Laboratory of Pulmonary Pathobiology , National Institute of Environmental Health Sciences , Research Triangle Park , North Carolina , USA .
J Biol Chem 270 : 16236-42 ( 1995)
Abstract
Alpha 2-Macroglobulin ( alpha 2M ) is a multifunctional secreted glycoprotein that serves as a ubiquitous proteinase inhibitor and as a binding protein for platelet-derived growth factor ( PDGF ) BB and homologues of PDGF-BB secreted in culture by macrophages .
The interaction of alpha 2M with PDGF-A chain molecules has not been addressed .
This is a potentially important issue because fibroblasts and smooth muscle cells produce PDGF-AA , whereas macrophages produce mainly PDGF-BB .
Recombinant human 125I-PDGF-B chain molecules ( AB and BB ) bound to plasma-derived , native human , or bovine alpha 2M and trypsin-activated alpha 2M on Superose 6 fast protein liquid chromatography gel filtration and on nondenaturing polyacrylamide gel electrophoresis , whereas 125I-PDGF-AA did not .
Similar results were obtained with 125I-PDGF isoforms binding to immobilized bovine alpha 2M and alpha 2M-methylamine .
The same differential pattern of unlabeled PDGF isoforms binding to alpha 2M was observed by Western blotting of PDGF .
Human lung fibroblasts secreted alpha 2M as measured by Western blotting , and fibroblast-derived alpha 2M possessed the same differential binding pattern for PDGF isoforms as did plasma-derived alpha 2M .
The specific binding of PDGF-AB and -BB to these fibroblasts was inhibited by native bovine alpha 2M , although PDGF-AA binding was not affected .
Native alpha 2M preferentially blocked fibroblast chemotaxis to the PDGF-B chain dimers .
These data suggest that only PDGF-B chain dimers , such as those produced by macrophages or released from platelets , are regulated by alpha 2M and that PDGF-AA produced by fibroblasts and smooth muscle cells is not controlled by this cytokine-binding protein .