Signal transduction pathways involved in PDGF receptor-induced cellular responses

Abstract: Platelet-derived growth factor (PDGF), is a potent connective tissue cell mitogen and chemoattractant which binds to two structurally related receptors denoted PDGF α- and β- receptors. Upon binding of PDGF to the receptor, signaling molecules containing Src Homology 2 (SH2) domains are recruited to the activated receptor and mediate propagation of the signal. This thesis describes several such interactions and the resulting biological responses.The cytoplasmic tyrosine kinase Src has been implicated in mitogenic signaling induced by PDGF. However, PDGF α-receptors containing mutations of two tyrosine residues responsible for Src binding, could still mediate mitogenic-responses, despite decreased Src - PDGF α-receptor association.The lipid kinase phosphoinositide 3-kinase (PI3-kinase) has been reported to mediate reorganization of the cytoskeleton and to induce cell migration upon stimulation of cells with PDGF. Rac, a member of the Rho family of monomeric GTPases, acts downstream of PI3-K in reorganization of the cytoskeleton. By expression of dominant-negative or constitutively active mutants of Rac, and treatment of cells with wortmannin, a drug that inhibits PI3-K activity, the involvement of PI3-K and Rac in cell mobility could be established.PI3-K consists of two subunits, each of which exists in several isoforms. To identify the role of two specific isoforms of the catalytic PI3-K subunits in PDGF and insulin signaling, the effect of microinjection of neutralizing antibodies was studied. This approach showed that p110 a is required for PDGF-induced cytoskeletal changes, whereas p110β is not. In contrast, insulin-induced cytoskeletal changes involved p110β but not p110α.The SH2 domain-containing adaptor molecule Shb has been shown to be involved in signaling leading to programmed cell death, apoptosis. Shb could bind to both PDGF α- and β-receptors via its SH2 domain. Residue 720 in the kinase insert of the PDGF α-receptor was involved in this interaction. Overexpression of Shb led to increased apoptosis in cells cultured in low serum; treatment with PDGF-BB, but not PDGF-AA. rescued cells from apoptosis in a manner dependent on a functional Shb SH2 domain. Furthermore, the pattern of PDGF-induced cytoskeletal changes was changed in cells overexpressing Shb.