TNF-alpha induces MUC1 gene transcription in lung epithelial cells: its signaling pathway and biological implication

TNF-α is a key regulator of MUC1, an anti-inflammatory molecule, during airway Pseudomonas aeruginosa infection

Muc1 is a heterodimeric mucin that is expressed on the apical surface of airway epithelial cells as well as hematopoietic cells. Both in vivo and in vitro studies revealed that Muc1 suppresses inflammatory responses induced by Pseudomonas aeruginosa (PA). In this study, we sought to determine, using intact animals (C57BL/6 mice), whether the expression of Muc1 is important during airway PA infection, and how Muc1 levels are controlled during inflammation. Our results showed that: (1) Muc1 levels in the wild-type (WT) mice were initially low, but gradually increased after PA inhalation, reaching a peak on Day 2, remaining elevated until Day 4, and then gradually decreasing to basal levels on Day 7; (2) TNF receptor 1(-/-) mice failed to increase Muc1 levels after PA infection; (3) after PA inhalation, more inflammatory cells were present in the bronchoalveolar lavage fluid from either Muc1(-/-) or TNF receptor(-/-) mice compared with their WT control animals; (4) more apoptotic neutrophils were present in bronchoalveolar lavage fluid from WT mice compared with Muc1(-/-) mice. We conclude that Muc1(-/-) mice are more inflammatory than WT mice during airway PA infection as a result of both an increase in neutrophil influx and a decrease in neutrophil apoptosis. These results suggest that the up-regulation of Muc1 during airway PA infection might be crucial for suppressing excessive and prolonged inflammatory responses, and is induced mainly by TNF-α, the key proinflammatory mediator.

Anti-inflammatory effect of MUC1 during respiratory syncytial virus infection of lung epithelial cells in vitro

MUC1 is a transmembrane glycoprotein expressed on the apical surface of airway epithelial cells and plays an anti-inflammatory role during airway bacterial infection. In this study, we determined whether the anti-inflammatory effect of MUC1 is also operative during the respiratory syncytial virus (RSV) infection. The lung epithelial cell line A549 was treated with RSV, and the production of TNFalpha and the levels of MUC1 protein were monitored temporally during the course of infection by ELISA and Western blot analysis. Small inhibitory RNA (siRNA) transfection was utilized to assess the role of MUC1 in regulating RSV-mediated inflammatory responses by lung epithelial cells. Our results revealed that: 1) following RSV infection, an increase in MUC1 level was preceded by an increase in TNFalpha production and completely inhibited by soluble TNF receptor (TNFR); and 2) knockdown of MUC1 using MUC1 siRNA resulted in a greater increase in TNFalpha level following RSV infection compared with control siRNA treatment. We conclude that the RSV-induced increase in the TNFalpha levels upregulates MUC1 through its interaction with TNFR, which in turn suppresses further increase in TNFalpha by RSV, thus forming a negative feedback loop in the control of RSV-induced inflammation. This is the first demonstration showing that MUC1 can suppress the virus-induced inflammatory responses.

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.

Tumor necrosis factor-alpha-mediated regulation of the inositol 1,4,5-trisphosphate receptor promoter

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, has been implicated as a central mediator in multiple homeostatic and pathologic processes. Signaling cascades downstream of its cellular cognate receptors, as well as the resultant transcriptional responses have received intense interest in regards to how such signals impact cellular physiology. Notably, TNF-alpha was shown to potentiate neuronal Ca(2+) signaling by enhancing type-1 inositol 1,4,5-trisphosphate receptor (IP(3)R) steady-state mRNA levels. In the present study, we sought to determine the promoter region ultimately responsive to TNF-alpha exposure. We report that a sequence encompassing a specificity protein 1 (SP-1) binding site is necessary for TNF-alpha regulation. Electrophoretic mobility shift analysis demonstrated specific binding to this sequence, while site-directed mutagenesis of this site abrogated both JNK-mediated regulation as well as transcription factor binding. Expression of a dominant-negative SP-1 eliminated both the enhanced promoter activity and the elevated IP(3)R-mediated Ca(2+) signals observed with TNF-alpha exposure. Overall, these data delineate a key pathway by which TNF-alpha in a neuronal environment modulates IP(3)R expression and intracellular Ca(2+) homeostasis.

