Effect of receptor kinase inactivation on the rate of internalization and degradation of PDGF and the PDGF beta-receptor

The complementary DNAs for wildtype and tyrosine kinase-inactivated (K634A) forms of the PDGF beta-receptor were expressed in porcine aortic endothelial cells. We examined the internalization and degradation of ligands and receptors after exposure of receptor expressing cells to PDGF-BB, which binds to the beta-receptor with high affinity, and PDGF-AB, which binds with lower affinity. Cells expressing wildtype beta-receptors were able to internalize and degrade the receptor, as well as the ligand, after exposure to PDGF-BB or -AB. Cells expressing the kinase-inactivated mutant receptor also internalized and degraded both receptor and ligand, but with lower efficiency compared with the wildtype receptor cells. The degradation of either form of receptor was inhibited by treatment of the cells with the lysosomotropic drug chloroquine. Exposure of wildtype and K634A receptor expressing cells to PDGF-AB resulted in a twofold slower rate of internalization of this ligand as compared with PDGF-BB, whereas the relative rate of degradation was similar for the two ligands. Our data indicate that tyrosine kinase activity promotes, but is not a prerequisite for, ligand-induced internalization and degradation of the ligand-receptor complex.