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 fact or beta 1 (TGF-beta 1). The constitutively synthesized homologue of hu man 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, preg nancy and in certain pathological conditions. The physiological role o f rat alpha(2)M is not known. In this investigation, we demonstrated t hat rat alpha(1)M and rat alpha(2)M bind TGF-beta 1. The equilibrium d issociation constants (K-D) for the binding of TGF-beta 1 to the nativ e forms of alpha(1)M and alpha(2)M were 257 and 109 nM respectively. a lpha(1)M underwent conformational change when it reacted with methylam ine. The resulting product bound TGF-beta 1 with higher affinity (32 n M). Methylamine-treated rat alpha(2)M did not undergo conformational c hange and did not bind TGF-beta 1 with increased affinity. Previous st udies suggest that the native conformationmay be the principal form re sponsible for the cytokine-carrier activity of alpha(2)M in plasma and serum-supplemented cell culture medium. To confirm that native rat al pha(2)M is a more efficient TGF-beta 1 carrier than native alpha(1)M, fetal bovine heart endothelial cell (FBHE) proliferation assays were p erformed. TGF-beta 1 (5 pM) inhibited FBHE proliferation, and native a lpha(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 cha nge 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 carrier s of TGF-beta 1. The native form of rat alpha(2)M probably has a predo minant role, compared with native alpha(1)M, as a TGF-beta 1 carrier i n the plasma during the acute-phase response.