Vav2 activates Rac1, Cdc42, and RhoA downstream from growth factor receptors but not beta1 integrins.
Liu, B P and Burridge, K
Mol. Cell. Biol., 20: 7160-9 (2000)
1999
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The Rho family of GTPases plays a major role in the organization of the actin cytoskeleton. These G proteins are activated by guanine nucleotide exchange factors that stimulate the exchange of bound GDP for GTP. In their GTP-bound state, these G proteins interact with downstream effectors. Vav2 is an exchange factor for Rho family GTPases. It is a ubiquitously expressed homologue of Vav1, and like Vav1, it has previously been shown to be activated by tyrosine phosphorylation. Because Vav1 becomes tyrosine phosphorylated and activated following integrin engagement in hematopoietic cells, we investigated the tyrosine phosphorylation of Vav2 in response to integrin-mediated adhesion in fibroblasts and epithelial cells. However, no tyrosine phosphorylation of Vav2 was detected in response to integrin engagement. In contrast, treating cells with either epidermal growth factor or platelet-derived growth factor stimulated tyrosine phosphorylation of Vav2. We have examined the effects of overexpressing either wild-type or amino-terminally truncated (constitutively active) forms of Vav2 as fusion proteins with green fluorescent protein. Overexpression of either wild-type or constitutively active Vav2 resulted in prominent membrane ruffles and enhanced stress fibers. These cells revealed elevated rates of cell migration that were inhibited by expression of dominant negative forms of Rac1 and Cdc42. Using a binding assay to measure the activity of Rac1, Cdc42, and RhoA, we found that overexpression of Vav2 resulted in increased activity of each of these G proteins. Expression of a carboxy-terminal fragment of Vav2 decreased the elevation of Rac1 activity induced by epidermal growth factor, consistent with Vav2 mediating activation of Rac1 downstream from growth factor receptors. | 10982832
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Signaling through focal adhesion kinase.
Schlaepfer, D D, et al.
Prog. Biophys. Mol. Biol., 71: 435-78 (1999)
1998
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Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK-) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK- cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors. | 10354709
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