A functional soluble extracellular region of the platelet-derived growth factor (PDGF) beta-receptor antagonizes PDGF-stimulated responses.

The platelet-derived growth factor beta-receptor (PDGFr) is a 180-kDa transmembrane glycoprotein which binds BB-PDGF with high affinity. We have expressed the extracellular region of the receptor in Chinese hamster ovary cells using an expression vector that carries a dihydrofolate reductase gene as an amplifiable marker. Upon amplification of the receptor cDNA sequences by methotrexate a 110-kDa soluble form of the receptor extracellular region (XR) was secreted at 12 mg/liter. The soluble XR protein fully retained the high affinity specific binding of the intact PDGFr for BB-PDGF (apparent dissociation constant, 0.4 nM). In the presence of ligand the soluble XR protein formed complexes that migrated on sodium dodecyl sulfate gels at the size expected for dimers of the protein. When added to fibroblast cultures the soluble XR protein blocked the ability of BB-PDGF to stimulate DNA synthesis but did not alter the mitogenic effect of AA-PDGF. The XR fragment also inhibited the binding of BB-PDGF to PDGFr and the activation of PDGFr tyrosine kinase by BB-PDGF. Thus, the soluble extracellular region protein of the PDGFr binds BB-PDGF with high affinity and functions as a specific antagonist of BB-PDGF actions.