Sequence analysis of the 5' region of the human DF3 breast carcinoma-associated antigen gene

MUC1 Story: Great Expectations, Disappointments and the Renaissance

In the course of studying human mucin MUC1, the attitude towards this molecule has been changing time and again. Initially, the list of presumable functions of MUC1 was restricted to protecting and lubricating epithelium. To date, it is assumed to play an important role in cell signaling as well as in all stages of oncogenesis, from malignant cell transformation to tumor dissemination. The story of MUC1 is full of hopes and disappointments. However, the scientific interest to MUC1 has never waned, and the more profoundly it has been investigated, the clearer its hidden potential turned to be disclosed. The therapeutic potential of mucin MUC1 has already been noted by various scientific groups at the early stages of research. Over forty years ago, the first insights into MUC1 functions became a strong ground for considering this molecule as potential target for anticancer therapy. Therefore, this direction of research has always been of particular interest and practical importance. More than 200 papers on MUC1 were published in 2016; the majority of them are dedicated to MUC1-related anticancer diagnostics and therapeutics. Here we review the history of MUC1 studies from the very first attempts to reveal its functions to the ongoing renaissance.

Hormonal Regulation of MUC1 Expression

Several circulating mucinous markers, including CA 15.3, MCA, CA 459, CASA, and Truquant BR, are secreted products of the polymorphic MUC1 gene, and are used as diagnostic tools in patients with breast cancer. In clinical practice the measurement of the levels of these markers in the blood can give important information on the tumor's response to treatment and its biological behavior during disease monitoring. Since the marker levels reflect the activity of the tumor, it is important to know all factors influencing the production/secretion and the blood concentrations of MUC1 mucin. Recent findings suggest that MUC1 gene expression is regulated by steroid hormones and other substances present in the serum. Such observations are very important not only because of their biological significance but also for their clinical implications, as one approach to breast cancer therapy is based on chemical hormone manipulation. Nevertheless, we have preliminarily demonstrated that endocrine treatment in breast cancer patients does not influence the circulating CA 15.3 serum levels, so changes in marker levels are related only to the clinical evolution of the tumor.

Emerging potential of natural products for targeting mucins for therapy against inflammation and cancer

Deregulated mucin expression is a hallmark of several inflammatory and malignant pathologies. Emerging evidence suggests that, apart from biomarkers, these deregulated mucins are functional contributors to the pathogenesis in inflammation and cancer. Both overexpression and downregulation of mucins in various organ systems is associated with pathobiology of inflammation and cancer. Restoration of mucin homeostasis has become an important goal for therapy and management of such disorders has fueled the quest for selective mucomodulators. With improved understanding of mucin regulation and mechanistic insights into their pathobiological roles, there is optimism to find selective non-toxic agents capable of modulating mucin expression and function. Recently, natural compounds derived from dietary sources have drawn attention due to their anti-inflammatory and anti-oxidant properties and low toxicity. Considerable efforts have been directed towards evaluating dietary natural products as chemopreventive and therapeutic agents; identification, characterization and synthesis of their active compounds; and improving their delivery and bioavailability. We describe the current understanding of mucin regulation, rationale for targeting mucins with natural products and discuss some natural products that modulate mucin expression and functions. We further discuss the approaches and parameters that should guide future research to identify and evaluate selective natural mucomodulators for therapy.

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.

MUC1 and the MUCs: A Family of Human Mucins with Impact in Cancer Biology

Mucins represent a family of glycoproteins characterized by repeat domains and a dense O-glycosylation. During the last two decades, the gene and peptide structures of various mucins as well as their glycosylation states were partly elucidated. Characteristic tumor-associated alterations of the expression patterns and glycosylation profiles were observed in biochemical, immunochemical, and histological studies and are discussed in the light of efforts to use the most prominent member in this family, MUC1, as a tumor target in anti-tumor strategies. Within this context the present review, focusing on MUC1, describes recent work on the regulation of mucin biosynthesis by cytokines and hormones, the role of mucins in cell adhesion, and their interaction with the immune system. Important aspects of clinical diagnostics based on mucin antigens are discussed, including the application of tumor serum assays and the significance of numerous studies revealing correlations between the expression of peptide cores or mucin-associated carbohydrates and clinicopathological parameters like tumor progression and prognosis.

MUC3 and MCA serum levels and steroid receptor content in breast cancer

Mucins are an important class of complex glycoproteins expressed by many epithelial cells and their malignant counterparts. The aim of this study was to determine the serum levels of MUC3 and mucin-like carcinoma-associated antigen (MCA) in patients with primary breast cancer and to analyze the possible relationships between these two mucins and the steroid receptor status. The preoperative basal serum levels of MUC3 (ELISA assay with monoclonal antibody 1143/B7) and MCA (EIA assay with anti-MCA mouse monoclonal antibody b-12) were determined in 44 patients with breast cancer while estrogen receptor (ER) and progesterone receptor (PgR) levels were measured by the dextran-coated charcoal method in the cytosol of neoplastic tissue. MUC3 was expressed in 43/44 serum samples while high MCA serum levels were found in 16/44 only; the mean values of both markers did not correlate with menopausal status, tumor size, nodal involvement or ER. The only significant difference observed was a lower median value of MCA in patients with small tumors (T1-T2). No statistically significant correlation between MUC3 and MCA, MUC3 and ER or MCA and ER was observed; a statistically significant direct correlation between MUC3 and PgR+ status and a statistically significant inverse correlation between MCA and PgR+ were observed. Our results suggest that further investigations are necessary to establish whether progesterone can modulate MUC3 and MCA expression in breast cancer.

