Differentiation-associated expression of β-N-acetylgalactosaminylated N-linked oligosaccharides in mammary epithelial cells

Abstract

Not only mammalian pituitary glycoprotein hormones but also many glycoproteins from a variety of animal species have been shown to contain N-linked oligosaccharides with the GalNAcβ1→4GlcNAc structure. Two types of β-1,4-GalNAcT were found; one transfers N-acetylgalactosamine to acceptor oligosaccharides, which is stimulated by the hormone peptide and the other simply transfers sugar without such activation. In the case of bovine mammary membrane glycoproteins, the expression of β-N-acetylgalactosaminylated N-linked oligosaccharides was associated with the functional differentiation of the epithelial cells. In contrast, the expression level of such oligosaccharides was much reduced in glycoprotein samples from human breast tumors compared with those from the unaffected regions. These results strongly suggest that the β-N-acetylgalactosaminylation is one which is regulated under cellular differentiation and dedifferentiation of the mammary gland. Whether or not β-N-acetylgalactosaminylated N-linked oligosaccharides have unique functions in addition to clearance of the hormone from the circulation remains to be elucidated.