Integrin-dependent control of inositol lipid synthesis in vascular endothelial cells and smooth muscle cells.

Extracellular matrix (ECM) molecules, such as fibronectin (FN), regulate fibroblast sensitivity to soluble growth factors, in part, by controlling cellular levels of phosphatidylinositol bis-phosphate (PIP2), the substrate for phospholipase C-gamma (McNamee et al., 1993, J. Cell Biol. 121, 673-678). In the present study, we extended these investigations by exploring whether cells of the vascular wall also exhibit this response and analyzing the mechanism by which adhesion to ECM regulates intracellular PIP2 mass. Capillary endothelial cells, pulmonary vascular smooth muscle cells, and C3H 101/2 fibroblasts were all found to exhibit a similar two- to threefold increase in PIP2 mass within 3 h after binding to dishes coated with FN. Furthermore, similar effects were observed using dishes coated with a variety of different ECM molecules, including collagen types I and IV as well as a synthetic RGD-containing peptide. An increase in PIP2 mass also was produced when suspended cells bound to microbeads (4.5 micron diameter; coated with RGD-peptide or anti-integrin beta 1 antibody) that induce local integrin clustering and focal adhesion formation, independently of cell spreading. In contrast, neither binding of soluble FN nor binding of microbeads coated with ligands for other transmembrane surface receptors (e.g., acetylated low-density lipoprotein, antibodies against heparan sulfate) had any effect on PIP2 mass. While these results suggest that integrin clustering stimulates PIP2 synthesis, no change in total cellular or cytoskeletal-associated phosphatidylinositol-4-phosphate kinase (PIP kinase) activity could be detected when cells bound to immobilized integrin ligands. However, when focal adhesion complexes were isolated from these cells using a magnetic procedure (G. Plopper and D. E. Ingber, 1993, Biochem. Biophys. Res. Commun. 193, 571-578), this subfraction of the cytoskeleton was found to be enriched for PIP kinase activity by more than twofold relative to the whole cytoskeleton. These data suggest that ECM binding may increase PIP2 mass in vascular cells by clustering cell surface integrin receptors and activating cytoskeletal-associated PIP kinases locally within the focal adhesion complex.

Convergence of integrin and growth factor receptor signaling pathways within the focal adhesion complex.

Extracellular matrix controls capillary endothelial cell sensitivity to soluble mitogens by binding integrin receptors and thereby activating a chemical signaling response that rapidly integrates with growth factor-induced signaling mechanisms. Here we report that in addition to integrins, growth factor receptors and multiple molecules that transduce signals conveyed by both types of receptors are immobilized on the cytoskeleton (CSK) and spatially integrated within the focal adhesion complex (FAC) at the site of integrin binding. FACs were rapidly induced in round cells and physically isolated from the remainder of the CSK after detergent-extraction using magnetic microbeads coated with fibronectin or a synthetic RGD-containing peptide. Immunofluorescence microscopy revealed that multiple signaling molecules (e.g., pp60c-src, pp125FAK, phosphatidylinositol-3-kinase, phospholipase C-gamma, and Na+/H+ antiporter) involved in both integrin and growth factor receptor signaling pathways became associated with the CSK framework of the FAC within 15 min after binding to beads coated with integrin ligands. Recruitment of tyrosine kinases to the FAC was also accompanied by a local increase in tyrosine phosphorylation, as indicated by enhanced phosphotyrosine staining at the site of integrin binding. In contrast, neither recruitment of signaling molecules nor increased phosphotyrosine staining was observed when cells bound to beads coated with a control ligand (acetylated low density lipoprotein) that ligates transmembrane scavenger receptors, but does not induce FAC formation. Western blot analysis confirmed that FACs isolated using RGD-beads were enriched for pp60c-src, pp125FAK, phospholipase C-gamma, and the Na+/H+ antiporter when compared with intact CSK or basal cell surface preparations that retained lipid bilayer. Isolated FACs were also greatly enriched for the high affinity fibroblast growth factor receptor flg. Most importantly, isolated FACs continued to exhibit multiple chemical signaling activities in vitro, including protein tyrosine kinase activities (pp60c-src and pp125FAK) as well as the ability to undergo multiple sequential steps in the inositol lipid synthesis cascade. These data suggest that many of the chemical signaling events that are induced by integrins and growth factor receptors in capillary cells may effectively function in a "solid-state" on insoluble CSK scaffolds within the FAC and that the FAC may represent a major site for signal integration between these two regulatory pathways. Future investigations into the biochemical and biophysical basis of signal transduction may be facilitated by this method, which results in isolation of FACs that retain the CSK framework as well as multiple associated chemical signaling activities.

Adhesion to fibronectin stimulates inositol lipid synthesis and enhances PDGF-induced inositol lipid breakdown.

The aim of these experiments was to investigate whether inositol lipids might mediate some of the effects of extracellular matrix (ECM) on cellular form and functions. The lipid phosphatidylinositol bisphosphate (PIP2) plays a role in cytoskeletal regulation while its hydrolysis products, diacylglycerol and inositol triphosphate, serve as second messengers. We therefore measured the effect of adhesion to fibronectin (FN) on PIP2 and its hydrolysis products, in the presence and absence of the soluble mitogen PDGF. PDGF induced a threefold increase in release of water-soluble inositol phosphates in C3H 10T1/2 fibroblasts when cells were attached to FN, but had little effect in suspended cells. Suppression of inositol phosphate release in unattached cells was not due to dysfunction of the PDGF receptor or failure to activate phospholipase C-gamma; PDGF induced similar tyrosine phosphorylation of PLC-gamma under both conditions. By contrast, the total mass of phosphatidylinositol bisphosphate (PIP2), the substrate for PLC-gamma, was found to decrease by approximately 80% when cells were detached from their ECM attachments and placed in suspension in the absence of PDGF. PIP2 levels were restored when suspended cells were replated on FN, demonstrating that the effect was reversible. Furthermore, a dramatic increase in synthesis of PIP2 could be measured in cells within 2 min after reattachment to FN in the absence of PDGF. These results show that FN acts directly to stimulate PIP2 synthesis, and that it also enhances PIP2 hydrolysis in response to PDGF. The increase in PIP2 induced by adhesion may mediate some of the known effects of FN on cell shape and cytoskeletal organization, while regulation of inositol lipid hydrolysis may provide a means for integrating hormone- and ECM-dependent signaling pathways.