Demonstration of an alpha2-macroglobulin receptor in human fibroblasts, absent in tumor-derived cell lines.

Abstract

The binding and uptake by normal human fibroblasts of human a2-macroglobulin (aZM) and azM l trypsin, purified from plasma and labeled with lz51, were examined. Saturation of the uptake was reached at 200 gg of a2M/ml of medium, with a maximal rate of uptake of about lo6 molecules of azM/cell and /h. These parameters compared well with those obtained with complete serum. Measurements of binding of ‘251-az-macroglobulin to fibroblasts demonstrated the presence of a specific surface receptor. At 4”C, about 11,500 + 500 receptors/cell were detected with an apparent association constant for a2M l trypsin complexes of 0.54 nW’. Measurements of binding at 37°C did not yield valuable results but dilution with cold a2M* trypsin provided data which indicated that the number of receptors/cell was at least 6 times higher than at 4°C. These data suggested also that the affinity of the receptor for a2M complexes was highly temperature-dependent. Comparison of the kinetics of uptake and binding of azM* trypsin and “active” a2M indicated that the low binding and uptake observed with “active” azM were most likely due to a fraction of complexes and denaturated azM, always present in purified preparations of this inhibitor. Finally, evidence is provided that two tumor-derived cell lines, which were previously shown to have a low intracellular a2M content under steady state culture conditions, are devoid of the specific azM binding site at their plasma membrane.