Platelet-derived growth factor (PDGF) can cause the migration of fibroblasts. In order for the cells to move, there are major changes in the actin cytoskeleton: decreased stress fibers and focal adhesion sites, redistribution of actin to the cell cortex, and the formation of lamellipodia and filopodia. Jiménez et al. studied the mechanism by which PDGF signals to the actin cytoskeleton using transfected or microinjected cells. Transfection of dominant negative forms of the small guanosine trisphosphatases (GTPases) Rac or Cdc42 suggested that the PDGF receptor (PDGFR) couples to both of these GTPases to produce different effects on the cytoskeleton: Rac stimulates lamellipodia, Cdc42 stimulates stress fiber disassembly, filopodia, and decreased focal adhesion complexes. Transfection of cells with activated forms of Cdc42 or treatment of cells (transfected with Myc-tagged Cdc42) with PDGF stimulated a Cdc42-associated phosphatidylinositol 3-kinase (PI3K) activity. However, when cells were transfected with the regulatory subunit (p85) or a constitutively active catalytic subunit of PI3K, different phenotypes (decreased stress fibers and focal contacts for p85 subunit-expressing cells and increased formation of lamellipodia for catalytic subunit-expressing cells) were observed. Pharmacological inhibition of PI3K activity did not block the Cdc42-mediated cytoskeletal changes. Analysis of cells expressing mutants containing only the Cdc42 interaction domain or a p85 isoform that cannot bind Cdc42 suggests that the p85 subunit directly interacts with Cdc42 to mediate these cytoskeletal changes. Mutants of the PDGFR lacking critical residues for p85 interaction were also unable to respond to PDGF with Cdc42-mediated cytoskeletal rearrangements. These two pieces of evidence suggest that p85 must interact with Cdc42 and the PDGFR to mediate the Cdc42-controlled actin rearrangements. A final piece to the puzzle fell into place when N-WASP mutants blocked the Cdc42- and p85-induced actin rearrangements and inhibited PDGF-stimulated cell migration. Thus, the PDGF stimulates Cdc42 activity by a pathway that involves N-WASP and the p85 subunit of the PI3K, but is independent of the catalytic activity of PI3K.
Jiménez, C., Portela, R.A., Mellado, M., Rodríguez-Frade, J.M., Collard, J., Serrano, A., Martínez-A., C., Avila, J., and Carrera, A.C. (2000) Role of the PI3K regulatory subunit in the control of actin organization and cell migration. J. Cell Biol. 151: 249-261. [Abstract] [Full Text]