Hypoxia enhances MUC1 expression in a lung adenocarcinoma cell line

Expression of a transmembrane mucin MUC1 is emphasized in most cases of carcinoma. High expression of MUC1 is closely associated with cancer progression and metastasis, leading to poor prognosis. However, little is known about how MUC1 is overexpressed in malignant tumor. In this study, we demonstrated that: (1) Hypoxia, a typical feature of malignant tumor, enhanced the expression of MUC1 mRNA and protein in a human lung adenocarcinoma cell line; (2) the hypoxia-induced increase in MUC1 mRNA was mediated by the transcriptional activity of MUC1 promoter, but not mRNA stability. Moreover; (3) CoCl(2), an inducer of Hypoxia Inducible Factor (HIF)-1alpha, increased the expression of MUC1 mRNA; and (4) HIF-1alpha-targeted siRNA but not its control siRNA decreased hypoxia-induced MUC1 mRNA. These data suggest that hypoxia enhances the expression of MUC1 through the transcriptional regulation by HIF-1alpha in a human lung epithelial cell line.

MUC1 mucin interacts with calcium-modulating cyclophilin ligand

MUC1 is an integral membrane glycoprotein expressed on epithelial and hematopoietic cells with a COOH-terminus (CT) that mediates intracellular signal transduction. To better understand MUC1-dependent signaling, we searched for proteins binding to its CT using the yeast two-hybrid system with the MUC1 CT as bait and a human epithelial cell cDNA library as prey. Of the six positive clones identified, all encoded calcium-modulating cyclophilin ligand (CAML). The MUC1 CT interacted with CAML in transformed yeast cells as revealed by growth on selective media and in situ X-alpha-galactosidase activity. Binding of the MUC1 CT to CAML in human epithelial cells was confirmed by reciprocal coimmunoprecipitations, confocal microscopy, protein crosslinking, and coupled transcription/translation analyses. By deletion mutagenesis, the NH(2)-terminus of CAML was responsible for binding to the MUC1 CT. Finally, transfection of cells with plasmids encoding MUC1 and CAML increased intracellular calcium levels compared with cells transfected with either plasmid alone, suggesting a possible biological significance of the MUC1-CAML interaction.

MUC1 mucin: a peacemaker in the lung

MUC1 is a membrane-tethered mucin expressed on the surface of epithelial cells lining mucosal surfaces. Recent studies have begun to elucidate the physiologic function of MUC1 in the airways, pointing to an antiinflammatory role that is initiated late in the course of bacterial infection and is mediated through inhibition of TLR signaling. These new findings have great potential for clinical applications in controlling excessive and prolonged lung inflammation. This review briefly summarizes the protein structural features of MUC1 relevant to its function, the discovery of its antiinflammatory properties, and potential directions for future avenues of study.

Transcriptional control of the expression of MUC1

MUC1 is a multifunctional cell surface glycoprotein that modulates cell adhesion, protects mucosa from infection and enzymatic attack, lubricates cell surfaces, participates in multiple signal-transduction pathways and is overexpressed by many tumors. MUC1 levels change dynamically in various cellular contexts. The primary mechanism for controlling MUC1 expression appears to be transcriptional through a complex combination of often overlapping regulatory motifs that control both tissue specificity and overall rate of transcription. This review will summarize the current knowledge of the factors known to control MUC1 transcriptional regulation, including cytokines, steroid hormones and the growth factors they stimulate, as well as suggest how this information may be exploited in the future to control MUC1 expression in specific biological contexts.

Mucins in the mucosal barrier to infection

The mucosal tissues of the gastrointestinal, respiratory, reproductive, and urinary tracts, and the surface of the eye present an enormous surface area to the exterior environment. All of these tissues are covered with resident microbial flora, which vary considerably in composition and complexity. Mucosal tissues represent the site of infection or route of access for the majority of viruses, bacteria, yeast, protozoa, and multicellular parasites that cause human disease. Mucin glycoproteins are secreted in large quantities by mucosal epithelia, and cell surface mucins are a prominent feature of the apical glycocalyx of all mucosal epithelia. In this review, we highlight the central role played by mucins in accommodating the resident commensal flora and limiting infectious disease, interplay between underlying innate and adaptive immunity and mucins, and the strategies used by successful mucosal pathogens to subvert or avoid the mucin barrier, with a particular focus on bacteria.

MUC1 mucin is a negative regulator of toll-like receptor signaling

MUC1 (MUC1 in humans and Muc1 in nonhuman species) is a transmembrane mucin-like glycoprotein expressed in epithelial cells lining various mucosal surfaces as well as hematopoietic cells. Recently, we showed that Muc1(-/-) mice exhibited greater inflammatory responses to Pseudomonas aeruginosa or its flagellin compared with their wild-type littermates, and our studies with cultured cells revealed that MUC1/Muc1 suppressed the Toll-like receptor (TLR) 5 signaling pathway, suggesting its anti-inflammatory role. Here we demonstrate that other TLR signaling (TLR2, 3, 4, 7, and 9) is also suppressed by MUC1/Muc1. The results from this study suggest that MUC1/Muc1 may play a crucial role during airway infection and inflammation by various pathogenic bacteria and viruses.