Expression of M-N#1, a histo-blood group B-like antigen, is strongly up-regulated in nonapoptosing mammary epithelial cells during rat mammary gland involution.

Antibodies against the histo-blood group B-like antigen M-N#1 efficiently block the growth in vivo of rat mammary carcinoma cells that bear the antigen (Sleeman et al., 1999, Oncogene 18, 4485--4494). To try to understand the function of the M-N#1 antigen, we investigated when and where the antigen is expressed during the normal function of the rat mammary gland. Expression was virtually only seen during mammary gland involution. Here, strong expression of the antigen was observed in mammary epithelial cells, beginning around 2 days postweaning and lasting throughout the involution process. Dexamethasone treatment of animals postlactation inhibited alveolar collapse and remodeling in the mammary gland but inhibited neither the apoptosis of mammary epithelial cells nor the expression of the M-N#1 antigen. We show that up-regulation of carbohydrate antigens is not a general phenomenon during mammary gland involution, and thus that M-N#1 antigen expression is specifically regulated. Up-regulation of alpha(1,2)fucosyltransferase A, an enzyme required for M-N#1 antigen synthesis, is at least partly responsible for regulated M-N#1 antigen expression postlactation. Most significantly, we observed that the M-N#1 antigen is virtually exclusively expressed on nonapoptosing epithelial cells in the involuting mammary gland. These data suggest that M-N#1 antigen expression might either provide a survival function and/or be expressed in epithelial cells that are destined to grow and remodel mammary duct structures.