Alpha-Macroglobulin Secreted by Alveolar Macrophages Serves as a Binding Protein for a Macrophage-derived Homologue of Platelet-derived Growth Factor

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Abstract
Evidence is presented to support our hypothesis that an alpha-macroglobulin (alpha M) produced by lung macrophages serves as a specific binding protein for platelet-derived growth factor (PDGF) from these same macrophages. Culture medium "conditioned" by alveolar macrophages was fractionated by gel filtration according to molecular weight. Proteins larger than 200 kD were bound to greater than 50% of the macrophage-derived PDGF (MD-PDGF) that was extractable by 1 M acetic acid. Another approximately 25% was bound to fractions at approximately 150 kD, and approximately 20% remained unbound. The two high molecular weight fractions inhibited approximately 40% of specific [125I]PDGF binding to rat lung fibroblasts, whereas other fractions did not block PDGF binding to its receptor. Only the greater than 200 kD fractions inhibited the binding of PDGF antisera to purified human PDGF by 20% of control and exhibited specific complex formation and coelution on a gel filtration column with [125I]PDGF. The macrophage-derived alpha M (MD-alpha M) was separated from other macrophage-derived proteins by nickel-affinity chromatography and exhibited clear characteristics of alpha Ms, i.e., cross-reactivity with antibodies to human alpha 2-macroglobulin (alpha 2M) on immunoblots as well as gel migration corresponding to the electrophoretic mobility of the protease-bound "fast" and protease-unbound "slow" forms of human alpha 2M. Nickel-bound protein identified as an alpha M was bound to greater than 50% of the acid-extractable MD-PDGF in macrophage-conditioned medium, supporting the view that the greater than 200 kD protein separated by gel filtration is an alpha M.(ABSTRACT TRUNCATED AT 250 WORDS)