We have characterized the interaction of homodimeric porcine transforming growth factor-pl (TGF-PI) with affinity-purified human plasma fibronectin. Using a solidphase binding assay, we have demonstrated that TGF-01 binds to fibronectin immobilized on Immunlon ITM microtiter plates. TGF-Pl binding increased with time, reaching a plateau after 4-6 h, and was dependent upon the concentration of both labeled TGF-PI and immobilized fibronectin present. The binding of radiolabeled TGF-p1 to fibronectin was saturable and was reduced 75% in the presence of a 100-fold excess of unlabeled TGF-P1. TGF-01 bound to fibronectin with an association rate constant (y) of 2.96 x lo3 M-' s-l and did not readily dissociate under various conditions. The binding of TGF-P 1 to fibronectin was insensitive to variations in ionic strength over a range of 0.1-1.0 M NaCl and was relatively insensitive to divalent cation concentration in the range of 0.1-10.0 mM as well. These data suggest that the binding of TGF-Dl to fibronectin may not be dependent upon the interaction of charged amino acids within these two molecules. However, the binding of TGF-P1 to fibronectin was strongly pH-dependent and binding decreased dramatically below pH 4.0 and above pH 10.0, suggesting that charged amino acids may influence TGF-P1 /fibronectin interactions. The association of TGF-01 with immobilized fibronectin or other extracellular matrix components and subsequent dissociation under acidic conditions or by an as-yet-unidentified mechanism may play a role in the distribution and/or activity of this potent growth regulator at sites of tissue injury and inflammation in vivo.
Key words cell growth, cell differentiation, growth factor binding, extracellular matrix, fibroneetin
The association of polypeptide growth factors with components of the extracellular matrix may play an important role in the regulation of the distribution and/or activity of these growth factors. The binding of several growth factors to extracellular matrix proteins has been reported [ 11. Others, like basic fibroblast growth factor (bFGF) which is synthesized by endothelial cells, are reportedly incorporated into the pericellular matrix, presumably bound to heparan sulfate proteoglycans . The binding of bFGF to extracellular matrix heparan sulfate may limit its distribution and increase its