Estrogen receptor does not directly regulate the murine Muc-1 promoter

Muc-1 is a heavily O-glycosylated, type 1 membrane glycoprotein present on the surface of polarized secretory uterine epithelial cells. Previous studies have shown that treatment of ovariectomized mice with 17-beta-estradiol (E2) strongly induces Muc-1 mRNA expression in an estrogen receptor (ER)-mediated fashion in the uterus. In this study, the 5.4 kb Muc-1 gene promoter has been isolated from a mouse genomic library and the proximal 1.85 kb region has been sequenced. Sequence analysis revealed the presence of one potential full estrogen response element (ERE) (GCTCGCGGTGACC) located at -748 to -735 bp in the Muc-1 promoter and several potential ERE half sites. Electrophoretic mobility shift assays (EMSA) showed that neither ERalpha nor ERbeta bind efficiently to this sequence. Transient cotransfection assays using constructs containing various deletion mutations of the 5' Muc-1 flanking sequences showed that E2 had no direct stimulation on promoter-driven reporter in NMuMG cells or primary mouse uterine epithelial cells, but did stimulate a consensus ERE CAT-reporter gene activity. In addition, E2-treatment of Weg-ER cells, a mouse uterine epithelial cell line stably expressing human ERalpha, did not restore endogenous Muc-1 expression or activate Muc-1 promoter-driven CAT activity. These results indicate that regions of the Muc-1 gene promoter within -1838 to +43 bp do not respond to E2 and ER stimulation and that ER alone is not sufficient to restore Muc1 gene expression. Deletion analyses also revealed that the sequence between -73 and +43 bp of the Muc-1 promoter is the minimal promoter region required for maximal Muc-1 promoter activity. Collectively, these results demonstrate that ER does not directly regulate the 1.85 kb murine Muc-1 gene promoter. Therefore, E2 control of uterine Muc-1 gene expression is likely to be indirect, i.e. mediated by stromal cell-derived factors.

Interaction of the DF3/MUC1 breast carcinoma-associated antigen and beta-catenin in cell adhesion

The DF3/MUC1 mucin-like glycoprotein is aberrantly overexpressed in human breast carcinomas. The functional role of DF3 is unknown. The present studies demonstrate that DF3 associates with beta-catenin. Similar findings have been obtained for gamma-catenin but not alpha-catenin. DF3, like E-cadherin and the adenomatous polyposis coli gene product, contains an SXXXXXSSL site that is responsible for direct binding to beta-catenin. The results further demonstrate that interaction of DF3 and beta-catenin is dependent on cell adhesion. These findings and the role of beta-catenin in cell signaling support a role for DF3 in the adhesion of epithelial cells.

Detection of intraoperative tumor cell dissemination in patients with breast cancer by use of reverse transcription and polymerase chain reaction

BACKGROUND

Animal studies have shown that malignant cells are shed into the blood stream during surgical resection of a primary tumor and that this may enhance the development of metastases. The evidence for tumor cell dissemination during surgical manipulation of human cancer is unclear. We have applied the technique of reverse transcription and polymerase chain reaction to detect circulating tumor cells in peripheral venous blood of patients with breast cancer perioperatively.

METHODS

To target breast-specific gene transcription complementary DNA was prepared by reverse transcription of blood messenger RNA with oligonucleotide primers unique to CK18 and DF3 antigens. Preliminary assessment of specificity showed that the DF3 antigen was more suitable than CK18 for the purpose of this study. Assessment of sensitivity showed that as few as 10 tumor cells per 5 ml blood could be identified by this method. Peripheral blood samples were obtained by venepuncture from patients before, during, and 24 hours after breast surgery (nine malignant and three benign).

RESULTS

In the group of patients with malignant disease, tumor cells were detected in one patient before operation and four patients during operation. No tumor cells were detected in the postoperative samples nor in any of the samples of patients with benign disease.

CONCLUSIONS

These findings suggest that tumor manipulation during operation encourages tumor cell dissemination.

Does a novel form of the breast cancer marker protein, MUC1, act as a receptor molecule that modulates signal transduction?

