Human hepatocytes exhibit receptors for α₂‐macroglobulin and pregnancy zone protein‐proteinase complexes

Abstract

Hepatocytes were isolated by application of the two-step collagenase technique to pieces of human liver. 125I-labelled alpha 2-macroglobulin-trypsin complex bound to hepatocytes at 4 degrees C with a half time of approximately 4.5 h. At near equilibrium half of the receptors were saturated at an alpha 2-macroglobulin-trypsin complex concentration of about 60 pmol 1(-1) and the Scatchard plot was linear. Dissociation of the labelled complex was slow (T1/2 = 24 h) at low receptor occupancies. At high receptor occupancies dissociation was biphasic with a rate constant (K-1) for the initial rapid phase of about 2.4 x 10(-2) min-1. Labelled alpha 2-macroglobulin-trypsin complex bound at 4 degrees C was rapidly internalized at 37 degrees C (T1/2 = 1.9 min), and in 3.5 h approximately 10% of the label was released into the medium in a trichloroacetic acid-soluble form. At 37 degrees C, 125I alpha 2-macroglobulin-trypsin was taken up by hepatocytes and trichloroacetic acid soluble radioactivity appeared in the medium following a sigmoidal curve. Similar results were obtained with 125I-pregnancy zone protein-chymotrypsin complex. At 4 degrees C, hepatocytes bound nearly equal amounts of labelled alpha 2-macroglobulin-trypsin and pregnancy zone protein-chymotrypsin complex, and a large excess (100 nmol 1(-1) of one of the macroglobulins could almost completely abolish binding of trace amounts (5-20 pmol 1(-1] of the other. The present findings strongly suggest that the hepatocyte is of major importance for removal of alpha 2-macroglobulin- and pregnancy zone protein-proteinase complex in humans, in agreement with previous results in rats and mice.

The human alpha 2-macroglobulin receptor: identification of a 420-kD cell surface glycoprotein specific for the activated conformation of alpha 2-macroglobulin.

“…The requirement of metal ions for the binding of this protein to immobilized c~2M:Me is in agreement with the known requirement of metal ions for the binding of cz2M:Me and c~2M-proteinase complexes to cells (11,13,18,32,40,42,52). Many ligands, such as o~2M-proteinase complexes, that enter the cell by receptor-mediated endocytosis are delivered to lysosomes, while their receptors are recycled back to the cell surface (5).…”

Cloning of the human ?2-macroglobulin gene and detection of mutations in two functional domains: the bait region and the thiolester site

“…Localized intrapulmonary a2M synthesis under the control of cell-specific promotors could very efficiently regulate the activity of potentially damaging proteinases according to local requirements. Specific cellular receptors for a2M on macrophages and other cell types (Van Leuven et al 1986a, b;Petersen et al 1987;Munck-Petersen et al 1988b) bind a2M-proteinase complexes, which are then rapidly internalized and degraded. The investigation of the possible role of a2M in the pathogensis of pulmonary diseases is complicated by the likelihood of such compartmentalized processes.…”

Mannan‐Binding Protein Forms Complexes with α‐2‐Macroglobulin. A Proposed Model for the Interaction

“…The a2M-r'eceptor is found in high numbers on fibroblasts [27][28][29], monocyte-macrophages [30][31][32] and especially on hepatocytes [33][34][35]. The receptor, which is functionally dependent on Ca^"*^ [35,36], binds only to Q2M in complex with proteinase [25]. Bound complexes are internalized, separated from the recycling receptors in endocytotic vesicles and degraded in the lysosomes [30,37].…”