Engagement of integrin receptors with extracellular ligands gives rise to the formation of complex multiprotein structures that link the ECM to the cytoplasmic actin cytoskeleton. These adhesive complexes are dynamic, often heterogeneous structures, varying in size and organization. In motile cells, sites of adhesion within ®lopodia and lamellipodia are relatively small and transient and are referred to as`focal complexes,' whereas adhesions underlying the body of the cell and localized to the ends of actin stress ®bers are referred to as`focal adhesions'. Signal transduction through focal complexes and focal adhesions has been implicated in the regulation of a number of key cellular processes, including growth factor induced mitogenic signals, cell survival and cell locomotion. The formation and remodeling of focal contacts is a dynamic process under the regulation of protein tyrosine kinases and small GTPases of the Rho family. In this review, we consider the role of the focal complex associated protein tyrosine kinase, Focal Adhesion Kinase (FAK), in the regulation of cell movement with the emphasis on how FAK regulates the¯ow of signals from the ECM to the actin cytoskeleton. Oncogene (2000) 19, 5606 ± 5613.

Utilizing mutants of extracellular signal-regulated kinase 2 (ERK2) that are defective for intrinsic mitogenactivated protein kinase or ERK kinase (MEK) binding, we have identified a convergent signaling pathway that facilitates regulated MEK-ERK association and ERK activation. ERK2-⌬19-25 mutants defective in MEK binding could be phosphorylated in response to mitogens; however, signaling from the Raf-MEK pathway alone was insufficient to stimulate their phosphorylation in COS-1 cells. Phosphorylation of ERK2-⌬19-25 but not of wild-type ERK2 in response to Ras V12 was greatly inhibited by dominant-negative Rac. Activated forms of Rac and Cdc42 could enhance the association of wild-type ERK2 with MEK1 but not with MEK2 in serumstarved adherent cells. This effect was p21-activated kinase (PAK) dependent and required the putative PAK phosphorylation sites T292 and S298 of MEK1. In detached cells placed in suspension, ERK2 was complexed with MEK2 but not with MEK1. However, upon replating of cells onto a fibronectin matrix, there was a substantial induction of MEK1-ERK2 association and ERK activation, both of which could be inhibited by dominant-negative PAK1. These data show that Rac facilitates the assembly of a mitogen-activated protein kinase signaling complex required for ERK activation and that this facilitative signaling pathway is active during adhesion to the extracellular matrix. These findings reveal a novel mechanism by which adhesion and growth factor signals are integrated during ERK activation.The extracellular signal-regulated protein kinases (ERKs) are ubiquitous protein kinases that function downstream of the ras oncogene and are involved in cell proliferation, movement, and differentiation (35). Ras is activated in response to agonist stimulation and recruits Raf protein kinases to the plasma membrane (28,34,45), where Raf undergoes activation by a mechanism that is incompletely understood. Raf proteins phosphorylate and activate mitogen-activated protein kinase or ERK kinases (MEKs), which phosphorylate the two ERK proteins, ERK1 and ERK2, on a TEY sequence in the catalytic domain, resulting in ERK activation (24, 37). MEKs can serve as cytoplasmic anchors for the ERKs through a direct binding interaction, holding the ERKs in the cytoplasm at times when the signaling pathway is inactive (21). Stimulation of the pathway results in the phosphorylation of the ERKs and their dimerization (26) and translocation to the nucleus and other sites of action (10,23,29), where they phosphorylate the various substrates required for biological responses. Sustained ERK activation and nuclear translocation are required for S-phase entry and proliferation of fibroblasts (6, 36), and both aspects are dependent upon the integration of adhesion and growth factor signals (2, 40, 47). However, the mechanisms by which adhesion and growth factor signals are integrated to modify ERK function are presently unclear.MEK proteins bind directly to the ERKs through a region in the N terminus of MEK (3,4,21). However, other sequ...

Focal adhesion kinase (FAK) has been implicated in the regulation of cell migration. In addition, FAK expression is increased in a number of highly metastatic tumor cell lines. Therefore, we investigated the role of FAK in regulating migration of prostate carcinoma cell lines with increasing metastatic potential. We show that highly tumorigenic PC3 and DU145 cells exhibit intrinsic migratory capacity, while poorly tumorigenic LNCaP cells require a stimulus to migrate. Increased metastatic potential of PC3 and DU145 cells correlates with increased FAK expression, overall tyrosine phosphorylation and activity, as measured by autophosphorylation of tyrosine 397. However, in PC3 and DU145 cells, FAK autophosphorylation is adhesion dependent whereas a second site of tyrosine phosphorylation, tyrosine 861, a Src speci®c site, is uncoupled from adhesion-dependent signaling events. Finally, inhibiting the FAK/Src signal transduction pathway by over expressing FRNK (Focal adhesion kinase-Related Non-Kinase), an inhibitor of FAK activation, or treatment with PP2, a Src family kinase inhibitor, signi®cantly inhibited migration of prostate carcinoma cell lines, demonstrating that tumor cell migration continues to be dependent on signals emanating from this pathway.

Integrins are transmembrane proteins that serve as primary sensors of the extracellular matrix (ECM) environment. In response to interactions with the ECM, integrins initiate signaling pathways that regulate cell migration, growth, and survival. Advances in imaging have contributed to the understanding of the dynamic nature of these cell-ECM interactions and the complexes that form at these sites and have provided insights into their regulation and signal organizing functions.

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