Abstract. Episialin (MUC1) is a transmembrane molecule with a large mucin-like extracellular domain protruding high above the cell surface. The molecule is located at the apical side of most glandular epithelial cells, whereas in carcinoma cells it is often present at the entire surface and it is frequently expressed in abnormally large quantities. We have previously shown that overexpression of episialin reduces cell-cell interactions. Here we show that the integrinmediated adhesion to extracellular matrix of transfectants of a melanoma cell line (A375), a transformed epithelial cell line (MDCK-ras-e) and a human breast epithelial cell line (HBL-100) is reduced by high levels of episialin. This reduction can be reversed by inducing high avidity of the ffl integrins by mAb TS2/16 (at least for/~l-mediated adhesion). The adhesion can also be restored by redistribution of episialin on the cell surface by monoclonal antibodies into patches or caps. Similarly, capping of episialin on ZR-75-1 breast carcinoma cells, growing in suspension, caused adherence and spreading of these cells.We propose that there is a delicate balance between adhesion and anti-adhesion forces in episialin expressing cells, which can be shifted towards adhesion by strengthening the integrin-mediated adhesion, or towards anti-adhesion by increasing the level of expression of episialin.

Episialin (MUC1, PEM, EMA, CA15-3 antigen) is a sialylated, membrane-associated glycoprotein with an extended mucin-like ectodomain. This domain mainly consists of 30-90 homologous 20-amino acid repeats that are rich in 0-glycosylation sites (serines and threonines). It is likely that this part forms a polyproline ,B-turn helix. As a result, the ectodomain can protrude more than 200 nm above the cell surface, whereas most cell surface molecules do not exceed a length of 35 nm. Normally, episialin is present at the apical side of glandular epithelial cells. On carcinoma cells, however, it can be strongly overexpressed and it is often present over the entire cell surface. We have previously shown that episialin, if it is interspersed between adhesion molecules, nonspecifically reduces cell-cell and cell-extracellular matrix interactions in vitro and in vivo, presumably by steric hindrance caused by the extreme length and high density of the episialin molecules at the cell surface. To analyze the molecular mechanism for this anti-adhesion effect in more detail, we have now deleted an increasing number of repeats in the episialin cDNA and transfected the resulting mutants into murine L929 cells expressing the homophilic adhesion molecule E-cadherin. Here we show that the length of episialin is the dominant factor that determines the inhibition of E-cadherin-mediated cell-cell interactions. For the anti-adhesive effect mediated by the full length episialin, charge repulsion by negatively charged sialylated 0-linked glycans is far less important.

Patients with metastatic cancer commonly have increased serum galectin-3 concentrations, but it is not known whether this has any functional implications for cancer progression. We report that MUC1, a large transmembrane mucin protein that is overexpressed and aberrantly glycosylated in epithelial cancer, is a natural ligand for galectin-3. Recombinant galectin-3 at concentrations (0.2-1.0 g/ml) similar to those found in the sera of patients with metastatic cancer increased adhesion of MUC1-expressing human breast ( Galectin-3 is one of 15 known members of the galectin family of naturally occurring galactoside-binding lectins that are expressed intracellularly and extracellularly by many cell types (1). Galectin-3 concentrations are increased up to 5-fold in the sera of patients with breast, gastrointestinal, or lung cancer (2). Moreover, higher galectin-3 concentrations are seen in the sera of patients with metastatic disease than in the sera of patients with localized tumors (2). The source of increased circulating galectin-3 in cancer patients is not clear, but it is probably generated by tumor cells as well as by peritumoral inflammatory and stromal cells (2). It is not known whether this increased circulating galectin-3 has any functional implications for cancer progression.Cytoplasmic galectin-3 is known to be anti-apoptotic (3), whereas nuclear galectin-3 promotes pre-mRNA splicing (4, 5). Cell surface galectin-3 is involved in various cell-cell and cellmatrix interactions (1, 6, 7) and enhances cancer cell adhesion to and invasion through basement membrane by interacting with extracellular matrix proteins such as fibronectin, collagen, or laminin (1,8,9). Galectin-3 expressed on the endothelial cell surface has been shown to promote the adhesion of breast cancer cells to endothelium by interaction with cancer-associated Thomsen-Friedenreich (galactose ␤1,3N-acetylgalactosamine ␣Ϫ (TF)) 2 antigen expressed by unknown cell surface molecules (10 -14). TF antigen is the core I structure of mucin-type O-linked glycans, but in its simplest nonsialylated, nonextended form it acts as an oncofetal antigen, and its presence/expression is increased in malignant and premalignant epithelia (15).MUC1 (also known as episialin and DF3) is a large (M r Ͼ 250,000) transmembrane mucin protein expressed on the apical surface of most normal secretory epithelia including those in the mammary gland, and the gastrointestinal, respiratory, urinary, and reproductive tracts. The MUC1 extracellular domain consists of variable numbers of 20-amino acid tandem repeat peptides (VNTR) that are rich in serines, threonines, and prolines. These tandem repeat domains are heavily glycosylated

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We performed a high-throughput retroviral insertional mutagenesis screen in mouse mammary tumor virus (MMTV)-induced mammary tumors and identified 33 common insertion sites, of which 17 genes were previously not known to be associated with mammary cancer and 13 had not previously been linked to cancer in general. Although members of the Wnt and fibroblast growth factors (Fgf) families were frequently tagged, our exhaustive screening for MMTV insertion sites uncovered a new repertoire of candidate breast cancer oncogenes. We validated one of these genes, Rspo3, as an oncogene by overexpression in a p53-deficient mammary epithelial cell line. The human orthologs of the candidate oncogenes were frequently deregulated in human breast cancers and associated with several tumor parameters. Computational analysis of all MMTV-tagged genes uncovered specific gene families not previously associated with cancer and showed a significant overrepresentation of protein domains and signaling pathways mainly associated with development and growth factor signaling. Comparison of all tagged genes in MMTV and Moloney murine leukemia virus-induced malignancies showed that both viruses target mostly different genes that act predominantly in distinct pathways.