INTRODUCTION Platelet-derived growth factor (PDGF), a mitogen for mesenchymal cells, is a homo- or heterodimeric protein composed of disulfide-bonded A- and B-polypeptide chains (PDGF-AA, PDGF-BB and PDGF-AB). The PDGF isoforms elicit their effects on target cells by binding to two related tyrosine kinase receptors denoted α- and β-receptors (reviewed by Claesson-Welsh, 1994). The binding of PDGF induces receptor dimerization leading to activation of the intrinsic tyrosine kinase activity of the receptors. The kinase activity is exerted both on the receptor itself and on intracellular substrates. PDGF has been assigned in vivo functions in tissue repair and in embryonal development as well as in pathological processes, such as formation of atherosclerotic plaques (reviewed by Heldin and Westermark, 1996). In vitro, PDGF stimulates cell division, reorganization of actin microfilaments and chemotaxis of fibroblasts, smooth muscle cells and phagocytic cells. An essential feature of the chemotactic response is the formation of lamellipodia with membrane ruffling extending in the direction of movement. In adherent cells, membrane ruffling which is caused by reorganization of the actin filament system, appears in response to different growth factors (Ridley et al., 1992). Another aspect of actin reorganization is the formation of circular membrane ruffles, which is a transient response transduced by the PDGF β- receptor (Mellström et al., 1988). Previous studies have indicated a role for phosphatidylinositol 3′-kinase (PI3-kinase) in PDGF-induced actin reorganization and cell motility responses (Kundra et al., 1994; Wennström et al., 1994a,b). In addition, the small GTP-binding protein Rac, is a major effector of PI3-kinase in this pathway (Hawkins et al., 1995; Nobes et al., 1995). Thus, microinjection of dominant negative Rac1 (N17Rac1) has been shown to abolish PDGF-induced membrane ruffling, while injection of constitutively active 1973 Journal of Cell Science 111, 1973-1980 (1998) Printed in Great Britain © The Company of Biologists Limited 1998 JCS4567 In this study we demonstrate that stimulation with platelet- derived growth factor (PDGF) leads to a marked reorganization of the vimentin filaments in porcine aortic endothelial (PAE) cells ectopically expressing the PDGF β- receptor. Within 20 minutes after stimulation, the well- spread fine fibrillar vimentin was reorganized as the filaments aggregated into a dense coil around the nucleus. The solubility of vimentin upon Nonidet-P40-extraction of cells decreased considerably after PDGF stimulation, indicating that PDGF caused a redistribution of vimentin to a less soluble compartment. In addition, an increased tyrosine phosphorylation of vimentin was observed. The redistribution of vimentin was not a direct consequence of its tyrosine phosphorylation, since treatment of cells with an inhibitor for the cytoplasmic tyrosine kinase Src, attenuated phosphorylation but not redistribution of vimentin. These changes in the distribution of vimentin occurred in conjunction with reorganization of actin filaments. In PAE cells expressing a Y740/751F mutant receptor that is unable to bind and activate phosphatidylinositol 3′-kinase (PI3-kinase), the distribution of vimentin was virtually unaffected by PDGF stimulation. Thus, PI3-kinase is important for vimentin reorganization, in addition to its previously demonstrated role in actin reorganization. The small GTPase Rac has previously been shown to be involved downstream of PI3- kinase in the reorganization of actin filaments. In PAE cells overexpressing dominant negative Rac1 (N17Rac1), no change in the fine fibrillar vimentin network was seen after PDGF-BB stimulation, whereas in PAE cells overexpressing constitutively active Rac1 (V12Rac1), there was a dramatic change in vimentin filament organization independent of PDGF stimulation. These data indicate that PDGF causes a reorganization of microfilaments as well as intermediate filaments in its target cells and suggest an important role for Rac downstream of PI3-kinase in the PDGF stimulated reorganization of both actin and vimentin filaments. Key words: Platelet-derived growth factor (PDGF), Vimentin, Phosphatidylinositol 3′-kinase (PI3-kinase), Rac1, Src SUMMARY PDGF induces reorganization of vimentin filaments Sigrídur Valgeirsdóttir 1, *, Lena Claesson-Welsh 2 , Erik Bongcam-Rudloff 3 , Ulf Hellman 1 , Bengt Westermark 4 and Carl-Henrik Heldin 1 1 Ludwig Institute for Cancer Research, Biomedical Center, Uppsala, Sweden 2 Department of Medical Biochemistry and Microbiology, Biomedical Center, Uppsala, Sweden 3 Department of Cell Biology, The Wenner-Gren Institute, Stockholm, Sweden 4 Department of Pathology, Uppsala, Sweden *Author for correspondence Accepted 11 May; published on WWW 30 June 1998