Proteinases are isoform-specific regulators of the binding of transforming growth factor beta to alpha(2)-macroglobulin

alpha(2)-Macroglobulin (alpha(2)M) regulates growth and gene expression in many cell types by binding and neutralizing transforming growth factor beta (TGF-beta). In this study we characterized the effects of the serine proteinase, plasmin, on the interaction of alpha(2)M with TGF-beta 1 and TGF-beta 2. Binding of both TGF-beta isoforms to purified alpha(2)-plasmin complex was primarily non-covalent and reversible. The binding affinity of alpha(2)M for TGF-beta 1 was increased by plasmin; the K-d values were 320 and 84 nM for native alpha(2)M and alpha 2M-plasmin respectively. In contrast the affinity of alpha(2)M for TGF-beta 2 was decreased by plasmin; the K-d values were 14 and 80 nM for native alpha(2)M and alpha(2)M-plasmin, respectively. Thrombin decreased the affinity of alpha(2)M for TGF-beta 2 in a similar manner to plasmin. In assays of DNA synthesis in fetal bovine heart endothelial cells, native alpha(2)M neutralized the activity of exogenously added TGF-beta 2, whereas alpha(2)M-plasmin, at equivalent concentrations, had almost no effect. Native alpha(2)M and methylamine-modified alpha(2)M increased platelet-derived growth factor alpha-receptor expression in Vascular smooth-muscle cells, an activity attributed to the neutralization of autocrine TGF-beta activity, whereas alpha(2)M-plasmin was less effective at the same concentration. These studies demonstrate that the effects of proteinases on the cytokine-binding and cytokine-neutralizing activities of alpha(2)M are cytokine-dependent. By reacting with alpha(2)M, proteinases might regulate not only the availability of cytokines in the extracellular spaces but also the composition of the cytokine milieu.