two were negative for MUC2. In general, MUC1 expression was stronger than that of MUC2. Non-neoplastic salivary gland tissue revealed a moderate MUC1 and MUC2 expression in excretory ducts and a strong expression in striated ducts. The apical plasma membrane of some serous acini expressed MUC1. Mucous acini were negative for both antigens. No change in immunoreactivity was noted in cases of chronic sclerosing sialadenitis. In conclusion, the different expression pattern of MUC1 and MUC2 in salivary gland neoplasia may be of additional value for the classification of salivary gland tumors.

Electron microscopic immunogold localization of salivary mucins MG1 and MG2 in human submandibular and sublingual glands

The human salivary mucins MG1 and MG2 are well characterized biochemically and functionally. However, there is disagreement regarding their cellular and glandular sources. The aim of this study was to define the localization and distribution of these two mucins in human salivary glands using a postembedding immunogold labeling method. Normal salivary glands obtained at surgery were fixed in 3% paraformaldehyde-0.1% glutaraldehyde and embedded in Lowicryl K4M or LR Gold resin. Thin sections were labeled with rabbit antibodies to MG1 or to an N-terminal synthetic peptide of MG2, followed by gold-labeled goat anti-rabbit IgG. The granules of all mucous cells of the submandibular and sublingual glands were intensely reactive with anti-MG1. No reaction was detected in serous cells. With anti-MG2, the granules of both mucous and serous cells showed reactivity. The labeling was variable in both cell types, with mucous cells exhibiting a stronger reaction in some glands and serous cells in others. In serous granules, the electron-lucent regions were more reactive than the dense cores. Intercalated duct cells near the acini displayed both MG1 and MG2 reactivity in their apical granules. In addition, the basal and lateral membranes of intercalated duct cells were labeled with anti-MG2. These results confirm those of earlier studies on MG1 localization in mucous cells and suggest that MG2 is produced by both mucous and serous cells. They also indicate differences in protein expression patterns among salivary serous cells.

Expression of membrane-associated mucins MUC1 and MUC4 in major human salivary glands

Mucins are high molecular weight glycoproteins secreted by salivary glands and epithelial cells lining the digestive, respiratory, and reproductive tracts. These glycoproteins, encoded in at least 13 distinct human genes, can be subdivided into gel-forming and membrane-associated forms. The gel-forming mucin MUC5B is secreted by mucous acinar cells in major and minor salivary glands, but little is known about the expression pattern of membrane-associated mucins. In this study, RT-PCR and Northern blotting demonstrated the presence of transcripts for MUC1 and MUC4 in both parotid and submandibular glands, and in situ hybridization localized these transcripts to epithelial cells lining striated and excretory ducts and in some serous acinar cells. The same cellular distribution was observed by immunohistochemistry. Soluble forms of both mucins were detected in parotid secretion after immunoprecipitation with mucin-specific antibodies. These studies have shown that membrane-associated mucins are produced in both parotid and submandibular glands and that they are expressed in different cell types than gel-forming mucins. Although the function of these mucins in the oral cavity remains to be elucidated, it is possible that they both contribute to the epithelial protective mucin layer and act as receptors initiating one or more intracellular signal transduction pathways.