GLYCOSYLATION-DEPENDENT CELL-ADHESION MOLECULE-1 (GLYCAM-1) MUCIN IS EXPRESSED BY LACTATING MAMMARY-GLAND EPITHELIAL-CELLS AND IS PRESENT IN MILK

Glycosylation-dependent cell adhesion molecule 1 (GlyCAM 1) is a mucin like endothelial glycoprotein that acts as an adhesive ligand for L se lectin by presenting one or more O-linked carbohydrates to the lectin domain of this leukocyte cell surface selectin. The GlyCAM 1 glycoprot ein has been previously shown to be expressed specifically by the endo thelial cells of peripheral and mesenteric lymph nodes and in an unkno wn site in lung. Here we report that this protein is also expressed du ring lactation by mammary epithelial cells. Northern blot analysis has shown that the mRNA for GlyCAM 1 appears to be induced during pregnan cy in a manner similar to that previously described for hormonally ind uced milk proteins. In situ hybridization analysis reveals that the si te of GlyCAM 1 synthesis in the mammary gland is in the epithelial cel ls that produce these same milk proteins. Immunohistochemistry of mamm ary glands using antisera directed against GlyCAM 1 peptides demonstra tes that these eptihelial cells contain GlyCAM 1 protein, and that thi s protein is also found lumenally in the milk of the secreting mammary gland. Analysis of murine milk shows that immunoreactive GIyCAM 1 is found in the soluble whey fraction. Finally, labeling analysis of milk GlyCAM 1 has demonstrated that this form of the glycoprotein lacks th e sulfate-modified carbohydrate that has recently been shown to be req uired for the ligand binding activity to L selectin. The nonsulfated m ammary GlyCAM 1 is unable to interact with L selectin, consistent with the hypothesis that milk GlyCAM 1 has a different function than endot helial GlyCAM 1. These data thus suggest that milk GlyCAM 1 is a hormo nally regulated milk protein that is part of the milk mucin complex. I n addition, the finding that the mammary form of GlyCAM 1 contains dif ferent carbohydrate modifications than the endothelial form suggests t hat this glycoprotein may be a scaffold for carbohydrates that mediate functions in addition to cell adhesion.