Tyrosine kinase activity of purified recombinant cytoplasmic domain of platelet-derived growth factor beta-receptor (beta-PDGFR) and discovery of a novel inhibitor of receptor tyrosine kinases.

Aberrant expression of platelet-derived growth factor and its receptor (PDGFR) has been implicated in various human disorders, including cardiovascular disease and certain types of cancer. Inhibitors of the tyrosine kinase activity of PDGFR are leads in the development of novel agents to combat these diseases. We describe here a novel, potent inhibitor of PDGFR tyrosine kinase, 3-(4-dimethylamino-benzylidenyl)-2-indolinone (DMBI). The compound also inhibits signal transduction through fibroblast growth factor receptor 1 (FGFR1), but is not active towards epidermal growth factor receptor (EGFR) or c-Src tyrosine kinase. The activity of DMBI and other tyrosine kinase inhibitors was compared in a cell-based assay as well as in an assay based on purified recombinant platelet-derived growth factor beta-receptor (beta-PDGFR) lacking the transmembrane and ligand-binding domain. We showed that this truncated beta-PDGFR could dimerize, and that dimerization was required for tyrosine kinase activity. Tyrosine kinase activity was modulated by inhibitors of beta-PDGFR autophosphorylation in cells, but not by specific inhibitors of EGFR or c-Src tyrosine kinase. We conclude that beta-PDGFR lacking the transmembrane and ligand-binding domain retains the essential properties of the full-length receptor tyrosine kinase.