Identification of an alpha(2)-macroglobulin receptor in human mammary epithelial cells

Several cases of zinc (Zn) deficiency in human infants caused by abnormally low concentrations of Zn in breast milk were recently reported, the underl ying mechanism of which is not known. Alpha(2)-macroglobulin (alpha(2)-M), a major Zn-binding ligand in serum, presents a potential vehicle for mammar y Zn uptake. This study was conducted to determine if an alpha(2)-M recepto r 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 bi nding and uptake studies, alpha(2)-M, preactivated with methylamine and lab eled with I-125, 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 bindin g were measured by addition of a 100-fold molar excess of unlabeled alpha(2 )-M and serum albumin or lactoferrin, respectively. Binding at 4 degrees 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 bi nding site: a high affinity, saturable binding site and a low affinity, non saturable binding site. Uptake of alpha(2)-M at 37 degrees C was rapid and continuous over increasing concentrations of alpha(2)-M, and internalized a lpha(2)-M was rapidly degraded. Results from this study present evidence fo r receptor-mediated uptake of alpha(2)-M in human mammary epithelial cells, which in turn, provides a potential mechanism for Zn acquisition by the ce ll.