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Identification of an alpha 2-Macroglobulin Receptor in Human Mammary Epithelial Cells

Manuscript received 14 January 1998. Initial reviews completed 27 March 1998. Revision accepted 15 October 1998.

Donna Beshgetoor and Bo Lönnerdal

Department of Nutrition, University of California, Davis, CA 95616, U.S. Fax: (530) 752-8966 Tel: (530) 752-8347 E-mail: bllonnerdal{at}ucdavis.edu

Several cases of zinc (Zn) deficiency in human infants caused by abnormally low concentrations of Zn in breast milk were recently reported, the underlying mechanism of which is not known. Alpha2-macroglobulin (alpha 2-M), a major Zn-binding ligand in serum, presents a potential vehicle for mammary Zn uptake. This study was conducted to determine if an alpha 2-M receptor is present in human mammary epithelial cells, where it may be involved in the endocytosis of alpha 2-M into the mammary gland. Normal human mammary epithelial cells were grown to confluency in serum-free medium. For all binding and uptake studies, alpha 2-M, preactivated with methylamine and labeled with 125I, was added to cells for varied lengths of time to determine saturation over time and at varied concentrations to determine saturation over increasing concentration of ligand. Nonspecific and competitive binding were measured by addition of a 100-fold molar excess of unlabeled alpha 2-M and serum albumin or lactoferrin, respectively. Binding at 4°C was specific for alpha 2-M and approached saturation kinetics at 56 nmol/L. Scatchard plot analysis of the binding data demonstrated more than one binding site: a high affinity, saturable binding site and a low affinity, nonsaturable binding site. Uptake of alpha 2-M at 37°C was rapid and continuous over increasing concentrations of alpha 2-M, and internalized alpha 2-M was rapidly degraded. Results from this study present evidence for receptor-mediated uptake of alpha 2-M in human mammary epithelial cells, which in turn, provides a potential mechanism for Zn acquisition by the cell.

Key words: zinc, alpha 2-macroglobulin, alpha 2-macroglobulin receptor, mammary gland, humans.

The Journal of Nutrition Vol. 129 No. 1 January 1999, pp. 152-157
Copyright ©1999 by the American Society for Nutritional Sciences




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