Binding and degradation of α2-macroglobulin by cultured fibroblasts

Abstract

We studied the interactions of α 2-macroglobulin, a major protease inhibitor of plasma and of serum-containing culture medium, with cultured fibroblasts. Iodinated human α 2-macroglobulin bound specifically to washed cell layers of cultured human fibroblasts. At 0–4°C, binding was saturated at a concentration of 10–20 μg/ml. At 37°C, radiolabel appered in the medium in a form soluble in 10% trichloroacetic acid. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated that ingested iodinated α 2-macroglobulin transiently forms a complex with a trypsin-like protease. Indirect immunofluorescence demonstrated α 2-macroglobulin in vacuoles of fibroblasts grown in 10% human serum or incubated with purified α 2-macroglobulin. Fibroblasts transformed by SV-40 (VA-13 cells) bound and degraded less 125I-labeled α 2-macroglobulin than non-transformed fibroblasts and had fewer vacuoles containing α 2-macroglobulin. These observations indicate that cultured fibroblasts bind, take up by endocytosis, and degrade α 2-macroglobulin. Binding and endocytosis of α 2-macroglobulin by a cell may be a means of modulating proteases in the micro-environment of the cell and during endocytosis.