A mechanism for inhibition of E-cadherin-mediated cell-cell adhesion by the membrane-associated mucin episialin/MUC1.

Abstract

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.

Requirement of both mucins and proteoglycans in cell‐cell dissociation and invasiveness of colon carcinoma HT‐29 cells

“…Such mechanism could promote invasion through an impairment of E-cadherin function by steric hindrance, since MUC1 was previously shown to cause steric hindrance of adhesion molecules like Ecadherin in murine fibroblasts. 15 However, the fact that either the anti-MUC1 or the anti-MUC5AC antibody was able to abrogate cell invasiveness and restored the function of E-cadherin in HT-29 5M21 cells indicated that neither MUC1 nor MUC5AC could disturb by itself the function of E-cadherin, suggesting a somewhat cooperation between these 2 types of mucins towards the Ecadherin function. The interaction of both mucins at the periphery of the HT-29 5M21 cells was further shown by observing that the capping with an anti-mucin antibody (MUC1 or MUC5AC) resulted in the spontaneous capping of the other mucin in the absence of its specific antibody, showing the existence of an interaction between these mucins in the formation of the mucin layer at the periphery of the cells.…”

“…14 On the other hand, MUC1 was shown to cause steric hindrance of adhesion molecules like E-cadherin, through its large negatively charged extracellular domain, which contains many oligosaccharide side chains. 15,16 Proteoglycans were also shown to disturb the function of E-cadherin through steric hindrance in variants of the murine mammary gland cell line NMuMG and the canine epithelial kidney cell line MDCK. 17 During colorectal carcinogenesis, the expression of MUC1 was found upregulated, particularly in later stages of the disease.…”

Overexpression of MUC1 reconfigures the binding properties of tumor cells

“…This hypothesis is supported by findings demonstrating that cell density decreases and distance between cells increases with high-level expression of MUC1. 25 The mechanism by which overexpression of MUC1 affects other receptor-ligand interactions at the cell surface has been the subject of much speculation but little definitive investigation. One widely held hypothesis posits that heavily glycosylated forms of MUC1, by virtue of its mass (over 10 6 Daltons), extended structure and molecular volume, make a steric shield at the cell surface that acts in a nonspecific manner to block accessibility of other ligands…”

“…This hypothesis is supported by findings demonstrating that cell density decreases and distance between cells increases with high-level expression of MUC1. 25 The mechanism by which overexpression of MUC1 affects other receptor-ligand interactions at the cell surface has been the subject of much speculation but little definitive investigation. One widely held hypothesis posits that heavily glycosylated forms of MUC1, by virtue of its mass (over 10 6 Daltons), extended structure and molecular volume, make a steric shield at the cell surface that acts in a nonspecific manner to block accessibility of other ligands 25,26 A second hypothesis, which is investigated here, is that specific structures on MUC1, (protein, oligosaccharide, sulfated structures) interact specifically with ligands on the same cell surface to block their accessibility to binding by exogenous ligands or receptors including those on opposing cell surfaces or stromal structures.…”

“…25 The mechanism by which overexpression of MUC1 affects other receptor-ligand interactions at the cell surface has been the subject of much speculation but little definitive investigation. One widely held hypothesis posits that heavily glycosylated forms of MUC1, by virtue of its mass (over 10 6 Daltons), extended structure and molecular volume, make a steric shield at the cell surface that acts in a nonspecific manner to block accessibility of other ligands 25,26 A second hypothesis, which is investigated here, is that specific structures on MUC1, (protein, oligosaccharide, sulfated structures) interact specifically with ligands on the same cell surface to block their accessibility to binding by exogenous ligands or receptors including those on opposing cell surfaces or stromal structures. The latter hypothesis can be easily modified to explain how a molecule could show both adhesive and antiadhesive functions: antiadhesive functions would be mediated by binding to cell receptors/ligands on the same cell surface, and adhesive functions would be mediated by binding to receptors/ligands on opposing surfaces.…”