Glycosylation-dependent cell adhesion molecule 1 (GlyCAM 1) mucin is expressed by lactating mammary gland epithelial cells and is present in milk.

Abstract

Glycosylation-dependent cell adhesion molecule 1 (GlyCAM 1) is a mucinlike endothelial glycoprotein that acts as an adhesive ligand for L selectin by presenting one or more O-linked carbohydrates to the lectin domain of this leukocyte cell surface selectin. The GlyCAM 1 glycoprotein has been previously shown to be expressed specifically by the endothelial cells of peripheral and mesenteric lymph nodes and in an unknown site in lung. Here we report that this protein is also expressed during lactation by mammary epithelial cells. Northern blot analysis has shown that the mRNA for GlyCAM 1 appears to be induced during pregnancy in a manner similar to that previously described for hormonally induced milk proteins. In situ hybridization analysis reveals that the site of GlyCAM 1 synthesis in the mammary gland is in the epithelial cells that produce these same milk proteins. Immunohistochemistry of mammary glands using antisera directed against GlyCAM 1 peptides demonstrates that these epithelial cells contain GlyCAM 1 protein, and that this protein is also found lumenally in the milk of the secreting mammary gland. Analysis of murine milk shows that immunoreactive GlyCAM 1 is found in the soluble whey fraction. Finally, labeling analysis of milk GlyCAM 1 has demonstrated that this form of the glycoprotein lacks the sulfate-modified carbohydrate that has recently been shown to be required for the ligand binding activity to L selectin. The nonsulfated mammary GlyCAM 1 is unable to interact with L selectin, consistent with the hypothesis that milk GlyCAM 1 has a different function than endothelial GlyCAM 1. These data thus suggest that milk GlyCAM 1 is a hormonally regulated milk protein that is part of the milk mucin complex. In addition, the finding that the mammary form of GlyCAM 1 contains different carbohydrate modifications than the endothelial form suggests that this glycoprotein may be a scaffold for carbohydrates that mediate functions in addition to cell adhesion.