Specific signaling through heteromeric PDGF receptor complexes

Platelet-derived growth factor (PDGF) is a potent mitogen and chemoattractant for mesenchymal cells and exert its effect by binding to two structurally related receptor tyrosine kinases, denoted α- and β-receptors. PDGF binding induces dimerization of its receptors, both homo-and heterodimerization, leading to their autophosphorylation on tyrosine residues and binding of downstream signaling molecules. This thesis describes autophosphorylation and binding of signal transduction molecules to homo- and heterodimeric PDGF receptor complexes. Heterodimeric PDGF receptor complexes have been found to mediate a stronger mitogenic response than homodimeric receptor complexes. It was found that Tyr771 in the PDGF β-receptor was significantly less phosphorylated in the heterodimeric β-receptor compared to the homodimeric receptor, and this correlated with reduced binding of GTPase activating protein (GAP) for Ras and decreased activation of the Ras/Mitogen activated protein kinase pathway. The mechanism behind the lowered phosphorylation of Tyr771 in the heterodimeric PDGF β-receptor was investigated. It was found that the SH2 domain-containing tyrosine phosphatase SHP-2 was responsible, at least in part, for the dephosphorylation of Tyr771 in the heterodimeric β-receptor. PDGF-induced autophosphorylation of tyrosine residues in the receptors has been proposed to occur in trans between the receptor molecules in the dimers. We demonstrated by phosphopeptide mapping that all major autophosphorylation sites can be phosphorylated in trans, both in the PDGF α- and β-receptors. Analyses of the abilities of heterodimeric receptor complexes of one kinase-active and one kinase-inactive receptor to mediate mitogenicity, chemotaxis and activation of mitogen activated protein kinase revealed that the signaling capacities were retained. This illustrates a functional co-operation between the two receptor molecules in the dimer, where one receptor provides a functional kinase and the other acts as a substrate and provides docking sites for downstream signaling molecules. Elucidating the mechanisms behind the unique signaling properties of the heterodimeric PDGF receptor complex, two heterodimer-specific autophosphorylation sites, Tyr692 and Tyr970, were identified and found to interact with the low molecular weight protein tyrosine phosphatase (LMW-PTP). Mutation of Tyr692 or Tyr970 to phenylalanine residues did not affect PDGF-induced mitogenicity, but the Tyr692 to phenylalanine mutation reduced the chemotactic response mediated by the heterodimeric PDGF receptor complex. A mechanism for the lowered chemotactic response was found to involve an increased RasGAP binding and a decreased SHP-2 binding to the heterodimeric β-receptor