The 56 kd platelet-derived growth factor (PDGF)-related protein is phosphorylated and the most stable form in human glioma cells

We report herein the presence of a 56 kd platelet derived growth factor (PDGF)-related protein as a phosphorylated form in human glioma cells. The phosphorylation of the 56 kd form was found to be the longest of all PDGF-related proteins. By Western blotting using a monoclonal anti-PDGF B-chain, the 80 kd, 56 kd, 40 kd, 28 kd and 17 kd PDGF-related proteins were detected, while after treatment among the nitrocellulose membrane transblotted cell extracts with alkaline phosphatase, 40 kd was the most densely observed while the 56 kd and 80 kd PDGF-related proteins were also detected. In a 32P flush labeling study, it was revealed that PDGF-related proteins incorporated with 32P were detected at 28, 32, 35, 40, 56 and 80 kd but the 17 kd monomer was not labeled. Among the labeled PDGF-related proteins, the 56 kd PDGF-related protein alone remained intracellularly for at least 16 hours. These results indicated that the PDGF-related proteins in human glioma cells are synthesized in a phosphorylated form and partly remain in a 56 kd phosphorylated form intracellularly. The 56 kd form may thus be the most stable form and likely has a substantial biological effect.