Identification of a soluble receptor for platelet-derived growth factor in cell-conditioned medium and human plasma

We have discovered a soluble form of the platelet-derived growth factor (PDGF) alpha receptor, designated sPDGF-R alpha, that is produced by and secreted into the conditioned medium of the human osteosarcoma cell line, MG-63. Additionally, sPDGF-R alpha activity has been detected in normal human blood plasma and serum. We have achieved partial purification of this protein by column chromatography using three different affinity matrices: anti-PDGF-R alpha monoclonal antibody (mAb) 292.15-Sepharose, PDGF-BB-Sepharose, and wheat germ agglutinin-agarose. All three matrices have been shown to purify a 90-kDa protein that is recognized by mAbs specific for the PDGF-R alpha extracellular domain. sPDGF-R alpha is capable of binding PDGF ligand in solution and can compete with cell-associated PDGF receptors for ligand binding. We provide three pieces of data suggesting that the sPDGF-R alpha is generated by proteolytic clipping of the full-length PDGF-R alpha protein. First, the conditioned medium of an expression cell line transfected with a cDNA construct designed to produce only full-length PDGF-R alpha exhibits sPDGF-R alpha activity. Second, a truncated intracellular fragment of the PDGF-R alpha, presumably representing the intracellular counterpart of the clipped sPDGF-R alpha, can be immunoprecipitated from the MG-63 osteosarcoma cell extracts using antiserum raised against an intracellular portion of PDGF-R alpha. Finally, we have been unable to detect alternative splicing in the PDGF-R alpha transcript using reverse transcription-polymerase chain reaction.