Molecular analysis of a protein highly expressed in human breast cancer, indicates the presence of a polymorphic tandem repeat domain that encodes a conserved 20 amino acid repeat motif rich in serine and threonine residues that in the mature protein, designated MUC1, are linked via O-glycosidic linkages to sugar residues. Recent studies performed in our laboratory have led to the molecular characterization of a novel MUC1 repeat array minus mRNA, generated by an alternative splicing event that deletes the central tandem repeat array and its flanking sequences. The conceptually derived amino acid sequence of the novel MUC1 protein shows that it is identical with the previously reported transmembrane MUC1 amino acid sequence except for the deletion of the central 20 amino acid tandem repeat array and sequences immediately flanking the repeat array. This indicates that the novel MUC1 protein, which is devoid of the "hallmark" feature of mucins, the tandem repeat array, may be functionally different to the much larger, heavily glycosylated polymorphic repeat array containing MUC1 proteins, that affect cell-cell interactions. Based on an analysis of its peptide sequence, we propose the hypothesis that the novel MUC1 protein may act as a receptor molecule that modulates signal transduction. Preliminary experimental data supports this hypothesis. It appears, therefore, that the MUC1 gene is multifunctional with regard to its protein products- the repeat array containing MUC1 proteins may alter cellular adhesion processes whereas the novel MUC1 protein could be acting as a receptor-like molecule participating in signal transmission.

Characterization of cis-acting elements regulating transcription of the human DF3 breast carcinoma-associated antigen (MUC1) gene

The present studies have examined the sequences responsible for regulating transcription of the human DF3 breast carcinoma-associated antigen (MUC1) gene. A region 1656 base pairs upstream to the DF3 transcription initiation site was fused to the chloramphenicol acetyltransferase gene. Transient expression assays using a series of deleted constructs demonstrated that the region from position -618 contains the regulatory sequences necessary for DF3 transcription in human MCF-7 breast cancer cells. Further analysis with internal deletion vectors and heterologous promoter constructs indicated the involvement of cis-acting elements in the fragment extending from positions -598 to -485. By gel retardation and DNA footprinting, we have identified a protein in MCF-7 cells that recognizes sequences between positions -505 and -485. The results of Southwestern studies demonstrate that this protein has an apparent molecular mass of 45 kDa. Taken together, these results suggest that DF3 gene transcription is regulated by a previously undescribed transacting factor.

The mouse episialin (Muc1) gene and its promoter: rapid evolution of the repetitive domain in the protein

We have cloned the Muc1 gene of the mouse, encoding the murine equivalent of human episialin (also known as EMA or PEM), a mucin-like glycoprotein that is overexpressed in carcinoma cells. The extracellular domain of the mouse protein, that mainly consists of tandem repeats, contains 16 repeats of variable length and sequence, whereas the human protein usually contains between 30 and 90 nearly identical repeats. The murine repeats contain more potential O-glycan side chains and this may result in a more extended conformation of the murine protein. The transmembrane and cytoplasmic domains of the protein show about 90% conservation. The promoter region shows many conserved regions that could function as transcription factor binding sites.

Structure and expression of the human polymorphic epithelial mucin gene: an expressed VNTR unit

The human polymorphic epithelial mucin (PEM) is expressed apically by glandular epithelium and by the carcinomas that develop from these tissues. Previously isolated cDNA clones revealed that the core protein contained a large domain consisting of variable numbers of 60 bp tandem repeats (TR), making it an expressed minisatellite. We now report the full genomic sequence of the PEM gene, including 803 bp of 5' flanking sequence. The gene is composed of 7 exons and varies in size from approximately 4 to approximately 7 kb, depending on the number of tandem repeats in exon 2. Expression of PEM was obtained from a genomic clone in an Epstein-Barr virus based vector, after transfection into a human epithelial cell line, indicating the presence of effective regulatory sequences in this clone.

Transcriptional regulation of DF3 gene expression in human MCF-7 breast carcinoma cells

The DF3 gene codes for a high molecular weight human breast tumor-associated glycoprotein. The detection of this antigen in human milk has also suggested that its expression represents a differentiated function of mammary epithelium. The present studies have examined the regulation of DF3 gene expression in human MCF-7 breast carcinoma cells. These cells express two DF3 transcripts of 4.5 and 7.0 kb. Treatment of MCF-7 cells with 12-0-tetradecanoylphorbol-13-acetate (TPA) was associated with increases in levels of both DF3 mRNAs. When nuclear run-on assays were used, DF3 gene transcription was at low to undetectable levels in untreated MCF-7 cells and was increased after TPA exposure. TPA-induced increases in DF3 mRNA levels were also inhibited by actinomycin D (ACT). MCF-7 cells exposed to ACT further demonstrated that the half-lives of the 4.5 and 7.0 kb transcripts are 26 and 11 h, respectively. The results also demonstrate that the protein synthesis inhibitor, cycloheximide (CHX), increases DF3 mRNA levels in MCF-7 cells. These effects of CHX were sensitive to actinomycin D and not associated with stabilization of the DF3 transcripts. Taken together, these findings indicate that DF3 gene expression is controlled at a transcriptional level in TPA- and CHX-treated MCF-7 cells.