Showing papers by "Joan S. Brugge published in 1998"

Integrin-mediated Signals Regulated by Members of the Rho Family of GTPases

Abstract: The organization of the actin cytoskeleton can be regulated by soluble factors that trigger signal transduction events involving the Rho family of GTPases. Since adhesive interactions are also capable of organizing the actin-based cytoskeleton, we examined the role of Cdc42-, Rac-, and Rho-dependent signaling pathways in regulating the cytoskeleton during integrin-mediated adhesion and cell spreading using dominant-inhibitory mutants of these GTPases. When Rat1 cells initially adhere to the extracellular matrix protein fibronectin, punctate focal complexes form at the cell periphery. Concomitant with focal complex formation, we observed some phosphorylation of the focal adhesion kinase (FAK) and Src, which occurred independently of Rho family GTPases. However, subsequent phosphorylation of FAK and paxillin occurs in a Rho-dependent manner. Moreover, we found Rho dependence of the assembly of large focal adhesions from which actin stress fibers radiate. Initial adhesion to fibronectin also stimulates membrane ruffling; we show that this ruffling is independent of Rho but is dependent on both Cdc42 and Rac. Furthermore, we observed that Cdc42 controls the integrin-dependent activation of extracellular signal-regulated kinase 2 and of Akt, a kinase whose activity has been demonstrated to be dependent on phosphatidylinositol (PI) 3-kinase. Since Rac-dependent membrane ruffling can be stimulated by PI 3-kinase, it appears that Cdc42, PI 3-kinase, and Rac lie on a distinct pathway that regulates adhesion-induced membrane ruffling. In contrast to the differential regulation of integrin-mediated signaling by Cdc42, Rac, and Rho, we observed that all three GTPases regulate cell spreading, an event that may indirectly control cellular architecture. Therefore, several separable signaling pathways regulated by different members of the Rho family of GTPases converge to control adhesion-dependent changes in the organization of the cytoskeleton, changes that regulate cell morphology and behavior.

Casting light on focal adhesions.

Abstract: Oncogenic transformation by v-Src is associated with dramatic alterations in cell morphology, actin organization and anchorage-dependence. These changes correlate with tyrosine phosphorylation of multiple proteins that associate with cytoskeletal structures known as focal adhesions (FA) — membrane-associated sites where clustered integrin receptors couple extracellular matrix (ECM) proteins to intracellular cytoskeletal proteins and actin stress fibers (Fig. 1). Although the structural components of FAs are well-defined, the specific roles of individual Src substrates in integrin-mediated cytoskeletal rearrangements or Src-mediated transformation remain elusive. On page 361 of this issue, Hisamaru Hirai and coworkers demonstrate that the FA-associated protein p130Cas (or Cas, which is encoded by Crkas), is essential for myofibril organization in cardiac muscle, actin organization of stress fibers in cultured fibroblasts, and Src-induced morphological transformation and anchorage-independence. These studies implicate Cas in the regulation of actin organization and highlight the structural and functional similarities of FAs and muscle Z discs (to which actin fibers attach). In addition, they suggest that the role of Cas in Src transformation may involve actin organization.