Interactions between cells and the extracellular matrix coordinate signaling pathways that control various aspects of cellular behavior. Integrins sense the physical properties of the extracellular matrix and organize the cytoskeleton accordingly. In turn, this modulates signaling pathways that are triggered by various other transmembrane receptors and augments the cellular response to growth factors. Over the past years, it has become clear that there is extensive crosstalk between integrins, Src-family kinases and Rho-family GTPases at the heart of such adhesion signaling. In this Commentary, we discuss recent advances in our understanding of the dynamic regulation of the molecular connections between these three protein families. We also discuss how this signaling network can regulate a range of cellular processes that are important for normal tissue function and disease, including cell adhesion, spreading, migration and mechanotransduction.

We have studied the formation of different types of cell matrix adhesions in cells that bind to fibronectin via either α5β1 or αvβ3. In both cases, cell adhesion to fibronectin leads to a rapid decrease in RhoA activity. However, α5β1 but not αvβ3 supports high levels of RhoA activity at later stages of cell spreading, which are associated with a translocation of focal contacts to peripheral cell protrusions, recruitment of tensin into fibrillar adhesions, and fibronectin fibrillogenesis. Expression of an activated mutant of RhoA stimulates αvβ3-mediated fibrillogenesis. Despite the fact that α5β1-mediated adhesion to the central cell-binding domain of fibronectin supports activation of RhoA, other regions of fibronectin are required for the development of α5β1-mediated but not αvβ3-mediated focal contacts. Using chimeras of β1 and β3 subunits, we find that the extracellular domain of β1 controls RhoA activity. By expressing both β1 and β3 at high levels, we show that β1-mediated control of the levels of β3 is important for the distribution of focal contacts. Our findings demonstrate that the pattern of fibronectin receptors expressed on a cell dictates the ability of fibronectin to stimulate RhoA-mediated organization of cell matrix adhesions.

Cell proliferation is controlled not only by soluble mitogens but also by components of the extracellular matrix (ECM) such as ®bronectin, to which cells adhere via the integrin family of transmembrane receptors. Input from both growth factor receptors and integrins is required to stimulate progression through the G1 phase of the cell cycle, via induction of G1 cyclins and suppression of inhibitors of the G1 cyclin-dependent kinases. Extensive crosstalk takes place between integrin and growth factor receptor signaling pathways, and mitogenic signaling is weak and transient in the absence of integrin-mediated cell adhesion. In normal untransformed cells, all of the important mitogenic signal transduction cascades, namely those downstream of the Ras and Rho family small GTPases and the phosphoinositide 3-OH kinase-PKB/Akt pathway, are regulated by integrinmediated cell adhesion. As a result, these cells are anchorage-dependent for growth. In contrast, constitutive activity of each of these pathways has been reported in cancer cells, which not only reduces their mitogen dependence but also allows these cells to grow in an anchorage-independent fashion.

Cell migration is modulated by regulatory molecules such as growth factors, oncogenes, and the tumor suppressor PTEN. We previously described inhibition of cell migration by PTEN and restoration of motility by focal adhesion kinase (FAK) and p130 Crk-associated substrate (p130Cas). We now report a novel pathway regulating random cell motility involving Shc and mitogen-activated protein (MAP) kinase, which is downmodulated by PTEN and additive to a FAK pathway regulating directional migration. Overexpression of Shc or constitutively activated MEK1 in PTEN- reconstituted U87-MG cells stimulated integrin- mediated MAP kinase activation and cell migration. Conversely, overexpression of dominant negative Shc inhibited cell migration; Akt appeared uninvolved. PTEN directly dephosphorylated Shc. The migration induced by FAK or p130Cas was directionally persistent and involved extensive organization of actin microfilaments and focal adhesions. In contrast, Shc or MEK1 induced a random type of motility associated with less actin cytoskeletal and focal adhesion organization. These results identify two distinct, additive pathways regulating cell migration that are downregulated by tumor suppressor PTEN: one involves Shc, a MAP kinase pathway, and random migration, whereas the other involves FAK, p130Cas, more extensive actin cytoskeletal organization, focal contacts, and directionally persistent cell motility. Integration of these pathways provides an intracellular mechanism for regulating the speed and the directionality of cell migration.