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Differential binding and regulation of platelet-derived growth factor A and B chain isoforms by alpha 2-macroglobulin
J C Bonner1, A R Osornio-Vargas
1Laboratory of Pulmonary Pathobiology, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA.
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.
Insights
Alpha 2-Macroglobulin (alpha 2M) regulates platelet-derived growth factor (PDGF)-BB but not PDGF-AA. This proteinase inhibitor preferentially affects PDGF-B dimers, impacting fibroblast chemotaxis.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Alpha 2-Macroglobulin (alpha 2M) is a key proteinase inhibitor and binding protein for growth factors.
- Platelet-Derived Growth Factor (PDGF) exists in various isoforms (AA, AB, BB), with different cellular origins and functions.
- The interaction between alpha 2M and PDGF-AA has not been previously investigated.
Purpose of the Study:
- To investigate the binding specificity of alpha 2M to different PDGF isoforms (AA, AB, BB).
- To determine if alpha 2M regulates PDGF-AA, which is produced by fibroblasts and smooth muscle cells.
- To assess the functional consequences of alpha 2M binding on PDGF-mediated fibroblast activity.
Main Methods:
- Utilized radiolabeled PDGF isoforms (AA, AB, BB) for binding assays with purified alpha 2M.
- Employed gel filtration (Superose 6) and gel electrophoresis (nondenaturing polyacrylamide) to analyze binding.
- Performed Western blotting to detect PDGF isoform binding to immobilized alpha 2M and fibroblast-secreted alpha 2M.
- Assessed the effect of alpha 2M on PDGF-induced fibroblast chemotaxis.
Main Results:
- Alpha 2M specifically bound to PDGF-AB and PDGF-BB, but not to PDGF-AA.
- This differential binding pattern was consistent across various alpha 2M preparations (plasma-derived, trypsin-activated, immobilized) and PDGF isoforms.
- Fibroblast-derived alpha 2M exhibited the same binding preference for PDGF isoforms as plasma-derived alpha 2M.
- Native alpha 2M inhibited PDGF-AB and -BB binding to fibroblasts and preferentially blocked chemotaxis induced by PDGF-B chain dimers, without affecting PDGF-AA response.
Conclusions:
- Alpha 2-Macroglobulin selectively binds and regulates PDGF-B chain dimers (PDGF-AB and -BB).
- PDGF-AA, produced by fibroblasts and smooth muscle cells, is not controlled by alpha 2M.
- These findings highlight a specific mechanism by which alpha 2M modulates growth factor signaling based on PDGF isoform composition.