Compartmentalization of autocrine signal transduction pathways in Sis-transformed NIH 3T3 cells

The transforming protein of simian sarcoma virus is homologous to the platelet-derived growth factor (PDGF) B-chain. Fibroblasts transformed with simian sarcoma virus constitutively produce a growth factor that stimulates the endogenous tyrosine kinase of PDGF receptors in an autocrine manner. Autophosphorylation of PDGF receptors upon ligand stimulation provides binding sites for Src homology 2 domains of intracellular signaling molecules, which thereby become activated. We have characterized the PDGF receptor-mediated signal transduction in NIH 3T3 cells transformed with a PDGF B-chain cDNA (Sis 3T3 cells) in the absence and presence of suramin, a polyanionic compound that quenches PDGF-induced mitogenicity and reverts the transformed phenotype of the Sis 3T3 cells. Our data show that in the presence of suramin the general level of tyrosine phosphorylation was decreased. Nevertheless, autophosphorylated receptors complexed with substrates persisted in the cells. Suramin had no effect on activation of phosphatidylinositol 3'-kinase or on tyrosine phosphorylation of phospholipase C-gamma and GTPase-activating protein of Ras. On the other hand, kinase activation of Src and Raf-1, phosphorylation of protein-tyrosine phosphatase 1D/Syp and Shc, and complex formation with Grb2 were greatly diminished by suramin. A possible explanation for our findings is that different PDGF receptor-coupled signaling pathways are active in different structural or functional compartments in the cell. Those pathways that are not affected by suramin might elicit distinct cellular responses, which are not sufficient for growth and transformation.