Immunohistochemical Study of the Expression of Human Milk Fat Globule Membrane Glycoprotein 70¹ Free

Human milk fat globule membrane, which is said to derive from apical plasma membrane of secretory epithelial cells in breast, was analyzed by sodium dodecyl sulfate-two:dimensional gel electrophoresis. More than 35 components were detected in the gels. One of the major glycoproteins with an apparent molecular weight of 70,000, human milk fat globule membrane glycoprotein, was purified to homogeneity. The pattern of distribution of this glycoprotein in tissues was studied using polyclonal rabbit antibodies to the purified component. The localization of the antigen was accomplished by an indirect immunoperoxidase staining method. Normal mammary epithelial cells display this antigen mostly on the apical plasma membrane, whereas poorly differentiated breast carcinoma cells retained it predominantly in the cytoplasm. These observations suggest that the proper insertion of this glycoprotein into an apical membrane domain may be impaired in malignant tumor cells. In addition, a small population of tumor cells in each case examined failed to express detectable amounts of this component, indicating the presence of antigenic heterogeneity among the tumor cell population.