Integrin function: molecular hierarchies of cytoskeletal and signaling molecules.

Abstract

Abstract. Integrin receptors play important roles in or-ganizing the actin-containing cytoskeleton and in signal transduction from the extracellular matrix. The initial steps in integrin function can be analyzed experimentally using beads coated with ligands or anti-integrin antibodies to trigger rapid focal transmembrane responses. A hierarchy of transmembrane actions was identified in this study. Simple integrin aggregation triggered localized transmembrane accumulation of 20 signal transduction molecules, including RhoA, Racl, Ras, Raf, MEK, ERK, and JNK. In contrast, out of eight cytoskeletal molecules tested, only tensin coaccumulated. Integrin aggregation alone was also sufficient to induce rapid activation of the JNK pathway, with kinetics of activation different from those of ERK. The tyrosine kinase inhibitors herbimycin A or genistein blocked both the accumulation of 19 out of 20 signal transduction molecules and JNK-and ERK-mediated signaling. Cytochalasin D had identical effects, whereas three other tyrosine kinase inhibitors did not. The sole exception among signaling molecules was the kinase pp125 FAK which continued to coaggregate with a5131 integrins even in the presence of these inhibitors. Tyrosine kinase inhibition also failed to block the ability of ligand occupancy plus integrin aggregation to trigger transmembrane accumulation of the three cytoskeletal molecules talin, et-actinin, and vinculin; these molecules accumulated even in the presence of cytochalasin D. However, it was necessary to fulfill all four conditions, i.e., integrin aggregation, integrin occupancy, tyrosine kinase activity, and actin cytoskeletal integrity, to achieve integrin-mediated focal accumulation of other cytoskeletal molecules including F-actin and paxillin. Integrins therefore mediate a transmembrane hierarchy of molecular responses.I NTEGRIN receptors for extracellular matrix molecules play central and complex roles in cell interactions. They mediate cell adhesion, migration, and invasion, but they also have a multitude of intracellular effects on the organization of the actin-containing cytoskeleton as well as roles in a variety of signaling processes (for reviews see Hynes, 1992;Sastry and Horwitz, 1993;Juliano and Haskill, 1993;Gumbiner, 1993;Pavalko and Otey, 1994;Schaller and Parsons, 1994;Shattil et al., 1994a;Clark and Brugge, 1995). A complex series of steps leads from initial integrin interactions with an extracellular ligand to transmembrane effects on the localization of cytoskeletal molecules or signaling molecules, to the activation of signaling pathways, and to eventual regulation of gene expression. Identifying distinct mechanisms of integrin responses to extracellular stimuli and patterns in the classes of responding molecules will be crucial for understanding how integrins function.

Syndecans: multifunctional cell-surface co-receptors

“…Dimerization is a well-known mechanism for activation of receptor kinases [62]. Intramembrane clustering has also been shown to be important for activation of downstream activities of non-catalytic receptors [63,64] and cell-adhesion molecules [65][66][67][68].…”

“…The strength of adhesive interactions would also be enhanced by clustering of receptors in the membrane and stabilized by syndecan-cytoskeleton association (Figure 3b). Integrin binding and clustering are also linked to transduction of extracellular-matrix-dependent signalling events through activation of focal adhesion kinase and other pathways, and recruitment of cytoskeletal proteins to the membrane [68,115,116]. It is likely that syndecans contribute unique functional activities to the process of cell-matrix adhesion.…”

rsc: a novel oncogene with structural and functional homology with the gene family of exchange factors for Ral

“…The beads were washed four times with lysis bu er minus protease and phosphatase inhibitors and twice with kinase bu er (50 mM Tris pH 7.5, 10 mM MgCl 2 ). The kinase reaction was conducted by adding 25 ml of kinase bu er containing 5 mCi of [g 32 P]ATP (New England Nuclear) and 25 mg of Myelin Basic Protein (Sigma) to the beads, and incubating at 308C for 30 min (Miyamoto et al, 1995). The reaction was stopped by boiling the samples in loading bu er for 2 min.…”

Correlation between the tumoral expression of β3-integrin and outcome in cervical cancer patients who had undergone radiotherapy

“…The cytoplasmic tails of the b-subunit contain two highly conserved motifs (NPXY and NXXY), which play a major role in the regulation of integrin-mediated functions (Dedhar and Hannigan, 1996). At least 20 different signalling proteins can associate with activated integrin receptors (Miyamoto et al, 1995). These cooperative interactions set in train signalling cascades, which are involved in cell differentiation, adhesion, migration, invasion, proliferation, angiogenesis and survival (for reviews, see Varner and Cheresh, 1996;Dedhar, 1999;Brakebusch et al, 2002).…”