Biological Significance of Mucins Produced by Epithelial Cancer Cells

Abstract

Since normal epithelial cells exhibit a clear polarity, synthesized mucins are transported to the apical cell surface and become secretory or membrane-bound glycoproteins. Mucins are major glycoprotein components of mucus, covering the luminal surfaces of the epithelial respiratory, gastrointestinal, and reproductive tracts. Upon malignant transformation, mucins are secreted into the tumor tissues and/or the bloodstream of cancer patients because of loss of the polarity of epithelial tissues. Qualitative changes of mucins are detected in O-glycans expressed on core proteins. Such aberrant O-glycans are generally called cancer-associated carbohydrate antigens. Many mAbs against cancer-associated carbohydrate antigens have been produced and used for the detection of tumor markers in the bloodstream and for the determination of epitopic structures. It has been reported that patients with a higher amount of mucins in their bloodstream have a lower 5-year survival rate. However, little is known regarding the biological significance of mucins. Since many lectins are found on immune cells, we predicted that mucins may interact with these lectins. Generally, a single carbohydrate chain binds to a lectin so weakly that the interaction may be biologically ineffective but carbohydrate chains clustered on the core protein like mucins could bind to multiple lectins, thus resulting in high-affinity binding. Mucins generally possess so many tandem repeats that if there is a single binding site in the tandem repeat unit, many lectins expressed on immune cells may be bridged. If a lectin plays a role in signal transduction, it is expected to mediate a strong signal in immune cells, maybe leading to an effect on immune function.