DIFFERENCES IN THE BINDING OF TRANSFORMING GROWTH-FACTOR-BETA-1 TO THE ACUTE-PHASE REACTANT AND CONSTITUTIVELY SYNTHESIZED ALPHA-MACROGLOBULINS OF RAT

Human alpha(2)-macroglobulin (alpha(2)M) is a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor beta 1 (TGF-beta 1). The constitutively synthesized homologue of human alpha(2)M in the adult rat is alpha(1)M. Rat alpha(2)M is an acute-phase reactant, expressed at high levels in experimental trauma, pregnancy and in certain pathological conditions. The physiological role of rat alpha(2)M is not known. In this investigation, we demonstrated that rat alpha(1)M and rat alpha(2)M bind TGF-beta 1. The equilibrium dissociation constants (K-D) for the binding of TGF-beta 1 to the native forms of alpha(1)M and alpha(2)M were 257 and 109 nM respectively. alpha(1)M underwent conformational change when it reacted with methylamine. The resulting product bound TGF-beta 1 with higher affinity (32 nM). Methylamine-treated rat alpha(2)M did not undergo conformational change and did not bind TGF-beta 1 with increased affinity. Previous studies suggest that the native conformationmay be the principal form responsible for the cytokine-carrier activity of alpha(2)M in plasma and serum-supplemented cell culture medium. To confirm that native rat alpha(2)M is a more efficient TGF-beta 1 carrier than native alpha(1)M, fetal bovine heart endothelial cell (FBHE) proliferation assays were performed. TGF-beta 1 (5 pM) inhibited FBHE proliferation, and native alpha(2)M (0.3 mu M) counteracted this activity whereas alpha(1)M (0.3 mu M) had almost no effect. Rat alpha(2)M underwent conformational change when it reacted with plasmin incorporating 1.1 mol of plasmin/mol. alpha(2)M-plasmin bound TGF-beta 1; the K-D (61 nM) was lower (P < 0.01) than that determined for the native alpha M-TGF-beta 1 interaction. These studies demonstrate that both;at cc-macroglobulins are carriers of TGF-beta 1. The native form of rat alpha(2)M probably has a predominant role, compared with native alpha(1)M, as a TGF-beta 1 carrier in the plasma during the acute-phase response.